Amino Group Functionalized N-Heterocyclic 1,2,4 ... - ACS Publications

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Amino Group Functionalized N‑Heterocyclic 1,2,4-Triazole-Derived Carbenes: Structural Diversity of Rhodium(I) Complexes Jan Turek,† Illia Panov,‡ Michal Horácě k,§ Zdeněk Č ernošek,† Zdeňka Padělková,† and Aleš Růzǐ čka*,† †

Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10 Pardubice, Czech Republic ‡ Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10 Pardubice, Czech Republic § J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, 182 23 Prague, Czech Republic S Supporting Information *

ABSTRACT: The synthesis of the amino group functionalized NHC precursor 1-tert-butyl-4-(2-((dimethylamino)methyl)-phenyl)-3-phenyl-4H-1,2,4-triazol-1-ium perchlorate has been developed. The generation and bonding properties of the NHC ligand have been evaluated in reactions toward three Rh(I) complexes[Rh(COD)Cl]2, [Rh(cyclooctene)2Cl]2, and [Rh(ethylene)2Cl]2, respectively. For the first complex, [(NHC)RhCl(COD)], the coordination of the dangling amino group was not observed because of the fully occupied coordination neighborhood of the Rh atom. On the other hand, in the case of [(NHC)RhCl(ethylene)], [(NHC)RhCl(cyclooctene)], [(NHC)Rh(COD)]+[BF4]−, and [(NHC)RhCl(CO)] a strong intramolecular coordination of the amino nitrogen atom was revealed, thus forming the unusual seven-membered diazametallacycle. All of the products of these reactions were characterized in solution by NMR spectroscopy as well as in the solid state by X-ray diffraction analysis.

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alkenes, and alkynes,8 also using the asymmetric protocol.9 Transfer hydrogenation,10 hydrogenation of olefins,11 hydroformylation,12 Suzuki as well as other C−C coupling reactions,13 and addition of organoboronic acids to aldehydes have also been explored.14 On the other hand, only a few examples of catalytically active 1,2,4-triazole-derived Rh(I) complexes have been reported so far.15 In addition, rhodium(I) NHC complexes containing carbonyl ligands have been used for the electron-donating ability/strength evaluation of the particular NHCs using the modified procedure developed for Tolman electronic parameter (TEP) estimation.16 High interest in NHC−rhodium(I) chemistry led to the investigation of nearly all groups of normal5,17 and abnormal18 versions of stabilizing NHC ligands, including bis-carbenes,19 functionalized carbenes,20 and others.21 Within the whole class of NHC complexes the triazole-based complexes are less populated, most likely because of the prediction of lower σ-donating abilities in comparison to imidazole-based, mesoionic, or even CAAC carbenes.22 However, it is noteworthy that one of the earliest commercially available NHCs was the 1,2,4-triazol-5-ylidene introduced by Enders and co-workers,23 and it also has very recently been proven that the well-known and low-cost analytical reagent Nitron, containing a 1,2,4-triazol-5-ylidene backbone, has a tautomeric form which acts like a typical NHC ligand.24

INTRODUCTION The initial successful attempts to prepare N-heterocyclic carbenes (NHC)1,2 and their metal complexes3 attracted considerable attention of many research teams in both academia and industry. Since that time, many different variations of ligands (Chart 1; saturated Arduengo type (A), Chart 1. Most Commonly Used NHC Ligands

imidazole- (B), triazole- (C), and tetrazole-derived (D), mesoionic (E), and CAAC (F) carbenes) have been developed and subsequently used for complexation4 of almost all metalloids and metals of the periodic table. Selected complexes, mainly of the late transition metals, were tested for applications in organic chemistry transformations,5 for the development of new materials6 as well as in medicinal chemistry.6,7 The most studied metal, along with palladium, ruthenium, and gold, is rhodium, which was tested in many catalytic processes in the form of its metal complexes or its immobilized forms in both lower and higher oxidation states. Rhodium(I) complexes containing 1,3-disubstituted imidazolin-2-ylidenederived ligands were found to be efficient catalysts for hydrosilylation of unsaturated compounds, such as carbonyls, © 2013 American Chemical Society

Received: September 13, 2013 Published: November 6, 2013 7234

dx.doi.org/10.1021/om400912s | Organometallics 2013, 32, 7234−7240

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The functionalized type of NHC−metal complexes containing an adjacent coordination group could be introduced for hemilability, carriers of chirality, modification of bite angles, establishment of a specific ligand geometry, or simply finetuning of the electronic or steric properties of the ligand. By these functionalizations of the NHC skeleton new five- or sixmembered oxa-, aza-, phospha-, or sulfametallacycles are usually formed via a relatively weak coordination bond. Among all types of functionalized NHCs, the primary or secondary amine functionalized species have recently become the most promising. This was mainly due to the possible application of such NHC transition-metal complexes in catalysis.25 In this paper we report on the functionalization of a triazolebased NHC by the 2-[(dimethylamino)methyl]phenyl group, in order to use the extra stabilization energy gained by a hemilabile coordination of the dimethylamino group to the metal center and thus the formation of the seven-membered diazametallacycle containing an NHC backbone. The number of publications describing the unusual formation of the sevenmembered diazametallacycle is rather limited, dealing mostly with ruthenium25a,b,26 complexes and with very few other examples for palladium27 and iridium28 and only one rhodium complex.29

Scheme 2. General Procedure of Free Carbene Generation (3) and Subsequent Rh(I) Complex Formation (4a−c, 5)

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in good yield. The presence of a low-field doublet in the 13C NMR spectrum at 186.0 ppm with a characteristic coupling constant 1J(103Rh−13C) of 50 Hz supports the formation of a Rh−CNHC bond. The 1H NMR signals of methylene and methyl protons of the pendant amino group are shifted upfield in comparison to the free carbene, which leads to the assumption that the amino group stays uncoordinated. Structural determination of 4a confirmed the NMR conclusions (Figure 1). The X-ray structure shows the rhodium atom with a pseudo-square-planar arrangement of the ligands and a bond angle Cl−Rh−CNHC of 88.91(6)°. The Rh−CNHC bond length amounts to 2.053(2) Å, which is slightly elongated (approximately 0.04 Å) in comparison to those obtained for other (triazolinylildene)rhodium(I) complexes.31 The coordination sphere of the Rh atom is fully saturated by a chlorine atom and

RESULTS AND DISCUSSION The synthesis of the new NHC precursor 1-tert-butyl-4-(2((dimethylamino)methyl)phenyl)-3-phenyl-4H-1,2,4-triazol-1ium perchlorate (Scheme 1) proceeds in two steps with a Scheme 1. Synthesis of the Carbene Precursor 2

moderate overall yield (Scheme 1). In the first recyclization30 step, 2-phenyl-1,3,4-oxadiazole reacts at elevated temperature with 2-((dimethylamino)methyl)aniline in 1,2-dichlorobenzene (o-DCB) to afford N,N-dimethyl-1-(2-(3-phenyl-4H-1,2,4triazol-4-yl)phenyl)methanamine (1). Triazole 1 is then quarternized with 2-chloro-2-methylpropane under reflux conditions in acetic acid. The required triazolium salt 2 is finally recrystallized in form of a perchlorate as a white powder. The 1H and 13C NMR spectra of 2 showed characteristic peaks at 10.58 (N−CH−N) and 143.3 (N−CH−N), similar to those reported for related N-alkyl(aryl)triazolium salts.9 The formation of 2 was also confirmed by an X-ray diffraction analysis (Figure S1, Supporting Information). All distances and angles in the solid-state structure of 2 lie within the expected range. The direct synthesis of NHC−Rh complexes consists of the generation of the free carbene from the triazolium salt with a strong base (potassium tert-butoxide) and its subsequent in situ complexation with Rh(I) precursors (Scheme 2). Formation of the free carbene 3 can be unambiguously confirmed by the lack of an 1H NMR resonance for the imidazolium proton (N−CH−N) along with the low-fieldshifted resonance attributed to the the carbene carbon at 211.0 ppm in the 13C NMR spectrum. The first Rh complex (4a) of a newly developed NHC was prepared from the widely used [Rh(COD)Cl]2 as a yellow solid

Figure 1. Molecular structure (ORTEP 40% probability level) of 4a. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Rh1−C10 = 2.053(2), Rh1−C23 = 2.102(2), Rh1−C22 = 2.115(2), Rh-Cg(C22, C23) = 1.990(3), Rh1−C26 = 2.189(2), Rh1−C27 = 2.192(2), Rh−Cg(C26,C27) = 2.080(3), Rh1− Cl1 = 2.3919(6), C22−C23 = 1.392(4), C26−C27 = 1.374(4); C10− Rh1−Cl1 = 88.91(6), N4−C10−N2 = 102.55(19). 7235


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positions cis to the carbene atom are occupied by olefin substituents and rather strongly coordinated amino nitrogen atoms (Rh−N = 2.244(3) Å for 4b and 2.2672(18) Å for 4c). The significant carbene−metal bond shortening of about 0.1 Å in comparison to 4a was revealed (Rh−CNHC = 1.942(3) Å for 4b and 1.929(2) Å for 4c). These Rh−CNHC distances are comparable with those of recently reported NHC−Rh monoolefin complexes containing pyridine-type ligands, even though the pyridine donating groups in those complexes are in the position trans to the carbene ligand.32 The most widely used method to gain information about the donor strength of a particular carbene ligand involves comparing the stretching frequencies (or calculated TEP values from the νav(CO) signal) of the carbonyl ligands in their respective metal complexes.16 Therefore, 4a was treated with CO to afford the appropriate metal carbonyl 5 in almost quantitative yield. The 13C NMR spectrum of 5 displays a CNHC resonance at 179.9 ppm (d, 1JRhC = 58 Hz) along with one Rhcoupled CO resonance at 188.8 ppm (1JRhC = 78 Hz). The predicted formation of monocarbonyl complex 5 was also established by an X-ray structure determination (Figure 3).

a chelating COD ligand; therefore, the dangling amino group of the NHC ligand does not interact with the metal atom. Because one of our initial aims was to prepare complexes with extra stabilization via the intramolecular coordination of the amino function, we treated the free carbene with the analogous but less sterically demanding Rh precursors [Rh(ethylene)2Cl]2 and [Rh(cyclooctene)2Cl]2 (Scheme 2). The formation of the desired products 4b,c was again elucidated by the means of 1H and 13C NMR similarly to the case of 4a. 13C NMR resonances of the carbene atom revealed comparable values of the chemical shifts (187.0 ppm for 4b, 187.6 ppm for 4c) as well as the coupling constants (55 Hz for both 4b and 4c). In addition to that, 1H and 13C NMR spectra display very broad resonances for the nonequivalent olefinic protons and carbons, which are most likely caused by the fluxional behavior of the ethylene and cyclooctene units. Because of that, broad signals were observed in the 13C NMR spectra instead of the expected doublets with the characteristic 1 103 J( Rh−13C) coupling constant. The coordination of the dangling amino group is also clearly visible in the 1H NMR spectral pattern of methyl and methylene protons, where methyl protons appear as two singlets and methylene protons appear as an AB spin system. The same trend is visible in the 13 C NMR spectra, where two resonances, which are shifted slightly downfield in comparison to those for 4a, are present for methyl carbons. In both cases, single crystals suitable for X-ray structure determinations were obtained (Figure 2 for 4b and Figure S2

Figure 3. Molecular structure (ORTEP 40% probability level) of 5· C7H8. Hydrogen atoms, one molecule of toluene, and the second independent molecule of 5 are omitted for clarity. Selected interatomic distances (Å) and angles (deg) (data for the second independent molecule are given in brackets): Rh1−C10 = 1.990(3) [1.986(3)], Rh1−C22 = 1.797(3) [1.792(4)], Rh1−Cl1 = 2.3980(9) [2.3946(8)], Rh1−N1 = 2.221(3) [2.198(3)], C22−O1 = 1.149(4) [1.156(5)]; C22−Rh1−C10 = 91.74(14) [89.75(16)], C22−Rh1−Cl1 = 86.67(12) [88.70(12)], N1−Rh1−Cl1 = 91.45(7) [91.37(8)], C10− Rh1−N1 = 90.84(11) [90.24(13)], C10−Rh1−Cl1 = 174.08(10) [178.15(10)], N4−C10−N2 = 102.0(3) [102.0(3)].

Figure 2. Molecular structure (ORTEP 30% probability level) of 4b· THF. Hydrogen atoms and one molecule of THF are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Rh1−C1 = 1.942(3), Rh1−Cl1 = 2.4131(8), Rh1−N4 = 2.244(3), Rh1−C22 = 2.068(3), Rh1−C23 = 2.099(3), Rh-Cg(C22,C23) = 1.962(3), C22− C23 = 1.402(5); C1−Rh1−Cl1 = 171.97(8), C1−Rh1−N4 = 91.74(10), N4−Rh1−Cl1 = 90.60(7), N3−C1−N1 = 101.9(2).

The molecular structure of 5 resembles the previous pseudosquare-planar geometries of 4b,c with intramolecular coordination of the amino nitrogen. The NHC ligand is in a disposition trans to the chloro ligand, whereas the CO ligand is in a disposition trans to the amino-N donor atom. The Rh− CNHC bond distance of 1.990(3) Å is slightly elongated in comparison to those of 4b,c but is still shorter than in the case of 4a. On the other hand, the intramolecular coordination of the amino donor group is similar to that of 4b,c. A comparison with the reported structures33 revealed that the structure of 5 is interestingly much less distorted than all other complexes, adopting almost ideal 90° L−Rh−L′ angles.

(Supporting Information) for 4c). With the presence of monodentate substituents on Rh, the expected intramolecular coordination of the adjacent amino group was observed for both complexes 4b and 4c, thus completing the pseudo-squareplanar coordination neighborhood of the Rh atom. The slight distortions from the square-planar geometry are within the expected range for unsymmetrically substituted four-coordinate d8 species. In contrast to the case for 4a, chlorine atoms are disposed trans to the carbene unit with Cl−Rh−CNHC bond angles of 171.97(8)° (for 4b) and 165.98(6)° (for 4c). The 7236


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were calibrated to signals of THF-d8 (δ(13C) 67.21 or 25.31), C6D6 (δ(13C) 128.06), DMSO-d6 (δ(13C) 39.52), CDCl3 (δ(13C) 77.16), and CD2Cl2 (δ(13C) 53.84). All 13C NMR spectra were measured using standard proton-decoupled experiments, and CH and CH3 vs C and CH2 were differentiated by the help of APT methods.36 Elemental Analysis. The compositional analyses were determined under an inert atmosphere of argon on the automatic analyzer. Analysis could not be performed for compound 2 due to the possible explosive behavior of perchlorates. IR Spectroscopy. IR spectra of samples in KBr pellets were measured in an air-protecting cuvette on a FT IR spectrometer in the range 400−4000 cm−1. KBr pellets were prepared in a glovebox under purified nitrogen. Raman Spectroscopy. Raman spectra were measured on a Raman microscope (grating 600, objective 10×) equipped with a Peltier cooled Synapse CCD 1024. Raman scattering was excited using a Nd:YAG laser with an excitation wavelength of 532.09 nm and a laser power of 50 mW (exposition 10 s, accumulation 5×). X-ray Data Collection and Structure Solution Refinement. The X-ray data (see the Supporting Information, Table S1) for single crystals of all compounds were obtained at 150 K on a Nonius KappaCCD diffractometer with Mo Kα radiation (λ = 0.71073 Å), a graphite monochromator, and the ϕ and χ scan mode. Data reductions were performed with DENZO-SMN.37 The absorption was corrected by integration methods.38 Structures were solved by direct methods (Sir92)39 and refined by full matrix least squares based on F2 (SHELXL97).40 Hydrogen atoms were mostly localized on a difference Fourier map; however, to ensure uniformity of the treatment of the crystals, all hydrogen atoms were recalculated into idealized positions (riding model) and assigned temperature factors Uiso(H) = 1.2[Ueq(pivot atom)] or 1.5Ueq for the methyl moiety with C−H = 0.96, 0.97, and 0.93 Å for methyl, methylene, and hydrogen atoms in aromatic rings or the allyl moiety, respectively. Crystallographic data for structural analysis have been deposited with the Cambridge Crystallographic Data Centre. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EY, U.K. (fax, +44-1223336033; e-mail, [email protected]; www, http://www.ccdc.cam. ac.uk). CCDC deposition numbers: 956524 (for 5), 956525 (for 4b), 956526 (for 4c), 956527 (for 4a), 956528 (for 2). N,N-Dimethyl-1-(2-(3-phenyl-4H-1,2,4-triazol-4-yl)phenyl)methanamine (1). A solution of phenyloxadiazole (7.30 g; 50 mmol), trifluoroacetic acid (7.70 mL; 100 mmol), and 2[(dimethylamino)methyl]aniline (7.50 g; 50 mmol) in 15 mL of oDCB was stirred at 220 °C with a modified Dean−Stark trap. After 24 h of stirring, the reaction mixture was cooled and o-DCB was removed in vacuo, and the remaining residue was dissolved in 150 mL of DCM. The organic layer was washed afterward with 50 mL of 25% aqueous ammonia, water, and brine. After removal of all volatiles, the residue was purified by flash chromatography on silica gel (n-hexane/ethyl acetate 1/1, then ethyl acetate) to give the crude product, which was crystallized from n-hexane/ethyl acetate. Yield: 9.35 g (65%). 1H NMR (400 MHz, DMSO-d6): δ 8.77 (s, 1H, CH), 7.54−7.53 (m, 2H, ArH), 7.47−7.44 (m, 2H, ArH), 7.40−7.31 (m, 5H, ArH), 3.00 (s, 2H, NCH2), 1.91 (s, 6H, N(CH3)2). 13C{1H} NMR (100 MHz, DMSOd6): δ 154.9 (NCN), 152.5 (NCHN), 135.8 (ArC), 134.0 (ArC), 130.8 (ArC), 129.9 (ArC), 129.7 (ArC), 128.6 (ArC), 128.5 (ArC), 128.4 (ArC), 127.7 (ArC), 127.0 (ArC), 58.9 (NCH2), 44.9 (N(CH3)2). Anal. Calcd for C17H18N4: C, 73.35; H, 6.52; N, 20.13. Found: C, 73.41, H, 6.60, N, 20.18. Mp: 104−106 °C. 1-(tert-Butyl)-4-(2-((dimethylamino)methyl)phenyl)-3-phenyl-4H-1,2,4-triazol-1-ium Perchlorate (2). A mixture of 1 (6.95 g; 25 mmol), anhydrous sodium iodide (11.25 g, 75 mmol), and tertbutyl chloride (8.30 mL; 75 mmol) in 20 mL of acetic acid was refluxed for 36 h under an argon atmosphere. After the mixture was cooled to room temperature and all solvent was evaporated, the resulting residue was dissolved in 150 mL of boiling 20% aqueous ethanol. A small amount of sodium sulfite was added to the hot reaction mixture, which was then neutralized to pH 9 by addition of aqueous ammonia. Finally, hot filtration with 2 g of activated charcoal

The single-crystalline material was also studied by means of Raman and IR spectroscopy. Two very close resonances at 1967 and 1957 cm−1 in the IR spectra and at 1964 and 1955 cm−1 in the Raman spectra were found. These two ν(CO) resonances can be assigned to each of the two independent molecules in the crystal lattice of 5. Because the structure of 5 includes only one carbonyl group whose vibration is mainly influenced by the amino group in the trans position, an appropriate TEP value determination, which was developed for bis-carbonyl complexes, was not possible. However, a comparison with previously reported monocarbonyl complexes revealed that the ν(CO) value lies within the expected range.33 Another comparison could be made with the Rh(I) derivative of P,N-substituted indene, which also has the amino group in a disposition trans to the carbonyl group and therefore reveals similar values of Rh−N and Rh−C(CO) separations as well as similar ν(CO) values.34 Finally, the reaction of 4a with AgBF4 was performed, proving once more the flexibility of the ligand. The formation of the proposed complex 6 (Scheme 2) was again determined by 1H and 13C NMR spectroscopy, where both resonances due to NCH2 and NCH3 groups were shifted downfield in comparison to the resonances of the same groups in 4a in both 1H (significantly) and 13C (slightly) NMR spectra. Observed values are even higher than the same types of values found for 4b,c and 5, respectively. These findings together with approximately 3−6 ppm differences of signal values for the carbene carbon (182.8 ppm for 6 and 186 ppm for 4a) in 13C NMR spectra indicate the formation of an ionic structure32c with a strong intramolecular interaction of the pendant arm.

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CONCLUSIONS The new hybrid NHC ligand precursor containing an adjacent amino donor group has been synthesized and characterized. The subsequent complexation with the Rh(I) precursors revealed an expected versatility of the NHC ligand, which can act as a monodentate or bidentate ligand depending on the substitution pattern of the metal. Complexes 4b,c, 5, and 6, containing a strong intramolecular coordination of the pendant amino group, thus revealed the formation of the unusual sevenmembered diazametallacycle. This striking feature of the NHC ligand opens up the possibility of stabilizing various transition or non transition metals in their common as well as less common oxidation states. In addition, the variable bonding mode can play an important role in the interaction of the metal with a substrate in the catalytic cycle.

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EXPERIMENTAL SECTION

General Comments. All reactions leading to free carbene and its Rh(I) complexes were performed under an atmosphere of argon gas using standard Schlenk techniques. A solvent purification system was used to dry solvents, which were afterward degassed and stored under an argon atmosphere. Materials. 2-[(Dimethylamino)methyl]aniline was prepared according to the literature procedure.35 NMR Spectroscopy. Deuterated solvents (THF-d8, C6D6, CDCl3, and CD2Cl2) were distilled under reduced pressure, degassed, and finally stored over a K mirror or molecular sieves under an argon atmosphere. Solutions were obtained by dissolving approximately 40 mg of each compound in approximately 0.7 mL of deuterated solvent. Values of 1H chemical shifts were calibrated to the internal standard tetramethylsilane (δ(1H) 0.00) or to residual signals of THF-d8 (δ(1H) 3.58 or 1.72), C6D6 (δ(1H) 7.16), DMSO-d6 (δ(1H) 2.50), CDCl3 (δ(1H) 7.26), and CD2Cl2 (δ(1H) 5.32). Values of 13C chemical shifts 7237


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(C(CH3)3). Anal. Calcd for C23H30ClN4Rh: C, 55.15; H, 6.04; N, 11.19. Found: C, 55.20, H, 6.08; N, 11.24. Mp 230 °C dec. [(NHC)RhCl(cyclooctene)] (4c). A solution of free carbene 3 (99 mg, 0.29 mmol) in 20 mL of hexane was added to a suspension of [RhCl(cyclooctene)2]2 (106 mg, 0.15 mmol) in 10 mL of hexane. The reaction mixture was stirred overnight at room temperature. The resulting yellow precipitate was collected and dried in vacuo. Yield: 42%. 1H NMR (400 MHz, THF-d8): δ 7.62 (d, 3J = 7.3 Hz, 1H, ArH), 7.54 (t, 3J = 7.3 Hz, 1H, ArH), 7.38−7.34 (m, 2H, ArH), 7.27−7.23 (m, 2H, ArH), 7.14−7.09 (m, 3H, ArH), 4.11−4.07 (m, 1H, CHCOE), 3.40, 3.30 (AB spin system, Δδ = 0.09 ppm = 37 Hz, 2J = 12 Hz, 2H, NCH2), 3.27−3.23 (m, 1H, CHCOE), 2.86 (s, 1H, NCH3), 2.61 (s, 1H, NCH3), 2.19 (s, 9H, C(CH3)3), 1.58−1.23 (m, 12H, CH2 COE). 13 C{1H} NMR (100 MHz, THF-d8): δ 187.6 (d, 1JRh,C = 55 Hz, NCcarbN), 152.5 (NCN), 139.3 (ArC), 134.9 (ArC), 133.8 (ArC), 130.8 (ArC), 130.7 (ArC), 129.4 (ArC), 129.0 (ArC), 128.9 (ArC), 127.5 (ArC), 127.0 (ArC), 66.7 (NCH2), 62.9 (C(CH3)3), 50.5 (NCH3), 50.1 (NCH3), 49.3 (br s, CHCOE), 45.5 (br s, CHCOE), 31.7 (C(CH3)3), 30.0 (CH2 COE), 28.3 (CH2 COE), 27.5 (CH2 COE), 27.3 (CH2 COE), 27.0 (CH2 COE), 26.2 (CH2 COE). Anal. Calcd for C29H40ClN4Rh: C, 59.74; H, 6.92; N, 9.61. Found: C, 59.80, H, 6.97; N, 9.66. Mp 240 °C dec. [(NHC)RhCl(CO)] (5). CO gas was bubbled through a yellow solution of 3a (100 mg; 0.17 mmol) in 20 mL of dry CH2Cl2 at room temperature for 20 min without any significant color change. All volatiles were removed in vacuo, and the resulting product was washed with hexane to afford pure 5 as a yellow-brown powder. Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated toluene solution at −30 °C. Yield: 94%. 1H NMR (400 MHz, CD2Cl2): δ 7.54−7.51 (m, 1H, ArH), 7.48−7.46 (m, 1H, ArH), 7.43− 7.40 (m, 1H, ArH), 7.31 (t, 3J = 7.2 Hz 3H, ArH), 7.12−7.07 (m, 3H, ArH), 3.38, 3.16 (AB spin system, Δδ = 0.21 ppm = 86 Hz, 2J = 12 Hz, 2H, NCH2), 2.64 (s, 1H, NCH3), 2.56 (s, 1H, NCH3), 2.11 (s, 9H, C(CH3)3). 13C{1H} NMR (100 MHz, CD2Cl2): δ 188.8 (d, 1JRh,C = 78 Hz, CO), 179.9 (d, 1JRh,C = 58 Hz, NCcarbN), 152.0 (NCN), 138.0 (ArC), 133.0 (ArC), 132.5 (ArC), 130.9 (ArC), 130.3 (ArC), 129.4 (ArC), 129.0 (ArC), 128.9 (ArC), 127.1 (ArC), 125.0 (ArC), 65.4 (NCH2), 63.2 (C(CH3)3), 50.9 (NCH3), 48.9 (NCH3), 31.7 (C(CH3)3). IR (KBr): 1967 (ν(CO)), 1957 (ν(CO)) cm−1. Raman: 1964 (ν(CO)), 1955 (ν(CO)) cm − 1 . Anal. Calcd for C22H26ClN4ORh: C, 52.76; H, 5.23; N, 11.19. Found: C, 52.91; H, 5.30; N, 11.25. Mp 190 °C dec. [(NHC)Rh(COD)]+BF4− (6). A bright yellow solution of 3a (61 mg; 0.10 mmol) in 20 mL of dry THF was added to a suspension of AgBF4 (20 mg, 0.10 mmol) in 10 mL of THF at room temperature. The reaction mixture was stirred overnight, the solid residues were filtered off afterward, and the filtrate was evaporated to dryness. The crude product was finally washed with hexane to afford pure 6 in form of a yellow powder. Yield: 85%. 1H NMR (400 MHz, THF-d8): one 1H resonance of NCH2 obscured by THF-d8, δ 7.89 (d, 3J = 7.6 Hz, 1H, ArH), 7.67 (t, 3J = 7.6 Hz, 1H, ArH), 7.53−7.49 (m, 2H, ArH), 7.43− 7.41 (m, 1H, ArH), 7.35−7.27 (m, 4H, ArH), 6.00 (br s, 1H, CHCOD), 5.03−4.99 (m, 1H, CHCOD), 4.08−4.06 (m, 1H, CHCOD), 3.49 (part of the AB spin system, 2J = 13.1 Hz, 1H, NCH2), 3.33−3.28 (m, 1H, CHCOD), 3.16 (br s, 2H, CH2 COD), 2.83−2.80 (m, 2H, CH2 COD), 2.73 (s, 1H, NCH3), 2.59 (s, 1H, NCH3), 2.44 (br s, 2H, CH2 COD), 2.12 (s, 9H, C(CH3)3), 1.86 (br s, 2H, CH2 COD). 13C{1H} NMR (100 MHz, THF-d8): δ 182.8 (d, 1JRh,C = 52 Hz, NCcarbN), 153.4 (NCN), 143.5 (ArC), 138.4 (ArC), 134.0 (ArC), 133.9 (ArC), 131.6 (ArC), 130.6 (ArC), 129.9 (ArC), 129.3 (ArC), 127.3 (ArC), 125.8 (ArC), 98.9 (d, 1 JRh,C = 7.9 Hz, CHCOD), 96.5 (d, 1JRh,C = 8.4 Hz, CHCOD), 79.4 (d, 1 JRh,C = 13.7 Hz, CHCOD), 69.5 (d, 1JRh,C = 12.7 Hz, CHCOD), 65.5 (NCH2), 63.5 (C(CH3)3), 53.7 (NCH3), 49.9 (NCH3), 35.2 (CH2 COD), 33.2 (CH2 COD), 31.1 (C(CH3)3), 28.7 (CH2 COD), 28.4 (CH2 COD). Anal. Calcd for C29H38BF4N4Rh: C, 55.08; H, 6.06; N, 8.86. Found: C, 55.17; H, 6.15; N, 8.91. Mp: 230 °C dec.

was performed. A solution of sodium perchlorate (14.5 g; 100 mmol) in 25 mL of water was added to a hot filtrate, and the resulting reaction mixture was vigorously stirred overnight. The white precipitate was collected by filtration and washed with water. The product 2 was further purified by recrystallization in isopropyl alcohol and vacuumdried. Yield: 8.59 g (79%). 1H NMR (400 MHz, DMSO-d6): δ 10.58 (s, 1H, CH), 7.91−7.89 (m, 1H, ArH), 7.68−7.65 (m, 2H, ArH), 7.59−7.55 (m, 1H, ArH), 7.51−7.45 (m, 5H, ArH), 3.16−3.09 (br m, 2H, NCH2), 1.85 (s, 6H, N(CH3)2), 1.76 (s, 9H, C(CH3)3). 13C{1H} NMR (100 MHz, DMSO-d6): δ 153.2 (NCN), 143.3 (NCHN), 135.5 (ArC), 132.1 (ArC), 132.0 (ArC), 131.8 (ArC), 131.7 (ArC), 129.4 (ArC), 129.2 (ArC), 128.8 (ArC), 128.5 (ArC), 123.3 (ArC), 63.4 (NCH2), 60.2 (C(CH3)3), 44.7 (N(CH3)2), 28.2 (C(CH3)3). Mp: 217−219 °C. General Procedure for Free Carbene Generation (3) and for Preparation of [(NHC)RhCl(R)] Complexes 4a−c. Free carbene 3 was generated at room temperature from the reaction of 2 with tBuOK in dry toluene. After 1 h of stirring, the resulting salt was filtered off and the yellow filtrate was evaporated to dryness to afford free carbene 3 as a yellow oil. A Schlenk tube was charged with the starting Rh(I) complex ([Rh(COD)Cl] 2 , [Rh(ethylene) 2 Cl] 2 , or [Rh(cyclooctene)2Cl]2) and a small amount of dry THF (approximately 10 mL). Fresh free carbene 3 in 30 mL of dry THF was added dropwise afterward to a stirred suspension of Rh(I) complex at room temperature (molar ratio 2/Rh(I) complex 1.2/1). The reaction mixture was stirred overnight, the solid residues were filtered off afterward, and the filtrate was evaporated to dryness. The crude products were finally washed with hexane to give pure 4a−c in the form of a yellow to reddish brown powder. Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated toluene or THF solution at −30 °C under an argon atmosphere. (NHC) (3). Yield: 85%. 1H NMR (500 MHz, C6D6): δ 7.56−7.54 (m, 2H, ArH), 7.42 (d, 3J = 7.5 Hz, 1H, ArH), 7.08−7.03 (m, 2H, ArH), 6.96−6.93 (m, 3H, ArH), 6.92−6.89 (m, 1H, ArH), 3.64, 3.08 (AX spin system, Δδ = 0.56 ppm = 280 Hz, 2H, NCH2), 1.87 (s, 6H, N(CH3)2), 1.83 (s, 9H, C(CH3)3). 13C{1H} NMR (125 MHz, C6D6): δ 211.0 (NCcarbN), 152.0 (NCN), 139.9 (ArC), 137.0 (ArC), 129.3 (ArC), 128.9 (ArC), 128.8 (ArC), 128.6 (ArC), 128.4 (ArC), 128.3 (ArC), 127.8 (ArC), 121.5 (ArC), 60.1 (NCH2), 59.2 (C(CH3)3), 45.3 (N(CH3)2), 30.9 (C(CH3)3). [(NHC)RhCl(COD)] (4a). Yield: 68%. 1H NMR (400 MHz, CDCl3): δ 7.64−7.60 (m, 1H, ArH), 7.55−7.52 (m, 2H, ArH), 7.34−7.31 (m, 1H, ArH), 7.27−7.20 (m, 5H, ArH), 4.86 (br s, 2H, CHCOD), 3.38 (br s, 1H, CHCOD), 2.78−2.74 (m, 1H, CHCOD), 2.67, 2.52 (AB spin system Δδ = 0.15 ppm =60 Hz, 2J = 15.2 Hz, 2H, NCH2), 2.34−2.25 (m, 2H, CH2 COD), 2.11−2.10 (m, 10H, N(CH3)2 + CH2 COD), 1.84 (s, 9H, C(CH3)3), 1.59−1.51 (m, 1H, CH2 COD), 1.53 (br s, 1H, CH2 COD). 13C{1H} NMR (100 MHz, CDCl3): δ 186.0 (d, 1JRh,C = 50 Hz, NCcarbN), 152.1 (NCN), 137.1 (ArC), 134.6 (ArC), 131.9 (ArC), 130.3 (ArC), 129.7 (ArC), 128.7 (ArC), 128.5 (ArC), 128.0 (ArC), 127.8 (ArC), 126.6 (ArC), 96.7 (d, 1JRh,C = 7.7 Hz, CHCOD), 94.3 (d, 1 JRh,C = 7.5 Hz, CHCOD), 69.3 (d, 1JRh,C = 14.8 Hz, CHCOD), 68.1 (d, 1 JRh,C = 14.4 Hz, CHCOD), 62.5 (NCH2), 59.1 (C(CH3)3), 45.3 (N(CH3)2), 32.6 (CH2 COD), 31.6 (CH2 COD), 31.2 (C(CH3)3), 28.6 (CH2 COD), 28.4 (CH2 COD). Anal. Calcd for C29H38ClN4Rh: C, 59.95; H, 6.59; N, 9.64. Found: C, 60.01, H, 6.67; N, 9.73. Mp: 200−202 °C. [(NHC)RhCl(ethylene)] (4b). Yield: 65%. 1H NMR (400 MHz, THF-d8): δ 7.62 (d, 3J = 7.4 Hz, 1H, ArH), 7.53 (t d, 3J = 7.4 Hz, 4J = 1.2 Hz, 1H, ArH), 7.35 (t, 3J = 7.6 Hz, 2H, ArH), 7.25 (t, 3J = 6.8 Hz, 2H, ArH), 7.08 (t, 3J = 7.7 Hz, 3H, ArH), 3.41, 3.34 (AB spin system Δδ = 0.06 ppm =26 Hz, 2J = 12 Hz, 2H, NCH2), 2.98 (br s, 1H, CH2 ethylene), 2.84 (s, 1H, NCH3), 2.64 (s, 1H, NCH3), 2.47 (br s, 1H, CH2 ethylene), 2.38 (br s, 1H, CH2 ethylene), 2.19 (s, 9H, C(CH3)3), 1.46 (br s, 1H, CH2 ethylene). 13C{1H} NMR (100 MHz, THF-d8): one 13C resonance of NCH2 obscured by THF-d8, δ 187.0 (d, 1JRh,C = 55 Hz, NCcarbN), 152.3 (NCN), 139.0 (ArC), 134.6 (ArC), 133.5 (ArC), 130.6 (ArC), 129.6 (ArC), 129.3 (ArC), 128.9 (ArC), 128.8 (ArC), 127.1 (ArC), 126.9 (ArC), 62.0 (C(CH3)3), 50.4 (NCH3), 50.2 (NCH3), 45.4 (br s, CH2 ethylene), 36.5 (br s, CH2 ethylene), 31.4 7238


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ASSOCIATED CONTENT

S Supporting Information *

Figures giving the molecular structures of products 2 and 4c and CIF files and tables giving crystallographic data for 4a−c and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.R.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Grant Agency of the Czech Republic for its financial support (project no. P207/12/0223). Eva Koudelková (Department of Physical Chemistry, University of Pardubice) is gratefully acknowledged for her help with the CO experiment.

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REFERENCES

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