High-Yield Thermolytic Conversion of Imidazolium Salts into Arduengo

Article pubs.acs.org/Organometallics

High-Yield Thermolytic Conversion of Imidazolium Salts into Arduengo Carbene Adducts with BF3 and PF5 Chong Tian,† Wanli Nie,† Maxim V. Borzov,*,† and Pengfei Su‡ †

Key Laboratory of Synthetic and Natural Chemistry of the Ministry of Education, College of Chemistry and Material Science, North-West University, Taibai Bei avenue 229, Xi’an 710069, People's Republic of China ‡ Xi’an Modern Chemistry Research Institute, East Zhangba road 168, Xi’an 710065, People's Republic of China S Supporting Information *

ABSTRACT: Thermolysis of 1-ethyl-3-methyl-1H-imidazolium tetrafluoroborate (1) and 1,3-dimethyl-1H-imidazolium hexafluorophosphate (3) under reduced pressure eliminates HF to furnish the BF3 and PF5 adducts of the corresponding Arduengo carbenes (2 and 4) in high yields. The intuitively anticipated imidazole N adducts with BF3 and/or PF5 arising from elimination of alkyl fluorides are not detected at all. These observations represent the first examples of a direct bond rearrangement of the type [C−H] + E−F → [C−E] + H−F (E = B, P). DFT computational studies suggest a single-step mechanism for the reaction 1 → 2 + HF. Lower yield thermolysis of a 1,2,3-trimethyl-1H-imidazolium hexafluorophosphate (11) into [(1,3-dimethyl-1H-imidazolium-2-yl)methyl]pentafluorophosphate (12) supports the generality of this transformation.

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cation plays a role of an electrophile)14 or by a destruction of [Ag(AC)2][BF4]-type complexes with ZrCl4 (the latter serves as a both Ag+ and F− withdrawal reagent)1 have been shown to proceed, albeit in low yield. Recently we tested a variety of synthetic approaches to Cp− imidazole type ligands starting from 1-ethyl-3-methyl-1H-imidazolium tetrafluoroborate (1), which along with related N,N′-diorganosubstituted imidazolium salts represent well-known commercially available compounds finding a broad application in industry and laboratory techniques as so-called “ionic liquids”.15 In one of the experiments, we, somewhat serendipitously, observed signs of the degradation of 1 at elevated temperatures under reduced pressure. Being rather intrigued with this fact, we have undertaken a more detailed study of this phenomenon.

INTRODUCTION Arduengo carbene (AC)−borane1−7 and −phosphane4,8−10 adducts have been known since 1993 and 1997, respectively, and present rather well characterized compounds which exhibit properties of true aromatic systems.1 Some of the AC·BH3 adducts have been already used as reducing agents.6,11 The AC·BEt3 adduct may serve as a source of an AC ligand for organo-transition-metal syntheses.12 When the AC and borane are both sterically hindered, as in the case of 1,3-di-tert-butyl2,3-dihydro-1H-imidazol-2-ylidene and B(C6F5)3, formation of a frustrated Lewis acid/base pair capable of activating C−O (THF cleavage) and H−H bonds has been proved.13 Additionally, the ability of ACs to stabilize hindered borenium cations5 and BB double bonds3 is also known. AC−borane and −phosphane adducts are usually prepared in moderate to good yields from the corresponding free stable AC and a borane or a phosphane (or a direct precursor of the former). This approach is limited because it cannot be employed for ACs that are thermally unstable. For AC−borane adducts, modified approaches in which a free carbene is generated in situ in the presence of a borane were applied.7,12 Few example, AC−borane adduct formations by an ipso-SEAr substitution at a BAryl4− anion (H atom at the second position of an imidazolium © 2012 American Chemical Society

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RESULTS AND DISCUSSION Rather unexpectedly, at 300−400 °C and 1.5 × 10−3 Torr, tetrafluoroborate 1 decomposes to afford an almost quantitative yield of the AC−borane adduct (1-ethyl-3-methyl-2,3-dihydro1H-imidazol-2-ylidene-κC2)trifluoroboron (2), which distills under Received: November 5, 2011 Published: February 16, 2012 1751

dx.doi.org/10.1021/om201086d | Organometallics 2012, 31, 1751−1760

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and C6 all lay within this plane (deviations from PL1 are 0.010(4), −0.003 (4), and 0.059(4) Å, respectively). The fluorine atom F3 is oriented toward the C4 methylene group atom (N1/C1/B1/F3 torsion angle −10.8(3)). The H atoms at the NCH3 group are disordered between two positions (occupancies 0.72(3) and 0.28(3)). The C5/C4/N1 plane is nearly perpendicular to the PL1, as indicated by an interplane angle of 88.6(2)°. Analysis of the Cambridge Structural Database (CSD; version 5.27, release May 200916) reveals 19 structurally characterized imidazole-related AC−borane adducts (24 fragments). Among them, there are only four AC·BF3 type compounds1,4,17 (5 fragments), with {μ-2,2′-oxybis[(3-methyl-2,3-dihydro-1Himidazol-2-ylidene-1-yl-κC 2 )ethane]}bis[trifluoroborane] 1 being the closest analogue of 2. Interestingly, this compound1 actually presents two moieties of 2 linked together at the end ethyl group carbons with an −O− bridge, with all related distances, angles, and even torsion angles being quite similar to those observed in 2. Statistical analysis of the structural parameters of the known AC·BF3 type compounds1,4,17 reveals that all distances B1−F1(2,3), C1−B1, C1−N1(2), C2(3)−N1(2), C2−C3 and angles N1(2)−C1−B1, C1−N1(2)−C2(3), N1(2)−C2(3)−C3(2), N1−C1−N2 in 2 (similar bonds and angles are grouped) are close to the median values observed earlier (1.383, 1.644, 1.351, 1.385, 1.343 Å and 126.98, 110.29, 106.97, 105.60°, respectively). The unsymmetrical unit of 4 nearly adopts the C2v point group symmetry. The P atom in 4 is in an octahedral coordination environment, with all cis angles being 90° within 0.43°. All P−F bond lengths are nearly identical (range from 1.5916(18) through 1.5988(18) Å) and match the range reported for [1,3-bis(2,4,6-trimethylphenyl)-1H-imidazol-2ylidene]pentafluorophosphorus.4 The P−C bond length in 4 (1.874(2) Å) is also close to that observed for the 1,3-bis(2,4,6trimethylphenyl)-1H-imidazol-2-ylidene adducts with PF5 (1.898 Å)4 and PF4Ph (1.909 and 1.910 Å)10 and for the 4,5dimethyl-1,3-bis(1-methylethyl)-1H-imidazol-2-ylidene adduct with PO2Cl (1.844 Å).9 The imidazole ring in 4 is planar within 0.003 Å. The P atom and methyl group atoms deviate from this rms plane by 0.003(4), 0.012 (5), and 0.013(5) Å, respectively. The C−P−Ftrans angle is nearly linear (179.4(1)°). The four equatorial P−F bonds are oriented at almost exactly 45° with respect to the imidazole plane (45.52(11), 44.05(11),

the reaction conditions, and HF, which was trapped in a liquid nitrogen cooled finger. Moreover, thermolysis of 1,3-dimethyl1H-imidazolium hexafluorophosphate (3) similarly affords (1,3d i m e t h y l - 2 , 3 - d i h y d r o - 1 H - i m i d a z o l - 2 - y l i d e n e -κ C 2 ) pentafluorophosphorus (4) (see Scheme 1). This observed Scheme 1. Thermolysis of Salts 1 and 3

straightforward high-yield route to AC·BF3 and AC·PF5 adducts seems to be rather attractive for the case of sterically nonhindered carbenes. It is remarkable that the intuitively expected N→BF3 and N→PF5 adducts 5a,b and 6 could not be detected among the products, even in trace amounts. It is also noteworthy that all attempts to perform the analogous thermolytic reactions for the case of 1,2-dialkyl-1H-imidazolium salts with complex anions containing no F atoms yielded only complex mixtures of unidentified products, with no signs indicating the presence of AC adducts. Both AC adducts 2 and 4 are crystalline materials resistant to air and moisture and can even be recrystallized from boiling water without any signs of hydrolysis. Their molecular structures are depicted in Figure 1. Principal geometric parameters of these AC adducts (in comparison with the DFT-computed parameters for the optimized geometries of 1−4 and related systems, vide infra) are given in Tables 1 and 2, respectively. In compound 2, the imidazole ring is planar within 0.003 Å, with the C1 atom deviating the most from the PL1 plane (PL1 denotes the C1/N1/C2/C3/N2 rms plane). Atoms B1, C4,

Figure 1. Molecular structures of 2 (left) and 4 (right) with labeling. Thermal ellipsoids are shown at the 50% probability level. H atoms at C6 in 2 are disordered between two positions (site occupancies 0.72(3) and 0.28(3)). 1752


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Table 1. Principal Structural Parameters for 2 (X-ray Diffraction Analysis (Sample Crystal from Toluene) and DFT Computation Data for 2a,b), 1, TS, and the Hypothetical NHC·HF Adduct 7 (DFT Computation Data Only)a 2 (exptl) C1−N1 C1−N2 C2−N1 C3−N2 C2−C3 N1−C4 N2−C6 C4−C5 C1−B1 B1−F1 B1−F2 B1−F3 B1−F4 N1−C1−N2 N1−C1−B1 N2−C1−B1 C1−N1−C2 C1−N1−C4 C2−N1−C4 C1−N2−C3 C1−N2−C6 C3−N2−C6 N1−C2−C3 N2−C3−C2 N1−C4−C5 C1−B1−F1 C1−B1−F2 C1−B1−F3 F1−B1−F2 F1−B1−F3 F2−B1−F3 F4−B1−F1 F4−B1−F2 F4−B1−F3 C1−H1 H1−F N1−C1−H1 N2−C1−H1 C1−H1−F

2a (calcd)

2b (calcd)

1 (calcd)

TS (calcd)

7 (calcd)

1.342(2) 1.342(2) 1.378(3) 1.371(3) 1.325(3) 1.475(3) 1.465(3) 1.491(3) 1.644(3) 1.386(3) 1.379(3) 1.371(3)

1.349 43 1.348 81 1.379 47 1.382 67 1.352 72 1.473 49 1.464 34 1.524 08 1.673 24 1.392 94 1.397 65 1.397 33

1.347 93 1.347 88 1.382 15 1.379 60 1.353 69 1.474 36 1.463 77 1.523 58 1.669 17 1.395 83 1.399 36 1.392 85

105.821 123.416 130.736 110.322 125.436 124.209 110.200 126.663 123.133 106.845 106.811 112.391 110.078 106.598 106.743 111.225 111.243 110.750

105.938 130.344 123.714 110.082 126.169 123.695 110.372 124.826 124.752 106.928 106.678 112.395 106.680 106.825 109.436 110.743 111.656 111.270

1.354 20 1.354 16 1.383 96 1.384 12 1.353 70 1.464 46 1.457 04 1.525 24 3.192 26 1.320 41 1.314 52 1.310 84 3.544 15 104.056

1.354 11 1.353 92 1.385 71 1.385 73 1.353 22 1.463 81 1.455 86 1.525 30

105.46(16) 130.21(17) 124.32(16) 109.81(16) 126.25(17) 123.94(18) 110.39(17) 125.41(17) 124.16(18) 107.34(19) 106.99(19) 112.4 (2) 108.56(17) 108.60(18) 111.14(17) 109.51(18) 109.67(19) 109.33(19)

1.332 33 1.330 94 1.380 84 1.380 10 1.357 62 1.474 87 1.465 40 1.523 43 3.111 00 1.412 82 1.369 94b 1.422 92 1.433 54 108.864

108.468 124.464 126.997 108.601 124.399 126.734 107.076 106.991 112.523

111.554 123.501 124.898 111.599 123.673 124.727 106.437 106.354 112.720

111.613 123.336 125.007 111.677 123.511 124.809 106.405 106.313 112.741

112.069 108.047 111.606 107.065 111.110 108.047 1.07888 2.01743 126.292 124.733 137.475

119.291 119.565 120.986

1.63412 0.99880 128.228 126.708 177.813

103.990

1.61727 1.00511 128.263 127.746 179.618

a Distances are given in Å and angles in deg. B−F in BF3 (D3h) is 1.313 73 Å and in BF4− (Td) is 1.410 59 Å; H−F distance in monomeric HF is 0.924 19 Å (RB3LYP/6-311+G(2d,p) level of theory). bF2 atom is positioned trans to C1.

46.32(11), and 44.0(1)°). This feature was also observed earlier for analogues with octahedral P centers. All other data (GS-MS, IR, 1H, 13C{1H}, 11B, 31P, and 19F NMR, and elemental analyses) are fully consistent with the structures of 2 and 4, with the spectral parameters for both 2 and 4 being close to those previously reported for their 1,3-dimesityl counterparts.4 Figure 2 shows the characteristic 13C{1H} NMR signals of the quaternary carbon atoms in 2 and 4. However, compounds 2 and 4 themselves seem to be not as interesting as the thermolytic reactions that lead to their formation. To the best of our knowledge, this is the first case where a C-bound H atom and an F atom from either BF4− or PF6− undergo HF elimination with formation of a C−B or C−P bond. Distantly, it could resemble the case of an aromatic nucleophilic substitution, with the substrate and nucleophile roles played by rather untypical species and with a mechanism that remains to be unveiled. The nearly quantitative yields of 2

and 4 and the absence of N adducts 5a,b and 6 among the reaction products (which could be expected by an analogy with, for example, the thermolysis of trialkyloxonium tetrafluoroborates18) also required some reasonable accommodation. This prompted us to computationally study these reactions at the RB3LYP/6-311+G(2d,p) level of theory. Mechanistic studies and elucidation of kinetic parameters were performed for 1 → 2 + HF, 1 → 5a + EtF, 1 → 5b + MeF, and related reactions only. The less evident β-elimination reaction of 1 leading to ethene was also considered (for details on the latter model, see the Supporting Information). Optimized geometries of 3, 4, and 6 are provided in Figure 3. The thermochemistry diagram for 1 → 2 + HF and related reactions along with the optimized stationary and transition state geometries of the participants is given in Figure 4. Two stationary conformations for 2 with alternative orientations of the BF3 group (only one is depicted in Figure 4) 1753


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Table 2. Principal Structural Parameters for 4 (X-ray Diffraction Analysis and DFT Computation Data) and 3 (DFT Computation Data Only)a C1−N1 C1−N2 C2−N1 C3−N2 C2−C3 N1−C4 N2−C5 C1−P1 P1−F1 P1−F2 P1−F3 P1−F4 P1−F5 P1−F6 N1−C1−N2 N1−C1−P1 N2−C1−P1 C1−N1−C2 C1−N1−C4 C2−N1−C4 C1−N2−C3 C1−N2−C5 C3−N2−C5 N1−C2−C3 N2−C3−C2 C1−P1−F1 C1−P1−F2 C1−P1−F3 C1−P1−F4 C1−P1−F5 C1−P1−F6 F1−P1−F2 F1−P1−F3 F1−P1−F4 F1−P1−F5 F1−P1−F6 F2−P1−F3 F2−P1−F4 F2−P1−F5 F2−P1−F6 F3−P1−F4 F3−P1−F5 F3−P1−F6 F4−P1−F5 F4−P1−F6 F5−P1−F6 C1−H1 H1−F6 N1−C1−H1 N2−C1−H1 C1−H1−F6

4 (exptl)

4 (calcd)

3 (calcd)

1.345(3) 1.336(3) 1.379(4) 1.363(3) 1.331(5) 1.477(4) 1.475(4) 1.874(2) 1.5961(18) 1.5916(18) 1.5988(18) 1.5984(18) 1.5959(17)

1.348 46 1.348 46 1.378 64 1.378 65 1.351 97 1.468 52 1.468 52 1.913 77 1.640 15 1.636 70 1.640 12 1.636 65 1.597 79

106.4(2) 125.96(18) 127.68(18) 109.0(2) 128.6(2) 122.4(2) 107.3(2) 128.0(2) 121.9(2) 107.3(2) 107.3(3) 89.64(10) 89.8(1) 89.98(10) 90.22(10) 179.4(1)

106.377 126.811 126.812 109.789 127.562 122.622 109.789 127.561 122.622 107.023 107.023 88.523 88.489 88.524 88.493 179.996

90.48(11) 179.61(10) 90.32(11) 90.14(11)

90.167 177.047 89.755 91.477

89.57(11) 179.20(12) 89.64(11)

89.754 176.982 91.507

89.63(12) 90.24(11)

90.168 91.477

90.34(10)

91.511

1.332 23 1.330 97 1.381 63 1.380 66 1.357 02 1.465 04 1.465 97 3.567 69 1.665 63 1.606 43 1.609 73 1.673 25 1.606 70 1.655 94 108.797 119.806 110.153 108.561 124.685 126.696 108.608 124.590 126.571 106.985 107.049 54.860 122.248 121.907 54.800 126.857 49.881 90.245 176.691 87.521 89.885 87.978 92.401 177.031 92.197 90.296 89.766 91.998 90.021 89.756 87.671 176.723 1.07783 2.57876 126.000 125.098 90.465

Figure 2. 13C{1H} NMR quaternary carbon atom signals for 2 (top; δ 156.71 ppm, 1J(11B−13C) = 86.9 Hz, 2J(19F−13C) = 62.0 Hz) and 4 (bottom; δ 155.59 ppm, 1J(31P−13C) = 329.8 Hz, 2J(19Fcis−13C) = 63.0 Hz, 2J(19Ftrans−13C) = 0.0 Hz).

computed geometries and predicted spectral parameters for both 2 and 4, in general, are in a good agreement with the X-ray diffraction analyses (see Tables 1 and 2), IR (see the Supporting Information), and NMR (see Tables 3 and 4) data. The computed and experimental chemical shifts of the F atoms and the values of the direct spin−spin coupling constants (SSCC's) 1J(B−F) and 1J(P−F) disagree the most, which is not surprising due to a deliberate neglect of the solvent effects on the geometry of the most polar fragments in 2 and 4. For the case of the initial compound 1, geometries of the transition states that would be responsible for formation of the nonobserved N→BF3 adducts 5a,b (TS-a,b in Figure 4) are rather trivial and resemble the classical transition state for the SN2 substitution mechanism at a tetrahedral C atom. TS-a,b both exhibit sharp profiles (related imaginary frequencies 360.6i and 422.9i cm−1) and evidently correspond to a simultaneous cleavage of the N−C bond and one of the B−F bonds along with a formation of a C−F bond. The only transition state found for the actual 1 → 2 + HF reaction (TS in Figure 4) is absolutely different. It can be best described as an aggregate of a slightly distorted hypothetical AC·HF adduct (7 in Figure 4) and a planar BF3 molecule. The imaginary frequency related to TS is very low (31.1i cm−1), with contributions of N−H, H−F, and B−F interatomic distance changes into this imaginary vibrational mode being very small. The consistency of the TS with the 1 → 2 + HF reaction is proved by the intrinsic reaction coordinate (IRC) computation in both directions starting from the TS (see Figure 5). While in the reverse direction the computed IRC path leads to the initial salt 1, in the forward direction it reaches a formal H-bound 2·HF adduct which would be evidently unstable even at room temperature (yellow line in Figure 4). It is noteworthy that, in both directions, the bond rearrangement begins far away from the TS (both by energy (ΔESCF) and by the mass-weighed reaction coordinate (ζ)). The computational analysis of the decomposition of 1 reveals several remarkable details. First of all, intuitively expected N→ BF3 adducts 5a,b are really the most thermodynamically favored and chemically reasonable products for T = 25 to 190 °C (left side in Figure 4; difference in the free Gibbs energies ΔG(5a + EtF and 5b + MeF) − ΔG(2 + HF) equals −12.3 and −8.7 kcal mol−1 at 25 °C and −13.8 and −9.6 kcal mol−1 at 190 °C). For temperatures above 190 °C computational data predict dissociation of 5a,b into 1-methyl- or 1-ethyl-1H-imidazole

a

Distances are given in Å and angles in deg. P−F in PF5 (D3h) is 1.549 445 (apical) and 1.555 71 Å (equatorial) and in PF6− (Oh) is 1.636 27 Å; H−F distance in monomeric HF is 0.92419 Å (RB3LYP/6311+G(2d,p) level of theory).

negligibly differ in their SCF energies (ΔESCF = 3.5 cal mol−1), while the stationary conformation for 4 is unique. The 1754


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Figure 3. Stationary conformations of compounds 3 (left), 4 (middle), and 6 (right). The atom numbering in 4 is in accordance with that used in the X-ray diffraction analysis.

Figure 4. Thermochemistry diagram for the 1 → 2 + HF (red; the path related to T = 270 °C is highlighted), 1 → 5a + EtF (blue), 1 → 8a + EtF + BF3 (cyan), 1 → 5b + MeF (green), 1 → 8b + MeF +BF3 (light green), 1 → 2·HF (yellow), 1 → 7 + BF3 (pink), and 1 → 9 + HF + BF3 (brown) conversions within the temperature range of 25−450 °C. For all participants and within all temperature ranges modeled, the free Gibbs energies (ΔG) and free Gibbs activation energies (ΔG⧧) are referenced to that of 1. For the 1 → 2 + HF reaction, the inset logarithmic plot illustrates the dependence of the calculated rate constants (k, red curve) and half-conversion times (τ, blue curve) on temperature.

(8a,b) and BF3, with the ΔG(8a + EtF + BF3 and 8b + MeF + BF3) − ΔG(2 + HF) difference being even greater (−14.2 and −9.6 kcal mol−1 at 190 °C and −28.3 and −23.2 kcal mol−1 at 450 °C; see Figure 4). An alternative 1 → 8a + C2H4 + HF + BF3 conversion pathway is discussed in the Supporting Information. Similarly, 6 (25 through 130 °C) and 8a + PF5 (above 130 °C) are the most favored products for the thermolysis of 3, with ΔG(6 + MeF)−ΔG(4 + HF) being −4.4 and −5.3 kcal mol−1 at 25 and 130 °C and ΔG(8a + MeF + PF5) −ΔG(4 + HF) being −5.1 and −24.5 kcal mol−1 at 130 and 450 °C. Thus, were it not for the kinetic prohibition of an alkyl fluoride elimination, both AC adducts 2 and 4 would have no chance to be even detected among the products. Indeed, comparison of the free activation Gibbs energies (ΔG⧧) shows that TS-a,b are both systematically higher in energy than TS, with the smallest difference ΔG⧧(TS-a,b) − ΔG⧧(TS) observed at 230 °C (6.1 and 7.4 kcal mol−1; stepwise character of the ΔG⧧(T) for TS arises from the accounting for the free intrinsic rotation and is due to the increase of the

number of the accessible wells for rotation of the BF3 moiety in TS from one up to two and three at 250 and 310 °C). The highest estimated rates of formation for 5a,b relative to that of 2 are 0.011 and 0.004 (both at 450 °C). Dissociative mechanisms (via 7 + BF3 or via free carbene 9 + HF + BF3; see Figure 4) are inconsistent with experiment (HBF4 is not detected among the captured volatile products) and are both strongly disfavored by ΔG with respect to 2 + HF below 400 °C. However, at higher temperatures cleavage of the C−B bond in 2 is favored (the same is true for the C−P bond in 4). In practice, such a bond cleavage would mean formation of a free nonhindered AC 9 or its N,N′-dimethyl analogue 10, which will immediately undergo subsequent reactions. The signs of thermal decomposition (darkening of the melt and fuming) are really observed at ca. 390 °C for both 2 and 4, and at 420 °C they very quickly convert into black nonidentifiable materials (under an Ar atmosphere). Numerical estimates of ΔG(T), rate constant k(T), and halfconversion time τ(T) for 1 → 2 + HF (see the inset plot in 1755


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Table 3. Observed (CD3CN, 25 °C) and DFT Calculateda Chemical Shifts (δ/ppm) and SSCC's (J/Hz) for 2 δ nucleus

J

obsd

SSCC

obsdb

calcd

J(B,C) 1 J(B,F) 2 J(F,C1) 4 J(F,C4) 4 J(F,C6) 1 J(H2,C2) 1 J(H3,C3) 1 J(H4,C4) 1 J(H5,C5) 1 J(H6,C6) 3 J(H2,H3) 3 J(H4,H5) 5 J(H6,F)

86.9 37.2d 62.0 2.8 2.8 198.6 198.4 143.7 128.1 146.7 1.7 7.3 0.7

78.8 −88.4 60.0 2.7 4.1 184.4 184.5 134.2 122.2 133.7 1.5 6.5 0.89

calcd

B (B1) −0.20 F (F1−3) −138.99c 45.11 CH2CH3 (C4) 16.88 CH2CH3 (C5) 36.73 NCH3 (C6) BC (C1) 156.71 121.67 C4H (C2) 123.83 C5H (C3) 4.23 CH2CH3 (H4A,B) 1.36 CH2CH3 (H5A−C) 3.82 NCH3 (H6A−C) 7.11 C4H (H2) 7.17 C5H (H3)

11

is depicted in Figure 6. Principal geometric parameters for 12 are collected in Table 5. A molecule of zwitterionic compound 12 presents a combination of planar imidazolium and pseudo-octahedral phosphate moieties linked with a methylene group. Analysis of CSD16 reveals no close analogues of 12. Moreover, it seems to be the first structurally characterized compound where a PF5 group is linked to a tetrahedral C atom. In 12, the C−CH2− PF5 fragment adopts a conformation in which PF5 is the most remote from the imidazolium ring. Geometrical parameters for the imidazolium moiety are very close to those reported for 1,2,3-trimethyl-1H-imidazolium cation.19

−3.02 −170.07 48.82 17.90 35.80 171.06 123.34 123.99 4.69 1.37 3.72 6.72 6.79

1

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SUMMARY AND CONCLUSIONS In summary, our results show that 2 and 4 can be prepared in high yields directly from affordable imidazolium salts with BF4− and PF6− anions. We believe that this route may be applicable to other AC·BF3 and AC·PF5 adducts derived from sterically nonhindered Arduengo carbenes. Formation of compound 12 points to a rather general character of a direct [C−H] + EFn− → [C−EFn−1] + H−F thermolytic bond rearrangement (E = B, n = 4; P, n = 6), at least for organic cations with acidic CH groups. Both experimental and computational data suggest a single elementary step mechanism for this transformation. The systematic failure to perform analogous reactions for imidazolium salts with complex anions containing no F atoms may be indicative of the specific role played by a strongly charged but poorly nucleophilic fluorine atom. Further studies by our research team on the applicability of this reaction to other related organic substrates and F-containing complex anions are currently in progress.

a

Values are averaged over conformers 2a and 2b and chemically equivalent nuclei within them. bAbsolute values are given. cδ(F(10B)) −138.92 ppm. dFrom 11B NMR. From 19F NMR: 1J(11B,F) = 36.5 Hz, 1 10 J( B,F) = 12.0 Hz.

Table 4. Observed (CD3CN, 25 °C) and DFT Calculateda Chemical Shifts (δ/ppm) and SSCC's (J/Hz) for 4 δ nucleus

obsd

J calcd

P (P1) −151.20 −109.98 cis-F (F1−4) −55.20c −69.26 trans-F (F5) −73.51 −91.38 CH3 (C4,5) 38.75 39.93 PC (C1) 155.59 169.82 CH (C2,3) 123.84 123.82 CH3 (H4,5A−-C) 3.89 3.94 CH (H2,3) 7.12 6.67

SSCC 1

J(P,C1) 1 J(P,F-trans) 1 J(P,F-cis) 2 J(F,F-trans/cis) 2 J(F-trans,C1) 2 J(F-cis,C1) 4 J(F-cis,C4) 4 J(F-cis,C2) 3 J(P,C2) 4 J(P,H2) 4 J(P,H4) 5 J(F-cis,H4) 1 J(H2,C2) 1 J(H4,C4)

obsdb

calcd

329.8 294.67 753.1d −1207.7 779.2d −1006.0 50.4 −141.5 0.0 1.4 63.0e 52.5 4.3 4.7 1.3 1.1 10.4 9.9 3.1 2.7 1.0 −0.8 1.0 −0.2 202.5 187.2 143.8 135.5

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EXPERIMENTAL SECTION

General Considerations. Thermolytic experiments were performed in all-sealed glass vessels with application of a high-vacuum line (the residual pressure of noncondensable gases within (1.0−1.5) × 10−3 Torr; 1 Torr = 133 Pa). 1-Ethyl-3-methyl-1H-imidazolium tetrafluoroborate (1) and 1,3-dimethyl- (3), and 1,2,3-trimethyl-1Himidazolium (11) hexafluorophosphates were purchased from J&K Ultraf ine Material Co.. Degassed toluene and CD3CN were kept over Na−K alloy and/or CaH2, respectively, in evacuated reservoirs under Teflon needle valves and directly transferred by recondensation into reaction vessels or NMR sample tubes using the high-vacuum line. 1H, 13 C{1H}, 19F, 11B, and 31P NMR spectra, except for 13C{1H} and 31P NMR spectra for compound 12, were recorded on a Bruker Avance 500 instrument at 500, 125, 470, 160, and 202 MHz, respectively, in CD3CN at 25 °C. 13C{1H} and 31P NMR spectra for compound 12 were recorded on a Varian INOVA 400 instrument at 100 and 162 MHz under the same conditions and using the same sample. For 1H and 13C{1H} spectra, the solvent (δH 1.94 and δC 1.32) resonances were used as internal reference standards. For 19F, 11B, and 31P spectra, CFCl3, F3B·OEt2, and 85% aqueous H3PO4 were used as external reference standards. GC-mass spectra were measured on Agilent 6890 Series GC system equipped with HP 5973 mass-selective detector. IR spectra were recorded on a Bruker VERTEX 70 FT IR instrument. Elemental analyses were performed on a Vario ELIII CHNOS automated analyzer. (1-Ethyl-3-methyl-2,3-dihydro-1H-imidazol-2-ylidene-κC2)trifluoroboron (2). A 1.00 g amount (5.05 mmol) of 1 (colorless liquid at room temperature) was placed into a predried system for distillation of high-melting compounds, connected via a liquidN2-cooled trap to a high-vacuum line, evacuated, and heated with a stream of hot air under stirring. At ca. 300 °C gas evolution started. The quickly solidifying colorless distillate was collected in a receiver. The distillation was continued until no material remained in the starting vessel. The crude product (0.89 g, 99%) was collected and

a

Values are averaged over chemically equivalent nuclei within 4. aAbsolute values are given. cδ(F(13C)) −55.22 ppm. dFrom 31P NMR. From 19F NMR: 1J(P,F-trans) = 753.0 Hz, 1J(P,F-cis) = 777.2 Hz. eFrom 13C{1H} NMR. From 19F NMR: 2J(F-cis,C1) = 64.0 Hz.

b

Figure 4) agree well with the experiment. At 270 °C ΔG(T) goes to 0 and rapidly becomes negative at higher temperatures, while the reaction is already kinetically fast and apparently irreversible (HF is removed). Maintaining of 50−100 mg amounts of 1 at constant temperatures within a T = 250− 350 °C range in an evacuated, sealed-off vessel (30 mL) followed by rapid cooling allows us to detect only identical low contents of 2 for different temperatures. This may be due to an insufficient rate of “freezing” of the equilibrium under the cooling conditions and implicitly can prove the actual reversibility and fast character of both direct and reverse reactions within the mentioned temperature range. Of interest, thermolysis of 1,2,3-trimethyl-1H-imidazolium hexafluorophosphate (11) in a lower yield affords the zwitterionic product 12 (see Scheme 2). Its molecular structure 1756


Organometallics

Article

Figure 5. IRC profile from the TS (green) in the forward (blue, positive weighed reaction coordinate ζ) and reverse (red, negative ζ) directions. The relative SCF energy (ΔE) is referenced to that of 1. All structures are depicted in a projection along the N···N line.

Table 5. Principal Structural Parametersa for 12

Scheme 2. Thermolysis of Hexafluorophosphate 11

C6−C1 C1−N2 C1−N1 N2−C3 N2−C5 N1−C2 N1−C4 C3−C2 C6−P1 P1−F1 P1−F2 P1−F3 P1−F4 P1−F5 a

1.470(3) 1.333(4) 1.341(4) 1.396(4) 1.463(5) 1.365(5) 1.459(5) 1.329(5) 1.866(3) 1.6097(16) 1.604(3) 1.5784(18) 1.603(2) 1.6122(18)

N1−C1−N2 N1−C1−C6 N2−C1−C6 C1−N1−C2 C1−N1−C4 C2−N1−C4 C1−N2−C3 C1−N2−C5 C3−N2−C5 N1−C2−C3 N2−C3−C2 C1−C6−P1 C6−P1−F1 C6−P1−F2

107.3(2) 126.9(3) 125.8(3) 109.0(3) 125.3(3) 125.5(3) 109.0(3) 127.3(3) 123.7(3) 108.4(4) 106.3(4) 117.06(18) 177.6(1) 90.80(13)

C6−P1−F3 93.18(11) C6−P1−F4 91.66(14) C6−P1−F5 89.32(11) F1−P1−F2 88.59(12) F1−P1−F3 89.14(9) F1−P1−F4 88.90(12) F1−P1−F5 88.35(10) F2−P1−F3 90.70(15) F2−P1−F4 177.21(12) F2−P1−F5 88.59(15) F3−P1−F4 90.49(15) F3−P1−F5 177.41(11) F4−P1−F5 90.11(16)

Distances are given in Å and angles in deg.

from toluene. Mp: 237.5 C. MS (EI, 70 eV; m/z (%)): 203 (100) [M − F]+, 107 (38) [PF4]+, 96 (92) [M − PF5]•+, 95 (63) [M − PF5 − H]+. Anal. Calcd for C5H8PF5N2: C, 27.04; H, 3.63; N, 12.61. Found: C, 27.98; H, 3.15; N, 12.56. FTIR (neat, solid, ν/cm−1): 3176 (w), 3145 (w), 3000−2927 (vw; a set of broad bands), 1628 (w), 1581 (w), 1489 (m), 1448 (m-w), 1416 (m-w), 1354 (w), 1335 (w), 1240 (m), 1176 (m), 1097 (m-w), 1041 (w), 821 (vs), 788 (vs), 768 (vs), 748 (s), 712 (s), 673 (s). For 1H, 13C{1H}, 19F, and 31P NMR spectral parameters of 4 were comparable with the computed data; see Table 4. For views of the spectra, see the Supporting Information. For a view of the IR spectrum of 4 overlaid with the computed spectrum, see the Supporting Information. (1,3-Dimethyl-1H-imidazolium-2-yl)methylpentafluorophosphate (12). This compound was prepared similarly to 2 from 1.85 g (7.22 mmol) of 11 (white crystalline material). The brown solidified distillate was recrystallized from ca. 15 mL of hot methanol, which gave 0.77 g (45%) of pure 12. Mp: 218−219 °C. MS (EI, 70 eV; m/z (%)): 217 (91) [M − F]+, 197 (16) [M − F − HF]+, 110 (98) [M − PF5]•+, 109 (100) [M − PF5 − H]+, 107 (19) [PF4]+, 95 (60) [M − CH3PF5]+, 68 (61), 54 (42), 42 (76). Anal. Calcd for C6H10PF5N2: C, 30.52; H, 4.27; N, 11.86. Found: C, 30.62; H, 4.18; N, 11.76. FTIR (neat, solid, ν/cm−1): 3182 (w), 3157 (w), 3000−2927 (vw; a set of broad bands), 1587 (w), 1535 (w), 1512 (w), 1458 (w), 1429 (wq, sh), 1340 (w), 1292 (w), 1254 (m), 1221 (w), 1188 (m), 1101 (w), 1024 (w),

Figure 6. Molecular structure of 12 with labeling. Thermal ellipsoids are shown at the 50% probability level. recrystallized from hot water, which gave 0.85 g (95%) of 2 free of 1 as well-formed colorless crystals. Mp: 79−80 °C (from water). MS (EI, 70 eV; m/z (%)): 159 (78) [M − F]+, 131 (15) [M − F − C2H4]+, 110 (59) [M − BF3]•+, 82 (100) [M − BF3 − C2H4]•+. Anal. Calcd for C6H10BF3N2: C, 40.49; H, 5.66; N, 15.74. Found: C, 40.20; H, 5.36; N, 15.44. FTIR (neat, solid, ν/cm−1): 3172 (w), 3147 (w), 3000− 2927 (vw; a set of broad bands), 1489 (m), 1454 (m), 1421 (w), 1361 (w), 1342 (w), 1284 (w), 1230 (w), 1188 (m), 927 (vs), 854 (w), 802 (m), 764 (s), 732 (w), 719 (s), 658 (m). For 1H, 13C{1H}, 19F, and 11 B NMR, spectral parameters of 2 were comparable with the computed data; see Table 3. For views of the spectra, see the Supporting Information. For a view of the IR spectrum of 2 overlaid with the computed spectrum, see the Supporting Information. (1,3-Dimethyl-2,3-dihydro-1H-imidazol-2-ylidene-κC 2 )pentafluorophosphorus (4). This compound was prepared similarly to 2 from 1.50 g (6.20 mmol) of 3 (white crystalline material), which gave 1.29 g of product (94%) after crystallization 1757


Organometallics

Article

Table 6. Crystal Data, Data Collection, Structure Solution, and Refinement Parameters for 2, 4, and 12 formula fw cryst size, mm space group a, Å b, Å c, Å β, deg V, Å3 ρcalcd, g cm−3 μ, mm−1 F(000) index ranges

no. of measd, unique rflns; Rint no. of rflns with F2 > 2σ(F2) completeness to θ (θ, deg) transmissn coeff range no. of params R, Rwa (F2 > 2σ(F2)) R, Rwa (all data) Sa max, min diff map, e Å−3 a

2 (from toluene)

2 (from water)

4 (from ethanol)

12 (from methanol)

C6H10BF3N2 177.97 0.36 × 0.28 × 0.27 P21/c 7.2873(8) 7.7101(8) 15.1225(16) 101.933(2) 831.31(15) 1.422 0.130 368 −8 ≤ h ≤ 8 −9 ≤ k ≤ 7 −17 ≤ l ≤ 18 4144, 1547; 0.024 1038 0.998 (25.5) 0.954−0.965 138 0.0391, 0.0957 0.0685, 0.1046 1.065 0.166, −0.166

C6H10BF3N2 177.97 0.30 × 0.26 × 0.16 P21/c 7.2918(15) 7.7091(15) 15.126(3) 101.935(3) 831.9(3) 1.421 0.130 368 −7 ≤ h ≤ 8 −8 ≤ k ≤ 9 −18 ≤ l ≤ 16 4022, 1497; 0.023 1007 0.998 (25.1) 0.961−0.979 111 0.0417, 0.1027 0.0677, 0.1100 1.054 0.163, −0.156

C5H8F5N2P 222.10 0.36 × 0.21 × 0.12 P21/n 7.1365(8) 15.0684(16) 7.7793(9) 92.141(2) 835.97(16) 1.765 0.368 448 −8 ≤ h ≤ 8 0 ≤ k ≤ 18 0≤l≤9 1651, 1651; 0.000 1356 0.999 (26.0) 0.898−0.957 121 0.0394, 0.1239 0.0508, 0.1310 1.142 0.314, −0.358

C6H10F5N2P 236.13 0.38 × 0.30 × 0.06 P21/n 8.3723(12) 7.5087(10) 14.914(2) 94.799(4) 934.3(2) 1.679 0.335 480 −9 ≤ h ≤ 9 −8 ≤ k ≤ 8 −17 ≤ l ≤ 17 4232, 4233; 0.000 2948 0.996 (25.1) 0.883−0.980 130 0.0574, 0.1501 0.0924, 0.1659 1.063 0.473, −0.385

Conventional R = ∑||Fo| − |Fc||/∑|Fo|; Rw = [∑w(Fo2 − F2c)2/∑w(Fo2)2]1/2; S = [∑w(Fo2 − Fc2)2/ ((no. of data) − (no. of params))]1/2.

876 (w), 860 (m), 806 (s), 781 (s), 760 (vs), 729 (m), 658 (w), 621 (w). 1H NMR (δ/ppm): 3.02 (d quint, 2 H, 2J(PH) = 27.8 Hz, 3 J(FcisH) = 5.3 Hz, 1J(13CH) = 128.9 Hz, CH2), 3.72 (d, 6 H, 5 J(PH) = 1.7 Hz, 1J(13CH) = 143.3 Hz, CH3), 7.14 (d, 2 H, 5J(PH) = 0.7 Hz, 1J(13CH) = 201.3 Hz, CH). 13C{1H} NMR (δ/ppm): 33.31 (d quint, 1J(PC) = 241.8 Hz, 3J(FcisH) = 49.9 Hz, CH2), 35.70 (broad s, CH3), 122.24 (broad s, CH), 149.50 (broad s, C). 19F NMR (δ/ppm): −49.96 (ddt, 4 F, 1J(PFcis) = 833.0 Hz, 2J(FcisFtrans) = 40.3 Hz, 3J(FcisH) = 5.3 Hz, Fcis), −67.98 (d quint, 1 F, 1J(PFtrans) = 715.0 Hz, 3J(FcisFtrans) = 40.1 Hz, Ftrans). 31P NMR (δ/ppm): −135.54 (d quint t, 1J(PFtrans) = 715.1 Hz, 1J(PFcis) = 832.2 Hz, 2J(PH) = 27.6 Hz). For the views of the 1H, 13C{1H}, 19F, and 31P NMR and IR spectra of 12, see the Supporting Information. X-ray Crystallography. Colorless single crystals of 2 suitable for X-ray diffraction analyses were prepared by recrystallization from either boiling water (100 °C, in air) or hot toluene (in the absence of air). The nearly identical crystallographic data in the latter case were of slightly better quality and are referred to for the discussion. Colorless sample crystals of 4 and 12 suitable for X-ray diffraction analyses were grown from ethanol and methanol solutions, respectively. X-ray data were collected on a Bruker SMART APEXII diffractometer (graphitemonochromated Mo Kα radiation, 0.710 73 Å) using the ω scan mode at 296 K. In all cases, the absorption corrections were performed semiempirically from equivalents.20 All structures were solved by direct methods (SHELXS) and refined by full-matrix least squares on F2 (SHELXL).21 All crystals presented colorless blocks and belonged to the monoclinic crystal system with Z = 4 formula units per cell. The sample crystals of 4 and 12 contained nonmerohedral twin contaminants (dominant to minor component transform twin laws (matrices row by row): 0.999 82 −0.000 13 0.000 19 −0.000 62 −0.999 72 −0.001 11 −0.080 20 0.000 30 −0.999 54 and 0.485 95 −0.000 02 0.514 33 −0.000 13 −0.999 83 0.000 02 1.484 66 −0.000 30 −0.485 72). On data reduction, the generated *_m.mul files were processed with the TWINABS program (version 2008/2). The contributions of the minor components for 4 and 12 were estimated to be 23.15 and 45.49%. The structures were then solved with the detwinned HKLF 4 data files and finally refined with the HKLF 5 format data files with reflections from only the main component and

composites included. For 4, the data were merged accordingly to the point group 2/m (the BASF parameter converges to 0.224(4)). For 12, the data were not merged (the BASF parameter converges to 0.472(3)). In all cases, non-H atoms were refined anisotropically. For 2 (from toluene), all H atoms except of those of the disordered NCH3 group were found from difference Fourier synthesis and refined in an isotropic approximation. Except for those, all other H atoms were put into calculated positions and refined as riding atoms with distances C−H = 0.96 (CH3), 0.97 (CH2), 0.93 Å (CArH) and Uiso(H) = 1.5[Ueq(C)], 1.2[Ueq(C)], and 1.2[Ueq(C)], respectively. In all cases, no restraints were applied. For the other crystal data, data collection, structure solution and refinement parameters for 2, 4, and 12, see Table 6. Computational Details. All computations were made at the RB3LYP/6-311+G(2d,p) level of theory (restricted hybrid DFT method) with the GAUSSIAN03W program package22 for all compounds and transition states mentioned in the text except for compounds 11 and 12. Vibrational frequency analysis and wave function stability checks were performed for all optimized structures to prove their validity. If not explicitly specified, in all cases the default GAUSSIAN03W parameters were used. NMR shielding tensors were computed with the gauge-independent atomic orbital (GIAO) method23 at the same level of theory (scf=tight instruction was used). For comparison purposes, all chemical shifts were brought to the reference standards applied in the experiment (1H and 13C, TMS; 19F, CFCl3; 11B, Et2O·BF3; 31P, 85% aqueous H3PO4). For 1H, 13C, and 19F NMR spectra references, the built-in values of the GIAO isotropic components of the shielding tensors were used (31.8821, 182.4656, and 155.3161 ppm). For 11B and 31P NMR spectra referencing, geometries of Et2O·BF3 and PH3 were optimized at the same level of theory and the GIAO isotropic components of the shielding tensors for the nuclei of interest were calculated (99.1356 and 561.242 ppm, respectively). The reference value for 85% aqueous H3PO4 (321.242 ppm) was then determined by a simple addition of the known experimental chemical shift of PH3 (−240 ppm) to its calculated value. Spin−spin coupling constants were estimated as described in ref 24 (NMR=spinspin). 1758


Organometallics

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In the search for the TS, TS-a, and TS-b, the intrinsic QST3 algorithm of the GAUSSIAN03W program package was used, with starting geometries varied many times in each case. The found transition states are all unique ones (i.e., no intermediates or alternative conformations were found). For TS along with compounds 1, 3, and 2·HF, the final steps of optimizations were performed with calculation of the force constants at every iteration cycle (calcal keyword) due to a very flat character of the saddle or stationary points. The intrinsic reaction coordinate (IRC) computations were performed with a build-in GAUSSIAN03W algorithm25 in both “forward” and “reverse” directions starting from TS (the C1−B1 distance was used to define these directions within the phase instruction). The simultaneous presence of very flat and very steep segments in the reaction profile along with its extended character (range from < −30 through >+25 amu1/2 bohr) was a serious computational complication and caused failure of the IRC procedure with the default parameters. To overcome this difficulty, the step size along the IRC was increased from the default 0.1 up to 0.4 amu1/2 bohr (stepsize = 40) while the convergence criteria on max/rms force and max/rms displacement were tightened to 2.25 × 10−4, 1.50 × 10−4, 9.00 × 10−4, and 6.00 × 10−4, respectively (all in atomic units; iop(1/7=150) instruction; simple application of standard verytight keyword fails in the case). It is worth noting that despite the increased allowed numbers of points and cycles for each point optimization (maxpoints = 100 and maxcyc = 200), the IRC procedure was instable and had to be resumed twice in each of the directions. Thermochemistry analyses26,27 were performed for all compounds except for 11 and 12, and computed NMR reference standards within a 25−450 °C temperature range with corrections for the free intrinsic rotation28 (built-in routines of the GAUSSIAN03W program package). To eliminate an internal “bug” of the f req=hindrotor instruction, rotations around the cycle-forming bonds were forced to be “frozen by user”. Intrinsic redundant coordinates (all bonds, angles, and torsion angles) generated automatically by the routine were eliminated and then explicitly redefined (the modredundant instruction). Free and free activation Gibbs energies (ΔG(T), ΔG⧧(T)) were modeled at a pressure 0.001 atm (=0.76 Torr). Vibrational frequencies were scaled by an empirical factor of 0.9613.26

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REFERENCES

(1) Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H. Inorg. Chim. Acta 2003, 352, 143−150. (2) (a) Casely, I. J.; Liddle, S. T.; Blake, A. J.; Wilson, C.; Arnold, P. L. Chem. Commun. 2007, 5037−5039. (b) Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2008, 47, 7433−7437. (c) Kuhn, N.; Henkel, G.; Kratz, T.; Kreutzberg, J.; Boese, R.; Maulitz, A. H. Chem. Ber. 1993, 126, 2041−2045. (d) Weber, L.; Dobbert, E.; Stammler, H. G.; Neumann, B.; Boese, R.; Blaser, D. Chem. Ber./Recl. 1997, 130, 705− 710. (e) Zheng, X.; Herberich, G. E. Organometallics 2000, 19, 3751− 3753. (f) Ramnial, T.; Jong, H.; McKenzie, I. D.; Jennings, M.; Clyburne, J. A. C. Chem. Commun. 2003, 1722−1723. (g) Phillips, A. D.; Power, P. P. Acta Crystallogr. 2005, C61, o291−o293. (h) Arnold, P. L.; Blake, A. J.; Wilson, C. Chem. Eur. J. 2005, 11, 6095−6099. Arnold, P. L.; (i) Liddle, S. T.; Mungur, S. A.; Rodden, M. R.; Wilson, C. Spec. Publ. Chem. Soc. 2006, 305 (Recent Advances in ActinideScience), 195−197. (3) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.; Schaefer, H. F. I.; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2007, 129, 12412−12413. (4) Arduengo, A. J. III; Davidson, F.; Krafczyk, R.; Marshall, W. J.; Schmutzler, R. Monatsh. Chem. 2000, 131, 251−265. (5) Matsumoto, T.; Gabbai, F. P. Organometallics 2009, 28, 4252− 4253. (6) Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S.-H.; Curran, D. P.; Malacria, M.; Fensterbank, L.; Lacote, E. Chem. Eur. J. 2009, 15, 12937−12940. (7) Bissinger, P.; Braunschweig, H.; Kupfer, T.; Radacki, K. Organometallics 2010, 29, 3987−3990. (8) (a) Kuhn, N.; Eichele, K.; Walker, M.; Berends, T.; Minkwitz, R. Z. Anorg. Allg. Chem. 2002, 628, 2026−2032. (b) Arduengo, A. J., III; Calabrese, J. C.; Cowley, A. H.; Dias, H. V. R.; Goerlich, J. R.; (c) Marshall, W. J.; Riegel, B. Inorg. Chem. 1997, 36, 2151−2158. (d) Tumanskii, B.; Sheberla, D.; Molev, G.; Apeloig, Y. Angew. Chem., Int. Ed. 2007, 46, 7408−7411. (9) Kuhn, N.; Stroebele, M.; Walker, M. Z. Anorg. Allg. Chem. 2003, 629, 180−181. (10) Arduengo, A. J. III; Krafczyk, R.; Marshall, W. J.; Schmutzler, R. J. Am. Chem. Soc. 1997, 119, 3381−3382. (11) Solovyev, A.; Chu, Q.; Geib, S. J.; Fensterbank, L.; Malacria, M.; Lacote, E.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 15072−15080. (12) Yamaguchi, Y.; Kashiwabara, T.; Ogata, K.; Miura, Y.; Nakamura, Y.; Kobayashi, K.; Ito, T. Chem. Commun. 2004, 2160− 2161. (13) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.; Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428−7432. (14) van den Broeke, J.; Stam, M.; Lutz, M.; Kooijman, H.; Spek, A. L.; Deelman, B.-J.; van Koten, G. Eur. J. Inorg. Chem. 2003, 2798− 2811. (15) (a) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772−3789. (b) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667−3692. (c) Welton, T. Chem. Rev. 1999, 99, 2071−2084. (16) Allen, F. H. Acta Crystallogr. 2002, B58, 380−388. (17) Kuhn, N.; Fawzi, R.; Kotowski, H.; Steimann, M. Z. Kristallogr. 1997, 212, 2. (18) Meerwein, H.; Battenberg, E.; Gold, H.; Pfiel, E.; Willfang, G. J. Prakt. Chem. 1939, 154, 83−156. Jones, F. R.; Plesch, P. H. J. Chem. Soc., Dalton Trans. 1979, 927−932. (19) Stenzel, O.; Raubenheimer, H. G.; Esterhuysen, C. J. Chem. Soc., Dalton Trans. 2002, 1132−1138. (20) Sheldrick, G. M. SADABS and TWINABS; University of Göttingen, Göttingen, Germany, 1996. (21) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (22) Frisch, M. J. et al. Gaussian 03W, Revision 6.1; Gaussian, Inc., Pittsburgh, PA, 2003. (23) (a) Stephens, P. J.; Devlin, F. J.; Cheeseman, J. R.; Frisch, M. J.; Bortolini, O.; Besse, P. Chirality 2003, 15, S57−S64. Ruud, K.; (b) Helgaker, T.; Bak, K. L.; Jøergensen, P.; Jensen, H. J. A. J. Chem. Phys. 1993, 99, 3847−3859. (c) Wolinski, K.; Hinton, J. F.; Pulay, P.

ASSOCIATED CONTENT

* Supporting Information S

Figures, tables, text, and CIF and AVI files giving views of NMR and IR spectra, Cartesian coordinates for optimized geometries, energies and thermochemical computational results, full computational data on modeled IR and NMR spectral parameters, discussion of 1 → 8a + C2H4 + HF + BF3 conversion, summary of the IRC computation, additional thermochemistry plots, CIF files for compounds 2, 4, and 12, animation movies for the imaginary vibrational modes in all transition states, animation movie for the IRC path following, and the full reference for the Gaussian03W program package. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Project Nos. 20702041 and 21072157) and Shaanxi province Administration of Foreign Experts Bureau Foundation (Grant No. 20106100079) is gratefully acknowledged. 1759


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High-Yield Thermolytic Conversion of Imidazolium Salts into Arduengo

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