J. Phys. Chem. 1993,97, 1862-1867
1862
Hydrogen Bonding and the Structure of Substituted Quinomlines: Solid-state NMR and Single-Crystal X-ray Diffraction Studies Krzysztof Wozniak and Tadeusz M. Krygowski' Department of Chemistry, University of Warsaw, 02-093 Warszawa, ul. Pasteura 1, Poland
Eqgeniusz Grech Institute of Fundamental Chemistry, Technical University of Szczecin, 71-065 Szczecin, ul. Piastbw 42, Poland
Waclaw Kolodziejski and Jacek Illinowski' Department of Chemistry, University of Cambridge, Lenrfleld Road, Cambridge CB2 I E W,U.K. Received: October 28, 1992
2,3-DimethyIquinoxaline tetrafluoroborate (DMQH) and 2,3-(2-dipyridyl)-6,7-dimethylquinoxaline tetrafluoroborate (PDDQH) have been compared with their free parent bases 2,3-dimethylquinoxaline (DMQ) and 2,3-(2-dipyridyl)-6,7-dimethylquinoxaline(PDDQ) using singletrystal X-ray diffraction and 'Hand I3Csolidstate NMR with magic-angle spinning. Hydrogen bonding substantially affects 'hard" structural parameters such as bond lengths and angles, and this is reflected in the NMR spectsa, where significantchanges of chemical shift and splittings of the lines are observed. The structure and dynamics of the hydrogen bonds are discussed.
Introduction It is well-known that a solid-state NMR spectnunoften contains a greater number of lines than the solution spectrum of the same compound.' Tbis "solid-state effect" may originate from the structural inequivalence of moleaules in the unit cell or from deformations caused by crystal packing forces (broadly termed as molecular interactions in the solid state). The study of intermolecular interactions is often the aim for both the NMR spectroscopist and the crystallographer. This is the case with solid hydrogen-bonded crystals, which have been extensively studied by IH NMR2 and by X-ray diffraction (XRD).3 An interesting aspect of the molecular interactions concerns their effect on the molecular geometry in the crystal in comparison to that in solution.* Some believe that solid-state packing forces can affect only "softn parameters of molecular geometry, such as the torsional angles,while hard parameters, such as bond angles and bond lengths in aromatic compounds, are unaffectedas This concept has been questioned on the basis of analysis of model molecular structures determined to a very high preci~ion.~.~ For example, it has been found that hydrogen bonding between the pnitrosophenolate ion and water molecules in the crystal lattice substantially changes the bond lengths in the benzene ring.' We wish to report that the application of NMR and XRD in tandem yields important information on the dramatic distortions of the molecular backbone upon H-bond protonation of 2,3-dimethylquinoxalinetetrafluoroborate (DMQH) and 2,3-(2-dipyridyl)6,7-dimethylquinoxaline tetrafluoroborate (PDDQH) in comparison with their free parent bases 2,3-dimethylquinoxaline (DMQ)) and 2,3-(2-dipyridyl)-6,7-dimethylquinoxaline(PDDQ).
DMQH
c##
(SHELXS-86*) and refined using full-matrix least-squares refinement (SHELX-769). Heavy atoms were refined with anisotropic and H atoms with isotropic temperature factors. Atomic scattering factors were taken from ref 10. A difference between two geometric parameters was considered significant if it was higher than 3u, where u = (ul2 + and UI and u2 are the estimated standard deviations of the parameters.11 In order to make the assessment easier, the parameter differences given in the figures are also expressed in terms of the common standard deviation u (parameter value divided by u is given in brackets). Solid-state NMR. IH magic-angle-spinning (MAS) NMR spectra and I3C MAS NMR spectra with cross-polarization (CP) were recorded at 100.61 and 400.13 MHz, respectively, at 25 OC using a high-speed double-bearingprobehead and zirconia rotors spun in nitrogen gas. The IH spectra were measured with MAS at 13-1 5 kHz using 2 ~ ( 6 6pulses ~ ) and a 1-srecycle delay. The single contact I3C CP/MAS experiments were performed with 4-ms contact times for optimal contact time experiments and 50 HS for the short-contact-time experiments. The length of the 1H and 13C r / 2 pulses was typically 3 M, the recycle delay 10 s, and the MAS rate 8 W z . The I3C dipolar-dephased spectral2were recorded with a Sops delay prior to acquisition. Dipolar-dephascd experiments expose quaternary carbon lines, and the CP exper-
Experimental Section Reparation. The crystals of PDDQ and DMQH were precipitated from equimolar solutions of free amines and HBFd in butanol and then recrystallized from acetonitrile by slow isothermal evaporation of the solvent. X-rayDiffraction. Single-crystal X-ray diffraction measurements were carried out using the equipment and conditionslisted in Table I. The ~ 2 scan 9 mode was used, and the unit cell parameters were obtained by a least-squares refinement. The structures were solved using multisolution direct methods 0022-3654/93/2097-1862$O4.OO/0
(a
1993 American Chemical Society
Hydrogen Bonding and the Structure of Quinoxalines
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1863
TABLE I: Crystal Data for DMQ, DMQH, PDDQ, and PDDQH DMQ general formula molecular weight diffractometer
DMQH
CIOHION~ 158.202 Enraf-Nonius CAD 4 monoclinic
crystallogr group space group cell dimensions:
(4 c (4
7.641 (2) 9.896(1) 11.480(1) 99.10 (1) 857.12 (25) 4 1.226 Mo K a 0.699 336 29 1 623 0.0396
0
b (4
B (dee) cell volume (A') molec multiplicity 4 (g/cm3) radiation tempciatire (K) ' reflections observed final R factor H
C2oH16N4 312.37 Syntex P21
monoclinic
monoclinic
m / c
a / c
13.516(1) 6.003(2) 14.662(1) 109.58(1) 1121 4 1.458 Cu K a 11.53 504 29 1 2104 0.0586
16.474(1) 13.132(1) 7.638 (1) 99.24 (1) 1630.94 (24) 4 1.272 Cu Ka 5.782 656 29 1 1032 0.0450
PDDQH G O H 7N4BF4 I 400.I9 Enraf-Nonius CAD 4 monoclinic P2da
10.88(1) 14.33(1) 12.02(1) 101.17(2) 1838 (3) 4 1.446
Mo Ka 1.10 824 29 1 3113 0.0718 H
H
Q
P H
PDDQ
CIOHIINZBF~ 246.01 KUMA
H
H
H F
F
Figure 1. Thermal-ellipsoid plot of the DMQH molecule at 50% probability (ORTEP22).
iments with short contact time highlight lines from carbons with adjacent protons. Ordinary CP/MAS spectra contain all these lines. We note that ordinary spectra cannot be simply obtained by the addition of the respective dipolar-dephased and shortcontact-time spectra, since in both latter cases the relative line intensities are substantially distorted.
Results .ad Mscuasion X-ray Diffraction. We are primarily interested in the XRD results which are relevant to the interpretation of the NMR spectra.I3 Thus we have shown that for the quinoxalines studied all molecules in the unit cell are structurally equivalent. The hydrogen-bonded complexes of DMQH and PDDQH are shown in Figures 1 and 2. The orientation of BF4- is disordered in both crystals, since we found more than four possible positions for fluorine atoms and only one for boron. In DMQH the hydrogen bond involves one of the nitrogen atoms of the quinoxalinemoiety and the BF4- ion, while in PDDQH the hydrogen bond links the nitrogen atoms of neighbor pyridyl groups with the BF4- ion located further away. The latter structure is quite different from that found in solution, where PDDQ forms hydrogen bonds involving quinoxaline nitrogens with solvents.l4J5 However, an intramolecular hydrogen bond between the pyridyl nitrogen and the quinoxaline nitrogen16 or between the two pyridyl nitrogen atomsi7has been postulated. Hydrogen bonding in DMQH and PDDQH seems fairly complicated. In DMQH there are two fluorine atoms engaged in 80% (case I) and 20% (case 11). as indicated by the relative populations of the proton, which they share with one nitrogen atom of quinoxaline. The N-H, H-F, and N--F distances are 1.05(4), 1.74(4), and 2.731(4) A in case I and 1.05(4), 1.80(5), and 2.696(4) A in case 11. The NHF angle is 157(4)O and 142-
H
Figure 2. Thermal-ellipsoid plot of the PDDQH molecule at 50% probability (ORTEP22).
(3)' for cases I and 11,respectively. There is also a short contact of 2.25(3) A between the nitrogen atom involved in the hydrogen bond and the hydrogen atom of the neighboring methyl group. The C-H and C-N distances are 1.02(3) and 2.411(3) A, respectively, and the CHN angle is 87O. In PDDQH there are two proton locationsin the hydrogen bond with relative populations of 80% and 20%, corresponding to distances of 0.79(4) and 1.49(10) A from one nitrogen atom or of 1.80(4) and 1.08(11) A to the other, respectively. The N-.N distance is 2.574(4) A for both proton positions, while the NHN angles are 169(4)O and 172(4)O with 80% and 20% probability, respectively. Since the heavy atoms are displaced from the symmetry plane by not more than 0.05 A in all the compounds studied, the quinoxaline moiety is planar. The hydrogen bond forces the molecules to readjust their bond lengths and bond angles within the quinoxalineplane (Figures 3-6), thereby breaking molecular symmetry. We note that the pyridyl rings are twisted from this plane by the same 39.7O angle in DMQ, while in DMQH the hydrogen bond renders them inequivalent. The resulting twist angles are 2SS0 and 31.2O for the pyridyl groups affected more (ring A) and less (ring B) by the hydrogen bond, respectively (Figure S), and the distance between the pyridyl nitrogen atoms decreases from 3.863(2) A in DMQ to 2.574(4) A in DMQH. We note that bond lengths and bond angles in DMQ/DMQH, Le., hardstructural parameters, are substantiallyaffected (Figures 3 and 4). as are the bond angles in PDDQ/PDDQH (Figurm 5 and 6). In the latter case the changes of bond length are not significant (not shown). The effect of the hydrogen bond is felt even at remote sites: consider, for example, the bond angles of the carbon atoms 6 and 7 in the ring D of PDDQ (for numbering
Wozniak et al.
1864 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 H
(a1
H
Figure 5. Differences between the bond angles (deg) of PDDQH and PDDQ. Values in bracketsare expressed in terms of thecommon standard deviation. A proton of the N.-H-.N hydrogen bond is, on average, located closer to the pyridyl ring A.
Figure 3. Differences between the geometric parameters of DMQH and DMQ: (a) bond lengths (A), (b) bond angles (deg). Values in brackets are expressed in terms of the common standard deviation.
(a) 06
ob -12
06 1 1
ob - 1 1
0 1 -os
4 11 4
I
Figure 6. Differences within the pairsof corresponding bond angles (deg) of PDDQH, which are equal in the parent base PDDQ. Values in brackets are expressed in terms of the common standard deviation.
0
0.9 13.2)
-7.4 5.2 -1.4 1.9 -3.6 4.4 [5.0] [13.4](25.51 [31.1] [33.1][18.4]
0
H
Figure 4. Differences within the pairs of corresponding geometric parameters of DMQH, which are equal in the parent base DMQ: (a) bond lengths (A), (b) bondangles (deg). Valuesin brackets areexpressed in terms of the common standard deviation.
see the structures and Figure 5). The largest parameter changes are: 0.025 A for the bond length and 6.2O for the bond angle of DMQ/DMQH (Figure 3); 6.6O for the bond angle in PDDQ/ PDDQH (Figure 5). Solid-stateNMR. The 1H MAS NMR spectra shown in Figure 7 are sufficientlywell resolved to provide interesting information, despite the narrow chemical shift range inherent toproton NMR. Thus the lines of strongly acidic protons in DMQH and PDDQH appear at 14.4 and 21.3 ppm, respectively. These lines are, of course, missing from the spectra of the parent bases. In addition to the hydrogen bond deshielding, the unusually high chemical shift of the acidic proton in PDDQH is caused by the deshielding ring current of the two adjacent pyridyl groups. This is in
agreement with the XRD structure, in which the acidic proton takes part in hydrogen bonding between pyridyl nitrogens. In the spectra of DMQ and PDDQ there are methyl lines at 1.1 and 1.5 ppm, respectively. These experiencehigh-frequency shifts of 2.2 and 0.4 ppm in the corresponding saltsDMQH and PDDQH, respectively, which reflects the relative through-bond distance of the methyl groups from the centers of perturbation, i.e., from the protonation sites. Again, this is in accordance with the XRD results which show decreasing distortions of bond lengths and bond angles with increasing distance from the protonation site. Surprisingly, the aromatic lines from DMQ are partly resolved, although it is difficult to assign the peab at 9.6 and 6.4 ppm to particular 5 (8) and 6 (7) sites. The composite aromatic line from DMQH appears as a shoulder at ca. 7 ppm. PDDQ gives only one aromatic line at 7.6 ppm, which corresponds to that at 8.4 ppm in PDDQH. The chemical shift changes upon protonation observed in the proton spectra cannot be explained without a detailed analysis of the electron density redistribution. Suffice it to say that even the interpretation of chemical shielding in DMQ and PDDQ poses severe problems, since their solution and solid-state chemical shifts are very different.'* We only note that the effects of protonation on IH chemical shifts are so large that they must be caused by a substantial redistribution of electron density. The I3Cresonances from DMQ and PDDQ (Figures 8 and 9 ) can be easily assigned by comparison with solutionspectra (Tables I1 and 111) and by inspecting the dipolar-dephased and short-
Hydrogen Bonding and the Structure of Quinoxalines
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1865
71
20.0
_xJ 22.7
8.4
A
PDDQH
1.9
22.2
1.1
DMQ
9.6
6.4
A 3.3 A
25
20
15
10
5
0.0
-5
-10
ppm from TMS
i8
26
24
k
20
18
16
ppm from TMS
Figure 7. Proton NMR spectra ofquinoxalines: DMQH, PDDQH, DMQ, and PDDQ. Asterisk denotes a spinning sideband from the acidic proton line.
Figure 8. '3CMAS NMR lines from the methyl groups of quinoxalines: DMQH, PDDQH, DMQ, and PDDQ.
contact-time spectra (Figures 10 and 11). Carbons 2 and 3 in PDDQ are deshielded by comparison with those of 6,7-dimethylquinoxalineby the ring currents of the adjacent pyridyl groups. Protonation effects are far more pronounced in the 13C spectra (Figures 8 and 9) than in the 'H spectra (Figure 7). Methyl lines split into two components (Figure 8) separated by 2.5 ppm in the DMQ/DMQH case and by 2.1 ppm in the PDDQ/PDDQH case. The splittings are such that one of the components almost retains the position of the unperturbed resonance and the high-frequency components haveslightly higher intensity than the lower frequency ones. This characteristic splitting pattern can be recognized in the aromatic region only for the DMQ/DMQH case (Figure lo), especially in the dipolar-dephasedspectra. We note (Figure 3b) that the bond angles of DMQH located further from the hydrogen bond (upper part of the drawing) are changed less than their counterparts close to the perturbation (lower part of the drawing), which reflectsvery well the splitting pattern of concern. This observation can be readily explained by the dependence of both bond angles and chemical shift on carbon hybridization. The two doublets corresponding to resonances (5.8) and (6,7) in the short-contact-time spectrum of DMQH probably overlap giving rise to a specious triplet. This tentative assignment corresponds to the splitting of 8.3 and 4.0 ppm for carbons (5,8) and (6,7), respectively, in agreement with the XRD data which
show decreasing distortions of bond lengths and bond angles with the increasing distance from the protonation site. PDDQH gives aromatic resonances at 153.7, 148.2, 147.2, 145.0, 142.4, and 139.8 ppm (Figure 9). The short-contact-time spectra of PDDQ and PDDQH (Figure 11) suggest that upon protonation there is no splitting but a significant shift of the pyridyl lines. A splitting of the line from carbons 5 and 8 must be small or absent. It follows that line of carbon 1' does not split but only shifts from 157.4 to 153.7ppmconsideringthespectraofPDDQandPDDQH in Figure 11. Therefore, the lines at 148,145,142, and 140 ppm in the dipolar-dephased spectrum of PDDQH should all be assigned to the quinoxaline moiety. Since the line at 148 ppm has a complex structure (see overlapping resonances at 148.2 and 147.2 ppm in Figure 9), at least two of the lines from carbons (2,3), (9, lo), and (6.7) appear in the spectrum of PDDQH as overlapping doublets. We feel that the latter spectrum cannot be fully assigned without site-specific '3Clabeling. Finally, we address the question of why the carbon lines are split. Since, according to XRD, all the molecules of DMQH or PDDQH are equivalent in the unit cell, the splittings must be caused by the protonation and the consequent distortions of the molecular backbone (cf. Figures 2 4 ) . Three possibilities must be considered.
Wozniak et al.
1866 The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 149
138
r
PDDQ
1
126
8*7
124
5.8
140
157
DMQ short
CP contact time
129 6.7
6,7 + 5.8 (7)
DMQ
160
150
140
130
120
ti0
ppm from TMS
rso 140 130 ppm from TMS
120
Figure 9. Aromatic region of the 'IC MAS NMR spectra of quinoxalines: DMQH, PDDQH, DMQ, and PDDQ.
Figure 10. Short-contact-time and dipolar-dephased spectra of DMQH and DMQ in the aromatic region.
TABLE II: Asd ent of tbe Solid-state 'JC NMR Spectra of DMQ rod D M E and Reference Datal9 for DMQ in Solution'
TABLE IIk Asd t of tbe Solid-state "R Spectra of PDDQ m f e r e n c e to tbe Solution Data for 6.7-Mmetb~lauiaox1~ md for 2,2'-Bi~yrid~l~ ~~
~~
carbon CHI
DMQ
DMQI9
DMQH
22.2
22.91
20.4 22.9 119.9 128.2 (?) 128.2 (?) 132.2 128.2 142.9 154.1 159.6
5,8
121.9
128.15
61
129.1
128.59
9, 10 2.3
141.8 154.5
140.93 153.25
Question marks denote tentative assignments (see text).
First, the proton may spend most of the time close to the quinoxaline nitrogen in DMQH or one of the pyridyl nitrogens in PDDQH rendering the proton-acceptor molecule asymmetric and all the carbons in the quinoxaline moiety in DMQH or PDDQH and the pyridyl groups in PDDQH inquivalent. Such a highly asymmetric hydrogen bond can be readily ruled out for PDDQH, since the relevant pyridyl lines appear as singlets. The model fits DMQH, because it implies five carbon doublet lines. One component in the doublet should have a similar chemical
carbon CHI PDDQ ref 19
20.0 19.98
598 126.2 128.16
6 7 138.8 140.23
9, 10 140.4 141.74
2,3 149.6 143.77
carbon PDDQ ref 20
3'
5'
4'
6'
1'
123.9 121.4
124.5 124.0
138.3 137.2
149.2 149.4
157.4 156.4
shift to the unperturbed line, which would correspond to the less distorted carbon site in DMQH of the two formerly quivalent in DMQ. The intensity difference between the doublet components must be caused by relaxation, Le., for dipolar-dephased experiments from different T2 relaxation times. Such a structure for DMQH is quite probable, since the BFd- anion is a weak proton acceptor and therefore does not form hydrogen bonds in solution.*' Second, one can assume two different proton positions in the hydrogen bond with a double-minimum energy well and the frequency of proton jumps to be smaller than the splittings in the
Hydrogen Bonding and the Structure of Quinoxalines
of splittings for pyridyl lines in PDDQH cannot be accounted for in this model. The third model is similar to the sccond but postulates very fast proton jumps. Since in this case only four I3C lines from DMQH would be expected because of exchange averaging, this possibility must be rejected for DMQH. However, the model calls for the absence of splittings for pyridyl lines in PDDQH, since they should be blurred by the proton exchange within the N-*H.-N hydrogen bond linking the pyridyl rings. In order to account for the splitting of the quinoxalinelines, we must require that the pyridyl rings be, on the average, differently twisted out of the quinoxaline plane. This is the structure found by single crystal XRD.
PDDQ
127ppm
128
'IiI "'hri"'
PDDQH
The Journal of Physical Chemistry, Vol. 97, NO. 9. 1993 1867
short
CP
contact time
Acknowledgment. We are grateful to Unilever Research, Port Sunlight, for support.
References and Note
160
150
140
130
120
ppm from TMS Figure 11. Short-contact-timeand dipolar-dephased spectra of PDDQH and PDDQ in the aromatic region.
frequency units, Le., less than several hundred hertz. In this case one would predict separate carbon lines for both proton positions. Only this model requires a doublet for the acidic proton line, but this might be obscured by proton spin diffusion. The model implies that each carbon line of DMQ should split into threecomponents in DMQH (two when the proton is close to nitrogen and possibly one when it is away, that is close to BFd- anion), so it does not agree with the experimental results. Furthermore, the absence
(1) Fyfe, C. A. Solid Srare NMR for Chemists; CFC Press, 1984. (2) Harris, R. K.; Jackson, P.; Merwin, L. H.; Say, B. J.; Higele, G. J . Chem. Soc., Faraday Trans. I1988, 3649 and references cited therein. (3) Krygowski, T. M.; Kalinowski, M. K.; Turowska-Tyrk, I.; Hiberty, C.; Milart, P.; Silvwtro. A.; Topsom, R. D.; Dahene. S. Srruct. Chem. 1990, 2, 71. Wozniak, K.; Krygowski, T. M. J . Mol. Srrucr. 1989, 193, 81. Lipkowski, J.; Andretti, G. D.; Sgarabotto, P.Cryst. Srrucr. Commun. 1977, 6 , 197. (4) Dunitz, J. D. X-Ray AnalysisandtheSrrucrureofOrganic Molecules; Cornel1 University Press: Ithaca and London, 1978. (5) Kitajgorodski, A. J. Molecular Crysrals and Molecules; Academic Prws: New York, 1973. ( 6 ) Bernstein, J. International School of Crystallography, 11th Course, Lecture Notes: Eflect of Crystal Environment on Molecular Structure. Erice, 1985. (7) Krygowski, T. M.; Turowska-Tyrk, I. Chem. Phys. Lett. 1987,138, 90. (8) Sheldrick, G. M. SHELXS86-Program for Crystal Srrucrure Determination; University of Gottingen: Gottingen, Germany, 1986. (9) Sheldrick, G. M. SHELX-76 Program for Crystal Srrucrure Determination, University of Cambridge: Cambridge, 1976. ( 10) International Tables for X-Ray Crystallography; Birmingham, Kynoch Press (present distributor Kluver Academic Publishers: Dodrccht), 1974; Vol. IV. (1 1) Krygowski,T.M. CorrelationAnalysisinOrganicCrystalChemistry. In Progress in Physical Organic Chemistry; Taft, R . W., Ed.; J. Wiley Bt Sons: New York, 1990, Vol. 17, p 239. (12) Opella, S. J.; Frey. M. H. J. J . Am. Chem. Soc. 1979, 101, 5854. (13) Detailed XRD data arc available from the authors on rcquut. (14) Escuer, A.; Vicente, R.; Comas, T.; Ribas. J.; Gomez, M.;Solans, X . Inorg. Chim. Acta 1990, 177, 161. (15) Brzezinski, B.; Olejnik, J.; Zundel, G. Chem. Phys. Lrrr. 1987,135, 93. (16) Escuer. A.; Comas, T.; Ribas, J.; Vicente, R.; Solans, X.; Zanchini, C.; Gattcschi, D. Inorg. Chim. Acta 1989, 162,97. (17) Rillene. D. P.; Taghdiri, D. G.; Jones, D. S.; Keller, C. D.; Worl. L. A.; Meyer, T. J.; Levy, H. Inorg. Chem. 1987, 26, 578. (18) The Aldrich Library of N M R Specrra, 2nd ed.; Poucheret, Ch. J., Ed.; Aldrich Chemical Company, Inc.: Milwaukee, 1983, Vol. 2, p 759. (19) McNab, H. J . Chem. Soc., Perkin Trans. 11982, 357. (20) Marker, A.; Canty, A. J.; Brownlee, R. T. C. A u t . J. Chem. 1978, 31, 1255. (21) Dryjanski, P.; Kccki, Z. J . Mol. Struct. 1972, 12, 219. (22) Johnson, C. K. Report ORNL-5138; Oak Ridge National Laboratory: Oak Ridge TN, 1976.
