Competition Between ,,-Pincer and ,-Chelate Ligands in Platinum(II)

Replacement of the chloride ligand of PtCl{κ-,,-[py-CHR-py]} (R = H (), Me ()) and PtCl{κ-,,-[py-O-CH-O-py]} () by hydroxido gives Pt(OH){κ-,,-[py-CHR-py]} (R = H (), Me ()) and Pt(OH){κ-,,-[py-O-CH-O-py]} (). 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 and with 3-(2-pyridyl)pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt{κ-,,-[py-CHR-py]}{κ--[R'pz-py]} (R = H; R' = H (), Me (). R = Me; R' = H (), Me ()), displaying κ--pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N-to-N slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt{κ-,,-[py-CHR-py]}{κ--[CFpz-py]} (R = H (), Me ()) and Pt{κ-,,-[py-CHR-py]}{κ--[CFpz-py]} (R = H (), Me ()). 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 lead to equilibria between Pt{κ-,,-[pyO-CH-Opy]}{κ--[R'pz-py]} (R' = H (), Me ()) with a κ--pyridylpyrazolate anion, keeping the pincer coordination of the di(pyridyloxy)aryl ligand, and Pt{κ-,-[pyO-CH(Opy)]}{κ--[R'pz-py]} (R' = H (), Me ()) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt{κ-,,-[pyO-CH-Opy]}{κ--[CFpz-py]} (), Pt{κ-,,-[pyO-CH-Opy]}{κ--[CFpz-py]} (), and Pt{κ-,-[pyO-CH(Opy)]}{κ-,-[CFpz-py]} (). The N-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of - with 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole yield Pt{κ-,,-[py-CHR-py]}{κ--[(CF)C(py)HN]} (R = H (), Me ()) or Pt{κ-,,-[pyO-CH-Opy]}{κ--[(CF)C(py)HN]} (), displaying κ--pyrrolate coordination. Complexes - 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.