Introduction

Metal complexes based on lutidine-derived PNP pincer ligands have gained considerable attention due to their applications in organometallic chemistry and catalysis (Fig. 1a).¹ In these derivatives metal–ligand cooperativity, triggered by deprotonation of the methylene arms of the ligand accompanied by dearomatisation of the pyridine ring, has led to unique reactivity in the activation of a diversity of X–H (X = H, C, O, N) bonds. While significantly less studied, analogous complexes based on CNC pincers (C stands for a N-heterocyclic carbene, NHC) have also been described (Fig. 1b).^2–4 As shown with Ru–CNC complexes,^3,4 these derivatives can also be deprotonated at the methylene CH₂N bridges, and participate in metal–ligand cooperation processes. Furthermore, the larger Py–CH₂–NHC linkage, which forms 6-membered metallacycles upon coordination, confers a greater flexibility on the ligand in comparison to 5-membered rings formed in PNP pincers. This flexibility should permit stabilizing metal complexes in a variety of coordination geometries, a relevant issue in catalysis where the intermediates in the catalytic cycle may need to adopt different structural arrangements. For example, while PNP ligands have only exhibited meridional coordination modes, facial coordination of CNC ligands in Ru complexes has been observed.³ More remarkable, as demonstrated by the Pidko's group, is that the increased flexibility of the CNC pincer results in enhanced reactivities towards H₂ and CO₂ in comparison to Ru–PNP systems.^4b

In addition, non-symmetric pincer ligands, i.e. having two inequivalent flanking donor groups, allow for a larger electronic and steric diversity derived from the potential tuning of two different side donors.⁵ With respect to lutidine-derived pincer complexes, some examples of PNP′⁶ and CNC′^4a derivatives have been reported. Unsymmetrical PNX and CNX pincer complexes have also been described, although these derivatives are usually of the type PNN ¹ and CNN ⁷ where hemilabile coordination of the N-donor flanking group has been proposed. In marked contrast, complexes based on CNP ligands having a pyridine central moiety and in which the two side functionalities are two significantly different strong σ-donors, such as a phosphine and a NHC, have not been investigated. In fact, a limited number of hybrid tridentate ligands possessing both phosphine and NHC donors have been reported, and these have either a ligand backbone based on a 1,3-disubstituted phenyl ring,^8a or a different arrangement of the donor moieties, where the NHC group is the central unit of the pincer ligand.^8b–f Also, recently Danopoulos, Braunstein et al. have prepared Co and Cr complexes based on deprotonated NHC/phosphine mixed pincer ligands having a central picoline motif (Fig. 1c).⁹

Based on these precedents, we aimed to develop a new class of ligands having NHC and phosphine side donors and a lutidine central fragment (CNP, Fig. 1d). A fundamental difference of these ligands with the previous picoline-based pincer derivatives reported by Danopoulos, Braunstein et al. resides in the presence of a methylene linker between the pyridine and the NHC functionalities, which could also be susceptible to deprotonation and should enhance pincer flexibility. In this contribution, we report on the synthesis of the precursors of these ligands as well as their coordination to Pd and Ir complexes. In particular, the ability of CNP ligands to adapt to different coordination geometries and participate in ligand-assisted processes has been assessed.

Results and discussion

Syntheses of imidazolium salts 1

Syntheses of imidazolium salts 1a(Cl), 1b(Cl) and 1b(Br) were effected as shown in Scheme 1. Derivative 1a(Cl) was prepared by reaction of the corresponding 2-chloromethyl-6-imidazolylmethyl-pyridine with diphenyl phosphine in the presence of KO^tBu. Alternatively, in the case of salts 1b(Cl) and 1b(Br), higher product yields were obtained when diphenyl phosphine–borane adduct was used for the introduction of the P-donor fragment, followed by phosphine deprotection by simple treatment with refluxing MeOH. The CNP ligands precursors were obtained with moderate to good yields (55–85%) as white to brown solids.

Synthesis and deprotonation of Pd–CNP complex 2a

For an evaluation of the coordination capabilities of these new CNP ligands, we initially studied the formation of Pd derivatives. Thus, salt 1a(Cl) was reacted with Ag₂O in CH₂Cl₂, followed by addition of PdCl₂(cod) to yield complex 2a (Scheme 2).¹⁰ The spectroscopic data support the formation of a complex in which the CNP ligand is coordinated to the metal centre as a pincer. For example, its ¹H NMR spectrum shows distinctly two signals for the bridging methylenes. The CH₂P protons appear at 4.32 ppm (d, ²J_HP = 11.3 Hz), whereas the NCH₂ hydrogens produce a singlet at 6.05 ppm. The ¹³C{¹H} NMR spectrum exhibits a doublet at 166.4 ppm with a large ²J_CP (183 Hz, carbene carbon, C², of the NHC moiety), indicating the trans disposition of the NHC and phosphine moieties.

A single crystal X-ray diffraction study of 2a confirmed the proposed structure (Fig. 2). Thus, complex 2a in the solid state is comprised of a Pd atom in an square-planar coordination geometry, with the carbene and phosphine fragments of the pincer disposed trans to each other (C²(NHC)–Pd–P = 168.65°), and the chloride ligand trans to the pyridine (N(Py)–Pd–Cl = 175.03°). The NHC–Pd–Py chelate ring has a boat conformation as determined by the torsion angle C(14)–N(3)–Pd(1)–C(1) of 32.4°, whereas the 5-membered ring involving the phosphine donor has an envelope conformation with a C(18)–N(3)–Pd(1)–P(1) torsion angle of 28.4°.

Since it can be expected that CNP ligands can be deprotonated in either the CH₂P or CH₂–NHC arms, the acid/base responsiveness of 2a was tested by adding KHMDS to a suspension of the complex in THF.¹¹ In the ¹H NMR spectrum of the resulting product 3a, a significant up-field shift for the signals of the pyridine protons (5.55–6.46 ppm) in accord with the dearomatisation of this moiety is observed. Meanwhile, the CHP fragment produces a singlet signal at 3.41 ppm (integrating to 1H) in the ¹H NMR spectrum and a doublet at 63.3 ppm (¹J_CP = 66 Hz) in the ¹³C{¹H} NMR experiment, evidencing the selective deprotonation of the CNP ligand at the CH₂P arm.

Synthesis and structural features of Ir–CNP complexes

Reaction of imidazolium salts 1 with Ir(acac)(cod) provided cationic olefin complexes 4, isolated as yellow to orange solids in moderate to good yields (30–80%) (Scheme 3). These derivatives are stable in the solid state to the atmospheric agents, and have been fully characterised by NMR. For example, in the ¹H NMR spectrum of 4b(Br), the CH₂P protons are diastereotopic and appear as doublet of doublets at δ 3.36 (²J_HP = 2.1 Hz) and 4.17 ppm (²J_HH = 15.5 Hz and ²J_HP = 11.6 Hz). Similarly, protons for the CH₂N bridge produce two doublets at δ 5.65 and 6.91 (²J_HH = 14.1 Hz). The ¹³C{¹H} NMR spectrum shows a doublet at 164.8 ppm for the C² NHC carbon with a very small ²J_CP coupling constant of 8 Hz. These data suggest a cis coordination of the phosphine and NHC donors, also confirmed in the solid state by a single crystal X-ray diffraction study of 4b(BArF) (Fig. 3), obtained by anion exchange in complex 4b(Br) with NaBAr_F.

The structure of complex 4b(BArF) is best described as adopting a distorted trigonal bipyramidal geometry despite the acute P–Ir–N(Py) bond angle of 75.98(8)°. The CNP ligand exhibits a facial coordination, with the NHC donor in the apical position (P–Ir–C(NHC) angle of 95.61(10)°). In addition, the six-membered chelate ring involving the NHC and pyridine donors adopts a boat-like conformation as defined by the dihedral angle C(13)–N(3)–Ir(1)–C(1) of −50.6°, while the chelate ring containing the phosphine fragment exhibits an envelope conformation with a C(17)–N(3)–Ir(1)–P(1) angle of 29.5°. This facial coordination mode is unprecedented in M–PNP complexes and may be ascribed to the larger flexibility of the six-membered Py–M–NHC chelate ring, as previously observed in Ru–CNC complexes.³ In addition, the C_(axial)–N_(equatorial)–P_(equatorial) coordination mode of the pincer differs significantly from previously reported pentacoordinated d⁸ pincer complexes,¹² for which an eq–ax–eq distribution is usually observed.¹³ Finally, as observed with other pentacoordinated diolefin Ir complexes,¹³ the distance from the Ir atom to the centroid of the CC bonds is slightly longer for the alkene coordinated trans to the NHC than for the olefin placed in the meridional position (Δd_{(Ir-centroid CC)} = 0.14 Å).

As exemplified with complex 4a(Cl), a dynamic behaviour in solution for complexes 4 has been evidenced by NMR. VT-¹H NMR spectra of 4a(Cl) registered in the temperature range between 50 and −80 °C show sharp signals for the resonances attributable to the CNP ligand. In contrast, broad signals at 2.98 and 3.49 ppm, integrating for two protons each, are observed for the olefinic protons at 25 °C. ¹H–¹H COSY and ¹H–¹H NOESY experiments indicate that each signal is produced by protons of different olefinic moieties; i.e. H^b1,H^b2 and H^a1,H^a2 produce signals at 2.98 and 3.49 ppm, respectively (Fig. 4a). In addition, these signals broaden upon lowering the temperature and eventually split at temperatures below −25 °C into two sets of two signals each, appearing at δ 2.23 (H^b2) and 3.39 (H^b1), and 2.86 (H^a2) and 3.90 (H^a1), respectively. An approximate value of ΔG^‡ = 10.9 kcal mol⁻¹ at the coalescence temperature (244 K) can be estimated for the fluxional process. This dynamic behaviour can be ascribed to alkene site exchange allowed by the decoordination of the CC fragment trans to the NHC moiety to produce the distorted tetrahedral intermediate A, followed by re-coordination of the free olefin moiety to the opposite side without a net change of the fac coordination mode of the CNP ligand (Fig. 4a).

In addition, the ¹H,¹H-exchange spectroscopy (EXSY) experiment of 4a(Cl) registered at 50 °C demonstrates the existence of additional dynamic processes with higher energy barriers. Thus, intense exchange cross-peaks between the signals for the olefinic protons appearing at 2.98 and 3.49 ppm are observed, which can be explained by the formal rotation of the diolefin ligand allowed by the decoordination of one of the Ir–alkene bonds (Fig. 4a). Furthermore, the observation of strong correlation peaks between the resonances of the o-, m- and p-protons of one of the PPh groups with the aromatic protons of the other phenyl group, as well as between the signals of the methylene protons in each of the CH₂P and CH₂N arms, indicates that the CNP pincer in complexes 4 also undergoes structural changes, which could be assigned to a slow interconversion between the two enantiomeric forms of the complex. Previously, mirror-image isomer exchange involving a pseudo-Berry rotation has been observed for pentacoordinated pincer Ir complexes containing diolefin ligands.^13c However, this process should not be possible in complexes 4 due to the C_(axial)–N_(equatorial)–P_(equatorial) coordination mode of the pincer. Since, as discussed above, olefin decoordination seems facile, the observed fluxional process could likely involve the intermediacy of the square-planar structure B (Fig. 4b).

To evaluate the donating properties of the CNP ligands, we prepared the carbonyl derivative 5a(Cl) by bubbling CO through a CH₂Cl₂ solution of complex 4a(Cl) (Scheme 3). Signals of the bridging CH₂P and CH₂N protons in the ¹H NMR spectrum support a planar coordination of the CNP pincer. Thus, the CH₂P protons produce a doublet signal at 4.18 ppm (²J_HP = 10.0 Hz) while the CH₂N hydrogens appear as a singlet at 6.11 ppm. In the IR spectrum, the carbonyl ligand absorbs at 1985 cm⁻¹, which is a higher frequency than that corresponding to the (^tBu–PNP)–Ir analogue (1964 cm⁻¹),¹⁴ suggesting a lower electron density at the metal centre in the Ir–CNP system. The CO ligand is detected in the ¹³C{¹H} NMR spectrum by the appearance of a doublet signal at 177.2 ppm (²J_CP = 10 Hz), while the C² NHC carbon appears at 178.1 ppm (²J_CP = 99 Hz).

Interestingly, complex 5a(Cl) reversibly coordinates a new CO molecule yielding complex 6a(Cl), as inferred from the absorption bands corresponding to the CO ligands, which appear in the IR spectrum at 1946 and 2021 cm⁻¹.¹⁵ In the ¹H NMR spectrum of 6a(Cl), the presence of a singlet signal at 6.09 ppm (2H) for the CH₂N arm and a doublet at 4.29 ppm (2H, ²J_HP = 10.9 Hz) attributable to the CH₂P moiety suggests the existence of a symmetry plane containing the CNP–Ir coordination plane and points out to a meridional coordination of the pincer ligand,^13b at variance with the coordination geometry in the also pentacoordinated compounds 4.

As determined by VT-¹H NMR spectroscopy (see ESI†), carbonyl complexes 5a(Cl) and 6a(Cl) exhibit a dynamic behaviour in solution, which equilibrates the two otherwise diastereotopic hydrogens of both methylene bridges. In square-planar Pd ¹⁶ and octahedral Ru complexes incorporating CNC ligands,^3b similar dynamic processes have been ascribed to a slow interconversion between the two twisted conformations adopted by both C²(NHC)–N(Py)–M chelate rings of the pincer ligand. Similarly, the observed fluxionality in derivatives 5a(Cl) and 6a(Cl) can be attributed to the fast atropoisomerism between the two limiting enantiomeric forms shown in Fig. 5.

Complexes 4a(Cl) and 4b(Cl) react with H₂ producing the dihydrido derivatives 7 and cyclooctene (Scheme 3). At room temperature, the ¹H NMR spectrum of complex 7a(Cl) shows two doublets of doublets at −20.19 (²J_HP = 13.8 Hz, ²J_HH = 7.0 Hz) and −23.30 ppm (²J_HP = 18.9 Hz) due to the hydrido ligands placed trans to the pyridine and trans to the chloride, respectively. In the ¹³C{¹H} NMR spectrum, the C² NHC appears at 172.9 ppm as a doublet signal (²J_CP = 119 Hz). Exposure of a sample of 7a(Cl) in CD₂Cl₂ to deuterium gas (2 bar) or addition of CD₃OD causes fast H/D exchange of the hydrido ligands.

Furthermore, the structural features of complex 7a(Cl) have been studied in the solid state by single crystal X-ray diffraction (Fig. 6). This derivative has an octahedral geometry with the two hydrido ligands occupying mutually cis positions and the CNP ligand adopting a meridional coordination, as defined by the C(1)–Ir(1)–P(1) angle value of 166.5(3)°. The chelate ring incorporating the NHC fragment has a boat-like conformation as shown by the C(14)–N(3)–Ir(1)–C(1) torsion angle of −26.5(9)°, whereas an envelope conformation for the N(Py)–Ir–P ring is observed with a C(18)–N(3)–Ir(1)–P(1) dihedral angle of −14.9(8)°.

Deprotonation and ligand-assisted H₂ activation

We have also explored the deprotonation of the Ir–CNP complexes 4.¹⁷ Treatment of 4b(Br) with KO^tBu produces the selective deprotonation of the CH₂P arm (Scheme 4). The resulting complex 8b is characterised in the ¹H NMR spectrum by the presence of significantly high-field shifted signals for the pyridine protons (5.6–6.4 ppm), evidencing the dearomatisation of the pyridine ring. The CHP proton appears as a singlet at 3.86 ppm, while the CH₂–NHC hydrogens generate two doublets at 4.93 and 5.29 ppm (²J_HH = 13.6 Hz). In the ¹³C{¹H} NMR spectrum, the resonance caused by the C² NHC carbon appears as an overlapped doublet at 170.6 ppm. Although the J_CP value cannot be unambiguously calculated, a value of 2 to 20 Hz is estimated, suggesting a cis coordination of the phosphine and NHC fragments. This coordination mode is further supported by the existence of strong NOE contacts between the protons of the xylyl and PPh₂ groups.

Deprotonated complex 8b reacts with H₂ at 0 °C to produce the trihydrido derivative 9b in a ligand-assisted process leading to the re-aromatisation of the pyridine fragment. Complex 9b is only stable under an atmosphere of H₂ and can be also obtained by reaction of 7b(Cl) with KO^tBu followed by exposure to H₂. The trihydrido complex 9b shows in the ¹H NMR spectrum a doublet of doublets at −9.98 ppm (2H, ²J_HP = 18.2 Hz, ²J_HH = 4.8 Hz) due to the apical hydrido ligands and a doublet of triplets at −19.64 ppm (1H, ²J_HP = 14.4 Hz) produced by the hydride trans to the pyridine N. The C² of the NHC fragment appears in the ¹³C{¹H} NMR spectrum as a doublet at 176.9 ppm with a large ²J_CP (121 Hz), in agreement with a trans disposition of the NHC and phosphine donors of the CNP ligand.

Hydrogenation of ketones catalysed by Ir–CNP complexes

In order to assess the catalytic potential of these Ir–CNP complexes, their performance in the hydrogenation of ketones was studied (Table 1).¹⁸ In the presence of KO^tBu, complex 4a(Cl) smoothly catalysed the hydrogenation of acetophenone under 4 bar of H₂ at 30 °C in 2-methyltetrahydrofuran, using a S/C/B ratio of 100/1/15 (entry 1). By using the same pressure, catalyst loading could be decreased to a S/C ratio of 250 after heating to 60 °C (entry 2). Also, at this temperature, a lower H₂ pressure (1 bar) could be employed (entry 3). Under the latter conditions, complex 4b(Cl) was found to be slightly less active, whereas a significantly lower catalytic activity was obtained with the carbonyl derivative 5a(Cl) (entries 4 and 5). Finally, the hydrogenation of a series of ketones was performed with complex 4a(Cl). High conversions were obtained in the case of acetophenone derivatives substituted with p-methoxi, p-chloro and o-bromo substituents (entries 6–8). Alternatively, the presence of fluoro substituents seems somewhat detrimental since 2-fluoroacetophenone was reduced with a slightly lower yield (entry 9). Also, the hydrogenation of a cyclic ketone, α-tetralone, proceeded with a high conversion (entry 10).

Conclusions

In summary, Pd and Ir complexes based on novel lutidine-derived CNP pincer ligands have been synthesised. The flexibility of the chelating Py–CH₂–NHC fragment of the ligands allows for both facial and meridional coordination modes in five- and six-coordinated Ir–CNP complexes. Furthermore, selective deprotonation of the CH₂P arm in Ir–CNP complexes promotes ligand-assisted H–H activation, leading to active species in ketone hydrogenation. Further studies involving the application of metal complexes based on CNP ligands in X–H (X = H, C, heteroatom) bond activation and as catalysts in the (de)hydrogenation of polar substrates are currently in progress in our laboratory.

Experimental

General procedures

All reactions and manipulations were performed under nitrogen or argon, either in a Braun Labmaster 100 glovebox or using standard Schlenk-type techniques. All solvents were distilled under nitrogen with the following desiccants: sodium-benzophenone-ketyl for diethyl ether (Et₂O) and tetrahydrofuran (THF); sodium for hexane, pentane and toluene; CaH₂ for dichloromethane (CH₂Cl₂) and acetonitrile (CH₃CN); and NaOMe for methanol (MeOH). 1-(3,5-Dimethylphenyl)-1H-imidazole and 1-(2,4,6-trimethylphenyl)-1H-imidazole were prepared as previously described.¹⁹ Ir(acac)(cod)²⁰ and NaBAr_F ²¹ were synthesized according to literature procedures. All other reagents were purchased from commercial suppliers and used as received. NMR spectra were obtained on Bruker DPX-300, DRX-400, AVANCEIII/ASCEND 400R or DRX-500 spectrometers. ³¹P{¹H} NMR shifts were referenced to external 85% H₃PO₄, while ¹³C{¹H} and ¹H shifts were referenced to the residual signals of deuterated solvents. All data are reported in ppm downfield from Me₄Si. All NMR measurements were carried out at 25 °C, unless otherwise stated. NMR signal assignations were confirmed by 2D NMR spectroscopy (¹H–¹H COSY, ¹H–¹H NOESY, ¹H–¹³C HSQC and ¹H–¹³C HMBC). HRMS data were obtained on a JEOL JMS-SX 102A mass spectrometer at the Instrumental Services of Universidad de Sevilla (CITIUS). ESI-MS experiments were carried out in a Bruker 6000 apparatus by the Mass Spectrometry Service of the Instituto de Investigaciones Químicas. Elemental analyses were run by the Analytical Service of the Instituto de Investigaciones Químicas in a Leco TrueSpec CHN elemental analyzer. IR spectra were acquired on a Bruker Tensor 27 instrument.

Synthesis of imidazolium salts 1

Imidazolium salts 1 were synthesised in two steps from the corresponding 2,6-bis(halomethyl)pyridines and imidazoles as shown below.

[2-(Diphenylphosphinyl)methyl-6-(3-mesitylimidazolium-1-yl)methyl]pyridine chloride, 1a(Cl)

A solution of 2,6-bis(chloromethyl)pyridine (4.00 g, 22.7 mmol) and 1-mesityl-1H-imidazole (2.12 g, 11.4 mmol) in toluene (80 mL) was refluxed for 7 days. The precipitate was filtered, washed with cold THF (2 × 20 mL) and pentane (3 × 20 mL), and dried under vacuum to yield [2-chloromethyl-6-(3-mesitylimidazolium-1-yl)methyl]pyridine chloride as a brown solid (2.91 g, 70%). ¹H NMR (400 MHz, CDCl₃): δ 10.56 (s, 1H, H arom Imid), 8.13 (s, 1H, H arom Imid), 8.06 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.84 (dd, ³J_HH = 7.7 Hz, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.48 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.10 (s, 1H, H arom Imid), 6.99 (s, 2H, 2 H arom Mes), 6.24 (s, 2H, CH₂N), 4.63 (s, 2H, CH₂Cl), 2.33 (s, 3H, CH₃), 2.04 (s, 6H, 2 CH₃). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ 156.0 (C_q arom), 151.9 (C_q arom), 141.5 (C_q arom), 140.8 (CH arom), 138.9 (CH arom), 134.3 (2 C_q arom), 130.8 (C_q arom), 130.0 (2 CH arom), 125.1 (CH arom), 124.1 (CH arom), 124.0 (CH arom), 122.8 (CH arom), 52.3 (CH₂N), 45.1 (CH₂Cl), 21.2 (CH₃), 17.7 (2 CH₃). HRMS (ESI): m/z 326.1411 [(M − Cl)⁺] (exact mass calculated for C₁₉H₂₁ClN₃: 326.1424).

In subsequent step, to a solution of PPh₂H (1.08 g, 5.80 mmol) in THF (10 mL) was added a solution of KO^tBu (0.650 g, 5.80 mmol) in THF (10 mL). The resulting mixture was stirred for 5 min, and added to a solution of [2-chloromethyl-6-(3-mesitylimidazolium-1-yl)methyl]pyridine chloride (2.00 g, 5.52 mmol) in MeCN (40 mL). The suspension was stirred overnight, and MeOH (15 mL) was added to quench the reaction. Solvent was evaporated, and the residue was extracted with CH₂Cl₂ (2 × 15 mL). The solid obtained after removal of the solvent was washed with diethyl ether (3 × 20 mL) and pentane (3 × 20 mL). Imidazolium salt 1a(Cl) was isolated as a light brown solid (2.40 g, 85%). ¹H NMR (500 MHz, CDCl₃): δ 10.30 (s, 1H, H arom Imid), 7.85 (s, 1H, H arom Imid), 7.60 (d, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.54 (dd, ³J_HH = 7.6 Hz, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.36 (m, 4H, 4 H arom PPh), 7.28 (m, 6H, 6 H arom PPh), 7.12 (s, 1H, H arom Imid), 7.03 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.01 (s, 2H, 2 H arom Mes), 5.94 (s, 2H, CH₂N), 3.59 (s, 2H, CH₂P), 2.33 (s, 3H, CH₃), 2.01 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ −11.7. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 158.8 (d, J_CP = 8 Hz, C_q arom), 152.7 (C_q arom), 141.4 (C_q arom), 138.7 (d, J_CP = 3 Hz, CH arom), 138.5 (d, J_CP = 15 Hz, 2 C_q arom), 137.9 (CH arom), 134.7 (2 C_q arom), 133.0 (d, J_CP = 19 Hz, 4 CH arom), 131.2 (C_q arom), 129.9 (2 CH arom), 129.0 (2 CH arom), 128.7 (d, J_CP = 7 Hz, 4 CH arom), 124.0 (d, J_CP = 5 Hz, CH arom), 123.9 (CH arom), 122.7 (CH arom), 121.3 (d, J_CP = 2 Hz, CH arom), 53.7 (CH₂N), 38.3 (d, J_CP = 17 Hz, CH₂P), 21.1 (CH₃), 17.7 (2 CH₃). HRMS (ESI): m/z 476.2243 [(M − Cl)⁺] (exact mass calculated for C₃₁H₃₁N₃P: 476.2256).

[2-(Diphenylphosphinyl)methyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine chloride, 1b(Cl)

A solution of 2,6-bis(chloromethyl)pyridine (3.96 g, 22.5 mmol) and 1-xylyl-1H-imidazole (1.93 g, 11.2 mmol) in THF (20 mL) was heated to 45 °C for 7 days. The solution was reduced to half the initial volume by solvent evaporation, and Et₂O (10 mL) was added to precipitate the product. The solid was filtered, washed with Et₂O (2 × 10 mL) and pentane (3 × 5 mL) and dried under vacuum. [2-Chloromethyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine chloride was isolated as a white solid (2.21 g, 57%). ¹H NMR (400 MHz, CDCl₃): δ 11.09 (s, 1H, H arom Imid), 8.19 (d, ³J_HH = 7.2 Hz, 1H, H arom Py), 8.09 (s, 1H, H arom Imid), 7.89 (dd, ³J_HH = 8.0 Hz, ³J_HH = 7.2 Hz, 1H, H arom Py), 7.54 (m, 2H, H arom Py + H arom Imid), 7.23 (s, 2H, 2 H arom Xyl), 7.11 (s, 1H, H arom Xyl), 6.10 (s, 2H, CH₂N), 4.69 (s, 2H, CH₂Cl), 2.36 (s, 6H, 2 CH₃). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 156.1 (C_q arom), 151.7 (C_q arom), 140.9 (CH arom), 140.5 (2 C_q arom), 136.4 (CH arom), 134.3 (CH arom), 132.1 (CH arom), 125.2 (CH arom), 124.1 (C_q arom), 123.8 (CH arom), 120.31 (CH arom), 119.58 (2 CH arom), 52.7 (CH₂N), 45.4 (CH₂Cl), 21.4 (2 CH₃). HRMS (ESI): m/z 312.1256 [(M − Cl)⁺] (exact mass calculated for C₁₈H₁₉ClN₃: 312.1268).

In a subsequent step, to a solution of Ph₂P(BH₃)H (0.288 g, 1.44 mmol) in THF (10 mL) was added a solution of KO^tBu (0.161 g, 1.44 mmol) in THF (5 mL). The mixture was stirred for 10 min, and added to a suspension of [2-chloromethyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine chloride (0.500 g, 1.44 mmol) in MeCN (10 mL). The resulting suspension was stirred overnight, and MeOH (10 mL) was added to quench the reaction. The solvent was evaporated under vacuum, and the solid was extracted with CH₂Cl₂ (3 × 10 mL). Solvent removal followed by washings with Et₂O (2 × 10 mL) yields a light orange solid which should correspond to the borane adduct of 1b(Cl). This solid was dissolved in MeOH (10 mL), and the solution was transferred to a Fisher–Porter vessel and heated to 75 °C for 24 h. Volatiles were removed under vacuum, and MeOH (10 mL) was newly added and the previous procedure repeated. The resulting solid was washed with toluene (2 × 5 mL) and Et₂O (3 × 5 mL) to give an off-white solid (0.444 g, 62%). ¹H NMR (400 MHz, CD₂Cl₂): δ 11.28 (s, 1H, H arom Imid), 7.74 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.64 (dd, ³J_HH = 7.7 Hz, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.55 (d, ³J_HH = 1.1 Hz, 1H, H arom Imid), 7.44 (m, 4H, 4 H arom), 7.36 (m, 7H, 7 H arom), 7.29 (s, 2H, 2 H arom Xyl), 7.21 (s, 1H, H arom Xyl), 7.12 (d, ³J_HH = 7.8 Hz, 1H, H arom Py), 5.91 (s, 2H, CH₂N), 3.69 (s, 2H, CH₂P), 2.45 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ −11.8. ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): δ 159.0 (d, J_CP = 8 Hz, C_q arom), 152.6 (C_q arom), 141.0 (2 C_q arom), 138.5 (d, J_CP = 15 Hz, 2 C_q arom), 138.0 (CH arom), 136.6 (CH arom), 134.9 (C_q arom), 133.0 (d, J_CP = 19 Hz, 4 CH arom), 131.9 (CH arom), 129.1 (2 CH arom), 128.7 (d, J_CP = 7 Hz, 4 CH arom), 124.2 (d, J_CP = 5 Hz, CH arom), 123.7 (CH arom), 121.8 (CH arom), 120.2 (CH arom), 119.7 (2 CH arom), 54.0 (CH₂N), 38.3 (d, J_CP = 16 Hz, CH₂P), 21.3 (2 CH₃). HRMS (ESI): m/z 462.2082 [(M − Cl)⁺] (exact mass calculated for C₃₀H₂₉N₃P: 462.2094).

[2-(Diphenylphosphinyl)methyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine bromide, 1b(Br)

A solution of 1-(3,5-xylyl)-1H-imidazole (1.00 g, 5.81 mmol) in THF (20 mL) was added to a solution of 2,6-bis(bromomethyl)pyridine (3.08 g, 11.6 mmol) in THF (20 mL). The solution was stirred for 7 days at room temperature. The resulting precipitate was filtered, washed with cold THF (2 × 10 mL) and hexane (2 × 10 mL), and dried to give [2-bromomethyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine bromide as a light brown solid (2.00 g, 79%). ¹H NMR (500 MHz, CD₂Cl₂): δ 10.86 (s, 1H, H arom Imid), 7.98 (s, 1H, H arom Imid), 7.80 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.75 (s, 1H, H arom Imid), 7.72 (dd, ³J_HH = 7.7 Hz, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.42 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.34 (s, 2H, 2 H arom Xyl), 7.12 (s, 1H, H arom Xyl), 5.98 (s, 2H, CH₂N), 4.51 (s, 2H, CH₂Br), 2.36 (s, 6H, 2 CH₃). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ 157.4 (C_q arom), 152.9 (C_q arom), 141.0 (2 C_q arom), 138.9 (CH arom), 136.1 (CH arom), 134.7 (C_q arom), 132.0 (CH arom), 124.0 (CH arom), 123.9 (CH arom), 123.6 (CH arom), 120.7 (CH arom), 119.7 (2 CH arom), 53.9 (CH₂N), 34.0 (CH₂Br), 21.2 (2 CH₃). HRMS (ESI): m/z 356.0751 [(M − Br)⁺] (exact mass calculated for C₁₈H₁₉BrN₃: 356.0757).

In a subsequent step, to a solution of Ph₂P(BH₃)H (0.483 g, 2.42 mmol) in THF (10 mL) was added a solution of KO^tBu (0.271 g, 2.42 mmol) in THF (10 mL). The resulting mixture was stirred for 10 min, and added to a solution of [2-bromomethyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine bromide (1.01 g, 2.39 mmol) in MeCN (20 mL). The suspension was stirred overnight, and MeOH (15 mL) was added to quench the reaction. The solvent was evaporated under vacuum, and the resulting solid was extracted with CH₂Cl₂ (3 × 10 mL). Solvent removal followed by washings with Et₂O (2 × 10 mL) yields a light orange solid which should correspond to the borane adduct of 1b(Br). This solid was dissolved in MeOH (10 mL), and the solution was transferred to a Fisher–Porter vessel and heated to 75 °C for 24 h. Volatiles were removed under vacuum, and MeOH was newly added and the previous procedure was repeated. The resulting solid was washed with toluene (10 mL) and Et₂O (10 mL) to give an off-white solid (0.693 g, 55% yield). ¹H NMR (500 MHz, CD₂Cl₂): δ 10.77 (s, 1H, H arom Imid), 7.65 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.61 (t, ³J_HH = 1.2 Hz, 1H, H arom Imid), 7.56 (m, 2H, H arom Py + H arom Imid), 7.38 (m, 4H, 4 H arom PPh), 7.32 (s, 2H, 2 H arom Xyl), 7.29 (m, 6H, 6 H arom PPh), 7.13 (s, 1H, H arom Xyl), 7.07 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 5.84 (s, 2H, CH₂N), 3.62 (s, 2H, CH₂P), 2.37 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ −11.7. ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ 159.0 (d, J_CP = 8 Hz, C_q arom), 152.3 (C_q arom), 141.0 (2 C_q arom), 138.4 (d, J_CP = 15 Hz, 2 C_q arom), 137.9 (CH arom), 135.9 (CH arom), 134.8 (C_q arom), 132.9 (d, J_CP = 19 Hz, 4 CH arom), 131.9 (CH arom), 129.1 (2 CH arom), 128.7 (d, J_CP = 7 Hz, 4 CH arom), 124.2 (d, J_CP = 5 Hz, CH arom), 123.9 (CH arom), 121.6 (CH arom), 120.3 (CH arom), 119.7 (2 CH arom), 54.0 (CH₂N), 38.3 (d, J_CP = 16 Hz, CH₂P), 21.3 (2 CH₃). HRMS (ESI): m/z 462.2089 [(M − Br)⁺] (exact mass calculated for C₃₀H₂₉N₃P: 462.2099).

Synthesis of Pd–CNP complexes 2a and 3a

Complex 2a

A solution of 1a(Cl) (0.100 g, 0.20 mmol) in CH₂Cl₂ (7 mL) was added Ag₂O (0.049 g, 0.21 mmol). The suspension was stirred for 24 h, and filtered. To the resulting solution was added PdCl₂(cod) (0.057 g, 0.20 mmol). After stirring for 4 h, solvent was removed under vacuum, and the residue was washed with Et₂O (2 × 5 mL), extracted with CH₂Cl₂ (2 × 5 mL), and crystallized from a CH₂Cl₂/toluene solvent mixture. Pale yellow solid (0.096 g, 60%). Anal. calcd (%) for C₃₁H₃₀AgCl₃N₃PPd: C 46.8; H 3.8; N 5.3; found: C 47.2; H 3.7; N 4.8. ¹H NMR (500 MHz, CD₂Cl₂): δ 8.30 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 8.21 (s, 1H, H arom NHC), 8.05 (dd, ³J_HH = 7.5 Hz, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.92 (d, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.73 (dd, ³J_HP = 12.1 Hz, ³J_HH = 8.1 Hz, 4H, 4 H arom PPh), 7.58 (t, ³J_HH = 7.5 Hz, 2H, 2 H arom PPh), 7.50 (m, 4H, 4 H arom PPh₂), 7.03 (s, 2H, 2 H arom Mes), 6.95 (s, 1H, H arom NHC), 6.05 (s, 2H, CH₂N), 4.32 (d, ²J_HP = 11.3 Hz, 2H, CH₂P), 2.39 (s, 3H, CH₃), 2.17 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 34.8. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 166.4 (d, J_CP = 183 Hz, C-2 NHC), 161.9 (d, J_CP = 7 Hz, C_q arom), 155.1 (C_q arom), 142.1 (CH arom), 139.6 (C_q arom), 135.8 (C_q arom), 135.5 (2 C_q arom), 133.3 (d, J_CP = 11 Hz, 4 CH arom), 132.7 (2 CH arom), 129.7 (d, J_CP = 12 Hz, 4 CH arom), 129.0 (2 CH arom), 126.6 (d, J_CP = 46 Hz, 2 C_q arom), 126.4 (CH arom), 125.4 (d, J_CP = 10 Hz, CH arom), 124.0 (d, J_CP = 5 Hz, CH arom), 123.4 (d, J_CP = 5 Hz, CH arom), 55.2 (CH₂N), 41.7 (d, J_CP = 28 Hz, CH₂P), 21.3 (CH₃), 18.6 (2 CH₃). MS (ESI, CH₂Cl₂): m/z (%): 616 (100) [(M − AgCl₂)⁺].

Complex 3a

To a suspension of 2a (0.025 g, 0.04 mmol) in THF (2 mL) was added KHMDS (0.007 g, 0.04 mmol). The mixture was stirred for 30 min, and solvent was evaporated under reduced pressure. The residue was washed with Et₂O (2 × 2 mL) and extracted with THF (2 × 2 mL). Solvent removal under vacuum provides complex 3a as an orange solid (0.020 g, 85%). An analytical pure sample of 3a could not be obtained due to significant decomposition of the complex during purification.

¹H NMR (400 MHz, THF-d₈): δ 7.71 (m, 4H, 4 H arom PPh₂), 7.44 (s, 1H, H arom NHC), 7.22 (m, 6H, 6 H arom PPh₂), 6.98 (s, 1H, H arom NHC), 6.87 (s, 2H, 2 H arom Mes), 6.46 (ddd, ³J_HH = 8.9 Hz, ³J_HH = 6.4 Hz, ⁵J_HP = 2.8 Hz, 1H, H^c), 6.30 (d, ³J_HH = 8.7 Hz, 1H, H^b), 5.55 (d, ³J_HH = 6.3 Hz,1H, H^d), 4.87 (s, 2H, CH₂N), 3.41 (s, 1H, H^a), 2.30 (s, 3H, CH₃), 2.08 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (162 MHz, THF-d₈): δ 25.3. ¹³C{¹H} NMR (126 MHz, THF-d₈): δ 175.1 (d, J_CP = 166 Hz, C-2 NHC), 174.8 (d, J_CP = 26 Hz, C_q arom), 150.0 (C_q arom), 138.7 (C_q arom), 137.9 (C_q arom), 137.4 (d, J_CP = 8 Hz, 2 C_q arom), 136.2 (2 C_q arom), 133.3 (d, J_CP = 11 Hz, 4 CH arom), 132.7 (C^c), 129.6 (2 CH arom), 129.0 (2 CH arom), 128.2 (d, J_CP = 11 Hz, 4 CH arom), 123.4 (d, J_CP = 5 Hz, CH arom), 121.2 (d, J_CP = 4 Hz, CH arom), 117.0 (d, J_CP = 21 Hz, C^b), 102.9 (C^d), 63.3 (d, J_CP = 66 Hz, C^a), 56.9 (CH₂N), 21.2 (CH₃), 18.6 (2 CH₃).

Synthesis of Ir–CNP complexes 4–9

Complex 4a(Cl)

A solution of 1a(Cl) (0.769 g, 1.50 mmol) in CH₂Cl₂ (8 mL) was added to a solution of Ir(acac)(cod) (0.600 g, 1.50 mmol) in CH₂Cl₂ (8 mL). The resulting solution was stirred overnight. Solvent was evaporated, and the solid was recrystallized from cold THF. The obtained solid was washed with Et₂O (2 × 10 mL) and pentane (2 × 10 mL). Yellow solid (0.682 g, 56%). ¹H NMR (400 MHz, CD₂Cl₂): δ 8.46 (s, 1H, H arom NHC), 8.33 (d, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.88 (dd, ³J_HH = 7.5 Hz, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.79 (dd, ³J_HP = 8.5 Hz, ³J_HH = 8.5 Hz, 2H, 2 H arom PPh), 7.62 (m, 3H, 3 H arom), 7.38 (d, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.12 (d, ²J_HH = 14.0 Hz, 1H, NCHH), 7.08 (t, ³J_HH = 7.5 Hz, 1H, H arom), 6.89 (m, 3H, 3 H arom), 6.75 (s, 1H, H arom), 6.65 (s, 1H, 1H, H arom NHC), 5.87 (dd, ³J_HP = 8.0 Hz, ³J_HH = 8.0 Hz, 2H, 2 H arom PPh), 5.56 (d, ²J_HH = 14.0 Hz, 1H, NCHH), 3.97 (dd, ²J_HH = 14.8 Hz, ²J_HP = 11.5 Hz, 1H, PCHH), 3.45 (m, 3H, 2 CH = COD + PCHH), 2.93 (br, 2H, 2 CH = COD), 2.31 (s, 3H, CH₃), 2.05 (br, 4H, 4 CHH COD), 1.75 (s, 3H, CH₃), 1.71 (br, 2H, 2 CHH COD), 1.28 (br, 2H, 2 CHH COD), 0.98 (s, 3H, CH₃). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 16.9. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 164.4 (d, J_CP = 8 Hz, C-2 NHC), 160.1 (d, J_CP = 4 Hz, C_q arom), 158.5 (d, J_CP = 6 Hz, C_q arom), 139.5 (CH arom + C_q arom), 137.7 (C_q arom), 136.8 (d, J_CP = 18 Hz, C_q arom), 135.9 (C_q arom), 135.3 (C_q arom), 134.0 (CH arom), 133.8 (CH arom), 131.6 (d, J_CP = 2 Hz, CH arom), 130.3 (d, J_CP = 10 Hz, 2 CH arom), 130.1 (d, J_CP = 2 Hz, CH arom), 130.1 (d, J_CP = 39 Hz, C_q arom), 129.3 (d, J_CP = 10 Hz, 2 CH arom), 129.2 (d, J_CP = 8 Hz, 2 CH arom), 128.8 (d, J_CP = 10 Hz, 2 CH arom), 125.0 (d, J_CP = 5 Hz, CH arom), 124.7 (CH arom), 124.3 (CH arom), 123.6 (d, J_CP = 2 Hz, CH arom), 63.5 (br, 4 CH = COD), 59.5 (NCH₂), 44.3 (d, J_CP = 29 Hz, PCH₂), 37.7 (d, J_CP = 6 Hz, 2 CH₂ COD), 28.8 (br, 2 CH₂ COD), 21.0 (CH₃), 18.0 (CH₃), 17.5 (CH₃). MS (ESI, CH₂Cl₂): m/z (%): 776 (100) [(M − Cl)⁺] (fragmentation of ion m/z 776: 666 (100) [(M − HCl − C₈H₁₂)⁺]). HRMS (ESI): m/z 776.2740 [(M − Cl)⁺] (exact mass calculated for C₃₉H₄₂N₃IrP: 776.2746).

Complex 4b(Cl)

To a solution of Ir(acac)(cod) (0.269 g, 0.67 mmol) in CH₂Cl₂ (5 mL) was added 1b(Cl) (0.335 g, 0.67 mmol) in CH₂Cl₂ (8 mL), and the reaction mixture was stirred for 4 h. After solvent evaporation, the solid was extracted with CH₃CN (5 mL). The solvent was removed in vacuo, and the resulting solid was washed with toluene (3 × 3 mL) and Et₂O (3 × 5 mL), and dried. Orange-yellow solid (0.412 g, 77%). Anal. calcd (%) for C₃₈H₄₀ClIrN₃P·CH₂Cl₂: C 53.1, H 4.8, N 4.8; found: C 53.4, H 5.0, N 4.75. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.42 (d, ³J_HH = 4.0 Hz, 1H, H arom NHC), 8.32 (d, ³J_HH = 8.0 Hz, 1H, H arom Py), 7.90 (dd, ³J_HH = 8.0 Hz, ³J_HH = 8.0 Hz, 1H, H arom Py), 7.83 (ddd, ³J_HP = 10.4 Hz, ³J_HH = 8.0 Hz, ⁴J_HH = 1.6 Hz, 2H, 2 H arom PPh), 7.63 (m, 3H, 3 H arom PPh), 7.51 (d, ³J_HH = 8.0 Hz, 1H, H arom Py), 7.20 (d, ²J_HH = 12.0 Hz, 1H, NCHH), 7.05 (td, ³J_HH = 7.6 Hz, ⁵J_HP = 1.2, 1H, H arom PPh), 6.84 (d, ³J_HH = 4.0 Hz, 1H, H arom NHC), 6.83 (s, 1H, H arom Xyl), 6.76 (m, 2H, 2 H arom PPh), 6.53 (s, 2H, 2 H arom Xyl), 5.59 (d, ²J_HH = 12.0 Hz, 1H, NCHH), 5.43 (dd, ³J_HH = 7.7 Hz, ³J_HH = 7.7 Hz, 2H, 2 H arom PPh), 4.19 (dd, ²J_HH = 15.6 Hz, ²J_HP = 11.6, 1H, PCHH), 3.46 (br, 2H, 2 CH = COD), 3.35 (dd, ²J_HH = 15.6 Hz, ²J_HP = 3.2 Hz, 1H, PCHH), 2.96 (br, 2H, 2 CH = COD), 2.32 (m, 4H, 4 CHH COD), 2.12 (s, 6H, 2 CH₃), 1.94 (m, 2H, 2 CHH COD), 1.39 (m, 2H, 2 CHH COD). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 19.9. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 164.5 (d, J_CP = 8 Hz, C-2 NHC), 160.3 (d, J_CP = 4.0 Hz, C_q arom), 159.0 (d, J_CP = 6 Hz, C_q arom), 139.5 (2 C_q arom), 139.2 (C_q arom), 138.4 (CH arom), 135.2 (d, J_CP = 25 Hz, C_q arom), 134.4 (d, J_CP = 13 Hz, 2 CH arom), 131.8 (d, J_CP = 2 Hz, CH arom), 129.6 (CH arom), 129.4 (m, 3 CH arom), 129.2 (d, J_CP = 10 Hz, 2 CH arom), 129.0 (d, J_CP = 8 Hz, 2 CH arom), 128.5 (d, J_CP = 47 Hz, C_q arom), 124.6 (d, J_CP = 4 Hz, CH arom), 124.1 (CH arom), 123.4 (d, J_CP = 2 Hz, CH arom), 122.1 (2 CH arom), 121.8 (CH arom), 66.1 (br, 4 CH = COD), 59.1 (NCH₂), 44.7 (d, J_CP = 27 Hz, PCH₂), 38.4 (d, J_CP = 6 Hz, 2 CH₂ COD), 28.3 (2 CH₂ COD), 21.3 (2 CH₃).

Complex 4b(Br)

A solution of Ir(acac)(cod) (0.368 g, 0.92 mmol) in CH₂Cl₂ was added a solution of 1b(Br) (0.500 g, 0.92 mmol) in CH₂Cl₂, and the solution was stirred overnight. Solvent was evaporated, and the residue was extracted with CH₃CN (5 mL). The solution was brought to dryness, and the resulting solid was washed with toluene (7 mL) and Et₂O (7 mL) and dried. Pale orange solid (0.238 g, 31%). Anal. calcd (%) for C₃₈H₄₀BrIrN₃P: C 54.2, H 4.8, N 5.00; found: C 54.4, H 5.05, N 4.7. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.28 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 8.26 (d, ³J_HH = 1.5 Hz, 1H, H arom NHC), 7.92 (dd, ³J_HH = 7.7 Hz, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.83 (dd, ³J_HP = 8.9 Hz, ³J_HH = 8.9 Hz, 2H, 2 H arom PPh), 7.64 (m, 3H, 3 H arom PPh), 7.49 (d, ³J_HH = 7.8 Hz, 1H, H arom Py), 7.09 (t, ³J_HH = 7.5 Hz, 1H, H arom PPh), 6.91 (d, ²J_HH = 14.1 Hz, 1H, NCHH), 6.88 (d, ³J_HH = 1.5 Hz, 1H, H arom NHC), 6.85 (s, 1H, H arom Xyl), 6.79 (dd, ³J_HH = 6.6 Hz, ³J_HH = 6.6 Hz, 2H, 2 H arom PPh), 6.53 (s, 2H, 2 H arom Xyl), 5.65 (d, ²J_HH = 14.1 Hz, 1H, NCHH), 5.45 (dd, ³J_HH = 8.4 Hz, ³J_HH = 8.4 Hz, 2H, 2 H arom PPh), 4.17 (dd, ²J_HH = 15.5 Hz, ²J_HP = 11.6 Hz, 1H, PCHH), 3.49 (br, 2H, 2 CH = COD), 3.36 (dd, ²J_HH = 15.7 Hz, ²J_HP = 2.1 Hz, 1H, PCHH), 2.98 (br, 2H, 2 CH = COD), 2.35 (br, 4H, 2 CH₂ COD), 2.14 (s, 6H, 2 CH₃), 1.92 (br, 2H, CH₂ COD), 1.42 (br, 2H, CH₂ COD). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 20.0. ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): δ 164.8 (d, J_CP = 8 Hz, C-2 NHC), 160.4 (d, J_CP = 3 Hz, C_q arom), 158.8 (d, J_CP = 6 Hz, C_q arom), 139.6 (CH arom), 139.2 (C_q arom), 138.5 (2 C_q arom), 135.3 (d, J_CP = 25 Hz, C_q arom), 134.5 (d, J_CP = 13 Hz, 2 CH arom), 131.9 (CH arom), 129.8 (CH arom), 129.7 (CH arom), 129.5 (d, J_CP = 9 Hz, 2 CH arom), 129.3 (d, J_CP = 10 Hz, 2 CH arom), 129.1 (d, J_CP = 8 Hz, 2 CH arom), 128.6 (d, J_CP = 43 Hz, C_q arom), 124.7 (d, J_CP = 4 Hz, CH arom), 123.8 (CH arom), 123.4 (CH arom), 122.3 (2 CH arom), 122.2 (CH arom), 66.0 (br, 4 CH = COD), 59.4 (CH₂N), 44.7 (d, J_CP = 28 Hz, CH₂P), 38.5 (d, J_CP = 6 Hz, 2 CH₂ COD), 28.4 (2 CH₂ COD), 21.4 (2 CH₃). MS (ESI, CH₂Cl₂): m/z (%): 762 (100) [(M − Br)⁺] (fragmentation of ion m/z 762: 654 (100) [(M − Br − C₈H₁₂)⁺]).

Complex 4b(BArF)

A solution of 4b(Br) (0.100 g, 0.12 mmol) in CH₂Cl₂ (5 mL) was added to a solution of NaBAr_F (0.105 g, 0.12 mmol) in CH₂Cl₂ (5 mL). The resulting suspension was stirred for 4 h. The precipitate was filtered off, and the solvent was removed under vacuum to yield the complex as an orange solid (0.164 g, 85%). Crystals of complex 4b(BArF) suitable for X-ray diffraction analysis were grown by layering pentane over a CH₂Cl₂ solution. Anal. calcd (%) for C₇₀H₅₂BF₂₄IrN₃P: C 51.7, H 3.2, N 2.6; found: C 51.6, H 3.25, N 2.5. ¹H NMR (500 MHz, CD₂Cl₂): δ 7.86 (dd, ³J_HH = 7.6 Hz, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.81 (dd, ³J_HH = 8.5 Hz, ³J_HP = 6.7 Hz, 2H, 2 H arom PPh), 7.76 (s, 8H, 8 H arom BAr_F), 7.66 (m, 4H, 3 H arom PPh + H arom Py), 7.58 (s, 4H, 4 H arom BAr_F), 7.48 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.33 (d, ³J_HH = 2.0 Hz, 1H, H arom NHC), 7.11 (t, ³J_HH = 7.5 Hz, 1H, H arom PPh), 6.95 (d, ³J_HH = 2.0 Hz, 1H, H arom NHC), 6.93 (s, 1H, H arom Xyl), 6.82 (ddd, ³J_HH = 8.1 Hz, ³J_HH = 8.1 Hz, ⁴J_HP = 1.9 Hz, 2H, 2 H arom PPh), 6.49 (s, 2H, 2 H arom Xyl), 5.86 (d, ²J_HH = 14.2 Hz, 1H, NCHH), 5.48 (dd, ³J_HH = 8.7 Hz, ³J_HP = 8.7 Hz, 2H, 2 H arom PPh), 5.44 (d, ²J_HH = 14.2 Hz, 1H, NCHH), 4.16 (dd, ²J_HH = 15.6 Hz, ²J_HP = 11.3 Hz, 1H, PCHH), 3.55 (br, 2H, 2 CH = COD), 3.39 (dd, ²J_HH = 15.6 Hz, ²J_PH = 3.0 Hz, 1H, PCHH), 3.02 (br, 2H, 2 CH = COD), 2.39 (m, 4H, 4 CHH COD), 2.16 (s, 6H, 2 CH₃), 1.95 (m, 2H, 2 CHH COD), 1.49 (m, 2H, 2 CHH COD). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ 20.3. ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ 166.1 (d, J_CP = 8 Hz, C-2 NHC), 162.2 (q, J_CB = 50 Hz, 4 BC_q arom BAr_F), 161.5 (d, J_CP = 3 Hz, C_q arom), 157.5 (d, J_CP = 6 Hz, C_q arom), 139.7 (CH arom), 139.0 (d, J_CP = 10 Hz, C_q arom), 139.0 (2 C_q arom), 135.3 (m, 8 CH arom BAr_F), 135.1 (overlapped, C_q arom), 134.5 (d, J_CP = 14 Hz, 2 CH arom), 132.2 (d, J_CP = 2 Hz, CH arom), 130.5 (CH arom), 130.1 (d, J_CP = 2 Hz, CH arom), 129.7 (d, J_CP = 10 Hz, 2 CH arom), 129.5 (d, J_CP = 10 Hz, 2 CH arom), 129.3 (d, J_CP = 8 Hz, 2 CH arom), 129.3 (q, J_CF = 32 Hz, 8 C_q arom BAr_F), 128.2 (d, J_CP = 38 Hz, C_q arom), 125.3 (d, J_CP = 5 Hz, CH arom), 124.8 (q, J_CF = 272 Hz, 8 CF₃), 123.4 (CH arom), 122.6 (2 CH arom), 122.2 (CH arom), 122.0 (CH arom), 117.9 (m, 4 CH arom BAr_F), 66.2 (br, 4 CH = COD), 60.7 (NCH₂), 44.7 (d, J_CP = 28 Hz, PCH₂), 38.3 (d, J_CP = 6 Hz, 2 CH₂ COD), 28.6 (2 CH₂ COD), 21.4 (2 CH₃). MS (ESI, CH₂Cl₂): m/z (%): 762 (100) [(M − C₃₂H₁₂BF₂₄)⁺] (fragmentation of ion m/z 762: 654 (100) [(M − C₃₂H₁₂BF₂₄ − C₈H₁₂)⁺]).

Complex 5a(Cl)

A solution of 4a(Cl) (0.080 g, 0.10 mmol) in CH₂Cl₂ (10 mL) was bubbled with CO for 5 min, and the solvent was evaporated. The resulting solid was washed with Et₂O (2 × 10 mL) and pentane (2 × 10 mL), and crystallized from THF. Orange solid (0.048 g, 70%). Anal. calcd (%) for C₃₂H₃₀ClIrN₃OP: C 52.6, H 4.1, N 5.75; found: C 52.2, H 4.6, N 5.5. IR (CH₂Cl₂): 1985 cm⁻¹ (ν_CO). ¹H NMR (300 MHz, CD₂Cl₂): δ 8.41 (s, 1H, H arom NHC), 8.28 (d, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.97 (dd, ³J_HH = 7.6 Hz, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.77 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.61 (ddd, ³J_HP = 11.8 Hz, ³J_HH = 7.7 Hz, ⁴J_HH = 0.6 Hz, 4H, 4 H arom PPh), 7.46 (m, 6H, 6 H arom PPh), 7.02 (m, 3H, 2 H arom Mes + H arom NHC), 6.11 (s, 2H, CH₂N), 4.18 (d, ²J_HP = 10.0 Hz, 2H, CH₂P), 2.34 (s, 3H, CH₃), 2.12 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 45.7. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 178.1 (d, J_CP = 99 Hz, C-2 NHC), 177.2 (d, J_CP = 10 Hz, CO), 164.7 (d, J_CP = 7 Hz, C_q arom), 156.5 (C_q arom), 141.4 (CH arom), 140.1 (C_q arom), 136.3 (2 C_q arom), 135.7 (C_q arom), 133.2 (d, J_CP = 12 Hz, 4 CH arom), 131.9 (d, J_CP = 2 Hz, 2 CH arom), 130.3 (d, J_CP = 53 Hz, 2 C_q arom), 129.4 (d, J_CP = 11 Hz, 4 CH arom), 129.2 (2 CH arom), 125.4 (CH arom), 124.4 (d, J_CP = 10 Hz, CH arom), 123.6 (d, J_CP = 3 Hz, CH arom), 122.0 (d, J_CP = 3 Hz, CH arom), 54.6 (CH₂N), 42.7 (d, J_CP = 31 Hz, CH₂P), 21.2 (CH₃), 18.5 (2 CH₃).

Complex 6a(Cl)

In a J. Young valved NMR tube, a solution of 5a(Cl) (0.011 g, 0.01 mmol) in CD₂Cl₂ (0.7 mL) was pressurised with 1 bar of CO. The solution was analyzed by NMR spectroscopy. IR (CH₂Cl₂): 1946, 2021 cm⁻¹ (ν_CO). ¹H NMR (400 MHz, CD₂Cl₂): δ 8.57 (d, ³J_HH = 0.8 Hz, 1H, H arom NHC), 8.41 (d, ³J_HH = 7.5 Hz, 1H, H arom Py), 7.98 (dd, ³J_HH = 7.6 Hz, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.74 (d, ³J_HH = 7.7 Hz, 1H, H arom Py), 7.47 (m, 10H, 10 H arom PPh), 7.01 (d, ³J_HH = 0.8 Hz, 1H, H arom NHC), 6.98 (s, 2H, 2 H arom Mes), 6.09 (s, 2H, CH₂N), 4.29 (d, ²J_HP = 10.9 Hz, 2H, CH₂P), 2.32 (s, 3H, CH₃), 2.03 (s, 6H, 2 CH₃). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 26.8. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 177.4 (CO), 161.5 (C_q arom), 161.3 (d, J_CP = 94 Hz, C-2 NHC), 155.1 (C_q arom), 140.5 (CH arom), 140.2 (C_q arom), 136.1 (2 C_q arom), 135.7 (C_q arom), 132.5 (d, J_CP = 12 Hz, 4 CH arom), 131.1 (2 CH arom), 129.5 (d, J_CP = 11 Hz, 4 CH arom), 129.4 (d, J_CP = 55 Hz, 2 C_q arom), 129.4 (2 CH arom), 125.2 (CH arom), 124.4 (CH arom), 124.0 (d, J_CP = 9 Hz, CH arom), 122.8 (CH arom), 56.7 (CH₂N), 44.9 (d, J_CP = 37 Hz, CH₂P), 21.2 (CH₃), 18.1 (2 CH₃).

Complex 7a(Cl)

In a Fisher–Porter vessel, a solution of 4a(Cl) (0.120 g, 0.15 mmol) in CH₂Cl₂ (8 mL) was pressurised with 2 bar of H₂ and stirred overnight. The system was depressurised, solvent was evaporated and the residue was washed with Et₂O (2 × 10 mL) and pentane (2 × 10 mL). Yellow solid (0.083 g, 80%). Crystals of complex 7a(Cl) suitable for X-ray diffraction analysis were grown by layering hexane over a CH₂Cl₂ solution. Anal. calcd (%) for C₃₁H₃₂ClIrN₃P: C 52.8, H 4.6, N 5.9; found: C 52.9, H 4.7, N 5.4. ¹H NMR (400 MHz, CD₂Cl₂): δ 7.79 (dd, ³J_HH = 7.8 Hz, ³J_HH = 7.8 Hz, 1H, H arom Py), 7.73 (dd, ³J_HP = 10.6 Hz, ³J_HH = 7.9 Hz, 2H, 2 H arom PPh), 7.57 (d, ³J_HH = 7.8 Hz, 1H, H arom Py), 7.40 (m, 6H, 6 H arom), 7.33 (m, 4H, 4 H arom), 7.09 (s, 1H, H arom NHC), 7.02 (s, 1H, H arom Mes), 6.97 (s, 1H, H arom Mes), 6.70 (d, ²J_HH = 14.6 Hz, 1H, NCHH), 4.96 (d, ²J_HH = 14.6 Hz, 1H, NCHH), 4.41 (dd, ²J_HH = 16.3 Hz, ²J_HP = 10.4 Hz, 1H, PCHH), 3.38 (dd, ²J_HH = 16.3 Hz, ²J_PH = 9.5 Hz, 1H, PCHH), 2.40 (s, 3H, CH₃), 2.20 (s, 3H, CH₃), 1.92 (s, 3H, CH₃), −20.19 (dd, ²J_HP = 13.8 Hz, ²J_HH = 7.0 Hz, 1H, IrH trans to Py), −23.30 (dd, ²J_HP = 18.9 Hz, ²J_HH = 7.0 Hz, 1H, IrH cis to Py). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 26.8. ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): δ 172.9 (d, J_CP = 119 Hz, C-2 NHC), 164.8 (d, J_CP = 6 Hz, C_q arom), 156.1 (C_q arom), 138.6 (C_q arom), 138.2 (C_q arom), 136.9 (C_q arom), 136.5 (CH arom), 135.7 (d, J_CP = 50 Hz, C_q arom), 135.4 (C_q arom), 134.7 (d, J_CP = 13 Hz, 2 CH arom), 132.4 (d, J_CP = 11 Hz, 2 CH arom), 130.5 (d, J_CP = 2 Hz, CH arom), 129.5 (CH arom), 129.2 (CH arom), 128.6 (CH arom), 128.3 (d, J_CP = 10 Hz, 2 CH arom), 128.0 (d, J_CP = 9 Hz, 2 CH arom), 122.7 (CH arom), 122.3 (d, J_CP = 9 Hz, CH arom), 121.2 (d, J_CP = 4 Hz, CH arom), 120.4 (d, J_CP = 4 Hz, CH arom), 56.7 (CH₂N), 47.1 (d, J_CP = 33 Hz, CH₂P), 21.3 (CH₃), 18.9 (CH₃), 18.5 (CH₃). Signals for one quaternary aromatic carbon could not be identified.

Complex 7b(Cl)

In a Fisher–Porter vessel, a solution of 4b(Cl) (0.100 g, 0.12 mmol) in CH₂Cl₂ (5 mL) was pressurised with 5 bar of H₂ and heated to 50 °C. After 16 h, the system was cooled to room temperature and depressurised. The solvent was evaporated and the residue was washed with Et₂O (3 × 3 mL) and pentane (3 × 3 mL). Pale yellow solid (0.073 g, 85%). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.85 (m, 2H, 2 H arom PPh), 7.74 (dd, ³J_HH = 8.0 Hz, ³J_HH = 8.0 Hz, 1H, H arom Py), 7.56 (d, ³J_HH = 8.0 Hz, 1H, H arom Py), 7.47 (m, 4H, 4 H arom), 7.36 (m, 4H, 4 H arom), 7.29 (m, 3H, 3 H arom), 7.24 (d, ³J_HH = 1.5 Hz, 1H, H arom NHC), 7.18 (m, 1H, H arom), 7.11 (s, 1H, H arom), 6.53 (d, ²J_HH = 14.4 Hz, 1H, NCHH), 4.96 (d, ²J_HH = 14.8 Hz, 1H, NCHH), 4.49 (dd, ²J_HH = 16.4 Hz, ²J_HP = 10.4 Hz, 1H, CHHP), 3.54 (dd, ²J_HH = 16.6 Hz, ²J_HP = 10.0 Hz, 1H, CHHP), 2.44 (s, 6H, 2 CH₃), −19.73 (dd, ²J_HP = 16.6 Hz, ²J_HH = 7.2 Hz, 1H, IrH trans to Py), −23.24 (dd, ²J_HP = 18.8 Hz, ²J_HH = 7.2 Hz, 1H, IrH cis to Py). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 27.7. ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): δ 172.4 (d, J_CP = 122 Hz, C-2 NHC), 164.9 (d, J_CP = 5 Hz, C_q arom), 155.8 (C_q arom), 141.9 (C_q arom), 138.5 (2 C_q arom), 136.8 (2 CH arom), 136.3 (d, J_CP = 52 Hz, C_q arom), 134.6 (d, J_CP = 12 Hz, 2 CH arom), 134.2 (d, J_CP = 38 Hz, C_q arom), 132.9 (d, J_CP = 11 Hz, 2 CH arom), 130.7 (d, J_CP = 2 Hz, CH arom), 129.8 (d, J_CP = 1 Hz, CH arom), 128.6 (d, J_CP = 10 Hz, 2 CH arom), 128.2 (d, J_CP = 9 Hz, 2 CH arom), 125.7 (2 CH arom), 122.7 (CH arom), 122.6 (d, J_CP = 9.0 Hz, CH arom), 121.5 (d, J_CP = 4 Hz, CH arom), 121.4 (d, J_CP = 5 Hz, CH arom), 57.1 (CH₂N), 46.3 (d, J_CP = 32 Hz, CH₂P), 21.6 (2 CH₃). HRMS (ESI): m/z 656.1794 [(M − Cl)⁺] (exact mass calculated for C₃₀H₃₀N₃IrP: 656.1801).

Complex 8b

In a J. Young valved NMR tube, a suspension of 4b(Br) (0.030 g, 0.036 mmol) in THF-d₈ (0.7 mL) was treated with KO^tBu (0.004 g, 0.039 mmol) forming a dark red solution. The solution was kept to 0 °C to avoid thermal decomposition of the product. Satisfactory elemental analysis could not be obtained due to the low thermal stability of the product.

¹H NMR (400 MHz, THF-d₈, 273 K): δ 7.98 (ddd, ³J_HH = 7.9 Hz, ³J_HP = 7.9 Hz, ⁴J_HH = 1.6 Hz, 2H, 2 H arom PPh), 7.50 (d, ³J_HH = 2.0 Hz, 1H, H arom NHC), 7.45 (m, 3H, 3 H arom PPh), 7.22 (d, ³J_HH = 2.0 Hz, 1H, H arom NHC), 7.00 (s, 2H, 2 H arom Xyl), 6.92 (td, ³J_HH = 7.4 Hz, ⁵J_HP = 1.5 Hz, 1H, H arom PPh), 6.81 (ddd, ³J_HH = 7.7 Hz, ³J_HH = 7.7 Hz, ⁴J_HP = 1.4 Hz, 2H, 2 H arom PPh), 6.79 (s, 1H, H arom Xyl), 6.65 (dd, ³J_HH = 7.5 Hz, ³J_HP = 7.5 Hz, 2 H, 2 H arom PPh), 6.39 (ddd, ³J_HH = 8.5 Hz, ³J_HH = 6.3 Hz, ⁵J_HP = 1.9 Hz, 1H, H^c), 5.98 (d, ³J_HH = 8.6 Hz, 1H, H^b), 5.58 (d, ³J_HH = 6.2 Hz, 1H, H^d), 5.29 (d, ²J_HH = 13.6 Hz, 1H, NCHH), 4.93 (d, ²J_HH = 13.6 Hz, 1H, NCHH), 3.86 (s, 1H, H^a), 3.09 (br, 2H, 2 CH = COD), 2.62 (br, 2H, 2 CH = COD), 2.16 (s, 6H, 2 CH₃), 2.04 (br, 4H, 4 CHH COD), 1.89 (br, 2H, 2 CHH COD), 1.66 (br, 2H, 2 CHH COD). ³¹P{¹H} NMR (162 MHz, THF-d₈, 273 K): δ 17.7. ¹³C{¹H} NMR (101 MHz, THF-d₈, 273 K): δ 170.6 (m, C-2 NHC + C_q arom), 153.6 (d, J_CP = 6 Hz, C_q arom), 148.8 (d, J_CP = 15 Hz, C_q arom), 140.8 (C_q arom), 138.5 (2 C_q arom), 136.8 (d, J_CP = 53 Hz, C_q arom), 134.0 (d, J_CP = 10 Hz, 2 CH arom), 131.4 (d, J_CP = 2 Hz, C^c), 130.3 (d, J_CP = 11 Hz, 2 CH arom), 129.2 (m, 2 CH arom), 128.4 (d, J_CP = 8 Hz, 2 CH arom), 128.3 (d, J_CP = 8 Hz, 2 CH arom), 127.1 (CH arom), 122.6 (2 CH arom), 122.0 (CH arom), 121.9 (CH arom), 114.2 (d, J_CP = 14 Hz, C^b), 100.3 (C^d), 76.5 (d, J_CP = 59 Hz, C^a), 61.6 (CH₂N), 37.3 (br d, J_CP = 3 Hz, 2 CH₂ COD), 30.7 (br, 2 CH₂ COD), 21.5 (2 CH₃). Signals for the four olefinic carbons could not be identified probably due to significant line broadening. cis Coordination of the phosphine and NHC fragments of the CNP ligand is proposed on the basis of the following NOE contacts:

Complex 9b

In a J. Young valved NMR tube, a suspension of 4b(Br) (0.030 g, 0.036 mmol) in THF-d₈ (0.7 mL) cooled to 0 °C was treated with KO^tBu (0.004 g, 0.039 mmol). The NMR tube was charged with 5 bar of H₂ and kept to 0 °C to avoid thermal decomposition of the product. The resulting solution was analysed by NMR spectroscopy.

In a J. Young valved NMR tube, a suspension cooled to −20 °C of 7b(Cl) (0.012 g, 0.017 mmol) in THF-d₈ (0.7 mL) was treated with KO^tBu (0.002 g, 0.018 mmol). Immediately, the NMR tube was charged with 5 bar of H₂ and kept to 0 °C to avoid thermal decomposition of the product. After 1 h, the resulting solution was analysed by NMR spectroscopy.

¹H NMR (400 MHz, THF-d₈): δ 7.78 (dd, ³J_HP = 8.5 Hz, ³J_HH = 8.5 Hz, 4H, 4 H arom PPh), 7.82 (s, 2H, 2 H arom), 7.50 (dd, ³J_HH = 7.6 Hz, ³J_HH = 7.6 Hz, 1H, H arom Py), 7.35 (m, 2H, 2 H arom), 7.24 (m, 6H, 6 H arom), 7.11 (m, 1H, H arom), 7.07 (s, 1H, H arom NHC), 6.94 (s, 1H, H arom NHC), 5.18 (s, 2H, NCH₂), 3.98 (d, ²J_PH = 10.0 Hz, 2H, PCH₂), 2.38 (s, 6H, 2 CH₃), −9.98 (dd, ²J_HP = 18.2 Hz, ²J_HH = 4.8 Hz, 2H, IrH cis to Py), −19.64 (dt, ²J_HP = 14.4 Hz, ²J_HH = 4.8 Hz, 1H, IrH trans to Py). ³¹P{¹H} NMR (162 MHz, THF-d₈): δ 30.9. ¹³C{¹H} NMR (101 MHz, THF-d₈): δ 176.9 (d, J_CP = 121 Hz, C-2 NHC), 164.7 (d, J_CP = 6 Hz, C_q arom), 155.9 (C_q arom), 143.0 (C_q arom), 139.1 (d, J_CP = 42 Hz, 2 C_q arom), 137.2 (2 C_q arom), 134.3 (d, J_CP = 13 Hz, 4 CH arom), 134.1 (CH arom), 129.4 (2 CH arom), 128.2 (CH arom), 127.8 (d, J_CP = 10 Hz, 4 CH arom), 125.5 (2 CH arom), 121.3 (CH arom), 121.1 (d, J_CP = 9 Hz, CH arom), 120.5 (CH arom), 120.1 (d, J_CP = 4 Hz, CH arom), 59.9 (CH₂N), 49.2 (d, J_CP = 34 Hz, CH₂P), 21.2 (2 CH₃).

Representative procedure for ketone hydrogenation

In a glovebox, a Fischer–Porter vessel was charged with a solution of complex 4a(Cl) (2.0 mg, 2.5 μmol), KO^tBu (2.7 mg, 37 μmol) and acetophenone (30 μL, 0.26 mmol) in 2-methyltetrahydrofuran (2.0 mL). The reactor was purged three times with H₂, and finally pressurized to 1 bar and heated to 60 °C. After 16 h, the reactor was slowly cooled down to room temperature, the reaction solution was evaporated, and conversion was determined by ¹H NMR spectroscopy using mesitylene as internal standard.

Acknowledgements

Financial support (FEDER contribution) from the Spanish MINECO (CTQ2013-45011-P, CTQ2016-80814-R and CTQ2014-51912-REDC) is gratefully acknowledged. M. H. J. thanks SECITI-DF for a postdoctoral fellowship.

Notes and references

Footnote