Stability of Methyl Platinum Complexes in Water:  The Role of pH and Geometry

Abstract

2 , DPPBTS), to establish the stability of the platinum methyl bond in aqueous solution. From pH ) 3 to 14 there is no reaction other than ligand substitution of H 2 O or OHfor Cl -; there is no CH 3 OH elimination. At lower pH's protonolysis of the Pt-Me bond occurs, producing CH 4 . Dissolving PtMe 2 L 2 into a solution of HCl at pH ) 1 rapidly produces PtCl 2 L 2 for the bidentate ligands, but trans-Pt(Cl)(Me)L 2 is observed for the monodentate ligands. trans-PtClMe(TPPTS) 2 undergoes protonolysis very slowly, indicating an important role for geometry in protonolysis reactions. The protonolysis reactions occur stereospecifically but are sometimes accompanied by cistrans isomerization. The cis or trans thermodynamic preferences, cis-PtCl 2 L 2 , cis-PtMe 2 L 2 , cis-Pt(OH)(Me)L 2 , trans-Pt(Cl)(Me)L 2 , and trans-Pt(H 2 O)(Me)L 2 + , are not easily explained.Platinum and palladium complexes are catalysts for many reactions. [1][2][3][4] In a number of cases, the product formation involves β-hydride elimination; relatively few involve reductive elimination. A number of reactions such as hydrogenation, hydration, hydroamination, and methane to methanol could involve reductive elimination. Goldberg has shown that reductive elimination can occur from Pt(IV) complexes, with formation of carboncarbon or carbon-oxygen bonds. 5 Protonolysis reactions of Pt(II) alkyl complexes in CH 3 OH have been the subject of a few studies. Reactions of trans-Pt(Me)(X)-(PEt 3 ) 2 with triflic acid produced methane, possibly through a Pt(IV) hydride. Deuterium labeling showed extensive incorporation into the CH 4 . 6 A study of HBF 4 or HCl reaction with cis-PtR 2 (PEt 3 ) 2 in CH 3 OH showed cleavage of one Pt-R (R ) Me, Et, etc.) and a slow cis to trans isomerization. 7 In this protonolysis a 10 4 increase was shown for C 2 H 6 elimination over CH 4 . 7 Protonolysis studies on platinum(II) alkyls and aryls showed that acid strength was very important, halide sometimes had an effect, steric hindrance occurred, and aryl cleavage was slower than alkyl, but could not distinguish the site of protonolysis. 8 Florinated bidentate phosphines examined by Roddick and co-workers were much less prone to methyl protonolysis. 9 The dimethyl complexes required neat acids and the monomethyl complexes required neat acids at elevated temperatures. 9 The recent interest in water-soluble organometallic complexes as potential "green" approaches to catalysis raises questions regarding the stability of metal alkyl bonds in aqueous solutions. In this article, we use cis-PtMe 2 (TPPTS) 2 , trans-Pt(Cl)(Me)(TPPTS) 2 , PtMe 2 (LL), and Pt(Cl)(Me)(LL) (TPPTS ) P(m-C 6 H 4 SO 3 Na) 3 , LL ) (m-C 6 H 4 SO 3 Na) 2 PCH 2 CH 2 P(m-C 6 H 4 SO 3 Na) 2 , DPPETS; (m-C 6 H 4 SO 3 Na) 2 PCH 2 CH 2 CH 2 P(m-C 6 H 4 SO 3 Na) 2 , DP-PPTS; (m-C 6 H 4 SO 3 Na)PCH 2 CH 2 CH 2 CH 2 P(m-C 6 H 4 SO 3 -Na) 2 , DPPBTS) (throughout this article LL is generic for one of the three bidentate ligands and L 2 is generic for two TPPTS ligands or one of the bidentate ligands) to examin...

Water-Soluble Mono- and Dimethyl N-Heterocyclic Carbene Platinum(II) Complexes: Synthesis and Reactivity

“…The 1 H NMR spectra of complexes 2 remained unaltered 8 days after being dissolved in D 2 O under neutral or alkaline conditions (0.1 M NaOD) at room temperature. The stability of these complexes in water resembles that found by Atwood’s group for Pt(II) dimethyl complexes with sulfonated phosphanes . In contrast, it has been reported that PtMe 2 complexes with sulfonated iminopyridine ligands are quickly degraded under similar conditions by displacement of the didentate N,N-ligand by water .…”

“…Broadening of resonances was, however, observed in D 2 O for 3c and 3d . The concurrence of isomerization processes in solution is well known for other square-planar haloalkyl platinum(II) complexes. , …”

“…The concurrence of isomerization processes in solution is well known for other square-planar haloalkyl platinum(II) complexes. 37,40 The reaction of dimethyl complexes 2 with HBF 4 in water at room temperature also occurs rapidly and chemoselectively. After addition of the first equivalent of acid and subsequent …”

Organometallics in Superacidic Media: Characterization of Remarkably Stable Platinum–Methyl Bonds in HF/SbF5 Solution

“…What is the origin of the trans X ligand effect on protonolysis? The trans effect on protolytic stability in trans -(R 3 P) 2 Pt­(R)­(X) systems has been previously noted by Thorn, Atwood, and ourselves . We have noted that the Pt–Me 1 J CH value for the donor phosphine complex (dmpe)­Pt­(Me)­(O 2 CCF 3 ), 126 Hz, is significantly increased to 137–139 Hz in (dfepe)­Pt­(Me)­(X) systems (X = Cl, O 2 CCF 3 , OSO 2 F), reflecting substantial carbon rehybridization in response to decreased metal electron density.…”

27 Mechanisms of reactions in solution

“…212 Aqueous solutions of a range of complexes of the form [PtMe 2 L 2 ], where L represents a range of ligands of which P(m-C 6 H 4 SO 3 Na) 3 is typical, are stable to hydrolysis of the Pt-Me bond over pH range of 3-14. 213 However, below pH 3, protonolysis yielding CH 4 becomes a feature of the reactions. The mechanism of the protonolysis the Pt(II) bonds in [PtPh(2,6-C 6 H 3 (CH 2 NMe 2 )) 2 ] and [PtPh(Bu 2 bpy)] has been determined.…”