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Proton-switchable access to seven-coordinate ONNO dicarboxamide and NNNN dicarboxamidate rhenium oxo complexes provides a platform for understanding thermodynamics and bonding in pentagonal bipyramidal complexes. 

Great progress has been made in the past decade in the use of pincer-ligated transition metal complexes for the reduction of dinitrogen. Such complexes, however, have required 'pre-activation' by a strong reducing agent like Na/Hg or KC8 to achieve reductive N2 splitting. In this study, non-innocent pincer molybdenum(III) trihalide complexes, (PhPN5P)MoCl3 and (tBuPPHP)MoBr3, bearing acidic E-H (E = N or P) protons on the ligand periphery, have been utilized to investigate deprotonative N2 splitting. These complexes can be activated in the presence of KOtBu, without the need for a strong reductant. Reaction with KOtBu presumably affords (PhPN5P*)MoCl2 and (tBuPPP)MoBr2 respectively, through the loss of HX across the E-M bond. N2 binding at the vacant coordination site on the metal is followed by splitting of N2 to afford nitrides (PhPN5P*)MoVI(N)Cl2 and (tBuPPP)MoV(N)Br. Previous studies have demonstrated the reduction of molybdenum nitrides to ammonia in the presence of chem. reductants and proton sources but little is known about the relative reactivity of various nitrides and the detailed sequence of events leading to ammonia formation and regeneration of the active species. We, therefore, have begun an investigation of such catalytic cycles for ammonia formation. Mechanistic studies of the new complex (iPrPSP)Mo and of Nishibayashi's (tBuPNpyP)Mo systems, including DFT and electrochemical studies, revealed characteristic roles of the halides in splitting dinitrogen. A new pathway leading to the formation of ammonia and regeneration of the catalyst was elucidated.