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Inorganics
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Redox-Active Ligand Complexes
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Redox-Active Ligand Complexes

A special issue of Inorganics (ISSN 2304-6740). This special issue belongs to the section "Coordination Chemistry".

Deadline for manuscript submissions: closed (28 February 2022) | Viewed by 26990

Special Issue Editors

Prof. Dr. Kazuyuki Takahashi

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Guest Editor
Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe, Hyogo 657-8501, Japan
Interests: functional molecular materials; molecular conductors; molecular magnets; molecular dielectrics; molecular optical materials; spin-crossover; valence tautomerism; thermo- and photochromism; photoluminescence; chrage-transfer; proton-transfer; phase transition; crystal engineering
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Martin T. Lemaire

E-Mail Website
Guest Editor
Department of Chemistry, Brock University, St. Catharines, ON L2S 3A1, Canada
Interests: synthesis; redox-active ligands; stable radicals; molecule-based magnets; spin-crossover; high-spin molecules
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

A redox-active ligand is an electron donor, acceptor, or radical molecule that can coordinate to a metal ion. Complexes containing redox-active ligands possess properties derived from the redox-active ligand component as well as the metal ion, and so they are expected to exhibit a wide variety of physical properties, such as conductivity, magnetism, and dielectric and optical properties in the condensed phase. The electron transfer between a redox-active ligand and metal center induces various intriguing dynamic phenomena, such as valence tautomerism, and other electron-transfer-induced magnetic and dielectric transitions. Moreover, the recent development of electrically conducting metal–organic frameworks opens the possibility of using redox-active ligand complexes for novel practical applications (such as sensory materials, for example). As such, redox-active ligand chemistry has attracted significant attention not only within coordination chemistry research but also in materials science. This Special Issue aims to collect research and review contributions focused on recent advances in fundamentals and applications of redox-active ligand complexes. We invite you to contribute your research or review articles concerning redox-active ligand complexes, which we expect will make a great impact on the future direction of redox-active ligand chemistry.

Prof. Dr. Kazuyuki Takahashi
Prof. Dr. Martin T. Lemaire
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Inorganics is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • electron donor/acceptor ligands
  • radical ligands
  • metal coordination complexes
  • electrochemical properties
  • conducting properties
  • magnetic properties
  • dielectric properties
  • optical properties
  • electron/charge transfer
  • magnetic exchange
  • polarization
  • valence tautomerism
  • single molecule magnets
  • multifunctional materials

Published Papers (8 papers)

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Research

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15 pages, 3089 KiB  
Article
Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands
by Kazuyuki Takahashi, Takumi Noguchi, Keiji Ueda, Atsuhiro Miyawaki and Suguru Murata
Inorganics 2022, 10(6), 84; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics10060084 - 16 Jun 2022
Viewed by 1542
Abstract
To elucidate the oxidation behavior of the 2,2′-azobisphenolate (azp) ligand, a series of homoleptic 1:2 AlIII complexes of four azp derivatives (L1) with 5,5′-dichloro-, 5,5′-dimethyl-, 5,5′-di-t-butyl-, 3,3′,5,5′-tetramethyl-substituents and of one imino derivative (L2) were synthesized and obtained as TPP[Al(L)2 [...] Read more.
To elucidate the oxidation behavior of the 2,2′-azobisphenolate (azp) ligand, a series of homoleptic 1:2 AlIII complexes of four azp derivatives (L1) with 5,5′-dichloro-, 5,5′-dimethyl-, 5,5′-di-t-butyl-, 3,3′,5,5′-tetramethyl-substituents and of one imino derivative (L2) were synthesized and obtained as TPP[Al(L)2]·solvent (TPP = tetraphenylphosphonium ion). The X-ray crystal structure analyses showed that the two ONO-tridentate ligands were meridionally coordinated to a central AlIII ion in an almost perpendicular manner to give a homoleptic octahedral coordination structure in all the AlIII complexes. The proton nuclear magnetic resonance spectra suggested that all the AlIII complexes retained the homoleptic coordination structure in solution. From the cyclic voltammetry measurements in dichloromethane solutions, all the AlIII complexes with the azp ligands showed two partially reversible oxidation waves, and an additional reversible or partially reversible reduction wave. The substitution effects on the first oxidation and reduction peak potentials were revealed in the AlIII complexes with the azp ligands. On the other hand, the imino complex showed a partially reversible oxidation wave accompanying a film deposition. The density functional theory (DFT) calculations indicated that the molecular orbital (MO) coefficients of the frontier MOs in the AlIII complexes were present on the ligands and were absent on the AlIII ion. These results confirmed that the azp ligands are susceptible to oxidation and can give a relatively stable oxidation species depending upon substituent effects. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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7 pages, 1833 KiB  
Communication
Synthesis and Structure of a Ferrocenylsilane-Bridged Bisphosphine
by Takahiro Sasamori, Hiromu Ueno and Shogo Morisako
Inorganics 2022, 10(2), 22; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics10020022 - 15 Feb 2022
Viewed by 2337
Abstract
A bisferrocenylsilane-bridged bisphosphine, i.e., a bisphosphine bridged by bis(1’-dicyclohexylphosphino-1-ferrocenyl)dimethoxysilane, was synthesized and structurally characterized. Its redox behavior was examined by cyclic voltammetry and differential pulse voltammetry, which revealed two-step oxidation processes. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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10 pages, 2067 KiB  
Article
Facile Synthesis and Redox Behavior of an Overcrowded Spirogermabifluorene
by Shogo Morisako, Kohsuke Noro and Takahiro Sasamori
Inorganics 2021, 9(10), 75; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9100075 - 06 Oct 2021
Viewed by 2946
Abstract
A spirogermabifluorene that bears sterically demanding 3,3′,5,5′-tetra(t-butyl)-2,2′-biphenylene groups (1) was obtained from the reaction of in-situ-generated 2,2′-dilithiobiphenylene with GeCl2·(dioxane). The solid-state structure and the redox behavior of 1 were examined by single-crystal X-ray diffraction analysis and electrochemical [...] Read more.
A spirogermabifluorene that bears sterically demanding 3,3′,5,5′-tetra(t-butyl)-2,2′-biphenylene groups (1) was obtained from the reaction of in-situ-generated 2,2′-dilithiobiphenylene with GeCl2·(dioxane). The solid-state structure and the redox behavior of 1 were examined by single-crystal X-ray diffraction analysis and electrochemical measurements, respectively. The sterically hindered biphenyl ligands endow 1 with high redox stability and increased electron affinity. The experimental observations were corroborated by theoretical DFT calculations. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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19 pages, 6641 KiB  
Article
A Quasi-Intramolecular Solid-Phase Redox Reaction of Ammonia Ligands and Perchlorate Anion in Diamminesilver(I) Perchlorate
by Lara Alexandre Fogaça, Laura Bereczki, Vladimir M. Petruševski, Berta Barta-Holló, Fernanda Paiva Franguelli, Miklós Mohai, Kende Attila Béres, Istvan E Sajó, Imre Miklós Szilágyi and Laszlo Kotai
Inorganics 2021, 9(5), 38; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9050038 - 09 May 2021
Cited by 15 | Viewed by 3113
Abstract
The reaction of ammoniacal AgNO3 solution (or aq. solution of [Ag(NH3)2]NO3) with aq. NaClO4 resulted in [Ag(NH3)2]ClO4 (compound 1). Detailed spectroscopic (correlation analysis, IR, Raman, and UV) analyses were [...] Read more.
The reaction of ammoniacal AgNO3 solution (or aq. solution of [Ag(NH3)2]NO3) with aq. NaClO4 resulted in [Ag(NH3)2]ClO4 (compound 1). Detailed spectroscopic (correlation analysis, IR, Raman, and UV) analyses were performed on [Ag(NH3)2]ClO4. The temperature and enthalpy of phase change for compound 1 were determined to be 225.7 K and 103.04 kJ/mol, respectively. We found the thermal decomposition of [Ag(NH3)2]ClO4 involves a solid-phase quasi-intramolecular redox reaction between the perchlorate anion and ammonia ligand, resulting in lower valence chlorine oxyacid (chlorite, chlorate) components. We did not detect thermal ammonia loss during the formation of AgClO4. However, a redox reaction between the ammonia and perchlorate ion resulted in intermediates containing chlorate/chlorite, which disproportionated (either in the solid phase or in aqueous solutions after the dissolution of these decomposition intermediates in water) into AgCl and silver perchlorate. We propose that the solid phase AgCl-AgClO4 mixture eutectically melts, and the resulting AgClO4 decomposes in this melt into AgCl and O2. Thus, the final product of decomposition is AgCl, N2, and H2O. The intermediate (chlorite, chlorate) phases were identified by IR, XPS, and titrimetric methods. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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10 pages, 1355 KiB  
Article
Synthesis of a Ru(II) Complex with a Naphthoquinone-Annelated Imidazole Ligand Exhibiting Proton-Responsive Redox and Luminescent Behavior
by Takuya Shiga, Minami Tachibana, Hiroki Oshio and Masayuki Nihei
Inorganics 2021, 9(4), 24; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9040024 - 03 Apr 2021
Cited by 1 | Viewed by 2827
Abstract
A mononuclear ruthenium complex, [RuII(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 [...] Read more.
A mononuclear ruthenium complex, [RuII(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 V, −1.53 V, and −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, and two bpy/bpy•− redox couples, respectively. The redox potential of Ru(II)/Ru(III) was positively shifted upon the addition of trifluoromethanesulfonic acid due to protonation of the L− moiety, leading to stabilization of the Ru 4d orbital. In UV-vis absorption measurements for the Ru complex in acetonitrile, a metal-to-ligand charge transfer (MLCT) band was observed at 476 nm, which was shifted to 450 nm by protonation, which might be due to a decrease in the electron delocalization and stabilization of the π orbitals in L−. The blue shift of the MLCT band by protonation was associated with a shift of an emission band from 774 nm to 620 nm, which could be caused by the decreased electronic delocalization in the MLCT excited state. These electrochemical and spectroscopic changes were reversible for the protonation/deprotonation stimuli. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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10 pages, 1413 KiB  
Article
Synthesis and Physical Properties of Tetrathiafulvalene-8-Quinolinato Zinc(II) and Nickel(II) Complexes
by Keijiro Tsujimoto, Shinya Yamamoto and Hideki Fujiwara
Inorganics 2021, 9(2), 11; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9020011 - 01 Feb 2021
Cited by 2 | Viewed by 2508
Abstract
To develop donor–acceptor–donor (D–A–D) type new photo-electric conversion materials, new tetrathiafulvalene (TTF)-Mq2-TTF complexes 1 and 2 were synthesized, where two bis(n-hexylthio)tetrathiafulvalene moieties were attached to the Mq2 part (1: M = Zn, 2: M = [...] Read more.
To develop donor–acceptor–donor (D–A–D) type new photo-electric conversion materials, new tetrathiafulvalene (TTF)-Mq2-TTF complexes 1 and 2 were synthesized, where two bis(n-hexylthio)tetrathiafulvalene moieties were attached to the Mq2 part (1: M = Zn, 2: M = Ni, q = 8-quinolinato) through amide bonds. UV-Vis absorption spectra of these complexes showed strong and sharp absorption maxima at 268 nm and small absorption maxima around 410 nm, corresponding to those of Znq2 and Niq2 parts. Furthermore, complexes 1 and 2 exhibited absorption tails up to a much longer wavelength region of ca. 700 nm, suggesting the appearance of charge transfer absorption from TTF to the Mq2 parts. The photoelectrochemical measurements on the thin films of these complexes casted on ITO-coated glass substrates suggest that positive photocurrents can be generated by the photoinduced intramolecular electron transfer process between the TTF and Mq2 parts. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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Review

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28 pages, 6040 KiB  
Review
Molecular S = 2 High-Spin, S = 0 Low-Spin and S = 0 ⇄ 2 Spin-Transition/-Crossover Nickel(II)-Bis(nitroxide) Coordination Compounds
by Takayuki Ishida, Saki Ito, Yuta Homma and Yukiya Kyoden
Inorganics 2021, 9(2), 10; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics9020010 - 20 Jan 2021
Cited by 8 | Viewed by 3925
Abstract
Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a [...] Read more.
Heterospin systems have a great advantage in frontier orbital engineering since they utilize a wide diversity of paramagnetic chromophores and almost infinite combinations and mutual geometries. Strong exchange couplings are expected in 3d–2p heterospin compounds, where the nitroxide (aminoxyl) oxygen atom has a direct coordination bond with a nickel(II) ion. Complex formation of nickel(II) salts and tert-butyl 2-pyridyl nitroxides afforded a discrete 2p–3d–2p triad. Ferromagnetic coupling is favored when the magnetic orbitals, nickel(II) dσ and radical π*, are arranged in a strictly orthogonal fashion, namely, a planar coordination structure is characterized. In contrast, a severe twist around the coordination bond gives an orbital overlap, resulting in antiferromagnetic coupling. Non-chelatable nitroxide ligands are available for highly twisted and practically diamagnetic complexes. Here, the Ni–O–N–Csp2 torsion (dihedral) angle is supposed to be a useful metric to describe the nickel ion dislocated out of the radical π* nodal plane. Spin-transition complexes exhibited a planar coordination structure in a high-temperature phase and a nonplanar structure in a low-temperature phase. The gradual spin transition is described as a spin equilibrium obeying the van’t Hoff law. Density functional theory calculation indicates that the energy level crossing of the high- and low-spin states. The optimized structures of diamagnetic and high-spin states well agreed with the experimental large and small torsions, respectively. The novel mechanism of the present spin transition lies in the ferro-/antiferromagnetic coupling switch. The entropy-driven mechanism is plausible after combining the results of the related copper(II)-nitroxide compounds. Attention must be paid to the coupling parameter J as a variable of temperature in the magnetic analysis of such spin-transition materials. For future work, the exchange coupling may be tuned by chemical modification and external stimulus, because it has been clarified that the parameter is sensitive to the coordination structure and actually varies from 2J/kB = +400 K to −1400 K. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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27 pages, 9605 KiB  
Review
Prototype Material for New Strategy of Photon Energy Storage
by Toshio Naito
Inorganics 2020, 8(10), 53; https://0-doi-org.brum.beds.ac.uk/10.3390/inorganics8100053 - 25 Sep 2020
Cited by 5 | Viewed by 3125
Abstract
The smart utilization of photons is paid global attention from the viewpoint of renewable energy and information technology. However, it is still impossible to store photons as batteries and condensers do for electrons. All the present technologies utilize (the energy of) photons in [...] Read more.
The smart utilization of photons is paid global attention from the viewpoint of renewable energy and information technology. However, it is still impossible to store photons as batteries and condensers do for electrons. All the present technologies utilize (the energy of) photons in situ, such as solar panels, or in spontaneous relaxation processes, such as photoluminescence. If we can store the energy of photons over an arbitrary period and utilize them on demand, not only we will make an innovative progress in energy management, but we will also be able to replace a part of electrons by photons in the information technology for more efficient performance. In this article, we review a prototype of such a material including the current status of related research as well as where we are heading for. Full article
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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