Luminescent Gold(III) Alkynyl Complexes: Synthesis, Structural Characterization, and Luminescence Properties

Abstract

In contrast to the related gold(i) [1][2][3] and the isoelectronic platinum(ii) compounds, [4,5] which are known to show rich luminescence properties, luminescent gold(iii) compounds are rare, with very few exceptions that emit at room temperature in solution.[6] The reasons for the lack of luminescence in gold(iii) species are probably the presence of low-energy d-d ligand field (LF) states and the electrophilicity of the gold(iii) center. The presence of a nonemissive low-lying d-d state would quench the luminescence excited state by thermal equilibration or energy transfer.[7] Coupling of strong sdonating alkynyl ligands to gold(iii) should render the metal center more electron rich, with the additional advantage of raising the energy of the d-d states, which would result in enhanced luminescence by increasing the chances of populating the emissive state for the construction of luminescent organometallic materials. Despite the fact that alkynyl complexes of gold(i) [3] and the isoelectronic platinum(ii) [5] are known and have been shown to display rich photoluminescence properties, to our surprise, alkynyl complexes of gold(iii) are extremely rare.[8] Although there is increasing interest in the use of gold(iii) compounds for catalysis of organic synthetic reactions of alkynes, [9] the chemistry of gold(iii) alkynyls is essentially unexplored and underdeveloped, and their luminescence properties are virtually unknown.Herein we describe the synthesis of a novel series of biscyclometalated gold(iii) alkynyl complexes [Au(C^N^C)-(C CR)] [HC^N^CH = 2,6-diphenylpyridine, R = C 6 H 5 (1), C 6 H 4 -Cl-p (2), C 6 H 4 -OCH 3 -p (3), C 6 H 4 -NH 2 -p (4); HC^N^CH = 2,6-bis(4-tert-butylphenyl)pyridine, R = C 6 H 5 (5)], which are the first of their kind (Scheme 1). The molecular structure of 1 was determined by X-ray crystallography. The photophysical properties of 1-5 were also studied.Unlike most other gold(iii) compounds, which exhibit luminescence only at low temperature or are nonemissive, complexes 1-5 display luminescence in various media at both low and ambient temperature. Figure 1 shows the crystal structure of 1, in which the gold(iii) center adopts a distorted square-planar coordination geometry with C (9)

Syntheses and Photophysical Properties of Luminescent Mono-cyclometalated Gold(III) cis-Dialkynyl Complexes

“…These orbitals are quite susceptible for thermal population at ambient temperatures which then follows rapid nonradiative relaxation pathways. 2 Here in this work we report the syntheses of stable mononuclear cyclometalated cisgold(III)-σ-dialkynyl complexes and their luminescent properties. ABSTRACT: A series of novel luminescent neutral cyclometalated gold(III) complexes of the type cis-[(N ∧ C)Au(CtCR) 2 ] (R = aryl, silyl groups) having different cyclometalating cores (N ∧ C) have been synthesized by CuI promoted halide to alkynyl metathesis with NEt 3 as in situ deprotonating agent.…”

“…Notables among the few known examples include mononuclear gold(III) complexes containing one alkynyl ligand, viz., [Au(CtCCF 3 )Me 2 (L)] and [Au(CtCR)Me 2 (PPh 3 )], 10,11 [Au(C ∧ N ∧ C)(CtCR)], 2,12,13 and three alkynyl ligands [Au (CtCR) 3 4 ] þ [Au(CtCR) 4 ] À . 14,15 Dinuclear gold(I)Àgold(III) complexes bearing two alkynyl ligands, viz., [Au I (μ-{CH 2 }PPh 2 ) 2 Au III (CtCR) 2 ], have also been reported 16 (Figure 1). In general, gold(III) complexes tend to exhibit relatively large positive reduction potentials, 17,18 and the consequent instability is a probable reason for their scarcity.…”

Unique Metal‐Diyne, ‐Enyne, and ‐Enediyne Complexes: Part of the Remarkably Diverse World of Metal‐Alkyne Chemistry

“…In contrast to the related Au(I) (224,234,(241)(242)(243)(244)(245)(246)(247)(248)(249)(250)(251)(252)(253)(254)(255)(256), and the isoelectronic Pt(II) compounds (16,250,257,258) that show excellent luminescence properties, room temperature luminescent Au(III) compounds are rare with only a limited number of examples known (259)(260)(261). This same trend holds for alkynyl complexes; Au(I) (240-242, 246, 249) and Pt(II) species (16,262,263) are widely known and possess rich photoluminescence properties, while the number Au(III) examples is quite limited (257,259,264,265). The primary reason for this is the low-energy ligand-field states that are capable of quenching the emissive excited state (266).…”

“…Additionally, the ease in metal center reduction to Au(I) makes these complexes susceptible to participation in photoinduced electron transfer as a secondary quenching pathway to luminescence. One of the advantages of coupling strong s-donating alkynyl ligands to Au(III) is that the strong bonding interaction raises the energy of the ligand-field states, at the same time decreasing the electrophilicity of the Au(III) center, which enhances the overall luminescence of the compound (259,264). For example, cyclometalated Au(III)-alkynyl compounds 1-5 ( Fig.…”

Alkynyl derivatives of gold complexes containing C6H3-5-Me-2-EPh2 (E = P, As) ligands

“…In sharp contrast to gold (I), gold(III) alkynyl complexes are relatively uncommon and, to the best of our knowledge, there are no reports of gold(II) alkynyl complexes. Gold(III) complexes of the type [Au(C^CCF 3 )Me 2 (L)] (L ¼ PMe 2 Ph, PPh 3 ) [7e9] and [Au(C^CR)Me 2 (PPh3)] (R ¼ H, Me) have been prepared by Schmidbaur et al [10] and the Yam group reported the first examples of the luminescent behavior of gold(III) alkynyl complexes of the type [Au(C^N^C)(C^CR)] [HC^N^CH ¼ 2,6-diphenylpyridine, R ¼ Ph, C 6 H 4 -Cl-p, C 6 H 4 -OCH 3 -p, C 6 H 4 -NH 2 -p; HC^N^CH ¼ 2,6-bis(4-t-butylphenyl)pyridine, R ¼ Ph] [11]. A series of gold(I)-gold(III) complexes of the type [Au I (m-{CH 2 } 2 PPh 2 ) 2 Au III (C^CR) 2 ] [R]Ph, t Bu, SiMe 3 ] has also been reported, and represents the first fully characterized examples of gold complexes containing two alkynyl ligands bound to a gold(III) center [12].…”