Synthesis, Structures, and Electronic Properties of [8Fe-7S] Cluster Complexes Modeling the Nitrogenase P-Cluster

摘要

High-yield synthesis of the iron-sulfur cluster [{N(SiMe_3)_2}{SC(NMe_2)_2}Fe_4S_3]_2(μ6-S) {μ-N(SiMe_3)_2}_2 (1), which reproduces the [8Fe-7S] core structure of the nitrogenase PN-cluster, has been achieved via two pathways: (1) Fe{N(SiMe_3)_2}_2 4- HSTip (Tip = 2,4,6-~iPr_3C_6H_2) + tetramethylthiourea (SC(NMe_2)_2) + elemental sulfur (S_8); and (2) Fe_3{N(SiMe_3)_2}_2(μ-STip)_4 (2) + HSTip + SC(NMe_2)_2 + S_8. The thiourea and terminal amide ligands of 1 were found to be replaceable by thiolate ligands upon treatment with thiolate anions and thiols at -40 ℃, respectively, and a series of [8Fe-7S] clusters bearing two to four thiolate ligands have been synthesized and their structures were determined by X-ray analysis. The structures of these model [8Fe-7S] clusters all closely resemble that of Jthe reduced form of P-cluster (P~N) having 8Fe(II) centers, while their 6Fe(II)-2Fe(III) oxidation states correspond to the oxidized form of P-cluster (P~(ox)). The cyclic voltammograms of the [8Fe-7S] clusters reveal two quasi-reversible one-electron reduction processes, leading to the 8Fe(II) state that is the same as the P~N-cluster, and the synthetic models demonstrate the redox behavior between the two major oxidation states of the native P-cluster. Replacement of the SC(NMe_2)_2 ligands in 1 with thiolate anions led to more negative reduction potentials, while a slight positive shift occurred upon replacement of the terminal amide ligands with thiolates. The clusters 1, (NEt_4)_2[{N(SiMe_3)_2}(SC_6H_4-4-Me)Fe_4S_3]_2(μ_6-S)[μ-N(SiMe_3)_2}_2(3a),and[(SBtp){SC(NMe_2)_2}Fe_4S_3]_2(μ_6-S){μ-N(SiMe_3)_2}_2 (5; Btp = 2,6-(SiMe_3)_2C_6H_3) are EPR silent at 4-100 K, and their temperature-dependent magnetic moments indicate a singlet ground state with antiferromagnetic couplings among the iron centers. The ~(57)Fe Moessbauer spectra of these clusters are consistent with the 6Fe(II)-2Fe(III) oxidation state, each exhibiting two doublets with an intensity ratio of ca. 1:3, which are assignable to Fe(III) and Fe(II), respectively. Comparison of the quadrupole splittings for 1, 3a, and 5 has led to the conclusion that two Fe(III) sites of the clusters are the peripheral iron atoms.