Proton magnetic resonance spectra of porphyrins. Part VI. Complex formation between nickel(II)–mesoporphyrin IX dimethyl ester and piperidine

Abstract

Complex formation between nickel(II)–mesoporphyrin IX dimethyl ester and piperidine has been studied by ¹H n.m.r. resonance. The dependence of the chemical shifts of the porphyrin protons on the piperidine concentration indicates that only one ligand is added, and the alternation in the sign of the shifts of the meso- and β-CH protons suggests that delocalisation of the unpaired spin occurs mainly via a π-mechanism.

In order to obtain a quantitative interpretation of the results it is necessary to correct for simultaneous formation of a chloroform–piperidine complex. A separate study on this system gave K ca. 0·2 l. mole^–1 and a low-field complex shift of the chloroform proton of 2·08 p.p.m. The porphyrin shifts when combined with these results are consistent with a contact shift of the meso-protons of ca. 10³ Hz and an equilibrium constant for the 1 : 1 complex of ca. 10^–2 l. mole^–1.