Transcript RuP(OMe) 2

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
Since Demas and Adamson’s 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)3]+2 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.
RuP(O Me)2 vs. RuPO Me
[RuPOMe]
N
Ru
N
…………………………..
Me
P
P
Me O
Ru
O Me
2.0
RuPOMe
[2.85 x 10-3M]
RuP(OMe)2
438nm
1.5
1.0
0.5
Me O
OMe
0.0
350
Ru
N
N
N
N
OMe
OMe
O
N
400
450
500
550
600
650
700
750
Wavelength (nm)
N
N
Emission Concentration Dependence
RuP(O Me)6 in Acetone/ Ethanol 2:1 (v/ v) 298K
[RuP(OMe)6]
[RuP(OMe)2]
1.11 x 10-3M (424nm)
4.0
RuP(OMe)6
[3.0 x 10-3M]
Emission Intensity
3.5
Absorbance Spectra
2.22 x 10-4M (413nm)
3.0
2.5
4.44 x 10-5M (402nm)
2.0
1.5
1.0
0.5
0.0
360
RuPO Me [5 x 10 M]
EtO H/ MeO H 4:1 (v/ v)
2.5
3
2
2.5
MLCT
1.5
450nm
1
Absorbance
Absorbance
3
400
420
2
1.5
c
460
480
500
520
540
Conclusions
1
350
450
550
Wavelength (nm)
650
750
0
325
375
425
Wavelength (nm)
475
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.
20
MLCT
15
21.7kK
10
Molar Absorbance
10 -3 (cm-1.M -1 )
35
25
5
0
50
45
40
35
30
-3
Wavenumber 10 (cm-1)
25
20
31P
525
RuP(O Me)6 [7.32 x 10 -5 M] in CH3 CN
30
ppm
Wavelength (nm)
5.7 x 10-4M
2.3 x 10-4M
9.2 x 10-5M
3.7 x 10-5M
λmax
374nm → 380nm
440
0.5
0
250
380
Concentration Dependence RuP(O Me)2
EtO H/ Acetone 2:1 (v/ v)
-5
The synthesis of bis(2,2’-bipyridyl){diphenyl(2-methoxyphenyl)phosphine} ruthenium(II)
hexafluorophosphate [RuPOMe] has been reported by Rogers and Wolf.2
4’-Tolylterpyridylbis(2-methoxyphenyl)phenylphosphineruthenium(II) tetrafluoroborate
[RuP(OMe)2]
and
4’-tolylterpyridyltris(2,6-methoxyphenyl)phosphineruthenium(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 phosphineether 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.
MeO
MeO
PhP
Emission Intensity
N
0.5
Synthesis
RuPOMe
601nm
2.5
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-associative 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.
Room Temperature Emission
Hemilabile Complexes
Introduction
15
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.
31P
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.
NMR Studies
NMR Chemical Shifts (ppm)
RuPOMe
RuP(OMe)2
RuP(OMe)6
Free Ligand
-14.45
-31.7
-70.8
Complex
56.95
41.8
15.3
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.
Acknowledgements
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
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.