Cyano-bridged Homodinuclear Copper(II) Complexes

The synthesis and structural analysis (single crystal X-ray data) of two mononuclear ([Cu(L1)(CN)]BF4 and [Cu(L3)(CN)](BF4)) and three related, cyanide-bridged homodinuclear complexes ([{Cu(L1)}2(CN)](BF4)3·1.35 H2O, [{Cu(L2)}2(CN)](BF4)3 and [{Ni(L3)}2(CN)](BF4)3) with a tetradentate (L1) and two isomeric pentadentate bispidine ligands (L2, L3; bispidines are 3,7-diazabicyclo[3.3.1]nonane derivatives) are reported, together with experimental magnetic, electron paramagnetic resonance (EPR), and electronic spectroscopic data and a ligand-field-theory-based analysis. The temperature dependence of the magnetic susceptibilities and EPR transitions of the dicopper(II) complexes, together with the simulation of the EPR spectra of the mono- and dinuclear complexes leads to an anisotropic set of g- and A-values, zero-field splitting (ZFS) and magnetic exchange parameters (Cu1: gz = 2.055, gx = 2.096, gy = 2.260, Az = 8, Ax = 8, Ay = 195 × 10–4 cm–1, Cu2: g and A as for Cu1 but rotated by the Euler angles α = −6°, β = 100°, Dexc = −0.07 cm–1, Eexc/Dexc = 0.205 for [{Cu(L1)}2(CN)](BF4)3·1.35 H2O; Cu1,2: gz = 2.025, gx = 2.096, gy = 2.240, Az = 8, Ax = 8, Ay = 190 × 10–4cm–1, Dexc = −0.159 cm–1, Eexc/Dexc = 0.080 for [{Cu(L2)}2(CN)](BF4)3). Thorough ligand-field-theory-based analyses, involving all micro states and all relevant interactions (Jahn–Teller and spin–orbit coupling) and DFT calculations of the magnetic exchange leads to good agreement between the experimental observations and theoretical predictions. The direction of the symmetric magnetic anisotropy tensor Dexc in [{Cu(L2)}2(CN)](BF4)3 is close to the Cu···Cu vector (22°), that is, nearly perpendicular to the Jahn–Teller axis of each of the two CuII centers, and this reflects the crystallographically observed geometry. Antisymmetric exchange in [{Cu(L1)}2(CN)](BF4)3·1.35 H2O causes a mixing between the singlet ground state and the triplet excited state, and this also reflects the observed geometry with a rotation of the two CuII sites around the Cu···Cu axis.