Synthetic, Structural and High Field NMR Spectroscopic Studies of Arene-Chromium Complexes
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.