The Tungsten-Tungsten Triple Bond. 13. Bisalkyl Tetracarboxylates of Dimolybdenum and Ditungsten. Triple Bonds between Metal Atoms with the Valence Molecular Orbital Description π⁴ẟ²

Four synthetic procedures have been developed for the preparation of compounds of formula W₂R₂(0₂CR')₄: (1) W₂R₂(NMe₂)₄+ 4R'COOCOR'— W₂R₂(0₂CR')₄+ 4R'CONMe₂; (2) W₂R₂(OR“)₄+ 4R'COOH — W₂R₂(0₂CR')₄+ 4ROH; (3) W₂R₂(0₂CR')₄+ 4RCOOH — W₂R₂(0₂CR^//)₄+ 4R'COOH; (4) W₂R₆+ 4R'COOH — W₂R₂(0₂CR')₄+ 4RH. In all but one case (R = i-Bu) the R group lacks 0-hydrogen atoms, and representative combinations of R = Me, Ph, Bz (benzyl), p-tolyl, o-tolyl, np (np = neopentyl), and CH₂SiMe₃with R' = H, Me, CF₃, Et, Ph, p-Me0Ph t-Bu, mesityl, and CHPh₂have been obtained by reactions in hydrocarbon or CH₂C1₂solvents at or below room temperature. Reaction 1 has been most extensively employed and has been extended to the synthesis of Mo₂(np)₂(0₂CMe)₄. Limiting factors to the generalized syntheses of M₂R₂(0₂CR')₄compounds involve the ease of reductive elimination from the M₂⁶⁺center, which occurs more readily for M = Mo than M = W and (ii) for R = a 0-hydrogen-containing alkyl ligand relative to a 0-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(0₂CR')₄(M—M) compounds, including the formate, which has not previously been obtained by alternate methods. The compounds of formula M₂R₂(0₂CR')₄have 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, 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(0₂CMe)₂(0₂CCF₃)₂. In the solid state all of the structurally characterized compounds have a central M₂(0₂C)₄paddle-wheel core, typical of M₂(0₂CR')₄compounds 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(0₂CR')₄compounds, 2.18-2.20 A, are essentially identical with those in the d⁴-d⁴W₂(0₂CR')₄compounds while the W-C bonds are as expected, ca. 2.17-2.21 A. The Mo-Mo distance in Mo₂(np)₂(0₂CMe)₄, 2.13 A, is longer by 0.04 A than that found in Mo₂(0₂CMe)₄(M—M). The M-0 distances are essentially identical in the d³-d³and d⁴-d⁴compounds, ca. 2,08 A. The M-M distances in the new compounds are the shortest thus far reported for d³-d³dinuclear compounds of tungsten and molybdenum. In general, the 1H 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(0₂CMe)₄is spectroscopically analogous to W₂Me₂(0₂CNEt₂)₄, which has equatorially aligned W-C bonds with a C_2i;-W₂C₂(0₂C)₄core. For W₂Bz₂(0₂CEt)₄a mixture of the two isomers is present in solution and the equilibrium constant is solvent dependent: the relative concentration of the Civ” to Z>₄/rW₂C₂(0₂C)₄isomer is ca. 1:2 in CD₂C1₂and ca. 1:8 in toluene-d8 and benzene-d₆. Spectroscopic studies on the axially ligated compounds W₂R₂(0₂CR')₄indicate that the valence M-M MO description is 7t4<52,i.e., directly analogous to that of the M₂(0₂CR')₄compounds except with respect to the presence of the M-M a bond. This conclusion is reached by a comparison of the photoelectron spectra, the UV-visible spectra, and cyclic voltammetric studies. An assignment of the^:(5 — 5) transition is possible for both the d³-d³and d⁴-d⁴compounds, and the effect of conjugation of aromatic rings on the 5 —_toC0) MLCT transition is experimentally and theoretically clarified with respect to earlier studies on W₂(0₂CAr)₄(M—M) compounds. In the photoelectron spectra an ionization from one of the M-C a bonds is close in energy to the ionization from the M-M 7r bonds. The M-M bonds of valence MO configuration 7t452are remarkably short in terms of the simplistic view that occupation of M-M a and a* orbitals effectively leads to a cancellation of a-bonding contributions, e.g., as in He₂, a²a*². This is not the case in M₂R₂(0₂CR')₄compounds because M-C and M-M a and a* mixing occurs such that there is residual M-M cr bonding in the d³-d³dinuclear compounds. The magnitude of this residual M-M a bonding is greater for M = W than M = Mo because of relativistic effects. This is the first recognition of this phenomenon for M-M multiple bonds, and a comparison with the bonding in the diatomic molecule C₂and that in HC=CH is noted. Formally C₂has the MO configuration la²la*²2a²2a*²l7r⁴, i.e., a net double bond lacking a cr component, but s-p mixing effectively increases the bonding in 2a and decreases the antibonding in 2a*². In the M₂(0₂CR')₄compounds there are two significant MOs having M-M a character because of valence s and d_zz mixing. The bonding in the R-E=E-R units in acetylene (E = C) and in E₂R₂(0₂CR')₄compounds (E = Mo, W) has extensive mixing of E-R and E-E a and a* orbitals. The spectroscopic results are compared to a previous Xa-SW calculation [J. Am. Chem. Soc. 1985, 107, 4459].