Synthesis of Pd(II) complexes of unsymmetrical, hybrid selenoether and telluroether ligands: Isolation of tellura-palladacycles by fine tuning of intramolecular chalcogen bonding in hybrid telluroether ligands

Abstract

Two novel unsymmetrical selenoether ligands namely, N,N-dimethyl-2-(o-tolylselanyl)aniline (10) and 2-((2,6-dimethylphenyl)selanyl)-N,N-dimethylaniline (11) were synthesized from N,N-dimethylbenzylamine via the ortho-lithiation route. When ligands 10 and 11 were treated with Pd(COD)Cl2 (COD = 1,5-cyclooctadiene), the reaction afforded novel Pd(II) complexes of the form Pd(2-NMe2CH2C6H4)(R)SeCl2, where R = 2-MeC6H4 (16) and 2,6-MeC6H3 (17) respectively. Similarly, when unsymmetrical telluroether ligands, N,N-dimethyl-2-(phenyltellanyl)aniline (12), N,N-dimethyl-2-(o-tolyltellanyl)aniline (13), 2-((2,6-dimethylphenyl)tellanyl)-N,N-dimethylaniline (14) and 2-((2,6-diisopropylphenyl)tellanyl)-N,N-dimethylaniline (15), which contain one coordinating group with an sp3 N-donor atom, were treated with Pd(COD)Cl2, the palladium telluroether complexes of the form Pd(2-NMe2CH2C6H4)(R)TeCl2, where R = C6H5 (18), 2-MeC6H4 (19), 2,6-MeC6H3 (20) and 2,6-iPrC6H3 (21) respectively were obtained. The selenoether complexes 16 and 17 and telluroether complexes 18, 20 and 21 were characterized by single crystal X-ray diffraction studies. In the asymmetric unit of complex 18, two molecules are present and due to the presence of weak, intermolecular Te⋯Cl interactions, the two molecules exist as symmetrically related dimer. The N-donor substituents, which were involved in intramolecular chalcogen bonding (IChB) with the chalcogen atoms in the free ligand, made six-membered chelating rings in the complexes. Again, when bis(2-phenylazophenyl-C,N′)selenide 22 was treated with Pd(COD)Cl2, it afforded ortho C–H bond activated cyclopalladated complex 24. However, when bis(2-phenylazophenyl-C,N′)telluride 23 was treated with Pd(COD)Cl2, due to strong IChB from two N-donor substituents, the reaction afforded cleaved product, [2-(phenylazo)phenyl-C,N′]tellurenyl chloride, 25. All the metal complexes exhibited significant downfield chemical shifts in 77Se/125Te NMR spectra as compared to the corresponding free ligands. The downfield shifts in the 77Se/125Te NMR signal in the complexes might be attributed to the coordination of the chalcogen atoms to the electropositive Pd(II) center.