Iron(I) Complex Bearing an Open-Shell Diazenido Ligand

Low-valent transition-metal diazenido species are important
intermediates
in transition-metal-mediated dinitrogen reduction reactions. Isolable
complexes of the type unanimously feature closed-shell diazenido ligands.
Those bearing open-shell diazenido ligands have remained elusive.
Herein, we report the synthesis, characterization, and reactivity
of a d7 iron(I) complex featuring an open-shell
silyldiazenido ligand, [(ICy)Fe(NNSiiPr3)(η2:η2-dvtms)] (1, ICy = 1,3-dicyclohexylimidazole-2-ylidene, dvtms = divinyltetramethyldisiloxane).
Complex 1 is prepared in good yield by silylation of
the iron(-I)-N2 complex [K(18-crown-6)][(ICy)Fe(N2)(η2:η2-dvtms)] with iPr3SiOTf and has been fully characterized
by various spectroscopic methods. Theoretical studies, in combination
with characterization data, established an S = 1/2
ground spin-state for 1 that can best be described as
a quartet iron(I) center featuring an antiferromagnetically coupled
triplet silyldiazenido ligand. The diazenido and alkene ligands in 1 are labile, as indicated by the facile disproportionation
reaction of 1 at ambient temperature to transform into
the iron(II) bis(diazenido) species [(ICy)(NNSiiPr3)2Fe(dvtms)Fe(NNSiiPr3)2(ICy)] (2) and the
iron(0) species [(ICy)Fe(η2:η2-dvtms)]
and also the alkene-exchange reaction of 1 with PhCHCHBC8H14 to form [(ICy)Fe(NNSiiPr3)(η2-trans-PhCHCHBC8H14)] (3). Complex 1 is
light-sensitive. Upon photolysis, it undergoes a SiiPr3 radical-transfer reaction to yield [(ICy)Fe(σ:η2-MeCHSiMe2OSiMe2CHCHSiiPr3)] (4) and N2. The reactions of 1 with the trityl radical and organic
bromides yield iron(II) complexes, which indicates its reducing nature.
Moreover, 1 is a weak hydrogen-atom abstractor, as indicated
by its inertness toward HSi(SiMe3)3 and cyclohexa-1,4-diene
and the low calculated N–H bond dissociation energy (48 kcal/mol)
of its corresponding iron(II) iso-hydrazenido species.