Intramolecular P–H···H–Si Dihydrogen Bonding in the 5-Dimethylsilyl-9,9-dimethylxanthen-4-yl-diphenylphosphonium Cation

Abstract

The synthesis of 4-bromo-5-diphenylphosphino-9,9-dimethylxanthene (1a), 4-bromo-6-diphenylphosphinodibenzofuran (1b), 4-diphenylphosphino-5-dimethylsilyl-9,9-dimethylxanthene (2a), 4-diphenylphosphino-6-dimethylsilyldibenzofuran (2b), 5-dimethylsilyl-9,9-dimethylxanthen-4-yl-diphenylphosphonium tetrakis(pentafluorophenyl)borate [3a][B(C6F5)4], and 6-dimethylsilyldibenzofuran-4-yl-diphenylphosphonium tetrakis(pentafluorophenyl)borate [3b][B(C6F5)4] was reported. Unlike the benzofuran derivate [3b]+, the xanthene derivative [3a]+ shows an intramolecular dihydrogen bond of the type P–H···H–Si, which was studied by DFT calculation using a set of real-space bond indicators (RSBIs) derived from the Atoms-In-Molecules (AIM), Non-Covalent Interactions (NCI) indicator, and Electron Localizability Indicator (ELI-D) methods. Neither [3a]+ nor [3b]+ shows any propensity to release dihydrogen to give the 4-diphenylphosphino-9,9-dimethylxanthen-5-yl-dimethylsilyl cation [4a]+ or the 4-diphenylphosphino-dibenzofuran-5-yl-dimethylsilyl cation [4b]+, respectively.