Synthetic Models for the Active Site of the [FeFe]-Hydrogenase: Catalytic Proton Reduction and the Structure of the Doubly Protonated Intermediate

This report compares biomimetic hydrogen evolution reaction
catalysts
with and without the amine cofactor (adtNH): Fe2(adtNH)­(CO)2(dppv)2 (1NH) and Fe2(pdt)­(CO)2(dppv)2 (2) [(adtNH)2– = HN­(CH2S)22–, pdt2– = 1,3-(CH2)3S22–, and dppv = cis-C2H2(PPh2)2]. These compounds are spectroscopically, structurally,
and stereodynamically very similar but exhibit very different catalytic
properties. Protonation of 1NH and 2 gives three isomeric hydrides each, beginning
with the kinetically favored terminal hydride, which converts sequentially
to sym and unsym isomers of the
bridging hydrides. In the case of 1NH, the corresponding ammonium hydrides are also observed. In
the case of the terminal amine hydride [t-H1NH]­BF4, the ammonium/amine
hydride equilibrium is sensitive to counteranions and solvent. The
species [t-H1NH2]­(BF4)2 represents the first example
of a crystallographically characterized terminal hydride produced
by protonation. The NH---HFe distance of 1.88(7) Å indicates
dihydrogen-bonding. The bridging hydrides [μ-H1NH]+ and [μ-H2]+ reduce near −1.8 V, about 150 mV more negative
than the reductions of the terminal hydride [t-H1NH]+ and [t-H2]+ at −1.65 V. Reductions of the
amine hydrides [t-H1NH]+ and [t-H1NH2]2+ are irreversible. For the
pdt analogue, the [t-H2]+/0 couple is unaffected by weak acids (pKaMeCN = 15.3) but exhibits catalysis with HBF4·Et2O, albeit with a turnover frequency (TOF) around
4 s–1 and an overpotential greater than 1 V. The
voltammetry of [t-H1NH]+ is strongly affected by relatively weak acids
and proceeds at 5000 s–1 with an overpotential of
0.7 V. The ammonium hydride [t-H1NH2]2+ is a faster catalyst,
with an estimated TOF of 58 000 s–1 and an
overpotential of 0.5 V.