Trisilylation of a Formal Cr(-I) Bis-dinitrogen Complex

The long-sought goal of dinitrogen (N₂) fixation chemistry is to effect facile synthesis of nitrogen-containing chemicals directly from N₂. To date, electrophilic attack of coordinated N₂ ligands of transition metal dinitrogen complexes represents the most straightforward route to construct nitrogen-element bonds. Although electrophilic functionalization of monodinitrogen complexes to furnish diazenido and hydrazido species has been well-documented, for those complexes containing multiple N₂ moieties, analogous reactions invariably lead to N₂ extrusion and at most one N₂ can be derivatized. Herein, we describe electrophilic functionalization of a mononuclear formal chromium(-I) bis-dinitrogen complex supported by a PCP-pincer ligand. Specifically, treatment with TMSCl (TMS = SiMe₃) results in silylation of its two N₂ ligands and clean formation of a Cr(IV) complex bearing a diazenido and a hydrazido unit en route from a Cr(II) bis-diazenido intermediate, alongside a Cr(IV) monohydrazido species in a 1:1 ratio. This is the first time that functionalization of two N₂ ligands on a single metal center has been realized. In contrast, the reaction using TMSOTf instead of TMSCl as the electrophile generates a Cr(III) hydrazido complex. On the basis of detailed analyses of the electronic structure evolution during the trisilylation reaction using highly correlated wave function-based ab initio approaches, we identify a necessary condition for functionalization of two N₂ ligands, viz., bis-dinitrogen complexes ought to feature noncompetitive eight-electron M-(N₂)₂ π-backdonation. To satisfy this requirement, how to design appropriate dinitrogen complexes to effect this challenging transformation is discussed.