Extended metal atom chains

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Extended metal atom chains (EMACs) are molecules that consist of a linear string of directly bonded metal atoms, surrounded by organic ligands. They can be seen as the ultimate miniaturized versions of macroscopic wires, in which the piece of metal has been scaled down to a chain of single atoms. Because of their conducting properties, EMACs are considered useful as molecular wires or devices in a bottom-up approach to nanoelectronics.[1]

Contents

[edit] Structure

An EMAC molecule contains a linear string of transition metals (typically Cr, Co, Ni, or Cu) that are bonded to each other and are surrounded helically by four long organic molecules. The metal chains are capped at the ends by two anions, usually halides. The four organic ligands are made of repeating pyridylamido units, which contain nitrogen donor atoms. Each metal atom is six-coordinate, bonded to two other metals along the axis of the molecule (except terminal metals, which are bonded to one metal and one capping anion) and to four nitrogen atoms perpendicular to the axis.

During the synthesis of EMACs the organic ligands act as templates that bring individual metal ions together and align them into a linear string. The number of nitrogen atoms in the ligand determines exactly the number of metal atoms that will be incorporated into the chain. Thus, the synthesis yields molecular wires of very high purity and exact stoichiometry, which means that the total length of the molecule can be controlled with atomic precision. This feature, in combination with the fact that the molecules have well-defined ends, differentiates EMACs from other kinds of inorganic molecular wires: EMACs exist only as distinct molecular entities, they do not aggregate and they do not form periodic structures of repeating units.

Most known EMACs contain from three to nine metal atoms. The longest EMACs that have been constructed so far incorporate nine Ni atoms and have a length of approximately 2 nanometers, while it is estimated that chains with up to 17 metal atoms (4-5 nanometers) are readily accessible with currently available ligands.[2]

[edit] Early development and debate

The first EMACs with three metal atoms were synthesized in the early 1990's independently by the groups of Shie-Ming Peng (NTU) and F. Albert Cotton (Texas A&M), who coined the term extended metal atom chains. The cobalt-cointaing molecule Co3(dpa)4Cl2 (dpa=dipyridylamide) was synthesized by both research groups, but each proposed a different structure: the group from Taiwan reported an unsymmetric structure with a long and a short Co-Co bond, whereas the Texas group identified a symmetric structure with equal Co-Co bond lengths. This disagreement sparked a controversy that lasted for years, until it was realized that both forms of the molecule actually exist simultaneously. While this led to the realization that the compound can be used as a molecular switch, it also created a new problem since none of the recognized types of isomerism could explain the existence of a molecule in two structural forms that differ only in the length of one or more bonds (and not in their stereochemistry or connectivity of the atoms). The problem was finally resolved through a quantum chemical study by Pantazis and McGrady, who showed that the two structural forms result from different electronic configurations.[3] The Pantazis-McGrady model is currently used to understand the different electronic states and interpret the magnetic properties of EMACs.

[edit] Applications

The delocalized nature of the bonding along the metal string facilitates electron transport from one end of the molecule to the other, hence EMACs can be used as electrical conductors in nanocircuits. Moreover, conductance can be controlled and fine-tuned by oxidation or reduction of the metal chain, opening the way for the construction of molecular rheostats and switches or transistors. These possibilities have been demonstrated in two major practical applications:

  • single-molecule transistors incorporating the trinuclear dipyridylamido compounds Cu3(dpa)4Cl2 and Ni3(dpa)4Cl2 (dpa=dipyridylamide), fabricated on oxidized silicon substrates with aluminum gate electrodes.[4]
  • stochastic switches made of penta- and heptachromium EMACs attached to a gold surface.[5]

[edit] See also

[edit] References

  1. ^ F. Albert Cotton, Carlos A. Murillo and Richard A. Walton (eds.), Multiple Bonds Between Metal Atoms, 3rd edition, Springer (2005).
  2. ^ S.-M. Peng, C.-C. Wang, Y.-L. Jang, Y.-H. Chen, F.-Y. Li, C.-Y. Mou, M.-K. Leung (2000) “One-dimensional metal string complexes”, J. Magn. Magn. Matter, vol. 209, pp. 80-83. doi:10.1016/S0304-8853(99)00650-2 .
  3. ^ D. A. Pantazis, J. E. McGrady (2006) “A three-state model for the polymorphism in linear tricobalt compounds”, J. Am. Chem. Soc., vol. 128, pp. 4128-4135. doi:10.1021/ja0581402.
  4. ^ D.-H. Chae, J. F. Berry, S. Jung, F. A. Cotton, C. A. Murillo, Z. Yao (2006) “Vibrational Excitations in Single Trimetal-Molecule Transistors”, Nano Letters, vol. 6, pp. 165-168. doi:10.1021/nl0519027.
  5. ^ I-W. P. Chen, M.-D. Fu, W.-H. Tseng, J.-Y. Yu, S.-H. Wu, C.-J. Ku, C.-h. Chen, S.-M. Peng (2006) “Conductance and Stochastic Switching of Ligand-Supported Linear Chains of Metal Atoms”, Angew. Chem. Int. Ed., vol. 45, pp. 5814-5818. doi:10.1002/anie.200600800.