A study of the pyramidality index in tris(2,4,6-triisopropylphenyl)phosphonium perchlorate

Abstract

The title compound, C45H70P+·ClO4−or [Tripp3PH+][ClO4−], was produced from the perchlorate salt of the corresponding radical cation Tripp3P·+through very slow H-atom abstraction reactions in a solution of 1,2-dimethoxyethane (Tripp is 2,4,6-triisopropylphenyl). 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) Å]. 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 [P...H = 1.99 (2)–2.17 (2) Å]. The sum of the angles around the P atom [Σ(C—P—C) = 349.9 (6)°] is a convenient index of pyramidality for tricoordinate centres. This value is significantly larger than in analogous Mes3PH+salts (Mes is mesityl or 2,4,6-trimethylphenyl), for which the average of the three reported structures in the literature is Σ(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)°. The value of Σ(C—P—C) in the title compound is midway between that of the neutral phosphane Tripp3P [334.4 (6)°] and the phosphoniumyl radical cation Tripp3P·+[359.8 (2)°]. This geometrical feature provides additional support for the assignment as a phosphonium salt.