Click Reaction Driven, Highly Fluorescent Dinuclear Organogold(I) Complex Exhibits a Dual Role: A Rare Au···H Interaction and an Antiproliferative Agent

Recent Trends in Group 11 Hydrogen Bonding

Conventional hydrogen bonding (H-bonding) has been extensively studied in organic and biological systems. However, its role in transition metal chemistry, particularly with Group 11 metals (i. e. Cu, Ag, Au) as hydrogen bond acceptors, remains relatively unexplored. Through a combination of experimental techniques, such as Nuclear Magnetic Resonance (NMR), Infrared spectroscopy (IR), X-Ray Diffraction (XRD), and computational calculations, several aspects of H-bonding interactions with Group 11 metals are examined, shedding light on its impact on structural motifs and reactivity. These include bond strengths, geometries, and effects on electronic structures. Understanding the intricacies of hydrogen bonding within transition metal chemistry holds promise for various applications, including catalytic transformations, the construction of molecular assemblies, synthesis of complexes displaying anticancer activities, or luminescence applications (e. g. Thermally Activated Delayed Fluorescence, TADF). This review encompasses the most significant recent advances, challenges, and future prospects in this emerging field.

Deciphering the Primary Role of Au⋅⋅⋅H-X Hydrogen Bonding in Gold Catalysis

Au⋅⋅⋅H-X (X=N or C) hydrogen bonding is gaining increasing interest, both in the study of its intrinsic nature and in their operability in different fields. While the role of these interactions has been studied in the stabilization of gold(I) complexes, their role during the minimum free energy reaction pathway of a given catalytic process remains unexplored. We report herein that complex [Au(C≡CPh)(pip)] (pip=piperidine) catalyses the A3 -coupling reaction for the synthesis of propargylamines, thanks to the ability of Au(I) to promote weak hydrogen bonding interactions with the reactants along the free energy profile. Density Functional Theory (DFT) calculations show that these Au⋅⋅⋅H-X interactions play a directing role in the catalysed A3 -coupling. Topological non-covalent interactions (NCI), interaction region indicator (IRI) and quantum theory of atoms in molecules (QTAIM) analysis in real space of the electron density provide a description of these interactions accurately.

Isostructural Series of Ni(II), Pd(II), Pt(II), and Au(III) Azido Complexes with a N^C^N Pincer Ligand to Elucidate Trends in the iClick Reaction Kinetics and Structural Parameters of the Triazolato Products

An isoelectronic and isostructural series of cyclometalated azido complexes [M(N3)(dpb)] with M = Ni(II), Pd(II), Pt(II), and Au(III) based on the N^C^N pincer ligand 1,3-di(2-pyridyl)phenide (dpb) was characterized by X-ray diffraction analysis and investigated for reactivity in the iClick reaction with a wide range of internal and terminal alkynes by using 1H and 19F NMR spectroscopy. Reaction rate constants were found to increase with greater charge density in the order Ni(II) > Pd(II) > Pt(II) > Au(III). Terminal alkynes R-C≡C-R' with strongly electron-withdrawing groups R and R' exhibited faster kinetics than those with electron-donating substituents in the order CF3 > ketone > ester > H > phenyl ≫ amide, while R = CH3 resulted in complete loss of reactivity. Four symmetrical triazolato complexes [M(triazolatoCOOCH3,COOCH3)(dpb)] with M = Ni(II), Pd(II), Pt(II), and Au(III) as well as four nonsymmetrically substituted triazolato complexes [Pt(triazolatoR,R')(dpb)] originating from terminal and internal alkynes were shown by X-ray crystal structure analysis to exclusively feature N2-coordination of the five-membered ring ligand. However, the Pt(II) triazolato complexes exist as a mixture of N1- and N2-coordinated species in solution. Torsion angles between the mean planes of the N^C^N pincer and the triazolato ligand increase from a nearly coplanar to a perpendicular arrangement when going from Au(III)/Pt(II)/Pd(II) to Ni(II), while different substituents R and R' on the alkyne have no influence on the torsion angle and were rationalized by DFT calculations. Finally, a carbohydrate derivative obtained by glucuronic acid conjugation to methyl propiolate demonstrates the facile biofunctionalization of metal complexes via the iClick reaction.

Click-derived multifunctional metal complexes for diverse applications

The Click reaction that involves Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) serves as the most potent and highly dependable tool for the development of many complex architectures. It has paved the way for the synthesis of numerous drug molecules with enhanced synthetic flexibility, reliability, specificity and modularity. It is all about bringing two different molecular entities together to achieve the required molecular properties. The utilization of Click chemistry has been well demonstrated in organic synthesis, particularly in reactions that involve biocompatible precursors. In pharmaceutical research, Click chemistry is extensively utilized for drug delivery applications. The exhibited bio-compatibility and dormancy towards other biological components under cellular environments makes Click chemistry an identified boon in bio-medical research. In this review, various click-derived transition metal complexes are discussed in terms of their applications and uniqueness. The scope of this chemistry towards other streams of applied sciences is also discussed.

Heart of gold: enabling ligands for oxidative addition of haloorganics in Au(I)/Au(III) catalysed cross-coupling reactions

The field of Au-catalysis has been an area rich with new discoveries due to the unique properties of the lustrous element. In the past decade, developments in Au(I)/Au(III) cross-coupling methodology have been made possible with the use of external oxidants that facilitate the challenging oxidation of Au(I) to Au(III) in a stable and catalytically competent fashion. Until recently, Au-chemistry was not known to undergo catalytic transformations that feature oxidative addition of haloarenes like those that were made famous by transition metals such as Pd and Ni. The discovery that ligand modification could facilitate the oxidative addition of Au(I) with haloorganics to provide Au(III) intermediates that are competent in other areas of catalysis (i.e. Lewis acid catalysis) has revolutionized this field and has led to the invention of new cross-coupling methodology. The recent advances at the leading edge in the emerging field of Au(I)/Au(III) catalysis under redox-neutral conditions are highlighted.

Tri(N-carbazolyl)phosphine Gold(I) Complexes: Structural and Catalytic Activity Studies

Twelve tri(N-carbazolyl)phosphine gold(I) complexes, bearing both protonated and deuterated aryl phosphorous triamide-type ligands, have been synthesized and characterized. An elusive Au-H(D) interaction between the H(D) atoms of the tri(N-carbazolyl)phosphine ligand at the H-1(D-1) position of the carbazolyl ring and the central gold atom was observed. Complexes 5(H)/5(D) bearing the dibrominated tri(N-carbazolyl)phosphine ligand exhibit isotopic polymorphism, in which two dramatically different crystal-packing modes between the protonated and deuterated forms occur. The catalytic potential of these complexes has been showcased in the gold(I)-catalyzed glycosylation with glycosyl o-alkynylbenzoates as donors, with TON being up to 27 000.

Just Add the Gold: Aggregation-Induced-Emission Properties of Alkynylphosphinegold(I) Complexes Functionalized with Phenylene-Terpyridine Subunits

A series of organometallic complexes containing an alkynylphosphinegold(I) fragment and a phenylene-terpyridine moiety connected together by flexible linker have been prepared using the specially designed terpyridine ligands. The compounds were studied crystallographically to reveal that all of them contain a linearly coordinated Au(I) atom and a free terpyridine moiety. The different orientations of the molecules relative to each other in the solid state determine the multiple noncovalent interactions such as antiparallel ππ stacking, CH-π, and CH-Au, but no aurophilic interactions are realized. The organometallic Au(I) complexes obtained show fluorescence in the solution and dual singlet-triplet emission in the solid state. This means that their photophysical behavior is determined by both intermolecular lattice-defined interactions and Au(I) atom introduction. Density functional theory computational analysis supported the assignment of emission to intraligand electronic transitions only inside the phenylene-terpyridine part with no Au(I) involved. In addition, a study of the nature of the excited states for the "dimer" with an antiparallel orientation of the terpyridine fragment showed that this orientation leads to the generation of abstracted singlet and triplet states, lowering their energy in comparison with the monomer complex. Thus, the complexes obtained can be qualified as examples of Au(I)-containing organometallic aggregation-induced-emission luminogens.

A gold(I) 1,2,3-triazolylidene complex featuring the interaction between gold and methine hydrogen

A mesoionic N-heterocyclic carbene-gold(I) complex with a unique Au⋯H-C(methine) intramolecular hydrogen bonding interaction has been investigated in the solid state. The structure of this new neutral gold(I)-carbene was characterized by FT-IR and NMR spectroscopy, TGA, and X-ray diffraction techniques. Density functional theory (DFT) and atoms-in-molecule (AIM) analysis revealed that the gold-hydrogen bonding situation is more favored. Besides, the photophysical properties of the gold(I) complex were also investigated.

Group 11 metal complexes of the dinucleating triazole appended bisphosphine 1,4-bis(5-(diisopropylphosphaneyl)-1-phenyl-1H-1,2,3-triazol-4-yl)benzene

The synthesis of a triazole appended dinucleating bisphosphine 1,4-bis(5-(diisopropylphosphaneyl)-1-phenyl-1H-1,2,3-triazol-4-yl)benzene (2) and its coinage metal complexes are described. The dinucleating bisphosphine 2 was obtained by the temperature-controlled lithiation of 1,4-bis(1-phenyl-1H-1,2,3-triazol-4-yl)benzene (1a) and 1,4-bis(1-(2-bromophenyl)-1H-1,2,3-triazol-4-yl)benzene (1b) followed by the reaction with ⁱPr₂PCl. The reactions of 2 with copper(I) halides in 1 : 2 molar ratios yielded the [Cu(μ₂-X)]₂ dimeric complexes [{Cu(μ₂-X)}₂(PⁱPr₂N₃PhC₂)₂C₆H₄] (3, X = Cl; 4, X = Br; and 5, X = I), whereas the reaction of 2 with AgBr resulted in the formation of hetero-cubane complex [{Ag₄(μ₃-Br)₄}{(PⁱPr₂N₃PhC₂)₂C₆H₄}₂] (7). Similar reactions of 2 with AgX in 1 : 2 molar ratios yielded disilver complexes [{Ag(μ₂-X)}₂{(PⁱPr₂N₃PhC₂)₂C₆H₄}] (6, X = Cl and 8, X = I). Treatment of 2 with AgOAc in a 1 : 2 molar ratio afforded a dinuclear complex [Ag₂(μ₂-OAc)₂{(PⁱPr₂N₃PhC₂)₂(C₆H₄)}] (9) with one of the acetate ligands bridging the two metal centres in the side-on mode, whereas the other one adopting the end-on mode keeping the >CO group uncoordinated. The reaction of 2 with two equivalents of [AuCl(SMe₂)] afforded the digold complex [(AuClPⁱPr₂N₃PhC₂)₂C₆H₄] (10). The molecular structures of 2-5 and 7-10 were confirmed by single crystal X-ray analysis. Non-covalent interactions between Cu and C_arene were observed in the molecular structures of 3, 4 and 5. These weak interactions were also assessed by DFT calculations in terms of their non-covalent interaction plots (NCI) and QTAIM analyses.

Au⋅⋅⋅H-C Hydrogen Bonds as Design Principle in Gold(I) Catalysis

Secondary ligand-metal interactions are decisive in many catalytic transformations. While arene-gold interactions have repeatedly been reported as critical structural feature in many high-performance gold catalysts, we herein report that these interactions can also be replaced by Au⋅⋅⋅H-C hydrogen bonds without suffering any reduction in catalytic performance. Systematic experimental and computational studies on a series of ylide-substituted phosphines featuring either a PPh3 (Ph YPhos) or PCy3 (Cy YPhos) moiety showed that the arene-gold interaction in the aryl-substituted compounds is efficiently compensated by the formation of Au⋅⋅⋅H-C hydrogen bonds. The strongest interaction is found with the C-H moiety next to the onium center, which due to the polarization results in remarkably strong interactions with the shortest Au⋅⋅⋅H-C hydrogen bonds reported to date. Calorimetric studies on the formation of the gold complexes further confirmed that the Ph YPhos and Cy YPhos ligands form similarly stable complexes. Consequently, both ligands showed the same catalytic performance in the hydroamination, hydrophenoxylation and hydrocarboxylation of alkynes, thus demonstrating that Au⋅⋅⋅H-C hydrogen bonds are equally suited for the generation of highly effective gold catalysts than gold-arene interactions. The generality of this observation was confirmed by a comparative study between a biaryl phosphine ligand and its cyclohexyl-substituted derivative, which again showed identical catalytic performance. These observations clearly support Au⋅⋅⋅H-C hydrogen bonds as fundamental secondary interactions in gold catalysts, thus further increasing the number of design elements that can be used for future catalyst construction.

Hydrogen bonding to metals as a probe for an inverted ligand field

Electron-rich, late transition metals are known to act as hydrogen-bonding (HBd) acceptors. In this regard, Pt(ii) centres in square-planar environments are particularly efficient. It is however puzzling that no convincing experimental evidence is currently available for the isoelectronic neighbour Au(iii) being involved in HBd interactions. We report now on the synthesis and characterisation of two series of isoleptic and isoelectronic (d8) compounds [(CF3)3Pt(L)]- and (CF3)3Au(L), where the L ligands are based on the quinoline frame and have been selected to favour HBd with the metal centre. Strong HBd interactions were actually found in the Pt(ii) compounds, based on structural and spectroscopic evidence, and they were further confirmed by theoretical calculations. In contrast, no evidence was obtained in the Au(iii) case. In order to find the reason underlying this general disparity, we undertook a detailed theoretical analysis of the model systems [(CF3)3Pt(py)]- and (CF3)3Au(py). This study revealed that the filled dz2 orbital is the HOMO in the case of Pt(ii), but is buried in the lower energy levels in the case of Au(iii). The sharply different electronic configurations involve ligand-field inversion on going from Pt to the next element Au. This is not a gradual but an abrupt change, which invalidates Au(iii) as a HBd-acceptor wherever ligand-field inversion occurs.

Acridine N-Heterocyclic Carbene Gold(I) Compounds: Tuning from Yellow to Blue Luminescence

The synthesis and the luminescence features of three gold(I)-N-heterocyclic carbene (NHC) complexes are presented to study how the n-alkyl group can influence the luminescence properties in the crystalline state. The mononuclear gold(I)-NHC complexes, [(L¹ )Au(Cl)] (1), [(L² )Au(Cl)] (2), and [(L³ )Au(Cl)] (3) were isolated from the reactions between [(tht)AuCl] and corresponding NHC ligand precursors, [N-(9-acridinyl)-N'-(n-butyl)-imidazolium chloride, (L¹ .HCl)], [N-(9-acridinyl)-N'-(n-pentyl)-imidazolium chloride, (L² .HCl)] and [N-(9-acridinyl)-N'-(n-hexyl)-imidazolium chloride, (L³ .HCl)]. Their single-crystal X-ray analysis reveals the influence of the n-alkyl groups on solid-state packing. A comparison of the luminescence features of 1-3 with n-alkyl substituents is explored. The molecules 1-3 depicted blue emission in the solution state, while the yellow emission (for 1), greenish-yellow emission (for 2), and blue emission (for 3) in the crystalline phase. This paradigm emission shift arises from n-butyl to n-pentyl and n-hexyl in the crystalline state due to the carbon-carbon rotation of the n-alkyl group, which tends to promote unusual solid packing. Hence n-alkyl group adds a novel emission property in the crystalline state. Density Functional Theory and Time-Dependent Density Functional Theory calculations were carried out for monomeric complex, N-(9-acridinyl)-N'-(n-heptyl)imidazole-2-ylidene gold(I) chloride and dimeric complex, N-(9-acridinyl)-N'-(n-heptyl)imidazole-2-ylidene gold(I) chloride to understand the structural and electronic properties.