Accessing Unusual Reactivity through Chelation-Promoted Bond Weakening.

Highly reducing Sm(II) reductants and protic ligands were used as a platform to ascertain the relationship between low-valent metal-protic ligand affinity and degree of ligand X-H bond weakening with the goal of forming potent proton-coupled electron transfer (PCET) reductants. Among the Sm(II)-protic ligand reductant systems investigated, the samarium dibromide N-methylethanolamine (SmBr2-NMEA) reagent system displayed the best combination of metal-ligand affinity and stability against H2 evolution. The use of SmBr2-NMEA afforded the reduction of a range of substrates that are typically recalcitrant to single-electron reduction including alkynes, lactones, and arenes as stable as biphenyl. Moreover, the unique role of NMEA as a chelating ligand for Sm(II) was demonstrated by the reductive cyclization of unactivated esters bearing pendant olefins in contrast to the SmBr2-water-amine system. Finally, the SmBr2-NMEA reagent system was found to reduce substrates analogous to key intermediates in the nitrogen fixation process. These results reveal SmBr2-NMEA to be a powerful reductant for a wide range of challenging substrates and demonstrate the potential for the rational design of PCET reagents with exceptionally weak X-H bonds.

Coordination-Induced Bond Weakening.

Coordination-induced bond weakening is a phenomenon wherein ligand X-H bond homolysis occurs in concert with the energetically favorable oxidation of a coordinating metal complex. The coupling of these two processes enables thermodynamically favorable proton-coupled electron transfer reductions to form weak bonds upon formal hydrogen atom transfer to substrates. Moreover, systems utilizing coordination-induced bond weakening have been shown to facilitate the dehydrogenation of feedstock molecules including water, ammonia, and primary alcohols under mild conditions. The formation of exceptionally weak substrate X-H bonds via small molecule homolysis is a powerful strategy in synthesis and has been shown to enable nitrogen fixation under mild conditions. Coordination-induced bond weakening has also been identified as an integral process in biophotosynthesis and has promising applications in renewable chemical fuel storage systems. This review presents a discussion of the advances made in the study of coordination-induced bond weakening to date. Because of the broad range of metal and ligand species implicated in coordination-induced bond weakening, each literature report is discussed individually and ordered by the identity of the low-valent metal. We then offer mechanistic insights into the basis of coordination-induced bond weakening and conclude with a discussion of opportunities for further research into the development and applications of coordination-induced bond weakening systems.

Ammonia Solvation vs Aqueous Solvation of Samarium Diiodide. A Theoretical and Experimental Approach to Understanding Bond Activation Upon Coordination to Sm(II).

Coordination-induced desolvation or ligand displacement by cosolvents and additives is a key feature responsible for the reactivity of Sm(II)-based reagent systems. High-affinity proton donor cosolvents such as water and glycols also demonstrate coordination-induced bond weakening of the O-H bond, facilitating reduction of a broad range of substrates. In the present work, the coordination of ammonia to SmI2 was examined using Born-Oppenheimer molecular dynamics simulations and mechanistic studies, and the SmI2-ammonia system is compared to the SmI2-water system. The coordination number and reactivity of the SmI2-ammonia solvent system were found to be similar to those of SmI2-water but exhibited an order of magnitude greater rate of arene reduction by SmI2-ammonia than by SmI2-water at the same concentrations of cosolvent. In addition, upon coordination of ammonia to SmI2, the Sm(II)-ammonia solvate demonstrates one of the largest degrees of N-H bond weakening reported in the literature compared to known low-valent transition metal ammonia complexes.

Proton donor effects on the reactivity of SmI2. Experimental and theoretical studies on methanol solvation vs. aqueous solvation.

Proton donors are important components of many reactions mediated by samarium diiodide (SmI2). The addition of water to SmI2 creates a reagent system that enables the reduction of challenging substrates through proton-coupled electron-transfer (PCET). Simple alcohols such as methanol are often used successfully in reductions with SmI2 but often have reduced reactivity. The basis for the change in reactivity of SmI2-H2O and SmI2-MeOH is not apparent given the modest differences between water and methanol. A combination of Born-Oppenheimer molecular dynamics simulations and mechanistic experiments were performed to examine the differences between the reductants formed in situ for the SmI2-H2O and SmI2-MeOH systems. This work demonstrates that reduced coordination of MeOH to Sm(ii) results in a complex that reduces arenes through a sequential electron proton transfer at low concentrations and that this process is significantly slower than reduction by SmI2-H2O.