Formation in solution, preparation, crystal structure and magnetic characterization of di-µ-hydroxo-bis[diaqua(2,2′-bipyrimidine)copper(II)] diperchlorate dihydrate

Abstract

A new binuclear complex of formula [Cu₂(bipym)₂(H₂O)₄(OH)₂][ClO₄]₂·2H₂O (bipym = 2,2′-bi-pyrimidine) has been synthesised and its crystal structure determined by X-ray crystallographic methods. It crystallizes in the triclinic space group P, with a= 7.021(1), b= 10.004(1), c= 10.721(1)Å, α= 100.83(1), β= 95.80(1), γ= 99.05(1)° and Z= 1. Least-squares refinement of 2888 reflections with I > 3σ(I) and 256 parameters gave a final R′= 0.0522. The structure consists of discrete centrosymmetric di-µ-hydroxo-copper(II) dimers with 2,2′-bipyrimidine as outer ligand, weakly co-ordinated and crystallization water molecules, and unco-ordinated perchlorate anions. The co-ordination geometry around each copper(II) ion is approximately elongated tetragonal octahedral with the two bridging hydroxo groups and two nitrogen atoms of bipym building the equatorial plane and two water molecules occupying the axial positions. The Cu(1)–OH–Cu(1a) bridging angle and the intramolecular Cu(1)⋯ Cu(1a) separation are 95.0(1)° and 2.870(1)Å, respectively. The temperature dependence of the magnetic susceptibility, studied in the range 300–4 K, shows a relatively strong intramolecular ferromagnetic coupling (+147 cm^–1 for the singlet–triplet energy gap). Further evidence for the triplet ground state is provided by the variable-temperature ESR spectra. The strength of the exchange interaction is discussed on the basis of the structural features and correlated with reported magneto-structural data on parent double hydroxide-bridged copper(II) complexes. The formation of hydroxo complexes of [Cu(bipym)(H₂O)]²⁺ has been investigated by potentiometry in aqueous solutions [equation (i); log K₁=–10.350(1)(25 °C, 0.1 mol dm^–3 NaNO₃)]. 2[Cu(bipym)(H₂O)₂]²⁺ [graphic omitted] [Cu₂(bipym)₂(OH)₂]²⁺+ 2H⁺+ 2H₂O (i) The value of this stability constant is compared to those reported for related Cu^IIL complexes where L are bidentate N-donor ligands.