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

• Published in: Journal of the American Chemical Society 1998-03, Vol.120 (9), p.2071-2085
• Main Authors: Johnson, Adam R, Davis, William M, Cummins, Christopher C, Serron, Scafford, Nolan, Steven P, Musaev, Djamaladdin G, Morokuma, Keiji
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja971491z

The terminal chalcogenide complexes Mo(E)(N[R]Ar)3 (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2), where E = O, S, Se, and Te, were prepared by reaction of the three-coordinate complex Mo(N[R]Ar)3 with ONC5H5, S8 or SC2H4, Se, and Te/PEt3 in respective yields of 72, 63, 80, and 73%. The Mo(E)(N[R]Ar)3 complexes were studied by EPR, SQUID, cyclic voltammetry, 2H 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 (2H 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][O3SCF3] was prepared similarly (71% yield) upon treatment of Mo(O)(N[R]Ar)3 with [Cp2Fe][O3SCF3]. Small-scale experiments monitored by 1H NMR spectroscopy established that Mo(N[R]Ar)3 deoxygenates OSMe2, NO2, and SO2 but fails to deoxygenate CO2. Also essentially inert to Mo(N[R]Ar)3 were found to be OPPh3, t-BuNCO, and O2SMe2. Treatment of Mo(N[R]Ar)3 with Se2Ph2 provided Mo(SePh)(N[R]Ar)3 in 72% yield. Treatment of Mo(N[R]Ar)3 with CS2 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 + dE basis set) for the model complexes Mo(E)(NH2)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)(NH2)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.