Structural, Electrochemical, and Spectroscopic Investigation of Acetate Bridged Dinuclear Tetrakis-Schiff Base Macrocycles of Mn and Zn

Abstract

The synthesis of Mn2LAc+, Zn2LAc+, and H4L2+ is described, where L is a tetrakis-Schiff base macrocycle formed using 4-tert-butyl-2,6-diformylphenol and 2,2′-diamino-N-methyldiethylamine resulting in an N6O2 coordination environment. In Mn2LAc+ and Zn2LAc+, the two metal centers are bridged by an acetate ligand. [Mn2LAc](ClO4)·(DMF)0.5, [Mn2LAc](ClO4)·(ACN)0.5, and [Zn2LAc](PF6) crystallized in the space group P2(1)/c, with nearly identical unit-cell dimensions and geometric structures. Electrochemical analysis of Zn2LAc+, and H4L2+ by cyclic voltammetry (CV) revealed two irreversible anodic waves that were assigned to oxidations of the phenolate ligands. CVs of Mn2LAc+ displayed two chemically reversible anodic waves corresponding to Mn(II/III) oxidations, followed by irreversible oxidations of the phenolate ligands. Interfacial electron transfer rates for the single electron oxidations from Mn2(II)LAc+ to Mn(II)Mn(III)LAc2+ to Mn2(III)LAc+ determined from digital simulation of the CVs were 0.6 and 1.1 × 10(–3) cm s(–1), respectively. The sluggish interfacial electron transfer rates observed in electrochemical scans of Mn2LAc+ are consistent with broken symmetry density functional theory electronic structure calculations (B3LYP/6-311G(2df)/6-311G(d,p)) that predict large structural rearrangements of the Mn coordination environment upon oxidation to Mn(III) with associated Jahn–Teller distortions. Titration of Mn2LAc+, Zn2LAc+, and H4L2+ with NOPF6 in acetonitrile allowed for the isolation and spectroscopic examination of higher oxidations and were consistent with electrochemical assignments. The electrochemical and spectroscopic analysis of these complexes will aid in future studies involving electrocatalytic processes with related dinuclear macrocycles.