Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands

Selective, Base-Free Hydrogenation of Aldehydes Catalyzed by Ir Complexes Based on Proton-Responsive Lutidine-Derived CNP Ligands Práxedes Sánchez, Martín Hernández-Juárez, Nuria Rendón,*JoaquínLópez-Serrano, Eleuterio Alvarez, Margarita Paneque, and AndrésSuárez* Cite This: Organometallics 2021, 40, 1314−1327 Read Online ACCESS Metrics & More Article Recommendations * sıSupporting Information ABSTRACT: Metal catalysts based on ligands containing protonresponsive sites have found widespread applications in the hydrogenation of polar unsaturated substrates. In this contribution, Ir complexes incorporating lutidine-derived CNP (C = N-heterocyclic carbene, NHC; P = phosphine) pincer ligands with two nonequivalent Brønsted acid/base sites have been examined in the hydrogenation of aldehydes. To this end, Ir(CNP)H2Cl complexes were synthesized in two steps from the CNP ligand precursors and Ir(acac)(COD). These derivatives react with an excess of NaH to yield the trihydride derivatives Ir(CNP)H3, which were assessed as catalyst precursors in the hydrogenation of a series of aldehydes. The catalytic reactions were performed using commercial-grade substrates under neutral, mild conditions (0.1 mol % Ir-CNP; 4 bar H2, room temperature) with high conversions and selectivities for the reduction of the carbonyl function in the presence of other readily reducible groups such as CC, nitro, and halogens. Reaction of an Ir(CNP)H2Cl complex with base in the presence of an aromatic aldehyde produces the reversible formation of alkoxide Ir complexes in which the aldehyde is bound to the deprotonated pincer framework (CNP*) through the CH-NHC arm of the ligand. These species, along with a carboxylate complex resulting from the Ir mediated oxidation of the aldehyde by water, is observed in the reaction of Ir(CNP)H3with benzaldehyde. Finally, investigation of the mechanism of the hydrogenation of aldehydes has been carried out by means of DFT calculations considering the involvement of each arm of the Ir-CNP/CNP*derivatives. Calculations support a mechanism in which the catalyst switches its metal−ligand cooperation sites to follow the lowest energy pathway for each step of the catalytic cycle. ■INTRODUCTION Metal−ligand cooperation is a valuable concept for the design of novel catalysts for the development of sustainable, atomeconomical catalytic hydrogenation and dehydrogenation reactions of polar substrates. 1 A prominent class of metal− ligand complexes containing Brønsted acid/base functionalities are those based on lutidine-derived PNX (P = phosphine, X = phosphine or a hemilabile N-donor) pincer ligands (Figure 1). 2 In these systems, the acidic −CH2−methylene arms can participate in reversible proton transfer processes leading to deprotonated metal species that are capable to activate H−Y (Y = H, heteroatom) bonds. An interesting modification of the PNX ligands resides in the substitution of the P-donors by N- heterocyclic carbenes (NHCs) of similar electron-donor strength (Figure 1). 3−5 Metal complexes based on these CNX (C = NHC, X = NHC or N-donor) ligands are also readily deprotonated at the methylene linkers and can participate in ligand-assisted processes. Moreover, this modification offers larger flexibility to the ligand due to the Received: February 24, 2021 Published: April 22, 2021 Figure 1. General structures of metal complexes with lutidine-derived PNP, CNC, and CNP ligands. Article pubs.acs.org/Organometallics © 2021 American Chemical Society 1314 https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 Downloaded via UNIV DE SEVILLA on September 9, 2022 at 07:21:59 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. formation of 6-membered chelates upon coordination of the larger Py−CH2−NHC linkage in comparison to the 5- membered rings provided by the PNX pincers. This higher flexibility has been shown in Ru−CNC*(*denotes a deprotonated lutidine-derived pincer) complexes to provide enhanced reactivities toward the ligand-assisted activation of H2and the binding of CO2to the deprotonated metal−ligand framework in comparison to analogous Ru−PNP*systems. 3 A further approach to the development of improved catalytic systems that has been limitedly explored consists of the use of ligands incorporating two different Brønsted acid/base sites. 6 The underlying idea of this strategy is that the catalyst can switch its metal−ligand cooperation mode to choose the lowest energetically accessible pathway for each step of the catalytic cycle. Thus, metal complexes based on lutidinederived ligands containing secondary amines as flanking donors have provided efficient catalysts in a variety of (de)hydrogenations of polar substrates. 7−9 Since these derivatives contain two acidic functionalities, they might exhibit metal−ligand cooperation based on both pyridine aromatization/dearomatization and amine-metal/amido-metal interconversion. 10 Another of such potentially dual-mode systems are metal complexes incorporating ligands based on a 2-hydroxypyridine fragment and a picoline arm. 11 However, in all the above-mentioned systems the large difference in basicity between both proton-responsive sites impedes changes in the metal−ligand cooperation modes, and the reactions proceed predominantly via a single form of metal−ligand cooperation (i.e., amine/amide interconversion). 12 Recent research from our group has focused on the study of nonsymmetric lutidine-derived pincer ligands, i.e., having two inequivalent flanking donor groups, 13 particularly of the CNP class (Figure 1). 14−16 The presence of two significantly different strong σ-donors in the CNP pincer, such as a phosphine and a NHC, allows a larger electronic and steric diversity of the ligands, and facilitates a direct comparison of the relative acid/base reactivities of the NHC- and phosphinebound methylene arms. Herein, we report the synthesis of Ir complexes incorporating new CNP ligands having di-tert-butylphosphino side arms and their application to the chemoselective hydrogenation of commercial-grade aldehydes under neutral conditions. Aldehyde hydrogenation represents an environmentally friendly alternative to the use of alumino- and borohydrides commonly used in organic synthesis laboratories and industrial settings for the reduction of carbonyl groups. 17 Although hydrogenation of carbonyl derivatives, particularly ketones, is usually regarded as a well-developed technology, relatively few examples of catalytic systems have been described for the hydrogenation of aldehydes to primary alcohols, 18,19 most of them requiring previous extensive substrate purification and the use of a base as a catalyst activator, which is an important limitation to the reaction scope due to the sensitivity of aldehydes to basic conditions. Finally, experimental and theoretical mechanistic studies with the Ir-CNP complexes indicate that different acid/ base sites of the CNP ligands are involved in the individual steps of the catalytic cycle. ■RESULTS AND DISCUSSION Ligand Synthesis. Alkylation of di-tert-butylphosphine with the imidazolium salt 1a in CH2Cl2,followedby deprotonation of the resulting phosphonium salt 2a·HCl after treatment with phenethyldiethylamine ScavengePore resin provided the imidazolium salt 2a (Scheme 1). The CNP ligand precursor 2a, which produces a broad resonance at 37.1 ppm in the 31P{1H} NMR spectrum, was isolated with moderate purity (65−77% as determined by NMR), although it was employed without further purification for the synthesis of Ir-CNP complexes. An analogous procedure was used for the preparation of the ligand precursor 2b. Synthesis of Iridium Complexes. The preparation of Ir complexes containing the CNP ligands was initially assayed by the reaction of the imidazolium salt 2a with Ir(acac)(COD). 14 This procedure led to the formation of the fully characterized complex 3(Scheme 2). In the 1H NMR spectrum, this derivative exhibits a doublet signal in the hydride region at −24.5 ppm (2JHP = 17 Hz), and only two resonances (integrating for 3H each) in the region expected for the methyl substituents of the mesityl group (2.35 and 2.25 ppm). Moreover, two other resonances integrating for 1H are observed at 3.95 (d, 2JHH = 11.3 Hz) and 2.91 ppm (dd, 2JHH = 11.2 Hz, 3JHP = 9.1 Hz) that exhibit cross-peaks signals in the 1H−13C HSQC experiment with a carbon broad resonance appearing at δC−9.6 ppm, indicative of the presence of an Ir−C bond. Overall, these data points to the metalation of one of the CH3groups of the mesityl fragment, 20 as confirmed by a single-crystal X-ray diffraction analysis of the complex (Figure 2). Derivative 3possesses, in the solid state, a slightly distorted octahedral geometry with the metalated CNP ligand adopting a planar coordination with C(1)−Ir(1)−P(1) and C(10)−Ir(1)−N(3) angles of 166.8°and 172.8°, respectively. Moreover, the existence of a hydrogen bond between the chloride ligand and the axial hydrogen of the CH2N arm is evident from the H···Cl distance of 2.6 Å, which is shorter than the sum of van der Waals radii of H and Cl (2.9−3.0 Å). 21 This interaction is also manifested in the significant deshielding of the doublet resonance corresponding to the axial CHHN hydrogen in the 1H NMR spectrum (δH 7.07 ppm, 2JHH = 14.6 Hz). Overnight exposure to H2(4 bar) of a solution heated to 65 °Cof3in CH2Cl2produced the formation of the chlorodihydride complex 4a, which was isolated quantitatively as a yellow solid (Scheme 2). The hydride region of the 1H NMR spectrum of 4a shows two doublets of doublets at −20.36 (2JHP = 14.4 Hz, 2JHH = 6.5 Hz) and −24.89 ppm (2JHP = 17.2 Hz, 2JHH = 6.5 Hz) attributable to the IrH trans and cis to the pyridine fragment, respectively. In the 13C{1H} NMR experiment, the C-2 NHC was observed as a doublet resonance appearing to 173.2 ppm with a large JCP coupling constant of 115 Hz, suggesting trans coordination of the NHC and phosphine fragments of the pincer. Pressurization of a solution of 4a in CD2Cl2at room temperature with deuterium gas (3 bar) produced after 5 h the complete H/D scrambling of the hydrido ligands. However, deuterium incorporation at Scheme 1. Synthesis of Ligand Precursors 2 Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1315 the methylene arms of the pincer was not observed even after prolonged heating of the sample to 50 °C (72 h). On the other hand, reaction of the xylyl substituted salt 2b with Ir(acac)(COD) in CH2Cl2followed by exposure to H2(4 bar) at room temperature produced the expected dihydride complex 4b (Scheme 3). 14 Interestingly, formation of 4b was accompanied of the hydrogenation of the imidazolylidene ligand fragment yielding an inseparable mixture of 4b and 5 formed in an 8:2 ratio. The addition of dihydrogen to the C C double bond of the imidazolylidene moiety was readily deduced from the observation of new multiplet signals at 3.91 (integrating to 2H) and 4.05 and 4.30 ppm (1H each) in the 1H NMR spectrum assignable to the −CH2CH2−linkage of the imidazolidinylidene donor. The hydride ligands of 5 produce two doublet of doublets appearing at −20.41 (2JHP = 14.5 Hz, 2JHH = 5.9 Hz) and −24.40 ppm (2JHP = 17.8 Hz, 2JHH = 5.9 Hz) that correspond to the equatorial and axial hydrido ligands, respectively. Hydrogenation of an imidazolylidene ligand has previously been observed in a related system. 15 Finally, in order to obtain trihydride Ir-CNP complexes that may serve as hydrogenation catalysts, complex 4a was treated with an excess of NaH in THF leading to the formation of 6a, which is stable in the solid state under atmospheric conditions for extended periods of time, albeit readily reacts with chlorinated solvents leading to the regeneration of 4a (Scheme 2). The apical hydrido ligands of 6a produce in the 1H NMR spectrum a doublet of doublets at −10.14 ppm (2JHP = 15.6 Hz, 2JHH = 5.4 Hz), whereas a doublet of triplets appearing at −20.03 ppm (2JHP = 13.8 Hz, 2JHH = 5.4 Hz) can be ascribed to the meridional IrH hydrogen. Similarly, reaction of the mixture of 4b and 5with NaH produced the formation of an 8:2 mixture of 6b and 7(Scheme 3). Upon exposure of a toluene-d8solution of the trihydride derivative 6a to D2(5 bar, r.t., 20 h), the hydrido ligands and the methylene bridges underwent partial H/D exchange, suggestive of the reversible exchange of free D2with a η2-H2 ligand resulting from the intramolecular protonation of Ir−H hydrogens by protons of both the P-CH2and NHC-CH2 methylene fragments (Scheme 4). 22 This observation can be regarded as an indirect evidence of ligand-assisted H−H activation. Hydrogenation of Aldehydes. In order to assess the catalytic potential of the trihydride Ir−CNP complexes, their performance in the chemoselective hydrogenation of commercial-grade aldehydes was studied (Table 1). Using a catalyst loading of 0.5 mol %, complex 6a smoothly catalyzed the hydrogenation of benzaldehyde under 4 bar of H2at 25 °Cin 2-methyltetrahydrofuran 23 with a conversion of 70% (entry 1). The catalysts mixture 6b/7was also examined under the same conditions providing a similar catalytic activity (entry 2). By using higher concentrations of substrate and lower catalyst loadings (0.1 mol %) of 6a, benzaldehyde was hydrogenated with 96% conversion (entry 3). Under the latter reaction conditions, a series of substituted aromatic aldehydes was tested in order to determine the chemoselectivity and functional group tolerance of the catalytic system. The hydrogenation of substrates bearing halogen substituents was carried out with high conversions, and no hydrogenolysis of the C−X bond was detected (entries 4 and 5). Similarly, 4- nitrobenzaldehyde was selectivity hydrogenated to the corresponding alcohol without observable reduction of the nitro group (entry 6), whereas trifluoromethyl substituted benzaldehyde was reduced with 76% conversion (entry 7). Scheme 2. Synthesis of Complexes 3, 4a, and 6a Figure 2. ORTEP drawing at 30% ellipsoid probability of complex 3. Most hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: Ir(1)−C(1) 1.980(5), Ir(1)−N(3) 2.139(4), Ir(1)−P(1) 2.32271(12), Ir(1)−C(10) 2.085(5), Ir(1)− Cl(1) 2.5616(11), C(1)−Ir(1)−P(1) 166.79(16), C(1)−Ir(1)− C(10) 83.44(18), C(10)−Ir(1)−N(3) 172.79(16), C(1)−Ir(1)− N(3) 89.47(17), P(1)−Ir(1)−N(3) 82.24(11), C(10)−Ir(1)−P(1) 104.97(13). Scheme 3. Synthesis of Complexes 4b, 5, 6b, and 7 Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1316 Moreover, hydrogenation of vanillin was accomplished with complete conversion (entry 8). Finally, the hydrogenation of alkyl substituted aldehydes was examined using complex 6a. Hexanal, 2-methylpentanal, and cyclohexanecarboxaldehyde were hydrogenated with conversions higher than 91% (entries 9−11). Since the chemoselective reduction of aldehydes containing aCC bond is a relevant process in the industrial production of fine chemicals and pharmaceutical compounds, 24 the hydrogenation of a series of CC unsaturated aldehydes was next examined with 6a (Table 2). The reduction of transcinnamaldehyde, containing a conjugated CC double bond, to the corresponding alcohol was performed with high conversion and elevated selectivity using the previously optimized reaction conditions (entry 1). Instead, the hydrogenation of trans-2-hexen-1-al required an increase of the catalyst loading to 0.5 mol % to obtain an admissible conversion (entry 2). The hydrogenation of citronellal took place with complete conversion and a selectivity of 99% (entry 3), whereas cis/trans isomerization of the CC double bond was not observed in the reaction of citral, a 1:0.6 mixture of geranial and neral (entry 4). Mechanistic Studies. Initially, the reactivity of the Ir complex 4a with base in the presence of aldehyde was investigated to get further insight into the acid−base properties of the methylene bridges of the lutidine-derived CNP ligand. 14−16 Addition of potassium bis(trimethylsilyl)amide (KHMDS) or tBuOK (1.1 equiv) to a solution of 4a and benzaldehyde (3 equiv) in THF-d8brought about the formation of two diastereomeric dihydride species, 8aMand 8am, in a 9.4:0.6 ratio (Scheme 5). These derivatives are formally derived from the addition of the aldehyde to the Ir- CNP*framework deprotonated at the NHC-bound arm. Similarly, complex 4a in the presence of KHMDS (1.1 equiv) reacted with p-bromobenzaldehyde in THF-d8, yielding the isomers 8bMand 8bm, also formed in a 9.4:0.6 ratio. Complexes based on deprotonated PNP*and CNC* lutidine-derived pincer ligands have been shown to exhibit Frustrated Lewis Pair-type reactivity toward electrophiles such as CO2, carbonyl compounds, nitriles, and imines. 3,5,25 In the 1H NMR spectrum, the hydrido ligands of the major complex 8aMproduce two doublets of doublets at −19.39 (2JHP = 14.2 Hz, 2JHH = 7.8 Hz) and −24.33 (2JHP = 18.2 Hz, 2JHH = 7.8 Hz) ppm, whereas 8amalso shows two doublet of doublets for the IrH hydrogens appearing at −20.06 (2JHP = 13.7 Hz, 2JHH = 7.4 Hz) and −24.11 (2JHP = 18.5 Hz, 2JHH = 7.4 Hz) ppm. The presence of an intense cross-peak signal in the 1H−1H NOESY spectrum between one of the resonances corresponding to the CH2P arm (δ2.95 ppm) and one of the hydrido ligands (δ−24.33 ppm) of the major isomer 8aM allows to assign the resonance appearing at higher field in the hydride region to the apical hydride. Moreover, the formation of the new carbon−carbon bond is deduced from the appearance of a singlet at 4.90 ppm corresponding to the alkoxide −C(Ph)H−O−Ir fragment, which exhibits an intense cross-peak with the singlet resonance due to the CHN moiety (δH5.64 ppm) in the 1H−1H NOESY spectrum. The former Scheme 4. Deuteration of 6a with D2and Proposed Mechanism for the Observed H/D Exchanges Table 1. Hydrogenation of Aldehydes Catalyzed by Ir-CNP Complexes a a Reaction conditions, unless otherwise noted: 4 bar of H2,2- methyltetrahydrofuran, 0.1 mol % Ir-CNP, 25 °C, 24 h. [S] = 2.0 M. Conversion was determined by 1HNMRspectroscopyusing mesitylene as internal standard. b 0.5 mol % Ir-CNP, [S] = 0.3 M, 6 h. Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1317 signal is significantly shifted upfield with respect to the chemical shift of the carbonyl hydrogen of the free aldehyde (δH10.0 ppm), as expected from a change in the sp2 hybridization of the carbonyl carbon to sp3.Further confirmation of the assignment of this signal was obtained from the existence of an exchange cross-peak in the 1H−1H EXSY spectrum with the resonance of the carbonyl proton of free benzaldehyde, also suggesting the reversible exchange between coordinated and free benzaldehyde. The relative stabilities of 8aMand 8amwere studied using DFT calculations (B3LYP-D3, 6-31g(d,p)/SDD). The product of the exo coordination of benzaldehyde to the deprotonated Ir-CNP*framework was found to be 0.9 kcal mol−1more stable than the result of binding of the aldehyde through the opposite face (endo), indicating that the major complex 8M should coordinate the aldehyde exo, whereas 8mcorresponds to the endo coordination of the carbonyl compound. Further confirmation of this assignment was obtained from the observation of a strong cross-peak due to the resonances of the CHO−Ir moiety and one of the tBu groups of 8aMin the 1H−1H NOESY spectrum. Moreover, calculations also showed that adducts resulting from the coordination of benzaldehyde to the Ir-CNP*framework deprotonated at the phosphine arm are 14.0 (exo isomer) and 8.7 kcal/mol (endo isomer) higher in energy with respect to 8aM. The lower stability of these isomers can be attributed to the higher steric hindrance caused by the bulky di(tert-butyl)phosphino donor fragment. The structure of complex 8bMin the solid state was corroborated by the X-ray diffraction analysis of a single crystal (Figure 3). This is composed by a distorted octahedral Ir(III) complex with the pincer adopting a meridional coordination and the hydrido ligands in mutually cis positions. The C−C bond distance between the carbonyl carbon and the CH arm (C14−C13 = 1.591 Å) is characteristic for a C−C single bond, whereas the C14−O1 of 1.369 Å exhibits a typical value for single C−O binding. The Ir1−O1 bond (2.219 Å) is in the range of other Ir-OR bond distances. 26 Addition at room temperature of benzaldehyde(1.8 equiv) to a THF-d8solution of complexes 8b gave within minutes a mixture of 8aMand 8bM,afurtherindicationforthe reversibility of the formation of the C−C and O−Ir bonds. Relative thermodynamic stabilities of complexes 8aMand 8bM were determined from the observed equilibrium 8aM+pbromobenzaldehyde ⇌8bM+ benzaldehyde by using 1H NMR spectroscopy. From the calculated 298Keq = 7.2, a stability order of the adducts 8bM>8aMwas estimated, as expected from the higher electrophilicity of the carbonyl carbon of p-bromobenzaldehyde. Since aldehyde coordination to the Ir-CNP*framework deprotonated at the NHC-arm (ANHC) was found to be reversible, the study of the isomerization of ANHC to the Ir- CNP*species deprotonated at the P-bound bridge (AP) was deemed interesting. To this end, a solution of the in situ generated complex 8a was treated with MeOD-d3producing the deuteration of the CH-NHC methyne and CH2−P methylene bridges (Scheme 6). As previously shown above with the deuteration of 6a with D2, this result can be regarded as an indirect proof of the feasibility of the interconversion between the two tautomeric forms of Ir(CNP*)H2,ANHC and AP(see below). 16 Next, aiming to elucidate the species participating in the hydrogenation of aldehydes catalyzed by Ir-CNP complexes, Table 2. Hydrogenation of Unsaturated Aldehydes Catalyzed by Complex 6a a a Reaction conditions, unless otherwise noted: 4 bar of H2,2- methyltetrahydrofuran, 0.1 mol % 6a,25°C, 24 h. [S] = 2.0 M. Conversion was determined by 1HNMRspectroscopyusing mesitylene as internal standard. b 0.5 mol % 6a, [S] = 1.0 M. c Geranial/neral ratio = 1:0.6. Scheme 5. Formation of Complexes 8 Figure 3. ORTEP drawing at 30% ellipsoid probability of complex 8bM. Most hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: Ir(1)−C(1) 2.005(11), Ir(1)−N(3) 2.142(10), Ir(1)−O(1) 2.219(7), Ir(1)−P(1) 2.283(3), C(13)− C(14) 1.591(16), C(14)−O(1) 1.369(13), C(1)−Ir(1)−P(1) 168.5(3), C(1)−Ir(1)−O(1) 82.6(4), O(1)−Ir(1)−N(3) 82.4(3), C(1)−Ir(1)−N(3) 90.2(4), P(1)−Ir(1)−N(3) 81.2(3). Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1318 the reaction of the trihydride complex 6a with benzaldehyde (4.0 equiv) in THF-d8was examined by NMR spectroscopy (Scheme 7). In the 1H NMR spectrum, signals attributable to the formation of 8aM(85%), a new species 9(15%), and benzyl alcohol (1 equiv with respect to 8aM) were observed. Complex 9was isolated from the reaction mixture and characterized by NMR spectroscopy and X-ray diffraction techniques as a η1-carboxylate Ir(CNP)(PhCO2)H2complex. In the solid state, the structure of 9shows an Ir center in an octahedral geometry in which the pincer ligand is meridionally coordinated, and the apical coordination sites are occupied by one hydrido and the carboxylate ligands (Figure 4). The Ir(1)−O(1) bond length (2.225(2) Å) lies in the range of other Ir−O distances in complexes with carboxylate ligands coordinated trans to H. 26 Moreover, the carboxylate group shows little change with respect to the free benzoate, as reflected in the O(1)−C(28)−O(2) angle of 127.7(4)°, and in the similar values of both oxygen−carbon bond distances (O(1)−C(28) = 1.262(4) Å; O(2)−C(28) = 1.242(5) Å). The 1H NMR spectrum of 9in CD2Cl2shows, in the hydride region, two doublets of doublets at −19.91 (2JHP = 14.4 Hz, 2JHH = 6.7 Hz) and −26.85 (2JHP = 17.1 Hz, 2JHH = 6.7 Hz) ppm, corresponding to the IrH hydrogens trans and cis to the pyridine fragment, respectively. Relevant signals of 9in the 13C{1H} NMR spectrum include a doublet at δ174.7 ppm (JCP = 116 Hz) produced by the C-2 NHC carbon, and a resonance at δ171.0 ppm due to the carboxylic carbon. In the 1H−1H exchange spectroscopy (EXSY) spectrum of 9 registered at 25 °C intense exchange cross-peaks are observed between the signals corresponding to the hydride ligands, as well as between the resonances of the diastereotopic CH2−P and CH2−NHC hydrogens. This observation points to the existence of an exchange process that should involve the reversible decoordination of the benzoate ligand facilitated by the large trans influence of the apical hydride, or the reversible elimination of benzoic acid after intramolecular protonation of the carboxylate ligand by the methylene linkers of the pincer (see the Supporting Information). As formation of complex 9was thought to take place by an Ir promoted aldehyde oxidation by adventitious water present in the commercial aldehyde, 27 an independent synthesis of this derivative was sought. Addition of deionized H2O to a heated THF solution of 8aM/8am(formed in situ from 4a, KHMDS and a 3-fold excess of benzaldehyde) produced the initially dark red solution to turn gradually to yellow, bringing about the clean formation of 9(Scheme 8). However, in order to rule out the role of adventitious atmospheric oxygen in the formation of 9, 28 a THF solution of complex 8aMwas heated under air leading to a complex reaction mixture. This suggests that O2is not the oxygen source in the reaction. Nonetheless, to further demonstrate that water is the oxygen donor, synthesis of 18O-labeled 9was carried out using H218O. The high resolution MS spectrum (ESI ionization, positive mode) of the complex exhibits signals at m/z= 750.2790, 752.2836, and 754.2880, which according to their isotopic distribution can be assigned to [Ir(CNP)H(PhC16O2)]+, [Ir(CNP)H- (PhC16O18O)]+, and [Ir(CNP)H(PhC18O2)]+, respectively. Similarly, in the negative mode of the HRMS spectrum, signals at m/z= 121.0295, 123.0337, and 125.0380, corresponding to [PhC16O2]+, [PhC16O18O]+, and [PhC18O2]+, respectively, are obtained. These analyses confirmed 18O incorporation into the final product, and demonstrate that water is the oxygen source in the synthesis of the carboxylate complex 9. Finally, the reaction of a mixture of complexes 8aMand 9, obtained from the reaction of 6a with benzaldehyde (4 equiv), Scheme 6. Deuteration of 8a with MeOD-d3 Scheme 7. Reaction of 6a with Benzaldehyde Figure 4. ORTEP drawing at 30% ellipsoid probability of complex 9. Most hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: Ir(1)−C(1) 2.016(4), Ir(1)−N(3) 2.173(3), Ir(1)−O(1) 2.225(2), Ir(1)−P(1) 2.2772(9), C(28)− O(1) 1.262(4), C(1)−Ir(1)−P(1) 165.83(10), C(1)−Ir(1)−O(1) 91.19(12), O(1)−Ir(1)−N(3) 88.32(10), C(1)−Ir(1)−N(3) 88.42(13), P(1)−Ir(1)−N(3) 82.04(8). Scheme 8. Synthesis of Complex 9 Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1319 with H2(2 bar) was followed up by 1H and 31P{1H} NMR spectroscopy. Hydrogenation of benzaldehyde was ensued, being adduct 8aMand carboxylate 9the major observable Ir species until complete conversion of the aldehyde took place and complex 8aMwas transformed to the trihydride derivative 6a (see Supporting Information). Moreover, a control experiment involving the hydrogenation of p-bromobenzalde- hyde (room temperature, 3 bar of H2, tetrahydrofuran, 4 h) catalyzed by 9(1.0 mol %) demonstrated that the carboxylate complex is also a competent catalyst precursor. 19 DFT Calculations. The assumption of an outer-sphere, ligand-assisted mechanism for the hydrogenation of aldehydes catalyzed by complexes 6seems plausible on the basis of the experimental results discussed above, albeit different pathways involving the CH2−P and CH2−N methylene linkers of the pincer can be hypothesized. Therefore, we deemed it interesting to explore theoretically the mechanism of the hydrogenation of aldehydes by complexes 6by the performance of DFT calculations (B3LYP-D3, 6-31g(d,p)/SDD). Initially, since complex 8aMwas experimentally determined to be a suitable catalyst precursor, the dissociation of benzaldehyde from 8aMleading to the formation of intermediate ANHC, which is deprotonated at the NHC-arm of the pincer, was examined. The formation of 8aMwas found to be exergonic by 11.0 kcal/mol. Moreover, in addition to ANHC, the existence of the isomer AP, deprotonated at the P- bridge of the pincer, can also be hypothesized. 16 This derivative was found to be 1.0 kcal/mol more stable than ANHC. Calculations of the catalytic cycle were commenced by considering the heterolytic H2cleavage from the η2-H2adducts AP·H2and ANHC·H2, which are 7.9 and 7.2 kcal/mol less stable than AP+H 2and ANHC +H 2, respectively (Figure 5). This process takes place through concerted four-membered transition states (TSA(NHC)→6a and TSA(P)→6a), having energy barriers (ΔG‡) of 7.0 and 12.1 kcal/mol, respectively (Figure 6). The hydrogenation of APand ANHC is energetically favorable, yielding 6a with energy returns of 11.9 and 12.9 kcal/mol, respectively. Next, ligand-assisted hydrogenation of benzaldehyde by 6a was calculated to proceed with the participation of the CH2−P arm of the pincer as the proton source to yield benzyl alcohol Figure 5. DFT calculated free energy (ΔGin THF, kcal/mol) profile and reaction steps of the hydrogenation of benzaldehyde catalyzed by 6a. Note that the origin of energies is AP+H 2+ benzaldehyde (red line) and ANHC +H 2+ benzaldehyde (blue line). Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1320 and regenerate the deprotonated complex AP. This step is endergonic by 10.1 kcal/mol from 6a (+ benzaldehyde), while the reverse step is barrierless, as indicated by relaxed potential energy (PES) scans (see the Supporting Information). Conversely, calculations have showed a stepwise hydrogen transfer when the CH2−NHC arm of the Ir complex 6a was the proton source. This process takes place through the initial formation of the less stable intermediate BNHC after hydride attack to the carbonyl carbon (in this case, a TS was located for the formation of BNHC, and dissociation of the aldehyde to regenerate 6a was shown to be almost barrierless). Subsequent proton abstraction from the CH2−NHC bridge of the pincer yields benzyl alcohol and ANHC. This step has a barrier of 6.3 kcal/mol, amounting the overall energy barrier from 6a to 19.4 kcal/mol. The full hydrogen transfer from 6a to the aldehyde to regenerate ANHC is endergonic by 11.1 kcal/mol. The observed differences in the hydrogen transfer to the aldehyde involving the methylene CH2−P and CH2−NHC arms of the catalyst can be ascribed to the slightly higher acidic character of the phosphine containing arm. Overall, DFT calculations of the mechanism of the hydrogenation of aldehydes catalyzed by the Ir-CNP complex 6a show that H2activation is easier for the deprotonated species ANHC than for AP(ΔΔG‡= 5.8 kcal/mol), whereas a more favorable process takes place when the methylene CH2− P bridge of the catalyst is involved in the hydrogen transfer step. Although different mechanisms for the interconversion of APand ANHC can be invoked, a potential pathway could involve proton transfer assisted by protic molecules (water or alcohol) (see the Supporting Information). 16,29 The likeliness of such a process has been indirectly evidenced by the observed deuteration of the CH-NHC methyne and CH2−P methylene bridges of 8a with MeOD-d3(see above). ■CONCLUSIONS We have demonstrated that lutidine-derived CNP iridium complexes, having two nonequivalent Brønsted acid/base sites, are efficient catalysts in the hydrogenation of commercial-grade aldehydes. The catalytic reactions are readily performed under neutral, mild (25 °C, 4 bar H2) conditions using low catalyst loadings (0.1 mol %) with high selectivities toward the reduction of the carbonyl group in the presence of other reducible functions. Experimental and theoretical studies support that both CH2−NHC and CH2−P methylene bridges of the pincer can participate in ligand-assisted processes. However, while participation of the NHC-arm of the pincer is favored in the H2activation step of the catalytic cycle, hydrogen transfer to the aldehyde is easier when the process is assisted by the CH2−P bridge. Since, according to experimental results, interconversion between the two deprotonated forms of the catalyst (APand ANHC) seems facile, the lowest energy pathway in the catalytic cycle involves a switch of the metal−ligand cooperation site. Due to the widespread use of metal complexes containing Brønsted acid/ base functionalities in hydrogenation and dehydrogenation reactions, we think that the above-mentioned approach (i.e., use of two nonequivalent proton-responsive sites with similar acid/base properties) could be a fruitful approach to catalyst development. ■EXPERIMENTAL SECTION General Information. All reactions and manipulations were performed under nitrogen or argon atmosphere, either in a Braun Labmaster 100 glovebox or using standard Schlenk-type techniques. All solvents were dried and distilled under nitrogen, using the following desiccants: sodium-benzophenone-ketyl for diethyl ether (Et2O), tetrahydrofuran (THF), and 2-methyltetrahydrofuran (2Me- THF); sodium for pentane and toluene; and CaH2for dichloromethane (CH2Cl2). The NMR experiments were carried out on Bruker DPX-300, DRX-400, DRX-500, and AdvanceIII-400/R spectrometers. The 1H and 13C spectra were referenced to external SiMe4using the residual proton peaks of the deuterated solvent as internal standards, while 31P was referenced to external 85% H3PO4. NMR signal assignations were made by routine one- and twodimensional experiments, including 1H−1H COSY, 1H−1H NOESY, 1H−13C HSQC, 1H−13C HMBC, and 1H{31P} NMR spectroscopies. All NMR measurements were carried out at 25 °C. Elemental analyses were run by the Analytical Service of the Instituto de Investigaciones Qui micas in a Leco TrueSpec CHN elemental analyzer. High resolution mass spectra (HRMS) data were obtained using a Jeol JMS-SX 102A mass spectrometer at the Analytical Services of the Universidad de Sevilla (CITIUS). Infrared spectra were recorded on a Bruker Tensor 27 instrument. Ir(acac)(COD) 30 and salts 1a,b 14 were synthesized according to reported methods. All other reagents were purchased from commercial suppliers and used as received. Imidazolium Salt 2a. A solution of 1a (0.800 g, 2.21 mmol) and tBu2PH (1.29 g, 8.80 mmol) in CH2Cl2(20 mL) was heated to 65 °C for 6 days. The solvent was evaporated, and the residue was washed with THF (15 mL), and dried under a vacuum. ScavengePore phenethyl diethylamine (3.500 g; base loading: 0.66 mmol/g) was added to a solution of the solid in CH2Cl2(30 mL), and the mixture was stirred for 1 h and filtered. The solution was brought to dryness, and the residue was washed with Et2O (15 mL), and dried under a vacuum (0.509 g). The compound was obtained with a purity of 77% as determined by 1H NMR spectroscopy, and used for the synthesis of the Ir complex 3without further purification. 1H NMR (400 MHz, CD2Cl2)δ10.84 (s, 1H, H arom Imid), 8.07 (s, 1H, H arom), 7.65 (m, 2H, 2 H arom), 7.40 (m, 1H, H arom), 7.15 (m, 1H, H arom), 7.04 (m, 2H, 2 H arom Mes), 6.02 (s, 2H, CH2N), 3.04 (br, 2H, CH2P), 2.34 (s, 3H, CH3), 2.07 (s, 6H, 2 CH3), 1.11 (d, 3JHP = 11.0 Hz, 18H, 2 C(CH3)3). 31P{1H} NMR (162 MHz, CD2Cl2)δ37.1 (br). 13C{1H} NMR (101 MHz, CD2Cl2)δ163.2 (Cqarom), 159.2 (Cqarom), 152.5 (Cqarom), 139.4 (CH arom), 138.0 (CH arom), Figure 6. Views of the DFT-optimized geometries of the transition states TSA(NHC)·H2→6a (top) and TSA(P)·H2→6a (bottom). Methyl groups of the mesityl (top) and t-Bu (bottom) fragments have been omitted for clarity. Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1321 134.8 (2 Cqarom), 131.4 (Cqarom), 130.1 (2 CH arom), 124.5 (CH arom), 123.9 (CH arom), 122.7 (CH arom), 121.1 (CH arom), 53.8 (overlapped with the residual solvent signal, CH2N), 36.5 (d, JCP =59 Hz, 2 C(CH3)3), 32.2 (d, JCP = 19 Hz, CH2P), 29.7 (d, JCP = 12 Hz, 2 C(CH3)3), 21.2 (CH3), 17.8 (2 CH3). HRMS (ESI) m/z436.2869 [(M−Cl)+] (exact mass calculated for C27H39N3P: 436.2882). Imidazolium Salt 2b·HCl. A solution of 1b (0.700 g, 2.00 mmol) and tBu2PH (1.17 g, 8.00 mmol) in CH2Cl2(10 mL) was heated to 65 °C for 6 days. The solvent was evaporated, and the residue was washed with THF (15 mL) and dried under a vacuum giving rise to a light brown solid (0.883 g) that was characterized as the hydrochloride salt of 2b. The compound was obtained with a purity of 65% as determined by 1H NMR spectroscopy, and used without further purification for the synthesis of the Ir complex 4.1H NMR (400 MHz, CD2Cl2)δ11.90 (s, 1H, H arom Imid), 8.93 (d, 1JHP = 495 Hz, 1H, PH), 8.89 (s, 1H, H arom Imid), 7.99 (d, 3JHH = 7.6 Hz, 1H, H arom Py), 7.78 (m, 2H, H arom Py + H arom Imid), 7.71 (d, 3JHH = 7.8 Hz, 1H, H arom Py), 7.52 (s, 2H, 2 H arom Xyl), 7.18 (s, 1H, H arom Xyl), 6.06 (s, 2H, CH2N), 4.01 (d, 2JHP = 12.1 Hz, 2H, CH2P), 2.44 (s, 6H, 2 CH3), 1.47 (d, 3JHP = 16.0 Hz, 18H, 2 C(CH3)3). 31P{1H} NMR (162 MHz, CD2Cl2)δ32.5 (br). 13C{1H} NMR (101 MHz, CD2Cl2)δ153.0 (Cqarom), 152.5 (d, JCP = 7 Hz, Cqarom), 140.7 (2 Cqarom), 139.3 (CH arom), 137.1 (CH arom), 134.5 (Cqarom), 131.4 (CH arom), 125.0 (CH arom), 124.8 (d, JCP = 6 Hz, CH arom), 123.2 (CH arom), 119.7 (CH arom), 119.0 (2 CH arom), 52.7 (CH2N), 32.8 (d, JCP = 33 Hz, 2 C(CH3)3), 27.3 (2 C(CH3)3), 24.2 (d, JCP = 41 Hz, CH2P), 21.0 (2 CH3). HRMS (ESI) m/z422.2718 [(M−HCl−Cl)+] (exact mass calculated for C26H37N3P: 422.2725). Complex 3. A dichloromethane (25 mL) solution of 2a (0.700 g, 1.38 mmol) and Ir(acac)(COD) (0.495 g, 1.24 mmol) was stirred for 3 days. The formed yellow precipitate was filtered, washed with Et2O (2 ×10 mL) and pentane (2 ×10 mL), and dried under a vacuum. Light yellow solid (0.431 g, 47%). Crystals of 3suitable for X-ray diffraction analysis were grown from a saturated solution of the complex in CH2Cl2. Anal. Calcd (%) for C27H38ClIrN3P: C 48.89, H 5.78, N 6.34. Found: C 48.99, H 5.94, N 6.06. IR (nujol) 2214 (νIrH) cm−1.1H NMR (500 MHz, CD2Cl2)δ7.64 (s, 1H, H arom NHC), 7.63 (dd, 3JHH = 7.8 Hz, 3JHH = 7.8 Hz, 1H, H arom Py), 7.41 (d, 3JHH = 7.8 Hz, 1H, H arom Py), 7.24 (s, 1H, H arom NHC), 7.20 (d, 3JHH = 7.8 Hz, 1H, H arom Py), 7.07 (d, 2JHH = 14.5 Hz, 1H, CHHN), 6.98 (s, 1H, H arom Mes), 6.71 (s, 1H, H arom Mes), 4.90 (d, 2JHH = 14.5 Hz, 1H, CHHN), 3.95 (d, 2JHH = 11.3 Hz, 1H, CHHIr), 3.58 (dd, 2JHH = 16.5 Hz, 2JHP = 8.7 Hz, 1H, CHHP), 2.91 (dd, 2JHH = 11.2 Hz, 3JHP = 9.1 Hz, 1H, CHHIr), 2.70 (dd, 2JHH = 16.5 Hz, 2JHP = 7.2 Hz, 1H, CHHP), 2.35 (s, 3H, CH3Mes), 2.25 (s, 3H, CH3Mes), 1.40 (d, 3JHP = 12.4 Hz, 9H, C(CH3)3), 1.34 (d, 3JHP = 12.5 Hz, 9H, C(CH3)3), −24.47 (d, 2JHP = 17.0 Hz, 1H, IrH). 31P{1H} NMR (202 MHz, CD2Cl2)δ54.6. 13C{1H} NMR (125 MHz, CD2Cl2)δ176.1 (d, JCP = 118 Hz, C-2 NHC), 166.9 (d, JCP = 5 Hz, Cqarom), 156.3 (Cqarom), 151.4 (Cqarom), 136.5 (Cqarom), 135.7 (CH arom), 135.2 (Cqarom), 128.0 (CH arom), 127.7 (Cqarom), 127.2 (CH arom), 122.6 (CH arom), 121.7 (d, JCP = 7 Hz, CH arom), 119.8 (d, JCP = 4 Hz, CH arom), 119.6 (d, JCP = 4 Hz, CH arom), 56.9 (CH2N), 40.0 (d, JCP = 23 Hz, CH2P), 38.0 (d, JCP = 9 Hz, C(CH3)3), 33.2 (d, JCP = 20 Hz, C(CH3)3), 30.8 (d, JCP = 4 Hz, C(CH3)3), 29.0 (d, JCP = 5 Hz, C(CH3)3), 22.2 (CH3Mes), 20.9 (CH3Mes), −9.6 (br, CH2Ir). Complex 4a. In a Fisher−Porter vessel, a suspension of 3(0.120 g, 0.18 mmol) in CH2Cl2(20 mL) was pressurized with 4 bar of H2 and heated to 65 °C overnight. The system was depressurized, solvent was evaporated and the residue was washed with pentane (2 ×10 mL). Yellow solid (0.115 g, 96%). Anal. Calcd (%) for C27H40ClIrN3P: C 48.75, H 6.06, N 6.32. Found: C 49.02, H 5.77, N 6.35. IR (nujol) ν= 2235 (νIrH), 2141 (νIrH)cm −1.1H NMR (400 MHz, CD2Cl2)δ7.69 (dd, 3JHH = 7.8 Hz, 3JHH = 7.8 Hz, 1H, H arom Py), 7.46 (d, 3JHH = 7.8 Hz, 1H, H arom Py), 7.30 (m, 2H, H arom Py + H arom NHC), 7.03 (s, 1H, H arom Mes), 6.95 (d, 2JHH = 14.4 Hz, 1H, CHHN), 6.92 (s, 1H, H arom Mes), 6.88 (s, 1H, H arom NHC), 4.92 (d, 2JHH = 14.4 Hz, 1H, CHHN), 3.58 (dd, 2JHH = 16.5 Hz, 2JHP = 8.7 Hz, 1H, CHHP), 2.78 (dd, 2JHH = 16.5 Hz, 2JHP = 7.6 Hz, 1H, CHHP), 2.37 (s, 3H, CH3), 2.18 (s, 3H, CH3), 1.92 (s, 3H, CH3), 1.25 (d, 3JHP = 12.6 Hz, 9H, C(CH3)3), 1.20 (d, 3JHP = 12.5 Hz, 9H, C(CH3)3), −20.36 (dd, 2JHP = 14.4 Hz, 2JHH = 6.5 Hz, 1H, IrH trans to Py), −24.89 (dd, 2JHP = 17.2 Hz, 2JHH = 6.5 Hz, 1H, IrH cis to Py). 31P{1H} NMR (122 MHz, CD2Cl2)δ64.8. 13C{1H} NMR (101 MHz, CD2Cl2)δ173.2 (d, JCP = 115 Hz, C-2 NHC), 167.0 (d, JCP = 4 Hz, Cqarom), 156.4 (Cqarom), 138.4 (Cqarom), 138.2 (Cqarom), 136.7 (Cqarom), 135.8 (CH arom), 135.4 (Cqarom), 128.9 (CH arom), 128.3 (CH arom), 122.1 (CH arom), 121.5 (d, JCP = 8 Hz, CH arom), 121.0 (d, JCP = 3 Hz, CH arom), 119.8 (d, JCP = 4 Hz, CH arom), 56.5 (CH2N), 39.4 (d, JCP = 23 Hz, C(CH3)), 38.1 (d, JCP = 10 Hz, CH2P), 32.4 (d, JCP = 24 Hz, C(CH3)), 30.6 (d, JCP = 3 Hz, 3 C(CH3)), 29.1 (d, JCP = 4 Hz, 3 C(CH3)), 21.3 (CH3), 18.8 (CH3), 18.1 (CH3). Complexes 4b and 5. Dichloromethane (8 mL) was added to a mixture of 2b·HCl (0.155 g, 0.31 mmol) and phenethyldiethylamine ScavengePore resin (0.850 g; base loading: 0.66 mmol/g), and the suspension was stirred for 1 h. The mixture was filtered through a short pad of Celite and the solvent was evaporated. A solution of Ir(acac)(COD) (0.125 g, 0.31 mmol) in THF (5 mL) was added to the residue, and the mixture was stirred overnight. The solvent was evaporated, and the resulting solid was washed with pentane (2 ×5 mL). In a Fisher−Porter vessel, a solution of the obtained solid in CH2Cl2(20 mL) was pressurized with 4 bar of H2, and heated to 65 °C overnight. The system was depressurized, solvent was evaporated, and the residue was washed with pentane (2 ×10 mL). The compounds 4b and 5, formed in an approximated ratio of 8:2, were isolated as a yellow solid. Satisfactory elemental analysis for this solid could not be obtained (0.064 g, 32%). NMR spectroscopy and HRMS data for complex 4b:1H NMR (500 MHz, CD2Cl2)δ7.70 (dd, 3JHH = 7.7 Hz, 3JHH = 7.7 Hz, 1H, H arom Py), 7.47 (d, 3JHH = 7.8 Hz, 1H, H arom Py), 7.28 (m, 3H, 2 H arom Xyl + H arom Py), 7.23 (d, 3JHH = 1.7 Hz, 1H, H arom NHC), 7.17 (d, 3JHH = 1.7 Hz, 1H, H arom NHC), 7.08 (s, 1H, H arom Xyl), 6.94 (d, 2JHH = 14.4 Hz, 1H, CHHN), 4.87 (d, 2JHH = 14.4 Hz, 1H, CHHN), 3.64 (dd, 2JHH = 16.7 Hz, 2JHP = 8.7 Hz, 1H, CHHP), 2.89 (dd, 2JHH = 16.6 Hz, 2JHP = 7.8 Hz, 1H, CHHP), 2.39 (s, 6H, 2 CH3), 1.33 (d, 3JHP = 12.4 Hz, 9H, C(CH3)3), 1.27 (d, 3JHP = 12.7 Hz, 9H, C(CH3)3), −20.13 (dd, 2JHP = 14.7 Hz, 2JHH = 6.3 Hz, 1H, IrH trans to Py), −24.67 (dd, 2JHP = 17.4 Hz, 2JHH = 6.3 Hz, 1H, IrH cis to Py). 31P{1H} NMR (162 MHz, THF-d8)δ64.6. 13C{1H} NMR (101 MHz, THF-d8)δ172.2 (d, JCP = 115 Hz, C-2 NHC), 166.7 (Cq arom), 156.9 (Cqarom), 142.7 (Cqarom), 137.6 (2 Cqarom), 136.3 (CH arom), 128.9 (CH arom), 125.8 (2 CH arom), 122.5 (m, 2 CH arom), 121.7 (CH arom), 120.6 (CH arom), 55.8 (CH2N), 39.3 (d, JCP = 23 Hz, C(CH3)3), 38.2 (d, JCP = 11 Hz, CH2P), 33.0 (d, JCP = 24 Hz, C(CH3)3), 31.0 (3 C(CH3)3), 29.6 (3 C(CH3)3), 21.2 (2 Ar− CH3). HRMS (ESI) m/z616.2420 [(M−Cl)+] (exact mass calculated for C26H38IrN3P: 616.2433). NMR spectroscopy and HRMS data for complex 5:1H NMR (500 MHz, CD2Cl2)δ7.67 (dd, 3JHH = 7.6 Hz, 3JHH = 7.6 Hz, 1H, H arom Py), 7.43 (d, 3JHH = 7.8 Hz, 1H, H arom Py), 7.32 (s, 1H, H arom Xyl), 7.19 (m, 3H, 3 H arom), 6.36 (d, 2JHH = 14.6 Hz, 1H, CHHN), 4.30 (m, 1H, CHH NHC), 4.17 (d, 2JHH = 14.7 Hz, 1H, CHHN), 4.05 (m, 1H, CHH NHC), 3.91 (m, 2H, 2 CHH NHC), 3.61 (overlapped m, 1H, CHHP), 2.88 (overlapped m, 1H, CHHP), 2.34 (s, 6H, 2 CH3), 1.29 (d, 3JHP = 12.0 Hz, 9H, C(CH3)3), 1.22 (d, 3JHP = 12.8 Hz, 9H, C(CH3)3), −20.41 (dd, 2JHP = 14.5 Hz, 2JHH = 5.9 Hz, 1H, IrH trans to Py), −24.40 (dd, 2JHP = 17.8 Hz, 2JHH = 5.9 Hz, 1H, IrH cis to Py). 31P{1H} NMR (162 MHz, THF-d8)δ64.2. HRMS (ESI) m/z618.2563 [(M−Cl)+] (exact mass calculated for C26H40IrN3P: 618.2584). Complex 6a. Tetrahydrofuran (8 mL) was added to a mixture of complex 4a (0.095 g, 0.14 mmol) and NaH (0.070 g, 2.9 mmol), and the resulting suspension was heated to 40 °C for 2 days. The solid was filtered off, and the solution was brought to dryness and extracted with Et2O(2×10 mL). The solvent was evaporated, and the residue was extracted with Et2O(2×8 mL) and dried under a vacuum. Yellow solid (0.079 g, 88%). Anal. Calcd (%) for C27H41IrN3P: C Organometallics pubs.acs.org/Organometallics Article https://doi.org/10.1021/acs.organomet.1c00109 Organometallics 2021, 40, 1314−1327 1322