Neutral benzoquinolate cyclometalated platinum(II) complexes as precursors in the preparation of luminescent Pt–Ag complexes

Abstract

Complexes [Pt(C6F5)(bzq)(CNR)] {R=tBu (1), 2-naphthyl (2), 2,6-Me2Ph (3)} have been prepared by replacement of the labile acetone in [Pt(C6F5)(bzq)(Me2CO)] with the corresponding isocyanide ligand. The structures 1 and 3 (X-ray) confirms their square planar geometry, with the bzq ligand coplanar to the metal plane. The reactions of 1–3 toward AgClO4 lead to the trinuclear complexes [{Pt(C6F5)(bzq)(CNR)}2Ag]ClO4 {R=tBu (4), 2-naphthyl (5)} or the tetranuclear [{Pt(C6F5)(bzq)(CN-2,6-Me2Ph)Ag}2](ClO4)2 (6). The structures of these three polynuclear complexes have been established by X-ray diffraction studies. Two pseudo-polymorphs of 4 (4a and 4b), only differing in the crystallization solvent, have been found. 4a/b and 5 are trinuclear Pt–Ag–Pt complexes with a “sandwich” disposition. In all three cases, two “Pt(C6F5)(bzq)(CNR)” fragments are linked by a silver atom through Pt→Ag bonds of donor acceptor nature and the silver center establishes a short η1 interaction with the Cipso of the bzq ligands. Furthermore, the study of the structure of 6 reveals two “(C6F5)(bzq)(CN-2,6-Me2Ph)PtAg” subunits, each containing a Pt→Ag bond related by an inversion center and held together through η2 interactions established by the silver atom of one unit and one of the aromatic rings of the bzq ligand of the other. The reaction of 1–3 with [Ag(OClO3)(PPh3)] produces [(C6F5)(bzq)(CNR)PtAg(PPh3)]ClO4 {R=tBu, (7), 2-naphthyl (8), 2,6-Me2Ph (9)}. These dinuclear clusters contain a Pt→Ag bond and show dynamic processes in solution (NMR) which involve the rupture and formation of these interactions. The X-ray structure of 7 confirms the presence of a Pt–Ag bond and a η1 interaction with the Cipso of the bzq ligand (2.514(6) Å). All the crystal structures determined show π⋯π interactions of the bzq which stack in a parallel fashion with interplanar distances of ca. 3.3–3.4 Å. In 5 and 6 the aromatic fragments of the isocyanide ligands also take part in these π⋯π interactions. Absorption and emission properties of all the complexes 1–9 have been studied and explained with the aid of TD-DFT theoretical calculations.Compounds 1–9 are not emissive in solution but they are in rigid matrix. In glassy 2-Me-THF or CH2Cl2 at 77K the Pt/Ag compounds 4–9 at any concentration (10−3M, 10−4M, 10−5M) show the same emission spectra as their corresponding starting complexes 1–3, in line with the rupture of the Pt-Ag bonds. In the solid state only the dinuclear compounds [Pt(C6F5)(bzq)(CN-tBu)AgPPh3]ClO4 (7) and [Pt(C6F5)(bzq)(CN-2,6-Me2Ph)AgPPh3]ClO4 (9) together with the tetranuclear complex [{Pt(C6F5)(bzq)(CN-2,6-Me2Ph)Ag}2](ClO4)2 (6) are emissive at room temperature showing bright greenish (7, 9) or yellowish (6) phosphorescence. In all three compounds a significant contribution of M′ orbitals (AgPPh3/Ag) to the frontier orbitals (FO) has been observed, and their main emissions seem to arise from mixed excited states 3ILCT (π–π∗ (bzq))/3MM′LCT (Pt/AgPPh3→bzq) (7, 9) or 3LMM′CT [(bzq)→Pt/Ag]/3ILCT [π–π∗ (bzq)] (6) in nature.