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

• Published in: Inorganica Chimica Acta 1999-11, Vol.295 (1), p.56-63
• Main Authors: Granifo, Juan, Bird, Steven J, Orrell, Keith G, Osborne, Anthony G, Šik, Vladimir
• Format: Article
• Language: English
• Subjects: Dynamic NMR; Pyridine complexes; Rhenium complexes
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(99)00306-0

The complexes fac-[ReCl(CO) 3L] [L= ortho or para substituted methyl or tbutyl aromatic derivatives of 2,6-bis[1-(phenylimino)ethyl]pyridine (BIP)] were synthesised. In solution all complexes exist as bis-imine E, E and E,Z isomers, with the former greatly predominating (>85% in most cases). 1,4-Metallotropic shifts of the ReCl(CO) 3 moiety between the ligand N donor pairs occur in all cases. In the 1H NMR spectra of the methyl- and the para- tBu aromatic derivatives of BIP these shifts were detected as exchange between equivalent E, E forms of the complexes. In the case of the ortho- tBu derivative, where the E, Z-isomer was more in evidence (21% at 303 K), E, E⇌ E, Z isomerisation occurs rather more readily than the accompanying metallotropic shifts. Rates and activation energies for the detected dynamic processes were based on NMR bandshape analysis. The results indicated that Me and tBu substitution of the phenyl rings in BIP had a negligible effect on the Re IN 1,4-metallotropic shifts, whereas ortho substitution of the rings, particularly by tBu had a considerable influence on the relative proportions of E,E and E,Z conformers, presumably because of steric effects.