Mechanisms of Substitution Reactions on Cyclometallated Platinum(IV) Complexes:  “Quasi-labile” Systems

Abstract

The substitution reactions of SMe 2 by phosphines (PMePh 2 , PEtPh 2 , PPh 3 , P(4-MeC 6 H 4 ) 3 , P(3-MeC 6 H 4 ) 3 , PCy 3 ) on Pt IV complexes having a cyclometalated imine ligand, two methyl groups in a cis-geometrical arrangement, a halogen, and a dimethyl sulfide as ligands, [Pt(CkN)(CH 3 ) 2 (X)(SMe 2 )], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines and the X and CkN ligands. In all cases, a limiting dissociative mechanism has been found, where the dissociation of the SMe 2 ligand corresponds to the rate-determining step. The pentacoordinated species formed behaves as a true pentacoordinated Pt IV compound in a steady-state concentration, given the solvent independence of the rate constant. The X-ray crystal structures of two of the dimethyl sulfide complexes and a derivative of the pentacoordinate intermediate have been determined. Differences in the individual rate constants for the entrance of the phosphine ligand can only be estimated as reactivity ratios. In all cases an effect of the phosphine size is detected, indicating that an associative step takes place from the pentacoordinated intermediate. The nature of the CkN imine and X ligands produces differences in the dimethyl sulfide dissociation reactions rates, which can be quantified by the corresponding ∆S q values (72, 64, 48, 31, and 78 J K -1 mol -1 for CkN/X being C

Mechanism of the Competition between Phenyl Insertion and Ligand Reductive Elimination on a Hindered Platinum(IV) Cyclometalated Complex

“…NMR The cyclometalated complexes 1Bzl, 1Me, and 1CH 2 Mes have been prepared by oxidative addition of the C-Br bond of the corresponding 2-BrC 6 H 4 CHNZ imines to the cis-[Pt(Ph) 2 (SMe 2 ) 2 ] complex. 21,24,25 Their 1 H NMR spectra agree with the formulation expected (Table S4); in all cases the proton NMR spectra of the complexes indicated the coexistence of mer (Ph/Ph/SMe 2 ) and fac (Ph/Ph/SMe 2 ) isomeric forms of the compounds in the reaction mixture. 20,29 No elemental analyses were carried out on these complexes, given their high reactivity to produce immediate parallel insertion and reductive elimination reactions, which prevented the existence of a 100% pure sample of the dimethyl sulfide derivatives.…”

“…20 In general, the systems behaved in a dissociatively (k 1 , k -1 , k 2 ) activated quasi-labile way. 21,22 The establishment of the first associatively activated substitution reaction on an octahedral Pt(IV) complex (k 2 ′) has been achieved for the R ) Me, Y 4 ) F 4 , L ) SMe 2 , E ) PMePh 2 , PEtPh 2 , PPh 3 systems. 23 During the studies of the reaction depicted in Scheme 1 with R ) Ph, Y 4 ) H 4 , X ) Br, S ) SMe 2 , the surprising formation of a seven-membered metallacycle via a formal insertion of a phenyl ring in a cyclometalated Pt-C aryl bond has been established.…”

Cyclometallated platinum(IV) compounds as promising antitumour agents

“…and 4g induce significant changes in the mobility of plasmid DNA, altering the DNA tertiary structure as the standard reference cisplatin, although at higher concentrations (see Table 2). Although there is not a clear evidence, the obtained results point to the fact that the presence of bulky equatorial substituents such as aryl ligands (2d and 2f) instead of smaller ligands (2a-2c) and of iodido or bromido (4g, 4h) axial ligands rather than chlorido (4i) favour the covalent binding to DNA in agreement with a dissociatively activated substitution mechanism for this type of compounds [111].…”

Kinetico-mechanistic studies on the formation of seven-membered [C,N]-platinacycles: the effect of methyl or fluoro substituents on the aryl ancillary ligands

“…This isomeric distribution has already been observed for complexes of the same family,25,26 even the isomerization on substitution of the dialkylsulfide by phosphine has been quantified, as well as the formation of dimeric species. 25,27 Scrambling between these two isomeric forms is known to be fast on the NMR time-scale for the bis-methyl analogue of 52-HBrH and, consequently, should not be relevant to the follow up 5 → 6 reductive elimination.26 Effectively the disappearance of the two isomers of species 5 occurs simultaneously, indicating its rapid scrambling. From this point the appearance of two reductive eliminated forms of complex 6 at rather different chemical shifts is evident in the 1H NMR spectra for compounds 51-MeBrF and 52-MeBrF.…”