Mechanochemical synthesis in copper(ii) halide/pyridine systems: single crystal X-ray diffraction and IR spectroscopic studies

Abstract

Whereas complexes of divalent metal halides (X = Cl, Br, I) with/from pyridine commonly crystallise as trans-[M(py)₄X₂]·2py, M on a site of 222 symmetry in space group Ccca, true for CuCl₂ and CuBr₂ in particular, the copper(II) iodide adduct is of the form [Cu(py)₄I]I·2py, Cu on a site of mm2 symmetry in space group Cmcm, and five-coordinate (square-pyramidal), the same cationic species also being found in 2[Cu(py)₄I](I₃)·[(py)₂Cu(μ-I)₂Cu(py)₂] (structurally defined). Bromide or N-thiocyanate may be substituted for the unbound iodide ion in the solvated salt, resulting in complexes which crystallize in space group Ccca, but with both anions and the metal atom disordered. In [Cu(py)₄(I₃)₂], a pair of long Cu⋯I contacts approach a square-planar Cu(py)₄ array. Assignments of the ν(CuN) and ν(CuX) (X = Br, I, SCN) bands in the far-IR spectra are made, the latter with the aid of analogous assignments for [Cu(py)₂X₂] (X = Cl, Br), which show a dependence of ν(CuX) on the Cu–X bond length that is very similar to that determined previously for copper(I) halide complexes. The structure of the adventitious complex [(trans-)(H₂O)(py)₄CuClCu(py)₄](I₃)₃·H₂O is also recorded, with six- and five-coordinate copper atoms; rational synthesis provides [{Cu(py)₄}₂(μ-Cl)](I₃)₃·H₂O with one water molecule less. In [{Cu(py)₄Cl}_(∞|∞)](I₃)·3py, square pyramidal [Cu(py)₄Cl]⁺ cations, assisted by Cl⋯Cu interactions, stack to give rise to infinite polymeric strings. Several of these compounds were prepared mechanochemically, illustrating the applicability of this method to syntheses involving redox reactions as well as to complex syntheses involving up to five components. The totality of results demonstrates that the [Cu^II(py)₄] entity can be stabilized in an unexpectedly diverse range of mononuclear and multinuclear complexes through the presence of lattice pyridine molecules, the bulky triiodide ion, or a combination of both.