Competition between N,C,N‑Pincer and N,N‑Chelate Ligands in Platinum(II)

Replacement of the chloride ligand of PtCl­{κ3-N,C,N-[py-C6HR2-py]} (R = H (1), Me (2)) and PtCl­{κ3-N,C,N-[py-O-C6H3-O-py]} (3) by hydroxido gives Pt­(OH)­{κ3-N,C,N-[py-C6HR2-py]} (R = H (4), Me (5)) and Pt­(OH)­{κ3-N,C,N-[py-O-C6H3-O-py]} (6). These compounds promote deprotonation of 3-(2-pyridyl)­pyrazole, 3-(2-pyridyl)-5-methylpyrazole, 3-(2-pyridyl)-5-trifluoromethylpyrazole, and 2-(2-pyridyl)-3,5-bis­(trifluoromethyl)­pyrrole. The coordination of the anions generates square-planar derivatives, which in solution exist as a unique species or equilibria between isomers. Reactions of 4 and 5 with 3-(2-pyridyl)­pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt­{κ3-N,C,N-[py-C6HR2-py]}­{κ1-N1-[R′pz-py]} (R = H; R′ = H (7), Me (8). R = Me; R′ = H (9), Me (10)), displaying κ1-N1-pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N1-to-N2 slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt­{κ3-N,C,N-[py-C6HR2-py]}­{κ1-N1-[CF3pz-py]} (R = H (11a), Me (12a)) and Pt­{κ3-N,C,N-[py-C6HR2-py]}­{κ1-N2-[CF3pz-py]} (R = H (11b), Me (12b)). 1,3-Bis­(2-pyridyloxy)­phenyl allows the chelating coordination of the incoming anions. Deprotonations of 3-(2-pyridyl)­pyrazole and its substituted 5-methyl counterpart promoted by 6 lead to equilibria between Pt­{κ3-N,C,N-[pyO-C6H3-Opy]}­{κ1-N1-[R′pz-py]} (R′ = H (13a), Me (14a)) with a κ-N1-pyridylpyrazolate anion, keeping the pincer coordination of the di­(pyridyloxy)­aryl ligand, and Pt­{κ2-N,C-[pyO-C6H3(Opy)]}­{κ2-N,N-[R′pz-py]} (R′ = H (13c), Me (14c)) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt­{κ3-N,C,N-[pyO-C6H3-Opy]}­{κ1-N1-[CF3pz-py]} (15a), Pt­{κ3-N,C,N-[pyO-C6H3-Opy]}­{κ1-N2-[CF3pz-py]} (15b), and Pt­{κ2-N,C-[pyO-C6H3(Opy)]}­{κ2-N,N-[CF3pz-py]} (15c). The N1-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of 4–6 with 2-(2-pyridyl)-3,5-bis­(trifluoromethyl)­pyrrole yield Pt­{κ3-N,C,N-[py-C6HR2-py]}­{κ1-N1-[(CF3)2C4(py)­HN]} (R = H (16), Me (17)) or Pt­{κ3-N,C,N-[pyO-C6H3-Opy]}­{κ1-N1-[(CF3)2C4(py)­HN]} (18), displaying κ1-N1-pyrrolate coordination. Complexes 7–10 are efficient green phosphorescent emitters (488–576 nm). In poly­(methyl methacrylate) (PMMA) films and in dichloromethane, they experience self-quenching, due to molecular stacking. Aggregation occurs through aromatic π–π interactions, reinforced by weak platinum–platinum interactions.