σ-Bond Activation of Small Molecules and Reactions Catalyzed by Transition-Metal Complexes: Theoretical Understanding of Electronic Processes

Abstract

σ-Bond activations of R1-R2 and R1-X1 (R1, R2 = H, alkyl, aromatics, etc.; X1 = electronegative group) by transition-metal complexes are classified into two main categories: σ-bond activation by a metal (M) center and that by a metal-ligand bond. The former is classified into two subcategories: concerted oxidative addition to M and stepwise oxidative addition via nucleophilic attack of M. The latter is also classified into two subcategories: heterolytic activaton by M-X2 (X2 = anion ligand) and oxidative addition to M-L (L = neutral ligand). In the concerted oxidative addition, charge transfer (CT) occurs from the M d orbital to the σ* antibonding orbital of R1-R2, the clear evidence of which is presented here. The concerted oxidative additions of Ph-CN, Me-CN, and Ph-Cl to a nickel(0) complex are discussed as examples. The stepwise oxidative addition occurs through nucleophilic attack of M to R1-X1 to form an ion-pair intermediate. In the nucleophilic attack, CT occurs from the M dσ to either the σ* orbital or empty pπ orbital of R1-X1. Solvation plays a crucial role in stabilizing the transition state and ion-pair intermediate. The oxidative addition reactions of Ph-I, CH3-Br, and Br2B(OSiH3) to platinum(0), platinum(II), and palladium(0) complexes are discussed. In the heterolytic activation of R1-R2 by an M-X2 bond, R1 and R2 are bound with M and X2, respectively, indicating that R1 becomes anion-like and R2 becomes cation-like. CT mainly occurs from the X2 ligand to the σ* antibonding orbital of R1-R2 and also from R1 to the M empty d orbital. In the oxidative addition to an M-L moiety, R1 is bound with M, R2 is bound with L, and thus-formed L-R2 is bound with M. The oxidative addition reaction of the Si-H bond of silane to Cp2Zr(C2H4) and that of the H-H bond of H2 to Ni[MesB(o-Ph2PC6H4)2] are discussed as examples. The importance of the σ-bond activation in such catalytic reactions as nickel(0)-catalyzed phenylcyanation of alkyne, nickel(0)-catalyzed carboxylation of phenyl chloride, ruthenium(II)-catalyzed hydrogenation of carbon dioxide, and the Hiyama cross-coupling reaction is discussed based on theoretical studies.

Transition‐Metal‐Mediated Cleavage of Fluoro‐Silanes under Mild Conditions

“…Because of this characteristicf eature, this type of s-bond metathesis reaction is named "heterolytic sbond activation". [15] In the SiÀFb ond activation by the iridium hydride complex B1,t he iridium center forms an ew bond with the electropositives ilicon atom and the hydridel igand forms an ew bond with the electronegative Fa tom, as shown in Figure 3. This situation is opposite to that of normal"hetero- www.chemeurj.org lytic s-bond activation".…”

“…Three mechanistic scenarios are conceivable to account for the SiÀF s-bond activation by iridium hydride 2 ( Figure 2): www.chemeurj.org a) concerted oxidative addition, b) S N 2t ype reaction, and c) sbond metathesis. [15] To determine the pathway through which the reaction occurs, density functionalt heory (DFT) calculations were performed at the B3PW91 [16] level of theory using model compounds in which the phenyl groups at phosphorus were replaced by methyl groups. [18] Consideringt hese results, the initial step in the reactioni sp robably the formation of A1 or B1, upon displacemento ft wo PPh 3 at Ir by the phosphine sidearms of 1a (Figure 3).…”

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

“…The authors confirmed the electrophilic nature of the active oxidizing species. The latter demonstrated high 3˝/2˝ratios (40)(41)(42)(43)(44)(45)(46)(47)(48)(49), alcohol/ketone ratios > 5, and high stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane [187]; kinetic studies revealed a sizable kinetic isotopic effect (k H /k D 4.5-4.7), indicating that that the H atom lies on the reaction coordinate and participates in the C-H bond breaking step. Altogether, the experimental data provided indirect evidence in favor of the participation of a high-valent (presumably oxometal(V) [188]) species, ruling out the contribution of a free-radical-driven oxidation pathway.…”

Ironing out the photochemical and spin-crossover behavior of Fe(II) coordination compounds with computational chemistry

“…Transition metal complexes can serve a myriad of functions. These include, but are not limited to, chemotherapeutic agents [1][2][3], CO 2 activation [4][5][6][7], artificial water splitting [8][9][10][11][12], C-H and C-C activation [13][14][15][16][17], and catalysis of numerous other organic transformations [18][19][20][21]. This diversity in function is the result of stark differences in the electronic structure of transition metal complexes in contrast to the chemistry exhibited by main group elements.…”