Highly Reactive Cyclic(alkyl)(amino) Carbene- and N-Heterocyclic

Aug 30, 2018 - Synopsis. The first examples of cyclic(alkyl)(amino) carbene (CAAC)-bismuth complexes have been structurally characterized and are ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Highly Reactive Cyclic(alkyl)(amino) Carbene- and N‑Heterocyclic Carbene-Bismuth(III) Complexes: Synthesis, Structure, and Computations Guocang Wang,† Lucas A. Freeman,† Diane A. Dickie,† Réka Mokrai,‡ Zoltán Benkő ,‡ and Robert J. Gilliard, Jr.*,†

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†

Department of Chemistry, University of Virginia, 409 McCormick Road, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States ‡ Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szent Gellért tér 4, Budapest, 1111, Hungary S Supporting Information *

ABSTRACT: Cyclic(alkyl)(amino) carbene (CAAC)-stabilized complexes of phosphorus, one of the lightest group 15 elements, are well-established and can often be obtained in high yields. In contrast, analogous CAAC compounds of bismuth, the heaviest nonradioactive member of group 15, are unknown. Indeed, reactivity increases as you descend the group, and as a result there are only a few examples of N-heterocyclic carbene (NHC)bismuth complexes. Moreover, activated bismuth compounds often readily extrude bismuth metal, making isolation of stable complexes highly challenging. We report that CAACs react with phenylbismuth dichloride (PhBiCl2) to afford Et2CAAC-Bi(Ph)Cl2 and CyCAAC-Bi(Ph)Cl2. Significantly, these complexes represent the first structurally characterized examples of CAAC-coordination to bismuth. The CAAC-stabilized bismuth compounds can also be obtained from air-stable salts, [Et2CAAC-H]22+ [Cl2(Ph)Bi(μ-Cl2)Bi(Ph)Cl2]2− and [CyCAAC-H]22+ [Cl2(Ph)Bi(μ-Cl2)Bi(Ph)Cl2]2−, by deprotonation with potassium bis(trimethylsilyl)amide, K[N(SiMe3)2]. The electronic effects of the ligand on the bismuth center were investigated by comparing the CAAC-Bi(Ph)Cl2 complexes to the NHC analogues, SIPr-Bi(Ph)Cl2(THF) and IPr-Bi(Ph)Cl2 (SIPr = 1,3bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Interestingly, the “normal” IPr-Bi(Ph)Cl2 slowly isomerizes to the “abnormal” carbene complex, Cl2(Ph)Bi-IPr-H, at −37 °C. In the solid-state, the CAAC-, NHC-, and abnormal NHC-bismuth compounds exhibit Bi atomic centers in unique coordination environments. The complexes were fully characterized by NMR, elemental analysis, and single crystal X-ray diffraction studies. In addition, the bonding was probed by natural bond orbital (NBO) calculations.

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INTRODUCTION It has been 27 years since the seminal discovery of Arduengo’s crystalline N-heterocyclic carbene (NHC).1 Since then, NHCs have become one of the most popular classes of ligands used in synthetic chemistry.2 In main-group chemistry, NHCs have served as ligands in catalysis and as a means to isolate highly reactive molecules with unusual bonding.3 Of particular interest over the past decade have been the syntheses of carbene-stabilized homo- and heteronuclear bonds, radicals, and related main-group molecules that were otherwise inaccessible.3 It is noteworthy that the synthesis of most of these molecules began with a carbene-main-group element halide complex. In group 15 chemistry, there are hundreds of examples of carbene-phosphorus complexes,4 several arsenic5 and antimony6 complexes have also been reported. However, bismuth has remained a challenge with only two reports in the literature.7,8 As you descend the group, the atomic size © XXXX American Chemical Society

increases resulting in poorer orbital overlap, and isolation of stable carbene-element molecules becomes more challenging. For example, Dutton reported that NHCs react with BiCl3 to form the NHC-BiCl3 adduct (A) (Figure 1).7 This compound was obtained in 56% yield, while analogous NHC complexes of PCl3 and AsCl3 were obtained quantitatively, ca. 97% yield.9,10 In two steps starting from the corresponding free NHC, the triflate-substituted compound (B) was also isolated.7 Recently, Goicoechea prepared the bismuth salt (C) by combining a mixture of NHC-BiBr3 (D) and AlBr3.8 We are interested in the synthesis of heavier main-group oxo complexes for C−H bond activation. Inspired by Evans and coworkers11 who proposed that a reactive Bi−O intermediate facilitated C−H activation, we sought to utilize carbenes to Received: June 29, 2018

A

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Reaction of Cyclic(Alkyl)(Amino) Carbenes with Phenylbismuth Dichloride

Figure 1. (A−D) Structurally characterized NHC-bismuth complexes.

isolate low-valent bismuth complexes that could undergo oxidation to form stable molecules with a {carbene−Bi−O} moiety. Realizing that these types of bismuth complexes would be highly reactive, we used the strongly σ-donating and πaccepting cyclic(alkyl)(amino) carbene (CAAC), which has been rapidly emerging since the original report by Bertrand.12 These CAAC ligands have facilitated the isolation of a number of compounds where NHCs have been insufficient.13 Our initial efforts using this reduction−oxidation strategy starting from CAAC and BiCl3 did not yield the desired product. Consistent with the challenges reported by Dutton with NHCs,7 we observed that a number of reactions favored the formation of black precipitate, which was attributed to metallic bismuth. Indeed, mixtures of free CAAC and BiCl3 were unstable and generated significant amounts of bismuth metal prior to the addition of other reagents. This unusual reactivity, compared to lighter group 15 analogues,3−6 prompted us to study the coordination chemistry of carbenes toward phenylbismuth dichloride (PhBiCl2) with the hypothesis that the addition of a phenyl group would enhance stability. Herein, we report the synthesis, molecular structures, and computations of Et2 CAAC-Bi(Ph)Cl2 (1), CyCAAC-Bi(Ph)Cl2 (2), [Et2CAACH]22+ [Cl2(Ph)Bi(μ-Cl2)Bi(Ph)Cl2]2− (3), [CyCAAC-H]22+ [Cl2(Ph)Bi(μ-Cl2)Bi(Ph)Cl2]2− (4), SIPr-Bi(Ph)Cl2 (5), IPrBi(Ph)Cl2 (6), and Cl2(Ph)Bi-IPr-H (7). Notably, compounds 1 and 2 represent the first examples of cyclic(alkyl)(amino) carbene-bismuth complexes. Moreover, together with 5, 6, and 7, these are the first carbene−Bi complexes where a sigmabonded aryl group is attached to bismuth metal.

due to the steric demand of the iPr and Me groups on the cyclohexyl substituent, which prevents complexation. While pure 1 has limited stability in the solid state, compound 2 is stable. However, some decomposition to bismuth metal and free CAAC is observed in THF solutions of 1 and 2 above −20 °C. Despite this, we found the synthesis of 1 and 2 to be higher yielding and more stable than the corresponding CAAC complexes of bismuth(III) chloride. Yellow air- and moisture-sensitive block-shaped crystals of 1 suitable for a single crystal X-ray diffraction study were grown from a THF/toluene (1:2) mixture at room temperature. The asymmetric unit of 1 shows a tetracoordinate bismuth in a seesaw environment, with two Bi−Cl bonds and two Bi−C bonds. Examination of the packing diagram reveals an additional Bi−Cl interaction, stabilizing 1 as a dimer with pentacoordinate bismuth centers in a square pyramidal arrangement (Figure 2). Consequently, there are significant differences in the Bi−Cl bond lengths [Bi1−Cl1: 2.7654(5), Bi1−Cl2: 2.6281(5), Bi1−C1′: 3.2729(6)]. Notably, the C1− Bi1 bond length of 2.457(2) Å is the longest of all reported Ccarbene−Bi bonds (2.35−2.418 Å),7,8 which could be a contributing factor to the instability of 1. The C1−Bi1 bond is also significantly longer than the C23−Bi1 bond [2.273(2) Å], which is indicative of the dative character of the CAAC→ Bi interaction. Yellow rectangular prism-shaped single crystals of compound 2 suitable for X-ray diffraction analysis were obtained from a solution of THF/hexanes (1:2) at room temperature. The molecular structure of 2 shows a CyCAAC coordinated to a Bi(Ph)Cl2 moiety (Figure 3). As with 1, compound 2 is dimeric in the solid-state. The pentacoordinate square pyramidal geometry at the Bi1 center is completed by an interaction with a second CyCAAC-Bi(Ph)Cl2 unit. Compound 2 possesses an intermolecular Bi1−C1′ bond [3.1276(7) Å] that is shorter than that in 1 [3.2729(6)]. Unlike compound 1, compound 2 has intramolecular Bi−Cl bond lengths that are nearly equal [Bi1−Cl1: 2.6991(7), Bi1−Cl2: 2.7042(7) Å]. The C1−Bi1 bond length in compound 2 [2.412(2) Å] is

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RESULTS AND DISCUSSION A THF solution of PhBiCl2 was added to Et2CAAC in diethyl ether at −37 °C (Scheme 1). Upon warming to room temperature, a yellow solid precipitated out of solution. The 1 H NMR spectrum of the solid in THF-d8 revealed a characteristic singlet at 1.91 ppm for the CH2 protons of the five-membered heterocyclic ring, compared to 1.71 ppm for the free Et2CAAC. This new peak was attributed to compound 1, the first CAAC-bismuth complex, which was isolated in 45% yield. Compound 2, containing the larger CyCAAC, was prepared in a similar fashion in 50% yield. The 1H resonance for the CH2 protons of 2 is shifted significantly downfield to 2.16 ppm from 1.76 ppm for the free CyCAAC. Interestingly, reaction of the bulky Cy*CAAC with PhBiCl2 did not produce a precipitate, and 1H NMR studies of the reaction mixture showed free Cy*CAAC and PhBiCl2 starting materials. This is B

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Å] is slightly larger than that in 1 [92.95(6) Å], while the N1− C1−Bi1 angle in 2 [115.0(1)°] is slightly smaller than in 1 [118.81(1)°]. Interestingly, with the C1→Bi1 axis of 1 as a reference, the larger and more rigid cyclohexyl group in 2 promotes the anticlockwise diagonal rotation of the Bi(Ph)Cl2 unit. During our studies we determined that [Et2CAAC-H]+[Cl]− and [CyCAAC-H]+[Cl]−salts react with phenylbismuth dichloride to afford compounds 3 or 4, respectively (Scheme 2). Air-stable 3 and 4 can be deprotonated with potassium bis(trimethylsilyl)amide to give 1 or 2. However, the direct reaction of CAAC with PhBiCl2 leads to easier isolation of the pure compounds. Colorless block-shaped single crystals of 3 and 4 were obtained from a DCM/Et2O (1:1) mixture (Figures 4 and 5). In contrast to the long intermolecular Bi1′− Cl1 distances in 1 [3.2729(6)] and 2 [3.1276(7)], the bridging Bi−Cl bonds in 3 [Bi1−Cl1: 2.8460(6), Bi1−Cl1′: 2.8505(6)] and 4 [Bi1−Cl1: 2.8337(8), Bi1−Cl1′: 2.8378(9)] are significantly shorter. After isolating CAAC-Bi(Ph)Cl2 complexes, we sought to compare the structures and properties of 1 and 2 to their NHC analogues. THF solutions of SIPr and PhBiCl2 were cooled to −37 °C, combined, and allowed to react overnight (Scheme 3). The 1H NMR spectrum of the reaction mixture revealed a characteristic singlet at 4.19 ppm for the CH2 protons of the five-membered heterocyclic ring (compared to 3.79 ppm for the free SIPr), and the multiplet for the methine proton of the Dipp group was shifted downfield from 3.24 to 3.57 ppm. This new species was attributed to compound 5, which was isolated in 24% yield. The stability of compound 5 is dependent on the weakly bound THF. Loss of THF results in the decomposition of 5 and the formation of bismuth metal and free SIPr. Colorless rectangular prism-shaped single crystals of compound 5 suitable for X-ray diffraction analysis were obtained from a THF/hexanes (2:1) solution at −37 °C. The structure of compound 5 shows a pentacoordinate bismuth center adopting a distorted square pyramidal geometry (Figure 6). The C1−Bi1 bond length of 2.428(3) Å in compound 5 is between the C1−Bi1 bonds in 1 [2.457(2) Å] and 2 [2.412(2) Å]. The Bi−Cl bond lengths in compound 5 [Bi1−Cl1: 2.688(1), Bi1−Cl2: 2.665(1) Å] are comparable to CAAC complexes 1 [Bi1−Cl1: 2.7653(5), Bi1−Cl2: 2.6282(5) Å] and 2 [Bi1−Cl1: 2.6991(7) Å, Bi1−Cl2: 2.7042(7) Å]. The weak dative-type Bi−OTHF bond in compound 5 [2.814(2) Å] is longer than that in PhBiCl2(THF)14 [2.608(7) Å], but in the range of reported Bi−OTHF bonds [2.404(7)−3.016(9) Å].15,16 To investigate the effect changes to the electronics of imidazole ring have on the bismuth center, we sought to isolate the unsaturated carbene analogue of 5. Therefore, IPr and PhBiCl2 were combined in THF-d8 at −37 °C. 1H NMR

Figure 2. Molecular structure of 1 (thermal ellipsoids at 50% probability; H atoms and a cocrystallized toluene molecule are omitted for clarity). Selected bond distances (Å) and angles (deg): C1−N1: 1.304(2); C1−C2: 1.532(2); C1−Bi1: 2.4566(15); Bi1− C23: 2.2732(16); Bi1−Cl1: 2.7654(5); Bi1−Cl2: 2.6281(5); Bi1--Cl1′: 3.2729(6); N1−C1−Bi1: 118.81(11); C1−Bi1−C23: 92.95(5), C1−Bi1−Cl1: 91.08(4); C1−Bi1−Cl2: 94.86(4).

Figure 3. Molecular structure of 2 (thermal ellipsoids at 50% probability; H atoms omitted for clarity). Selected bond distances (Å) and angles (deg): C1−N1: 1.304(3); C1−C2: 1.519(3); C1−Bi1: 2.4123(19); B1−C24: 2.267(2); Bi1−Cl1: 2.6991(7); Bi1−Cl2: 2.7042(6); Bi1---Cl1′: 3.1276(7); N1−C1−Bi1: 115.02(14); C1− Bi1−C24: 94.79(7), C1−Bi1−Cl1: 96.65(5); C1−Bi1−Cl2: 80.52(5).

slightly shorter than in 1 [2.457(2)]. However, there is no significant difference in the Bi−CPh bond lengths [2.273(2) Å (1); 2.267(2) Å (2)]. The C1−Bi1−C24 angle in 2 [94.79(7)

Scheme 2. Synthesis of Aldiminium-Bismuth Salts, Transformation to Cyclic(Alkyl)(Amino) Carbene-Bismuth Coordination Complexes

C

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Molecular structure of 3 (thermal ellipsoids at 50% probability; H atoms omitted for clarity; only the major position of the disordered Et group shown). Selected bond distances (Å): Bi1−Cl1: 2.8460(6); Bi1−Cl2: 2.6180(6); Bi1−Cl3: 2.6344(6); Bi1−C23: 2.263(2).

Figure 5. Molecular structure of 4 (thermal ellipsoids at 50% probability; H atoms omitted for clarity). Selected bond distances (Å): Bi1−Cl1: 2.8377(8); Bi1−Cl2: 2.6229(9); Bi1−Cl3: 2.6384(8); Bi1−C24: 2.245(3).

Scheme 3. Reaction of N-Heterocyclic Carbenes with Phenylbismuth Dichloride

Figure 6. Molecular structure of 5 (thermal ellipsoids at 50% probability; H atoms and noncoordinated THF omitted for clarity). Selected bond distances (Å) and angles (deg): C1−N1: 1.327(4); C1−N2: 1.337(4); C2−C3: 1.507(4); C1−Bi1: 2.428(3); B1−C28: 2.253(3); Bi1−Cl1: 2.6881(9); Bi1−Cl2: 2.6649(9); Bi1−O1: 2.814(2); N1−C1−Bi1: 115.92(19); C1−Bi1−C28: 98.61(10), C1−Bi1−Cl1: 96.14(7); C1−Bi1−Cl2: 82.76(7); C28−Bi1−O1: 82.07(9); C1−Bi1−O1: 174.12(9).

described above, the Bi1 atom in 6 achieves pentacoordinate geometry via a weak interaction with the phenyl ring of an adjacent IPr ligand (Bi1−centroid = 3.500 Å) instead of with a nearby chloride. In contrast, Bi2 does form an intermolecular Bi2−Cl2 interaction (Figure 7). The C1−Bi1 and C34−Bi2 bonds in 6 [2.342(6) Å, 2.367(6) Å, respectively] are slightly shorter than in 5 [2.428(3) Å]. In notable contrast to the C2− C3 single bond in compound 5 [1.507(4) Å], the shortened C2−C3 and C35−C36 bonds in 6 [1.345(8) Å, 1.340(8) Å respectively] unambiguously confirm the unsaturated nature of the carbene backbone. This unique bonding arrangement is also evident in the C1−Bi1−Cl1 and C34−Bi2−Cl3 angles of 6 [88.99(15)°, 81.95(14)°, respectively], which are significantly smaller than the comparable C1−Bi1−Cl1 angle in 5 [96.14°]. The different coordination modes observed for compounds 5 and 6 may be attributed to the properties of the ligands. While there is a relatively small difference in the donor strength of the saturated (SIPr) and unsaturated (IPr) carbene ligands, SIPr is a stronger π-acceptor. During the process of growing single crystals of 6 from a THF/hexanes (2:1) mixture at −37 °C, compound 7 was

studies revealed a downfield shift to 3.08 ppm for the methine proton of the Dipp groups (2.88 ppm for free IPr ligand), indicating the formation of a new product. The solvent was removed to afford a white solid, and after recrystallization from THF/hexanes (2:1) compound 6 was isolated in 25% yield. While a THF solution of compound 6 is relatively stable at low temperature, at room temperature, 6 decomposes to produce metallic bismuth. Air and moisture-sensitive colorless crystals of 6 suitable for a single crystal X-ray diffraction study were obtained from a THF/hexanes (2:1) mixture at −37 °C. It is noteworthy that compound 6 has two crystallographically distinct molecules in the asymmetric unit. Unlike the complexes that have been D

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

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intermolecular interactions are instead C−H···Cl hydrogen bonds between the former carbenic carbon and the chloride with a slightly longer Bi1−Cl1 bond [2.703(2) Å]. There was no evidence of compound 7 in the original NMR spectrum obtained from the reaction of IPr with PhBiCl2. Subsequent 1H NMR analysis of the solution from which 6 and 7 were isolated showed that the conversion of 6 to 7 in THF at −37 °C after 2 weeks was less than 5%. Attempts were made to promote the conversion on a preparative scale; however, in solution 6 is highly reactive and decomposes to bismuth metal and unidentified products. Consequently, compound 7 could only be isolated for crystallographic studies (Figure 8). Although the mechanism for the isomerization is unclear, Goicoechea observed the transformation of IPr-EBr3 (E = Sb, Bi) to the corresponding abnormal carbene complexes at 75 °C.8 To gain insight into the structure and properties of these complexes, we performed density functional theory (DFT) calculations on simplified systems (Dipp and Et groups were replaced by methyl groups indicated as 1M, 5M, and 6M for 1, 5, and 6, respectively) at the ωB97XD/cc-pVDZ(-PP) level of theory (see Table 2 and Supporting Information for details). The complex formation energy and Gibbs free energy for 1M are more negative compared to the analogous NHC complexes. The data are in agreement with the slightly shortened Bi−Ccarbene distance of 1M (2.34 Å) compared to 5M (2.36 Å) and 6M (2.37 Å). In all three complexes the bond between Bi and the carbenic C atoms is clearly weaker than the one between the Bi and C atoms of the Ph groups (cf. 2.34 Å vs 2.27 Å in case of 1M) and can be described as dative type. Indeed, NBO calculations show a high contribution of the C atom in this bond (ca. 80% in all three complexes). Besides the small differences in the Bi−Ccarbene bond lengths the other geometrical parameters are similar for 1M, 5M, 6M, and correspond to the experimental data. There is a marked contrast between the parameters of the complexes compared to the uncomplexed PhBiCl2. The Bi−Cl bonds elongate and weaken upon coordination (e.g., in the case of 1M the bond length increases from 2.48 Å in BiPhCl2 to 2.70 Å in the complex, and the Wiberg bond index (WBI) value decreases from 0.74 to 0.43). The Bi−CPh bond stays practically unchanged. Similar changes can be observed in the charge distribution, which was determined by natural population analysis (NPA). The partial charges of the Cl atoms become more negative by ca. 0.10−0.15 e (e.g., −0.51 e in BiPhCl2 and −0.61 e in 1M). In the complexes, the net charge donation from the carbene to the BiPhCl2 moiety (Δq, calculated from the sum of partial charges within the BiPhCl2 fragment) amounts to ca. −0.25−(−0.36) e. However, it is noteworthy that the charges of the Bi atoms and phenyl groups show no significant change upon coordination. Thus, we conclude that the donation from the carbene disperses mainly on the Cl atoms.

Figure 7. Molecular structure of 6 (thermal ellipsoids at 50% probability; H atoms and cocrystallized THF omitted for clarity). Selected bond distances (Å) and angles (deg): C1−N1: 1.356(8); C1−N2: 1.354(7); C2−C3: 1.345(8) C1−Bi1: 2.342(6); Bi1−C28: 2.266(6); Bi1−Cl1: 2.6739(16); Bi1−Cl2: 2.7044(15); C34−N3: 1.346(7); C34−N4: 1.354(7); C35−C36: 1.340(8) C34−Bi2: 2.367(6); Bi2−C61: 2.250(6); Bi2−Cl3: 2.7021(16); Bi2−Cl4: 2.6462(15); Bi2−Cl2′: 3.4661(15); N1−C1−Bi1: 132.4(4); N2− C1−Bi1: 120.1(4); C1−Bi1−C28: 103.5(2), C1−Bi1−Cl1: 88.99(15); C1−Bi1−Cl2: 82.14(15); N3−C34−Bi2: 118.6(4); N4−C34−Bi2: 134.9(4); C34−Bi2−C61: 99.9(2), C34−Bi2−Cl3: 81.95(14); C34−Bi2−Cl4: 87.93(14).

Figure 8. Molecular structure of 7 (thermal ellipsoids at 50% probability; H atoms and cocrystallized THF omitted for clarity; only the major position of the disordered Dipp group shown). Selected bond distances (Å) and angles (deg): C1−N1: 1.346(10); C1−N2: 1.335(10); C2−C3: 1.347(11); N2−C3: 1.405(9); C2−Bi1: 2.248(8); Bi1−Cl1: 2.703(2); Bi1−Cl2: 2.645(2); Bi1−C28: 2.260(9); C34−N3: 1.339(10); C34−N4: 1.302(10); C35−C36: 1.366(11); C35−Bi2: 2.218(7); N4−C36: 1.389(9); Bi1−Cl3: 2.759(2); Bi2−Cl4: 2.637(2); Cl1−H34: 3.428(8) N1−C2−Bi1: 125.6(5); C2−Bi1−C28: 90.0(3), C35−Bi2−C61: 93.0(3); N3− C35−Bi2: 125.1(5).

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obtained as colorless prism-shaped single crystals whose structure is shown in Figure 8. The IPr ligand in 7 is coordinated to bismuth “abnormally” through one of the backbone carbon atoms instead of the “normal” carbenic carbon.17 The Bi−CIPr [C2−Bi1: 2.248(8) Å] and Bi−CPh [Bi1−C28: 2.260(9) Å] bonds are equal within experimental error, which is quite different from the other compounds described in this report. Also notable is that there are no intermolecular interactions involving the Bi atoms in 7. Therefore, the Bi atomic centers are truly tetracoordinate with seesaw molecular geometry. The only significant

CONCLUSION We have synthesized and structurally characterized the first examples of CAAC-bismuth complexes. To the best of our knowledge, these Bi compounds represent the heaviest metal to which a CAAC has been coordinated and the longest CAAC-p-block element bonds on record. We have also investigated their stability by comparing them with their NHC analogues, which were structurally unique. All of these compounds are highly reactive in solution at room temperE

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Bond Distances (Å) and Angles (deg) for Compounds 1−7 1

2

3

5

6

7

2.428(3)

2.342(6)

2.245(3)

2.253(3)

2.266(6)

2.6180(6) 2.6344(6)

2.6229(9) 2.6384(8)

2.6649(9) 2.6881(9)

2.6739(16) 2.7044(15)

2.218(7) 2.218(7) 2.248(8) 2.260(9) 2.254(8) 2.645(2) 2.703(2) 2.637(2) 2.759(2)

1.279(3)

1.273(3)

1.327(4)

1.356(8)

Bi−Ccarbene

2.4566(15)

2.4123(19)

Bi−CPh

2.2732(16)

2.267(2)

2.263(2)

Bi−Cl

2.6281(5) 2.7654(5)

2.6991(7) 2.7042(6)

Bi1---C1′ C1−N1

3.2729(6) 1.304(2)

3.1276(7) 1.304(3)

173.831(13)

174.629(19)

Cl1−Bi1−Cl2

4

174.12(9)

SHELXTL Software Package within OLEX2. Severely disordered solvent in 4 was accounted for using the Platon Squeeze. Other solvent and/or main molecule disorder in 1, 3, and 5−7 was modeled with constraints and restraints as needed on the thermal displacement parameters or bond distances as needed. Deuterated solvents were purchased from Acros Organics and Cambridge Isotope Laboratories. Elemental analyses were performed at Robertson Microlit Laboratories, Ledgewood, NJ, USA. Et2CAAC,18 CyCAAC,12 Cy*CAAC,12 SIPr,19 IPr,19 and PhBiCl214 were prepared using literature procedures. Synthesis of Compound 1. To a dry Et2O (15 mL) solution of Et2 CAAC (314 mg, 1.0 mmol) in a 100 mL Schlenk flask at −37 °C was added a solution of PhBiCl2 (357 mg, 1.0 mmol) in dry THF (5 mL). The reaction was allowed to warm to room temperature and stirring was continued for 2 h. The yellow precipitate was collected by filtration. The solid was washed with 2 × 10 mL of Et2O to yield compound 1 as an air- and moisture-sensitive yellow solid (302 mg, 45% yield). Yellow crystals suitable for X-ray diffraction studies were obtained from a THF/toluene (1:2) mixture at room temperature. 1H NMR (800.13 MHz, THF-d8, 298 K): δ = 9.66 (d, JHH = 7.8 Hz, 2H, Hortho-Ph), 7.70 (t, JHH = 7.8 Hz, 2H, Hmeta-Ph), 7.47 (d, JHH = 7.6 Hz, 2H, Hmeta-dipp), 7.36 (t, JHH = 7.8 Hz, 1H, Hpara-dipp), 7.32 (t, JHH = 7.7 Hz, 1H, Hpara-Ph), 3.11 (sept, JHH = 7.6 Hz, 2H, CHCH3), 1.91 (s, 2H, CH2), 1.78 (m, 2H, CH2CH3), 1.69 (m, 2H, CH2CH3), 1.44 (s, 6 H, CCH3), 1.28 (d, JHH = 7.0 Hz, 12H, CHCH3), 0.92 (t, JHH = 16.0 Hz, 6H, CH3). No sufficiently resolved 13C data could be obtained. m.p.: decomposed at 140 °C. Anal. Calcd for C28H40BiCl2N: C, 50.16; H, 6.01; N, 2.09%. Found: C, 49.66; H, 5.92; N, 1.95%. Synthesis of Compound 2. To a dry Et2O (15 mL) solution of Cy CAAC (350 mg, 1.07 mmol) in a 100 mL Schlenk flask at −37 °C was added a solution of PhBiCl2 (461 mg, 1.07 mmol) in dry THF (5 mL). The reaction was allowed to warm to room temperature, and stirring was continued for 2 h. The yellow solid was collected by filtration. The solid was washed with 2 × 10 mL of Et2O to yield compound 2 as an air- and moisture-sensitive yellow solid (335 mg, 50% yield). Yellow crystals suitable for X-ray diffraction studies were obtained from a THF/hexane (1:2) layered solvent at room temperature. 1H NMR (800.13 MHz, THF-d8, 298 K): δ = 9.60 (d, JHH = 7.8 Hz, 2H, Hortho-Ph), 7.70 (t, JHH = 7.8 Hz, 2H, Hmeta-Ph), 7.49 (t, JHH = 7.8 Hz, 1H, Hpara-Dipp), 7.41 (d, JHH = 7.7 Hz, 2H, Hmeta-Dipp), 7.38 (t, JHH = 7.7 Hz, 1H, Hpara-Ph), 3.21 (sept, JHH = 7.7 Hz, 2H, CHCH3), 2.16 (s, 2H, CH2), 2.02 (dt, JHH = 16.0 Hz, 2H, H2CCy), 1.60 (s, 6 H, CCH3), 1.58 (d, JHH = 16.0 Hz, 2H, H2CCy), 1.51 (d, JHH = 8.0 Hz, 6 H, CHCH3), 1.45 (t, JHH = 16.0 Hz, 2H, H2CCy), 1.33 (d, JHH = 16.0 Hz, 2H, H2CCy), 1.25 (d, JHH = 8.0 Hz, 6H, CHCH3), 1.17 (t, JHH = 16.0 Hz, 2H, H2CCy). No sufficiently resolved 13C data could be obtained. m.p.: decomposed at 150 °C. Anal. Calcd for C29H40BiCl2N: C, 51.03; H, 5.91; N, 2.05%. Found: C, 51.02; H, 5.73; N, 2.01%. Synthesis of Compound 3. Solid [Et2CAAC-H][Cl] (700 mg, 2.0 mmol) was added to a dry THF (20 mL) solution of PhBiCl2 (714 mg, 2.0 mmol) at room temperature, and the reaction was

Table 2. Formation Energies (ΔE) and Gibbs Free Energies (ΔG) in kcal/mol, Geometrical Parameters (Bond Length in Å, Wiberg Bond Indices), NPA Partial Charges (q) in Electrons (q(Ph) is Calculated as Sum of Partial Charges within the Ph Group) and Net Charge Transfer in Electrons BiPhCl2 ΔE ΔG Bi−Ccarbene (Å) Bi−CPh (Å) Bi−Cl (Å) Bi−Ccarbene WBI Bi−CPh WBI Bi−Cl WBI q(Bi) q(Cl) q(Ph) Δq

2.25 2.48 0.80 0.74 1.42 −0.51 −0.40

1M

5M

6M

−44.1 −27.8 2.34 2.27 2.70 0.61 0.77 0.43 1.40 −0.61 −0.43 −0.25

−35.9 −19.3 2.37 2.27 2.69 0.60 0.78 0.45 1.41 −0.66 −0.45 −0.36

−38.5 −21.9 2.36 2.27 2.70 0.61 0.78 0.44 1.41 −0.67 −0.42 −0.35

ature. To gain insight into the structure and bonding of these complexes, we performed DFT calculations that indicated the bond between Bi and the carbenic carbon atoms is relatively weak and can be described as dative type. Indeed, NBO calculations show a significant contribution of the carbon atom in Ccarbene−Bi bonds. The charges of the Bi atoms and phenyl groups show no significant change upon coordination, and the donation from the carbene disperses mainly on the Cl atoms. Studies are underway regarding the utilization of these molecules as precursors for catalytic reactions.

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169.43(5)

1.346(10) 1.339(10) 173.80(7)

EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out under an atmosphere of purified nitrogen or argon using standard Schlenk techniques or in a MBRAUN LABmaster glovebox equipped with a −37 °C freezer. Dichloromethane was purified by distillation over calcium hydride. All other solvents were distillated over sodium/ benzophenone. Glassware was oven-dried at 190 °C overnight. The NMR spectra were recorded at room temperature on a Bruker Avance 600 MHz (1H: 600.13 MHz and 13C: 150.90 MHz) and 800 MHz spectrometer (1H: 800.13 MHz and 13C: 201.193 MHz). Proton and carbon chemical shifts are reported in ppm and are referenced using the residual proton and carbon signals of the deuterated solvent (1H; THF-d8, δ 3.58, 1.72, 13C; THF-d8, δ 67.21, 25.31; 1H; CD2Cl2, δ 5.32, 13C; CD2Cl2, δ 53.84). Single crystal X-ray diffraction data were collected on a Bruker Kappa APEXII Duo diffractometer running the APEX3 software suite with an Incoatec Microfocus IμS (Cu Kα, λ = 1.54178 Å) for 7 and Mo Kα fine-focus sealed tube (λ = 0.71073 Å) for 1−6. The structures were solved and refined using the Bruker F

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

G

reflns collected independent reflns data/restraints/parameters GOF on F2 R1 (I > 2σ(I)) wR2 (all data)

formula FW (g/mol) temp (K) λ (Å) size (mm) crystal habit crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (g/cm3) μ (mm−1) θ range (deg) index ranges 5594.0(11) 8 1.621 6.512 2.01−29.60 −21 ≤ h ≤ 21 −25 ≤ k ≤ 25 −26 ≤ l ≤ 26 66941 7857 [R(int) = 0.0349] 7857/0/304 1.077 0.0205 0.0440

2 C29H40BiCl2N 682.50 150(2) 0.71073 0.170 × 0.419 × 0.532 yellow plate orthorhombic Pbca 15.5216(18) 18.638(2) 19.337(2)

1

C63H88Bi2Cl4N2 1433.11 150(2) 0.71073 0.174 × 0.248 × 0.330 yellow block triclinic P1̅ 9.8980(9) 11.3694(10) 13.9324(12) 91.716(2) 105.8690(10) 92.865(2) 1504.7(2) 1 1.582 6.056 1.52−29.61 −13 ≤ h ≤ 13 −15 ≤ k ≤ 15 −19 ≤ l ≤ 19 33079 8464 [R(int) = 0.0211] 8464/57/361 1.064 0.0151 0.0352 2890.7(2) 4 1.624 6.393 1.78−29.60 −13 ≤ h ≤ 15 −20 ≤ k ≤ 20 −22 ≤ l ≤ 24 36687 8127 [R(int) = 0.0321] 8127/0/330 1.017 0.0216 0.0428

91.4990(10)

C28H41BiCl3N 706.95 150(2) 0.71073 0.145 × 0.197 × 0.257 colorless rod monoclinic P21/c 10.9034(5) 14.8793(7) 17.8238(9)

3

Table 3. Data Collection and Structure Refinement Details for Compounds 1−7 4 C29H41BiCl3N 718.96 150(2) 0.71073 0.188 × 0.201 × 0.218 yellow block triclinic P1̅ 9.5792(19) 13.796(3) 14.149(3) 69.947(3) 77.962(3) 89.182(3) 1714.5(6) 2 1.393 5.391 1.57−29.61 −13 ≤ h ≤ 13 −19 ≤ k ≤ 19 −19 ≤ l ≤ 19 36386 9643 [R(int) = 0.0331] 9643/0/313 1.019 0.0239 0.0554

5 C86H126Bi2Cl4N4O5 1855.66 150(2) 0.71073 0.181 × 0.305 × 0.486 colorless block triclinic P1̅ 9.1566(11) 10.3219(12) 22.723(3) 82.292(3) 79.850(3) 85.928(4) 2092.4(4) 1 1.473 4.379 1.83−29.63 −12 ≤ h ≤ 12 −14 ≤ k ≤ 14 −31 ≤ l ≤ 31 43624 11771 [R(int) = 0.0502] 11771/10/456 1.077 0.0345 0.0614

6 C37H49BiCl2N2O 817.66 100(2) 0.71073 0.076 × 0.081 × 0.453 colorless rod triclinic P1̅ 12.2394(11) 16.2233(14) 18.8318(17) 96.976(3) 106.912(3) 91.761(3) 3542.5(5) 4 1.533 5.158 1.27−26.47 −15 ≤ h ≤ 15 −20 ≤ k ≤ 19 −23 ≤ l ≤ 23 78351 14576 [R(int) = 0.1198] 14576/10/777 0.999 0.0384 0.0837

7

13532.6(7) 4 1.499 11.857 2.14−68.37 −31 ≤ h ≤ 31 −14 ≤ k ≤ 14 −52 ≤ l ≤ 51 55211 12399 [R(int) = 0.1169] 12399/0/675 1.000 0.0506 0.1254

107.163(2)

C136H172Bi4Cl8N8O 3054.33 100(2) 1.54178 0.043 × 0.057 × 0.091 colorless plate monoclinic I2/a 26.4694(6) 12.3660(4) 43.2705(12)

Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

JHH = 6.5 Hz, 2H, CHCH3), 1.78 (m, 4H, THF), 1.36 (d, JHH = 6.5 Hz, 6 H, CHCH3), 1.07 (d, JHH = 6.0 Hz, 6H, CHCH3); 13C{1H} NMR (150.90 MHz, THF-d8, 298 K): δ = 187.5 (N = CH), 146.9, 140.1, 135.5, 131.2, 127.1, 126.6, 124.4, 68.0, 29.3, 26.0, 23.4 ppm. Anal. Calcd for C33H42BiCl2N2: C, 53.09; H, 5.67; N, 3.75. Found: C, 53.58; H, 5.74; N, 3.74%.

stirred overnight. The resulting white precipitate was collected by filtration. The solid was washed with 2 × 5 mL of THF to yield compound 3 as an air-stable white solid (1.13 g, 80% yield). Colorless crystals suitable for X-ray diffraction studies were obtained from a CH2Cl2/Et2O (1:1) mixture solvents at room temperature. 1H NMR (800.13 MHz, CD2Cl2, 298 K): δ = 9.22 (d, JHH = 7.8 Hz, 2H, HorthoPh), 9.02 (s, 1 H, CH=N), 7.78 (t, JHH = 7.8 Hz, 2H, Hmeta-Ph), 7.59 (t, JHH = 7.8 Hz, 1H, Hpara-Dipp), 7.39 (d, JHH = 7.8 Hz, 2H, HmetaDipp), 7.32 (t, JHH = 7.8 Hz, 1H, Hpara-Ph), 2.66 (sept, JHH = 7.8 Hz, 2H, CHCH3), 2.38 (s, 2H, CH2), 1.97 (m, 4H, CH2CH3), 1.55 (s, 6 H, CCH3), 1.37 (d, JHH = 7.6 Hz, 6 H, CHCH3), 1.15 (d, JHH = 6.8 Hz, 6H, CHCH3), 1.09 (t, JHH = 7.4 Hz, 6H, CH2CH3); 13C{1H} NMR (201.193 MHz, CD2Cl2, 298 K): δ = 192.1 (CH = N), 144.7, 140.3, 133.0, 132.5, 129.3, 127.4, 125.9, 84.5, 57.3, 43.5, 30.2, 28.9, 27.0, 22.2, 9.5 ppm. m.p.: 190−192 °C. Anal. Calcd for C28H41BiCl3N•0.5CH2Cl2: C, 45.68; H, 5.65; N, 1.87. Found: C, 45.82; H, 5.48; N, 1.72%. Synthesis of Compound 4. Solid [CyCAAC-H][Cl] (724 mg, 2.0 mmol) was added to a dry THF (20 mL) solution of PhBiCl2 (714 mg, 2.0 mmol) at room temperature, and the reaction was stirred overnight. The resulting white solid was collected by filtration. The solid was washed with 2 × 5 mL of THF to yield compound 4 as a white solid (1.12 g, 78% yield). Colorless crystals suitable for X-ray diffraction studies were obtained from a CH2Cl2/Et2O (1:1) mixture at room temperature. 1H NMR (800.13 MHz, CD2Cl2, 298 K): δ = 9.22 (d, JHH = 7.8 Hz, 2H, Hortho-Ph), 9.01 (s, 1 H, CH = N), 7.78 (t, JHH = 7.6 Hz, 2H, Hmeta-Ph), 7.58 (t, JHH = 7.8 Hz, 1H, Hpara-Dipp), 7.39 (d, JHH = 7.9 Hz, 2H, Hmeta-Dipp), 7.33 (t, JHH = 6.2 Hz, 1H, Hpara-Ph), 2.62 (sept, JHH = 7.8 Hz, 2H, CHCH3), 2.44 (s, 2H, CH2), 2.08 (m, 2H, H2CCy), 1.67−1.80 (m, 6H, H2CCy), 1.53 (s, 6 H, CCH3), 1.48 (m, 2H, H2CCy), 1.35 (d, JHH = 6.7 Hz, 6 H, CHCH3), 1.14 (d, JHH = 6.8 Hz, 6H, CHCH3); 13C{1H} NMR (201.193 MHz, CD2Cl2, 298 K): δ = 190.3 (CH = N), 144.7, 140.2, 132.9, 132.5, 129.1, 127.4, 125.9, 84.2, 53.4, 45.7, 34.3, 30.1, 29.1, 26.8, 24.5, 22.2, 21.6 ppm. m.p.: 220−222 °C. Anal. Calcd for C29H41BiCl3N•0.5CH2Cl2: C, 46.53; H, 5.56; N, 1.84. Found: C, 46.39; H, 5.70; N, 1.93%. Synthesis of Compound 5. A solution of PhBiCl2 (174 mg, 0.487 mmol) in dry THF (5 mL) was prepared in a 20 mL scintillation vial. A second solution of SIPr (190 mg, 0.488 mmol) in dry THF (5 mL) was prepared. These solutions were cooled to −37 °C, combined, and allowed to react overnight at −37 °C. Solvent was removed in vacuo to yield a white solid. Recrystallization from a saturated solution of THF/hexanes (2:1) yielded colorless block crystals suitable for X-ray diffraction (86 mg, 24%). 1H NMR (600.16 MHz, THF-d8, 298 K) δ = 8.84 (dd, JHH = 8.0, 1.2 Hz, 2H, Hortho-Ph), 7.22 (t, JHH = 7.8 Hz, 2H, Hmeta-Ph), 7.20 (t, JHH = 7.6 Hz, 2H, HparaDipp), 7.09 (d, JHH = 7.7 Hz, 4H, Hmeta-Dipp), 6.98 (tt, JHH = 7.4, 1.2 Hz, 1H, Hpara-Ph), 4.19 (s, 4H, CHimidazole), 3.61 (m, 4H, THF) 3.57 (m, 4H, CH(CH3)2), 1.77 (m, 4H, THF), 1.44 (d, JHH = 6.5 Hz, 12H, CH3), 1.18 (d, JHH = 6.8 Hz, 12H, CH3).13C NMR (150.90 MHz, THF-d8, 298 K) δ = 148.2, 140.4, 135.3, 131.4, 130.8, 127.3, 125.0, 68.2, 55.9, 29.6, 27.1, 26.4, 24.2. m.p.: decomposed at 180 °C. No consistent elemental analysis data could be obtained due to the high reactivity of 5 upon loss of the coordinating THF. Synthesis of Compound 6 and 7. To a solution of PhBiCl2 (46 mg, 0.107 mmol) in dry THF-d8 (0.6 mL) at −37 °C was added solid IPr (45 mg, 0.116 mmol). The clear colorless solution was kept at −37 °C overnight. The solvent was removed in vacuo to yield a white solid. Recrystallization from a saturated solution of THF/hexanes (2:1) yielded colorless small crystals of compound 6 (21 mg, 25%). Colorless crystals of compound 7 suitable for X-ray diffraction studies were obtained from the same solution. The conversion of 6 to 7 was determined to be less than 5% and thus far isolation attempts on preparative scale has proved difficult. Therefore, the characterization of 7 was limited to crystallographic studies. For 6: 1H NMR (600.16 MHz, THF-d8, 298 K): δ = 8.92 (d, JHH = 7.7 Hz, 2H, Hortho-Ph), 7.62 (s, 2 H, CHimidazole), 7.34 (t, JHH = 7.8 Hz, 2H, Hmeta-Ph), 7.25 (t, JHH = 7.6 Hz, 1H, Hpara-dipp), 7.19 (d, JHH = 12.0 Hz, 2H, Hmeta-dipp), 7.02 (t, JHH = 8.0 Hz, 1H, Hpara-Ph), 3.62 (m, 4H, THF), 3.08 (sept,

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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.inorgchem.8b01813. (PDF) Accession Codes

CCDC 1852318−1852324 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]. ORCID

Diane A. Dickie: 0000-0003-0939-3309 Robert J. Gilliard, Jr.: 0000-0002-8830-1064 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the University of Virginia for support of this work. Z.B. acknowledges the Hungarian National Research, Development, and Innovation Office (NKFIH, PD 116329).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01813 Inorg. Chem. XXXX, XXX, XXX−XXX