Hypercoordinate Silacycloalkanes - American Chemical Society

found in the literature, there is only one report on a crystal- lographically characterized .... Allg. Chem. 1997, 623, 633. (d) Day, R. O.; Sreelatha...
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Organometallics 2009, 28, 5459–5465 DOI: 10.1021/om900636e

5459

Hypercoordinate Silacycloalkanes: Step-by-Step Tuning of NfSi Interactions§ Erica Brendler,† Erik W€achtler,‡ and J€ org Wagler*,‡ †

Institut f€ ur Analytische Chemie and ‡Institut f€ ur Anorganische Chemie, Technische Universit€ at Bergakademie Freiberg, D-09596 Freiberg, Germany Received July 20, 2009

The silacycloalkanes (ON)2Si(CH2)n (ON = 8-oxyquinolinate, n = 3, 4, 5, 6) reveal stepwise decreasing NfSi coordination with increasing ring size. Whereas for n = 3 and 4 hexacoordinate silicon compounds were found at room temperature, n = 5 supports an equilibrium that allowed for the isolation of two coordination isomers (CN 4 and 6) as crystalline solids, and n = 6 causes the equilibrium to shift toward the tetracoordinate Si compound.

The class of hypercoordinate silicon compounds1 includes a great number of molecules comprising formally dative NfSi interactions, which were shown to cover a wide range of Si-N distances. The additional N-donor action served for both new insights into nucleophilic substitution mechanisms2 and a source of hot debate whether these Si-N bonds should be referred to as “covalent” or “dative”.3 Whereas experimental4 and computational5 approaches were addressed to finding an answer to this question, smart syntheses of Si compounds bearing subtly tunable ligand systems were chosen to probe the flexibility of the formally dative NfSi bond.1a,6 An entirely different approach, also serving the latter purpose, was addressed in our herein presented study: §

Dedicated to Prof. Gerhard Roewer on the occasion of his 70th birthday. *Corresponding author. E-mail: [email protected]. (1) For recent reviews on hypercoordinate silicon complexes see for example: (a) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303. (b) Kost, D.; Kalikhman, I. Adv. Organomet. Chem. 2004, 50, 1. (c) Tacke, R.; P€ ulm, M.; Wagner, B. Adv. Organomet. Chem. 1999, 44, 221. (2) (a) Brendler, E.; Heine, T.; Hill, A. F.; Wagler, J. Z. Anorg. Allg. Chem. 2009, 635, 1300. (b) Bassindale, A. R.; Parker, D. J.; Taylor, P. G.; Turtle, R. Z. Anorg. Allg. Chem. 2009, 635, 1288. (c) Gostevskii, B.; Silbert, G.; Adear, K.; Sivaramakrishna, A.; Stalke, D.; Deuerlein, S.; Kocher, N.; Voronkov, M. G.; Kalikhman, I.; Kost, D. Organometallics 2005, 24, 2913. (3) Schiemenz, G. P. Z. Naturforsch. B 2006, 61, 535. (4) (a) Kocher, N.; Henn, J.; Gostevskii, B.; Kost, D.; Kalikhman, I.; Engels, B.; Stalke, D. J. Am. Chem. Soc. 2004, 126, 5563. (b) Korlyukov, A. A.; Lyssenko, K. A.; Antipin, M. Yu.; Kirin, V. N.; Chernyshev, E. A.; Knyazev, S. P. Inorg. Chem. 2002, 41, 5043. (5) (a) Fester, G. W.; Wagler, J.; Brendler, E.; B€ ohme, U.; Gerlach, D.; Kroke, E. J. Am. Chem. Soc. 2009, 131, 6855. (b) Fester, G. W.; Wagler, J.; Brendler, E.; B€ ohme, U.; Roewer, G.; Kroke, E. Chem.;Eur. J. 2008, 14, 3164. (6) (a) Kalikhman, I.; Kertsnus-Banchik, E.; Gostevskii, B.; Kocher, N.; Stalke, D.; Kost, D. Organometallics 2009, 28, 512. (b) Sergani, S.; Kalikhman, I.; Yakubovich, S.; Kost, D. Organometallics 2007, 26, 5799. (c) Kalikhman, I.; Gostevskii, B.; Botoshansky, M.; Kaftory, M.; Tessier, C. A.; Panzner, M. J.; Youngs, W. J.; Kost, D. Organometallics 2006, 25, 1252. (7) (a) Gerlach, D.; Brendler, E.; Heine, T.; Wagler, J. Organometallics 2007, 26, 234. (b) Wagler, J.; B€ohme, U.; Brendler, E.; Blaurock, S.; Roewer, G. Z. Anorg. Allg. Chem. 2005, 631, 2907. (c) Gostevskii, B.; Kalikhman, I.; Tessier, C. A.; Panzner, M. J.; Youngs, W. J.; Kost, D. Organometallics 2005, 24, 5786. (d) Spiniello, M.; White, J. M. Organometallics 2000, 19, 1350. (e) Karsch, H. H.; Richter, R.; Witt, E. J. Organomet. Chem. 1996, 521, 185. r 2009 American Chemical Society

We were aiming for subtle tuning of the steric and electronic nature of an Si atom, thus influencing NfSi coordination, by ring size variation within a silacycloalkane series. Whereas some scattered examples of molecular structures of hypercoordinate silacyclobutanes7 and -pentanes8 can be found in the literature, there is only one report on a crystallographically characterized pentacoordinate silacyclohexane8d and, to our best knowledge, none regarding any higher-coordinate silacycloheptane. Hence, no systematic investigation of the correlation silacycloalkane ring size versus NfSi coordination had been carried out yet. In our former studies 8-oxyquinolinate, although conformationally rigid in itself, proved to be a flexible ligand by means of its capability of adapting to various coordination modes within the Si-coordination sphere.9 Whereas the SiPhMe moiety allows for Si hexacoordination with two oxinate ligands (compound I, Scheme 1), the related SiMe2 compound (II) exhibits a tetracoordinate silicon atom both in the solid state and in solution. With the lower steric demand about the Si atom (compared with two Me groups) and a variable degree of the so-called ring-strain-release Lewis acidity,7a,10 silacycloalkanes were chosen to approach intermediate coordination patterns between I and II. For that purpose 1,1-dichlorosilacycloalkanes An (n = 1-4), accessible via procedures published in the literature,11 were reacted with 8-oxyquinolinate derivatives (Scheme 2). (Note: although various attempts were made to improve the yield of 1,1dichlorosilacycloheptane, it did not exceed 12%. The isolation and structural characterization of 1,1,8,8-tetrachloro-1,8-disilacyclotetradecane provides an answer as to which side products are formed.12) Whereas 1,1-dichlorosilacyclobutane (8) (a) B€ ohme, U.; Foehn, I. C. Acta Crystallogr. C 2007, 63, o613. (b) Maaranen, J.; Andell, O. S.; Vanne, T.; Mutikainen, E. J. Organomet. Chem. 2006, 691, 240. (c) Hensen, K.; Gebhardt, F.; Bolte, M. Z. Anorg. Allg. Chem. 1997, 623, 633. (d) Day, R. O.; Sreelatha, C.; Deiters, J. A.; Johnson, S. E.; Holmes, J. M.; Howe, L.; Holmes, R. R. Organometallics 1991, 10, 1758. (9) (a) Wagler, J.; Gerlach, D.; Roewer, G. Chem. Heterocycl. Compd. 2006, 42, 1557. (b) Wagler, J.; Schley, M.; Gerlach, D.; B€ohme, U.; Brendler, E.; Roewer, G. Z. Naturforsch. B 2005, 60, 1054. (10) (a) Denmark, S. E. J. Org. Chem. 2009, 74, 2915. (b) Denmark, S. E.; Regens, C. S. Acc. Chem. Res. 2008, 41, 1486. Published on Web 09/03/2009

pubs.acs.org/Organometallics

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Organometallics, Vol. 28, No. 18, 2009

Brendler et al.

Scheme 1

Scheme 2

Figure 1. 29Si NMR spectra of 3 in THF-d8 at various temperatures (from top: 228, 233, 243, 253, 263, 273, 283, 293 K).

and -pentane delivered exclusively hexacoordinate silicon compounds 16 and 26, respectively [δSi = -129.9 ppm (16, solid state), -103.3 ppm (26, solid state), -100.3 ppm (26, THF), -98.6 ppm (26, CDCl3)], the related silacyclohexane produced a 29Si NMR signal characteristic of a tetracoordinate Si compound (-20.2 ppm at 20 °C in CDCl3). This chemical shift was shown to be strongly dependent on both the solvent and temperature (-45.3 ppm at 20 °C in THF-d8, Figure 1). From the VT NMR spectra (in THF-d8) the thermodynamic data for the formation of the hexa- from the tetracoordinate Si

compound were derived from ln K versus 1/T plots (Figure 2, 34 f 36: ΔH -23.5 kJ/mol, ΔS -85.8 J/mol K; 44 f 46: H -9.0 kJ/mol, ΔS -55.7 J/mol K). Complexation of the silacyclohexane (and the related -heptane system) with oxinato N atoms proved noticeably less exothermic than the reaction between phenylsilicon compounds and imine ligands, which involves ca. -30 to -35 kJ/mol for the formation of one N-Si bond.13 A still decreasing ring-strain-release Lewis acidity upon ring size extension can be considered as the reason for the less pronounced exothermic N-Si bond formation reaction for 4 (compared with 3). The ratio of the components in equilibrium in a solution of 3 at ca. 5 °C, i.e., comprising reasonable amounts of both the tetra- and the hexacoordinate Si compound, gave rise to the accidental isolation of two different solids from two independent syntheses. Whereas one of them consists of the hexacoordinate Si complex 36, the other one was found to be its isomer 34 with tetracoordinate Si atom. Although the 29Si NMR shift of the related silacycloheptane is also temperature dependent (Figure 2), even at -50 °C the tetracoordinate Si compound 44 is predominant and was the only isomer isolated as a crystalline solid. The molecular structures of 16, 26, 36, 34, and 44 were determined crystallographically (Figure 3). Selected bond lengths are presented in Table 1.

(11) 1,1-Dichlorosilacycloalkanes were prepared according to procedures described in: (a) Laane, J. J. Am. Chem. Soc. 1967, 89, 1144. (b) West, R. J. Am. Chem. Soc. 1954, 76, 6012. The use of bromoalkyl starting materials (for ring-closure reactions or Grignard reagent syntheses) is a potential source for the partial formation of 1-bromo-1-chloro-substituted silacycloalkanes, which cannot easily be removed from the 1,1-dichlorosilacycloalkane by distillation. In the case of 1,1-dichlorosilacyclopentane, -hexane, and -heptane we found a content of less than 10% of the 1-bromo1-chlorosilacycloalkane in the final product (29Si NMR spectroscopically). These silanes were used as obtained for the preparation of the respective oxinato silicon complexes, a slight excess of the respective silane providing for complete reaction of the 8-oxyquinoline used. In case of 1,1-dichlorosilacyclobutane, however, 29Si NMR spectra indicated the presence of ca. 30% of the 1-bromo-1-chloro derivative. This mixture was transformed into pure 1,1-dichlorosilacyclobutane by stirring with freshly prepared silver chloride for 5 days. (AgCl was precipitated from AgNO3 and NaCl solutions under exclusion of light, followed by repeated decantation and washing with water as well as ethanol and acetone and finally drying at 120 °C for 1 h.) Distillation of the silacyclobutane thus obtained afforded 1,1-dichlorosilacyclobutane of high purity.

(12) Neither the addition of SiCl4 to an ice-cooled two-phase ether solution of hexamethylene-dimagnesium bromide nor simultaneous addition of an ether solution of SiCl4 and a one-phase hexamethylene-dimagnesium bromide solution in ether/thf into vigorously stirred ether under reflux afforded more than 12% yield of 1,1-dichlorosilacycloheptane. This silacycloalkane was separated by vacuum distillation. Contrary to the literature11 we have added hexadecane to the crude product in order to improve the distillation conditions. Following the distillation of 1,1-dichlorosilacycloheptane at 38-48 °C (2 mmHg) the hexadecane was distilled off at 105-112 °C (2 mmHg). From this fraction some colorless crystals of 1,1,8,8-tetrachloro-1,8-disilacyclotetradecane formed within 1 day. Crystal structure analysis: C12H24Cl4Si2, CCDC-740866, T 95(2) K; monoclinic, P21/c; a 6.9533(1) A˚, b 10.0966(2) A˚, c 12.6146(2) A˚, β 98.807(1)°; V 875.16(3) A˚-3; Z 2; μ(Mo KR) 0.796 mm-1; θmax 50°; 57 495 reflections (9154 unique, Rint 0.0228), 130 parameters, GoF 1.051, R1/wR2 [I > 2σ(I)] 0.0210/0.0579, R1/wR2 (all data) 0.0296/0.0604, residual electron density (highest peak, deepest hole) 0.454, -0.413 e A˚-3 (13) (a) Lippe, K.; Gerlach, D.; Kroke, E.; Wagler, J. Organometallics 2009, 28, 621. (b) Wagler, J.; B€ohme, U.; Brendler, E.; Roewer, G. Organometallics 2005, 24, 1348.

Article

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Table 1. Selected Bond Lengths [A˚] and Angles [deg] of Compounds 16, 26, 36, 34, and 44a 16 (296 K) 16 (100 K) 26 3 (THF)0.5

Figure 2. ln(K x4fx6) vs 1/T plot of the 29Si VT NMR data of x = 3 (diamonds) and x = 4 (squares) in THF-d8 (linearization of the data of 3: ln(K) = 2829.1T-1 - 10.3200, R2 = 0.9994; and of 4: ln(K ) = 1086.6T -1 - 6.7048, R2 = 0.9991).

Si-N

2.035(1)

2.026(1)

2.073(2) 2.063(2)

Si-O

1.800(1)

1.804(1)

1.807(1) 1.798(1)

Si-C

1.905(2)

1.910(1)

1.902(2) 1.898(2)

C-Si-C 78.6(1)

79.1(1)

92.7(1)

N-Si-N 86.4(1)

86.0(1)

84.0(1)

O-Si-O 164.4(1)

164.6(1)

163.1(1)

36 2.117(1) 2.099(1) 2.090(1) 2.126(1) 1.792(1) 1.800(1) 1.789(1) 1.788(1) 1.897(2) 1.903(2) 1.904(2) 1.905(2) 99.3(1) 100.8(1) 80.6(1) 81.4(1) 161.0(1) 162.4(1)

34

44

3.991(3) 4.023(2) 2.758(3) 2.744(2) 1.670(2) 1.671(1) 1.666(2) 1.667(1) 1.849(3) 1.851(2) 1.848(3) 1.851(2) 111.6(2) 120.1(1)

99.2(1) 99.8(1)

a In the order of the atomic labels in Figure 3. For compound 36 the data of the second molecule in the asymmetric unit are also included.

Figure 3. Molecular structures of (from left) 16 at room temperature, 26 (top), 36, 34 (middle), 2a, 44 (bottom) in the crystal (ORTEP diagram with 30% probability ellipsoids, hydrogen atoms omitted, selected atoms labeled, capping of tetrahedral faces indicated with thin arrows for 34 and 44). The asymmetric unit of 16 consists of 1/2 molecule. The asymmetric unit of 36 comprises two molecules, only one of which is depicted. The structure of 26 bears 1/2 THF molecule in the asymmetric unit, which is disordered by symmetry (2-fold axis) and not depicted here.

In the three silacycloalkanes comprising hexacoordinate Si atoms the oxinato ligand adapts the N-trans-C arrangement, which has also been found for compounds such as I and its SiPh29 and SiMeCl analogues.14 As expected initially, the N-Si bond length responds to the ring size of the silacycloalkane (16: 2.035(1); 26: 2.063(2), 2.073(2); 36: 2.099(1), 2.117(1), 2.090(1), 2.126(1) A˚). Despite the variable N-Si bond lengths, the overall coordination geometries about the Si atoms remain very similar: The trans-Si-N-situated Si-C (14) Klebe, G.; TranQui, D. Acta Crystallogr. C 1984, 40, 476.

bonds do not show any significant alterations (range 1.897(2)-1.905(2) A˚) nor do the Si-O bond lengths vary in an unusual manner (range 1.788(1)-1.807(1) A˚). The angle N-Si-N, however, responds notably to increasing silacycloalkane ring size (i.e., increasing C-Si-C angle), thus being compressed. Much more striking appears the transition from compound 36 to its isomer 34, comprising a tetracoordinate Si atom. Whereas the SiMe2 analogue II exhibited a bicapped tetrahedral coordination sphere about silicon (Si-N separations ca. 2.78 A˚),9b compound 34 (and also 44) presents the Si atom housed within a monocapped tetrahedral arrangement (Si-Ncap/Si 3 3 3 Nremote separations 34: 2.758(3)/3.991(3), 44: 2.744(2)/4.023(2) A˚). This capping causes a widening of the three basal angles with respect to the capped tetrahedal face (to 347.7° and 348.3°, respectively) and a slight high-field shift in the solid state 29Si NMR spectrum (-16.0 and -7.3 ppm, respectively) with respect to phenoxy derivatives lacking this additional donor property [1,1-di(4-tertbutylphenoxy)silacyclohexane: -11.2 ppm; 1,1di(4-tertbutylphenoxy)silacycloheptane: -2.8 ppm]. Thus, compounds 34 and 44 represent another coordination mode of 8-oxyquinolinato-Si compounds that has not been reported in the literature yet. The silacyclohexane ring in both structures 36 and 34 adapts the chair conformation, which proved to be the predominant conformation over twist and boat for the parent H-substituted silacyclohexane, as shown by Arnason.15 Although comprising a seven-membered ring, the conformation of 44 mimics that of 34 by adopting a quasichair referring to the bond Si1-C24 of 44 as a substitute for atom Si1 in compound 34, and the opposite carbon atom C21 is still folded toward the capping N atom N2. The larger ring system, however, results in a significant widening of the C-Si-C angle (111.6(2)° in 34, 120.1(1)° in 44), thus demonstrating at least the greater steric demand of the silacycloheptane backbone over the silicon atom, whereas discussion of ring strain with reference to these angles would remain speculative. The variation of the C-Si-C angles along the series 16, 26, 36 (78.6(1)°; 92.7(1)°; 99.3(1)° and 100.8(1)°, (15) Arnason, I.; Thorarinsson, G. K.; Matern, E. Z. Anorg. Allg. Chem. 2000, 626, 853.

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Organometallics, Vol. 28, No. 18, 2009 Scheme 3

respectively), however, hints at the development of Silocated ring strain upon increasing silacycloalkane ring size greater than the silacyclopentane, thus supporting the nonoctahedral coordination patterns for the silacyclohexane and -heptane. Nevertheless, in addition to the above-mentioned temperature-dependent NMR properties of compound 4, its reactivity also hints at intermediate hexacoordination of its Si atom in solution, as shown in the following. In solution compounds 16, 26, 3, and even 4 proved unstable when exposed to light over several days; that is, the initial yellow color of the solutions turned red. Some hexacoordinate Si compounds comprising Si-C bonds have already been proven capable of shifting an Si-bound organyl substituent to the imine carbon atom of an Si-coordinated imine or diazo group (Scheme 3),13a,16 whereas related pentacoordinate silicon complexes did not exhibit such a reactivity pattern.17 Although 8-oxyquinolinate cannot be considered an ordinary imine due to the embedding of its formal imine moiety in a π-conjugated aromatic system, the same kind of rearrangement reaction was shown to occur in the above hypercoordinate silicon complexes upon UV irradiation (Scheme 4). The pentacoordinate Si compounds 1a, 2a, 3a, and even 4a were detected by 29Si NMR spectroscopy in THF solutions of the respective precursors upon UV irradiation (δSi = -71.8, -64.7, -65.8, -75.7 ppm, respectively). In addition, some single crystals of 2a were obtained and the identity of this compound was proven by X-ray crystallography (Figure 3). The Si-N bonds (Si1-N1 1.783(2); Si1-N2 2.094(2) A˚) occupy the axial positions with respect to a distorted trigonal-bipyramidal coordination sphere (angle N1-Si1-N2 163.7(1)°). The equatorially situated Si-O bonds (Si1-O1 1.696(1), Si1-O2 1.710(1) A˚) are very similar in length but noticeably shorter than their counterparts in the hexacoordinate Si compound 26 (1.798(1) and 1.807(1) A˚). Also remarkably shorter than in its precursor is the remaining Si-C bond (1.854(2) A˚ in 2a, 1.898(2) and 1.902(2) A˚ in 26), thus demonstrating a decrease in Si-C bond activation upon decrease of the coordination number. In conclusion we have shown that 8-oxyquinolinate-substituted silacycloalkanes (ON)2Si(CH2)n with four-, five-, (16) (a) Wagler, J.; Roewer, G.; Gerlach, D. Z. Anorg. Allg. Chem. 2009, 635, 1279. (b) Kalikhman, I.; Gostevskii, B.; Kertsnus, E.; Deuerlein, S.; Stalke, D.; Botoshansky, M.; Kost, D. J. Phys. Org. Chem. 2008, 21, 1029. (c) Wagler, J.; Doert, T.; Roewer, G. Angew. Chem., Int. Ed. 2004, 43, 2441. (d) Kano, N.; Yamamura, M.; Kawashima, T. J. Am. Chem. Soc. 2004, 126, 6250. (17) Wagler, J.; Brendler, E. Z. Naturforsch. B 2007, 62, 225.

Brendler et al. Scheme 4

six-, and seven-membered ring systems proved capable of NfSi coordination, furnishing a hexacoordinate Si environment with gradually decreasing N-Si bond strength upon ring extension. This Si hexacoordination was proven activation for photostimulated silacycloalkane ring-opening (Si-C bond cleavage) under formation of Si complexes comprising Si(C,N,O) chelating 2-substituted 1,2-dihydro8-oxyquinolinato ligands. Whereas this rearrangement gave rise to the X-ray structural characterization of the first oxinato-Si complex 2a, bearing a pentacoordinate Si atom, the rather moderate NfSi coordination in the 1,1-bis(8oxyquinolinato)silacyclohexane 3 allowed for the isolation of two coordination isomers, one with hexa- and the other with tetracoordinate Si atoms in the solid state. To the best of our knowledge, this is the first report on the isolation and structural characterization of two such isomers.

Experimental Section General Considerations. All manipulations were routinely carried out under a dry argon atmosphere using standard Schlenk techniques, with dried solvents that were distilled from sodium benzophenone ketyl (diethyl ether, THF, hexane) or dried over 3 A˚ molecular sieves (chloroform). Routine 1H, 13C, and 29Si NMR spectra (of solutions) were obtained at 21 °C using a Bruker DPX 400 spectrometer equipped with a 10 mm probe. VT NMR spectra of compounds 3 and 4 were recorded on a Bruker AVANCE III 500 spectrometer with a 5 mm probe. Chemical shifts (δ) are given relative to internal SiMe4. 29Si CP/MAS spectra were recorded on a Bruker AVANCE 400 MHz WB spectrometer using a 7 mm rotor with KelF insert. Single-crystal X-ray structure analyses were carried out on a Bruker X8 APEX II CCD diffractometer using Mo KR radiation (λ = 0.71073 A˚). The structures were solved by direct methods (SHELXS-97) and refined with full-matrix least-squares method (refinement of F2 against all reflections with SHELXL-97). All non-hydrogen atoms were anisotropically refined. C-bonded hydrogen atoms were placed in idealized positions and refined isotropically except for 1,1,8,8,-tetrachloro-1,8-disilacyclotetradecane, the hydrogen atoms of which were refined isotropically without any restraints. Syntheses and NMR Spectroscopic Data of Compounds 1, 2, 3, 4, 1a, 2a, 3a, and 4a. Synthesis of 1,1-Bis-K(O,N)-(oxinato)silacyclobutane (16). To a vigorously stirred solution of 8-oxyquinoline (1.50 g, 10.3 mmol) in tetrahydrofuran (thf, 40 mL) were added triethylamine (1.57 g, 15.4 mmol) and chlorotrimethylsilane (1.30 g, 12.0 mmol). Stirring at ambient temperature was continued for 1 h, whereupon the triethylamine hydrochloride was removed by filtration and washing with thf (15 mL). From the combined filtrate and washings the solvent was removed under reduced pressure and the remaining oil was dissolved in chloroform (20 mL). This solution was cooled in an ice bath, whereupon 1,1-dichlorosilacyclobutane11 (0.87 g, 6.17 mmol) was added. The resulting yellow solution was stored in a dark place at room temperature for 1 day followed by storage at 6 °C

Article for 1 week. The yellow crystalline product was then filtered off, washed with chloroform (2 mL), and dried in a vacuum. Yield: 1.00 g (2.79 mmol, 54%). Upon heating (up to 300 °C), this compound decomposes without melting, and upon exposure to light, this yellow compound turns brown on the surface of the crystals. Compound 16 exhibits poor solubility in solvents such as thf and chloroform, thus rendering solution NMR spectroscopy unsuitable for characterization. Anal. Found (%): C 70.25, H 5.17, N 7.67. Calcd for C21H18N2O2Si: C 70.36, H 5.06, N 7.82. 29Si CP/MAS NMR (79,5 MHz, νspin = 4 kHz): δiso -129.9. Crystal structure analysis of 16: C21H18N2O2Si, CCDC740865, T 296(2) K; monoclinic, C2/c; a 16.961(3) A˚, b 7.6099(12) A˚, c 15.524(3) A˚, β 121.917(5)°; V 1700.8(5) A˚-3; Z 4; μ(Mo KR) 0.157 mm-1; θmax 27°; 9067 reflections (1854 unique, Rint 0.0424), 119 parameters, GoF 1.051, R1/wR2 [I > 2σ(I)] 0.0392/0.0970, R1/wR2 (all data) 0.0555/0.1031, residual electron density (highest peak, deepest hole) 0.201, -0.222 e A˚-3. Since both the Si atom and the silacyclobutane-β-C atom are located on a 2-fold axis, rendering the silacyclobutane ring disordered by symmetry, a second data set was collected at 100 K: C21H18N2O2Si, CCDC-744880, T 100(2) K; monoclinic, C2/c; a 16.8161(3) A˚, b 7.5069(2) A˚, c 15.4108(4) A˚, β 122.062(1)°; V 1648.68(7) A˚-3; Z 4; μ(Mo KR) 0.162 mm-1; θmax 30°; 15 944 reflections (2384 unique, Rint 0.0348), 123 parameters, GoF 1.076, R1/wR2 [I > 2σ(I)] 0.0371/0.0948, R1/wR2 (all data) 0.0474/ 0.0994, residual electron density (highest peak, deepest hole) 0.401, -0.257 e A˚-3. This low-temperature data set allowed for the refinement of the slightly disordered silacyclobutane by split positions for the β-C atom. The silacyclobutane ring was thus found to comprise a puckering angle j of 7.1(3)°. Synthesis of 1,1-Bis-K(O,N)-(oxinato)silacyclopentane thf-solvate [26 3 0.5(thf)]. To a magnetically stirred solution of 8-oxyquinoline (1.50 g, 10.3 mmol) and triethylamine (1.57 g, 15.4 mmol) in thf (50 mL) was added 1,1-dichlorosilacyclopentane11 (0.88 g, 5.65 mmol) dropwise. Stirring at ambient temperature was continued for 1 h, whereupon the triethylamine hydrochloride precipitate was filtered off and washed with thf (10 mL). The volume of the combined filtrate and washings was then diminished to ca. 10 mL by removal of solvent under reduced pressure. Upon storage at room temperature (in a dark place) yellow crystals formed, which were separated from the supernatant by filtration and washing with thf (2 mL). To the filtrate was added diethyl ether (2 mL), and this solution was stored at 6 °C for 1 week to yield a second fraction of the product, which was also removed from the supernatant by filtration and washing with thf (2 mL). Yield (of the combined fractions, which were briefly dried in a vacuum): 0.85 g (2.08 mmol, 41%), yellow crystals that do not show any striking color change upon storage in a glass Schlenk tube exposed to visible light. Anal. Found (%): C 70.57, H 5.94, N 6.83. Calcd for C24H24N2O2,5Si: C 70.55, H 5.92, N 6.86. 1H NMR (CDCl3, 400 MHz): δ 0.34 (m, 4 H, SiCH2), 1.70 (m, 4 H, SiCH2CH2), 7.12, 7.21, 7.26, 7.57, 8.09 (mm, 10 H, ar), 8.63 (m, 2 H, NdCH). 13C NMR (CDCl3, 100 MHz): δ 20.8 (SiCH2), 26.2 (SiCH2CH2), 111.3, 113.2, 121.7, 129.1, 130.6, 135.8, 138.2, 141.5, 155.1. 29Si NMR (79,5 MHz): δ -98.6 (CDCl3), -100.3 (THF), 29Si CP/MAS NMR (79,5 MHz, νspin = 4 kHz): δiso -103.3. Crystal structure analysis of (26)2 3 THF: C48H48N4O5Si2, CCDC-740864, T 296(2) K; monoclinic, C2/c; a 32.107(2) A˚, b 8.2085(6) A˚, c 19.9681(15) A˚, β 127.929(3)°; V 4151.1(5) A˚-3; Z 4; μ(Mo KR) 0.139 mm-1; θmax 25°; 18 967 reflections (3643 unique, Rint 0.0476), 289 parameters, GoF 1.023, R1/wR2 [I > 2σ(I)] 0.0407/0.0877, R1/wR2 (all data) 0.0854/0.0985, residual electron density (highest peak, deepest hole) 0.164, -0.240 e A˚-3. Synthesis of 1,1-Bis(oxinato)silacyclohexane (isomers 34 and 36). To a magnetically stirred ice-bath-cooled solution of 8-oxyquinoline (1.50 g, 10.3 mmol) and triethylamine (1.57 g, 15.4 mmol) in thf (40 mL) was added 1,1-dichlorosilacyclohexane11 (1.00 g, 5.9 mmol) dropwise. Stirring was continued at ambient temperature for 1 h, whereupon the triethylamine hydrochloride precipitate was filtered off and washed with thf (10 mL). The solvent

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from the combined filtrate and washings was then removed in a vacuum, and the remaining oil was dissolved in diethyl ether (2 mL) and stored at 6 °C for 1 week. One such synthesis then afforded yellow crystals of isomer 36, and an analogous batch produced colorless crystals of isomer 34. In both cases the crystals were separated from the supernatant by decantation followed by washing with a mixture of 1 mL of diethyl ether plus 1 mL of pentane and drying under vacuum. Yield: 36 1.00 g (2.59 mmol, 50%). Mp: 72-75 °C. Anal. Found (%): C 71.54, H 5.86, N 7.10. Calcd for C23H22N2O2Si: C 71.47, H 5.74, N 7.25. 29Si CP/MAS NMR (79.5 MHz, νspin = 4 kHz): δiso -112.0, -114.0. Yield: 34 1.35 g (3.50 mmol, 68%). Mp: 69-72 °C. Anal. Found (%): C 71.43, H 5.72, N 7.17. Calcd for C23H22N2O2Si: C 71.47, H 5.74, N 7.25. 29Si CP/MAS NMR (79.5 MHz, νspin = 4 kHz): δiso -16.1. In solution these compounds produce identical NMR spectra. 1 H NMR (CDCl3, 400 MHz): δ 1.15 (m, 4 H, SiCH2); 1.54 (m, 2 H, SiCH2CH2CH2); 1.89 (m, 4 H, SiCH2CH2); 7.27, 7.37, 8.05 (mm, 10 H, ar); 8.67 (m, 2 H, NdCH); 13C NMR (CDCl3, 100 MHz): δ 15.4 (SiCH2), 25.1 (SiCH2CH2), 29.6 (SiCH2CH2CH2), 117.1, 120.1, 121.3, 127.2, 129.5, 135.8, 141.0, 148.0, 151.9. 29Si NMR (79,5 MHz): δ -20.2 (CDCl3), -47.2 (THF). Crystal structure analyses: 36, C23H22N2O2Si, CCDC-740860, T 296(2) K; triclinic, P1; a 9.8534(4) A˚, b 14.1842(5) A˚, c 15.1915(6) A˚, R 74.658(2)°, β 86.37(2)°, γ 72.272(2)°; V 1950.16(13) A˚-3; Z 4; μ(Mo KR) 0.142 mm-1; θmax 28°; 36 418 reflections (9284 unique, Rint 0.0247), 505 parameters, GoF 1.067, R1/wR2 [I > 2σ(I)] 0.0396/0.1027, R1/wR2 (all data) 0.0617/0.1115, residual electron density (highest peak, deepest hole) 0.294, -0.264 e A˚-3. 34: C23H22N2O2Si, CCDC-740863, T 296(2) K; monoclinic, P21/c [twin (100)(010)(00-1) with populations 62.5% and 37.5%]; a 16.861(3) A˚, b 15.362(3) A˚, c 7.5845(13) A˚, β 90.275(6)°; V 1964.5(6) A˚-3; Z 4; μ(Mo KR) 0.141 mm-1; θmax 25°; 25 423 reflections (3422 unique, Rint 0.0310), 254 parameters, GoF 1.127, R1/wR2 [I > 2σ(I)] 0.0443/0.0991, R1/wR2 (all data) 0.0608/0.1052, residual electron density (highest peak, deepest hole) 0.160, -0.263 e A˚-3. Synthesis of 1,1-Bis(oxinato)silacycloheptane (44). To a magnetically stirred ice-bath-cooled solution of 8-oxyquinoline (1.50 g, 10.3 mmol) and triethylamine (1.57 g, 15.4 mmol) in thf (40 mL) was added 1,1-dichlorosilacycloheptane11 (1.00 g, 5.50 mmol) dropwise. Stirring was continued at ambient temperature for 1 h, whereupon the triethylamine hydrochloride precipitate was filtered off and washed with thf (10 mL). The solvent from the combined filtrate and washings was then removed in a vacuum, and the remaining oil was dissolved in toluene (3 mL) to give a white precipitate, which dissolved when the mixture was heated to reflux. Upon cooling to room temperature, colorless crystals of 44 formed, which were filtered off, washed with toluene (6 mL), and dried in a vacuum. Yield: 1.16 g (2.90 mmol, 55%). Mp: 123-127 °C. Anal. Found (%): C 72.31, H 6.10, N 6.86. Calcd for C24H24N2O2Si: C 71.97, H 6.04, N 6.99. 1H NMR (CDCl3, 400 MHz): δ 1.21 (m, 4 H, SiCH2), 1.68, 1.83 (2 m, 4 H þ 4 H, SiCH2CH2CH2), 7.34, 7.40, 8.09 (mm, 10 H, ar), 8.77 (m, 2 H, NdCH). 13C NMR (CDCl3, 100 MHz): δ 15.7 (SiCH2), 23.0 (SiCH2CH2), 32.0 (SiCH2CH2CH2), 117.5, 120.3, 121.2, 127.1, 129.6, 135.7, 141.4, 148.5, 151.9. 29Si NMR (CDCl3, 79,5 MHz): δ -3.1. 29Si CP/MAS NMR (79.5 MHz, νspin = 4 kHz): δiso -7.3. Crystal structure analysis of 44: C24H24N2O2Si, CCDC-740862, T 296(2) K; monoclinic, P21/c [twin (100)(010)(00-1) with populations 65% and 35%]; a 17.0154(7) A˚, b 16.2456(6) A˚, c 7.2695(3) A˚, β 90.012(2)°; V 2009.47(14) A˚-3; Z 4; μ(Mo KR) 0.140 mm-1; θmax 26°; 15 794 reflections (3931 unique, Rint 0.0290), 263 parameters, GoF 1.042, R1/wR2 [I > 2σ(I)] 0.0344/0.0761, R1/ wR2 (all data) 0.0463/0.0798, residual electron density (highest peak, deepest hole) 0.156, -0.188 e A˚-3. Exposure of 16, 26, 3, and 4 (as solution or suspension in THF) to UV (medium-pressure Hg lamp, λmax = 365-436 nm) in a

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Table 2. Conditions and Results of the UV-Irradiation Experiments with 1,1-Bis(oxinato)silacycloalkanesa starting material

m/g

V(thf)/mL

soln/suspn

t/h

δSi (crude prod in THF)

16 26 3 0.5thf 34 44

1.4 1.7 1.4 1.2

100 120 120 100

suspension solution solution solution

4 3.5 3.5 4.5

-71.8 -64.7 -65.8 -75.7

a Residual starting material was detected by 29Si NMR spectroscopy upon irradiation of 4.

water-cooled Schlenk-type reactor (temperature of cooling water ca. 15-20 °C) as in Table 2. All those experiments afforded red-colored solutions upon irradiation of the initially yellow-colored solutions or suspensions: From the solution obtained by irradiation of 26 3 0.5thf the solvent was removed in a vacuum, and the solid residue was recrystallized from THF (3 mL) and diethyl ether (3 mL) to yield the rearrangement product 2a as a red crystalline solid. Yield: 0.22 g (0.59 mmol, 14%). Mp: 122-127 °C. Anal. Found (%): C 70.14, H 5.52, N 7.34. Calcd for C22H20N2O2Si: C 70.49, H 5.41, N 7.52. 1H NMR (THF-d8, 500 MHz): δ 0.57, 1.20, 1.28, 1.55, 1.81, 1.92 (mm, 8 H, -(CH2)4-), 4.88 (m, 1 H, N-CH-CH2-), 5.34, 6.09, 6.21, 6.28, 6.38, 7.21, 7.45, 7.59, 7.76, 8.54, 8.98 (mm, 11 H, aryl and alkenyl). 13C NMR (THF-d8, 126 MHz): δ 22.9 (Si-CH2CH2CH2CH2CH-N), 25.9 (Si-CH2CH2CH2CH2CHN), 29.3 (Si-CH2CH2CH2CH2CH-N), 41.3 (Si-CH2CH2CH2CH2CH-N), 55.8 (Si-CH2CH2CH2CH2CH-N), 111.1, 112.5, 113.5, 116.4, 117.4, 118.0, 124.0, 124.7, 125.3, 129.1, 130.9, 137.0, 140.0, 140.1, 145.2, 145.5, 152.2. 29Si NMR (THF-d8, 99,4 MHz): δ -64.7. Crystal structure analysis of 2a: C22H20N2O2Si, CCDC-740861, T 296(2) K; monoclinic, P21/c; a 15.2850(12) A˚, b 9.7919(6) A˚, c 13.3812(10) A˚, β 110.943(2)°; V 1870.4(2) A˚-3; Z 4; μ(Mo KR) 0.145 mm-1; θmax 25°; 12 402 reflections (3289 unique, Rint 0.0297), 244 parameters, GoF 1.052, R1/wR2 [I > 2σ(I)] 0.0379/0.0873, R1/wR2 (all data) 0.0617/0.0952, residual electron density (highest peak, deepest hole) 0.225, -0.224 e A˚-3. VT NMR Studies of Coordination Equilibria of Compounds 3 and 4. Compounds 3 and 4 undergo rapid Si-N bond dissociation and bond formation, which can be monitored by 29Si NMR spectroscopy as the 29Si resonance signal shifts to higher field at lower temperatures, hence indicating the support of Si-N bond formation upon cooling. The 29Si chemical shift observed (δeq), which depends on the coordination number of the Si atom (4 vs 6), is determined as a function of the fractions of the tetraand the hexacoordinate silicon complex (x4 and x6, respectively), which would produce resonance signals at δ4 and δ6, respectively.

δeq ¼ x4 ðδ4 Þ þ x6 ðδ6 Þ ¼ x4 ðδ4 Þþ½ð1 -x4 Þðδ6 Þ; x4 ¼ ðδeq -δ6 Þ=ðδ4 -δ6 Þ Hence, the chemical shift observed corresponds to the equilibrium constant K6 for the formation of the hexacoordinate Si complex.

K6 ¼ x6 =x4 ¼ ð1 -x4 Þ=x4 ¼ f1 -½ðδeq -δ6 Þ=ðδ4 -δ6 Þg=½ðδeq -δ6 Þ=ðδ4 -δ6 Þ ¼ ðδ4 -δeq Þ=ðδeq -δ6 Þ Whereas δ(T) was observed NMR spectroscopically, the 29Si NMR shifts of the purely tetra- and hexacoordinate species in the same solvent were not known. Thus, suitable approximations for δ4(3), δ4(4), δ6(3), and δ6(4) had to be made in order to obtain reasonable equilibrium constants K6. For the equilibrium between 34 and 36 the limiting value δ6 was

Table 3. 29Si NMR Shifts (δ in ppm) of Selected Silacycloalkane Derivatives silacyclohexane silacycloheptane Δδ 1,1-dichloro28.6 1-chloro-1-(4-tertbutylphenoxy)8.6 tert -11.2 1,1-di(4- butylphenoxy)4 4 -16.0 3 and 4 in the solid state

36.7 16.6 -2.8 -7.3

þ8.1 þ8.0 þ8.4 þ8.7

derived from solid state NMR spectra. The crystal structure of 36 comprises two independent molecules, which give rise to two 29Si CP/MAS NMR signals (at -112 and -114 ppm). The signal with the maximum shielding (-114 ppm) was therefore chosen to represent δ6. (It was not possible to observe such a far high-field-shifted signal in solution.) Below -45 °C (δeq reached -102.5 ppm) the signal collapsed because of precipitation of 36 from the NMR solution. Vice versa, it proved even more difficult to decide which chemical shift could be regarded as representative for δ4. In the solid state 34 exhibits a capped tetrahedron, the capping of which might already cause a slight high-field shift of the 29Si NMR signal (34 in the solid state: δiso -16.0 ppm). Hence, 1,1-di(4-tertbutylphenoxy)silacyclohexane, a 1,1-bis(aryloxy)silacyclohexane that lacks any additional donor function, was synthesized from 1,1-dichlorosilacyclohexane: The desired amounts of the chlorosilane, 4-tertbutylphenol, and triethylamine were mixed in THF at room temperature, and the hydrochloride precipitate was removed by filtration and washed with THF. From the filtrate and washings the solvent was removed in a vacuum and the residue was dissolved in CDCl3 as a nondonor solvent for NMR analysis. δSi = -11.2 ppm. Thus, the limiting chemical shifts for the equilibrium between 34 and 36 were set at δ4 = -11.2 and δ6 = -114.0.

For the equilibrium between 44 and 46 the limiting chemical shifts δ4 and δ6 were estimated as follows: Whereas in analogy to 34 the chemical shift of 1,1-di(4-tertbutylphenoxy)silacycloheptane (δSi = -2.8 ppm) was determined experimentally and considered a representative value for δ4, there was no solid state NMR signal available for isomer 46. A selection of corresponding silacyclohexane and -heptane derivatives with their 29Si NMR shifts is listed in Table 3. From reactions of the 1,1-dichlorosilacycloalkanes with less than 2 equiv of 4-tertbutylphenol (in the presence of Et3N, as pointed out above) the NMR data of the silacycloalkanes bearing both an aryloxy and a Cl substituent were accessible. One can see that the transition from the silacyclohexane to the silacycloheptane system is accompanied by a representative shift of the 29Si NMR signal to lower field by ca. 8.2 ppm. Thus, the value δ6(3) = -114.0 “corrected” by this downfield shift (δ6(4) = -105.8) was considered a suitable constant for the evaluation of the coordination equilibrium between 44 and 46. We did not consider a computational estimation of δ6(4) since both the quantum chemical optimization of its molecular structure and the calculation of its 29Si NMR shift would comprise various errors, whereas comparison of the experimentally determined chemical shifts of the 1,1-dichlorosilacycloalkanes of ring size 4, 5, 6, and 7 (δSi = þ18.2, þ45.3, þ28.6, þ36.7, respectively, with Δδ = þ27.1, -16.7, þ8.1) and of the hexacoordinate silicon complexes 16, 26, and 36 in the solid state (δSi = -129.9, -103.3, -114.0, respectively, Δδ = þ26.6, -10.7) supports the concept of alternating chemical shifts to hold for both tetra- and hexacoordinate silacycloalkanes, thus allowing for a rough approximation of δ6(4). Whereas the VT NMR spectra of compound 3 show homogeneous behavior throughout the chemical shift range from tetra- to hexacoordinate Si complexes, one has to exclude a pentacoordinate silacycloheptane “45” from playing a dominant role in the equilibrium between 44 and 46. Two approaches

Article served the purpose to rule out significant contributions of a potential species “45”: (i) Instead of δ6(4) = -105.8 ppm chemical shift values representative of pentacoordinate silicon complexes (δ5 = -50, -60, -70 ppm) were used for determination of K and linearization of ln K versus 1/T. Whereas the ln K versus 1/T analysis of an equilibrium between 44 and 46 (using δ6(4) = -105.8 ppm) results in an R2-factor of 0.9991 for the linearization, the approach using a pentacoordinate Si complex as limiting species results in linearization of lower quality (R2 = 0.9975, 0.9982 and 0.9986, respectively). (ii) The molecular geometries of three conformers of compound 4 were optimized at the MPW1PW91/6-311G(2d,p) level of theory (a: starting from the crystal structure data of 36, ring extension by one CH2 group, b: starting from the crystal structure data of 44, c: starting from optimized a, followed by distortion toward an alternative pentacoordinate Si complex with an axial-equatorial silacycloheptane ring and an axial Si-N bond). Whereas a indeed converged to a six-coordinate Si complex (which can be regarded as 46), which is structurally related to 36 (Si-N separations 2.15 and 2.18 A˚), b and c converged to capped tetrahedral Si compounds with Si-Ncap distances of 2.83 and 2.95 A˚, respectively. Among these three conformations a and b appear to be the favored ones, thus being in accord with the crystallographic data of 34, 36, and 44 (relative energies in kcal/mol: a

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0.36, b 0.00, c 4.06). Furthermore, single-point energy calculations of a, b, and c at the MP2/6-31G(d,p) level proved a more stable than b and c by 7.5 and 12.5 kcal/mol, respectively.

Acknowledgment. TU Bergakademie Freiberg and Deutsche Forschungsgemeinschaft (DFG) are acknowledged for financial support. We thank Daniela Gerlach, Institut f€ ur Anorganische Chemie, TU Bergakademie Freiberg, for computational analyses of the above-mentioned alternative penta- and hexacoordinate silacycloheptane conformers. Supporting Information Available: X-ray crystallographic files in CIF format (for structure determinations of 1,1,8,8tetrachloro-1,8-disilacyclotetradecane and compounds 16, 26, 2a, 36, 34, and 44) and a pdf document containing details of the syntheses of 1,1-dichlorosilacycloheptane and 1,1,8,8-tetrachloro-1,8-disilacyclotetradecane as well as an ORTEP drawing of 1,1,8,8-tetrachloro-1,8-disilacyclotetradecane in the crystal structure and further details of the analyses of the coordination equilibria (34 vs 36) and (44 vs 46) by VT NMR spectroscopy are deposited with the ACS. This information is available free of charge via the Internet at http://pubs.acs.org.