Anion Recognition by Organometallic Calixarenes - ACS Publications

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

Anion Recognition by Organometallic Calixarenes: Analysis from Relativistic DFT Calculations Alexandre O. Ortolan,† Ina Øestrøm,† Giovanni F. Caramori,*,† Renato L. T. Parreira,‡ Alvaro Muñoz-Castro,§ and F. Matthias Bickelhaupt∥,⊥

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†

Departamento de Química, Universidade Federal de Santa Catarina, Campus Universitário Trindade, CP 476, Florianópolis, SC, 88040-900, Brazil ‡ Núcleo de Pesquisas em Ciências Exatas e Tecnológicas, Universidade de Franca, 14404-600, Franca, SP, Brazil § Lab. de Química Inorgánica y Materiales Moleculares, Universidad Autonoma de Chile, Llano Subercaceaux 2801, San Miguel, Santiago, 8910060, Chile ∥ Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands ⊥ Institute of Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands S Supporting Information *

ABSTRACT: The physical nature of the noncovalent interactions involved in anion recognition was investigated in the context of metalated calix[4]arene hosts, employing Kohn−Sham molecular orbital (KS-MO) theory, in conjunction with a canonical energy decomposition analysis, at the dispersion-corrected DFT level of theory. Computed data evidence that the most stable host−guest bonding occurs in ruthenium complexed hosts, followed by technetium and molybdenum metalated macrocyclic receptors. Furthermore, the guest’s steric fit in the host scaffold is a selective and crucial criterion to the anion recognition. Our analyses reveal that coordinated charged metals provide a larger electrostatic stabilization to anion recognition, shifting the calixarenes cavity toward an electron deficient acidic character. This study contributes to the design and development of new organometallic macrocyclic hosts with increased anion recognition specificity.

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INTRODUCTION The development of synthetic molecular receptors desired for efficient anion recognition and sensing capabilities has become an important field in supramolecular chemistry.1 This importance is related to the enormous variety of environmental, chemical, and biological processes in which anions are involved, where more versatile hosts are relevant for selective complexation and recognition of anionic guest species.2,3 Regardless of the immense progress in this field, recognition of anions is still a challenge, especially in aqueous and biological media.4 In this sense, “tuning” noncovalent interactions (NCI) in order to enhance such nonbonded stabilizing interactions is a potential method to design anion receptors. Generally, NCI are classified according to their physico-chemical nature, encompassing multipole−multipole interactions,5 hydrogen bonds,6−10 π-stacking,11,12 cation−π,13−17 anion−π,18−22 among other interactions.23,24 In this sense, molecular calixarenes constitute a suitable family of macrocyclic oligomers for anion recognition, owing to their convenient cavity shape.25−28 Specifically, [1n]metacyclophanes, in which the aromatic units of the calixarenes are linked by methylene bridges in ortho position (Figure 1).29,30 Their typical framework, which resembles a cone shaped calix, allows their use for a wide range of © XXXX American Chemical Society

applications, such as selective removal of ions from aqueous media,31−33 as drug carrier agents,34 and in the preparation of synthetic ionic channels,35,36 pores and membranes,37−41 besides their use in catalysis.42−45 Atwood and co-workers46 reported in 1995 the synthesis and characterization of a series of bi- and tetrametalated macrocyclic complexes soluble in aqueous media, based on calix[4]arene and its derivatives (Figure 1).46,47 The tetrametalated complex arises from capping the four aromatic faces of the macrocyclic with an (η6-arene)Ru2+ fragment, and a concomitant loss of two phenolic protons, yielding the formation of a complex with a 6+ net charge (Figure 1a). The authors also reported a bimetalated calixarene formed by the coordination of two ruthenium ions in two opposite faces of the macrocyclic cavity, leading to the loss of a phenolic proton, hence resulting in a complex with an overall charge of 3+ (Figure 1b).46,47 Such species were isolated and characterized as a salt of BF4− and [H(CF3CO2)4]−, for the bimetallic complex, and the tetrametalated complex was obtained as a salt of BF4−, CF3SO3−, HSO4−+SO42−, and PF6−.46,47 Received: May 6, 2018

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DOI: 10.1021/acs.organomet.8b00292 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 1. Schematic representation of complexes (a) [Ru(η6-p-cymene)4(η6:η6:η6:η6-calix[4]arene-2H)]6+ and (b) [Ru(η6-p-cymene)2(η6:η6calix[4]arene-1H)]3+.

Figure 2. Schematic representation of optimized hosts 1−8 at the ZORA-BP86-D3(BJ)/TZ2P level of theory. [Ru(η6-Bzn)4(η6:η6:η6:η6calix[4]arene-2H)]·(BF4)6 (1); [Ru(η6-Bzn)2(η6:η6-calix[4]arene)]·(BF4)4 (2); [Ru(η6-Bzn)2(η6:η6-calix[4]arene-1H)]·(BF4)3 (3); [Ru(η6Bzn)2(η6:η6-calix[4]arene-2H)]·(BF4)2 (4); [Mo(η6-Bzn)4(η6:η6:η6:η6-calix[4]arene)] (5); [Tc(η6-Bzn)4(η6:η6:η6:η6-calix[4]arene)]·(BF4)4 (6); [Tc(η6-Bzn)4(η6:η6:η6:η6-calix[4]arene-1H)]·(BF4)3 (7); and [Tc(η6-Bzn)4(η6:η6:η6:η6-calix[4]arene-2H)]·(BF4)2 (8).

These species, schematically shown in Figure 1, are watersoluble and air-stable complexes, possessing an inner hydrophobic cavity, suggested to be suitable to incorporate anionic species via noncovalent interactions, especially small inorganic anions.46,47 The single crystal X-ray analysis confirmed their host capability in recognizing BF4−, SO42−, and I−.47 Moreover, 1 H NMR titration in aqueous media demonstrated that such compounds are also able to recognize halide guests, in which the binding constants (100−550 M−1) decreased following the order Cl− < Br− < I−.47 These findings suggest that the host’s cavity steric fit is selective to certain anions.47 Besides the ruthenium tetrametalated calixarene, rhodium and iridium complexes were also obtained by direct complexation, confirming the rich synthetic versatility of these molecular calixarenes, allowing incorporating several metal fragments in the outer face of the aromatic rings.46,47

Herein, in order to gain more insights into the physical nature of the NCI governing the host−guest capabilities of the metallic complexes reported by Steed et al.,46 we use bi- and tetrametalated host calixarenes 1−8 models (Figure 2), as representative species designed based on the experimental data available in the literature,46,47 which is crucial for further development of versatile anion host receptors. To guideline this study, structures 1−8 are classified in four different coordination models, depicted as 1, 2−4, 5, and 6−8. These host molecules are a streamlined version of Steed’s systems,46 where the p-cymene units are replaced by benzene units to facilitate theoretical analysis. In addition, the location and nature of involved counterions BF4− were chosen from the original X-ray crystallographic arrangement in order to keep the net charge neutral.47 In host−guest complexes 1−8 (Figure 2), structures 1 and 3 are directly connected to the experimentally characterized B


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Organometallics

where ΔVNelstat is the electrostatic energy of the host−guest system, in which the anion guest was replaced by its isoelectronic neutral analogue (BF4− by CF4 and halogens by the noble gases). The geometry optimization, analytical vibrational frequencies,67−69 the KS-MO/EDA,59−61 the Hirshfeld charge analysis,70−72 and the NPA charges73 from NBO v.6.073−75 were performed at the 2016 version of the ADF software package.76−78

organometallic calixarenes,46,47 in contrast to the remaining structures (2, 4−8, which are included in order to evaluate and understand the following aspects in the further anion complexation: (i) the influence of the metal charge; (ii) its quantity in the host−guest system; and the (iii) protonation/ deprotonation effects of the phenolic groups. The outer face host’s coordination take into account the following metals: ruthenium(II) (1−4), molybdenum(0) (5), and technetium(I) (6−8). Where, in compound 1, four (η6arene)Ru2+ coordinate the arene units of the host, whereas, in structures 2−4, this number reduces to two (η6-arene)Ru2+ units. The different charges displayed by coordinated metals to the macrocyclic framework may lead to uncommon noncovalent interactions, which also provides more insight into the electrostatic effects of the 2+ charge of the isoelectronic ruthenium analogues Mo(0) and Tc(I). The choice of Mo(0) and Tc(I) is based in their related d6 closed-shell electronic structure to metallocene Ru(II) derivatives. Furthermore, the effect of successive loss of phenolic protons in compounds ranging from 2−4, and 6−8, is reported. These two series of compounds vary in the nature of the metal, and its quantity: two Ru(II) in 2−4, and four Tc(I) in 6−8. Compound 5 is coordinated with the neutral Mo(0) atom; hence, no counterions are needed to equalize the net charge of the host. Moreover, its phenolic hydroxyls are kept protonated; otherwise, a negative charge buildup would disable the host ability to interact with anions. In this way, the present paper intends to shed light on the field of anion recognition, by evaluating different aspects for design of versatile anion receptors toward improved novel anion hosts, evaluating the influence of different transition metal fragments coordinated to the calixarene framework, in variable quantities and charge, for further explorative synthesis efforts.

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RESULTS AND DISCUSSION Metalated Calixarenes Hosts. Our host−guest model complexes are based on the X-ray structures reported by Atwood and co-workers.46,47 In order to maintain an overall neutral charge in the studied systems, BF4− counterions contributing as extra negative charges were conveniently removed from the host crystallographic structure previous to geometrical optimization, for each specific system. Setting the same charge to the hosts enables the comprehension of the role of each receptor scaffold in a nonbiased way, providing a true bonding situation in which no electrostatic overcompensation effects take place. Different initial positions of the counterion species may lead to small changes in the hosts’ electronic structure. However, since, in our comparative study, all systems are derived from the X-ray structures, the systematic error introduced by BF4− is minimized. For instance, in host 1, in which four ruthenium 2+ moieties provide a 6+ net charge to the calixarene, three BF4− counterions were kept as in the X-ray data structure, adjacent to the metal ions within the lattice,46 and the three BF4− remaining were reflected to the opposite side of the host, balancing the negative/positive charge distribution. Following the same procedure, two opposite anions were removed to provide structures 2 and 6. Moreover, removal of three BF4− provided the hosts 3 and 7; the removal of four anions, hosts 4 and 8; and when all six anions are removed, the host 5. Host structures 1−8 are shown in Figure 2, and selected geometrical parameters are presented in Tables S1 and S2 (Supporting Information). In contrast to the metal free calixarene, the tetrametalated ruthenium complexes inherit an increased ring acidity, and spontaneously lose two phenolic protons, as 1.47 Since bimetallic ruthenium complexes were also reported,47 we proposed the hosts 2−4, in which two of the phenolic units are capped with (η6-arene)Ru2+ moieties. Experimentally, from the 1 H NMR and crystallographic analysis, the bimetallic species reported the absence of one phenolic proton, represented exactly as in 3.47 In order to highlight this proton loss effect on the host−guest complexes, we proposed hosts 2−4, ranging the number of protons in the oxygen atoms of the calixarene lower ring. In host 2, all phenolic rings are protonated, forming a hydrogen bonding network between the four O-H groups. Similarly, in 3 and 4, one and two protons were removed, respectively. We reinforce that the overall charges were kept neutral by placing conveniently BF4− counterions among the host structure, as found in reported crystallographic data for these complexes.46,47 Iridium(II) and rhodium(II) counterparts have been also reported, in a similar way to the ruthenium(II) analogues; however, their open-shell structures makes difficult further analysis for straight comparison.47 Thus, we set to find related metal coordination with the aromatic faces of calixarenes, resulted in host complexes with molybdenum(0) and technetium(I). The differently charged metals coordinated in the macrocyclic framework may result in different magnitude

COMPUTATIONAL METHODS

The calculations were performed with the GGA functional of Becke and Perdew (BP86),48,49 with Grimme’s dispersion correction and Becke−Johnson damping functions, D3(BJ).50−54 The relativistically optimized triple-ζ quality TZ2P basis set55 was employed, in conjunction to the scalar ZORA Hamiltonian.56−58 All host−guest complexes reported herein were verified as true minima in the potential energy surface through the absence of negative eigenvalues in the Hessian matrix. The noncovalent interactions in the host−guest systems were investigated through Kohn−Sham Molecular Orbitals (KS-MO) in conjunction with a canonical Energy Decomposition Analysis (EDA).59−61 In this scheme, the total interaction energy (ΔEint) of the interacting host−guest fragments is decomposed into meaningful contributions: Pauli repulsion (ΔEPauli), electrostatic (ΔVelstat), orbital interaction (ΔEoi), and dispersion (ΔEdisp) energy terms ΔEint = ΔVelstat + ΔE Pauli + ΔEoi + ΔEdisp

(1)

The geometrical deformation of the individual fragments is expressed as the preparation energy, ΔEprep, which, when added to the ΔEint, provides the bond dissociation energy. Further details about the EDA methodology are found in the literature.59,62,63 The ion−dipole (i−d)64−66 contributions to the ΔVelstat term, besides the higher-order terms, such as dipole−dipole, dipole− quadrupole, etc., were calculated as

ÄÅ É ÅÅ ij ΔV N yzÑÑÑÑ Å i−d = ÅÅÅ1 − jjjj elstat zzzzÑÑÑ·100% ÅÅ Ñ ÅÅÇ k ΔVelstat {ÑÑÑÖ

(2) C


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Organometallics

Figure 3. Views of the molecular electrostatic potential maps of host structures 1−8. For each species, three different views are given: perspective (top large image), upper (left bottom), and lower views (right bottom). Color scale shown in a.u. Surface contour value of 0.01 a.u.

above all aromatic structure (Figure 3). Bimetallic ruthenium hosts (2−4) presented the higher acidity in regions where the arene units are coordinated with a metallic cation (depicted in blue), in contrast to the noncoordinated aromatic rings (green regions, Figure 3). Furthermore, the technetium ion host 6 presents an increased acidic cavity than its molybdenum neutral analogue 5, indicating that, even though the entire system net charge for both complexes was kept the same, the presence of a positively charged metal on the outer face of the aromatic units of the cavity is capable of shifting its electronic character. Thus, charged metal cations are able to modulate the π-acidity of the host structure. Protonated phenolic groups also have a significant importance in the electrostatic potential of the cavity. The presence of the hydrogen of the phenolic hydroxyl makes the calixarene lower ring with a more acidic character, which can be seen following the MEP of compounds 2−4 and 6−8. Such a deprotonation effect on the MEPs is related to the metal capacity to accept electronic density in order to stabilize the negative charge bared by the oxygen groups from the cavity lower ring (Table S3). This drives the deprotonated hosts to be less acidic than their respective protonated counterparts. This trend can be seen in the receptors with all protonated phenolic groups (2 and 6), which are more acidic than their deprotonated analogues (3, 4, and 7, 8, respectively) (Figure 3). Host−Guest Systems Structures. Atwood and coworkers47 reported a series of 1H NMR titration studies carried out to assess the degree of anion affinity in solution. They related the binding of Cl−, Br−, I−, and NO3− species with the bi- and tetrametalated ruthenium hosts π-acidic cavities.46,47 Accordingly, we analyzed BF4− as the guest for the hosts 1−8, as well as the halogens Cl−, Br−, I− as guests exclusively for host 1. The nitrate anion was excluded from the studied guests, since it was experimentally reported that, in aqueous solution, its coordination is done with the NO3− lying between the two metallic centers on the outside of the

and nature of noncovalent interactions. In addition to the advantage of dealing with isoelectronic species, their d6 closedshell electronic structures are feasible to compute, when compared to other open-shell structures (such as Ir(II) and Rh(II) derivatives). Therefore, these complexes with metals in different oxidation states provide insight into the electrostatic effects of the 2+ charge of the (η6-arene)Ru2+ groups in hosts 1−4. On this wise, the neutral Mo0 host 5 and the Tc+ complexes 6−8 were proposed. Due to the metal neutral charge of compound 5, and the absence of counterions to balance overall charge, all phenolic hydroxyls are protonated. In contrast, compound 6, where all of the four phenolic groups are protonated, four BF4− are placed along the host scaffold, due to the charged metallic ion. The same rationale follows in 7 and 8, in which one and two protons were removed, respectively, and the net charge was kept neutral with the addition of the right amount of BF4− counterions. The molecular electrostatic potential (MEP) of the hosts 1− 8 (Figure 3) shows that all structural modifications have substantial influence on the electron density of the cavity, reflecting directly in its π-acidity. The inclusion of (η6arene)Tc+ at the aromatic moieties of calixarene resulted in the host cavity to be more acidic than the respective (η6arene)Mo0 coordinated complexes. This effect is much more pronounced on going from technetium to ruthenium complexes, especially in regions above the coordinated (η6arene)Ru2+ group. In positively charged metal ions, such as (η6-arene)Tc+ and (η6-arene)Ru2+, the metal is able to accept electron density from the calixarene fragment, as shown by the NPA charges (Table S3, Supporting Information). This charge transfer decreases the electron density on the macrocyclic cavity, leading to an increased acidity, as indicated by MEP maps. Unlike charged metal species, the neutral molybdenum atoms, instead of accepting charge, donate electron density to the cavity, which enhances its π-basic character (Table S3). In this sense, host 1 presented the most significant acidic cavity over all structures, as indicated by the blue surface regions D


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Figure 4. Perspective, upper, and lower views of the minimum structures of compounds 1·Cl−−8·BF4−, optimized at the ZORA-BP86-D3(BJ)/ TZ2P level of theory.

Table 1. Energy Decomposition Analysis (kcal mol−1) and the Hirshfeld Fragment Charge Analysis (a.u.) for Compounds of the Halogen Series 1·X− (X− = Cl−, Br−, and I−) and 1·BF4−−8·BF4−a 1·Cl− 1·Br− 1·I− 1·BF4− 2·BF4− 3·BF4− 4·BF4− 5·BF4− 6·BF4− 7·BF4− 8·BF4−

ΔEint

ΔEprep

ΔEPauli

ΔVelstat

i−d (%)

−77.5 −78.0 −73.8 −67.0 −57.0 −45.8 −27.8 −15.0 −50.0 −39.1 −28.0

6.1 12.0 11.3 11.9 3.9 1.9 2.8 1.4 3.0 2.9 1.8

45.2 54.9 61.3 29.5 34.5 28.4 29.9 22.2 31.5 24.4 20.0

−68.5 (56%) −78.4 (59%) −81.6 (60%) −56.0 (58%) −51.8 (57%) −38.8 (52%) −21.6 (37%) −1.0 (3%) −40.7 (50%) −26.8 (42%) −13.7 (29%)

89 87 86 82 75 75 53 71 67 48

ΔEoi −40.3 −37.0 −31.3 −25.1 −24.2 −22.2 −22.5 −22.4 −25.1 −22.7 −21.0

(33%) (28%) (23%) (26%) (26%) (30%) (39%) (60%) (31%) (36%) (44%)

ΔEdisp −14.1 −17.5 −22.2 −15.4 −15.5 −13.2 −13.6 −13.8 −15.7 −14.0 −13.3

(11%) (13%) (16%) (16%) (17%) (18%) (24%) (37%) (19%) (22%) (28%)

q1

q2

−0.830 −0.870 −0.895 −0.904 −0.893 −0.896 −0.896 −0.922 −0.908 −0.916 −0.925

−0.172 −0.130 −0.105 −0.096 −0.107 −0.104 −0.104 −0.079 −0.093 −0.085 −0.075

Values in parentheses correspond to the percentage of each stabilizing contribution (ΔVelstat + ΔEoi + ΔEdisp = 100%).

a

(Figure S1 − Supporting Information) reveals that the chosen level of theory (ZORA-BP86-D3/TZ2P) successfully reproduces several experimental47 bond lengths and angles, which are all reported in Table S1. The ZORA-BP86-D3/TZ2P scheme was chosen based in its previous success to reproduce experimental geometries in our studies with heterocalixarenes,27 metalated heterocalixarenes,28 and also with other organometallic complexes of ruthenium(II).79

macrocyclic, instead to entering within the cavity.47 The choice of BF4− as the “model-guest” was based on the availability of the crystallographic data.47 The minima structures of host− guest complexes 1·Cl−−8·BF4− are shown in Figure 4, and selected geometrical parameters are presented in Tables S1 and S2. Direct comparison between 1·BF4− with the X-ray of [Ru(η 6 -p-cymene) 4 (η 6 :η 6 :η 6 :η 6 -calix[4]arene-2H)]·(BF 4 ) 6 E


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well as the dispersion interactions, ΔEdisp. This high stabilization of electrostatic nature is in agreement with the very acidic nature of the aromatic units of the host 1, as shown in the MEPs (Figure 3). For instance, the ΔVelstat range of −68.5 to −81.6 kcal mol−1 in 1·Cl− and 1·I−, and contributes to aproximately 56−60% of the total ΔEint stabilization energies. The ΔEdisp accounts for 11−16% of the total bonding stabilization, ranging from −14.1 to −22.2 kcal mol−1 in 1·Cl− and 1·I−. Small guests such as Cl− provide more stabilizing ΔEoi interactions. Included in the ΔEoi term are the donor−acceptor interactions between the occupied and virtual molecular orbitals of the hosts fragments, such as the charge-transfer and inner-fragment polarization. As already pointed out, the chloride guests have a better fit in the host’s cavity, which causes the orbital interactions to be much more effective. In order to fit into the host’s cavity, larger guests, such bromide and iodide, experience more repulsive interactions between the occupied orbitals of the interacting fragments, as indicated by the larger ΔEPauli energy term. Orbital interactions accounts for 23−33% of the total bonding stabilization. Larger ΔEoi values are related to a larger charge transfer, as shown by the Hirshfeld charge analysis (Table 1), which indicates a transference of 0.172, 0.130, and 0.105 e− in 1·Cl−, 1·Br−, and 1·I−. The orbital interaction diagram for the host−guest interaction in 1·Cl− (Figure 5) exemplifies the origin of the

The host−guest minima structures show that the halogen anions reach great depths inside the center of the calixarene cavity. Similarly, guest BF4− presented one fluorine atom pointing toward the center of the macrocyclic cavity, whereas its three remaining fluorine atoms were slightly above the upper rim level, pointing toward the hydrogen atoms of the arene units. The form which the anions fit inside the host cavity of 1, together with the short host−guest distances obtained, indicates that a strong interaction takes place. As pointed out by Atwood and co-workers,46 the calixarene cavity exhibits an excellent size complementarity for small anions, allowing the guests to be included profoundly inside the host bowl. The distance between the anion and the centroid of the calixarene lower ring (distance i−X, Table S1) clearly demonstrates how deep the guest can be inserted within the cavity. For instance, X−i distances of complexes ranged from 2.365 (2·BF4−) to 2.981 Å (3·BF4−), and from 2.971 (1·Cl−) to 3.621 Å (1·I−) in the halogen guest complexes. Noncovalent Bonding Analysis. To provide a deeper insight about the nature and magnitude of the host−guest interaction, all systems were analyzed by means of Kohn− Sham Molecular Orbitals (KS-MO) coupled to a canonical Energy Decomposition Analysis (EDA),59−61 considering the negatively charged hosts Cl−, Br−, I−, and BF4−, and the neutral hosts 1−8 as closed-shell interacting fragments. The most stable bonding interaction belongs to the ruthenium(II) complexes, 1·Cl−−1·BF4− and 2·BF4−−4·BF4−, followed by technetium(I) and, the least stable situation, in the case of the molybdenum(0) complexes (6·BF4−−8·BF4− and 5· BF4−) (Table 1). The largest interaction arises in 1·Br−, followed by 1·Cl− and 1·I−. All halogen guests stabilizing bonding energies are larger than tetrafluoroborate guest in 1· BF4−. This trend is obtained due to the anion steric fit into the host cavity; i.e., the best interaction results when the anion is small enough to fit into the cavity, but large enough to efficiently interact with the host π−acid framework, as in 1·Br−. However, a large scaffold distortion occurs in the calixarene host in order to behold the bromide guest inside its cavity (see ΔEPauli). The distortion in the host’s geometry, in the case of 1· Br−, is followed by a larger preparation energy (ΔEprep), resulting in a lower bond dissociation energy (BDE) than is found in 1·Cl−. Hence, in contrast to the larger stabilizing interaction energy found in 1·Br−, the amount of energy that the host expends in order to accept the larger bromide guest leads the cavity to be more selective to chloride anions. Furthermore, the chloride anion remains more deeply inserted inside the host cavity, than the bromide and iodide anions (i− X parameter, Table S1). For instance, the ΔEPauli repulsion for complexes 1·Cl−−1·I− increases according to the ionic radius of the halogen guests, 45.2, 54.9, and 61.3 kcal mol−1 for 1·Cl−, 1·Br−, and 1·I−, respectively. The decreasing series in BDE (Cl− > Br− > I−) agrees with the experimentally reported trend in the solution binding constants.47 As an illustration, the ΔEint for 1·Br−, 1·I−, 1·Cl−, and 1·BF4− are −78.0, −77.5, −73.8, and −67.0 kcal mol−1. Since their ΔEprep are 12.0, 11.3, 6.1, and 11.9 kcal mol−1, the BDE is predicted as 66.0, 71.4, 62.5, and 55.1 kcal mol−1, respectively. Host−guest complexes 1·Cl−−1·BF4− are mostly stabilized by the electrostatic interactions between the fragments, as indicated by the large ΔVelstat energy term. In the case of halogen guests, increasing the ionic radius of the anion reflects in a more significant contribution of the ΔVelstat to the ΔEint, as

Figure 5. Schematic orbital-interaction diagram for complex 1·Cl−, based on quantitative Kohn−Sham MO analyses. The LUMO of the complex is a a nonparticipating calixarene aromatic π* orbital, left out for clarity. MOs shown in blue/red indicate double occupied, whereas in cyan/orange empty virtual orbitals.

orbital stabilization pointed by the ΔEoi term. In this scheme, the fully occupied 3p orbitals of the chloride guest interact with the virtual π* orbitals of the calixarene host, delocalized above the entire macrocyclic structure. The host’s LUMOs are formed by the combination of 2p carbon atoms of the calixarene moiety and the 4d orbitals of the ruthenium(II) ion (Figure 5). The weight contributions from the 2p(carbon)/ 4d(ruthenium) orbitals to the host LUMO, LUMO+1, and LUMO+2 are 51/49%, 53/47%, and 54/46%, respectively. As depicted in Figure 5, both the frontier orbitals of 1·Cl− are delocalized through the host−guest structures. For F


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Organometallics instance, the HOMOs of 1·Cl− are mainly composed by 3p occupied orbitals of chloride ion (91%, 89%, and 88% of Cl− orbitals in HOMO-n, n = 2, 1, 0, respectively), whereas the LUMOs are mostly from the calixarene virtual orbitals. The large energy difference between the host and guest orbitals confirms the low contribution of the orbital term in EDA analysis. These interactions in 1·Cl− reflects in a small charge transfer from the guest to the host, 0.172 e (Hirshfeld charge analysis, Table 1). Lower ΔEoi and charge transfer than 1·Cl− are observed in complexes 1·Br−−8·BF4−. The different electronic nature of the metallic species affects the magnitude of the host−guest interaction due to their large contribution to the ΔVelstat component of the bonding. The overall neutral charge of the metal in 5 produces a ΔVelstat bonding component only slightly stabilizing (−1.0 kcal mol−1), when compared with 6+ net charge of host 1 (−56.0 kcal mol−1). Intermediate values of ΔVelstat are seen in 6·BF4−−8· BF4− due to a direct effect of (η6-arene)Tc+ moieties in the host scaffold. The coordinations of positively charged technetium(I) and ruthenium(II) ions to the macrocyclic lead to very acidic host cavities (Figure 3), especially due their ability to drain charge from the aromatic units (NPA charges, Table S2). In contrast, the neutral molybdenum(0) metal provides additional electronic density to the calixarene host (NPA charges, Table S2), substantially reducing the effectiveness of the cavity to interact with anionic species. For instance, ΔVelstat is the main component of ΔEint in 1·Cl−−1·BF4−, providing 56−60% of bonding stabilization. In 6·BF4−−8·BF4−, the contribution reduces to 29−50%, whereas, in 5·BF4−, only 3% of bonding stabilization is provided by ΔVelstat. Reducing the number of coordinated metals in the host structure also results in a lower ΔVelstat stabilizing contribution. Therefore, a small stabilization of the ΔEint is seen for 2·BF4−− 4·BF4− when compared to 1·BF4−. Deprotonation of phenolic groups on going from 2·BF4− to 4·BF4− decreases the ΔVelstat value, as a result of an increased negative charge that is distributed among the host framework. Furthermore, almost no change in the magnitude of the ΔEoi term is noticed. The role of the metal and its unique electronic character in the ΔVelstat term is directly related to its capacity to withdraw the electronic density from the calixarene cavity. Hence, four ruthenium(II) atoms of 1 are capable of accepting more electronic charge than only two atoms as in 2−4. As an example, the ΔEoi stabilization values ranged from −22.2 (3· BF4−) to −24.2 kcal mol−1 (2·BF4−), whereas the ΔVelstat contribution to the bonding escalated between 37% and 57%, ranging from −21.6 to −51.8 kcal mol−1 for compounds 4· BF4− and 2·BF4−, respectively. The large magnitude of the ΔVelstat component, especially in the four Ru2+ metalated host 1, provided the most stabilizing bonding scenario among all studied host−guest complexes. The ion−dipole (i−d %)64−66 represents the ion−dipole contribution of the ΔVelstat term, besides the higher-order terms (dipole−dipole, dipole−quadrupole, etc.) and is obtained according to eq 2. The ΔVNelstat term is obtained in an EDA analysis where an isoelectronic neutral analogue replaces the anion guest, e.g., CF4 instead BF4−, and halogens instead of the noble gases. As the charge of the coordinated metal increases, a larger ion−dipole contribution to the ΔVelstat stabilization is seen (Table 1). Thus, the anion recognition capability of metalated calixarene hosts increases with coordination of molybdenum(0), technetium(I), and

ruthenium(II). The ion−dipole contribution to the ΔVelstat term in 1·Cl−−1·BF4− ranged between 82% and 89%. A smaller i−d % contribution was observed in 2·BF4−−4·BF4−, ranging from 53% to 75%, whereas, in 6·BF4−−8·BF4−, the i−d % ranged between 48% and 71%. A low ion−dipole (i−d %) contribution suggests a lower anion recognition specificity, but the measure of ion−dipole contribution over the larger multipole terms through the eq 2 is limited to systems in which the ΔVelstat have a significant stabilizing contribution to the total interaction energy. Interestingly, in 5·BF4−, where the metallic atom coordinated to the calixarene arenic units is neutral, Mo0, a larger ΔVelstat interaction with the neutral CF4 (−8.5 kcal mol−1) is observed, in contrast to the negatively charged BF4− (−1.0 kcal mol−1), suggesting that this host is not adequate to recognize anions by means of electrostatic interactions. This finding agrees with the MEP of the structure 5, in which a very lower electrostatic potential is seen in the host cavity (Figure 3). To confirm this hypothesis, an additional EDA analysis was performed for a hypothetical isoelectronic cationic host−guest system 5·NF4+. The ΔVelstat energy term indicates that the guest NF4+ provides an even more stabilizing situation: −17.5 kcal mol−1. These findings can be rationalized in terms of the coordinated metal, Mo0, which is neutral and not only reduces the host → Mo0 charge donation but also amplifies the Mo0 → host backdonation, and as a consequence, the capacity of aromatic portions in the host cavity to recognize anions becomes less favorable, while it configures a more feasible situation to interact with neutral or cationic guests, as revealed by the results. The effect of successive loss of phenolic proton on the anion−π interaction can be understood by looking at 2·BF4−− 4·BF4− and 6·BF4−−8·BF4− complexes. The two major differences between compounds 2·BF4−/6·BF4−, 3·BF4−/7· BF4−, and 4·BF4−/8·BF4− are the nature and the quantity of the employed metallic species, i.e., two Ru(II) versus four Tc(I), respectively. Since the (η6-arene)Ru2+ and (η6-arene)Tc+ moieties contribute with the same additional positive charge to the host cavity, comparing 2·BF4−−4·BF4− and 6· BF4−−8·BF4− enables one to understand the amount of stabilization provided by each metallic ion to the host system, while the host cavities negative charge increases with deprotonation. The loss of one proton takes place in 3·BF4− and 7·BF4−, followed by a second proton in 4·BF4− and 8· BF4−, compared to the fully protonated 2·BF4− and 6·BF4−. In addition, the number of deprotonated phenolic groups rules the amount of BF4− counterions added to the system to let the host with a neutral charge. Hence, four BF4− are added in 2· BF4− and 6·BF4−, three BF4− in 3·BF4− and 7·BF4−, and two BF4− in 4·BF4− and 8·BF4−. The ΔEint in both Ru(II) and Tc(I) complexes series decreases as the phenolic hydroxyls are deprotonated, since the negative charge generated in the oxygen of the acidic hydroxyls must be stabilized, consequently increasing the electron density of the host cavity. However, comparing the ΔEint values in 2·BF4−−4·BF4− and 6·BF4−−7· BF4− (Table 1), it is clear that Ru(II) complexes are better hosts for anionic species. An exception is found for the pair 4· BF4−/8·BF4−, in which a slightly more stabilizing ΔEint is found for the Tc(I) counterpart. In practical terms, the obtained results indicate that the anion−π interaction is more effective on the calixarene framework coordinated with two Ru(II) ions, than with four Tc(I), while the host’s phenolic groups are deprotonated. This provides valuable information in terms of G


Article

Organometallics

Our bonding analyses based on our quantum chemical analyses of a series of model host−guest complexes using dispersion-corrected DFT show that the strengthening effect on the host−guest affinity is caused by the ability of the coordinating metal species to accept charge density from the calixarene’s electron rich aromatic units, making the cavity more π-acidic. The preference for smaller anions, i.e., the more stabilizing host−guest interaction for the same, is the result of a better steric fit inside the calixarene bowl. Such a better fit implies less geometrical deformation of the host cavity and thus less destabilizing deformation strain upon accommodating the anion. The most stabilizing electrostatic contribution between host and guest was found in the case of ruthenium(II) and technetium(I) systems. Orbital interactions provide further stabilization. They arise from donor−acceptor interactions between the guest’s HOMO and the empty π*-type orbitals in the calixarene bowl. Dispersion stabilization contributes ca. 13−22% to the total bond energy. Our results highlight the enormous anion recognition capability of organometallic calixarenes, displaying a rich versatility and possibilities for encouraging further explorative synthesis efforts. We envisage that the insights obtained will contribute to a more rational design of new hosts for anion recognition with tuned selectivity and complexation efficiency.

atom economy, especially when attempting to obtain the most cost-effective host for a specific anion. The fact that the calixarenes coordinated with Ru(II) are better hosts for anions than with Tc(I) is related to the electrostatic nature of the host cavity, which becomes more π-acidic with the presence of two Ru(II), than coordinated to four Tc(I) ions. Values of ΔEint for 2·BF4−−4·BF4− (Table 1) ranged from −57.0 to −27.8 kcal mol−1, whereas, in 6·BF4−−8·BF4−, the interaction varies from −50.0 to −28.0 kcal mol−1. The ΔΔEint difference of −0.2 kcal mol−1 between 4·BF4− and 8·BF4− is explained by the different values of the ΔEPauli term, 29.9 and 20.0 kcal mol−1, respectively. In compound 5·BF4−, four neutral metallic species are coordinated to the calixarene framework. To relate the obtained results of this host−guest system, we also performed an EDA analysis of a BF4− interacting with the free calix[4]arene molecule. The results are summarized in Figure S2 (Supporting Information). The ΔEint for calix[4]arene·BF4− is −7.8 kcal mol−1, almost half of the interaction obtained for 5·BF4−, and only 12% of 1·BF4−. These findings indicate that even the complexation with a neutral metal, such as molybdenum(0), is a feasible strategy to enhance the anion−π interactions between the host−guest system. The rationale behind this is that the metal coordination allows not only a larger electrostatic interaction but also a larger charge transfer from the anion guest to the macrocyclic host structure. The calculated calix[4]arene·BF4− displays ΔEPauli (20.6 kcal mol−1) and ΔEdisp values (−13.3 kcal mol−1), which are similar to those of the metalated series (1·BF4−−8·BF4−). The ΔVelstat and the ΔEoi, on the other hand, are less stabilizing, −0.5 and −14.6 kcal mol−1, respectively. From the Hirshfeld charge analysis, the charge transfer estimate for this host−guest complex is only 0.071 e, the lowest value from all systems reported herein. In summary, the most stable noncovalent bonding was found in the calixarene host complexed with Ru2+, followed by Tc+ and, the least favorable, Mo0. This trend is directly related to the capacity of the metal to withdraw the electronic density of the macrocyclic cavity, as well as the quantity of coordinated metals. This finding agrees with the smaller interaction observed in host−guest complexes when a reduced amount of Ru(II) coordinates the aromatic moieties of the anion receptor. Metal coordination to the calixarene structure offers a electrostatic stabilization even when the metallic species in the complex is neutral. The ion−dipole contribution to the electrostatic stabilization indicates that inclusion of Ru2+ provides the best recognition specificity to anions among the studied systems. Large stabilizing interactions were found to be dependent on the guest capacity to fit sterically inside the cavity, resulting in a larger stabilizing electrostatic character. Significant orbital interactions were also observed involving the occupied HOMO’s orbitals of the guests, and the LUMO’s orbitals of the hosts. Deprotonation of the phenolic groups on the lower rim of the calix-shaped host overloads the receptor’s cavity with electron density, which is better administrated to accept an anion with two Ru(II) in contrast to four Tc(I).

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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.8b00292. Schematic representation of all host−guest complexes reported herein. Comparison between the optimized and experimental X-ray structures of 1·BF4−. Selected geometrical parameters. Selected O-H lower ring vibrational frequencies. Averaged NPA charges for selected groups. Minima structure for calix[4]arene··· BF4−, including selected results from the EDA analysis (PDF) Molecular geometry (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alexandre O. Ortolan: 0000-0002-7073-8647 Ina Øestrøm: 0000-0001-7159-2574 Giovanni F. Caramori: 0000-0002-6455-7831 Renato L. T. Parreira: 0000-0002-5623-9833 Alvaro Muñoz-Castro: 0000-0001-5949-9449 F. Matthias Bickelhaupt: 0000-0003-4655-7747 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.O.O. thanks CNPq for his Ph.D. scholarship (grant 142339/ 2015-6). G.F.C. thanks CNPq (grants 302408/2014-2 and 311963/2017-0) for the research fellowship. I.Ø. thanks CAPES for a Master’s scholarship (grant 1732086). R.L.T.P. thanks FAPESP (grant 2011/07623-8) and CAPES (Science without Borders program, grant 88881.068346/2014-01) for

CONCLUDING REMARKS The resulting anion−π interactions in host−guest complexes obtained from organometallic calix[4]arenes are strengthened and can be efficiently tuned through the proper choice of the exo-coordinating metal fragment to the host’s arene moieties. We find a preference for binding smaller anions. H


Article

Organometallics financial support. F.M.B. thanks The Netherlands Organisation for Scientific Research (NWO) for financial support. A.M.-C. is thankful for the financial support from FONDECYT 1180683.

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Anion Recognition by Organometallic Calixarenes - ACS Publications

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