Spectroscopy, photoredox properties and X-ray crystal structures of triangular gold(I) and silver(I) phosphine complexes

Abstract

The reaction of HC(PPh₂)₃ with K[AuCl₄] in the presence of 2,2′-thiodiethanol in methanol yielded [Au₃{HC(PPh₂)₃}₂Cl]²⁺. The related [Ag₃{HC(PPh₂)₃}₂]³⁺ was prepared by the reaction of HC(PPh₂)₃ with Ag(CF₃SO₃) in dichloromethane. Both complex cations were isolated as perchlorate salts. The X-ray structures of [Au₃{HC(PPh₂)₃}₂Cl][ClO₄]₂ and [Ag₃{HC(PPh₂)₃}₂][ClO₄]₃ have been determined: [Au₃{HC(PPh₂)₃}₂Cl][ClO₄]₂·2MeCN, monoclinic, space group C2/c(no. 15), a= 26.466(3), b= 12.741(2), c= 23.012(5)Å, β= 98.89(1)° and Z= 4; [Ag₃{HC(PPh₂)₃}₂][ClO₄]₃·2MeCN, monoclinic, space group, P2₁/n, a= 14.597(4), b= 23.047(2), c= 22.485(3)Å, β= 91.49(2)° and Z= 4. Both [Au₃{HC(PPh₂)₃}₂Cl]²⁺ and [Ag₃{HC(PPh₂)₃}₂]³⁺ consist of three metal atoms arranged in a nearly equilateral triangle with the M–M–M angles close to 60°. The measured intramolecular Ag–Ag and Au–Au distances of 3.1618(5)–3.2228(9) and 2.9220(8)–3.0889(8)Å respectively, indicate that the Au–Au bonding interaction is much stronger than that for Ag–Ag. The UV/VIS spectrum of an acetonitrile solution of [Au₃{HC(PPh₂)₃}₂Cl]²⁺ exhibits intense absorption bands at 270 and 290 nm, assignable to the d_σ·→ p_σ transitions. For [Ag₃{HC(PPh₂)₃}₂]³⁺ the UV/VIS spectrum is virtually identical to that of the free HC(PPh₂)₃. Excitation of a degassed acetonitrile solution of [Au₃{HC(PPh₂)₃}₂Cl]²⁺ at 300–400 nm at room temperature leads to an observed photoluminescence centred at ca. 537 nm with a lifetime of 11 µs [ϕ=(1.0 ± 0.1)× 10^–3]. The excited-state redox potential, [Au₃{HC(PPh₂)₃}₂Cl]³⁺+ e^–→[Au₃{HC(PPh₂)₃}₂Cl]²⁺*, determined by quenching studies with a series of pyridinium acceptors, is –1.6(2) V vs. saturated sodium chloride calomel electrode (SSCE).