Extended metal atom chains
Encyclopedia
Extended metal atom chains (EMACs) are molecules that consist of a linear string of directly bonded metal atoms, surrounded by organic ligand
s. These compounds represent the smallest molecular wires. Although such species have no applications, they are researched for the bottom-up
approach to nanoelectronics
.
s (typically Cr
, Co
, Ni
, or Cu
) that are bonded to each other and are surrounded helically by four long organic ligand
s. The metal chains are capped at the ends by anions, usually halide
s. The four organic ligand
s 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.
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 nine 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.
) and F. Albert Cotton
(Texas A&M
), who coined the term extended metal atom chains. The cobalt-containing molecule Co3(dpa)4Cl2 (dpa = 2,2'-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 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 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. The Pantazis-McGrady model is currently used to understand the different electronic states and interpret the magnetic
properties of EMACs.
s 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, switch
es, and transistor
s. These possibilities have been demonstrated:
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. These compounds represent the smallest molecular wires. Although such species have no applications, they are researched for the bottom-up
Top-down and bottom-up design
Top–down and bottom–up are strategies of information processing and knowledge ordering, mostly involving software, but also other humanistic and scientific theories . In practice, they can be seen as a style of thinking and teaching...
approach to nanoelectronics
Nanoelectronics
Nanoelectronics refer to the use of nanotechnology on electronic components, especially transistors. Although the term nanotechnology is generally defined as utilizing technology less than 100 nm in size, nanoelectronics often refer to transistor devices that are so small that inter-atomic...
.
Structure
An EMAC molecule contains a linear string of transition metalTransition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s (typically Cr
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, Co
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, Ni
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, or Cu
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
) that are bonded to each other and are surrounded helically by four long organic ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. The metal chains are capped at the ends by anions, usually halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s. The four organic ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s are made of repeating pyridylamido units, which contain nitrogen donor atoms. Each metal atom is six-coordinate
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
, 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 nine 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.
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 (NTUNTU
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 television broadcaster...
) and F. Albert Cotton
F. Albert Cotton
Frank Albert Cotton was the W.T. Doherty-Welch Foundation Chair and Distinguished Professor of Chemistry at Texas A&M University. He authored over 1700 scientific articles. Cotton was recognized for his research on the chemistry of the transition metals.-Education:Frank Albert Cotton was born on...
(Texas A&M
Texas A&M University
Texas A&M University is a coeducational public research university located in College Station, Texas . It is the flagship institution of the Texas A&M University System. The sixth-largest university in the United States, A&M's enrollment for Fall 2011 was over 50,000 for the first time in school...
), who coined the term extended metal atom chains. The cobalt-containing molecule Co3(dpa)4Cl2 (dpa = 2,2'-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 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 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
Quantum chemistry
Quantum chemistry is a branch of chemistry whose primary focus is the application of quantum mechanics in physical models and experiments of chemical systems...
study by Pantazis and McGrady, who showed that the two structural forms result from different electronic configurations. The Pantazis-McGrady model is currently used to understand the different electronic states and interpret the magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
properties of EMACs.
Potential applications
EMACs have no commercial applications, but they are of potential use as electrical conductorElectrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
s in nanocircuits
Nanocircuitry
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 such progressively smaller circuits, more can be fitted on...
. Moreover, conductance can be controlled and fine-tuned by oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
or reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of the metal chain, opening the way for the construction of molecular rheostats, switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es, and transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
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 gateField-effect transistorThe field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
electrodes. - "stochastic switches" made of penta- and heptachromium EMACs attached to a gold surface.