Structures and Electronic Properties of Si-Substituted Benzenes and

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Structures and Electronic Properties of Si-Substituted Benzenes and Their Transition-Metal Complexes Vaisakh Mohan† and Ayan Datta*,‡ †

National Institute of Science Education and Research, Institute of Physics Campus, Bhubaneswar-751005, Orissa, India and School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Thiruvananthapuram-695016, Kerala, India

‡

ABSTRACT Structural and electronic properties for a series of silicon-substituted benzenes (CnSimH6, where n = 0-6, m = 0-6, and n þ m = 6) are studied through density functional theory calculations. Benzene is found to sustain its planarity up to two Si substitutions for all isomers. For three Si substitutions, only the 1,3,5alternate structure (6) is planar, while for four Si substitution, only the 2,3,5,6 structure (10) is planar. Further Si substitution makes all the isomers for the rings nonplanar, which eventually leads to the fully puckered C3v structure for hexasilabenzene (13). The reorganization energies for these molecules are sufficiently low to be favorably utilized for hole conduction. All the molecules form very stable full-sandwich and half-sandwich complexes of the type η6-(CnSimH6)2Cr and η6-(CnSimH6)Cr(CO)3. The binding energies for these complexes increase with increase in the number of Si atoms in the rings. Strategies are proposed for experimental design of extended sheets of silicenes and mixed C/Si graphenes through transition-metal complexation of the six-membered rings. SECTION Dynamics, Clusters, Excited States

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he chemistry and physics of new forms of carbon have been in the frontline of research in nanotechnology for the last two decades.1,2 Interest in carbon-based materials arises primarily because of their easy tailoribility, rational structural design, and versatile state of aggregation at various length and time scales.3,4 Contrary to the huge interest in carbon-based nanomaterials, the chemistry of silicon, belonging to the same group in the periodic table, is not very well studied.5 The lack of interest in Si-based nanomaterials maybe attributed to the unavailability of planar sp2-like bonding environment. Si prefers an sp3 bonding environment. Hence, ideal two-dimensional sheets of silicenes undergo puckering in the six-membered rings to form ordered ripples (unpublished results). Thus, the basic building block of fullerenes, graphenes, and nanotubes, namely, benzene, is mostly confined to the carbon family. However, the past few years has witnessed a renewed interest in Si-based nanomaterials for applications as electronic and optical materials. All-Si fullerenes and quantum dots of various nuclearities and shapes have been synthesized.6,7 Many new structures of all-Si clusters have been proposed based on computational studies mainly through density functional theory (DFT) calculations.8,9 However, a rational design for truly all-Si-based nanomaterials requires a bottomup approach starting from a fundamental unit like benzene. The all-Si analogue of benzene, hexasilabenzene, is well characterized through computational studies.10,11 Unlike benzene, it has a puckered C3v geometry. The puckering distortion is a consequence of the weak Si-Si π-bonding, which results

r 2009 American Chemical Society

in the electron-electron interactions being overcome by the electron-nuclear interactions and a strong Jahn-Teller distortion. It is interesting to study the trade-off between the planarity of the six-membered ring and the ratio of C and Si for mixed carbo-sila benzenes. In this manuscript, we critically study the effect of substitution of C by Si in the six-membered π-framework to their structural and electronic properties. We also propose mechanisms for stabilizing such partially Si/all-Si rings through complexation to transition metals such as Cr. We predict the structures and stabilities of all the Si analogues of silico-transition-metal complexes. The structures for all the clusters were optimized at the DFT level using the hybrid B3LYP functional at the 6-31þG(d) basis set level. All the calculations are performed using the Gaussian 03 set of codes.12 Frequency calculations were performed for removal of vibrational instabilities in all the structures. In Figure 1, the structures for the various clusters considered in the study are shown. Benzene (1) with a D6h symmetry still retains it planarity with a single substitution of C by a Si atom (2). A search for such a molecule in the Cambridge Crystallographic Database (CCDC)13 leads to five structures.14 Interestingly, all the structures have a planar six-membered motif, suggesting reliability in the level of our calculations. The computed (experimental)15 C-Si bond-lengths are found to Received Date: October 2, 2009 Accepted Date: November 2, 2009 Published on Web Date: November 10, 2009

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DOI: 10.1021/jz900080q |J. Phys. Chem. Lett. 2010, 1, 136–140

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energy isomers. Of them, only the 1,3,5-alternately substituted structure (6) is found to be planar. The origin for the nonplanarity of the 1,2,3-substituted isomer (7) and the 1,2,4substituted isomer (8) is traced to the stronger steric repulsion between the bulkier Si atoms, when nearest neighbor to each other. This is also supported by the larger mean puckering angle, j = 9.31° in 7 (two nearest-neighbor Si-Si interactions) compared to j = 5.41° in 8 (one nearest-neighbor Si-Si interaction). The stabilities of the isomers decrease in the following order: 6 (0.0 kcal/mol) > 7 (þ4.8 kcal/mol) > 8 (þ11.5 kcal/mol). The greater stability of 6 is understood on the basis of its planar structure, which allows delocalizations similar to borazine, albeit lesser than benzene. For the tetra-substituted benzenes, the stabilities of the isomers decrease in the following order: 11 (0.0 kcal/mol, j = 15.3°) > 9 (þ4.2 kcal/mol, j = 18.03°) > 10 (þ9.5 kcal/mol, j = 0°). Interestingly, even though 10 has a perfectly planar structure, it is the least stable of all the isomers. This suggests that in tetrasubstituted isomers, delocalization of π electrons across the ring is minimal, and even a planar structure is insufficient to ensure stability through aromaticity. The lack of aromaticity 10 is easily understood on the basis of the very long Si 3 3 3 Si bond of 2.22 Å, which breaks π conjugation across the verticle mean plane of the ring. The penta-siliconsubstituted benzene (12) is nonplanar with j = 26.0°. The hexa-Si-substituted benzene (13) has chairlike distorted structure with j = 37.1°. It is interesting to study the effect of Si substitution in benzene on its reorganization energies (λ).18 The hole and electron reorganization energies (λhole and λelectron) are defined as λhole [λelectron] = ionization energy (vertical) - ionization energy (adiabatic) and [|electron affinity (vertical) electron affinity (adiabatic)|], respectively. λhole (λelectron) for the molecules are 1 (0.15 eV, 0.11 eV), 2 (0.08 eV, 0.18 eV), 3 (0.09 eV, 1.10 eV), 4 (0.12 eV, 2.53 eV), 5 (0.06 eV, 1.24 eV), 6 (0.17 eV, 0.54 eV), 7 (0.18 eV, 0.79 eV), 8 (0.10 eV, 2.45 eV), 9 (0.09 eV, 0.61 eV), 10 (0.12 eV, 0.38 eV), 11 (0.29 eV, 0.53 eV), 12 (0.16 eV, 0.48 eV), and 13 (3.44 eV, 0.43 eV), respectively. Benzene, 1, has an amphiphilic behavior with comparable λhole and λelectron. However, most of the Si-substututed benzenes (with the exception of hexasilabenzene, 13) have smaller λhole, suggesting that partial doping of benzene with Si makes the molecules predominatingly hole conductors. 13 is a poor hole conductor but a moderate electron conductor. The origin of the large (small) cation (anion) reorganization energies in hexasilabenzene is understood on the basis of the delocalization of the π-electrons in the rings. For the cation, the C3v puckering distortions lead to poor overlap between the 3pz orbitals in comparison to that for benzene (as seen in Figure 2), which increases the λhole. However, for the anion, the additional electron is delocalized over five Si atoms, which stabilizes the anion and thus reduces the λelectron. One of the well-established strategies to stabilize new molecules is through the coordination of the molecules as ligands to transition metals through the formation of organometallic complexes. Benzene forms stable full-sandwich and half-sandwich organometallic complexes like η6- (C6H6)2Cr and η6-(C6H6)Cr(CO)3.19 The exceptional stability of these complexes is understood on the basis of the 18-electron rule,

Figure 1. Minimum energy structures for six-membered rings for homogeneous and heterogeneous C/Si nuclearities.

be 1.774 Å (1.765(4) Å and 1.770(4) Å), indicating equal C-Si bonds in even the experimental molecules. Also, a bond-length of ∼1.77 Å indicates that the C-Si bonds essentially have an intermediate character between a double bond and single bond (∼CdSi = 1.70 Å and ∼C-Si = 1.89 Å) and the π electrons are fairly well delocalized along the ring. This is also supported by almost all equal computed (experimental) C-C bond lengths of 1.402 Å and 1.405 Å (1.381(6)1.399(6) Å). Previous computational study of the nucleusindependent chemical shift (NICS) (at the GIAO-SCF/6-31þ G(d) level) in 2 are also indicative of its strong aromatic behavior.16,17 Substituting the second silicon in benzene leads to three different isomers, namely, the ortho (3), meta (4), and para (5) forms. Interestingly, all the three isomers are planar, and the stabilities follow the order: 4 (0.0 kcal/mol) > 3 (þ0.9 kcal/mol) > 5 (þ11.4 kcal/mol). Such a variation in the stability for the isomers is understood on the basis of the lower stability of conjugation along a SidSi bond compared to that of a CdC bond. For the meta isomer, no-conjugation exists between the two Si atoms, while for both the ortho and para forms, π-conjugation is allowed, the extent of conjugation being more for the para isomer. For the trisubstituted benzene, C3Si3H6, various structures are considered, and the structures 6, 7, and 8 are the lowest


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Figure 4. Ground-state minimum energy structures for the halfsandwich complexes for (CnSimH6)Cr(CO)3 (n þ m = 6; n = 0-6; m = 0-6).

Figure 2. HOMO of benzene (I) and SOMO of (II) hexasilabenzene cation and (III) hexasilabenzene anion.

theory, which has been shown to be quite satisfactory for calculation of metal complexes.21 The binding energies for 14, 15, 16, 17, 18, 19, and 20 are -12.5, -29.2, -41.3, -50.0, -51.9, -53.1, and -58.2 kcal/mol, respectively. Interestingly, in all the complexes, the most stable isomers correspond to nearest-neighbor presence of the Si atoms. For example, the ortho-isomer complex (16) is more stable than the meta or the para isomers. Also, Si atoms on the top and bottom rings of the full-sandwich complexes prefer closer proximity, and thus, in all the complexes other than 18, fully eclipsed orientation of the Si-substituted rings is preferred. For example, the eclipsed conformer (15) is 3.0 kcal/mol more stable than its staggered form. The exceptional structure of 18 with twisted alternate rings arises as a result of its skewed structure that leads to steric repulsion in the fully eclipsed conformer. For the all-Si-substituted full-sandwich complex (20), the chairlike structure of the Si6H6 is conserved. For the half-sandwich complexes, the binding energies for 21, 22, 23, 24, 25, 26 and 27 are -51.8, -58.8, -63.6, -66.6, -68.5, -69.0, and -69.7 kcal/mol, respectively. Similar to the full-sandwich complexes, the chair-type structure of Si6H6 is also maintained in (Si6H6)Cr(CO)3 (27). In Figure 5, the profiles for the variation in the binding energies in the full and half-sandwich complexes with the sequential substitution of C by Si in the rings are shown. The halfsandwich complexes are more stable than the full-sandwich complexes for the same degree of substitution. This is understood based on the fact that metal (Cr(0)) to ligand (CnSimH6) backbonding increases in the half-sandwich complexes as a result of the strong σ-donating and π-accepting abilities of the three carbonyl groups. Also, the sequential substitution of C by Si stabilize the complexes. Thus, Si-substituted benzenes form more stable complexes with the transition metals than their organic counterparts. In conclusion, we have for the first time, performed an extensive search for various possible structures of sila-substituted benzenes. Interestingly, benzene can easily maintain

Figure 3. Ground-state minimum energy structures for the full-sandwich complexes for (CnSimH6)2Cr (n þ m = 6; n = 0-6; m = 0-6).

leading to the formation of a closed shell configuration.20 We have modeled structures for similar complexes using the mixed C-Si rings and the all-Si-rings. All the possible conformations for the Si-substituted rings in complexes were considered. Those reported in Figure 3 and Figure 4 for the full and half-sandwich complexes, respectively, are the lowest energy geometries. The binding energies for the complexes are calculated as ΔE (full-sandwich) = Ecomplex - 2Eligand ECr(0) and ΔE (half-sandwich) = Ecomplex - Eligand - ECr(CO)3. The lowest energy spin-state for the Cr(0) atom corresponds to a septet state (S = 3), while Cr(CO)3 is diamagnetic in the ground state. All the calculations for the transition-metal complexes are performed at the B3PW91/LANL2DZ level of


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Figure 5. Variation in the binding energies (in kcal/mol) with sequential substitution of C by Si in full and half sandwich complexes (n = 0 represents conventional organometallic complexes; n = 6 represents all-silicon organometallic complexes).

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its planarity for substitution of two C by Si. However, as the number of Si-atoms increases in the ring, its starts to pucker. The hole reorganization energies for these molecules are quite comparable to that of benzene, suggesting that extended materials designed from these motif might be encouraging hole conduction materials. All these complexes form very stable complexes with transition metals. On the basis of our computation study, we predict that doping silabenzene rings with transition metals might be the ideal strategy to stabilize new low-dimensional silicenes and mixed Si/C graphenes. We look forward for an experimental realization of our suggestions.

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SUPPORTING INFORMATION AVAILABLE Cartesian coordinates, energies, harmonic frequencies for Si/C derivatives of various nuclearities, and complete ref 12. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. E-mail: ayan@ iisertvm.ac.in.

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ACKNOWLEDGMENT A.D. thanks DST Fast Track Scheme (Govt.

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of India) for partial research funding. (14)

REFERENCES (1)

(2)

(15)

(a) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191. (b) Chen, F.; Tao, N. J. Electron Transport in Single Molecules: From Benzene to Graphenes. Acc. Chem. Res. 2009, 42, 429–438. (c) Allen, M. J.; Tung, V. C.; Karner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. [Online early access]. DOI: 10.1021/ cr900070d. Published Online: July 17, 2009. http://pubs.acs.org/ doi/abs/10.1021/cr900070d (accessed September 20, 2009). (a) Castro Neto, A. H.; Guinea, F.; Peres, N. M.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109–162. (b) Westervelt, R. M. Graphene


(16)

(17)

139

Nanoelectronics. Science 2008, 320, 324–325. (c) Service, R. F. Carbon Sheets an Atom Thick Give Rise to Graphene Dreams. Science 2009, 324, 875–877. Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C.Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. Endo, M.; Iijima, S.; Dresselhaus, M. S. Carbon Nanotubes; Pergamon: Oxford, 1997. Organosilicon Chemistry: From Molecules to Materials; Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim, Germany, 2005; Vols. 1-6. (a) Belomoin, G.; Alsalhi, M.; Aqi, A. A.; Nayfeh, M. H. X-ray Structure Factors for Si-Nanoparticles. J. Appl. Phys. 2004, 95, 5019–5022. (b) Zhang, X.; Brynda, M.; Britt, R. D.; Carroll, E. C.; Larsen, D. S.; Louie, A. Y.; Kauzlarich, S. M. Synthesis and Characterization of Manganese-Doped Silicon Nanoparticles: Bifunctional Paramagnetic-Optical Nanomaterial. J. Am. Chem. Soc. 2007, 129, 10668–10669. (a) Vaic, M. R.; Sprujit, E.; Lagen, B. v.; Cola, L. D.; Zuilhof, H. Alkyl-Functionalized Oxide-Free Silicon Nanoparticles: Synthesis and Optical Properties. Small 2008, 4, 1835– 1841. (b) Liao, Y.-C.; Roberts, J. T. Self-Assembly of Organic Monolayers on Aerosolized Silicon Nanoparticles. J. Am. Chem. Soc. 2006, 128, 9061–9065. (c) Li, Z. F.; Swihart, M. T.; Ruckenstein, E. Luminesecent Silicon Nanoparticles Capped by Conductive Polyaniline through the Self-Assembly Method. Langmuir 2004, 20, 1963–1971. (a) Golcoechea, J. M.; Sevov, S. C. Naked Deltahedral Silicon Clusters in Solutions: Synthesis and Characterization of Si93and Si52-. J. Am. Chem. Soc. 2004, 126, 6860–6861. (b) Singh, A. K.; Kumar, V.; Briere, T. M.; Kawazoe, Y. Cluster Assembled Metal Encapsulated Thin Nanotubes of Silicon. Nano Lett. 2002, 2, 1243–1248. (a) Yoo, S.; Zhao, J.; Wang, J.; Zeng, X. C. Endohedral Silicon Fullerenes SiN (27 e N e 29). J. Am. Chem. Soc. 2004, 126, 13845–13849. (b) Hossain, D.; Hagelberg, F.; Pittman, C. U.; Saebo, S. Structures and Stabilities of Clusters of Si12, Si18, and Si20 Containing Endohedral Charged and Neutral Atomic Species. J. Phys. Chem. C 2007, 111, 13864–13871. Nagase, S.; Teramae, H.; Kudo, T. Hexasilabenzene (Si6H6). Is the Benzene-Like D6h Structure Stable? .J. Chem. Phys. 198786, 45134517 Clabo, D. A., Jr.; Schaefer, H. F., III. The Silicon Analog of Benzene - Hexasilabenzene (Si6H6). J. Chem. Phys. 1986, 84, 1664–1669. Frisch, M. J. et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2003. Allen, F. H. The Cambridge Structural Database: A Quarter of a Million Crystal Structures and Rising. Acta. Cryst. B 2002, 58, 380–388. CCDC codes for the crystals: GILPAT, KEVRAE, NEYNUA, QIZTIC, and YAZQAS. Wakita, K.; Tokitoh, N.; Okazaki, R.; Takagi, N.; Nagase, S. Crystal Structure of a Stable Silabenzene and its Photochemical Valence Isomerization into Corresponding Silabenzvalene. J. Am. Chem. Soc. 2000, 122, 5648–5649. Tokitoh, N.; Wakita, K.; Okazaki, R.; Nagase, S.; Schleyer, P. V. R.; Jiao, H. A. A Stable Neutral Silaaromatic Compound, 2-{2,4,6-Tris[bis(trimethylsilyl)methyl]phenyl}-2-silanaphthalene. J. Am. Chem. Soc. 1997, 119, 6951–6952. (a) Miyajima, K.; Nakajima, A.; Yabushita, S.; Knickelbein, M. B.; Kaya, K. Ferromagnetism in One-Dimensional Vanadium-Benzene Sandwich Clusters. J. Am. Chem. Soc. 2004, 126, 13202–13203. (b) Miyajima, K.; Yabushita, S.;


pubs.acs.org/JPCL

(18)

(19) (20)

(21)

Knickelbein, M. B.; Miyajima, K. Stern-Gerlach Experiments of One-Dimensional Metal-Benzene Sandwich Clusters: Mn(C6H6)m (M = Al, Sc, Ti, and V). J. Am. Chem. Soc. 2007, 129, 8473–8480. (c) Rehaman, A.; Datta, A.; Mallajosyula, S. S.; Pati, S. K. Quantifying Aromaticity at the Molecular and Supramolecular Limits: Comparing Homonuclear, Heteronuclear, and H-Bonded Systems. J. Chem. Theory Comput. 2006, 2, 30–36. (d) Datta, A.; Mallajosyula, S. S.; Pati, S. K. Nonlocal Electronic Distribution in Metallic Clusters: A Critical Examination of Aromatic Stabilization. Acc. Chem. Res. 2007, 40, 213–221. (e) Datta, A.; Pati, S. K. New Examples of Metalloaromatic Al-Clusters: (Al4M4)Fe(CO)3 (M = Li, Na and K). Chem. Commun. 2005, 5032–5034. (f) Datta, A.; Pati, S. K. Limit to Puckering of Benzene with Sterically Crowded Molecules: Hexaferrocenylbenzene. Chem. Phys. Lett. 2006, 433, 67–70. (a) Deng, W. -Q; Goddard, W. A., III. Prediction of Hole Mobilities in Oligoacene Organic Semiconductor from Quantum Mechanical Calculations. J. Phys. Chem. B 2004, 108, 8614–8621. (b) Datta, A.; Mohakud, S.; Pati, S. K. Electron and Hole Mobilities in Polymorphs of Benzene and Naphthalene: Role of Intermolecular Interactions. J. Chem. Phys. 2007, 126, 144710–144717. Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley: New York, 2009. Huheey, J. E.; Keiter, E.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and Reactivity, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 1997. Szabo, K. J.; Hupe, E.; Larsson, A. L. E. Stereoelectronic Control on the Kinetic Stability of β-Acetoxy-Substituted (η3-Allyl)palladium Complexes in a Mild Acidic Medium. Organometallics 1997, 16, 3779–3785.


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Structures and Electronic Properties of Si-Substituted Benzenes and

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