Article pubs.acs.org/JACS
Transuranic Hybrid Materials: Crystallographic and Computational Metrics of Supramolecular Assembly Robert G. Surbella III,† Lucas C. Ducati,‡ Kristi L. Pellegrini,§ Bruce K. McNamara,§ Jochen Autschbach,∥ Jon M. Schwantes,§ and Christopher L. Cahill*,† †
Department of Chemistry, The George Washington University, 800 22nd Street NW, Washington, D.C. 20052, United States Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes 748, São Paulo 05508-000, Brazil § Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, United States ∥ Department of Chemistry, University at Buffalo, State University of New York, 312 Natural Sciences Complex, Buffalo, New York 14260, United States ‡
S Supporting Information *
ABSTRACT: Assembly of a family of 12 supramolecular compounds containing [AnO2Cl4]2− (An = U, Np, Pu), via hydrogen and halogen bonds donated by substituted 4-Xpyridinium cations (X = H, Cl, Br, I), is reported. These materials were prepared from a room-temperature synthesis wherein crystallization of unhydrolyzed and valence-pure [An(VI)O2Cl4]2− (An = U, Np, Pu) tectons is the norm. We present a hierarchy of assembly criteria based on crystallographic observations and subsequently quantify the strengths of the non-covalent interactions using Kohn−Sham density functional calculations. We provide, for the first time, a detailed description of the electrostatic potentials of the actinyl tetrahalide dianions and reconcile crystallographically observed structural motifs and non-covalent interaction acceptor−donor pairings. Our findings indicate that the average electrostatic potential across the halogen ligands (the acceptors) changes by only ∼2 kJ mol−1 across the AnO22+ series, indicating that the magnitude of the potential is independent of the metal center. The role of the cation is therefore critical in directing structural motifs and dictating the resulting hydrogen and halogen bond strengths, the former being stronger due to the positive charge centralized on the pyridyl nitrogen, N−H+. Subsequent analyses using the quantum theory of atoms in molecules and natural bond orbital approaches support this conclusion and highlight the structuredirecting role of the cations. Whereas one can infer that Columbic attraction is the driver for assembly, the contribution of the non-covalent interaction is to direct the molecular-level arrangement (or disposition) of the tectons.
■
INTRODUCTION There is an emerging recognition of the role(s) that noncovalent interactions play in assembly and functionality of materials. Biochemists, for example, pursue functional supramolecular coordination complexes with applications spanning biochemical machines,1,2 sensors,3,4 and drug delivery5,6 to materials that can mimic natural proteins7 and function as biocompatible polymers.8−10 Likewise, materials chemists strive to harness these weak interactions to prepare functional materials with a wide breadth of potential applications as adaptive materials,11,12 smart sensors13,14 and optics,15,16 functional surfaces,17,18 and conductive nanomaterials.19,20 In contrast, the self-assembly of actinide materials is far less mature, and as such, our efforts are focused on exploring solidstate supramolecular chemistry in the 5f block by delineating assembly criteria and developing structure−property relationships. Supramolecular assembly has been demonstrated to be a robust and viable route to prepare large single crystals of © 2017 American Chemical Society
uranyl-bearing hybrid materials, and it has been used to assemble discrete molecular building units or “tectons” ([UO2Cl4]2− or [UO2Br4]2−, for example) into extended structures via halogen and/or hydrogen bond-donating pyridinium cations.21,22 This strategy is dependent upon the ability to promote a desired and reproducible uranyl tecton that is appropriately functionalized to accept non-covalent interactions. Generation of such a tecton is a challenge in and of itself, owing to the tendency of the uranyl23,24 (and, more broadly, actinyl)25 cations to hydrolyze to form secondary building units of higher nuclearity. For this reason, we have developed a synthetic strategy wherein acidic media, rich in coordinating anions (Cl− and Br−, for instance), can thwart metal-ion hydrolysis and promote the formation of the [UO2X4]2− anion almost exclusively.26,27 This species is a prime candidate for supramolecular assembly, as the Received: June 1, 2017 Published: July 12, 2017 10843
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
that can readily accept hydrogen and halogen bonds. Next, the strengths of these non-covalent interactions are assessed crystallographically and ranked (where possible) according to their relative interaction distances. Moreover, we have calculated the energies of the non-covalent interactions (NCIs) using density functional theory (DFT) in order to quantitatively evaluate the hierarchy of NCI strengths as put forth crystallographically. Finally, the NCIs were analyzed using the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) approaches, the results of which are correlated with the electrostatic potential maps and DFT calculations to highlight the role of both electrostatics and the non-covalent interaction in driving and directing molecular assembly. In totality, these efforts provide a foundation upon which the assembly of molecular actinide and organic species can be delineated and quantified.
four equatorial halide ligands readily accept a range of noncovalent interactions, such as halogen and hydrogen bonds.26,27 Moreover, one can judiciously select a molecular cation with a desired non-covalent interaction donor to generate systematic families of materials within which assembly criteria and strengths of interactions may be delineated. Knowledge of these criteria is a pre-requisite for introducing a degree of control over self-assembly such that customizable molecular building units can be purposefully combined using a preselected set of non-covalent interactions. In this contribution, we report an extension of our supramolecular assembly strategies to include the isostructural neptunyl and plutonyl tetrachlorides, [AnO2Cl4]2−, where An = Np(VI) or Pu(VI). As with the uranyl, the NpO22+ and PuO22+ cations are susceptible to metal-ion hydrolysis, yet more important is the challenge presented by the redox chemistry of aqueous Np and Pu species in acidic media.28 For instance, previous efforts pairing the [PuO2Cl4]2− cation with bipyridinium cations led to the incorporation of [Pu(IV)Cl6]2− in some compounds, as stabilizing a desired oxidation state over the course of synthesis proved non-trivial.29 We have consequently amended our synthetic strategy to include a non-coordinating chemical oxidant (NaBrO3) to prepare a single Np(VI)O22+- or Pu(VI)O22+-containing tecton for assembly with charge-balancing and hydrogen and/or halogen bond-donating 4-X-pyridinium cations (Scheme 1).
■
EXPERIMENTAL SECTION
Synthesis and Single-Crystal X-ray Diffraction. Compounds 1−12: (C5H5NX)2[AnO2Cl4] (X = H, Cl, Br, I; An = U, Np, Pu). The syntheses and structures of 1−4 are reported elsewhere,21,22 yet a procedure outlining their syntheses is included in the Supporting Information. Single crystals of 5−12 were grown by slow evaporation from acidic aqueous, chloride-rich media that was prepared using a valence pure NpO22+ or PuO22+ chloride stock solution and two molar equivalents (based on Np or Pu) of the appropriate 4-X-pyridine. Preparation of the stock solutions and additional synthetic details are reported in the Supporting Information. Crystals of 5−12 were harvested from their mother liquors and characterized via single-crystal X-ray diffraction. Selected crystallographic information for 5−12 are provided in Table 1, whereas details pertaining to the data collections and structural refinements are provided in the Supporting Information. Computational Details. Electrostatic Potentials. The atomic coordinates of the [AnO2Cl4]2− (from 3, 7, and 11) and 4-Xpyridinium (X = H, Cl, Br, I) (from 1−4) tectons were obtained directly from the crystallographic data, and the electrostatic potential (ESP) was calculated with Kohn−Sham (KS) DFT. The KS calculations were performed using the Gaussian 09 (rev. D.01) program38 using the M06-2X39 functional and the following basis sets: U, Np, and Pu, 60MWB-SEG+ECP-60MWB;40−42 Cl and Br, def2TZVP; I, def2-TZVP+ECP; C, N, and O, def2-TZVP; and H, def2SVP.43 The ESP, V (r), can be used to analyze the formation of noncovalent interactions in the crystalline state, such as hydrogen and halogen bonds.44,45 The electrostatic potential of a molecule at a point r is described by eq 1, where ZA is the charge on a nucleus A, located at RA, and ρ (r) is the electron density of the molecule.
Scheme 1. 4-X-Pyridinium (X = H, Cl, Br, I) Cations Featured in This Study
This synthetic effort not only advances our understanding of transuranic crystal chemistry, which is underdeveloped, but also presents an opportunity to develop structural and electronic trends across the U, Np, and Pu series. Establishing periodic trends is, of course, important on a fundamental level, but it is also vital for exploiting those similarities or differences at an applied level, for instance, in separation30,31 or remediation32,33 chemistries. For these reasons, we have prepared and structurally characterized a family of 12 (eight newly reported here) materials, (C5H5NX)2[AnO2Cl4], where An = U(VI), Np(VI), and Pu(VI); X = H, Cl, Br, and I. The principle motivations for this effort are to probe the supramolecular interactions in [AnO2Cl4]2− systems and to assess assembly criteria such that a comprehensive set of “rules” may be delineated. As such, the structural motifs across compounds 1−12 are described (from X-ray diffraction data) and subsequently delineated using four computational approaches. We first demonstrate a rationale for the crystallographically observed non-covalent interaction acceptor−donor pairings using density functional calculations of electrostatic potential maps in a manner consistent with works by Brammer,34 Politzer,35 and others in the actinide field.36,37 Here, the magnitude and distribution of the electrostatic potential about the [AnO2Cl4]2− tectons, for instance, are useful in identifying suitable regions of the molecular species
V (r) =
∑ A
ZA − |RA − r|
∫ |rρ′ (−r′)r| dV ′
(1)
The ESP is given here in atomic units (au) and refers to a fictitious positive probe charge of 1 au placed at position r. A negative/positive potential indicates an electron-rich/deficient region. Information about the response of the molecule to the presence of the probe charge is not contained in the ESP and its graphical visualizations. In order to deduce chemically relevant information, the ESP is usually mapped onto a van der Waals surface or an isodensity surface. In this work, we generated electron density isosurfaces at 0.001 au (electrons per bohr3), based upon a recommendation by Bader et al.46 as this isosurface encompasses most of the integrated electron density and is considered to occur at distances from the nuclei that are most relevant for evaluating non-covalent interactions.44,45 Quantifying Non-covalent Interaction Strengths. The strengths of the hydrogen and halogen bonding interactions in 1−12 were quantified using DFT calculations with counterpoise corrections for the basis set superposition error (BSSE), with the same functional and basis sets as used for the ESPs. The crystal structures were truncated 10844
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society Table 1. Selected Crystallographic Information of Compounds 1 and 5−12 formula formula mass temp (K) λ (Kα) crystal color and habit size crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Dcalc (Mg m−3) Z μ (mm−1) no. of reflections measured no. of independent reflections Rint final R1 values (I > 2σ(I)) final wR2 (F2) values (I > 2σ(I)) goodness of fit on F2 CCDC number
1
5
6
7
8
(C5H6N)2[UO2Cl4] 572.05 296(2)K 0.71073 green prismatic 0.100 × 0.75 × 0.50 triclinic P1̅ 7.7185(5) 10.2938(6) 11.3249(7) 78.2560(10) 72.0860(10) 75.8620(10) 822.16(9) 2.311 2 10.518 12301 4627 0.0254 0.0249 0.0552
(C5H6N)2[NpO2Cl4] 571.02 296(2) 0.56086 yellow-green prismatic 0.361 × 0.267 × 0.252 triclinic P1̅ 7.7161(6) 10.2869(9) 11.2740(9) 78.003(2) 71.909(2) 75.712(2) 815.91(12) 2.324 2 7.698 33886 6244 0.0430 0.0310 0.0562
(C5H5NCl)2[NpO2Cl4] 639.90 296(2) 0.71073 yellow-green prismatic 0.229 × 0.053 × 0.032 monoclinic C2/m 15.7687(10) 6.7068(4) 9.0971(6) 90.00 105.585(2) 90.0 926.71(10) 2.293 2 6.473 7582 1307 0.0382 0.0248 0.0457
(C5H5NBr)2[NpO2Cl4] 728.82 296(2) 0.56086 yellow-green prismatic 0.345 × 0.183 × 0.128 monoclinic C2/m 15.9996(11) 8.6659(6) 6.8488(5) 90.00 102.023(2) 90.00 928.76(11) 2.606 2 9.067 33516 2393 0.0361 0.0165 0.0332
(C5H5NI)2[NpO2Cl4] 822.80 296(2) 0.71073 yellow-green prismatic 0.135 × 0.122 × 0.076 monoclinic C2/m 16.3553(8) 8.6680(4) 6.9455(3) 90.00 102.3940(10) 90.00 961.70(8) 2.841 2 9.171 19190 1687 0.0488 0.0333 0.0590
1.000 1553661
1.078 1553662
1.073 1553663
1.037 1553664
1.191 1553665
formula formula mass temp (K) λ (Kα) crystal color and habit size crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Dcalc (Mg m−3) Z μ (mm−1) no. of reflections measured no. of independent reflections Rint final R1 values (I > 2σ(I)) final wR2 (F2) values (I > 2σ(I)) goodness of fit on F2 CCDC number
9
10
11
12
(C5H6N)2[PuO2Cl4] 576.02 296(2) 0.71073 yellow-orange prismatic 0.291 × 0.165 × 0.070 triclinic P1̅ 7.7417(4) 10.3137(5) 11.2637(6) 77.9360(10) 71.7730(10) 75.4320(10) 818.42(7) 2.337 2 4.677 31093 4630 0.0269 0.0208 0.0467 1.056 1553666
(C5H5NCl)2[PuO2Cl4] 644.90 296(2) 0.56086 yellow prismatic 0.253 × 0.088 × 0.062 monoclinic C2/m 15.8025(11) 6.7045(5) 9.1106(6) 90.00 105.384(2) 90.00 930.66(11) 2.301 2 5.348 12508 2407 0.0396 0.0322 0.0669 1.083 1553667
(C5H5NBr)2[PuO2Cl4] 733.82 296(2) 0.56086 yellow prismatic 0.283 × 0.277 × 0.126 monoclinic C2/m 16.1488(9) 8.7340(5) 6.7757(4) 90.00 101.912(2) 90.00 935.09(9) 2.606 2 7.465 10959 1863 0.0276 0.0176 0.0414 1.143 1553668
(C5H5NI)2[PuO2Cl4] 827.80 296(2) 0.56086 yellow-orange prismatic 0.204 × 0.143 × 0.122 monoclinic C2/m 16.4644(11) 8.7019(5) 6.8794(5) 90.00 102.267(2) 90.00 963.12(11) 2.854 2 6.725 9893 1570 0.0287 0.0202 0.0512 1.107 1553669
into fragments: the [AnO2Cl4]2− building unit(s) and the 4-Xpyridinium cation(s) (Figures S4−S8). The attraction energies between the fragments were in turn calculated (results are given in in kJ mol−1) and represent the sum of the electrostatic (Coulombic) attraction of the [AnO2Cl4]2− anions and 4-X-pyridinium cations and the non-covalent interactions. The errors associated with the DFT and ESP calculations are 1−2 kcal mol−1 or 4−8 kJ mol−1. It is also
important to keep in mind that electron density reorganization takes place when fragments are brought together. The halogen bonds were subjected to a natural bond orbital (NBO) analysis in order to explore the contribution of hyperconjugation (i.e., charge transfer), to the strength and nature of the interaction. The quantum theory of atoms in molecules (QTAIM) analysis was performed to characterize the electron density topology and assess the 10845
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
equatorial chloro ligands or “yl” oxo groups may influence the actinyl vibrational modes. Structure Types I−IV. Structure Type I: Compounds 1, 5, and 9. The structure of 1 contains two crystallographically unique uranyl tetrachloro and pyridinium tectons, the assembly of which can be described as two unique supramolecular layers that stack in an ABAB fashion (Figure 2). In layer A, the [UO2Cl4]2− tectons align along the [100] direction and are linked into chains via bridging bifurcated hydrogen bonds stemming from the pyridinium cations (Figure 3). The relevant interaction distances and angles are N1Hn1···Cl3 3.357(5) Å and ∠N1Hn1···Cl3 113.3°; N1Hn1···Cl4 3.573(5) Å and ∠N1Hn1···Cl4 122.7°. The pyridinium cations also form offset π-interactions that “zip” the adjacent chains together to form sheets in the (−110) plane. Parameters relevant to the offset π-interaction are provided in Table S2. In layer B, the [UO2Cl4]2− tectons are flanked by the pyridinium cations, which each form a bridging bifurcated hydrogen bond between two chloro ligands (Cl1 and Cl1′) of neighboring uranyl BUs (Figure 3). The relevant hydrogen bond distances and angles are N2Hn2···Cl1 3.432(5) Å and ∠N2Hn2···Cl1 116.8°; N2Hn2···Cl1′ 3.616(5) Å and ∠N2 Hn2···Cl1 130.4°. The pyridinium cations also form offset πinteractions to form sheets in the (−110) plane. The [NpO2Cl4]2− analogue of structure type I (compound 5) is isomorphous with both the [UO2Cl4]2− and [PuO2Cl4]2− materials, yet the stacking sequence differs. In 5, a single sheet, containing two crystallographically unique pyridinium and [NpO2Cl4]2− tectons stack to form a layered structure (Figure S2). The pyridinium cations participate in bridging bifurcated hydrogen bonding and offset π-interactions that facilitate the formation of the single sheet (Tables S2 and S3). Structure Type II: Compound 2. Structure type II features only a single example, compound 2, (C5H5NCl)2[UO2Cl4], and is the only instance where progression across the period (from [UO2Cl4]2− to [NpO2Cl4]2− to [PuO2Cl4]2−) in which a fixed 4-XPyH cation does not result in isomorphism. Compound 2 is described below, despite being reported elsewhere,21 to provide context for the ensuing discussion of structural systematics across 1−12. Compound 2 crystallizes in the space group P1̅ and consists of supramolecular chains that contain [UO2Cl4]2− and 4-ClPyH tectons (Figure 4). The 4-ClPyH cations form bifurcated hydrogen bonds to the chloro ligands of the [UO2Cl4]2− dianions: N1H1n···Cl1 3.356(3) Å and ∠N1Hn1···Cl1 113°; N1Hn1···Cl2 3.462(3) Å and ∠N1Hn1···Cl2 138°. The chloro substituents in turn form CCl···Cl halogen bonds (C3Cl3···Cl1 3.314(1)Å and ∠C3Cl3···Cl1 163.5(1)°) to the chloro ligands of a neighboring [UO2Cl4]2− tecton to form chains (Figure 4).
degree of covalency of each halogen and hydrogen bonding interaction. Both the interaction energy calculations and the NBO analysis were performed using Gaussian 09 (rev. D.01)38 whereas the QTAIM analysis was performed using AIMALL.47 The geometric parameters of, and intermolecular distances between, the molecular fragments used in all calculations were taken directly from the experimental diffraction data and not optimized.
■
RESULTS Assembling the [UO2Cl4]2−, [NpO2Cl4]2−, and [PuO2Cl4]2− anions with 4-X-pyridinium (X = H, Cl, Br, I) cations formed a total of 12 compounds that can each be categorized into one of four structure types. A thorough description of the actinyl tetrachloro dianions, along with one compound representative of each structure type is provided in the following section to capture the relevant non-covalent interactions and modes of assembly. A summary of the structure types is provided in Table 2, whereas a comprehensive list of non-covalent interaction types and parameters is provided in Tables S2−S6. Table 2. Compounds 1−12, Categorized into Four Structures Types [UO2Cl4]2− 4-HPyH 4-ClPyH 4-BrPyH 4-IPyH
1: 2: 3: 4:
Type Type Type Type
I II IV IV
[NpO2Cl4]2−
[PuO2Cl4]2−
Type Type Type Type
9: Type I 10: Type III 11: Type IV 12: Type IV
5: 6: 7: 8:
I III IV IV
Structural Descriptions: The [AnO2Cl4]2− (An = U(VI), Np(VI), Pu(VI)) Tectons. The square bipyramidal actinyl tetrachloro dianions each feature identical connectivity and consist of a linear actinyl cation (AnO22+) that is coordinated equatorially by four chloro ligands at average distances of U Cl 2.671 Å, NpCl 2.654 Å, and PuCl 2.658 Å (Figure 1). The average AnO bond distances in 1−12 decreases across the period: [UO2Cl4]2− (1.765 Å UO) > [NpO2Cl4]2− (1.748 Å NpO) > [PuO2Cl4]2− (1.735 Å PuO). The contraction in the “yl” bond lengths is explained by the trend of decreasing ionic radii across the 5f elements (i.e., the actinide contraction)48−50 and is consistent with An = O bond distances reported for the uranyl,51,52 neptunyl,53,54 and plutonyl55,56 tetrachloride dianions in other systems. The actinyl (AnO) bonds were probed by ATR-IR spectroscopy (Figures S21−S32 and Table S10) as the energy of the asymmetric stretching frequency (ν3) was expected to decrease with decreasing “yl” bond length across the series, UO22+ > NpO22+ > PuO22+.57,58 A clear trend in the ν3 vibrational energies across 1−12 was not evident, however, yet one may speculate that second sphere interactions with the
Figure 1. Left to right: [UO2Cl4]2−, [NpO2Cl4]2−, and [PuO2Cl4]2− tectons as yellow, green, and red polyhedra. The chloro ligands are shown as green spheres, whereas the axial oxygen atoms are red. 10846
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
Figure 2. Compound 1, (C5H6N)2[UO2Cl4], can be described as two unique supramolecular sheets (A and B) that alternate to form an ABA layered structure.
Figure 3. Pyridinium cations and [UO2Cl4]2− dianions in 1 from two unique layers A (left) and B (right) via bridging bifurcated hydrogen bonds and offset π-interactions.
Figure 4. A chain motif is observed in 2 as hydrogen and halogen bonds link the 4-ClPyH cations and [UO2Cl4]2− anions together. This is the only example in this series where structure type II is observed. Redrawn from ref 21.
bonds donated by flanking 4-BrPyH cations. These bridging bifurcated hydrogen bonds are symmetric, with interaction distances and angles at N1Hn1···Cl2 3.250(2) Å and ∠N1 Hn1···Cl2 139°, respectively. The bromo substituent of the 4BrPyH cations links adjacent and out-of-plane chains into sheets via C 3 Br 1 ···Cl 1 halogen bonds: C 3 Br 1 ···Cl 1 3.3281(9) Å and ∠C3Br1···Cl1 179.10(8)°.
Structure Type III: Compounds 6 and 10. Compound 6, (C5H5Cl)2[NpO2Cl4], crystallizes in the space group C2/m and consists of [NpO2Cl4]2− anions that are charge balanced by 4ClPyH cations. Two 4-ClPyH cations form linear NH···Cl hydrogen bonds (N1H5···Cl2 3.163(3) Å and ∠N1H5···Cl2 158(4)°) to the chloro ligands of each [NpO2Cl4]2− dianion (Figure 5). The 4-ClPyH cations also participate in offset πinteractions, with their neighboring and out-of-plane symmetry equivalents, to form infinite cationic stacks along the [010] direction. Relevant interactions parameter are provided in Table S2. The hydrogen bonding and offset π-interaction motifs assemble the 4-ClPyH and [NpO2Cl4]2− tectons into supramolecular sheets in the (201) plane (Figure 5). Structure Type IV: Compounds 3, 4, 7, 8, 11, and 12. Compound 7, (C5H5NBr)2[NpO2Cl4], crystallizes in the space group C2/m, and the structure can be described as consisting of supramolecular sheets in the (201) plane (Figure 6). The neptunyl tetrachloro dianions align along the [010] direction and form chains via bridging bifurcated NH···Cl hydrogen
■
DISCUSSION Acceptor and Donor Characteristics. We present here for the first time a detailed description of the isodensity surfacemapped ESPs of the actinyl tetrahalide dianions, the chloro and oxo ligands of which may serve as acceptor sites of noncovalent interactions from 4-X-pyridinium (X = H, Cl, Br, I) donors. The ESPs of the [AnO2Cl4]2− and 4XPyH+ tectons are overall negative and positive (respectively) due to their charges, and their magnitudes and distribution are used to rank the strengths of the acceptor and donor sites (and the resulting NCIs), as well as to reconcile observed pairings across 1−12. 10847
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
Figure 5. 4-ClPyH cations in 6 form linear NH···Cl hydrogen bonds to the [NpO2Cl4]2− anions and offset-π interactions to their symmetry equivalents to form sheets in the (201) plane.
Figure 6. 4-BrPyH cations link the [NpO2Cl4]2− tectons together in 7 via bridging bifurcated NH···Cl hydrogen bonds and CBr···Cl halogen bonds to form sheets.
Figure 7. Electrostatic potential of the [AnO2Cl4]2− (An = U, Np, Pu; left-to-right) tectons are mapped onto a 0.001 au isodensity surface. Several regions of interest are highlighted. The color scale ranges from dark blue to red, which represent potentials of −652 and −786 kJ mol−1, respectively. The electrostatic potential is overall negative due to the charge of the complex ion.
10848
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
mol−1 as the AnO22+ center is varied, indicating the potential values are not very sensitive to the nature of the metal center. The magnitude of the ESP and steric accessibility of the chloro ligands (perpendicular to the An−Cl bond) render this area as the primary halogen and hydrogen bond acceptor site in 1−12. Moreover, the rather invariant ESP values of this region across the [AnO2Cl4] 2− series likely explains the frequency of isomorphism within this family. [AnO2Cl4]2− Tectons. The “yl” Oxygen Atoms. The magnitude of the electrostatic potential on the isodensity surface around the oxo ligands (coaxial to the An = O bond) decreases with the ionic radii of the central metal species: U(VI) > Np(VI) > Pu(VI). The electrostatic potential values are [UO2Cl4]2−, −700 kJ mol−1 > [NpO2Cl4]2−, −691 kJ mol−1 > [PuO2Cl4]2−, −679 kJ mol−1 (Figure 7 and Table 3). The decreasingly negative potential at the “yl” oxygen atoms indicates that the propensity of these ligands to accept NCIs, such as hydrogen or halogen bonds, also decreases across the period as increasingly electrophilic sites become less prone to accept non-covalent interactions. Whereas this characterization may be less useful to the discussion of compounds 1− 12 as the “yl” oxo atoms do not participate in any non-covalent interactions, this information is relevant in the context of several related systems wherein the nominally terminal oxo ligands accept halogen bonds62 or can be “activated” to form additional covalent linkages.63−65 Systematically harnessing the oxo atoms as non-covalent interaction acceptors in the self-assembly of uranyl materials is an active area of research within our own group66−68 and has also been a focus elsewhere as an opportunity to enhance actinide separations.69,70 Region between the Cl Ligands. The most negative electrostatic potential is located on the isodensity surface between the Cl ligands of the [AnO2Cl4]2− dianion (Table 3 and Figure 7). This region is also the most sterically hindered and therefore not preferred as an acceptor of non-covalent interactions in this family. This region is described in detail here, however, as it can accept non-covalent interactions and is, in fact, the norm when pairing the [UO2Cl4]2−, [UO2Br4]2− or [PuO2Cl4]2− dianions, with linear hydrogen bond-donating bipyridinium cations.36,37,39 The magnitude of the ESP in the region between the Cl ligands increases (becomes more negative) across the period: [UO2Cl4]2−, −776 kJ mol−1 < [NpO2Cl4]2−, −783 kJ mol−1 < [PuO2Cl4]2−, −786 kJ mol−1.
[AnO2Cl4]2− Tectons. The Chloro Ligands. The equatorial chloro ligands exhibit two distinct regions of differing ESP when coordinated to the AnO22+ cation, one of which is wellsuited to accept NCIs whereas the other is not. A large ESP resides on the chloro ligands in the region perpendicular to the An−Cl bond (the acceptor sites) relative to the areas coaxial to the An−Cl bond, which are electrostatically deficient (Figure 7). This is consistent with observations by Brammer et al. where the ability of metal-bound halogens to function as good acceptors of hydrogen and halogen bonds was rationalized by a similar electrostatic anisotropy that was ultimately induced by metal−ligand coordination.34,59,60 The electron-deficient region of the chloro ligands in the direction coaxial to the An−Cl bonds increase slightly (becomes more negative) across the period, [UO2Cl4]2−, −652 kJ mol −1 < [NpO 2 Cl 4 ] 2− , −656 kJ mol −1 < [PuO2Cl4]2−, −657 kJ mol−1, and feature the smallest ESP of the entire [AnO2Cl4]2− anions; these are therefore the worst potential acceptor sites (Figure 7 and Table 3). This trend of Table 3. Selected Electrostatic Potential Values in kJ mol−1 of the [AnO2Cl4]2− Tectons in Different Regions on the Isodensity Surfaces region
[UO2Cl4]2−
[NpO2Cl4]2−
[PuO2Cl4]2−
coaxial with the AnO bond between Cl ligands coaxial with the AnCl bond
−700 −776 −652
−691 −783 −656
−679 −786 −657
slightly decreasing ESP on the chloro ligands (yet, admittedly within error) from U(VI) < Np(VI) < Pu(VI) correlates with the reported effective nuclear charges of the actinyl cations as they decrease from UO22+, Zeff = +3.2 < NpO22+, Zeff = +3.0 < PuO22+, Zeff = +2.9.61 The chloro ligands exhibit a more negative ESP perpendicular to the An−Cl bonds due to the high concentration of electrons and, thus, are well-suited to accept NCIs (Table S7 and Figure 8). The ESP across this region varies as it is most negative on the surface between the ligands and becomes increasingly less negative toward the apex of the ligand. The potential is reported as a range from [UO2Cl4]2−, −776 to −664 kJ mol−1; [NpO2Cl4]2−, −782 to −660 kJ mol−1; [PuO2Cl4]2−, −784 to −662 kJ mol−1 (Figure 8). The average electrostatic potential across the halogen ligands, however, changes by only ∼2 kJ
Figure 8. Electrostatic potential maps of the [UO2Cl4]2− and [PuO2Cl4]2− tectons illustrate the similarity of the non-covalent interaction acceptor sites of the two dianions. 10849
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
Figure 9. Left to right: Electrostatic potential maps of the 4-HPyH, 4-ClPyH, 4-BrPyH, and 4-IPyH cations. The color scale ranges from dark blue to red, which represents potentials of 624 and 294 kJ mol−1, respectively.
from Cl < Br < I as the magnitude of the electrostatic potential increases accordingly: +362 kJ mol−1 < +385 kJ mol−1 < +412 kJ mol−1 (Table 4). Having characterized the electrostatic potential at the noncovalent interaction acceptors (Cl ligands) and donors (pyridyl nitrogen and halo substituents), we are now able to reconcile the observed acceptor−donor pairings across 1−12. The similarity in the electrostatic potential of the hydrogen and halogen bond donor sites across the [AnO2Cl4]2− series not only explains the high degree of isomorphism in this family but also suggests the strengths of the non-covalent interaction is dictated largely by the pyridinium cations. We can therefore expect the strength of the hydrogen bonding interactions to increase from 4-IPyH < 4-HPyH < 4-BrPyH < 4-ClPyH whereas the strength of the halogen bonds should increase 4ClPyH < 4-BrPyH < 4-IPyH. Going forward, this information is used to support both the crystallographic and computational discussions aimed at quantifying the strengths of the NCIs. The metric that will guide the crystallographic assessment will be relative or normalized interaction distances, i.e., shorter distances (d) equate to stronger interactions, using the van der Waals radii of the acceptor and donor atoms (eq 2).73 We will therefore equate greater overlap in the van der Waals radii of the acceptor (ra) and donor (rd) atoms with increased noncovalent interaction strength. We use the vdW radii values as recommended by Bondi (N, 1.55 Å; O, 1.52 Å; Cl, 1.75 Å; Br, 1.85 Å; I, 1.98 Å).74
This observation is consistent with the decreasing effective metal charges across the “yl” cations and also with the decreasing electrostatic potential residing on the “yl” oxos, from U(VI) > Np(VI) > Pu(VI), which leads to an accumulation of electron density around the metal in the equatorial plane. 4-X-Pyridinium Cations (X = H, Cl, Br, I). The electrostatic potential of each 4-X-pyridinium cation is positive everywhere on the isodensity surface, owing to their overall positive charges, with the most residing around each respective pyridyl nitrogen (i.e., is the hydrogen bond donor; Figure 9 and Table 4). The potential at the N−H+ on each of the two Table 4. Electrostatic potential values in kJ mol−1 in selected regions of the isodensity surfaces of the 4-X-Pyridinium cations from 1−4 region cation
σ-hole
N−H+
+362 +385 +412
from layer A: +628 from layer B: +627 +631 +629 +623
pyridinium 4-chloropyridinium 4-bromopyridinium 4-iodopyridinium
crystallographically unique 4-HPyH cations (in 1) is +628 and +627 kJ mol−1, respectively. The potential at the N−H+ moiety of the 4-XPyH (X = Cl, Br, I) cations decreases slightly as the electron-donating ability of the halogen substituent increases: Cl (+631 kJ mol−1), Br (+629 kJ mol−1), and I (+623 kJ mol−1). These values (albeit within error) indicate the hydrogen bond donor strength of each cation should increase from 4-IPyH < 4-HPyH < 4-BrPyH < 4-ClPyH. The ESP at the halogen substituents is anisotropic, as polarization of the halogen by the protonated pyridyl ring results in the formation of a σ-hole coaxial to the C−X bond (Figure 9). A σ-hole is typically described as a closed or semiclosed region of more positive electrostatic potential on the outermost surface of an atom that is involved in a polarizing covalent bond, whereas the “sides” of the atom have a more negative potential.45,71 The magnitude of the ESP at the σ-hole can be used as an indicator of halogen bond strength.72 We can therefore expect the strength of the halogen bonds to increase
% vdW = d /(ra + rd)
(2)
Ranking Interaction Strengths Crystallographically. Compounds 1−4: (C5H5NX)2[UO2Cl4] (X = H, Cl, Br, I). Compounds 1−4 feature three different varieties of bifurcated NH···Cl hydrogen bonds and are representative of three structure types: type I (1), type II (2), and type IV (3 and 4). The NH···Cl hydrogen bonds in 1, 3, and 4 bridge and assemble the [UO2Cl4]2− anions into chains and ultimately sheets. The interactions in 1 are asymmetric, meaning the N H···Cl interaction distance to each acceptor is different, whereas those in 3 and 4 are symmetric (Figures 3 and 6). The NH···Cl hydrogen bonds in 2 do not bridge as both acceptor ligands are on the same [UO2Cl4]2− tecton. Comparing the relative interaction distances across 1−4 reveals 10850
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society Table 5. Calculated Hydrogen Bond (HB) Strengths in kJ mol−1 from the Structures of 1−4, 5−8, and 9−12 energy per HB cluster
[UO2Cl4]2−
[NpO2Cl4]2−
(4-HPyH)2[AnO2Cl4]
1, layer A: −542 layer B: −532 2, −564 3, −528 4, −521
5, layer A: −549 layer B: −540 6, −566 7, −530 8, −522
(4-ClPyH)2[AnO2Cl4] (4-BrPyH)4[AnO2Cl4] (4-IPyH)4[AnO2Cl4]
[PuO2Cl4]2− 9, layer A: layer B: 10, 11, 12,
−542 −534 −572 −515 −523
in 3, 7, and 11 are 98.7%, 98.9%, and 98.6%. These observations are consistent with the similar magnitudes of the electrostatic potential at the non-covalent interaction acceptor site (extension of the Cl ligand perpendicular to the An−Cl bond) on the [AnO2Cl4]2− tectons across the period U, Np, and Pu. In fact, the relative interaction strength of any noncovalent interaction (hydrogen or halogen bond) within a given structure type does not change if the cation is kept constant while the [AnO2Cl4]2− building unit is changed. Ranking Interaction Strengths Computationally. We have established trends of relative NCI strengths and, with the aid of the ESP maps, have also identified several potential shortcomings when using a purely crystallographic approach for the analysis of hydrogen bonding interactions with differing geometries. As such, the energies of the hydrogen and halogen bond interactions were calculated and reported in kJ mol−1. We should comment, however, that we have thus far oversimplified our terminology, and in reality, all of the interactions mentioned herein should be properly termed “chargeassisted”75,77,78 as the Coulombic contribution (i.e., attraction between the cation and anion) to these interactions is dominant. The clearly defined hydrogen and halogen bonding motifs, however, cannot be ignored and are indeed significant. We highlight this now such that our calculated interaction energies (between −418 and −586 kJ mol−1) can be kept in perspective as these values are much more energetic than a typical neutral halogen or hydrogen bond, which are generally in the range of ∼0.2−62 kJ mol−1 (Table S9).75,77,79 More specifically, however, we note that the energies of the chargeassisted hydrogen and halogen bonding interactions calculated herein are consistent with those determined by Awwadi et al. in similar copper halide and 4-halopyrindinium, (C5H5NX)2[CuX′4] (X = Cl, Br, I and X′ = Cl, Br), supramolecular systems.80 Compounds 1−4: (C5H5NX)2[UO2Cl4] (X = H, Cl, Br, I). Recall that our crystallographic metrics led us to rank the hydrogen bonds in 1−4 as increasing from 1 < 2 < 3 ≈ 4. This is not substantiated computationally as the energies of the hydrogen bonds increase in the order 4 (−520.8 kJ mol−1) < 3 (−528.2 kJ mol−1) < 1 (−531.8 and −541.4 kJ mol−1) < 2 (−563.3 kJ mol−1) (Table 5). The computationally derived hydrogen bond energies are consistent with the expected ranking of strengths as put forth by analysis of the electrostatic potential at the N−H+ donors: 4 (4IPyH, +623 kJ mol−1) < 3 (4BrPyH, +629 kJ mol−1) < 1 (4HPyH, +630 kJ mol−1) < 2 (4ClPyH, +631 kJ mol−1). The need for a quantitative metric to rank interaction strengths is realized here, as comparison of geometrically different interaction types on the basis of interaction distances can be misleading. It is worth mentioning that NH···Cl hydrogen bonds in 3 and 4 are crystallographically indistinguishable as their interaction distances are not statistically different, yet computationally we do observe a 7.4 kJ mol−1 difference in their
an increase in the overlap of the vdW radii of the acceptor (Cl) and donor (N) from 1 < 2 < 3 ≈ 4 (Table S4). This crystallographic ranking of strength is not consistent with the expected ranking based on the electrostatic potential at the N− H + donors: 4 (4IPyH, +623 kJ mol−1) < 3 (4BrPyH, +629 kJ mol−1) < 1 (4HPyH, +630 kJ mol−1) < 2 (4ClPyH, +631 kJ mol−1), where the higher potential should equate to a stronger donor. This inconsistency highlights a potential limitation of utilizing vdW radii in comparing non-covalent interactions of differing geometry. Compounds 2−4 feature CX···ClU (X = Cl, Br, I) halogen bonding interactions that exhibit a trend of increasing overlap in the vdW radii of the acceptor (Cl) and donor (X) atoms as X increases Cl < Br < I (Table S4). The relative halogen bond strengths therefore increase from 2 < 3 < 4, which is consistent with the increasing electrostatic potential of the σ-hole belonging to each halogen donor: 2 (Cl, +362 kJ mol−1) < 3 (Br, +385 kJ mol−1) < 4 (I, +412 kJ mol−1) (Figure 9). These results are consistent with well-established trends of halogen bonding wherein strength increases with polarizability of the donor atom.75,76 Compounds 5−8: (C5H5NX)2[NpO2Cl4] (X = H, Cl, Br, I). Compounds 5−8 feature three varieties of NH···Cl hydrogen bonds (two bifurcated and one linear) and are representative of three structure types: type I (5), type III (6), and type IV (7 and 8). The NH···Cl hydrogen bonding interactions in 5, 7, and 8 are analogous to those in the corresponding uranyl analogues whereas those in 6 are linear, with a single chloro ligand of a [NpO2Cl4]2− tecton serving as an acceptor. The N−H···Cl hydrogen bonds increase in relative strength (based on vdW overlap) from 5 < 7 ≈ 8 < 6 (Table S5). This ranking is once again not consistent with an electrostatic argument based on the magnitudes of the electrostatic potential at each pyridyl nitrogen (Table 4). Compounds 7 and 8 feature CX···ClNp (X = Br, I) halogen bonds that increase in relative strength from 7 < 8. Once again, the overlap of the vdW radii of the donor and acceptor increases with a larger, more polarizable halogen bond donor that also exhibits a greater electrostatic potential at the σhole: 7, 92.4% vdW (4BrPyH, +385 kJ mol−1) < 8, 87.8% vdW (4IPyH, +412 kJ mol−1) (Table S5). The hydrogen and halogen bonding interactions in 9−12, (C5H5NX)[PuO2Cl4], are summarized in Table 8 (below) and follow trends identical to those outlined for 5−8. Structure Type IV: (C5H5NX)2[AnO2Cl4] (An = U, Np, Pu ; X = Br, I). Compounds 3, 4, 7, 8, 11, and 12 feature bifurcated bridging NH···Cl hydrogen bonds and CX···ClAn halogen bonds that ultimately link the [AnO2Cl4]2− tectons into sheets (Figure 6). The relative strengths of the NH···Cl hydrogen and CX···Cl halogen bonds featured in structure type IV are rather invariant when the cation is kept constant and the An(VI) metal center changes from U, Np, Pu. For instance, the percentage of overlap in the vdW radii of N and Cl 10851
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
of the 4-BrPyH cations around the [PuO2Cl4]2− anions used in this calculation is consistent with the arrangement used in the calculation featuring the [UO2Cl4]2− and [NpO2Cl4]2− tectons (Figure S8). The relative strength of the CX···ClAn (X = Br, I ; An = Np, Pu) halogen bonds in 7, 8, (C5H5NX)2[NpO2Cl4], and 11, 12, (C5H5NX)2[PuO2Cl4], increased from 7 < 8 and 11 < 12. The calculated energies of these interactions are consistent with this ranking: 7, −396 kJ mol−1 < 8, −419 kJ mol−1 and 11, −396 kJ mol−1 < 12, −419 kJ mol−1 (Table 6). Probing the Nature of Specific Non-covalent Interactions. The CX···ClU (X = Cl, Br, I) halogen and N H···ClU hydrogen bonds in 2−4 were characterized using QTAIM and NBO analysis to evaluate the NCIs in a manner that is somewhat independent of the Columbic attraction.81 Our goal was to answer the question, “Are the non-covalent interactions merely a consequence of a predominately electrostatic packing or do they exert their own influence?” First, the topological properties of the electron density, ρ(r), at bond critical pointsthe region between two interacting atoms where the electron density reaches a minimum 46 is characterized in order to establish whether the non-covalent interactions are attractive and help reconcile their influence over the manner in which the charged tectons are organized. Second, we use the NBO analysis to provide evidence that charge transfer plays a role in the mechanism responsible for the formation of the halogen bonds. QTAIM: Compounds 2−4. A bond critical point (BCP) was located for each CX···ClU halogen and NH···Cl hydrogen bonding interaction in 2−4 (Table 7). A BCP indicates the presence of an interaction, meaning these noncovalent interactions are relevant and are not simply a consequence of crystal packing. The value of the electron density, ρ, at the BCPs of the C-X···Cl−U halogen bonds is small and positive, and increases from X = Cl (9.7 × 10 −3 AU) < Br (11.1 × 10 −3 AU) < I (14.8 × 10 −3 AU). The small but positive values of the Laplacian (∇ 2ρ) and near zero but negative values of the electronic energy density (H) are consistent with the signs (positive and negative, respectively) of values derived in related systems that describe “partially covalent” or charge-assisted hydrogen and halogen bonds.82,83 The values the ρ, ∇2ρ, and H of the halogen bonding interactions are, however, much smaller than what would be expected of a strong halogen bond with covalent character. Moreover, the negative ratio of the kinetic (G) and potential (V) energy density of each CX···ClU halogen bond is slightly greater than one, which is consistent with a weak electrostatic non-covalent interaction.84 We can conclude that the halogen bonds in 2−4 are weak and electrostatic in nature and increase in strength from 2 < 3 < 4. Our assessment of the
interaction energies (Table 5). Without wishing to overstate these observations, it is of note that crystallographically indistinguishable interactions are, in fact, computationally discernible in this system. We ranked the strengths of the CX···ClU (X = Cl, Br, I) halogen bonds in 1−4 as increasing from 2, Cl < 3, Br < 4, I. This is consistent with the computational results as the energies of the halogen bonds do increase: 2, −356 kJ mol−1 < 3, −396 kJ mol−1 < 4, −424 kJ mol−1 (Table 6). The agreement between both approaches suggests that crystallography is best utilized in assigning relative strengths when the geometry and type of interaction are the same. Table 6. Halogen Bond (XB) Strengths in kJ mol−1 from the Structures of 2−4, 7,8, and 11,12 compound, cluster
cluster energy
energy per XB interaction
2, (4-ClPyH)2[UO2Cl4] 3, (4-BrPyH)2[UO2Cl4] 4, (4-IPyH)2[UO2Cl4] 7, (4-BrPyH)2[NpO2Cl4] 8, (4-IPyH)2[NpO2Cl4] 11, (4-BrPyH)2[PuO2Cl4] 12, (4-IPyH)2[PuO2Cl4]
−712 −792 −848 −792 −838 −792 −838
−356 −396 −424 −396 −419 −396 −419
Compounds 5−12: (C5H5NX)2[AnO2Cl4] (An = Np and Pu; X = H, Cl, Br, I). The crystallographic analysis of the noncovalent interactions in 5−8, (C5H5NX)2[NpO2Cl4], and 9− 12, (C5H5NX)2[PuO2Cl4], revealed identical trends wherein the relative strength of the hydrogen bonds increased from 5 < 7 ≈ 8 < 6 (and 9 < 11 ≈ 12 < 10), whereas halogen bond strength increased from 7 < 8 (and 11 < 12). Once again, the computational results are not in agreement with this ranking. The energy of the NH···Cl hydrogen bonds in the [NpO2Cl4]2−-containing series increase from 8, −522 kJ mol−1 < 7, −530 kJ mol−1 < 5, −540 and −549 kJ mol−1 < 6, −566 kJ mol−1 (Table 5). This computationally derived ranking is consistent with the expected ranking based upon the electrostatic potential on each of the N−H+ donors (Table 4). This trend is also observed in the [PuO2Cl4]2−-containing series: 11, −515 kJ mol−1 < 12, −528 kJ mol−1 < 9, −534 and −542 kJ mol−1 < 10, −572 kJ mol−1; however, we note a seemingly anomalously low NH···Cl hydrogen bond energy in 11 that deviates from this trend. This example perhaps (again) highlights the importance and influence of geometric arrangement, as subsequent calculations using two different 4BrPyH orientations about the [PuO2Cl4]2− tectons yielded NH···Cl strengths of −535 and −530 kJ mol−1, respectively (Figure S8). For consistency, the lower (than expected) value of −515 kJ mol−1 is reported, as the geometry and distribution
Table 7. Topological Parameters of Electron Density: Electron Density (ρ), Its Laplacian (∇2ρ), Local Kinetic Energy Density (G), Potential Energy Density (V) and Electronic Energy Density (H) Parameters All in 103 au compd, BCP 2, 3, 4, 2, 3, 4, a
CCl···ClU CBr···ClU CI···ClU NH···ClU NH···ClUa NH···ClUa
ρ
∇2ρ
ε
H
V
G
−G/V
9.7 11.1 14.8 11.6 12.2 12.1
32.6 33.6 40.4 34.2 38.6 38.9
0.0270 0.0380 0.0403 0.0225 0.0276 0.0294
−1.7 −1.4 −1.2 −0.7 −0.9 −0.9
−4.8 −5.6 −7.7 −7.1 −7.9 −8.0
6.5 7.0 8.9 7.8 8.8 8.9
1.4 1.3 1.2 1.1 1.1 1.1
A bridging bifurcated hydrogen bond with two acceptor sites. 10852
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
Figure 10. NpCl···Cl−Np close contacts between the [NpO2Cl4]2− building units in 7 are highlighted. The interaction energy is positive and therefore not attractive.
CX···Cl halogen bonds by the QTAIM approach is consistent with those of related systems across the halogen bonding community85−88 and are fully consistent with the ESP data for 2−4. Similar trends are observed in the analysis of the NH···Cl hydrogen bonds as the value of the electron density and Laplacian at each BCP is small: 2, 11.6 × 10−3 au < 4, 12.1 × 10−3 au < 3, 12.2 × 10−3 au (Table 7). These results, when taken in context with the near zero values of the electronic energy density (H) associated with each hydrogen bond, are consistent with weak and electrostatic interactions. Natural Bond Orbital (NBO) Analysis: Compounds 2− 4. The second order perturbation energy of each halogen bond in 2−4 indicates charge density donation by the hyperconjugation interaction between the Cl lone pairs of the [UO2Cl4]2− tectons and the σ-antibonding orbital of the halogen donor (on the 4-X-pyridinium cations) has a minor, yet notable contribution to the energy of the interaction. The energy associated with the nacceptor(Cl) → σ*donor(X) charge transfer increases from 2, 20.9 kJ mol−1 < 3, 39.3 kJ mol−1 < 4, 70.3 kJ mol−1 (Table S8), which is consistent with both the total interaction energies and the ESP magnitude of the halogen σ-hole. The contribution of charge transfer to the overall interaction energy is small, yet is a clear component to the mechanism governing the formation of the halogen bonds. The NBO and QTAIM analysis supports our conclusion that the Columbic attraction between the [UO2Cl4]2− (and indirectly [AnO2Cl4]2− as well) anions and 4-X-pyridinium cations is the dominant contributor to the overall interaction strengths (Tables 5 and 6). This analysis also demonstrates that the hydrogen and halogen bonding interactions themselves do contribute to the overall non-covalent interaction energies, as the presence of BCPs and the analysis thereof, combined with the evidence for charge donation via hyperconjugation, all support this conclusion. More broadly, within the context of the well-defined noncovalent interaction acceptor and donor regions of the [AnO2Cl4]2− and 4-X-pyridiniun tectons as posited by the ESP maps, one can infer that although the Columbic attraction is the driver for assembly, the contribution of the non-covalent interaction is to direct the molecular-level arrangement of the tectons. Limitations of a Purely Crystallographic Assessment of Packing. The [AnO2Cl4]2− anions in 3,4, 7,8, and 11,12 (all structure type IV) align in a linear fashion such that two of the chloro ligands of each anion is oriented directly at those of a neighboring tecton (Figure 10). This is of note as the van der Waals radii of each chloro ligand overlaps by 3−6%, and the AnCl···Cl angle of ∼180° suggests the presence of an attractive interaction (Table 8). The “acceptor” and “donor”
Table 8. AnCl···ClAn Interaction Distances and Energies in Compounds 3,4, 7,8, and 11,12 4-BrPyH
[UO2Cl4]2− [NpO2Cl4]2− [PuO2Cl4]2−
4-IPyH
distance, Å
energy, kJ mol−1
distance, Å
energy, kJ mol−1
3: 3.346(2) 7: 3.3556(8) 11: 3.403(1)
+655 +657 +654
4: 3.300(1) 8: 3.359(2) 12: 3.388(2)
+659 +657 +654
regions of the chloro ligands are identical and lie coaxial to the An−Cl bond. Recall, this region is the most electrostatically deficient (i.e., the most positive region of the anion), yet the electrostatic potential in the region is negative: [UO2Cl4]2−, −652 kJ mol−1, [NpO2Cl4]2−, −655 kJ mol−1, [PuO2Cl4]2−, −657 kJ mol−1 (Table 3). These interactions must therefore be unattractive, as regions of negative potential repel one another. The strengths of these interactions were calculated and are, in fact, unattractive, as the average AnCl···ClAn interaction energy in 3,4, 7,8, and 11,12 is +653 kJ mol−1 (Table 8). This is significant as the ability to discern whether an interaction is attractive or unattractive crystallographically is not always trivial. These results also raise the question of why this energetically unfavorable arrangement is observed so broadly. The answer is likely simple in that the cumulative total of attractive and unattractive energies must result in an overall favorable arrangement. For example, in compound 7, each [NpO2Cl4]2− tecton participates in attractive (two NH···Cl hydrogen and two CBr···Cl halogen bonds) and repulsive (two NpCl···ClNp short contacts) non-covalent interactions (Figure 6). The collective sum of the energies (or strengths) of both the attractive and repulsive components is −537 kJ mol−1 and suggests an energetically favorable arrangement. This is of course a gross oversimplification of the forces (long- and short-range) that ultimately contribute to crystal packing, as we have only considered six interactions, in an isolated pairwise fashion no less. We highlight this as a first approximation that can explain seemingly anomalous or counterintuitive packing motifs.
■
CONCLUSIONS We have developed a synthetic strategy that is appropriate for restricting speciation and adjusting and maintaining the oxidation state of aqueous neptunium and plutonium species. As a result, large single crystals of eight novel transuranic materials, (C5H5NX)2[AnO2Cl4] (An = Np, Pu; X = H, Cl, Br, I), assembled by hydrogen and halogen bond-donating pyridinium cations have been prepared and structurally characterized. Crystallographic analysis and comparison with the analogous [UO2Cl4]2− materials revealed a series of 10853
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society
National Security Internship Program (NSIP) at Pacific Northwest National Laboratory and for funding from the National Technical Nuclear Forensics Center, Part of the Domestic Nuclear Detection Office within the Department of Homeland Security, and conducted at the U.S. Department of Energy’s Pacific Northwest National Laboratory, which is operated for DOE by Battelle under Contract DE-AC0576RL1830. The computations were carried out by L.C.D. and J.A. with support from the DOE Heavy Elements Program grant DE-SC0001136. L.C.D. is also grateful for fellowships from the São Paulo Research Foundation (FAPESP) (2014/ 21930-9) and the Ministry of Science, Technology, Innovation and Communications (CNPq) (202068/2015-3). The authors are grateful to Korey P. Carter for assistance with ATIR measurements, which were collected at the ND Energy Center for Infrared microscope time. We are also grateful to Dr. Ginger E. Sigmon for contributions to the ATIR and single-crystal Xray diffraction data collections.
isomorphous structure types with identical non-covalent acceptor−donor pairings and structural motifs. Electrostatic potential maps were calculated for the [AnO2Cl4]2− anions (An = U, Np, Pu) and, in conjunction with the crystallographic data, reveal that the potential at the halogen and hydrogen bond acceptor sites (the Cl ligands) is surprisingly invariant and independent of the An(VI) center. The ESPs at the AnO22+ “yl” oxo atoms suggest a slight decrease in the propensity to accept NCIs across the period, but are also relatively invariant. The strengths of the non-covalent interactions were quantified using density functional calculations, and the energies of the hydrogen and halogen bonds were found to decrease and increase (respectively) as the substituent on the pyridinium was changed from Cl to Br to I. The interaction energies are consistent with the findings of the NBO and QTAIM analyses in that the crystallographically observed motifs and noncovalent interaction strengths are influenced most significantly by the hydrogen and halogen bond donors. This level of analysis has brought supramolecular assembly of actinyl chlorides to par with analogous studies within the d-block, and also highlights the difficulty of distinguishing the [AnO2Cl4]2− anions chemically.
■
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05689. The structural data of 1 and 5−12 at 296(2) K and 6−8 at 110(2) K can be requested from the Cambridge Structure Data Base, reference codes 1553661−1553672, at https://www.ccdc.cam. ac.uk/. Synthetic notes; single-crystal X-ray diffraction sample preparation; 110(2) K structural determination of 6−8; comparison of compound 1 at 296(2) and 100(2) K; additional figures of compound 5; select non-covalent interaction parameters for compounds 1, 5, 6, 9, and 10; additional electrostatic surface potential information; figures of the geometries used in the calculation of non-covalent interaction strength; thermal ellipsoidal representations of the title compounds; and ATIR spectra of 1−12, including Figures S1−S32 and Tables S1−S22 (PDF) X-ray crystallographic data for 1, 5−12, and 6−8 at 110(2) K (CIF)
■
REFERENCES
(1) Ariga, K.; Mori, T.; Ishihara, S.; Kawakami, K.; Hill, J. P. Chem. Mater. 2014, 26, 519. (2) Jena, B. P. Exp. Biol. Med. 2005, 230, 307. (3) Cook, T. R.; Vajpayee, V.; Lee, M. H.; Stang, P. J.; Chi, K.-W. Acc. Chem. Res. 2013, 46, 2464. (4) Lu, L.; Chen, W. Nanoscale 2011, 3, 2412. (5) Wang, L.; Li, L.-l.; Fan, Y.-s.; Wang, H. Adv. Mater. 2013, 25, 3888. (6) Gheybi, H.; Adeli, M. Polym. Chem. 2015, 6, 2580. (7) Boersma, A. J.; Roelfes, G. Nat. Chem. 2015, 7, 277. (8) Webber, M. J.; Appel, E. A.; Meijer, E. W.; Langer, R. Nat. Mater. 2016, 15, 13. (9) Boekhoven, J.; Stupp, S. I. Adv. Mater. 2014, 26, 1642. (10) Martina, M.; Hutmacher, D. W. Polym. Int. 2007, 56, 145. (11) Liu, K.; Kang, Y.; Wang, Z.; Zhang, X. Adv. Mater. 2013, 25, 5530. (12) Li, C.-P.; Chen, J.; Liu, C.-S.; Du, M. Chem. Commun. 2015, 51, 2768. (13) Martínez-Máñez, R.; Sancenón, F.; Hecht, M.; Biyikal, M.; Rurack, K. Anal. Bioanal. Chem. 2011, 399, 55. (14) Yarimaga, O.; Jaworski, J.; Yoon, B.; Kim, J.-M. Chem. Commun. 2012, 48, 2469. (15) Priimagi, A.; Cavallo, G.; Metrangolo, P.; Resnati, G. Acc. Chem. Res. 2013, 46, 2686. (16) You, L.; Zha, D.; Anslyn, E. V. Chem. Rev. 2015, 115, 7840. (17) Berger, G.; Soubhye, J.; Meyer, F. Polym. Chem. 2015, 6, 3559. (18) Ariga, K.; Yamauchi, Y.; Rydzek, G.; Ji, Q.; Yonamine, Y.; Wu, K. C.-W.; Hill, J. P. Chem. Lett. 2014, 43, 36. (19) Faul, C. F. J. Acc. Chem. Res. 2014, 47, 3428. (20) Saeki, A.; Koizumi, Y.; Aida, T.; Seki, S. Acc. Chem. Res. 2012, 45, 1193. (21) Andrews, M. B.; Cahill, C. L. Dalton Trans. 2012, 41, 3911. (22) Surbella, R. G., III; Andrews, M. B.; Cahill, C. L. J. Solid State Chem. 2016, 236, 257. (23) Andrews, M. B.; Cahill, C. L. Chem. Rev. 2013, 113, 1121. (24) Burns, P. C. Rev. Mineral Geochem. 1999, 38, 23. (25) Knope, K. E.; Soderholm, L. Chem. Rev. 2013, 113, 944. (26) Deifel, N. P.; Cahill, C. L. CrystEngComm 2009, 11, 2739. (27) Deifel, N. P.; Cahill, C. L. C. R. Chim. 2010, 13, 747. (28) Kihara, S.; Yoshida, Z.; Aoyagi, H.; Maeda, K.; Shirai, O.; Kitatsuji, Y.; Yoshida, Y. Pure Appl. Chem. 1999, 71, 1771. (29) Wilson, R. E.; Schnaars, D. D.; Andrews, M. B.; Cahill, C. L. Inorg. Chem. 2014, 53, 383. (30) Nash, K. L.; Choppin, G. R. Sep. Sci. Technol. 1997, 32, 255. (31) Gorden, A. E. V.; Xu, J.; Raymond, K. N.; Durbin, P. Chem. Rev. 2003, 103, 4207.
AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Lucas C. Ducati: 0000-0002-6539-4325 Jochen Autschbach: 0000-0001-9392-877X Christopher L. Cahill: 0000-0002-2015-3595 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The primary source of support for this research is the U.S. Department of Energy, Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Sciences, Office of Science, Heavy Elements Program, under grant DE-FG02-05ER15736 at GWU. R.G.S. acknowledges an Internship through the 10854
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
Article
Journal of the American Chemical Society (32) Maher, K.; Bargar, J. R.; Brown, G. E. Inorg. Chem. 2013, 52, 3510. (33) Francis, A. J. J. Alloys Compd. 1998, 271−273, 78. (34) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1, 277. (35) Politzer, P.; Murray, J. S. Cryst. Growth Des. 2015, 15, 3767. (36) Cannes, C.; Le Naour, C.; Moisy, P.; Guilbaud, P. Inorg. Chem. 2013, 52, 11218. (37) Kumar, N.; Seminario, J. M. J. Phys. Chem. C 2013, 117, 24033. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 09, Revision D.01, Gaussian, Inc: Wallingford, CT, 2009. (39) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (40) Küchle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535. (41) Cao, X.; Dolg, M.; Stoll, H. J. Chem. Phys. 2003, 118, 487. (42) Cao, X.; Dolg, M. J. Mol. Struct.: THEOCHEM 2004, 673, 203. (43) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. (44) Murray, J. S.; Politzer, P. Croat. Chem. Acta 2009, 82, 267. (45) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol. Model. 2007, 13, 291. (46) Bader, R. F. W.; Carroll, M. T.; Cheeseman, J. R.; Chang, C. J. Am. Chem. Soc. 1987, 109, 7968. (47) Keith, T. A. AIMAll, Version 17.01.25; Gristmill Software: Overland Park, KS, 2017; aim.tkgristmill.com. (48) Cotton, S. Lanthanide and Actinide Chemistry; John Wiley & Sons, Ltd.: New York, 2006; p 145. (49) Denning, R. G. Complexes, Clusters and Crystal Chemistry; Springer: Berlin/Heidelberg, 1992; p 215. (50) Seth, M.; Dolg, M.; Fulde, P.; Schwerdtfeger, P. J. Am. Chem. Soc. 1995, 117, 6597. (51) Schnaars, D. D.; Wilson, R. E. Inorg. Chem. 2013, 52, 14138. (52) Lhoste, J.; Henry, N.; Loiseau, T.; Guyot, Y.; Abraham, F. Polyhedron 2013, 50, 321. (53) Wilkerson, M. P.; Dewey, H. J.; Gordon, P. L.; Scott, B. L. J. Chem. Crystallogr. 2004, 34, 807. (54) Cornet, S. M.; Redmond, M. P.; Collison, D.; Sharrad, C. A.; Helliwell, M.; Warren, J. C. R. Chim. 2010, 13, 832. (55) Staritzky, E.; Singer, J. Acta Crystallogr. 1952, 5, 536. (56) Berthon, C.; Boubals, N.; Charushnikova, I. A.; Collison, D.; Cornet, S. M.; Den Auwer, C.; Gaunt, A. J.; Kaltsoyannis, N.; May, I.; Petit, S.; Redmond, M. P.; Reilly, S. D.; Scott, B. L. Inorg. Chem. 2010, 49, 9554. (57) Bean, A. C.; Scott, B. L.; Albrecht-Schmitt, T. E.; Runde, W. Inorg. Chem. 2003, 42, 5632. (58) Hay, P. J.; Martin, R. L.; Schreckenbach, G. J. Phys. Chem. A 2000, 104, 6259. (59) Brammer, L. Dalton T 2003, 3145. (60) Brammer, L.; Minguez Espallargas, G.; Libri, S. CrystEngComm 2008, 10, 1712. (61) Choppin, G. R.; Rao, L. F. Radiochim. Acta 1984, 37, 143. (62) Surbella, R. G., III; Cahill, C. L. CrystEngComm 2014, 16, 2352. (63) Sarsfield, M. J.; Helliwell, M. J. Am. Chem. Soc. 2004, 126, 1036. (64) Arnold, P. L.; Pécharman, A.-F.; Hollis, E.; Yahia, A.; Maron, L.; Parsons, S.; Love, J. B. Nat. Chem. 2010, 2, 1056. (65) Fortier, S.; Hayton, T. W. Coord. Chem. Rev. 2010, 254, 197.
(66) Carter, K. P.; Kalaj, M.; Cahill, C. L. Inorg. Chem. Front. 2017, 4, 65. (67) Carter, K. P.; Kalaj, M.; Cahill, C. L. Eur. J. Inorg. Chem. 2016, 2016, 126. (68) Carter, K. P.; Cahill, C. L. Inorg. Chem. Front. 2015, 2, 141. (69) Franczyk, T. S.; Czerwinski, K. R.; Raymond, K. N. J. Am. Chem. Soc. 1992, 114, 8138. (70) Walton, P. H.; Raymond, K. N. Inorg. Chim. Acta 1995, 240, 593. (71) Wang, H.; Wang, W.; Jin, W. J. Chem. Rev. 2016, 116, 5072. (72) Politzer, P.; Murray, J. S.; Clark, T. Phys. Chem. Chem. Phys. 2013, 15, 11178. (73) Lommerse, J. P. M.; Stone, A. J.; Taylor, R.; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108. (74) Bondi, A. J. Phys. Chem. 1964, 68, 441. (75) Gilday, L. C.; Robinson, S. W.; Barendt, T. A.; Langton, M. J.; Mullaney, B. R.; Beer, P. D. Chem. Rev. 2015, 115, 7118. (76) Cavallo, G.; Metrangolo, P.; Milani, R.; Pilati, T.; Priimagi, A.; Resnati, G.; Terraneo, G. Chem. Rev. 2016, 116, 2478. (77) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. ́ (78) Lieffrig, J.; Jeannin, O.; Frąckowiak, A.; Olejniczak, I.; Swietlik, R.; Dahaoui, S.; Aubert, E.; Espinosa, E.; Auban-Senzier, P.; Fourmigué, M. Chem. - Eur. J. 2013, 19, 14804. (79) Grabowski, S. J. J. Phys. Org. Chem. 2004, 17, 18. (80) Awwadi, F. F.; Taher, D.; Haddad, S. F.; Turnbull, M. M. Cryst. Growth Des. 2014, 14, 1961. (81) Bankiewicz, B.; Palusiak, M. Comput. Theor. Chem. 2011, 966, 113. (82) Grabowski, S. J.; Sokalski, W. A.; Dyguda, E.; Leszczyński, J. J. Phys. Chem. B 2006, 110, 6444. (83) Mohan, N.; Suresh, C. H. J. Phys. Chem. A 2014, 118, 1697. (84) Cremer, D.; Kraka, E. Angew. Chem., Int. Ed. Engl. 1984, 23, 627. (85) Grabowski, S. J. Phys. Chem. Chem. Phys. 2013, 15, 7249. (86) Wang, Y.-H.; Lu, Y.-X.; Zou, J.-W.; Yu, Q.-S. Int. J. Quantum Chem. 2008, 108, 90. (87) Lu, Y.-X.; Zou, J.-W.; Wang, Y.-H.; Yu, Q.-S. J. Mol. Struct.: THEOCHEM 2006, 776, 83. (88) Lu, Y.-X.; Zou, J.-W.; Wang, Y.-H.; Jiang, Y.-J.; Yu, Q.-S. J. Phys. Chem. A 2007, 111, 10781.
10855
DOI: 10.1021/jacs.7b05689 J. Am. Chem. Soc. 2017, 139, 10843−10855
