ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

ion and completion of the cyclometalation reaction are feasible and produce the cyclometalated complex 205. Notably, addition of a strong acid reverts the cyclometalation and regenerates the agostic species 204. Small changes in the rhodium precursor significantly modify the product outcome and lead at fast rates to the cyclometalated complex 203, that is, the final oxidative addition product S. Similarly, alkene C-H bond activation has been observed to occur readily in a PCP ligand setting.348 Milstein and co-workers developed a procedure for preparing the cyclometalated PCP, PCN, or PCO pincer complexes 209 by rhodium-mediated cleavage of a strong Caryl-Calkyl bond (Scheme 87).32,349,350 Kinetically, Calkyl-H bond activation is competitive and complex 208 is formed as a second product at essentially similar rates. However, C-C bond cleavage appears to be the thermodynamically more favored process. Obviously, the higher energy required for Caryl-Calkyl bond activation (bond dissociation energy 427 kJ mol-1) compared to C-H bond breaking (368 kJ mol-1) is compensated by the stronger Rh-Caryl bond and the formation of two fiverather than six-membered metallacycles. Because rhodium-mediated Caryl-Calkyl and Calkyl-H bond activations in 206 are thermodynamically and kinetically very similar processes,351 subtle changes in the ligand parameters strongly affect the reaction outcome. For example, PPh2 donors induce C-H bond cleavage to afford 208d exclusively at ambient temperatures. C-C activation and formation of 209d is only induced upon heating under H2 atmosphere,352 presumably because these conditions allow the product selectivity to be controlled thermodynamically. In contrast, bulky PtBu2 donors as in 206a promote C-C bond activation already at room temperature, and the aryl complex 209a is the only product observed. Similarly, replacing one phosphine donor by a harder NEt2 group or the use of less basic phosphinite donors results in selective C-C bond cleavage at room temperature. These preferences suggest steric factors to play a more dominant role than electronic differences in determining the C-C vs C-H selectivity. Steric congestion induced by the donor substituents has been proposed to arrange the orbitals of the metal center to be directed toward the C-C bond rather than toward the C-H bond.353 Kinetic investigations have identified the coordination compound 207 as a common intermediate of Calkyl-H and the Caryl-Calkyl bond activation. On the basis of the similar rates for both cleavage processes and because of the absence of any detectable intermediate, formation of 207 and not C-H or C-C bond activation appears to be rate-limiting. While displacement of a weakly bound olefin ligand in the metal precursor may be fast, formation of the 8-membered metallacycle in 207 may be less favored, especially due to the potential for forming diand polymeric coordination compounds. It is worth noting that, upon substituting the PCP pincer ligand to a PCN donor array, the coordination complex 207c becomes stable at low temperature and has been spectroscopically characterized.354 As a consequence, the rate-determining step is shifted to a later stage on the reaction coordinate. Notably, slight modifications in the metal precursor, that is, utilization of [Rh(C2H4)(CO)(solv)2]BF4 instead of [RhCl(C2H4)2]2, alters the stability of the intermediates and produces complex 210 (Scheme 88).355 This complex may be represented by an agostic interaction of the C-C bond with the rhodium center, or alternatively by an arenium structure including an sp3-hybridized ipso carbon. Because of only weak Cipso-Rh interactions in solution (e.g., the absence of any 103Rh coupling), the agostic description is favored. The different product outcome has been presumed to originate from the reduced electron density at the cationic rhodium(I) center due to the strongly withdrawing CO ligand and the relatively weak Rh-N bond. This configuration Scheme 87 Scheme 86 Cyclometalation Using d-Block Transition Metals Chemical Reviews, 2010, Vol. 110, No. 2 603

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Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

Kinetic Isotope Effects in the Study of Organometallic Reaction Mechanisms

Trans-effects in octahedral transition metal complexes

Publisher Summary This chapter discusses the X-ray structural analyses of organolithium compounds. Lithium compounds, perhaps the most widely used organometallic synthetic intermediates, have fascinating structures. Nevertheless, organic chemistry textbooks often treat organolithium compounds merely as “carbanions” or as monomers. The chemical behavior of lithium compounds cannot be understood without the knowledge of the structures, the degree of aggregation, and the extent and nature of solvation. “Terminally bonded” organolithium compounds afford, in principle, the simplest examples of lithium–carbon σ–bonds. This class comprises lithium compounds with a single contact between lithium and a carbon atom. The dimer of lithiobicyclo butane, and of lithiodimethylsulfide, illustrates the bridging between sp 3 -hybridized carbon atoms while phenyllithium exemplifies bridging between sp 2 carbon atoms. A few organolithium compounds crystallize in aggregates with more than four subunits. Such compounds, derived from substituted acetylenic substrates (excluding simple acetylides), form oligomeric structures involving bridging lithium atoms and possible interactions of the lithium atoms with acetylenic π system. The benzophenonedilithium compound 50, formed by the reduction of benzophenone with lithium metal, crystallizes as a dimer. A large number of mixed-metal organometallic compounds have been prepared in which the hydrocarbon ligands bridge the lithium and the other metal atoms. A number of lithium/transition-metal compounds involving metal–metal multiple bonds as well as lithium–alkyl group interactions have been structurally characterized. The structure of lithium compounds without lithium–carbon bonds summarizes the structures of some lithium compounds that do not involve Li–C contacts.

X-ray structural analyses of organolithium compounds

Thermal rearrangement of diaryl(2,2′-bipyridyl)platinum(II) complexes normally proceeds via consecutive 3-metallations of the bipyridyl ligand with hydrogen migration and elimination of two equivalents of arene, while, in the presence of free pyridine, the process can be interrupted at a monoarylplatinum intermediate by formation of a dinuclear pyridine adduct whose molecular structure has been determined.

‘Roll-over’ 3-metallation of co-ordinated 2,2′-bipyridyl in the thermal rearrangement of diary(bipyridyl)platinum(II) complexes: molecular structure of (µ-bidyl)[PtPh(Butpy)]2

Organometallic and organobimetallic complexes of platinum(II) containing 2,2′-bipyrimidyl: syntheses and spectroscopic characteristics

Mechanisms of thermolytic rearrangement of dineophylplatinum(II) complexes via intramolecular carbon-hydrogen bond activation

By the interaction either of transition-metal dialkylamides with bis(t-butyl)methanol or of the halide with the lithium alkoxide, a series of neutral alkoxides or lithium alkoxo-metallate anions have been prepared for chromium(III,IV), manganese(II), iron(III), cobalt(II), and copper(I). Electron paramagnetic resonance and electronic absorption spectra are reported. The crystal and molecular structures of the complexes LiCr(OCHBut2)4·thf(1), Cr(OCHBut2)4(2), and LiFe(OCHBut2)4·But2CHOH (3) have been determined by an X-ray crystallographic study. Compound (1) is triclinic, space group P, with a= 19.986(2), b= 11.618(2), c= 11.390(1)A, α= 117.61(1), β= 76.45(1), γ= 93.11(1)°, and Z= 2. Compound (2) is monoclinic, space group C2/c, with a= 20.249(3), b= 10.748(1), c= 20.275(2)A, β= 116.68(1)°, and Z= 4. Compound (3) is monoclinic, space group P21/n, with a= 18.308(3), b= 22.505(4), c= 11.979(5)A, β= 94.20(1)°, and Z= 4. The structures were determined using data measured on an automatic diffractometer and refined by least squares to R values of 0.068 (3 396 observed data), 0.067 (2 057 data), and 0.061 (4 879 data) for (1), (2), and (3) respectively. All structures contain a distorted tetrahedrally co-ordinated Cr or Fe atom; and in (1) and (3), the lithium ion is incorporated into the MO4 co-ordination sphere, and bridges two of the four alkoxo-oxygen atoms. The transition-metal geometries are discussed in terms of steric electronic factors.

Synthesis and properties of bis(t-butyl)methoxides of chromium(III,IV), manganese(II), iron(III), cobalt(II), and copper(I). The crystal and molecular structures of lithium tetrakis[bis(t-butyl)methoxo]chromate(III)–tetrahydrofuran (1/1), tetrakis[bis(t-butyl)methoxo]chromium(IV), and lithium tetrakis[bis(t-butyl)methoxo]ferrate(III)–bis(t-butyl)-methanol (1/1)

G. Brent Young

Papers

‘Roll-over’ 3-metallation of co-ordinated 2,2′-bipyridyl in the thermal rearrangement of diary(bipyridyl)platinum(II) complexes: molecular structure of (µ-bidyl)[PtPh(Butpy)]2

Organometallic and organobimetallic complexes of platinum(II) containing 2,2′-bipyrimidyl: syntheses and spectroscopic characteristics

Mechanisms of thermolytic rearrangement of dineophylplatinum(II) complexes via intramolecular carbon-hydrogen bond activation

Mechanisms of Thermolytic Rearrangement of cis-Bis(silylmethyl)platinum(II) Complexes: .beta.-Carbon Transfer Predominates over Hydrogen Transfer