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Rhodium Complexes Bearing PAlP Pincer Ligands Naofumi Hara, Teruhiko Saito, Kazuhiko Semba, Nishamol Kuriakose, Hong Zheng, Shigeyoshi Sakaki, and Yoshiaki Nakao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04199 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018
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Rhodium Complexes Bearing PAlP Pincer Ligands Naofumi Hara,† Teruhiko Saito,† Kazuhiko Semba,† Nishamol Kuriakose,‡ Hong Zheng,‡ Shigeyoshi Sakaki,‡,* and Yoshiaki Nakao†,* of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan, and ‡Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103, Japan †Department
Supporting Information Placeholder Abstract: We report rhodium complexes bearing PAlP pincer ligands with an X-type aluminyl moiety. IR spectroscopy and singlecrystal X-ray diffraction analysis of a carbonyl complex exhibit the considerable -donating ability of the aluminyl ligand, whose Lewis acidity is confirmed through coordination of pyridine to the aluminum center. The X-type PAlP–Rh complexes catalyze C2selective mono-alkylation of pyridine with alkenes. Group 13 X-type ligands that form a σ-bond to a transition metal center have received considerable attention as they formally possess a lone pair of electrons and a vacant π-orbital, resulting in unique bifunctional nucleophilic/electrophilic properties1 to cooperate with transition metal centers. Recently, such reactive species have been integrated into multi-dentate supporting ligands for transition metals.2 For example, Yamashita and Nozaki reported the synthesis and the reactivity of group 9 transition metal complexes that bear tridentate pincer-type ligands with an X-type boryl donor (PBP) ligand [Chart 1(a)].2a–d Remarkably, the corresponding PBP–Rh complex can activate carbon–carbon bonds of cyclobutanone derivatives at room temperature, demonstrating the high reactivity of the rhodium center resulting possibly from the highly electron-donating nature of the PBP pincer ligand.2d They also revealed a strong trans influence of the PBP ligand by X-ray diffraction analysis. Furthermore, nickel and cobalt complexes bearing the ligand have been shown to catalyze hydrogenation of olefins through a unique mode of H2 activation by the B–Co/Ni bonds.2e Compared with the rich chemistry of boryl ligands, analogous X-type aluminyl ligands have received less attention, probably due to prospective synthetic difficulties associated with their highly ionic character.3 To the best of our knowledge, merely four examples of X-type aluminyl ligands have been reported [Chart 1(b)].4 A very recent example by Takaya and Iwasawa is particularly intriguing because the palladium complex bearing an aluminyl pincer ligand shows very high activity towards catalytic reduction of CO2.4d The σ-donation3b of aluminyl ligands in combination with Lewis acidity possibly higher than that of boryl ligand should enable cooperative and/or site-selective activation of organic compounds by transition metal centers to result in unique and useful transformations. Aiming at the synthesis of such transition metal complexes, we designed a new diphosphine ligand with an X-type aluminyl moiety bearing a tridentate NNN backbone (PAlP), which was based on related triphosphine ligands introduced by Lu.5a This ligand should be able to stabilize the highly ionic aluminum moiety, considering that its structure should be sufficiently flexible to accommodate both tetra- and pentacoordinate aluminum species. Herein, we report the synthesis, characteristics and reactivity of rhodium complexes bearing an Xtype PAlP pincer ligand [Chart 1(c)].5
Chart 1. Transition Metal Complexes with X-type Group 13 Ligands.
The reaction of diphosphine 1 with 2.2 equivalents of tBuLi in Et2O at room temperature afforded lithiated 1, which was subsequently added to a suspension of AlCl3 in toluene to furnish aluminum-containing diphosphine 2 in 80% yield (Scheme 1). The single-crystal X-ray diffraction analysis of 2 showed that the aluminum center adopted distorted trigonal bi-pyramidal geometry composed of three nitrogen, one chlorine, and one phosphorus atoms. A resonance at 10.6 ppm was observed in 31P{1H} NMR at room temperature in C6D6 due to a fast equilibrium between coordination and dissociation of the phosphines to the aluminum center.6 The reaction between 2 and 0.50 equivalents of [Rh(nbd)(μCl)]2 (nbd = 2,5-norbornadiene) at 80 °C afforded Al/Rh heterobimetallic complex 3 (Scheme 1). 31P{1H} NMR of 3 in CDCl3 afforded two broad signals at room temperature and four magnetically inequivalent phosphorus signals at 60.0/68.7 ppm (JP–Rh = 163/166, JP–P = 28/26 Hz, major) and 57.5/69.6 ppm (JP–Rh = 161/163, JP–P = 31/28 Hz, minor) at –40 °C.7 These results indicated that there was a slow equilibrium in an NMR time-scale. DFT calculations suggested they were derived from two unsymmetrical structures of 3, which could be interconverted because of a low energy difference of 2.7 kcal/mol (Figure S4). The crystal structure of 3 was unequivocally determined by X-ray diffraction analysis, demonstrating its symmetrical structure in a solid state due possibly to better packing. The Al–Rh bond [2.5487(8) Å] is longer than that of a reported rhodium complex bearing a typical Z-type aluminum ligand (2.425 Å).5b The Wiberg bond index of the Al–Rh bond in 3 was calculated to be 0.27, which could support the bonding interaction but was smaller than that (0.39) of the reported complex.5b The relatively smaller value can be ascribed to charge-transfer interaction between two nitrogen and aluminum, increasing the electron density of aluminum to weaken the Al–Rh bond (Figure S5 and Table S1). Besides the Wiberg bond index, the geometry of aluminum and rhodium centers could also support the bonding interaction to conclude that 3 has a Z-type aluminum center.
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Scheme 1. Preparation of Rhodium Complex Bearing an X-type Aluminyl Ligand and Crystal Structure of 2 and 3
forded dicarbonyl complex 5 (Scheme 2). 13C{1H} NMR showed two kinds of doublets of triplets at 202.3 ppm (dt, JRh–C = 73 Hz, JP–C = 20 Hz) and 207.7 ppm (dt, JRh–C = 58 Hz, JP–C = 12 Hz). The IR spectroscopy of 5 featured two absorption bands at 1973 cm–1 and 1907 cm–1, which were assigned to the carbonyl ligands. These suggested high electron density of the rhodium center in 5, compared with carbonyl vibration frequencies reported of dicarbonyl PSiP–rhodium (νCO = 2025/1974 cm–1)2f and monocarbonyl PBP-rhodium (νCO = 1933 cm–1).2c The solid structure of 5 determined by single-crystal X-ray diffraction analysis again revealed the X-type coordination of the aluminyl ligand. The aluminum and rhodium centers adopt distorted tetrahedral and distorted trigonal bi-pyramidal geometries, respectively. The Al–Rh bond length [2.4378 Å] of 5 is considerably shorter than that of 3, and the Wiberg bond index of 5 (0.47) is larger than that of 3 (0.27).9 The Rh–C1 distance is 1.930 Å, suggesting that the PAlP ligand has almost the same trans-influence as the PSiP ligand, in which the distance of Rh–CO bond trans to silyl is reported to be 1.9397(3) Å.2f Scheme 2. Synthesis and Crystal Structure of 5
Crystal structures of 2 and 3 (atomic displacement parameters set at 30% probability; all hydrogen atoms and solvent molecules are omitted for clarity). See Table S7 for details. Reduction of 3 with 2.0 equivalents of KC8 in the presence of 3.0 equivalents of norbornadiene afforded rhodium complex 4 (Scheme 1),8 in which the aluminum center served as an X-type aluminyl ligand. A signal at ppm (JP–Rh = 153 Hz) was majorly observed in 31P{1H} NMR, while unidentified signals at ppm (JP–Rh = 140, 144 Hz) were also noted as minor components, which could be a structural or conformational isomer of 4 (vide infra). 1H NMR showed two kinds of olefinic protons of the coordinating norbornadiene at 3.04 and 5.32 ppm, suggesting monodentate coordination. Unfortunately, the single crystals of 4 suitable for X-ray diffraction analysis could not be obtained. Thus, we conducted DFT calculations to gain some more insights into the structural and electronical characters of 4. A calculated structure of 4 exhibited a substantial bonding overlap between the rhodium d and the lone pair orbitals of aluminum (Figure S13), suggesting a covalent bond between them. It was also supported by the Wiberg bond index (0.45).9 As the d orbital (–5.4 eV) of neutral [Rh(nbd)] is energetically located below that of the valence orbital of aluminum (–3.4 eV) of the PAlP ligand, the Al–Rh bond is polarized (Al+/Rh−) (Figure S14). This coordination mode is unusual and thus represents another characteristic of the complex. It should furthermore be noted that the coordinating double bond of the norbornadiene ligand is not located trans to aluminum (side view of 4 in Figure S17), but above it, which is uncommon for such diene ligands due probably to the transinfluence by aluminum. The PiPr2 moieties are located not to be trans each other, with the PRhP angle being 107.92° (Figure S13). This should be attributed to geometrical flexibility imposed by the Al–N–CH2–P chain structure. As the coordinating double bond prefers strong -back donation from the rhodium(I) center, it occupies the coordination site trans to the phosphines to overlap well with the d orbital, which is destabilized by two phosphines bearing the narrow PRhP angle.10 We then investigated the derivatization of 4 to further gain experimental evidences and characteristics of X-type PAlP–Rh complexes. Exposure of 4 under a CO atmosphere (1.0 atm) af-
Crystal structure of 5 (atomic displacement parameters set at 30% probability; all hydrogen atoms and solvent molecules are omitted for clarity). See Table S8 for details. To further understand the electronic character of the X-type PAlP pincer ligand, we conducted a series of DFT calculations. Initially, we examined L1 as a model for the PAlP ligand, in which the PiPr2 moieties were omitted to focus on the frontier orbitals on the aluminum atom [Figure 1(a)]. The HOMO of L1 is located on the aluminum center [Figure 1(b)] and corresponds to a lone pair orbital.11 The empty 3p orbital on the aluminum atom, which is oriented perpendicular to the N1AlN3 plane, is identified as the LUMO+9, which participates in the coordination of Lewis bases (vide infra).12 Another characteristic is the Al+–N– bonds, which are heavily polarized due to the large difference of electronegativity between aluminum and nitrogen [Figure 1(c)]. Considering these aspects, L1 should be best represented by its canonical diamido–Al+ resonance structure [Figure 1(d)]. A similar electronic structure was previously proposed for the anionic fivemembered aluminum(I) heterocycle [: Al{N(H)C(H)}2].3c,13
Figure 1. Coordination geometry (a), frontier orbitals (b), electron distribution (c), and dominant canonical form (d) for the anionic aluminum model ligand L1.
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Journal of the American Chemical Society To confirm the Lewis acidity of the aluminum in the X-type PAlP ligand, the reaction of 4 with pyridine was conducted. Small broad multiplet signals were observed in the 31P{1H} NMR, and the subsequent addition of HSiEt3 to the mixture afforded rhodium complex 6, that was characterized by 1H, 13C and 31P NMR spectroscopy and single-crystal X-ray diffraction analysis, through the oxidative addition of the C(2)–H bond of pyridine coordinating to the aluminum center (Scheme 3). Hydrosilylation and hydropyridylation (vide infra) of the norbornadiene ligand were also detected by a GC-MS analysis of the reaction mixture. Similar reaction was recently reported by Ozerov with an iridium complex bearing an X-type boryl ligand.2g Rhodium hydride was observed at –7.17 ppm (td, JP–H = 24 Hz, JRh–H = 11 Hz) in 1H NMR and 2048 cm–1 in IR spectrums.14 13C{1H} NMR revealed a resonance at 224.6 ppm (dt, JRh–C = 34 Hz, JP–C = 9.3 Hz,), which was assigned to the pyridyl C-2 bonding to the rhodium center. Finally, the crystal structure of 6 was confirmed by X-ray diffraction analysis. The formation of 6 reveals that the PAlP pincer ligand has enough Lewis acidity to interact with a Lewis base and that the rhodium center is electron-rich enough to activate a proximal C–H bond. A pure sample of 6 was treated with norbornadiene to show NMR spectra similar to those obtained with the reduction of 3 (Eq. S3).
Scheme 4. C2-selective mono-alkylation of pyridine catalyzed by 6
Scheme 3. Synthesis and Crystal Structure of 6
ASSOCIATED CONTENT
aDetermined
by 1H NMR.
In conclusion, we have successfully synthesized and characterized rhodium complexes bearing the X-type PAlP ligand, demonstrating that the newly developed ligand has moderate Lewis acidity as well as possible -donating nature to enable a catalytic C2-selective mono-alkylation of pyridine with alkenes through the oxidative addition of C(2)–H bond to rhodium(I). Further studies on the development of other catalytic reactions are ongoing in our laboratory.
Supporting Insformation: Detailed experimental procedures, as well as spectroscopic and analytical data. CIF files containing crystallographic data for 2, 3, 5, and 6. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Crystal structure of 6 (atomic displacement parameters set at 30% probability; all hydrogen atoms and solvent molecules are omitted for clarity). See Table S8 for details. The observed stoichiometric oxidative addition of C(2)–H bond of pyridne prompted us to investigate possible C2-selective functionalizations of pyridine by 6 as a catalyst. As a model reaction, we conducted the reaction of pyridine with alkenes to expect the C2-selective alkylation,15 which generally requires a substituent at the C6 position to hamper deactivation of metal catalysts by coordination of pyridine.15a Meanwhile, the reaction with parent pyridine often gives a mixture of C2- and C6-dialkylation products with a limited scope of alkenes.15b,15c The reaction of pyridine (7) with styrene (8a) in the presence of 5 mol% of 6 at 110 °C for 20 h afforded a mixture of C2-alkylated products (linear/branch = 1/4) in 80% yield (Scheme 4). Although the yields of the corresponding products were moderate, 1-octene (8b) and trimethylvinylsilane (8c) were also viable substrates to give linear alkylation products.16 Trace amounts of dialkylation products were observed only in the case of 8a. Use of complex 2 and 3 or [RhCl(nbd)]2, in combination with several different phosphine ligands and AlEt2Cl as an external Lewis acid, did not afford the product, whereas 4 gave alkylation products in similar yields (Table S2–S4). NMR studies and the reaction using 7-d5 suggested that 6 was the resting state and that the exclusive C(2)–H bond activation was followed by alkene insertion in a reversible manner (See Supporting Information) .
Corresponding Author
[email protected] [email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the “JST CREST program Grant Number JPMJCR14L3 in Establishment of Molecular Technology towards the Creation of New Functions”
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8) We failed to obtain complexes similar to 4 through the reduction in the presence of other alkene ligands such as 1,5-cyclooctadiene and cyclooctene. 9) Wiberg bond indexes of complex 4, 5, and 6 are shown in Figure S17. 10) (a) Hoffmann, R.; Chen, M. M.–L.; Thorn, D. L. Inorg. Chem. 1977, 16, 503. (b) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J. Am. Chem. Soc. 1979, 101, 3801. (c) Sakaki, S.; Hori, K.; Ohyoshi, A. Inorg. Chem. 1978, 17, 3183. (d) Sakaki, S.; Tsuru, N.; Ohkubo, K. J. Phys. Chem. 1980, 84, 3390. (e) Sakaki, S.; Mizoe, N.; Musashi, Y.; Biswas, B.; Sugimoto, M. J. Phys. Chem. A 1998, 102, 8027. 11) DFT calculations were carried out using the B3PW91-D3 functional for geometry optimizations, and the wB97XD functional for the evaluation of the bonding situation and the relative stability. 12) Other HOMO and LUMO orbitals of L1 are shown in Figures S15 and S16. 13) For a very recent study on aluminyl anion, see: Hicks, J.; Vasko, P.; Goicoechea, J. M.; Aldridge, S. Nature 2018, 557, 92. 14) For reported NMR and/or IR spectrum of rhodium hydride complexes, see: (a) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J. J. Am. Chem. Soc. 1987, 109, 2803. (b) Nishihara, Y.; Takemura, M.; Osakada, K. Organometallics 2002, 21, 825. (c) Nozaki K.; Matsuo, T.; Shibahara, F.; Hiyama, T. Organometallics 2003, 22, 594. 15) (a) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2007, 129, 5332. (b) Guan, B.-T.; Hou, Z. J. Am. Chem. Soc. 2011, 133, 18086. (c) Tran, G.; Hesp, K. D.; Mascitti, V.; Ellman, J. A. Angew. Chem. Int. Ed. 2017, 56, 5899. 16) Other alkenes including cyclohexene and methyl acrylate and 2picoline did not participate in the alkylation reaction, whereas 3picoline gave an alkylation product in poor yield. Studies to further optimize the catalyst structure and expand the substrate scope are under investigation.
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