Laura E. Harrington

Sterically hindered organometallics: multi-n-rotor (n=5, 6 and 7) molecular propellers and the search for correlated rotations

Abstract For the past several years, we have focused our attention on the design, syntheses and analyses of the dynamic behaviour of sterically encumbered complexes of the type (CnRn)MLx, where R=alkyl or aryl, n=5, 6 or 7, and MLx is an organometallic fragment. As archetypes of propellers or gear-wheels, the challenge is to construct molecules in which the internal rotations are restricted, and possibly even correlated. The incorporation of appropriate probes allows the independent measurement, by variable-temperature NMR spectroscopy, of the associated rotational barriers in these chiral arrays. In recent studies, we have successfully devised five-, six- and seven-membered ring systems that include naphthyl and other bulky organic substituents, and have further shown that an organometallic addendum can be used to monitor the stereodynamics of such conformationally flexible architectures.

Rhodium complexes of 3,4-di(β-naphthyl)-2,5-diarylcyclopentadienone: indirect detection of slowed naphthyl rotation

Abstract In the solid state, [(acetylacetonate)-3,4-di(β-naphthyl)-2,5-diarylcyclopentadienone]rhodium(I) ( 4a , aryl=phenyl, 4b , aryl= m -xylyl) exist as head-to-tail dimers in which each rhodium is bonded to the γ-carbon of the acetylacetonate ligand in the other half of the molecule. These dimers are also favoured in solution at low temperature, and restricted naphthyl rotation results in the formation of numerous conformers whereby the naphthyls can adopt either proximal or distal orientations. These rotamers can be detected by observation of the 1 H- and 13 C-NMR methyl resonances in the acetylacetonate moiety. Triphenylphosphine cleaves the dimers ( 4a and 4b ) to give the corresponding monomers, (C 4 Ar 2 (naphthyl) 2 CO)Rh(acac)PPh 3 ( 5a and 5b ), and the crystal structure of 5a exhibits a proximal/distal disorder of one naphthyl substituent. The rotamers of 5a and 5b are also present in solution, and the variable-temperature 31 P-NMR spectra yield activation enthalpies of 8.2±0.5 kcal mol −1 for naphthyl rotation in each compound.

5,7-Di-2-pyridyl-2,3-dihydro­thieno[3,4-b][1,4]dioxine

The title compound, C16H12N2O2S, was prepared by a Neigishi cross-coupling reaction to investigate the coordination chemistry of thiophene-containing ligands. In the molecule, the pyridine rings are twisted from the thiophene ring by 20.6 (1) and 4.1 (2)°. The six-membered dihydrodioxine ring is in a half-chair conformation.

N′-(3-Thienylmethyl­ene)pyridine-2-carbohydrazide

The title compound, C11H9N3OS, was prepared to investigate the coordination chemistry of thiophene-containing ligands as precursors to interesting metallopolymers. The molecule is nearly planar. The angle between the thiophene and pyridine rings is 8.63 (4)° and features the expected trans configuration about the imine bond. The structure is stabilized by a weak intermolecular N—H⋯O hydrogen bond. The distance between centroids of adjacent thiophene rings [3.67 (8) Å] suggests the presence of π–π interactions.