Synthesis, Structure, and Ligand Redistribution Equilibria of Mixed Ligand Complexes of the Heavier Group 2 Elements Containing Pyrazolato andβ‐Diketiminato Ligands

Treatment of M[N(SiMe 3 ) 2 ] 2 (THF) 2 (M = Ca, Sr, Ba) with various stoichiometries of N ‐ tert ‐butyl‐4‐( tert ‐butylimino)‐2‐penten‐2‐amine (L t Bu H) and 3,5‐di‐ tert ‐butylpyrazole ( t Bu 2 pzH) afforded [(η 2 ‐ t Bu 2 pz)Ca(μ‐η 5 :η 2 ‐ t Bu 2 pz)(μ‐η 2 :η 2 ‐ t Bu 2 pz)Ca(η 5 ‐L t Bu )] (17 %), [Sr(μ‐η 5 :η 2 ‐ t Bu 2 pz)(η 5 ‐L t Bu )] 2 (50 %), and [Ba(μ‐η 5 :η 2 ‐ t Bu 2 pz)(η 5 ‐L t Bu )] 2 (66 %) as colorless crystalline solids. The formulations of the new complexes were assigned from spectral and analytical data and by X‐ray crystal structure determinations. In the solid state, [(η 2 ‐ t Bu 2 pz)Ca(μ‐η 5 :η 2 ‐ t Bu 2 pz)(μ‐η 2 :η 2 ‐ t Bu 2 pz)Ca(η 5 ‐L t Bu )] exists as a dimer that is held together by μ‐η 5 :η 2 ‐ and μ‐η 2 :η 2 ‐pyrazolato ligands, with a terminal η 2 ‐ t Bu 2 pz ligand on one calcium ion and an η 5 ‐L t Bu ligand on the other. The dimeric structures of [Sr(μ‐η 5 :η 2 ‐ t Bu 2 pz)(η 5 ‐L t Bu )] 2 and [Ba(μ‐η 5 :η 2 ‐ t Bu 2 pz)(η 5 ‐L t Bu )] 2 are connected by two μ‐η 5 :η 2 ‐pyrazolato ligands, and the coordination sphere of each metal ion is capped with an η 5 ‐L t Bu ligand. In toluene solution, [(η 2 ‐ t Bu 2 pz)Ca(μ‐η 5 :η 2 ‐ t Bu 2 pz)(μ‐η 2 :η 2 ‐ t Bu 2 pz)Ca(η 5 ‐L t Bu )] exists as an equilibrium mixture of at least four compounds. In toluene solution, [Sr(μ‐η 5 :η 2 ‐ t Bu 2 pz)(η 5 ‐L t Bu )] 2 exists in equilibrium with Sr(η 5 ‐L t Bu ) 2 and Sr 4 ( t Bu 2 pz) 8 . A van’t Hoff analysis of this equilibrium between 46 and 104 °C afforded Δ H ° = 22.2 ± 0.7 kcal/mol, Δ S ° = 43.7 ± 1.9 cal/mol · K, and Δ G °(298 K) = 9.2 ± 0.9 kcal/mol. [Ba(μ‐η 5 :η 2 ‐ t Bu 2 pz)(η 5 ‐L t Bu )] 2 exists in toluene solution as a single, pure species between –60 and +80 °C. The overall results suggest that the η 5 ‐L t Bu ligand is better at saturating the coordination sphere of the metal ion to which it coordinates than is the t Bu 2 pz ligand, and is suggested as a more promising ligand for the construction of practical, volatile chemical vapor deposition film growth precursors. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)