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Title: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
Author(s):Trinh, Brian Bang
Director of Research:Girolami, Gregory S
Doctoral Committee Chair(s):Girolami, Gregory S
Doctoral Committee Member(s):Burke, Martin D; Fout, Alison R; Rauchfuss, Thomas B
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Transition metal alkane complexes
NMR spectroscopy
isotopic perturbation of resonance
PCP pincer complexes
transition metal nitride precursors
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. Improvements were made to the procedures for the synthesis of (RPCP)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. Procedures for the synthesis of(tBuPCP)TiCl2 and (tBuPCP)CrCl2(THF), where (tBuPCP) is 2,6-bis(di-tert-butylphosphinomethyl)phenyl, were developed. The titanium compound reacts with LiBH4 to afford (tBuPCP)Ti(BH4)2. (tBuPCP)CrCl2(THF) can be desolvated to afford (tBuPCP)CrCl2 by application of heat under vacuum. (tBuPCP)CrCl2 undergoes reduction and decomposition when treated with LiBH4 but affords the chromium(II) product (tBuPCP)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.
Issue Date:2020-07-10
Rights Information:Copyright 2020 Brian B. Trinh
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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