Synthesis and characterization of dirhodium(II,II) formamidinate complexes containing short-bite nitrogen ligands

The Rh24+ complex [Rh2(form)2(O2CCF3)2 (H2O)2] (form=N,N′-di-p-tolylformamidinate anion) easily reacts with the short-bite ligand pyrido[2,3-b]pyrazine, molar ratio 1:2, yielding the complex [Rh2(form)2(pyrido[2,3-b]pyrazine)2 (O2CCF3)2] which has been characterized by conductivity measurements and IR and 1H NMR spectroscopy. The complex exhibits a "lantern" type structure with the two nitrogen ligands coordinated, in a cisoid arrangement, at the equatorial positions across the dirhodium unit while the axial sites are occupied by two monoligated trifluoroacetate groups. Conductivity and IR data show that in DMSO the complex undergoes dissociation of both the axial ligands leading to the species [Rh2(form)2(pyrido[2,3-b]pyrazine)2(DMSO) 2]2+. The parent compound reacts also with 2-chloro-5,7-dimethyl-1,8-naphtyridine and 2,7-dichloro-5-methyl-1,8-naphtyridine leading to [Rh2(form)2(O2CCF3) (C10H9N2O)(H2O)] and [Rh2(form)2(O2CCF3) (C9H6N2ClO)(H2O)], respectively, in which the dirhodium unit is supported by three different bridging ligands. Microanalysis, 1H NMR and X-ray data unambiguously show that the reaction proceeds, unexpectedly, with elimination of a trifluoroacetate group and the substitution of a chlorine atom of the naphtyridine ligands by oxygen.