Iron chelates of 2,2'-bidipyrrin: stable analogues of the labile iron bilins.

A unique series of halogenidoiron(III) complexes of the open-chain tetrapyrrolic ligand 2,2'-bidipyrrin (bpd) ([FeX(bdp)] X=F, Cl, Br, I) was prepared from simple pyrrolic and bipyrrolic precursors and iron chloride by a one-pot condensation/metalation strategy, followed by salt metathesis with CsF, LiBr, or NaI. Crystallographic analysis revealed that in all cases the 2,2'-bidipyrrin ligand is forced to reside in a helical conformation when bound to the iron atom. Whereas the extremely sensitive fluorido derivative was isolated as a CsF adduct and forms 1D polymeric chains in the solid state, the more stable chlorido, bromido, and iodido derivatives crystallize as discrete monomeric molecules with a distorted pentacoordinate iron(III) ion in an intermediate spin ground state. Magnetic susceptibility measurements and Mössbauer data of the compounds are in agreement with this interpretation. In solution, however, all the compounds are pentacoordinate with the iron atom in the high-spin (S=5/2) state and dynamic with respect to helix inversion. In the presence of air, the iron chelates react stepwise with the nucleophiles methanol and imidazolate at the tetrapyrrole terminal alpha,omega-positions, presumably through the hexacoordinate species [Fe(bdp)(MeOH)2]+ and [Fe(im)2-(bdp)](-), respectively. The successive increase of strain at these positions results in increasingly labile intermediates that spontaneously release the iron ion from the mono- or disubstituted tetrapyrrole ligands.