Alkali Metal Ions Dictate the Structure and Reactivity of an Iron(II) Imido Complex

The presence of redox innocent metal ions has been proposed to modulate the reactivity of metal ligand multiple bonds; however, insight from structure/function relationships is limited. Here, alkali metal reduction of the Fe(III) imido complex [Ph₂B(^tBuIm)₂Fe═NDipp] (1) provides the series of structurally characterized Fe(II) imido complexes [Ph₂B(^tBuIm)₂Fe═NDippLi(THF)₂] (2), [Ph₂B(^tBuIm)₂Fe═NDippNa(THF)₃] (3), and [Ph₂B(^tBuIm)₂Fe═NDippK]₂ (4), in which the alkali metal cations coordinate the imido ligand. Structural investigations demonstrate that the alkali metal ions modestly lengthen the Fe═N bond distance from that in the charge separated complex [Ph₂B(^tBuIm)₂Fe═NDipp][K(18-C-6)THF₂] (5), with the longest bond observed for the smallest alkali metal ion. In contrast to 5, the imido ligands in 2-4 can be protonated and alkylated to afford Fe(II) amido complexes. Combined experimental and computational studies reveal that the alkali metal polarizes the Fe═N bond, and the basicity of imido ligand increases according to 5 < 4 ≈ 3 < 2. The basicity of the imido ligands influences the relative rates of reaction with 1,4-cyclohexadiene, specifically by gating access to complex 5, which is the species that is active for HAT. All complexes 2-4 react with benzophenone form metastable Fe(II) intermediates that subsequently eliminate the metathesis product Ph₂C═NDipp, with relative rates dependent on the alkali metal ion. By contrast, the same reaction with 5 does not lead to the formation of Ph₂C═NDipp. These results demonstrate that the coordination of alkali metal ions dictate both the structure and reactivity of the imido ligand and moreover can direct the reactivity of reaction intermediates.