Light-induced reactions of trans-chloro(cyanomethyl)bis(triphenylphosphine) palladium(II) and the crystal and molecular structures of its acetone, acetonitrile, and benzene solvates

Abstract

X-Ray structure determinations have been completed for the acetone, acetonitrile, and benzene solvates of [PdCl(CH2CN)(PPh3)2](2). Crystals of (2)· acetone and (2)· acetonitrile are isomorphous, monoclinic, space group P21/n with four formula units in the unit cell. The acetone solvate has cell data : a= 12.679(4), b= 14.553(1), c= 19.506(5)A, and β= 94.72(1)°; for the acetonitrile solvate a= 12.581(2), b= 14.323(2), c= 19.359(3)A, and β= 93.85(1)°. The (2)· benzene crystals are also monoclinic, space group P21/a, with four formula units in a unit cell of dimensions a= 22.140(3), b= 10.058(1), c= 18.699(6)A, and β= 114.78(2)°. The structures were solved by the heavy-atom method and refined by full-matrix least squares calculations to R= 0.026, 0.028, and 0.050 for 4 907 2,998, and 3 664 observed reflections for the acetone, acetonitrile, and benzene solvates, respectively. In all three complexes molecule (2) has slightly distorted square planar co-ordination with Pd–P 2.319–2.342(1), Pd–Cl 2.369(2)–2.394(1), and Pd–C 2.063(2)–2.088(6)A. In (2)· acetone and (2)· acetonitrile the solvent molecules are trapped in polar pockets whereas in (2)· benzene the benzenes of solvation are in non-polar channels. The structures show little evidence of an interaction of the nitrile carbon atom with the metal atom nor any obvious structural features that might explain the photochromic behaviour of crystalline (2). Irradiation of (2) with u.v. or tungsten lamps, either in the solid state or in solution, leads to similar products. In the presence of oxygen, in benzene, the principal products are trans-[PdCl2(PPh3)2](3), [{PdCl(CH2CN)(PPh3)}n](4). [{PdCl2(PPh3)}2](5), PPh3O (6), and acetonitrile. The accumulation of (4) and (5) results from a loss of PPh3[from (2) and (3) respectively], the oxidation of which is probably catalysed by a reduced palladium species generated by photolysis of (2). In the absence of oxygen, in benzene, the only products identified were(3) and acetonitrile whilst in chloroform essentially quantitative conversion of (2) into (3), acetonitrile, and some succinonitrile is observed. Although an e.s.r. investigation showed that radicals are generated upon irradiation of (2) the principal mode of formation of acetonitrile appears to involve proton abstraction from traces of water in the solvent. The red colour assumed by crystalline (2) upon irradiation is limited to the surface. Similar colours have been obtained by heating or irradiating (2) in solution and by treating ethanolic solutions of (3) with base. It is suggested that reduced palladium species are responsible for these colours.