Aromatic Metalated Benzenes C - American Chemical Society

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Loading Aromatic Six-Membered Carbocyclic Rings with Coinage Metals: Aromatic Metalated Benzenes C6M6 and 1,3,5‑C6H3M3 (M = Cu, Ag, Au) Exhibiting Intriguing Properties Athanassios C. Tsipis* Laboratory of Inorganic and General Chemistry, Department of Chemistry, University of Ioannina, 451 10 Ioannina, Greece S Supporting Information *

ABSTRACT: The trimerization of coinage metal acetylides MCCH and MCCM (M = Cu, Ag, Au) to form 1,3,5C6H3M3 and C6M6 aromatic metalated benzenes, respectively, is investigated using electronic structure calculation methods. The structural, energetic, magnetotropic, and spectroscopic properties of the aromatic coinage metalated benzenes are analyzed at the PBE0/Def2-QZVPP level of theory and compared to the respective properties of the unsubstituted archetype aromatic benzene molecule. These exotic molecules exhibit unique properties which are of interest for potential application in several technological issues. + 2 + 2] trimerization of ethyne to benzene yielded ΔH° = −140.2 kcal/mol and ΔH°,⧧ = 53.1 kcal/mol. The predicted standard enthalpies of trimerization of coinage metal acetylides calculated at the PBE0/Def2-QZVPP level of theory are compiled in Table 1. For the sake of comparison the standard enthalpy of trimerization of ethyne calculated at the same level of theory is also included in Table 1.

M

etal−carbon binary junctions have been found to exhibit interesting properties, such as Schottky barrier rectification, optical and tunneling devices, and chemical protectors against oxidation. Exotic molecule based nanocrystals of metal acetylides have recently been synthesized and fully characterized.1 The explosive gold(I) acetylide C2Au2 was described over a century ago,2 and both the analogous copper and silver compounds and the alkynyls RCCCu(Ag) are well-known substances. The coinage metal group organoacetylides are important from practical and theoretical viewpoints. For instance, the copper acetylides display photoconductivity,3 and yet the structure of such acetylides is not sufficiently understood. The insolubility of these compounds may be evidence of their polymeric nature. The complete vibrational spectra of solid organoacetylides (RCCCu)x and (RCCAg)x have been recorded, and the assignments are given of several bands.4 It has also been known for a long time that metal acetylides selfassemble into ultrathin nanowires which are initially semiconducting.1a Coinage metal acetylides and particularly copper acetylides have been used in organic synthesis since the synthesis of the dicopper acetylide by Böttger5 in 1859. However, despite their widespread use in synthesis, little is known about the fundamental properties of individual coinage metal acetylides, perhaps because of their explosive, as well as elusive, chemical behavior. Therefore, we thought it would be advisable to investigate the fundamental properties of MCCH and MC CM (M = Cu, Ag, Au) acetylides, giving emphasis to their trimerization products that could result from thermally allowed pericyclic [2 + 2 + 2] cycloadditions of coinage metal acetylides in analogy to the thermally allowed pericyclic [2 + 2 + 2] cycloaddition of ethyne to form benzene. CCSD(T)/6-311G(d,p)//QCISD/6-311G(d,p) calculations6 on the concerted [2 © 2012 American Chemical Society

Table 1. Standard Enthalpies (in kcal/mol) of Coinage Metal Substituted Acetylide Trimerization Computed at the PBE0/ Def2-QZVPP Level acetylide

product

ΔH°

HCCH CuCCH AgCCH AuCCH CuCCCu AgCCAg AuCCAu

C6H6 1,3,5-C6H3Cu3 1,3,5-C6H3Ag3 1,3,5-C6H3Au3 C6Cu6 C6Ag6 C6Au6

−161.5 −120.3 −127.7 −151.9 −83.4 −97.9 −140.9

On thermodynamic grounds, 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds are consistently predicted to be the preferred products of MCCH and MCCM trimerizations, respectively. As in the case of trimerization of ethyne to benzene, all trimerization reactions of coinage metal substituted acetylides are exothermic. The estimated standard enthalpies decrease upon substitution of the hydrogen atoms of acetylene by coinage metal atoms and increase along the Cu-, Ag-, and Au-substituted derivatives. On the basis of the thermodynamic data, the Received: August 9, 2012 Published: October 5, 2012 7206

dx.doi.org/10.1021/om3007695 | Organometallics 2012, 31, 7206−7212

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trimerization of coinage metal substituted acetylides would be expected to be feasible. However, as the respective genuine transition states (TSs) cannot be located and the planar pseudoTSs that possess several imaginary vibrational frequencies are associated with very high reaction barriers, the concerted pericyclic [2 + 2 + 2] mechanism can be ruled out for these reactions. Probably the formation of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds proceeds through a radical mechanism analogous to the radical mechanism proposed for the formation of the benzene molecule. It is worth noting that in the trimerization of dichloroacetylene, ClCCCl, the tentative D3h structure of the C6Cl6 TS showed six imaginary vibrational frequencies at the BLYP/6-311G(d,p) level.6 The same holds true for the D3h structures of the C6M6 TSs studied herein. Attempts to locate the genuine TSs exhibiting lower symmetry have failed. All calculations were performed using the Gaussian 09 program suite.7 The geometries of all stationary points were fully optimized, without symmetry constraints, employing the 1997 hybrid functional of Perdew, Burke, and Ernzerhof8 as implemented in the Gaussian09 program suite.7 This functional uses 25% exchange and 75% correlation weighting and is denoted as PBE0. For the geometry optimizations we have used the Def2QZVPP basis for all atoms. Hereafter the method used in DFT calculations is abbreviated as PBE0/Def2-QZVPP. All stationary points have been identified as minima (number of imaginary frequencies Nimag=0). The natural bond orbital (NBO) population analysis was performed using Weinhold’s methodology.9 Magnetic shielding tensors have been computed with the GIAO (gauge-including atomic orbitals) DFT method10 as implemented in the Gaussian09 series of programs7 employing the PBE0 functional. Nucleus-independent chemical shift (NICS) values were computed at the same level according to the procedure described by Schleyer et al.11 The magnetic shielding tensor element was calculated for a ghost atom, Bq, located at the center of the ring. Negative (diatropic) NICS values indicate aromaticity, while positive (paratropic) values imply antiaromaticity. The LOL (localized orbital locator) plots were obtained employing the Multiwfn software version 2.2.1.12 The LOL function relies on the consideration of the electron kinetic energy density and reveals slow electron regions as compared to the uniform electron gas (LOL > 0.5). The equilibrium structures of the coinage metalated benzenes and the respective monomers optimized at the PBE0/Def2QZVPP level of theory are depicted in Figure 1. Perusal of Figure 1 reveals that the structure of the coinage metalated benzenes closely resembles the structure of the archetype benzene, but with a small expansion of the carbocyclic ring. In both series of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) molecules the expansion of the carbocyclic ring follows the trend Cu > Ag > Au. A similar trend is followed by the CC bond length of the respective MCCH and MCCM monomers. The estimated Wiberg bond orders (WBOs) for the C−C bonds in the MCCH and MCCM monomers (2.891−2.982) illustrate their triple-bond character. The WBO(C−C) values of 1.399− 1.430 in the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) molecules confirm their partial double-bond character. On the other hand, the WBO(M−C) values of 0.605−0.680 for M = Cu, 0.630−0.777 for M = Ag, and 0.889−0.950 for M = Au are indicative of a significant covalent character for the Cu−C and Ag−C bonds and strong covalent interactions for the Au−C bonds. The metal atoms in the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) aromatics and the respective MCCH and MCCM

Figure 1. Equilibrium structures (bond lengths in Ǻ ), natural atomic charges (in red), Wiberg bond Orders (in blue), and six-center bond orders of 1,3,5-C6H3M3, C6M6 (M = Cu, Ag, Au), and the archetype C6H6 aromatics and the respective MCCH and MCCM monomers calculated at the PBE0/Def2-QZVPP level of theory.

monomers acquire positive natural atomic charges of 0.43−0.69, 0.39−0.67, and 0.23−0.44 e in copper, silver, and gold derivatives, respectively. Inspection of the natural atomic charges on M and C atoms of the M−C bonds reveal the contribution of electrostatic interactions is higher in the Cu−C and Ag−C bonds than in the Au−C bonds. According to the six-center bond order (6c-BO) analysis of the coinage metalated benzenes the carbocyclic ring of C6Cu6 is more stable than that of 1,3,5C6H3Cu3 (0.0514 vs 0.0464). However, the opposite holds true for the carbocyclic rings of the silver and gold metalated benzenes. The spatial organization of the bonding mechanism in the coinage metalated benzenes can easily be recognized by the cut-plane localized orbital locator (LOL) representations depicted schematically for C6H6, C6H3Ag3, and C6Ag6 in Figure 2. The LOL representations for the C6H3Cu3, C6Cu6, C6H3Au3 and C6Au6 molecules are analogous (Figure S1, Supporting Information). Identification of LOL basins such as atomic shells, bonds, and lone electron pairs are reflective13 of the nature of the electronic structure of the investigated systems. The slowest electron regions (LOL > 0.5) are located between the bonded atoms which correspond to a typical 2c-2e bonding situation (shown in yellow-red). The pale green LOL basins (LOL ≈ 0.5) characterize faster moving electrons (delocalized electrons) typical of multicenter bonding. Lone pairs are visible as deep red sickles on the atoms. The remaining colors (lighter blue, blue, and deep blue), corresponding to 0.0 < LOL < 0.5, represent regions in space which are increasingly avoided by electrons, such as the space far away from nuclei and the space between the shells of the atoms. 7207


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Figure 2. Cut-plane LOL profiles on the ring plane and on a plane 1 Ǻ above the ring plane of C6H6, 1,3,5-C6H3Ag3, and C6Ag6.

weaker is the electron delocalization on the outer all-metal rings in the C6M6 (M = Cu, Ag, Au) coinage metalated benzenes (pale blue). The most interesting finding is the weak metallophilic interactions that exist in the C6M6 coinage metalated benzenes, as is reflected by the very small WBO(M···M) values of 0.091, 0.091, and 0.112 for M = Cu, Ag, Au, respectively. The weak van der Waals interactions are clearly visualized in the 3D plots of the reduced density gradient (RDG) given in Figure 3.

A significant amount of electron density is accumulated between the M and C atoms of the M−C bonds (0.20−0.40 e), which increases along the series Cu−C < Ag−C < Au−C. This is consistent with the covalent character of the Au−C bond being higher than that of the Ag−C and Cu−C bonds. It is worth noting the delocalized electron density over the six-membered carbocyclic ring shown by the LOL representation on the plane(1) 1 Ǻ above the carbocyclic ring plane (Figure 2). Much 7208


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Figure 3. 3D plots of the reduced density gradient (RDG) (isosurface 0.700) for the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) coinage metalated benzenes and the archetype benzene molecule.

Figure 4. NICSzz-scan profiles of 1,3,5-C6H3M3, C6M6 (M = Cu, Ag, Au), and the archetype C6H6 aromatics calculated at the GIAO/PBE0/Def2QZVPP level of theory.

The isosurfaces show nonbonded overlap (steric repulsions) within the six-membered rings of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) coinage metalated benzenes and the archetype benzene molecule. Moreover, the appearance of small diskshaped RDG domains (shown in green) in the RDG isosurfaces12,14 in the region between the peripheral M atoms of the C6M6 (M = Cu, Ag, Au) coinage metalated benzenes demonstrate the weak interactions (metallophilicity) between the peripheral M atoms. Note the absence of small disk-shaped RDG domains in the region between M and H atoms in the 1,3,5C6H3M (M = Cu, Ag, Au) coinage metalated benzenes, illustrating the absence of M···H interactions in these molecules. The M···M interactions in the C6M6 (M = Cu, Ag, Au) compounds lead to the formation of homocentric six-membered all-metal rings (dual ring systems) which provide a second channel, strictly orthogonal for symmetry reasons to the carbocyclic ring channel, for electron delocalization. The new channel of electron delocalization is evidenced by the NICSzzscan profiles11,15 shown in Figure 4, whereas the most salient features of the NICSzz-scan curves are compiled in Table 2. It can be observed from Table 2 and Figure 4 that the diatropicity (aromaticity) of the 1,3,5-C6H3M3 coinage meta-

Table 2. NICSzz Values (in ppm) Calculated at the Ring Center (NICSzz(0)) and 1.0 Å above the Ring Center (NICSzz(1)) along with the 13C NMR Chemical Shifts (δ, ppm)a for 1,3,5-C6H3M3, C6M6 (M = Cu, Ag, Au), and the Archetype C6H6 Aromatics Calculated at the GIAO/PBE0/ Def2-QZVPP Level of Theory compd

NICSzz(0)

NICSzz(1)

δ(13C−M)

C6H6 C6H3Cu3 C6Cu6 C6H3Ag3 C6Ag6 C6H3Au3 C6Au6

−16.4 −14.9 −39.1 −16.1 −46.9 −13.7 −23.5

−30.4 −25.7 −27.4 −25.2 −28.8 −24.3 −23.4

165.2 236.2 171.5 243.6 158.9 181.9

δ(13C−H) 136.0 161.0 159.4 151.6

a The 13C chemical shifts are equal to the difference between the 13C shielding of the TMS external reference standard (σiso = 187.5 ppm) and the 13C shielding tensors of the molecule.

lated benzenes is lower than the diatropicity of the archetype benzene molecule, probably due to the deformation of the delocalized electron density induced by the coinage metal 7209


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Table 3. Estimated ΔH1, ΔH2, and ΔH3 (in kcal mol−1) Values for the Dissociations (i), (ii), and (iii), Respectively, of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds Computed at the PBE0/Def2-QZVPP Level

substituents (compare the shapes of the π MOs contributing to diatropicity shown in Figure S2 (Supporting Information)). From a phenomenological point of view the coinage metal substituents at the 1,3,5-positions of the six-membered carbocyclic ring render the delocalized π electrons slower than in the benzene aromatic. The slight expansion of the carbocyclic ring in the 1,3,5-C6H3M3 coinage metalated benzenes could also contribute to lowering the diatropicity with respect to the benzene archetype. On the other hand, the fully substituted C6M6 coinage metalated benzenes exhibit a slightly lower π aromaticity associated with the carbocyclic rings relative to the π aromaticity of benzene, probably due to the above-evidenced expansion of the carbocyclic rings (see Figure 1) and an extra ring current attributed to σ delocalization within the array of peripheral coinage metal substituents. The fact that the NICSzz-scan profiles have a minimum in the molecular plane (cf. the NICSzz(0) values) also points to an origin within the σ delocalization for the additional ring current. Furthermore, the strong upfield shifts of the 13C NMR signals of 45.9−107.6 ppm further substantiate the additional σ current present in the fully substituted C6M6 coinage metalated benzenes. A number of MOs support the σ delocalization within the array of peripheral coinage metal substituents (Figure S2). Comparison of the NICSzz-scan profiles of C6Au6 and C6H6 points out once again the behavior of Au is like that of hydrogen.16 It is worth noting that a thorough analysis of the bonding of simple model aurocarbons such as CAu4, C2Au2, C2Au4, C2Au6, and C6Au6 using DFT methods was reported by Pyykkö et al.16a In summary, the fully substituted C6M6 coinage metalated benzenes are new examples of molecular systems displaying two orthogonal ring currents (πtype ring current associated with the carbocyclic ring and σ-type ring current associated with the array of peripheral coinage metal substituents). Let us now examine the stability of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds with respect to the following dissociation processes: 1,3,5‐C6H3M3 → 3MCCH and C6M 6 → 3MCCM

ΔH1a

ΔH2b

ΔH3c

C6H6 1,3,5-C6H3Cu3 1,3,5-C6H3Ag3 1,3,5-C6H3Au3 C6Cu6 C6Ag6 C6Au6

−161.5 −120.3 −127.7 −151.9 −83.4 −97.9 −140.9

−936.1 −825.5 −846.5 −806.1 −715.2 −757.1 −677.5

−1348.4 −1196.1 −1164.4 −1216.2 −1044.0 −980.6 −1085.3

a ΔH1 for dissociation (i). dissociation (iii).

ΔH2 for dissociation (ii). cΔH3 for

Table 4. Principal Singlet−Singlet Optical Transitions in the Absorption Spectra of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) Coinage Metal Benzenes and the Archetype Benzene Molecule Calculated at the TD-DFT-CAM-B3LYP/Def2QZVPP Level compd C6H6 C6H3Cu3

ΔH1 C6H3Ag3

ΔH2 (ii) C6H3Au3

1,3,5‐C6H3M3 → 6C + 3M + 3H and C6M6 → 6C + 6M ΔH3

b

based on the estimated atomization energies follows the trend (including the archetype benzene molecule) C6H6 > 1,3,5C6H3Au3 > 1,3,5-C6H3Cu3 > 1,3,5-C6H3Ag3 > C6Au6 > C6Cu6 > C6Ag6. The principal singlet−singlet optical transitions in the absorption spectra of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) coinage metalated benzenes and the archetype benzene molecule calculated at the TD-DFT-CAM-B3LYP/Def2QZVPP level along with their assignments are compiled in Table 4.

(i)

1,3,5‐C6H3M3 → 3CM + 3CH and C6M6 → 6CM

compd

(iii)

The estimated ΔH values as the difference between the energies of the dissociation products in their ground states and the energies of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds are compiled in Table 3. It can be seen that the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds are predicted to be bound with respect to (i) their dissociation to MCCH or MCCM monomers, (ii) their dissociation to the CM and CH fragments, and (iii) their dissociation to the constituent atoms (atomization energies). Note that the formation of 1,3,5-C6H3M3 (M = Cu, Ag, Au) compounds from the association of the MCCH monomers corresponds to a more exothermic process than the formation of the C6M6 compounds from the association of the MCCM monomers. The same holds true for the formation of the 1,3,5C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds from the association of the CM and CH species. Generally, the stability of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) compounds

C6Cu6

C6Ag6

C6Au6

excitation

λ (nm)

OS, f (au)

HOMO,-1 → LUMO, L+1 H-2,-3 → L+3,+4 HOMO,-1 → LUMO H-5 → LUMO H-4 → L+1,+2 HOMO,-1 → LUMO HOMO,-1 → L+6,+7 H-2,-3 → L+5 HOMO,-1 → L+8 H-2,-3 → LUMO H-2,-3 → L+3,+4 HOMO,-1 → L+7 HOMO,-1 → L+7 HOMO,-1 → L+1,+2 HOMO,-1 → L+3 HOMO,-1 → L+3 HOMO,-1 → L+1,+2 HOMO,-1 → L+3 HOMO,-1 → L+6 H-1,-2 → L+1,+2 H-3,-4 → L+1,+2 H-1,-2 → L+3

173.4 117.8 464.0 395.4 254.6 421.3 251.2 214.9 204.9 391.3 220.4 203.1 193.0 534.0 463.4 393.5 536.3 399.3 320.0 428.9 391.6 312.6

0.618 0.171 0.165 0.011 0.138 0.319 0.200 0.311 0.140 0.214 0.149 0.480 0.226 0.210 0.015 0.035 0.226 0.109 0.366 0.236 0.015 0.008

The TD-DFT-CAM-B3LYP/Def2-QZVPP calculated absorption spectrum of benzene in the gas phase showed two strong absorption bands in the UVC region with peak maxima at 118 and 173 nm. The experimental Sn−S1 absorption spectrum of benzene in the gas phase shows three peaks located at 7.0 eV (177 nm), 7.8 eV (159 nm), and 9.4 eV (132 nm),17 while the 7210


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Figure 5. Absorption spectra of the 1,3,5-C6H3M3 and C6M6 (M = Cu, Ag, Au) coinage metalated benzenes and the archetype benzene molecule calculated at the TD-DFT/CAM-B3LYP/Def2-QZVPP level.

allowed 1Elu transitions computed at the CASPT2F level absorb at 7.06 eV (176 nm).18 The excellent performance of the TDDFT-CAM-B3LYP level for the calculation of the absorption spectrum of benzene is worth noting. The most salient feature of the absorption spectra of the 1,3,5C6H3M3 (M = Cu, Ag, Au) coinage metalated benzenes is the very strong absorption band occurring in the UVB region for 1,3,5-C6H3Au3 and in the visible region for the 1,3,5-C6H3Cu3 and 1,3,5-C6H3Ag3 molecules. The blue shifts of these bands along the series of the 1,3,5-C6H3M3 (M = Cu, Ag, Au) benzenes arise from the electronic transitions associated with translationally allowed excitations that contribute to the induced conventional π current in the carbocyclic rings (Figure 5). Interestingly, the absorption spectra of the fully substituted coinage metalated benzenes are very simple, showing two or three strong absorption bands in the UVA and visible regions of the spectra. The three fully substituted coinage metalated benzenes showed different patterns of the absorption spectra (Figure 5). Thus, C6Cu6 showed one strong and two weaker absorption bands with peak maxima at 534, 464, and 393 nm, respectively. Moreover, the HOMO,-1 → LUMO excitation corresponds to a dipole-forbidden transition absorbing in the visible region (732 nm, f = 0.000 au). It is worth noting that all

the above transitions being translationally allowed (Tx, Ty) give rise15 to the π-type ring current associated with the carbocyclic ring and σ-type ring current associated with the array of peripheral copper substituents. The HOMO and H-1 are mainly localized on the carbocyclic ring, while L+1, L+2, and L+3 are primarily localized on the peripheral array of copper substituents (Figure S2). These transitions could be assigned as charge transfer (CT) transitions. The absorption spectrum of C6Ag6 showed three bands. The band with the highest intensity has a peak maximum at 319 nm, while the weaker bands absorb in the visible region, having peak maxima at 399 and 536 nm, respectively. The last two absorption bands correspond to the absorption bands of the copper compound and along with the first stronger transition are assigned as CT transitions. The HOMO,-1 → LUMO excitation corresponds to a dipole-forbidden transition absorbing in the visible region (778 nm, f = 0.000 au), red-shifted with respect to the analogous band in the copper derivative. Finally, the absorption spectrum of C6Au6 also showed three bands, but in contrast with the spectrum of C6Ag6 the band with the highest intensity has a peak maximum at 429 nm (visible region). The HOMO → LUMO excitation corresponds to a dipole-forbidden transition absorbing in the visible region (687 7211


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nm, f = 0.000 au), blue-shifted with respect to the analogous band in the copper derivative. The most important properties of the coinage metalated benzenes with potential technological interest (application in flexible, lightweight, and low-cost electronic devices such as light emitting diodes (LEDs), field-effect transistors (FETs), lasers, and photovoltaic cells) are the high static polarizabilities and low HOMO−LUMO gaps (Table 5) in comparison to the

compd

C6H3Cu3 C6Cu6 C6H3Ag3 C6Ag6 C6H3Au3 C6Au6

αxx 78.9 (79.16)a [79.50]b 223.0 385.8 251.2 438.3 234.2 373.3

αyy 78.9 (79.16) [79.50] 223.0 386.0 251.2 438.7 234.2 373.3

azz 41.7 (44.13) [42.24] 103.0 155.1 112.0 168.7 113.6 164.0

α̅ 67.3 (67.48) [67.08] 183.0 309.0 204.8 349.7 194.0 303.5

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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εH−εL 7.087 3.692 2.260 3.540 2.058 4.262 2.593

a

Figures in parentheses are experimental values taken from ref 19. Figures in brackets are SDQ-MP4/A theoretical values taken from ref 20.

b

corresponding properties of the archetype benzene molecule and organic polymers used in the fabrication of high-performance optoelectronic devices. Furthermore, the absorption spectra of the coinage metalated benzenes show considerable overlaps with the emission spectra of well-established host materials, thus providing efficient Förster transfer. Inspection of Table 5 reveals the excellent performance of the CAM-B3LYP functional for the calculation of the static polarizabilities of benzene; the CAMB3LYP estimated static polarizabilities are in excellent agreement with the experimental and high-quality theoretical (SDQ-MP4/ A) data available. In summary, the coinage metalated benzenes are predicted to be stable aromatic molecules that can be viably synthesized, exhibiting intriguing properties with potential technological interest. The coinage metal substituents offer synthetic flexibility, for they constitute coordinatively unsaturated metal centers capable of interacting with a variety of ligands, yielding extended and/or dendritic structures. Furthermore, the present findings may not only expand the aromaticity concept to dual-ring systems involving two orthogonal delocalization channels but may also indicate whole classes of new coinage metalated benzenes (mono-, di-, tri-, tetra-, and pentasubstituted), opening a new chemistry for the coinage metalated benzenes and other aromatic hydrocarbons.

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Table 5. Static Polarizabilities α (in au) of the Coinage Metalated Benzenes and the Archetype C6H6 Aromatic Calculated at the CAM-B3LYP/Def2-QZVPP Level along with the HOMO−LUMO Energy Gaps (εH−εL, in eV) C6H6

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and tables giving the complete ref 7, cut-plane LOL profiles of C6H3Cu3, C6Cu6, C6H3Au3, and C6Au6 (Figure S1), 3D plots of the MOs of C6H6, C6M6 (M = Cu, Ag, Au) and C6H3Au3 (Figure S2), Cartesian coordinates and energies (in hartrees), NICS-scan data (Tables S2−S8), and absorption spectra for the “frozen” peripheral M3, H3M3, and M6 fragments (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. 7212


Aromatic Metalated Benzenes C - American Chemical Society

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