Functional Model for the [Fe] Hydrogenase Inspired by the Frustrated Lewis Pair Concept

[Fe] hydrogenase (Hmd) catalyzes
the heterolytic splitting of H2 by using, in its active
site, a unique organometallic iron-guanylylpyridinol
(FeGP) cofactor and, as a hydride acceptor, the substrate methenyltetrahydromethanopterin
(methenyl-H4MPT+). The combination FeGP/methenyl-H4MPT+ and its reactivity bear resemblance to the
concept of frustrated Lewis pairs (FLPs), some of which have been
shown to heterolytically activate H2. The present work
exploits this interpretation of Hmd reactivity by using the combination
of Lewis basic ruthenium metalates, namely K­[CpRu­(CO)2]
(KRp) and a related polymeric Cp/Ru/CO compound (Rs), with the new imidazolinium salt 1,3-bis­(2,6-difluorophenyl)-2-(4-tolyl)­imidazolinium
bromide ([TolImF4]+Br–) that was designed to emulate
the hydride acceptor properties of methenyl-H4MPT+. Solid-state structures of [TolImF4]+Br– and
the corresponding imidazolidine HTolImF4 reveal that the heterocycle
undergoes similar structural changes as in the biological substrate.
DFT calculations indicate that heterolytic splitting of dihydrogen
by the FLP Rp–/[TolImF4]+ is exothermic, but the formation
of the initial Lewis pair should be unfavorable in polar solvents.
Consequently the combination Rp–/[TolImF4]+ does
not react with H2 but leads instead to side products from
nucleophilic substitution (k = 4 × 10–2 L mol –1 s–1 at room temperature).
In contrast, the heterogeneous combination Rs/[TolImF4]+ does split H2 heterolytically
to give HTolImF4 and HRuCp­(CO)2 (HRp) or DTolImF4 and DRp when using D2. The reaction has been followed by 1H/2H and 19F NMR spectroscopy as well as by IR spectroscopy and reaches
96% conversion after 1 d. Formation of HTolImF4 under these conditions
demonstrates that superelectrophilic activation by protonation, which
has been proposed for methenyl-H4MPT+ to increase
its carbocationic character, is not necessarily required for an imidazolinium
ion to serve as a hydride acceptor. This unprecedented functional
model for the [Fe] hydrogenase, using a Lewis acidic imidazolinium
salt as a biomimetic hydride acceptor in combination with an organometallic
Lewis base, may provide new inspiration for biomimetic H2 activation.