Cyano-bridged homodinuclear copper(II) complexes

Abstract

The synthesis and structural analysis (single crystal X-ray data) of two mononuclear ([Cu(L(1))(CN)]BF(4) and [Cu(L(3))(CN)](BF(4))) and three related, cyanide-bridged homodinuclear complexes ([{Cu(L(1))}(2)(CN)](BF(4))(3)·1.35 H(2)O, [{Cu(L(2))}(2)(CN)](BF(4))(3) and [{Ni(L(3))}(2)(CN)](BF(4))(3)) with a tetradentate (L(1)) and two isomeric pentadentate bispidine ligands (L(2), L(3); 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: g(z) = 2.055, g(x) = 2.096, g(y) = 2.260, A(z) = 8, A(x) = 8, A(y) = 195 × 10(-4) cm(-1), Cu2: g and A as for Cu(1) but rotated by the Euler angles α = -6°, β = 100°, D(exc) = -0.07 cm(-1), E(exc)/D(exc) = 0.205 for [{Cu(L(1))}(2)(CN)](BF(4))(3)·1.35 H(2)O; Cu1,2: g(z) = 2.025, g(x) = 2.096, g(y) = 2.240, A(z) = 8, A(x) = 8, A(y) = 190 × 10(-4)cm(-1), D(exc) = -0.159 cm(-1), E(exc)/D(exc) = 0.080 for [{Cu(L(2))}(2)(CN)](BF(4))(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 D(exc) in [{Cu(L(2))}(2)(CN)](BF(4))(3) is close to the Cu···Cu vector (22°), that is, nearly perpendicular to the Jahn-Teller axis of each of the two Cu(II) centers, and this reflects the crystallographically observed geometry. Antisymmetric exchange in [{Cu(L(1))}(2)(CN)](BF(4))(3)·1.35 H(2)O 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 Cu(II) sites around the Cu···Cu axis.