Substituent Effect in 2-Benzoylmethylenequinoline Difluoroborates

Article pubs.acs.org/JPCA

Substituent Effect in 2‑Benzoylmethylenequinoline Difluoroborates Exhibiting Through-Space Couplings. Multinuclear Magnetic Resonance, X‑ray Diffraction, and Computational Study Anna Zakrzewska,† Erkki Kolehmainen,‡ Arto Valkonen,‡ Esa Haapaniemi,‡ Kari Rissanen,‡ Lilianna Chęcińska,§ and Borys Ośmiałowski*,† †

Faculty of Technology and Chemical Engineering, University of Technology and Life Sciences, Seminaryjna 3, PL-85-326 Bydgoszcz, Poland ‡ Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland § Structural Chemistry and Crystallography Group, University of Łódź, Pomorska 163/165, PL-90-236 Łódź, Poland S Supporting Information *

ABSTRACT: The series of nine 2-benzoylmethylenequinoline difluoroborates have been synthesized and characterized by multinuclear magnetic resonance, X-ray diffraction (XRD), and computational methods. The through-space spin−spin couplings between 19F and 1H/13C nuclei have been observed in solution. The NMR chemical shifts have been correlated to the Hammett substituent constants. The crystal structures of six compounds have been solved by XRD. For two derivatives the X-ray wave function refinement was performed to evaluate the character of bonds in the NBF2O moiety by topological and integrated bond descriptors.

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INTRODUCTION

In structurally related ligands such as 2-methylenecarbonylpyridine derivatives three prototropic tautomeric forms exist (Scheme 1), i.e., enolimine (O), ketoimine (K), and

In various fluorescence-based analytical techniques, boroncontaining compounds are widely used. Among these the BODIPY dyes play a special role in chemodosimetry,1 biolabeling,2,3 NIR spectroscopy probes,4−7 solar cells8,9 in photodynamic therapy,10 and other11,12 applications as they are relatively easy to functionalize.13,14 Their chemical structure and luminescent properties are often studied in solution and in the solid state.15−18 The BODIPY core contains two pyrrole units, and its structure is symmetric with one covalent and one dative N → B bond (DB) where the real structure is an average of two identical forms and both N−B distances are the same in solid state.19−22 In addition to BODIPYs, the most common structures are those containing NBF2N,23,24 NBF2O,25−28 OBF2O,29−32 and NBF2S33 moieties where to forms are different in compounds that carry an unsymmetrically chelated BF2 moiety. It was shown34 that the β-ketoiminate difluoroborate exists entirely as an alkoxy-imine-BF2 form in the solid state25,26,33,35,36 (Chart 1).

Scheme 1. Tautomeric Equilibrium in 2-Methylenecarbonyl Heterocycles

enaminone (E). Therefore it is interesting to clarify whether a corresponding nonprototropic tautomerism exists also in BF2 moiety containing compounds since both the proton and BF2 moiety can be regarded as Lewis acids. The tautomeric equilibrium between E and K depends strongly on the substituent.37 The equilibrium toward O or E forms is shifted by benzoannulation of the heterocycle. In 2phenacylpyridines38,39 and 3-phenacylisoquinolines40 the ketoimine (K) coexists with enolimine (O) in solution, while in 2quinoline,37 1-isoquinoline, and 6-phenanthridine derivatives40 the equilibrium is ketoimine (K)−enamine (E). In the solid state, however, only one form is present.37−39 The position of the BF2 moiety in crystalline β-ketoiminate25 is in contradiction

Chart 1. Two Forms in β-Ketoiminate Difluoroborate Moiety

Received: November 8, 2012 Revised: December 18, 2012 Published: December 19, 2012 © 2012 American Chemical Society

252

dx.doi.org/10.1021/jp311072q | J. Phys. Chem. A 2013, 117, 252−256

The Journal of Physical Chemistry A

Article

Chart 2. Structure and Atom Numbering in 1−9

Table 1. HOMA Indices for Rings A, B, and Ca

to the position of the proton in the protonated analogues.41 The present study of 2-benzoylmethylenequinoline difluoroborates 1−9 (Chart 2) is a continuation of our studies on the substituent effects in related molecules.37,38,40,42 It is expected that the substituent effect may be observed in 1−9 due to higher rigidity of the ring formed by the stronger interaction of dative bond character than that of the intramolecular hydrogen bonding in parent 1,2-dihydro-2-benzoylmethylenequinolines.37 In addition, these compounds exhibit strong fluorescence, which is now under detailed study. However, before that a detailed study on the ground state properties of 1−9 were conducted with a set of compounds carrying different substituents from a strongly electron accepting one (CF3) to a strongly electron donating one (NMe2). In order to gain further insight into the bonding and electronic properties of these difluoroboron chelate complexes, six single crystal X-ray structures have been solved and two of them are forwarded for X-ray wave function refinement to evaluate the character of bonds in the NBF2O moiety by topological and integrated bond descriptors.

compound

ring A

ring B

ring C

1 4 5 6 7 8b

0.8729 0.8832 0.9058 0.8882 0.8989 0.8935

0.7549 0.7400 0.7573 0.7810 0.7696 0.7496

0.6268 0.7129 0.7128 0.6614 0.6676 0.7434

a

See Chart 2 for ring labels. bHOMA indices averaged from the data of two molecules that are present in the unit cell.

lowest for 8. The same is true for HOMAring B − HOMAring C, while HOMAring A − HOMAring B is almost constant. These results suggest that the electron delocalization in a ring containing the BF2 moiety depends on the substituent R (Chart 2), but unfortunately no good correlation with Hammett constants was found. In order to complete the conventional structure analysis, the experimental electron-density distribution was analyzed for two derivatives (ORTEP plots in Chart 3): 1 (4-CF3) and 5 (H). Based on the low-temperature diffraction data, X-ray wave function refinement47 was performed. This is a two-step approach, combining the Hirshfeld atom refinement48 (first step) and X-ray constrained wave function fitting49,50 (second step), which affords a wave function constrained to the experimental diffraction data. Such a wave function can be subsequently analyzed topologically by the “quantum theory of atoms in molecules” (QTAIM) approach,51 which enables a quantitative interpretation of atomic and bonding properties. Within this work, the molecular electron-density distribution was particularly investigated for electronic effects around the boron atom. Examination of the wave function for the NBF2O kernel by the QTAIM approach indicates that the interaction of B−N is dative while those of B−O and B−F are single polar covalent bonds. Table S11 in the Supporting Information lists the values of experimental electron density and its corresponding Laplacian at the critical points of all B−X (X = F, N, O) bonds. At first glance, the electron density is more or less around 1 e Å−3, whereas the Laplacian (positive in all cases) is clearly pronounced for B−F bonds, for B−O is slightly smaller, but for B−N is relatively small. In the same time, the ellipticity shows a comparable but inverse trend: the values decrease from the highest for B−N to the lowest for B−F, but these do not exceed 0.07 (Table S12 in the Supporting Information). The values of ellipticity very close to zero confirm that all bonds are single (a reference value of ε = 0 is for a perfectly isolated C−C single bond). The delocalization index52,53 is of the same order of magnitude for all investigated connections, and its low value (∼0.3) points toward low electron sharing between the boron atom and its neighbors (a reference value of δA,B = 1 is for a single Lewis pair equally shared between two atoms A and B).

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EXPERIMENTAL METHODS The compounds were obtained from 2-methylenebenzoylquinolines37 as described earlier.43 The standard structural determinations and X-ray wave function refinement (XWR) were performed as described in the Supporting Information. All NMR spectra have been run with a Bruker Avance DRX500 FT NMR spectrometer equipped with either an inverse detection BBI probe head with a z-gradient coil or a direct observation BBO probe head. For details see the Supporting Information.

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RESULTS AND DISCUSSION The XRD data for 1 and 4−8 show that the B−O distance is shorter than that of B−N. Further, the average C11−C12 bond length is 1.354 Å while that of C12−O13 is 1.328 Å. Thus the C11−C12 bond is in the range between distances for aromatic and typical double bonds,44 while C12−O13 is longer than a typical CO double bond (1.230−1.260 Å).44 The shorter N−O distance in the studied molecules in comparison with non-BF2 derivatives37,38 shows the NBF2O bridge is more rigid and N → B interaction is stronger than respective interactions in compounds with intramolecular hydrogen bonding.37,38 The bond lengths of the rings A−C (Chart 2) were used to calculate the harmonic oscillator model of aromaticity (HOMA)45,46 index (see the Supporting Information for a detailed HOMA index explanation). This index was developed in order to test the degree of electron delocalization in organic molecules, taking the value for benzene (HOMA = 1) as a reference. Table 1 collects the HOMA values. These values show that the delocalization is higher in ring A than in the others. Except for 8 the HOMA for ring C is significantly lower than that for B. It is interesting that the difference HOMAring A − HOMAring C is highest for 1 and 253



Article

Chart 3. ORTEP68 Plots of 1 and 5

Table 2. Through-Space (TSJ) and 1J(11B,19F) Coupling Constants [Hz]

An analysis of the integrated QTAIM charges confirmed that the fluorine (∼ −0.8e), oxygen, and nitrogen (∼ −1.2e) atoms are negatively polarized and the B atom is positively charged (∼ +2.3e). Although the F atom has a much higher electronegativity than the N and O atoms, the relative position of the bond critical point (BCP) measured by the d1/d relation (d1, the distance from BCP to the electronegative atom X = F, N, O; d, distance B−X) varies in the narrow range of 0.66−0.68. These findings agree very well with those obtained from theoretical calculations for an isolated molecule at the MP2/6311+G(2d,2p) level of theory for the investigated group of compounds and with published results for small Lewis acid− base adducts (especially ammonia trifluoroborane H3N·BF3),54 where the B−F and B−N bonds were analyzed by means of QTAIM and ELI-D methods for gas-phase and crystal environments. Considering all the above-mentioned observations, one can state that the B−N bond is a dative bond and the B−O and B−F bonds are single polar covalent bonds. Summarizing all investigated types of bonds, B−F, B−N, and B−O are partially ionic, which is reflected in low electrondensity values at BCPs, the positive sign of the Laplacian, and a low delocalization index. On the other hand, the covalent contribution is proved by the negative value of the total energy density over the electron-density ratio (H/ρBCP) (Table S12 in the Supporting Information); however, a higher degree of covalency is observed for the dative N−B bond (in this case it is supported by the value of the Laplacian close to zero) and may be due to the lower electronegativity difference between these atoms than that of the B−O bond or the B−F bond. The 1H, 13C, 15N, 19F, and 11B NMR spectra that confirm the structures of 1−9 are maintained in solution. The resonance of H8 at ca. 8.7 ppm (a similar aryl proton shift was observed in BODIPYs55) shows a doublet of triplets, while the respective nucleus in the ligand (1,2-dihydro-2-benzoylmethylenequinoline) gave doublet at ca. 7.4 ppm.37 In 1−9 the shape of the signals suggests through-space spin−spin coupling and the relatively higher chemical shift suggests H···F hydrogen bonding. The deconvolution of H8 signals gave values of TS 1 J( H8,19F) (Table 2). These values are smaller than that in 1-fluorobenzo[c]phenanthrenes (ca. 8.2 Hz56) and might be due to the larger distance57,58 between H8 and F in 2-benzoylmethylenequinoline difluoroborates. On the other hand, this value is clearly higher than that of a coupling between methyl protons and fluorine in 2,6-diethylfluorobenzene (ca. 0.3 Hz59). The through-space couplings were previously reported for the studies of the conformational preferences of BODIPY dyes55,60,61 and can be observed only in molecules where the fluorine is in close proximity to another atom. According to calculations at the MP2/6-311+G(2d,2p) level the H8−F distance is 2.310 Å (average), while C8−F is 2.934 Å and C9−F is 2.998 Å. These values are in agreement with the X-ray structure (2.946 and 3.004 Å (average) for C8−F and C9−F in

subst 4-NMe2 4-OMe 4-Me 3-Me H 4-Br 3-Br 3-F 4-CF3

TS

J(1H8,19F) 2.21 2.23 2.19 2.23 2.36 2.28 2.27 2.26 2.19

TS

J(13C8,19F)

TS

J(13C9,19F)

7.77 7.69 7.81 7.81 7.81 7.94 7.81 7.56 7.81

a

Due to the limited resolution of the missing.

2.39 2.39 2.90 2.77 2.90 a 2.77 a 2.52

1

J(11B,19F) 18.95 18.47 18.23 18.39 18.23 18.07 18.15 18.15 18.07

13

C spectra, the couplings are

5, respectively, Supporting Information). Moreover, the resonances for 13C8 (123 ppm), 13C9 (139 ppm), and 11B (2.1 ppm) nuclei are triplets due to coupling to two equivalent (I = 1/2) fluorine nuclei (Chart 4 and Supporting Chart 4. Subspectra of 3-Me (6) and 4-CF3 (1) Derivativesa

a

See Chart 2 for atom numbers.

Information). In contrast, the resonance for the C2 carbon is only a singlet. This is understandable since nitrogen and oxygen atoms, which are on the coupling route, can weaken the scalar couplings. Table 1 collects the TSJ(13C,19F) and 1J(11B,19F) couplings. Except for 4-CF3 (1) and 3-F (2) derivatives the H8 signal is a doublet of triplets (Chart 4c). For 3-F derivative the doublet is observed, while for the 4-CF3 derivative the doublet of triplets is again split (Chart 4d) by the long-range effect of the 254



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fluorine atoms of the CF3 group. This suggests that the double splitting of H8 by fluorines in 1 (from BF2 and CF3 groups) is due to the through-space and long-range through-bond coupling at a time as observed before for J(19F,19F).62 The 1H, 13C, 15N, and 19F chemical shifts (Supporting Information) correlate with Hammett constants63 (except C3, C5, and C9, which are practically constant). The same is true for 1J(11B,19F) (R = 0.929, Supporting Information), while 11B chemical shifts do not show such a correlation. This gives the basis to conclude that the substituent effect is transmitted to the F−B bond through-space, which is quite rare.64,65 This effect may be explained by the fact that the H8···F hydrogen bond is formed by weakly (C−H) and highly (F−B) polarized moieties. The 1J(11B,19F) coupling constants in currently studied molecules are larger than that in BF 3 ·NMe 3 (1J(11B,19F) = 12.6 Hz66) but smaller than in BODIPY dyes (1J(11B,19F) = 31−33 Hz61,67). The currently observed TS 13 J( C8,19F) values are larger than in BODIPY derivatives TS 13 19 ( J( C, F) = ca. 2 Hz)61 but the TSJ(13C9,19F) couplings are very close. It is worth noting that the geometries of BODIPY dyes and the currently studied molecules are different; i.e., the six-membered heterocyclic ring instead of five-membered ring (BODIPYs) is present. This results in different interatomic distances and angles.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS Our NMR spectral and X-ray structural observations on nine variously substituted 2-benzoylmethylenequinoline difluoroborates suggest a covalent character for the B−O bond and a dative one for B → N. A detailed list of these observations is (a) the X-ray structural B−N/O interatomic distances, (b) properties of bond critical points (BCPs) in the NBF2O moiety, (c) 15N and 13C NMR chemical shifts, and (d) the double character and single character of the C11−C12 and C12−O13 bonds, respectively. The prevalence of a through-space coupling mechanism between 19F and C8 and C9 is obvious because 3 19 13 J( F, C8) and 3J(19F,13C9) spin−spin couplings are resolved whereas 3J(19F,13C2) and 3J(19F,13C12) are not visible. The higher TSJ(19F,13C8) than TSJ(19F,13C9) can be explained via the the different bond angles B−N−C8 vs B−N−C9. In summary, we present a novel data set suggesting that compounds carrying the NBF2O structural fragment exist preferably as an alkoxyimine form in the solid state and in solution having all bonds of the boron atom polarized. Except for the 13C NMR chemical shifts of C3, C9, and C10 and that of 11B which are practically constant, all other 1H, 13C, 15N, and 19F NMR shifts correlate with the substituent character of the phenyl ring with R = 0.91−0.99. These data have revealed the general picture of the ground state of these fluorescent molecules and will hopefully help in the interpretation of the results from photochemical measurements, which are at the moment under progress.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Detailed information on the single crystal X-ray structure analysis and NMR spectral measurements (NMR parameters, comment on 15N chemical shifts and correlation charts), computations (Cartesians, properties of BCPs, chosen interatomic distances), melting points, and reaction yields. This material is available free of charge via the Internet at http://pubs.acs.org. 255



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Substituent Effect in 2-Benzoylmethylenequinoline Difluoroborates

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