Diyne and Phosphorus−Carbon Bond Reactivity in the Reaction between [Co2(CO)6]2(PhC4Ph) and 2,3-Bis(diphenylphosphino)maleic Anhydride (bma). Syntheses, Molecular Orbital Properties, and X-ray Diffraction Structures of Co2(CO)4[μ-η2:η2:η1:η1-(Z)-Ph2P(Ph)CC(PhC2)- CC(Ph2P)C(O)OC(O)], Co2(CO)2(bma)2·CH2Cl2, and Co2(CO)2(bma)[μ-CC(Ph2P)C(O)OC(O)](μ2-Ph2P)·C6H14

Abstract

Thermolysis (>80 °C) of the diyne complex [Co2(CO)6]2(PhC4Ph) (1) with 2,3-bis(diphenylphosphino)maleic anhydride (bma) affords the new compounds Co2(CO)4[μ-η2:η2:η1:η1-(Z)-Ph2P(Ph)CC(PhC2)CC(Ph2P)C(O)OC(O)] (3), Co2(CO)2(bma)2 (4), and Co2(CO)2(bma)[μ-CC(Ph2P)C(O)OC(O)](μ2-Ph2P) (5) in low yields, while [Co2(CO)6]2(PhC4Ph) reacts with added bma in either refluxing CH2Cl2 or in the presence of Me3NO to give the thermally sensitive complex [Co2(CO)4(bma)(PhC4Ph)Co2(CO)6] (2). Independent experiments reveal that 3 arises from 2 by loss of the Co2(CO)6 group, coupled with PC bond cleavage and diyne functionalization by the transient phosphido and maleic anhydride moieties, and the reaction between 2 and excess bma leads to both 4 and 5, with 5 originating from 4. The kinetics for the reaction of 4 to 5 have been measured by UV−vis spectroscopy, and on the basis of the first-order rate constants and the activation parameters (ΔH⧧ = 27.0 ± 0.6 kcal mol-1 and ΔS⧧ = 1.0 ± 0.3 eu), a mechanism involving dissociative CO loss as the rate-determining step is presented. Binuclear 4 is extremely photosensitive and is converted cleanly to 5 by 366 nm light with a quantum efficiency of 0.0043. Compounds 2−5 have been isolated and characterized in solution by IR and 31P NMR spectroscopy. The solid-state structures of 3−5 have been established by X-ray crystallography. The X-ray structure of 4 reveals the presence of two bma ligands that are attached to the Co2(CO)2 unit in a head-to-tail fashion via the PPh2 groups and maleic anhydride π bond, and the X-ray structure of 5 supports the existence of a μ2-PPh2 moiety and a noncomplexed maleic anhydride π bond, the result of PC(maleic anhydride) bond activation. The redox properties of 2−5 were explored by cyclic voltammetry in CH2Cl2, and the oxidation/reduction behavior is discussed with respect to redox stabilization that each complex experiences, as modulated by the bma ligand(s). The orbital composition of the HOMO and LUMO levels in 3−5 has been studied by extended Hückel calculations, and the data are discussed relative to the observed electrochemistry. A plausible mechanism for the activation of Co2(CO)2(bma)2 and the formation of Co2(CO)2(bma)[μ-CC(Ph2P)C(O)OC(O)](μ2-Ph2P) is presented.