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

The nature of the (Na–DB30C10) + complex has been studied by 23 Na NMR in four non‐aqueous solvents: nitromethane (NM), acetonitrile (AN), acetone (AC) and pyridine (PY). The equilibrium constant of formation ( K f ) of (Na‐DB30C10) + in AC, AN and PY was determined at 300 K: log K f = 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 k c (the rate constant for the formation of the complex) > 5 × 10 7 l mol −1 s −1 . Evidence was found for the formation of a 2 : 1 (Na 2 –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.