Structure of Alkali Biphenyl Ion Pairs in Solution and in the Solid State

NMR experiments on sodium biphenyl (NaBp), potassium biphenyl (KBp) and ribidium biphenyl (RbBp), dissolved in various solvents are summarized with emphasis on the structural information which can be obtained from such experiments. The crystal structures of NaBp.2Tg (Tg = triglyme), KBp.2Ttg and RbBp.2Ttg (Ttg tetraglyme) are discussed, in relation to the NMR results obtained for these systems in solution. In the solid state these crystals may be considered to be built up of solvent separated ion pairs. Magnetic experi— ments are reported for single crystals of the three systems. Susceptibility measurements revealed a ferromagnetic coupling in NaBp.2Tg and KBp.2Ttg and an antiferromagnetic coupling in RbBp.2Ttg. All crystals exhibit an exchange narrowed ESR line, with an orientation dependent linewidth. From the resonance positions the molecular g tensor of the biphenyl anion could be derived. The orientation dependent linewidth is due to dipolar interactions between the spins. A quantitative explanation of it could not be given at present. INTRODUCTION The early work of Fuoss (1954, ref. 1) and Winstein (1954, ref. 2) suggested that ion pairs may exist in solution in two different distinct forms. In later work it has become customary to refer to them as the loose or solvent separated and the tight or contact ion pairs. During the last two decades these species have been studied in great detail, using optical and magnetic resonance techniques, and also conductivity methods. Especially in numerous experiments carried out by Szwarc et al. (3) it has been shown that ion pairs are well defined chemical species with their own physical properties. In 1970 Canters et al. (4) succeeded in preparing single crystals of the paramagnetic alkali biphenyl ion pairs. The crystal structure of one of its representatives viz, the rubidium biphenyl ion pair was recently solved at this laboratory (5). It appeared that the structure could be characterized as a solvent separated ion pair, in which the cation is completely surrounded by two tetraglyme molecules. Stucky et al. (6) reported the structures of some diamagnetic aromatic ion pairs, using tetramethylethylenediamine and bisquinuclidine as coordinating molecules. In these complexes, the cation is coordinated to the aromatic moiety and to the nitrogen atoms of the coordinating molecules, and hence these species belong to the category of contact ion pairs. Another class of interesting single crystals of diamagnetic ion pairs is formed by the alkali complexes derived from cyclooctatetraene and its derivatives (7). The crystal structures reveal that the alkali ions are located above the center of the planar eight—membered ring, at a distance of about 2.5 A, thus showing that these crystals also belong to the class of contact ion pair structures. In this paper we report the results of magnetic and X—ray diffraction measurements obtained on three alkali biphenyl (Bp) systems, viz. NaBp in triglyme (Tg), KBp in tetraglyme (Ttg) and RbBp in tetraglyme (Ttg). The chemical formula of Tg and Ttg is: CH3O(CH2CH2O)nCH3 with n = 3 and 4, respectively. For all the three systems single crystals could be prepared, with composition NaBp.2Tg, KBp.2Ttg and RbBp.2Ttg. The crystal structure of NaBp.2Tg has been solved recently and has not been reported yet. In this study the results of experiments on these systems in solution and in the solid state will be discussed. Special attention will be paid to the structural parameters of the three systems in the solid state in particular to those of the NaBp system. The characteristics of the latter will be compared with those of the KBp and RbBp systems.