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

Abstract

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.