Hexacoordinate Bonding and Aromaticity in Silicon Phthalocyanine

Nov 24, 2010 - (11-13) Such molecules are viewed to be hypervalent because the central silicons are thought to bear more than eight electrons in the v...
0 downloads 10 Views 360KB Size
Download PDF
J. Phys. Chem. A 2010, 114, 13257–13267

13257

Hexacoordinate Bonding and Aromaticity in Silicon Phthalocyanine Yang Yang Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106 ReceiVed: September 28, 2010; ReVised Manuscript ReceiVed: NoVember 4, 2010

Si-E bondings in hexacoordinate silicon phthalocyanine were analyzed using bond order (BO), energy partition, atoms in molecules (AIM), electron localization function (ELF), and localized orbital locator (LOL). Bond models were proposed to explain differences between hexacoordinate and tetracoordinate Si-E bondings. Aromaticity of silicon phthalocyanine was investigated using nucleus-independent chemical shift (NICS), harmonic oscillator model of aromaticity (HOMA), conceptual density functional theory (DFT), ring critical point (RCP) descriptors, and delocalization index (DI). Structure, energy, bonding, and aromaticity of tetracoordinate silicon phthalocyanine were studied and compared with hexacoordinate one. 1. Introduction Phthalocyanines form an important class of tetrapyrrole macrocycles and have wide applications in medical science1-3 and material science4,5 because of their extensive π-electron systems. However, the physical studies of unsubstituted phthalocyanines have been made less amenable due to their notoriously poor solubility6,7 and tendency to aggregate in solution.8 Phthalocyanines with ligands bear good solubility in various organic solvents.9 In the meantime, the axial ligands, providing steric hindrance, strongly encourage disaggregation.9 Silicon phthalocyanines are thus of particular interest because various terminated groups can be attached to the axial positions above and below the aromatic plane. To the best of our knowledge, however, no hexacoordinate carbon phthalocyanine has been reported experimentally, though it could be a potential analogue of silicon phthalocyanine, if it exists. This has been, at least partly, attributed to the smaller size of the carbon atom, which does not fit the phthalocyanine cavity.10 It should also be pointed out that the coordination difference between carbon and silicon could account for the absence of carbon phthalocyanine. Carbon is known to have a coordination number of generally no more than 4 while the coordination number of silicon in many molecules is more than 4, though they both have only four valence electrons.11-13 Therefore, an investigation of hexacoordinate Si-E bondings in phthalocyanine is expected to contribute to a greater understanding of the molecular structure of silicon phthalocyanine and the coordination of silicon. The molecular properties of a large number of pentacoordinate and hexacoordinate silicon molecules have been studied both theoretically and experimentally.11-13 Such molecules are viewed to be hypervalent because the central silicons are thought to bear more than eight electrons in the valence shell.11-13 The Si-N bonding in pentacoordinate silatrane has been extensively studied.14,15 The properties of this bonding are strongly related to the cage effect. It was pointed out that silatrane can exist in an exo form in which the Si-N bonding is unnecessary to support the molecular framework because of the cage effect.14 Si-N bondings in silicon phthalocyanine are affected by probably an even stronger ring effect and thus are different from most other Si-N bondings. Not all four Si-N bondings are necessary to fix the position of silicon in phthalocyanine. Here, * E-mail: [email protected].

“bond” and “bonding” are used as different concepts although they sometimes refer to an identical meaning. A bond is a significant interatomic interaction which is well-defined by conventional models such as molecular orbital theory and valence bond theory, while all considerable interatomic interactions are called bondings. The length of a normal single Si-N bond in tetracoordinate molecules is about 1.74 Å, and the length distribution in various chemical environments is tight.16,17 On the other hand, many Si-N bondings in hypervalent molecules, which are the fifth or the sixth Si-E bondings and are often referred to as dative bondings, are different. Their lengths cover a large span, from 1.9 to 2.7 Å and probably broader,16 and are very sensitive to both intramolecular16-18 and intermolecular environments.18,19 Dative Si-N bondings are soft because only a small amount of energy is required to change their bond lengths.20,21 Moreover, the formation and deformation of dative Si-N bondings are flexible, strongly affected by chemical environment, and even reversible.22-25 The reversibility of dative Si-N bondings leads to reversible equilibrium between isomers with different coordination numbers.23-25 Si-N bonding in silicon phthalocyanine, however, is apparently different from the above two and cannot be divided into either group. To date, the crystal structures of many silicon phthalocyanines have been reported26-40 and the Si-N bond lengths in these phthalocyanines are all about 1.9 Å, rather than 1.7 Å, which distinguishes them from normal single bonds. Moreover, their small length variation is comparable to a normal single bond, which also distinguishes them from dative bondings. Unlike other hypervalent silicon molecules with soft, flexible, and even reversible molecular structure, the octahedron surrounding around silicon is very rigid. The ligation between silicon and ligands in silicon phthalocyanines are strong enough to resist most environmental effects. These make silicon phthalocyanine different from many hypervalent silicon molecules and investigation of Si-E (E ) N, Cl) bondings in silicon phthalocyanine meaningful. It was shown that an understanding of aromaticity could provide additional information about the nature of relevant chemical bondings.41 Furthermore, as referred to in the beginning, many molecular properties and applications of silicon phthalocyanines are governed by their π-electron systems. One property closely related to π electrons is aromaticity. It is expected that a detailed understanding of aromaticity would be helpful to the exploration and further application of phthalocyanine.

10.1021/jp109278v  2010 American Chemical Society Published on Web 11/24/2010

13258

J. Phys. Chem. A, Vol. 114, No. 50, 2010

Yang

Figure 1. Chemical structures of SiPcCl2, 1; SiNcCl2, 2; GePcCl2, 3; GeNcCl2, 4, Si(NH2)4, 5; and SiCl4, 6. The bondings studied are marked in red. The bonding atoms are colored. 7 is created by removing two chlorine atoms from 1, and 8 is created by removing two chloride anions from 1.

This paper aims to get an understanding of chemical bonding and aromaticity in silicon phthalocyanines, using silicon phthalocyanine dichloride (SiPcCl2), 1 (Figure 1), as an example. Its analogues, silicon naphthalocyanine dichloride (SiNcCl2), 2, germanium phthalocyanine dichloride (GePcCl2), 3, and germanium naphthalocyanine dichloride (GeNcCl2), 4, were also studied (Figure 1). For comparison, two corresponding tetracoordinate silicon examples, Si(NH2)4, 5, and SiCl4, 6, were analyzed using parallel routes. The tetracoordinate neutral and positive charged silicon phthalocyanines, 7 and 8, were also investigated and compared to 1-6.

LOL were plotted using Multiwfn.54 Topological analysis of ELF was carried out in Topmod.55 Atoms-in-molecules (AIM), ELF, and LOL analyses imported the wave functions generated by Gaussian at the B3LYP/6-31G(d) level. Nucleus-independent chemical shift (NICS) values were obtained at the nonweighted ring geometric center and the point 1 Å above the center unless otherwise noted. NMR calculations were carried out at 6-31G(d), 6-31+G(d), and 6-311+G(d,p) basis sets, and the gauge independent atomic orbital (GIAO) method56 was employed. Delocalization indices were calculated using AIM2000. The parameters of AIM and ELF topological analyses were set so that the step size was 0.1 au.

2. Computations The B3LYP method, a hybrid density functional, has been shown to give satisfactory results for pyrrole macrocycles42-44 and has been used throughout. The 6-31G(d) basis set was used for optimization. The stationary points of 1-8 were all confirmed to be true energy minima by frequency check (no imaginary frequency). Restricted B3LYP was used for closed shell, and unrestricted B3LYP was used for open shell. Optimization, frequency, NMR, and Mayer bond order45 were performed using Gaussian 09 package.46 Wiberg47 and atomatom overlap-weighted natural atomic orbital (NAO)48 bond orders were computed with NBO 3.049 implemented in Gaussian 09. Energy partition was performed in MOPAC200950 using PM651 semiempirical method, the most recently developed semiempirical method based on experimental and ab initio data from over 9000 compounds. The search of bond critical points (BCPs) and topological analysis of BCPs were done in AIM2000.52 The three-dimensional electron localization function (ELF) and localized orbital locator (LOL) grids were obtained using CheckDen53 and visualized using Gaussview 3.09. The one-dimensional lines and two-dimensional maps of ELF and

3. Results and Discussion 3.1. Structures. The phthalocyanine and naphthalocyanine rings are approximately composed of four symmetric subunits. The Si-N bond lengths in 1 and 2 are 1.93 and 1.93 Å, respectively; see Table 1. The Si-Cl bond lengths in 1 and 2 are 2.21 and 2.21 Å. On the other hand, the Si-N bond length in 5 is 1.74 Å and the Si-Cl bond length in 6 is 2.05 Å, both significantly shorter than the corresponding ones in 1 and 2. Compared to 5 and 6, the increase of bond lengths in 1 and 2 suggests the decrease of bond strength in them. More elongation of Si-N bond length than of Si-Cl bond length suggests more weakening of Si-N bonding. Since the Si-E bondings in 5 and 6 serve as examples of formal single bonds, Si-E bondings in 1 and 2 should be weaker than single bonds. The Ge-E bond lengths in 3 and 4 are close to each other, similar to the case between 1 and 2, suggesting an insignificant relationship between the aromatic ring size and M-E (M ) Si, Ge) bond lengths. The Ge-Cl bonding is 0.10 Å longer than the Si-Cl bonding. This elongation is close to the covalent atomic radii difference between germanium and silicon, 0.11

Hexacoordinate Bonding in Silicon Phthalocyanine

J. Phys. Chem. A, Vol. 114, No. 50, 2010 13259

TABLE 1: Bond Lengths, Bond Orders, and Diatomic Energies of 1-6 bond order atomic pair Mayer Wiberg NAO avg length (Å) energy (eV) 1 1 2 2 3 3 4 4 5 6

Si-N Si-Cl Si-N Si-Cl Ge-N Ge-Cl Ge-N Ge-Cl Si-N Si-Cl

0.51 0.94 0.50 0.95 0.67 0.98 0.66 0.98 1.07 1.13

0.40 0.69 0.40 0.70 0.40 0.67 0.40 0.67 0.70 0.90

0.48 0.66 0.48 0.67 0.47 0.62 0.46 0.63 0.73 0.79

0.46 0.76 0.46 0.77 0.51 0.76 0.51 0.76 0.83 0.94

1.93 2.21 1.93 2.21 1.98 2.31 1.99 2.31 1.74 2.05

-9.8 -8.7 -9.7 -8.7 -9.9 -8.6 -9.8 -8.6 -15.4 -10.7

Å. In contrast, the Ge-N bonding is only 0.05 Å longer than the Si-N bonding, which can be attributed to the constraint from the ring cavity. The silicon tetrapyrrole model is thus more suitable for our studies because it is “freer” from the pressure of ring cavity and more “original” when being used for comparison with corresponding tetracoordinate references. 3.2. Bond Orders (BO). Mayer, Wiberg, and NAO bond orders of 1-6 are listed in Table 1. Multiple types of bond orders were used to check the reliability of each one. Generally, Mayer bond orders are modestly larger than the other two, which occur more frequently in heterodiatomic bondings with large electronegativity differences between bonding atoms.57 Mayer bond order has been demonstrated to be useful in a number of transition metal oxide and chloride molecules.58,59 This seems to imply that Wiberg and NAO bond orders tend to moderately underestimate bond orders for highly polar covalent bondings. Nevertheless, the tendencies of the three bond orders agree with each other. The bond orders of Si-E bondings in 1 and 2 are smaller than the corresponding ones in 5 and 6. The bond orders of Si-N bondings in 1 and 2 are on average 57% of the ones in 5 and the bond orders of Si-Cl bondings in 1 and 2 are on average 81% of the ones in 6. This is consistent with elongation of the Si-N and Si-Cl bondings in 1 and 2 and larger elongation of the Si-N bondings. For all M-E bond orders, little difference was found between 1 and 2, and between 3 and 4, which further suggests that there is insignificant relationship between M-E bondings and aromatic ring size. 3.3. Energy Partition. In semiempirical quantum chemistry methods, the total molecular energy can be partitioned into a one-center term (Ei) and a two-center term (Eij):

Etotal )

∑ Ei + ∑ Eij i

(1)

i 1/2 are often associated with localized orbitals and, thus, shared interactions. For the case of silicon phthalocyanine dichloride, a ν(r) isovalue around 0.55 is suitable for graphical representation of localization domains. LOL provides more distinguishable diagrams than ELF. Most ELF localization domains show up at high isovalues within a narrow range. On the other hand, appearances of LOL localization domains cover a much broader isovalue range. Here, along with the decrease of isovalues, different LOL localization domains show up sequentially: C-H > C-C > C-N > Si-N. Two-dimensional filled-color diagrams of ELF and LOL of 1 were plotted along the phthalocyanine plane (Figure S4 in the Supporting Information). Redder regions represent higher values, and bluer regions represent lower values. The ELF range was set to be [0, 1], and the LOL range was set to be [0, 0.75]. LOL apparently provides a much better distinction between different bonding domains. In addition, LOL provides a better distinction for different atomic shells in a onedimensional plotting. Peaks corresponding to core electrons and bonding electrons, and peaks corresponding to different shells, are more distinguishable in LOL plotting (Figure S5 in the Supporting Information). In the LOL one-dimensional plotting,

13262

J. Phys. Chem. A, Vol. 114, No. 50, 2010

the innermost core peak is high and sharp, other core peaks are medium and sharp, and bonding peaks are medium and broad. 3.7. Bond Models for Si-E Bondings. The concept of hypervalency, in some senses, refers to all six Si-E bondings as regular two-center-two-electron (2c-2e) bonds. To accommodate six bonding ligands, the hexacoordinate silicon was proposed to bear sp3d2 hybridization. On the basis of this explanation, the central silicon has 12 surrounding electrons and disobeys the octet rule. However, the energy gap between d and p orbitals is so large that it is difficult to compensate hybridization promotion energy during bond formation.89 Moreover, theoretical studies indicate the insignificant participation of d orbitals.89,90 In contrast to this conventional valence bond explanation, a three-center-four-electron (3c-4e) bond was proposed.89 Three collinear atomic orbitals combine and result in one bonding, one nonbonding, and one antibonding orbitals. The bonding and nonbonding orbitals are doubly occupied by four electrons. Only the bonding electron pair is responsible for binding the three atoms together. The nonbonding electron pair mainly concentrate around ligands due to a large electronegativity difference between the central atom and the ligand. On the basis of this explanation, there are two 2c-2e bonds and two 3c-4e bonds in 1. The two nonbonding electrons in each 3c-4e bond are not counted as being shared by the central atom, and hexacoordinate silicon is not an exception to the octet rule. Therefore, the concept of hypervalency for hexacoordinate silicon phthalocyanine is more likely to be a conventionally nominal rather than a truly meaningful concept. Since the nonbonding electrons contribute little to the bond order and bond strength, the whole 3c-4e bond is approximately as strong as a single bond and each Si-N bonding can be viewed as a halfbond or a single-electron bond.91 An investigation of multicenter π bonds suggests that bond strength is determined by the number of centers rather than the number of electron pairs.92 An octahedral molecule AB6 has six equivalent A-B bondings. The nature of these six bondings should thus be the same. This can be achieved when the final structure is explained as resonance between three contributing structures with equivalent contributions, each with two 2c-2e bonds and two 3c-4e bonds at different axial positions. Therefore, each A-B bonding is considered as a combination of 1/3 3c-4e bond (or 2/3 singleelectron bond) and 1/3 2c-2e bond. For an ABnC6-n molecule, contributions from different contributing structures could be different. Both Si-N and Si-Cl bondings in 1 are weaker than 2c-2e bonds in 5 and 6 and can be viewed as mixtures of a 3c-4e bond and a 2c-2e bond. Here we made a rough estimation based on previous percentage data of bond order, diatomic energy, FBCP, and DI and the fact that the sum of six Si-E bondings can be viewed as two 2c-2e bonds and two 3c-4e bonds (or four single-electron bonds). Approximately, a Si-N bonding in 1 is composed of 80% 3c-4e bond and 20% 2c-2e bond, compared to 40% 3c-4e bond and 60% 2c-2e bond composition in a Si-Cl bonding. ELF studies show that Si-E bonding attractors are close to N/Cl attractors, which is opposite with BCPs. Both ELF and LOL studies suggest Si-E bonding domains merge with domains at the N/Cl side first when the isovalues are reduced. When η(r) ) 0.8, Si-N bonding domains apparently bend toward nitrogen. These are consistent with the 3c-4e model, in which two nonbonding electrons mainly reside around N/Cl and act like a lone pair. Despite some similarity between the 3c-2e bond and the 3c-4e bond, they are different from an ELF view. The synaptic order of valence basins, defined as the number of core basins they share a boundary with, is useful in characterizing the nature

Yang of valence basins. A monosynaptic basin corresponds to a lone pair, a disynaptic basin corresponds to two-center bonds, and polysynaptic basins correspond to multicenter bonds.93 The 3c-2e bonds in diborane are characterized by trisynaptic basins.94 However, two disynaptic basins rather than one trisynaptic basin were found for each collinear Si-E bonding pair, which is presumed to bear contributions from 3c-4e bond. Furthermore, no trisynaptic basins were found in XeF2, a classic example with a 3c-4e bond.93 Further studies are necessary to resolve this issue. 3.8. Nature of Si-E Bondings. Strengths of six Si-E bondings in 1 are in the same scale. Four Si-N bondings are equivalent due to the rigid ring effect. Different methods provide convergent results about the hexacoordinate bondings. Although hexacoordinate Si-E bondings are weaker than their tetracoordinate counterpart, their natures do not vary based on AIM and ELF descriptors. AIM results suggest Si-E bondings to be an intermediate type between closed-shell interaction and shared interaction. ELF and LOL results indicate that Si-E bondings are shared interactions, though their electron localizations are not as strong as other bondings in 1. The results from AIM and ELF/LOL actually agree with each other though they appear to be different. The connection between AIM and ELF/ LOL is the kinetic energy density. AIM results show that Si-E bonding is dominated by extraction of electron density (positive Laplacian) and excess of potential energy density (negative local energy density). In other words, Si-E bonding has relatively small kinetic energy density. On the other side, the ELF/LOL Si-E bonding domains correspond to low kinetic energy term in eqs 3a-3c, 4a, and 4b (high ELF/LOL values correspond to “slow” electrons). The electrons must be “slow” to localize at the bonding region, and the driving force for a shared interaction is to lower kinetic energy density. Most bondings are actually hybrids of shared interaction and closed-shell interaction, and pure ones are mostly restricted to homodiatomic bondings. Therefore, the classifications of AIM and ELF/LOL are just relative. The difference between them is that AIM classification is based on two criteria (Laplacian and energy density), while ELF/LOL classification is only based on kinetic energy density. Combining results from AIM and ELF/LOL, the Si-E bondings in 1, 2, 5, and 6 (and probably in many other molecules) have a significant contribution from shared interaction and also considerable contribution from closed-shell interaction. They are characterized by a moderately positive Laplacian, moderately small kinetic energy density, moderately large electron localization, and moderately negative local energy density. 3.9. Aromaticity. Aromaticity, like chemical bonding, is one of the most frequently referred to concepts in chemistry. However, unlike chemical bonding, aromaticity is a multidimensional property and results from different criteria might be divergent.95 Therefore, it is important to include multiple criteria when characterizing aromaticity. The harmonic oscillator model of aromaticity (HOMA),96 a structural-based index, is based on the idea that aromaticity leads to equalization of involved bond lengths and is defined as n

HOMA ) 1 -

R (R - Ri)2 n i)1 opt

∑

(5)

where n is the number of bonds taken into account and R is a normalization constant, which is 257.7 for CC bonds and 93.52 for CN bonds. Ropt is the optimal bond length (1.388 Å for CC bonds and 1.334 Å for CN bonds), which makes HOMA equal

Hexacoordinate Bonding in Silicon Phthalocyanine

J. Phys. Chem. A, Vol. 114, No. 50, 2010 13263

TABLE 4: NICS(0) and NICS(1) Values Calculated with Various Basis Sets (ppm) NICS(0) pyrrole benzene isoindole isoindoline benzoisoindole benzoisoindoline 1 2

NICS(1)

ring

6-31G(d)

6-31+G(d)

6-311+G(d,p)

6-31G(d)

6-31+G(d)

6-311+G(d,p)

A B A B A B C A B C A B D A B C D

-15.64 -9.67 -18.22 -8.29 -2.65 -9.58 -19.47 -11.70 -6.93 -2.15 -10.03 -9.78 -3.58 -9.20 -16.08 -1.76 -10.35 -9.31 -13.62

-13.69 -7.97 -16.52 -6.75 -2.48 -8.23 -17.61 -10.30 -5.47 -1.82 -8.56 -8.39 -3.04 -7.96 -14.83 -1.13 -8.84 -8.13 -12.27

-13.68 -8.04 -16.31 -6.62 -2.76 -7.95 -17.58 -10.21 -5.41 -2.22 -8.55 -8.31 -2.87 -7.60 -14.58 -1.03 -8.78 -7.97 -12.12

-11.46 -11.20 -14.17 -9.46 -1.32 -10.87 -15.32 -12.52 -8.85 -1.03 -11.38 -11.39 -6.10 -10.93 -14.78 -4.50 -11.80 -11.14 -12.69

-10.01 -10.12 -12.74 -8.35 -0.83 -9.94 -13.72 -11.23 -7.69 -0.40 -10.11 -10.24 -5.39 -9.94 -13.56 -3.68 -10.56 -10.09 -11.47

-10.15 -10.19 -12.82 -8.53 -1.20 -9.89 -13.86 -11.41 -7.76 -0.86 -10.26 -10.31 -5.31 -9.88 -13.42 -3.68 -10.68 -10.06 -11.31

to unity for a system with all bonds equal to the optimal one. More recently, nucleus-independent chemical shift (NICS),97 a magnetic-based index, takes advantage of the ring current effect, and is defined as the negative value of the absolute magnetic shielding at the ring center or the point 1 Å above the ring center. In conceptual density functional theory (DFT),98 the chemical hardness η, defined as η ) I - A, where I ) -EHOMO and A ) -ELUMO, is helpful to understanding aromaticity.99 A large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap corresponds to high stability and low reactivity and is often connected to high aromaticity. On the basis of this, a T-index was proposed, which is defined as themultipleofthenumberofconjugatedatomsandHOMO-LUMO gap, since the HOMO-LUMO gap is generally smaller for larger molecules.100 Other conceptual DFT quantities such as the chemical potential and electrophilicity index also help describe molecular properties. The chemical potential µ, negative of the electronegativity χ, is calculated by µ ) -1/2(I + A). The electrophilicity index, ω, can be expressed as ω ) µ2/2η. In AIM theory, a closed ring is characterized by a ring critical point (RCP), a (3, +1) electron density saddle point. Descriptors at the RCP were proposed to be electronic aromatic indices.101-103 In benzene, the DI between para-related carbons is larger than that between meta-related carbons despite larger distance between para-related carbons. This is regarded as evidence of aromatic delocalization. A related electronic-based aromatic index, para-related DI (PDI), is defined as the mean of three DIs of para-related carbons in a given six-member ring.95 When it comes to five-member rings such as pyrrole, the DI difference between a formal single and a formal double C-C bond, ∆DI, was used for evaluation of aromaticity.95 Since aromatic delocalization tends to lead to equivalence between a formal single bond and a formal double bond in an aromatic ring, the ∆DI value is small for a highly aromatic ring. NICS values are somewhat sensitive to the basis set,97 so several different basis sets were used for evaluation (Table 4). NICS(0) and NICS(1) values change significantly from 6-31G(d) to 6-31+G(d), suggesting the importance of including a diffuse function in studying NICS. The differences between 6-31+G(d) and 6-311+G(d,p), however, are small, indicating 6-31+G(d) to be a suitable basis set for large systems to which applying 6-311+G(d,p) is very time-consuming. Generally, results from different basis sets provide a consistent tendency.

NICS values of ring B (Figure 2) in isoindoline and 1 are similar to that of benzene, while the NICS value of ring B in isoindole is significantly smaller (only magnitudes were used for comparison since all signs are negative in this section; Figure S6 and Table S2 in the Supporting Information). Likewise, NICS values of ring C in benzoisoindoline and 2 are similar to that of benzene, but the one in benzoisoindole is significantly smaller than that of benzene (Figure S7 and Table S3 in the Supporting Information). Moreover, NICS values of ring B in benzoisoindoline and 2 are slightly larger than that of benzene, but the one in benzoisoindole is significantly larger than that of benzene. On the basis of local NICS values, it seems that the C8NH4 moiety (moiety I) of 1 and the C12NH6 moiety (moiety II) of 2 are more similar to isoindoline and benzoisoindoline, rather than to isoindole and benzoisoindole. In addition, benzo rings in isoindoline, benzoisoindoline, 1, and 2 are more similar to individual benzene rings than the ones in isoindole and benisoindole. This is not surprising for isoindoline and benzoisoindoline because their saturated five-member rings have small effect on the aromatic system. However, it is interesting that moiety I and moiety II, bearing structures similar to those of isoindole and benzoisoindole, have NICS values similar to those of isoindoline and benzoisoindoline. Moreover, NICS values of ring A in 1 and 2 are much smaller than those of unsaturated ring A in isoindole and benzoisoindole, but comparable to those of the saturated ring A in isoindoline and benzoisoindoline (Figure S8 and Table S2 in the Supporting Information), further supporting differences between moiety I (moiety II) and isoindole (benzoisoindole). Compared to NICS(0), NICS(1) values are bigger in all six-member rings

Figure 2. Lettering scheme for aromatic rings. Isoindole and isoindoline contain ring A and ring B. Benzoisoindole and benzoisoindoline contain ring A, ring B, and ring C. 1 contains ring A, ring B, and ring D. 2 contains ring A, ring B, ring C, and ring D.

13264

J. Phys. Chem. A, Vol. 114, No. 50, 2010

Yang

TABLE 5: Electronic- and Structural-Based Aromaticity Indices descriptors at ring critical points (au) pyrrole benzene isoindole isoindoline benzoisoindole benzoisoindoline 1 2

ring

F

λ1

λ2

λ3

∇2 F

G(r)

V(r)

E(r)

HOMA

A B A B A B C A B C A B A B C

0.0484 0.0200 0.0472 0.0192 0.0393 0.0203 0.0468 0.0188 0.0189 0.0390 0.0196 0.0194 0.0450 0.0205 0.0445 0.0198 0.0192

0.208 0.089 0.194 0.076 0.156 0.084 0.191 0.075 0.079 0.155 0.081 0.082 0.200 0.079 0.197 0.077 0.081

0.225 0.089 0.230 0.088 0.185 0.093 0.230 0.083 0.083 0.183 0.087 0.086 0.200 0.098 0.198 0.091 0.085

-0.0513 -0.0149 -0.0512 -0.0142 -0.0403 -0.0151 -0.0511 -0.0141 -0.0140 -0.0401 -0.0146 -0.0145 -0.0475 -0.0158 -0.0472 -0.0153 -0.0145

0.3817 0.1625 0.3726 0.1498 0.3008 0.1625 0.3691 0.1439 0.1477 0.2977 0.1533 0.1533 0.3525 0.1617 0.3482 0.1527 0.1518

0.0838 0.0315 0.0812 0.0289 0.0636 0.0315 0.0803 0.0277 0.0285 0.0628 0.0296 0.0296 0.0767 0.0315 0.0756 0.0296 0.0293

-0.0721 -0.0224 -0.0692 -0.0204 -0.0519 -0.0224 -0.0684 -0.0194 -0.0200 -0.0513 -0.0209 -0.0209 -0.0653 -0.0226 -0.0642 -0.0210 -0.0207

0.0117 0.0091 0.0120 0.0085 0.0116 0.0091 0.0120 0.0083 0.0085 0.0116 0.0087 0.0087 0.0114 0.0089 0.0114 0.0086 0.0086

0.85 0.98 0.74 0.63 -1.28 0.98 0.68 0.53 0.47 -1.33 0.73 0.76 0.51 0.96 0.47 0.74 0.69

but smaller in most five-member rings (Table S3 in the Supporting Information). The exceptions are ring A in 1 and 2, in which NICS(1) > NICS(0). This behavior of five-member rings in 1 and 2 differentiates them from other aromatic molecules in Table 4. This leads to the conclusion that moiety I (moiety II) is definitely different from isoindole (benzoisoindole) and is probably also different from isoindoline (benzoisoindoline). The bridging aza nitrogens could play a very important role in affecting the molecular property. Furthermore, a phthalocyanine cannot be simply viewed as four isoindole or isoindoline subunits bridged by nitrogens. It has been proposed that the electronic structure of phthalocyanine should be viewed as joining four benzene rings to the central tetraazaporphyrin core rather than bridging four isoindole subunits with nitrogens because electronic characteristics of benzene are conserved.104 This is consistent with NICS analysis of phthalocyanine. NICS values of ring D, the six-member Si-N-CR-N-CR-N ring in 1 and 2, were also computed. The values are surprisingly high (higher than 10), corresponding to strong aromaticity. The strong aromaticity may partly be attributed to the large aromatic ring,18 the annulene ring. However, NICS is a local aromatic index because contributions from remote molecular parts are small.105 Therefore, the local six-member ring rather than the global ring should be the major contribution to this high NICS value. The two Si-N bondings, with some π character, may contribute somewhat to the high NICS value. However, the π character in Si-N bonding is much less than a formal double bond. Results from previous sections suggest that Si-N bondings are only a little involved in the π system. A high NICS value of the six-member ring Mg-N-CR-N-CR-N in magnesium phthalocyanine was reported,106 which suggests that the N-CR-N-CR-N moiety is probably to be the determinant. Therefore, the Si-N bondings can be described as homoaromatic bonding and the six-member ring is a monohomoaromatic ring. The structural-based index, HOMA, clearly indicates that ring A in isoindoline and benzoisoindoline are nonaromatic (Table 5 and Table S4 in the Supporting Information). HOMA values of ring A in 1 and 2 are around 0.5, smaller than HOMA values of ring A in isoindole and benzoisoindole. The HOMA values of ring B in 1, however, is similar to that of ring B in isoindoline and benzene, with HOMA values almost equal to 1. The almost unity HOMA value of ring B in 1 seems to suggest that the neighboring ring A is saturated (Figure S9 in the Supporting Information), while the fact is that HOMA of ring A in 1 is close to that of ring A in

PDI

∆DI 0.177

0.105(0) 0.186 0.068(34) 0.340 0.100(3) 0.160 0.064(49) 0.061(37) 0.240 0.072(26) 0.076(25) 0.174 0.081(10) 0.080 0.061(27) 0.068(26)

isoindole (Figure S10 in the Supporting Information). This indicates that moiety I is different from both isoindole and isoindoline. Likewise, HOMA of ring A in 2 is close to that of ring A in isoindole but HOMA of ring B in 2 is close to that of ring B in isoindoline. Generally, HOMA of an aromatic ring in a system composed of more unsaturated rings is smaller (Figures S9-S11 in the Supporting Information). Therefore, HOMA of pyrrole is significantly higher than those of other five-member rings and HOMA of benzene is the highest among all six-member rings in Table 5. This is different from NICS, which shows a ring in a molecule with more than one unsaturated ring may have an NICS value higher than the original individual ring. The electronic-based index, PDI, is roughly consistent with HOMA, suggesting benzene, ring B in isoindoline, and ring B in 1 to be the most aromatic rings. High PDI values are accompanied by low standard deviations. DI values of unsubstitued para-related carbon pairs, Cγ1-Cγ2 and Cε1-Cε2 (Figure 2), are always higher than those of the other two in the given six-member rings, and are always around 0.1. The major differences of PDI values come from the other two carbon pairs, for example, Cβ1-Cδ2 and Cβ2-Cδ1 in ring B (Figure 2). ∆DI values in the two saturated rings, isoindoline and benzoisoindoline, are the highest two. Generally, ∆DI values are smaller for systems with more conjugated rings. Among the RCP descriptors, λ3, the negative curvature, was proposed to be the best descriptor for aromaticity because it correlates with the electron distribution normal to the aromatic ring.101 A more negative value indicates higher electron concentration near the RCP and stronger aromaticity. RCP descriptors of five-member rings are significantly different from descriptors of six-member rings (Table 5). This means the comparison of RCP descriptors between six-member rings and five-member rings is meaningless. Generally, stronger aromaticity (based on results from NICS, HOMA, and PDI) corresponds to a higher RCP electron density, larger Laplacian, larger kinetic energy density, and more negative potential energy density. The RCP electron density, curvatures, Laplacian, kinetic energy density, and potential energy density of ring A in isoindoline and benzoisoindoline are apparently different from the rest in Table S5 in the Supporting Information, representing little aromaticity. Furthermore, the local energy density is not suitable as a criterion for aromaticity since it is almost the same for all rings listed in Table S5 in the Supporting Information and is incapable of distinguishing nonaromatic and aromatic rings.

Hexacoordinate Bonding in Silicon Phthalocyanine

J. Phys. Chem. A, Vol. 114, No. 50, 2010 13265

TABLE 6: Conceptional DFT Descriptors of 1-4 (au) 1 2 3 4

HOMO

LUMO

η

T-index

µ

ω

-0.192 -0.174 -0.195 -0.176

-0.115 -0.109 -0.116 -0.110

0.078 0.065 0.078 0.066

3.12 3.66 3.12 3.68

-0.154 -0.141 -0.156 -0.143

0.151 0.153 0.155 0.155

Geometric and electronic aromatic indices provide more consistent results with each other than with the magnetic index. For example, NICS studies suggest that moiety I of phthalocyanine is closer to isoindoline but geometric and electronic indices suggest that moiety I of phthalocyanine is closer to isoindole. Geometric and electronic indices indicate individual benzene and pyrrole rings to be the most aromatic, but NICS indicates that rings in larger aromatic systems are sometimes more aromatic than the original individual rings. The HOMO-LUMO gaps are almost the same for 1 and 3, and for 2 and 4 (Table 6), which is not surprising because both HOMO and LUMO bear the most contributions from the aromatic rings. Both HOMO and LUMO are higher in naphthalocyanine than in phthalocyanine, and the HOMO-LUMO gap is smaller for naphthalocyanine ring, which is consistent with its larger size. If all the carbon and nitrogen atoms are presumed to participate in the conjugation, then the numbers of conjugated atoms for 1 and 2 are 40 and 56, respectively. The T-indices for 1 and 2 are thus 3.12 and 3.66, respectively. From a global aromatic consideration, naphthalocyanine, the larger one, is more aromatic based on this index. Electronegativity of phthalocyanine is slightly higher than that of naphthalocyanine. Electrophilicities of the two are similar to each other. 3.10. Tetracoordinate Silicon Phthalocyanine. Tetracoordinate GePc has been synthesized but not well characterized,106 and the crystal structure of Ge(TPP) (TPP ) tetraphenylporphyrin) has been reported.107 However, to the best of our knowledge, structures of tetracoordinate silicon phthalocyanine are still experimentally unknown. Axial substitution should be accompanied by axial cleavage, and tetracoordinate silicon phthalocyanine could possibly be intermediate during this process. Therefore, it is expected that a theoretical investigation of such molecules would be instructive. It has been shown in previous sections that Si-Cl bondings bear both covalent components (shared interaction) and ionic components (closedshell interaction). Therefore, both neutral SiPc, 7, and positively

charged SiPc2+, 8, were optimized. Compared to 1, 7 has two fewer chlorine atoms and 8 has two fewer chloride anions. Both singlet (7A and 8A) and triplet (7B and 8B) states converged at C2V geometry, with two nitrogens at the trans position (NR) bending upward and the other two (Nβ) bending downward. Instead of a planar structure, an approximately tetrahedral conformation is adopted here. The structures of 7A, 7B, 8A, and 8B are somewhat similar. The Si-N bondings are all about 1.8 Å, shorter than the ones in hexacoordinate 1 and relatively close to the formal single Si-N bond in 5 (Table 7). NR-Si-NR and Nβ-Si-Nβ angles are around 160°, and NR-Si-Nβ angles are slightly more than 90°. BO and AIM analyses suggest that Si-N bondings in 7 and 8 are slightly weaker than the ones in 5 but apparently stronger than the ones in 1 and 2 (Table 8). 7A and 8A are 13 and 65 kJ/mol more stable than 7B and 8B, respectively. If all electrons are presumed to belong to original atoms, 7 has two more electrons localizing around silicon than 8. Nevertheless, the bonding situations are similar for 7A, 7B, 8A, and 8B (Table 8). It could thus be possible that, instead of locating around silicon, the additional two electrons in 7 delocalize over the aromatic ring. Consistent with this, ELF inspections of 7A and 7B find no lone pair around silicon. Natural bond orbital (NBO) analysis reveals similar partial charges carried by silicon and nitrogen in 7 and 8, showing roughly the same amount of electrons localizing around silicon and contributing to Si-N bonding in 7 and 8. Further evidence were obtained from NICS values calculated at the points 1, 2, 3, and 4 Å above the central silicon at the B3LYP/ 6-311+G(d,p) level. It turns out that 7B and 8A are aromatic and 7A and 8B are antiaromatic, which suggests that 7 contains 4N π electrons and 8 contains 4N + 2 π electrons. It is worth noting that the Hu¨ckel aromaticity rule should reverse from the singlet state to the triplet state.108 Only eight electrons are around Si-N bondings in 7 and the other two combine with original 4N + 2 π electrons to form 4N + 4 electrons, which reverses aromaticity compared to 8. SiPc(L)2106 and Si(TPP)(L)2109 (L ) py, THF) have been reported to be antiaromatic, which suggests that SiPc(L)2106 can be viewed as a 7A subunit and two ligands bound together through intermolecular ligation. Therefore, such hexacoordinate silicon macrocycles are expected to bear similar molecular properties with tetracoordinate silicon macrocycles. The oxidation state of phthalocyanine ring in 7 is -4, and it was reported that there is no EPR signal for

TABLE 7: Structures, NBO Charges, and NICS Values for Tetracoordinate Silicon Phthalocyanines 7 and 8 bond length (Å) 7A 7B 8A 8B

bond angle (deg)

NBO charge

NICS (ppm)

Si-NR

Si-Nβ

NR-Si-NR

Nβ-Si-Nβ

NR-Si-Nβ

Si

NR

Nβ

NICS(1)

NICS(2)

NICS(3)

NICS(4)

1.78 1.80 1.81 1.79

1.81 1.80 1.81 1.80

158 161 163 161

160 161 163 161

92 92 91 92

2.26 2.25 2.26 2.27

-0.89 -0.84 -0.83 -0.87

-0.80 -0.84 -0.83 -0.82

34.4 -10.9 -9.5 26.1

19.8 -7.5 -6.2 15.7

19.8 -5.2 -4.4 9.0

6.7 -3.6 -3.1 5.3

TABLE 8: Bond Orders and AIM Descriptors of Tetracoordinate Silicon Phthalocyanines 7 and 8 bond orders 7A 7A 7B 7B 8A 8A 8B 8B

Si-NR Si-Nβ Si-NR Si-Nβ Si-NR Si-Nβ Si-NR Si-Nβ

AIM descriptors (au)

Mayer

Wiberg

NAO

F

λ1

λ2

λ3

G(r)

V(r)

E(r)

dSi

dN

0.88 0.93 0.91 0.91 0.89 0.89 0.88 0.91

0.62 0.64 0.63 0.63 0.63 0.63 0.62 0.64

0.65 0.67 0.66 0.66 0.65 0.65 0.64 0.66

0.114 0.110 0.111 0.111 0.110 0.110 0.113 0.110

-0.186 -0.173 -0.176 -0.176 -0.173 -0.173 -0.182 -0.174

-0.156 -0.152 -0.151 -0.151 -0.149 -0.149 -0.155 -0.151

0.872 0.802 0.816 0.816 0.794 0.794 0.851 0.806

0.178 0.163 0.167 0.167 0.162 0.162 0.173 0.164

-0.223 -0.207 -0.211 -0.211 -0.207 -0.207 -0.218 -0.208

-0.045 -0.044 -0.044 -0.044 -0.044 -0.044 -0.045 -0.044

1.33 1.34 1.34 1.34 1.34 1.34 1.33 1.34

2.04 2.07 2.06 2.06 2.07 2.07 2.05 2.07

13266

J. Phys. Chem. A, Vol. 114, No. 50, 2010

Pc(-4).110,111 Two more electrons in Pc(-4) are paired in the lower orbital derived from Jahn-Teller splitting of the original eg LUMO of Pc(-2).110,111 The larger singlet-triplet energy gap of 8 is not surprising in view of stabilized aromatic 8A and destabilized antiaromatic 8B. Likewise, the smaller singlet-triplet energy gap of 7 is correlated with destabilization of antiaromatic 7A and stabilization of aromatic 7B. It is thus possible to change the singlet/triplet gap and even reverse the sequence of singlet and triplet states by modifying the coordination of the central atom. On the basis of data from the bond length and angle, bond order, NBO charge, and AIM, differences between Si-NR and Si-Nβ in aromatic 7B and 8A are much smaller than the ones in antiaromatic 7A and 8B (Tables 7 and 8). This is consistent with aromaticity leading to equalization of the four subunits. 4. Conclusions Si-E bondings and aromaticity in hexacoordinate 1 have been studied and compared to references. The principal conclusions are as follows: 1. Six Si-E bondings in hexacoordinate 1 exist based on AIM and ELF/LOL criteria. They bear a significant contribution from shared interaction and considerable contribution from closed-shell interaction. In other words, they are highly polar covalent bonds. Based on AIM and ELF/LOL criteria, the natures of Si-E bondings in hexacoordinate 1 and 2 and tetracoordinate 5-8 are identical. 2. Si-E bondings in 1 are weakened compared to a conventionally formal single bond. Si-N bondings are weakened more than Si-Cl bondings. The relative strength of Si-E bonding compared to that of a formal single bond would be approximately 60% for Si-N bondings and approximately 80% for Si-Cl bondings. 3. Si-E bondings in 1 and 2 are almost identical. Ge-E bondings in 3 and 4 are also almost identical. M-E bondings are little involved in the π-electron system. AIM and ELF studies support the existence of π components of Si-N bondings in 1. On the other hand, Si-Cl bondings are cylindrical. Compared to ELF, LOL provides a more decisive distinction between different attractors. Electronegativity affects the ellipticity, polarity, and DI of M-E bondings. 4. The 3c-4e bond model provides reasonable explanations for hexacoordinate Si-E bondings. Silicon in 1 still obeys the octet rule, and the concept of hypervalence for 1 is nominal rather than meaningful. Si-E bondings are viewed as the result of resonance between 3c-4e bonds and 2c-2e bonds. ELF and LOL suggest that the behaviors of Si-E bonding domains somewhat resemble lone pair domains. ELF results from synaptic order reveal the difference between 3c-4e bond and 3c-2e bond and do not support the existence of a three-center bond in 1. 5. Hexacoordinate Si-E bondings were studied using different methods. These methods provide convergent results. On the other hand, aromaticities of 1, 2, and references were studied with multiple indices. These criteria provide somewhat divergent details. Moiety I in 1 is different from both isoindole and isoindoline based on aromaticity judgments. Si-N bondings in 1 are homoaromatic bondings. 6. Tetracoordinate 7 and 8 are distorted from planar structure. Si-N bondings in 7 and 8 are significantly stronger than the ones in 1 and 2. In 7, two electrons originally belonging to silicon delocalize over the phthalocyanine ring instead of localizing around silicon and reverse aromaticity compared to 8. The singlet-triplet energy gap in 8 is significantly larger

Yang than that in 7, consistent with aromaticity prediction. Aromaticity is the reason leading to Si-N bonding equalizations in 7B and 8A. Acknowledgment. The author thanks Dr. Malcolm Kenney for helpful discussion and the Case High Performance Computing Cluster for allocation of computing time. Supporting Information Available: Figures S1-S11, Tables S1-S5, and complete ref 46. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bonnett, H. Chem. Soc. ReV. 1995, 24, 19–33. (2) Macdonald, I. J.; Dougherty, T. J. J. Porphyrins Phthalocyanines 2001, 5, 105–129. (3) Allen, C. M.; Sharman, W. M.; Van Lier, J. E. J. Porphyrins Phthalocyanines 2001, 5, 161–169. (4) Could, R. D. Coord. Chem. ReV. 1996, 156, 237–274. (5) Mckeown, N. B. J. Mater. Chem. 2000, 10, 1979–1995. (6) Konami, H.; Hatano, M. Chem. Lett. 1988, 1359–1362. (7) Li, Z.; Lieberman, M. Inorg. Chem. 2001, 40, 932–939. (8) George, R. D.; Snow, A. W.; Shirk, J. S.; Barger, W. R. J. Porphyrins Phthalocyanines 1998, 2, 1–7. (9) Rihter, B. D.; Bohorquez, M. D.; Rodgers, M. A. J.; Kenney, M. E. Photochem. Photobiol. 1992, 55, 677–680. (10) Zdravkovski, D.; Milletti, M. C. J. Mol. Struct. (THEOCHEM) 2005, 717, 85–89. (11) Voronkov, M. G.; Trofimova, O. M.; Bolgova, Y. I.; Chernov, N. F. Russ. Chem. ReV. 2007, 76, 825–845. (12) Chuit, C.; Corriu, R. J. P.; Reye, C.; Young, J. C. Chem. ReV. 1993, 93, 1371–1448. (13) Tandura, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr. Chem. 1986, 131, 99–189. (14) Dillen, J. J. Phys. Chem. A 2004, 108, 4971–4977. (15) Trofimov, A. B.; Zakrzewski, V. G.; Dolgounitcheva, O.; Ortiz, J. V.; Sidorkin, V. F.; Belogolova, E. F.; Belogolov, M.; Pestunovich, V. A. J. Am. Chem. Soc. 2005, 127, 986–995. (16) Haaland, A. Angew. Chem., Int. Ed. 1989, 28, 992–1007. (17) Anglada, J. M.; Bo, C.; Bofill, J. M.; Crehuet, R.; Poblet, J. M. Organometallics 1999, 18, 5584–5593. (18) Hagemann, M.; Mix, A.; Berger, R. J. F.; Pape, T.; Mitzel, N. W. Inorg. Chem. 2008, 47, 10554–10564. (19) Gordon, M. S.; Carroll, M. T.; Jensen, J. H.; Davis, L. P.; Burggraf, L. W.; Guidry, R. M. Organometallics 1991, 10, 2657–2660. (20) Chernyshev, E. A.; Knyazev, S. P.; Kirin, V. N.; Vasilev, I. M.; Alekseev, N. V. Russ. J. Gen. Chem. 2004, 74, 58–65. (21) Narula, S. P.; Shankar, R.; Kumar, M.; Chadha, R. K.; Janaik, C. Inorg. Chem. 1997, 36, 1268–1273. (22) Hagemann, M.; Mix, A.; Berger, R. J. F.; Pape, T.; Mitzel, N. W. Inorg. Chem. 2008, 47, 10554–10564. (23) Wagler, J.; Bohme, U.; Brendler, E.; Roewer, G. Organometallics 2005, 24, 1348–1350. (24) Nakash, M.; Goldvaser, M. J. Am. Chem. Soc. 2004, 126, 3436– 3437. (25) Brendler, E.; Wachtler, E.; Wagler, J. Organometallics 2009, 28, 5459–5465. (26) Barker, C. A.; Findlay, K. S.; Bettington, S.; Batsanov, A. S.; Perepichka, I. F.; Bryce, M. R.; Beeby, A. Tetrahedron 2006, 62, 9433– 9439. (27) Brewis, M.; Clarkson, G. J.; Helliwell, M.; Holder, A. M.; McKeown, N. B. Chem.sEur. J. 2000, 6, 4630–4636. (28) Lo, P.-C.; Huang, J.-D.; Cheng, D. Y. Y.; Chan, E. Y. M.; Fang, W.-P.; Ko, W. H.; Ng, D. K. P. Chem.sEur. J. 2004, 10, 4831–4838. (29) Sosa-Sanchez, J. L.; Sosa-Sanchez, A.; Farfan, N.; ZamudioRivera, L. S.; Lopez-Mendoza, G.; Flores, J. P.; Beltran, H. I. Chem.sEur. J. 2005, 11, 4263–4273. (30) Leng, X.; Ng, D. K. P. Eur. J. Inorg. Chem. 2007, 4615–4620. (31) Silver, J.; Sosa-Sanchez, J. L.; Frampton, C. S. Inorg. Chem. 1998, 37, 411–417. (32) Mooney, J. R.; Choy, C. K.; Knox, K.; Kenney, M. E. J. Am. Chem. Soc. 1975, 97, 3033–3038. (33) Ciliberto, E.; Doris, K. A.; Pietro, W. J.; Reisner, G. M.; Ellis, D. E.; Fragala, R.; Herbstein, F. H.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1984, 106, 7748–7761. (34) Sasa, N.; Okada, K.; Nakamura, K.; Okada, S. J. Mol. Struct. 1998, 446, 163–178. (35) Decreau, R.; Julliard, M.; Giorgi, M. Acta Crystallogr. 1999, C55, 1717–1719.

Hexacoordinate Bonding in Silicon Phthalocyanine (36) Silver, J.; Frampton, C. S.; Fern, G. R.; Davies, D. A.; Miller, J. R.; Soda-Sanchez, J. L. Inorg. Chem. 2001, 40, 5434–5439. (37) Farren, C.; FitzGerald, S.; Bryce, M. R.; Beeby, A.; Batsanov, A. S. J. Chem. Soc., Perkin Trans. 2 2002, 1, 59–66. (38) Brewis, M.; Helliwell, M.; McKeown, N. B. Tetrahedron 2003, 59, 3863–3872. (39) Choi, M. T. M.; Choi, C.-F.; Ng, D. K. P. Tetrahedron 2004, 60, 6889–6894. (40) Cammidge, A. N.; Nekelson, F.; Helliwell, M. A.; Cook, M. J. J. Am. Chem. Soc. 2005, 127, 16382–16383. (41) Rzepa, H. S. Nat. Chem. 2009, 1, 510–512. (42) Strenalyuk, T.; Samdal, S.; Volden, H. V. J. Phys. Chem. A 2007, 111, 12011–12018. (43) Samdal, S.; Volden, H. V.; Ferro, V. R.; de la Vega, J. M. G.; Gonzalez-Rodriguez, D.; Torres, T. J. Phys. Chem. A 2007, 111, 4542– 4550. (44) Strenalyuk, T.; Samdal, S.; Volden, H. V. J. Phys. Chem. A 2008, 112, 9075–9082. (45) Mayer, I. Chem. Phys. Lett. 1983, 97, 270–274. (46) Frisch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (47) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096. (48) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735–746. (49) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1. (50) Steward, J. J. P. MOPAC2009 ; Stewart Computational Chemistry: Colorado Springs, CO, http://OpenMOPAC.net. (51) Stewart, J. J. P. J. Mol. Model. 2007, 13, 1173–1213. (52) Biegler-Konig, F.; Schonbohm, J. J. Comput. Chem. 2002, 23, 1489–1494. (53) Pacios, L. F.; Fernandez, A. J. Mol. Graphics Modell. 2009, 28, 102–112. (54) Lu, T. Multiwfn, version 1.4; http://multiwfn.codeplex.com/. (55) Noury, S.; Krokidis, X.; Fuster, F.; Silvi, B. Comput. Chem. 1999, 23, 597–604. (56) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. J. Chem. Phys. 1996, 104, 5497–509. (57) Olah, J.; Blockhuys, F.; Veszpremi, T.; Alsenoy, C. V. Eur. J. Inorg. Chem. 2006, 69–77. (58) Bridgeman, A. J.; Cavigliasso, G.; Ireland, L. R.; Rothery, J. J. Chem. Soc., Dalton Trans. 2001, 2095–2108. (59) Bridgeman, A. J.; Cavigliasso, G. J. Chem. Soc., Dalton Trans. 2001, 3556–3563. (60) Mizuguchi, J. J. Phys. Chem. A 2001, 105, 10719–10722. (61) Matsumoto, S.; Tokunaga, W.; Miura, H.; Mizuguchi, J. Bull. Chem. Soc. Jpn. 2001, 74, 471–477. (62) Sanda, F.; Takata, T.; Endo, T. Macromolecules 1994, 27, 3986– 3991. (63) Sanchez, M.; Ruette, F. J. Mol. Struct. (THEOCHEM) 1992, 254, 335–342. (64) Bader, R. F. W.; Nguyen-Dang, T. T.; Tal, Y. Rep. Prog. Phys. 1981, 44, 893–948. (65) Bader, R. F. W. Acc. Chem. Res. 1985, 18, 9–15. (66) Bader, R. F. W. J. Phys. Chem. A 1998, 102, 7314–7323. (67) Cremer, D.; Kraka, E. Angew. Chem., Int. Ed. 1984, 23, 627– 628. (68) Nakanishi, W.; Hayashi, S.; Narahara, K. J. Phys. Chem. A 2008, 112, 13593–13599. (69) Bader, R. F. W. J. Phys. Chem. A 1998, 102, 7314–7323. (70) Kochera, N.; Selinkaa, C.; Leussera, D.; Kostb, D.; Kalikhmanb, I.; Stalkea, D. Z. Anorg. Allg. Chem. 2004, 630, 1777–1793. (71) Gibbs, G. V.; Jayatilaka, D.; Spackman, M. A.; Cox, D. F.; Rosso, K. M. J. Phys. Chem. A 2006, 110, 12678–12683. (72) Korlyukov, A. A.; Lyssenko, K. A.; Antipin, M. Y.; Kirin, V. N.; Chernyshev, E. A.; Knyazev, S. P. Inorg. Chem. 2002, 41, 5043–5051.

J. Phys. Chem. A, Vol. 114, No. 50, 2010 13267 (73) Fester, G. W.; Wagler, J.; Brendler, E.; Bohme, U.; Gerlach, D.; Kroke, E. J. Am. Chem. Soc. 2009, 131, 6855–6864. (74) Lopez, C. S.; Faza, O. N.; Cossio, F. P.; York, D. M.; de Lera, A. R. Chem.sEur. J. 2005, 11, 1734–1738. (75) Sanderson, R. T. J. Am. Chem. Soc. 1983, 105, 2259–2261. (76) Fradera, X.; Austen, M. A.; Bader, R. F. W. J. Phys. Chem. A 1999, 103, 304–314. (77) Matta, C. F.; Hernandez-Trujillo, J. J. Phys. Chem. A 2003, 107, 7496–7504. (78) Matito, E.; Poater, J.; Sola, M.; Duran, M.; Salvador, P. J. Phys. Chem. A 2005, 109, 9904–9910. (79) Chesnut, D. B. Heteroat. Chem. 2003, 14, 175–185. (80) Chesnut, D. B. Chem. Phys. 2001, 271, 9–16. (81) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397– 5403. (82) Savin, A.; Jepsen, O.; Flad, J.; Andersen, O. K.; Preuss, H.; von Schnering, H. G. Angew. Chem., Int. Ed. 1992, 31, 187–188. (83) Savin, A.; Silvi, B.; Colonna, F. Can. J. Chem. 1996, 74, 1088– 1096. (84) Fuster, F.; Sevin, A.; Silvi, B. J. Phys. Chem. A 2000, 104, 852– 858. (85) Silvi, B.; Savin, A. Nature 1994, 371, 683–686. (86) Schmider, H. L.; Becke, A. D. J. Mol. Struct. (THEOCHEM) 2000, 527, 51–61. (87) Tsirelson, V.; Stash, A. Acta Crystallogr. 2002, B58, 780–785. (88) Jacobsen, H. Can. J. Chem. 2008, 86, 695–702. (89) Cheung, Y.-S.; Ng, C.-Y.; Chiu, S.-W.; Li, W.-K. J. Mol. Struct. (THEOCHEM) 2003, 623, 1–10. (90) Walsh, R. Acc. Chem. Res. 1981, 14, 246–252. (91) Tramsek, M.; Zemva, B. Acta Chim. SloV. 2006, 53, 105–116. (92) Ostby, K.-A.; Haaland, A.; Gundersen, G.; Noth, H. Organometallics 2005, 24, 5318–5328. (93) Silvi, B. J. Mol. Struct. 2002, 614, 3–10. (94) Alikhani, M. E.; Fuster, F.; Silvi, B. Struct. Chem. 2005, 16, 203– 210. (95) Poater, J.; Fradera, X.; Duran, M.; Sola, M. Chem.sEur. J. 2003, 9, 400–406. (96) Krygowski, T. M. J. Chem. Inf. Comput. Sci. 1993, 33, 70–78. (97) Schleyer, P. V.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema Hommes, N. J. R. J. Am. Chem. Soc. 1996, 118, 6317–6318. (98) Geerlings, P.; De Proft, F.; Langenaeker, W. Chem. ReV. 2003, 103, 1793–1873. (99) De Proft, F.; Geerlings, P. Chem. ReV. 2001, 101, 1451–1464. (100) Aihara, J.-I. Theor. Chem. Acc. 1999, 102, 134–138. (101) Howard, S. T.; Krygowski, T. M. Can. J. Chem. 1997, 75, 1174– 1181. (102) Mohajeri, A.; Ashrafi, A. Chem. Phys. Lett. 2008, 458, 378–383. (103) Mandado, M.; Bultinck, P.; Gonzalez-Moa, M. J.; Mosquera, R. A. Chem. Phys. Lett. 2006, 433, 5–9. (104) Orti, E.; Bredas, J. L. J. Chem. Phys. 1988, 89, 1009–1016. (105) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. R. Chem. ReV. 2005, 105, 3842–3888. (106) Cissell, J. A.; Vaid, T. P.; DiPasquale, A. G.; Rheingold, A. L. Inorg. Chem. 2007, 46, 7713–7715. (107) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. J. Am. Chem. Soc. 2007, 129, 7841–7847. (108) Gogonea, V.; Schleyer, R. v. R.; Schreiner, P. R. Angew. Chem., Int. Ed 1998, 37, 1945–1948. (109) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 12212–12213. (110) Mack, J.; Stillman, M. J. Inorg. Chem. 1997, 36, 413–425. (111) Mack, J.; Stillman, M. J. J. Porphyrins Phthalocyanines 2001, 5, 67–76.

JP109278V