Synthesis and characterization of photosensitive, dinuclear palladium(I) complexes with 1,1′-bis(diphenylphosphino)ferrocene (dppf), [Pd2(dppf)2(RNC)2]2+ (R=xylyl and mesityl)

Reactions of the dinuclear palladium(I) complex, [Pd2(RNC)6](PF6)2 (R=2,6-xylyl (Xyl), 2,4,6-mesityl (Mes)), with 1,1′-bis(diphenylphosphino)ferrocene (dppf) gave dipalladium(I) complexes with dppf ligands, [Pd2(dppf)2(RNC)2](PF6)2 (1, R=Xyl, 66%; 2, R=Mes, 18%), which were characterized by elemental analysis, 1H- and 31P-NMR spectroscopy, IR and UV–vis absorption spectroscopic analyses, and cyclic voltammetry. The structure of 1 was characterized by X-ray crystallography. The cation of compound 1 is composed of two Pd(I) atoms joined by a PdPd σ-bond (2.602(1) Å), and each palladium ion has a square planar structure ligated by a terminal 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° and an average Pd⋯Fe distance of 4.236 Å. The cyclopentadienyl rings of dppf ligands are in staggered form. The 1H- and 31P-NMR and the electronic absorption spectra of 1 and 2 indicated that the metalmetal bonded 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 electronic absorption spectral changes and might generate a cation radical, [Pd(dppf)(RNC)]+. The cyclic voltammograms of 1 and 2 in acetonitrile solution showed two successive quasi-reversible oxidation waves at E1/2=0.60, 0.72 V (vs. Ag/AgPF6) (1) and 0.62, 0.73 V (2) and an irreversible reduction wave at E1/2=−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 demonstrated that a charge-transfer communication between the Fe centers occurred through the PdPd single bond.