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. 2011 Jun 1;133(21):8098-101.
doi: 10.1021/ja201731q. Epub 2011 May 6.

Mild redox complementation enables H2 activation by [FeFe]-hydrogenase models

James M Camara  1 , Thomas B Rauchfuss
Affiliations

Mild redox complementation enables H2 activation by [FeFe]-hydrogenase models

James M Camara et al. J Am Chem Soc. .

Abstract

Mild oxidants such as [Fe(C(5)Me(5))(2)](+) accelerate the activation of H(2) by [Fe(2)[(SCH(2))(2)NBn](CO)(3)(dppv)(PMe(3))](+) ([1](+)), despite the fact that the ferrocenium cation is incapable of oxidizing [1](+). The reaction is first-order in [1](+) and [H(2)] but independent of the E(1/2) and concentration of the oxidant. The analogous reaction occurs with D(2) and proceeds with an inverse kinetic isotope effect of 0.75(8). The activation of H(2) is further enhanced with the tetracarbonyl [Fe(2)[(SCH(2))(2)NBn](CO)(4)(dppn)](+) ([2](+)), the first crystallographically characterized model for the H(ox) state of the active site containing an amine cofactor. These studies point to rate-determining binding of H(2) followed by proton-coupled electron transfer. Relative to that by [1](+), the rate of H(2) activation by [2](+)/Fc(+) is enhanced by a factor of 10(4) at 25 °C.

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Figures

Figure 1
Figure 1
Active site of the Hox state of [FeFe]-hydrogenase (left) and the model [1]+ (right, R = CH2Ph).
Figure 2
Figure 2
Structure of the cation in [Fe2[(SCH2)2NBn](CO)4(dppn)]BArF4. Selected bond lengths: (Fe-N) = 3.234(3), (Fe-Fe) = 2.568(1), (Fe-P) = 2.231(1) Å.
Figure 3
Figure 3
IR spectra of a CH2Cl2 solution of [2]BArF4 and [Fc]BArF4 before (black) and 5 (red) and 14 min. (blue) after introducing H (1atm).
Figure 4
Figure 4
X-Band EPR spectrum of [2]BArF4 (110 K, 1:1 CH2Cl2:toluene frozen solution). Parameters: gz = 2.1260 with Az(31P) = 78 MHz, gx ≈ gy = 2.0165 with Ax(31P) ≈ Av(31P) = 79 MHz.
Scheme 1
Scheme 1
Activation of H2 by [1]+ and Cp*2Fe+ to form [1H]+.
Scheme 2
Scheme 2
The rate law for oxidation of H2 by [1]+ and ferrocenium is consistent with the rate determining step being H2 addition (step 1) or redox/heterolysis of the H2 complex (step 2).
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