Preparation, Characterization, and Reactivity of Dinitrogen Molybdenum Complexes with Bis(diphenylphosphino)amine Derivative Ligands that Form a Unique 4‑Membered P–N–P Chelate Ring

Five dinitrogen–molybdenum complexes bearing bis­(diphenylphosphino)­amine
derivative ligands (LR) that
form a unique 4-membered P–N–P chelate ring, trans-[Mo­(N2)2(LR)2] (2R: R = Ph, Xy, p-MeOPh, 3,5-iPr2Ph, iPr), were prepared for the purpose
of binding a dinitrogen molecule. The corresponding two dichloride-molybdenum
complexes, trans-[MoCl2(LR)2] (1R: R = Ph, Xy), were also prepared as comparisons. FT-IR
spectra of 2R were measured and
compared the ν­(NN) values. Moreover, X-ray crystal structure
determination of 1R (R = Ph,
Xy) and 2R (R = Xy, 3,5-iPr2Ph) is performed. These experimental results
indicated that the coordinated dinitrogen molecule gets easily influenced
by the N-substitutent of diphosphinoamine ligand. In addition, to
investigate the effect of the properties of the diphosphinoamine ligand
for the dinitrogen molybdenum complexes, we performed DFT calculations
that focused on the difference of N-substituent, the dihedral angle
between P–N–P plane and N-substituent aryl group, and
the P–N–P bite angle. This calculation revealed that
the competition between the back-donation from metal to dinitrogen
and that from metal to ligand was affected by P–N–P
bite angle and the dihedral angle of N-substituent of ligand. In order
to examine the reactivity with respect to conversion of dinitrogen
to ammonia, protonation and trimethylsilylation reactions of the coordinated
dinitrogens were carried out for 2R.