High- and Low-Spin Six-Coordinate Complexes of Iron(II) with a Saturated Tetradentate Macrocyclic Ligand

Six-coordinate tetragonal complexes of iron(II) have been synthesized with a 14-membered, fully saturated, macrocyclic, tetradentate ligand having nitrogen donors. Theligandis meso-5,5,7,12,12,14-hexamethy1-1,4,8,11-tetraazacyclotetradecane (abbreviated [14]aneN.) and the complexes are of the general formula FeII([14]aneN4)X2, where X- = CN-, NO₂-, NCS-, C1-, Br-, I-, CH₃Co₂-, and BF4-. Salts of the cation FeII([14]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 CH₃GN., 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-ttransfer bands permits the study of the d-d spectra of these complexes, from which we estimate Dqxy low spin = 2100 cm-1 and B = 540 cm-1 (for the acetonitrile complex of Fe([14]aneN4)2+). The remaining derivatives constitute a series of high-spin six-coordinate iron(II) complexes showing the spectrochemical series CH₃Co₂- > C1- > Br- > I- > BF4-. From the spectra Dqxy high spin = 1400 cm-1, so that these systems have made it possible to compare the high- and low-spin Dq values for a single métal ion-ligand combination in fixed geometry (tetragonal). The NCS- (and possibly the NOa- 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 viewis supported by the fact that high spin is greater toward Fe2+ for the saturated ligand [14]aneN4 than for the corresponding diimine or tetraimine macrocycles, although the ligand fields of the latter two have both been proven to be greater than that of [14]aneN4.