New C∧N∧C Bis-Cyclometalated Platinum(II) Complexes: Synthesis, Structures, and Photophysical Properties

The reaction of a new ligand, ethyl 2,6-diphenylisonicotinate (EtO2C–C∧N∧C–H2), with K2PtCl4 in acetic acid affords the monocyclometalated complex [{(EtO2C–C∧N∧C–H)­Pt­(μ-Cl)}2] (1), which transforms to the bis-cyclometalated derivative [Pt­(EtO2C–C∧N∧C)­(DMSO)] (2) when heated in hot DMSO. Complex 2 is the precursor for preparing a new series of neutral mononuclear bis-cyclometalated complexes [Pt­(EtO2C–C∧N∧C)­(L)] (L = tht (3), PPh3 (4), CN-tBu (5), py (6), py-tBu (7), py-NH2 (8), py-CN (9), py-CONH2 (10)). These new complexes have been characterized spectroscopically, and structures of 2–10 have been determined crystallographically. Within each crystal structure the individual molecules pack in a head-to-tail arrangement. Noncovalent interactions, including π···π, C–H···O, C–H···N, N–H···Pt, N–H···π, C–H···π, and N–H···O, contribute significantly to the supramolecular structures displayed by these complexes in the solid state. All complexes display UV–vis absorptions in dichloromethane solution. Excitation and emission studies as well as lifetime measurements are described and can be correlated to the solid-state structures of the complexes. DFT and TDDFT computational studies have been performed on 5 and 8 which support the conclusions drawn from the photophysical studies.