A Homologous Series of Base‐Free Organo(alkaline‐earth) Metallocenes: Synthesis and Molecular Structures of [1,2,4‐(SiMe₃)₃C₅H₂]₂(Ca, Sr, Ba)

Abstract

Bis[1,2,4‐tris(trimethylsilyl)cyclopentadienyl] complexes [1,2,4‐(SiMe3)3C5H2]2Ae [(Cp3Si)2Ae; Ae = Ca (1), Sr (2), Ba (3)] are isolated from the 2:1 reaction of K[Cp3Si] and AeI2 in diethyl ether. Under the same reaction conditions used for the heavier metallocenes, the corresponding (Cp3Si)2Be compound was not formed, although calculations suggest that the beryllocene would be sterically feasible. Compound 1 crystallizes as a bent monomer from hexanes (ring centroid−Ca−ring centroid = 166.7°) with two η5‐Cp3Si ligands on the calcium atom [Ca−C(av) = 2.63(2) Å]. Metallocene 2, the first crystallographically characterized example of an unsolvated strontocene, is also isolated as a bent monomer (159.4°) from hexanes with two η5‐Cp3Si ligands on the strontium atom [Sr−C(av) = 2.819(2) Å]. Compound 3 crystallizes on sublimation as a coordination dimer in which each barium atom is flanked by two η5‐Cp3Si rings [Ba−C(av) = 3.01(2) Å] and one methyl group from a neighboring Cp′ ring; the intermolecular Ba···C(methyl)′ distance is 3.275(6) Å. The amount of bending observed in 1−3 and other Group‐2 metallocenes varies with the metals, the steric bulk of the cyclopentadienyl ligands, and crystal packing effects, and is not easily predictable. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Stability of cyclopentadienyl aryloxide complexes of calcium and barium

“…K[Cp 3Si ] [11], (Cp 3Si )2Ca [9] and IBa(BHT)(thf)3 [12] were prepared as previously described. K[Odipp] and KCp* (Cp* = C5Me5) were prepared from potassium hydride and HOdipp or HCp*, respectively.…”

“…(Cp 3Si )Ca(Odipp)(thf) 2 was characterized with spectroscopic methods and X-ray crystallography. The 1 H NMR resonances for the [Cp 3Si ] − and [Odipp] − ligands are shifted from those of the corresponding homoleptic compounds [9]. As with the related mono(ring) alkaline-earth iodide (Cp 3Si )CaI(thf), the aryloxide derivative is resistant to ligand redistribution in both THF and aromatics.…”

“…Archetypal examples include Cp CaI(thf) n (Cp = C 5 Me 5 or C 5 Me 4 Et) complexes, which undergo Schlenk-type rearrangement in THF solution to form 2:1:1 mixtures of Cp CaI(thf) n , Cp 2 Ca, and CaI 2 , respectively [7,8]. The position of equilibrium can be shifted somewhat by the solvent (aromatic solvents favor the heteroleptic species) and by the use of "encapsulating" cyclopentadienyl rings such C 5 (i-Pr) 4 H (=Cp 4i ) [1,3], C 5 (SiMe 3 ) 3 H 2 (=Cp 3Si ) [2] and C 5 (t-Bu) 3 H 2 [9], which sterically interfere with the rearrangement process. The identity of the X ligand in Cp CaX complexes also affects the position of Schlenk equilibrium, and the present work demonstrates how the use of aryloxide ligands can make possible the isolation of rearrangement-resistant complexes in the absence of encapsulating Cp rings or with easily displaceable Cp rings such as Cp 3Si .…”

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

“…Most of the base free metallocenes of heavier alkaline-earth metals (Ca, Sr, Ba) have structures with non parallel rings in the solid state and as well in the gas phase [11][12][13][14][15][16][17]. The only reported exception, Ba(i-Pr 5 C 5 ) 2 [10] with Cp(centroid)-M-Cp(centroid) angle of 180°, is a compound where the metal has sterically extremely crowded environment.…”

“…Complexes M(t-Bu 3 C 5 H 2 ) 2 (M = Sr, Ba) had the best thermal stability, and it was also noticed that this ligand can be prepared conveniently in one pot synthesis [8] with good yield. Some other recently reported strontocenes and barocenes with even more bulky ligands are Ba(i-Pr 4 C 5 H 4 ) 2 [9], M(i-Pr 5 C 5 ) 2 [10], and M((Me 3 Si) 3 C 5 H 2 ) 2 (M = Sr, Ba) [11]. These compounds have not been tested as possible ALD precursors.…”