Osmium alkane complexes: Synthesis, characterization, and isotopic labeling studies

Capra, Nicolas Edward

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https://hdl.handle.net/2142/120502 Copy

Description

Title

Osmium alkane complexes: Synthesis, characterization, and isotopic labeling studies

Author(s)

Capra, Nicolas Edward

Issue Date

2023-03-27

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

Girolami, Gregory S

Doctoral Committee Chair(s)

Girolami, Gregory S

Committee Member(s)

• Vura-Weis, Josh
• Olshansky, Lisa
• Fout, Alison

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Transition metal alkane complexes
• NMR spectroscopy
• isotopic perturbation of resonance
• alkane activation
• osmium complexes

Abstract

Treatment of the osmium(II) bromide complex (C5Me5)Os(dfmpm)Br, where dfmpm = (F3C)2PCH2P(CF3)2, in hot toluene with an excess of dipropyl- or dibutylzinc affords the osmium(II) alkyls (C5Me5)Os(dfmpm)(CH2CH2CH3) and (C5Me5)Os(dfmpm)(CH2CH2CH2CH3). Di(2‑methylpropyl)zinc affords a mixture of (C5Me5)Os(dfmpm)(CH2CH(CH3)2) and the β‑hydrogen elimination product (C5Me5)Os(dfmpm)H. Treating this crude product with iodine and triethylamine converts the hydride to the iodide complex (C5Me5)Os(dfmpm)I, from which the osmium 2-methylpropyl complex can be isolated by chromatography. This procedure can also be used to purify the previously reported osmium ethyl complex (C5Me5)Os(dfmpm)(CH2CH3). Low-temperature protonation of (C5Me5)Os(dfmpm)R, where R = CH2CH2CH3, CH2CH2CH2CH3, or CH2CH(CH3)2, in CDFCl2 with an excess of trifluoromethanesulfonic acid generates the corresponding σ-propane, σ-butane, and σ-(2-methyl)propane complexes, which are stable for hours at and below -110 °C. Each alkane ligand binds to the metal center through a single methyl group which has a 1H NMR chemical shift between δ -2.20 and -2.25 and a 3JHH constant that is about 1 Hz smaller than in the free alkane. The proximal and distal methyl groups of the bound propane and butane ligands exchange with one another, and a similar exchange process also occurs in the ethane analogue. For the ethane and butane complexes, ΔG‡ for exchange of the proximal and distal methyl groups at ‑115 °C is about 11 kcal mol‑1. The activation parameters for dissociation of each C1-C4 alkane ligand from the (C5Me5)Os(dfmpm)+ fragment have been determined; for all five complexes, ΔHdiss‡ is 16(3) kcal mol-1 and ΔSdiss‡ is 18(14) cal mol-1 K-1. A positive entropy of activation is expected for these dissociation processes. At -100 °C, ΔGdiss‡ is 12.79(9) kcal mol-1 for methane, 13.18(10) kcal mol‑1 for ethane, 13.30(10) kcal mol-1 for propane, 13.30(9) kcal mol-1 for butane, and 13.16(10) kcal mol-1 for 2‑methylpropane. The coordination modes of the propane and butane ligands have been determined through isotopic perturbation of equilibrium (IPE) experiments. The bound methyl group of the propane and butane ligands coordinates through one “bridging” hydrogen (i.e., κ1-H or η2-C,H); two “terminal” hydrogens of the bound methyl group do not interact with osmium. 1JCH for the bridging C-H bond is 75(5) Hz in propane and 76(2) Hz in butane; whereas for the two terminal C-H bonds, 1JCH is 143(3) Hz in propane and 143(2) Hz in butane. The chemical shift of the bridging hydrogen is δ -10.60(6) in propane and δ -10.36(7) in butane; the chemical shifts of the two terminal hydrogens are δ 1.93(3) and δ 1.84(4), respectively. As a result of zero-point energy effects, there is a difference in energy ΔE between the M···H‑CDR2 and M···D-CHR2 structures of these complexes; ΔE is 0.314(3) kcal mol‑1 for the propane complex and 0.325(3) kcal mol‑1 for the butane complex. For the ethane, propane, butane, and 2‑methylpropane complexes, the rotational exchange of the bridging and terminal hydrogens of the bound methyl group begins to slow down at -110 °C, and the corresponding barrier is about 0.5 kcal mol-1 higher than the rotational barrier of the corresponding free alkane. The trends in ΔGdiss‡, 1JCH, and ΔE indicate that methane is slightly less strongly bound and slightly less structurally perturbed than the other alkane ligands studied. These trends are nearly in agreement with the polarizabilities and the C-H orbital energies of the alkanes. However, the differences in 1JCH between the bound and free alkanes are too large to be due solely to dispersion forces (bond polarization or van der Waals interactions), so there must be considerable covalent d-σ mixing between osmium and the bridging C‑H bond. Differences in this covalent interaction are the most likely cause of the weaker binding of methane in this system.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

© 2023 Nicolas Edward Capra

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