Alkali metal cation effects for rapid C-H activation by iron(0) complexes.

Ion pairing with alkali metal cations offers a novel way to enhance the rate of C-H activation. This strategy is demonstrated here using diketiminate-supported iron(0) complexes, which previously were shown to activate the C-H bond of benzene slowly (hours at room temperature) and did not reach completion before decomposition. However, by removing a crown ether that sequesters the alkali metal countercation, there is rapid C-H oxidative addition in less than a minute at ambient temperature, showing that alkali cation effects can strongly influence C-H activation, even with the more weakly interacting Rb and Cs. The products from C-H activation are unusual dimeric iron(ii) phenyl hydrides bridged by K+, Rb+, or Cs+ cations. Because locating bridging hydrides in the middle of heavy atoms is challenging, neutron diffraction was used to conclusively locate the bridging hydride atoms for the Cs derivative. These results establish "alkali control" as a method for tuning the rate and outcome of C-H activation reactions by anionic iron complexes.