This paper reports an integrated chemical/physicochemical investigation of the aqueous chemistry of Cp2TiCl2, Cp2VCl2, and Cp2ZrCl2, employing high-field FT NMR, chloride potentiometry, pH titrimetry, and electrical conductivity. Experimental conditions include those employed previously to study the hydrolysis of cis-dichlorodiammineplatinum(II) (“cisplatin”) and those approximating physiological. In unbuffered aqueous 0.32 M KNO3 solution at 37 °C, the order of decreasing hydrolytic stability of the M-(η5-C5H5) bond in the Cp2MCl2 complexes (monitored by high-field FT NMR) was found to be V (kinitiai≤ 3.0 × 10-3 h-1) > Ti (kinitia| = 6.4 (1) × 10-3 h-1) ≫ Zr (knitial = 3.8 (1) × 10-2h-1). Of the three complexes studied, only Cp2VCl2 was found to possess a stable M-(η5-C5H5) bond at physiological pH. Addition of any of the Cp2MCl2 complexes to water (pure or 0.32 M KNO3) results in rapid chloride ion dissociation with approximate half-lives for the loss of the second chloride (the first is too rapid to measure) of 50 min (M = Ti), 30 min (M = Zr), and 24 min (M = V) in contrast to the relatively slow chloride hydrolysis observed with cisplatin. Chloride dissociation is also more extensive than that in cisplatin. Equilibrium chloride ion concentration measurements indicate that the equilibrium constant (K1) for the first Cp2MCl2 chloride dissociation is too large to measure, while K2 = 4.2 (2.7) × 10-2M (M = Ti) and 2.7 (1.2) × 10-3 M (M = V). Titrimetric studies indicate that the acidity of the Cp2M2+-bound water molecules is uniformly more acidic than the metal-bound water molecules of cis-[Pt(NH3)2(H2O)2]2+ (Ti, pKa= 3.5 (5) and 4.35 (9); V, pKa = 4.73 (3) and 5.15 (13)). Implications of these results for the observed biological activity of the Cp2MCl2 complexes are briefly discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry