Reaction Mechanism for Olefin Exchange at Chloro Ethene Complexes of Platinum(II)

Abstract

Complex equilibria in methanol/chloroform/dichloromethane solutions containing Zeise's anion, [PtCl(3)(C(2)H(4))](-) (1), the solvento species, trans-[PtCl(2)(C(2)H(4))(MeOH)] (2), and the dinuclear complex, trans-[PtCl(2)(C(2)H(4))](2) (3), have been studied by UV-vis, (1)H, and (195)Pt NMR spectroscopy, giving average values of K(Cl) = (1.6 +/- 0.2)10(3) M(-)(1) and K(S) = (0.16 +/- 0.02) M(-)(1) for the equilibrium constants between 2 and 1 and 3 and 2, respectively. The bridged complex 3 is completely split into monomeric solvento complexes 2 in methanol and in chloroform or dichloromethane solutions with [MeOH] > 0.5 M. Ethene exchange at the mononuclear complexes 1 and 2 was studied by (1)H NMR line-broadening experiments in methanol-d(4). Observed overall exchange rate constants decrease with an increase in free chloride concentration due to the displacement of the rapid equilibrium between 1 and 2 toward the more slowly exchanging parent chloro complex 1. Ethene exchange rate constants at 298 K for complexes 1 and 2 are k(ex1) = (2.1 +/- 0.1)10(3) M(-)(1) s(-)(1)and k(ex2) = (5.0 +/- 0.2)10(5) M(-)(1) s(-)(1), respectively, with corresponding activation parameters DeltaH(1)() = 19.1 +/- 0.3 kJ mol(-)(1), DeltaS(1)() = -117 +/- 1 J K(-)(1) mol(-)(1), DeltaH(2)() = 10.2 +/- 0.4 kJ mol(-)(1), and DeltaS(2)() = -102 +/- 2 J K(-)(1) mol(-)(1). The activation process is largely entropy controlled; the enthalpy contributions only amounting to approximately 30% of the free energy of activation. Ethene exchange takes place via associative attack by the entering olefin at the labile site trans to the coordinated ethene, which is either occupied by a chloride or a methanol molecule in the ground state. The intimate mechanism might involve a two-step process via trans-[PtCl(2)(C(2)H(4))(2)] in steady state or a concerted process via a pentacoordinated transition state with two ethene molecules bound to the platinum(II).

Ligand Substitution Dynamics in Metal Complexes

“…158 Because of the strong trans labilizing effect of h 2 -C 2 H 4 , Zeise's anion exchanges bound and free ethene rapidly in methanol (k 2 ¼ 2.1  10 3 L/ (mol s) at 298 K). 159 The static effect of ethene on the trans-Pt-Cl bond length is very small, presumably because s electron release and p à electron acceptance by h 2 -C 2 H 4 counteract each other, indicating that the origin of the labilization lies in stabilization of the associative transition state rather than any weakening of the trans-Pt-Cl bond in the ground state.…”

Bridge-splitting of trans-[PtCl2(η2-CH2CH2)]2 by weak nucleophiles: Crystal and molecular structure of trans-[PtCl2(η2-CH2CH2)(MeCN)]

“…Reaction of 1 with ethene in acetone at À80°C results in the formation of a yellow unstable complex, trans-[PtCl 2 (g 2 -CH 2 @CH 2 ) 2 ] [2]. This species has been postulated as a short-lived intermediate in the reaction mechanism for ethene exchange at [PtCl 3 (g 2 -CH 2 @CH 2 )] À [3,4], and in non-co-ordinating solvents like chloroform, it is formed as a product of bridge-splitting of 1 by ethene. Upon standing at room temperature under an ethene atmosphere the white, moderately stable compound, cis-[PtCl 2 (g 2 -CH 2 @CH 2 ) 2 ], can be obtained [5].…”

“…Due to the gaseous nature of ethene, and the increase in reactivity, studies of 1 are considerably more complicated. We succeeded in obtaining values for the bridge-splitting equilibrium constants, according to Scheme 1, for MeOH [4] and ethene [5] and now extended this work to illustrate the quantitative experimental studies between 1 and THF and CH 3 CN. These solvents are representative of weak nucleophiles enabling the investigation of these highly reactive systems using simple, conventional UV-Vis measurements [7].…”

“…By comparing the bridge-splitting equilibrium constants for trans-[PtCl 2 (g 2 -CH 2 @CH 2 )] 2 with L, to form trans-[PtCl 2 (g 2 -CH 2 @ CH 2 )(L)], of 0.13 ± 0.01 (MeOH) [4], 6.8 ± 0.6 (CH 2 @CH 2 ) [5], 0.0289 ± 0.0007 (THF) and 3601 ± 215 (MeCN) mol À1 dm 3 , the difference in the coordinating ability of the various nucleophiles to the Pt(II) metal centre is quantitatively defined.…”

Bridge splitting of trans-[PtCl2(C2H4)]2 by ethene using a simple combined NMR–UV/vis cell: Crystal and molecular structure of cis-[PtCl2(C2H4)2]

“…The reaction of 1 with ethene in acetone at À80°C yields a yellow, unstable complex, postulated to be trans-[PtCl 2 (C 2 H 4 ) 2 ], while upon standing at room temperature a white insoluble compound, assumed to be cis-[PtCl 2 (C 2 H 4 ) 2 ] [4], is obtained. Trans-[PtCl 2 (C 2 H 4 ) 2 ] (2), not yet characterised, has been postulated as a shortlived intermediate in the reaction mechanism for ethene exchange at [PtCl 3 (C 2 H 4 )] À [5,6], and in non-coordinating solvents like chloroform, it is formed as a product of bridge splitting of 1 by ethene. Similarly, bridge splitting of trans-[PtCl 2 (CH 2 CHC 6 H 5 )] 2 by CH 2 CHC 6 H 5 in chloroform is reported to produce trans-[PtCl 2 (CH 2 CHC 6 H 5 ) 2 ] with an equilibrium constant of 0.0235 ± 0.0003 mol À1 dm 3 [7].…”