Molybdenum(VI) complexes of a few pyrimidine derived ligands and the study of metal mediated CN bond cleavage resulting in ligand transformation during complex formation

The syntheses of dioxomolybdenum(VI) complexes were attempted using the Schiff base ligands HL1 and HL2. The ligands were prepared by the condensation of 4,6-dimethyl 2-hydrazino pyrimidine with salicyl aldehyde (for HL1) and o-hydroxy acetophenone (for HL2). When MoO2(acac)2 was used as a metal precursor, HL1 yielded the normal ligand exchange product [MoO2(L1)Cl] (1) in a medium acidified by HCl. However, instead of the normal product, HL2 produced two bis chloro dioxomolybdenum(VI) complexes [MoO2(L3)Cl2] (2) and [Mo2O5(L3)Cl2] (3) in the same reaction medium. Here, L3 is a bidentate neutral ligand 2-(3,5-dimethyl-1-pyrazolyl) 4,6-dimethyl pyrimidine, which is afforded by a transformation of the used ligand. This ligand transformation reaction is probably triggered by molybdenum mediated CN bond cleavage in the hydrazone part of HL2 followed by cyclization with acetylacetone released from the metal precursor.Again, when MoO2(acac)2 was reacted with another related ligand H2L1′, a bis salicyl hydrazone of 2,4-dihydrazino pyrimidine, in acidic medium, [MoO2(L4)Cl] (4) was produced as the only product. Here, also a similar ligand transformation, but only to a partial extent, leads to the formation of the ligand HL4, the salicyl hydrazone of 2-hydrazino-4-(3,5-dimethyl-1-pyrazolyl) pyrimidine. HL4, thus produced in the reaction medium binds in a monoanionic fashion to the MoO22+ core to form the complex [MoO2(L4)Cl] (4). The complexes are characterized by elemental analyses, electronic spectra, IR, 1H NMR, magnetic measurements, EPR and by cyclic voltammetry. Complexes 1, 2, 3 and 4, as well as the ligand HL2, have been crystallographically characterized in order to probe into the structures and the ligand transformation process in a comprehensive way.

The syntheses of three mononuclear dioxomolybdenum (VI) complexes and a binuclear oxo μ-oxo molybdenum(VI) complex have been described. Three of these four complexes are the result of an in situ transformation of the used pyrimidine derived Schiff base ligands. The ligand transformation may have been initiated by Mo mediated CN bond cleavage.Figure optionsDownload as PowerPoint slide