STRUCTURES OF MERCURY MERCAPTIDES

Transcription

1 STRUCTURES OF MERCURY MERCAPTIDES PART 11. X-RAY STRUCTURAL ANALYSIS OF MERCURY ETHYLMERCAPTIDE D. C. BRADLEY~ AND N. R. KUNCHUR~ Department of Chemistry, University of Western Ontario, London, Ontario Received November 16, 1964 ABSTRACT The unit cell parameters for Hg(sC~H5)s were determined by X-ray diffraction: a = 7.54 f 0.02; b = 4.87 f 0.01; c = f 0.04 A; p = 85". The unit cell contains four formula units, and a complete structural analysis revealed a molecular lattice with insignificant intermolecular bonding involving mercury and sulfur. Interatomic distances (excluding C-H) were determined. INTRODUCTION This work forms a part of the programme of studying the coordination con~pounds of mercaptans with mercury. Recent work (I) showed that mercury nlethyl~nercaptide had an unusual structure and it seemed worthwhile to investigate the structure of nlercury ethylmercaptide. The crystal structure Hg(S.C2Hj)2 was partially solved by Wells (2) but his results were qualitative. The ~vork described in the present paper was carried out to locate all atoins (except hydrogens) in order to establish the stereochenlistry of mercury in this con~pound and to obtain reliable values for the dimensions of the molecule. EXPERIMENTAL Mercury ethylmercaptide H~(S.CBH~)B was prepared by Wertheim's method (3) and purified by crystallization from ethanol. It forms thin plates on (001) outlined by (110) faces. With Cu I<, radiation (A = A) the compound had a high absorption coefficient of 421 cm-l. Unfortunately the thinness and fragility of the crystals prevented them from being ground into spheres or cylinders to facilitate the absorption correction. The thin plates were cut into needles with square cross-section of 0.1 mm. Absorption corrections were applied assuming that the needles were cylinders of diameter 0.1 mm. The cell parameters were a = 7.54 f 0.02 A; b = 4.87 f 0.01 A; c = f 0.04 A; P = 85"; Z = 4; denled = 2.45 g cm-i; dabs = 2.47 g cm-l; space group = Cc. The intensity data on h01 and Okl reflections were collected using the multifilm technique and a range of exposure times on a Stoe Weissenberg camera. Intensities were estimated visually and corrected for Lorentz and polarization factors. The relative FO values were scaled by Wilson's method, and in the final stages the scaling was improved by comparison with the calculated structure amplitudes. Strz~ctz~re Determination Although the systematic absences correspond to the space groups Cc and C2/c, the latter space group was discarded on the basis of the intensities of the reflections hkl with 1 odd. The space group Cc has no special positions and so all atoms have to be placed in general positions. h01 and Okl, being the best resolved projections, were considered for structure determination. Patterson syntheses computed for these projections gave the following coordinates for the mercury atom. r/a Y /b Z/C Hg The contribution of mercury to the structure amplitude is such that it has only a real component for hkl with 1 even and only an imaginary component for hkl with 1 odd. A mercury-phased Fourier has, therefore, a higher symmetry than the real symmetry with the result that for every peak in a k01 Fourier map, another peak related to the first by a center of symmetry (which is at origin where mercury is placed) would be produced. So if the distribution of two sulfur atoms and four carbon atoms around mercury is acentric, four peaks for two sulfur atoms and eight peaks for four carbon atoms would be produced. Since the stereochemistry of the mercury mercaptide molecule was not known, the problem of locating the light sulfur and carbon atoms was solved by the following procedure. lpresent address: Chentistry Department, Qzleen Mary College, London, England. 2Present address: Center for Crystallographic Research, Roswell Park Meelnorial Institzlte, Bz~falo, N. 1: Correspondence regarding this co?niizz~nication skoz~ld be sent to this address. Canadian Journal of Chemistry. Volume 43 (1965) 2786

2 BRADLEY AND KUNCHUR: STRUCTURES OF MERCURY MERCAPTIDES. PART II 2787 The values for atomic scattering factors for all atoms were taken from ref. 4. A mean isotropic temperature factor B = 2.8 AZ was applied to the scattering curves of all atoms. Fob, were brought on the absolute scale by il'ilson's method. Both the h01 and Okl Patterson projections showed clearly peaks corresponding to mercury-sulfur interactions. Since the coordinates of lnercury were known, the coordinates of sulfur were derived. Structure factors were calculated for mercury and one sulfur atom only. For the reflections hkl with 1 even, both mercury and sulfur contributed to the real part of the structure amplitude and only sulfur contributed to the imaginary part. Contribution of mercury was subtracted from the real part while the imaginary part was not changed since it did not have any contribution from mercury. Phase angles were calculated using the modified realcomponent and the unmodified imaginary component. Thesephaseangleswere used for computing a mercury difference Fourier. The coefficients used in this Fourier were obtained by subtracting the contribution of mercury from the Fob, which were brought on an absolute scale. A similar procedure was carried out for reflection hkl with 1 odd. Only sulfur contributed to the real part and both sulfur and mercury contributed to the imaginary part of these reflections. The 1201 and Okl lnercury difference Fourier done in this manner revealed the positions of all sulfur and carboll atoms. New phase angles were calculated considering all light atoms in the structure factor calculations, and a second cycle of lnercury difference Fourier syntheses was carried out. rl third cycle was repeated so that the atomic coordinates of all atoms were further refined. The 'y' coordinate of mercury was also refined during these successive Fourier syntheses. The final agreement index for various groups of reflections is shown in Table I. The final mercury difference Fourier projections upon (010) and (001) are shown in Figs. 1 and 3. The final coordinates are listed in Table I1 while the observed and calculated structure factors including the phase angles are listed in Table IV. The lowest value of FO which could be observe3 was 8, and hence unobserved reflections are indicated as < 8. The agreement index stated in Table I takes into account observed reflections as well as observed and unobserved reflections. TABLE I Agreement Agreement No. of index for index for No. of observed observed observed and Type of observed and unobserved reflections unobserved reflections reflections reflections only reflections &01 55 h01 32 Okl (= 2n) 24 Okl (= 2n + 1) 22 TABLE I1 TABLE I11 Interatomic distances and bond angles Distance (A) Angles (O) Hgi-SI 2.45 I-Ig~Slcl 106 Hg1-s~ 2.45 HglSK3 106 Hg1-SB 3.56 SICICZ 103 Hgl-Sd 3.53 SZCG I16 SI-C HglSaIlg, 100 SZ-C~ 1.65 HglSlHg~ 99 C1-CZ 1.54 SaHglSl 84 Cx-Ca 1.84 S~H~ZSI 83 Hgi... Hgl 4.87f0.02 Hgl... HgZ 4.65f

3 CANADIAN JOURNAL OF CHEMISTRY. VOL G5 TABLE IV

4 BRADLEY AND KUKCHUR: STRUCTURES OF MERCURY MERCAPTIDES. PART I1 TABLE IV (Continued) h k 1 Fo FO Alpha < <

5 CANADIAN JOURNAL OF CHEMISTRY. VOL. 43, 1965 TABLE IV (Concluded) h k I Fo Fa Alpha < < < Diffraction Effects and tlze Accz~racy of Coordinates The diffraction effects due to the presence of heavy mercury atom were eliminated by doing mercury difference Fourier syntheses. A rough estimate regarding the accuracy of the coordinates was obtained by comparing the common '2' coordinates of the atoms in the two projections. The deviations in the coordinates of mercury and sulfur are found to be 0.01 a and 0.05 A respectively. All carbon atoms excepting carbon C4 correspond to reasonable bond lengths and bond angles within the molecule. The bond length C3-C4 of 1.84 a is much longer than one would expect. On the whole one can state that all carbon atoms have been located and thus the stereochemistry of the mercury mercaptide molecule was defined a '12 FIG. 1. Electron density projection along b axis. Contours are drawn at equal arbitrary intervals. FIG. 2. Interpretation diagram for Fig. 1.

6 BRADLEY AND KUNCHUR: STRUCTURES OF MERCURY MERCAPTIDES. PART I1 FIG. 3. Electron density projection along a axis. Contours are drawn at equal arbitrary intervals. FIG. 4. Interpretation diagram for Fig. 3. RESULTS AND DISCUSSION The various bond lengths and bond angles obtained froin the final coordinates are shown in Table 111. The arrangement of the molecules in the (h01) and (Okl) projections can be seen in Figs. 2 and 4 respectively. In the crystal structure a simple inolecule C2HzS-Hg-SC2Hj contains two Hg-S bonds which are linear within the experimental errors, the Hg-S distance being 2.45 A for both these bonds. There are four more sulfur atoms arranged at a distance of 3.54 A from each nlercury arranged in a square planar configuration. The short Hg-S bonds are perpendicular to this plane so that a mercury atoin is surrounded by six sulfur atoms in a distorted octahedral arrangement. The low melting point of this coinpound (76 "C) suggests that the sulfur atoms at 3.53 A and 3.56 A froin mercury do not give rise to strong interinolecular bonds. A comparison of this compound with inercury methylillercaptide (I) shows that the next longer Hg-S bond lengths of 3.25 A in the latter are slightly shorter than in the for~ner. The higher melting point of Hg(SCH3)2 (175") suggests that the secondary interactions are of greater significance in this compound. Also the larger size of the ethylmercaptide group leads to a looser packing of the inercury ethylmercaptide inolecules and hence the crystal structure is essentially illolecular in type. It is noteworthy that the estimated sum of the Van der

7 2702 C.1NADIAN JOURNAL OF CHEMISTRY. VOL Waal's radii of mercury sulfur of 3.35 A is greater than the secondary Hg... S bond lengths in Hg(SCH3)Z but less than those in Hg(SC2HG)z. ACKNOWLEDGMENTS We are indebted to the U.S. Office of Naval Research for supporting this research. We thank Mr. W. Wilson of the Coinputation Center for much help in writing the programs in Fortran IV for IBM 7040 compiiter and Dr. J. Hart for providing the computing facilities. REFERENCES 1. D. C. BRADLEY and N. R. KUNCHUR. J. Chem. Phys. 40, 2258 (1964). 2. A. F. WELLS. Z. Icrist. 96, 435 (193'7). 3. E. WERTHEIM. J. Atn. Chem. Soc. 51,3661 (1929). 4. INTERNATIONAL TABLES FOR X-RAY CRYSTALLOGRAPHY. Vo Birmingham : Icynoch Press