Steric Congestion at, and Proximity to, a Ferrous Center Leads to Hydration of α‑Nitrile Substituents Forming Coordinated Carboxamides

The question of the conversion of
nitrile groups into amides (nitrile hydration) by action of water
in mild and eco-compatible conditions and in the presence of iron
is addressed in this article. We come back to the only known example
of hydration of a nitrile function into carboxamide by a ferrous [Fe­(II)]
center in particularly mild conditions and very efficiently and demonstrate
that these unusual conditions result from the occurrence of steric
stress at the reaction site and formation of a more stable end product.
Two bis­(cyano-substituted) (tris 2-pyridyl methyl amine) ligands have
been prepared, and the structures of the corresponding FeCl2 complexes are reported, both in the solid state and in solution.
These two ligands only differ by the position of the nitrile group
on the tripod in the α and β position, respectively, with
respect to the pyridine nitrogen. In any case, intramolecular coordination
is impossible. Upon action of water, the nitrile groups are hydrated
however only if they are located in the α position. The fact
that the β-substituted β-(NC)2TPAFeCl2 complex is not water sensitive suggests that the reaction proceeds
in an intramolecular way at the vicinity of the metal center. In the
bis α-substituted α-(NC)2TPAFeCl2 complex, both functions are converted in a very clean fashion, pointing
out that this complex exhibits ligand flexibility and is not deactivated
after the first hydration. At a preparative scale, this reaction allows
the one-pot conversion of the bis­(cyano-substituted) tripod into a
bis­(amido-substituted) one in particularly mild conditions with a
very good yield. Additionally, the XRD structure of a ferric compound
in which the two carboxamido ligands are bound to the metal in a seven-coordinate
environment is reported.