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.

One-pot reactions between the [MnO(OCPh)(py)] triangular precursors and either CaBr·xHO or CaCl·6HO, in the presence of salicylhydroxamic acid (shaH), have afforded the heterometallic complexes [MnCa(OCPh)(shi)(HO)(MeCO)] (1) and (pyH)[MnMnCaCl(OCPh)(shi)(py)] (2), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO)·4HO/Ca(NO)·4HO and Mn(OCPh)·2HO/Ca(ClO)·4HO "metal blends" and shaH, in the presence of external base NEt, led to the new complexes (NHEt)[MnMnCa(OEt)(shi)(EtOH)] (3) and (NHEt)[MnCa(CO)(shi)] (4), respectively. In all reported compounds, the anion of the tetradentate (N,O,O,O)-chelating/bridging ligand salicylhydroxime (shi), resulting from the in situ metal-ion-assisted amide-iminol tautomerism of shaH, 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 MnCa octahedron bound to an additional Mn unit, 3 consists of a Mn "ring" surrounding a Ca atom, and 4 adopts a rectangular cuboidal motif of eight Mn atoms accommodating two Ca 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.

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A new {Dy5} cluster compound has been synthesized and structurally characterized from the initial use of the Schiff base ligand N-naphthalidene-2-amino-5-chlorophenol (nacpH2) in coordination chemistry. The 1:1 reaction between Dy(hpd)3∙2H2O and nacpH2, in a solvent mixture comprising CH2Cl2 and MeOH, afforded orange crystals of [Dy5(OH)2(hpd)3(nacp)5(MeOH)5] (1) in 70% yield, where hpd− is the anion of 3,5-heptadione. The {Dy5} complex can be described as two vertical {Dy3(μ3-OH)}8+ triangles sharing a common vertex; such a metal topology is unprecedented in 4f-metal cluster chemistry. Direct current (dc) magnetic susceptibility studies revealed the presence of some weak ferromagnetic exchange interactions between the five DyIII ions at low temperatures. Alternating current (ac) magnetic susceptibility measurements at zero applied dc field showed that complex 1∙3MeOH∙CH2Cl2 exhibits temperature- and frequency-dependent out-of-phase signals below ~20 K, characteristics of a single-molecule magnet (SMM). The resulting relaxation times were used to construct an Arrhenius-type plot and determine an effective energy barrier, Ueff, of 100 K for the magnetization reversal. The application of a small dc field of 200 Oe resulted in the surpassing of the quantum tunneling process and subsequently the increase of the Ueff to a value of 170 K. The reported results are part of a long-term program aiming at the preparation of structurally and magnetically interesting lanthanide complexes bearing various Schiff base chelating/bridging ligands.

The self-assembly reaction of MnCl ⋅4H O, acenaphthenequinone dioxime (acndH ) and NEt has yielded an unprecedented, linear {Mn Mn } complex with an S=5 spin ground state and non-SMM behavior. The targeted replacement of the central Mn ion with Gd and Dy 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.