Synthesis and characterization of photosensitive, dinuclear palladium(I) complexes with 1,1 '-bis(diphenylphosphino)ferrocene (dppf), [Pd-2(dppf)(2)(RNC)(2)](2+) (R = xylyl and mesityl)

Reactions of the dinuclear palladium(I) complex, [Pd-2(RNC)(6)](PF6)(2) (R = 2,6-xylyl (Xyl), 2,4,6-mesityl (Mes)), with 1,1'-bis-(diphenylphosphino)f errocene (dppf) gave dipalladium(I) complexes with dppf ligands, [Pd-2(dppf )(2)(RNC)(2)](PF6)(2) (1, R = Xyl, 66%; 2, R = Mes, 18%), which were charac terized by elemental analysis, H-1- and P-31-NMR spectroscopy, IR and UV-vi s absorption spectroscopic analyses, and cyclic voltammetry. The structure of 1 was characterized by X-ray crystallography. The cation of compound 1 i s composed of two Pd(I) atoms joined by a Pd-Pd sigma-bond (2.602(1) Angstr om) and each palladium ion has a square planar structure ligated by a termi nal isocyanide, two P atoms of dppf, and the neighboring Pd atom. The dppf ligands chelate to the metal with an average P-Pd-P bite angle of 99.19 deg rees and an average Pd Fe distance of 4.236 Angstrom. The cyclopentadienyl rings of dppf ligands are in staggered form. The H-1- and P-31-NMR and the electronic absorption spectra of 1 and 2 indicated that the metal-metal bon ded structure as observed in the crystal of 1 was retained in the solution. Complexes 1 and 2 were extremely photosensitive, and underwent a homolytic cleavage even under a room light. The reaction was monitored by the electr onic absorption spectral changes and might generate a cation radical, [Pd(d ppf)(ANC)](+). The cyclic voltammograms of 1 and 2 in acetonitrile solution showed two successive quasi-reversible oxidation waves at E-1/2 = 0.60, 0. 72 V (vs. Ag/AgPF6) (1) and 0.62, 0.73 V (2) and an irreversible reduction wave at E-1/2 = -1.23 V (1) and -1.22 V (2). The former oxidation waves can be assigned to Fe(II)/Fe(III) processes of the two ferrocenyl groups and d emonstrated that a charge-transfer communication between the Fe centers occ urred through the Pd-Pd single bond. (C) 1999 Elsevier Science S.A. All rig hts reserved.