Generation and Oligomerization ofN-Ferrocenyl Ketenimines via Open-Shell Intermediates

Abstract

In the presence of oxidant (Ag­[SbF6]) and base, N-ferrocenyl thioamide Fc-NHC­(S)­CH3 (H-1; Fc = Fe­(η5-C5H5)­(η5-C5H4)) converts in an unexpected multistep reaction sequence to a novel N,S-heterocyclic ring, which initiates an oligomerization reaction. Key intermediates toward the resulting complicated material are Ag 6 (1) 6 silver clusters of the anionic N,S-chelating ligand 1 − and EPR-active piano stool complexes resulting from ring-slipped cyclopentadienyl ligands, as well as electrophilic N-ferrocenyl ketenimine Fc-NCCH2 (2) and its ferrocenium cation 2 •+ formed by hydrosulfide elimination. Mechanistic insight is achieved using X-ray diffraction and mass spectrometry, as well as EPR and NMR spectroscopic studies, combined with DFT calculations. In addition to the fundamental mechanistic insight, the results could have implications for smart oligomers/polymers, heterocycle synthesis, and controlled-release materials.

A Ferrocenyl Amino Substituted Stannylene as an Intramolecular Fe→Sn Lewis Adduct

“…Our group and others have shown, that the electron‐rich, sterically demanding and redox‐active ferrocenyl moiety is able to form strong XH ··· Fe hydrogen bonds (X = O, N) . We suppose that ferrocene could act both as a Lewis base beyond this Brønstedt‐type basicity.…”

Unexpected C–C Bond Formation with a Ferrocenyl Fischer Carbene Complex

“…Possibly, II can also form directly from Fc‐NH 2 1 assisted by the evolving O1H1 ··· Fe2 hydrogen bond. XH ··· Fe hydrogen bonds are meanwhile a well‐established structural motif , . The XH ··· Fe bond dissociation energy can amount up to 13 kJ · mol –1 .…”

“…[27] A fully analogous reaction (without the intermediate alkylation step) might be operative here. Amino ferrocene, although a weak nucleophile, attacks a carbonyl ligand cis to the carbene ligand forming zwitterion I (Scheme 2 [26,[42][43][44][45] The XH•••Fe bond dissociation energy can amount up to 13 kJ•mol -1 . Consequently, the ferrocenyl moiety might assist the nucleophilic attack acting as an internal base.…”

Redox‐Controlled Stabilization of an Open‐Shell Intermediate in a Bioinspired Enzyme Model

“…According to the electrochemical data of 1 , the comproportionation constant for 1 + amounts to K C = 72 at 298 K. The overall oxidation state of 1 n + in solution is conveniently monitored by paramagnetic 1 H NMR spectroscopy with the largest paramagnetic chemical shifts being found for the fully oxidized species 1 2+ , as expected for ferrocenium ions (Supporting Information, Figure S2) . Due to the typically rapid self‐exchange reaction between Fc and Fc + ( k ex = 9.1 × 10 6 –1.25 × 10 7 m –1 s –1 ), averaged concentration‐weighted proton chemical shifts are observed . This 1 H NMR spectroscopic monitoring of the redox state allows controlled OAT/IET reactivity studies of 1 , 1 + or 1 2+ with phosphanes as substrates.…”

“…The absence of Fc + units in INT1‐Mo + should also preclude side product formation due to nucleophilic attack at the iron center or at the Cp rings. After treating 1 + with one equivalent of PMe 3 in THF, the reaction mixture was rapidly frozen to 77 K. Recording an EPR spectrum of this solution gave a rhombic EPR resonance with g 1,2,3 = 1.9698, 1.9538, 1.9375 characteristic for Mo V[21] and excluding the presence of ferrocenium ions , , , . An additional coupling to an I = 1 / 2 nucleus with A 1,2,3 = 0.5, 1.5, 0.7 mT and to 95/97 Mo with A 1,2,3 = 4.7 mT (not optimized) has also been included, yet these many parameters should not be over‐interpreted, as the orientations of g and A tensors in Euler space might also be relevant leading to over‐parametrization (Figure ).…”