NUCLEAR MAGNETIC R:~SONANCE S'I'UDH~S OF BORON TRIFLUORIDE AND SOME OF ITS COMPLEXES NUCLEAR MAGNETIC RESONANCE STUDIES OF BORON 'l1RIFLUORIDE AND SOME OF ITS COMPLEXES by JOI-frJ ST.2;PH_c;N HARTMAN, B.Sc. 1 M.Sc. A Thesis Submitted to the Faculty of Graduate Studies in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy McMaster University January, 1967 DOCTOR OF PHILOSOPHY (1967) McM.J'l.STER UNIVERSITY (Chemistry) Hamilton, Ontario. TITLE: Nuclear Magnetic Resonance Studies of Boron Trifluoride and Some of Its Complexes AUTHOR: John Stephen Hartman, B.Sc. (Queen's University), M.Sc. (University of Ottav,a) SUPVERVISOR: Professor R. J. Gillespie NUMB8R OF PJ\GES: xi, 231 SCOPE Al\JD com:ENTS: Nuclear magnetic resonance spectroscopy has been used to study donor'.'"acceptor complexes of boron trifluoride with ketones, water, and methanol. Various NMR techniques have been applied which, in certain favourable cases, hDve yielded information on the stoichiometry, struc­ ture, relative stabilities, and reactions of the complexes in soluUon. A new method is proposed for the detection of BF complexes having low 3 formation constants. The me.gni tude and sign of the B-F. coupling constant in the tetra­ fluoroborate anion have been found to be dependent on the solvent. This solvent dependence is interpreted in terms of salvation and association of the ion in solution. 19 10 F spectra of BF over a ranc;e of temperatures agree well 3 with line shapes calculated, using Pople's expressions, for coupling of fluorine-19 with boron-10 (1=3) which is undergoing partial quadru­ pole relaxation. ii ACKNO\'/LEDGEMI~NTS The author wishes to thank his research director, Professor R. J. Gjllespie, for h:i.s ndvice and encoura.gement throue;hout this work. Thanks are also due to Dr. U.R.K. Rao v1ho collaborated in 10 the preparation of BF and provided the computer programme used 3 in calculating the spectra of this compound, to Dr. W. A. Whitla for advice and assistance in the operation of a high-vacuum system, to Mr. C. Schonfeld and Mr. J. Bacon for assistance with the NMR spectrometer, and to Professor G. W. King for providing the sample of lOBF3"CaF • In addition thanks are due to a number of fellow 2 students, post-doctoral fellows, and faculty members of the Depart­ ment of Chemistry for helpful discussions and assistance. The financial assist::mce which made this work possible is gratefully acknowledged. The National Research Council of Canada provided Scholarships for 1963-65, and the Department of University Affairs, Province of Ontario provided an Ontario Graduate Fellowship for 1965-66. iii TABLE OF CONTENTS CHAPT:SR I: Introduction•••• . 1 A. Nuclear Magnetic Resonance. 1 B. Donor-Acceptor Complexes of the Boron Trihalides • • • • • • • 12 C. High-Resolution NMR Studies of Boron Trihalide Complexes. • • • • • . 18 CHAPTEI< II: Experimental•••••••••••••••••• CHAPTER III: Dilute Solutions of Boron Trifluoride in Acetone. • • 40 Introduction . 40 Results and Discussion. 41 (i) Room Temperature l9F Spectra. 41 (ii) Low Temperature 19F Spectra of Fresh Solutions•.•••••••• 42 (iii) Low Temperature l9F Spectra of Partially Decomposed Solutions. • • • • • • • • • 46 (iv) Identification of l9F Peaks. 53 (v) Changes in the Rate of the Condensation Reaction . (vi) Kinetics of BF Exchange 64 3 . • . Theory•••• e • . 72 CHAPT.w IV: ?ne Adduct H o.BF •••••••••••••• 78 2 3 Introduction•••• . 78 Results and Discussion. 80 iv CHAPTER V: Exchange Reactions in the System Boron Tri­ fluoride-Methanol. A Modified·Interpre­ tation. • • • . 92 Introduction. 92 Discussion. 93 Results and Interpretation. 96 CHAPTER VI: BF, Exchange in Ketone.BF Complexes in 3 Methylene Chloride Solvent. • •• 103 Introduction. 103 Results and Discussion. • 103 (i) Room Temperature Proton Spectra•• 103 (ii) Low Temperature Proton Spectra •• 106 (iii) Fluorine-19 Spectra•• . 113 (iv) Boron-11 Spectra•••• 120 CHAPTER VII: Exchange of Fluorine among Boron Atoms in Methylene Chloride Solutions of Ketone.BF Complexes••••••••••••••••••3 123 Introduction••••••• 123 Results and Discussion. 123 (i) General. • • • • 123 (ii) Kinetic Studies•• 128 (iii) Possible Mechanisms for the Fluorine Scrambling••• 132 CHAPTER VIII:. BF - Catalysed Aldol Condensation Reactions 3 in Methylene Chloride Solutions of Acetone and of Mesityl Oxide. 137 Introduction••• 137 v CHAP'l'ER VIII Results and Di3cussion. 137 (continued) (i) Solutions of Simple Ke tones and BFy • 137 (ii) Solutions of Mesityl Oxide, Water, and BF3" . 139 (iii) Forward and Reverse Aldol l~eactions. 145 CHAPTBR IX: Weak Complexes of Boron Trifluoride. Introduction••• Results and Discussion••• (i) Boron Trifluoride in Fluorosulphuric Acid. 154 (ii) Boron Trifluoride in Methylene Chloride and other 11 Inert11 Solvents•• . • 158 (iii) Diphenyl Ether and Boron Trifluoride in Methylene Chloride. 163 Conclusion••••• CHAPTER X: Solvent Effects on the Boron-Fluorine Coupling Constant in the Tetrafluoroborate Ii.nion. • • 167 Introduction. Results•• . .. 171 (i) General. 171 (ii) Variation of J in AgBF in the ?/ater­ 4 Acetoni..trile System. • . • • ••• 172 (iii) Variation of J in AgBF in the \'later­ 4 Acetone and Acetonitrile-Acetone Systems•• . 177 (iv) Vario tion:::; in J with Temper':l.ture • • 178 Discussion•••• vi CHAPTER X (i) Change of Sign of J. • ~ • • (continued) (ii) Structural Interpretation of the Variations in J ••••••• . (iii) Correlation with the Solvent Dielectric Constant••• 190 (iv) Col1apse of B-F Splittings in Solvents of Low Dielectric Constant: Evidence for Quadrupole Relaxation••• 192 (v) The Exchange of Fluorine anong Boron Atoms. : • • • • • • • • • ••• 196 CHAPTER XI: Quadrupole Relaxation in a Nucleus of Spin 1=3. The 19F Nl1R Spectrum of lOBF • • • • • 201 3 Introduction. 201 Theory. • • 202 Results and Discussion. 209 Calculations••••••• 219 BIBLIOGR;,_PHY . 221 APPENDIX 228 vii LIST OF TABLES Table I Spin States of Three Equivalent Spin-1/2 Nuclei 6 II -80° l9F Chemical Shifts of Partially Decomposed Solutions of BF in Acetone 50 3 III Relative Areas of ''Free" and "Complexed" Methyl Peaks in Methanol-BF Solutions in Sulphur Dioxide 3 JV Proton Shifts to Low Field, and Changes in Ja.[3' on Complexation of Ketones with BF 106 3 V -80° Proton Chemical Shifts in Hethylene Chloride Solutions of Mesityl Oxide 142 VI Room Ter.,perature Proton ChemicnJ Shifts in Hethylene Chloride.Solutions after Reaction at 100° VII J Values in AgBF Solutions in '.'later, 11 19 4 Ace~;nifriie, and the 50:50 Mole Percent Solvent Mixture 175 VIII Correlation of 19F NMR spectral features with solvent dielectric constant 191 viii LIST OF FIGURES 1. Spin states of the J\X system 5 19 2. Room tern"9erature F spectra of BF in acetone 3 3. -80° l9,.,~ spectra_ of f res hly prepared so 1u·ionst · of BF in acetone 45 3 19 4. Low Temperature F spectra of a 1.SM solution of BF 2 in acetone, after one hour of reaction at room J temperature 47 5. l9F spectra between -26° and +27° of a 1. 5M solution of BF~ in acetone, after one hour of reaction at room temperature 48 6. -80° l9-F spectra of partially decomposed solutions of isotopically substituted BF in acetone 52 3 7. -80° l9F spectra of a l.lM solutfon of BF _in acetone after reaction at room temperature for various3 lengths of time 54 8. Plot of the changes in relative areas of the -80° l9F peaks (Figure 7) with reaction time at +22° 9. -80° l9F spectra of a solution of mesityl oxide and BF in acetone 59 3 10. -80° l9F spectra of a solution of BF~ in acetone-d , 6 before and after the addition of phorone 61 19 l]. Co:'.lputer fit of some F spectra of a partially de­ composed 1. 5M solution of BF in acetone 69 3 19 12. Com1mter fit of some F spectra of a partially de­ composed l.2M solution of BF in acetone-d6 71 3 13. -80° spectra of an acetone solution of water and BF , in the molar ratio 1:2 3 81 -80° snectra of acetone solutions of water and BF , 3 such th?- t [H o] ( ( BF ] 84 2 3 Plot of methyl chemical shifts vs. acid-base ratio for methano1-BF solutions in sulphur dioxide 99 3 ix Variation of ketone chemical shifts with the acid-base ratio, [BF ]/(ketone] 3 l?. Proton spectra of a methylene chloride solution of MIK and BF , in the molnr ratio 2:1 108 3 18. Arrhenius plot for BF exchange among donor sites 7 in a solution of acetone and BF in a 2:1 molar ratio in methyJ.ene chloride 3 111 19. Arrhenius plot for BF-z exchange between 1:1 complex and free BF in a soltition of pinacolone and BF in a 1:2 molar 3ratio in methylene chloride 3 11? 20. Variation of boron-11 peak widths with temperature 121 21. Collapse of the isotope shift in a methylene chloride solution of pinacolone and BF , of isotope ratio 3 lOB: llB == 52:48 125 22. Variation of reJntive J. 9F chemic,,1 shift of MIK.BF and H 0.BF with temperature 3 129 2 3 23. Arrhenius plot for fluorine exchange between MIK.BF and H 0.BF in methylene chloride 3 130 2 3 24. -80° proton spectra of the rethyl region of solutions of mesityl oxide in methylene chloride 25. Room temperature proton spectra of the methyl region of a solution of mesityl oxide, water,and BF in methylene chloride after various times of re~ction 147 26. Variation of 19F peak widths at half-height with temperature in BF~ solutions in methylene chloride and fluorosulphur1c acid 27. Variation of 19F peak widths at half-:10i€_';ht with teriperc:i.ture in BF solutions in methylene chloride, chloroform, and t~luene 161 28. Variation of l9F neak widths at half-hcie;ht v,ith temnera tures in methylene chloride solutions of di­ phenyl ether and BF 164 3 29.