The Tungsten-Tungsten Triple Bond. 13. Bisalkyl Tetracarboxylates of Dimolybdenum and Ditungsten. Triple Bonds between Metal Atoms with the Valence Molecular Orbital Description ?4δ2

Article abstract of DOI:10.1021/ja00256a038

Four synthetic procedures have been developed for the preparation of compounds of formula W2R2(O2CR')4: (1) W2R2(NMe2)4 + 4R'COOCOR' -> W2R2(O2CR')4 + 4R'CONMe2; (2) W2R2(OR'')4 + 4R'COOH -> W2R2(O2CR')4 + 4R''OH; (3) W2R2(O2CR')4 + 4R''COOH -> W2R2(O2CR'')4 + 4R'COOH; (4) W2R6 + 4R'COOH -> W2R2(O2CR')4 + 4RH.In all but one case (R = i-Bu) the R group lacks β-hydrogen atoms, and representative combinations of R = Me, Ph, Bz (benzyl), p-tolyl, o-tolyl, np (np = neopentyl), and CH2SiMe3 with R' = H, Me, CF3, Et, Ph, p-MeOPh, t-Bu, mesityl, and CHPh2 have been obtained by reactions in hydrocarbon or CH2Cl2 solvents at or below room temperature.Reaction 1 has been most extensively employed and has been extended to the synthesis of Mo2(np)2(O2CMe)4.Limiting factors to the generalized syntheses of M2R2(O2CR')4 compounds involve the ease of reductive elimination from the M2⁶⁺ center, which occurs more readily (i) for M = Mo than M = W and (ii) for R = a β-hydrogen-containing alkyl ligand relative to a β-hydrogen-lacking (stabilized) ligand.In the case of the latter, reductive elimination is still possible by M-C bond homolysis, which may be thermally or photochemically induced.Photolysis allows for the generalized syntheses of W2(O2CR')4(M<*>M) compounds, including the formate, which has not previously been obtained by alternate methods.The compounds of formula M2R2(O2CR')4 have been characterized by IR spectroscopy, mass spectroscopy, NMR studies, cyclic voltammetry, UV-visible spectroscopy, and photoelectron spectroscopy, and, in certain cases, single-crystal X-ray studies have been carried out: M = Mo, R = np, R' Me; M = W, (i) R = Bz, R' = Et; (ii) R = np, R' = Et; (iii) R = np, R' = Ph; (iv) R = np, R' = H; (v) R = np, R' = Me and CF3, viz., W2(np)2(O2CMe)2(O2CCF3)2.In the solid state all of the structurally characterized compounds have a central M2(O2C)4 paddle-wheel core, typical of M2(O2CR')4 compounds with M-M quadruple bonds, supplemented by axially aligned M-C(alkyl/aryl) bonds.Of particular note is the fact that the W-W distances in W2R2(O2CR')4 compounds, 2.18-2.20 Angstroem, are essentially identical with those in the d⁴-d⁴ W2(O2CR')4 compounds while the W-C bonds are as expected, ca. 2.17-2.21 Angstroem.The Mo-Mo distance in Mo2(np)2(O2CMe)4, 2.13 Angstroem, is longer by 0.04 Angstroem than that found in Mo2(O2CMe)4(M<*>M).The M-O distances are essentially identical in the d³-d³ andd⁴-d⁴ compounds, ca. 2.08 Angstroem.The M-M distances in the new compounds are the shortest thus far reported for d³-d³ dinuclear compounds of tungsten and molybdenum.In general, the (1)H NMR studies indicate the geometry found in the solid-state is maintained in solution but, for less bulky combinations of R and R', an alternate isomer is present; e.g., W2Me2(O2CMe)4 is spectroscopically analogous to W2Me2(O2CNEt2)4, which has equatorially aligned W-C bonds with a C_2υ-W2C2(O2C)4 core.For W2Bz2(O2CEt)4 a mixture of the two isomers is present in solution and the equilibrium constant is solvent dependent: the relative...