Ferro- and Antiferromagnetic Coupling through 1,n-Dithiosquarate Bridges (n = 2, 3) in Copper(II) Dimers. Syntheses, Crystal Structures, and Magnetic Properties of [Cu2(bpca)2(1,2-dtsq)(H2O)]·2H2O and [Cu2(bpca)2(1,3-dtsq)]·2H2O

The preparation, crystal structures, and variable-temperature magnetic susceptibility data for two dithiosquarate-bridged copper(II) complexes of formulas [Cu2(bpca)2(1,2-dtsq)(H2O)]·2H2O (1) and [Cu2(bpca)2(1,3-dtsq)]·2H2O (2) (where bpca = the bis(2-pyridylcarbonyl)amide anion, 1,2-dtsq = dianion of 3,4-dimercapto-3-cyclobutene-1,2-dione, and 1,3-dtsq = dianion of 3-hydroxy-4-mercapto-2-thioxo-3-cyclobuten-1-one) are reported herein. 1 and 2 crystallize in the triclinic system, space group P1̄, with a = 9.3243(4) Å, b = 10.9036(5) Å, c = 15.5432(4) Å, α = 90.276(2)°, β = 100.009(2)°, γ = 94.139(2)°, and Z = 2 for 1 and a = 7.656(2) Å, b = 9.351(3) Å, c = 11.587(2) Å, α = 109.02(2)°, β = 97.40(2)°, γ = 110.27(2)°, and Z = 1 for 2. Their structures consist of the neutral dithiosquarate-bridged copper(II) units [Cu2(bpca)2(1,2-dtsq)(H2O)] (1) and [Cu2(bpca)2(1,3-dtsq)] (2) and water molecules of crystallization. The copper environment is distorted square pyramidal CuN3SO in both compounds. Bpca acts as a tridentate end-cap ligand with its three nitrogen atoms occupying three of the four basal positions. 1,3-Dithiosquarate adopts a centrosymmetric bis-bidentate bridging mode with the sulfur atom filling the fourth equatorial position around each copper atom and the oxygen at the apical site. 1,2-Dithiosquarate acts also as a bridging ligand but in an asymmetrical manner, chelating through the two sulfur atoms at one copper atom and unidentate through one sulfur at the other copper atom. The bridging sulfur occupies one equatorial position at a copper atom and the apical site at the other one. The metal−metal separations through the extended 1,2- and 1,3-dtsq bridges are 5.1654(8) and 7.212(3) Å, respectively. The exchange pathways accounting for the intermediate ferro- (1, J = +32.4 cm-1) and antiferromagnetic (2, J = −33.5 cm-1) couplings (Ĥ = −JŜ A·Ŝ B) observed in these dithiosquarate-bridged compounds are analyzed and discussed in the light of density functional theory (DFT) calculations.