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Synthesis, Structural, and Photophysical Studies of Phenylquinoline and Phenylquinolinyl Alkynyl Based Pt(II) Complexes

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posted on 23.06.2014, 00:00 by Elena Lalinde, M. Teresa Moreno, Santiago Ruiz, Sergio Sánchez
We present the synthesis, characterization, and photophysical properties of two series of alkynyl–platinum­(II) complexes containing the phenylquinoline (Hpq) or the cyclometalated phenylquinolinyl (pq) ligand. Unusual monoanionic complexes (NBu4)­[Pt­(CCR)3­(Hpq)] (R = Tol 1, C6H4OMe-3 2, C6H4CF3-4 3) were generated in moderate yield by treatment of [Pt­(pq)­(μ-Cl)]2 with an excess of LiCCR (1:8), followed by subsequent treatment with NBu4 in iPrOH. However, a similar reaction with LiCCtBu evolves with formation of [Pt­(pq)­(μ–κCα:​η2-CCtBu)]2 4. Related dimers [Pt­(pq)­(μ–κCα:​η2-CCR)]2 (R = Tol 5, C6H4OMe-3 6, C6H4CF3-4 7) were successfully prepared by modification of the reaction conditions. Single-crystal studies were performed on 2, 4, and 5, revealing the presence of intramolecular (H···Pt and H···CC) and intermolecular Omethoxy···HHpq contacts in 2 and the formation of butterfly structures in 4 and 5. In the tBu derivative 4, a shorter intramolecular Pt···Pt separation was found [3.0936(3) Å in 4 vs 3.2109(3) Å in 5], which is related to the nature of η2-alkynyl linkages (η2-asymmetrical in 4 vs η2-symmetrical in 5). Anionic 13 display bright emissions in rigid media (solid, glass) with resolved side vibronic bands at 77 K. The energy of the emissions (supported by computational studies on 2) allows us to ascribe these emissions primarily to a mixed alkynyl–metal to the phenylquinoline 3[(L′ + M)­LCT] excited state, being perturbed in 2 (solid) by the intermolecular Omethoxy···HHpq contacts. Whereas diplatinum complexes 57 have no visible emission at 298 K in the solid state, 4 shows a structureless band (578 nm), which is significantly red-shifted at 77 K (595 nm). Theoretical studies suggest a notable distortion in the excited state in relation to S0 and identify the emission within the dimetallacycle Pt2(CC)2 with alkynyl to pq charge-transfer 3[(MM + L′)­LCT] [with minor 3IL (pq) contribution], whereas, in 5, the emission is tentatively ascribed to a mixture of 3L′LCT with minor 3MMLCT character. The emission profiles observed in glassy solution for all 47 complexes (556–562 nm, CH2Cl2) and in the solid state for 67 are consistent with those exhibited by mononuclear related chromophores 3[(L′ + M)­LCT] (L′ = CCR, L = pq), suggesting negligible contribution of the Pt···Pt interaction.