1998 J. Am. Chem. SOC. 1993, 115, 1998-2005 Gas-Phase Chemistry of HSiS- and HSiNH-: Ions Related to Silathioformaldehyde and the Silaazomethine of Formaldehyde Robert Damrauer,* Michele Krempp, and Richard A. J. O'Hair Contribution from the Department of Chemistry, University of Colorado at Denver, P.O. Box 1733364, Denver, Colorado 8021 7-3364, and Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309-021 5. Received July 20, 1992 Abstract: Anions related to silathioformaldehyde (HzSi=S) and the silaazomethine of formaldehyde (H,Si=NH) have been prepared and studied using flowing afterglow selected ion flow tube techniques. The connectivitiesof these anions are shown in experimental studies to be [HSiSI- and [HSiNHl-. In addition, ab initio computational studies demonstrate that these are the most stable anion isomers. These anions are prepared by both collisionally-induced dissociation and direct source reactions. The anions resulting from the later preparations have been submitted to reaction chemistry studies with C02, COS, CSz, SOz, 02, and C6Fs. Comparisons with earlier studies of HSiO- are made in terms of a simple mechanistic picture. Introduction Low-valent silicon-containing compounds have been widely studied in recent years both experimentally'2 and comp~tationally.~ Studies of silicon-arbon, silicon-xygen, silicon-nitrogen, and siliconsilicon multiply bonded compounds have demonstrated that such species are generally stable, but are highly reactive unless encumbered by sterically demanding substituents.l Although our understanding of such species has certainly increased in recent years, our knowledge is still primative compared to that accu- mulated over the years on analogous carbon species. We have been interested in low-valent silicon-containing species for some time and have used ion-molecule reaction chemistry to study simple silicon-containing anions whose conjugate acids are highly reactive, low-valent silicon-containing Our previously reported work has concentrated on anions (conjugate bases) of (CH3)2Si,4 (CH3)2Si=CH2,5 Si=CH2,6 H2Si=0,7 CH3(H)Si=0,* and CH30(H)Si=0.8 These studies have the advantage that many simple silicon-containing anions are readily prepared in the gas phase, even though their conjugate acids would be expected to be exceptionally reactive in either the gas or condensed phase. Thus, in studying the reaction chemistry of simple anions unencumbered by bulky substituents in the gas phase, we have not only a measure of the reactivity of such anions, but an indirect probe of their corresponding conjugate acids. The flowing afterglow selected ion flow tube (FA-SIFT) technique which we use allows the selection of ions from complex reaction mixture^.^.^ Because reaction chemistry studies occur under single-collision conditions, stoichiometric relationships can be determined (excluding, of course, the many collisions that ions and neutrals undergo with neutral buffer gas; see Experimental Section). In addition, we can measure reaction rates and detect multiple reaction pathways. In favorable cases, thermochemical properties such as heats of formation of anions and their corre- sponding conjugate acids, electron affinities of corresponding radicals, and gas-phase acidity of the conjugate acids can be determined as well.7 In relating the chemistry of anions and their conjugate acid species, it is important to realize that the structure of the acid is not always known. For example, the conjugate acid of the anionic species containing two hydrogens, one silicon, and one carbon (designated [Hz, Si, C]; see note at the end of para- graph) could be either H miH, HzC=Si, or C=SiHz. When we study the reactions of [H, Si, C]- with acids, we are uncertain of the structure of the reaction product with acids unless other information is available. Thus, linking our experimental studies with computational work carried out both by other workersI0 and us7 has proven advantageous in sorting out structural questions for such species. (General formulations like [H2,Si, C] and [H, Si, C]- will be used for compounds and anions whose compositions are known, but whose structures are not. When the structure is *Address correspondence to this author at the University of Colorado at Denver. 0002-7863/93/1515-1998%04.00/0 known, the connectivity of its atoms will be clearly designated. For example, the [H2CSi] formulation indicates that two hy- drogens are bound to carbon which is bonded to silicon. Similarly, [HSiOI- represents a typical anion where the negative charge location is unknown, but the connectivity is. HSiO- indicates known connectivity with negative charge localization on oxygen.) In this paper we describe experimental and computational studies on [H, Si, SI- and [H2,Si, N]-. These studies are sum- marized in the context of our more general work on the anions [H,, Si, XI- and the parent conjugate acid compounds [H,,.+l, Si, XI, where we have now studied X = C for n = 1,6 X = 0 for n = 1,7 X = S for n = 1, and X = N for n = 2. The work on the structure and reactivity of the two anions and their conjugate acids discussed herein further probes the relationship between silicon and carbon (in these cases between the silicon analogues of thi- oformaldehyde and azomethine). Experimental Section All experiments were carried out at room temperature in the flowing afterglow selected ion flow tube (FA-SIFT). Although this instrument has been described in detail previously,'' a brief summary of its operation in these experiments is appropriate for readers who are not familiar with the technique. The FA-SIFT consists of four sections: a flow tube for ion preparation (A), an ion separation and purification region (B), a second flow tube for studying the chemical reactions of the ions selected (C), and finally an ion detection region (D). In the first flow tube (A), for example, ions like HS- (prepared from HO- and CS2) or H2N- (prepared by electron impact of NH,) are entrained in a rapidly flowing helium stream (0.3 Torr). Phenylsilane is added downstream through a movable inlet producing the major products shown in eq 1 and 2. C6H,SiH3 + HS- - H3SiS-+ C& (1) CBHSSiH3 + H2N- - H3SiNH-+ CBH, (2) (1) Raabe, G.; Michl, J. In The Chemistry ofFuncfiona1 Groups; Patai, S., Rappoport, Z., Eds.; John Wiley: New York, 1989; pp 1015-1142. This review gives an excellent collection of relevant silicon references up through early 1989. (2) OHair, R. A. J.; Krempp, M.; Damrauer, R.; DePuy, C. H. Inorg. Chem. 1992, 31, 2092-6. This paper gives a collection of relevant references for other than silicon low-valent species. (3) Apeloig, Y. In The Chemistry of Functional Groups; Patai, S., Rap poport, Z., Eds.; John Wiley: New York, 1989; pp 57-225. (4) Damrauer, R.; DePuy, C. H.; Davidson, I. M. T.; Hughes, K. J. Or- ganometallics 1986, 5, 2054-7. (5) Damrauer, R.; DePuy, C. H.; Davidson, I. M. T.; Hughes, K. J. Or- ganometallics 1986, 5, 2050-4. (6) Damrauer, R.; DePuy, C. H.; Barlow, S. E.; Gronert, S. J. Am. Chem. SOC. 1988, 110,2005-6. (7) Gronert, S.; OHair, R. A. J.; Prodnuk, S.; Slilzle, D.; Damrauer, R.; DePuy, C. H. J. Am. Chem. SOC. 1990, 112, 997-1003. (8) Damrauer, R.; Krempp, M. Organometallics 1990, 9, 999-1004. (9) Damrauer, R.; Krempp, M.; Schmidt, M. W.; Gordon, M. S. J. Am. Chem. SOC. 1991, 113, 2393-2400. (10) Schmidt, M. W.; Gordon, M. S. J. Am. Chem. Soc. 1991, 113, (1 1) Van Doren, J. M.; Barlow, S. E.; Depuy, C. H.; Bierbaum, V. M. Int. 5244-8. J. Mass Spectrom. Ion Processes 1987, 81, 85-100. 0 1993 American Chemical Society