Communication pubs.acs.org/IC
Pyrazinetetracarboxamide: A Duplex Ligand for Palladium(II) Jessica Lohrman, Hanumaiah Telikepalli, Thomas S. Johnson, Timothy A. Jackson, Victor W. Day, and Kristin Bowman-James* Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States S Supporting Information *
assembly of a “supramolecular cruciform”.6 Both homonuclear and heteronuclear palladium(II) and platinum(II) dipincers were reported by van Koten (Figure 1b),7 and the platinum(II) complex was later used by Shionoya to fabricate an elegant “molecular double ball bearing” (Figure 1c).8 Although there have been reports of dimetallic 2,3-dicarboxamide-substituted pyrazine complexes,9−13 to our knowledge, there have been no reports of dimetallic pyrazinetetracarboxamide complexes. Here we report a new addition to the duplex pincer family, tetraethylpyrazine-2,3,5,6-tetracarboxamide and its dipalladium(II) complex (Scheme 1). In the complex, two short O···H+···O hydrogen bonds between adjacent amide carbonyl groups serve as the charge balance for two coordinated acetate ions.
ABSTRACT: Tetraethylpyrazine-2,3,5,6-tetracarboxamide forms a dipalladium(II) complex with acetates occupying the fourth coordination sites of the two bound metal ions. Crystallographic results indicate that the “duplex” dipincer has captured two protons that serve as the counterions. The protons lie between adjacent amide carbonyl groups with very short O···O distances of 2.435(5) Å. In the free base, the adjacent carbonyl groups are farther apart, averaging 3.196(3) Å. While the dipalladium(II) complexes stack in an ordered stepwise fashion along the a axis, the free base molecules stack on top of each other, with each pincer rotated by about 60° from the one below.
Scheme 1. Synthesis of the Dipalladium(II) Complex 2
P
incer ligands possess clawlike tridentate grips that provide ideal semirigid frameworks for planar coordination to transition metals. In most cases, the central donor electrons are supplied by a phenyl or pyridyl ring. This class of ligands was first reported in the 1970s.1,2 Pincers are now found in numerous applications in biomimetic, catalytic, and materials chemistry,3,4 gaining popularity by the ease with which their chemistry can be fine-tuned through donor and peripheral group modifications. While most pincer complexes are monometallic, 1,4-dimetalated phenyl-based “duplex” pincers can be obtained by functionalization of the central ring system with two additional donor groups (Figure 1).5−8 Reports of dimetalated phenyl-based pincers are somewhat rare. An SCS platinum(II) dipincer first reported by Loeb and Shimizu (Figure 1a)5 was later used by Bunz and Weck in the
The ligand can be synthesized in almost 90% yield by heating the methyl ester of the pyrazinetetracarboxylate precursor with ethylamine in a sealed vial for 48 h. The dipalladium(II) complex 2 was obtained after 24 h by reacting a suspension of 1 in CH3CN with a slight excess of 2 equiv of Pd(OAc)2 at room temperature, followed by recrystallization from CH3OH. Crystals suitable for X-ray analysis were obtained for both 1 and 2 by slow evaporation from CH3OH. The dipalladium complex 2 crystallizes in the triclinic space group P1̅. The palladium(II) ions are bound in a tridentate fashion to the deprotonated amide and pyrazine nitrogen atoms as well as a monodentate-bound acetate ion at the fourth coordination site. The resulting overall dinegative charge is counterbalanced by two protons that lie between adjacent amide carbonyl groups. The O···O distances of 2.435(5) Å are significantly below the usual O···O hydrogen bond distances of ca. 2.7−2.8 Å. The O···H distances are not equal, with the proton residing closer to O1 (0.764 Å) than O2 (1.680 Å). The pincers stack in a staggered stepwise fashion along the a axis (Figure 2b,c). The closest intermolecular interactions are between the vertical acetate CO oxygen atoms and neighboring pyrazine carbon atoms [C2 and C3 at 2.721(5)
Figure 1. Dimetalated pincer complexes: (a) SCS pincer of Loeb and Shimizu; (b) van Koten’s NCN pincer; (c) NCN pincer used by Shionoya in a molecular “ball bearing”. The overall charges on parts a and b are not shown and depend on the charge on the ligand, L. © XXXX American Chemical Society
Received: March 8, 2016
A
DOI: 10.1021/acs.inorgchem.6b00594 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
also shifted to lower energies in the complex compared to the free ligand (from 1653 to 1603 cm−1; see Figure S5). Duplicate ligand resonance signals are observed in both the 1H and 13C NMR spectra (see Figures S3 and S4). Variabletemperature NMR spectra in DMSO-d6 result in a broadening and eventual coalescence of the duplicate signals as the temperature is raised from room temperature to 100 °C (see Figure S6). We suspect that the duplicate signals could be due to an equilibrium process, for example, between a coordinated protonated acetate (i.e., acetic acid) and the proton in the short O···H+···O hydrogen bond or possibly a ligand isomerization/ tautomerization process. We have observed an NMR-detectable rotational barrier for pendant groups in stereoisomers in a naphthalene-appended palladium(II) dicarboxamide pincer.14 However, because of the larger steric “footprint” of naphthalene, more restricted rotation would be anticipated. We are continuing to explore the cause of this phenomenon. The structure of the less rigid free base 1 provides an interesting comparison to the palladium(II) complex 2. The free base crystallizes in the triclinic space group P1̅ and contains 11 crystallographically independent pincers, one of which is shown (Figure 3a, molecule G). For nine of the independent pincers, one or more of the ethyl groups are disordered.
Figure 2. (a) Overhead perspective view of the dipalladium complex, 2, with numbering scheme; (b) side view of two adjacent columnar stacks; and (c) overhead packing view down adjacent columns.
and 2.852(5) Å, respectively], plus a slightly longer distance to the pyrazine nitrogen atom at 3.052(5) Å. The very short hydrogen-bonded counterions add to the unusual nature of the structure and were found after the acetates were unambiguously identified. When no other obvious counterions were found in the crystal lattice, the residual electron density was located between the adjacent carbonyl groups and assigned as hydrogen atoms. In subsequent least-squares cycles, the hydrogen atoms refined successfully to give an R1 value of 0.035. Similar short O···H···O distances ranging from 2.35 to 2.45 Å have been observed for 2,3-pyrazinedicarboxamide complexes,10,12,13 which has been attributed to an amide−iminol tautomerism [RN(H)C(O)R′ ↔ RNC(OH)R′].10,12,13 Because of this delocalization effect, the N−C bond is usually shorter and the CO bond is somewhat longer in complexes compared to the free ligand, as noted by Stoeckli-Evans in comparison studies.13 We also see a slight shortening of the C−N distance and a more substantial elongation of the CO bond in 2 compared with the average of the 11 independent molecules (vide infra) in 1, from an average of 1.320(4) and 1.227(3) Å in 1 to 1.299(5) and 1.278(5) Å in 2 for the C−N and C−O distances, respectively (see Table S2). Density functional theory (DFT) computations were employed to determine the expected C−N and CO distances for a bona fide tautomer of the dipincer free base 1, with two amides in their iminol forms. The results are summarized in the Supporting Information (Figure S21 and Table S3). The iminol tautomer of 1 shows a compressed C−N distance of 0.286 Å, consistent with the expected CN double bond. This distance is 0.034 Å shorter than that calculated for 2 and 0.064 Å less than that of the dipincer free base. Moreover, relative to a DFT structure of the free base, the iminol tautomer shows OC elongation of 0.10 Å (Table S3), which is larger than the 0.06 Å CO elongation observed in both the experimental and DFT structures of 2. Thus, the dipincer ligand in 2 possesses character intermediate between that of the amide and iminol tautomers. Other experimental evidence for the unusually short hydrogen bond comes from NMR in dimethyl-d6 sulfoxide (DMSO-d6). A significant downfield-shifted signal (19.57 ppm) is observed (see Figure S3), as anticipated for very short hydrogen bonds. The signal is absent in CD3OD, where the proton might more readily undergo deuterium exchange. The carbonyl stretch in the IR is
Figure 3. DFT-optimized structures of (a) the pincer free base 1, (b) a tautomer of 1, and (c) the dipalladium(II) complex. Key metric parameters for these species are provided in Table S3.
The distances between the amide carbonyl oxygen atoms, which for most of the independent molecules point toward each other, range from 3.129(3) to 3.241(3) Å with an average of 3.196(3) Å. In the pincer shown in Figure 3a, the O···O distances are 3.184(3) and 3.203(3) Å. The pincers stack in columns, each neighbor slightly offset as a result of the columnar hydrogenbond glue, which consists of two intermolecular NH···O and two O···HN bonds from each molecule to its neighbors above and below (Figure 3b,c). Tetracarboxamide-substituted six-membered rings are not common. The phenyl analogues, 1,2,4,5-phenyltetracarboxamides, are known as the pyromellitamides. They were first reported in 1914 as precursors to phthalocyanines;15 however, few subsequent reports exist.16−18 Several years ago we also reported pyromellitamides as capstones for cyclophane-based capsules as anion hosts.19 In these studies anions were found to bind in crevices on the outside of the capsules. An interesting twist to the pyromellitamide chemistry was reported by Thordarson and co-workers in 2007, who noted gel B
DOI: 10.1021/acs.inorgchem.6b00594 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
(2) (a) van Koten, G.; Jastrzebski, J. T. B. H.; Noltes, J. G.; Spek, A. L.; Schoone, J. C. J. Organomet. Chem. 1978, 148, 233−245. (b) van Koten, G.; Timmer, K.; Noltes, J. G.; Spek, A. L. J. Chem. Soc., Chem. Commun. 1978, 250−252. (3) The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. M., Eds.; Elsevier: Amsterdam, The Netherlands, 2007. (4) Albrecht, M.; van Koten, G. Angew. Chem. 2001, 113, 3866−3898; Angew. Chem., Int. Ed. 2001, 40, 3750−3781. (b) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759−1792. (c) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239−2246. (d) Selander, N.; Szabó, K. J. Chem. Rev. 2011, 111, 2048−2076. (e) Albrecht, M.; Lindner, M. M. Dalton Trans. 2011, 40, 8733−8744. (f) Zargarian, D.; Castonguay, A.; Spasyuk, D. M. Top. Organomet. Chem. 2013, 40, 131−174. (g) van Koten, G. J. Organomet. Chem. 2013, 730, 156−164. (5) Loeb, S. J.; Shimizu, G. K. H. J. Chem. Soc., Chem. Commun. 1993, 1395−1397. (6) Gerhardt, W. W.; Zucchero, J.; Wilson, J. N.; South, C. R.; Bunz, U. H. F.; Weck, M. Chem. Commun. 2006, 2141−2143. (7) Steenwinkel, P.; Kooijman, H.; Smeets, W. J. J.; Spek, A. L.; Grove, D. M.; van Koten, G. Organometallics 1998, 17, 5411−5426. (8) Hiraoka, S.; Hisanaga, Y.; Shiro, M.; Shionoya, M. Angew. Chem., Int. Ed. 2010, 49, 1669−1673. (9) Fleischer, E. B.; Lawson, M. B. Inorg. Chem. 1972, 11, 2772−2775. (10) Fleischer, E. B.; Jeter, D.; Florian, R. Inorg. Chem. 1974, 13, 1042− 1047. (11) Mallik, I.; Mallik, S. Synlett 1996, 1996, 734−736. (12) (a) Hausmann, J.; Jameson, G. B.; Brooker, S. Chem. Commun. 2003, 2992−2993. (b) Klingele, J.; Boas, J. F.; Pilbrow, J. R.; Moubaraki, B.; Murray, K. S.; Berry, K. J.; Hunter, K. A.; Jameson, G. B.; Boyd, P. D. W.; Brooker, S. Dalton Trans. 2007, 633−645. (13) (a) Cati, D. S.; Stoeckli-Evans, H. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, 60, m174−m176. (b) Cati, D. S.; Stoeckli-Evans, H. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, 60, o210−o212. (c) Cati, D. S.; Ribas, J.; Ribas-Ariño, J.; Stoeckli-Evans, H. Inorg. Chem. 2004, 43, 1021−1030. (14) Wang, Q.-Q.; Begum, R. A.; Day, V. W.; Bowman-James, K. J. Am. Chem. Soc. 2013, 135, 17193−17199. (15) Meyer, H.; Steiner, K. Monatsh. Chem. 1914, 35, 391−405. (16) (a) Lawton, E. A.; McRitchie, D. D. J. Org. Chem. 1959, 24, 26. (b) Imai, Y. J. Polym. Sci., Part B: Polym. Lett. 1970, 8, 555−558. (c) Saini, A. K.; Carlin, C. M.; Patterson, H. H. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 419−427. (d) Kaupp, G.; Schmeyers, J.; Boy, J. Tetrahedron 2000, 56, 6899−6911. (e) Watson, K. G.; Cameron, R.; Fenton, R. J.; Gower, D.; Hamilton, S.; Jin, B.; Krippner, G. Y.; Luttick, A.; McConnell, D.; MacDonald, S. J. F.; Mason, A. M.; Nguyen, V.; Tucker, S. P.; Wu, W.-Y. Bioorg. Med. Chem. Lett. 2004, 14, 1589−1592. (17) Webb, J. E. A.; Crossley, M. J.; Turner, P.; Thordarson, P. J. Am. Chem. Soc. 2007, 129, 7155−7162. (18) (a) Tong, K. W. K.; Dehn, S.; Webb, J. E. A.; Nakamura, K.; Braet, F.; Thordarson, P. Langmuir 2009, 25, 8586−8592. (b) Jamieson, A.; Tong, K. W. K.; Hamilton, W. A.; He, L.; James, M.; Thordarson, P. Langmuir 2014, 30, 13987−13993. (c) Dehn, S.; Tong, K. W. K.; Clady, R. G. C.; Owen, D. M.; Gaus, K.; Schmidt, T. W.; Braet, F.; Thordarson, P. New J. Chem. 2011, 35, 1466−1471. (d) Yepuri, N. R.; Jamieson, S. A.; Darwish, T. A.; Rawal, A.; Hook, J. M.; Thordarson, P.; Holden, P. J.; James, M. Tetrahedron Lett. 2013, 54, 2538−2541. (19) Kang, S. O.; Day, V. W.; Bowman-James, K. Org. Lett. 2008, 10, 2677−2680.
formation in N,N′,N″,N‴-1,2,4,5-tetrakis(ethylhexanoate)pyromellitamide. They also found that small anions such as bromide and chloride could have a negative cooperativity influence, resulting in gel destruction.17 A crystal structure of the ethylhexanoate-appended pyromellitamide was in many respects similar to that of 1, including relatively short O···O separations (3.13 Å) and columnar stacking. However, rather than hydrogen bonds directly binding the pyrazine units in the present structure, interstitial water molecules serve as bridges between pincers in the pyromellitamide corollary.17 Since their first report, the Thordarson group has extended this chemistry to other pyromellitamide systems.18 We are also beginning to explore whether these simple pyrazinetetracarboxamides will reveal interesting soft material behavior and have observed gel formation in both the free bases and, under certain instances, the palladium(II) complexes. This chemistry is currently under investigation in our group and will be reported elsewhere. In conclusion, both intermetallic electronic communication through conjugated systems and the influence of very short hydrogen-bonded distances are of great interest to the chemical community. This readily obtainable, new class of ditopic duplex pyrazinetetracarboxamides opens the possibility of exploring these interesting phenomena in a single molecular framework. The duplex pyrazine may also lead to possible nonmetallic and/ or bimetallic gels. A better understanding of the influence of the very short hydrogen bond between adjacent carbonyl groups could be influential in applications in supramolecular chemistry including areas of bimetallic catalysis and responsive materials.
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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.6b00594. Synthetic details, 1H and 13C NMR and ESI-MS spectra of 1 and 2, and information on the crystallographic and DFT studies including thermal ellipsoid plots (PDF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (Grant DE-SC0010555), for support of this work and the National Science Foundation (Grant CHE0923449) for the purchase of the X-ray diffractometer. The authors also thank Drs. Justin Douglas and Sarah Neuenswander of the Nuclear Magnetic Resonance Laboratory, University of Kansas, and Dr. Pedro Metola, currently at the University of Texas at Austin, for their assistance and helpful discussions, and Dr. Cynthia Day for assistance in X-ray structure searches.
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REFERENCES
(1) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020− 1024. C
DOI: 10.1021/acs.inorgchem.6b00594 Inorg. Chem. XXXX, XXX, XXX−XXX
