Calorimetric studies of stereoselective vanadyl(IV) tartrate reactions

Abstract

A ΔH of −1·46±0·10 kcal/mole has been determined for the reaction 1 2 [( VO) 2(d- C 4 H 2 O 6) 2] 4− + 1 2 [( VO) 2(l- C 4 H 2 O 6) 2] 4− → [( VO) 2(d- C 4 H 2 O 6)(l- C 4 H 2 O 6)] 4− (“C 4H 2O 6”=tartrato(4-) ligand) in aqueous solution at 298°K in the concentration region [V(IV)]=0·1 to 0·2 M. That the ΔH value observed for the reaction [(VO) 2( l-C 4H 2O 6) 2] 4− + d-C 4H 4O 6 2− → [(VO) 2( d-C 4H 2O 6)( l-C 4H 2O 6)] 4− + l-C 4H 4O 6 2− in the presence of excess ligand is less negative than that observed for the ligand exchange reaction between the binuclear enantiomeric complexes indicates the presence of an exothermic side reaction—probably [(VO) 2(C 4H 2O 6) 2] 4− + 2C 4H 4O 6 2− → 2[(VO)(C 4H 3O 6) 2] 4−. A barrier to rotation of 3 kcal/mole has been estimated for the tartrato(4-) bridging ligand to account for the enthalpy difference between the active and racemic binuclear vanadyl(IV) tartrate(4-) complexes. A value of 24 is calculated for the equilibrium constant of the ligand exchange reaction between the enantiomeric vanadyl(IV) binuclear complexes from the experimental ΔH and the predicted ΔS of 1·37 cal/mole-deg.