chemical lab report:synthesis and properties of a cobalt cage complex
TRANSCRIPT
Awad Albalwi St.No:3343297 Chem991, Expt 3, Lab Report
Synthesis and Properties of a Cobalt Cage Complex
Abstract
In this experiment [Co(diNOsar)]Br3 is synthesised from [Co(en)3]Br3 in the presence of formaldehyde, nitromethane and sodium carbonate. A yield of
36.7% was obtained for the [Co(diNOsar)]Br3.The electrochemical properties of Co(diNOsar)]Br3, [Co(en)3]Br3 and [Co(sepulchrate)]Br3 were investigated by using cyclic voltammetry . The CV spectra of the [Co(en)3]Br3 complex has shown clearly reduction of the Co(III) ion to Co(II) ion .However, there is no
reversible oxidation of the Co(II) ion to Co(III) ion. The reason for irreversible
oxidation the Co(II) ion might due to the Co(II) complex exchanging ligands.
The CV spectra of Co(diNOsar)]Br3 and [Co(sepulchrate)]Br3 has shown both reduction and oxidation of the Co ion occurred indicating that the reactive Co(II) ion was stabilised whilst bound inside the cage. The UVVis spectra of
the complexes showed that the wavelength of maximum absorption was
nearly similar for the compounds indicating that the electronic state and
geometry around the metal ion remained unchanged between the complexes.
Finally 1H NMR was investigated for the structure of the Co(diNOsar)]Br3 complex and allowed the symmetry of the complex to be assigned as D3h.
Introduction
Cage Ligands are three dimensional multidentate ligands that are capable of
encapsulating metal ions. Metal ions that are bound inside cage complex are
often rendered inactive even for very reactive metal ions such as Co(II). The
structure and type of ligands used to form a cage complex can be
manipulated to produce cage compounds that exhibit selective binding
properties towards certain metal ions. Recently there has been much interest
in the synthesis of cage complexes for their many potential uses including
metal ion recovery, controlled drug delivery, nuclear medicine and chelation
therapy1. One of the most famous is the cage complex [Co(diNOsar)]Br3 which is
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synthesis from [Co(en)3]3+, formaldehyde and ammonia. This type of synthesis introduces a new of class of coordination compounds as direct descendants
from classical complexes. The synthesis exhibits a high degree of
steroselectivity and would not be possible without the presence of the metal
ion. The metal ion acts to allow the preparation of the cage complex through
the thermodynamic and kinetic template effects. These effects bring the
intermediate into a conformation that allows the final product to form and in
some cases through binding to the complex shifts the equilibrium such to
produce more product.
In this work [Co(diNOsar)]3+, a complex closely resembling [Co(sepulchrate)]3+ is synthesised from [Co(en)3]3+, nitromethane and aqueous formaldehyde. The structure of the prepared complex, [Co(sepulchrate)]3+ and [Co(en)3]3+ will be investigated using cyclic voltammetry to examine the electrochemical properties of the complexes and UVVis spectrophotometry to
investigate the electronic properties of the complexes.
Aims
● Synthesise [Co(diNOsar]3+ from [Co(en)3] in the presence of Na2CO3, nitromethane and formaldehyde.
● Investigate the electronic properties of [Co(diNOsar)]3+, [Co(sepulchrate)]3+ and [Co(en)]3+ using UVVis spectrophotometry and the electrochemistry of the complexes using circular
voltammetry (CV).
Methods See the Chem301 2010 lab manual1.
Results
The yield of [Co(diNOsar)]Br3 was calculated as follows: MW of [Co(en)3]Br3 = 479 gm/mol MW of [Co(diNOsar)]Br3 = 674 gm/mol Mass [Co(en)3]Br3 used = 2.517 g = 0.00526 moles Theoretical yield of [Co(diNOsar)]Br3 = 0.00535 moles = 3.59g
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Mass [Co(diNOsar)]Br3 obtained = 1.32g % Yield = 1.32/3.59 x 100% = 36.7%
Compound �at Absorbance maxima 1 (nm) �at Absorbance maxima 2 (nm) [Co(en)3]Br3 469 345[Co(diNOsar)]Br3 485 340 Table.1 / the UVVis spectra data for the complexes .
Figure1/The CV spectra for the three compounds are attached with the
assignment.
Cyclic voltammetry part:
Current(uA)
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Figure2/ the electrochemical properties of [ Co (sep)3]3+
Current(uA)
Figure3/ the electrochemical properties of [ Co (en)3]3+
Figure4/ the electrochemical properties of [ Co (dinNOsar)3]3+
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Figure5/ the electrochemical properties of [ Co (dinNOsar)3]3+NMR part: NMR spectrum:
Figure6/
Questions/Discussion
A yield of 36.7% for the synthesised cage complex [Co(diNOsar)]Br3 was obtained. Reasons for the low yield include the difficulty in getting all of the
product crystallizing out of solution . The yield may be improved by using
more dilute reaction conditions
The reaction mechanism for the synthesis of [Co(diNOsar)]3+ is
provided in (scheme.1). The presence of the metal ion is very imprtant in
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order for the synthesis of [Co(diNOsar)]3+ to occur. The coordination sphere of
the metal ion would hold the reaction groups in the correct positions for
cyclization reaction. The metal ion acts as a “template” . Formation of the
cage complex is impossible in the absence of the metal ion and where the
metal ion acts by coordinating the reactants5 . The metal imposes a specific
geometry on the ligands due to the formation of a complex with the metal.
This brings the ligands in close to allow them to react and hence form the
cage. The presence of the metal ion also provides a favourable equilibrium
which favours the formation of the cage over the reactants.
Scheme 1Mechanism for the formation of [Co(diNOsar)]3+
The wavelengths at maximum absorption for the two complexes investigated
are provided in table 1 in the results section. The UVVis spectra of all
compounds shows the presence of two absorbance bands. The absorbance
bands are found to occur at similar wavelengths for all two compounds.The
absorption bands are very influenced by the environment. The position of
absorbance bands in inorganic complexes is dependant on the geometry and
electronic nature of the metal ion. As the absorbance bands for two
compounds occur at similar wavelengths it suggests that the geometry and
electronic nature of the Cobalt ion at the centre of the complexes is similar in
the two complexes. This indicates that in the two complexes the cobalt ion is
present in the +3 oxidation state and the ligands are bound to the ion in an
octahedral case. The absorbance values at the peak absorbance wavelengths
are also found to vary between the two complexes with the cage
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complexes.figure1. The reasons for this difference in absorbance values may
be due to greater distortion of the octahedral geometry in the cage
compounds the [Co(en)3]3+ complex compared to the [Co(diNOsar)]3+ complex.
Cyclic Voltammetry (CV) is a technique used for measuring the
electrochemical properties of a variety of compounds including metal
complexs. The CV spectrum of [Co(en)3]Br3 has shown that a reduction peak at around 580mV (figure.3). This peak corresponds to the reduction of Co(III)
to Co(II). The CV spectrum of [Co(en)3]Br3 does not show a oxidation peak for the conversion of Co(II) back to Co(III). This suggests that in the case of
[Co(en)3]Br3 the oxidation of the cobalt ion is not reversible indicating that the [Co(en)3]+ ion is not stable. The reason for this might be understood by considering the nature of the ethylenediamine ligands. As the Co(III) ions are
reduced resulting in the formation of the [Co(en)3]+ species the ethylenediamine ligands are exchanged for water ligands causing the
oxidation to be irreversible resulting in the unsymmetrical CV spectrum. The
Co(III) oxidation state is a low spin d6 system and is tightly bound resulting in it not readily giving up the ethylenediamine ligands. Once the reactive high
spin d7 Co(II) ion is formed the ligands are readily exchanged due to the CoN bonds constantly breaking.
The CV spectrum of [Co(sep)]3+ has showna symmetric spectrum showing both oxidation and reduction peaks for the Cobalt ion(Figure.2). This
suggests that when bound inside the cage complex the Co(II) ion is stable
within the CV time scale and does not readily exchange ligands as was the
case with [Co(en)3]3+. In order for the Co(II) in the sepulchrate cage to exchange ligands all 6 CoN bonds would need to break at the same time in
order for a new ligands such as water bind to the metal. This however is very
unlikely and as a result the unstable Co(II) is kinetically stabilised whilst bound
inside the cage even though it is not considered to be thermodynamically
stable.
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The CV spectrum for [Co(diNOsar)]3+ also shows both oxidation and reduction peaks for the cobalt ion indicating that the Co(II) ion is also
kinetically stabilised whilst bound inside the diNOsar cage. much interest in
the areas of metal recovery and chelation therapy1 due to their ability to strongly bind to certain metal ions.
Question:
The 1H NMR spectrum of [Co(diNOsar)]Br3 was used to investigate the structure of the complex. The 1H NMR spectrum has shown two overlapping quartets that result from the presence of an AB and AA’BB’ spin systems
(figure6)4. The AB quartet is consists of the two sets of peaks centred at 4 and 3.5 ppm. This pattern arises from the coupling between the protons nearest to
the NO2 groups. The pattern appears as a quartet due to the presence of some second order spectra arising from the distance between the two
doublets being quite small. .
The other quartet containing the peaks centred at 3.1ppm and 3.7ppm
constitute and AA ’BB’ spin system. This splitting pattern is also due to the
presence of some second order spectra resulting from the overlapping some
of the peaks. This AA’BB” system is due to two magnetically nonequivalent
protons attached to the same carbon atoms on the ethylenediamine straps
coupling to each other.
Conclusion
[Co(diNOsar)]Br3 was successfully synthesised from [Co(en)3]Br3. The electronic properties of these two complexes along with [Co(sep)]Br3 were successfully investigated using UVVis spectroscopy and cyclic voltammetry..
An understanding of the required reaction conditions needed in order to
synthesis complexes with good yields obtained as was an understanding of
the usefulness of UVVis and CV in investigating complexes.
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References
1 2010 Chem301 manual lab for experiment 3 . 2 I.I. Creaser, J.MacB. Harrowfield, A.J. Herlt, A.M. Sargeson, J.
Springborg, R.J. Geue and M.R. Snow, J. Amer. Chem. Soc., 99, 3181 (1977).
3 R.J. Geue, T.W. Hambley, J.MacB. Harrowfield, A.M.
Sargeson and M.R. Snow, J. Amer. Chem. Soc., 106, 5478 (1984). 4
Harrodield,J M, Lawrance, G A, & 1985, “ Sargeson,A M, FacilSeynthesoisfa MacrobicycHliec xaamine Gobalt(llC[ ompleBxa seodn TrisIEthylenediaminelGolUbaltfl” , J Chemical Education, 62 ,9 , 804806. 5Emelens,H,J & Sharpe,A,G . Advances in inorganic chemistry and radiochemistry.
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