Stabilization of diazene in Fe(II)-sulfur model complexes relevant for nitrogenase activity. I. A new approach to the evaluation of intramolecular hydrogen bond energies

A key step in the biological nitrogen fixation problem is the transfer of p rotons and electrons onto inert molecular nitrogen. A first intermediate wi ll then be diazene (diimide), N2H2, which is thermodynamically unstable wit h respect to dissociation into N-2 and H-2 in the gas phase. Thus, diazene must be stabilized such that the reduction of a complex binding an activate d nitrogen becomes energetically feasible. A considerable contribution to t his stabilization has been attributed to hydrogen bonds of the type N-H . . .S. We investigate the strength of these hydrogen bonds in two model compo unds. Since the contribution of an intramolecular hydrogen bridge to the to tal binding energy of a molecule is not a well-defined concept, it is neces sary to define a suitable descriptor for this quantity. We present a new ap proach of estimating hydrogen-bond energies from two-center shared-electron numbers obtained from density functional calculations. Our approach is par ticularly designed for highly complex systems such as transition-metal comp lexes with large coordination spheres.