title: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Lethbridge, Lethbridge, AB, Canada T1K 3M4. boere@uleth.ca
authors.forename: René T
authors.initials: RT
authors.lastname: Boeré
authors.forename: Yuankui
authors.initials: Y
authors.lastname: Zhang
keywords: crystal structure; phosphoniumyl radical cations; pyramidality index; triarylphosphonium ions