Synthesis and dynamic NMR studies of stereochemical non-rigidity in rhenium(I) complexes of 2,6-bis[1-(phenylimino)ethyl]pyridine derivatives

The complexes fac-[ReCl(CO)(3)L] [L=ortho or para substituted methyl or 'bu tyl aromatic derivatives of 2,6-bis[1-(phenylimino)ethyl]pyridine (BIP)] we re synthesised. In solution all complexes exist as bis-imine E,E and E,Z is omers, with the former greatly predominating (>85% in most cases). 1,4-Meta llotropic shifts of the ReCl(CO)(3) moiety between the ligand N donor pairs occur in all cases. In the H-1 NMR spectra of the methyl- and the para-Bu- t aromatic derivatives of BIP these shifts were detected as exchange betwee n equivalent E,E forms of the complexes. In the case of the ortho-Bu-t deri vative, where the E,Z-isomer was more in evidence (21% at 303 K), E,E rever sible arrow E,Z isomerisation occurs rather more readily than the accompany ing metallotropic shifts. Rates and activation energies for the detected dy namic processes were based on NMR bandshape analysis. The results indicated that Me and Bu-t substitution of the phenyl rings in BIP had a negligible effect on the Re-t-N 1,4-metallotropic shifts, whereas ortho substitution o f the rings, particularly by Bu-t had a considerable influence on the relat ive proportions of E,E and E,Z conformers, presumably because of steric eff ects. (C) 1999 Elsevier Science S.A. All rights reserved.