New cyclometallated precursors of unsubstituted N-phenylpyrazole [{Pd(phpz)(μ-X)}2] (X = AcO or OH) and study of their reactivity towards selected ligands

Abstract

A new acetate-bridged dinuclear palladacycle with unsubstituted N-phenylpyrazole [{Pd(phpz)(μ-AcO)}₂] 1 has been isolated and characterised, including an X-ray diffraction study. A survey of the Cambridge Structural Database (CSD) v. 5.31 looking for analogous dimeric C^N cyclopalladated complexes has been done, exploring the incidence of cisoid/transoid arrangements, the preferred conformation of the eight-membered ring formed in the double bridge, the Pd–Pd distance and the main factors that affect it. The reaction of 1 with NBu₄OH yielded [{Pd(phpz)(μ-OH)}₂] 2 that has shown to be a complementary precursor of 1 in terms of acid/base reactivity. In this sense, both 1 and 2 are also well differentiated from halide precursors available to date. The preparation of selected complexes with potential applications in several fields, [Pd(phpz)(O^N)] O^N = N-p-chlorophenylsalycilaldiminate (N-pClsal) 3, picolinic acid (pic) 4; 8-hydroxiquinolinate (oxin) 5; 2-pyrrole-carboxaldeydate (2-pcal) 6, [Pd(phpz)(O^O)] O^O = salycilaldehydate (sal) 7 acetylacetonate (acac) 8, [{Pd(phpz)(μ-N^S)}₂] N^S = 2-mercapto-1-methylimidazolate (SMeimz) 11; [{Pd(phpz)(μ-N^O)}₂] N^O = succinimidate (succ) 12; [{Pd(phpz)(μ-N^N)}₂] (N^N = pyrazolate (pz) 13, has been achieved using 1 or 2 as starting materials in acid/base reactions. Dithiocarbamate [Pd(phpz)(S₂CNEt₂)] 9 and dithiophosphate [Pd(phpz){S(S)P(OEt)₂}] 10 derivatives have been synthesised in related reactions, and the reactivity of 1 against neutral phoshine ligands has also been tested with the preparation of [Pd(phpz)(AcO)(PPh₃)] 14. The crystal structures of compounds 7, 9, 11, 12 and 13 (this one obtained from a powder sample using synchrotron radiation) have also been established, and together with 1 are the first examples of complexes containing unsubstituted N-phenylpyrazole as cyclometallated backbone that have been deposited to date on the Cambridge Structural Database.