Synthesis and Application of Pyrrole-Based PNPâIr Complexes to

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

Synthesis and Application of Pyrrole-Based PNP−Ir Complexes to Catalytic Transfer Dehydrogenation of Cyclooctane Shin Nakayama,† Shogo Morisako,‡ and Makoto Yamashita*,‡ †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27, Kasuga, Bunkyo-ku, 112-8551 Tokyo, Japan ‡ Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan S Supporting Information *

ABSTRACT: A series of tBu- and iPr-substituted PNP−pincer Ir and Rh complexes with pyrrole-based core were synthesized and characterized. The structures of the obtained complexes were varied depending on the size of alkyl substituents and ligands other than PNP ligand. All of them exhibit low activity toward transfer dehydrogenation of cyclooctane.

■

INTRODUCTION Multidentate ligands are widely utilized in organometallic chemistry and catalysis because of their ability to construct unique environment around the transition metal atom. Among them, pincer ligand, a tridentate ligand with meridional coordination mode, is one of the most promising compounds for application to catalysis by utilizing the thermal stability of pincer complexes.1 As one of the most important reactions in industry, dehydrogenation of alkanes to produce the corresponding alkenes has been widely studied with pincer−Ir complexes2 after the seminal discovery of homogeneous Ir− phosphine complexes3 and their improved version with a pincer ligand.4 To improve catalytic activity for dehydrogenation of alkane, three different modifications of the most active PCP−Ir catalyst systems have been extensively investigated, i.e., (1) tuning electronics and sterics of the pincer ligand with a variety of substituents,5 (2) using other metals rather than iridium,4a,6 and (3) using other coordinating atoms rather than phosphorus and carbon atoms.7 Among the reported pincer complexes being active toward dehydrogenation of alkane, a POCSP−Ir complex possessing a carbon-based PCP ligand with oxygenand sulfur-tethered side arms was recently reported to be the most active catalyst system at 200 °C.5l Recently, we and others have explored synthesis, structure, properties, and catalytic activity of late metal complexes possessing a boron-based PBP pincer ligand.8 Some of these PBP complexes exhibited a moderate catalytic activity toward dehydrogenation of alkane.8i,j,w Since the substituent effect of sp2-hybridized boryl ligand as σ-donor9 and π-acceptor10 would not match the thermodynamic requirement for the C(sp3)−H bond cleavage,11 we turned our attention to the other ligand system. Considering the anionic nitrogen atom would have opposite substituent effect, i.e., σ-acceptor and π-donor, anionic nitrogen-based PNP pincer Ir system would have higher catalytic activity for dehydrogenation of alkane. However, recently reported anionic PNP−Ir system A having carbazolide© XXXX American Chemical Society

based PNP ligand exhibited no catalytic activity for dehydrogenation of alkanes (Chart 1), although PNP−Rh Chart 1. Examples of Anionic PNP−Pincer Complexes

system B with the same ligand exhibited good catalytic activity.6f,g A slightly different anionic PNP ligand system, C, possessing a pyrrole core has been reported to be used for the preparation of the corresponding PNP−pincer metal complexes (Scheme 1).12 However, there has no report about their catalytic application toward dehydrogenation of alkane. Herein, Scheme 1. Previously Reported Synthesis of i-Pr Substituted PNP−Ir Complexes

Received: February 3, 2018

A

DOI: 10.1021/acs.organomet.8b00072 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics we report synthesis of pyrrole-based PNP−pincer Ir and Rh complexes and their catalytic activity toward dehydrogenation of cyclooctane, which is a landmark reaction to compare catalytic activity among the different catalyst system.

■

RESULTS AND DISCUSSION A solution of previously reported [iPr-PNP]Ir(cod) complex 2a-Ir12a was heated under atmospheric pressure of H2 to afford dinuclear Ir(III) complex 4a-Ir (Scheme 2). The 31P NMR Scheme 2. Synthesis of Dinuclear iPr-Substituted PNP−Ir Dihydride Complex 4a-Ir and Generation of the Corresponding Mononuclear Complex 5a-Ir

Figure 1. Molecular structure of 4a-Ir (thermal ellipsoid at 50% probability; hydrogen atoms except hydride ligand and cocrystallized hexane molecule omitted for clarity). Selected bond distances (Å) and angles (deg): Ir1---Ir1* 2.7677(5), Ir1−P1 2.2756(11), Ir1−P2* 2.2872(10), Ir1−N1 2.117(3), Ir1−H1 1.82(4), Ir1−H1A 1.87(4), Ir1−H1B 1.91(4); N1−Ir1−P1 79.42(9), N1−Ir1−P2* 164.73(9), P1−Ir1−P2* 106.55(4).

spectra of 4a-Ir exhibited two singlet signals at 2.64 and 51.05 ppm,13 in which the former signal may correspond to the phosphorus nucleus located trans to hydride, stronger transinfluencing ligand.14 The 1H NMR spectrum of 4a-Ir showed two inequivalent pyrrole CH signals in the aromatic region and three characteristic broad hydride signals at −18.85 (broad multiplet), −12.67 (broad triplet of triplet, J = 65, 21 Hz), and −3.71 (broad multiplet) ppm with integral ratio of 2:1:1. The 1 H{31P} NMR spectrum (decoupling of 31P nuclei) exhibited simplified hydride signals to a broad doublet, a broad singlet, and a broad triplet. Judging from the integral ratio, two hydride ligands (−18.85 ppm) are equivalent and coupled with one hydride ligand. The multiplicity of the signal at −12.67 ppm in 1 H NMR spectrum indicates that this hydride ligand is coupled with two P nuclei: one phosphorus atom located at trans position to the hydride ligand and the other at cis position. Similarly, the hydride ligand resonating at −3.71 ppm was coupled with two magnetically equivalent hydride ligands as confirmed by 1H{31P} NMR spectrum. The dinuclear structure of 4a-Ir was confirmed by single-crystal X-ray diffraction analysis (Figure 1). Two phosphorus atoms in the iPr-PNP ligand coordinated to two different Ir atoms to form the dinuclear structure. As a result, the pyrrolide nitrogen atom located in the trans position to the phosphorus atom of the other ligand. The differential Fourier map indicated the existence of four hydride ligands. Two of four hydride ligands bridge two Ir atoms and the remaining two hydride ligands exist as a terminal hydride. The positions of all four hydride ligands are consistent with the 1H and 1H{31P} NMR spectra. The IR spectrum of 4a-Ir exhibited characteristic Ir−H vibrations for terminal Ir−H (at 2127 cm−1) and bridging Ir−H−Ir moieties (around 1100 cm−1), which were also supported by the vibrational analysis with DFT calculations (see SI). To confirm the potential dissociation of dinuclear structure of 4a-Ir to the corresponding mononuclear structure, VT NMR experiments were performed. Heating of 4a-Ir in toluene at 108 °C for 2.5 h led to a quantitative conversion of 4a-Ir. The 1H NMR spectrum of the crude product in C6D6 exhibited a characteristic hydride signal at δH −22.62 ppm and C2v symmetrical

pattern of the remaining signals. The long T1 (3.95 s)15 also supported the conclusion that two hydrogen atoms existed as hydride ligand.16 Thus, we tentatively assigned this species as mononuclear dihydride complex 5a-Ir. All the effort for recrystallization of 5a-Ir afforded single crystals of 4a-Ir, probably due to existence of an equilibrium between 4a-Ir and 5a-Ir and the higher crystallinity of 4a-Ir. Previously reported tBu-substituted PNP ligand 1b12b was also treated with iPrMgCl·LiCl and [IrCl(coe)2]2 to afford [tBu-PNP]Ir(coe) complex 2b-Ir (Scheme 3). Subsequent Scheme 3. Synthesis of t-Bu Substituted PNP−Ir Complexes 2b-Ir and 5b-Ir

exposure of 2b-Ir toward H2 resulted in the formation of mononuclear [tBu-PNP]IrH2 5b-Ir as a stable compound in contrast to the case of 5a-Ir probably due to the difference in steric hindrance around Ir center. The 31P NMR spectrum of 2b-Ir exhibited two distinct doublet signals with almost similar chemical shifts (δP 50.7 and 51.8) and an identical coupling constants of 342 Hz. Geminal protons at allylic position were inequivalent in the 1H NMR spectrum of 2b-Ir probably due to the orthogonal relationship between the coordinating of CC bond in COE ligand to Ir and the PNP−Ir plane. Single-crystal X-ray diffraction analysis of 2b-Ir revealed the mononuclear structure with orthogonally oriented CC bond in the COE ligand to the PNP−Ir plane (Figure 2). A characteristic CC vibration (1240 cm−1) was observed in the IR spectrum, which could be assigned by DFT calculations (see SI). The 31P NMR B


Article

Organometallics

Figure 2. Molecular structure of 2b-Ir (thermal ellipsoid at 50% probability; one of two independent molecules and hydrogen atoms omitted for clarity). Selected bond distances (Å) and angles (deg) [parameters of second independent molecules are in brackets]: Ir1−P1 2.411(2) [Ir2−P4 2.392(3)], Ir1−P2 2.299(2) [Ir2−P3 2.308(2)], Ir1−N1 2.042(7) [Ir2−N2 2.026(8)], Ir1−C23 2.173(8) [Ir2−C53 2.167(9)], Ir1−C24 2.176(9) [Ir2−C54 2.162(9)], C23−C24 1.437(12) [C53−C54 1.417(13)]; P1−Ir1−P2 157.93(8) [P3−Ir2− P4 158.15(9)], N1−Ir1−P1 78.9(2) [N2−Ir2−P4 78.8(2)], N1−Ir1− P2 79.6(2) [N2−Ir2−P3 79.7(2)].

Figure 3. Molecular structure of 5b-Ir (thermal ellipsoid at 50% probability; hydrogen atoms except hydride ligand omitted for clarity). Selected bond distances (Å) and angles (deg): Ir1−P1 2.2947(9), Ir1−P2 2.3009(9), Ir1−N1 2.011(3), Ir1−H1 1.49(3), Ir1−H1A 1.54(3); P1−Ir1−P2 164.60(3), N1−Ir1−P1 82.27(8), N1−Ir1−P2 82.35(8).

Scheme 4. Synthesis of i-Pr Substituted PNP−Rh Complexes 6a-Rh and 4a-Rha

spectrum of 5b-Ir exhibited one singlet signal, which is being consistent with mononuclear structure. The simple 1H NMR spectrum of 5b-Ir showed one tBu signal (36H integral ratio), one methylene signal (4H), and one CH signal (2H), indicating that the molecular structure should have C2v symmetry. The X-ray crystallographic analysis confirmed the C2v symmetrical structure of 5b-Ir with two hydride ligands which could be located on the differential Fourier map (Figure 3). The T1 measurement (398 ms)15 also supported the hydridic feature of two hydrogen atoms.16 Two strong Ir−H vibrations at 2147 and 2170 cm−1, which were assigned by DFT calculations, also supported the existence of two hydride ligands. Complexation of Rh with iPr-PNP ligand 1a was also performed by a treatment of 1a with LiHMDS and [RhCl(C2H4)2]2 to give a mononuclear [iPr-PNP]Rh(C2H4) complex 6a-Rh (Scheme 4). Single-crystal X-ray diffraction analysis revealed C2v-symmetrical structure of 6a-Rh (Figure 4). The 31 P NMR spectrum of 6a-Rh exhibited one doublet signal having a coupling with Rh nucleus and the 1H NMR spectra of 6a-Rh included characteristic 4H signal of coordinating ethylene ligand, reflecting the C2v-symmetrical structure of 6a-Rh. Reaction of 6a-Rh with H2 afforded a dinuclear complex 4a-Rh (Scheme 4), as confirmed by X-ray analysis (Figure 5). In contrast to the case of 6a-Rh, the 31P NMR spectrum of 4aRh showed two doublet signals at δP 41.3 (1JRhP = 124 Hz) and 69.4 ppm (1JRhP = 169 Hz).13 Each doublet has different coupling constant to Rh, indicating the environment of two magnetically inequivalent phosphorus atoms as confirmed by X-ray analysis. Differential Fourier map in X-ray analysis indicated the presence of one terminal hydride ligand and one bridging hydride ligand on Rh. Because the 1H NMR spectrum could be observed, 4a-Rh can be considered a diamagnetic

a

LiHMDS: LiN(SiMe3)2.

compound, consisting of Rh(I) and Rh(III) centers17 or two Rh(II) centers with Rh−Rh bonding.18 Two hydride signals in the 1H NMR spectrum separately resonated as quintet of triplet signals, indicating the following two things: (1) coupling to four P atoms and two Rh atoms and (2) rapid equilibrium for migration of terminal hydride ligand between Rh(I)−Rh(III) structures 4a-Rh and 4a′-Rh (Scheme 5a) or an existence of a slightly more stable Rh(II)−Rh(II) structure 4a-Rhsym in solution (Scheme 5b). The tBu-substituted PNP−pincer ligand 1b was also introduced to Rh by a treatment with LiHMDS and [RhCl(C2H4)2]2 to result in a formation of a mononuclear [tBu-PNP]Rh(C2H4) complex 6b-Rh (Scheme 6). Exposing a solution of 6b-Rh toward H2 afforded the corresponding mononuclear dihydrogen [tBu-PNP]Rh(H2) complex 5b-Rh. Both 6b-Rh and 5b-Rh exhibited one doublet in the 31P NMR spectra and C2v symmetrical pattern of the PNP ligand in the 1 H and 13C NMR spectra. Ethylene ligand in 6b-Rh resonated at 3.45 ppm as a doublet of triplet signal in the 1H NMR spectrum and at 44.97 ppm as a doublet signal in the 13C NMR spectrum. In the case of 5b-Rh, a hydride signal (doublet of triplet, 2H) was found at −11.5 ppm in the 1H NMR spectrum. Single crystal X-ray diffraction analysis revealed the monomeric C


Article

Organometallics

Scheme 5. Potential Equilibrium of 4a-Rh in Solution

Scheme 6. Synthesis of t-Bu Substituted PNP−Rh Complexes 6b-Rh and 5b-Rh Figure 4. Molecular structure of 6a-Rh (thermal ellipsoid at 50% probability; hydrogen atoms and cocrystallized toluene molecule omitted for clarity). Selected bond distances (Å) and angles (deg): Rh1−P1 2.3049(15), Rh1−P2 2.3026(16), Rh1−N1 2.011(5), Rh1− C1 2.135(6), Rh1−C2 2.141(6), C1−C2 1.367(10); P1−Rh1−P2 161.83(6), N1−Rh1−P1 81.26(14), N1−Rh1−P2 80.68(14).

Figure 5. Molecular structure of 4a-Rh (thermal ellipsoid at 50% probability; hydrogen atoms omitted for clarity). Selected bond distances (Å) and angles (deg): Rh1---Rh2 2.8268(13), Rh1−P1 2.2877(16), Rh1−P4 2.2596(19), Rh2−P2 2.268(2), Rh2−P3 2.2900(15), Rh1−N1 2.080(4), Rh2−N2 2.094(4), Rh1−H69 1.88(5), Rh2−H69 1.78(5), Rh1−H70 1.52(5); P1−Rh1−P4 101.61(4), N1−Rh1−P1 81.22(10), N1−Rh1−P4 164.10(10), P2− Rh2−P3 101.94(4), N2−Rh2−P3 81.51(9), N2−Rh2−P2 165.64(9).

Figure 6. Molecular structure of 6b-Rh (thermal ellipsoid at 50% probability; hydrogen atoms omitted for clarity). Selected bond distances (Å) and angles (deg): Rh1−P1 2.3289(11), Rh1−P2 2.3297(10), Rh1−N1 2.020(3), Rh1−C23 2.143(4), Rh1−C24 2.146(4), C23−C24 1.369(7); P1−Rh1−P2 159.82(4), N1−Rh1− P1 79.85(10), N1−Rh1−P2 80.16(10).

Ir complexes exhibited higher activity than those of the corresponding Rh complexes having the same set of ligand, which is in contrast to the case of carbazolide-based system.6f,g The highest activity was recorded with 3a-Ir at 220 °C although the activity was moderate among the reported catalysts. It should be noted that the most active catalyst, 3a-Ir, could catalyze dehydrogenation of n-octane with very low TON of 4 at 180−220 °C.

structures of 6b-Rh and 5b-Rh (Figures 6 and 7). In the solid state, ethylene ligand in 6b-Rh is in orthogonal relationship with [PNP]Rh plane. Two peaks in the differential Fourier map were assigned as dihydrogen ligand [H44−H45 0.97(5) Å] in 5b-Rh. Existence of two hydrogen atoms as dihydrogen ligand was confirmed with its short T1 (66 ms).15,16 All the [PNP] complexes obtained in this study were subjected to transfer dehydrogenation of cyclooctane as a catalyst in the presence of tert-butylethylene (Table 1). Except for 5b-Rh, all the complexes showed no catalytic activity at 140 °C. Similar to the case of previously reported catalysts,2−7 elevating temperature improved catalytic activity in all cases. All

■

CONCLUSION In conclusion, we synthesized and characterized a series of tBuand iPr-substituted PNP−pincer Ir and Rh complexes having D


Article

Organometallics

without any purification. H2 gas was purified by passing through deoxygenating and dehydrating columns (Nikka Seiko). [Ir(coe)2Cl]2,19 [Rh(C2H4)2Cl]2,20 1a,12a 1b,12b and 2a-Ir12a were synthesized according to the literature procedure. NMR spectra were recorded on JEOL ECA (500 MHz for 1H, 126 MHz for 13C. 203 MHz for 31P) and JEOL ECA (400 MHz for 1H) spectrometers. Chemical shifts are reported in δ (ppm) relative to the residual protiated solvent for 13C and external H3PO4 for 31P used as reference. Elemental analysis were performed at the A Rabbit Science Co., Ltd. IR spectra were obtained on Agilent Cary 630 FTIR (Agilent Technologies, Inc.) with ATR attachment. Melting points (mp) were determined with a MPA100 OptiMelt instrument (Tokyo Instruments, Inc.) and are uncorrected. X-ray crystallographic analyses were performed on a VariMax/Saturn CCD diffractometer. Mass spectra were measured on a JEOL JMS-700 mass spectrometer. Synthesis of 4a-Ir. In a 10 mL J. Young tube, a solution of 2a-Ir (778 mg 1.24 mmol, 10.0 mL of benzene) was frozen by cooling with liquid N2, and then the headspace of the tube was evacuated. The headspace of the tube was then backfilled with gaseous H2. The mixture was allowed to warm to room temperature, then was stirred at room temperature for 18 h. Volatiles were removed under reduced pressure to afford black solids. The black solids were recrystallized from a concentrated toluene solution layered with n-hexane as a dihydride dinuclear complex 5 (286 mg, 44%). 1H NMR (500 MHz, C6D6) δ −18.85 (br m, 2H, hydride, becoming a broad doublet with J = 7 Hz under 31P decoupling), −12.67 (br tt, J = 65, 21 Hz, 1H, hydride, becoming a broad singlet under 31P decoupling), −3.71 (br dt, J = 7, 7 Hz, 1H, hydride, becoming a broad triplet with J = 9 Hz under 31P decoupling), 0.70 (dd, 3JPH = 15 Hz, 3JHH = 7 Hz, 6H, CH3), 0.76 (dd, 3JPH = 12 Hz, 3JHH 7 Hz, 6H), 0.81 (dd, 3JPH = 15 Hz, 3JHH = 7 Hz, 6H, CH3), 0.84 (dd, 3JPH = 14 Hz, 3JHH 7 Hz, 6H, CH3), 0.99 (dd, 3JPH = 14 Hz, 3JHH = 7 Hz, 6H, CH3), 1.02 (dd, 3JPH = 11 Hz, 3JHH = 7 Hz, 7H, CH3), 1.26 (dd, 3JPH = 13 Hz, 3JHH = 7 Hz, 6H, CH3), 1.31 (dd, 3JPH = 16 Hz, 3JHH = 7 Hz, 6H, CH3), 1.48−1.58 (m, 2H), 1.65−1.82 (m, 4H), 1.94−2.04 (m, 2H), 2.45 (dd, 2JPH = 12 Hz, 2JHH= 15H, C−CH2-P), 3.06−3.14 (m, 2H, C−CH2−P), 3.62 (d, 2JPH = 9 Hz, 4H, C−CH2−P), 6.31 (s, 2H, pyr-H), 6.36 (s, 2H, pyr-H). 31P NMR (203 MHz, C6D6) δ 2.64 (s), 51.05 (s). 13C NMR (101 MHz, CDCl3) δ 16.21 (d, 2JPC = 7 Hz, CH3), 16.54 (t, 2JPC = 3 Hz, CH3), 17.32 (s, CH3), 17.94 (d, 2JPC = 3 Hz, CH3), 18.14 (s, CH3), 19.83 (s, CH3), 20.41 (s, CH3), 20.61 (s, CH3), 21.55 (t, 1JPC = 15 Hz, P−CH− CH3), 27.38 (d, 1JPC = 15 Hz, C−CH2−P), 27.38 (s, CH3), 27.60 (s, CH3), 29.17 (t, 1JPC = 15 Hz, P−CH−CH3), 30.87 (d, 1JPC = 40 Hz, P−CH−CH3), 31.87 (d, 1JPC = 32 Hz, C−CH2−P), 98.45 (dd, J = 4 Hz, 8 Hz, pyrrole-βC), 105.66 (t, J = 4 Hz, pyrrole-βC), 127.35 (dd, J = 6, 2 Hz, pyrrole-αC), 136.03 (s, pyrrole-αC); mp 163.5−165.2 °C (dec.). IR(ATR) 1114, 1087, 1071, 2127 cm−1. HRMS(FAB+): Calcd for C36H72N2P4Ir2 [M]: 1042.3904. Found 1042.3907. See the Supporting Information for NMR spectra of the isolated complex. Generation of 5a-Ir from 4a-Ir. A solution of 4a-Ir (10.0 mg 9.60 μmol, 600 μL of toluene) in a J. Young NMR tube was heated at 108 °C for 2.5 h. After heating, all the volatiles were removed under reduced pressure. Then, C6D6 (600 μL) was added to the residue. The resulting NMR sample was subjected to NMR measurements to show nearly quantitative generation of 5a-Ir (see the Supporting Information for NMR spectra). 1H NMR (400 MHz, toluene-d8) δ −22.62 (t, J = 9 Hz, 1H), 0.85 (dt, J = 7, 7 Hz, 12H), 1.03 (dt, J = 7, 7 Hz, 12H), 1.69−1.82 (m, 4H), 2.96 (t, J = 4 Hz, 4H), 6.55 (s, 2H); 31P NMR (203 MHz, C6D6) δ 70.7 (s). Synthesis of 2b-Ir. To a precooled (−35 °C) solution of 1b (86.2 mg, 255 μmol, 3.00 mL of THF) in a 15.0 mL vial, a THF solution of i PrMgCl·LiCl (1.00 M, 224 μL, 224 μmol) was added dropwise at −35 °C. After stirring the resulting solution for 30 min, the solution was allowed to warm to room temperature. The reaction mixture was added dropwise to a solution of [Ir(coe)2Cl]2 (100 mg, 112 μmol, 3.00 mL of THF) in a 15.0 mL vial at room temperature. The resulting solution was stirred at room temperature for 3.5 h. The brown solution was filtered through a pad of Celite to give a brown filtrate. Removal of volatiles from the filtrate gave black solids. After dissolution of the black solid in toluene, 1,4-dioxane (50 μL) was added to the mixture.

Figure 7. Molecular structure of 5b-Rh (thermal ellipsoid at 50% probability; hydrogen atoms except hydride ligand omitted for clarity). Selected bond distances (Å) and angles (deg): Rh1−P1 2.2962(9), Rh1−P2 2.3009(8), Rh1−N1 2.004(3), Rh1−H44 1.74(3), Rh1−H45 1.70(4); P1−Rh1−P2 162.16(3), N1−Rh1−P1 82.20(7), N1−Rh1− P2 81.42(8).

Table 1. Catalytic Activity (TON) in the Transfer Dehydrogenation of Cyclooctane Using Pyrrole-Based PNP−Ir or PNP−Rh Complexes as Catalysta

catalyst temp (°C)

3a-Ir

4a-Ir

2b-Ir

5b-Ir

6a-Rh

4a-Rh

6b-Rh

5b-Rh

140 160 180 200 220

0 4 8 17 28

0 6 9 20 18

0 3 4 6 7

0 3 7 9 10

0 0 0 5 11

0 0 5 5 5

0 0 4 5 6

4 3 5 5 12

a

TONs were estimated by GC with internal standard (n-dodecane).

pyrrole-based backbone. The structures of the obtained complexes were varied depending on the size of alkyl substituents and ligands other than PNP ligand to form mono- or dinuclear complexes. All the newly obtained complexes were slightly active toward transfer dehydrogenation of cyclooctane in the presence of tert-butylethylene as a hydrogen acceptor.

■

EXPERIMENTAL SECTION

General Procedures. All the preparations and manipulations involving air- and moisture-sensitive compounds were carried out under Argon atmosphere by using Schlenk and glovebox (Miwa MFG, KIYON) technique. All glassware were dried for 20 min in the 250 °C oven before use. Et2O (Kanto Chemical, dehydrated), toluene (Kanto Chemical, dehydrated), and n-hexane (Kanto Chemical, dehydrated) were purified by passing through a solvent purification system (Grass Contour). Benzene (Kanto Chemical, dehydrated) and dioxane (Kanto Chemical, dehydrated) were dried over Na/K alloy with stirring and were filtered though a pad of oven-baked alumina before use. C6D6 was dried by distillation over sodium benzophenone followed by vacuum transfer. Cyclooctane and tert-butylethylene were used after degassing with freeze−pump−thaw cycles and passing through a pad of alumina. LiHMDS and n-dodecane were used E


Article

Organometallics The resulting suspension was centrifuged, and the supernatant was collected. Then, the remaining residue was extracted with hexane. After combining the supernatant and hexane solution, volatiles were removed under reduced pressure. The residue was recrystallized from n-hexane to give red solids (53.2 mg, 77.6 μmol, 35%). 1H NMR (400 MHz, C6D6) δ 1.17 (dd, 3JPH = 10 Hz, 5JPH = 2 Hz, 18H, CH3), 1.23 (dd, 3JPH = 9 Hz, 5JPH = 2 Hz, 18H, CH3), 1.43−1.62 (m, 4H, 2CH2), 1.70−1.86 (m, 6H, 2CH2 + 2allylic CHH), 2.54 (ddt, J = 13, 4, 4 Hz, 2H, 2allylic CHH), 2.99 (d, 2JPH = 7 Hz, 2H, C−CH2−P), 3.19 (d, J = 6 Hz, 2H, C−CH2−P), 3.45 (dt, 2JPH = 7, 3JHH Hz, 2H, −CH=CH−), 6.41 (d, 3JHH = 3 Hz, 1H, pyr-H), 6.46 (d, 3JHH = 3 Hz, 1H, pyr-H). 31 P NMR (162 MHz,C6D6) δ 50.7 (d, 2JPP = 342 Hz, 1P), 51.8 (d, 2JPP = 342 Hz, 1P). 13C NMR (101 MHz, C6D6) δ 27.01 (dd, J = 23, 3 Hz, C−CH2−P), 27.26 (s, −CH2−CH2−CH2−), 29.69 (dd, J = 19, 3 Hz, C−CH2−P), 30.23 (s, CH3), 31.31 (s, CH3), 33.24 (s, −CH2−CH2− CH2−), 35.37 (dd, J = 6, 3 Hz, P−C−(CH3)3), 36.46 (d, J = 18 Hz, −CH−CH2−CH2−), 36.48 (dd, J = 6, 3 Hz, P−C−(CH3)3), 44.37 (s, −CHCH−), 102.75 (d, J = 6 Hz, pyr-βC), 102.99 (d, J = 8 Hz, pyrβC), 137.97 (t, J = 3 Hz, pyr-αC), 138.96 (t, J = 4 Hz, pyr-αC). Mp 164.1−167.8 °C (dec.). IR(ATR) 1240 cm−1 (CC). Anal. Calcd for C30H56NP2Ir: C, 52.61; H, 8.24; N, 2.04. Found: C, 52.68; H, 8.48; N, 2.08. Synthesis of 5b-Ir. In a 50 mL J. Young tube, a solution of complex 2b-Ir (26.7 mg, 39.0 μmol, 1.50 mL of toluene) was frozen by cooling with liquid N2, and then the headspace of the tube was evacuated. The headspace of the tube was then backfilled with gaseous H2. After thawing the frozen solution, the resulting mixture was allowed to warm to room temperature. The reaction mixture was stirred at 70 °C for 20 h. Volatiles were removed under reduced pressure afforded black solids. The black solids were recrystallized by layering a concentrated toluene solution with n-hexane to give 5b-Ir (5.2 mg, 9.0 μmol, 23%). 1H NMR (400 MHz, C6D6) δ −22.77 (t, 3 JPH = 9 Hz, 2H, hydride), 1.18 (vt, 3JPH = 7 Hz, 36H, CH3), 3.16 (vt, 2 JPH = 4 Hz, 4H, C−CH2−P), 6.55 (s, 2H, pyr-H). 31P NMR (162 MHz, C6D6) δ 89.0 (s). 13C NMR (101 MHz, C6D6) δ 27.57 (t, 1JPC = 12 Hz, C−CH2−P), 29.98 (t, 2JPC = 3 Hz, CH3), 35.11 (t, 1JPC = 11 Hz, P−C(CH3)3), 105.05 (t, 3JPC = 5 Hz, pyrrole-βC), 144.35 (t, 2JPC = 5 Hz, pyrrole-αC). Mp 188.5−190.4 °C (dec.). IR(ATR) 2147 (Ir− H), 2170 cm−1 (Ir−H). Anal. Calcd for C22H44NP2Ir: C, 45.81; H, 7.69; N, 2.43. Found: C, 45.69; H, 7.78; N, 2.38. Synthesis of 6a-Rh. To a solution of 1a (300 mg, 0.916 mmol, 4.00 mL of toluene) in a 15 mL vial was added LiHMDS (161 mg, 0.962 mmol) as a solid at room temperature. After dissolving LiHMDS with shaking at room temperature, the resulting mixture was added to a solution of [Rh(C2H4)2Cl]2 (178 mg, 0.458 mmol, 3.00 mL of toluene) in a 15 mL vial. The resulting mixture was stirred at room temperature for 18 h. After the reaction mixture was filtered through a pad of Celite, volatiles were removed from the filtrate to afford brown solids. The residue was recrystallized by layering toluene solution with n-hexane at −35 °C to give yellow crystals of 6a-Rh (286 mg, 0.625 mmol, 44%). 1H NMR (400 MHz, C6D6) δ 0.91 (d of vt, 3JPH = 7 Hz, 3 JHH = 7 Hz, 12H, CH3), 1.01 (d of vt, 3JPH = 7 Hz, 3JHH = 7 Hz, 12H, CH3), 1.64−1.75 (sept of vt, 2JPH = 2 Hz, 3JHH = 7 Hz, 4H, P− CHMe2), 2.83 (vt, 2JPH = 4 Hz, 4H, C−CH2−P), 3.16 (d of t, 3JPH = 4 Hz, 2JRhH = 2 Hz, 4H, C2H4), 6.43 (s, 2H, pyr-H). 31P NMR (162 MHz, C6D6) δ 58.8 (d, 1JRhP = 135 Hz). 13C NMR (101 MHz, C6D6) δ 17.92 (s, CH3), 18.82 (t, 2JPC = 3 Hz, CH3), 24.53 (t, 1JPC = 10 Hz, P−CH(CH3)3), 25.68 (t, 1JPC = 10 Hz, C−CH2−P), 44.10 (dt, J = 11 Hz, 2 Hz, C2H4), 103.40 (t, 3JPC = 5 Hz, pyr-βC), 137.73 (dt, J = 6 Hz, 3 Hz, pyr-αC). Mp 115.7−116.4 °C (dec.). Anal. Calcd for C20H38NP2Rh: C, 52.52; H, 8.37; N, 3.06. Found: C, 52.84; H, 8.76; N, 3.13. Synthesis of 4a-Rh. In a 10 mL J. Young tube, a solution of 6a-Rh (50.2 mg, 110 μmol, 2.00 mL of toluene) was frozen by cooling with liquid N2, and then the headspace of the tube was evacuated. The headspace of the tube was then backfilled with gaseous H2. After thawing the frozen solution, the resulting mixture was allowed to warm to room temperature. The reaction mixture was stirred at 110 °C for 20 h. Volatiles were removed from the filtrate to afford brown solids.

The brown solids were recrystallized by layering toluene solution with n-hexane at −35 °C to give yellow crystals of 4a-Rh (18.0 mg, 20.9 μmol, 38%). 1H NMR (500 MHz, C6D6, see the Supporting Information for 1H NMR spectrum) δ −19.36 (qt, 1JRhH = 30 Hz, 2 JPH = 7 Hz, 1H, hydride), −10.15 (qt, 1JRhH = 70 Hz, 2JPH = 20 Hz, 1H, hydride), 0.74 (dd, 3JPH = 14 Hz, 3JHH = 7 Hz, 6H, CH3), 0.867 (dd, 3JPH = 15 Hz, 3JHH = 7 Hz, 12H), 0.874 (dd, 3JPH = 17 Hz, 3JHH = 7 Hz, 6H), 0.94 (dd, 3JPH = 12 Hz, 3JHH = 7 Hz, 6H, CH3), 1.21 (dd, 3 JPH = 10 Hz, 3JHH = 7 Hz, 6H, CH3), 1.40 (dd, 3JPH = 16 Hz, 3JHH = 7 Hz, 6H, CH3), 1.54 (dd, 3JPH = 14 Hz, 3JHH = 7 Hz, 6H, CH3), 1.60− 1.71 (m, 2H, P−CH−CH3), 1.77−1.89 (m, 2H, P−CH−CH3), 1.90− 2.07 (m, 4H, P−CH−CH3), 2.60 (dd, 2JPH = 14 Hz, 2JHH = 7 Hz, 2H, C−CH2−P), 3.15 (d, 2JPH = 13 Hz, 2H, C−CH2−P), 3.35 (dd, 3JPH = 14 Hz, 3JHH = 7 Hz, 2H, C−CH2−P), 3.78 (d, 3JPH = 13 Hz, 2H, C− CH2−P), 6.31 (s, 2H, pyr-H), 6.31 (s, 2H, pyr-H). 31P NMR (203 MHz, C6D6) δ 41.3 (d, 1JRhP = 124 Hz, 1P), 69.4 ppm (d, 1JRhP = 169 Hz, 1P). 13C NMR (101 MHz, C6D6) δ 16.91 (d, 2JPC = 6 Hz), 17.54 (d, 2JPC = 4 Hz), 19.05 (d, 2JPC = 5 Hz), 19.56 (d, 2JPC = 4 Hz), 20.08 (d, 2JPC = 3 Hz), 20.43 (d, 2JPC = 6 Hz), 21.55 (d, 2JPC = 4 Hz), 24.69 (d, 1JPC = 18 Hz), 27.58 (d, 1JPC = 22 Hz), 29.38 (d, 1JPC = 22 Hz), 29.83 (d, 1JPC = 15 Hz), 30.19 (d, 1JPC = 17 Hz), 32.09 (d, 1JPC = 23 Hz), 101.08 (d, JPC = 11 Hz, pyr-βC), 108.91 (t, JPC = 4 Hz, pyr-βC), 128.589 (s, pyr-αC), 139.089 (d, JPC = 5 Hz, pyr-αC). Mp 166.9− 169.0 °C (dec.). IR(ATR) 1101 cm−1 (symmetric and antisymmetric Rh−H). Anal. Calcd for C36H70N2P4Rh2: C, 50.24; H, 8.20; N, 3.25. Found: C, 50.63; H, 8.34; N, 3.28. Synthesis of 6b-Rh. To a solution of 2b (86.2 mg, 255 μmol, 1.55 mL of toluene) in a 15.0 mL vial was added LiHMDS (39.8 mg, 238 μmol) as a solid at room temperature. The reaction mixture was added dropwise to a solution of [Rh(C2H4)2Cl]2 (44.0 mg, 113 μmol, 1.50 mL of toluene) in a 15.0 mL vial at room temperature. The resulting solution was stirred at room temperature for 2 h. The brown solution was filtered through a pad of Celite to give a brown filtrate. Removal of volatiles from the filtrate gave brown solids. The solids were recrystallized from toluene to give yellow crystals of 6b-Rh (53.2 mg, 77.6 μmol, 35%). 1H NMR (400 MHz, C6D6) δ 1.13 (vt, 3JPH = 6 Hz, 36H, CH3), 2.97 (vt, 2JPH = 4 Hz, 4H, C−CH2−P), 3.45 (d of t, 2 JRhH = 5 Hz, 3JPH = 2 Hz, 4H, C2H4), 6.41 (s, 2H, pyr−CH). 13C NMR (101 MHz, C6D6) δ 26.75 (t, 1JPC = 9 Hz, C−CH2−P), 30.12 (vt, 2JPC = 2.9 Hz, CH3), 35.85 (t, 1JPC = 5.3 Hz, P−C(CH3)3), 44.97 (d, 1JRhC = 12 Hz, C2H4), 102.74 (t, 3JPC = 5 Hz, pyrrole-βC), 137.79 (td, 2JRhC = 3, 2JPC = 8 Hz, pyrrole-αC). 31P NMR (162 MHz, C6D6) δ 70.70 (d, 1JRhP = 135 Hz). Mp 196.4−199.2 °C. IR(ATR) 1255 cm−1 (CC). Anal. Calcd for C24H46NP2Rh: C, 56.14; H, 9.03; N, 2.73. Found: C, 55.99; H, 9.37; N, 2.89. Synthesis of 5b-Rh. In a 10 mL J. Young tube, a solution of 6b-Rh (10.2 mg, 19.9 μmol, 1.00 mL of toluene) was frozen by cooling with liquid N2, and then the headspace of the tube was evacuated. The headspace of the tube was then backfilled with gaseous H2. After thawing the frozen solution, the resulting mixture was allowed to warm to room temperature. The reaction mixture was stirred at 80 °C for 26 h. After filtration of the reaction mixture through a pad of Celite, volatiles were removed from the filtrate. The residue was recrystallized by layering toluene solution with n-hexane at −35 °C to give yellow crystals of 5b-Rh (3.3 mg, 6.8 μmol, 34%). 1H NMR (400 MHz, C6D6) δ −11.45 (dt, 1JRhH = 21 Hz, 2JPH 7 Hz, 2H, hydride), 1.14 (vt, 3 JPH = 7 Hz, 36H, CH3), 3.07 (vt, 2JPH = 4 Hz, 4H, C−CH2−P), 6.43 (s, 2H aromatic CH). 31P NMR (203 MHz, C6D6) δ 97.68 (d, 1JRhP = 134 Hz). 13C NMR (101 MHz, C6D6) δ 26.08 (t, 1JPC = 9 Hz, C− CH2−P), 29.50 (t, 2JPC = 4 Hz, CH3), 33.84 (t, 1JPC = 8 Hz, P− C(CH3)3), 103.20 (t, 3JPC = 5 Hz, pyrrole-βC), 139.35 (d, 2JPC = 3 Hz, pyrrole-αC). Mp 128.2−137.3 °C. IR(ATR) 2112 (Rh−H), 1940 cm−1 (Rh−H). Anal. Calcd for C22H44NP2Rh: C, 54.21; H, 9.10; N, 2.87. Found: C, 53.93; H, 9.19; N, 3.06. General Procedure for Catalytic Transfer Dehydrogenation of Cyclooctane. In a glovebox, catalyst (3.00 μmol) was dissolved in a solution consisting of cyclooctane (3000 equiv., 9.00 mmol) and H2 acceptor (tert-butylethylene, 600 equiv, 9.00 or 1.80 mmol) in a 10.0 mL J. Young tube. After bringing the tube out from the glovebox, the tube was heated by using an aluminum-block stirrer. After the F


Article

Organometallics

by Ruthenium Pincer Complexes. Chem. Rev. 2014, 114, 12024− 12087. (c) Selander, N.; Szabó, K. J. Catalysis by Palladium Pincer Complexes. Chem. Rev. 2011, 111, 2048−2076. (d) The Chemistry of Pincer Compounds; Elsevier: Oxford, 2007. (e) Kumar, A.; Bhatti, T. M.; Goldman, A. S. Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes. Chem. Rev. 2017, 117, 12357− 12384. (2) (a) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer Complexes. Chem. Rev. 2011, 111, 1761−1779. (b) Kumar, A.; Goldman, A. S. Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes. In The Privileged Pincer-Metal Platform: Coordination Chemistry & Applications; van Koten, G., Gossage, R. A., Eds.; Springer International Publishing: Cham, 2016; pp 307−334;. (c) Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Alkane Metathesis by Tandem Alkane-Dehydrogenation−Olefin-Metathesis Catalysis and Related Chemistry. Acc. Chem. Res. 2012, 45, 947−958. (d) Zhang, Y.; Yao, W.; Fang, H.; Hu, A.; Huang, Z. Catalytic alkane dehydrogenations. Sci. Bull. 2015, 60, 1316−1331. (e) Tang, X.; Jia, X.; Huang, Z. Challenges and opportunities for alkane functionalisation using molecular catalysts. Chem. Sci. 2018, 9, 288−299. (3) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. Iridium complexes in alkane dehydrogenation. J. Am. Chem. Soc. 1979, 101, 7738−7740. (4) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. A highly active alkane dehydrogenation catalyst: stabilization of dihydrido rhodium and iridium complexes by a P-C-P pincer ligand. Chem. Commun. 1996, 2083−2084. (b) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.; Jensen, C. M. Catalytic dehydrogenation of cycloalkanes to arenes by a dihydrido iridium P-C-P pincer complex. J. Am. Chem. Soc. 1997, 119, 840−841. (c) Gupta, M.; Kaska, W. C.; Jensen, C. M. Catalytic dehydrogenation of ethylbenzene and tetrahydrofuran by a dihydrido iridium P-C-P pincer complex. Chem. Commun. 1997, 461−462. (d) Xu, W. W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska, W. C.; KroghJespersen, K.; Goldman, A. S. Thermochemical alkane dehydrogenation catalyzed in solution without the use of a hydrogen acceptor. Chem. Commun. 1997, 2273−2274. (e) Liu, F. C.; Goldman, A. S. Efficient thermochemical alkane dehydrogenation and isomerization catalyzed by an iridium pincer complex. Chem. Commun. 1999, 655−656. (5) (a) Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S. Highly Effective Pincer-Ligated Iridium Catalysts for Alkane Dehydrogenation. DFT Calculations of Relevant Thermodynamic, Kinetic, and Spectroscopic Properties. J. Am. Chem. Soc. 2004, 126, 13044−13053. (b) Huang, Z.; Brookhart, M.; Goldman, A. S.; Kundu, S.; Ray, A.; Scott, S. L.; Vicente, B. C. Highly Active and Recyclable Heterogeneous Iridium Pincer Catalysts for Transfer Dehydrogenation of Alkanes. Adv. Synth. Catal. 2009, 351, 188−206. (c) Kuklin, S. A.; Sheloumov, A. M.; Dolgushin, F. M.; Ezernitskaya, M. G.; Peregudov, A. S.; Petrovskii, P. V.; Koridze, A. A. Highly Active Iridium Catalysts for Alkane Dehydrogenation. Synthesis and Properties of Iridium Bis(phosphine) Pincer Complexes Based on Ferrocene and Ruthenocene. Organometallics 2006, 25, 5466−5476. (d) Bézier, D.; Brookhart, M. Applications of PC(sp3)P Iridium Complexes in Transfer Dehydrogenation of Alkanes. ACS Catal. 2014, 4, 3411−3420. (e) Kovalenko, O. O.; Wendt, O. F. An electron poor iridium pincer complex for catalytic alkane dehydrogenation. Dalton Trans. 2016, 45, 15963−15969. (f) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.; Fan, H.-J.; Hall, M. B. Thermally Stable Homogeneous Catalysts for Alkane Dehydrogenation. Angew. Chem., Int. Ed. 2001, 40, 3596−3600. (g) Göttker-Schnetmann, I.; Brookhart, M. Mechanistic Studies of the Transfer Dehydrogenation of Cyclooctane Catalyzed by Iridium Bis(phosphinite) p-XPCP Pincer Complexes. J. Am. Chem. Soc. 2004, 126, 9330−9338. (h) GöttkerSchnetmann, I.; White, P.; Brookhart, M. Iridium bis(phosphinite) pXPCP pincer complexes: Highly active catalysts for the transfer dehydrogenation of alkanes. J. Am. Chem. Soc. 2004, 126, 1804−1811. (i) Böhnke, J.; Braunschweig, H.; Constantinidis, P.; Dellermann, T.; Ewing, W. C.; Fischer, I.; Hammond, K.; Hupp, F.; Mies, J.; Schmitt,

indicated time, the tube was cooled down to room temperature. An internal standard (n-dodecane, 1.00 mmol) was added to the reaction mixture and then the sample was analyzed by gas chromatography to estimate yields with a precalculated GC factor toward authentic samples. Details for X-ray Crystallography. Details of the crystal data and a summary of the intensity data collection parameters for 4a-Ir, 2b-Ir, 5b-Ir, 6a-Rh, 4a-Rh, 6b-Rh, and 5b-Rh are listed in Table S1. In each case, a suitable crystal was mounted with a mineral oil to the glass fiber and transferred to the goniometer of a VariMax Saturn CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71075 Å). All the following procedure for analysis, Yadokari-XG 2009 was used as a graphical interface.21 The structures were solved by direct method with (SIR-2014 and SIR-97)22 and refined by full-matrix least-squares techniques against F2 (SHELXL-2014).23 The intensities were corrected for Lorentz and polarization effects or NUMABS program (Rigaku 2005). The non-hydrogen atoms were refined anisotropically. Hydrogen atoms except hydride ligand were placed using AFIX instructions. The hydride ligands were refined isotropically. The molecular structures were drawn by using ORTEP-III program.24 Relatively low R1 value of 6a-Rh would be attributed to the low Rint value. Computational Details. All calculations were performed with Gaussian 09 (rev. D.01) software package.25 Both the molecular geometries of 9 and 10 were optimized without constraints at the B3LYP level26 with LanL2DZ/6-31+G* basis sets.27 The obtained vibrational frequencies were scaled with a factor of 0.9723.28 The optimized structures are attached as Supporting Information in “.xyz” format, which can be viewed with Mercury software.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00072. Computational details (PDF) DFT coordinates (XYZ) Accession Codes

CCDC 1821770−1821776 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.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Makoto Yamashita: 0000-0002-3665-5311 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by JST-CREST 14529307 from JST. We thank Prof. Tamejiro Hiyama (Chuo University) for providing an access to an X-ray diffractometer. Theoretical calculations were carried out using resources of the Research Center for Computational Science, Okazaki, Japan.

■

REFERENCES

(1) (a) Szabó, K. J.; Wendt, O. F. Pincer and Pincer-Type Complexes: Applications in Organic Synthesis and Catalysis; Wiley-VCH: Weinheim, 2014. (b) Gunanathan, C.; Milstein, D. Bond Activation and Catalysis G


Article

Organometallics

Isomerization. Organometallics 2012, 31, 7359−7367. (e) Chianese, A. R.; Drance, M. J.; Jensen, K. H.; McCollom, S. P.; Yusufova, N.; Shaner, S. E.; Shopov, D. Y.; Tendler, J. A. Acceptorless Alkane Dehydrogenation Catalyzed by Iridium CCC-Pincer Complexes. Organometallics 2014, 33, 457−464. (8) (a) Segawa, Y.; Yamashita, M.; Nozaki, K. Syntheses of PBP Pincer Iridium Complexes: A Supporting Boryl Ligand. J. Am. Chem. Soc. 2009, 131, 9201−9203. (b) Segawa, Y.; Yamashita, M.; Nozaki, K. Diphenylphosphino- or Dicyclohexylphosphino-Tethered Boryl Pincer Ligands: Syntheses of PBP Iridium(III) Complexes and Their Conversion to Iridium-Ethylene Complexes. Organometallics 2009, 28, 6234−6242. (c) Hasegawa, M.; Segawa, Y.; Yamashita, M.; Nozaki, K. Isolation of a PBP-Pincer Rhodium Complex Stabilized by an Intermolecular C-H σ-Coordination as the Fourth Ligand. Angew. Chem., Int. Ed. 2012, 51, 6956−6960. (d) Masuda, Y.; Hasegawa, M.; Yamashita, M.; Nozaki, K.; Ishida, N.; Murakami, M. Oxidative Addition of a Strained C−C Bond onto Electron-Rich Rhodium(I) at Room Temperature. J. Am. Chem. Soc. 2013, 135, 7142−7145. (e) Miyada, T.; Yamashita, M. Oxygenation of a Ruthenium Complex Bearing a PBP-Pincer Ligand Inducing the Formation of a Boronato Ligand with a Weak Ru−O Bond. Organometallics 2013, 32, 5281− 5284. (f) Ogawa, H.; Yamashita, M. Platinum complexes bearing a boron-based PBP pincer ligand: synthesis, structure, and application as a catalyst for hydrosilylation of 1-decene. Dalton Trans. 2013, 42, 625− 629. (g) Miyada, T.; Kwan, H. E.; Yamashita, M. Synthesis, Structure, and Bonding Properties of Ruthenium Complexes Possessing a BoronBased PBP Pincer Ligand and Their Application for Catalytic Hydrogenation. Organometallics 2014, 33, 6760−6770. (h) Ogawa, H.; Yamashita, M. Trial for anti-Markovnikov Hydration of 1-Decene Using Platinum Complexes Bearing a PBP Pincer Ligand, Inducing Alkene Isomerization and Decomposition of PBP Ligand. Chem. Lett. 2014, 43, 664−666. (i) Tanoue, K.; Yamashita, M. Synthesis of Pincer Iridium Complexes Bearing a Boron Atom and iPr-Substituted Phosphorus Atoms: Application to Catalytic Transfer Dehydrogenation of Alkanes. Organometallics 2015, 34, 4011−4017. (j) Kwan, E. H.; Kawai, Y. J.; Kamakura, S.; Yamashita, M. A long-tethered (P-B-P)pincer ligand: synthesis, complexation, and application to catalytic dehydrogenation of alkanes. Dalton Trans. 2016, 45, 15931−15941. (k) Yamashita, M. The Organometallic Chemistry of BoronContaining Pincer Ligands based on Diazaboroles and Carboranes. Bull. Chem. Soc. Jpn. 2016, 89, 269−281. (l) Kwan, E. H.; Ogawa, H.; Yamashita, M. A Highly Active PBP−Iridium Catalyst for the Dehydrogenation of Dimethylamine−Borane: Catalytic Performance and Mechanism. ChemCatChem 2017, 9, 2457−2462. (m) Hill, A. F.; Lee, S. B.; Park, J.; Shang, R.; Willis, A. C. Analogies between Metallaboratranes, Triboronates, and Boron Pincer Ligand Complexes†. Organometallics 2010, 29, 5661−5669. (n) Hill, A. F.; McQueen, C. M. A. Arrested B−H Activation en Route to Installation of a PBP Pincer Ligand on Ruthenium and Osmium. Organometallics 2014, 33, 1977−1985. (o) Lin, T.-P.; Peters, J. C. Boryl-Mediated Reversible H2 Activation at Cobalt: Catalytic Hydrogenation, Dehydrogenation, and Transfer Hydrogenation. J. Am. Chem. Soc. 2013, 135, 15310−15313. (p) Lin, T.-P.; Peters, J. C. Boryl−Metal Bonds Facilitate Cobalt/Nickel-Catalyzed Olefin Hydrogenation. J. Am. Chem. Soc. 2014, 136, 13672−13683. (q) Ganguly, G.; Malakar, T.; Paul, A. Theoretical Studies on the Mechanism of Homogeneous Catalytic Olefin Hydrogenation and Amine−Borane Dehydrogenation by a Versatile Boryl-Ligand-Based Cobalt Catalyst. ACS Catal. 2015, 5, 2754−2769. (r) Curado, N.; Maya, C.; Lopez-Serrano, J.; Rodriguez, A. Boryl-assisted hydrogenolysis of a nickel-methyl bond. Chem. Commun. 2014, 50, 15718−15721. (s) Rios, P.; Curado, N.; LopezSerrano, J.; Rodriguez, A. Selective reduction of carbon dioxide to bis(silyl)acetal catalyzed by a PBP-supported nickel complex. Chem. Commun. 2016, 52, 2114−2117. (t) Ríos, P.; Rodríguez, A.; LópezSerrano, J. Mechanistic Studies on the Selective Reduction of CO2 to the Aldehyde Level by a Bis(phosphino)boryl (PBP)-Supported Nickel Complex. ACS Catal. 2016, 6, 5715−5723. (u) Shih, W.-C.; Gu, W.; MacInnis, M. C.; Timpa, S. D.; Bhuvanesh, N.; Zhou, J.; Ozerov, O. V. Facile Insertion of Rh and Ir into a Boron−Phenyl

H.-C.; Vargas, A. Experimental Assessment of the Strengths of B−B Triple Bonds. J. Am. Chem. Soc. 2015, 137, 1766−1769. (j) Ahuja, R.; Punji, B.; Findlater, M.; Supplee, C.; Schinski, W.; Brookhart, M.; Goldman, A. S. Catalytic dehydroaromatization of n-alkanes by pincerligated iridium complexes. Nat. Chem. 2011, 3, 167−171. (k) NawaraHultzsch, A. J.; Hackenberg, J. D.; Punji, B.; Supplee, C.; Emge, T. J.; Bailey, B. C.; Schrock, R. R.; Brookhart, M.; Goldman, A. S. Rational Design of Highly Active ″Hybrid″ Phosphine−Phosphinite Pincer Iridium Catalysts for Alkane Metathesis. ACS Catal. 2013, 3, 2505− 2514. (l) Yao, W.; Zhang, Y.; Jia, X.; Huang, Z. Selective Catalytic Transfer Dehydrogenation of Alkanes and Heterocycles by an Iridium Pincer Complex. Angew. Chem., Int. Ed. 2014, 53, 1390−1394. (m) Yao, W.; Jia, X.; Leng, X.; Goldman, A. S.; Brookhart, M.; Huang, Z. Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes. Polyhedron 2016, 116, 12−19. (n) Morales-Morales, D.; Redón, R. o.; Yung, C.; Jensen, C. M. Dehydrogenation of alkanes catalyzed by an iridium phosphinito PCP pincer complex. Inorg. Chim. Acta 2004, 357, 2953−2956. (o) Shi, Y.; Suguri, T.; Dohi, C.; Yamada, H.; Kojima, S.; Yamamoto, Y. Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes: Synthesis and Application of Novel 7− 6−7 Ring-Based Pincer Iridium Complexes. Chem. - Eur. J. 2013, 19, 10672−10689. (p) Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.; Warmuth, R.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S. Rational Design and Synthesis of Highly Active Pincer-Iridium Catalysts for Alkane Dehydrogenation. Organometallics 2009, 28, 5432−5444. (q) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Acceptor PCP Pincer Iridium(I) Chemistry: Stabilization of Nonmeridional PCP Coordination Geometries. Organometallics 2011, 30, 697−711. (r) Adams, J. J.; Lau, A.; Arulsamy, N.; Roddick, D. M. Acceptor PCP Pincer Iridium Chemistry: (CF3PCP)IrIII Coordination Properties. Organometallics 2011, 30, 689−696. (6) (a) Gruver, B. C.; Adams, J. J.; Warner, S. J.; Arulsamy, N.; Roddick, D. M. Acceptor Pincer Chemistry of Ruthenium: Catalytic Alkane Dehydrogenation by (CF3PCP)Ru(cod)(H). Organometallics 2011, 30, 5133−5140. (b) Adams, J. J.; Gruver, B. C.; Donohoue, R.; Arulsamy, N.; Roddick, D. M. Acceptor pincer Ru(II) chemistry. Dalton Trans. 2012, 41, 12601−12611. (c) Haibach, M. C.; Wang, D. Y.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. (POP)Rh pincer hydride complexes: unusual reactivity and selectivity in oxidative addition and olefin insertion reactions. Chem. Sci. 2013, 4, 3683−3692. (d) Gruver, B. C.; Adams, J. J.; Arulsamy, N.; Roddick, D. M. Acceptor Pincer Chemistry of Osmium: Catalytic Alkane Dehydrogenation by (CF3PCP)Os(cod)(H). Organometallics 2013, 32, 6468−6475. (e) Zhang, Y.; Fang, H.; Yao, W.; Leng, X.; Huang, Z. Synthesis of Pincer Hydrido Ruthenium Olefin Complexes for Catalytic Alkane Dehydrogenation. Organometallics 2016, 35, 181−188. (f) Cheng, C.; Kim, B. G.; Guironnet, D.; Brookhart, M.; Guan, C.; Wang, D. Y.; Krogh-Jespersen, K.; Goldman, A. S. Synthesis and Characterization of Carbazolide-Based Iridium PNP Pincer Complexes. Mechanistic and Computational Investigation of Alkene Hydrogenation: Evidence for an Ir(III)/Ir(V)/Ir(III) Catalytic Cycle. J. Am. Chem. Soc. 2014, 136, 6672−6683. (g) Bezier, D.; Guan, C.; Krogh-Jespersen, K.; Goldman, A. S.; Brookhart, M. Experimental and computational study of alkane dehydrogenation catalyzed by a carbazolide-based rhodium PNP pincer complex. Chem. Sci. 2016, 7, 2579−2586. (7) (a) Jia, X.; Zhang, L.; Qin, C.; Leng, X.; Huang, Z. Iridium complexes of new NCP pincer ligands: catalytic alkane dehydrogenation and alkene isomerization. Chem. Commun. 2014, 50, 11056− 11059. (b) Brayton, D. F.; Beaumont, P. R.; Fukushima, E. Y.; Sartain, H. T.; Morales-Morales, D.; Jensen, C. M. Synthesis, Characterization, and Dehydrogenation Activity of an Iridium Arsenic Based Pincer Catalyst. Organometallics 2014, 33, 5198−5202. (c) Chianese, A. R.; Mo, A.; Lampland, N. L.; Swartz, R. L.; Bremer, P. T. Iridium Complexes of CCC-Pincer N-Heterocyclic Carbene Ligands: Synthesis and Catalytic C−H Functionalization. Organometallics 2010, 29, 3019−3026. (d) Chianese, A. R.; Shaner, S. E.; Tendler, J. A.; Pudalov, D. M.; Shopov, D. Y.; Kim, D.; Rogers, S. L.; Mo, A. Iridium Complexes of Bulky CCC-Pincer N-Heterocyclic Carbene Ligands: Steric Control of Coordination Number and Catalytic Alkene H


Article

Organometallics

(15) Estimated by using “inversion recovery” method at 22 °C. (16) (a) Morris, R. H. Dihydrogen, dihydride and in between: NMR and structural properties of iron group complexes. Coord. Chem. Rev. 2008, 252, 2381−2394. (b) Crabtree, R. H. Dihydrogen complexes: some structural and chemical studies. Acc. Chem. Res. 1990, 23, 95− 101. (c) Ammann, C.; Isaia, F.; Pregosin, P. S. 1H bridging and terminal hydride T1 values. Comments on classical vs non-classical hydride identification using T1 criteria. Magn. Reson. Chem. 1988, 26, 236−238. (17) Meier, E. B.; Burch, R. R.; Muetterties, E. L.; Day, V. W. A dinuclear rhodium complex with an octahedral rhodium(III) and a square-planar rhodium(I) center. J. Am. Chem. Soc. 1982, 104, 2661− 2663. (18) Lefort, L.; Crane, T. W.; Farwell, M. D.; Baruch, D. M.; Kaeuper, J. A.; Lachicotte, R. J.; Jones, W. D. Synthesis and Reactions of Cp-Linked Phosphine Complexes of Rhodium. Organometallics 1998, 17, 3889−3899. (19) Herde, J. L.; Lambert, J. C.; Senoff, C. V.; Cushing, M. A. Cyclooctene and 1,5-Cyclooctadiene Complexes of Iridium(I). Inorg. Synth. 2007, 18−20. (20) Cramer, R.; McCleverty, J. A.; Bray, J. Di-μ-chlorotetrakis(ethylene)dirhodium(I), 2,4-Pentanedionatobis(ethyl-ene)rhodium(I), and Di-μ-chlorotetracarbonyldirhodium(I). Inorg. Synth. 2007, 14−18. (21) Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses. Nippon Kessho Gakkaishi 2009, 51, 218−224. (22) (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR97: a new tool for crystal structure determination and refinement. J. Appl. Crystallogr. 1999, 32, 115−119. (b) Burla, M. C.; Caliandro, R.; Carrozzini, B.; Cascarano, G. L.; Cuocci, C.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori, G. Crystal structure determination and refinement via SIR2014. J. Appl. Crystallogr. 2015, 48, 306−309. (23) (a) Sheldrick, G. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (b) Sheldrick, G. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (24) Farrugia, L. J. ORTEP-3 for Windows - a version of ORTEP-III with a Graphical User Interface (GUI). J. Appl. Crystallogr. 1997, 30, 565−565. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (26) (a) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (b) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (c) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results obtained with the correlation energy density functionals of becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200−206. (d) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.

Bond, Leading to Boryl/Bis(phosphine) PBP Pincer Complexes. J. Am. Chem. Soc. 2016, 138, 2086−2089. (v) Shih, W.-C.; Gu, W.; MacInnis, M. C.; Herbert, D. E.; Ozerov, O. V. Boryl/Borane Interconversion and Diversity of Binding Modes of Oxygenous Ligands in PBP Pincer Complexes of Rhodium. Organometallics 2017, 36, 1718−1726. (w) Shih, W.-C.; Ozerov, O. V. Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation. Organometallics 2017, 36, 228−233. (x) Shih, W.-C.; Ozerov, O. V. Selective ortho C− H Activation of Pyridines Directed by Lewis Acidic Boron of PBP Pincer Iridium Complexes. J. Am. Chem. Soc. 2017, 139, 17297− 17300. (y) Gratz, M.; Backer, A.; Vondung, L.; Maser, L.; Reincke, A.; Langer, R. Donor ligands based on tricoordinate boron formed by BH-activation of bis(phosphine)boronium salts. Chem. Commun. 2017, 53, 7230−7233. (z) Eleazer, B. J.; Smith, M. D.; Peryshkov, D. V. POBOP pincer complexes of nickel(II): Synthesis and BH activation of the carborane ligand upon oxidation with iodine. J. Organomet. Chem. 2017, 829, 42−47. (aa) Schuhknecht, D.; Ritter, F.; Tauchert, M. E. Isolation and properties of a palladium PBP pincer complex featuring an ambiphilic boryl site. Chem. Commun. 2016, 52, 11823− 11826. (ab) Spokoyny, A. M.; Reuter, M. G.; Stern, C. L.; Ratner, M. A.; Seideman, T.; Mirkin, C. A. Carborane-Based Pincers: Synthesis and Structure of SeBSe and SBS Pd(II) Complexes. J. Am. Chem. Soc. 2009, 131, 9482−9483. (ac) El-Zaria, M. E.; Arii, H.; Nakamura, H. mCarborane-Based Chiral NBN Pincer-Metal Complexes: Synthesis, Structure, and Application in Asymmetric Catalysis. Inorg. Chem. 2011, 50, 4149−4161. (ad) Tsang, M. Y.; Viñas, C.; Teixidor, F.; Planas, J. G.; Conde, N.; SanMartin, R.; Herrero, M. T.; Domínguez, E.; Lledós, A.; Vidossich, P.; Choquesillo-Lazarte, D. Synthesis, Structure, and Catalytic Applications for ortho- and meta-Carboranyl Based NBN Pincer-Pd Complexes. Inorg. Chem. 2014, 53, 9284−9295. (9) Zhu, J.; Lin, Z. Y.; Marder, T. B. Trans influence of boryl ligands and comparison with C, Si, and Sn ligands. Inorg. Chem. 2005, 44, 9384−9390. (10) (a) Asakawa, H.; Lee, K.-H.; Furukawa, K.; Lin, Z.; Yamashita, M. Lowering the Reduction Potential of a Boron Compound by Means of the Substituent Effect of the Boryl Group: One-Electron Reduction of an Unsymmetrical Diborane(4). Chem. - Eur. J. 2015, 21, 4267−4271. (b) Asami, S.-s.; Okamoto, M.; Suzuki, K.; Yamashita, M. A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction with n-Butyllithium To Form a Stabilized Adduct by pπ-pπ Interaction. Angew. Chem., Int. Ed. 2016, 55, 12827−12831. (c) Asami, S.-s.; Ishida, S.; Iwamoto, T.; Suzuki, K.; Yamashita, M. Isolation and Characterization of Radical Anions Derived from a Boryl-Substituted Diphosphene. Angew. Chem., Int. Ed. 2017, 56, 1658−1662. (d) Asami, S.-s.; Suzuki, K.; Yamashita, M. Two-electron Reduction of Diboryldiphosphene: Formation of Intramolecular Charge-transfer Complex of Dianionic B−=P−P = B− Species. Chem. Lett. 2017, 46, 686−689. (11) Wang, D. Y.; Choliy, Y.; Haibach, M. C.; Hartwig, J. F.; KroghJespersen, K.; Goldman, A. S. Assessment of the Electronic Factors Determining the Thermodynamics of “Oxidative Addition” of C−H and N−H Bonds to Ir(I) Complexes. J. Am. Chem. Soc. 2016, 138, 149−163. (12) (a) Kessler, J. A.; Iluc, V. M. Ag(I) and Tl(I) Precursors as Transfer Agents of a Pyrrole-Based Pincer Ligand to Late Transition Metals. Inorg. Chem. 2014, 53, 12360−12371. (b) Venkanna, G. T.; Arman, H. D.; Tonzetich, Z. J. Catalytic C−S Cross-Coupling Reactions Employing Ni Complexes of Pyrrole-Based Pincer Ligands. ACS Catal. 2014, 4, 2941−2950. (13) The broadening factor of 5 Hz would hide cis 31P−31P coupling of ca. 20−40 Hz. (14) It was reported that the phosphorus atoms trans to hydride resonated at higher field than that for the phosphorus atoms trans to lower-trans-influencing ligand. See Dioumaev, V. K.; Procopio, L. J.; Carroll, P. J.; Berry, D. H. Synthesis and Reactivity of Silyl Ruthenium Complexes: The Importance of Trans Effects in C−H Activation, Si− C Bond Formation, and Dehydrogenative Coupling of Silanes. J. Am. Chem. Soc. 2003, 125, 8043−8058. I


Article

Organometallics (27) (a) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (b) Wadt, W. R.; Hay, P. J. Ab initio effective core potentials for molecular calculations. Potentials for main group elements Na to Bi. J. Chem. Phys. 1985, 82, 284−298. (c) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299−310. (d) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calculations; Elsevier: Amsterdam, 1984. (28) Sierraalta, A.; Martorell, G.; Ehrmann, E.; Añez, R. Improvement of scale factors for harmonic vibrational frequency calculations using new polarization functions. Int. J. Quantum Chem. 2008, 108, 1036−1043.

J


Synthesis and Application of Pyrrole-Based PNPâIr Complexes to

Recommend Documents

Synthesis and Application of Pyrrole-Based PNPâIr Complexes to