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

Abstract

The syntheses of dioxomolybdenum(VI) complexes were attempted using the Schiff base ligands HL 1 and HL 2. The ligands were prepared by the condensation of 4,6-dimethyl 2-hydrazino pyrimidine with salicyl aldehyde (for HL 1) and o-hydroxy acetophenone (for HL 2). When MoO 2(acac) 2 was used as a metal precursor, HL 1 yielded the normal ligand exchange product [MoO 2(L 1)Cl] ( 1) in a medium acidified by HCl. However, instead of the normal product, HL 2 produced two bis chloro dioxomolybdenum(VI) complexes [MoO 2(L 3)Cl 2] ( 2) and [Mo 2O 5(L 3)Cl 2] ( 3) in the same reaction medium. Here, L 3 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 C N bond cleavage in the hydrazone part of HL 2 followed by cyclization with acetylacetone released from the metal precursor. Again, when MoO 2(acac) 2 was reacted with another related ligand H 2 L 1 ′ , a bis salicyl hydrazone of 2,4-dihydrazino pyrimidine, in acidic medium, [MoO 2(L 4)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 HL 4, the salicyl hydrazone of 2-hydrazino-4-(3,5-dimethyl-1-pyrazolyl) pyrimidine. HL 4, thus produced in the reaction medium binds in a monoanionic fashion to the MoO 2 2+ core to form the complex [MoO 2(L 4)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 HL 2, have been crystallographically characterized in order to probe into the structures and the ligand transformation process in a comprehensive way.