Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 117

Figure 9 Structures of catalytically active synthetic small molecule hydrogenase mimics.
(a) derives from reference [ 78 ]; (b) derives from reference [ 80 ].
Historically [FeFe] hydrogenases have been the focus of synthetic analogue
studies [ 74 ], but highly active Ni enzyme analogues have been created recently
[ 76 , 77 ]. Studies on both systems have highlighted the mechanistic importance of
building a proton transfer site close to the H 2 -activating metal center in a H 2 catalyst
such as a hydrogenase. In the [FeFe] hydrogenases it has long been postulated that
the active site bridging dithiolate must be a dithiomethylamine rather than
containing a central bridging carbon or oxygen because it was proposed that the
nitrogen would play an essential role in holding a proton during catalysis. This
theory was finally proven in a recent synthetic analogue-based study [ 78 ]. Three
different [FeFe] enzyme active site mimics, possessing either a nitrogen- or carbon-
or oxygen-capped dithiolate ligand, were transported into a [4Fe4S] cluster
containing hydrogenase apo-protein and only the nitrogen-containing molecule
yielded a catalytically active H-cluster-containing enzyme (Figure 9 ). Similarly,
in Ni model compounds the engineering of a second coordination sphere that can
function as a proton relay has dramatically accelerated the rate of H 2 evolution
(Figure 9 )[ 79 , 80 ].