A Silsesquioxane-Based Diphosphinite Ligand: Synthesis, DFT Study, and Coordination Chemistry

The incompletely condensed silsesquioxane disilanol (c-C5H9)7Si7O9(OH)2OSiMePh2 has
been used as a backbone for the synthesis of the diphosphinite ligand (c-C5H9)7Si7O9(OPPh2)2OSiMePh2 (1), based on a silsesquioxane framework. By reaction with black selenium, the
corresponding selenide (2) was obtained, showing a JSe-P value of 815 Hz in the 31P 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)2OPH2 (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)7Si7O9OSiMePh2). 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)4R(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.