Cyclopalladation versus hydroxylation. A case of pH dependence

Abstract

1-Alkyl-2-(naphthyl-α-azo)imidazoles (NaiR, 2) (C 10H 7NNC 3H 2N 21R; R=Me ( a), Et ( b), PhCH 2 ( c)) have been reacted with Na 2PdCl 4 in MeOH or MeCN solutions of Pd(MeCN) 2Cl 2 to synthesise Pd(NaiR)Cl 2 ( 3). The reaction of Pd(OAc) 2 in boiling benzene with NaiR followed by the addition of LiCl has resulted in the synthesis of the cyclopalladated complex Pd(NaiRH)Cl ( 4). The ligand, NaiR, acts as a N, N′-bidentate chelator while NaiRH acts as a tridentate N, N′, C-cyclometallating ligand. The infrared spectra of 3 exhibit two PdCl stretches correspond to a cis-PdCl 2 geometry, and a single ν(PdCl) band in 4 suggests one PdCl bond. Cyclopalladation is supported by a single crystal X-ray crystal structural study of Pd(NaiEtH)Cl ( 4b) and the metallation takes place at C(8′)-position. The solution of Pd(NaiR)Cl 2 ( 3) is also irreversibly transformed into Pd(NaiRH)Cl ( 4) when the pH is adjusted to 4.5–6.0 by NaOAc or other bases (NaOMe, NaOH, LiOH, Li 2CO 3 etc.). At higher pH values (8–10) the reaction shows the chelative hydroxylation at the C(2′)-site to synthesise Pd(NaiRO)Cl ( 5). The structure of the hydroxylated blue product is also supported by a single crystal X-ray crystal structure of Pd(NaiEtO)Cl ( 5b). The reaction of Pd(NaiR)Cl 2 in MeCN with dilute sodium hydroxide in air, or aqueous silver nitrate under boiling conditions, or its treatment with Tollen's reagent in MeCN solution under ambient conditions has also yielded the hydroxylated product. All the compounds have been characterised by elemental analyses, IR, UV–vis and 1H-NMR data. The solution spectral behaviour has been interpreted by EHMO calculations.