Sublimation enthalpy of tetrakis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)lanthanide(III)-potassium complexes, KLn(C5HF6O2)4: Effect of lanthanide contraction

Abstract

The mass spectra of tetrakis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)lanthanide(III)-potassium complexes KLn(C5HF6O2)4, where Ln = La, Gd, Lu, (hereinafter referred to as KLn(hfa)4), have been recorded at different temperatures. The presence of the high-intensity ions KLn(hfa)3+ generated by elimination of one ligand from the parent species due to electron ionization indicates that the saturated vapours of all three complexes consist of the only KLn(hfa)4 species. The sublimation enthalpies were determined by effusion Knudsen method in frame of second law of thermodynamics. The vaporization was studied over temperature ranges 414 ÷ 471 К for KLa(hfa)4, 401 ÷ 453 К for KGd(hfa)4, and 399 ÷ 442 К for KLu(hfa)4. The values of ΔsubH°(T) equal 170.6 ± 2.1, 166.4 ± 3.6, and 161.8 ± 2.3 kJ/mol for the complexes of La, Gd and Lu respectively.DFT/PBE0 calculations show, that the shortening of the MO bond length due to lanthanide contraction is accompanied by a decrease in the negative net charge on the oxygen atoms bounded with potassium. The potassium ions play the role of the bridges between the [Ln(hfa)4]− anions in the crystal, and weakening the KO ionic bonds can explain the decrease in the enthalpy of sublimation of the lutetium complex in comparison with the lanthanum one.