Molybdenum complexes as models of nitrogenase

Four new series of bis(dinitrogen) complexes of molybdenum were prepared using oxygen-, nitrogen-, phosphorus- and sulfur-containing ligands. The sodium amalgam reduction of MoCl$\sb3$(trident) where trident = (PPh$\sb2$CH$\sb2$CH$\sb2$)$\sb2$O (POP), (PPh$\sb2$CH$\sb2$CH$\sb2$)$\sb2$NR (R = H (PNHP); R = Me(PNMeP)), (PPh$\sb2$PCH$\sb2$CH$\sb2$CH$\sb2$)S (dpps) and (PPh$\sb2$CH$\sb2$CH$\sb2$CH$\sb2$)$\sb2$PPh (ttp) in the presence of PR$\sb3$ = PMe$\sb2$Ph, PMePH$\sb2$ and PPh$\sb3$ gave the products trans- Mo(N$\sb2$)$\sb2$(trident)PR$\sb3$. trans- Mo(N$\sb2$)$\sb2$(dpps)PR$\sb3$ represents the first example of any metal complex with a prior-coordinated sulfur atom binding dinitrogen. All except the PNRP series were protonated by neat HBr and HCl to give the corresponding hydrazido(2-) complexes (MoX(NNH$\sb2$)(trident)PR$\sb3$) X. When the bis(dinitrogen) complexes are allowed to react exhaustively with acid in solution, varying amounts of dinitrogen, ammonia and hydrazine are given depending on solvent. The ttp and dpps systems led to the isolation and then preparation of new hydrazido(2-) complexes as MoX$\sb2$(NNH$\sb2$)(trident) where X = Cl and Br. Those of the ttp series were separated into syn and anti isomers. The acid reaction of anti-MoBr$\sb2$(NNH$\sb2$)ttp gives increased ammonia yields in the presence of the reducing agents SnCl$\sb2$ and VCl$\sb3$. The complex trans-Mo(N$\sb2$)$\sb2$(POP)PPh$\sb3$ reacts with HBr in Ch$\sb2$Cl$\sb2$ to give 1.5 mole dinitrogen prior to the isolation of a new intermediate. This compound give 0.5 mole hydrazine upon hydrolysis and 0.5 mole dinitrogen upon reaction with DMSO. A structure containing a bridging N$\sb2$H$\sb{\rm x}$ ligand is proposed for this compound. A new nitrido complex, MoCl$\sb2$N(triphos), where triphos = (PH$\sb2$CH$\sb2$CH$\sb2$)$\sb2$PPh, was prepared. This complex reacts with HCl in CH$\sb2$Cl$\sb2$ to give 70% ammonia