Ladder-like [CrCu] coordination polymers containing unique bridging modes of [Cr(C₂O₄)₃]^3- and Cr₂O₇²

Three heterometallic one-dimensional (1D) coordination polymers {A[CrCu₂(bpy)₂(C₂O₄)₄]·H₂O}_n [A = K⁺ (1) and NH₄⁺ (2); bpy = 2,2'-bipyridine] and [(Cr₂O₇)Cu₂(C₂O₄)(phen)₂]_n (3; phen = 1,10-phenanthroline) with uncommon topology have been synthesized using a building block approach and characterized by single-crystal X-ray diffraction, IR and impedance spectroscopies, magnetization measurements, and DFT calculations. Due to the partial decomposition of the building block [Cr(C₂O₄)₃]^3-, all three compounds contain oxalate-bridged [Cu₂(L)₂(μ-C₂O₄)]²⁺ units [L = bpy (1 and 2) and phen (3)]. In compounds 1 and 2 these cations are mutually connected through oxalate groups from [Cr(C₂O₄)₃]^3-, thus forming ladder-like topologies. Unusually, three different bridging modes of the oxalate ligand are observed in these chains. In compound 3 copper(ii) ions from cationic units are bridged through the oxygen atoms of Cr₂O₇^2- anions in a novel ladder-like mode. Very strong antiferromagnetic coupling observed in all three compounds, determined from the magnetization measurements and confirmed by DFT calculations (J = -343, -371 and -340 cm^-1 for 1, 2 and 3, respectively), appears between two copper(ii) ions interacting through the oxalate bridge.