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

Abstract

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.