Synthesis and solution fluxionality of rhenium(I) carbonyl complexes of 2,4,6-tris(pyrazolyl)pyrimidines (L), [ReX(CO)(3)L] (X=Cl, Br or I). Isolation and identification of the dinuclear complex [{ReBr(CO)(3)}(2)L]

2,4,6-Tris(pyrazolyl)pyrimidines reacted with pentacarbonylhalogenorhenium(I) complexes to form stable mixtures of bidentate, chelate complexes of types fac-[ReBr(CO)(3)(tpzp)], fac-[ReBr(CO)(3) (tmpzp)] and fac-[ReX(CO)(3)(tdmpzp)] (X=Cl, Br or I) where tpzp=2,4,6-tris(pyrazol-1-yl)pyrimidine, tmpzp=2,4,6-tris(4-methylpyrazol-1-yl)pyrimidine and tdmpzp-2,4,6-tris(3,5-dimethylpyrazol-1-yl)pyrimidine. In solution the complexes involving co-ordination with the 2-pyrazolyl ring nitrogen rather than the 4- or 6-pyrazolyl ring nitrogens strongly predominated (>91%). Fluxional exchange between the two types of co-ordination species of tmpzp and tdmpzp was detected and measured quantitatively by dynamic NMR methods. Activation energies Delta G double dagger (298 K) for the fluxions were in the range 53-74 kJ mol(-1). Dinuclear transition-metal complexes of type [{ReBr(CO)(3))(2)L] (L=tmpzp of tdmpzp) were also isolated. Structures were attributed to these non-fluxional complexes in solution.