Synthesis and Characterization of Functionalized [4Fe-4S] Cubane Clusters and Linkage to Metalloporphryrins as Catalytic Sites.

[摘要] Iron-sulfur clusters and hemes are found throughout biological systems in a variety of unique and important enzymatic systems. Only one type of enzyme contains both a heme and an iron-sulfur cluster at the active site of the enzyme, sulfite and nitrite reductase (SIR/NIR). The assimilatory variety of these enzymes is capable of both sulfite and nitrite reduction to produce sulfide and ammonia, respectively. As these enzymes function at ambient temperature and pressure and perform a six-electron reduction without substrate dissociation, they provide an attractive blueprint for biologically inspired multi-electron catalysts. The enzyme active site contains a [4Fe-4S] cluster which supplies electrons to a catalytic heme center through an axially coordinated sulfide bridge. The goal of this research is to synthesize complexes biologically inspired by SIR/NIR for the purpose of reductive catalysis. The three key structural features for the first generation complex design include (i) a metalloporphyrin, for use as a catalytic site, (ii) a [4Fe-4S] cluster, for use as an electron reservoir, and (iii) a bridging ligand or atom to link the catalytic and electron storage components. Two designs were formulated using different forms of connectivity for the bridging component: a) axial coordination of an iron-sulfur cluster via a bridging ligand to a heme complex, and b) covalent ligation of the iron-sulfur cluster directly to the functionalized porphyrin ligand. The axially bound model developed here was optimized to accomplish the optimal binding of the heme and [4Fe-4S] cluster by modification of the electron density of the heme iron by the addition of electron withdrawing groups to the porphyrin, and by evaluation of different types of organic bridges to connect these components. The covalently bound model utilizes a singly functionalized porphyrin ligand intended for direct binding to an iron-sulfur cluster for electron transfer from the cluster to the heme iron via the porphyrin ligand. The covalent bound model is likely the more viable route for further progress. In the process of synthesizing and testing different types of functionalized [4Fe-4S] clusters, an interesting, sulfide-bridged cluster was discovered. The properties of the sulfide-bridged double cubane cluster with pyridylthiolate ligands were then investigated.

[发布机构] University of Michigan

[效力级别] Iron-sulfur Clusters

[关键词] Bioinorganic Chemistry;Iron-sulfur Clusters;Metalloporphyrins;Chemistry;Science;Chemistry