Iodophosphonium salt structures: homonuclear cation-anion interactions leading to supramolecular assemblies

The structures of iodophosphonium cations RnPI4-n+ depend significantly on the nature of their counteranions, which act as nucleophiles towards electrophilic iodine atoms bonded to the formally charged phosphorus atom. This nucleophilic attack leads to P-I bond lengthening. that can be understood as consequence of (n --> sigma*) donor-acceptor interactions, i.e. population of the sigma* energy level of the attacked P-I bond. For a given cation, anion 'iodophilicity' correlates well with P-I and I-I distances: the phosphane and the anion compete for coordination with the central linearly coordinated iodine atom. When different iodophosphonium cations are compared, however, P-III-I correlations are not always that straightforward, since specific effects of the peculiar substituents like size, electronic properties and packing preferences do also play a role. In di- and triiodophosphonium ions, bi- and trifunctionality of the soft Lewis acids R2PI2+ and RPI3- and the ability of iodide ions to bridge up to five cations allows the formation of rings, chains, columnar, layer and 3D net structures, which are all due to I ... I interactions. Comparison of several structures involving the same cation confirms. that I-3(-) is a much weaker donor than I-, and indicates, that in solid compounds bridging I- anions 'spread' their donor ability over several iodophosphonium accepters; i.e. the individual cation is less affected. This allows to understand, why compounds RPI4 (R = Me, i-Pr, t-Bu) are stable as solids, but dissociate in inert solvents into RPI2 and molecular iodine. (C) 1999 Elsevier Science S.A. All rights reserved.