Low-coordinate iron(II) amido complexes of β-diketiminates:: Synthesis, structure, and reactivity

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes (LFeNHAr)-Fe-R (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and H-1 NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes (LFeNHAr)-Fe-R may be oxidized using AgOTf, and the products (LFe)-Fe-R(NHAr)(OTf) are characterized with F-19 NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography, In the structures of the iron(III) complexes (LFe)-Fe-R(NHAr)(OTf) and (LFe)-Fe-R(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not pi-bonding. The reactions of the amido group of (LFeNHAr)-Fe-R with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H2NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.