Pyridine Ligands

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Pyridine Ligands
2
and the Stability of
Birju Patel Johns Hopkins University December
19, 2007
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Cyclam-Chelated
Advanced Inorganic Chemistry Lab Professor
Justine Roth TAs Ankur Gupta and Simone
Novaes-Card
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Ruthenium(II) Complexes
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Hypothesis

• Since the macrocycle effect confers thermodynamic
  stability on Ruthenium(II) complexes, we expect
  to be able to measure this stability as it is
  affected by the steric tension caused by both
  bulky and bridged ligands through spectroscopic
  analysis (UV and 1H NMR). In doing so, this
  experiment also hopes to synthesize a new
  bridged/macrocycle Ruthenium(II) complex which
  can be useful for modelling other thermodynamic
  qualities of second row transition metals.

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Chemical Background
Cyclam (14aneN4)
Chelate
1,4,8,11-Tetraazacyclotetradecane (CAS 295-37-4)
2,3-DPP
Bridging Ligand
2,3-Bis(2-pyridyl)pyrazine (CAS 35005-96-3)
Bpy
Non-bridging Ligand
2,2-Bipyridyl (CAS 366-18-7)
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RuIICl2(cyclam)
(µ-2,3-DPP)RuII (cyclam)2
(DPP)RuII (cyclam)2
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Ruthenium Chemistry

• Ruthenium(II) complexes are interesting catalysts
  for their photophysical and redox properties5
• There has been increasing interest in
  supramolecular chemistry, especially in the
  complexes as ligands and complexes as metals
  approach, which have given insights into energy
  migration patterns in the visible range6
• RuIICl2(macrocycle) are stable as cis-compounds
  and undergo high rates of chloride ligand
  substitution7 this stability is mostly due to
  the chelate effect
• Steric effects in the trans compound have been
  observed by cyclic voltammetry2 these studies
  also showed stability encouraged by the larger
  size of RuII versus RuIII

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Analytic Background

• UV will most likely show bpy-centered p ? p
  transitions4 in the UV region (280 nm). Visible
  range spectrum transitions in the range of 500 nm
  will be Ru-Bridging Ligand CT and below 400 nm
  will be Ru-bpy CT
• Bulkier ligands will cause UV-Vis ?max to
  increase lower energy transition from eg
• 1H NMR data should show shielding of the cyclam
  hydrogens when steric tension plays a role
  through bulky/bridged ligands8

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Method

• Synthesis of Tetra(triphenylphosphine)ruthenium(II
  ) dichloride (method adapted from 1, 2, 3)
• Reflux Ruthenium trichloride trihydrate (0.2 g)
  in methanol (50 ml) and a sixfold excess (1.2 g)
  of triphenylphosphine under argon for 3 hours
  vacuum filter
• Synthesis of cis-Ru(cyclam)Cl2
• Add 0.6g Tetra(triphenylphosphine)ruthenium(II)
  dichloride to 0.1g cyclam in 30 ml benzene and
  heat the solution for 20 h at 45C
• Vacuum filter and recrystallize with hot
  methanol-water
• Measure UV-Vis and 1H NMR spectra in benzene
  solvent
• Synthesis of µ-2,3-DPPcis-Ru(cyclam)2Cl4
• Reflux 0.05g cis-Ru(cyclam)Cl2 with 0.03g DPP in
  15ml EtOH for 2 h
• Vacuum filter and wash with ethanol
• Measure UV-Vis and 1H NMR spectra in benzene
  solvent
• Synthesis of cis-Ru(bpy)(cyclam)Cl2
• Reflux 0.05g cis-Ru(cyclam)Cl2 with 0.02g bpy in
  15ml EtOh for 2 h
• Vacuum filter and wash with ethanol
• Measure UV-Vis and 1H NMR spectra in benzene
  solvent

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Results
Yield (actual / ) UV ?max (nm) UV Peak Drop Off ? (nm)
RuIICl2(cyclam) 0.1143 g/ 56 281 nm 325 nm
(DPP)RuII (cyclam)2 0 g / 0 285 nm 345 nm
RuII(bpy)(cyclam) 0 g / 0 280 nm 325 nm 350 nm
NMR Peaks (d) NMR Peaks (d)
RuIICl2(cyclam) 1.542 0.400 0.295 -0.002
(DPP)RuII (cyclam)2 3.326 (b) 0.938 (t) 0.415 0.307 0.012
RuII(bpy)(cyclam) 1.556 0.438 0.309 0.014
(b) broad (t) triplet
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RuIICl2(cyclam)
UV
1H NMR
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(µ-2,3-DPP)RuII (cyclam)2
UV
1H NMR
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RuII(bpy)(cyclam)
UV
1H NMR
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Discussion

• Yield was much lower than expected. Product had
  to be flushed out of filter paper, straight into
  NMR tube. Low yield could be representative of
  thermodynamic difficulty of coordinating such
  bulky ligands although our macrocycle was small
  on purpose or small scale of reaction
  performed. Less than half a millimole of starting
  reagent was produced.

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UV-Vis Discussion

UV-Vis data showed peaks only in the high-energy
UV region of the spectrum. Since Ruthenium(II) is
d6, this would be expected only of molecule with
bpy-ligands however, presence of these peaks in
the RuCl2(cyclam) molecule suggests MLCT to the
cyclam molecule. Higher wavelength UV represents
weaker bonding in the ligand field. Data shows
this with redshifts in ?max and broadening of the
peak (dropoff point is at a higher wavelength).
Thus, steric effects cause tension and lower
energy UV-Vis absorption.
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NMR Discussion
Computational expectations for 1H NMR spectra
show downfield peaks (7-9 ppm) we would expect
from the pyridine rings. These were crowded over
by the benzene solvent NMR peaks would
theoretically be more deshielded than what is
shown in the experimental data. We infer this
means that cyclam is a more stable macrocycle
than computationally predicted. Data shows bpy
to cause more steric tension than DPP, as
evidenced by deshielded cyclam hydrogens
(coordinated nitrogens draw more electron density
from cyclam hydrogens when it is more closely
bound to Ruthenium(II)). However, the broad peak
around 3 ppm and triplet near 1 ppm look at out
of place. These are possibly DPP-related signals
or contaminants, such as free DPP in the solution.
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Conclusion

• We were able to synthesize our compounds but at
  very low yields. UV-Vis and 1H NMR data allowed
  us some insights into the stability of the
  bridged and bulky complexes, but the data does
  not seem to corroborate what we expected. This
  may be due to interesting and complex stabilities
  formed by our ligands.
• First, however, we want to confirm that we have
  actually produced our target complexes, so it
  would be best to synthesize the compounds in
  greater mass and analyze by mass spectroscopy. IR
  spectra would be useful for better insights into
  coordination geometry. Analysis by cyclic
  voltammetry and improved methods of synthesis
  would be avenues to pursue if we wanted to
  continue this work in the macrocyclic and steric
  effects.

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