Biphenyl tricarbonylchromium complexes. Part 7. Preparation, static and dynamic stereochemistry of mono- and bis-tricarbonylchromium complexes of o,o′-bridged biphenyls

Abstract

Mono- and bis-Cr(CO)₃ complexes of 9,10-dihydrophenanthrene (1) and the corresponding ketone (3)(with a CH₂COCH₂ bridge) and their o,o′-dimethyl derivatives (2) and (4) have been prepared. Compounds (1) and (3) give exo-complexes whereas (2a) and (4) afford a 50:50 mixture of exo- and endo-isomers separable by medium pressure chromatography. According to lanthanide induced shift n.m.r. spectroscopy (LIS) complexation does not change the torsional angles of the biphenyl system to a significant degree. Isomeric ratios and relative (exo, endo) configurations were determined from ¹H n.m.r. spectra. The exo⇌endo equilibria are determined by steric interactions between bridge, ring (methyl) protons and the Cr(CO)₃ groups. Kinetic studies on the interconversion rate of the isomeric monocomplexes of dimethyldihydrophenanthrene (2) revealed that complexation decreases the inversion barrier from ca. 97 [in (2)] to ca. 91 kJ mol^–1(in the complex).