Isocyanide Ligand Tuning of Rhenium Imido N−H BDFE Values for Catalytic Dinitrogen Reduction

During N 2 reduction cycles that involve transition-metal-mediated N 2 splitting to nitrides, it is often thermodynamically difficult to form the first N–H bond on the way to ammonia. Increasing this N–H bond dissociation free energy (BDFE) is challenging because changes in the pK a and redox potential typically counteract one another. Here, we report that pincer-ligated rhenium complexes containing isocyanide ligands have dramatically higher imido N–H BDFE values than analogous chloride, cyanide, and pyridine complexes. Systematic electronic and steric variation of the isocyanides revealed that the Re V/IV reduction potentials for the nitride complexes can be varied over a range of ca. 2 V with more muted changes in the Re IV N–H pK a values, resulting in large N–H BDFE values that can be tuned over a range of 16 kcal/mol. Using this strategy, several new Re catalysts for N 2 reduction to NH 3 were discovered, including an alkyl isocyanide complex that produced up to 22 equiv of ammonia per Re center at room temperature using SmI 2 as reductant and ethylene glycol as a proton source. The results provide guidance on ligand tuning for forming N–H bonds from nitrides as part of homogeneous catalytic cycles for conversion of N 2 to NH 3 .