Four-Coordinate Molybdenum Chalcogenide Complexes Relevant to Nitrous Oxide N−N Bond Cleavage by Three-Coordinate Molybdenum(III): Synthesis, Characterization, Reactivity, and Thermochemistry

The terminal chalcogenide complexes Mo(E)(N[R]Ar) 3 (R = C(CD 3 ) 2 CH 3, Ar = 3,5-C 6 H 3 Me 2 ), where E = O, S, Se, and Te, were prepared by reaction of the three-coordinate complex Mo(N[R]Ar) 3 with ONC 5 H 5, S 8 or SC 2 H 4, Se, and Te/PEt 3 in respective yields of 72, 63, 80, and 73%. The Mo(E)(N[R]Ar) 3 complexes were studied by EPR, SQUID, cyclic voltammetry, 2 H NMR spectroscopy, and single-crystal X-ray diffraction. Thermolysis of each Mo(E)(N[R]Ar) 3 complex resulted in (formal) tert -butyl radical elimination giving molybdenum(VI) chalcogenide complexes Mo(E)(NAr)(N[R]Ar) 2 in yields of 85 (E = O), 84 (E = S), 64 (E = Se) and 40% (E = Te). tert -Butyl elimination kinetics were monitored ( 2 H NMR) over a 62−104 °C temperature range for Mo(O)(N[R]Ar) 3, and from 66 to 93 °C for Mo(S)(N[R]Ar) 3; in both cases, a first-order decay was observed. Treatment of Mo(O)(N[R]Ar) 3 with iodine (0.5 equiv) provided [Mo(O)(N[R]Ar) 3 ][I] in 88% yield. The triflate salt [Mo(O)(N[R]Ar) 3 ][O 3 SCF 3 ] was prepared similarly (71% yield) upon treatment of Mo(O)(N[R]Ar) 3 with [Cp 2 Fe][O 3 SCF 3 ]. Small-scale experiments monitored by 1 H NMR spectroscopy established that Mo(N[R]Ar) 3 deoxygenates OSMe 2, NO 2, and SO 2 but fails to deoxygenate CO 2 . Also essentially inert to Mo(N[R]Ar) 3 were found to be OPPh 3, t -BuNCO, and O 2 SMe 2 . Treatment of Mo(N[R]Ar) 3 with Se 2 Ph 2 provided Mo(SePh)(N[R]Ar) 3 in 72% yield. Treatment of Mo(N[R]Ar) 3 with CS 2 led to Mo(S)(N[R]Ar) 3 and (μ-CS)[Mo(N[R]Ar) 3 ] 2; the latter was isolated in 42% yield and was the subject of an X-ray diffraction study. Bond dissociation enthalpies D (MoE) for Mo(E)(N[R]Ar) 3 (E = O and S) were experimentally determined to be 155.6 ± 1.6 and 104.4 ± 1.2 kcal mol - 1, respectively. MoE bond lengths predicted by density functional B3LYP calculations (lanl2dz + d E basis set) for the model complexes Mo(E)(NH 2 ) 3 (E = O, S, Se, and Te) were found to compare favorably with the experimentally determined MoE bond lengths. Predicted bond dissociation enthalpies D (MoE) for the hypothetical complexes Mo(E)(NH 2 ) 3 are 91 (E = Se) and 71 (E = Te) kcal mol - 1 . A key finding is that Mo(N[R]Ar) 3 selectively splits the nitrous oxide N−N bond to give Mo(N)(N[R]Ar) 3 and Mo(NO)(N[R]Ar) 3, despite the fact that the oxo complex Mo(O)(N[R]Ar) 3 possesses a very strong Mo−O bond and can be prepared by an alternate route.