Central imidazolidine ring hydrolysis of a binucleating amine phenol ligand during complex formation with manganese(III): synthesis, structure and electron transfer properties of mononuclear MnN 4O 2 complex

Abstract

The reaction of μ-bis(tetradentate) ligand H 3L ′, formed from imidazolidine ring insertion within the parent hexadentate Schiff base ligand of salicylaldehyde and triethylenetetramine, with manganese(II) salt in air spontaneously transforms the binucleating ligand, with respect to the removal of the phenol substituted imidazolidine ring, affording the mononuclear manganese(III) complex of the parent hexadentate ligand (H 2L). The mononuclear complex has MnN 4O 2 coordination sphere as established from a crystal structure determination. The imine nitrogen pairs are coordinated trans to each other in [Mn IIIL] +. The Mn–N(imine), Mn–N(amine), and Mn–O(phenol) distances are, respectively, 2.001 (6), 2.246 (8) and 1.949 (6) Å. The mononuclear complex has a magnetic moment corresponding to the high-spin 3d 4 configuration. In dimethylformamide solution, two quasireversible couples for manganese(IV)–manganese(III) and manganese(III)–manganese(II) are observed in cyclic voltammetry with E 1/2 values of 0.66 and 0.02 V versus SCE, respectively. One-electron nature of these two couples can be verified from current height and room-temperature solution coulometry data. The X-band EPR spectrum of the coulometrically oxidized and frozen methanol–toluene solution of [Mn IIIL] + consists of weak and strong resonances at g∼4 and g∼2, respectively, the latter resonance shows 55Mn hyperfine lines and forbidden lines are also resolved. The broad shoulder around 615 nm ( ε ∼ 200 l mol −1cm −1) may be assigned to a d-d transition for Mn(III) in pseudotetragonal environment.