N-Cobaltocenium Amide as Reactive Nucleophilic Reagent for Donor–Acceptor Bimetallocenes

Abstract

Deprotonation of the aminocobaltocenium ion [Cc-NH2]+ ([H-1] + ) generates the nucleophilic imine CcNH (1). Reaction of 1 with acid chlorides R–COCl (R = Ph, Fc, and Cc+) yields the reference amide [Ph-CO-NH-Cc]+ (2 + ) and the amide-linked hetero- and homobimetallocenes [Fc-CO-NH-Cc]+ (3 + ) and [Cc-CO-NH-Cc]2+ (4 2+ ), respectively. Cation–anion interactions of charged amides 2 + –4 2+ in the solid state and in solution are probed by single crystal X-ray diffraction and NMR and IR spectroscopy. Intramolecular metal–metal interactions in donor–acceptor heterobimetallocene 3 + and in mixed-valent homobimetallocene 4 + (prepared electrochemically) are discussed within the Marcus–Hush framework aided by spectroelectrochemical experiments and time-dependent density functional theory calculations.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor–Donor–Donor Triad Based on Diarylboryl and Triarylamine Units

“…The radical cations of bis­(NAr 3 ) derivatives, including compound 1 (Figure a), are some of the most intensively investigated MV compounds (where NAr 3 = triarylamine) , that have deepened fundamental knowledge, including through-bond and through-space pathways , and the effects of π-bridge structures and topologies on charge transfer (CT) mechanisms. Recently, responsiveness of bis­(NAr 3 ) •+ derivatives (and other MV compounds) to external stimuli such as light and protons has been examined, which has revealed new phenomena (i.e., photoswitchable mixed valency − and proton-coupled mixed valency − ). Although counterions influence CT rates and IVCT energies, their control has not received much attention .…”

Direct Amination of Cobaltocenium Hexafluoridophosphate via Vicarious Nucleophilic Substitution

“…9 Furthermore, the Heinze group has recently used deprotonated aminocobaltocenium hexafluoridophosphate in amide-coupling reactions to generate bioorganometallic, redox-responsive, short peptides. 10 In related work, Heck and co-workers have prepared the corresponding 1-amino-1′,2′,3′,4′,5′-pentamethylcobaltocenium hexafluoridophosphate 11 and nitropentamethylcobaltocenium hexafluoridophosphate 11 that served as a valuable synthon for nucleophilic substitution reactions with water, alcohols or primary amines. 12…”

“…9 Furthermore, the Heinze group has recently used deprotonated aminocobaltocenium hexafluoridophosphate in amidecoupling reactions to generate bioorganometallic, redoxresponsive, short peptides. 10 In related work, Heck and coworkers have prepared the corresponding 1-amino-1′,2′,3′,4′,5′-pentamethylcobaltocenium hexafluoridophosphate 11 and nitropentamethylcobaltocenium hexafluoridophosphate 11 that served as a valuable synthon for nucleophilic substitution reactions with water, alcohols or primary amines. 12 From a synthetic chemist's viewpoint, our best preparation of aminocobaltocenium hexafluoridophosphate, 3 based in part on early work of Sheats and Rausch, 13 involves a rather laborious multistep protocol via cobaltocenium/carboxylic acid/acid chloride/acyl azide/Curtius rearrangement.…”

“…For example, no reaction at all was observed in the VNS standard solvent dimethyl sulfoxide, 16 and variations in stoichiometry or temperature proved less productive. Mechanistically (Scheme 1), this amination consists most likely of a nucleophilic addition of a zwitterionic trimethylhydraziniumide at cobaltocenium, giving first a cyclopentadienyl substituted cyclopentadiene endo-hydride cobalt(I) complex, followed by base-promoted hydride removal by the internal trimethylamine moiety, thereby generating an η 4 -cyclopentadienimine cobalt(I) complex 10 that is finally protonated under hapticity change from η 4 to η 5 to afford the desired aminocobaltocenium salt.…”

“…These last two compounds are highly useful new organometallic precursors for further functionalization, including nucleophilic aromatic substitution, copper­(I)-catalyzed azide–alkyne [3 + 2] cycloaddition (CuAAC-click chemistry), transition-metal-centered cycloaddition to afford cobaltoceniumyl-substituted tetraaza-1,3-butadiene (tetrazene) complexes, and palladium-catalyzed carbon–carbon cross-coupling (Suzuki reaction) . Furthermore, the Heinze group has recently used deprotonated aminocobaltocenium hexafluoridophosphate in amide-coupling reactions to generate bioorganometallic, redox-responsive, short peptides . In related work, Heck and co-workers have prepared the corresponding 1-amino-1′,2′,3′,4′,5′-pentamethylcobaltocenium hexafluoridophosphate and nitropentamethylcobaltocenium hexafluoridophosphate that served as a valuable synthon for nucleophilic substitution reactions with water, alcohols or primary amines …”