Jeremy M. Rawson

Thiazyl radicals: old materials for new molecular devices

This review describes recent advances in the development of thiazyl radicals, particularly dithiadiazolyl (RCNSSN) and dithiazolyl (RCSNSCR) as molecule-based materials. Particular emphasis is placed on their potential applications as molecular conductors, magnets and switchable materials and incorporates an overview of the structure–property relationships required in these molecular systems.

The coordination chemistry of 2-pyridone and its derivatives

Abstract This review covers the coordination chemistry of 2-pyridone (2-hydroxypyridine) and its derivatives from 1968 to 1993. The derivatives studied have chiefly been 2-pyridone itself and those substituted in the 6-position of the ring, in particular 6-methyl-2-pyridone and 6-chloro-2-pyridone. These ligands have found their major usage as 1,3-bridging ligands akin to carboxylates. A large number of dimeric complexes have been synthesized with elements including Cr, Mo, W, Tc, Re, Ru, Os, Rh, Ir, Pd, Pt and Cu, and in which the metal-metal bond order varies from four to zero. The structural chemistry of these dimeric complexes is discussed, as are physical studies designed to increase the understanding of metal-metal bonding. Larger polymetallic arrays can be made with first-row metals such as chromium, cobalt and copper. Work on platinum complexes of 2-pyridone has been important in modelling the interaction of cis -[Pt(NH 3 ) 2 CI 2 ] with uracil nucleobases; this work is also reviewed. Dimeric rhodium(I) and iridium(I) complexes display interesting photochemistry, and this work is described. A recent development is the use of 2-pyridones as bridging ligands in heterometallic assemblies, especially of copper and lanthanoids and Group 2 metals.

C–H Bond Activation by Radical Ion Pairs Derived from R3P/Al(C6F5)3 Frustrated Lewis Pairs and N2O

Al(C6F5)3/R3P [R = tert-butyl (tBu), mesityl (Mes), naphthyl (Nap)] frustrated Lewis pairs react with N2O to form species having the formula R3P(N2O)Al(C6F5)3, which react with additional alane to generate proposed frustrated radical ion pairs formulated as [R3P·][(μ-O·)(Al(C6F5)3)2] that can activate C-H bonds. For R = tBu, C-H activation of a tBu group affords [tBu2PMe(C(CH2)Me)][(μ-OH)(Al(C6F5)3)2]. In the case of R = Mes, the radical cation salt [Mes3P·][(μ-HO)(Al(C6F5)3)2] is isolated, while for R = Nap, the activation of toluene and bromobenzene gives [(Nap)3PCH2Ph][(μ-OH)(Al(C6F5)3)2] and [(Nap)3PC6H4Br][(μ-HO)(Al(C6F5)3)2], respectively.

Benzo-fused dithiazolyl radicals: from chemical curiosities to materials chemistry

Abstract Synthetic routes to benzo-fused 1,2,3- and 1,3,2-dithiazolylium salts are described. Their chemical stabilities and physical properties, especially their redox behaviour, are discussed. One electron reduction yields the corresponding benzo fused 1,2,3- and 1,3,2-dithiazolyl radicals. The electronic properties of the dithiazolyl ring are compared with a series of isoelectronic sulphur–nitrogen rings including dithiadiazolyl and trithiadiazolylium radicals. The ability to tune the redox behaviour of the dithiazolyl ring by changing the substituents, coupled with the lower dimerisation energy afforded by delocalisation of the unpaired electron makes these molecules attractive building blocks for the construction of molecular conductors and magnets. Recent results in this area are summarised.

Hydrothermal synthesis and magnetic properties of novel Mn(II) and Zn(II) materials with thiolato-carboxylate donor ligand frameworks

The hydrothermal reaction of thiosalicylic acid, (C(6)H(4)(CO(2)H)(SH)-1,2) with manganese(III) acetate leads to formation of the coordination solid [Mn(5)((C(6)H(4)(CO(2))(S)-1,2)(2))(4)(mu3-OH)2] (1) via a redox reaction, where resulting manganese(II) centres are coordinated by oxygen donor atoms and S-S disulfide bridge formation is simultaneously observed. Reaction of the same ligand under similar conditions with zinc(II) chloride yields the layered coordination solid [Zn(C(6)H(4)(CO(2))(S)-1,2)] (2). Hydrothermal treatment of manganese(III) acetate with 2-mercaptonicotinic acid, (NC(5)H(3)(SH)(CO(2)H)-2,3) was found to produce the 1-dimensional chain structure [Mn(2)((NC(5)H(3)(S)(CO(2))-2,3)(2))(2)(OH(2))(4)].4H(2)O (3) which also exhibits disulfide bridge formation and oxygen-only metal interactions. Compound 3 has been studied by thermogravimetric analysis and indicates sequential loss of lattice and coordinated water, prior to more comprehensive ligand fragmentation at elevated temperatures. The magnetic behaviour of 1 and 3 has been investigated and both exhibit antiferromagnetic interactions. The magnetic behaviour of 1 has been modelled as two corner-sharing isosceles triangles whilst 3 has been modelled as a 1-dimensional chain.

Catalytic dehydrocoupling of Me2NHBH3 with Al(NMe2)3.

The catalytic dehydrocoupling reaction of Me(2)NHBH(3) with Al(NMe(2))(3) gives the dimer [Me(2)NBH(2)](2) and the chain [(Me(2)N)(2)BH], involving the thermally-stable Al(III) hydride catalyst [{(Me(2)N)(2)BH(2)}(2)AlH].

Magnetic Properties of Thiazyl Radicals

A series of thiazyl radicals related to the trithiadiazolylium radical cation, S3N 2 +• are described. In many instances the compounds exist as spin-paired singlets which are consequently diamagnetic. However, when the dimerisation process can be inhibited, these open shell molecules exhibit very strong exchange interactions, characterised by Weiss constants, θ, up to 102 K. Magneto-structural correlations show that, in the majority of instances, the magnetic exchange interactions are propagated via close intermolecular S···N and S···S contacts (typically in the region 3.1–3.7 A). Examples of thiazyl radicals exhibiting magnetic ordering temperatures in excess of 50 K are described. In addition, the phenomenon of bistability (in which both open-shell monomeric and closed-shell dimeric forms are stable over the same temperature range) is discussed and examples of thiazyl radicals exhibiting bistability up to room temperature described.

Characterization and Magnetic Properties of a "Super Stable" Radical 1,3-Diphenyl-7-trifluoromethyl-1,4-dihydro-1,2,4-benzotriazin-4-yl

1,3-Diphenyl-7-trifluoromethyl-1,4-dihydro-1,2,4-benzotriazin-4-yl (4), prepared in high yield via the catalytic oxidation of the corresponding amidrazone 5 by using Pd/C (1.6 mol %) and 1,8-diazabicyclo[5.4.0]undec-7-ene (0.1 equiv) in air, is stable in dichloromethane solutions in the presence of MnO(2) and KMnO(4). Furthermore, radical 4 is thermally stable well past its melting point (160-161 °C) with a decomposition onset temperature of 288 °C. X-ray studies show that radical 4 packs in equidistant slipped π-stacks along the a axis. Cyclic voltammetry shows two fully reversible waves, corresponding to the -1/0, 0/+1 processes. EPR studies indicate that the spin density is mainly delocalized on the triazinyl fragment of the heterocycle. Magnetic susceptibility measurements in the 5-300 K region showed that the radical obeys Curie-Weiss behavior down to 10 K (C = 0.376 emu·K·mol(-1) and θ = +1.41 K) consistent with weak ferromagnetic interactions between S = 1/2 radicals. Subsequent fitting of the magnetic data to a 1D ferromagnetic chain model provided an excellent fit (g = 2.00, J/k = +1.49 K) down to 10 K but failed to reproduce the subsequent decrease in χT at lower temperatures, which has been ascribed to the onset of weaker antiferromagnetic interactions between ferromagnetic chains.

Single-source materials for metal-doped titanium oxide: Syntheses, structures, and properties of a series of heterometallic transition-metal titanium oxo cages

Titanium dioxide (TiO(2)) doped with transition-metal ions (M) has potentially broad applications in photocatalysis, photovoltaics, and photosensors. One approach to these materials is through controlled hydrolysis of well-defined transition-metal titanium oxo cage compounds. However, to date very few such cages have been unequivocally characterized, a situation which we have sought to address here with the development of a simple synthetic approach which allows the incorporation of a range of metal ions into titanium oxo cage arrangements. The solvothermal reactions of Ti(OEt)(4) with transition-metal dichlorides (M(II)Cl(2), M = Co, Zn, Fe, Cu) give the heterometallic transition-metal titanium oxo cages [Ti(4)O(OEt)(15)(MCl)] [M = Co (2), Zn (3), Fe (4), Cu (5)], having similar MTi(4)(μ(4)-O) structural arrangements involving ion pairing of [Ti(4)O(OEt)(15)](-) anion units with MCl(+) fragments. In the case of the reaction of MnCl(2), however, two Mn(II) ions are incorporated into this framework, giving the hexanuclear Mn(2)Ti(4)(μ(4)-O) cage [Ti(4)O(OEt)(15)(Mn(2)Cl(3))] (6) in which the MCl(+) fragments in 2-5 are replaced by a [ClMn(μ-Cl)MnCl](+) unit. Emphasizing that the nature of the heterometallic cage is dependent on the metal ion (M) present, the reaction of Ti(OEt)(4) with NiCl(2) gives [Ti(2)(OEt)(9)(NiCl)](2) (7), which has a dimeric Ni(μ-Cl)(2)Ni bridged arrangement arising from the association of [Ti(2)(OEt)(9)](-) ions with NiCl(+) units. The syntheses, solid-state structures, spectroscopic and magnetic properties of 2-7 are presented, a first step toward their applications as precursor materials.

Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet

The crystal structure of the spin-canted antiferromagnet beta-p-NCC(6)F(4)CNSSN* at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first-principles bottom-up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=-33.8 cm(-1)), which generates a three-dimensional diamond-like magnetic topology within the crystal. The computed magnetic susceptibility, chi(T), which was determined by using this magnetic topology, quantitatively reproduces the experimental features observed above 36 K. Owing to the anisotropic contraction of the crystal lattice, both the geometry of the intermolecular contacts at 12 K and the microscopic J(AB) radical-radical magnetic interactions change: the J(d1) radical-radical interaction becomes even more antiferromagnetic (-43.2 cm(-1)) and two additional ferromagnetic interactions appear (+7.6 and +7.3 cm(-1)). Consequently, the magnetic topologies of the 12 and 160 K structures differ: the 12 K magnetic topology exhibits two ferromagnetic sublattices that are antiferromagnetically coupled. The chi(T) curve, computed below 36 K at the limit of zero magnetic field by using the 12 K magnetic topology, reproduces the shape of the residual magnetic susceptibility (having subtracted the contribution to the magnetization arising from spin canting). The evolution of these two ferromagnetic J(AB) contributions explains the change in the slope of the residual magnetic susceptibility in the low-temperature region.