Zinc complexes of 3-pyrimidinyl-benzo-1,2,4-thiadiazine

Abstract

The synthesis of the novel redox active ligand 3-(2′,6′-pyrimidine)-benzo-1,2,4-thiadiazine (pmbtdaH) is reported. The radical pmbtda can be prepared by in situ 1e− oxidation and its radical character confirmed by EPR spectroscopy, along with DFT calculations of the spin density distribution. Reaction of pmbtdaH with ZnCl2·2H2O in a 1:1 mol ratio afforded the dinuclear complex Zn2Cl4(pmbtdaH)2 (1) in which the two trigonal bipyramidal metal centres are linked via μ2-bridging Cl− ligands while the pmbtdaH ligand binds in an N,N′-chelate fashion via N(4) and the pyrimidinyl N(1) atom. The outcome of the reaction of pmbtdaH with Zn(hfac)2 (hfac = hexafluoroacetylacetonate) proved sensitive to the crystallization method; the mononuclear octahedral complex Zn(hfac)2(pmbtdaH) (2) in which the ligand coordinates in a similar N,N′-chelate fashion to that observed in 1 was isolated by layering a concentrated CH2Cl2 solution with hexane. Conversely the same reaction mixture afforded the tetranuclear complex Zn4(hfac)6(pmbtdaox)2 (3) when left undisturbed in a sealed vial. In 3 the ligand is both deprotonated and oxidized forming an anionic ligand in which there are two potential N,N′-chelate pockets and an additional S-oxide donor. The pmbtdaox− ligand bridges between a Zn(hfac)2 and a Zn(hfac) unit to form Zn2(hfac)3(pmbtdaox). Two such Zn2(hfac)3(pmbtdaox) units dimerise through the S-oxide oxygen atom to form a centrosymmetric tetranuclear Zn4(hfac)6(pmbtdaox)2 cluster. The structures of 1–3 have been determined by X-ray diffraction, elemental analysis, IR and UV–Vis spectroscopies as well as 1H NMR spectroscopy for complex 2.