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Isolation and Characterization of Single and Sulfide‐Bridged Double [4Fe–4S] Cubane Clusters with 4‐Pyridinethiolato Ligands
Author(s) -
Gerlach Deidra L.,
Coucouvanis Dimitri,
Kampf Jeff,
Lehnert Nicolai
Publication year - 2013
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201300802
Subject(s) - cubane , chemistry , electron paramagnetic resonance , crystallography , sulfide , ligand (biochemistry) , sulfur , cluster (spacecraft) , stereochemistry , nuclear magnetic resonance spectroscopy , crystal structure , nuclear magnetic resonance , biochemistry , physics , receptor , organic chemistry , programming language , computer science
Cuboidal iron–sulfur clusters, [4Fe–4S], are important electron‐transfer (ET) sites in biology. In addition, more complex structures, usually consisting of modified or fused cubane clusters, are used as active sites in many important enzymes. For example, the Fe–Mo cofactor (FeMoco) of nitrogenase contains two fused cubanes. Here, we report the synthesis of three new para ‐pyridylthiolate ligated iron–sulfur cubane clusters, two single clusters (Bu 4 N) 2 [Fe 4 S 4 (SMePy) 4 ] and (Bu 4 N) 2 [Fe 4 S 4 (SPy) 4 ], and the sulfide‐bridged double cubane (Bu 4 N) 4 [{Fe 4 S 4 (SPy) 3 } 2 S] with 4‐pyridinethiolato exogenous ligands. The properties of these clusters were then explored by 1 H NMR, IR, and UV/Vis spectroscopy, cyclic voltammetry (CV), and X‐ray crystallography. Importantly, (Bu 4 N) 4 [{Fe 4 S 4 (SPy) 3 } 2 S] is the first example of a crystallographically characterized sulfide‐bridged double cubane with all‐thiolato exogenous ligands that is not supported by a large encapsulating ligand. This cluster shows a bridging Fe–S–Fe angle of 104°, its other structural parameters are in close agreement with those of the single‐cluster analog (Bu 4 N) 2 [Fe 4 S 4 (SPy) 4 ]. Finally, the one‐electron‐reduced forms of (Bu 4 N) 2 [Fe 4 S 4 (SPy) 4 ] and (Bu 4 N) 4 [{Fe 4 S 4 (SPy) 3 } 2 S] were studied by low‐temperature electron paramagnetic resonance (EPR) spectroscopy. Both clusters exhibit reversible one‐electron reductions at –401 and –528 mV [vs. the normal hydrogen electrode (NHE)], respectively. The one‐electron‐reduced forms of both clusters show S = 1/2 ground states as evident from EPR spectroscopy at liquid‐helium temperature. The temperature‐dependent data for the double cubane further indicate that the extra electron is trapped in one of the clusters of the dimer and that a low‐lying excited state is likely present in this complex, close in energy to the ground state.