Thiourea and pyridine derivatives as entering and leaving groups in reversible chelate ring-opening and ring-closure substitution reactions of trans-dichlorobis(O-dimethylaminophenyldimethylarsine-NAs)-rhodium(III) complexes

Abstract

The kinetics of the reversible reaction (i) have been studied in methanol [X = thiourea, NN′-diphenylthiourea, trans-[Rh(L)₂Cl₂]⁺+ X ⇌mer-[Rh(L)(L′)Cl₂X]⁺(i), pyridine (py), 3Me-py, 3Cl-py, 3CN-py, 4CN-py, 2Me-py, 2,4Me₂-py, or 2,4,6Me₃-py; L and L′=o-dimethylaminophenyldimethylarsine-NAs and -As respectively]. The system obeys rate law (ii). The rate constant of the –d[Rh(L)₂Cl₂⁺]dt=k_(t→m)[Rh(L)₂Cl₂⁺][X]–k_(m→t)[Rh(L)(L′)Cl₂X⁺](ii), forward reaction is hardly affected by the nature of the entering group, apart from ortho-substituted pyridines which exhibit comparatively low reactivity. The rate constant of the reverse reaction is strongly dependent on the leaving group X. A linear relation between log k_(m→t), and pK_a of the pyridines which are not ortho-substituted indicates that the rate of the mer→trans conversion is largely governed by Rh–X bond rupture. A linear free-energy relation of type (iii) with a value of α of 1.2 correlates the free energy of activation of this conversion with the ΔG_(m→t)^‡=αΔG_(m→t)^⊖+β(iii), standard free energy of the reaction. This result is taken as an indication that there is little interaction between Rh^III and X in the transition state.