Download Synthesis and Characterization of PhCC-Ru-L-Ru

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RWU STUDENT RESEARCH
PROPOSAL
Name of applicant:
Lindsey Sesin
General area of research:
Inorganic Chemistry
Title of research project:
The study of ruthenium dimers based on trans(dppm)2ClRuCCPh.
Abstract
The purpose of this project is to synthesize linear ruthenium complexes and study the
interactions between bridged ruthenium metal centers. This will be accomplished by
synthesizing systems based on the dimeric compound trans-trans-( R-PhCC-Ru(dppm)2BL-Ru(dppm)2-CCPh-R’), where BL are the bridging ligands pyrazine, 4,4’ bipyridine,
4,4’ bipyridyl ethane, 4,4’ bipyridyl ethene, and 4,4’ bipyridyl ethyne. The R and R’
subtituents in the para position on the ligand formally trans to BL are various electron
donating and electron accepting groups. Multinuclear complexes based on trans-trans-(
R-PhCC-Ru(dppm)2 are of interest because its linear geometry allows potential fabrication
of, and subsequent investigation of, extended molecular architectures that may ultimately
find utility in a wide variety of molecular electronic devices.
Date of submission of proposal to committee:
November 30, 2005
Amount of funding required:
$1004.00
Final Report Due:
November 31, 2006
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Introduction
Many research groups have investigated the ability of transition metal complexes
to transfer electrons from both their ground and electronically exited states.1 One
particularly interesting result is the use of ruthenium-based chromphores to convert light
energy into electrical energy or stored chemical energy.1 Since complexes based on
ruthenium exhibit various stable oxidation states, absorbs light, and is synthetically
versatile, it is an attractive complex for a wide variety of studies.2 Research has shown
ruthenium’s ability to function efficiently in liquid-based photovolatic cells by absorbing
light and transferring electrons into conduction bands of solid state semiconductors.3 This
has been an important factor in advancing the studies of converting solar energy into
electricity. Transition metals in general have been linked together with a wide variety of
bridging ligands.4 Many groups have shown that the π network of orbitals on the bridging
ligand provides a pathway for delocalization of electron density between the metal and
ligand creating an efficient mechanism for communication between two or more metal
centers.4 In addition, it has also been postulated that geometrically linear ruthenium
bridged ligand complexes, when properly exploited, can display a more efficient transfer
of electrons than those with non-linear geometries.5
Over the past five years, chemistry students at RWU have made significant
progress exploring a related ruthenium complex, trans-[Cl(pyridine)4RuL]+ (Fig 1A). A
previous study carried out by by Amanda Kuber (RWU ‘01) examined different
ruthenium complexes where L = pyridine, pyrazine, 4, 4’-bipyridine. However, Kuber’s
attempts to remove the chloride ligand trans to the L ligand were unsuccessful because of
pyridine’s inherent ability to remove large amounts of electrons density from the
ruthenium metal center. This makes it virtually impossible to remove the chloride ligand
from the highly positive ruthenium center without decomposing the complex. Changing
pyridine ligands to diphenylphosphino methane (dppm) ligands dramatically alters the
electronics of the system to our advantage. (Fig. 1B)
Py
Cl
Ru
Cl
Ph2P
Py
Py
L
Py
A
PPh2
Ru
Ph2P
L
PPh2
B
Figure 1. trans-LRuCl2py4 (A) where L = pyridine, prazine or 4,4’-bipyridine and
trans-L-RuCl(dppm)2 (B) where L = phenylacetylide ligand
Synthetically altering the electronic nature of the ruthenium center has allowed us
to successfully remove the chloride ligand. This has been demonstrated recently by Mr.
Jason Hill (RWU ’05). Mr. Hill explored the rate in which the chloride ligand can be
extracted from the ruthenium metal center in trans-(ClRu(dppm)2CCPh) by reacting the
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halide containing metal complex with Tl+ ion. (Fig. 2). Understanding the kinetics of this
complex is an important foundation for my research because it is has not only elucidated
the kinetic parameters associated with removing the chloride ligand but has
unequivocally demonstrated that it is possible to remove the halide without decomposing
the metal complex.
PPh2
Cl
Tl+
PPh2
Ph2P
Ru
C
PPh2
C
R
Ph2P
Abstraction
S
Ru
C
C
R + TlCl
S= solvent
Ph2P
PPh2
Ph2P
Figure 2. Abstraction of Cl- using Tl
+
Mr. Hill’s results elegantly revealed that the abstraction of the trans chloride
ligand depended linearly on the electron accepting/donating characteristics of the para
substituent (R) attached to the phenyl acetylide ligand (Fig. 3). Mr. Hill’s results are
useful because it provides insight into the role R plays in the ability to remove the
chloride ligand.
H
C
C
R
R=NH2, H, Cl, COH, CN, NO2
Figure 3. Phenyl acetylene ligand with para- substituent R
The results of this study strongly suggest that the nature of the electronic
communication between two bridged ruthenium complexes can be modulated by the R
substituent (Fig. 3). Thus, it is of interest to evaluate the extent of electronic interactions
between ruthenium dimers such as the ones proposed and to determine how R impacts the
distribution of electron density in the molecule.
My specific research will synthesize and investigate various dimeric Ru(dppm)
complexes that are tied together with a bridging ligand. An important part of this study
will be the successful abstraction of the trans-chloride ligand attached to the metal
ruthenium with Ag+ followed by reactions with bridging ligands that can link the metal
centers. This work has not been previously attempted here at RWU. Mr. Hill
demonstrated that the chloride can be removed and that the open coordination site could
be replaced with a solvent molecule (acetonitrile) but he did not generate any other
monomeric or dimeric complexes based on the Ru(dppm)2 metal centers.
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Once the trans-chloride ligand is abstracted, a desired ligand containing an
immine nitrogen (pyridine, pyrazine, etc..) will replace it to confirm that the ruthenium
complex will not decompose. The lone pair of electrons on nitrogen allows the pyridyl
ligand to coordinate to metal ions. Also, the anti bonding  -type orbitals on the nitrogen,
that caused much grief in previous studies, in this case will actually increase the strength
of the resulting M-L bond thus stabilizing our desired product. Once pyridine is
successfully attached to the ruthenium complex without destroying the compound, other
proposed bridging ligands (Fig 4) will be attempted.
If I am able to make significant progress on this project, I am hoping to present
this research at the National ACS conference in Chicago, Illinois in the Spring of 2007.
N
N
C
H2
C
H2
N
4,4' bipyridyl ethane
N
C
H
C
H
N
4,4' bipyridyl ethene
N
C
C
N
4,4' bipyridyl ethyne
N
pyrazine
N
N
4,4'-bipyridine
Figure 4. Commercially available bridging ligands
Detailed Project Description
The starting material, trans-(ClRu(dppm)2CCPh-4-R) ,will be prepared from
published procedures.7 R is NH2, H, Cl, COH, CN, or NO2.
Each trans-(RuCl(dppm)2CCPh-4-R) monomer will then be reacted with Ag+ to
remove the chloride ligand as shown in figure 5. Results for varying substituent R will
be studied and documented to find the best way to remove the chloride ion.
PPh2
PPh2
Ph2P
Ph2P
Abstraction
Cl
Ru
C
Ag
C
S
R
Ru
C
S= solvent
+
PPh2
Ph2P
PPh2
Ph2P
Figure 5. Abstraction of Cl- using Ag+
C
R + AgCl
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Subsequent addition of 0.5 equivalents of a desirable bridging ligand (Fig. 4)
should result in a high yield of the proposed bridged species. Products will be purified
via column chromatography and recrystallized from DMF/Et2O solutions using standard
procedures.
One desired product in particular will be:
PPh2
N
Ph2P
R
PPh2
C C Ru
Ph2P
Ru C C
R
PPh2 Ph2P
N
PPh2
Ph2P
O
Figure 6. trans-trans-[Ru(CCHC6H4-4-R]Cl(dppm)2 bridged
with 1,5-dipyridyl-1,4-pentadiene-3-one
The bridging ligand 1,5-dipyridyl-1,4-pentadiene-3-one (Fig. 6) is the subject of
another proposal (Ramkamur ’06/ von Riesen) being submitted concurrently. This
bridging ligand is intriguing as it should exhibit proton induced switching by taking
advantage of the pH induced keto/enol equilibria. Protonation of the keto-oxygen should
disrupt the conjugation between the metal centers thus diminishing the extent of
electron interaction that can take place between ruthenium centers. This should be
evident in both the spectroscopy of the dimer as well as in the electrochemical behavior
(cyclic voltammograms) of the complex.
Research Methods and Procedures
Using a published procedure and Mr. Hill’s lab notebook containing his kinetics
study of Ru(dppm), [Ru(CCHC6H4-4-R]Cl(dppm)2 will be obtained through a series of
successive reactions with toluene, [RuCl2(MeSO)4], dppm, CH2Cl2, 4-HCCC6H4R,
NH4PF6, NEt3 and hexane, which are all available through the Aldrich Chemical
Company. All reactions take place under positive pressure of argon gas. All reactions
will be studied using RWU’s instruments.
Laboratory Space Required
All procedures will be carried out in the lab facilities at RWU.
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References
1. Roundhill, D.M. Photochemistry and Photophysics of Coordination Compounds,
Wiley, New York, 1994.
2. Juris, A.; Campogna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Cord. Chem.
Rev., 1988, 84, 85.
3. Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes,
Academic Press, London, 1992
4. McGrady J.; Lovell T.; Stranger R.; Humphrey M.G. Organometallics. 1997,
16,4004.
5. Lampert, C.M., Solar Energy Materials and Solar Cells, 1994, 32, 221
6. Miessler, G.; Tarr, D.; Inorganic Chemistry, 3rd Ed., Pearson Prentice Hall, 2004.
7. Zakeeruddin, S.; Nazeeruddin, M.; Rotzinger, F.; Kalyanasundaram, K.;Gratzel,
M.; Inor. Chem. Rev., 1988, 84, 85.
8. McDonagh, A.M.; Deeble, G.J.; Hurst, S.; Cifuentes, M.P.; Humphrey, M.G.; J
Chem. Ed. 2001, 78, 232.
9. Choua, S.; Jovaiti, A.; Geoffroy, M.; Physical Chemistry Chemical Physics, 1999,
1, 3557.