New reactivity for U

Replacing the conventional ligands of uranium metallocenes by a group with a 'soft–hard–soft' coordination environment leads to new reactivity patterns and structures for low- and high-valent uranium ions

Credit: © 2009 Wiley

Actinide chemistry is largely based on metallocene complexes, but many of these compounds are unstable because the cyclopentadienyl ligands form dimers during oxidation reactions. Motivated by previous successes stabilizing highly reactive groups on lanthanides and transition metals, Jaqueline Kiplinger and colleagues at Los Alamos National Laboratory have avoided this problem by using a ligand combining 'soft' and 'hard' coordination environments¹.

The researchers replaced the conventional bis(cyclopentadienyl) framework of a classic uranium metallocene — which acts as a synthetic equivalent for uranium(II) when in the presence of graphite intercalation compound KC₈ — with a set of two phosphorus–nitrogen–phosphorus pincer compounds. This new uranium(II) synthon showed different reactivity patterns, resulting in the first hydrazonido complex of an actinide. This features a C=N–N group where one nitrogen forms a double bond and the other a dative bond with the uranium centre. The synthon also reacted with the strong oxidant pyridine oxide leading to a uranium(VI) complex that cannot be obtained with metallocenes because the cyclopentadienyl ligands are released as dimers under the same conditions.

These new actinide structures, and reactivity patterns, arise because of the various coordination possibilities provided by the soft–hard–soft PNP pincer ligand as well as the steric crowding and electronic density at the metal centre.