Since Demas and Adamson

1
Hemilabile Coordination Complexes as a Tool for
Small Molecule Sensing Anthony Tomcykoski,
Wayne E. Jones Jr. Department of Chemistry, SUNY
at Binghamton, Binghamton, NY 13902

Room Temperature Emission
Hemilabile Complexes
Introduction
Since Demas and Adamsons introduction of
tris(2,2-bipyrdine)ruthenium(II) as a
photosensitizer, numerous applications have been
developed to take advantage of its rich
photophysical properties.1 In the ground state,
Ru(bpy)32 is a chiral molecule with ? and L
isomers. Upon absorption of electromagnetic
radiation in the solar spectrum, various
electronic excited states are observed. Of
particular interest is the spin-forbidden
transition state (3MLCT) having a long excited
state lifetime in solution (5µs) that radiatively
decays in high yield. Utilizing this complex as a
luminescent model plays a significant role in
developing fluorescent chemosensors. Hemilabile
ligands have been of great interest to chemists
working toward the development of molecular
sensors. Hemilabile coordination is found to
occur amongst polydentate ligands that contain
both chemically inert and labile sites bound to a
metal center. In the presence of molecules with a
strong affinity to the metal center, an exchange
reaction can occur in which dissociative-associati
ve and interchange mechanisms have been proposed.
After reacting, the hemilabile ligand will remain
tethered in close proximity to the metal center
due to the inert binding position. Upon
coordination by a competing molecule, the
photophysical properties of the complex as a
whole will change resulting in a signal that can
be monitored. Of the three classes of
chemosensors, chromophoric, potentiometric, and
fluorescent, this research aims at the latter as
the means by which the signal is obtained. We
have been investigating a series of hemilabile
coordination complexes which contain polypyridyl
chromophoric ligands centered on ruthenium. These
systems show promise as chemosensors due to
electronic transitions to the chromophoric
polypyridyl ligand. In terms of hemilabile
coordination, various phosphine-ether ligands
have been explored as ancillary ligands due to
the inert phosphine binding site and labile ether
binding site. Phosphine-ether ligands previously
have been shown to exhibit reversible binding in
the presence of small polar molecules such as
acetonitrile, acetone, and water. With many
possible applications, the use of these complexes
as humidity sensors is the driving force within
the scope of this research. In addition, the
tridentate complexes are designed in a manner to
serve as receptor units in conjugated polymer
systems. An application of this type would allow
for the construction of thin film sensors
suitable for practical devices.
RuPOMe
RuPOMe 2.85 x 10-3M
RuPOMe 601nm RuP(OMe)2 438nm
RuP(OMe)6 3.0 x 10-3M
RuP(OMe)6
RuP(OMe)2
1.11 x 10-3M (424nm) 2.22 x 10-4M
(413nm) 4.44 x 10-5M (402nm)
Absorbance Spectra
ppm
Conclusions
The blue shift in emission spectra implies that
the energy gap between the excited state and
ground state increases. This can be described
based on destabilization of the LUMO by
introduction of electron donating groups. The
presence of methoxy groups on the ancillary
ligand shifts the electron density towards the
metal center resulting in an increasing frequency
in transitions.
5.7 x 10-4M 2.3 x 10-4M 9.2 x 10-5M 3.7 x 10-5M
The photophysics of hemilabile coordination
complexes are dependent upon concentration. The
complexes RuPOMe, RuP(OMe)2, and RuP(OMe)6 show
blue shifting emission spectra with lmax of 601,
438, and 424nm respectively. RuP(OMe)6 shows a
blue shift in the emission band with decreasing
solution concentrations. The main differences
between the bidentate and tridentate ligands are
seen as a blue shift in emission spectra.
Although the complexes studied are luminescent at
room temperature, terpyridyl complexes in general
have a much shorter excited state lifetime. This
photophysical property makes bipyridyl
chromophores slightly more appealing for
practical use. Finally, the 31P NMR spectra show
unique complex resonances alluring to the fact of
coordination through phosphorous.
MLCT 450nm
Synthesis
31P NMR Studies
Other means must be employed to convincingly show
the relative amount of each specie present in
solution. Quantitative NMR techniques may be used
to show abundance of a specific nuclei of
interest. Given that phosphorous nuclei are
spin-½ with 100 natural abundance, 31P NMR can
be used to show various complex forms in a given
sample solution. The primary interest for
acquiring 31P NMR spectra is to show coordination
through phosphorous. Further tests need to be
performed to assign all peaks observed, but the
resonance furthest upfield can be labeled as the
ether-bound complex. The least chemically
shielded nuclei resonate at a higher frequency,
and this is the case for the bound phosphine
ligand with coordinate covalent bonds to
ruthenium through methoxy groups.
The synthesis of bis(2,2-bipyridyl)diphenyl(2-me
thoxyphenyl)phosphine ruthenium(II)
hexafluorophosphate RuPOMe has been reported by
Rogers and Wolf.2 4-Tolylterpyridylbis(2-methoxyp
henyl)phenylphosphineruthenium(II)
tetrafluoroborate RuP(OMe)2 and
4-tolylterpyridyltris(2,6-methoxyphenyl)phosphine
ruthenium(II) tetrafluoroborate RuP(OMe)6 are
prepared by reacting one equivalent of the
terpyridyl ligand and LiCl with RuCl3.nH2O in
N,N-dimethylformamide at reflux for 48 hours. The
solution is cooled to room temperature to which
is added an equal volume of acetone. The reaction
mixture is refrigerated overnight and then
filtered through fritted glass. The crystals are
stored and used as the starting material for
coordination by the tridentate hemilabile ligand.
Ru(ttpy)Cl3 is dissolved in acetone and to the
mixture is added three equivalents of AgBF4 to
stir overnight under a steady stream of nitrogen.
Upon filtration of solid AgCl, one equivalent of
the appropriate phosphine-ether ligand is added
to the acetonated complex and allowed to react at
reflux for 48 hours under N2 atmosphere. The
mixture is cooled with any solid impurities being
separated by filtration. The reaction mixture is
evaporated to dryness and the crystals stored in
a desiccator.
Acknowledgements
MLCT 21.7kK
A.T. and W.E.J. thank The Research Foundation and
The Chemistry Department for financial support. A
special thanks goes to Dr. Jürgen Schulte and Dr.
Justin Martin for instrumental support and useful
discussions.
References
31P NMR Chemical Shifts (ppm)

1. Demas, J.N. Adamson, A.W. J. Am. Chem. Soc.
   1971, 93, 1800-1801
2. Rogers, C.W. Wolf, M.O. Chem. Commun. 1999,
   2297-2298
3. Angell, S.E. Zhang, Y. Rogers, C.W. Wolf,
   M.O. Jones, W.E. Inorg. Chem. 2005, 44,
   7377-7384.

For RuP(OMe)6, the band at 2.17 x 104 cm-1
(460nm) can be identified as a metal-to-ligand
charge transfer (MLCT) with a molar absorptivity
of 1.52 x 103 cm-1.M-1. The bottom spectrum is
displayed in in terms of wavenumber (cm-1) to
show a linear relationship to energy.
Free Ligand Complex
RuPOMe -14.45 56.95
RuP(OMe)2 -31.7 41.8
RuP(OMe)6 -70.8 15.3
..