Valence Tautomerism in D‐Block Complexes

Abstract

The need for both improved energy efficiency of information communication technologies, and increased data storage density to meet the requirements of contemporary society, has enhanced interest in the miniaturization of electronic devices. The ultimate in miniaturization is a single molecule and the possibility of utilizing individual molecules as device components has afforded the scientific field of molecular materials. So‐called “smart” materials respond to an external stimulus and such stimuli‐response can be achieved at the molecular level. Molecular materials that can be switched between forms with different electronic structures in response to an external stimulus offer the possibility of controlled switching between species with different physical and electronic properties, including most obviously, color. The phenomenon of valence tautomerism (VT) involves stimulated intramolecular electron transfer between a redox‐active metal and ligand, sometimes accompanied by a spin transition at the metal center. While best known for cobalt–dioxolene systems, other redox‐active metal and ligand combinations can also display VT. Known since 1980, in recent times, the field of VT has expanded in a range of directions, spanning from enhancing understanding of the fundamental science and development of new examples, to incorporation of VT species into technologically relevant media, including nanoparticles, thin films, and deposition on surfaces. Particularly exciting are developments in the use of density functional theory calculations, which now allow for accurate evaluation of the likelihood and characteristics of potential VT transitions in yet‐to‐be synthesized compounds. This article presents the fundamental aspects of valence tautomerism in d‐block complexes before comprehensibly describing recent developments and possible future implications.

A Convenient DFT-Based Strategy for Predicting Transition Temperatures of Valence Tautomeric Molecular Switches

“…In contrast, application of DFT to Fe(II) SCO has seen greater development, with several reported efforts to computationally determine whether SCO will occur. 42−48 In a landmark study, Brooker et al developed a simple method to determine transition temperatures of Fe(II) SCO complexes with azinesubstituted 1,2,4-triazole ligands, based on a correlation of the solution T 1/2 values with observed and DFT calculated 15 N NMR chemical shift values for ligand resonances. 49 It is desirable from both environmental and cost standpoints that computational chemistry becomes an essential tool for screening candidate compounds for a targeted property prior to synthesis and experimental investigation.…”

“…Metal complexes that interconvert between low spin (LS) and high spin (HS) state in response to a stimulus, for example, upon heating and cooling, have provided numerous examples of switchable spin crossover (SCO) systems. , Another class of switchable molecules that offers diverse possibilities for applications are those that exhibit valence tautomerism (VT), involving stimulated intramolecular electron transfer between a redox-active metal and coordinated ligand. − Valence tautomeric interconversions can be triggered by stimuli such as temperature, light, magnetic fields, or pressure, and VT has been reported for complexes of various metals, including transition, main group, and lanthanoid-metals. The range of metal–ligand combinations and the intrinsic redox-activity suggests avenues for multifunctional materials that are not available with SCO compounds.…”

“…Cobalt complexes incoporating dioxolene (diox) ligands in the catecholate (cat 2– ) or semiquinonate (sq •– ) oxidation states provide the most numerous examples of VT. − The first family of such complexes to be studied were the [Co­(dbdiox)­(dbsq)­(N 2 L)] (dbdiox/dbsq •– = 3,5-di- tert -butyldioxolene/semiquinonate; N 2 L = diimine; Chart ) series, with thermal VT reported for the 2,2′-bipyridyl (bpy) analogue by Buchanan and Pierpont in 1980 . Valence tautomerism in these complexes involves a concerted spin transition and thus interconversion between the LS [Co­(dbcat)­(dbsq)­(N 2 L)] tautomer at low temperature and the HS [Co­(dbsq) 2 (N 2 L)] tautomer at high temperature.…”

“…Density functional theory (DFT) , has become an invaluable tool for chemists to study the structure and properties of molecules. Since the first application of DFT to the [Co­(dbdiox)­(dbsq)­(N 2 L)] family of VT complexes in 1997, DFT approaches have been employed on a number of occasions to rationalize experimental observations. ,,− However, VT systems are challenging to model using DFT methods: they are open-shell species, highly functional dependent, and involve a change in spin state. − As a result, accurate DFT analysis of VT is nontrivial. Recently, DFT methods have also been employed to predict candidate VT compounds, which were subsequently verified to exhibit VT experimentally, including by some of us. − In other cases, DFT predictions are yet to be confirmed via experiments.…”

Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism

“…In certain circumstances, such complexes can be stimulated to switch between states through intramolecular metal−ligand charge transfer in a process known as valence tautomerism (VT). 16,19 As a consequence of the complex interplay between metal ion and ligand redox states, valence tautomeric complexes offer fascinating potential for redox (electrical) control of materials properties when integrated into devices. 9,20−23 Cobalt complexes have proven capable of exhibiting either SCO or VT in the presence of a suitable ligand environment.…”