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

High-yield synthesis of the iron-sulfur cluster [{N(SiMe3)(2)}{SC(NMe2)(2))Fe4S3](2)(mu(6)-S) {mu-N(SiMe3)(2))(2) (1), which reproduces the [8Fe-7S] core structure of the nitrogenase P-N-cluster, has been achieved via two pathways: (1) Fe{N(SiMe3)(2)}(2) + HSTip (Tip = 2,4,6-(Pr3C6H2)-Pr-i) + tetramethylthiourea (SC(NMe2)(2)) + elemental sulfur (S-8); and (2) Fe-3{N(SiMe3)(2)}(2)(mu-STip)(4) (2) + HSTip + SC(NMe2)(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 degrees C, 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 the 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(NMe2)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, (NEt4)(2)[{N(SiMe3)(2)}(SC6H4-4-Me)Fe4S3](2)(mu(6)-S){mu-N(SiMe3)(2)}(2) (3a), and [(SBtp){SC(NMe2)(2)}Fe4S3](2)(mu(6)-S){mu-N(SiMe3)(2)}(2) (5; Btp = 2;6-(SiMe3)(2)C6H3) 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 Fe-57 Mossbauer 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.