Crystal structures and thermal properties of Ba(1,2,4-t-Bu3C5H2)2 and Sr(1,2,4-t-Bu3C5H2)2: Precursors for atomic layer deposition

Sterically hindered Lewis base free bis(1,2,4-tri-tert-butylcyclopentadienyl)strontium (1) and bis(1,2,4-tri-tert-butylcyclopentadienyl)barium (2) were synthesized using the common metathesis route and characterized with NMR, MS, TGA/SDTA and XRD. Compound 1 crystallized as a monomer with typical bent structure. Asymmetric unit contains two independent slightly different Sr(t-Bu3C5H2)2 molecules with Cp(centroid)–Sr–Cp(centroid) angles of 165.1° and 169.4°. Depending on the way of crystallization two polymorphs (2a and 2b) were observed for Ba(t-Bu3C5H2)2. On sublimation Ba(t-Bu3C5H2)2 crystallizes as chains in which one methyl group of each Ba(t-Bu3C5H2)2 unit interacts with neighboring Ba(t-Bu3C5H2)2 unit’s barium atom. Slow crystallization of waxy evaporation residue of toluene solution results in monoclinic crystals (2b) whose asymmetric unit contains four slightly different individual Ba(t-Bu3C5H2)2 molecules with Cp(centroid)–Ba–Cp(centroid) angles of 161.3–164.9°. Both compounds prepared are volatile, thermally stable and reactive and thus suitable precursors for atomic layer deposition of thin films.