Activation of unsaturated small molecules by bio-relevant multinuclear metal-sulfur clusters

Abstract

The high affinity of sulfur (S) for transition metal ions (M), as well as the flexible M−S bonding and bridging modes, enable the formation of a wide range of multi-metallic compounds called metal-sulfur (M−S) clusters. Even though M−S clusters tolerate various nuclearities in principle, the number of metals in molecular entities, particularly those applicable in catalytic reactions, are typically 4 or lower, while the highest number of 8 has been found in the biological N2-reducing catalyst. Other than the N2 reduction, M−S clusters in nature have been found in the conversion of small molecules such as CO2, CO, and H2. With the detailed structures of these native M−S clusters elucidated by recent biochemical studies, we can take steps toward the development of artificial catalysts superior to enzymes and/or those for biologically inaccessible reactions, while another direction is to deeply understand the enzymatic reactions and their relationship between the M−S structures. Here we review the progress in the reaction studies on synthetic M−S clusters, focusing on their utility in the activation and transformation of small molecules. The main part of this review has been divided into three sections: i) transformations of N2 and related small molecules, ii) reduction of CO2, and iii) transformations of organic small molecules. Remarkable recent progress includes the development of catalytic N2-silylation toward N(SiMe3)3 and CO2 reduction toward C1–C4 hydrocarbons. Tandem reduction and N-methylation of nitroarenes have also been accomplished in the last decade. Through the review process, the development of catalytic reactions by M−S clusters was found to be still challenging. Collaborative works among chemists who can synthesize M−S clusters, those who are skilled in developing homogeneous catalytic reactions, those having spectroscopic techniques, and theoretical chemists may provide new insights into this field.