Organometallic and coordination chemistry of carbon nanomaterials

Carbon is the element of life. Because of its ability to form a multitude of stable bonds with other carbon atoms and many other elements in the periodic table an enormous and structurally diverse library of organic compounds can be formed. As a consequence carbon provides the chemical basis for life on Earth. Stable covalent bonds are also responsible for the rich tapestry of inorganic forms of carbon which are the carbon allotropes. The two classical forms, namely, graphite and diamond allotropes have been known to mankind for a very long time. Only in the last few decades new synthetic carbon allotropes such as fullerenes, carbon nanotubes and graphene have been discovered. The hexagonal network of graphene sheets or nanotube cylinders, or the polyhedra of fullerenes consisting of pentagons and hexagons of carbon are highly aesthetically pleasing, providing inspiration both to artists and scientists. The shape of carbon nanotubes also influences their outstanding physical properties, such as mechanical strength, electrical and heat conductance that are already outperforming many traditional materials. Optical properties and the electron acceptor behaviour of fullerenes make them excellent components for photovoltaic devices. Nanodiamonds show promise in photoluminescence and surface coatings. The electronic properties of atomically thin structures of graphene have recently unleashed a wave of discoveries important for future nanoelectronics and spintronics.

Transition metals in form of nanoparticles are the catalysts of the nanotube formation process, while transition metal surfaces are essential for the formation of graphene. In both cases metal–carbon interactions are controlling the structures and thus the electronic properties of the carbon nanostructures being formed. Similarly, in order to harness their functional properties the inorganic nanocarbons must be connected or interfaced with the macro world. For instance, field effect transistors exploiting the charge mobility and electronic band gap of carbon nanotubes rely on the interface between the carbon atoms of the nanotube and the metal nanoelectrodes connecting the device. The type and quality of the carbon nanotube–metal bonding crucially determine the functional properties of such transistors, sometimes dominating over the intrinsic properties of the nanotube itself.

Individual metal atoms, cations, clusters or even nanoparticles coupled to carbon nanostructures introduce new properties either through covalent metal–carbon bonds or coordination bonds via a ligand group, charge transfer, electrostatic or van der Waals interactions. Many of these properties are not present in all-carbon materials. For example, fullerenes and nanotubes are able to bind metals either directly through M–C bonds or via functional groups containing nitrogen or oxygen donor atoms, or can entrap metal atoms in the internal cavities of the fullerene cages, as elegantly exemplified with the endohedral metallofullerenes. Metals can bring magnetic, optical and redox activities into the carbon nanostructures, thus broadening the spectrum of their practical applications. Endohedral metallofullerenes, for example, can offer fascinating magnetic and photoluminescent properties, not available in metal-free fullerenes, which can be exploited in magnetic resonance imaging and quantum information processing respectively. Considering that many transition metals exhibit excellent catalytic properties, metal nanoparticles anchored on carbon nanotube surfaces are utilised as new heterogeneous catalysts, whereas metal clusters embedded within nanotube cavities give rise to nano-reactors where chemical processes can be controlled with exquisite precision yielding new molecular products, inaccessible by other means.

All the exciting aspects of carbon nanostructures mentioned above ultimately rely on the fundamental principles of inorganic chemistry which describe the mechanisms of bonding with metals. While the chemistry of carbon nanostructures is still hindered by many practical challenges, including their structural complexity, polydispersity and lack of purity, understanding of interactions and bonding with metals is already making big steps forward and opening up new horizons for these fascinating materials.

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