Organometallic Isocyanocyclopentadienides: A Combined Synthetic, Spectroscopic, Structural, Electrochemical, and Theoretical Investigation

This article reports on the chemistry of two organometallic isocyanocyclopentadienides,
which represent an emerging new class of aromatic isocyanides incorporating nonbenzenoid
π-systems. Interaction of aminoferrocene with a mixture of phenyl formate/phenol followed
by subsequent dehydration of the resulting ferrocenylformamide with POCl3 produced air
and thermally stable isocyanoferrocene (CNFc, Fc = ferrocenyl) in a high yield. Treating
lithiocymantrene, LiCm (Cm = (η5-C5H4)Mn(CO)3), with tosyl azide afforded thermally
sensitive cymantrenyl azide. Without isolation, CmN3 was reduced by NaBH4 to form
aminocymantrene, which was converted into air stable but thermally and light sensitive
isocyanocymantrene, CNCm. Combining 6 equiv of CNR (R = Fc, Cm) with bis(naphthalene)chromium(0) afforded Cr(CNR)6. Successive one-electron oxidations of Cr(CNR)6 with Ag+
produced the corresponding paramagnetic [Cr(CNR)6]+ and [Cr(CNR)6]2+. The compounds
[Cr(CNR)6]0,1+,2+ (R = Fc, Cm) are remarkable due to the incorporation of seven transition
metal atoms within relatively compact ML6 motifs. The physical, chemical, electrochemical,
and spectroscopic properties of the structurally characterized series [Cr(CNFc)6]0,1+,2+ indicate
that the electronic influence of the ferrocenyl moiety, often compared to an alkyl group, is
in fact more similar to that of aryl substituents. Electrochemical properties of [Cr(CNR)6]0,1+,2+
(R = Fc, Cm) are consistent with isocyanocymantrene being a substantially stronger π-acid
than isocyanoferrocene. This conclusion was unambiguously corroborated by a DFT analysis
of the Frontier molecular orbitals of CNFc and CNCm. Unpaired spin delocalization within
odd-atom, nonbenzenoid aromatic π-systems of [Cr(CNR)6]1+,2+ (R = Fc, Cm) was studied
by multinuclear paramagnetic NMR and contrasted with patterns observed for similar
complexes incorporating benzenoid and even-atom, nonbenzenoid aromatic moieties.