Valence Tautomerism: New Challenges for Electroactive Ligands

Abstract

Valence tautomeric complexes combine redox-active ligands and transition metal ions with two or more accessible oxidation states, exhibiting two nearly degenerated electronic states with localized electronic structures. Charge distribution in such electronic isomers has an appreciable sensitivity to the environment so an external perturbation, like photons, temperature and/or pressure, may lead to an intramolecular electron transfer between both redox active units and therefore to a reversible interconversion between the two degenerated electronic states. Moreover, since each electronic isomer exhibits different optical, electronic and/or magnetic properties, these complexes are being proposed as candi-

A Valence Tautomeric Dinuclear Copper Tetrakisguanidine Complex

“…In the past, several transition-metal complexes with, for example, odioxolene ligands have demonstrated valence tautomerism. [2][3][4][5][6] The electronic structure is often influenced by a number of parameters, such as temperature, pressure, modifications of the coligands, the environment (solid state or solution), the solvent, and for charged complexes, even the counterions. [7][8][9][10] Most studied valence tautomeric transition-metal complexes are based on cobalt, [11] for which the change from the Co II high-spin to Co III low-spin state is associated with a relative high barrier.…”

Valence‐Tautomeric Metal–Organic Nanoparticles

“…[9,10] Inspired by this last approach, here we show how using the appropriate multitopic organic ligand makes it possible to structure well-known functional building blocks in the form of spherical particles. Polymerization of electroactive {Co(3,5-dbsq)(3,5-dbcat)} units, [14] where 3,5-dbsq À and 3,5-dbcat 2À are respectively the semiquinonate radical and catecholate forms of 3,5-di-tert-butyl-1,2-benzoquinone (dbq), with the ligand 1,4-bis(imidazol-1-ylmethyl)benzene (bix) [15,16] Since each electronic isomer has a different magnetic moment and optical properties, these complexes are candidates for use in future molecular electronic devices and switches, [17,18] and therefore serve as an excellent model system to create the first functional NMOPs that exhibit valence tautomerism.In a typical experiment, an aqueous solution of Co(CH 3 COO) 2 ·4 H 2 O was added to an ethanolic solution of 3,5-di-tert-butyl-1,2-catechol and bix with vigorous stirring at room temperature, and a very intense blue color developed. Blue nanoparticles were precipitated by fast addition of water to the mixture.…”

“…Increasing the temperature induces a gradual increase of the magnetic moment up to a c M T = 0.6 emu K mol À1 at 275 K. Then, a further increase of the temperature induces an abrupt transition over a narrow temperature range of 40-50 K up to a maximum value of c M T = 2.6 emu K mol À1 at 340 K. Such a c M T value is in agreement with conversion to the high-spin isomer hs-[Co II -(3,5-dbsq) 2 ] bearing one S = 3/2 (catechol) and two S = 1/2 (semiquinone) independent units. [15] The magnetic properties of the nanoparticles were directly measured on an aqueous colloidal suspension (76 AE 9 nm in diameter) located in a sealed tube to ensure that the magnetic properties correspond to the behavior of dispersed nanoparticles (Figure 4). The stability of the nanospheres during the magnetic measurements was confirmed by SEM before and after the measurements.…”

Optically induced valence tautomeric interconversion in cobalt dioxolene complexes

“…They are some dithienylethylene derivatives, 8,9 tetracyanoethylene organometallic compounds, 10 prussian blue analogues, 11,12 spin-crossover 13,14 and valence tautomeric complexes. [15][16][17][18][19][20][21][22] As a general paradigm, all of these systems characterized by a spin ground state S and a orbital momentum L, can be switched in their respective metastable states with spin state S' and orbital momentum L' upon optical irradiation. The scheme of the interconversion process is shown in Figure 1.…”

“…[15][16][17][18][19][20][21][22] Among all the metal dioxolene complexes undergoing valence tautomerism, cobalt derivatives are undoubtedly the most attractive. In all of the reported examples the interconversion involves an intramolecular electron transfer from a coordinated dioxolene ligand to a six-coordinate diamagnetic, low spin (ls) Co III (t 2g 6 (O h )), yielding a Co II coordinated to the one electron oxidized dioxolene species (Scheme 1).…”