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

[Fe] hydrogenase (Hmd) catalyzes the heterolytic splitting of H-2 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 ([(Tol)Im(F4)]Br-+(-)) that was designed to emulate the hydride acceptor properties of methenyl-H4MPT+. Solid-state structures of [(Tol)Im(F4)]Br-+(-) and the corresponding imidazolidine H(Tol)Im(F4) 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(-)/[(Tol)Im(F4)](+) is exothermic, but the formation of the initial Lewis pair should be unfavorable in polar solvents. Consequently the combination Rp(-)/[(Tol)Im(F4)](+) does not react with H-2 but leads instead to side products from nucleophilic substitution (k = 4 x 10(-2) L mol (-1) s(-1) at room temperature). In contrast, the heterogeneous combination Rs/[(Tol)Im(F4)](+) does split H-2 heterolytically to give H(Tol)Im(F4) and HRuCp(CO)(2) (HRp) or D(Tol)Im(F4) and DRp when using D-2. The reaction has been followed by H-1/H-2 and F-19 NMR spectroscopy as well as by IR spectroscopy and reaches 96% conversion after 1 d. Formation of H(Tol)Im(F4) 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 H-2 activation.