Pseudo-rotation mechanism for fast olefin exchange and substitution processes at orthometalated C,N-complexes of platinum(ii)

Abstract

Bridge splitting in chloroform of the orthometalated chloro-bridged complex [Pt(μ-Cl)(2-Me₂NCH₂C₆H₄)]₂ (1), with ethene, cyclooctene, allyl alcohol and phosphine according to 1 + 2L → 2[PtCl(2-Me₂NCH₂C₆H₄)(L)], where L = C₂H₄ (3a), C₈H₁₄, (3b), CH₂CHCH₂OH (3c), and PPh₃ (4a and 4b) gives monomeric species with L coordinated trans or cis to aryl. With olefins the thermodynamically stable isomer with L coordinated cis to aryl is formed directly without an observable intermediate. With phosphine and pyridine, the kinetically controlled trans-product isomerizes slowly to the more stable cis-isomer. Bridge splitting by olefins is slow and first-order in 1 and L, with largely negative ΔS^‡. Substitution of ethene cis to aryl by cyclooctene and allyl alcohol to form 3b and 3c, and substitution of cot from 3b by allyl alcohol to form 3c are first order in olefin and complex, ca. six orders of magnitude faster than bridge cleavage due to a large decrease in ΔH^‡, and with largely negative ΔS^‡. Cyclooctene exchange at 3b is first-order with respect to free cyclooctene and platinum complex. All experimental data for olefin substitution and exchange are compatible with a concerted substitution/isomerization process via a turnstile twist pseudo-rotation in a short-lived labile five-coordinated intermediate, involving initial attack on the labile coordination position trans to the σ-bonded aryl. Bridge-cleavage reactions of the analogous bridged complexes occur similarly, but are much slower because of their ground-state stabilization and steric hindrance.