Synthesis and Characterization of Sterically Encumbered β-Ketoiminate Complexes of Iron(ii) and Zinc(ii)

Abstract

The synthesis, structure, and spectroscopic signatures of a series of four-coordinate iron(II) complexes of β-ketoiminates and their zinc(II) analogues are presented. An unusual five-coordinate iron(II) triflate with three oxygen bound protonated β-ketoimines is also synthesized and structurally characterized. Single-crystal X-ray crystallographic analysis reveals that the deprotonated bis(chelate)metal complexes are four-coordinate with various degrees of distortion depending on the degree of steric bulk and the electronics of the metal center. Each of the high-spin iron(II) centers exhibits multiple electronic transitions including ligand π to π*, metal-to-ligand charge transfer, and spin-forbidden d-d bands. The ¹H NMR spectra of the paramagnetic high-spin iron(II) centers are assigned on the basis of chemical shifts, longitudinal relaxation times (T₁), relative integrations, and substitution of the ligands. The electrochemical studies support variations in the ligand strength. Parallel mode EPR measurements for the isopropyl substituted ligand complex of iron(II) show low-field resonances (g > 9.5) indicative of complex aggregation or crystallite formation. No suitable solvent system or glassing mixture was found to remedy this phenomenon. However, the bulkier diisopropylphenyl substituted ligand exhibits an integer spin signal consistent with an isolated iron(II) center [S = 2; D = −7.1 ± 0.8 cm⁻¹; E/D = 0.1]. A tentative molecular orbital diagram is assembled.