chemistry of chromotropic acid. 1 h and 13 c...
TRANSCRIPT
Chemistry of chromotropic acid. 'H and 13c NMR spectroscopy of chromotropic acid and its derivatives
PARIS E. GEORGHIOU,~ CHI KEUNG (JIMMY) Ho, AND CHESTER R. JABLONSKI Department of Chemistry, Memorial University of Newfoundland, St. John's, Nfld., Canada A l B 3x7
Received November 9, 1990
This paper is dedicated to Dr. Hugh J . Anderson on the occasion of his 65th birthday
PARIS E. GEORGHIOU, CHI KEUNG (JIMMY) Ho, and CHESTER R. JABLONSKI. Can. J. Chem. 69, 1207 (1991). The 'H and I3C NMR spectra of chromotropic acid (CTA) (4,5-dihydroxy-2,7-naphthalenedisulphonic acid) have been
unambiguously assigned. Proton NOED spectra were used to show the proximity of both H-3 and H-6 and the hydroxyl groups. Two-dimensional 'H-'~c NMR correlation spectra of CTA, of its corresponding diacetoxy derivative, and of 3-bromo- and 3,6-dibromo-CTA support the assignments. A regioselective deuterium exchange reaction of the C-3 and C-6 protons of CTA with deuterium oxide was observed during the NMR experiments. This latter finding is strongly indicative of the mode of formation, and of the nature of the chromogen formed in the reaction of CTA with formaldehyde in the well-known CTA-formaldehyde analytical reaction.
Key words: chromotropic acid, 3-bromochromotropic acid, 3,6-dibromochromotropic acid.
PARIS E. GEORGHIOU, CHI KEUNG (JIMMY) HO et CHESTER R. JABLONSKI. Can. J . Chem. 69, 1207 (1991) On a fait un attribution non-ambigue des spectres RMN du 'H et du 13c de l'acide chromotropique (ACT) (acide 4,5-
dihydroxy-naphtalbne-2,7-disulfonique). On a utilist les spectres d'EOND du proton pour dtmontrer que les H-3 et H-6 ainsi que les groupements hydroxyles sont a proximitt les uns des autres. Les spectres RMN 'H-I3C de corrtlation bidimensionnelle de I'ACT, de son dtrivt diacttoxylt et des acides 3-bromo- et 3,6-dibromo-chromotropiques sont en accord avec les attributions. Au cours des exptriences de RMN en prtsence d'oxyde de deuttrium, on a observt une reaction d'tchange rtgiostlective du deuttrium avec les protons en C-3 et en C-6 de I'ACT. Cette observation est un indice prtcieux du mode de formation et de la nature du chromogtne form6 lors de la rtaction de I'ACT avec le formaldthyde au cours de la reaction analytique bien connue de 1'ACT-formaldthyde.
Mots clPs : acide chromotropique, acide 3-bromochromotropique, acide 3,6-dibromochromotropique. [Traduit par la rtdaction]
Introduction Chromotropic acid (CTA) 1 (4,5-dihydroxy-2,7-naphthal-
enedisulphonic acid) is employed as an analytical reagent and as an intermediate in the syntheses of numerous azo dyes, which are used extensively both in the dye industry and in analytical chemistry (1-3). In North America it has found widespread use as the chromogen-forming reagent in the analysis of formaldehyde using the method recommended by the National Institute for Occupational Safety and Health (NIOSH), Method P&CAM 125 (4, 5). The exact nature of the chromogen and its formation by the reaction of CTA with formaldehyde has, however, not been unambiguously proven. The most often quoted structure (4-8) for the chromogen formed between CTA and formaldehyde has been the para,para-quinoidal adduct 2, but no direct proof of this structure has ever been presented. The structure is based on an analogy to the reaction of formaldehyde with aromatic compounds in the LeRosen test (8).
Kame1 and Wizinger (9) proposed the mono-cationic dibenz- oxanthylium structure 3 as an alternative. This structure is the one that is most consistent with the chemistry of the reaction (10). As part of our ongoing studies of methods for the analysis of formaldehyde we were interested in understanding the chemistry of the formaldehyde-CTA reaction through the use of NMR spectroscopy. Surprisingly, considering its widespread use, there does not appear to be an unambiguous assignment of either its 'H or 13C NMR spectrum in the literature. Lajunen et al. (11) reported a 13C NMR study of CTA, but the 13C NMR spectrum was not definitively assigned. More recently, Free- man et al. (12) described the 250-MHz 'H NMR spectrum of CTA and other naphthalene-based dyestuff intermediates
' ~ u t h o r to whom correspondence may be addressed.
with assignments based on comparisons with mono-substituted naphthalenes. This paper describes our findings using through- bond and through-space NMR correlation techniques, which allowed the unambiguous assignment of the 'H and 13C NMR spectra of CTA and its mono and dibromo derivatives. These assignments enabled us to propose an hypothesis for the chemistry of the CTA-formaldehyde chromogen-forming reaction (10).
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1208 CAN. J. CHEM. VOL. 69, 1991
Experimental 'H NMR spectra were recorded with the following instruments at the
designated frequencies: Varian Anaspect EM 360 (60 MHz), Bruker WP-80 (80 MHz), or GE GN-300NB (300 MHz) spectrometers. I3C NMR spectra were recorded with the Bruker WP-80 (20.1 MHz) and (or) the GE GN-300NB (75.47 MHz) instrument. The solvent used was either deuterium oxide or water as noted in the experimental details. Proton nuclear Overhauser effect difference (NOED) spectra were determined under steady state conditions (13) on a GE 300-NB instrument using a set of 16K interleaved experiments of 8 transients cycled 12 to 16 times through the list of decoupling frequencies. In each experiment the decoupler was on in continuous wave (CW) mode for 6 s with sufficient attenuation to give an approximate 70-90% reduction in intensity of the irradiated peak. A 24 s delay preceded each frequency change. A set of four "dummy" scans was &nployed to equilibrate the spins prior to data acquisition. No relaxation delay was applied between successive scans of a given frequency. Difference spectra were obtained on zero-filled 32K data tables that had been digitally filtered with a 1-2 Hz exponential line-broadening function.
Ultraviolet and visible spectra were determined on a Unicam SP.800 ultraviolet spectrophotometer. Infrared spectra were recorded on a Perkin-Elmer 1320 infrared spectrophotometer using KBr disks. Sephadex LH-20 purchased from Pharmacia Fine Chemicals was used in the gel permeation chromatography.
Purijication of 4,5-dihydroxy-2,7-naphthalenedisulphonic acid di- sodium salt ( I )
The material purchased from Sigma Chemical Co. was specified as being only 98% pure. Purified 1 was obtained by dissolving 20 g of the Sigma product in 100 mL of water in a 125-mL Erlenmeyer flask on a hot plate. A solution of 20 g of sodium chloride in 50 mL of hot water was added to the chromotropic acid solution until a precipitate formed. The flask was stoppered and the solution was allowed to first crystallize at room temperature and then at low temperature in a refrigerator. The crystals were filtered with suction and dried in air. The product was almost colourless: UV (H20) A,, absorbances: 316, 332, 346 nm. IR (KBr) v,,: 3500 (-S020-H and ArO-H), 1200 (S=O) cm-'. 'H NMR (300 MHz; D20, TSP internal standard) 6 ppm: 7.20 (d, J = 1.4 Hz, lH, Ar-H), 7.92 (d, J = 1.4 Hz, lH, Ar-H). I3C NMR and APT (D20) 6 ppm: 106.44 (C-3 and C-6), 116.18 (C-lo), 118.12 (C-1 and C-8), 134.64 (C-9), 141.36 (C-2 and C-7), 153.76 (C-4 and C-5). 2-D 'H-'~c correlation (D20): 7.20 ppm correlates with 106.44 (C-3 and C-6), 7.92 ppm correlates with 118.12 (C-l and C-8).
Deuterium exchange experiments with chromotropic acid In a typical experiment, an NMR tube was charged with a solution
containing 67 mg of purified 1 in 0.30 mL of deuterium oxide. A small amount of DSS as internal standard was added to the solution. The NMR tube was heated in a boiling water bath. 'H NMR spectra were recorded at 10 min intervals until the signal at 7.20 ppm completely disappeared and the signal at 7.92 ppm collapsed to a sharp singlet. The deuterium exchange could be reversed by adding 0.1 mL of water to the NMR tube. The I3C NMR spectra were recorded for a freshly prepared deuterium oxide solution of purified 1, and following heating of the NMR tube in a boiling water bath for 15 min. Side-by-side comparison of the spectra indicate that the intensity of the 106.44ppm signal (C-3 and C-6) was reduced by over 90%. All other signals were unchanged.
4,5-Diacetoxy-2,7-naphthalenedisulphonic acid disodium salt (4) In a 25-mL Erlenmeyer flask, 1.0 g (2.68 mmol) of chromotropic
acid disodium salt was dissolved in 3.0 mL of 3.0 M aqueous sodium hydroxide. To this solution was added approximately 2 g of crushed ice, followed by 0.6 mL of acetic anhydride. The mixture was stirred for 5-10 min with ice-bath cooling. The diacetate separated out as colourless, crystalline needles. It was filtered by suction, washed with portions of absolute ethanol, and dried in air. Recrystallization from water afforded 0.52 g (40%) of crystals having a decomposition point at 320°C. X-ray crystallographic analysis revealed that the compound was a dihydrate: UV (H20) A,,, absorbances: 315, 329, 339 nm.
'H NMR (DzO) 6 ppm: 2.43 (s, 3H, CH3-), 7.72 (d, J = 2 Hz, lH, Ar-H), 8.52 (d, J = 2 Hz, lH, Ar-H).
3,6-Dibromo-4,5-dihydroxy-2,7-naphthalenedisulphonic acid diso- dium salt (5a)
In a 25-mL Erlenmeyer flask, 1.9 g (5.1 mmol) of chromotropic acid disodium salt was mixed with 12 mL of concentrated sulphuric acid (96%, d = 1.84 g/mL). The mixture was heated and stirred at 80°C until all the solid dissolved. The solution was cooled to approximately 15°C and, with continuous cooling and stirring, a solution of 0.8 g (5.0 mmol) of bromine in 3.0 mL of glacial acetic acid was added dropwise over 1 h. After the addition of bromine, the solution was stirred and cooled for an additional 3 h. To this solution, approximately 15 g of ice was added. The white precipitate was filtered off with suction, using a sintered glass funnel. The residue was dissolved in a minimal amount of water at room temperature. To this aqueous solution one half of its volume of concentrated hydrochloric acid solution was added. The grey-white leaflets were suction filtered using a sintered glass funnel, and were press-dried. The residue was further dried under vacuum to afford 1.25 g (46%) of 5a, which decomposed above 40°C with liberation of bromine: UV (H20) A,,, absorbances: 340,358, 375 nm. 'H NMR (D20) 6 ppm: 8.10 (s, Ar-H). I3C NMR (D20) 6 ppm: 102.44 (C-3 and C-6), 116.26 (C-lo), 122.04 (C-1 and C-8), 130.74 (C-9), 140.11 (C-2 and C-7), 149.62 (C-4 and C-5). 2-D ' H - I ~ C correlation (D20): 8.10ppm correlates with 122.04 (C-1 and C-8); APT (D20): 122.04 ppm (C-1 and C-8, one attached proton).
3-Bromo-4,5-dihydroxy -2,7-naphthalenedisulphonic acid disodium salt (6)
In a 25-mL Erlenmeyer flask, 1.9 g (5.1 rnrnol) of chromotropic acid disodium salt was mixed with 12 mL of concentrated sulphuric acid (96%, d = 1.84 g/mL). The mixture was heated and stirred at 80°C until all the solid dissolved. The solution was cooled to approximately 15°C and, with continuous cooling and stirring, a solution of 0.4 g (2.5 mmol) of bromine in 3.0 mL of glacial acetic acid was added dropwise over 3 h. After the addition of bromine, the solution was stirred and cooled for an additional 1 h. To the solution approximately 15 g of ice was added. The white precipitate was suction-filtered using a sintered glass funnel. The residue was dissolved in a minimal amount of water at room temperature. To this aqueous solution one half of its volume of concentrated hydrochloric acid solution was added. The grey-white leaflets were once again filtered off using suction and a sintered glass funnel, and were press-dried. The residue was further dried under vacuum to afford 0.98 g (40%) of 6, which decomposed at 40°C with liberation of bromine: UV (H20) A,,, absorbances: 316, 336,348 nrn. 'H NMR (D20) 6 ppm: 7.17 (d, J = 2 Hz, lH, Ar-H), 7.80 (d, J = 2 Hz, IH, Ar-H), 8.10 (s, lH, Ar-H). I3C NMR (D20) 6 ppm: 100.51 (C-3), 107.06 (C-6), 115.96 (C-lo), 118.62 (C-8), 121.33 (C-1), 132.63 (C-9), 140.17 (C-7), 141.28 (C-2), 151.35 (C-4), 152.23 (C-5).
Results and discussion The 300-MHz 'H NMR spectrum of CTA shows two
AX-type doublets (7.92, 7.20 ppm, J = 1.4 Hz), which is consistent for structure 1 assuming negligible interannular coupling (14). An unambiguous assignment of this spectrum, however, cannot be made on the basis of neighbouring group effects alone. Of the two types of protons on the CTA molecule, one type (on C-3, or C-6) is ortho to the sulphonic acid group, and the other type (on C- 1, or C-8) is para to the hydroxyl but ortho to the sulphonic acid group. Since these two types of protons are both ortho to the sulphonic acid group, the effect of the electron-withdrawing sulphonic acid group should be the same on both of them. The hydroxyl group on the other hand is ortho to the proton on C-3 (or C-6) and para to the proton on C- 1 (or C-8). It has been shown that the ortho and para position protons in phenol are equally affected by the hydroxyl group and have similar chemical shifts (15). In naphthalene, however,
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GEORGHIOU ET AL.
due to ring current effects (15), the a protons, which are closer to both rings, appear downfield (7.81 ppm) with respect to the p protons (7.46 pprn). By analogy with naphthalene therefore, the signals at 7.92 pprn could be tentatively assigned to the proton on C-1 (and C-8) and the signals at 7.20 pprn can be assigned to the proton on C-3 (and C-6). Nevertheless, the assignment of chemical shifts using only the arguments made above or those made by Freeman et al. (12) is clearly not sufficient for a multi-substituted naphthalene derivative such as CTA.
During the 'H NMR experiments on CTA in deuterium oxide it was observed that the signals at 7.20 pprn gradually diminished in intensity. The signals ultimately disappeared completely and the doublet at 7.92 pprn was converted into a sharp singlet. This specific exchange could be reproduced within 0.5 h at 90°C. That a simple hydrogen-deuterium exchange occurred was established by regenerating the original spectrum by the addition of water to the solution contained in the NMR tube. These findings could be accounted for by the pathway shown in Scheme 1.
Even though Scheme 1 shows exchange of the C-3 and C-6 protons, the alternative exchange of the C-1 and C-8 protons could, in principle, be equally likely. This ambiguity is of importance in understanding the reactions of CTA, thus it was necessary to definitively assign the 'H NMR spectrum of CTA.
The corresponding diacetate of CTA, 4, was synthesized to provide additional evidence for the assignment of the 'H NMR spectrum of CTA. The proximity of the methyl protons of the acetoxy group to the proton on C-3 (or C-6) suggested that a nuclear Overhauser effect (NOE) (16) in the 'H NMR spectrum could be observed. The 300-MHz 'H NMR spectrum of 4 consists of a single signal at 2.19 pprn due to the acetate methyl groups and a pair of sharp AX-type doublets at 8.27 and 7.48 pprn ( J - 1.6 Hz). Assignment of the low-field signals to the C-1, C-8 protons and the C-3, C-6 protons respectively was established by NOED experiments conducted at 300 MHz. Irradiation of the signal at 7.48 pprn enhanced the acetate signals whereas irradiation of the 8.27 pprn signal did not. The assignment of the 7.48 pprn signal to the proton on C-1 (C-8) was confirmed by irradiating the acetate signal, which resulted in a clearly enhanced signal at 7.48 pprn relative to that at 8.27 ppm. Thus, the higher-field signal is due to the protons ortho to the acetoxy group. Two-dimensional 'H-13C NMR chemical shift correlation (16) spectra provided additional evidence for these assignments. These results suggest that the
higher-field signal that undergoes deuterium exchange in CTA is due to the protons ortho to the hydroxyl group, C-3 and C-6.
Proton NOED spectra of CTA itself confirmed the above assignments by unambiguously showing that the higher-field resonance is proximal to the hydroxyl group. The experiments were conducted in DMSO-d6 solution to slow exchange at the hydroxyl site. To confirm that the assignment of the peaks in DMSO-d6 is the same as those in D20, a dilution study was conducted in DMSO-d6/D20. No change in the relative positions of the two low-field signals was observed. Figure 1 shows a small positive enhancement of the hydroxyl resonance on partial saturation of the higher-field peak. The correlation was confirmed by observation of a selective enhancement of the high-field resonance when the hydroxyl peak was saturated.
Lajunen et al. (11) assigned the 25-MHz 13C NMR spectrum of CTA in D,O, which consisted of six signals (see Table l ) , to their respective carbon atoms by analyzing the response of the 13C chemical shifts to varying pH. The largest effect on the chemical shift was observed for the C-4, C-5, and C-10 signals when the pH was changed from 4.5 to 8.0. At basic pH one of the hydroxyl groups is dissociated into the corresponding oxyanion. Dissociation of the second hydroxyl group, however, is not possible in aqueous solution since it is very strongly hydrogen-bonded to the neighbouring oxyanion via an intra- molecular six-membered ring. Nevertheless, none of their assignments can be considered to be unambiguous.
We wished to assign the 13c NMR spectrum of CTA using more direct evidence based on through-bond scalar C-H coupling interactions. The 75-MHz broad band decoupled spectrum of CTA in DMSO-d6 consists of six signals with approximately the same chemical shifts (Table 1) as noted by Lajunen in D20. Our experiments confirmed their original assignments. A 2-D HETCOR spectrum optimized for one- bond coupling revealed that the lower-field 'H signal at 7.92 pprn assigned to the proton on C- 1 (and C-8) was correlated with the signal at 117.50 ppm. The 'H signal at 7.20 pprn assigned to the proton on C-3 (and C-6) was correlated with the 13c signal at 108.22 ppm.
The proton-coupled 13C NMR spectrum of CTA revealed that the signal due to C-3 (and C-6) was split into a doublet of doublets ( ' J = 162.4Hz; 3J = 6.3Hz), the signal due to C-10 was split into an approximate quintet ( 3 ~ = 5.9 Hz), and that due to C-1 (and C-8) was split into a doublet of triplets ( 'J = 165.1 Hz; 3~ = 5.0 HZ). The C-9 signal appeared as an apparent triplet ('J = 2.5 Hz), the signal due to C-2 (and C-7) was an
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CAN. J. CHEM. VOL. 69, 1991
I " " I " " I " " I " " I " " 1 " " ~ " " ~ " " I " " ~ 10 9 8 7 6 5 4 3 2 1 PPM
FIG. I. Proton NOED spectra of chromotropic acid in DMSO-d6: (a) reference spectrum; (b) saturation ( x 256) of 6.02 pprn signal showing positive enhancement of 9.98 pprn signal; (c) saturation ( x 256) of 6.60ppm signal indicating no enhancement of 9.98 pprn signal.
TABLE 1. 13C NMR chemical shifts for chromotropic acid (1) and derivatives (Sa, 6)
Compound (solvent) C- 1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 Reference
1 (D20) 118.12 141.36 106.44 153.76 153.76 106.44 141.36 118.12 134.64 116.18 Thiswork 1 (DMSO-d6) 117.50 147.22 108.22 154.98 154.98 108.22 147.22 117.50 135.97 115.65 This work 1 (I3201 119.4 142.7 107.8 154.8 154.8 107.8 142.7 119.4 135.9 117.8 11 5 a (D20) 122.04 140.11 102.44 149.62 149.62 102.44 140.1 1 122.04 130.74 116.26 This work 6 (D20) 121.33 141.28 100.51 151.35 152.23 107.06 140.17 118.62 132.63 115.96 This work
unresolved multiplet, and the C-4 (and C-5) signal was split into an apparent doublet ( 2 ~ = 1.7 HZ).
Confirmation of the 13C NMR assignments was obtained from two long-range HETCOR experiments (17) optimized for couplings of 10.0 and 7.7 Hz respectively. The 'H signal of H-3 (and H-6) at 7.20 pprn was correlated ( 2 ~ H , C ) with the 13C signals due to C-4 (and C-5), C-2 (and C-7), as well as ( 3 ~ ~ , C ) with C-1 (and C-8) and C-10. The 'H signal of H-1 (and H-8) at 7.92 pprn was correlated ( 2 ~ H , C ) with the 13C signals due to C-9, as well as (3JH,C) with C-1 (and C-8), C-10 and C-3 (and C-6).
The 13C NMR of the deuterium-exchanged CTA in D20 revealed that the intensity of the signal assigned to C-3 (and C-6) was diminished by over 90%. All other signals were unchanged. Thus it is the C-3 (and C-6) of CTA and not C-1 (and C-8) which are labile.
The 3,6-dibromo, 3,6-dichloro, and 3,6-diiodo derivatives Sa, b,c have been reported in the literature (1 8) and their NMR spectra were of interest for comparative purposes. Attempts to
synthesize these dihalogenated derivatives using the method of Maslow ska and Duda ( 18) were unsuccessful. The procedures reported by Kuznetsov and Basargin (19) for the synthesis of the monobrominated 3-bromo-CTA 6 afforded only the dibrominated product 5a . Modification of their procedure, however, afforded 6 in approximately 40% yields. The 'H NMR spectrum of 5a consisted of a single line at 8.10 ppm; that of 6 showed three signals in the aromatic region: one singlet at 8.10 pprn corresponding to the proton on the brominated ring and an AX pattern (7.17 and 7.80 ppm, J - 2 Hz) for the non-brominated ring. The 13C NMR spectra of 5 a and 6 consisted of six and 10 signals respectively and their chemical shifts are summarized in Table 1.
The 2-D 'H-13c HETCOR spectrum of 6 revealed that the singlet at 8.10 pprn in the 'H NMR spectrum is correlated ('JH,,-) with the 13C signal at 121.33 ppm, which is due to C-1 of 6. The doublet at 7.80 pprn ( J = 2 Hz) in the 'H NMR spectrum is correlated ('JH,,-) with the 13C signal at 118.62 ppm, which can be assigned to C-8. The doublet in the proton
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GEORGHIOU ET AL. 121 1
s ectrum at 7.17 ppm (J = 2 Hz) is correlated ( I J ~ , ~ ) with the !i' ' C signal at 107.06 ppm, which is due to C-6. It is interesting that the deuterium exchange reaction at
C-3 and C-6 observed with CTA in deuterium oxide is regio- selective. This could be due to intermolecular assistance by the proximal hydroxyl groups, or due to steric inaccessibility of the C-1 and C-8 positions. The NMR spectra of Sa, 6, and CTA that follow from the evidence presented here provide essential information for interpreting the spectra observed when the reaction of CTA and formaldehyde was monitored directly by NMR (10). O n the basis of these in situ experiments and calibration-line studies, it was concluded that the mono-cationic dibenzoxanthylium structure 3 for the chromogen is most consistent with the chemistry of the reaction (10).
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TABLE 2. Bond lengths (A) with estimated standard deviations in parentheses
Length Length
Bond Uncorr. Corr. Bond Uncorr. Corr.
TABLE 3. Bond angles (deg) with estimated standard deviations in parentheses
Bonds Angle (deg) Bonds Angle (deg)
Bond lengths (corrected and uncorrected) and angles are given in TABLE 4. Intra-annular torsion angles (deg) standard Tables 2 and 3 and intra-annular torsion angles in Table 4. Hydrogen deviations in parentheses atom parameters, anisotropic thermal parameters, bond lengths and angles involving hydrogen, torsion angles, and structure factors have Atoms Torsion angle (deg) been d e p ~ s i t e d . ~
C(2)-O(1)-C(1)-N 1.4(2) Results and discussion (33)-N -C(l)-O(1) -6.4(2)
As already suggested by the spectroscopic data and ele- mental composition, the crystallographic analysis confirms formula 4, and thus shows the possibility of cyclocondensation of N-(2-hydroxyalky1)hydroxamic acids in the presence of Lewis acid boron compounds even when chelate-forming reactions such as those observed for the salicylohydroxamate 6 (2) and the hydroxyacetohydroxamate 7 (3) are not involved. The resulting open-chain BF,-adduct 4 (Fig. 1) is a stable compound and represents a new type of N-oxide boron com- plex with a boron,nitrogen-(1,3)betaine structure (7).
The 0(2)BF, grouping in 4 has a distorted tetrahedral geometry with bond angles at boron ranging from 103.9(2)0 for F(1)-B-O(2) to 113.3(2)" for F(1)-B-F(2). BF, itself has a trigonal planar structure both in the gas phase (8) and in the
'Supplementary material mentioned in the text may be purchased solid state (9, 10). Tetracoordination of the boron a t y leads from the Depository of Unpublished Data, Document Delivery, CISTI, to lengthen in^ of the B-F bonds in 4 (mean 1.382 A (corr.) National Research Council of Canada, Ottawa, KIA OS2. and 1.361 A (uncorr.)) compared to those reported for
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FIG. 1. Stereoview of 4; 50% probability thermal ellipsoids are shown for the non-hydrogen atoms.
crystalline BF,: mean values of 1.278 A (9) and 1.289 A (10). This is consistent with a weakening of the pp(m)-interaction between boron and the fluorine ligands upon complex forma- tion. The mean B-F distance in 4 is greater than the cor- responding mean B-F bond length (1.345 A (uncorr.)) report- ed for pyridine-BF, (11). This could result from stronger coordination in the N-oxide BF,-adduct 4 than in pyridine-BF,. To the best of our knowledge, there are no other structures of heterocyclic N-oxide BF, complexes in the literature. It is probably safe to assume that the 0-B coordinative bond in zwitterionic N-0-B complexes like 4 is generally stronger than the N-B bonds of comparable, but more sterically hindered, N-B adducts.
The libration-corrected 0-B bond length in 4 (1.530 A) is in good agreement with the 0-B distance of 1.533 A derived from semiempirical MNDO calculations on the BF, complex of an acetaldehyde adduct of pyridine, 9 (12), which corresponds to an analo ous B,N-(1,4)betaine structure. An 0-B distance of 1.54 $. has been calculated for the BF, complex of dimethylsulfoxide (13). While there is a paucity of structural information reported for BF, complexes with basic oxides (14), the crystal structure of the related benzaldehyde-BF, adduct features an 0-B bond length of 1.591 A (15). The 0-B distance in 4 is very similar to the (C)F-B distance of 1.528(8) A recently reported for an isoelectronic F,B-FC linkage in a binuclear ferracyclopentadiene derivative (16). The average terminal B-F distance of 1.37(1) A for the F,B-FC adduct is similar to that in 4 (see above).
The five-membered 2-oxazoline ring is nearly planar (maximum deviation from the mean plane is 0.070(2) A for C(3)) and has a flattened C(3)-envelope conformation. The phenyl ring is not coplanar with the oxazoline ring (dihedral angle between normals to the mean planes = 28.8") in spite of conjugation or possible m-interaction between the imidate moiety and the aromatic ring system. The C(1)-C(9) bond length of 1.468 is somewhat longer than the comparable C(sp2)-C(ar) distance of 1.450 A in compound 6 (2), in which the phenyl ring is held in an approximately coplanar orientation with respect to the oxazoline ring by chelate formation via the
ortho-phenolic ligand. The cyclohexane ring in 4 has a relatively undistorted chair conformation (see Table 4), with the oxygen atom of the spiro-linked oxazoline ring occupying an equatorial position and the methylene substituent (C(3)) in an axial position.
As observed earlier for compounds 6 (2) and 7 (3), the C(1)-N bond in 4 has distinct double bond character. The bond length of 1.3 17 A corresponds to a m-bond order between 0.7 and 0.8 on the basis of a bond length vs. bond order plot (17), and resembles the C=N+ bond distance in 2 (1) and other boron chelates of hydroxamic acids such as 8 (18) and related hydroxamates (19-23). The double bond character of the iminium C=N+ bond in the open-chain B,N-betaine 4 appears to be somewhat reduced relative to that in the cyclic B,N-betaines 6 (1.294 A) and 7 (1.281 A). The C(1)-O(1) bond in 4 (1.323 A) is equal in length t? the corresponding bond in the oxazoline moiety of 6 (1.323 A) and similar to the same bond in the oxazoline ring of 7 (1.310 A), indicative of greatly reduced double bond character of the imidate C-0 bonds in all three compounds. The N-0 bond distance in 4 (1.371 A) is similar to those in 6 (1.369 A) and 7 (1.367 A), and can also be compared with that in the difluoroboron chelate 8 (1.374 A).
In the solid state, the shortest intermolecular contacts may represent three weak C-H...F interactions (H..-F = 2.47(3), 2.49(3), and 2.55(2) A). All other intermolecular distances correspond to normal van der Waals interactions.
Acknowledgements
We thank the Natural Sciences and Engineering Research Council of Canada and the Fonds der Chemischen Industrie, Frankfurt am Main, for financial support.
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1216 CAN. J. CHEM. VOL. 69, 1991
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