The document discusses bifunctional catalysts for the oxidation of water or organic substrates. The goal is to create a water oxidizing catalyst using ruthenium and bifunctional ligands to transfer protons during oxidation. The catalyst would use sunlight as an energy source to produce hydrogen fuel by splitting water. It may also help oxidize alkane substrates like methane through C-H bond oxidation. The author plans to synthesize analogs of an NHC/pyridyl ligand with different pendant base groups and test their ability to catalyze these oxidation reactions.

1. Bifunctional Catalysts for Selective
Oxidation of Water or Organic
Substrates
A. Two Major Long Term Goals
The oxidation reaction that splits water into
H+
and O2 is worth exploring in artificial
systems to produce cheap and clean H2 fuel.
The challenge is to create a water oxidizing
catalyst (WOC) capable of having high
turnover numbers. The WOC system I will
be studying has ruthenium (Ru) as the metal
and bifunctional ligands to help transfer
protons during the oxidation process (Eq.1).
Sunlight is the ideal energy source, since it
is free and abundant; however, first the
necessary step to harness the sun’s energy is
to create a fast, long-lived WOC. Figure 1
shows a commonly accepted mechanism for
water oxidation. It is typically accepted that
the challenge lies in transitioning from step
4 to 5. Our strategically designed ligands
may be able to effectively transfer protons in
any or all of the steps that involve proton
loss.
Eq. 1 2 H2O → O2 + 4 H+
+ 4 e-
Figure 1
In addition, we hypothesize that proton-
transferring ligands could help other
oxidation reactions. A very attractive
reaction is alkane oxidation during the
synthesis of organic molecules, selectively
targeting C-H bonds, as can be seen in the
related reaction (Eq. 2) with X representing
a halogen or oxygen.
Eq. 2 C-H + X-H  C-X + 2H+
+ 2e-
Chen and White’s work on targeting
methylene (C-H to C-OH) bonds for
oxidation has shown that it is possible to
mimic enzymatic oxidation with an iron
catalyst with similar selectivity and
predictability. [1]
In fact, there are several
recent published examples of WOC used for
selective alkane oxidation,[2,3]
along with
examples of water oxidation reactions that
lead to the carbons of the WOC becoming
oxidized.[4,5,6]
B. Previous Experiments and Preliminary
Results
Studies done by the T.J. Meyer group on
ruthenium-based complexes (7) on water
oxidation have shown increased rates of
catalysis in the presence of a weak base. It
is believed that weak bases in solution help
increase the rate of the O-O bond-forming
step. However, if too much base is added
then the effect becomes detrimental, causing
the WOC to cease functioning.[7]
To
improve on this, it is our hypothesis that
attaching pendent base groups to the
bipyridine ligand, as shown in 8 (Figure 2),
will not only significantly increase the
WOC’s performance, but also prevent
catalyst inhibition because the base will be
held away from the metal. Thus far, four
analogs of 8 have been tested showing the
crucial role of the pendent base and how it is
able to help facilitate the loss of electrons
from the WOC. Our new strategy is to
improve the WOC by replacing a pyridine
with an N-heterocyclic carbene (NHC)
RuV
O
RuII
O
RuIII
OH
RuIV
O
H H
-e-
-H+
-e-
-H+
-e-
O
H
H
RuIII
O
RuIV
O-e-
-H+-H+
OH O-
+H2O
-O2
1 2 3 4 5 6

2. while maintaining the presence of a pendent
base.(9).
Figure 2
Figure 3
Figure 4
C. Personal Contribution and Research
Plan
My work in the Grotjahn lab will involve
synthesizing analogs of the NHC/pyridyl
ligand in 9 with different pendent base
groups (B = OMe, OH, NH2, COO-
) on the
pyridine. Once these ligands are
synthesized, their corresponding ruthenium
complexes can be made. Varying the
ligands will give each complex different
electrochemical properties and potentially
change the steric interactions between the
ligand and the metal. The most optimal
ligand will have a weak base that transfers
protons effectively but is incapable of
binding to the metal. We need to make and
test a variety of ligands in order to determine
what is best.
In order to make all the analogs of the ligand
(9), based on previous experiments in the
Grotjahn lab, a three step approach (Figure
5) is expected to lead to all of the desired
ligands.[8]
Figure 5
The T.J. Meyer group installed an NHC
bearing a pyridyl substituent without a base
(9 with B = H). An NHC is a stronger base
than a pyridine, causing an increased trans
effect that will allow easier separation of O2
from 6. They did this in 2 steps from
imidazolium salt[7]
(14, B=H) and we will
follow their procedure.
There are several ways to test for oxygen
evolution and the Grotjahn lab has
experience with them.[7]
In the area of alkane oxidation, substrates
such as cis-decalin or cis-1,4-
dimethylcyclohexane will be tested as used
by Crabtree’s group.[2]
We can look not
only for C-H oxidation to C-OH but also
whether there is retention of configuration,
which would be significant evidence against
carbocation or radical reactions, The studies
done by Zhou et al. have shown that metal
catalysts are essential to forming reasonable
quantities of the desired product and the
N
N
N
Ru
X
N
N
7
N
N
N
Ru
X
N
N
8
B
B
N
N
N
Ru
X
N
N
N
B
9
N
N
H
NaH
N
N
N
CH3X
N+
N
N
X-
N
N
N
Br
Na
B
B B

3. retention of stereochemistry. Without a
catalyst the reaction was very low yielding
and gave a ratio of 1.5:1 of the possible cis
and trans stereoisomers. Addition of the
catalyst caused a dramatic increase of
product and boosted the ratio to 300:1,[2]
which is what we hope to see.
D. Conclusion
The Grotjahn group are experts at both
designing and synthesizing ligands for
proton transfer. It is expected we will make
a significant contribution to two fields,
oxidation of water and oxidation of organic
compounds. The latter reaction would be
useful for the syntheses of organic
molecules.
References
[1] M.S. Chen, M.C. White, Science 327, 566 (2010).
[2] Zhou, M.; Schley, N. D.; Crabtree, R. H., "Cp*
Iridium Complexes Give Catalytic Alkane
Hydroxylation with Retention of Stereochemistry," J.
Am. Chem. Soc. 2010, 132, 12550-12551.
[3] Zhou, M.; Hintermair, U.; Hashiguchi, B. G.; Parent,
A. R.; Hashmi, S. M.; Elimelech, M.; Periana, R. A.;
Brudvig, G. W.; Crabtree, R. H., "Cp* Iridium
Precatalysts for Selective C-H Oxidation with
Sodium Periodate As the Terminal Oxidant,"
Organometallics 2013, 32, 957-965.
[4] Savini, A.; Belanzoni, P.; Bellachioma, G.;
Zuccaccia, C.; Zuccaccia, D.; Macchioni, A.,
"Activity and degradation pathways of pentamethyl-
cyclopentadienyl-iridium catalysts for water
oxidation," Green Chem. 2011, 13, 3360-3374.
[5] Zuccaccia, C.; Bellachioma, G.; Bolano, S.;
Rocchigiani, L.; Savini, A.; Macchioni, A., "An
NMR Study of the Oxidative Degradation of Cp*Ir
Catalysts for Water Oxidation: Evidence for a
Preliminary Attack on the Quaternary Carbon Atom
of the -C-CH3 Moiety," Eur. J. Inorg. Chem. 2012,
2012, 1462-1468.
[6] Meyer,T.J., “The art of splitting water,” Nature
(London, United Kingdom) 2008, 451, 778-779.
[7] Grotjahn, D.B.; Brown, D. B.; Martin, J. K.;
Marelius, D. M.; Abadjian, M. C.; Tran, H. N.;
Kalyuzhny, G.; Vecchio, K. S.; Specht, Z. S.; Cortes-
Llamas, S. A.; Mirando-Soto, V.; Niekerk, C.;
Moore, C. E.; Rheingold, A. L., “Evolution of
iridium-based molecular catalysts during water
oxidation with ceric ammonium nitrate,” J. Am.
Chem. Soc., 2011, 133, 19024-19027
[8] Specht, Z.; Cortes-Llamas, S.; Tran, H.; van Niekerk,
C.; Rancudo, K.; Golen, J.; Moore, C.; Rheingold,
A.; Dwyer, T.; Grotjahn, D., “Enabling
Bifunctionality and Hemilability of N-Heteroaryl
NHC Complexes,” Eur. J. Inorg. Chem. 2011, 17,
6606-6608