Inspection of the current list shows that papers on
superconductivity occupy four slots, and
graphene
accounts for the remainder. The former group of papers is about the
remarkable iron/lanthanum superconductor which was first discussed
in the
Nov/Dec
2008 issue of Science Watch. The latter group were
dealt with in the
Jan/Feb
2010 issue, but among them now is one that is new to the list:
paper #7. This reports a chemical method, using hydrazine, for
reducing graphite oxide to graphene sheets which exhibit
semiconducting behavior.
Although the glamour of superconductors and graphene electrodes
holds center stage, some excellent conventional chemistry is
waiting in the wings at position #14: R.A. Kelly, et al.,
Organometallics, 27(2): 202-10, 28 January 2008, with 18
citations this period and 66 overall. This paper concerns iridium
complexes with N-heterocyclic carbene (NHC) ligands and represents
the collective work of researchers in chemistry departments at the
University of New Orleans (Louisiana), the Institute of Chemical
Research of Catalonia (Spain), the University of Salerno (Italy),
and the University of Miami (Florida). The lead author of the paper
is
Steven Nolan, who is now at the University of St. Andrews in
Scotland.
Organometallic Chemistry and Catalysis.
Figure from the
Nolan Group, University
of St. Andrews, Scotland, U.K.
Carbenes are rather remarkable entities, with a central carbon atom
with two covalent bonds plus an electron pair, in theory making
them ideal ligands. (All the NHCs of paper #14 are heterocyclic
molecules with two nitrogens attached to the carbene carbon.) These
carbene complexes of transition metals have attracted considerable
interest as catalysts for various reactions, including olefin
metathesis, hydrogenation, hydrosilation, cross-coupling, and
isomerization. Although carbene complexes are overshadowed by the
more common phosphine complexes, there is now evidence that they
are actually better in many respects. Not only are they are more
stable towards air and moisture, but some of them have unique
catalytic abilities.
The NHCs were first isolated in 1991 but have gained popularity
greatly since then, and Nolan has shown just how versatile their
ruthenium, palladium, and copper complexes are as catalysts. When
he turned his attention to iridium-NHC complexes things became
clearer as to why they are superior, and as a result this work is
now being highly cited.
Paper #14 concerns the iridium complexes of nine different NHCs of
the form [(NHC)Ir(CO)2Cl] which were produced in yields
as high as 90%. They were made via the corresponding cyclooctadiene
complexes. The focus of #14 is the carbonyl stretching frequencies
of the products, and these are used to examine and rank the
electron-donating strength of the NHC entities. The result reveals
that all are more strongly donating as ligands than even the
strongest donating tertiary phosphines. What is also shown is that
the other constituents of the heterocyclic ring of the NHC have
only minor influences on the molecule’s electron donating
power. The carbene with bulky adamantyl groups attached to the two
nitrogens gave the lowest stretching frequencies of the CO bonds,
and the reason seems to be their steric requirements.
So why is this paper being strongly cited now?
Says Nolan, "It deals with fundamental properties of a ligand
family that is becoming very popular in organometallic chemistry
and homogeneous catalysis. Some of these NHC complexes (Ru, Pd, Cu,
and Au) can now be synthesized on a large scale and are
commercially available. I believe we are addressing some
fundamental concepts that are widely applicable to these important
areas. It is our hope that a basic understanding of this ligand
family will help move towards a sustainable green chemical
industry."
Currently Nolan is directing his research toward understanding and
maybe even predicting the reactivity of transition metal complexes.
If he succeeds, he believes it will then be possible to design
catalysts with improved performance in the many areas where they
are being used and possibly even designing novel chemical
transformations.
Since paper #14 appeared, Nolan has moved on to studying NHC
complexes of gold. In his paper in the Journal of the American
Chemical Society (N. Marion, et al., 131[2]: 448-9,
2009), he shows that these will act as catalysts for the hydration
of alkynes, and in acid-free conditions, and at levels of only
parts per million. In Chemistry – A European Journal
(R.S. Ramon, et al., 15(35): 8695-7, 2009), he looks at
the way such gold catalysts permit the hydration of nitriles to
form amides.
Dr. John Emsley is at the Department of Chemistry,
Cambridge University, U.K.
C. de la Cruz, et al., "Magnetic order
close to superconductivity in the iron-based layered
LaO1-xFx FeAs systems,"Nature, 453(7197): 899-902, 12 June 2008. [6
U.S. and China institutions] *311WV
C. Lee, et al., "Measurement of the
elastic properties and intrinsic strength of monolayer
graphene,"Science, 321(5887): 385-8,
18 July 2008. [Columbia U., New York, NY] *327FB
31
†
5
H. Takahashi, et al.,
"Superconductivity at 43 K in an iron-based
layered compound LaO1-xFx
FeAs,"Nature, 453(7193): 376-8, 15
May 2008. [Nihon U., Tokyo, Japan; Tokyo Inst.
Technol., Japan] *301AI
29
3
6
D.C. Elias, et al., "Control of
graphene’s properties by reversible
hydrogenation: Evidence for graphane,"Science, 323(5914): 610-3, 30 January 2009.
[U. Manchester, U.K.; Inst. Microelectronics Tech.,
Chernogolovka, Russia; Cambridge U., U.K.; U. Nijmegen,
Netherlands] *400JB
23
†
7
S. Gilje, et al., "A chemical route to
graphene for device applications,"Nano
Letters, 7(11): 3394-8, November 2007. [U. Calif.,
Los Angeles] *232ZI
22
†
8
R.R. Nair, et al., "Fine structure
constant defines visual transparency of
graphene,"Science, 320(5881): 1308,
6 June 2008. [U. Manchester, U.K.; U. Minho, Braga,
Portugal] *309DO
22
†
9
A.A. Balandin, et al., "Superior
thermal conductivity of single-layer
graphene,"Nano Letters, 8(3): 902-7,
March 2008. [U. Calif., Riverside] *273QS
21
†
10
C. Gomez-Navarro, et al., "Electronic
transport properties of individual chemically reduced
graphene oxide sheets,"Nano Letters,
7(11): 3499-503, November 2007. [Max Planck Inst. Solid
State Res., Stuttgart, Germany; U. Siegen, Germany;
EPFL, Lausanne, Switzerland] *232ZI