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

Abstract

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 d 7 iron(I) complex featuring an open-shell silyldiazenido ligand, [(ICy)Fe(NNSi i Pr 3 )-(η 2 :η 2 -dvtms)] (1, ICy = 1,3-dicyclohexylimidazole-2-ylidene, dvtms = divinyltetramethyldisiloxane). Complex 1 is prepared in good yield by silylation of the iron(-I)-N 2 complex [K(18-crown-6)][(ICy)Fe-(N 2 )(η 2 :η 2 -dvtms)] with i Pr 3 SiOTf and has been fully characterized by various spectroscopic methods. Theoretical studies, in combination with characterization data, established an S = 1/2 ground spinstate 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)(NNSi i Pr 3 ) 2 Fe(dvtms)Fe(NNSi i Pr 3 ) 2 (ICy)] ( 2) and the iron(0) species [(ICy)Fe(η 2 :η 2 -dvtms)] and also the alkene-exchange reaction of 1 with PhCH�CHBC 8 H 14 to form [(ICy)Fe(NNSi i Pr 3 )(η 2 -trans-PhCH�CHBC 8 H 14 )] (3). Complex 1 is light-sensitive. Upon photolysis, it undergoes a Si i Pr 3 radical-transfer reaction to yield [(ICy)Fe(σ:η 2 -MeCHSiMe 2 OSiMe 2 CH�CHSi i Pr 3 )] (4) and N 2 . 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(SiMe 3 ) 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.