PI-COORDINATION OF ARENES TO PHOSPHORUS - MODELS OF STABLE PI-COMPLEX INTERMEDIATES IN ELECTROPHILIC AROMATIC-SUBSTITUTION

A series of salts containing cations of the form [Mes*NP(arene)](+) (arene = benzene, toluene, mesitylene) have been isolated and structurally characterized, representing the first arene pi-complexes of phosphorus. X-ray crystal structures reveal hexahapto association of the arene to the phosphorus center in all cases with the distance between phosphorus and the centroid of the arene decreasing from benzene [P-benzene(centroid), 2.820(4) Angstrom] to toluene [P-toluene(centroid), 2.767(7) Angstrom] to mesitylene [P-mesitylene(centroid), 2.687(4) Angstrom] in the heptachlorodigallate (Ga2Cl7-) salts {crystal data: [Mes*Np(benzene)][GaCl4], Pbca, a = 24.948(4) Angstrom, b = 20.195(4) Angstrom, c = 11.953(3) Angstrom, V = 6022(4) Angstrom(3), Z = 8, D-c = 1.279(1) Mg m(-3) R = 12.126(5) Angstrom, V = 6173(8) Angstrom(3), Z = 8, D-c = 1.278(2) Mg m(-3), R = 0.0760, R(w) = 0.0760; [Mes*NP(benzene)][Ga2Cl7], P ($) over bar 1, a = 11.579(2) Angstrom, b = 16.350(3) Angstrom, c = 11.085(3) Angstrom, alpha = 90.74(2)degrees, beta = 118.00(2)degrees, gamma = 73.53(2)degrees, V = 1559(1) Angstrom(3), Z = 2, D-c = 1.428(1) Mg m(-3), R = 0.0386, R(w) = 0.0386; [Mes*NP(toluene)][Ga2Cl7], P ($) over bar 1, a = 11.571(4) Angstrom, b = 16.335(5) Angstrom, c = 11.326(3) Angstrom, alpha = 91.47(4)degrees, beta = 117.87(2)degrees, gamma = 105.34(3)degrees, V = 1797(1) Angstrom(3), Z = 2, D-c = 1.423(1) Mg m(-3), R = 0.0464, R(w) = 0.0464; [Mes*NP(mesitylene)][Ga2Cl7], P ($) over bar 1, a = 12.119(3) Angstrom, b = 16.575(4) Angstrom, c = 11.309(3) Angstrom, alpha = 93.22(2)degrees, beta = 115.62(2)degrees, gamma = 107.64(2)degrees, V = 1905(1) Angstrom(3), Z = 2, D-c = 1.391(1) Mg m(-3), R = 0.0420, R(w) = 0.0450}. In the solid state and in dichloromethane solution, the C-13 and H-1 NMR chemical shifts for the arene units of the complexes are significantly deshielded with respect to those for the corresponding free arene. In addition, there are notable differences in the positions of characteristic combination bands in the IR spectra (1700-2000 cm(-1)). The new cations are viewed as models for pi-complex intermediates of electrophilic aromatic substitution.