Synthesis of Iridium(III) and Rhodium(III) Complexes Bearing C8

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

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Synthesis of Iridium(III) and Rhodium(III) Complexes Bearing C8Metalated Theophylline Ligands by Directed C−H Activation Tristan Tsai Yuan Tan 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: The directed C−H activation at the C8 position of N-donor tethered theophylline with iridium(III) and rhodium(III) is presented. The donor strength of the Ntethered donor group has been varied. Proligands bearing a strongly donating imidazolin-2-ylidene or the weaker donating pyridine group were both metalated under similar conditions, suggesting that the electron density at the metal center does not play a significant role in the C−H activation step, which was concluded to proceed via a carboxylate-assisted route. The synthesis and characterization of iridium(III) and rhodium(III) complexes bearing chelating CNHC^Cazolato ligands (M = Ir: [4], M = Rh: [5]) and Npyridine^Cazolato ligands (M = Ir: [7], M = Rh: [8]) are reported. In addition, the NHC complexes which are the precursors to the CNHC^Cazolato complexes (M = Ir: [2], M = Rh: [3]) were isolated and characterized.

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for this field to gain traction. To date, complexes bearing Cmetalated azolato ligands of type C are generally accessible via deprotonation of pNHC ligands.7−15 In addition, complexes of type C can be synthesized directly, either via the oxidative addition of the C2−X (X = halide) bond of a neutral azole to a low valent metal16−18 or via the directed C−H activation of Ndonor substituted neutral azoles.19,20a,b Less common methods for the preparation of C-azolato complexes comprise the transmetalation of lithiated azoles20c and the reaction of metal azido complexes with isocyanides.20d The C−H activation in N-donor-functionalized azoles is of interest, as this reaction resembles the orthometalation.21−26 Various mechanisms for this C−H activation have been proposed27−31 including mechanisms for the C−H activation leading to pNHC complexes.4 On the basis of the established protocols for orthometalation, we sought to develop a synthetic route to complexes bearing azolato ligands. Contrary to classical NHCs with varying electronics,32−35 known pNHCs and C-metalated azolato ligands have mainly been obtained from imidazoles8,9 or benzimidazoles.7,10,14 Herein, we report the synthesis of IrIII and RhIII complexes bearing C8-metalated theophylline ligands via the activation of the C8−H bond of donor-functionalized theophylline. We also address the mechanism of the C8−H activation most likely proceeding a carboxylate-assisted reaction.

INTRODUCTION While the vast majority of N-heterocyclic carbenes (NHCs) employed in organometallic coordination chemistry act as spectator ligands (Figure 1, A) that do not participate directly

Figure 1. Complexes bearing classical NHC (A), pNHC (B), and Cmetalated azolato ligands (C).

in chemical transformations,1,2 the development of NHCs exhibiting a non-innocent or cooperative functionality has gained significant attention in recent years.3 One important class of such ligands are the protic NHCs (pNHCs) that feature an acidic proton at one of the nitrogen atoms of the diaminoheterocycle (Figure 1, B)4−6 and their deprotonated derivatives featuring a basic ring-nitrogen atom (Figure 1, C).4−6 Complexes of these ligands are of particular interest to us, since they have been shown to react cooperatively together with the metal center they coordinate to activate molecules such as dihydrogen,7−9 alkynes,9 and even carbon dioxide.10 Furthermore, complexes bearing pNHCs (B) or C-metalated azolato ligands (C) have been shown to be catalytically active in the hydrogenation of polar, unsaturated bonds.8,11−14 While complexes bearing C-metalated azolato ligands are an emerging class of compounds, their chemistry is still relatively unexplored compared to that of complexes bearing classical, “Arduengo” type NHCs. More generalized synthetic procedures for the preparation of such complexes and a better mechanistic understanding of their reactions are still required © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis of Complexes Bearing CNHC^Cazolato Ligands. We initially sought an N-donor-functionalized Received: March 18, 2019

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DOI: 10.1021/acs.organomet.9b00184 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

conditions, even after extended reaction times in the presence of an excess of NaOAc.

theophylline derivative. The donor in this derivative is required to bind to the metal center, allowing for a subsequent activation of the theophylline C8−H bond. An N-heterocyclic carbene was identified as a suitable donor. Consequently, we prepared the imidazolium/azole proligand 1-Cl. Compound 1Cl was synthesized in a three step sequence starting from theophylline (Scheme 1). Nucleophilic addition of theophyl-

Scheme 2. Synthesis of NHC Complexes [2] and [3]

Scheme 1. Synthesis of Proligand 1-Cl

Complexes [2] and [3] were both isolated and characterized by NMR spectroscopy, high resolution ESI mass spectrometry, and single crystal X-ray diffraction studies (see the Supporting Information for NMR spectra). Interestingly, the 1H NMR spectrum of complex [2] revealed that the two protons of the methylene bridge (C12) are diastereotopic and their resonances were detected as two doublets at δ 7.51 and 6.49 ppm (2JHH = 12.5 Hz). Considering that complex [2] does not feature any chiral center, the diastereotopicity of the methylene protons would imply a restricted rotation about the Ir−CNHC (C13) bond. This type of hindered rotation at ambient temperature might also explain that no activation of the C8−H bond occurs. Studies of related complexes suggest that the restricted rotation about the M−CNHC bonds is solely due to steric reasons.37,38 The 1H NMR spectrum of complex [3] shows that the protons of the methylene bridge are also diastereotopic and their resonances were detected as broad signals at δ = 7.70 and 6.52 ppm. These resonances, however, were too broad for the geminal coupling to be resolved. The 13C{1H} spectrum of complex [2] showed the resonance of the NHC carbon atom C13 coordinated to the iridium(III) center at δ 156.3 ppm, while the 13C{1H} spectrum of complex [3] exhibited the resonance of the NHC carbon atom coordinated to the rhodium(III) center as a doublet at δ 170.8 ppm ( 1 J CRh = 56.9 Hz). Upon deprotonation and metal coordination, the resonances for these carbon atoms are shifted significantly downfield compared to the C13 resonance in salt 1-Cl at δ 143.5 ppm. The 13C{1H} resonances for the theophylline C8 carbon atoms ([2]: δ = 144.7 ppm; [3]: δ = 144.9 ppm) are only slightly shifted downfield compared to the resonance for the theophylline C8 carbon atom in 1-Cl (δ = 137.4 ppm). Slow vapor diffusion of n-pentane into a saturated chloroform solution of the respective complexes yielded single crystals of composition [2]·CHCl3 and [3]·CHCl3. The molecular structures of both complexes are shown in Figure 2. The structure analyses revealed that no interaction between the C8−H bonds and the metal centers exists in both complexes. The Ir−CNHC bond length in [2] measures 2.047(3) Å, and the Rh−CNHC bond length in complex [3] measures 2.056(3) Å. These values fall in the typical range for Ir−CNHC and Rh−CNHC bond distances.20 Upon deprotonation and metalation, the N10−C13−N11 angles shrinks significantly to 103.4(3)° and 103.4(2)° for [2] and [3] compared to the magnitude of this angle observed in proligand 1-Cl (108.02(10)°; see the Supporting Information). This behavior is in accord with previous observations.2 The N7−

line to formaldehyde gave N7-hydroxymethyl theophylline in good yield.36 The reaction proceeded regioselectively at the N7 position, and no N9 alkylated product was observed. The subsequent reaction of N7-hydroxymethyl theophylline with thionyl chloride yielded 7-chloromethyl theophylline. The imidazolium/azole salt 1-Cl was finally obtained as a colorless solid from the reaction of 7-chloromethyl theophylline with Nmethylimidazole. Proligand 1-Cl was characterized by NMR spectroscopy, high resolution ESI-mass spectrometry, and single crystal X-ray diffraction (see the Supporting Information for NMR spectra and crystallography data of 1-Cl). The 1H NMR spectrum of 1-Cl (in DMSO-d6) features the resonances for the N−CH−N protons within the diaminoheterocycles at δ 9.41 (H13) and 8.62 ppm (H8) in accord with the cationic nature of the imidazolium ring. Similarly, the resonances for the carbon atoms C13 and C8 were detected in the 13C{1H} NMR spectrum at δ 143.5 (C13) and 137.4 ppm (C8), respectively. The molecular structure determination reveals an N−C13−N angle of 108.02(10)°, which is typical for imidazolium cations,2 while the N−C8−N angle of 112.94(9)° is significantly larger (see the Supporting Information). Both the imidazolium moiety and the neutral theophylline moiety of proligand 1-Cl could undergo N−CH−N deprotonation. However, it can be expected that the deprotonation of the imidazolium group proceeds much faster than that of the azole group due to the cationic nature of the former. Deprotonation of the imidazolium group of 1-Cl generates an NHC which could bind to a metal center and act as a directing group to bring the metal into proximity of the theophylline C8−H bond. Therefore, we attempted to regioselectively metalate the imidazolium group of 1-Cl. Reaction of 1 equiv of proligand 1-Cl with 0.5 equiv of either [IrCl2Cp*]2 or [RhCl2Cp*]2 and an excess of NaOAc (4 equiv) in acetonitrile at ambient temperature proceeded under selective deprotonation of the imidazolium moiety and coordination of the resulting imidazolin-2-ylidene to the metal center, giving the complexes [2] and [3] in good yields (Scheme 2). No activation of the C8−H bond of the theophylline group was detected under these reaction B


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Scheme 3. Synthesis of CNHC^Cazolato Chelate Complexes [4] and [5]

acetate assisted deprotonation/metalation reaction mechanism.31b Formation of the CNHC^Cazolato chelate complexes was confirmed by NMR spectroscopy, high resolution ESI mass spectrometry, and single crystal X-ray crystallography. The 1H NMR spectrum of [4] exhibited resonances of the diastereotopic protons of the methylene bridge as sharp doublets at δ 6.79 and 5.52 ppm (2JHH = 12.0 Hz). The diastereotopic methylene protons of [5] were detected in the 1H NMR spectrum as doublets at δ 6.84 and 5.54 ppm (2JHH = 12.5 Hz). The 13C{1H} NMR spectrum of [4] exhibited the resonance of the azolato carbon atom of the theophylline moiety C8 at δ 159.4 ppm and that of the NHC carbon atom C13 at δ 153.4 ppm. The corresponding 13C{1H} NMR resonances for [5] were observed as doublets at δ 177.1 ppm (C8, 1JCRh = 45.8 Hz) and at δ 171.8 (C13, 1JCRh = 53.1 Hz). The 13C NMR resonances were unambiguously assigned by means of heteronuclear correlation 2D NMR experiments (see the Supporting Information). The molecular structures of complexes [4] and [5] were determined by single crystal X-ray diffraction. Slow vapor diffusion of n-pentane into saturated chloroform solutions of the complexes yielded single crystals of composition [4]· 1.5CHCl3·H2O and [5]·1.5CHCl3·H2O, respectively. The results of the structure determinations are depicted in Figure 3. The structure analyses revealed a significant decrease in the M−CNHC bond lengths upon formation of the six-membered chelate rings (Ir1−C13 2.047(3) Å for [2] to Ir1−C13 2.008(2) Å for [4] and Rh1−C13 2.056(3) Å for [3] to Rh1− C13 2.003(2) Å for [5]). In addition, the M−CNHC bond lengths ([4]: 2.008(2) Å, [5]: 2.003(2) Å) are slightly shorter than the M−Cazolato bond lengths ([4]: 2.022(2) Å, [5]: 2.016(2) Å).20a The shorter Rh−CNHC bond in comparison to the Rh−Cazolato bond in the solid-state molecular structures is in agreement with the observation of a higher 1JCRh coupling constant for the Rh−CNHC resonance (1JCRh = 53.1 Hz) compared to the Rh−Cazolato resonance (1JCRh = 45.8 Hz) in the 13C{1H} NMR spectrum. The additional metric parameters in [4] and [5] fall in the range previously observed for related complexes. While the C13−N distances in the NHC heterocycle are, as expected, almost identical, similar behavior for the C18−N distances was noted for the azolato ligand. This might be attributed to the electron-poor nature of the theophylline derived azolato ligand, while azolato ligands obtained from more electron-rich precursors such as adenine normally feature distinctly different N−Cazolato separations.18 ́ 5 Grotjahn,8,9 Previously, studies by Kuwata and Ikaryiya, 14,15a 4,7,19 Cossiart, and by us have demonstrated that, in the absence of a base, donor-tethered azoles can tautomerize to pNHC ligands in the presence of a metal center at elevated

Figure 2. Molecular structures of complexes [2] in [2]·CHCl3 (top) and [3] in [3]·CHCl3 (bottom). Ellipsoids are drawn at 50% probability, and H atoms have been omitted for clarity. Selected bond lengths [Å] and angles [deg]: [2] [[3]]: M−Cl1 2.4214(8) [2.4206(7)], M−Cl2 2.4254(7) [2.4195(7)], M−C13 2.047(3) [2.056(3)], range M−C Cp* 2.144(3)−2.245(3) [2.143(3)− 2.243(3)], N7−C8 1.352(5) [1.357(4)], N9−C8 1.329(4) [1.332(4)], N10−C13 1.366(4) [1.370(3)], N11−C13 1.359(4) [1.355(4)]; Cl1−M−Cl2 85.30(3) [87.32(3)], Cl1−M−C13 93.78(9) [92.61(8)], Cl2−M−C13 91.59(9) [94.88(8)], N7−C8− N9 113.2(3) [113.6(3)], N10−C13−N11 103.4(3) [103.4(2)].

C8−N9 angles in [2] and [3] are not affected by the C13 metalation and are almost unchanged from the value observed in the proligand 1-Cl. At this point, it can be concluded that, even in the presence of an excess of NaOAc, only the imidazolium moiety of proligand 1-Cl is deprotonated and metalated at ambient temperature. The restricted rotation about the M−CNHC bond in both [2] and [3], as demonstrated by NMR spectroscopy and X-ray diffraction studies, may also contribute to the lack of activation observed for the C8−H bond. We assumed that elevated temperatures may enable rotation about the M−C13 bond and bring the theophylline moiety into closer proximity to the metal center. Metalation of the theophylline C8 carbon atom might then proceed by oxidative addition of the C8−H bond as previously described for related complexes19 or, in the presence of NaOAc, by an acetate assisted reaction.20a Indeed, heating of a mixture of proligand 1-Cl, [MCl2Cp*]2 (M = Ir, Ru), and an excess of NaOAc for 12 h at 80 °C resulted in the formation of complexes [4] and [5] bearing a CNHC^Cazolato chelate ligand. Under these reaction conditions and in contrast to the conditions leading to [2] and [3], both the C13−H bond of the imidazolium moiety and the C8−H bond of the theophylline moiety are activated and both diaminoheterocycles are metalated in excellent yields (Scheme 3). No protonation of the ring-nitrogen atom of the theophylline group was observed. This together with the presence of an excess of NaOAc during the synthesis indicates to us that the possible oxidative addition of the C8−H bond plays no role and complexes [4] and [5] are formed by an C


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effect on the formation of the theophylline derived azolato donor. Synthesis of Complexes Bearing a Npyridine^Cazolato Chelate Ligand. The pyridine tethered proligand 6 was synthesized by alkylation of the N7 position of theophylline with 2 (bromomethyl)pyridine (Scheme 4). The alkylation of Scheme 4. Synthesis of Ligand Precursor 6

theophylline was regioselective, and no N9 alkylated product was observed. Proligand 6 was isolated in moderate yield and characterized by NMR spectroscopy, single crystal X-ray diffraction, and high resolution ESI mass spectrometry (see the Supporting Information for NMR spectra and crystallography data). Using reaction conditions similar to those employed for the preparation of the CNHC^Cazolato chelate complexes [4] and [5], the nitrogen atom and the C8 position of proligand 6 were readily metalated and iridium(III) complex [7] and rhodium(III) complex [8] were isolated in excellent yields (Scheme 5).

Figure 3. Molecular structures of complexes [4] in [4]·1.5CHCl3· H2O (top) and [5] in [5]·1.5CHCl3·H2O (bottom). Ellipsoids are drawn at 50% probability, and H atoms have been omitted for clarity. Selected bond lengths [Å] and angles [deg]: [4] [[5]]: M−Cl1 2.4408(5) [2.4403(5)], M−C8 2.022(2) [2.016(2)], M−C13 2.008(2) [2.003(2)], range M−C Cp * 2.139(2)−2.256(2) [2.134(2)−2.253(2)], N7−C8 1.365(2) [1.365(2)], N9−C8 1.361(3) [1.357(2)], N10−C13 1.356(3) [1.351(3)], N11−C13 1.353(3) [1.356(3)]; Cl1−M−C8 91.02(5) [95.15(5)], Cl1−M− C13 90.90(6) [92.59(6)], C8−M−C13 85.17(8) [85.19(8)], N7− C8−N9 109.5(2) [109.4(2)], N10−C13−N11 104.3(2) [104.6(2)].

Scheme 5. Synthesis of Complexes Bearing N^C Chelating Ligands [7] and [8]

temperatures. One proposed mechanism for this tautomerization comprises the oxidative addition of the NC−HN bond of the azole to the metal center, followed by the reductive elimination of a proton4,19 which protonates the azolato ringnitrogen atom to give a pNHC ligand. In the present study, the presence of sodium acetate in the reaction mixture allows for a “redox-neutral” mechanism,25 where the two basic oxygen atoms of the acetate ligand provide a suitable route for the proton transfer. Previously, studies have shown that the C2 position of an electron-deficient dichloroazole could be deprotonated cleanly in the presence of sodium acetate and rhodium(III).20a In order to support our hypothesis that the theophylline C8−H bond was activated by an acetate-assisted mechanism, a weaker σ donor could be attached to the theophylline. If this substitution does not affect the formation of the azolato ligand, any oxidative addition pathway could be ruled out. Therefore, a pyridine tethered theophylline was synthesized and the C8 metalation of the theophylline moiety was subsequently attempted. Since pyridine is a much weaker donor than imidazoline-2-ylidene,32,35 the C8−H oxidative addition should now require harsher reaction conditions. Conversely, the acetate assisted C−H activation has been shown to be facilitated by various metal centers with varying electronics,28,29 and hence the pyridine tether should have no

Interestingly, six-membered cyclometalated rings with nitrogen donors are quite rare,21,24 and [RhCl2Cp*]2 was previously found not to react at all with 2 benzylpyridine.24 The higher acidity of the C8−H bond of theophylline could account for the formation of complexes [7] and [8] obtained under reaction condition where other aryl C−H bonds are unreactive. Complexes [7] and [8] were characterized by NMR spectroscopy, high resolution ESI mass spectrometry, and single crystal X-ray diffraction. The 13C{1H} spectrum of [7] showed the resonance of the azolato C8 carbon atom at δ 164.9 ppm, downfield shifted compared to the resonance of the C8 carbon in [4] (δ 159.4 ppm). This downfield shift reflects the higher acidity of the iridium(III) center coordinated by the pyridine donor in [7] compared to that in [4] coordinated by the more electron-rich NHC donor.32 For the rhodium complex [8], the resonance of the theophylline C8 carbon atom was observed at δ 178.7 ppm (1JCRh = 44.6 Hz). The difference in the chemical shifts of the C8 carbon resonance between [8] and [5] is smaller than that for the iridium complexes [7] and [4], most likely due to the reduced acidity of RhIII compared to IrIII.24 In addition, a 2JCRh coupling constant of 1.5 Hz was observed for the resonances of the ortho pyridine carbon atom C17 in rhodium complex [8]. D


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synthesis of complexes bearing azolato ligands with varying electronics at the metal center.

Single crystals of composition [7]·CHCl3 and [8]·CHCl3 were grown by vapor diffusion of n-pentane into a saturated chloroform solution of the respective complexes. The molecular structures of the complexes are shown in Figure 4.

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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 at 298 K on Bruker AVANCE I 400 or Bruker AVANCE III 400 spectrometers. Chemical shifts (δ) are expressed in ppm downfield from SiMe4 using the residual protonated solvent signal as an internal standard. Coupling constants are expressed in Hz. For the assignments of the NMR resonances, see the numbering at the molecular plots. Mass spectra were obtained with an Orbitrap LTQ XL spectrometer (Thermo Scientific). Compounds N7hydroxymethyltheophylline,36 [IrCp*Cl2]2,39 and [RhCp*Cl2]240 were prepared as previously described. Synthesis of N7-Chloromethyltheophylline.

Thionyl chloride (10.5 mL, 145 mmol) was added to a suspension of N7-hydroxymethyltheophylline (5.05 g, 24.0 mmol) in dry dichloromethane (60 mL). The reaction mixture turned into a colorless solution upon addition of the thionyl chloride. The mixture was stirred for 12 h. Over this period, a white solid precipitated from the initially clear solution. Water (100 mL) was then added slowly to quench residual thionyl chloride, and the mixture was neutralized by slowly adding Na2CO3 until the pH was neutral. The N7chloromethyltheophylline was extracted with dichoromethane (3 × 20 mL). The combined organic fractions were dried over anhydrous magnesium sulfate and the solvent was removed under reduced pressure to give N7-chloromethyltheophylline as a colorless solid. Yield: 5.232 g (22.9 mmol, 95%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.79 (s, 1H, H8), 6.07 (s, 2H, H12), 3.58 (s, 3H, H11), 3.40 (s, 3H, H10). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 154.8 (C6), 151.5 (C2), 149.2 (C4), 141.7 (C8), 105.8 (C5), 52.1 (C12), 29.9 (C11), 28.0 (C10). MS (EI, 20 eV): m/z (%) = 228 (100) [M]+. Anal. Calcd: C, 42.02; H, 3.97; N, 24.51%. Found: C, 42.23; H, 3.88; N, 24.66%. Synthesis of 1-Cl.

Figure 4. Molecular structure of [7] in [7]·CHCl3 (top) and [8] in [8]·CHCl3 (bottom). Ellipsoids are drawn at 50% probability, and H atoms have been omitted for clarity. Selected bond lengths [Å] and angles [deg]: [7] [[8]]: M−Cl 2.4083(12) [2.4017(13)], M−N10 2.111(4) [2.108(5)], M−C8 2.001(5) [1.986(5)], range M−CCp* 2.155(5)−2.249(5) [2.156(5)−2.248(5)], N7−C8 1.369(6) [1.363(7)], N9−C8 1.355(7) [1.353(7)]; Cl−M−N10 87.65(11) [89.91(12)], Cl−M−C8 88.06(14) [88.89(15)], N10−M−C8 87.18(18) [87.6(2)], N7−C8−N9 111.5(4) [112.2(5)].

The structure analyses showed that the M−C8 bond lengths in [7] and [8] (2.001(5) and 1.986(5) Å) are shorter than the M−C8 bond lengths in the imidazoline-2-ylidene tethered complexes [4] and [5] (2.022(2) and 2.016(2) Å). Together with the 13C{1H} NMR data, we take this as an additional indication for a higher acidity of the metal centers in the C^N chelate complexes [7] and [8] compared to the C^C chelate complexes [4] and [5]. Considering that the strong σ-donating NHC and the weaker σ-donating pyridine group had little effect on the reaction conditions required for the metalation of the C8 position of theophylline, it appears that the electron richness of the metal center plays no role in the formation of the C^N and C^C chelate complexes. This makes an oxidative addition scenario for the formation of these complexes very unlikely.31b

A sample of N7-chloromethyltheophylline (2.3 g, 10.1 mmol) was suspended in dichloromethane (5 mL), and 1-methylimidazole (1.2 g, 15.0 mmol) was added. The suspension was stirred at ambient temperature for 1 h. The solvent was then removed under reduced pressure to give a colorless powder. The powder was washed with a small amount of acetone (3 × 2 mL) and diethyl ether (10 mL) to remove excess 1-methylimidazole and dried under vacuum to give 1Cl. Yield: 3.0 g (9.7 mmol, 97%). 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 9.41 (s, 1H, H13), 8.62 (s, 1H, H8), 7.97 (d, 3JHH = 1.8 Hz, 1H, H14), 7.74 (d, 3JHH = 1.8 Hz, 1H, H15), 6.71 (s, 2H, H12), 3.87 (s, 3H, H16), 3.43 (s, 3H, H11), 3.25 (s, 3H, H10). 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) = 154.5 (C6), 150.8 (C2), 148.7 (C4), 143.5 (C13), 137.4 (C8), 124.1 (C14), 121.6 (C15), 105.2 (C5), 56.2 (C12), 36.0 (C16), 29.5 (C11), 27.6 (C10). HRMS (ESI, positive ions): m/z (%) = 275.1252 (100, calculated for [1]+ 275.1256).

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CONCLUSION Iridium(III) and rhodium(III) complexes bearing donor tethered C-metalated azolato donors have been obtained by the acetate-assisted, C−H activation of the C8 position of theophylline. The acetate-assisted C−H activation methodology can possibly be extended to other metals that mediate C−H activations, such as palladium and ruthenium.28,29 In addition, we have demonstrated that the nature of the Ntethered donor can be varied from a strongly donating NHC to a weaker donating pyridine group, thus allowing for the E


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Organometallics Synthesis of [2].

from dark orange to light orange during this time. The solvent was removed under reduced pressure, and chloroform (5 mL) was added. The resulting suspension was filtered through Celite, and the residue was rinsed with 5 mL of chloroform. The solvent of the combined organic phases was removed under reduced pressure to give [4] as a light orange solid. Yield: 16 mg (0.025 mmol, 96%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.12 (d, 3JHH = 2.0 Hz, 1H, H14), 7.01 (d, 3 JHH = 2.0 Hz, 1H, H15), 6.79 (d, 2JHH = 12.0 Hz, 1H, H12), 5.52 (d, 2 JHH = 12.0 Hz, 1H, H12), 3.89 (s, 3H, H16), 3.63 (s, 3H, H11), 3.37 (s, 3H, H10), 1.78 (s, 15H, H18). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 159.4 (C8), 154.1 (C6), 153.4 (C13), 151.7(C2), 151.4 (C4), 122.4 (C15), 120.3 (C14), 107.1 (C5), 92.4 (C17), 58.7 (C12), 37.2 (C16), 30.1(C11), 27.6 (C10), 9.5 (C18). HRMS (ESI, positive ions): m/z (%) = 637.1656 (100, calculated for for [[4] + H]+ = 637.1662). Synthesis of [5].

A mixture of 1-Cl (8 mg, 0.026 mmol), [IrCl2Cp*]2 (10 mg, 0.013 mmol), and sodium acetate (10 mg, 0.12 mmol) in acetonitrile (10 mL) was stirred for 12 h at ambient temperature. The solvent was then removed under reduced pressure, and chloroform (5 mL) was added. The resulting suspension was filtered through Celite, and the residue was rinsed with chloroform (5 mL). The solvent of the combined organic phases was removed under reduced pressure to give [2] as a dark orange solid. Yield: 15 mg (0.022 mmol, 85%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.90 (s, 1H, H8), 7.51 (d, 2JHH = 12.5 Hz, 1H, H12), 7.17 (d, 3JHH = 2.2 Hz, 1H, H14), 6.95 (d, 3JHH = 2.2 Hz, 1H, H15), 6.49 (d, 2JHH = 12.5 Hz, 1H, H12), 4.01 (s, 3H, H16), 3.61 (s, 3H, H11), 3.46 (s, 3H, H10), 1.64 (s, 15H, H18). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 156.3 (C13), 155.9 (C6), 151.4 (C2), 149.3 (C4), 144.7 (C8), 124.8 (C15), 120.9 (C14), 105.9 (C5), 89.4 (C17), 57.6 (C12), 38.8(C16), 30.0(C11), 28.0(C10), 9.1 (C18). HRMS (ESI, positive ions): m/z (%) = 637.1646 (100, calculated for [[2] − Cl]+ = 637.1662). Synthesis of [3].

A mixture of 1-Cl (10 mg, 0.032 mmol), [RhCl2Cp*]2 (10 mg, 0.016 mmol), and sodium acetate (10 mg, 0.12 mmol) in acetonitrile (10 mL) was stirred for 12 h at 80 °C. The color of the solution changed from dark orange to light orange after the reaction was complete. The solvent was removed under reduced pressure, and 5 mL of chloroform was added. The resulting suspension was filtered through Celite, and the residue was rinsed with 5 mL of chloroform. The solvent of the combined organic phases was removed under reduced pressure to give [5] as a light orange solid. Yield: 17 mg (0.029 mmol, 91%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.15 (d, 3JHH = 2.0, 1H, H14), 7.03 (d, 3JHH = 2.0, 1H, H15), 6.84 (d, 2JHH = 12.5 Hz, 1H, H12), 5.55 (d, 2 JHH = 12.5 Hz, 1H, H12), 3.95 (s, 3H, H16), 3.65 (s, 3H, H11), 3.36 (s, 3H, H10), 1.71 (s, 15H, H18). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 177.1 (d, 1JCRh = 45.8 Hz, C8), 171.8 (d, 1JCRh = 53.1 Hz, C13), 153.9 (C6), 151.7 (C2), 151.0 (d, 3JCRh = 1.9 Hz, C4), 123.1 (C15), 121.2 (C14), 107.7 (C5), 98.9 (d, 1JCRh = 5.1 Hz, C17), 58.4 (C12), 37.8 (C16), 30.1 (C11), 27.5 (C10), 9.7 (C18). HRMS (ESI, positive ions): m/z (%) = 547.1089 (100, calculated for [[5] + H]+ = 547.1096). Synthesis of 6.

A mixture of 1-Cl (10 mg, 0.032 mmol), [RhCl2Cp*]2 (10 mg, 0.016 mmol), and sodium acetate (10 mg, 0.12 mmol) in acetonitrile (10 mL) was stirred for 12 h at ambient temperature. The solvent was then removed under reduced pressure, and chloroform (5 mL) was added. The resulting suspension was filtered through Celite, and the residue was rinsed with chloroform (5 mL). The solvent of the combined organic phases was removed under reduced pressure to give [3] as a dark orange solid. Yield: 15 mg (0.026 mmol, 81%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.94 (s, 1H, H8), 7.70 (br, 1H, H12), 7.29 (d, 3JHH = 1.7 Hz, 1H, H14), 7.02 (d, 3JHH = 1.7 Hz, 1H, H15), 6.52 (br, 1H, H12), 4.07 (s, 3H, H16), 3.61 (s, 3H, H11), 3.46 (s, 3H, H10), 1.63 (s, 15H, H18). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 170.8 (d, 1JCRh = 56.9 Hz, C13), 155.9 (C6), 151.4 (C2), 149.3 (C4), 144.9 (C8), 125.8 (C15), 122.1 (C14), 105.9 (C5), 96.8 (d, 1JCRh = 7.0 Hz, C17), 57.5 (C12), 39.3 (C16), 30.0 (C11), 28.1 (C10), 9.4 (C18). HRMS (ESI, positive ions): m/z (%) = 547.1089 (100, calculated for [[3] − Cl]+ = 547.1095). Synthesis of [4].

Theophylline (1.0 g, 5.6 mmol) and potassium carbonate (3.0 g, 22 mmol) were suspended in N,N-dimethylformamide (10 mL) and stirred for 3 h at room temperature. Subsequently, 2-(bromomethyl)pyridine hydrobromide (1.5 g, 5.9 mmol) was then added to the suspension. The mixture immediately turned from colorless to pink. The reaction mixture was stirred for 12 h at ambient temperature. The volume of N,N-dimethylformamide was then reduced to about 1 mL under reduced pressure, and water (10 mL) was added, causing 6 to precipitate as a white powder. Compound 6 was collected by filtration, rinsed with a small amount of water (10 mL), and dried thoroughly under reduced pressure. Yield: 1.0 g (3.7 mmol, 66%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.55 (d, 3JHH = 4.9, 1H, H17), 7.83 (s, 1H, H8), 7.69 (td, 3JHH = 7.7, 4JHH 1.5 Hz, 1H, H15), 7.44 (d, 3 JHH = 7.7 Hz, 1H, H14), 7.24 (dd, 3JHH = 7.7, 3JHH = 4.9 Hz, 1H, H16), 5.59 (s, 2H, H12), 3.58 (s, 3H, H11), 3.38 (s, 3H, H10). 13 C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 155.3 (C6), 154.5

A mixture of 1-Cl (8 mg, 0.026 mmol), [IrCl2Cp*]2 (10 mg, 0.013 mmol), and sodium acetate (10 mg, 0.12 mmol) in acetonitrile (10 mL) was stirred for 12 h at 80 °C. The color of the solution changed F


Article

Organometallics

solutions were found with SHELXT (intrinsic phasing)42a and were refined with SHELXL42b 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. Crystal Data for [2]·CHCl3. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of n-pentane into a concentrated solution of [2] in chloroform. Formula C23H30N6Cl5IrO2, M = 791.98, orange block, 0.24 × 0.20 × 0.10 mm3, triclinic, space group P1̅, a = 8.6332(2), b = 8.6606(2), c = 21.2728(6) Å, α = 79.081(2), β = 80.155(2), γ = 61.5070(1)°, V = 1366.58(6) Å3, Z = 2, rcalcd = 1.925 g·cm−3, μ = 5.409 mm−1, 47 306 intensities measured in the range 3.9° ≤ 2Θ ≤ 63.1°, 8925 independent intensities (Rint = 0.0641), 8324 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.375 ≤ T ≤ 0.746), refinement of 342 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0304, wR = 0.0719 [I ≥ 2σ(I)], R = 0.0337, wR = 0.0734 (all data). The asymmetric unit contains one formula unit of [2] and one molecule of CHCl3. Crystal Data for [3]·CHCl3. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of n-pentane into a concentrated solution of [3] in chloroform. Formula C23H30N6Cl5O2Rh, M = 702.69, orange block, 0.32 × 0.20 × 0.04 mm3, triclinic, space group P1̅, a = 8.6070(3), b = 8.6109(3), c = 21.2868(6) Å, α = 79.266(2), β = 80.161(2), γ = 61.520(2)°, V = 1356.30(8) Å3, Z = 2, rcalcd = 1.721 g·cm−3, μ = 1.157 mm−1, 109 684 intensities measured in the range 5.6° ≤ 2Θ ≤ 66.4°, 10 371 independent intensities (Rint = 0.0796), 8397 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.608 ≤ T ≤ 0.746), refinement of 342 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0467, wR = 0.1161 [I ≥ 2σ(I)], R = 0.0640, wR = 0.1266 (all data). The asymmetric unit contains one formula unit of [2] and one molecule of CHCl3. Crystal Data for [4]·1.5CHCl3·H2O. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of n-pentane into a concentrated solution of [4] in chloroform. Formula C23.5H31.5N6Cl5.5IrO3, M = 833.22, yellow block, 0.40 × 0.30 × 0.27 mm3, triclinic, space group P1̅, a = 9.2353(4), b = 11.6685(4), c = 15.5801(6) Å, α = 94.579(2), β = 106.367(2), γ = 108.705(2)°, V = 1498.82(10) Å3, Z = 2, rcalcd = 1.846 g·cm−3, μ = 4.982 mm−1, 106 350 intensities measured in the range 4.8° ≤ 2Θ ≤ 73.4°, 14 346 independent intensities (Rint = 0.0364), 13 029 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.388 ≤ T ≤ 0.747), refinement of 378 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0279, wR = 0.0602 [I ≥ 2σ(I)], R = 0.0333, wR = 0.0621 (all data). The asymmetric unit contains one formula unit of [4], one CHCl3 molecule (SOF = 1), one CHCl3 molecule (SOF = 1/2), and one water molecule. Crystal Data for [5]·1.5CHCl3·H2O. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of n-pentane into a concentrated solution of [5] in chloroform. Formula C23.5H31.5N6Cl5.5O3Rh, M = 743.93, orange block, 0.40 × 0.04 × 0.02 mm3, triclinic, space group P1̅, a = 9.2524(3), b = 11.6644(4), c = 15.5429(5) Å, α = 94.321(2), β = 106.716(2), γ = 108.667(2)°, V = 1495.92(9) Å3, Z = 2, rcalcd = 1.652 g·cm−3, μ = 1.100 mm−1, 117 406 intensities measured in the range 3.8° ≤ 2Θ ≤ 66.3°, 11 353 independent intensities (Rint = 0.0681), 9622 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.621 ≤ T ≤ 0.747), refinement of 384 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0375, wR = 0.0888 [I ≥ 2σ(I)], R = 0.0489, wR = 0.0970 (all data). The asymmetric unit contains one formula unit of [4], one CHCl3 molecule (SOF = 1), one CHCl3 molecule (SOF = 1/2), and one water molecule. Crystal Data for [7]·CHCl3. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of n-pentane into a concentrated solution of [7] in chloroform. Formula C24H28N5Cl4-

(C17), 151.7 (C2), 149.6 (C13), 148.9 (C4), 141.9 (C8), 137.5 (C15), 123.5 (C16), 122.9 (C14), 106.7 (C5), 51.3 (C12), 29.8 (C11), 28.0 (C10). HRMS (ESI, positive ions): m/z (%) = 272.1140 (100, calculated for [6 + H]+ = 272.1147). Synthesis of [7].

A sample of compound 6 (14 mg, 0.05 mmol) and [IrCp*Cl2]2 (20 mg, 0.025 mmol) were dissolved in acetonitrile (10 mL). Sodium acetate (20 mg, 0.24 mmol) was added, and the mixture was heated to 80 °C for 12 h. After cooling to ambient temperature, the solvent was removed under reduced pressure and chloroform (10 mL) was added. The suspension was filtered through Celite, and the residue was rinsed with additional chloroform (5 mL). The solvent from the combined organic phases was removed under reduced pressure to give [7] as a brown-orange solid. Yield: 25 mg (0.04 mmol, 80%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.00 (dd, 3JHH = 5.8, 4JHH = 1.6 Hz, 1H, H17), 7.80 (td, 3JHH = 7.5, 4JHH = 1.6 Hz, 1H, H15), 7.53 (d, 3 JHH = 7.4 Hz, 1H, H14), 7.30 (t, 3JHH = 7.5, H16), 6.25 (d, 2JHH = 15.5 Hz, 1H, H12), 4.66 (d, 2JHH = 15.5 Hz, 1H, H12), 3.67 (s, 3H, H11), 3.39 (s, 3H, H10), 1.69 (s, 15H, H19). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 164.9 (C8), 157.5 (C17), 156.1 (C13), 154.4 (C6), 151.8 (C2), 151.8 (C4), 139.0 (C15), 125.7 (C16), 124.5 (C14), 107.9 (C5), 89.9 (C18), 50.2 (C12), 30.3 (C11), 27.6 (C10), 9.2 (C19). HRMS (ESI, positive ions): m/z (%) = 598.1789 (100, calculated for [[7] − Cl]+ = 598.1794). Synthesis of [8].

A sample of compound 6 (18 mg, 0.066 mmol) and [RhCp*Cl2]2 (20 mg, 0.032 mmol) were dissolved in acetonitrile (10 mL). Sodium acetate (20 mg, 0.24 mmol) was added, and the mixture was heated to 80 °C for 12 h. After cooling to ambient temperature, the solvent was removed under reduced pressure and chloroform (10 mL) was added. The suspension was filtered through Celite, and the residue was rinsed with additional chloroform (5 mL). The solvent from the combined organic phases was removed under reduced pressure to give [8] as a dark red solid. Yield: 32 mg (91%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.07 (dd, 3JHH = 5.7, 4JHH= 1.6 Hz, 1H, H17), 7.82 (td, 3JHH = 7.5, 4JHH = 1.6 Hz, 1H, H15), 7.50 (d, 3JHH = 7.5 Hz, 1H, H14), 7.36 (td, 3JHH = 7.5, 4JHH = 1.6 Hz, 1H, H16), 6.30 (d, 2 JHH = 15.6 Hz, 1H, H12), 4.73 (d, 2JHH = 15.6 Hz, 1H, H12), 3.66 (s, 3H, H11), 3.39 (s, 3H, H10), 1.64 (s, 15H, H19). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) = 178.7 (d, 1JCRh = 44.6 Hz, C8), 156.6 (d, 2JCRh = 1.5 Hz, C17), 156.4 (C13), 154.3 (C6), 151.8 (C2), 151.2 (C4), 138.8 (C15), 125.0 (C16), 124.7 (C14), 108.8 (C5), 97.3 (d, 1 JCRh = 6.5 Hz, C18), 49.4 (C12), 30.1 (C11), 27.6 (C10), 9.4 (C19). HRMS (ESI, positive ions): m/z (%) = 544.0981 (100, calculated for [[8] − Cl]+ = 544.0986). X-ray Crystallography. Single crystals of [2]·CHCl3, [3]·CHCl3, [4]·1.5CHCl3·H2O, [5]·1.5CHCl3·H2O, [7]·CHCl3, and [8]·CHCl3 were analyzed by X-ray diffraction, and the data are presented below (for crystallographic data of 1-Cl and 6, see the Supporting Information). X-ray diffraction data were collected at T = 100(2) K with a Bruker AXS APEX II CCD or a Bruker PHOTON II CPAD diffractometer equipped with a microsource using graphite-monochromated Mo Kα radiation (l = 0.71073 Å). Semiempirical multiscan absorption corrections were applied to all data sets.41 Structure G


Article

Organometallics IrO2, M = 752.51, orange needle, 0.27 × 0.09 × 0.08 mm3, monoclinic, space group Pc, a = 11.8977(2), b = 13.6971(2), c = 8.4702(1) Å, β = 102.2010(10)°, V = 1349.16(3) Å3, Z = 2, rcalcd = 1.852 g·cm−3, μ = 5.376 mm−1, 32 723 intensities measured in the range 4.6° ≤ 2Θ ≤ 62.0°, 8532 independent intensities (Rint = 0.0193), 8451 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.546 ≤ T ≤ 0.746), refinement of 332 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0281, wR = 0.0699 [I ≥ 2σ(I)], R = 0.0284, wR = 0.0701 (all data). The asymmetric unit contains one formula unit of [7] and one CHCl3 molecule. Crystal Data for [8]·CHCl3. Crystals suitable for an X-ray diffraction study were obtained by slow diffusion of n-pentane into a concentrated solution of [8] in chloroform. Formula C24H28N5Cl4O2Rh, M = 663.25, orange needle, 0.40 × 0.10 × 0.10 mm3, monoclinic, space group Pc, a = 11.9246(5), b = 13.6863(6), c = 8.4464(1) Å, β = 102.3500(10)°, V = 1346.58(10) Å3, Z = 2, rcalcd = 1.636 g·cm−3, μ = 1.063 mm−1, 27 729 intensities measured in the range 4.6° ≤ 2Θ ≤ 61.2, 8193 independent intensities (Rint = 0.0474), 7618 observed intensities [I ≥ 2σ(I)], semiempirical absorption correction (0.625 ≤ T ≤ 0.850), refinement of 332 parameters against all |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions, R = 0.0492, wR = 0.0533 [I ≥ 2σ(I)], R = 0.1199, wR = 0.1220 (all data). The asymmetric unit contains one formula unit of [8] and one CHCl3 molecule.

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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.9b00184. Crystallographic data for 1-Cl, 6 and NMR spectra of all new complexes (PDF) Accession Codes

CCDC 1899999−1900006 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 Notes

The authors declare no competing financial interest.

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

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REFERENCES

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Synthesis of Iridium(III) and Rhodium(III) Complexes Bearing C8

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