Title: Fluxional Behavior of η<sup>1</sup>,η<sup>2</sup>-Cyclooctenylpalladium Acetylacetonate and Related Complexes
Abstract: The cyclooctenylpalladium complexes [Pd(η1,η2-C8H12·R)(acac)] exhibit fluxional behavior in solution that equilibrates the acac methyl environments. Free energies of activation for the process have been determined by variable-temperature NMR spectroscopy. These appear to show no dependence on the nature of exo substituents, and they are higher for bulky endo groups. Lower ΔG⧧ values are obtained in more polar solvents or with added phosphines, suggesting the operation of an associative mechanism. In contrast, the platinum complex [Pt(η1,η2-C8H12·OMe)(acac)] is static up to 380 K, as is the C,N-bonded palladium species [Pd(C6H4NNC6H5-2)(acac)]. These are fluxional, however, in the presence of a tertiary phosphine. The hexafluoroacetylacetonate (hfac) complex [Pd(η1,η2-C8H12·OMe)(hfac)] exhibits a lower value of ΔG⧧ than its acac analogue, with or without added PEt3. When a phosphine is added to one of the metal complexes, low-temperature 31P NMR studies reveal equilibria involving a phosphine-ligated species, the starting acac complex, and free phosphine. The position of equilibrium depends on the nature of the R group and on the phosphine. The rearrangement is proposed to take place from a five-coordinate species, either by a series of pseudorotations or by way of a turnstile mechanism. Although it is not possible to distinguish between these two, the simplicity of the latter in this case is particularly attractive.
Publication Year: 1998
Publication Date: 1998-09-19
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 11
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