Diamagnetic molybdenum nitride complexes supported by diligating tripodal triamido-phosphine ligands as precursors to paramagnetic phosphine donors

Reduction of highly bulky triphenolamine molybdenum nitrido and chloride complexes

Transition metal nitrides are key intermediates in the catalytic reduction of dinitrogen to ammonia. To date, transition metal nitride complexes with the triphenolamine (TPA) ligand have not been reported and the system with the ligand has been much less studied for ammonia formation compared with other systems. Herein, we report a series of molybdenum complexes supported by a sterically demanding TPA ligand, including a nitrido complex NMo(TPA). We achieved the stoichiometric conversion of the nitride moiety into ammonia under ambient conditions by adding proton and electron sources to NMo(TPA). However, the catalytic turnover for N₂ reduction to ammonia was not observed in the triphenolamine ligand system unlike the Schrock system-triamidoamine ligand. Density functional theory calculation revealed that the molybdenum center favors binding NH₃ over N₂ by 16.9 kcal mol^-1 and the structural lability of the trigonal bipyramidal (TBP) molybdenum complex seems to prevent catalytic turnover. Our systematic study showed that the electronegativity and bond length of ancillary ligands determine the preference between N₂ and NH₃, suggesting a systematic design strategy for improvement.

Cyclometalated titanium and zirconium complexes stabilised by a new silylmethylene-linked tetradentate triamidophosphine

The silylmethylene-linked triamidophosphine P(CH2SiMe2NHPh)3 was isolated in form of its tri-lithium salt Li3[P(CH2SiMe2NPh)3]·2.5Et2O (1·2.5Et2O) and employed for the synthesis of titanium and zirconium complexes. Starting from 1·2.5Et2O, the chlorido complexes [κ(4)-N,N,N,P-PN3]MCl (4-M, M = Ti, Zr) were prepared and examined with respect to alkylation. Upon reaction of 4-Ti with (trimethylsilyl)methyl lithium, intra-ligand cyclometalation at one of the ortho-N-phenyl positions was observed and the resulting thermally stable titanazetidine [κ(5)-N,N,N,P,C-N2P(NC)]Ti (5-Ti) isolated. Similarly, the related zirconazetidine [κ(5)-N,N,N,P,C-N2P(NC)]Zr(THF) (5-Zr) was isolated upon reaction of 4-Zr with Bn2Mg(THF)2. Using LiCH2PMe2 as alkyl transfer reagent, both complexes 4-M were converted to the corresponding phosphametallacyclopropanes [κ(4)-N,N,N,P-PN3]M(κ(2)-C,P-CH2PMe2) (7-M, M = Ti, Zr). Upon gentle heating, complexes 7-M were cleanly converted to the cyclometalated species 5-M and trimethylphosphine. These results are discussed in the context of related amidophosphine and triamidoamine complexes.