Donor-Acceptor Complexes between Simple Phosphines. First Structural Data for an Almost Forgotten Class of Compounds

Abstract

Lewis acid-base adducts between simple phosphines have been structurally characterized for the first time in the form of (Me3P)PBr3 (1) and (Me3P)PBr2Bz′ (2; Bz′ = 3,5-dimethylbenzyl). On the basis of its solid-state structure the bonding situation in 1 may be described as intermediate between a neutral donor-acceptor complex containing hypervalent phosphorus(III) and an ion pair, according to the resonance formulae Me3P → PBr3 ↔ [Me3 P-PBr2]+Br- . The weak (incomplete) bond formation to the third bromine atom clearly reflects the reluctance of trivalent phosphorus to accommodate more than four electron pairs in its valence shell. (Crystal data for 1: monoclinic P21/n, a = 7.142(3), b = 16.844(2), c = 8.139(4) Å, ß = 106.24(2)°, Z = 4). As a result of the inferior acceptor properties of PBr2Bz′ as compared to PBr3 , 2 has more ion pair character [Me3P-PBrBz′]+Br- than 1. (Crystal data for 2: orthorhombic P 212121, a = 7.063(2), b = 8.578(2), c = 25.158(4) Å, Z = 4). Both compounds have short P-P bonds (1: 2.264(2); 2: 2.236(2) Å). The results are compared with previous IR/Raman and NMR investigations on 1 : 1 and 2 : 1 adducts between organophosphines (as donors) and phosphorus trihalides (as acceptors).