Electronic Lability in a Dinuclear Cobalt–Bis(dioxolene) Complex

One-Step versus Two-Step Valence Tautomeric Transitions in Tetraoxolene-Bridged Dinuclear Cobalt Compounds

The syntheses of valence tautomeric compounds with multistep transitions using new redox-active ligands are the long-term goal of the field of bistable materials. The redox-active tetraoxolene ligand, 2,7-di-tert-butylpyrene-4,5,9,10-tetraone (pyrene^Q-Q), is now developed to synthesize a pair of dinuclear compounds {[CoL₂]₂(pyrene^Sq-Sq)}[Co(CO)₄]₂·xCH₂Cl₂·2C₆H₅CH₃ (1, x = 2, L = 1,10-phenanthroline, phen; 2, x = 1.5, L = 2,2'-bipyridine, bpy). Variable-temperature magnetic susceptibilities and single-crystal X-ray diffraction measurements indicate a partial one-step valence tautomeric transition for 1 and a rare two-step valence tautomeric transition for 2, respectively. DFT calculation results are consistent with the experimental data, revealing the correlation between thermodynamic parameters and the one-step/two-step valence tautomeric behaviors.

Quantum Chemical Study of Spin Transitions in the Bimetallic Fe/Co Complexes with the Bis(catecholate) Bridging Ligand

Abstract

The computational modeling of the spatial and electronic structures, energy characteristics, and magnetic properties of the bimetallic iron and cobalt complexes with 9,10-dimethyl-9,10-ethano-9,10-dihydro-2,3,6,7-tetrahydroxyanthracene and terminal tris(2-pyridylmethyl)amine bases is performed using the density functional theory method (DFT UTPSSh/6-311++G(d,p)). The chosen tetradentate redox ligand is shown to be a promising precursor for the production of magnetically active compounds. The calculations make it possible to establish a relationship between the relative energies of the electronic isomers of the complexes and the structures of the ancillary N-donor moieties. The coordination compounds prone to the manifestation of spin transitions accompanied by a change in the magnetic properties are revealed.

Valence tautomerism in a [2 × 2] Co4 grid complex containing a ditopic arylazo ligand

We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

Rational Design of Electronically Labile Dinuclear Fe and Co complexes with 1,10-Phenanthroline-5,6-Diimine: A DFT study

A series of coordination compounds of redox-active 1,10-phenanthroline-5,6-diimine with Co^II bis-diketonates and Fe^II dihydrobis(pyrazolyl)borates has been computationally designed by means of density functional theory (DFT UB3LYP*/6-311++G(d,p)) calculations of their electronic structure, energy characteristics, and magnetic properties. Four types of complexes differing by the nature and position of the terminal metal-centered fragments have been considered. The performed systematic calculations have revealed the systems capable of undergoing thermally initiated spin-state switching rearrangements, including those governed by the synchronized mechanisms of spin crossover and valence tautomerism. The predicted magnetic characteristics allow one to consider the dinuclear cobalt complexes and heterometallic Co/Fe compounds with 1,10-phenanthroline-5,6-diimine as building blocks for molecular and quantum electronics devices. © 2019 Wiley Periodicals, Inc.

Valence tautomerism and spin crossover in pyridinophane–cobalt–dioxolene complexes: an experimental and computational study

Variation of alkyl substituents on pyridinophane ancillary ligands governs temperature-dependent valence tautomeric or spin crossover equilibria in a family of cobalt–dioxolene complexes.

The structurally variable network of spin couplings and migrating paramagnetic centers in binuclear o-quinone CoII complexes with biradical acene linkers: a computational DFT study

A series of new magnetically-active coordination compounds comprising binuclear mixed-ligand complexes of cobalt bis-diketonates with acene linkers functionalized by two redox-active o-quinone moieties has been designed by means of density functional theory (DFT UB3LYP*/6-311++G(d,p)) calculations of their electronic structure, energy characteristics and magnetic properties. Two types of redox-active ligands include those with an acene linker bridging two o-benzoquinone fragments and the ligands containing an integrated π-conjugated system formed by annulation of o-quinone rings to the polycyclic core. The calculations reveal the dependence of spin density distribution in the compounds under study on the type of ligand. The considered binuclear CoII diketonate adducts manifest the capability of undergoing one- and two-step spin transitions induced by intramolecular electron transfers between the metal ions and the ligand system. An increase in the number of condensed rings of the acene linker promotes the stabilization of the biradicaloid state of the linker and enhances exchange interactions between all paramagnetic centers of the complexes giving rise to the formation of a flexible spin coupling network. The predicted unusual magnetic properties make the binuclear cobalt complexes with di-o-quinone ligand containing acene linker groups the promising building blocks for molecular electronic and spintronic devices.

Dinuclear adducts of di-o-iminoquinone ligands with CoII diketonates: computational insights into two-step valence tautomeric rearrangements

Density functional theory (DFT) computational modeling [B3LYP*/6-311++G(d,p)] of a series of potentially valence tautomeric (VT) dinuclear 2:1 adducts of Co^II bis-acetylacetonate, bis-trifluoroacetylacetonate and bis-hexafluoroacetylacetonate with redox-active tetradentate di-o-iminoquinone ligands has been performed. The significant energy preference of the low-spin electromeric forms of the complexes on the basis of Co^II bis-acetylacetonate points to a low probability of VT in these compounds. Electron-withdrawing CF₃-groups in the diketonate moiety of the mixed-ligand complexes promote narrowing of the energy gaps between the electronic states, which allows one-step VT rearrangements to be expected in the adducts of Co^II bis-trifluoromethylacetylacetonate. The most suitable energy parameters for the occurrence of two-step VT conversion (stability of the adduct with respect to dissociation into the components, small relative energies of electromeric forms and thermally achievable energy barriers to intramolecular electron transfers estimated as minimum energy crossing points) are found for the adduct of di-o-iminoquinone with Co^II bis-trifluoroacetylacetonates comprising a diphenylene linker.

New homodinuclear tris(3-alkylpyrazolyl)borate complexes of CoII and NiII with a tetraacetylethane dianion as a bridging ligand

Polynuclear complexes and coordination polymers of 3d metals have attracted significant interest evoked by a number of their unique properties. One of the most common approaches to the directed synthesis of coordination polymers is the linking of pre-prepared discrete coordination units by polydentate ligands. The formation of polynuclear complexes is usually a spontaneous process and precise prediction of the products of such reactions is virtually impossible in most cases. Tris(pyrazolyl)borates (Tp) act as tripodal `capping' ligands which form stable complexes with 3d metal ions. In such 1:1 compounds, three metal-ion coordination sites are occupied by N atoms from a Tp anion. This limits the number of remaining coordination sites, and thus the number of additional ligands which may coordinate, and opens an attractive approach for the directed design of desirable structures by exploiting ligands with appropriate composition and topology. In the present study, Tp anions with neopentyl [Tp<sup>Np</sup>, tris(3-neopentylpyrazolyl)borate] and cyclohexyl [Tp<sup>Cy</sup>, tris(3-cyclohexylpyrazolyl)borate] substituents were used as `capping' ligands and the dianion of tetraacetylethane (3,4-diacetylhexa-2,4-diene-2,5-diolate, tae^2-) was employed as a bridge. The dinuclear complexes (μ-3,4-diacetylhexa-2,4-diene-2,5-diolato-κ⁴O²,O³:O⁴,O⁵)bis{[tris(3-cyclohexyl-1H-pyrazol-1-yl-κN²)borato]cobalt(II)} acetonitrile disolvate, [Co₂(C₂₇H₄₀BN₆)₂(C₁₀H₁₂O₄)]·2CH₃CN, (I)·2CH₃CN, and (μ-3,4-diacetylhexa-2,4-diene-2,5-diolato-κ⁴O²,O³:O⁴,O⁵)bis{[tris(3-neopentyl-1H-pyrazol-1-yl-κN²)borato]nickel(II)}, [Ni₂(C₂₄H₄₀BN₆)₂(C₁₀H₁₂O₄)], (II), were synthesized by the reaction of the mononuclear complexes Tp^CyCoCl or Tp^NpNiCl with H₂tae (3,4-diacetylhexane-2,5-dione or tetraacetylethane) in the presence of NEt₃ as base. Compounds (I) and (II) were characterized by mass spectrometry, elemental analysis, and X-ray crystallography. They possess similar molecular structures, X-ray diffraction revealing them to be dinuclear in nature and composed of discrete Tp-M units in which two metal ions are linked by a tae^2- dianion. Each metal ion possesses a five-coordinate square-pyramidal environment. The interplanar angles between the acetylacetonate fragments are significantly smaller than the near-90° values commonly observed.