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

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

Palladium Complexes of N-Heterocyclic Carbenes as Catalysts for Cross-Coupling Reactions—A Synthetic Chemist's Perspective

Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

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Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

N-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5) react with Ni(CO)4 to give the saturated tricarbonyl complexes Ni(CO)3(IMes) (8), Ni(CO)3(SIMes) (9), Ni(CO)3(IPr) (10), Ni(CO)3(SIPr) (11), and Ni(CO)3(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)3(PR3) by recording their νCO stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1−5 are surprisingly small. Novel, unsaturated Ni(CO)2(IAd) (13) and Ni(CO)2(ItBu) (14) compounds are obtained from the reaction of Ni(CO)4 with IAd (6) and ItBu (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni−C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni(0) and Ni(II) complexes. Computational studies on compounds 8−14 are in line wi...

Steric and electronic properties of N-heterocyclic carbenes (NHC) : A detailed study on their interaction with Ni(CO)4

Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

The synthesis and characterization of three- and four-coordinate Ni(CO)n(NHC) (n = 2, 3; NHC = N-heterocyclic carbene) complexes are reported. Reactions with CO of the Ni(CO)2(NHC) complexes lead to the quantitative formation of Ni(CO)4. Investigation of this reaction under equilibrium conditions allows for the determination of Ni−NHC bond dissociation energies.

Stable, three-coordinate Ni(CO)(2)(NHC) (NHC = N-heterocyclic carbene) complexes enabling the determination of Ni-NHC bond energies.

Rhodium alkoxide complexes : formation of an unusually strong intermolecular hydrogen bond in (PMe3)3Rh-Otol(HOtol)

Reaction of Mo(N[R]Ar)3 (R = tBu or C(CD3)2CH3) with N2O gives rise exclusively to a 1:1 mixture of nitride NMo(N[R]Ar)3 and nitrosyl ONMo(N[R]Ar)3, rather than the known oxo complex OMo(N[R]Ar)3 and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)3 with N2O and, independently, the heat of reaction of Mo(N[R]Ar)3 with NO. Derived from the latter measurements is an estimate (155.3 ± 3.3 kcal·mol-1) of the molybdenum−nitrogen bond dissociation enthalpy for the terminal nitrido complex, NMo(N[R]Ar)3. Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar)3 shows that the nitrous oxide N−N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)3 and N2O, consistent with a mechanism featuring post-rate-determining dinuclear N−N bond scission, but also consistent with cleavage of the N−N bond at a single metal center in a mechan...

Carl D. Hoff

Papers

Steric and electronic properties of N-heterocyclic carbenes (NHC) : A detailed study on their interaction with Ni(CO)4

Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

Stable, three-coordinate Ni(CO)(2)(NHC) (NHC = N-heterocyclic carbene) complexes enabling the determination of Ni-NHC bond energies.

Rhodium alkoxide complexes : formation of an unusually strong intermolecular hydrogen bond in (PMe3)3Rh-Otol(HOtol)

On the origin of selective nitrous oxide N-N bond cleavage by three-coordinate molybdenum(III) complexes.