In continuation of our work on Wanzlick/Arduengo carbenes containing redox-active ferrocenyl substituents we report on the synthesis of N,N‘-diferrocenyl imidazol(in)ium salts as precursors of imidazol(in)-2-ylidenes. The necessary starting material for this chemistry is aminoferrocene, which was prepared by an improved and large-scale synthesis by the sequence solid lithioferrocene, iodoferrocene, N-ferrocenylphthalimide, aminoferrocene. The preparation of N,N‘-diferrocenyl heterocycles involves condensation of aminoferrocene with glyoxal to afford N,N‘-diferrocenyldiazabutadiene [Fc-DAB], reduction, condensation with formaldehyde, and oxidation with trityl salts to yield N,N‘-diferrocenylimidazol(in)ium salts. In situ deprotonation and trapping with electrophiles yielded the expected metal complexes and derivatives in some cases [Ag+ or S8], but attempted reaction with other transition metals [e.g., Pd(II)] failed to give the corresponding complexes, due to (i) steric hindrance by the two N-ferrocenyl substituents, (ii) reduced acidity of the imidazol(in)ium precursors, and (iii) inaccessibility of the free carbenes. Spectroscopic [IR, Raman, UV−vis, MS, NMR (1H, 13C, 109Ag)], structural [X-ray], and electrochemical [CV] properties are reported and compared to those of other N-heterocyclic carbene derivatives.

Cumulene sp-carbon molecular wires C 2 [Fc(Ph)CdC(Ph)Fc] up to C 7 [Fc(Ph)CdCdCd CdCdCdC(Ph)Fc] endcapped by two electroactive ferrocenyl groups are presented in this report. Synthetically, ferrocenyl cumulenes can be built-up by a modular strategy using C 1 synthon ferrocenyl(phenyl)ketone as starting material with various acetylenic/propargylic/ homopropargylic C 2 -C 5 reagents, taking into account and exploiting the efficient stabilization of an electron-deficient carbenium center by an adjacent ferrocenyl moiety. With increasing cumulene chain length the reactivity of cumulenes increases considerably, indicating steric protection as the main requirement for bulk stability. Even cumulenes C 2 , C 4 , and C 6 are conjugated "molecular wires" effecting electronic communication between the terminal ferrocenyl substituents, whereas odd cumulenes C 3 , C 5 , and C 7 are nonconjugated and electronically decoupled due to their orthogonal terminal π-systems. Electrochemically, separate redox waves can be detected up to a C 6 cumulene spacer, but the electronic communication between the endcapping redox-active ferrocenyl substituents decreases with increasing cumulene length.

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Methylated ferrocenes are useful building blocks for novel materials in molecular electronics with advantageous properties in comparison to normal ferrocene derivatives. The presence of nine, eight, or five methyl substituents leads to (i) a decrease in oxidation potential, (ii) amplified donor capacity with correspondingly increased stability of the ferrocenium salts, and, most significantly, (iii) increased solubility. A modular synthetic approach based on standard Wittig chemistry affords π-conjugated soluble nona-, octa-, and pentamethylated biferrocenes, bridged by up to five vinylene−phenylene subunits. These biferrocenes with spacers are “molecular wires” with a metal-to-metal distance of up to 40 Å and an effective conjugation pathway of up to 50 Å.

Cobaltocenium carboxylic acid (carboxycobaltocenium) hexafluorophosphate, a key compound for other monofunctionalized cobaltocenium salts, has been synthesized in >70% overall yield starting from cobaltocenium hexafluorophosphate by a synthetic sequence involving (i) nucleophilic addition of lithium (trimethylsilyl)ethynide, (ii) hydride removal by tritylium hexafluorophosphate, and (iii) oxidative cleavage of the alkynyl substituent by potassium permanganate.

A convenient access to a triad of triazoles with ferrocenyl and cobaltoceniumyl substituents is reported. N‐Alkylation, deprotonation and metalation with CuI/AgI/AuI synthons affords the heteroleptic triazolylidene complexes. Due to the combination of neutral, electron‐donating ferrocenyl substituents and cationic, strongly electron‐withdrawing cobaltocenium substituents, the mesoionic carbene (MIC) ligands of these complexes are electronically interesting “push–pull”, “pull–push” and “pull–pull” metalloligands with further switchable redox states based on their fully reversible FeII/FeIII, (ferrocene/ferrocenium) and CoIII/CoII, (cobaltocenium/cobaltocene) redox couples. These are the first examples of metal complexes of (di)cationic NHC ligands based on cobaltoceniumyl substituents. DFT calculated Tolman electronic parameter (TEP) of the new MIC ligands, show these metalloligands to be extremely electron‐poor NHCs with properties unmatched in other carbene chemistry. Utilization of these multimetallic electronically tunable compounds in catalytic oxazoline synthesis and in antitumor studies are presented. Remarkably, 1 mol % of the AuI complex with the dicationic MIC ligand displays full catalytic conversion, without the need for any other additives, in less than 2 hours at ambient temperatures. These results thus firmly establish these new classes of cobaltoceniumyl based (di)cationic MIC ligands as prominent players in several branches of chemistry.

Monofunctionalized cobaltocenium salts are obtained for the first time from cobaltoceniumdiazonium bis(hexafluorophosphate) with various nucleophiles via Sandmeyer-type and related reactions. For successful conversions, reaction conditions are quite critical: either standard solution chemistry in nitromethane or solvent-free ball milling proved necessary, depending on the type of reactant. By this synthetic approach valuable synthons such as iodocobaltocenium and azidocobaltocenium salts are accessible that open up new vistas in cobaltocenium chemistry. Spectroscopic characterization by NMR, IR, HRMS, and single-crystal structure analysis as well as the results of electrochemical studies are reported. Derivatives with two reversible reductions show the expected relation of the half-wave potentials with the Hammett substituent parameter σ p of the respective substituent with a slightly larger slope for the first reduction. The carboxylic acid (reductive deprotonation of the −COOH functionality), the iodo (protodehalogenation), and the azido derivatives undergo irreversible subsequent reactions after primary reduction.

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