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Synthesis and characterization of anionic transition metal isothiocyanate complexes prepared from metal powders and thiourea
Inorganica Chimica Acta 338 (2002) 99 /104 www.elsevier.com/locate/ica
Synthesis and characterization of anionic transition metal isothiocyanate complexes prepared from metal powders and thiourea Jerry D. Harris a,b, William E. Eckles a, Aloysius F. Hepp b,*, Stan A. Duraj a,*, Phillip E. Fanwick c b
a Department of Chemistry, Cleveland State University, Cleveland, OH 44115, USA Thin-Film Technology Group, NASA Glenn Research Center, Mail Stop 302-1, 21000 Brookpark Road, Cleveland, OH 44135, USA c Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA
Received 12 December 2001; accepted 4 March 2002
Abstract Three new isothiocyanate-4-methylpyridine anionic compounds were prepared by refluxing metal powders (Mn, Fe, and Cu) with thiourea in 4-methylpyridine (g-picoline). The isothiocyanate ligand is believed to be generated in situ by the isomerization of thiourea to NH4SCN at reflux temperatures. The complexes (Hpic)2[Mn(NCS)4(pic)2] ×/2pic (1), (Hpic)2[Fe(NCS)4(pic)2]×/2pic (2), and (Hpic)[Cu(NCS)3(pic)2] ×/pic (3) (where pic /g-picoline) were characterized by single crystal X-ray crystallography. Compounds 1 and 2 are isostructural with four equatorially bound isothiocyanate ligands and two axially bound g-picoline molecules. Compound 3 is a five-coordinate Cu(II) molecule with a distorted square pyramidal geometry. Coordinated picoline and two isothiocyanates form the basal plane and the remaining isothiocyanate is bound at the apex. The structural data for compounds 1, 2, and 3 are presented. Published by Elsevier Science B.V. Keywords: Thiocyanate; Isothiocyanate; Methylpyridine; Picoline; Coordination; Crystal structures
1. Introduction One of the major obstacles to developing lightweight, flexible, thin-film solar cells is the unavailability of lightweight substrate or superstrate materials that are compatible with current deposition techniques. There are two solutions for this problem: (1) develop new substrate (or superstrate) materials that are compatible with current deposition techniques; or (2) develop new deposition techniques that are compatible with existing materials [1]. NASA Glenn Research Center has decided to focus on the later approach. Our objective is a chemically based approach to developing processes and precursor materials that will produce high-efficiency devices at temperatures below 400 8C [2 /6]. Depositions at low temperatures minimize problems associated with differences in the coeffi* Corresponding authors. Tel.: /1-216-433 3835; fax: /1-216-433 6106 (A.F.H.). Tel.: /1-216-687 2454 (S.A.D.) E-mail addresses: [email protected] (A.F. Hepp), [email protected] (S.A. Duraj). 0020-1693/02/$ - see front matter. Published by Elsevier Science B.V. PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 9 0 3 - 9
cients of thermal expansion between the substrate and thin-film devices and/or decomposition of the substrate. Low temperature processing also enables device fabrication on metal coated polymer substrates such as KaptonTM, a space qualified polyimide. Organometallic molecules typically have low decomposition temperatures and are ideal candidates as single-source precursor molecules for material deposition at low temperatures [7]. In the course of our research, a series of reactions were performed in 4-methylpyridine, also known as gpicoline (pic), where thiourea and a metal powder were refluxed for extended periods of time. Three new isothiocyanate complexes were prepared and structurally characterized. These complexes, a manganese and an iron N-bound tetrakis(isothiocyanate) and a five-coordinate copper N-bound tris(isothiocyanate) are the first anionic isothiocyanate-4-methylpyridine /metal complexes ever reported. The synthetic protocol differs from the more common methods of introducing the thiocyanate ligand into the coordination sphere of the metal by treating soluble metal species with alkali metal
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thiocyanates, ammonium thiocyanate or thiocyanogen in solution.
2. Experimental 2.1. Techniques and materials Manipulation of moisture- and air-sensitive materials were performed under an Ar atmosphere employing standard Schlenk techniques and a double-manifold line. Solids were manipulated in an Ar filled glovebox. Solvents were freshly distilled from sodium benzophenone ketyl under N2 prior to use. The metal powders (Strem Chemicals Inc.), thiourea and Celite (Aldrich Chemical Company) were used without further purification. 2.2. Synthesis of (H-pic)2[Mn(NCS)4(pic)2] ×/2pic (1) Manganese powder (0.27 g, 4.9 mmol), thiourea (1.90 g, 25.0 mmol) and 4-methylpyridine (25 ml) were added to a round bottom flask. After refluxing overnight all of the metal was consumed, yielding a red /brown solution. The solution was allowed to cool to room temperature (r.t.) and left undisturbed for approximately 4 weeks, during which time only a few small crystals appeared. The solution was then filtered through Celite and layered with hexanes. Large crystals grew within several days. 2.3. Synthesis of (H-pic)2[Fe(NCS)4(pic)2] ×/2pic (2) Iron powder (0.28 g, 5.0 mmol), thiourea (1.90 g, 25.0 mmol) and 4-methylpyridine (25 ml) were added to a round bottom flask. Upon refluxing, a yellow solution formed within a few hours and turned red after refluxing overnight. The solution refluxed for 4 days, and then was cooled and layered with 25 ml of hexanes. High quality, dark red crystals grew in a few days. 2.4. Synthesis of (H-pic)[Cu(NCS)3(pic)2] ×/pic (3) Copper powder (0.32 g, 5.0 mmol), thiourea (1.90 g, 25.0 mmol), and 4-methylpyridine (25 ml) were added to a round bottom flask. After refluxing for 2 days, all of the metal powder was consumed. The orange solution was allowed to cool to r.t., filtered through Celite and layered with 30 ml of hexanes. X-ray quality yellow needle and plate shaped crystals grew in 3 days. 2.5. X-ray structure determinations Single crystal diffraction data were collected on an Enraf /Nonius CAD4 automated diffractometer with graphite monochromated Mo Ka radiation. Crystals
were either mounted on a glass fiber (1) or sealed in capillaries (2 and 3). Data was collected at r.t. using v / 2u scans. Initial cell parameters were determined from least-square refinements of 25 high angle reflections obtained from an automatic centering program. To monitor crystal integrity during data collection, three check reflections were monitored periodically (every 83/ 150 min, depending on data set) during the data collections. Check reflections for compound 1 revealed an intensity loss of 70.8% during data collection, so a linear decay correction was applied to the data set. No other data set showed any crystal degradation during data collection. All data sets were corrected for Lp effects, and with the exception of compound 1 an empirical absorption correction was applied to each data set by the use of DIFABS [8]. Azimuthal scans of several reflections for compound 1 indicated the crystal needed no absorption correction. All three structures were solved using SHELX-86 [9] and refined using MOLEN [10]. Initial position of heavy atoms in each of the structures were located using the Patterson method, and the remaining atoms were located in successive leastsquare refinements. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were either located or placed on the appropriate carbon atoms using the standard riding model. Relevant crystallographic data are summarized in Table 1.
3. Results and discussion In our initial publication, we reported that reactions with manganese, iron and Cu in g-picoline yielded the previously reported compounds [Mn(NCS)2(pic)4]×/pic, [Fe(NCS)2(pic)4]×/pic, [Cu(NCS)2(pic)4]×/2/3pic×/1/3H2O, and [Cu(NCS)(pic)2]n [11], where the composition of the products was determined by the stoichiometry of the metal powders and thiourea used. In the work reported here, all metals were reacted with excess thiourea, yielding the first anionic isothiocyanate-4methylpyridine /metal complexes to be reported. Isostructural compounds 1 and 2 consists of sixcoordinate metal centers with four equatorially bound isothiocyanate ligands and two axially bound 4-methylpyridine molecules to give an octahedral geometry. The unit cell for each compound also contains two protonated 4-methylpyridine molecules as counterions and two intrained solvent molecules. The products are shown in Figs. 1 and 2. Axially bound picoline molecules are coplanar, consistent with an inversion center being located at the metal atom sites. In the solid, the metal and the four isothiocyanate ligands form sheets parallel to the a /b plane, located at z /0 and 1/2. The picoline rings extend above and below the metalisothiocyanate layers. The rings of the bound picoline interleave with the rings of the intrained solvent and
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Table 1 Crystallographic data for (Hpic)2[Mn(NCS)4(pic)2]× 2pic (1), (Hpic)2[Fe(NCS)4(pic)2]× 2pic (2), and (Hpic)[Cu(NCS)3(pic)2]× pic (3) Compound
1
2
3
Molecular formula Formula weight Temperature (K) ˚) Wavelength (A Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Dcalc (g cm 3) m (cm 1) Crystal size (mm) 2umax (8) Scan method Observed [I ] 3s (I )] Data/parameters R Rw Goodness-of-fit ˚ 3) Max. residual peak (e A
C40H44MnN10S4 848.06 293 0.71073 P 21/n (no. 14) 12.691(3) 12.338(1) 14.478(4) 90 90.44(2) 90 2266(1) 2 1.24 4.94 0.50 0.38 0.20 45 v /2u 1849 3129/254 0.042 0.050 1.267 0.54
C40H44FeN10S4 848.97 293 0.71073 P 21/n (no. 14) 12.632(1) 12.335(2) 14.484(1) 90 90.424(7) 90 2256.8(7) 2 1.249 5.48 0.40 0.38 0.35 45 v /2u 1941 3114/250 0.037 0.046 1.375 0.36
C27H29CuN7S3 611.31 293 0.71073 Cc (no. 9) 14.656(1) 15.635(2) 14.390(1) 90 112.89 90 3038(1) 4 1.34 9.45 0.46 0.44 0.22 46 v /2u 1734 2201/341 0.045 0.054 1.657 0.46
Fig. 1. ORTEP diagram of the [Mn(NCS)4(pic)2]2 (1) molecule with hydrogens. Atoms are represented by ellipsoids at 50% probability.
Fig. 2. ORTEP diagram of the [Fe(NCS)4(pic)2]2 (2) molecule with hydrogens. Atoms are represented by ellipsoids at 50% probability.
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counterion molecules to yield efficient packing in the solid. In the manganese compound 1, Mn/N distances ˚ for the bound were found to be 2.05(4) and 2.214(4) A ˚ isothiocyanate and 2.306(4) A for the coordinated solvent (Table 2). In the iron compound 2, correspond˚ for ing Fe /N distances measure 2.132(3) and 2.155(4) A ˚ the bound isothiocyanate and 2.243(3) A for the coordinated 4-methylpyridine (Table 3). The isothiocyanate ligands in both compounds are nearly linear, with N /C /S bond angles ranging from 179.0(4) to 179.3(5)8 in the Mn compound and 179.4(3) to 179.5(3)8 in the Fe compound. Metal /N/S angles range from 165.9(4) to 174.1(4)8 and 166.2(3) to 174.3(3)8 for compounds 1 and 2, respectively. All bond distances and angles in these compounds are consistent with the similar Mn2 and Fe2 neutral compounds of [Mn(NCS)2(4-MePy)4] and [Fe(NCS)2(4-MePy)4] [11 /13]. Reactions of thiourea with Cu metal yielded a fivecoordinate Cu(II) molecule. The unit cell for compound 3 consists of the Cu-containing molecule [Cu(NCS)3(pic)2] , a protonated picoline molecule serving as the counterion, and a trapped 4-methylpyridine molecule. The Cu-containing molecule has a distorted square pyramidal geometry, with the coordinated picoline and two isothiocyanates forming the basal plane and the remaining isothiocyanate bound at the apex (Fig. 3). The compound has a layered structure with the Cu atoms and isothiocyanate molecules lying in planes parallel to the (010) plane and located at z /0.25 and 0.75. The coordinated picoline molecules extend above and below the Cu-isothiocyanate layers and interleave with bound picoline from neighboring layers and occluded solvent molecules for efficient packing in the solid. Cu /Npicoline bond distances are 2.039(7) and ˚ (Table 4). Two of the Cu /NCS bond 2.050(7) A ˚ , but the third is distances are 1.928(9) and 1.968(9) A ˚ 2.23(1) A. The elongated Cu /N7 bond is for the
Table 3 ˚ ) and angles (8) with estimated standard Selected bond lengths (A deviations for (Hpic)2[Fe(NCS)4(pic)2]× 2pic (2) Bond lengths Fe N(1) Fe N(2) Fe N(31) N(1) C(1) N(2) C(2) C(1) S(1) C(2) S(2)
2.155(4) 2.132(3) 2.243(3) 1.157(5) 1.151(4) 1.625(5) 1.630(4)
Bond angles Fe N(1) C(1) Fe N(2) C(2) N(1) C(1) S(1) N(2) C(2) S(2) N(1) Fe N(2) N(1) Mn N(31) N(2) Mn N(31)
166.2(3) 174.3(3) 179.5(3) 179.4(3) 91.5(1) 88.4(1) 89.5(1)
Table 2 ˚ ) and angles (8) with estimated standard Selected bond lengths (A deviations for (Hpic)2[Mn(NCS)4(pic)2]× 2pic (1) Bond lengths Mn N(1) Mn N(2) Mn N(11) N(1) C(1) N(2) C(2) C(1) S(1) C(2) S(2)
2.214(4) 2.205(4) 2.306(4) 1.146(5) 1.145(5) 1.636(6) 1.628(5)
Bond angles Mn N(1) C(1) Mn N(2) C(2) N(1) C(1) S(1) N(2) C(2) S(2) N(1) Mn N(2) N(1) Mn N(11) N(2) Mn N(11)
165.9(4) 174.1(4) 179.3(5) 179.0(4) 88.0(1) 88.7(1) 90.7(1)
Fig. 3. ORTEP diagram of the [Cu(NCS)3(pic)2] (3) molecule with hydrogens. Atoms are represented by ellipsoids at 50% probability.
isothiocyanate molecule bound at the apical position and arises from long range electronic interactions with neighboring Cu atoms. The apical isothiocyanate is positioned such that one of the lone pair of electrons on the sulfur atom is directed at the vacant site of the Cu on a neighboring [Cu(NCS)3(pic)2] molecule. The Cu /S7
J.D. Harris et al. / Inorganica Chimica Acta 338 (2002) 99 /104 Table 4 ˚ ) and angles (8) with estimated standard Selected bond lengths (A deviations for (Hpic)[Cu(NCS)3(pic)2]× pic (3) Bond lengths Cu N(1) Cu N(2) Cu N(7) Cu N(31) Cu N(41) Cu S(7)
1.928(9) 1.968(9) 2.23(1) 2.050(7) 2.039(7) 3.402(4)
N(1) C(1) N(2) C(2) N(7) C(7) C(1) S(1) C(2) S(2) C(7) S(7)
1.15(1) 1.15(1) 1.17(1) 1.60(1) 1.61(1) 1.59(1)
Bond angles Cu N(1) C(1) Cu N(2) C(2) Cu N(7) C(7) Cu S(7) C(7) N(1) Cu N(2) N(7) Cu S(7) N(7) Cu N(1) N(7) Cu N(2) N(7) Cu N(31) N(7) Cu N(41) N(1) Cu N(31)
175.0(8) 167.9(8) 149.4(9) 142.6(4) 165.4(4) 174.4(3) 99.1(4) 95.4(4) 93.6(3) 93.3(3) 90.7(3)
N(1) Cu N(41) N(1) Cu S(7) N(2) Cu N(31) N(2) Cu N(41) N(2) Cu S(7) N(31) Cu S(7) N(41) Cu S(7) N(1) C(1) S(1) N(2) C(2) S(2) N(7) C(7) S(7)
90.7(3) 85.8(3) 88.5(3) 88.4(3) 79.6(3) 88.9(2) 84.0(2) 179.0(1) 176.7(9) 179.4(9)
˚ . Although this distance for this interaction is 3.402(4) A distance is outside the Cu /sulfur van der Waals distance ˚ ), there undoubtedly are some long range (3.25 A electrostatic interactions between the two atoms. This interaction allows the molecules to form one-dimensional ‘chains’ that run perpendicular to the (101) plane. In addition, steric crowding limits the Cu centers from moving closer together and allowing the Cu /S or Cu / NCS bond to shorten. Long Cu /S and elongated apical Cu /NCS bonds in pseudo five-coordinate Cu-isothiocyanate compounds have been previously observed. In [Cu(NCS)2(3,4-dimepy)3] and [Cu(NCS)2(3-mepy)3] [14], the three substituted pyridine molecules and one of the isothiocyanate ligands form the basal plane of a distorted square based pyramid, with the remaining isothiocyanate bound at the apex. In these molecules the apical isothiocyanate bridges to another molecule at the vacant Cu site through the sulfur with bond distances of ˚ , respectively for the two mole3.41(1) and 3.62(1) A cules. The apical Cu /N bonds for the two molecules are ˚ , respectively. In the extended 2.22(2) and 2.219(1) A structure of [Cu(NCS)(cyclam)](SCN) [15], (where cyclam is 1,4,8,11-tetraazacyclotetradecane), a similar apical Cu /NCS bond lengthening is observed. In the molecule, the cyclam forms the base of a square based pyramid, with the isothiocyanate ligand at the apex. Again the thiocyanate molecule bridges to a neighboring molecule at the vacant Cu site, giving Cu /NCS and Cu / ˚ , respecS bond distances of 2.430(4) and 3.015(1) A tively. All three isothiocyanate groups in [Cu(NCS)3(pic)2] are nearly linear, with the N/C /S bond angle ranging from 176.7(9) to 179.4(9)8. The Cu /N/C bond angles
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are 167.9(9) and 175.0(8)8 for the terminal isothiocyanates and 149.4(9)8 for the bridging isothiocyanate. The Cu /S /C bond angle for the bridging isothiocyanate is 142.6(4)8. In the [Cu(NCS)3(pic)2] chains, the planes formed by the Cu(NCS)3 units rock back and forth around the chain axis with a dihedral angle between alternating planes of approximately 308. All other bond distances and angles are as expected and compare well with other Cu-isothiocyanate-substituted pyridine compounds. Traditionally, thiocyanate and isothiocyanate compounds have been prepared in solution by reacting metal salts with thiocyanate salts and donor ligands or from M(SCN)n and donor ligands. However, the products reported here were prepared by refluxing metal powders with excess thiourea in 4-methylpyridine. Several early reactions were attempted using pyridine, but no crystalline products were formed. The reactions appear to be quantitative, and in a few cases after crystallization, what remains is a flask full of crystals and a nearly colorless solution. Although the reactions are a ‘one pot synthesis,’ the mechanism for the reaction is not completely understood. It is well documented that thiourea can be prepared from solutions of NH4SCN, with reported yields ranging from 25 to 95% [16,17] and that the equilibrium concentration ratio of NH4SCN to thiourea (NH4SCN:S /C(NH2)2) is approximately 0.75:0.25 in propanol and butanol at temperatures near the boiling point of 4-methylpyridine (145 8C) [18]. Therefore, the most probable pathway for the reactions is that under picoline reflux conditions a portion of the thiourea is converted to the salt and the metal reacts with the thiocyanate ions present in solution and forces the isomerization reaction (Eq. (1)) to the right. SC(NH2 )2 XSCN NH4
(1)
However, this pathway fails to account for the observed oxidation of the metals during the reaction. A potential source of metal oxidation is impurities in the solvent, but all solvents were rigorously dried and distilled under nitrogen prior to use. Another possibility is contamination of the metal powders with metal oxide impurities; the isolated products are simply the result of the oxidized metal reacting with the isothiocyanate ions in solution and leaving the unoxidized metal unreacted. This scenario also seems unlikely given the metal powders were completely consumed during the reactions. One last possibility to consider is the reduction of NH4 or protonated g-picoline at the reflux temperatures of 4-methylpyridine to yield NH3 and H2 or gpicoline and H2, which would require one electron per NH4 or Hpic that is reduced. Some empirical evidence does support the hypothesis; during the early stages of many of the reactions, evolution of gas was observed. However, no attempt was made to isolate or
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characterize the gas. Any NH3 or H2 formed during the reaction would out-gas from the solution during reflux conditions and thus pull the reaction to the right. Furthermore, with the exception of Cu, all metals that have yielded crystalline products have a negative aqueous standard reduction potential, compared to the reduction potential of hydrogen, (0.0 V, E 8) [19]. In summary, the synthetic pathway is not yet fully understood, but it has yielded three new compounds. The synthetic mechanism and the oxidation of the metal powders are currently under investigation, and the results will be reported in a future communication.
4. Conclusion During our effort to prepare new single-source precursor molecules that could be used to deposit device quality material at low temperatures, three new anionic metal/isothiocyanate-4-methylpyridine complexes were synthesized and structurally characterized. They were prepared by refluxing the metal powders (Mn, Fe or Cu) with excess thiourea in 4-methylpyridine, rather than the standard synthetic protocol of reacting metal salts with thiocyanate salts or thiocyanogen.
5. Supplementary material A complete listing of anisotropic displacement parameters, interatomic distances, angles, hydrogen coordinates and displacement parameters and structure factor amplitudes for the reported compounds are available upon request from the authors. Atomic coordinates for the structural analyses have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 175214 /175216 for compounds 1, 2 and 3. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: /44-1223-336-033; e-mail: deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements We gratefully acknowledge the National Aeronautics and Space Administration for its support through grants NCC3-817, NCC3-869 and NAG3-2484, and the NASA Glenn Research Center Director’s Discretionary Fund.
References [1] K.K. Banger, J.D. Harris, J.E. Cowen, A.F. Hepp, Thin Solid Films 403 /404 (2001) 390. [2] J.D. Harris, R.P. Raffaelle, K.K. Banger, M.A. Smith, D.A. Scheiman, A.F. Hepp, Proc. of the 17th Space Photovoltaic Research and Technology Conference, Brook Park, OH, September 11 /13, 2001. [3] J.D. Harris, D.G. Hehemann, J.E. Cowen, A.F. Hepp, R.P. Raffaelle, J.A. Hollingsworth, Proc. of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, September 17 / 22, 2000, pp. 563 /566. [4] J.A. Hollingsworth, A.F. Hepp, W.E. Buhro, Chem. Vap. Deposition 5 (1999) 105. [5] J.A. Hollingsworth, W.F. Buhro, A.F. Hepp, P.P. Jenkins, M.A. Stan, Mater. Res. Soc. Symp. Proc. 495 (1998) 171. [6] D.O. Henderson, R. Mu, A. Ueda, M.H. Wu, E.M. Gordon, Y.S. Tung, M. Huang, J. Keay, L.C. Feldman, J.A. Hollingsworth, W.E. Buhro, J.D. Harris, A.F. Hepp, R.P. Raffaelle, Mater. Design 22 (2001) 585. [7] K.K. Banger, J. Cowen, A.F. Hepp, Chem. Mater. 13 (2001) 3827. [8] N. Walker, D. Stuart, Acta Crystallogr., Sect. A 39 (1983) 158. [9] G.M. Sheldrick, SHELXS-86, Program for Crystal Structure Solution, University of Go¨ttingen, Go¨ttingen, Germany, 1986. [10] C.K. Fair, MOLEN, An Interactive Intelligent System for Crystal Structure Analysis, Enraf /Nonius, Delft, The Netherlands, 1990. [11] J.D. Harris, W.E. Eckles, A.F. Hepp, S.A. Duraj, P.E. Fanwick, J. Richardson, E.M. Gordon, Mater. Design 22 (2001) 625. [12] N.V. Pervukhina, N.V. Podberezskaya, I.V. Davydova, N.V. Kislykh, J. Incl. Phenom. 13 (1992) 9. [13] J. Lipkowski, J. Incl. Phenom. 8 (1990) 439. [14] M. Kabesova´, Z. Kozı´skova´, Collect. Czech. Chem. Commun. 54 (1989) 1800. [15] T.-H. Lu, T.H. Tahirov, Y.-L. Liu, C.-S. Chung, C.-C. Huang, Y.-S. Hong, Acta Crystallogr., C 52 (1996) 1093. [16] R.E. Powers, J. Mitchell, Preparation of Thiourea, Patent # 2,506,596, Koppers Company, Inc., USA, 1951. [17] H.J. Arnikar, A.H. Kapadi, D.G. Naik, J.G. Chandwadkar, J. Radioanal. Nucl. Chem. 185 (1994) 227. [18] G.H. Burrows, J. Am. Chem. Soc. 46 (1924) 1623. [19] R.C. Weast, M.J. Astle, W.H. Beyer, Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1986.
Synthesis and characterization of anionic transition metal isothiocyanate complexes prepared from metal powders and thiourea Jerry D. Harris a,b, William E. Eckles a, Aloysius F. Hepp b,*, Stan A. Duraj a,*, Phillip E. Fanwick c b
a Department of Chemistry, Cleveland State University, Cleveland, OH 44115, USA Thin-Film Technology Group, NASA Glenn Research Center, Mail Stop 302-1, 21000 Brookpark Road, Cleveland, OH 44135, USA c Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA
Received 12 December 2001; accepted 4 March 2002
Abstract Three new isothiocyanate-4-methylpyridine anionic compounds were prepared by refluxing metal powders (Mn, Fe, and Cu) with thiourea in 4-methylpyridine (g-picoline). The isothiocyanate ligand is believed to be generated in situ by the isomerization of thiourea to NH4SCN at reflux temperatures. The complexes (Hpic)2[Mn(NCS)4(pic)2] ×/2pic (1), (Hpic)2[Fe(NCS)4(pic)2]×/2pic (2), and (Hpic)[Cu(NCS)3(pic)2] ×/pic (3) (where pic /g-picoline) were characterized by single crystal X-ray crystallography. Compounds 1 and 2 are isostructural with four equatorially bound isothiocyanate ligands and two axially bound g-picoline molecules. Compound 3 is a five-coordinate Cu(II) molecule with a distorted square pyramidal geometry. Coordinated picoline and two isothiocyanates form the basal plane and the remaining isothiocyanate is bound at the apex. The structural data for compounds 1, 2, and 3 are presented. Published by Elsevier Science B.V. Keywords: Thiocyanate; Isothiocyanate; Methylpyridine; Picoline; Coordination; Crystal structures
1. Introduction One of the major obstacles to developing lightweight, flexible, thin-film solar cells is the unavailability of lightweight substrate or superstrate materials that are compatible with current deposition techniques. There are two solutions for this problem: (1) develop new substrate (or superstrate) materials that are compatible with current deposition techniques; or (2) develop new deposition techniques that are compatible with existing materials [1]. NASA Glenn Research Center has decided to focus on the later approach. Our objective is a chemically based approach to developing processes and precursor materials that will produce high-efficiency devices at temperatures below 400 8C [2 /6]. Depositions at low temperatures minimize problems associated with differences in the coeffi* Corresponding authors. Tel.: /1-216-433 3835; fax: /1-216-433 6106 (A.F.H.). Tel.: /1-216-687 2454 (S.A.D.) E-mail addresses: [email protected] (A.F. Hepp), [email protected] (S.A. Duraj). 0020-1693/02/$ - see front matter. Published by Elsevier Science B.V. PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 9 0 3 - 9
cients of thermal expansion between the substrate and thin-film devices and/or decomposition of the substrate. Low temperature processing also enables device fabrication on metal coated polymer substrates such as KaptonTM, a space qualified polyimide. Organometallic molecules typically have low decomposition temperatures and are ideal candidates as single-source precursor molecules for material deposition at low temperatures [7]. In the course of our research, a series of reactions were performed in 4-methylpyridine, also known as gpicoline (pic), where thiourea and a metal powder were refluxed for extended periods of time. Three new isothiocyanate complexes were prepared and structurally characterized. These complexes, a manganese and an iron N-bound tetrakis(isothiocyanate) and a five-coordinate copper N-bound tris(isothiocyanate) are the first anionic isothiocyanate-4-methylpyridine /metal complexes ever reported. The synthetic protocol differs from the more common methods of introducing the thiocyanate ligand into the coordination sphere of the metal by treating soluble metal species with alkali metal
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thiocyanates, ammonium thiocyanate or thiocyanogen in solution.
2. Experimental 2.1. Techniques and materials Manipulation of moisture- and air-sensitive materials were performed under an Ar atmosphere employing standard Schlenk techniques and a double-manifold line. Solids were manipulated in an Ar filled glovebox. Solvents were freshly distilled from sodium benzophenone ketyl under N2 prior to use. The metal powders (Strem Chemicals Inc.), thiourea and Celite (Aldrich Chemical Company) were used without further purification. 2.2. Synthesis of (H-pic)2[Mn(NCS)4(pic)2] ×/2pic (1) Manganese powder (0.27 g, 4.9 mmol), thiourea (1.90 g, 25.0 mmol) and 4-methylpyridine (25 ml) were added to a round bottom flask. After refluxing overnight all of the metal was consumed, yielding a red /brown solution. The solution was allowed to cool to room temperature (r.t.) and left undisturbed for approximately 4 weeks, during which time only a few small crystals appeared. The solution was then filtered through Celite and layered with hexanes. Large crystals grew within several days. 2.3. Synthesis of (H-pic)2[Fe(NCS)4(pic)2] ×/2pic (2) Iron powder (0.28 g, 5.0 mmol), thiourea (1.90 g, 25.0 mmol) and 4-methylpyridine (25 ml) were added to a round bottom flask. Upon refluxing, a yellow solution formed within a few hours and turned red after refluxing overnight. The solution refluxed for 4 days, and then was cooled and layered with 25 ml of hexanes. High quality, dark red crystals grew in a few days. 2.4. Synthesis of (H-pic)[Cu(NCS)3(pic)2] ×/pic (3) Copper powder (0.32 g, 5.0 mmol), thiourea (1.90 g, 25.0 mmol), and 4-methylpyridine (25 ml) were added to a round bottom flask. After refluxing for 2 days, all of the metal powder was consumed. The orange solution was allowed to cool to r.t., filtered through Celite and layered with 30 ml of hexanes. X-ray quality yellow needle and plate shaped crystals grew in 3 days. 2.5. X-ray structure determinations Single crystal diffraction data were collected on an Enraf /Nonius CAD4 automated diffractometer with graphite monochromated Mo Ka radiation. Crystals
were either mounted on a glass fiber (1) or sealed in capillaries (2 and 3). Data was collected at r.t. using v / 2u scans. Initial cell parameters were determined from least-square refinements of 25 high angle reflections obtained from an automatic centering program. To monitor crystal integrity during data collection, three check reflections were monitored periodically (every 83/ 150 min, depending on data set) during the data collections. Check reflections for compound 1 revealed an intensity loss of 70.8% during data collection, so a linear decay correction was applied to the data set. No other data set showed any crystal degradation during data collection. All data sets were corrected for Lp effects, and with the exception of compound 1 an empirical absorption correction was applied to each data set by the use of DIFABS [8]. Azimuthal scans of several reflections for compound 1 indicated the crystal needed no absorption correction. All three structures were solved using SHELX-86 [9] and refined using MOLEN [10]. Initial position of heavy atoms in each of the structures were located using the Patterson method, and the remaining atoms were located in successive leastsquare refinements. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were either located or placed on the appropriate carbon atoms using the standard riding model. Relevant crystallographic data are summarized in Table 1.
3. Results and discussion In our initial publication, we reported that reactions with manganese, iron and Cu in g-picoline yielded the previously reported compounds [Mn(NCS)2(pic)4]×/pic, [Fe(NCS)2(pic)4]×/pic, [Cu(NCS)2(pic)4]×/2/3pic×/1/3H2O, and [Cu(NCS)(pic)2]n [11], where the composition of the products was determined by the stoichiometry of the metal powders and thiourea used. In the work reported here, all metals were reacted with excess thiourea, yielding the first anionic isothiocyanate-4methylpyridine /metal complexes to be reported. Isostructural compounds 1 and 2 consists of sixcoordinate metal centers with four equatorially bound isothiocyanate ligands and two axially bound 4-methylpyridine molecules to give an octahedral geometry. The unit cell for each compound also contains two protonated 4-methylpyridine molecules as counterions and two intrained solvent molecules. The products are shown in Figs. 1 and 2. Axially bound picoline molecules are coplanar, consistent with an inversion center being located at the metal atom sites. In the solid, the metal and the four isothiocyanate ligands form sheets parallel to the a /b plane, located at z /0 and 1/2. The picoline rings extend above and below the metalisothiocyanate layers. The rings of the bound picoline interleave with the rings of the intrained solvent and
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Table 1 Crystallographic data for (Hpic)2[Mn(NCS)4(pic)2]× 2pic (1), (Hpic)2[Fe(NCS)4(pic)2]× 2pic (2), and (Hpic)[Cu(NCS)3(pic)2]× pic (3) Compound
1
2
3
Molecular formula Formula weight Temperature (K) ˚) Wavelength (A Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Dcalc (g cm 3) m (cm 1) Crystal size (mm) 2umax (8) Scan method Observed [I ] 3s (I )] Data/parameters R Rw Goodness-of-fit ˚ 3) Max. residual peak (e A
C40H44MnN10S4 848.06 293 0.71073 P 21/n (no. 14) 12.691(3) 12.338(1) 14.478(4) 90 90.44(2) 90 2266(1) 2 1.24 4.94 0.50 0.38 0.20 45 v /2u 1849 3129/254 0.042 0.050 1.267 0.54
C40H44FeN10S4 848.97 293 0.71073 P 21/n (no. 14) 12.632(1) 12.335(2) 14.484(1) 90 90.424(7) 90 2256.8(7) 2 1.249 5.48 0.40 0.38 0.35 45 v /2u 1941 3114/250 0.037 0.046 1.375 0.36
C27H29CuN7S3 611.31 293 0.71073 Cc (no. 9) 14.656(1) 15.635(2) 14.390(1) 90 112.89 90 3038(1) 4 1.34 9.45 0.46 0.44 0.22 46 v /2u 1734 2201/341 0.045 0.054 1.657 0.46
Fig. 1. ORTEP diagram of the [Mn(NCS)4(pic)2]2 (1) molecule with hydrogens. Atoms are represented by ellipsoids at 50% probability.
Fig. 2. ORTEP diagram of the [Fe(NCS)4(pic)2]2 (2) molecule with hydrogens. Atoms are represented by ellipsoids at 50% probability.
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counterion molecules to yield efficient packing in the solid. In the manganese compound 1, Mn/N distances ˚ for the bound were found to be 2.05(4) and 2.214(4) A ˚ isothiocyanate and 2.306(4) A for the coordinated solvent (Table 2). In the iron compound 2, correspond˚ for ing Fe /N distances measure 2.132(3) and 2.155(4) A ˚ the bound isothiocyanate and 2.243(3) A for the coordinated 4-methylpyridine (Table 3). The isothiocyanate ligands in both compounds are nearly linear, with N /C /S bond angles ranging from 179.0(4) to 179.3(5)8 in the Mn compound and 179.4(3) to 179.5(3)8 in the Fe compound. Metal /N/S angles range from 165.9(4) to 174.1(4)8 and 166.2(3) to 174.3(3)8 for compounds 1 and 2, respectively. All bond distances and angles in these compounds are consistent with the similar Mn2 and Fe2 neutral compounds of [Mn(NCS)2(4-MePy)4] and [Fe(NCS)2(4-MePy)4] [11 /13]. Reactions of thiourea with Cu metal yielded a fivecoordinate Cu(II) molecule. The unit cell for compound 3 consists of the Cu-containing molecule [Cu(NCS)3(pic)2] , a protonated picoline molecule serving as the counterion, and a trapped 4-methylpyridine molecule. The Cu-containing molecule has a distorted square pyramidal geometry, with the coordinated picoline and two isothiocyanates forming the basal plane and the remaining isothiocyanate bound at the apex (Fig. 3). The compound has a layered structure with the Cu atoms and isothiocyanate molecules lying in planes parallel to the (010) plane and located at z /0.25 and 0.75. The coordinated picoline molecules extend above and below the Cu-isothiocyanate layers and interleave with bound picoline from neighboring layers and occluded solvent molecules for efficient packing in the solid. Cu /Npicoline bond distances are 2.039(7) and ˚ (Table 4). Two of the Cu /NCS bond 2.050(7) A ˚ , but the third is distances are 1.928(9) and 1.968(9) A ˚ 2.23(1) A. The elongated Cu /N7 bond is for the
Table 3 ˚ ) and angles (8) with estimated standard Selected bond lengths (A deviations for (Hpic)2[Fe(NCS)4(pic)2]× 2pic (2) Bond lengths Fe N(1) Fe N(2) Fe N(31) N(1) C(1) N(2) C(2) C(1) S(1) C(2) S(2)
2.155(4) 2.132(3) 2.243(3) 1.157(5) 1.151(4) 1.625(5) 1.630(4)
Bond angles Fe N(1) C(1) Fe N(2) C(2) N(1) C(1) S(1) N(2) C(2) S(2) N(1) Fe N(2) N(1) Mn N(31) N(2) Mn N(31)
166.2(3) 174.3(3) 179.5(3) 179.4(3) 91.5(1) 88.4(1) 89.5(1)
Table 2 ˚ ) and angles (8) with estimated standard Selected bond lengths (A deviations for (Hpic)2[Mn(NCS)4(pic)2]× 2pic (1) Bond lengths Mn N(1) Mn N(2) Mn N(11) N(1) C(1) N(2) C(2) C(1) S(1) C(2) S(2)
2.214(4) 2.205(4) 2.306(4) 1.146(5) 1.145(5) 1.636(6) 1.628(5)
Bond angles Mn N(1) C(1) Mn N(2) C(2) N(1) C(1) S(1) N(2) C(2) S(2) N(1) Mn N(2) N(1) Mn N(11) N(2) Mn N(11)
165.9(4) 174.1(4) 179.3(5) 179.0(4) 88.0(1) 88.7(1) 90.7(1)
Fig. 3. ORTEP diagram of the [Cu(NCS)3(pic)2] (3) molecule with hydrogens. Atoms are represented by ellipsoids at 50% probability.
isothiocyanate molecule bound at the apical position and arises from long range electronic interactions with neighboring Cu atoms. The apical isothiocyanate is positioned such that one of the lone pair of electrons on the sulfur atom is directed at the vacant site of the Cu on a neighboring [Cu(NCS)3(pic)2] molecule. The Cu /S7
J.D. Harris et al. / Inorganica Chimica Acta 338 (2002) 99 /104 Table 4 ˚ ) and angles (8) with estimated standard Selected bond lengths (A deviations for (Hpic)[Cu(NCS)3(pic)2]× pic (3) Bond lengths Cu N(1) Cu N(2) Cu N(7) Cu N(31) Cu N(41) Cu S(7)
1.928(9) 1.968(9) 2.23(1) 2.050(7) 2.039(7) 3.402(4)
N(1) C(1) N(2) C(2) N(7) C(7) C(1) S(1) C(2) S(2) C(7) S(7)
1.15(1) 1.15(1) 1.17(1) 1.60(1) 1.61(1) 1.59(1)
Bond angles Cu N(1) C(1) Cu N(2) C(2) Cu N(7) C(7) Cu S(7) C(7) N(1) Cu N(2) N(7) Cu S(7) N(7) Cu N(1) N(7) Cu N(2) N(7) Cu N(31) N(7) Cu N(41) N(1) Cu N(31)
175.0(8) 167.9(8) 149.4(9) 142.6(4) 165.4(4) 174.4(3) 99.1(4) 95.4(4) 93.6(3) 93.3(3) 90.7(3)
N(1) Cu N(41) N(1) Cu S(7) N(2) Cu N(31) N(2) Cu N(41) N(2) Cu S(7) N(31) Cu S(7) N(41) Cu S(7) N(1) C(1) S(1) N(2) C(2) S(2) N(7) C(7) S(7)
90.7(3) 85.8(3) 88.5(3) 88.4(3) 79.6(3) 88.9(2) 84.0(2) 179.0(1) 176.7(9) 179.4(9)
˚ . Although this distance for this interaction is 3.402(4) A distance is outside the Cu /sulfur van der Waals distance ˚ ), there undoubtedly are some long range (3.25 A electrostatic interactions between the two atoms. This interaction allows the molecules to form one-dimensional ‘chains’ that run perpendicular to the (101) plane. In addition, steric crowding limits the Cu centers from moving closer together and allowing the Cu /S or Cu / NCS bond to shorten. Long Cu /S and elongated apical Cu /NCS bonds in pseudo five-coordinate Cu-isothiocyanate compounds have been previously observed. In [Cu(NCS)2(3,4-dimepy)3] and [Cu(NCS)2(3-mepy)3] [14], the three substituted pyridine molecules and one of the isothiocyanate ligands form the basal plane of a distorted square based pyramid, with the remaining isothiocyanate bound at the apex. In these molecules the apical isothiocyanate bridges to another molecule at the vacant Cu site through the sulfur with bond distances of ˚ , respectively for the two mole3.41(1) and 3.62(1) A cules. The apical Cu /N bonds for the two molecules are ˚ , respectively. In the extended 2.22(2) and 2.219(1) A structure of [Cu(NCS)(cyclam)](SCN) [15], (where cyclam is 1,4,8,11-tetraazacyclotetradecane), a similar apical Cu /NCS bond lengthening is observed. In the molecule, the cyclam forms the base of a square based pyramid, with the isothiocyanate ligand at the apex. Again the thiocyanate molecule bridges to a neighboring molecule at the vacant Cu site, giving Cu /NCS and Cu / ˚ , respecS bond distances of 2.430(4) and 3.015(1) A tively. All three isothiocyanate groups in [Cu(NCS)3(pic)2] are nearly linear, with the N/C /S bond angle ranging from 176.7(9) to 179.4(9)8. The Cu /N/C bond angles
103
are 167.9(9) and 175.0(8)8 for the terminal isothiocyanates and 149.4(9)8 for the bridging isothiocyanate. The Cu /S /C bond angle for the bridging isothiocyanate is 142.6(4)8. In the [Cu(NCS)3(pic)2] chains, the planes formed by the Cu(NCS)3 units rock back and forth around the chain axis with a dihedral angle between alternating planes of approximately 308. All other bond distances and angles are as expected and compare well with other Cu-isothiocyanate-substituted pyridine compounds. Traditionally, thiocyanate and isothiocyanate compounds have been prepared in solution by reacting metal salts with thiocyanate salts and donor ligands or from M(SCN)n and donor ligands. However, the products reported here were prepared by refluxing metal powders with excess thiourea in 4-methylpyridine. Several early reactions were attempted using pyridine, but no crystalline products were formed. The reactions appear to be quantitative, and in a few cases after crystallization, what remains is a flask full of crystals and a nearly colorless solution. Although the reactions are a ‘one pot synthesis,’ the mechanism for the reaction is not completely understood. It is well documented that thiourea can be prepared from solutions of NH4SCN, with reported yields ranging from 25 to 95% [16,17] and that the equilibrium concentration ratio of NH4SCN to thiourea (NH4SCN:S /C(NH2)2) is approximately 0.75:0.25 in propanol and butanol at temperatures near the boiling point of 4-methylpyridine (145 8C) [18]. Therefore, the most probable pathway for the reactions is that under picoline reflux conditions a portion of the thiourea is converted to the salt and the metal reacts with the thiocyanate ions present in solution and forces the isomerization reaction (Eq. (1)) to the right. SC(NH2 )2 XSCN NH4
(1)
However, this pathway fails to account for the observed oxidation of the metals during the reaction. A potential source of metal oxidation is impurities in the solvent, but all solvents were rigorously dried and distilled under nitrogen prior to use. Another possibility is contamination of the metal powders with metal oxide impurities; the isolated products are simply the result of the oxidized metal reacting with the isothiocyanate ions in solution and leaving the unoxidized metal unreacted. This scenario also seems unlikely given the metal powders were completely consumed during the reactions. One last possibility to consider is the reduction of NH4 or protonated g-picoline at the reflux temperatures of 4-methylpyridine to yield NH3 and H2 or gpicoline and H2, which would require one electron per NH4 or Hpic that is reduced. Some empirical evidence does support the hypothesis; during the early stages of many of the reactions, evolution of gas was observed. However, no attempt was made to isolate or
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characterize the gas. Any NH3 or H2 formed during the reaction would out-gas from the solution during reflux conditions and thus pull the reaction to the right. Furthermore, with the exception of Cu, all metals that have yielded crystalline products have a negative aqueous standard reduction potential, compared to the reduction potential of hydrogen, (0.0 V, E 8) [19]. In summary, the synthetic pathway is not yet fully understood, but it has yielded three new compounds. The synthetic mechanism and the oxidation of the metal powders are currently under investigation, and the results will be reported in a future communication.
4. Conclusion During our effort to prepare new single-source precursor molecules that could be used to deposit device quality material at low temperatures, three new anionic metal/isothiocyanate-4-methylpyridine complexes were synthesized and structurally characterized. They were prepared by refluxing the metal powders (Mn, Fe or Cu) with excess thiourea in 4-methylpyridine, rather than the standard synthetic protocol of reacting metal salts with thiocyanate salts or thiocyanogen.
5. Supplementary material A complete listing of anisotropic displacement parameters, interatomic distances, angles, hydrogen coordinates and displacement parameters and structure factor amplitudes for the reported compounds are available upon request from the authors. Atomic coordinates for the structural analyses have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 175214 /175216 for compounds 1, 2 and 3. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: /44-1223-336-033; e-mail: deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements We gratefully acknowledge the National Aeronautics and Space Administration for its support through grants NCC3-817, NCC3-869 and NAG3-2484, and the NASA Glenn Research Center Director’s Discretionary Fund.
References [1] K.K. Banger, J.D. Harris, J.E. Cowen, A.F. Hepp, Thin Solid Films 403 /404 (2001) 390. [2] J.D. Harris, R.P. Raffaelle, K.K. Banger, M.A. Smith, D.A. Scheiman, A.F. Hepp, Proc. of the 17th Space Photovoltaic Research and Technology Conference, Brook Park, OH, September 11 /13, 2001. [3] J.D. Harris, D.G. Hehemann, J.E. Cowen, A.F. Hepp, R.P. Raffaelle, J.A. Hollingsworth, Proc. of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, September 17 / 22, 2000, pp. 563 /566. [4] J.A. Hollingsworth, A.F. Hepp, W.E. Buhro, Chem. Vap. Deposition 5 (1999) 105. [5] J.A. Hollingsworth, W.F. Buhro, A.F. Hepp, P.P. Jenkins, M.A. Stan, Mater. Res. Soc. Symp. Proc. 495 (1998) 171. [6] D.O. Henderson, R. Mu, A. Ueda, M.H. Wu, E.M. Gordon, Y.S. Tung, M. Huang, J. Keay, L.C. Feldman, J.A. Hollingsworth, W.E. Buhro, J.D. Harris, A.F. Hepp, R.P. Raffaelle, Mater. Design 22 (2001) 585. [7] K.K. Banger, J. Cowen, A.F. Hepp, Chem. Mater. 13 (2001) 3827. [8] N. Walker, D. Stuart, Acta Crystallogr., Sect. A 39 (1983) 158. [9] G.M. Sheldrick, SHELXS-86, Program for Crystal Structure Solution, University of Go¨ttingen, Go¨ttingen, Germany, 1986. [10] C.K. Fair, MOLEN, An Interactive Intelligent System for Crystal Structure Analysis, Enraf /Nonius, Delft, The Netherlands, 1990. [11] J.D. Harris, W.E. Eckles, A.F. Hepp, S.A. Duraj, P.E. Fanwick, J. Richardson, E.M. Gordon, Mater. Design 22 (2001) 625. [12] N.V. Pervukhina, N.V. Podberezskaya, I.V. Davydova, N.V. Kislykh, J. Incl. Phenom. 13 (1992) 9. [13] J. Lipkowski, J. Incl. Phenom. 8 (1990) 439. [14] M. Kabesova´, Z. Kozı´skova´, Collect. Czech. Chem. Commun. 54 (1989) 1800. [15] T.-H. Lu, T.H. Tahirov, Y.-L. Liu, C.-S. Chung, C.-C. Huang, Y.-S. Hong, Acta Crystallogr., C 52 (1996) 1093. [16] R.E. Powers, J. Mitchell, Preparation of Thiourea, Patent # 2,506,596, Koppers Company, Inc., USA, 1951. [17] H.J. Arnikar, A.H. Kapadi, D.G. Naik, J.G. Chandwadkar, J. Radioanal. Nucl. Chem. 185 (1994) 227. [18] G.H. Burrows, J. Am. Chem. Soc. 46 (1924) 1623. [19] R.C. Weast, M.J. Astle, W.H. Beyer, Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1986.