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

Abstract

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.

Experimental and computational studies on the formation of cyanate from early metal terminal nitrido ligands and carbon monoxide

“…This value is lower than the magnetic susceptibility determined by variable-temperature SQUID magnetometry measurements (μ eff of 2.66μ B , see below), however differences between solution and the solid state such as this have been previously observed with Mo complexes in similar ligand fields. 20,21 DFT calculations of the paramagnetic NMR shifts 22,23 for triplet 5 are in good agreement (slope = 0.74, correlation (r 2 ) = 0.997) with the observed resonances ( Table 1). The assignments are further confirmed by linewidth calculations which correlated well (r 2 = 0.999, slope = 0.57) with the experimental linewidths ( Table 1) and indicate that the observed linewidths are dominated by the dipolar contribution-in other words the linewidths in this case are a function of the hydrogen atom distances from the metal center.…”

Bioorganometallic chemistry of molybdenocene dichloride and its derivatives

“…While based on thermodynamic considerations, the Mo(IV)-O bond strength in Cp 2 Mo 2+ is stronger than Mo(IV)-S [48,49], the Mo(IV)-O bond is more labile [47]. Thus, the objective of these two compounds (Cp 2 Mo(malonate) and [Cp 2 Mo(maltolato)]Cl) was also to elucidate the role of the oxygen chelating ligands, the aqueous stability and charge on the molybdenocene cytotoxic activity.…”

Functionalization of Complexed N2O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

“…11,12,18,22−31 Thus, the use of N 2 O as an oxidizing substrate in transition metal chemistry is often hampered by a combination of kinetic inertness and inevitable oxidation of the metal center, concomitant with entropy-assisted loss of a thermodynamically stable molecule such as dinitrogen. 32 …”