RESUMO
The self-assembly reaction of MnCl2 â 4H2 O, acenaphthenequinone dioxime (acndH2 ) and NEt3 has yielded an unprecedented, linear {MnIV2 MnIII } complex with an S=5 spin ground state and non-SMM behavior. The targeted replacement of the central MnIII ion with GdIII and DyIII ions has successfully increased the S and turned on the SMM dynamics without affecting the core structure and the nature of the magnetic exchange interactions.
RESUMO
One-pot reactions between the [Mn3O(O2CPh)6(py)x]+/0 triangular precursors and either CaBr2·xH2O or CaCl2·6H2O, in the presence of salicylhydroxamic acid (shaH2), have afforded the heterometallic complexes [MnIII4Ca2(O2CPh)4(shi)4(H2O)3(Me2CO)] (1) and (pyH)[MnII2MnIII4Ca2Cl2(O2CPh)7(shi)4(py)4] (2), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO3)2·4H2O/Ca(NO3)2·4H2O and Mn(O2CPh)2·2H2O/Ca(ClO4)2·4H2O "metal blends" and shaH2, in the presence of external base NEt3, led to the new complexes (NHEt3)2[MnIII4MnIV4Ca(OEt)2(shi)10(EtOH)2] (3) and (NHEt3)4[MnIII8Ca2(CO3)4(shi)8] (4), respectively. In all reported compounds, the anion of the tetradentate (N,O,O,O)-chelating/bridging ligand salicylhydroxime (shi3-), resulting from the in situ metal-ion-assisted amide-iminol tautomerism of shaH2, was found to bridge both Mn and Ca atoms. Complexes 1-4 exhibit a variety of different structures, metal stoichiometries, and Mn oxidation-state descriptions; 1 possesses an overall octahedral metal arrangement, 2 can be described as a Mn4Ca2 octahedron bound to an additional Mn2 unit, 3 consists of a Mn8 "ring" surrounding a CaII atom, and 4 adopts a rectangular cuboidal motif of eight Mn atoms accommodating two CaII atoms. Solid-state direct-current magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the Mn centers, leading to S = 0 spin ground-state values for all complexes. From a bioinorganic chemistry perspective, the reported compounds may demonstrate some relevance to both high-valent scheme (3) and lower-oxidation-level species (1, 2, and 4) of the catalytic cycle of the oxygen-evolving complex.
RESUMO
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 Mn(III) atoms at the square base and a Ca(II) atom occupying the apical site. Peripheral ligation about the core is provided by four η(1):η(1):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 Mn(III) 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.