Oxidative addition of n-alkyl halides to diimine–dialkylplatinum(ii) complexes: a closer look at the kinetic behaviors

Abstract

The n-alkyl halides, RX, were oxidatively added to the platina(II)cyclopentane complexes [Pt[(CH2)4](NN)], in which NN = bpy (2,2'-bipyridyl) or phen (1,10-phenanthroline), to give the platinum(IV) complexes [PtRX[(CH2)4](NN)], R = Et and X = Br or I; R = nBu and X = I, 1-3. The same reactions with the analogous dimethyl complex [PtMe2(bpy)] gave the expected platinum(IV) complexes [PtRXMe2(bpy)], R = Et or nPr and X = Br or I; R = nBu and X = I, 4-8. Kinetics of the reactions in benzene and acetone was studied using UV-vis spectrophotometery and a common S(N)2 mechanism was suggested for each case. The platina(ii)cyclopentane complexes reacted faster than the corresponding dimethyl analogs by a factor of 2-3. This is described as being due to a lower positive charge, calculated by density functional theory (DFT), on the platinum atom of [Pt[(CH)2)4](bpy)] compared with that on the platinum atom of the dimethyl analog [PtMe2(bpy)]. The values of DeltaDeltaS(double dagger) = DeltaS(double dagger)(acetone) - DeltaS(double dagger)(benzene) were found to be either positive or negative in different reactions and this is related to the solvation of the corresponding alkyl halide. It is suggested that in these reactions of RX reagents, for a given X, the electronic effects of the R group are mainly responsible for the change in the rates of the reactions and the bulkiness of the group is far less important.

Reaction of dimethylplatinum(II) complexes with PhCH2CH2Br: Comparative reactivity with CH3CH2Br and PhCH2Br and synthesis of Pt(IV) complexes

“…However, although the steric effect of the R group to some extent influences the reaction rate, it does not seem to be reasonable to attribute the order of reactivity to an increase in steric effects on going from R = CH 3 to R = Ph. The calculated results are in accord with experimental findings (in which the rate of reaction of 1a in acetone at 20°C with CH 3 CH 2 Br (k 2 = 1.43 × 10 −3 l mol −1 s −1 ) is nearly 10 4 times slower than that with PhCH 2 Br (k 2 = 11.61 l mol −1 s −1 )), [19] confirming that in this case the electronic nature of the R group is mainly responsible for the reactivity of bromide reagents and the influence due to bulkiness of the R groups is probably much less effective.…”

“…It is interesting to compare the behaviours of RCH 2 Br (R = CH 3 , Ph and PhCH 2 ) in their oxidative addition reactions with the organoplatinum(II) complex 1a with an emphasis on the differences between the effect of the R group of alkyl halides. Considering that second-order reactions for the reaction with CH 3 CH 2 Br and PhCH 2 Br are proposed to occur by the S N 2 mechanism, [18,19] we theoretically consider the S N 2 mechanism for oxidative addition of Pt(II) complex 1a with PhCH 2 CH 2 Br to conduct a systematic comparison based on the effect of R group on the rate of oxidative addition. The possible structures of the reactants, transition states, intermediates and products for the reaction of complex 1a with RCH 2 Br (R = CH 3 , Ph and PhCH 2 ) are shown in Scheme 2 along with their abbreviated names for ease of discussion.…”

“…[1,6,[19][20][21][22][23][24][25][26][27] This involves nucleophilic substitution of halide by Pt complex to form a Pt-C bond, followed by subsequent coordination of halide to form a six-coordinate octahedral alkyl product. It is interesting to compare the behaviours of RCH 2 Br (R = CH 3 , Ph and PhCH 2 ) in their oxidative addition reactions with the organoplatinum(II) complex 1a with an emphasis on the differences between the effect of the R group of alkyl halides.…”

Uncommon Solvent Effect in Oxidative Addition of MeI to a New Dinuclear Platinum Complex Containing a Platina(II)cyclopentane Moiety

“…Factors influencing the rate of the reactions and the kinetic parameters are also established. [1,2] Despite this, similar oxidative addition reactions involving dinuclear organoplatinum(II) complexes have not been investigated much. [4,5] These reactions could be important, as cooperative steric and/or electronic effects between the two adjacent metal centers may give rise to reaction pathways or products that are not possible in the kinetic data revealed that the reactions in nonpolar benzene or slightly polar CH 2 Cl 2 proceeded in two steps, each following a common S N 2 mechanism.…”

“…[1,2] This type of reaction plays a fundamental role in many industrially important catalytic reactions.…”