Anion–π, lone‐pair–π, π–π and hydrogen‐bonding interactions in a Cu^II complex of 2‐picolinate and protonated 4,4′‐bipyridine: crystal structure and theoretical studies

Abstract

A Cu^II complex of protonated 4,4′-bipyridine (Hbyp) and 2-picolinate (pic), [Cu₂(pic)₃(Hbyp)(H₂O)(ClO₄)₂], has been synthesised and characterised by single-crystal X-ray analysis. The structure consists of two copper atoms that have different environments, bridged by a carboxylate group. The equatorial plane is formed by the two bidentate picolinate groups in one Cu^II and one picolinate, one monodentate 4,4′-bipyridyl ligand and a water molecule in the other. Each copper atom is also weakly bonded to a perchlorate anion in an axial position. One of the coordinated perchlorate groups displays anion–π interaction with the coordinated pyridine ring. The noncoordinated carboxylate oxygen is involved in lone-pair (l.p.)–π interaction with the protonated pyridine ring. In addition there are π–π and H-bonding interactions in the structure. Bader's theory of “atoms in molecules” (AIM) is used to characterise the anion–π and l.p.–π interactions observed in the solid state. A high-level ab initio study (RI-MP2/aug-cc-pVTZ level of theory) has been performed to analyse the anion–π binding affinity of the pyridine ring when it is coordinated to a transition metal and also when the other pyridine ring of the 4,4′-bipyridine moiety is protonated. Theoretical investigations support the experimental findings of an intricate network of intermolecular interactions, which is characterised in the studied complex and also indicate that protonation as well as coordination to the transition metal have important roles in influencing the π-binding properties of the aromatic ring.