Reactions of acids with naphthyridine-functionalized ferrocenes: protonation and metal extrusion

Abstract

Reaction of 1,8-naphthyrid-2-yl-ferrocene (FcNP) with a variety of acids affords protonated salts at first, whereas longer reaction time leads to partial demetalation of FcNP resulting in a series of Fe complexes. The corresponding salts [FcNP·H][X] (X = BF₄ or CF₃SO₃ (1)) are isolated for HBF₄ and CF₃SO₃H. Reaction of FcNP with equimolar amount of CF₃CO₂H for 12 h affords a neutral complex [Fe(FcNP)₂(O₂CCF₃)₂(OH₂)₂] (2). Use of excess acid gave a trinuclear FeII complex [Fe₃(H₂O)₂(O₂CCF₃)₈(FcNP·H)₂] (3). Three linear iron atoms are held together by four bridging trifluoroacetates and two aqua ligands in a symmetric fashion. Reaction with ethereal solution of HCl afforded [(FcNP·H)₃(Cl)][FeCl₄]₂ (4) irrespective of the amount of the acid used. Even the picric acid (HPic) led to metal extrusion giving rise to [Fe₂(Cl)₂(FcNP)₂(Pic)₂] (5) when crystallized from dichloromethane. Metal extrusion was also observed for CF₃SO₃H, but an analytically pure compound could not be isolated. The demetalation reaction proceeds with an initial proton attack to the distal nitrogen of the NP unit. Subsequently, coordination of the conjugate base to the electrophilic Fe facilitates the release of Cp rings from metal. The conjugate base plays an important role in the demetalation process and favors the isolation of the Fe complex as well. The 1,1′-bis(1,8-naphthyrid-2-yl)ferrocene (FcNP₂) does not undergo demetalation under identical conditions. Two NP units share one positive charge causing the Fe-Cp bonds weakened to an extent that is not sufficient for demetalation. X-ray structure of the monoprotonated FcNP₂ reveals a discrete dimer [(FcNP₂·H)]₂[OTf]₂ (6) supported by two N–H···N hydrogen bonds. Crystal packing and dispersive forces associated with intra- and intermolecular π–π stacking interactions (NP···NP and Cp···NP) allow the formation of the dimer in the solid-state. The protonation and demetalation reactions of FcNP and FcNP₂ with a variety of acids are reported.