Di- and tri-methylplatinum (IV) complexes with tridentate amino acid ligands

Abstract

Reactions of the ligands iminodiacetic acid (H2ida), N-(methylimino)diacetic acid (H2mida), N-(phosphonomethyl)glycine (H3impa, glyphosate), iminodimethylenephosphonic acid (H4idmp) and (methylimino)dimethylenephosphonic acid (H4midmp) with fac- [Pt(CH3)2Br(H20)3]+ and cis-[{Pt(CH3)2(OH)2(H20)1.5}n] have been investigated. As well, reactions of the phosphonate ligands with fac- [Pt(CH3)3(H20)3]+ and PtC162- have been studied. The ligands H2L (L = ida, mida) reacted with a solution of [{Pt(CH3)2(OH)(H2O)1.5}n] in alkali to give initially [Pt(CH3)2(OH)2(L-N,)c)2- in which the ligand coordinates didentate trans to the methyl groups. Ring closure to form [Pt(CH3)2(OH)(L-N,OC,Oc’)] was very slow under alkaline conditions, but was much faster in acid solution, to give [Pt(CH3)2(H20)(L-N,Oc, Oc’)] (N trans to methyl). fac-[Pt(CH3)2Br(H20)3]+ with HL- gave [Pt(CH3)2Br(L-N,Oc,Oc’)]- (N trans to methyl). Irradiation with a mercury lamp caused facile isomerization to the isomer with N trans to bromide. This reaction occurred without dissociation of L2-. A side reaction at high pH produced [Pt(CH3)2(OH)(L-N,Oc,Oc’)]- (N trans to hydroxide). The colourless aqua complexes [Pt(CH3)2(H20)(L)] were photoisomerized much less efficiently than the pale yellow bromo complexes. Thermal isomerizations were very much slower. Pure solids K[Pt(CH3)2Br(L-N,Oc,Oc’)]) have been isolated by HPLC. fac-[Pt(CH3)3(H20)3]+ with Himpa2- gave [Pt(CH3)3(Himpa-N,Oc,Op)]. Reaction of glyphosate with [{Pt(CH3)2(OH)2(H20)1.5}n] in alkali gave a mixture of two isomers [Pt(CH3)2(OH)2(impa-N,Op)]3- and [Pt(CH3)2(OH)2(impa- N,Oc)]3- with the ligand coordinated didentate trans to the methyl groups. The corresponding complexes [Pt(CH3)2(OH)(impa-N,Oc,Op)]2- with glyphosate tridentate formed slowly under alkaline conditions (with the isomer having Oc trans to methyl favoured): in acid solution ring closure to form isomers of [Pt(CH3)2(H20)(Himpa-N,Oc,Op)] was much faster and crystals of the favoured isomer with Op trans to methyl were deposited. X-ray crystallography confirmed the structure. When the solid was dissolved in alkali and the solution allowed to stand, isomerization of [Pt(CH3)2(OH)(impa-N,Oc,Op)]2- to the isomer with N and Oc trans to methyl occurred. fac-[Pt(CH3)2Br(H20)3]+ with glyphosate gave two isomers of [Pt(CH3)2Br(Himpa-N,Oc,Op)]”. At high pH, where coordinated phosphonate is deprotonated, the preferred isomer had N and Oc trans to methyl, while at low pH, where coordinated phosphonate is protonated, the preferred isomer had N and Oc trans to methyl. The crystal structure of the silver salt of the latter isomer has been determined by X-ray diffraction. UV irradiation of isomers [Pt(CH3)2Br(impa-N,Oc,Op)]2- (N trans to CH3) gave the thermodynamically most stable third isomer, with N trans to bromide. Acidification and addition of silver ion gave crystals of Ag3[Pt(CH3)2Br(Himpa)][Pt(CH3)2Br(impa)].1.5H20, whose structure has been determined. Reaction of H4idmp with fac-[Pt(CH3)3(D2O)3]+ in D2O gave a complex with the ligand coordinated tridentate, but broadening of NMR peaks from the ligand protons was interpreted in terms of a rapid process in which metal-oxygen bonds are ruptured. [Pt(CH3)2Br(H2O)3]+ with H2idmp2- at pH 4 gave [Pt(CH3)2Br(H2idmp-N,Op,Op)] on long standing. cis-[pt(CH3)2(OD)4]2-“ with midmp4- in D2O at pD 12.5 gave [Pt(CH3)2(OD)2(midmp-N,Op]4-, but when the solution was acidified, the ligand dissociated. The relative instability of complexes with iminodimethylenephosphonate ligands coordinated tridentate may be due to steric interactions between phosphonate oxygen atoms and other ligands bound to the metal.Attempts to prepare a cobalt(III)-glyphosate complex were unsuccessful. trans-[Co(mida)2] has been prepared by reaction of Na3[Cp(CO3)3]. 3H2O with the ligand in an ice-cold acidic aqueous medium. Reaction of non-organometallic PtC162- with glyphosate gave predominantly [PtC13(Himpa-N,Oc,Op)]'. Continued heating of this system resulted in the formation of platinum(II)-glyphosate complexes and the appearance of phosphoric acid, H2C(OH)2 and aminomethylphosphonic acid as degradation products of glyphosate. An internal oxidation-reduction reaction in the platinum(IV) complex has been proposed.