The Synthesis and Electronic Structure of a Novel [Ni‘S₄’Fe₂(CO)₆] Radical Cluster: Implications for the Active Site of the [NiFe] Hydrogenases

Abstract

A novel [NiS4Fe2(CO)6]cluster (1: 'S(4)'=(CH(3)C(6)H(3)S(2))(2)(CH(2))(3)) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at -1.31 V versus Fc(+)/Fc to generate the EPR-active monoanion 1(-). Multifrequency Q-, X- and S-band EPR spectra of (61)Ni-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single (61)Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g(1) and A(1), and g(3) and A(3) to reproduce all of the features in the S- and X-band spectra. This behaviour provides a rare example of the detection and measurement of non-coincidence effects from frozen-solution EPR spectra without the need for single-crystal measurements, and in which the S-band experiment is sensitive to the non-coincidence. An analysis of the EPR spectra of 1(-) reveals a 24 % Ni contribution to the SOMO in 1(-), supporting a delocalisation of the spin-density across the NiFe(2) cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, nu(CO), shift to lower frequency by about 70 cm(-1) when 1 is reduced to 1(-). Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1(-) and suggest that the SOMO is delocalised over the whole cluster, but with little S-centre participation. This electronic structure contrasts with that of the Ni-A, -B, -C and -L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO.

Synthetic Models for the Active Sites of Nickel‐Containing Enzymes

“…Similar results were also found by the group of Schröder with a bisthiolate-bisthioether ligand [82]. For the NiFe 2 complex 21 with this latter ligand, the resulting species obtained after one-electron reduction showed ESR hyperfi ne coupling when enriched with 61 Ni, and an average shift of 70 cm Ϫ1 for the CO bands in the IR spectrum.…”

Hydrogen Evolution Catalyzed by Cobaloximes

“…[16][17][18][19]34 Despite the hundreds of structural models developed to date, few have been reported to show electrocatalytic activity related to proton reduction or hydrogen oxidation. While upwards of 50 structural mimics of [NiFe] hydrogenases have been reported, initially only biomimetic complexes that deviated significantly from the enzyme structure displayed any electrocatalytic behavior, including a trinuclear {Ni'S 2 'Fe 2 } system [35][36] and a few Ni-Ru complexes [37][38][39][40][41][42] …”

Thiolate‐Bridged Iron–Nickel Models for the Active Site of [NiFe] Hydrogenase

“…[31] On the other hand, a team of chemists in the UK including Schröder found that complex [(CO) 6 12 (Figure 9). [32] Similarly, Sellmann and co-workers used Ni('S 4 ') {21; 'S 4 ' = 1,2-bis(2-mercaptophenylthio)ethane} and Ni('S 4 C 3 Me 2 ') {22; 'S 4 C 3 Me 2 ' = 1,3-bis(2-mercaptophenylthio)-2,2-dimethylpropane} as precursors for the analogous Fe 2 Ni complexes [(CO) 6 Fe 2 ('S 4 ')Ni] (30) and [(CO) 6 Fe 2 ('S 4 C 3 Me 2 ')Ni] (31), respectively ( Figure 9). [33] The Ni-Fe [2.4789(9)-2.5038 (4) [25] Chemical reduction of 29 with Cp 2 Co (or electrochemical reduction of 29) leads to the formation of [29] -, which is EPR active and shows an interaction between the unpaired electron and the nickel nucleus upon 61 Ni (I = 3/2) enrichment.…”

“…Schröder and co-workers used trinuclear Fe 2 Ni complex 29 [32] as a catalyst for the electrochemical reduction of H + . [60] than 1 h. On the basis of DFT calculations and spectroscopic evidence, the SOMO of [29] -containing the unpaired electron was suggested to be delocalized over the Ni and the two Fe atoms, implicating metal-based protonation during catalysis.…”