Cooperative C-H Bond Activation by a Low-Spin d6 Iron-Aluminum Complex.

ABSTRACT

The reactions of transition metal complexes underpin numerous synthetic processes and catalytic transformations. Typically, this reactivity involves the participation of empty and filled molecular orbitals centered on the transition metal. Kinetically stabilized species, such as octahedral low-spin d6 transition metal complexes, are not expected to participate directly in these reactions. However, novel approaches that exploit metal-ligand cooperativity offer an opportunity to challenge these preconceptions. Here, we show that inclusion of an aluminum-based ligand into the coordination sphere of neutral low-spin d6 iron complex leads to unexpected reactivity. Complexes featuring an unsupported Fe-Al bond are capable of the intermolecular C-H bond activation of pyridines. Mechanistic analysis suggests that C-H activation proceeds through a reductive deprotonation in which the two metal centers (Fe and Al) act like a frustrated Lewis pair. The key to this behavior is a ground state destabilization of the d6 iron complex, brought about by the inclusion of the electropositive aluminum-based ligand. These findings have immediate implications for the design of reagents and catalysts based on first-row transition metals.