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 C2i;-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 dzz 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].
ASJC Scopus subject areas
- Colloid and Surface Chemistry