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

## TL;DR

This study explores how different ligands compete to bind with platinum(II) and identifies new efficient green light-emitting compounds.

## Contribution

The paper establishes coordination priorities between pincer and chelate ligands in platinum(II) and isolates new phosphorescent complexes.

## Key findings

- Pyridylpyrazolates are better chelate ligands than pyridylpyrrolates.
- Complexes 7–10 are efficient green phosphorescent emitters (488–576 nm).
- Molecular stacking in PMMA films causes self-quenching due to π–π and Pt–Pt interactions.

## Abstract

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.

A study about the competition between
the N,C,N-pincer
ligands 1,3-bis(2-pyridyl)phenyl
and 1,3-bis(2-pyridyloxy)phenyl with the N,N-chelate ligands pyridylpyrazolate or pyridylpyrrolate
in platinum(II) is presented. The study has allowed us to establish
coordination priorities between these classes of ligands, isolate
complexes with unusual coordination modes, and discover new highly
efficient green phosphorescent emitters.