Published on Web 11/23/2002
Tris(trifluoromethyl)borane Carbonyl, (CF3)3BCOsSynthesis, Physical, Chemical and Spectroscopic Properties, Gas Phase, and Solid State Structure Maik Finze,† Eduard Bernhardt,† Annegret Terheiden,† Michael Berkei,† Helge Willner,*,† Dines Christen,§ Heinz Oberhammer,*,§ and Friedhelm Aubke‡ Contribution from the Fakulta¨ t 4, Anorganische Chemie, Gerhard Mercator UniVersita¨ t Duisburg, Lotharstrasse 1, D-47048 Duisburg, Germany, Institut fu¨ r Physikalische und Theoretische Chemie, UniVersita¨ t Tu¨ bingen, 72076 Tu¨ bingen, Germany, Department of Chemistry, The UniVersity of British Columbia, VancouVer, British Columbia, V6T1Z1, Canada Received July 19, 2002
Abstract: Tris(trifluoromethyl)borane carbonyl, (CF3)3BCO, is obtained in high yield by the solvolysis of K[B(CF3)4] in concentrated sulfuric acid. The in situ hydrolysis of a single bonded CF3 group is found to be a simple, unprecedented route to a new borane carbonyl. The related, thermally unstable borane carbonyl, (C6F5)3BCO, is synthesized for comparison purposes by the isolation of (C6F5)3B in a matrix of solid CO at 16 K and subsequent evaporation of excess CO at 40 K. The colorless liquid and vapor of (CF3)3BCO decomposes slowly at room temperature. In the gas phase t1/2 is found to be 45 min. In the presence of a large excess of 13CO, the carbonyl substituent at boron undergoes exchange, which follows a first-order rate law. Its temperature dependence yields an activation energy (EA) of 112 kJ mol-1. Low-pressure flash thermolysis of (CF3)3BCO with subsequent isolation of the products in low-temperature matrixes, indicates a lower thermal stability of the (CF3)3B fragment, than is found for (CF3)3BCO. Toward nucleophiles (CF3)3BCO reacts in two different ways: Depending on the nucleophilicity of the reagent and the stability of the adducts formed, nucleophilic substitution of CO or nucleophilic addition to the C atom of the carbonyl group are observed. A number of examples for both reaction types are presented in an overview. The molecular structure of (CF3)3BCO in the gas phase is obtained by a combined microwave-electron diffraction analysis and in the solid state by single-crystal X-ray diffraction. The molecule possesses C3 symmetry, since the three CF3 groups are rotated off the two possible positions required for C3v symmetry. All bond parameters, determined in the gas phase or in the solid state, are within their standard deviations in fair agreement, except for internuclear distances most noticeably the B-CO bond lengths, which is 1.69(2) Å in the solid state and 1.617(12) Å in the gas phase. A corresponding shift of ν(CO) from 2267 cm-1 in the solid state to 2251 cm-1 in the gas phase is noted in the vibrational spectra. The structural and vibrational study is supported by DFT calculations, which provide, in addition to the equilibrium structure, confirmation of experimental vibrational wavenumbers, IR-band intensities, atomic charge distribution, the dipole moment, the B-CO bond energy, and energies for the elimination of CF2 from (CF3)xBF3-x, x ) 1-3. In the vibrational analysis 21 of the expected 26 fundamentals are observed experimentally. The 11B-, 13C-, and 19F-NMR data, as well as the structural parameters of (CF3)3BCO, are compared with those of related compounds.
Introduction
Borane carbonyls can be viewed on account of their high CO stretching wavenumbers as main group analogues of σ-bonded transition metal carbonyl cations.1,2 In both classes of carbonyls, CO is σ-bonded and the CO bond is polarized by * To whom correspondence should be addressed. E-mail:
[email protected]. H. Oberhammer: Telephone: 49-7071-295490. Fax: 49-7071295490. E-mail:
[email protected]. F. Aubke: Telephone: 1-604-822-2847. E-mail:
[email protected]. † Gerhard Mercator Universita ¨ t Duisburg. ‡ The University of British Columbia. § Universita ¨ t Tu¨bingen. (1) Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 24022425. (2) Willner, H.; Aubke, F. In Inorg. Chem. Hightlights; Meyer, G., Naumann, D., Wesemann, L., Eds.; Wiley-VCH: Weinheim Germany, 2002; p 195. 10.1021/ja0209924 CCC: $22.00 © 2002 American Chemical Society
positively charged central atoms, resulting in a strengthening of the CO bond by electrostatic contribution.3,4 The simplest example of a borane carbonyl, H3BCO, has been known since 1937.5 The compound has been extensively characterized by experimental, structural, and theoretical means.6-9 Approxi(3) Goldman, A. S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1996, 118, 12 15912 166. (4) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J.; Ziegler, T. Inorg. Chem. 1997, 36, 5031. (5) Burg, A. B.; Schlesinger, H. I. J. Am. Chem. Soc. 1937, 59, 780-787. (6) Gmelins Handbuch der Anorganischen Chemie, BorVerbindungen, Teil 10; 1980; 1st. Suppl. Volume 1., 1983; 2nd. Suppl. Volume 1, 1987; 3rd. Suppl. Volume 1, 1994; 4th. Suppl. Volume 1a, and 1996, ed.; Volume 1b 1; Springer-Verlag: Berlin, Heidelberg, New York, 1976; Vol. 37. (7) Venkatachar, A. C.; Taylor, R. C.; Kuczkowski, R. L. J. Mol. Struct. 1977, 38, 17. (8) Jones, L. H.; Taylor, R. C.; Paine, R. T. J. Chem. Phys. 1979, 70, 749. (9) Bauschlicher, C. W.; Ricca, A. Chem. Phys. Lett. 1995, 237, 14. J. AM. CHEM. SOC. 2002, 124, 15385-15398
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mately twenty additional borane carbonyl derivatives have become known in the meantime. They are synthesized primarily by addition of CO to suitable boranes and boron subhalides.6 We have recently reported in a preliminary communication on the synthesis of tris(trifluoromethyl)borane carbonyl, (CF3)3BCO,10 by solvolysis of the [B(CF3)4]- anion11 in concentrated sulfuric acid. This reaction was found accidentaly by testing the limits of stability of the very weak coordinating [B(CF3)4]anion. The reaction proceeds in a series of ligand transformations, starting from K[B(CN)4]12 according to (1), featuring a single C-bonded ligand.
During these transformations, the B-C bonds remain intact, whereas the substituents at the central boron atom are altered first by perfluorination11 and then by partial hydrolysis.10 The in situ generation of a B-CO group observed,10 is unprecedented. The new borane carbonyl is initially identified and characterized by a molecular mass determination, its IRspectrum (where ν(CO) is unprecedentedly high with 2252 cm-1), NMR-data and a limited number of characteristic reactions. In this publication, we want to report (i) on details of the synthesis of (CF3)3BCO and its thermal behavior, studied by gas phase kinetics, isotopic exchange reactions and low pressure flash thermolysis, with the decomposition products trapped in an Ar-matrix and studied by IR spectroscopy, (ii) the synthesis of the previously unknown reference compound (C6F5)3BCO by trapping the Lewis acid in a CO matrix, (iii) present an overview of a number of chemical reactions, that involve either nucleophilic substitution of the CO ligand or nucleophilic addition to the C atom of the carbonyl group, (iv) the molecular structure of (CF3)3BCO in the gas phase, obtained by electron diffraction combined with microwave spectroscopy, (v) the molecular structure of (CF3)3BCO, obtained by single-crystal X-ray diffraction, (vi) vibrational analysis of (CF3)3BCO and DFT calculations, and (vii) a complete characterization of (CF3)3BCO by heteronuclear NMR methods. The comprehensive characterization of (CF3)3BCO described here, allows a meaningful and informed comparison to other known carbonyl boranes5-9 and also to σ-bonded metal carbonyl cations.1,2 Experimental Section General Procedures and Reagents (a) Apparatus. Volatile materials were manipulated in a glass vacuum-line, equipped with two capacity pressure gauges (221 AHS-1000 and 221 AHS-100, MKS Baratron, Burlington, MA) and consisting of three U-traps and valves with PTFE stems (Young, London UK). The vacuum line was connected to an IR cell (Optical path length 200 mm, Si windows, 0.5 mm thick), contained in the sample compartment of a FTIR instrument. Because of this arrangement, it was possible to follow the course of the reactions closely and to monitor the purification processes of the products. Products were stored in flame-sealed glass ampules under (10) Terheiden, A.; Bernhardt, E.; Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 2002, 41, 799. (11) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Bu¨rger, H. Chem.Eur. J. 2001, 7, 4696. (12) Bernhardt, E.; Henkel, G.; Willner, H. Z. Anorg. Allg. Chem. 2000, 626, 560-568. 15386 J. AM. CHEM. SOC.
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liquid nitrogen in a storage Dewar vessel. The ampules were opened and resealed using an ampule key.13 The melting point of (CF3)3BCO was determined on samples placed inside 6 mm o.d. glass tubes, using a slowly heated stirred water bath, in the temperature range of 5-10 °C. Vapor pressures were measured in a small trap, using the above-mentioned capacitance manometer (AHS-100) in the temperature range between -45 to +10 °C. Five different cold bathes with ethanol as coolant were used to adjust the temperature as quickly as possible to prevent decomposition. Measurements at different temperatures were repeated several times on purified samples to obtain averaged vapor pressure data. For kinetic measurements, a small glass vessel (1 cm o.d., 10 cm length) equipped with a glass valve with a PTFE stem (Young, London UK) was used. For each run 0.015 mmol (CF3)3BCO and 0.8 mmol 13 CO were transferred into the evacuated vessel by cooling it to -196 °C. The vessel was placed in a thermostated water bath for the desired time period. The exchange process was quenched by cooling the cell to -196 °C. The excess of 13CO was recovered by cryopumping the gas into a vessel filled with molecular sieve (5 Å) kept at -196 °C. Subsequently, at -78 °C the decomposition products were removed in dynamic vacuum. By warming the vessel to room temperature, the solid residue was finally evaporated into an evacuated IR-cell and the ν(CO) absorbance ratio A(ν13CO)/A(νCO) was measured. This procedure was performed on 20 samples at bath temperatures of 10.9, 21.0, 25.2, 30.0, and 35.1 °C, respectively. For each temperature, four different reaction times were chosen (between 7 and 300 min). The potentiometric titration was performed on a 0.1 molar aqueous solution of (CF3)3BCO and was repeated several times. Matrix-isolated samples were prepared by passing a gas stream of Ar, Ne, or N2 (∼3 mmol h-1) over the sample placed in a small U-trap in front of the matrix support. A U-trap containing (CF3)3BCO or (C6F5)3B (in this case CO was used as matrix gas) was kept at -82 or +35 °C, respectively. In a stainless steel vacuum line with a volume of 1.1 L, a small amount of a 1:1 CO/BF3 mixture (ca. 0.1 mmol) was mixed with Ar at a 1:500 ratio. About 1 mmol of this mixture was directly deposited via a stainless steel capillary on the matrix support, kept at 16 K. Details of the matrix apparatus have been described elsewhere.14 (b) Chemicals. CO (standard grade, Messer-Griesheim, Krefeld, Germany), 13CO (> 99% isotopic enrichement, Deutero GmbH, Kastellaun, Germany) and C18O (> 99% Ventron, Numbai, India), as well as all standard chemicals and solvents were obtained from commercial sources. H3BCO was prepared from B2H6 and CO as reported.6 K[B(CF3)4] was synthesized as described previously from K[B(CN)4].11 (c) Synthetic Reactions. (1) (CF3)3BCO. The synthesis was performed in a “V-shaped” reaction vessel, equipped with a valve with a PTFE stem (Young, London) and fitted with two 100 mL roundbottom flasks. Solid K[B(CF3)4] (0.5 g/15.3 mmol) was dissolved in 20 mL of Et2O and transferred into one of the round-bottom flasks, that contained a PTFE-coated magnetic stirring bar. Ether was removed under reduced pressure and 25 mL of concentrated H2SO4 were placed into the second round-bottom flask. The reaction vessel was attached to a glass vacuum line with a series of three U-traps and evacuated. H2SO4 was added in 3 portions to K[B(CF3)4] at room temperature, and the resulting mixture was stirred vigorously. Gas evolution occurred immediately from the suspension. In the 1st U-trap, a liquid, identified as HSO3F, that had formed during the reaction, was separated at -30 °C. In the 2nd trap (CF3)3BCO was collected at -100 °C. Highly volatile byproducts such as BF3 were solidified at -196 °C in the 3rd U-trap. After completing the addition of H2SO4, the reaction mixture was stirred at room temperature until no further gas evolution was observed. The temperature was raised to 45 °C, and the contents of (13) Gombler, W.; Willner, H. J. Phys. E: Sci. Instruments 1987, 20, 1286. (14) Argu¨ello, G. A.; Grothe, H.; Kronberg, M.; Willner, H.; Mack, H. G. J. Phys. Chem. 1995, 99, 17525.
(CF3)3BCO−Synthesis the flask were stirred for approximately 7 h, until the reaction mixture became clear. 3.31 g of pure (CF3)3BCO (13.5 mmol) were obtained from the crude product after trap-to-trap distillation at -85 °C which corresponds to a yield of 88%. For the syntheses of (CF3)3B13CO and (CF3)3BC18O, a 50 mL roundbottom flask, fitted with a glass valve (Young, London) was charged with 0.24 mmol (CF3)3BCO and 5 mmol of 13CO or C18O by vacuum line transfer. The reaction was allowed to proceed at 35 °C 40 min and after subsequent cooling to -196 °C, the excess of isotopically enriched CO was recovered by cryopumping onto molecular sieve (5 Å) at -196 °C. The residue (0.12 mmol) was evaporated and trapped in vacuo at -78 °C. It contained about 50% of (CF3)3 B13CO or (CF3)3BC18O, respectively. (2) K2[(CF3)3BCO2]. A 50 mL round-bottom flask equipped with a valve with a PTFE stem (Young, London), fitted with a PTFE-coated magnetic stirring bar, was charged with 765 mg of (CF3)3BCO (3.1 mmol). About 10 mL of distilled water were condensed in vacuo, into the flask, kept at -196 °C. The mixture was allowed to warm to room temperature. Under vigorous stirring, 1.74 g of KOH (31 mmol) dissolved in 5 mL of distilled water was added to the clear colorless solution. After removing most of the water under reduced pressure, the reaction mixture was extracted with CH3CN three times, in portions of 100, 100, and 50 mL. The collected CH3CN layers were combined, dried over K2CO3 and filtered through a fine glass frit. Even after filtration, the organic solution was cloudy. After removing all volatiles in vacuo, 780 mg (2.30 mmol) 74% yield of white K2[(CF3)3BCO2] were obtained that showed no impurities in the NMR spectra. (3) [Pr3NH][(CF3)3BC(O)OH]. 780 mg of K2[(CF3)3BCO2] (2.30 mmol) were dissolved in 20 mL distilled water. Under vigorous stirring, 2 mL (24 mmol) of concentrated HCl and 0.05 mL (5 mmol) of trin-propyl ammine were added to the clear colorless solution. Immediately after addition small quantities of an off white solid formed and a small amount of the residual ammine was observed on the bottom of the flask. The mixture was extracted with CH2Cl2 three times (100 mL, 50 mL and 20 mL, respectively). The collected organic phases were combined, dried over MgSO4 and filtered through a fine glass frit, and the solvent was removed under reduced pressure to yield 794 mg of [Pr3NH][(CF3)3BC(O)OH] (2.12 mmol) 92%. NMR data of the [Pr3NH]+ cation: 1H NMR (300.13 MHz, CD3CN, 25 °C, TMS) δ 9.42 ppm (s, 1 H), 2,96 ppm (t, 6 H), 1,81-1,67 ppm (m, 6H), 0,95 ppm (t, 6H); 13C{1H}-NMR (125.758 MHz, CD3CN, 25 °C, TMS) δ 55.12 ppm (s, 1C), δ 17.76 ppm (s, 1C), δ 11.23 ppm (s, 1C). (4) (CF3)3BNCCD3. 130 mg (CF3)3BCO (0.5 mmol) were transferred in vacuo into a 5 mm o.d. NMR tube, equipped with a rotational symmetrical valve with a PTFE stem (Young, London).15 A 1.5-mL portion of dry CD3CN were condensed to the borane carbonyl and the reaction mixture was warmed to room temperature. Gas evolution was observed in the NMR tube. After 30 min at room temperature, NMR spectra of the mixture were recorded. Nearly pure (CF3)3BNCCD3 was identified that contained a small amount (∼4%, 19F NMR) of (CF3)3BC(OH)2 as the only impurity. (d) Preparation of Single-Crystals of (CF3)3BCO. Approximately 10 mg (CF3)3BCO were transferred in vacuo into a small glass ampule (6 mm o.d., 20 cm length) kept at -196 °C. Volatile impurities were removed under reduced pressure at -75 °C and the sealed ampule was stored at -70 °C for 6 d. At this temperature, the vapor pressure of (CF3)3BCO was sufficient (10-2 mbar) for a slow crystallization process. To prevent decomposition of the single-crystals, used in the subsequent X-ray diffraction analysis, the crystals were placed on a copper trough,16 cooled to -70 °C. During the preparation the trough was flushed with dry nitrogen. Suitable crystals were selected under a polarizing microscope and fitted into glass capillaries (o.d. 0.1 to 0.3 mm). The capillaries were sealed off at both ends to give glass cylinders of (15) Gombler, W.; Willner, H. International Laboratory 1984, 84. (16) Veith, M.; Ba¨ringhausen, H. Acta Crystallogr., Sect. B: Struct. Sci. 1974, 30, 1806.
ARTICLES Table 1. Crystallographic Data of (CF3)3BCO at 143 K
compound empirical formula formula weight crystal system, space group unit cell dimensions: a [Å] b [Å] c [Å] β [°] unit cell volume: V [Å3] Z value Fcalc [g m-3] R1, (I > 2σ(I))a R1, (all data)a wR2, (all data)b
(CF3)3BCO C4BF9O 245.85 monoclinic, P21/c (No.14) 7.254(1) 9.959(2) 10.793(2) 91.18(3) 779.5(2) 4 2.094 0.1271 0.1651 0.3937
a R ) (∑||F | - |F |)/∑|F |. b R ) [∑w(F 2 - F 2)2/∑wF 2]1/2, weight 1 o c o w o c o scheme w ) [σ2(Fo) + (aP)2 + bP]-1, P ) (max(0, Fo2) + 2Fc2)/3, a ) 0.1823, b ) 8.1774.
approximately 20 mm length. The glass cylinders, containing the crystals, were attached with wax onto goniometer heads. (e) Instrumentation. (I) Single-Crystal X-ray Diffraction. Diffraction data were collected at -143 K on a Nonius diffractometer with a CCD camera using Mo KR radiation (λ ) 0.710 69 Å) and a graphite monochromator. No transformations of the crystal during cooling from -70 to -130 °C were observed optically, but the reflexes became sharper on cooling. At the end of the data collection, the first intensity measurement was repeated, which demonstrated the stability of the crystal during the X-ray diffraction analysis. The intensity data were subsequently corrected with the SCALEPACK program.17 The structure was solved in P21/c (No. 14) by direct methods with SHELXS-9718,19 and refined with anisotropic temperature factors.20 A summary of experimental details and crystal data is collected in Table 1. (II) Electron Diffraction. Electron diffraction intensities were recorded on Kodak Electron Image plates (13 × 18 cm) with a KDG2 Diffraktograph21 at 25 and 50 cm nozzle-to-plate distances and with an accelerating voltage of about 60 kV. The (CF3)3BCO sample was kept at -6 °C (sublimation pressure about 10 mbar) and the inlet nozzle was at room temperature. The photographic plates were analyzed by the usual methods22 and averaged molecular intensities in the s-ranges 2-18 and 8-35 Å-1 (s ) (4π/λ) sin θ/2, λ is the electron wavelength and θ is the scattering angle) are shown in Figure 1. (III) Microwave Spectroscopy. The microwave spectrum was recorded in the X- and K-bands with a conventional stark spectrometer (modulation frequency 50 kHz). The sample must be allowed to flow continuously through the cell, which was accomplished by cooling the sample to -50 °C, the cell to -30 °C and adjusting the valves accordingly. Broad band as well as high resolution spectra were recorded. Because the broad band spectra were recorded to monitor and ascertain the sample flow through the cell, only the high-resolution spectra were chosen for a subsequent discussion. (IV) Vibrational Spectroscopy. (a) Gas-phase infrared spectra were recorded with a resolution of 2 cm-1 in the range 4000-50 cm-1 on a Bruker IFS 66v FTIR instrument. Matrix infrared spectra were recorded using another Bruker IFS 66v FT spectrometer in the reflectance mode employing a transfer optic. A DTGS detector, together with a KBr/Ge beam splitter was used in the region of 5000-400 cm-1. In this region, 64 scans were co-added for each spectrum using an apodized resolution of 1.2 or 0.3 cm-1. A Ge-coated 6 µm Mylar beam splitter and a far(17) Otwinowsky, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. (18) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 467. (19) Sheldrick, G. M. SHELXTL, Release 5.1 Software Reference Manual; Bruker AXS, Inc.: Madison, Wisconsin, USA, 1997. (20) Sheldrick, G. M. Universita¨t Go¨ttingen, 1997. (21) Oberhammer, H. Molecular Structure by Diffraction Methods; The Chemical Society: London, 1976; Vol. 4. (22) Oberhammer, H.; Gombler, W.; Willner, H. J. Mol. Struct. 1981, 70, 273. J. AM. CHEM. SOC.
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[B(CF3)4](solv)- + H3O+ 9 8 conc. H SO 2
4
(CF3)3BCO(g) + 3HF(solv) (2)
Figure 1. Experimental (dots) and calculated (full line) molecular intensities for long (top) and short (bottom) nozzle-to-plate distances and residuals of (CF3)3BCO.
IR DTGS detector were used in the region of 650-80 cm-1. In this region, 128 scans were co-added for each spectrum, using an apodized resolution of 0.5 cm-1. (b) Raman spectra of solid (CF3)3BCO, deposited from the gas phase on a metal finger at -196 °C in high vacuum, were recorded with a resolution of 2 cm-1 on a Bruker RFS 100/S FT Raman spectrometer using the 1064 nm excitation (500 mw) of a Nd: YAG laser (DPY 301 II-N-OEM-500, Coherent, Lu¨beck, Germany). (V) NMR Spectroscopy. 1H-, 19F-, and 11B-NMR spectra were recorded at room temperature on a Bruker Avance DRX-300 spectrometer operating at 300.13, 282.41 or 96.92 MHz for 1H-, 19F-, and 11B-nuclei, respectively. 13C-NMR spectroscopic studies were performed at room temperature or at -10 °C on a Bruker Avance DRX-500 spectrometer, operating at 125.758 MHz. The NMR signals were referenced against TMS and CFCl3 as internal standards and BF3‚OEt2 as external standard. Concentrations of the investigated samples were in the range of 0.1-1 mol L-1. (CF3)3BCO samples for NMR spectroscopic studies were prepared in 5 mm NMR tubes, equipped with special valves with PTFE stems (Young, London).15 Dry SO2 or CD2Cl2 were used as solvents. NMR samples in SO2 were investigated at -10 °C whereas spectra in CD2Cl2 were recorded at room temperature. Salts were dissolved in CD3CN and transferred into 5 mm o.d. NMR tubes and investigated at room temperature. (VI) UV Spectroscopy. UV spectra of gaseous samples in a glass cell (optical path length 10 cm) equipped with quartz windows (Suprasil, Heraeus, Hanau, Germany) were recorded on a Perkin-Elmer Lambda 900 spectrometer in the spectral range of 190-350 nm. Pressures were measured with a capacitance manometer (122 A-100, MKS Baratron, Burlington, MA). To eliminate absorption from atmospheric O2, the monochromator and the housing of the gas cell were flushed with dry N2. The uncertainties of IR and UV absorption cross sections (determined on base e) are estimated to be 5 to 10%. (VII) DSC Measurements. Thermo-analytical measurements were made with a Netzsch DSC204 instrument. Temperature and sensitivity calibrations in the temperature range of 20-500 °C were carried out with naphthalene, benzoic acid, KNO3, AgNO3, LiNO3, and CsCl. About 5-10 mg of the solid samples were weighed and contained in sealed aluminum crucibles. They were studied in the temperature range of 20-500 °C with a heating rate of 5 K min-1; throughout this process, the furnace was flushed with dry nitrogen. For the evaluation of the output, the Netzsch Protens 4.0 software was employed.
Results and Discussion
Synthetic Aspects and Thermal Properties of (CF3)3BCO. The synthesis of (CF3)3BCO involves the in situ generation of coordinated CO by the solvolysis of a single trifluoromethyl group10 of the recently reported salt K[B(CF3)4]11 in concentrated H2SO4 (96%). The formation reaction may be formulated as 15388 J. AM. CHEM. SOC.
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The new compound is isolated on account of its volatility. The byproduct HF is converted in H2SO4 into HSO3F, which is detected among the less volatile products of a trap-to-trap condensation. Precedents for the acid hydrolysis of CF3 groups which result in the formation of coordinated CO are reported for various complexes of Mn,23 Fe,23 Mo,23 Ru24, and Pt25,26 according to eq 3 H+
H2O
8 [M ) CF2]+ 9 8 [M-CO]+ M-CF3 9 2HF HF M ) Mn|, Fe|, Mo|, Ru|, Pt|
(3)
The reported hydrolysis of CF3-substituted aryl derivatives in concentrated sulfuric acid under more severe conditions (>100 °C, >6 h) to carboxy derivatives is a more remote precedent.27 However because in concentrated H2SO4, the oxonium ion,28 H3O+, is found to exist, its involvement in the formation of (CF3)3BCO (see eq 1) is very likely. A potential alternate route to (CF3)3BCO, the CO addition to the Lewis acid B(CF3)3 is not feasible because the latter compound is so far not known, despite considerable synthetic efforts.29 In addition, attempts to add CO to the related Lewis acid B(C6F5)3,30,31 which is known since 1963, have been unsuccessful,32 for reasons which will be discussed below. Hence, the synthesis of (CF3)3BCO described here, is both unique and unprecedented. The new compound (CF3)3BCO is a clear, colorless liquid at room temperature, with a melting point of 9 ( 1 °C. At this temperature, the vapor pressure is 38 mbar and the sublimation vapor pressure curve is given by the expression ln(p) ) -6162/T + 25.5 (p in mbar, T in K). Above its melting point, (CF3)3BCO decomposes rapidly, so that a reliable vapor pressure curve is not obtainable. The thermal decay of (CF3)3BCO in the gas phase follows a first-order rate law with t1/2 ) 45 min. The decomposition products CO and BF3 are identified by IR spectroscopy. Additional IR bands at 1378, 1327, 1244, 1222, 1150, 1130 (sh), 1068, 991, 937, 876, 751, 605 cm-1 and new signals in the NMR spectra at δ(11B) ) +19.4 and δ(19F) ) -75.6, -124.7, and -124.3 ppm indicate the formation of CF3BF2 in addition to further, unidentified CF-compounds. Hence, the rate determining step appears to be the dissociation of the B-CO bond. The resulting B(CF3)3-fragment decomposes fast (23) Richmond, T. G.; Crespi, A. M.; Shriver, D. F. Organometallics 1984, 3, 314. (24) Clark, G. R.; Hoskins, S. V.; Roper, W. R. J. Organomet. Chem. 1982, 234, C9. (25) Michelin, R. A.; Facchin, G. J. Organomet. Chem. 1985, 279, C25. (26) Appleton, T. G.; Berry, R. D.; Hall, J. R.; Neale, D. W. J. Organomet. Chem. 1989, 364, 249. (27) Baasner, B.; Hagemann, H.; Tatlow, J. C.; 4 ed.; Houben-Weyl, 2000; Vol. E10b/Part 2, p 418. (28) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, UK, 1984. (29) Ansorge, A.; Brauer, D. J.; Bu¨rger, H.; Krumm, B.; Pawelke, G. J. Organomet. Chem. 1993, 446, 25-35. (30) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc. 1963, 212. (31) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245. (32) Jacobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.; Meyer, O. Organometallics 1999, 18, 1724-1735.
(CF3)3BCO−Synthesis
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Table 2. IR Bands of Ar Matrix Isolated Products, Formed by Low Pressure Flash Thermolysis of (CF3)3BCO
a
band positiona
Ib
assignment
2138 1510 1498 1458 1451 1447 1404 1381 1372 1337
0.062 0.050 0.017 0.190 0.025 0.079 0.018 0.015 0.067 0.023
CO CF310BF2 10BF 3 CF311BF2 11BF 3
C2F4
band positiona
Ib
1328 1230 1221 1211 1207 1180 1148 1143 1135 1115
0.090 0.153 0.196 0.129 0.178 0.083 0.085 0.102 0.076 0.072
assignment
CF2 C2F4
band positiona
Ib
1102 984 919 755 712 695 676 605 559 525
0.42 0.065 0.005 0.013 0.004 0.020 0.003 0.021 0.010 0.016
assignment
CF2
CF310BF2 CF311BF2 11BF 3
Most intensive matrix site, cm-1. b Absorbance units.
at room temperature, so that the proposed overall decomposition is formulated as shown in eq 4
(CF3)3BCO h CO + (CF3)3B f CO + CF3BF2 + BF3 + further products (4) In contrast, BH3CO is found to decompose in the same IR cell at 28 °C in a different manner. The initial decomposition rate (t1/2 ≈ 100 min), which is comparable to that of (CF3)3BCO, decreases gradually, as equilibrium (5) is approached
2BH3CO h B2H6 + 2CO
(5)
Low pressure flash thermolysis of thermally labile compounds such as (CF3)3BCO, followed by subsequent quenching of the products in low-temperature matrixes, allows the detection of short-lived intermediates by IR spectroscopy.14,33 This technique is utilized here, to obtain further information, regarding the first step in the mono-molecular dissociation of (CF3)3BCO. Initial decomposition products, isolated in an Ar matrix, are observed by passing highly diluted (CF3)3BCO and Ar, through a heated spray-on nozzle at 100 °C. At about 180 °C, the initial amount of (CF3)3BCO is completely depleted. All matrix IR bands of the decomposition products are simultaneously increased in intensity. The same experiments were repeated by using N2 as matrix gas in order to form the isoelectronic (CF3)3BN2 molecule. However, this attempt failed. The “new” bands observed in an Ar matrix are listed in Table 2. By comparison to reference matrix IR spectra of authentic samples, the main products CO, BF3, CF2, and minor amounts of C2F4 are identified. The molar ratio CO/BF3 of 4:1 is estimated from a reference spectrum of a CO/BF3 mixture, isolated in an Ar matrix. All additional, unassigned bands in Table 2 can be attributed to a mixture of CF3BF2, (CF3)2BF and possibly minor amounts of B(CF3)3. From these observations, the following reaction pathway shown in eq 6 is proposed slow
(CF3)3BCO y\z CO + (CF3)3B
(6)
followed by a series of fast reactions as displayed in eq 7
8 (CF3)2BF 9 8 CF3BF2 9 8 BF3 (CF3)3B 9 -CF -CF -CF 2
2
(7)
2
This implies that the activation energy for the CF2 elimination of B(CF3)3 is lower, than the B-CO bond energy. Indeed DFT calculations predict for each step in eq 7 values of about 80 kJ (33) Sander, S.; Pernice, H.; Willner, H. Chem.-Eur. J. 2000, 6, 3645.
Figure 2. Arrhenius plot of the (1st order).
13CO
exchange reaction with (CF3)3BCO
mol-1, whereas the B-CO bond energy is estimated to be 114 kJ mol-1 (see below). The thermal decomposition of (CF3)3BCO is in the gas-phase retarded by the presence of CO and in solution by inert aprotic solvents (SO2, CH2Cl2). In the latter case, the solvation of the electrophilic CO carbon atom (see below) by polar solvent molecules, seems to be responsible for a strengthening of the B-CO bond. In the presence of CO the primary step in the decomposition (6) appears to be suppressed, by the backward reaction. The presence of low concentrations of the free Lewis acid (CF3)3B in equilibrium with CO is confirmed by the use of isotopically enriched 13CO and C18O
(CF3)3BCO + 13CO h (CF3)3B13CO + CO
(8)
which permits the syntheses the 13CO and C18O enriched borane carbonyls. With 13CO in a large excess, the exchange process (8) is of first order. The exchange rates at 308.3, 303.2, 298.4, 294.2, and 284.1 K are quantitatively determined to be k ) 4.025, 1.967, 0.9605, 0.4777, and 0.1005 × 10-4 s-1, respectively, by using the observed absorbance ratio Z ) A(ν13CO)/A(νCO) and the equation ln(1 + Z) ) kt + B. The corresponding Arrhenius plot of these data is displayed in Figure 2. From the mean square regression slope, ln(k) ) -13423 T-1 + 35.7, the activation energy is calculated to be EA ) 112 ( 1 kJ mol-1. This value is in good agreement with the theoretically predicted B-CO bond dissociation energy of 114 kJ mol-1 (see below). Hence, the activation energy for the B-CO bond cleavage seems to be nearly identical with the B-CO bond energy. The high value of the frequency factor of 3.2 × 1015 s-1 may be due to the high CO pressure of 2.5 bar in kinetic experiments. J. AM. CHEM. SOC.
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Finze et al.
Table 3. Thermal and Other Properties of Selected Borane Carbonyl Derivativesa compd
Tdecomp. [°C]b
ref
D (B−C) [kJ mol-1]
ref
r(B−C) [Å]
ref
νCO [cm-1]
ref
F3BCO (C6F5)3BCO (CF3)3BCO H3BCO (BF2)3BCO (BCl2)3BCO 1,10-B10H8(CO)2 1,12-B12H10(CO)2
-200 -120 0 10 20 20 200 400
6
7.6 (38) 112 90
34 32
2.89 (1.61) 1.62 1.53 1.52 1.54
35 32
36
7 38 38
1.54
41
2151 2230 2252 2165 2162 2176 2147 2210
c c c
38 38 40 40
c d
c
c c
37 39 38 40 40
a In parentheses: calculated values. b Estimated by behavior described in the Literature. c This work. d Best estimate from five experimental values according to reference.32
Scheme 1. Some Selected Chemical Reactions of (CF3)3BCO
The thermal properties of (CF3)3BCO and selected characteristic features are listed in Table 3 and compared to those of other borane carbonyls. In this comparison, the properties of the so far unknown32 (C6F5)3BCO are of interest. Because any experimental data of this borane carbonyl are unavailable, the compound is synthesized by isolation of (C6F5)3B molecules in solid CO, followed by subsequent slow evaporation of excess CO at 40 K. The solid film of (C6F5)3BCO, formed in this manner, exhibits ν(CO) at 2230 cm-1 and loses CO at about -120 °C. As can be seen in Table 3, the thermal stabilities of borane carbonyls vary widely and range from -200 °C for F3BCO to 400 °C for 1,2-B12H10(CO)2.11 As far as appropriate data are available, the stabilities seem to correlate well with the D(B-CO) bond energies. The (B-CO) bond lengths for the less stable species are quite large, but in more stable compounds, the (B-CO) bond lengths are invariant and do not appear to reflect the thermal stabilities of the respective species very well. For ν(CO) stretching wavenumbers, which will be discussed later, a simple correlation is not immediately obvious. The bond-forming reaction between Lewis acids of the type R3B and Lewis base including CO has recently been analyzed in some detail.32 It is concluded that the energy required to distort the R3B unit from its planar ground state to a pyramidal geometry, formed in the R3B-CO complex, is an important factor. It can hence be argued, that (CF3)3BCO should be more stable than (C6F5)3BCO. To obtain pyramidal (C6F5)3B, required for bonding to CO, more energy is needed than for (CF3)3B, on account of the bulkier C6F5 groups and the possibility, of 15390 J. AM. CHEM. SOC.
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π-interactions of the aryl π-system with the central boron atom in the trigonally coordinated parent compound. However, the synthetic route to (CF3)3BCO by partial hydrolysis of a CF3 group does not require a change in ground state and the difference between (CF3)3BCO and (C6F5)3BCO is reflected in the markedly different thermal stabilities of both compounds. In the closo-boranes the pyramidal R3B fragments are already present and hence very strong B-CO bonds result and carbonyl boranes of unusually high thermal stability.40 Chemical Properties of (CF3)3BCOsAn Overview. Reactions of (CF3)3BCO with nucleophiles can proceed in two different ways; (i) the attacking nucleophile can react with the carbon atom in an addition to the carbonyl group, which remains bound to the boron atom or (ii) the nucleophile can substitute CO on the central boron atom. Whether nucleophilic addition (i) or substitution (ii) occurs, depends on the relative stability of the resulting products and the nucleophilicities of the attacking reagents. Because, as discussed above, dissociation of (CF3)3BCO under reaction conditions is slight, only small amounts of decomposition products are observed and all reactions studied are fairly clean. As can be seen from an overview of the reactions we have studied so far in Scheme 1, addition reactions to the carbon atom of the CO substituent dominate, where the incoming nucleophile forms stable bonds with carbon. Examples are the reactions with methyllithium, methanol, water, KF, or liquid ammonia. The reactions with CH3CN and [B(CN)4]- result in substitution of the carbonyl by the stronger nucleophile. The
(CF3)3BCO−Synthesis
ARTICLES
reaction of (CF3)3BCO with NOCl also proceeds by exchange of the CO-ligand, giving rise to a novel, convenient synthesis of [(CF3)3BCl]-.42 In reactions, where the CO-ligand is replaced, (CF3)3BCO can be viewed as a synthon, used in place of the unknown Lewis acid (CF3)3B. Most of the reactions shown in Scheme 1 are fast because of the high reactivity of the borane carbonyl. Hence, the number of byproducts is usually small and in some cases negligible. On account of the highly reactive character of (CF3)3BCO, the number of solvents, suitable as reaction media, is limited. Especially in SO2 and CH2Cl2, (CF3)3BCO is stable and can be stored over a period of one week without any notable decomposition. Unfortunately, reactions with strong bases cannot be performed in these two solvents. In Et2O, (CF3)3BCO is stable at -78 °C for a few hours and reactions with organometallic reagents usually give high yields. THF reacts with (CF3)3BCO readily at -78 °C in a ring opening reaction, that gives a complex mixture of anionic esters of the type [(CF3)3BC(O)OR]and other not identified decomposition products. Ring opening reactions of THF with strong Lewis acids are well-known in the literature.43 An interesting reaction is observed between (CF3)3BCO and water. Upon dissolving (CF3)3BCO in H2O the dihydroxy carbene complex, (CF3)3BC(OH)2, is obtained, according to eq 9
(CF3)3BCO + H2O f (CF3)3BC(OH)2
(9)
which will undergo ionic dissociation in water (eq 10) -H+
-H+
z [(CF3)3BC(O)OH]- y\ z (CF3)3BC(OH)2 y\ + + +H
+H
[(CF3)3BCO2]2- (10) A potentiometric titration of (CF3)3BCO in water reveals pKa values to be
