Multimetallic Cooperativity in Activation of Dinitrogen at Iron-Potassium Sites

The reaction of soluble iron-oxygen-potassium assemblies with N₂ gives insight into the mechanisms of multimetallic N₂ coordination. We report a series of very electron-rich three-coordinate, β-diketiminate-supported iron(I) phenoxide complexes, which are metastable but have been characterized under Ar by both crystallography and solution methods. Both monomeric and dimeric Fe-OPh-K compounds have been characterized, and their iron environments are very similar in the solid and solution states. In the dimer, potassium ions hold together the phenoxide oxygens and aryl rings of the two halves, to give a flexible diiron core. The reactions of the monomeric and dimeric iron(I) compounds with N₂ are surprisingly different: the mononuclear iron(I) complexes give no reaction with N₂, but the dimeric Fe₂K₂ complex reacts rapidly to give a diiron-N₂ product. Computational studies show that the key to the rapid N₂ reaction of the dimer is the preorganization of the two iron atoms. Thus, cooperation between Fe (which weakens the N-N bond) and K (which orients the Fe atoms) can be used to create a low-energy pathway for N₂ reactions.