Two polymorph modifications of tris(hexafluoroacetylacetonato)iron(III) revealed: is that common for other trivalent metals?

Abstract

A long-standing issue about the correct identification of an important starting reagent, iron(III) hexafluoroacetylacetonate, Fe(hfac)₃ (1), has been resolved. The tris-chelated mononuclear complex was found to crystallize in two polymorph modifications which can be assigned as the low-temperature (1-L) monoclinic P2₁/n and the high-temperature (1-H) trigonal P-3. Low-temperature polymorph 1-L was found to transform to 1-H upon sublimation at 44 °C. Two modifications are clearly distinguished by powder X-ray diffraction (PXRD), single-crystal X-ray diffraction, differential scanning calorimetry (DSC), and melting-point measurements. On the other hand, the two forms share similar characteristics in direct analysis in real-time mass spectrometry (DART-MS), attenuated total reflection (ATR) spectroscopy, and some physical properties, such as color, volatility, sensitivity, and solubility. Analysis of the literature and some of our preliminary data strongly suggest that the appearance of two polymorph modifications for trivalent metal (both transition and main group) hexafluoroacetylacetonates is a common case for several largely used complexes not yet accounted for in the crystallographic databases.