Electrochemical Studies

1
Analysis of Bridgehead Effects on
FeFe-Hydrogenase Active Site Steric Bulk at
Nitrogen versus Carbon
Danielle J. Crouthers, David G. Munoz, Jason A.
Denny, and Marcetta Y. Darensbourg Texas AM
University, College Station, TX 77843
1H and 13C Variable Temperature NMR
Essential Features of FeFe-Hydrogenase Active
Site
13C NMR CD2Cl2
1H NMR CD2Cl2

• Open Site site for proton oxidative addition or
  dihydrogen binding
• Azadithiolate Linker relays protons to and from
  the iron distal to the 4Fe4S cluster
• Diatomic Ligands stabilize the redox states of
  the irons
• 4Fe4S Cluster redox active shuttle of electrons

• FeFe-hydrogenases are key enzymes in energy
  metabolism, catalyzing the reversible
  interconversion of protons and electrons into
  hydrogen under mild conditions (pH 7 E - 400
  mV).1
• The synthesis of small molecule models of the
  FeFe-H2ase active site is driven by the desire
  to better understand a unique biological system
  capable of producing hydrogen at rates comparable
  to platinum.
• Questions Can FeFe-hydrogenase active site
  models define the role of nitrogen at the bridge
  head? Is there a correlation between the Fe(CO)3
  rotor fluxionality and catalytic efficiency?

Energy Barrier for CO Site Exchange3,4
Comparison of Carbon and Nitrogen Bridgehead2
Complex Tcoal ?G (kJ/mol) ?G (kcal/mol) ?G calculated
edt 0 C 50.7 12.1 14.3
pdt -60 C 43.5 10.4 12.1
dmpdt -87 C 31 7.4 10.0
NMe -40 C 45.7 10.9 13.8
NtBu -30 C 46 11 15.0
NPh - - - 11.4
disulfide -60 C 38.3 9.2 11.5

• The energy barriers are calculated using
  datafrom 13C VTNMR, looking at peak separation
  and the coalescence temperature.
• Analysis of the carbon bridgehead complexes
  finds that steric bulk on the bridgehead lowers
  the energy for rotation however, steric bulk
  at the nitrogen bridgehead has little effect for
  Ralkyl and a greater effect for Rphenyl.

NH
NMe
NtBu
NPh
Complex ?(CO) IR (cm-1) Fe-Fe (Å) Flap Anglea () Torsionb () C/N--Fec
Pdt 2076, 2035, 2005, 1992, 1981 2.5105(8) 137.09 0.0(2) 3.498
dmpdt 2075, 2034, 2005, 1992, 1980 2.4939(4) 135.74 6.5(2) 3.735
NH 2075, 2036, 2007, 1990, 1981 2.5150(3) 131.95 0.00(9) 3.481
NMe 2075, 2036, 2002, 1990, 1984 2.4924(7) 122.26 0.0(4) 3.587
NtBu 2075, 2036, 2002, 1994, 1982 2.5172(9) 118.46 6.1(2) 3.320
NPh 2074, 2039, 1999, 1990, 1981 2.5047(6) 123.66 20.1(2) 3.48
Electrochemical Studies
PDT
DMPDT
The diiron complexes were studied in
acetonitrile with addition of acetic acid. The
complexes exhibit an increase in current with
addition of acetic acid at two events past the
first reduction. The nitrogen bridgehead
complexes show a 2- fold increase in the current
compared to the carbon bridgehead complexes at
the first catalytic event. NtBu shows a 1.5-fold
increase compared to the other hexacarbonyl
complexes studied at the second catalytic event.
Synthesis of Azadithiolate Disubstituted Complexes
NtBu
NMe
First Catalytic Peak Comparison
Second Catalytic Peak Comparison
1PMe3
2PMe3
3PMe3
Comparison of Disubstituted Structures
Conclusions

• Incorporation of nitrogen in the bridgehead has
  no effect on the vibrational spectra compared to
  carbon and only a minimal effect on the solid
  state molecular structure.
• Addition of steric bulk to a carbon bridgehead
  increases the torsion angle of the complex
  however addition of steric bulk to a pyramidal
  nitrogen has little effect on the torsion angle
  due to the direction the steric bulk is pointed.
  Steric bulk on a planar nitrogen increases the
  torsion angel similar to the carbon bridgehead
  complexes.
• Analysis of the hexacarbonyl complexes does not
  reveal any correlation between the Fe(CO)3 rotor
  fluxionality and catalytic efficiency.

Complex Fe-Fe (Å) Flap Anglea () Torsionb () C/N--Fec (Å)
pdt(PMe3)2 2.5554(2) 129.9 9.1(5) 3.449
dmpdt(PMe3)2 2.5690(7) 135.74 28.9(3) 3.731
NMe(PMe3)2 2.526(1) 122.24 2.1(3) 3.396
NtBu(PMe3)2 2.5860(2) 118.46 1.0(9) 3.298
NPh(PMe3)2 2.573(4) 121.28 10(2) 3.428
Acknowledgements
References
1) Pandy, A. S. et al. J. Am. Chem. Soc. 2008,
130, 4533. 2) Li, H. et al. J. Am. Chem. Soc.
2002, 124, 726. 3) Singleton, M. L. et al. C. R.
Chimie 2008, 11, 861. 4) Lyon, E. J. et al. J.
Am. Chem. Soc. 2001, 123, 3268.
MYD Research Group National
Science Foundation Robert A. Welch Foundation