6 Features of Active Sites of [FeFe]- and [NiFe]-H 2 ases
Built into Synthetic Analogues for Hydrogen Production
or Activation
While reductive elimination of two H• atoms forming H 2 has its overall rewards in
the Nitrogenase cycle, proton/hydride coupling/decoupling of H 2 is less energy
intensive by ca. 30 kcal/mol [12]. As illustrated in Fig. 6 the assistance of a
strategically positioned base is key to lowering the barrier to heterolytic cleavage
and the obvious base in the [FeFe]-H 2 ase active site is the secondary amine in the
bridgehead of the azadithiolate linker between the two irons [94]. This pendant base
within a 6-membered ring reminded DuBois and colleagues of tetradentate
bis-phosphine ligands (P 2 N 2 ligands), inspiring the development of a family of Ni
Fig. 10 a the reductive elimination (re) of H 2 during the N 2 fixation and b the proposed “push–
pull” model that illustrates the proposal that re is necessary for N 2 binding and activation [89].
Graphic a is reproduced with permission from Ref. [90]
Fig. 11 HER versus N 2 RR in model complexes (E = B, C or Si) for the active sites of
Nitrogenase [93]
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