Reversible temperature-induced polymorphic phase transitions of [Y(OAr)] and [Ce(OAr)] (Ar = 2,6-Bu-4-MeCH): interconversions between pyramidal and planar geometries

In studying the crystal structure of [Y(O-2,6- t Bu 2 -4-MeC 6 H 2 ) 3 ] [ 1 ] at different temperatures, three phase transitions were observed. Three polymorphs contained a 1 : 1 ratio of planar and pyramidal isomers of 1 with an increasing number of molecules in the asymmetric unit, but the fourth polymorph at 100 K had a 3 : 2 ratio favouring the planar geometry. These phase transitions were reversible and involved very subtle changes in the molecular conformation, which must happen in situ over all the molecules in the crystal. For [Ce(O-2,6- t Bu 2 -4-MeC 6 H 2 ) 3 ] [ 2 ] with one 4f-electron, a disorder/order transition was observed, similar to the first phase transition for 1 , with two different molecules observed in the asymmetric unit at 100 K {Σ(O-Ln-O) = 341.3° and 351.7°}. The study of these polymorphic systems has been complemented by computational studies in the gas phase and solid state that confirm that these complexes lie in a very shallow potential energy surface that allow molecules to transition between planar and pyramidal structures upon a change in temperature. Exploring the balance of energetics between planar and pyramidal forms of [Y(O-2,6- t Bu 2 -4-MePh) 3 ] and related complexes.