Nucleophilic Substitution at a Coordinatively Saturated Five-Membered NHC · Haloborane Centre

: In this paper, we have used a saturated five-membered N-Heterocyclic carbene (5SIDipp = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidine) for the synthesis of SNHC-haloboranes adducts and their further nucleophilic substitutions to put unusual functional groups at the central boron atom. The reaction of 5-SIDipp with RBCl 2 yields Lewis-base adducts, 5-SIDipp · RBCl 2 [R = H ( 1 ), Ph ( 2 )]. The hydrolysis of 1 gives the NHC stabilized boric acid, 5-SIDipp · B(OH) 3 ( 3 ), selectively. Replacement of chlorine atoms from 1 and 2 with one equivalent of AgOTf led to the formation of 5-SIDipp · HBCl(OTf) ( 4 ) and 5-SIDipp · PhBCl(OTf) ( 5a ), where all the substituents on the boron atoms are different. The addition of two equivalents of AgNO 3 to 2 leads to the formation of rare di -nitro substituted 5-SIDipp · BPh(NO 3 ) 2 ( 6 ). Further, the reaction of 5-SIDipp with B(C 6 F 5 ) 3 in tetrahydrofuran and diethyl ether shows a frustrated Lewis pair type small molecule activated products, 7 and 8 .


Results and Discussion
In our previous work, we have reported the first carbene MeBCl2 adduct via salt metathesis procedure [8]. In this work, we have prepared the 5SIDipp·haloborane adducts and shown their reactivities towards nucleophilic substitution. The addition of BHCl2·dioxane in the solution of 5SIDipp in n-hexane gives the white precipitation of 5-SIDipp·BHCl2, 1 at room temperature (Scheme 1). The precipitate was further dissolved in toluene and dichloromethane to afford colorless crystals of 1 at −36 °C. The 11 B NMR spectrum of 1 displays a resonance at 6.9 ppm as a sharp singlet. The backbone four protons appeared at 4.08 ppm in the 1 H NMR spectrum. 1 is characterized by single-crystal X-ray diffraction studies ( Figure 1). 1 is crystalized in the monoclinic P21/n space group. The B-C bond length in 1 [1.628(7) Å] is in good accordance with the previously reported 5-SIDipp·MeBCl2 [1.6261 (19) Å], but considerably longer compared to that in the 5-SIDipp·BH3 [1.593(4) Å] [8]. The increase in the bond length can be ascribed to the enhancement of steric hindrance at the central boron atom. The average B-Cl distance is 1.86 Å, which matches with the previously reported carbene-haloborane adducts (NHC·BCl3, NHC·BRCl2, and NHC·BR2Cl).  Inorganics 2022, 10, x FOR PEER REVIEW 2 of 13 5SIDipp·BPhCl2, 2 and studied their substitution reactions with AgOTf, AgNO3, and water. Furthermore, we have shown that the combination 5-SIDipp and B(C6F5)3 led to the activation of THF and diethyl ether via frustrated Lewis pair (FLP) way. Our results are reported herein.

Results and Discussion
In our previous work, we have reported the first carbene MeBCl2 adduct via salt metathesis procedure [8]. In this work, we have prepared the 5SIDipp·haloborane adducts and shown their reactivities towards nucleophilic substitution. The addition of BHCl2·dioxane in the solution of 5SIDipp in n-hexane gives the white precipitation of 5-SIDipp·BHCl2, 1 at room temperature (Scheme 1). The precipitate was further dissolved in toluene and dichloromethane to afford colorless crystals of 1 at −36 °C. The 11 B NMR spectrum of 1 displays a resonance at 6.9 ppm as a sharp singlet. The backbone four protons appeared at 4.08 ppm in the 1 H NMR spectrum. 1 is characterized by single-crystal X-ray diffraction studies ( Figure 1). 1 is crystalized in the monoclinic P21/n space group. The B-C bond length in 1 [1.628(7) Å] is in good accordance with the previously reported 5-SIDipp·MeBCl2 [1.6261 (19) Å], but considerably longer compared to that in the 5-SIDipp·BH3 [1.593(4) Å] [8]. The increase in the bond length can be ascribed to the enhancement of steric hindrance at the central boron atom. The average B-Cl distance is 1.86 Å, which matches with the previously reported carbene-haloborane adducts (NHC·BCl3, NHC·BRCl2, and NHC·BR2Cl).   The reaction of 1.1 equivalent of PhBCl 2 with 5-SIDipp in n-hexane gives an immediate white precipitate formation of 5-SIDipp·PhBCl 2 (2) (Scheme 1). The 11 B NMR spectrum of 2 shows one resonance at 1.8 ppm. 2 crystallizes in the monoclinic P2 1 /n space group ( Figure 2). The carbene carbon atom C5 is tri-coordinated and features a trigonal-planar geometry, and the boron atom connected with the C5 atom adopts a tetrahedral geometry. The B−C NHC bond distance is 1.6661(10) Å, which is slightly longer in comparison to that in 1 due to the steric congestion of the phenyl group at the boron center. The B-Cl bonds are 1.8992(8) Å and 1.8698(8) Å, which match with the previously reported B-Cl bond distance [8].
The reaction of 1.1 equivalent of PhBCl2 with 5-SIDipp in n-hexane gives a immediate white precipitate formation of 5-SIDipp·PhBCl2 (2) (Scheme 1). The 11 B NM spectrum of 2 shows one resonance at 1.8 ppm. 2 crystallizes in the monoclinic P21/n spac group ( Figure 2). The carbene carbon atom C5 is tri-coordinated and features a trigona planar geometry, and the boron atom connected with the C5 atom adopts a tetrahedra geometry. The B−CNHC bond distance is 1.6661(10) Å, which is slightly longer i comparison to that in 1 due to the steric congestion of the phenyl group at the boro center. The B-Cl bonds are 1.8992(8) Å and 1.8698(8) Å, which match with the previousl reported B-Cl bond distance [8].  Treatment of 1.05 equivalents of water in a dichloromethane solution of 1 hydrolyzes all the B-H and B-Cl bonds and forms NHC-stabilized boric acid, 5-SIDipp·B(OH) 3 , 3 (Scheme 2) exclusively. In our earlier work, we have reported the isolation of 6-SIDipp·B(OH) 3 as a minor product from the reaction of Br 2 /H 2 O with 6-SIDipp·BH 3 [9]. Replacement of the chloride and the hydride groups by hydroxide moieties in 3 is accompanied by an upfield shift in the 11 B NMR spectrum (−1.6 ppm) from that of 1. 3 crystallizes in the monoclinic P2 1 /c space group (Figure 3). The central boron-carbon distance is 1.650(5) Å, which is marginally shorter compared to that in 6-SIDipp·B(OH) 3 . The average B-(OH) bond distances are 1.38 Å.
Further, we added silver triflate to a dichloromethane solution of 1 at −78 • C, which replaced one of the labile chlorine atoms by the triflate group (Scheme 2). In the 11 B NMR spectrum of 4, the resonance for the central boron atom appears at −3.4 ppm. The resonance at −76.7 ppm in the 19 F NMR is characteristic of the triflate group attached to the central boron atom. Colorless crystals of 4 suitable for X-ray diffraction studies were grown from a saturated toluene solution at 4 • C. The constitution of 4 was authenticated by a single-crystal X-ray study ( Figure 4). 4 crystallizes in the monoclinic space group P2 1 /n. The relevant bond length and angles are given in the legend of  [8].
Treatment of one and two equivalents of AgOTf and AgNO 3 with 2 in toluene afforded mono-triflate and di-nitro substituted 5-SIDipp·boranes, respectively. Substitution of one and two chlorine atoms from the tetra coordinated boron atom of 2 resulted in the formation of 5-SIDipp·BPhCl(OTf), 5a, and 5-SIDipp-BPh(ONO 2 ) 2 , 6 (Scheme 3). The functional groups such as nitrate and triflate are rarely found to bind with the boron atom [5,9,[20][21][22][23][24][25][26]. 5a crystallizes in the monoclinic P2 1 /n space group ( Figure 5). The boron atom lies on a tetrahedral geometry, which can be confirmed from the bond angles around the boron atoms in 5a (C5-B2-Cl1 100.54(11), C5-B2-O1 106.51 (13), and C5-B2-C7 118.55 (14)). The B-C NHC bond length in 5a (1.648(2) Å) is in well agreement with that in 2. The B-O and B-Cl bonds in 5a is almost orthogonal to the plane (torsion angle: N1-C5-B2-Cl1 76.96(17) • , N2-C5-B2-Cl1 −89.6(2) • and torsion angle: N1-C5-B2-O1 −170.96 (14) • , N2-C5-B2-O1 22.5(2) • , respectively). However, the spectroscopic characterization of 5a becomes complicated because of solvent-mediated slow hydrolysis. The only signal in the 11 B NMR spectra of the product, 5b appears at 30.9 ppm as a singlet, which is indicative of a three-coordinated boron center, instead of a four-coordinated boron, as expected in 5a. We regrow the crystals from the NMR tube and realized that there is hydrolysis taking place at the B-Cl bond with adventitious water leading to 5-SIDipp stabilized borenium cation, with a triflate as a counter anion (5b). The constitution of 5b rationalizes the resonance at 30.9 ppm in the 11 B NMR spectrum. Although the formation of the 5b clearly can be seen from the molecular structure ( Figure S2), but due to low-quality data we refrain from discussing its structural parameters. However, even after repeated attempts, we were unable to stop this hydrolysis and hence, could not characterize 5a spectroscopically. The CF 3 group of the triflate moiety in 5b resonates at −78.6 ppm in the 19 F NMR, which is slightly different from the resonances of triflates bound to the boron atom (−76.7 ppm in 4) and is characteristic of the free triflate anion.
In the 11 B|NMR, 6 shows resonance at 4.2 ppm, shifted slight low-field with respect to that in 2 (1.9 ppm), presumably due to electron-withdrawing nature of the ONO 2 moieties. The solid-state structure of 6 also was confirmed by X-ray crystal analysis. 6 crystallizes in the monoclinic P2 1 /c space group ( Figure 6) and important structural parameters are given in the legend of Figure 6.    Treatment of one and two equivalents of AgOTf and AgNO3 with 2 in toluene afforded mono-triflate and di-nitro substituted 5-SIDipp·boranes, respectively. Substitution of one and two chlorine atoms from the tetra coordinated boron atom of 2 resulted in the formation of 5-SIDipp·BPhCl(OTf), 5a, and 5-SIDipp-BPh(ONO2)2, 6 (Scheme 3). The functional groups such as nitrate and triflate are rarely found to bind with the boron atom [5,9,20−26]. 5a crystallizes in the monoclinic P21/n space group ( Figure 5). The boron atom  In the 11 B NMR, 6 shows resonance at 4.2 ppm, shifted slight low-field with respec to that in 2 (1.9 ppm), presumably due to electron-withdrawing nature of the ONO2 moi eties. The solid-state structure of 6 also was confirmed by X-ray crystal analysis. 6 crystal lizes in the monoclinic P21/c space group ( Figure 6) and important structural parameter are given in the legend of Figure 6.  In the 11 B NMR, 6 shows resonance at 4.2 ppm, shifted slight low-field with respect to that in 2 (1.9 ppm), presumably due to electron-withdrawing nature of the ONO2 moieties. The solid-state structure of 6 also was confirmed by X-ray crystal analysis. 6 crystallizes in the monoclinic P21/c space group ( Figure 6) and important structural parameters are given in the legend of Figure 6. The combination of N-heterocyclic carbene and B(C6F5)3 has been exploited in FLP chemistry [27]. In our previous work, we have demonstrated the adduct formation between 5-SIDipp and B(C6F5)3 [8]. We have prepared the adduct in toluene/n-hexane. When we performed the same reaction in THF or diethyl ether, it led to the activation of those etheral solvents. The THF solution of the 5-SIDipp·B(C6F5)3 was kept for 12 h at room temperature, which afforded the zwitterionic species 7 in quantitative yield as a white solid (Scheme 4). The molecular structure of 7 was additionally established by X-ray diffraction analysis (Figure 7). 7 crystallizes in the triclinic P1 space group. One of the C-O bonds in the THF molecule is cleaved, and as a result, the THF ring becomes acyclic and inserts between the Lewis pairs. Similar to the case for 5-SIDipp·B(C6F5)3, 7 is not stable in solution at room temperature, so we were unable to satisfactorily characterize it by NMR spectroscopy. The 11 B NMR resonance at −2.8 ppm is similar to those established for the tetracoordinated boron compounds. The C5 atom adopts a trigonal planar geometry, which is confirmed by the sum of the bond angles [N1-C5-N2 112.29(15)°, N1-C5-C6 125.21(15)°, N2-C5-C6 122.50(14)°]. The C5-C6 bond distance is marginally shorter compared to the adjacent C-C bond (C5-C6 1.501(2) Å and C6-C7 1.539(2) Å). The boron atom adopts a tetrahedral geometry. The C-O (1.404(2) Å) and the B-O (1.453(2) Å) bond distances are similar to the other previously reported structures [28]. This reactivity was extended to diethyl ether (DEE), which resulted in the isolation of imidazolinium salt with a borate counter-anion (8). This compound presumably results from activation of the C-O bond of the diethyl ether with concomitant elimination of two ethylene molecules (Scheme 4). A signal at 8.9 ppm in the 1 H NMR spectrum confirms the presence of an imidazolinium cation. The 11 B NMR displays the characteristic resonance at −4.2 ppm, which can be assigned to a tetrahedral [(HO)B(C6F5)3] anion. X-ray crystallographic analysis later confirmed the structure of 8 (Figure 8). The combination of N-heterocyclic carbene and B(C 6 F 5 ) 3 has been exploited in FLP chemistry [27]. In our previous work, we have demonstrated the adduct formation between 5-SIDipp and B(C 6 F 5 ) 3 [8]. We have prepared the adduct in toluene/n-hexane. When we performed the same reaction in THF or diethyl ether, it led to the activation of those etheral solvents. The THF solution of the 5-SIDipp·B(C 6 F 5 ) 3 was kept for 12 h at room temperature, which afforded the zwitterionic species 7 in quantitative yield as a white solid (Scheme 4). The molecular structure of 7 was additionally established by X-ray diffraction analysis ( Figure 7). 7 crystallizes in the triclinic P1 space group. One of the C-O bonds in the THF molecule is cleaved, and as a result, the THF ring becomes acyclic and inserts between the Lewis pairs. Similar to the case for 5-SIDipp·B(C 6 F 5 ) 3 , 7 is not stable in solution at room temperature, so we were unable to satisfactorily characterize it by NMR spectroscopy. The 11 B NMR resonance at −2.8 ppm is similar to those established for the tetra-coordinated boron compounds. The C5 atom adopts a trigonal planar geometry, which is confirmed by the sum of the bond angles [N1-C5-N2 112.29 (15)

Conclusions
In summary, we have prepared 5-SIDipp·haloboranes adducts, 5-SIDipp·HBCl2 (1), and 5-SIDipp·PhBCl2 (2) and shown the selective nucleophilic substitution at the tetracoordinated boron center to obtain several boranes with rare functional groups such as -ONO2, -OTf, etc. The treatment of one equivalent of AgOTf with 1 and 2 led to the formation of haloboranes, 5-SIDipp·BHCl(OTf), 4 and 5-SIDipp·BPhCl(OTf), 5a, respectively, where all three substituents of the boron atom are different. 5a was found to be unstable and undergoes hydrolysis in the presence of adventitious water to give hydroxyborenium cation, 5b. The treatment of two equivalents of AgNO3 forms a rare di-nitrate substituted NHC-coordinated borane (6). The combination of 5-SIDipp and B(C6F5)3 were shown to affect the C-O bond cleavage differently for THF and diethyl ether. As expected, the 5-SIDipp/B(C6F5)3 combination in THF resulted in ring opening of the THF to produce borate 7. In the case of diethyl ether, the rupture of the C-O bond takes place along with the elimination of two molecules of ethylene, leading to the formation of an imidazolinium cation with tris (pentafluorophenyl) hydroxy borate as the counter anion (8).

General Procedures and Instrumentation
All manipulations were carried out in an inert atmosphere of argon using standard Schlenk techniques and in argon filled glove box. The solvents, especially toluene, tetrahydrofuran, dichloromethane, and n-hexane were purified by MBRAUN solvent purification system MB SPS-800. Other chemicals were purchased from Sigma Aldrich and TCI Chemicals and were used without further purification. The starting material, 5-SIDipp, was synthesized by using the literature procedure [29]. 1 H, 13 C, 11 B NMR, and 19 F spectra were recorded in CDCl3, using Bruker Avance DPX 400, or a Bruker Avance DPX 500 spectrometer. CDCl3 was dried by distillation over CaH2. Chemical shifts (δ) are given in ppm. NMR spectra were referenced to external SiMe4 ( 1 H and 13 C), BF3·OEt2 ( 11 B), CFCl3 ( 19 F) respectively. 1: A slightly excess of BHCl2·dioxane (0.20 mL, 0.58 mmol) was added to a 10 mL hexane solution of 5-SIDipp (0.20 g, 0.48 mmol) at room temperature in a Schlenk flask.

Conclusions
In summary, we have prepared 5-SIDipp·haloboranes adducts, 5-SIDipp·HBCl 2 (1), and 5-SIDipp·PhBCl 2 (2) and shown the selective nucleophilic substitution at the tetracoordinated boron center to obtain several boranes with rare functional groups such as -ONO 2 , -OTf, etc. The treatment of one equivalent of AgOTf with 1 and 2 led to the formation of haloboranes, 5-SIDipp·BHCl(OTf), 4 and 5-SIDipp·BPhCl(OTf), 5a, respectively, where all three substituents of the boron atom are different. 5a was found to be unstable and undergoes hydrolysis in the presence of adventitious water to give hydroxyborenium cation, 5b. The treatment of two equivalents of AgNO 3 forms a rare di-nitrate substituted NHC-coordinated borane (6). The combination of 5-SIDipp and B(C 6 F 5 ) 3 were shown to affect the C-O bond cleavage differently for THF and diethyl ether. As expected, the 5-SIDipp/B(C 6 F 5 ) 3 combination in THF resulted in ring opening of the THF to produce borate 7. In the case of diethyl ether, the rupture of the C-O bond takes place along with the elimination of two molecules of ethylene, leading to the formation of an imidazolinium cation with tris (pentafluorophenyl) hydroxy borate as the counter anion (8).

General Procedures and Instrumentation
All manipulations were carried out in an inert atmosphere of argon using standard Schlenk techniques and in argon filled glove box. The solvents, especially toluene, tetrahydrofuran, dichloromethane, and n-hexane were purified by MBRAUN solvent purification system MB SPS-800. Other chemicals were purchased from Sigma Aldrich and TCI Chemicals and were used without further purification. The starting material, 5-SIDipp, was synthesized by using the literature procedure [29]. 1 H, 13 C, 11 B NMR, and 19 F spectra were recorded in CDCl 3 , using Bruker Avance DPX 400, or a Bruker Avance DPX 500 spectrometer. CDCl 3 was dried by distillation over CaH 2 . Chemical shifts (δ) are given in ppm. NMR spectra were referenced to external SiMe 4 ( 1 H and 13 C), BF 3 ·OEt 2 ( 11 B), CFCl 3 ( 19 F) respectively.

4:
A DCM solution (20 mL) of 1 (0.47 g, 1 mmol) was added dropwise to a DCM solution (20 mL) of previously weighed AgOTf (0.25 g, 1 mmol) at −78 • C in the absence of light. A white precipitate of AgCl was formed immediately, and it was filtered through frit filtration after the reaction mixture was warmed to room temperature. The colorless toluene solution was concentrated (5 mL) and kept for crystallization at 4 • C, which afforded colorless crystals of 4 after 1−2 day(s). Yield = 0.48 g (82%  Figure S13). 5b: A toluene solution (20 mL) of 2 (0.55 g, 1 mmol) was added dropwise to a toluene solution (20 mL) of previously weighed AgOTf (0.25 g, 1 mmol) at −30 • C in the absence of light. A white precipitate of AgCl formed immediately, and it was filtered through frit filtration after the reaction mixture was warmed to room temperature. The colorless toluene solution was concentrated (5 mL) and was kept for crystallization at 4 • C, which afforded colorless crystals of 5b after 1 day. Yield = 0.25 g (45%).

7:
A THF solution (5 mL) of 5-SIDipp (0.382 g, 1 mmol) was added dropwise to a THF solution (20 mL) of previously weighed B(C 6 F 5 ) 3 (0.512 g, 1 mmol) at room temperature. The reaction mixture turned to a clear colorless solution immediately and run for 12 h. The solution was dried completely and 3 mL of toluene solution was added to dissolve the white solid product. Colorless crystals of 7 were afforded after keeping the toluene solution for crystallization at 4 • C after 2 days. Yield = 0.45 g (46%). The formation of 7 was accompanied by some other side products, which could not be identified. Hence, we did not have a spectroscopically pure product to record the 1 H and 13 C NMR. 11 B{ 1 H} NMR (128 MHz, 298 K, CDCl 3 ): δ = −2.8 (s, 1 B, B(C 6 F 5 ) 3 ) ppm ( Figure S23). 8: A diethyl ether solution (5 mL) of 5-SIDipp (0.382 g, 1 mmol) was added dropwise to a diethyl ether solution (5 mL) of previously weighed B(C 6 F 5 ) 3 (0.512 g, 1 mmol) at room temperature. The reaction mixture turned to a clear colorless solution immediately and run for 12 h. The solution was dried completely and 3 mL of toluene solution was added to dissolve the white solid product. Colorless crystals of 8 were afforded after keeping the toluene solution for crystallization at 4 • C after 1 day. Yield = 0.78 g (85%). Data Availability Statement: All the data reported in the study can be found in the supporting information and from the CCDC repository with accession numbers: 2180189-2180191, 2180197-2180200, 2180457.