Review
Valence tautomerism in metal complexes: Stimulated and reversible intramolecular electron transfer between metal centers and organic ligands

https://doi.org/10.1016/j.ccr.2014.01.014Get rights and content

Highlights

  • Valence tautomeric (VT) transitions in metal complexes involve a stimulated intramolecular electron transfer between ligand and metal.

  • Cobalt-dioxolene complexes and complexes with other redox-active metal–ligand combinations display VT transitions.

  • Valence tautomerism has been reported for mononuclear, polynuclear and polymeric complexes.

  • The combination of VT behavior with other physical and chemical properties can generate bifunctional molecule-based materials.

Abstract

The defining characteristic of a valence tautomeric (VT) transition in a metal complex with a redox-active organic ligand is a stimulated intramolecular electron transfer between metal and ligand. Reversible VT behavior is well established for octahedral cobalt complexes with o-dioxolene ligands, with other combinations of metal centers and organic ligands also capable of exhibiting the phenomenon. Although the thermodynamic basis of VT transitions in cobalt-dioxolene systems has been understood for some time, it has not necessarily been elucidated for the less common systems. In general a number of factors additional to the redox-active metal–ligand unit influence the manifestation of the VT transition. These include the ancillary ligands, counterions, solvent molecules and intermolecular interactions, with cooperativity between VT molecules important for hysteresis and bistability. Recent advances in the field include the development of new metal–ligand combinations that can display VT transitions and the recognition that stimuli in addition to heat, light and pressure can induce the transitions. Considerable effort is also currently being directed toward assembling multiple VT moieties into polynuclear and polymeric complexes and combining VT behavior with other physical and chemical properties to generate bifunctional molecule-based materials.

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.

  1. Download : Download high-res image (124KB)
  2. Download : Download full-size image

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.

References (130)

  • E. Evangelio et al.

    C. R. Chim.

    (2008)
  • W. Kaim et al.

    Coord. Chem. Rev.

    (2010)
  • C.G. Pierpont

    Coord. Chem. Rev.

    (2001)
  • A. Dei et al.

    Inorg. Chim. Acta

    (2008)
  • S. Goswami et al.

    Inorg. Chim. Acta

    (2011)
  • P. Gütlich et al.

    Coord. Chem. Rev.

    (2005)
  • I. Markevtsev et al.

    J. Magn. Magn. Mater.

    (2006)
  • A. Cui et al.

    J. Photochem. Photobiol.

    (2004)
  • C. Carbonera et al.

    Inorg. Chim. Acta

    (2007)
  • T. Yokoyama et al.

    Chem. Phys. Lett.

    (2001)
  • O. Sato et al.

    J. Photochem. Photobiol. A: Chem.

    (2002)
  • A. Beni et al.

    Chem. Phys. Lett.

    (2006)
  • A. Bencini et al.

    J. Mol. Struct.

    (2003)
  • E. Evangelio et al.

    Solid State Sci.

    (2009)
  • F. Yu et al.

    Inorg. Chim. Acta

    (2012)
  • O. Sato et al.

    Inorg. Chim. Acta

    (2008)
  • A. Panja

    Inorg. Chem. Commun.

    (2012)
  • I. Ando et al.

    Inorg. Chim. Acta

    (2012)
  • K. Dunbar et al.
  • G.N. Newton et al.

    Eur. J. Inorg. Chem.

    (2011)
  • D.N. Hendrickson et al.

    Top. Curr. Chem.

    (2004)
  • A. Dei et al.

    Acc. Chem. Res.

    (2004)
  • E. Evangelio et al.

    Eur. J. Inorg. Chem.

    (2005)
  • O. Sato et al.

    Angew. Chem. Int. Ed.

    (2007)
  • O. Sato et al.

    Acc. Chem. Res.

    (2007)
  • A. Dei et al.

    Appl. Magn. Reson.

    (2010)
  • C. Boskovic
  • I. Ratera et al.

    Chem. Soc. Rev.

    (2012)
  • W.I. Dzik et al.

    Angew. Chem. Int. Ed.

    (2011)
  • A.F. Heyduk et al.

    Inorg. Chem.

    (2011)
  • A.L. Smith et al.

    J. Am. Chem. Soc.

    (2010)
  • O.R. Luca et al.

    Chem. Soc. Rev.

    (2013)
  • T. Wada et al.

    Angew. Chem. Int. Ed.

    (2000)
  • S. Ghosh et al.

    Angew. Chem. Int. Ed.

    (2012)
  • P. Gütlich et al.

    Angew. Chem. Int. Ed.

    (1997)
  • D.A. Shultz
  • R.M. Buchanan et al.

    J. Am. Chem. Soc.

    (1980)
  • A. Panja

    RSC Adv.

    (2013)
  • F. Renz

    J. Phys.: Conf. Ser.

    (2010)
  • D.M. Adams et al.

    Angew. Chem. Int. Ed. Engl.

    (1995)
  • C. Roux et al.

    Inorg. Chem.

    (1996)
  • A. Caneschi et al.

    Chem. Eur. J.

    (2001)
  • A.K. Zvezdin et al.
  • G. Poneti et al.

    Angew. Chem. Int. Ed.

    (2010)
  • O.-S. Jung et al.

    Inorg. Chem.

    (1997)
  • S. Bin-Salamon et al.

    J. Am. Chem. Soc.

    (2005)
  • A. Caneschi et al.

    Inorg. Chem.

    (1998)
  • A. Droghetti et al.

    Phys. Rev. Lett.

    (2011)
  • Cited by (0)

    View full text