Selective carbonylation of benzene to benzaldehyde using a phosphorus–nitrogen PN3P–rhodium(I) complex

Chunhui Zhou , Jinsong Hu , Yuan Wang , Changguang Yao , Priyanka Chakraborty , Huaifeng Li , Chao Guan , Mei-Hui Huang and Kuo-Wei Huang *
KAUST Catalysis Center and Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia. E-mail: hkw@kaust.edu.sa

Received 18th August 2018 , Accepted 7th November 2018

First published on 7th November 2018


A PN3P pincer ligand (1) bearing dicyclopentylphosphine substituents reacts with [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) to produce the complex (PN3P)RhCl (2). Treatment of a benzene solution of 2 with KN(SiMe3)2 stimulates a dearomatization process, and C–H activation of benzene is achieved through the rearomatization of the central pyridine ring. This deprotonation/reprotonation of the NH arm of 2 gives the phenyl complex (PN3P)Rh(C6H5) (3). The subsequent introduction of CO gas into 3 yields the benzoyl complex (PN3P)RhCO (C6H5) (4), which can release benzaldehyde upon treatment with diluted HCl solution and regenerates 2.


Introduction

The C–H bond activation and functionalization of both saturated and unsaturated hydrocarbons constitute a remarkably important topic of current chemistry.1 As a simple aromatic aldehyde, benzaldehyde is utilized for a variety of applications in cosmetics, e.g., as a denaturant, a flavoring agent, a fragrance, and an effective precursor for selective hydrogenation into benzyl alcohol and other derivatives.2 The chemical reactivity of benzene is not high compared to other aromatic compounds; hence, the selective transformation into benzaldehyde by direct carbonylation of benzene is an area of great interest.3 For such a process, the carbonyl phosphine metal complexes with d8 Ir(I), Rh(I) and Ru(0) centers are among the most effective under irradiation conditions.4 IrBr(CO)(Ph2PCH2CH2PPh2),5 RhCl(CO)(PPh3)2,6 RhCl(CO)(PMe3)2,7 and Ru(CO)4(PPh3)8 are found to be photochemically reactive for inserting carbon monoxide (CO) directly into the C–H bond of benzene. Besides utilizing these classical d8 metal complexes, processes such as using ionic liquids9 and zeolite systems10 to accomplish the carbonylation of benzene have also been explored. Nevertheless, the transformation of benzene into benzaldehyde under mild conditions with a high selectivity and yield remains challenging.

Pincer complexes are composed of tridentate ligands and metal centers, where the ligands enforce the meridional geometry upon complexation with metals. Compared to other transition metal compounds, the diverse structures of pincer ligands offer a convenient platform to manipulate the steric and electronic properties of the corresponding pincer complexes.11 Among various pincer systems, the pyridine-based ligands have attracted tremendous attention.12 The Milstein group has introduced the concept of dearomatization/rearomatization for a wide variety of dehydrogenative/hydrogenative catalytic reactions and bond activation processes.11c,13 We have also demonstrated that related PN3P pincer complexes containing the NH arms instead of the CH2 spacers (first prepared by Haupt in 198714) undergo similar metal–ligand cooperation (MLC) reactions. Compared to the Milstein system, we have established that this seemingly small change in the arm from CH2 to NH resulted in unique catalytic reactivities,15 and thermodynamic and kinetic properties.16

The neutral complex (PN3P)RhICl reported by our group is capable of activating benzene and H2 effectively under mild conditions with a suitable base.16b The bond activation of C(sp2)–H and H–H through MLC furnished (PN3P)RhI(C6H5) and (PN3P)RhIH. The dearomatized intermediate (PN3P*)RhI could be trapped by CO to form the stable carbonyl complex (PN3P*)RhICO. However, upon our further examination, benzene carbonylation could not be achieved by reacting (PN3P)RhICl with CO gas and benzene together. Recently, Milstein and co-workers reported an analogous PNP–rhodium-hydride complex for a full stoichiometric cycle for the formylation of benzene using a CO2 synthon (Fig. 1).17 While it was not a catalytic system, it illustrated a process of CO2 splitting via MLC. The resulting metal carbonyl complex reacted with benzene to yield a benzoyl complex under UV irradiation. In the presence of p-toluenesulfonic (TsOH), release of benzaldehyde was observed.


image file: c8qo00892b-f1.tif
Fig. 1 Formylation of benzene to produce benzaldehyde based on PNxP–Rh complexes.

In this article, based on our Rh chemistry16b on the C–H activation of benzene, we demonstrate the synthesis and characterization of a new series of PN3P rhodium(I) complexes by replacing the substituent group on the P atoms from tBu (tert-butyl group) to cPe (cyclopentyl). The complex (PN3P)RhCl fulfills the process of benzene carbonylation into benzaldehyde by directly combining benzene and CO gas under mild conditions with a base (Fig. 1). To the best of our knowledge, this is the first example of the selective carbonylation of benzene into benzaldehyde without UV irradiation.

Results and discussion

Synthesis of the PN3P ligand (1)

PN3P ligand 1, with the cyclopentyl substituents attached at the phosphorus atom, was obtained via the modification procedure previously reported.16b The reaction of 2,6-diaminopyridine, triethylamine (Et3N, 2 equiv.), and chlorodicyclopentyl-phosphine (2 equiv.) was performed in dry tetrahydrofuran (THF) at 50 °C under an argon atmosphere, and recrystallization of the saturated pentane solution afforded the targeted PN3P ligand 1. The 31P NMR spectrum of ligand 1 showed a singlet at 38.7 ppm, suggesting that the two P atoms were magnetically equivalent.

Synthesis of the complex (PN3P)RhCl (2)

1 reacted with [Rh(COD)Cl]2 (COD = cyclooctadiene, 0.5 equiv.) under argon in THF to produce (PN3P)RhCl (complex 2). The 31P NMR spectrum displayed a doublet at 91.81 ppm (J = 154.5 Hz, 2P), and a singlet at 4.20 ppm was ascribed to the N–H protons of the arms in the 1H NMR spectrum. Thus, 2 exhibited a symmetric arrangement.

Synthesis of the complex (PN3P)Rh (C6H5) (3)

To examine the reactivity of complex 2 with benzene, a suspension of 2 was treated with KN(SiMe3)2 (1 equiv.) in dry benzene at room temperature. The brown organic suspension in benzene became a homogeneous crimson solution of (PN3P)Rh(C6H5) (3), presumably via a dearomatized intermediate (Scheme 1),16b though it was not detected spectroscopically. Complex 3 was well-characterized by NMR spectroscopy and X-ray diffraction analysis. A doublet at 94.22 ppm (J = 178.2 Hz, 2P) in the 31P NMR spectrum indicated the equivalence of the two P atoms. Suitable crystals of 3 were obtained by slowly evaporating the saturated toluene solution of 3 at room temperature (Fig. 2). The X-ray crystallography analysis revealed that the Rh(I) center was tetracoordinated, and the angles of P1–Rh1–P2 and N2–Rh1–C26 were 164.01(3)° and 177.18(11)°, respectively. The geometry was somewhat distorted square-planar around the Rh(I) center 3. The dihedral angle of the pyridine plane in the PN3P ligand and the incoming phenyl plane was 82.628(101)°, almost perpendicular to the pyridine ring.
image file: c8qo00892b-s1.tif
Scheme 1 Complex 2 reacts with benzene and carbonylation of benzene by inserting CO directly.

image file: c8qo00892b-f2.tif
Fig. 2 Molecular structure of complex 3, with thermal ellipsoids at 30% probability. Selected bond lengths [Å] and bond angles [°]: C4–N1, 1.3677(44); C3–N3, 1.3720(45); Rh1–N2, 2.0956(22); Rh1–C26, 2.0456(27); Rh1–P1, 2.2446(9); Rh1–P2, 2.2395(7); P1–Rh1–P2, 164.01(3); N2–Rh1–C26, 177.18(11); N2–Rh1–P2, 82.27(7); P2–Rh1–C26, 98.00(7); C26–Rh1–P1, 97.99(7); P1–Rh1–N2, 81.75(7).

Inserting CO directly to form complex (PN3P)RhCOC6H5 (4)

Upon introducing CO gas into the benzene solution of complex 3 at room temperature, the benzoyl complex 4 was gradually formed with an NMR conversion of 95% after 3 hours (Scheme 1). The crystal of 4 was also obtained using a similar method to that of complex 3, and the structure was elucidated clearly by X-ray crystallography (Fig. 3). Complex 4 showed a singlet proton signal in the 1H NMR spectrum at 5.42 ppm, consistent with a symmetrical NH proton and indicating that the ligand backbone of complex 4 remained unchanged compared to 3, which was accompanied by a doublet in the 31P NMR spectrum (97.67 ppm, J = 188.8 Hz, 2P). The 1H NMR and 31P {1H} NMR spectra of 4 showed downfield chemical shifts compared to those of 3 upon inserting the CO gas to the Rh-phenyl moiety. The X-ray crystallography analysis of 4 revealed that the angles of N1–Rh1–C25 and P2–Rh1–P3 were 176.74(7)° and 160.531(18)°, respectively, smaller than those of 3, demonstrating a more distorted square-planar geometry around the Rh(I) centre as a result of the CO insertion.
image file: c8qo00892b-f3.tif
Fig. 3 Molecular structure of 4, with thermal ellipsoids at 30% probability. Selected bond lengths [Å] and bond angles [°]: C1–N2, 1.3784(26); C5–N3, 1.3590(25); Rh1–N1, 2.1160(17); Rh1–C25, 1.9766(21); Rh1–P2, 2.2571(4); Rh1–P3, 2.2382(4); N1–Rh1–C25, 176.74(7); P2–Rh1–P3, 160.531(18); N1–Rh1–P2, 81.66(4); P2–Rh1–C25, 98.35(6); C25–Rh1–P3, 98.07(6); P3–Rh1–N1, 81.35(4).

Releasing benzaldehyde from complex 4

With complex 4 in hand, we sought to complete a stoichiometric process via the release of benzaldehyde from the benzoyl complex 4. We introduced different gases into complex 4 (H2, CO2, or a combination thereof), but no free benzaldehyde was detected. Different protic agents such as alcohols (methanol, ethanol, or trifluoroethanol) and water were tested, but still no acylated product was obtained from complex 4. Strong organic protic acids, such as trifluoromethanesulfonic acid, diphenylacetic acid, and p-toluenesulfonic acid (TsOH) could successfully achieve a partial discharge of benzaldehyde from 4, with approx. 30–50% yields. Finally, it was discovered that a diluted HCl solution (1 mmol mL−1) served as the best choice for this system, with a yield of 90%.

Upon mixing equimolar amounts of diluted HCl and a benzene solution of 4, an obvious color change occurred from reddish brown to light yellow. More interestingly, NMR spectroscopy revealed that a compound was produced, which exhibited the same 1H and 31P NMR signals as those of 2. After removing the benzaldehyde product, complex 2 was isolated in over 80% yield and upon treatment with KN(SiMe3)2 (1 equiv.) in benzene, complex 3 could be formed.

Even though a catalytic process was not fulfilled yet, a synthetic cycle was realized by combining the individual stoichiometric steps without the need of irradiation for benzene activation in other analogous systems of d8 Ir(I), Rh(I), and Ru(0) carbonyl phosphine complexes and Milstein's PNP rhodium(I) complex (Scheme 2). An overall yield of 90% of benzaldehyde was achieved with over 80% of Rh–Cl 2 recovered.


image file: c8qo00892b-s2.tif
Scheme 2 Stoichiometric cycle of the formylation of benzene with CO based on complex 2.

Conclusion

We have demonstrated that the PN3P-pincer complex (PN3P)RhCl (2) was capable of activating C–H in benzene to form the phenyl complex (PN3P)Rh(C6H5) (3) at room temperature with KN(SiMe3)2 as the base. More interestingly, a benzoyl complex 4 was obtained by treating a benzene solution of complex 3 with CO gas. In a diluted-HCl environment, a high yield of 90% benzaldehyde was obtained with the concomitant regeneration of complex 2. Thus, a synthetic cycle for the selective carbonylation of benzene utilizing CO was achieved. To the best of our knowledge, this is the first selective carbonylation of benzene into benzaldehyde accomplished by directly inserting carbon monoxide without irradiation. The extension of the substrate scope to afford different aldehydes and the potential transformation of this synthetic cycle into a catalytic reaction are currently under investigation in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the King Abdullah University of Science and Technology (KAUST) is acknowledged. This work was dedicated to Professor Xiyan Lu on the occasion of his 90th birthday.

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Footnote

Electronic supplementary information (ESI) available: Detailed descriptions of the preparation and characterization of compounds 1–4. CCDC 1846987 and 1846989. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qo00892b

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