A silsesquioxane-based diphosphinite ligand: Synthesis, DFT study, and coordination chemistry

The incompletely condensed silsesquioxane disilanol (c-C5H9)(7)Si7O9(OH)(2)OSiMePh2 has been used as a backbone for the synthesis of the diphosphinite ligand (C-C5H9)(7)Si7O9(OPPh2)(2)-OSiMePh2 (1), based on a silsesquioxane framework. By reaction with black selenium, the corresponding selenide (2) was obtained, showing a J(Se-P) value of 815 Hz in the P-31 NMR spectrum. DFT calculations established a good insight into the electron density of the P atoms present in the two model compounds CH3OPPh2 (3) and Ph2P(HSiOH)(2)OPH2 (4). The Mulliken charge distributions show a clear electron-withdrawing effect of the siloxy group, which is also present in diphosphinite ligand 1. By reaction of compound 1 with PdCl2-(C6H5CN)(2), the palladium complex [PdCl2R(OPPh2)(2)] (5) was obtained (R = (C-C5H9)(7)Si7O9-OSiMePh2). From a similar reaction of 1 with PtCl2(cod), the platinum analogue [PtCl2R(OPPh2)(2)] (6) could be isolated. The equimolar reaction Of Mo(CO)(4)(pip)(2) (pip = piperidine) with 1 yielded the molybdenum complex [Mo(CO)(4)R(OPPh2)(2)] (7). Ligand 1 showed a clear tendency to coordinate in a cis fashion for all complexes 5-7, as was determined by NMR spectroscopy and X-ray crystallography. However, the analogous reaction of 1 with RhCl(CO)(2) dimer yielded the mononuclear trans-[RhCl(CO)(1)] (8). Complexes 5-8 have been structurally characterized as the first examples of transition-metal complexes with a silsesquioxane-based bidentate phosphinite ligand.