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.