Silyl- and\nGermyl-Substituted Boranes: Synthesis and\nInvestigation as Potential Atomic Layer Deposition Precursors

Boranes featuring bulky hypersilyl or supersilyl groups\nand/or\nsterically unencumbered trimethylgermyl substituents were synthesized\nfor investigation as potential precursors for atomic layer deposition\n(ALD) of elemental boron. The envisaged ALD process would employ a\nboron trihalide coreactant, exploiting the formation of strong silicon-halogen\nand germanium-halogen bonds as a driving force. The alkali metal silyl\nand germyl compounds hypersilyl lithium, {(Me<sub>3</sub>Si)<sub>3</sub>Si}Li(THF)<sub>3</sub> (<b>1</b>), supersilyl sodium, (<sup><i>t</i></sup>Bu<sub>3</sub>Si)Na(THF)<sub><i>n</i></sub> (<b>2</b>, <i>n</i> = 2–3), and trimethylgermyl\nlithium, {Me<sub>3</sub>GeLi(THF)<sub>2</sub>}<sub>2</sub> (<b>3</b>), were used for the synthesis of the silyl- and germyl-substituted\nboranes in this work. Compounds <b>1</b> and <b>2</b> were\nsynthesized as previously reported, and compound <b>3</b> was\nisolated from the reaction of trimethylgermane with <i>tert</i>-butyl lithium. Compounds <b>2</b> and <b>3</b> were\ncrystallographically characterized. Reaction of B(NMe<sub>2</sub>)Cl<sub>2</sub> with 2 equiv of <b>1</b> afforded previously reported\n{(Me<sub>3</sub>Si)<sub>3</sub>Si}<sub>2</sub>B(NMe<sub>2</sub>) (<b>4</b>), whereas reactions of B(NMe<sub>2</sub>)Cl<sub>2</sub> or\n{B(NMe<sub>2</sub>)F<sub>2</sub>}<sub>2</sub> with excess <b>2</b> only afforded the monosilyl boranes (<sup><i>t</i></sup>Bu<sub>3</sub>Si)B(NMe<sub>2</sub>)X {X = Cl (<b>5</b>) and\nF (<b>6</b>)}. Reaction of <b>5</b> with 0.5 equiv of\n{Me<sub>3</sub>GeLi(THF)<sub>2</sub>}<sub>2</sub> (<b>3</b>)\nprovided the first example of a mixed silyl/germyl-substituted borane,\n(<sup><i>t</i></sup>Bu<sub>3</sub>Si)(Me<sub>3</sub>Ge)B(NMe<sub>2</sub>) (<b>7</b>). Attempts to synthesize (Me<sub>3</sub>Ge)<sub>2</sub>B(NMe<sub>2</sub>) from the 1:1 reaction of B(NMe<sub>2</sub>)Cl<sub>2</sub> with {Me<sub>3</sub>GeLi(THF)<sub>2</sub>}<sub>2</sub> afforded a mixture of two major products, one of which was\nidentified as the tri(germyl)(amido)borate {(Me<sub>3</sub>Ge)<sub>3</sub>B(NMe<sub>2</sub>)}Li(THF)<sub>2</sub> (<b>8</b>); compound <b>8</b> was isolated from the 1:1.5 reaction. Reaction of more sterically\nencumbered B(TMP)Cl<sub>2</sub> with 1 equiv of {Me<sub>3</sub>GeLi(THF)<sub>2</sub>}<sub>2</sub> afforded the di(germyl)(amido)borane (Me<sub>3</sub>Ge)<sub>2</sub>B(TMP) (<b>9</b>). Boranes <b>4</b>, <b>7</b>, and <b>9</b> and borate <b>8</b> were\ncrystallographically characterized. The thermal stability and volatility\nof boranes <b>4</b>, <b>7</b>, and <b>9</b> was\nevaluated, the solution reactivity of <b>4</b> and <b>7</b> with boron trihalides was assessed, and ALD was attempted using <b>4</b> in combination with BCl<sub>3</sub> and BBr<sub>3</sub> at\n150 and 300 °C.