Synthetically Reversible, Proton-Mediated Nitrite N-O Bond Cleavage at a Dicopper Site

A monocationic dicopper(I,I) nitrite complex [Cu₂(μ-κ¹:κ¹-O₂N)DPFN][NTf₂] (2) (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine, NTf₂^- = N(SO₂CF₃)₂^-), was synthesized by treatment of a dicopper acetonitrile complex, [Cu₂(μ-MeCN)DPFN][NTf₂]₂ (1), with tetrabutylammonium nitrite ([nBu₄N][NO₂]). DFT calculations indicate that 2 is one of three linkage isomers that are close in energy and presumably accessible in solution. Reaction of the μ-κ¹:κ¹-O₂N complex with p-TolSH produces nitrous acid (HONO) and the corresponding dicopper thiolate species via an acid-base exchange reaction. Notably, treatment of 2 with HNTf₂ results in N-O bond cleavage in the putative, HONO-ligated complex to form the more thermodynamically favorable nitrosyl-bridged dicopper complex [Cu₂(μ-NO)(μ-OH)DPFN][NTf₂]₂ (4). This scission can be reversed via deprotonation of the hydroxy ligand with KOBu. X-ray diffraction studies confirmed the solid-state molecular structures of 2 and 4. DFT calculations were used to construct a reaction coordinate diagram detailing formation of the μ-NO complex and to describe its electronic structure. The nitrosyl ligand in 4 is chemically labile, as demonstrated by its ready displacement in reactions with CO or NO₂^-.