Combined experimental and computational study of TpRu{P(pyr)3}(NCMe)Me (pyr = N-pyrrolyl):: Inter- and intramolecular activation of C-H bonds and the impact of sterics on catalytic hydroarylation of olefins

Complexes of the type TpRu{P(pyr)(3)}(L)R IL = PPh3 or NCMe; R = Cl, OTf (OTf = trifluoromethanesulfonate), Me, or Ph; Tp = hydridotris(pyrazolyl)borate; pyr = N-pyrrolyl} and TpRu{kappa(2)'-P,C-P(pyr)(2)(NC4H3)]NCMe have been synthesized and isolated. TpRu{P(pyr)(3)}(NCMe)Me initiates intermolecular C-H activation of benzene to form TpRu{P(pyr)(3)}(NCMe)Ph and, in the absence of benzene, intramolecular C-H activation of a pyrrolyl ring to form the cyclometalated species TpRu{kappa(2)-P,C-P(pyr)(2)(NC4H3)}NCMe. TpRu{P(pyr)(3)}(NCMe)Ph catalyzes the hydrophenylation of ethylene in benzene to produce ethylbenzene in low yields. Experimental and computational analyses of the hydrophenylation of ethylene by TpRu{P(pyr)(3)}(NCMe)Ph suggest that inefficient catalysis is not due to difficulty in the C-H activation of benzene by the active catalyst species, but rather likely arises from the steric bulk of the tris-N-pyrrolyl phosphine ligand, which inhibits coordination of ethylene and thus thwarts C-C bond formation.