The Species of Fehling's Solution

A species model for the copper(II)/L-(+)-tartaric acid (LH2, tartH(2)) aqueous system between pH 1.9 and 12.3 has been established by potentiometric, UV/Vis spectroscopic, solubility and crystallographic studies. Eight species were detected, including the oligonuclear major species Cu8L6H-10 in the neutral region and the mononuclear major species CuL2H-4 in the alkaline region, the latter being associated with Fehling's solution. The octanuclear complex was isolated as its lithium salt Li-7[Cu-8(L-tartH(-2))(4)(L-tartH(-1))(2)(H2O)(6)]NO3 center dot 19H(2)O (1). The "Fehling species" CuL2H-4 was crystallized as its sodium, potassium and caesium salts, namely K2Na4[Cu(L-tartH(-2))(2)]center dot 12H(2)O (2), Na-6[Cu(L-tartH-2)(2)]center dot 9H(2)O (3), Na-6[Cu(L-tartH-2)(2)]center dot 14H(2)O (4), and Cs-6[Cu(L-tartH-2)(2)]center dot 8H(2)O (5). Each of them shows a distorted square-planar environment of copper(II), similar to the related non-chiral cuprate [Cu(rac-tartH(-2)-kappa O-2(2), O-3)(2)](6-) (S. Albrecht, P. Klufers, Z. An-org. Allg. Chem. 2013, 639, 280-284). By comparing the components of the UV/Vis spectra of Fehling's solution and the solids, we confirmed the identity of the major solution species.