Fe(IV) alkylidenes are actually Fe(II), and a related octahedral Fe(II) “alkylidene” is a conjugated vinyl complex

Electronic structure calculations on previously synthesized [mer-{κ-C,N,C-(2-C6H4)CHN(1,2-C6H4)C(iPr)}Fe(PMe3)3][BArF4] (1) and mer,trans-{κ-C,N,C-(2-C6H4)CH(Bn)N(1,2-C6H4)C(iPr)}Fe(PMe3)2N2 (4-Bn) reveal that 1 is best interpreted as Fe(II) bound to a carbenium ion, while 4-Bn has an imino-vinyl unit bound to a ferrous center. Attempts to prepare LnFeCHR species capable of olefin metathesis or carbene transfer were obviated by alternative reactivity, although precursors mer,trans-{κ3-N,N,C-(2-py)CHNCH2CHCH}Fe(PMe3)2CH3 (6), [mer,trans-{κ3-N,N,C-2-py-CHNCH2CHCH}Fe(PMe3)2CH3] [PF6−] (8), mer,trans-{κ3-N,N,C-2-py-CHNCH2CHCH}Fe(PMe3)2I (9), mer-{κ3-N,N,C-(2-pyridyl) CHNCHCHCH}Fe(PMe3)3 (11), and mer-{κ3-C,N,C-(2-C6H4)CHNCH2CHCH}Fe(PMe3)3 (12) were prepared. Related phenyl derivatives, mer,trans-{κ3-N,N,C-(2-py)CHNC(Me)2CHCH}Fe(PMe3)2CH3 (13) and [mer,trans-{κ3-N,N,C-(2-py)CHNC(Me)2CHCH}Fe(PMe3)2N2][BArF4] (15) were prepared to test additional stability issues. Compounds 9 and 11 were structurally characterized, and metrical parameters of the latter, in addition to electronic structure calculations, suggest that it is not an alkylidene, but an Fe(II) center coordinated by a conjugated vinyl ligand with extensive delocalization in its π-system.

Calculations on previously synthesized “Fe(IV) alkylidenes” are more consistent with Fe(II) centers bound to carbenium and oxidized chelate ligands. Attempts to prepare LnFeCHR species failed due unsuccessful protonation of precursor vinyl complexes, but the approach did yield an unusual “Fe(II) alkylidene” that was shown to be a delocalized vinyl complex.Figure optionsDownload as PowerPoint slide