Tag Archives: Chemistry

This chemistry talk is Chinese to me… Here’s the article.

The industrial preparation of many
chemicals relies on the unparalleled
rate and selectivity enhancements
offered by metal compounds in solution. In
many cases, the best catalysts rely on the
scarcest elements, such as rhodium, iridium,
and platinum. The cost of these materials has
long driven efforts to make soluble catalysts
out of cheaper, more Earth-abundant metals
( 1), often by modifying their reactivity with
their surrounding ligands. This is especially
true for catalyzing reduction-oxidation, or
redox, reactions, which are critical not only
in catalysis but in energy generation and storage.
Such reactions usually change the oxidation
state of the metal in solution. We discuss
why there can be advantages to having the
redox changes occur in the ligands instead.
One major obstacle in replacing noble
metals with more common ones stems from
the differences in electronic structure. A
noble metal like platinum often favors twoelectron
redox changes to promote bondmaking
and breaking events. For the base
metals, one-electron redox changes occur
more frequently and present challenges for
controlling reactivity and stabilizing or maintaining
the function of the catalyst.
To mimic noble metals, one-electron redox
changes must be suppressed and two-electron
redox events facilitated. Most ligands used in
inorganic chemistry, such as ammonia or triphenylphosphine,
are not “redox-active”—
the energy needed to oxidize or reduce them
by even one electron is much greater than
that needed to change the oxidation state of
the metal, so changes in electronic structure
occur at the metal. Redox-active, or “noninnocent,”
ligands ( 2, 3) have more energetically
accessible levels that allow redox reactions
to change their charge state. For example,
NO may bind as a cation in a linear geometry
or an anion with a bent geometry.
Redox-active ligands have long been recognized
in coordination chemistry. Gray and
co-workers determined that square-planar
cobalt ( 4) and nickel ( 5) dithiolene complexes
were best described as metal(II) compounds
with two ligand radical anions, rather than the
metal in the +4 oxidation state and –2 ligands.
Catecholates and diimines also have a distinctive
ability to form radical species, which
normally would be unstable in solution, when
they are bound to metal centers. The extended
network of bonds in these ligands allow
them not only to stabilize radical species but
also to facilitate reversible reactions with the
metal center that may involve radical formation.
Spectroscopic, structural, and magnetic
data, theoretical modeling, and patterns of
reactivity are often needed to assign the true
electronic structure description of a transition
metal complex ( 6).
The use of metal complexes with radicals
on the supporting ligands as catalysts also
draws inspiration from enzymatic reactions
of certain metalloproteins ( 7). One of the
best understood examples is galactose oxidase,
which performs the two-electron oxidation
of alcohols to aldehydes. A Cu(II) ion is
coordinated to a modifi ed tyrosyl radical, and
this intricate bonding situation gives rise to
the function of the enzyme ( (see the fi gure,
panel A). This phenomenon may be pervasive
in metal-containing redox proteins.
However, it may be diffi cult to detect when
two radical ligands are present, because they
may strongly couple through a central metal
ion and not produce a distinctive signature in
spectroscopic studies ( 9).
For synthetic iron catalysis, the bis(imino)
pyridine family of ligands, pioneered in
base-metal olefin polymerization catalysis
by Brookhart and co-workers ( 10) and Gibson
and co-workers ( 11), can coax the metal
into the appropriate electronic confi guration
to engage in chemistry equal or superior to
reactions catalyzed by precious metals. The
ligand is stable in four chemically accessible
oxidation levels (neutral, as well as mono-,
di-, or trianions). The mono- and trianions are
radicals—they have an odd electron and a
spin state of 1/2—whereas the two electrons
of the dianion may spin-pair to form a singlet
ground state or stay unpaired and form a triplet
ground state ( 12, 13).
Many catalysts perform just one type
of reaction, but the iron complex (iPrPDI)
Fe(N2)2, can be used in a number of reactions,
such as the hydrogenation and hydrosilylation
of olefi ns ( 14), as well as the cyclization
of enynes and diynes ( 15) (see the fi gure,
panel B; iPr is isopropyl, and PDI is a pyridinediimine
ligand). A combination of spectroscopic
techniques and density functional theory
calculations established that this formally
iron(0) compound (with a neutral ligand) has
the physical oxidation state of an intermediate
spin iron(II) compound, where the metal
has transferred two electrons to the iPrPDI
ligand ( 16, 17).
We illustrate the role of