Luminescent PtAg Clusters Based on Neutral Benzoquinolate Cyclometalated Platinum Complexes

Complexes of the type [Pt(C6F5)(bzq)L] {bzq = 7,8-benzoquinolinate; L = PPh3 (2), 2,6-diphenylpyridine (pyPh(2), 3), tetrahydrothiophene (tht, 4), MeCN (5)} were prepared by replacing the acetone ligand in [Pt(C6F5)(bzq)(Me2CO)] (1) with the corresponding L ligand. The structures of complexes 2-4 were established by X-ray diffraction. Despite their neutral nature, complexes 1-5 react with AgClO4 in a 2:1 molar ratio to give the corresponding trinuclear complexes [{Pt(C6F5)(bzq)L}(2)Ag]ClO4 {L = Me2CO (6), PPh3 (7), pyPh(2) (8), tht (9), MeCN (10)}, which contain Pt -> Ag dative bonds. The structures of complexes 7, 9, and 10 were established by X-ray diffraction, which confirmed the existence of Pt-Ag bonds (ca. 2.8 angstrom) and short eta(1) bonding Ag-C interactions with the C-ipso atom of bzq (ca. 2.4 angstrom). Moreover, complexes 3, 4, 7, 9, and 10 show intermolecular pi center dot center dot center dot pi interactions between the aromatic rings of the bzq ligands (separations of ca. 3.5 angstrom). The reactions of 1-5 with [Ag(PPh3)(OClO3)] in a 1:1 molar ratio proceed with interchange of the ligands between the metals and the formation of [{Pt(bzq)(C6F5)(PPh3)}(2)Ag]ClO4 (7) and [AgL2]ClO4 (X-ray). Only in the case of L = pyPh2 (3) was the dinuclear complex [(C6F5)(bzq)(PPh3)PtAg(pyPh(2))]ClO4 (11) identified, and its structure was determined by X-ray diffraction. Complex 11 contains a Pt -> Ag [2.8147(1) angstrom] bond and a eta(1)-AgCipso(bzq) interaction [2.293(1) angstrom]. The electronic absorption and luminescence behaviors of 1-11 were investigated. The lower-lying absorption bands of the mononuclear complexes are ascribed to ligand-centered [(IL)-I-1, pi-pi* (bzq)] character mixed with some metal-to-ligand charge transfer [(MLCT)-M-1 5d(Pt)->pi*(bzq)]. For the trinuclear complexes, these bands are assigned to (ILCT)-I-1/(MMLCT)-M-1 [pi-pi*(bzq)]/{d/s(Pt,Ag)}->pi*(bzq)] transitions and to mixed (MLCT)-M-1/L-1 ' LCT [MLCT 5d(Pt)->pi*(bzq)]/[L ' LCT, Ar-f -> bzq] for dinuclear complex 11 on the basis of time-dependent TD (DFT) calculations carried out on 2, 4, 5, 9, 10, and 11Me in CH2Cl2. Only 2 and the heteronuclear compounds are emissive in the solid state at room temperature; however, all of the complexes are emissive at 77 K (solids and glasses). In each case, the main phosphorescent emission seems to be due to a transition similar in character to the lowest-energy electronic absorption.