Electrochemical and spectroelectrochemical studies on rhenium complexes

A series of binary ligand complexes [ReCl_6-n(py)_n]^z (where n=0 to 6, z is the charge on the complex and py=pyridine) have been chemically and electrochemically synthesised via reduction-induced substitution reactions from [ReCl₆]^2-. These complexes have been characterised using electrochemical and spectroelectrochemical techniques. Seven members of the series, namely, [ReCl₅py]^1-, trans-, cis- [ReCl₄(py)₂]^0,-1, mer-[ReCl₃(py)₃], trans[ReCl₂(py)₄]¹⁺, [ReCl(py)₅]²⁺ and [Re(py)₆]²⁺, have been structurally characterised by single crystal X-ray diffraction studies. Extended Hückel Molecular Orbital calculations have been performed on the complexes to elucidate which orientation of the pyridine ring(s) gives rise to the most stable structure. The theoretically and experimentally determined structures agree well. The complexes exhibit a wide range of redox processes spanning from Re^II/I to Re^V/IV based couples. The electrochemical data obtained from these complexes have been used to fit two ligand additivity models, those of Bursten and Lever. Practically, Lever's model is more convenient to use than Bursten's model while the latter is more successful in correcting for isomer effects. The linear relationship between the half-wave potentials for the redox couples of the series and either stoichiommetry, n, or the sum of Lever's ligand parameters ΣE_L indicates that the six ligands around the metal centre individually influence the electronic character of the metal centre. Comparison of the UV/Vis/NIR spectra of [ReCl_6-n(py)_n]^z complexes, varying n and z, has resulted in detailed assignments of the charge transfer bands to electronic transitions, including the elucidation of d–d transitions within the non-degenerate t_2g orbitals. To explore how a ligand alters the stability and reactivity of a complex and hence probe the influence of ligand substitution on the electronic structure of the metal centre in the complex, three monosubstituted chlororhenium complexes, [ReCl₅L]^1- (L=benzonitrile, acetonitrile and pyridine), have been electrochemically and spectroelectrochemically studied. The structure of the complex [ReCl₅(NCPh)]^1- has been determined by single crystal X-ray diffraction studies. The three complexes undergo reduction induced substitution reactions which have been fully investigated using several electrochemical techniques.