Heavy Element Metallacycles: Insights into the Nature of HostâGuest

Article pubs.acs.org/JPCC

Heavy Element Metallacycles: Insights into the Nature of Host−Guest Interactions Involving Dihalide Mercuramacrocycle Complexes Miguel Ponce-Vargas† and Alvaro Muñoz-Castro*,†,‡ †

Doctorado en Fisicoquímica Molecular, Relativistic Molecular Physics Group (ReMoPhys), Universidad Andres Bello, República 275, Santiago, Chile ‡ Dirección de Postgrado e Investigación, Universidad Autonoma de Chile, Carlos Antúnez 1920, Santiago, Chile S Supporting Information *

ABSTRACT: Host−guest chemistry is a relevant issue in materials science, which encompasses the study of highly structured molecular frameworks composed of at least two complementary entities associated through noncovalent interactions, where structures involving several metallic centers, namely, metallacycles, acting as host species, offer significant advantages over organic systems due to the high versatility of their binding sites in terms of ion recognition. In this context, we study via relativistic density functional calculations the host−guest formation of systems involving a heavy element metallacycle, [HgC(CF3)2]5, which binds to several halide anions to give [(HgC(CF3)2)5 2X]2 (X = Cl, Br, I). Our results reveal an interesting case where the expected soft acid−soft base pair is not the more stable situation. Instead, a surprising hard−soft pair arises as the preferred species, with stronger forces toward Cl− than those corresponding to I− by about 24 kcal/mol. To understand such a situation, the use of a detailed analysis of the energy decomposition analysis (EDA) terms suggests the electrostatic character of the host−guest pair, which is ruled by the ion−dipole term by about 97%, favoring the inclusion of the hard base, namely, Cl−, instead of the softer counterpart, I−. The current approach allows determining the role of certain Coulombic terms in the electrostatic nature of the interaction, leading to a clear rationalization of the soft−soft or hard−soft preferences into the formation of host−guest pairs, which can be extended to the study of the behavior of several organic or inorganic systems.

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INTRODUCTION

Among the wide range of metallacycles, mercuramacrocycles constitute an interesting case study as they present highly polarizable mercury(II) centers in a cyclic array, which can be considered as soft acids under the Pearson HSAB theory.22 For instance, several mercuramacrocycles, such as mercuracarborands,23−25 have been reported as host species with the mercury centers acting as electrophilic binding sites, forming two almost colinear primary σ bonds, and showing appreciable Lewis acidity. In the same way, mercuraazametallamacrocycles26,27 have shown the ability to trap Cu(I) and Ag(I) ions denoting strong Hg(II)···Cu(I)···Hg(II) and Hg(II)···Ag(I)···Hg(II) metallophilic interactions in a linear conformation. Similar host capabilities have been exhibited by perfluorated mercuramacrocycles, presenting a large number of electron-withdrawing fluorine atoms that increase the Lewis acidity of the metallic ring structure, thereby enhancing the anion receptor character of such systems. Among them, trimeric perfluoroortho-phenylene mercury(II), (o-C6F4Hg)3, can interact with arenes,28 crown ethers,29 thiacrown ethers,30 and many other species31 via noncovalent forces, whereas cyclic pentameric perfluoroisopropylidenemercury, [Hg(C(CF3)2]5, characterized

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Since their discovery in 1967 by Pedersen, great interest has been devoted to study and synthesize host−guest systems given their increasing relevance in materials science.3−8 Such systems can be defined as highly structured molecular frameworks composed of at least two complementary entities associated through noncovalent interactions, where the host possesses convergent binding sites and steric features that complement the guest entity.9−11 In this context, metallacycles offer significant advantages over organic systems in terms of ion recognition, as their preference for a specific guest can be adjusted in several ways, such as by modifying the cavity size, through the variation of the metallic centers and the nature of the organic ligands,12,13 and by the functionalization of such ligands with electron-withdrawing groups becoming more acidic than the metallic centers and thus the entire host cavity.14 Furthermore, the large range of different coordination numbers displayed by the metallic centers, in contrast to the limitations concerning the coordination of organic hosts,15 and the multidentate nature and preorganization exhibited by the cavity represent significant advantages.16 All these factors lead to the development of inorganic cages17 and metallacycles,18 toward the design of multimetallic catalysts19 and enzyme mimics,20 among other applications in materials science.21 © XXXX American Chemical Society

Received: September 12, 2014 Revised: November 12, 2014

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dx.doi.org/10.1021/jp5092625 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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tion. Finally, by using a scheme that distributes charges over all atoms, the atomic multipoles are reproduced exactly. To keep the atomic multipoles as local as possible, a weight function that falls off rapidly is used. The graphical representation of the quadrupole tensor (Θ) was obtained based on the method employed by Autschbach and co-workers,48 considering a function written in spherical coordinates representing the f(r) = ∑ijrirjΘij expression centered at the respective nucleus, depicting its angular dependence and sign. The noncovalent interaction (NCI) analysis was carried out by using the NCIPLOT program developed by Weitao Yang and co-workers49,50 and the NCI Milano program developed by Saleh and co-workers,51,52 both based on the analysis of electron density descriptors. All isosurfaces and figures have been drawn by using the software packages Chemcraft 53 and Visual Molecular Dynamics (VMD).54

by Antipin et al. is able to couple simultaneously two halide ions, generating a bipyramidal central core,32,33 which represents an interesting model for understanding noncovalent interactions and the nature of the host−guest interaction involving heavy elements. The understanding of intermolecular forces can be extended by studying the molecular charge distribution via multipole moments.34,35 In this context, the work about binding forces between trinuclear silver pyrazolate metallacycles and acetonitrile molecules36 where an electron-deficient region at the center of the metallacycles is available to significant quadrupole−dipole interactions with Lewis-basic acetonitrile molecules and the fluorinated benzene solubility study, rationalized in terms of molecular dipole and quadrupole moments,37 represent interesting contributions. Hence, we suggest that a reasonable manner to study the electronic cloud departure experienced by mercuramacrocycles with the guest entry, which accounts for higher-order interactions such as dipole−quadrupole, quadrupole−quadrupole, etc., via dipole moment vectors and quadrupole moment tensors located at the binding sites.35 As part of our current interest in acquiring a deeper knowledge of host−guest coupling concerning metallacycles,38 here we evaluate the interaction involving a series of heavy element macrocyles involving dihalide pentameric perfluoroisopropylidenemercury complexes, [(HgC(CF3)2)52X]2− (X = Cl, Br, I), where the Hg(II) centers constitute soft acids according to the hard and soft acid and bases (HSAB) principle,22 and with halide species going from hard (Cl−) to soft (I−) bases. This study is conducted by using relativistic density functional methods in conjunction with energy decomposition analysis according to the Morokuma scheme. In addition, a multipole moments study and a noncovalent index (NCI) analysis were carried out to unravel the nature of the host−guest coupling.

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RESULTS AND DISCUSSION The calculated structures for [HgC(CF3)2]5 (1) and [(HgC(CF3)2)5 2X]2 (1-2X) (X = Cl, Br, I) are shown in Figure 1.

Figure 1. Schematic representation of the structures for mercuramacrocycle [HgC(CF3)2]5 (1) and the [HgC(CF3)22X]52 (1-2X) series (color code: Hg, purple; C, gray; F, green).

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Geometry optimizations were carried out without any symmetry constraint, resulting in a D5h structure with the mercury centers lying in the same plane, depicting almost linear C−Hg−C inner angles (178.3°) within the ring and Hg−C− Hg angles (109.7°) very close to those typical of tetrahedral carbon atoms. The obtained structures lie in a stationary point on the potential energy surface (PES) as given from the vibrational analyses, which describes negative frequencies (∼20 cm−1) corresponding to the rotation of the fluoromethyl groups. Relevant structural parameters are summarized in Table 1. The good agreement between the available experimental data32,55,56 and calculated structures suggests methods here used are reliable in the optimization of this type of metallacycle. The inclusion of the halogen ions by reacting 1 and the respective [PPh4]+X− salt results in a structure where two anions are located above and below the Hg5 plane (Figure 1), as has been reported by Shur and co-workers.32 All the calculated complexes derived from 1 belong to the D5h symmetry point group, which exhibit a slight increase in the Hg−Hg distances from 3.331 to 3.425 Å (Table 1), which increases the diameter formed by the Hg5 ring. The X···Hg distances are shorter than the sum of the respective van der Waals radii of the isolated atoms (Hg−Cl = 3.9 Å, Hg−Br = 4.0 Å, Hg−I = 4.2 Å)57 suggesting an interaction involving more than weak forces between the mercury centers and the halide guests. The electronic structure of [(HgC(CF3)2)52Cl]2− (1-2Cl) is schematically given in Figure 2. The relevant Hg5 core based levels of [HgC(CF3)2]5 are described under the planar

COMPUTATIONAL DETAILS Relativistic density functional theory39 calculations were done using the ADF 2012.02 code,40 via the scalar ZORA Hamiltonian. All the electrons were treated variationally employing a triple-ξ Slater basis set plus a polarization function (STO-TZP) within the meta-generalized gradient approximation of Tao, Perdew, Staroverov, and Scuseria (TPSS).41 The TPSS functional depends on density, ρ(r), density gradient, ∇ρ(r), and the kinetic energy density, 1/2∑|∇φKS(r)|2, where φKS represents the Kohn−Sham orbitals. The performance of the TPSS functional has been evaluated in the calculation of dissociation energies and geometries of hydrogen-bonded complexes,42 binding energies of transition metal dimers,43 and excitation energies of small molecules and atoms.44 Geometry optimizations were done via the analytical energy gradient method implemented by Verluis and Ziegler.45 In order to consider long-range interactions, the dispersion Grimme correction46 was added for both geometry optimizations and energy decomposition analysis. Similar results were obtained by using the TZP-ZORA/PBE level of theory; thus, hereafter all the results refer to the TZP-ZORA/TPSS level of theory. The local multipole moments calculation is conducted in three stages according to the method developed by Swart et al.,47 where at the first step the molecular charge density is expressed as a sum of atomic densities. Then, a set of atomic multipoles is defined from such atomic densities and used to obtain the electrostatic potential outside the charge distribuB



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Table 1. Calculated Distances (Å) and Angles (degrees) for Host and Dihalide Mercuramacrocycle Complexes [(HgC(CF3)2)5X2]2−, X = Cl, Br, I

(HgC(CF3)2)5 1 calcd

exptl

5.66 3.33 2.11 105.0 184.0 -

5.667 3.331 2.163 109.7 181.7 -

5.63 3.310 2.103 103.8 185.6 3.221 1.568 3.253

exptl Hg5 diam.b Hg−Hg Hg−C Θ33 in order to describe the larger component by Θ33 (Table 4). The graphical representation of Θjk is given in Figure 4, denoting a nonaxial symmetry (Θ11 ≠ Θ22 ≠ Θ33), where the

The natural population analysis allows us to evaluate the charge distribution in the resulting host−guest pair (Table 3) accounting for the ΔEorb term, in agreement with the molecular orbital diagram in Figure 2. As a result, a net charge transfer can be seen from the halide ions toward the mercury centers denoted by a charge of about −0.87 e̅ over the ions rather than their formal −1 charge, thus donating about 0.13 e̅ toward the host structure. Going from 1-2Cl to 1-2I, the charge transfer slightly increases in the series. The charge distribution at the ZORA/TPSS level is given by the [Hg5]5.6+[C(CF3)2]5.6− form, similarly to the ZORA/PBE calculations.66 As a consequence of the inclusion of the halide centers, the electron-withdrawing group receives more electronic charge denoted by the rise from −5.6 to −6.17 e̅ of the fragment [C(CF3)2] (Table 3), leading to a more positively charged metallic core. Thus, the charge donation from the halide centers is distributed mainly over the trifluoromethyl ligands. The natural valence configuration of the Hg(II) centers depicts a 6s1.025d9.85 for 1, which denotes a slight variation to 6s0.945d9.85, remaining similar along the halide series. In order to gain a deeper understanding of the main stabilizing interaction energy term and the preference of the heavy element metallacycle containing soft-acid centers (Hg(II)), for hard bases (Cl−), we evaluate the contribution from different Coulombic-type interactions to the electrostatic term (ΔEelec).65 Since the stabilizing nature of ΔEelec is given by ion−dipole, ion−quadrupole, and higher-order interactions such as dipole−dipole, dipole−quadrupole, etc., an analysis based on the comparison with hypothetical dinoble gas (Ng) mercuramacrocycle analogues, namely, 1-2Ng, neutral species, and isoelectronic with the corresponding halide guests (Table S4, Supporting Information), is conducted.38 This approach enables an assessment of the interaction energy neglecting the ion−dipole, ion−quadrupole, etc. contribution to the electrostatic term allowing us to evaluate the role of higher-order interactions of lesser magnitude, which contribute to the preference in the formation of soft acid−soft base pairs.38 In the 1-2Ng series (Table S5, Supporting Information) the electrostatic contribution decreases significantly ranging in the hypothetical noble gas counterparts from −4.29 to −9.35 kcal mol−1, in contrast to the halide case which ranges from −162.02 to −185.99 kcal mol−1, as a consequence of the absence of ion−dipole interactions. To quantify the relevance of ion−dipole interaction over the higher-order electrostatic terms, the ΔEelec terms corresponding to 1-2Ng and 1-2X are related according to (1 − (NgΔEelec/XΔEelec))%, providing a simple parameter for the contribution of the halide ionic character to the electrostatic term. This treatment reveals the chloride complex as the system with the major ion−dipole contribution (97.35%) to ΔEelec, followed by the bromide (96.11%) and iodide systems (94.97%), denoting the quite small contribution (less than 5%) from other Coulombic terms, suggesting that the

Figure 4. Quadrupole moment tensor (Θ) representation for a Hg(II) center, in the studied systems. E



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Figure 5. NCI analysis of the studied complexes, where blue regions denote strong stabilizing forces and green regions denote weak interactions.

Θ11 and Θ33 components are located within the Hg5 plane. It can be seen through the series that spatial distribution of such tensors does not vary to a large extent, in agreement with the small contribution ( 0) 27; thus, the ρ*sign (λ2) ranges from negative to positive values according to the nature of the noncovalent interactions. The NCI analysis for the studied [(HgC(CF3)2)52X]2− series is presented in Figure 4, with stabilizing forces denoted as blue regions (λ2 < 0) located between the mercury centers and the halide guests species. These forces could be related with the match between the electron-deficient region of the metallic centers and the negative charged halide guests, in agreement to the discussion given above. The same type of stabilizing interactions have been reported in the mercury complexes [Hg(X)3]− (X = F, Cl, Br).50 Additionally, in all studied systems, weak forces denoted as green regions can be observed between the carbon atoms belonging to the ring and the guest species. Only, in 1-2I, weak interactions between the fluorine atoms and the iodide guests can be advised from the NCI analysis.

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CONCLUSIONS The study of the formation of host−guest systems involving the heavy element metallacycles, [HgC(CF3)2]5, namely, [(HgC(CF3)2)5 2X]2 (X = Cl, Br, I), reveals an interesting case where the expected soft−soft acid−bases soft acid−soft bases pair is not the more stable situation, revealing a surprising hard−soft coupling as the preferred host−guest species. The interaction with Cl− is more favored than that corresponding with I−, by an energy amount of about 24.33 kcal/mol. To understand such a situation, the use of a detailed analysis of the energy decomposition terms suggests the electrostatic character of the host−guest pair, which is ruled by the ion−dipole term by about 97%, favoring the inclusion of the hard base, namely, Cl−, instead of the softer guest, I−. The C−Hg−C bond which stabilizes the Hg 5 ring determines the shape of the local quadrupole moment at the binding site leading to an in-plane distribution, decreasing the possible dipole−quadrupole relevant for the soft−soft preference, denoting the versatility of metallic binding centers, which can be tailored by both stabilizing ligands and the arrival of guest moieties. The current approach allows determining the role of certain Coulombic terms to the electrostatic interaction nature, leading to a clear rationalization of the soft−soft or hard−soft preferences into the formation of host−guest pairs, which can be extended to the study of the behavior of several inorganic and organic hosts.

∇ρ 1 2(3π 2)1/3 ρ 4/3

where s(ρ) exhibits small values in regions where both covalent bonding and noncovalent interactions are located. To distinguish the nature of the interaction, each point in this region is correlated with the second eigenvalue of the electron F



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ASSOCIATED CONTENT

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AUTHOR INFORMATION

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S Supporting Information *

Electrostatic potential map for 1-2X and, −CF3, −CH3, and −H host derivatives, selected calculated distances (Å) and angles (degrees) for the hypothetical 1-X and 1-2Ng mercuramacrocycle complexes, respective energy decomposition analysis, and angular dependence of the quadrupole tensor. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Phone: +56 02 2 7927218. E-mail: alvaro.munoz@ uautonoma.cl. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank FONDECYT Grant 1140359 and PROJECT MILLENNIUM No. RC120001, for financial support. M.P.-V. acknowledges CONICYT 63130036 Doctoral fellowship and UNAB-DI-403-13/I. The authors thank Prof. Vladimir Shur for his kindness to provide the experimental crystallographic data for [(HgC(CF3)2)52X]2− (X = Cl, Br, I).

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