ReviewValence tautomerism in metal complexes: Stimulated and reversible intramolecular electron transfer between metal centers and organic ligands
Graphical abstract
Valence tautomeric (VT) transitions involve stimulated intramolecular electron transfer between a redox-active metal center and a redox-active organic ligand; reversible VT transitions induced by temperature, pressure, light-irradiation and other stimuli have been observed for systems involving d- and f-block metals with a variety of redox-active ligands. This review focuses on VT transitions involving new metal–ligand combinations, transitions in polynuclear and polymeric complexes and bifunctional materials that combine VT transitions with other chemical and physical properties.
Introduction
Of critical importance for the miniaturization of functional materials are molecular and molecule-based materials that can be switched between distinguishable electronic states by application of an external stimulus. Systems that can be switched in this way include spin crossover (SCO) complexes [1], [2], heterometallic complexes that exhibit electron transfer coupled spin transitions (ETCST), also known as charge transfer induced spin transitions (CTIST) [3], [4], and valence tautomeric (VT) complexes. A spin state transition at a metal center gives rise to the switching in SCO complexes, while ETCST/CTIST involve a concerted intramolecular electron transfer between different metal centers and metal-based spin transition. Valence tautomeric transitions similarly involve a stimulated intramolecular electron transfer, but in this case between a redox-active ligand and a redox-active metal center, giving rise to two different valence tautomers or redox isomers [5], [6], [7], [8], [9], [10], [11], [12]. In some cases, a spin transition at the metal center also takes place. Valence tautomeric transitions have been observed in the solid state and solution, for complexes with metals including vanadium, manganese, iron, cobalt, nickel, copper, ruthenium and ytterbium; and with a variety of redox-active ligand types, including o-dioxolenes, o-diimines, o-amino-phenolates, phenoxyl ligands, porphyrins and polychlorotriphenylmethyl radicals. Like SCO and ETCST/CTIST transitions, VT transitions are stimulated by the variation of an external parameter and can be induced thermally, by application of pressure or a magnetic field, or by irradiation with visible light or soft X-rays. In some cases, thermally induced VT transitions are accompanied by coordination/elimination of additional ancillary ligands, resulting in tautomeric forms with different coordination numbers. Such systems may be considered to not exhibit valence tautomerism in the strict sense, despite the reversibility associated with the intramolecular electron transfer. This is the case for the vanadium and nickel complexes discussed in this review, which have nevertheless been included for the sake of completeness. Rarely, chemical tuning of an ancillary ligand can alone induce a VT transition, without thermal equilibration.
By definition, VT complexes incorporate redox-active ligands, which thus must have an accessible radical form. Radical organic ligands are themselves of interest as building blocks for molecular materials [13], while the “non-innocence” of redox-active ligands has been of increasing interest in both organometallic and coordination chemistry with reference to catalysis, bioinorganic chemistry and molecular materials [14], [15], [16], [17], [18], [19], [20]. Valence tautomeric transitions, like SCO and ETCST/CTIST transitions, are typically accompanied by distinct and reversible changes in structural, magnetic and optical properties. It is the resulting effective switchability of these properties that may be exploited in future molecule-based materials for display devices, data storage, sensors and molecular electronics or spintronics. In this context, of particular interest are bistable materials that display a hysteretic VT transition around room temperature, with a wide hysteresis loop. Also of potential interest are systems in which a relatively long-lived metastable state can be produced (e.g. photo-generated) at accessible temperatures [11].
The last comprehensive review of the research field of valence tautomerism by Hendrickson and Pierpont appeared in the 2004 issues of Topics in Current Chemistry dedicated to spin transitions [5]. Since that time, a handful of other reviews have been published, which have focused on different aspects of valence tautomerism [6], [7], [8], [9], [10], [11], [12], although these mainly deal with cobalt-dioxolene complexes. Several earlier reviews of the field may also be of interest [21], [22], [23]. The scope of the present work is to provide a broad-ranging update of advances in the field of valence tautomerism since 2004, with a particular emphasis on systems other than cobalt-dioxolenes.
Section snippets
Valence tautomerism in cobalt-dioxolene complexes
Since the first report in 1980 of a VT transition for the complex [Co(2,2′-bpy)(3,5-dbsq)(3,5-dbdiox)] (3,5-dbsq = 3,5-di-tert-butyl semiquinone, 3,5-dbdiox = 3,5-di-tert-butyl dioxolene, 2,2′-bpy = 2,2′-bipyridine), octahedral cobalt complexes with o-dioxolene (diox) ligands have been the predominant subject of investigations into the phenomenon [24]. Fundamental research using mononuclear complexes to understand the origins of the VT transition and then tune its characteristics have in part been
Valence tautomerism in dioxolene complexes of metals other than cobalt
Cobalt-dioxolene complexes are by far the most well-established VT systems and have provided the most examples for study. This is largely due to the low spin to high spin transition that accompanies the electron transfer and enhances the vibrational and electronic components of the entropy contribution to the Gibbs free energy for the thermally induced process, which is unique to the cobalt case. However, as detailed below, VT transitions have also been reported for dioxolene complexes of
Valence tautomerism in complexes with redox-active ligands other than dioxolenes
Reports of valence tautomeric transitions induced by a physical stimulus in metal complexes with redox-active ligands other than o-dioxolenes are far fewer in number than those for dioxolene complexes. Those few reports do present very diverse classes of redox-active ligands, appropriate for different metal centers and redox states, as well as a variety of ancillary ligands.
Valence tautomerism in coordination polymers
From molecular polynuclear complexes, the logical next step to achieving intramolecular cooperativity associated with a VT transition is through the development of VT coordination polymers. Certainly numerous one-, two- and three-dimensional SCO coordination polymers have been reported and ETCST/CTIST was first probed in coordination polymers based on Prussian Blue [115], [3]. In contrast to these related materials, VT coordination polymers are relatively scarce, with only a handful of
Bifunctional valence tautomeric complexes
Molecular or molecule-based materials are an emerging class of nanoscale functional materials of interest for diverse future applications. Materials derived from discrete molecules offer several advantages, including their amenability to multifunctionalization through standard synthetic methodologies using ambient conditions. Thus the combination of valence tautomerism with other chemical or physical properties is being pursued as part of the development of bi- or multifunctional molecular
Concluding remarks
Valence tautomeric transitions in cobalt-dioxolene systems have now been explored for some 30 years, which has allowed the development of a solid understanding of their physicochemical origins. An appreciation of the role of apparently secondary influences, such as the ancillary ligands, intermolecular interactions, crystal packing and solvation effects, has also emerged over time. A highlight in this regard comes from separate studies from the groups of Sato and Shultz that have shown the
Acknowledgements
Ho-Chol Chang, Igor Fedushkin, Natia Frank and David Shultz are thanked for their generous provision of figures.
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