Fluxional behaviour in η3-allyl complexes of cr, mo and w as shown by magnetisation transfer difference spectroscopy (MTDS)

Abstract

(η3-allyl)4M2 (M  Cr, I;M  Mo, II) exist in solution as mixtures of two isomers (Ia/Ib  60/40; IIa/IIb  90/10). Each isomer has two inequivalent allyl groups which are trans to one another and bridge the two metal atoms. The other allyl groups are bonded to each one of the metals. In the major isomer these two allyl groups are cis, whereas in the minor isomer they are trans. At temperatures above 30°C for I (8O°C for II), interconversion of the isomers Ia and Ib (IIa and IIb) can be detected by magnetisation transfer difference spectroscopy (MTDS) (1H NMR 400 MHz). This interconversion proceeds by exchange of syn (anti) protons of the non-bridging allyl groups of Ia (IIa) with syn (anti) protons of the non-bridging allyl groups of Ib (IIb). In addition, in Ia and Ib syn—syn (anti—anti) exchange of the protons of the bridging allyl groups can be detected. (η3-allyl)4M (M  Mo, III; M  W, IV) shows fluxional behaviour at temperatures above 70°C for III (90°C for IV). Syn—anti and also syn—syn (anti—anti) exchange takes place. Each of these processes can be monitored separately by MTDS.