Delving into the complex picture of Ti(IV)–citrate speciation in aqueous media: Synthetic, structural, and electrochemical considerations in mononuclear Ti(IV) complexes containing variably deprotonated citrate ligands

Abstract

The aqueous reaction of TiCl 4 with citric acid at pH ∼ 4 (KOH), led to the surprising isolation of a species assembly K 3[Ti(C 6H 6O 7) 2(C 6H 5O 7)] · K 4[Ti(C 6H 5O 7) 2(C 6H 6O 7)] · 10H 2O ( 1). The same system at pH ∼ 3 (neocuproine), led to the crystalline material (C 14H 13N 2) 2[Ti(C 6H 6O 7) 3] · 5H 2O ( 2), while at pH 5.0 (NaOH), afforded Na 3[Ti(C 6H 6O 7) 2(C 6H 5O 7)] · 9H 2O ( 3). Analytical, spectroscopic and structural characterization of 1, 2 and 3 revealed their distinct nature exemplified by mononuclear complexes bearing variably deprotonated citrates bound to Ti(IV). Solid-state 13C MAS NMR spectroscopy in concert with solution 13C and 1H NMR on 3 provided ample evidence for the existence of bound citrates of distinct coordination mode to the metal ion. Cyclic voltammetry defined the electrochemical signature of complex 2, thereby projecting the physicochemical profile of the species formulated by the aforementioned properties. Comparison of cyclic voltammetric data on available discrete Ti(IV)–citrate species depicts the electrochemical profile and an E 1/2 value trend of the species in that binary system’s aqueous speciation, further substantiating the redox behavior of mononuclear Ti(IV)–citrate species in a pH-sensitive fashion. Collectively, the well-defined discrete species in 1– 3 reflect and corroborate a synthetically challenging yet complex pH-specific picture of the aqueous Ti(IV) chemistry with the physiological citric acid, and shed light on the pH-dependent speciation in the binary Ti(IV)–citrate system.