The two-halide super exchange pathway in Cu(II) chains: synthesis, structure, and magnetic behavior of (2-X-3-methylpyridinium) ₂ [CuX′ ₄ ] (X, X′ = Cl, Br)

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Reaction of CuX′2ꞏnH2O (X′ = Br, Cl) with 2-X-3-methylpyridine (X = Br, Cl) in aqueous acid gave (2-chloro-3-methylpyridinium)2[CuCl4] (1), (2-bromo-3-methylpyridinium)2[CuCl4] (2), (2-chloro-3-methylpyridinium)2[CuBr4] (3), (2-bromo-3-methylpyridinium)2[CuBr4] (4), while the reaction of 2-chloro-3-methylpyridinium bromide with CuBr2 in acetonitrile yielded the copper(II) trimer (2-chloro-3-methylpyridinium)2[Cu3Br8] (5). All compounds were characterized by single crystal X-ray diffraction. 1–4 crystalize in the monoclinic space group P21/n with a square planar arrangement of the halide ions about the copper center, which is located on a crystallographic inversion center. 5 crystallizes in the monoclinic space group P21/c and exhibits stacks of Cu3Br82− ions. Variable field and temperature magnetic measurements on 1–4 show modest antiferromagnetic interactions with the tetrabromidocuprates exhibiting stronger interactions. Fits to the uniform magnetic chain model yielded: 1, Curie constant (CC) = 0.438(2) emu-K/mol-Oe, J/kB = −4.63(5) K; 2, CC = 0.431(1) emu-K/mol-Oe, J/kB = −4.53(3) K; 3, CC = 0.415(2) emu-K/mol-Oe, J/kB = −21.6(2) K; 4, CC = 0.408(2) emu-K/mol-Oe, J/kB = −23.9(6) K. A Curie-Weiss correction employed in the fitting shows the chains to be highly magnetically isolated.