Dihydrogen bond between a bridging hydride and coordinated HNMe2 : solid state and solution characterization

The addition of one equivalent of dimethylamine (DMA) to the 44 valence-electron triangular cluster anion [Re3(μ3-H)(μ-H)3(CO)9]– (1) affords the novel unsaturated derivative [Re3(μ- H)4(CO)9(DMA)]– (2, 46 valence electrons) which contains a dimethylamine molecule terminally coordinated to a cluster vertex. Theoretical calculations reveal that in the more stable conformation the dimethylamine NH proton directs towards the hydride bridging the opposite cluster edge in syn position, the close proximity of the ligands bound to the cluster surface allowing the formation of an unconventional N–H…(μ-H)Re2 hydrogen bond. The presence of this conformation in the solid state has been proven by an X-ray structural analysis of crystalline [PPh4]2. Spectroscopic evidences (IR and NMR) indicate that the dihydrogen bond is maintained also in solution and, by the evaluation of the proton spin-lattice relaxation rates at variable temperature, a good estimation of the H…H distance in solution have been determined.