Synthesis and Reactivity of IrIII Complexes Bearing C-Metalated

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Synthesis and Reactivity of IrIII Complexes Bearing C‑Metalated Pyrazolato Ligands Tristan Tsai Yuan Tan, Sabrina Schick, and F. Ekkehardt Hahn* Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany

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S Supporting Information *

ABSTRACT: A bidentate triazolinylidene-pyrazole chelate ligand was metalated with [IrCp*Cl2]2 to give C,N-chelate complex [4]I. The N1-metalated pyrazole subsequently underwent a rollover metalation to give the complex with C5,C4-metalated pyrazolato ligand [5]. The reactivity of [5] toward insertion and protonation was investigated. Complex [5] was found to react with CO to give IrIII carbonyl complex [6]I, which subsequently reacted under migratory CO insertion into the Ir−C(pyrazolato) bond to give [7]. The reaction of [5] with the terminal alkyne methyl propiolate yielded 1,2 insertion product [8], featuring a 7-membered C,C-chelate ring.

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INTRODUCTION C-Metalated azolates can be viewed as anionic congeners of Nheterocyclic carbene (NHC) ligands, where one of the alkylated/arylated nitrogen atoms of the NHC is replaced by a free-base nitrogen atom. Different types of neutral (or zwitterionic) NHC ligands (Figure 1a) have become

Complexes of imidazolin-2-ylidenes (Figure 1a-I) are by far the most studied NHC complexes.1 The chemistry of mesoionic carbenes (MICs), namely imidazolin-4-ylidenes (Figure 1a-II) and 1,2,3-triazolin-5-ylidenes (Figure 1a-III), where one stabilizing nitrogen atom is moved away from the carbene carbon atom, has been explored less.5 In pyrazolin-4ylidenes (Figure 1a-IV),6 also named “bent allenes” when exocyclic heteroatoms are present,7 both heteroatoms are moved away from the carbene carbon atom. Complexes of these ligands are rarer still, in spite of their exceptionally strong donor strength. As one would expect, the attention received by anionic azolato ligands correlates with that of their parent NHCs. While C2-metalated imidazolates (Figure 1a-I)8 and C2metalated benzimidazolates3,9 have received some attention in recent years, only few reports of C4(5)-metalated imidazolates (Figure 1b-II)10 and C5-metalated 1,2,3-triazolates (Figure 1bIII)11 have appeared in the literature. Finally, reports of C4-metalated pyrazolato ligands (Figure 1b-IV) are exceedingly rare. To date, complexes bearing such ligands have been obtained via the auration12 or mercuration13 of pyrazole, the Ru mediated cyclization of alkylnylhydrazone,14 and most recently from the C−H oxidative addition of pyrazole to Os(II).15 Pyrazolin-4-ylidenes and C4metalated pyrazolates have the least heteroatom stabilization of all C-metalated azoles. The C4 carbon atom of pyrazolato ligands is particularly nucleophilic and the parent pyrazoles are challenging to deprotonate,6 which is one reason why complexes with such ligands are so rare. Focusing our

Figure 1. Schematic representation of (a) NHC complexes and (b) C-metalated azolato, triazolato and pyrazolato ligands.

ubiquitous spectator ligands in organometallic chemistry where they mainly act as strong σ-donors.1 Conversely, their more reactive anionic derivatives which include C-metalated azolates (Figure 1b)2 and their conjugate acids, the protic NHCs (pNHCs),2c,d have received less attention until recently. We became interested in developing synthetic methodologies leading to complexes bearing azolato ligands and in exploring their reactivity. Complexes bearing C-metalated azolato ligands have shown metal−ligand bifunctional reactivity2a,b and are important intermediates in transition metal catalyzed reactions.3,4 © XXXX American Chemical Society

Received: November 25, 2018

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DOI: 10.1021/acs.organomet.8b00856 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics attention on these complexes, we became interested in extending our synthetic methodologies for C2-metalated imidazolates (Figure 1b-I) to C4-metalated pyrazolates (Figure 1b-IV) via metal-mediated C−H activation reactions,4,8,9 hereby providing a better access to these unique complexes and allowing the exploration of the reactivity of the Cmetalated pyrazolato ligands. Herein we report the synthesis of a triazolium/pyrazole proligand 3-I. Upon metalation of the triazolinylidene obtained after C5 deprotonation, this group can act as a directing group for the C−H activation at the pyrazole. Using this ligand, an IrIII complex bearing a C4-metalated pyrazolato ligand via a “rollover” metalation17c was synthesized. We also explored the reactivity of this complex in order to determine the carbene or carbanionic character of the C-metalated pyrazolato ligand.

Scheme 2. Synthesis of Complexes [4]X and [5]

CH3CN suitable for an X-ray diffraction analysis were obtained by slow diffusion of diethyl ether into an acetonitrile solution of the complex. The molecular structure of cation [4]+ is shown in Figure 2. Complex cation [4]+ exhibits the classical

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RESULTS AND DISCUSSION Synthesis of Iridium(III) Pyrazolato Complex [5]. Ligand precursor 3-I was synthesized via a three-step procedure, starting from 3-amino-5-methylpyrazole (Scheme 1). Triazole/pyrazole skeleton 1 was obtained in excellent yield via the transamination of N,N-dimethylformamide azine16 with the primary amine of the 3-amino-5-methylpyrazole. Scheme 1. Synthesis of Ligand Precursor 3-I

Figure 2. Molecular structure of [4]+ in [4]I·CH3CN (ellipsoids drawn at 50% probability, hydrogen atoms have been omitted). Selected bond lengths [Å] and angles [°]: Ir1−I1 2.68591(14), Ir1− N4 2.124(2), Ir1−C1 2.016(2), range Ir1−CCp* 2.155(2)−2.251(2), N4−N5 1.359(2), N4−C4 1.336(3), C4−C5 1.387(3), C5−C6 1.387(3); C1−Ir1−N4 74.82(7), C4−N4−N5 105.0(2), N1−C1− N2 103.2(2), C4−C5−C6 103.8(2).

piano stool geometry. The Ir−CNHC bond length (2.016(2) Å) falls in the range previously observed for related 1,2,4-triazolin5-ylidene iridium(III) complexes.18b,c Upon heating of a mixture of [4]I and sodium acetate in acetonitrile to 80 °C, a second species was observed, and its formation could be monitored by thin-layer chromatography (Rf = 0.25, ethyl acetate). This species was later isolated in larger amounts by column chromatography and shown to be C4-metalated rollover product [5] (Scheme 2). The yield of [5] could be improved by the addition of potassium carbonate as a second base as well as by extending the reaction time to 4 days. Monitoring of the crude reaction mixture by 1H NMR spectroscopy revealed that after 4 days about 90% of [4]I had converted to [5]. A longer reaction time, however, did not increase the conversion. The connectivity in complex [5] was established by 1H and 13 C{1H} NMR spectroscopy and by single crystal X-ray diffraction analysis (Figure 3). The 13C NMR spectrum of [5] (in CDCl3) showed the resonance for the metalated pyrazolato carbon atom at δ 99.3 ppm, downfield from the resonance for this carbon atom in the N-metalated complex [4]I (96.1 ppm). The CNHC resonance also shifted slightly downfield from δ 164.1 ppm in[4]I to δ 165.7 ppm in [5].5e,f The structure analysis revealed that the endocyclic C−C bond lengths in the carbon-metalated pyrazolato ligand of [5] (1.391(2)−1.400(2) Å) do not change significantly upon

Methylation of the pyrazole moiety of 1 at the N1 position gave 2 in good yield. During the reaction, the N2 methylated pyrazole formed as a minor impurity which was separated from 2 by recrystallization. However, the mixture of isomers could also be used directly without purification for the subsequent methylation of the triazole without any detriment to the yield or purity of triazolium salt 3-I. We envisioned that upon triazolium deprotonation in 3-I the formed 1,2,4-triazolin-5-ylidene ligand could coordinate to a metal center and act as a directing group for the C−H activation at the C4 position of the pyrazole. Related C,C chelates have been synthesized by rollover metalation of selected pyridines.17 Reaction of 3-I with 0.5 equiv of [IrCp*Cl2]2 and sodium acetate in acetonitrile at room temperature yielded the C,Nchelate complex [4]I (Scheme 2). Under these reaction conditions, no C∧C chelate was detected, and complex [4]I was isolated in quantitative yield. While C,N-chelate complexes like [4]I are not unprecedented, this precursor for a subsequent rollover C4-metalation of the pyrazole provides useful data for comparison. The 13C NMR resonance of the CNHC atom was detected at 164.1 ppm, within the range of previously reported 1,2,4-triazolin-5ylidene iridium(III) complexes.18 Single crystals of [4]I· B


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Organometallics

ligand by CO. Full conversion of [5] to cationic carbonyl complex [6]I was completed after 2 days (Scheme 3). Scheme 3. Reaction of [5] with CO

Figure 3. Molecular structure of [5] (ellipsoids drawn at 50% probability, hydrogen atoms have been omitted). Selected bond lengths [Å] and angles [°]: Ir1−I1 2.70189(12), Ir1−C1 2.012(2), Ir1−C5 2.065(2), range Ir1−CCp* 2.157(2)(2)−2.245(2), N4−N5 1.361(2), N4−C4 1.326(2), C4−C5 1.400(2), C5−C6 1.391(2); C1−Ir1−C5 77.31(6), C4−N4−N5 101.09(13), N1−C1−N2 102.52(14), C4−C5−C6 100.84(14).

The 13C{H} NMR spectrum of [6]I showed the resonances for the pyrazolato and carbene carbon atoms bound to the metal center significantly upfield shifted compared to the neutral complex [5] ([5]: δ(Cpyrazolato) 99.3 ppm, δ(CNHC) 165.7 ppm; [6]I: δ(Cpyrazolato) 85.6 ppm, δ(CNHC) 151.0 ppm). The resonance for the carbonyl carbon atom was recorded at δ 162.1 ppm. The IR stretching mode was recorded at ν = 2041 cm−1 in the range observed for similar IrIII complexes bearing a Cp* ligand and a bidentate orthometalated ligand (ν = 2042 cm−1) .21a Diffusion of pentane into a chloroform solution of [6]I at −40 °C yielded pale yellow single crystals of composition [6]I· 2CHCl3·0.5C5H12. The X-ray diffraction analysis revealed the molecular structure of cation [6]+ depicted in Figure 4. The

rollover metalation ([4]I: (1.387(3) Å) or in comparison to the ligand precursor 3-I (1.387(2)−1.393(2) Å, see the Supporting Information). The C4−C5−C6 bond angle in the pyrazolate donor in [5] (100.84(14)°) is significantly smaller than the equivalent angle in the N-metalated pyrazole in [4]I (103.8(2)°). A similar decrease of the endocyclic in C−C−C angle has been observed by Esteruelas and co-workers upon C-metalation of a pyrazole.15 The Ir−C1 (2.012(2) Å) bond length is shorter than the Ir−C5 bond distance(2.065(2) Å), and the bite angle of the C,C-chelate ligand in [5] (77.31(6)°) is larger than the bite angle of the C,N-chelate ligand in complex cation [4]+ (74.82(7)°). Reaction of [5] with HBF4. We are interested in the reactivity of pyrazolato complex [5] featuring a free-base ringnitrogen atom in comparison to related complexes bearing NHCs with an unsubstituted ring-nitrogen atom. Various complexes bearing C2-metalated imidazolato and benzimidazolato ligands with a free-base ring-nitrogen atom have been shown to behave as anionic NHCs, with the negative charge mainly located at the unsubstituted ring-nitrogen atom.3,8e,f,9a Such anionic imidazolates react with electrophiles such as H+, alkyl halides, or transition metals at the unsubstituted ringnitrogen atoms instead of the metalated carbon atom. However, decreasing of the amount of heteroatom stabilization in MICs and remote carbenes is known to destabilize the HOMO of the carbene carbon atom.5,19 Therefore, it appeared reasonable to assume that C4-metalated pyrazolato ligands would be highly carbanionic in character. To test this hypothesis, complex [5] was reacted with HBF4· Et2O in dichloromethane. An immediate darkening of originally yellow solution was observed upon addition of the acid. The 1H NMR spectrum indicated that complex [4]BF4 had formed by protonation of the pyrazolato carbon atom (Scheme 2). Protonation of the ring-nitrogen atom, as was observed upon reaction of C2-metalated anionic NHCs with acids8b,9a,c was not observed. Complex [4]BF4 displays a lower solubility in organic solvents compared to [4]I but the complex cation [4]+ is identical in both salts as was demonstrated by an X-ray diffraction analysis (see the Supporting Information). Reaction of [5] with Carbon Monoxide. We next investigated the reactivity of [5] with CO. An NMR scale experiment using [5] in CD3CN and 2 bar of CO in a Young NMR tube at 80 °C indicated simple substitution of the iodo

Figure 4. Molecular structure of [6]+ in [6]I·2CHCl3·0.5C5H12 (ellipsoids drawn at 50% probability, hydrogen atoms have been omitted). Selected bond lengths [Å] and angles [°]: Ir1−C1 2.060(2), Ir1−C5 2.087(2), Ir1−C9 1.879(2), range Ir1−CCp* 2.229(2)(2)−2.255(2), N4−N5 1.362(3), N4−C4 1.326(3), C4− C5 1.385(3), C5−C6 1.391(3); C1−Ir1−C5 76.96(8), C4−N4−N5 101.9(2), N1−C1−N2 102.8(2), C4−C5−C6 102.0(2).

substitution of the iodo ligand in [5] for a carbonyl ligand in [6]+ and the concurrent increase of the electrophilicity of the iridium(III) center led to a significant expansion of all comparable Ir−C bond distances ([5]: Ir−C1 2.012(2) Å, Ir−C5 2.065(2) Å; [6]+: Ir−C1 2.060(2) Å, Ir−C5 2.087(2) Å). The Ir−C9 distance measures 1.879(2) Å). Complex cation [6]+ adopts the classical piano stool geometry with the bite angle of the C,C-chelate ligand in [5] essential identical to the equivalent angle in [6]+. C


Article

Organometallics Compound [6]I is stable in acetonitrile or chloroform solution under an argon atmosphere, with no changes detectable in the 1H NMR spectrum over several months. However, while trying to obtain single crystals of [6]I by diffusion of pentane into a dichloromethane solution of the compound, dark orange crystals were obtained. These differed in color from the pale yellow crystals of [6]I grown from chloroform. Initially, we assumed that a different polymorph of [6]I had formed in the dichloromethane/pentane solvent mixture. However, an X-ray diffraction study with the orange crystals revealed that these consist of the CO insertion product [7] (Scheme 3). The molecular structure of [7] is shown in Figure 5.

Scheme 4. Reaction of [5] with Methyl Propiolate

carbon atom bearing the ester group is bound to iridium. The same regioselectivity (coordination of the sterically more demanding substituted alkyne carbon atom to iridium) has also been observed for the 1,2-insertion of alkynes into the aryliridium(III) bond.21f,i Complex [8] was characterized by NMR spectroscopy and mass spectrometry. The 13C NMR resonances for the inserted alkyne were recorded at δ 134.3 ppm (CC−Ir) and δ 124.8 ppm (CC−pyrazolato). Crystals of [8] were obtained by diffusion of n-hexane into a chloroform solution of the complex. The X-ray diffraction structure determination revealed a complex with piano stool geometry (Figure 6). The bite angle of the C,C-chelate ligand (92.71(7)°) is slightly larger than the equivalent angle in [7] and significantly larger than in [5].

Figure 5. Molecular structure of [7] (ellipsoids drawn at 50% probability, hydrogen atoms have been omitted). Selected bond lengths [Å] and angles[°]: Ir1−I1 2.7000(2), Ir1−C1 2.003(2), Ir1− C9 2.021(2), range Ir1−CCp* 2.168(2)(2)−2.326(2), N4−N5 1.366(3), N4−C4 1.324(3), C4−C5 1.402(3), C5−C6 1.400(3); C1−Ir1−C9 91.11(9), C4−N4−N5 102.6(2), N1−C1−N2 101.8(2), C4−C5−C6 102.6(2).

Complex [7] proved stable in the solid state in air where the crystals did not degrade over a week. However, upon dissolving the crystals in chloroform for an NMR characterization, [7] underwent spontaneous decarbonylation and reverted to original pyrazolato complex [5]. Fortunately, the decarbonylation is slow enough that both 1H and 13C NMR spectra could be recorded. The acyl carbon atom resonated at δ 207.1 ppm. However, the ESI mass spectrum of [7] could not be obtained, and only the completely decarbonylated compound [5] was observed. A related CO insertion and decorbonylation has been described for C,N-cyclometalated iridium(III) complexes by Jones and co-workers20 and the insertion of isocyanides has also been described,6e further demonstrating the highly carbanionic character of the pyrazolato donor in [7]. Reactivity of [5] with a Terminal Alkyne. Iridium(III) complexes bearing C-metalated imidazolato ligands with an unsubstituted ring-nitrogen atom have been shown to react with terminal alkynes.8a In this reaction, the imidazolato ringnitrogen atom acts as a base for the deprotonation of the alkyne to give the pNHC/acetylide complex.8a Contrary to this observation, most cyclometalated aryl−iridium(III) complexes react with alkynes with insertion of the alkyne into the CAr−Ir bond.20,21 Reaction of [5] with 1 equiv of methyl propiolate in methanol yielded alkyne 1,2-insertion product [8] as an airstable, orange solid in good yield (Scheme 4). No iridium acetylide reaction product was detected, in contrast to the reactivity of iridium complexes with imidazolato ligands. Only one of the possible regioisomers was obtained where the

Figure 6. Molecular structure of [8] (ellipsoids drawn at 50% probability, hydrogen atoms have been omitted). Selected bond lengths [Å] and angles[°]: Ir1−I1 2.73680(14), Ir1−C1 2.009(2), Ir1−C10 2.066(2), range Ir1−CCp* 2.066(2)(2)−2.241(2), N4−N5 1.357(2), N4−C4 1.327(2), C4−C5 1.403(2), C5−C6 1.393(2), C5−C9 1.448(2), C9−C10 1.348(2); C1−Ir1−C10 92.71(7), C4− N4−N5 103.58(15), N1−C1−N2 101.90(14), C4−C5−C6 102.89(15), C5−C9−C10 127.2(2), Ir1−C10−C9 128.93(13).

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CONCLUSION Iridium(III) complexes bearing C-metalated pyrazolato ligands have been obtained by acetate-assisted rollover metalation from complexes with N-metalated pyrazole ligands. The rollover methodology can likely be extended to other transition metals that mediate C−H activation4 in pyrazole ligands thereby opening new avenues for the exploration of pyrazolato complexes. The reactivity of the C-metalated pyrazolato ligand toward protonation and migratory insertion was probed showing typical carbanion character of the pyrazolato ligand, thereby contrasting the reactivity of C-metalated imidazolates and benzimidazolates which exhibit anionic carbene character. Further investigations into the reactivity of IrIII−pyrazolato D


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Organometallics

NMR (400 MHz, DMSO-d6): δ 10.68 (s, 1H, H1), 9.70 (s, 1H, H2), 6.75 (s, 1H, H5), 4.11 (s, 3H, H3), 3.81 (s, 3H, H8), 2.36 (s, 3H, H7). 13C{1H} NMR (100 MHz, DMSO-d6): δ 142.4 (C6), 141.5 (C2), 140.7 (C1), 138.8 (C4), 97.1 (C5), 38.9 (C3), 36.6 (C8), 10.9 (C7). The signal for C3 overlaps with the DMSO signal; however, it can be detected using 1H,13C HSQC NMR spectroscopy. HRMS (ESI, positive ions): m/z 178.1081 (calcd for [3]+ 178.1093) Synthesis of [4]I.

complexes and toward the preparation pyrazolato complexes from other transition metals are in progress.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out under an argon atmosphere unless stated otherwise. 1H and 13C{1H} NMR spectra were measured on a Bruker AVANCE I 400 or a Bruker AVANCE III 400 spectrometer. Chemical shifts (δ) are expressed in ppm relative to SiMe4 using the residual protonated solvent signal as an internal standard. For the assignments of the NMR resonances see the numbering at the molecular plots. Coupling constants are expressed in Hz. Mass spectra were obtained with an Orbitrap LTQ XL spectrometer (Thermo Scientific). Infrared spectra were recorded with a Bruker ALPHA II FTIR Spectrometer. [IrCp*Cl2]222 and N,Ndimethylformamide azine16 were prepared as described. Synthesis of 4-(5-Methyl-1H-pyrazol-3-yl)-4H-1,2,4-triazole 1.

Samples of 3-I (20 mg, 0.066 mmol), [IrCp*Cl2]2 (26 mg, 0.033 mmol), sodium acetate (20 mg, 0.24 mmol), and potassium iodide (20 mg, 0.12 mmol) were stirred in 5 mL of acetonitrile overnight at ambient temperature. The solvent was then removed in vacuo. The solid residue was dissolved in chloroform (5 mL) and filtered through a short plug of Celite. The solvent was then removed giving [4]I as an orange solid. Yield: 50 mg (0.066 mmol, 99%). 1H NMR (400 MHz, CDCl3): δ 9.79 (s, 1H, H2), 7.29 (s, 1H, H5), 4.09 (s, 3H, H3), 3.84 (s, 3H, H8), 2.59 (s, 3H, H7), 1.99 (s, 15H, H10). 13C{1H} NMR (100 MHz, CDCl3): δ 164.1 (C1), 147.1 (C6), 145.0 (C4), 140.3 (C2), 96.1 (C5), 93.0 (C9), 40.0 (C3), 38.2 (C8), 13.6 (C7), 11.1 (C10). HRMS (ESI, positive ions): m/z 632.0847 (calcd for [4]+ 632.0826). Synthesis of [5].

Samples of 3-amino-5-methyl-1H-pyrazole (2.9 g, 30 mmol, 1 equiv), N,N-dimethylformamide azine (6.4 g, 45 mmol, 1.5 equiv), and ptoluenesulfonic acid monohydrate (0.6 g, 3 mmol) were dissolved in 30 mL of toluene, and the mixture was heated under an argon atmosphere to reflux for 2 days. A bubbler outlet was installed to the setup. After cooling to ambient temperature, the white precipitate which formed was collected by filtration and washed with cold dichloromethane (5 mL). Yield: 4.2 g (28 mmol, 93%). 1H NMR (400 MHz, DMSO-d6): δ 12.79 (s br, 1H, NH), 8.96 (s, 2H, H1), 6.36 (s, 1H, H3), 2.28 (s, 3H, H5). 13C{1H} NMR (100 MHz, DMSO-d6): δ 142.9 (C2), 140.9 (C4), 140.7 (C1), 95.2 (C3), 10.6 (C5). HRMS (ESI, positive ions): m/z 172.0590 (calcd for [1 + Na]+ 172.0594). Synthesis of 4-(1,5-Dimethyl-1H-pyrazol-3-yl)-4H-1,2,4-triazole 2.

Samples of 3-I (40 mg, 0.13 mmol), [IrCp*Cl2]2 (50 mg, 0.063 mmol), sodium acetate (50 mg, 0.61 mmol), and potassium carbonate (50 mg, 0.36 mmol) were heated under reflux in acetonitrile (10 mL) for 4 days. Subsequently, the reaction mixture was cooled to ambient temperature, and potassium iodide (50 mg, 30 mmol) was added. The solvent was reduced in vacuo to about 1 mL, and the mixture was loaded onto a 5 cm column of silica. Complex [5] was eluted with ethyl acetate. Removal of the solvent gave [5] as an orange crystalline solid. Yield 69 mg (0.11 mmol, 85%). 1H NMR (400 MHz, CDCl3): δ 8.34 (s, 1H, H2), 4.04 (s, 3H, H3), 3.76 (s, 3H, H8), 2.38 (s, 3H, H7, 1.95 (s, 15H, H10). 13C{1H} NMR (100 MHz, CDCl3): δ 165.7 (C1), 154.2 (C4), 141.3 (C6), 135.7 (C2), 99.3 (C5), 91.0 (C9), 39.1 (C3), 36.5 (C8), 12.9 (C7), 10.6 (C10). HRMS (ESI, positive ions): m/z 504.1727 (calcd for [5 − I]+ 504.1739). Synthesis of [4]BF4.

A suspension of 1 (2 g, 13.4 mmol) and potassium hydroxide (4g, 71.4 mmol) was stirred in 10 mL of acetonitrile for 2 h at ambient temperature. Subsequently, methyl iodide (2.8 g, 20 mmol) was added, and the reaction mixture was stirred for 16 h at ambient temperature. The solvent was then removed under reduced pressure, and 10 mL of dichloromethane was added. Insolubles were separated by filtration and washed with an additional 10 mL of dichloromethane. The volume of solvent was reduced to about 4 mL. The resulting solution was placed in a vial and layered carefully with nhexane. Colorless crystalline needles of 2 formed after the two solvent layers had homogenized. The crystals were collected, washed with nhexane, and dried in vacuo. Yield: 1.6 g (9.8 mmol, 73%). 1H NMR (400 MHz, CDCl3): δ 8.52 (s, 2H, H1), 6.06 (s, 1H, H3), 3.78 (s, 3H, H6), 2.32 (s, 3H, H5). 13C{1H} NMR (100 MHz, CDCl3): δ 142.7 (C2), 141.2 (C4), 140.3 (C1), 96.2 (C3), 36.3 (C6), 11.3 (C5). HRMS (ESI, positive ions): m/z 349.1604 (calcd for [(2)2 + Na]+ 349.1613). Synthesis of 4-(1,5-Dimethyl-1H-pyrazol-3-yl)-1-methyl1,2,4-triazolium 3-I.

A sample of [5] (10 mg, 0.016 mmol) was dissolved in dichloromethane (5 mL) and HBF4·Et2O (5 μL, 0.02 mmol). The pale yellow solution darkened upon this addition. The volume of solvent was reduced to about 1 mL, and 10 mL of diethyl ether was added. An orange precipitate formed which was collected by filtration and washed with diethyl ether. Yield: 10 mg (0.014 mmol, 88%). 1H NMR (400 MHz, DMSO-d6): δ 9.62 (s, 1H, H2), 6.77 (s, 1H, H5), 4.06 (s, 3H, H3), 3.77 (s, 3H, H8), 2.54 (s, 3H, H7), 1.93 (s, 15H, H10). 13C{1H} NMR (100 MHz, DMSO-d6): δ 163.4 (C1), 146.7 (C6), 144.3 (C4), 139.6 (C2), 93.9 (C5), 92.8 (C9), 39.4 (C3), 37.4

Compound 2 (500 mg, 3.1 mmol) and iodomethane (500 mg, 3.5 mmol) were dissolved in acetonitrile (5 mL) and heated at 80 °C for 12 h. After cooling to ambient temperature, a small amount of white precipitate had formed. Compound [3]I was precipitated from the solution by addition of diethyl ether (10 mL). It was isolated by filtration as an off-white powder. Yield: 600 mg (1.97 mmol, 64%). 1H E


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Organometallics

X-ray Crystallography. X-ray diffraction data were collected at T = 100 K with a Bruker AXS APEXII CCD diffractometer equipped with a microsource using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Semiempirical multiscan absorption corrections were applied to all data sets.23 Structure solutions were found with SHELXT (intrinsic phasing)24 and were refined with SHELXL25 against |F2| of all data using first isotropic and later anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms were added to the structure models on calculated positions. All crystallographic data are available in the Supporting Information.

(C8), 12.3 (C7), 10.0 (C10). HRMS (ESI, positive ions): m/z 632.0859 (calcd for [4]+ 632.0856). Synthesis of [6]I.

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A sample of [5] (20 mg, 0.032 mmol) was dissolved in CD3CN (1 mL) and loaded into a Young NMR tube. The solvent was flashfrozen in liquid nitrogen, and the tube was evacuated and refilled with CO (2 bar). The NMR tube was then sealed and heated in an oil bath behind a blast shield at 80 °C for 2 days. (Caution: Closed systems under positive pressure are potentially explosive and should always be handled behind a blast shield while wearing appropriate safety equipment.) The reaction progress was monitored by NMR spectroscopy. After 2 days, about 99% conversion had been reached. The solvent was then removed, and [6]I was purified by recrystallization from CHCl3/pentane. Yield 19 mg (0.029 mmol, 90%). 1H NMR (400 MHz, CDCl3): δ 8.59 (s, 1H, H2), 4.28 (s, 3H, H3), 3.83 (s, 3H, H8), 2.40 (s, 3H, H7), 2.19 (s, 15H, H11). 13C{1H} NMR (100 MHz, CDCl3): δ 162.1 (C9), 152.8 (C4), 151.0 (C1), 142.4 (C6), 137.2 (C2), 102.1 (C10), 85.6 (C5) 41.0 (C3), 37.3 (C8), 12.7 (C7), 10.8 (C11). HRMS (ESI, positive ions): m/z 532.1682 (calcd for [6]+ 532.1689). IR (KBr): ν 2041 (s, CO). Synthesis of [7].

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00856. NMR spectra and crystallographic details for all new compounds (PDF) Accession Codes

CCDC 1880345−1880353 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tristan Tsai Yuan Tan: 0000-0001-5391-7232 F. Ekkehardt Hahn: 0000-0002-2807-7232

A sample of [6]I (10 mg) was dissolved in dichloromethane (1 mL) in an NMR tube and layered with n-hexane. Over the course of 2 weeks, orange crystals of [7] grew. The crystals were collected with a spatula and rinsed with n-hexane. Compound [7] underwent decarbonylation in solution. Therefore, the NMR spectra contained a mixture of [7] and some [5]. Only the resonances for [7] are listed here, but the resonances for both compounds are depicted in Figures S16 and S17. 1H NMR (400 MHz, CDCl3): δ 8.67 (s, 1H, H2), 4.01 (s, 3H, H3), 3.71 (s, 3H, H8), 2.49 (s, 3H, H7), 1.77 (s, 15H, H11). 13 C{1H} NMR (100 MHz, CDCl3): δ 207.1 (C9), 160.0 (C1), 140.4 (C4), 137.3 (C2), 136.7 (C6), 95.1 (C10), 89.0 (C5), 41.1 (C3), 35.9 (C8), 10.5(C7), 9.8 (C11). Synthesis of [8].

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Deutsche Forschungsgemeinschaft (SFB 858 and IRTG 2027). REFERENCES

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A sample of [5] (32 mg, 0.051 mmol) was dissolved in methanol (5 mL), and methyl propiolate (5 μL, 0.06 mmol) was added. The reaction mixture was stirred overnight. Over this period, an orange precipitate formed. The volume of solvent was reduced to about 1 mL, and the orange precipitate was collected by filtration. Yield 30 mg (0.042 mmol, 82%). 1H NMR (400 MHz, CDCl3): δ 8.35 (s, 1H, H2), 7.06 (s, 1H, H9), 4.15 (s, 3H, H3), 3.76 (s, 3H, H8), 3.64 (s, 3H, H12), 2.26 (s, 3H, H7), 1.62 (s, 15H, H14). 13C{1H} NMR (100 MHz, CDCl3): δ 180.1 (C11), 158.3 (C1), 141.3 (C2), 140.6 (C4), 138.0 (C6), 134.3 (C10), 124.8 (C9), 112.2 (C5), 91.8 (C13), 50.5 (C12), 44.0 (C3), 36.4 (C8), 9.5 (C7), 9.1 (C14). HRMS (ESI, positive ions): m/z 588.1949 (calcd for [8 − I]+ 588.1951). F


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