Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

Geoffrey M. Chambers, Mioy T. Huynh, Yulong Li, Sharon Hammes-Schiffer, Thomas B. Rauchfuss, Edward Reijerse, Wolfgang Lubitz
2015 Inorganic Chemistry  
A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni I/II (SCys) 2 and Fe II (CN) 2 CO sites are represented with (RC 5 H 4 )Ni I/II and Fe II (diphos)(CO) modules, where diphos = 1,2-C 2 H 4 (PPh 2 ) 2 (dppe) or cis-1,2-C 2 H 2 (PPh 2 ) 2 (dppv). The two bridging thiolate ligands are represented by CH 2 (CH 2 S) 2 2− (pdt 2− ), Me 2 C(CH 2 S) 2 2− (Me 2 pdt 2− ), and (C 6 H 5 S) 2 2− . The reaction of Fe(pdt)(CO) 2 (dppe) and [(C 5 H 5 ) 3 Ni 2 ]BF 4
more » ... affords [(C 5 H 5 )Ni(pdt)Fe(dppe)-(CO)]BF 4 ([1a]BF 4 ). Monocarbonyl [1a]BF 4 features an S = 0 Ni II Fe II center with five-coordinated iron, as proposed for the Ni-SI a state of the enzyme. One-electron reduction of [1a] + affords the S = 1 / 2 derivative [1a] 0 , which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni I Fe II compound. The Ni I Fe II assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni II Fe I -based models. Compound [1a] 0 reacts with strong acids to liberate 0.5 equiv of H 2 and regenerate [1a] + , indicating that H 2 evolution is catalyzed by [1a] 0 . DFT calculations were used to investigate the pathway for H 2 evolution and revealed that the mechanism can proceed through two isomers of [1a] 0 that differ in the stereochemistry of the Fe(dppe)CO center. Calculations suggest that protonation of [1a] 0 (both isomers) affords Ni III -H-Fe II intermediates, which represent mimics of the Ni-C state of the enzyme. Beyond complementing our understanding of the enzyme, synthetic models could underpin the development of new catalysts for the processing of H 2 . 1,2 The possibility of connecting synthetic complexes to enzyme states was established with [(dppe)Ni(pdt)(μ-H)Fe(CO) 3 ] +.14,21 A related nickel-iron hydride [(amine) 2 Ni(SR) 2 (μ-H)FeL 3 ] + catalyzes the reduction of redox dyes by H 2 , a characteristic assay for the hydrogenases. 15 With a Ni II -H-Fe II core, these synthetic complexes represent early models for the Ni-R state of the protein, despite discrepancies (Table 1). 16 Concurrent with the development of these diamagnetic models, 21,22 efforts have been made to replicate the paramagnetic states of the enzyme. The two most prominent paramagnetic states are Ni-C and Ni-L. Invoked in all catalytic mechanisms is Ni-C, 6,7,23-25 which features a hydride bridging the Ni III and Fe II centers. No synthetic analogues of this nickel(III) hydride have been detected, but this paper provides a first glimpse of a synthetic model. With a Ni I Fe II core, Ni-L is typically generated by low-temperature photolysis of Ni-C, and recent evidence points to its role in catalysis. 26,27 These S = 1 / 2 states deviate most strongly from the conventional chemistry of dimetal systems and therefore offer the greatest opportunity for the discovery of fundamentally new reactivity. The first reported S = 1 / 2 NiFe complex was [(diphos)Ni(pdt)Fe-(CO) 2 L] + . Spectroscopic, crystallographic, and computational analyses show, however, that these cations are described as Ni II Fe I Chambers et al.
doi:10.1021/acs.inorgchem.5b01662 pmid:26421729 pmcid:PMC4807737 fatcat:rheqlszlozggji6v2ihghg6hvm