Pyridine Ligands

and the Stability ofBirju Patel

Johns Hopkins UniversityDecember 19, 2007

Cyclam-Chelated

Advanced Inorganic Chemistry LabProfessor Justine RothTAs Ankur Gupta and Simone Novaes-Card

Ruthenium(II) Complexes

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.

Chemical Background

Cyclam(14aneN4)

2,3-DPP

Bpy

1,4,8,11-Tetraazacyclotetradecane (CAS 295-37-4)

2,3-Bis(2-pyridyl)pyrazine (CAS 35005-96-3)

2,2’-Bipyridyl (CAS 366-18-7)

Chelate

Bridging Ligand

Non-bridging Ligand

RuIICl2(cyclam)

(μ-2,3-DPP)[RuII (cyclam)]2

(DPP)[RuII (cyclam)]2

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

Analytic Background

• UV will most likely show bpy-centered π π* 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 λmax 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

MethodSynthesis 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 45°C

• Vacuum filter and recrystallize with hot methanol-water• Measure UV-Vis and 1H NMR spectra in benzene solventSynthesis of μ-2,3-DPP[cis-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 solventSynthesis 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

ResultsYield (actual / %) UV λmax (nm) UV Peak Drop Off λ (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 (δ)

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

RuIICl2(cyclam)

UV

1H NMR

(μ-2,3-DPP)[RuII (cyclam)]2

UV

1H NMR

RuII(bpy)(cyclam)

UV

1H NMR

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.

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 λmax and broadening of the peak (dropoff point is at a higher wavelength). Thus, steric effects cause tension and lower energy UV-Vis absorption.

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.

RuIICl2(cyclam)

(μ-2,3-DPP)[RuII (cyclam)]2

RuII(bpy)(cyclam)

NMR Discussion

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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