Preparation and characterization of asymmetric α-alkoxy dipyrrin ligands and their metal complexes

Abstract

Asymmetric α-substituted dipyrrins have been synthesized and characterized. The compounds were formed by a metal mediated reaction involving a single alkoxy group substituted into the α-position of an α,β-unsubstituted dipyrrin. An α-methoxy dipyrrin, 5-(4-cyanophenyl)-1-methoxydipyrrin (α-OMe-4-cydpm), was prepared from 5-(4-cyanophenyl)-4,6-dipyrromethane. Methoxy, ethoxy, and propoxy derivatives (α-OMe-4-mecdpm, α-OEt-4-mecdpm, α-OPr-4-mecdpm) of 5-(4-methoxycarbonylphenyl)-4,6-dipyrromethane have also been prepared. A homoleptic, bis(1-methoxy)dipyrrinato zinc(II) complex, [Zn(α-OMe-4-mecdpm)₂], has been synthesized, as has a heteroleptic cobalt(III) complex with one α-OMe-4-cydpm ligand and two unsubstituted 5-(4-cyanophenyl)dipyrrin (4-cydpm) ligands ([Co(α-OMe-4-cydpm)(4-cydpm)₂]). The rotational barrier of the meso-aryl substituent of [Zn(α-OMe-4-mecdpm)₂] was found to be 17.3 kcal mol⁻¹ by variable-temperature NMR spectroscopy. The compounds α-OMe-4-cydpm and [Zn(α-OMe-4-mecdpm)₂] have also been characterized by X-ray diffraction. The formation of the new dipyrrin derivatives is shown to be general and can be performed on dipyrrins with various meso-aryl substitutents, with a variety of alcohols, and can be promoted by several metal salts.