High- and low-spin complexes with similar, ligands. II. Iron(II) complexes with sterically hindered analogues of 2,2'-bipyridyl

Benzo substitution cis to the nitrogen of 2,2'-bipyridyl (bipy), to form 2-(2'-pyridyl)quinoline (pq), transforms the tris-complex with iron(11) from low spin in Fe(bipy)32+ to essentially high spin but near the crossover in Fe(pq)32+. For this complex, the ligand field splitting near the crossover, Δc is estimated from magnetic and spectral data as c. 12000 cm-l. A similar value but 150 cm-l lower is estimated for the analogous complex with the sterically related ligand 2-methyl-1,10-phenanthroline. This apparent difference in Δ values could arise from a direct change in ligand field potential, or from other factors, such as a change in distortion effects. Due to steric hindrance, pq prefers to form bis-complexes Fe(pq)2X2, pseudooctahedral and high spin, with suitable monodentates, X = Cl2 Br, NCS, ClO4. Double benzo substitution of bipy to form 2,2'-biquinolyl (biq) so increases steric hindrance that the tris-complex could not be formed. Instead biq forms pseudotetrahedral complexes Fe(biq)22+ and Fe(biq)X2 (X = Cl, Br).