Preparation and Magnetic Properties of Iron(3+) Spin-Crossover Complexes Bearing a Thiophene Substituent: Toward Multifunctional Metallopolymers

The synthesis of a new 3-ethynylthienyl-substituted QsaIH ligand (QsalH is the short form for N-(8-quinolyl)salicylaldimine) (ThEAsalH 3), and the preparation, electronic, and magnetic properties of three homoleptic and cationic iron(3+) complexes containing this ligand with PF6- 4, SCN- 5, and ClO4- 6 counteranions are reported. In all three complexes a spin-crossover is observed in the solid state by variable temperature magnetic susceptibility measurements and Mossbauer spectroscopy, indicating that the synthetic modification of the QsalH ligand has not significantly altered the electronics at the metal center. This includes the observation of a very rare S = 5/2 to 312 spin-crossover in a non-porphyrin iron(3+) complex 5. The molecular structure and magnetic properties of an unusual iron(2+) complex 7 generated by reduction of complex 6 serendipitously during a recrystallization attempt in aerobic acetone solution is also reported: Complexes 4-6 feature iron(3+) reduction and oxidation of the thiophene ring at potentials of approximately -0.7 and +1.2 V (vs Fc), respectively.