²³Na nuclear magnetic resonance study of the complexation of sodium tetraphenylborate by [5,5]‐dibenzo‐30‐crown‐10 in nitromethane, acetonitrile, acetone and pyridine

Abstract

The nature of the (Na–DB30C10)+ complex has been studied by 23Na NMR in four non‐aqueous solvents: nitromethane (NM), acetonitrile (AN), acetone (AC) and pyridine (PY). The equilibrium constant of formation (Kf) of (Na‐DB30C10)+ in AC, AN and PY was determined at 300 K: log Kf = 3·9 ± 0·3, 3·4 ± 0·5 and 3·0 ± 0·1, respectively. In cases such as in pyridine, where exact values of the rate constants for the chemical exchange could not be calculated, limiting values were determined giving k−1 (the rate constant for complex dissociation) > 6·6 × 10−4 s−1 and kc (the rate constant for the formation of the complex) > 5 × 107 l mol−1 s−1. Evidence was found for the formation of a 2 : 1 (Na2–DB30C10)+2 complex in nitromethane. It has been previously shown that for (Na–DB24C8)+ the first coordination sphere of Na+ is exclusively filled by the oxygens of the crown ether, whereas for (Na–DB18C6)+ there is participation of at least one solvent molecule. The first coordination sphere of Na+ in (Na–DB30C10)+ consists of one or several molecules of solvent in addition to a number of oxygens from the crown ether. The soft wrapping of the Na+ by DB30C10 is in constrast with the hard wrapping of Na+ by DB24C8 and of K+ by DB30C10.