Molecular Co(II) and Co(III) Heteroarylalkenolates as Efficient Precursors for Chemical Vapor Deposition of Co₃O₄ Nanowires

Two new cobalt precursors, Co^II(PyCHCOCF₃)₂­(DMAP)₂ (1) and Co^III(Py­CHCOCF₃)₃ (2), based on Co­(II) and Co­(III) centers were synthesized using a redox active ligand system. The different chemical configurations of 1 and 2 and differential valence states of cobalt were confirmed by crystal structure determination and comprehensive analytical studies. Whereas 1 could not be studied by NMR due to the paramagnetic nature of the central atom, 2 was unambiguously characterized by multinuclear 1D and 2D NMR experiments in solution. Both compounds are efficient precursors for catalyst-free growth of Co₃O₄ nanowires on Si and Al₂O₃ substrates by a chemical vapor deposition process. The different valence states of cobalt species influenced their chemical decomposition pathways in the gas phase; for instance, 1 was partially oxidized (Co²⁺ → Co³⁺), and 2 underwent reduction (Co³⁺ → Co²⁺) to form pure cobaltite in both cases that verified the metal–ligand redox interplay. Co₃O₄ nanowires with nanometric diameters (50–100 nm) were obtained irrespective of the chosen cobalt precursor. Investigations on the humidity sensing behavior of CVD deposits demonstrated their potential as promising sensor materials.