A comparative study of the structures and reactivity of cyclometallated platinum compounds of N-benzylidenebenzylamines and cycloplatination of a primary amine

Abstract

The reaction of cis-[PtCl₂(dmso)₂] with ligands 4-ClC₆H₄CHNCH₂C₆H₅ (1a) and 4-ClC₆H₄CHNCH₂(4-ClC₆H₄) (1b) in the presence of sodium acetate and using either methanol or toluene as solvent produced the corresponding five-membered endo-metallacycles [PtCl{(4-ClC₆H₃)CHNCH₂C₆H₅}{SOMe₂}] (2a) and [PtCl{(4-ClC₆H₃)CHNCH₂(4′-ClC₆H₄)}{SOMe₂}] (2b). An analogous reaction for ligands 2,6-Cl₂C₆H₃CHNCH₂C₆H₅ (1c) and 2,6-Cl₂C₆H₃CHNCH₂(4-ClC₆H₄) (1d) produced five-membered exo-metallacycles [PtCl{(2,6-Cl₂C₆H₃)CHNCH₂C₆H₄}{SOMe₂}] (2c) and [PtCl{(2,6-Cl₂C₆H₃)CHNCH₂(4′-ClC₆H₃)}{SOMe₂}] (2d) when the reaction was carried out in methanol and seven-membered endo-platinacycles [PtCl{(MeC₆H₃)ClC₆H₃CHNCH₂C₆H₄}{SOMe₂}] (3c) and [PtCl{(MeC₆H₃)ClC₆H₃CHNCH₂(4′-ClC₆H₃)}{SOMe₂}] (3d) when toluene was used as a solvent. The reaction of 2,4,6-(CH₃)₃C₆H₂CHNCH₂(4-ClC₆H₄) (1e) produced in both solvents an exo-platinacycle [PtCl{(2,4,6-(CH₃)₃C₆H₂)CHNCH₂(4′-ClC₆H₃)}{SO(CH₃)₂}] (2e). Cyclometallation of 4-chlorobenzylamine was also achieved to produce compound [PtCl{(4-ClC₆H₃)CH₂NH₂}{SOMe₂}] (2g). The reactions of endo- and exo-metallacycles with phosphines evidenced the higher lability of the Pt–N bond in exo-metallacycles while a comparative analysis of the crystal structures points out a certain degree of aromaticity in the endo-metallacycle.