Electrochemical and spectroscopic detection of self-association of octa-alkyl phthalocyaninato cadmium compounds into dimeric species

The solution phase behaviour of non-peripherally substituted octa-hexyl cadmium phthalocyanine 3 and peripherally substituted octa-2-ethylhexyl cadmium phthalocyanine 4 has been investigated in fresh solutions of CH₂Cl₂, CHCl₃-d₁ and THF/THF-d₈ using ¹H NMR spectrometry, UV-Vis spectroscopy, cyclic voltammetry, square wave voltammetry and linear sweep voltammetry. The compounds show an unexpected propensity to form dimeric species in CH₂Cl₂ and CHCl₃-d₁, and, in the case of 4, also to a lesser extent in THF/THF-d₈. This phenomenon is not observed for their metal-free analogues 1 or 2. The electrochemical results provide particularly strong evidence for the dimeric structures. In particular both the first one-electron oxidation and one-electron reduction waves for 3 and 4, unlike those of 1 and 2, are split. This is consistent with sequential oxidation/reduction of the two Pc ligands within a dimer. The dimeric species are likely to be the immediate precursors of the recently discovered bis-cadmium tris-phthalocyanine triple-decker sandwich complexes 5 and 6 formed from 3 and 4 over a period of time. The electrochemical data for compounds 1–4 also show that (i) relative to the metal-free phthalocyanines, the cadmium phthalocyanines exhibit smaller formal reduction potentials for all but one of the observed electron transfer processes and (ii) the electron transfer processes associated with the peripherally substituted compounds, 2 and 4, are observed at more positive potentials than those for the corresponding non-peripherally substituted analogues 1 and 3.