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

Liquid-like properties of cyclopentadienyl complexes of barium: molecular dynamics simulations of nanoscale droplets

Cyclopentadienyl complexes of barium have great utility in materials science and engineering, in particular, as precursors in the atomic layer deposition processes, which are required to be fluidic as well as thermally stable and volatile. Here, we investigated the liquid-like properties of cyclopentadienyl barium complexes including (Me₅C₅)₂Ba, (^tBu₃C₅H₂)₂Ba, (ⁱPr₄C₅H)₂Ba, (ⁱPr₅C₅)₂Ba, and [(SiMe₃)₃C₅H₂]₂Ba, using molecular dynamics simulations of nanoscale droplets. The compounds were modeled using a recently developed generic force field, GFN-FF. Nanoscale droplets with about 5.0 nm diameters were formed by aggregating 96 molecules of each compound. Simulation results reveal that substituting methyl groups of (Me₅C₅)₂Ba with other alkyl and silyl moieties has a non-negligible effect on the intra- and intermolecular structure and dynamics. In particular, in contrast to more flexible (Me₅C₅)₂Ba, the substitution with five iso-propyl groups to form (ⁱPr₅C₅)₂Ba adds rigidity to the complex with restricted orientational fluctuations for two cyclopentadienyl ligands and arranges molecules parallel to each other with greater probability. In addition, comparison between (^tBu₃C₅H₂)₂Ba, with three tert-butyl groups, and its silyl analogue, [(SiMe₃)₃C₅H₂]₂Ba, reveals that intermolecular interactions between the molecules with silyl groups are softer than those with tert-butyl groups and result in broader radial distribution functions, whereas the dynamic properties are similar for both compounds. This work suggests that molecular dynamics simulations contribute to molecular-level understanding of the effect of chemical substitution in organometallic compounds on the intra- and intermolecular properties of molecular liquids.

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

The electron-rich cyclopentadienyl and the analogous indenyl and fluorenyl ligands (collectively denoted here as Cp') have been impactful in stabilizing electron-deficient metal centers including the highly electropositive alkaline earths. Being in the s-block, the group 2 metals follow a major periodic variation in their atomic and ionic properties which is reflected in those Cp' compounds. This article presents an overview of this class of compounds for all the five metals from beryllium to barium (radium is excluded for its radioactivity), highlighting their systematic variation.

Synthesis and Crystal Structures of New Strontium Complexes with Aminoalkoxy and β-Diketonato Ligands

New heteroleptic strontium complexes were synthesized using substitution reaction of bis(trimethylsilyl)amide of Sr(btsa)₂·2DME with aminoalkoxide and β-diketonate ligands. The complexes [Sr(bdmp)(btsa)]₂·2THF (1), [Sr(bdeamp)(btsa)]₂ (2), [Sr(dadamb)(btsa)]₂ (3), [Sr(bdmp)(hfac)]₃ (4), [Sr(bdeamp)(hfac)]₃ (5), [Sr(dadamb)(hfac)]₃ (6), and [Sr₃(dadamb)₄(tmhd)₂] (7) were prepared and characterized by means of various analysis techniques such as Fourier transform infrared, NMR, thermogravimetric analysis, and elemental analysis. Complexes 1-3 were further structurally confirmed by single-crystal X-ray crystallography, and they displayed dimeric structures in which strontium atoms were connected by alkoxide oxygen atoms of the μ₂ type. Compound 1 has a trigonal prismatic structure, whereas 2 and 3 have a distorted square pyramidal structure. In complexes 5-7, trimeric structures were obtained with strontium atoms connected by μ₃-O bonds of alkoxide oxygen atoms and μ₂-O bonds of alkoxide and β-diketonate oxygen atoms. The crystal structures of 5, 6, and 7 showed distorted capped octahedral geometry, while 7 (middle Sr atom) displayed a distorted trigonal prism geometry. Complexes 5-7 displayed ∼70% mass loss in the temperature range from 25 to 315 °C.

Heteroleptic strontium complexes stabilized by donor-functionalized alkoxide and β-diketonate ligands

Heteroleptic complexes of strontium () were prepared by employing β-diketonates and donor-functionalized alkoxides as coordinating ligands. The results illustrate the effect of alkoxide substituent groups on the overall structures of the complexes. The presence of a terminal methoxy group in the alkoxide ligands leads to the formation of trimeric complexes , whereas the substituents on the amine nitrogen prove to have less influence in determining the structure. The attempts to increase steric bulkiness of the aminoalkoxide ligands by introducing ethyl groups on the amine nitrogen and to the alkoxy carbon did not lead to a structural change from the dimeric form in but resulted in structurally interesting strontium complexes. In trimeric complexes , the three strontium atoms were held together by two μ3-O bonds using alkoxide oxygen atoms and two μ2-O bonds using a combination of alkoxide and β-diketonate ligand oxygens. The strontium metal centers in these complexes exhibit seven-coordination states in and , whereas exhibits one six-coordinated and two seven-coordinated strontium metals in its structure. All of the complexes were characterized using FT-NMR, FT-IR, elemental analyses, and thermogravimetric (TG) analyses.

Synthesis of new heteroleptic strontium complexes

A series of heteroleptic strontium complexes (1-9) using a combination of different aminoalkoxides and 2,2,6,6-tetramethyl-3,5-heptanedionate (tmhd) were prepared to examine the effect of the bulkiness and coordination ability of the aminoalkoxide ligand in these complexes as well as potential strontium precursors. All complexes were characterized by FT-IR, FT-NMR, elemental analyses, and thermo-gravimetric (TG) analyses. The crystal structure analyses of 1, 2, 4, and 5 demonstrate their stability in the dimer form and the unwillingness of the strontium atom to form more than six coordination bonds in these complexes. The complex 5 shows an unusual picture: the existence of one hexa-coordinated and one penta-coordinated strontium atom side by side in its dimer structure. The introduction of ether groups as coordination sites in complexes 6-9 led to a decrease in steric hindrance which resulted in the formation of the complex 7 as a tetramer. The complex 7 shows a unique Sr4O4 cubane core where oxygen atoms undergo μ3-bridging between strontium atoms. The TG analyses show that the complexes exhibit a step-wise decomposition character, with the major mass losses in the region 150-400 °C.

New volatile strontium and barium imidazolate complexes for the deposition of group 2 metal oxides

We report the synthesis, characterization, and experimental density function theory-derived properties of new volatile strontium and barium imidazolate complexes, which under atomic layer deposition conditions using ozone as a reagent can deposit crystalline strontium oxide at 375 °C.