A series of discrete dinuclear
cyano-bridged complexes have been synthesised and their properties investigated by X-ray crystallography, electronic spectroscopy, infrared spectroscopy and electrochemistry.
The dinuclear complexes consist of pentadentate-coordinated macrocyclic cobalt complexes bearing hexacyanometallates (iron, ruthenium and cobalt) in their sixth coordination site. The Co111-Fe11 and Co111-Ru11
complexes exhibit metal-to-metal charge transfer (MMCT, Fe ® Co and Ru ® Co) transitions in the visible region. Metal-to-metal charge transfer is not observed in the Co111- Co111 complexes since both metal centres are in identical oxidation states. Crystal structures were obtained for the dinuclear complexes trans-[L14CoNCRu(CN)5] (in two polymorphic forms), trans-[L14CoNCCo(CN)5] and trans-[L15CoNCCo(CN)5], where L14 = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine and L15 = 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine.
An analysis of the MMCT bands based on the theories of Marcus and Hush was conducted. The effects of structural changes to the complex (macrocyclic ring size and configuration, ligand donor atoms, MMCT donor metal centre) and changes in the environment (pH, pressure, temperature, solvent) on the redox potentials of the metal centres and the MMCT transition energy were studied. The investigation revealed that a degree of tuning of the spectral properties of the mixed valence complexes is possible by altering the redox
potentials of the metal centres.
A complication to the direct comparison of the results obtained from spectroscopy and electrochemistry is the existence of two spin states in the Co11 centre of the MMCT product. A low-spin (t2g6 eg1) Co11
centre is formed in the spin-allowed electronic transition, whereas a high-spin (t2g5 eg2) Co11 centre is evidently formed upon electrochemical reduction. This is manifest in the pressure and temperature dependent studies, where the difference between the redox potentials changes to a greater extent with temperature and pressure than does the energy of the MMCT transition.
Transient absorption measurements on the complexes trans-[L14CoNCFe(CN)5] and trans-[L14SCoNCFe(CN)5] (where L14S = 6-methyl-1,11-dithia-4,8- diazacyclotetradecan-6-amine) were conducted at low temperature in the nanosecond time-domain. The most prominent features of these spectra were LMCT bands due
to the Fe111 chromophore produced by MMCT excitation. Pump-probe experiments were also conducted at room temperature in the femtosecond time-domain, probing at 400 and 530 nm. Rapid back-electron transfer from the low-spin-Co11-Fe111 state was observed following excitation into the MMCT band with lifetimes of 0.8 and 1.3 ps for trans-[L14CoNCFe(CN)5] and trans-[L14SCoNCFe(CN)5], respectively. The observation of excited state lifetimes at low temperature on the nanosecond time scale, given the more rapid relaxation at room temperature, suggests a significant population of the high-spin- Co11-Fe111 state is formed, for which
back-electron transfer is relatively slow compared to the corresponding low-spin state.
Two crystal structures were also obtained for the mononuclear precursor, cis- [CoL14SCl]2+, with different counter ions to the previously published structure (dichloro and chloroperchlorate). The cis/trans isomerisation reactions of [CoL14OH]2+, and [CoL14SOH]2+, were also investigated, and found to be
catalysed in the presence of base.