Magnetic properties of 4,4′-bipiperidinediium tetrahalocuprates: crystal structures of 4,4′-bipiperidinediium tetrabromocuprate(II) monohydrate and bis(4,4′-bipiperidinediium) hexabromodicuprate(I)

Reaction of 4,4′-bipiperidine (bipip) with copper(II) bromide in aqueous HBr yielded the salts (H₂bipip)CuBr₄·H₂O (2) and (H₂bipip)₂Cu₂Br₆ (3). Compound 2 crystallizes in the space group P2 ₁ /m and forms alternating layers of the H₂bipip cations and CuBr₄ anions with the water molecules located in the anionic layer. Compound 3 contains Cu(I) and crystallizes in the space group P2 ₁ /c and comprises stacks of H₂bipip cations alternating with stacks of anions. Temperature-dependent magnetic measurements on 2 and its chloride analog, 1, show distinctly different behaviors supported by the two-halide superexchange pathway. The chloride compound (1) is best described as weakly interacting antiferromagnetic dimers (J ~ −13 K), while the bromide compound (2) exhibits very weak ferromagnetic interactions best modeled as a uniform chain with J = 2.45 (9) K (using the H = −J _ABΣS _A·S _B Hamiltonian).