Structural Diversities in Heterometallic Mn–Ca Cluster Chemistry from the Use of Salicylhydroxamic Acid: {MnIII4Ca2}, {MnII/III6Ca2}, {MnIII/IV8Ca}, and {MnIII8Ca2} Complexes with Relevance to Both High- and Low-Valent States of the Oxygen-Evolving Complex

Constructing "Closed" and "Open" {Mn8} Clusters

Use of the 1,3,5-tri(2-hydroxyethyl)-1,3,5-triazacyclohexane ligand, LH₃, in manganese chemistry affords access to two structurally related {Mn₈} clusters: a "closed" {Mn^III ₆Mn^II ₂} puckered square wheel of formula [Mn₈L₂(LH)O₃(OH)₂(MeO)₂Br(imH)(H₂O)₃](Br)₃ (1; imH = imidazole) and an "open" {Mn^III ₈} rod of formula [Mn^ΙΙΙ ₈L₂O₄(aibH)₂(aib)₂(MeO)₆(MeOH)₂](NO₃)₂ (2, aibH = 2-amino-isobutyric acid). In each case the triaza ligands, L/LH, direct the formation of {Mn₃} triangles with their N atoms preferentially bonding to the Jahn-Teller axes of the Mn^III ions. Subsequent self-assembly is dependent on the anion of the Mn salt and the identity of the organic coligand employed-the terminally bonded imidazole and the chelating/bridging amino acid. The {Mn₃} triangles fold up on themselves in 1, forming a wheel. However, the syn, syn-bridging carboxylates in 2 prevent this from happening, instead directing the formation of a linear rod. Magnetic susceptibility and magnetization measurements reveal competing ferro- and antiferromagnetic interactions in both complexes, the exchange being somewhat weaker in 1 due to the presence of Mn^II ions.

Synthesis, Structural and Physicochemical Characterization of a Titanium(IV) Compound with the Hydroxamate Ligand N,2-Dihydroxybenzamide

The siderophore organic ligand N,2-dihydroxybenzamide (H2dihybe) incorporates the hydroxamate group, in addition to the phenoxy group in the ortho-position and reveals a very rich coordination chemistry with potential applications in medicine, materials, and physical sciences. The reaction of H2dihybe with TiCl4 in methyl alcohol and KOH yielded the tetranuclear titanium oxo-cluster (TOC) [TiIV4(μ-O)2(HOCH3)4(μ-Hdihybe)4(Hdihybe)4]Cl4∙10H2O∙12CH3OH (1). The titanium compound was characterized by single-crystal X-ray structure analysis, ESI-MS, 13C, and 1H NMR spectroscopy, solid-state and solution UV-Vis, IR vibrational, and luminescence spectroscopies and molecular orbital calculations. The inorganic core Ti4(μ-O)2 of 1 constitutes a rare structural motif for discrete TiIV4 oxo-clusters. High-resolution ESI-MS studies of 1 in methyl alcohol revealed the presence of isotopic distribution patterns which can be attributed to the tetranuclear clusters containing the inorganic core {Ti4(μ-O)2}. Solid-state IR spectroscopy of 1 showed the presence of an intense band at ~800 cm-1 which is absent in the spectrum of the H2dihybe and was attributed to the high-energy ν(Ti2-μ-O) stretching mode. The ν(C=O) in 1 is red-shifted by ~10 cm-1, while the ν(N-O) is blue-shifted by ~20 cm-1 in comparison to H2dihybe. Density Functional Theory (DFT) calculations reveal that in the experimental and theoretically predicted IR absorbance spectra of the ligand and Ti-complex, the main bands observed in the experimental spectra are also present in the calculated spectra supporting the proposed structural model. 1H and 13C NMR solution (CD3OD) studies of 1 reveal that it retains its integrity in CD3OD. The observed NMR changes upon addition of base to a CD3OD solution of 1, are due to an acid-base equilibrium and not a change in the TiIV coordination environment while the decrease in the complex's lability is due to the improved electron-donating properties which arise from the ligand deprotonation. Luminescence spectroscopic studies of 1 in solution reveal a dual narrow luminescence at different excitation wavelengths. The TOC 1 exhibits a band-gap of 1.98 eV which renders it a promising candidate for photocatalytic investigations.

Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown-3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form

We report on the isolation of a new family of μ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada^2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{Mn^II(tpada)}₃(M)(H₂O)_n]²⁺ (Mn₃M) (M = Mn²⁺ (n = 0), Ca²⁺ (n = 1), or Sr²⁺ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada^2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn²⁺ or an alkaline earth metal, Ca²⁺ or Sr²⁺. Mn₃Ca and {Mn₃Sr}₂ join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four -[Sr-O]- repeating unit was also isolated by self-assembly of the tpada^2- ligand with Sr²⁺. This complex, namely, [Sr(tpada)(OH₂)]₄ (Sr₄), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr²⁺. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, Mn^II₃M, as well as in the three-electrons oxidized state, Mn^III₃M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

Pentanuclear clusters resembling the cubane-dangler connectivity in the native oxygen-evolving center of photosystem II

A series of pentametallic "cubane-plus-dangler" complexes have been target synthesized. Among them, the [Fe3Ni2] aggregate strongly resembled the native oxygen-evolving center by mimicking the "cubane-plus-dangler" skeleton, the aqua binding site, and the connectivity between the pendent ion and the parent cubane. Our synthetic strategy that uses tri-substituted methanol as the "cubane-generator" and carboxylate as the pendant ligand provides a feasible approach for accessing model compounds of biological catalyst systems.

Synthesis and crystal structure of two manganese-based 12-metallacrown-4 complexes: Na2(3-chlorobenzoate)2[12-MCMn(III)N(shi)-4](DMF)6 and MnNa(3-chlorobenzoate)3[12-MCMn(III)N(shi)-4](DMF)(H2O)4·4DMF·0.72H2O

The metallacrown (MC) complexes Na₂(3-chloro­benzoate)₂[12-MC_{Mn(III)N(shi)}-4](DMF)₆, 1, and MnNa(3-chloro­benzoate)₃[12-MC_{Mn(III)N(shi)}-4](DMF)(H₂O)₄·4DMF·0.72H₂O, 2, where shi³⁻ is salicyl­hydroximate and DMF is N,N-di­methyl­formamide, both have an overall square shape due to the presence of four ring Mn^III ions and four shi³⁻ ligands. The two MC complexes bind different cations in the central cavity of the mol­ecule: two Na⁺ ions in 1 and one Mn^II ion and one Na⁺ ion in 2.

Similar synthetic schemes yield two different metallacrown (MC) complexes: bis­(μ-3-chloro­benzoato)hexa­kis­(di­methyl­formamide)­tetra­kis­(μ₄-N,2-dioxido­benzene-1-carboximidato)tetra­manganese(III)disodium(I), [Mn₄Na₂(C₇H₄ClO₂)₂(C₇H₄NO₃)₄(C₃H₇NO)₆] or Na₂(3-chloro­benzoate)₂[12-MC_{Mn(III)N(shi)}-4](DMF)₆, 1, and tetra-μ-aqua-tris­(μ-3-chloro­benzoato)(di­methyl­formamide)­tetra­kis­(μ₄-N,2-dioxido­benzene-1-carboximidato)penta­manganese(III)sodi­um(I) di­methyl­formamide tetra­solvate 0.72-hydrate, [Mn₅Na(C₇H₄ClO₂)₃(C₇H₄NO₃)₄(C₃H₇NO)(H₂O)₄]·4C₃H₇NO·0.718H₂O or MnNa(3-chloro­benzo­ate)₃[12-MC_{Mn(III)N(shi)}-4](DMF)(H₂O)₄·4DMF·0.72H₂O, 2, where shi³⁻ is salicyl­hydrox­imate and DMF is N,N-di­methyl­formamide. Both complexes have the same framework consisting of four Mn^III ions in the MC ring and four shi³⁻ ligands, resulting in an overall square-shaped mol­ecule. The Mn^III ions are either five- or six-coordinate with elongated bond lengths in the apical or axial direction, respectively. The structure of 1 is nearly planar, and the MC binds two Na⁺ ions on opposite faces of the MC central cavity. The 3-chloro­benzoate anions also bind on opposite faces of the MC and form bridges between the central Na⁺ ions and the ring Mn^III ions. For 1 the metallacrown mol­ecule, except for the central Na⁺ ion, exhibits whole mol­ecule disorder over two sets of sites. Both moieties are centrosymmetric and are related to each other by a pseudo-mirror operation with opposite sense of rotation around the Na⋯Na axis. The occupancy ratio of the main disorder of the metallacrown mol­ecules and 3-chloro­benzoate anions refined to 0.9276 (9):0.0724 (9). The structure of 2 is slightly domed, and the MC binds both an Mn^II ion and an Na⁺ ion in the MC central cavity. The Mn^II ion is located on the convex side of the MC, while the Na⁺ ion binds to the concave side. Complex 2 represents the first instance of a [12-MC_{Mn(III)N(shi)}-4] mol­ecule binding both 3d transition metal and alkali metal ions in the central cavity. In addition, three 3-chloro­benzoate anions bind on the convex side of the MC and connect the Mn^II ion to three of the ring Mn^III ions.

The first amino acid bound manganese–calcium clusters: a {[MnIII3Ca]2} methylalanine complex, and a [MnIII6Ca] trigonal prism

The use of methylalanine in manganese/calcium chemistry has led to the synthesis and characterization of the first manganese/calcium amino acid containing polynuclear clusters.

Wheel-like 6 luminescent lanthanide complexes covering the visible and near-infrared domains

Luminescence properties of wheel-like lanthanide complexes.

Synthesis of a new acetate bridged Cu(ii) building block generated 1D polymer and studies on structural, magnetic, antibacterial and anticancer properties

Novel 1D polymer of formula [Cu₃(Py)₆(μ-CH₃COO)₄(I₃)₂]_n having asymmetric unit {[Cu_0.75(CH₃COO)₁(Py)_1.5]·(I₃)_0.5}_n and S = 1/2 shows bacterial growth inhibition and ROS mediated chemotherapy.

Synthesis, Characterization and Magnetic Studies of a Tetranuclear Manganese(II/IV) Compound Incorporating an Amino-Alcohol Derived Schiff Base

A new tetranuclear mixed-valence manganese(II/IV) compound [MnIIMnIV3(μ-Cl)3(µ3-O)(L)3] (1) (where H3L = (3E)-3-((Z)-4-hydroxy-4-phenylbut-3-en-2-ylideneamino)propane-1,2-diol) has been synthesized and characterized by different physicochemical methods. Single crystal X-ray diffraction analysis reveals that 1 is a tetrahedral cluster consisting of a Mn4Cl3O4 core in which the only Mn(II) ion is joined through three μ2-O bridges to an equilateral triangle of Mn(IV) ions, which are connected by a μ3-O and three μ2-Cl bridges. The redox behavior of 1 was studied by cyclic voltammetry. Variable temperature magnetic susceptibility measurements of 1 revealed predominant antiferromagnetic coupling inside the Mn4Cl3O4 cluster.