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Silyl- and Germyl-Substituted Boranes: Synthesis and Investigation as Potential Atomic Layer Deposition Precursors
Majeda Al Hareri1, Patricio Romero2, James F Britten3
1Department of Chemistry, McMaster University, 1280 Main St. West, Hamilton, Ontario L8S 4M1, Canada.
Abstract:
Boranes featuring bulky hypersilyl or supersilyl groups and/or sterically unencumbered trimethylgermyl substituents were synthesized for investigation as potential precursors for atomic layer deposition (ALD) of elemental boron. The envisaged ALD process would employ a boron trihalide coreactant, exploiting the formation of strong silicon-halogen and germanium-halogen bonds as a driving force. The alkali metal silyl and germyl compounds hypersilyl lithium, {(Me3Si)3Si}Li(THF)3 (1), supersilyl sodium, (Bu3Si)Na(THF) (2, n = 2-3), and trimethylgermyl lithium, {Me3GeLi(THF)2}2 (3), were used for the synthesis of the silyl- and germyl-substituted boranes in this work. Compounds 1 and 2 were synthesized as previously reported, and compound 3 was isolated from the reaction of trimethylgermane with tert-butyl lithium. Compounds 2 and 3 were crystallographically characterized. Reaction of B(NMe2)Cl2 with 2 equiv of 1 afforded previously reported {(Me3Si)3Si}2B(NMe2) (4), whereas reactions of B(NMe2)Cl2 or {B(NMe2)F2}2 with excess 2 only afforded the monosilyl boranes (Bu3Si)B(NMe2)X {X = Cl (5) and F (6)}. Reaction of 5 with 0.5 equiv of {Me3GeLi(THF)2}2 (3) provided the first example of a mixed silyl/germyl-substituted borane, (Bu3Si)(Me3Ge)B(NMe2) (7). Attempts to synthesize (Me3Ge)2B(NMe2) from the 1:1 reaction of B(NMe2)Cl2 with {Me3GeLi(THF)2}2 afforded a mixture of two major products, one of which was identified as the tri(germyl)(amido)borate {(Me3Ge)3B(NMe2)}Li(THF)2 (8); compound 8 was isolated from the 1:1.5 reaction. Reaction of more sterically encumbered B(TMP)Cl2 with 1 equiv of {Me3GeLi(THF)2}2 afforded the di(germyl)(amido)borane (Me3Ge)2B(TMP) (9). Boranes 4, 7, and 9 and borate 8 were crystallographically characterized. The thermal stability and volatility of boranes 4, 7, and 9 was evaluated, the solution reactivity of 4 and 7 with boron trihalides was assessed, and ALD was attempted using 4 in combination with BCl3 and BBr3 at 150 and 300 °C.
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