Redox coupled-spin crossover in cobalt β-diketonate complexes: Structural, electrochemical and computational studies

Structural, electrochemical, spectroelectrochemical, magnetic and spectroscopic studies are reported for the octahedral cobalt β-diketonate complexes, [Co(β-diketonate)2(N–N)] {β-diketonate = 2,2,6,6-tetramethylheptane-3,5-dionate (tmhd); N–N = 1,10-phenanthroline (phen) 1, 2,2′-bipyridine (2,2′-bpy) 2 and dimethylaminoethylamine (dmae) 3; β-diketonate = 1,3-diphenylpropane-1,3-dionate (dbm); N–N = phen 4, 2,2′-bpy 5, dmae 6}. X-ray crystallographic studies of the redox pair [Co(tmhd)2(2,2′-bipy)]0/+2/2+ show a shortening of the Co-ligand bond lengths by between 0.18 and 0.22 Å upon oxidation and a significantly more regular octahedral geometry around the cobalt in the cation consistent with spin crossover in addition to a change in oxidation state. Cyclic voltammetry of 1–6 reveals an irreversible one-electron oxidation to CoIII with large peak separations between the oxidation and reduction peaks, indicative of redox coupled-spin crossover (RCSCO); i.e. [Co(β-diketonate)2(N–N)] (S = 3/2) ↔ [Co(β-diketonate)2(N–N)]+ + e− (S = 0). Moreover, the complexes represent rare examples of RCSCO species with a CoO4N2 coordination sphere. The tmhd complexes are more easily oxidized than the respective dbm analogues with the oxidation peak potentials in the order bipy < phen < dmae. Oxidation of 1–6 with AgBF4 yields the corresponding CoIII cations, [Co(β-diketonate)2(N–N)]BF41+–6+ which has been confirmed by 1H NMR spectroscopy. Spectroelectrochemistry of the redox pairs [Co(β-diketonate)2(N–N)]0/+ is consistent with the isolated compounds being identical to the species formed at the electrode. Theoretical studies reveal that the SOMO is essentially metal d-orbital and β-diketonate based, consistent with the strong effect of the β-diketonate ligand on the oxidation potential. In addition, there are substantial changes in the relative stabilities of the various spin states compared with [Co(tacn)2]2+/3+ such that the high spin states become more accessible. The above results are consistent with a square scheme mechanism.

Cobalt β-diketonates complexes, [Co(β-diketonate)2(N–N)]0/+ have been prepared in which cyclic voltammetric studies reveal redox-coupled spin crossover possibly via a high spin CoIII intermediate. Structural studies and DFT calculations are presented to support these findings.Figure optionsDownload as PowerPoint slideHighlights
► [Co(β-diketonate)2(N–N)]0/+ complexes exhibit redox coupled-spin crossover.
► Structure of the redox pair [Co(β-tmhd)2(2,2′-bipy)]0/+ reported.
► Significant structural changes occur upon oxidation.
► Cyclic voltammetry suggests a square scheme mechanism.
► DFT calculations confirm the presence high energy spin state intermediates.