And 5-Halodecaboranes: Selective Syntheses from Closo-B10H10(2-) and Use As Polyborane Building Blocks" (2010)
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University of Pennsylvania ScholarlyCommons Publicly Accessible Penn Dissertations Fall 2010 6- and 5-Halodecaboranes: Selective Syntheses From ClOSO- B10H10(2-) and Use as Polyborane Building Blocks William C. Ewing University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/edissertations Part of the Inorganic Chemistry Commons Recommended Citation Ewing, William C., "6- and 5-Halodecaboranes: Selective Syntheses From ClOSO-B10H10(2-) and Use as Polyborane Building Blocks" (2010). Publicly Accessible Penn Dissertations. 261. https://repository.upenn.edu/edissertations/261 This paper is posted at ScholarlyCommons. https://repository.upenn.edu/edissertations/261 For more information, please contact [email protected]. 6- and 5-Halodecaboranes: Selective Syntheses From ClOSO-B10H10(2-) and Use as Polyborane Building Blocks Abstract Decaborane halogenated in the 6-position has been synthesized in high yields via the super-acid induced cage-opening reactions of closo-B10H10(2-) salts. These 6-halogenated compounds were then isomerized to their 5-substituted isomers through base catalysis. The isomerization was driven by the energy differences between the anionic-forms of each respective isomer. These reactions provided 5-halodeboranes in high yields. The bridging-hydrogens of the halodecaboranyl anions were fluxional at a range of temperatures. Variable-temperature NMR studies supported computationally proposed fluxional mechanisms. Both 5- and 6-halodecaboranes were reacted with alcohols yielding boranyl ethers. The mechanisms of substitution, where reactions with 6- and 5-halodecaboranes yielded 5- and 6-boranyl ethers, respectively, were explained computationally and confirmed through isotopic-labeling studies. The regeneration of the polymeric products of ammonia-borane dehydrogenation was carried out through a process that included digestion of the polymer, complexation of the digestate with a base, reduction of B-X bonds to B-H bonds, and finally displacement of the base with ammonia. While digestion schemes proved unable to digest all forms of the dehydrogenated materials, portions of the polymer digested to boron-trihalides were quantitatively regenerated to ammonia borane, with complete separation and collection of by-products. Degree Type Dissertation Degree Name Doctor of Philosophy (PhD) First Advisor Larry G. Sneddon Keywords Substituted Decaboranes, Halogenated Decaboranes, Polyboranes, Hydrogen Storage, Boranyl Ethers, Variable Temperature NMR Subject Categories Chemistry | Inorganic Chemistry This dissertation is available at ScholarlyCommons: https://repository.upenn.edu/edissertations/261 6- AND 5-HALODECABORANES: SELECTIVE SYNTHESES FROM 2 closo-B10H10 Ø AND USE AS POLYBORANE BUILDING BLOCKS William C. Ewing A DISSERTATION in Chemistry Presented to the Faculties of the University of Pennsylvania in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy 2010 _________________________________ Professor Larry G. Sneddon Supervisor of Dissertation ________________________________ Professor Gary Molander Graduate Group Chair Committee Members: Dr. I. Dmochowski, Professor of Chemistry Dr. D. Berry, Professor of Chemistry Dr. P. Walsh, Professor of Chemistry Acknowledgment I would like to gratefully acknowledge Professor James E. Davis and Professor Larry G. Sneddon, the men responsible for my discovery and re-discovery of chemistry. Similar acknowledgement is deserved of my parents, Peter and Deborah Ewing, whose support, if not patience, has been unwavering. Acknowledgement is also due the friends and colleagues, too numerous to mention, known to spike the too-oft bland punch served at the cantina of the day-to-day. “But then they danced down the street like dingledodies, and I shambled after as I've been doing all my life after people who interest me, because the only people for me are the mad ones, the ones who are mad to live, mad to talk, mad to be saved, desirous of everything at the same time, the ones who never yawn or say a commonplace thing, but burn, burn, burn, like fabulous yellow roman candles exploding like spiders across the stars and in the middle you see the blue centerlight pop and everybody goes ‘Awww!’” - J. Kerouac ii ABSTRACT 6- AND 5-HALODECABORANES: SELECTIVE SYNTHESES FROM 2 closo-B10H10 Ø AND USE AS POLYBORANE BUILDING BLOCKS William C. Ewing Larry G. Sneddon Decaborane halogenated at the 6-position (6-X-B10H13, X = Cl, Br, I) was 2 synthesized through cage-opening reactions of closo-B10H10 Ø induced by treatment with hydrogen halides absorbed in ionic liquid mixtures known to greatly enhance HX acidity. The previously unknown member of the 6-halogenated series, 6-F-B10H13, was 2 synthesized in excellent yields by reaction of the closo-B10H10 Ø with triflic acid in the 2 presence of 1-fluoropentane. Triflic acid also induced the cage-opening of closo-B10H10 Ø to 6-Cl-B10H13 when performed in CH2Cl2. The 6-halogenated isomers were used as starting materials in the syntheses of 5- X-B10H13 (X = Cl, Br, I). Base catalyzed isomerization reactions yielded equilibrium mixtures of 5-X-B10H13 and 6-X-B10H13 with ratios dictated by the free energy difference between the respective anions, 5-X-B10H12Ø and 6-X-B10H12Ø, strongly favoring the 5- substituted isomer. Pure 5-X-B10H13 was isolated in good yields by selective crystallization or by chromatography. These syntheses represent a significant step forward in the chemistry of this isomer, and provide the first path by which these compounds may be synthesized in useful quantities. iii Calculations on the 5- and 6-halogenated anions identified fluxional processes by which bridging hydrogens move about the open face of the cage, explaining the results of variable-temperature NMR studies on these compounds. Variable-temperature NMR was also used to validate similar processes previously proposed for the parent B10H13Ø. As proof of the utility of 5-X-B10H13 and 6-X-B10H13 as starting materials for further functionalization a new class of boranyl ether compounds, selectively substituted at the 6- or 5-position, were synthesized using the reaction of the halogenated species with alcohols. A range of alcohols, with various pendant functional groups, were tethered to the cage via a rare B-O-C organic/inorganic ether linkage. The regiochemistry of the reaction was unique in that reaction of 5-X-B10H13 with alcohols selectively yielded 6-RO-B10H13 ethers, and 6-X-B10H13 with the same alcohols yielded 5-RO-B10H13 compounds. A plausible reaction mechanism explaining this regiochemistry was found using DFT calculations, which subsequent experiments with deuterated starting materials supported. iv Table of Contents Chapter 1. Introduction and Outline of the Dissertation 1.1. Outline of the dissertation 1 1.2. Introduction: decaborate and decaborane 4 2 1.2.1. closo-B10H10 Ø 4 1.2.1.1. Syntheses 6 1.2.1.2. Electronic structure and bonding 6 1.2.1.3. Substitution reactions 9 1.2.2. nido-B10H14 11 1.2.2.1. Syntheses 12 1.2.2.2. Electronic structure and bonding 14 1.2.2.3. Substitution of a terminal B-H 15 1.2.2.4. 6,9-L2-B10H12 17 1.2.2.5. Incorporation of heteroatoms 18 1.3. Conclusion 20 1.4. References 20 v Chapter 2. New High Yield Synthetic Routes Salts to the Complete Series of 6-X- B10H13 Halodecaboranes (X = F, Cl, Br, I) via Super-Acid Induced 2- Cage-Opening Reactions of closo-B10H10 2.1. Introduction 26 2.2. Experimental 28 2.3. Results and Discussion 50 2.3.1. Super-acidic Ionic Liquid Synthesis and Characterization of 6-X-B10H13 50 2.3.2. Triflic-acid Based Synthesis and Characterization of 6-X-B10H13 57 2.3.3. Is Hawthorne’s Cage-Opening Mechanism Valid? 61 2.4. Conclusions 67 2.5. References 68 Chapter 3. Efficient Syntheses of 5-X-B10H13 Halodecaboranes via the Photochemical (X = I) and/or Base-Catalyzed (X = Cl, Br, I) Isomerization Reactions of 6-X-B10H13 3.1. Introduction 73 3.2. Experimental 74 3.3. Results and Discussion 122 3.3.1. Photochemical Isomerization of 6I to 5I 122 3.3.2. Base Catalyzed Isomerizations of 6-X-B10H13 and 5-X-B10H13 123 vi 3.3.3. Isomerization of 6-X-9-R-B10H12 145 3.4. Conclusions 149 3.5. References 150 Chapter 4. Probing the Fluxional Behavior of B10H13Ø and Halogenated Derivatives by Variable Temperature 11B NMR 4.1. Introduction 154 4.2. Experimental 158 4.3. Results and Discussion 169 4.3.1. Fluxional Properties of the B10H13Ø anion 169 í 4.3.2. Fluxional Properties of the 5- and 6-X-B10H12 anions 174 4.4. Conclusions 182 4.5. References 183 Chapter 5. An Inorganic Analog of the SN2’ Reaction: Nucleophilic Attack of Alcohols on Haloboranes to Yield Boranyl Ethers 5.1. Introduction 186 5.2. Experimental 189 5.3. Results and Discussion 248 5.3.1. Syntheses of 6OR and 5OR 248 5.3.2. Characterization of 6OR and 5OR 252 5.3.3. Crystallographic Structure Determinations of 5- and 6-OR 267 5.3.4. Computational Exploration of the Reaction Mechanism 277 vii 5.3.5. Substitution Reactions on Deuterated 6Br 285 5.4. Conclusions 291 5.5. References 291 Chapter 6. Regeneration of Spent Ammonia Borane Hydrogen Fuels 6.1. Introduction 296 6.2. Experimental 300 6.3. Results and Discussion 307 6.3.1 Digestion of Spent Fuels with Acids 309 6.3.1.1. Digestion with Oxyacids 311 6.3.1.2. Digestion with Haloacids 314 6.3.1.2.1. Digestion in Superacidic Ionic Liquid 314 6.3.1.2.2. Digestion in Mixtures of AlBr3/HBr 318 6.3.2. Recuction B-X and B-O Bonds 321 6.3.2.1. Reduction of TFA-digestate with Alane 322 6.3.2.2. Reduction of BBr3 to AB 324 6.3.2.3. Separation of Products from the Reduction of BBr3 328 6.3.2.4. Displacement of Sulfide with Ammonia 330 6.4.