Synthesis, Structure, and Characterization of a Phenolate-Bridged Dimanganese(II) Complex of a Dinucleating Phenol Ligand

The dinucleating phenol ligand LOH (LOH = 2, 6-bis((bis(2-(2-pyridyl)ethyl)amino)methyl)phenol (1)) is reacted with MnCIO₄)₂.6H₂O in the presence of excess acetate anion and a base (NEt₃) in methanol to produce the dinuclear Mnⁿ2 complex [Mn₂(LO)(OAc)₂]ClO₄ (5) which crystallizes in the monoclinic space group P2₁\n with unit cell dimensions a = 11.227 (3) k, b = 21.815 (8) Å c = 17.731 (7) Å α = 90.00 (3)°, β = 102.45 (3)°, and γ = 90.00 (3)°, with four molecules per unit cell. The structure shows that two acetate anions and a phenoxy oxygen bridge the manganese ions. The [Mn₂(LO)(μ-OAc)₂]⁺ cation has a noncrystallographic C₂ axis with the two Mn(II) ions in a nearly equivalent distorted octahedron coordination environment. Cyclic voltammetry of 5 in acetonitrile shows a reversible peak at 0.73 V and an irreversible peak at 1.28 V (vs Ag/AgCl). The two redox potentials assigned to the Mn^II₂/Mn^IIMn¹¹¹ and the Mn^IIMn^III/Mn^III2 redox processes are significantly higher than the potentials assigned to the same processes in the analogous but mixed-valent Mn^IIMn^III complexes showing that ligand 1 stabilizes the lower oxidation states. This is attributed to the six-membered chelate rings in 5 vs five-membered rings found in the mixed-valent complexes. Complex 5 is the first example of a structurally characterized stable discrete dinuclear Mn^II₂ complex in this series of analogous complexes. The EPR spectrum of the crystals of 5 at 77 K shows six peaks with no hyperfine structures whereas in DMF/methanol frozen glass it shows a set of six peaks centered near g = 2 with an 11-line hyperfine structure (J = 43 G) superimposed and observable on the two lower field peaks. The hyperfine structure is attributed to the isotropic coupling to the nuclei of the two equivalent Mn(II) ions (total nuclear spin = 5. A peak for the ΔM_s = 2 forbidden transition at half-field and two outer flanking peaks are also observed. A superimposed six-peak hyperfine structure observed at g = 2 (J = 80 G) is due to impurity caused by monomeric Mn(II) species.