A one-pot reduction route to bimetallic manganese 1,8-naphthyridine complexes

Abstract

Reaction of the dinucleating ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (^MeL) with the Mn^I and Mn^II precursors MnBr(CO)₅ and MnCl₂ resulted in the formation of the monometallic complexes [MnBr(CO)₃(^MeL)] (1) and [MnCl₂(^MeL)] (3). In both cases, formation of bimetallic manganese complexes could be achieved by reduction with KC₈, yielding the carbonyl-bridged complex [Mn₂(CO)₆(^MeL)] (2) and the helicate complex [Mn₂(^MeL)₂] (4), respectively. EPR results demonstrate that 4 represents a novel, weakly antiferromagnetically coupled homovalent dimer (J = −0.85 cm⁻¹). The two formally Mn⁰ ions are both high spin (S = 3/2) and exhibit a zero-field splitting of ≈1 cm⁻¹, suggesting reduction of the complex is substantially ligand centered, and may be better described as a Mn^II complex coupled to two open shell singlet ligands [Mn^II₂(^MeL²⁻)₂]. X-ray crystallography, UV-Vis spectroscopy and DFT analysis support this finding.