Comparison of steric and electronic requirements for C-C and C-H bond activation. Chelating vs nonchelating case

C-H bond activation was observed in a novel PCO ligand 1 (C6H(CH3)(3)(CH2OC H3)(CH2P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)]BF4 (sol v = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscop ically. Two products were formed in each case, namely those containing unco ordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectivel y. Upon heating of the C-H activation products in methanol at 70 degreesC, C-C bond activation takes place. Solvent evaporation under vacuum at room t emperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH3)Rh(C6H(CH3)(2)(CH2OCH3)(CH2P(t-Bu)(2))BF4), was characterize d by X-ray crystallography, which revealed a square pyramidal geometry with the BF4- anion coordinated to the metal. Comparison to the structurally si milar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanis m was studied computationally by means of a two-layer ONIOM model, using bo th the B3LYP and mPW1K exchange-correlation functionals and a variety of ba sis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpar t, The calculations reveal that the electronic requirements for both C-C an d C-H activation are essentially the same (14e intermediates are the key on es). On the other hand, the steric requirements differ significantly, and c helation appears to play an important role in C-C bond activation.