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(μ-H)Os₃(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) Å for this "unconventional" H⋯H interaction; theoretical calculations suggested an H⋯H distance of 1.89 Å 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 Å. 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 v(N-H) stretching in H(μ-H)Os₃(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.