(4-Acyl-5-pyrazolonato)titanium derivatives: Oligomerization, hydrolysis, voltammetry, and DFT study.

Twenty 4-acyl-5-pyrazolonato (Q) Ti derivatives of varied nuclearity were synthesized from Ti(OR)4 or TiCl4 and characterized with spectroscopic methods (IR, NMR, ESI-MS). While Ti-(beta-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 derivs. by ESI-MS, which also indicates a varied nuclearity for a given species, e.g., the isolated polynuclear [Q2Ti-m-O]n has several n values. Mononuclear Ti complexes were obtained under rigorous anhydrous conditions. The cis structures of mononuclear (QT)2Ti(OCH3)2, QT = 3-methyl-4-(neopentylcarbonyl)-1-phenylpyrazol-5-onato were 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 (QT)2TiCl2 cis isomers show similar energies. Voltammetry of mononuclear (QT)2Ti(OPr)2 and the antitumor tetranuclear compd. [(QB)2Ti-m-O]4, (QB = 4-benzoyl-3-methyl-1-phenylpyrazol-5-onato) indicate that the Ti(IV) is less prone to reduction to Ti(III) in the latter (Epc for the Ti(IV)/Ti(III) couple is -1.71 V and -1.46 V vs. 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.