Date of Award
2-1990
Degree Type
Thesis
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
Supervisor
Professor M.J. McGlinchey
Abstract
The reaction of chromium hexacarbonyl with arenes yields organometallic complexes of the type (arene)Cr(CO)₃. Complexation to naturally occurring chiral molecules gives diastereomers which can be differentiated by NMR spectroscopy. Typically, the α- and β-isomers of (methyl ℴ-methylpodocarpate)Cr(CO)₃ have been assigned by using ¹H two-dimensional NMR techniques at 500 MHz.
Tricarbonylchromium complexes of 1,3,5-triphenylbenzene and of hexaphenylbenzene (HPB) have been characterized by NMR spectroscopy. The mono-, bis- and tris-Cr(CO)₃ complexes of C₆H₃Ph₃ exhibit rapid phenyl rotation but (hexaphenylbenzene) Cr(CO)₃ displayed restricted rotation. At -80°C, the chromium-bonded phenyl is effectively orthogonal to the central ring and ΔG# for the fluxional process is = 12.2 kcal molˉ¹ (≈ 51 kJ molˉ¹), a value much lower than those of non-complexed hexa-aryl benzenes bearing ortho or meta substituents.
Sterically crowded chromium complexes of hexaethylbenzene (HEB), (HEB)Cr(CO)₂NO⁺ and (HEB)Cr(CO)(CS)(NO)⁺, exhibit restricted rotation about the arene-chromium bond below -100°C. In the variable-temperature ¹³C NMR spectra of (HEB)Cr(CO)(CS)(NO)⁺ each room temperature singlet observed for the methyl, methylene and arene ring carbons is split into six equally intense resonances at -105°C. This provides the first unequivocal demonstration of slowed tripodal rotation in a chromium complex. Two fluxional processes are observed; the lower energy one (ΔG# ≈ 9.4 kcal molˉ¹ or 39 kJ molˉ¹) involves intereonversion within each distal and within each proximal set. The second process (ΔG# ≈ 11.4 kcal molˉ¹ or 48 kJ molˉ¹) requires exchange between proximal and distal environments.
The protonation of (HEB)M(CO)₃, M = Cr or W, yields species in which the proton directly bonded to the metal atom approaches along the three-fold axis of the molecule. In less sterically crowded systems the HM(CO)₃ fragment is fluxional and adopts a square-based pyramidal geometry.
Recommended Citation
Mailvaganam, Bavani, "Synthetic, Structural and High Field NMR Spectroscopic Studies of Arene-Chromium Complexes" (1990). Open Access Dissertations and Theses. Paper 1876.
http://digitalcommons.mcmaster.ca/opendissertations/1876