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Vibrational Spectra and Conformations of Tris(perfluoroviny1)borane J. D. Odom," E. J. Stampf,' J. R. Durig," V. F. Kalasinsky,* Department of Chemistty, University of South Carolina, Columbia, South Carolina 29208
and S. Rlethmlller Chemistry Deparhnent, Virginia Military Institute, Lexington, Virginia 24450 (Received September 72, 1977) Publication costs assisted by the University of South Carolina
The infrared spectra (3000-200 cm-l) of gaseous and solid tris(perfluoroviny1)borane and the Raman spectra (300-100 cm-') for all three physical states have been recorded. Spectral changes that occur upon solidification indicate the presence of two conformers in the fluid phases. Since there are no apparent exclusions between the infrared and Raman spectra,a planar C3h conformation can be ruled out as a possibility. A detailed vibrational assignment is proposed for the more stable form on the basis of C3molecular symmetry. A variable temperature study failed to give an energy difference between the two conformers apparently because of the tendency of the sample to form a glass.
Introduction Vinylboranes form an interesting group of compounds because of the possibility of overlap between the olefinic T orbitals and the empty p-orbital of boron. For example, 13C NMR studies3 of trivinylborane and vinylhaloboranes suggest a degree of delocalization through the B-C bond. The planar s t r u ~ t u r of e ~vinyldifluoroborane ~~ is consistent with this interpretation. On the basis of electron diffraction measurements, trivinylborane has been reported to be planar! and vibrational data have been reported to support this ~ o n t e n t i o n . ~However, more detailed vibrational studies indicate the presence of multiple conformers in the fluid states and a single planar structure in the crystalline Fluorine substituted vinyl analogues have also been studied with regard to the question of delocalization. The preparation and characterization of a number of perfluorovinylboranes have been reported p r e v i o ~ s l y . ~ - ' ~ Vibrational spectra have been measured for perfluorovinyldifluoroborane and perfluorovinyldichloroborane~10~13 and the data are consistent with planar structures for these two compounds.13 Nuclear magnetic resonance spectra of some perfluorovinylboranes have also been reported,ll and the complexity of the spectrum for tris(perfluoroviny1)borane was attributed to long-range spin-spin coupling between fluorine atoms in separate perfluorovinyl groups. Since only limited infrared dataQhave been presented for tris(perfluoroviny1)borane and no structural data have appeared, we have undertaken a detailed infrared and Raman study of this compound with particular emphasis upon the conformational aspects of the problem. Experimental Section The synthesis and purification of B(C2F3)3was carried out in a conventional high-vacuum system employing greaseless stopcocks. All ground glass apparatus which came into contact with halogenated compounds was lubricated with Halocarbon vacuum grease. Bis(perfluoroviny1)dimethyltin was prepared according to a published procedure14 from (CH3)2SnC12(Alfa), Mg powder (Alfa), and C2F3Br (PCR). Perfluorovinyldichloroborane was prepared by the method of Stafford and StoneQ and its purity was checked by vapor pressure measurement (observed, 96 Torr at 0 "C; lit.,9 97.6 Torr at 0 "C) and IlB NMR spectroscopy" (31.3 ppm deshielded from BF3.0(C2H5)2 ) . 0022-3654/78/2082-0308$0 1.OO/O
Tris(perfluoroviny1)borane was prepared by reaction of C12BC2F3and (CH3)2Sn(C2F3)2 using a slight excess of the borane. In a typical reaction 6.70 mmol of C2F3BC12and 6.23 mmol of (CH3)2Sn(CzF3)2 were condensed into a glass reaction vessel fitted with a greaseless stopcock. The vessel was removed from the vacuum system and agitated on a mechanical shaker in a -112 "C slush bath (CS,). Over a period of several hours the bath warmed to -70 " C and reaction appeared complete. The vessel was warmed to room temperature and all volatile materials were pumped onto the vacuum line and separated on a low-temperature vacuum fractionation c01umn.l~ Purity was checked by vapor pressure measurementQand llB NMR spectroscoPY.12 Raman spectra were recorded on a Cary Model 82 spectrophotometer equipped with either a Coherent Radiation Laboratories Model 53A or a Spectra Physics Model 171 argon ion laser. The 5145-A line was used for excitation, and the power at the sample was varied between 0.5 and 2 W depending upon the physical state being examined. The spectra of the vapor were recorded using standard Cary multipass optics and gas cells equipped with greaseless stopcocks. The Raman spectra of crystalline and low-temperature liquid samples were obtained in either a cell of the Miller-Harney design16 or in a Cryogenic Technology Inc. Spectrim cryostat with a Lake Shore Cryotronics Model DTL 500 temperature controller. Raman spectra are shown in Figures 1 and 2. Infrared spectra were recorded from 3000 to 200 cm-l using a Perkin-Elmer Model 621 spectrophotometer. A Beckman IR-11 was used to verify the data between 400 and 200 cm-l. Atmospheric water was removed from the spectrometer housings by flushing with dry nitrogen. The instruments were calibrated using standard gases17 and solutions.18 In the mid-infrared region cesium iodide plates were used as window material, whereas polyethylene was employed for the far-infrared measurements. Spectra of the solid were obtained in a liquid nitrogen-cooled coldfinger cell having a CsI support. Infrared spectra are shown in Figures 3 and 4.
Results and Vibrational Assignments The vibrational modes of tris(perfluoroviny1)borane can be represented as follows for the various possible symmetries: C3h: 9A'(R) + 5A"(IR) + 10E'(R, IR) + 4E"(R) 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 309
Vibrational Spectra and Conformations of Tris(perfluoroviny1)borane
v l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
i
l
1500 1000 500 WAVENUMBER (CM-') Figure 3. Infrared spectra of tris(perfluoroviny1)borane: (A) vapor, (B)
solid.
I w'i''b-+-J-l , I , 1500 1000 500 WAVE NUMBER (CM'l) 1
,
,A-+--Yw IJ
Flgure 1. Raman spectra of tris(perfluoroviny1)borane: (A) vapor, (B)
liquid, and (C) annealed solid.
1200
1000
800
WAVENUMBER (cm-1)
Flgure 4. Infrared spectra of solid tris(perfluoroviny1)boraneobtained
at various stages of annealing.
+
WAVENUMBER (CM") Figure 2. Raman spectra of liquid tris(perfluoroviny1)borane at temperatures above the melting point.
C3v: 9AI(R, IR)
Cs: 14A(R, IR)
+ 4A,(-) + 14E(R, IR) + 14E(R, IR)
C,: 24A'(R, IR) 18A"(R, IR) To distinguish among these symmetries we must consider the vibrational spectra in some detail. The selection rules for Csh symmetry allow some exclusions between the infrared and Raman spectra. In particular, there should be 13 Raman lines which have no corresponding infrared bands and likewise there are five vibrations which should be active only in the infrared. For the other symmetries, all vibrations which are allowed in the infrared spectra are also allowed in the Raman spectra, and the differences will be in the number of polarized Raman lines. When the infrared and Raman spectra of the fluid phases of tris(perfluoroviny1)borane are compared it is
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TABLE I: Observed Infrared and Raman Frequencies (cm-l ) of Tris(perfluorovinyl)boranea Infrared Solid 1750 w 1731 m 1721 m, sh 1706 s, sh 1675 vs, vb 1600 vw 1383 m, sh 1370 s, sh 1361 s 1346 m, sh 1338 m, sh 1315 s, b 1290 s
Gas 1745 m 1737 m
Solid
Raman Liauid
Gas
Assignment and approximate description
1729 m 1722 s
1711 vs, p
1708 s, p 1705 s, p
C=C sym stretch (isomer 11) v 1 C=C sym stretch (A)
1670 vs, vb
1 6 8 1 vs
1670 dp, s
1678 s, dp
v i s C=C antisym stretch
1363 s
1363 vw
1365 vw, dp
1339 vw
1333 vw, p
1341 w w
Overtone or combination (2 X 667?)
1293 vw
1305 vw, dp
1310 vw, b
vl,
1211 w, p 1200 w, p
CF stretch (isomer 11) u , CF, antisym stretch (A) vI1 BC, antisym stretch (E)
1312 s 1299 s 1281 m, sh
v 1 6 CF, antisym stretch
(E)
(E)
CF stretch (E)
1242 vw, p 1198 s 1125 s 1095 m 1079 m 1062 m 891 m 874 s 850 s 831 s 690 w 682 w 674 m, sh 671 m 666 m 632 m 618 w 612 w 585 w 524 m 508 m 406 m 348 m 322 w 314 w 275 vw 260 w 230 w 213 s
1208 s 1197 m, sh 1135 vs 1114 m, sh 1110 m
1194 w 1189 w 1127 vw
'Ig3 w, d p 1129 vw, dp
1095 vw
1104 vw, dp
1081 m 1068 m 1045 m 896 w 891 w 880 m 876 m 855 s 834 s 811 m 700 w 695 w 685 m 680 m
1072 s
1078 m, p 1063 m, p
890 vw
890 vw, p
CF, stretch, loB
874 w
877 w, p 854 w, p 830 vw, dp
v 4 CF, stretch (A) CF, stretch (isomer 11) v I 9 CF, stretch (E)
830 vw
683 vw 670 w 666 w
647 w 631 w
521 w 510 vw 405 m
1081 m, p 1067 m, p
Fermi resonance with 1129 (507 t 585?) v g CF stretch (A) CF, sym stretch (isomer 11)
634 m 629 m 616 m 611 w 579 vw 525 w 506 w 402 vw 397 vw 347 w 320 w 313 w 303 w 273 w 229 w 196 w 183 w 161 m
667 w, p
667 w
v6
625 s, dp
633 m, dp
uz0 CF,
614 m, dp
BC, sym stretch (A) rock (E)
CF, wag (E) rock (A) v 6 CF, wag (A) u , CF, scissors (A) v Z 2 CF, scissors ( E ) v21
v g CF,
584 w, p 525 w, p 507 w, dp 395 w, dp 342 w, sh, p
400 vw 340 vw
v Z 3 CF bend (E) v g CF bend (A)
314 m, p
310 m, p
v , CF ~ u Z 4 CF
272 w, sh 253 w, p 228 w, p 210 vw, dp 193 vw, p 1 5 5 m, p
255 vw 225 vw
150 m, p
bend (A) bend (E) uZ1CCB bend ( E ) (isomer 11) v l , CCB bend (A) (isomer 11) u l , CBC bend (A) v Z 6 CBC bend ( E ) v i , C=C twist (A)
Abbreviations used: sym, symmetric; antisym, antisymmetric; s, strong; m, medium, w, weak; v, very; b, broad; sh, shoulder; p, polarized; dp, depolarized. a
clear that there are no exclusions, and thus, a conformer possessing planar C 3 h symmetry can be ruled out. It is interesting to note that the Raman spectra of the annealed solid are different from the spectra of the vapor and liquid (Figure 1)in that the lines at 1211,1063,854,253, and 210 cm-l have disappeared. Since other features in the spectra of the fluid and solid phases are virtually unchanged we conclude that these lines which disappear are attributable to a second, higher energy conformer of tris(perfluor0viny1)borane. In the infrared spectrum of the low-temperature solid the bands associated with the second conformer persist; however, in Figure 4 it can be seen that
the bands at 1062 and 850 cm-l decrease in intensity relative to neighboring bands as the solid is allowed to anneal. The time involved in the annealing process is considerable and after a few hours a nonvolatile product is formed on the CsI support plate. The solid sample whose Raman spectra are shown in Figure 1was annealed in a sealed glass capillary and no degradation of the sample was observed. We feel that the infrared spectra shown in Figure 4 confirm the evidence for a second conformer which is apparent in the Raman spectra. In Table I we have indicated the bands which would disappear in the annealed solid and are attributable to the
Vibrational Spectra and Conformations Of Tris(perfluoroviny1)borane
less stable conformer. The remaining bands arise from vibrations of the more stable conformer of tris(perfluorovinyl)borane, and there appear to be a t least 13 polarized Raman lines in the spectrum of the liquid. These data are most consistent with a structure having C3 symmetry, probably one in which the perfluorovinyl groups are noncoplanar as a result of a twist about each B-C bond. The conformation of the higher energy form cannot be definitely determined, since only a few of its vibrational frequencies have been observed. The assignments of the normal frequencies of vibration of the more stable form of tris(perfluoroviny1)borane are shown in detail in Table I. Most of these assignments are straightforward once the symmetry has been determined; in addition, depolarization ratios were particularly helpful. The C-F stretching modes appear to be near the ranges for the group frequencies as discussed by Stafford and Stonelo for a number of perfluorovinyl-metal compounds. The next highest frequencies are in the 830-900-~m-~ region, and it is difficult to come to a conclusion about their assignments. The intensities of these bands in the infrared spectra indicate that they are fundamentals and we have thus assigned them as CF2 stretching modes as in the fluoroethylene~.~~ The alternative of assigning these bands as CF2 bending motions would require that these normal modes are -200 cm-l higher than the corresponding motions in perfluorovinyldihaloboranes” which seems highly unlikely. The CF2 bending modes seem to be in the “n0rrna1’~range around 600 cm-l. The order of these frequencies are the same as those given for the corresponding motions for the perfluorovinyldihaloboranes, but these descriptions are only approximate since strong mixing is expected. The BC3 motions have frequencies similar to those found in trimethylboraneZ0 and trivinylborane.8 The only vibrational frequencies we have not observed are those for ~ 1 4 ~, 2 7 and , V28, the B-C torsion (A), the C=C twist (E), and the B-C torsion (E), respectively.
Discussion Neither of the two conformers of tris(perfluoroviny1)borane is planar as might be expected based on the structure of trivinylborane.6v8 Even though the fluorine atoms on adjacent perfluorovinyl groups are separated by more than twice their van der Waals’ radii, the twist about the B-C bonds must relieve unfavorable steric and/or electronic interactions. The “twisted” C3 conformation is a geometry that severly limits the amount of pr-p?r overlap that is possible in this molecule. We must conclude, then, that there is little, if any, multiple bond character in the B-C linkage. The actual value of the twist angle must be determined before the possible multiple bond character can be discussed more quantitatively. We have obtained evidence for a second conformer but have been unable to characterize it experimentally. The intensity of the “second conformer” bands would indicate a small energy difference between the geometrical isomers. Additionally a small energy difference would enhance the tendency of the molecule to form a glass and make the sample difficult to anneal. When the sample contained in a Pyrex capillary was cooled quickly to 20 K, we observed spectra that were essentially identical with the spectra of the room-temperature liquid. However, there is little doubt that a conformer with C3 symmetry exists in all phases and that it is the stable one in the well-annealed crystalline solid. Another interesting fact is that the conformer bands that are disappearing with
The Journal of Physical Chemistry, Vol. 8.2,No. 3, 7978 3911
annealing of the crystalline solid appear to be stronger in the fluid phases than their counterparts for the C3 conformer, e.g., the lb81/1068 and 865/834 cm-l pairs in the infrared spectrum of the gas and the 854/830 cm-l pair in the Raman spectrum of the liquid. These intensities may indicate that the C3 conformer is not the thermodynamically preferred conformer in the fluid phases. The intensities depend upon the dipole changes and polarizability changes that occur during the vibration and since the magnitudes of these terms are not known the absolute intensities are of limited consequence. However, the differences in the relative intensities as a function of temperature would answer this question, but unfortunately our variable temperature data me complicated by the glass formation. Tho complex I9F NMR spectra for tris(perfluoroviny1)boranell c m shed no light on the conformation, since the appearance of the spectra would be very similar for C3, C3h,and C3”symmetries. On the NMR time scale an average conformation is probably observed, and this means that the barrier to interconversion between conformers is fairly low (less than 4 kcal/mol). Since the planar conformation is not favored, we might expect that the conversion from the higher energy conformer to the lower energy one does not proceed through a planar intermediate. For a low barrier and small energy separation we might expect further that the second conformer is one in which the twist angle about each B-C bond is 90” (relative to the planar form). This conformation would have C3” symmetry. There is no direct evidence to support this, but on the basis of the vibrational and NMR data and the physical nature of the motions involved, we prefer C3”symmetry over symmetry for the second conformer.
Acknowledgment. The authors gratefully acknowledge the support from the National Science Foundation through Grants No. CHE 74-84805-AO2 and No. MPS 74-12241-A0.
References and Notes Present address: Department of Chemistry, Lander College, Greenwood, S.C. Present address: Department of Chemlstry, Mississippi State University, Mississippi State, Miss. 39762. b. W. Hall, J. D. mom, and P. B. Ellis, J. Am. Chem. Soc., 97, 4527 (1975). J. R. Durig, R. 0.Carter, and J. D. Qdom, Imrg. Cbem., 13, 701 (1974). J. R. Durig, L. W. Hall, R. 0. Carter, C. J. Wurrey, V. F. Kalasinsky, and J. D. Odom, J . Pbys. Cbem., 80, 1188 (1976). A. Ford, B. Beagley, W. Reade, and I. A. Steer, J. Moi. Struct., 24, 131 (1975). A. K. Holliday, W. Reade, K. R. Seddon, and I, A. Steer, J . Organomefal. Cbem., 67, 1 (1971). J. D. Odam, L. W. Hall, S. Riethmiller, and J. R. Durig, Inorg. Chem., 13, 170 (1974). S. L. Stafford and F. G. A. Stone, J. Am. Cbem. SQC.,82, 6238 (1960). S. L. Stafford and F. G. A. Stone, Spectrochlm. Acta, 17, 412 (1961). T. D. Coyle, S. L. Stafford, and F. G. A. Stone, Spectrochirn. Acta, 17, 968 (1961). E. J. Stampf and J. D. Mom, J. Orgaiiomefal. Chem., 108, 1 (1976). J. R. Durig, E. J. Stampf, J. D. Odom, and V. F. Kalasinsky, hnorg. Chem., 16, 2895 (1977). H. D. Kaesz, S. L. Stafford, and F. G. A. Stone, J . Am. Chern. Soc., 82, 6232 (1960). J. Dobson and R. Schaeffer, Inorg. Chem., 9, 2183 (1970). F. A. Miller and B. M. Harney, Appl. Specirosc., 24, 291 (1970). IUPAC, “Tables of Wavenumbers for the Calibration of Infrared Spectrometers”, Butterworths. Washington, D.C., 1961. R. N. Jones and A. Nadeau, Spectrochim. Acta, 20, 1175 (1964). D, E. Mann, N. Acquista, and E. K. Plyler, J. Cbem. Phys,, 22, 1586 (1954); R. Therrner and J. R. Nielson, /bid., 30, 98 (1959). L. A. Woodward, J. R. Hail, R. N. Dixon, and N. Sheppasd, Spectrochim. Acta, 15, 249 (1959); R. J. O’Brien and Q.A. Ozin, J. C h m . Soc. A , 1136 (1971).
