Polyhedron Vol. 6, No. 3, PP. 577-581, 1987 Printed in Great Britain 0277~5387/87 $3.00 + .oO 0 1987 Pergamon Journals Ltd zyxwvutsrq POTASSIUM-39 NUCLEAR MAGNETIC RESONANCE STUDY OF THE COMPLEXATION OF POTASSIUM THIOCYANATE BY DIBENZO-30-CROWN-10 IN NITROMETHANE HARALD D. H. STOVER, MARC ROBILLARD and CHRISTIAN DETELLIER* Ottawa-Carleton Chemistry Institute, University of Ottawa Campus, Ottawa(Ont.) KlN 9B4, Canada (Received 25 April 1986; accepted 15 August 1986) Abstract-A 2 : 1 potassium : dibenzo-30-crown-10 complex is formed in nitromethane sol- ution. Taking into account the ion-pairing equilibrium of KSCN in that solvent, an equi- librium constant for the 2 : 1 complex formation (& = 52 f 5) has been calculated. The characteristic K-39 relaxation rate and chemical shift of the 2 : 1 complex are compatible with an open structure of the crown, in which the two potassium cations are exposed to the solvent molecules and the thiocyanate counteranion. Large crown ethers are known to wrap around alkali metal cations in both solid state and solution. The wrapping of dibenzo-30-crown-10 (DB30ClO) around the potassium cation has been shown by the X-ray crystal structures of the potassium iodide DB30ClO’ and of the potassium thiocyanate DB30C 10 complexes. *The main difference between the X-ray structures of the uncomplexed and the potassium complexed DB30ClO consists in the con- version of four anti C-O conformations of the uncomplexed crown into four gauche C-O con- formations in the complex. ’ In solution, evidence has been given by H-l and C-13 NMR that the K+-DB30ClO complex has the same configuration as in the crystal, with complete desolvation of the cation upon complexation. 3*4 The Na+-DB30ClO complex displays a different con- figuration3 and complexes with other stoichio- metries (2: 1 and 3 : 2) are formed as we11.4*5 Dibenzo-24-crown-8 (DB24C8) wraps around the sodium cation, expelling the solvent molecules from the first solvation shell.6 With tetraphenylborate as the counteranion, (n + 1) : n Na+ : DB24C8 aggre- gates are formed to a small extent.7’8 Wrapping of ligands around the alkali metal cations is also observed in the case of acyclic polyethers. 9*10 *Author to whom correspondence should be addressed. K-39 is a quadrupolar nucleus (I = 3/2) whose receptivity is only 2.69 compared to C-13. This explains why potassium has been much less used than sodium for direct metal cation NMR studies. ’ ’ Shih and Popov have published a K-39 NMR study of potassium salts in non-aqueous solvents. ’ 2 Kin- etics studies of the complexation of the potassium cation with dibenzo-18-crown-613 and 18-crown- 6i4 have been carried out in various solvents. Neurohr et al. I ’ studied the complexation of K+ with several ionophores using K-39 NMR and the K- anion has been observed both in solution and in crystalline potassides.‘6,‘7 We report here a K-39 NMR study of the com- plexation of K+ by DB30ClO in nitromethane. We show that a 2 : 1 K+ : DB30ClO complex coexists with the 1 : 1 complex in nitromethane solutions. Taking into account the ion-pairing equilibrium of KSCN in that solvent, we calculate an equilibrium constant for the 2: 1 complex formation and the characteristic values of the relaxation rate and chemical shift of the 2 : 1 complex. EXPERIMENTAL After recrystallization from ethylacetate, dibenzo- 30-crown-10 (Parish Chem.) was vacuum-dried at 60°C for several hours. Potassium thiocyanate (Aldrich, 98% + ) was dried under vacuum at 60°C for at least three hours prior to use. Nitromethane 577