Amido Complexes of Iridium with a PNP Pincer Ligand: Reactivity

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

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Amido Complexes of Iridium with a PNP Pincer Ligand: Reactivity toward Alkynes and Hydroamination Catalysis Pablo Hermosilla,‡ Pablo Loṕ ez,‡ Pilar García-Orduña,‡ Fernando J. Lahoz,‡ Víctor Polo,§ and Miguel A. Casado*,‡ ‡

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Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Departamento de Química Inorgánica, CSIC-Universidad de Zaragoza, c/Pedro Cerbuna 12 50009, Zaragoza, Spain § Departamento de Química Física and Instituto de Biocomputación y Física de los Sistemas Complejos (BIFI), Universidad de Zaragoza, c/Pedro Cerbuna 12 50009, Zaragoza, Spain S Supporting Information *

ABSTRACT: The pincer ligand HN(CH2CH2PPh2)2 (1; PNHP) reacted with [{Ir(μ-X)(cod)}2] (X = Cl, OMe), affording complexes [fac-(PNHP)Ir(cod)]Cl (2) and [fac(PNP)Ir(cod)] (3), respectively. The X-ray molecular structure of 2 showed that the PNP ligand coordinates in a facial fashion, with the N atom in an axial site and both P atoms coordinated in the equatorial plane. Compound 1 is able to protonate the hydroxo bridges in the complex [{Ir(μOH)(coe)2}2] forming the new amido complex [mer-(PNP)Ir(coe)] (4). Complex 4 is an extremely air sensitive compound, as confirmed by the isolation of the oxo complex [mer-(PNP)Ir(η2-O2)] (8) from its interaction with air. Protonation of 4 with HBF4 afforded the corresponding amino complex [mer-(PNHP)Ir(coe)]BF4 (5), whose molecular structure enlightened by Xray crystallography confirmed the PNP ligand to be coordinated in a meridional fashion. The coe ligand in 4 is tightly bonded to iridium; however, under an atmosphere of ethylene at 60 °C or with acrylonitrile at 70 °C complex 4 exchanges the olefin, affording compounds [mer-(PNP)Ir(η2-C2H4)] (6) and [mer-(PNP)Ir(η2-C2H3CN)] (7), respectively. Interaction of 4 with alkynes depends on the nature of the substrate; therefore, methyl phenylpropiolate reacted with 4, affording the adduct [mer(PNP)Ir(η2-PhCCC(O)OMe)] (9), while the parent acetylene undergoes a double C−H activation, affording the Ir(III) complex [fac-(PNHP)IrH(CCH)2] (10). A DFT theoretical analysis of this transformation supports a metal−ligand cooperation mechanism. The reaction starts by deprotonation of an alkyne moiety by the PNP ligand followed by oxidative addition of the C−H bond to the metal of a second alkyne molecule. Additionally, we have tested complex 4 as a catalyst for the addition of gaseous ammonia to activated unsaturated substrates. A DFT theoretical analysis disclosed the operative mechanism on these organic transformations, which starts with a nucleophilic attack of ammonia to the bound alkyne, hydrogen migration to the metal, and reductive elimination steps.

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INTRODUCTION In contrast to early-transition-metal amides,1 late-transitionmetal amido complexes are less common. However, latetransition-metal amido complexes still attract a great deal of interest partially because of their potential participation in catalytic hydroamination processes,2 C−N coupling reactions,3 and transfer hydrogenation to unsaturated substrates,4 among other organic transformations.5 The main difference between early- and late-transition-metal amide species relies on the nature of the M−N bond. In the former cases favorable M−N π bonding explains the abundance of such species, while in the case of late metals repulsive filled pπ−dπ electronic repulsions are expected, a situation that accounts for the relative scarceness of such late-transition-metal amido species.6 Furthermore, amido ligands with C−H bonds in the α position are potentially able to undergo β-hydride elimination processes leading to imine formation.7 Pincer constructs constitute a very important family of polyfunctional ligands whose architectures can be tailored by ligand design; they have attracted enormous interest during the © XXXX American Chemical Society

last few years in the context of organometallic chemistry, partially due to their strong coordination to metals that renders reactive metallic fragments which have been found to be operative in a number of catalytic organic transformations,8 especially those pincer systems in combination with ruthenium.9 The intrinsic structure of these pincer ligands is a key factor that has allowed the development of the late-metal amido organometallic chemistry, precisely those systems that share a common amido center (Figure 1).10 In this context, PNP ligands based on a pyridine moiety have become quite popular because of their participation in metal−ligand cooperative catalysis via aromatization/dearomatization of the pyridinic ring, which in turn involves a change concerning the electronic distribution at the N atom (amine versus amido coordination).11 The application of PNP ligands embedded into an aliphatic array in the context of bond activation and homogeneous catalysis was first pioneered by Fryzuk et al., Received: May 30, 2018

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DOI: 10.1021/acs.organomet.8b00365 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 1. Different types of amido pincer ligands.

who synthesized a disilylamido ligand that allowed the isolation of stable amido complexes of Ir, Ni, and Pd by the assistance of the coordination of two phosphine fragments to the respective metals.12 The use of alkylic PNP n,ionic ligands of the type [N(CH2CH2PR2)2]− (R = alkyl, aryl) in combination with late transition metals usually leads to systems with a reactivity centered at the nitrogen atom in terms of basicity and nucleophilicity.13 These species are excellent candidates for exhibiting metal−ligand cooperativity. Along this line, there are recent examples of late-transitionmetal/PNP systems active in the catalytic dehydrogenation of alcohols,14 acceptorless dehydrogenative coupling of alcohols to esters,15 hydrogenation of amides16 and esters,17 Nalkylation of amines with alcohols,18 and α-alkylation of ketones with primary alcohol,s19 among other interesting catalytic transformations.20 Most of the aforementioned catalytic processes have been found to proceed through metal−ligand cooperation.21 In this paper we report on the ability of the known PNHP pincer ligand (PNHP = HN(CH2CH2PPh2)2 (1))22 to stabilize reactive iridium amine and amido complexes, some of which readily activate C−H bonds of unsaturated substrates. Additionally, we disclose the catalytic activity of an Ir(I) PNPbased complex in the hydroamination of activated substrates with ammonia.

Figure 2. Molecular structure of the cation of complex 2, [fac(PNHP)Ir(cod)]+. Hydrogen atoms (except that bound to an N atom) have been omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for Complex 2 Ir−P1 Ir−P2 Ir−N P1−Ir−P2 P1−Ir−N P1−Ir−Ct1 P1−Ir−Ct2 P2−Ir−N

2.3316(5) 2.3251(5) 2.1554(15) 104.017(16) 81.90(4) 131.85(5) 100.55(5) 82.74(4)

Ir−Ct1a Ir−Ct2a P2−Ir−Ct1 P2−Ir−Ct2 N−Ir−Ct1 N−Ir−Ct2 Ct1-Ir-Ct2

2.0290(18) 2.0702(20) 121.57(6) 101.60(6) 88.83(7) 174.24(7) 85.66(8)

a Ct1 and Ct2 represent the midpoints of the C(1)C(2) and C(5)C(6) bonds, respectively.

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RESULTS AND DISCUSSION The known P2,N-donor pincer ligand HN(CH2CH2Ph2)2 (1; PNHP) was prepared by in high yield following a modified established procedure and isolated as a viscous oil.23 Treatment of the chloro-bridged complex [{Ir(μ-Cl)(cod)}2] (cod = 1,5-cyclooctadiene) with 1 in a 1:2 molar ratio in toluene produced a white crystalline solid in good yield (69%). This has been fully characterized as the cationic complex [fac(PNHP)Ir(cod)]Cl (2). The slow diffusion of diethyl ether into a concentrated solution of 2 in acetone produced white single crystals, which allowed the study of its molecular structure by X-ray methods. The molecular structure of the cation of complex 2 is shown in Figure 2, and Table 1 collects selected geometrical parameters. Complex 2 is cationic, with the iridium atom in a slightly distorted trigonal bipyramidal geometry, with the apical sites occupied by the N atom and a double bond of the diolefin and the axial positions fulfilled by the phosphorus atoms of the pincer ligand and the other alkene arm of the cod fragment. The tridentate ligand is coordinated in a facial manner to iridium, with a P−Ir−P angle of 104.017(16)°, the smallest angle in the equatorial plane, most probably due to the intrinsic architecture of the ligand which prevents further opening. The tridentate coordination of PNHP ligands leads to the formation of two highly puckered five-membered Ir−P− C−C−N iridacycles (Ir−P1−C21−C22−N, Q = 0.5270(16) Å, ϕ = −117.74(17)°, 2T3/E3 conformation; Ir−P2−C24−

C23−N, Q = 0.4434(18) Å, ϕ = 108.78(16)°, E 4 conformation).24 The amine N atom exhibits bond angles larger than the 109.5° ideal value for a perfect tetrahedron (Ir−N−C23 112.72(11)°, Ir−N−C22 116.01(11)°, and C22−N−C23 111.17(14)°). The sum of these bond angles (339.9(2)°) evidenced the pyramidalization of the nitrogen atom, located at 2.1554(15) Å from the metal atom. The larger trans influence exerted by N atom, in comparison to that of the P1 atom, induce Ir−olefin bond distances to be dissimilar, with Ir−Ct2 (2.0702(20) Å) slightly longer than the Ir−Ct1 bond (2.0290(18) Å). The hydrogen atom of the amine establishes a hydrogen bond with the chloride anion, characterized by a 3.186(2) Å N···Cl interatomic distance and a 165(2)° N−H···Cl angle (see Figure S30). On the whole, the molecular structure of 2 exhibits features in common with those of the related complex [HN(2-PPh2-4-Me-C6H3)2Ir(cod)]Cl.25 Complex 2 was characterized in solution by means of NMR spectroscopy and mass spectrometry. The 31P{1H} NMR spectrum of 2 at room temperature showed a sole singlet at δ 14.4 ppm, which indicated Cs symmetry in solution. The 1H NMR spectrum of 2 in deuterated acetone at room temperature showed the olefinic protons of the cod ligand as a broad resonance centered at δ 3.93 ppm, signaling that the complex undergoes a dynamic process in solution. Therefore, we performed a VT NMR study, finding out that at 233 K the B


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Organometallics fluxional movement was almost frozen (see Figure S1). At this temperature, the olefinic protons were observed as two separated broad signals, a situation that fits with the bpt geometry observed in the solid state. The 13C{1H}-APT NMR spectrum acquired at 233 K showed the CH2P (δ 32.4 ppm) and CH2N (δ 57.4 ppm) carbons of each arm chemically equivalent, respectively, which confirms the presence of the Cs symmetry plane contained in the Ir−N axis. Additionally, the mass ESI+ spectrum of 2 showed a peak at m/z 742.2333 that corresponded to the calculated value. We took advantage of the presence of the reactive NH proton in 1 to attempt its deprotonation with the complex [{Ir(μ-OMe)(cod)}2], which bears highly basic ligands that are able to deprotonate weakly acidic protons in a number of cases.26 In fact, the complex [{Ir(μ-OMe)(cod)}2] was reacted with 1 in a 1:2 molar ratio, affording a dark red solid characterized as the complex [fac(PNP)Ir(cod)] (3), where the NH proton of 1 readily protonated the methoxo bridging ligands (Scheme 1). Monitoring of the reaction in deuterated toluene by NMR techniques showed the clean formation of 3 along with released methanol.

P-containing complexes, as confirmed upon inspection of the P{1H} NMR spectra. Due to these complications, we decided to shift to the additive deprotonation approach employed in the synthesis of complex 3. For that purpose we reacted the known hydroxobridged complex [{Ir(μ−OH)(coe)2}2]28 with 1 in a 1:2 molar ratio in toluene, which led to the isolation of an orange solid characterized as the compound [mer-(PNP)Ir(coe)] (4) with a 64% yield (Scheme 1). On the whole, the structural information gathered indicates that complex 4 is square planar, where the PNP ligand is κ2P,κN-coordinated in a meridional fashion and the coe ligand is coordinated trans to N. The ESI+ mass spectrum of 4 gave a peak at m/z 744.2498 that corresponded to the molecular ion [(PNP)Ir(coe)]+, suggesting that there is only one coe molecule coordinated to iridium. VT NMR studies on 4 in toluene-d8 showed that, as opposed to diolefin complexes 2 and 3, it was static within the range of temperature studied (193−293 K). The static behavior of 4 does not involve steric factors, since molecular modeling studies clearly indicated that rotation of the coe ligand around the N−Ir axis is not blocked by the steric pressure of the phenyl groups of the pincer ligand (vide infra). An inspection of the 1H−13C HSQC NMR spectrum of 4 showed the carbons of the CH2N fragments as two distinct multiplets at δ(13C) 59.3 and 60.2 ppm, which correlated with complex signals at δ(1H) 3.05 and 3.22 ppm. The same situation was observed for the CH2P fragments, information which revealed the lack of a symmetry plane that would relate both alkylic arms in 4. In accord with this, the 31P{1H} NMR spectrum of 4 showed an AB system centered at δ 43.6 ppm (2JP−P = 390 Hz). The olefinic protons of the coe olefin were at δ(1H) 2.18 ppm, a resonance that correlated to a sole multiplet at δ(13C) 37.7 ppm in the 1H−13C HSQC NMR spectrum, signaling the presence of a symmetry plane. The low chemical shift of the olefinic protons (and carbons) can be tentatively explained in terms of a strong π back-donation from the filled d orbitals of the metal to the olefin, which involves a reduction of the CC bond order. This electron donation is in part due to the presence of an amidic N atom, which on the NMR time scale displays a planar geometry in which the lone electron pair should be involved in the Ir−N bond.29 This situation explains the lack of reactivity of 4 with electrophiles such as methyl triflate and trifluoroacetic acid, which we expected to methylate (or protonate) the amidic nitrogen, as has already been observed in a related system.30 However, the reaction of 4 with a powerful electrophile such as tetrafluoroboric acid proceeded cleanly through protonation at the nitrogen atom (instead of protonation at the metal) and allowed the isolation of the cationic complex [mer-(PNHP)Ir(coe)]BF4 (5) in good yield. Deep red single crystals of 5 were grown by slow diffusion of hexanes into a concentrated solution of 5 in CH2Cl2, which were subjected to an X-ray structural analysis; the molecular structure of the cation 5+ is depicted in Figure 3, and bond lengths and angles characterizing the metal coordination sphere are reported in Table 2. Deviations from an ideal square-planar geometry in cation 5+ may be related to geometrical restraints of the tridentate coordination of the PNHP ligand, whose P−Ir−N angles values (83.1(3) and 82.2(3)°) are similar to those found in complex 2. The pincer ligand in 5+ is found to be mercoordinated to iridium, with the phosphine groups trans to each other, while the coe ligand fills the fourth coordination site, trans to the nitrogen atom. The Ir−P−C−C−N iridacycle 31

Scheme 1. Synthesis of Complexes 3 and 4

The solubility of 3 in aromatic solvents suggested a neutral nature and allowed its spectroscopic characterization by NMR techniques, which in general showed a fluxionality pattern at room temperature similar to that observed for complex 2. The 1 H NMR spectrum of 3 showed broad bands at room temperature for all the protons of the molecule. However, at 253 K the spectrum dramatically changed, showing a new pattern of resonances that reflected a trigonal-bipyramidal geometry around iridium and a Cs symmetry in solution (see Figure S2). An alternative synthetic route toward compound 3 relies on the relative acidity of the NH proton in 2, which in the presence of triethylamine cleanly afforded complex 3 upon removal of HNEt3Cl. Next we turned our attention to iridium complexes bearing simple olefins, looking for the formation of 16 e− unsaturated species from 1, which we expected to be more reactive. Specifically, we carried out reactions of 1 with the known chloro-bridged complex [{Ir(μ-Cl)(coe)2}2] and the compound [Ir(acetone)2(coe)2]PF6 (coe = cyclooctene) in several solvents, all of which led invariably to mixtures, as revealed by NMR spectroscopy of the isolated solids, which showed hydrido signals most probably as a consequence of C−H activation processes from the released coe, as has already been observed in related systems.27 Along this line, deprotonation of 1 with strong bases (tBuOK, MeLi) and further addition of [{Ir(μ-Cl)(coe)2}2] led again to the formation of mixtures of C


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Organometallics

However, the pattern of resonances from the coe ligand did not fit with the apparent Cs symmetry of 5 in solution. Only one resonance was observed for the CH protons at δ 3.33 ppm, which correlated to a sole signal at δ 60.0 ppm in the 13 C{1H}-APT NMR spectrum, reflecting some degree of symmetry that seems to relate both sides of the olefin. One could argue that flexibility of the ligand backbone could give rise to this phenomenon; however, protonation of the nitrogen atom avoids inversion at nitrogen, and subsequently the pucker chirality of the tridentate ligand is fixed. Therefore, the observed situation is probably the consequence of fluxional processes in solution, most probably related to a fast coe rotation around the Ir−N axis (as indicated by the solid-state structure), which did not freeze even at 183 K in CD2Cl2.31 Along this line, from an electronic perspective the drastic downfield shift of the resonances of the CH fragments of the coe molecule in 5 in comparison to those for amido complex 4 (Δ(δ(1H)) = 1.15 ppm; Δ(δ(13C)) = 22.3 ppm) is indicative of a much stronger π back-donation from the N atom to the metal in 4 in comparison to amine complex 5. This electronic arrangement in 4 enhances the binding of the coe olefin in comparison to 5. In order to explore the reactivity of olefin complex 4, we carried out a series of reactions with distinct substrates. For instance, addition of phosphanes to solutions of 4 did not induce any transformations, which is surprising since it is wellknown that π-coordinated olefins easily exchange with strongly electron donating phosphines, a chemical behavior that reinforces the existence of a strong Ir−olefin bond. Along this line, a number of amines (including ammonia) did not react with 4, even under harsh conditions. Stirring toluene solutions of 4 under an atmosphere of ethylene (1 bar) at 60 °C for 3 h in a J. Young pressure tube or with acrylonitrile at 70 °C afforded cleanly the corresponding olefin adducts [mer(PNP)Ir(η2-olefin)] (olefin = ethylene (6), acrylonitrile (7)). Complex 6 reflected a C2h symmetry at all temperature ranges (δ(31P) 42.5 ppm), where the η2-bound ethylene was observed as a virtual triplet at δ(1H) 1.99 ppm. Multinuclear NMR spectra of 7 in C6D6 at variable temperature is coherent with a square-planar geometry, in which the PNP ligand is mercoordinated. As in complexes 4 and 6, the symmetry plane containing both P and N atoms suggests some degree of planarization at nitrogen; additionally, the 31P{1H} NMR spectrum of 7 showed an AB system with a large coupling constant (2JP−P = 385 Hz). The loss of the symmetry plane in the molecule clearly indicates that the olefin is static at all ranges of temperature, most probably due to the strong binding to the metal, as suggested by the low chemical shifts observed for the olefin in both the 1H and 13C{1H} NMR spectra. For preparative purposes, the synthesis of adducts 6 and 7 must be carried out in an oxygen-free solvent. For example, when the solvent is not deoxygenated by means of three freeze and thaw cycles, stirring of 4 under an atmosphere of ethylene at 60 °C afforded a dark greenish solid, which was found to be a 1:1 mixture of complex 6 and a new species. This were further characterized as the oxo complex [mer-(PNP)Ir(η2O2)] (8),32 a clear indication that olefin complex 4 is highly sensitive toward oxygen, an observation that led to synthesizing oxo compound 8 simply by stirring solutions of 4 in air. Analysis of 8 by NMR spectroscopy showed only resonances assignable to the PNP ligand skeleton, two multiplets due to the fragments CH2N and CH2P, which correlated with two

Figure 3. Molecular structure of the cation [mer-(PNHP)Ir(coe)]+ (5+). For clarity, only the H atom bound to N and the major component of the disordered coe fragment have been included.

Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 5 Ir−P1 Ir−P2 P1−Ir−P2 P1−Ir−N P2−Ir−N

2.278(3) 2.301(3) 163.21(12) 83.1(3) 82.2(3)

Ir−N Ir−Ct1a P1−Ir−Cta P2−Ir−Cta Cta−Ir-N

2.144(9) 2.0619(1) 105.3(4) 82.2(3) 169.6(4)

a

Ct1 is the centroid of the olefinic bond of the coe ligand (main component).

ring puckering amplitudes (Ir−P1−C21−C22−N, Q = 0.491(3) Å, ϕ = −87.9(6)°, T3 conformation; Ir−P2−C24− C23−N, Q = 0.490(3) Å, ϕ = 93.0(6)°, 3T4 conformation)24 are similar to those observed in complex 2. Pyramidalization of the nitrogen atom in 5+ is analogous to that observed in 2, as evidenced by the sum of the bonding angles (342.1(13)°). The observed Ir−N distance (2.144(9) Å) is comparable to that of complex 2 and similar to other bond distances reported in related Ir-based PNHP pincer systems. For example, 2.176(3), 2.131(4), and 2.121(6) Å Ir−N bond lengths have been reported in [Ir(PMe3)(PNHPiPr)]+, [Ir(C2H4)(PNHPiPr)]+ and [Ir(CO(PNHPiPr)]+ cations. Slight differences among these values may be attributed to different trans influences, exerted by phosphine,30 ethylene,27a and CO27a ligands, respectively. These large distances (in comparison to those observed in Ir(PNP) fragments) have been related to the weak coordinative M−N bonding.27a Interestingly, the coe fragment in 5+ has been found to be disordered. The structural model includes this ligand in two sets of positions (see Figure S30) roughly related by a 180° rotation about the Ir−N axis. This feature may indicate that steric requirements resulting from the tridentate ligand coordination are not restrictive enough to fix the coe position in this complex. The 1H NMR spectrum of 5 in CD2Cl2 clearly showed a static compound at all ranges of temperature studied (see Figure S5); the NH proton was observed as a broad triplet at δ 5.80 ppm, which correlated with a resonance at 57.2 ppm in the 1H−15N HMBC spectrum. The 31P{1H} NMR spectrum of 5 showed a singlet at δ 34.1 ppm, which indicated the existence of a plane of symmetry that relates both alkylic arms of the PNP ligand, as confirmed by the 1H−13C HSQC NMR spectrum of 5, which reflected this structural arrangement. Pyramidalization at the N atom upon protonation becomes evident, since the original symmetry plane observed in 4 (which contains both P and N atoms) is not present in 5. D


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Organometallics triplets at δ 66.2 and 32.7 ppm in the 13C{1H} NMR spectrum, respectively. The pattern of the multinuclear NMR spectra of 8 indicated a C2h symmetry in solution; along this line, the 31 1 P{ H} NMR spectrum of 8 showed a sole singlet at δ 19.2 ppm. The presence of the coordinated dioxygen ligand was detected clearly in the mass spectrum of 8, which showed a peak at m/z 666.1316 that corresponded to the molecular ion [(PNP)Ir(O2)]+; additionally, microanalytical data obtained on 8 agreed with the proposed formulation. The efficiency of olefin coordination with deprotonated ligand 1 led us to explore the reactivity of 4 with alkynes. For that purpose, we reacted complex 4 with methyl phenylpropiolate to give an orange solid in moderate yield, characterized as the complex [(PNP)Ir(η2-PhCCC(O)OMe)] (9). In contrast, treatment of a solution of 4 in toluene under an atmosphere of acetylene (1 bar) in a Kontes pressure tube for 2 h at room temperature afforded a light yellow solid in 58% yield, further characterized as the Ir(III) complex [fac-(PNHP)IrH(CCH)2] (10) (Scheme 2).

enlightened its static nature in solution. A hydrido ligand was observed at δ −10.15 ppm as a doublet of doublets (2JH−P1 = 21 Hz, 2JH−P2 = 161 Hz), which located it trans to P2 and cis to P1 (Scheme 2), while the NH proton was observed at δ 5.26 ppm and the alkynyl protons in 10 were easily distinguished by their different multiplicity. The 31P{1H} NMR spectrum of 10 showed two doublets with a low coupling constant which indicated that they were mutually cis and therefore signaled a fac coordination of the PNHP ligand. The off-resonance 31P NMR spectrum of 10 split both signals into a well-resolved doublet of doublets (2JP1−H = 20 Hz; 2JP2−H = 149 Hz), which allowed us to unambiguously establish the stereochemistry of 10. The 13C{1H}-APT NMR spectrum of 10 showed four wellseparated signals for the carbons of the alkynyl fragments, in which the respective magnitude of the coupling constants allowed their correct assignment (see the Experimental Section and Figure S17). The spectroscopic information gathered for adduct 9 indicated a structure different from that assigned to complex 10. The 31P{1H} NMR spectrum of 9 showed a sole singlet, indicating some degree of symmetry which related both alkylic arms of the ligand, a situation confirmed by the observation of sole signals for the CH2N and CH2P carbons in the 13C{1H}APT NMR spectrum, respectively. Additionally, we observed the C(sp) carbons of the coordinated alkyne as two distinct multiplets at δ 106.9 and 67.9 ppm, with small coupling constants (2JC−P = 2−3 Hz). On the whole, the Cs symmetry in solution observed for 9 is coherent with a square-planar geometry, in which the PNP ligand is coordinated in a meridional fashion to iridium and the alkyne π-coordinated out of plane.33 However, we cannot exclude some distortion of this planar geometry, since there are several related iridium molecular structures with a distorted-tetrahedral environment.34 In order to understand the reaction of complex 4 with terminal alkynes, theoretical calculations at the DFT level were carried out on a model system, replacing the phenyl ligands by methyl groups. The Gibbs energetic profile for the reaction is

Scheme 2. Synthesis of Complexes 6−10

The pattern of the 1H NMR spectrum of 10 in CD2Cl2 did not change with the temperature and showed eight different resonances assigned to the protons from the alkylic chains of the ligand in the range of δ 2.28−3.47 ppm, which indicated that 10 did not have any elements of symmetry and

Figure 4. Gibbs energy profile (in kcal mol−1, relative to A and isolated molecules) for the formation of 10. E


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Organometallics shown in Figure 4. Starting from structure A, direct oxidative addition of the C−H bond to the Ir(I) center (green path), upon replacement of coe by the alkyne, requires an activation energy of 35.6 kcal mol−1, which is not affordable under the experimental conditions. Alternatively, coordination of a second alkyne molecule to the metal may take place through TSBD, leading to the planar-bipyramidal structure D. The transformation of D into F requires the consecutive C− H bond activation of two acetylene molecules by the metallic center and by assistance of the N atom of the pincer ligand. The two possible reaction pathways, namely (i) oxidative addition first followed by a ligand-assisted process (red path in Figure 4) and (ii) a ligand-assisted process followed by oxidative addition (blue path in Figure 4) have been calculated. Path i is characterized by the affordable structure TSDE showing an overall energetic barrier of 23.6 kcal mol−1. The hydride intermediate E can deprotonate the coordinated alkyne through TSEF, forming the observed product F. The process is strongly exergonic, −29.6 kcal mol−1. The alternative path ii starts by activation of the C−H bond by the N atom of the ligand as shown by TSDE′, presenting an overall energetic barrier of 20.8 kcal mol−1. The formed alkynyl intermediate E′ may undergo oxidative addition of the Ir atom through TSEF′, showing an energetic barrier of 13.3 kcal mol−1, to yield the final product F. Catalytic Activity of Complex 4 in the Hydroamination of Activated Substrates with Ammonia. The catalytic activity of the complex [mer-(PNP)Ir(coe)] (4) for the addition of ammonia to unsaturated substrates was evaluated. In general, catalytic reactions were first monitored in J. Young pressure NMR tubes with a catalyst loading of 5 mol % and the specific substrate, and these mixtures were then stirred at 60 °C in C6D6 under an ammonia atmosphere. Nonactivated alkenes (styrene, 1-hexene) or alkynes (phenylacetylene, 3-hexyne) were checked, and no hydroamination products were obtained under the catalytic conditions employed. In the case of terminal activated alkenes (acrylonitrile and methyl acrylate), the transformations were complete within 6 h. The analysis of pure samples of the products of the catalytic reactions by multinuclear NMR spectroscopy showed the formation of mixtures of the corresponding secondary and tertiary amines, while no traces of the primary amines were observed (see Table 3). These organic compounds are the result of the addition of ammonia to the alkenes. Additionally, the absence of the primary amines (H2NCH2CH2R; R = CN, CO2Me) shows that they are more nucleophilic than ammonia itself, and therefore they react more quickly with olefins than gaseous NH3.35 However, internal alkenes (crotonitrile, methyl crotonate) did not undergo any transformations under the above catalytic conditions. We performed some catalytic addition of ammonia on activated alkynes, such as dimethyl acetylenedicarboxylate (DMAD) and methyl phenylpropiolate. First, we ran blank experiments (without catalyst 4), finding that while no reaction between ammonia and methyl phenylpropiolate was observed at 60 °C upon 24 h, DMAD reacted with ammonia at 60 °C for 20 h to yield pure (Z)dimethyl 2-aminofumarate quantitatively. When the reaction was carried out with catalyst 4 the transformation was complete upon 20 h, affording pure enamine (E)-dimethyl 2aminofumarate selectively.36 In the case of methyl phenylpropiolate, the catalytic reaction afforded (Z)-methyl 3 amino-

Table 3. Hydroamination of Activated Unsaturated Substrates with NH3 Catalyzed by 4a

HA product (mol %)b entry

substrate

n=1

n=2

yield (%)c

1 2 3 4

CH2CHCN CH2CHCO2Me DMAD PhCCCO2Me

70 57

30 33

83 88 93 91

Catalytic reactions were carried out in toluene (3 mL) at 60 °C for 20 h using 5 mol % of 4 and gaseous ammonia (3 atm). bCalculated by NMR integration. cYield after workup. a

3-phenylacrylate selectively. No further addition of ammonia across the double bond of the enamines was observed. DFT calculations were performed on the catalytic cycle to shed light into the reaction mechanism. As in the previous DFT calculations, phenyl groups attached to the organometallic complex were replaced by methyl substituents for computational efficiency and two ammonia molecules were employed in the calculations instead of one in order to get a more realistic description of charged species and proton transfer processes. The Gibbs energetic profile is shown in Figure 5. Hence, the catalytic cycle starts by nucleophilic attack of ammonia to the coordinated alkyne as shown by TSGH, leading to intermediate H. Then, proton transfer from the protonated amine to the metal occurs through intermediate TSHI. This transition state determines the overall energetic barrier for the catalytic cycle, 28.5 kcal mol−1, relative to G. The proton is transferred to the metal, forming intermediate I, which can be described as an Ir(III) metal displaying trigonalbipyramidal arrangement. Finally, reductive elimination of the alkene yields the observed product and regenerates the catalyst. An alternative proposal, starting by the oxidative addition of a molecule of ammonia by the metal has also been calculated (see the Supporting Information) but the energetic barrier is excessively high.

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CONCLUSIONS In this contribution we have utilized a known PNP pincer ligand to stabilize new Ir(I) amido olefin complexes, in which the ligand may adopt both mer or fac coordination depending on the nature of the olefin. In general, the spectroscopic information gathered for the amido complexes characterized indicates a strong binding of the corresponding olefin (coe, ethylene, acrylonitrile), favored by a strong π back-donation from the metal induced by the amidic nitrogen atom of the ligand, which on the NMR time scale seems to be planar, most probably as a consequence of the participation of the free electron pair in the Ir−N bond. This electronic arrangement situation changes upon protonation at nitrogen with a strong electrophile, affording the corresponding amine organometallic salt. In these species the geometry of the nitrogen atom is tetrahedral and there is no inversion in solution; in this case the olefin (coe) is more labile and the olefin seems to undergo a fast rotation in solution. The amido complex exchanges the F


Article

Organometallics

Figure 5. Gibbs energy profile (in kcal mol−1, relative to A and isolated molecules) for the catalytic cycle. [{Ir(μ−OH)(coe)2}2]28 were prepared according to published procedures. Synthesis of PNHP (1). To a suspension of potassium tertbutoxide (6.71 g, 59.8 mmol) in THF (30 mL) was added pure diphenylphosphane (4 mL, 23.0 mmol) via syringe, giving a deep red solution which was stirred for 20 min. Then, solid bis(2-chloroethyl)amine hydrochloride (2.05 g, 11.45 mmol) was added to the mixture and the resulting solution was stirred under reflux for 10 h. Upon reaching room temperature, the reaction mixture was quenched with a solution of 1 M hydrochloric acid (10 mL, 10 mmol), leading to a two-phase system. The organic layer was separated, dried over anhydrous MgSO4, and then dried under vacuum, affording a whitish, viscous oil. Yield: 4.83 g (88%). 1H NMR (400 MHz, C6D6, 298 K): δ 7.33−7.46 (set of m, 20 H; PPh2), 2.75 (dd, 4H, 3JH−H = 7 Hz, 3JH−H = 16 Hz; CH2N), 2.24 (m, 4H; CH2P), 1.39 (br s, 1H; NH). 31P{1H} NMR (162 MHz, C6D6, 298 K): δ −20.6 (s). Synthesis of [fac-(PNHP)Ir(cod)]Cl (2).

coe olefin with alkynes, where the internal substrate coordinates out of plane and the parent acetylene undergoes two consecutive C−H activation processes, affording a hydrido Ir(III) complex where the PNHP ligand coordinates in a mer fashion, following a mechanism based on the concept of metal−ligand cooperation. The reaction starts by deprotonation of a second alkyne molecule by the PNP ligand and it is followed by oxidative addition of the C−H bond to the metal. Additionally, we have shown that an amido coe complex is active in the catalytic hydroamination of activated substrates. The operative mechanism studied by theoretical methods shows that these organic transformations follow a nucleophilic attack of ammonia to the bound alkyne, hydrogen migration to the metal, and reductive elimination steps.

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EXPERIMENTAL SECTION

Scientific Equipment. C, H, and N analyses were carried out in a PerkinElmer 2400 Series II CHNS/O analyzer. IR spectra of solid samples were recorded with a PerkinElmer 100 FT-IR spectrometer (4000−400 cm−1) equipped with attenuated total reflectance. 1H, 31 1 P{ H}, and 13C{1H} NMR spectra were recorded on Bruker Avance 300 (300.13, 121.42, and 75.48 MHz) and Bruker Avance 400 (400.16, 161.99, and 100.61 MHz) spectrometers. NMR chemical shifts are reported in ppm relative to tetramethylsilane and are referenced to partially deuterated solvent resonances. Coupling constants (J) are given in hertz. Spectral assignments were achieved by a combination of 1H−1H COSY, 13C-APT, 1H−13C HSQC, and 1 H−15N HMBC NMR experiments. ESI+ mass spectra were obtained on a Bruker MICROFLEX spectrometer using 1,8-dihydroxy-9,10dihydroanthracen-9-one (DIT, dithranol), as matrix. GC-MS analyses were recorded on an Agilent 5973 mass selective detector interfaced to an Agilent 6890 series gas chromatograph system, using a HP-5MS 5% phenyl methyl siloxane column (30 m × 250 mm with a 0.25 mm film thickness). Synthesis. All experiments were carried out under an atmosphere of argon using Schlenk techniques. Solvents were distilled immediately prior to use from the appropriate drying agents or obtained from a Solvent Purification System (Innovative Technologies). Oxygen-free solvents were employed throughout. CD2Cl2 and (CD3)2CO were dried using activated molecular sieves, while C6D6 and toluene-d8 were dried over a solid Na/K amalgam. Gaseous ethylene, acetylene, and ammonia were purchased from Air Liquide. The complexes [{Ir(μ-Cl)(cod)}2],37 [{Ir(μ-OMe)(cod)}2],38 and

To a solution of 1 (51 mg, 0.11 mmol) in toluene (5 mL) was added solid [{Ir(μ-Cl)(cod)}2] (35 mg, 0.05 mmol). After the mixture was stirred for 1 h, a white microcrystalline solid crystallized out of the solution, which was isolated by filtration with a cannula, washed with diethyl ether, and dried under vacuum. Yield: 56 mg (69%). Anal. Calcd for C36H41ClIrNP2: C, 55.62; H, 5.32; N, 1.80. Found: C, 55.65; H, 5.25; N, 1.74. IR (solid, cm−1): ν(N−H), 3049 (s), 3217 (s), 3356 (s). 1H NMR (400 MHz, (CD3)2CO, 298 K): δ 8.08 (br, 1H; NH), 7.13−7.70 (set of m, 20H; PPh2), 3.93 (br, 4H; = CH cod), 3.33 (m, 2H; CH2N), 2.86 (m, 4H; CH2N + CH2P), 2.56 (m, 2H; CH2P), 1.63 (m, 4H), 1.44 (m, 4H) (CH2 cod). 1H NMR (400 MHz, (CD3)2CO, 233 K): δ 8.19 (br, 1H; NH), 7.13−7.63 (set of m, 20H; PPh2), 4.16 (m, 2H), 3.53 (m, 2H) (CH cod), 3.28 (m, 2H), 2.97 (m, 2H) (CH2N), 2.86 (m, 2H), 2.56 (m, 2H) (CH2P), 1.83 (m, 2H), 1.51 (m, 2H), 1.33 (m, 4H) (CH2 cod). 31P{1H} NMR (162 MHz, (CD3)2CO, 233 K): δ 14.4 (s). 13C{1H} NMR (100 MHz, (CD3)2CO, 233 K): δ 132.3 (d, 2JC−P = 5 Hz), 132.2 (d, 2JC−P = 5 Hz), 131.3 (d, 2JC−P = 5 Hz), 131.2 (d, 2JC−P = 5 Hz) (Co PPh2), 130.0 (s), 129.7 (s) (Cp PPh2), 128.7 (m; Cm PPh2), 65.0 (m), 53.1 (m) (CH cod), 57.4 (m; CH2N), 32.4 (m; CH2P), 31.1−31.5 (set G


Article

Organometallics of s; CH2 cod). Mass Calcd for C36H41IrNP2: 741.8823. MS (ESI+): m/z 742.2333 (100%; M+ − 1H). Synthesis of [fac-(PNP)Ir(cod)] (3).

removed by vacuum, affording an intense orange powder. Yield: 0.07 g (78%). 1H NMR (300 MHz, CD2Cl2, 298 K): δ 8.19 (m, 4H; Ho PPh2), 7.50−7.64 (set of m, 16H; Ho + Hm + Hp PPh2), 5.60 (m, 1H; NH), 3.51 (m, 2H; CH2N), 3.33 (m, 2H; CH coe), 2.85 (m, 2H), 2.50 (m, 2H) (CH2P), 2.24 (m, 2H; CH2N), 1.85 (m, 2H), 1.18 (m, 4H), 0.90 (m, 6H) (CH2 coe). 31P{1H} NMR (121 MHz, CD2Cl2, 298 K): δ 34.1 (s). 13C{1H}-APT NMR (75 MHz, CD2Cl2, 298 K): δ 135.1, 132.0 (m; Co PPh2), 131.5, 130.5 (s; Cp PPh2), 128.7−128−9 (set of signals, Cp + Cm PPh2), 60.0 (s; CH coe), 52.5 (m; CH2N), 34.8 (m; CH2P), 32.0 (m; CH2CH coe), 31.4, 26.0 (s; CH2 coe). 1 H−15N HMBC NMR (41 MHz, CD2Cl2, 298 K): δ 57.2. Mass Calcd for C36H43IrNP2: 743.8903. MS (ESI+): m/z 744.2495 (100%; M+ + 1H). Synthesis of [mer-(PNP)Ir(η2-C2H4)] (6).

To a solution of 1 (0.18 g, 0.41 mmol) in toluene (10 mL) was added yellow solid [{Ir(μ-OMe)(cod)}2] (0.13 g, 0.20 mmol), giving a red solution within 45 min. The volume of the solvent was reduced to ca. 2 mL by vacuum, and the addition of hexanes induced the precipitation of a dark red solid. This was collected via filtration with a cannula, washed with hexanes, and then vacuum-dried. Yield: 0.24 g (82%). Anal. Calcd for C36H40IrNP2: C, 58.36; H, 5.44; N, 1.89. Found: C, 58.29; H, 5.31; N, 1.84. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.09−7.57 (set of m, 20H; PPh2), 3.90 (br, 4H; = CH cod), 3.35 (m, 2H), 2.84 (m, 2H) (CH2N), 2.76 (m, 2H), 2.31 (m, 2H) (CH2P), 1.46 (br, 8H; CH2 cod). 1H NMR (400 MHz, CD2Cl2, 253 K): δ 7.56−6.91 (set of m, 20H; PPh2), 3.93 (br, 2H), 3.65 (br, 2H) (CH cod), 3.26 (br, 2H), 2.84 (br, 2H) (CH2N), 2.76 (m, 2H), 2.29 (m, 2H) (CH2P), 1.92 (br, 2H), 1.57 (br, 2H), 1.27 (br, 4H) (CH2 cod). 31P{1H} NMR (162 MHz, CD2Cl2, 253 K): δ 14.7 (s). 13C{1H} NMR (100 MHz, CD2Cl2, 253 K): δ 138.3 (m), 134.1 (m) (Cipso PPh2), 132.3 (d, 2JC−P = 5 Hz), 132.2 (d, 2JC−P = 5 Hz), 131.2 (d, 2JC−P = 5 Hz), 131.2 (d, 2JC−P = 5 Hz) (Co PPh2), 130.0 (s), 129.7 (s) (Cp PPh2), 128.6 (m; Cm PPh2), 65.3 (m), 54.0 (m) (CH cod), 56.9 (m; CH2N), 33.1 (m; CH2P), 31.3 (br; CH2 cod). Mass Calcd for C36H40IrNP2: 740.8744. MS (ESI+): m/z 742.2369 (100%; M+ + 1H). Synthesis of [mer-(PNP)Ir(coe)] (4).

A solution of 4 (65 mg, 0.08 mmol) in oxygen-free toluene (5 mL) was transferred to a pressure Kontes tube, and then it was bubbled with gaseous ethylene (1 bar) for 2 min. After the pressure reactor was sealed, the solution was stirred at 60 °C, for 3 h leading to an orange solution. This was filtered to eliminate some solid impurities, and then the solvent was removed by vacuum, yielding an orange powder. Yield: 40 mg (76%). Anal. Calcd for C30H32IrNP2: C, 54.53; H, 4.88; N, 2.12. Found: C, 54.46; H, 4.91; N, 2.23. 1H NMR (300 MHz, C6D6, 298 K): δ 7.11−8.23 (set of m, 20H; PPh2), 3.57 (m, 4H; CH2N), 2.58 (m, 4H; CH2P), 1.99 (t, 3JH−P = 4 Hz, 4H; CH η2-C2H4). 31P{1H} NMR (121 MHz, C6D6, 298 K): δ 42.5 (s). 13 C{1H} NMR (75 MHz, C6D6, 298 K): δ 128.3−136.6 (set of m; PPh2), 61.3 (m; CH2N), 36.9 (m; CH2P), 18.0 (m; η2-C2H4). Mass Calcd for C30H32IrNP2: 660.745. MS (ESI+): m/z 660.789 (100%; M+). Synthesis of [mer-(PNP)Ir(η2-C2H3CN)] (7).

To a solution of 1 (0.51 g, 1.15 mmol) in toluene (10 mL) was added yellow solid [{Ir(μ−OH)(coe)2}2] (0.36 g, 0.42 mmol), and the resulting mixture was stirred for 1 h at room temperature, affording a colorless solution. Removal of the solvent to ca. 1 mL by vacuum and further addition of hexanes gave a whitish precipitate, which was subsequently filtered, washed with hexanes, and then dried under vacuum. Yield: 0.55 g (64%). Anal. Calcd for C36H42IrNP2: C, 58.20; H, 5.70; N, 1.89. Found: C, 58.15; H, 5.55; N, 1.81. 1H NMR (300 MHz, toluene-d8, 253 K): δ 7.90 (m, 2H), 7.68 (m, 2H) (Ho PPh2), 6.93−7.09 (set of m, 16H; Ho + Hm + Hp PPh2), 3.22 (m, 2H), 3.05 (m, 2H) (CH2N), 2.38 (m, 2H), 2.29 (m, 2H) (CH2P), 2.18 (m, 2H; CH coe), 1.38 (m, 12H; CH2 coe). 31P{1H} NMR (121 MHz, toluene-d8, 298 K): δ 43.6 (AB system, 2JP−P = 390 Hz). 13C{1H} NMR (75 MHz, toluene-d8, 253 K): δ 135.3 (d, 1JC−P = 20 Hz), 135.0 (d, 1JC−P = 20 Hz), 132.4 (d, 1JC−P = 20 Hz), 132.0 (d, 1JC−P = 20 Hz) (Cipso PPh2), 134.1 (d, 2JC−P = 5 Hz), 134.0 (d, 2JC−P = 5 Hz), 133.4 (d, 2JC−P = 5 Hz), 133.3 (d, 2JC−P = 5 Hz), 133.1 (d, 2JC−P = 6 Hz), 133.0 (d, 2JC−P = 6 Hz) (Co PPh2), 130.0 (s), 129.9 (s), 129.4 (s), 129.2 (s) (Cp PPh2), 127.8−128.3 (set of m; Cm PPh2), 60.2 (m), 59.3 (m) (CH2N), 39.4 (m), 35.7 (m) (CH2P), 37.7 (CH coe), 25.3−30.1 (set of s, CH2 coe). Mass Calcd for C36H42IrNP2: 742.8903. MS (ESI+): m/z 744.2498 (100%; M+ + 1H). Synthesis of [mer-(PNHP)Ir(coe)]BF4 (5).

A solution of 4 (49 mg, 0.067 mmol) in toluene (5 mL) was treated with neat acrylonitrile (12 mg, 14 μL, 0.206 mmol), and the resulting solution was stirred at 70 °C for 2 h. The volatiles were removed by vacuum, leading to the isolation of an orange solid. Yield: 38 mg (82%). 1H NMR (400 MHz, C6D6, 298 K): δ 8.29 (m, 2H), 7.88 (m, 4H), 7.43 (m, 2H) (Ho PPh2), 7.07−7.36 (set of m, 12H; Hm + Hp PPh2), 3.34 (m, 4H; CH2N), 2.65 (m, 1H), 2.36 (m, 3H) (CH2P), 2.00 (m, 1H; CHCN), 1.69 (m, 1H), 1.47 (m, 1H) (CH2) (acrylonitrile). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 43.7 (AB system, 2JP−P = 385 Hz). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 135.7 (dd, 2JC−P = 9 Hz, 4JC−P = 6 Hz), 133.9 (dd, 2JC−P = 8 Hz, 4 JC−P = 5 Hz), 132.8 (dd, 2JC−P = 6 Hz, 4JC−P = 4 Hz), 132.5 (dd, 2 JC−P = 7 Hz, 4JC−P = 5 Hz) (Co PPh2), 128.2−130.4 (set of signals, Cm + Cp PPh2), 126.5 (t, 3JC−P = 3 Hz; CN), 61.7, 61.2 (m; CH2N), 38.4 (dd, 1JC−P = 20 Hz; 3JC−P = 11 Hz), 36.1 (dd, 1JC−P = 20 Hz; 3 JC−P = 11 Hz) (CH2P), 17.4 (m; CH2), −4.6 (t, 2JC−P = 1 Hz;  CHCN). Mass Calcd for C31H31IrN2P2: 685.7560. MS (ESI+): m/z 685.8754. Synthesis of [mer-(PNP)Ir(η2-O2)] (8).

A solution of 4 (0.10 g, 0.13 mmol) in toluene (5 mL) was stirred in air. After 30 min, a green solid began crystallizing out of the solution, which was isolated by filtration, washed with hexanes, and then dried under vacuum. Yield: 39 mg (45%). Anal. Calcd for C28H28IrNO2P2: C, 50.60; H, 4.25; N, 2.11. Found: C, 50.45; H, 4.11; N, 2.01. 1H NMR (300 MHz, C6D6, 298 K): δ 7.11−8.23 (set of m, 20H; PPh2), 2.89 (m, 4H; CH2N), 2.29 (m, 4H; CH2P). 31P{1H} NMR (121 MHz, C6D6, 298 K): δ 19.2 (s). 13C{1H} NMR (75 MHz, C6D6, 298

To a solution of 4 (0.08 g, 0.1 mmol) in toluene (5 mL) was added HBF4·Et2O (16 μL, 0.12 mmol) at 213 K, and the resulting mixture was stirred for 10 min, affording an orange solution which was allowed to reach room temperature. The solution was filtered via cannula to remove some solid impurities, and then the solvent was H


Article

Organometallics K): δ 128.3−136.6 (set of m; PPh2), 66.2 (t, 3JH−P = 4 Hz; CH2N), 32.7 (t, 3JH−P = 15 Hz; CH2P). Mass Calcd for C28H28IrNO2P2: 664.6920. MS (ESI+): m/z 666.1316 (100%; M+ + 1H). Synthesis of [mer-(PNP)Ir(η2-PhCCC(O)OMe)] (9).

For preparative purposes, we performed the experiments as follows. A medium Fisher−Porter pressure reactor was charged with the corresponding substrate (2.15 mmol), the catalyst 4 (80 mg, 0.1 mmol) ,and toluene (3 mL), and then the mixture was cooled to −95 °C. The reactor was evacuated and then charged with anhydrous ammonia. The resulting mixture was stirred for 20 h at 60 °C, where the pressure of ammonia was 3 atm. When the mixture was cooled to room temperature, the volatiles were removed by vacuum, and the crude oils isolated were transferred to a short glass column packed with silica gel and then eluted with CH2Cl2; upon evaporation of the solvent heavy oils were isolated, which were further analyzed by GCMS and 1H, 13C{1H}-APT, and 1H−13C HSQC NMR techniques (see the Supporting Information). Crystal Structure Determination of Complexes 2 and 5. Xray diffraction data were collected on a Smart APEX Bruker diffractometer at 100(2) K, using a narrow oscillation frame (Δω = 0.3°) with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Intensities were integrated and corrected for absorption correction with SAINT39 and SADABS40 programs, integrated in the APEX2 package. The structures were solved by direct methods with the SHELXS41 program and refined by full-matrix least-squares refinements in F2 with SHELXL42 included in the WinGX43 package. Special details about the refinement are listed below. Crystal data for 2: C36H41ClIrNP2·2CH2Cl2; Mr = 947.14; white prism, 0.240 × 0.300 × 0.310 mm; triclinic, P1̅; a = 10.8841(5) Å, b = 11.4719(5) Å, c = 16.4525(8) Å; α = 86.6180(5)°, β = 76.3103(4)°, γ = 72.2091(4)°; V = 1900.28(15) Å3; Z = 2; ρcalc = 1.655 g cm−3; μ = 3.978 cm−1; minimum and maximum transmission factors 0.2958 and 0.4523; 2θmax = 57.368°; 46713 reflections collected; 9200 unique reflections (Rint = 0.0264); number of data/restraints/parameters 9200/1/588; final GOF = 1.040; R1 = 0.0172 (8974 reflections, I > 2σ(I)), wR2 = 0.0433 for all data; largest difference peak 1.758 e Å−3. Hydrogen atoms of the solvent were included in the model in calculated positions and refined with a riding model. The rest of the hydrogen atoms were included in observed positions and refined with a restraint in a C−H bond length. Crystal data for 5: C36H43BF4IrNP2·2CH2Cl2; Mr = 1000.51; orange needle, 0.056 × 0.090 × 0.120 mm; monoclinic, P21/n; a = 10.6822(8) Å, b = 20.5768(15) Å, c = 17.9348(13) Å; β = 106.708(2)°; V = 3775.7(5) Å3; Z = 4; ρcalc = 1.760 g cm−3; μ = 3.955 cm−1; minimum and maximum transmission factors 0.5060 and 0.7786; 2θmax = 56.688°; 39503 reflections collected; 9238 unique reflections (Rint = 0.0876); number of data/restraints/parameters 9238/0/383; final GOF = 1.191; R1 = 0.0816 (6719 reflections, I > 2σ(I)), wR2 = 0.2083 for all data; largest difference peak 3.444 e Å−3. The coe fragment of the metal complex was found to be disordered. Two sets of positions and complementary occupancy factors (0.53/0.47(4)) were used in the refinement. Two fluorine atoms of the counterion were also disordered. Solvent was also found to be disordered, and its refinement led to unrealistic parameters. Therefore, the contribution of the solvent to the calculated structure factors was calculated with the SQUEEZE program.44 CCDC 1846013−1846014 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc. cam.ac.uk/structures. Computational Details. All DFT theoretical calculations were performed using the Gaussian09 program package (Gaussian 09, Revision D.01, see the Supporting Information for full citation). The B3LYP method45 was employed, including the D3 dispersion correction scheme developed by Grimme46 using the Becke−Johnson damping47 for both energies and gradient calculations in conjunction with the “ultrafine” grid. The def2-SVP basis set48 was selected for all atoms for geometry optimizations, single-point calculations being performed with the def2-TZVP basis set to refine energy results. The nature of the stationary points was confirmed by analytical frequency analysis, and transition states were characterized by calculation of reaction paths following the intrinsic reaction coordinate. All reported energies are Gibbs free energies referred to a 1m standard state at

A solution of 4 (0.10 g, 0.13 mmol) in oxygen-free toluene (5 mL) was treated with neat methyl phenylpropiolate (26 mg, 0.16 mmol) via syringe, and the resulting orange solution was stirred at 60 °C for 1 h. This was filtered to eliminate some solid impurities, and the solvent was removed by vacuum, leading to the isolation of an orange solid. Yield: 79 mg (77%). 1H NMR (300 MHz, C6D6, 298 K): δ 6.77−8.14 (set of m, 25H; PPh2 + Ph), 3.55 (s, 3H; OMe), 3.41(m, 4H; CH2N), 2.48 (m, 2H) 2.33 (m, 2H) (CH2P). 31P{1H} NMR (121 MHz, C6D6, 298 K): δ 32.5 (s). 13C{1H} NMR (75 MHz, C6D6, 298 K): δ 155.7 (s; CO), 134.7 (m; Co), 133.9 (d, 1JC−P = 46 Hz), 133.5 (d, 1 JC−P = 53 Hz) (Cipso), 130.8 (s), 132.0 (m; Co), 130.2, 129.9, 129.0, 128.4, 127.3 (set of s; Co + Cp Ph), 106.9 (t, 2JC−P = 3 Hz;  CC(O)OMe), 67.9 (t, 2JC−P = 2 Hz; ≡CPh), 63.4 (m; CH2N), 51.9 (s; OMe), 36.6 (m; CH2P). Mass Calcd for C38H36IrNO2P2: 792.8625. MS (ESI+): m/z 793.2345. Synthesis of [fac-(PNHP)IrH(CCH)2] (10).

A solution of 4 (80 mg, 0.10 mmol) in toluene (5 mL) was transferred to a pressure Kontes tube, and then it was bubbled with gaseous acetylene (1 bar) for 10 min. The pressure tube was sealed, and the solution was stirred for 2 h at room temperature. A light yellow solid crystallized out of the solution, and then it was collected by filtration via cannula, washed with hexanes, and vacuum-dried. Yield: 42 mg (58%). Anal. Calcd for C32H32IrNP2: C, 56.13; H, 4.71; N, 2.05. Found: C, 56.01; H, 4.89; N, 2.14. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 8.16 (m, 4H), 8.02 (m, 4H) (Ho PPh2), 6.81−7.42 (set of m, 12H; Hm + Hp PPh2), 5.26 (br, 1H; NH), 3.47 (m, 1H; CH2N), 3.17 (m, 1H), 3.10 (m, 1H) (CH2N), 2.72 (m, 2H; CH2N1 + CH2P1), 2.58 (m, 1H; CH2P1), 2.46 (m, 1H; CH2P2), 2.28 (m, 1H), (CH2P2), 1.86 (t, 4JH2−P1 = 2 Hz, 1H; H2), 1.84 (dd, 4JH1−P1 = 5 Hz, 4 JH1−P2 = 3 Hz; H1), −10.15 (dd, 2JH−P1 = 21 Hz, 2JH−P2 = 161 Hz; Ir−H). 31P{1H} NMR (162 MHz, CD2Cl2, 298 K): δ 6.1 (d, 2JP−P = 11 Hz), 0.4 (d, 2JP−P = 11 Hz). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 137.2 (dd, 1JC−P = 54 Hz, 3JC−P = 5 Hz; Cipso PPh2), 134.2 (d, 2JC−P = 10 Hz), 133.9 (d, 2JC−P = 10 Hz) (Co PPh2), 133.5 (m; Cipso PPh2), 127.5−130.4 (set of m, Cm + Cp PPh2), 91.2 (dd, 2JC−P1 = 30 Hz, 2JC−P2 = 3 Hz; C1), 89.6 (m; C3), 86.6 (dd, 2JC−P1 = 116 Hz, 2 JC−P2 = 13 Hz; C2), 85.7 (t, 3JC−P = 3 Hz; C4), 54.6 (d, 2JC−P = 7 Hz; CH2N), 52.9 (d, 2JC−P = 7 Hz; CH2N), 30.5 (d, 1JC−P = 29 Hz; CH2P1), 30.3 (d, 1JC−P = 32 Hz; CH2P2). Mass Calcd for C32H32IrNP2: 684.7679. MS (ESI+): m/z 684.7534 (100%; M+ + 1H). Procedure for the Ir-Catalyzed Addition of Ammonia to Activated Unsaturated Substrates. The NMR monitoring of the catalytic reactions was carried out as follows. An oxygen-free J. Young pressure NMR tube (Wilmad) was charged with the olefin (0.67 mmol), solid 4 (25 mg, 0.033 mmol), and C6D6 (0.4 mL). Then the tube was frozen, subjected to three freeze and thaw cycles, and subsequently anhydrous ammonia was introduced. At this point, the molar ratio of the olefin versus ammonia was deduced by integrating the corresponding resonances in the 1H NMR spectra of each sample. The reaction tube was sealed and then placed in an oil bath at 60 °C for 20 h. After the reaction mixture was cooled, the products formed were analyzed by 1H NMR spectroscopy. I


Article

Organometallics 298.15 K, with the contribution to the translational entropy removed, as indicated by Morokuma et al.49

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00365. Multinuclear NMR spectra of complexes 2−10 and amines, hydrogen bonding in complex 2, disorder of the coe ligand in 5+, DFT absolute and relative energies, and geometrical representation of all DFT calculated structures (PDF). Accession Codes

CCDC 1846013−1846014 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.A.C.: [email protected]. ORCID

Fernando J. Lahoz: 0000-0001-8054-2237 Víctor Polo: 0000-0001-5823-7965 Miguel A. Casado: 0000-0003-1707-3022 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their appreciation for the financial support of MINECO/FEDER project CTQ2015-67366-P and DGA/ FSE (group E42_17R). V.P. thankfully acknowledges the resources from the supercomputers “Memento” and “Terminus” and the technical expertise and assistance provided by the Institute for Biocomputation and Physics of Complex Systems (BIFI)−Universidad de Zaragoza. P.G.-O. acknowledges the CSIC and European Social Fund for a PTA contract.

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REFERENCES

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Amido Complexes of Iridium with a PNP Pincer Ligand: Reactivity

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