© 2020 Brian B. Trinh LOW-TEMPERATURE PROTONATION STUDIES OF TRANSITION METAL ALKYL COMPLEXES AND THE SYNTHESIS AND CHARACTERIZATION OF EARLY TRANSITION METAL PCP PINCER COMPLEXES AND TWO-COORDINATE ALKYLAMIDO COMPLEXES BY BRIAN B. TRINH DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry in the Graduate College of the University of Illinois at Urbana-Champaign, 2020 Urbana, Illinois Doctoral Committee: Professor Gregory S. Girolami, Chair Professor Martin D. Burke Associate Professor Alison R. Fout Professor Thomas B. Rauchfuss ABSTRACT We recently reported NMR studies of an osmium alkane complex, + [Cp*Os(DFMPM)(CH4)] , where Cp* is pentamethylcyclopentadienyl and DFMPM is bis(bis(trifluoromethyl)phosphino)methane. As part of an effort to continue these studies, the procedures to synthesize the DFMPM ligand and the Cp*Os(DFMPM)Br intermediate have been improved. A laboratory-scale procedure was developed to synthesize diethylzinc in which the α- CH2 positions are isotopically labeled with carbon-13 and/or deuterium; these reagents were then employed to synthesize isotopically labeled analogs of Cp*Os(DFMPM)Et. Protonation of Cp*Os(DFMPM)Et with HOTf in CDCl2F at -130 °C affords the alkane complex [Cp*Os(DFMPM)(CH3CH3)][OTf]. The dissociation of the ethane ligand follows first- order kinetics characterized by activation parameters of ΔH‡ = 14.4 ± 4.2 kcal/mol and ΔS‡ = 7 ± 20 cal mol-1 K-1. These values are within error for those of the methane analog, ΔH‡ = 14.9 ± 1.5 kcal/mol and ΔS‡ = 12.3 ± 8.8 cal mol-1 K-1. IPR studies of the ethane complex indicate that the ethane coordinates to [Cp*Os(DFMPM)]+ through a single hydrogen atom in either an η2- or κ1- fashion, depending on whether or not the carbon atom is significantly involved in the bonding interaction. The IPR studies afford chemical shifts of δT = 1.99 ± 0.17 and δB = -10.99 ± 0.32 for the terminal and bridging hydrogen atoms of the α-methyl group of the ethane ligand, respectively. δT for the ethane complex is significantly deshielded compared to the methane analog (δT = 0.39 ± 0.05) and δB for the ethane complex is significantly shielded compared to the methane analog (δB = -8.92 ± 0.17). These data support the hypothesis that the chemical shifts of the alkane ligand are significantly affected by diamagnetic anisotropy. The upfield chemical shift of the α-carbon in the 13C NMR spectra of the ethane and methane complexes can be explained in the same way, and are not necessarily indicative of a bonding interaction with osmium. ii R Improvements were made to the procedures for the synthesis of ( PCP)M(CH3), where RPCP is a 2,6-bis(dialkylphosphinomethyl)phenyl pincer ligand and M is a group 10 transition metal. Complexes where R is isopropyl were prepared with nickel, palladium, and platinum. Additionally, complexes where R is t-butyl or adamantyl were prepared for nickel. Protonation of these complexes with HOTf in CDCl2F did not afford observable alkane complexes. In the cases of nickel and palladium, free alkane was generated even at -130 °C; the bonding interaction between the metal and the alkane is evidently very weak. In the case of platinum, protonation was slow until the reaction was warmed to -60 °C, at which point free alkane was generated; no alkyl/hydride or alkane coordination complexes could be observed. More electron-rich metal centers may be required to prepare alkane complexes of group 10 transition metals that can be studied by NMR spectroscopy. tBu tBu tBu Procedures for the synthesis of( PCP)TiCl2 and ( PCP)CrCl2(THF), where ( PCP) is 2,6-bis(di-tert-butylphosphinomethyl)phenyl, were developed. The titanium compound reacts tBu tBu with LiBH4 to afford ( PCP)Ti(BH4)2. ( PCP)CrCl2(THF) can be desolvated to afford tBu tBu ( PCP)CrCl2 by application of heat under vacuum. ( PCP)CrCl2 undergoes reduction and tBu decomposition when treated with LiBH4 but affords the chromium(II) product ( PCP)CrCl when reduced by KC8. All compounds were characterized crystallographically. A multi-gram-scale synthetic procedure was developed for Mn(TMP)2, where TMP is 2,2,6,6-tetramethylpiperidide). Mn(TMP)2 is a potential precursor compound for the chemical vapor deposition of manganese nitride films. The compound was crystallographically characterized. The solid state structure of Mn(TMP)2 is isomorphous to that of its iron analog Fe(TMP)2. iii ACKNOWLEDGMENTS The journey to obtaining a Ph.D. is a long and arduous road that no one completes alone. The list of people who have supported me so that I can get this far is too long for me to put on paper. Thank you for all of your love and support. Having said that, it would behoove me to make a few special mentions. First, I would like to mention my committee members: Professor Girolami, Professor Burke, Associate Professor Fout, and Professor Rauchfuss. I was able to collect the data presented in this dissertation thanks to your continued support and guidance. How you managed to assist me while simultaneously guiding all of the other students who have you on their committees is a testament to your inclusion in the ranks of the world’s best. You are amazing. To the former Girolami group members, thank you for all of your work. Your accumulated knowledge and wisdom was the foundation upon which I, and future Girolami members, could build upon. You also provided me with invaluable examples for mentorship. I will carry your lessons with me as I go on to teach others. To the current Girolami group members, you are a bundle of curiosity. Thank you for making CLSL more than a workplace. I look forward to seeing what you accomplish both in and out of the labs. To the administrative assistants of the Department of Chemistry, you are magicians. You break through interpersonal barriers for breakfast. Your positivity is a beacon of hope for those of us slogging through the challenges associated with research. I don’t know how the department would function without you. iv To my family and friends, you mean the world to me. Thank you for giving me stability and encouragement. You stood by my side when I needed you most and celebrated my accomplishments with me. Xiling, you are my best friend and love. You picked me up during my darkest times. You call me out when I’m being stubborn and help me tackle my problems head on. Your influence has made me a better person. I’m excited to see how we develop through a lifetime of adventure together. I love you. v TABLE OF CONTENTS CHAPTER 1: ISOTOPIC PERTUBATION OF RESONANCE AND THE PREPARATION OF ISOTOPICALLY LABELED DIALKYLZINC REAGENTS .......................................................................................................1 CHAPTER 2: SYNTHESIS AND CHARACTERIZATION OF AN OSMIUM ETHANE COMPLEX .....................................................................................14 CHAPTER 3: LOW-TEMPERATURE PROTONATION STUDIES OF METHYL COMPLEXES OF GROUP 10 TRANSITION METALS ..............................78 CHAPTER 4: SYNTHESIS AND CHARACTERIZATION OF PCP PINCER COMPLEXES OF TITANIUM AND CHROMIUM ....................................102 CHAPTER 5: IMPROVED SYNTHESIS OF BIS(2,2,6,6-TETRAMETHYLPIPERI- DIDO) COMPOUNDS OF MANGANESE(II) AND IRON(II) ...................150 APPENDIX A: IPR ANALYSIS OF METAL-BOUND ALKANES ....................................173 APPENDIX B: PREPARATION OF NMR SAMPLES FOR LOW-TEMPERATURE PROTONATION WITH TRIFLIC ACID .....................................................186 vi CHAPTER 1 ISOTOPIC PERTUBATION OF RESONANCE AND THE PREPARATION OF ISOTOPICALLY LABELED DIALKYLZINC REAGENTS Introduction Isotopic labeling is a powerful tool for understanding chemical reactions. Isotopic labeling experiments can identify which bonds are made or broken during a chemical reaction, elucidate which bonds are involved in the rate-limiting step of a reaction, and give information about chemical structure. Particularly relevant to the last of these applications is isotopic perturbation of resonance (IPR). IPR can be used to determine the nature of agostic interactions with metal centers.1 Our group has previously used IPR to determine which of the following coordination modes describes the structure of methane coordinated to a transition metal center: Figure 1.1: The four coordination modes of methane to a single metal center. If the coordinated methane is frozen in place, the coordination mode of the bound methane can be determined simply from the relative 1H NMR integrals of the resonances due to the hydrogen atoms in the terminal C-H positions versus the C-H positions that bridge to the metal in the 1H NMR spectrum. However, if the terminal and bridging C-H positions are in rapid exchange on the NMR timescale, the resonance for the terminal and bridging C-H positions will coalesce into a single resonance positioned at the weighted average chemical shift. Because experimentally 1 only one resonance is seen, determination of the coordination mode by integration is not possible, and instead the coordination mode must be determined by other means. The observed chemical shift of the bound methane in the 1H NMR spectrum is a weighted average of the chemical shifts of the terminal (δT) and bridging (δB) C-H positions. Because C-D bonds have a lower zero-point energy than analogous C-H bonds, and the zero point energy difference is larger if the C-H stretching frequency is larger, deuterium atoms in a bound methane complex will prefer to reside in sites characterized by higher-frequency C-H stretching modes, i.e., in terminal positions as opposed to bridging positions. As a result, replacing increasing numbers of hydrogen atoms in the bound methane with deuterium will change the weight-averaged 1H NMR chemical shift of the remaining hydrogen atoms on the bound methane towards δB. How much the chemical shift changes with deuterium incorporation depends on (1) the difference between δT and δB, (2) the energy difference between having hydrogen or deuterium in the bridging site(s) (ΔEBI), (3) the temperature, and (4) the coordination mode of the methane.