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(CD3)(2)CH3, 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, S-8 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 (H-2 NMR) over a 62-104 degrees C temperature range for Mo(O)(N[R]Ar)(3), and from 66 to 93 degrees 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 H-1 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 (mu-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)(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)(NIR]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.

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