Coordination polymers based on trinuclear heterometallic pivalates and polypyridines: Synthesis, structure, sorption and magnetic properties

It was shown that interaction of trinuclear pivalates Fe2MO(Piv)6(Hpiv)3 (M = NiII or CoII, Piv = (CH3)3CCO2−) with pyridine-containing ligands – trans-bis-1,2-(4-pyridyl)ethylene (trans-dpe), bis-1,3-(4-pyridyl)propane (py2Pn) resulted in formation of 2D porous coordination polymers [Fe2NiO(Piv)6(trans-dpe)1.5]n (1), [Fe2CoO(Piv)6(trans-dpe)1.5]n (2) and 1D polymers [{Fe2CoO(Piv)6(cis-dpe)}2(trans-dpe)]n (3), [Fe2NiO(Piv)6(py2Pn)(DMF)]n (4), where cis-dpe is cis-bis-1,2-(4-pyridyl)ethylene, formed due to in situ isomerization of trans-dpe. Interaction of Fe2NiO(Piv)6(Hpiv)3 with bis-2,3-(4-pyridyl)-2,3-dihydroxybutane led to in situ destruction of this ligand and isolation of trinuclear complex Fe2NiO(Piv)6(pyCOCH3)3 (5). X-ray structures of all five complexes were determined; compounds 1 and 2 were found to be isostructural. It was shown that crystal structures of [Fe2MO(Piv)6(trans-dpe)1.5]n complexes significantly changed at temperature variation. Sorption capacity of 1 and 2 on N2, H2 at 78 K (total volume of pores VT less than 0.045 cm3/g) was much lower than sorption capacity on methanol and ethanol at 293 K (VT from 0.16 cm3/g to 0.37 cm3/g), which can be explained by flexibility of crystal structure of these coordination polymers. Magnetic properties of 1 and 2 were simulated taking into account exchange interactions between metal ions and zero-field splitting of NiII or CoII. Exchange coupling between trinuclear units Fe2MO(Piv)6 was negligibly small.