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. The ligand is meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (abbreviated [14]aneN₄) and the complexes are of the general formula Fe^II([14]aneN₄)X₂, where X^- = CN^-, NO₂^-, NCS^-, Cl^-, Br^-, I^-, CH₃CO₂^-, and BF₄^-. Salts of the cation Fe^II([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 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 Dq^xy_{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 > Cl^- > Br^- > I^- > BF₄^-. From the spectra Dq^xy_{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 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 Dq^xy_{high spin} greater toward Fe²⁺ for the saturated ligand [14]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 [14]aneN₄.