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.