New square S=1/2 Heisenberg antiferromagnetic lattices: Pyridinium tetrahalocuprates and bispyrazinecopper(II) tetrafluoroborate

As part of a program to create new low dimensional S=1/2 antiferromagnetic Heisenberg compounds with moderate exchange, we have prepared four new copper complexes with square magnetic lattices. The complexes L-2[CuX4] have been prepared (L = 2-amino-5-methylpyridinium, X = Cl, Br; L = 2-amino-5-chloropyridinium, X = Br). The compounds are isomorphous, giving monoclinic crystals; space group C2/c. The tetrahalometallate ions lie in C-centered layers, forming a square magnetic lattice. The layers are separated by the organic cations. Preliminary analysis of the magnetic data shows that the exchange constant, J, increases in the order Br > Cl and 2-amino-5-chloropyridinium > 2-amino-5-methylpyridinium. A new member of the pyrazine bridged family M(pyrazine)(2)X-2 has also been prepared and studied. Cu(pz)(2) (BF4)(2) also crystallizes in the space group C2/c. Analysis of the temperature dependence of the susceptibility yields and exchange value of -7.98 K, similar to that found for the known Cu(pz)(2)(ClO4)(2) complex.