Synthesis and Characterization of Iron(II) Quinaldate Complexes

Abstract

Treatment of iron(II) chloride or iron(II) bromide with two equivalents of sodium quinaldate (qn = quinaldate or C 10 H 6 NO 2 − ) yields the coordinatively unsaturated mononuclear iron(II) quinaldate complexes Na [Fe II (qn) 2 Cl]·DMF and Na [Fe II (qn) 2 Br]·DMF, respectively. When a similar synthesis is carried out using iron(II) triflate, a solvent derived linear tri-iron(II) complex, [Fe II 3 (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 520-550 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 1 H NMR spectra of these paramagnetic high-spin iron(II) complexes are not consistent with retention of the solid state structures in DMF solution. The chemical shifts, longitudinal relaxation times (T 1 ), relative integrations, and substitution of the quinaldate ligands provide a means to fully assign the 1 H NMR spectra of the paramagnetic materials. These spectra are consistent with coordination equilibria between five-and six-coordinate species in 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, as well as to catalytic applications.In the central nervous system, the majority of tryptophan catabolism occurs via the kynurenine pathway.1 Several of the quinoline and pyridine carboxylic acid metabolites formed are important signaling agents in neuronal pathways which are vital to the onset of certain diseases. For instance, kynurenic acid (4-hydroxyquinoline-2-carboxylic acid) plays a role in glutamate signaling in the brain and is found at elevated levels in people with schizophrenia.2 Increases in iron concentrations and picolinic acid have been observed in Alzheimer's disease.3 , 4 The number of quinoline-2-carboxylic acids found in this pathway and their involvement in brain tumor nueropathology led us to examine the iron(II) coordination chemistry of these metabolites.Since the initial recognition that quinaldic acid (quinoline-2-carboxylic acid, Hqn) can form complexes with iron(II), 5 the coordination chemistry of quinaldate with iron(II) has received a great deal of attention. Many of the early studies on the determination of binding constants were carried out in water,6 -8 and provide the foundation for the use of quinaldic acid in quantitative determinations of metal content via gravimetric analysis.9 , 10 Reaction of an iron (II) salt with either quinaldic acid or sodium quinaldate initially yields a red complex, [Fe (qn...

A mononuclear carboxylate-rich oxoiron(iv) complex: a structural and functional mimic of TauD intermediate ‘J’

“…Similar observations have been made elsewhere. 64, 65 The structure of 3(b)-Cl , with two equatorial carboxylates cis to a labile ligand site, demonstrates that by careful ligand design complexes that act as structural mimics of the active site of mononuclear nonheme iron enzymes can be obtained (Scheme 1, Figure 2). …”

Beyond the Simple Copper(II) Coordination Chemistry with Quinaldinate and Secondary Amines

“…Structurally characterized complexes with quinaldinate reveal a bidentate chelating manner through pyridine nitrogen and carboxylate oxygen as a prevailing coordination mode [12][13][14][15][16][17][18][19][20][21][22]. Examples of a bridging mode through two or all three donor atoms are not very common [23][24][25][26]. The quinaldinate was introduced into our reaction system through the [Cu(quin)2(H2O)] starting material, one of the rare copper(II) quinaldinate compounds known prior to this report [27].…”

Spectroscopic and Structural Characterization, Enzyme Inhibitions, and Antioxidant Effects of New Ru(II) and Ni(II) Complexes of Schiff Base

“…In the literature, detailed crystallographic data related to similar complexes can be found in the following references. [42] Antioxidant Studies…”