Tuning the potential of redox-active diphosphine ligands based on the alkyne complexes [Tp*W(CO)L{η2-C2(PPh2)2}]

Abstract

Eleven complexes with the general formula [Tp*W(CO)L{η²-C₂(PPh₂)₂}]ⁿ⁺ {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)-borate, L = F⁻, Cl⁻, Br⁻, I⁻, MeS⁻, PhS⁻, pyCH₂S⁻, CN⁻ and TfO⁻; n = 0 and L = CH₃CN and pyridine; n = 1} have been synthesized and fully characterized. Depending on L, the oxidation process from W(II) to W(III) is detected between −0.28 and +0.55 V vs. Fc/Fc⁺ and the spectroscopic properties (X-ray, IR, and NMR) are influenced according to the electron-rich or electron-poor character of the central metal. The basicity of the alkyne complex-based phosphine groups was estimated by the ³¹P/⁷⁷Se coupling method of the corresponding diselenides. Selected examples of the dppa-complex ligands were converted into the corresponding κ²-PdCl₂ chelate complexes and employed in a Sonogashira reaction in order to estimate the effect of L on the catalytic behaviour of the dinuclear complexes. While the spectroscopic properties show a good correlation with the redox potential in a mostly linear fashion, catalytic activity is influenced only slightly. The effect of PdCl₂ coordination on the alkyne complex is evident from the W(II)/W(III) redox potentials measured by cyclic voltammetry supported by a change of the CO stretching frequency in IR. A comparison of the molecular structures of the alkyne complexes with terminal phosphine groups and the PdCl₂ chelate complexes all determined by XRD shows the essential flexibility of the bend-back angles in the alkyne complex moiety.