Oxidative Addition of Ethyl Iodide to a Dimethylplatinum(II) Complex: Unusually Large Kinetic Isotope Effects and Their Transition-State Implications

Abstract

The mechanism of oxidative addition of ethyl iodide to [PtMe 2 (2,2 0 -bipyridine)], 1, has been investigated by product analysis and by study of secondary deuterium kinetic isotope effects (KIEs), using the reagents C 2 H 5 I, C 2 D 5 I, CH 3 CD 2 I, and CD 3 CH 2 I. The reactions in acetone and benzene give [PtIMe 2 Et(bipy)], mostly as the product of trans oxidative addition, but with some of the isomeric product of cis oxidative addition and some [PtIMe 3 (bipy)], resulting from methyl group transfer. The reaction in benzene is light-sensitive, giving additional major products [PtI 2 Me 2 (bipy)] and [PtIMe 2 -(OOEt)(bipy)], as well as several minor products indicative of a photochemically initiated free-radical reaction. No H/D exchange within the ethyl group was observed in any of the products. The dark reactions in acetone and benzene follow second-order kinetics, with large negative values of the entropy of activation, indicating the S N 2 mechanism of oxidative addition of ethyl iodide to 1. However, for reaction with C 2 H 5 I vs C 2 D 5 I, values of the KIE k H /k D range from 1.32 to 1.72 in acetone and from 1.44 to 1.90 in benzene solution, and studies with CH 3 CD 2 I and CD 3 CH 2 I show that Rand β-deuterium KIEs make about equal contributions to the overall KIE. These are the first reports of isotope effects on the rate of oxidative addition reactions of ethyl halides, and the high values of the secondary deuterium KIE were unexpected for the S N 2 mechanism. Possible reasons for these observations are discussed. Scheme 2. a Free-Radical Chain Mechanism of Oxidative Addition with Possible Formation of Alkylperoxo Complexes of Platinum(IV) a R = i-Pr.

Secondary kinetic deuterium isotope effect in oxidative addition reaction of cycloplatinated(II) complexes with MeI

“…So these results in general are consistent with our recent studies indicating that nature of the diimine (or its substituted version) peripheral ligands (steric and electronic) may only have a mild influence on the KIE values [13e16]. We, however, have lately reported that KIE values in reactions of the Pt(II) complex [PtMe 2 (2,2 0 -bipyridine)] with CH 2 RX reagents (in which R ¼ H [14], CH 3 [13], or Ph [16]) are significantly dependent on R and a high value is obtained when R is CH 3 or Ph. The latter cases have been described [13,16] as the presence of bulkier alkyl halides and creation of more room for the CaÀH(D) out-of-plane bending vibration [34].…”

“…The reactions of many platinum(II) complexes with MeI have been extensively investigated both theoretically and experimentally [6,7,17,22] and in all cases, an S N 2 mechanism has been suggested. Operation of S N 2 mechanism in oxidative addition of alky halides to some organoplatinum(II) complexes has also been confirmed by secondary deuterium KIE study involving the reaction of MeI/CD 3 I or C 2 H 5 I/C 2 D 5 I [13,15].…”

“…We, however, have lately reported that KIE values in reactions of the Pt(II) complex [PtMe 2 (2,2 0 -bipyridine)] with CH 2 RX reagents (in which R ¼ H [14], CH 3 [13], or Ph [16]) are significantly dependent on R and a high value is obtained when R is CH 3 or Ph. The latter cases have been described [13,16] as the presence of bulkier alkyl halides and creation of more room for the CaÀH(D) out-of-plane bending vibration [34]. We therefore have come to the conclusion that in oxidation addition reactions of organoplatinum(II) complexes with alkyl halides, nature of the Pt complex has a minor influence, while that of bulkiness of the alkyl halides has a significant influence on the resulting KIE values.…”

Oxidative addition of MeI to some cyclometalated organoplatinum(II) complexes: Kinetics and mechanism

“…Alternatively, M can be isomerised by pseudo-rotation to give N [22], which can also be trapped by iodide to give 3. We suggest that formation of the thermodynamic isomer 3 is favoured over the kinetic isomer 2 as in former the more sterically demanding L ligands are situated in the "axial" positions as compared with the latter in which they are located in the "equatorial" position.…”

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

“…[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.…”