DIRECT EVIDENCE FOR A RATE-DETERMINING CHELATE RING-CLOSURE MECHANISMIN 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 ha s been carried out in dichloromethane solution. The contemporary prese nce in the starting substrate of a labile (SEt(2)) and a relatively in ert (CO) ligand in trans to firmly bonded phenyl groups makes it possi ble to distinguish three different reactivity patterns, according to t he nature of the bidentate ligands used. Relatively stable open-ring s pecies 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 consider able encumbrance. For the other dinitrogen ligands, the relative magni tude 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 st arting 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'-tetrameth ylethylenediamine 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 follow ed 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 follo ws the fast entry of the first end of the ligand. The substrate shows a remarkable selectivity between the two nucleophilic ends of an unsym metrical bidentate ligand with a clear preference for the most basic a nd 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 N MR spectra and their spectroscopic characteristics are compared to tho se of the parent amine and phosphine compounds. The final chelate comp ounds [PtPh(2)(N-N)] were either isolated as solids or characterised i n solution.