Crown Ether Assisted Convex Cesium Binding to a Sumanenyl Bowl

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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Crown Ether Assisted Convex Cesium Binding to a Sumanenyl Bowl Sarah N. Spisak,† Jingbai Li,‡ Andrey Yu. Rogachev,*,‡ Zheng Wei,† Toru Amaya,§ Toshikazu Hirao,∥ and Marina A. Petrukhina*,† †

Department of Chemistry, University at Albany, State University of New York, 1400 Washington Avenue, Albany, New York 12222, United States ‡ Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States § Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan ∥ The Institute of Scientific and Industrial Research, Osaka University, Mihoga-oka, Ibaraki, Osaka 567-0047, Japan S Supporting Information *

ABSTRACT: The first convex cesium-bound complex with a bowl-shaped carbanion, namely [{Cs+(dc-18-crown-6)}(C21H11−)] (1), has been isolated and crystallographically characterized. In the structure of 1, the cesium ion capped by dicyclohexano-18-crown-6 ether coordinates to the deprotonated five-membered ring of a sumanenyl bowl in an exo fashion with the Cs···C distances ranging from 3.315(5) to 3.609(6) Å. Theoretical modeling, including a topological QTAIM approach, reveals the cumulative effect of multiple weak CH···π interactions between the cationic {Cs+(dc-18-crown-6)} moiety and the convex surface of the C21H11− anion, explaining the formation and stability of the product.

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INTRODUCTION Bowl-shaped polycyclic aromatic hydrocarbons (also referred to as open geodesic polyarenes,1 fullerene fragments,2 buckybowls,3 molecular bowls,4 or π bowls5) constitute a unique class of π ligands with open access to both convex (exo) and concave (endo) faces. Exploration of the binding abilities of these ligands with extended and curved π surfaces revealed a broad variety of coordination possibilities.6 However, control of endo vs exo metal binding in such systems presents a significant synthetic challenge.7 This issue has been broadly addressed computationally,8 but crystallographically confirmed examples of selective concave metal placement are rare and can only be exemplified by such complexes as [CpM(C21H12)]PF6 (M = Fe, Ru).9 Recently, we have found that corannulene (C20H10), the smallest and the most studied curved fragment of C60-fullerene, tends to show almost exclusively an exo coordination mode in reactions with alkali metals.10 The only exception known so far is the cesium cation, which exhibits preferential concave placement in the negatively charged corannulene bowl (Scheme 1). 11 This binding stems from enhanced electrostatic interactions between the large cesium ion and concave surface of corannulene anions, as revealed computationally.11a Sumanene (C21H12; Scheme 2a) with a deeper and more rigid carbon framework12 in comparison to corannulene13 is expected to exhibit a pronounced concave metal ion binding tendency.14 This was recently illustrated by unique doubleconcave cesium ion encapsulation between two sumanenyl anions in the [Cs(C21H11−)2]− sandwich (Scheme 2b).14b In contrast to the broadly studied corannulene carbanions,15 only three crystallographically characterized examples have been reported for the sumanenyl monoanion, which has been isolated in its “naked” form with a sodium countercation14b and in a complexed form with CpZr or Cp*Zr units.16 © XXXX American Chemical Society

Scheme 1. (a) Corannulene and (b) Space-Filling Model of Cs+ Ion Dished Up in the Monoreduced Corannulene Bowl11a

Scheme 2. (a) Sumanene and (b) Space-Filling Model of Cs+ Ion Encapsulated between Two Sumanenyl Anions14b

Herein, we set out to further investigate the ligating properties of sumanene toward a large cesium ion and report its first convex coordination to the negatively charged bowlshaped carbanion. To support our experimental findings, we performed a comprehensive theoretical analysis of interactions between the cesium cation and sumanenyl anion, aiming at elucidating their nature and energetics. In addition to our Received: October 24, 2017

A

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Figure 1. (a) Molecular structure of 1 and (b) space-filling model of Cs+ ion binding to C21H11−.

Table 1. Key Distances (in Å)a in [{Cs+(dc-18-crown6)}(C21H11−)] (1), [Na(18-crown-6)(THF)2]+[C21H11−] ([Na-C21H11]), and [Na(18-crown6)(THF)2]+[Cs(C21H11−)2]− ([Cs(C21H11)2]−)14b

standard set of tools, which includes detailed EDA and NBO analysis, Bader’s topological quantum-mechanical approach of quantum theory of atoms in molecule (QTAIM17) has also been utilized. This in-depth computational study allowed us to quantify weak interactions found in this system, such as CH···π, and to demonstrate their dominating cumulative effect on the overall structure.

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RESULTS AND DISCUSSION Reaction of sumanene with Cs metal (1.5 equiv) in THF at room temperature readily proceeds to the monoanionic stage, which can be confirmed by both NMR and UV−vis spectroscopy measurements (see Figures S1 and S2 in the Supporting Information). Notably, the reaction of metallic cesium with sumanene proceeds much more quickly than that with sodium metal.14b In order to facilitate crystallization of the product, we used dicyclohexano-18-crown-6 ether (dc-18crown-6) and isolated the resulting orange plate-shaped crystals in 60% yield. The X-ray diffraction analysis confirmed the formation of a contact ion pair crystallized with one interstitial THF molecule, namely [{Cs+(dc-18-crown-6)}(C21H11−)]· THF (1·THF). In the structure of 1, the Cs+ cation is bound to the convex face of the sumanenyl monoanion at the deprotonated (D) fivemembered-ring site (Figure 1). This binding is in agreement with the negative charge localization in the D ring, as shown by the molecular electrostatic potential map of the C21H11− anion.14b The distance from the center of the D ring to the cesium ion is 3.209(6) Å with the corresponding Cs···C bond distances ranging from 3.316(5) to 3.609(6) Å. For comparison, the analogous distance to the D ring of C21H11− in the doubly concave bound product [Cs+(C21H11−)2]− is notably shorter (3.089(12) Å).14b The coordination sphere of the large Cs+ ion is completed by six O atoms of dicyclohexano-18-crown-6 with the Cs···O bond distances (3.031(4)−3.118(4) Å) being comparable to those observed in other crown-wrapped cesium cations.11,18 The geometric parameters of the sumanenyl bowl in 1 are close to those found in the structurally characterized “naked” and complexed monoanions (Table 1), indicating that convex binding of the cesium ion does not impose additional structural changes on the molecular structure of C21H11−. Exo vs Endo Binding. Energetics. In order to understand the experimentally observed unprecedented exo coordination of cesium ion toward the bowl-shaped sumanenyl monoanion, we turned to an in-depth theoretical analysis of this system. First, two possible coordination modes of Cs+ ion toward the

a b c d e f g h i j k l m n o Cs···C(η5 centroid)

1

[Na-C21H11]

[Cs(C21H11)2]−

1.433(10) 1.429(9) 1.397(9) 1.412(9) 1.553(9) 1.548(9) 1.387(9) 1.427(9) 1.399(9) 1.376(9) 1.436(9) 1.379(9) 1.389(9) 1.403(9) 1.432(9) 3.209(6)

1.439(3) 1.430(3) 1.409(3) 1.416(3) 1.553(3) 1.551(3) 1.394(3) 1.424(3) 1.395(3) 1.379(3) 1.437(3) 1.381(3) 1.390(3) 1.404(3) 1.425(3)

1.073(9)

1.11(3)

1.453(16) 1.402(16) 1.390(17) 1.395(17) 1.529(17) 1.525(17) 1.393(17) 1.430(17) 1.403(17) 1.405(17) 1.437(17) 1.391(17) 1.408(17) 1.369(17) 1.467(17) 3.089(12) 3.415(10) 3.051(10) 3.292(11) 1.12(2)

Cs···C(η6 centroid) bowl depth a

Values are averaged.

C21H11− bowl, namely exo (1a) and endo (1b) (Figure 2), have been compared computationally. Second, the influence of the cyclohexyl groups on the stability and coordination preference of the resulting isomers has been evaluated by modeling the corresponding adducts with the parent 18-crown-6 ether exo (2a) and endo isomers (2b) (Figure 3). B

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Figure 2. Exo (left, 1a) and endo (right, 1b) adducts of {Cs+(dc-18-crown-6)} with sumanenyl anion.

Figure 3. Exo (left, 2a) and endo (right, 2b) adducts of {Cs+(18-crown-6)} with sumanenyl anion.

Table 2. Calculated Bonding Energies at Different Levels of Theory (in kcal/mol) compound method a

PBE0 xDH-PBE0a DLPNO-CCSD(T)b B2PLYP-D3b a

1a exo

1b endo

Δ(exo−endo)

2a exo

2b endo

Δ(exo−endo)

−72.91 −83.20 −81.51 −83.88

−66.12 −74.01 −69.62 −73.67

−6.79 −9.19 −11.89 −10.21

−72.17 −79.19 −81.19 −78.14

−72.12 −81.80 −84.13 −79.57

−0.05 +2.61 +2.94 +1.43

PBE0/def2-TZVP(Cs)/cc-pVDZ(C,H,O)//cc-pVTZ(C,H,O). bPBE0/TZVP(all atoms)/ZORA along with RIJCOSX acceleration technique.

Geometry optimization of isomers 1a,b revealed that both correspond to local minima on the potential energy surface. However, the exo adduct was found to be significantly more stable at all levels of theory (Table 2). Moreover, subsequent accounting for noncovalent interactions (mainly of the CH···π type) through different highly accurate theoretical approaches (xDH-PBE0, DLPNO-CCSD(T), and B2PLYP-D3; see Table 2) indicates their very important contribution to the total stability of the products. At the same time, 2a,b with an unsubstituted 18-crown-6 ether capping ligand for the cesium ion show a weak but notable preference for endo coordination (Table 2). Interestingly, the trend becomes clear only after taking into consideration noncovalent interactions, whereas the standard hybrid functional PBE0 does not provide a clear confirmation (the exo−endo gap is as small as −0.05 kcal/mol at this level of theory). This finding illuminates again the importance of considering noncovalent interactions in these systems. Previously, we found that the PBE0 hybrid functional, although very accurate in reproducing geometrical features of bowlshaped π systems, may fail in evaluation of their energetics.19 Thus, the use of computational techniques that are more sensitive to noncovalent interactions is required for providing reliable results in bonding energy. Bonding Energy Decomposition Analysis. For the next step, an energy decomposition analysis (EDA) was performed for all systems under consideration. The EDA analysis allows one to quantify different components of bonding and to

translate the results into chemical language. A comparison of EDA results for 1a,b (Table 3) reveals that both attractive Table 3. EDA Analysis of Systems 1 and 2 compound param

1a exo

1b endo

2a exo

2b endo

ΔEint ΔEorb ΔEelstat ΔEPauli −De ΔEprep

−73.30 −17.05 −74.23 +17.99 −70.35 +2.95

−69.75 −16.45 −71.58 +18.27 −63.14 +6.61

−72.93 −15.62 −74.38 +17.07 −69.97 +2.96

−74.56 −18.10 −75.16 +18.69 −69.62 +4.94

components (ΔEorb and ΔEelstat, which can be interpreted as covalent and ionic components of the bonding, respectively) are notably larger for the exo isomer. At the same time, the repulsive contribution (ΔEPauli) was calculated to be the largest in the endo derivative. Altogether, these two factors make the bonding between the dc-18-crown-6 ether capped cesium cation and sumanenyl anion in 1a to be stronger than that in 1b. Replacement of the dc-substituted crown ether with its parent 18-crown-6 results in equalization of the bond energy components for exo and endo isomers. Consequently, no preference in stability can be given to any isomer. This theoretical result may be related to the experimental difficulty in crystallization of the resulting product in the presence of 18C

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Figure 4. Bond critical points along with bond paths in cationic {Cs+(dc-18-crown-6)} (left) and {Cs+(18-crown-6)} (right) moieties. Ring and cage critical points are omitted for clarity.

Figure 5. Bond critical points along with bond paths for 1a (left) and 1b (right). Arrows point to BCPs corresponding to Cs···C bonding contacts; c1−c7 denote CH···π bonding contacts. Ring and cage critical points are omitted for clarity.

Figure 6. Bond critical points along with bond paths for 2a (left) and 2b (right). Arrows point to BCPs corresponding to Cs···C bonding contacts; c1−c7 and o1 and o2 denote CH···π and CH···O bonding contacts, respectively. Ring and cage critical points are omitted for clarity.

crown-6 ether. Our numerous attempts to isolate the {Cs+(18crown-6)} product with sumanenyl anion, using a very broad range of crystallization conditions, produced only ill-diffracting products. Cs−O and Cs−C bonding. Next, we turned to a topological analysis of electron density, ρb, for all target compounds. First, the nature and energetics of the Cs−O bonding were considered (Tables S9−S14 in the Supporting Information). Topological analysis of electron density for {Cs+(dc-18-crown6)} and {Cs+(18-crown-6)} moieties reveals the presence of six bond critical points (BCPs) of (3,−1) type corresponding to six Cs−O bonds (Figure 4). Topological parameters such as the Laplacian (∇2ρb), total energy density as well as its components, kinetic (G(rb)) and potential energy (V(rb)) densities, and binding degree parameter (I(rb)) calculated for these BCPs unambiguously indicate the closed-shell type of interaction between a metal center and oxygen atoms of capping crown ether ligands (see

details in the Supporting Information). This type of interaction accompanied by relatively small values of electron density is usually associated with ionic character of the bond, whereas a negative value of the Laplacian together with a high value of electron density is attributed to a distinct covalent character (so-called shared interaction). In accordance with Espinosa’s evaluation scheme,20 energies of such Cs−O bonds are within a narrow interval of 2.00−3.50 kcal/mol (see the Supporting Information for details). Moreover, the total strength of these interactions was calculated to be essentially the same for both systems (−17.05 kcal/mol for {Cs+(dc-18-crown-6)} and −17.01 kcal/mol for {Cs+(18-crown-6)}). Thus, the effect of the crown ether nature on the stability of the resulting cationic moieties with Cs+ seems to be negligible. Interestingly, the strength of Cs−O interactions significantly decreases upon complexation of cesium cations with a sumanenyl monoanion (Figure 5 and Tables S9−S14 in the Supporting Information). The effect is more pronounced in the D

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CONCLUSIONS The in-depth theoretical investigation performed clearly shows that the addition of nonrigid cyclohexyl groups to the 18crown-6 skeleton leads to a notable strengthening of CH···π interactions between the cationic {Cs+(dc-18-crown-6)} moiety and the convex π-face of the sumanenyl anion and, consequently, to stabilization of the exo coordination mode observed in the [{Cs+(dc-18-crown-6)}(C21H11−)] complex that was synthesized and crystallographically characterized in this work. This finding illustrates the importance of careful consideration of such weak intermolecular interactions in crystal engineering, as they may provide a cumulative effect responsible for the formation and stability of the specific product.

case of endo isomers, in which the total strength of Cs−O interactions goes down to −9.41 kcal/mol for 1b (vs −13.64 kcal/mol for 1a) and −9.60 kcal/mol for 2b (vs −13.34 kcal/ mol for 2a). This finding points out the significant geometrical perturbation of cationic moieties upon formation of the endo products with a C21H11− bowl, which results in weakening of the Cs−O interactions. Interaction between the cesium cation and sumanenyl bowl can be described in terms of QTAIM as the presence of corresponding bond critical points between Cs and C atoms of the C21H11− anion. Indeed, two such BCPs were found in the case of exo isomers 1a and 2a (Figures 5 and 6). In contrast, endo derivatives 1b and 2b have only one BCP. Subsequent evaluation of the strength of these contacts revealed that they fall in a narrow range of 2.02−2.39 kcal/mol. Thus, bonding between the exo surface of a sumanenyl monoanion and cesium ion is twice as strong as that with the endo surface. Moreover, the nature of the crown ether has almost no influence on bonding (total strengths are −4.14 and −4.35 kcal/mol for 1a and 2a, respectively, and −2.38 and −2.34 kcal/mol for 1b and 2b, respectively). C−H···π Contacts. Considering the potential importance of CH···π contacts in these systems, a detailed analysis of such interactions in terms of topological quantum theory (QTAIM) was performed for all adducts under consideration. These interactions as well as those of the CH···O type can be easily interpreted as closed-shell interactions on the basis of analysis of calculated topological parameters summarized in Tables S15 and S16 in the Supporting Information. Seven BCPs corresponding to the CH···π contacts were found for 1a (Figure 5 and Table S15 in the Supporting Information). In contrast, its endo isomer 1b shows only two such contacts. Specifically, these contacts are found between the C−H bonds of one cyclohexyl group of dc-18-crown-6 and the peripheral six-membered rings of the sumanenyl anion. An evaluation of their energetics revealed a total stabilization effect of −2.82 kcal/mol for 1a and only −0.81 kcal/mol for 1b, thus confirming the energetic preference of exo over endo coordination in the case of dc-18-crown-6 capped cesium complexes. A similar analysis of 2a,b reveals their notable difference from 1a,b (Table S15 in the Supporting Information). In 2, the exo isomer shows only two CH···π contacts (with a total stabilization effect of 1.96 kcal/mol), while the endo derivative exhibits three such interactions (2.43 kcal/mol total) and three additional CH···O bonding contacts (0.90 kcal/mol total). Altogether, those should provide a significant energetic preference for the endo coordination of the cesium cation by the sumanenyl anion in this system. Summarizing results obtained by topological analysis of electron density in the products of the bowl-shaped sumanenyl monoanion with {Cs+(dc-18-crown-6)} and {Cs+(18-crown6)} cations, one can conclude that the coordination preference is mainly ruled by a net of weak interactions such as CH···π. Other types of bonding observed in these systems, namely Cs− O and Cs−C, are found to be essentially the same for the corresponding exo and endo isomers. Although the energy of each individual CH···π interaction is rather small, the total energy of combined interactions becomes significant. For comparison, the energy of CH···π interactions in the classic Tshaped benzene dimer was calculated to be only −0.48 kJ/mol (−0.12 kcal/mol),21 but that interaction is known to define the overall crystal structure.22

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out using break-and-seal and glovebox techniques under an atmosphere of argon. THF and hexanes were purchased from Pharmco-Aaper and were dried over Na/benzophenone and distilled prior to use. Cesium metal was purchased from Sigma-Aldrich and used as received. Dicyclohexano-18-crown-6 ether was purchased from VWR and dried over P2O5 in vacuo for 24 h. Sumanene (C21H12) was prepared as described previously23 and sublimed in vacuo in a sealed glass ampule (ca. 15 cm length, 1.1 cm o.d.) at 160−180 °C prior to use. The UV−vis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. The NMR spectra were measured on a Bruker AC-400 spectrometer at 400 MHz for 1H and were referenced to the resonances of the corresponding solvent used. The extreme air and moisture sensitivity of 1, along with the presence of interstitial THF molecules, prevented us from obtaining elemental analysis data. Preparation and Crystallization of 1. THF (1.4 mL) was added to a system containing sumanene (3 mg, 0.011 mmol), dicyclohexano18-crown-6 (excess, 6−7 equiv), and Cs metal (2.3 mg, 0.017 mmol). The initial suspension was colorless with a tint of yellow. In ca. 5 min, as the sumanene became completely dissolved, the mixture turned pale orange. The mixture was stirred for an additional 5 min at room temperature, and then it was filtered. The orange filtrate was layered with hexanes (0.8 mL) and placed at 10 °C. Orange plate-shaped crystals were present in moderate yield after 4 days. Yield: 5.7 mg, 60%. UV−vis (THF, nm): 413, 483. 1H NMR (400 MHz, THF-d8, 25 °C, ppm): δ 0.9 (8H, dc-18-crown-6), 1.3 (8H, dc-18-crown-6), 2.8 (2H, C21H11−), 3.4 (12H, dc-18-crown-6), 4.0 (8H, dc-18-crown-6), 4.4 (2H, C21H11−), 5.8 (1H, C21H11−), 6.7 (6H, C21H11−). Crystal Structure Determination and Refinement of 1. Data collection was performed on a Bruker D8 VENTURE X-ray diffractometer with a PHOTON 100 CMOS detector equipped with a Mo-target X-ray tube (λ = 0.71073 Å) at T = 100(2) K. Data reduction and integration were performed with the Bruker software package SAINT (version 8.38A).24 Data were corrected for absorption effects using the empirical methods as implemented in SADABS (version 2016/2).25 The structure was solved by SHELXT26 and refined by full-matrix least-squares procedures using the Bruker SHELXTL (version 2016/6)27 software package. All non-hydrogen atoms (including those in disordered parts) were refined anisotropically. The H atoms were included at calculated positions and refined as riders with Uiso(H) = 1.2[Ueq(C)] and Uiso(H) = 1.5[Ueq(C)]. The THF molecule was found to be disordered and was modeled with two orientations with relative occupancies of 0.53:0.47. The geometries of the disordered parts were restrained to be similar. The anisotropic displacement parameters of the disordered molecules in the direction of the bonds were restrained to be equal with a standard uncertainty of 0.01 Å2. They were also restrained to have the same Uij components, with a standard uncertainty of 0.04 Å2. Calculation Details. All geometry optimizations were performed at the PBE0 level of theory. Atoms of organic ligands were described with help of cc-pVTZ quality basis sets, whereas for the Cs atom the def2-TZVP basis set accompanied by effective core potential was used. E

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No symmetry restrictions were applied. All calculations were done using the Firefly program.28 The optimized geometry was then used for detailing the electronic structure with the help of the NBO technique,29 using the NBO 6.0 program suite.30 Evaluation of energetics was performed using highly accurate theoretical approaches (xDH-PBE0, DLPNO-CCSD(T), and B2PLYP-D3). xDH-PBE0 calculations were carried out with the same basis sets as those used for geometry optimization (by the Firefly program). Full-electron relativistically recontracted basis sets of tripleζ quality were utilized for all atoms in the case of B2PLYP-D3 and DLPNO-CCSD(T) calculations in combination with RIJCOSX31 acceleration approximation (by the ORCA program32). A topological analysis of electron density (based on the converged PBE0/ccpVTZ(C,H,O)/def2-TZVP(+ECP)(Cs) wave function) was performed within the QTAIM approach using the AIMALL program package.33 See the Supporting Information for more details.

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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.organomet.7b00782. Computational details, NBO charges, selected topological parameters etc., UV−vis and NMR spectra, and X-ray structural data of 1 (PDF) Cartesian coordinates for the calculated structures (XYZ) Accession Codes

CCDC 1570294 contains 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 data_ [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

*A.Y.R.: e-mail, [email protected]; tel, + 1 312 567 3151. *M.A.P.: e-mail, [email protected]; fax, +1 518 442 3462; tel, + 1 518 442 4406. ORCID

Marina A. Petrukhina: 0000-0003-0221-7900 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation CHE1608628 (M.A.P.), Illinois Institute of Technology (start-up funds, A.Y.R.), and 381688-FSU/Chemring/DOD-DOTC (A.Y.R) is gratefully acknowledged. This work was also partially supported by JST, ACT-C, Japan (Grant Number JPMJCR12Z3, T.H).

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00782 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00782 Organometallics XXXX, XXX, XXX−XXX