Triazine Annelated NHC Featuring Unprecedented Coordination

Article pubs.acs.org/Organometallics

Triazine Annelated NHC Featuring Unprecedented Coordination Versatility Dominik M. Buck and Doris Kunz* Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany S Supporting Information *

ABSTRACT: The new N-heterocyclic carbene ligand 3 is presented, which features both a weak σ-donor and a pronounced π-acceptor character. This electronic feature is realized by a side-on annelation of a bis(imidazo)triazine fragment. The coordination chemistry shows the ability of the species to act as a monodentate as well as a chelating and a bridging ligand. The last coordination mode is realized in the solid state of the complex [Rh(cod)(3)]BF4, in which three fragments are assembled around a BF4− fragment via anion−π interactions with the triazine moieties.

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electronic properties of carbene D by a π−π cyclophane interaction6 and showed that this can alter the outcome of a catalytic reaction.7 Hence, we were curious as to whether a modulation of the overall electron-donating effect could also be achieved by direct annelation of an electron-poor aromatic ring: keeping a low σdonor character and concurrently increasing the π-acceptor character. Therefore, we needed to synthesize NHCs annelated to electron-deficient triazines and investigate the properties of the resulting ligand and its metal complexes.

INTRODUCTION Since the first isolation of a “bottle-able” imidazol-2-ylidene by Arduengo and co-workers,1 N-heterocyclic carbenes (NHCs) have risen from an underestimated curiosity to a mature ligand class. They are a tool for both stabilizing reactive intermediates and providing catalytic activity by their steric and electronic effects on the transition metal.2 Thus, the electronic properties of NHC ligands have been subject to both numerous experimental and theoretical articles. Although the most prominent imidazole-derived carbenes, namely imidazolidinylidenes A and benzimidazolinylidenes B as well as imidazolinylidenes C, have been very well investigated (Figure 1),

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RESULTS AND DISCUSSION Synthesis of NHC Precursor. When synthesizing triimidazotriazine 1 according to a literature protocol by pyrolysis of blue copper(II) imidazolate at >260 °C in a sublimation apparatus,8 we stumbled upon the hitherto unidentified isomeric byproduct 2, which we report in the following to be an isomeric triimidazotriazine and a promising precursor for a side-on triazine annelated imidazolinylidene (Scheme 1). Separation by column chromatography and fractional sublimation provided 1 and the desired byproduct 2 in 24% and 5% yields, respectively. Scheme 1. Two Isomeric Triimidazotriazines 1 and 2 and Regioselective Alkylation of 2 to Imidazolium Salt 3·HBF4 with Meerwein Salt

Figure 1. Trends of the σ-donating and π-accepting character of imidazole-derived N-heterocyclic carbenes A−E.

comparably little work has been devoted to the research of 1,5- or 3,4-annelated NHCs of typed D and E,3 which were first explored by the group of Weiss and later by Lassaletta, Glorius, and ourselves.4 The σ-donor character weakens in the order from A to E, and the π-acceptor character follows the same trend due to the increase of π-electron density in the aromatic system of the ligand. These two opposing effects cause the net overall donating character of the carbene to remain almost constant.5 Fürstner and co-workers moduled the overall © XXXX American Chemical Society

Received: September 21, 2015

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DOI: 10.1021/acs.organomet.5b00805 Organometallics XXXX, XXX, XXX−XXX

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Organometallics While the C3h-symmetric compound 1 shows only two doublets in the 1H NMR spectrum at 7.93 and 7.26 ppm and three signals in the 13C NMR spectrum at 128.2 and 111.6 ppm as well as at 135.6 ppm for the quaternary carbon atom, the symmetry in the byproduct is reduced so that the annelated rings become chemically inequivalent. On the basis of 1D and 2D NMR spectra, we can conclude that one imidazole ring is isomerized in 2, showing a 1,5- instead of a 1,2-annelation. The 1 H NMR spectrum of 2 shows six doublets of equal intensity, of which the signals of the isomerized ring are shifted to lower field (8.48 and 7.44 ppm) in comparison to the signals of the nonisomerized imidazole moieties (8.01, 7.91, 7.24, and 7.23 ppm). In the 13C NMR spectrum the expected nine signals are observed. For the 1,2-annelated imidazole rings the two sets of signals are very similar (128.0 and 113.2 ppm as well as 127.9 and 111.9 ppm for the tertiary carbons and 134.4 and 134.3 ppm for the quaternary carbon). This accounts also for the signals of the tertiary carbon atoms of the 1,5-annelated ring at 127.2 and 110.8 ppm, while the signal for the quaternary carbon atom is shifted upfield to 125.1 ppm. DFT calculations (BP86/def-TZVP) reveal that the 1,5-annelation product 2 is about 19 kJ/mol higher in energy than the C3h-symmetric triazine derivative 1. Formation of the energetically unfavored product 2 can be rationalized by an attack of the imidazole’s C4/5 backbone instead of the more reactive C2 atom. Reaction of 2 with 1.2 equiv of triethyloxonium tetrafluoroborate at −60 °C resulted in exclusive alkylation of the nitrogen atom of the 1,5-annelated imidazole moiety so that 3· HBF4 could be isolated as a white solid in 89% yield. The regioselective alkylation is confirmed by the 1H NMR spectrum. The cationic charge of the imidazolium salt leads to a deshielding of the formamidinium hydrogen signal (H-11), resulting in a downfield shift of almost 2 ppm to a characteristic 10.39 ppm. The signal for H-9 is shifted by 1 ppm to 8.44 ppm in comparison to 2. As expected, the proton signals of the other rings are less influenced by the cationic charge; therefore, they are shifted downfield only by a maximum of 0.2 ppm and lie pairwise close to each other. Assignment of the signals of these chemically inequivalent protons was possible by observing an NOE for the H-9 and H-7 signals. Electronic Properties of Ligand 3. The very large 1JCH coupling constant of 230.4 Hz for the imidazolium proton (H11) in 3·HBF4 was determined from a nondecoupled 13C NMR spectrum. In accordance with Bent’s rule,9 it indicates a very high s-character of the formamidinium carbon hybrid orbital forming this C−H bond and seems to be typical for side-on annelated imidazolium salts. Nonnenmacher has determined a value of 226 Hz for the monopyrido annelated imidazolium salt D·HX,10 and Weiss et al. have reported a value of 232 Hz for the dipyrido annelated imidazolium salt E·HX.4a In contrast, the imidazolium salts of the carbene species A−C show lower coupling constants (e.g.: SIMes·HCl, 206 Hz; BIiPr·HBr, 218 Hz; IMes·HCl, 225 Hz).11 As imidazolium salts can be regarded as carbenes coordinated to a proton, these 1JCH coupling constants correlate inversely with the σ-donor strength of the respective carbenes (the σ-donor strength decreases with an increasing s-character of the carbene σ orbital). Therefore, the triazine annelated carbene 3 is expected to exhibit a rather weak σ-donor character that has strength comparable to that of dipyrido annelated carbene E (Figure 1). To also elucidate the π-acceptor character of 3, we decided to synthesize the respective selenourea 4 (Scheme 2), as the 77Se NMR chemical shift of selenoureas has been established as a

Scheme 2. Synthesis of Selenourea 4 via in Situ Generation of Carbene 3

reliable measure to access the π-accepting properties of NHCs.11,12 In analogy to literature procedures,11,12b we obtained selenourea 4 in 82% yield and determined a 77Se NMR chemical shift of 163 ppm for 4. Interestingly, this value lies in the range of the chemical shifts for selenoureas derived from saturated NHCs A (ca. 110−190 ppm), whereas those of comparable carbenes B (BIiPr·Se, 67 ppm) and C (ca. −22 to 110 ppm) show chemical shifts at higher field as well as the selenourea of carbene type E (−56 ppm) that was identified to exhibit even a π-donor character.13 Thus, we conclude that carbene 3 has both a weak σ-donor and a pronounced πacceptor character, which is very similar to the situation observed in phosphine ligands. However, the steric properties of 3 differ significantly from those of phosphines. So far, we demonstrated that a side-on annelation at the imidazolinylidene moiety in NHCs exerts a strong effect on the π-accepting properties of the NHC, depending on the electron richness of the annelated ring, while the weak σ-donor character remains unaffected. Thus, we realized an imidazolium salt whose corresponding carbene shows both a very weak σ-donor and a pronounced π-acceptor character by annelation with an electron-poor triazine ring. By introduction of a CF3 substituent at one of the nitrogen atoms of benzimidazolinylidenes B, Togni and co-workers have realized a similar electronic motive recently.14 All attempts to synthesize the free carbene 3 by deprotonation of the imidazolium salt 3·HBF4 by strong bases have failed so far. Therefore, we prepared metal complexes directly from the imidazolium salt to see how this new carbene’s properties will influence the bonding to metal atoms. Metal Complexes of Carbene 3. Silver carbene complexes are valuable NHC transfer agents,15 and at the same time it was shown that a metal to NHC ligand back-bonding contributes significantly to the orbital interaction energies in the electronrich coinage-metal complexes.16 Following the approach by Lin, we reacted imidazolium salt 3·HBF4 with silver(I) oxide in acetonitrile and obtained the desired silver carbene complex [Ag(3)2]BF4 (5) as a white precipitate (Scheme 3). However, separation from excess silver(I) oxide proved to be difficult, due Scheme 3. Synthesis of the Bis(NHC) Silver Complex 5

B

DOI: 10.1021/acs.organomet.5b00805 Organometallics XXXX, XXX, XXX−XXX

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leads to eight carbon signals in total: four at 98.7, 97.4, 70.6, and 68.7 ppm (CH), in which the Rh−C coupling is not resolved due to a dynamic behavior, and four broad signals at 33.0 (2×), 29.8, and 28.9 ppm (CH2). Single crystals were grown by slow diffusion of pentane in a solution of 6 in dichloromethane at room temperature. The molecular structure (Figure 2) features a rather short Rh−carbene bond (2.011(2)

to the insolubility of both Ag2O and complex 5 in common organic solvents. Therefore, we reacted imidazolium salt 3· HBF4 with potassium acetate in the presence of 0.5 equiv of AgBF4 in acetonitrile and obtained complex 5 along with KBF4. Complex 5 is hardly soluble in organic solvents except for DMSO. Complex 5 was identified by the 13C NMR resonance of the carbene carbon at 170.6 ppm, a rather low chemical shift for [Ag(NHC)2]+ complexes.17 Due to the absence of free Ag+ ions in solution, an intermolecular exchange of the Ag+ ions is avoided, so that the carbene signal shows the typical coupling pattern of two doublets due to the coupling with the 107Ag and 109 Ag nuclei. The 1JAgC coupling constants of 193 and 222 Hz, respectively, are to the best of our knowledge the largest coupling constants observed for [Ag(NHC)2]+ complexes.17 This confirms again the high s-character of the σ-orbital of carbene 3. Ag(NHC) complexes are known for their weak bonds, and due to the only weak σ-donor character of the NHC ligand, we assume that the complex gains some stability also by the π-acceptor character of the carbene ligand 3. In DMSO the complex 5 degrades within days, but the stability was sufficient to obtain a HR-ESI+ mass spectrum which showed a signal at m/z 559.09801 corresponding to the [Ag(3)2]+ fragment with the expected isotope pattern. Next, we tackled the synthesis of Rh complexes of 3 in a similar fashion. Stoichiometric amounts of [Rh(μ-Cl)(cod)]2, 3·HBF4, and KOAc were suspended in acetonitrile and sonicated for 5 min to obtain a solution. Upon agitation a yellow precipitate composed of [Rh(3)(Cl)(cod)] (6) and KBF4 formed (Scheme 4). Most of the KBF4 was separated by

Figure 2. Molecular structure of complex 6. Atoms are shown with anisotropic atomic displacement parameters at the 50% probability level. Hydrogen atoms are omitted for clarity.

Å) and a comparably acute N−C−N angle (103.20(16)°). Such a small angle is typically found for Rh(Cl)(COD) complexes for pyrido annelated carbenes D and E13 as well as for triazole derived carbenes19 and indicates an acute angle also for the free carbene. By vigorous stirring of a degassed suspension of 6 in THF under an atmosphere of carbon monoxide, we were able to exchange the cod ligand for two CO molecules. The resulting complex [Rh(3)(Cl)(CO)2] (7) was isolated as a reddish solid in 68% yield. Characterization by IR spectroscopy provided the symmetric and asymmetric stretching modes of the coordinated CO ligands (2083 and 2004 cm−1 in CH2Cl2, respectively), which corresponds20 to a Tolman electronic parameter (TEP) for 3 of 2055 cm−1. The TEP value also confirms the electronpoor nature of 3 and underlines its overall electronic similarity to typical trialkylphosphines (TEP values: PCy3, 2056 cm−1; PtBu3, 2056 cm−1).21 In order to see whether carbene ligand 3 could also serve as a chelating ligand, we synthesized its Rh(cod) complex with the weakly coordinating BF4− counterion. Instead of using AgBF4 as a chloride abstractor for complex 6 and facing the separation problem of AgCl formed, we started from imidazolium salt 3· HBF4 and used [Rh(μ-OMe)(cod)]2 as both base and rhodium source (Scheme 4). In contrast to the case for compound 6, the resulting complex 8 is soluble in acetonitrile and only slightly soluble in dichloromethane, and only four signals for the cod ligand are observed in the 1H and 13C NMR spectra (1H, four broad singlets in a 1:1:2:2 ratio at 4.92, 3.83, 2.52, and 2.05 ppm; 13C, 96.6, 77.6, 32.7, and 29.8 ppm), which is indicative of a Cs-symmetric complex. This geometry is realized if the square-planar coordination sphere of Rh(I) is completed by the coordination of N1 from the neighboring imidazo moiety. Except for a splitting of the signals of the endo and exo protons of the cod CH2 groups, no change or line broadening is observed in the 1H NMR spectrum upon cooling to −40 °C. Thus, carbene 3 adopts a chelating coordination mode. This is surprising, as the basicity of this nitrogen atom is very low, as was already obvious from the alkylation results forming 3· HBF4. For the comparable triazine 1, a pKa of 1 was determined for the corresponding acid.22 DFT calculations also suggest a chelating coordination for the cationic complex fragment; however, this can be expected in the gas phase.

Scheme 4. Synthesis of Rhodium cod Complexes of Carbene Ligand 3, Which Can Serve as a Monodentate (in 6), Chelating (in 8) or Bridging Ligand (in 9)

dissolving the residue in a minimal amount of dichloromethane followed by filtration and removal of the solvent. A 31% yield of complex 6 was obtained as a yellow solid that still contained 7% of KBF4. The 13C NMR spectrum of 6 in CD2Cl2 confirms the formation of a rhodium carbene complex with a characteristic doublet at 181.9 ppm. The chemical shift and the 1JRhC coupling constant of 52.2 Hz are typical for NHC complexes of type C, while those of types D (172.3 ppm; 51 Hz)3 and E (162.9 ppm; 52.8 Hz)18 show lower chemical shifts. Due to the asymmetric substitution pattern at the imidazolinylidene moiety the Rh complex is C1 symmetric, so that the cod ligand C

DOI: 10.1021/acs.organomet.5b00805 Organometallics XXXX, XXX, XXX−XXX

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interactions could be realized in the future and could be an interesting feature for catalytic applications.

Upon cooling of a saturated solution of 8 in a mixture of acetonitrile and diethyl ether to −35 °C, single crystals suitable for X-ray diffraction analysis were obtained. The yellow hexagonal prisms lose cocrystallized solvent upon preparation and fracture cleanly perpendicular to the hexagonal axis of the prism.23 The structure was solved in the hexagonal space group P63/ m and shows one moiety of [Rh(3)(cod)]BF4 in the asymmetric unit. The trimer 9 is formed, in which the monomers are assembled in a conical shape around the 3-fold axis (Figure 3). Surprisingly, 3 no longer serves as a chelating

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CONCLUSIONS In summary, we have presented the electronic properties and versatile coordination abilities of a bisimidazotriazine annelated carbene ligand, which is the first example of a triazine annelated N-heterocyclic carbene. We have shown that the 1,5-annelation of the imidazolinylidene is the source of its weak σ-donor character as in pyrido annelated species, while in contrast, the electron poor π-system of the triazine moiety leads to a pronounced π-acceptor ability. Despite the very low basicity of the nitrogen atoms of the two additional imidazo moieties, they can still serve as donor sites if stronger ligands, e.g. chlorido ligands, are absent. In case of tetrafluoroborate as the counterion the carbene ligand takes up a κ-C,N chelating binding mode in solution, while in the solid state one tetrafluoroborate counterion serves as a template via anion−π interactions, leading to a trimeric assembly of the complex, with the carbene ligands in a κ-C,N bridging fashion.

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EXPERIMENTAL SECTION

All reactions were carried out under an inert argon atmosphere in dried and degassed solvents using standard Schlenk techniques, unless otherwise stated. Compounds sensitive to moisture and air were handled in an MBraun glovebox with an argon atmosphere. Solvents were dried using standard procedures. Copper(II) imidazolate8 and [Rh(μ-OMe)(cod)]226 were prepared according to literature procedures. Chemicals purchased from commercial suppliers were used as received without further purification. NMR spectra were recorded using Bruker instruments (AVII+400, DRX-250, and AVII+500). 1H and 13C NMR spectra were referenced to TMS on the basis of the (residual) signal of the deuterated solvent. 77 Se NMR shifts were calibrated toward Ph2Se2 (463 ppm) in CDCl327 as an external standard. Assignments were made on the basis of 2DNMR experiments. If an unambiguous assignment was not possible, relative assignments are given (e.g., C3/7 is next to C2/6, whereas C7/3 is next to C6/2). Elemental analyses were measured using a varioMicro Cube built by Elementar Analysensysteme GmbH. Mass spectra were recorded using different instruments depending on the ionization method (FAB, Finnigan MAT TSQ 70 Triplequadrupole; ESI, Bruker Daltonics exquire 3000plus; HR-ESI, Bruker Daltonics MAXIS 4G ESI/TOF-MS). IR spectra were recorded with a Bruker Vertex70 instrument. All measurements were performed at the University of Tübingen. Synthesis of Triimidazo[1,2-a:1′,2′-c:1″,5″-e][1,3,5]triazine (2). On the basis of a known procedure for the synthesis of triazine 1,8 a sublimation apparatus was charged with thoroughly dried copper(II) imidazolate (10.3 g, 52.3 mmol) and evacuated to a pressure of 6 × 10−3 mbar. With dynamic vacuum applied, this apparatus was immersed in a Wood’s metal bath preheated to 260 °C. The blue copper(II) imidazolate slowly converted to a highly viscous dark mass while a white solid deposited on the cooling finger. After 4 h at 260 °C the temperature was slowly increased to 300 °C within 2 h. After an additional 1 h at 300 °C the heating bath was removed and the apparatus was vented with argon and cooled to room temperature overnight. The white sublimate (2.55 g) was dissolved in refluxing toluene (15 mL per gram) unter ambient conditions. Upon cooling, crystals of imidazole separated from the solution and were removed by filtration. The solvent of the filtrate was removed with a rotary evaporator, and the off-white residue was purified by column chromatography (silica gel/acetone). Mixed fractions of 2 and imidazole were purified by subliming off imidazole in vacuo at 60 °C for 24 h. The product was obtained as a white solid (87 mg, 5%). Analytically pure samples were obtained by resublimation at 100−105 °C and 7 × 10−3 mbar.

Figure 3. Molecular structure of compound 9 (trimer of 8) in the crystalline state. Atoms are shown with anisotropic atomic displacement parameters at the 50% probability level. Hydrogen atoms, two BF4− counterions, and one acetonitrile molecule are omitted for clarity. The view is in the direction of the cone end.

but as a bridging ligand, by coordinating to a second Rh center via N5. The center of the cone is occupied by a BF4− ion situated exactly on the 3-fold axis with the tetrahedral plane of the anion facing the triazine plane. The boron atom is located exactly above the center of the triazine ring and the fluorine above the middle of each triazine C−N bond. The distances between the triazine plane and the fluorine atoms (3.011, 2.994, and 2.996 Å) are all below the sum of the van der Waals radii of N and F and are below the mean distance (3.060 Å) reported for anion−π interactions between 1,3,5-triazine and BF4−.24,25 Thus, the BF4− ion serves as a template for the formation of 9 in the crystalline state. It is one of the few structural characterized BF4−−π interactions without H−F bridges. However, N−H bridges are present at the cone end of the trimeric assembly (N1−H3′ = 2.584 Å, N1−C3′ = 3.238 Å, N1−H3′−C3′ = 116.47°). At the open end of the cone, the BF4− ion is in short contact with 1 equiv of acetonitrile. The second BF4− ion shows hydrogen bridges to imidazo CH bonds at the open side of the cone (F21−H7 = 2.441 and F21−H9 = 2.478 Å). This structure demonstrates that the triazine moiety of the coordinated NHC 3 is still electron poor and thus able to interact with anions in an attractive fashion. The modulation of the electronic properties of NHCs by such anion−π D

DOI: 10.1021/acs.organomet.5b00805 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Rf (silica gel/acetone) = 0.32. 1H NMR (400 MHz, DMSO-d6, δ): 8.48 (d, 4J = 0.8 Hz, 1H, 11-H), 8.01 (d, 3J = 1.6 Hz, 1H, 3/7-H), 7.91 (d, 3J = 1.6 Hz, 1H, 7/3-H), 7.44 (d, 4J = 0.8 Hz, 1H, 9-H), 7.24 (d, 3J = 1.6 Hz, 1H, 2/6-H), 7.23 (d, 3J = 1.6 Hz, 1H, 6/2-H). 13C{1H} NMR (100.6 MHz, DMSO-d6, δ): 134.4 (C12a/4a), 134.3 (C4a/ 12a), 128.0 (C2/6), 127.9 (C6/2), 127.2 (C11), 125.1 (C8a), 113.2 (C3/7), 111.9 (C7/3), 110.8 (C9). Anal. Calcd for C9H6N6: C, 54.54; H, 3.05; N, 42.41. Found: C, 54.77; H, 2.72; N, 42.57. Synthesis of Imidazolium Salt 3·HBF4. In a Schlenk tube 2 (201 mg, 1.01 mmol) was dissolved in dichloromethane (12 mL) at room temperature and cooled to −60 °C. To the resulting suspension was added a precooled solution of triethyloxonium tetrafluoroborate (Et3OBF4) (231 mg, 1.22 mmol, 1.2 equiv) in dichloromethane (3 mL). This mixture was stirred at −60 °C for 3 h and then brought to room temperature within 2.25 h. After it was stirred overnight at room temperature, the reaction mixture was quenched by the addition of 1 mL of ethanol. The white precipitate that formed was separated by filtration and washed with 2 mL of dichloromethane. After drying in vacuo for several hours, imidazolium salt 3·HBF4 was obtained as a white solid (285 mg, 89%). 1 H NMR (400 MHz, DMSO-d6, δ): 10.39 (d, 4J = 1.7 Hz, 1H, 11H), 8.44 (d, 4J = 1.7 Hz, 1H, 9-H), 8.21 (d, 3J = 1.6 Hz, 1H, 3-H), 8.11 (d, 3J = 1.7 Hz, 1H, 7-H), 7.47 (d, 3J = 1.6 Hz, 1H, 2-H), 7.40 (d, 3J = 1.7 Hz, 1H, 6-H), 4.50 (q, 3J = 7.2 Hz, 2H, −CH2CH3), 1.56 (t, 3J = 7.2 Hz, 3H, −CH2CH3). 1H NMR (400 MHz, CD3CN, δ): 9.43 (s, 1H, 11-H), 7.96 (d, 3J = 1.7 Hz, 1H, 3-H), 7.93 (s, 1H, 9-H), 7.77 (d, 3 J = 1.7 Hz, 1H, 7-H), 7.40 (d, 3J = 1.7 Hz, 1H, 2-H), 7.33 (d, 3J = 1.7 Hz, 1H, 6-H), 4.46 (q, 3J = 7.2 Hz, 2H, −CH2CH3), 1.62 (t, 3J = 7.2 Hz, 3H, −CH2CH3). 13C{1H} NMR (100.61 MHz, DMSO-d6, δ): 134.6 (C4a), 132.2 (C12a), 129.2 (C2), 128.9 (C6), 127.0 (C11), 126.3 (C8a), 113.8 (C3), 113.7 (C7), 105.4 (C9), 46.3 (CH2), 15.1 (CH3). 13C NMR (62.90 MHz, DMSO-d6, δ): 134.6 (dd, 3JCH = 13.1, 8.5 Hz, C4a), 132.2 (dd, 3JCH = 13.5, 8.1 Hz, C12a), 129.2 (dd, 1JCH = 196.1 Hz, 2JCH = 9.4 Hz, C2), 128.9 (dd, 1JCH = 196.7 Hz, 2JCH = 9.4 Hz, C6), 127.0 (ddt, 1JCH = 230.4 Hz, 3JCH = 4.7, 4.7 Hz, C11), 126.3 (dd, 2JCH = 8.1 Hz, 3JCH = 8.1 Hz, C8a), 113.8 (dd, 1JCH = 201.1 Hz, 2 JCH = 16.5 Hz, C3), 113.7 (dd, 1JCH = 202.1 Hz, 2JCH = 16.2 Hz, C7), 105.4 (ddt, 1JCH = 210.5 Hz, 3JCH = 4.0, 4.0 Hz, C9), 46.3 (tq, 1JCH = 145.6 Hz, 2JCH = 4.7 Hz, CH2), 15.1 (qt, 1JCH = 128.9 Hz, 2JCH = 3.4 Hz, CH3). MS (FAB+, 3-NBA, m/z): 227.2 [M − BF4]+, 541.2 [2M − BF4]+. Anal. Calcd for C11H11N6BF4: C, 42.07; H, 3.53; N, 26.76. Found: C, 41.53; H, 3.04; N, 26.40. Synthesis of Selenourea 4. On the basis of the literature procedures11,12b imidazolium salt 3·HBF4 (34.4 mg, 109 μmol) was suspended in 3 mL of tetrahydrofuran along with gray selenium (28.2 mg, 357 μmol, 3.3 equiv). After sonication of this mixture for 2 min, it was cooled to −35 °C and a solution of KOtBu (15.0 mg, 134 μmol, 1.22 equiv) in 1 mL of tetrahydrofuran was added. The reaction mixture was held at −35 °C for 30 min and then was brought slowly to room temperature. Stirring was continued overnight before the solvent was removed in vacuo and the residue suspended in 5 mL of dichloromethane. The suspension was filtered through a pad of Celite to remove excess selenium, and the clear yellow filtrate was evaporated in vacuo to yield selenourea 4 as a beige solid (27.8 mg, 83%). 1 H NMR (250 MHz, CDCl3, δ): 7.70 (d, 3J = 1.7 Hz, 1 H, 3/7-H), 7.39 (d, 3J = 1.7 Hz, 1 H, 2/6-H), 7.28 (d, 3J = 1.7 Hz, 1 H, 7/3-H), 7.21 (d, 3J = 1.7 Hz, 1 H, 6/2-H), 7.10 (s, 1 H, 9-H), 4.37 (q, 3J = 7.4 Hz, 2 H, −CH2CH3), 1.49 (t, 3J = 7.4 Hz, 3 H, −CH2CH3). 13C{1H} NMR (100.6 MHz, CDCl3, δ): 151.1 (C11), 134.7 (C12a/4a), 134.6 (C4a/12a), 129.8 (C6/2), 129.1 (C2/6), 123.2 (C8a), 111.5 (C7/3), 111.4 (C3/7), 100.8 (C9), 45.1 (CH2), 13.8 (CH3). 77Se{1H} NMR (47.7 MHz, CDCl3, δ): 163 (C11Se). Synthesis of Silver Complex [Ag(3)2]BF4 (5). A mixture of 3· HBF4 (30.0 mg, 95.5 μmol), AgBF4 (9.3 mg, 48 μmol, 0.5 equiv), and potassium acetate (10.3 mg, 105 μmol, 1.1 equiv) was dissolved in 6 mL of acetonitrile. Upon sonication for 10 min a voluminous colorless precipitate formed. This mixure was stirred for an additional 4 h at room temperature. The solid was separated by filtration, washed three times with diethyl ether (3 mL each), and dried in vacuo. The

colorless solid (30.9 mg) still contained small amounts of KBF4. It is hardly soluble in common organic solvents except for DMSO. 1 H NMR (400 MHz, DMSO-d6, δ): 8.15 (s, 1H, 9-H), 8.00 (d, 3J = 1.2 Hz, 1H, 3-H), 7.91 (d, 3J = 1.6 Hz, 1H, 7-H), 7.30 (d, 3J = 1.2 Hz, 1H, 2-H), 7.27 (d, 3J = 1.6 Hz, 1H, 6-H), 4.58 (q, 3J = 7.1 Hz, 2H, CH2CH3), 1.61 (t, 3J = 7.1 Hz, 3H, −CH2CH3). 13C{1H} NMR (100.6 MHz, DMSO-d6, δ): 170.6 (dd, 1J107AgC = 193 Hz, 1J109AgC = 222 Hz, C11), 135.8 (C12a), 134.6 (C4a), 128.5 (C2), 127.8 (C6), 126.8 (d, 3JAgC = 5.2 Hz*, C8a), 113.5 (C7), 112.9 (C3), 105.1 (d, 3JAgC = 1.4 Hz*, C9), 48.3 (CH2), 16.7 (CH3) (* indicates that the different coupling constants for the silver isotopes 107Ag and 109Ag are not resolved). HRMS (ESI+, m/z): 559.09801 [M − BF4]+; calculated 559.09794. Synthesis of [Rh(cod)(3)Cl] (6). A Schlenk tube was charged with 3·HBF4 (100 mg, 318 μmol), [Rh(μ-Cl)(cod)]2 (78.5 mg, 159 μmol, 0.5 equiv) and potassium acetate (34.4 mg, 350 μmol, 1.1 equiv). This mixture was suspended in 6 mL of acetonitrile and sonicated for 5 min, whereupon a yellow precipitate was formed. The mixture was stirred at room temperature for one additional day before the solid was separated by filtration and washed three times with acetonitrile (14 mL in total). The dried solid was redissolved in 2 mL of dichloromethane and filtered through a pad of Celite. The filter was washed with 1 mL of dichloromethane. Removal of the solvent and subsequent drying in vacuo yielded a yellow powder (50.0 mg), which contains about 7% of KBF4 according to elemental analysis. Yield: 31%. 1 H NMR (400 MHz, CD2Cl2, δ): 7.72 (d, 3J = 1.6 Hz, 1H, 3-H), 7.42 (d, 3J = 1.6 Hz, 1H, 2-H), 7.28 (s, 1H, 9-H), 7.27 (d, 3J = 1.6 Hz, 1H, 7-H), 7.10 (d, 3J = 1.6 Hz, 1H, 6-H), 5.13−4.90 (br m, 2H, CHcod), 4.97 (q, 3J = 7.4 Hz, 2H, −CH2CH3), 3.55−3.32 (br m, 2H, CHcod), 2.73−2.32 (br m, 4H, CH2 cod), 2.09−1.84 (br m, 4H, CH2 cod), 1.65 (t, 3J = 7.4 Hz, 3H, −CH2CH3). 13C{1H} NMR (100.6 MHz, CD2Cl2, δ): 181.9 (d, 1JRhC = 52.2 Hz, C11), 135.9 (C12a), 135.1 (C4a), 129.8 (C6), 129.4 (C2), 127.2 (d, 3JRhC = 0.9 Hz, C8a), 112.4 (C3), 112.3 (C7), 103.4 (C9), 98.7 (br s, CHcod), 97.4 (br s, CHcod), 70.6 (br s, CHcod), 68.7 (br s, CHcod), 49.0 (CH2), 33.0 (br s, 2CH2 cod), 29.8 (br s, CH2 cod), 28.9 (br s, CH2 cod), 16.2 (CH3). HRMS (ESI+, m/z): 437.09540 [M − Cl]+; calculated 437.09555. Anal. Calcd for C19H22N6RhCl + 7.3% KBF4: C, 47.28; H, 4.59; N, 17.41. Found: C, 47.28; H, 4.33; N, 17.47. Crystals suitable for X-ray structure analysis were obtained from slow diffusion of pentane into a solution of the complex in dichloromethane. Synthesis of [Rh(CO)2(3)Cl] (7). The cod-ligated complex 6 (25.7 mg, 54.4 μmol) was suspended in 3.5 mL of THF and degassed. Vigorous stirring for 2 h under an atmosphere of carbon monoxide resulted in the formation of a clear yellow solution. After removal of the solvent in vacuo the remaining solid was washed with pentane (three times, 5 mL each) and subsequently dried in vacuo to yield 7 as a reddish powder (15.6 mg, 68%). 1 H NMR (400 MHz, THF-d8, δ): 7.83 (br s, 1H), 7.79 (s, 1H), 7.57 (d, 3J = 1.1 Hz, 1H), 7.26 (d, 3J = 1.5 Hz, 1H), 7.16 (d, 3J = 1.1 Hz, 1H), 4.78−4.55 (m, 2H, −CH2CH3), 1.59 (t, 3J = 7.3 Hz, 3H, −CH2CH3). 13C{1H} NMR (62.9 MHz, THF-d8, δ): 188.3 (d, 1JRhC = 55.5 Hz, CO), 184.5 (d, 1JRhC = 73.4 Hz, CO), 136.9 (C12a/4a), 136.5 (C4a/12a), 130.1 (C6/2), 129.4 (C2/6), 129.0 (C8a), 113.3 (C3/7), 113.0 (C7/3), 105.2 (C9), 49.7 (CH2), 16.3 (CH3), the signal of carbene atom C11 could not be detected. IR (CH2Cl2): νsym(CO) 2083 cm−1, νasym(CO) 2004 cm−1. IR (KBr): νsym(CO) 2075 cm−1, νasym(CO) 2007 cm−1. Synthesis of [Rh(cod)(3)]BF4 (8). In a Schlenk tube were suspended 3·HBF4 (100 mg, 318 μmol) and [Rh(μ-OMe)(cod)]2 (77.1 mg, 159 μmol, 0.5 equiv) in 10 mL of acetonitrile. This mixture was sonicated for 5 min and vigorously stirred for 23 h at room temperature. The resulting solution was brought to dryness, and the resulting solid was washed three times with 10 mL portions of Et2O and dried again in vacuo. The resulting orange solid was washed three times with a total of 3 mL of dichloromethane. The yellow residue was redissolved in 5 mL of acetonitrile and filtered. After removal of the solvent the obtained yellow solid was dried in vacuo at 60 °C for several hours. Yield: 126 mg (75%). E

DOI: 10.1021/acs.organomet.5b00805 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics H NMR (400 MHz, CD3CN, δ): 7.86 (d, 3J = 1.7 Hz, 1H, 3-H), 7.56 (s, 1H, 9-H), 7.52 (d, 3J = 1.7 Hz, 1H, 7-H), 7.41 (d, 3J = 1.7 Hz, 1 H, 2-H), 7.21 (d, 3J = 1.7 Hz, 1H, 6-H), 4.92 (br s, 2H, CHcod), 4.81 (q, 3J = 7.4 Hz, 2H, −CH2CH3), 3.86 (br s, 2H, CHcod), 2.52 (br s, 4H, CH2 cod), 2.06 (br s, 4H, CH2 cod), 1.61 (t, 3J = 7.4 Hz, 3H, −CH2CH3). 13C{1H} NMR (62.9 MHz, CD3CN, δ): 174.2 (d, 1JRhC = 50.9 Hz, C11), 137.0 (C12a), 136.0 (C4a), 130.1 (C6), 129.5 (C2), 128.6 (C8a), 113.9 (C3), 113.4 (C7), 105.6 (C9), 96.5 (br s, CHcod), 78.2 (br s, CHcod), 49.2 (CH2), 32.6 (br s, CH2 cod), 29.8 (br s, CH2 cod), 16.4 (CH3). 19F{1H} NMR (376.5 MHz, CD3CN, δ): 151.8 (s, BF4). MS (ESI+, m/z): 437.1 [M − BF4]+. Anal. Calcd for C19H22N6RhBF4: C, 43.54; H, 4.23; N, 16.03. Found: C, 42.83; H, 3.62; N, 15.55. Crystals suitable for X-ray structure analysis were obtained by cooling a saturated solution (at room temperature) of the complex in a mixture of acetonitrile and Et2O to −35 °C. The obtained molecular structure reveals trimer 9 in the solid state. 1

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00805. CCDC 1426098 (6) and 1426097 (9) also contain supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. NMR spectra of all new compounds as well as the details and graphics of all calculated structures (PDF) Cartesian coordinates (XYZ) Experimental and crystal data for 6 (CIF) Experimental and crystal data for 9(CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for D.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support of this work by the CarlZeiss-Stiftung (Ph.D. fellowship for D.M.B.) and to the BMBF and MWK-BW (Professorinnenprogramm).

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00805 Organometallics XXXX, XXX, XXX−XXX