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

The synthesis of Mn
2
LAc
+, Zn
2
LAc
+, and H
4
L
2+ 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 Mn
2
LAc
+ and Zn
2
LAc
+, 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 P21/c, with nearly identical unit-cell dimensions and geometric structures.
Electrochemical analysis of Zn
2
LAc
+, and H
4
L
2+ by cyclic
voltammetry (CV) revealed two irreversible anodic waves that were
assigned to oxidations of the phenolate ligands. CVs of Mn
2
LAc
+ displayed two chemically reversible anodic waves corresponding to
MnII/III oxidations, followed by irreversible oxidations
of the phenolate ligands. Interfacial electron transfer rates for
the single electron oxidations from Mn
2
II
LAc
+ to Mn
II
Mn
III
LAc
2+ to Mn
2
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 Mn
2
LAc
+ 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 MnIII with associated Jahn–Teller
distortions. Titration of Mn
2
LAc
+, Zn
2
LAc
+, and H
4
L
2+ 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.