Study on thermodynamic property of pyrrolylaldiminate dialkyl-aluminum (Methyl- and Ethyl- substituted) complexes

Pyrrolylaldiminate dialkyl-aluminum (methyl- and ethyl- substituted) complexes with different substituents were prepared and characterized. Single-crystal X-ray diffraction proved that compound 2A [(PyrImEt)AlMe2] is a monomer with a planar five-membered ring structure. The optimized structure of density functional theory (DFT) shows that compound 5A [(PyrImcPr)AlMe2] and compound 6A [(PyrImiPr)AlMe2] is a monomer and planar five-membered ring structure. Pyrrolylaldiminate dialkyl-aluminum complexes have fluorescence properties and accord with the characteristics of rigid planar complexes. Based on the monomer and the planar structure of pyrrolylaldiminate dialkyl-aluminum complexes, we further studied the complex's thermodynamic properties from the aspects of thermogravimetric (TGA), Differential Scanning Calorimeter (DSC), and InP-T−1 (Vapor pressure-temperature functions). The results of TGA and DSC showed that the pyrrolylaldiminate dialkyl-aluminum complex with planar structure has good thermal stability. When controlling the chain length of substituents, dimethyl-aluminum complexes with low melting points can be prepared. Substituents’ effect on the complex's volatilization was investigated according to the vapor pressure-temperature function. Overall, Tvapor pressure at P=1Torr of complexes with small molecular weight is low (Tvapor pressure at P=1Torr of pyrrolylaldiminate dimethyl-aluminum complexes is 60–85 °C, and Tvapor pressure at P=1Torr of pyrrolylaldiminate diethyl‑aluminum complexes is 75–103 °C). Pyrrolylaldiminate dialkyl-aluminum complexes have good thermal stability and volatility, consistent with the requirements of atomic layer deposition. The effect of substituents on the enthalpy of evaporation (ΔHvapour) of dimethyl-aluminum [56.83–89.80 kJ.mol−1] and diethyl‑aluminum complexes [76.52–83.58 kJ.mol−1] is different, and ΔHvapour of dimethyl-aluminum complexes is more easily affected by substituents. For diethyl‑aluminum complexes, the flexible substituent has little effect on ΔHvapour of the complexes.