Iron‐Only Hydrogenase Active Site Models Containing a Cysteinyl Group Coordinated through Its Sulfur Atom to One Iron Atom of the Diiron Subsite

We have developed a simple and convenient method for the synthesis of the first H‐cluster models in which a L ‐cysteinyl group is coordinated to one of the two iron atoms of the diiron subsite through its sulfur atom. This synthetic method includes (i) treatment of the Boc‐protected L ‐cysteine ester Boc‐NHCH(CH 2 SH)CO 2 Et ( 1 , Boc = tert ‐butoxycarbonyl) with EtONa to give the L ‐cysteinyl mercaptide NaSCH 2 CH(NH‐Boc)CO 2 Et ( 2 ); (ii) further treatment of 2 with [Cp(CO) 2 FeI] to produce the metallothioether ligand Cp(CO) 2 FeSCH 2 CH(NH‐Boc)CO 2 Et ( 3 ); and (iii) treatment of the parent diiron complex [Fe 2 (μ‐SCH 2 ) 2 CH 2 (CO) 6 ] ( 4 ), [Fe 2 (μ‐SCH 2 ) 2 N( t Bu)(CO) 6 ] ( 5 ), or [Fe 2 (μ‐SCH 2 ) 2 N(C 6 H 4 OMe‐ p )(CO) 6 ] ( 6 ) with Me 3 NO · 2H 2 O followed by ligand 3 to afford the target model compounds [Fe 2 (μ‐SCH 2 ) 2 CH 2 (CO) 5 (ligand 3 )] ( 7 ), [Fe 2 (μ‐SCH 2 ) 2 N( t Bu)(CO) 5 (ligand 3 )] ( 8 ), or [Fe 2 (μ‐SCH 2 ) 2 N(C 6 H 4 OMe‐ p )(CO) 5 (ligand 3 )] ( 9 ), respectively. All the new compounds 2 , 3 , and 7 – 9 have been characterized by elemental analysis and various spectroscopic techniques. The X‐ray diffraction analysis of 8 has confirmed that these models contain a cysteinyl sulfur atom not only coordinated to one Fe atom of the diiron subsite, but also to the Fe atom of the Cp(CO) 2 Fe moiety to form the linkage [Fe Cp ‐(μ‐cysteinyl‐S)‐Fe subsite ], which is similar to [Fe cubane ‐(μ‐cysteinyl‐S)‐Fe subsite ] found in natural enzymes. In addition, spectroscopic and electrochemical measurements have further demonstrated that the linkage [Fe Cp ‐(μ‐cysteinyl‐S)‐Fe subsite ] can provide substantial electronic communication between the diiron subsite and the Cp(CO) 2 Fe moiety. Under electrochemical conditions, 8 has been shown to be a catalyst for HOAc proton reduction to dihydrogen, and a new type of E*2E2C mechanism for this catalytic reaction is suggested.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)