The isolation, characterisation, gas phase electron diffraction and crystal structure of the thermally stable radical [CF3CSNSCCF3]˙

Scott Brownridge; Hongbin Du; Shirley A. Fairhurst; Robert C. Haddon; Heinz Oberhammer; Simon Parsons; Jack Passmore; Melbourne J. Schriver; Leslie H. Sutcliffe; Nicholas P. C. Westwood

doi:10.1039/B001489N

The isolation, characterisation, gas phase electron diffraction and crystal structure of the thermally stable radical [CF₃CSNSCCF₃]˙†

Scott Brownridge,^a Hongbin Du,^a Shirley A. Fairhurst,^b Robert C. Haddon,^c Heinz Oberhammer,^d Simon Parsons,^e Jack Passmore,*^a Melbourne J. Schriver,^f Leslie H. Sutcliffe^g and Nicholas P. C. Westwood^h

Author affiliations

* Corresponding authors

^a Department of Chemistry, University of New Brunswick, Post Office Box 45222, Fredericton, New Brunswick, Canada
E-mail: passmore@unb.ca

^b Nitrogen Fixation Laboratory, John Innes Centre, Norwich, UK

^c Department of Chemistry and Physics, and Advanced Carbon Materials Center, University of Kentucky, Lexington, Kentucky, USA

^d Institut fur Physikalische und Theoretische Chemie, Universitat Tuebingen, Auf der Morganstralle 8, Tuebingen 1, Germany

^e Department of Chemistry, University of Edinburgh, Edinburgh, UK

^f Department of Science, Atlantic Baptist University, P.O. Box 6004, Moncton, New Brunswick, Canada

^g Chemistry and Physics Departments, University of Surrey, Guildford, Surrey, UK

^h Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada

Abstract

The first structural characterisation of a heterocyclic free radical in all phases has been achieved. 4,5-Bis(trifluoromethyl)-1,3,2-dithiazol-2-yl, [CF₃ [upper bond 1 start] CSNSC [upper bond 1 end] CF₃]˙ 4d, a 7π radical, was prepared quantitatively on reduction of [4d][AsF₆] with various reducing agents. It is a blue gas over a green paramagnetic liquid [¹⁹F NMR δ −59.7], which freezes to a diamagnetic black-green solid. There is a remarkably large volume increase on melting implying that the liquid consists of discrete monomeric radicals, and consistently there is no tendency to dimerise in CCl₃F solution and the vibrational spectra attributable to the monomer are essentially identical in all phases. The thermodynamic properties obtained for the vapourisation, sublimation and melting processes are very similar to those of related diamagnetic materials. 4d is remarkably thermochemically robust giving decomposition products identical to those from photochemical decomposition, a quantitative mixture of CF₃ [upper bond 1 start] CSSC [upper bond 1 end] CF₃ (and its oligomers) and CF₃CNSNCCF₃. The reaction chemistry is described. The crystal structure shows radical 4d to be a planar heterocyclic ring which associates in the solid as a diamagnetic tetramer. The structure of the gaseous monomer determined by electron diffraction is very similar to that in the solid phase. Comprehensive physical measurements were made including the UV-visible spectra of the bright blue solutions, variable temperature magnetic susceptibility and quantitative variable temperature ESR spectra both of which confirm a phase change from a diamagnetic solid to a paramagnetic liquid on melting, the vibrational spectra, and the photoelectron spectrum of the gaseous free radical. A study of the behavior of R [upper bond 1 start] CSNSC [upper bond 1 end] R˙ (R = H, CH₃ or CF₃) given by the reduction of RCSNSCR⁺ on increase of concentrations of radical is reported, including the isolation of HCSNSCCF₃˙ at low temperature. The experimental and calculated [UMPW1PW91/6-31+G*] structures of 4d are in good agreement. These and related calculations also support the interpretation of the vibrational and photoelectron spectra, and the thermodynamic properties of 4d, as well as provide possible explanations for the relative stabilities of R [upper bond 1 start] CSNSC [upper bond 1 end] R˙, and imply that the weak intramolecular interactions in 4d in the solid state are largely ionic S^δ+⋯N^δ− interactions.

Journal of the Chemical Society, Dalton Transactions

The isolation, characterisation, gas phase electron diffraction and crystal structure of the thermally stable radical [CF₃CSNSCCF₃]˙†

Abstract

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The isolation, characterisation, gas phase electron diffraction and crystal structure of the thermally stable radical [CF₃CSNSCCF₃]˙

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Journal of the Chemical Society, Dalton Transactions