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Iridium is very important element among the all transition metals with highest reported oxidation state i.e. +9 in gas phase existing species IrO4 +. Instead of its less reactivity, it forms number of compounds having oxidation states between-3 to +9. It is second known densest element after osmium. Till now its toxicity and environmental impact is not much more reported and thus it may be use as green element in various fields of its application. Reason behinds it's less toxicity and environmental impact may be due to its less reactivity and solubility. Corrosion and heat resistant properties of Iridium makes it much more useful element for alloying purpose. Iridium is the member of platinum family and used as catalyst due to its variable oxidation states. Iridium(III) complexes show great catalytic activity in both the acidic and basic medium for various organic as well as inorganic chemical transformations. Catalyst may be defined as the substance which can increases the rate of reaction of a specific chemical reaction without changing its own composition. Iridium is only one reported catalyst which is able to capture the sunlight and convert it into the chemical energy. Thus, it may be used in artificial photosynthesis process to solve our future food problem. Instead of these advantage, Iridium chemistry and its catalytic activity is not much reviewed till date, therefore, present review includes a brief introduction about chemistry and catalytic application of Iridium, which proof itself a boon for beginners to start their research career in the field of Iridium chemistry.
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Green Chemistry & Technology Letters
Vol 2, No 4, December 2016, pg. 206-210
eISSN: 2455-3611, DoI: 10.18510/gctl.2016.247
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206
IRIDIUM CHEMISTRY AND ITS CATALYTIC APPLICATIONS: A BRIEF
REVIEW
SANTOSH BAHADUR SINGH
Department of Chemistry, National Institute of Technology Raipur, Raipur-492010, Chhattisgarh (India)
Corresponding author email id: singhsbnitrr15@gmail.com
Article History: Received on 05 th March 2016, Revised on 17th September 2016, Published on 10th December 2016
Abstract
Iridium is very important element among the all transition metals with highest reported oxidation state i.e. +9 in gas phase existing
species IrO4+. Instead of its less reactivity, it forms number of compounds having oxidation states between -3 to +9. It is second known
densest element after osmium. Till now its toxicity and environmental impact is not much more reported and thus it may be use as green
element in various fields of its application. Reason behinds it’s less toxicity and environmental impact may be due to its less react ivity
and solubility. Corrosion and heat resistant properties of Iridium makes it much more useful element for alloying purpose. Iridium is
the member of platinum family and used as catalyst due to its variable oxidation states. Iridium(III) complexes show great catalytic
activity in both the acidic and basic medium for various organic as well as inorganic chemical transformations. Catalyst may be defined
as the substance which can increases the rate of reaction of a specific chemical reaction without changing its own compositio n. Iridium is
only one reported catalyst which is able to capture the sunlight and convert it into the chemical energy. Thus, it may be used in artificial
photosynthesis process to solve our future food problem. Instead of these advantage, Iridium chemistry and its catalytic activity is not
much reviewed till date, therefore, present review includes a brief introduction about chemistry and catalytic application of Iridium,
which proof itself a boon for beginners to start their research career in the field of Iridium chemistry.
Keywords: Iridium, Oxidation State, Catalysis, Photosynthesis, Alloys, Environmental Impact.
INTRODUCTION
Iridium (Ir) is one of most important rare element among nine rarest elements [i.e. Ruthenium (Ru), Rhodium (Rh), Palladium
(Pd), Tellurium (Te), Rhenium (Re), Osmium (Os), Iridium (Ir), Platinum (Pt) and Gold (Au)] present in the Earth’s crust. It
usually occurs in nature as an uncombined element or in natural alloys, i.e. especially the osmium-iridium alloys, osmiridium
(osmium rich) and iridosmium (iridium rich). Noble metals include the transition metals of Platinum family i.e. element of VIIIB
groups namely Iron, Cobalt, Nickel, Ruthenium, Rhodium, Palladium, Osmium, Iridium and Platinum. Noble metals means less
reactive elements, in fact, iridium is the most corrosion resistant metal known. Natural sources of Iridium are mainly found in
Canada, South Africa, Russia and State of Alaska. Smithson Tennant (1761-1815) discovered the Iridium (in 1803), and named it
Iridium (based on the name of Greek Goddess ‘Iris’, which symbol is aRainbow). Tennant choose this mainly due to various
colours of Iridium compounds like Iridium potassium chloride (K2IrCl6) is dark red, Iridium tri-bromide (IrBr3) is olive-green and
Iridium tri-chloride (IrCl3) is dark-green to blue-black [1]. Catalytic chemistry of Iridium started in 1960 by Lauri Vaska [2]
[Vaska L (1968) Accounts Chem Res 1:335] with his studies on IrCl(CO)(PPh3)2complex, later on it is known as “Vaska’s
complex. This was the first study that gives a major and different role to Iridium for Organometallic chemists to understand the
oxidative addition, a fundamental step in homogeneous catalysis. Wang et al. (2014) reported an iridium containing compound
(iridium tetraoxide cation; IrO4+) in gas phase [3]. This is highest experimentally known formal oxidation state of any chemical
species till date. Present review mainly explores the basic chemistry of Iridium and its applications.
IRIDIUM CHEMISTRY
Iridium is one the most important element of the platinum group elements. It has an electronic configuration of
1s22s22p63s23p63d104s24p64d105s25p65d76s2. It has widest range of oxidation states i.e. -3 to +9 among the all transition metals.
With this unique electronic configuration iridium shows various unique properties, that explores it broad area applications i.e.
industrial, medical and catalysis [4,5,6,7]. Key process in photosynthesis is the photolysis of water means breaking of water
molecules in hydrogen and oxygen [8,9]. Sheehan et al. (2015) reported the iridium as an effective molecular catalyst for water
oxidation [10]. Thus, iridium becomes a most promising catalyst to solve food crisis (via catalyzing artificial photosynthesis) and
energy problem (via production of hydrogen as alternating source of energy). A brief summary of iridium chemistry is shown
below in diagrammatic scheme-1.
Green Chemistry & Technology Letters
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eISSN: 2455-3611, DoI: 10.18510/gctl.2016.247
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207
Rh
Os Pt
Mt
Applications Oxidation states
Isotopes
Physical properties
77Ir192.217
[Xe]4f145d76s2
Group VIII
Period 6
Iridium from the
name of Greek
Goddess 'Iris'
Discovered by Smithson
Tennat in 1803.
Natural isotopes and abundance (%)
77Ir191 37.3 %
77Ir193 62.7 %
Oxidation states with example
-3 [Ir(CO)3]3-
-1 [Ir(CO)3(PPh3)]-
0 [Ir4(CO)12]
+1 [Ir(CO)Cl(PPh3)2]
+2 [IrCl2]
+3 [IrCl3]
+4 [IrO2]
+5 [Ir4F20]
+6 [IrF6]
+7 [(n2-O2) IrO2]+
+8 [IrO4]
+9 [IrO4]+
Melting point 2719K
Boiling point 4403K
Iridium (second densest element) is a very hard, brittle and silvery-white transition metal. It is known as the most corrosion
resistant metal even at temperature as high as 2000oC
Due to its widest range of oxidation states (from -3 to +9) and
its unique chemical and physical properties it shows wonderful catalytic properties.
Scheme-1: A breif summary of Iridium Chemistry
1. Industrial
2. Medical
3. Catalysis
APPLICATIONS OF IRIDIUM
Iridium and its complexes have wide range of application in various field of science. Catalysis is one most important application
field of Iridium and its complexes i.e. catalyzed tritium labeling process for targeted drug design, catalyzed various organic
reactions including hydrogenation, hydrogen-transfer reactions, functionalisation of C-H bonds, allylic substitution, 1,3-dipolar
cycloadditions, catalyzed fine chemical synthesis, etc. Other applications of iridium and its complexes includes (i) used as
anticancer agent, (ii) used in energy production, (iii) used in electrical and electrochemical processes and (iv) used for fixation of
atmospheric carbon dioxide (CO2) in useful products [5,6,10,11,12]. Scheme-2 includes brief information about various
applications of iridium and iridium-based complexes.
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Catalysis
Mode of Action: Enhance the
rate of reaction by optinglower
activation energy pathway.
Anticancereous activity [Due
close structural and electronic
similarities with PtII cis-platin]
Mode of Action: Attacks on
DNA and alter the redox status
of affected cells.
Energy Production [Production of
hydrogen as alternating source of renewable
enegry]
Mode of Action: By catalyzing
electrochemical oxidation of water
Electrical and electrochemical use
Mode of Action: Due a wide range of
oxidation state it is capable to form
various alloys with other metals
according to their redox potentials.
Fixation of atmospheric CO2
Mode of Action: By simple
molecular arrangement on an
iridium hydride catalyst, CO2can
be transform into HCOO-(formate)
a precursor of methanol.
Ir
Scheme-2: Various applications of Iridium and its complexes
CATALYSIS BY IRIDIUM
Iridium has been reported as a versatile catalyst to catalyze various chemical reactions in both the acidic and basic medium
[13,14]. Iridium and its complexes have been used as homogeneous catalyst, nano-catalyst and bio-catalyst.
Iridium as homogeneous catalyst
Iridium is a good homogeneous catalyst and has a wide range of applications [6,7] which is summarized in Scheme -3 given
below:
Ir as homogeneous catalyst
Hydrogenation
It is the one of the most important
method for reduction of C=C,
C=N and C=O double bonds in
organic synthesis. Iridium and its
complexes play an important in
organic synthesis through
hydrogenation in
presence/absence of selective
legands i.e. P, N&P andC&N.
Hydrogen transfer reactions
It provides an alternative to direct
hydrogenation for reduction of a
wide range of substrates i.e. carbonyl
compounds, imines, alkenes, arenes
and CO2etc. Iridium complexes acts
as one of most important catalyst for
catalyzing hydrogen transfer
reactions.
Carbon-carbon bond formation
It is the key process in organic synthesis
to synthesize the a large no of organic
molecules. Iridium shows great efficiency
to catalyze C-C bond formation via
hygrogenation and hydrogen transfer
reactions
Carbon-hydrogen bond fuctionalization
It is also one of the most important process that
holds enormous potential value in virtually every
sphere of organic synthesis. First examples of
oxidative addition of CH bonds is shown by
Iridium. This addition is key to iridiums leading
role in alkane dehydrogenation and related
reactions. Catalysts based on iridium have also
proven highly effective for valuable borylations
of CH bonds and, to a lesser extent, for CSi
coupling.
Allylic substitution reactions
A number of nucleophile containing C,
N & O reacts with allylic esters in
presence of iridium catalyst and forms
branched allylic sustitution products.
Iridium-catalyzed asymmetric allylic
substitution has become a valuable
method to prepare products from the
addition of nucleophiles at the more
substituted carbon of an allyl unit
1,3-Dipolar Cycloaddition
Reactions
These are atom economic
processes and are important for
synthesis of heterocyclic
compounds. Ir(III) plays
significant role in catalysis of
1,3-Dipolar cycloaddition
reactions.
Scheme-3: Various fields of applications of Iridium as homogeneous catalyst
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209
Iridium as nano-catalyst
Transition metal nanoparticles with controlled diameter, size, and shape show better efficiency of catalysis to catalyze various
chemical reactions in comparison to bulk materials. Recent reporting on catalytic applications of iridium nanoparticles explores its
important in the field of nanocatalysis [7].
Bayram et al. (2010) reported the complete hydrogenation of benzene at room temperature and mild pressure by using zero-valent
iridium nanoparticles [15].
Ir(0) nanoparticles
Room temperature
Mild Pressure
Rueping et al. (2011) reported the synthesis of stabilizer free iridium coated carbon nano-tubes (Ir@CNT) and their catalytic
applications in hydrogenation of N-heterocyclic compounds [16].
NN
H
Iridium coated
carbon nanotubes
60 oC, 2.25 h,
190 bar H2&
Acetone
Iridium as bio-catalyst
Catalytic activity of iridium and its complexes also makes them an attractive bio-catalyst which can catalyze some specific
biological reactions [5,17,18]. Some of them illustrated below:
NAD+NADH
Reduction with formate
IrIII Cp* Complex
Environmental Impact of Iridium
There is no more reporting till date about any health benefits or risks associated with iridium and its complexes on human be ings
as well as on the environments.
CONCLUSION
Iridium and its complexes have a wide range catalytic activity to catalyze number of organic and inorganic transformations. It is
most promising metal as bio-catalyst, catalyst for artificial photosynthesis and catalyst for water oxidation for hydrogen
production in near future. Present review highlights the fundamentals of iridium chemistry and explores its applications in various
dimensions and inspiring the further research to extend the iridium chemistry.
ACKNOWLEDGEMENT
I am very grateful to Professor Praveen Kumar Tandon, Department of Chemistry, University of Allahabad, Allahabad, who
introduced me to chemical kinetics, catalysis and organic synthesis which I opted as my research fields. It was his valuable
guidance and encouragement which inspired me and generated confidence in me right from the beginning of my research career to
point at which I can write these lines. Further, I would like to express my sincere thanks to Professor (Mrs.) Fahmida Khan, Head,
Department of Chemistry, National Institute of Technology Raipur, Raipur, for her moral support, inspiration and continuous
blessing and providing departmental facilities.
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Vol 2, No 4, December 2016, pg. 206-210
eISSN: 2455-3611, DoI: 10.18510/gctl.2016.247
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210
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4736474
BIOGRAPHY:
Dr. Santosh Bahadur Singh received his D.Phil. Degree in 2010 at University of Allahabad, Allahabad,
under the supervision of Professor Praveen K. Tandon. Subsequently he was a Research Associate of
CSIR, New Delhi at University of Allahabad, Allahabad. From July 2015 to till date he is working as
Faculty (Temp.) in Department of Chemistry, National Institute of Technology, Raipur (C.G.). He is a life
member of Materials research society of India. His research interests are centered on the chemistry of
surface in adsorption, nano-catalysis, chemical kinetics, water remediation and oxidative transformation of
organic compounds.
... Iridium and Osmium: Iridium is the only metal known to be corrosion resistant and resist extreme temperatures even up to 2000 °C [294]. Since iridium is less reactive, it is generally considered to be biocompatible. ...
... It is the second densest metal with an elemental density of 22.56 g/cm 3 [295]. Iridium is commonly applied in the field of catalysis, energy production, artificial photosynthesis, and electrochemical processes to fix atmospheric carbon dioxide in products and as anticancer agent [294]. Synthesis of iridium nanoparticles through natural mode is still in its infancy and only a single report was identified through an extensive study of the literature. ...
... In fact, Ir exhibits a unique set of properties almost meeting the key requirements in a wide range of highly demanding applications [26], such as the high melting temperature (2446°C) [27], high specific strength at high temperature, remarkable oxidation and corrosion resistance. To now, Ir is already used in catalysis [28,29], to manufacture high temperature crucibles, for dental implants and jewellery, for sensors [30] and electronic devices working at high temperatures (dimensionally stable anodes) [31]. Ir is used also as hardening agent for other metals, (such as Pt [32]) and multicomponent alloys [33]. ...
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