Electrochemical Studies

Analysis of Bridgehead Effects on [FeFe]-Hydrogenase Active Site: Analysis of Bridgehead Effects on [FeFe]-Hydrogenase Active Site: Steric Bulk at Nitrogen versus CarbonSteric Bulk at Nitrogen versus Carbon

Danielle J. Crouthers, David G. Munoz, Jason A. Denny, and Marcetta Y. Darensbourg*Texas A&M University, College Station, TX 77843

AcknowledgementsMYD Research Group

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National Science Foundation

Robert A. Welch Foundation

References1) 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.

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.

1H and 13C Variable Temperature NMR

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.

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.

Essential Features ofEssential Features of [FeFe]-Hydrogenase Active Site [FeFe]-Hydrogenase Active Site

Synthesis of Azadithiolate Disubstituted ComplexesSynthesis of Azadithiolate Disubstituted Complexes

Complex ν(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) 3.498dmpdt 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) 3.481NMe 2075, 2036, 2002, 1990, 1984 2.4924(7) 122.26 0.0(4) 3.587NtBu 2075, 2036, 2002, 1994, 1982 2.5172(9) 118.46 6.1(2) 3.320NPh 2074, 2039, 1999, 1990, 1981 2.5047(6) 123.66 20.1(2) 3.48

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

1PMe3 2PMe3 3PMe3

Comparison of Carbon and Nitrogen Comparison of Carbon and Nitrogen BridgeheadBridgehead22

< < < < <

7.4 9.2 10.4 10.9 11 12.1

25 ° C

0 °-10 °

-20 °

-30 °

-40 °

-50 °

0 °

-20 °-30 °

-40 °

-50 °

-60 °

-70 °

-80 °

-10 °

20 °• 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

1H NMRCD2Cl2

13C NMRCD2Cl2

Fe FeCC

C

C

SSC

N

C

O

O

O

O

R

OO

Fe FeCC

C

C

SSC

N

C

O

O

O

O

R

OO

ap

ba

ba

ba

ba

ap

Energy Barrier for CO Site ExchangeEnergy Barrier for CO Site Exchange3,43,4

Complex Tcoal ΔG‡

(kJ/mol)ΔG‡

(kcal/mol)

ΔG‡

calculated

edt 0 °C 50.7 12.1 14.3pdt -60 °C 43.5 10.4 12.1

dmpdt -87 °C 31 7.4 10.0NMe -40 °C 45.7 10.9 13.8NtBu -30 °C 46 11 15.0NPh - - - 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 R=alkyl and a greater effect for R=phenyl.

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 R=alkyl and a greater effect for R=phenyl.

Comparison of Disubstituted Structures Comparison of Disubstituted Structures

NH NMe NtBu NPh

First Catalytic Peak Comparison Second Catalytic Peak Comparison

PDT

NtBuNMe

DMPDT