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

Giada Finocchio,Silvia Rizzato,Leonardo Lo Presti,Giovanni Macetti,Gers Tusha

doi:10.3390/cryst10060477

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.

Highlights

Because of its versatility, Cu2+ has been defined as a chameleon in coordination chemistry [1].The stereochemistry of copper complexes is frequently dominated by a subtle interplay of Jahn–Teller, dynamic and steric effects [2,3] that cause it to depart from that expected based on a simple crystal field framework [4]
In the two days, a number of yellow, tiny acicular crystals appeared in the bulk liquor. Their quality was generally low; preliminary single-crystal X-ray diffraction analysis showed that they consisted of a copper-containing P21 /c monoclinic phase, which will be described in detail in a forthcoming study on the reactivity of such complexes
CDCQB crystal is made by centrosymmetric neutral building blocks, each formed by two inversion-related units.by

Summary

Introduction

Cu2+ has been defined as a chameleon in coordination chemistry [1].The stereochemistry of copper complexes is frequently dominated by a subtle interplay of Jahn–Teller, dynamic and steric effects [2,3] that cause it to depart from that expected based on a simple crystal field framework [4]. The discovery of an inverted ligand field in Cu-based coordination compounds [5,6,7], prototyped by the square planar anion Cu(CF3 )4 − , deepened the general understanding of coordinative bonding [8] and provided new ways to exploit the reactivity of unusual chemical states of both the metal and the ligands. In a recent review [8], Hoffmann and co-workers pointed out that late transition elements like Cu and Zn lie on the borderline of what they define as “transition metal thinking” and “main group thinking”. Crystals 2020, 10, x FOR PEER REVIEW and electronic properties of the complex. This kind of analysis definitely suggests the existence of a

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