Effect of Sulfonamide and Carboxamide Ligands on the Structural Diversity of Bimetallic Rh^II–Rh^II Cores: Exploring the Catalytic Activity of These Newly Synthesized Rh₂ Complexes

Abstract

A new class of dirhodium(II) complexes with tethered sulfonamide and carboxamide ligands was synthesized and characterized. A new type of coordination mode was found for the quinoline moiety containing a sulfonamide ligand, which afforded the axially coordination-free bimetallic dirhodium complexes. Studies were conducted on the catalytic properties of these complexes for cyclopropanation reactions, and the findings indicate that a free axial coordination site is crucial for achieving a high degree of reactivity.

An Unusual Perpendicular Metallacycle Intermediate is the Origin of Branch Selectivity in the Rh(II)-Catalyzed C–H Alkylation of Aryl Sulfonamides with Vinylsilanes

“…In the course of our previous experimental mechanistic study of the branch-selective Rh­(II)-catalyzed C–H alkylation of aryl sulfonamides with vinylsilanes, a novel twisted paddlewheel-shaped rhodium complex 1a with two sulfonamide molecules coordinated to the dirhodium core was isolated as a catalytically active intermediate (Figure ). , The structure of the complex, which was determined by X-ray crystallography, showed that each substrate chelates to each rhodium atom by the aminoquinoline moiety rather than to the two rhodium atoms as a bridge ligand. Based on the nature of dimeric rhodium­(II) complexes in C–H functionalization reactions, as determined by both experimental and computational studies, we carried out an in-depth study of the reactivity of the isolated species and the branch selectivity of this reaction.…”

“…As described above, a twisted paddlewheel complex 1a was only isolated with the sulfonamide substrate in our previous study, and the cis-or trans-oriented tethered paddlewheel complexes 1b and 1c were not observed. 16 The relative Gibbs free energies of 1b and 1c were exceptionally high compared to 1a (10.7 and 7.4 kcal mol −1 , respectively), which is consistent with the fact that we were unable to observe or isolate these complexes. Due to the tetrahedral geometry of the sulfonamide moiety, the phenyl group is positioned away from the dirhodium center in 1a and does not provide any interactions, which leaves free axial coordination sites.…”

“…As described above, a twisted paddlewheel complex 1a was only isolated with the sulfonamide substrate in our previous study, and the cis - or trans -oriented tethered paddlewheel complexes 1b and 1c were not observed . The relative Gibbs free energies of 1b and 1c were exceptionally high compared to 1a (10.7 and 7.4 kcal mol –1 , respectively), which is consistent with the fact that we were unable to observe or isolate these complexes.…”

Melding of Experiment and Theory Illuminates Mechanisms of Metal-Catalyzed Rearrangements: Computational Approaches and Caveats

“…62,70 Preliminary evidence from our work indicates that including explicit solvent at the presumably vacant axial position of dirhodium complexes can modulate reactivity of dirhodium tetracarboxylates, which aligns with a number of experimental studies that show that axial coordination can influence electronic communication between the two Rh atoms in these complexes. [136][137][138][139][140] In this case, however, the question to be addressed with microsolvation was well-defined; it was specific to a particular position where solvent could bind. Any electron donor (including reactants) could, in principle, coordinate the vacant, axial coordination site of dirhodium complexes, which may (or may not) have a non-negligible effect on reaction barriers.…”

Effect of Tethered, Axially Coordinated Ligands (TACLs) on Dirhodium(II,II) Catalyzed Cyclopropanation: A Linear Free Energy Relationship Study

“…Experimentally, studying axial coordination by tethering them to bridging ligands has emerged as a viable strategy to circumvent the weak interaction associated with axial ligands. − This provides a bias for the site of reactivity and ensures a high local concentration for axial coordination. We have investigated the role of axial coordination by designing bridging ligands that have pendant thioether moieties that are capable of axial coordination (complexes 1 – 6 , Figure ).…”