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)

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Reactions of the dinuclear palladium(I) complex, [Pd 2 (RNC) 6 ](PF 6 ) 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, [Pd 2 (dppf) 2 (RNC) 2 ](PF 6 ) 2 ( 1 , R=Xyl, 66%; 2 , R=Mes, 18%), which were characterized by elemental analysis, 1 H- and 31 P-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) A), 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 A. The cyclopentadienyl rings of dppf ligands are in staggered form. The 1 H- and 31 P-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 E 1/2 =0.60, 0.72 V (vs. Ag/AgPF 6 ) ( 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 demonstrated that a charge-transfer communication between the Fe centers occurred through the PdPd single bond.