Communication pubs.acs.org/IC
Protic Ruthenium Tris(pyrazol-3-ylmethyl)amine Complexes Featuring a Hydrogen-Bonding Network in the Second Coordination Sphere Hiroaki Yamagishi,† Shohei Nabeya,† Takao Ikariya,*,† and Shigeki Kuwata*,†,‡ †
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ‡ PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
the multiple intramolecular H-bond network. However, their catalytic applications9 as well as complexation to heavier, πelectron-rich transition metals3 remain surprisingly rare. The versatility of the pyridine-armed tripodal tris(pyridylmethyl)amine (TPA) ligands in bioinorganic modeling and oxidation catalysis10 prompted us to scrutinize the coordination chemistry of a proton-responsive analogue of TPA, tris(pyrazol-3ylmethyl)amine ligand LH3, in which the three β-protic pyrazoles11−13 would serve as rigid proton-delivering groups to the coordination site with a larger space (Chart 1). We describe here the synthesis and structures of LH3 complexes of ruthenium and explore the catalytic role of the protic units in the second coordination sphere. The LH3-type ligand 1 [R = C6H2Me3-2,4,6 (Mes)] was newly synthesized by 3-fold Claisen condensation of a nitrilotriacetic acid ester and subsequent dehydrative condensation with hydrazine.14 When 1·0.5H2O was treated with chlorido complex [(η6-C6H6)RuCl2]2 in an Ru/1 ratio of 1:1, the cationic ruthenium(II) complex [{RuCl(LH3)}2(μ2-Cl)]Cl (2) was obtained as shown in eq 1. X-ray analysis revealed the dinuclear
ABSTRACT: We synthesized ruthenium complexes bearing a tris(pyrazol-3-ylmethyl)amine ligand LH3 and revealed that this tripodal ligand allows predictable accumulation of three proton-delivering NH groups around a coordination site. The Brønsted acidity of the NH groups in LH3 led to the formation of multiple hydrogen bonds with the substrate ligand and deprotonation. The chlorido complex ligated by LH3 catalyzed disproportionation of 1,2-diphenylhydrazine.
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trict regulation of hydrogen-bond (H-bond) formation and proton transfer between the substrate molecule and ancillary ligands in the second coordination sphere is an emerging research area because such interactions have been implicated in many biological transformations catalyzed by metalloenzymes under mild conditions.1 Multidentate ligands with three or more proton-responsive arms provide attractive scaffolds to replicate the active site structures with complicated H-bond networks and their functions. Chart 1 illustrates representative examples of C3symmetric tripodal ligands furnished with proton-responsive functional groups;2−8 this class of ligands effectively accumulate H-bonding interactions around a coordination site trans to the amine donor and allow the isolation of reactive species thanks to Chart 1
structure of 2 (Figure 1a). The cation has an approximate C2 axis passing through the bridging chlorido ligand. The two octahedral Ru(LH3) units are further linked by the H-bond network involving the protic pyrazole arms, terminal chlorido ligands, a chloride counteranion, and a solvating water molecule. These noncovalent interactions obviously contribute to stabilize the monochlorido-bridged structure of 2, which sharply contrasts with the structures of the aprotic mononuclear RuCl2(tpa) and dinuclear {Ru(tpa)}2(μ2-Cl)22+ complexes reported so far.15,16 The presence of the Brønsted acidic NH groups is also evidenced Received: September 3, 2015
© XXXX American Chemical Society
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DOI: 10.1021/acs.inorgchem.5b02044 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 1. Crystal structures of (a) 2·H2O·3C2H4Cl2·3Et2O, (b) 3, and (c) 5·0.5Et2O. The CH hydrogen atoms, solvating molecules, and mesityl groups, except for the ipso carbon atoms (for 2), as well as the minor components of the disordered chloride counteranion in 5 are omitted for clarity. Ellipsoids are drawn at the 30% probability level.
by the 1H NMR spectrum, which displays three distinct low-field resonances at δ 12.65, 13.64, and 13.86, in agreement with the dinuclear structure with an intramolecular H-bond network in solution. Despite the presence of multiple interactions between the two Ru(LH3) units, the chlorido-bridged dinuclear core in 2 is split by dative ligands to give mononuclear Ru(LH3) complexes. Treatment of 2 with an excess of triphenylphosphine led to formation of the mononuclear phosphine complex [RuCl(PPh3)(LH3)]Cl (3), as shown in Scheme 1. The 1H NMR
The proton-donating ability of the LH3 ligand in 3 was further demonstrated by a deprotonation experiment. Upon reaction of 3 with an equimolar amount of a base, the uncharged pyrazolato−bis(pyrazole) complex [RuCl(PPh3)(LH2)] (4) was obtained (Scheme 1). In the 1H NMR spectrum of 4, two of the three mesityl groups are equivalent, while the NH signals could not be assigned even in the lower temperature. As expected, the addition of 1 equiv of an acid cleanly regenerated the cationic tris(pyrazole) complex 3. To explore metal−ligand cooperative reactivity in the Ru(LH3) platform, we next examined the reaction of the chlorido complex 2 with a hydrazine. The addition of a slight excess of 1,2-diphenylhydrazine to 2 afforded the aniline complex [RuCl(PhNH2)(LH3)]Cl (5), as shown in Scheme 1.18 The concurrent formation of equimolar amounts of free aniline and azobenzene per 5 was confirmed by the 1H NMR spectrum.19 Figure 1c depicts the crystal structure of 5. All of the three NH groups are engaged in substrate binding through NH···Cl Hbonds and an NH···π interaction (NH1···centroid: 2.316 Å). The N−N bond cleavage proved to be catalytic when 20 equiv of 1,2-diphenylhydrazine was used (Scheme 2).20,21 Part of the
Scheme 1. Reactions of 2 (R = Mes)
Scheme 2. Catalytic Disproportionation of 1,2Diphenylhydrazine with 2
a b
spectrum of 3 exhibits two NH resonances in a 2:1 intensity ratio, which were identified by their exchange with added D2O. In line with the suggested Cs symmetry of 3, the methylene hydrogen atoms give rise to one 2H singlet and two geminally coupled resonances with 2H intensity each. One of the latter signals is further split to double doublets with a remote 4JPH coupling, indicating the trans orientation of the phosphine and amine ligands with rigid five-membered chelate arms. In the crystal of 3, the chloride counteranion is located between two pyrazole rings with short NH···Cl2 distances of 2.398 and 3.074 Å to suggest the presence of H-bonds (Figure 1b), although one of the N− H···Cl angles is fairly narrow [around H3, 95.3(2)°]. In addition, the N5−H3 unit pointing almost perpendicular to a phenyl group (C52−C57) in the phosphine ligand with a close NH··· centroid contact of 2.721 Å is involved in an NH···π interaction.17
In moles per mole of ruthenium in 2. Determined by 1H NMR. NaBArF4 (2 equiv per 2) was added.
produced aniline would be trapped in 5 featuring an LH3···Cl··· H2NPh H-bond network. Actually, removal of the chloride anion by the addition of NaBArF4 [ArF = C6H3(CF3)2-3,5] noticeably accelerated the reaction. In addition, TPA complex [Ru(tpa)(MeCN)2](SbF6)215 exhibited no catalytic activity even in the presence of external pyrazole. These observations clearly indicate the significance of the protic ligand in the catalysis. Still, we need further investigation to determine whether the reaction occurs through 2H+/2e− shuttling12 or one-electron steps with intermediary nitrogen radicals.19c To summarize, we have demonstrated that the pyrazole-armed tripodal amine ligand LH3 provides a rigid coordination environment surrounded by three protic NH groups. The unique orientation of the Brønsted acidic NH groups in the second coordination sphere, different from those in the B
DOI: 10.1021/acs.inorgchem.5b02044 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
(10) (a) Puri, M.; Que, L., Jr. Acc. Chem. Res. 2015, 48, 2443−2452. (b) Ishizuka, T.; Ohzu, S.; Kojima, T. Synlett 2014, 25, 1667−1679. (c) Mandon, D.; Jaafar, H.; Thibon, A. New J. Chem. 2011, 35, 1986− 2000. (d) Blackman, A. G. Eur. J. Inorg. Chem. 2008, 2633−2647. (e) Berreau, L. M. Comments Inorg. Chem. 2007, 28, 123−171. (f) Suzuki, M. Acc. Chem. Res. 2007, 40, 609−617. (g) Kruppa, M.; König, B. Chem. Rev. 2006, 106, 3520−3560. (h) Karlin, K. D.; Lee, D.H.; Obias, H. V.; Humphreys, K. J. Pure Appl. Chem. 1998, 70, 855−862. (11) (a) Kuwata, S.; Ikariya, T. Chem. Commun. 2014, 50, 14290− 14300. (b) Kuwata, S.; Ikariya, T. Chem. - Eur. J. 2011, 17, 3542−3556. (c) Pérez, J.; Riera, L. Eur. J. Inorg. Chem. 2009, 4913−4925. (d) Grotjahn, D. B. Dalton Trans. 2008, 46, 6497−6508. (12) Umehara, K.; Kuwata, S.; Ikariya, T. J. Am. Chem. Soc. 2013, 135, 6754−6757. (13) Toda, T.; Kuwata, S.; Ikariya, T. Z. Anorg. Allg. Chem. 2015, 641, 2135−2139. (14) For R = H, see: Juanes, O.; De Mendoza, J.; Rodríguez-Ubis, J. C. J. Chem. Soc., Chem. Commun. 1985, 1765−1766. (15) Whiteoak, C. J.; Nobbs, J. D.; Kiryushchenkov, E.; Pagano, S.; White, A. J. P.; Britovsek, G. J. P. Inorg. Chem. 2013, 52, 7000−7009. (16) Kojima, T.; Amano, T.; Ishii, Y.; Ohba, M.; Okaue, Y.; Matsuda, Y. Inorg. Chem. 1998, 37, 4076−4085. (17) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; Oxford University Press: Oxford, U.K., 1999. (18) No intermediate was observed even when the Ru/substrate ratio was reduced to 1:1. (19) For disproportionation of 1,2-diphenylhydrazine, see: (a) Nakajima, Y.; Suzuki, H. Organometallics 2005, 24, 1860−1866. (b) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 2728−2729. (c) Milsmann, C.; Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 12099−12107. (d) Hamilton, C. R.; Gau, M. R.; Baglia, R. A.; McWilliams, S. F.; Zdilla, M. J. J. Am. Chem. Soc. 2014, 136, 17974−17986. (20) For catalytic disproportionation of hydrazines, see ref 12. (21) The observed amount of aniline was lower than that expected (e.g., 6.0−7.0 mol in the reaction without NaBArF4) probably because of evaporation of the reaction mixture before NMR measurements.
precedents shown in Chart 1 as well as the related pincer-type complex,12 allow three-point noncovalent interactions with larger substrates such as aniline. Cooperation of the protic ligand LH 3 and π-basic ruthenium center in catalytic disproportionation of 1,2-diphenylhydrazine has been suggested by a control experiment using an aprotic TPA analogue. Further studies on the mechanistic details along with ligation of LH3 as a protic TPA to other metals are currently underway.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02044. Experimental procedures (PDF) X-ray crystallographic data in CIF format for 2, 3, and 5 (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for a Grant-in-Aid for Scientific Research (S) (Grant 22225004) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and The Sumitomo Foundation (to S.K.). This work was partially supported by the PRESTO program on “Molecular Technology and Creation of New Functions” from JST (to S.K.).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b02044 Inorg. Chem. XXXX, XXX, XXX−XXX
