Structural study and solution integrity of dioxomolybdenum(VI) complexes with tridentate Schiff base and azole ligands

Four new molybdenum complexes [Mo ^VIO ₂(L ¹)(Him)] (1), [Mo ^VIO ₂(L ¹)(3-MepzH] (2), [Mo ^VIO ₂(L ²)(3-MepzH)] (3), and [(Mo ^VIO ₂) ₂(μ-L ³)(MeOH) ₂] (4) were synthesized and characterized by IR, NMR, ESI-MS, and single-crystal structure analysis [H ₂L ¹ = 2-(salicylideneamino)-2-methyl-1-propanol, H ₂L ² = 2-(3-methoxysalicylideneamino)-2-methyl-1-propanol, H ₄L ³ = 1,7-bis(salicylidene)dihydrazide malonic acid, Him = imidazole and 3-MepzH = 3-methylpyrazole]. In all four structures the molybdenum atom has a distorted octahedral coordination with the three meridional donor atoms from the Schiff base di- or tetraanion (L ^1,2) ^2-/(L ³) ^4- and one oxo group occupying the sites of the equatorial plane. The other oxo group and the azole or methanol molecule occupy the apical sites. In 1-3 two centrosymmetrically related molecules form a hydrogen-bonded pair through the (azole)N-H···O(alkoxo) interaction. Additional crystal packing appears to be controlled mostly by π stacking between the aromatic rings of the salicyl moiety. ESI-MS investigations reveal that the integrity of complexes 1-4 is largely retained in methanol solution. At the same time evidence is provided that di- to tetranuclear oligomers of formula [{Mo ^VIO ₂(L)} _x] and [{Mo ^VIO ₂(L)} _x(3-MepzH)] with L = L ¹,L ², x = 2,3,4 are present simultaneously with 2 and 3 in methanol solution, respectively the tetranuclear species [{(Mo ^VIO ₂) ₂(L ³)} ₂] with 4.