Synthesis, Electronic Structure, and Structural Characterization of the New, “Non-Innocent” 4,5-Dithio-Catecholate Ligand, Its Metal Complexes, and Their Oxidized 4,5-Dithio-o-quinone Derivatives

Proton-electron-coupled functionalities of conductivity, magnetism, and optical properties in molecular crystals

Proton-electron-coupled reactions, specifically proton-coupled electron transfer (PCET), in biological and chemical processes have been extensively investigated for use in a wide variety of applications, including energy conversion and storage. However, the exploration of the functionalities of the conductivity, magnetism, and dielectrics by proton-electron coupling in molecular materials is challenging. Dynamic and static proton-electron-coupled functionalities are to be expected. This feature article highlights the recent progress in the development of functionalities of dynamic proton-electron coupling in molecular materials. Herein, single-unit conductivity by self-doping, quantum spin liquid state coupled with quantum fluctuation of protons, switching of conductivity and magnetism triggered by the disorder-order transition of deuterons, and their external responses under pressure and in the presence of an electric field are introduced. In addition, as for the functionalities of proton-d/π-electron coupling in metal dithiolene complexes, magnetic switching with multiple PCET and vapochromism induced by electron transfer through hydrogen-bond (H-bond) formation is introduced experimentally and theoretically. We also outlined the basic and applied issues and potential challenges for development of proton-electron-coupled molecular materials, functionalities, and devices.

Proton and Electron Transfer in the Formation of a Copper Dithiolene-Based Coordination Polymer

Metal bis(dithiolene) complexes are promising building blocks for electrically conductive coordination polymers. N-Heterocyclic dithiolene complexes allow their cross-linking via the coordination of N-donor atoms to additional transition metal ions. In this study, we present the formal copper(II) and copper(III) 6,7-quinoxalinedithiolene complexes [Cu(qdt)₂]^- and [Cu(qdt)₂]^2- (qdt^2-: 6,7-quinoxalinedithiolate), as well as the 2D coordination polymer Cu[Cu(Hqdt)(qdt)] (3). The dithiolene complexes were isolated as (Bu₄N)₂[Cu(qdt)₂] (1), Na[Cu(qdt)₂]·4H₂O (2a), [Na(acetone)₄][Cu(qdt)₂] (2b), and [Ni(MeOH)₆][Cu(qdt)₂]₂·2H₂O (2c). Their crystal structures reveal nearly planar complexes with a high tendency of π-stacking. For a better understanding of their coordination behavior, the electronic properties are investigated by UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT simulations. The synthesis of the 2D coordination polymer 3 involves the reduction and protonation of the monoanionic copper(III) complex. A combination of powder X-ray diffraction, magnetic susceptibility measurements, as well as IR and EPR spectroscopy confirm that formal [Cu^II(Hqdt)(qdt)]^- units link trigonal planar copper(I) atoms to a dense 2D coordination polymer. The electrical conductivity of 3 at room temperature is 2 × 10^-7 S/cm. Temperature dependent conductivity measurements confirm the semiconducting behavior of 3 with an Arrhenius derived activation energy of 0.33 eV. The strong absorption of 3 in the visible and NIR regions of the spectrum is caused by the small optical band gap of E_g,opt = 0.65 eV, determined by diffuse reflectance spectroscopy. This study sheds light on the coordination chemistry of N-heterocyclic dithiolene complexes and may serve as a reference for the future design and synthesis of dithiolene-based coordination polymers with interesting electrical and magnetic properties.

Thiosemiquinoid Radical-Bridged CrIII2 Complexes with Strong Magnetic Exchange Coupling

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)₂Cr₂(^RL^4-)]²⁺ (^OLH₄ = 1,2,4,5-tetrahydroxybenzene, ^OSLH₄ = 1,2-dithio-4,5-dihydroxybenzene, ^SLH₄ = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between Cr^III centers in these complexes, with exchange constants of J = -2.83(3) (^OL^4-), -2.28(5) (^OSL^4-), and -1.80(2) (^SL^4-) cm^-1. Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)₂Cr₂(^RL^3-•)]³⁺. Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) (^OL^3-•), - 401(8) (^OSL^3-•), and -487(8) (^SL^3-•) cm^-1. These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

Construction of three-dimensional anionic molecular frameworks based on hydrogen-bonded metal dithiolene complexes and the crystal solvent effect

Three-dimensional hydrogen-bonded anionic molecular frameworks based on a metal dithiolene complex were constructed with a significant solvent effect.

HOMO stabilisation in π-extended dibenzotetrathiafulvalene derivatives for their application in organic field-effect transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB-TTF) central core and a 2,1,3-chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB-TTF, have been synthesised as active materials for organic field-effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron-withdrawing 2,1,3-chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution-processed single-crystal transistors exhibit high performance with a hole mobility up to 0.04 cm(2)  V(-1)  s(-1) as well as good ambient stability.

Sterically hindered o-quinone annulated with dithiete: a molecule comprising diolate and dithiolate coordination sites

A new stable sterically hindered o-quinone annelated with a 1,2-dithiete ring was prepared by using mild conditions. The skeleton of the compound comprises diolate and dithiolate functions that have the potential to bind metals leading to the corresponding complexes. The reactivity of this compound as a ligand with respect to both coordination sites was studied. Reactions with metals indicate that the o-quinone function is reduced in the first stage to give semiquinonate and catecholate complexes. The dithiolate coordination site was involved in the reaction in a few cases only after diolate was bound. A trinuclear manganese complex with coordination on both sites was obtained and characterized by EPR spectroscopy. The electrochemical study of this quinone fused with dithiete is reported.