Synthesis and elaboration of the dinuclear iron-imide cluster core [Fe2(μ-NR)2]2+

The protolysis of mononuclear ferric amide precursors FeCl[N(SiMe3)(2)](2)(THF) (1) or [FeCl2{N(SiMe3)(2)}(2)](-) (2) by primary amines provides, under suitable conditions, an effective route to dinuclear weak-field ferric-imide clusters with [Fe-2(mu-NR)(2)](2+) cores. In the synthesis of known arylimide clusters [Fe-2(mu-NAr)(2)Cl-4](2-) (Ar = Ph, p-Tol, Mes) from 2, the counterion has a major effect on selectivity and yield, and the use of quaternary ammonium salts affords a substantial improvement over earlier, Li+-based chemistry. The new tert-butylimide core is obtained by protolysis of 1 with excess (BuNH2)-Bu-t to give crystalline cis-Fe-2(mu-(NBu)-Bu-t)(2)Cl-2((NH2Bu)-Bu-t)(2) (9). Complex 9 can be transformed to other dinuclear species through substitution of the terminal amines by pyridines, PEt3, or chloride, or through protolysis of bridging alkylimides by arylamines, allowing isolation of trans-Fe-2(mu-(NBu)-Bu-t)(2)Cl-2(DMAP)(2) (DMAP = 4-dimethylaminopyridine), cis-Fe-2(mu-(NBu)-Bu-t)(2)Cl-2(PEt3)(2), [Fe-2(mu-(NBu)-Bu-t)(2)Cl-4](-), and trans-Fe-2(mu-NPh)(2)Cl-2((NH2Bu)-Bu-t)(2). The susceptibility of alkyl substituents to beta-elimination appears to limit the general applicability of protolytic cluster assembly using alkylamines. The dinuclear clusters have been characterized by X-ray, spectroscopic, and electrochemical analyses.