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 p-p* and the low energy absorptions originate from intraligand p-p* with a small admixture of metal-to-ligand charge-transfer transitions.



Keywords and Phrases

Activation analysis; Chelation; Chemical bonds; Energy absorption; Isomers; Palladium; Palladium compounds; Regioselectivity; C-h activation; Coordination sphere; Cyclometallation; High-energy absorption; Metal to ligand charge transfers; Neutral Lewis basis; Single crystal x-ray structures; Time dependent density functional theory; Naphthalene; C-H activation; Cyclometallation; Palladium

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