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

Abstract

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-methylpyrazole-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 ΔG^‡(298 K) for the fluxions were in the range 53–74 kJ mol^–1. Dinuclear transition-metal complexes of type [{ReBr(CO)₃}₂L](L = tmpzp or tdmpzp) were also isolated. Structures were attributed to these nonfluxional complexes in solution.