Oxygen atom transfer in the reaction between hexakis(dimethyl-tert-butylsiloxy)ditungsten and nitric oxide. A remarkable difference in the reactivity of the tungsten-tungsten triple bond as a function of the attendant ligands: Bu tO versus Bu tMe 2SiO

Abstract

The siloxy complex W 2(OSiMe 2Bu t) 6 ( I) and NO react in hydrocarbon solvents in the presence of pyridine (py) to give the oxo tungsten compounds W(O)(OSiMe 2Bu t) 4(py) ( II), W 2(O) 4(OSiMe 2Bu t) 4(py) 2 ( III) and W(NO)(OSiMe 2Bu t) 3(py) 2 ( IV). The relative amounts of the oxo compounds II and III to the nitrosyl complex IV depends upon the reaction temperature with low temperatures (−72 °C) favoring the nitrosyl derivative IV. An intermediate in the reaction is formulated as W 2(μ-O)(OSiMe 2Bu t) 6(py) 2 and is formed along with N 2O after the coupling of two nitrosyl ligands. The N 2O liberated in the reaction is then also active in oxygen atom transfer to yield II and III along with N 2. Compounds II, III and IV are inert with respect to further reactions with NO and N 2O under the conditions leading to their formation. An alternative synthesis of IV involves the reaction between W(NO)(OBu t) 3(py) and Bu tMe 2SiOH (3 equiv.) in the presence of pyridine. Compounds II, III and IV have been characterized by single crystal X-ray crystallography, 1H and 13C NMR spectroscopy, IR spectroscopy and elemental analysis. Reactions employing 15N 18O have been monitored by 15N NMR spectroscopy and the products analyzed by mass spectrometry and IR spectroscopy. Compound II contains a distorted octahedral geometry about tungsten with trans oxo and pyridine ligands: WO=1.68(1), WO(siloxide)=1.90(1) (av.), WN=2.44(1) Å. Compound III involves an edge-shared bioctahedron with terminal and bridging oxo ligands: WO(oxo)=1.72(1), Wμ-oxo=1.95(1) (av.), WO(siloxide)=1.89(1) (av.) and WN=2.42(1) Å. Compound IV is pseudo-octahedral with trans nitrosyl and pyridine ligands: WN(nitrosyl)=1.74(1), WN(pyridine- trans)=2.34(1), WN(pyridine- cis)=2.29(1), WO=1.59(1) (av.) Å. A derivative of the intermediate W 2(μ-O)(OSiMe 2Bu t) 6(py) 2 was characterized by X-ray crystallography as W 2(μ-O)(μ-OBu t)(OSiMe 2Bu t) 5(py) 2 ( V). Compound V contains bridging oxo and t-butoxide ligands that span a formal tungsten-tungsten double bond, WW=2.488(1) Å. In contrast to the above Mo 2(OSiMe 2Bu t) 6 ( VI) (made by the addition of Bu tMe 2SiOH (6 equiv.) to Mo 2(OBu t) 6) and NO (>2 equiv.) react in hydrocarbon solutions to give [Mo(NO)(OSiMe 2Bu t) 3] 2 ( VII), an analogue of the previously structurally characterized compound [Mo(NO)(OBu t) 3] 2 that contains a centrosymmetric (ON)(O 2M(μ- O) 2MO 2(NO) skeleton with a linear MNO moiety and no MM bond. Crystallographic data for the compounds: II: space group P 1 , a=17.414(5), b=22.679(7), c=11.519(3) Å, α=99.77(2), β=102.15(2), γ=108.13(1)°, V=4089.00 Å 3, Z=4; III: space group P 1 , a=10.708(3), b=12.469(4), c=10.185(3) Å, α=107.44(1), β=111.56(1), γ=90.53(2)°, V=1195.75 Å 3, Z=1; IV: space group P2 1/ c, a=14.049(3), b=10.832(2), c=24.807(5) Å, β=102.00(1)°, V=3692.74 Å 3, Z=4; V: space group P 1 , a=14.881(3), b=17.245(3), c=12.830 Å, α=91.20(1), β=102.23(1), γ=76.34(1)°, V=3125.26 Å 3, Z=2.