RESUMO
Cubane-type clusters [Fe(4)S(4)(SR*)(4)](2-) containing chiral thiolate ligands with R* = CH(Me)Ph (1), CH(2)CH(Me)Et (2), and CH(2)CH(OH)CH(2)OH (3) have been prepared by ligand substitution in the reaction systems [Fe(4)S(4)(SEt)(4)]/R*SH (1-3, acetonitrile) and [Fe(4)S(4)Cl(4)](2-)/NaSR*(3, Me(2)SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe(4)S(4)L(4-n)(SR*)(n)](2-) (L = SEt, Cl) with n = 1-4. Clusters 1 and 2 were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (n = 1) and seven diastereomers and their enantiomers (n = 2-4). Reactions were monitored by isotropically shifted (1)H NMR spectra in acetonitrile or Me(2)SO. In systems affording 1 and 2 as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et(4)N)(2)[1] afforded 1-[SS(RS)(RS)] in which two sites are disordered because of occupancy of R and S ligands. Similarly, (Et(4)N)(2)[2] led to 2-[SSSS], a consequence of spontaneous resolution upon crystallization. The clusters 3-[RRRR] and 3-[SSSS] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with n = 1-4 by appearance of four pairs of diastereotopic SCH(2) signals in both acetonitrile and Me(2)SO reaction systems. Identical spectra were obtained with racemic, R-(-), and S-(+) thiols, indicating that ligand-ligand interactions are too weak to allow detection of diastereomers (e.g., [SSSS] vs [SSRR]). The stability of 3 in Me(2)SO/H(2)O media is described.
Assuntos
Química Bioinorgânica/métodos , Ferro/química , Compostos Organometálicos/síntese química , Compostos de Sulfidrila/química , Enxofre/química , alfa-Ciclodextrinas/química , Acetonitrilas/química , Cristalização , Cristalografia por Raios X , Dimetil Sulfóxido/química , Ferro/metabolismo , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Conformação Molecular , Compostos Organometálicos/análise , Oxirredução , Estereoisomerismo , Enxofre/metabolismoRESUMO
The stability of cubane-type [Fe4S4(SR)4](2-) clusters in mixed organic/aqueous solvents was examined as an initial step in the development of stable water-soluble cluster compounds possibly suitable for reconstitution of scaffold proteins in protein biosynthesis. The research involves primarily spectrophotometric assessment of stability in 20-80% Me2SO/aqueous media (v/v), from which it was found that conventional clusters tend to be stable for up to 12h in 60% Me2SO but are much less stable at higher aqueous content. α-Cyclodextrin mono- and dithioesters and thiols were prepared as ligand precursors for cluster binding, which was demonstrated by spectroscopic methods. A potentially bidentate cyclodextrin dithiolate was found to be relatively effective for cluster stabilization in 40% Me2SO, suggesting (together with earlier results) that other exceptionally large thiolate ligands may promote cluster stability in aqueous media.
Assuntos
Complexos de Coordenação/química , Ferro/química , Enxofre/química , Materiais Biomiméticos/síntese química , Materiais Biomiméticos/química , Dimetil Sulfóxido/química , Interações Hidrofóbicas e Hidrofílicas , Proteínas Ferro-Enxofre/síntese química , Proteínas Ferro-Enxofre/química , Ligantes , Modelos Moleculares , Estrôncio/química , alfa-Ciclodextrinas/químicaRESUMO
An extensive series of 3:1 site-differentiated cubane-type clusters [Fe(4)S(4)(PPr(i)(3))(3)L] (L = Cl(-), Br(-), I(-), RO(-), RS(-), RSe(-)) has been prepared in 40-80% yield by two methods: ligand substitution of [Fe(4)S(4)(PPr(i)(3))(4)](1+) in tetrahydrofuran (THF)/acetonitrile by reaction with monoanions, and reductive cleavage of ligand substrates (RSSR, RSeSeR, I(2)) by the all-ferrous clusters [Fe(8)S(8)(PPr(i)(3))(6)]/[Fe(16)S(16)(PPr(i)(3))(8)] in THF. These neutral clusters are stable and do not undergo ligand redistribution reactions involving charged species in benzene and THF solutions. X-ray structural studies confirm the cubane stereochemistry but with substantial and variable distortions of the [Fe(4)S(4)](1+) core from idealized cubic core geometry. Based on Fe-S bond lengths, seven clusters were found to have compressed tetragonal distortions (4 short and 8 long bonds), and the remaining seven display other types of distortions with different combinations of long, short, and intermediate bond lengths. These results further emphasize the facile deformabililty of this core oxidation state previously observed in [Fe(4)S(4)(SR)(4)](3-) clusters. The Fe(2.25+) mean oxidation state was demonstrated from (57)Fe isomer shifts, and the appearance of two quadrupole doublets arises from the spin-coupled |9/2,4,1/2> state. The S = 1/2 ground state was further supported by electron paramagnetic resonance spectra and magnetic susceptibility data.