Coordination Chemistry of Bidentate Bis(NHC) Ligands with Two

Jul 29, 2014 - Jean-François Longevial , Adam Langlois , Antoine Buisson , Charles H. Devillers , Sébastien Clément , Arie van der Lee , Pierre D. ...
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Coordination Chemistry of Bidentate Bis(NHC) Ligands with Two Different NHC Donors Sabrina Schick, Tania Pape, and F. Ekkehardt Hahn* Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: Reaction of N-aminobenzimidazole 1 or N-aminoimidazole 2 with N,Nbis(dimethyleneamin)azine 3 yielded the biazoles 4-(benzimidazol-1-yl)-4H-[1,2,4]triazole 4 and 4-(imidazol-1-yl)-4H-[1,2,4]triazole 5. Double N,N-alkylation yielded the new bis(NHC) precursors 1-methyl-4-(3-methylbenzimidazol-1-yl)-4H-[1,2,4]triazolium tetraffluoroborate [6a](BF4)2, 1-ethyl-4-(3-ethylbenzimidazol-1-yl)-4H-[1,2,4]triazolium tetrafluoroborate [6b](BF4)2, and 1-methyl-4-(3-methylimidazol-1-yl)-4H-[1,2,4]triazolium tetrafluoroborate [7](BF4)2, each featuring a triazolium moiety in addition to a benzimidazolium or imidazolium group linked together by a N−N bond. The diazolium salts react with Pd(OAc)2 to give the bis(NHC) complexes [8a](BF4)2, [8b](BF4)2, and [9](BF4)2, each bearing an unsymmetrical triazolylidene/ benzimidazolylidene or triazolylidene/imidazolylidene dicarbene ligand coordinated in a chelating fashion to the metal center.

1. INTRODUCTION

ligands C and D only yield dinuclear ones caused by the orientation of the carbene donors. Of particular interest are bis(NHC) ligands with a direct link between the heterocycles as such ligands, based on the short separation of the NHC donors, might be able to coordinate and stabilize dinuclear metal complex fragments featuring an M−M bond. Such ligands of type E featuring two 1,2,4-triazolylidenes were first introduced by Crabtree and Peris et al. in 2007.11 Later, related bis(NHC) ligands of type F with two directly linked 1,2,3triazolin-4-ylidenes were introduced.12 Finally, the direct linkage of two imidazolin-2-ylidenes was achieved with bis(NHC) G.13 The first complex with a heteroatom stabilized di(NHC) ligand H was synthesized by Tschugajeff et al. in 1925.14 Here, two acyclic diaminocarbene donors are directly linked via the nitrogen atoms to each other. The short distance between the two carbene carbon atoms did not prevent the formation of a chelate complex.15 Similar observations were made for the bis(NHC) ligands E−G that normally form chelate complexes in spite of the short linker between the carbene donors.11−13 As an exception, ligand E has been shown to react with [RhI(CO)2(OAc)]2 to yield a dinuclear RhII−RhII complex with one dicarbene ligand bridging the two rhodium atoms.11a The bis(NHC) ligand G reacts with CuI, AgI, or AuI to give dinuclear complexes with a short M−M separation, while the reaction with [Rh(μ-Cl)(cod)]2 yields a RhI chelate complex.13 Only few bis(NHC) ligands with a direct link between the heterocycles are known, and all of the known compounds feature two identical NHC donors.11−13 We became interested in this ligand type in our search for bidentate bis(NHC) ligands. Here, we report the preparation of N,N-connected

Since the isolation of the first stable N-heterocyclic carbene (NHC) in 1991 by Arduengo et al.,1 NHCs have developed into an important type of ligand in organometallic chemistry.2 A large number of NHC ligand precursors as well as various NHCs derived from imidazole,3 benzimidazole,4,5 or triazole6 have been generated over the last two decades. Among these are bidentate ligands with one NHC moiety and an additional heteroatom donor (A).7 In addition, various bis(NHC) ligands linked via the ring-nitrogen atoms (B) with different types of linkers between the NHC donors are known (Figure 1)8 next to benzobiscarbenes (C)9 and dicarbenes derived from trisubstituted triazoles (D).10 While ligands of type B can form both mononuclear chelates and dinuclear complexes,

Figure 1. Selection of bidentate, heteroatom stabilized bis(carbene) ligands. © 2014 American Chemical Society

Received: May 23, 2014 Published: July 29, 2014 4035

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oriented in a coplanar fashion, but twisted along the N3−N4 bond with a dihedral angle C2−N3−N4−C3 of 76.2°. This value falls in the range previously observed for N−N linked bis(triazoles) (92.7°).18 Because of the similarity of the two heterocycles making up 5, the potential for crystallographic disorder exists. Such a disorder was, however, not observed, and the thermal displacement parameters for all atoms are rather similar, confirming the proper assignment of atoms. The azole/triazole compounds 4 and 5 are rapidly double Nalkylated in good yield in acetonitrile by reaction with trialkyloxonium tetrafluoroborate (R = Me, Et) to give the benzimidazolium/triazolium salts ([6a](BF4)2 and [6b](BF4)2) and the imidazolium/triazolium salt [7](BF4)2 (Scheme 1). These salts were fully characterized by NMR spectroscopy, and the spectroscopic data for [6a](BF4)2 are exemplarily discussed here. The 1H NMR spectrum of [6a](BF4)2 shows two singlets at δ = 10.82 ppm (N−NCHN−Me triazolium moiety) and δ = 10.22 ppm (N−NCHN−Me benzimidazolium moiety) for the most acidic protons of the heterocycles. The resonance for the NCHN H3 proton of the triazolium moiety was observed at δ = 9.81 ppm. Two resonances for the carbon atoms of the N− NCHN−Me groups were detected in the 13C{1H} NMR spectrum at δ = 143.9 ppm for the benzimidazolium moiety19 and at δ = 144.4 ppm for the triazolium moiety. The resonance for the carbon atom of the remaining CH group of the triazolium moiety (C3) is obstructed by the resonances for N− NCHN−Me carbon atoms. Composition and connectivity [6a](BF4)2 have been confirmed by an X-ray diffraction analysis with crystals obtained by slow diffusion of hexane into a saturated acetonitrile solution of the compound. The molecular structure of the dication [6a]2+ is depicted in Figure 3. Metric parameters for the

unsymmetrical diazolium salts composed of either a 1,2,4triazolium and an imidazolium or a 1,2,4-triazolium and a benzimidazolium moiety. In addition, the coordination chemistry with PdII of the new dicarbene ligands obtained from the asymmetrical diazolium salts is described.

2. RESULTS AND DISCUSSION The azole/triazole derivatives 4 and 5 have been generated by transamination of N,N-dimethylformamide azine 316 with the primary amine function of the N-aminoazoles17 1 and 2 in equimolar amounts (Scheme 1). The bicyclic compounds 4 and Scheme 1. Synthesis of the Diazolium Salts [6a](BF4)2, [6b](BF4)2, and [7](BF4)2

5 were obtained as colorless solids. Compound 5, for example, was identified by 1H NMR spectroscopy showing the resonances for the NCHN protons at δ = 9.14 ppm (2H, triazole) and δ = 8.22 ppm (1H, imidazole) and by 13C{1H} NMR spectroscopy where the resonances for the two chemically different NCN carbon atoms were observed at δ = 142.7 ppm (triazole) δ = 136.7 ppm (imidazole). In addition to the NMR spectroscopic characterization, the molecular structure of compound 5 was determined. Single crystals of 5 suitable for an X-ray diffraction study were obtained by slow diffusion of diethyl ether into a saturated acetonitrile solution of 5. The molecular structure is depicted in Figure 2, confirming the formation of the bicyclic compound. Bond lengths and angles in 5 fall in the expected range. The N−C−N angles at the potential carbene precursor carbon atoms C1, C2, and C3 are rather similar and fall in the small range of 109.4(2)−110.0(2)°. The two heterocycles are not

Figure 3. Molecular structure of the dication [6]2+ (50% displacement ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): N1−N3 1.379(2), N1−C1 1.345(3), N2−C1 1.312(3), N3−C8 1.341(3), N3−C9 1.377(3), N4−C8 1.297(3); N1−C1−N2 108.7(2), N3−C8−N4 106.1(2), C1−N1− N3 123.5(2), C8−N3−N1 124.5(2).

dication fall in the range previously observed for related 1,2,4triazolium or benzimidazolium salts.18,19 The length of the N− N bond linking the heterocycles does not change significantly due to the double N-methylation (1.386(3) Å in 5; 1.379(2) Å in [6]2+). As expected, the N3−C8−N4 angle in [6]2+ is smaller (106.1(2)°) than the equivalent angle in the nonmethylated compound 5 (N1−C2−N3 110.0(2)°). In accord with the observations for the imidazole/triazole 5, the two heterocycles in [6a]2+ are twisted along the N1−N3 bond with a dihedral angle C8−N3−N1−C1 of 86.1°. Planarization by computational methods of the hypothetical dicarbene ligand resulting from double deprotonation of [6]2+ (dihedral angle C8−N3−N1−C1 0°) reveals that the carbene electron pairs

Figure 2. Molecular structure of the imidazole/triazole 5 (50% displacement ellipsoids; hydrogen atoms have been omitted). Selected bond lengths (Å) and angles (deg): N1−N2 1.408(3), N1−C2 1.311(3), N2−C1 1.309(3), N3−C1 1.371(3), N3−C2 1.363(3), N3− N4 1.386(3), N4−C3 1.369(3), N4−C4 1.383(3), N5−C3 1.321(3), N5−C5 1.389(3), C4−C5 1.346(3); N2−C1−N3 109.4(2), N1−C2− N3 110.0(2), N4−C3−N5 109.8(2). 4036

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observed of the diazolium NHC precursor [6a](BF4)2, thus indicating a successful double deprotonation. The signal for the remaining triazole proton (δ = 10.27 ppm) is significantly downfield shifted compared to the equivalent resonance for the azolium ligand precursor salt (δ = 9.81 ppm). In the 13C{1H} NMR spectrum of [8a](BF4)2, two signals at δ = 153.2 ppm and δ = 147.8 ppm are observed, which were assigned to the carbene carbon atoms (benzimidazolin-2-ylidene and 1,2,4triazolin-5-ylidene). The downfield shift for the resonance of the metalated NCN carbon atom of the benzimidazolin-2ylidene moiety is more pronounced than that for the 1,2,4triazolin-5-ylidene, in accord with previous observations.19 Slow diffusion of diethyl ether into a solution of [8a](BF4)2 in a solvent mixture of acetonitrile and DMSO afforded crystals of [8a′](BF4)2 suitable for an X-ray diffraction study. Cation [8a′]2+ differs form [8a]2+ by substitution of one acetonitrile ligand for a DMSO molecule at the metal center. The molecular structure analysis (Figure 4) shows that the N,N-

would intersect at an angle of only 40°, thus not favoring the formation of chelate complexes. As was seen for [6a](BF4)2 and [6b](BF4)2, the two carbon atoms of the triazolium ring become chemically inequivalent after N-quaternization, and the same observation was made after formation of [7](BF4)2 from 5. The 1H NMR spectrum of [7](BF4)2 shows two singlets at δ = 10.74 ppm (N−NCHN− Me triazolium moiety) and at δ = 9.74 ppm (N−NCHN−Me imidazolium moiety). Resonances at δ = 9.71, 8.21, and 7.99 ppm are assigned to the protons of the triazolium and imidazolium backbone. Two signals at δ = 144.0 and 138.6 ppm were observed in the 13C{1H} NMR spectrum for the N− NCHN−Me carbon atoms of the triazolium moiety, while the resonance for the N−NCHN−Me and NCHN carbon atoms of the imidazolium group was detected at δ = 143.8 ppm. The resonances for the backbone carbon atoms of the imidazolium group were found at δ = 123.5 and 123.1 ppm. For the synthesis of the dicarbene chelate complexes [8a](BF4)2, [8b](BF4)2, and [9](BF4)2, the salts [6a](BF4)2, [6b](BF4)2, and [7](BF4)2, respectively, and Pd(OAc)2 were dissolved in acetonitrile and heated under reflux for 12 h. (Scheme 2). The palladium complexes were obtained in good yields of 89−95%. Scheme 2. Synthesis of the Palladium Complexes [8a](BF4)2, [8b](BF4)2, and [9](BF4)2

Figure 4. Molecular structure of the dication [8a′]2+ in [8a′](BF4)2· DMSO (50% displacement ellipsoids; hydrogen atoms have been omitted). Selected bond lengths (Å) and angles (deg): Pd−O1 2.080(4), Pd−N6 2.040(5), Pd−C1 1.978(6), Pd−C9 1.968(6), N1− C1 1.354(7), N2−C1 1.354(7), N2−N3 1.376(6), N3−C9 1.364(7), N5−C9 1.323(7); O1−Pd−N6 88.8(2), O1−Pd−C1 173.9(2), O1− Pd−C9 94.4(2), N6−Pd−C1 97.2(2), N6−Pd−C9 176.6(2), C1− Pd−C9 79.5(2), N1−C1−N2 105.4(5), N3−C9−N5 102.1(5), C1− N2−N3 114.4(4), C9−N3−N2 115.6(4).

linked bis(NHC) ligand coordinates to the metal center in a chelating fashion in spite of the short linker between the NHC donors. A similar formation of chelate complexes has been observed for N,N-linked bis(triazolylidene) dicarbene ligands.11,12 The coordination geometry around the palladium atom in [8b′] differs significantly from a square-planar arrangement. This is mainly caused by the small bite angle of the bis(NHC) ligand, which measures only 79.5(2)°. Consequently, the N6− Pd−C1 and O1−Pd−C9 angles expanded to 97.2(2)° and 94.4(2)°, respectively. Upon formation of the five-membered chelate ring, the C−N−N angles involving the N−N bridge (C1−N2−N3 114.4(4)° and C9−N3−N2 115.6(4)°) become more acute than was observed in the ligand precursor salt [6a](BF 4 ) 2 (C1−N1−N3 123.5(2)° and C8−N3−N1 124.5(2)°), and the two heterocycles are oriented in an almost coplanar fashion in the palladium complex. In spite of the presence of two different NHC donors, the Pd−C bond lengths do not differ significantly (Pd−C1 1.978(6) Å,19,22 Pd−C9 (1.968(6) Å11b). This rather unexpected observation might be due to the coordination of different solvent molecules in a

The palladium chelate complexes [8a](BF4)2, [8b](BF4)2, and [9](BF4)2 each feature two different NHC donors derived from azole and triazole skeletons. Such complexes are rare, but interesting, as they allow for a direct comparison of the spectroscopic and metric parameters associated with the two different NHC ligands. The first complexes containing two different, azole derived NHCs were synthesized in 2009.20 Shortly thereafter, the first chelating coordinated bis(NHC) ligands featuring two different (imidazolin-2-ylidene and benzimidazolin-2-ylidene) NHCs were described.21 Bis(NHC) ligands containing carbene donors derived from heterocycles with differing numbers of nitrogen atoms, as depicted in Scheme 2, have, to the best of our knowledge, not been described yet. The 1H NMR spectrum of compound [8a](BF4)2 lacks the two resonances δ = 10.82 ppm and δ = 10.22 ppm that were 4037

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trans-position to the NHC donors, which makes it impossible to directly compare the Pd−CNHC bond lengths. The 1H and 13C{1H} NMR spectra of compound [8b](BF4)2 are similar to those obtained for [8a](BF4)2 and are consistent with the proposed molecular structure (Scheme 2). Two resonances at δ = 150.1 ppm and δ = 144.7 ppm were recorded in the 13C{1H} NMR spectrum for the CNHC resonances of the benzimidazolin-2-ylidene and the 1,2,4-triazolin-5-ylidene moiety, respectively. Formation of [8b](BF4)2 was also indicated by the observation of a strong peak at m/z = 214.5442 in the HRMS (ESI positive ions, calcd for [8b]2+ 214.5444). Single crystals of [8b](BF4)2 suitable for an X-ray diffraction study were grown by slow diffusion of diethyl ether into a solution of the compound in acetonitrile. Contrary to the situation in [8a](BF4)2/[8a′](BF4)2 where an acetonitrile ligand, at least in the crystallized compound, was substituted for a DMSO ligand, complex cation [8b]2+ features acetonitrile ligands trans to both of the NHC donors (Figure 5), thereby allowing for a direct comparison of the metric parameters associated with the NHCs.

NCHN resonance for the 1,2,4-triazolin-5-ylidene (δ = 9.11 ppm) and two CH resonances for the imidazolin-2-ylidene backbone (δ = 8.03 and 7.36 ppm) skeleton were observed. The 13C{1H} NMR spectrum shows the expected two resonances for the carbene carbon atoms at δ = 152.8 ppm (imidazolin-2-ylidene) and δ = 144.5 ppm (1,2,4-triazolin-5ylidene). Formation of [9](BF4)2 was also indicated by the observation of a strong peak at m/z = 175.5204 in the HRMS spectrum (ESI positive ions, calcd for [9]2+ 175.5214). Single crystals suitable for an X-ray diffraction study of [9](BF4)2·0.5MeCN were grown from a concentrated solution of [9](BF4)2 in acetonitrile. The asymmetric unit contains one formula unit with the acetonitrile molecule residing on a special position and shared between two asymmetric units (Figure 6).

Figure 6. Molecular structure of the dication [9]2+ in [9](BF4)2· 0.5CH3CN (50% displacement ellipsoids; hydrogen atoms have been omitted). Selected bond lengths (Å) and angles (deg): Pd−N6 2.042(2), Pd−N7 2.044(2), Pd−C1 1.979(2), Pd−C5 1.966(2), N1− C1 1.331(3), N2−C1 1.346(3), N2−N3 1.373(3), N3−C5 1.349(3), N5−C5 1.330(3); N6−Pd−N7 87.81(8), N6−Pd−C1 97.06(9), N6− Pd−C5 175.31(9), N7−Pd−C1 175.11(10), N7−Pd−C5 96.34(9), C1−Pd−C5 78.81(10), N1−C1−N2 103.9(2), N3−C5−N5 102.4(2).

As in the case of [8a′]2+ and [8b]2+, dication [9]2+ features a distorted square-planar geometry with coplanar arranged NHC donors and the C1−Pd−C5 angle (78.81(10)°) deviating most significantly from the ideal arrangement. The Pd−C1 and Pd−C5 bond lengths of 1.979(2) and 1.966(2) Å, respectively, fall in the expected range.8a,11b They are, however, significantly different with the shorter separation observed for the Pd−Ctriazolylidene. This is surprising, as triazolin5-ylidenes are normally weaker donors than imidazolin-2ylidenes. Additional factors such as the π-acceptor capability and cross-conjugation across both heterocycles may be of importance here. The difference in the Pd−Ccarbene bond lengths is not reflected in the Pd−N bond distances as they are essentially identical.

Figure 5. Molecular structure of the dication [8b]2+ in [8b](BF4)2 (50% displacement ellipsoids; hydrogen atoms have been omitted). Selected bond lengths (Å) and angles (deg): Pd−N6 2.068(2), Pd− N7 2.047(2), Pd−C1 1.978(2), Pd−C3 1.984(2), N1−C1 1.353(2), N2−C1 1.326(2), N1−N4 1.374(2), N4−C3 1.355(2), N5−C3 1.335(2); N6−Pd−N7 84.21(6), N6−Pd−C1 98.78(6), N6−Pd−C3 173.47(6), N7−Pd−C1 176.03(6), N7−Pd−C3 97.60(6), C1−Pd− C3 79.13(6), N1−C1−N2 103.08(13), N4−C3−N5 105.82(13).

Cation [8b]2+ features a distorted square-planar coordinated palladium atom. The C1−Pd−C3 angle (79.13(6)°) deviates most significantly from the ideal value. The benzimidazolin-2ylidene and the 1,2,4-triazolin-5-ylidene rings are oriented almost coplanar with a dihedral angle of only 0.2°. The Pd− CNHC bond lengths are identical within experimental error (Pd−C1 1.978(2) Å, Pd−C3 1.984(2) Å). This similarity, however, is not indicative of similar donor abilities of the two NHCs as the Pd−Ccarbene distances are influenced by both the σ-donor and the π-acceptor properties of a given NHC ligand. In addition, potential cross-conjugation across both heterocycles might be a factor further complicating the situation. It appears that the benzimidazolin-2-ylidene features a stronger trans influence than the 1,2,4-triazolin-5-ylidene, as can be concluded from the longer Pd−N6 (2.068(2) Å) distance compared to the Pd−N7 (2.047(2) Å) distance. The 1H NMR spectrum of complex [9](BF4)2, featuring the imidazole/triazole derived bis(NHC) ligand, also reveals the absence of two NCHN resonances attributable to the ligand precursor [7](BF4)2 (δ = 10.74 and 9.74 ppm). Only one

3. CONCLUSIONS We present the preparation of N,N-linked diazolium salts featuring a triazolium and either a benzimidazolium ([6a](BF4)2, [6b](BF4)2) or an imidazolium ([7](BF4)2) group obtained by transamination of N-aminoazoles with N,Ndimethylformamide azine and subsequent N-quaternization of the initially obtained diazoles. The diazolium salts featuring two different N,N-linked azolium heterocycles react with Pd(OAc)2 to give the bis(NHC) complexes ([8a](BF4)2, [8b](BF4)2, and [9](BF4)2) containing two different NHC donors coordinated to the same metal center. The coordinated bis(NHC) ligands feature a rather small bite angle of about 79°. Slightly different Pd−CNHC bond distances were only detected in the case of the imidazolylidene/triazolylidene bis(NHC) ligand. 4038

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suspended in MeCN (10 mL). The mixture was heated under reflux for 2 days. Over this period, [6b](BF4)2 precipitated from the reaction mixture as a solid. The precipitate was collected by filtration, washed with a solvent mixture of MeCN:CH2Cl2 (1:5), and dried in vacuo to give [6b](BF4)2 as a colorless solid. Yield: 0.920 g (2.21 mmol, 68%) 1 H NMR (400 MHz, DMSO-d6): δ 10.88 (s, 1H, N−NCHN−Et triazolium), 10.30 (s, 1H, N−NCHN−Et benzimidazolium), 9.85 (s, 1H, NCHN triazolium), 8.35−8.32 (m, 1H, Ar-H), 8.14−8.12 (m, 1H, Ar-H), 7.93−7.90 (m, 2H, Ar-H), 4.80 (q, 2H, 3JHH = 7.2 Hz, NCH2) 4.68 (q, 2H, 3JHH = 7.2 Hz, NCH2), 1.62 (t, 3H, 3JHH = 7.2 Hz, NCH2CH3), 1.60 ppm (t, 3H, 3JHH = 7.2 Hz, NCH2CH3). 13C{1H} NMR (100.6 MHz, DMSO-d6): δ 144.0 (N−NCHN−Et triazolium), 143.6 (N−NCHN−Et benzimidazolium), 143.3 (NCHN triazolium), 130.4, 128.7, 128.5, 128.2, 114.5, 112.6 (Ar-C), 48.6 (NCH2), 43.5 (NCH2), 13.9 (NCH2CH3), 13.3 ppm (NCH2CH3). HRMS (ESI, positive ions): m/z (%) = 121.5736 (100) [6b]2+ (calcd for [6b]+ 121.5742). N-Methylimidazolium-1-methyl-1,2,4-triazolium Bis(tetrafluoroborate) [7](BF4)2. Compound 5 (300 mg, 2.22 mmol) and trimethyloxonium tetrafluoroborate (718 mg, 4.85 mmol) were suspended in MeCN (10 mL). The mixture was heated under reflux for 2 days. During this time, a colorless precipitate had formed. The precipitate was collected by filtration, washed three times with dichloromethane, and dried in vacuo, giving [7](BF4)2 as a colorless solid. Yield: 693 mg (2.05 mmol, 92%) 1H NMR (400 MHz, DMSOd6): δ 10.74 (s,1H, N−NCHN−Me triazolium), 9.74 (s, 1H, N− NCHN−Me imidazolium), 9.71 (s, 1H, NCHN triazolium), 8.21 (s, 1H, NNCHCHN), 7.99 (s, 1H, NNCHCHN), 4.27 (s, 3H, NCH3), 4.07 ppm (s, 3H, NCH3). 13C{1H} NMR (100.6 MHz, DMSO-d6): δ 144.0 (N−NCHN−Me triazolium), 143.8 (N−NCHN−Me imidazolium), 138.6 (NCHN triazolium), 123.5 (NNCHCHN), 123.1 (NNCHCHN), 39.8 (NCH3), 37.1 ppm (NCH3). HRMS (ESI, positive ions) m/z (%): 164.0938 (100) [7 − H]+ (calcd for [7 − H]+ 164.0936). N-Methylbenzimidazolin-2-ylidene-1-methyl-1,2,4-triazolin5-ylidene Complex [8a](BF4)2. A mixture of Pd(OAc)2 (43 mg, 0.19 mmol) and salt [6a](BF4)2 (76 mg, 0.20 mmol) was heated under reflux in acetonitrile (7 mL) for 12 h. The initially observed orange suspension turned into a light yellow solution over this period. After cooling to ambient temperature and removal of the volatiles in vacuo, the solid reaction product was washed several times with diethyl ether (5 mL each) and dried in vacuo to give [8](BF4)2 as a light yellow solid. Yield: 104 mg (0.18 mmol, 95%). 1H NMR (400 MHz, DMSOd6): δ 10.27 (s, 1H, NCHN triazolylidene), 8.37 (d, 1H, 3JHH = 7.9 Hz, Ar-H), 8.01 (d, 1H, 3JHH = 7.9 Hz, Ar-H), 7.71−7.66 (m, 2H, Ar-H), 4.20 (s, 3H, CH3), 4.17 (s, 3H, CH3), 1.96 ppm (s, 6H, CH3CN). 13 C{1H} NMR (100.6 MHz, DMSO-d6): δ 153.2 (NCN benzimidazolin-2-ylidene), 147.8 (NCN triazolin-5-ylidene), 134.8 (NCHN triazolylidene), 132.5, 126.5, 125.8, 125.3 (Ar-C), 118.7 (CH3CN), 113.2, 111.3 (Ar-C), 40.3 (NCH3), 39.4 (NCH3), 1.83 ppm (CH3CN). HRMS (ESI, positive ions) m/z (%): 200.5310 (100) [8a]2+ (calcd for [8a]2+ 200.5293). N-Ethylbenzimidazolin-2-ylidene-1-ethyl-1,2,4-triazolin-5ylidene Complex [8b](BF4)2. A mixture of Pd(OAc)2 (43 mg, 0.19 mmol) and the precursor [6b](BF4)2 (106 mg, 0.25 mmol) was heated under reflux in acetonitrile (7 mL) for 12 h. The initially orange suspension turned into a colorless solution over this period. After cooling to ambient temperature and removal of the volatiles in vacuo, the solid reaction product was washed with diethyl ether (5 mL each) and dried in vacuo to give [8b](BF4)2 as a light yellow solid. Yield: 100 mg (0.17 mmol, 89%). 1H NMR (400 MHz, CD3CN): δ 9.47 (s, 1H, NCHN triazolin-5-ylidene), 8.08 (d, 1H, 3JHH = 7.2 Hz, Ar-H), 7.94 (d, 1H, 3JHH = 7.2 Hz, Ar-H), 7.71−7.66 (m, 2H, Ar-H), 4.62 (q, 2H, 3 JHH = 7.3 Hz, NCH2), 4.49 (q, 2H, 3JHH = 7.3 Hz, NCH2), 1.96 (s, 6H, CH3CN), 1.58 ppm (t, 6H, 3JHH = 7.3 Hz, NCH2CH3 one signal for both ethyl groups). 13C{1H} NMR (100.6 MHz, CD3CN): δ 150.1 (NCN benzimidazolin-2-ylidene), 144.7 (NCN triazolin-5-ylidene), 135.2 (NCHN triazolylidene), 132.7, 128.5, 127.8, 126.7 (Ar-C), 118.4, (CH3CN), 114.4, 112.2 (Ar-C), 50.1 (NCH2), 44.6 (NCH2), 15.9 (NCH2CH3), 15.4 (NCH2CH3), 1.8 ppm (CH3CN). HRMS

4. EXPERIMENTAL SECTION General Procedures. All manipulations were carried out under an argon atmosphere using conventional Schlenk techniques or in a glovebox. Solvents were dried and freshly distilled by standard methods prior to use. 1H and 13C{H} NMR spectra were measured on a Bruker AVANCE I 400 or a Bruker AVANCE III 400 spectrometer. Chemical shifts (δ) are expressed in ppm downfield from tetramethylsilane using the residual protonated solvent signal as internal standard. Coupling constants are expressed in Hertz. Mass spectra were obtained with an Orbitrap LTQ XL (Thermo Scientific) or a MicroTof (Bruker Daltonics) spectrometer. The preparations of N,N-dimethylformamide azine,16 N-amino-benzimidazole (1),17 and 1(1,2,4-triazol-4-yl)-1H-benzimidazol (4)17 were carried out as described. Consistent microanalytical data were difficult to obtain due to the presence of fluorine in all ligand azolium salts and metal complexes. ESI HRMS data as well as 1H and 13C{1H} NMR spectroscopic data for these compounds are provided instead (see the Supporting Information). N-Aminoimidazole (2). To a solution containing imidazole (6.37 g, 0.094 mol) and potassium hydroxide (24.5 g, 0.437 mol) in water (250 mL) at 80 °C was added hydroxylamine-O-sulfonic acid (12.8 g, 0.113 mol) in small portions over a period of 1 h. The solution was stirred for a further hour and then concentrated to 150 mL under reduced pressure. A solid precipitated, which was removed by filtration. The solution was extracted 10 times with ethyl acetate (50 mL each). The organic phase was dried with MgSO4, and the solvent was removed under reduced pressure. The orange residue was purified by column chromatography (SiO2, ethyl acetate). Yield: 1.64 g (19.7 mmol, 21.0%). 1H NMR (400 MHz, CDCl3): δ 7.76 (s, 1H, NCHN), 7.12 (m, 2H, NCHCHN), 4.77 (s, 2H, NH2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 136.0 (NCHN), 125.9 (NH2NCHCHN), 121.7 (NH2NCHCHN). MS (MALDI-TOF, positive ions): m/z (%) = 84 (100) [2 + H]+. Anal. Calcd for C3H5N3: C, 43.36; H, 6.06; N, 50.58%. Found: C, 43.42; H, 6.12; N, 50.46%. N4-(Imidazole-1-yl)-4H-[1,2,4]triazole 5. Samples of compounds 2 (830 mg, 10.0 mmol), 3 (2.13 g, 15.0 mmol), and ptoluene sulfonic acid (0.4 g) were dissolved in toluene (20 mL). The solution was heated under reflux for 3 days. After cooling to ambient temperature, the formed precipitate was isolated by filtration, washed with dichloromethane (10 mL) and diethyl ether (20 mL), and dried in vacuo, giving 5 as a colorless solid. Yield: 486 mg (3.6 mmol, 36%). 1 H NMR (400 MHz, DMSO-d6): δ 9.14 (s, 2H, NCHN triazole), 8.22 (s, 1H, NCHN imidazole), 7.12 (s, 1H, NNCHCHN), 7.08 ppm (s, 1H, NNCHCHN). 13C{1H} NMR (100.6 MHz, DMSO-d6): δ 142.7 (NCHN triazole), 136.7 (NCHN imidazole), 127.9 (NNCHCHN), 121.3 ppm (NNCHCHN). MS (EI): m/z (%) = 135 (100, [5]+), 68 (6, [triazole − H]+). Anal. Calcd for C5H5N5: C, 44.44; H, 3.73; N, 51.84%. Found: C, 44.71; H, 3.85; N, 51.44%. N-Methylbenzimidazolium-1-methyl-1,2,4-triazolium Bis(tetrafluoroborate) [6a](BF4)2. Compound 4 (814 mg, 4.40 mmol) and trimethyloxonium tetrafluoroborate (1.436 g, 9.71 mmol) were suspended in acetonitrile (10 mL). The mixture was heated under reflux for 12 h. Over this period, [6a](BF4)2 precipitated from the reaction mixture as a solid. The solid was collected by filtration, washed three times with small amounts of CH2Cl2, and dried in vacuo to give [6a](BF4)2 as a slightly yellow solid. Yield: 1.522 g (3.92 mmol, 89%) 1H NMR (400 MHz, DMSO-d6): δ 10.82 (s, 1H, N−NCHN−Me triazolium), 10.22 (s, 1H, N−NCHN−Me benzimidazolium), 9.81 (s, 1H, NCHN triazolium), 8.27−8.24 (m, 1H, Ar-H), 8.09−8.07 (m, 1H, Ar-H), 7.93−7.90 (m, 2H, Ar-H), 4.35 (s, 3H, NCH3), 4.33 ppm (s, 3H, NCH3). 13C{1H} NMR (100.6 MHz, DMSO-d6): δ 144.4 (N−NCHN−Me triazolium), 143.9 (N− NCHN−Me benzimidazolium and C3 triazolium), 130.3, 129.4, 128.7, 128.1, 114.4, 112.2 (Ar-C), 39.8 (NCH3), 34.3 ppm (NCH3). HRMS (ESI, positive ions): m/z (%) = 214.1084 (100) [[6a] − H]+ (calcd for [[6a] − H]+ 214.1093). N-Ethylbenzimidazolium-1-ethyl-1,2,4-triazolium Bis(tetrafluoroborate) [6b](BF4)2. Compound 4 (600 mg, 3.24 mmol) and triethyloxonium tetrafluoroborate (1.228 g, 6.46 mmol) were 4039

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Organometallics

Article

046 measured intensities (4.2° ≤ 2θ ≤ 61.0°), semiempirical absorption correction (0.856 ≤ T ≤ 0.961), 6135 independent (Rint = 0.0255) and 5771 observed intensities (I ≥ 2σ(I)), refinement of 279 parameters against |F2| of all measured intensities. R = 0.0249, wR = 0.0587, Rall = 0.0274, wRall = 0.0579.

(ESI, positive ions): m/z (%) = 214.5442 (100) [8b]2+ (calcd for [8b]2+ 214.5444). N-Methylimidazolin-2-ylidene-1-methyl-1,2,4-triazolin-5-ylidene Complex [9](BF4)2. A mixture of Pd(OAc)2 (87 mg, 0.39 mmol) and the salt [7](BF4)2 (131 mg, 0.39 mmol) was heated under refluxed in acetoniotrile (7 mL) for 12 h. The orange suspension turned into a colorless solution over this period. After cooling to ambient temperature and removal of the volatiles in vacuo, the solid reaction product was washed several times with dichloromethane (5 mL each) and dried in vacuo to give [9](BF4)2 as a colorless solid. Yield: 195 mg (0.37 mmol, 95%). 1H NMR (400 MHz, DMSO-d6): δ 9.11 (s, 1H, NCHN triazolylidene), 8.03 (s, 1H, NNCHCHN), 7.36 (s, 1H, NNCHCHN), 4.13 (s, 3H, NCH3), 3.94 (s, 3H, NCH3), 1.96 ppm (s, 6H, CH3CN). 13C{1H} NMR (100.6 MHz, DMSO-d6): δ 152.8 (NCN imidazolin-2-ylidene), 144.5 (NCN triazolin-5-ylidene), 134.7 (NCHN triazolylidene), 125.8 (NNCHCHN), 118.7 (CH3CN), 115.3 (NNCHCHN), 41.4 (NCH3), 39.3 (NCH3), 1.8 ppm (CH3CN). HRMS (ESI, positive ions): m/z (%) = 175.5204 (100) [9]2+ (calcd for [9]2+ 175.5214). X-ray Diffraction Studies. X-ray diffraction data for all compounds were collected at T = 153(2) K with a Bruker AXS APEX CCD diffractometer equipped with a rotation anode using graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å) for [8a′](BF4)2·DMSO, [8b](BF4)2, and [9](BF4)2·0.5MeCN or Cu−Kα radiation (λ = 1.54178 Å) for 5 and [6a](BF4)2. Diffraction data were collected over the full sphere and were corrected for absorption. Structure solutions were found with the SHELXS-9723 package using direct methods and were refined with SHELXL-9723 against |F2| using first isotropic and later anisotropic thermal parameters. Hydrogen atoms were added to the structure models on calculated positions. 5. C5H5N5, M = 135.14, colorless crystal, 0.38 × 0.20 × 0.07 mm3, monoclinic, space group P21/c, a = 9.957(2) Å, b = 7.126(2) Å, c = 8.899(2) Å, β = 101.318(14)°, V = 619.2(2) Å3, Z = 4, ρcalc = 1.450 g· cm−3, μ = 0.851 mm−1, ω- and φ-scans, 2041 measured intensities (15.4° ≤ 2θ ≤ 130.0°), semiempirical absorption correction (0.738 ≤ T ≤ 0.943), 906 independent (Rint = 0.0459) and 752 observed intensities (I ≥ 2σ(I)), refinement of 92 parameters against |F2| of all measured intensities. R = 0.0572, wR = 0.1624, Rall = 0.0637, wRall = 0.1682. [6](BF4)2. C11H13N5B2F8, M = 388.88, colorless crystal, 0.15 × 0.12 × 0.10 mm3, monoclinic, space group P21/n, a = 13.2878(5) Å, b = 7.7937(3) Å, c = 16.7579(5) Å, β = 112.115(2)°, V = 1607.79(10) Å3, Z = 4, ρcalc = 1.607 g·cm−3, μ = 1.475 mm−1, ω- and φ-scans, 8995 measured intensities (7.3° ≤ 2θ ≤ 142.0°), semiempirical absorption correction (0.809 ≤ T ≤ 0.866), 2985 independent (Rint = 0.0422) and 2530 observed intensities (I ≥ 2σ(I)), refinement of 237 parameters against |F2| of all measured intensities. R = 0.0514, wR = 0.1453, Rall = 0.0579, wRall = 0.1508. [8a′](BF4)2. C17H26N6B2F8O2PdS2, M = 690.58, colorless crystal, 0.13 × 0.03 × 0.02 mm3, monoclinic, space group P21/n, a = 6.3861(5) Å, b = 14.5693(11) Å, c = 28.194(2) Å, β = 90.958(2)°, V = 2622.8(4) Å3, Z = 4, ρcalc = 1.749 g·cm−3, μ = 0.952 mm−1, ω- and φscans, 23 027 measured intensities (2.9° ≤ 2θ ≤ 55.0°), semiempirical absorption correction (0.886 ≤ T ≤ 0.981), 6022 independent (Rint = 0.0984) and 3420 observed intensities (I ≥ 2σ(I)), refinement of 359 parameters against |F2| of all measured intensities. R = 0.0538, wR = 0.1097, Rall = 0.1221, wRall = 0.1358. [8b](BF4)2. C17H21N7B2F8Pd, M = 603.43, colorless crystal, 0.46 × 0.15 × 0.14 mm3, monoclinic, space group P21/n, a = 15.4586(9) Å, b = 8.4466(5) Å, c = 18.5053(11) Å, β = 104.607(3)°, V = 2338.2(2) Å3, Z = 4, ρcalc = 1.714 g·cm−3, μ = 0.878 mm−1, ω- and φ-scans, 40 773 measured intensities (3.1° ≤ 2θ ≤ 63.7°), semiempirical absorption correction (0.689 ≤ T ≤ 0.900), 7493 independent (Rint = 0.0299) and 6791 observed intensities (I ≥ 2σ(I)), refinement of 320 parameters against |F2| of all measured intensities. R = 0.0245, wR = 0.0734, Rall = 0.0304, wRall = 0.0877. [9](BF4)2·0.5MeCN. C12H16.5N7.5B2F8Pd, M = 545.85, colorless crystal, 0.16 × 0.13 × 0.04 mm3, orthorhombic, space group Fdd2, a = 25.5241(10) Å, b = 28.4837(11) Å, c = 11.1776(4) Å, V = 8126.3(5) Å3, Z = 16, ρcalc = 1.785 g·cm−3, μ = 1.000 mm−1, ω- and φ-scans, 24

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files for 5, [6a](BF4)2, [8a′](BF4)2, [8b](BF4)2, and [9](BF4)2·0.5CH3CN (CIF files) and 1H and 13 C{1H} NMR spectra for selected compounds. 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: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (SFB 858) for financial support. REFERENCES

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