Synthesis, NMR, and X-ray Studies of Iridium Dihydride C,N and N,P

Feb 26, 2016 - The iridium(III) dihydride complexes [Ir(H)2(L1)(6,6′-bi-2-picoline)]BArF (5; L1 = (S)-1-[2-(2-adamantan-2-yl-4,5-dihydrooxazol-4-yl)...
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Synthesis, NMR, and X‑ray Studies of Iridium Dihydride C,N and N,P Ligand Complexes Stefan Gruber†,* Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland S Supporting Information *

ABSTRACT: The iridium(III) dihydride complexes [Ir(H)2(L1)(6,6′-bi-2-picoline)]BArF (5; L1 = (S)-1-[2-(2adamantan-2-yl-4,5-dihydrooxazol-4-yl)-ethyl]-3-(2,6diisopropylphenyl)-1,2-dihydroimidazol-2-ylidene, BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) and [Ir(H)2(L2)(6,6′-bi-2-picoline)]BArF (6; L2 = (R)-2-((di-tertbutylphosphanyl)methyl)-4-phenyl-4,5-dihydrooxazole) were prepared from the corresponding [Ir(COD)(L)]BArF precursors by treatment with 6,6′-bi-2-picoline under H2 and characterized by 2D NMR spectroscopy and X-ray diffraction. In solution, the N,P complex 6 exists as two isomeric dihydride species (6a and 6b) that are in rapid equilibrium at room temperature. Furthermore, the X-ray structures for [Ir(COD)(L1)]BArF (1) and [Ir(COD)(L2)]BArF (2) are reported. The structural comparison of the solid-state structures of the iridium(I) precursor 1 and the iridium(III) dihydride complex 5 revealed a significant change in the backbone geometry of the C,N ligand. The original U-shaped conformation of the ligand switches to an S-shaped conformation, and therefore, the substituent in the oxazoline ring occupies different quadrants in the iridium coordination sphere. Notable in this context is the finding that a similar switch in the ligand backbone was observed for the C,N iridium(III) dihydride olefin species 3 ([Ir(H)2[(E)-1-methyl-4-(1-phenylprop-1-en-2-yl)benzene-D5](L1)]BArF), which represents a catalytically competent intermediate.

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INTRODUCTION An increasing number of iridium catalysts based on chiral C,N or N,P ligands are finding applications in the asymmetric hydrogenation of olefins and have enhanced the scope of this transformation.1 In contrast to rhodium- or rutheniumdiphosphine catalysts, they do not require substrates bearing a coordinating functional group adjacent to the CC bond to achieve high levels of enantiomeric excess. However, the mechanistic details of the iridium-catalyzed asymmetric CC bond hydrogenation remain to be clarified, and therefore, a more detailed knowledge about the reaction pathway and the enantioselectivity-determining step is necessary. In early work, Crabtree et al.2 and Chin et al.3 were able to identify the monodentate N and P derived iridium(III) dihydride complexes [Ir(H)2(COD)(PCy3)(py)]PF6 and [Ir(H)2(COD)(PPh3)(PhCN)]ClO4, respectively, at low temperature by 1H and 31P NMR after treating the corresponding [Ir(COD)(P)(N)]+ precursors with hydrogen gas. Later studies by Pfaltz et al.4 have shown that the chiral bidentate N,P hydrogenation catalyst [Ir(COD)(PHOX)]BArF (BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) also forms [Ir(H)2(COD)(PHOX)]BArF after treatment with hydrogen gas at 233 K. Upon warming under hydrogen gas to 273 K, formation of two isomeric iridium(III) dihydride complexes [Ir(H)2(PHOX)(THF)2]BArF and cyclooctane was observed. Further warming to room temperature led to the formation of an inactive trinuclear hydride complex.5 In 2013, Gruber et al.6 © XXXX American Chemical Society

reported reliable procedures for the formation and isolation of hydride-bridged dinuclear iridium(III) tetrahydride complexes with chiral bidentate N,P and C,N ligands in dichloromethane under hydrogen gas. Both N,P and C,N dinuclear iridium(III) hydride complexes were studied by X-ray analysis and showed high catalytic activity, presumably via the formation of an active mononuclear species under the reaction conditions. More recently, Gridnev et al.7 have shown by NMR studies that the bidentate N,P derived complex [Ir(COD)(BiphPHOX)]BArF reacts with hydrogen gas at low temperature to form two isomeric hydride-bridged dinuclear iridium pentahydride complexes. These equilibrate in solution via a hydride-bridged dinuclear iridium tetrahydride complex to four isomeric mononuclear dihydride iridium(III) complexes [Ir(H)2(BiphPHOX)]BArF. At the same time,8 Gruber and Pfaltz experimentally identified and characterized previously elusive9 bidentate C,N and N,P derived iridium(III) dihydride olefin complexes, which are catalytically competent intermediates, by NMR spectroscopy in CD2Cl2 (see Scheme 1). Treatment of iridium(I) complexes 1 and 2 with hydrogen gas in the presence of (E)-1-methyl-4-(1-phenylprop-1-en-2yl)benzene-D5 at 238 and 233 K, respectively, led to the formation of iridium(III) dihydride olefin complexes. In the case of the C,N complex 1, a single isomer of the dihydride Received: December 3, 2015

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C,N based intermediate 3 exclusively afforded a new species 5. The 1H NMR spectrum showed two new hydride signals as doublets (2J(H,H) = 7.3 Hz) at −21.82 and −28.45 ppm, whereas the hydride signals for intermediate 3 at −15.56 and −42.64 ppm have disappeared. Furthermore, the 1H NMR spectrum revealed that the alkene was released from intermediate 3 as the signal for the olefinic proton at 4.83 ppm has shifted to 6.82 ppm (free olefin). Complex 5 can also be prepared and isolated in 88% yield by treating a dichloromethane solution of 1 and 10 equiv of 6,6′-bi-2picoline with hydrogen gas for 4 h at room temperature (Scheme 2).12 The structure was assigned on the basis of 2D NMR studies and X-ray analysis (vide infra). When intermediates 4a and 4b were treated with 6,6′-bi-2picoline, the 1H NMR spectrum revealed the formation of two new species 6a and 6b in a ratio of ∼1.9:1. These two complexes 6a and 6b could be isolated in 97% yield with the identical ratio by treating a dichloromethane solution of 2 and 2 equiv of 6,6′-bi-2-picoline with hydrogen gas for 2 h at room temperature.13 In the 1H NMR spectrum at 253 K, the two hydrides appeared as two doublets of doublets at −23.26 and −25.55 ppm for the major isomer (2J(H,H) = 8.1 Hz) and at −23.34 and −25.80 ppm for the minor isomer (2J(H,H) = 8.2 Hz), respectively. The 2J(H,P) values of 15−20 Hz for both isomers confirmed that both hydrides were located cis to the phosphorus atom.14 On the basis of 2D NMR studies, the 3D structures for 6a (major) and 6b (minor) were assigned as shown in Scheme 2. The two isomers differ in the position of the hydride and N atom of the bipicoline ligand in the apical position. A clear distinction between isomer 6a and 6b can be made based on the presence or absence of the 1H NOE contacts between either the protons at the stereogenic carbon center or the phenyl group of the chiral ligand and the apical hydrides (see Figure 1). As observed for iridium(III) dihydride olefin intermediates 4a and 4b,8 the NOESY spectrum of 6a and 6b showed an exchange process between these two complexes at 295 K. The data reveal that the apical hydride (Ha) of 6a exchanges selectively with the equatorial hydride (Hb*) of 6b and that Hb is exchanging with Ha* (Figure 2). However, the question of the mechanism of the dynamics remains open.15 Solid-State Structures. Crystals of complexes 1 and 2 suitable for X-ray diffraction were obtained from hexane/ dichloromethane solutions. These complexes are very efficient catalysts for the asymmetric hydrogenation of unfunctionalized olefins and have been previously characterized;16 however,

Scheme 1. Selection of C,N and N,P Derived Iridium Complexes

iridium olefin complex 3 was identified, and the threedimensional structure was elucidated by 2D NMR studies at low temperature. For the reaction with the N,P iridium complex 2, it was demonstrated that two isomeric iridium(III) dihydride olefin complexes, 4a and 4b, were formed that coordinate the olefin with the opposite enantioface and are in rapid equilibrium at 253 K. However, these species are not stable in solution above 263 K, which makes it difficult to study their chemistry. Computational studies10 have as well provided evidence that such iridium(III) dihydride olefin species are important intermediates in the catalytic olefin hydrogenation cycle. Therefore, iridium(III) dihydride complexes continue to be of considerable interest and further studies will be necessary for a better understanding of their chemistry. Herein, further studies are reported focusing on the synthesis and characterization of new C,N and N,P derived iridium(III) dihydride complexes that are formed under hydrogen gas. Furthermore, structural differences between the ligand backbones of the iridium(I) precursors and the iridium(III) dihydride complexes are discussed based on the comparison of the solid-state structures.

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RESULTS AND DISCUSSION Reactions of Intermediates 3 and 4 with 6,6′-Bi-2picoline. In an effort to study the chemistry of iridium(III) dihydride complexes,11 intermediates 3 and 4 (prepared at low temperature in CD2Cl2)8 were allowed to react with 5 equiv of 6,6′-bi-2-picoline at 233 K in an NMR tube. The reaction of the

Scheme 2. Synthesis of C,N and N,P Derived Iridium(III) Dihydride Complexes

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bite angle (91.9(2)°) is slightly smaller than that in the latter (94.14(16)°). More interestingly, the superposition (N1, Ir1, and C8) of the solid-state structures 1 and 5 reveals a significant switch in the backbone geometry of the C,N ligand (Figure 4). In structure 1, the ligand adopts a U-shaped17 conformation in which the imidazolidine and oxazoline substituents point above the N1−Ir1−C8 plane. In contrast, in structure 5, the ligand exhibits an S-shaped conformation with the oxazoline substituent pointing below the N1−Ir1−C8 plane. These findings may be relevant with respect to the structural orientation of the C,N ligand in the catalytic cycle, which could have an impact on the enantioselectivity-determining step. Therefore, it was investigated if a similar switch in the C,N backbone could be elucidated by 2D NMR spectroscopy for iridium(III) dihydride olefin intermediate 3. Inspection of the 2D NOESY of intermediate 3 showed weak NOE contacts from the proton 5-Ha to the adamantyl protons (see Figure 5a). The absence of NOE contacts between the protons at carbon C-4 and the adamantyl protons accounts for a similar flip in the C,N backbone. Furthermore, 2D NOESY analysis of the previously reported6 dinuclear iridium hydride complex 7, which can be converted to an active species under hydrogenation conditions, revealed weak NOE contacts from the proton 5-Ha to the adamantyl protons (see Figure 5b). Inspection of the reported6 solid-state structure of the related t Bu dinuclear iridium hydride analogue clearly verified a switch in the C,N backbone. These results demonstrate that the change in the oxidation state from iridium(I) to iridium(III) (upon oxidative addition of H2) and the associated switch from a pseudo-square-planar to an octahedral structure can induce a significant change of the quadrant occupation in the iridium coordination sphere. On the other hand, a DFT study by Pfaltz et al.4 on an [Ir(PHOX)(COD)]+ precursor and the corresponding hydrogen activated dihydride structures [Ir(H)2(PHOX)(COD)]+ and [Ir(H)2(PHOX)(solvent)2]+ did not show a significant conformational change of this type. Several other computational studies10 have been performed on activated dihydride structures of the formula [Ir(H)2(alkene)(H2)(ligand’)]+ or [Ir(H)2(alkene)(ligand)]+; however, no solid-state structures or calculations on the precursors were reported. Therefore, further studies would be necessary to clarify whether such conformational changes also occur in other catalyst systems. The solid-state structure of 6 reveals that the inner coordination sphere of the iridium atom contains the bipicoline and the chelating N,P ligand, with a similar bite angle of the bipicoline ligand as that described for structure 5. The hydride ligands were not located in the final difference density map, but two apparent vacant sites in the coordination sphere of Ir1 trans to the nitrogen atoms N1 and N2 represent the positions of the two hydrides, in agreement with the solution structure described above for 6a, the major isomer of 6. The superposition (P1, Ir1, and N1; see Figure 6) of the solidstate structures 2 and 6a reveals a less significant structural change in the backbone than that observed for the C,N complexes. This finding is in agreement with the fact that the N,P ligand has definitively less degrees of freedom in the backbone than the C,N ligand. As a result, the four quadrants remain occupied in a similar way to that observed in the solidstate structure of precursor 2.

Figure 1. Section of the 2D NOESY spectrum showing a selected contact of the apical hydride for the major 6a (red) and minor 6b (blue) species to the ligand.

Figure 2. Section of the phase sensitive 2D NOESY spectrum showing the exchange cross-peaks (red) between the hydrides Ha and Hb* as well as between Hb and Ha* (295 K, 500 MHz, CD2Cl2).

solid-state structures have not been reported to date. Crystal structures of the cations of these complexes are shown in Figure 3 with selected bond lengths and bond angles in the caption. The inner coordination sphere of the iridium atom in complexes 1 and 2 contains the COD and the chelating C,N and N,P ligand, respectively, and these structures can be described as pseudo-square planar. Crystals of the iridium(III) dihydride complexes 5 and 6 suitable for X-ray diffraction were obtained from hexane/ dichloromethane solutions (Figure 3, bottom). The inner coordination sphere of the iridium atom in the solid-state structure of 5 contains the bipicoline and the chelating C,N ligand, with a relative short bite angle of 76.9° for N4−Ir−N5. The hydride ligands were not located in the final difference density map, but two apparent vacant sites in the coordination sphere of Ir1 trans to the nitrogen atoms N1 and N4 represent the positions of the two hydrides, in agreement with the solution structure described above. Comparison of solid-state structures 1 and 5 reveals that, in the former, the N1−Ir1−C8 C

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Figure 3. ORTEP view of the cations of salts 1 (a), 2 (b), 5 (c), and 6a (d). Hydrogen atoms and solvent molecules are omitted for clarity, and thermal ellipsoids are set to 30% probability. Selected bond lengths (Å) and bond angles (deg) for cation 1: Ir1−N1, 2.131(4); Ir1−C8, 2.077(6); N1−Ir1−C8, 91.9(2). Selected bond lengths (Å) and bond angles (deg) for cation 2: Ir1−N1, 2.106(2); Ir1−P1, 2.3251(7); N1−Ir1−P1, 81.27(7). Selected bond lengths (Å) and bond angles (deg) for cation 5: Ir1−N1, 2.284(4); Ir1−C8, 2.003(4); Ir1−N4, 2.205(4); Ir1−N5, 2.126(4); N1− Ir1−C8, 94.14(16); N4−Ir1−N5, 76.85(16). Selected bond lengths (Å) and bond angles (deg) for cation 6a: Ir1−N1, 2.197(6); Ir1−P1, 2.2354(19); Ir1−N2, 2.271(6); Ir1−N3, 2.116(5); N1−Ir1−P1, 81.61(17); N2−Ir1−N3, 75.0(2).

shown to decompose readily, making their chemistry difficult to study. Herein is shown that iridium(III) dihydride complexes 5 and 6 can be prepared at room temperature in dichloromethane under hydrogenation conditions in the presence of 6,6′-bi-2-picoline and characterized by NMR and X-ray analysis. In solution, the N,P complex 6 exists as two isomeric dihydride species (6a and 6b) that are in rapid equilibrium at room temperature. In contrast, only one isomeric form was observed for the C,N derived iridium(III) dihydride complex 5 in solution. Complexes 5 and 6 represent rare examples18 of iridium(III) dihydride C,N and N,P ligand complexes that are derived from catalytically active precursors16 and have been characterized by X-ray analysis. Furthermore, crystal structures of complexes 1 and 2, which serve as precatalysts, are reported. Comparison of the solidstate structures of the precursor 1 and the dihydride complex 5 showed a significant switch in the backbone geometry of the C,N ligand. The original U-shaped conformation of the ligand switches to an S-shaped conformation, and therefore, the substituent in the oxazoline ring occupies different quadrants in the iridium coordination sphere. Notable in this context is the finding that a similar conformational switch was observed in the C,N iridium(III) dihydride olefin intermediate 3 and the dinuclear C,N iridium(III) hydride complex 7. Since this backbone switch may influence the enantioselectivity-determining step, this observation could be relevant for the development

Figure 4. Superposition (N1−Ir1−C8) of the cations of the solid-state structures of 1 (red) and 5 (blue).

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CONCLUSIONS Previous studies of C,N and N,P derived iridium(III) dihydride olefin complexes have shown that these species can be characterized by 2D NMR spectroscopy at low temperature.8 At room temperature, however, these intermediates were D

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materials were purchased from commercial sources and used as received. [Ir(L1)(COD)]BArF (1),16a [Ir(L2)(COD)]BArF (2),16b [Ir(H)2[(E)-1-methyl-4-(1-phenylprop-1-en-2-yl)benzene-D5](L1)]BArF (3),8 [Ir(H)2[(E)-1-methyl-4-(1-phenylprop-1-en-2-yl)benzeneD5](L2)]BArF (4),8 and [IrH(L1)(μ-H)]2(BArF)2 (7)6 were prepared according to literature procedures. 1H, 13C, 31P, and 2D NMR spectra were recorded with a Bruker Advance DRX-500 (500 MHz) spectrometer. Chemical shifts (δ) are given in ppm and referenced relative to TMS for 1H and 13C NMR spectra and H3PO4 for 31P NMR spectra. 1H 2D NOESY spectra were acquired using a standard three-pulse sequence and a mixing time of 700 ms. X-ray Analyses. Data sets were obtained using a Bruker Kappa Apex2 diffractometer (graphite monochromated Mo Kα radiation, λ = 0.71073 Å and Cu Kα radiation, λ = 1.54178 Å) at 123 K. The structures were solved by direct methods using the program SIR92.19 CRYSTALS20 was used for structure refinement. The crystallographic data and R values of the full-matrix least-squares refinements are given in Tables S4 (1), S5 (2), S6 (5), and S7 (6a), respectively (see the Supporting Information). All atoms except hydrogen atoms and atoms of disordered molecules were refined with anisotropic displacement parameters. In general, H atoms were placed at calculated positions based on stereochemical considerations and refined according to the riding model. Synthesis of [Ir(H)2(L1)(6,6′-bi-2-picoline)](BArF) (5). Iridium complex 1 (40.0 mg, 24.6 μmol, 1.0 equiv) and 6,6′-dimethyl-2,2′bipyridine (45.4 mg, 246.4 μmol, 10.0 equiv) were added to a Schlenk tube and dissolved in CH2Cl2 (2.0 mL). The Schlenk tube was placed in liquid N2 until the yellow solution froze. The Schlenk tube was then evacuated and purged with hydrogen gas (balloon), and the solution was allowed to warm to room temperature. After stirring for 4 h at that temperature, the solvent was removed under vacuum. The residue was washed with hexane (3 × 5 mL) and dried at 0.3 mbar to afford salt 5 as an orange solid (37 mg, 88%). A CH2Cl2 solution (0.5 mL) of this solid (10 mg) was layered with hexane (2 mL) and stored at 4 °C, to afford crystals of 5 suitable for X-ray diffraction. HR ESI-MS (CH2Cl2, 333 K): Calcd m/z for ([C42H55IrN5O]+: 838.4033; Found: 838.4035. 1 H NMR (500 MHz, CD2Cl2, 295 K): δ: 7.96−7.94 (m, 2H, C-5′-H + C-8′-H), 7.81−7.76 (m, 2H, C-4′-H + C-9′-H, 7.73 (br s, 8H, BArFH), 7.56 (br s, 4H, BArF-H), 7.45−7.42 (m, 2H, C-3′-H + C-10′-H), 7.29 (t, 1H, J1 = 7.8 Hz, C-12-H), 7.14 (dd, 1H, J1 = 7.8 Hz, J2 = 1.2 Hz, C-11-H), 7.10 (d, 1H, J1 = 1.9 Hz, C-6-H), 6.98 (dd, 1H, J1 = 7.8 Hz, J2 = 1.2 Hz, C-13-H), 6.91 (d, 1H, J1 = 1.9 Hz, C-7-H), 4.85−4.80 (m, 2H, C-3-H + C-5-Ha), 4.15 (dd, 1H, J1 = 8.7 Hz, J2 = 7.2 Hz, C-2Ha), 4.07 (dd, 1H, J1 = 8.7 Hz, J2 = 1.5 Hz, C-2-Hb), 4.02 (ddd, 1H, J1 = 13.6 Hz, J2 = 6.5 Hz, J3 = 3.2 Hz, C-5-Hb), 2.76 (s, 3H, C-1′-H), 2.67 (s, 3H, C-12′-H), 2.43−2.35 (m, 1H, C-18-H), 2.29−2.21 (m, 2H, C15-H + C-4-Ha), 1.79 (ddt, 1H, J1 = 13.6 Hz, J2 = 11.1 Hz, J3 = 3.2 Hz, C-4-Hb), 1.41−1.13 (m, 12H, C-23-H, C-24-H, C-22-H + C-17-H), 1.14−1.05 (m, 9H, C-22-H, C-24-H + C-16-H), 0.96 (d, 3H, J1 = 6.9 Hz, C-19-H), 0.36 (d, 3H, J1 = 6.9 Hz, C-20-H), −21.86 (d, 1H, J1 = 7.3 Hz, Ha), −28.63 (d, 1H, J1 = 7.3 Hz, Hb). 13C{1H} NMR (125 MHz, CD2Cl2, 295 K): δ: 180.1 (C-1), 162.2 (q, 1JCB = 50 Hz, CBArF), 162.4 (C-2′), 161.6 (C-11′), 159.6 (C-6′), 158.3 (C-7′), 158.1 (C-8), 146.4 (C-10), 145.3 (C-14), 138.5 (C-9), 138.1 (C-4′), 137.7 (C-9′), 135.2 (C-BArF), 129.6 (C-12), 129.2 (q, 2JCF = 32 Hz, C-BArF), 127.2 (C-10′), 127.0 (C-3′), 125.0 (q, 1JCF = 272 Hz, C-BArF), 124.6 (C-11), 123.9 (C-13), 123.8 (C-7), 121.4 (C-5′), 121.1 (C-6), 120.9 (C-8′), 117.9 (br s, C-BArF), 73.2 (C-3), 72.2 (C-2), 46.6 (C-5), 38.7 (C-22), 36.8 (C-21), 36.4 (C-4), 36.1 (C-24), 30.9 (C-1′), 29.0 (C-18), 29.0 (C-15), 27.7 (C-23), 26.7 (C-12′), 25.3 (C-16), 24.7 (C-19), 22.9 (C-17), 21.4 (C-20). Synthesis of [Ir(H)2(L2)(6,6′-bi-2-picoline)](BArF) (6). Iridium complex 2 (40.0 mg, 27.2 μmol, 1.0 equiv) and 6,6′-dimethyl-2,2′bipyridine (10.0 mg, 54.5 μmol, 2.0 equiv) were added to a Schlenk tube and dissolved in CH2Cl2 (2.0 mL). The Schlenk tube was placed in liquid N2 until the yellow solution froze. The Schlenk tube was then evacuated and purged with hydrogen gas (balloon), and the solution was allowed to warm to room temperature. After stirring for 2 h at that temperature, the solvent was removed under vacuum. The residue was washed with hexane (3 × 4 mL) and dried at 0.3 mbar to afford salt 6

Figure 5. Section of the 2D NOESY spectrum showing selected weak contacts from the proton 5-Ha to the adamantyl protons at carbon C22 (red) for intermediate 3 (a) and complex 7 (b), respectively.

Figure 6. Superposition (N1−Ir1−C8) of the cations of the solid-state structures of 2 (red) and 6a (blue).

of improved C,N iridium catalysts that induce higher enantioselectivities. In contrast, the superposition of the N,P solid-state structures 2 and 6a did not reveal such a significant change in the orientation of the ligand.

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EXPERIMENTAL SECTION

General Information. All reactions and manipulations with air- or moisture-sensitive materials were carried out under an atmosphere of argon using standard Schlenk techniques. Schlenk tubes were heated in an oven (120 °C) and then dried under high vacuum. CH2Cl2 was dried over CaH2 and then distilled under argon. Dry hexane was purchased in septum-sealed bottles from Aldrich and stored over 4 Å molecular sieves under argon. CD2Cl2 was distilled (bulb-to-bulb) over CaH2, degassed by three freeze−pump−thaw cycles, and stored over 4 Å molecular sieves under argon. All commercially available starting E

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Organometallics Present Address

as a yellow solid (41 mg, 97%). A CH2Cl2 solution (0.5 mL) of this solid (10 mg) was layered with hexane (2 mL) and stored at 4 °C, to afford crystals of 6 suitable for X-ray diffraction. HR ESI-MS (CH2Cl2, 333 K): Calcd m/z for ([C30H42N3OPIr]+: 684.2691; Found: 684.2690. 1H NMR spectroscopy of the crude material indicates clean formation of two isomers of [Ir(H)2(L2)(6,6′-bi-2-picoline)]BArF in the ratio of 1.0/0.53, which are in equilibrium at 295 K as verified by 1H 2D exchange spectroscopy. 6a: 1H NMR (500 MHz, CD2Cl2, 253 K): 7.79−7.63 (m, 12H, C-9′-H, BArF-H, C-4′-H, C-5′-H and C-8′-H, overlap with minor-C-5′-H and minor-C-4′-H), 7.56 (br s, 4H, BArF-H), 7.42 (d, 1H, J = 7.7 Hz, C-10′-H, overlap with minor-C10′-H), 7.26 (dd, 1H, J = 7.5 Hz, J = 1.2 Hz, C-3′-H), 6.70−6.67 (m, 1H, C-12-H), 6.63−6.61 (m, 2H, C-10-H), 6.53−6.49 (m, 2H, C-11H), 5.02−4.95 (m, 2H, C-8-H and C-7-Hb), 4.60−4.52 (m, 1H, C-7Ha overlap with minor-C-8-H), 3.08 (dd, 1H, J = 18.6 Hz, J = 9.7 Hz, C-5-Hb), 3.00 (br dd, 1H, 2JHP = 18.8 Hz, J = 6.0 Hz, C-5-Ha), 2.91 (br s, 3H, C-12′-H, overlap with minor-C-5-Hb), 2.70 (s, 3H, C-1′-H), 1.30 (d, 9H, 3JHP = 13.3 Hz, C-3-H, overlap with minor-C-1-H), 1.16 (br d, 9H, 3JHP = 14.1 Hz, C-1-H overlap with minor-C-3-H), −23.26 (dd, 1H, 2JHP = 16.0 Hz, J = 8.1 Hz, Ir-Hb), −25.55 (dd, 1H, 2JHP = 33.6 Hz, J = 8.1 Hz, Ir-Ha). 13C{1H}-NMR (125 MHz, CD2Cl2, 253 K): δ: 176.2 (d, 2JCP = 12 Hz, C-6), 161.9 (q, 1JCB = 50 Hz, C-BArF), 161.0 (C-11′), 160.5 (C-2′), 158.4 (C-7′), 158.1 (C-6′), 138.2 (C-4′), 137.7 (C-9′), 136.6 (C-9), 134.9 (C-BArF), 128.8 (C-12), 128.4 (q, 2 JCF = 32 Hz, C-BArF), 127.7 (C-11), 127.3 (C-10), 125.9 (C-10′), 125.5 (C-3′), 124.6 (q, 1JCF = 272 Hz, C-BArF), 120.3 (C-8′), 119.1 (C-5′), 117.6 (br s, C-BArF), 77.1 (C-7), 71.1 (C-8), 39.6 (d, JCP = 39 Hz, C-4), 32.9 (d, JCP = 23 Hz, C-2), 31.6 (C-12′), 29.6 (C-1′), 29.1 (d, JCP = 5 Hz, C-1), 27.6 (br s, C-3), 25.9 (d, JCP = 18 Hz, C-5). 31P{1H}NMR (202 MHz, CD2Cl2, 253 K): δ: 45.7; 6b: 1H NMR (500 MHz, CD2Cl2, 253 K): δ: 7.93 (d, 1H, J = 7.8 Hz, C-8′-H), 7.87 (t, 1H, J = 7.8 Hz, C-9′-H), 7.79−7.63 (m, 10H, C-5′-H, BArF-H and C-4′-H, overlap with major-C-9′-H, major-C-4′-H, major-C-5′-H and major-C8′-H, 7.56 (br s, 4H, BArF-H), 7.39 (d, 1H, J = 7.6 Hz, C-10′-H, overlap with major-C-10′-H), 7.32−7.29 (m, 1H, C-12-H), 7.18 (t, 2H, J = 7.8 Hz, C-11-H), 7.09 (d, 1H, J = 7.8 Hz, C-3′-H), 6.73 (br d, 2H, = 7.4 Hz, C-10-H), 4.80 (dd, 1H, J = 8.9 Hz, J = 8.8 Hz, C-7-Hb), 4.60−4.52 (m, 1H, C-8-H overlap with major-C-7-Ha), 4.48 (dd, 1H, J = 8.8 Hz, J = 8.0 Hz, C-7-Hb), 3.16 (dd, 1H, 2JHP = 18.8 Hz, J = 9.0 Hz, C-5-Ha), 2.94−2.89 (m, 1H, C-5-Hb, overlap with major-C-12′H), 2.68 (s, 3H, C-12′-H), 1.75 (s, 3H, C-1′-H), 1.31 (d, 9H, 3JHP = 14.0 Hz, C-1-H, overlap with major-C-3-H), 1.16 (br d, 9H, 3JHP = 14.1 Hz, C-3-H overlap with major-C-1-H), −23.34 (dd, 1H, 2JHP = 15.2 Hz, J = 8.2 Hz, Ir-Hb), −25.80 (dd, 1H, 2JHP = 34.0 Hz, J = 8.2 Hz, Ir-Ha). 13C{1H}-NMR (125 MHz, CD2Cl2, 253 K): δ: 177.1 (d, 2 JCP = 12 Hz, C-6), 161.9 (q, 1JCB = 50 Hz, C-BArF), 162.8 (C-2′), 161.5 (C-11′), 159.1 (C-7′), 158.2 (C-6′), 139.2 (C-9′), 138.9 (C-9), 137.8 (C-4′), 134.9 (C-BArF), 129.5 (C-11), 129.4 (C-12), 128.4 (q, 2 JCF = 32 Hz, C-BArF), 127.5 (C-10), 125.9 (C-10′), 125.8 (C-3′), 124.6 (q, 1JCF = 272 Hz, C-BArF), 120.5 (C-5′), 119.3 (C-8′), 117.6 (br s, C-BArF), 78.2 (C-7), 70.0 (C-8), 39.0 (d, JCP = 39 Hz, C-2), 33.2 (d, JCP = 22 Hz, C-4), 30.0 (C-1′), 29.0 (d, JCP = 4 Hz, C-3), 28.2 (C-12′), 27.9 (br s, C-1), 25.8 (d, JCP = 21 Hz, C-5). 31P{1H}-NMR (202 MHz, CD2Cl2, 253 K): δ: 45.2.

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†

University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, OX1 3TA, Oxford, United Kingdom. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I would like to thank Prof. Dr. Andreas Pfaltz for his generous support, for inspiring discussions, and for the possibility to perform this work in his laboratory. Dr. Markus Neuburger (Laboratory for Chemical Crystallography, University of Basel) is thanked for his assistance with crystal structure analysis. Financial support from the University of Basel is gratefully acknowledged.

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ABBREVIATIONS L1 = (S)-1-[2-(2-adamantan-2-yl-4,5-dihydrooxazol-4-yl)ethyl]-3-(2,6-diisopropylphenyl)-1,2-dihydroimidazol-2-ylidene; L2 = (R)-2-((di-tert-butylphosphanyl)methyl)-4-phenyl4,5-dihydrooxazole; COD = 1,5-cyclooctadiene; BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; NHC = Nheterocyclic carbene; Cy = cyclohexyl; py = pyridine; PHOX = phoshinooxazoline

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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.5b00989. Additional spectroscopic data and complete labeling of the complexes (PDF) Crystallographic data for 1, 2, 3, and 4 (CIF)

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00989 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chem. Soc. 2004, 126, 16688−16689. (e) Cui, X.; Fan, Y.; Hall, M. B.; Burgess, K. Chem. - Eur. J. 2005, 11, 6859−6868. (f) Hopmann, K. H.; Bayer, A. Organometallics 2011, 30, 2483−2497. (g) Hopmann, K. H.; Frediani, L.; Bayer, A. Organometallics 2014, 33, 2790−2797. (h) Sparta, M.; Riplinger, C.; Neese, F. J. J. Chem. Theory Comput. 2014, 10, 1099−1108. (i) Borràs, C.; Biosca, M.; Pàmies, O.; Diéguez, M. Organometallics 2015, 34, 5321−5334. (11) For studies of bidentate C,N and N,P derived iridium(III) dihydride solvent complexes in CD3CN, see ref 6. For studies of bidentate N,P derived iridium(III) dihydride solvent complexes in THF-d8, see ref 4 and: Schramm, Y.; Barrios-Landeros, F.; Pfaltz, A. Chem. Sci. 2013, 4, 2760−2766. (12) At room temperature, the hydride signals appeared at −21.86 and −28.63 ppm. Gruber et al. have already reported the synthesis of the tBu analogue; however, no solid-state structure was obtained. See ref 6. (13) Analogues structures of the type [Ir(H)2(N,P)(N,N)]+ have been described in the literature; however, they do not exist as isomeric species and no solid-state structures are given. See: Drago, D.; Pregosin, P. S.; Pfaltz, A. Chem. Commun. 2002, 286−287. (14) For an orientation trans to a phosphorus atom, a coupling constant of 100−200 MHz would be expected. See: Pregosin, P. S. NMR in Organometallic Chemistry; Wiley-VCH: Weinheim, 2012. (15) One of the reviewers pointed out that, on the basis of the data, the following statements can be made: (1) There is no molecular hydrogen dissociation, as this would lead to random exchange, and (2) there is no simple hydrogen rotation (i.e., dihydride converted to bound molecular hydrogen - molecular hydrogen rotation and reformation of the dihydride), since this would lead to intramolecular exchange between the two nonequivalent hydrides and this is not observed. (16) (a) Perry, M. C.; Cui, X.; Powell, M. T.; Hou, D.-R.; Reibenspies, J. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125, 113−123. (b) Schrems, M. G.; Neumann, E.; Pfaltz, A. Angew. Chem., Int. Ed. 2007, 46, 8274−8276. (17) This structural description was used by Burgess and co-workers for similar complexes of the type [Ir(COD)(C,N)]BArF; see ref 16a. (18) For the solid-state structure of a related N,P iridium(III) dihydride complex with the formula [IrCl(H)2(PHOX)(PPh3)], see: Carmona, D.; Ferrer, J.; Lorenzo, M.; Santander, M.; Ponz, S.; Lahoz, F. J.; Lopez, J. A.; Oro, L. A. Chem. Commun. 2002, 870−871. (19) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (20) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.

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DOI: 10.1021/acs.organomet.5b00989 Organometallics XXXX, XXX, XXX−XXX