Download Syntheses, Structures and Photophysical Properties of Metal

This study involved the syntheses of fluorescent
transition metal complexes that bear luminophores.
Syntheses, Structures and Photophysical
Properties of Metal Carbonyl Clusters with
Dansyl and Acridone Luminophores
Wai-Yeung Wong, Ka-Ho Choi, and Zhenyang Lin
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An overview of what will be discussed:
• The syntheses:
• A new series of fluorescent dinuclear and trinuclear
metal complexes bearing dansyl and acridone
entities were produced.
• Used groups 8 and 9 transition metal elements with
acetylenes
• Properties of the new compounds:
• Spectroscopic
• Structural
• photophysical
Luminophore: atomic grouping in a molecular entity that increases
its ability to emit light (ie. when its electronic excitation is
associated with a given emission band that is approx. localized.
Origin: L. Lumen, light, + G. Phoros, bearing
In this study, dansyl and acridone luminophores were used
Importance: wide applications in environmental and biological
sciences, and molecular electronics
In fact, fluorescent methylmercury acetylides (containing acridone
and dansyl) have been synthesized which could provide methods
for detecting methylmercury in environmental and biological
samples.
Methylmercury: health risk to humans and wildlife that consume
large quantities of mercury contaminated fish.
How they synthesized the fluorescent
complexes :
• Fluorescent-labeled alkyne ligands were reacted with metal
carbonyl compounds
• Dansyl (R1) and Acridone (R2) are the fluorescent labels.
• Dansyl: 5-(dimethylamino)napthalene-1-sulfonyl
• Acridone: structure on next page
• These were incorporated into the following derivitized
molecules (alkyne ligands).
• 5-(dimethylamino)-N-(2-propynyl)-1naphthalenesulfonamide (I)
• 10-(2-propynyl)-9-acridone (II)
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5-(dimethylamino)-N-(2-propynyl)-1-naphthalenesulfonamide (I)
Structures I and II are alkyne ligands:
10-(2-propynyl)-9-acridone (II)
• Reaction of alkynes (RC=CR) in organometallic
complexes:
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Has 2 occupied π-MO’s
Can potentially donate 2- or 4- electrons
The C=C bond length usually lengthens upon complexation
Bond length in complexes: ~124-137pm (indicates a weaker bond,
with loss of triple bond character)
– The C=C bond length of a free alkyne~120pm
– There are many known mono- and polynuclear organometallic
complexes involving alkyne ligands
– This study involves polynuclear complexes.
The alkyne ligands bearing the luminophores were
reacted with the following metal carbonyl
compounds:
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[Co2(CO)8]
[Et3NH][Fe2(CO)6(µ-CO)(µ-StBu)
[Ru3(CO)10(NCMe)2]
[Os3(CO)10(NCMe)2]
• Structures I and II provide a way of attaching a fluorescent
label to a metal core via an acetylide linkage.
8 new cluster complexes were obtained:
[Co2(CO)8( µ-η2-RCH2CCH) R=R1,R2
(1,2)
[Fe2(CO)6(µ-StBu)(µ-η2-RCH2C=CH2)]
(3,4)
[Os3(CO)9(µ-CO)(µ3- η2-RCH2CCH)]
(5,6)
[Ru3(CO)9(µ-CO)(µ3- η2-RCH2CCH)]
(7,8)
4 x 2(using R1 and R2) complexes were produced.
Total=8
Recall: R1 represents dansyl, R2 Acridone (the
luminophores)
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The syntheses:
• The fluorescent dinuclear colbalt complexes (1,2) were
prepared using one equivalent of I and II, respectively
[Co2(CO)8]
+ RCH2C=CH
Reaction performed at 0 degress celsius
[Co2(CO)8( µ-η2-RCH2CCH)
The iron carbonyl salt, [Et3NH][Fe2(CO)6(µ-CO)(µ-StBu)] was
thermally treated with I and II:
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σ,π-vinyl complexes (3,4) were obtained
Vinyl: from organic chemistry, a vinyl group is CH2=CHσ,π refers to bonding that occurs (more later)
[Fe2(CO)6(µ-StBu)(µ-η2-RCH2C=CH2)]
The reaction of the alkynes (I and II) with
[Ru3(CO)10(NCMe)2]& [Os3(CO)10(NCMe)2]:
• Displacement of MeCN by the terminal alkynes occurred
at room temperature
• And the following Ru and the Os complexes were
produced :
[M3(CO)9(µ-CO)(µ3- η2-RCH2CCH)] where M = (Ru, Os)
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These new complexes have been fully characterized
by:
The New Compounds:
• Purified by preparative TLC on silica
• Isolated as:
– orange to red solids (1-4)
– yellow solids (5-8)
• (1-8)- high solubility in dichloromethane
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FTIR
H NMR spectroscopy
UV/Vis spectroscopy
FABMS (fast atom bombardment mass spec)
Also, single x-ray structural analyses and molecular
orbital calculations were performed on selected
complexes (1,2,4 and 8 from previous slide)
The results of IR, HNMR, Mass spectrometry &
Absorption/Emission Spectra:
• A little bit about IR:
- organic molecules (ie. the alkyne ligands) are constantly
vibrating.
- Each stretching/bending vibration occurs at a specific
frequency depending on what kind of bond(s) are involved.
- The molecule will absorb the energy of the IR radiation if
it matches the frequency of one of its vibrations (molecule
will stretch and bend more)
- In this way, different kinds of bonds can be identified by
the specific wavenumber (1/cm) of light that is absorbed.
The IR spectrum of the new series of fluorescent complexes
indicated the following:
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The disappearance of the ν(C=C) and ν(=C-H) bands showed that the
acetylenic unit (-CH2C=CH) must be coordinated to the metal carbonyl
fragments. This was observed for all of the new complexes (1-8)
• The Ru (5,6) and Os (7,8) complexes displayed a broad
peaks at 1880 1/cm and 1848 1/cm. These peaks indicate
the presence of a bridging carbonyl group in each case.
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HNMR spectra:
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NMR spectroscopy- absorption of radiation to determine structural
feature of organic compounds. HNMR specifically determines C-H
framework.
• Resonances stemming from the coordinated dansyl and
acridone group show:
– Signals for the –C=CH protons in I and II were absent
– Presence of singlet peak was resolved for –CCH protons (except in
3 & 4)
– 3&4 have vinyl protons- were shown as doublets
Mass spectroscopic data:
• A 1:1 metal/ligand stoichiometry was established from the
parent ion peaks (for each of the new compounds)
Electronic Absorption and Luminescence Spectra:
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Luminescence: is the emission of light by a substance caused by
physical or chemical means.
In this case, the free alkynes absorb light in the near-UV region which
are a result of π- π* transitions in the ligand framework. (remember:
the ligand framework contains the luminophores ;acridone and dansyl)
The absorption of light promotes an electron into a higher energy
state/level
Since the excited electronic states are unstable, the electron eventually
falls back to a lower energy state and loses its excess energy.
These molecules are capable of emitting some of this energy as light
(fluorescence).
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-Complexes 1-8 were excited in CH2Cl2 which resulted in blue to
greenish-blue luminescence (emission) bands.
-The peaks of these bands were in the range of 506-509 nm for the
dansyl derivatives while the adridone derivatives had peaks in the 425431 nm range.
-The complexed fluorophores were found to have very similar
emission wavelengths to the uncomplexed ligands (I and II).
-However, an important finding was that the fluorescence quantum
yields of the metal complexes were less than I and II by one to two
orders of magnitude.
- Quantum yield: # of species removed/number of photons absorbed.
-this decrease in luminescence intensity is known as quenching and
may be due to the heavy-atom effect, energy transfers or electrontransfer mechanism.
-The excitation and photoluminescence spectra of 1 and 2 are
represented on the following slide
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506-509 nm
(dansyl luminophore)
425-431 nm
(acridone luminophore)
Now, an overview of what was determined via crystal
structure analyses and molecular orbital calculations:
X-ray crystallography:
-a standard technique used for solving crystal structures
-good quality crystals of the compounds are required for x-ray diffraction
studies
-By using this technique, the experimental geometries of 1,2,4,and 8 were
determined.
-For 1 and 2 (dinuclear cobalt complexes), the structural features are
similar
This is structure 1. Here it can be seen that the Co(1) and Co(2)
axis forms a pseudo-tetrahedron with the C=C bond of the alkyl ligand.
-The C=C vector lies ppd to the Co-Co vector
-Co-Co separation: 2.492 A
-C(7)-C(8) bond length: 1.341 A
-suggests loss of triple bond character of alkyne
ligand upon complexation
- actual triple bond length ~1.20A
-Structure 1 is shown on next slide.
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This is the structure of 4 (the dinuclear iron complex).
-the Fe-Fe edge is bridged by both the thiolate and the µ-σ-vinyl group
-Fe(2)-S(1) length shorter then Fe(1)-S(1) -this asymmetric bridge is typical for diiron
compounds
-However, the C(7)-S(1) bond is slightly longer than typical C-S bonds indicating
delocalization of electron density (from thiolate bridge to metal core).
-The C(11)-C(12) bond is bound to Fe(2) by π-interaction
-bound to Fe(1) via σ –interaction
-the C(11)-C(12) bond
length 1.37A characteristic of
vinylic coordination
This is the structure of 8 (the trinuclear osmium complex):
-the three Os atoms are coordinated to the alkyne ligand by 2 σ bonds through Os(1) and
Os(3) and a π bond to Os(2).
-the C(11) –C(12) bridges the Os(1)-Os(3) bond
-a 4 membered ring is formed,
making a 58.1 degree dihedral
angle with Os3 triangle
The x-ray data provided the experimental geometries of the compounds.
Molecular orbital calculations were then performed based on these
geometries.
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Common features for the 4 complexes (1,2,4,&8):
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The molecular orbitals in the frontier region show very similar characteristics
HOMO-LUMO gaps (for the 4 clusters) are close to each other
HOMO corresponds to pi-bonding in the aromatic unit (from dansyl/acridone)
in alkyne ligand
LUMO corresponds to metal-metal antibonding
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In comparison to the uncomplexed alkyne ligand (I):
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HOMO-a contribution from the p-pi orbital of NMe2 along with the pibonding within the aromatic unit.
LUMO-the π* orbital of the aromatic moiety (fragment).
The energy gap: 33,792 1/cm
HOMO-LUMO energy gap of complex 1 was calculated to be 23, 356
1/cm
This energy gap is less than I.
Thus, the bridging of the alkyne ligand significantly affects the orbital
structure in the HOMO-LUMO region.
From the fact that the LUMO’s for the new complexes involve metalmetal antibonding (possibility of fragmentation), the life-time of the
excited state of these complexes is shortened compared to the
luminophore itself.
From the earlier discussion of the absorption/emission spectra, we
know that there was a strong quenching of the fluorescence intensity
upon comlexation of the alkynes.
– LUMO’s for I (alkyne): corresponds to the π* orbital of the aromatic
moiety (fragment). Longer-lived excited state
– LUMO’s for complex: metal- metal antibonding. Shorter-lived
excited state.
Wong, Choi and Lin suggest that poor fluorescence intensity is a
result of the shortened lifetime of the excited states in the new
complexes.
In Conclusion:
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A new series of fluorescent-labeled metal clusters was successfully
synthesized.
The metal complexes (groups8, 9) containing dansyl and acridone
luminophores were obtained in moderate yield.
They were characterized spectroscopically (IR, Mass Spec,
Absorption/ emission spec. etc.) and with x-ray crystallography, and
molecular orbital calculations
The new complexes emit blue to blue-green light (in solution at room
temp.)
The luminescence bands were typical of the fluorophores (although
they had been quenched considerably)
• They demonstrated how luminescence properties of
fluorophores can be altered in presence of metal cluster
cores.
• The understanding of the quenching processes will be
important in the development of new molecular
luminescent sensing systems.
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