Synthesis, Characterization, and Complexation of Tetraarylborates with Aromatic Cations and Their Use in Chemical Sensors Terhi Alaviuhkola,* [a] Johan Bobacka, [b] Maija Nissinen, [c] Kari Rissanen, [c] Ari Ivaska, [b] and Jouni Pursiainen [a] Introduction Tetraphenylborate was introduced in analytical chemistry in the 1940s, when it was used for the precipitation of mono- cations such as those of potassium, rubidium, cesium, and quaternary ammonium compounds. [1] Tetraphenylborate is an example of a simple organic precipitating reagent, but there have also been numerous studies of other substituted borates that form saltlike precipitates. [2] The investigation of borate complexes containing organic cations as guest mole- cules has not attracted a great deal of interest, although a variety of substituted borates bearing aryl groups have been synthesized. [3, 4] Electrochemical sensors constitute an impor- tant group of chemical sensors that are attractive for practi- cal applications allowing the use of small-size, portable, and low-cost instrumentation. [5, 6] Potentiometric ion sensors (ion-selective electrodes, ISEs) based on neutral or charged carriers (receptors, ionophores) are one of the oldest and most successful types of chemical sensors in routine use today, especially in clinical analysis. [7] These ion sensors are normally based on plasticized polymer membranes contain- ing specific ionophores and ionic additives. [8] Tetraphenylborate derivatives have been used as ionic ad- ditives in ISEs, initially just to reduce the anionic interfer- ence observed in the presence of lipophilic anions (Donnan exclusion). [8] They cannot form specific, strong ion pairs due to their shielded negative charge, but they nevertheless seem to play an active role as complexing agents, in addition to bringing negative charge to the sensor surface. For exam- ple, it has recently been shown that the Hg 2 + interference on Ag + -ISEs can be reduced by six orders of magnitude when replacing tetrakis(4-chlorophenyl)borate by a weakly Abstract: Five aromatic borate anions, namely tetrakis(4-phenoxyphenyl)bo- rate (1), tetrakis(biphenyl)borate (2), tetrakis(2-naphthyl)borate (3), tetra- kis(4-phenylphenol)borate (4), and tet- rakis(4-phenoxy)borate (5), have been prepared and tested as ion-recognition sites in chemical sensors for certain ar- omatic cations and metal ions. To gain further insight into the complexation of the cations, some complexes have been prepared and structurally character- ized. The complexation behavior of 1 and 2 towards N-methylpyridinium (6), 1-ethyl-4-(methoxycarbonyl)pyridinium (7), tropylium (8), imidazolium (9), and 1-methylimidazolium (10) cations has been studied, and the stability con- stants of the complexes of 1 with cat- ions 6 and 8 have been measured to compare them with the values for the previously studied complexes of tetra- phenylborate. The structures of the borate anions and their complexes have been characterized by NMR and mass spectrometric methods. X-ray crystal structures have been deter- mined for potassium tetrakis(4- phenoxyphenyl)borate (K + ·1), N-meth- ylpyridinium tetrakis(4-phenoxyphen- yl)borate (6·1), 1-ethyl-4-(methoxycar- bonyl)pyridinium tetrakis(4-phenoxy- phenyl)borate (7·1), tropylium tetra- kis(4-phenoxyphenyl)borate (8·1), and imidazolium tetrakis(biphenyl)borate (9·2). The results show that borate de- rivatives are potential candidates for a completely new family of charged car- riers for use in cation-selective electro- des. Keywords: aromatic cations · borates · charged carrier · ion-selec- tive electrode · molecular recogni- tion · noncovalent interactions [a] T. Alaviuhkola, Prof. Dr. J. Pursiainen University of Oulu, Department of Chemistry P.O. Box 3000, 90014 Oulu (Finland) Fax: (+ 358) 8-553-1603 E-mail: Terhi.Alaviuhkola@oulu.fi [b] Dr. J. Bobacka, Prof. Dr. A. Ivaska bo Akademi University, Process Chemistry Centre c/o Laboratory of Analytical Chemistry, 20500 bo-Turku (Finland) [c] Dr. M. Nissinen, Prof. Dr. K. Rissanen University of Jyväskylä, Department of Chemistry P.O. Box 35, 40351 Jyväskylä (Finland) Chem. Eur. J. 2005, 11, 2071 – 2080 DOI: 10.1002/chem.200400992  2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2071 FULL PAPER