Carboxylate and Alkyl Carbonate Coordination at the Hydrophobic Binding Site of Redox-Active Dicobalt Amine Thiophenolate Complexes

A series of new dicobalt complexes of the permethylated macrocyclic hexaamine dithiophenolate ligand H2LMe
have been prepared and investigated in the context of ligand binding and oxidation state changes. The octadentate
ligand is an effective dinucleating ligand that supports the formation of bioctahedral complexes with a central
N3Co(μ-SR)2(μ-X)CoN3 core structure, leaving a free bridging position X for the coordination of the substrates. The
acetato- and cinnamato-bridged complexes [(LMe)CoII2(μ-O2CMe)]+ (2) and [(LMe)CoII2(μ-O2CCHCHPh)]+ (5) were
prepared by reaction of the μ-Cl complex [(LMe)CoII2(μ-Cl)]+ (1) with the corresponding sodium carboxylates in
methanol. The electrochemical properties of these and of the methyl carbonate complex [(LMe)CoII2(μ-O2COMe)]+
(8) were also investigated. All complexes undergo two stepwise oxidations at ca. E11/2 = +0.22 and at E21/2 = ca.
+0.60 V vs SCE, affording the mixed-valent complexes [(LMe)CoIICoIII(μ-O2CR)]2+ (3, 6, 9) and the fully oxidized
CoIIICoIII forms [(LMe)CoIII2(μ-O2CR)]3+ (4, 7, 10), respectively. Compounds 3, 6, 9 and 4, 7, 10 refer to acetato-,
cinnamato-, and methylcarbonato species, respectively. The CoIICoIII compounds were prepared by comproportionation
of the respective CoII2 and CoIII2 compounds. The CoIIICoIII species were prepared by bromine oxidation of the
CoIICoII forms. The crystal structures of complexes 2·BPh4·MeCN, 3·(I3)2, 5·BPh4·2MeCN, 6·(ClO4)2·EtOH, 7·(ClO4)3·MeCN·(H2O)3, and 9·(ClO4)2·(MeOH)2·H2O were determined by single-crystal X-ray crystallography at 210
K. The oxidations occur without gross structural changes of the parent complexes. The CoIICoIII complexes are
composed of high-spin CoII (d7) and low-spin CoIII (d6) ions. The CoIIICoIII complexes are diamagnetic. The oxidation
reactions affect the binding mode of the substrates. In the CoII2 and CoIICoIII forms the carboxylates bridge the two
Co2+ ions in a symmetric μ-1,3 fashion with uniform C−O bond distances, whereas asymmetric bridging modes,
with one short CO and one long C−O distance, are adopted in the fully oxidized species. This is consistent with
the observed shifts in vibrational frequencies for νas(C−O) and νs(C−O) across the series.