16-Electron pentadienyl- and cyclopentadienyl-ruthenium half-sandwich complexes with bis(imidazol-2-imine) ligands and their use in catalytic transfer hydrogenation

Abstract

Bis(η⁵-2,4-dimethylpentadienyl)ruthenium(II), [(η⁵-C₇H₁₁)₂Ru] (1, “open ruthenocene”), which has become accessible in high yield and large quantities via an isoprene-derived diallyl ruthenium(IV) complex, can be converted into the protonated open ruthenocene 2 by treatment with HBF₄ and subsequently into the protonated half-open ruthenocene 3 by reaction with cyclopentadiene. The electronic structure of 3 was studied by DFT methods, revealing that the CH-agostic complex [(η⁵-C₅H₅)Ru{(1-4η)-C₇H₁₂-η²-C⁵,H⁵}]BF₄ (A) represents the global minimum, which is 3.7 kcal mol⁻¹ lower in energy than the hydride complex [(η⁵-C₅H₅)RuH(η⁵-C₇H₁₁)]BF₄ (B). 2 and 3 were treated with the ligands N,N′-bis(1,3,4,5-tetramethylimidazolin-2-ylidene)-1,2-ethanediamine (BL^Me) and N,N′-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,2-ethanediamine (BL^iPr) to afford the cationic 16-electron pentadienyl and cyclopentadienyl complexes [(η⁵-C₇H₁₁)Ru(BL^R)]BF₄ (4a, R = Me; 4b, R = iPr) and [(η⁵-C₅H₅)Ru(BL^R)]BF₄ (5a, R = Me; 5b, R = iPr). All complexes catalyse the transfer hydrogenation of acetophenone in isopropanol, and the most active complex 4a in this reaction was employed for the hydrogenation of a broader range of aliphatic and aromatic ketones.

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Introduction

Imidazolin-2-imine ligands such as BL^Me and BL^iPr,^1,2 and related guanidine-type ligands,³ have found widespread use in organometallic and coordination chemistry and proved to be suitable ancillary ligands for applications in homogeneous catalysis.⁴ Because the imidazole moiety can efficiently stabilise a positive charge (Scheme 1),⁵ these diimine species are highly basic and may serve as strong N-donor ligands towards transition metals, as was demonstrated e.g. by the isolation of stable 16-electron half-sandwich complexes of the type [(η⁵-C₅Me₅)Ru(BL^R)]⁺,^6,7 [(η⁷-C₇H₇)Mo(BL^R)]⁺,⁸ and [(η⁶-arene)Ru(BL^R)]²⁺ (R = Me, iPr; arene = cymene, benzene).⁹ The mono- and dicationic cyclopentadienyl- and arene-ruthenium complexes, which are isolable even in the presence of chloride counterions,^6,9 were employed as catalysts for the transfer hydrogenation of acetophenone, with isopropanol serving as the hydrogen source.⁹

Scheme 1 Mesomeric structures for the bis(imidazolin-2-imine) ligands BL^R (R = Me, iPr).

To vary the half-sandwich ruthenium moiety in these coordinatively unsaturated and catalytically active complexes,¹⁰ we chose bis(η⁵-2,4-dimethylpentadienyl)ruthenium(II), [(η⁵-C₇H₁₁)₂Ru] (1, “open ruthenocene”),¹¹ as a suitable starting material, since this compound has recently become conveniently accessible in high yield and large quantities via an isoprene-derived diallyl ruthenium(IV) complex.^12–14 Previously, a related protocol had directly furnished the protonated open ruthenocene 2,¹⁵ which was also obtained by treatment of 1 with HBF₄ (Scheme 2).¹⁶ It was also briefly mentioned that the reaction of 2 with cyclopentadiene affords the corresponding cationic Cp complex 3, but to the best of our knowledge, no experimental details were published.¹⁷ Alternatively, 3 can be prepared by the reaction of the half-open ruthenocene (η⁵-2,4-dimethylpentadienyl)(η⁵-cyclopentadienyl)ruthenium(II), [(η⁵-C₅H₅)Ru(η⁵-C₇H₁₁)], with HBF₄.¹⁸ Complex 2 served as a versatile starting material for the preparation of numerous half-sandwich 2,4-dimethylpentadienyl-ruthenium complexes by displacement of the diene ligand,^12,16,19,20 and consequently we expected that both 2 and 3 would readily react with the diimine ligands BL^R (R = Me, iPr) to afford complexes of type 4 and 5, respectively (Scheme 2).²¹ Thus, the preparation and characterisation of these complexes and their use in hydrogen transfer catalysis is described herein. In addition, an electronic structure calculation of 3 together with full experimental details for its preparation from 2 are reported.

Scheme 2 Preparation of pentadienyl and cyclopentadienyl ruthenium half-sandwich complexes from protonated open and half-open ruthenocenes.

Results and discussion

Preparation and electronic structure of the half-open ruthenocene 3

The open ruthenocene 1 was treated with HBF₄·Et₂O in diethyl ether solution as previously described to afford 2 as a yellow crystalline solid in quantitative yield.¹⁶ Dropwise addition of freshly distilled cyclopentadiene to an acetone solution of 2 at −75 to −80 °C gave 3 as a pale yellow solid in 80% yield after stirring at room temperature and precipitation with diethyl ether (Scheme 2).¹⁷ It should be noted that this method is generally applicable and provides e.g. the corresponding methyl- and tert-butylcyclopentadienyl complexes by reaction with the corresponding cyclopentadiene derivatives.²¹ The NMR spectroscopic data of 3 are identical to those reported previously, with the ¹H NMR spectrum displaying signals at ca. −10.1, −0.3 and 3.0 ppm in the temperature range between −10 and −80 °C,^17,18 which can be assigned to the RuH and to the endo- and exo-CH₂ hydrogen atoms of an agostic ruthenium species (see the ESI†). Accordingly, methyl rotation, i.e. H exchange within an agostic CH₃ group, is frozen out in this temperature range on the NMR time scale, and a detailed dynamic NMR study derived a barrier of ΔG^‡ = 12.6 kcal mol⁻¹ for this process. In contrast, 1,5-H transfer cannot be frozen out (vide supra), and an upper limit of ca. 7.2 kcal mol⁻¹ was estimated for the barrier of the latter process.^17,22

To support the experimental findings and to further characterise the electronic structure of the protonated half-open ruthenocene, DFT calculations were carried out for the cation in 3 at the B3LYP/6-311G(d,p) level of theory. Two minimum structures A and B were located, which are connected by a low-energy transition state TS (Fig. 1). The global minimum A represents an agostic ruthenium complex [(η⁵-C₅H₅)Ru{(1-4η)-C₇H₁₂-η²-C⁵,H⁵)}]BF₄ ²³ with Ru–C5 and Ru–H5 distances of 2.38 and 1.85 Å.²⁴ The agostic C–H bond is elongated (1.18 Å), and the C4–C5 bond is significantly longer than the other C–C bonds (1.49 vs. 1.42, 1.44 and 1.41 Å) within the pentadienyl ligand. Structure B is 3.7 kcal mol⁻¹ higher in energy and represents the hydride complex [(η⁵-C₅H₅)RuH(η⁵-C₇H₁₁)] with a Ru–H bond of 1.57 Å. The pentadienyl ligand is symmetrically bound, with long C1–H and C5–H distances of ca. 2.20 Å. A and B are connected by a transition state (TS) that is 5.1 and 1.4 kcal mol⁻¹ higher in energy than A and B, respectively. Naturally, B can be regarded as an intermediate in the intraligand 1,5-H transfer from C1 to C5, and the low calculated barrier for this process is in agreement with the observation that this process is not frozen out in an accessible temperature range on the NMR time scale.¹⁷

Fig. 1 Potential-energy profile for 1,5-intraligand H transfer in the protonated half-open ruthenocene 3. Values correspond to ΔG_{298 K} = B3LYP/6-311G(d,p) Gibbs free energies at 298 K. For values derived with the B97-D and M06 functionals, see ESI.† TS = transition state.

Preparation of pentadienyl and cyclopentadienyl ruthenium bis(imidazolin-2-imine) complexes

The reaction of the protonated open and half-open ruthenocenes 2 and 3 with the ligands BL^Me and BL^iPr in acetonitrile afforded the half-sandwich pentadienyl and cyclopentadienyl complexes [(η⁵-C₇H₁₁)Ru(BL^R)]BF₄ (4a, R = Me; 4b, R = iPr) and [(η⁵-C₅H₅)Ru(BL^R)]BF₄ (5a, R = Me; 5b, R = iPr) as purple solids in good yield (ca. 80%, Scheme 2). These reactions are likely to proceed via the cationic tris(acetonitrile) complexes [(η⁵-C₇H₁₁)Ru(CH₃CN)₃]⁺ and [(η⁵-C₅H₅)Ru(CH₃CN)₃]⁺,^19,25 which are conveniently accessible from 2 and 3.²¹ NMR spectra recorded in acetone-d₆ gave no indication for the formation of stable solvate complexes with tightly bound acetonitrile or acetone ligands. The ¹H NMR spectra of complexes 4 show the expected signals for the 2,4-dimethylpentadienyl ligand, with a singlet at ca. 4.9 ppm for the 3-CH and doublets at ca. 2.0 and −0.9 ppm (J_H,H = 2.5 Hz) for the exo- and endo-1,5-CH hydrogen atoms. The ¹H NMR spectra of complexes 5 display a singlet at ca. 3.52 ppm for the C₅H₅ moieties, which is at higher field than usually observed for cyclopentadienyl-ruthenium half-sandwich complexes and could be ascribed to shielding by the imidazole rings.²⁴ Resonances assigned to the BL^Me and BL^iPr ligands in 4 and 5 are similar to those reported for the corresponding [(η⁵-C₅Me₅)Ru(BL^R)]⁺ ions.⁶

The crystal structures of 4a·THF, 4b and 5b were determined by X-ray diffraction analyses, confirming in all cases the formation of 16-electron half-sandwich complexes with two-legged piano-stool (pseudo-trigonal planar) geometries around the ruthenium atoms (Fig. 2–4). Interestingly, two different polymorphs of 4a·THF (A and B) were characterised, which both crystallise in the triclinic space group P1̄. While the metal–carbon and metal–nitrogen bond lengths are similar (Table 1), the structures differ in the orientation of the pentadienyl ligand, which renders the molecules either almost C_s- (A, Fig. 2, top) or C₁-symmetric (B, Fig. 2, bottom). These conformations are similar to those found in bis(pentadienyl) complexes, which may exhibit symmetric syn- and anti-eclipsed or asymmetric gauche-eclipsed conformations.²⁶ The latter orientation is also found for the two independent molecules in 4b (Fig. 3, only one cation shown). It should be noted that 4a and 4b represent, to the best of our knowledge, the first structurally characterised half-sandwich pentadienyl ruthenium complexes with a (formal) electron count of 16, and previously described pentadienyl species are usually 18-electron complexes with common three-legged piano-stool (pseudo-tetrahedral) geometries.^16,19,27,285a and 5b are also very rare examples of 16-electron half-sandwich ruthenium complexes bearing the (η⁵-C₅H₅)Ru unit, with [(η⁵-C₅H₅)Ru(tmeda)][BAr^F₄] and [(η⁵-C₅H₅)Ru(dppe)][GaI₄] representing the only previously characterised systems (tmeda = Me₂NCH₂CH₂NMe₂, dppe = Ph₂PCH₂CH₂PPh₂).^29,30 In contrast, a significantly larger number of 16-electron complexes are known that contain the sterically more demanding (η⁵-C₅Me₅)Ru fragment.^31–33

Fig. 2 ORTEP diagram of the cation in 4a·THF in polymorphs A (top) and B (bottom) with thermal displacement parameters drawn at 50% probability (see Table 1 for bond lengths and angles).

Fig. 3 ORTEP diagram of the cation in 4b (only one of the two independent cations is shown) with thermal displacement parameters drawn at 50% probability (see Table 1 for bond lengths and angles).

Fig. 4 ORTEP diagram of the cation in 5b with thermal displacement parameters drawn at 50% probability (see Table 1 for bond lengths and angles).

Table 1 Selected bond lengths (Å) and angles (°) for pentadienyl and cyclopentadienyl ruthenium bis(imidazolin-2-imine) complexes

	4a·THF (polymorph A)	4a·THF (polymorph B)	4b ^a	5b
a The asymmetric unit contains two independent molecules with very similar structural parameters; the values of molecule 1 are shown. b Angle between the RuN2 and imidazole ring planes. c ρ = 2a/(b + c), with a, b, and c representing the exo- (a) and endocyclic (b, c) bonds within the CN3 guanidine moiety.
Ru–N1	2.0529(17)	2.1262(12)	2.123(2)	2.0601(15)
Ru–N2	2.0627(17)	2.0155(12)	2.009(2)	2.0423(15)
Ru–C	2.128(2)–2.159(2)	2.0928(14)–2.1850(14)	2.104(3)–2.153(3)	2.1135(19)–2.1569(19)
C3–N1	1.344(3)	1.3266(18)	1.336(3)	1.348(2)
C3–N3	1.358(3)	1.3620(18)	1.360(3)	1.362(2)
C3–N4	1.351(3)	1.3554(18)	1.360(3)	1.355(2)
C10/C14–N2	1.359(3)	1.3530(18)	1.362(3)	1.353(2)
C10/C14–N5	1.345(3)	1.3472(18)	1.348(3)	1.351(2)
C10/C14–N6	1.351(3)	1.3528(18)	1.355(3)	1.362(2)
N1–Ru–N2	78.52(7)	77.79(5)	78.85(8)	77.31(6)
(N–Ru–N)/(C3N2)^b	69.5; 86.7	62.8; 69.9	79.1; 78.4	83.9; 82.6
ρ	0.992; 1.008	0.976; 1.002	0.982; 1.008	0.992; 0.997

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Similarly to the previously reported complexes [(η⁵-C₅Me₅)Ru(BL^R)]⁺ and [(η⁶-arene)Ru(BL^R)]²⁺,^6,9 the stability of 4 and 5 can be ascribed to the strong π-electron-releasing ability of the BL^R ligands as illustrated by the ylidic mesomeric structure in Scheme 1, which consistently affords short Ru–N bond lengths in the range 2.01–2.13 Å (Table 1). Charge separation and delocalisation can also be clearly deduced from the observation of large dihedral angles between the imidazole and N–Ru–N planes, which rules out the possibility of any substantial π-interaction between the imidazole rings and the nitrogen atoms. As a consequence, the three C–N bond lengths within the imidazolin-2-imine (or guanidine) CN₃ unit are almost equal, and this structural feature can be illustrated by the parameter ρ = 2a/(b + c), with a, b, and c representing the exo- (a) and endocyclic (b, c) bonds.³⁴ In agreement with efficient charge delocalisation, ρ values between 0.98 and 1.01 are found (Table 1).

Catalytic transfer hydrogenation of ketones

Catalytic transfer hydrogenation has become a standard method for the reduction of ketones to secondary alcohols,³⁵ with half-sandwich ruthenium complexes representing the most widely studied catalyst systems.³⁶ Coordinatively unsaturated species are considered as important reaction intermediates, and therefore, the complexes 4 and 5 can be expected to serve as hydrogen transfer catalysts. The hydrogenation of acetophenone was chosen as a test reaction, and the activity of the complexes 4a, 4b, 5a and 5b was studied in boiling 2-propanol (b.p. = 82 °C) with 1 mol% catalyst loading and potassium hydroxide (10 mol%) as the base. The reaction progress was monitored by gas chromatography (GC), and the results are summarized in Table 2. The cyclopentadienyl complexes 5a and 5b display activities similar to those previously reported for the related complexes [(η⁵-C₅Me₅)Ru(BL^R)]X and [(η⁶-arene)Ru(BL^R)]X₂ (X = Cl or weakly-coordinating counterion),⁹ whereas the pentadienyl complexes 4a and 4b show higher activities. The most active complex 4a was further employed as a catalyst under the same conditions for the reduction of a range of aliphatic and aromatic ketones to the corresponding secondary alcohols. In all cases, full conversion was achieved in less than one hour (Table 3).

Table 2 Catalytic transfer hydrogenation of acetophenone^a

Substrate	Catalyst	Time [h]	Conversion [%]	TOF [h^−1]
a Conditions: 0.02 mmol cat., 0.2 mmol KOH, 10 mL iPrOH, 2 mmol acetophenone, temperature: 82 °C, the formation of 2-phenylethanol is monitored by gas chromatography.
Acetophenone	4a	0.5	99	198
Acetophenone	4b	0.75	98	131
Acetophenone	5a	3	86	29
Acetophenone	5b	4	100	25

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Table 3 Catalytic transfer hydrogenation of various ketones with 4a^a

Substrate	Catalyst	Time [min]	Conversion [%]	TOF [h^−1]
a Conditions: 0.02 mmol cat., 0.2 mmol KOH, 10 mL iPrOH, 2 mmol ketone, temperature: 82 °C, the formation of the respective secondary alcohol is monitored by gas chromatography.
3-Pentanone	4a	40	100	150
2-Methylbutanone	4a	60	99	99
Cyclohexanone	4a	10	100	600
Cycloheptanone	4a	40	97	146
4-Heptanone	4a	60	96	96
p-Chloroacetophenone	4a	30	100	200
Benzophenone	4a	60	98	98
1-Acetonaphthone	4a	20	96	288

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Conclusions

This paper provides further evidence for the ability of the bis(imidazolin-2-imine) ligands BL^Me and BL^iPr to stabilise coordinatively unsaturated transition metal complexes, furnishing the complexes [(η⁵-C₇H₁₁)Ru(BL^R)]BF₄ (4a, R = Me; 4b, R = iPr) and [(η⁵-C₅H₅)Ru(BL^R)]BF₄ (5a, R = Me; 5b, R = iPr) as rare examples of 16-electron ruthenium complexes containing pentadienyl- or cyclopentadienyl (C₅H₅) ligands. These complexes were prepared from bis(η⁵-2,4-dimethylpentadienyl)ruthenium(II), [(η⁵-C₇H₁₁)₂Ru] (1, “open ruthenocene”) via the protonated open or half-open ruthenocenes 2 and 3, and this route provides a general and convenient pathway to the important class of pentadienyl and cyclopentadienyl ruthenium half-sandwich complexes.

Experimental

General

All operations were performed in an atmosphere of dry argon using Schlenk and vacuum techniques. All solvents were purified by standard methods and distilled prior to use. ¹H and ¹³C NMR spectra were recorded on a Bruker DPX 400 device. The chemical shifts are given in ppm relative to tetramethylsilane (TMS) as internal standard. The spin coupling patterns are indicated as s (singlet), d (doublet), m (multiplet), sept (septet) and br (broad, for unresolved signals). Elemental analyses (C, H, N) were measured on an Elementar Vario EL III CHNS elemental analyser. 1,¹³2,^14,16 BL^Me,⁶ and BL^iPr,⁶ were prepared according to published procedures.

[(η⁵-C₅H₅)Ru{(1-4η)-C₇H₁₂-η²-C⁵,H⁵}]BF₄ (3, “protonated half-open ruthenocene”). A solution of the protonated open ruthenocene 2 (190 mg, 0.50 mmol) in acetone (7.5 mL) was cooled to −80 °C. A solution of freshly distilled cyclopentadiene (42 μL, 0.50 mmol) in acetone (7.5 mL) was added dropwise at −75 to −80 °C over a period of 20 min. The solution was allowed to warm to room temperature, concentrated to ca. 3 mL, and the product was precipitated with diethyl ether (15 mL). The supernatant solution was removed via syringe, and the precipitate was washed with diethyl ether (3 × 5 mL). The precipitate was dried in vacuo to yield the product as a pale yellow solid (138 mg, 0.40 mmol, 80%). The spectroscopic data are identical to those previously published.^17,18 A variable-temperature NMR study is shown in the ESI.†

[(η⁵-C₇H₁₁)Ru(BL^Me)]BF₄ (4a). A solution of BL^Me (82.2 mg, 0.27 mmol) in acetonitrile (5 mL) was added dropwise to an orange suspension of 2 (100.0 mg, 0.26 mmol) in acetonitrile (10 mL) at room temperature. During the addition of the ligand, the reaction mixture turned purple and the solution was stirred overnight. The solvent was removed under reduced pressure. The residue was dissolved in 5 mL THF and the product precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a purple solid (123.2 mg, 81%). Anal. Calcd for C₂₃H₃₉BF₄N₆Ru: C, 47.02; H, 6.69; N, 14.31. Found: C, 46.92; H, 6.80; N, 13.70. ¹H NMR (400 MHz, CD₂Cl₂, 25 °C): δ = 4.92 (s, 1 H, CH), 3.57 (s, 12 H, NCH₃), 2.81 (s, 4 H, C₂H₄), 2.19 (s, 12 H, BL^Me-CCH₃), 2.00 (d, J_H,H = 2.5 Hz, 2 H, CH₂-exo), 1.49 (s, 6 H, C₇H₁₁–CH₃), −0.86 (d, J_H,H = 2.5 Hz, 2 H, CH₂-endo) ppm. ¹³C{¹H} NMR (100 MHz, CD₂Cl₂, 25 °C): δ = 155.9 (NCN), 119.6 (NCCH₃), 87.0 (C₇H₁₁–CCH₃), 83.7 (CH), 56.3 (C₂H₄), 38.8 (CH₂), 31.2 (NCH₃), 24.8 (C₇H₁₁–CH₃), 9.0 (BL^Me-CCH₃) ppm.

[(η⁵-C₇H₁₁)Ru(BL^iPr)]BF₄ (4b). A solution of BL^iPr (112.5 mg, 0.27 mmol) in acetonitrile (5 mL) was added dropwise to an orange suspension of 2 (100.0 mg, 0.26 mmol) in acetonitrile (10 mL) at room temperature. During the addition of the ligand the reaction mixture turned purple and the solution was stirred overnight. The solvent was removed under reduced pressure. The residue was dissolved in 5 mL THF and the product precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a purple solid (144.6 mg, 81%). Anal. Calc. for C₃₁H₅₅BF₄N₆Ru: C, 53.22; H, 7.92; N, 12.01. Found: C, 53.26; H, 8.12; N, 12.28. ¹H NMR (400 MHz, CD₂Cl₂, 25 °C) δ = 5.35 (sept, ³J_H,H = 7.1 Hz, 4 H, NCH,), 5.01 (s, 1 H, CH), 2.78 (s, 4 H, C₂H₄), 2.29 (s, 12 H, BL^iPr-CCH₃), 2.01 (d, J_H,H = 2.5 Hz, 2 H, CH₂-exo), 1.60 (d, 12 H, CHCH₃, ³J_H,H = 7.1 Hz), 1.60 (s, 6 H, C₇H₁₁–CH₃), 1.43 (d, 12 H, CHCH₃, ³J_H,H = 7.1 Hz), −0.95 (d, J_H,H = 2.5 Hz, 2 H, CH₂-endo) ppm. ¹³C{¹H} NMR (100 MHz, CD₂Cl₂, 25 °C) δ = 155.7 (NCN), 120.6 (NCCH₃), 87.2 (C₇H₁₁–CCH₃), 85.6 (CH), 58.9 (C₂H₄), 48.9 (NCH), 36.6 (CH₂), 24.7 (C₇H₁₁–CCH₃), 22.7 (CHCH₃), 21.8 (CHCH₃), 10.4 (BL^iPr-CCH₃) ppm.

[(η⁵-C₅H₅)Ru(BL^Me)]BF₄ (5a). A solution of BL^Me (54.8 mg, 0.18 mmol) in acetonitrile (5 mL) was added dropwise to an orange suspension of 3 (59.6 mg, 0.17 mmol) in acetonitrile (10 mL) at room temperature. During the addition of the ligand, the reaction mixture turned purple and the solution was stirred overnight. The solvent was removed under reduced pressure. The residue was dissolved in 5 mL THF and the product precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a purple solid (63.0 mg, 65%). Anal. Calcd for C₂₅H₃₅BF₄N₆Ru: C, 45.25; H, 5.97; N, 15.08. Found: C, 45.15; H, 6.13; N, 14.44. ¹H NMR (400 MHz, acetone-d₆, 25 °C): δ = 3.71 (s, 12 H, NCH₃), 3.53 (s, 5 H, C₅H₅), 2.74 (s, 4 H, C₂H₄), 2.28 (s, 12 H, CCH₃) ppm. ¹³C NMR (100 MHz, acetone-d₆, 25 °C): δ = 158.0 (NCN), 120.0 (NCCH₃), 58.3 (C₅H₅), 55.6 (C₂H₄), 31.6 (NCH₃), 8.7 (CCH₃) ppm.

[(η⁵-C₅H₅)Ru(BL^iPr)]BF₄ (5b). A solution of BL^iPr (100.0 mg, 0.24 mmol) in acetonitrile (5 mL) was added dropwise to an orange suspension of 3 (80.3 mg, 0.23 mmol) in acetonitrile (10 mL) at room temperature. During the addition of the ligand the reaction mixture turned purple, and the solution was stirred overnight. The solvent was removed under reduced pressure. The residue was dissolved in 5 mL THF and the product precipitated with n-hexane (30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo afforded the product as a purple solid (128.2 mg, 83%). Anal. Calcd for C₂₉H₄₉BF₄N₆Ru: C, 52.02; H, 7.38; N, 12.55. Found: C, 51.98; H, 7.59; N, 12.40. ¹H NMR (400 MHz, acetone-d₆, 25 °C): δ = 5.40 (sept, ³J_H,H = 7.1 Hz, 4 H, NCH), 3.52 (s, 5 H, C₅H₅), 2.72 (s, 4 H, C₂H₄), 2.41 (s, 12 H, CCH₃), 1.68 (d, 12 H, CHCH₃, ³J_H,H = 7.1 Hz), 1.56 (d, 12 H, CHCH₃, ³J_H,H = 7.1 Hz), ppm. ¹³C NMR (100 MHz, acetone-d₆, 25 °C): δ = 156.9 (NCN), 121.1 (NCCH₃), 58.3 (C₅H₅), 57.8 (C₂H₄), 49.6 (NCH), 22.1 (CHCH₃), 21.7 (CHCH₃), 10.2 (CCH₃) ppm.

General procedure for transfer hydrogenation

Potassium hydroxide (11.2 mg, 0.2 mmol) was added to the catalyst solution (0.02 mmol, 1 mol%) in 2-propanol (10 mL) at room temperature. The solution was heated to 81 °C, and the ketone (2 mmol, 0.2 M) was added. Approximately 0.1 mL samples were regularly taken and filtered through the short silica gel column by using diethyl ether as a solvent. The reaction progress was monitored by gas chromatography (GC).

Single-crystal X-ray structure determinations

Numerical data are summarized in Table 4. Crystals were mounted in inert oil on glass fibres. Intensity data were recorded at low temperature on an Oxford Diffraction Xcalibur E diffractometer using monochromated Mo Kα radiation. Absorption corrections were performed on the basis of multi-scans. Structures were refined anisotropically on F² using the program SHELXL-97.³⁷ Methyls were refined as idealized rigid groups allowed to rotate but not tip; other hydrogens were included using a riding model starting from calculated positions. Exceptions and special features: For both polymorphs of 4a, the hydrogens at C17, C19 and C21 were refined freely (but with DFIX distance restraints). The THF of 4a (polymorph A) and the methylene bridge (C41–C42) of 4b are disordered over two positions. The refinements were stabilized using similarity restraints, but the dimensions of disordered groups should be interpreted with caution. For 5b, the tetrafluoroborate anions lie with their B atoms on crystallographic twofold axes.

Table 4 Crystallographic data^a

	4a·THF (polym. A)	4a·THF (polym. B)	4b	5b
a CCDC 1054781–1054784 contain the supplementary crystallographic data for this paper.
Empirical formula	C27H47BF4N6ORu	C27H47BF4N6ORu	C31H55BF4N6Ru	C29H49BF4N6Ru
Formula weight	659.59	659.59	699.69	669.62
Space group	P1̄	P1̄	P21/c	P2/c
a [Å]	8.0534(2)	7.4989(5)	15.7948(2)	18.4902(4)
b [Å]	13.6581(4)	11.8261(4)	14.0426(4)	11.6717(2)
c [Å]	14.1481(4)	17.6250(8)	31.4970(9)	15.5756(2)
α [°]	100.097(2)	100.627(3)	90	90
β [°]	97.841(2)	95.959(4)	99.176(2)	110.188(2)
γ [°]	96.840(2)	97.082(4)	90	90
V [Å^3]	1501.38(7)	1511.62(13)	6896.6(3)	3154.89(10)
Z	2	2	8	4
T [K]	100(2)	100(2)	100(2)	100(2)
λ [Å]	0.71073	0.71073	0.71073	0.71073
ρ calc[g cm^–3]	1.459	1.449	1.348	1.410
μ [mm^–1]	0.58	0.58	0.51	0.55
Reflections collected	42 715	60 750	128 239	94 761
Independent reflections	7446 [Rint = 0.052]	9616 [Rint = 0.041]	17 068 [Rint = 0.036]	6935 [Rint = 0.053]
Goodness of fit on F^2	0.89	0.94	1.11	0.89
R(Fo) [I > 2σ(I)]	0.0313	0.0258	0.0418	0.0260
wR (Fo^2) (all refl.)	0.0621	0.0554	0.0971	0.0546
Δρ [e Å^–3]	0.63/−0.57	0.74/−0.63	1.10/−0.99	0.46/−0.52

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Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through grant Ta 189/6-3. We thank Dr C. G. Daniliuc and Dr C. G. Hrib for recording the X-ray data.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Selected NMR spectra and computational details. CCDC 1054781–1054784. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/C5DT01080B

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