Acta Crystallographica Section C

Crystal Structure Communications

Volume 69, Part 9 (September 2013)

organic compounds

fn3145 scheme

Acta Cryst. (2013). C69, 1051-1054    [ doi:10.1107/S0108270113020647 ]

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

R. T. Boeré and Y. Zhang

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 [[Sigma](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 [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)°. The value of [Sigma](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.

Formula: C45H70P+·ClO4-

Keywords: crystal structure; triarylphosphonium ions; pyramidality index; phosphoniumyl radical cations.

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