Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Characterizing Hydrogen-Bond Interactions in Pyrazinetetracarboxamide Complexes: Insights from Experimental and Quantum Topological Analyses Jessica Lohrman,† Erik A. Vázquez-Montelongo,‡ Subhamay Pramanik,† Victor W. Day,† Mark A. Hix,‡ Kristin Bowman-James,*,† and G. Andrés Cisneros*,‡ †
Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States Department of Chemistry, University of North Texas, 1508 W Mulberry Street, Denton, Texas 76201, United States
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‡
S Supporting Information *
ABSTRACT: Experimental and topological analyses of dipalladium(II) complexes with pyrazinetetracarboxamide ligands containing tetraethyl (1), tetrahexyl (2), and tetrakis(2-hydroxyethyl) ethyl ether (3) are described. The presence of two very short O---O distances between adjacent amide carbonyl groups in the pincer complexes revealed two protons, which necessitated two additional anions to satisfy charge requirements. The results of the crystal structures indicate carbonyl O---O separations approaching that of low barrier hydrogen bonds, ranging from 2.413(5) to 2.430(3) Å. Solution studies and quantum topological analyses, the latter including electron localization function, noncovalent interaction, and Bader’s quantum theory of atoms in molecules, were carried out to probe the nature of the short hydrogen bonds and the influence of the ligand environment on their strength. Findings indicated that the ligand field, and, in particular, the counterion at the fourth coordination site, may play a subtle role in determining the degree of covalent association of the bridging protons with one or the other carbonyl groups.
Figure 1. Pyrazinetetracarboxamide pincer complexes with ethyl (L1), hexyl (L2), and 2-hydroxyethyl ethyl ether (L3) substituents.
influence of the pendant chains on the solubilities. We were also curious as to whether the short O···H+···O HBs found for the palladium(II) complex of H4L1 would also be observed with the new ligands. We have now obtained crystallographic confirmation of these short HBs for the two new complexes. To better characterize the nature of these HBs, here we present electron localization function (ELF),12−14 noncovalent interaction (NCI),15,16 and Bader’s quantum theory of atoms in molecules (QTAIM)17−22 analyses to investigate the nature of these interactions. One of us has previously shown that distinct HBs such as low-barrier hydrogen bonds (LBHBs) can be characterized by means of ELF analysis.23,24 Additionally, combined ELF/NCI calculations are powerful tools that can provide insight into unconventional chemical bonding and weak NCIs in chemical and biological systems.25−28 Pyrazine-2,3,5,6-tetracarboxamides are relatively easy to prepare from the tetrasubstituted methyl ester. The ethyl substituent gives the largest yield at almost 87% for the final step. The palladium(II) complexes were made as previously described for the acetate, with the exception that K2PdCl4 was used as the starting material for reaction with H4L3. Crystals of 1 and 3 suitable for X-ray diffraction were grown by slow evaporation from methanol, while crystals of 2 were obtained by vapor diffusion of ether into a CH2Cl2 solution of 2. All three palladium(II) complexes contain hydrogen atoms between each pair of adjacent amide carbonyl groups. The O--O distance varies only slightly, with O···H+···O distances equal to 2.430(5) Å for 1, 2.427(4) and 2.426(4) Å for 2, and 2.413(3) Å for 3. The O−H−O angles are not far from linear,
T
ridentate pincers are known for aggressive binding of transition-metal ions.1 Nonetheless, reports of 1,4dimetalated pincers are rare.2−5 The SCS diplatinum(II) pincer of Loeb and Shimizu2 and palladium(II) and platinum(II) complexes with NCN amine-based dipincers by van Koten et al.,3 Weck et al.,4 and Zhang and Lei5 are representative of known dimetalated pincer complexes. While these complexes contain 1,4-dimetalated benzenes, to our knowledge, we were the first to report dimetalated pyrazinetetracarboxamides.6 Of particular interest in our previously reported dipalladium(II) structure was the presence of two very short hydrogen bonds (HBs) between the 2,3- and 5,6-adjacent carboxamide oxygen atoms, at 2.430(5) Å. Similar phenomena were found for transition-metal complexes with pyrazine-2,3-dicarboxamides7−11 and attributed to a delocalized intermediate between amidate and iminolate tautomers.8,11 In an extension of our previous study with the tetraethylsubstituted dipincer H4L1, we introduced extended hydrophobic chains, tetrahexyl (H4L2), and hydrophilic chains, 2hydroxyethyl ethyl ether (H4L3) (Figure 1), to explore the © XXXX American Chemical Society
Received: March 16, 2018
A
DOI: 10.1021/acs.inorgchem.8b00627 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry 169(7), 174(6) (average of two), and 171(8)°, for 1−3, respectively. The protons in each case were located in difference Fourier maps and refined isotropically. While it is true that the electron density between the two oxygen atoms was located and refined, the exact location of the hydrogen atom should not be taken as being highly accurate. Of the three complexes, in 3, the protons were closest to the center between the two adjacent carbonyl groups, compared to 1 and 2 (Figure 2). The presence of this very short O···H+···O
Figure 3. (a) Intermolecular stacking of the acetate complexes, as shown for 2 and (b) zigzag hydrophilic-hydrophobic chains of 3.
Figure 4 (combined ELF/NCI analyses are provided in Figures S24 and S25). Typically, hydrogen basins arising from ELF analyses are represented as covalent bonds (disynaptic basins), with the basin shared between the hydrogen atom and a heavy atom.7,8 Strong interactions such as LBHBs are calculated by ELF to be valence basins associated with one single hydrogen atom (monosynaptic basin).11 In the case of complex 1, the basin associated with the hydrogen atoms involved in the HB is seen to be shared (disynaptic basin) with the corresponding oxygen V(H1,O1) and V(H2,O1′). For complex 2, one of the hydrogen atoms is associated with a monosynaptic basin [V(H1)], while the other is associated with a disynaptic one [V(H2,O4)]. Conversely, the basins in complex 3 are associated exclusively with the hydrogen atoms for both HBs, V(H1) and V(H2). The combined ELF/NCI analysis (Figures S24 and S25) is consistent with the above results, except for complex 2. For complex 1, NCI analysis shows a dark blue surface for both HBs, indicating a strong HB for both interactions. In the case of 2, disynaptic basin V(H2,O4) is predicted, although no strong interacting surface is observed between H2 and O4 from NCI analysis. For complex 3, no surfaces are observed between the hydrogen atom and any of the oxygen atoms. Thus, the combined ELF/NCI results indicate that in 1 the interaction is basically short, but not LBHB; in 2, one of the O···H+···O HB interactions is indicative of an LBHB, while the other is not; in 3, with the shortest O---O distance, both interactions are indicative of LBHBs. It should be noted that crystallographic symmetry restrictions mandate that both HBs should be the same in 1 and 3 because no minimizations were performed. The results of ELF population and distributed multipole analyses for selected basins are described below. Population and multipole analyses for complex 3 (Table S5) show that the first and second polar moments for the lone-pair basins that are directly interacting with the hydrogen atom are significantly larger than the ones pointing away from the HB interaction region. This is an indication of the strong polarization on the oxygen atoms induced by the LBHB. In addition, population analysis shows that each of the oxygen atoms that share the hydrogen atom for complex 3 has a similar electron population (O2 = 4.03 and 1.62; O1 = 3.73 and 1.98); on the other hand, complex 1 (Table S3) does not (O2 = 2.81 and 2.99; O1 =
Figure 2. Overhead perspective views with selected labels of 1 (a), 2 (b), and 3 (c).
hydrogen bond is also manifested in solution, with sharp signals integrating to two protons at 19.43 ppm in CDCl3 for 2 and 19.60 ppm in DMSO-d6 for 3 (Figures S7 and S14). Selected bond lengths (Table S1) within the tautomeric regions are commensurate with approaching an intermediate between the amidate and iminolate tautomers. The amide C− O and C−N distances lie within narrow ranges, and average 1.280(5) and 1.300(5) Å, respectively, which is intermediate between single- and double-bond distances (Table S1). Other spectral measurements confirm the influence of the intermediate electron delocalization. In the solid-state IR spectra, the carbonyl stretches shift to lower energies for each of the complexes compared to the free ligand by about 50 cm−1 for 16 and 2, and 60 cm−1 for 3 (Figures S7 and S14). Intermolecular interactions between the nonbonded carboxylate oxygen atoms of the acetate and pyrazine π systems of neighboring complexes above and below in the stacked array are seen for 1 and 2, with distances ranging from 2.669(5) to 2.853(5) Å (Figure 3a). The crystallographic packing for 3 is influenced by the presence of the extended hydrophilic chain (Figure 3b). Two opposing chains each capture a water molecule, which then forms very weak HB interactions (around 3.0 Å) with the adjacent hydroxyl group and oxygen atoms of two other neighboring complexes. The small differences in the O---O distances do not indicate significant intermolecular influence. Although in our original report of these duplex pyrazine pincers we did preliminary density functional theory studies, those were primarily performed to investigate the possibility that 1 was a result of the reversion of the ligand to the iminol(ate) tautomeric form.6 Those results indicated that 1 possessed a structure intermediate between the amidate and iminolate forms. In this contribution, we have performed quantum topological analyses to better understand the character of the observed HBs and, in particular, localization of the electrons based on the crystallographic findings. ELF topological analyses of complexes 1−3 based on the geometries obtained from the crystal structures are shown in B
DOI: 10.1021/acs.inorgchem.8b00627 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 4. Closeup of ELF analysis for the two HB regions for complexes 1 (a), 2 (b), and 3 (c). Atoms involved in the HBs as well as lone pairs and covalent bond basins labeled for clarity. Isosurface cutoff for ELF = 0.85.
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3.86). This further suggests the presence of a covalent bond between H1 and O1 for complex 1. For complex 2 (Table S4), populations for the monosynaptic and disynaptic basins are similar to those for complexes 3 and 1, respectively. Complementary QTAIM analysis was carried out on complexes 1−3 (Tables S6−S8) to corroborate the results from ELF analysis. It has been previously reported that analysis of the density and Laplacian of the density at the bond critical points (BCPs) can be used to investigate different HBs.29−32 Covalent bonds (shared interactions) possess negative values for ∇2ρ(r) and small values for ρ(r). Closed-shell interactions, like HBs, also have small ρ(r) but positive ∇2ρ(r) values. For complex 1 (Table S6), the ∇2ρ(r) values for the BCPs located between H1−O1 and H2−O1′ were negative, while the ∇2ρ(r) values for the BCPs at O1−H2 and O2′−H2 are positive. For complex 3 (Table S8), all of the ∇2ρ(r) values were negative. For complex 2 (Table S7), the value of ∇2ρ(r) for BCPs corresponding to H1−O2, O3−H1, and H2−O4 is negative, while the one corresponding to O1−H2 is positive. These results are consistent with ELF analysis and indicate that complex 1 exhibits a short HB, while 3 exhibits LBHBs; for complex 2, one of the HB corresponds to an LBHB and the other to a short HB. In summary, the duplex pincers provide an ideal platform for exploring the delicate interplay between the ligand strength and acid−base properties. The short O···H+···O HBs almost certainly possess high pKa values, although with the exception of 3, the complexes are not soluble in water. Similar short bonds have been observed in cyclopentadiene esters, where the driving force appears to be the formation of the highly stable aromatic cyclopentadienides.33 In that case, metal-ion coordination is not needed to trigger the HB formation. Furthermore, solution and solid-state experimental studies indicate that the intramolecular HBs may depend on other factors, including pincer pendant groups and fourth coordination site ancillary ligands. Computational findings indicate that localization or delocalization of the electrons within these very short HBs depends on very subtle differences in O---O separations, which, in turn, depends on the surrounding ligand field. We are continuing in our exploration of these HB phenomena.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00627. Synthesis and characterization details, 1H and 13C NMR, FT-IR, and ESI-MS of 2 and 3, information on the crystallographic studies including additional packing diagrams, and additional details of the ELF, NCI, and QTAIM studies (PDF) Accession Codes
CCDC 1825173−1825174 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Mark A. Hix: 0000-0003-3371-0116 Kristin Bowman-James: 0000-0001-6329-5262 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS E.A.V.-M., M.A.H., and G.A.C. acknowledge the Department of Chemistry, University of North Texas, for use of the HPC cluster CRUNTCH3 and the National Science Foundation (NSF) for Grant CHE-1531468. E.A.V.M. thanks CONACyT for financial support. K.B-J. thanks the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (Grant DE-SC0018629), for support of this work and the NSF (Grant CHE-0923449) for the purchase of the X-ray diffractometer. C
DOI: 10.1021/acs.inorgchem.8b00627 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.8b00627 Inorg. Chem. XXXX, XXX, XXX−XXX
