Syntheses, Structures, and Polymorphism of β‐Diketonato Complexes – Co(thd)₃

Abstract

Abstract Oxidation of Co(thd) 2 dissolved in different solvents has been investigated in air and oxygen atmosphere. In oxygen atmosphere and at the boiling point of the solvents this treatment leads to oxidation of Co II to Co III , but also to degradation of some of the thd ligands and formation of a new mixed‐ligand complex. Three pure‐cultivated crystalline Co(thd) 3 phases are reported: 1 (room‐temperature phase), 2 (low‐temperature phase), and 3 (metastable phase) and in addition there exists an amorphous Co(thd) 3 phase ( 4 ) with approximate composition Co(thd) 3 · x H(thd); x = 0.06. Reaction of metal(II) oxides ( M O, M = Mn, Fe, and Co) with H(thd) under air or O 2 atmosphere is an easy direct route to M (thd) 3 complexes. Structure determinations are reported for Co(thd) 3 ( 1 – 3 ) based on single‐crystal X‐ray diffraction data. Modification 1 crystallizes in space group $P{\bar 3}c \rm 1$ with a = b = 18.8100(10), c = 18.815(2) Å at 295 K; R ( wR 2) = 0.180, modification 2 in space group C 2/ c with a = 28.007(12), b = 18.482(8), c = 21.356(9) Å, β = 97.999(5)° at 100 K; R ( wR 2) =0.211, and modification 3 in space group Pnma with a = 19.2394(15), b = 18.8795(15), c = 10.7808(8) Å at 100 K; R ( wR 2) = 0.193. The molecular structures of 1 – 3 all comprise a central Co atom octahedrally co‐ordinated by the ketonato O atoms of three thd ligands. The transformation between modifications 1 and 2 is of a fully reversible second‐order character. Modifications 1 and 3 are, on the other hand, related by a quasi‐reversible cycle. Heat treatment (specifically sublimation) of 1 leads to 3 whereas re‐crystallization or prolonged storage at room temperature is required to regenerate 1 . Co(thd) 3 has sufficient thermal stability to permit sublimation without degradation. The various forms of Co(thd) 3 are all diamagnetic, viz. a confirmation of the Co III valence state.