Transition metal and actinide complexes with unsymmetrical alkyl chelates

Lastowski, Robert Joseph

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https://hdl.handle.net/2142/125716 Copy

Description

Title

Transition metal and actinide complexes with unsymmetrical alkyl chelates

Author(s)

Lastowski, Robert Joseph

Issue Date

2024-07-11

Director of Research (if dissertation) or Advisor (if thesis)

Girolami, Gregory S

Doctoral Committee Chair(s)

Girolami, Gregory S

Committee Member(s)

• Abelson, John R
• Mirica, Liviu M
• Vura-Weis, Josh

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• inorganic chemistry
• organometallic chemistry
• ligand field theory
• magnetic anisotropy
• chemical vapor deposition

Abstract

This thesis describes the synthesis and study of several transition and actinide metal complexes containing unsymmetrical alkyl chelate ligands that bind to a metal through one sp3-hybridized carbon atom and either one BH3 or NMe2 group. The electronic, fluxional, and magnetic properties of these compounds are described, and the potential applications of some complexes as chemical vapor deposition (CVD) precursors are investigated. Chapter 1 is a brief review of the syntheses, structures, reactivities, and applications of transition metal complexes bearing unsymmetrical alkyl chelating ligands. Chapter 2 describes new compounds of stoichiometry M(CH2NMe2BH3)3 (M = Ti, Cr, and Co), each of which contains three chelating boranatodimethylaminomethyl (BDAM) ligands. In all three compounds, the BDAM anion, which is isoelectronic and isostructural with the neopentyl group, is bound to the metal center at one end by a metal–carbon σ bond and at the other by one three-center M–H–B interaction. The crystal structures show that the d1 titanium(III) compound is trigonal prismatic (or eight-coordinate, if two longer-ranged M···H interactions with the BH3 groups are included), whereas the d3 chromium(III) compound and the d6 cobalt(III) compounds are both fac-octahedral. The Cr and Co compounds exhibit two rapid dynamic processes in solution: exchange between the Δ and Λ enantiomers and exchange of the terminal and bridging hydrogen atoms on boron. For the Co complex, the barrier for Δ/Λ exchange (ΔG⧧298 = 10.1 kcal mol–1) is significantly smaller than those seen in other octahedral cobalt(III) compounds; DFT calculations suggest that Bailar twist and dissociative pathways for Δ/Λ exchange are both possible mechanisms. The UV-vis absorption spectra of the cobalt(III) and chromium(III) species show that the ligand field splittings Δo caused by the M–H–B interactions are unexpectedly large, thus placing them high on the spectrochemical series (near ammonia and alkyl groups); their nephelauxetic effect is also large. The DFT calculations suggest that these properties of M–H–B interactions are in part a consequence of their three-center nature, which delocalizes electron density away from the metal center and reduces electron-electron repulsions. Chapter 3 describes the synthesis and characterization of two actinide alkyls: thorium and uranium compounds of stoichiometry M(CH2NMe2BH3)4 (M = Th, U) where CH2NMe2BH3– is the boronatodimethylaminomethyl group (abbreviated BDAM). The crystal structures of these compounds show that the coordination geometry of the metal center can be described in terms of a D2d dodecahedron, whose eight vertices are defined by the four metal-bound carbon atoms and the four boron atoms of the BH3 groups. Two of the BH3 groups in each compound bind to the metal centers in a κ2 fashion (i.e., by means of two B–H–M bridges) and two in a κ1 fashion. Thus, both compounds can also be described as ten-coordinate in which four carbon atoms and six hydrogen atoms form the inner coordination sphere. The structures of both complexes and the nature of the BH3–M interactions were investigated computationally. The NMR spectra of the diamagnetic thorium(IV) compound and paramagnetic uranium(IV) compound are discussed; both compounds are fluxional in solution even at −124 °C. Variable temperature magnetization studies of the paramagnetic uranium(IV) compound in the solid state are consistent with its tetravalent oxidation state. Chapter 4 describes the synthesis and characterization of the quadruply-bonded dimer Mo2(CH2NMe2BH3)4 in which each molybdenum(II) center is bound to two chelating boronatodimethylaminomethyl (BDAM) ligands. The BDAM anions, which are isoelectronic and isostructural with neopentyl groups, bind to the metal at one end by a metal–carbon σ bond and at the other by a three-center M–H–B interaction. Each BDAM ligand chelates to a single Mo atom so that the metal-metal bond is unbridged; the Mo-Mo distance is 2.114(2) Å. Solid-state structural and solution NMR data, analyzed via McConnell’s equation, gave a value for the magnetic anisotropy of the Mo-Mo quadruple bond in Mo2(BDAM)4 that is significantly smaller than those of Mo-Mo quadruple bonds reported for some other molecules. We used DFT-based calculations to show that the magnetic anisotropies associated with ligands (such as chloride groups and aryl rings) can greatly affect the NMR chemical shifts of reporter groups, so that ignoring their contributions leads to significant over-estimates of the anisotropy due just to the metal-metal bond. We propose a method to quantify and correct for the magnetic anisotropy effects arising from the ligands. Application of this method to Mo2(BDAM)4 indicates that the anisotropy arising from the BDAM ligands is relatively small, probably because they have low polarizabilities and do not engage in π bonding: our best estimate of magnetic anisotropy of the Mo-Mo quadruple bond in this molecule is about -800 × 10-36 m3 molecule-1. Chapter 5 describes the synthesis and characterization of three new compounds of the stoichiometry M[(CH2)3NMe2]2, where M = Ni, Pd, or Pt, each of which contains two chelating 3-dimethylamino-1-propyl ligands. All three compounds are isostructural in the solid state and adopt square-planar geometries as expected for four-coordinate d8 metal ions bound to strong field ligands. The nickel(II) and palladium(II) compounds decompose above -78 and 0 °C, respectively, but the platinum(II) compound has a thermolysis onset temperature of 130 °C. The Pd and Pt complexes are dynamic in solution: they undergo ring inversion with activation parameters of ΔHǂ = 4.9 ± 0.3 kcal mol-1 and ΔSǂ = -9.9 ± 1.9 cal mol-1 K-1 for the Pd compound and ΔHǂ = 4.7 ± 0.2 kcal mol-1 and ΔSǂ = -12.1 ± 1.6 cal mol-1 K-1 for the Pt compound. These parameters correspond to free energies of activation for 2 and 3 of ΔGǂ = 7.9 ± 0.1 and 8.3 ± 0.1 kcal mol-1, respectively, at 298 K. The Pt complex is relatively volatile and sublimes at 40 °C and 5 mTorr. In benzene solution, the Pt compound thermolyzes primarily through β-hydrogen elimination to form dimethyl(propenyl)amine and a platinum(II) hydrido aminopropyl intermediate, followed by reductive elimination from the latter to form dimethyl(propyl)amine and Pt0; 80 ± 10% of the hydrogen atoms and 75 ± 5% of the carbon atoms present in the precursor can be accounted for in the byproducts. The thermolysis of 3 in C6D6 follows first-order kinetics, with an activation free energy ΔGǂ of 29.9 ± 0.1 kcal mol-1 at 110 °C. Under CVD conditions, thin films grown from the Pt compound at 200 °C appear metallic and contain nanocrystalline Pt; analysis of the film growth byproducts by NMR spectroscopy suggest that the main decomposition pathway involves β-hydrogen elimination and reductive elimination steps, as seen in solution.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125716

Copyright and License Information

Copyright 2024 Robert Lastowski

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