COORDINATION CHEMISTRY &
CATION- BINDING HOSTS
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Presentation Outline
Coordination chemistry
Crown ethers
Other ethers
Summary of content
Selectivity of cation complexation
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Today`s Discussion
It`s crucial to know and understand the concepts!
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The study of supramolecular complexes of metal cations is the coordination of relatively labile metal ions and relatively elaborate ligands.
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1-A. LIGANDS
- A ligand is an ion or a molecule which is bonded to the central atom(s) in a complex compound.
- Ions or molecules which act as ligands are Lewis bases (electron donors; often lone pair donors) and are capable of independent existence.
a ligand
central atom
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In [Fe(CN)6]3−, the cyanide ligands would be removed as CN- because this has a closed valence electron shell containing 8 electrons (both for N and C).
As a result, the oxidation state of the cyanide is thus –1, or (-I) and that of ion is +3, or (III). These are usually written as CN(-I) and Fe(III).
1-B. OXIDATION STATE
In [Cu(EDTA)]2−, the oxidation state of EDTA is -4, thus the oxidation state of copper is +2 or Cu(II).
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1-C. BONDING
Bonding in coordination complexes ranges from entirely ionic ion-dipole type interactions in which a ligand lone pair of electrons forms a dative bond to a positively charged metal cation to entirely covalent in which there is significant orbital overlap between metal and ligand valence orbitals.

Classification:
Small, highly charged metal ions, and those with closed valence shells such as Na+ or Al3+ tend to form more ionic compounds.

Metals in lower oxidation states or those with unfilled sub-shells such as the transition metals tend to form more covalent complexes.
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1-D. COORDINATION NUMBER
Definition: coordination number (CN) is the number of atoms or ligating groups (such as chloride Cl-) bound to a metal.

CN = 4
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C2v
Heptafluorotantalate (TaF72−) Heptafluoroniobate (NbF72−)
1-D. COORDINATION NUMBER
1-D. COORDINATION NUMBER
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Coordination numbers 10 and 11 are unique to complexes involving lanthanides and actinides.
Coordination number 12 has a structure that is involved in boron chemistry, the icosahedron.
Coordination number 15 is the highest reported coordination number currently, being described for [PbHe15]2+
Answer: {3, 5}
1-E. HARD-SOLF ACID BASE THEORY
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Hard acids: the acceptor atom is of high positive charge, small size, and does not have easily excited outer electrons, i.e. non-polarizable.
Examples: H+, Al3+, Sn4+ (high oxidation states of the transition metals)
Soft acids: the acceptor atom is of low positive charge, large size, and has several easily excited outer electrons, i.e. it is polarizable.
Examples: Ag+, Cu+, Tl+, Hg2+ (low oxidation states of the transition metals)
Hard bases: the donor atom is of low polarizability, high electronegativity, hard to reduce, and associated with empty orbitals of high energy and hence inaccessible. Examples: F-, OH-
Soft bases: the donor atom is of high polarizability, low electronegativity, easily oxidized and associated with empty, low-lying orbitals.
Examples: I-, Ph3P, R2S, H-.
The Principle of HSAB states that HAs form more stable complexes with HBs while SBs form more stable complexes with SAs.
In the point of view of molecular orbitals, it terms hard molecules have a large gap between HOMO and LUMO, whereas soft molecules have a small HOMO-LUMO gap.
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1-E. HARD-SOLF ACID BASE THEORY
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1.F. OPTICAL ISOMERISM
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1.F. OPTICAL ISOMERISM
Theory: Chiral compounds synthesized from achiral starting materials and reagents are generally racemic (i.e. a 50:50 mixture of enantiomers). Resolution is the separation of racemates into their component enantiomers. Since enantiomers have identical physical properties, such as solubility and melting point, resolution is extremely difficult. Diastereomers, on the other hand, have different physical properties, and this fact is used to achieve resolution of racemates. Reaction of a racemate with an enantiomerically pure chiral reagent gives a mixture of diastereomers, which can be separated.
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1.F. OPTICAL ISOMERISM
Application: Resolution of tris(ethylenediamine)cobalt(III) chloride
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2. CROWN ETHERS
The crown ethers are among the simplest and most appealing macrocyclic ligands, and are ubiquitous in supramolecular chemistry as hosts for both metallic and organic cations.
They consist simply of a cyclic array of ether oxygen atoms linked by organic spacers, typically —CH2CH2— groups.
The discovery of this crown ether gained a Nobel prize in 1987 for Charles Pedersen.
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In trying to carry out the synthesis of the linear diol (3.3) as a ligand for the catalytic vanadyl ion, Pedersen carried out the reaction in Scheme 3.2. Unknown to Pedersen, his starting material was slightly contaminated by some free catechol (3.2). The resulting product was a mixture of the desired compound (3.3) along with a small amount of dibenzo[18]crown-6, formed in only 0.4 % yield. Then, he isolated and characterized this small amount of by-product.
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The compound dissolved sparingly in methanol, but its solubility was enhanced significantly on addition of alkali metal salts. He concluded that ‘the potassium ion had fallen into the hole at the centre of the molecule’. Pedersen gave the name ‘crown ethers’ because of the crown-like shape of the capsular complex.
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Pedersen described a total of six different methods of crown ether synthesis:
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3. OTHER ETHERS
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Depends on the purposes, we need to synthesize the corresponding compounds:
For example, if we want to capture only Cu+ but on Na+ or Ca2+ ions in the cell, we need to design a probe which can selectively bind to Cu+ but not the other ions.
Another example is in complex extraction, to specifically extract Fe3+ out of solution of organic solvent, a metal chelator such as [EDTA4-] may be a good choice. Since the binding affinity of it to Fe3+ is very high, and moreover, the complex formed is [Fe(EDTA)]- which is charged and not soluble in organic solvent. Thus, it can easily be separated.
Also, the below figure shows stability of different ether complexes which is also a factor to choose a proper ether.




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One application of such ether structure appears in fluorescent probes
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4. SELECTIVITY OF CATION COMPLEXATION
Size complementarity between cation versus host cavity.
Electronic complementarity between the cation and host binding sites (cf. HSAB).
Electrostatic charge.
Solvent (polarity, hydrogen bonding and coordinating ability.
Degree of host preorganization
Enthalpy and entropy contributions to the cation–host interaction
Cation and host free energies of solvation.
Nature of the counter-anion and its interactions with solvent and the cation
Cation binding kinetics.
Chelate ring size and donor group orientation.
A successful host exhibits a strong affinity for one particular guest and a much lower affinity. The selectivity is governed by an enormous number of factors as listed below:
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The size complementarity between cation versus host cavity is one of the major factors:
Coordination chemistry is the study of supramolecular complexes.
A ligand is an ion or a molecule which is bonded to the central atom, in cation binding complex, it can be a crown ether, podands, lariats or other types of ethers.
Coordination number of complexes are various, from small numbers such as 2, 4, 6, to even very higher numbers 10, 12, 15.
HSAB theory gives an idea of which couple of metal and ligand forms can form a complex.
Coordination complex can be chiral. The enantiomers can be separated by resolution.
Crown ethers, podands, lariats and many other types of ethers are synthesized for selective binding to specific metal ions.
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SUMMARY OF CONTENT
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