Tropolone and its N-derivatives isopropylaminotropone and diisopropylaminotroponimine react with [Cp*Ru(CH 3 CN) 3 ]PF 6 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 C 5 /C 7 π-complexes, [Cp*Ru-(C 7 H 5 )O 2 H] + (4), [Cp*Ru(C 7 H 5 )O(N i Pr)H] + (5), and [Cp*Ru(C 7 H 5 )(N i Pr) 2 H] + (6), are air-stable 18-electron metallocenes without precedence. In solution, NMR spectroscopy proves the principal π-coordination of the substituted η 7 -cycloheptatrienyl ligands, whereas in the solid state a η 5 -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 = Cu 2+ , 10−12) or tetrametallic (M = Fe 3+ , 13) complex cations. Their single-crystal structure analyses show square-planar or square-pyramidal (M = Cu 2+ ) or octahedral (M = Fe 3+ ) coordination motifs with peripheral, η 5 -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 Cu 2+ 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.

Oxidative addition of cobaltoceniumdiazonium bis(hexafluoridophosphate) with (pseudo)halide aurates gave gold(III) complexes containing zwitterionic cobaltoceniumide as a ligand. Its selenium derivative, cobaltoceniumselenolate, was obtained by an electrophilic aromatic substitution reaction of iodocobaltocenium iodide with Na2Se. Spectroscopic and structural data in combination with DFT calculations showed that this cobaltocenylidene species is a mesoionic carbene quite different from common N‐heterocyclic carbenes. Its ligand properties (TEP, singlet‐triplet gap, nucleophilicity, π‐acidity, Brønsted basicity) are in part comparable to those of cyclic (amino)(alkyl/aryl)carbenes. Electrochemistry data showed that the mesoionic cobaltoceniumides are more electron‐rich than their parent ferrocenes. The reversible reduction of the tricyanido gold complex appears 50 mV negative of the cobaltocenium/cobaltocene couple, whereas that of the selenide derivative is shifted cathodically by 550 mV.

Cobaltocenium carbaldehyde (formylcobaltocenium) hexafluoridophosphate, a long sought-after functionalized cobaltocenium salt, is accessible from cobaltocenium carboxylic acid by a three-step synthetic sequence involving (i) chlorination to the acid chloride, (ii) copper-borohydride reduction to the hydroxymethyl derivative, and (iii) Dess–Martin oxidation to the title compound. Due to the strongly electron-withdrawing cationic cobaltocenium moiety, no standard aldehyde reactivity is observed. Instead, nucleophilic addition followed by haloform-type cleavage prevails, thereby ruling out common useful aldehyde derivatization. One-electron reduction of cobaltocenium carbaldehyde hexafluoridophosphate affords the deep-blue, isolable cobaltocene carbaldehyde 19-valence-electron radical whose spin density is located fully at cobalt and not at the formyl carbon atom. 1H/13C NMR, IR, EPR spectroscopy, high-resolution mass spectrometry, cyclic voltammetry, single crystal structure analysis (XRD), and density functional theory are applied to characterize these unusual formyl-cobaltocenium/cobaltocene compounds.