Oxygen insertion in a carbon–phosphorus bond of the phenylethynyl-di-(tert-butyl)-phosphine bridged dicobalt complex: exploring the nature of oxygen migration using DFT

Abstract

In the process of isolation under aerobic conditions phenylethynyl-di-(tert-butyl)-phosphine bridged dicobalt complex [(µ-PPh₂CH₂PPh₂)Co₂(CO)₄(µ,η-PhCCP(t-Bu)₂)] 4a underwent a partial oxidation. The identity of the oxidized product, [(µ-PPh₂CH₂PPh₂)Co₂(CO)₄(µ,η-PhCC–O–P(O)(t-Bu)₂)] 5, was established by spectroscopic means as well as the single-crystal X-ray diffraction method. This is the first crystallographic evidence that unambiguously supports the formation of an organometallic version of a phosphinate ester. The mechanism for the formation of 5 from 4a was proposed, and its validity was examined by DFT means. For the purpose of comparison, a similar mechanism illustrating the transformation of PhCCP(t-Bu)₂1O into PhCC–O–P(O)(t-Bu)₂5O, the organic counterpart of 5, was examined by the same method. It was found that the metal fragment is indeed capable of assisting the oxidation process by lowering the activation energy, although the effect is small. The impact of the presence of an electron-withdrawing substituent such as a fluorine atom in the alkynylphosphine was also investigated. Results demonstrated that the conversion of fluorine-substituted phosphines to the corresponding phosphinate esters can be achieved more readily. In addition, the energy barrier for the reaction of a phosphine with dioxygen yielding the phosphine oxide was calculated to be much lower than that on the way to the phosphinate ester.