1840
Organometallics 1995, 14, 1840-1843
Synthesis and X-ray Structure of Intramolecularly Coordinated Silyl Cations Johannes Belzner,*lt Dirk Schar,? Boris 0. Kneise1,t and Regine Herbst-Irmert Institut fur Organische Chemie der Georg-August-Universitat Gottingen, Tammannstrasse 2, 0-37077 Gottingen, Germany, and Znstitut fur Anorganische Chemie der Georg-August-Universitat Gottingen, Tammannstrasse 4, 0-37077 Gottingen, Germany Received December 6, 1994@ Bis[2-((dimethylamino)methyl)phenyl]silyltriflate and [2-((dimethylamino)methyl)phenyllphenylsilyl triflate were prepared by treatment of the corresponding dihydrosilanes with trimethylsilyl triflate in excellent yield. X-ray crystallography shows [Ar2SiHl+[OS02CFJ(Ar = 2-(Me2NCH2)C6H4;triclinic, Pi, a = 8.421(2) b = 9.892(3) A, c = 13.183(4) A, a = 82.78(2)",8 , = 74.00(1)", y = 75.84(2)", 2 = 2) to exist in the solid state as a separated ion pair with a pentacoordinated cationic silicon center, whereas [Ar(Ph)SiHl+[OS02CF31(monoclinic, P2,/n, a = 8.923(2) A, b = 9.957(2) A, c = 20.132(3) ,8 = 98.74(1)", 2 = 4) forms a tight ion pair, in which the triflate anion occupies one axial position of the trigonal bipyramid around silicon.
A,
A,
While trivalent silyl cations are stable in the gas phase,l the electron-deficient silicon center almost inevitably undergoes coordinative interaction with either its counterion or with the solvent in the solid or in solution.2 The formation of intermolecular complexes between silyl cations and Lewis bases in solution has been well inve~tigated;~ furthermore, some solid-state structures of such intermolecularly coordinated silyl cations were determined.4 Recently, Corriu5 provided some evidence for the formation of an intramolecularly pentacoordinated silyl cation (siliconium ion) using the tridentate 2,6-bis((dimethylamino)methyl)phenylsubstituent. In contrast, Willcott6 proved unambiguously that a quite similar compound exists in solution as an equilibrium mixture of two tetracoordinated structures. Very recently, Corriu7 published the first X-ray structure of an intramolecularly coordinated siliconium ion using the conformationally rigid 84dimethylamino)naphthalene substituent; herein we report the synthesis and the solid-state structure of two siliconium ions bearing the 2-((dimethylamino)methyl)phenylsubstituInstitut fur Organische Chemie der Georg-August-Universitat Gottingen. 3 Institut fur Anorganische Chemie der Georg-August-Universitat Gottingen. Abstract published in Advance ACS Abstracts, March 1, 1995. (1)See e.g.: Schwarz, H. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; pp 446-450, and references cited therein. ( 2 )See e.g.: (a) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430-2443. (b) Prakash, G. K. S.; Keyaniyan, S.; Aniszfeld, R.; Heiliger, L.; Olah, G . A.; Stevens, R. C.; Choi, H.-K.; Bau, R. J . Am. Chem. Soc. 1987,109, 5123-5126. (c) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993,262, 402-404. ( 3 )See e.g.: Bassindale, A. R.; Stout, T. Tetrahedron Lett. 1985, 26, 3403-3406 and references cited therein. ( 4 )(a) Hensen, K.; Zengerly, T.; Pickel, P.; Klebe, G. Angew. Chem. l983,95,739;Angew.Chem.,Int. Ed. Engl. 1983,22,725. (b) Hensen, K.; Zengerly, T.; Muller, T.; Pickel, P. 2.Anorg. Allg. Chem. 1988,558, 21-27. ( 5 )( a ) Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reye, C. Angew. Chem. 1993,105, 1372-1375; Angew. Chem., Int. E d . Engl. 1993,32,13111314. (b) Carre, F.; Chuit, C.; Corriu, R. J. P.; Mehdi, A,; Reye, C. Angew. Chem. 1994, 106, 1152-1154; Angew. Chem., Int. Ed. Engl. 1994,33, 1097. (6) Benin, V. A.; Martin, J. C.; Willcott, M. R. Tetrahedron Lett. 1994, 35, 2133-2136. (7) Breliere, C.; Carre, R.; Corriu, R.; Wong Chi Man, M. J . Chem. SOC.,Chem. Commun. 1994, 2333-2334. @
ent, thus showing that, against first assumption^,^^ this substituent is highly suitable to stabilize a cationic silicon center via intramolecular coordination. When bis[2-((dimethylamino)methyl)phenyllsilane(la) was treated with trimethylsilyl triflate, bis[2-((dimethylamino)methyl)phenyllsilyl triflate (218was isolated as a white solid in excellent yield; its solubility properties (insoluble in hexane, diethyl ether, and benzene, soluble in dichloromethane and chloroform) are in good agreement with an ionic structure.
I+
la
2
The lH NMR spectrum provides evidence for a strong coordination of both NMez groups to the silicon center: the methyl protons of the dimethylamino group as well as the benzylic protons are chemically inequivalent to each other and give rise to two singlets at 6 2.56 and 2.76 as well as a characteristic AB pattern at 6 4.27 and 4.33. Of further diagnostic value for the structural elucidation of 2 is the 29Si-1H coupling constant of 272 Hz, which is significantly increased compared to that found for covalent la and does reflect the enhanced s character of the Si-H bond, as is expected for sp2hybridized ~ i l i c o n .The ~ SiH signal, whose 29Sisatellites allow the determination of the 29Si-1H coupling constant, is found at 6 4.60, thus showing an upfield shift of about 0.25 ppm in comparison t o the starting silane la. The same trend is observed in the 29Si NMR spectrum: hydride abstraction from la is accompanied (8) Alternatively, 2 may be prepared by reaction of cyclotrisilane (ArZSi)s with trimethylsilyl triflate. ( 9 )Jutzi, P.; Bunte, E.-A. Angew. Chem. 1992, 104, 1636-1638; Angew. Chem., Int. Ed. Engl. 1992,31, 1605.
0276-733319512314-1840$09.00/0 0 1995 American Chemical Society
Intramolecularly Coordinated Silyl Cations
Organometallics, Vol. 14, No. 4, 1995 1841
F121
ciw c1191
Fi3
a121
Fill ciiar
CI191
Figure 1. Crystal structure of 2. Hydrogen atoms not bonded to silicon are omitted for clarity; displacement ellipsoids are at the 50% probability level. Table 1. Selected Bond Lengths (A) and Bond Angles (deg) in Triflate 2 Si(l)-C(ll) Si(l)-N(21)
1.874(2) Si(l)-C(21) 2.072(2) Si(1)-N(11)
C(l l)-Si(l)-C(21) H(l)-Si(l)-C(ll) H(l)-Si(l)-N(ll) C(ll)-Si(l)-N(ll) C(21)-Si(l)-N(21)
119.8(1) 123.8(9) 86.8(9) 84.3(1) 83.7(1)
1.882(2) Si(1)-H(1) 2.052(2) C(2l)-Si(l)-H( 1) N(Il)-Si(l)-N(21) C(2l)-Si(l)-N(ll) H(l)-Si(l)-N(21) C(ll)-Si(l)-N(21)
1.34(2) 116.3(9) 171.2(1) 98.9(1) 84.5(9) 101.8(1)
by an upfield shift of the 29Si NMR resonance in 2 (&la)-45.0; d(2) -51.6). Thus, upon conversion of the dihydrosilane l a to the triflate 2, NMR spectroscopic behavior is found which parallels the observations made for 8-(dimethylamino)naphthalene-~ubstituted silanes7 but is in contrast with that reported for the 2,6-bis((dimethy1amino)methyl)phenyl-substitutedsystem; in this case, the 'H NMR signal of the SiH proton and the 29SiNMR signal are shifted downfield upon going from the dihydrosilane to the corresponding siliconium Colorless crystals of 2 suitable for X-ray analysis (Table 1)were obtained by crystallization from dichloromethane/diethyl ether (1O:l). The X-ray structure (Figure 1)clearly proves the ionic structure of 2. There exists no interaction between the silicon center and the triflate anion: the shortest silicon-oxygen distance is 4.165 which is greater by far than the sum of the van der Waals radii of both elements (3.62 Ahlo The coordination geometry around silicon is that of a slightly distorted trigonal bipyramid, in which both dimethylamino groups occupy the axial positions. The Si-N distances are 2.052(2) and 2.072(2)A,respectively, and resemble those found in the intermolecularly pentacoordinated imidazole complex 3 (2.034(3) and 2.005(3) A),4b These distances are significantly longer than a covalent Si-N bond distance (1.70-1.76 AI1' but smaller than the dative Si-N bond distance in neutral pentacoordinated silicon compounds bearing the 2-((dimethy1amino)methyl)phenyl substituent (e.g. 2.291(2) A in lb).I2H(l), C(11), and C(21) form the equatorial plane; their bond angles to the central silicon atom sum up to
Fi31
Figure 2. Crystal structure of 4. Hydrogen atoms not bonded to silicon are omitted for clarity; displacement ellipsoids are at the 50% probability level. Table 2. Selected Bond Lengths (A) and Bond Angles (deg) in Triflate 4 Si(l)-C(Il) Si( 1)-O( 1) S(1)-0(2)
1.866(2) Si(l)-C(21) 1.857(2) Si(1)-H(1) 1.951(1) Si( 1)-N( 11) 2.052(2) S( 1)-0( 1) 1.421(2) S(1)-0(3) 1.422(2)
C(ll)-Si(l)-C(21) H( 1)-Si( 1)-C( 11) H(1)-Si( 1)-0(1) C(ll)-Si(l)-O(l) C(21)-Si(l)-N(ll) Si(l)-C(ll)-C(l2) Si(l)-C(21)-C(22)
116.4(1) 127.6(8) 88.2(9) 93.5(1) 96.2(1) 113.7(1) 122.3(1)
C(2l)-Si(l)-H(l) N( 11)-Si( 1)-0( 1) C(2l)-Si(l)-O(l) H(1)-Si(1)-N(l1) C(11)-Si(1)-N(l1) Si(1)-C(l1)-C(l6) Si(l)-C(21)-C(26)
1.35(2) 1.486(1) 116.0(8) 174.3(1) 89.4(1) 88.5(9) 84.9(1) 128.0(1) 120.2(1)
360(2)", thus manifesting the total flattening of the silicon center.
r
Ma
+
C' NMez
A,
(10) Bondi, A. J.Phys. Chem. 1964, 68, 441-451. (11)Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai, s.,Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; p 254. (12)Probst, R.; Leis, C . ; Gamper, S.; Herdtweck, E.; Zybill, L.; Auner, N. Angew. Chem. 1991, 103, 1155-1157; Angew. Chem., Int. Ed. Engl. 1991,30,1132-1134.
3
lb
In an analogous manner, [2-((dimethylamino)methyl)phenyllphenylsilyl triflate' (4) was prepared from the corresponding dihydrosilane 55ain 91%yield. Again, the
4
5
solubility properties as well as the increased 29Si-1H coupling constant (290 Hz)argue for an ionic structure, which was eventually established for the solid state by single-crystal X-ray diffraction (Figure 2). The silicon center of 4 is pentacoordinated with Si(l),C(11), C(21),
Belzner et al.
1842 Organometallics, Vol. 14, No. 4, 1995 and H( 1) forming the central plane of a trigonal bipyramid (sum of the bond angles 360(2)"). The twist angle between this plane and the phenyl substituent amounts to 18.2", whereas the 2-((dimethylamino)methyl)phenyl substituent is located almost perpendicular to the central plane (twist angle 79. 1"). Correspondingly, the dimethylamino group is brought into an apical position. In order to accomplish an optimal coordination of the lone pair at nitrogen to the silicon center, the bond angles around C(11) show, in contrast to the angles around C(21), appreciable deviation from 120": the endocyclic angle Si(l)-C(ll)-C(l2) is reduced to 113.7(I)', whereas the exocyclic angle Si(l)-C(ll)-C(l6) is widened up to 128.0(1)". The resulting Si-N distance is 2.052(2) which is similar to those found in 2; however, the second axial position of the trigonal bipyramid is now occupied by the triflate anion. The Si-0 distance of 1.951(1) is distinctly longer than typical covalent Si-0 bond lengths, which range between 1.61 and 1.74 A,13 and is elongated significantly even in comparison t o the Si-0 distance in protonated fBu3SiOH (1.779(9)All4 as well as to the exceptionally long Si-0 bond (1.853(5) A) found in a rutheniumsubstituted silyl triflate.15 Thus, 4 may be described as a tight ion pair between an intramolecularly coordinated silyl cation and a triflate anion. However, there is also some covalent bonding interaction between Si(1)and O(1) in 4 present, which is reflected by the lengthening of the S(1)-0(1)(1.486(1)A)bond relative to the S(1)-0(2) and S(1)-0(3) bonds (1.421(2) and 1.422(2) A, respectively). It is of interest to note that 4 does not form a tetracoordinated silyl cation, as is known for the reaction products of MesSiI with pyridine4a or of MesSiCl with N-methylimida~ole,~~ but prefers pentacoordination with inclusion of the triflate anion into its coordination sphere. Experiments which focus on to the synthesis of a tetrahedral silyl cation by substituting the triflate anion in 4 by a less nucleophilic counterion are under way.
Table 3. Summary of Crystal Data, Details of Intensity Collection, and Least-Squares Refinement Parameters for 2 and 4 2 empirical formula Mr cryst size (mm) cryst syst space group a (A) b c (A) a (deg)
(4)
A,
A
Experimental Section IH NMR and 13C NMR spectra were recorded on a Bruker AM 250 ('H NMR, 250 MHz; 13C NMR, 62.9MHz). C,, CH, CH2, and CH3 were determined using the DEFT pulse sequence. 29SiNMR spectra were recorded on a Bruker AMX 300 (59.6 MHz) using a refocused INEPT pulse sequence. Chemical shifts refer to 6 ~ 0.0. s Mass spectra were recorded on a Varian MAT 311 A. Melting points are uncorrected. Elemental analyses were performed at Mikroanalytisches Labor der Georg-August-UniversitatGottingen. All manipulations were carried out under an inert argon atmosphere using carefully dried glassware. Ethereal solvents used were dried by refluxing over sodium and distilled immediately before use; dichloromethane was dried using molecular sieves (4A). Bis[2-((dimethylamino)methyl)phenyllsilylM a t e (2). To a stirred solution of 775 mg (2.6mmol) of la16in 20 mL of (13) Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; p 263. (14)Xie, 2.; Bau, R.; Reed, C. A. J . Chem. Soc., Chem. Commun. 1994, 2519-2520. (15)Straus, D. A,; Zhang, C.; Quimbita, G. E.; Grumbine, S. D.; Heyn, R. H.; Tilley, D.; Rheingold, A. L.; Geib, S. J. J . A m . Chem. SOC. 1990, 112, 2673-2681. (16) Auner, N.; Probst, R.; Hahn, F.; Herdtweck, E. J . Organomet. Chem. 1993,459,25-41.
no. of rflns coll no. of indep rflns R(int) no. of data no. of params
S gl gz
R1 ( F > 40(F)) wR2 (all data) largest diff peak (e A-2) largest diff hole (e A-3)
4
C I ~ H Z S N ~ O ~ F ~ C16HlRNOd5SiS S~S 446.56 389.46 0.7 x 0.6 x 0.6 0.9 x 0.8 x 0.8 monoclinic triclinic pi p211n 8.421 (2) 8.923 (2) 9.957 (2) 9.892 (3) 20.132 (3) 13.183 (4) 82.78 (2) 90 74.00 (1) 98.74(1) 75.84(2) 90 10213 5 ) 1767.9(6) 2 4 1.452 1.463 0.268 0.296 808 468 8 5 20 5 50 8 5 20 5 50 -9 5 h 5 10 -10 e h c 10 -ll
