Two different (oxalato)(bipyridine)copper(II) complexes in one single crystal. Crystal structures and magnetic properties of [Cu2(bipy)2(H2O)2(C2O4)]X2·[Cu(bipy)(C2O4)](X = NO3–, BF4– or ClO4–)

Abstract

Three complexes of formula [Cu₂(bipy)₂(H₂O)₂(C₂O₄)]X₂·[Cu(bipy)(C₂O₄)](bipy = 2,2′-bipyridine; C₂O₄^2–= oxalate, X = NO₃^–1, BF₄^–2 or ClO₄^–3 have been synthesised and their crystal structures determined by single-crystal X-ray diffraction methods at room temperature. They are all isostructural and crystallize in the monoclinic system, space group C2/c, Z= 4, with a= 21.739(2), b= 10.458(1), c= 16.023(2)Å, β= 95.69(1)° for 1, a= 22.740(5), b= 10.508(1), c= 16.129(2)Å, β= 92.32(1)° for 2, and a= 22.819(3), b= 10.583(2), c= 16.389(2)Å, β= 91.52(2)° for 3. The crystal structures of 1 and 2 were determined from Patterson and Fourier analysis and refined by full-matrix least-squares techniques, whereas that of 3 was solved by direct methods. Final values of the reliability factors R(R′) were 0.029 (0.051) for 1, 0.033 (0.046) for 2 and 0.057 (0.059) for 3 with 3290, 3526 and 1972 observed reflections respectively. Their structures consist of cationic centrosymmetric dinuclear [Cu₂(bipy)₂(H₂O)₂(C₂O₄)]²⁺ units, neutral axiosymmetric mononuclear [Cu(bipy)(C₂O₄)] entities and either NO₃^–, BF₄^– or ClO₄^– as counter ion. Each copper atom of the dinuclear species is in a square-pyramidal environment with two oxalate oxygen and two bipyridine nitrogen atoms as a base and a water molecule at the apical position. The copper atom of the mononuclear complex is in a slightly tetrahedrally distorted square comprised of two bipyridine nitrogen and two oxalate oxygen atoms. In both complexes one or two more distant atoms of the counter ion completes a (5 + 1) or a (4 + 2) copper co-ordination, respectively. The mono- and di-nuclear entities form an alternating chain via weak interactions through counter ions and copper atoms. Variable-temperature (20–300 K) magnetic susceptibility measurements revealed a strong antiferromagnetic interaction within the dinuclear unit, the singlet–triplet energy gap being –386, –378 and –376 cm^–1 for 1, 2 and 3, respectively. The χ_MT versus T curve for all three complexes exhibits a plateau at T < 80 K which corresponds to the Curie law expected for the mononuclear complex. The magnitude of the exchange coupling in this series has been analysed in the framework of a simple orbital model.