Bridge vs\nTerminal Cyano-coordination in Binuclear\nCobalt Porphyrin Dimers: Interplay of Electrons between Metal and\nLigand and Spin-Coupling via Bridge

Three cyano-coordinated cobalt porphyrin dimers were\nsynthesized\nand thoroughly characterized. The X-ray structure of the complexes\nreveals that cyanide binds in a terminal fashion in both the <i>anti</i> and <i>trans</i> isomers of ethane- and ethylene-bridged\ncobalt porphyrin dimers, while in the <i>cis</i> ethylene-bridged\ndimer, cyanides bind in both terminal and bridging modes. The nonconjugated\nethane-bridged complex stabilizes exclusively a diamagnetic metal-centered\noxidation of type Co<sup>III</sup>(por)(CN)<sub>2</sub> both in the\nsolid and in solution. In contrast, the complexes with the conjugated\nethylene-bridge contain signatures of both paramagnetic ligand-centered\noxidation of the type Co<sup>II</sup>(por<sup>•+</sup>)(CN)<sub>2</sub> and diamagnetic metal-centered oxidation of type Co<sup>III</sup>(por)(CN)<sub>2</sub> with the metal-centered oxidized species being\nthe major component in the solid state as observed in XPS, while the\nligand-centered oxidized species are present in a significant amount\nin solution. <sup>1</sup>H NMR spectrum in solution displays two set\nof signals corresponding to the simultaneous presence of both the\ndiamagnetic and paramagnetic species. EPR and magnetic investigation\nreveal that there is a moderate ferromagnetic coupling between the\nunpaired electrons of the low-spin Co<sup>II</sup> center and the\nporphyrin π-cation radical in Co<sup>II</sup>(por<sup>•+</sup>)(CN)<sub>2</sub> species as well as an antiferromagnetic coupling\nbetween the two Co<sup>II</sup>(por<sup>•+</sup>) units through\nthe ethylene and CN bridges.