Bonding and activation of N2 in Mo(0) complexes supported by hybrid tripod ligands with mixed dialkylphosphine/diarylphosphine donor groups: interplay of steric and electronic factors.

Molybdenum dinitrogen complexes are presented which are supported by novel hybrid tripod ligands of the type Me-C(CH2PPh2)2(CH2P(i)Pr2) (trpd-1) and H-C(CH2PPh2)(CH2P(i)Pr2)2 (trpd-2) having mixed dialkylphosphine/diarylphosphine donor groups. Reaction of the ligand trpd-1 with [MoI3(thf)3] followed by sodium amalgam reduction in the presence of the dppm gives the dinitrogen complex [Mo(N2)(trpd-1)(dmpm)] where trpd-1 is coordinated in a κ(3) fashion. The complex exhibits a moderate activation of N2 which enables its protonation under retention of the pentaphosphine ligation. Replacement of dmpm by the sterically more demanding coligand dppm is found to hamper coordination of N2 and leads to [Mo(trpd-1)(dppm)], the first structurally characterized five-coordinate Mo(0) complex with a phosphine-only ligand sphere. Employing the ligand trpd-2 along with the diphosphines dmpm and dppm in an analogous synthetic route results in a mixture of the bis(dinitrogen) complexes trans-[Mo(N2)2(κ(2)-trpd-2)(diphosphine)] and trans-[Mo(N2)2(iso-κ(2)-trpd-2)(diphosphine)] where the tripod ligand trpd-2 coordinates with two phosphine arms and one phosphine group (PPh2 or P(i)Pr2, respectively) is free. Similar results are obtained with the pure alkyl- and arylphosphine tripod ligands H-C(CH2P(i)Pr2)3 (trpd-3) and H-C(CH2PPh2)3 (tdppmm), leading to trans-[Mo(N2)2(κ(2)-trpd-3)(diphos)] and trans-[Mo(N2)2(κ(2)-tdppmm)(dmpm)], respectively. The electronic and steric reasons for the experimental findings are considered, and the implications of the results for the area of synthetic nitrogen fixation with molybdenum phosphine systems are discussed.