Journal of Chemistry and Chemical Sciences, Vol.7(11), 1022-1025, November 2017 ISSN 2229-760X (Print) (An International Research Journal), www.chemistry-journal.org ISSN 2319-7625 (Online)
Versatile Applications of Lanthanides-Catalysis
Wakeel Ahmed Dar 1 and Ruqiya Bhat 2
1Department of Chemical Sciences, Jamia Millia Islamia, New Delhi-110025, INDIA. 2Department of Physics, Kashmir University, J&K., INDIA.
(Received on: October 15, 2017)
ABSTRACT
The lanthanides are well known for their luminescence and magnetic properties. This article briefs about the use of lanthanides catalysts. The catalytic function of lanthanide is important as they have neither restricted geometry not limited coordination number. Their size and shape plays a very vital role in elucidating the catalytic functional properties. These properties of lanthanides are useful for coordinating the chiral ligands around the Ln(III) ions, allowing designing of highly stylish complexes. These catalytic properties make these lanthanides as functional molecules of new science world. The goal of this article to present a briefing about catalytic properties of lanthanides. The lanthanides till date have been used in many organic reaction synthesis. The photocycloaddition reactions are initial also by lanthanides. The briefing is given below. This article also pays the a little insight regarding luminescence and their complexes. The lanthanide are hard acids (lewis acids) and are soluble in water also. This feature allows them to perform their catalysis not only in organic solvent but in water too. The reactions of varying systems can be performed and in future they may play an important applications related to drug designing.
Keywords: Lanthanides, Paramagnetic NMR, photoluminescence, absorption, energy level.
INTRODUCTION
Lanthanide complexes have been widely investigated over the past four decades because of the unique optical and magnetic properties of the lanthanide ions. 1,2 The trivalent lanthanides constitute a series of hard lewis acids with a similar chemical behavior. Strategies
1022 Wakeel Ahmed Dar, et al., J. Chem. & Cheml. Sci. Vol.7(11), 1022-1025 (2017) for the design of stable trivalent lanthanide complexes relies on the use of various ligands such as polyaminopolycarboxylates 3, cryptands 4, calixarenes 5, podands 6, helicates 7 and among others the complexes with β-diketones 8 are the most popular and intensively studied lanthanide complexes. The tris( β-diketonate) lanthanide complexes possess significant thermodynamic stability and higher volatility. The tris( β-diketonate) lanthanide complexes are coordinatively unsaturated coordinated to solvent molecules. The solvent molecule can be replaced by one or two additional ligands that act as lewis bases, and form adducts with tris(β-diketonate) complexes because of the tendency of the rare-earth ion to expand its coordination sphere 8. The Lewis bases that form adducts with rare-earth tris( β-diketonates) are mostly N, N-donor ligands such as 2, 2 ʹ-bipyrimidine, 2, 2 ʹ-bipyridyl, 2, 2 ʹ, 6 ʹ, 2 ʹʹ -terpyridyl 9, bathophenanthroline 10 , 2-(2- pyridyl)benzimidazole 11 etc. The simplest of non-chelating heterocyclic ammine is pyrazine which can be employed to synthesize mononuclear or dinuclear complexes. The 1H NMR represents a fruitful technique in determining the structure of lanthanide complexes. The most members of the series are paramagnetic (with the exception of La(III) and Lu(III)) with short electron relaxation times (10 -4 s) and some of these have been tested as shift reagents 12 . In 1969 lanthanide induced shift reagent was first reported by Hinckley in simplifying the unresolved 1H NMR resonances in low field NMR spectra of organic molecules in non- aqueous solvent 13 . It was a major advance in the field of shift reagents and since then it has been established that Ln(III) complexes result in far less line width broadening and generate shifts which are predominantly pseudocontact in nature 14 . The lanthanides as catalyst are the hot topics of nowadays. In this paper we report recent applications of lanthanides in catalysis. Organic reactions are usually initiated by catalysts to form an important class of new molecules. The p-block and d-block elements have been largely studied for use as catalysts and the use of lanthanides as catalysts are limited. The use of lanthanides as catalyst in asymmetric synthesis was first published by Danishefsky et al ., (1983). Since, the lanthanide complexes are chiral and achiral, chiral complexes find their extended applications in asymmetric synthesis. The coordination number of lanthanide complexes is not limited since it can range from 6-12. They are hard acids and tend to bond with organic molecules containing O, N and S. This property is highly fruitful for assembling organic ligands around the lanthanide ion. The active site of lanthanide is responsible for catalytic activities. Lanthanide complexes find their use in cyclo-addition reactions, [2+2] cycloaddition reactions. The visible light was used for synthesis of symmetric [2+2] photocycloadditions of unsaturated ketones using transition metal and lanthanide catalyst viz. [Ru(bpy) 3]Cl 2 and [Eu(OTF) 3] combined with shiff’s base ligand (chiral) 15 . Lanthanide catalysed aldol reactions (direct aldol reactions), Mukaiyama-aldol reactions, Nitroaldol reactions, lanthanide catalysts for epoxidations are also reported.
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Figure 1: Schematic Representation of Lanthanide being used as Catalyst in Organic Synthesis
CONCLUSIONS
This article demonstrates that lanthanide complexes may prove to be more effective for catalyst studies and has gained new development in the organic synthesis reactions, because of their large size and unique coordination number. The coordination sites of lanthanides are available for accommodating large number of ligands as compared to transition metals and this feature allows the design of stylish complexes. Lanthanides being lewis acids are soluble in water also and various catalytic functions of lanthanides can be performed in water in addition to organic solvents. This will form the basis of the lanthanide systems as environmental friendly catalysts for various reaction systems and their increased applications in coming times.
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