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Article

Syntheses and Applications of Symmetrical Dinuclear Half-Sandwich Ruthenium(II)–Dipicolinamide Complexes as Catalysts in the Transfer Hydrogenation of Ketones

by
Robert Tettey Kumah
,
Sabathile Thandeka Mvelase
and
Stephen Otieno Ojwach
*
School of Physics and Chemistry, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01, Scottsville 3209, South Africa
*
Author to whom correspondence should be addressed.
Inorganics 2022, 10(11), 190; https://doi.org/10.3390/inorganics10110190
Submission received: 28 September 2022 / Revised: 22 October 2022 / Accepted: 24 October 2022 / Published: 29 October 2022
(This article belongs to the Special Issue Metal-Catalyzed (De)Hydrogenation, Hydrogen Transfer and Hydroelementation Reactions)
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Abstract

:
The treatment of [Ru(η6-p-cymene)Cl2]2 with N,N’-(1,2-phenylene)dipicolinamide (H2L1) afforded the double salt complex [{Ru(η6-p-cymene)2-µ-Cl}L1][Ru(η6-p-cymene)Cl3], (Ru1) in moderate yields. Separately, the reactions of ligands (H2L1), N,N’-(4,5 dimethyl-1,2-phenylene)dipicolinamide (H2L2), and N,N’-(4-methoxy-1,2-phenylene)dipicolinamide (H2L3) with the [Ru(η6-p-cymene)Cl2]2 in the presence of KPF6 afforded the respective dinuclear half-sandwich Ru(II) complexes [{(Ru(η6-p-cymene)2--µ-Cl}L1][PF6] (Ru2), [{(Ru(η6-p-cymene)2-µ-Cl}L2][PF6] (Ru3), and [{(Ru(η6-p-cymene)2-µ-Cl}L3][PF6] (Ru4). NMR and FT-IR spectroscopies, ESI-MS spectrometry, and elemental analyses were used to establish the molecular structures of the new dinuclear ruthenium(II) complexes. Single crystal X-ray crystallography was used to confirm the piano-stool geometry of the dinuclear complexes Ru1 and Ru4, as containing N^N chelated ligand and bridging chlorido ligands in each Ru(II) atom. The complexes (Ru1-Ru4) showed good catalytic activities at low catalyst concentrations of 0.005 mol% in the transfer hydrogenation of a wide range of ketone substrates.

Graphical Abstract

1. Introduction

Transition metal catalysed transfer hydrogenation (TH) reactions have provided a versatile and efficient protocol for the syntheses of valuable bulk and fine chemicals [1,2]. Ever since the first (S)-BINAP/diamine-Ru(II) complexes were employed as catalysts in the transfer hydrogenation (TH) of ketones by Noyori and Ikariya [3], a plethora of transition-metal-based catalysts have been developed for transfer hydrogenation reactions [4,5]. While a number of mononuclear-metal-based complexes have been shown to give promising catalytic activities in the TH of, for example, ketones, the use of multinuclear analogues is still in its infancy [6]. Thus, the development of multinuclear complexes to mediate these TH reactions is beginning to grain traction with the aim of enhancing catalytic activity and stability [7,8,9]. Few examples of multinuclear complexes based on Ru(II) [5,7], Ir(I/III) [10,11,12], and Rh(II) [13] metals have so far been reported in the TH of ketones. Many of these complexes are derived from N-heterocarbene (NHCs) [11], Schiff bases [14], and phosphinite–Schiff base ligands [15].
In the design of multinuclear catalysts, key factors such as the electronic property and adaptability of a chelating ligand are considered [15]. For example, a ligand framework bearing multi-donor sites often favours the stabilisation of polynuclear complexes [16,17]. Another versatile method of synthesising multinuclear complexes is the one-pot coordination of polydentate ligands by a metal salt. This strategy is essentially viable in terms of the atom economy, the yield, the compatibility of the metal centres and coordination sites, and the ease of coordination between the metal atom and the ligand framework [17].
Following these synthetic protocols, a number of polynuclear complexes anchored on N^N^N donor ligands have been developed and shown to give promising catalytic activities in TH reactions [7,8,18]. In one such approach, a dinuclear Ru(II)-N^N^N complex, bearing a 4,4′-(CH2)3-bipyridine linker, was reported to give high catalytic activity (TOF up to 1.4 × 107 h−1) towards the TH of ketones [7]. Recently, tri- and hexanuclear ruthenium(II) complexes obtained by assembling 16-e− Ru(II) pyrazolyl-imidazolyl- units with polypyridines, also display TOFs up to 7.1 × 106 h−1 in the TH of ketones [7]. Having been encouraged by our earlier findings on the synthesis and applications of mononuclear carboxamide Ru(II) complexes in the TH of ketones [19], herein, we report the synthesis, structural elucidation, and applications of symmetrical dinuclear piano-stool ruthenium(II)-dipicolinamide complexes as catalysts in the TH of ketones.

2. Results and Discussion

2.1. Synthesis and Characterisation of Ligands and Complexes

The multifunctional (pyridyl)carboxamide ligands, N,N′-(1,2-phenylene)dipicolinamide (H2L1), N,N’-(4-methoxy-1,2-phenylene)dipicolinamide (H2L2), and N,N’-(4,5 dimethyl-1,2-phenylene)dipicolinamide (H2L3) were synthesised following a modified procedure from the literature [20,21,22,23,24]. The synthetic details and spectroscopic data of the dicarboxamide ligands are given as supplementary data, ESI†. The treatment of the [Ru(η6-p-cymene)Cl2]2 precursor with the dipicolamide ligand H2L1 in the presence of sodium methoxide (NaOMe) resulted in the formation of the dinuclear Ru(II) complex [{Ru(η6-p-cymene)2-µ-Cl}L1][Ru(η6-p-cymene)Cl3] (Ru1), as illustrated in Scheme 1. The reactions of the [RuCl2(η6-p-cymene)]2 precursor with the ligands H2L1, H2L2, and H2L3 in the presence of KPF6 gave the cationic complexes [{(Ru(η6-p-cymene)2--µ-Cl}L1][PF6] (Ru2), [{(Ru(η6-p-cymene)2-µ-Cl}L2][PF6] (Ru3), and [{(Ru(η6-p-cymene)2-µ-Cl}L3][PF6] (Ru4), respectively (Scheme 1).
All the isolated complexes were characterised using NMR spectroscopy, mass spectrometry, FT-IR spectroscopy, microanalysis, and single-crystal X-ray crystallography (Ru1 and Ru3). The formation of the dinuclear Ru(II) complexes Ru1–Ru4 was confirmed by comparing their 1H NMR spectroscopic data to their corresponding free ligands H2L1–H2L3 (Figures S1–S7). For instance, in the 1H NMR spectrum ligand H2L1, the signal assigned to the amide proton (N-H) observed at δ: 10.17 ppm disappeared upon coordination to form the corresponding complex Ru2 (Figure S1 vs. Figure S5). Similar 1 H NMR spectral data were observed for the other complexes Ru1, Ru3, and Ru4 (Figures S1–S7, ESI†). This was an indicative of the deprotonation of the amide protons prior to complexation, as has been observed in other related complexes [24,25], and was consistent with the solid-state structures of complexes Ru1 and Ru4 (Figure 1 and Figure 2). In addition, the 1H NMR spectra of complexes Ru1–Ru4 displayed signals of the pyridine protons downfield (7.80–9.40 ppm) compared to 8.69–7.45 ppm in the free ligands (HL1–HL3). 13C{1H} NMR spectroscopy was also useful in establishing the formation of the dinuclear Ru(II) complexes. For example, the carbonyl carbon signals in complexes Ru1–Ru4 were observed downfield in comparison with the signals of the corresponding free ligands (Figures S8–S14). For instance, the 13C{1H} NMR spectrum of complex Ru2 showed the carbonyl (C=O) signal downfield at 169.6 ppm (Figure S12) compared to the signal at 163.3 ppm for its free ligand H2L2 (Figure S8). This trend is reasonable, since the C=O motif is within the Ru(II) coordination sphere and is likely to be electron-deficient (ligands predominantly sigma-donors). 31P{1H} NMR spectra of Ru2–Ru4 exhibited a septet signal between ~131 and ~157 ppm (Figures S15 and S17) and established the presence of the PF6− counter-anion in these compounds, as depicted in Scheme 1. This was further supported by 19F NMR spectroscopic data which displayed doublet signals in the range of −69 ppm to −71 ppm (Figures S18 and S20).
The successful formation of complexes Ru1–Ru4 was further supported by comparing their FT-IR spectra to their respective free ligands H2L1–H2L3 (Figures S21–S27). As noted in the 1H NMR spectroscopic data, the sharp signal of the amidic N–H functional group in H2L2, recorded at 3325 cm−1, disappeared in the spectrum of the corresponding complex Ru3. Similar observations were recorded in the FT-IR spectroscopic data of the other complexes Ru1, Ru2, and Ru4. Furthermore, the FT-IR spectroscopic data of complexes Ru1–Ru4 (Figures S24–S27) showed the (C=O) signals at lower frequencies (1618–1620 cm−1) compared to their corresponding free ligands H2L1–H2L3 (1664–1688 cm−1). These could be assigned to the resonance enhancement within the deprotonated ligand leading to the weakening of the carbonyl (C=O) group in the coordinated ligands [22,23,26]. Mass spectrometry was also employed to elucidate the molecular compositions of complexes Ru1–Ru4. The compounds gave base peaks at m/z = 825 (Ru1 and Ru2), 853 (Ru3), and 851(Ru4), corresponding to the parent cations [M]+, signifying the stability of the compounds. In addition, the experimental isotopic mass distributions correlated well with the theoretical patterns (Figures S31–S34, ESI†). The elemental analyses data of the complexes tallied well with the proposed empirical formulae, as shown in Scheme 1.

2.2. Molecular Structures of Ruthenium(II) Complexes Ru1 and Ru3

The slow diffusion of diethyl ether into dichloromethane solutions of complexes Ru1 and Ru4 gave single crystals suitable for X-ray analyses. Figure 1 and Figure 2 show the molecular structure of the cationic complexes Ru1 and Ru4, respectively, while the crystallographic data are shown in Table 1. The packing diagrams and structures of the compouds showing the counter anions and solvent molecules are given in Figures S35–S38. Complexes Ru1 and Ru4 crystallise in triclinic and monoclinic systems with P-1 and P21/c, space groups, respectively. The net charge on the cationic species in Ru1 and Ru4 are balanced by the counter-anions [Ru(η6-p-cymene)Cl3]− and [PF6]−, respectively. The half-sandwich complexes Ru1 and Ru4 exhibit three-legged piano stool geometry (Figure 1 and Figure 2), which is typical of [Ru(η6-p-cymene)(L)Cl]+ complexes. In the coordination sphere of both compounds, Ru1 and Ru4, the p-cymene ring resides on the apex, whereas the bridging chlorido ligands, Npyridine ^ and Namidate chelate, constitute the base of the piano stool. In the two complexes (Ru1 and Ru4), the five-membered chelating rings have an average bite angle for Npyridine -Ru- Namidate of 77.18 (18)° Å. The two Ru(II) atom centres in both compounds are separated by an average distance of 4.167 (5) Å, which is relatively shorter than the average 5.613 ± 18 Å calculated for 9 half-sandwich dinuclear ruthenium(II) complexes [27]. The average bond distance for Ru-Npyridine and Ru-Namidate of 2.093 (2) Å and 2.109 (2) Å in compounds Ru1 and Ru4 are within the mean bond length for Ru-Npyridine = 2.090 (14) Å and Ru-Namidate of 2.083 (22) Å, obtained from three (3) piano-stool Ru(II) complexes [27]. The p-cymene rings in the compounds are almost planar, with the Ru(II) atoms at 3.205(11) Å distance from the centroid of the p-cymene rings, and are comparable to the average 3.182(18) Å calculated for 18 half-sandwich Ru(II) structures [27].

2.3. Transfer Hydrogenation of Ketones

To explore the feasibility of the dinuclear Ru(II) compounds (Ru1–Ru4) to mediate the TH of ketones, acetophenone (1.12 mL, 1.00 mmol) and KtBuO (0.4 mol%) were used as model substrate and base, respectively (Table 2). ESI Figures S39–S46 show the 1H NMR spectral data of the crude TH mixtures used to determine the respective percentage conversions and yields with time. In the presence of KtBuO (1.00 mL of 0.04 M in 2-propanol), complex Ru2 (5.5 × 10−4 mol%, 550 ppm) achieved percentage yields of 98% corresponding to a TOF of 2.2 × 102 h−1 in the TH of acetophenone in 6 h (Table 2, entry 7). To succinctly verify the role played by complex Ru2 and the KtBuO base, we carried out some control experiments. Firstly, a Ru-catalyst-free experiment employing only the KtBuO base was performed, and afforded negligible percentage yields of 1% within 6 h (Table 2, entry 1), consistent with the findings of Tenorio and co-workers [28]. In another set of control experiments, a base-free reaction gave lower percentage yields of 30% in 6 h (Table 2, entry 2). From these control results, it is therefore plausible to assign the higher percentage yields to the ruthenium(II) complexes in these TH reactions.
Having established the viability of complex Ru2 to mediate the catalytic transfer of acetophenone, we then studied the effect of catalyst concentration by varying the catalyst loadings from 0.0025 to 0.055 mol% (Table 2, entries 3–7). From the results, while a general increase in catalyst loading resulted in higher percentage yields, lower catalytic activities (TOFs and TONs) were observed (Figure S47). For instance, an increase in the catalyst loading, from 0.0025 to 0.055 mol%, was accompanied by an increase in the percentage yields, from 39% to 98%, but a decrease in TOF from 2.60 × 103 h−1 to 2.2 × 102 h−1 (Table 2, entries 3 vs. 7); this is in line with the previous reports of Yu and co-workers, and has been attributed to the lower magnitudes of catalytic activities at higher catalyst loadings [7]. The nature of the base is known to greatly influence the performance of the metal catalysts in the TH of ketones. Thus using catalyst Ru2, we tested the various bases K2CO3 and KOH, and established the order of catalytic activity as K2CO3 < KOH < tBuOK (Table 2, entry 4, 8 and 9), consistent with the relative strengths of the bases [9].

2.3.1. Influence of Catalyst Structure on the TH of Acetophenone

Having established the optimised reaction parameters (catalyst loading, 5.00 × 10−3 mol% (50 ppm); tBuOK, 4.0 mol% and temperature, 82 °C), we sought to investigate the effects of the catalyst structure/ligand motif on the transfer hydrogenation of ketones. In general, all the pre-catalysts showed appreciable catalytic activities (TOFs between 1.72 × 103 h−1 and 1.97 × 103 h−1), comparable to the phosphine–amide ruthenium(II) complexes reported in the literature [20]. As demonstrated in Figure 3 and Figure S48 and Table 3, complexes Ru3 and Ru4, bearing methyl and methoxy electron-donating groups on the phenyl linker, exhibited slightly higher catalytic activities compared to the unsubstituted analogue Ru2 (Table 3, entries 2–4). For example, complex Ru3 (methyl) displayed a TOF of 3.13 × 103 h−1 (kobs = 1.73 × 10−1 ± 0.12) compared to the TOF of 2.87 × 103 h−1 (kobs of 1.69 x10−1 (±0.03) h−1) recorded for the unsubstituted Ru2, (Table 3, entries 3 vs. 4). While electron-donating groups are expected to result in lower catalytic activity due to the diminished electrophilicity of the metal centre, the higher catalytic activities of complexes Ru3 and Ru4 could be assigned to the improved stability of the resultant active species [29,30,31]. Interestingly, complex Ru1 gave the highest catalytic activity, which may be ascribed to the presence of the [Ru(p-cymene)Cl3]− counter-anion, which, on its own, could act as a catalyst (double catalyst) in the TH of acetophenone (Table 3, entry 1).
The catalytic activities of complexes Ru1–Ru4 (TOF of up to 3.27 × 103 h−1) compared poorly with some of the highly active multinuclear Ru(II)-based catalysts which demonstrated TOFs up to 1.0 × 106 h−1, as reported in the literature [7,9,18]. While the carboxamide ligands have the propensity to stabilise the Ru(II) complexes, the relatively lower catalytic activities observed for complexes Ru1–Ru4 could be linked to the larger internuclear distance between the two metal centres, thus hindering the mutual interactions between the two metal centres [30,31]. On a positive note, the complexes showed higher catalytic activities compared to reported half-sandwich nitrogen-donor ruthenium(II) complexes, where TOFs of 5 × 102 h−1 were recorded [5,32,33,34,35,36].

2.3.2. Investigation of the Ketone Substrate Scope

To study the applicability of the catalysts in the TH of a wide range of ketones, catalysts including heteroaromatic and aliphatic ketones were investigated using complex Ru2 (Table 4). From the Table 4, acetophenone derivatives containing electron-withdrawing substituents displayed higher percentage yields. For example, 2-chloroacetophenone and 4-chloroacetophenone furnished a percentage yield of 99% in 4 h compared to acetophenone, which attained 86% in 6 h (Table 4, entries 1 vs. 2 and 3). This observation could be explained by the reduction in electron density on the carbonyl carbon, thus facilitating nucleophilic attack [9,37]. In sharp contrast, the introduction of electron-donating groups led to diminished catalytic activities of complex Ru2. As an illustration, 4-amino acetophenone exhibited lower percentage yields of 78% compared to that of acetophenone, at 86% (Table 4, entries 1 vs. 9). The trend of reactivity of the substrates were not significantly affected by the position of the electron-donating groups on the phenyl ring. For instance, 2-methyl acetophenone and 4-methyl acetophenone exhibited comparable percentage yields of 81 and 79% (Table 4, entries 5 vs. 6). This trend points to electronic factors in the substrates playing a key role in controlling the catalytic activities of complex Ru2. With steric effect, 2-methyl acetophenone (more sterically demanding) would be expected to give lower percentage yields than 4-methyl acetophenone. Indeed, this hypothesis was augmented by the lower percentage yields of 79% (6 h) reported for 4-methyl acetophenone compared to percentage yields of 99% (4 h) recorded for of 4-chloroacetophenone (Table 4, entries 3 and 6). Similar trends were previously reported by Chai et al., using dinuclear ruthenium(II) complexes supported on tridentate nitrogen-donor ligands [9].
Acetophenone derivatives containing polyaromatic groups such as 2-acetylnaphthalene also realised decent percentage yields of 88% comparable to yields of 86% recorded for acetophenone (Table 4, entries 1 vs. 10, 11) which are in line with the findings of Chai et al. [9]. More significantly, heteroaromatic acetophenone derivatives such as 1-acetyl imidazole also afforded moderate percentage yields of 76% (Table 4, entries 12, 13). The relatively lower yields realised for 1-acetyl imidazole compared to acetophenone (86%) could be assigned to the irreversible binding of the N-donor atoms of the hetero-atoms of the substrates to the active Ru(II) centre [7]. Interestingly, and contrary to the known trends, aliphatic ketones such as 2-pentanone were also reduced with comparable percentage yields of 88% to acetophenone of 86% (Table 4, entries 1 vs. 14). In an earlier report by Liu et al. [7], aliphatic ketones showed lower catalytic activities compared to acetophenone [38]. The reasons for this behaviour of complex Ru2 (higher catalytic activities for aliphatic ketones) is still not clear to us at this stage.

3. Conclusions

In conclusion, four cationic dinuclear Ru(II) complexes ligated on carboxamide ligands have been structurally characterised and studied as pre-catalysts in the TH of ketones. The ruthenium complexes (Ru1 and Ru4) exhibit three-legged piano stool geometry around their Ru(II) atoms, in which the coordination sphere consists of N^O chelate, one bridging chlorido, and η6-p-cymene ligands. The Ru(II) compounds formed active catalysts in the TH of ketone substrates, giving moderate to high catalytic activities of TONs of up to 1.96 × 103 at very low catalyst concentrations. The complexes bearing electron-donating groups (Ru3 and Ru4) on the ligand backbone were more active than their unsubstituted counterparts (Ru2). Additionally, ketone substrates containing electron-donating groups and heteroatoms afforded lower percentage yields when compared to substrates bearing electron-withdrawing groups. Thus, the stability of the complexes and the nature of the electronic properties of the ketone substrates appeared to regulate the catalytic performances of these complexes in the TH.

4. Experimental Section

4.1. Materials and Instrumentations

All synthetic manipulations, unless otherwise stated, were carried out using the standard Schlenk technique under an inert atmosphere. All solvents were distilled and dried according to standard purification procedures [39]. Starting materials, 2-picolinic acid, p-phenylenediamine, o-phenylenediamine, 4-methoxy benzene-1,2-diamine, 4,5-diamethylbenzene-1,2-diamine, triphenylphosphate, sodium methoxide and [Ru(η6-p-cymene)Cl2]2 precursors were purchased from Sigma Aldrich and used without further purification. The carboxamide ligands, N, N’-(1,2-phenylene)dipicolinamide (H2L2), N, N’-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L2), and N, N’-(4-methoxy-1,2-phenylene)dipicolinamide (H2L3) were prepared following the literature procedure [21,22,23,24], and Nuclear Magnetic Resonance (NMR) spectra were acquired on a Bruker Avance Ultrashield 400 MHz spectrometer using d-CDCl3 and d6-DMSO as solvents at room temperature. The chemical shift values of 1H, 13C{1H}, 19F and 31P{1H} NMR are recorded in parts per million (ppm) relative to TMS with the residual solvent peak as an internal reference and coupling constants are measured in Hertz (Hz) [40]. Mass spectrometry and elemental analyses were performed on a micro-mass LCT premier mass spectrometer and Flash 2000 Thermo scientific analyser, respectively.

4.2. X-ray Data Collection, Structure, and Refinement

A single crystal suitable for X-ray diffraction analysis was mounted on a glass fibre with epoxy cement. The crystals have been cut to size (less than collimator cross-section diameter). The X-ray data of the complexes were collected on the Bruker Apex-II CCD at 100 K and the graphite monochrome Cu-Kα radiation at 0.71073 Å. Structures were initially resolved by direct method programs (SIR-92) and further refined by the full-matrix least-squares techniques on F2 using SHELXL-2015 [41]; all calculations were manipulated using the WinGX-2018 crystallographic package. A SADABS semi-empirical multi-scan absorption correction was applied to the data. Direct methods, SHELXS-2015 and WinGX-2018, were used to solve the structure. All non-hydrogen atoms were located in the difference density map and refined anisotropically with SHELX-2015. All hydrogen atoms were included as idealised contributors in the least-squares process. The positions of all hydrogen atoms were calculated using a standard riding model with C–Haromatic distances of 0.93 Å and Uiso = 1.2 Ueq and CHmethylene distances of 0.99 Å and Uiso = 1.2 Ueq and C–Hmethyl distances of 0.98 Å and Uiso = 1.5 Ueq.

4.3. Synthesis of Dinuclear Ruthenium(II) Carboxamide Complexes

4.3.1. [{Ru(η6-p-cymene)2-μ-Cl}2L1][Ru(η6-p-cymene)Cl3] (Ru1)

To a solution of dichloro-ruthenium p-cymene dimer, [Ru(η6-p-cymene)Cl2]2 (0.10 g, 0.16 mmol) in the mixed methanol and chloroform (1/1, 10/10 mL), H2L1 (0.06 g, 0.16 mmol) and sodium methoxide, NaOMe (0.02 g, 0.32 mmol) were added, and the resultant mixture was reacted at 25 °C for 18 h. The resultant orange suspension was evaporated, and the crude product was dissolved in dichloromethane and filtered over celite. The filtrate was then concentrated to about 3 mL, diethyl ether (20 mL) was added, and the mixture filtered and dried in vacuum to obtain an orange compound. Yield: 0.16 g (86%). 1H NMR (400 MHz, CDCl3) δ: 9.40 (d, 3JHH = 5.6, 2Hpyridine), 8.28(t, 3JHH = 7.6, 2Hpyrdine), 8.11(d, 3JHH = 7.6, 2Hpyridine), 7.88(t, 3JHH = 8.0, 2Hpyridine), 7.56(dd, 3JHH = 3.6, 2Hbenzene), 7.27(dd, 3JHH = 3.6, 2Hbenzene), 5.62(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.52(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.30(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.05(d, 3JHH = 4.4 Hz, 2Hpcymene), 2.12(m, 3JHH = 6.8 Hz, 2Hpcymene), 1.19 (s, 6-Hpcymene), 0.98 (d, 3JHH = 6.8 Hz, 6Hpcymene), 0.81 (d, 3JHH = 6.8, 6Hpcymene). 13C{1H} NMR (101 MHz, CDCl3) δ:162.8, 154.1, 143.7, 139.2, 130.1, 128.9, 125.8, 124.9, 89.1, 81.4, 30.6, 22.1, 18.5. ESI-MS (m/z) at 852[M+, 100%]. HR-MS (ESI): m/z 823.0958 [M+], calcd for C38H40N4O2ClO2Ru2 823.0927. FT-IR (cm−1): (νC=O)amidate = 1617.97; Anal. Calcd for C48H54Cl4N4O2ClRu3: C, 49.53; H, 4.68; N, 4.81. Found: C, 49.33.; H, 4.71; N, 4.57.

4.3.2. [{Ru(η6-p-cymene)2-μ-Cl}2L1][PF6] (Ru2)

To [Ru(η6-p-cymene)Cl2]2 (0.10 g, 0.16 mmol) in a methanol and chloroform solvent system (1/1, 10/10 mL), H2L1 (0.06 g, 0.16mmol) and sodium methoxide, NaOMe (0.01 g, 0.32 mmol) were added, and reacted at room temperature for 12 h. KPF6 (0.03 g, 0.16 mmol) was added, and the solution was stirred for 6 h. The resultant orange suspension was evaporated, and the crude substance was dissolved in dichloromethane and filtered over celite. The filtrate was then concentrated to about 3 mL, diethyl ether (20 mL) was added, and it was filtered and then dried in vacuum. An orange compound was obtained. Yield: 0.14 g (88%). 1H NMR (400 MHz, CDCl3) δ: 9.40 (d, 3JHH = 5.6, 2Hpyridine), 8.28(t, 3JHH = 7.6, 2Hpyrdine), 8.11(d, 3JHH = 7.6, 2Hpyridine), 7.87(t, 3JHH = 8.0, 2Hpyridine), 7.56(dd, 3JHH = 3.6, 2Hbenzene), 7.26(dd, 3JHH = 3.6, 2Hbenzene), 5.61(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.52(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.34(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.05(d, 3JHH = 4.4 Hz, 2Hpcymene), 2.12(m, 3JHH = 6.8 Hz, 2Hpcymene), 1.49 (s, 6-Hpcymene), 0.98(d, 3JHH = 6.8, 6Hpcymene), 0.80(d, 3JHH= 6.8, 6Hpcymene). 13C{1H} NMR (101 MHz, CDCl3) δ:162.6, 154.1, 143.7, 139.2, 130.2, 128.9, 125.8, 124.9, 81.6, 30.6, 22.1, 18.7. ESI-MS (m/z); 823 [M+, 100%]. HR-MS (ESI): m/z 823.0958 [M+], calcd for C38H40N4O2ClO2Ru2 823.0927. FT-IR (cm−1): (νC=O)amidate = 1618.76. Anal. Cald. for C38H40ClN4O2Ru2PF6: C, 47.18; H, 4.17; N, 5.79. Found: C, 47.31; H, 3.94; N, 5.42.

4.3.3. [{Ru(η6-p-cymene)2-μ-Cl}2L2][PF6] (Ru3)

[Ru(η6-p-cymene)Cl2]2 (0.10 g, 0.16 mmol), H2L2 (0.06 g, 0.16 mmol), NaOMe (0.02 g, 0.32 mmol) and KPF6 (0.03 g, 0.16 mmol). An orange compound was obtained. Yield: 0.11 g (72%). 1H NMR (400 MHz, CDCl3) δ: 9.40 (d, 3JHH = 5.2, 2Hpyridine), 8.28(t, 3JHH = 7.2, 2Hpyrdine), 8.08(d, 3JHH = 7.6, 2Hpyridine), 7.80(t, 3JHH = 8.0, 2Hpyridine), 7.30(s, 2Hbenzene), 5.61(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.49(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.38(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.05(d, 3JHH = 4.4 Hz, 2Hpcymene), 2.12(m, 3JHH = 6.6 Hz, 2Hpcymene), 3.31 (s, CH3), 2.32 (s, 6Hpcymene), 0.98(d, 3JHH = 6.6, 6Hpcymene), 0.80 (d, 3JHH = 6.6, 6Hpcymene). 13C{1H} NMR (101 MHz, CDCl3) δ:162.5, 154.5, 141.0, 139.3, 133.3, 130.5, 129.0, 126.4, 125.9, 81.3, 36.4, 31.4, 24.1, 19.6. ESI-MS (m/z); 851 [M+, 100%]. HR-MS (ESI): m/z 851.1271 [M+], calcd for C40H44ClN4O2Ru2 81.1240. FT-IR (cm−1): (νC=O)amidate = 1619.97. Anal. Cald. for C40H44ClN4O2Ru2PF6: C, 48.27; H, 4.46; N, 5.63. Found: C, 48.28; H, 4.42; N, 5.37.

4.3.4. [{Ru(η6-p-cymene)2-μ-Cl}2L3][PF6] (Ru4)

[Ru(η6-p-cymene)Cl2]2 (0.10 g, 0.16 mmol), H2L3 (0.06 g, 0.16 mmol), NaOMe (0.02 g, 0.32 mmol) and KPF6 (0.03 g, 0.16 mmol). Orange compound was obtained. Yield: 0.13 g (88%). 1H NMR (400 MHz, CDCl3) δ: 9.41 (m, 2Hpyridine), 8.26(m, 2Hpyrdine), 8.13(m, 2Hpyridine), 7.78(m, 2Hpyridine), 7.46(d, 3JHH = 4.6 Hz, 2Hbenzene), 7.08(d, 3JHH = 4.6 Hz, 2Hbenzene), 7.06(s, 1Hbenzene), 5.61(d, 3JHH = 4.6Hz, 2Hpcymene), 5.49(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.38(d, 3JHH = 4.4 Hz, 2Hpcymene), 5.05(d, 3JHH = 4.4 Hz, 2Hpcymene), 2.12(m, 3JHH = 6.6 Hz, 2Hpcymene), 3.31 (s, OCH3), 2.32 (s, 6Hpcymene), 0.98(d, 3JHH = 6.6, 6Hpcymene), 0.80 (d, 3JHH = 6.6, 6Hpcymene). 13C NMR (101 MHz, CDCl3) δ: 169.5, 155.4, 141.0, 154.2, 144.8, 140.6, 137.4, 129.3, 128.4, 126.6, 115.09, 110.7, 86.8, 82.8, 81.3, 55.6, 20.7, 23.1, 20.9, 18.1. ESI-MS m/z (%); 823 [M+, 100%]. FT-IR (cm−1): (νC=O)amidate = 1619.11. Anal. Cald. for C41H46ClN4O3Ru2PF6: C, 48.03; H, 4.52; N, 5.46. Found: C, 48.14; H, 4.38; N, 5.81.

4.3.5. Transfer Hydrogenation of Experiments

A modified transfer hydrogenation of ketones produced was as followed: a stock solution of the ruthenium(II) complex, for example Ru1 (0.0010 M) was prepared in 10.0 mL isopropanol. To a solution of acetophenone (1.15 mL, 1.00 mmol), KtBuO (1 mL, 0.04 M in iPrOH) and Ru1 (550 ppm) were added and diluted with 2.5 mL of pure iPrOH. The resultant solution was then refluxed at 82 °C, during which about 0.1 mL aliquot of the crude mixture was taken at regular time intervals. The percentage conversions and yields were determined using 1H NMR spectroscopy by comparing the intensity of the methyl signals of acetophenone (s, δH: 2.59 ppm) to those of the 1-phenyl ethanol (d, δH: 1.49 ppm) of the crude products [42,43,44,45].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics10110190/s1, Crystallographic data are deposited with the Cambridge Crystallographic Data Centre, CCDC deposition numbers: 217198 and 217201. ESI (electronic supplementary information) contains the spectroscopic data used in this article. Figures S1–S7: 1H NMR spectra data of ligands and Ru(II) complexes. Figures S8–S14: 13C NMR spectra of the ligands and Ru(II) complexes. Figures S15–S17: 31P NMR spectra of complexes Ru2–Ru4. Figures S18–S20: 19F NMR spectra of complexes Ru2–Ru4. Figures S21–S27: FT-IR spectra of the ligands and Ru(II) complexes. Figures S28–S34: ESI-MS spectra of ligands and Ru(II) complexes. Figures S35–S38: Molecular structures and packing diagrams of complexes Ru1 and Ru4. Figures S39–S46: 1H NMR spectra of TH aliquots taken at different time intervals for determination of percentage yields. Figure S47: A plot of Conversion, TOF vs catalyst loading used to determine optimised reaction conditions. Figure S48: The plot of In[Ac.]t/[Ac.]0 vs. time for determination of kobs of the catalysts.

Author Contributions

R.T.K.: Investigation, experimental work, validation, software, formalization of analyses, and drafting of the original manuscript. S.T.M.: Investigation, experimental work, validation. S.O.O.: Supervision, resources, review, and editing of the final draft, corresponding author. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data available as Supplementary Materials.

Acknowledgments

Thank you to the University of KwaZulu-Natal for providing financial assistance and to Leigh Hunter for solving the structures of complexes Ru1 and Ru4.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 10 00190 sch001 550
Scheme 1. Preparation of dinuclear piano-stool ruthenium(II) compounds Ru1–Ru4.
Scheme 1. Preparation of dinuclear piano-stool ruthenium(II) compounds Ru1–Ru4.
Inorganics 10 00190 sch001
Inorganics 10 00190 g001 550
Figure 1. Molecular structure of Ru1, with ellipsoids drawn at 50% probability level. Hydrogen atoms, CH2Cl2 solvent and [RuCl3(p-cymene)]- counter-anion have been omitted for clarity. Selected bond lengths (Å): N(1)-Ru(1), 2.104(4); N(2)-Ru(1), 2.094(4); Ru(1)-Cl(1), 2.4526(12); Ru(1)-Ru(2), 4.168(5). Selected bond angles (°): N(1)-Ru(1)-N(2), 77.55(15); N(1)-Ru(1)-Cl(1), 85.37(11); N(2)-Ru(1)-Cl(1), 86.96(11); Ru(1)-Cl(1)-Ru(2), 115.68(4).
Figure 1. Molecular structure of Ru1, with ellipsoids drawn at 50% probability level. Hydrogen atoms, CH2Cl2 solvent and [RuCl3(p-cymene)]- counter-anion have been omitted for clarity. Selected bond lengths (Å): N(1)-Ru(1), 2.104(4); N(2)-Ru(1), 2.094(4); Ru(1)-Cl(1), 2.4526(12); Ru(1)-Ru(2), 4.168(5). Selected bond angles (°): N(1)-Ru(1)-N(2), 77.55(15); N(1)-Ru(1)-Cl(1), 85.37(11); N(2)-Ru(1)-Cl(1), 86.96(11); Ru(1)-Cl(1)-Ru(2), 115.68(4).
Inorganics 10 00190 g001
Inorganics 10 00190 g002 550
Figure 2. Molecular structure of Ru4, with ellipsoids drawn at 50% probability level. Hydrogen atoms and [PF6]– counter-anion are omitted for clarity. Selected bond distances (Å): N(1)-Ru(1), 2.097(2); N(2)-Ru(1), 2.081(2); Ru(1)-Cl(1), 2.4627(7); Ru(1)-Ru(2), 4.168(5). Selected bond angles (°): N(2)-Ru(1)-N(1), 77.54(9); N(1)-Ru(1)-Cl(1), 83.23(7); N(2)-Ru-Cl(1), 85.79(6).
Figure 2. Molecular structure of Ru4, with ellipsoids drawn at 50% probability level. Hydrogen atoms and [PF6]– counter-anion are omitted for clarity. Selected bond distances (Å): N(1)-Ru(1), 2.097(2); N(2)-Ru(1), 2.081(2); Ru(1)-Cl(1), 2.4627(7); Ru(1)-Ru(2), 4.168(5). Selected bond angles (°): N(2)-Ru(1)-N(1), 77.54(9); N(1)-Ru(1)-Cl(1), 83.23(7); N(2)-Ru-Cl(1), 85.79(6).
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Inorganics 10 00190 g003 550
Figure 3. Time-dependent transfer hydrogenation reaction of acetophenone catalysed by Ru1–Ru4, 5.00 × 10−3 mol% (50 ppm); tBuOK: 4.0 mol%; temp. 82 °C). (a) Yield (%) vs. time graph, (b) −In[Ac.]t/[Ac.]0 vs. time plot of the Ru1–Ru4 catalysts (Ac.—acetophenone).
Figure 3. Time-dependent transfer hydrogenation reaction of acetophenone catalysed by Ru1–Ru4, 5.00 × 10−3 mol% (50 ppm); tBuOK: 4.0 mol%; temp. 82 °C). (a) Yield (%) vs. time graph, (b) −In[Ac.]t/[Ac.]0 vs. time plot of the Ru1–Ru4 catalysts (Ac.—acetophenone).
Inorganics 10 00190 g003
Table
Table 1. Summary of crystallographic parameters and refinement data.
Table 1. Summary of crystallographic parameters and refinement data.
ParametersRu1Ru4
Empirical formulaC48H54Cl4N4O2Ru3+[CH2Cl2]C39H42ClF6N4O3PRu2
  Formula weight1418.74997.32
  Temperature/K100 (2)100 (2)
  Wavelength(Å)1.541781.54170
  Crystal systemTriclinicMonoclinic
  Space groupP-1P21/c
  Unit cell dimensions;
  a13.121 (2)Å15.3868 (4)
  b15.820 (3)Å19.3230 (5)
  c16.506 (4)Å13.7638 (4)
  α61.430 (6)◦90
  β71.530 (11)◦101.771 (1)
  ɣ72.142 (7)◦90
Volume2803.9 (10)Å34006.18 (19)
Z24
Density (calculated) / Mg/m31.680 1.654
Absorption coefficient/ mm−111.1867.713
F(000)1424.0200080
Crystal Size (mm3)0.14 × 0.095 × 0.070.15 × 0.15 × 0.15
Theta range for data collection71.96168.225
  Reflections collected110367297
  Completeness 97.6%99.4%
  Refinement methodFull-matrix least-square on F2Full-matrix least-square on F2
  Goodness-of-fit on F21.0761.057
  Final R indices [I>2sigma(I)]R1 = 0.0511, wR2 = 0.1447R1 = 0.0278, wR2 = 0.0782
  R indices (all data)R1 = 0.0567, wR2 = 0.1509R1 = 0.0282, wR2 = 0.0785
Largest diff. peak and hole/eA−31.55/−2.571.42/−0.50
Table
Table 2. Effects of catalyst concentration and base on transfer hydrogenation of acetophenone using complex Ru2 as a catalyst a.
Table 2. Effects of catalyst concentration and base on transfer hydrogenation of acetophenone using complex Ru2 as a catalyst a.
Inorganics 10 00190 i001
EntryCatalyst Loading/× 10−3 mol% (ppm)Baseb Conversion[%]Yield%TON × 104TOF × 103/h−1
1-KtBuO21--
25.00 (50)-32300.641.07
32.50 (25)KtBuO41391.562.60
45.00 (50)KtBuO86851.722.87
515.00 (150)KtBuO95940.631.05
625.00 (250)KtBuO96950.260.43
755.00 (550)KtBuO99980.130.22
85.00 (50)KOH76761.712.85
95.00 (50)K2CO329280.580.97
a Conditions: 1.0 mmol acetophenone, 0.4 mol%, tBuOK in 1.00 mL iPrOH, temperature 80 °C, time = 6 h. b Determined by 1H NMR spectroscopy. Anisole was used as an internal standard. Turnover number (TON) = moles of product formed/moles of catalyst used. Turnover frequency (TOF) = moles of product formed/moles of catalyst/time (h). Experiments were performed in triplicate to ensure reproducibility (standard deviation, S.D = ±1.0).
Table
Table 3. Transfer hydrogenation of acetophenone catalysed by Ru1–Ru4 complexes: Effect of catalyst structure a.
Table 3. Transfer hydrogenation of acetophenone catalysed by Ru1–Ru4 complexes: Effect of catalyst structure a.
Inorganics 10 00190 i002
EntryCatalystb Conv. [%]b Yield [%]TON × 104TOF/h−1 x 103kobs × 10−1/h−1
1[RuCl2(p-cymene)2]12110.020.03--
2Ru199981.963.271.97
3Ru286851.722.871.69
4Ru396941.883.131.76
5Ru492921.843.071.73
a Conditions: acetophenone: 1.00 mmol; [Ru]: 5.00 × 10−3 mol% (50 ppm); tBuOK: 4.00 mol% in 2.5 mL and diluted with 1.00 mL IPrOH, temperature 80 °C, 6 h. b Determined by 1H NMR spectroscopy. Anisole was used as an internal standard. c [Ru] = 5.5 × 10−2 mol% (550 ppm). Turnover number (TON) = moles of product formed/moles of catalyst, [Ru] = concentration of the catalyst. Turnover frequency (TOF) = moles of product formed/moles of catalyst/time (h), [Ru] = concentration of the catalyst. Experiments were performed in triplicate to ensure reproducibility, S.D = ±1.0).
Table
Table 4. Investigation of substrate scope using complex Ru2 as a catalyst a.
Table 4. Investigation of substrate scope using complex Ru2 as a catalyst a.
EntryKetoneb Yield (%)EntryKetoneb Yield%
1Inorganics 10 00190 i003868Inorganics 10 00190 i00475
2 *Inorganics 10 00190 i005999Inorganics 10 00190 i00678
3 *Inorganics 10 00190 i0079910Inorganics 10 00190 i00888
4Inorganics 10 00190 i0097011Inorganics 10 00190 i01084
5Inorganics 10 00190 i0118112Inorganics 10 00190 i01276
6Inorganics 10 00190 i0137913Inorganics 10 00190 i01473
7Inorganics 10 00190 i0157814Inorganics 10 00190 i01688
a Conditions: 1.0 mmol acetophenone, tBuOK (4.00 mol%) in 2.5 mL and diluted with 1.00 mL iPrOH, Ru2, 5.00 × 10−3 mol% (50 ppm) temperature 80 °C, 6 h. b Determined by 1H NMR spectroscopy. Anisole was used as an internal standard. * Reaction time = 4 h. (All experiments were carried outs in triplicate to ensure reproducibility, S.D = ±1.0).
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Kumah, R.T.; Mvelase, S.T.; Ojwach, S.O. Syntheses and Applications of Symmetrical Dinuclear Half-Sandwich Ruthenium(II)–Dipicolinamide Complexes as Catalysts in the Transfer Hydrogenation of Ketones. Inorganics 2022, 10, 190. https://doi.org/10.3390/inorganics10110190

AMA Style

Kumah RT, Mvelase ST, Ojwach SO. Syntheses and Applications of Symmetrical Dinuclear Half-Sandwich Ruthenium(II)–Dipicolinamide Complexes as Catalysts in the Transfer Hydrogenation of Ketones. Inorganics. 2022; 10(11):190. https://doi.org/10.3390/inorganics10110190

Chicago/Turabian Style

Kumah, Robert Tettey, Sabathile Thandeka Mvelase, and Stephen Otieno Ojwach. 2022. "Syntheses and Applications of Symmetrical Dinuclear Half-Sandwich Ruthenium(II)–Dipicolinamide Complexes as Catalysts in the Transfer Hydrogenation of Ketones" Inorganics 10, no. 11: 190. https://doi.org/10.3390/inorganics10110190

APA Style

Kumah, R. T., Mvelase, S. T., & Ojwach, S. O. (2022). Syntheses and Applications of Symmetrical Dinuclear Half-Sandwich Ruthenium(II)–Dipicolinamide Complexes as Catalysts in the Transfer Hydrogenation of Ketones. Inorganics, 10(11), 190. https://doi.org/10.3390/inorganics10110190

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