Abstract:

The title compound, C45H70P+center dot ClO4-or [Tripp(3)PH(+)][ClO4-], was produced from the perchlorate salt of the corresponding radical cation Tripp(3)P.(+) through very slow H-atom abstraction reactions in a solution of 1,2-dimethoxyethane (Tripp is 2,4,6triisopropylphenyl). The H atom on the P atom was located in a difference map and was freely refined with an isotropic displacement consistent with full occupancy [P-H = 1.37 (3) A]. It is the most sterically congested triarylphosphonium salt yet to be reported and has a ` propeller' arrangement of the three Tripp groups around the P atom, with the protonation site located along the molecular threefold axis. There are short contacts between the flanking isopropyl methine H atoms and the P atom [Ph3H = 1.99 (2)2.17 (2) A]. The sum of the angles around the P atom [Sigma(C-P-C) = 349.9 (6) angstrom] is a convenient index of pyramidality for tricoordinate centres. This value is significantly larger than in analogous Mes3PH + salts (Mes is mesityl or 2,4,6trimethylphenyl), for which the average of the three reported structures in the literature is Sigma (C-P-C) = 345.3 (6) . For comparison, in the ubiquitous Ph3PH+ salts, this parameter has a typical average value of only 333.3 (9)degrees. The value of Sigma (CP- C) in the title compound is midway between that of the neutral phosphane Tripp3P [334.4 (6)degrees] and the phosphoniumyl radical cation Tripp3P(+) [359.8 (2)degrees]. This geometrical feature provides additional support for the assignment as a phosphonium salt.