Steric and Acidity Control in Hydrogen Bonding and Proton Transfer to trans-W(N 2 ) 2 (dppe) 2 .

The interaction of trans-W(N2 )2 (dppe)2 (1; dppe = 1,2-bis(diphenylphosphino)ethane) with relatively weak acids (p-nitrophenol, fluorinated alcohols, CF3 COOH) was studied by means of variable temperature IR and NMR spectroscopy and complemented by DFT/B3PW91-D3 calculations. The results show, for the first time, the formation of a hydrogen bond to the coordinated dinitrogen, W-N≡N···H-O, that is preferred over H-bonding to the metal atom, W···H-O, despite the higher proton affinity of the latter. Protonation of the core metal-the undesirable side step in the conversion of N2 to NH3 -can be avoided by using weaker and, more importantly, bulkier acids.