Gold(I) Alkynyls Supported by Mono- and Bidentate NHC Ligands: Luminescence and Isolation of Unprecedented Ionic Complexes

Reactions of NHC·HX (NHC = 1-benzyl-3-methylbenzimidazol-2-ylidene, X = Br^–, PF₆^–) and (AuCCR)_<i>n</i> (R = Ph, C₃H₆OH) in the presence of Cs₂CO₃ initially afford compounds of the general formula [(NHC)₂Au]₂[(RC₂)₂Au]­X, which can be isolated by crystallization. With increased reaction time, only the expected mononuclear complexes of the type [NHCAuCCR] are produced. The crystal structure of [(NHC)₂Au]₂[(PhC₂)₂Au]­PF₆ reveals an unprecedented triple-decker array upheld by a remarkably short (2.9375(7) Å) unsupported Au···Au···Au contact. The mononuclear complex [NHCAuCCPh] was found to crystallize as three distinct polymorphs and a pseudopolymorph, which depending on the intermolecular Au···Au distances emit blue, green, or yellow light. Two synthetic approaches were employed for the preparation of a series of dinuclear NHC-ligated Au­(I) alkynyl complexes of the general formula [NHC-(CH₂)_<i>n</i>-NHC­(AuCCR)₂], where NHC = <i>N</i>-benzylbenzimidazol-2-ylidene, R = Ph, C₃H₆OH, C₆H₁₀OH, and <i>n</i> = 1–3. In solution, the complexes with aliphatic substituents on the alkynyl fragment are nonemissive, whereas their phenyl-bearing congeners demonstrate characteristic metal-perturbed ³[IL­(CCPh)] emission. In the solid state, a clear correlation between intermolecular aurophilic interactions and luminescence was established, including their role in the luminescent thermochromism of the phenylalkynyl complexes. The relationship between the Au···Au distance and emission energy was found to be <i>inverse</i>: i.e., the shorter the aurophilic contact, the higher the emission energy. We tentatively attribute this behavior to a smaller extent of excited-state distortion for a structure with a shorter Au···Au separation

Aurophilicity in Action: Fine-Tuning the Gold(I)–Gold(I) Distance in the Excited State To Modulate the Emission in a Series of Dinuclear Homoleptic Gold(I)–NHC Complexes

The solution-state emission profiles of a series of dinuclear Au­(I) complexes <b>4</b>–<b>6</b> of the general formula Au₂(NHC-(CH₂)_<i>n</i>-NHC)₂Br₂, where NHC = <i>N</i>-benzylbenzimidazol-2-ylidene and <i>n</i> = 1–3, were found to be markedly different from each other and dependent on the presence of excess bromide. The addition of excess bromide to the solutions of <b>4</b> and <b>6</b> leads to red shifts of ca. 60 nm, and in the case of <b>5</b>, which is nonemissive when neat, green luminescence emerges. A detailed computational study undertaken to rationalize the observed behavior revealed the determining role aurophilicity plays in the photophysics of these compounds, and the formation of exciplexes between the complex cations and solvent molecules or counterions was demonstrated to significantly decrease the Au–Au distance in the triplet excited state. A direct dependence of the emission wavelength on the strength of the intracationic aurophilic contact allows for a controlled manipulation of the emission energy by varying the linker length of a diNHC ligand and by judicial choice of counterions or solvent. Such unique stimuli-responsive solution-state behavior is of interest to prospective applications in medical diagnostics, bioimaging, and sensing. In the solid, the investigated complexes are intensely phosphorescent and, notably, <b>5</b> and <b>6</b> exhibit reversible luminescent mechanochromism arising from amorphization accompanied by the loss of co-crystallized methanol molecules. The mechano-responsive properties are also likely to be related to changes in bromide coordination and the ensuing alterations of intramolecular aurophilic interactions. Somewhat surprisingly, the photophysics of NHC ligand precursors <b>2</b> and <b>3</b> is related to the formation of ground-state associates with bromide counterions through hydrogen bonding, whereas <b>1</b> does not appear to bind its counterions

Coordination to Imidazole Ring Switches on Phosphorescence of Platinum Cyclometalated Complexes: The Route to Selective Labeling of Peptides and Proteins via Histidine Residues

In this study, we have shown that substitution of chloride ligand for imidazole (Im) ring in the cyclometalated platinum complex Pt­(phpy)­(PPh₃)Cl (<b>1</b>; phpy, 2-phenylpyridine; PPh₃, triphenylphosphine), which is nonemissive in solution, switches on phosphorescence of the resulting compound. Crystallographic and nuclear magnetic resonance (NMR) spectroscopic studies of the substitution product showed that the luminescence ignition is a result of Im coordination to give the [Pt­(phpy)­(Im)­(PPh₃)]Cl complex. The other imidazole-containing biomolecules, such as histidine and histidine-containing peptides and proteins, also trigger luminescence of the substitution products. The complex <b>1</b> proved to be highly selective toward the imidazole ring coordination that allows site-specific labeling of peptides and proteins with <b>1</b> using the route, which is orthogonal to the common bioconjugation schemes via lysine, aspartic and glutamic acids, or cysteine and does not require any preliminary modification of a biomolecule. The utility of this approach was demonstrated on (i) site-specific modification of the ubiquitin, a small protein that contains only one His residue in its sequence, and (ii) preparation of nonaggregated HSA-based Pt phosphorescent probe. The latter particles easily internalize into the live HeLa cells and display a high potential for live-cell phosphorescence lifetime imaging (PLIM) as well as for advanced correlation PLIM and FLIM experiments