Stepwise construction of a 4-hydroxyphenyl functionalized O,N,N-tridentate ferrocene-containing enaminone: Spectral, analytical and structural studies

Tetrahydropyranylation of methyl-4-hydroxybenzoate proceeds with formation of its corresponding THP aromatic ether THP–O–p-C6H4CO2CH3 (1; THP = tetrahydropyranyl, C5H9O). Reaction with in situ generated carbanion of acetylferrocene gave the protected organometallic complex Fc–C(O)CHC(OH)(p-C6H4–O–THP) (2, Fc(η5–C5H5)Fe(η5–C5H4)) featuring keto-enol functionality; subsequent acidic treatment of 2 afforded the 4-hydroxyphenyl substituted counterpart Fc–C(O)CHC(OH)(p-C6H4OH) (3) that exists as a 73/27 mixture of keto-enol/β-diketone tautomers in DMSO solution. The functionalized tridentate metalloligand Fc–C(O)CHC(p-C6H4OH)N(H)CH2CH2NH2 (4) incorporating O,N,N-donors is readily obtained upon Schiff base condensation of 3 with 1,2-diaminoethane. Complex 4 exists exclusively as its ferrocenyl-enaminone tautomer in solution and in the solid state. Compounds 1–4 were characterized by NMR (1H, 13C) and IR spectroscopy as well as by elemental analysis and single-crystal X-ray diffraction.

Graphical abstractThe 4-hydroxyphenyl functionalized O,N,N-tridentate metalloligand (illustrated) is readily prepared by treatment of its β-diketone precursor with ethylenediamine in refluxing ethanol, and exists exclusively as its ferrocenyl-enaminone tautomer in solution and in the solid state.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Stepwise construction of a functionalized O,N,N-tridentate metalloligand. ► Crystal structures of the ferrocenyl-enaminone and of its two β-diketone precursors. ► Only ferrocenyl-enaminone tautomer does exist in solution and in the solid state. ► 4-Hydroxyphenol functionalization achieved by tetrahydropyranylation protection. ► Crystal structure of the protected tetrahydropyranyl aromatic ether precursor.