Antonio Alberola

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.

Synthesis and magnetic properties of the novel dithiadiazolyl radical, p-NCC6F4C6F4CNSSN*.

The dithiadiazolyl radical p-NCC6F4C6F4CNSSN* (4) retains its monomeric nature in the solid state with molecules linked together into chains via supramolecular CN-S interactions. Variable temperature magnetic studies on 4 show that it behaves as a near-ideal Curie paramagnet (|theta| less than 0.1 K), indicating negligible intermolecular exchange. The effective magnetic moment (1.78 micro(B)) is temperature independent and in excellent agreement with the value expected for an S = 1/2 paramagnet with g = 2.01(1.74 micro(B)). The lack of exchange coupling between radicals is attributed to the absence of significant orbital overlap between radical centres.

Molecular tailoring of spin-transition materials: preparation, crystal structure and magnetism of trifluoromethyl-pyridyl-1,3,2-dithiazolyl

The α-phase of pyridyl-1,3,2-dithiazolyl, pyDTA, has previously been shown to exhibit a distorted π-stacked structure which is diamagnetic at low temperature. Unlike other π-stacked dithiazolyl radicals it does not undergo a first order phase transition to a regular π-stacked paramagnetic phase even upon warming to its melting point. Here we show that incorporation of a trifluoromethyl group destabilises the diamagnetic π*–π* dimer and the title compound (1) exhibits a spin-transition (without hysteresis) at 50 K.

Weakening of the π*–π* dimerisation in 1,2,3,5-dithiadiazolyl radicals: structural, EPR, magnetic and computational studies of dichlorophenyl dithiadiazolyls, Cl2C6H3CNSSN

A series of dichlorophenyl dithiadiazolyls (DCP-DTDA) have been prepared and structurally characterized by X-ray diffraction (1 = 2,3-DCP-DTDA; 2 = 2,4-DCP-DTDA; 3 = 2,5-DCP-DTDA, 4 = 3,4-DCP-DTDA and 5 = 3,5-DCP-DTDA). All the radicals adopt cis-cofacial π*–π* dimers with intra-dimer S⋯S contacts in the range 3.119(8)–3.300(5) A. The propensity for chloro-aromatics to adopt a β-sheet structure assists formation of lamellar structures in 1–5 with intra-stack Cl⋯Cl contacts in the range 3.44(1)–3.82(1) A. Perpendicular to the stacking direction, the packing is dominated by formation of intermolecular S⋯Cl contacts. Variable-temperature magnetic susceptibility measurements on 1–5 in the range 5–300 K reveal the onset of paramagnetism around ca. 200 K. Solid-state EPR studies reveal that this paramagnetism is associated with thermal population of a triplet excited state. A good fit to the Bleaney–Bowers model determined the exchange interactions (2J) between the radicals in the dimers to be in the range −904 to −1529 cm−1. Computational studies of the dimerisation process indicate that at intra-dimer S⋯S distances in excess of 3.2 A the closed-shell singlet becomes destabilized with respect to both the open-shell singlet and triplet state configurations.

Crystal structures and magnetic properties of a sterically encumbered dithiadiazolyl radical, 2,4,6-(F3C)3C6H2CNSSN..

The sterically protected dithiadiazolyl radical (F3C)3C6H2CNSSN. (1) crystallises in two polymorphs: 1alpha, comprised of monomeric units and 1beta, containing a mixture of both pi*-pi* dimers and S = (1/2) monomers; whilst both polymorphs exhibit similar structure-directing motifs, the variation in packing leads to different magnetic behaviour.

A new Co(II) coordination solid with mixed oxygen, carboxylate, pyridine and thiolate donors exhibiting canted antiferromagnetism with TC≈ 68 K

Reaction of Co(II) chloride with the sodium salt of 2-mercaptonicotinic acid in water at 200 °C results in the formation of Co4(2-mna)4(H2O), which orders as a canted antiferromagnet at 68 K.

Synthesis and characterisation of tetrafluorobenzo-1,3,2-dithiazolyl

Heterocyclic dithiazolyl (DTA) radicals recently have been shown to exhibit unusual physical properties including high magnetic ordering temperatures, as in the case of benzo-1,3,2-dithiadiazolyl1 (BDTA) (Tc = 11 K) and display of bistability2 at room temperature in trithiatriazapentalene (TTTA) among others. A new DTA salt [TFB-DTA]Cl (1) was prepared from C6F4(SH)2 using standard synthetic methods.2 Crystals were grown by sublimation (Figure 1). The radical was generated in situ by reduction of the salt with Ag powder in CH2Cl2. The X-band EPR spectrum of the radical exhibited a triplet of triplets due to coupling of the unpaired spin to one 14N nucleus (I = 1) and two 19F (I = 1/2) (giso = 2.0044, aiso = 11.0 G, aiso = 1.5 G). Further resolution of coupling to the remaining two F atoms could not be achieved (Figure 2).

An unusual single-crystal-to-single-crystal (2 + 2) photocyclisation reaction of a TTF-arylnitrile derivative†

4-([2,2′-Bi(1,3-dithiolylidene)]-4-yl)benzonitrile undergoes a [2 + 2] photocycloaddition reaction upon irradiation with polychromatic light, an unusual single-crystal-to-single-crystal transformation for a TTF derivative. In contrast, the closely related pyridylnitrile derivative adopts a different packing motif and is stable to light under the same conditions.