Benzoquinolateplatinum(ii) complexes as building blocks in the synthesis of Pt–Ag extended structures

Abstract

The reaction between (NBu₄)[Pt(bzq)(C₆F₅)₂] (1, bzq = 7,8-benzoquinolate) and AgClO₄ in a 1 : 1 molar ratio, in acetone, gives the polymer [{Pt(bzq)(C₆F₅)₂}Ag]_n (2). The reaction of 2 with equimolecular amounts of PPh₃ and SC₄H₈ (tht) produces the bimetallic complexes [{Pt(bzq)(C₆F₅)₂}AgL] (L = PPh₃ (3), tht (4)). For L = py, decomposition takes place and [Pt(bzq)(C₆F₅)py] (5) is obtained. All these complexes have been characterized by X-ray diffraction. The most interesting features of complexes 2–4 is the presence of Pt–Ag bonds, with Pt–Ag distances of ca. 2.75 Å. Besides, the silver centres establish short η¹ bonding interactions with the C_ipso of the bzq ligands, with distances Ag–C of ca. 2.45 Å. Complex 2 is a one-dimensional infinite chain in which the fragments “Pt(bzq)(C₆F₅)₂⁻” and Ag⁺ alternate. On the other hand, complexes 1 and 3–5 show intermolecular pairing through π⋯π interactions between the aromatic rings of the bzq ligand, having interplanar separations of ca. 3.5 Å. Complex 2 dissolves in donor solvents (acetone, THF) as discrete bimetallic solvated fragments [{Pt(bzq)(C₆F₅)₂}AgS_n] (S = solvent), similar to complexes 3 and 4. The persistence of the Pt–Ag bond in 2–4, supported by multinuclear NMR spectroscopy, causes a significant blue-shift in the lowest-lying absorption in relation to 1. This fact is attributed (TD-DFT) to a remarkable modification of the orbitals contributing to the HOMO, which changes the character of the transition from ¹LC/¹MLCT in 1 to admixture ¹L′LCT/¹MLCT in the bimetallic complexes. The low energy feature (490–530 nm) of 2 in solid state is attributed to CT from the Pt fragments to the Ag centers. Complexes 2–4 are only emissive in rigid media (solid and glasses). In the solid state, the metallic chain 2 exhibits a bright orange emission (560 nm, 298 K; 590 nm, 77 K), assigned to an excited state involving charge transfer from the platinum fragment with a remarkable contribution of C₆F₅ (Ar_f) rings to the Pt–Ag bond (³LMM′CT/³L′M′CT). However, 3 and 4 exhibit in solid state at 298 K a vibronic band, which is clearly resolved in two close non-equilibrated bands at 77 K in 3, tentatively ascribed to a mixture of ³MLCT/³L′LCT transitions modified by the formation of the Pt–Ag bond. In glassy solution (77 K) 2–4 display a vibronic emission ascribed primarily to ³LC character.