Electrochemical Studies 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 A&M University, College Station, TX Acknowledgements MYD Research Group $$$$$$$$$$$$$$$$$$$$ National Science Foundation Robert A. Welch Foundation References 1) Pandy, A. S. et al. J. Am. Chem. Soc. 2008, 130, ) Li, H. et al. J. Am. Chem. Soc. 2002, 124, ) Singleton, M. L. et al. C. R. Chimie 2008, 11, ) Lyon, E. J. et al. J. Am. Chem. Soc. 2001, 123, 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. 1 H and 13 C 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. Essential Features of [FeFe]-Hydrogenase Active Site [FeFe]-Hydrogenase Active Site Synthesis of Azadithiolate Disubstituted Complexes Complexν(CO) IR (cm -1 )Fe-Fe (Å) Flap Angle a (°) Torsion b (°) C/N-- Fe c Pdt2076, 2035, 2005, 1992, (8) (2)3.498 dmpdt2075, 2034, 2005, 1992, (4) (2)3.735 NH2075, 2036, 2007, 1990, (3) (9)3.481 NMe2075, 2036, 2002, 1990, (7) (4)3.587 NtBu2075, 2036, 2002, 1994, (9) (2)3.320 NPh2074, 2039, 1999, 1990, (6) (2)3.48 ComplexFe-Fe (Å) Flap Angle a (°) Torsion b (°) C/N--Fe c (Å) pdt(PMe 3 ) (2) (5)3.449 dmpdt(PMe 3 ) (7) (3)3.731 NMe(PMe 3 ) (1) (3)3.396 N t Bu(PMe 3 ) (2) (9)3.298 NPh(PMe 3 ) (4) (2) PMe3 2 PMe3 3 PMe3 Comparison of Carbon and Nitrogen Bridgehead 2 <<<<< ° 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 1 H NMR CD 2 Cl 2 13 C NMR CD 2 Cl 2 Energy Barrier for CO Site Exchange 3,4 ComplexT coal ΔG ‡ (kJ/mol ) ΔG ‡ (kcal/mol ) ΔG ‡ calculate d edt0 °C pdt-60 °C dmpdt-87 °C NMe-40 °C NtBu-30 °C NPh disulfide-60 °C  The energy barriers are calculated using datafrom 13 C 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 13 C 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 NHNMeNtBuNPh First Catalytic Peak ComparisonSecond Catalytic Peak Comparison PDT NtBuNMe DMPDT