4-Acyl-5-pyrazolonato)titanium Derivatives: Oligomerization, Hydrolysis, Voltammetry and DFT study

Twenty 4-acyl-5-pyrazolonato (Q) titanium derivatives of varied nuclearity have been synthesized from Ti(OR) 4 or TiCl 4 and characterized with spectroscopic methods (IR, NMR, ESI-MS). While Ti-(β-diketonato) cleavage is not seen in isolated solids, Ti-O(alkoxy) (or Ti-Cl) bonds cleave upon hydrolysis, leading to several structural forms, including oligomers. Ionic Q species with no Ti, i.e., obtained after Ti-Q cleavage, are seen for some Ti-Q derivatives by ESI-MS, which also indicates a varied nuclearity for a given species, e.g., the isolated polynuclear [Q 2Ti-μ-O] n has several "n" values. Mononuclear Ti complexes are obtained under rigorous anhydrous conditions. The cis structures of the mononuclear species (Q T) 2Ti(OCH 3) 2, Q T = 3-methyl-4-(neopentylcarbonyl)-1-phenylpyrazol-5-onato have been analyzed with DFT methods. A trans influence is a major driving force that accounts for several sets of Ti-O bonds. One of the cis stereoisomers is 56 kcal/mol higher in energy than the other two. In contrast, all (Q T) 2TiCl 2 cis isomers show similar energies. Voltammetry of the mononuclear species (Q T) 2Ti(OnPr) 2 and the antitumor tetranuclear compound [(Q B) 2Ti-μ-O] 4, (Q B = 4-benzoyl-3-methyl-1-phenylpyrazol-5-onato) indicate that the Ti IV is less prone to reduction to Ti III in the latter (Ep c for the Ti IV/Ti III couple is -1.71 V and -1.46 V versus Fc +/Fc, respectively). Potential antitumor compounds having a Ti/Q ratio of 1:1 do not disproportionate, unlike the equivalent acetylacetonato derivatives, and are water-soluble