{FeCp}+-Induced Hexafunctionalization of Hexamethylbenzene with Dendrons for the Direct Synthesis of Redox-Active Iron-Centered Metallodendrimers

Abstract

Phenol tri- and nonaallyl dendrons (3 and 7, respectively) were functionalized at the focal position to give the new triallyl dendrons 4 and 6 and the nonaallyl dendrons 11 and 13 that contain a iodoalkyl or a bromobenzyl termini. All these dendrons were used for the [FeCp]+-induced hexafunctionalization of hexamethylbenzene in [FeCp(eta6-C6Me6)][PF6] (1) under mild conditions in the presence of KOH. These reactions directly yielded the 18-allyl and 54-allyl dendrimers 9, 10, and 14 with a [FeCp(eta6-arene)]+ unit located at the dendrimer core. Cyclic voltammetry studies were recorded in THF and DMF with these metallodendrimers and compared with those of analogous dendrimers or complexes of smaller size that contain a [FeCp(eta6-arene)]+ unit at the core. The decreased rate of heterogeneous electron transfer when the dendritic size increases first disclosed by Diederich and Gross is confirmed. The variation of the redox potential of the Fe(II/I) redox system with increasing dendritic size is negligible even in a solvent of high dielectric constant such as DMF. This trend is attributed to fact that the involved "redox" orbital is buried on the metal center, well protected by the shell of alkyl chains (electron-reservoir nature), unlike in ferrocene. The chemical irreversibility increases in THF as the dendrimer size increases, due to more facile ligand substitution with THF at the 19-electron level when the chain bulk increases.

Organo-Iron Benzaldehyde-Hydrazone Complexes. Synthesis, Characterization, Electrochemical, and Structural Studies

“…Benzylic C−H bond activation in cationic arene-cyclopentadienyliron complexes and subsequent bond formation of the resulting neutral cyclohexadienyl species with various elements is a well-established general and useful procedure for effecting modifications on the structure of the arene ligand. 2b,, It is made possible by the enhancement of the acidity of the benzylic protons in the cationic complex. For instance, the p K a in DMSO has indeed been found to be about 14 units lower for the 18-electron complexes [CpM(η 6 -C 6 Me 6 )] + PF 6 - (M = Fe, Ru; p K a = 29) than for the free arene (p K a = 43). − On the other hand, several authors 15a,21-23 have also demonstrated that deprotonation could occur at the amino substituent on the arene ligand and that subsequent treatment with an excess of alkylating reagents give rise to formation of new N−C bonds.…”

Cationic organoiron polyelectrolyte three‐arm stars

“…Metal‐containing stars and dendrimers have been the focus of many research groups 19–32. Several different metals have been incorporated into macromolecules, including iron,33–36 ruthenium,37–42 and platinum 43, 44.…”

Ruthenium-Coordinated Spirolactams via Intramolecular Nucleophilic Addition to η6-Arene Metal Complexes

“…In particular, (arene)Cr(CO) 3 complexes have emerged as versatile synthetic intermediates that are amenable to stereocontrolled side-chain functionalization and nucleophilic aromatic addition/substitution (S N Ar) processes . In addition to chromium, isoelectronic arene−metal complexes of (CO) 3 Mn(I) and cyclopentadienyl (Cp) Fe(II) also have been frequently employed in the context of complex molecule construction. , In contrast, the chemistry of related (η 6 -arene)CpRu(II) derivatives has not been extensively developed . Importantly, however, the CpRu(II) fragment offers several distinct advantages as an arene activating organometallic moiety in that arene ruthenium complexes can be prepared in high yield using exceedingly mild reaction conditions and coordination of the CpRu group is compatible with a wide range of preexisting arene ligand functionality.…”