Six-coordinate tetragonal complexes of iron(II) have been synthesized with a 14-membered, fully saturated macrocyclic, tetradentate ligand having nitrogen donors. The ligand is meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (abbreviated aneN4) and the complexes are of the general formula FeII(aneN4)X2, where X- = CN-, NO2-, NCS-, Cl-, Br-, I-, CH3CO2-, and BF4-. Salts of the cation FeII(aneN4)(CH3CN)22+ have also been studied. The iron(II) atom exists predominantly in the low-spin form when the axial ligands are relatively strong. In spectrochemical order the examples are CN-, NO2-, NCS-, or CH3CN. These compounds are the first examples of stabilization of the low-spin state of Fe2+ with saturated nitrogen as the most abundant donors, an accomplishment attributable to the enhanced ligand field strength due to the constrictive effect of the mechanically confining in-plane macrocyclic ligand. The absence of low-energy charge-transfer bands permits the study of the d-d spectra of these complexes, from which we estimate Dqxylow spin = 2100 cm-1 and B = 540 cm-1 (for the acetonitrile complex of Fe(aneN4)2+). The remaining derivatives constitute a series of high-spin six-coordinate iron(II) complexes showing the spectrochemical series CH3CO2 > Cl- > Br- > I- > BF4-. From the spectra Dqxyhigh spin = 1400 cm-1, so that these systems have made it possible to compare the high- and low-spin Dq values for a single metal ion-ligand combination in fixed geometry (tetragonal). The NCS- (and possibly the NO2- and CH3CN) derivative exists as a mixture of spin states. It is inferred that other 14-membered macrocycles having unsaturated donors produce five-coordinated high-spin structures because the high-spin metal ion is too large to remain symmetrically encompassed by the ring. This view is supported by the fact that Dqxyhigh spin greater toward Fe2+ for the saturated ligand aneN4 than for the corresponding diimine or tetraimme macrocycles, although the ligand fields of the latter two have both been proven to be greater than that of aneN4.
|Original language||English (US)|
|Number of pages||10|
|State||Published - 1972|
ASJC Scopus subject areas
- Inorganic Chemistry