[M(C₅O₅)₂(H₂O)_n]²⁻ as a Building Block for Hetero- and Homo-bimetallic Coordination Polymers: From 1D Chains to 3D Supramolecular Architectures

A series of one-dimensional (1D) coordination polymers, {[Cu(en)₂][M(C₅O₅)₂(H₂O)_m]·xH₂O}_n, (M = Mn (1), m = 4, x = 2; M = Co (2), Ni (3), Cu (4), Zn (5), m = 2, x = 0; en = ethylenediamine; C₅O₅²⁻ = croconate, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione), using [M(C₅O₅)₂(H₂O)_n]²⁻ dianion as a building block has been synthesized and characterized by the X-ray diffraction method. Structural determination reveals that, in compounds 1−5, one [M(C₅O₅)₂(H₂O)_n]²⁻ dianion coordinates via two oxygen (croconate) donors to the axial sites of two Cu(II) centers of [Cu(en)₂]²⁺ cations to form one-dimensional (1D) linear (1, 3 and 4) or zigzag (2 and 5) chains of alternating [M(C₅O₅)₂(H₂O)_n]²⁻ and [Cu(en)₂]²⁺ units. The croconate acts as a bridging ligand with three coordination modes as μ_1,3-bismonodentate for 1, μ_1,2,3-bidentate/monodentate for 2 and 5, and μ_1,2,4-bidentate/monodentate for 3 and 4, respectively. In addition, a byproduct, [Cu(en)(C₅O₅)(H₂O)]₂ (6), which is cocrystallized with 4, shows a dimeric structure formed through the croconate bridge with μ_1,1,2-bidentate/monodentate coordination mode. All of their 1D frameworks (1−5) and dimeric structure (6) are extended to three-dimensional (3D) supramolecular architectures by the intermolecular π−π interaction among croconates and intermolecular hydrogen bonds among the en, croconate, and coordinated water molecules.