Comparison of Steric and Electronic Requirements for C−C and C−H Bond Activation. Chelating vs Nonchelating Case

Abstract

C-H bond activation was observed in a novel PCO ligand 1 (C(6)H(CH(3))(3)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)()]BF(4) (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 degrees C, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH(3))Rh(C(6)H(CH(3))(2)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))BF(4)), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF(4)(-) anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation.

CC versus CH Activation and versus Agostic CC Interaction Controlled by Electron Density at the Metal Center

“…While studying the activation of strong bonds, we found that cationic unsaturated Rh I precursors can undergo carbon ± carbon bond activation upon coordination to PCP, [7] PCN, [9] or PCO [5] ligands. Thus, the PCN ligand 2 reacted with the cationic complex [Rh(C 2 H 4 ) 2 (solv) n ]BF 4 (4) [10] to quantitatively yield the CÀC activation product 5 as the only product under very mild conditions (Scheme 2)…”

“…Thus, we have demonstrated that upon coordination to bischelating PCX (X P, [3] N, [4] O [5] ) ligand systems (Scheme 1) a metal center can be directed to the proximity of an unstrained, ™hidden∫ CÀC bond, resulting in thermodynamically and kinetically favored CÀC bond activation. [6] Moreover, utilizing these PCX systems, a remarkable degree of control over metal insertion into strong CÀH versus CÀC bonds can be achieved by choice of solvent, [7] metal, or ligand set.…”

Cleavage of unreactive bonds with pincer metal complexes

“…33 With the related PCO pincer ligand precursor 19, C-H activation is kinetically preferred at room temperature (Scheme 9). 34 However, this 65 reaction is not selective and produces a mixture of 20 and 21, indicating that both C-H bonds in ortho position to the phosphine are activated. Upon heating, C-C bond activation takes place exclusively at the position ortho to both the O and P donor groups to afford 22.…”

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

“…356 With the PCO pincer ligand precursor 206c, cyclometalation via C-H activation is kinetically preferred at room temperature (Scheme 89). 357 However, this reaction is not selective and produces a mixture of 212 and 213.…”