Article pubs.acs.org/Organometallics
A Cationic N‑Heterocyclic Carbene Containing an Ammonium Moiety Mirko Ruamps, Noel̈ Lugan, and Vincent César* LCC-CNRS, Université de Toulouse, INPT, UPS, 205 route de Narbonne, 31077 Toulouse cedex 4, France S Supporting Information *
ABSTRACT: The dicationic 4-ammonio-substituted imidazolium salt (2·H)(OTf)2 is readily obtained from its 4-amino-substituted imidazolium precursor (1·H)(OTf) by methylation of the exocyclic nitrogen. The cationic NHC (2)(OTf) is generated in situ by deprotonation of (2· H)(OTf)2 with 1 equiv of potassium bis(trimethylsilyl)amide and is trapped with sulfur or selenium. The direct linkage of the cationic ammonium moiety to the carbenic heterocycle through a σ bond has a profound effect on the electronic properties of the NHC by significantly decreasing its σ-donor ability and, to a lesser extent, increasing its π-acidity. These features make the 4-ammonio-substituted NHC 2+ an overall weak electron donor. To evaluate the coordination abilities of carbene 2+, several transition-metal complexes supported by this ligand have been prepared, including the cationic rhodium(I) complexes [RhCl(cod)(2)](OTf) and [RhCl(CO)2(2)](OTf), the cationic palladium(II) complexes [PdCl(η3allyl)(2)](OTf) and [PdCl2(CH3CN)(2)](OTf), the zwitterionic palladium(II) complex [PdCl3(2)], and the tricationic silver(I) complex [Ag(2)2](OTf)3.
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system of the heterocyclic carbenes.9,10 Both carbenes were shown to be poor electron-donor ligands and to display a significant π-acceptor character, as the presence of the cationic charge decreases the energy level of the antibonding π* orbital of the NHCs. In line with our research interests on backbone-functionalized NHCs, we recently reported the synthesis of 4- and 4,5dimethylamino-substituted imidazol-2-ylidenes and their use as highly efficient ligands in organometallic catalysis.11,12 We next envisioned that the formal quaternization of the remote amino group would lead to the cationic NHC C featuring an imidazol2-ylidene linked to a cationic ammonium moiety through a single σ C−N bond. In contrast to the antecedent cationic NHCs A and B, this specific connection was expected to have an effect mainly on the σ-donation of the carbene center.13 In 2016, Weigand and co-workers reported the cationic NHC D being substituted by a phosphonium group, which is a heavier analogue of carbene C.14 This prompted us to disclose our own results on the synthesis, electronic properties, and coordination ability of the NHC C.
INTRODUCTION Over the last two decades, N-heterocyclic carbenes (NHCs) have emerged as a privileged class of ligands in the field of organometallic chemistry,1 with successful applications in the field of homogeneous catalysis,2 or as luminescent materials3 and other functional materials.4 Reasons for these everincreasing developments can be found in the unique stereoelectronic properties of the NHCs: namely, a strong σ-donation and an efficient protection of the metal center thanks to the nitrogen-attached side arms surrounding it. Consequently, a major thread of research in NHC chemistry focuses on the fine tuning of NHCs to expand their application scope.5,6 More specifically, introducing a remote anionic moiety into the NHC backbone leading to anionic NHCs has emerged as an efficient and promising strategy to increase their electron-donating properties and to unveil new reactivities.7 Conversely, it can be anticipated that the grafting of a cationic moiety onto the heterocyclic structure should lead to a diminished electronic donicity of the NHCs. Still, only a few examples of such cationic NHCs have been reported to date (Chart 1).8 These include the cationic NHCs A and B, in which the cationic charge brought by the organometallic fragment [(C5Me5)Ru]+ or by the fused pyridinium moiety, respectively, is part of the π
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RESULTS AND DISCUSSION The dicationic imidazolium (2·H)(OTf)2 was synthesized in 71% isolated yield upon quaternization of the exocyclic nitrogen atom of the corresponding 4-(dimethylamino)imidazolium triflate (1·H)(OTf) precursor with an excess of methyl triflate (5 equiv) at high temperature (130 °C) (Scheme 1). These harsh conditions were found necessary for a complete methylation, revealing the poor nucleophilicity of the exocyclic nitrogen atom in (1·H)(OTf), as already pointed out by Weiss and Huber in a similar system.12
Chart 1. Previously Reported Cationic N-Heterocyclic Carbenes A, B, and D and System of Study C
Received: January 10, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
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well with the 1JCH coupling constant between the carbon and the hydrogen atoms of the precarbenic position in the imidazolium precursors, with higher 1JCH coupling constants corresponding to poorer σ-donor carbenes due to a more pronounced s-orbital character of the C−H bond.18 Here, 1JCH values of 225 and 231 Hz were measured for (1·H)(OTf) and (2·H)(OTf)2, respectively. By comparison of these values with that obtained for the unsubstituted (IMes)·HCl (1JCH = 225 Hz) parent species, it appears that the NMe2 substitution has basically no effect on the σ-donation of the carbene and that the subsequent cationization induces a clear decrease of the σdonor properties of the corresponding NHC. Carbene 2+ thus appears as the weakest σ-donor NHC reported up to date, along with its analogous phosphonium counterpart D recently reported by Weigand and co-workers (1JCH = 233 Hz),14 the introduction of the cationic ammonium moiety having a stronger inductive withdrawing effect than two neutral chlorine atoms in IPrCl2 (1JCH = 229 Hz).17a On the other hand, the 77Se chemical shifts in NHC-Se adducts were shown to be a reliable scale of the π-acidity of the NHCs. Indeed, it reflects the relative weights of the two canonical structures of NHC−Se adducts (i.e., a simple donation from the Lewis basic NHC to the Lewis acidic Se in the case of purely σ-donor NHCs leading to low δ(77Se) chemical shifts), and a more double bonded selenourea, in which a lone pair of the Se can back-donate in the low-lying π* orbital of a more π accepting NHC leading to higher δ(77Se) chemical shifts.17 In the present case, the chemical shift of the selenium atom in [4](OTf) was recorded at δ(77Se) 102.2 ppm, while the corresponding signal in the neutral adduct 1·Se was found at δ(77Se) 42.8 ppm, revealing a more pronounced π-acidic character of the carbene 2+ in comparison to carbene 1. However, this value remains relatively low by comparison with those of other structurally related NHCsδ(77Se) 116 ppm for SIMes, 174 ppm for IPrCl2 or 184 ppm for the amino-amidocarbene SIMesO 17indicating that the methylation of the exocyclic nitrogen has only a small influence on the π-system of the NHC. We next turned our attention toward the possible use of cationic NHC 2+ as supporting ligand in transition-metal complexes and started our investigations with rhodium(I) (Scheme 2). The treatment of the imidazolium ditriflate (2· H)(OTf)2 with 0.5 mol equiv of the in situ generated [Rh(N(SiMe3)2)(cod)]2 precursor at low temperature led to the formation of the ionic rhodium complex [5](OTf) in good yield (81%). The complex was fully characterized by spectroscopic and analytical techniques. The coordinated carbene atom resonates at δ 193.6 ppm (1JRh−C = 53.4 Hz) in the 13C NMR spectrum, which is significantly shifted to low field in comparison to the corresponding carbenic signal in the neutral [RhCl(1)(cod)] complex (δ(N2C) 180.7 ppm, 1JRh−C = 52.6 Hz) previously reported by our group.11a Even if the 13C NMR chemical shifts of carbene carbon atoms in NHCs do not directly correlate to the electronic donicity of the corresponding NHC and are subject to variability depending to other factors,19 this shift can be reasonably attributed to a decrease in the electron density on the carbene center due to the electronwithdrawing effect of the cationic ammonium moiety. Crystals of [5](OTf) suitable for X-ray diffraction analysis were grown by slow diffusion of Et2O into a saturated solution in acetonitrile. As shown in Figure 2, the rhodium(I) center displays a square-planar geometry, in which the ligand 2+ is positioned nearly orthogonally to the coordination plane (dihedral angle {N1a−C1a−Rh1a−Cl1a} = 64.78°), a feature
Scheme 1. Synthesis of the Dicationic Imidazolium Triflate (2·H)(OTf)2 and of Its Thio- and Selenourea Derivatives [3](OTf) and [4](OTf)a
a
Mes = 2,4,6-trimethylphenyl.
The dicationic nature of (2·H)2+ led to a poor solubility of the imidazolium ditriflate (2·H)(OTf)2 in chlorinated solvents (CH2Cl2, CDCl3), in contrast to the high solubility properties of (1·H)(OTf) in these solvents. In addition, the introduction of the second positive charge induced a strong and characteristic deshielding of the protons in positions 2 and 5 of the imidazolium ring in the 1H NMR spectra from δ 8.29 and 6.99 ppm in (1·H)(OTf) to δ 8.99 and 8.44 ppm in (2·H)(OTf)2, respectively. The molecular structure of (2·H)(OTf)2 was established by a single-crystal X-ray diffraction analysis (Figure 1). Of note is the lengthening of the C2−N3 bond induced by
Figure 1. Molecular structure of the cationic part ([2·H]2+) of the dicationic imidazolium triflate [2·H](OTf)2 (ellipsoids drawn at the 30% probability level). Hydrogen atoms, except those on the imidazolium ring, and acetonitrile solvent have been omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−N1 1.3357(19), C1−N2 1.3264(18), N1−C2 1.3972(18), N2−C3 1.3803(18), C2−C3 1.353(2), C2−N3 1.4639(18); N1−C1−N2 109.27(12).
the quaternization of the N3 nitrogen atom from 1.386(3) Å in (1·H)(OTf) to 1.4639(18) Å in (2·H)(OTf)2,15 the length of the latter being in the typical range of a CAr−N+ bond (1.465 Å).16 The cationic free NHC [2](OTf) was quantitatively generated by treatment of (2·H)(OTf)2 with 1 equiv of potassium bis(trimethylsilyl)amide (KHMDS) at 0 °C. To confirm its formulation as an NHC, 2+ was trapped with S8 or metallic Se to give the cationic thio- and selenoureas [3](OTf) and [4](OTf), respectively (Scheme 1). At that point, a first estimation of the σ-donor and π-acceptor properties of the cationic NHC 2+ could be performed following the approach recently described by Ganter and co-workers.17 On one hand, it has been shown that the σ-donation abilities of NHCs correlate B
DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
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gaseous CO through a solution of [5](OTf) in dichloromethane. The overall electronic donation of cationic carbene 2+ could then be assessed by recording the IR spectrum of [6](OTf) in solution in CH2Cl2 and measuring the frequencies of the two stretching vibrations of the coordinated carbonyl ligands, which were found at ν 2002.4 and 2086.4 cm−1. By application of the linear correlation developed by Plenio and Nolan, these frequencies were converted to a TEP value for 2+ (TEP: Tolman electronic parameter) of 2055.7 cm−1, which has to be compared to the TEP value of 2048.6 cm−1 recorded for its neutral analogue 1. This shift in TEP values of 7.1 cm−1 indicates that the incorporation of the cationic charge into the structure of 2+ renders it a very poor overall electron-donating NHC. In particular, its electronic donation appears to be even weaker than that of the 4,5-dichloro-substituted IMesCl2 (TEP = 2054.2 cm−1)21 and is almost of the same magnitude as that of the five-membered amino-amido-carbenes (TEP = 2056− 2057 cm−1).20b However, in contrast to the latter NHCs which are π-acidic, 2+ is a very weak σ-donor NHC ligand. By analogy to anionic NHCs in which the anionic charge on the backbone allowed the easy generation of zwitterionic complexes composed of a cationic metallic center and of an anionic NHC,22 we came to consider that a similar charge effect could occur with this cationic NHC to generate a zwitterionic complex consisting of the cationic NHC 2+ and of a formal anionic metal center through electrostatic stabilization. While such zwitterionic assemblies have been recently isolated using α-cationic phosphines,23 only very few examples including a NHC have been reported and they all show a cationic charge disconnected from the carbenic heterocycle.24 To test this possibility, we targeted the synthesis of the zwitterionic palladate complex [(2+)PdCl3−] (9), as several occurrences of [(NHC)PdX3]− anionic complexes could be found in the literature.24,25 First, the complex [7](OTf) was synthesized in 65% yield by reacting the precursor [PdCl(η3-allyl)]2 with 2 equiv of the in situ generated [2](OTf) carbene. In a following step, the allyl ligand was displaced by treating [7](OTf) with an excess of HCl in CH2Cl2. The orange precipitate which appeared along the reaction course was formulated as the dimeric species [Pd(μ-Cl)Cl(2)]2(OTf)2 according to literature data11a,26 and also because its dissolution in CH3CN yielded the complex [8](OTf) in a 84% isolated yield. The latter
Scheme 2. Preparation of Complexes Containing the Cationic NHC 2+
consistent with the reported solid-state structures of known [RhCl(NHC)(cod)] complexes. The Rh1a−C1a bond length of 2.057(4) Å compares well with those observed in related IMes-based neutral rhodium complexes.11a,20 The COD complex [5](OTf) was then converted to the dicarbonyl complex [6](OTf) in 86% isolated yield by passing a stream of
Figure 2. Molecular structures of the cationic parts 5+, 8+, and 93+ of the corresponding complexes [5](OTf), [8](OTf), and [10](OTf)3. Ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Bonds and distances are reported in Table 1. C
DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Cations 5+, 8+, and 93+ 5+ a Rh1a−C1a C1a−N1a C1a−N2a N1a−C2a N2a−C3a C2a−C3a C3a−N3a N1a−C1a−N2a C1a−Rh1a−Cl1a a
8+ a 2.057(4) 1.372(5) 1.367(4) 1.380(4) 1.405(4) 1.337(5) 1.466(4) 104.2(3) 64.78
Pd1a−C1a C1a−N1a C1a−N2a N1a−C2a N2a−C3a C2a−C3a C3a−N3a N1a−C1a−N2a N1a−C1a−Pd1a−Cl1a
103+ 1.964(3) 1.353(4) 1.362(4) 1.385(4) 1.394(4) 1.353(5) 1.466(4) 105.7(3) 88.95
Ag1−C1 C1−N2 C1−N1 N2−C3 N1−C2 C2−C3 C2−N3 N1−C1−N2 C1−Ag1−C7 N1−C1−C7−N4
2.065(3) 1.351(3) 1.356(3) 1.374(3) 1.394(3) 1.340(4) 1.468(3) 105.0(2) 178.09(12) 134.49
Ag1−C7 C7−N5 C7−N4 N5−C9 N4−C8 C8−C9 C8−N6 N4−C7−N5
2.067(3) 1.346(4) 1.354(4) 1.375(4) 1.401(3) 1.343(4) 1.461(4) 105.5(2)
Complexes [5](OTf) and [8](OTf) crystallize with two independent molecules in the asymmetric unit, displaying similar geometrical data.
hindrance provided by the N-aryl groups, since the NHC D+ is substituted by two very bulky 2,6-diisopropylphenyl groups.
complex was fully characterized by spectroscopic and analytical techniques. The 13C{1H} NMR spectrum in particular showed a singlet at δ(N2C) 158.7 ppm, a value typical for the NHC carbon atom in Pd-NHC complexes. The molecular structure of [8](OTf) was then firmly confirmed by a single-crystal X-ray diffraction experiment. The cationic part is depicted in Figure 2, and selected bond distances and angles are given in Table 1. The coordination geometry around the palladium center is square-planar with the acetonitrile being in a trans position relative to the NHC 2+ ligand. Metrical parameters around the palladium center are otherwise totally comparable to those observed in other neutral trans-PdX2(CH3CN)(NHC) complexes. 27 Adding 1 equiv of (PPN)Cl (PPN + = bis(triphenylphosphine)iminium) to a solution of [8](OTf) in CD3CN led to the appearance of a second set of signals in the 1 H NMR spectrum after 5 min and finally to the formation of orange crystals on the walls of the NMR tube after 1 day. We tentatively formulate the new complex as the zwitterionic complex [PdCl3(2)] (9) resulting from the displacement of the acetonitrile ligand by a chloride anion. The isolated compound was extremely difficult to characterize, since it is insoluble in most of the common solvents (CH2Cl2, CHCl3, MeOH, EtOH, CH3CN, THF, ...) except in DMSO-d6, in which NMR analysis could finally be carried out. Significantly, the silent 19F NMR spectrum indicated the absence of a triflate anion in 9. The 1H NMR was here again composed of two sets of signals corresponding to 9 and to the complex [PdCl2(dmso)(2)]Cl (9c), in which the DMSO solvent displaced one chlorido ligand from the palladium coordination sphere. Adding sequentially 2 equiv of (PPN)Cl to the NMR tube showed that complexes 9 and 9′ are in an equilibrium relationship (see the Supporting Information). Eventually, the tricationic silver(I) complex [10](OTf)3 was easily obtained in good yield (73%) by simply reacting the imidazolium salt (2·H)(OTf)2 with 0.6 equiv of Ag2O and was fully characterized by spectroscopic, analytical, and crystallographic techniques (Figure 2 and Table 1). Complex [10](OTf)3 is the second example of a tricationic bis(NHC)Ag complex after the complex [Ag(D)2](OTf)3 reported by Weigand and co-workers in their recent publication.14 Noteworthy, the carbene carbon atoms resonate in the same range in the 13C NMR spectra for both complexes (δ(N2C) 188.0 ppm for [10](OTf)3 vs δ(N2C) 186.5 ppm for [Ag(D)2](OTf)3). However, the Ag−CNHC bonds in 103+ (Ag1−C1 = 2.065(3) Å, Ag1−C7 = 2.067(3) Å) are markedly shorter than those in Ag(D)23+ (Ag−CNHC = 2.130(4) and 2.149(4) Å). This shortening can be ascribed to the lowering of the steric
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CONCLUSION In summary, we have disclosed a new class of cationic NHC based on an imidazol-2-ylidene whose position 4 is substituted by an ammonium moiety. The direct grafting of such a cationic moiety onto the carbenic heterocycle through a σ bond significantly reduces the σ-donation properties of the NHC, while its π acidity is less affected but nevertheless increases slightly. These results are in sharp contrast to Ganter’s previous reports on cationic NHCs, in which the cationic charge is included in the π system of the NHC and illustrate the importance of the design of the NHC structure for a rational tuning of the electronic properties of NHCs. Eventually, this ammonio-substituted NHC nicely complements the work by Weigand and co-workers on phosphonio-derived NHCs and lays the foundation for new directions in NHC design. We are currently exploring alternative pathways for the design of new NHC structures in our laboratory.
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EXPERIMENTAL SECTION
General Information. All manipulations were performed under an inert atmosphere of dry nitrogen by using standard vacuum line and Schlenk tube techniques. Dry and oxygen-free organic solvents (THF, Et2O, CH2Cl2, toluene, pentane) were obtained using a LabSolv (Innovative Technology) solvent purification system. 1,3-Dimesityl-4(dimethylamino)imidazolium triflate ((1·H)OTf) was synthesized as previously described by our group. All other reagent-grade chemicals were purchased from commercial sources and used as received. Chromatographic purification of the compounds was performed on silica gel (SiO2, 63−200 μm) or deactivated aluminum oxide (neutral Al2O3, Brockmann type III (4.7% water), 50−200 μm) flushed with nitrogen just before use. 1H, 31P, and 13C NMR spectra were obtained on Bruker Avance 400, Avance III HD 400, and Avance 500 spectrometers and were referenced relative to the residual signals of the deuterated solvents (1H and 13C) and to 85% H3PO4 (31P, external standard).28 When necessary, additional information on the carbon signal attribution was obtained using 13C{1H,31P}, J-modulated spin− echo (JMOD) 13C{1H}, 1H−13C HMQC, and/or HMBC experiments. Mass spectra (ESI mode) were obtained using a Xevo G2 QTof (Waters) spectrometer and were performed by the mass spectrometry service of the “Institut de chimie de Toulouse”. Elemental analyses were carried out by the elemental analysis service of the LCC (Toulouse, France) using a PerkinElmer 2400 series II analyzer. The bulk purity of all new compounds was established by NMR spectroscopy and elemental analyses. 1,3-Dimesitylimidazolium-4-(trimethylammonium) Ditriflate ((2·H)(OTf)2). Methyl triflate (1.1 mL, 10 mmol, 5 equiv) was added D
DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
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Organometallics to a solution of (1·H)OTf (1.0 g, 2.0 mmol) in chlorobenzene (15 mL). After it was stirred for 24 h at 130 °C, the reaction mixture was cooled at room temperature and a white-brown precipitate was obtained. Chlorobenzene and excess methyl triflate were removed under vacuum. The obtained product was washed three times with a CHCl3/Et2O (1/5) mixture and dried under high vacuum to obtain a white-gray powder (940 mg, 71%). Crystals suitable for an X-ray diffraction experiment were grown by slow liquid diffusion of Et2O in a saturated acetonitrile solution of (2·H)(OTf)2. 1H NMR (400 MHz, CD3CN): δ 8.98 (d, J = 1.8 Hz, 1H, N2CH), 8.44 (d, J = 1.8 Hz, 1H, CHIm), 7.29 (s, 2H, CHMes), 7.19 (s, 2H, CHMes), 3.65 (s, 9H, N(CH3)3), 2.42 (s, 3H, CH3 para), 2.39 (s, 3H, CH3 para), 2.22 (s, 6H, CH3 ortho), 2.18 (s, 6H, CH3 ortho). 13C{1H} NMR (101 MHz, CD3CN): δ 145.1, 143.6 (CAr), 142.1 (N2CH), 136.8, 136.37, 135.5 (CAr), 132.1, 130.9 (CHMes), 128.3 (CAr), 122.8 (CH Im), 12 2.0 (q, JCF = 321 Hz, CF3SO3), 58.3 (N(CH3)3), 21.3, 21.2 (CH3 para), 18.5, 17.9 (CH3 ortho). MS (ESI, positive mode): m/z (%) 512 (7) [M − OTf]+, 362 (7) [M − 2 OTf − H]+, 244 (7) [M − 2 OTf − Mes]+, 181.6 (100) [M − 2 OTf]2+. Anal. Calcd for C26H33F6N3O6S2 (MW = 661.67 g mol−1) + 0.45 CH3CN: C, 47.50, H, 5.09, N, 7.10. found: C, 47.20; H, 4.91; N, 7.33 (despite considerable drying under high vacuum, acetonitrile could not be removed in totality from the sample). 1,3-Dimesityl-2-thio-4-(trimethylammonio)imidazoline Triflate ([3](OTf)). A solution of KHMDS (0.5 M in toluene, 200 μL, 0.10 mmol, 1.0 equiv) was added dropwise to a solution of (2· H)(OTf)2 (66.2 mg, 0.10 mmol) in THF (4 mL) at 0 °C. After 30 min, S8 (5 mg, 0.15 mmol, 1.5 equiv) was added as a solid and the solution was slowly warmed up to room temperature overnight. After evaporation of THF the crude product was dissolved in dichloromethane and washed twice with water to remove KOTf. The organic phase was dried over Na2SO4, filtered, and concentrated to about 1 mL. Precipitation with diethyl ether and filtration led to the pure product as a slightly yellow powder (37 mg, 68%). 1H NMR (400 MHz, CD3CN): δ 7.47 (s, 1H, CHIm), 7.17 (s, 2H, CHMes), 7.08 (s, 2H, CHMes), 3.44 (s, 9H, N(CH3)3), 2.38 (s, 3H, CH3 para), 2.35 (s, 3H, CH3 para), 2.13 (s, 12H, CH3 ortho). 13C{1H} NMR (101 MHz, CD3CN): δ 167.2 (N2CS), 142.5, 141.1, 138.3, 136.7, 133.66, 131.7 (CAr), 131.1, 130.1 (CHMes), 122.1 (q, JCF = 321 Hz, CF3SO3), 115.6 (CH Im), 57.7 (N(CH3)3), 21.3, 21.2 (CH3 para), 18.6, 18.0 (CH3 ortho). MS (ESI, positive mode): m/z (%) 394 (100) [M − OTf]+. HRMS (ESI): m/z calcd for C24H32N3S (M − OTf), 394.2317; found, 394.2312. εr = 1.3 ppm. The purity of compound [3](OTf) was evidenced by its 1H and 13C NMR spectra. 1,3-Dimesityl-2-seleno-4-(trimethylammonio)imidazoline Triflate ([4](OTf)). A solution of KHMDS (0.5 M in toluene, 400 μL, 0.20 mmol, 1.0 equiv) was added dropwise to a solution of (2· H)(OTf)2 (132.3 mg, 0.2 mmol) in THF (4 mL) at 0 °C. After 30 min, selenium (19.8 mg, 0.25 mmol, 1.25 equiv) was added as a solid and the solution was slowly warmed to room temperature overnight. After evaporation of THF the crude product was dissolved in acetonitrile and filtered over Celite to remove the remaining selenium. The solution was evaporated and the product dissolved in dichloromethane and washed twice with water to remove KOTf. The organic phase was dried over Na2SO4, filtered, and concentrated to about 1 mL. Precipitation with diethyl ether and filtration led to the pure product as a brown powder (92.1 mg, 78%). 1H NMR (300 MHz, CD3CN): δ 7.65 (s, 1H, CHIm), 7.18 (s, 2H, CHMes), 7.08 (s, 2H, CHMes), 3.46 (s, 9H, N(CH3)3), 2.39 (s, 3H, CH3 para), 2.35 (s, 3H, CH3 para), 2.12 (s, 12H, CH3 ortho). 13C{1H} NMR (75 MHz, CD3CN): δ 162.7 (N2CSe), 141.6, 140.3, 137.2, 135.5 (CAr), 130.3, 129.1 (CHMes), 116.7 (CH Im), 56.9 (N(CH3)3), 20.3, 20.1 (CH3 para), 17.9, 17.2 (CH3 ortho). 77Se{1H} NMR (95 MHz, acetone-d6): δ 102.2. MS (ESI, positive mode): m/z (%) 442 (100) [M − OTf ]+. HRMS (ESI): m/z calcd for C24H32N380Se, 442.1761; found, 442.1760. εr = 0.2 ppm. [4](OTf) was assessed to be >98% pure by 1H NMR spectroscopy. 1,3-Dimesityl-4-(dimethylamino)imidazolin-2-selenone (1· Se). A solution of KHMDS (0.5 M in toluene, 980 μL, 0.49 mmol, 1.1 equiv) was added dropwise to a solution of (1·H)(OTf) (223 mg, 0.45 mmol) in THF (10 mL) at 0 °C. After 30 min, solid selenium
(excess, ca. 75 mg) was added as a solid and the ice−water bath was removed. After 2 h, the solution was filtered through a plug of Celite and silica gel and rinsed with CH2Cl2 until the solution was colorless. The solvent was evaporated under reduced pressure, and the residue was washed with pentane to give a gray powder (185 mg, 96%). 1H NMR (400 MHz, CDCl3): δ 7.01 (s, 2H, CHMes), 6.99 (s, 2H, CHMes), 6.28 (s, 1H, CHIm), 2.51 (s, 6H, N(CH3)2), 2.34 (s, 3H, CH3 para), 2.33 (s, 3H, CH3 para), 2.16 (s, 6H, CH3 ortho), 2.15 (s, 6H, CH3 ortho). 13 C{1H} NMR (101 MHz, CDCl3): δ 154.4 (NCN), 144.4, 139.3, 139.1, 136.1, 135.5, 134.7, 132.9 (C Ar), 129.6, 129.3 (CHAr), 103.9 (CH Im‑5), 42.9 (N(CH3)2), 21.4, 21.3 (CH3 para), 18.4, 18.2 (CH3 ortho). 77Se{1H} NMR (95 MHz, CDCl3): δ 32.2. 77Se{1H} NMR (95 MHz, acetone-d6): δ 42.8. MS (ESI): m/z (%) 428 (100) [M + H]+. HRMS (ESI): m/z calcd for C23H30N380Se, 428.1605; found, 428.1599. εr = 1.4 ppm. Anal. Calcd for C23H29N3Se (MW = 426.47): C, 64.78; H, 6.85; N, 9.85. Found: C, 64.70; H, 6.50; N, 9.74. Chloro(η4-cycloocta-1,5-diene)(1,3-dimesityl-4(trimethylammonio)imidazol-2-ylidene)rhodium(I) Triflate ([5](OTf)). A solution of KHMDS (0.5 M in toluene, 630 μL, 0.315 mmol, 1.05 equiv) was added dropwise to a solution of [RhCl(1,5cod)]2 (74 mg, 0.15 mmol, 0.50 equiv) in THF (5 mL) at −80 °C. After 30 min, solid (2·H)(OTf)2 (198.5 mg, 0.30 mmol, 1.0 equiv) was added as a solid and the solution was warmed overnight. After 12 h, all volatiles were removed in vacuo and the crude residue was purified by flash chromatography on a short silica pad (SiO2, pure CH2Cl2 then CH2Cl2/MeOH 95/5) to give a yellow powder (185 mg, 81%). Crystals suitable for an X-ray diffraction experiment were obtained by slow liquid diffusion of diethyl ether in a solution of [5](OTf) in acetonitrile. 1H NMR (400 MHz, CDCl3): δ 7.55 (s, 1H, CHIm), 7.16 (s, 1H, CHMes), 7.10 (s, 1H, CHMes), 7.02 (s, 1H, CHMes), 6.99 (s, 1H, CHMes), 4.59−4.45 (m, 2H, CHCOD), 3.56 (s, 9H, N(CH3)3), 3.44 (br, 1H, CHCOD), 3.10 (br, 1H, CHCOD), 2.45 (s, 3H, CH3 ortho), 2.42 (s, 3H, CH3 ortho), 2.36 (s, 6H, CH3 ortho), 2.07 (s, 3H, CH3 para), 2.06 (s, 3H, CH3 para), 1.88−1.65 (m, 4H, CH2 COD), 1.60− 1.39 (m, 4H, CH2 COD). 13C{1H} NMR (101 MHz, CDCl3): δ 193.6 (d, JCRh = 53.4 Hz, N2C), 141.5, 139.7, 138.1, 137.23, 136.8, 136.1, 135.2, 134.6, 132.5 (CAr), 131.3, 130.1, 129.8, 128.7 (CH Mes), 120.4 (q, JCF = 320.2 Hz, CF3SO3−), 120.1 (CH Im), 97.5 (d, JCRh = 7.2 Hz, CHCOD), 69.4 (d, JCRh = 14.0 Hz, CHCOD), 68.0 (d, JCRh = 14.0 Hz, CHCOD), 57.4 (N(CH3)3), 33.1, 32.3, 28.8, 27.9 (CH2 COD), 21.8 (CH3 ortho), 21.3, 21.25 (CH3 para), 20.0, 19.4, 18.5 (CH3 ortho). MS (ESI, positive mode): m/z (%) 608 (100) [M − OTf]+. HRMS (ESI): m/z calcd for C32H44N3Cl103Rh (M − OTf), 608.2279; found, 608.2288. εr = 1.5 ppm, [5](OTf) was isolated as a pure compound, as evidenced by its 1H and 13C NMR spectra. Chlorodicarbonyl(1,3-dimesityl-4-(trimethylammonio)imidazol-2-ylidene)rhodium(I) Triflate ([6](OTf)). CO gas was bubbled into a solution of [5](OTf) (50 mg, 66 μmol) in CH2Cl2 (5 mL) for 10 min, during which time the color changed from bright yellow to very pale yellow. After 30 min, all volatiles were removed under vacuum and the residue was washed with pentane (2 × 5 mL) to yield after drying a pale yellow powder (40.7 mg, 86%). 1H NMR (400 MHz, CDCl3): δ 7.72 (s, 1H, CHIm), 7.10 (s, 2H, CHMes), 6.97 (s, 2H, CHMes), 3.63 (s, 9H, N(CH3)3), 2.41 (s, 3H, CH3 para), 2.33 (s, 3H, CH3 para), 2.21 (s, 6H, CH3 ortho), 2.19 (s, 6H, CH3 ortho). 13C{1H} NMR (101 MHz, CDCl3): δ 185.8 (d, J = 46.4 Hz, N2C), 184.0 (d, J = 55.0 Hz, Rh-CO), 182.3 (d, J = 73.5 Hz, Rh-CO), 142.3, 140.4, 137.2, 136.15, 135.1, 134.2, 131.7 (CAr and CIm), 131.0, 129.7 (CH Mes), 120.4 (q, JCF = 320.2 Hz, CF3SO3), 120.5 (CHIm), 57.4 (N(CH3)3), 21.4, 21.3 (CH3 para), 19.9, 18.6, (CH3 ortho). MS (ESI, positive mode): m/z (%) 569 (24) [M − OTf − CO − CH3CN]+, 556 (100) [M TfO]+. IR (CH2Cl2): νCO 2002.4, 2086.4 cm−1. [6](OTf) was assessed to be >95% pure by 1H NMR spectroscopy, with the main impurity being residual pentane (see the Supporting Information). Chloro(η3-2-propen-1-yl)(1,3-dimesityl-4(trimethylammonio)imidazol-2-ylidene)palladium(II) Triflate ([7](OTf)). A solution of KHMDS (0.5 M in toluene, 630 μL, 0.315 mmol, 1.05 equiv) was added dropwise to a solution of (2·H)(OTf)2 (198.5 mg, 0.30 mmol) in THF (5 mL) at −20 °C. After 30 min, solid [PdCl(allyl)]2 (54.9 mg, 0.15 mmol, 0.5 equiv) was added as a solid E
DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
1.0 equiv) and Ag2O (41.7 mg, 0.18 mmol, 0.6 equiv), and the reaction mixture was stirred overnight in the absence of light. The obtained mixture was then filtered over Celite and evaporated. The product was redissolved in acetone, and diethyl ether was added until saturation. The clear solution was put in the deep freezer at −25 °C and pure product was obtained as colorless crystals (130 mg, 73%). Single crystals suitable for an X-ray diffraction experiment were grown by slow diffusion of diethyl ether in a solution of [10](OTf)3 in CH3CN. 1 H NMR (400 MHz, CD3CN): δ 7.87 (d, JH−Ag = 1.9 Hz, 2H, CHIm), 7.12 (s, 4H, CHMes), 7.06 (s, 4H, CHMes), 3.44 (s, 18H, N(CH3)3), 2.47 (s, 6H, CH3 para), 2.44 (s, 6H, CH3 para), 1.76 (s, 12H, CH3 ortho), 1.71 (s, 12H, CH3 ortho). 13C{1H} NMR (101 MHz, CD3CN): δ 188.0 (d, JC109Ag = 217.2 Hz, N2C), 188.0 (d, JC107Ag = 186.7 Hz, N2C), 137.1 (d, JCAg = 8.5 Hz, CIm‑4), 143.1, 141.7, 136.6, 135.5, 135.0, 132.6 (CAr), 131.6, 130.4 (CHMes), 122.0 (q, JCF = 328.2 Hz, CF3SO3), 120.7 (d, JCAg = 4.6 Hz, CHIm), 57.9 (N(CH3)3), 21.4, 21.3 (CH3 para), 18.3, 17.6 (CH3 ortho). MS (ESI, positive mode): m/z (%) 620 (10) [Ag(IMes(NMe3)3)(OTf)]+, 362 (18) [(IMes(NMe3)3)]+, 277 (100) [M − 3 OTf]3+. HRMS (ESI): m/z calcd for [C48H64AgN6]3+ (M − 3 OTf), 277.1414; found, 277.1415. εr = 0.36 ppm; Bulk purity of >99.5% for compound [10](OTf)3 was evidenced by its 1H and 13C NMR spectra. Details of X-ray Diffraction Experiments for [2·H](OTf)2, [5](OTf), [8](OTf), and [9](OTf)3. Single-crystal X-ray diffraction data collections were carried out on a Bruker D8/APEX II/Incoatec IμS Microsource diffractometer (graphite monochromator, Mo Kα radiation, λ = 0.71073 Å). All calculations were performed on a PCcompatible computer using the WinGX system.29 The structures were solved using the SIR92 program,30 which revealed in each instance the position of most of the non-hydrogen atoms. All of the remaining nonhydrogen atoms were located by the usual combination of full-matrix least-squares refinement and difference electron density syntheses using the SHELX program.31 Atomic scattering factors were taken from the usual tabulations. Anomalous dispersion terms for S, Rh, Pd, and/or Ag were included in Fc. All non-hydrogen atoms were allowed to vibrate anisotropically. The hydrogen atoms were generally set in idealized positions (R3CH, C−H = 0.96 Å; R2CH2, C−H = 0.97 Å; RCH3, C−H = 0.98 Å; C(sp2)−H = 0.93 Å; Uiso 1.2 or 1.5 times greater than the Ueq value of the carbon atom to which the hydrogen atom is attached) and their positions refined as “riding” atoms, except for H31, H32, H35, and H36 in the structure of [5](OTf), whose positions were inferred from a residual electron density map and refined with Uiso arbitrarily set at 0.05 Å2. After the initial structure solution for complex [5](OTf) was completed, it was found that approximatively 15% of the total cell volume was filled with disordered solvent molecules, which, however, could not be modeled in terms of discrete molecules. The disordered solvent contribution was then subtracted from the observed data using the SQUEEZE procedure.32 CCDC 1523976−1523979 contain supplementary crystallographic data for the three structures unveiled in this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
and the cold bath was removed. After 12 h, all volatiles were removed in vacuo and the crude residue was purified by flash chromatography on a short silica pad (SiO2, CH2Cl2 to CH2Cl2/MeOH 95/5) to give a white powder (140 mg, 65%). 1H NMR (400 MHz, CD3CN): δ 7.83 (s, 1H, CHIm), 7.16 (s, 1H, CHMes), 7.14 (s, 1H, CHMes), 7.06 (s, 1H, CHMes), 7.04 (s, 1H, CHMes), 5.06−4.80 (m, 1H, CHallyl), 3.66 (dd, J = 7.5, 2.3 Hz, 1H, CH2 allyl), 3.48 (s, 10H, 9H for N(CH3)3) and 1H for CH2 allyl), 2.58 (d, J = 13.3 Hz, 1H, CH2 allyl), 2.37 (s, 3H, CH3 para), 2.34 (s, 3H, CH3 para), 2.27 (s, 3H, CH3 ortho), 2.24 (s, 6H, CH3 ortho), 2.22 (s, 3H, CH3 ortho), 1.85 (d, J = 12.1 Hz, 1H, CH2 allyl). 13C{1H} NMR (101 MHz, CD3CN): δ 193.1 (N2C), 142.3, 140.9, 137.8, 137.6, 136.5, 136.3, 136.0, 133.4 (CAr), 131.2, 131.1, 130.1 (CHMes), 120.9 (CH Im), 115.8 (CHAllyl), 72.2 (CH2 Allyl), 58.1 (N-CH3), 53.4 (CH2 Allyl), 21.3, 21.2 (CH3 para), 19.8, 19.7, 18.74, 18.61 (CH3 ortho). MS (ESI, positive mode): m/z (%) 546 (100) [M − OTf]+. Anal. Calcd for C28H37ClF3N3O3PdS (MW = 694.55): C, 48.42; H, 5.37; N, 6.05. Found: C, 48.03; H, 5.25; N, 5.98. Dichloro(acetonitrile)(1,3-dimesityl-4-(trimethylammonio)imidazol-2-ylidene)palladium(II) Triflate ([8](OTf)). HCl (4 M in dioxane, 0.5 mL) was added to a solution of 7 (43.0 mg, 0.062 mmol) in CH2Cl2 (2 mL), and the solution was heated at 40 °C overnight. A red phase separated from the reaction mixture. The volatiles were removed in vacuo, and the residue was washed with Et2O (2 × 5 mL) using an ultrasonic bath to yield a finely divided orange powder, which was dissolved in acetonitrile (1 mL). After 5 min, all volatiles were removed to yield an orange powder (38 mg, 84%). Crystals suitable for an X-ray diffraction experiment were obtained by slow liquid diffusion of diethyl ether into a saturated solution of [8](OTf) in acetonitrile. 1 H NMR (400 MHz, CD3CN): δ 7.86 (s, 1H, CHIm), 7.24 (s, 2H, CHMes), 7.13 (s, 2H, CHMes), 3.44 (s, 9H, N(CH3)3), 2.45 (s, 3H, CH3 para), 2.41 (s, 3H, CH3 para), 2.32 (s, 6H, CH3 ortho), 2.31 (s, 6H, CH3 ortho). 13C{1H} NMR (101 MHz, CD3CN): δ 158.7 (N2C), 142.9, 141.5, 138.6, 137.7, 137.3, 135.2, 132.1 (CAr and CN Im), 131.5, 130.3 (CHMes), 122.4 (CH Im), 58.3 (N(CH3)3), 21.3, 21.2 (CH3 para), 19.4 (CH3 ortho). MS (ESI, positive mode): m/z (%) 581 (10) [M − OTf]+, 540 (100) [M − OTf − CH3CN]+. HRMS (ESI): m/z calcd for C26H35N4Cl2104Pd, 577.1279; found, 577.1280. εr = 0.2 ppm. The purity of compound [8](OTf) was evidenced by its 1H and 13C NMR spectra. Trichloro(1,3-dimesityl-4-(trimethylammonio)imidazol-2ylidene)palladate(II) ([9]). (PPN)Cl (26 mg, 0.045 mmol, 1.05 equiv) was added at room temperature to a solution of [8](OTf) (32 mg, 0.044 mmol) in CH3CN (2 mL), and the reaction mixture was stirred overnight. An orange precipitate appeared along the reaction course. The supernatant solution was removed through a filtering cannula, and the solid residue was washed with CH2Cl2 (2 × 3 mL) was dried under vacuum to yield a pale yellow powder (23.1 mg, 92%). Complex 9 was found to be insoluble in water and in organic solvents such as chlorinated solvents, CH3CN, THF, Et2O, MeOH, and EtOH and could be dissolved only in DMSO. Two complexes were observed when recording the NMR spectra of 9 in DMSO-d6, 9 and [PdCl2(dmso)(2)]Cl (9′), in which one chloride ligand has been displaced by DMSO. 1H NMR (300 MHz, DMSO-d6): δ 8.51 (s, 1H, CHIm, 9′), 8.33 (s, 1H, CHIm, 9), 7.23 (s, 2H, CHMes, 9′), 7.18 (s, 2H, CHMes, 9), 7.13 (s, 2H, CHMes, 9′), 7.08 (s, 2H, CHMes, 9), 3.49 (s, 9H, N(CH3)3, 9′), 3.45 (s, 9H, N(CH3)3, 9), 2.37 (s, 6H, CH3 para), 2.32 (s, 6H, CH3 para), 2.32−2.27 (m, 24H, CH3 ortho). 13C{1H} NMR (75.5 MHz, DMSO-d6): δ 163.8 (N2C), 140.6, 140.1, 139.3, 138.9, 137.7, 137.4, 136.8, 136.1, 136.0, 135.1, 131.7, 131.2 (CAr and CN Im), 130.1 (CHMes, 9′), 130.0 (CHMes, 9), 129.0 (CHMes, 9′), 128.8 (CHMes, 9), 121.9 (CHIm, 9′), 121.1 (CHIm, 9), 56.7 (N(CH3)3), 20.8 (CH3 Mes, 9), 20.7 (CH3 Mes, 9′), 20.7 (CH3 Mes, 9), 20.6 (CH3 Mes, 9), 20.4 (CH3 Mes, 9′), 19.1 (CH3 Mes, 9), 18.8 (CH3 Mes, 9′). Anal. Calcd for C24H32Cl3N3Pd + CH3CN + 0.1 CH2Cl2: C, 50.17; H, 5.68; N, 8.97. found: C, 50.04; H, 5.51, N; 9.34 (one cocrystallizing molecule of acetonitrile was present, and residual CH2Cl2 could not be removed totally after washing and drying). Bis(1,3-dimesityl-4-(trimethylammonio)imidazol-2-ylidene)silver(I) Tris(triflate) ([10](OTf)3). Acetonitrile (2 mL, nondistilled) was added to solid imidazolium (2·H)(OTf)2 (198.5 mg, 0.3 mmol,
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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.7b00017.
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NMR spectra (PDF) Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for V.C.:
[email protected]. ORCID
Noël Lugan: 0000-0002-3744-5252 Vincent César: 0000-0002-6203-6434 F
DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support by the CNRS and the Région Midi-Pyrénées for a Ph.D. grant to M.R. REFERENCES
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DOI: 10.1021/acs.organomet.7b00017 Organometallics XXXX, XXX, XXX−XXX
