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Multidecker Bis(benzene)chromium: Opportunities for Design of Rigid and Highly Flexible Molecular Wires Jun Jiang,† Joshua R. Smith,‡ Yi Luo,*,† Helena Grennberg,*,§ and Henrik Ottosson*,§ Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, 106 91 Stockholm, Sweden, Department of Chemistry, Humboldt State UniVersity, Arcata, California 95521, United States, and Department of Biochemistry and Organic Chemistry, Box 576, Uppsala UniVersity, 751 23 Uppsala, Sweden ReceiVed: October 12, 2010
On the basis of density functional theory calculations, we have designed three classes of multidecker bis(benzene)chromium molecular wires with -(arene-chromium(0)-arene)- sandwich complexes as monomer units. The arene fragments of the wires are either [2.2]paracyclophane (class-1), biphenylene (class-2), or biphenyl (class-3) compounds with two strongly coupled benzene rings. The wires are rigid (class-1) or highly flexible (class-3), and they are realistic synthetic targets as the bonding at each Cr(0) atom satisfies the 18electron rule. The Cr(0) atoms couple strongly with the arene units giving a “quasi-band” that stems from the highest occupied molecular orbital (HOMO) of the monomers, a HOMO sub-band in which the orbitals are highly delocalized indicating metal/π-conjugation. Moreover, the HOMO energies are close to the Fermi energy of the metal electrodes used (Zn(111)), and therefore, injected electrons can easily tunnel through the wires. The metal of the electrodes was selected so that its Fermi level is located slightly above the HOMO energies of the wires. High conductivity and very slow decay of conductance with increased length are found for all three wire classes, making them suitable for molecular electronics applications. Class-2 and class-3 wires display high conformational flexibilities and, simultaneously, only modest conformational dependence of the conductance. These wires therefore function as molecular electrical cords, i.e., molecules which are easily twisted and coiled and for which the conductance displays only modest conformational dependence. SCHEME 1
Introduction Metal wires only one atom wide enjoy a particular attraction to nanotechnology because they represent the thinnest possible electrical cords. Numerous such wires have been studied computationally as well as experimentally.1-4 However, these wires are only realizable at experimental conditions that are hard to access, and the utilization suffers significantly from their low stability and low flexibility. Small molecular scaffolds that keep the metal atoms in arrangements where they still interact, directly or indirectly, and that bring stability and flexibility to the wire are thus highly desirable. The area of one-dimensional molecular transition metal wires has now seen a revival,5-9 as exemplified by trimetal-molecule wires, transistors, and spin filters.9-11 However, a hitherto unexplored criterion for the molecular wire design is to maintain good charge conduction for all available conformations of the molecular wire. Ideally, the conductance of such wires is conformationally independent so that they behave as molecular analogues of macroscopic electrical cords. To design these wires, an intermediate linker unit is required that brings conformational flexibility to the wire and that simultaneously allows the metal atoms to interact strongly. Recently, several examples of transition-metal-polypyridyl wires have been demonstrated.12,13 The photoinduced electron and energy transfer properties of the parent hexacoordinated ruthenium(II) building block is well-known, as are those of a multitude of supramolecular assemblies thereof.14 A drawback * Corresponding author. E-mail:
[email protected]. † Royal Institute of Technology. ‡ Humboldt State University. § Uppsala University.
of these very stable coordination compounds is the lack of direct metal/π-interactions.15 An interesting alternative compound class having direct metal/ π-interaction could be wires based on transition metal sandwich complexes. However, the direct stacking of metals and arenes into -(metal-arene)n- sequences does not lead to the desired stability because the metal atoms in this molecular arrangement are coordinatively unsaturated as the individual repeat units of the -(metal-arene)- do not fulfill the 18-electron rule. This is in stark contrast to the situation in ferrocene and bis(benzene)chromium where the bonding to the metal is coordinatively satisfied (Scheme 1).16 In fact, no -(metal-arene)n- multidecker sandwich complex has been identified that displays long-term persistence at ambient conditions, as opposed to ferrocene and bis(benzene)chromium which are stable when heated above 200 °C. So far, evidence for the formation of cluster compounds with presumable -(metal-arene)n- multidecker sandwich structures comes exclusively from gas-phase studies.17-22 A large number of computational studies have also been performed,23 and it is clear from experiments as well as computations that they have interesting electronic features.21,23 Although it is doubtful if the earlier investigated -(metal-arene)n- wires with coordinatively unsaturated metal
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Jiang et al. In properly designed π-conjugated molecular wires, the conductance can decay very slowly with length,12,35-37 and electrons can even be transported over distances as long as 40 nm.12 The tailored design of such molecular wires is of key importance, and an ideal molecular electronics toolbox should contain a set of different wire types, some rigid with high directionality and some very flexible that are equally conductive in each conformation that they adopt. We now postulate that a set of different oligomeric multidecker sandwich complexes which satisfy the 18-electron rule can provide such a set of wires. To test our hypothesis, we have carried out quantum chemical investigations of the conducting abilities of the three wire classes of Figure 1A, as well as their variation in conductance with conformation. Computational Methods
Figure 1. (A) Three classes of multidecker bis(benzene)chromium wires investigated, in which vectors o-c and o′-c′ represent the molecular axis of the subunits, and (B) electric circuit model of a multidecker wire between Zn(111) electrodes.
atoms can exist as persistent molecules at ambient conditions, and even less be exploited in applications, the general structural motif is highly interesting. In the present study, we have used quantum chemical calculations to design synthetically realistic multidecker sandwich wires which are constructed so that each benzene ring only binds to one Cr(0) atom. Each repeat unit therefore satisfies the 18-electron rule (Scheme 1). If the arene ligands of two adjacent repeat units in addition are strongly coupled electronically, good conduction through the -(arene-metal-arene)n- motifs should result. Three classes of oligomers 1-3 with such motifs were considered (Figure 1A), and it is noteworthy that a few of the shortest class-2 and class-3 oligomers previously have been generated and found to be persistent.24-26 For example, the class-3 dimer, µ-(η6:η6-biphenyl)-bis[(η6-biphenyl)chromium], was stable when sublimed at 180 °C.24 The interactions in class-2 and class-3 wires rely on π-conjugation, whereas the interaction in class-1 wires relies on through-space π-π interaction. Indeed, polymers having alternate [2.2]paracyclophane and flourene repeat units displayed an extended conjugation via the π-π stacking of the [2.2]paracyclophane unit,27 and the charge transport mediated by a [2.2]paracyclophane core inserted in a phenylenevinylene wire is significant and of comparable order to that through a regular π-conjugated benzene.28-30 [2.2]Paracyclophane can also be functionalized,31 as can biphenylene and biphenyl, and this could allow incorporation of our wires into more complex molecular and supramolecular structures. The multidecker bis(benzene)chromium nanowires made up of n units of these three classes of monomers are here termed as [class-1]n, [class-2]n, and [class-3]n. The aryl subunits in these wires are readily available, and they are perfectly stable at ambient conditions and at elevated temperatures.32 Our proposed wires will also have good conformational flexibility for rotation about the arene-metal-arene axis, similar to other sandwich transition metal complexes. For instance, the rotational barrier for ferrocene is found to be about 0.9 and 0.75 kcal/mol in experiments33 and theoretical calculations,34 respectively.
Using the GAUSSIAN03 program package38 and our recently developed central insertion scheme (CIS),39 we have calculated optimal geometries and electronic structures of the designed multidecker bis(benzene)chromium nanowires from 0 to 60 nm, at hybrid density functional theory (DFT) B3LYP level.40 The C and H atoms were described with the 6-31G basis set,41 while the CEP-121G effective core potential and basis set was used to describe the Cr atoms.42 A generalized quantum chemical Green’s function approach,43 implemented in the QCME program (Quantum Chemistry for Molecular Electronics),44 has been used to compute the conducting properties of these molecular wires connected to Zn(111) electrodes at both ends. Our choice of Zn as the electrode material was based on the best match of the HOMO band of the wires with the Fermi level of the metal. It was found that the HOMO levels are well above the Fermi level of gold, and such electrodes would therefore lead to oxidation of our molecular wires. Instead of calculating transport through the oxidized wires, we decided to explore the neutral wires. These will also be significantly more stable than the oxidized ones. To focus on the electron tunneling mechanism through the wires, we propose here weak electrode-molecule couplings. The molecular wires are therefore physisorbed via the hydrogen atoms and the terminal benzene rings to the metal electrodes, as shown in Figure 1B. In this case, the hybridization between electronic structures of the wires and electrodes can be neglected. In principle, our theoretical model considers a system that consists of two semi-infinite electron reservoirs, namely, the source (S) and the drain (D), connected by a molecule (M). The transition matrix element from the source to the drain is written as
T(Ei) )
∑ ∑ VJSVDK ∑ 〈J|η〉〈η|K〉 zi - εη J
K
(1)
η
where J and K run over all atomic sites, which are denoted as 1, 2, ..., N, where sites 1 and N are two end sites of the molecule that connect with two electron reservoirs. VJS (VDK) represents the coupling between atomic site J (K) and reservoirs S (D). Orbital |η〉 is the eigenstate of the Hamiltonian Hf of a finite system with the molecule sandwiched between two clusters of metal atoms. The tunneling current density from state k (Ek) of the source to some state q (Eq) of the drain is
Multidecker Bis(benzene)chromium
iSD )
2π p
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∑ f(Ek) × [1 - f(Eq)]|T|2 × δ(Ek - Eq) k,q
(2) where f(E) is the Fermi distribution function, which is dependent on the applied voltage. The net current through the molecular junction can then be computed in regard to applied bias. Results and Discussion One could reason that all three wires will have similar stability, even though wires of class-1 likely will be the most facile to generate synthetically. The class-1 oligomers will display the lowest conformational flexibility and are therefore the most suitable for probing the potential wire properties. Properties of Class-1 Wires. We define the conformations of the wires as [class-1]n-R, where R represents the angle between the molecular axis of two adjacent segments, i.e., the angle between the vectors o-c and o′-c′ in Figure 1A. The lowest-energy conformation of class-1 is [class-1]n-60 so that the class-1 wires have a helical structure. Furthermore, for [class1]8-60 the computed binding energy between the Cr atoms and [2.2]paracyclophanes (the energy difference between the bonded system and separated ones45) is 89.2 kcal/mol, a sufficiently large value to hinder the dissociation of the multidecker sandwich complexes.45 Starting from [class-1]4-60 and [class-1]8-60, we have computed the energy cost to change the angle R from 0° to 90°, to test the flexibility. For [class-1]8, the barrier for simultaneous rotation about each unit is found to be 2.3 kcal/ mol per unit (Table 1) and is even lower for rotation about a single unit. Therefore, the class-1 wires will be very flexible in 1D space. On the other hand, extensive bending about the Cr atom of the central arene-Cr-arene unit of [class-1]8 is energetically very disfavored (Table 1). The electronic properties of class-1 wires have also been investigated. The calculated frontier molecular orbitals (MO) of the octamer [class-1]8-60 are illustrated in Figure 2. One can see that the highest occupied molecular orbital (HOMO) of [class-1]8-60 is delocalized smoothly, on both the [2.2]paracyclophane segments and the Cr atoms. Thus, the HOMO reveals clear conjugation through the sandwich complexes, which favors electron transport. However, the lowest unoccupied molecular orbital (LUMO) is distributed on the [2.2]paracyclophane segments and with negligible contributions from the Cr atoms. The electronic structure of even longer class-1 wires is represented as the density of states (DOS) of the semi-infinite [class-1]96-60 nanowire (60.9 nm) in Figure 3B. We found the HOMO and LUMO of [class-1]n always to be located around -3.6 and -0.3 eV, respectively. Furthermore, around the HOMO (LUMO) energy, there is an energy sub-band that consists of many occupied (unoccupied) MOs, namely, the HOMO sub-band (LUMO sub-band). The MOs of the HOMO sub-band (LUMO sub-band) have similar distribution patterns to that of HOMO (LUMO). An interesting observation is that the HOMO sub-band of a [class-1]n system (with (n-1) Cr atoms) always consists of 6(n-1) orbitals. The same feature also appears for the [class-2]n and [class-3]n wires (vide infra), suggesting the same high conjugation of their HOMO sub-bands. The Fermi level of the bulk Zn is at approximately -3.63 eV, and our B3LYP/6-31G hybrid DFT calculation on a model molecule of a benzene ring physically adsorbed to the Zn(111) surface suggests a Fermi level at about -3.48 eV. The electrode Fermi level is close to the occupied MOs of the molecular wire. It is expected that the occupied MOs of the HOMO sub-band
TABLE 1: Computed Relative Energies and Conductances at Zero Bias of [Class-1]8 Wires with Different Conformations in Which [Class-1]8-r Represents Rotation Angle r and [Class-1]8-60-B-i Represents the Bending Angle i of [Class-1]8-60a conformation
energy (kcal/mol)
conductance (µS)
[class-1]8-0 [class-1]8-15 [class-1]8-30 [class-1]8-45 [class-1]8-60 [class-1]8-75 [class-1]8-90 [class-1]8-60-B-15 [class-1]8-60-B-30
1.5 1.4 1.0 0.3 0.0 1.3 2.3 7.1 28.6
3.953 2.477 1.956 1.477 1.434 1.041 0.521 0.434 0.217
a Here R represents the angle between the vectors o-c and o′-c′ in Figure 1A, and i represents the bending angle of o-Cr-o′ in Figure 1A.
Figure 2. HOMO and LUMO of all three wires classes (octamers): [class-1]8-60, [class-2]8, and [class-3]8.
will serve as conducting channels for electron transport. Because of the highly conjugated metal/π-system in the HOMO subband, good conductivity can be expected.35-37 With the QCME program,44 the conducting properties of [class-1]n have been examined. The computed conductance at zero external bias for the octamer [class-1]8-60 is 1.434 µS. The conductances of the other conformations ([class-1]8-R) are all close to that of [class1]8-60, and the largest deviation is found for [class-1]8-0 with a zero bias conductance at 3.953 µS. We have also tested the conducting performance of class-1 wires under deformation situations, such as compressing, stretching, mismatching, and bending, and it is demonstrated that small deformation (within 0.4 Å) of one subunit of [class-1]8-60 can at most change the conductance at zero bias from 1.434 to 0.434 µS (see Supporting Information for further details). Thus, in contrast to the fact that conductance of nanowires can change in magnitude due to small changes of geometry,46 the conductance of class-1 wires maintains the magnitude. Consequently, class-1 wires are very flexible for rotation about their principal axis; however, they maintain a stable conducting performance. One of the most important goals of molecular electronics is the search for molecular wires that transport charge over extended distances.35,37,36,47,48 In general, the conductance of a molecular wire decays exponentially with the length, following the formula S ) S0 exp(-βL), where S0 is the prefactor of conductance S, β is the decay factor, and L represents the wire length. Most organic molecular wires have β values larger than 2.0 nm-1.47 For example, our previous calculations with the same
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Figure 3. (A) Conformation of three classes of multidecker bis(benzene)chromium wires. (B) Density of states (DOS) of all three wire classes with semi-infinite length, in which dashed lines represent the Fermi level of the Zn(111) electrodes.
Figure 4. Conductance (in log scale) of the three wires as a function of the wire length.
approach as employed herein gave β values for alkane chains as 2.0-4.0 nm-1 and as 4.0 nm-1 for polyphenyl wires.48 The corresponding experimental studies provided β values of 3.5-14.0 nm-1 for alkane chains and 3.5-6.1 nm-1 for polyphenyl wires.47 Recently, systems with strong π-conjugation have been reported with extraordinarily low β values; e.g., porphyrins35 and oligo-p-phenylenevinylene (oPPV)37 wires can have β values as low as 0.4 and 0.1 nm-1, respectively. To investigate the length dependence of class-1 wires, we have computed the zero bias conductance of a series of wires from [class-1]4 (1.9 nm) to [class-1]30 (18.6 nm), with the results shown in Figure 4. Here, one can clearly see that the conductance length dependence of class-1 wires follows the exponential decay rule. The decay factor β of class-1 wires is found to be about 0.68 nm-1, i.e., a low value which can be ascribed to the highly conjugated metal/π-system. Class-1 wires are therefore ideal candidates for molecular electronics, particularly as they display very good directionality because of their rigid rod structure. Properties of Class-2 and -3 Wires. With the same theoretical methods, class-2 and -3 wires have been investigated in detail. In contrast to class-1, the class-2 wires display 2Dconformational flexibility within each bis(benzene)chromium unit, whereas class-3 wires have full 3D conformational flexibility as there is also the possibility for rotation between the phenyl groups within each biphenyl segment. It is found in
calculations that the geometric minimum of the class-2 and -3 wires corresponds to linear arrangements of the multidecker complexes as displayed in Figure 1A. Most of the electronic properties of class-2 and -3 wires are similar to those of class-1 wires. The binding energy between the Cr atoms and the arene subunits of (class-2)8 and (class-3)8 wires is 90.5 and 88.9 kcal/ mol, respectively, making both of them strongly bonded and realistic. This is also in line with previous syntheses of class-2 and class-3 dimers.24-26 The calculated frontier orbitals of the octamers [class-2]8 and [class-3]8 are displayed in Figure 2, and one can see that the HOMOs are strongly delocalized and involve both metal and arene units, while the LUMOs are only distributed on the arene units. The DOS of the semi-infinite nanowires [class-2]100 (54.1 nm) and [class-3]100 (55.7 nm) are shown in Figure 3B, and they also show features of HOMO sub-bands very close to the Zn Fermi level which suggests that the occupied MOs of the HOMO sub-bands are good conducting channels for electron transport. The [class-2]n systems are 2D flexible as the angles between two adjacent arene-Cr-arene segments can vary from 0 to 90° with an energy cost of less than 1.0 kcal/mol. For the octamer system [class-2]8, the largest deviation of zero bias conductance due to the variations of the angle between two subunits range from 0.033 µS for [class-2]8-0 to 0.023 µS for [class-2]8-45. Thus, the class-2 wires also have a relatively stable conducting performance when the conformations are changed. As noted above, [class-3]n systems have 3D flexibility as they can twist about the phenyl-phenyl linkage within each biphenyl segment. The largest energy costs to change the rotation and the twist angles are found to be less than 1 and 14 kcal/mol, respectively, suggesting a reasonable flexibility. The stability in the conducting performance of class-3 wires has also been demonstrated by calculations for the octamer [class-3]8. The variation of the rotation angles between the subunits can at most change the zero bias conductance of [class-3]8 from 0.363 to 0.436 µS. The largest deviation of the zero bias conductance resulting from changing the twist angle of the phenyl-phenyl linkage of [class-3]8 is 0.242 µS and is observed when the twist angle was set to 30°. Thus, the change of conductance in regard to the twist angle of class-3 wires does not follow the cosinesquared relation obtained in the experimental study of biphenyl junctions,49 implying that electron transport in the multidecker bis(benzene)chromium wires is more complicated than in smaller biphenyl wires.
Multidecker Bis(benzene)chromium Finally, we have computed the zero bias conductance of class-2 and -3 wires with lengths ranging from 1.6 nm ([class2]4 and [class-3]4) to 16 nm ([class-2]30 and [class-3]30), with results given in Figure 4. Here, one can see that the length dependences of all three classes of wires follow the exponential decay rule. The β values of the class-1, -2, and -3 wires are 0.68, 0.68, and 0.79 nm-1, respectively, suggesting that all three wires are nanowires with good conducting performances. Interestingly, the ratio of the β values between class-2 and -3 (0.68: 0.70) equals the ratio of their subunit length (0.547 nm: 0.563 nm), which implies that class-2 and -3 have the same conductance decay factors in regard to the number of monomer units. On the other hand, very different conductance prefactors S0 have been found in the three classes (Figure 4), and these follow the order class-1 > class-3 > class-2, consistent with the order of the conductance of the wires at the same lengths. It is noteworthy that the ratios of the three conductance prefactors S0 are proportional to the square of the DOS of HOMO subbands. Interestingly, all three wires [class-1, -2, -3]n, with the same number of n, have the same number of MOs (6(n-1)) in their HOMO sub-bands. It can be estimated that the DOS of the HOMO sub-band is associated with the reciprocal of the sub-bandwidth. With this assumption, we estimate the ratio of the square of the DOS for class-1, -2, and -3 wires to be 4.4: 1.0:2.0, which has the same intensity order as the conductance prefactors S0 of 20.8:0.76:6.16. Therefore, we conclude that electron tunneling through the occupied MOs of the HOMO sub-bands dominates the electron transport processes in our wires. The low conductance decay factor in regard to length of the multidecker bis(benzene)chromium wires can be ascribed to the highly conjugated π-systems in their HOMO sub-bands. Conclusions and Outlook An ideal molecular wire predicted by quantum chemical calculations should be realistic,50 highly conducting, and either rigid or highly flexible. Those criteria are all fulfilled by the multidecker bis(benzene)chromium wires designed herein through a DFT computational investigation. The multidecker wires should, according to our calculations, be chemically stable to decomposition, have either rigid or high conformational flexibility, and have good and stable conducting performances with regard to conformational variation. Compared to molecular wires such as alkanes and polyphenyl chains, our designed multidecker wires have very low β decay factors with increased length, and thus they represent ideal wires to be used in molecular electronics. We are presently investigating their syntheses. It should be noted that there are ample possibilities for structural variability in designing suitable multidecker bis(benzene)chromium wires as the construction is not restricted to the three arene units mentioned above. The three arene units can be further functionalized, e.g., as recently summarized by Hopf for the [2.2]paracyclophane,31 and there are also options for other linker units. For example, properly chosen terphenylenes or dibenzofulvenes51 instead of the biphenylene and biphenyl linker units of class-2 and -3 wires could allow for wider band gap tuning. The general multidecker sandwich complex motif presented here could thus open up for the tailored design of a large series of different types of realistic molecular wires. Acknowledgment. We are grateful to the Carl Trygger Foundation for a postdoctoral fellowship for JJ and for financial support from the Swedish research council (Vetenskapsrådet) and the Uppsala University KoF initiative in molecular electronics.
J. Phys. Chem. C, Vol. 115, No. 3, 2011 789 Supporting Information Available: Descriptions of how the electronic structure changes with the geometry and length of the class-1, -2, and -3 wires, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Agrait, N.; Yeyati, A. L.; van Ruitenbeek, J. M. Phys. Rep. 2003, 377, 81. (2) Hoffmann, R.; Weissenberger, D.; Hawecker, J.; Stoffler, D. Appl. Phys. Lett. 2008, 93, 043118. (3) Pascula, J. I.; Mendez, J.; Gomezherrero, J.; Baro, A. M.; Garcia, N.; Landman, U.; Luedtke, W. D.; Bogachek, E. N.; Cheng, H. P. Science 1995, 267, 1793. (4) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (5) Bera, J. K.; Dunbar, K. R. Angew. Chem., Int. Ed. 2002, 41, 4453. (6) Berry, J. F.; Cotton, F. A.; Daniels, L. M.; Murillo, C. A. J. Am. Chem. Soc. 2002, 124, 3212. (7) Berry, J. F.; Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 2418. (8) Berry, J. F.; Cotton, F. A.; Lei, P.; Lu, T.; Murillo, C. A. Inorg. Chem. 2003, 42, 3534. (9) Chae, D.-H.; Berry, J. F.; Jung, S.; Cotton, F. A.; Murillo, F. A.; Yao, Z. Nano Lett. 2006, 6, 165. (10) Tsai, T.-W.; Huang, Q.-R.; Peng, S.-H.; Jin, B. Y. J. Phys. Chem. C 2010, 114, 3561. (11) Georgiev, V. P.; McGrady, J. E. Inorg. Chem. 2010, 49, 5591. (12) Tuccitto, N.; Ferri, V.; Cavazzini, M.; Quici, S.; Zhavnerko, G.; Licciardello, A.; Rampi, M. A. Nat. Mater. 2008, 8, 41. (13) Flores-Torres, S.; Hutchinson, G. R.; Soltzberg, L. J.; Abrua, H. D. J. Am. Chem. Soc. 2006, 128, 1513. (14) Armaroli, N. Photochem. Photobiol. Sci. 2003, 2, 73. (15) (a) Chae, D.-H.; Berry, J. F.; Jung, S.; Cotton, F. A.; Murillo, C. A.; Yao, Z. Nano Lett. 2006, 6, 165. (b) Berry, J. F.; Cotton, F. A.; Lu, T.; Murillo, C. A.; Roberts, B. K.; Wang, X. J. Am. Chem. Soc. 2004, 126, 7082. (16) Tolman, C. A. Chem. Soc. ReV. 1972, 72, 337. (17) Schildcrout, S. M. J. Am. Chem. Soc. 1973, 95, 3846. (18) Kurikawa, T.; Negishi, Y.; Hayakawa, F.; Nagao, S.; Miyajima, K.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 1998, 120, 11766. (19) Kurikawa, T.; Takeda, H.; Hirano, M.; Judai, K.; Arita, T.; Nagao, S.; Nakajima, A.; Kaya, K. Organometallics 1999, 18, 143. (20) Nagao, S.; Kato, A.; Nakajima, A.; Kaya, K. J. Am. Chem. Soc. 2000, 122, 4221. (21) Miyajima, K.; Muraoka, K.; Hashimoto, M.; Yasuike, T.; Yabushita, S.; Nakajima, A. J. Phys. Chem. A 2002, 106, 10777. (22) Hosoya, N.; Takegami, R.; Suzumura, J.-i.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, A. J. Phys. Chem. A 2005, 109, 9. (23) (a) Wang, L.; Cai, Z. X.; Wang, J. Y.; Lu, J.; Luo, G. F.; Lai, L.; Zhou, J.; Qin, R.; Gao, Z. X.; Yu, D. P.; Li, G. P.; Mei, W. N.; Sanvito, S. Nano Lett. 2008, 8, 3640. (b) Zhang, X.; Wang, J.; Gao, Y.; Zeng, X. C. ACS Nano 2009, 3, 537. (c) Wu, J.-C.; Wang, X.-F.; Zhou, L.; Da, H.-X.; Lim, K. H.; Yang, S.-W.; Li, Z.-Y. J. Phys. Chem. C 2009, 113, 7913. (d) Zhu, L.; Wang, J. J. Phys. Chem. C 2009, 113, 8767. (e) Xu, K.; Huang, J.; Lei, S.; Su, H.; Boey, F. Y. C.; Li, Q.; Yang, J. J. Chem. Phys. 2009, 131, 104704. (f) Zhang, X.; Ng, M.-F.; Wang, Y.; Wang, J.; Yang, S.-W. ACS Nano 2009, 3, 2515. (g) Wu, X.; Zeng, C. J. Am. Chem. Soc. 2009, 131, 14246. (h) Martı´nez, J. I.; Garcı´a-Lastra, J. M.; Lo´pez, M. J.; Alonso, J. A. J. Chem. Phys. 2010, 132, 044314. (i) Yi, Z.; Shen, X.; Sun, L.; Shen, Z.; Hou, S.; Sanvito, S. ACS Nano 2010, 4, 2274. (j) Shen, L.; Jin, H.; Ligatchev, V.; Yang, S.-W.; Sullivan, M. B.; Feng, Y. Phys. Chem. Chem. Phys. 2010, 12, 4555. (k) Zhang, Z.; Wu, X.; Guo, W.; Zeng, X. C. J. Am. Chem. Soc. 2010, 132, 10215. (l) Huang, J.; Li, Q.; Xu, K.; Su, H.; Yang, J. J. Phys. Chem. C 2010, 114, 11946. (24) Elschenbroich, C.; Heck, J. J. Am. Chem. Soc. 1979, 191, 6773. (25) Ceccon, A.; Gambaro, A.; Romanin, A. M.; Venzo, A. J. Organomet. Chem. 1982, 239, 345. (26) Elschenbroich, Heck, J.; Massa, W.; Birkhahn, Chem. Ber. 1990, 123, 2331. (27) Morisaki, Y.; Chujo, Y. Bull. Chem. Soc. Jpn. 2004, 78, 288. (28) Seferos, D. S.; Trammell, S. A.; Bazan, G. C.; Kushmerick, J. G. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8821. (29) Wu, S. M.; Gonzalez, M. T.; Huber, R.; Grunder, S.; Mayor, M.; Schonenberger, C.; Calame, M. Nature Nanotechnol. 2008, 3, 569. (30) Nguyen, P.; Gomez-Elipe, P.; Manners, I. Chem. ReV. 1999, 99, 1515. (31) Hopf, H. Angew. Chem., Int. Ed. 2008, 47, 2. (32) The pyrolysis of biphenyl at 420-465 °C produces benzene, terphenyls, and quaterphenyls, over an activation barrier of 72.1 kcal/mol as found by: Proksch, E.; Strigl, A.; Wagner-Loeffler, M.; Szinovatz, W.
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