NMR spectra of the porphyrins. Part 40. Self-aggregation in zinc(II) and nickel(II) 2-vinylphyllperythrins

Abstract

The large concentration dependence of the ¹H NMR spectrum of nickel(II) 2-vinylphylloerythrin has been recorded and analysed in terms of both monomer–dimer and monomer–dimer–trimer equilibria. The equilibrium constant obtained from these analyses is ca. 250 dm³ mol^–1 which is large enough to give significant concentration shifts at concentrations of 2 × 10^–4 mol dm^–3.

The complexation shifts are almost identical to those obtained previously for zinc(II) 2-vinylphyllo-erythrin, showing, in this porphyrin, the independence of the aggregate structure to the central metal atom.

The aggregation shifts obtained were analysed using a previous well-defined ring-current model and considering all possible modes of aggregation, including face-to-face and back-to-face structures. The aggregation shifts are best reproduced on the basis of a model in which the porphyrin ring current is reduced by ca. 5% on aggregation. In this model the separation of the porphyrin planes is ca. 5.0 Å with some lateral displacement. Both face-to-face and back-to-face structures give almost identical calculated shifts in complete agreement with the observed shifts.