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Article

Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation

by
Xiaofan Jia
,
Songyuan Tian
,
Philip J. Shivokevich
,
W. Dean Harman
,
Diane A. Dickie
and
T. Brent Gunnoe
*
Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA
*
Author to whom correspondence should be addressed.
Inorganics 2022, 10(6), 76; https://doi.org/10.3390/inorganics10060076
Submission received: 10 May 2022 / Revised: 23 May 2022 / Accepted: 27 May 2022 / Published: 31 May 2022
(This article belongs to the Special Issue Inorganics: 10th Anniversary)
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Versions Notes

Abstract

:
Ruthenium(II) complexes with the general formula TpRu(L)(NCMe)Ph (Tp = hydrido(trispyrazolyl)borate, L = CO, PMe3, P(OCH2)3CEt, P(pyr)3, P(OCH2)2(O)CCH3) have previously been shown to catalyze arene alkylation via Ru-mediated arene C–H activation including the conversion of benzene and ethylene to ethylbenzene. Previous studies have suggested that the catalytic performance of these TpRu(II) catalysts increases with reduced electron-density at the Ru center. Herein, three new structurally related Ru(II) complexes are synthesized, characterized, and studied for possible catalytic benzene ethylation. TpRu(NO)Ph2 exhibited low stability due to the facile elimination of biphenyl. The Ru(II) complex (TpBr3)Ru(NCMe)(P(OCH2)3CEt)Ph (TpBr3 = hydridotris(3,4,5-tribromopyrazol-1-yl)borate) showed no catalytic activity for the conversion of benzene and ethylene to ethylbenzene, likely due to the steric bulk introduced by the bromine substituents. (Ttz)Ru(NCMe)(P(OCH2)3CEt)Ph (Ttz = hydridotris(1,2,4-triazol-1-yl)borate) catalyzed approximately 150 turnover numbers (TONs) of ethylbenzene at 120 °C in the presence of Lewis acid additives. Here, we compare the activity and features of catalysis using (Ttz)Ru(NCMe)(P(OCH2)3CEt)Ph to previously reported catalysis based on TpRu(L)(NCMe)Ph catalyst precursors.

Graphical Abstract

1. Introduction

Alkyl arenes serve as precursors for a wide range of products, including polymers, pharmaceuticals, and surfactants. For example, ethylbenzene is produced on a scale of approximately 40 million tons each year [1]. The current industrial synthesis of alkyl arenes is accomplished either by a traditional Friedel–Crafts alkylation, which is catalyzed by a Lewis acid (e.g., AlCl3) in the presence of a Brønsted acid (e.g., HF), or by using an acidic zeolite catalyst [2,3,4,5,6]. Due to the nature of the acid-mediated electrophilic arene substitution reaction, important reaction outcomes are dictated by the mechanism of the reaction. For example, the alkyl arene product is generally more reactive than the arene substrate, which can result in polyalkylation at even low or modest conversions [3]. Moreover, reactions using α-olefins exclusively give products with x-aryl alkanes where x ≥ 2. The selective synthesis of 1-aryl alkanes is not viable with current commercial catalytic processes for arene alkylation [7,8,9,10].
Catalytic arene alkylation has been reported using molecular complexes and homogeneous catalysts based on iridium [11,12], platinum [13,14,15,16,17,18,19], and ruthenium [20,21,22,23,24,25,26,27,28], and direct arene alkenylation has been reported using rhodium [29,30,31,32,33,34,35,36,37,38,39], ruthenium [40,41], and palladium [42,43] catalysts. These catalytic processes provide routes for the synthesis of alkyl and alkenyl arenes that provide complementary selectivity to acid-catalyzed arene alkylation reactions [3]. Recent success for Rh-catalyzed arene alkenylation to give anti-Markovnikov products and the newly reported Ni catalysis by the Hartwig and Eisenstein groups for the formation of 1-aryl alkanes or alkenes, which we have termed super linear alkyl benzenes to differentiate from linear alkyl benzenes that are linear alkanes with internal phenyl substitution, suggest significant opportunities for new selectivity using molecular transition metal-based catalysts [7,9,10,37,44].
We have reported a series of studies on arene alkylation using Ru(II) catalyst precursors with the general formula TpRu(L)(NCMe)Ph (L = CO, PMe3, P(OCH2)3CEt, P(pyr)3 or P(OCH2)2(O)CCH3; Tp = hydridotris(pyrazolyl)borate) including the catalytic conversion of ethylene and benzene to ethylbenzene. In these studies, we have discovered that the electron density of the Ru center has a significant influence on the catalyst performance and, in particular, catalyst longevity [25]. We have demonstrated that more strongly electron-withdrawing ligands L (e.g., L = CO) for catalyst precursors of the type TpRu(L)(NCMe)Ph increase catalyst longevity. Moreover, we have demonstrated that a cationic Ru(II) complex gives an increase in turnover number compared to catalysis with a closely related charge neutral Ru(II) catalyst precursor [21].
We sought to probe additional improvements in Ru catalyzed arene alkylation by introducing new ligands that reduced Ru-centered electron density (Scheme 1). Previously, our group determined and compared the electron density of the Ru(II) center using Ru(III/II) redox potentials from cyclic voltammetry [23,24,25]. A variety of ligands, including carbon monoxide, phosphines (PMe3, P(Pyr)3 where Pyr = N-pyrrolyl), and phosphites (P(OCH2)3CEt, P(OCH2)2(O)CCH3) with varying steric and electronic structures were installed as the L ligand on catalyst precursors of the type TpRu(L)(NCMe)Ph [26]. The most electron-poor complex among the series, TpRu(CO)(NCMe)Ph, proved to be the longest-lived catalyst [27]. The linear nitrosyl ligand, formally NO+, is an analog of CO that is generally considered to be a stronger π-acceptor than CO. Thus, we considered that the cationic complex [TpRu(NO)(NCMe)Ph]+ would have a more electron-deficient Ru(II) center than TpRu(CO)(NCMe)Ph, and may potentially lead to enhanced catalytic performance.
We have demonstrated that a cationic Ru(II) complex supported by a tris(pyrazolyl)alkane ligand [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] (HC(pz5)3 = tris(5-methyl-pyrazolyl)methane; BAr’4 = tetrakis [3,5-bis(trifluoromethyl)phenyl]borate) successfully increased the TONs (turnover numbers) for an ethylene hydrophenylation over 30-fold compared to TpRu(P(OCH2)3Et)(NCMe)Ph [21,26]. We found that it was necessary to install methyl groups into the pyrazolyl-alkane ligand to inhibit Ru–Npyrazolyl bond cleavage and intramolecular pyrazolyl C–H activation [45].
With the success of [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] for catalytic hydrophenylation of ethylene, we sought to explore related motifs that might provide reduced electron density at Ru(II). For example, tris(triazolyl)borate (Ttz) ligands have a similar coordination mode to Tp and HC(pz5)3 [46,47,48], but, compared with the trispyrazolyl borate/methane ligands, the more electronegative nitrogen on the triazole ring may decrease the donor ability of the ligand and lead to a more electron-deficient Ru(II) complex. Moreover, acid additives might bind to a free nitrogen atom of the tris(triazolyl)borate ligand to further reduce its donor ability. Moreover, installation of inductively-electron-withdrawing groups (e.g., Br) on the pyrazole ring provides another strategy to obtain electron-deficient Ru(II) complexes. Thus, we considered that by replacing the hydrogen atoms on the pyrazole rings of Tp by bromine, TpBr3, could reduce the donor ability, and allow access to less electron-rich Ru(II) complexes than TpRu(L)(NCMe)Ph (Scheme 1).

2. Results and Discussion

2.1. Attempted Synthesis of Complexes That Provide Access to [TpRu(NO)Ph]+

We attempted to synthesize a precursor to [TpRu(NO)(Ph)]+ by first preparing TpRu(NO)(OTf)Ph (OTf = trifluoromethanesulfonate) as shown in Scheme 2. TpRu(NO)Cl2 (1) was synthesized following a reported procedure with slight modification (see Section 4) [49,50]. The dichloride complex 1 was characterized by IR and 1H NMR spectroscopy. The IR spectrum of 1 exhibited an intense absorption at 1898 cm−1, which is due to NO stretching, and in the 1H NMR spectrum of 1, resonances corresponding to the three Tp-pyrazolyl groups in 1 appear as two sets of signals with a 1:2 integration ratio, which is consistent with the expected mirror plane of symmetry (Figure S1). The complex TpRu(NO)Ph2 (2) was synthesized based on a reported procedure [50] from the reaction between 1 and diphenyl zinc in THF with a 54% isolated yield. The diphenyl complex 2 was characterized by IR and 1H NMR spectroscopy (Figure S2). The IR spectrum of 2 exhibited an intense band at 1826 cm−1, indicating the retention of the nitrosyl ligand. The νNO of 2 is higher energy than that of Cp*Ru(NO)Ph2 (Cp * = pentamethylcyclopentadiene; 1755 cm−1) [51], demonstrating a smaller degree of π-backdonation from Ru to the NO+ ligand by the Ru(II) center supported with Tp [52].
Crystals obtained by the slow diffusion of hexanes into a methylene chloride (DCM) solution of 2 were suitable for a single crystal X-ray diffraction study, and the resulting solid-state structure of the complex is shown in Figure 1. However, at room temperature, both solid 2 and a DCM solution of 2 decompose to form a black solid with biphenyl observed as a decomposition product by 1H NMR spectroscopy (Scheme 3; Figure S3). Although a detailed reaction pathway for and the final Ru product from the conversion of 2 to form biphenyl was not elucidated, it is possible that an initial reductive elimination of two phenyl groups initiates the decomposition of the Ru complex 2 (Scheme 3). It has been reported by Bergman and coworkers that heating Cp*Ru(NO)Ph2 in benzene produces biphenyl and the NO-bridged Ru complexes [Cp*Ru(μ-NO)]2 and [Cp*Ru(μ-NO)(Ph)]2 (Scheme 3) [51]. With a structurally more bulky, but electronically less donating Tp ligand, complex 2 might be more likely to undergo the reductive elimination reaction than Cp*Ru(NO)Ph2 under similar conditions [53].
Although at room temperature, complex 2 is not stable, it can be stored at −40 °C with no biphenyl formation after at least 2 days. Adding a cold DCM solution of 0.9 equivalents of HOTf into a DCM solution of 2 at −78 °C under inert atmosphere results in a color change from yellow to orange, which likely results from protonation of one of the phenyl groups to form TpRu(NO)(Ph)(OTf) (3). Complex 3 was isolated as a tan solid by precipitation at low temperature (−78 °C) with an 80% isolated yield (Scheme 2). The 1H NMR spectrum of 3 is consistent with the formation of an asymmetric complex with nine resonances due to the Tp ligand (Figure S4).
Unfortunately, the replacement of a phenyl group with OTf to form complex 3 decreases the stability of the Ru complex. The decomposition of 3 is faster than 2, and after just a few minutes at room temperature, a pink CDCl3 solution of 3 turns dark brown. After 10 min at room temperature, the 1H NMR spectrum (CDCl3) of 3 reveals multiple new resonances due to decomposition (Figure S5). Thus, the instability of complex 3 prevented studies of catalytic olefin hydroarylation.

2.2. Ruthenium(II) Complexes Supported by TpBr3 and Ttz Ligands

We synthesized and isolated (TpBr3)Ru(P(OCH2)3CEt)(NCMe)Ph (4) and (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5) (Scheme 4). In order to confirm the successful installation of the phenyl ligand in complex 4, the reaction of 4 with a Brønsted acid was performed based on the assumption that the phenyl group could be protonated to form free benzene (Scheme S1). Adding one equivalent of trifluoroacetic acid to a CDCl3 solution of 4 resulted in the disappearance of all five phenyl resonances (Figure S12), and the concomitant formation of a resonance consistent with the formation of free benzene at 7.35 ppm, which provided further confirmation of the identity of complex 4.
For TpRu(L)(NCMe)Ph complexes, we observed that rapid rotation of the phenyl ligand on the NMR timescale at room temperature resulted in time averaging of the ortho and meta hydrogen atoms. As a result, the phenyl group in these Ru(II) complexes resonate as three peaks in 1H NMR spectrum (Figure 2) [21,26] In contrast, the room temperature 1H NMR spectrum of complex 4 shows restricted rotation of the phenyl ligand where the five hydrogen atoms are not equivalent (i.e., five distinct resonances are observed for the phenyl ligand of (4)). Variable temperature NMR was used to probe the rotation of the Ru–Ph bond in 4.
The 1H NMR spectra of complex 4 in DMSO-d6 were acquired at temperatures between 20 °C and 100 °C (Figure S13). Four resonances due to the ortho and meta-H atoms are observed at low temperature, but these resonances broaden upon heating, indicating a likely increase in the rate of rotation of the phenyl ligand. At room temperature, the chemical shift difference between the two doublets for the ortho protons in 4 is 870 Hz, and the difference between the triplets for the meta protons is 150 Hz. As the temperature is raised from 20 °C to 100 °C, the pairs of signals for the ortho and meta protons broaden and average to a broad peak at 6.60 ppm. Based on the slow exchange chemical shift difference and coalescence temperature (Tc) of 80 °C for the resonances of the ortho protons, a ΔG⧧ of 14.3 kcal/mol is calculated for the activation barrier for phenyl rotation.
The hydrophenylation of ethylene was attempted using 4 as the catalyst under different ethylene pressures and reaction temperatures. Under all conditions (Table S1), no evidence of catalytic activity was observed. The lack of reactivity is likely due to the large sterically bulky bromine substituents on the Tp ligand, which might prevent olefin coordination.
The 1H NMR spectrum of complex 5 is shown in Figure S8. There are six resonances due to the three triazolyl groups, as expected for an asymmetric complex. The cyclic voltammetry of complex 5 shows a reversible Ru(III/II) oxidation at 0.80 V (vs. NHE), which is a +0.25 V shift compared to the previously reported complex TpRu(P(OCH2)3CEt)(NCMe)Ph (Ru(III/II) E1/2 = 0.55 V) [25], indicating the extra N atom on the nitrogen heterocycle ring of complex 5 decreases the donor ability of the poly(triazolyl) ligand compared to Tp, and results in a less electron-rich Ru center. The Ru(III/II) redox potential of 0.80 V for 5 is nearly the same (only 0.02 V more negative) as the Ru(III/II) potential observed for the cationic Ru(II) complex [(HC(pz5)3Ru(P(OCH2)3)CEt)(NCMe)Ph][BAr’4] [21], which is reported to be the longest lived of our Ru(II) catalysts for ethylene hydrophenylation [21].
Complex 5 was tested as a catalyst for ethylene hydrophenylation reactions. Due to the poor solubility of complex 5 in benzene, catalysis was performed using a low catalyst loading where we observed full dissolution under catalytic conditions. The results of catalysis at different temperatures are shown in Figure 3. At 75 °C, the rate of the reaction is slow, and the catalytic turnovers (TOs) reached ~8 after 44 h, after which, time catalysis ceased, indicating a likely deactivation of complex 5. The catalytic performance of 5 increased as the temperature was increased above 90 °C, and the initial TOFs (average TOs after 10 h were used to calculate TOFs) of the reaction at 90 °C, 105 °C, and 120 °C were 4.5 × 10−4, 6.3 × 10−4 and 1.6 × 10−3 s−1, respectively. However, the high reaction temperatures also led to rapid catalyst deactivation, leading to shortened longevity of 44 h for the reaction at 90 °C, 31 h for the reaction at 105 °C, and 15 h for the 120 °C reaction. Under the conditions studied, optimal catalysis occurs at 120 °C to give a total turnover number (TON) of approximately 50 after 20 h, but the majority of the catalysis (TON = 46) is complete after 10 h.
Some recent studies have explored the modulation of the σ-donor properties of Ttz via chemical interactions of the exo-nitrogen (4-N) (Figure 4). Harman, Machan, and co-workers have studied the cyclic voltammetry of [TtzMo(CO)3]− and [TtzW(CO)3]− with the presence of Brønsted acid. They observed the protonation of the exo-4-N lone pairs of the triazole ring in Ttz ligand, and the single electron oxidation of these species consistent with a proton-coupled electron transfer (PCET) pathway [54]. Work by Papish and co-workers found that ligand protonation of Ttz complexes of first-row transition metals can have a significant effect on the electron density at the metal center [55,56,57,58,59,60], and that these properties can be used to influence catalytic behavior. Thus, the influence of acid on the catalytic performance of 5 was investigated.
Table 1 shows the result of catalytic ethylene hydrophenylation using 5 with the presence of additives. The addition of Brønsted acid (entries 1 and 2) led to decomposition of the catalyst, and no catalytic turnovers were observed. Lewis acids, such as LiNTf2, NaNTf2, and KNTf2 (entries 3–5), were shown to positively influence the catalysis, with LiNTf2 showing an approximate 100% increase compared to catalysis with no additive (TON = 60). This might be explained by the weak coordination of Li+ with the exo-nitrogen (4-N) on the triazole. Moreover, the coordination of NTf2 in place of NCMe could play a role in the catalysis, although we did not find any evidence of a Ru–NTf2 complex. After 6 h of reaction, 147 TON of ethylbenzene with a TOF of 1.0 × 10−3 s−1 were produced from the reaction of 5 and LiBArF4, indicating that Li+ is responsible for more than a doubled catalytic performance enhancement.

3. Summary and Conclusions

In this work, we have synthesized three electron-deficient Ru(II) catalyst precursors that are supported by Tp or related ligands. These precursors include: (1) a TpRu(II) catalyst with a nitrosyl (formally NO+) ligand, which underwent a facile reductive elimination of biphenyl that resulted in catalyst decomposition; (2) a Ru complex supported by TpBr3 that exhibited no catalytic reactivity (likely due to its steric hinderance); (3) a Ru complex supported by the Ttz ligand that shows activity as a catalyst for the ethylene hydrophenylation reaction. Compared to our previous Ru(II) catalyst, TpRu(P(OCH2)3CEt)(NCMe)Ph, (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5) gives a greater than seven-fold increase in TON (147 vs. 20) with the addition of LiBAr’F4 as an additive. This is consistent with our previous observation that the decrease of electron density at the Ru(II) center can lead to slower catalyst deactivation.

4. Experimental Section

General Considerations. Unless otherwise noted, all synthetic procedures were performed under anaerobic conditions in a dinitrogen-filled glovebox, or by using standard Schlenk techniques. Glovebox purity was maintained by periodic nitrogen purges, and was monitored by an oxygen analyzer (O2 < 15 ppm for all reactions). Tetrahydrofuran (THF) was dried by distillation from sodium/benzophenone. Benzene, n-pentane, and CH2Cl2 were purified by passage through a column of activated alumina. Hexanes, CDCl3, CD2Cl2, and DMSO-d6 were used as received, and stored under a dinitrogen atmosphere over 4 Å molecular sieves, and ethanol was used as received. 1H NMR spectra were recorded on a Varian 600, Varian 500 MHz, or a Bruker 600 MHz or 800 MHz spectrometer. All 1H spectra are referenced against residual proton signals (1H NMR) of the deuterated solvents. 31P{1H} NMR spectra were obtained on a Varian 500 MHz (operating frequency = 201 MHz) or Varian 600 MHz (operating frequency = 243 MHz) spectrometer, and referenced against an external standard of H3PO4 (δ = 0). GC/MS was performed using a Shimadzu GCMS-QP2010 Plus system with a 30 m × 0.25 mm capillary column with Rxi-5ms with 0.25 μm film thickness using the electron impact ionization method. GC/FID was performed using a Shimadzu GC-2014 system with a 30 m × 0.25 mm DB-5ms capillary column with 0.25 μm film thickness. FID response factors for other products were determined in a similar fashion, using authentic standards of products. Electrochemical experiments were performed under a nitrogen atmosphere using a BAS Epsilon potentiostat. Cyclic voltammograms were recorded in CH3CN using a standard three-electrode cell from 0.4 V to +1.2 V with a glassy carbon working electrode and tetrabutylammonium hexafluorophosphate as the electrolyte. Tetrabutylammonium hexafluorophosphate was recrystallized and dried under dynamic vacuum at 110 °C for 48 h prior to use. All potentials are reported versus NHE (normal hydrogen electrode), using ferrocene as the internal standard. IR spectra were obtained on a Shimadzu IRAffinity-1 Fourier transform infrared spectrometer. All other reagents were used as received from commercial sources. The preparation, isolation, and characterization of [(η6-p-cymene)Ru(Br)(μ-Br)]2, Ph2Mg[THF]2, (η6-p-cymene)Ru(P(OCH2)3CEt)(Br)Ph, and KTtz (potassium hydridotris(1,2,4-triazol-1-yl)borate) were performed according to the literature procedures [21,26,46,54,61,62]. Elemental analyses were performed by Atlantic Microlab, Inc. X-ray diffraction studies were performed on a Bruker Kappa APEXII Duo system equipped with a fine-focus sealed tube (Mo Kα, λ = 0.71073 Å) and a graphite monochromator.
TpRu(NO)Cl2 (1). This is a modified procedure based on the literature [50]. A solution of Ru(NO)Cl3 monohydrate (2.37g, 10 mmol) and two equivalents of KTp (5.04 g, 20 mmol) in 150 mL of ethanol was stirred under a dinitrogen atmosphere for two days to yield a dark brownish red mixture. The resulting suspension was filtered through a two-inch pad of Celite, and the filtrate was concentrated to dryness under reduced pressure to get a solid crude product. The resulting solid was dissolved in CH2Cl2 and purified by column chromatography using a 5-inch plug of silica gel. The pink solution that eluted first is unreacted Ru(NO)Cl3. The second fraction from the column is TpRu(NO)Cl2, which was brown. The solvent was then removed under reduced pressure to yield 2.2 g (53%) of complex 1 as a purple solid. 1H NMR (600 MHz, CDCl3) δ 8.15 (dd, 3JHH = 2.3, 3JHH = 0.7 Hz, 1H, pyrazyl 3-H), 7.91 (d, 3JHH = 2.2 Hz, 2H, pyrazyl 3-H), 7.80 (dd, 3JHH = 2.5, 4JHH = 0.7 Hz, 2H, pyrazyl 5-H), 7.59 (dd, 3JHH = 2.4, 4JHH = 0.8 Hz, 1H, pyrazyl 5-H), 6.43 (t, 3JHH = 2.4 Hz, 2H, pyrazyl 4-H), 6.27 (t, 3JHH = 2.4 Hz, 1H, pyrazyl 4-H). IR (KBr): νNO = 1898 cm−1.
TpRu(NO)Ph2 (2). This is a modified procedure based on the literature [50]. Stirring TpRu(NO)Cl2 (1) (415 mg, 1 mmol) in THF under dinitrogen at −78 °C results in a light purple solution. A cold THF solution of four equivalents of Ph2Zn (880 mg, 4 mmol) was added dropwise to the solution of 1. After the addition of the Ph2Zn, the reaction was stirred at −78 °C for an additional 2 h. Then, the reaction mixture was allowed to warm to 0 °C and stirring was maintained for 48 h. The reaction mixture turned brown during this time period. Next, the mixture was filtered through Celite, and the solvent was removed under reduced pressure. The resulting solid was dissolved in CH2Cl2 and eluted through a plug of silica gel. The brown solution of TpRu(NO)Ph2 (2), which eluted after complex 1, was collected, and the solvent was removed under reduced pressure to give a product with 54% isolated yield (270 mg). 1H NMR (600 MHz, CDCl3) δ 7.78 (dd, 3JHH = 2.4, 4JHH = 0.7 Hz, 2H, pyrazyl 3-H), 7.64 (dd, 3JHH = 2.2, 4JHH = 0.7 Hz, 1H, pyrazyl 3-H), 7.39 (dd, 3JHH = 2.0, 4JHH = 0.6 Hz, 2H, pyrazyl 4-H), 7.20–7.14 (m, 4H, phenyl ortho-H), 7.11 (dd, 3JHH = 2.2, 4JHH = 0.7 Hz, 1H, pyrazyl 4-H), 7.09–7.03 (m, 4H, phenyl, meta-H), 7.02–6.96 (m, 2H, phenyl para-H), 6.26 (t, 3JHH = 2.1 Hz, 2H, pyrazyl 5-H), 6.10 (t, 3JHH = 2.3 Hz, 1H, pyrazyl 5-H). IR (KBr): νNO = 1826 cm−1.
TpRu(NO)(OTf)Ph (3). Adding a cold CH2Cl2 solution of 0.9 equivalents of HOTf (27.1 mg, 0.18 mmol) into a CH2Cl2 solution of TpRu(NO)Ph2 (2) (100 mg, 0.2 mmol) at −78 °C under inert atmosphere resulted in protonation of one of the phenyl groups. The reaction was performed at −78 °C for 2 h, and then allowed to warm to room temperature. A color change of the reaction solution from light brownish red to purple was observed during warming. The solvent was removed under reduced pressure, and the resulting solid was dissolved in DCM and passed through a plug of silica gel using DCM as eluent. The light pink solution was collected, and the solvent was evaporated to obtain TpRu(NO)(OTf)Ph (3) in 80% yield (91.2 mg). 1H NMR (600 MHz, CDCl3) δ 8.38 (d, 3JHH = 2.1 Hz, 1H, pyrazyl 3-H), 7.89 (dd, 3JHH = 2.6, 0.7 Hz, 1H, pyrazyl 3-H), 7.76 (dd, J = 2.4, 0.7 Hz, 1H, pyrazyl 3-H), 7.70 (dd, J = 2.4, 0.7 Hz, 1H, pyrazyl 4-H), 7.63–7.56 (m, 2H, phenyl ortho-H), 7.48–7.42 (m, 2H, phenyl meta-H), 7.39–7.32 (m, 1H, phenyl para-H), 6.95 (dd, 3JHH = 2.7, 4JHH = 1.3 Hz, 1H, pyrazyl 4-H), 6.94 (dd, 3JHH = 2.0, 4JHH = 1.3 Hz, 1H, pyrazyl 4-H), 6.39 (t, 3JHH = 2.2 Hz, 1H, pyrazyl 5-H), 6.36 (t, 3JHH = 2.4 Hz, 1H, pyrazyl 5-H), 6.19 (t, 3JHH = 2.3 Hz, 1H, pyrazyl 5-H).
(TpBr3)Ru(P(OCH2)3CEt)(NCMe)Ph (4). The complex (η6-p-cymene)Ru(P(OCH2)3CEt)(Br)Ph (0.55 g, 1.0 mmol) was dissolved in NCMe (20 mL), added to a pressure tube, and heated for 3.5 h at 70 °C. The reaction was allowed to cool to room temperature. The mixture was filtered through Celite, and the filtrate was concentrated to dryness, yielding the putative complex (NCMe)3Ru(P(OCH2)3CEt)(Br)Ph.3 The resulting solid was taken up in CH2Cl2 (10 mL) and added to a 50 mL thick-wall glass pressure tube with TlTpBr3 (1.24 g, 1.1 mmol) in DMSO (10 mL). The solution was heated to 70 °C for 15 h, after which, it was cooled to room temperature and filtered through Celite. The volatiles were removed from the filtrate under reduced pressure. Benzene was added, and the mixture was stirred for 10 min. The solution was filtered through Celite, and the filtrate was discarded. The remaining white solid was dissolved in CH2Cl2 and filtered through Celite. The filtrate was concentrated to 2 mL, and hexanes were added to induce precipitation. The colorless precipitate was collected on a fine porosity frit. The solid was washed with pentane and dried in vacuo to yield 1.2 g of tan solid (44%). 1H NMR (600 MHz, DMSO-d6) δ 7.56 (d, 3JHH = 7.6 Hz, 1H, phenyl ortho-H), 6.74 (vt, 1H, phenyl meta-H), 6.59 (tt, 3JHH = 7.1, 4JHH = 1.3 Hz, 1H, phenyl para-H), 6.49 (vt, 1H, phenyl meta-H), 6.11 (d, 3JHH = 7.8 Hz, 1H, phenyl ortho-H), 4.18 (d, 3JPH = 3.9 Hz, 6H, P(OCH2)3CCH2CH3), 2.40 (s, 3H, NCCH3), 1.19 (q, 3JHH = 7.6 Hz, 2H, P(OCH2)3CCH2CH3), 0.77 (t, 3JHH = 7.7 Hz, 3H, P(OCH2)3CCH2CH3). 31P{1H} NMR (121 MHz, DMSO-d6) δ 143.1.
(Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5). The complex (η6-p-cymene)Ru(P(OCH2)3CEt)(Br)Ph (0.55 g, 1.0 mmol) was dissolved in NCMe (20 mL), added to a pressure tube, and heated for 3.5 h at 70 °C. The reaction was allowed to cool to room temperature. The mixture was filtered through Celite, and the filtrate was concentrated to dryness, yielding the putative complex (NCMe)3Ru(P(OCH2)3CEt)(Br)Ph.3 The resulting solid was taken up in CH2Cl2 (10 mL) and added to a 50 mL thick-wall glass pressure tube with KTtz (0.28 g, 1.1 mmol) in CH2Cl2 (10 mL). The solution was heated to 70 °C for 15 h, after which, it was cooled to room temperature and filtered through Celite. The volatiles were removed from the filtrate under reduced pressure. Benzene was added, and the mixture was stirred for 10 min. The solution was filtered through Celite, and the filtrate was discarded. The remaining white solid was dissolved in CH2Cl2 and filtered through Celite. The filtrate was concentrated to 2 mL, and hexanes were added to induce precipitation. The white precipitate was collected on a fine porosity frit. The solid was washed with pentane and dried in vacuo to yield an off-white solid (262 mg, 43%). 1H NMR (600 MHz, CDCl3) δ 8.32 (d, 3JHH = 2.4 Hz, 2H, triazole-H), 8.28 (s, 1H, triazole-H), 8.23 (s, 1H, triazole-H), 7.92 (s, 1H, triazole-H), 7.54 (s, 1H, triazole-H), 6.99–6.96 (m, 2H, phenyl-H), 6.89–6.80 (m, 3H, phenyl-H), 4.18 (dt, 3JPH = 5.2, 4JHH = 2.3 Hz, 6H, P(OCH2)3CCH2CH3), 2.31 (s, 3H, NCCH3), 1.19 (q, 3JHH = 7.7 Hz, 2H, P(OCH2)3CCH2CH3), 0.82 (t, 3JHH = 7.7 Hz, 3H, P(OCH2)3CCH2CH3). 31P{1H} NMR (121 MHz, CD2Cl2) δ 133.3. Analyzed and calculated for C20H26O3N10PBRu: C, 40.21; H, 4.39; N, 8.04. Found: C, 40.23; H, 4.29; N, 8.17.
Catalytic Oxidative Hydrophenylation of Ethylene using (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5). A representative catalytic reaction is described. A stock solution containing complex 5 (0.023 mmol), hexamethylbenzene (74.5 mg, 0.46 mmol), and benzene (200 mL) was prepared in a volumetric flask. Thick-walled Fisher-Porter reactors were charged with stock solution (10 mL). The vessels were sealed, pressurized with 15 psig of ethylene, and subsequently stirred and heated to the desired temperature (75, 90, 105, and 120 °C) in an oil bath. The reaction was sampled every 2 h. At each time point, the reactors were cooled to room temperature, sampled, recharged with ethylene, and heated. Aliquots of the reaction mixture were analyzed by GC/FID using relative peak areas versus the internal standard.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics10060076/s1, Figure S1: 1H NMR spectrum of TpRu(NO)Cl2 (1) in CDCl3; Figure S2: 1H NMR spectrum of TpRu(NO)Ph2 (2) in CDCl3; Figure S3: 1H NMR spectrum of TpRu(NO)Ph2 (2) in CDCl3 after 24 hours at room temperature, the production of biphenyl (integrated) is observed; Figure S4: 1H NMR spectrum of TpRu(NO)(OTf)Ph (3) in CDCl3; Figure S5: 1H NMR spectrum of TpRu(NO)(OTf)Ph (3) in CDCl3 after 10 minutes at room temperature; Figure S6: 1H NMR spectrum of (TpBr3)RuP(OCH2)3CEt(NCMe)Ph (4) in DMSO-d6; Figure S7: 31P{1H} NMR spectrum of (TpBr3)RuP(OCH2)3CEt(NCMe)Ph (4) in DMSO-d6; Figure S8: 1H NMR spectrum of (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5) in CDCl3; Figure S9: 31P{1H} NMR spectrum of (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5) in CDCl3; Figure S10: Solid state IR spectrum of TpRu(NO)Cl2 (1); Figure S11: Solid state IR spectrum of TpRu(NO)Ph2 (2); Scheme S1. Protonation of (TpBr3)RuP(OCH2)3CEt(NCMe)Ph (4) by HTFA (trifluroracetic acid); Figure S12: 1H NMR spectra of protonation of (TpBr3)RuP(OCH2)3CEt(NCMe)Ph (4) using HTFA; Figure S13: Variable temperature 1H NMR spectra of (TpBr3)RuP(OCH2)3CEt(NCMe)Ph (4) from 20 °C (bottom) to 100 °C (top) in DMSO-d6; Figure S14: CVs of complexes 5 in NCMe; Figure S15: Calibration curve for ethylbenzene quantification on GC-FID; Figure S16: Representative GC-FID chromatogram of a reaction mixture from ethylene hydrophenylation reaction under anaerobic conditions; Table S1: Crystallographic data for TpRu(NO)Ph2 (2).

Author Contributions

X.J. and S.T. performed the synthesis and characterization for the ruthenium complexes; P.J.S. synthesized the Ttz ligand. D.A.D. performed the X-ray data collection and refinement. X.J., W.D.H. and T.B.G. analyzed the data and wrote the paper. T.B.G. administered and supervised the project. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (DE-SC0000776).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

CCDC 2170944 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (accessed on 24 May 2022).

Acknowledgments

We thank Pedro Perez (Departamento de Química y Ciencia de los Materiales Universidad de Huelva) for providing a sample of the TpBr3 ligand. We thank Ke Zhang for acquiring IR spectra of complexes 1 and 2.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 10 00076 sch001 550
Scheme 1. New Ru(II) complexes with reduced electron density compared to TpRu(L)(NCMe)Ph complexes.
Scheme 1. New Ru(II) complexes with reduced electron density compared to TpRu(L)(NCMe)Ph complexes.
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Inorganics 10 00076 sch002 550
Scheme 2. Proposed synthetic route for TpRu(NO)(OTf)Ph (3).
Scheme 2. Proposed synthetic route for TpRu(NO)(OTf)Ph (3).
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Figure 1. ORTEP of TpRu(NO)Ph2 (2) with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity. Only one of the three crystallographically independent but chemically identical molecules in the asymmetric unit is shown. Selected bond lengths (Å) and bond angles (°): Ru(1)-C(10) 2.098(6), Ru(1)-C(16) 2.084(7), Ru(1)-N(7) 1.728(5), N(7)-O(1) 1.151(6), Ru(1)-N(1) 2.119(5), Ru(1)-N(3) 2.174(5), Ru(1)-N(5) 2.167(6); Ru(1)-N(7)-O(1) 177.5(5), C(10)-Ru(1)-C(16) 93.8(2), N(1)-Ru(1)-N(3) 83.70(17), N(1)-Ru(1)-N(5) 81.95(19), N(3)-Ru(1)-N(5) 86.5(2).
Figure 1. ORTEP of TpRu(NO)Ph2 (2) with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity. Only one of the three crystallographically independent but chemically identical molecules in the asymmetric unit is shown. Selected bond lengths (Å) and bond angles (°): Ru(1)-C(10) 2.098(6), Ru(1)-C(16) 2.084(7), Ru(1)-N(7) 1.728(5), N(7)-O(1) 1.151(6), Ru(1)-N(1) 2.119(5), Ru(1)-N(3) 2.174(5), Ru(1)-N(5) 2.167(6); Ru(1)-N(7)-O(1) 177.5(5), C(10)-Ru(1)-C(16) 93.8(2), N(1)-Ru(1)-N(3) 83.70(17), N(1)-Ru(1)-N(5) 81.95(19), N(3)-Ru(1)-N(5) 86.5(2).
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Scheme 3. (a) Net reductive elimination of biphenyl from TpRu(NO)Ph2 (2) and (b) previously reported biphenyl formation from Cp*Ru(NO)Ph2 [51].
Scheme 3. (a) Net reductive elimination of biphenyl from TpRu(NO)Ph2 (2) and (b) previously reported biphenyl formation from Cp*Ru(NO)Ph2 [51].
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Scheme 4. Synthetic routes for the preparation of (TpBr3)Ru(P(OCH2)3CEt)(NCMe)Ph (4) and (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5).
Scheme 4. Synthetic routes for the preparation of (TpBr3)Ru(P(OCH2)3CEt)(NCMe)Ph (4) and (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5).
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Figure 2. Rotation of phenyl group in the TpRu(L)(NCMe)Ph complexes; when the rotation of the Ru-C is rapid, time average of the HO and HO’, HM, and HM’ is observed.
Figure 2. Rotation of phenyl group in the TpRu(L)(NCMe)Ph complexes; when the rotation of the Ru-C is rapid, time average of the HO and HO’, HM, and HM’ is observed.
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Figure 3. Ethylene hydrophenylation catalyzed by (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5) at different temperatures. Conditions: 10 mL benzene solution of 0.00025 mol% complex 5, 15 psig ethylene; 75, 90, 105, and 120 °C. Error bars represent standard deviations from at least three independent experiments.
Figure 3. Ethylene hydrophenylation catalyzed by (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5) at different temperatures. Conditions: 10 mL benzene solution of 0.00025 mol% complex 5, 15 psig ethylene; 75, 90, 105, and 120 °C. Error bars represent standard deviations from at least three independent experiments.
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Figure 4. The possible interaction between triazole ring and Lewis acid (LA).
Figure 4. The possible interaction between triazole ring and Lewis acid (LA).
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Table
Table 1. Effect of additives for ethylene hydrophenylation catalyzed by (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5).
Table 1. Effect of additives for ethylene hydrophenylation catalyzed by (Ttz)Ru(P(OCH2)3CEt)(NCMe)Ph (5).
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EntryAdditives aTON of Ethylbenzene
1HNTf2 b−
2HBArF’4−
3LiNTf2120(6)
4NaNTf291(3)
5KNTf254(9)
6LiBArF’4147(12)
7AlMe3−
Reactions were performed with 0.00025 mol % of complex 5 dissolved in benzene with hexamethylbenzene as an internal standard at 90 °C with 15 psig of ethylene. TONs were determined by GC-FID after 40 h, and the average of three independent experiments. a Three equivalents of additive were used in each reaction. b NTf2 = bis(trifluoromethane)sulfonimide.
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Jia, X.; Tian, S.; Shivokevich, P.J.; Harman, W.D.; Dickie, D.A.; Gunnoe, T.B. Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation. Inorganics 2022, 10, 76. https://doi.org/10.3390/inorganics10060076

AMA Style

Jia X, Tian S, Shivokevich PJ, Harman WD, Dickie DA, Gunnoe TB. Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation. Inorganics. 2022; 10(6):76. https://doi.org/10.3390/inorganics10060076

Chicago/Turabian Style

Jia, Xiaofan, Songyuan Tian, Philip J. Shivokevich, W. Dean Harman, Diane A. Dickie, and T. Brent Gunnoe. 2022. "Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation" Inorganics 10, no. 6: 76. https://doi.org/10.3390/inorganics10060076

APA Style

Jia, X., Tian, S., Shivokevich, P. J., Harman, W. D., Dickie, D. A., & Gunnoe, T. B. (2022). Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation. Inorganics, 10(6), 76. https://doi.org/10.3390/inorganics10060076

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