In organometallic chemistry, extended metal atom chains (EMACs) are molecules that consist of a linear chain of directly bonded metal atoms, surrounded by organic

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

s. These compounds represent the smallest

molecular wire Molecular wires (or sometimes called molecular nanowires) are molecular chains that conduct electric current. They are the proposed building blocks for molecular electronic devices. Their typical diameters are less than three nanometers, while th ...

s. Such species are researched for the bottom-up approach to

nanoelectronics Nanoelectronics refers to the use of nanotechnology in electronic components. The term covers a diverse set of devices and materials, with the common characteristic that they are so small that inter-atomic interactions and quantum mechanical ...

, although no applications are near term.

Structure

An EMAC molecule contains a linear string of

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that c ...

s (typically Cr, Co, Ni, or Cu) that are bonded to each other and surrounded helically by organic

s. The metal chains are usually end-capped by anions, usually

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a f ...

s. The organic ligands often pyridylamide,

pyridone Pyridone may refer to several organic compounds with the formula : * 2-Pyridone * 3-Pyridone *4-Pyridone 4-Pyridone is an organic compound with the formula . It is a colorless solid. The compound exists in equilibrium with a minor tautomer, pyr ...

naphthyridine Diazanaphthalenes are a class of aromatic heterocyclic chemical compounds that have the formula C8H6N2. They consist of a naphthalene double ring in which two of the carbon atoms have been replaced with nitrogen atoms. There are ten positional isom ...

, or their derivatives. 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. The organic ligands template the formation of the chains by bringing the metal ions together and aligning them into a linear string. The number of nitrogen atoms in the ligand determines the number of metal atoms that will be incorporated into the chain. Thus, the synthesis yields molecular wires of predetermined length. This feature, in combination with the fact that the molecules have well-defined ends, differentiates EMACs from other kinds of 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 eleven Ni atoms and have a length of approximately 2 nanometers, although it is estimated that chains with up to 17 metal atoms (4-5 nanometers) could be accessed with currently available ligands. In contrast with EMACs, linear chain compounds are infinite in length. They are not terminated with capping ligands.

Early development and debate

The first EMACs with three metal atoms were synthesized in the early 1990s independently by the groups of

Shie-Ming Peng Shie-Ming Peng (; born 2 March 1949) is a Taiwanese chemist. Peng attended National Hsinchu Senior High School and graduated from National Taiwan University prior to completing a doctorate from the University of Chicago. He returned to teach at NTU ...

(

NTU Ntu is a village in Nagaland, India. NTU may stand for: * National Taxpayers Union, a nonprofit organization in the United States * National Team Unity, a political party in Trinidad and Tobago * National Television Company of Ukraine, a Ukrainian ...

) and

F. Albert Cotton Frank Albert Cotton FRS (April 9, 1930 – February 20, 2007) was an American chemist. He was the W.T. Doherty-Welch Foundation Chair and Distinguished Professor of Chemistry at Texas A&M University. He authored over 1600 scientific articles. C ...

(

Texas A&M Texas A&M University (Texas A&M, A&M, or TAMU) is a public university, public, Land-grant university, land-grant, research university in College Station, Texas. It was founded in 1876 and became the flagship institution of the Texas A&M Unive ...

), who coined the term ''extended metal atom chains''. The cobalt-containing molecule Co₃(dpa)₄Cl₂ (dpa =

2,2'-dipyridylamide 2,2′-Dipyridylamine is an organic compound with the formula (C5H4N)2NH. It consists of a pair of 2-pyridyl groups (C5H4N) linked to a secondary amine. The compound forms a range of coordination complexes. Its conjugate base, 2,2′-dipyridyla ...

) 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 debate 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 In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

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 configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom ...

s. The Pantazis-McGrady model is currently used to understand the different electronic states and interpret the

magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...

properties of EMACs.

Potential applications

EMACs have no commercial applications, but they are of potential use as

electrical conductor In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gen ...

s in

nanocircuits Nanocircuits are electrical circuits operating on the nanometer scale. This is well into the quantum realm, where quantum mechanical effects become very important. One nanometer is equal to 10−9 meters or a row of 10 hydrogen atoms. With suc ...

. Moreover, conductance can be controlled and fine-tuned by

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

or reduction of the metal chain, opening the way for the construction of molecular

rheostat A potentiometer is a three- terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...

es, and

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s. These possibilities have been demonstrated: * "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 A gate or gateway is a point of entry to or from a space enclosed by walls. The word derived from old Norse "gat" meaning road or path; But other terms include '' yett and port''. The concept originally referred to the gap or hole in the wal ...

electrodes. * "stochastic switches" made of penta- and heptachromium EMACs attached to a gold surface.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. .

References

{{Reflist Nanotechnology

Structure

Early development and debate

Potential applications

See also

References