Steric Effects of the Spin State of Iron(II) in Complexes of Substituted Bipyridine Derivatives

Iron(II) and nickel(II) complexes of dimethyl 2,2′-bipyridine-3,3′-dicarboxylate (dmbc), 1,1′-biisoquinoline (biq), 6,6′-dimethyl-2,2′-bipyridine (dmbpy) and 4,4′6,6′-tetramethyl-2,2′- bipyridine (tmbpy) have been prepared. [FeN6]2+ derivatives of dmbc and biq were isolated as solid salts. These show anomalous temperature dependence of their magnetism and Mossbauer spectra which has been interpreted in terms of a thermally induced singlet (1A1) ↔ quintet (5T2) transition. The transition is also observed for [Fe(biq)3][BF4]2 in solution and has been analysed in terms of a thermal equilibrium involving the two spin states. Electronic spectral data for the [NiN6]2+ derivatives indicate values for the field strength of these ligands which are consistent with the appearance of the transition in the iron(II) species. Both dmbpy and tmbpy yield only bis(ligand) complexes with either iron(II) or nickel(II). Infrared spectral data suggest that in these complexes the associated perchlorate or fluoroborate anions are coordinated, which leads to distorted six-coordinate structures. A similar iron(II) complex was isolated with 2,9-dimethyl-1,10-phenanthroline.