Syntheses, crystal structures and magnetic properties of copper(II) polynuclear and dinuclear compounds with 2,3-bis(2-pyridyl)pyrazine (dpp) and pseudohalide as ligands

The preparation, crystal structures and magnetic properties of four heteroleptic copper(II) complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) and azide, cyanate or thiocyanate as ligands are reported, [Cu(dpp)(N3)2]n (1), [Cu(dpp)(NCO)2]n (2), [Cu(dpp)(NCS)2]2 (3) and [Cu(H2O)(dpp)(NCS)2]2·2H2O (4). Compounds 1 and 2 are isomorphous, triclinic, space group P1̄, and consist of mononuclear building blocks featuring copper atoms with close to square planar coordination geometries. The mononuclear units are, however, associated into chains through weak axial Cu–N bonds formed by end-on asymmetrically bridging azido/cyanato groups and by pyridyl nitrogen atoms. Taking these contacts into account, copper may be described as elongated octahedral. Compound 3 is monoclinic, space group P21/c. The mononuclear building blocks are similar to those in 1 and 2, but in this case the association between these units is such as to form dinuclear molecules through end-to-end bridging thiocyanate with weak axial CuS bonds, yielding a square pyramidal environment of copper. Compound 4 is triclinic, space group P1̄, and features a mononuclear unit which contains an apically coordinated water molecule. This leads to a significantly weaker intermolecular Cu⋯S contact as compared to that found in 3, but an association into dinuclear units is structurally and magnetically evident. The shortest Cu⋯Cu separations occur across the azido, cyanato or thiocyanato bridges, respectively, which are coordinated equatorially to one unit and axially to the neighbouring unit; 3.568(1) (1), 3.547(1) (2), 5.432(1) (3) and 5.371(1) Å (4). Variable temperature magnetic susceptibility measurements on compounds 1, 2 and 4 reveal weak antiferromagnetic coupling across the azido, cyanato and thiocyanato bridges, respectively (J-values of −2.3(1) (1), −1.0(1) (2) and −1.0(1) (4) cm−1).