Structural and spectroscopic study of the dihydrogen bond in an imine triosmium complex

The presence of an intramolecular XH...HM interaction between the imine proton donor and the terminal hydride in H(mu-H)Os-3(CO)(10)(HN=CPh2) has been investigated by X-ray analysis, NMR and IR spectroscopy and theoretical calculations. The localization of the hydrogen atoms in the crystal structure yielded a H...H distance of 1.79(6) Angstrom for this "unconventional" H...H interaction; theoretical calculations suggested an H...H distance of 1.89 Angstrom in the solid state. A NMR determination of the interproton distance, obtained from the isolation of the selective H,H dipolar contribution to the hydride relaxation time, afforded a value of 2.00 +/- 0.05 Angstrom. The difference between NMR and solid state determinations may be explained on the basis of the occurrence, in solution, of a large amplitude oscillatory motion of the imine ligand along the N-Os coordination axis. Further evidence of the presence of the favorable N-H...H-M intramolecular hydrogen bond interaction has been obtained from the red shift of the nu(N-H) stretching in H(mu-H)Os-3(CO)(10)(HN=CPh2) with respect to that of the related Os-3(CO)(11)(HN=CPh2) compound. DFT(B3LYP) calculations gave results in agreement with the experimental findings and allowed further insight into the nature of the N-H...H-M dihydrogen bond, pinpointing the electrostatic nature of this interaction and the role of the high polarizability of the Os-H bond.