Preparation and structural characterization of a series of ternary palladium(II) binuclear complexes containing triazolopyrimidinate bridges

The reaction of the [Pd(H 2 O) 2 (L–L)][NO 3 ] 2 electrophiles (L–L = ethane-1,2-diamine 1, propane-1,3-diamine 2, or 2,2′-bipyridine 3) with 4,7-dihydro-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine (Hmtpo) in 1∶1 molar ratio, in aqueous media (pH 7–8), yielded palladium(II) binuclear complexes of general formula [Pd 2 (µ-mtpo-N 3 ,N 4 ) 2 (L–L) 2 ][NO 3 ] 2 . The compounds have been structurally characterized by one- and two-dimensional 1 H and 13 C NMR spectroscopy and single-crystal X-ray diffraction (1 and 3). The crystal structures of both compounds show that the Pd atoms are located in nearly square-planar PdN 4 environments. Two mtpo ligands arranged in a head-to-tail orientation bridge the two metal centres, giving rise to short intermetallic contacts. The Pd · · · Pd separation appears to be modulated by the interactions between the auxiliary ligands L–L in adjacent co-ordination planes. Thus, substitution of ethane-1,2-diamine by planar 2,2′-bipyridine is responsible for a shortening of the intermetallic separation from 3.225(1) Å in compound 1 to 3.034(1) Å in 3. The NMR data suggest that the structures are preserved in solution.