Organometallics 1995, 14, 1518-1521
1518
Synthesis and Characterization of New Silyl Niobocene Complexes. X-ray Molecular Structure of do Nb(115-C5H4SiMe3)2(H)2(SiPh~H) Antonio Antifiolo,t Fernando Carrillo,? Mariano Fajardo,$ Antonio Otero,**+ Mauricio Lanfranchi,g and Maria Angela Pellinghellig Departamento de Quimica Inorganica, Organica y Bioquimica, Facultad de Quimicas, Campus Universitario, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain, Departamento de Quimica Inorganica, Campus Universitario, Universidad de Alcala, 28871 Alcala de Henares, Spain, and Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universitci degli Studi di Parma, Centro di Studio per la Strutturistica Diffrattometrica del CNR, Viale delle Scienze 78,I-43100 Parma, Italy Received August 3, 1994@ Summary: Thermal treatment of hW$-CsH&iMedkH)o3 (1) with organosilicon hydrides (HSiRd affords bis-
((trimethylsily1)cyclopentadienyl)niobium dihydride silyl complexes, Nb(y5-CsH*SiMe3)2(H)z(SiRd), SiR3 = SiMezPh (2), SiMePhz (3), SiPhHz (41, SiPhzH (8,or SiPhs (6), in excellent yields. Spectroscopic data indicate the presence of only one of two possible structural isomers in which the silyl substituent is central in the equatorial plane with a symmetrical structure. Compound 5 has been characterized by X-ray diffraction: monoclinic s ace group P21ln with a = 13.233(4)A, b = 20.843(6) c = 11.043(7)A, p = 99.17(2)",V = 3007(2) A3, Z = 4, Dcalcd= 1.221g / m L , R = 0.0543,and R, = 0.0570.The coordination polyhedron may be described as a distorted tetrahedron with a centered edge. The complex adopts a "bent sandwich" coordination with the two hydrides flanking either side of the Nb-Si bond (2.616(3)A).
coupling constants, in agreement with the importance of a near dihydrogen state. This has led us to study the behavior of Nb(y5-C5H.&iMe3)2(H)3(1) toward organosilicon hydrides, HSiR3.
Results and Discussion We have found 1 reacts readily with several organosilicon hydrides to give niobocene silyl dihydride species (eq 21, with 2,SiR3 = SiMeaPh; 3,SiR3 = SiMePh2; 4,
Introduction In the last few years, the oxidative addition of organosilicon hydrides to coordinatively unsaturated hydride group 5 metal complexes has been successfully applied to the preparation of silyl derivatives.' Using this method, several silyl complexes, M(y5-C5H5)2(H)2(SiR3), have been preparedla,b?efrom trihydride metallocene derivatives of niobium and tantalum, M(y5C5H&(H)3 (M = Nb, Ta), eq 1. The process presumably
M(y5-C5H5)2(H)2(SiR3) (1) proceeds by reductive elimination of H2 from the trihydride to give M(y5-C5H5)H,which then adds the silane. We have previously described2 the anomalous spectroscopic properties of various niobocene trihydride complexes bearing electron-withdrawingsubstituents, SiMe3, in the cyclopentadienyl rings, which show large H-H + Universidad
de Castilla-La Mancha.
* Universidad de Alcall.
UniversitB. degli Studi di Parma. Abstract published in Advance ACS Abstracts, November 15,1994. (1)(a) Allison, J. S.; Aylett, B. J.; Colquhoun, H. M. J. Organomet. Chem. 1978,112,67. (b) Curtis, M. D.; Bell, L. G.; Buttler, W. M. Organometallics 1986,4,701. (c) Berry, D. H.; Jiang, Q. J.Am. Chem. SOC.1987,109,6211.(d) Aitken, C.; Barry, J.-P.; Gauvin, F.; Harrod, J. F.; Malek, A.; Rousseau, D. Organometallics 1989,8,1732.(e) Jiang, Q.;Carroll, P. J.; Berry, D. H. Organometallics 1991,10, 3648. (2)Antiriolo, A,; Chaudret, B.; Commenges, G.; Fajardo, M.; Jalbn, F.; Morris, R. H.; Otero, A.; Schweitzer, C. T. J. Chem. Soc., Chem. Commun. 1988,1210. 8
@
L SiMeo
SiR3 = SiPhH2; 5,SiR3 = SiPh2H; and 6,SiR3 = SiPh3. The initial step presumably is formation of the 16e coordinativelyunsaturated Nb(y5-C5€&SiMe3)2Hby thermolytic loss of H2. This then is followed by oxidative addition of the organosilicon hydride so that a closedshell 18e configuration in the silyl dihydride derivative is achieved. The formation of the intermediate species Nb(y5-C5H5-,R,)2(H) in the preparation of complexes L = n-acid ligand, from such as Nb(y5-C5H5-,~)~(H)(L), a mixture of Nb(y5-C5H5-,R,)2(H)3 and L via thermal or photochemical treatments3 has been proposed previously. Although the preparative method has been previouslylb~d~e employed in the synthesis of several families of tantalocene silyl hydride derivatives, only two related and niobocene complexes, Nb(y5-CgH5)2(H)2(SiPhMe2) Nb(y5-C5H5)2(H)2(SiMe20SiMe3), prepared by Curtis and co-workers,lbhave been described. Complexes 2-6 crystallize as air-sensitive microcrystalline materials (3)See, for example: (a)Doherty, N. M.; Bercaw, J. E. J.Am. Chem. SOC.1986,107,2670. (b) Burger, B. J.; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J. E. J.Am. Chem. SOC.1988,110,3314. (c) Antiiiolo, A.;Fajardo, M.; Jalbn, F. A.; Lbpez-Mardomingo, C.; Otero, A.; SanzBernab6, C. J. Organomet. Chem. 1989,369,187.
0276-733319512314-1518$09.00/0 0 1995 American Chemical Society
Notes
Organometallics, Vol. 14, No. 3, 1995 1519
from hexane or ethanol. Their stability to air is increased by the presence of phenyl substituents in the silyl group, so that complex 6 survives several hours of exposure to air. Attempts to prepare a complex with EtsSiH were not successful. A complex mixture of products was obtained which could not be characterized. A similar observation had been made previouslylb with Nb(q5-C&MH)3. Complexes 2-6 have been characterized spectroscopically. Their IR spectra show a weak single, broad band at -1700 cm-l assigned to 4NbH). In addition, the IR spectra of 4 and 5, which contain hydrogen on the silyl groups, exhibit 4%-H) as a medium single band a t 2957 and 2040 cm-l, respectively. The 'H NMR spectra of the complexes clearly show that of the possible symmetrical or unsymmetrical and Nb(q6-C5H4isomers Nb(q5-C5H4SiMe3)2(H)(SiR3)(H) SiMe&(H)(H)(SiR3), only the symmetrical one was present in solution. In fact, the spectra show a broad singlet a t 6 --4.50 for the two equivalent hydride ligands in accordance with a time-averaged C.2, symmetry with the silyl group in the central position. The cyclopentadienyl rings are equivalent, showing two broad resonances (see Experimental Section)for an A2B2 spin system, which indicates a symmetrical disposition on the niobium center with a rapid rotation of the SiR3 around the Nb-Si bond. This symmetrical disposition has been confirmed by means of the corresponding 13C NMR spectra in which the three expected resonances for the cyclopentadienyl carbon atoms are present (see Experimental Section). The thermodynamically more favorable symmetrical isomer is obtained under our experimental conditions. Attempts to detect the unsymmetrical isomer were carried out by recording the lH NMR spectra for 2-6 at low temperature, but in all cases isomerization was never observed. 29SiNMR data can be used to detect the presence of a H-Si interaction in the silyl hydride derivative^.^ Corriu and co-workers5 showed that the 'J(Si,H) for the H-Si interaction is -65 Hz, thus appearing between that characteristic of classical silyl hydrides (-6 Hz) and that of free silanes (-200 Hz). The 29Si NMR spectrum of 6 shows a doublet at 6 23.8 for the silyl group with lJ(Si,H) = 177 Hz. This coupling disappears when the hydrogen of the silyl group is irradiated. In the lH NMR spectrum, the coupling also is observed for the resonance of this hydrogen, and in addition, the line width of the signals for the two hydride ligands (-6 Hz) is characteristic of classical silyl hydrides. In the 29SiNMR spectrum of 3,where there is no hydrogen in the silyl group, the resonance for this group appears as a singlet at 6 26.7. These spectroscopic data allow us to establish the classical nature of these silyl hydrides. In order to confirm this, we determined the molecular structure of 5. X-ray Stnrcture of Nb(~s-C&SiM~)s(H)z(SiPh~ (5). Figure 1provides a view of complex 5 which adopts a familiar "bent sandwich" coordination. The two centroids of the two Cp' rings and the two hydrides surround the niobium atom at the vertices of an irregular tetrahedron. Moreover, the metal atom interacts with the Si atom of the Si(1)HPhz moiety, the NbSi(1)bond roughly bisects the H(lT)-Nb-H(BT) bond angle, and the four atoms Nb, Si(l),H(lT), and H(2T)
are roughly coplanar (maximum deviation 0.29(6)A for H(2T)). The coordination polyhedron thus can be described as a tetrahedron with a centered edge. Therefore, this complex belongs to the class of bent metallocene complexes containing their ligands in the equatorial wedge with the two hydrides flanking either side of the Nb-Si bond in the symmetrical isomeric form. The Nb-Si(1) distance, 2.616(3) A, corresponds to a single, covalent bond and is shorter than that observed (Nb-Si = 2.669(1)A). This in [Cp2Nb(SiMe3)(q2-C2H)16 shortening can be due to the different oxidation state of the niobium atoms. Silyl complexes of tantalum have received relatively more study, but only two molecular structures of silyl hydride complexes of tantalum have been reported, [Cp2Ta(H)2(SiMe2Ph)llband [CpzTa(H)(SiMe2H)21.le The two hydride ligands were clearly located in 5. The two H(1T) and H(2T) atoms are at 1.77(6)and 1.76(6) A from Nb with a bond angle of 115.4(26)". They adopt a symmetrical position with respect t o the Si(1) atom, and the two Nb-H(lTkSi(1) and Nb-H(2TkSi(1) angles (76.9(20), 81.6(22)") are similar. Unfortunately, the hydride ligands were not locatedlb or not clearly located1" in the two above-cited silyl complexes of tantalum and hence we cannot make any comparison. On the basis of Schubert's work,4 it seems reasonable to assume no significant Si-H interaction at distances greater than 2.00 A, but can we exclude any interaction between the hydrides and the silicon atom on the basis of van der Waals radii? The two cyclopentadienyl rings are nearly eclipsed, while the two SiMe3 groups are trans to each other. The angle subtended by the two Cp' ring centroids (141.3(4)")is greater than that observed in [CpzTa(H)2(SiMez-
(4)Schubert, U.Adu. Organomet. Chem. 1990,30,151. ( 5 ) Colomer, K.; Con-iu, R. J. P.; Mazin, C.; Vioux, A. Inorg. Chem. 1982,21, 368.
( 6 )Arnold, J.; Tilley, T.D.; Rheingold, A. L.; G i b , S. J. Organometallics 1987,6,473.
Figure 1. View of the molecular structure of [Cp'zNb(H)z(SiHPhz)] with the atom-numbering scheme. Selected distances (A): Nb-CE(1) 2.064(9); Nb-CE(2) 2.050(10); Nb-H(lT) 1.77(6);"b-H(2T) 1.76(6);Nb-Si(1) 2.616(3); Si(lbH(1T) 2.37(6);Si(lFH(2T) 2.21(6). Selected angles (deg): CE(lI-Nb-CE(2) 141.3(4);CE(2)-Nb-H(1T) 110.7(18); CE(1)-Nb-Si(1) 115.2(3);CE(2)-Nb-H(2T) 101.2(18);CE(l)-Nb-H(lT) 88.9(18);Si(l)-Nb-H(lT) 62.0(19); CE(1)-Nb-H(2T) 99.6(19);Si(l)-Nb-H(2T) 56.6(18);CE(2)-Nb-Si(l) 103.5(3);H(lT)-Nb-H(BT) 115.4(26). CE(1) and CE(2) are the centroids of the C(13)-C(17)and C(21)-C(25)Cp' rings, respectively.
1520 Organometallics, Vol. 14,No. 3, 1995 Table 1. Crystallographic Data for Compound [Cp’zNb(H)2(SiHPhdI formula fw diffractometer radiation cryst syst space group a, A
b, 8,
c, A
h deg
v,A 3
Z g Cm-3 F(000) cryst size, mm p(Cu Ka), cm-’ temp, K scan speed, deg min-’ scan width, deg scan mode 20 range, deg reflns measd no. of unique total data no. of unique obsd data [ I 2 2u(I)] transm factors ( m a , min) function minimized weighting scheme w = k/[u2(Fo) gFo2] no. of variables max shifverror, final cycle madmin diff peaks, e/A3 goodness of fit R, Rwa Dcdcd.
+
CzsH39Si3Nb 552.781 Siemens AED nickel-filtered Cu Ka, I. = 1.541 838 monoclinic P21ln 13.233(4) 20.843(6) 11.043(7) 99.17(2) 3007(2) 4 1.22 1 1160 0.18 x 0.19 x 0.20 44.90 295 3-9 1.20 0.142 tan e
me
+
6-140 h,k,fl 5700 2505 1.000, 0.759 Ew(lFoI
- IFcl)2
k = 1.1167, g = 0.0004 328 0.63 0.421-0.60 1.5952 0.0543, 0.0570
‘R = ZllFoI - lFclI/ElFol. Rw = [Zw(lFol - IFc1)21Z~(Fo)zl”~.
Ph)l and [CpzTa(H)(SiMeeH)zI(138.0(2), 1380.0’). The two centroids, the Nb and Si(1) atoms, are coplanar as in the dihydride silyl tantalum complex and the asymmetrical angles around the metal atoms are due to steric hindrance between the C(13)-C(17) Cp’ ring and the C(7)-C(12) phenyl ring (C(7)-H(17) = 2.69(9), and C(7)-C(17) = 3.30(1) A).
Experimental Section General Procedures. All operations were performed under an inert atmosphere using standard vacuum line (Schlenk) techniques. Solvents were purified by distillation from appropriate drying agents before use. NMR spectra were obtained on a Varian Unity FT-300 instrument. IR spectra were recorded as Nujol mulls between CsI plates (in the region between 4000 and 200 cm-l) on a Perkin-Elmer PE 883 IR spectrometer. Elemental analyses were performed on a Perkin-Elmer 2400 microanalyzer. Nb(q5-C5H4SiMe3)z(H)3was prepared as described earlier.2 Organosilicon hydrides (Aldrich and Fluka) were used without purification. Nb(~5-CaH4SiMes)a(H)a(Si&),Si& = SiMeSh (2);SiRs = SiMePha (3);SiRs = SiPhHz (4); Si% = S i P h a (5); and SiRs = SiPh (6). To a solution of Nb(q5-C5H4SiMe3)~(H)3 (1; 300 mg, 0.81 mmol) in 20 mL of toluene was added RsSiH (0.81 mmol) by syringe. The solution was warmed t o 65 “C and stirred for 3 h. The resulting red-brown solution was filtered and evaporated to dryness. The brown, oily residue was extracted with 20 mL of ethanol. After concentration and cooling, white or pale yellow crystals of complexes 2-6 were obtained. 2: pale yellow crystals, 88%yield. IR (Nujol; cm-l): 1685, v(Nb-H). ‘H NMR (CfiDs;6): -4.96 (s, br, 2H, Nb-H), 0.13 (9, 18H, C5HSiMea), 0.74 (s, 6H, SiMezPh), 4.20 (s, 4H), 4.90 (s,4H) (q5-C5H4,exact assignment not possible), 7.10-7.70 (m, 5H, SiMeSh). 13C{lH} NMR (CsDs; 6): 0.40 (SiMea), 19.2
Notes
Table 2. Atomic Coordinates ( x 104) and Isotropic Thermal Parameters (A2x 104) with ESDs in Parentheses for the Non-Hydrogen Atoms of Compound [Cp’W(H)2(SiHPh2)] xla
ylb
ZIC
Ua
5133.8(6) 6636(2) 3442(2) 6545(3) 7467(7) 8499(8) 9142(9) 8783(11) 7792(10) 7142(9) 6237(7) 6208(9) 5894(11) 5617(15) 5624(13) 5945(9) 3647(7) 333l(7) 3516(8) 3924(8) 4046(7) 4124(9) 20 18(7) 3889(10) 6098(8) 6663(8) 6115(9) 5224(9) 5200(9) 6768( 12) 7769(18) 5554( 17)
2297.8(4) 2793( 1) 3769(1) 693(2) 3441(5) 3476(6) 3941(7) 4355(6) 4333(5) 3868(5) 3150(6) 3797(7) 4031(9) 3607( 12) 2986( 11) 2740(7) 2964(4) 2366(6) 1876(6) 2146(6) 2801(6) 4375(5) 3904(5) 3804(5) 1427(5) 2006(5) 2390(5) 2079(5) 1496(5) 43(6) 839(7) 396(8)
3710.2(7) 274 l(2) 3799(3) 3980(4) 3637(8) 351 l(10) 4105(12) 4842( 13) 5022(11) 4414(10) 1173(9) 921(11) -262(15) - 1184(15) -955( 15) 184(11) 3 190(8) 3622(10) 2790( 11) 186%10) 2083(9) 3006( 11) 3447( 11) 5467(9) 4688(9) 501 l(9) 5716(8) 5837(9) 5211(10) 5 121(15) 3503(22) 2829( 15)
689(3) 768(10) 863(12) 1357(19) 767(38) 1049(5 1) 1085(60) 1167(65) 1143(58) 1018(51) 894(44) 1184(59) 1701(87) 1939(124) 1659(103) 1190(57) 709(34) 875(40) 921(47) 971(50) 849(44) 1267(58) 1276(58) 1380(63) 887(44) 832(42) 862(43) 935(47) 927(47) 1997(98) 4155(213) 3474( 160)
“Equivalent isotropic U , defined as one-third of the trace of the orthogonalized Uij tensor.
(SiMezPh),89.2,91.6,94.0 (C&SiMe3), 128,133.5 (SiMeSh). Anal. Calcd for C~dH39Si3Nb:C, 58.29; H, 5.87. Found: C, 58.45; H, 5.91. 3: pale yellow crystals, 87% yield. IR (Nujol; cm-l): 1726, v(Nb-H). lH NMR (C&; 6): -4.66 (s, br, 2H, Nb-H), 0.03 (9, 18H, C5H&iMe3), 0.77 (9, 3H, SiMePhz), 4.20 (s, 4H), 4.90 (s, 4H) (q5-C5H4,exact assignment not possible), 7.10-7.84 (m, 10H, SiMePhz). l3C(lH} NMR (C&; 6): 0.33 (SiMes), 19.2 (SiMePhz), 91.7, 94.0, 94.7 (C&SiMe3), 129.7, 134.7, 150.0 (SiMePhz). 29SiNMR (CsDs; 6, ref TMS): -3.0 (9, CsHaiMea), 26.7 (s, SiMe Phz). Anal. Calcd for CzgH41Si3Nb: C, 62.59; H, 5.57. Found: C, 62.69; H, 5.64. 4: white crystals, 92% yield. IR (Nujol; cm-l): 2057, v(SiH); 1729, v(Nb-H). ‘H NMR (CsDs; 6): -4.90 (s, br, 2H, NbH), 0.13 (s, 18H, C5H&iMe3), 4.10 (s, 4H), 4.90 (8, 4H) (q5C5H4, exact assignment not possible), 5.24 (2H, SiPhHz), 7.107.97 (m, 5H, SiPh,Hz). 13C{lH)NMR (CsDs; 6): 0.44 (SiMes), 91.5, 93.0, 96.8 (C&3iMe3), 135.6, 146.5 (SiPhHz). Anal. Calcd for CzzH35Si3Nb: C, 56.65; H, 5.36. Found: C, 56.70; H, 5.23. 5: white crystals, 93% yield. IR (Nujol; cm-l): 2040, v(SiHI; 1728,v(Nb-H). lH NMR (CfiDfi;6): -4.64 (8,br, 2H, NbH), 0.14 (9, 18H, CsH&iMes), 4.10 (8,4H), 4.90 (s, 4H) (q5C5H4, exact assignment not possible) 5.86 (lH, SiPhzH), 7.107.90 (m, 10H, SiPhzH). l3C(lH} NMR (CeD6; 6): 0.55 (SiMea), 91.9, 94.3, 94.9 (C&&SiMes),135.6, 146.8 (SiPhzH). 29SiNMR (CsD6; 6, referenced TMS): -0.3 ( 8 , C5H&iMe3), 23.8 (d, l J g i ~ = 177 Hz, SiPhzH). Anal. Calcd for c~&&Nb: C, 61.99; H, 5.35. Found: C, 62.06; H, 5.43. 6: white crystals, 98% yield. IR (Nujol,cm-l): 1720, v(NbH). ‘H NMR (C&; 6): -4.27 (s, br, ZH, Nb-HI, 0.06 (s,18H, C5H&iMed, 4.10 (9, 4H), 4.90 (9, 4H) (q5-C5H4,exact assignment not possible), 7.10-7.97 (m, 15H, SiPh3). 13C{lH}(C&; 6): -0.55 (SiMes), 90.5, 92.1, 95.6 (C&SiMe3), 134.6, 145.5 (SiPh3). Anal. Calcd for C34H43Si3Nb: C, 66.02; H, 5.33. Found: C, 66.10; H, 5.29.
Notes X-ray Data Collection, Structure Determination, and Refinement of Nb(~6-CsH4SiMes)a(H)a(SiPhaH) (5). Crystals suitable for X-ray analysis were obtained by recrystallization from ethanol. A single crystal was sealed in a Lindemann capillary under dry nitrogen and used for data collection. Table 1contains a summary of data collection conditions and results. Lattice parameters were determined from a leastsquares refinement of 30 reflection settings (20 < 0 < 37 deg) obtained from an automatic centering routine. Three standard reflections were monitored every 50 measurements; no significant decay was noticed over the time of data collection.The individual profiles have been analyzed following the method of Lehmann and L a r ~ e n . Intensities ~ were corrected for Lorentz and polarization effects. A correction for absorption was applied.* Only the observed reflections were used in the structural solution and refinements. The structure was solved by Patterson and Fourier methods and refined by full-matrix least squares, first with isotropic thermal parameters and then with anisotropic thermal parameters for all the non-hydrogen atoms. The methyl carbon atoms bonded to Si(3) show very large thermal parameters with the major axis roughly in the plane of the three carbon atoms. Attempts to resolve a possible disorder of this group were unsuccessful. The two niobium hydrides, the hydrogen atom of Si(1) and the hydrogen atoms of the Cp’ rings were clearly located in the hlp map and refined isotropically. The positional parameters of the two hydride ligands were tested by means of a “potential ener& technique using the HYDEX p r o g ~ a m .All ~ the other hydrogen atoms were placed at their geometrically calculated positions ( d c - ~ (7) Lehmann, M. S.; Larsen, F. K. Acta Crystallogr., Sect. A 1974, 30, 580. (8) (a) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983,39, 158. (b) Ugozzoli, F. Comput. Chem. 1987,11,109.
Organometallics, Vol. 14, No. 3, 1995
1521
= 1.00 A) and refined “riding” on the corresponding carbon
atoms, with isotropic thermal parameters. The analytical scattering factors, corrected for the real and imaginary parts of anomalous dispersion, were taken from ref 10. All calculations were carried out on the GOULD POWERNODE 6040 and ENCORE 91 computers of the Centro di Studio per la Strutturistica Dfiattometrica del CNR, Parma, using the S H E W 86 and SHEIXS-76 systems of crystallographic computer programs.ll Table 2 provides the atomic coordinates.
Acknowledgment. A.A., F.C., M.F., and A.O. gratefully acknowledge financial support from DGICyT (Grant PB92-0715) of Spain. M.L. and M.A.P. gratefully acknowledge financial support from the Minister0 dell’ Universita della Ricerca Scientifica e Tecnologica (MURST) and Consiglio Nazionale delle Ricerche (CNR) (Rome, Italy). SupplementaryMaterial Available: Tables of hydrogen atom coordinates (Table SI), anisotropic thermal parameters for the non-hydrogen atoms (Table SII), complete bond distances and angles (Table III), and least-squares planes and lines (Table SIV)(12 pages). Ordering information is given on any current masthead page. A list of structure factors is available upon request from the authors. OM940617N (9) Orpen, A. G. J. Chem. Soc., Dalton Trans. 1980,2509.
(10)International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. N . (11)Sheldrick, G. M. SHELXS-86 Program for the Solution of Crystal Structures; University of Giittingen, Gbttingen, Germany, 1986. SHELX-76 Program for Crystal Structure Determination; University of Cambridge, Cambridge, England, 1976.
