Synthesis, structure, and properties of a pentanuclear cobalt(III) coordination cluster

A tetra Co(II/III) complex with an open cubane Co4O4 core and square-pyramidal Co(II) and octahedral Co(III) centres: bifunctional electrocatalytic activity towards water splitting at neutral pH

The reaction of 2,6-diformyl-4-methylphenol, 4-methoxybenzoylhydrazine and Co(OAc)₂·4H₂O in 1 : 2 : 2 mole ratio in methanol under aerobic conditions produced in 61% yield a tetranuclear complex having the molecular formula [Co^IICo^III(μ-OAc)(μ₃-OH)(μ-L)]₂ where OAc^- and L^3- represent acetate and N',N''-(5-methyl-2-oxido-1,3-phenylene)bis(methan-1-yl-1-ylidene)bis(4-methoxybenzoylhydrazonate), respectively. The elemental analysis and the mass spectrometric data confirmed the molecular formula of the complex. It is electrically non-conducting and paramagnetic. The complex crystallized as acetonitrile solvate. The X-ray structure shows that each Co(II) centre has a distorted square-pyramidal NO₄ coordination sphere, while each Co(III) centre is in a distorted octahedral NO₅ environment. The four metal atoms and the four bridging O-atoms form an open cubane type Co₄O₄ motif. In the crystal lattice, self-assembly of the solvated complex via intermolecular O-H⋯O interaction leads to a two-dimensional network structure. The infrared and electronic spectroscopic features of the complex are consistent with its molecular structure. Cryomagnetic measurements together with theoretical calculations suggest the presence of easy-axis anisotropy for the square-pyramidal Co(II) centres. The complex is redox-active and displays metal centred oxidation and reduction responses on the anodic and cathodic sides, respectively, of the Ag/AgCl electrode. Bifunctional heterogeneous electrocatalytic activity of the complex towards O₂ and H₂ evolution reactions (OER and HER) in neutral aqueous medium has been explored in detail.

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

The reaction of organic molecules mediated by a metal center (template synthesis) can result in a final connectivity that may differ from the one obtained in the absence of the metal. The condensation of carbonyl fragments with primary amines form C=N iminic bonds, the so-called Schiff bases, which can act as ligands for the templating metal center by means of the lone pair on the nitrogen atom. This review focuses on the template methods for the reaction between a carbonyl compound (mainly salicylaldehyde) and a primary aliphatic diamine able to prevent the double condensation on both amine groups and obtain tridentate N2O ligands. These adducts, still having one free amino group, can further react, yielding tetradentate salen-type Schiff base ligands. A screening over the transition metals able to show such a template effect will be presented, with particular attention to copper(II), together with their peculiar reactivity and the available crystal structure of the metal complexes and related coordination geometries.

Conductive Self-Assembled Monolayers of Paramagnetic {CoII Co 4 III } and { Co 4 II Co 2 III } Coordination Clusters on Gold Surfaces

Two polynuclear cobalt(II,III) complexes, [Co₅(N₃)₄(N-n-bda)₄(bza·SMe)₂] (1) and [Co₆(N₃)₄(N-n-bda)₂(bza·SMe)₅(MeOH)₄]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H₂bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C₂-symmetric {Co^IICo4III} core of two isosceles Co₃ triangles with perpendicularly oriented planes, sharing a central, high-spin Co^II ion residing in a distorted tetrahedral coordination environment. This central Co^II ion is connected to four outer, octahedrally coordinated low-spin Co^III ions via oxo bridges. Compound 2 comprises a semi-circular {Co4IICo2III} motif of four non-interacting high-spin Co^II and two low-spin Co^III centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {Cu^IILn^III} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co₅}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral Co^II centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co₁₀(N₃)₂(N-n-bda)₆(bza·SMe)₆] (3), containing 10 virtually non-interacting high-spin Co^II centers.

Mononuclear Ru(II) Complexes of an Arene and Asymmetrically Substituted 2,2'-Bipyridine Ligands: Photophysics, Computation, and NLO Properties

By using monosubstituted 2,2'-bipyridine asymmetric ancillary ligands with different electron donor moieties and an arene ligand (p-cymene), we successfully designed and synthesized six Ru(II) compounds (RuBPY1-6) that belong to a piano-stool-type system. The NLO properties of the synthesized complexes have been studied in both solution and the solid state. The electronic spectra of these compounds show a broad feature with two absorption bands in the visible window (350-650 nm). RuBPY1-6 complexes exhibit NIR emission spectra in the solution state (at >720 nm), the maxima of which are bathochromically shifted in comparison to those of the concerned ligands. Interestingly, compounds RuBPY1-6 show NIR emission in their solid state too. Title compounds RuBPY1-6 have lifetimes in the range of 0.2 to 0.9 ns. An important feature of this work is the π-association of the p-cymene ligand to Ru(II) in the synthesized complexes; the π complex is formed by breaking the symmetry of p-cymene, found in the starting precursor (Ru₂ dimer). This has been established by NMR spectral studies along with DFT calculations on the ¹H NMR spectra. We could derive the molecular structure of the cationic part of this system by density functional theory (DFT), associated with ¹H NMR spectral studies. The minimum energy structures for RuBPY1 and RuBPY2 have been optimized at DFT/B3LYP along with the LANL2DZ basis set for ruthenium atoms. These optimized structures are further considered to calculate the excited state properties using the TDDFT method. The electrochemical studies of the complexes, investigated in acetonitrile solution, show that this system is associated with a well-defined Ru(III)/Ru(II) reversible couple, rarely observed for a Ru(II) piano-stool-type compound, along with a feature of irreversible ligand oxidation. The absorption cross-section values, obtained from the two-photon absorption studies of title compounds RuBPY1-6, are worth reporting and lie in the range of 3-28 GM (in the femtosecond case).

Trapping of a Pseudotetrahedral CoIIO4 Core in Mixed-Valence Mixed-Geometry [Co5] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism

A systematic one-step one-pot multicomponent reaction of Co(ClO₄)₂·6H₂O, H₃L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO₂Na; R = CH₃, C₂H₅) resulted in the two structurally novel coordination aggregates [Co^IICo^III₄L₂(μ_1,3-O₂CCH₃)₂(μ-OH)₂](ClO₄)₄·4H₂O (1) and [Co^IICo^III₄L₂(μ_1,3-O₂CC₂H₅)₂(μ-OH)(μ-OMe)](ClO₄)₄·5H₂O (2). At room temperature, reactions of H₃L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {Co^II₂} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(μ_1,3-O₂CCH₃)(μ-OH/OMe)Co₂}⁺ fragments were utilized to trap a central Co^II ion during the formation of [Co₅] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo- T _d Co^II ion present. The structural distortion leads to an easy-axis magnetic anisotropy ( D = -31.31 cm^-1 for 1 and -21.88 cm^-1 for 2) and a small but non-negligible transverse component ( E/ D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O-Co-O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co₂(μ-L)} hosts controls the distortion of the central {CoO₄} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.