Electronic and Steric Effects in Rollover C-H Bond Activation

Steric and electronic factors in rollover C-H bond activation of substituted 2,2'-bipyridines, mediated by platinum(II), have been investigated by comparing the influence of two substituents, CH3 and CF3, on the progress of the reaction. The substituents were chosen to have similar steric hindrance but different electronic effects and were placed in position 6 (i.e., near one of the nitrogen atoms) or in position 5, which allows, in part, electronic and steric influence to be distinguished. The ligands studied, 6-methyl-2,2'-bipyridine, 5-methyl-2,2'-bipyridine, 6-trifluoromethyl-2,2'-bipyridine, and 5-trifluoromethyl-2,2'-bipyridine, were compared to unsubstituted 2,2'-bipyridine in the reaction with the electron-rich complex [Pt(Me)(2)(DMSO)(2)]. The electron-withdrawing CF3 group was found to have a significant effect in accelerating the cyclometalation reaction. The substituent in position 6 influences the stability of the intermediate adduct [Pt(N,N)(Me)(2)] (N,N = chelated bipyridine), as indicated by density functional theory calculations. The steric hindrance of substituted bipyridines was also evaluated by defining and measuring the angle zeta in [Pt(N,N)(Me)(2)] adducts. The presence of a substituent in position 6 causes destabilization of the adduct, acceleration of the cyclometalation reaction, and regioselectivity of C-H bond activation.