FULL PAPER DOI: 10.1002/ejoc.200700349 Chlorodifluorothioacetic Acid, CF 2 ClC(O)SH: Synthesis, Characterization, X-ray Structure and Conformational Properties Mauricio F. Erben, [a][‡] Roland Boese, [b] Helge Willner, [c] and Carlos O. Della Védova* [a,d][‡] Keywords: Sulfur / Thiocarboxylic acids / Conformation analysis / Structure elucidation / Ab initio calculations Chlorodifluoroacetic anhydride, [CF 2 ClC(O)] 2 O, and chloro- difluoroacetyl chloride, CF 2 ClC(O)Cl, react with hydrogen sulfide to yield chlorodifluorothioacetic acid, CF 2 ClC(O)SH. This novel thiocarboxylic acid has been obtained in pure form and characterized by NMR, IR and Raman spectroscopy. By extrapolation of the vapour pressure curve, log p = 6.102– 2068/T (p [bar], T [K]), the boiling point was determined to be 66(1) °C. The melting point of the pure substance is –83(2) °C. The conformational properties of the gas-phase molecule have been studied by vibrational spectroscopy [IR (vapour), Raman (liquid)] and quantum chemical calculations [B3LYP and MP2 methods with 6-311++G(d,p) and 6- 311++G(3df,2p) basis sets]. The most stable form has C 1 sym- metry with a synperiplanar (syn) orientation of the C=O double bond with respect to the S–H single bond whereas in the chlorodifluoroacetyl group the gauche orientation of the Introduction Thioacetic acid, CH 3 C(O)SH, was first obtained in 1854 by Kekulé [1] and represents the first known organic acid containing sulfur. [2,3] Currently this compound is widely used in chemical synthesis as a reagent for the introduction of the thiol group into organic molecules, being the most common and versatile reagent for the free-radical addition of a thiol precursor to terminal alkenes. [4] Halothioacetic acids of the type CX 3 C(O)SH (X = halogen) are also very well known. For instance, the addition of trichloro- [a] CEQUINOR (CONICET-UNLP), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115 (B1900AJL), C. C. 962, La Plata, Buenos Aires, República Argentina Fax: +54-221-425-9485 E-mail: carlosdv@quimica.unlp.edu.ar [b] Institut für Anorganische Chemie, Universität Duisburg-Essen, Universitätsstr. 5–7, 45117 Essen, Germany [c] FB C, Anorganische Chemie, Bergische Universität Wuppertal, Gaußstrasse 20, 47097 Wuppertal, Germany [d] Laboratorio de Servicios a la Industria y al Sistema Científico (UNLP-CIC-CONICET), Camino Centenario, Gonnet, Buenos Aires, República Ar- gentina [‡] Mauricio F. Erben and Carlos O. Della Védova are members of the Carrera del Investigador of CONICET, República Ar- gentina. Supporting information for this article is available on the WWW under http://www.eurjoc.org or from the author. Eur. J. Org. Chem. 2007, 4917–4926 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 4917 ClC–C=O moiety is preferred (gauche-syn form). The analy- sis of the IR spectrum of gaseous CF 2 ClC(O)SH suggests the presence of two other conformations in equilibrium at room temperature. These forms have been fully characterized by high-level quantum chemical calculations. The structure of a single crystal, grown by a miniature zone-melting procedure using focused infrared laser radiation, was determined by X- ray diffraction analysis at low temperature. The crystalline solid [monoclinic, space group C2/c, a = 17.6165(14), b = 5.8848(5), c = 11.0877(9) Å, β = 113.5330(11)°] consists exclu- sively of molecules adopting the gauche-syn conformation. These molecules associate, forming pairs of cyclic dimers in the crystal through S–H···O=C hydrogen bonds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) thioacetic acid to olefins was studied early on by Cun- neen. [5] More recently, CCl 3 C(O)SH has been used as a do- nor ligand to form stable gold(I) complexes. [6] With a sim- ilar purpose, the reactivity of group-12 metal thiocarboxyl- ate species has been used to prepare valuable metal sulfide materials by thiocarboxylic anhydride elimination. [7] Tri- fluorothioacetic acid, CF 3 C(O)SH, was prepared in 1960 by Sheppard and Muetterties [8] and its chemistry was further explored by Rochat and Gard. [9] As has been pointed out by these authors, the presence of the –SH group offers a way of preparing new sulfenyl halides and sulfides that con- tain the trifluoroacetyl group. While the molecular structure and conformational prop- erties of carboxylic acids have been widely studied, analo- gous thiocarboxylic species have attracted less attention. In principle, two planar conformations, with a syn and anti orientation of the C=O bond with respect to the S–H bond, are feasible for the –C(O)SH moiety (Scheme 1). At this point, CH 3 C(O)SH is by far the most-studied thiocarbox- ylic acid species. The molecular structure of gaseous CH 3 C(O)SH in its electronic ground state was studied early on by Gordy. By gas electron diffraction, the geometrical parameters of the heavy atoms could be estimated. [10] Furthermore, several studies of the vibrational spectra have been carried out with the aim of seeking evidence of a pos- sible thiol/thione tautomeric equilibrium. [11,12] Noe [13] in- vestigated the conformational equilibrium around the C–S