The Complexed 1,3-Diphospha-2-Arsa-Allyl Radical and its Cationic and Anionic Derivatives

Abstract

Photolysis of [Cp*As2] (1 a) in the presence of Mes*PPMes* (Mes*=2,4,6-tri-tert-butylphenyl) leads to the novel 1,3-diphospha-2-arsaallyl radical [(CO)5W(μ,η2:η1-P2AsMes*2)W(CO)4] (2 a). The frontier orbitals of the radical 2 a are indicative of a stable π-allylic system that is only marginally influenced by the d orbitals of the two tungsten atoms. The SOMO and the corresponding spin density distribution of the radical 2 a show that the unpaired electron is preferentially located at the two equivalent terminal phosphorus atoms, which has been confirmed by EPR spectroscopy. The protonated derivative of 2 a, the complex [(CO)5W(μ,η2:η1-P2As(H)Mes*2)W(CO)4] (6 a) is formed during chromatographic workup, whereas the additional products [Mes*PPMes*] as the Z-isomer (3) and the E-isomer (4), and [As23] (5) are produced as a result of a decomposition reaction of radical 2 a. Reduction of radical 2 a yields the stable anion [(CO)5W(μ,η2:η1-P2AsMes*2)W(CO)4]− in 7 a, whereas upon oxidation the corresponding cationic complex [(CO)5W(μ,η2:η1-P2AsMes*2)W(CO)4][SbF6] (8 a) is formed, which is only stable at low temperatures in solution. Compounds 2 a, 7 a, and 8 a represent the hitherto elusive complexed redox congeners of the diphospha-arsa-allyl system. The analogous oxidation of the triphosphaallyl radical [(CO)5W(μ,η2:η1- P3Mes*2)W(CO)4] (2 b) also leads to an allyl cation, which decomposes under CH activation to the phosphine derivative [(CO)5WW(CO)4] (9), in which a CH bond of a methyl group of the Mes* substituent has been activated. All new products have been characterized by NMR spectrometry and IR spectroscopy, and compounds 2 a, 3, 6 a, 7 a, and 9 by X-ray diffraction analysis.