Mo/WN2 and N2H2 complexes with trans nitrile ligands: electronic structure, spectroscopic properties and relevance to nitrogen fixation

Molybdenum and tungsten dinitrogen complexes with trans nitrile groups are of significant interest to synthetic nitrogen fixation as they can be protonated to the corresponding N2H2 complexes with retention of the trans ligand. This is in contrast to their bis(dinitrogen) counterparts where one N2 group is exchanged by a ligand X deriving from the acid used for protonation (X=Hal−, HSO4 − etc.). Here the first crystal structure of a tungsten(0) dinitrogen nitrile complex having a P4 ligand set is presented. The electronic and vibrational properties of this and analogous N2 and N2H2 systems with trans nitrile ligands are investigated using IR and Raman spectroscopies coupled to DFT calculations. Force constants are evaluated from experimental frequencies and isotope shifts by the QCA–NCA procedure developed earlier [N. Lehnert, F. Tuczek, Inorg. Chem. 38 (1999) 1659]. The resulting electronic structure descriptions are compared with those obtained earlier for the bis(dinitrogen) and trans fluoro N2H2 complexes, respectively [N. Lehnert, F. Tuczek, Inorg. Chem. 38 (1999) 1671]. Importantly, the N2 ligand is found to be activated to a higher degree in the trans nitrilo as compared with the corresponding bis(dinitrogen) system. On the other hand, the N2H2 ligand is less activated in the trans nitrilo than in the analogous trans fluoro complexes. Further, bonding of the nitrile group becomes labile at the N2H2 stage of N2 reduction. These results are interpreted based on the electronic donor/acceptor properties of the respective trans ligands, and the consequences regarding the reactivity of these systems towards further protonation and their potential use for synthetic nitrogen fixation are discussed.