Ar. Johnson et al., 4-COORDINATE MOLYBDENUM CHALCOGENIDE COMPLEXES RELEVANT TO NITROUS-OXIDE N-N BOND-CLEAVAGE BY 3-COORDINATE MOLYBDENUM(III) - SYNTHESIS, CHARACTERIZATION, REACTIVITY, AND THERMOCHEMISTRY, Journal of the American Chemical Society, 120(9), 1998, pp. 2071-2085
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 reac
tion 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%. T
he Mo(E)(N[R]Ar)(3) complexes were studied by EPR, SQUID, cyclic volta
mmetry, 2H NMR spectroscopy, and single-crystal X-ray diffraction. The
rmolysis of each Mo(E)(N[R]Ar)(3) complex resulted in (formal) tert-bu
tyl radical elimination giving molybdenum(VI) chalcogenide complexes M
o(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), a
nd from 66 to 93 degrees C for Mo(S)(N[R]Ar)(3); in both cases, a firs
t-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) upo
n treatment of Mo(O)(N[R]Ar)(3) with [Cp2Fe][O3SCF3]. Small-scale expe
riments 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, a
nd 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 d
issociation 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 m
ol(-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 di
ssociation 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 tha
t 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.