Spin Delocalization in the Copper(I) Complexes of Bis(verdazyl) Diradicals

Stable organic radicals - a material platform for developing molecular quantum technologies

An electron spin is a natural candidate for a quantum bit - the quantum information storage unit. Stable organic radicals, consisting of unpaired electron spins, can thus be explored for the development of quantum science and technologies, owing to their excellent chemical tunability and their great promise for scalability. The molecular network formed by the stable organic radicals can be used for the design of spin-based quantum computing circuits. Here the state-of-the-art development of stable organic radicals is reviewed from a variety of perspectives. The categories of stable organic radicals are discussed, emphasizing on the π-conjugated radical networks. The applications of the stable organic radicals to quantum communications, quantum computing and quantum sensing are reviewed. The quantum teleportation based on the donor-acceptor-radical molecular system is reviewed. For controllable quantum gate operations, the spin dynamics in a bi-radical molecule driven by a triplet is discussed. Quantum sensing of lithium ions using stable organic radicals is realized for the development of new energy materials. Quantum timing and quantum imaging are still unexplored by using stable organic radicals. In conclusion, stable organic radicals, especially the π-conjugated radical networks, can make a great new contribution to the development of quantum technologies.

Cyclic [Cu-biRadical]2 Secondary Building Unit in 2p-3d and 2p-3d-4f Complexes: Crystal Structure and Magnetic Properties

Employing the new nitronyl nitroxide biradical ligand biNIT-3Py-5-Ph (2-(5-phenyl-3-pyridyl)-bis(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)), a 16-spin Cu-radical complex, [Cu8(biNIT-3Py-5-Ph)4(hfac)16] 1, and three 2p-3d-4f chain complexes, {[Ln(hfac)3][Cu(hfac)2]2(biNIT-3Py-5-Ph)2}n (LnⅢ= Gd 2, Tb 3, Dy 4; hfac = hexafluoroacetylacetonate), have been prepared and characterized. X-ray crystallographic analysis revealed in all derivatives a common cyclic [Cu-biNIT]2 secondary building unit in which two bi-NIT-3Py-5-Ph biradical ligands and two CuII ions are associated via the pyridine N atoms and NO units. For complex 1, two such units assemble with four additional CuII ions to form a discrete complex involving 16 S = 1/2 spin centers. For complexes 2–4, the [Cu-biNIT]2 units are linked by LnIII ions via NO groups in a 1D coordination polymer. Magnetic studies show that the coordination of the aminoxyl groups with Cu or Ln ions results in behaviors combining ferromagnetic and antiferromagnetic interactions. No slow magnetic relaxation behavior was observed for Tb and Dy derivatives.

A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox-Active Ligands

Two homoleptic copper(II) complexes [Cu(L1)2 ] and [Cu(L2)2 ] with anionic redox-active ligands were synthesised, one with urea azine (L1) and the other with thio-urea azine (L2) ligands. One-electron oxidation of the complexes initiates an unprecedented redox-induced electron transfer process, leading to monocationic copper(I) complexes [Cu(L1)2 ]+ and [Cu(L2)2 ]+ with two oxidised ligands. While [Cu(L1)2 ]+ is best described as a CuI complex with two neutral radical ligands that couple antiferromagnetically, [Cu(L2)2 ]+ is a CuI complex with two clearly different ligand units in the solid state and with a magnetic susceptibility close to a diamagnetic compound. Further one-electron oxidation of the complex with L1 ligands results in a dication [Cu(L1)2 ]2+ , best described as a CuI complex with a twofold oxidised, monocationic ligand and a neutral radical ligand. The stability in at least three redox states, the accumulation of spin density at the ligands and the facile ligand-metal electron transfer make these complexes highly attractive for a variety of applications; here the catalytic aerobic oxidation of alcohols to aldehydes is tested.

Syntheses, structures and magnetic properties of the lanthanide complexes of the pyrimidyl-substituted nitronyl nitroxide radical

Four 2p-4f Ln^III-radical complexes, [(NIT-2-Pm)Ln(hfac)₃]·0.5C₇H₁₆ (NIT-2-Pm = 2-pyrimidyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide, hfac = hexafluoroacetylacetonato, Ln = Tb (1), Dy (2), Ho (3) and Er (4)), and four 4f-2p-4f Ln^III-radical-Ln^III complexes, [(μ-NIT-2-Pm)Ln₂(hfac)₆(H₂O)₂]·0.5C₇H₁₆ (Ln = Tb (5), Dy (6), Ho (7) and Er (8)) have been synthesized and characterized structurally and magnetically. These compounds can be selectively obtained by controlling the reaction ratio of Ln(hfac)₃·2H₂O to the radical ligand NIT-2-Pm. The crystal structures show that in the former four complexes 1-4, the NIT-2-Pm radical acts as a terminal bidentate ligand chelating to one Ln^III ion, while in 5-8, the NIT-2-Pm acts as a bridging ligand linking two Ln^III ions to form a binuclear three-spin system. Magnetic studies revealed that complexes 1-4 and 6 show frequency-dependent ac magnetic susceptibilities, suggesting a possible single-molecule magnet behavior. To the best of our knowledge, complexes 3 and 4 are the first Ho-NIT and Er-NIT compounds showing slow magnetic relaxation. Compounds 5-8 represent a rare family of compounds showing the NIT bridged 4f-2p-4f three-spin motif, while complex 6 is a rare NIT bridged multinuclear lanthanide compound possessing SMM-like behaviour. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these compounds suggests weak antiferromagnetic coupling between the Ln^III and NIT radical in 1-8 and weak ferromagnetic Ln^III-Ln^III interactions in 5-8.

2p–3d–4f hetero-tri-spin molecule-based magnetic compounds

The most up-to-date advances in the design and synthesis of molecular magnetic materials by means of a 2p–3d–4f strategy are reviewed.

Azo anion radical complex of rhodium as a molecular memory switching device: isolation, characterization, and evaluation of current-voltage characteristics

Two rare examples of azo anion diradical complexes of Rh(III) are reported. These complexes showed excellent memory switching properties with a large ON/OFF ratio and are suitable for RAM/ROM applications. Their electronic structures have been elucidated using a host of physical methods, including X-ray crystallography, variable-temperature magnetic susceptibility measurement, cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory. The results indicate a predominant triplet state description of the systems with two ferromagnetically coupled radicals.

Playing with organic radicals as building blocks for functional molecular materials

The literature has shown numerous contributions on the synthesis and physicochemical properties of persistent organic radicals but there are a lesser number of reports about their use as building blocks for obtaining molecular magnetic materials exhibiting an additional and useful physical property or function. These materials show promise for applications in spintronics as well as bistable memory devices and sensing materials. This critical review provides an up-to-date survey to this new generation of multifunctional magnetic materials. For this, a detailed revision of the most common families of persistent organic radicals-nitroxide, triphenylmethyl, verdazyl, phenalenyl, and dithiadiazolyl-so far reported will be presented, classified into three different sections: materials with magnetic, conducting and optical properties. An additional section reporting switchable materials based on these radicals is presented (257 references).

Strong ferromagnetic metal-ligand exchange in a nickel bis(3,5-dipyridylverdazyl) complex

A new 1,5-dipyridyl verdazyl, synthesized from the corresponding dipyridyl hydrazone, coordinates nickel(ii) to form a structurally characterized, pseudooctahedral complex analogous to Ni(terpy)(2)(2+). The unusually short Ni-verdazyl distance results in strong ferromagnetic exchange (J(Ni-rad) = +300, J(rad-rad) = +160 cm(-1)) between all three paramagnetic species along with a metal-ligand charge transfer band in the electronic spectrum.

Mn(II) and Cu(II) Complexes of a Dithiadiazolyl Radical Ligand: Monomer/Dimer Equilibria in Solution

Complexes of the 4-(2'-pyridyl)-1,2,3,5-dithiadiazolyl radical bidentate ligand with bis(hexafluoroacetylacetonato)manganese(II) and with bis(hexafluoroacetylacetonato)copper(II) have been prepared. Unlike the previously reported cobalt(II) complex, these complexes form dimers via intermolecular S...S contacts in the solid state. The spectroscopic and magnetic properties of these species in the solid state and in solution are reported and compared to the previously reported Co(II) complex, with emphasis on the elucidation of the a monomer/dimer equilibrium in the solution. The electrochemical properties of these species in solution are also presented and discussed.

The 2003 CSC Pure or Applied Inorganic Chemistry Award Lecture Adventures in stable radical chemistry

Stable radicals are of intense fundamental interest because they challenge conventional bonding paradigms, and they find a wide range of uses ranging from organic and polymer synthesis to biological and medicinal applications to materials science. Yet the directed synthesis and study of stable radicals for either fundamental or applied purposes are rarely pursued strategies. This Award Lecture describes my research group's efforts in exploratory and targeted research focusing on stable radical design.Key words: stable radicals, verdazyls, molecular magnetism.

Synthesis of Multitopic Verdazyl Radical Ligands. Paramagnetic Supramolecular Synthons

[structure: see text] The syntheses of several verdazyl radical and diradicals containing pyridine-based multitopic coordination sites are described. These compounds were designed to be paramagnetic analogues of oligopyridine metallosupramolecular building blocks.

Molecular Paramagnetic Semiconductor: Crystal Structures and Magnetic and Conducting Properties of the Ni(dmit)2 Salts of 6-Oxoverdazyl Radical Cations (dmit = 1,3-Dithiol-2-thione-4,5-dithiolate)

Four kinds of 1:1 and 1:3 salts of 3-[4-(trimethylammonio)phenyl]-1,5-diphenyl-6-oxoverdazyl radical cation ([1](+)) and its mono- and dimethyl derivatives ([2](+) and [3](+)) with Ni(dmit)(2) anions (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) ([1](+)[Ni(dmit)(2)](-) (4), [2](+)[Ni(dmit)(2)](-) (5), [3](+)[Ni(dmit)(2)](-) (6), and [1](+)[Ni(dmit)(2)](3)(-) (7)) have been prepared, and the magnetic susceptibilities (chi(M)) have been measured between 1.8 and 300 K. The chi(M) values of salts 5 and 7 can be well reproduced by the sum of the contributions from (i). a Curie-Weiss system with a Curie constant of 0.376 (K emu)/mol and negative Weiss constants (THETAV;) of -0.4 and -1.7 K and (ii). a dimer system with strong negative exchange interactions of 2J/k(B) = -354 and -258 K, respectively. The dimer formations in Ni(dmit)(2) anions have been ascertained by the crystal structure analyses of salts 4-6. In salts 4 and 6, Ni(dmit)(2) dimer molecules are sandwiched between two verdazyl cations, indicating the formation of a linear tetramer in 4 and 6. The magnetic susceptibility data for salts 4 and 6 have been fitted to a linear tetramer model using an end exchange interaction of 2J(1)/k(B) = -600 K and a central interaction of 2J(2)/k(B) = -280 K for 4 and 2J(1)/k(B) = -30 K and 2J(2)/k(B) = -580 K for 6, respectively. The results of the temperature dependence of the g(T) value in salts 4-6 obtained by ESR measurement also support the above analyses. The 1:1 salts 4-6 are insulators. On the other hand, the conductivity of the 1:3 salt 7 at 20 degrees C was sigma = 0.10 S cm(-)(1) with an activation energy E(A) = 0.099 eV, showing the semiconductor property. Salt 7 is a new molecular paramagnetic semiconductor.

Verdazyl radicals as oligopyridine mimics: structures and magnetic properties of M(II) complexes of 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (M = Mn, Ni, Cu, Zn)

The verdazyl radical 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (3) was prepared, and its homoleptic metal complexes M(3)(2)(2+).2X(-) (5, M = Mn(II); 6, M = Ni(II); 7, M = Cu(II); 8, M = Zn(II); X = ClO(4), PF(6)) were characterized by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Relevant crystallographic parameters are as follows: 5, monoclinic space group Pna2(1), a = 18.755(4) A, b = 11.154(3) A, c = 16.594(4) A, alpha = 90.00 degrees, beta = 90.00 degrees, gamma = 90.00 degrees, V = 3471.4(13) A(3), and Z = 4; 7, triclinic space group Ponedblac;, a = 9.4638(18) A, b = 9.8442(19) A, c = 18.769(4) A, alpha = 103.746(3) degrees, beta = 92.925(3) degrees, gamma = 94.869(3) degrees, V = 1687.8(6) A(3), and Z = 2; 8, triclinic space group Ponedblac;, a = 9.4858(14) A, b = 9.7919(14) A, c = 18.889(3) A, alpha = 104.196(3) degrees, beta = 92.855(3) degrees, gamma = 94.216(3) degrees, V = 1692.1(4) A(3), and Z = 2. In all cases, the two verdazyl-based ligands bind almost perpendicular to each other in meridional positions, yielding pseudooctahedral metal complexes whose general structural features are strongly reminiscent of metal bis(terpyridine) complexes. The intramolecular metal-verdazyl magnetic exchange coupling is strongly ferromagnetic in 6 (J(Ni-vd) = +240 cm(-1)), and strongly antiferromagnetic in 5 (J(Mn-vd) = -93 cm(-1)). Complex 7 exhibits weak ferromagnetic coupling (J(Cu-vd) = -4.5 cm(-1)). Intramolecular radical-radical coupling in the zinc complex 8 was found to be weakly antiferromagnetic (J(vd-vd) = -8 cm(-1)). Intramolecular radical-radical exchange was generally weak in the four metal complexes, ranging from -10 cm(-1) (for 5) to +2 cm(-1) (for 7). The low-temperature magnetic behavior of 7 and 8 is complex, possibly arising from a combination of intra- and intermolecular interactions.