Carbon-13 Solid-State NMR Studies on Synthetic Model Compounds of [4Fe−4S] Clusters in the 2+ State

Carbon-13 solid-state NMR is used to determine, via the temperature dependence of paramagnetic shifts, the spin ladder of magnetic levels and the related spin exchange parameters in [4Fe−4S] 2+ clusters. This study has been carried out between 180 and 330 K in three model compounds of the active sites of 4Fe−4S proteins: [(C 2 D 5 ) 4 N] 2 [Fe 4 S 4 (S 13 CD 2 C 6 D 5 ) 4 ], [(C 2 H 5 ) 4 N] 2 [Fe 4 S 4 (SC(CH 3 ) 3 ) 4 ], and [(C 2 H 5 ) 4 N] 2 [Fe 4 Se 4 (SC(CH 3 ) 3 ) 4 ]. The temperature dependencies of both the isotropic and anisotropic parts of the 13 C paramagnetic shift tensors have been measured and analyzed. Antiferromagnetic exchange coupling constants among iron atoms of J ≈ 430, 380, and 370 cm -1 have been deduced from fits of their temperature dependencies. Additional contributions of the double-exchange term B favoring electronic delocalization and of a Δ J term associated with the compression of the cubane geometry are also discussed. We also show that connections between these measurements and those of magnetic susceptibility are especially close in the case of the [4Fe−4S] 2+ redox state.