Transcript Cu II complex

Ligands that Favor/Force the Formation
of Tetrahedral Complexes with an
Application in Bioinorganic Chemistry
Marion E. Cass, Carleton College
Michael J. Stevenson, Dartmouth College
Molly L. Croteau, Dartmouth College
Created by Michael J. Stevenson, Dartmouth College ([email protected]) Molly L. Croteau, Dartmouth College
([email protected]) and Marion E. Cass, Carleton College ([email protected]), and posted on VIPEr on April 18,
2016. Copyright Michael J. Stevenson, Molly L. Croteau, and Marion E. Cass. This work is licensed under the Creative Commons
Attribution Non-commercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
• dmp = 2,9-dimethyl-1,10-phenanthroline
• Bidentate ligand w planar 1,10-phenanthroline
•
M
methyl groups block the ability of a second
dmp or other ligands to bind in the same plane
• Ni(II) d8 forms a tetrahedral complex:
Not a Square Planar Complex
•
Pt(II) distorts to avoid tetrahedral coordination
NiIIdmpI2
PtIIdmpI2
For all 3D JSmol movable images addressed in this LO see:
http://www.people.carleton.edu/~mcass/1-InorganicC351/jsmol/LigandsFavoringTetrahedralGeometry.htm
A Bio-inorganic Application
Dean Wilcox and his students at Dartmouth College carry out careful
thermodynamics measurements to examine metal binding in metalloproteins
Molly Croteau examines the binding of metal ions to Azurin (a blue copper electron
transport protein). In azurin the copper metal ion shuttles between Cu(I) and Cu(II) in
order to provide electrons to cytochrome c oxidase in bacterial cells. Binding of copper in
both oxidation states to the apo-azurin is essential for understanding how this protein
tunes its reduction potential to participate with cytochrome c oxidase in vivo.
Michael Stevenson studies the binding of various metal ions to the copper
metallochaperone protein HAH1. The reducing environment of the cell makes the
predominant copper oxidation state to be Cu(I). HAH1 is finely tuned to bind Cu(I) and
transport it through the cytosol for delivery to the trans Golgi network. Competition
by other metal ions provides insight into ferreting out the structure/thermodynamic
relationship that provides the selectivity for Cu(I). It also provides insight into
potential mechanisms of heavy metal toxicity in this and other biochemical pathways.
In both instances, it is crucial to know the oxidation state of the
metal being delivered during a carefully controlled experiment.
For Cu(I), this is not an easy experimental task.
Apo-protein
Cu(I)Ln +
Desired Experiment
Measure Thermodynamics
of Cu(I) binding to the
protein in question
Cu(I)-protein
Experimental Challenge
Solution
Cu(I) salts tend to be insoluble
Find a ligand that will create a soluble Cu(I)Ln complex
Other species in solution (H2O, Buffer, Anions,
etc) compete for the Cu(I)
Find a ligand that will bind more strongly than H2O,
Cu(I)Ln complexes can disproportionate to
form Cu(0) and Cu(II)Ln species
Find a ligand that forms a relatively stable CuILn complex
to disfavor the disproportion reaction
Buffer, Anions etc)
2CuILn ⇌ Cu(0) + CuIILn
Cu(I) complexes can be oxidized in the
presence of O2
CuILn + O2  CuIILn
Again use a ligand that forms a relatively stable CuILn complex
However most importantly, work in an O2 free
environment
+ nL
Ligand 1: Me6Trien
CuIMe6Trien will not disproportionate
2CuIL +
Cu(0) + CuIIL2+
And in fact: the reverse comproportion reaction is the preferred preparation method
CuII + Cu(0) + excess Me6Trien
in O2
If O2 free
CuIL is the predominant
Species in solution
For 3D JSmol movable images see:
http://www.people.carleton.edu/~mcass/1-Inorganic-C351/jsmol/LigandsFavoringTetrahedralGeometry.htm
2 CuIL+
The Cu(I) will
oxidize to Cu(II)
and will pick up
an additional L:
anion or solvent
Ligand 2: BCA
[CuIBCA2]3-will not disproportionate
2[CuIBCA2]3-
Cu(0) + [CuIIBCA2]2-
CuI complex
CuII complex
in O2
The CuII complex is believed to have a similar
3D structure to the Cu(I) complex. The crystal
structure of the complex with one BCA analog
and two Cl- Ligands is shown below
If O2 free
[CuIBCA2]3- is the
predominant
species in soln
For 3D JSmol movable images see:
http://www.people.carleton.edu/~mcass/1-Inorganic-C351/jsmol/LigandsFavoringTetrahedralGeometry.htm
Ligand 3: BCS
[CuIBCS2]3-will not disproportionate
2[CuIBCS2]3-
Cu(0) + [CuIIBCS2]2-
CuI complex
CuII complex
in O2
If O2 free
[CuIBCS2]3- is the
predominant
species in soln
For 3D JSmol movable images see:
http://www.people.carleton.edu/~mcass/1-Inorganic-C351/jsmol/LigandsFavoringTetrahedralGeometry.htm
In Summary: All Three ligands form relatively
stable CuI complexes in the absence of O2
Me6Trien
BCA
BCS
1. What are the similarities in the 3
complexes?
2. Suggest why the CuI complexes are
“relatively stable” (meaning relative to
its CuII complex under the controlled
experimental conditions).
3. Suggest why they are soluble in H2O.
4. It turns out for the reaction:
CuI + n L  CuILn
Kf(CuIMe6Trien) < Kf(CuIBCA2) < Kf(CuIBCS2)
Suggest why it is useful to have 3
ligands with 3 differing formation
constants. If you had to rationalize the
relative order of the formation
constants what would you suggest?
5. When oxidized, the CuII complex of
Me6Trien has a different geometry
than the CuI complex. Suggest why this
occurs with Me6Trien but the distortion
is less pronounced with BCA or BCS?
References and Resources
• D.K. Johnson, M. J. Stevenson, Z.A. Almadidy, S.E. Jenkins, D. E. Wilcox, N.
E. Grossoeheme; “Stabilization of Cu(I) for binding and calorimetric
measurements in aqueous solution” Dalton Transactions, 2015, Vol 44,
Issue 37, p. 16494-16505 (DOI 10.1039/c5dt02689)
• A very neat application of the shuttling of the CuI/CuII complexes with
Me6Trien and other ligands is for use in radical polymerization reactions:
See G. Kickelbick, T. Pintauerb and K. Matyjaszewski; “Structural
comparison of CuII complexes in atom transfer radical polymerization”
New J. Chem, 2002, 26, p. 462-468 (DOI 10.1039/b105454f)