Fundamental properties of small molecule models of Fe-only hydrogenase: computations relative to the definition of an entatic state in the active site
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Well-studied organometallic complexes (-SRS)Fe2(CO)6 that serve as structural models of the active site of Fe-only hydrogenases have been employed in DFT computational studies with the goal of understanding the fundamental nature of the active site of this biological catalyst. Intramolecular CO site exchange processes, experimentally observable in variable temperature (VT) NMR studies were modeled. The transition state structure of the Fe(CO)3 unit rotation looks very similar to the structure that the active site has adopted in the protein environment. That is, a semi-bridging CO is formed upon Fe(CO)3 rotation partially disrupting the Fe-Fe bonding interaction and leaving an open site trans to this semi-bridging CO. The CN-/CO substitution reaction of these complexes which yields the disubstituted derivatives, (-SRS)[Fe(CO)2(CN)]22-, was also examined as experimental results found a complicated, R-dependent, reactivity pattern for the second CN- addition. The connection of the above rotation process to the CN-/CO substitution was supported by the fact that an intermediate with a -CO group, like that resulting from the Fe(CO)3 unit rotation, is formed upon CN- attack. The assumption that the Fe(CO)3 rotational barrier is an important contributor to the overall activation energy of CN- attack, explains the experimental observation that generally the second CN- addition finds a lower Fe(CO)3 rotational barrier due to the presence of the already coordinated CN- ligand. 2003 Elsevier Science B.V. All rights reserved.