Square-planar metal(II) complexes containing ester functionalised bis(phosphino) amines: Mild P-N methanolysis and C-arene-H cyclometallation

Abstract

The synthesis of three new ester functionalised bis(phosphino)amines Ph2PN{R}PPh2 [R = C6H4(3-CO2Me) 1a; C6H3(3,5-CO2Me)(2) 1b; C6H4(4-CO2Me) 1c] upon stoichiometric reaction of Ph2PCl and the appropriate H2N{R} in Et2O is described. Reaction of 1 equiv. of 1a c with MCl2(cod) (M = Pt, Pd) in CH2Cl2 afforded the dichlorometal(II) complexes PtCl2(1a) 2a, PtCl2(1b) 2b, PtCl2(1c) 2c and PdCl2(1b) 2'b respectively. The corresponding dibromo (and diiodo) platinum(II) complexes 3b (and 4b) were synthesised, in > 80% isolated yields, from PtBr2(cod) or PtI2(cod). When a suspension of 2b in MeOH was stirred at r.t. for ca. 16 h the mixed complex cis-PtCl2[Ph2PNH{R}](Ph2POMe) 5a [R = C6H3(3,5-CO2Me)(2)] was cleanly generated. Metathesis of 5a using excess NaBr or NaI in MeOH/acetone afforded cisPtBr(2)[Ph2PNH{R}](Ph2POMe) 5b or cis-PtI2[Ph2PNH{R}](Ph2POMe) 5c. Methanolysis of PtCl2(1a) 2a at ambient temperature afforded, in low yield (18%), the regiospecific P, C-orthometallated complex cis-PtCl [Ph2PNH{C6H3(3-CO2Me)}](Ph2POMe) 6. We speculate the C - H activated complex 6 is obtained via initial formation of cis-PtCl2[Ph2PNH{C6H4(3-CO2Me)}](Ph2POMe) 7. A similar observation was also found using 2c whereupon examination of the isolated solid, by P-31{H-1} NMR spectroscopy, revealed formation of three complexes namely orthometallated cis-PtCl[Ph2PNH{C6H3(4-CO2Me)}](Ph2POMe) 9, cis-PtCl[Ph2PNH{C6H4(4-CO2Me)}](Ph2POMe) 10 and cis-PtCl2(Ph2POMe)(2) 8. All new compounds reported here have been characterised by multinuclear NMR and IR spectroscopy, microanalysis and in six cases by single crystal X-ray crystallography. The X-ray structure of cis-PtCl[Ph2PNH{C6H3(3-CO2Me)}]( Ph2POMe) 6 revealed selective C - H cycloplatination at the 6-position (as opposed to the 2-position) of the N-arene ring.