KINETIC-STUDY OF BETA-HYDRIDE ELIMINATION OF MONOALKYL COMPLEXES OF PLATINUM(II) - EFFECTS OF VARYING THE ALKYL CHAIN-LENGTH OR THE CIS GROUP IN THE REACTION OF CIS-BIS(TRIETHYLPHOSPHINE) (ALKYL)(HALO OR PSEUDOHALO)PLATINUM(II) COMPLEXES

The monoalkyl complexes CIS-[Pt(PEt3)2(R)Br] (R = C2H5, C2D5, N-C3H7, n-C4H9) and cIs-[Pt(PEt3)2(n-C4H9)X] (X = Cl, Br, I, NO2, N3, SCN, SeCN) undergo a facile β-hydride elimination in acetone, yielding trans-[Pt(PEt3)2HX] and olefins. No alkanes are produced in these reactions that go to completion and are unaffected by the presence of an excess of halide ion in solution that serves to prevent a possible concurrent geometrical isomerization. The corresponding trans-monoalkyl species are stable under the same experimental conditions. The systems were characterized by 1H and 31P NMR. The rates of thermal decomposition were obtained by GLC, measuring the relative amounts of volatile products at various times. The ethyl complex decomposes at a rate ten times slower than that of the n-propyl and n-butyl analogues. For the complexes cis-[Pt(PEt3)2(R)Br], the activation parameters are as follows: R = C2H5, ΔH* = 101 ± 2 kJ mol-1, ΔS* = +5 ± 4 J K-1 mol-1; R = n-C3H7, ΔH* = 91 ± 4 kJ mol-1, ΔS* =-7 ± 10 J K-1 mol-1; R = n-C4H., ΔH* = 90 ± 2 kJ mol-1, ΔS* =-4 ± 4 J K-1 mol-1; R = C2D5, ΔH* = 99 ± 2 kj mol-1, ΔS* =-10 ± 5 J K-1 mol-1. At 298.16 K, the kinetic isotope effect (kd(C2H5)/kd(C2D5)) is 3.1. The rates of decomposition of the complexes cis-[Pt(PEt3)2(n-C4H9)X] are strongly dependent on the nature of the X group, the overall difference of reactivity being at least 4 orders of magnitude between the first and the last members of the series. The reactivity sequence X = N3 < NO2 < Cl < NCS < Br < NCSe < I correlates well with some NMR parameters (δ(1H), δ(195Pt), and 1J(PtP)) of the trans-[Pt-(PEt3)2HX] hydride products. The distribution of the olefin products, 1-butene, m-2-butene, and trans-2-butene is also dependent on the nature of X. The most probable mechanism for the thermolysis involves fast and reversible β-hydride elimination and olefin insertion in a pre-rate-determining step, followed by slow olefin loss from a 5-coordinate [PtL2(H)(olefin)X] intermediate. © 1990, American Chemical Society. All rights reserved.