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Open AccessArticle

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

1
Department of Chemistry, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy
2
Laboratoire de Physique et Chimie Théoriques, Université de Lorraine and CNRS, 1 Boulevard Arago, F-57078 Metz, France
*
Author to whom correspondence should be addressed.
Crystals 2020, 10(6), 477; https://doi.org/10.3390/cryst10060477
Received: 18 May 2020 / Revised: 27 May 2020 / Accepted: 30 May 2020 / Published: 4 June 2020
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. View Full-Text
Keywords: copper; inverted ligand field; ligand field theory; experimental charge density; quantum theory of atoms in molecules; 4,7-dichloroquinoline copper; inverted ligand field; ligand field theory; experimental charge density; quantum theory of atoms in molecules; 4,7-dichloroquinoline
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MDPI and ACS Style

Finocchio, G.; Rizzato, S.; Macetti, G.; Tusha, G.; Lo Presti, L. Unravelling the Chemistry of the [Cu(4,7-Dichloroquinoline)2Br2]2 Dimeric Complex through Structural Analysis: A Borderline Ligand Field Case. Crystals 2020, 10, 477.

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