posted on 2015-03-02, 00:00authored byAlysha A. Alaimo, Daisuke Takahashi, Luís Cunha-Silva, George Christou, Theocharis C. Stamatatos
The one-pot reactions
between Mn(ClO4)2·6H2O, Ca(ClO4)2·4H2O, and the potentially tetradentate
(N,O,O,O) chelating/bridging ligand salicylhydroxime (shiH3), resulting from the in situ metal ion-assisted amide–iminol
tautomerism of salicylhydroxamic acid in the presence of various fluorescence
carboxylate groups and base NEt3, afford a family of structurally
similar {Mn4Ca} clusters with distorted square pyramidal
topology. The reported complexes (NHEt3)2[Mn4Ca(L1)4(shi)4] (1), (NHEt3)2[Mn4Ca(L2)4(shi)4] (2), (NHEt3)5[Mn4Ca(L2)4(shi)4(shiH2)2](ClO4) (3), and (NHEt3)2[Mn4Ca(L3)4(shi)4] (4) contain a similar [Mn4Ca(μ-NO)4]10+ core of four MnIII atoms at the
square base and a CaII atom occupying the apical site.
Peripheral ligation about the core is provided by four η1:η1:μ carboxylate groups of the anions
of 2-naphthoic acid (L1–), 9-anthracenecarboxylic
acid (L2–), and 1-pyrenecarboxylic acid (L3–). Solid-state direct current magnetic susceptibility
studies revealed the presence of predominant antiferromagnetic exchange
interactions between the 4 MnIII centers, which were primarily
quantified by using a simple 1-J fit model to give S = 0 spin ground states with low-lying excited states close
in energy to the ground state. Solution studies in solvent MeCN were
carried out on all complexes and confirmed their structural integrity.
Cyclic voltammetry studies showed a similar well-defined reversible
oxidation and an irreversible reduction for all complexes, thus establishing
their redox potency and electrochemical efficiency. Emission studies
in solution proved the optical activity of all compounds, with the
observed “blue” emission peaks attributed to the π-rich
chromophores of the organic fluorescence ligands. The combined results
demonstrate the ability of shiH3 and fluorescence carboxylates
to yield new heterometallic Mn/Ca clusters with (i) the same Mn/Ca
ratio as the oxygen-evolving complex of Photosystem II, (ii) structural
stability in solution, and (iii) a pronounced redox and optical activity.