CARBON-MONOXIDE REACTIONS OF THE FLUXIONAL PHOSPHINE COMPLEX (ETA-3-PR3)MO(CO)3 (R=2,4,6-TRIMETHOXYPHENYL)

The syntheses, spectral properties, and structures of complexes (eta(x)-TMPP)Mo(CO)y (TMPP = tris(2,4,6-trimethoxyphenyl)phosphine; x = y = 3, 1; x = 2, y = 4, 2; x = 1, y = 5, 3) are described. Compounds 1-3 exhibit octahedral geometries with the TMPP ligand bound to the metal in monodentate, bidentate and tridentate coordination modes, respectively. These compounds constitute the first homologous series that demonstrates all three possible binding modes of the TMPP ligand and the first such series involving group VI tri-, tetra-, and pentacarbonyl complexes supported by an identical ligand that changes hapticity. Compound 1, (eta3-TMPP)Mo(CO)3, was isolated from the reaction of (cht)Mo(CO)3 (cht = cycloheptatriene, C7H8) with 1 equiv of TMPP. The molecule crystallizes as a CH2Cl2 solvate in the monoclinic space group P2(1)/a: a = 17.155(8) angstrom, b = 12.019(4) angstrom, c = 16.985(6) angstrom, beta = 95.69(3)degrees, V = 3485(2) angstrom3, and Z = 4. The structure was refined to least-squares residuals of R = 0.069 (R(w) = 0.084) and a goodness-of-fit index of 2.09. Compound 2, (eta2-TMPP)MO(CO)4, was isolated in high yield from solutions of 1 treated with CO and in lower yield from solutions of 1 purged with N2 or H-2 which facilitate dissociation of CO. Compound 2 crystallizes in the monoclinic space group P2(1)/n: a = 12.222(3) angstrom, b = 17.756(8) angstrom, c = 15.342(2) angstrom, beta = 97.94(2)degrees, V = 3297(2) angstrom3, and Z = 4. The structure was refined to residuals of R = 0.032 (R(w) = 0.046) and goodness-of-fit = 1.29. (eta1-TMPP)Mo(CO)5, 3, was prepared by reaction of 2 with CO and by workup of CO saturated solutions of 1 under a CO atmosphere; the conversion between 2 and 3 is a reversible process. Compound 3 crystallizes in the monoclinic space group P2(1)/n: a = 11.232(3) angstrom, b = 23.250(7) angstrom, c = 13.515(4) angstrom, beta = 103.79(2)degrees, V = 3428(2) angstrom3, and Z = 4. The structure was refined to least-squares residuals of R = 0.032 (R(w) = 0.043) and goodness-of-fit = 1.94. The ether interactions in 1 and 2 are fluxional, as confirmed by variable-temperature H-1 NMR spectroscopy whereas 3 exhibits H-1 NMR properties in accord with a freely rotating monodentate phosphine ligand. H-1 NMR spectroscopy is a powerful tool for the assignment of structure in these systems, with the solution behavior correlating well with the solid-state structures. The syntheses, spectroscopic properties, and structural details of this series of related compounds will be discussed and compared to related compounds in the literature.