Reactions Involving Carbon Dioxide and Mixed Amido-Phosphido Dinuclear Compounds: M2(NME2)4(PR2)2(MM), Where M = Mo and W. Comparative Study of the Insertion of Carbon Dioxide into Metal-Nitrogen and Metal-Phosphorus Bonds

Article abstract of DOI:10.1021/ja00203a014

1,2-M2(NM2)4(P(t-Bu)2)2 compounds react with CO2 (>4 equiv) to yield initially M2(O2CNMe2)2(O2CP(t-Bu)2)2(NMe2)2 where M = Mo and W.The reaction occurs rapidly at room temperature in solution and even as a gas/solid heterogeneous reaction.The molybdenum compound is labile to ligand-exchange reactions in solution, and attempts to grow good quality crystals yielded Mo2(O2CP(t-Bu)2)4*2C6H6, a compound having a Mo-Mo quadruple bond supported by a paddle-wheel arrangement of four phosphinecarboxylate ligands that span the M-M bond.The compound W2(O2CNMe2)2(O2CP(t-Bu)2)2(NMe2)2 has been characterized in the solid state and shown to contain a (W<*>W)(6+) central unit, 2.29 (1) Angstroem, with a pair of O2CNMe2 ligands that span the W-W bond.The phosphinecarboxylates form one short, 2.10 (1) Angstroem, and one long 2.67 (1) Angstroem, W-O bond.The coordination at each tungsten atom involves a distorted square-planar WO3N moiety with an additional weak O---W bond (2.67 (1) Angstroem) along the W-W axis.A related reaction occurs between 1,2-W2(NMe2)4(PCy2)2 and CO2, initially giving W2(O2CNMe2)2(O2CPCy2)2(NMe2)2, where Cy = cyclohexyl, which appears, based on NMR data, to be structurally related to W2(O2CNMe2)2(O2CP(t-Bu2)2(NMe2)2.In solution an isomerization occurs to give one of two isomers that are formulated as having phosphinecarboxylates that bridge the (W<*>W)(6+) unit by use of P- and O-to-W bonds.Mechanistic studies employing the use of (13)CO2 have established that at low temperatures CO2 preferentially attacks the phosphido ligand (with respect to the amido ligands) giving an initially P-bound phosphinecarboxylate that then rearranges to the O-bound form of the ligand.Comparisons are made withearlier work involving the reactions between metal amides and CO2.Crystal data (i) for Mo2(O2CP(t-Bu)2)4*2C6H6 at -155 deg C: a = 13.731 (21) angstroem, b = 12.789 (17) angstroem, c = 15.835 (23) Angstroem, β = 94.86 (8) deg, Z = 2, d_calcd = 1.324 g cm^-3, space group P2₁/n. (ii) for W2(O2CNMe2)2(O2CP(t-Bu)2)2(NMe2)2 at -151 deg C: a = 15.455 (7) Angstroem, b = 15.786 (7) Angstroem, c = 8.502 (4) Angstroem, α = 97.22 (2) deg, β = 97.86 (2) deg, γ = 101.22 (2) deg, Z = 2, d_calcd = 1.686 g cm^-3, space group P<*>.