Analysis of the Aromaticity of Five-Membered Heterometallacycles

Article pubs.acs.org/Organometallics

Analysis of the Aromaticity of Five-Membered Heterometallacycles Containing Os, Ru, Rh, and Ir Rafael Islas,*,†,‡ Jordi Poater,† and Miquel Solà*,† †

Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Catalonia, Spain ‡ Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, C.P. 36050, México S Supporting Information *

ABSTRACT: We study the molecular structure and aromaticity in a series of experimental and new in silico designed five-membered heterometallacycles with general formula M(XC3H3)(PH3)2, where M = OsH3, OsCl3, OsCl2, RuCl2, RhCl2 or IrCl2 and X = NH, O, S, CH−, or CH+. The electron delocalization of the five-membered rings in these complexes is analyzed using the induced magnetic field, NICS, and MCI descriptors of aromaticity. Our results indicate that the five-membered rings in all complexes with X = NH, O, S, and CH− have a low aromatic character denoted by nonintense diatropic behavior and low MCI values. Five-membered rings in complexes with X = CH+ are clearly paratropic and antiaromatic according to MCI values with the exception of M = OsCl3. The reason for this exception is discussed.

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benzene,16 and Bleeke et al. published the experimental detection of iridabenzenes, the second largest family of metallabenzenes.17−19 In those organometallic compounds aromaticity has been studied.17,20−22 Thus, Iron found that NICS23−26 values of some metallabenzenes such as (C5H5Ir)(PH3)3, (C5H5Pt)Cp, and [(C5H5Pt)(PH3)3]+ compare well with those of benzene, suggesting that they may be aromatic.27 Moreover, Fernández and Frenking studied the π-bonding strength in metallabenzene compounds and the corresponding acylic reference molecules, and they concluded that metallabenzenes should be considered as aromatic compounds.28 Heterometallabenzenes are obtained when in a metallabenzene a CH unit is substituted by a heteroatom, such as oxygen or nitrogen.29 In heterometallabenzenes both the transition metal and the heteroatom reside in the aromatic ring and include systems such as metallapyryliums,30,31 metallapyridines,32 and metallathiabenzenes,33 among others. Few theoretical studies on heterometallabenzenes have been carried out until now. Very recently, some of us worked34 on the study of the stability of ortho-, meta-, and paraheterometallabenzenes and concluded that meta isomers for Ir and Rh metallapyridines and the ortho isomers for Ir, Rh, Ru, and Os metallaphosphinines are the most stable. On the other hand, the ortho and meta compounds in Ru and Os metallapyridines are isoenergetic. The reasons for the relative stability of the different isomers were analyzed through an energy decomposition analysis.34

INTRODUCTION The planarity of its ring and the equalization of the C−C ring bond lengths, its low and selective reactivity, its high stability with respect to linear isomers, and its particular spectroscopic and magnetic properties make benzene the archetype of aromaticity. In this system, the π-electrons are distortive in the sense that they favor the D3h symmetric structure of benzene over the D6h one. This was proposed first by Berry1 in an attempt to account for the observed increased frequency of the b2u Kekulé vibrational mode when going from the ground 1 A1g to the first 1B2u excited state.2−4 This idea was reinforced later on by the work of Haas and Zilberg,5,6 and the theory on the distortive character of π-electrons in aromatic systems gained support over the years, especially thanks to the work of Hiberty, Shaik, and co-workers, and others.7,8 More recently, Pierrefixe and Bickelhaupt provided further support to the idea that the regular geometry of benzene is due to the σ- and not to the π-electrons, the latter having a slight tendency to localize double bonds.9,10 When a CH unit in benzene is substituted by a transition metal atom, a metallabenzene is obtained.11,12 Thorn and Hoffmann predicted theoretically the existence of these species in 1979.13 In that seminal work they proposed that the electronic delocalization is the mechanism to stabilize three hypothetical classes of metallabenzene compounds. In 1982, the first metallabenzene, an osmabenzene, was synthesized by Roper et al.14 In that work, they reported the structural analysis based on X-ray diffraction, and they established the planarity of the ring, like in benzene. Years later, Stone et al. synthesized and reported ferrobenzene,15 Ernst et al. reported molybdena© 2014 American Chemical Society

Received: February 3, 2014 Published: March 27, 2014 1762

dx.doi.org/10.1021/om500119c | Organometallics 2014, 33, 1762−1773

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Figure 1. Models of osmatricyclic nitrogen-containing compounds by Esteruelas et al.38,91

Also, five-membered rings (5-MRs) containing transition metals have been studied, principally those based on pyrrole, called metallapyrroles, such as iridapyrroles35,36 and osmapyrroles.37−39 In this case substitution of a CH unit by a metal in a cyclopentadiene ring (or even disubstitution)40 decreases the ring strain owing to the longer M−C bond lengths, thus stabilizing the antiaromatic and strained 5-MRs.41,42 An interesting example is the metallametallocenes of the type CpM(C4H4M′)Cp (M, M′ = Fe, Co, Ni) that were discovered by Reppe and Vetter43 and, recently, studied theoretically by Schaefer III et al.44 The main difference between aromatic metallacycles and the traditional aromatic organic rings is that π-bonding in the former involves the metal d-orbitals. The substitution of one atom of the ring in an aromatic organic molecule by an isoelectronic inorganic fragment with similar shape and approximate energy in their respective frontier orbitals can be explained by the isolobal analogy proposed by Hoffmann.45 It is possible to say the isolobal substitution in classic aromatic organic rings conserve the electronic delocalization and, as a consequence, the aromaticity.46−48 Some authors, such as Jia et al.,49 suggested that these metallabenzenes are aromatic with eight π-electrons, as opposed to Hückel’s rule of (4n + 2) πelectrons. Other authors support this idea, arguing that these compounds present Möbius aromaticity,42,50,51 which arises from the incorporation of d-orbitals of the transition metal atoms into the π-orbital ring system (the spatial arrangement of the molecular orbitals is characteristic for Möbius strips, hence the name). Other authors classify these metallacycles as six or 10 π-electron systems, the classification depending on the degree of participation of the metal orbitals in the π-molecular orbitals (MOs).22,27,28,34,52−54 In recent years, Esteruelas et al. worked both experimentally and theoretically on osmabicycles, aromatic compounds where one Os atom is present in five-membered metallacycles (see Figure 1).55 They emphasized the interaction of some d-orbitals of the metal, specifically dxz and dyz, with the π-orbitals of the planar organic fragment.38 Furthermore, the bond lengths of the cycles are between those of single and double bonds, as expected in delocalized systems. To confirm the aromatic character, NICS24,25,56 was computed in OsH2(κ-N,N-oHNC6H4NH)-(PMe3)2.55 NICS(0) and NICS(1) values obtained in the 5- and 6-MRs of that compound denote an aromatic character.55 The aim of this work is to analyze the aromaticity of new in silico monocycles reminiscent of the systems reported by

Esteruelas et al. that contain a 5-MR with one metal transition atom and one heteroatom. Also, we want to study systems obtained by isoelectronic substitution, or as Boldyrev and Olsen called it, by electronic transmutation,57 making substitutions of chemical entities with the same number of valence electrons. There are only a few studies on the molecular structure and aromaticity of five-membered heterometallacycles.24,55 The present work aims to fill this void by studying the electron delocalization of five-membered heterometallacycles using different descriptors. For the analysis of aromaticity we employed the magnetic indexes Bindz58,59 and NICS. In previous works, we have shown that the z component of the induced magnetic field (Bindz), equivalent to NICSzz,60 is a good descriptor for the magnetic response of nonclassical (anti)aromatic compounds or where NICS is not a good criterion;25,26,61,62 hence, the main discussion of the possible aromatic behavior is based on the Bindz values. This methodology has been employed in different systems such as boron rings,63 metallic clusters such as Al42− salts,64 molecular stars,65 molecular motors,66 molecules with a planar tetracoordinated,67,68 pentacoordinated,69 or even hexacoordinated carbon,70 and classic systems such as benzene,71 borazine,72 and other organic compounds.73 For completeness, the electronic-based multicenter index of aromaticity (MCI) was also calculated.74−84 MCI was proven to work properly in metallacycles85 and, in particular, in heterometallabenzenes.34

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METHODS SECTION

The geometries were optimized and characterized with Gaussian 0386 using the functional B3PW9187,88 and the def2-TZVP basis set for all the atoms.89 Pseudopotentials for Os, Ru, Rh, and Ir were considered.90 Harmonic frequency computations were performed at the same level to characterize the nature of all stationary points. Moreover, we checked the stability of the wave functions for the complexes studied and found that wave functions obtained are indeed minima except for complexes 42, 44, 45, 55, and 65 (vide infra). For these complexes, there is a UHF instability. In these cases, however, we have preferred to report the spin-restricted closed-shell results for two reasons: first, to make a comparison of all systems at the same level of calculation and, second, to avoid possible spin contamination problems. We took the geometry of the models employed by Esteruelas et al. in the theoretical part of their study, in which they replaced the PiPr3 units by PH3 and used H instead of the p-tolyl groups attached to the N atoms of the 5-MRs (see Figure 1).38,91 1763


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Model 1 is formed by three fused rings, two of them five-membered and containing the two Os atoms. Each of the three rings contains one nitrogen atom. This diosmatricyclic structure is planar, and the molecular orbital analysis indicates the presence of 15 π-MOs. Model 2 is a tricyclic osmium compound where the central 5-MR contains the unique Os atom, and the other two rings are six-membered, one of them having two nitrogen atoms. This molecule presents a planar 5MR. Both structures are models considered in the computational studies of their respective works.38,91 We focused our study on one of the 5-MRs of 1 for which we substituted the free valences with two hydrogen atoms (see Scheme 1). The NH group was schematically substituted by isovalent O, S, and

Scheme 2. Positions Where Shielding Tensors Were Computed for Species 11

Scheme 1. From the Model of the Real Molecule to the Analyzed Five-Membered Metallacycles

where P(A) stands for the n! permutations of the elements in the string {A}. The MCI index has been successfully applied to a broad number of situations, from simple organic compounds to complex allmetal clusters with multiple aromaticity.74−84 For planar species, Sij(Ak) = 0 for i ∈ σ and j ∈ π-orbital symmetries and, therefore, the MCI can be exactly split into σ- and π-contributions, namely, MCIσ and MCIπ, respectively. The numerical integrations over the atomic domains were carried out within the “fuzzy atom” framework,94 using the Becke-ρ partitioning scheme95 with the APOST-3D program.96 The Iring and MCI indexes were obtained with the ESI-3D program97 at the same B3PW91/def2-TZVP level of theory.

CH−. CH+ substitution was also considered. All the new systems were considered as a set of metallacycles, and from that group we built another five new sets of five molecules, changing the H atoms bonded to the Os by Cl atoms or changing the transition metal atom (osmium) by Ru, Rh, or Ir. With this schematic substitution we propose 30 molecules, in which the electronic delocalization was analyzed with the Bindz and NICS profiles and multicenter electron delocalization indexes. As said before, in aromatic compounds the electronic delocalization generates an equalization of the bond distances in the ring. In the molecules reported here, there are two C−C bonds in the 5-MRs, which are expected to be equal (or close to being) in aromatic rings in the signal of electronic delocalization. The shielding tensors employed for the calculation of the Bindz and NICS were computed with the GIAO (gauge-including atomic orbitals) method as implemented in Gaussian 03 employing B3PW91/def2-TZVP for all atoms and including pseudopotentials for Os, Ru, Rh, and Ir. The inclusion of the pseudopotentials is necessary because the shielding tensors are sensitive to relativistic effects, as Castro et al. reported in 2010, when they studied a series of spherenes.92 In their words, “any prediction of electron delocalization containing heavy elements without considering adequate treatment of relativistic effects may lead to an erroneous chemical interpretation.”92 The 5-MR lies in the xy plane, and its geometrical center corresponds to the origin of the Cartesian coordinate system. The second-rank shielding tensors were computed along the z axis, above and below of the ring plane. In Scheme 2, the positions where shielding tensors were computed are represented by small light blue spheres. The electron delocalization multicenter index was used as an electronic index of aromaticity. MCI stemmed from the Iring index that was defined by Giambiagi in 2000,93 as

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RESULTS AND DISCUSSION Structural Analysis. All geometries depicted in Figure 2 are minima. The ring bond lengths are shown in Table 1. The general formula for all molecules studied is M(XC3H3)(PH3)2, where M = OsH3, OsCl3, OsCl2, RuCl2, RhCl2, or IrCl2 and X = NH, O, S, CH−, or CH+. In this work, we reoptimized the geometries of the models 1 and 2 with B3PW91/def2-TZVP to compare their angles and bond lengths with those of the new systems proposed in this work. Also, we computed the shielding tensors of these models with the same functional and basis set. As mentioned in the introduction, the bond length equalization is related with the delocalization of conjugated cyclic systems, benzene as the best example. Some aromatic indexes are based on this geometrical parameter, for example, HOMA,98 which reports variations in the bond lengths compared with some reference, generally, benzene. In this work we also look at bond lengths (d3 and d4, vide infra) as evidence of electronic delocalization. For the molecules in set 1, the transition metal atom is coordinated to three hydride and two phosphine ligands as well as to the 5-MR. All molecules in the set are valence isoelectronic, except for the cationic organometallic structure 15, which has two valence electrons less. The HOMO for the valence isoelectronic molecules 11 to 14 is a π-MO, which is unoccupied and becomes the LUMO in the cationic system (X = CH+). Therefore, 15 has a double-occupied π-orbital less. The consequence of the absence of that MO is the nonplanar geometry of the 5-MR in 15 (see Figure 3). The lengths of the

OCC

∑

Iring(A) =

Si1i2(A1) Si2i3(A 2 ) ... Sini1(A n)

i1, i2 , i3 ,..., in

where Sij(Ak) is the overlap between occupied MOs i and j within the domain of atom k. In this formula it is considered that the ring is formed by atoms in the string {A} = {A1, A2, ..., An}. Extension of this Iring index of Giambiagi by Bultinck and co-workers resulted in the socalled MCI index: OCC

MCI(A) =

∑ Iring(A) = ∑ ∑ P(A)

Si1i2(A 2 ) ... Sini1(A n)

P(A) i1, i2 , i3 ,..., in

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Figure 2. Geometries and labels of the species studied at the B3PW91/def2-TZVP level of theory.

or CH−. When X = S, the d1 bond length (2.444 Å) is the largest of set 1. On the other hand, the shortest bond length is

Os−X bonds, denoted as d1 in Table 1, are in the same range as Os−N bond lengths of 1 and 2 complexes when X = NH, O, 1765


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Table 1. Bond Lengths (in Å) of the Five-Membered Rings of All Complexes Studiedc

Figure 3. Selected angles and dihedral planes (in degrees) of 11: P(1)−Os−P(2) 169.6, Os−N−C(3) 117.8, N−C(3)−C(1) 116.5, C(3)−C(1)−C(2) 114.1, C(1)−C(2)−Os 117.6, C(2)−Os−N 74.0, Os−N−C(3)−C(1) 0.0, Os−N−C(3)−H(4) 180.0, N−C(3)−C(1)− C(2) 0.0, C(3)−C(1)−C(2)−H(2) 180.0, H(3)−Os−C(2)−H(2) 0.0; and 15: P(1)−Os−P(2) 166.8, Os−C(3)−C(4) 118.3, C(3)− C(4)−C(1) 113.0, C(1)−C(2)−Os 118.3, C(2)−Os−C(3) 75.8, Os− C(3)−C(4)−C(1) 9.4, Os−C(3)−C(4)−H(4) 172.4, C(3)−C(4)− C(1)−C(2) 0.1, C(4)−C(1)−C(2)−H(2) 162.4, H(3)−Os−C(2)− H(2) 16.6.

the 5-MR. The steric repulsion generated by the Cl atoms not only produces the deformation of the rings, which are not perfectly planar, but also changes the P(1)−Os−P(2) angles. For example, in the metallathiophenes 13 and 23, the angles are 167.1° and 148.0°, respectively. Also, in set 2, all systems are isoelectronic, except for the molecule with X = CH+, which has two electrons less than the rest of the molecules. Compared with the bond lengths of set 1, the presence of the three Cl atoms reduces all Os−X bond distances, except for X = CH+, for which the Os−X bond length increases by 0.125 Å. Also, the Os−C bonds are shorter than those in set 1, but the reduction is not so drastic. The X−C(1), C(1)−C(2), and the C(2)− C(3) bond distances change only slightly. If one Cl− anion is removed from the molecules of set 2, the ring strain is released and structures with planar rings are obtained. In this new set (set 3), the osmium is coordinated to only two Cl atoms and two PH3 units (Figure 2). The Os−C bond lengths in set 3 molecules are shorter than in set 2. The P(1)−Os−P(2) angles have been relaxed: for 33 the angle is 170.7°, 22° bigger than compound 23. For 35, the two Cl atoms lie in the same plane as the ring plane. In set 4 Os atoms of set 3 are replaced by Ru atoms. This substitution does not affect the bond lengths of the 5-MR that are similar to set 3. Moreover, all geometries contain planar 5MRs. The complex with X = CH+, 45, is not isoelectronic to the rest of the molecules of set 4, but contains a planar ring like all the other ruthenium complexes. Set 5 corresponds to rhodium complexes bonded to two chlorine anions, two PH3 units, and the 5-MR. The bond lengths of these complexes are in the same range as those of analogous compounds in set 4. Also, the largest lengths of Rh− X and X−C bonds are obtained when X = S. The rings are planar, except for 55, which presents a non-totally planar ring core. The dihedral angle C(1)−C(2)−C(3)−C(4) is 12°, meanwhile, the same dihedral is 0.1° in 54. Finally, set 6 consists of iridacycles. The bond lengths of 61, 62, and 63 are in good agreement with the experimental values reported by Bleeke17,99 (see Table 2) for similar complexes. The largest Ir−X bond is found for X = S and, in the other extreme, the shortest is with X = CH+. Also the sulfur compound exhibits the largest Ir−C length bond. The only

a

For 1, experimental bond lengths are from a 1,7-diosma-2,4,6-triaza-sindacene complex. C−C bond lengths lie in between 1.454 and 1.420 Å.91 bFor 2, experimental bond lengths are from a 6-osmapyrimido[2,1a]isoindole derivate.38 cThe models 1 and 2 are included, and their bond lengths without parentheses are from the geometries reoptimized with B3PW91/def2-TZVP for this work. The bond lengths in parentheses are from the experimental geometries reported by Esteruelas et al. In the picture, M represents the metals bonded to hydrogen or chlorine atoms, and X represents the heteroatoms and the CH− and CH+ units.

2.021 Å for X = CH+. The X−C bond lengths are shorter than in 1 and 2, with X = NH and O, and larger when X = S and CH+. When X = CH− the bond length is in the range of the N− C bond length of 1 and 2. The C−C bond distances, denoted as d3 in Table 1, are shorter when X = S and CH+ and in the range when X = NH, O, and CH−. For all d4 distances, except for the CH+ system, the bond lengths are shorter than reference complexes 1 and 2. The bond length difference between d3 and d4 in Table 1 is a measure of bond length equalization in the 5MR. It can be considered a measure of structural aromaticity. According to the results for set 1 (but also for the rest of the sets), the most similar d3 and d4 bond lengths correspond to X = S and the most different to X = CH−. The lowest d3−d4 absolute value is found in complex 1. For set 2 the three hydrogen atoms coordinated to the osmium were replaced with three Cl atoms. The presence of more voluminous substituents generates a steric repulsion that explains why the three Cl atoms are not in the same plane as 1766


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Table 2. Selected Bond Lengths (in Å) for Iridapyrroles, Iridafurans, and Iridathiophenes

a

At the B3PW91/def2-TZVP level used in this work. bref 17. cRef 99.

nonplanar ring core is found in 65, the C(1)−C(2)−C(3)C(4) dihedral angle being 10.7° in comparison to 0.1° for 64. Aromaticity Analysis. A. Aromaticity from Magnetic Descriptors. The analysis of (anti)aromaticity in terms of the magnetic response was done with Bindz and NICS, two widely used and accepted aromaticity indicators in the chemical literature. Negative (positive) values of Bindz and NICS denote aromaticity (antiaromaticity). The more negative the values, the more aromatic the ring is. In a previous work, we showed the importance of being careful in the interpretation of the values of NICS at ring centers (NICS(0)) determined by the nonweighted mean of the heavy-atom coordinates.61 NICS(0) values may contain important spurious contributions from the in-plane tensor components that are not related to aromaticity, as Lazzaretti established in 2000.100 NICS measured 1 Å above the center of the ring (NICS(1)) reflects better the aromaticity patterns, because at 1 Å the effects of the σ-bonding contributions are diminished. Furthermore, we also calculated the out-of-plane component of the NICS(1) value, NICSzz(1), which is considered to describe the π-electron effects even more accurately and is judged to be a better descriptor of aromaticity.101 Some of us reported in 2007 the tendency of the NICS profiles when the benzene ring is coordinated to a chromium tricarbonyl complex.102 In that work, we proved that the NICS(0) value yields the erroneous conclusion that the benzene ring is more aromatic when it is coordinated to Cr(CO)3 because this NICS(0) is altered by the magnetic field created by the electrons involved in the bonding between the ring and Cr atom in the (η6-C6H6)Cr(CO)3 complex. On the other hand, we showed that the NICS(0)zz value provides the correct trend, i.e., a reduction of the aromaticity of the benzene ring when coordinated to the metal.80 For this reason, for the present work we preferred to employ Bindz (=NICSzz) profiles to discuss aromaticity trends. However, NICS profiles are also available in the Supporting Information. Only for set 1 both profiles are discussed. First, we analyze the two models studied by Esteruelas et al.38,91 to have a starting point of reference. Figure 4 shows the Bindz and NICS profiles computed for the 5- and 6-MRs of 1. The molecular plane is perpendicular to the z axis, and both profiles are symmetric along this axis. NICS profiles indicate that both rings can be considered aromatic due to their NICS negative values. The 5-MR is less aromatic than the pyridinic ring. For the pyridinic ring, NICS(1) highlights the most negative value, −8 ppm, and for the osmium-containing ring NICS(1) is around −3 ppm. Therefore, NICS assigns to this latter ring a low- or nonaromatic character.103

Figure 4. Bindz and NICS profiles computed in the axis perpendicular to the five- and six-membered rings in the diosmatricyclic nitrogencontaining system, studied by Esteruelas et al.38,91 The molecular plane lies perpendicular to the z axis of the Cartesian coordinate system. The points where the shielding tensors were calculated are represented by small blue spheres.

On the other hand, Bindz profiles present paratropic regions in the center of the rings (5 and 25 ppm for the 6- and 5-MRs), and these positive values decrease rapidly to negative values. The most diatropic regions are around 1.5 Å from the plane with −16 ppm for the 6-MR and −7 ppm for the 5-MR. The change from diatropic to paratropic character with the distance was already previously observed.25,104 NICS and Bindz profiles indicate that the most aromatic ring is the 6-MR. The osmiumcontaining ring can be considered as low- or nonaromatic due to small values of NICS and the intense paratropic region localized in the center of the ring as denoted by Bindz. The osmapolycycle, 2, has been also analyzed with the two magnetic indexes (Figure 5a), and its response is similar to that calculated in the diosmatricyclic species: small negative values for NICS with the 5-MR being less aromatic than the other two 6-MRs. For those hexagonal structures, denoted in Figure 5a as rings A and C, the magnetic response is basically equivalent even though they are constituted by different atoms. The profiles of the z component of the induced magnetic field present the same tendency as in 1. The osmium-containing ring presents an intense paratropic region (29 ppm) around the molecular center and the smallest value (−6 ppm) at 2 Å from the plane ring. Rings A and C are paratropic in the ring center (5 and 3 ppm, respectively). The highest diatropic response (around −20 ppm) is at 1 Å above and below the plane where the molecule lies. Rings A and C can be considered aromatic, 1767


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Figure 5. (a) Bindz and NICS profiles computed in the axis perpendicular to the five- and six-membered rings in the osmatricyclic nitrogen-containing system studied by Esteruelas et al.38,91 The molecular plane lies perpendicular to the z axis of the Cartesian coordinates system. (b) Bindz and NICS profiles computed in the axis perpendicular to the ring planes. The molecular plane lies perpendicular to the z axis of the Cartesian coordinate system. The profile was computed along the axis with the highest symmetry. Shielding tensors were computed at the B3PW91/def2-TZVP level of theory.

Figure 6. Occupied molecular π-orbitals of 11.

are less aromatic than benzene, pyrrole, or pyridine. Benzene is the archetype of the organic aromaticity, and the other two are classical aromatic compounds containing heteroatoms. NICS profiles of benzene and pyridine present a small diminution at the center. The pyrrole profile presents a more intense diatropic behavior in that position (Figure 5b). Meanwhile,

whereas ring B has low or nonaromatic character. Figures 4 and 5a show the same behavior around the ring centers with strong paratropic values and considerable diatropic regions at 1 Å in the z axis direction. NICS profiles calculated in Figures 4 and 5a reveal low aromatic character in 1 and 2. The 5-MRs present more positive values than the 6-MRs. But both types of rings 1768


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Bindz reveals an intense paratropic region in the center of each ring in 1 and 2. Particularly, the 5-MRs present values higher than 25 ppm. The positive values of the z component of Bind in the plane of the ring can be a consequence of the presence of the Os atom (similar to what was found in the (η6C6H6)Cr(CO)3complex102). Around 1 Å above and below the molecular plane, both 5- and 6-MRs present negative Bindz values related with diatropic currents, associated with aromatic compounds. The 6-MRs present the most intense diatropic region at approximately 1 Å from the molecular plane. The Bindz profiles of 5- and 6-MRs analyzed in 1 and 2 have similar shapes to those of benzene, pyrrole, and pyridine (Figure 5b), but with positive values around the ring center that decrease along the z axis until R is ca. 1.5 Å. For the molecules proposed here, an inspection of the occupied MOs highlights the participation of the metal dorbitals of the transition metal in the π-MOs system. For example, Figure 6 shows the participation of the dxz- and dyzorbitals of the osmium atom in the π-MOs of complex 11. The existence of five valence π-MOs indicates this compound is aromatic with 4n + 2π electrons (n = 2). Figure 7 depicts the plots of NICS curves of set 1. As can be seen in this figure, 15 has a paratropic behavior (NICS(0) = 17

Figure 8. Bindz profiles computed in the axis perpendicular to the fivemembered rings of molecules of set 1.

compound with Bindz = 40 ppm in the ring center, coinciding with its NICS profile and its structural parameters (it is not a perfectly planar structure). Contrary to species 11 to 14, 15 has a bond length d4 larger than d3. This can be easily explained from the occupied π-MOs in Figure 6. Since 15 has two electrons less, the HOMO orbital in Figure 6 remains nonoccupied in complex 15. Lack of occupancy of this orbital explains the lengthening of the d4 bond and the shortening of d3. Moreover, the loss of these two π-electrons concurs with the change in the aromatic character when going from complexes 11−14 (slightly aromatic) to 15 (antiaromatic). For set 2, profiles of the Bindz collected in Figure 9 are less symmetric than set 1 profiles. Complex 25 protrudes from the

Figure 7. NICS profiles computed in the axis perpendicular to the fivemembered rings of the molecules of set 1.

ppm) related to an antiaromatic character. The other four molecules of set 1 present small diatropic values in their respective molecular planes, but around 1 Å above and below the ring, they present negative values of NICS related to aromaticity. The tendency of these four compounds matches in shape with the graphics plotted in Figures 4 and 5a. On the other hand, profiles of Bindz (Figure 8) show positive values around the ring center and negative values at 1 Å from the center, except for 15. Bindz profiles of set 1 coincide with plots of Figures 4 and 5a, specifically with the 5-MRs. This distribution is similar in shape to the magnetic response of classic organic aromatic compounds, such as benzene, pyrrole, and pyridine (see Figure 5b). Interestingly, the most aromatic compound according to Bindz profiles is 13, and this species is also the one having the lowest d3−d4 differences (see Table 2). Among pyrrole, furan, and thiophene, the latter is also considered the most aromatic.105 Considering species 11 to 14, 14 is the system with the lowest aromatic character by Bindz profiles and also has the largest d3−d4 difference. The highest value of the z component of the induced magnetic field is present in 15, which can be considered as an antiaromatic

Figure 9. Bindz profiles computed in the axis perpendicular to the fivemembered rings of the molecules of set 2.

rest because of the position of one PH3 substituent, which lies close to the axis where the profile was computed. The Bindz profile is similar to the other complexes of this set, although it is a nonperfect planar structure. In this case, the five π-MOs of 25 are occupied, and this is the reason that this metallaorganic complex shows similar magnetic response to the other compounds of this set. The π-MO occupation in 25 also explains why in this complex d3 is larger than d4, at variance with complex 15. An intense paratropic region is present around the ring center, and a strong diatropic region is found at 1 Å from the molecular plane of each molecule. The low symmetry is a consequence of the more voluminous chlorine atoms, compared to H. However, the values of the Bindz are 1769


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more negative than those reported in set 1, indicating set 2 is more diatropic at 1 Å above and below the molecular plane. The Bindz profiles of sets 3 and 4 (Figures 10 and 11, respectively) reveal similar tendencies for osmium and


The profiles of set 6 present paratropic values in the center of the rings; especially 65 has the most positive Bindz value of 29 ppm, followed by 61 with 16 ppm (Figure 13). At 1.5 Å from

Figure 10. profiles computed in the axis perpendicular to the fivemembered rings of the molecules of set 3. Bindz



ruthenium metallacycles. The systems with four carbon atoms and charged positively are antiaromatic, with values around 40 ppm for 35 and 45 ppm for 45 at the ring center. As for 15, d4 is larger than d3 due to the nonoccupancy of the π-MO with antibonding d3 and bonding d4 combinations. The difference from the other systems of the same set at the same point is about 35−40 ppm, which is the largest difference in all the systems analyzed in the present work. On the other hand, the neutral four-carbon structures present the same tendency as the rest of the molecules in their respective groups. The sulfurcontaining compounds show the most diatropic behavior in both cases (Os and Ru) if they are compared with the other complexes of sets 3 and 4. Bindz profiles of set 5 (Figure 12) present a similar tendency to the other sets, the difference between the system with X = CH+ with the rest of the molecules of each set being less pronounced than for the previous sets. The magnetic response, studied with NICS (Figure S4) and Bindz (Figure 12), points to a moderate electronic delocalization. A small displacement is found in 54, originated by one PH3 oriented below the ring toward the z axis.

the molecular plane, 63 and 64 present the most significant diatropic values, at −11 ppm. The magnetic response of set 6 is in the same range as set 5, around 15 ppm at the ring center and −10 ppm at 1.5 Å. Only when X = CH+, 55 and 65, the Bindz values at the center of the rings are bigger than the other complexes, at 27 and 29 ppm, respectively. The chemistry of the iridacycles has been studied by Bleeke et al.,17 and they reported an extensive work on the synthesis and experimental characterization of those compounds.99 Some of their 1H NMR results indicate the aromatic character of the iridacycles.99 Our calculations reveal a small diatropic response of the iridacompounds analyzed. B. Aromaticity from Electronic Descriptors. With the aim to complement the above aromaticity analysis based on magnetic measures, we have also calculated the electronic-based multicenter index of aromaticity, MCI (results presented in Table 3).74−84 For both compounds 1 and 2, MCI values confirm that 6-MRs are quite aromatic (0.022−0.039 e), whereas 5-MRs are basically nonaromatic (0.002−0.004 e). In addition, the aromatic character of these 6-MRs is lower than that of benzene (0.054 e) or pyridine (0.051 e), and the same happens for the 5-MRs with respect to pyrrole (0.022 e). Like 1770


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synthesized tricyclic structures (1 and 2) were studied with Bindz and NICS profiles as well as with MCI results. Our calculations indicate a nonintense diatropic character and relatively low MCI values for the rings in 1 and 2. The sixmembered rings in 1 and 2 are more aromatic than the fivemembered rings. The latter have to be considered as rings with low aromatic character. For all the complexes with X = NH, O, S, and CH−, and also for 25, the magnetic response and MCI values are similar to those found in their respective isoelectronic compounds, and, in general, the complexes reported present the same tendency as the five-membered cycles of 1 and 2. The systems with X = CH+ are clearly paratropic systems and antiaromatic according MCI values, except for 25. This exception can be explained with the argument that five molecular orbitals with π-symmetry are doubly occupied in 25, like for all systems in set 2. The rest of the complexes can be catalogued as nonaromatic or lowaromatic compounds.103

Table 3. MCI Values (in Electrons) of the Systems under Analysisa

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

a

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For comparison, MCI for benzene (0.054 e), pyridine (0.051 e), pyrrole (0.022 e), furan (0.007 e), and thiophene (0.031 e).

AUTHOR INFORMATION

Corresponding Authors

for benzene, we found that in all these complexes MCI is almost equal to MCIπ, MCIσ being very close to zero. Therefore, electron delocalization in these complexes corresponds basically to π-electrons. If we now focus on set 1, MCI confirms that 15 is antiaromatic, whereas the rest present a low aromaticity, with 13 as the most aromatic. For set 2, system 25 is less antiaromatic than 15, getting closer to the rest of systems, which present a low aromaticity (the trend delivered by MCI here differs a bit as compared to that derived from Bindz). For sets 3 and 4, MCI finds systems 35 and 45 as the most antiaromatic along the whole series, with the largest difference compared to the rest. In general, MCI results indicate that compounds of sets 3 and 4, which correspond to the complexes with the largest positive charges, are the least aromatic. For all sets, the systems with X = S are the most aromatic in each series. As a whole, MCI calculations support the aromaticity analysis based on magnetic measures, reinforcing the conclusions obtained in the previous section.

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Research Executive Agency of the European Research Council for financial support through the PIRSES-GA-2009-247671 project of the FP7-PEOPLE2009-IRSES program. R.I. thanks the same organization for the postdoctoral fellowship received from the same project. M.S. and J.P. are grateful to the Ministerio de Ciencia e Innovación (MICINN, project numbers CTQ2011-23156/BQU and CTQ2011-25086), the Generalitat de Catalunya (project numbers 2009SGR637 and 2009SGR528, Xarxa de Referència en Quı ́mica Teòrica i Computacional, and ICREA Academia 2009 prize for M.S.), and the FEDER fund (European Fund for Regional Development) for the grant UNGI08-4E-003. The Centre de Serveis Cientıfí cs i Acadèmics de Catalunya (CESCA) is acknowledged for a generous allocation of computer time.

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CONCLUSIONS We performed isoelectronic substitutions to design fivemembered rings containing different transition metal atoms (M = Os, Ru, Rh, and Ir) and heteroatoms (X = NH, O, and S, as well as CH−). Also, we included in the analysis the twoelectron-deficient substituent X = CH+ to compare the effects of nonisoelectronic entities. The structural results found in this work are in good agreement with some previous experimental values. The best example is the set of iridacycles reported experimentally.38,91 All the isoelectronic complexes show similar structural framework, and mostly all five-membered rings are (quasi)planar. The induced magnetic field and NICS indexes were computed to quantify the importance of the electronic delocalization in the stability of these compounds. It is necessary to mention that aromaticity is a complex multifold phenomenon that cannot be summarized by only the magnetic response and structural behavior, and, for this reason, MCI values were also computed. For comparison, the models of the

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