SIGNIFICANT ANTIFERROMAGNETIC EXCHANGE IN COPPER(II) AND NICKEL(II) DINUCLEAR COMPLEXES OF THE SUBSTITUTED PYRAZINE LIGAND 2,3,5,6-TETRA(2-PYRIDYL)PYRAZINE (TPPZ) - MAGNETIC-PROPERTIES, SYNTHESIS AND CRYSTAL-STRUCTURE

The dinuclear compounds [Cu-2(TPPZ)Cl-4]. 5H(2)O (1), [Ni-2(TPPZ)(H2O) (6)](NO3)(4) . 2.5H(2)O (3) and [Ni-2(TPPZ)(H2O)(6)]-(squarate)(2) . 5 H(2)O (4), where TPPZ is the ligand 2,3,5,6-tetra(2-pyridyl) pyrazine, have been synthesised and characterised crystallographically. [Cu-2(T PPZ)Cl-4]. 5H(2)O (1) crystallises in the monoclinic system, space gro up P2(1)/c, a=15.030(3), b=13.224(2), c=15.320(2) Angstrom, beta = 105 .40(1)degrees, V= 2935.62(8) Angstrom(3), Z=4, R1 = 0.035 (observed da ta). [Ni-2(TPPZ) (H2O)(6)] (NO3)(4) . 2.5H(2)O (3) crystallises in the monoclinic system, space group I2/a, a = 20.660(2), b = 13.724(1), c= 25.436(3) Angstrom, beta = 91.89(1)degrees, V= 7208.1(12) Angstrom(3) , Z= 8, R1 = 0.070 (observed data). [Ni-2(TPPZ)(H2O)(6)] (squarate), 5 H(2)O (4) crystallises in the monoclinic system, space group A2/n, a=1 9.014(2), b = 10.518(1), c = 19.131(2) Angstrom, beta = 94.22(1)degree s, V= 3815.6(7) Angstrom(3), Z= 4, R1 = 0.035 (observed data). Magneti c measurements show significant antiferromagnetic coupling for pyrazin e bridged compounds: 2J = -34.1 cm(-1), g = 2.05 for 1; 2J= -7.6 cm(-1 ), D = -13.3 cm(-1), g=2.16 for 3; 2J= -7.5 cm(-1), D= -10.4 cm(-1), g =2.17 for 4. The magnetic behaviour of the related published compound [Cu-2(TPPZ)(H2O)(4)] (ClO4)(4) . 2H(2)O (2) has also been measured: 2J = -61.1 cm(-1), g=2.13. The possibility of a sigma superexchange mecha nism in these compounds is discussed.