Abstract:
The gas phase thermal decomposition of the acetylacetonate, 1,1,1,5,5,5- hexafluoroacetylacetonate and 2,2,6,6-tetramethyl-3,5-heptanedionate complexes of barium, copper(II) and yttrium has been investigated using the technique of Infrared Laser Powered Homogeneous Pyrolysis (IR LPHP). Identification of decomposition product species was facilitated through the use of Fourier Transform Infrared spectroscopy and combined Gas Chromatography-Mass Spectrometry. Matrix isolation studies were employed to detect and identify short-lived species produced during decomposition of the p-diketone ligands, while semi-empirical calculations using PC Spartan Plus were used to evaluate relative activation energies of possible reaction pathways. Laser pyrolysis of the individual p-diketones was conducted to assist in interpreting the decomposition of the metal complexes. All three P-diketones underwent distinct routes of decomposition. Thermolysis of acetylacetone proceeds via two parallel molecular elimination pathways: the enol tautomer eliminates H2O to produce 2-methylfuran, while the keto tautomer affords acetone and ketene. 1,1,1,5,5,5- Hexafluoroacetylacetone undergoes HF elimination with subsequent ring closure to yield 2,2,-difluoro-5-(trifluoromethyl)furan-3(2H)-one. In contrast, 2,2,6,6-tetramethyl- 3,5-heptanedione decomposes via bond homolysis, the resulting radicals lose CO or ketene to produce /-butyl radicals, which undergo disproportionation and combination reactions. With the exception of copper bis(acetylacetonate), all the metal p-diketonates studied decompose with formation of the corresponding p-diketone. The suggested mechanism involves intramolecular H-atom transfer from either the adjoining ligand or coordinated water molecule, depending on whether the complex is anhydrous or hydrated. Instead of undergoing molecular decomposition, the copper bis(acetyacetonate) complex is believed to dissociate into Cu and acetylacetonyl radicals.
