Valence tautomerism in dinuclear cobalt dioxolene complexes with non-innocent behaving bridging ligands
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Abstract
Valence tautomers represent one of the three major classes of inorganic switchable and bistable compounds, alongside spin-crossover compounds and mixed-valence compounds. Valence tautomerism describes an intramolecular electron transfer between an electron-donor and an intramolecular electron transfer between an electron-donor and an electron-acceptor fragment, induced by an external stimulus, which can proceed reversibly. In most cases, a metal centre and an organic ligand constitute the corresponding redox pair. This gives rise to two discrete redox isomers with distinct charge distributions as well as distinct physical, optical, and spectroscopic properties.
For thermally induced valence tautomerism to occur, the complex compounds involved must meet certain requirements. The organic ligand must exhibit non-innocent behaviour with respect to its redox properties. The valence orbitals of the donor and acceptor fragments must be energetically very similar. The intramolecular coupling must be strong enough to allow electron transfer, yet weak enough to preserve two discrete states and prevent their electronic structures from being obscured by delocalization or orbital mixing.
Owing to their ability to respond to external stimuli with a change in their properties, valence tautomers are ideal candidates for highly miniaturized electronic devices. Dinuclear compounds additionally offer the possibility of addressing up to three distinct states, enabling the realization of more complex logic gates. Cobalt-dioxolene compounds have emerged as a particularly suitable model system. Their valence orbitals lie energetically very close to one another, and through judicious choice of ligands, a spin-crossover phenomenon can additionally occur at the valence tautomeric transition. Furthermore, the valence tautomeric transition in this system can be conveniently monitored by magnetometry, which is rarely the case of other metal centres.
The present work systematically investigates, using a piperazine-based bridging ligand, the effects of various modifications on the valence tautomeric transition and its transition profile. The electronic modification of the dioxolene ligands, the steric and electronic modification of the bridging ligand, packing effects and lattice solvent effects through variation of the counter-anions, as well as the effect of altered electronic communication within the bridging unit were examined. In a first step, the influence of tetrahalogenated dioxolenes on the valence tautomeric transition was studied, the redox potential of the catecholate/semiquinonate pair was modulated, and a systematic anion study was conducted to investigate packing effects. Subsequently, the bridging ligand was modified with electron-donating and electron-withdrawing substituents to fine-tune the Co(II)/Co(III) redox potential. Through a carefully balanced interplay of steric substituents on the bridging ligand, the Co(II)semiquinonate radical state could be preferentially stabilised. In combination with an electronically moderately tuned 4,5-dichlorocatecholate, it was possible for the first time to prepare a dinuclear valence tautomeric complex bearing an innocent bridging ligand that undergoes a complete and reversible transition. In the final step, the aliphatic bridging ligand, which affords low electronic communication between the cobalt centres, was replaced by an aromatic pyrazine unit providing enhanced electronic communication. Preliminary results demonstrate that valence tautomerism can likewise be realised with the new system.