The Boat Conformation in Pyrazaboles. A ... - ACS Publications

The Boat Conformation in Pyrazaboles. A Theoretical and Experimental Study† Emma Cavero,# Raquel Giménez,*,# Santiago Uriel,‡ Eduardo Beltrán,# José Luis Serrano,# Ibon Alkorta,*,§ and José Elguero§

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 838–847

Departamento de Química Orgánica y Química Física, Área de Química Orgánica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragón, UniVersidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, Centro Politécnico Superior-Instituto de Ciencia de Materiales de Aragón, UniVersidad de Zaragoza-CSIC, 50015 Zaragoza, Spain, and Instituto de Química Médica, CSIC, C/ Juan de la CierVa, 3, 28006 Madrid, Spain ReceiVed March 26, 2007; ReVised Manuscript ReceiVed NoVember 19, 2007

ABSTRACT: Experimental data are combined with theoretical calculations to study the stability of the boat conformation of the pyrazabole ring. For the experimental studies, new BH2- and BF2-pyrazaboles disubstituted with iodo or ethynyl groups at the 2and 6-positions have been prepared and structurally characterized. We have found different molecular structures and crystal packings depending on the substituents. The iodo derivatives have a bent molecular shape due to the boat conformation of the pyrazabole ring, and the molecules are arranged in stacks with the same conformation (all-up or all-down boat conformation) along the crystallographic b axis. Stacks of molecules with the same conformation interact in a plane by means of iodo-iodo short contacts. However, the ethynyl derivatives are formed by bent-shaped molecules for BH2 and planar structures for BF2. Density functional theory and ab initio calculations have been performed on these compounds and on unsubstituted analogues to understand the effect of substituents on the conformation, inversion barrier, and NMR chemical shifts. The results of these calculations were compared with the experimental results. Theoretically, the boat conformation is an energy minimum, with the planar and chair conformations as transition states in an energy diagram. In solution, the compounds are in a boat conformation (with a boat-to-boat dynamic equilibrium) irrespective of their crystal structures. Introduction Boron-nitrogen adduct bonds give rise to sophisticated molecular architectures, and their directionality has evolved as an ideal tool for coordination chemistry, crystal engineering, and supramolecular chemistry.1 One example is the structure of the pyrazabole ring, a heterocycle that results from the dimerization of two pyrazoles by a double BH2 bridge. Pyrazabole has been known since 1966 (Figure 1, compound 1), when Trofimenko reported it contemporaneously with the poly(pyrazolyl)borate ions and their free acids.2a,b Many derivatives of pyrazabole exist, with forms substituted both at the boron centers and at the carbon atoms. A survey of the Chemical Abstracts between 1987 and 2006 shows that 41 documents (publications and patents) concern pyrazaboles, far fewer than those concerning polypyrazolylborates2c,d (also called scorpionates, with nearly 1 000 references in the same period). The most significant publications on pyrazaboles include a review by Niedenzu3 with X-ray structure determinations, a publication concerning NMR properties in the solid state,4 and another reporting some ab initio calculations.5 Pyrazaboles crystallize mainly in the boat conformation for the B2N4 ring. However, in some cases these systems have been found with chair and planar forms, but a relationship has yet to be found between the nature of the substituents at the boron or at the pyrazole ring and the ring conformation (Figure 2). Serwatowski et al.4 reported that 4,4,8,8-tetraethylpyrazabole (Figure 1, R1 ) H, R2 ) C2H5) displays a phase transition in †

Dedicated to Dr. Swiatoslaw Trofimenko, an outstanding chemist. * To whom correspondence should be addressed. (R.G.) Phone: +34-976762277. Fax: +34-976-762686. E-mail: [email protected]. (I.A.) Phone: +3491-5622900Ext 411. E-mail: [email protected]. # Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragón. ‡ Centro Politécnico Superior-Instituto de Ciencia de Materiales de Aragón. § Instituto de Química Médica.

the solid state, which corresponds to the rotation of the ethyl groups attached to boron and to a boat-boat B2N4 ring inversion. Differential scanning calorimetry (DSC) measurements provided evidence that the barrier is around 28.6 kJ mol-1. Alvarado et al.5 calculated [B3LYP/6-31+G(d,p)] the geometry of pyrazabole (1) itself to have C2V symmetry restrictions and found good agreement with the experimental X-ray geometry reported by Niedenzu et al.6 Apart from these structural studies, pyrazaboles have been included quite recently as part of materials with diverse functionalities like those of luminescent polymers, which contain the 4,4,8,8-tetraethylpyrazabole ring,7 as bridges in ferrocene derivatives with redox properties8 and as the core of liquid crystals.9 In the latter case, our group has observed that the molecules have a bent shape due to the boat conformation of the B2N4 ring9b,c and display mesomorphic behavior at room temperature.9a,b The boat conformation of the pyrazabole ring is of particular interest as the molecules in the boat conformation possess a dipole moment in the bent direction, which is appropriate for the design of new polar materials. As part of our research in materials, we are interested in studying the stability of the boat conformation. For this reason, we have synthesized new BH2- and BF2-pyrazaboles substituted with iodo or ethynyl groups due to the relatively easy postfunctionalization (Figure 1, compounds 3–6) and studied their crystal structures. Moreover, the dipole moment should increase in the bent direction in the BF2 derivatives with respect to the BH2 analogues. We also studied their solution structures and compared these with the results of theoretical calculations (density functional theory (DFT) and ab initio methods) to understand the role that the substituents have on the conformation, inversion barrier, and NMR shifts. For com-

10.1021/cg0702939 CCC: $40.75  2008 American Chemical Society Published on Web 02/16/2008

Theoretical and Experimental Study of Pyrazaboles

Figure 1. Structures of the pyrazaboles studied in this work.

parative purposes, we have also included the calculations for the related unsubstituted compounds 1 and 2. Experimental Section Materials and Methods. All reagents and solvents were commercially available from Aldrich. Boranes (borane trimethylamine and boron trifluoride etherate) were handled under an argon atmosphere and dissolved in dry solvents. Caution: Dihydrogen and HF are evolved in these reactions! FT-IR spectra were recorded on a Nicolet Avatar FTIR spectrophotometer using KBr pellets. 1H, 13C, 19F, and 11B NMR spectra were recorded on a Bruker Avance 400 spectrometer (9.4 T, 400.13 MHz for 1H, 100.62 MHz for 13C, 376 MHz for 19F and 128 MHz for 11B) on samples in CDCl3 or in toluene-d8. Melting points were measured with a Stuart melting point apparatus. Elemental analyses were performed with a Perkin-Elmer 240 analyzer. Crystal Structure Determination. X-ray quality single crystals were obtained by slow evaporation of solutions of the compounds in toluene (compounds 3, 5, 6) or hexane (compound 4). A summary of crystal data and refinement parameters is given in Table 1. Diffraction data of 3, 4, and 5 were collected on a Siemens P4 diffractometer at 295 K, and data of 6 were collected on an Oxford Diffraction Xcalibur diffractometer at 295 K and 150 K. Graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) was used, and structures were solved by direct methods. All non-hydrogen atoms were refined anisotropically by the full-matrix least-squares method on F2. The hydrogen atoms were generated geometrically. Solution and refinement were performed with the software package Bruker SHELXTL.10 Theoretical Calculations. The geometries of the structures were optimized at the B3LYP/6-31G*, B3LYP/6-311+G**, and MP2/631G* computational levels11–14 within the Gaussian-03 package.15 In the case of iodine derivatives, the DGDZVP basis set was used for the iodine atoms and the 6-311+G** was used for the remaining ones.16 Frequency calculations at the corresponding computational level were carried out to evaluate the minimum or TS nature of the geometries obtained. The absolute chemical shieldings were evaluated at the B3LYP/6-311+G**//B3LYP/6-311+G** computational level (B3LYP/ DGDZVP in the case of compounds 3 and 4) with the GIAO method.17 The effect of water as a solvent on the relative stability of the conformation was carried out using the PCM method.18 Preparation of the Compounds. 2,6-Diiodopyrazabole (3). A solution of 4-iodopyrazole (10 g, 52 mmol) and borane trimethylamine complex (3.7 g, 52 mmol) in dry toluene (100 mL) was heated under reflux for 3 h. The solvent was removed and the product was purified by column chromatography on silica gel (35–70 µm) using hexane/ dichloromethane 7/3 as the eluent. White solid. Yield: 62%. mp 171 °C. Anal. Calcd. for C6H8B2I2N4: C 17.51; H 1.96; N 13.61. Found: C 17.53; H 1.98; N 13.67. NMR (Table 2). IR (Table 3). 4,4,8,8-Tetrafluoro-2,6-diiodopyrazabole (4). Boron trifluoride etherate (11 mL, 87.5 mmol) was added dropwise to a flame-dried flask containing a solution of 2,6-diiodopyrazabole (3) (720 mg, 1.75 mmol) in dry THF (20 mL). The mixture was heated under reflux for 20 h. The reaction mixture was poured into a solution of saturated sodium bicarbonate (50 mL). Dichloromethane (50 mL) was added and the organic layer was extracted, washed with water, dried over anhydrous magnesium sulfate, and evaporated to dryness. The solid was purified by recrystallization from ethanol. Yield: 13%. White solid. mp 227 °C (dec). Anal. Calcd. for C6H4B2F4I2N4: C 14.90; H 0.83; N 11.59. Found: C 14.83; H 0.89; N 11.57. NMR (Table 2). IR (Table 3). 2,6-Bis(ethynyl)pyrazabole (5). A Schlenk tube containing 2,6diiodopyrazabole (3) (2.22 g, 5.42 mmol), dichlorobis(triphenylphosphine)palladium(II) (0.038 g, 5.42 × 10-5 mol), copper(I) iodide (0.021 g, 1.08 × 10-4 mol), and dry THF (100 mL) was degassed on a vacuum

Crystal Growth & Design, Vol. 8, No. 3, 2008 839 line by repeatedly alternating between an argon atmosphere and vacuum. Diisopropylamine (10 mL) was then added. Finally, a solution of trimethylsilylacetylene (1.16 g, 11.4 mmol) was added dropwise, and the reaction mixture was stirred at room temperature for 24 h. The reaction mixture was then filtered though a pad of Celite, and ethyl acetate (30 mL) was used to wash any additional product through the Celite pad. The filtrate was then evaporated to dryness. The resulting oil was dissolved in THF (50 mL), and tetrabutylammonium fluoride was added (TBAF 3H2O, 2.9 g, 10.84 mmol). The mixture was stirred for 1 h and poured into a mixture of diethyl ether/water (2/1), and the organic layer was extracted, dried over anhydrous magnesium sulfate, and evaporated to dryness. The product was purified by column chromatography on silica gel (35–70 µm) using hexane/dichloromethane 9/1 as the eluent and by recrystallization from hexane. Yield 58%. White solid. mp 204 °C (dec). Anal. Calcd. for C10H10B2N4: C 57.79; H 4.85; N 26.96. Found: C 57.78; H 4.81; N 26.96. NMR (Table 2). IR (Table 3). 4,4,8,8-Tetrafluoro-2,6-bis(ethynyl)pyrazabole (6). This compound was prepared by following the procedure described for 5, starting from 4,4,8,8-tetrafluoro-2,6-diiodopyrazabole (3). The product was purified by column chromatography on silica gel (35–70 µm) using hexane/ dichloromethane 4/1 as the eluent and by recrystallization from toluene. Yield: 30%. White solid. mp 227 °C (dec). Anal. Calcd. for C10H6B2F4N4: C 42.93; H 2.16; N 20.02. Found: C 42.89; H 2.10; N 19.98. NMR (Table 2). IR (Table 3).

Results and Discussion Synthesis and Characterization. The synthetic pathways followed in this work are shown in Scheme 1. 2,6-Diiodopyrazabole (3) was prepared by the reaction of 4-iodopyrazole with borane trimethylamine complex as described in the literature.1b The BF2 derivative 4 was synthesized from 3 by a substitution reaction with boron trifluoride etherate in refluxing THF, by adapting a procedure described previously19 to overcome the formation of the 4,4-difluoro-2,6-diiodopyrazabole intermediate. Compounds 5 and 6 were obtained by a cross-coupling reaction between compounds 3 or 4 with trimethylsilylacetylene. We have observed that these compounds are stable under Sonogashira coupling conditions and show similar reactivity to pyrazoles.20 The resulting trimethylsilylacetylene-substituted rings were not isolated but deprotected with tetrabutylammonium fluoride (TBAF) to yield compounds 5 and 6. The two steps, that is, coupling and deprotection, gave an overall isolated yield of 30–60%. NMR data for the different nuclei from chloroform solutions are gathered in Table 2. Only a singlet is observed in the aromatic region of the 1H NMR spectra as the compounds are highly symmetrical. This signal appears at lower fields in the BF2 derivatives (ca. 0.4 ppm) than in the BH2 analogues as a consequence of the stronger electron-withdrawing character of the fluoro-substituent. This effect is also detected for the acetylenic proton of compounds 5 and 6, albeit to a lesser extent (0.2 ppm). Protons bound to boron appear as a distorted quadruplet in the 3–4 ppm region due to coupling to 11B (natural abundance 80.42%, I ) 3/2). The signal sharpens considerably and appears as a singlet in the 1H{11B} decoupling experiments, in which coupling to 10B (natural abundance 19.58%, I ) 3) is not observed. The appearance of the protons bound to boron changes with temperature (Figure 3). For example, with compound 5 at high temperatures (373 K, toluene-d8) the signal appears as a distorted quadruplet that changes gradually to a broad doublet (293 K, chloroform-d) and collapses to a broad singlet below 273 K (Figure 3). In an effort to estimate 1J(11B, 1 H), we considered that the signal is a partially relaxed quadruplet in which the outer lines move toward the inner lines, which remain almost constant in their position until the splitting collapses, as reported by Claramunt et al.21 The distance between

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Figure 2. Optimized structures of the three conformations of 2. Left: boat conformation. Middle: chair conformation. Right: planar conformation. Table 1. Crystal Data and Refinement for Compounds 3–6 compound chem formula crystal size, mm3 crystal system space group formula weight T, K θ range data collect, ° a, Å b, Å c, Å R, ° β, ° γ, ° Z F(000) V, Å3 Dcalc, g cm-3 µ, mm-1 collected reflns unique reflns data/restraints/params final R indices [I > 2σ(I)] R indices (all data)

3 C6H8B2I2N4 0.23 × 0.19 × 0.16 monoclinic C2/c 411.58 295 3.01–27.49 23.493(3) 4.4270(10) 13.654(2) 90.00 117.810(10) 90.00 4 752 1256.0(4) 2.177 4.976 1913 1423 (Rint ) 0.0785) 1423/0/81 R1 ) 0.0444, wR2 ) 0.1128 R1 ) 0.0650, wR2 ) 0.1432

4 C6H4B2F4I2N4 0.14 × 0.12 × 0.08 monoclinic C2/c 483.55 295 1.95–26.36 24.106(9) 4.717(2) 13.759(5) 90.00 119.86(2) 90.00 4 880 1356.8(9) 2.367 4.667 1848 1380 (Rint )0.0524) 1380/0/90 R1 ) 0.0583, wR2 ) 0.1592 R1 ) 0.0645, wR2 ) 0.1700

5

6

C10H10B2N4 0.21 × 0.19 × 0.12 monoclinic P2(1)/m 207.84 295 2.42–25.99 4.2370(10) 15.905(3) 8.4140(10) 90.00 92.020(10) 90.00 2 216 566.66(18) 1.218 0.075 1314 1157 (Rint ) 0.0188) 1157/0/100 R1 ) 0.0414, wR2 ) 0.0952 R1 ) 0.0655, wR2 ) 1091

6

C10H6B2F4N4 0.44 × 0.25 × 0.19 monoclinic C2/m 279.81 295 2.84–27.46 10.974(3) 9.604(4) 5.5258(10) 90.00 100.205(18) 90.00 2 280 573.2(3) 1.621 0.145 5818 691 (Rint ) 0.0178) 691/0/58 R1 ) 0.0281, wR2 ) 0.0800 R1 ) 0.0331, wR2 ) 0.0825

C10H6B2F4N4 0.44 × 0.25 × 0.19 monoclinic C2/m 279.81 150 2.86–26.37 10.9005(18) 9.5590(19) 5.461(3) 90.00 101.39(3) 90.00 2 280 557.8(4) 1.666 0.149 2970 605 (Rint ) 0.0183) 605/0/58 R1 ) 0.0251, wR2 ) 0.0714 R1 ) 0.0283, wR2 ) 0.0731

Table 2. NMR Data at r.t.a compound

a

1

13

H NMR

3

7.65 (pz-3,5)

4

8.04 (pz-3,5)

5

7.73 (pz-3,5) 3.01 (C≡CH)

6

8.14 (pz-3,5) 3.20 (C≡CH)

C NMR

139.8 (pz-3,5) 55.7 (pz-4) 140.5 (pz-3,5) 58.4 (pz-4) 138.1 (pz-3,5) 102.6 (pz-4) 79.3 (C≡CH) 138.5 (pz-3,5) 105.0 (pz-4) 81.4 (C≡CH) 71.2 (C≡CH)

11

B NMR

-8.5 (br s) -0.4 [t, J(11B,19F) ) 20] -8.6 (br s) -0.3 [t, 1J(11B,19F) ) 18]

1

H or

19

F NMR of BR2

3.5 [q, 1J(11B,1H) ) 130] 149.0 [q, 1J(11B,19F) ) 20] 3.5 [q, 1J(11B,1H) ) 130] 149.9 [q, 1J(11B,19F) ) 18]

δ, ppm; J, Hz. Table 3. IR Data (Wavenumbers, cm-1)

compound H-C≡C 3 4 5 6

3300 3297

arC-H 3135, 3125 3144, 3134 3134 3135

B-R2

C≡C

pyrazabole ring

2457, 2421 1664, 1509, 1380 1149 1720, 1530, 1388 2446 2119 1698, 1550, 1373 1136 2127 1739, 1569, 1388, 1365

the inner lines was taken as the coupling constant and gave a value of 1J(11B, 1H) ) 130 Hz. In the decoupled experiments, we did not observe a 2J(1H, 1H) coupling at any temperature; this coupling would be expected to be about 4.5 Hz.22 Therefore, the two protons bound to boron are isocronous or equivalent on the NMR time scale due to a fast equilibrium in solution. With respect to the 13C NMR spectra, it is worth noting that the carbon atoms at the 2- and 6-positions appear highly shielded in the iodo derivatives (55–60 ppm) compared to the ethynyl

derivatives (102–105 ppm). The effect of the fluoro-substitution is not as marked as in the 1H NMR spectra. In the 11B NMR spectra of the BH2 derivatives a broad singlet centered at ca. -8.5 ppm is observed. However, a broad triplet is seen in the BF2 derivatives at ca. -0.4 ppm, with a coupling constant 1J(11B, 19F) of 15–20 Hz. Accordingly, in the 19F NMR spectra of compounds 4 and 6 an almost ideal quartet with a similar coupling constant is found due to coupling to 11B. This indicates that the two fluorine nuclei in the BF2 group are equivalent on the NMR time scale. The FTIR spectra show several B-H stretching bands at 2450 cm-1 (Table 3), and B-F stretching bands have been assigned in the 1150–1136 cm-1 region. Csp2-H stretching bands corresponding to the pyrazabole ring are also observed, in this case as a doublet at 3135 cm-1 for the iodo-substituted compounds or as a single band for the ethynyl compounds. In


Crystal Growth & Design, Vol. 8, No. 3, 2008 841

Scheme 1. Synthetic Pathway to the Substituted Pyrazaboles

these last two derivatives the bands corresponding to the triple bond and terminal C-H of the ethynyl substituent are clearly detected. Crystal Structures Description of the Structures and Packing. X-ray quality single crystals were obtained for all the new compounds 3–6. All of them crystallize in the monoclinic system (collection data and parameters are gathered in Table 1) and display differences depending on the substituents R1 and R2 (Figure 4). Iodo-substituted compounds 3 and 4 crystallize in the same space group. Both molecular structures are bent in shape due to the boat conformation of the pyrazabole ring, and have a binary symmetry axis in the bent direction. The boron atoms are in a distorted tetrahedral coordination with an N-B-N angle of 104.0(6)° for compound 3, and an N-B-N of 104.1(4)° and F-B-F angle of 112.5(5)° for compound 4. This means that one R2 substituent is in a pseudoaxial position and the other in a pseudoequatorial position. The mean planes that contain each pyrazole ring (C3N2 rings) are arranged at an angle of 129.1° in compound 3 and 127.2° in compound 4. Therefore, the BF2 compound displays a more marked bent-shape than the BH2 derivative. Compound 3 stacks along the b axis in such a way that each stack consists of molecules with the same bent conformation.

Figure 3. Temperature-dependent 1H NMR spectra (a) and 1H{11B} NMR spectra (b) of compound 5. All spectra were measured in CDCl3 solutions - except for the 373 K spectrum, which was measured in a toluene-d8 solution.

These stacks interact with neighboring ones in a plane by means of iodine-iodine short contacts. In this way planes of molecules with the same boat geometry are formed. Iodo-iodo distances are 3.96 Å, and the angle formed by three interacting iodines is 68° (Figure 5). Compound 4 also packs in a similar way, with iodo-iodo short contacts between neighboring stacks of molecules forming planes of molecules with the same boat conformation (Figure 6). However, the presence of fluoro-substituents also gives rise to C-H · · · F interactions between the equatorial F atom of one molecule and the C-H of the pyrazabole ring of a neighboring plane of molecules, thus generating a network with small voids. These short contacts were measured as F · · · H 2.49(8) Å and F · · · C 3.252(7) Å. The F · · · H distance found is 0.28 Å less than the sum of H and F van der Waals radii, and slightly larger than the reported for (Z)-1,3-difluorobuta-1,3-diene, where a F-H distance of 2.40 Å has been attributed to a hydrogen bond.23 Compound 5 adopts a bent-shape due to the boat conformation of the B2N4 central ring. The boron atoms exhibit a distorted tetrahedral coordination with an N-B-N angle of 104.72(19)°, similar to compound 3. The mean plane containing the two pyrazole rings (C3N2 rings) form an angle of 134°, larger than in compounds 3 and 4. Molecules with the same boat geometry are stacked along the a axis (Figure 7) in parallel planes. The molecules that are situated in the same plane display the same boat geometry (all-up or all-down), and planes of opposite geometry are intercalated. Structure of compound 6 has been registered at 295 K and 150 K. In both cases we have found that compound 6 crystallizes in the C2/m spatial group, displaying slight differences in bond lengths and angles between the two sets of data. Surprisingly, this molecule exhibits a planar conformation in which all the atoms of the pyrazabole ring are situated in the same plane, and the fluoro-substituents are arranged above or below that plane, giving rise to a very symmetrical molecule. The boron atoms are in a nearly tetrahedral coordination, with an N-B-N angle of 107.86(12)° and F-B-F of 110.60(14)°. The molecular packing is also different from the rest of the compounds studied as these molecules do not stack but arrange in planes consisting of interdigitated molecules (Figure 8). We did not observe short contacts involving the fluoro-substituents in this case. Comparison of Structures. All structures studied display the boat conformation except for compound 6, which crystallizes in a planar conformation. For comparative purposes, representative averaged distances and angles for compounds 3 to 6 are shown schematically in Figure 9a. The boron atom has a distorted tetrahedral coordination in the bent structures 3–5, with the R2-B-R2 angles larger than the N-B-N angles. However, in the planar structure of compound 6 these angles are closer to an ideal tetrahedral coordination. It is interesting to note a relationship between the N-B-N angle and the conformation of the B2N4 ring. In the boat compounds the angle is around 104°, whereas in the planar compound it is 108°, closer to the ideal tetrahedron. We also observed slight differences in the sizes of the rings (pyrazole rings and B2N4 ring) depending on the substituents


Cavero et al.

Figure 4. ORTEP molecular drawings for the new compounds solved at 295 K. Selected bond distances (Å) and angles (°): 3: N(1)-B(1) 1.596(10), N(1)-N(2) 1.374(8), N(1)-N(2)-B(1) 120.7(6), N(2)-B(1)-N(1a) 104.0(6). 4: N(1)-B(1) 1.614(8), N(1)-N(2) 1.380(6), B(1)-F(1) 1.379(8), N(2)-N(1)-B(1) 120.4(5), N(2a)-B(1)-N(1) 104.1(4), F(1)-B(1)-F(2) 112.5(5). 5: N(1)-B(1) 1.570(2), N(1)-N(2) 1.3666(19), N(2)-N(1)-B(1) 120.73(16), N(1)-B(1)-N(1a) 104.72(19). 6: N(1)-B(1) 1.5579(13), N(1)-N(1a) 1.3553(18), F(1)-B(1) 1.3651(12), N(1a)-N(1)-B(1) 126.07(6), N(1b)-B(1)-N(1) 107.86(12), F(1)-B(1)-F(1b) 110.60(14).

Figure 5. Packing of compound 3 showing the I · · · I interactions.

at the pyrazole ring (R1) and at the boron atom (R2) by comparing the bond distances. In general, the iodo-substituted compounds have larger bond distances than other pyrazaboles. The replacement of hydrogen by fluorine (cf. compounds 3 and 4) gives rise to a slight increase in the size of the rings, but the replacement of iodine by ethynyl (cf. compounds 3 and 5) gives rise to a slight decrease in the ring sizes. Moreover, for the BF2 compounds the rings are smaller in the planar compound (6) than in the bent compound (4). In a literature survey we found that pyrazaboles mainly crystallize with the B2N4 ring in the boat conformation. However, depending on the substituent, planar or chair forms have also been described. We selected for comparative purposes

the structures of four analogues, a BH2 compound 1,6 a BF2 compound 2,19 and similar BCl224 and BBr26 compounds. Some relevant data for these structures are shown in Figure 9b. The first two structures are bent with the angle increasing from 134.7° to 137.9° on going from BH2 to BF2; the other two compounds are planar. It can be seen that an increase in the size of the substituent at the boron (H < F < Cl < Br) tends to open the angle and flatten the structure, probably due to steric effects of the groups in axial position. To release these effects an opening of the N-B-N and closing of R2-B-R2 takes place. The relationship between the N-B-N angle and the ring conformation observed with the previous structures (3–6) is also applicable, being closer to the ideal tetrahedral coordination in



Figure 6. Packing of compound 4 showing the CH · · · F interactions.

Figure 7. Packing of compound 5.

Figure 8. Packing of compound 6.

the planar structures. It is also interesting to note that the C-C, C-N, and N-N bond distances in compounds 1 and 2 are shorter than in the iodo-containing compounds 3 and 4. Therefore, the presence and nature of substituents in the 2- and 6-positions of the pyrazabole ring also influence the conformation.

For compounds 3 and 4, an increase in the size of the structure due to the presence of iodo-substituents means that the bent geometry of the molecule can be stable, even with fluorosubstituents, without increasing the angle. Indeed, a reduction of the bent angle takes place on going from 3 to 4, which is an


Cavero et al.

Figure 9. Averaged bond distances (Å) and angles (°) of pyrazaboles. (a) Crystallized in this work. (b) Previously described in the literature.

indication that packing effects could be more important that molecular electronic or steric effects in the solid state. Theoretical Calculations Geometries and Barriers. The molecules shown in Figure 1 have been studied in three conformations: boat, planar, and chair (see Figure 2 for an example of each conformation). At all the computational levels, both in vacuo and in the water solution model and for all the molecules, the boat conformation is the minimum, the chair is the transition state connecting the two possible boats, and the planar conformation (very close in energy to the chair) is a second -order saddle-point transition state that connects the two chair conformations. The energy differences between the three conformations considered are small, as can be seen in Table 4. The angles formed by the pyrazole rings in the boat conformation are gathered in Table 5. In all cases, the fluoro derivatives show larger angles (more planar) than the corresponding hydrogenated counterparts in all the theoretical calculations. The experimental data for R1 ) H are well reproduced by the calculations. Note that the stacking effect of the experimental R1 ) I (the pyrazabole systems are stacked in the X-ray structures) could affect the angle between the pyrazole rings. The calculated values are different from the experimental ones (Table 5), not only for compound 6 but also for the relative angles of the remaining five compounds, for which a clear relationship is not observed between calculated and experimental values.

Table 4. Relative Energies (kJ mol-1) of the Three Conformations Considered compound

conformation

1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6

boat planar chair boat planar chair boata planara chaira boata planara chaira boat planar chair boat planar chair

a

B3LYP/ 6-31G*

B3LYP/ 6-311+G**

MP2/ 6-31G*

0.0 22.6 22.0 0.0 17.0 15.9

0.0 22.4 21.0 0.0 9.3 9.1 0.0 23.2 21.8 0.0 9.9 9.7 0.0 23.3 21.8 0.0 10.4 10.1

0.0 31.8 28.0 0.0 21.6 18.3

0.0 23.4 22.7 0.0 18.4 17.0

The DGDZVP basis set has been used for the iodine atoms.

The inclusion of water as a solvent has a small effect on the relative energies of the three conformations considered (Table 6). The dipole moments of the boat conformation are gathered in Table 5. Because of the symmetry of the conformation, the only component of the dipole moment that is not zero corresponds to the symmetry axis along the bent direction. In all



Planar: Erel(B3LYP/6-311+G**) )

Table 5. Angle between the Pyrazole Rings in the Boat Conformation B3LYP/ 6-31G* compound 1 2 3a 4a 5 6 a

B3LYP/ 6-311+G**

angle (°)

µ (D)

134.7 138.0

2.83 3.86

134.4 136.9

2.34 3.60

angle (°)

µ (D)

136.1 138.0 134.8 143.3 135.8 143.7

2.65 3.83 1.68 3.23 2.06 3.56

MP2/ 6-31G*

22.3 + 0.55R1 - 3.3R2, n ) 8, r2 ) 1.000

experimental

angle (°)

µ (D)

129.1 134.8

2.99 4.10

Chair: Erel(B3LYP/6-311+G**) ) 21.0 + 0.4R1 - 2.9R2, n ) 8, r2 ) 1.000

angle (°)

21.3 + 0.53R1(I) - 2.9R2(I), n ) 8, r2 ) 0.991

The DGDZVP basis set has been used for the iodine atoms.

Table 6. Relative Energies (kJ mol-1) of the Three Configurations in the Water Solvent Model at the B3LYP/6-31G* Computational Level conformation

PCM-B3LYP/6-31G*

5 5 5 6 6 6

boat planar chair boat planar chair

0.0 25.0 24.0 0.0 17.4 17.0

cases, the BF2 derivatives show larger dipole moments (1.2 D larger on average) than the corresponding BH2 compounds. The Erel values can be analyzed using a Free-Wilson matrix.25 For R1 the Erel value is 0 when the substituent is H and 2 when it is iodo or ethynyl (as there are two substituents on the 2- and 6-position of the pyrazabole); and for R2, 0 for H and 4 for F (there are four fluorine atoms on boron). The results are as follows: Planar + chair: Erel(B3LYP/6-311+G**) ) 21.6 + 0.48R1 3.1R2, n ) 8, r2 ) 0.993

(3)

Planar + chair: Erel(B3LYP/DGDZVP) )

134.7 137.9 129.1 127.2 134.0 180

compound

(2)

(1)

(4)

The ethynyl- and fluoro-substituents cannot be compared, but clearly the effects on the conformation are more important on the boron (R2, positions 4 and 8 of the pyrazabole ring) than on the carbon (R1, positions 2 and 6). An ethynyl group stabilizes the boat by 0.5 kJ mol-1, while an F substituent destabilizes the boat by 3.1 kJ mol-1. If we consider that the chair is the transition state, then the inversion barrier is increased by 0.4 kJ mol-1 by each ethynyl group and decreased by 2.9 kJ mol-1 per fluorine atom. The effect of the iodo substituent is similar to that of the ethynyl one, that is, about 0.5 kJ mol-1. It is worth noting that the barrier for 1, corresponding to the chair, calculated at the MP2/6-31G* level (28.0 kJ mol-1, Table 4) agrees with the experiments reported by Serwatowski et al.4 in solid state by 13C CPMAS NMR for 4,4,8,8-tetraethylpyrazabole (Figure 1, R1 ) H, R2 ) C2H5). A representation of the situation for compound 1, as calculated at the MP2/ 6–31G* level, is provided in Figure 10, which shows the minima (boat), the TS (chair), and the planar conformation as a secondorder transition state (saddle point). Even the highest calculated barrier (28.0 kJ mol-1) is in the zone of rapid phenomena in DNMR. NMR Chemical Shifts. The calculated absolute chemical shielding values, σ, are shown in Table 7 (those corresponding to R1 ) H are reported in the Supporting Information). To compare the absolute shieldings shown in Table 7 with the experimental data in Table 2 we require the σ values of the reference compounds calculated at the same level (GIAO-

Table 7. Absolute Chemical Shieldings (σ, ppm) at the GIAO/B3LYP/6-311+G** Computational Levela

compound 3

compound 4

atom

boat

chair

planar

boat

chair

planar

exp 3

exp 4

1 B (2) 7 C (8,9,10) 13 H (14,15,16) average R2 (17,18,19,20)

-7.5 138.5 7.60 3.96

-7.3 139.0 7.59 4.10

-7.4 137.5 7.53 4.28

-1.5 139.4 7.91 149.4

-1.5 140.4 7.95 141.1

-1.5 140.0 7.94 140.2

-8.5 139.8 7.65 3.5

-0.4 140.5 8.04 149.0

compound 5

compound 6

atom

boat

chair

planar

boat

chair

planar

exp 5

exp 6

1 B (2) 7 C (8,9,10) 11 C (12) 13 H (14,15,16) average R2 (17,18,19,20) 21 C (24) 22 C (25) 23 H (26)

-7.6 138.2 103.9 7.76 3.97 72.7 80.0 2.70

-7.6 138.8 105.5 7.74 4.10 72.7 81.0 2.71

-7.5 137.2 105.5 7.65 4.30 73.2 81.9 2.69

-1.6 139.0 105.5 8.07 149.7 71.0 82.0 2.78

-1.6 140.1 106.8 8.08 141.9 71.0 82.8 2.78

-1.5 139.5 106.7 8.05 140.4 71.0 82.8 2.78

-8.6 138.1 102.6 7.73 3.5

-0.3 138.5 105.0 8.14 149.9 71.2 81.4 3.20

a

79.3 3.01

For compounds 3 and 4 (R1 ) I), the DGDZVP basis set has been used for the iodine atoms. Atom numbering is shown in the figure.


Cavero et al.

conformation with a rapid boat-to-boat inversion in the NMR time scale in agreement with the low calculated barriers. In the solid state, all BH2 compounds are bent in shape, but in the case of the BF2 compounds we obtained both bent and planar conformations depending on the substituents, indicating that substitution at the boron, stacking effects, and short contact interactions are decisive for the final molecular conformation.

Figure 10. Energy diagram for the conformations of pyrazabole 1 at the MP2/6-31G* level.

B3LYP/6-311++G**):26 1H NMR, TMS, δ ) 0.00 ppm, σ ) 31.97 ppm; 13C NMR, TMS, δ ) 0.00 ppm, σ ) 184.75 ppm; 11 B NMR, BF3OEt2, δ ) 0.00 ppm, σ ) 101.95 (we also considered BH4-, δ ) -38.0 ppm, σ ) 154.16 ppm); 19F NMR, CFCl3, δ ) 0.00 ppm, σ ) 153.70 ppm (we also considered BF3OEt2, δ ) -153.0 ppm, σ ) 334.39 ppm). Although the calculated σ values for the three conformations are rather similar (19F shows the largest differences), the correlation coefficients are always better using the absolute shieldings of the boat conformation (Table 7: r2 ) 0.9999, 0.9984, and 0.9980 for the boat, chair, and planar conformations, proving that the behavior in solution corresponds to this structure (we have averaged the calculated σ values corresponding to the boat-to-boat inversion). 1

H NMR, δ (ppm) ) 31.8 - 1.00σ (ppm), n ) 9, r2 ) 0.989 (5) C NMR, δ (ppm) ) 175.4 - 0.95σ (ppm), n ) 11,

13

r2 ) 1.000

(6)

B NMR, δ (ppm) ) 76.7 - 0.75σ (ppm), n ) 6,

11

r2 ) 0.995

(7)

F NMR, δ (ppm) ) -129.9 + 0.85σ (ppm), n ) 4,

19

r2 ) 1.000 (8) Thus, the comparison of calculated absolute shielding with experimental chemical shifts pointed out a rapid boat-to-boat inversion in the NMR time scale. Conclusion Theoretical calculations show that all the pyrazaboles studied have a boat conformation as an energy minimum, with the chair and planar conformations as transition states of different order in the energy diagram of Figure 10. The boat conformation exhibits a molecular dipolar moment in the direction of the bend angle; therefore, these molecules could be of interest in the preparation of new bent polar materials. Future work will be directed in this line of research. The calculations performed also indicate that carbon substitution at positions 2 and 6 stabilizes the boat conformation. Analysis of the calculated NMR chemical shifts and comparison with the experimental NMR data (Figure 3) lead to the conclusion that in solution all compounds are in a boat

Acknowledgment. We thank the Ministerio de Educación y Ciencia and FEDER (projects MAT2003-07806-CO2-01, MAT2006-13571-CO2-01, programas Ramón y Cajal and FPU), Comunidad Autónoma de Madrid (Project MADRISOLAR, ref S-0505/PPQ/0225), and the Gobierno de Aragón for financial support. The authors thank Dr. P. Romero for assistance with the NMR experiments. Thanks are also given to the CTI (CSIC) for allocation of computer time. Supporting Information Available: Complete ref 15. Crystallographic information files (CIF) for compounds 3–6. Total energy of the structures calculated at the B3LYP/6-311+G** computational level. Cartesian coordinates and frequencies of the calculated structures at the B3LYP/6-311+G** computational level. σ values for Table 7 and δ values for compounds 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG0702939

The Boat Conformation in Pyrazaboles. A ... - ACS Publications

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