Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis, Reactions, and Structures of Heterocycle-Tethered Boranes and Their Precursors Vasudevan Subramaniyan and Ganesan Mani* Department of Chemistry, Indian Institute of Technology - Kharagpur, Kharagpur, West Bengal, India 721 302 S Supporting Information *
ABSTRACT: Heterocycle-tethered organoboranes generally dimerize by using the heteroatom present in them. When this does not take place, some interesting molecules or properties are bound to be observed. This is demonstrated in this work, as well. The lithium salt of 1,2-dimethylimidazole reacts with Et2B(OMe) or 9BBN-OMe to give the air-stable intermolecular adduct of 2(dialkylborylmethyl)-1-methylimidazole (alkyl = Et (1) and 9-BBN (2)) after treatment with BF3·OEt2. These two dimers exist in two stereoisomers: clockwise and counterclockwise, as shown by their X-ray structures. On the contrary, in the analogous reaction between the lithium salt of 1,3,5-trimethylpyrazole and Et2B(OMe) in the presence of BF3·OEt2, an air- and moisture-sensitive adduct, 1-(diethylborylmethyl)-3,5-dimethylpyrazole·BF2(OMe) (3), was isolated as a liquid in an excellent yield, instead of its dimer. Its structure is based on 1H, 19F, 11B, and HRMS methods and supported by anion binding studies. Consequently, the reactive neat liquid 3 decomposes in air to give the bicyclo[3.3.1]nonane-like neutral bicycle 4. Notably, a CH2Cl2 solution of 3 decomposes differently to give the OH-bridged cationic analogous bicycle 5. In addition, their precursors, [2(diethylborylmethyl)-1-methylimidazole·LiOMe(1,2-dimethylimidazole)]2 6, [(1-(diethylborylmethyl)-3,5-dimethylpyrazole· LiOMe)2THF] 7, and [(1-(diethylborylmethyl)-3,5-dimethylpyrazole·LiOMe(DMAP)2] 8, were also isolated and structurally characterized. In the crystal lattices of structures of 4, 5, and 7, both enantiomers RR and SS are present.
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INTRODUCTION Organoboranes containing heterocycles quench their Lewis acidity by forming either inter-1−3 or intramolecular4,5 adducts depending upon the steric hindrance of groups present in them. This has been demonstrated by Yalpani and co-workers by synthesizing a series of dialkylpyrazolylboranes. While the less sterically demanding group substituted pyrazolylboranes formed dimers,6,7 the bulky group substituted ones remained as monomers.8 This property has largely been used in developing an array of fluorescent compounds for optoelectronic applications.9 In addition, very remarkably, when the acidity of the boron atom is not quenched, wonderful frustrated Lewis pairs (FLP) are formed.10 Given all these, in general, the boron chemistry in recent times has centered around the task of tuning the Lewis acidity of the boron atom caused by its empty p orbital for different application purposes including sensing fluoride ions at parts per million levels.11,12 Bourissou and co-workers have reported the synthesis of (2picolyl)BCy2, which exists as a dimer in the solid state. However, the solution study showed that it is in a dynamic equilibrium with its monomer.13 Hoefelmeyer and co-workers have reported the reactive dimers of 2-(picolyl)boranes and their reactions to give products formed by the 1,2-addition of the B−C(picolyl) bond across the double bond in nitriles, ketones, aldehydes, and amides.14 Mitzel and co-workers have reported the synthesis of a series of 2-borylmethylpyridine derivatives. They found that they are monomers with the boron Lewis acidity quenched via intramolecular B−N bond © XXXX American Chemical Society
formations. In addition, conversely, they found the formation of the frustrated Lewis pairs when the pyridine ring contains tert-butyl and long chain boryl groups in the 2 and 6 positions of the pyridine.15 Very recently, Wang and co-workers have reported the synthesis of 2-bis(2,4,6-tris(trifluoromethyl)phenyl)borylmethylpyridine derivatives. They are monomers and have been found to be intramolecular FLPs.16 Sugihara et al. have reported the cyclic tetramer structure of 3(diethylboryl)pyridine.17 Following this, Wakabayashi and coworkers have reported the self-assembled structures of various dialkylborylpyridine derivatives.18−20 Herein, we report the synthesis and structure−property relationship of new dialkylborylmethylimidazole and -pyrazole derivatives. We also report the structure of their precursors and reactions to give new neutral and cationic bicyclo[3.3.1]nonane-like bicycles.
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RESULTS AND DISCUSSION Imidazole-Tethered Boranes. As shown in Scheme 1, treatment of Et2B(OMe) or 9-BBN-OMe with 1 equiv of the lithium salt of 1,2-dimethylimidazole generated in situ at low temperature followed by treatment with BF3·OEt2 afforded compound 1 or 2, which was isolated as a colorless crystalline solid in 32 and 53% yields, respectively. These are the intermolecular adducts of 2-(dialkylborylmethyl)-1-methylimidazole, and their low yields are attributed to the metalation that Received: October 14, 2017
A
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 1. Synthesis of 2-(Dialkylborylmethyl)-1methylimidazole Dimers
Figure 1. X-ray structure of 1: (a) clockwise and (b) counterclockwise stereoisomers. These isomers differ from each other by the direction in which two diethylborylmethylimidazole units are connected to form the dimer. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−B1 1.667(4), C6−B2 1.624(4), C8−B2 1.624(4), C10−B2 1.668(4), C15−B1 1.648(4), C17−B1 1.606(4), N2−B2 1.621(4), N4−B1 1.634(4), C2−C1−B1 116.3(2), C17−B1−N4 106.8(2), C17−B1−C15 113.1(2), N4−B1−C15 107.2(2), C17−B1−C1 111.1(2), N4−B1−C1 108.4(2), C15−B1− C1 109.9(2), N2−B2−C8 107.1(2), N2−B2−C6 109.0(2), C8−B2− C6 112.0(2), N2−B2−C10 108.4(2), C8−B2−C10 111.3(2), C6− B2−C10 109.0(2).
can also occur at the five position of the imidazole ring.21,22 These dimers are moderately air stable but decompose slowly in solution. The 1H NMR spectrum of 1 in CDCl3 showed broad resonances for the methylene protons, indicating the ring fluxionality of the molecule in solution. However, the two different imidazole CH protons appear as two doublets with a small coupling constant (1.8 Hz). Further, the 13C NMR spectrum showed seven signals including two broad signals due to the boron-bound methylene carbons and is consistent with the seven different carbon atoms in the structure. Conversely, the 1H NMR spectrum of 2 in CDCl3 featured an AB spin pattern for the diastereotopic methylene protons, in addition to two doublets for the imidazole CH protons. In contrast to 1, the 13C NMR spectrum showed a sharp signal for the boronbound methylene carbons. All these indicate the rigidity of the molecule in solution because of the steric hindrance of the 9BBN groups. Further, the tetracoordinated boron atoms in 1 and 2 are supported by their 11B NMR spectra displaying broad peaks at δ = −3.4 and −3.7 ppm, respectively. Furthermore, we also tested the robustness of the dimers at higher temperature. The 11B NMR spectrum of 1 in CDCl3 at 50 °C showed no change, indicating the rigidity of the boat shape conformation, which does not dissociate into its monomers at this elevated temperature. Compounds 1 and 2 crystallize in the monoclinic P21/c and the tetragonal P43 space groups, respectively. The asymmetric unit of 1 contains one molecule, whereas two molecules constitute the asymmetric unit of 2. Their X-ray structures given in Figure 1 and Figure 2 revealed that the dimer adopted the eight-membered boat shape conformation. Two such conformations are present in each of these crystal lattices. They are nonsuperimposable stereoisomers and differ from each other in their sequence in which the monomer units are linked via two B−N bonds to form the eight-membered ring. In one isomer, the two B−N bonds are in the clockwise arrangement and, in the other, in the counterclockwise fashion, as shown in Figure 1 and Figure 2. This is similar to the stereoisomers reported by Hoefelmeyer and co-workers.14 The geometry around each boron atom is a distorted tetrahedral geometry, with the ring inner angle formed by N, B, and C
Figure 2. X-ray structure of 2: (a) clockwise and (b) counterclockwise stereoisomers. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) for one of the molecules in the crystal lattice: C1−B1 1.617(6), C5−B1 1.628(6), C9−B1 1.676(6), C14−B2 1.624(6), C18−B2 1.633(7), C22−B2 1.673(6), N2−B2 1.626(6), N4−B1 1.631(5), C22−C23 1.477(5), C9−C10 1.469(6), C1−B1−C5 103.7(3), C1−B1−N4 112.4(3), C5−B1−N4 110.3(3), C1−B1−C9 110.5(3), C5−B1−C9 114.9(3), N4−B1−C9 105.2(3), C14−B2−N2 113.2(3), C14−B2−C18 103.3(3), N2−B2−C18 109.4(4), C14−B2−C22 110.5(4), N2−B2−C22 105.3(3), C18− B2−C22 115.4(4).
atoms being smaller (for example N4−B1−C1 = 108.4(2)°) than the outer angle formed by the ethyl carbon atoms (for example, C15−B1−C17 = 113.1(2)°). In addition, for both molecules, the ring C−B bond distances are longer (1.667(6) to 1.686(6) Å) than the outer alkyl C−B distances (1.606(4) to 1.648(4) Å). The angle between the two imidazole ring mean planes is 71.4° in 1, which is larger than those (43.4 and 46.9°) found for the clockwise and counterclockwise isomers of 2. B
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
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slightly longer than the average Li−O distance (1.865(9) Å) found in the reported lithium phenolate complex. Pyrazole-Tethered Borane and Its Precursors. Having found the dimeric structures for the imidazole-tethered boranes, we focused our attention on heterocycle molecules containing two nitrogens present adjacent to each other so that dimerization can be prevented owing to steric crowding. In line with this, an unexpected interesting diboron compound 4 containing the methoxydifluoroborane moiety was isolated in excellent yield from the reaction between the in situ generated lithium salt of 1,3,5-trimethylpyrazole and 1 equiv of Et2B(OMe) followed by treatment with BF3·OEt2 and vacuum distillation. Its mechanism of formation is proposed in Scheme 2. The added Lewis acid does not remove LiOMe from the
This difference is due to the presence of bulky 9-BBN groups in 2, which squeeze the ring. In the course of our study, we also isolated the product formed before the addition of Lewis acid BF3·OEt2. As shown in Scheme 1, the dilithium complex 3 was isolated as colorless crystals from the reaction mixture. These crystals are air sensitive and formed in very poor yield, which precluded us from characterizations by spectroscopic methods. However, its structure was determined by X-ray diffraction method. Complex 3 crystallizes in the triclinic P1̅ space group with a half molecule in the asymmetric unit. The whole molecule was generated by the inversion center located in the center of the four-membered ring formed by two imidazole nitrogens and two lithium atoms. An ORTEP diagram along with important bond lengths and angles is given in Figure 3. This dilithium
Scheme 2. Synthesis of 1-(Diethylborylmethyl)-3,5dimethylpyrazole-BF2(OMe) Adduct 4 and Its Precursors
Figure 3. X-ray structure of the LiOMe adduct of the monomer, 2(diethylborylmethyl)-1-methylimidazole, 3. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): C4− C5 1.469(5), C5−B1 1.661(5), C6−B1 1.648(5), C8−B1 1.642(6), C10−O1 1.421(4), B1−O1 1.515(5), N1−Li1′ 2.125(6), N1−Li1 2.139(6), N3−Li1 2.050(7), O1−Li1 1.891(6), Li1−N1′ 2.125(6), Li1···Li1′ 2.736(11), C4−C5−B1 119.5(3), O1−B1−C8 111.4(3), O1−B1−C6 112.0(3), C8−B1−C6 108.6(3), O1−B1−C5 105.0(3) C8−B1−C5 107.5(3), C6−B1−C5 112.2(3), Li1′−N1−Li1 79.8(2), C10−O1−B1 116.4(3), C10−O1−Li1 115.1(3), B1−O1−Li1 127.2(3), O1−Li1−N3 114.9(3), O1−Li1−N1′ 112.3(3), N3−Li1− N1′ 113.5(3), N1′−Li1−N1 100.2(2).
adduct 5 (see below for its structure) and liberate the monomer, diethylborylmethyl-3,5-dimethylpyrazole 7, for subsequent dimerizaiton to proceed; instead, BF3·OEt2 loses one of its fluorine atoms as LiF probably because of the reactive three-coordinate lithium atom present in the structure of 5 and leads to the formation of 4. To our surprise, 4 is a viscous liquid, and its structure is based on NMR and HRMS methods, which is supported by anion binding studies. The molecular ion peak at m/z 179.1736 (calcd 179.1714) corresponds to the [M − BF2(OMe) + H]+ ion, confirming the presence of pyrazolylmethylborane 7 in 4. The 1H NMR spectrum of 4 in CDCl3 features a singlet at δ = 3.55 ppm owing to the presence of the methoxy methyl group, which is supported by the 13C NMR spectrum with a triplet at δ = 50.4 ppm owing to the coupling with the B atom. However, the integrated intensity of the OMe singlet does not match with those of the other peaks and appeared a puzzle to us. Further, the 11B NMR spectrum of 4 in CDCl3 shows two signals because of the presence of two different boron environments.
complex is formed by the bridging coordination mode of the LiOMe adduct of the monomer, 2-(diethylborylmethyl)-1methylimidazole, and stabilized by the coordination of two unreacted 1,2-dimethylimidazoles. The two methoxyborate anions lie in a trans fashion about the two lithium atoms with the imidazole nitrogen bridge bonded to the two lithium atoms. The geometry around each lithium atom is a distorted tetrahedral geometry. The Li···Li distance of 2.736(11) Å is longer than those (2.395(5),23 2.444(15),24 and 2.495(8) Å25) reported for the analogous dimeric lithium complexes. It is very interesting to note that the geometry around the methoxy oxygen atom is planar with an average angle of 119.5°, indicating the change in hybridization of the oxygen atom from sp3 to sp2 caused by the donation of lone pair of electrons from the filled oxygen p orbital to the empty boron p orbitals. This planar geometry is similar to the lithium phenolate complex previously reported.26 The O1−Li1 distance of 1.891(6) Å is C
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
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Organometallics The triplet at δ = 0.1 ppm with J(BF) = 21.4 Hz indicates the presence of the BF2 group, and the broad signal at δ = 9.3 ppm is assigned to the diethylboron group. This is supported by the 19 F NMR spectrum in which a quartet at δ = −146.7 ppm appeared with almost the same coupling constant, J(BF) = 21.3 Hz. In addition, importantly, it also shows additional multiplets with lower intensity values. This is not due to some impurities or cannot be ignored because they appeared every time when we recorded for different batches of samples. Hence, this suggests the presence of other compounds having alternative structures such as the structural isomer I and the dimer II, as shown in Chart 1, and they can be in equilibrium with 4. The
peaks. Further, irrespective of what the structure is for 4, all of them (4, I, or II) could give the 1:1 complex 9 as the final product. As shown in Scheme 3, the initial 1 equiv of F− ion Scheme 3. Anion Binding Study: Reaction of F− Ion as Its nBu4N+ Salt with 4 To Give the 1:1 Complex among Other Compounds in THF or CDCl3
Chart 1. Alternative Structures for 4
liberates the BF2(OMe) part from 4 owing to the high basicity of the F− ion and gives the [BF3(OMe)]− ate complex 8. In the presence of a second equiv of F− ion, the 1:1 complex [PzMe2CH2BEt2F]− 9 is formed from the pyrazolylmethylborane 7 formed in the first step. Furthermore, the titration spectra showed that the binding of F− ion occurs via slow complexation equilibria.28 Interestingly, the multiple products formed in the reaction of 4 with F− ion were supported by the negative ion mode ESI− MS and 19F NMR spectra (Figure 4). The ESI(−) spectrum of
structure I shows the fluoride bridging, instead of methoxy oxygen, and is similar to the proposed structure for B(OMe)F2.27 There are two fluorine environments in I, which can give rise to two multiplets in the 19F NMR spectrum. So we assign the unassigned multiplets in the 19F NMR spectrum to the presence of I and II or its higher cyclic homologues existing together with 4 as a viscous liquid. To support further the existence of other structural compounds, the 1H NMR spectrum was recorded for different concentrations of 4 using the sample of one batch. While the higher concentration sample (0.12 M in CDCl3; see Supporting Information (SI), Figure S14c) displayed a spectrum appearing very similar to the other previously recorded spectra which showed the expected number of peaks (see SI, Figure S12), the lower concentration sample (0.04 M in CDCl3; see SI, Figure S14a) showed additional peaks in every region as compared to the former one. These additional peaks could be because of the presence of at least two compounds like I and II. In addition, this is also reflected in the 19F NMR spectra of 4 recorded for two different concentrations (0.024 and 0.20 M in CDCl3; see SI, Figure S23). These spectra are similar but differ with respect to peak intensity values. All these indicate the occurrence of concentration-induced aggregation in which 4 possesses different structures like I and II. As a result, the 1H NMR spectrum of 4 is concentration dependent, and higher concentration samples show merged peaks, and their integrated intensity values do not match each other well. To support the proposed other possible structures for 4, an anion binding study was carried out by the 1H NMR titration method. Incremental additions of a solution of Bu4NF in CDCl3 to a solution of 4 in CDCl3 showed the appearance of new resonances together with those of 4 (see SI, Figure S59). For example, three new additional peaks appeared after the addition of 0.4 equiv of F− ion in the pyrazole ring CH resonance region. This trend continued until the addition of 2 equiv of F− ion, after which, interestingly, only one broad peak appeared. This supports the proposed structures for 4 because in low concentrations these alternative structures exist in significant quantities, as shown by the 1H NMR spectra discussed above. Each one can react with F− ions to give new
Figure 4. 19F NMR spectra of 4 and BF2(OMe) in the presence of F− ion in THF: (a) 4, (b) 4 with 1 equiv of F− ion, (c) 4 with 2 equiv of F− ion, (d) 4 with 4 equiv of F− ion, (e) BF2(OMe) with 1 equiv of F− ion, and (f) BF2(OMe).
a solution of 4 in CH3CN/THF containing ∼0.5 equiv of Bu4NF features the molecular ion peaks at m/z 99.0226 (calcd 99.0229) for the [BF3(OMe)]− anion 8, m/z 87.0032 (calcd 87.0035) for BF4−, and m/z 197.1618 (calcd 197.1631) for the 1:1 complex [PzMe2CH2BEt2F]− 9. Further, the 19F NMR spectrum of 4 in THF in the presence of 4 equiv of Bu4NF showed signals at δ = −116.5 ppm as a broad resonance for the excess F− ion, −143.2 ppm as a quartet with J(BF) = 15.8 Hz, D
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
[PzMe2CH2BEt2Cl]−, and [PzMe2CH2BEt2Br]− were formed, as shown by their negative ion mode ESI−MS spectra. In addition, we also isolated the lithium methoxide adduct of the free borane 7 which existed before the addition of BF3·OEt2 in the synthesis of 4 (Scheme 2). The reaction between the in situ generated lithium salt of 1,3,5-trimethylpyrazole and 1 equiv of Et2B(OMe) in THF gave the dilithium complex 5 as crystals in 73% yield. In an analogous manner, when the same reaction mixture was treated with DMAP, the mononuclear lithium complex 6 containing two DMAPs was isolated as crystals in 80% yield. In the presence of strong donor DMAP, the dilithium complex 5 split into two monomers, which are subsequently stabilized by the coordination of DMAP. The 1H NMR spectrum of 5 in CDCl3 showed the methoxy resonance at δ = 3.25 ppm and the pyrazolylmethylene resonance at δ = 3.01 ppm, which are slightly shifted upfield as compared to 4. The 11B NMR spectrum showed the presence of the tetracoordinated boron atom. The structures of both complexes were confirmed by X-ray diffraction methods. Moreover, in separate reactions, treatment of 5 or 6 with 2 or 3 equiv of BF3· OEt2 gave the liquid compound 4, as shown by their 1H NMR spectra, proving that both complexes 5 and 6 are precursors for 4. Complex 5 crystallizes in the triclinic P1̅ space group, and one molecule is present in the asymmetric unit. An ORTEP diagram is given in Figure 5 and revealed that an unsymmetrical
−150.9 ppm, −151.8 ppm, and −188.7 ppm as broad resonances (Figure 4d). To confirm the resonance due to the F− ion complex 9, the 19F NMR spectrum was recorded for BF2(OMe), which was prepared from the reaction between 2 equiv of BF3·OEt2 and BOMe3,27,29,30 in the presence of the F− ion in THF. BF2(OMe) gave a single resonance at δ = −156.6 ppm in THF (Figure 4f), but in the presence of 1 or 2 equiv of F− ion, four major resonances at δ = −120.3, −143.2, −150.9, and −151.9 ppm appear (Figure 4e). While the peak at −120.3 ppm is due to the excess F− ion, two of the remaining three signals are due to the BF3(OMe)− and BF4− salts formed in the reaction between BF2(OMe) and F− ion. The signals that appeared for this reaction (Figure 4e) have also appeared in the spectrum of 4 in the presence of 4 equiv of F− ion in THF except the peak around 188 ppm (Figure 4d). The formation of [BF3(OMe)]− and BF4− ions in the reaction between BF2(OMe) and F− ion was also confirmed by the negative ion mode ESI−MS spectrum recorded for BF2(OMe) in the presence of 2 equiv of F− ion in CH3CN (see SI, Figure S35). In addition, the X-ray structure of the protonated bicycle 11 contains a BF4− ion (see below, Scheme 4). Hence, it is Scheme 4. Synthesis of the Bicyclo[3.3.1]nonane-like Neutral (10) and Cationic (11) Bicycles
Figure 5. X-ray structure of the dilithium complex 5. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): O1−Li2 1.921(5), O1−Li1 2.022(5), O2−Li2 1.919(5), O2− Li1 2.010(4), B1−O2 1.582(3), B2−O1 1.567(3), N2−Li1 2.023(5), N3−Li1 2.029(5), O3−Li2 1.916(5), Li1···Li2 2.601(6), O3−Li2−O2 132.4(2), O3−Li2−O1 126.8(3), O2−Li2−O1 100.4(2), Li2−O2− Li1 82.89(19), Li2−O1−Li1 82.51(19), N2−Li1−N3 107.8(2), O2− Li1−O1 94.07(18).
binuclear lithium complex consists of two lithium atoms bridged by two pyrazolylmethyl(methyoxy)borate anion and one THF. The pyrazole nitrogens from both ligands remain coordinated to the same lithium atom (Li1), and the two methoxy oxygens are bridge bonded to two lithium atoms, forming a four-membered Li2O2 ring. The methoxy methyl groups are dispositioned trans to each other about this ring plane, and the geometry around the methoxy oxygen atom is a distorted tetrahedral geometry. This is in contrast to the planar geometry of the methoxy oxygen atom in the dilithium complex 3. Each methoxy oxygen atom is bonded to four different groups and hence is a chiral center. Analysis of the two molecules present in the packing diagram revealed that one has RR and the other has SS configuration, as shown in Figure 5. Although three-coordinate chiral oxygen molecules have been reported,31,32 as far as we know, four-coordinate oxygen
plausible that similar decomposition could take place for 4 in the presence of F− ion to give the resonances in the region of −143 to −152 ppm, where BF2(OMe) appears in the presence of the F− ion. Therefore, the resonance at −188.7 ppm in Figure 4c,d is assigned to the F− ion complex 9. In addition, it is also noticed that the same sample with 1 equiv of F− ion does not show resonances around δ = −120 and −188 ppm, indicating that the added F− ion preferentially reacts first with the BF2(OMe) part in 4 and the 1:1 complex 9 forms after 2 equiv of F− ion addition, supporting the multiple equilibria shown in Scheme 3. Further, similar study in CDCl3 was also carried out. Upon addition of 3 equiv of F− ion (see SI, Figure S28c), the same spectral pattern including the peak for the 1:1 complex 9 appeared. Furthermore, when 4 was treated with CN−, Cl−, and Br− ions as their nBu4N+ salts in THF, the analogous 1:1 complexes [PzMe2CH2BEt2CN]−, E
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
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molecule which reacts with two molecules of the free borane 7 to give the bicycle 10 with concomitant formation of two ethane molecules. The ring C−B bond distances are slightly longer than the ethyl C−B bond distances. However, these distances remain shorter than the corresponding ring C−B bond distances found in 1 and 2, indicating the ring strain in the bicycle. The B1−O1−B2 angle of 118.22(14)° and the average B−O distances of 1.417(2) Å are similar to those found in the bicyclo[3.3.1]nonane-like B2(μ-O)Et2(7-azain)2 structure reported by Wang.33 However, this B1−O1−B2 angle is smaller than that found in the 2,2′-biimidazole stabilized polycyclic molecule reported by Niedenzu et al.34 In addition, this structure is similar to B2N4Se-bicyclo[2.2.1]heptane reported by Yalpani et al.35 Interestingly, in contrast to the decomposition of the neat liquid 4 in air, the dichloromethane solution of 4 in open air decomposed differently and yielded the monocationic bicyclic compound 11 as crystals in 77% yield (Scheme 4). It contains the BF4− counteranion which must originate from BF2(OMe) present in 4, and as far as we know, it represents the first cationic bicycle of its type. The pattern of the 1H NMR spectrum of 11 in acetone-d6 appears similar to that of 10 but shows different chemical shifts. For example, the AB quartet of the ring methylene protons appears in the deshielded region at δ = 3.58 and 3.43 ppm with J(HAHB) = 14.0 Hz. The Fourier transform infrared (FT-IR) spectrum shows a band at 3176 cm−1 for the OH group. A broad singlet at δ = 4.2 ppm and a sharp singlet at δ = −1.0 ppm in the 11B NMR spectrum support the presence of two different tetravalent boron atoms in the structure. The 19F NMR shows a singlet at δ = −151.0 ppm for the presence of the BF4− anion. Further, treatment of 11 with excess KHCO3 in CH2Cl2 gave back the neutral bicycle 10, as shown by the 1H NMR spectrum. The X-ray structure of 11 is given in Figure 7 with selected bond distances and angles. Although the core structure is similar to 10, it is a salt with the protonated bridging oxygen atom. The bridging B−O bond distances (O1−B2 = 1.508(3) Å and O1−B1 = 1.522(3) Å) are longer than those in 10 because of protonation, which, however, does not affect the B2−O1−B1 angle of 118.1°; it is almost the same as in 10. In
chirality has not been reported. The molecule has one approximate C2 axis passing through two lithium atoms. Interestingly, one lithium atom Li1 is four-coordinate and adopts a distorted tetrahedral geometry, whereas the other atom Li2 is three-coordinate and possesses the planar geometry with the THF O3−Li2−O1(O2) angles being greater than the O2−Li2−O1 angle (100.4(2)°). As a result, the ring Li1−O distances (2.010(4) and 2.022(5) Å) are longer than the Li2− O distances (1.921(5) and 1.919(5) Å). The Li···Li distance of 2.601(6) Å is considerably lower than 2.736(11) Å found in complex 3; however, it remains longer than those in several reported complexes.23−25 The X-ray structure of complex 6 (see SI, Figure S61) revealed the six-membered chelate ring formed by the pyrazolylmethyl(methoxy)borate anion with the lithium atom containing two DMAP molecules. The geometry around the bridge bonded oxygen atom is planar with an average angle of 118.7°, resembling the oxygen atom in structure 3. The B1−O1 and Li1−O1 distances are similar to those in 3. The oxygen atom is located 0.181 Å from the mean plane formed by Li, B, and C atoms. The tetracoordinated lithium atom adopts a distorted tetrahedral geometry with the O1−Li1−N2 angle of 101.9(6)°, which is smaller than the N3−Li1−N5 angle, 115.6(6)°. Synthesis and Characterization of Boron Bicycles. The adduct 4 is an air-sensitive compound and readily reacts with air and gave the neutral bicycle 10 as crystals in very good yield (Scheme 4). In the 1H NMR spectrum of 10 in acetone-d6, the ring methylene protons gives an AB quartet at δ = 2.91 and 3.03 ppm with J(HAHB) = 13.3 Hz, indicating the rigidity of the molecule in solution. In the 11B NMR spectrum, a singlet at δ = 5.8 ppm suggests the presence of tetravalent boron atoms having equivalent environments. Bicycle 10 crystallizes in the triclinic P1̅ space group with one molecule in the asymmetric unit. The X-ray structure is given in Figure 6 along with important bond lengths and angles. It revealed two boron atoms bridged by two pyrazolylmethyl groups and one oxygen atom. In addition, each boron atom is bonded to one ethyl group. It possesses a butterfly shape with an oxygen atom as the head and the two pyrazole rings as the wings. The bridging oxygen atom could come from a water
Figure 7. X-ray structure of the protonated cationic bicycle compound 11. Most hydrogen atoms and the disordered BF4− anion are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−B1 1.602(4), C7−B2 1.599(4), C9−B2 1.598(4), C15−B1 1.593(4), N2− B2 1.615(3), N3−B1 1.598(3), O1−B2 1.508(3), O1−B1 1.522(3), B2−O1−B1 118.11(19), O1−B1−C15 108.7(2), O(1)−B(1)−N(3) 103.74(18), C(15)−B(1)−N(3) 113.5(2), O(1)−B(1)−C(1) 106.77(19), C(15)−B(1)−C(1) 113.5(2), N3−B1−C1 110.0(2), O1−B2−C7 109.8(2), O1−B2−C9 104.9(2), C7−B2−C9 114.0(2), O1−B2−N2 105.66(18), C7−B2−N2 113.2(2), C9−B2−N2 108.6(2).
Figure 6. X-ray structure of the neutral bicycle 10. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): O1−B1 1.414(2), O1−B2 1.421(2), N4−B1 1.696(2), N2−B2 1.683(3), B2−C11 1.629(3), B2−C9 1.607(3), B1−C3 1.618(3), B1−C2 1.609(3), B1−O1−B2 118.22(14), O1−B2−C2 112.29(16), O1−B2−C9 112.54(16), C2−B1−C3 112.27(15), O1−B1−N4 106.44(13), C2−B1−N4 109.34(14), C3−B1−N4 105.53(14), N1− C3−B1 112.83(14), C2−B1−C3 112.27(15), O1−B2−N2 106.76(14), C11−B2−N2 105.72(15), C9−B2−N2 108.81(15), N3−C11−B2 112.86(13). F
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
C18H34B2N4: C, 65.89; H, 10.44; N, 17.08. Found: C, 65.13; H, 10.79; N, 16.92. Although the elemental analysis values of H and N support the purity, the obtained lower value of C can be because of incomplete combustion.37,38 However, the purity was confirmed by 1H and 13C spectra. Synthesis of 2-(9-Borabicyclo[3.3.1]non-9-ylmethyl)-1methyl-1H-imidazole Dimer, 2. Synthesis of 2 was carried out by the above-mentioned procedure using 1,2-dimethylimidazole (0.5 mL, 5.64 mmol), n-BuLi (3.9 mL, 6.20 mmol, 1.6 M in hexane), 9methoxy-9-BBN (0.86 g, 5.64 mmol), and BF3·OEt2 (0.7 mL, 5.67 mmol) in THF. After removing all volatiles from the reaction mixture, the residue was extracted with dichloromethane (25 mL × 3). Again the solvent was removed under vacuum, and the resulting residue was loaded onto a silica gel filled column. Elution using petroleum ether/ ethyl acetate (2:1) gave the first fraction from which the solvent was removed to give colorless crystalline solid 2 (0.65 g, 1.50 mmol). Yield: 53%. Suitable single crystals for X-ray diffraction study were obtained by slow evaporation of dichloromethane solution of 2. 1H NMR (CDCl3): δ 6.63 (br s, 1H, imidazole CH), 6.33 (br s, 1H, imidazole CH), 3.45 (s, 3H, NMe), 2.52 (d, J = 13.6, 2H, CH2), 2.19 (d, J = 13.6, 2H, CH2), 1.94−1.26 (m, 26H, 9-BBN), 0.87 (s, 2H, 9BNN). 13C{1H} NMR (CDCl3): δ 158.5, 121.2, 117.0, 33.7, 31.8, 31.3, 31.2, 24.6, 23.9. 11B NMR (CDCl3): δ − 3.7 (s). FT-IR (KBr, cm−1): ν = 2912 (m), 2849 (s), 1561 (w), 1492 (s), 1449 (w), 1314 (w), 1284 (w), 1206 (m), 1163 (s), 1130 (s), 1077 (m), 1035 (w), 985 (w), 939 (s), 903 (m), 856 (w), 824 (w) 716 (s), 545 (w), 462 (w). HRMS (+ESI): m/z calcd for [monomer + H+] C13H22BN2+ 217.1871, found 217.1874. Anal. Calcd for C26H42B2N4: C, 72.24; H, 9.79; N, 12.96. Found: C, 70.88; H, 10.01; N, 12.93. Similar to compound 1, the elemental analysis values of H and N support the purity, and the obtained lower value of C can be because of incomplete combustion.37,38 In this case also, the purity was confirmed by 1H and 13 C spectra. Synthesis of [2-(Diethylborylmethyl)-1-methylimidazole· LiOMe(1,2-dimethylimidazole)]2, 3. To a solution of 1,2dimethylimidazole (1 mL, 11.28 mmol) in THF (30 mL) was added n-BuLi (3.9 mL, 6.2 mmol, 1.6 M in hexane) at −114 °C. The solution was stirred for 1 h and then Et2B(OMe) (6.2 mL, 6.2 mmol, 1 M in THF) was added at −114 °C. The reaction mixture was allowed to come to room temperature and stirred overnight. The concentrated reaction mixture was cooled to −16 °C to give only a few crystals of 3. Other characterizations could not be done because of its very poor yield. Synthesis of 1-Diethylborylmethyl-3,5-dimethyl-1H-pyrazole·BF2(OMe), 4. Synthesis of 4 was carried out by following the procedure of 1 using 1,3,5-trimethylpyrazole (2.0 g, 18.2 mmol), nBuLi (12.5 mL, 19.97 mmol, 1.6 M in hexane), Et2B(OMe) (20.00 mL, 19.80 mmol, 1 M in THF), and BF3·OEt2 (2.5 mL, 20.25 mmol) in THF. After extraction and filtration, the petroleum ether was removed under vacuum, and the resulting oily residue was distilled under reduced pressure to give 4 as a viscous liquid (4.1 g, 15.9 mmol). Yield: 87%. 1H NMR (CDCl3): δ 5.93 (s, 1H, pyrazole CH), 3.55 (s, 3H, OMe), 3.24 (s, 2H, CH2), 2.36 (s, 3H, pyrazole Me), 2.24 (s, 3H, pyrazole Me), 0.66 (t, J = 8, 6H, BCH2CH3), 0.33−0.22 (m, 4H, BCH2CH3). 13C{1H} NMR (CDCl3): δ 145.0, 142.5, 106.8, 50.4 (t, 3J(CF) = 4.8), 42.6, 19.1, 14.0, 12.5 (t, J = 4.6), 11.8, 9.5. 11B NMR (CDCl3): δ 9.3 (br s), 0.12 (t, J(BF) = 21.4). FT-IR (ATR, cm−1): ν = 2947 (m), 2909 (m), 2869 (m), 2820 (w), 1556 (m), 1480 (m), 1424 (m), 1404 (m), 1315 (m), 1287 (w), 1223 (w), 1183 (m), 1146 (m), 1104 (s), 1018 (s), 992 (s), 933 (m), 878 (s), 848 (m), 810 (s), 732 (s) 672 (w), 639 (w), 536 (w), 494 (w). HRMS (+ESI): m/z calcd for [M − BF2(OMe) + H+] C9H18BN2+ 179.1714, found 179.1736. Synthesis of [(1-(Diethylborylmethyl)-3,5-dimethylpyrazole· LiOMe)2THF], 5. To a solution of 1,3,5-trimethylpyrazole (0.50 g, 4.50 mmol) in THF (20 mL) at −114 °C was added n-BuLi (3.1 mL, 4.99 mmol, 1.6 M in hexanes). The solution was stirred for 1 h, and then Et2B(OMe) (4.5 mL, 4.5 mmol, 1 M in THF) was added at −114 °C. The resulting solution was allowed to warm to room temperature and stirred for 14 h. All solvents were removed by vacuum, and the resulting residue was extracted with warm petroleum ether (3 × 30
addition, the ring B−N and B−C bond distances are shorter than those found in 10. Each boron atom in 10 or 11 is bonded to four different groups and hence is a chiral center. The two stereoisomers present in their packing diagrams are shown in Figure 6 and Figure 7, with their configurations: RR and SS. In addition, they can also be viewed as clockwise and counterclockwise isomers as discussed for 1 or 2.
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CONCLUSIONS In summary, hitherto unknown imidazole and pyrazoletethered boranes were synthesized and structurally characterized. While the imidazole-tethered boranes dimerize, the pyrazoleborane derivative does not dimerize probably owing to the methyl group present next to the Lewis base N atom; instead the interesting BF2(OMe) adduct was isolated as a clear liquid. Consequently, the reactive pyrazolylmethylborane adduct gave two new bicycles upon exposure to the air, and their structures were established by X-ray diffraction studies. The alternative possible structures for the liquid compound 4 were proposed based on spectroscopic and anion binding studies. Besides, the precursors of both the heterocycle boranes were structurally characterized. Further study to isolate free boranes using steric hindrances is underway.
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EXPERIMENTAL SECTION
General Procedure. All reactions were carried out under a nitrogen atmosphere using standard Schlenk line techniques or nitrogen-filled glovebox. Petroleum ether (bp 40−60 °C) and other solvents were distilled under N2 atmosphere according to the standard procedures. 9-Methoxy-9-BBN was synthesized from 9-BBN by the literature procedure.36 Other chemicals were obtained from commercial sources and used as received. 1H (400 MHz), 13C (102.6 MHz), 19F (376.5 MHz), and 11B (128.3 MHz) NMR spectra were recorded at room temperature. 1H NMR chemical shifts are referenced with respect to the chemical shift of the residual protons present in the deuterated solvents. 19F NMR spectra were recorded on a 400 MHz spectrometer operating at 376.5 MHz, for which 0.05% trifluorotoluene in CDCl3 was used as an external reference resonating at −62.71 ppm. Chemical shifts are in parts per million, and coupling constants are in hertz. FT-IR and attenuated total reflectance (ATR) spectra were recorded using PerkinElmer Spectrum Rx. Highresolution mass spectra (ESI) were recorded using the Xevo G2 Tof mass spectrometer (Waters). Synthesis of 2-Diethylborylmethyl-1-methyl-1H-imidazole Dimer, 1. To a solution of 1,2-dimethylimidazole (2 mL, 22.5 mmol) in THF (30 mL) was added n-BuLi (15.5 mL, 24.8 mmol, 1.6 M in hexane) at −114 °C. The solution was stirred for 1 h, and then Et2B(OMe) (24.7 mL, 24.8 mmol, 1 M in THF) was added at −114 °C. The reaction mixture was allowed to come to room temperature and stirred overnight. Then BF3·OEt2 (3.05 mL, 24.7 mmol) was dropwise added and stirred for an additional 2 h. The solvent was removed under vacuum, and the resulting residue was extracted with petroleum ether (50 mL × 3) and filtered. The solvent was removed from filtrate to give 1 as a colorless solid. The solid was dissolved in petroleum ether (∼5 mL) and cooled to −16 °C to give colorless crystals of 1 (1.2 g, 3.65 mmol). Yield: 32%. 1H NMR (CDCl3): δ 6.68 (d, J = 1.8, 2H, imidazole CH), 6.45 (d, J = 1.8, 2H, imidazole CH), 3.54 (s, 6H, imidazole NMe), 2.07 (br s, 4H, ring CH2), 0.63 (br s, 12H, BCH2CH3), 0.29 (br s, 8H, BCH2CH3). 13C{1H} NMR (CDCl3): δ 156.3, 121.7, 117.2, 34.2, 21.5, 15.2, 9.9. 11B NMR (CDCl3): δ − 3.40. FT-IR (KBr, cm−1): ν = 3366 (m, br), 3145 (m), 2938 (m), 2872(m), 2826 (w), 1663 (w), 1393 (s), 1367 (w), 1353 (s), 1278 (m), 1219 (m), 1169 (m), 1123 (s), 1084 (m), 1044 (s), 988 (m), 912 (w), 841 (m), 765 (w), 726 (s), 676 (m), 643 (w), 521 (w), 505 (w), 459 (w) 430 (w). HRMS (+ESI): m/z calcd for [monomer + H+] C9H18BN2+ 165.1558, found 165.1555. Anal. Calcd for G
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics mL) and filtered. The solvent was removed from the filtrate, and the resulting oily residue was dissolved again in petroleum ether (10 mL), which was cooled to −16 °C for 1 week to give colorless crystals of 5. The crystals were separated and dried under vacuum (0.83 g, 1.65 mmol). Yield: 73%. The purity was confirmed by NMR data. 1H NMR (CDCl3): δ 5.70 (s, 2H, pyrazole CH), 3.81 (m, 4H, THF), 3.25 (s, 6H, OCH3), 3.01 (s, 4H, BCH2N), 2.18 (s, 6H, pyrazole CH3), 2.08 (s, 6H, pyrazole CH3), 1.89 (m, 4H, THF), 0.71 (t, 3J(HH) = 6, 12H, ethyl CH3), 0.13 (m, 3J(HH) = 8, 8H, ethyl CH2). 11B NMR (CDCl3): δ 2.1 (br s). FT-IR (ATR, cm−1): ν = 2938 (s), 2901 (s), 2862 (s), 1544 (m), 1460 (m), 1425 (m), 1388 (m), 1299 (m), 1263 (m), 1208 (w), 1085 (m), 1043 (s), 1020 (s), 980 (m), 888 (m), 862 (m), 802 (m), 770 (s), 710 (m), 664 (m), 642 (m), 624 (m), 527 (m), 458 (m). HRMS (+ESI): m/z calcd for [7 + H]+ C10H20BN2+ 179.1714, found 179.1718. Synthesis of [1-(Diethylborylmethyl)-3,5-dimethylpyrazole· LiOMe(DMAP)2], 6. To a solution of 1,3,5-trimethylpyrazole (0.18 g, 1.60 mmol) in THF (20 mL) at −114 °C was added n-BuLi (1.1 mL, 1.8 mmol, 1.6 M in hexanes). The solution was stirred for 1 h, and then Et2B(OMe) (1.6 mL, 1.6 mmol, 1 M in THF) was added at −114 °C. The resulting solution was allowed to warm to room temperature and stirred for 14 h. To the resulting solution was added 4(dimethylamino)pyridine (0.39 g, 3.19 mmol) and stirred for 3 h. All solvents were removed under vacuum, and the resulting residue was extracted with warm petroleum ether (3 × 30 mL) and filtered. The solvent was removed from the filtrate, and the resulting oily residue was dissolved again in petroleum ether (10 mL), which was cooled to −16 °C for 1 day to give colorless crystals of 6. The crystals were separated and dried under vacuum (0.60 g, 1.30 mmol). Yield = 80%. The purity was confirmed by NMR data. 1H NMR (CDCl3): δ = 8.22 (d, 2H, J(HH) = 4, DMAP CH), 7.86 (d, J(HH) = 8, 2H, DMAP CH), 6.48 (q, 4H, J(HH) = 6, DMAP CH), 5.61 (s, 1H, pyrazole CH), 3.48 (s, 3H, OCH3), 3.27 (s, 2H, pyrazole CH2), 3.08(s, 6, DMAP N− CH3), 3.00 (s, 6H, DMAP NCH3), 2.05 (s, 3H, pyrazole CH3), 2.02 (s, 3H, pyrazole CH3), 0.67 (t, J(HH) = 8, 6H, ethyl CH3), 0.51−0.35 (m, 4H, ethyl CH2). 13C{1H} NMR (100 MHz, CDCl3): δ = 154.7, 154.3, 148.9, 144.7, 143.9, 137.5, 106.4, 105.9, 102.6, 49.1, 39.2, 38.9, 13.5, 13.2, 12.8, 11.5, 10.5, 9.5, 1.0. 11B NMR (CDCl3): δ −1.7 (br s). FT-IR (ATR, cm−1): ν = 2947 (m), 2909 (m), 2869 (m), 2820 (w), 1556 (m), 1480 (w), 1458 (w), 1424 (w), 1404 (m), 1315 (w), 1287 (w), 1223 (w), 1183 (m), 1146 (m), 1104 (s), 1056 (m), 1018 (s), 992 (m), 878 (s), 848 (m), 810 (m), 732 (m), 672 (w), 639 (w), 592 (w), 536 (w), 494 (w). HRMS (+ESI): m/z calcd for [7 + H]+ C10H20BN2+ 179.1714, found 179.1745; m/z calcd for [10 + H]+ C16H29B2N4O+ 315.2522, found 315.2545. Reaction of 5 with BF3·OEt2. To a THF solution of the dilithium compound 5 (0.21 g, 0.42 mmol) was added BF3·OEt2 (0.1 mL, 0.81 mmol) and stirred for 6 h. The volatiles were removed under vacuum at room temperature, and the resulting residue was extracted with petroleum ether. The solvent was removed under vacuum to give the colorless liquid 4 (0.20 g, 0.77 mmol). Yield: 92%. Its 1H NMR in CDCl3 is the same as that of 4 obtained earlier. Reaction of 6 with BF3·OEt2. This reaction was carried out by the above-mentioned procedure using 6 (0.50 g, 1.08 mmol) and BF3· OEt2 (0.4 mL, 3.24 mmol). The solution was stirred for 24 h, and 0.18 g (0.70 mmol) of 4 was obtained. Yield: 65%. Its 1H NMR in CDCl3 is the same as that of 4 obtained earlier. Synthesis of the Neutral Bicycle [B2(μ-O)Et2(CH2pzMe2)2], 10. A vial containing the adduct 4 (0.20 g, 0.78 mmol) was kept open in the air for 2 h. Colorless crystals of 10 were formed, which were washed with petroleum ether and dried under vacuum (0.11 g, 0.35 mmol). Yield: 91%. 1H NMR (CDCl3): δ 5.77 (s, 2H pyrazole CH), 3.03 (d, J(HAHB) = 13.3, 2H, CH2), 2.91 (d, J(HAHB) = 13.3, 2H, CH2), 2.28 (s, 6H, pyrazole Me), 2.12 (s, 6H, pyrazole Me), 0.75 (t, J(HH) = 7.2, 6H, BCH2CH3), 0.69 (m, 4H, BCH2). 13C NMR (CDCl3): δ 142.6, 141.4, 105.7, 43.3, 14.2, 13.7, 11.3, 8.6. 11B NMR (CDCl3): δ 5.8 (br s). FT-IR (KBr, cm−1): ν = 2926 (s), 2739 (w), 2700 (w), 2599 (w), 2420(w), 1551 (s), 1458 (s), 1388 (s), 1308 (s), 1274 (m), 1249 (m), 1226 (m), 1161 (s), 1123 (s), 1047 (s), 979 (m), 915 (m), 859 (s), 794 (m), 766 (s), 709 (m), 659 (m), 605 (s), 586
(m), 542 (m), 527 (m), 484 (m), 439 (s). HRMS (+ESI): m/z calcd for [M + H+] C16H29B2N4O+ 315.2522, found 315.2521. Anal. Calcd for C16H28B2N4O: C, 61.19; H, 8.99; N, 17.84. Found: C, 61.51; H, 9.41; N, 17.89. This is the best obtained value for CHN analysis in which only the H value slightly exceeds the acceptable value of 0.4% difference. However, the purity was confirmed by 1H and 13C NMR data. Synthesis of the Cationic Bicycle [B2(μ-OH)Et2(CH2pzMe2)2]+BF4−, 11. A CH2Cl2 solution of 4 (0.20 g, 0.78 mmol) was allowed to evaporate slowly in the air at room temperature. Colorless crystals of 11 were formed. After 24 h, the crystals were separated, washed with THF, and dried under vacuum (0.12 g, 0.30 mmol). Yield: 77%. 1H NMR (acetone-d6): δ 6.27 (s, 2H, pyrazole CH), 3.58 (d, J(HAHB) = 16, 2H, CH2), 3.43 (d, J(HAHB) = 12, 2H, CH2), 2.42 (s, 6H, pyrazole Me), 2.31 (s, 6H, pyrazole Me), 0.94 (m, 2H, BCH2), 0.81 (m, 2H, B−CH2), 0.75 (t, J(HH) = 6, 6H, BCH2CH3). 13C NMR (CDCl3): δ 144.4, 108.0, 43.4, 13.6, 11.6, 7.6. 11 B NMR (CDCl3): δ 4.2 (br s), −1.0 (s). 19F NMR (CDCl3): δ = 151.0 (s). FT-IR (KBr, cm−1): δ = 3172 (w), 2951 (w), 2928 (w), 2876 (w), 1554 (m), 1462 (m), 1419(m), 1386 (m), 1346 (m), 1324(m), 1284(w), 1245 (w), 1080 (s), 966 (s), 890 (m), 774 (m), 699 (w), 610 (w), 554 (w), 525 (w), 413 (w). HRMS (+ESI): m/z calcd for [M − BF4−]+ C16H29B2N4O+ 315.2522, found 315.2552. HRMS (−ESI): calcd for [BF4−] 87.0035, found 87.0032. NMR Titrations for Anion Binding Study. Using a 10 μL Hamilton gastight syringe, all the titrations were carried out by adding an incremental amount of anions (F− and CN−) (2 μL, 1 × 10−3 mmol, 0.195 equiv) as their n-Bu4N+ salts in CDCl3 (0.5 M) to a NMR tube containing the receptor 4 (0.0013 g, 0.005 mmol) in CDCl3 (0.5 mL). Spectra were recorded immediately after each addition. X-ray Crystallography. Suitable single crystals of 1, 2, 3, 5, 6, 10, and 11 were grown from the solvents mentioned in their respective synthetic procedures. Single-crystal X-ray diffraction data collections were performed using Bruker APEX-II CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The space group for every structure was obtained by the XPREP program. The structures were then solved by SIR-9239 or SHELXT-9740 available in WinGX, which successfully located most of the nonhydrogen atoms. Subsequently, least-squares refinements were carried out on F2 using SHELXL-2014/741 to locate the remaining nonhydrogen atoms. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon atoms were fixed in calculated positions. In the case of structure 11, the OH hydrogen was located from the difference Fourier map. The disordered BF4− anion was handled with EADP refinement. The refinement data for all the structures are given in Supporting Information (Table S1).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00766. NMR, IR, and HRMS spectra (PDF) Accession Codes
CCDC 1580808−1580814 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +91 3222 282320. Fax: +91 3222 282252. H
DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics ORCID
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Ganesan Mani: 0000-0002-0782-6484 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the DST and CSIR (New Delhi, India) for financial support and for the X-ray and NMR facilities.
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DOI: 10.1021/acs.organomet.7b00766 Organometallics XXXX, XXX, XXX−XXX
