Cyanide-Bridged FeIII–CuII Complexes: Jahn–Teller Isomerism and Its Influence on the Magnetic Properties

We report here the synthesis and characterization of four dinuclear cyanide-bridged FeIII–CuII complexes, based on a tetra- or a pentadentate bispidine ligand (L1 or L2, respectively; bispidines are 3,7-diazabiyclo[3.3.1]­nonane derivatives) coordinated to the CuII center, and a tridentate bipyridineamide (bpca) coordinated to the low-spin FeIII site, with cyanide groups completing the two coordination spheres, one of them bridging between the two metal ions. The four structurally characterized complexes [{Fe­(bpca)­(CN)3}­{Cu­(L1·H2O)}]­BF4, [{Fe­(bpca)­(CN)3}­{Cu­(L2)}]­[Fe­(bpca)­(CN)3]·5H2O, [{Fe­(bpca)­(CN)3}­{Cu­(L2·MeOH)}]­PF6·MeOH·H2O, and [{Fe­(bpca)­(CN)3}­{Cu­(L2)}]­PF6·2H2O belong to different structural isomers. The most important differences are structurally and electronically enforced (direction of the pseudo-Jahn–Teller mode) strong or weak interactions of the copper­(II) center with the cyanide bridge. The related strength of the magnetic coupling of the two centers is analyzed with a combination of experimental magnetic, electron paramagnetic resonance (EPR), electronic spectroscopic data together with a ligand-field theory- and density functional theory (DFT)-based analysis.