π‑Complexes of Tropolone and Its N‑Derivatives: Ambidentate [O,O]/[N,O]/[N,N]-Cycloheptatrienyl Pentamethylcyclopentadienyl Ruthenium Sandwich Complexes

Tropolone and its N-derivatives isopropylaminotropone
and diisopropylaminotroponimine
react with [Cp*Ru­(CH3CN)3]­PF6 as
the source of the electron-rich Cp*Ru+ synthon in a simple
“capping reaction” to the first cyclopentadienyl/cycloheptatrienyl
sandwich complexes containing two vicinal oxo or amino substituents.
These heteroleptic, cationic C5/C7 π-complexes,
[Cp*Ru­(C7H5)­O2H]+ (4), [Cp*Ru­(C7H5)­O­(NiPr)­H]+ (5), and [Cp*Ru­(C7H5)­(NiPr)2H]+ (6), are air-stable
18-electron metallocenes without precedence. In solution, NMR spectroscopy
proves the principal π-coordination of the substituted η7-cycloheptatrienyl ligands, whereas in the solid state a η5-cycloheptatrienyl coordination mode with bent oxo/imino moieties
prevails, as shown by X-ray single-crystal structure analyses. Chemically,
these compounds are the conjugate Brønsted acids of neutral [O,O]-,
[N,O]-, or [N,N]-metalloligands. Reaction with metal acetates or acetylacetonates
by in situ deprotonation and complex formation gives
access to trimetallic (M = Cu2+, 10–12) or tetrametallic (M = Fe3+, 13) complex cations. Their single-crystal structure analyses show square-planar
or square-pyramidal (M = Cu2+) or octahedral (M = Fe3+) coordination motifs with peripheral, η5-coordinated Cp*Ru moieties. Electrochemical studies on the trorucenes 4–6 showed a likely metal-based chemically
reversible (4) or irreversible one-electron reduction
(5, 6) as well as an irreversible one-electron
oxidation for the N-substituted compounds 5 and 6. The behavior of the heterotri- and tetrametallic complexes 10, 11, and 13 was rationalized
by a combination of cyclic and square-wave voltammetry as well as
the combination of chronocoulometry and linear-sweep voltammetry and
by comparison with the mononuclear copper or iron tropolone and topolonimine
precursors 14–16. These studies indicate
that in 10 and 11 the trorucene moieties
are reduced first in two coincident or slightly separated one-electron
reductions with the Cu2+ reduction at very negative potential,
whereas in 13 the central Fe­(III)-tris­(troponolato) moiety
is reduced prior to the trorucene appendices. An in vitro anticancer screening in MDA-MB-231 breast adenocarcinoma and HT-29
colon carcinoma cell lines showed enhanced antiproliferative activity
for the [O,O]-coordinated iron and copper complexes.