Anion-Directed Self-Assembly of Unusual Discrete and One-Dimensional Copper(II) Complexes of 3,6-Bis(2′-pyridyl)pyridazine

The preparation and crystal structures of six new copper­(II) compounds of formula [Cu­(dppn)₂(ClO₄)]­ClO₄ (1), [Cu₂(dppn)­(OH)­(ClO₄)₃(H₂O)₃]·H₂O (2), [Cu₂(dppn)₂(H₂O)₂]­(ClO₄)₄ (3), [Cu₂(dppn)₂(ClO₄)₄] (4), {[Cu₂(dppn)­(N₃)₄]·H₂O/[Cu₂(dppn)­(N₃)₄(H₂O)]}_n (5) and {[Cu₂(dppn)­(OH)­(dca)₃]·H₂O}_n (6) [dppn = 3,6-bis­(2′-pyridyl)­pyridazine and dca = dicyanamide] are reported. 1 is a mononuclear complex where two bidentate dppn molecules and a monodentate perchlorate build an intermediate square pyramidal/trigonal bipyramidal (sp/tbp) five-coordinate environment around the copper­(II) ion, the overall positive charge being balanced by a free perchlorate anion. 2–4 are dinuclear complexes with either one (2) or two bis-bidentate dppn ligands (3, 4) and a single hydroxo bridge (2). One (2) or two water molecules (3) and three (2) or four perchlorate groups (4) complete the metal coordination sphere, describing elongated octahedral (2 and 4) or intermediate sp/tbp environments (3). Free perchlorate anions neutralize the positive charge of the dicopper­(II) unit in 3. 3 and 4 are noteworthy because 1:1 Cu­(II)-dppn complexes have never been reported. The simultaneous use of dppn and potentially bridging coligands such as azide and dca led to the chain compounds 5 and 6. Two forms, {[Cu₂(dppn)­(N₃)₄]·H₂O}_n (5A) and {[Cu₂(dppn)­(N₃)₄(H₂O)]}_n (5B), were noted to coexist in the crystals of complex 5, sharing a common [Cu₂(dppn)­(N₃)₄] dicopper­(II) fragment. 5A consists of a step-like one-dimensional polymer in which the dinuclear entities are linked through single end-to-end azido bridges, while a supramolecular one-dimensional motif is present in 5B, where the Cu–(μ_1,3-N₃)–Cu link is replaced by a Cu–Ow···(N₃)–Cu hydrogen-bonding interaction. Similarly, [Cu₂(dppn)­(μ-OH)­(dca)₂] dicopper­(II) fragments in 6 are connected via single μ_1,5-dca bridges. However an intrachain hydrogen bond is also present in this case, contributing to define a ladder-like motif instead. The variable-temperature magnetic study of 2, 3, 5 and 6 reveals the occurrence of very strong antiferromagnetic interactions between the copper­(II) ions [2, 3 and 6 being practically diamagnetic at room temperature and J = −598(5) cm^–1 for 5; H = −JS_A · S_B]. The remarkable efficiency of the double (3) and single diazine (2, 5 and 6) and single hydroxo (2 and 6) or end-on azido (5) groups to mediate electronic effects when acting as bridges between the copper­(II) ions [values of the copper–copper separation of 3.346(1) (2), 3.785(2) (3), 3.469(1) (5) and 3.368(1) Å (6)] accounts for the strong magnetic couplings observed in these compounds.