Enhancement of axial coordination by β-substitutions on Ni IITPP

Abstract

The coordination ability of solvent molecules in metalloporphyrins is directly dependent on the nature of the central metal ion and on its valency. Because of the conjugated nature of the porphyrin ring system, electron donating or withdrawing substituents on the periphery of the molecule have been shown to affect the basicity of the porphyrin nitrogens [1, 2]. This, in turn, often affects the visible absorption spectra, redox potentials and axial ligation reactions of metalloporphyrins complexes [3–6]. Earlier spectrometric measurements on Ni(II) unsubstituted and para-substituted porphyrins showed the weak complexation of Ni(II) with strong Lewis bases [3]. Using electrochemical measurements on NiTPP (1) and NiTPP (CN) 4 ( 2) we demonstrate that the influence of β-substituted cyano-groups is particularly striking when compared to the very weak (on quasiabsence) interaction of Ni(II) with Lewis bases, in the unsubstituted porphyrin. In DMF, the potential E 1 2 shifts (for the first reduction step of 2), with increasing pyridine concentration, and reveals the formation of NiTPP(CN) 4(PY) n complexes ( 3), according to the following equilibria: ▪ The very similar values of K 1 and K 2 were determined by using the relation of Kuta [7]: ▪ If water, instead of Py, is added, nickel in 2 forms only the monoliganded complex Ni(TPP(CN) 4(H 2O) ( 4). In the same experimental conditions the redox potential of 1 does not shift significantly with neither pyridine nor water. These results agree with independent spectrometric measurements [6] carried out with pyridine in chloroform solution. On kinetic point of view, a slow down is observed for the heterogeneous electrochemical rate constants of the electron transfer rate corresponding to the order of the calculated values for the equilibrium constants.