SYNTHESIS AND CRYSTAL-STRUCTURES OF 2 DINUCLEAR COPPER(II) COMPLEXES WITH 2,3-BIS(2-PYRIDYL)PYRAZINE AS BRIDGING LIGAND

Two dinuclear copper(II) complexes with 2,3-bis(2-pyridyl)pyrazine (dp p) as bridging ligand, Cu-2(dpp)(sq)(2)(H2O)(6) (1) and [Cu-2(dpp)(cr) (2)(H2O)(3)]. 2H(2)O (2) [dpp=2,3-bis(2-pyridyl)pyrazine, sq=C4O42- di anion of squaric acid, cr=C5O52- dianion of croconic acid] have been s ynthesized and structurally characterized by X-ray diffraction methods . Crystals of 1 are monoclinic, space group C2/c, with a=13.687(3), b= 8.169(4), c=21.619(4) Angstrom, beta=92.32(3)degrees and Z=4; crystals of 2 are triclinic, space group P (1) over bar, with a=8.654(1), b=9. 702(1), c=16.156(1) Angstrom, alpha=94.63(1), beta=99.75(1), gamma=98. 93(1)degrees and Z=2. In both compounds dpp exhibits the bis(chelating ) coordination mode; in 1 the dpp ligand is situated on a two-fold rot ation axis, while in 2 there is no such crystallographic symmetry rest riction. Squarate acts as a monodentate, terminal ligand (1) while cro conate acts as chelating ligand (2). Water molecules constitute the re maining ligands in both compounds. In 1 the copper atom is six coordin ated and displays a distorted compressed octahedral geometry, the shor t bonds being formed to a pyridyl nitrogen and a squarate oxygen atom. In 2 the two crystallographically independent copper atoms display si gnificantly different coordination geometries; Cu(1) has a distorted, elongated octahedral environment where the long bonds are formed to th e pyrazine nitrogen and a croconate oxygen atom, while Cu(2) is five-c oordinated, square pyramidal, with a water molecule in apical position and short equatorial bonds to both pyridyl and pyrazine nitrogen atom s and two croconate oxygen atoms. The intramolecular metal-metal separ ations are 6.993(2) and 6.913(1) A in 1 and 2, respectively. The magne tic behaviour of 1 has been investigated over the temperature range 4. 2-100 K. Fitting of the magnetic susceptibility data by a simple Blean y-Bowers expression yields a value for the singlet-triplet energy gap (J) of -1.2 cm(-1), corresponding to a very weak antiferromagnetic int eraction.