Introduction
BIP with imino-C and imino-N substituents
Bis(imino)pyridine (BIP) with imino-C (SC) and imino-N (SN) substituents.
Bis(imino)pyridine ligands are an important class of ligands that are currently being investigated around the globe for their properties in numerous fields of industry. Their tridentate nitrogen donor complexes, with different transition metals, give these ligands unique electronic and redox properties.
The unique catalytic properties and the flexibility of these substances are not limited to the catalysis of ethylene, on which they were initially discovered, but they have within a decade expanded to embrace many other chemical mediated processes, such as the reduction of ketones via hydrogenation, metathesis etc.
However, their properties are yet not fully known and there is still some optimization to be done for every potential application, while still, an integrated cost-efficient and productive catalyst remains to be found
Preparation (Synthesis) of BIPs
BIP synthesis from the condensation of diacetylpyridine (Sc=CH3) or 2,6-pyridine carboxaldehyde (Sc=H), or 2,6-dibenzoyl-pyridine (Sc=phenyl), with an aniline (SN= aryl) or generally another amine (SNNH2).
Generally, synthesis of BIPs proceeds with condensation of an amine with for example 2,6-diacetylpyridine, which is the most popular method for synthesis of BIPs.
Different amines and ligand modifications have been utilised for this purpose giving a variety of products with different properties and conformations.
Bis(aryl-imino) pyridine ligands can be synthesized from the condensation reaction of 2,6-pyridinedicarboxaldehyde or 2,6-diacetylpyridine or 2,6-dibenzoylpyridine with anilines in alcohol or CH2Cl231 solution, at elevated temperatures.
It is possible to condensate amino-heterocycles such as amino-pyrrolyl, triazolyl, carbazoles and indolyls as N-substituents with the condensation reaction with 2,6-diacetylpyridine to form N-azolyl BIP structures with N-N bonding.
Liu et al. have condensed 2,6-diacetylpyridine with the hydrazine of oxamic acid to obtain bis (semioxamazide-imino) pyridine at 55oC in a methanol solution.
Other Syntheses: Synthesis of 2-(chloro-substituted-1H-benzoimidazol-2-yl)-6- (1-aryl-iminoethyl) pyridines (L1-L6) (Reaction 7).
Complexes of BIP with Fe and Co
These complexes can be prepared in numerous ways and with a varying number of metal substituents. Typically, the synthesis of the iron and cobalt halide systems proceeds by treatment of BIP with the respective metal halide (e.g. FeCl2, CoCl2, FeCl3) salts in THF, or n-butanol (boiling at 80oC for about 10min. or CH2Cl231,40 for about 2hrs,25 or at 40oC for about 1hrs). The complex formed is paramagnetic, coloured, high-spin species, pentacoordinate in most cases, with pseudo-square-pyramidal geometry, with a nearly perpendicular arrangement of the aryl, rings relative to the square plane.
Preparation of dimeric bis(aryl-imino) pyridine iron dinitrogen compounds with aryl substituents smaller than isopropyl from a related precursor. FeBr2 may be achieved by stirring the THF solutions of these precursors in the presence of naphthalene resulting in the iron dinitrogen compounds.
A straightforward method for the synthesis of bis(imino)pyridine iron dialkyl compounds is by substitution of the pyridine ligands in (py)2Fe(CH2SiMe3)2 with the appropriate chelate
Applications of BIP Fe and Co complexes
Polymerization of ethylene to polyethene
Polymerization of ethylene to polyethene proceeds via a Cossee-type propagation mechanism, involving the migratory insertion of ethylene into a metal alkyl bond and in the case of FeII, iron(II) alkyl cations act as propagating species.
It is terminated through:
- A bimolecular β-H transfer to metal (BHT), or monomer
- A less common chain transfer to aluminium
Fe and Co halide complexes of BIP with aryl groups with substituents in the ortho position upon activation with MAO, give an active ethylene polymerization catalyst converting ethylene to highly linear polyethene. The reaction is affected by the type of metal-ligand, metal substituent, structure, reaction conditions, branching, and molecular weight distribution.
Oligomerization of ethylene to α-olefins
α-olefins in the range C6-C20 are used as comonomers in the polymerization of ethylene to give linear low-density polyethene or for the preparation of detergents and synthetic lubricants.
They are also used to prepare detergents and synthetic lubricants. The reaction mechanism is not well understood. The reaction is affected by steric effects and the electronic factor of ligands.
Other factors influencing the reaction include metal activities, reaction conditions, molecular weight distribution, and cocatalysts.
Polymerization of Butadiene to polybutadiene
Polybutadiene is a material that finds extensive and diverse applications in the tire industry but studies on butadiene polymerization, remain scarce. In butadiene polymerization, selective cis-1,4 polymerization of butadiene is of prime importance since the elastomeric rubbery product, cis-1,4-polybutadiene, constitutes one of the major ingredients in the tire industry. The structure of the active centre is the determining factor for the activity, molecular weight, and stereochemistry of polymerization.
FeII pyridinediimine systems were first reported by the tire and rubber company Goodyear for the co-dimerization of butadiene and ethylene.
Hydrosilylation of aldehydes and ketones
Reduction of C=O bonds to alcohols is an important reaction in organic synthesis with hydrosilylation being among the most useful reduction methods. Due to chemoselectivity issues and severe reaction conditions with other reaction routes (metal hydrides), there is a serious demand for new, mild, and chemoselective reducing methods.
Due to chemoselectivity issues and severe reaction conditions with other reaction routes (metal hydrides), there is a serious demand for new, mild, and chemoselective reducing methods.
Iron dialkyl ligands are used in the hydrosilylation process because:
- BIPs are tolerant in many functional groups
- BIPs are some of the most active iron-based reduction catalysts
Acetylene polymerization
Polyacetylene gives rise to metallic conductivity after suitable doping and is one of the most important conjugated polymers. The macroscopic morphology of the polymer produced in that way is dependent on the reaction conditions.
Stirring has been found important for film formation as it increases the yield. The rate of polymerization has been found to decrease with time due to the slow diffusion of acetylene through the gel. With time, the insoluble polymer rapidly encapsulates the catalyst, preventing further polymerization.
Copolymerization with Hexene
Late transition metal catalysts tolerate heteroatomic functionalities, opening up the possibility for copolymerization of olefins with other monomers to produce linear or branched systems. The copolymerization of ethylene with other α-olefins comonomers is an important industrial process for the control of the densities of the produced polymers.
BIP with Ni
Synthesis of Ni complexes with BArIP is generally similar to Fe and Co complexes. That is the reaction of the relevant ligand with the respective nickel salt (NiCl2) in boiling THF.
Applications of BIP-Ni
- Oligomerization of ethylene
The reaction is increased by ethylene pressure, temperature and cocatalyst to metal ratio.
- Butadiene polymerization
Occurs similarly to that of Fe and Co BIP complexes.
- Norbornene polymerization
Several parameters, including the nature of catalytic precursors, the concentration of MAO, reaction temperature and reaction time have some effect on the activities of the complexes and the molecular weight of the PNB.
The bicyclo[2,2,1]hept-2-ene, known as norbornene, polymerizes to give products that exhibit strong thermal stability, have excellent dielectric properties (low dielectric constant, excellent transparency, small optical dielectric loss), optical transparency, large refractive index, and unusual transport properties, mechanical properties, high chemical resistance, high glass transition and decomposition temperatures, good UV resistance, and low water uptake, which can be of use in microelectronics applications
BIP complexed to Mn
Similar to Fe, Co and Ni, halide complexes, adding an Mn metal centre in BIP is accomplished with the addition of the Mn halide in the respective BIP ligand in refluxing acetonitrile in yields ranging 40-77% or THF in higher yields (80-95%). Toluene solution can also be used, same as reaction with manganese perchlorate.
Applications BIP-Mn
- Superoxide-Dismutase
The superoxide anion is formed during respiration.
Dismutase enzymes catalyze excess superoxide as it is harmful at high levels.
- Electrochemical applications
Electronically conducting polymers are used in various devices.
- Polymerization of ethylene
The Mn ligand redox properties are widely used industrially.
All mammalian life consumes oxygen as the ultimate oxidant supporting cellular respiration with a considerable portion of the oxygen metabolized through its superoxide anion. The superoxide anion is the one-electron reduction product of molecular oxygen formed as a byproduct of normal cellular respiration.
In organisms, superoxide dismutase enzymes (SOD-oxidoreductase enzymes Mn, Fe, Cu, Zn, Ni-based) present in cell mitochondria, plasma, and the extracellular spaces in eukaryotic and some prokaryotic cells, catalyze the disproportionation of superoxide to dioxygen and hydrogen peroxide, which then is decomposed via catalase to water and dioxygen. The inability of the human body to adequately control and limit the overproduction of superoxide anion.
Electronically conducting polymers have been studied extensively owing to their potential applications in energy conversion devices, sensors, electrochromic displays, microelectronic devices, electrocatalysis, military etc.
Fe(II)- and Co(II) complexes bearing bis(imino)-pyridine and related ligands are famous for their efficient ligand framework in the polymerization of ethene at high activities and adjustable product properties. On the other hand, despite all BIPs being redox-active, it is the Mn ligand redox properties, which enable its application on different industrial aspects.
Conclusion
In this paper, bis(imino)pyridine and methods for its synthesis, syntheses of its complexes with Fe, Co, Ni and Mn and the applications of these complexes in different chemical processes have been covered.
The success with BIPs in the polymerization of ethylene is due mostly to the ease of structural modification (steric control) of these compounds which allows for greater control in the MW distributions and properties of final product and optimization possibilities (via cocatalysts temperature, metal centre etc.) in the production line.
Despite the much-sought high productivities of BIP for ethylene polymerization and oligomerization and its cost-effectiveness for this application, the BIP as a catalyst for that industry still needs to be tested/ studied or modified.
Owing to their tridentate system their unique properties, structural flexibility and previous (even if recent) experience of BIPs in the PE industry, it was possible to use the BIP ligands in a range of applications.