DOI: 10.1002/chem.201402080
Communication
& Synthetic Methods
Ruthenium Complexes of an Abnormally Bound, Anionic N-Heterocyclic Carbene Conor Pranckevicius and Douglas W. Stephan*[a] Abstract: The abnormally bound, anionic NHC–borane complex [Ru(IDipp-BF3)(p-cymene)Cl]2 (4; IDipp-BF3 = 1,3(2,6-iPr2C6H3)2-2-BF3(C3HN2)-4-yl) was synthesized by transmetalation from Li[(IDipp-BF3)2Ag]. Addition of donors gave species of the form [Ru(IDipp-BF3)(p-cymene)(L)Cl], whereas halide abstraction with Ag(Et2O)[B(C6F5)4] gave C H activation of the methine position of the IDipp BF3 ligand.
N-heterocyclic carbenes (NHCs) have been responsible for significant advances in transition-metal, organic, and main-group chemistry.[1] Their tunable steric demands together with their strong donor ability have proven to be useful in numerous catalytic transformations, notably, in olefin metathesis[2] and in cross-coupling reactions.[3] The utility of NHCs as organometallic ligands is often due to their ability to stabilize highly coordinatively unsaturated transition-metal species. In this vein, an interesting extension to this ligand class involves the notion of generating anionic NHCs. Anionic dicarbenes were first isolated by Robinson and co-workers in 2010 by lithiation of 1,3-(2,6iPr2C6H3)2(C3H2N2) (IDipp) at the abnormal position.[4] Selective coordination of a neutral electrophile to the abnormal position allows isolation of the anionic NHC as its Li salt.[1k, 4–5] To date, anionic NHCs and their metal complexes have been accessed by deprotonation of neutral N-heterocycles,[6] incorporating anionic functionalities in the NHC backbone,[7] C H activation,[8] and by reduction of a NHC in the presence of a metal source.[9] These reactions often require that the metals involved are either stable to reducing conditions, or are tolerant of the strong basicity of the anionic NHC alkali salt. Seeking to explore the strong electron-donor properties offered by anionic NHCs, we envisioned binding a metal at the abnormal position[6g, h, 10] of an anionic carbene would further enhance the donor ability. Herein, we report first examples of mono- and dinuclear RuII complexes containing abnormally bound anionic NHCs. These species were obtained by a convenient and gentle transmetalation route employing an Ag complex. The subsequent reactivity of the Ru-anionic NHC complex with donors and a halide abstractor is also described.
Our initial investigation involved examining the reactions of the lithiated IDipp-BF3 adduct (2), with a number of common Ru starting materials; however, this led only to intractable mixtures. A strategy frequently used in the synthesis of Ru–NHC complexes is the use of an NHC–Ag complex as a transmetalating agent,[11] and we next investigated whether this strategy could be extended to anionic abnormal carbenes. It was found that stirring suspension of Li(IDipp-BF3) with AgCl in THF cleanly generated a species of the formula Li[(IDipp-BF3)2Ag] (3). No reduction to Ag metal was observed. Crystals of the THF solvate [(THF)3Li][(IDipp-BF3)2Ag] were grown from THF/pentane, and were analyzed by single-crystal X-ray diffraction. The geometry at Ag is linear with coordination by two NHCs through the abnormal positions. The average Ag C distance is 2.090(3) . There is a F Li dative interaction in the solid state with a bond length of 1.882(6) . In [D8]THF solution, C–109Ag and C–107Ag coupling to the a carbon is apparent in the
Scheme 1. Synthesis of compounds 2–4.
[a] C. Pranckevicius, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George St., Toronto, Ontario, M5S3H6 (Canada) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402080. Chem. Eur. J. 2014, 20, 6597 – 6602
Figure 1. Molecular structure of 3. iPr groups and hydrogens atoms were removed for clarity.
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C NMR spectrum with coupling constants of 184 and 159 Hz, respectively (Scheme 1 and Figure 1). Stirring equimolar amounts of 3 and [Ru(p-cymene)Cl2]2 in THF for 16 h resulted in a dark red solution, which upon purification gave crystals of dark red [Ru(IDipp-BF3)(p-cymene)Cl]2 (4) in 63 % yield. Compound 4 is a rare example of an abnormally bound Ru NHC species,[8, 12] and the first example of a Ru complex containing a nonbridging anionic NHC. In [D8]THF solution, four doublets were observed in the alkyl region of the 1 H NMR spectrum, corresponding to the four inequivalent iPr
Figure 3. Variable-temperature 1H NMR (CD2Cl2) spectrum of 4, aromatic region.
Figure 2. Molecular structure of 4. iPr groups and hydrogens atoms were removed for clarity.
groups. Broad resonances are attributable to p-cymene protons. A single-crystal X-ray analysis (Figure 2) confirmed the dimeric nature of 4, in which the two Ru centers are bridged by chloride atoms. The coordination sphere is completed by the p-cymene and the anionic carbene ligand. The Ru C distance for the carbene donor was determined to be 2.119(2) , slightly longer than the length of 2.084(7) reported for the abnormal NHC complex [Ru(p-cymene)(1,3-dimethyl-2-phenylimidazol-4yl)Cl2].[12a] This lengthening may be attributed to steric factors. Interestingly, a CD2Cl2 solution of 4 at room temperature gave rise to a broad, largely featureless 1H NMR spectrum. Upon cooling to 0 8C, the signals sharpen to reveal the presence of two species in a 1:0.66 ratio. Further cooling to 20 and 40 8C altered this ratio to 1:0.60 and 1:0.50, respectively, consistent with an equilibrium between two species. Each species contains an h6 p-cymene ring and an abnormally bound IDipp-BF3 ligand. Upon further cooling to 80 8C, signals for the minor component broaden into the baseline of the NMR spectrum, whereas sharper resonances were observed for the major component (Figure 3 and the Supporting Information). This behavior is believed to be the result of monomer/dimer equilibrium with the dominant species being the dimer. The observed broadening an 80 8C in the minor species is attributed to the wagging of the chloride ligand in monomeric [Ru(IDippBF3)(p-cymene)Cl] between two magnetically inequivalent positions on the NMR timescale. However, the possibility Chem. Eur. J. 2014, 20, 6597 – 6602
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Scheme 2. Synthesis of compounds 5–7.
of conformational isomers of dimeric 4 giving rise to the observed equilibrium cannot be ruled out (Scheme 2). The dimeric 4 readily reacted with monodentate ligands including PPh3 and CO to give the compounds [Ru(IDipp-BF3)(pcymene)(PPh3)Cl] (5) and [Ru(IDipp-BF3)(p-cymene)(CO)Cl] (6), respectively, which were characterized crystallographically (see the Supporting Information). Complex 5 displayed ten doublets in the aliphatic region of the 1H NMR spectrum, indicative of restricted rotation of all five iPr groups, presumably resulting from steric congestion in the ligand field. The carbonyl complex 6 displayed a CO stretching frequency at u˜ = 2008 cm 1. Ru C bond lengths are 2.118(3) and 2.0682(13) in 5 and 6, respectively. Compound 4 also reacted with Ag(Et2O)[B(C6F5)4] in CH2Cl2, resulting in an immediate color change from dark red to orange, and the precipitation of AgCl from solution. The fil-
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Communication tered reaction mixture was layered with hexamethyldisiloxane, and pale orange crystals of a new species 7 were formed over a 24 hour period. The 19F NMR spectrum was consistent with the 1:1 ratio of B(C6F5)4 and IDipp-BF3 moieties. 1H NMR data for 7 revealed protonation of the IDipp-BF3 ligand at the abnormal position had occurred, although the two backbone protons remained inequivalent, as are all of the aryl and alkyl protons in the IDipp-BF3 ligand. Notably, one of the iPr methine protons of the ligand was absent. A doublet was observed at d = 2.66 ppm integrating to 1 H, and was observed to couple to a doublet of doublets at d = 7.61 ppm, also 1 H, as indicated by a correlation spectroscopy (COSY) experiment. Additionally, there is a broad upfield singlet at 3.30 ppm integrating to 3 H, suggesting an agostic interaction between Ru and one of the iPr methyl groups. Collectively, these data suggest that proton transfer had occurred between the abnormal position on the NHC and one of the iPr methine positions, and part of one of the Dipp rings has now coordinated to Ru. A molecular structure of 7 revealed that cationic Ru center is coordinated to h6 to a p-cymene ligand and an iPr group depro-
Figure 4. Molecular structure of the cation of 7. Hydrogen atoms shown were located in the electron-density map and freely refined, other hydrogen atoms were removed for clarity.
tonated at the methine position on IDipp-BF3 (Figure 4). This unusual binding is stabilized by an h2-interaction with the adjacent phenyl ring and an agostic interactions with the adjacent CH3 group. The crystallographic data revealed the Ru C distance for the deprotonated carbon is 2.104(3) , whereas the corresponding distances to the adjacent aryl and methyl carbons are 2.236(3), 2.234(3), and 2.342(3) , respectively. The Caryl Caryl bond length on the h2-coordianted ring is elongated to 1.448(4) . The hydrogens on the agostically coordinated methyl group were located in the electron-density map and give rise to a Ru H distance of 1.85(5) . The formation of 7 suggests that upon abstraction of the halide from Ru by Ag(Et2O)[B(C6F5)4], the cationic Ru center mediates proton migration from the methine carbon to the carbene carbon. The iPr-deprotonated carbene is trapped by coordinatively unsaturated Ru, affording the unique binding mode observed. Presumably, this rearrangement is facilitated Chem. Eur. J. 2014, 20, 6597 – 6602
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in this case by the basicity of the anionic carbene and the partial coordination of the arene ring to Ru. Bertrand and coworkers have observed that their free abnormal NHC decomposed by a related proton-transfer reaction in the presence of a Lewis acid or by heating.[10b] In conclusion, this work has demonstrated that transmetalation of an anionic NHC ligand from Ag provides a viable access to a Ru dimer containing the anionic NHC ligand. Subsequent reactions with donors effect dimer scission, whereas abstraction of halide results in a unique binding mode of the carbene by deprotonation of the iPr fragment of the N-aryl substituent. This new reactivity stands in contrast to neutral NHCs and other carbene donors, and suggests that anionic NHC complexes may provide access to further unique reactivity.
Experimental Section General comments All manipulations were carried out under an atmosphere of dry, O2-free N2 employing a Vacuum Atmospheres glovebox or a Schlenk vacuum line. Solvents were purified with a Grubbs-type column system manufactured by Innovative Technology and dispensed into thick-walled Straus flasks equipped with Teflon-valve stopcocks. Deuterated dichloromethane was distilled under reduced pressure from CaH2 and degassed by successive freeze/ pump/thaw cycles. Deuterated THF was distilled from purple sodium benzophenone ketyl. 1 H, 13C, 19F, 11B, 31P, and 7 Li NMR spectra were recorded at 25 8C on Bruker 400 MHz, Agilent DD2 500 MHz, or Agilent DD2 600 MHz spectrometers, unless otherwise noted. Chemical shifts are reported in parts per million are given relative to SiMe4 and referenced to the residual solvent signal. Elemental analyses were performed in house employing a PerkinElmer CHN Analyzer. In some cases, elemental analyses were reproducibly low on carbon content, presumably due to the formation of metal carbides during combustion. IR spectra were collected on a PerkinElmer Spectrum One FTIR instrument. Compound [Ru(p-cymene)Cl2]2, nBuLi solution, BF3(OEt)2, PPh3, and CO were purchased from Sigma–Aldrich or Strem chemicals and used without further purification. Complex Ag(Et2O)[B(C6F5)4][13] and IDipp[14] were synthesized according to literature procedures.
Synthesis of IDipp BF3 (1) IDipp (1.045 g, 2.69 mmol) was dissolved in diethyl ether (10 mL), and BF3(Et2O) (0.362 mL, 2.93 mmol) was added to the solution in a dropwise fashion with stirring. A white crystalline precipitate formed immediately, and n-pentane (10 mL) was added to the mixture. After stirring for 10 min, the precipitate was collected by filtration, washed with pentane (2 10 mL), and dried under vacuum (1.165 g, 95 %). 1H NMR (400 MHz, [D8]THF): d = 7.52 (s, 2 H, NCH 2), 7.44 (t, 3JH H = 8 Hz, 2 H, p-Dipp), 7.29 (d, 3JH H = 8 Hz, 4 H, mDipp), 2.57 (septet, 3JH H = 7 Hz, 4 H, CH(CH3)2), 1.27 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.17 ppm (d, 3JH H = 7 Hz, 6 H, CH(CH3)2); 13C NMR (125 MHz, [D8]THF): 146.28 (ipso-iPr), 134.88 (ipso), 130.76 (p-Dipp), 124.93 (NCH), 124.27(m-Dipp), 29.70 (CH(CH3)), 25.10 (CH(CH3)), 23.17 ppm (CH(CH3)); 19F NMR (376.5 MHz, [D8]THF): d = 140.01 ppm (1:1:1:1 q, 1JB F = 33 Hz, BF3); 11B NMR (128.4 MHz, [D8]THF): d = 0.89 ppm (q, 1JB F = 33 Hz, BF3); elemental analysis calcd for (456.39): C, 71.05; H, 7.95; N, 6.14; found: C, 71.35; H, 7.93; N, 6.14.
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Communication Synthesis of (THF)xLi(IDippBF3) (2) A solution of 1 (1.163 g, 2.55 mmol) in THF (10 mL) was cooled to 40 8C, and a pre-cooled nBuLi solution (1.70 mL, 1.6 m in hexane, 2.72 mmol) was added. The stirred mixture was allowed to warm to RT over the course of 1 h to give a white precipitate. n-Pentane (10 mL) was then added, the mixture was shaken, and the supernatant decanted. The white solid obtained was washed with n-pentane (4 10 mL) and dried under vacuum (1.540 g, 97 %). The product was stored in a 40 8C freezer under N2. 1H NMR (400 MHz, [D8]THF): d = 7.27 (t, 3JH H = 8 Hz, 1 H, p-Dipp), 7.20 (dd, 3JH H = 7 Hz, 8 Hz, 1 H, p-Dipp), 7.15 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 7.11 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 6.27 (s, 1 H, NCH), 2.79 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 2.70 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 1.22 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.21 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.13 (d, 3 JH H = 7 Hz, 6 H, CH(CH3)2), 1.10 ppm (d, 3JH H = 7 Hz, 6 H, CH(CH3)2); 13 C NMR (125 MHz, [D8]THF): d = 146.42 (ipso Ar-iPr), 146.11 (ipso Ar-iPr), 142.30 (ipso), 137.16 (ipso), 129.80 (NCH), 128.57 (p-Dipp), 127.43 (p-Dipp), 123.14 (m-Dipp), 122.80 (m-Dipp), 28.90 (CH(CH3)), 28.81 (CH(CH3)), 25.46 (CH(CH3)), 24.89 (CH(CH3)), 23.48 (CH(CH3)), 23.18 ppm (CH(CH3)); 19F NMR (376.5 MHz, [D8]THF): d = 141.29 ppm (1:1:1:1 br q, 1JB F = 36 Hz, BF3); 11B NMR (128.4 MHz, [D8]THF): d = 2.26 ppm (q, 1JB F = 38 Hz, BF3); 7Li NMR (155.5 MHz, [D8]THF): d = 1.25 ppm (s); elemental analyses was not possible due to the extreme air sensitivity of 2.
Synthesis of Li[Ag(IDippBF3)2] (3) To a suspension of 2 (1.180 g, 1.90 mmol) in THF (10 mL) was added anhydrous AgCl (0.245 g, 1.72 mmol), and the mixture was stirred for 16 h in the dark. The resulting suspension was filtered through Celite pad, and the solvent was removed in vacuo. Toluene (10 mL) was added and stirred for 10 min, the mixture was then filtered from LiCl, and concentrated under vacuum. The resulting colorless oil was triturated in n-pentane, giving a white solid, which was dried in vacuo (0.806 g, 83 %). Crystals of [Ag(IDippBF3)2][Li(THF)3] were grown from slow diffusion of n-pentane into a THF solution. 1H NMR (400 MHz, [D8]THF): d = 7.30 (t, 3JH H = 8 Hz, 2 H, p-Dipp), 7.17 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 7.02 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 6.33 (d, J = 2 Hz, 1 H, NCH), 2.60 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 2.56 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 1.18 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.13 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.10 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.00 ppm (d, 3JH H = 7 Hz, 6 H, CH(CH3)2); 13C NMR (125 MHz, [D8]THF): d = 161.54 (pseudo dd, 1 JC 109Ag = 184 Hz, 1JC 107Ag = 159 Hz, NCAg), 146.39 (ipso Ar-iPr), 145.92 (ipso Ar-iPr), 140.59 (ipso), 136.16 (ipso), 129.39 (pseudo d, 2 JC Ag = 15 Hz, NCH), 129.25 (p-Dipp), 128.41 (p-Dipp), 123.46 (mDipp), 123.13 (m-Dipp), 28.99 (CH(CH3)), 28.94 (CH(CH3)), 25.45 (CH(CH3)), 25.27 (CH(CH3)), 23.33 (CH(CH3)), 23.21 ppm (CH(CH3)); 19 F NMR (376.5 MHz, [D8]THF): d = 139.5– 139.9 ppm (br m, BF3); 11 B NMR (128.4 MHz, [D8]THF): d = 1.0– 1.6 ppm (br m, BF3); 7Li NMR (155.5 MHz, [D8]THF): d = 0.96 ppm (s); elemental analysis calcd for C54H70AgB2F6LiN4 (1025.58): C, 63.24; H, 6.88; N 5.46; found: C, 63.48; H, 6.80; N, 5.28.
Synthesis of [Ru(p-cymene)(IDippBF3)Cl]2 (4) To a solution of 3 (145 mg, 0.141 mmol) in THF was added [Ru(pcymene)Cl2]2 (75 mg, 0.122 mmol), and the mixture was stirred for 16 h at 25 8C, resulting a dark red solution and the precipitation of AgCl. The solution was filtered and concentrated under vacuum. The red oil obtained was dissolved in toluene (10 mL) and stirred for 2 h, affording a red precipitate. CH2Cl2 (8 mL) was added, and the resultant suspension was stirred for 2 h and then filtered from Chem. Eur. J. 2014, 20, 6597 – 6602
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LiCl. The mixture was concentrated in vacuo, and Et2O (10 mL) added to cause the precipitation of red microcrystals. The solid was washed with Et2O (3 5 mL) and dried. To remove final traces of LiCl, the solid was dissolved in dichloromethane, filtered, the product precipitated with n-pentane, and dried under high vacuum (124 mg, 63 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of n-pentane in to a CH2Cl2 solution. 1H NMR (600 MHz, [D8]THF): d = 7.45 (t, 3JH H = 7 Hz, 1 H, pDipp), 7.34 (d, 3JH H = 7 Hz, 2 H, m-Dipp), 7.28 (t, 3JH H = 7 Hz, 1 H, pDipp), 7.13 (d, 3JH H = 7 Hz, 2 H, m-Dipp), 6.85 (br s, 1 H, NCH), 5.50 (br, 2 H, p-cymeneAr), 4.60 (br, 2 H, p-cymeneAr), 2.90 (br, 2 H, CH(CH3)2), 2.71–2.53 (br m, 3 H, CH(CH3)2), 1.66 (s, 3 H, p-cymeneMe), 1.41 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.27 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.21 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.09 (d, 3JH H = 6.8 Hz, 6 H, CH(CH3)2), 0.97 ppm (br, 6 H, p-cymene CH(CH3)2); 13C NMR (125 MHz, CD2Cl2): d = 130.05 (p-Dipp), 130.00 (p-Dipp), 124.49 (br, m-Dipp), 124.19, 124.10 (br, m-Dipp), 30.60, 29.23, 28.97, 25.15, 23.25 ppm (br); 19F NMR (376.5 MHz, [D8]THF): d = 136.4– 137.0 ppm (br m, BF3); 11B NMR (128.4 MHz, [D8]THF): d = 0.51 ppm (br q, 1JB F = 35 Hz, BF3); elemental analysis calcd for C74H98B2Cl2F6N4Ru2·2 (CH2Cl2) (1622.12): C, 56.27; H, 6.35, N, 3.45; found: C, 56.12; H, 6.12; N, 3.41.
Synthesis of [Ru(IDippBF3)(p-cymene)(PPh3)Cl] (5) PPh3 (16 mg, 0.061 mmol) was added to a dark red solution of 3 (43 mg, 0.030 mmol) in CH2Cl2 (2 mL), resulting in an immediate color change to orange. The mixture was filtered and concentrated to 1 mL. n-Pentane (15 mL) was added to cause the precipitation of an orange-brown solid, which was collected, washed with npentane (3 5 mL), and dried under high vacuum (45 mg, 76 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of pentane in to a CH2Cl2 solution. 1H NMR (400 MHz, CD2Cl2): d = 7.46–7.26 (m, 18 H, PPh3 + p-Dipp 2 + mDipp 1), 7.18 (dd, 3JH H = 8 Hz, 4JH H = 1.3 Hz, 1 H, m-Dipp), 7.13 (dd, 3JH H = 8 Hz, 4JH H = 2 Hz, 1 H, m-Dipp), 7.10 (dd, 3JH H = 8 Hz, 4 JH H = 2 Hz, 1 H, m-Dipp), 6.03 (s, 1 H, NCH), 5.81 (d, 3JH H = 6 Hz, pcymeneAr), 5.47 (apparent t, 3JH H = 5 Hz), 4.91 (d, 3JH H = 6 Hz, pcymeneAr), 3.92 (d, 3JH H = 6 Hz, p-cymeneAr), 2.65 (septet, 2 H, 3 JH H = 7 Hz, 2 H, CH(CH3)2), 2.53 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.45 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.32 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.12 (s, 3 H, p-cymeneMe), 1.61 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.33 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.31 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.24 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.09 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.08 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 0.96 (d, 3 JH H = 7 Hz, 3 H, CH(CH3)2), 0.92 (apparent t, 3JH H = 6 Hz, 6 H, CH(CH3)2), 0.31 ppm (d, 3JH H = 7 Hz, 3 H, CH(CH3)2); 13C NMR (125 MHz, CD2Cl2): d = 149.20 (ipso Ar-iPr), 146.92 (ipso Ar-iPr), 146.74 (ipso Ar-iPr), 145.56 (ipso Ar-iPr), 138.40 (ipso), 134.26 (br, PPh3), 133.70 (ipso), 130.74 (br, PPh3), 129.63 (p-Dipp), 129.26 (pDipp), 128.92 (d, 3JP C = 8.9 Hz, NCH), 128.82 (d, 3JP C = 1.3 Hz, ipso cymeneiPr), 128.33 (br, PPh3), 124.03 (m-Dipp), 123.49 (m-Dipp), 123.24 (m-Dipp), 122.43 (m-Dipp), 99.71 (d, 3JP C = 5 Hz, p-cymeneAr), 88.99 (d, 3JP C = 20 Hz, p-cymeneAr), 87.77 (p-cymeneAr), 79.95 (d, 3JP C = 13 Hz, p-cymeneAr), 31.11 (CH(CH3)2), 30.00 (CH(CH3)2), 29.54 (CH(CH3)2), 29.98 (CH(CH3)2), 28.83 (CH(CH3)2), 27.45 (CH(CH3)2), 27.25 (CH(CH3)2), 26.93 (CH(CH3)2), 24.73 (CH(CH3)2), 24.69 (CH(CH3)2), 24.24 (CH(CH3)2), 22.61 (CH(CH3)2), 22.36 (CH(CH3)2), 22.05 (CH(CH3)2), 21.26 (CH(CH3)2), 17.45 ppm (p-cymeneMe); 31P NMR (162.0 MHz, CD2Cl2): d = 30.27 ppm (s); 19F NMR (CD2Cl2): 134.04 ppm (1:1:1:1 br q, 1JB F = 33 Hz, BF3); 11B NMR (128.4 MHz, CD2Cl2): d = 0.66 ppm (br q, 1JB F = 37 Hz, BF3); elemental analysis calcd for C55H64BClF3N2Ru (988.41): C, 66.83; H, 6.53, N, 2.83; found: C, 66.12; H, 6.24; N, 2.80.
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Communication Synthesis of [Ru(IDippBF3)(p-cymene)(CO)Cl] (6) A deep red solution of 3 (53 mg, 0.036 mmol) in CH2Cl2 (2 mL) was backfilled with 1 atm of CO, resulting in an immediate color change to orange. The mixture was filtered and concentrated to 1 mL. n-Pentane (15 mL) was added to precipitate the product from solution as a pale yellow solid, which was collected, washed with n-pentane (3 5 mL), and dried under high vacuum (38 mg. 69 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of n-pentane into a CH2Cl2 solution. 1H NMR (400 MHz, CD2Cl2): d = 7.47 (t, 3JH H = 8 Hz, 2 H, p-Dipp), 7.30 (d, 3 JH H = 8 Hz, 1 H, m-Dipp), 7.27 (d, 3JH H = 7 Hz, 1 H, m-Dipp), 7.24 (d, 3JH H = 8 Hz, 1 H, m-Dipp), 7.23 (d, 3JH H = 8 Hz, 1 H, m-Dipp), 6.67 (s, 1 H, NCH), 6.28 (d, 3JH H = 7 Hz, 1 H, p-cymeneAr), 5.88 (apparent t, 3JH H = 8 Hz, 2 H, p-cymeneAr), 5.37 (d, 3JH H = 6 Hz, 1 H, pcymeneAr), 2.84 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.67 (septet, 3 JH H = 7 Hz, 1 H, CH(CH3)2), 2.54–2.42 (m, 2 H, 2 CH(CH3)2), 2.31 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.13 (s, 3 H, p-cymeneMe), 1.33– 1.19 (m, 21 H, 7 CH(CH3)), 1.15 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.12 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.07 ppm (d, 3JH H = 7 Hz, 3 H, CH(CH3)2); 13C NMR (125 MHz, CD2Cl2): d = 193.60 (CO), 148.72 (ipso Ar-iPr), 147.30 (ipso Ar-iPr), 146.16 (ipso-Ar-iPr), 145.09 (ipso Ar-iPr), 137.80 (ipso), 134.35 (ipso), 130.54 (p-Dipp), 130.16 (NCH), 130.15 (p-Dipp), 124.99 (m-Dipp), 123.94 (m-Dipp), 123.74 (m-Dipp), 123.37 (m-Dipp), 121.82 (ipso-cymeneiPr), 116.83 (ipso-cymeneMe), 108.49 (p-cymeneAr), 99.27 (p-cymeneAr), 95.05 (p-cymeneAr), 90.24 (p-cymeneAr), 31.84 (CH(CH3)2), 29.17 (CH(CH3)2), 29.14 (CH(CH3)2), 29.01 (CH(CH3)2), 26.84 (CH(CH3)2), 26.30 (CH(CH3)2), 24.73 (CH(CH3)2), 24.63 (CH(CH3)2), 24.16 (CH(CH3)2), 23.90 (CH(CH3)2), 22.81 (CH(CH3)2), 22.70 (CH(CH3)2), 22.63 (CH(CH3)2), 22.49 (CH(CH3)2), 19.52 ppm (p-cymeneMe); 19F NMR (376.5 MHz, CD2Cl2): d = 137.11 ppm (1:1:1:1 q, 1JB F = 34 Hz, BF3); 11B NMR (128.4 MHz, CD2Cl2): d = 0.75 (q, 1JB F = 37 Hz, BF3); IR (KBr): u˜ = 2008 cm 1 (s, CO); elemental analysis calcd for C39H49BClF3N2ORu (754.14): C, 60.52; H, 6.55; N, 3.71; found: C, 58.82; H, 6.16; N, 3.55.
Ar-iPr), 145.29 (ipso Ar-iPr), 140.54 (p-Dipp), 138.6 (br d, 1JC F = 243 Hz, p-C6F5), 136.7 (br d, 1JC F = 246 Hz, m-C6F5), 132.85, 131.44 (p-Dipp), 127.04, 125.32 (NCH), 125.07 (NCH), 124.49 (m-Dipp), 124.45 (m-Dipp), 124.2 (br, ipso-C6F5), 124.03 (m-Dipp), 114.90 (ipsocymeneiPr), 104.54 (ipso-cymeneMe), 100.54 ([Ru]CC(CH3)2), 89.72 (q, JC F = 3 Hz, cymeneAr), 86.95 (q, JC F = 3 Hz, p-cymeneAr), 81.07 (m, p-cymeneAr), 80.88 (p-cymeneAr), 69.48 ([Ru]CC(CH3)2), 65.31 ([Ru] mDipp), 30.87 (CH(CH3)2), 29.67 (CH(CH3)2), 29.23 (CH(CH3)2), 25.34 (CH(CH3)2), 25.28 (CH(CH3)2), 24.26 (CH(CH3)2), 23.38 (CH(CH3)2), 22.61 (CH(CH3)2), 22.38 (CH(CH3)2), 22.05 (CH(CH3)2), 21.06 (CH(CH3)2), 20.88 ([Ru]CCH3), 17.73 (p-cymeneMe), 2.51 ppm ([Ru]CH3); 19F NMR (376.5 MHz, CD2Cl2): d = 133.09 (m, 8F, o-C6F5), 137.56 (1:1:1:1 q, 1JB F = 34 Hz, 3F, BF3), 163.63 (t, 3JF F = 20.1 Hz, 4F, p-C6F5), 167.48 (t, 3JF F = 17.9 Hz, 8F, m-C6F5); 11B NMR (128.4 MHz, CD2Cl2): d = 0.66 (q, 1JB F = 34 Hz, BF3), 16.69 ppm (s, B(C6F5)4); elemental analysis for C61H49B2F23N2Ru (1369.71): C, 53.49; H, 3.61; N, 2.05; found: C, 53.31; H, 3.58; N, 2.41.
Acknowledgements The authors gratefully acknowledge the financial support of NSERC of Canada. D.W.S. is also grateful for the award of a Canada Research Chair. C.P. would like to gratefully acknowledge a Queen Elizabeth II/Edwin Walter and Margery Warren Scholarship in Science and Technology for funding. Keywords: abnormal NHCs · anionic NHCs · C H activation · carbenes · ruthenium
Synthesis of [Ru(IDippBF3)(p-cymene)][B(C6F5)4] (7) Compound 3 (44 mg, 0.030 mmol) was dissolved in CH2Cl2 (2 mL), and Ag(Et2O)[B(C6F5)4] (52 mg, 0.061 mmol) was added, causing an immediate color change to orange and the precipitation of AgCl. The mixture was stirred for 30 min at 25 8C and was filtered. The solution was concentrated in vacuo to 2 mL, and (Me3Si)2O (2 mL) was layered on top. After 24 h, pale orange needles of 7 separated from solution. The solid was collected by filtration, washed with Et2O, and dried under high vacuum (36 mg, 43 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of Et2O in to a CH2Cl2 solution. 1H NMR (600 MHz, CD2Cl2): d = 7.61 (dd, 3JH H = 6, 9 Hz, 1 H, p-Dipp), 7.59 (t, 3JH H = 8 Hz, 1 H, p-Dipp), 7.47 (d, 3JH H = 9 Hz, 1 H, m-Dipp), 7.39 (dd, 3JH H = 8 Hz, 4JH H = 1 Hz, 1 H, m-Dipp), 7.37 (dd, 3JH H = 8 Hz, 4JH H = 1 Hz, 1 H, m-Dipp), 7.29 (d, 3JH H = 2 Hz, 1 H, NCH), 7.21 (d, 3JH H = 2 Hz, 1 H, NCH), 6.35 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 6.29 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 4.59 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 4.51 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 2.67 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 2.66 (d, 3 JH H = 6 Hz, 1 H, [Ru]m-Dipp), 2.53 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.32 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.29 (s, 3 H, [Ru]CCH3), 2.05 (s, 3 H, p-cymeneMe), 1.41 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.35 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.32 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.30 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.21 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.20 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.17 (d, 3 JH H = 7 Hz, 3 H, CH(CH3)2), 1.04 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 3.30 ppm (br s, 3 H, [Ru]CH3); 13C NMR (200 MHz, CD2Cl2): d = 152.14 (ipso Ar-iPr), 148.5 (br d, 1JC F = 241 Hz, o-C6F5), 146.24 (ipso Chem. Eur. J. 2014, 20, 6597 – 6602
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Received: February 7, 2014 Published online on April 29, 2014
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& Synthetic Methods
Ruthenium Complexes of an Abnormally Bound, Anionic N-Heterocyclic Carbene Conor Pranckevicius and Douglas W. Stephan*[a] Abstract: The abnormally bound, anionic NHC–borane complex [Ru(IDipp-BF3)(p-cymene)Cl]2 (4; IDipp-BF3 = 1,3(2,6-iPr2C6H3)2-2-BF3(C3HN2)-4-yl) was synthesized by transmetalation from Li[(IDipp-BF3)2Ag]. Addition of donors gave species of the form [Ru(IDipp-BF3)(p-cymene)(L)Cl], whereas halide abstraction with Ag(Et2O)[B(C6F5)4] gave C H activation of the methine position of the IDipp BF3 ligand.
N-heterocyclic carbenes (NHCs) have been responsible for significant advances in transition-metal, organic, and main-group chemistry.[1] Their tunable steric demands together with their strong donor ability have proven to be useful in numerous catalytic transformations, notably, in olefin metathesis[2] and in cross-coupling reactions.[3] The utility of NHCs as organometallic ligands is often due to their ability to stabilize highly coordinatively unsaturated transition-metal species. In this vein, an interesting extension to this ligand class involves the notion of generating anionic NHCs. Anionic dicarbenes were first isolated by Robinson and co-workers in 2010 by lithiation of 1,3-(2,6iPr2C6H3)2(C3H2N2) (IDipp) at the abnormal position.[4] Selective coordination of a neutral electrophile to the abnormal position allows isolation of the anionic NHC as its Li salt.[1k, 4–5] To date, anionic NHCs and their metal complexes have been accessed by deprotonation of neutral N-heterocycles,[6] incorporating anionic functionalities in the NHC backbone,[7] C H activation,[8] and by reduction of a NHC in the presence of a metal source.[9] These reactions often require that the metals involved are either stable to reducing conditions, or are tolerant of the strong basicity of the anionic NHC alkali salt. Seeking to explore the strong electron-donor properties offered by anionic NHCs, we envisioned binding a metal at the abnormal position[6g, h, 10] of an anionic carbene would further enhance the donor ability. Herein, we report first examples of mono- and dinuclear RuII complexes containing abnormally bound anionic NHCs. These species were obtained by a convenient and gentle transmetalation route employing an Ag complex. The subsequent reactivity of the Ru-anionic NHC complex with donors and a halide abstractor is also described.
Our initial investigation involved examining the reactions of the lithiated IDipp-BF3 adduct (2), with a number of common Ru starting materials; however, this led only to intractable mixtures. A strategy frequently used in the synthesis of Ru–NHC complexes is the use of an NHC–Ag complex as a transmetalating agent,[11] and we next investigated whether this strategy could be extended to anionic abnormal carbenes. It was found that stirring suspension of Li(IDipp-BF3) with AgCl in THF cleanly generated a species of the formula Li[(IDipp-BF3)2Ag] (3). No reduction to Ag metal was observed. Crystals of the THF solvate [(THF)3Li][(IDipp-BF3)2Ag] were grown from THF/pentane, and were analyzed by single-crystal X-ray diffraction. The geometry at Ag is linear with coordination by two NHCs through the abnormal positions. The average Ag C distance is 2.090(3) . There is a F Li dative interaction in the solid state with a bond length of 1.882(6) . In [D8]THF solution, C–109Ag and C–107Ag coupling to the a carbon is apparent in the
Scheme 1. Synthesis of compounds 2–4.
[a] C. Pranckevicius, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George St., Toronto, Ontario, M5S3H6 (Canada) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402080. Chem. Eur. J. 2014, 20, 6597 – 6602
Figure 1. Molecular structure of 3. iPr groups and hydrogens atoms were removed for clarity.
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C NMR spectrum with coupling constants of 184 and 159 Hz, respectively (Scheme 1 and Figure 1). Stirring equimolar amounts of 3 and [Ru(p-cymene)Cl2]2 in THF for 16 h resulted in a dark red solution, which upon purification gave crystals of dark red [Ru(IDipp-BF3)(p-cymene)Cl]2 (4) in 63 % yield. Compound 4 is a rare example of an abnormally bound Ru NHC species,[8, 12] and the first example of a Ru complex containing a nonbridging anionic NHC. In [D8]THF solution, four doublets were observed in the alkyl region of the 1 H NMR spectrum, corresponding to the four inequivalent iPr
Figure 3. Variable-temperature 1H NMR (CD2Cl2) spectrum of 4, aromatic region.
Figure 2. Molecular structure of 4. iPr groups and hydrogens atoms were removed for clarity.
groups. Broad resonances are attributable to p-cymene protons. A single-crystal X-ray analysis (Figure 2) confirmed the dimeric nature of 4, in which the two Ru centers are bridged by chloride atoms. The coordination sphere is completed by the p-cymene and the anionic carbene ligand. The Ru C distance for the carbene donor was determined to be 2.119(2) , slightly longer than the length of 2.084(7) reported for the abnormal NHC complex [Ru(p-cymene)(1,3-dimethyl-2-phenylimidazol-4yl)Cl2].[12a] This lengthening may be attributed to steric factors. Interestingly, a CD2Cl2 solution of 4 at room temperature gave rise to a broad, largely featureless 1H NMR spectrum. Upon cooling to 0 8C, the signals sharpen to reveal the presence of two species in a 1:0.66 ratio. Further cooling to 20 and 40 8C altered this ratio to 1:0.60 and 1:0.50, respectively, consistent with an equilibrium between two species. Each species contains an h6 p-cymene ring and an abnormally bound IDipp-BF3 ligand. Upon further cooling to 80 8C, signals for the minor component broaden into the baseline of the NMR spectrum, whereas sharper resonances were observed for the major component (Figure 3 and the Supporting Information). This behavior is believed to be the result of monomer/dimer equilibrium with the dominant species being the dimer. The observed broadening an 80 8C in the minor species is attributed to the wagging of the chloride ligand in monomeric [Ru(IDippBF3)(p-cymene)Cl] between two magnetically inequivalent positions on the NMR timescale. However, the possibility Chem. Eur. J. 2014, 20, 6597 – 6602
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Scheme 2. Synthesis of compounds 5–7.
of conformational isomers of dimeric 4 giving rise to the observed equilibrium cannot be ruled out (Scheme 2). The dimeric 4 readily reacted with monodentate ligands including PPh3 and CO to give the compounds [Ru(IDipp-BF3)(pcymene)(PPh3)Cl] (5) and [Ru(IDipp-BF3)(p-cymene)(CO)Cl] (6), respectively, which were characterized crystallographically (see the Supporting Information). Complex 5 displayed ten doublets in the aliphatic region of the 1H NMR spectrum, indicative of restricted rotation of all five iPr groups, presumably resulting from steric congestion in the ligand field. The carbonyl complex 6 displayed a CO stretching frequency at u˜ = 2008 cm 1. Ru C bond lengths are 2.118(3) and 2.0682(13) in 5 and 6, respectively. Compound 4 also reacted with Ag(Et2O)[B(C6F5)4] in CH2Cl2, resulting in an immediate color change from dark red to orange, and the precipitation of AgCl from solution. The fil-
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Communication tered reaction mixture was layered with hexamethyldisiloxane, and pale orange crystals of a new species 7 were formed over a 24 hour period. The 19F NMR spectrum was consistent with the 1:1 ratio of B(C6F5)4 and IDipp-BF3 moieties. 1H NMR data for 7 revealed protonation of the IDipp-BF3 ligand at the abnormal position had occurred, although the two backbone protons remained inequivalent, as are all of the aryl and alkyl protons in the IDipp-BF3 ligand. Notably, one of the iPr methine protons of the ligand was absent. A doublet was observed at d = 2.66 ppm integrating to 1 H, and was observed to couple to a doublet of doublets at d = 7.61 ppm, also 1 H, as indicated by a correlation spectroscopy (COSY) experiment. Additionally, there is a broad upfield singlet at 3.30 ppm integrating to 3 H, suggesting an agostic interaction between Ru and one of the iPr methyl groups. Collectively, these data suggest that proton transfer had occurred between the abnormal position on the NHC and one of the iPr methine positions, and part of one of the Dipp rings has now coordinated to Ru. A molecular structure of 7 revealed that cationic Ru center is coordinated to h6 to a p-cymene ligand and an iPr group depro-
Figure 4. Molecular structure of the cation of 7. Hydrogen atoms shown were located in the electron-density map and freely refined, other hydrogen atoms were removed for clarity.
tonated at the methine position on IDipp-BF3 (Figure 4). This unusual binding is stabilized by an h2-interaction with the adjacent phenyl ring and an agostic interactions with the adjacent CH3 group. The crystallographic data revealed the Ru C distance for the deprotonated carbon is 2.104(3) , whereas the corresponding distances to the adjacent aryl and methyl carbons are 2.236(3), 2.234(3), and 2.342(3) , respectively. The Caryl Caryl bond length on the h2-coordianted ring is elongated to 1.448(4) . The hydrogens on the agostically coordinated methyl group were located in the electron-density map and give rise to a Ru H distance of 1.85(5) . The formation of 7 suggests that upon abstraction of the halide from Ru by Ag(Et2O)[B(C6F5)4], the cationic Ru center mediates proton migration from the methine carbon to the carbene carbon. The iPr-deprotonated carbene is trapped by coordinatively unsaturated Ru, affording the unique binding mode observed. Presumably, this rearrangement is facilitated Chem. Eur. J. 2014, 20, 6597 – 6602
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in this case by the basicity of the anionic carbene and the partial coordination of the arene ring to Ru. Bertrand and coworkers have observed that their free abnormal NHC decomposed by a related proton-transfer reaction in the presence of a Lewis acid or by heating.[10b] In conclusion, this work has demonstrated that transmetalation of an anionic NHC ligand from Ag provides a viable access to a Ru dimer containing the anionic NHC ligand. Subsequent reactions with donors effect dimer scission, whereas abstraction of halide results in a unique binding mode of the carbene by deprotonation of the iPr fragment of the N-aryl substituent. This new reactivity stands in contrast to neutral NHCs and other carbene donors, and suggests that anionic NHC complexes may provide access to further unique reactivity.
Experimental Section General comments All manipulations were carried out under an atmosphere of dry, O2-free N2 employing a Vacuum Atmospheres glovebox or a Schlenk vacuum line. Solvents were purified with a Grubbs-type column system manufactured by Innovative Technology and dispensed into thick-walled Straus flasks equipped with Teflon-valve stopcocks. Deuterated dichloromethane was distilled under reduced pressure from CaH2 and degassed by successive freeze/ pump/thaw cycles. Deuterated THF was distilled from purple sodium benzophenone ketyl. 1 H, 13C, 19F, 11B, 31P, and 7 Li NMR spectra were recorded at 25 8C on Bruker 400 MHz, Agilent DD2 500 MHz, or Agilent DD2 600 MHz spectrometers, unless otherwise noted. Chemical shifts are reported in parts per million are given relative to SiMe4 and referenced to the residual solvent signal. Elemental analyses were performed in house employing a PerkinElmer CHN Analyzer. In some cases, elemental analyses were reproducibly low on carbon content, presumably due to the formation of metal carbides during combustion. IR spectra were collected on a PerkinElmer Spectrum One FTIR instrument. Compound [Ru(p-cymene)Cl2]2, nBuLi solution, BF3(OEt)2, PPh3, and CO were purchased from Sigma–Aldrich or Strem chemicals and used without further purification. Complex Ag(Et2O)[B(C6F5)4][13] and IDipp[14] were synthesized according to literature procedures.
Synthesis of IDipp BF3 (1) IDipp (1.045 g, 2.69 mmol) was dissolved in diethyl ether (10 mL), and BF3(Et2O) (0.362 mL, 2.93 mmol) was added to the solution in a dropwise fashion with stirring. A white crystalline precipitate formed immediately, and n-pentane (10 mL) was added to the mixture. After stirring for 10 min, the precipitate was collected by filtration, washed with pentane (2 10 mL), and dried under vacuum (1.165 g, 95 %). 1H NMR (400 MHz, [D8]THF): d = 7.52 (s, 2 H, NCH 2), 7.44 (t, 3JH H = 8 Hz, 2 H, p-Dipp), 7.29 (d, 3JH H = 8 Hz, 4 H, mDipp), 2.57 (septet, 3JH H = 7 Hz, 4 H, CH(CH3)2), 1.27 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.17 ppm (d, 3JH H = 7 Hz, 6 H, CH(CH3)2); 13C NMR (125 MHz, [D8]THF): 146.28 (ipso-iPr), 134.88 (ipso), 130.76 (p-Dipp), 124.93 (NCH), 124.27(m-Dipp), 29.70 (CH(CH3)), 25.10 (CH(CH3)), 23.17 ppm (CH(CH3)); 19F NMR (376.5 MHz, [D8]THF): d = 140.01 ppm (1:1:1:1 q, 1JB F = 33 Hz, BF3); 11B NMR (128.4 MHz, [D8]THF): d = 0.89 ppm (q, 1JB F = 33 Hz, BF3); elemental analysis calcd for (456.39): C, 71.05; H, 7.95; N, 6.14; found: C, 71.35; H, 7.93; N, 6.14.
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Communication Synthesis of (THF)xLi(IDippBF3) (2) A solution of 1 (1.163 g, 2.55 mmol) in THF (10 mL) was cooled to 40 8C, and a pre-cooled nBuLi solution (1.70 mL, 1.6 m in hexane, 2.72 mmol) was added. The stirred mixture was allowed to warm to RT over the course of 1 h to give a white precipitate. n-Pentane (10 mL) was then added, the mixture was shaken, and the supernatant decanted. The white solid obtained was washed with n-pentane (4 10 mL) and dried under vacuum (1.540 g, 97 %). The product was stored in a 40 8C freezer under N2. 1H NMR (400 MHz, [D8]THF): d = 7.27 (t, 3JH H = 8 Hz, 1 H, p-Dipp), 7.20 (dd, 3JH H = 7 Hz, 8 Hz, 1 H, p-Dipp), 7.15 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 7.11 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 6.27 (s, 1 H, NCH), 2.79 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 2.70 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 1.22 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.21 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.13 (d, 3 JH H = 7 Hz, 6 H, CH(CH3)2), 1.10 ppm (d, 3JH H = 7 Hz, 6 H, CH(CH3)2); 13 C NMR (125 MHz, [D8]THF): d = 146.42 (ipso Ar-iPr), 146.11 (ipso Ar-iPr), 142.30 (ipso), 137.16 (ipso), 129.80 (NCH), 128.57 (p-Dipp), 127.43 (p-Dipp), 123.14 (m-Dipp), 122.80 (m-Dipp), 28.90 (CH(CH3)), 28.81 (CH(CH3)), 25.46 (CH(CH3)), 24.89 (CH(CH3)), 23.48 (CH(CH3)), 23.18 ppm (CH(CH3)); 19F NMR (376.5 MHz, [D8]THF): d = 141.29 ppm (1:1:1:1 br q, 1JB F = 36 Hz, BF3); 11B NMR (128.4 MHz, [D8]THF): d = 2.26 ppm (q, 1JB F = 38 Hz, BF3); 7Li NMR (155.5 MHz, [D8]THF): d = 1.25 ppm (s); elemental analyses was not possible due to the extreme air sensitivity of 2.
Synthesis of Li[Ag(IDippBF3)2] (3) To a suspension of 2 (1.180 g, 1.90 mmol) in THF (10 mL) was added anhydrous AgCl (0.245 g, 1.72 mmol), and the mixture was stirred for 16 h in the dark. The resulting suspension was filtered through Celite pad, and the solvent was removed in vacuo. Toluene (10 mL) was added and stirred for 10 min, the mixture was then filtered from LiCl, and concentrated under vacuum. The resulting colorless oil was triturated in n-pentane, giving a white solid, which was dried in vacuo (0.806 g, 83 %). Crystals of [Ag(IDippBF3)2][Li(THF)3] were grown from slow diffusion of n-pentane into a THF solution. 1H NMR (400 MHz, [D8]THF): d = 7.30 (t, 3JH H = 8 Hz, 2 H, p-Dipp), 7.17 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 7.02 (d, 3JH H = 8 Hz, 2 H, m-Dipp), 6.33 (d, J = 2 Hz, 1 H, NCH), 2.60 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 2.56 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 1.18 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.13 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.10 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.00 ppm (d, 3JH H = 7 Hz, 6 H, CH(CH3)2); 13C NMR (125 MHz, [D8]THF): d = 161.54 (pseudo dd, 1 JC 109Ag = 184 Hz, 1JC 107Ag = 159 Hz, NCAg), 146.39 (ipso Ar-iPr), 145.92 (ipso Ar-iPr), 140.59 (ipso), 136.16 (ipso), 129.39 (pseudo d, 2 JC Ag = 15 Hz, NCH), 129.25 (p-Dipp), 128.41 (p-Dipp), 123.46 (mDipp), 123.13 (m-Dipp), 28.99 (CH(CH3)), 28.94 (CH(CH3)), 25.45 (CH(CH3)), 25.27 (CH(CH3)), 23.33 (CH(CH3)), 23.21 ppm (CH(CH3)); 19 F NMR (376.5 MHz, [D8]THF): d = 139.5– 139.9 ppm (br m, BF3); 11 B NMR (128.4 MHz, [D8]THF): d = 1.0– 1.6 ppm (br m, BF3); 7Li NMR (155.5 MHz, [D8]THF): d = 0.96 ppm (s); elemental analysis calcd for C54H70AgB2F6LiN4 (1025.58): C, 63.24; H, 6.88; N 5.46; found: C, 63.48; H, 6.80; N, 5.28.
Synthesis of [Ru(p-cymene)(IDippBF3)Cl]2 (4) To a solution of 3 (145 mg, 0.141 mmol) in THF was added [Ru(pcymene)Cl2]2 (75 mg, 0.122 mmol), and the mixture was stirred for 16 h at 25 8C, resulting a dark red solution and the precipitation of AgCl. The solution was filtered and concentrated under vacuum. The red oil obtained was dissolved in toluene (10 mL) and stirred for 2 h, affording a red precipitate. CH2Cl2 (8 mL) was added, and the resultant suspension was stirred for 2 h and then filtered from Chem. Eur. J. 2014, 20, 6597 – 6602
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LiCl. The mixture was concentrated in vacuo, and Et2O (10 mL) added to cause the precipitation of red microcrystals. The solid was washed with Et2O (3 5 mL) and dried. To remove final traces of LiCl, the solid was dissolved in dichloromethane, filtered, the product precipitated with n-pentane, and dried under high vacuum (124 mg, 63 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of n-pentane in to a CH2Cl2 solution. 1H NMR (600 MHz, [D8]THF): d = 7.45 (t, 3JH H = 7 Hz, 1 H, pDipp), 7.34 (d, 3JH H = 7 Hz, 2 H, m-Dipp), 7.28 (t, 3JH H = 7 Hz, 1 H, pDipp), 7.13 (d, 3JH H = 7 Hz, 2 H, m-Dipp), 6.85 (br s, 1 H, NCH), 5.50 (br, 2 H, p-cymeneAr), 4.60 (br, 2 H, p-cymeneAr), 2.90 (br, 2 H, CH(CH3)2), 2.71–2.53 (br m, 3 H, CH(CH3)2), 1.66 (s, 3 H, p-cymeneMe), 1.41 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.27 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.21 (d, 3JH H = 7 Hz, 6 H, CH(CH3)2), 1.09 (d, 3JH H = 6.8 Hz, 6 H, CH(CH3)2), 0.97 ppm (br, 6 H, p-cymene CH(CH3)2); 13C NMR (125 MHz, CD2Cl2): d = 130.05 (p-Dipp), 130.00 (p-Dipp), 124.49 (br, m-Dipp), 124.19, 124.10 (br, m-Dipp), 30.60, 29.23, 28.97, 25.15, 23.25 ppm (br); 19F NMR (376.5 MHz, [D8]THF): d = 136.4– 137.0 ppm (br m, BF3); 11B NMR (128.4 MHz, [D8]THF): d = 0.51 ppm (br q, 1JB F = 35 Hz, BF3); elemental analysis calcd for C74H98B2Cl2F6N4Ru2·2 (CH2Cl2) (1622.12): C, 56.27; H, 6.35, N, 3.45; found: C, 56.12; H, 6.12; N, 3.41.
Synthesis of [Ru(IDippBF3)(p-cymene)(PPh3)Cl] (5) PPh3 (16 mg, 0.061 mmol) was added to a dark red solution of 3 (43 mg, 0.030 mmol) in CH2Cl2 (2 mL), resulting in an immediate color change to orange. The mixture was filtered and concentrated to 1 mL. n-Pentane (15 mL) was added to cause the precipitation of an orange-brown solid, which was collected, washed with npentane (3 5 mL), and dried under high vacuum (45 mg, 76 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of pentane in to a CH2Cl2 solution. 1H NMR (400 MHz, CD2Cl2): d = 7.46–7.26 (m, 18 H, PPh3 + p-Dipp 2 + mDipp 1), 7.18 (dd, 3JH H = 8 Hz, 4JH H = 1.3 Hz, 1 H, m-Dipp), 7.13 (dd, 3JH H = 8 Hz, 4JH H = 2 Hz, 1 H, m-Dipp), 7.10 (dd, 3JH H = 8 Hz, 4 JH H = 2 Hz, 1 H, m-Dipp), 6.03 (s, 1 H, NCH), 5.81 (d, 3JH H = 6 Hz, pcymeneAr), 5.47 (apparent t, 3JH H = 5 Hz), 4.91 (d, 3JH H = 6 Hz, pcymeneAr), 3.92 (d, 3JH H = 6 Hz, p-cymeneAr), 2.65 (septet, 2 H, 3 JH H = 7 Hz, 2 H, CH(CH3)2), 2.53 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.45 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.32 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.12 (s, 3 H, p-cymeneMe), 1.61 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.33 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.31 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.24 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.09 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.08 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 0.96 (d, 3 JH H = 7 Hz, 3 H, CH(CH3)2), 0.92 (apparent t, 3JH H = 6 Hz, 6 H, CH(CH3)2), 0.31 ppm (d, 3JH H = 7 Hz, 3 H, CH(CH3)2); 13C NMR (125 MHz, CD2Cl2): d = 149.20 (ipso Ar-iPr), 146.92 (ipso Ar-iPr), 146.74 (ipso Ar-iPr), 145.56 (ipso Ar-iPr), 138.40 (ipso), 134.26 (br, PPh3), 133.70 (ipso), 130.74 (br, PPh3), 129.63 (p-Dipp), 129.26 (pDipp), 128.92 (d, 3JP C = 8.9 Hz, NCH), 128.82 (d, 3JP C = 1.3 Hz, ipso cymeneiPr), 128.33 (br, PPh3), 124.03 (m-Dipp), 123.49 (m-Dipp), 123.24 (m-Dipp), 122.43 (m-Dipp), 99.71 (d, 3JP C = 5 Hz, p-cymeneAr), 88.99 (d, 3JP C = 20 Hz, p-cymeneAr), 87.77 (p-cymeneAr), 79.95 (d, 3JP C = 13 Hz, p-cymeneAr), 31.11 (CH(CH3)2), 30.00 (CH(CH3)2), 29.54 (CH(CH3)2), 29.98 (CH(CH3)2), 28.83 (CH(CH3)2), 27.45 (CH(CH3)2), 27.25 (CH(CH3)2), 26.93 (CH(CH3)2), 24.73 (CH(CH3)2), 24.69 (CH(CH3)2), 24.24 (CH(CH3)2), 22.61 (CH(CH3)2), 22.36 (CH(CH3)2), 22.05 (CH(CH3)2), 21.26 (CH(CH3)2), 17.45 ppm (p-cymeneMe); 31P NMR (162.0 MHz, CD2Cl2): d = 30.27 ppm (s); 19F NMR (CD2Cl2): 134.04 ppm (1:1:1:1 br q, 1JB F = 33 Hz, BF3); 11B NMR (128.4 MHz, CD2Cl2): d = 0.66 ppm (br q, 1JB F = 37 Hz, BF3); elemental analysis calcd for C55H64BClF3N2Ru (988.41): C, 66.83; H, 6.53, N, 2.83; found: C, 66.12; H, 6.24; N, 2.80.
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Communication Synthesis of [Ru(IDippBF3)(p-cymene)(CO)Cl] (6) A deep red solution of 3 (53 mg, 0.036 mmol) in CH2Cl2 (2 mL) was backfilled with 1 atm of CO, resulting in an immediate color change to orange. The mixture was filtered and concentrated to 1 mL. n-Pentane (15 mL) was added to precipitate the product from solution as a pale yellow solid, which was collected, washed with n-pentane (3 5 mL), and dried under high vacuum (38 mg. 69 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of n-pentane into a CH2Cl2 solution. 1H NMR (400 MHz, CD2Cl2): d = 7.47 (t, 3JH H = 8 Hz, 2 H, p-Dipp), 7.30 (d, 3 JH H = 8 Hz, 1 H, m-Dipp), 7.27 (d, 3JH H = 7 Hz, 1 H, m-Dipp), 7.24 (d, 3JH H = 8 Hz, 1 H, m-Dipp), 7.23 (d, 3JH H = 8 Hz, 1 H, m-Dipp), 6.67 (s, 1 H, NCH), 6.28 (d, 3JH H = 7 Hz, 1 H, p-cymeneAr), 5.88 (apparent t, 3JH H = 8 Hz, 2 H, p-cymeneAr), 5.37 (d, 3JH H = 6 Hz, 1 H, pcymeneAr), 2.84 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.67 (septet, 3 JH H = 7 Hz, 1 H, CH(CH3)2), 2.54–2.42 (m, 2 H, 2 CH(CH3)2), 2.31 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.13 (s, 3 H, p-cymeneMe), 1.33– 1.19 (m, 21 H, 7 CH(CH3)), 1.15 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.12 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.07 ppm (d, 3JH H = 7 Hz, 3 H, CH(CH3)2); 13C NMR (125 MHz, CD2Cl2): d = 193.60 (CO), 148.72 (ipso Ar-iPr), 147.30 (ipso Ar-iPr), 146.16 (ipso-Ar-iPr), 145.09 (ipso Ar-iPr), 137.80 (ipso), 134.35 (ipso), 130.54 (p-Dipp), 130.16 (NCH), 130.15 (p-Dipp), 124.99 (m-Dipp), 123.94 (m-Dipp), 123.74 (m-Dipp), 123.37 (m-Dipp), 121.82 (ipso-cymeneiPr), 116.83 (ipso-cymeneMe), 108.49 (p-cymeneAr), 99.27 (p-cymeneAr), 95.05 (p-cymeneAr), 90.24 (p-cymeneAr), 31.84 (CH(CH3)2), 29.17 (CH(CH3)2), 29.14 (CH(CH3)2), 29.01 (CH(CH3)2), 26.84 (CH(CH3)2), 26.30 (CH(CH3)2), 24.73 (CH(CH3)2), 24.63 (CH(CH3)2), 24.16 (CH(CH3)2), 23.90 (CH(CH3)2), 22.81 (CH(CH3)2), 22.70 (CH(CH3)2), 22.63 (CH(CH3)2), 22.49 (CH(CH3)2), 19.52 ppm (p-cymeneMe); 19F NMR (376.5 MHz, CD2Cl2): d = 137.11 ppm (1:1:1:1 q, 1JB F = 34 Hz, BF3); 11B NMR (128.4 MHz, CD2Cl2): d = 0.75 (q, 1JB F = 37 Hz, BF3); IR (KBr): u˜ = 2008 cm 1 (s, CO); elemental analysis calcd for C39H49BClF3N2ORu (754.14): C, 60.52; H, 6.55; N, 3.71; found: C, 58.82; H, 6.16; N, 3.55.
Ar-iPr), 145.29 (ipso Ar-iPr), 140.54 (p-Dipp), 138.6 (br d, 1JC F = 243 Hz, p-C6F5), 136.7 (br d, 1JC F = 246 Hz, m-C6F5), 132.85, 131.44 (p-Dipp), 127.04, 125.32 (NCH), 125.07 (NCH), 124.49 (m-Dipp), 124.45 (m-Dipp), 124.2 (br, ipso-C6F5), 124.03 (m-Dipp), 114.90 (ipsocymeneiPr), 104.54 (ipso-cymeneMe), 100.54 ([Ru]CC(CH3)2), 89.72 (q, JC F = 3 Hz, cymeneAr), 86.95 (q, JC F = 3 Hz, p-cymeneAr), 81.07 (m, p-cymeneAr), 80.88 (p-cymeneAr), 69.48 ([Ru]CC(CH3)2), 65.31 ([Ru] mDipp), 30.87 (CH(CH3)2), 29.67 (CH(CH3)2), 29.23 (CH(CH3)2), 25.34 (CH(CH3)2), 25.28 (CH(CH3)2), 24.26 (CH(CH3)2), 23.38 (CH(CH3)2), 22.61 (CH(CH3)2), 22.38 (CH(CH3)2), 22.05 (CH(CH3)2), 21.06 (CH(CH3)2), 20.88 ([Ru]CCH3), 17.73 (p-cymeneMe), 2.51 ppm ([Ru]CH3); 19F NMR (376.5 MHz, CD2Cl2): d = 133.09 (m, 8F, o-C6F5), 137.56 (1:1:1:1 q, 1JB F = 34 Hz, 3F, BF3), 163.63 (t, 3JF F = 20.1 Hz, 4F, p-C6F5), 167.48 (t, 3JF F = 17.9 Hz, 8F, m-C6F5); 11B NMR (128.4 MHz, CD2Cl2): d = 0.66 (q, 1JB F = 34 Hz, BF3), 16.69 ppm (s, B(C6F5)4); elemental analysis for C61H49B2F23N2Ru (1369.71): C, 53.49; H, 3.61; N, 2.05; found: C, 53.31; H, 3.58; N, 2.41.
Acknowledgements The authors gratefully acknowledge the financial support of NSERC of Canada. D.W.S. is also grateful for the award of a Canada Research Chair. C.P. would like to gratefully acknowledge a Queen Elizabeth II/Edwin Walter and Margery Warren Scholarship in Science and Technology for funding. Keywords: abnormal NHCs · anionic NHCs · C H activation · carbenes · ruthenium
Synthesis of [Ru(IDippBF3)(p-cymene)][B(C6F5)4] (7) Compound 3 (44 mg, 0.030 mmol) was dissolved in CH2Cl2 (2 mL), and Ag(Et2O)[B(C6F5)4] (52 mg, 0.061 mmol) was added, causing an immediate color change to orange and the precipitation of AgCl. The mixture was stirred for 30 min at 25 8C and was filtered. The solution was concentrated in vacuo to 2 mL, and (Me3Si)2O (2 mL) was layered on top. After 24 h, pale orange needles of 7 separated from solution. The solid was collected by filtration, washed with Et2O, and dried under high vacuum (36 mg, 43 %). Crystals suitable for X-ray diffraction analysis were grown from slow diffusion of Et2O in to a CH2Cl2 solution. 1H NMR (600 MHz, CD2Cl2): d = 7.61 (dd, 3JH H = 6, 9 Hz, 1 H, p-Dipp), 7.59 (t, 3JH H = 8 Hz, 1 H, p-Dipp), 7.47 (d, 3JH H = 9 Hz, 1 H, m-Dipp), 7.39 (dd, 3JH H = 8 Hz, 4JH H = 1 Hz, 1 H, m-Dipp), 7.37 (dd, 3JH H = 8 Hz, 4JH H = 1 Hz, 1 H, m-Dipp), 7.29 (d, 3JH H = 2 Hz, 1 H, NCH), 7.21 (d, 3JH H = 2 Hz, 1 H, NCH), 6.35 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 6.29 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 4.59 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 4.51 (d, 3JH H = 6 Hz, 1 H, p-cymeneAr), 2.67 (septet, 3JH H = 7 Hz, 2 H, CH(CH3)2), 2.66 (d, 3 JH H = 6 Hz, 1 H, [Ru]m-Dipp), 2.53 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.32 (septet, 3JH H = 7 Hz, 1 H, CH(CH3)2), 2.29 (s, 3 H, [Ru]CCH3), 2.05 (s, 3 H, p-cymeneMe), 1.41 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.35 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.32 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.30 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.21 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.20 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 1.17 (d, 3 JH H = 7 Hz, 3 H, CH(CH3)2), 1.04 (d, 3JH H = 7 Hz, 3 H, CH(CH3)2), 3.30 ppm (br s, 3 H, [Ru]CH3); 13C NMR (200 MHz, CD2Cl2): d = 152.14 (ipso Ar-iPr), 148.5 (br d, 1JC F = 241 Hz, o-C6F5), 146.24 (ipso Chem. Eur. J. 2014, 20, 6597 – 6602
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Received: February 7, 2014 Published online on April 29, 2014
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