DOI: 10.1002/chem.201403179
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& Pincer Complexes
Unsymmetrical Pincer-Type Ruthenium Complex Containing bProtic Pyrazole and N-Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere Tatsuro Toda, Shigeki Kuwata,* and Takao Ikariya*[a] electronic N-heterocyclic carbene (protic NHC)[4, 17–21] arms. The proton- and electron-donating ability of the two protic NH groups at the b-positions to the metal was evaluated.
Abstract: A reaction of a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine with [RuCl2(PPh3)3] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer-type ruthenium complex 2 containing a protic pyrazole and N-heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC–pyrazolato complex 3, indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2-propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal–ligand cooperation.
An unprecedented protic NHC–pyrazole hybrid 2 was obtained by chelation-assisted tautomerization of the imidazole group[4, 17–20, 22] in a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine as shown in Equation (1).
Proton-responsive ligands surrounding a metal play crucial roles in metal–ligand bifunctional catalysis.[1] A variety of protic ligands, such as amines,[2, 3] pyrazoles,[4–9] imidazoles,[10] oximes,[11, 12] pyridinols,[13] and picoline/lutidine-based chelates,[14] have been used to facilitate the substrate binding, activation, and transformation through noncovalent interactions. Although most of the artificial bifunctional catalysts have only one kind of such protic ligands, metalloenzymes in Nature generally possess two or more structurally differentiated cooperating groups with various ranges of proton affinity in the second coordination sphere, which accelerate biological transformations in a delicate manner.[15] During our continuing study on the metal–ligand bifunctional catalysts,[3–7, 16, 17] we have recently synthesized a symmetrical pincer-type ruthenium complex 1 featuring two protic pyrazole arms,[5] and also demonstrated that its iron analogue catalyzes disproportionation of hydrazine through bidirectional, two-proton-coupled two-electron transfer.[6] Differentiation of the cooperating protic sites would be beneficial to more efficient substrate recognition, proton transfer, and stabilization of the reaction intermediates. Herein, we describe the synthesis and structure of the unsymmetrical variant 2, which contains a protic pyrazole and iso-
The 1H NMR spectrum of 2 displays two NH singlets at d = 10.41 and 9.53 ppm, which were identified by their facile exchange with added D2O. In the 13C{1H} NMR spectrum, the carbene resonance appeared at d = 182.6 ppm as a triplet split by the two phosphorous nuclei. The mer-, tridentate ligation has been confirmed by X-ray analysis of 2 as depicted in Figure 1.[23] The NHC and pyrazole protons are engaged in a hydrogen-bonding interaction with the chlorido ligand and the solvating methanol, respectively, in line with the expected Brønsted acidity of these protons.[4, 18] In addition, one of the pyridylmethylenic hydrogen atoms lies close to the hexafluorophosphate counteranion, which implies that this methylene group could also be acidic due to potential dearomatization of the pyridine moiety.[14] The cyclic voltammogram of 2 displayed a reversible one-electron oxidation, in which E1/2 = 0.49 V (vs. Fc0/ + ). The oxidation potential is close to that of the bis(pyrazole) analogue 1 (0.51 V), indicating that the electrondonating properties of the protic NHC and pyrazole arms are comparable. When the protic NHC–pyrazole complex 2 was treated with an equimolar amount of a base, deprotonation of the pyrazole arm occurred to give the NHC–pyrazolato complex 3 [Eq. (2)].[25]
[a] T. Toda, Prof. Dr. S. Kuwata, Prof. Dr. T. Ikariya Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology O-okayama, Meguro-ku, Tokyo 152-8552 (Japan) Fax: (+ 81) 3-5734-2637 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403179. Chem. Eur. J. 2014, 20, 1 – 5
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Figure 2. Crystal structure of 3·0.5CH2Cl2. One of the two crystallographically independent molecules is shown. Hydrogen atoms, except for the NH group, as well as the solvating molecule, were omitted for clarity. Ellipsoids are drawn at the 30 % probability level. Selected interatomic distances []: Cl1···N3 3.180(4)/3.226(4) and Cl1···H1 2.646/2.732.
Figure 1. Crystal structure of 2·CH3OH. Hydrogen atoms, except for the NH and methylene groups, were omitted for clarity. Ellipsoids are drawn at the 30 % probability level. Selected interatomic distances []: O1···N1 2.811(4), O1···H1 1.904, Cl1···N3 3.201(3), Cl1···H2 2.719, F1···C9 3.346(4), F1···H8 2.437, F2···C9 3.397(4), and F2···H8 2.524.
Table 1. Selected interatomic distances [] and angles [8] in 2 and 3.
Ru1 Cl1 Ru1 N2 Ru1 C10 N1 N2 N2 C3 N3 C10 N4 C10 N2-N1-C1 N1-N2-C3 C10-N3-C12 N3-C10-N4
Protonation of the pyrazolato complex 3 cleanly regenerated the pyrazole complex 2, showing that the deprotonation is reversible. A low-field 31P-coupled triplet at d = 189.6 ppm in the 13 1 C{ H} NMR spectrum of 3, as well as the D2O-exchangable singlet at d = 9.76 ppm in the 1H NMR, indicated that the protic NHC moiety in 2 remained after the deprotonation. Deprotonation also led to a negative shift of the oxidation potential (E1/2 = 0.04 V vs. Fc0/ + ). The crystal structure of the pyrazolato complex 3 is shown in Figure 2.[23] The b-nitrogen atom in the NHC unit lies close to the chlorido ligand (N3···Cl1 3.203 , mean of two crystallographically independent molecules), whereas that in the pyrazole unit is more separated (N1···Cl1 3.887 , mean) in agreement with the deprotonation of the pyrazole arm, as well as the presence of an intermolecular hydrogen bond between the protic NHC and chlorido ligand. The decreased N2-N1-C1 angle in the pyrazole ring [106.78 (mean) vs. 111.8(3)8 in 2] is also diagnostic of the NH deprotonation.[4] In contrast, the metrical parameters in the NHC ring of 2 and 3 are almost identical as summarized in Table 1. The selective deprotonation demonstrates the stronger Brønsted acidity of the pyrazole group than the isoelectronic NHC unit in 2.[26] Next, we examined deprotonation of the remaining protic NHC ligand. Reaction of the protic NHC–pyrazole complex 2 with two equivalents of a base under dihydrogen led to the formation of the hydrido complex 4 as shown in Scheme 1. The 1H NMR spectrum of 4 displays a hydrido resonance at d = 11.40 ppm, as well as a low-field singlet at d = 7.83 ppm. The &
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2·CH3OH
3·0.5 CH2Cl2 Molecule A
Molecule B
2.451(1) 2.125(3) 1.980(3) 1.342(4) 1.345(4) 1.354(4) 1.354(4) 111.8(3) 106.3(3) 113.0(3) 103.0(2)
2.464(1) 2.129(4) 1.986(4) 1.345(5) 1.374(5) 1.338(5) 1.374(5) 106.5(4) 110.2(3) 113.7(4) 103.4(4)
2.458(1) 2.119(3) 1.990(5) 1.354(5) 1.374(5) 1.367(6) 1.363(6) 106.9(3) 108.9(3) 111.6(4) 104.6(4)
Scheme 1. Formation of hydrido complex 4.
latter signal could be assigned to the protic NHC group, because the 13C{1H} NMR spectrum substantiates the presence of the carbene carbon (d = 191.7 ppm, t). Scheme 1 illustrates the 2
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Communication possible mechanism for formation of 4. Twofold deprotonation of 2 would generate a coordinatively unsaturated imidazolyl– pyrazolato complex 5 via the monodeprotonated complex 3.[27] Subsequent heterolytic cleavage of dihydrogen by the cooperation of Lewis acidic metal center and Brønsted basic imidazolyl group[28] would give the hydrido complex 4. Deprotonation of 2 in 2-propanol also resulted in the formation of 4 (Scheme 1) presumably through hydrogen transfer from 2propanol to 5 in a concerted or stepwise manner. In pioneering studies, Grotjahn and co-workers described hydrogenation and transfer hydrogenation of half-sandwich-type imidazolyl complexes.[19] The stronger Brønsted basicity of the imidazolyl group in 5 compared to the pyrazolato ligands is inferred by similar dehydrochlorination of the bis(pyrazole) complex 1 in methanol, which gave a methanol adduct [RuL(CH3OH)(PPh3)2] (L = doubly deprotonated bis(pyrazole) pincer ligand) instead of the hydrido–pyrazole complex corresponding to 4.[5] On the other hand, preliminary experiments on catalytic hydrogenation of acetophenone suggested that the protic NHC–pyrazole hybrid 2 is less active than the bis(pyrazole) complex 1.[29] The lower activity may be due to the smaller polarization of the bNH group and hydrido ligand in 4. In summary, we have synthesized a new unsymmetrical pincer-type complex 2 containing two b-NH groups with different Brønsted acidity through cyclometalation of an appropriately designed pyrazole–imidazole hybrid ligand. Deprotonation experiments clearly revealed that the protic NHC unit in 2 is less acidic than the pyrazole unit, whereas the electron-donating characters of these N-heterocycles are almost the same on the basis of the electrochemical measurements. In addition, the fully deprotonated complex 5 containing two different Brønsted basic sites in the second coordination sphere promoted heterolytic cleavage of H2 and oxidation of 2-propanol through the metal–ligand cooperation. The present study provides the first direct comparison of the properties of these two isoelectronic b-protic ligands and gives insights to the future design of the metal–ligand bifunctional catalysts. Investigations to explore the catalytic application of these multiprotonresponsive complexes are in progress.
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Experimental Section Details on the reaction procedures, characterization data, and structural determination for 2 and 3 are given in the Supporting and CCDC-997068 Information. CCDC-997067 (2·CH3OH) (3·0.5 CH2Cl2) contain 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/ data_request/cif.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (S; no. 22225004) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and The Sumitomo Foundation (S.K.).
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[24] [25]
[26]
[27]
[28]
[29]
monoclinic, P21/n, a = 19.248(5), b = 22.739(6), c = 23.947(6) , b = 90.993(3)8, V = 10480(5) 3, Z = 8, 1calcd = 1.318 g cm 3, m = 0.504 mm 1, F000 = 4312, transmission factors 0.865–0.961, reflection measured 116752, independent reflections 23951, no. of valuables 1309, Rint = 0.0687, R1 = 0.0696 [I > 2s(I)], wR2 = 0.1713 (all data), residual electron density 2.26/ 2.27 e 3. Data was collected at 93 K with graphite-monochromated MoKa radiation (l = 0.7107 ) to a maximum 2q value of 558. Intensity data were corrected for Lorentz polarization effects and for absorption. Structure solution and refinements were performed with the CrystalStructure program package.[24] The structures were refined against F2. The hydroxy hydrogen atom in the solvating methanol in 2·CH3OH was isotropically refined, whereas the rest hydrogen atoms, except for those in the disordered components, were included in the refinements by using a riding model. CrystalStructure 4.0, Crystal Structure Analysis Package, Rigaku Corporation, Tokyo 196 – 8666, Japan, 2000 – 2010. Complex 2 can also be deprotonated with triethylamine (pKa of BH + = 9.00 in DMSO) in CD2Cl2, whereas 2 seems inert toward 2,6-lutidine (4.46). A 1H NMR experiment revealed that the bis(pyrazole)complex 1 protonates the NHC–pyrazolato complex 3, indicating the Brønsted acidity of 1 superior to that of 2. The pKa values of [RuL(NH3)5]3 + (L = a protic NHC and pyrazole) are reported to be 11.0 and 5.98, respectively: a) M. F. Tweedle, H. Taube, Inorg. Chem. 1982, 21, 3361 – 3371; b) C. R. Johnson, W. W. Henderson, R. E. Shepherd, Inorg. Chem. 1984, 23, 2754 – 2763. Preliminary deprotonation experiments of 2 with two equivalents of KOtBu in toluene under nitrogen led to the formation of a dinitrogen adduct 5–N2, which was characterized by the n˜ (NN) band (2156 cm 1) in the IR spectrum. For heterolytic cleavage of dihydrogen by metal–ligand cooperation, see: a) G. J. Kubas, J. Organomet. Chem. 2014, 751, 33 – 49; b) R. H. Crabtree, New J. Chem. 2011, 35, 18 – 23; c) J. C. Gordon, G. J. Kubas, Organometallics 2010, 29, 4682 – 4701. Thiel and co-workers reported catalytic hydrogenation and transfer hydrogenation of acetophenone with an n-butyl analogue of 1,[8] whereas catalytic transfer hydrogenation with a pincer-type (protic pyrazole)(aprotic NHC) – ruthenium complexes has been described: F. Zeng, Z. Yu, Organometallics 2008, 27, 6025 – 6028.
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COMMUNICATION & Pincer Complexes T. Toda, S. Kuwata,* T. Ikariya* && – &&
Make a difference! A pincer-type ruthenium complex with two different types of NH groups at the b-positions to the metal was synthesized (see scheme). Deprotonation experiments revealed
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that protic NHC arm is less acidic than the pyrazole one. H2 heterolysis and oxidation of 2-propanol through the cooperation of the metal and deprotonated NHC is also described.
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Unsymmetrical Pincer-Type Ruthenium Complex Containing b-Protic Pyrazole and N-Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere
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Unsymmetrical Pincer-Type Ruthenium Complex Containing bProtic Pyrazole and N-Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere Tatsuro Toda, Shigeki Kuwata,* and Takao Ikariya*[a] electronic N-heterocyclic carbene (protic NHC)[4, 17–21] arms. The proton- and electron-donating ability of the two protic NH groups at the b-positions to the metal was evaluated.
Abstract: A reaction of a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine with [RuCl2(PPh3)3] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer-type ruthenium complex 2 containing a protic pyrazole and N-heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC–pyrazolato complex 3, indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H2 or in 2-propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal–ligand cooperation.
An unprecedented protic NHC–pyrazole hybrid 2 was obtained by chelation-assisted tautomerization of the imidazole group[4, 17–20, 22] in a 2-(imidazol-1-yl)methyl-6-(pyrazol-3-yl)pyridine as shown in Equation (1).
Proton-responsive ligands surrounding a metal play crucial roles in metal–ligand bifunctional catalysis.[1] A variety of protic ligands, such as amines,[2, 3] pyrazoles,[4–9] imidazoles,[10] oximes,[11, 12] pyridinols,[13] and picoline/lutidine-based chelates,[14] have been used to facilitate the substrate binding, activation, and transformation through noncovalent interactions. Although most of the artificial bifunctional catalysts have only one kind of such protic ligands, metalloenzymes in Nature generally possess two or more structurally differentiated cooperating groups with various ranges of proton affinity in the second coordination sphere, which accelerate biological transformations in a delicate manner.[15] During our continuing study on the metal–ligand bifunctional catalysts,[3–7, 16, 17] we have recently synthesized a symmetrical pincer-type ruthenium complex 1 featuring two protic pyrazole arms,[5] and also demonstrated that its iron analogue catalyzes disproportionation of hydrazine through bidirectional, two-proton-coupled two-electron transfer.[6] Differentiation of the cooperating protic sites would be beneficial to more efficient substrate recognition, proton transfer, and stabilization of the reaction intermediates. Herein, we describe the synthesis and structure of the unsymmetrical variant 2, which contains a protic pyrazole and iso-
The 1H NMR spectrum of 2 displays two NH singlets at d = 10.41 and 9.53 ppm, which were identified by their facile exchange with added D2O. In the 13C{1H} NMR spectrum, the carbene resonance appeared at d = 182.6 ppm as a triplet split by the two phosphorous nuclei. The mer-, tridentate ligation has been confirmed by X-ray analysis of 2 as depicted in Figure 1.[23] The NHC and pyrazole protons are engaged in a hydrogen-bonding interaction with the chlorido ligand and the solvating methanol, respectively, in line with the expected Brønsted acidity of these protons.[4, 18] In addition, one of the pyridylmethylenic hydrogen atoms lies close to the hexafluorophosphate counteranion, which implies that this methylene group could also be acidic due to potential dearomatization of the pyridine moiety.[14] The cyclic voltammogram of 2 displayed a reversible one-electron oxidation, in which E1/2 = 0.49 V (vs. Fc0/ + ). The oxidation potential is close to that of the bis(pyrazole) analogue 1 (0.51 V), indicating that the electrondonating properties of the protic NHC and pyrazole arms are comparable. When the protic NHC–pyrazole complex 2 was treated with an equimolar amount of a base, deprotonation of the pyrazole arm occurred to give the NHC–pyrazolato complex 3 [Eq. (2)].[25]
[a] T. Toda, Prof. Dr. S. Kuwata, Prof. Dr. T. Ikariya Department of Applied Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology O-okayama, Meguro-ku, Tokyo 152-8552 (Japan) Fax: (+ 81) 3-5734-2637 E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403179. Chem. Eur. J. 2014, 20, 1 – 5
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Figure 2. Crystal structure of 3·0.5CH2Cl2. One of the two crystallographically independent molecules is shown. Hydrogen atoms, except for the NH group, as well as the solvating molecule, were omitted for clarity. Ellipsoids are drawn at the 30 % probability level. Selected interatomic distances []: Cl1···N3 3.180(4)/3.226(4) and Cl1···H1 2.646/2.732.
Figure 1. Crystal structure of 2·CH3OH. Hydrogen atoms, except for the NH and methylene groups, were omitted for clarity. Ellipsoids are drawn at the 30 % probability level. Selected interatomic distances []: O1···N1 2.811(4), O1···H1 1.904, Cl1···N3 3.201(3), Cl1···H2 2.719, F1···C9 3.346(4), F1···H8 2.437, F2···C9 3.397(4), and F2···H8 2.524.
Table 1. Selected interatomic distances [] and angles [8] in 2 and 3.
Ru1 Cl1 Ru1 N2 Ru1 C10 N1 N2 N2 C3 N3 C10 N4 C10 N2-N1-C1 N1-N2-C3 C10-N3-C12 N3-C10-N4
Protonation of the pyrazolato complex 3 cleanly regenerated the pyrazole complex 2, showing that the deprotonation is reversible. A low-field 31P-coupled triplet at d = 189.6 ppm in the 13 1 C{ H} NMR spectrum of 3, as well as the D2O-exchangable singlet at d = 9.76 ppm in the 1H NMR, indicated that the protic NHC moiety in 2 remained after the deprotonation. Deprotonation also led to a negative shift of the oxidation potential (E1/2 = 0.04 V vs. Fc0/ + ). The crystal structure of the pyrazolato complex 3 is shown in Figure 2.[23] The b-nitrogen atom in the NHC unit lies close to the chlorido ligand (N3···Cl1 3.203 , mean of two crystallographically independent molecules), whereas that in the pyrazole unit is more separated (N1···Cl1 3.887 , mean) in agreement with the deprotonation of the pyrazole arm, as well as the presence of an intermolecular hydrogen bond between the protic NHC and chlorido ligand. The decreased N2-N1-C1 angle in the pyrazole ring [106.78 (mean) vs. 111.8(3)8 in 2] is also diagnostic of the NH deprotonation.[4] In contrast, the metrical parameters in the NHC ring of 2 and 3 are almost identical as summarized in Table 1. The selective deprotonation demonstrates the stronger Brønsted acidity of the pyrazole group than the isoelectronic NHC unit in 2.[26] Next, we examined deprotonation of the remaining protic NHC ligand. Reaction of the protic NHC–pyrazole complex 2 with two equivalents of a base under dihydrogen led to the formation of the hydrido complex 4 as shown in Scheme 1. The 1H NMR spectrum of 4 displays a hydrido resonance at d = 11.40 ppm, as well as a low-field singlet at d = 7.83 ppm. The &
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2·CH3OH
3·0.5 CH2Cl2 Molecule A
Molecule B
2.451(1) 2.125(3) 1.980(3) 1.342(4) 1.345(4) 1.354(4) 1.354(4) 111.8(3) 106.3(3) 113.0(3) 103.0(2)
2.464(1) 2.129(4) 1.986(4) 1.345(5) 1.374(5) 1.338(5) 1.374(5) 106.5(4) 110.2(3) 113.7(4) 103.4(4)
2.458(1) 2.119(3) 1.990(5) 1.354(5) 1.374(5) 1.367(6) 1.363(6) 106.9(3) 108.9(3) 111.6(4) 104.6(4)
Scheme 1. Formation of hydrido complex 4.
latter signal could be assigned to the protic NHC group, because the 13C{1H} NMR spectrum substantiates the presence of the carbene carbon (d = 191.7 ppm, t). Scheme 1 illustrates the 2
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Communication possible mechanism for formation of 4. Twofold deprotonation of 2 would generate a coordinatively unsaturated imidazolyl– pyrazolato complex 5 via the monodeprotonated complex 3.[27] Subsequent heterolytic cleavage of dihydrogen by the cooperation of Lewis acidic metal center and Brønsted basic imidazolyl group[28] would give the hydrido complex 4. Deprotonation of 2 in 2-propanol also resulted in the formation of 4 (Scheme 1) presumably through hydrogen transfer from 2propanol to 5 in a concerted or stepwise manner. In pioneering studies, Grotjahn and co-workers described hydrogenation and transfer hydrogenation of half-sandwich-type imidazolyl complexes.[19] The stronger Brønsted basicity of the imidazolyl group in 5 compared to the pyrazolato ligands is inferred by similar dehydrochlorination of the bis(pyrazole) complex 1 in methanol, which gave a methanol adduct [RuL(CH3OH)(PPh3)2] (L = doubly deprotonated bis(pyrazole) pincer ligand) instead of the hydrido–pyrazole complex corresponding to 4.[5] On the other hand, preliminary experiments on catalytic hydrogenation of acetophenone suggested that the protic NHC–pyrazole hybrid 2 is less active than the bis(pyrazole) complex 1.[29] The lower activity may be due to the smaller polarization of the bNH group and hydrido ligand in 4. In summary, we have synthesized a new unsymmetrical pincer-type complex 2 containing two b-NH groups with different Brønsted acidity through cyclometalation of an appropriately designed pyrazole–imidazole hybrid ligand. Deprotonation experiments clearly revealed that the protic NHC unit in 2 is less acidic than the pyrazole unit, whereas the electron-donating characters of these N-heterocycles are almost the same on the basis of the electrochemical measurements. In addition, the fully deprotonated complex 5 containing two different Brønsted basic sites in the second coordination sphere promoted heterolytic cleavage of H2 and oxidation of 2-propanol through the metal–ligand cooperation. The present study provides the first direct comparison of the properties of these two isoelectronic b-protic ligands and gives insights to the future design of the metal–ligand bifunctional catalysts. Investigations to explore the catalytic application of these multiprotonresponsive complexes are in progress.
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Experimental Section Details on the reaction procedures, characterization data, and structural determination for 2 and 3 are given in the Supporting and CCDC-997068 Information. CCDC-997067 (2·CH3OH) (3·0.5 CH2Cl2) contain 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/ data_request/cif.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (S; no. 22225004) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and The Sumitomo Foundation (S.K.).
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[24] [25]
[26]
[27]
[28]
[29]
monoclinic, P21/n, a = 19.248(5), b = 22.739(6), c = 23.947(6) , b = 90.993(3)8, V = 10480(5) 3, Z = 8, 1calcd = 1.318 g cm 3, m = 0.504 mm 1, F000 = 4312, transmission factors 0.865–0.961, reflection measured 116752, independent reflections 23951, no. of valuables 1309, Rint = 0.0687, R1 = 0.0696 [I > 2s(I)], wR2 = 0.1713 (all data), residual electron density 2.26/ 2.27 e 3. Data was collected at 93 K with graphite-monochromated MoKa radiation (l = 0.7107 ) to a maximum 2q value of 558. Intensity data were corrected for Lorentz polarization effects and for absorption. Structure solution and refinements were performed with the CrystalStructure program package.[24] The structures were refined against F2. The hydroxy hydrogen atom in the solvating methanol in 2·CH3OH was isotropically refined, whereas the rest hydrogen atoms, except for those in the disordered components, were included in the refinements by using a riding model. CrystalStructure 4.0, Crystal Structure Analysis Package, Rigaku Corporation, Tokyo 196 – 8666, Japan, 2000 – 2010. Complex 2 can also be deprotonated with triethylamine (pKa of BH + = 9.00 in DMSO) in CD2Cl2, whereas 2 seems inert toward 2,6-lutidine (4.46). A 1H NMR experiment revealed that the bis(pyrazole)complex 1 protonates the NHC–pyrazolato complex 3, indicating the Brønsted acidity of 1 superior to that of 2. The pKa values of [RuL(NH3)5]3 + (L = a protic NHC and pyrazole) are reported to be 11.0 and 5.98, respectively: a) M. F. Tweedle, H. Taube, Inorg. Chem. 1982, 21, 3361 – 3371; b) C. R. Johnson, W. W. Henderson, R. E. Shepherd, Inorg. Chem. 1984, 23, 2754 – 2763. Preliminary deprotonation experiments of 2 with two equivalents of KOtBu in toluene under nitrogen led to the formation of a dinitrogen adduct 5–N2, which was characterized by the n˜ (NN) band (2156 cm 1) in the IR spectrum. For heterolytic cleavage of dihydrogen by metal–ligand cooperation, see: a) G. J. Kubas, J. Organomet. Chem. 2014, 751, 33 – 49; b) R. H. Crabtree, New J. Chem. 2011, 35, 18 – 23; c) J. C. Gordon, G. J. Kubas, Organometallics 2010, 29, 4682 – 4701. Thiel and co-workers reported catalytic hydrogenation and transfer hydrogenation of acetophenone with an n-butyl analogue of 1,[8] whereas catalytic transfer hydrogenation with a pincer-type (protic pyrazole)(aprotic NHC) – ruthenium complexes has been described: F. Zeng, Z. Yu, Organometallics 2008, 27, 6025 – 6028.
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COMMUNICATION & Pincer Complexes T. Toda, S. Kuwata,* T. Ikariya* && – &&
Make a difference! A pincer-type ruthenium complex with two different types of NH groups at the b-positions to the metal was synthesized (see scheme). Deprotonation experiments revealed
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that protic NHC arm is less acidic than the pyrazole one. H2 heterolysis and oxidation of 2-propanol through the cooperation of the metal and deprotonated NHC is also described.
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Unsymmetrical Pincer-Type Ruthenium Complex Containing b-Protic Pyrazole and N-Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere
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