Syntheses, Crystal Structures, Magnetic Properties, and EPR Spectra of Tetranuclear Copper(II) Complexes Featuring Pairs of “Roof-Shaped” Cu2X2 Dimers with Hydroxide, Methoxide, and Azide Bridges

Hydroxo- and methoxo-bridged tetranuclear copper(II) complexes of the tetramacrocyclic ligand 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene (Ldur), have been prepared from [Cu4Ldur(H2O)8](ClO4)8·9H2O (1). Addition of base to an aqueous solution of 1 gave [Cu4Ldur(μ2-OH)4](ClO4)4 (2). Diffusion of MeOH into a DMF solution of 2 produces [Cu4Ldur(μ2-OMe)4](ClO4)4·HClO4·2/3MeOH (3), a complex which hydrolyzes on exposure to moisture regenerating 2. The structurally related azido-bridged complex, [Cu4Ldur(μ2-N3)4](PF6)4·4H2O·6CH3CN (4), was produced by reaction of Ldur with 4 molar equiv of Cu(OAc)2·H2O and NaN3 in the presence of excess KPF6. Compounds 2 − 4 crystallize in the triclinic space group P1̄ (No. 2) with a = 10.248(1) Å, b = 12.130(2) Å, c = 14.353(2) Å, α = 82.23(1)°, β = 80.79(1)°, γ = 65.71(1)°, and Z = 1 for 2, a = 10.2985(4) Å, b = 12.1182(4) Å, c = 13.9705(3) Å, α = 89.978(2)°, β = 82.038(2)°, γ = 65.095(2)°, and Z = 1 for 3, and a = 12.059(2) Å, b = 12.554(2) Å, c = 14.051(2) Å, α = 91.85(1)°, β = 98.22(1)°, γ = 105.62(1)°, and Z = 1 for 4. The complexes feature pairs of isolated dibridged copper(II) dimers with “roof-shaped” Cu2(μ2-X)2 cores (X = OH-, OMe-, N3 -), as indicated by the dihedral angle between the two CuX2 planes (159° for 2, 161° for 3, and 153° for 4). This leads to Cu···Cu distances of 2.940(4) Å for 2, 2.962(1) Å for 3, and 3.006(5) Å for 4. Variable-temperature magnetic susceptibility measurements indicate weak antiferromagnetic coupling (J = −27 cm-1) for the hydroxo-bridged copper(II) centers in 2 and very strong antiferromagnetic coupling (J = −269 cm-1) for the methoxo-bridged copper(II) centers in 3. Pairs of copper(II) centers in 4 display the strongest ferromagnetic interaction (J = 94 cm-1) reported thus far for bis(μ2-1,1-azido)-bridged dicopper units. Spectral measurements on a neat powdered sample of 4 at 33.9 GHz or 90 Ghz confirm the spin-triplet ground state for the azido-bridged copper(II) pairs.