Aromatic and aldehyde carbon-hydrogen bond activation at cationic Rh(III) centers. Evaluation of electronic substituent effects on aldehyde binding and C-H oxidative addition

Cationic rhodium methyl complexes, [Cp-*(PMe3)Rh(Me)(CH2Cl2)]BAr4' (1) and [Cp-*(P(OMe)(3))Rh(Me)(CH2Cl2)]BAr4' (3), react with benzene to yield the corresponding phenyl complexes, [Cp-*(PMe3)Rh(Ph)(CH2Cl2)]BAr4' (6) and [Cp-*(P(OMe)(3))Rh(Ph)(CH2Cl2)]BAr4' (7). First-order rate constants observed in 1.1 M benzene in CD2Cl2 at 25 degreesC are (2.1 +/- 0.2) x 10(-5) s(-1) and (1.9 +/- 0.2) x 10(-5) s(-1), respectively. Reactions of 1 and 3 with p-X-substituted benzaldehydes (X = -CF3, -CH3, and -OMe) initially produce the sigma-aldehyde adducts, [Cp-*(L)Rh(Me)(p-XC6H4CHO)]BAr4' (L = PMe3 (15), P(OMe)(3) (16)). Exchange of free with bound aldehyde occurs via a dissociative process and quantitative NMR rate measurements show that complexes of 1 exchange faster than those of 3 and that less basic aldehydes exchange faster than more basic aldehydes (p-CF3C6H4CHO > p-CH3C6H4CHO > p-CH3OC6H4CHO). The aldehyde adducts undergo C-H bond activation to produce initially methane plus acyl aldehyde adducts, [Cp-*(L)Rh(C(O)C6H4X)(p - XC6H4CHO)]BAr4' (L = PMe3 (17), P(OMe)(3) (18)). Rates of C-H activation are correlated with aldehyde exchange rates; activation barriers of weakly bound aldehydes are lower than more strongly bound aldehydes. In the case of L = PMe3, decarbonylation of the aldehyde adducts occurs cleanly to form aryl carbonyl complexes, [Cp-*(PMe3)Rh(C6H4X)(CO)]BAr4'. For L = P(OMe)(3), decarbonylation is a more complicated process; some intermediates and products have been identified by NMR spectroscopy. (C) 2004 Elsevier Ltd. All rights reserved.