Inorganic click (iClick) synthesis of heterotrinuclear Pt(II)/Au(I)2 complexes

Electrospray Mass Spectrometry to Study Combinatorial iClick Reactions and Multiplexed Kinetics of [Ru(N3)(N∧N)(terpy)]PF6 with Alkynes of Different Steric and Electronic Demand

In a combinatorial approach, a family of ruthenium(II) azido complexes [Ru(N₃)(N∧N)(terpy)]PF₆ with terpy = 2,2':6',2″-terpyridine and N∧N as a bidentate chelator derived from 2,2'-biypridine and its 4,4'-disubstituted derivatives, 2,2'-bipyrimidine, and 1,10-phenanthroline were reacted with different internal and terminal alkynes to give access to a total of 7 × 7 = 49 triazolato complexes in a room-temperature catalyst-free iClick reaction. The reactants were mixed in a repurposed high-performance liquid chromatography (HPLC) autosampler, and the reaction progress was monitored by direct injection into an electrospray mass spectrometer. The ratio of the peak intensities of [Ru(N₃)(N∧N)(terpy)]⁺ and [Ru(triazolato)(N∧N)(terpy)]⁺ was converted to a colored heat map for facile visual inspection of the conversion ratio. By automated multiple injections of the reaction mixture in fixed time intervals and plotting peak intensities over reaction time, pseudo-first-order rate constants were easily determined. Finally, nonoverlapping isotope patterns of the azido starting materials and triazolato products enabled multiplexed parallel determination of rate constants for four different ruthenium(II) azido complexes from a single sample vial, thereby reducing experiment time by 75%.

iClick synthesis of network metallopolymers

Described is an approach to preparing the first iClick network metallopolymers with porous properties. Treating digoldazido complex 2-AuN3 with trigoldacetylide 3-AuPPh3 or 3-AuPEt3, trialkyne 3-H, tetragoldacetylide 4-AuPPh3, or tetraalkyne 4-H in CH₂Cl₂ affords five iClick network metallopolymers 5-AuPPh3, 5-AuPEt3, 5-H, 6-AuPPh3, and 6-H. Confirmation of the iClick network metallopolymers comes from FTIR, ¹³C solid-state cross-coupling magic angle spinning (CPMAS) NMR spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and nitrogen and CO₂ sorption analysis. Employing model complexes 7-AuPPh3, 7-AuPEt3, 7-H, 8-AuPPh3, and 8-H provides structural insights due to the insolubility of iClick network metallopolymers.

Heterobimetallic Gold/Ruthenium Complexes Synthesized via Post-functionalization and Applied in Dual Photoredox Gold Catalysis

The synthesis of heterobimetallic Au^I /Ru^II complexes of the general formula syn- and anti-[{AuCl}(L1∩L2){Ru(bpy)₂ }][PF₆ ]₂ is reported. The ditopic bridging ligand L1∩L2 refers to a P,N hybrid ligand composed of phosphine and bipyridine substructures, which was obtained via a post-functionalization strategy based on Diels-Alder reaction between a phosphole and a maleimide moiety. It was found that the stereochemistry at the phosphorus atom of the resulting 7-phosphanorbornene backbone can be controlled by executing the metal coordination and the cycloaddition reaction in a different order. All precursors, as well as the mono- and multimetallic complexes, were isolated and fully characterized by various spectroscopic methods such as NMR, IR, and UV-vis spectroscopy as well as cyclic voltammetry. Photophysical measurements show efficient phosphorescence for the investigated monometallic complex anti-[(L1∩L2){Ru(bpy)₂ }][PF₆ ]₂ and the bimetallic analogue syn-[{AuCl}(L1∩L2){Ru(bpy)₂ }][PF₆ ]₂ , thus indicating a small influence of the {AuCl} fragment on the photoluminescence properties. The heterobimetallic Au^I /Ru^II complexes syn- and anti-[{AuCl}(L1∩L2){Ru(bpy)₂ }][PF₆ ]₂ are both active catalysts in the P-arylation of aryldiazonium salts promoted by visible light with H-phosphonate affording arylphosphonates in yields of up to 91 %. Both dinuclear complexes outperform their monometallic counterparts.

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.

SPAAC iClick: progress towards a bioorthogonal reaction in-corporating metal ions

Combining strain-promoted azide-alkyne cycloaddition (SPAAC) and inorganic click (iClick) reactivity provides access to metal 1,2,3-triazolates. Experimental and computational insights demonstrate that iClick reactivity of the tested metal azides (LM-N₃, M = Au, W, Re, Ru and Pt) depends on the accessibility of the azide functionality rather than electronic effects imparted by the metal. SPAAC iClick reactivity with cyclooctyne is observed when the azide functionality is sterically unencumbered, e.g. [Au(N₃)(PPh₃)] (Au-N3), [W(η³-allyl)(N₃)(bpy)(CO)₂] (W-N3), and [Re(N₃)(bpy)(CO)₃] [bpy = 2,2'-bipyridine] (Re-N3). Increased steric bulk and/or preequilibria with high activation barriers prevent SPAAC iClick reactivity for the complexes [Ru(N₃)(Tp)(PPh₃)₂] [Tp = tris(pyrazolyl)borate] (Ru-N3), [Pt(N₃)(CH₃)(PⁱPr₃)₂] [ⁱPr = isopropyl] (Pt(II)-N3), and [Pt(N₃)(CH₃)₃]₄ ((PtN3)4). Based on these computational insights, the SPAAC iClick reactivity of [Pt(N₃)(CH₃)₃(P(CH₃)₃)₂] (Pt(IV)-N3) was successfully predicted.

Solvent-Dependent Reactivity and Photochemistry of Dinuclear and Mononuclear Platinum(IV) Azido Triazaolato Complexes

Reaction between the platinum(IV) azido complex trans,trans,trans-[Pt(py)2(N3)2(OH)2] (1) and 1,4-diphenyl-2-butyne-1,4-dione 2 in MeCN produces the intermediate peroxide-bridged dimeric platinum(IV) azido triazolato species (5), which has been characterised by X-ray crystallography. However, if the reaction between 1 and 2 is conducted in MeOH it results in decomposition. Over time in MeCN, dimer (5) converts into mononuclear complexes trans,trans,trans-[Pt(py)2(N3)(triazole)(OH)2] (3 a/3 b), which are in dynamic exchange. If resuspended in protic solvents (MeOH,H2O), 3 a/3 b undergo a slow (22 d) irreversible rearrangement to a cyclised platinum(IV) species 4 which contains a formally N,O-chelated ligand. Conversion of 3 a/3 b to 4 in d 4-MeOH can be accelerated (384x) by irradiation with visible light, although continued irradiation also produces N3 . and OH. radicals, and the [4-N3]+ species can be readily detected by ESI-MS. Solvent choice significantly effects both the cycloaddition reaction between 1 and 2, and the stability of the resultant complexes.

Hexavanadate-Organogold(I) Hybrid Compounds: Synthesis by the Azide-Alkyne Cycloaddition and Density Functional Theory Study of an Intriguing Electron Density Distribution

The fully oxidized Lindqvist-type hexavanadate compounds decorated by phosphine-derivatized Au(I) moieties oriented in a transoid fashion (n-Bu₄N)₂[V₆O₁₃{(OCH₂)₃CCH₂(N₃C₂C₆H₅)AuP(C₆H₄OMe)₃}₂] (POM_N^Au) and (n-Bu₄N)₂[V₆O₁₃{(OCH₂)₃CCH₂OCH₂(C₂N₃H)AuP(C₆H₄OMe)₃}₂] (POM_C^Au) have been prepared by azide-alkyne cycloaddition reactions and characterized by various techniques, including NMR, IR, and UV/vis spectroscopy and electrospray ionization mass spectrometry. Electronic structure calculations unveil the potential of these model hybrid junctions for application in controlled charge-transport experiments on substrate surfaces.

Exploiting azide-alkyne click chemistry in the synthesis, tracking and targeting of platinum anticancer complexes

Click chemistry is fundamentally important to medicinal chemistry and chemical biology. It represents a powerful and versatile tool, which can be exploited to develop novel Pt-based anticancer drugs and to better understand the biological effects of Pt-based anticancer drugs at a cellular level. Innovative azide-alkyne cycloaddition-based approaches are being used to functionalise Pt-based complexes with biomolecules to enhance tumour targeting. Valuable information in relation to the mechanisms of action and resistance of Pt-based drugs is also being revealed through click-based detection, isolation and tracking of Pt drug surrogates in biological and cellular environments. Although less well-explored, inorganic Pt-click reactions enable synthesis of novel (potentially multimetallic) Pt complexes and provide plausible routes to introduce functional groups and monitoring Pt-azido drug localisation.

An Application Exploiting Aurophilic Bonding and iClick to Produce White Light Emitting Materials

The paper focuses on exploiting aurophilic bonding to produce white light emitting materials. Inorganic Click (iClick) is employed to link two or four Au(I) metal ions through a triazolate bridge. Depending on the choice of phosphine ligand (PEt₃ or PPh₃), dinuclear Au₂-FO or tetranuclear Au₄-FO complexes can be controllably synthesized (FO = 2-(9,9-dioctylfluoreneyl-)). The iClick products Au₂-FO and Au₄-FO are characterized by combustion analysis and multinuclear NMR, TOCSY 1D, ¹H-¹³C gHMBC, and ¹H-¹³C gHSQC. In addition, the photophysical properties of Au₂-FO and Au₄-FO were examined in THF solution. Transient absorption spectroscopy was employed to elucidate the excited state features of the gold compounds. Solution processed OLEDs were fabricated and characterized, which gave white light electroluminescence with CIE coordinates (0.34, 0.36), as seen referenced to CIE standard illuminant D65 (0.31, 0.32). TDDFT computational analysis of Au₂-FO and Au₄-FO reveals the origin of light emission. In the case of Au₄-FO, direct excitation leads to increased aurophilic bonding in the excited state, and as a result the emission profile is broadened to cover a larger region of the visible spectrum, thus giving white light emission. Designing molecules that can access or increase aurophilic bonding in the excited state provides another tool for fine-tuning the emission profiles of gold complexes.

A novel Pt(iv) mono azido mono triazolato complex evolves azidyl radicals following irradiation with visible light

A novel Pt^IV azido triazolato complex exists as an equilibrium between two species in d₃-MeCN and evolves azide radicals (but not hydroxide radicals) when irradiated with visible light.

The platinum(iv) azido complex trans,trans,trans-[Pt^IV(N₃)₂(OH)₂(py)₂] (1) undergoes cycloaddition with 1,4-diphenyl-2-butyne-1,4-dione (2) under mild, catalyst-free conditions, affording a number of mono and bis click products. The major mono click product (3) exists in MeCN as an equilibrium mixture between two species; 3a and 3b rapidly interconvert through nucleophilic attack of the axial Pt–OH group at the adjacent Ph–CO group. The kinetic and thermodynamic parameters for this interconversion have been measured by selective saturation-transfer NMR spectroscopic experiments and are consistent with cyclisation at the Pt centre. Complex 3b was also characterised by X-ray crystallography. Visible light irradiation (440–480 nm) of 3 in d₃-MeCN produces azidyl radicals (N₃˙), as demonstrated by EPR spin-trapping with DMPO; no generation of hydroxyl radicals was observed. ¹H–¹⁹⁵Pt HMBC NMR confirmed that the photoproducts were Pt^IV rather than Pt^II species, and HPLC was consistent with these being [3–N₃]⁺ species; no facile photoejection of the triazolato ligand was observed, consistent with MS/MS fragmentation of 3. When 3 was irradiated in the presence of 5′-GMP, no 5′-GMP photoproducts were observed, suggesting that complex 3 is likely to exhibit significantly simplified biological activity (release of azidyl radicals but not DNA binding) compared with complex 1.

Platinum(iv) azido complexes undergo copper-free click reactions with alkynes

We report our investigations into the first examples of copper-free 1,3-dipolar cycloaddition (click) reactions of electrophiles with a PtIV azido complex. The Pt-IV azido complex trans, trans, trans-[PtIV(py)2(N3)2(OH)2] (1) was reactive towards dimethyl acetylenedicarboxylate (DMAD) (2), diethyl acetylenedicarboxylate DEACD (3), N-[(1R,8S,9s)-bicyclo[6.1.0]non-4-yn-9-ylmethyloxycarbonyl]-1,8-diamino-3,6-dioxaoctane (BCN) (11) and dibenzocyclooctyne-amine (DBCO) (12) resulting in formation of the corresponding mono (a) and bis-substituted (b) complexes. Complexes of 2 undergo further reactions between the Pt centre and the carbonyl group to form 2a' and 2b'. This is not seen for the products of the corresponding PtII azido complex trans-[Pt(py)2(N3)2] with acetylene 2. Novel complexes 2a', 2b', 11a and 11b have been characterised by multinuclear NMR, IR and UV-vis spectroscopy and ESI-MS. These reactions represent new synthetic routes to novel Pt(iv) complexes.

A new synthetic route to in-chain metallopolymers via copper(i) catalyzed azide-platinum-acetylide iClick

The first example of an in-chain metallo-poly(triazolate) synthesized by CuAAC is reported. Azido-platinum-acetylide (A-M-B) monomers are catalytically polymerized with copper(i) acetate to yield 1,2,3-triazolate linked Pt(ii) units. The metallopolymers are characterized by multinuclear NMR, IR, UV/Vis, GPC, and MS.

Catalyst-free room-temperature iClick reaction of molybdenum(ii) and tungsten(ii) azide complexes with electron-poor alkynes: structural preferences and kinetic studies

Two isostructural and isoelectronic group VI azide complexes of the general formula [M(η³-allyl)(N₃)(bpy)(CO)₂] with M = Mo, W and bpy = 2,2'-bipyridine were prepared and fully characterized, including X-ray structure analysis. Both reacted smoothly with electron-poor alkynes such as dimethyl acetylenedicarboxylate (DMAD) and 4,4,4-trifluoro-2-butynoic acid ethyl ester in a catalyst-free room-temperature iClick [3 + 2] cycloaddition reaction. Reaction with phenyl(trifluoromethyl)acetylene, on the other hand, did not lead to any product formation. X-ray structures of the four triazolate complexes isolated showed the monodentate ligand to be N2-coordinated in all cases, which requires a 1,2-shift of the nitrogen from the terminal azide to the triazolate cycloaddition product. On the other hand, a ¹⁹F NMR spectroscopic study of the reaction of the fluorinated alkyne with the tungsten azide complex at 27 °C allowed detection of the N1-coordinated intermediate. With this method, the second-order rate constant was determined as (7.3 ± 0.1) × 10^-2 M^-1 s^-1, which compares favorably with that of first-generation compounds such as difluorocyclooctyne (DIFO) used in the strain-promoted azide-alkyne cycloaddition (SPAAC). In contrast, the reaction of the molybdenum analogue was too fast to be studied with NMR methods. Alternatively, solution IR studies revealed pseudo-first order rate constants of 0.4 to 6.5 × 10^-3 s^-1, which increased in the order of Mo > W and F₃C-C[triple bond, length as m-dash]C-COOEt > DMAD.

On the Mechanism of Copper(I)-Catalyzed Azide-Alkyne Cycloaddition

The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction regiospecifically produces 1,4-disubstituted-1,2,3-triazole molecules. This heterocycle formation chemistry has high tolerance to reaction conditions and substrate structures. Therefore, it has been practiced not only within, but also far beyond the area of heterocyclic chemistry. Herein, the mechanistic understanding of CuAAC is summarized, with a particular emphasis on the significance of copper/azide interactions. Our analysis concludes that the formation of the azide/copper(I) acetylide complex in the early stage of the reaction dictates the reaction rate. The subsequent triazole ring-formation step is fast and consequently possibly kinetically invisible. Therefore, structures of substrates and copper catalysts, as well as other reaction variables that are conducive to the formation of the copper/alkyne/azide ternary complex predisposed for cycloaddition would result in highly efficient CuAAC reactions. Specifically, terminal alkynes with relatively low pKa values and an inclination to engage in π-backbonding with copper(I), azides with ancillary copper-binding ligands (aka chelating azides), and copper catalysts that resist aggregation, balance redox activity with Lewis acidity, and allow for dinuclear cooperative catalysis are favored in CuAAC reactions. Brief discussions on the mechanistic aspects of internal alkyne-involved CuAAC reactions are also included, based on the relatively limited data that are available at this point.

Photophysical and photochemical studies of tricarbonyl rhenium(i) N-heterocyclic carbene complexes containing azide and triazolate ligands

Rhenium NHC complexes bound to azide anions readily react with alkynes to form the corresponding triazolate complexes, a new class of photochemically active species.

N-heterocyclic carbene gold(I) and silver(I) complexes bearing functional groups for bio-conjugation

This work describes several synthetic approaches to append organic functional groups to gold and silver N-heterocyclic carbene (NHC) complexes suitable for applications in biomolecule conjugation. Carboxylate appended NHC ligands (3) lead to unstable Au(I) complexes that convert into bis-NHC species (4). A benzyl protected carboxylate NHC-Au(I) complex 2 was synthesized but deprotection to produce the carboxylic acid functionality could not be achieved. A small library of new alkyne functionalized NHC proligands were synthesized and used for subsequent silver and gold metalation reactions. The alkyne appended NHC gold complex 13 readily reacts with benzyl azide in a copper catalyzed azide-alkyne cycloaddition reaction to form the triazole appended NHC gold complex 14. Cell cytotoxicity studies were performed on DLD-1 (colorectal adenocarcinoma), Hep-G2 (hepatocellular carcinoma), MCF-7 (breast adenocarcinoma), CCRF-CEM (human T-Cell leukemia), and HEK (human embryonic kidney). Complete spectroscopic characterization of the ligands and complexes was achieved using (1)H and (13)C NMR, gHMBC, ESI-MS, and combustion analysis.

Organogold oligomers: exploiting iClick and aurophilic cluster formation to prepare solution stable Au4 repeating units

Cluster formation via multiple gold–gold bonds provides sufficient thermodynamic driving force to overcome entropic penalties to link multiple units and create solution stable organogold oligomers.

Au-iClick mirrors the mechanism of copper catalyzed azide–alkyne cycloaddition (CuAAC)

Isolated digold-triazolate products and first-order kinetic profiles for Au–acetylide/azide reactants in iClick provide compelling support for two copper ions in CuAAC.