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Ligand-Enforced Isomeric Low-Spin and High-Spin Cobalt(II) Complexes
Masnun Naher1, Blake J P Connolly1, Lawrence R Gahan1
1School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane 4072, Australia.
None:
In contrast to their hexaaminecobalt(III) analogues, which all possess a low-spin d6 ground state, all known hexaaminecobalt(II) complexes exhibit a high-spin d7 electronic configuration, and these compounds have been central to the development of electron transfer theories. Here, we report the isolation and structural and spectroscopic characterization of the first low-spin hexaaaminecobalt(II) complex [Co(trans-diammac)](ClO4)2·H2O (trans-diammac = trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine). The origin of this unprecedented finding is a ligand enforced metal ion compression, which leads to short Co-N coordinate bonds and spin pairing. The stereoisomeric [Co(cis-diammac)]2+ (cis-diammac = cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine), where isomerism is dictated by the organic ligand, is high-spin and exhibits typical properties of a hexaaminecobalt(II) complex. A combination of single-crystal X-ray crystallography, UV-vis spectroscopy, EPR (CW and pulse) spectroscopy, and solution magnetic measurements have been used to provide a unique stereoisomeric pairing where isomerism dictates a change in the electronic ground state of the complex.
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