Iridium PC sp3 P-type Complexes with a ... - ACS Publications

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Iridium PCsp3P‑type Complexes with a Hemilabile Anisole Tether Dominic C. Babbini and Vlad M. Iluc* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: A series of iridium PCsp3P complexes based on bis(2-diisopropylphosphinophenyl)-2-anisoylmethane (PCanisHP) is reported. (PCanisP)Ir(H)Cl was generated from the C−H activation of the backbone by [Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene), while the dihydride (PCanisP)Ir(H)2 was generated by hydride metathesis from (PCanisP)Ir(H)Cl. Both complexes are 18e octahedral complexes and water stable. The hemilability of the anisole tether was probed using CO and PMe3; multiple isomers, in which the anisole substituent was displaced, were generated, showing the flexibility of the ligand backbone. (PCanisP)Ir(H)2 showed deuterium incorporation in the hydride, backbone, and anisole positions upon moderate heating in C6D6. Both (PCanisP)Ir(H)Cl and (PCanisP)Ir(H)2 were precatalysts for transfer dehydrogenation of cyclooctane under moderate conditions.

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INTRODUCTION Metal−ligand cooperation has been of much interest recently, as it allows the observation of unique reactivity in several organometallic systems.1−16 For example, a new class of ruthenium PNP-type pincer catalysts, which exhibit metal− ligand cooperation, is capable of splitting water into O2 and H2.17 The metal−ligand cooperation in these compounds enhances their reactivity toward bond activation1 and increases the diversity of possible transformations.18 Iridium complexes are active in a multitude of transformations such as hydrogenation and dehydrogenation,19 CO and CO2 insertion,20 borylation,21−23 and olefin isomerization.24 Complexes supported by PCP-type pincers possess both inherent stability and good catalytic activity due to the strong planar chelation and the ability to vary the electronic properties of the ligand.25 Unlike their PCsp2P counterparts, examples of PCsp3P-type pincers are less common.26−42 Even though the stability of the resulting complexes is often attributed to the rigidity of the backbone, the ability of such frameworks to adopt different conformations may increase the versatility of specific catalytic systems. In addition, iridium complexes supported by a flexible PCsp3P ligand, which incorporates a tethered anisole and is capable of hemilability, may provide enhancements to catalytic behavior and bond activation via metal−ligand cooperation. Herein, we describe a synthetic procedure for a PCsp3P scaffold that exhibits a weak internal ligand interaction with iridium and its influence on the reactivity of the respective metal complexes.

was reduced using hydroiodic acid (Scheme 1) to give bis(2bromophenyl)-2-anisoylmethane (2). Compound 2, in turn, Scheme 1. Synthesis of PCanisHP (3)

reacted with n-BuLi, followed by the addition of iPr2PCl to afford the substituted bis(2-diisopropylphosphinophenyl)-2anisoylmethane (PCanisHP, 3) in good yield (Scheme 1). Compound 3 exhibits a phosphine-coupled triplet at 7.81 ppm (4JHP = 6.6 Hz) in its 1H NMR spectrum corresponding to the triaryl methine position and broad peaks from 2.05 to 0.93 ppm corresponding to the phosphine isopropyl groups. A broad singlet at −6.32 ppm in the 31P spectrum was assigned to the two phosphine environments. The broad peaks for the isopropyl groups observed in the 1H and 31P NMR spectra

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RESULTS AND DISCUSSION Synthesis and Characterization of Iridium Complexes. The novel PCsp3P scaffold was synthesized starting with the reaction of 2,2′-dibromobenzophenone with anisoyllithium to form bis(2-bromophenyl)-2-anisoylmethanol (1). This carbinol © XXXX American Chemical Society

Received: February 27, 2015

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DOI: 10.1021/acs.organomet.5b00165 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

geometric parameters (Figure 2). Both molecules exhibit a square-planar geometry around the iridium center with the

likely represent an averaged signal that characterizes an inversion of the helicity of the triarylmethane moiety. Single crystals were grown from a saturated n-pentane solution (Figure 1). Compound 3 has a propeller-type structure with

Figure 2. Thermal ellipsoid (50% probability level) representation of one of the two independent molecules in the unit cell of 4. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Ir(1)−P(11) 2.346(2), Ir(1)−Cl(1) 2.348(3), Ir(1)− C(11) 2.133(10), Ir(1)−C(12) 2.088(10), Ir(1)−C(15) 2.147(9), Ir(1)−C(16) 2.173(9); P(11)−Ir(1)−Cl(1) 91.28(9).

Figure 1. Thermal ellipsoid (50% probability level) representation of 3. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): P(1)−C(12) 1.8517(15), C(22)−P(2) 1.811(3); C(31)−C−C(21) 110.31(12), C(31)−C−C(11) 111.96(11), C(21)− C−C(11) 112.16(11).

typical CAr−P distances (1.8517(15) and 1.811(3) Å) in comparison to a similar PCsp3P scaffold (CAr−P 1.838 and 1.843 Å)2 and a tetrahedral geometry around the triaryl methine carbon, with C−C−C angles between 110.31(12) and 112.16(11)°. Two unique phosphine environments are observed in the solid state and represent the two averaged phosphine environments observed in the 31P NMR spectrum. The reaction of 3 with 1/2 equiv of [Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene) at 25 °C for 2 h led to the monocoordinated species (κ1-PCanisHP)Ir(COD)Cl (4; Scheme 2). The 1H

chloride ligand cis to one phosphine arm, P(21)−Ir(2)−Cl(2) = 90.00(8)° and P(11)−Ir(1)−Cl(1) = 91.28(9)°, and typical metal−ligand distances, Ir(1)−P(11) = 2.346(2) Å, Ir(2)− P(21) = 2.341(2) Å, Ir(1)−Cl(1) = 2.348(3) Å, and Ir(2)− Cl(2) = 2.358(2) Å. It was reasoned that a hydride complex would allow an easier activation of the backbone than would the corresponding chloride. The synthesis of a corresponding hydride was accomplished by the metathesis reaction of 4 with 1 equiv of LiEt3BH in Et2O at −35 °C for 30 min (Scheme 2). In the resulting complex, (κ2-PCanisHP)Ir(COD)H (5), iridium coordinates to the supporting ligand in a κ2 fashion through both phosphorus atoms with a trigonal-bipyramidal geometry at iridium. This structure is characterized in solution by asymmetric phosphine environments that exhibit two 31P NMR resonances, one as a broad singlet at 4.87 ppm and the other as a doublet at 2.66 ppm (2JPP = 17.0 Hz). A lack of resolution of the P−P coupling in the singlet resonance could be attributed to the exchange of two unique phosphine environments on one of the phosphine substituents. An upfield resonance, representing the hydride environment, was observed in the 1H NMR spectrum as a doublet of doublets at −14.23 ppm (2JHP = 26.3, 22.6 Hz), and a downfield resonance representing the backbone methine was observed as a doublet of doublets at 7.68 ppm (4JHP = 5.8, 2.7 Hz). X-ray crystallography supported the solution structure assignments with a P(1)−Ir−P(2) angle of 100.971(16)° and Ir−P(1) and Ir−P(2) distances of 2.3732(5) and 2.3543(5) Å, respectively (Figure 3). The hydride ligand was found in the electron density map and occupies an axial position relative to the trigonal plane. Unfortunately, heating of 5 in C6D6 at 80 °C in order to promote backbone activation led to decomposition over several days. However, heating 4 at 80 °C for 6 days led to the oxidative addition of the methine C−H bond to form (PCanisP)Ir(H)Cl (6; Scheme 2) quantitatively. Complex 6 can also be formed by the direct reaction of 3 with

Scheme 2. Synthesis of 4−6

NMR spectrum of 4 exhibits a doublet of doublets at 8.06 ppm (4JHP = 13.2 and 7.7 Hz) corresponding to the methine position of the backbone coupling to two inequivalent phosphine environments. This assignment is supported by the presence of two singlets in the 31P NMR spectrum at 49.87 and −12.40 ppm corresponding to the bound and free phosphine donors. X-ray diffraction studies show that 4 crystallizes with two independent molecules in the unit cell that have similar B


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Organometallics

the electron difference map at 1.42(4) Å from iridium, is found trans to the anisole substituent, consistent with the fact that the two strong σ donors (R3C− and H−) occupy positions trans with respect to the weaker σ donors (Oether and Cl−). Even though the hydride was found in the electron density map, the Ir−H distance is shorter than expected at 1.42(4) Å. This is likely due to the large librational motion of the hydride ligand and may not be representative of the true Ir−H distance. It is of note that the methine carbon is nearly tetrahedral, with angles ranging from 108.3(3) to 112.3(3)°, whereas previously characterized triarylmethane PCsp3P-type complexes exhibited highly distorted tetrahedral geometries with angles as high as 129°.45 The formation of the dihydride complex (PCanisP)Ir(H)2 (7) was accomplished via a salt metathesis reaction using NaAlH4 (eq 1). Since any persistent Cl− would allow the slow reversion

Figure 3. Thermal ellipsoid (50% probability level) representation of 5. Most hydrogen atoms are omitted, and isopropyl substituents are shown in wireframe for clarity. Selected distances (Å) and angles (deg): Ir−P(1) 2.3732(5), Ir−P(2) 2.3543(5), Ir−C(61) 2.2581(18), Ir−C(62) 2.2574(18), Ir−C(65) 2.1525(18), Ir−C(66) 2.1405(18); P(1)−Ir−P(2) 100.971(16)°.

[Ir(COD)Cl]2 at 80 °C for 6 days in benzene with a 90% yield (Scheme 2). The 1H NMR spectrum of 6 features a symmetric phosphine-coupled triplet at −29.85 ppm (2JHP = 16.9 Hz) corresponding to the hydride ligand. The corresponding 31P spectrum shows a singlet at 37.77 ppm assigned to the symmetrical phosphine donors. Single crystals of 6 were grown from the diffusion of npentane into a saturated toluene solution (Figure 4). The solid-

Figure 4. Thermal ellipsoid (50% probability level) representation of 6. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Ir−C 2.119(3), Ir−O 2.249(2), Ir−P(1) 2.2684(9), Ir−P(2) 2.3032(9), Ir−Cl 2.4541(9); P(1)−Ir−P(2) 154.55(3), C− Ir−Cl 171.46(9).

of 7 back to 6, special care was taken to remove all NaCl formed in the reaction. The 1H NMR spectrum exhibited two upfield hydride peaks as broad triplets of doublets at −11.39 (2JHP = 13.2 Hz, 2JHH = 7.0 Hz) and −31.85 ppm (2JHP = 17.8 Hz, 2JHH = 6.6 Hz) as a consequence of coupling to two symmetrical phosphines and to each other. The upfield resonance at −11.39 ppm is attributed to the hydride trans to the sp3 backbone carbon, and the resonance at −31.9 ppm is attributed to the hydride trans to the methoxy oxygen. The drastic difference in chemical shift is associated with the trans substituent: the moderately π donating ether ligand shifts the trans hydride resonance upfield.44,46 The presence of symmetrical phosphines is also supported by a singlet in the corresponding 31P NMR spectrum at 58.6 ppm. Compound 7 was easily recrystallized from a saturated hexanes solution at −35 °C as light yellow crystals. The solid-state molecular structure (Figure 5) exhibits a distorted-octahedral geometry around iridium with the angle P(1)−Ir−P(2) = 152.55(2)°, distances Ir−P(1) = 2.2529(7) and Ir−P(2) = 2.2748(7) Å, and a slightly elongated distance Ir−O(3) = 2.2948(16) Å in comparison to compound 6. Both hydride ligands were found in the electron density map in a cis orientation relative to one another. Reactivity Studies. Heating 7 in C6D6 at 80 °C for a prolonged time (up to 18 days) did not lead to the reductive elimination of dihydrogen; however, the incorporation of deuterium was observed in the hydride positions, the anisole methyl positions, and backbone aromatic positions (eq 2). This

state molecular structure displays a distorted octahedral geometry around the iridium center with a P(1)−Ir−P(2) angle of 154.55(3)° and a C−Ir−Cl angle of 171.46(9)°. The Ir−O distance of 2.249(2) Å indicates coordination of the anisole oxygen.43,44 The distances Ir−P(1) = 2.2684(9) Å, Ir− P(2) = 2.3032(9) Å, and Ir−Cl = 2.4541(9) Å are in the common range for iridium PCsp3P pincer complexes.45 The chloride ligand occupies the trans position with respect to the central sp3 carbon, while the hydride ligand, which was found in

process was monitored by 1H NMR spectroscopy, which indicated a broadening and gradual loss of the hydride C


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Organometallics Scheme 3. Synthesis of 8−11

Figure 5. Thermal ellipsoid (50% probability level) representation of 7. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Ir−P(1) 2.2529(7), Ir−P(2) 2.2748(7), Ir−C 2.186(2), Ir−O(3) 2.2948(16); P(1)−Ir−P(2) 152.55(2), C−Ir− O(3) 75.39(7).

mutually trans orientation.58 The carbonyl carbon was observed in the 13C NMR spectrum as an unresolved doublet of doublets (apparent triplet) at 183.1 ppm (2JPC = 4.67 Hz) and exhibits a CO stretching frequency at 1950.6 cm−1 in the corresponding IR spectrum. X-ray diffraction allowed the determination of the solid-state molecular structure of 8 (Figure 6) and indicates a

resonances corresponding to (PCanisP)Ir(H)(D) and the initial formation of a deuterium-coupled multiplet at 3.71 ppm corresponding to partial deuteration of the anisole CH3 group. The complete loss of hydride, anisole, and several backbone aromatic peaks was apparent after 4 days. The formation of the deuterated complex (PCanis(D)P)Ir(D)2 (7-d) was confirmed by the corresponding 2H NMR spectrum, which showed a singlet at 3.90 ppm for OCD3 and two broad singlets at −11.84 and −31.98 ppm for the two deuteride ligands. The deuterium incorporation was found to be reversible on heating in C6H6 for a similar time with no decomposition. Hydrogen−deuterium exchange between aromatic C−H bonds and deuterated solvents was previously observed47−50 for iridium dihydride complexes,51 and the α-activation of alkyl ethers has been well studied,52,53 including deuterium incorporation.54,55 At this time, it is still unclear if this reaction takes place through an intramolecular or intermolecular process. The intramolecular mechanism is more likely,55−57 due to the proximity of the methyl group to the iridium center, and it likely involves metal−ligand cooperation (Schemes S1 and S3 in the Supporting Information). Due to inaccessibility, the incorporation of deuterium on the phenyl rings of the ligand must take place through an intermolecular mechanism (Schemes S1 and S2 in the Supporting Information).51 In order to probe the hemilability of the anisole donor, a slight excess of CO was added to a solution of 7 in THF and led to the formation of a new complex, (mer-PCanisP)Ir(CO)H2 (8, Scheme 3). The 1H NMR spectrum of 8 exhibits a doublet of doublets of doublets at −11.18 ppm (2JHP = 25.5, 15.4 Hz, 2 JHH = 4.3 Hz) and a triplet of doublets at −14.22 ppm (2JHP = 13.3 Hz, 2JHH = 4.4 Hz). The splitting of the first hydride peak (−11.18 ppm) is attributed to coupling with two inequivalent phosphines and the other hydride. The upfield shift at −14.22 ppm was an apparent triplet of doublets due to the nearly identical phosphine coupling constants as well as coupling to the other hydride. Hydride coupling suggests nonsymmetrical phosphine environments; this interpretation is supported by the appearance of two doublets in the corresponding 31P NMR spectrum at 59.95 ppm (2JPP = 272.3 Hz) and 42.61 ppm (2JPP = 272.4 Hz), both exhibiting a coupling suggestive of a

Figure 6. Thermal ellipsoid (50% probability level) representation of 8. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Ir−P(1) 2.2689(16), Ir−P(2) 2.2796(16), Ir−C 2.244(6), Ir−C(1) 1.881(7), C(1)−O(1) 1.148(7); C(1)−Ir−C 98.8(2), P(1)−Ir−P(2) 158.90(6).

distorted-octahedral geometry around iridium with the angle P(1)−Ir−P(2) = 158.90(6)°, the angle C(1)−Ir−C = 98.8(2)°, the distance Ir−P(1) = 2.2689(16) Å, the distance Ir−P(2) = 2.2796(16) Å, the distance Ir−C = 2.244(6) Å, and the carbonyl distance Ir−C(1) = 1.881(7) Å. Both hydride ligands were found in the electron density map and retain the relative cis orientation observed for 7. Heating 8 at 80 °C for 7 days led to its isomerization to ( facPCanisP)Ir(CO)H2 (9; Scheme 3) as well as some significant decomposition, which was attributed to the reductive elimination of H2. As a consequence, heating under an H2 atmosphere allowed the full conversion of 8 to 9 without D


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Organometallics

DIPPX ligand adopting a fac orientation in an intermediate structure.60 This intermediate was proposed and spectroscopically observed in an analogous system using variable-temperature NMR experiments.61 Isomerization has also been attributed to reversible oxidative addition/reductive elimination of H2. In the DIPPX system, the trans rearrangement of the hydrides is most favorable due to the rigidity of the backbone and instability of the intermediate; however, in our system the more flexible backbone allows the more geometrically optimized fac orientation, which preserves the cis dihydride. Preservation of the cis dihydride orientation is considered important in dehydrogenation catalytic cycles.62 These observations describe the ability of the ligand to dissociate through the weakly binding anisole substituent and open a coordination site to allow the approach of a substrate, as well as the flexibility of the backbone, which could allow unique orientations to be accessible without the loss of the chelate ligand. Displacement of the anisole substituent by trimethylphosphine was also studied. The reaction of 7 with PMe3 in C6D6 at 25 °C for 5 min (Scheme 3) leads to a 1:3 mixture of two isomers, (mer-PCanisP)Ir(H)2PMe3 (10) and ( fac-PCanisP)Ir(H)2PMe3 (11). Because both compounds had comparable solubility properties and decomposed on silica gel, they could not be separated; however, they were characterized by multinuclear NMR spectroscopy. A manual crystal separation allowed the structural characterization of 11 (Figure 8).

decomposition. Isomerization of 8 involved the rearrangement of the carbonyl ligand trans to the backbone sp3 carbon as well as a rearrangement of the pincer ligand from a meridional to a facial binding mode. Complex 9 shows two doublets of doublets at −7.52 ppm (2JHP = 127.2, 20.2 Hz) and −10.12 ppm (2JHP = 127.9, 18.3 Hz) in its 1H NMR spectrum that represent the two asymmetric hydrides, which both couple cis and trans to the two nonsymmetrical phosphines. No H−H coupling between hydrides was resolved, indicating a small coupling constant. The nonequivalence of the phosphines was further evidenced by the observation of two doublets in the corresponding 31P NMR spectrum at 42.62 ppm (2JPP = 16.2 Hz) and 28.98 ppm (2JPP = 17.7 Hz). The carbonyl carbon was observed in the 13C NMR spectrum as an apparent triplet at 176.73 ppm (2JPC = 4.3 Hz) and characterized by an IR stretching frequency of 1920.8 cm−1. The solid-state molecular structure of 9 (Figure 7) indicates a distorted-octahedral

Figure 7. Thermal ellipsoid (50% probability level) representation of 9. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Ir−C 2.211(3), Ir−C(4) 1.866(3), Ir−P(1) 2.3321(7), Ir−P(2) 2.3174(7), C(4)−O(4) 1.151(4); P(2)−Ir−P(1) 106.73(3), C(4)−Ir−C 174.61(12), Ir−C(4)−O(4) 176.9(3).

geometry around iridium with the angle P(1)−Ir−P(2) = 106.73(3)°; both hydride ligands were found in the electron density map and retain their relative cis orientation. The shift of the carbonyl stretching frequency from 1950.6 cm−1 in 8 to 1920.8 cm−1 suggests a stronger metal interaction in the trans to the backbone orientation due to stronger π back-donation. This could account for the thermodynamic preference of the fac isomer but was not observed structurally. The Ir−C(4) distance was 1.866(3) Å, in comparison to 1.881(7) Å for 8, and the C(4)−O(4) distance was 1.151(4) Å, in comparison to 1.148(7) Å for 8. Analogous rearrangements have been described with the carbonyl dihydride (DIPPX)Ir(H)2(CO) (DIPPX = diisopropylphosphino-m-xylene), in which the carbonyl ligand rearranges similarly from a cis to a trans position with respect to the backbone carbon donor.59 In (DIPPX)Ir(H)2(CO), the two hydrides also rearrange from cis to trans with respect to each other to obtain the thermodynamically favored orientation, in contrast to our system, which maintains a mutually cis dihydride orientation. That rearrangement was attributed to a nondissociative trigonal twist, with the

Figure 8. Thermal ellipsoid (50% probability level) representation of 11. Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Ir−C 2.2616(15), Ir−P(1) 2.3173(6), Ir−P(2) 2.2723(5), Ir−P(4) 2.3082(6); P(2)−Ir−P(1) 103.302(18), P(2)− Ir−P(4) 148.036(16), P(4)−Ir−P(1) 107.406(16), C−Ir−P(4) 108.45(4).

Complex 10 is analogous to 8 and retains the mer orientation of the backbone as well as the cis dihydride orientation. Characterization by 1H NMR spectroscopy of 10 shows two hydride resonances, one at −14.12 ppm as a doublet of doublets of doublets (2JHP = 41.9, 29.3, 14.5 Hz) and another at −16.74 ppm as a complex multiplet. Assignment of the cis orientation of the two hydrides is justified by the large coupling constant (2JHP = 41.9 Hz) of the first hydride resonance that is E


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Organometallics indicative of trans H−P coupling. In the 31P NMR spectrum of 10, two downfield doublets of doublets, having both cis and trans coupling, appear at 53.58 ppm (2JPP = 310.6, 19.1 Hz) and 32.88 ppm (2JPP = 310.5, 15.1 Hz) consistent with a meridional geometry of the backbone. A third upfield doublet of doublets was also observed for PMe3 at −60.48 ppm as an apparent triplet exhibiting cis coupling (2JPP = 16.14 Hz). In the other isomer (11), the backbone occupies a facial orientation and PMe3 is trans to one of the phosphine donors of the pincer ligand. The hydrides also retain their cis orientation. This cis dihydride fac backbone orientation is consistent with 31P NMR resonances found at 36.31 ppm, as a doublet of doublets (2JPP = 329.1, 14.8 Hz) with both cis and trans phosphine coupling, and at 31.62 ppm (2JPP = 19.0, 15.4 Hz, 2JPH = 7.3 Hz), as a doublet of doublets of doublets, which contains a selectively resolved trans hydride coupling as well as two cis phosphine couplings. A resonance at −55.83 ppm (2JPP = 329.1, 18.6 Hz) was assigned to PMe3, and it indicates both cis and trans phosphorus coupling. Two upfield hydride resonances were observed in the 1H NMR spectrum of 11. A doublet of doublets of doublets of doublets was observed at −11.49 ppm (2JHP = 120.3, 25.5, 20.0 Hz, 2JHH = 4.4 Hz) and exhibited coupling with the trans and the two cis phosphines as well as the other hydride. A second hydride resonance was also observed as an apparent doublet of triplets of doublets at −16.41 ppm (2JHP = 23.0, 11.9 Hz, 2JHH = 4.7 Hz), consistent with coupling between the three cis phosphines as well as the cis hydride. Complex 11 was also characterized by X-ray diffraction (Figure 8) and exhibits a distorted-octahedral geometry with a fac orientation of the backbone PCP fragment (P(1)−Ir−P(2) = 103.302(18)°) and PMe3 in a trans-like orientation to one of the backbone phosphine donors (P(2)−Ir−P(4) = 148.036(16)°). The two hydride ligands were found in the electron density map and have a relative cis orientation to each other. The two isomers do not interchange thermally, suggesting that the formation of 10 is not a prerequisite for the formation of 11. The isomer containing PMe3 in the trans orientation versus the backbone carbon was not observed, even though the trans isomer was observed in complex 9. It is likely that the stronger π-acceptor ligand CO cannot occupy a position trans to the other phosphine π acceptors, while the weaker π acceptor PMe3 is capable of this type of coordination. Transfer Dehydrogenation Studies. Transfer dehydrogenation of cyclooctane has often been used as a benchmark catalytic reaction for iridium PCP-type complexes; therefore, we decided to investigate this transformation using the newly synthesized iridium complexes. A coordinatively unsaturated catalyst was generated through the dehydrohalogenation of (PCanisP)Ir(H)Cl (6) using an excess of NaOtBu or directly from (PCanisP)Ir(H)2 (7) in the presence of an acceptor (NBE = norbornene or TBE = tert-butylethylene). Reactions were tested using a 1:1 molar ratio of cyclooctane to acceptor and showed low activity for both compounds (Table 1). This may be attributed to high concentrations of the olefin, as this has been shown to limit the reactivity of the catalyst in this transformation.63 Therefore, reactions were also run at a low concentration of olefin and the results are described in Table 2. Although the activity increased in comparison to that of previous reactions, our systems display fairly low activity for this transformation in comparison to other PCsp3P iridium examples.64,65 Reactions that took place under an argon

Table 1. Transfer Dehydrogenation of Cyclooctane at High Olefin Concentrationa

entry

cat.

olefin

base

amt, mol %

Ar/N2

TONb

1 2 3 4 5 6 7 8 9

6 6 6 6 7 7 7 7 7

TBE TBE NBE NBE TBE TBE TBE NBE NBE

NaOtBu NaOtBu NaOtBu NaOtBu

0.1 0.1 0.1 0.1 0.1 0.1 1 0.1 0.1

N2 Ar N2 Ar N2 Ar N2 N2 Ar

1.5 2.0 1.1 2.3 4.5 6.1 3.8 1.4 1.4

a

Reaction mixtures consisted of 17.0 mmol of COA and 17.0 mmol of acceptor and were heated to 150 °C for 24 h. bBased on total number of double bonds formed to catalyst.

atmosphere showed a small increase in activity, likely due to coordination of dinitrogen, which has been observed to inhibit previous iridium catalysts.63 A higher conversion was observed at higher catalyst loadings. Complex 7 is slightly better than 6, possibly due to side reactions with NaOtBu. A reaction using KN(TMS)2 as a base to dehydrohalogenate complex 6 did not lead to any alkene products. Higher reactivity was observed when TBE was used instead of NBE, likely a consequence of steric differences between the two acceptors. It is likely that the mechanism is similar to those for previously described, analogous examples, wherein the coordinatively unsaturated iridium(I) species is the active catalyst.62,66 The dissociation of anisole in (PCanisP)IrH2 (7) is necessary for the alkene insertion to take place and might limit the reactivity of the system. The subsequent reductive elimination can generate the coordinatively unsaturated (PCanisP)Ir fragment. This active species should perform the C−H activation of cyclooctane to form the (PCanisP)Ir(H)(cyclooctyl) intermediate, which could then undergo β-hydride elimination to form cyclooctene and 7, followed by the hydrogenation of norbornene to norbornane.

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CONCLUSIONS A novel PCsp3P scaffold for iridium complexes was developed. Substitution reactions with strongly coordinating ligands showed the hemilability and flexibility of the ligand, since the anisole group can be readily displaced and the backbone can adopt both meridional and facial geometries. Deuterium incorporation into the ligand and hydride positions of 7 suggests a ligand activation process. Transfer dehydrogenation of cyclooctane is achieved through a coordinatively unsaturated species obtained by the dehydrohalogenation of 6 with a strong base or by the dehydrogenation of 7 with an acceptor. The (PCanisP)Ir complexes discussed herein represent relatively air and water stable precursors for active Ir(I) complexes.

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EXPERIMENTAL SECTION

General Conditions. All air-sensitive manipulations were conducted in a glovebox or using Schlenk techniques under vacuum or an inert atmosphere of nitrogen or argon. The reagent 2,2′dibromobenzophenone was prepared following previous methods.67 The solvents THF, Et2O, toluene, n-pentane, and hexanes were dried using a solvent column purification system.68 The solvents C6H6 and C6D6 were dried over CaH2. All other reagents were purchased commercially and used without further purification unless otherwise F


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Organometallics Table 2. Transfer Dehydrogenation of Cyclooctane at Low Olefin Concentrationa

entry

cat.

olefin

base

amt, mol %

Ar/N2

conversion,b %

1 2 3 4 5 6 7 8 9 10 11 12 13

6 6 6 6 6 6 6 7 7 7 7 7 7

TBE NBE NBE TBE TBE TBE NBE TBE NBE NBE TBE TBE TBE

NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu KN(TMS)2

0.1 1 1 1 1 5 1 0.1 1 1 1 1 5

Ar N2 Ar N2 Ar Ar Ar Ar N2 Ar N2 Ar Ar

2σ(I) and R1 = 0.0748, wR2 = 0.0723 for all 5505 data. Residual electron density (e Å−3) max/min: 0.984/−0.870. X-ray Crystal Structure of (fac-PCanisP)Ir(CO)H2 (9). X-rayquality single crystals were obtained from a concentrated n-pentane solution at −35 °C in the glovebox. The resulting crystals were light yellow blocks. Crystal and refinement data for 9: C33H45IrO2P2; Mr = 727.83; monoclinic; space group P21/n; a = 10.8877(8) Å; b = 18.6255(13) Å; c = 15.2210(11) Å; α = 90°; β = 95.1787(13)°; γ = 90°; V = 3074.0(4) Å3; Z = 4; T = 120(2) K; λ = 0.71073 Å; μ = 4.475 mm−1; dcalc = 1.573 g cm−3; 33996 reflections collected; 5402 unique reflections (Rint = 0.0294); R1 = 0.0178, wR2 = 0.0388 for 4873 data with I > 2σ(I) and R1 = 0.0220, wR2 = 0.0397 for all 5402 data. Residual electron density (e Å−3) max/min: 0.368/−0.392. X-ray Crystal Structure of (cis-fac-PCanisP)Ir(H)2PMe3 (11). Xray-quality single crystals were obtained by manual separation of a concentrated n-pentane solution of a mixture of 10 and 11 at −35 °C in the glovebox. The resulting crystals were light yellow blocks. Crystal and refinement data for 11: C35H54IrOP3; Mr = 775.89; monoclinic; space group P21/n; a = 10.903(2) Å; b = 20.135(4) Å; c = 15.620(3) Å; α = 90°; β = 94.941(3)°; γ = 90°; V = 3416.4(11) Å3; Z = 4; T = 120(2) K; λ = 0.71073 Å; μ = 4.075 mm−1; dcalc = 1.509 g cm−3; 34798 reflections collected; 8477 unique reflections (Rint = 0.0167); R1 = 0.0144, wR2 = 0.0329 for 7924 data with I > 2σ(I) and R1 = 0.0165, wR2 = 0.0336 for all 8477 data. Residual electron density (e Å−3) max/min: 1.212/−1.323.

Hz, P(backbone, fac)), −55.83 (dd, 2JPP = 329.1, 18.8 Hz, PMe3(fac)), −60.49 (app t, 2JPP = 16.1 Hz, PMe3(mer)). Anal. Calcd for C33H54IrOP3: C, 54.18; H, 7.01. Found: C, 54.31; H, 7.12. General Method for Transfer Dehydrogenation of Cyclooctane at High Olefin Concentration. A solution of 6 and NaOtBu (10 equiv) or 7 (0.1 or 1 mol % with respect to the acceptor) was combined with norbornene or tert-butylethylene (17 mmol) in 17 mmol of cyclooctane in a Schlenk tube under nitrogen or argon. The reaction mixture was heated in an oil bath at 150 °C for 24 h. A 0.1 mL aliquot of the resulting solution was added to 0.6 mL of a 0.2 M solution of trimethoxybenzene (as a standard) in CDCl3, and the percent conversion was determined by integration of the olefinic region in the 1H NMR spectrum. General Method for Transfer Dehydrogenation of Cyclooctane at Low Olefin Concentration. A solution of 6 and NaOtBu (10 equiv) or 7 (0.1, 1, or 5 mol % with respect to the acceptor) was combined with norbornene or tert-butylethylene (2 mmol) in 4 mL of cyclooctane in a Schlenk tube under nitrogen or argon. The reaction mixture was heated in an oil bath at 150 °C for 24 h. A 0.1 mL aliquot of the resulting solution was added to 0.6 mL of a 0.2 M solution of trimethoxybenzene (as a standard) in CDCl3, and the percent conversion was determined by integration of the olefinic region in the 1H NMR spectrum. X-ray Crystal Structure of (PCanisHP) (3). X-ray-quality single crystals were obtained from a concentrated n-pentane solution at −35 °C in the glovebox. The resulting crystals were colorless blocks. Crystal and refinement data for 3: C32H44OP2; Mr = 506.61; monoclinic; space group P2/c; a = 11.2267(9) Å; b = 13.8576(11) Å; c = 20.1437(16) Å; α = 90°; β = 106.046(2)°; γ = 90°; V = 3011.8(4) Å3; Z = 4; T = 120(2) K; λ = 0.71073 Å; μ = 0.166 mm−1; dcalc = 1.117 g cm−3; 34582 reflections collected; 5025 unique reflections (Rint = 0.0341); R1 = 0.0338, wR2 = 0.0817 for 4350 data with I > 2σ(I) and R1 = 0.0419, wR2 = 0.0886 for all 5025 data. Residual electron density (e Å−3) max/min: 0.303/−0.228. X-ray Crystal Structure of (κ1-PCanisHP)Ir(COD)Cl (4). X-rayquality single crystals were obtained from a concentrated n-pentane solution at −35 °C in the glovebox. The resulting crystals were light yellow shards. Crystal and refinement data for 4: C40H56ClIrOP2; Mr = 842.44; triclinic; space group P1̅; a = 14.286(3) Å; b = 17.903(4) Å; c = 18.047(4) Å; α = 100.617(6)°; β = 105.155(7)°; γ = 108.879(6)°; V = 4028.8(17) Å3; Z = 4; T = 120(2) K; λ = 0.71073 Å; μ = 3.487 mm−1; dcalc = 1.389 g cm−3; 54455 reflections collected; 16593 unique reflections (Rint = 0.1032); R1 = 0.0666, wR2 = 0.1556 for 10945 data with I > 2σ(I) and R1 = 0.1080, wR2 = 0.1720 for all 16593 data. Residual electron density (e Å−3) max/min: 2.758/−3.709. X-ray Crystal Structure of (κ2-PCanisHP)Ir(COD)H (5). X-rayquality single crystals were obtained from a concentrated n-pentane solution at −35 °C in the glovebox. The resulting crystals were bright yellow blocks. Crystal and refinement data for 5: C43H60IrOP2; Mr = 847.05; triclinic; space group P1̅; a = 10.3914(9) Å; b = 10.5136(9) Å; c = 18.8352(15) Å; α = 93.4220(10)°; β = 91.7780(10)°; γ = 114.5150(10)°; V = 1865.4(3) Å3; Z = 2; T = 120(2) K; λ = 0.71073 Å; μ = 3.697 mm−1; dcalc = 1.508 g cm−3; 26780 reflections collected; 6582 unique reflections (Rint = 0.0117); R1 = 0.0129, wR2 = 0.0298 for 6502 data with I > 2σ(I) and R1 = 0.0132, wR2 = 0.0303 for all 6582 data. Residual electron density (e Å−3) max/min: 0.676/−0.467. X-ray Crystal Structure of (PCanisP)Ir(H)Cl (6). X-ray-quality single crystals were obtained from a concentrated toluene solution layered with n-pentane at −35 °C in the glovebox. The resulting crystals were bright orange-yellow blocks. Crystal and refinement data for 6: C32H44ClIrOP2; Mr = 734.26; triclinic; space group P1̅; a = 10.3699(9) Å; b = 10.6122(9) Å; c = 15.2348(13) Å; α = 76.7410(10)°; β = 86.0090(10)°; γ = 67.4240(10)°; V = 1506.5(2) Å3; Z = 2; T = 120(2) K; λ = 0.71073 Å; μ = 4.650 mm−1; dcalc = 1.619 g cm−3; 26526 reflections collected; 7526 unique reflections (Rint = 0.0443); R1 = 0.0292, wR2 = 0.0623 for 6723 data with I > 2σ(I) and R1 = 0.0363, wR2 = 0.0665 for all 7526 data. Residual electron density (e Å−3) max/min: 2.159/−1.253. X-ray Crystal Structure of (PCanisP)Ir(H)2 (7). X-ray-quality single crystals were obtained from a concentrated n-pentane solution

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving NMR spectra for compounds 1−11, IR spectra for 8 and 9, and crystallographic data for compounds 3−9 and 11. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00165.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.M.I.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Allen Oliver for assistance with the crystallography studies and the ND Energy Material Characterization facility for the use of the FT-IR spectrometer. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (ACS PRF # 53536-DNI3). J


Article

Organometallics

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(38) Allen, O. R.; Field, L. D.; Magill, A. M.; Vuong, K. Q.; Bhadbhade, M. M.; Dalgarno, S. J. Organometallics 2011, 30, 6433. (39) Boyd, L. M.; Clark, G. R.; Roper, W. R. J. Organomet. Chem. 1990, 397, 209. (40) Ruhland, K.; Herdtweck, E. Adv. Synth. Catal. 2005, 347, 398. (41) Lesueur, W.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1997, 36, 3354. (42) Weng, W.; Chen, C.-H.; Foxman, B. M.; Ozerov, O. V. Organometallics 2007, 26, 3315. (43) Pontiggia, A. J.; Chaplin, A. B.; Weller, A. S. J. Organomet. Chem. 2011, 696, 2870. (44) Barquı ́n, M.; Ciganda, R.; Garralda, M. A.; Ibarlucea, L.; Mendicute-Fierro, C.; Rodrı ́guez-Dieǵ uez, A.; Seco, J. M. Eur. J. Inorg. Chem. 2013, 2013, 1225. (45) Azerraf, C.; Gelman, D. Organometallics 2009, 28, 6578. (46) Werner, H.; Schulz, M.; Windmueller, B. Organometallics 1995, 14, 3659. (47) Zhou, J.; Hartwig, J. F. Angew. Chem. 2008, 120, 5867. (48) Morales-Morales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M. Organometallics 2001, 20, 1144. (49) Faller, J. W.; Felkin, H. Organometallics 1985, 4, 1488. (50) Lehman, M. C.; Gary, J. B.; Boyle, P. D.; Sanford, M. S.; Ison, E. A. ACS Catal. 2013, 3, 2304. (51) Iluc, V. M.; Fedorov, A.; Grubbs, R. H. Organometallics 2011, 31, 39. (52) Conejero, S.; Paneque, M.; Poveda, M. L.; Santos, L. L.; Carmona, E. Acc. Chem. Res. 2010, 43, 572. (53) Whited, M. T.; Grubbs, R. H. Acc. Chem. Res. 2009, 42, 1607. (54) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 2092. (55) Santos, L. L.; Mereiter, K.; Paneque, M.; Slugovc, C.; Carmona, E. New J. Chem. 2003, 27, 107. (56) Lee, D.-H.; Chen, J.; Faller, J. W.; Crabtree, R. H. Chem. Commun. 2001, 213. (57) Campos, J.; Espada, M. F.; López-Serrano, J.; Carmona, E. Inorg. Chem. 2013, 52, 6694. (58) Musa, S.; Fronton, S.; Vaccaro, L.; Gelman, D. Organometallics 2013, 32, 3069. (59) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997, 16, 3786. (60) Li, S.; Hall, M. B. Organometallics 1999, 18, 5682. (61) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2011, 30, 697. (62) Li, S.; Hall, M. B. Organometallics 2001, 20, 2153. (63) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1996, 2083. (64) Bézier, D.; Brookhart, M. ACS Catal. 2014, 4, 3411. (65) Polukeev, A. V.; Gritcenko, R.; Jonasson, K. J.; Wendt, O. F. Polyhedron 2014, 84, 63. (66) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.; Fan, H.-J.; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40, 3596. (67) Bickelhaupt, F.; Jongsma, C.; de Koe, P.; Lourens, R.; Mast, N. R.; van Mourik, G. L.; Vermeer, H.; Weustink, R. J. M. Tetrahedron 1976, 32, 1921. (68) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518.

REFERENCES

(1) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588. (2) Burford, R. J.; Piers, W. E.; Parvez, M. Organometallics 2012, 31, 2949. (3) Noyori, R.; Sandoval, C. A.; Muñiz, K.; Ohkuma, T. Philos. Trans. R. Soc. A 2005, 363, 901. (4) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1997, 36, 285. (5) Shvo, Y.; Czarkie, D. J. Organomet. Chem. 1986, 315, C25. (6) Karvembu, R.; Prabhakaran, R.; Natarajan, K. Coord. Chem. Rev. 2005, 249, 911. (7) Fujita, K.-i.; Tanino, N.; Yamaguchi, R. Org. Lett. 2006, 9, 109. (8) Zweifel, T.; Naubron, J.-V.; Grützmacher, H. Angew. Chem., Int. Ed. 2009, 48, 559. (9) Musa, S.; Shaposhnikov, I.; Cohen, S.; Gelman, D. Angew. Chem., Int. Ed. 2011, 50, 3533. (10) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840. (11) Gunanathan, C.; Gnanaprakasam, B.; Iron, M. A.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2010, 132, 14763. (12) Stepowska, E.; Jiang, H.; Song, D. Chem. Commun. 2010, 46, 556. (13) Bertoli, M.; Choualeb, A.; Lough, A. J.; Moore, B.; Spasyuk, D.; Gusev, D. G. Organometallics 2011, 30, 3479. (14) Käß, M.; Friedrich, A.; Drees, M.; Schneider, S. Angew. Chem., Int. Ed. 2009, 48, 905. (15) Askevold, B.; Nieto, J. T.; Tussupbayev, S.; Diefenbach, M.; Herdtweck, E.; Holthausen, M. C.; Schneider, S. Nat. Chem. 2011, 3, 532. (16) Comanescu, C. C.; Iluc, V. M. Organometallics 2014, 33, 6059. (17) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74. (18) Gunanathan, C.; Milstein, D. Chem. Rev. 2014, 114, 12024. (19) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761. (20) Lee, D. W.; Jensen, C. M.; Morales-Morales, D. Organometallics 2003, 22, 4744. (21) Chianese, A. R.; Mo, A.; Lampland, N. L.; Swartz, R. L.; Bremer, P. T. Organometallics 2010, 29, 3019. (22) Lee, C.-I.; Zhou, J.; Ozerov, O. V. J. Am. Chem. Soc. 2013, 135, 3560. (23) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith, M. R. Science 2002, 295, 305. (24) Biswas, S.; Huang, Z.; Choliy, Y.; Wang, D. Y.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2012, 134, 13276. (25) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (26) Azerraf, C.; Shpruhman, A.; Gelman, D. Chem. Commun. 2009, 466. (27) Azerraf, C.; Gelman, D. Chem. - Eur. J. 2008, 14, 10364. (28) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2013, 136, 1105. (29) Olsson, D.; Arunachalampillai, A.; Wendt, O. F. Dalton Trans. 2007, 5427. (30) Nemeh, S.; Flesher, R. J.; Gierling, K.; Maichle-Mössmer, C.; Mayer, H. A.; Kaska, W. C. Organometallics 1998, 17, 2003. (31) Gerber, R.; Fox, T.; Frech, C. M. Chem. - Eur. J. 2010, 16, 6771. (32) Tauchert, M. E.; Warth, D. C. M.; Braun, S. M.; Gruber, I.; Ziesak, A.; Rominger, F.; Hofmann, P. Organometallics 2011, 30, 2790. (33) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 11687. (34) Hill, A. F.; McQueen, C. M. A. Organometallics 2012, 31, 8051. (35) Bennett, M. A.; Clark, P. W. J. Organomet. Chem. 1976, 110, 367. (36) Crocker, C.; Empsall, H. D.; Errington, R. J.; Hyde, E. M.; McDonald, W. S.; Markham, R.; Norton, M. C.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Dalton Trans. 1982, 1217. (37) Crocker, C.; Errington, R. J.; McDonald, W. S.; Odell, K. J.; Shaw, B. L.; Goodfellow, R. J. J. Chem. Soc., Chem. Commun. 1979, 498. K


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