DIRECT EVIDENCE FOR A RATE-DETERMINING CHELATE RING-CLOSURE MECHANISM IN THE REACTION OF A PLATINUM(II) COMPLEX WITH BIDENTATE NITROGEN LIGANDS

A spectrophotometric and H-1 NMR kinetic study of the reactions of cis-[PtPh(2)(CO)(SEt(2))] with an extended series of dinitrogen chelating ligands (N-N) of widely different steric and electronic properties has been carried out in dichloromethane solution. The contemporary presence in the starting substrate of a labile (SEt(2)) and a relatively inert (CO) ligand in trans to firmly bonded phenyl groups makes it possible to distinguish three different reactivity patterns, according to the nature of the bidentate ligands used. Relatively stable open-ring species of the type cis-[PtPh(2)(CO)(N-N)] were isolated with the long chain ligand 1,4-diaminobutane and with N-phenyl-1,2-diaminoethane in which one of the two nitrogens exhibits very low basicity and considerable encumbrance. For the other dinitrogen ligands, the relative magnitude of the rates of attack at the metal and of ring closure dictates whether the chelate formation is seen as a simple conversion of the starting complex into the final che]ate [PtPh(2)(N-N)] or as a two stage process with evidence for the formation of an open-ring species. With rigid or highly sterically hindered ligands such as 2,2'-dipyridyl, 1,10-phenanthroline, dicyclohexylethylenediimine or N,N,N',N'-tetramethylethylenediamine a single stage process from the starting complex to the final chelate compound was observed. The rate determining step is thought to involve slow dissociation of SEt(2) from the complex followed by fast ring closing. With 1,2-diaminoethane, 1,3-diaminopropane, 2-aminomethylpyridine, 2-aminoethylpyridine, 2,2'-dipyridylamine and 2,2'-dipyridylsulfide a slow ring closure of the open-ring complex follows the fast entry of the first end of the ligand. The substrate shows a remarkable selectivity between the two nucleophilic ends of an unsymmetrical bidentate ligand with a clear preference for the most basic and the less hindered end. Open-ring reaction intermediates cis-[PtPh(2)(CO)(N-N)] were characterised in solution by their IR, H-1 and C-13 NMR spectra and their spectroscopic characteristics are compared to those of the parent amine and phosphine compounds. The final chelate compounds [PtPh(2)(N-N)] were either isolated as solids or characterised in solution.