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

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 1–3 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 5–7 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 4–7 complexes (556–562 nm, CH2Cl2) and in the solid state for 6–7 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.