Synthesis and Structure of a Mixed-Ligand Dinuclear Ruthenium

Communication pubs.acs.org/Organometallics

Synthesis and Structure of a Mixed-Ligand Dinuclear Ruthenium Trihydrido Complex Supported by Cn* and Cp* Ligands (Cn* = 1,4,7Trimethyl-1,4,7-triazacyclononane, Cp* = η5‑C5Me5): Enhancement of Reactivity toward CO2 by Introduction of the Cn* Ligand Kyo Namura, Masato Ohashi, and Hiroharu Suzuki* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: The novel mixed-ligand diruthenium trihydrido complex Cn*Ru(μ-H)3RuCp* (2; Cn* = 1,4,7-trimethyl1,4,7-triazacyclononane, Cp* = η5-C5Me5) was synthesized, and its structure was determined by an X-ray diffraction study. Density functional theory (DFT) calculations for 2 revealed that the electron density at the Cn*-ligated ruthenium atom is higher than that at the Cp*-ligated ruthenium atom. Whereas the unmixed Cp*,Cp*-ligated Cp*Ru(μ-H)4RuCp* (5) exhibited no reactivity toward CO2 at 20 atm, the mixed-ligand complex 2 reacted with CO2 smoothly at atmospheric pressure and afforded the bis(μ-formato) complex Cn*Ru(μ-η1:η1-O2CH)2(μH)RuCp* (4) quantitatively.

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functional theory (DFT) calculations, and the effects of the different ligands were evaluated using these methods. Furthermore, the introduction of the Cn* ligand increased the electron density at the ruthenium center, and as a result complex 2 should exhibit a high reactivity toward CO2 to give the bis(μ-formato) complex Cn*Ru(μ-η 1 :η 1 -O 2 CH) 2 (μH)RuCp* (4) at atmospheric pressure. The addition of excess potassium hydride to a suspension of [Cn*RuH(H2)2](PF6) (1)6 in THF afforded a pale yellow solution, to which a slurry of [Cp*RuCl]49 in THF was added dropwise. During the addition, the solution turned purple and mixed-ligand complex 2 was formed. Extraction with toluene afforded analytically pure complex 2 in 91% yield (Scheme 1).10

ransition-metal clusters often exhibit unique reactivity, the so-called “multimetallic activation”, stemming from multiple coordination of substrates.1 We have synthesized dinuclear to pentanuclear transition-metal polyhydrido clusters supported by substituted cyclopentadienyl groups,2 which undergo smooth cleavage of thermodynamically robust bonds such as C−H bonds in alkanes3 and N−H bonds in ammonia.4 Recently, we reported the synthesis and reactivity of mixedligand heterobimetallic complexes supported by 1,4,7-trimethyl1,4,7-triazacyclononane (Cn*)5 and Cp* (C5Me5) ligands: namely, [Cn*Ru(μ-H)3IrCp*]+ and [Cn*Ru(μH)3OsHCp*]+.6 Both Cn* and Cp* are coordinated to the metal center in a facial geometry and formally donate six electrons on the ionic model. However, Cn* is a neutral ligand, while Cp* is an anionic ligand.7 In addition, because Cn* has more σ-donor character than Cp* because of almost no πacceptor character, substitution of Cn* for Cp* increases the electron density at the metal center and enhances the nucleophilicity of the cluster. However, in heterobimetallic clusters with mixed-ligand systems, it is difficult to evaluate clearly the influence of each ligand separately because the frameworks of these complexes are composed of different metals. In this context, we attempted the synthesis of a homometallic complex with a mixed-ligand system, Cn* and Cp*. Close investigation of the structure, electronic character, and reactivity of the mixed-ligand homometallic complex enables clear evaluation of the influence of different ligands on the multimetallic core.8 In this paper, we report the synthesis of the Cn*−Cp* mixed-ligand diruthenium trihydrido complex Cn*Ru(μH)3RuCp* (2). The structure and electronic properties of 2 were established by an X-ray diffraction study and density © 2012 American Chemical Society

Scheme 1. Preparation of the Mixed-Ligand Complex 2

Complex 2 was identified on the basis of NMR spectroscopy. The 1H NMR spectrum of 2 in THF-d8 showed three sharp singlets at δ −21.53 (3H), 1.84 (15H), and 3.41 (9H), stemming from the bridging hydride, the Cp* group, and the methyl groups in the Cn* ligand, respectively. The intensity Received: June 6, 2012 Published: August 20, 2012 5979

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Cn* ligand significantly increases the electron density at Ru1. Furthermore, the three hydrido ligands possess small positive charges (+0.05). The shortening of the three Ru1−H bonds reflects the fact that Ru1 is more electronegative than Ru2. The reaction of 1 with excess potassium hydride was carried out in an NMR tube. The 1H NMR spectrum of a mixture of 1 and excess potassium hydride showed the complete consumption of 1 and the clean formation of the deprotonated mononuclear complex Cn*RuH4 (3). Even in the presence of excess potassium hydride, the deprotonation of 3 did not proceed. The 1H NMR spectrum of 3 in THF-d8 showed two sharp singlets at δ −16.11 (4H) and 3.21 (9H) stemming from the hydrido ligands and the methyl groups in the Cn* ligand, respectively. Complex 3 underwent an H/D exchange reaction with D2 at 40 °C. In this reaction, the formation of the isotopomer 3-dn (n = 1−3) was observed, which indicates that 3 is a mononuclear tetrahydrido complex (see the Supporting Information). The formation of 3 strongly suggests that the reaction of 3 with [Cp*RuCl]4 results in the formation of the monocationic dinuclear tetrahydride intermediate A, which affords 2 by deprotonation (Scheme 2).14 Complex 3 was not isolated because of difficulty in its separation from potassium hexafluorophosphate.

ratio of these signals indicates that complex 2 includes a Cn* ligand and a Cp* ligand in the molecule. The X-ray diffraction study of 2 unambiguously demonstrated a dinuclear structure (Figure 1). The three bridging

Figure 1. ORTEP drawing of 2 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru1− Ru2, 2.4676(6); Ru1−N1, 2.166(5); Ru1−N2, 2.164(6); Ru1−N3, 2.151(6); Ru2−CP, 1.792; Ru1−Ru2−CP, 178.2 (CP = Cp* centroid).

hydrido ligands occupied the positions trans to the three nitrogen atoms in the Cn* ligand. The coordination geometries at Ru1 and Ru2 in 2 can be described as octahedral and a threelegged-stool-like arrangement, respectively. The Ru1−Ru2 distance of 2.4676(6) Å is comparable to the metal−metal bond lengths of a 30-electron diruthenium trihydrido complex with hexamethylbenzene ligands, [{(η 6 -C 6 Me 6 )Ru} 2 (μH)3](PF6) (2.4681(4) Å),11 and a diruthenium tetrahydrido complex with Cp* ligands, Cp*Ru(μ-H)4RuCp* (5) (2.463(1) Å).2a Density functional theory (DFT, B3PW91 level12) calculations were performed for 2, and the resulting optimized structure of 2 is shown in Figure 2. The average metal−hydrido bond lengths (Ru1−H(av) = 1.725 Å, Ru2−H(av) = 1.893 Å) indicate the significant elongation of the Ru2−H bonds. Natural population analysis13 revealed that the natural charge at Ru1 (−0.654) is more negative than that at Ru2 (−0.317), which indicates that the introduction of the highly σ-donating

Scheme 2. Possible Formation Process of Complex 2

The application of carbon dioxide (CO2) to synthetic chemistry as a source of a C1 building block is important in order to utilize carbon resources effectively.15 Tuning the electronic environment of the metal center by the selection of appropriate ligands and metals enables uptake of CO2 because the oxygen atoms of CO2 are weakly Lewis basic and the carbon atom is weakly Lewis acidic.16 Because the electron density at the metal center was increased by the introduction of the Cn* ligand, the insertion of CO2 into the Ru−H bonds of 2 proceeded smoothly. When CO2 (1 atm) was introduced into the THF solution of 2 at room temperature, the solution immediately turned yellow and the bis(μ-formato) complex Cn*Ru(μ-η 1 :η 1 -O 2 CH) 2 (μH)RuCp* (4) was formed quantitatively (Scheme 3).17 The 1H NMR spectrum of 4 in THF-d8 showed sharp singlets at δ −17.62 (1H), 1.59 (15H), and 7.85 (2H); these were assigned to the bridging hydrido ligand, the Cp* group, and the bridging formato ligands, respectively. The methyl signals of the Cn* ligands appeared at δ 2.91 (6H) and 2.92 (3H), which indicates that 4 has Cs symmetry. In the 13C NMR spectrum, the formate carbon appeared at δ 175.1 (d, JCH = 194 Hz).

Figure 2. Optimized structure of 2 at the B3PW91 level. Hydrogen atoms on carbons are omitted for clarity. Natural charges are underlined. Bond distances (Å) and angles (deg): Ru1−Ru2, 2.4792; Ru1−N1, 2.2067; Ru1−N2, 2.2057; Ru1−N3, 2.2018; Ru2− CP, 1.7669; Ru1−H1, 1.7267; Ru1−H2, 1.7224; Ru1−H3, 1.7249; Ru2−H1, 1.8892; Ru2−H2, 1.8971; Ru2−H3, 1.8948; Ru1−Ru2−CP, 179.82 (CP = Cp* centroid). 5980

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Scheme 3. Reaction of 2 with CO2

Scheme 4. Preparation of the Bis(μ-formato) Complex 6

The X-ray diffraction study of 4 demonstrated a dinuclear structure containing two bridging formato ligands. One of the two crystallographically independent molecules of 4 is shown in Figure 3. The coordination geometries at Ru1 and Ru2 in 4 can

Many transition-metal formato complexes synthesized by the insertion of CO2 into M−H bonds have been reported.15a,e However, the preparation of a μ-formato complex by the insertion of CO2 into a metal-bridging hydride is relatively rare. To date, dinuclear iron,20 rare-earth-metal,21 zinc,22 and titanium23 μ-formato complexes obtained by CO2 insertion into the μ-hydrido ligands have been reported. To the best of our knowledge, complex 4 is the first example of a diruthenium μ-formato complex prepared by the insertion of CO2 into a metal-bridging hydride. In summary, we synthesized the novel mixed-ligand diruthenium trihydrido complex Cn*Ru(μ-H)3RuCp* (2) in good yield through the reaction of Cn*RuH4 (3) with [Cp*RuCl]4. Natural population analysis of 2 revealed that the Cn*-ligated ruthenium atom possesses higher electron density than the Cp*-ligated atom and that electronic anisotropy is induced by different ligands. Furthermore, complex 2 indicated high reactivity toward CO2, because the introduction of the highly σ-donating Cn* ligand increased the electron density at the reaction field and enhanced the nucleophilicity of the complex. Whereas the Cp*,Cp*-ligated complex Cp*Ru(μ-H)4RuCp* (5) indicated no reactivity toward CO2 even at 20 atm, the Cn*,Cp*-ligated complex 2 reacted with CO2 smoothly at atmospheric pressure and afforded the bis(μ-formato) complex Cn*Ru(μ-η 1 :η 1 O2CH)2(μ-H)RuCp* (4) quantitatively. Further studies on the reactivities and the coupling reactions of CO2 with organic substrates by 2 and 4 are now being investigated, and the results will be reported in due course.

Figure 3. ORTEP drawing of 4 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru1− Ru2, 3.0892(6); Ru1−O1, 2.105(4); Ru1−O3, 2.128(4); Ru2−O2, 2.187(4); Ru2−O4, 2.192(4); O1−C1, 1.254(7); O2−C1, 1.233(7); O3−C2, 1.240(7); O4−C2, 1.228(7); Ru2−CP, 1.739; O1−C1−O2, 128.4(5); O3−C2−O4, 128.3(6) (CP = Cp* centroid).

be described as octahedral and a three-legged-stool-like arrangement, respectively. Moreover, the electronic anisotropy in the metal centers, namely the unsymmetrical electron distribution, affected the structures of formato ligands. The average Ru1O−C distance (1.249 Å) was longer than the average Ru2O−C distance (1.232 Å) by 0.02 Å. The optimized structure of 4 at the B3PW91 level shows similar structural features (see the Supporting Information). The slight elongation of Ru1O−C bonds could reflect that Ru1O−C bonds are more reduced than Ru2O−C bonds because the electron donation by the Cn* ligand to Ru1 is stronger than that by the Cp* ligand to Ru2. A mixed-ligand diiron bis(μacetato) μ-oxo complex supported by Cn* and tris(pyrazolyl)borate ligands was reported by Slep et al., and its bridging acetato ligands showed a similar structural feature.18 The metal−metal bond length of 4 (Ru1−Ru2(av) = 3.088) was elongated by 0.62 Å because of the insertion of two CO2 molecules. We previously reported that the Cp*,Cp*-ligated complex Cp*Ru(μ-H)4RuCp* (5) reacts with formic acid and affords the bis(μ-formato) complex Cp*Ru(μ-η 1 :η 1 -O 2 CH) 2 (μH)2RuCp* (6) quantitatively (Scheme 4).19 Whereas complex 5 had no reactivity toward CO2 even at 20 atm at room temperature and a formato complex such as complex 6 was not formed at all, the Cn*,Cp*-ligated complex 2 reacted with CO2 smoothly at atmospheric pressure. Because the introduction of the highly σ-donating Cn* ligand increased the electron density at the ruthenium center and enhanced the nucleophilicity of the complex, complex 2 demonstrated a greater affinity toward CO2 than 5.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Text, figures, tables, and CIF files giving synthetic details for compounds 3, H/D exchange experiment for 3, NMR spectra of 3, details for DFT calculations, and X-ray crystallographic data for 2 and 4. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research is supported by Grant No. 18105002 (Scientific Research (S)) from the Japan Society of the Promotion of Science. One of the authors, K. N., is supported by JSPS (Grant-in-Aid for JSPS fellows). 5981

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(14) We reported the Cn*Ru−Cp*Os analogue of intermediate A.6 (15) (a) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrmann, W. A.; Kühn, F. E. Angew. Chem., Int. Ed. 2011, 50, 8510. (b) Riduan, S. N.; Zhang, Y. Dalton Trans. 2010, 39, 3347. (c) Sakakura, T.; Choi, J. C.; Yasuda, H. Chem. Rev. 2007, 107, 2365. (d) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388. (e) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95, 259. (16) (a) Leitner, W. Coord. Chem. Rev. 1996, 153, 257. (b) Gibson, D. H. Chem. Rev. 1996, 96, 2063. (17) Synthesis of 4: a 50 mL Schlenk tube was filled with 2 (111.8 mg, 218.5 μmol) and THF (3.0 mL). After the solution was evacuated at −78 °C, CO2 (1 atm) was introduced at room temperature. Immediately, the solution turned yellow and the solution was stirred for 5 min. Removal of the solvent under reduced pressure afforded 4 (131.5 mg, 219.3 μmol, 100% yield) as a yellow solid. Single crystals for X-ray analysis were obtained from THF at −30 °C. Data for 4 are as follows. 1H NMR (400 MHz, THF-d8, room temperature): δ 7.85 (s, 2H, μ-O2CH), 3.33−3.26 (m, 2H, NCH2), 2.923 (s, 3H, NCH3), 2.917 (s, 6H, NCH3), 2.99−2.84 (m, 6H, NCH2) 2.71−2.56 (m, 4H, NCH2), 1.59 (s, 15H, Cp*), −17.62 (s, 1H, RuH). 13C NMR (100 MHz, THF-d8, room temperature): δ 175.1 (d, JCH = 194 Hz, μO2CH), 70.8 (s, C5Me5), 62.7 (t, JCH = 133 Hz, NCH2), 60.7 (t, JCH = 131 Hz, NCH2), 59.3 (t, JCH = 139 Hz, NCH2), 56.6 (q, JCH = 136 Hz, NCH3), 49.4 (q, JCH = 135 Hz, NCH3), 11.4 (q, JCH = 124 Hz, C5Me5). Anal. Calcd for C21H39N3O4Ru2: C, 42.06; H, 6.55; N, 7.01. Found: C, 42.21; H, 6.68; N, 6.92. (18) Slep, L. D.; Mijovilovich, A.; Meyer-Klaucke, W.; Weyhermüller, T.; Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 15554. (19) Suzuki, H.; Kakigano, T.; Igarashi, M.; Tanaka, M.; Moro-oka, Y. J. Chem. Soc., Chem. Commun. 1991, 283. (20) Yu, Y.; Sadique, A. R.; Smith, J. M.; Dugan, T. R.; Cowley, R. E.; Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.; Holland, P. L. J. Am. Chem. Soc. 2008, 130, 6624. (21) Cui, D.; Nishiura, M.; Tardif, O.; Hou, Z. Organometallics 2008, 27, 2428. (22) Schulz, A.; Eisenmann, T.; Schmidt, S.; Bläser, D.; Westphal, U.; Boese, R. Chem. Commun. 2010, 46, 7226. (23) Ni, J.; Qiu, Y.; Cox, T. M.; Jones, C. A.; Berry, C.; Melon, L. Organometallics 1996, 15, 2428.

REFERENCES

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