High- and low-spin six-coordinate complexes of iron(II) with a saturated tetradentate macrocyclic ligand

James C. Dabrowiak, Philip H. Merrell, Daryle H. Busch

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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 FeII([14]aneN4)X2, where X- = CN-, NO2-, NCS-, Cl-, 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 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([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 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 [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.

Original languageEnglish (US)
Pages (from-to)1979-1988
Number of pages10
JournalInorganic Chemistry
Issue number9
StatePublished - 1972

ASJC Scopus subject areas

  • Inorganic Chemistry


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