Synthesis, coordination behavior and structural features of chiral iron(ii) PNP diferrocene complexes

Abstract

Five new chiral PNP ferrocene ligands with either an imine or amine nitrogen coordination site were synthesized. Only the imine type ligands formed Fe(II) complexes with the general formula [Fe(PNP)X₂] (X = Cl, Br). In the solid state these complexes adopt a tetrahedral geometry with the PNP ligand coordinated in a κ²P,N-fashion with the one pendant-arm and the other not coordinated, as determined by X-ray crystallography and Mössbauer spectroscopy. The complexes are paramagnetic with a quintet ground state. In solution there is an equilibrium between [Fe(κ³P,N,P-PNP)X₂] and [Fe(κ²P,N-PNP)X₂] complexes. Boronation of the non-coordinated arm shifts the equilibrium towards the four-coordinate complex [Fe(κ²P,N-PNP^BH₃)Br₂]. DFT calculations are consistent with the experimental results and indicate that the experimentally observed κ² isomer is thermodynamically the most stable. In a CO atmosphere, [Fe(PNP)(CO)₂Br]Br was formed rather than [Fe(PNP)(CO)Br₂].