Multiple Coordination Geometries Research Articles

ChemInform Abstract: Unique Colours of 3d‐Transition‐Metal‐Substituted Lyonsite Molybdates and Their Derivatives: The Role of Multiple Coordination Geometries and Metal‐to‐Metal Charge Transfer.

AbstractThe compounds Li3Al1‐xMIIIx (MoO4)3 (0 < x ≤ 1.0; MIII: Cr, Fe) and Li3‐xAl1‐xMII2x (MoO4)3 (0 < x ≤ 1.0; MII: Co, Ni, Cu) are synthesized by solid state reaction of stoichiometric mixtures of the constituent binaries at around 700 °C in air, and characterized by powder XRD, and UV/VIS diffuse reflectance spectroscopy.

Unique Colours of 3d‐Transition‐Metal‐Substituted Lyonsite Molybdates and Their Derivatives: The Role of Multiple Coordination Geometries and Metal‐to‐Metal Charge Transfer

We describe the synthesis, crystal structures, and optical absorption spectra/colours of 3d transition‐metal‐substituted lyonsite‐type oxides Li3Al1–xMIIIx(MoO4)3 (0 < x ≤ 1.0; MIII = Cr, Fe) and Li3–xAl1–xMII2x(MoO4)3 (0 < x ≤ 1.0; MII = Co, Ni, Cu). The oxides were readily synthesized by the solid‐state reaction of stoichiometric mixtures of the constituent binaries at around 700 °C in air. The crystal structures determined by Rietveld refinement of PXRD data revealed that in the smaller trivalent‐metal‐substituted lyonsite oxides, the MIII ions occupy the octahedral (8d, 4c) sites and the lithium ions occur exclusively at the trigonal‐prismatic (4c) site in the orthorhombic (Pnma) structure. On the other hand, larger divalent‐metal (CoII/CuII)‐substituted derivatives show occupancy of the CoII/CuII ions at both the octahedral and trigonal‐prismatic sites. We have investigated the colours and optical absorption spectra of Li3Al1–xMIIIx(MoO4)3 (MIII = Cr, Fe) and Li3–xAl1–xMII2x(MoO4)3 (MII = Co, Ni, Cu) and interpreted the results in terms of the average crystal field strengths experienced by MIII/MII ions in multiple coordination geometries. We have also identified the role of metal‐to‐metal charge transfer (MMCT) from the partially filled transition metal 3d orbitals to the empty Mo 4d orbitals in the resulting colours of these oxides.

Identification of different coordination geometries by XAFS in copper(II) complexes with trimesic acid

X-ray absorption spectroscopy (XAS) is very useful in revealing the information about geometric and electronic structure of a transition-metal absorber and thus commonly used for determination of metal-ligand coordination. But XAFS analysis becomes difficult if differently coordinated metal centers are present in a system. In the present investigation, existence of distinct coordination geometries around metal centres have been studied by XAFS in a series of trimesic acid Cu(II) complexes. The complexes studied are: Cu3(tma)2(im)6 8H2O (1), Cu3(tma)2(mim)6 17H2O (2), Cu3(tma)2(tmen)3 8.5H2O (3), Cu3(tma) (pmd)3 6H2O (ClO4)3 (4) and Cu3(tma)2 3H2O (5). These complexes have not only Cu metal centres with different coordination but in complexes 1–3, there are multiple coordination geometries present around Cu centres. Using XANES spectra, different coordination geometries present in these complexes have been identified. The variation observed in the pre-edge features and edge features have been correlated with the distortion of the specific coordination environment around Cu centres in the complexes. XANES spectra have been calculated for the distinct metal centres present in the complexes by employing ab-initio calculations. These individual spectra have been used to resolve the spectral contribution of the Cu centres to the particular XANES features exhibited by the experimental spectra of the multinuclear complexes. Also, the variation in the 4p density of states have been calculated for the different Cu centres and then correlated with the features originated from corresponding coordination of Cu. Thus, these spectral features have been successfully utilized to detect the presence of the discrete metal centres in a system. The inferences about the coordination geometry have been supported by EXAFS analysis which has been used to determine the structural parameters for these complexes.

Structural characterization and photophysical properties of a luminescent trinuclear zinc(II) complex exhibiting multiple coordination geometries

A luminescent trinuclear zinc(II) complex, tris(μ-2,3-toluenedithiolato)-1:2κ2S1;1:2κ2S1′, 2:3κ2S2;3κS2′, 1:3κ2S3;3κS3′-bis(2,2′-bypyridine)-1κ2N,N′,2κ2N,N′-trizinc, containing 3,4-dimercaptotoluene (tdt) and 2,2′-bipyridine (bpy) has been synthesized and found to have the trinuclear formulation Zn3(tdt)3(bpy)2 (I). The complex crystallizes in the monoclinic space group P21/n with a = 11.7247(12), b = 20.134(2), c = 17.7376(18) A and β = 103.743(2)∘. The complex has two heteroleptic zinc(II) centers exhibiting similar distorted trigonal bipyramidal geometries with the third homoleptic zinc(II) center displaying a flattened tetrahedral geometry. Present in the solid state are both intra- and intermolecular π–π interactions. The complex is emissive at 77 K in the solid state with an emission maximum at 574± 2 nm with a luminescence lifetime of 7.4± 0.4 ns. The unstructured emission is assigned as a metal-mediated ligand-to-ligand π–π* charge transfer transition.

Multiple Coordination Geometries Supported by Methylene-Linked Guanidines

The methylene-linked bicyclic guanidine based on the 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) framework provides a versatile platform able to support trigonal-planar, tetrahedral, and square-planar metal centers.

Docking studies of matrix metalloproteinase inhibitors: zinc parameter optimization to improve the binding free energy prediction

Docking of metalloproteinase inhibitors remains a challenge due to the zinc multiple coordination geometries and the lack of appropriate force field parameters to model the metal/ligand interactions. In this study, we explore the docking accuracy and scoring reliability for the docking of matrix metalloproteinase (MMP) inhibitors using AutoDock 3.0. Potential problems associated with zinc ion were investigated by docking 16 matrix metalloproteinase ligands to their crystal structures. A good coordination between the zinc binding group (ZBG) and the zinc was shown to be a prerequisite for the ligand to fit the binding site. A simplex optimization of zinc parameters, including zinc radius, well depth, and zinc charges, was performed utilizing the 14 MMP complexes with good docking. The use of optimized zinc parameters (zinc radius: 0.87 Å; well depth: 0.35 kcal/mol; and zinc charges: +0.95 e) shows improvement in both docking accuracy at the zinc binding site and the prediction of binding free energies. Although further improvement in the docking procedure, particularly the scoring function is needed, optimization of zinc parameters provides an efficient way to improve the performance of AutoDock as a drug discovery tool.