Metal complexes of benzodiazepines. Part 2. The reaction of 1,4-benzodiazepines with halide-bridged complexes of palladium(II)[Pd2X4(PPrn3)2](X = Cl or l)

Abstract

1,4-Benzodiazepines cleave the halide-bridged complexes [Pd₂X₄(PPrⁿ₃)₂](X = Cl or l) to give monomeric complexes in which the benzodiazepine is presumably co-ordinated through N(4) of the heterocyclic ring. The reaction between [Pd₂I₄(PPrⁿ₃)₂] and 7-chloro-1,3-dihydro-1-methyl-5-phenyl-, 5-(2-chlorophenyl)-1,3-dihydro-7-nitro-, 1,3-dihydro-7-nitro-5-phenyl-, and 7-bromo-1,3-dihydro-5-(2-pyridyl)-2H-1,4-benzodiazepin-2-one and 7-chloro-2,3-dihydro-1-methyl-5-phenyl-1,4-benzodiazepine has been studied kinetically and thermodynamically in CHCl₃. The stability of the monomeric complexes formed varies considerably throughout the series of ligands and is much lower when the benzodiazepine is in the 2-keto form. The nucleophilic activities of these substances, as expressed by the second-order rate constants k₂ corresponding to direct attack at palladium in the cleavage reaction, are similar but much lower, for steric reasons, than that of pyridine. The difference in stability of the various complexes is attributed mainly to the difference in the rate constants corresponding to the reverse reaction, i.e. the release of the co-ordinated ligand.