Reactions of Acids with Naphthyridine-Functionalized Ferrocenes: Protonation and Metal Extrusion

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.