Ligand effects on the structures and magnetic properties of tricyanomethanide-containing copper(ii) complexes

Abstract

The preparation, crystal structure and magnetic properties of four heteroleptic copper(II) complexes with the tricyanomethanide (tcm⁻) and the heterocyclic nitrogen donors 3,6-bis(2-pyridyl)pyridazine (dppn), 2,5-bis(2-pyridyl)pyrazine (2,5-dpp), 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) and 2,3-bis(2-pyridyl)quinoxaline (2,3-dpq) are reported, {[Cu₂(dppn)(OH)(tcm)₂]·tcm}_n (1), {[Cu(2,5-dpp)(tcm)]·tcm}_n (2), {[Cu₂(2,3-dpp)₂(tcm)₃(H₂O)_0.5]·tcm·0.5H₂O}_n (3) and [Cu(2,3-dpq)(tcm)₂]_n (4). 1 has a ladder-like structure with single µ-1,5-tcm ligands forming the sides and a bis-bidentate dppn and a single µ-hydroxo providing the rung. Each copper atom in 1 exhibits a distorted square pyramidal CuN₄O surrounding: the basal plane is built by the hydroxo-oxygen, a nitrile-nitrogen atom from a tcm group and one pyrazine and a pyridyl nitrogen atoms from the dppn whereas the apical position is filled by a nitrile-nitrogen atom from a symmetry-related tcm ligand. The structures of 2–4 consists of zig-zag (2 and 3)/linear (4) chains of copper(II) ions which are bridged by either bis-bidentate 2,5-dpp (2) and 2,3-dpp (3) molecules or single µ-1,5-tcm (4) groups. The copper atoms in 2 and 4 are five coordinated with distorted trigonal bipyramidal (2) and square pyramidal (4) CuN₅ surroundings. The axial positions in 2 are occupied by two pyridyl-nitrogen atoms from two 2,5-dpp ligands whereas the trigonal plane is built by a nitrile-nitrogen from a terminally bound tcm group and two pyrazine nitrogen atoms from two 2,5-dpp molecules. The basal plane in 4 is defined by a pyridyl and a pyrazine nitrogen atoms from the bidentate 2,3-dpq ligand and two nitrile nitrogen atoms from two tcm groups (one terminal and the other bridging) whereas the apical position is filled by a nitrile nitrogen from another tcm ligand. The crystallographically independent copper atoms in 3 [Cu(1) and Cu(2)] exhibit elongated octahedral geometries being defined by four nitrogen atoms from two 2,3-dpp groups [Cu(1) and Cu(2)] either two terminally bound tcm ligands [Cu(1)] or a water molecule and a monodentate tcm ligand [Cu(2)] in cis positions. Magnetic susceptibility measurements for 1–4 in the temperature range 1.9–295 K reveal the occurrence of strong [J ca. −1000 cm⁻¹ (1); Ĥ = −JŜ_A · Ŝ_B] and weak [J = −0.13 (2), −0.67 (3) and −0.18 cm⁻¹ (4); Ĥ = −J ∑_IŜ_i · Ŝ_i+1] antiferromagnetic interactions in agreement with the different nature of the exchange pathways involved, diazine and single µ-hydroxo (1) and the extended 2,5-dpp (2), 2,3-dpp (3) and single µ-1,5-tcm (4) bridges with copper–copper separations of 3.363(8) (1), 7.111(1) (2), 6.823(1) and 7.056(1) (3) and 7.446(1) Å (4).