Melbourne J. Schriver

Paramagnetic liquids: the preparation and characterisation of the thermally stable radical ButCNSNS· and its quantitative photochemically symmetry allowed rearrangement to a second stable radical ButCNSSN

The new thermally stable paramagnetic liquid 5-t-butyl-1,3,2,4-dithiadiazolyl has been isolated in the dark and quantitatively photochemically isomerised to the paramagnetic liquid 5-t-butyl-2,3,1,4-dithiadiazolyl (non-systematic numbering for ease of comparison).

The high yield preparation, characterisation, and gas phase structure of the thermally stable CF3CSNSCCF3·, 4,5-bis(trifluoromethyl)-1,3,2-dithiazolyl and the X-ray crystal structure of benzo-1,3,2-dithiazolyl

The very thermally stable, but photochemically sensitive radical, 4,5-bis(trifluoromethyl)-1,3,2-dithiazolyl has been prepared, isolated, and fully characterised including a gas phase structure, and found to be paramagnetic in the liquid state at room temperature; the X-ray structure of benzo-1,3,2-dithiazolyl has been obtained for comparison.

Thermal hysteresis in dithiadiazolyl and dithiazolyl radicals induced by supercooling of paramagnetic liquids close to room temperature: a study of F3CCNSSN and an interpretation of the behaviour of F3CCSNSCCF3

The trifluoromethyl-substituted dithiadiazolyl and dithiazolyl radicals, F3CCNSSN (1) and F3CCSNSCCF3 (2) associate through pi*-pi* covalent and electrostatic S delta+...N delta- interactions in the solid state, but melt with a dramatic volume increase to generate paramagnetic liquids; these radicals exhibit thermal hysteresis, which arises through a meta-stable super-cooled liquid state, close to room temperature.

The kinetics of the quantitative, symmetry allowed, reverse electron demand cycloadditions of the pseudo-1,3-dipole SNS+ with alkynes and nitriles; the preparation and X-ray crystal structures of NCCSNSCHAsF6 and SNSNC–CNSNS(AsF6)2: the precursor to a new class of S2N2C–CN2S2n+(n= 0,1,2) bicyclics

The rates of cycloaddition reactions of SNSAsF6 with alkynes and nitriles increase as the ionization potential (Ei) of the triple bond decreases, and accordingly reaction with HCCCN occurs preferentially at the CC bond; however, it reacts with NCCN quantitatively to give the dication ([graphic omitted])22+, from which several members of a new family of S2N2C–CN2S2n+(n= 0,1,2) bicyclics have been prepared.

Synthesis of (TDAE)(O2SSO2)(s) and discovery of (TDAE)(O2SSSSO2)(s) containing the first polythionite, [O2SSSSO2]2–

Gaseous SO2 reacts with tetrakis(dimethylamino)ethylene (TDAE) in acetonitrile in a 2:1 stoichiometric ratio to give analytically pure insoluble purple (TDAE)(O2SSO2) (1) in about 80% yield. Crystals of (TDAE)(O2SSSSO2) (2) were obtained from orange solution over the purple solid. The Raman spectrum of [TDAE](2+) was established using (TDAE)(A) salts [A = 2Br(-), 2Br(-)·2H2O (X-ray), 2[Br3](-) (X-ray)]. Vibrational spectroscopy showed that [O2SSO2](2-) in 1 has C2h geometry. The X-ray structure of 2 showed that it contained [O2SSSSO2](2-), the first example of a new class of sulfur oxyanions, the polythionites. The geometry of [O2SSSSO2](2-) consists of S2 with an S-S bond length of 2.003(1) Å connected to two terminal SO2 moieties by much longer S-S bonds of 2.337(1) Å. Calculations (B3PW91/6-311+G(3df)) show that the structural units in [O2SSSSO2](2-) are joined by the interaction of electrons in two mutually perpendicular π* SOMOs of the triplet-state diradical S2 with unpaired electrons in the π*-antibonding orbitals of the two terminal [SO2](•-) and polarized to delocalize the negative charge equally onto the three fragments. Thermodynamic estimates show 2 to be stable with respect to loss of sulfur and formation of 1, in contrast to [O2SSSSO2](2-) salts of small cations that are unstable toward the related dissociation. Reaction of TDAE with an excess of liquid SO2 led to (TDAE)(O3SOSO3)·SO2 (preliminary X-ray, Raman), (TDAE)(O3SSSSO3)·2SO2 (preliminary X-ray, Raman), and (TDAE)(O3SSO2) (Raman).

The general quantitative cycloaddition reactions of the dithianitronium cation with olefins: the preparation of 1,3,2-dithiazolium and 1,4-dithia-7-azanorbornylium cations and the crystal and molecular structure of 1,4-dithia-7-azanorbornylium hexafluoroarsenate(V)

A number of olefins react quantitatively with S2NAsF6via a concerted symmetry allowed cycloaddition to give 1 : 1 and 2 : 1 stoicheiometric cationic products: the generality of reaction is rationalised and the X-ray crystal structure determination of the 1,4-dithia-7-azanorbornylium cation is reported.

Preparation and characterization of 1,3,2-dithiazolidine and 1,4-dithia-7-azabicyclo[2.2.1]heptane cations, and a mechanistic study of the cycloaddition reactions of alkenes with SNS

The cation SNS+(as the AsF6– salt) underwent quantitative concerted symmetry-allowed cycloaddition reactions with alkenes [C2H4, trans- and cis-MeHCCHMe, H2CCMe2, MeHCCH2, Me2CCMe2 and norbornene (bicyclo[2.2.1]hept-2-ene)] to give 1,3,2-dithiazolidine cations 1, which in a second quantitative concerted symmetry-allowed cycloaddition reaction with another alkene molecule gave 1,4-dithia-7-azabicyclo[2.2.1]heptane cations 2(with the exception of Me2CCMe2). The cycloadducts were characterized by elemental analyses and IR and NMR (1H, 13C, 14N) spectroscopies. The vibrational spectra were assigned with the aid of frequencies obtained by ab initio(RHF/6–31G*) calculations. When alkene = C2H4 the calculated geometry of 2 was in good agreement with that obtained from its crystal structure reported previously; that of 1 correlates well with the experimental data (IR, Fourier-transform Raman, NMR). Kinetic studies showed that the rate constants of the first cycloaddition of SNS+ to C2H4 are comparable with those of nitrile and alkyne cycloadditions, indicating that the cycloaddition proceeds via the interaction of the highest occupied molecular orbital of the alkene and the lowest unoccupied one of SNS+ as was previously observed for various nitriles and alkynes. The second cycloaddition leads to stereospecific 2, except for H2CCHMe. Contrary to the prediction of a simple frontier molecular model, the rate of the second cycloaddition was faster than the first for C2H4, cis-MeHCCHMe, and H2CCMe2 and strongly dependent on the steric activity of the alkene. It is proposed that the second cycloaddition likely occurs via a concerted and synchronous pathway.

Preparation of 1,3,2-dithiazolium hexafluoroarsenate(V), preparation and crystal structures of 5-methyl-1,3,2,4-dithiadiazolium and 4-methyl-1,3,2-dithiazolium hexafluoroarsenate(V) and the reduction of these salts to stable free radicals

Crystalline CH3[graphic omitted] AsF6–, CH3[graphic omitted] AsF6–, and [graphic omitted] AsF6– were prepared in essentially quantitative yield by the reaction of S2NAsF6 with CH3CN, CH3CCH, and HCCH respectively, in sulphur dioxide solution. The compounds have been characterised by elemental analysis, i.r. and mass spectrometry. The compound CH3[graphic omitted] AsF6– crystallizes in the monoclinic space group P21/c, with unit-cell dimensions a= 8.182(7), b= 9.822(3), c= 11.515(7)A, β= 110.91(6)°, and Z= 4. The structure was solved from low-temperature diffractometer data by direct methods and refined by least-squares techniques to a final R of 0.12 for 893 observed reflections. The structure consists of discrete, planar CH3[graphic omitted] cations and AsF6– anions with some cation–anion interactions. The compound CH3[graphic omitted] AsF6– crystallizes in the monoclinic space group P21/c, with unit-cell dimensions a= 8.539(5), b= 9.941 (2), c= 12.053(5)A, β= 116.69(4)°, and Z= 4. The structure was solved by direct methods and refined by least-squares techniques to a final R of 0.049 for 1 077 observed reflections. The structure contains the hitherto unknown CH3[graphic omitted] cation and the AsF6– anion with some cation–anion interaction. Both CH3[graphic omitted] and [graphic omitted] (R = H or CH3) can be regarded as 6π cyclic systems. The cations have been reduced chemically and electrolytically to form long-lived free radicals identified by their e.s.r. spectra; INDO calculations have yielded values of hyperfine coupling constants in good agreement with those obtained experimentally.

Absorption of SO2(g) by TDAE[O2SSO2](s) to Give TDAE[O2SS(O)2SO2](s): Related Reactions of [NR4]2[O2SSO2](s) (R = CH3, C2H5)

One mole equivalent of gaseous SO2 is absorbed by purple TDAE[O2SSO2](s), producing red, essentially spectroscopically pure TDAE[O2SS(O)2SO2](s); under prolonged evacuation, the product loses SO2(g), regenerating TDAE[O2SSO2](s). Similarly, [NR4]2[O2SS(O)2SO2](s) (R = Et, Me) can be prepared, albeit at lower purity, from the corresponding tetraalkylammonium dithionites (prepared by a modification of the known [NEt4]2[O2SSO2](s) preparation). While the [NEt4](+) salt is stable at rt; the [NMe4](+) salt has only limited stability at -78 °C. Vibrational spectra assignments for the anion in these salts were distinctly different from those for the anion in salts containing the long-known [O3SSSO3](2-) dianion, the most thermodynamically stable form of [S3O6](2-) (we prepared TDAE[O3SSSO3]·H2O(s) and obtained its structure by X-ray diffraction and vibrational analyses). The best fit between the calculated ((B3PW91/6-311+G(3df) and PBE0/6-311G(d)) and experimental vibrational spectra were obtained with the dianion having the [O2SS(O)2SO2](2-) structure. Vibrational analyses of the three [O2SS(O)2SO2](2-) salts prepared in this work showed that the corresponding [O3SSO2](2-) salts were present as a ubiquitous decomposition product. The formation of these new [O2SS(O)2SO2](2-) dianion salts was predicted to be favorable for [NMe4](+) and larger cations using a combination of theoretical calculations (B3PW91/6-311+G(3df)) and volume based thermodynamics (VBT). Similar methods accounted for the greater stabilities of the TDAE(2+) and [NEt4](+) salts of [O2SS(O)2SO2](2-) compared to [NMe4]2[O2SS(O)2SO2](s) toward irreversible decomposition to the corresponding [O3SSO2](2-) salts. These salts represent the first known examples of a new class of poly(sulfur dioxide) dianion, [SO2]n(2-) in which n > 2.

The energetics of formation and X-ray crystal structure of SNSNS(AsF6)2, containing the lattice-stabilized aromatic 6π 1,3,4,2,5-trithiadiazolium(2+) cation formed by the crystal-lattice-energy-driven, symmetry-allowed cycloaddition of SN+ and SNS+

We report the preparation, vibrational spectra and X-ray crystal structure of [graphic omitted](AsF6)2, containing the 6π 1,3,4,2,5-trithiadiazolium(2+) cation, which dissociates in solution to SN+ and SNS+, consistent with ab initio 6–31G* calculations (estimated gas-phase dissociation enthalpy: –400 kJ mol–1); the [graphic omitted]2+ ring represents a local energy minimum, and the cycloaddition of SN+ and SNS+ is driven in the solid state by the high lattice energy of the 1 : 2 salt.