Synthesis and Characterization of Iron(II) Quinaldate Complexes

Treatment of iron(II) chloride or iron(II) bromide with 2 equiv of sodium quinaldate (qn = quinaldate or C10H6NO2−) yields the coordinatively unsaturated mononuclear iron(II) quinaldate complexes Na[FeII(qn)2Cl]·DMF and Na[FeII(qn)2Br]·DMF (DMF = N,N-dimethylformamide), respectively. When a similar synthesis is carried out using iron(II) triflate, a solvent-derived linear triiron(II) complex, [FeII3(qn)6(DMF)2], with two five-coordinate iron(II) centers and a single six-coordinate iron(II) center is obtained. Each of these species has been characterized using X-ray diffraction. The vibrational features of these complexes are consistent with the observed solid-state structures. Each of these compounds exhibits an iron(II)-to-quinaldate (π*) charge-transfer band between 520 and 550 nm. These metal-to-ligand charge-transfer bands are sensitive to substitution of the quinaldates as well as alteration of the first coordination sphere ligands. However, the 1H NMR spectra of these paramagnetic high-spin iron(II) complexes are not consistent with retention of the solid-state structures in a DMF solution. The chemical shifts, longitudinal relaxation times (T1), relative integrations, and substitution of the quinaldate ligands provide a means to fully assign the 1H NMR spectra of the paramagnetic materials. These spectra are consistent with coordination equilibria between five- and six-coordinate species in a DMF solution. Electrochemical studies are reported to place these oxygen-sensitive compounds in a broader context with other iron(II) compounds. Iron complexes of bidentate quinoline-2-carboxylate-derived ligands are germane to metabolic pathways, environmental remediation, and catalytic applications.