Absence of 1,3-metallotropic shifts in M(CO)5 complexes of pyrimidine and 1,3,5-triazine (M = W, Mo, Cr)

M(CO)(5)L complexes with M=W-0, Mo-0 and Cr-0 and L = pyrimidine and 1,3,5-triazine were prepared by photochemical generation from the parent M(CO)(6) complexes in acetone. All six complexes are not fluxional among available N-donor sites from room temperature up to ca. 60 degrees C in CDCl3, and do not undergo rapid exchange with external excess free ligand. Thus, as in the case of the W(CO)(5)L complexes, a one-bond migration between N sites in a diazine occurs readily for pyridazine (e.g. a 1,2-metallotropic shift has been reported), but two-bond migrations for a 1,3-metallotropic shift is not seen with pyrimidine or triazine, just as the three-bond migration for a 1,4-metallotropic shift was not observed previously for the pyrazine complexes. The decomposition processes of the M(CO)(5)L complexes in CDCl3, with pyrazines is known to follow the order Cr (minutes lifetime) much greater than Mo (several hours) > W (days to weeks duration), but for the pyrimidine and 1,3,5-triazine complexes Cr and W are of comparable stability (stable for 1-2 weeks), and more so than the Mo analogue (stable for about 7-8 h). H-1 NMR data on all six complexes show that protons ct to the site of metallation are shifted downfield by 0.27-0.36 ppm compared to the free ligand. The magnitude of Delta delta for the effect of metals on the chemical shift follow the order W > Mo > Cr. The more remote H4 protons of pyrimidine or triazine exhibit the smallest downfield shift upon metallation by the M(CO)(5) moiety. For the Cr(CO)(5) derivative the H4 proton of pyrimidine and H5 of triazine are shifted upfield. The upfield shift effect for the H5, near absence of a net effect for H4 of pyrimidine, and the upfield shift observed for H4 of the triazine complex of the Cr series are attributed mainly to a strong resonance component with d pi back-donation for the Cr derivative, as well as a possible TIP influence on the chemical shift parameter, contributions that are attenuated for the Mo and W complexes with higher energy excited states. No evidence for eta(2)-type coordination for these d(6) complexes was obtained in contrast to Ru(II) d(6) chemical relatives which adopt eta(2) coordination to pyrimidines. (C) 1999 Elsevier Science S.A. All rights reserved.