Unravelling the Chemistry of the [Cu(4,7-Dichloroquinoline)2Br2]2 Dimeric Complex through Structural Analysis: A Borderline Ligand Field Case

Abstract

Large dark prismatic crystals (P 1 ¯ ) consisting of closely packed centrosymmetric [Cu(4,7-dichloroquinoline)2]2Br4 binuclear units are formed when 4,7-dichloroquinoline (DCQ, C9H5NCl2) binds copper(II). Cu2+ adopts a strongly distorted square pyramidal coordination geometry, perturbed by electrostatic interactions with two axial μ–Br ligands acting as highly asymmetric bridges. It is shown that, as electronic states of ligands are higher in energy than the metal ones, antibonding orbitals bear significant ligand-like character and electronic charge is partially transferred from inner-sphere coordinated halogen atoms to copper. Overall, the title compound sits on the Hoffman’s border between main group and transition chemistry, with non-negligible contributions of the ligands to the frontier orbitals. The relative energy placement of metal and ligand states determines an internal redox process, where the metal is slightly reduced at the expense of partial oxidation of the bromide ligands. In fact, the crystal structure is partially disordered due to the substitution of some penta-coordinated Cu(II) centers with tetra-coordinated Cu(I) ions. The geometry of the complex is rationalized in terms of electrostatic-driven distortions from an ideal octahedral prototype. Implications on the reactivity of Cu(II)–quinoline complexes are discussed.

Experimental Charge Density Analysis and Electrostatic Properties of Crystalline 1,3-Bis(Dimethylamino)Squaraine and Its Dihydrate from Low Temperature (T = 18 and 20 K) XRD Data

“…When theoretical EDDs are compared to experimental results, it is often said, correctly, that the former are mostly affected by the choice of quantum mechanical method, the latter by quality of the sample, treatment of the data, and choice of multipolar model (to cite only a few sources of error). In all our EDD studies so far (starting with the one of crystalline citrinin at 19 K) [44], we have found that reliable modelling of H atoms requires anisotropic displacement parameters (ADPs), and poles up to the quadrupole level [45,46].…”

Molecular Dynamics Simulation of Molecular Crystals under Anisotropic Compression: Bulk and Directional Effects in Anthracene and Paracetamol

“…X-ray crystallography has given for half a century a fundamental contribution to the quantitative understanding of molecular recognition and crystal packing. For all its merits, it does however provide only a space–time average picture, a limitation that obscures entropic and vibrational effects on cohesive energies. − Dynamic information from standard X-ray diffraction experiments is confined to atomic displacement parameters (ADPs), whose interpretation is not always immediate in terms of fundamental physics. , Accounting for dynamic effects is obviously desirable in principle and also for cutting-edge applications like, for example, energy transduction and shape-changing materials . Moreover, thermal expansion coefficient and isothermal compressibility rely on a dynamic interplay among intermolecular interaction forces, often comprehensively dubbed noncovalent interactions (NCIs).…”

Anharmonic Thermal Motion Modelling in the Experimental XRD Charge Density Determination of 1-Methyluracil at T = 23 K

“…As for the anharmonicity of thermal motion, the relevance of its inclusion in ADP models when analyzing EDDs in crystals is still an open question, the uncertainty being partly related to the fact that very asphericity in atomic charge densities can be due to genuine anharmonic thermal motion and/or to chemical bonding [ 19 ], as well as to static or dynamic disorder [ 20 , 21 ]. Long ago, at an international symposium on the accuracy in structure factor measurement, Werner F. Kuhs presented [ 22 ] a review on the properties of the most widely used formalisms to describe anharmonicity in crystallographic structure analysis and later published a paper [ 23 ] on both the theoretical and experimental aspects of generalized ADPs.…”