Synthesis and Protonation of an Encumbered Iron Tetraisocyanide Dianion

Reported here are synthetic studies probing highly reduced iron centers in an encumbering tetraisocyano ligand environment. Treatment of FeCl2 with sodium amalgam in the presence of 2 equiv of the m-terphenyl isocyanide CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) produces the disodium tetraisocyanoferrate Na2[Fe­(CNArMes2)4]. Structural characterization of Na2[Fe­(CNArMes2)4] revealed a tight ion pair, with the Fe center adopting a tetrahedral coordination geometry consistent with a d10 metal center. Attempts to disrupt the cation–anion contacts in Na2[Fe­(CNArMes2)4] with cation-sequestration reagents lead to decomposition, except for the case of 18-crown-6, where a mononuclear complex featuring a dianionic 1-azabenz­[b]­azulene ligand was isolated in low yield. Formation of this 1-azabenz­[b]­azulene is rationalized to proceed by an aza-Büchner ring expansion of a CNArMes2 ligand mediated by a coordinatively unsaturated Fe center. Disodium tetraisocyanoferrate Na2[Fe­(CNArMes2)4] is readily protonated by trimethylsilanol (HOSiMe3) to produce the monohydride ferrate salt, Na­[HFe­(CNArMes2)4], the anionic portion of which serves as an isocyano analogue of the hydrido-tetracarbonyl metalate [HFe­(CO)4]−. Treatment of Na­[HFe­(CNArMes2)4] with methyl triflate (MeOTf; OTf = [O3SCF3]−) at low temperature in the presence of dinitrogen yields the five-coordinate Fe(0) complex Fe­(N2)­(CNArMes2)4. The formation of Fe­(N2)­(CNArMes2)4 in this reaction is consistent with the intermediacy of the neutral tetraisocyanide Fe­(CNArMes2)4. The decomposition of Fe­(N2)­(CNArMes2)4 to the dimeric complex [Fe­(η6-(Mes)-μ2-C-CNArMes)]2 and a seven-membered cyclic imine derived from a CNArMes2 ligand is presented and provides insight into the ability of CNArMes2 and related m-terphenyl isocyanides to stabilize zerovalent four-coordinate iron complexes in a strongly π-acidic ligand field.