- Probing substituent effects on the activation of H2 by phosphorus and boron frustrated Lewis pairs
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The impact of substituent changes on phosphorus and boron-containing frustrated Lewis pairs (FLPs) has been examined. The phosphites (RO) 3P R = Me, Ph form classical Lewis acid-base adducts of the formula (RO)3PB(C6F5)3 R = Me 1; R = Ph 2, whereas P(O-2,4-tBu2C6H3) 3 and P(O-2,6-Me2C6H3)3 generate FLPs. Nonetheless, these latter combinations do not react with H 2. The more basic phosphinite tBu2POR, R = tBu 3 reacts with B(C6F5)3 to give (tBu2(H)PO) B(C6F5)36. The related species tBu 2POR, R = Ph 4; 2,6-Me2C6H35 showed no reaction with B(C6F5)3 but the FLPs react under H2 (4 atm) to give [tBu2P(OR)H][HB(C 6F5)3] R = Ph 7 and 2,6-Me2C 6H38. Similarly, tBu2PCl in combination with B(C6F5)3, generates an FLP that upon addition of H2, gives [tBu2PH2][ClB(C6F 5)3] 9 albeit in low yield. The diborane 1,4-(C 6F5)2B(C6F4)B(C 6F5)2 in combination with either tBu 3P or (C6H2Me3)3P generates FLPs that react with H2 to give [R3PH] 2[1,4-(C6F5)2HB(C6F 4)BH(C6F5)2] (R = tBu 11, C 6H2Me312). Similarly PhB(C6F 5)2 and tBu3P react with H2 giving [tBu3PH][HBPh(C6F5)2] 13. The combination of B(OC6F5)3 and PtBu3 also generate an FLP which reacts with H2 to give [HPtBu 3][B(OC6F5)4] 14, the product of substituent redistribution. The boronic esters, (C6H 4O2)BC6F515, (C6H 3FO2)BC6F516 and (C 6F4O2)BC6F517, and the borate esters B(OC6H3(CF3)2) 318, B(OC6H2F3)319 and B(OC6H4CF3)320 were prepared and shown to generate FLPs with tBu3P or (C6H 2Me3)3P. Nonetheless, no reaction with H 2 was observed for 15-17. Collectively these data suggest that there is a threshold of combined Lewis acidity and basicity that is required to effect the splitting of H2.
- Neu, Rebecca C.,Ouyang, Eva Y.,Geier, Stephen J.,Zhao, Xiaoxi,Ramos, Alberto,Stephan, Douglas W.
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p. 4285 - 4294
(2010/07/04)
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- Synthesis of a series of fluorinated boronate compounds and their use as additives in lithium battery electrolytes
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A new series of anion receptors based on boronate compounds have been synthesized. These compounds can be used as anion receptors in lithium battery electrolytes. The so-called boronate means that the compounds contain a boron bonded with two oxygen atoms and one carbon atom. This series includes various boronate compounds with different fluorinated aryl and fluorinated alkyl groups. When these anion receptors are used as additives in 1,2-dimethoxyethane (DME) solutions containing various lithium salts, the ionic conductivities of these solutions are greatly increased. The electrolytes tested in this study were DME solutions containing the following lithium salts: LiF, CF3COOLi, and C2F5COOLi. Without the additive, the solubility of LiF in DME (and all other nonaqueous solvents) is very low. With some of these boronate compounds as additives, LiF solutions in DME with concentration as high as 1 M were obtained. The solubilities of the other salts were also increased by these additives. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy studies show that I- anions are complexed with these compounds in DME solutions containing LiI salts. The degree of complexation is also closely related to the structures of the fluorinated aryl and alkyl groups which act as electron-withdrawing groups. The NEXAFS results are in good agreement with ionic conductivity studies.
- Lee,Ma,Yang,Sun,McBreen
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p. A1429-A1435
(2008/10/09)
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- Synthesis and study of new cyclic boronate additives for lithium battery electrolytes
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Two novel boronate compounds, 2-(pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole (1) and 2-(pentafluorophenyl)-4,4,5,5-tetrakis(trifluoromethyl)-1,3,2-dioxaborolane (2), have been synthesized as additives for lithium battery electrolytes. These cyclic boronate compounds have a much more significant effect on conductivity enhancement of LiF salt in dimethoxyethene (DME) or ethyl carbonate-dimethyl carbonate (EC-DMC) than either borane or borate additives we previously synthesized. The conductivity of a composite electrolyte containing compound 1 and LiF has reached 9.54 × 10-3 S/cm in DME and 4.79 × 10-3 S/cm in EC-DMC (1:2). This is due to the lower molecular weight and less steric hindrance effects of compound 1. In the case of compound 2, the enhanced performance also comes from the improved solubility in polar solvents. Composite electrolytes containing LiF and either compound 1 or compound 2 have excellent electrochemical stability in the EC-DMC solvent, with respective electrochemical windows of 4.05 and 5.1 V. The composite electrolyte containing LiF and compound 2 shows high cycling efficiency and cyclability in both Li/LiMn2O4 and Li/LiNi0.8Co0.2O2 cells.
- Lee,Sun,Yang,McBreen
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p. A1460-A1465
(2008/10/08)
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