ANIONIC AND STERIC FACTORS GOVERNING COORDINATIVE UNSATURATION AT CARBENIC PHOSPHENIUM CENTERS

Tetrachlorogallate and tetraphenylborate salts of the phosphenium cations [(iPr2N)2P]+ (1) and [MeNCH2CH2N(Me)P]+ (2) are examined. Spectroscopic characterization confirms the ionic formulation for all compounds except for 2[BPh4], which exists as the covalent alternative phosphine-borane MeNCH2CH2N(Me)P(Ph)-BPh3 (5) (crystal data: C28H30BN2P, triclinic, P1BAR, a = 9.7 10(1) angstrom, b = 14.962(2) angstrom, c = 9.610(2) angstrom, alpha = 105.45(1)degrees, beta = 113.59(1)degrees, gamma = 92.73(1)degrees, Z = 2). The ionic borate 1[BPh4] represents the first isolated phosphenium salt containing a ''noncoordinating anion'', and its stability with respect to the covalent alternative analogue 5 is postulated to rely upon the steric bulkiness of both cation and anion. Compound 1 [BPh4] reacts with chlorinated solvents within hours to give a number of products, one of which has been identified as the phosphonium salt [(iPr2N)2P(Cl)CH2Cl][BPh4] (8[BPh4]) (crystal data: C37H50BCl2N2P, monoclinic, P2(1)/n, a = 10.049(3) angstrom, b = 21.794(3) angstrom, c = 16.391(3) angstrom, beta = 92.78(2)degrees, Z = 4), the result of an oxidative addition of CH2Cl2 to the phosphenium center. In contrast, tetrachlorogallate salts of 1 and 2 are indefinitely stable in CH2Cl2 and CHCl3 solutions. The crystal structures of 1[GaCl4] (crystal data: C12H28Cl4GaN2P, tetragonal, I4(1)cd, a = 20.014(4) angstrom, b = 20.014(4) angstrom, c = 21.466(6) angstrom, Z = 16) and 2[GaCl4] (crystal data: C4H10C14GaN2P, monoclinic, P2(1) a = 6.489(3) angstrom, b = 13.893(5) angstrom, c = 7.066(5) angstrom, beta = 92.58(5)degrees, Z = 2) reveal similar anionic arrays around the cation despite different space groups and cation size. The closest contacts occur between the phosphorus and chlorine centers but are in excess of the van der Waals radii. Nevertheless, electrostatic interactions are believed to be maintained to some degree in solution and are responsible for protection of the cationic center. Such interactions are expected to be weaker in the tetraphenylborate salt, which renders the phosphorus center susceptible to attack by solvent molecules that are small enough to infiltrate the steric shield allowing for formation of 8 and other products.