Low-oxidation state dirhodium complex produced by four-electron reduction of dirhodium(ii) complex supported by a flexible macrocyclic ligand

Abstract

Dinuclear rhodium complexes represent a distinct class of transition-metal compounds, exhibiting unique reactivity enabled by close metal–metal proximity. However, despite extensive investigations, access to highly reduced Rh₂ complexes has remained a challenge due to their intrinsic instability. Herein, we report the synthesis and characterization of a series of dirhodium complexes in the formal oxidation states of Rh^II₂, Rh^I₂, and Rh⁰₂, supported by a flexible macrocyclic bis(pyridinediimine) (PDI₂) ligand. Single-crystal X-ray diffraction analysis revealed the crystal structure of the Rh₂ complexes. The PDI₂ ligand system flexibly adapts both structure and electronics across redox states, facilitating reversible multielectron interconversion. Structural analysis, UV-vis-NIR spectroscopy, Rh L₃-edge absorption spectroscopy, and DFT calculations collectively demonstrate that the Rh^I₂ and Rh⁰₂ complexes are stabilized by delocalization of electron density onto the redox-active PDI₂ ligand. Moreover, molecular orbital analysis indicates that the Rh⁰₂ complex features a Rh–Rh bond mediated by 5p orbital interactions. This work offers fundamental insights into the electronic structure and bonding of dirhodium complexes in low oxidation states and expands the design principles for redox-active bimetallic systems.