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12 Topics in Organometallic Chemistry Editorial Board: J.M. Brown · P.H. Dixneuf · A. Fürstner · L.S. Hegedus P. Hofmann · P. Knochel · S. Murai · M. Reetz · G. van Koten Palladium in Organic Synthesis Volume Editor: J. Tsuji Vol. 14, 2005 Metal Carbenes in Organic Synthesis Volume Editor: K. H. Dötz Vol. 13, 2004 Theoretical Aspects of Transition Metal Catalysis Volume Editor: G. Frenking Vol. 12, 2005 Ruthenium Catalysts and Fine Chemistry Volume Editors: C. Bruneau, P.H. Dixneuf Vol. 11, 2004 New Aspects of Zirconium Containing Organic Compounds Volume Editor: I. Marek Vol. 10, 2005 CVD Precursors Volume Editor: R. Fischer Vol. 9, 2005 Metallocenes in Regio- and Stereoselective Synthesis Volume Editor: T. Takahashi Vol. 8, 2005 Transition Metal Arene pp-Complexes in Organic Synthesis and Catalysis Volume Editor: E.P. Kündig Vol. 7, 2004 Organometallics in Process Chemistry Volume Editor: R.D. Larsen Vol. 6, 2004 Organolithiums in Enantioselective Synthesis Volume Editor: D.M. Hodgson Vol. 5, 2003 Organometallic Bonding and Reactivity: Fundamental Studies Volume Editors: J.M. Brown, P. Hofmann Vol. 4, 1999 Activation of Unreactive Bonds and Organic Synthesis Volume Editor: S. Murai Vol. 3, 1999 Lanthanides: Chemistry and Use in Organic Synthesis Volume Editor: S. Kobayashi Vol. 2, 1999 Alkene Metathesis in Organic Synthesis Volume Editor: A. Fürstner Vol. 1, 1998 Topics in Organometallic Chemistry Recently Published and Forthcoming Volumes Theorectical Aspects of Transition Metal Catalysis Volume Editor : G. Frenking With contributions by D. V. Deubel · G. Drudis-Sole · G. Frenking · A. Lledos · C. Loschen F. Maseras · A. Michalak · K. Morokuma · G. Musaev S. Sakaki · V. Staemmler · S. Tobisch · G. Ujaque · T. Ziegler 23 The series Topics in Organometallic Chemistry presents critical overviews of research results in organometal- lic chemistry, where new developments are having a significant influence on such diverse areas as organic synthesis, pharmaceutical research, biology, polymer research and materials science. Thus the scope of coverage includes a broad range of topics of pure and applied organometallic chemistry. Coverage is designed for a broad academic and industrial scientific readership starting at the graduate level, who want to be informed about new developments of progress and trends in this increasinly interdisciplinary field. Where appropriate, theoretical and mechanistic aspects are included in order to help the reader understand the underlying principles involved. The individual volumes are thematic and the contributions are invited by the volumes editors. In references Topics in Organometallic Chemistry is abbreviated Top. Organomet. 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Typesetting: Fotosatz-Service Köhler GmbH, Würzburg Production editor: Christiane Messerschmidt, Rheinau Cover: design & production GmbH, Heidelberg Printed on acid-free paper 02/3141 me – 5 4 3 2 1 0 Volume Editor Professor Dr. Gernot Frenking Philipps-Universität Marburg Fachbereich Chemie Lahnberge 35032 Marburg, Germany frenking@chemie.uni-marburg.de Editorial Board Prof. John M. Brown Dyson Perrins Laboratory South Parks Road Oxford OX1 3QY john.brown@chem.ox.ac.uk Prof. Alois Fürstner Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Mühlheim an der Ruhr, Germany fuerstner@mpi-muelheim.mpg.de Prof. Peter Hofmann Organisch-Chemisches Institut Universität Heidelberg Im Neuenheimer Feld 270 69120 Heidelberg, Germany ph@phindigo.oci.uni-heidelberg.de Prof. Gerard van Koten Department of Metal-Mediated Synthesis Debye Research Institute Utrecht University Padualaan 8 3584 CA Utrecht, The Netherlands vankoten@xray.chem.ruu.nl Prof. Manfred Reetz Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Mülheim an der Ruhr, Germany reetz@mpi.muelheim.mpg.de Prof. Pierre H. Dixneuf Campus de Beaulieu Université de Rennes 1 Av. du Gl Leclerc 35042 Rennes Cedex, France Pierre.Dixneuf@univ-rennes1.fr Prof. Louis S. Hegedus Department of Chemistry Colorado State University Fort Collins, Colorado 80523-1872, USA hegedus@lamar. colostate.edu Prof. Paul Knochel Fachbereich Chemie Ludwig-Maximilians-Universität Butenandstr. 5–13 Gebäude F 81377 München, Germany knoch@cup.uni-muenchen.de Prof. Shinji Murai Faculty of Engineering Department of Applied Chemistry Osaka University Yamadaoka 2-1, Suita-shi Osaka 565, Japan murai@chem.eng.osaka-u.ac.jp For all customers who have a standing order to Topics in Organometallic Chemistry, we offer the electronic version via SpringerLink free of charge. Please contact your librarian who can receive a password for free access to the full articles by registration at: springerlink.com If you do not have a subscription, you can still view the tables of contents of the volumes and the abstract of each article by going to the SpringerLink Homepage,clicking on “Browse by Online Libraries”, then “Chemical Sciences”, and finally choose Topics in Organometallic Chemistry. You will find information about the – Editorial Board – Aims and Scope – Instructions for Authors – Sample Contribution at springeronline.com using the search function. Topics in Organometallic Chemistry is also Available Electronically Preface It has been stated in the past that the search for new catalysts has more the character of an art than a science discipline. This is because there was usually more speculation than true knowledge about the reaction mechanisms of catalytic processes. Even the identity of the catalytically active species was frequently not known, which is the reason that systematic testing of all pos- sibly interesting compounds for catalytic reactions was carried out. This is costly and time consuming. The situation has changed in the last decade because much progress has been made in understanding the mechanisms of many catalytic reactions. Besides sophisticated experimental tools, quantum chemical calculations of transition states and reaction intermediates played a prominent role in gaining much better insight into the fundamentals of transition metal catalysis. Estimating solvent effects and the calculation of spectroscopic data are now routinely included in many theoretical studies. Although the designof new catalytically active species is still largely a trial- and-error process, modern research is guided by theoretical calculations in the search for new catalysts, which helps researchers to focus on more promis- ing compounds. The progress in quantum chemical method development has led to the present situation where theory and experiment are synergistically used in an unprecedented manner. In particular, the calculation of transition metal compounds is no longer a too-difficult task for quantum chemistry because efficient methods are available for dealing with many-electron atoms and with relativistic effects. The seven articles in this volume do not provide a comprehensive view of theoretical investigations of catalytic reactions, because the field has expanded already beyond the scope that can be covered in one book. The contributions written by experts in the field exemplarily demonstrate the strength but also the present limitations of quantum chemical methods for giving insights into the mechanism of transition-metal mediated reactions. Because the develop- ment of new theoretical methods is still a very active research area, much progress can be expected in the coming years. Marburg, Germany, February 2005 Gernot Frenking Preface Contents Transition Metal Catalyzed ss-Bond Activation and Formation Reactions D. G. Musaev · K. Morokuma . . . . . . . . . . . . . . . . . . . . . . . . . 1 Theoretical Studies of C-H ss-Bond Activation and Related Reactions by Transition-Metal Complexes S. Sakaki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Enantioselectivity in the Dihydroxylation of Alkenes by Osmium Complexes G. Drudis-Solé · G. Ujaque · F. Maseras · A. Lledós . . . . . . . . . . . . . 79 Organometallacycles as Intermediates in Oxygen-Transfer Reactions. Reality or Fiction? D. V. Deubel · C.Loschen · G. Frenking . . . . . . . . . . . . . . . . . . . . 109 Late Transition Metals as Homo- and Co-Polymerization Catalysts A. Michalak · T. Ziegler . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Co-Oligomerization of 1,3-Butadiene and Ethylene Promoted by Zerovalent ‘Bare’ Nickel Complexes S. Tobisch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 The Cluster Approach for the Adsorption of Small Molecules on Oxide Surfaces V. Staemmler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Author Index Volume 1-14 . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Transition Metal Catalyzed ss-Bond Activation and Formation Reactions Djamaladdin G. Musaev ( ) · Keiji Morokuma Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, 1515 Dickey Dr., Atlanta GA 30322, USA dmusaev@emory.edu, morokuma@emory.edu 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 The Role of the Lower-Lying Electron States of Transition Metal Cations in Oxidative Addition of the ss-Bonds (such as H-H, C-H and C-C) . . . . . . . 2 3 Role of Cooperative Effects in the Transition Metal Clusters . . . . . . . . . . 6 3.1 Reaction of Pt and Pd Metal Atoms with H2/CH4 Molecules . . . . . . . . . . . 7 3.2 Reaction of Pd2 and Pt2 Dimers with H2/CH4 Molecules . . . . . . . . . . . . . 9 4 ss-Bond Activation via Nucleophilic Mechanism: the Role of Redox Activity of the Transition Metal Center – Hydrocarbon Hydroxylation by Methanemonooxygenase (MMO) . . . . . . . . . . . . . . . . . . . . . . . 10 5 Vinyl-Vinyl Coupling on Late Transition Metals Through C-C Reductive Elimination Mechanism . . . . . . . . . . . . . . . . 17 5.1 Reductive Elimination from PtIV Halogen Complexes [Pt(CH=CH2)2X4]2– (X=Cl, Br, I) . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2 Reductive Elimination from Mixed PtIV Complexes [Pt{cis-/trans-(CH=CH2)2(PH3)2}Cl2] . . . . . . . . . . . . . . . . . . . . . . . 19 5.3 Reductive Elimination from PtII Halogen Complexes [Pt(CH=CH2)2X2]2– (X=Cl, Br, I) . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.4 Reductive Elimination from PtII Complexes with Amine and Phosphine Ligands [Pt(CH=CH2)2X2] (X=NH3, PH3) . . . . . . . . . . . . 21 5.5 Reductive Elimination from PdIV Complexes [Pd(CH=CH2)2X4]2– (X=Cl, Br, I) 23 5.6 Reductive Elimination from Mixed PdIV Complex [Pd{trans-(CH=CH2)2(PH3)2}Cl2] . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.7 Reductive Elimination from PdII Halogen Complexes [Pd(CH=CH2)2X2]2– (X=Cl, Br, I) . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.8 Reductive Elimination from PdII Complexes with Nitrogen and Phosphine Ligands [Pd(CH=CH2)2X2] (X=NH3, PH3) . . . . . . . . . . . . 23 5.9 Reductive Elimination from RhIII, IrIII, RuII and OsII Complexes . . . . . . . . . 24 5.10 General Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 5.11 Comparison of the Vinyl-Vinyl (Csp2-Csp2) and Alkyl-Alkyl (Csp3-Csp3) Reductive Elimination . . . . . . . . . . . . . . . . 26 6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Topics Organomet Chem (2005) 12: 1– 30 DOI 10.1007/b104397 © Springer-Verlag Berlin Heidelberg 2005 Abstract The factors controlling the transition metal catalyzed s-bond (including H-H, C-H and C-C) activation and formation, the fundamental steps of many chemical transfor- mations, were analyzed. It was demonstrated that in the mono-nuclear transition metal systems the (1) availability of the lower lying s1dn–1 and s0dn states of the transition metal atoms, and (2) nature of the ligands facilitating the reduction of the energy gap between the different oxidative states of the transition metal centers are very crucial. Meanwhile, in the transition metal clusters the “cooperative” (or “cluster”) effects play important roles in the catalytic activities of these clusters. Another important factor affecting the catalytic activity of the transition metal systems shown to be their redox activity. Keywords s-Bond activation and formation · Transition metal systems · Catalytic activity 1 Introduction Sigma-bond (including H-H, C-H and C-C) activation and formation are fundamental steps of many chemical transformations and have been subject of numerous review articles [1]. It is well accepted that certain transition metal complexes significantly facilitate the s-bond activation/formation steps, which may occur via various mechanisms, including oxidative addition/reductive elimination, metathesis and nucleophilic attack. However, the factors affecting H-H, C-H and C-C activation/formation still need to be clarified in detail. In this chapter we intend to analyze some factors that control the catalytic activ- ity of transition metal complexes toward H-H, C-H and C-C bond activation/ formation. Namely, we elucidate the role of (a) lower-lying electronic states of transition metal cations/atoms, (b) cooperative effects in transition metal clus- ters, (c) redox activity of the transition metal centers, and (d) the role of metal and ligand effects in vinyl-vinyl coupling. 2 The Role of the Lower-Lying Electron States of Transition Metal Cations in Oxidative Addition of the ss-Bonds (such as H-H, C-H and C-C) The study of gas-phase activation of H-H, C-H and C-C bonds of the hydro- gen molecule and saturated hydrocarbons, respectively, by bare transition metal atoms and cations is very attractive for getting insight to the mecha- nisms and factors (such as nature of metal atoms and their lower-lying electronic states) controlling catalytic activities of transition metal complexes. Such studies, which are free fromthe ligand and solvent effects, have been subject of many experimental [2] and theoretical [3] papers in the past 10–15 years. Experimental studies indicate that reaction of some transition metal cations (such as Fe+, Co+, and Rh+) with methane exclusively leads to the ion-molecule complex M+(CH4), while others (such as Sc+ and Ir+) pro- 2 D. G. Musaev · K. Morokuma Transition Metal Catalyzed s-Bond Activation and Formation Reactions 3 Fi g. 1 Po te nt ia l e ne rg y pr of ile o ft he r ea ct io n Ir + + C H 4 ca lc ul at ed a t t he M R- SD C I- C A SS C F le ve l o ft he or y ceed further via oxidative addition mechanism and leads to hydrido-metal- methyl and/or MCH2++H2 products. In order to find some insight to the dif- ference in the reactivity of early and late, as well as first-, second- and third- row transition metal cations (TMCs), we have studied the mechanism of the reaction of M+ (M=Sc, Fe, Co, Rh and Ir) with CH4 at the CASSCF and MR-SDCI levels of theory in conjunction with large basis sets. The results of these studies have been published elsewhere [4]. Here we discuss general trends, factors controlling reactivity of the transition metal cations toward s-bonds, and predict the most favorable metal cations that can efficiently insert into s-bonds. As expected, the first step of the reaction M++CH4 is the formation of ion-molecule complex M+(CH4) (see Fig. 1, which, as an example, includes the potential energy surface of the reaction Ir++CH4 at the several lower-lying electronic states of the Ir cation). Our calculations show that these complexes are structurally non-rigid, where M+ can nearly freely rotate around the CH4 molecule by the pathways (C2v)´(C3v, TS)´(C2v)´… and/or (C3v)´(C2v, TS)´(C3v)´…, depending on the nature of metal atom and the electronic state of the complex M+(CH4). These complexes are stable by 21.9 (M=Sc), 15.5 (13.7±0.8) (M=Fe), 21.4 (22.9±0.7) (M=Co), 16.8 (M=Rh), and 20.7 (M=Ir) kcal/mol relative to the ground state dissociation limit M++CH4 (experimen- tal values are given in parentheses). From the resultant M+(CH4) complex the reaction proceeds via the C-H bond activation transition state (TS) to give the hydrido-metal-methyl cation complex, HMCH3+. In this step the C-H s-bond is broken and M-H and M-CH3 bonds are formed.Also, the oxidation number of the M-center increases by two. In order to analyze the reactivity of TMCs toward C-H (as well as H-H and C-C) bond, one has to elucidate the factors controlling thermodynamics and kinetics of the reaction M+(CH4)ÆHMCH3+. Our [4] and other [3] studies have shown that thermodynamics of the reaction M+(CH4)ÆHMCH3+ is controlled by the two factors. The first factor is the availability of the s1dn–1 state of the cation M+, which is expected to be the dominating bonding state in the resultant HMCH3+ complex. The second factor is the loss of exchange energy (the loss of high-spin coupling (exchange energy) between valence electrons on the unsaturated transition-metal ion subsequent to the formation of covalent metal-ligand bonds) upon formation of M-H and M-CH3 bonds [5]. Upon formation of M-H and M-CH3 bonds, which stabilize the system, the loss of exchange energy occurs and counteracts the stabilization. Thus, if the s1dn–1 configuration of the cation M+ is the energetically most favor- able one (or easily available, i.e. the promotion energy from the ground state to the excited s1dn–1 state is small) and the loss of exchange energy for formation of two, M-H and M-CH3, bonds in the s1dn–1 state is small, the reaction M+(CH4) ÆHMCH3+ is thermodynamically favorable. Taking into account these factors, one can easily explain the calculated trends in the exothermicity of the reaction M+(CH4)ÆHMCH3+, and predict thermodynamically the most favorable reac- tion M+(CH4)ÆHMCH3+. 4 D. G. Musaev · K. Morokuma Our studies show that the reaction M+(CH4)ÆHMCH3+ is endothermic by 20.3, 32.3, 37.7 and 40.3 kcal/mol for M=Sc, Fe, Co, and Rh, respectively, while it is exothermic by 8.7 kcal/mol for M=Ir. This trend in the energy of the reac- tion M+(CH4)ÆHMCH3+ can be qualitatively explained in terms of the energy gap between the lower lying s0dn and s1dn–1 states of the metal cations and the necessary exchange energy loss for formation of two covalent bonds to the s1dn–1 state. Indeed, the s1dn–1 state is a ground state for Ir+, and thus Ir+ can easily form two M-L s-bonds.While the ground states of the Sc and Fe cations are the s1dn–1 states, these cations need 3.7 and 41.4 kcal/mol energy (exchange energy loss for formation of two covalent bonds to the s1dn–1 state) to form two M-L bonds. Meanwhile the ground electronic configurations of Co and Rh are the s0dn states, and the calculated exchange energy loss for formation of two covalent bonds to the s1dn–1 state plus s0d8Æs1d7 promotion energy are 39.2 and 77.5 kcal/mol for Co+ and Rh+, respectively. Thus, the calculated trend in the energy of the reaction M+(CH4)ÆHMCH3+, Ir (–6.6)<Sc (20.3)<Fe (32.3)<Co (37.7)<Rh (40.3), is in a qualitatively agreement with the calculated exchange energy loss for formation of two covalent bonds plus the cost of promotion to the s1dn–1 state: Ir<Sc (3.7)<Fe+ (41.4)~Co+ (39.2)<Rh+ (77.5) [5]. As the exchange plus promotion costs for formation of two bonds increases via Sc+ (3.7)<Ti+ (13.3)<V+ (32.9)<Cr+ (72.7) and then decreases via Mn+ (51.6)>Ni+ (41.6)~Fe+ (41.4)>Co+ (39.2) (in kcal/mol) [5],we expect that the thermody- namic stability of insertion product to decrease via M=Sc+>Ti+>V+>Cr+ and increase Mn+<Ni+~Fe+~Co+. Furthermore, it is well established that the s1dn–1 state becomes the most favorable, and the M-L bond strength significantly in- creases for the third-row transition metal cations. Therefore, one may expect that exothermicity of the reaction M+(CH4)ÆHMCH3+ will significantly increase upon going from the first- and second-row TMCs to the third-row. Meanwhile, the kinetic stability (existence of the C-H bond activation TS and the barrier height) of the HMCH3+ complexes is mainly controlled by: (1) the endothermicity of reaction M+(CH4)ÆHMCH3+. The large endothermicity of reaction reduces the barrier for the reverse reaction HMCH3+ÆM+(CH4), and makes HMCH3+ unstable relative to M+(CH4).As noted above, our studies show that the reaction M+(CH4)ÆHMCH3+ is endothermic by 20.3, 32.3, 37.7 and 40.3 kcal/mol for M=Sc, Fe, Co, and Rh respectively, while it is 8.7 kcal/mol exothermic for M=Ir. (2) The availability of the s0dn electronic configuration of the metal center. It is well known that upon oxidative addition of the C-H/H-H s-bond to transition metal center a charge transfer from the doubly occupied C-H/H-H s-orbitals to the s (s and ds) orbitals of metal center (called “dona- tion”) and from metal p-orbitals to the s* antibonding orbital of the C-H/H-H bond (called “back donation”) takes place (see Scheme 1). These interactions are efficient when the metal center has empty (or partially empty) s-type s and ds, orbitals and occupied dp orbitals. Since Fe+, Co+, Rh+, and Ir+ (and all late transition metal atoms) have double occupied dp orbitals but Sc+ (and all early transition metal atoms) has none, the “back donation” effect is expected to be larger for late transition metals compared to the early ones. Transition Metal Catalyzed s-Bond Activation and Formation Reactions 5 In contrast, transition metal cations such as Co+ and Rh+, have the s0dn ground state with empty s-orbital as opposed to Sc+, Ir+, and Fe+, and one may expect the strong s(C-H)Æ(s,ds) (M) donation effects for the former cations. Thus, availability of the s0dn state (with doubly occupied dp and empty s-orbitals) for Co+ and Rh+, as well as Fe+, facilitates both “donation” and “back donation”effects and makes the s-bond activation significantly easier for these cations compared to Sc+ and Ir+. This statement is consistent with the calculated trends; the reverse reaction HMCH3+ÆM+(CH4) occurs without barrier and is con- trolled by thermodynamic factors for M=Fe+, Co+ and Rh+. Meanwhile reaction M+(CH4)ÆHMCH3+ occurs with energetic barrier of 38.5 and 2.1 kcal/mol, for M=Sc and Ir, and is controlled by both thermodynamic and kinetic factors. The reverse reaction HMCH3+ÆM+(CH4) occurs with 18.2 and 10.8 kcal/mol barri- ers for M=Sc and Ir, respectively. On the basis of these discussions, we conclude that: (1) The s0dn state of the TMC favors the formation of ion-molecule complexes, while the s1dn–1 state leads to formation of oxidative addition product, (2) the availability of both s1dn–1 and s0dn configurations of transition metal cations (and atoms) is absolutely neces- sary for their reactivity toward s-bonds (such as H-H, C-H, C-C, O-H, and N-H), and (3) all early first-row (Sc+, Ti+ and V+) transition metals cations, having empty s or d orbitals with a1 symmetry, as well as many third-row (Hf +, Ta+, W+, Re+, Os+, Ir+ and Pt+) TMCs, can easily activate s-bonds in the gas phase, and stabilize the oxidative addition product complexes. 3 Role of Cooperative Effects in the Transition Metal Clusters In this section we expand the conclusions of the previous section to bare transi- tion metal clusters in order to test them again and to identify another,“coopera- tive”(or “cluster”) effect that affects the reactivity of transition metals. In practice, transition metals are important ingredients of heterogeneous and nano-catalysts, therefore clear understanding of their reactivity at electronic level is essential to unravel the secret of their catalytic activities.Diverse classes of experimental and theoretical studies already have provided a wealth of information concerning the electronic structure, spectroscopic as well as dynamic properties of variety types of clusters, including Ptn [6], Pdn [7], Fen+ [8], Con+ [9], and Nbn+ [10]. 6 D. G. Musaev · K. Morokuma Scheme 1 Schematic presentation of “donation” and “back-donation” contributions on M-(HX) interaction In particular, in the experimental work of Cox et al. [6, 7], the measured rate constants of CH4 and H2 activation by unsupported Pt and Pd clusters (n=6~24) show a large variation as functions of the cluster size. In the case of Pt clusters, it was found that dimer through pentamer were the most reactive, while the reactivity dropped significantly starting from Pt6. The single Pt atom was less reactive compared to Pt2–5 by an order of magnitude, and the bulk was less reactive by at least several orders of magnitude. In the case of Pd clusters, it was found that the activation rate constants for both H-H and C-H bonds show sig- nificant oscillation in terms of the cluster size. The peak value of the measured rate constant is around n=8 and 10, and the minimum rate constants have been observed for n=3 and n=9. Understanding the size-dependence of reactivity of clusters has become one of the most fascinating and intriguing issues in cluster chemistry [11]. To unravel the reason behind the observed variation of reactivity as a func- tion of cluster size [6, 7], we have chosen to study the detailed mechanism of H2 and CH4 activation on small Ptn and Pdn (n=1–6) clusters. Results of these studies have already been published [12]. Here, we intend to analyze the factors controlling the reactivity of these clusters. 3.1 Reaction of Pt and Pd Metal Atoms with H2/CH4 Molecules First of all, we recall briefly the electronic structure and reactivity of Pd and Pt atoms, shown in the previous section, since they are the fundamental building blocks of the clusters and their characteristics have a major influence on the properties of clusters. According to a large number of theoretical as well as experimental studies, Pd and Pt atoms have very different electronic structures and consequently distinct reactivities. The ground state of Pd atom has a closed- shell s0d10 configuration, where the open-shell s1d9(3D) state is 21.9 kcal/mol above [13]. Therefore, based on the conclusions of the previous section, one may expect that the Pd atom cannot break the H-H or C-H bonds in H2 and CH4 and rather forms molecular complexes Pd(H2) and Pd(CH4), respectively. The calculated Pd-H2 bond energy is 16.2 kcal/mol. For the Pt atom, the s1d9(3D) state is the ground electronic state, while the s0d10 configuration is 11.1 kcal/mol higher in energy [13]. Consequently, it has been observed that Pt atom breaks the H-H and C-H bond, in agreement with the conclusions of the previous sec- tion. Since the ground state of Pt atom is a triplet but resultant HPtH or HPtCH3 is a singlet, a curve crossing from the triplet to the singlet state is required, and the minimum crossing point can be viewed as the transition state for the acti- vation process starting from the ground electronic state atom (Fig. 2a). The binding energy of H2 and CH4 to the Pt atoms is 47.4 kcal/mol and 34.3 kcal/mol, respectively. Thus, the calculated results for the reactions Pd+H2/CH4 and Pt+H2/CH4 once again confirm our conclusions from the previous section. Transition Metal Catalyzed s-Bond Activation and Formation Reactions 7 8 D. G. Musaev · K. Morokuma Fi g. 2a –d Po te nt ia l e ne rg y pr of ile o ft he r ea ct io ns :a Pt + H 2/ C H 4; b Pd 2+ H 2; c Pd 2+ C H 4; d Pt 2+ C H 4 3.2 Reaction of Pd2 and Pt2 Dimers with H2/CH4 Molecules As seen in the potential energy profile of these reactions, shown in Fig. 2b–d, both Pd2 and Pt2 activate H-H/C-H bonds with very small barrier. In reaction Pt2+H2/CH4, H-H/C-H activation preferentially takes place on a single metal atom.Afterwards, one of the H atoms migrates to the other Pt atom over a neg- ligible isomerization barrier. On both the singlet and the triplet state, H-H activation is expected to be barrierless, while C-H activation has a distinct barrier on the triplet state for reaction starting from the ground triplet state Pt2. Nevertheless, since the barrier for C-H activation on the triplet state is small and lower than the expected minimum of seams of crossing (MSX) between singlet and triplet in the Pt-CH4 system, it is expected that Pt2 activates the C-H bond in CH4 with a faster rate than the Pt atom, which is in accord with the experimental observation of Cox et al. [6, 7]. Calculations show that the ground state of the reactants Pd2+H2/CH4 is a triplet state, while the product complexes have a singlet ground state. Therefore, one may expect that the reaction proceeds either on the excited singlet state surface or through the minimum of triplet-singlet seams of crossing. On the singlet state potential energy surface of Pd2+H2/CH4 (see Fig. 2b,c) the reaction is downhill without activation transition state. Meanwhile, the calculated triplet-singlet MSX lies lower in energy than the triplet state transition state. Therefore, the H2/CH4 activation by Pd2 starting from the triplet ground state dimer is expected to proceed via an intersystem crossing mechanism with very small barrier. Interestingly, the activation of s-bonds occurs only upon per- pendicular approach of H-H/C-H bonds to the Pd-Pd bond. Thus, in contrast to the single atom case where Pd and H2/CH4 form only a molecular complex and no H-H/C-H bond cleavage occur, two Pd atoms work “cooperatively” and readily break H2/CH4. This “cooperative” mechanism for H2/CH4 activation on Pd2 is different from the case of Pt2+H2/CH4. In Pt2 dimer H2/CH4 activation takes place preferentially on a single atom, while in Pd2 dimer it occurs on the Pd-Pd bond. Moreover, in the final activation products, H/CH3 groups prefer the bridged sites ofPds, but are localized on metal sites in Pt2. Those results can be rationalized as the following, as illustrated in Scheme 2. The singlet state Pd2 consists of mainly two s0d10 Pd atoms, and the LUMO sg has a correct symmetry to accept electron density effectively from the H2/CH4 s orbital upon perpendicular approach.As a result, the activation takes place preferentially in this approach. In the case of Pt2+H2/CH4 reaction, the metal HOMO and LUMO are of localized metal d character (as established in a number of studies of metal clusters, the s-s contribution in the metal-metal bonding is dominant, while d-d interaction is weak). Therefore, the HOMO and LUMO of triplet Pt2 are all of localized d characters, while the s and s* orbitals that contain large s characters are much lower and higher in energy, respectively, and therefore activation takes place preferentially on a single atom (rather than on the Pt-Pt bond, where the strongest HOMO/LUMO interaction between the metal and H2 is expected. Transition Metal Catalyzed s-Bond Activation and Formation Reactions 9 10 D. G. Musaev · K. Morokuma Scheme 2 Orbital diagram of triplet Pd2 and Pt2, MO energies (in hartree) are calculated at the full valence (20el./12orb.) CASSCF level In the final singlet products, Pt2(H)2/Pt2(H)(CH3) and Pd2(H)2/Pd2(H)(CH3), the metal dimers actually are in their triplet configurations. In the case of Pt2, the metal-metal s bonding orbital is low in energy, and the Pt-H/CH3 bond has large d character. Therefore, the H/CH3 groups in Pt2(H)2/Pt2(H)(CH3) do not like the bridged sites, but rather localize on each Pt atom. In the case of Pd2, in its triplet electronic state, the metal-metal s bonding orbital is the HOMO. Therefore, both the CH3/H-Pd-Pd bonding and antibonding orbitals have much metal s component. As a result, the H/CH3 ligands prefer the bridged sites rather than the localized metal sites. Thus, our studies of the reactivity of Pd/Pt clusters with H2/CH4 molecules clearly show a “cooperative” effect that could play a significant role in the re- activity of the transition metal clusters. Thus, the catalytically inactive metal atoms could form very active clusters ! 4 ss-Bond Activation via Nucleophilic Mechanism: the Role of Redox Activity of the Transition Metal Center – Hydrocarbon Hydroxylation by Methanemonooxygenase (MMO) Another important factor controlling the reactivity of transition metal cen- ters toward s-bond is their redox activity. Indeed, it is well established that transition metal centers with low redox potential can be active catalysts [14]. For example, let us discuss the reactivity in hydrocarbon hydroxylation by Methanemonooxygenase (MMO). MMO, one of the members of diiron-containing metalloenzyme family, is an enzyme that catalyzes methane oxidation reaction, i.e. conversion of the inert methane molecule to methanol [15]. During this reaction two reducing equivalents from HAD(P)H are utilized to split the O-O bond of O2. One O atom is reduced to water by 2-electron reduction, while the second is incorporated into the substrate to yield methanol: CH4 + O2 + NAD(P)H + H+ Æ CH3OH + NAD(P)+ + H2O Experimental studies [16] show that the best-characterized forms of the solu- ble MMO (sMMO) contain three protein components: hydroxylase (MMOH), so-called B component (MMOB) and reductase (MMOR), each of which is re- quired for efficient substrate hydroxylation coupled to NADH oxidation. The hydroxylase, MMOH, which binds O2 and substrate and catalyzes the oxidation, is a hydroxyl-bridged binuclear iron cluster. In the resting state of MMOH (MMOHox), the diiron cluster is in the diferric state [FeIII-FeIII], and can accept one or two electrons to generate the mixed-valence [FeIII-FeII] or diferrous state [FeII-FeII], respectively. The diferrous state of hydroxylase (MMOHred) is the only one capable of reacting with dioxygen and initiating the catalytic cycle. X-ray crystallographic studies of the enzyme from Methylococcus capsula- tus (Bath) [17] and Methylosinus trichosporium OB3b [18] have unveiled the coordination environment of the Fe centers of MMOHox and MMOHred. According to these studies, in MMOHox each Fe center has six-coordinate octahedral environment (see Scheme 3). Fe ions are bridged by a hydroxy ion, a bidentate Glu g-carboxylate and a water molecule (or another carboxylate). In addition, each Fe ion is coordinated by one His nitrogen ligand and one monodentate Glu carboxylate. The two Fe centers are different from each other in that one of them (Fe2) has an additional monodentate glutamate carboxylate, while the other Fe (Fe1) has one additional water molecule. Upon reduction, one of carboxylate ligands undergoes a so-called “1,2-carboxylate shift” from being a terminal, monodentate ligand bound to Fe2 to a monodentate, bridging ligand between the two irons, with the second oxygen of this carboxylate also coordinated to Fe2. In addition, the hydroxyl bridge is lost, and the other hydroxyl/water bridge shifts from serving as a bridge to being terminally bound to Fe1. Also, the terminal water bound to Fe1 in the oxidized form of MMOH seems to move out upon reduction of the cluster. Thus, in reduced form of MMOHred the ligand environment of Fe ions becomes effectively five coor- dinated, which is reasonable since this is the form of the cluster that activates dioxygen. It was established that MMOHred reacts very fast with O2 and forms a metastable, so-called compound O, which spontaneously converts to another compound called P (see Scheme 4). Spectroscopic studies [19] indicate that compound P is a peroxide species, where both oxygens are bound symmetri- cally to the irons. Compound P spontaneously converts to compound Q, which was proposed to contain two antiferromagnetically coupled high-spin Fe(IV) Transition Metal Catalyzed s-Bond Activation and Formation Reactions 11 12 D. G. Musaev · K. Morokuma Sc he m e 3 St ru ct ur al r ep re se nt at io n of th e bi nu cl ea r Fe c en te r of di fe rr ic M M O H ox an d di fe rr ou s M M O H re d (s ee [1 5] ) Transition Metal Catalyzed s-Bond Activation and Formation Reactions 13 Scheme 4 Experimentally proposed catalytic cycle of MMO (see [15]) centers. EXAFS and spectroscopic studies [20, 21] of compound Q, trapped from M. trichosporium OB3b and M. capsulatus, have demonstrated that com- pound Q has diamond core, (FeIV)2(m-O)2 structure with one short (1.77 Å) and one long (2.05 Å) Fe-O bond per Fe atom and a short Fe-Fe distance of 2.46 Å. Compound Q has been proposed to be the key oxidizing species for MMO. In the literature there have been several computational attempts [22–25] to elucidate mechanism of methane oxidation by intermediate Q. Our results [25] show that reaction proceeds via the mechanism presented in Fig. 3. Later, this mechanism was validated by several times and currently is well accepted. As seen in Fig. 3, reaction of compound Q (modeled as structure I) with methane starts with coordination of CH4. In general, the CH4 molecule could coordinate to I via two distinct pathways: O-side and N-side. The O-side path- way corresponds to the coordination of the methane molecule from the side where the two Glu (carboxylate) located, while the N-side pathway corresponds to the coordination of CH4 from the two His (imidazole) side. Our calculations show that both pathways proceed via very similar transition states and inter- mediates, and the N-side pathway is thermodynamically and kinetically more favorable than O-side. In spite of this, in this paper we base our discussions only on the O-side mechanism because it is believe to correspond to the process occurring in the protein. The coordination of CH4 to complex I leads to the methane-Qcomplex, structure II. The interaction between methane and struc- ture I (compound Q) is extremely weak; the complexation energy is calculated (relative to the corresponding reactants) to be 0.7 and 0.3 kcal/mol for the 9A and 11A state, respectively. Because of unfavorable zero point energy and entropy factors, it is most likely that the complex II does not exist in reality, and therefore we will not discuss it. Results in Table 1 show that the 9A state of structure I very qualitatively is the Fe(IV)-Fe(IV) complex. On the other hand, in the 11A state it is the Fe(IV)- Fe(III) mixed valence species, where Fe2 is in the Fe(III) state with five spins that are heavily delocalized onto O2. The calculated fact that I_9 (here and below in A_B, A is the structure, while B is the electronic state) is lower in energy than I_11 suggests that Fe(IV)-Fe(IV) is the preferred state for complex I (and com- pound Q). This is consistent with the experimental conclusions [21]. The activation of the methane C-H bond takes place on the diamond oxygen O1. At the TS1, structure III, the C-H bond to be broken is elongated from 1.089 Å in II to 1.271 Å and 1.296 Å in transition states III_9 and III_11. Fur- thermore, the O-H bond is nearly formed, with distance of 1.250 Å and 1.241 Å at the TSs, compared to 0.983 Å and 0.978 Å in products IV_9 and IV_11, respectively. These geometrical changes indicate clearly that III_9 and III_11 are the TSs corresponding to the H abstraction process. The H-abstraction barriers are calculated to be 23.2 and 19.0 kcal/mol for the 9A and 11A states, respectively, relative to the corresponding CH4 complexes II_9 and II_11, re- spectively. These values of the barrier are in reasonable agreement with avail- able experimental estimates, 14–18 kcal/mol [23]. The spin densities for TS1, III, and product IV are found to be similar to each other within their respective 9A and 11A states (see Scheme 5 and Table 1). Furthermore, the spin densities are nearly identical between 9A and 11A, except for those on the O2..H..CH3 frag- 14 D. G. Musaev · K. Morokuma Fig. 3 The potential energy profile (in kcal/mol) for both the 9A state and the 11A state of the methane activation reaction via O-site pathways: (NH2)(H2O)Fe(m-O)2(h2-COO)2Fe(H2O)- (NH2)+CH4Æ(NH2)(H2O)Fe(m-O)(HOCH3)(h2-HCOO)2Fe(H2O)(NH2) Transition Metal Catalyzed s-Bond Activation and Formation Reactions 15 Scheme 5 The spin recoupling scheme in the intermediates of the reaction Table 1 Relative (in kcal/mol, relative to the 9A reactants) energies, and Mulliken atomic spin densities (in e) of various intermediates and transition states, for 9A and 11A states, for the reaction of the complex I with molecule of methanea. The numbers after slash are relative to the 11A reactants Structures DE (in Atomic spin densities (in e) kcal/mol) LnFe2 LnFe1 O2 O1 Hb CH3c 9A state I+CH4 0.0 3.44 3.55 0.44 0.30 – – II –1.5 3.43 3.52 0.44 0.31 – – III 23.2 4.59 3.54 0.40 –0.37 0.06 –0.55 IV 11.3 4.64 3.51 0.43 0.08 0.00 –0.99 V 20.6 4.58 3.23 0.38 0.20 0.00 –0.73 VI –41.8 4.54 2.92 0.35 0.00 0.00 0.00 VII –34.3 4.49 2.95 0.38 0.00 0.00 0.00 IVd 23.7 4.52 1.69 0.56 0.07 0.00 0.99 11A state I 5.4/0.0 4.62 3.47 1.03 0.43 – – II 4.2/–1.2 4.62 3.47 1.03 0.43 – – III 24.4/19.0 4.63 3.52 0.43 0.55 –0.07 0.59 IV –11.3/5.9 4.67 3.58 0.43 0.10 0.00 0.98 V 18.6/13.2 4.57 3.97 0.50 0.01 0.01 0.81 VI –53.9/–59.3 4.48 4.59 0.66 0.00 0.00 0.00 VII –46.7/–52.1 4.54 4.54 0.67 0.00 0.00 0.00 IVd –15.5/10.1 4.65 4.54 0.42 0.08 0.00 0.99 a Here, LnFe stands for the (H2O)(NH2)Fe-fragment. This table does not include the portion of spin densities located on the bridging carboxylate ligands, each of which may have about 0.10–0.15e spin. b H atom located between O2 and CH3 fragments. c The number for the entire CH3 fragment. ment. For the CH3 groupitself, the total Mulliken charge (not given in Table 1) is at most +0.03e for both the 9A and 11A states and the spin densities on this group for 9A and 11A are of same magnitude but of opposite sign. One can interpret all these values in the following way. In both TSs, III_9 and III_11, a radical center begins to develop on the CH3 group, with spin densities of –0.46e and +0.52e, respectively, and in both intermediates, IV_9 and IV_11, the CH3 group is now a real radical with spin densities of –0.98e and 1.00e, respectively. The spin densities on Fe1 and Fe2 in IV_9 and IV_11, which formally can be written as L4Fe(m-O)(m-OH)FeL4 with the methyl radical only weakly interact- ing via a C...HO interaction, can qualitatively be considered to correspond to Fe(IV) with four spins and Fe(III) with five spins, respectively. In going from II_9 to III_9, the very qualitative formal oxidation state of Fe2 changes from Fe(IV) to Fe(III), while from II_11 (which is already Fe(III)) to III_11, no such change is required. Since the two Fe centers are coupled ferromagnetically in both 9A and 11A states, the spin of the CH3 radical in both III and IV has to couple antiferromagnetically (with negative spin) and ferromagnetically (with position spin) to make the total spin 2S+1 equal to 9 and 11, respectively. In the radical complexes IV_9 and IV_11 the interaction of the CH3 radical with the two iron atoms is very weak and, therefore, their total energies are nearly iden- tical. It is quite interesting that we find that a mixed valence state is responsible for the methane oxidation reaction. The present spin density analysis clearly demonstrates that (1) the methane oxidation proceeds via a bound-radical mechanism, and (2) the first electron transfer from substrate to Fe-centers occurs through the TS1 and is completed at the resultant bound-radical complex IV. The second electron transfer from the substrate to Fe-centers starts with the recombination of methyl radical with the bridging hydroxyl ligand at the tran- sition state, TS2, structure V. Indeed, our results show that in the 11A state upon going from IV_11 to the methanol complex VI_11, Fe1 changes its formal oxi- dation state from Fe(IV) with four spins to Fe(III) with five spins, while the spin density on the methyl radical is completely annihilated upon forming a covalent bond between CH3 and OH. The transition state V_11 has a spin distribution between that of IV_11 and VI_11. On the other hand, in the 9A state upon going from IV_9 to the methanol complex VI_9, the spin density on Fe1 is reduced by about 0.5, corresponding to the disappearance of roughly one unpaired electron. Since Fe(V) is not a stable species, it is most likely Fe1 changed its formal oxi- dation state from Fe(IV) with four spins to Fe(III) with five formal d electrons. Because of the restriction 2S+1=9, i.e., the total number of unpaired electrons must be 8 within the Fe(III)-Fe(III) core, Fe1 in VI_9 chose to form one d-lone pair with only three spins remaining. This complex VI_9 is thus higher in en- ergy than the corresponding complex VI_11 in violation of the Hund rule. The barrier heights for the CH3 addition to the hydroxyl ligand calculated relative to the intermediate IV_9 and IV_11 are 9.3 and 7.3 kcal/mol for the 9A and 11A states, respectively. Obviously, this step of the reaction is not rate-de- termining, and can occur rather fast. Overcoming the barriers at TS2 leads to the complexes methanol-complexes VI, L4Fe(OHCH3)(m-O)FeL4, and completes 16 D. G. Musaev · K. Morokuma Transition Metal Catalyzed s-Bond Activation and Formation Reactions 17 5 Vinyl-Vinyl Coupling on Late Transition Metals Through C-C Reductive Elimination Mechanism Another important s-bond activation/formation process discussed in this article is vinyl-vinyl coupling, shown in Scheme 7.Vinyl-vinyl coupling opens a convenient route to conjugated 1,3-dienes and is widely employed in many catalytic coupling reactions. The great potential of the field is still undercontinuous development [26, 27] and, therefore, elucidation of the C-C bond formation mechanism and the factors controlling it are very crucial. In litera- ture, numerous mechanistic studies on C-C reductive elimination and reverse process, oxidative addition (C-C bond activation), have been reported for di- Scheme 6 The redox cycle of Fe-centers during hydrocarbon hydroxylation by MMO the second electron transfer process. The overall reaction I + CH4ÆVI is calculated to be exothermic by 34.3 and 46.8 kcal/mol for the 9Aand 11A states, respectively. The final step of elimination of the methanol molecule and regen- eration of the enzyme could be a complex process, and possibilities of different mechanisms exist but have not yet been studied computationally. Thus, these results clearly show that the Fe-centers are not directly involved in methane C-H bond cleavage. However, they play a crucial role in the methane oxidation process, by accepting two electrons from the substrate. Having low oxidation potential for Fe-centers is definitely an important factor in this process and significantly facilitates it. Thus, during the hydrocarbon hydrox- ylation by MMO, the Fe centers undergo multiple reduction and oxidation steps; at first, MMOHox [ Fe(III)-Fe(III) state] should be reduced (2-electron reduction) to MMOHred [Fe(II)-Fe(II) state], then it should be oxidized (4-elec- tron oxidation) by O2 molecule to [Fe(IV)-Fe(IV) state], after which it again is reduced by substrate to Fe(IV)-Fe(III) state and Fe(III)-Fe(III) states (see Scheme 6). Thus facile oxidation and reduction of Fe-center plays a crucial role in the hydrocarbon hydroxylation by MMO. alkyl or mixed complexes of Pt [28–34], Pd [31, 35], Rh [33], Ru [32] and Ir [33]. Also, several theoretical investigations on C-C bond activation by Pt [36–39], Pd [36–40], Rh [40–43] and Ir [42, 44] have been performed. However, these studies were made for Csp3-Csp3 and Csp3-Caryl (in most cases methyl-methyl and methyl-aryl, respectively) bonds, and no theoretical study appears to have been carried out for reductive elimination of unsaturated vinyl ligands leading to a conjugated product. Therefore, we have published [45] two theoretical (DFT and ONIOM) papers on the mechanism of vinyl-vinyl C-C reductive elimina- tion reaction on the transition metal complexes, major conclusions of which are summarized here and compared with other C-C coupling processes. We also intend to analyze the factors controlling these reactions. 5.1 Reductive Elimination from PtIV Halogen Complexes [Pt(CH=CH2)2X4]2– (X=Cl, Br, I) As shown in Fig. 4, our calculations showed that the reaction involving vinyl ligands may proceed via two different transition states, 1_TS and 1A_TS, which possess s-cis and s-trans configuration around the central single bond, respec- tively. Their corresponding energies are given in Table 2. We note here that at the B3LYP level of theory, the calculated relative energies of s-trans, s-gauche and s-cis conformers are 0.0, 3.5 and 4.0 kcal/mol respectively, in agreement with earlier findings. Reductive elimination through the s-trans pathway retains C2 symmetry and has to overcome the activation barrier of 29.4 kcal/mol. On the other hand, ini- tial complex (1A_Init) and transition state (1A_TS) for s-cis pathway belong to Cs point group and the activation energy is 4.9 kcal/mol higher than for the for- mer (Table 2). In the latter process, the product buta-1,3-diene will be released in the s-gauche (C2) form. Reductive elimination reactions are exothermic by 17.0 and 19.9 kcal/mol for s-cis and s-trans pathways, respectively (Table 2). Energy difference between the initial bis-s-vinyl complexes 1_Init and 1A_Init is only 0.6 kcal/mol, with the former being more stable. The relative order of the transition states (1_TS and 1A_TS) reflects the stability of corre- sponding buta-1,3-diene isomers in a free form. Thus, s-cis transition states are expected to lie higher in energy and we will continue the study following the s-trans pathway only. 18 D. G. Musaev · K. Morokuma Scheme 7 Vinyl-vinyl coupling reaction on transition metal complexes Transition Metal Catalyzed s-Bond Activation and Formation Reactions 19 Fig. 4 S-cis and s-trans reductive elimination pathways from octahedral complexes Upon examination of the C-C reductive elimination reaction from a series of halogen complexes (PtIV: X=Cl, Br, I; Table 2), a clear trend is observed; the heavier the ligand, the smaller the activation barrier, and the larger the exother- micity of the process. Therefore, the easier reductive elimination reaction should be expected from iodide complexes of PtIV with the activation energy of only 25.0 kcal/mol and reaction energy of –30.3 kcal/mol. 5.2 Reductive Elimination from Mixed PtIV Complexes [Pt{cis-/trans- (CH=CH2)2(PH3)2}Cl2] Introducing phosphine ligand either in cis or trans position to s-vinyl groups favors C-C bond formation (Table 2). However, in the case trans substitution (4) the influence is much larger,DE≠=18.5 kcal/mol, compared to DE≠=27.6 kcal/mol for cis derivative (5) and DE≠=29.4 kcal/mol for the chloride complex (1). It is found that lower activation barriers are accompanied by higher reaction exothermicity (cf. DEfor 1, 4 and 5 in Table 2). 20 D. G. Musaev · K. Morokuma Ta bl e 2 A ct iv at io n (D E≠ ,D H ≠ , DG ≠ ) a nd r ea ct io n en er gi es ( DE ,D H ,D G ) o ft he C -C r ed uc ti ve e lim in at io n st ag e in th e ga s ph as e (i n kc al /m ol )a N o. Sy st em b DE ≠ DE DH ≠ DH DG ≠ DG 1 Pt (I V ), X = C l 29 .4 –1 9. 9 28 .6 –1 9. 4 29 .2 –3 0. 6 1A Pt (I V ), X = C l, s- ci sc 34 .3 –1 7. 0 34 .0 –1 6. 0 33 .2 –2 8. 0 2 Pt (I V ), X = Br 27 .2 –2 5. 3 26 .4 –2 4. 8 26 .5 –3 5. 9 3 Pt (I V ), X = I 25 .0 –3 0. 3 24 .2 –2 9. 9 24 .3 –4 1. 4 4 Pt (I V ), X 1, X 2= PH 3; X 3, X 4= C l; 18 .5 –4 0. 4 17 .5 –3 9. 9 18 .0 –5 2. 2 5 Pt (I V ), X 1, X 2= C l; X 3, X 4= PH 3; 27 .6 –3 3. 2 26 .5 –3 3. 5 27 .6 –4 6. 6 6 Pt (I I) ,X = C l 40 .6 –3 .7 /2 6. 1 39 .1 –2 .9 /2 7. 6 39 .0 –3 .0 /1 7. 9 7 Pt (I I) ,X = Br 35 .4 –8 .3 /2 0. 6 34 .2 –7 .3 /2 1. 9 34 .0 –7 .5 /1 2. 6 8 Pt (I I) ,X = I 31 .2 –1 2. 3/ 13 .0 30 .0 –1 1. 3/ 14 .3 29 .9 –1 1. 5/ 5. 0 9 Pt (I I) ,X = N H 3 35 .2 –8 .5 /0 .9 33 .7 –7 .8 /1 .5 32 .2 –9 .3 /– 10 .0 10 Pt (I I) ,X = PH 3 19 .3 –2 7. 4/ –1 7. 8 18 .2 –2 6. 4/ –1 7. 5 18 .5 –2 6. 9/ –2 9. 0 11 Pd (I V ), X = Br 12 .9 –5 1. 3 12 .5 –5 0. 5 12 .9 –6 1. 2 12 Pd (I V ), X = I 11 .4 –5 5. 4 10 .9 –5 4. 6 11 .3 –6 5. 8 13 Pd (I V ), X 1, X 2= PH 3; X 3, X 4= C l; 7. 3 –5 7. 4 6. 6 –5 6. 3 7. 2 –6 7. 4 14 Pd (I I) ,X = C l 18 .9 –2 2. 2/ 2. 3 17 .8 –2 1. 1/ 4. 2 18 .2 –2 1. 2/ –5 .2 15 Pd (I I) ,X = Br 15 .0 –2 6. 9/ –4 .8 14 .0 –2 5. 6/ –3 .1 14 .2 –2 6. 0/ –1 2. 2 16 Pd (I I) ,X = I 11 .9 –3 0. 8/ –1 2. 0 11 .0 –29. 5/ –1 0. 4 11 .0 –2 9. 9/ –1 9. 4 17 Pd (I I) ,X = N H 3 15 .3 –2 5. 7/ –1 5. 4 14 .3 –2 4. 8/ –1 4. 7 13 .6 –2 6. 0/ –2 5. 5 18 Pd (I I) ,X = PH 3 6. 8 –4 2. 3/ –3 3. 8 5. 9 –4 1. 0/ –3 3. 3 6. 1 –4 2. 1/ –4 4. 4 19 [R hI II (C H = C H 2) 2( PH 3) 3C l] 17 .8 –2 9. 1 16 .8 –2 8. 8 18 .2 –4 1. 0 20 [I rI II (C H = C H 2) 2( PH 3) 3C l] 28 .5 –1 3. 8 27 .3 –1 3. 7 28 .4 –2 6. 4 21 [R uI I ( C H = C H 2) 2( PH 3) 3C l] 28 .1 –4 4. 8/ 23 .2 27 .6 –4 2. 8/ 21 .9 29 .7 –3 9. 0/ 13 .0 22 [O sI I ( C H = C H 2) 2( PH 3) 3C l] 34 .1 –4 2. 2/ 35 .0 33 .5 –4 0. 1/ 34 .2 35 .5 –3 6. 2/ 23 .4 a DE ≠ = E( T S) –E (I ni t) ;f or M IV co m pl ex es :D E= E( Pr od )– E( In it ); fo r M II co m pl ex es :D E= E( p- C om p) –E (I ni t) ,w hi le t he v al ue s af te r/ co rr es po nd t o DE = E( Pr od )– E( In it ). T he s am e is a pp lic ab le a ls o fo r en th al pi es a nd fr ee e ne rg ie s. b Tw o vi ny l l ig an ds a re ci st o ea ch o th er ,X 1 an d X 2 ar e tr an st o vi ny l l ig an ds ,a nd X 3 an d X 4 ar e tr an s t o ea ch o th er . c R ed uc ti ve e lim in at io n th ro ug h th e s- ci s p at hw ay (s ee F ig .4 ); al l t he o th er c al cu la ti on s w er e do ne fo r th e s- tr an s p at hw ay . Transition Metal Catalyzed s-Bond Activation and Formation Reactions 21 Reactions of both 4 and 5 lead to the same products, trans-[Pt(PH3)2Cl2] and buta-1,3-diene. Therefore, the difference in reaction energies (DE) reflects the relative stability of initial bis-s-vinyl derivatives; 5_Init is thermodynamically more stable by 7.2 kcal/mol than 4_Init.Very likely, destabilization of 4_Init due to mutual trans orientation of vinyl and phosphine ligands is responsible for facilitating the reductive elimination process 4_InitÆÆ4_TSÆÆ4_Prod. 5.3 Reductive Elimination from PtII Halogen Complexes [Pt(CH=CH2)2X2]2– (X=Cl, Br, I) C-C bond formation initiated from the square planar bis-s-vinyl complexes (6, 7 and 8) also proceeds through the three-centered transition state (Fig. 5). Upon metal-carbon s-bond breakage and reductive elimination, the coordination vacancy becomes available at the metal center and a p-complex of buta-1,3-di- ene is formed. Double bond coordination (h2-C=C) to the metal atom is preferred in this case, while the structure with central C-C bond coordination has an imaginary frequency corresponding to h2–C–CÆh2–C=C rearrange- ment. The same trend as noted above for PtIV is observed for the series of PtII complexes (6 to 8, Table 2); heavier halogen ligands make C-C bond formation easier by decreasing the activation energy and favoring the process thermo- dynamically. However, comparing the C-C reductive elimination reaction from PtII (6–8, Table 2) complexes with those of PtIV (1–3, Table 2) complexes, one can deduce a clear preference of the higher oxidation state. For the PtIV derivatives the activation barriers are lower by 11.2–6.2 kcal/mol and the processes are more exothermic by 16.2–18.0 kcal/mol than the corresponding PtII species. 5.4 Reductive Elimination from PtII Complexes with Amine and Phosphine Ligands [Pt(CH=CH2)2X2] (X=NH3, PH3) The energetics of the C-C bond formation reaction starting from the amine complex 9 (DE≠=35.2 and DE=–8.5 kcal/mol) are similar to those for the bro- mide derivatives (DE≠=35.4 and DE=–8.3 kcal/mol, Table 2). However, reductive elimination reaction from phosphine complex 10 is much easier with the activation energy of only 19.3 kcal/mol and reaction exothermicity of –27.4 kcal/mol (Table 2). This barrier is only slightly higher than that for the corresponding PtIV phosphine complex (4, Table 2). In addi- tion, Pt(PH3)2 can be considered as a rather stable product in contrast to Pt(NH3)2 and PtX22– (X=Cl, Br, I). The finding is consistent with the experi- mental experience that phosphine is often added to reaction mixtures to keep the catalyst from decomposing so that the process can take place under homogeneous conditions [1a, 28, 29, 46]. Obviously, phosphine derivatives are the best candidates for reductive elimination process among the PtII com- plexes. 22 D. G. Musaev · K. Morokuma Fi g. 5 S- tr an sr ed uc ti ve e lim in at io n pa th w ay fr om s qu ar e pl an ar c om pl ex es 5.5 Reductive Elimination from PdIV Complexes [Pd(CH=CH2)2X4]2– (X=Cl, Br, I) In the PdIV complexes, the C-C bond formation takes place rather easily with very low barriers of 12.9 and 11.4 kcal/mol for 11 (X=Br) and 12 (X=I), respec- tively (Table 2). These are about 14 kcal/mol lower than for the corresponding PtIV complexes, while the exothermicity of the reactions is increased by as much as 26 kcal/mol. The high exothermicity suggests very early transition state struc- tures. The results (see Table 2) show that reductive elimination from PdIV com- plexes would proceed most easily among the MII and MIV halogen derivatives (M=Pt, Pd). 5.6 Reductive Elimination from Mixed PdIV Complex [Pd{trans-(CH=CH2)2(PH3)2}Cl2] We excluded from consideration the complexes with PH3 ligands located cis to s-bonded carbon atoms, since this type of PtIV derivatives (5, Table 2) showed very small effect of PH3 on reaction energetics. Previous PtIV/PtII results (4 and 10, Table 2) suggest that phosphines trans to carbons should significantly facilitate the reductive elimination process. This trend seems to be rather gen- eral and applicable to PdIV complexes as well; the corresponding PdIV complex 13 with phosphines trans to carbons has a low activation barrier for C-C bond formation of only 7.3 kcal/mol. 5.7 Reductive Elimination from PdII Halogen Complexes [Pd(CH=CH2)2X2]2– (X=Cl, Br, I) Within the series of PdII halogen complexes 14 (X=Cl), 15 (X=Br) and 16 (X=I), the expected trends in the energies and geometry changes are again found; the heavier the halogen, the smaller the activation energy and the earlier the transition state. The smallest barrier is calculated for iodide complex DE≠= 11.9 kcal/mol and the largest 18.9 kcal/mol for chloride, with bromide in-be- tween, 15.0 kcal/mol. Increase in activation energies is accompanied by de- crease in exothermicity (Table 2).As discussed earlier for the corresponding Pt compounds, reductive elimination reaction from PdII halogen complexes also gives anionic Pd0 species [47]. 5.8 Reductive Elimination from PdII Complexes with Nitrogen and Phosphine Ligands [Pd(CH=CH2)2X2] (X=NH3, PH3) The activation and reaction energies for the complexes with X=NH3 and X=Br (17 and 15 in Table 2) are similar: DE≠=15.0 and 15.3 kcal/mol and DE=–26.9 and –25.7 kcal/mol for X=Br and X=NH3, respectively. Thus, both complexes would show a similar reactivity in the reductive elimination process. However, Transition Metal Catalyzed s-Bond Activation and Formation Reactions 23 the stability of [Pd0Br2]2– and [Pd0(NH3)2] relative to the corresponding p-com- plex differs significantly, the former being ca. 11 kcal/mol less stable than the latter.For the PdII phosphine complex 18 (X=PH3), vinyl-vinyl coupling reaction is very easy, with the barrier of only 6.8 kcal/mol, the smallest barrier reported in this article. 5.9 Reductive Elimination from RhIII, IrIII, RuII and OsII Complexes We extended the study of C-C reductive elimination reactions to other members of late transition metals in order to find possible alternatives to Pd/Pt complexes for catalytic coupling reactions. The calculations were performed only for the corresponding phosphine complexes, for which experimental precedents for the reaction were reported. In the case of Rh and Ir derivatives, an extra s-bonded ligand has to be added to maintain correct oxidation state, because RhII and IrII compounds are rarely known [48]. The chloride has been chosen for this purpose. Thus the model compounds studied are 19 [RhIII(CH=CH2)2(PH3)3Cl], 20 [IrIII(CH=CH2)2(PH3)3Cl], 21 [RuII(CH=CH2)2(PH3)3],and 22 [OsII(CH=CH2)2- (PH3)3]. As in the previous cases (Figs. 4 and 5), vinyl-vinyl coupling for these com- pounds also occurs through the three-centered transition states. The smallest ac- tivation barrier among these four compounds is found for RhIII19 (17.8 kcal/mol), while those for the other complexes are considerably higher (28.1–34.1 kcal/mol). To summarize, the present calculations show that RhIII-based complexes can be considered as possible catalysts for vinyl-vinyl reductive elimination, while IrIII, RuII and OsII analogs are likely to be less active. Once again, our calculations shown that the second raw metals have lower barriers as well as higher exother- micity than the third row metals within each subgroup RhIII>IrIII and RuII>OsII. 5.10 General Discussion Thus, our calculations, in an agreement with the available experiments, have demonstrated that the reactivity of the [M(CH=CH2)2Xn] complexes (for M=Pd and Pt) in C-C bond formation depends on the nature of the ligand X, and decreases in the order: X=PH3>I>Br,NH3>Cl for both MIV and MII oxidation states. In all the cases, phosphine ligands decrease the activation barriers and increase the exothermicity of the reaction. The present results also show that all the second row metals (Ru, Rh and Pd) show the lower barriers and higher exothermicity than the corresponding third raw metals (Os, Ir and Pt) within each subgroup (Table 2), i.e. the reactivity of the studied complexes decreases via PdII>PtII, PdIV>PtIV, RhIII>IrIII, and RuII>OsII. The complexes of PtIV are more reactive than corresponding complexes of PtII. Similar results have been obtained for Pd complexes, while for them this effect is less pronounced. Con- sidering the most reactive phosphine complexes, the following overall relative 24 D. G. Musaev · K. Morokuma reactivity order in vinyl-vinyl coupling reaction may be suggested for these metals: PdIV, PdII>PtIV, PtII. Thus, Pd complexes are suggested to be the most reactive for this reaction. Furthermore, these results again demonstrate the importance of the elec- tronic configuration of the metal for s–bond activation/formation reaction.As pointed out earlier, the change in the degree of oxidation of metal atoms dur- ing the oxidative addition/reductive elimination reactions could be described in terms of the promotion of electronic configuration of metal atoms. In the M0, MII, and MIV complexes, both Pt and Pd atoms possess d10, s1d9 and s2d8 elec- tronic configurations, respectively.13 In M0 no covalent bonds are possible, since all five d orbitals are doubly occupied. In contrast, MII and MIV atoms can make two and four covalent bonds, respectively, through the hybrid s and d orbitals. The energy differences between the lower-lying electronic configurations of Pd and Pt atoms, and the calculated average reaction energies for C-C reductive elimination reactions of the Pd/Pt-complexes are given in Table 3. As seen from Tables 3 and 2, the concept of lower lying electronic configu- rations provides reliable qualitative description of the systems studied. In particular, (i) MIVÆMII reductive elimination is always more exothermic than MIIÆM0, which is consistent with the calculated s2d8Æs1d9 and s1d9Æd10 pro- motion energies, and (ii) both PdIVÆPdII and PdIIÆPd0 processes are more exothermic compared to PtIVÆPtII and PtIIÆPt0, which is again agreed with the calculated s2d8Æs1d9 and s1d9Æd10 promotion energies of Pd and Pt atoms. In agreement with the Hammond postulate, (i) the activation energies are lower when the reaction starts from MIV derivatives than from MII, and (ii) C-C bond formation involving palladium complexes requires significantly smaller bar- riers than with platinum. Transition Metal Catalyzed s-Bond Activation and Formation Reactions 25 Table 3 Promotion and average reaction energies (in kcal/mol) for C-C reductive elimina- tion reactions from platinum and palladium complexesa,b Reaction Atomic energy DEaverage differencec X=Cl, Br, I X=PH3, NH3d Pd Pt Pd Pt Pd Pt MIVÆMII –56.0 –14.8 –53.4 –27.8 –57.4 –40.4 (s2d8Æs1d9) (11,12) (2,3) MIIÆM0 –21.9 11.1 –4.8 19.9 –33.8 –17.8 (s1d9Æs0d10) (14–16) (6–8) [–15.4] [0.9] a The compounds used for averaging are given in parentheses (see Table 2 for energies). b For PtII and PdII, DE with respect to the final products (MX2+diene), rather than to p-com- plexes is used. c Experimentally determined. d Values for X=NH3 are given in brackets. Similar considerations are also applicable for the reductive elimination reactions for the other platinum group metals. Our calculations on reductive elimination reactions have shown an energetic preference of RhIII com- pounds to IrIII, RuII and OsII compounds. The fact can be rationalized taking into account that the only exothermic promotion energy of –37.6 kcal/mol13 is expected for s2d7Æs1d8 (RhIIIÆRhI). In contrast, s2d7Æs1d8(IrIIIÆIrI) and s1d7Æd8 (RuIIÆRu0) are endothermic by 25.1 and 9.2 kcal/mol respec- tively [13]. 5.11 Comparison of the Vinyl-Vinyl (Csp2-Csp2) and Alkyl-Alkyl (Csp3-Csp3) Reductive Elimination Comparing Csp2-Csp2 reductive elimination with the similar process involving alkyl groups (Csp3-Csp3), one may clearly note the same qualitative trends. Experimental studies have shown that dimethyl complexes of Pt(II) eliminate ethane much easier than their Pt(II) analogs [30, 35], and reactivity of Pt(IV) methyl complexes is higher than Pt(II) [30]. These results are in line with our findings (Table 2). However, the absolute values of activation energies computed for vinyl-vinyl coupling are significantly lower than that for methyl-methyl coupling. Particu- larly, DE≠=49.8 kcal/mol (MP4//MP2 level) [39a], DE≠=60.8 kcal/mol (MP4//HF) [39b,c], and DH≠=41.1 kcal/mol (GVB) [37a] were reported for ethane reductive elimination from [PtII(CH3)2(PH3)2]. The values are much higher than DE≠=1 9.3 kcal/mol (DH≠=18.2 kcal/mol) calculated in the present work for vinyl-vinyl coupling from [PtII(CH=CH2)2(PH3)2]. Ethane elimination from [PdII(CH3)2- (PH3)2] was found to proceed with DH≠=10 kcal/mol (GVB) [37a] and DE≠=26.3 kcal/mol (MP4//HF) [39c], while our value for [PtII(CH=CH2)2(PH3)2] is again lower DE≠=6.8 kcal/mol (DH≠=5.9 kcal/mol). Similar relationships are found for the ethane reductive elimination from [RhIII(CH3)2(PH3)Cp] DE≠= 65.6 kcal/mol (MP2//HF) [43] and [PtIV(CH3)2(PH3)2Cl2] DH≠=34.2 kcal/mol (GVB) [37a] as compared to [RhIII(CH=CH2)2(PH3)2Cl] DE≠=17.8 kcal/mol and [PtIV(CH=CH2)2(PH3)2Cl2] DH≠=17.5 kcal/mol given in the present work. In addition, vinyl-vinyl coupling is generally much more exothermic than methyl- methyl coupling [37a, 39, 43].The differences may come from the relative stability in the products; D(C-C) in buta-1,3-diene, 115.8 kcal/mol, is consid- erably larger than in ethane, 90.0 kcal/mol [49]. Thus, the vinyl-vinyl coupling is energetically more favored than the methyl-methyl reductive elimination. These theoreticalresults fairly well agree with experimental findings, which point out that practical implementation of Csp3–Csp3 coupling is rather prob- lematic due to slow reductive elimination [50], in contrast to the processes involving vinyl groups. 26 D. G. Musaev · K. Morokuma 6 Concluding Remarks Above we have presented four different factors that control the catalytic activ- ity of transition metals toward s-bonds. In the mono-nuclear transition metal systems (1) the availability of the lower lying s1dn–1 and s0dn states of the tran- sition metal atoms, and (2) the nature of the ligands facilitating the reduction of the energy gap between the different oxidative states of the transition metal centers are very crucial. Meanwhile, as was demonstrated, in the transition metal clusters the “cooperative” (or “cluster”) effects play important roles in the catalytic activities of these clusters. Another factor, which could be very im- portant for catalytic activity of the transition metal systems is shown to be their redox activity. However, those four factors are definitely not the only ones that play crucial roles in the catalytic activity of transition metal systems with s-bonds. The transition metal catalyzed s-bond activation and formation are very complex processes and need more detailed investigations. 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(a) Basch H, Mogi K, Musaev DG, Morokuma K (1999) J Am Chem Soc 121:7249; (b) Mo- rokuma K, Musaev DG, Vreven T, Basch H, Torrent M, Khoroshun DV (2001) IBM J Res Dev 45:367; (c) Torrent M, Musaev DG, Basch H, Morokuma K (2001) J Phys Chem B 105:4453; (d) Basch H, Musaev DG, Mogi K, Morokuma K (2001) J Phys Chem B 105:8452; (e) Torrent M, Mogi K, Basch H, Musaev DG, Morokuma K (2001) J Phys Chem B, 105:8616; (f) Basch H, Musaev DG, Mogi K, Morokuma K (2001) J Phys Chem A 105:3615; (g) Torrent M, Musaev DG, Morokuma K (2001) J Phys Chem B 105:322; (h) Torrent M, Vreven T, Musaev DG, Morokuma K, Farkas O, Schlegel HB (2002) J Am Chem Soc 124:192; (i) Torrent M, Musaev DG, Basch H, Morokuma K (2002) J Comput Chem 23:59 26. For some recent examples of vinyl-vinyl coupling see: (a) Gallagher WP, Terstiege I, Maleczka RE (2001) J Am Chem Soc 123:3194;
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