5-Ferrocenyl-2,2′-bipyridine ligands: synthesis, palladium(II) and copper(I) complexes, optical and electrochemical properties

Two ferrocene-appended 2,2'-bipyridine ligands, 5-ferrocenyl-2,2'-bipyridine (Fc-bipy) and 5-ferrocenyl-5'- methyl-2,2'-bipyridine (Fc-bipy-Me) metalloligands were synthesised using palladium(0) catalysed Suzuki-Miyaura cross-coupling reaction. Their coordination chemistry with palladium(II) and copper(I) ions was examined. The molecular structures of several complexes, including [Pd(Fc-bipy-Me)(Cl)(2)], [Pd(Fc-bipy)(py)(2)](SbF6)(2), [Pd(Fc-bipy)(DMAP)(2)](SbF6)(2), [Pd(Fc-bipy-Me)(DMAP)(2)](SbF6)(2), and [Cu(Fc-bipy)( 6,6'-dimesityl-2,2'-bipyridine)](PF6) and [Cu(Fc-bipy-Me)(6,6'-dimesityl-2,2'-bipyridine)](PF6) were determined using X-ray crystallography. H-1 and C-13 NMR spectra and high resolution electrospray mass spectrometry (HR-ES-MS) experiments confirmed that the majority of the complexes are stable in solution. The exceptions were [Pd(Fc-bipy)(DMAP)(2)](SbF6)(2) and [Pd(Fc-bipy-Me)(DMAP)(2)](SbF6)(2) which were found to disproportionate to give equilibrium mixtures containing [Pd(Fc-bipy)(2)](SbF6)(2) and [Pd(DMAP)(4)](SbF6)(2). The optical absorption and electrochemical properties of these complexes were compared to those of the parent ligands. The 5-ferrocenyl-2,2'-bipyridine ligands are yellow/orange in colour with the expected weak d-d transition found at similar to 450 nm. The absorption bands of the palladium(II) and copper(I) complexes are red-shifted relative to the free ligands and show enhanced molar absorptivity. The complexes all display strong MLCT absorptions in the visible spectrum, the palladium(II) compounds are deep purple (lambda(max) similar to 540-560 nm), while the copper(I) complexes are redbrown (lambda(max) similar to 470 nm). Electrochemical investigations showed that all the compounds displayed the expected chemically reversible one-electron oxidation of the ferrocenyl group. The reduction potential of the redox-active ferrocene unit was anodically shifted, relative to the free ligand, upon complexation to the metal ions. The copper(I) complexes displayed an additional reversible oxidation wave that was assigned as a Cu(II)/Cu(I) redox process.