Article pubs.acs.org/Organometallics
Ruthenophanes: Evaluating Cation−π Interactions in [Ru(η6‑C16H12R4)(NH3)3]2+/3+ Complexes. A Computational Insight Giovanni F. Caramori,*,†,‡ Leone C. Garcia,† Diego M. Andrada,‡ and Gernot Frenking‡ †
Departamento de Quı ́mica, Universidade Federal de Santa Catarina, Campus Universitário Trindade, CP 476, Florianópolis, Santa Catarina 88040-900, Brazil ‡ Fachbereich Chemie, Philipps-Universität-Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany S Supporting Information *
ABSTRACT: The nature of cation−π interactions in a set of [Ru(η6-C16H12R4)(NH3)3]2+3+ (R = F, CN, CH3, and others), complexes was investigated with Su−Li energy decomposition analysis and the natural orbitals for chemical valence and the extended transition state method EDA-NOCV. The long-distance effects of electron-donating and electron-withdrawing substituents as well as protonation of the ipso carbon on the nature of cation−π interactions were investigated. Both energy decomposition analyses, Su−Li EDA and EDA-NOCV, are in total agreement, showing that the presence of electron-donating substituents such as CH3, NH2, and H3CO tends to stabilize the ruthenium−arene interaction while electron-withdrawing substituents such as F, CN, and NO2 tend to weaken such interactions. The electrostatic component of the ruthenium−arene interaction is the most affected by the substitution, despite the fact that the covalent character is much more significant than the electrostatic character. EDA-NOCV reveals that the most important orbital stabilization comes from donation and back-donation between the interacting fragments, while the σ density deformations present a moderate contribution to total orbital stabilization energy in ruthenium−arene interactions of complexes 1−8.
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INTRODUCTION
intramolecular Wurtz coupling. However, the interest of chemists in cyclophanes was kindled in the last 40 years by the pioneering works of Brown4 and Cram.5 The intrinsic structural features of [2n]cyclophanes make them attractive probes to investigate and to comprehend the interplay between strain versus aromaticity,6 the role of transannular effects (through-bond and through-space interactions),7−10 the effects of substituents in transannular π−π interactions,9,10 chemical shifts in aromatic systems,11 intramolecular charge transfer processes,12−14 and transannular reactions.15 Cyclophanes have also been employed as selective catalysts, as auxiliaries in asymmetric synthesis,16 as catalysts that simulate enzymatic functions, 17 in supramolecular chemistry,18 in optoelectronic devices,19,20 as precursors of nonlinear optical materials,21 and in the preparation of artificial receptors.22,23 The occurrence of transannular interactions9,10 in cyclophanes has been detected not only by photoelectrons spectroscopy and by electronic spin resonance (ESR)24,25 but also by other spectroscopic analyses such UV−vis and NMR.26 These transannular interactions between the aromatic decks give a unique π-electron system, which is present not only in double-deck systems but also in multilayered [2 n ]cyclophanes.27
The term cyclophane denotes a class of compounds in which benzene rings are connected by carbon bridges.1 [2.2]Cyclophanes are the simplest [2n]cyclophanes, which comprise two benzene rings connected by n-ethano bridges, where 2 ≤ n ≤ 6 (Figure 1).2 In 1899, Pellegrin3 published the first synthesis of a cyclophane, obtaining the [2.2]metacyclophane from an
Received: March 3, 2014 Published: April 18, 2014
Figure 1. Examples of [2n]cyclophanes. © 2014 American Chemical Society
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In principle, cyclophanes28−35 could be employed to prepare polymers presenting a π-electron system delocalized over a great many benzene decks within the whole molecule, although the synthesis of such polymers has been shown to be prohibitive.36 An alternative way to obtain such systems was to employ [2n]cyclophanes in conjunction with transition-metal complexes, since the former species are perfectly feasible for transition-metal coordination due to the presence of arene decks (Figure 2).36 After the discovery of the Creutz−Taube
Figure 2. Schematic representation of a polymer structure alternating [2.2]paracyclophane units and transition metals of mixed valence.
Figure 3. Schematic representation of the [Ru(η6-C16H12R4)(NH3)3]2+/3+ complexes 1 (R1 = R2 = R3 = R4 = H), 2 (R1 = R2 = R3 = R4 = F), 2a (R1 = R4 = H; R2 = R3 = F), 3 (R1 = R2 = R3 = R4 = CN), 3a (R1 = R4 = H; R2 = R3 = CN) 4 (R1 = R2 = R3 = R4 = CH3), 4a (R1 = R4 = H; R2 = R3 = CH3), 5 (R1 = R4 = H; R2 = R3 = NO2), 6 (R1 = R4 = H; R2 = R3 = NH2), 7 (R1 = R4 = H; R2 = R3 = OCH3), and 8 (R1 = R2 = R3 = R4 = H and ipso carbon is protonated).
compound,37 interest in obtaining mixed-valence binuclear compounds has increased dramatically.36 In this scenario, Boekelheide had an outstanding contribution especially to transition-metal cyclophane chemistry during the 1980s and 1990s, in particular regarding the preparation of ruthenium complexes containing [2n]cyclophanes (namely ruthenophanes) and their electrochemical, structural, and spectroscopic properties, therefore providing important insights into the design of electrically conducting polymers.36,38−46 Boekelheide has shown that it is possible to prepare binuclear ruthenophanes with mixed valence, in which two-electron intervalence transfer takes place.45 Cyclophanes, in general, have the ability to strongly coordinate with transition metals, acting as π ligands.2,47−50 This ability is enhanced by the π−π interactions between the aromatic moieties.1 However, either the π−π interactions or the π-electron-donating ability can be affected by substitution or by protonation.51,52 Interactions involving cyclophanes and metallic cations can be classified as cation−π interactions,41−43,53,54 which have a fundamental importance in molecular recognition,55 selectivity and transportation of ions through trans-membrane channels,56 protein−ligand interactions,57 biomimetic catalysis,58,59 and others. This work investigates whether long-distance substitution and ipso carbon protonation at the [2.2]cyclophane moiety have, in fact, any direct influence on the nature of ruthenium− arene interactions in ruthenium(II) complexes containing cyclophanes as π ligands. An understanding of the physical origins of such interactions and an evaluation of how they are affected by electron-donating and electron-withdrawing groups, particularly in the aromatic moiety that is not directly interacting with the cation,7 are crucial for the design of novel ruthenophane derivatives. To achieve such aim, the simple piano stool model complexes [Ru(η6-C16H12R4)(NH3)3]2+/3+ were employed (Figures 3 and 4), which have an arrangement of ligands similar to that observed in several complexes such [Ru(η6-C16H16)Cl2L], [Ru(η6-C16H16)ClL2]+, [Ru(η6-C16H16)L3]2+, [Ru(η6-C16H16)(NH3)3]2+, and [Ru(η6C16H16)2]2+, previously synthesized and characterized.40,60−63 The changes in aromaticity due to the metal coordination and the presence of different substituents were evaluated with the HOMA criterion and its components EN and GEO.64 The nature of the ruthenium−arene interaction in [Ru(η6-
C16H12R4)(NH3)3]2+ complexes was characterized using two different EDA procedures, the energy decomposition analysis of Su and Li,65 Su−Li EDA, and the energy decomposition analysis based on the natural orbitals for chemical valence and the extended transition state method, EDA-NOCV.66 The former method decomposes the interaction energy into physically meaningful terms such as electrostatic, exchangerepulsion, polarization, induction, and short-range dispersion contributions, while the latter not only decomposes the interaction energy into chemically meaningful contributions such as electrostatic, Pauli repulsion, and orbital interaction but also provides an energetic estimation of the deformation densities related to the electron flow channels for bonding between the interacting fragments.
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COMPUTATIONAL METHODS
The geometries of complexes 1−8 (Figure 4) were optimized without constraints at the nonlocal DFT level of theory,67,68 by using the exchange functional of Becke69 and the correlation functional of Perdew70 in conjunction with the atom pairwise dispersion correction71−73 BP86-D3, and the Ahlrichs triple-ζ-quality basis set def2-TZVP.74 The BP86-D3/def2-TZVP model was employed in conjunction with the required ECP75 for ruthenium and scalar relativistic effects by using the zero-order regular approximation, ZORA.76 All geometry optimizations were performed employing the ORCA package.77 The model BP86-D3/def2-TZVP used for geometry optimization has provided data in excellent agreement with the available X-ray structure data of [Ru(η6-C6H6)Cl2(PPh3)]+,62 as shown in the Supporting Information (Table S1 and Figure S1). Molecular orbital and deformation density pictures were obtained with the ADFview program, a single integrated graphical user interface (GUI) of ADF2013. All reported structures 1−8 were characterized as minima on the potential energy surfaces by the absence of imaginary eigenvalues in the Hessian matrix. The changes in aromaticity were evaluated by the harmonic oscillator model of aromaticity (HOMA) and its two terms describing the decrease in aromaticity due to the bond length elongation (EN) and due to the bond length alternation (GEO).64 HOMA and its components were calculated by using geometries of 1−8 optimized by the BP86-D3/def2-TZVP model. The energy decomposition analysis developed by Su and Li, Su−Li EDA, 65 was employed to characterize the ruthenium−arene 2302
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Figure 4. Optimized structures of the [Ru(η6-C16H12R4)(NH3)3]2+/3+ model complexes 1 (R1 = R2 = R3 = R4 = H), 2 (R1 = R2 = R3 = R4 = F), 3 (R1 = R2 = R3 = R4 = CN), 4 (R1 = R2 = R3 = R4 = CH3), 5 (R1 = R4 = H; R2 = R3 = NO2), 6 (R1 = R4 = H; R2 = R3 = NH2), 7 (R1 = R4 = H; R2 = R3 = OCH3), and 8 (R1 = R2 = R3 = R4 = H and ipso carbon is protonated) at the BP86-D3/def2-TZVP level of theory. interaction in [Ru(η6-C16H12R4)(NH3)3]2+/3+ complexes 1−8 by considering [Ru(NH3)3]2+ and [C16H12R4]0/+ as interacting closeshell fragments and by employing the MP278/def2-TZVP74 model.
Su−Li EDA calculations were carried out as implemented in the quantum chemistry package GAMESS-US.79−81 The EDA-NOCV66 approach was also employed to characterize the orbital contributions 2303
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Table 1. Selected Geometric Parameters of Optimized Structures of 1−8a at the BP86-D3/def2-TZVP Level of Theory dihedral angle (deg)b
distance (Å) compd
A−B
Ru−A
C5C13C14C11
C6C15C16C12
C1C5C3C4
C7C11C9C10
1 2 2ac 3 3a 4 4a 5 6 7 8
2.784/3.003 2.840/2.946 2.889/2.981 2.882/2.976 2.928/2.997 2.859/3.022 2.898/3.010 2.902/2.997 2.853/2.990 2.846/2.981 2.998/2.987
1.690 1.687 1.702 1.685 1.702 1.698 1.705 1.689 1.707 1.693 1.705
−0.6/−0.6 −0.9/−1.0 9.2/17.5 2.0/2.3 12.7/18.0 20.7/25.1 13.1/17.2 17.0/17.4 22.2/23.2 14.2/20.8 −17.4/−25.2
−0.7/−0.9 −0.7/−1.3 8.8/17.8 2.4/2.7 12.9/18.1 12.1/25.0 12.8/17.3 10.4/25.8 16.8/23.5 14.1/17.3 −22.8/−31.8 distance (Å)
18.5/14.0 18.0/13.3 20.6/14.5 18.5/14.0 20.3/14.4 21.9/14.8 21.0/14.1 19.0/13.9 26.9/14.3 19.1/14.6 21.2/17.6
−13.2/−14.0 −9.0/−10.6 −11.3/−11.7 −11.0/−14.4 −14.3/−14.5 −15.9/−17.3 −13.7/−14.6 −13.9/−15.2 −12.4/−13.4 −11.3/−12.5 −11.6/−12.3
1 2 2a 3 3a 4 4a 5 6 7 8
Ru−C1
Ru−C2
Ru−C3
Ru−C4
Ru−C5
Ru−C6
2.168 2.170 2.178 2.167 2.174 2.180 2.175 2.167 2.189 2.174 2.182
2.123 2.121 2.119 2.111 2.114 2.109 2.124 2.114 2.093 2.116 2.114
2.177 2.179 2.186 2.184 2.195 2.183 2.188 2.187 2.191 2.181 2.201
2.127 2.124 2.129 2.120 2.121 2.131 2.125 2.116 2.122 2.130 2.116
2.363 2.359 2.397 2.367 2.404 2.399 2.404 2.380 2.462 2.372 2.412
2.260 2.260 2.270 2.258 2.269 2.249 2.268 2.255 2.231 2.260 2.265
a For the atom numbering, see complex 1, Figure 4. bValues in italics depict geometric parameters of the isolated [2.2]paracyclophanes. cCompounds 2a−4a are the disubstituted analogues of 2−4.
to the interaction between the fragments [Ru(NH3)3]2+ and [C16H12R4]0/+ in [Ru(η6-C16H12R4)(NH3)3]2+/3+ complexes 1−8, by employing the Becke-Perdew exchange-correlation functional with an inclusion of the dispersion correction (BP86-D3). The triple-ζ STO basis set TZ2P+,82 in conjunction with the zero-order regular approximation (ZORA), is implemented in ADF2013 software.83,84 A detailed description of Su−Li EDA and EDA-NOCV approaches is provided in the Supporting Information.
not cause large steric congestion such as F, CN, and CH3, both the tetrasubstituted (2−4) and disubstituted analogues (2a− 4a) were taken into account, while for substituents that induce more steric congestion such as NO2, NH2, and OCH3, only the disubstituted analogues were considered (5−7). According to Table 1, the dihedral angle of ring A (Figure 4), C1C5C3C4, provides a measurement of ring A distortion from planarity. For the isolated cyclophane derivatives, such distortion ranges between ∼14 and 15°, except for the protonated species. The coordination with [Ru(NH3)3]2+ tends to push the deviation from planarity in ring A up, as indicated by the dihedral C1C5C3C4 values, which range from 18 to 27°, suggesting that the π density is pushed still more to the external face of the ring A toward the ruthenium ion, increasing then the boatlike deformation in this ring. The deformation in ring A with the coordination depends on both the pattern of substitution and the nature of the substituent. For instance, the largest values of dihedral angle C1C5C3C4 are observed for electron-donating groups such as CH3 and NH2, while for electron-withdrawing groups the smallest values are observed. In addition, on going from tetrasubstituted to disubstituted analogues, considerable differences can be observed. For example, on going from 2 to 2a and from 3 to 3a, an increase of the dihedral C1C5C3C4 from 18.0 to 20.6° and from 18.5 to 20.3° is observed, respectively. On the other hand, on going from 4 to 4a a decrease in the C1C5C3C4 dihedral angle is observed, indicating that in the presence of four electron-withdrawing groups the distortion of ring A is smaller than that in the presence of two. On the other hand, in the presence of four electron-donating groups such as CH3 the distortion is more accentuated than that in the presence of two groups (Table 1). In fact, such deformations
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RESULTS AND DISCUSSION Geometries. [2.2]Paracyclophanes have a peculiar structure, presenting two conformationally constrained benzene rings in close proximity due to the short bridges tethering the aromatic rings (Figure 1). As a consequence, repulsive interactions occur between the π clouds in the inter-ring region, pushing the π density to the external faces of the benzene rings, causing a boatlike deformation in both rings. In comparison to benzene, cyclophanes thereby are better π donors, exhibiting a higher tendency to form π complexes with acceptor molecules.7,85 In fact, both rings are twisted and distorted relative to one another, which relieves the π-cloud repulsion but introduces additional strain. Such electronic repulsion is also disclosed by the distortion of bridges from a parallel to a twisted arrangement, as reported in the literature,6,9,10,86 which shows that the minimum-energy geometry of the [2.2]paracyclophane is of D2 symmetry, reduced from D2h symmetry by torsion strain. The binding of [2.2]paracyclophane with the [Ru(NH3)3]2+ cation affects the strain induced by the tethers considerably. Table 1 compiles important geometric parameters for both the isolated [2.2]paracyclophane ligands and the model complexes 1−8. It is important to emphasize that for substituents that do 2304
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Figure 5. Evaluation of HOMA, EN, and GEO aromaticity indexes in rings A and B of complexes 1−8.
the bridge tension. For instance, for complex 2, the dihedral values (C5C13C14C11 and C6C15C16C12) are −0.9 and −0.8°, while for 2a, the values are 9.2 and 8.8°, respectively. In contrast, in the presence of two CH3 groups (4a), the dihedral C5C13C14C11 angle is 13.1°, but in the presence of four CH3 groups, this value increases to 20.5°. The results show that, by duplicating the number of electron-donating and electronwithdrawing groups in B, an increase and a decrease in the bridge tension is observed, respectively. As observed in previously reported structures containing metal-coordinated cyclophane ligands,60,87−91 there are two long (average 2.389 and 2.254 Å) and four short (average 2.174, 2.185, 2.112, and 2.123 Å) Ru−C bonds. These differences arise from the shallow boat conformation of the aromatic cyclophane decks. The geometry regarding the ruthenium ion moiety consists of a distorted tetrahedron, with the cyclophane and the other ligands adopting a piano-stool configuration. The distance from the ruthenium ion to the centroid of the coordinated ring is on average 1.7 Å, in good agreement with many reported distances, which range from 1.64 to 1.79 Å.92−97 The results presented show that the coordination and the presence of different substituents on ring B affects the charge distribution and consequently the tension on the cyclophane structure, while the Ru−A distances (the distance between the Ru atom and the centroid of ring A) are not affected considerably, with values ranging around 1.7 Å. Aromaticity. As previously reported by Caramori et al.,6 the [2.2]paracyclophane is slightly less aromatic than benzene, xylenes, and other isomers such as [2.2]metacyclophane and [2.2]metaparacyclophane, as a direct consequence of the enhanced boatlike deformation of the rings in [2.2]paracyclophane. The harmonic oscillator model of aromaticity (HOMA) and its components (EN and GEO) were calculated for complexes 1−8 in order to evaluate both effects, the metal coordination and the nature and quantity of substituents on the aromaticity of [2.2]paracyclophane. Figure 5 graphically illustrates the profiles of HOMA, EN, and GEO. As observed for complexes 1−7, ring B is locally more aromatic than ring A,
are also confirmed by the HOMA, EN, and GEO values, which indicate a considerable decrease of the aromaticity in ring A. According to Figure 5, HOMA in ring B ranges from 0.80 to 0.85, while for ring A HOMA values range from 0.64 to 0.71, considering that the maximum value of HOMA for benzene is ∼1.64 Such a decrease in the aromaticity of ring A is due to the bond elongation term (EN), which presents values ranging from 0.29 to 0.36 (Figure 5). These findings are in full agreement with the increase of the boatlike deformation in this ring. On the other hand, the bond alternation term (GEO) does not present any significant contribution to the decrease of aromaticity in ring A. In the substituted ring B, a decrease of the nonplanarity is observed when the dihedral angle C7C11C9C10 values in isolated cyclophanes and in complexes 1−8 are compared, indicating that the coordination minimizes the repulsion between the π clouds of cyclophane rings, since it pushes up the π density to the external face of ring A, as also confirmed by the inter-ring distances A−B, which shows a slight decrease after the coordination, and by the contours of the deformation density channels obtained with the EDA-NOCV approach (see Figure 10). As reported previously,6,9,10 the dihedral angles in ethano bridges reflect the transannular interactions between the aromatic moieties and the strain induced by the tethers. The results show clearly that after the coordination with [Ru(NH3)3]2+ ion the dihedral angles of the ethano bridges, C5C13C14C11 and C6C15C16C12, are considerably reduced, indicating a decrease in the bridge strain. The results also reveal that the nature and the pattern of substitution in B (Figure 4) can affect the transannular interactions between the aromatic moieties and consequently the cyclophane tension. According to Table 1, the largest dihedral angle values, C5C13C14C11 and C6C15C16C12, are observed when electrondonating substituents such as CH3, NH2, and CH3O are employed, while in the case of electron-withdrawing substituents such as CN and F, the smallest values are observed. In fact, not only the nature but also the quantity of electrondonating or electron-withdrawing groups has a direct effect on 2305
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Table 2. Energy Decomposition Analysis, Su−Li EDA (kcal mol−1), for Complexes 1−8 at the MP2/def2-TZVP Level of Theory and NPA Charges of the Interacting Fragments [Ru(NH3)3]2+ (q1) and [C16H12R4]0/+ (q2) 1 ΔEint
−163.0
ΔEex
−260.4
ΔErep
534.7
ΔEPaulia
274.3
ΔEele
−138.5 (31.7%)
ΔEpol
−214.8
ΔEdisp
−84.0
ΔEorb tot
−298.8 (68.3%)
ΔEprep
0.028
Dec
163.0
q1 q2
1.316 0.684
2 −151.4 −156.1b −258.6 −252.3 531.3 518.0 272.7 265.7 −127.0 (29.9%) −129.3 (30.7%) −212.1 −209.8 −85.0 -82.7 −297.1 (70.1%) −292.5 (69.3%) 0.026 0.029 151.4 156.1 1.359 0.641
3 −141.2 −149.7 −257.0 −251.9 529.3 517.7 272.3 265.8 −114.6 −122.4 −213.3 −209.9 −85.6 -83.2 −298.9 −293.1 0.028 0.029 141.2 149.7 1.394 0.606
(27.7%) (29.5%)
(72.3%) (70.5%)
4 −166.0 −164.2 −258.8 −253.1 531.3 519.4 272.5 266.3 −139.8 −136.1 −215.9 −212.6 −82.8 -81.8 −298.7 −294.4 0.029 0.030 166.0 164.2 1.302 0.698
5
(31.9%) (31.6%)
(68.1%) (68.4%)
6
7
8
−150.9
−172.9
−168.8
−30.5
−258.6
−259.2
−259.6
−242.9
532.0
532.4
532.6
502.0
273.4
273.2
273.0
259.1
−124.7 (29.4%)
−148.2 (33.2%)
−143.1 (32.4%)
−3.2 (1.1%)
−213.9
−215.8
−215.4
−203.7
−85.7
−82.1
−83.3
−82.7
−299.6 (70.6%)
−297.9 (66.8%)
−298.7 (67.6%)
−286.4 (98.9%)
0.029
0.032
0.029
0.028
150.9
172.9
168.8
30.5
1.363 0.637
1.275 0.725
1.308 0.692
1.475 1.525
a
On the basis of previously observed connections and similarities between Su−Li EDA and EDA-NOCV, the following relations are observed: pol + ΔEdisp. bValues in italics describe the Su−Li EDA terms of disubstituted analogues (2a−4a) of 2−4. ΔEPauli = ΔEex + ΔErep and ΔEorb tot = ΔE c The bonding dissociation energy is calculated as follows: −De = ΔEint + ΔEprep.
framework, BP86-D3/TZ2P+, since it is not possible to perform EDA-NOCV using post-HF methods. Our aim is not to compare the performances of ab initio or DFT methods but to provide information regarding the nature of ruthenium− arene interactions in ruthenophanes by using different but complementary approaches, in which the interaction energy components present connections and similarities. Initially, the Su−Li EDA scheme was employed to investigate whether or not the substitution at ring B (Figure 4) of the [2.2]paracyclophane ligand affects the ruthenium−arene interactions in complexes 1−8. In this case, [Ru(NH3)3]2+ and [C16H12R4]0/+ were considered as interacting fragments. Table 2 compiles the Su−Li EDA results, which show that in comparison with the nonsubstituted complex 1 (ΔEint = −163.7 kcal mol−1) the presence of electron-donating substituents such as CH3, NH2, and H3CO tends to strengthen the ruthenium−arene interaction while electron-withdrawing substituents such as F, CN, and NO2 tend to weaken such an interaction. The interaction energy values, ΔEint, for complexes containing electron-donating substituents range from −166.0 to −172.0 kcal mol−1, while for complexes with electronwithdrawing groups these values range from −141.2 to −151.4 kcal mol−1. The substitution at ring B affects mainly the electrostatic component of the ruthenium−arene interaction (Table 2 and Figure 6). According to Su−Li EDA, the covalent character is more significant than the electrostatic in ruthenium−arene interactions, as revealed by the magnitude of polarization (ΔEpol), exchange (ΔEex), and ΔEorb tot terms. The presence of electron-donating substituents attributed to complexes 4, 6, and 7 leads not only to an additional stabilization in electrostatic and polarization components (ΔEele and ΔEpol) but also to a reduction of the repulsion term (ΔErep), since the exchange (ΔEex) becomes less stabilizing in
indicating that the metal coordination reduces the local aromaticity considerably. The reduction of the aromaticity in A is due to the large increase of the C−C bond elongation (EN), which ranges between 0.3 and 0.4. In comparison with ring B, the EN values fluctuate between 0.03 and 0.18. It is important to note that the term GEO, which describes the decrease in aromaticity due to the bond length alternation, is not affected by the metal coordination, and all observed values are close to 0. For that reason they are not plotted in Figure 5. A remarkable result is that the nature and amount of substituents employed do not have a distinct effect on the local aromaticity of A or B, except in the case of the substituents OCH3 and CH3, which increase the aromaticity of both rings slightly in comparison with 1. For the other substituents, the effect on HOMA is merely subtle and electron-donating groups tend to increase the local aromaticity in A or B, while electron-withdrawing groups tend to diminish it. For instance, the decrease in the aromaticity in B for 2, 2a, and 5 is due to the increase on the bond length alternation (GEO), while for 3 and 4 it is due to the increase of the bond length elongation (EN). The most significant change in the aromaticity of B is observed when the ipso carbon is protonated. The protonation reduces the aromaticity drastically because both the bond length alternation and elongation significantly increase. Bonding Analysis. Two types of energy decomposition were employed for analyzing the ruthenium−arene interaction energies in ruthenophanes 1−8, the Su−Li EDA65 and the EDA-NOCV.66 The Su−Li energy decomposition was carried out by employing the MP2/def2-TZVP level of theory. The choice of this model is rooted in the fact that, in Su−Li EDA, CCSD(T) and MP2 interaction energies are very similar.65 On the other hand, EDA-NOCV was carried out within the DFT 2306
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destabilization of the electrostatic (ΔEele = −3.2 kcal mol−1) and exchange components (ΔEex = −242.9 kcal mol−1), as expected, since in this case both interacting fragments are charged ([Ru(NH3)3]2+ and [C16H17]+). As shown in Figure 6, the electrostatic component (ΔEele) is the most sensitive to the substituent effects, which can be related with the changes on quadrupole moment at ring B (the component perpendicular to the ring, Qzz(2)). According to the values reported in Table 3, the presence of electron-withdrawing substituents can reduce a negative quadrupole value or still turn it into a positive value, as observed for complexes 3 and 8. In Table 2, the values depicted in italics describe the Su−Li EDA for the disubstituted analogues (2a−4a) of 2−4. The results show clearly that by reducing the quantity of electronwithdrawing groups such as F and CN a significant increase in the total ruthenium−arene bonding energy is observed. For instance, on going from 2 to 2a, ΔEint changes from −151.4 to −156.1 kcal mol−1. A similar situation is also observed on going from 3 to 3a; in this case, ΔEint changes from −141.2 to −149.7 kcal mol−1. The changes in ΔEint are a direct consequence of the decrease in the ΔErep and the increase in the stabilization of ΔEele. On the other hand, a reduction in the quantity of electron-donating groups has an opposite effect. In fact, when the quantity of CH3 groups is reduced from four to two, the
Figure 6. Changes of Su−Li EDA components in relation to 1.
the presence of such substituents (Table 2 and Figure 6). Such behavior is reversed when electron-withdrawing substituents are present, as in complexes 2, 3, and 5, where a destabilization of electrostatic (ΔEele), polarization (ΔEpol), and exchange (ΔEex) components and consequently a stabilization of repulsion terms (ΔErep) in ruthenium−arene interactions are observed (Table 2 and Figure 6). The protonation of the ipso carbon has a very peculiar effect, resulting in the most significant decay in the total ruthenium−arene interaction (ΔEint = −30.5 kcal mol−1), especially due to the large
Table 3. EDA-NOCV of [Ru(NH3)3]2+ (q1) and [C16H12R4]0/+ (q2) in [Ru(η6-C16H12R4)(NH3)3]2+/3+ Complexes 1−8 (kcal mol−1), by Employing BP86-D3/TZ2P+ as the Level of Theory 1
a
ΔEint
−140.9
ΔEPauli
296.9
ΔEele
−184.2 (42.0%)
ΔEorb tot
−244.7 (58.0%)
ΔEorb 1
−79.7 (32.6%)
ΔEorb 2
−58.7 (24.0%)
ΔEorb 3
−30.1 (12.3%)
ΔEorb 4
−18.8 (7.7%)
ΔEorb 5
−13.6 (5.6%)
ΔEorb 6
−11.3 (4.6%)
ΔEorb res
−32.4 (13.2%)
ΔEdisp
−9.0
q1
1.687
q2
0.313
Qzz(2)
−13.84
2
3
4
−129.2 −135.0a 295.2 287.8 −173.8 (41.9%) −174.8 (42.2%) −241.3 (58.1%) −239.2 (57.8%) −77.3 (32.0%) −77.8 (32.5%) −57.3 (23.7%) −56.4 (23.6%) −30.3 (12.6%) −30.3 (12.7%) −19.1 (7.9%) −18.2 (7.6%) −13.7 (5.7%) −13.5 (5.6%) −11.3 (4.7%) −11.3 (4.7%)
−119.0 −127.9 294.8 288.2 −161.9 (40.0%) −168.4 (41.3%) −242.7 (60.0%) −239.0 (58.7%) −76.9 (31.7%) −76.6 (32.0%) −55.9 (23.0%) −55.4 (23.2%) −31.1 (12.8%) −31.0 (13.0%) −19.6 (8.0%) −18.5 (7.7%) −13.8 (5.7%) −13.6 (5.7%) −11.4 (4.7%) −11.3 (4.8%)
−145.2 −143.4 296.4 289.0 −186.3 (43.1%) −181.2 (42.8%) −246.2 (56.9%) −242.5 (57.2%) −77.6 (31.5%) −79.0 (32.6%) −60.0 (24.4%) −57.6 (23.8%) −31.0 (12.6%) −30.4 (12.5%) −18.0 (7.3%) −18.0 (7.4%) −13.8 (5.6%) −13.6 (5.6%) −11.2 (4.6%) −11.3 (4.7%)
−127.4
5 −152.9
6 −143.3
7 −8.1
8
296.3
296.7
295.8
279.5
−171.5 (41.4%)
−195.3 (44.2%)
−188.0 (43.4%)
−43.0 (15.4%)
−243.2 (58.6%)
−246.3 (55.8%)
−245.2 (56.6%)
−236.1 (84.6%)
−78.0 (32.1%)
−74.4 (30.2%)
−78.4 (32.0%)
−74.7 (31.6%)
−56.4 (23.2%)
−63.9 (26.0%)
−58.6 (23.9%)
−54.7 (23.1%)
−31.3 (12.8%)
−32.7 (13.3%)
−30.4 (12.4%)
−32.1 (13.6%)
−19.0 (7.8%)
−17.2 (7.0%)
−18.6 (7.6%)
−21.0 (8.9%)
−13.7 (5.7%)
−13.8 (5.6%)
−13.6 (5.6%)
−13.2 (5.6%)
−11.3 (4.6%)
−11.0 (4.3%)
−11.3 (4.6%)
−11.2 (4.7%)
−32.4 (13.4%) -31.7 (13.3%) −9.2 −8.8 1.716 1.698 0.284 0.302 −7.77 −11.90
−34.2 (14.1%) -32.6 (13.6%) −9.2 −8.8 1.744 1.717 0.256 0.283 10.40 3.20
−34.6 (14.0%) -32.6 (13.4%) −9.1 −8.7 1.660 1.670 0.340 0.330 −10.20 −11.40
−33.5 (13.8%)
−33.4 (13.6%)
−34.3 (13.9%)
−29.3 (12.5%)
−9.0
−8.9
−9.0
−8.5
1.718
1.643
1.675
1.791
0.282
0.357
0.325
1.209
−3.56
−9.54
−8.73
4.60
Values in italics describe the EDA-NOCV terms of disubstituted analogues (2a−4a) of 2−4. 2307
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Figure 7. Correlation diagram between the total interation energy ΔEint and Hammett constant σp.
Figure 8. Dependence of Su−Li EDA terms with the [Ru(NH3)3]2+ ↔ [C16H16]0 distance in 1.
In order to get more insight about the physical nature of ruthenium−arene interactions, the Su−Li EDA for complex 1 was performed by varying the interfragment distance ([Ru(NH3)3]2+ ↔ [C16H16]0). The equilibrium distance was changed by moving the fragments upward and downward and by employing regular increments on the interfragment distance, Δr(Å), as depicted in Figure 8. The proportional contribution of each term was then computed according to eq 1. The results show clearly that at the equilibrium distance, Δr = 0, the covalent character predominates in ruthenium−arene interaction, with exchange and polarization terms contributing approximately 37.5% and 32.5%, while the electrostatic and dispersion components contribute 19.5% and 10.5% to the total interaction energy, respectively. By diminishing the interfragment distance, Δr < 0, a slight decrease in the polarization, electrostatic, and dispersion contributions is observed, while a considerable increase of the exchange contribution is observed. By increasing the interfragments distance, Δr > 0, in equivalent increments of 0.1 Å, a decrease of the exchange contribution and an increase of polarization and electrostatic contributions are observed, while the dispersion contribution is kept almost
total ruthenium−arene bonding energy becomes less stable. For instance, on going from on going from 4 to 4a, ΔEint changes from −166.0 to −164.2 kcal mol−1. In this case, there is a destabilization of the electrostatic interaction, ΔEele, going from −139.8 to −136.1 kcal mol−1. The bonding dissociation energies (De) were calculated by evaluating the preparation energy (ΔEprep), which is the energy difference of the fragments between the values calculated with the frozen geometry of the complexes and those with the electronic reference state and the electronic ground state at the equilibrium geometry, to the interaction energy, ΔEint (see Table 2 footnote). Since the calculated preparation energies are very close to 0, the total interaction energy ΔEint is very close to the bonding dissociation energy De. The effect of the nature of the substituents on the magnitude of total ruthenium−arene bonding energy is well visualized in Figure 7, which plots ΔEint vs the Hammett constant. The Hammett plot shows clearly that electron-donating groups tend to strengthen the ruthenium−arene interaction whereas electron-withdrawing groups weaken this interaction. 2308
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Figure 9. Molecular orbital correlation diagram of 1, in which [Ru(NH3)3]2+ and [C16H16]0 are the interacting fragments.
constant. At Δr = 0.3 Å, the contributions of exchange and polarization are equivalent. The additional increase of the interfragment distance from 0.3 to 2.0 Å implies a dramatic decrease of exchange and dispersion contributions, which at 2.0 Å are smaller than 5%. On the other hand, a further increase of the of electrostatic and polarization contribution terms is observed when the interfragment distance increases, reaching at 2.0 Å values of around 45% and 50%. These results show clearly that around the equilibrium distance (rectangular area, Figure 8) the cation−π interaction in 1 is essentially held by the electron exchange and polarization, while the electrostatic and dispersion contributions can be considered moderate and tiny, respectively. % ΔEy =
ΔE ele
ΔEy + ΔE ex + ΔEpol + ΔE disp
than the electrostatic character (15.4−44.2%) (Table 3). Despite the difference between the absolute values provided by both decomposition schemes, which is a consequence of the different levels of theory employed, both analyses present coherent results in terms of the effect of substituents on ruthenium−arene interaction. As expected, EDA-NOCV shows that electron-donating substituents tend to stabilize the ruthenium−arene interactions, while electron-withdrawing substituents and the protonation of the ipso carbon weaken them. The large values for the orbital component (ΔEorb tot ) can be explained in terms of the low-lying acceptor orbitals of the charged fragment, [Ru(NH3)3]2+, strengthening the ligand− metal donation, [C16H16]0 → [Ru(NH3)3]2+, as can be observed in the molecular orbital correlation diagram (Figure 9). The covalent character of the ruthenium−arene interaction in 1 that emerges from the EDA-NOCV scheme makes clear that the dominant density deformation, Δρ1, arises particularly from both ligand−metal donation, [C16H16]0 → [Ru(NH3)3]2+, and back-donation, [Ru(NH3)3]2+ → [C16H16]0, between the interacting fragments (Figure 10 and Figure S2 (Supporting Information)), in which the amount of charge transferred is Δq1 = 1.129e. Such charge transfer corresponds to the highest energetic stabilization: ΔEorb 1 = −79.7, Δq1 = |ν1| = 1.129. This observation is in agreement with the correlation diagram of molecular orbitals (Figure 9) and with the SFO (symmetrized fragment orbital) contribution, which shows a density outflow from the HOMO of [C16H16]0 to the LUMO+1 of [Ru-
(1)
To complement the Su−Li EDA results, an EDA-NOCV analysis was employed in order to provide further information about the orbital contributions, ΔEorb i , in terms of charge flow, Δρi(r), since the polarization contribution, as shown by Su−Li EDA results, is very significant to the cation−π interactions in ruthenophanes 1−8 and is also directly related to the orbital component in th EDA-NOCV scheme, as has already been reported by Su and Li.65 The EDA-NOCV results are entirely in line with Su−Li EDA results, showing that in ruthenium− arene interactions the covalent character (55.8−84.6%) is larger 2309
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kcal mol−1, respectively. Two other less significant density deformations with contributions (ΔEorb and ΔEorb 5 6 ) ranging −1 from −13.0 to −11.0 kcal mol , respectively, characterize the σ contributions to the ruthenium−arene interactions, comprising 10% of the total orbital contribution. The other less significant orbital contributions, grouped into ΔEorb res , comprise density deformation related with density polarization from the cyclophane structure toward the external face of ring A. The EDA-NOCV results show clearly that the most significant density deformation channels, Δρi, involve density outflows and inflows that tend to push the density to the external face of the ring A toward the ruthenium ion, increasing then the boatlike deformation in this ring, as already observed from the geometrical parameters. The EDA-NOCV analysis for the disubstituted analogues (values depicted in italics; 2a−4a) of 2−4 is in line with the previous Su−Li EDA results, indicating that by reducing the quantity of electron-withdrawing groups such as F and CN a significant increase in the total ruthenium− arene bonding energy is observed, while a decrease of the bonding energy stabilization is observed when the quantity of the electon-donor group CH3 is diminished.
■
CONCLUSIONS In the present study, the nature of cation−π interactions in a set of ruthenophanes was investigated with Su−Li energy decomposition analysis (Su−Li EDA) and the EDA-NOCV approach, a combination of the extended transition state method (ETS) with the natural orbitals for chemical valence scheme (NOCV). The long-distance effects of electrondonating and electron-withdrawing substituents as well as the protonation of the ipso carbon on nature of cation−π interactions were investigated. The geometric parameters reveal that the coordination with [Ru(NH3)3]2+ tends to increase the nonplanarity feature of the cyclophane ring A directly coordinated to the ruthenium atom, suggesting that there is an outflow of the density to the external face of the ring A toward the ruthenium ion, increasing then the boatlike deformation in this ring. On the other hand, in the noncoordinated ring B, a decrease in the nonplanarity is observed, indicating that the coordination minimizes the repulsion between the π clouds of cyclophane decks. The Su−Li EDA shows that the presence of electron-donating substituents such as CH3, NH2, and H3CO tends to stabilize the ruthenium−arene interaction while electron-withdrawing substituents such as F, CN, and NO2 tend to weaken it. Su−Li EDA also shows that the substitution at ring B affects mainly the electrostatic component of the ruthenium−arene interaction, despite the fact that in ruthenium−arene interactions the covalent character is much more significant than the electrostatic character and that the exchange and polarization components are the most significant contributions to the total interaction energy. EDA-NOCV results are entirely in line with Su−Li EDA, showing that in ruthenium−arene interactions the covalent character is greater than the electrostatic character. EDA-NOCV results also show that the dominant density deformations (Δρi) that provide the most significant orbital stabilization come from donation and back-donation between the interacting fragments, while the σ density deformations present a moderate contribution to the total orbital stabilization energy in ruthenium−arene interactions of complexes 1−8. The EDA-NOCV reveals that the most important density deformation channels involve density outflows and inflows, yielding an accumulation of density on the external face of ring
Figure 10. Contours of deformation densities, Δρi(r), describing the interaction between the fragments [Ru(NH3)3]2+ and [C16H16]0 fragments in 1 and their corresponding energy (ΔEorb i , in kcal mol−1) and charge estimation (in au): (red surfaces) density outflow; (blue surfaces) density inflow (contour value 0.003).
(NH3)3]2+ and also density outflow from the occupied [Ru(NH3)3]2+ orbital (HOMO) to the unoccupied [C16H16]0 orbital (LUMO) (Figure S2, Supporting Information). The EDA-NOCV results show that the density deformation, Δρ1, is also present in all other complexes, 2-8, providing the highest energetic stabilization, ΔEorb 1 , whose magnitude is dependent on the nature of the substituent employed. Table 3 also compiles all the most significant orbital contributions (ΔEorb 1 ) to the ruthenium−arene interaction, showing that the four quantitatively most crucial contributions orb orb orb (ΔEorb 1 , ΔE2 , ΔE3 , ΔE4 ) characterize donation and/or back-donation channels, covering more than 76% of the total orbital contribution (Table 3, Figure 10, and Figures S2−S4 (Supporting Information)). For instance, in complex 1, the density deformation channel, Δρ2, presents an amount of charge transfer Δq2 = 1.051e and the energetic stabilization −1 ΔEorb 2 = −58.7 kcal mol . Δρ2 does not only characterize a donation channel but also comprises small density outflow from the occupied [Ru(NH3)3]2+ orbital (HOMO-2) to the unoccupied [C16H16]0 orbital (LUMO+1) and also a polarization density outflow from the HOMO to the LUMO of [C16H16]0 (Figure S4, Supporting Information). This density deformation channel is also present in complexes 2−8, with energetic stabilization values, ΔEorb 2 , depending on the nature of the substituent employed (Table 3). The two other density deformation channels (Δρ3 and Δρ4) provide energetic orb stabilization (ΔEorb 3 , ΔE4 ), with values ranging between −30.1 and −32.7 kcal mol−1 and between −17.2 and −21.0 2310
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A. The strength of this interaction depends on the nature of the substituent in ring B.
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ASSOCIATED CONTENT
* Supporting Information S
Text, figures, and an xyz file giving selected experimental and calculated parameters (bond lengths Å, bond angles (deg)) for [Ru(η6-C6H6)Cl2(PPh3)]+, contours of deformation densities, the main SFO contributions of the interacting fragments in 1, a detailed description of Su−Li EDA and EDA-NOCV approaches, and data for the calculated structures in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for G.F.C.:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the National Council for Scientific and Technological Development (CNPq) for financial support (grant 470985/2011-9) and the excellent service of the Hochschulrechenzentrum of the Philipps-Universität Marburg. G.F.C. thanks the Capes (Coordination for the Improvement of Higher Level -or Education- Personnel) for a postdoc research scholarship (grant 3181-13-8). D.M.A. thanks the Deutscher Akademischer Austauschdienst for a postdoctoral scholarship.
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dx.doi.org/10.1021/om500203u | Organometallics 2014, 33, 2301−2312
