Reaction of a Biomimetic Molybdenum Complex
with Carbon DioxideBy Michael Yanagisawa
Brown University ‘13
• Global levels of carbon dioxide (CO2) are rising, giving way to global climate change
• A goal worldwide: reduce CO2 levels› Less CO2 means less global warming
Ref. 1
• The challenge: CO2 is very stable› CO2 reduction gets rid of favorable linear
structure
› CO2 reduction is a thermodynamic uphill reaction
• Nature effectively reduces CO2
› Photosynthesis produces sugars from CO2
• Bio-inspired solutions seem promising
• The enzyme formate dehydrogenase (FDH) reversibly converts formate (HCO2
-) to CO2
› CO2 + H+ + 2e- → HCO2-
• Our goal: to mimic the active site of FDH to convert carbon dioxide into formate
› Formate can be used as a fuel source
• Recycling CO2 from a harmful product into a useful substrate could solve the CO2 problem
Ref. 2,3
• The active site of FDH consists of a molybdenum center with two pterin ligands
MoIV
O
HN
NH
HN
NH
O
S
SO
NH
HN NH
HN
O
S
S
SeH
Cys
+ formate
- CO2 and H+
MoVI
O
HN
NH
HN
NH
O
S
SO
NH
HN NH
HN
O
S
S
SeHCys
O
C O
H
Ref. 4
• Biomimetic complexes have been synthesized by other groups
• Characteristics› Mo metal center› Ditholene ligands
• For our complex, R = phenyl
MoIVR
R
R
R
O
Ref. 5
• Jun Seo of our group reacted a tungsten analogue with carbon dioxide› W and Mo have very similar properties
• The reaction formed a tungsten dimer
WS
S
S
S
O
CO2 (2 atm)
MeCN, 90°C
WS
S
O
WS
S
O
S
S
S
Ref. 6
• Our compound is [MoO(pdt)2]2-
• pdt = phenyl dithiolene
• What is the product of a reaction with CO2?
Research Question
MoIVPh
Ph
Ph
Ph
O
CO2 (2 atm)
heat?
• The dream: our complex reduces CO2 to formate like its inspiration, FDH
• The hope: CO2 binds onto the metal center, revealing an intermediate to forming formate
• Another possibility: a product analogous to Jun's product i.e. a Mo dimer
+
OH
O
P4S10
NiCl2
dioxaneNi
S
S
S
S
Ph
Ph
Ph
Ph
Mo(CO)3(MeCN)3 + Ni
S
S
S
S
Ph
Ph
Ph
Ph
DCM
MoS
S
S
S
Ph
Ph
Ph
Ph
OC CO
MoS
S
S
S
Ph
Ph
Ph
Ph
OC CO
MoIVS
S
S
S
Ph
Ph
Ph
Ph
O
Ref. 7
• 50 mL Schlenk flask filled with
• 20 mg [MoO(pdt)2]2- and
• 2 mL MeCN
• attached to a 100 mL CO2 bulb (2 atm)
• Reaction heated to 90°C and stirred 2 days
• After two days, we dried the product
• The crude product was rinsed with diethyl ether and collected (the “ether layer”)
• The remaining product (the “metal layer”) was then redried
Spectroscopy
To test the reaction, we used IR, UV-Vis, EPR, 1H and 13C NMR, GC/MS, and ESI
We also tried to crystallize the product for x-ray crystallography, but the product did not crystallize
We report IR, EPR, GC/MS, and ESI and their interesting interpretations
Characteristic Mo=O stretching visible Starting material νMoO = 886 cm-1
After reaction, stretching shifted left to product νMoO = 924 cm-1
For [MoVO(pdt)2]-, νMoO = 924 cm-1
Interpretation:› Our product has a Mo(V) center
Ref. 5b
No peaks present
Interpretation:› Our product has no unpaired electrons› IR shows evidence of Mo(V)› Conclusion: antiferromagnetic Mo(V) dimer
GC/MS of ether layer› Peaks at 51, 77, and 105› Matches up with diphenyl ethanedione
Interpretation:› In the reaction, some dithiolene ligand is
falling off the metal center and is part of the organic product
ESI of metal layer (still preliminary)› Peaks at 427 and 757
› Mass of [MoO(pdt)2]2- = 596.66
Interpretation:› The 757 peak again suggests some sort of
dimer. The 427 peak suggests a complementary metal center; 427 and 757 average to around the 596.
Conclusion
Our product is a Mo(V) antiferromagnetic dimer
MoIVPh
Ph
Ph
Ph
O
CO2 (2 atm)
heat?
Conclusion
Predicted product (MW ~ 757)
Where R1 + R2 may be:› A CO2 molecule
› O and S bridging atoms› Something else?
MoS
SPh
Ph
O
MoS
S Ph
Ph
O
R2
R1
Looking Forward
We would like to crystallize the product and identify the final product
Monoatomic Mo and W complexes have been known to dimerize Example: Jun’s W compound
Use labelled CO2 to track the oxygen atoms; start to deduce a mechanism
References
1. Crowley, T. J.; Berner, R. A. Science 2001, 292, 870-872.
2. Ha, S.; Dunbar, Z.; Masel, R.I. J. Power Sources 2006, 158, 129-136.
3. From a paper’s press notes: http://www.usu.edu/science/htm/one-step-closer-usu-biochemists-convert-greenhouse-gas-to-fuel/
4. Boyington, J. C.; Gladyshev, V. N.; Khangulov, S. V.; Stadtman, T. C.; Sun, P. D. Science 1997, 275, 1305-1308.
5. (a) Lim B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-273. (b) Lim B. S.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920-1930.
6. Paper in press.
7. (a) Tate, D. P.; Knipple, W. R.; Augi, J. M. Inorg. Chem. 1962, 1, 433-434. (b) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1965, 87, 1483-1489. (c) Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-273.
Acknowledgements
• Camly Tran• Dr. Eunsuk Kim• Jun Seo• The Kim lab• Brown University
› Undergraduate Teaching and Research Award for funding (summer 2012)