A Functional [NiFe]-Hydrogenase Model Compound That Undergoes Biologically Relevant Reversible Thiolate Protonation

Two model compounds of the active site of [NiFe]-hydrogenases
with
an unusual {S2Ni­(μ-S)­(μ-CO)­Fe­(CO)2S}-coordination environment around the metals are reported.
The neutral compound [Ni­(xbsms)­(μ-CO)­(μ-S)­Fe­(CO)2(‘S’)], (1) (H2xbsms = 1,2-bis­(4-mercapto-3,3-dimethyl-2-thiabutyl)­benzene)
is converted to [1H]­[BF4] by reversible protonation
using HBF4·Et2O. The protonation takes
place at the terminal thiolate sulfur atom that is coordinated to
nickel. Catalytic intermediates with a protonated terminal cysteinate
were suggested for the native protein but have not yet been confirmed
experimentally. [1H]­[BF4] is the first dinuclear
[NiFe] model compound for such a species. Both complexes have been
synthesized and characterized by X-ray crystallography, NMR-, FTIR-,
and 57Fe-Mössbauer spectroscopy as well as by electronic
absorption and resonance Raman spectroscopy. The experimental results
clearly show that the protonation has a significant impact on the
electronic structure of the iron center, although it takes place at
the nickel site. DFT calculations support the interpretation of the
spectroscopic data and indicate the presence of a bonding interaction
between the metal ions, which is relevant for the enzyme as well.
Electrochemical experiments show that both 1 and [1H]­[BF4] are active for electrocatalytic proton
reduction in aprotic solvents.