Metallacrown
Encyclopedia
Metallacrowns are a unique class of macrocyclic compounds
that consist of metal ions and solely or predominantly heteroatoms in the ring. Classically, metallacrowns contain an [M-N-O] repeat unit in the macrocycle. First discovered by Prof. Vincent L. Pecoraro and Myoung Soo Lah in 1989, metallacrowns are best described as inorganic analogues
of crown ether
s. To date, over 600 reports of metallacrown research have been published. Metallacrowns with sizes ranging from 8-MC-4 to 60-MC-20 have been synthesized.
.
, i.e. by dissolving the ligand in a solvent followed by the desired metal salt.
The first reported metallacrown was MnII(OAc)2(DMF)6[12-MCMn(III)N(shi)-4]. Metallacrowns can be prepared with a variety of metals in the ring and in a variety of ring sizes. In recent years, many other metallacrowns have been prepared, including 9-MC-3, 15-MC-5, and 18-MC-6. Ring size is controlled by a number of factors, such as the geometry of the ligand chelate ring, ring metal Jahn-Teller distortion
, central metal size, steric effects
, and stoichiometry
. Common ring metals have included V(III), Mn(III), Fe(III), Ni(II) and Cu(II). Hydroxamic acid
s, such as salicylhydroxamic acid
, and oximes are commonly utilized in metallacrown ligands.
. Metallacrowns typically contain fused chelate rings in their structure, which imparts them with substantial stability. Metallacrowns have been synthesized with substantial variety. Mixed ligand and mixed ring-metal, and mixed-oxidation state metallacrowns are known. Inverse metallacrowns have been reported that contain metal ions oriented towards the center of the ring. Metallacryptates, metallahelicates, and fused metallacrowns are known. Among the interesting features of metallacrowns are the similarities between certain structures and the corresponding crown ether. For example, in the 12-C-4, the cavity size is 2.79 Å and the bite distance is 0.6 Å. In the 12-MC-4, the cavity size is 2.67 Å and the bite distance is 0.5 Å.
as the central metal are potential MRI contrast agents. A lot of attention is focused on metallacrown molecular recognition
and host-guest chemistry
. Chelation
of heavy metals by 15-MC-5 complexes could be utilized in lanthanide separation or heavy metal sequestration. Metallacrown container molecules constructed from the 15-MC-5 structure type have been shown to selectively encapsulate carboxylate anions in hydrophobic cavities. A crystalline solid displaying second-harmonic generation was generated by including a nonlinear optical
chromophore in a chiral metallacrown compartment. Metallacrowns have also been utilized in the construction of microporous
. and mesoporous materials. In another potential application, some metallacrowns exhibit antibacterial activity.
Macrocycle
A macrocycle is, as defined by IUPAC, "a cyclic macromolecule or a macromolecular cyclic portion of a molecule." In the chemical literature, organic chemists may consider any molecule containing a ring of nine or more atoms to be a macrocycle...
that consist of metal ions and solely or predominantly heteroatoms in the ring. Classically, metallacrowns contain an [M-N-O] repeat unit in the macrocycle. First discovered by Prof. Vincent L. Pecoraro and Myoung Soo Lah in 1989, metallacrowns are best described as inorganic analogues
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
of crown ether
Crown ether
Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH2CH2O-. Important members of this series are the tetramer , the pentamer , and the hexamer...
s. To date, over 600 reports of metallacrown research have been published. Metallacrowns with sizes ranging from 8-MC-4 to 60-MC-20 have been synthesized.
Nomenclature
Metallacrown nomenclature has been developed to mimic the nomenclature of crown ethers, which are named by the total number of atoms in the ring, followed by “C” for “crown,” and the number of oxygen atoms in the ring. For example, 12-crown-4 or 12-C-4 describes Figure 2a. When naming metallacrowns, a similar format is followed. However, the C becomes “MC” for “metallacrown” and the “MC” is followed by the ring metal, other heteroatom, and the ligand used to make the metallacrown. For example, the metallacrown in Figure 2b is named [12-MCFe(III)N(shi)-4], where “shi” is the ligand, salicylhydroxamic acidSalicylhydroxamic acid
Salicylhydroxamic acid is a drug that is a potent and irreversible inhibitor of bacterial and plant urease usually used for urinary tract infections. The molecule is similar to urea but is not hydrolyzable by the urease enzyme. It is also a trypanocidal agent...
.
Preparation
Metallacrowns form via self-assemblySelf-assembly
Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction...
, i.e. by dissolving the ligand in a solvent followed by the desired metal salt.
The first reported metallacrown was MnII(OAc)2(DMF)6[12-MCMn(III)N(shi)-4]. Metallacrowns can be prepared with a variety of metals in the ring and in a variety of ring sizes. In recent years, many other metallacrowns have been prepared, including 9-MC-3, 15-MC-5, and 18-MC-6. Ring size is controlled by a number of factors, such as the geometry of the ligand chelate ring, ring metal Jahn-Teller distortion
Jahn-Teller effect
The Jahn–Teller effect, sometimes also known as Jahn–Teller distortion, or the Jahn–Teller theorem, describes the geometrical distortion of non-linear molecules under certain situations. This electronic effect is named after Hermann Arthur Jahn and Edward Teller, who proved, using group theory,...
, central metal size, steric effects
Steric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
, and stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
. Common ring metals have included V(III), Mn(III), Fe(III), Ni(II) and Cu(II). Hydroxamic acid
Hydroxamic acid
A hydroxamic acid is a class of chemical compounds sharing the same functional group in which an hydroxylamine is inserted into a carboxylic acid. Its general structure is R-CO-NH-OH, with an R as an organic residue, a CO as a carbonyl group, and a hydroxylamine as NH2-OH. They are used as metal...
s, such as salicylhydroxamic acid
Salicylhydroxamic acid
Salicylhydroxamic acid is a drug that is a potent and irreversible inhibitor of bacterial and plant urease usually used for urinary tract infections. The molecule is similar to urea but is not hydrolyzable by the urease enzyme. It is also a trypanocidal agent...
, and oximes are commonly utilized in metallacrown ligands.
Structure
Many structures have been characterized by single-crystal X-ray crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
. Metallacrowns typically contain fused chelate rings in their structure, which imparts them with substantial stability. Metallacrowns have been synthesized with substantial variety. Mixed ligand and mixed ring-metal, and mixed-oxidation state metallacrowns are known. Inverse metallacrowns have been reported that contain metal ions oriented towards the center of the ring. Metallacryptates, metallahelicates, and fused metallacrowns are known. Among the interesting features of metallacrowns are the similarities between certain structures and the corresponding crown ether. For example, in the 12-C-4, the cavity size is 2.79 Å and the bite distance is 0.6 Å. In the 12-MC-4, the cavity size is 2.67 Å and the bite distance is 0.5 Å.
Potential applications
Metallacrowns are most widely studied for their potential use as SMMs(single molecule magnets). Notably, the first mixed manganese-lanthanide SMM was a metallacrown. Metallacrowns with gadoliniumGadolinium
Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
as the central metal are potential MRI contrast agents. A lot of attention is focused on metallacrown molecular recognition
Molecular recognition
The term molecular recognition refers to the specific interaction between two or more molecules through noncovalent bonding such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, electrostatic and/or electromagnetic effects...
and host-guest chemistry
Host-guest chemistry
In supramolecular chemistry, host-guest chemistry describes complexes that are composed of two or more molecules or ions that are held together in unique structural relationships by forces other than those of full covalent bonds. Host-guest chemistry encompasses the idea of molecular recognition...
. Chelation
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....
of heavy metals by 15-MC-5 complexes could be utilized in lanthanide separation or heavy metal sequestration. Metallacrown container molecules constructed from the 15-MC-5 structure type have been shown to selectively encapsulate carboxylate anions in hydrophobic cavities. A crystalline solid displaying second-harmonic generation was generated by including a nonlinear optical
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
chromophore in a chiral metallacrown compartment. Metallacrowns have also been utilized in the construction of microporous
Microporous material
A microporous material is a material containing pores with diameters less than 2 nm.Porous materials are classified into several kinds by their size. According to IUPAC notation A microporous material is a material containing pores with diameters less than 2 nm.Porous materials are...
. and mesoporous materials. In another potential application, some metallacrowns exhibit antibacterial activity.