Regioselective and regiospecific C(naphthyl)–H bond activation: Isolation, characterization, crystal structure and TDDFT study of isomeric cyclopalladates

The C2(naphthyl)–H, C3(naphthyl)–H and C8(naphthyl)–H bonds of the naphthyl group present in a group of naphthylazo-2′-hydroxyarenes (H2L) have been activated at room temperature by palladium(II) and stable cyclopalladates of the type [PdL(B)] have been isolated in presence of neutral Lewis bases (B). The activation of C2(naphthyl)–H and C8(naphthyl)–H bonds of 1-(2′-hydroxynaphthylazo) naphthalene (H2L1) lead to the formation of isomeric cyclopalladates 2a & 2b respectively. The single crystal X-ray structures of both the isomers show the naphthylazonaphtholate is coordinated to palladium(II) as a dianionic terdentate C,N,O-donor and Lewis base B occupies the fourth position in the coordination sphere. The ortho-palladate (2a) contains both five-membered carbopalladacycle and azonaphtholato chelate ring whereas a five-membered carbopalladacycle and a six-membered azonaphtholato chelate ring are present in peri-palladate (2b). On the other hand, only C3(naphthyl)–H bond of 2-(2′-hydroxyarylazo)naphthalene (H2L2 & H2L3) has been found to be regiospecifically activated by palladium(II). The role of auxiliary donors on the regioselective and regiospecific C(naphthyl)–H bond activation and the rationale behind the formation of isomeric cyclopalladates have been discussed. All of the cyclopalladates absorb strongly in the ultraviolet and visible region. The Time-dependent density functional theory (TDDFT) calculations reveal that the high energy absorptions are predominantly due to intraligand π–π* and the low energy absorptions originate from intraligand π–π* with a small admixture of metal-to-ligand charge-transfer transitions.