Synthesis and Protonation of N‑Heterocyclic-Carbene-Supported Dinitrogen Complexes of Molybdenum(0)

The
N-heterocyclic-carbene-supported dinitrogen (N2)
complexes of molybdenum(0) trans-Mo­(N2)2(LR)4 (LR = 1,3-R2-4,5-dimethyl­imidazol-2-ylidene; 2a: R
= Me, 2b: R = Et) and mer-Mo­(N2)3(LR)3 (3: R = iPr) were synthesized from MoCl4(THF)2 and KC8 in the presence of LR under an atmosphere
of N2. In agreement with the characteristically strong
σ-donation from the N-heterocyclic carbenes to molybdenum, complexes 2a, 2b, and 3 exhibited lower N–N
stretching frequencies in their IR spectra and displayed longer N–N
as well as shorter Mo–N bond lengths in their molecular structures
relative to corresponding phosphine analogues. Although protonation
of the molybdenum-bound N2 ligands in these phosphine complexes
with H2O is unprecedented, it was accomplished for 2a, 2b, and 3 under concomitant
release of ammonia.