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(C 5 HF 6 O 2 ) 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 Δ sub H°(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 M O 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 K O ionic bonds can explain the decrease in the enthalpy of sublimation of the lutetium complex in comparison with the lanthanum one.