PHOSPHORUS CARBON BOND-CLEAVAGE AND TETRAHEDRANE CLUSTER ACTIVATION IN THE REACTION BETWEEN BIS(DIPHENYLPHOSPHINO)MALEIC ANHYDRIDE (BMA) AND PHCCO3(CO)9 - SYNTHESES, KINETIC-STUDIES, AND X-RAY-DIFFRACTION STRUCTURES OF PHCCO3(CO)7(BMA) AND CO3(CO)6(MU(2)-ETA(2),ETA(1)-C(PH)C=C(PPH2)C(O)OC(O))(MU(2)-PPH2)

The tricobalt cluster PhCCo3(CO)9 (1) reacts with the bidentate phosphine ligand 2,3-bis-(diphenylphosphino)maleic anhydride (bma) in the presence of added Me3NO to give the diphosphine-substituted cluster PhCCo3(CO)7(bma) (2). Cluster 2 is unstable in solution, readily losing CO to afford Co3(CO)6(mu2-mu2,eta1-C(Ph)C = C(PPh2)C(O)OC(O))(mu2-PPh2) (3) as the sole observed product. Both clusters have been isolated and characterized by IR and NMR (P-31 and C-13) spectroscopy. Variable-temperature P-31 NMR measurements on cluster 2 indicate that the bma ligand functions as both a bridging and a chelating ligand. At -97-degrees-C, P-31 NMR analysis of 2 reveals a K(eq) of 5.7 in favor of the bridged bma cluster. The bridged bma cluster 2 is the only observed species above -50-degrees-C. Clusters 2 and 3 have been structurally characterized by single-crystal X-ray diffraction analyses. 2 crystallizes in the orthorhombic space group Pna2(1): a = 20.488(2) angstrom, b = 10.620(1) angstrom, c = 17.665(1) angstrom, v = 3844 angstrom3, Z = 4, d(calc) = 1.604 g-cm-3; R = 0.0367, R(w) = 0.0391 for 1344 observed reflections. 3 crystallizes in the monoclinic space group P2(1)/n: a = 11.538(1) angstrom, b = 17.0754(8) angstrom, c = 19.506(1) angstrom, beta = 92.108(7)-degrees, V = 3840 angstrom3, Z = 4, d(calc) = 1.557 g.cm-3; R = 0.0392, R(w) = 0.0432 for 2012 observed reflections. The solid-state structure of 2 does not correspond to the major bridging isomer observed in solution but rather the minor chelating isomer. The presence of the new six-electron mu2-eta2,eta1-benzylidene(diphenylphosphino)maleic anhydride ligand in 3, which results from the coupling of the mu3-benzylidyne capping group in 2 with the bma ligand, is established by X-ray crystallography. The conversion of 2 to 3 followed first-order kinetics, with the reaction rates being independent of the nature of the reaction solvent and strongly suppressed by added CO, supporting dissociative CO loss as the rate-determining step. The activation parameters for CO loss were determined to be DELTAH(double-dagger) = 29.9 +/- 2.2 kcal/mol and DELTAS(double-dagger) = 21 +/- 6 eu. The reactivity of 2 in the cleavage of the olefinic phosphorus-carbon bond relative to the analogous diphosphine-substituted cluster PhCCo3(CO)7(cis-Ph2PCH=CHPPh2) is discussed, and plausible mechanisms for the chelate to bridge bma ligand exchange in 2 and the formation of cluster 3 are presented.