Low temperature thermal analysis of some boron halide complexes with weak Lewis bases Public Deposited

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  • Thermal analysis was used to investigate the following seven systems: H₂S-B₂F₄, SOF₂-B₂F₄, COC1₂-B₂F₄, SF₄-B₂F₄, Xe-BF₃, NF₃-BF₃, and NF₃-BC1₃. In two of the seven systems compound formation was observed. In the system, H₂S-B₂F₄ the compounds H₂S[superscript .]B₂F₄ and 2H₂S[superscript .]B₂F₄ are formed. The compound H₂S[superscript .]B₂F₄ has a melting point of -94.99 ± 0.3° C while the compound 2H₂S[superscript .]B₂F₄ melts at -100.3 ± 0.3° C. The system has three eutectics at 16.2, 35.2 and 50.5 mole percent B₂F₄ and at -107.4 ± 0.3° C, -100.4 ± 0.3° C, and -94.9 ± 0.3° C, respectively. In the system, SF₄-B₂F₄ the compound SF₄[superscript .]B₂F₄ is formed. The SF₄[superscript .]B₂F₄ slowly decomposes at room temperature. Two eutectics are found in the phase diagram at 3 ± 2 and 98 ± 2 mole percent B₂F₄ and at -116.3 ± 0.5° C and -59.8 ± 0.5° C, respectively. The system, SOF₂-B₂F₄ has a single eutectic at 24 mole percent B₂F₄ and a temperature of -141.7 ± 0.5° C. The two components are completely miscible. The system, COC1₂-B₂F₄ has a single eutectic at four mole percent B₂F₄ and a temperature of -132.5 ± 0.5° C. The two components are completely miscible. The compounds Xe and BF₃ are immiscible although BF₃ is slightly soluble in liquid Xe. Xe is completely insoluble in liquid BF₃. The compounds NF₃ and BF₃ are immiscible although NF₃ is slightly soluble in liquid BF₃. The solubility of BF₃ in liquid NF₃ was not investigated. The compounds NF₃ and BC1₃ are immiscible with NF₃ being very slightly soluble in liquid BC1₃. The solubility of BC1₃ in liquid NF₃ was not investigated. BF₃ is shown to displace B₂F₄ from the complex SF₄[superscript .]B₂F₄ in 93% yield. BF₃ is a stronger Lewis acid than either BC1₃ or B₂F₄ toward the Lewis bases SOF₂ and COC1₂ on the basis of compound formation. No conclusion can be drawn about the relative acid strengths of BC1₃ and B₂F₄ relative to these two bases. Based on acid displacement, BF₃ is a stronger Lewis acid than B₂F₄ relative to the Lewis base SF₄. Toward the Lewis base H₂S, the order of increasing Lewis acidity appears to be B₂F₄ < BF₃ < BC1₃, B₂C1₄. This is a qualitative order based on the height of the maximum between eutectics in the phase diagrams. Some difficulty was encountered with the synthesis of B₂F₄. Possible reasons for these difficulties are discussed. The generally weaker acidity of B₂F₄ as compared to BF₃ is accounted for on the basis of electronegativity and steric considerations.
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