A Comparison of Neutral Mg2+-Selective Ionophores in Solvent Polymeric Membranes: Complex Stoichiometry and Lipophilicity

Abstract

Mg 2+ -selective ionophores were intensively investigated in order to obtain Mg 2+ sensors sufficiently selective for the determinations of human specimens. From basic considerations discussed before, 1,2 electrically neutral lipophilic amide derivatives were considered to be attractive candidates for designing magnesium-selective ionophores and sensing layers. A set of amides were synthesized and investigated, whereof the di-and tripodal malondiamides showed attractive features. [3][4][5][6][7][8][9][10][11][12] The discrimination of calcium, potassium and sodium ions by these ionophores was of primary interest. For comparability of different ionophores and membranes, the procedures for determining the selectivity coefficients were standardized according to IUPAC. 2,13 Bis(malondiamides) were the first compounds which showed a pronounced preference of magnesium ions. [5][6][7] Membrane electrodes containing these ligands achieved selectivities in the range of log K pot Mg,Ca >-1.0 (SSM) in the presence of 120 mol% of lipophilic borate salt in solvent polymeric membranes.Tris(malondiamides) were expected to improve the discrimination of alkaline over alkaline earth ions, since Mg 2+ is typically involved in octahedral coordination and is supposed to form 1:1 complexes with a tripodal ligand. Using ionophores of this class, the electrodes selectively responded to Mg 2+ discriminating Ca 2+ by more than a factor of ten (SSM log K pot Mg,Ca <-1). Attaching three secondary malondiamide units to a benzene core e.g. in ETH 3832 (see Fig. 1), improved the selectivity for magnesium over calcium ions to a significant extent.9,10 The corresponding, more lipophilic adamantylamide derivative, ETH 5506, shows satisfying selectivity of Mg 2+ against Ca 2+ , K + and Na + (SSM, log K pot Mg,Ca =-1.8, log K pot Mg,K =-2.9 and log K pot Mg,Na = -4.1) for physiological applications. 2,11,12 Recently, malondiamide derivatives of diazacrown ethers were synthesized and studied by Suzuki et al. 14 Compared to ETH 5506, K22B5 shows even higher Mg 2+ selectivity against Ca 2+ (SSM, log K pot Mg,Ca =-2.5), but poor discrimination of monovalent cations (SSM, log K pot Mg,K =-1.5 and log K pot Mg,Na =-3.2). The addition of lipophilic borate salt (e.g. tetraphenylborate salt) to the neutral carrier-based membranes provides lipophilic anionic sites in the polymeric electrode membranes, which support the selective extraction of Mg 2+ , and have been shown also to be favourable in other aspects. 2,15,16 The concentration of anionic sites affects the selectivity of an electrode, primarily by controlling the concentrations of both the exchangeable cations, and the free ionophore available for complexation at the membrane/solution interface. The optimum composition of a Mg 2+ -selective membrane with respect to the borate/ligand ratio is estimated by taking the charge number of interfering ions and the theoretical stoichiometry of ion-ligand complex into account. 16 Referring to the theoretical treatment of the optimum ratio of borate to ligand,...

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