An inclusion compound is a complex in which one component (the ‘host’) forms the cavities either by hydrogen bonding or polymerization in which other components( the ‘guest’) are fixed. Two important types of inclusion complexes are the channel(canal) complexes and the clathrate (cage) complexes. Urea forms an inclusion complex with straight-chain alkanes. Urea crystallizes in a hexagonal lattice, in the presence of guest molecules and traps them in the cavity.
Channel complexes:
Channel complexes are those where the host crystallizes in a form with parallel, approximately cylindrical channels in which molecules of the guest are enclosed lengthwise. Urea normally forms a crystal structure that is closely packed, but in the presence of various straight-chain molecules, e.g., n-alkanes, n-alcohols, n-acids, n-esters, etc. urea crystallizes in a more open structure that contains long channels enclosing the guest molecules. Branched-chain and cyclic structures cannot ‘fit’ into these channels, and so this property affords a means of separating straight-chain from branched-chain compounds. The formula of the channel complexes is AnX, where ‘ A ‘ is the host molecule, ‘n‘, usually not a whole number, has values of 4 or more and increases as the length of B increases. It is important to note that if a molecule is sufficiently long, then ‘small’ side chains will not prevent such a molecule from behaving as a guest.
2. Clathrates:
When quinol is crystallized from a solution in water saturated with SO2, a quinol- SO2 complex is obtained. It has the molecular formula of the type nC6H4(OH)2⋅Z, where Z is one guest molecule and the value of ‘ n ‘ for the host molecule can vary over a range. Powell et al(1948), using x-ray analysis found that in these complexes the quinol molecules were linked together through hydrogen bonds to form giant molecules. The cavities in which enclosed the guest molecules. Powell named these complexes clathrates and showed that the cages must be large enough to contain the guest molecule and they must be so arranged that the guest molecules do not escape. These clathrates are stable, but the imprisoned molecules may be released either by melting or by means of an organic solvent that dissolve quinol.
Figure 1.09: Diagrammatic representation of clathrate formation and its decomposition.
Characteristic properties of Inclusion complexes :
These complexes are decomposed by melting or by dissolving in a suitable solvent.
Channel complex formation has been used to resolve racemic modifications, and also to separate geometrical isomers.
X-ray analysis of these channel complexes has shown that the diameter of the channel is about 5.0 A0.
There is no chemical affinity between the host and guest molecules in the inclusion compounds.
Examples:
1. Schiessler et al(1957) have shown that 2-methyldecane forms a channel complex with urea.
2. Cyclodextrin forms an orange-yellow crystal of inclusion complex with ferrocene when a solution of both compounds in a 2:1 ratio in water is boiled for 2 days and then allowed to stand for 10 hours at room temperature.
Charge Transfer Complexes:
Charge transfer complexes exist in different types of molecules, organic as well as inorganic, and in all phases of matter i.e. in solids, liquids, and even gases. e.g. Many d- and polynitro compounds form colored complexes with various aromatic hydrocarbons, phenols, etc. These complexes usually contain components in a 1:1 ratio. In inorganic chemistry, most of the charge-transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer band in transition metal complexes results from the movement of electrons between molecular orbitals(MO’s) that are predominantly metal in character and those that are predominantly ligand in character.
Complexes of this type are called ‘charge-transfer complexes‘ or ‘electron-donor-acceptor-complexes’ in which attraction between the components occurs due to an electronic transition into an excited electronic state, such that a fraction of electron charge is transferred between the components. The resulting electrostatic attraction stabilizes the molecular complex. The source molecule from which the charge is transferred is called the ‘electrons donor’ and the molecule which receives the charge is called ‘electrons acceptor, hence the alternate, ‘electron-donor-acceptor-complexes‘. The Colour of the complex develops due to the transfer of an electron from the highest occupied π – MO of one component to the lowest unoccupied π – MO of the other component. A charge-transfer complex may be regarded as a resonance hybrid of two contributing structures; a non-bonded (DA) held together by van der Waal forces and (D++A−), the bonding in which is very weak.
Let us consider the charge-transfer complex formed from 1,3,5-trinitrobenzene and mesitylene. X-ray analysis has shown that the two rings lie in parallel planes and overlap(partial). One molecule behaves as an electron-donor(D) and the other as an electron-acceptor(A). D has a low ionization potential and A has a low-energy vacant orbital and a high electron affinity. Owing to their spatial arrangements, overlap occurs between the MO’s, resulting in the transfer of electrons from the highest occupied π – orbital of (D) to the lowest unoccupied π – orbital of (A).
2. Quinhydrones are a group of colored substances formed from quinones and another aromatic compound. They are believed to be charge-transfer complexes. e.g. phenoquinone (structure I) is a 2:1 molar red charge-transfer complex, in which phenol acts as the electron donor and the quinone as the electron acceptor. but 1:1 molar adducts are also known. e.g. structure (II) is a 1:1 molar quinone-phenol charge-transfer complex.
3. An intense purple-colored iodine-starch complex is also an example of a charge-transfer complex.
Localized and Delocalized bonds :
Bonds in which the orbitals involved in overlapping are almost in fixed positions are called ‘localized bonds’ Sigma( σ) bonds formed by the overlapping of s−s,s−p, hybrid orbital −s, and hybrid orbital −p orbitals of the two atoms in a molecule, are localized bonds.
Whereas, Pi(π) bonds formed by the side-wise overlapping of p-p or p-d orbitals of the two adjacent atoms in a molecule are ‘delocalized bonds. A sigma bond is stronger than a pi bond. e.g., four C−H,σ – bonds formed by the overlapping of half-filled sp sp3 hybrid orbitals of a carbon atom with four half-filled 1 s atomic orbitals of hydrogens are localized bonds.
Figure 1.10: Formation of four sp3-s, localized C−H, σ – bonds in methane.
In 1,3-butadiene, each carbon atom is sp2 hybridized. The sp2 hybrid orbitals overlap with each other and with s- orbitals of the hydrogen atoms to form C−C, and C−H,σ− bonds. Since the bonds result from the overlap of sp2 hybrid orbitals, all carbon and hydrogen atom lie in one plane. In addition to these C−C, C−H, σ-bonds, each carbon atom in 1-3-butadiene possesses an unhybridized p – orbitals that are perpendicular to the plane of σ-bonds. The p – orbital of C−2 overlaps with the p – orbitals of C-1 and C−3 similarly, the p-orbital of C−3 overlaps with the p-orbitals of C−2 and C−4 to form two π-bonds. The overlap of p – orbitals of C−2 and C−3 in both directions, which allows the π-electrons to be spread over a large area due to delocalization. This delocalization of π-electrons is responsible for the greater stability of 1,3 -butadiene.
Figure 1.11: Formation of sp2−sp2, and sp2−s, localized C−C and C−H,σ – bonds and delocalized C-C, π – bonds in 1,3-butadiene.
