- Metal-oxo clusters can be described as soluble pieces of metal-oxide frameworks. Currently, metal-oxo clusters are being considered as solution-based precursors for extreme ultraviolet (EUV, 13.5 nm) photoresist materials, which are a crucial component in the fabrication of microelectronic devices. Two different photoresist precursor systems have been investigated – alkyltin clusters and zirconium peroxide clusters. Four new butyltin structures have been crystallized, all having the β or γ Keggin topology:
β-[(BuSn)₁₂(NaO₄)(OCH₃)₁₂ (O)₅ (OH)₇ ] (β-NaSn₁₂),
γ-[(BuSn)₁₂(NaO₄ )(OCH₃)₁₂(O)₅(OH)₇ ] (γ-NaSn₁₂),
γ-[(BuSn)₁₂(NaO₄ )(OCH₃)₁₁ (O)7(OH)₆(BuSnOCH₃)] (γ-NaSn₁₃),
β-[(BuSn)₁₂(CaO₄ )(OCH₃)₁₂(O)4(OH)₈]²⁺ (β-CaSn₁₂).
All four of these were synthesized by hydrolysis of BuSnCl₃ with either NaOH or Ca(OH)₂ in methanol. Two new zirconium peroxide structures have been characterized as well – an oxo-centered tetrahedron [Zr₄ (OH)₄ (μ-O₂)₂(μ₄ -O)(H₂O)₁₂ ](ClO₄ )₆.*xH₂O (ZrTd) and a 25-membered wheel structure [Zr₂₅O₁₀ (OH)₅₀ (O₂)5(H₂O)₄₀](ClO₄ )₁₀ *xH₂O (Zr₂₅). All of these new structures have been characterized extensively in solution by techniques including multinuclear NMR, small angle x-ray scattering (SAXS), and electrospray-ionization mass spectrometry (ESI-MS).
The alkyltin Keggin system is unique in that only the rarer β and γ isomeric forms have been crystallized. Furthermore, the Na-centered clusters have only been isolated as a mixture of β and γ isomers. We have therefore explored several factors which may influence the Keggin isomer such as the charge of the octahedral metal, the size and charge of the central metal, and the effects of aging, solvent, and heating. Changing the central metal from Na to Ca resulted in the stabilization of the β-isomer. Density functional theory computations ranked the relative stabilities of these clusters γ-CaSn₁₂ < γ-NaSn₁₂ < β-CaSn₁₂ < β-NaSn₁₂. Aging of Na- and Ca-centered clusters was studied in air by FT-IR and in organic solvents (C₆D₆, CDCl3₃, 9:1 C₆D₆:MeOD, 9:1 CDCl₃:MeOD) by ¹H NMR. Results showed hydrolysis of the bridging methoxy ligands due to ambient humidity or residual water in the organic solvents. Addition of excess deuterated methanol was also found to increase the rate of hydrolysis resulting in the following order: C₆D₆ < C₆D₆/MeOD < CDCl₃/MeOD ≈ CDCl₃. Characterization by variable-temperature ¹¹⁹Sn NMR showed that the formation of additional isomers can be promoted by heating solutions of NaSn₁₂ and CaSn₁₂ in C₆ D₆ . ²³Na NMR characterization of heated solutions of NaSn₁₂ showed five chemical shifts, corresponding to all five Keggin isomers simultaneously in solution.
The aqueous chemistry of zirconium is typically dominated by the ubiquitous square tetramer (Zr₄) which spontaneously assembles upon dissolution of zirconium oxyhalide salts at low pH. By exchanging the halide with perchlorate and adding peroxide to the solution, we were able to crystallize two new topologies. Adding 1:1 peroxide/Zr resulted in Zr25, and adding 10:1 peroxide/Zr resulted in ZrTd. To shed light on the role of peroxide in these systems, the reaction pathway was monitored by SAXS and pair distribution function (PDF). These studies revealed that when excess peroxide is present, in the case of ZrTd, a large cluster species initially forms in solution before breaking down into the smaller tetrahedral cluster. Conversely, without excess peroxide, small trimer and pentamer fragments are formed in solution which then assemble into Zr₂₅ in the solid state at the interface between crystal and solution. The highly acidic nature of this solution prevents formation of the large cluster in the solution state. The trimer and pentamer fragments have also been observed by ESI-MS and the intact Zr₂₅ is never observed in water by any solution characterization methods.