Syntheses, structures and magnetism of linear tri- and tetra-copper chains containing anions of N, N′-bis(pyrimidine-2-yl)formamidine

Reaction of Kpmf (pmf⁻ = anion of N, N′-bis(pyrimidyl-2-yl)formamidine, Hpmf) with divalent copper salt CuX₂ afforded the linear trinuclear complexes of the type [Cu₃(pmf)₄](X)₂ (X = BF₄, 1; NO₃, 2; ClO₄, 3), while reaction of Kpmf with monovalent copper salt CuX gave the linear tetranuclear complexes of the type Cu₄(pmf)₄X₂ (X = Cl, 4; Br, 5). The copper atoms of complexes 1–5 are helically bridged by four pmf⁻ ligands, resulting in three different coordination modes for the pmf⁻ ligands. In complexes 1–3, one pmf⁻ ligand adopts a new coordination mode with the two amine nitrogen atoms chelating to the central copper atom, while the other three feature chelation by one pyrimidyl and one adjacent amine nitrogen atoms. The Cu(ii)⋯Cu(ii) distances are 2.729(2) and 2.825(2) Å for 1, 2.762(1) and 2.832(1) Å for 2 and 2.732(1) and 2.827(1) Å for 3. In complexes 4 and 5, the pmf⁻ ligands are coordinated to the copper atoms in tetradentate fashion with each nitrogen atom coordinating to one Cu atom. The Cu⋯Cu distances are 2.580(1) and 2.549(1) Å for 4 and 2.582(1) and 2.561(1) Å for 5. Antiferromagnetic interactions between the copper ions are observed with calculated g and J values of 2.03(1) and −188(2) cm⁻¹ for 1, 2.09(1) and −268(3) cm⁻¹ for 2, and 2.09(1) and −486(2) cm⁻¹ for 5. By comparing the magnetic data it can be shown that the bonding mode of the pmf⁻ ligand is one of the important factors in determining the strengths of the Cu⋯Cu interactions in linear trinuclear and tetranuclear copper complexes.