Organometallics 2009, 28, 3105–3108
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Notes Reactions of C,N-chelated Tin(II) and Lead(II) Compounds with Zirconocene Dichloride Derivatives Josef Baresˇ,† Philippe Richard,‡ Philippe Meunier,‡ Nadine Pirio,‡ Zdenˇka Padeˇlkova´,† Zdeneˇk Cˇernosˇek,‡ Ivana Cı´sarˇova´,§ and Alesˇ Ru˚zˇicˇka*,† Department of General and Inorganic Chemistry, Faculty of Chemical Technology, UniVersity of Pardubice, na´m. Cˇs. legiı´ 565, CZ 532 10 Pardubice, Czech Republic, Institute de Chemie Moleculaire de l’UniVersite´ de Bourgogne (ICMUB-UMR 5260), 9 aVenue Alain SaVary, BP 47870, F-21078 Dijon Cedex, France, and Department of Inorganic Chemistry, Faculty of Natural Science, Charles UniVersity in Prague, HlaVoVa 2030, 128 40 Praha 2, Czech Republic ReceiVed NoVember 24, 2008 Summary: The reactions of a C,N-chelated ((LCN)2M, where LCN is 2-((dimethylamino)methyl)phenyl and M is Sn (1) or Pb (2)) stannylene and plumbylene with di-n-butylbis(η5-cyclopentadienyl)zirconium (3) were studied. In the case of the stannylene, the trinuclear carbene-like complex [(LCN)2Sn]2Cp2Zr (4) was isolated as the major product. The second product isolated resulted from transmetalation of the C,N-chelating ligand from tin to zirconium, followed by 1-butene coupling to the coordinated aryne (C,N-chelating ligand) (5). In the lead case, the major product was the complex 5. Byproducts were elemental lead, the free ligand (LCNH), butane, and 1-butene. The oxidation of 4 by oxygen gaVe the six-membered trioxa trizircona cyclic complex (6) and an eight-membered tetraoxa tetrastanna cyclic complex (7). Early work on compounds containing an organotin group bonded to a different metal has been largely limited to derivatives of the alkali metals and of the other elements of group 14, but later compounds with transition metals have been investigated extensively for their application in organic synthesis.1 Of interest are the parallels between the organometallic chemistry of the early transition metals and the corresponding groups in the p block of the periodic table. Clear examples of chemical analogies between group 4 organometallics and group 14 derivatives are rarer than group 3/13 and 5/15 examples, possibly because of the unusually diverse nature of the chemistry associated with the group 14 elements. Nonetheless, structural similarities exist between compounds of elements in the two groups. For example, Piers and co-workers have studied the reactivity of the heavier analogues of carbenes, Lappert′s stannylene, toward zirconocene derivatives.2 A hafnocene disilene complex having a trinuclear structure similar to that of the Piers complex was reported recently by Marschner.3 Here * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Pardubice. ‡ Institute de Chemie Moleculaire de l’Universite´ de Bourgogne. § Charles University in Prague. (1) The Chemistry of Organic Germanium, Tin and Lead Compounds; Rappoport, I. , Ed.; Wiley-VCH: Weinheim, Germany, 2002; Vols. 1 and 2, and references therein. (2) (a) Piers, W. E.; Whittal, R. M.; Ferguson, G.; Gallagher, J. F.; Froese, R. D. J.; Stronks, H. J.; Krygsman, P. H. Organometallics 1992, 11, 4015. (b) Whittal, R. M.; Fergusson, G.; Gallagher, J. F.; Piers, W. E. J. Am. Chem. Soc. 1991, 113, 9867. (3) Fischer, R.; Zirngast, M.; Flock, M.; Baumgartner, J.; Marschner, C. J. Am. Chem. Soc. 2005, 127, 70.
Chart 1
Scheme 1. Reactivity of Zirconocene 3 toward 1 and 2
we report analogous reactions of C,N-chelated stannylene (1)4 and plumbylene (2)5,6 (Chart 1).
Results and Discussion The reaction of di-n-butylzirconocene with stannylene (1) at low temperature leads to the formation of a red solution (λmax 285, 406, 476, and 687 nm), from which the trinuclear compound 4 (Scheme 1, Figure 1) was isolated in 64% yield based on the starting zirconocene. The compound was studied in both the solid state and solution by NMR, EPR, and UV-vis spectroscopy. The structure of 4, determined in an X-ray diffraction study (Figure 1), is similar to that of the previously reported complex of Piers,2 with the main difference being in the separation between tin atoms, which is 3.5567(17) Å in the case of 4 and 4.2364(13) Å for the Piers compound (C5H4CH3)2Zr{Sn[CH(SiMe3)2]2} (8).2 Such a shortening is probably caused (4) Angermund, K.; Jonas, K.; Kru¨ger, C.; Latten, J. L.; Tsay, Y.-H. J. Organomet. Chem. 1988, 353, 17. (5) De Wit, P.; Van der Kooi, H. O.; Wolters, J. J. Organomet. Chem. 1981, 216, C9. (6) Baresˇ, J.; Meunier, P.; Pirio, N.; Cı´sarˇova´, I.; Ru˚zˇicˇka, A. Manuscript in preparation.
10.1021/om801120a CCC: $40.75 2009 American Chemical Society Publication on Web 04/20/2009
3106 Organometallics, Vol. 28, No. 10, 2009
Notes
Figure 2. EHT calculation for (C5H5)2Zr[(SnH2)2(NH3)]2.
Figure 1. Molecular structure of 4 (ORTEP diagram, 50% probability level, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): Zr(1)-C(5) ) 2.477(3), Zr(1)-C(4) ) 2.484(3), Zr(1)-C(8) ) 2.504(3), Zr(1)-C(7) ) 2.508(3), Zr(1)-C(1) ) 2.512(3), Zr(1)-C(10) ) 2.516(3), Zr(1)-C(6) ) 2.516(3), Zr(1)-C(2) ) 2.517(3), Zr(1)-C(9) ) 2.529(3), Zr(1)-C(3) ) 2.532(3), Zr(1)-Sn(2) ) 2.8230(4), Zr(1)-Sn(1) ) 2.8614(4), Sn(1)-C(11) ) 2.200(3), Sn(1)-C(21) ) 2.210(3), Sn(2)-C(31) ) 2.183(3), Sn(2)-C(41) ) 2.215(3), Sn(1)-N(11) ) 2.627(3), Sn(1)-N(21) ) 4.887(3), Sn(2)-N(31) ) 2.640(3), Sn(2)-N(41) ) 4.862(3); Sn(2)-Zr(1)-Sn(1) ) 77.510(1), C(11)-Sn(1)-C(21) ) 96.63(10), C(11)-Sn(1)-Zr(1) ) 117.38(7), C(21)-Sn(1)-Zr(1) ) 142.15(8).
by a strong interaction of each tin atom with one of the intramolecularly donating amino groups and the resulting change of the tin coordination geometry from ψ-trigonal bipyramidal to tetrahedral. The distances between the only two coordinated nitrogen and tin atoms (2.627(3) and 2.640(3) Å) and the other angles and distances found in 4 are comparable to those found in the parent stannylene.3 The remaining nitrogen atoms, N(21) and N(31), are out of the tin primary coordination sphere. The SnThe Sn-Zr-Sn angle is somewhat sharper than that in 8 (Sn(2)-Zr(1)-Sn(1) ) 77.51(11)° for 4 and 95.06(4)° for 8). The rest of the distances and angles in 4 are very close to those in 8 (for example: Sn-Zr ) 2.8614(4) Å in 4 and 2.8715(11) Å in 8). In solution, one set of relatively sharp signals was found in the 1H NMR spectrum in toluene-d8. In the 119Sn NMR spectrum of 4 in toluene-d8, a sharp signal at 923 ppm was observed at room temperature. When the temperature of measurement was lowered to 220 K, the signal was split into two signals (938.6 and 873.2 ppm). The decoalescence of both 1 H and 119Sn signals is due to the nonequivalence of both tin nuclei and benzylic hydrogen atoms. The 119Sn NMR shifts are shifted downfield in comparison to those of the parent stannylene (148.1 ppm).3 This is in strong contrast to the findings of Piers,2 in which the Lappert stannylene resonates at 2328 ppm and the compound 8 at 1678 ppm. This opposite direction of the chemical shift is probably caused by the fact that, in stannylene 1, the tin atom is four-coordinate with two intramolecularly bonded nitrogen atoms and that there is only one coordination bond in 4. In contrast, in Lappert′s stannylene the monomer-dimer equilibrium involved only two bonding partners for the tin atom in the monomeric form. The structure of compound 4 was investigated by a theoretical approach. The HOMO of (C5H5)2Zr(SnH2)2 (analogue of 4) is shown in Figure S1 (Supporting Information). This complex was chosen as a model for 8 (the frontier orbital orders and compositions are the same in both compounds). This orbital is clearly metal-centered and is engaged in π donation from Zr to
Sn, as was indicated for 8.2 This π back-donation explains the observed short Zr-Sn distance of 2.87 Å. The largest overlap between the metal orbital and the Sn p orbital should lead to an Sn-Zr-Sn angle close to 90° for most efficient backbonding. The observed angle of 95.06(4)° for 8 could be explained by the effect of the bulky CH(SiMe3)2 substituents on the Sn atoms. The same electronic feature should be present in the Cp2Zr(CO)2 complex. Figure S2 (Supporting Information) represents the HOMO which clearly indicated a metal-centered orbital engaged in back-bonding toward the π* MO of the CO ligands. In this complex, the absence of steric hindrance leads to the optimal value of 90° for the C-Zr-C angle.7 The HOMO of (C5H5)2Zr[(SnH2)2(NH3)]2 as another analogue of 4 is shown in Figure 2. As for (C5H5)2Zr(SnH2)2, the frontier orbital order and compositions are the same in both compounds. The characteristics of this orbital are essentially the same as those observed for (C5H5)2Zr(SnH2)2: metal-centered and π-backbonding features. The bonding schemes are thus similar for (C5H5)2Zr(SnH2)2 and (C5H5)2Zr[(SnH2)2(NH3)2]. The backdonation leads (as in Cp2Zr(CO)2) to a short Zr-Sn distance (2.817 and 2.856 Å). However, the surprising geometrical feature is the very sharp Sn-Zr-Sn angle of 77.5°; this value is far from the optimal 90° one. This discrepancy could be explained by the role of the amine groups of the Sn substituents: the amine lone pairs, which interact with the vacant Sn p orbitals, interact also with the d metal orbital. However, as one can see in the figure, for the HOMO the overlap between the d orbital and the nitrogen lone pair linear combination is negative. Thus, the observed reduction of the Sn-Zr-Sn angle minimizes this antibonding interaction. Compound 4 is light-sensitive, exposure to light generating a minor paramagnetic species detected by its ESR spectrum with a central signal at g ) 1.9723. This signal has satellites, six bands due to Zr (41.0 G), two bands from the Sn nuclei (25.2 Gauss) (see Figure S5 in the Supporting Information), and the rest possibly from the 14N isotope. The amount of the paramagnetic species can be increased by irradiation with UV light, giving about 3% of conversion after 1 h. We can only speculate about the presence of a biradical species such as Cp2ZrSn(LCN)2Sn(LCN)2Zr(Cp)2 or Cp2ZrrSn(H)(LCN)2 similar to those proposed by Samuel for π adducts of unsaturated compounds.8 The oxidation of 4 by O2 gave two known products, the cyclozirconoxane 69aand the cyclostannoxane 7,9b in essentially (7) (a) Atwood, J. L.; Rogers, R. D.; Hunter, W. E.; Floriani, C.; Fachinetti, G.; Chiesi-Villa, A. Inorg. Chem. 1980, 19, 3812. For the first theoretical study see: (b) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729.
Notes
Organometallics, Vol. 28, No. 10, 2009 3107 Scheme 2. Oxidation of 4
quantitative yield (Scheme 2); these were identified by NMR spectroscopy and X-ray diffraction. The minor product of the reaction of stannylene 1 and in situ generated zirconocene is compound 5. The compound 5((C5H5)2Zr-c-{3-[-CH2(CH2CH3)CH]-2-[(CH3)2NCH2]C6H3-}) (Figure 3) is also the main product of the reaction of plumbylene 2 with in situ generated zirconocene. During this reaction plumbylene 2 is degraded to elemental lead and the free ligand LCNH. Compound 5 is formed by ortho metalation followed by aryne to 1-butene C-C coupling, as described many times in the literature for benzyne.10 A plausible mechanism of the formation of 5 based on literature data is given in Figure S4 (Supporting Information). Compound 5 is a zirconocene derivative with four formal Zr-C bonds and the coordination polyhedron (close to trigonal bipyramidal) completed by one intramolecular Zr-N coordination. The latter is a medium-strong bond in comparison to 32 examples in the Cambridge Structural Database, comparable, for example, to the zirconocene derivative of Majoral (2.404(3) Å).11 Compound 5 crystallizes in a chiral space group with central chirality at C11. The crystal structure of this compound was determined nine times with the same result, indicating that the R isomer is always formed. During the reaction a color change from deep red to light pink was observed. After the reaction, the 1H NMR spectrum revealed two sets of signals belonging to a mixture of diastereoisomers, in which C11 and the plane defined by phenyl ring are the stereogenic centers.
Figure 3. Molecular structure of 5 (ORTEP diagram, 50% probability level, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): Zr(1)-C(1) ) 2.310(5), Zr(1)-C(10) ) 2.377(4), Zr(1)-C(23) ) 2.522(4), Zr(1)-C(15) ) 2.527(4), Zr(1)-C(14) ) 2.541(5), Zr(1)-C(22) ) 2.542(4), Zr(1)-C(16) ) 2.548(5), Zr(1)-C(17) ) 2.551(5), Zr(1)-C(21) ) 2.554(4), Zr(1)-C(19) ) 2.560(4), Zr(1)-C(18) ) 2.563(5), Zr(1)-C(20) ) 2.581(5), Zr(1)-N(1) ) 2.652(3); C(1)-Zr(1)-C(10) ) 68.79(17), C(1)-Zr(1)-N(1) ) 66.60(11), C(10)-Zr(1)-N(1) ) 134.71(11).
Experimental Section General Methods. NMR Spectroscopy. Spectra of the studied compounds were measured in deuterated solvents (toluene-d8, benzene-d6) at 220-360 K on Bruker Avance 500 spectrometers in pulsed mode with Fourier transformation. 1H, 13C, and 119Sn NMR spectra were measured in a 5 mm diameter broadband tunable sampler. The values of 1H chemical shifts were calibrated to the inert standard of tetramethylsilane (δ(1H) 0.00 ppm) or to the residual signal of benzene (δ(1H) 7.16 ppm) or toluene (δ(1H) 2.09 ppm). The values of 13C chemical shifts were calibrated to the signal for benzene (δ(13C) 128.3 ppm) or toluene (δ(13C) 20.4 ppm). The values of 119Sn chemical shifts were calibrated to the signal of the external standard tetramethylstannane in a coaxial capillary (δ(119Sn) 0.00 ppm). Positive values of chemical shifts indicate a shift to lower field in comparison to the standard. The interaction constants were found in appropriate spectra measured with digital resolution better then 0.5 Hz/point. UV-Vis Spectroscopy. Electronic absorption spectra were measured on a Jasco V-570 double-beam UV/vis/near-IR spectrometer Measurement was done in the spectral region 850 - 250 nm, the 1 cm quartz cuvetttes were used. THF used as the solvent. ESR Spectroscopy. The ESR spectra of solutions were measured at ambient temperature at X-band (ν ∼9.5 GHz) using an ERS 221 spectrometer (Magnettech Berlin). A microwave power of 1 mW, sufficiently below the saturation power, was used. X-ray Diffraction Techniques. Single crystals of the studied compounds suitable for X-ray diffraction studies were obtained at -20 °C from the solution with various solvents (mainly diethyl ether). Crystallographic data were obtained on a Nonius KappaCDD or Kuma KM4CCD diffractometer with area detector, a source of Mo KR radiation, and a graphite monochromator. A single crystal was installed on a glass fiber in inert oil and measured at the wavelength 0.710 73 or 0.710 69 Å. The structures were solved by direct methods (SIR9212). All reflections were used in the structure refinement based on F2 by the full-matrix least-squares technique (SHELXL9713). Heavy atoms were refined anisotropically. Hydrogen atoms were mostly localized on a difference Fourier map; however, to ensure the uniformity of treatment of the crystal, all hydrogens were recalculated into idealized positions (riding model) and assigned temperature factors Uiso(H) ) 1.2[Ueq(pivot atom)] or 1.5 times the Ueq for the methyl moiety. Absorption corrections were carried on, using Gaussian integration from crystal shape.14 A full list of crystallographic data and parameters, including fractional coordinates, has been deposited at the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ,U.K.(fax,int.code+44(1223)336-033;e-mail,
[email protected]). CCDC deposition numbers: 706493 and 706494 for for 4 and 5, respectively. Preparation of Compound 4. Zirconocene dichloride (0.232 g, 0.794 mmol) was dissolved in 30 mL of THF, and 1.59 mmol (8) Samuel, E. Inorg. Chem. 1983, 22, 2967. (9) (a) Fachinetti, G.; Floriani, C.; Villa, A. C.; Guastini, C. J. Am. Chem. Soc. 1979, 101, 1767. (b) Padeˇlkova´, Z.; Nechaev, M. S.; Cˇernosˇek, Z.; Brus, J.; Ru˚zˇicˇka, A. Organometallics 2008, 27, 5303. (10) For similar compounds as products of aryne-olefin coupling see: (a) Buchwald, S. L.; Watson, B. T. J. Am. Chem. Soc. 1986, 108, 7411. (b) Erker, G.; Kropp, K. J. Am. Chem. Soc. 1979, 101, 3659. (c) Kropp, K.; Erker, G. Organometallics 1982, 1, 1246. (d) Cuny, G. D.; Gutierrez, A.; Buchwald, S. L. Organometallics 1991, 10, 537. (e) Majoral, J. P.; Meunier, P.; Igau, A.; Pirio, N.; Zablocka, M.; Skowronska, A.; Bredeau, S. Coord. Chem. ReV. 1998, 145, 178–180. (f) Buchwald, S. L.; Nielsen, R. B. Chem. ReV. 1988, 88, 1047. (11) Cadierno, V.; Zablocka, M.; Donnadieu, B.; Igau, A.; Majoral, J. P. Organometallics 1999, 18, 1882. (12) Altomare, A.; Cascarone, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 1045. (13) Sheldrick G. M. SHELXL-97, A Program for Crystal Structure Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997. (14) Clark, R. C.; Reid, J. S. Acta Crystallogr. 1995, A51, 887.
3108 Organometallics, Vol. 28, No. 10, 2009 of n-BuLi (1 mL, 1.6 M in hexanes) was added dropwise at -78 °C. The solution was stirred for 10 min, and the stannylene 1 in 10 mL of THF (0.615 g, 1.59 mmol) was added slowly over 30 min. After the mixture had been stirred overnight at room temperature, a darkening of the solution was observed. Afterward, volatiles were removed in vacuo and the remaining solid was extracted with 30 mL of Et2O. After filtration, about one-third of the volume was removed under vacuum and deep red microcrystals of 4 crystallized at -28 °C. These crystals were isolated by filtration in 64% yield (0.5 g). Mp: 174-176 °C. Anal. Calcd: C, 55.85; H, 5.3; N, 5.66. Found: C, 55.8; H, 5.3; N, 5.7. 1H NMR (500.13 MHz, toluene-d8, 295 K, ppm): δ 2.01 (s, N-CH3, 24H), 3.40 (s, N-CH2, 8H), 5.37 (s, Cp, 10H), 7.03 (d of d, H(4), J ) 7.2 Hz, 4H), 7.13 (d of d, H(5), J ) 7.0 Hz, 4H), 7.39 (d, H(3), J ) 7.3 Hz, 4H), 7.44 (d, H(6), J ) 7.0 Hz, 4H). 13C{1H} NMR (125.67 MHz, toluene-d8, 295 K, ppm): δ 45.3 (N-CH3), 66.8 (N-CH2), 91.0 (C5H5), aromatic C at 126.8, 126.9, 127.0, 138.2, 143.6, 162.6. 119Sn{1H} NMR (186.50 MHz, toluene-d8, 295 K, ppm): δ 922.9; at 350 K, 944.4; at 320 K, 934.5; at 300 K, 928; at 250 K, 942.2 (broad), 872.9 (broad); at 220 K, 938.6, 873.2. Crystallographic data for 4 · Et2O were obtained by crystallization from a saturated Et2O solution in a freezer at -28 °C: C50H68N4O1Sn2Zr, Mr ) 984.68, triclinic, P1j, a ) 10.5104(6) Å, b ) 11.8483(7) Å, c ) 20.4351(17) Å, R ) 86.818(5)°, β ) 87.759(6)°, γ ) 73.511(5)°, Z ) 2, V ) 2435.6(6) Å3, Dc ) 1.459 g cm-3, µ ) 1.266 mm-1, Tmin ) -0.573, Tmax ) 0.994; 27 986 reflections measured (θmax ) 26°), 4977 independent reflections (Rint ) 0.0940), 3644 reflections with I > 2σ(I), 241 parameters, S ) 1.024, R1(obsd data) ) 0.0265, wR2(all data) ) 0.0414; maximum, minimum residual electron density 0.982, -0.380 e Å-3. CCDC deposition number: 706493. Preparation of Compound 5. Compound 5 was prepared using the same procedure as for 4, but instead of stannylene 1, plumbylene 2 (0.754 g, 1.59 mmol) was used. After the reaction mixture had been stirred overnight, a light red solution was evaporated, the residue was extracted with 20 mL of Et2O, and this extract was filtered off. In the gray residue before the extraction elemental lead was identified by XRF (X-ray fluorescence) techniques. An adequate amount of dimethylbenzylamine was identified by GC/MS techniques in the mother liquor after the reaction. Colorless crystals of 5 crystallized from the filtrate in the freezer at -28 °C (yield 58% based on starting zirconocene dichloride). Mp: 187.5 °C. Anal. Calcd: C, 68.61; H, 5.26; N, 3.48. Found: C, 68.6; H, 5.2; N, 3.5. 1 H NMR (500.13 MHz, toluene-d8, 295 K, ppm): δ 0.93, 1.65 (d, anisochronous protons, CH2(1′), J ) 10.6 Hz, 2H), 1.27 (t, CH3(4′), J ) 7.3 Hz, 3H), 1.56, 2.35 (m, anisochronous protons, CH2(3′), 2H), 1.70 (s, N-CH3, 6H), 2.55 (d, AX spin system, N-CH2, J ) 12.7 Hz, 1H), 3.28 (m, CH(2′), 1H), 3.71 (d, AX spin system,
Notes N-CH2, J ) 12.7 Hz, 1H), 5.54 (s, Cp, 5H), 5.56 (s, Cp′, 5H), 6.81 (d, CH(3), 1H), 6.88 (d, CH(5), 1H), 7.05 (t, CH(4), 1H). 13 C{1H} NMR (125.67 MHz, toluene-d8, 295 K, ppm): 11.4 (C(4′)), 30.8 (C(3′)), 49.7 (C(1′)), 49.7 (N-CH3), 54.1 (C(2′)), 71.6 (N-CH2), 120.3 (C(3)), 123.3 (C(5)), 125.6 (C(4)), 140.3 (C(6)), 163.4 (C(2)), 181.4 (C(1)). Crystallographic data for 5 were obtained on colorless single crystals by crystallization from saturated hexane solution in a freezer at -28 °C: C23H29NZr, Mr ) 410.67, orthorhombic, P212121, a ) 8.360(2) Å, b ) 14.5139(3) Å, c ) 15.5511(4) Å, Z ) 4, V ) 1886.91(8) Å3, Dc ) 1.446 g.cm-3, µ ) 0.588 mm-1, Tmin ) -0.408, Tmax ) 0.470; 6521 reflections measured (θmax ) 25.03°), 3317 independent reflections (Rint ) 0.0299), 2986 reflections with I > 2σ(I), 227 parameters, S ) 1.0085, R1(obsd data) ) 0.0310, wR2(all data) ) 0.0650; maximum, minimum residual electron density 0.468, -0.412 e Å´-3. CCDC deposition number: 706494. Definitions: Rint ) ∑|Fo2 - Fo,mean2|/∑Fo2; S ) [∑(w(Fo2 - Fc2)2)/ (Ndiffrs - Nparams)]1/2, with weighting scheme w ) [σ2(Fo2) + (w1P)2 + w2P]-1, where P ) [max(Fo2) + 2Fc2]; R(F) ) ∑||Fo| - |Fc||/ ∑|Fo|, Rw(F2) ) [∑(w(Fo2 - Fc2)2)/(∑w(Fo2)2)]1/2. The oxidation of 4 was performed by bubbling of dried and CO2free air from the cylinder (GC quality) into an Et2O (30 mL) solution of 4 (0.5 g). The red color disappeared immediately after the introduction of the first bubbles of air, and compound 6 crystallized in almost quantitative yield when the initial solution was concentrated under vacuum to approximately 10 mL. After filtration and crystallization from Et2O (20 mL) the structure of a single-crystal material was determined by X-ray crystallographic techniques and compared to the data in ref 9a. Compound 7 crystallized from the mother liquor obtained after the oxidation in the freezer. NMR spectra and the measured unit cell parameters of obtained crystals were identical with those of the tetrameric stannoxane described in ref 9b.
Acknowledgment. The Pardubice group thanks the Grant Agency of the Czech Republic (Grant No. 203/07/0468), the Grant Agency of Czech Academy of Science (Grant No. KJB401550802), and the Ministry of Education of the Czech Republic (Grant No. VZ 0021627501) for financial support. Supporting Information Available: Text, figures, tables, and CIF files giving all experimental details, crystallographic data for compounds 4 and 5, theoretically constructed structures, and ESR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. OM801120A
