Synthesis and Reactivity of Ruthenium Azido Complexes Containing a Hydridotris(pyrazolyl)borate Ligand and Dimerization of Terminal Alkynes in Organic and Aqueous Media

Ligand-Dominated Activation of CO2 and CS2 by the Putative Nickel Phosphiniminato Intermediates

Room-temperature photoactivation of the first- and second-generation PN3P-pincer nickel azido complexes 1a and 1b in the presence of CO2 or CS2 afforded N-bound carbamates, dithiocarbamates, and isothiocyanates, providing insights into CO2 and CS2 activation and demonstrating how a seemingly small difference in the ligand structure significantly influences the reactivity. Theoretical calculations disclosed that the charge of the phosphorus atom plays a critical role in determining the nitrogen atom transfer to form a plausible nickel phosphiniminato intermediate.

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.

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.

[3 + 2] cycloaddition of azido-bridged molybdenum(ii) complex with nitriles and alkynes

The azido-bridged molybdenum complex [N(CH₃)₄][(μ_1,1-N₃)₃{Mo(η³-C₃H₅)(CO)₂}₂], 1, was synthesized and its reactions with unsaturated nitriles and alkynes were investigated. The isolated [3 + 2] cycloaddition products were the N(2), N(3) bound tetrazolate complexes [N(CH₃)₄][(μ_1,1-N₃)₂(μ-N₄C{R}-κ²N²:N³){Mo(η³-C₃H₅)(CO)₂}₂] (R = C(CN)C(CN)₂ (2), C₆H₄NO₂, (3)) and [N(CH₃)₄][(μ-N₄C{R}-κ²N²:N³)₂(μ_1,1-N₃){Mo(η³-C₃H₅)(CO)₂}₂] (R = C(CN)C(CN)₂ (4), C₆H₄NO₂ (5)), and the N(1), N(2) bound triazolate complexes [N(CH₃)₄][(μ-N₃C₂{R}₂-κ²N¹:N²)(μ_1,1-N₃)₂{Mo(η³-C₃H₅)(CO)₂}₂] (R = CO₂CH₃ (6) and R = CO₂CH₂CH₃ (7). The reactivity of these cycloaddition reactions could be determined by the electronic properties of both metal azide and dipolarophile. In the reaction of 1 with nitriles, at most two bridging azido groups can participate in the cycloaddition reactions and elevated temperature is required for the preparations of 3 and 5. In the case of alkynes, only one azido group is active for the reaction. These complexes are fluxional in solution, and isomers were found in 3 and 5. The molecular structures of the above complexes were determined by single-crystal X-ray diffraction analysis, which reveals a distorted octahedral geometry around each molybdenum atom, and the two metal atoms are connected through three bridging ligands. The formation of these heterocycles demonstrated the [3 + 2] cycloaddition reaction could also be applied to the less electron-rich azido-bridged molybdenum complex.

iClick Reactions of Square-Planar Palladium(II) and Platinum(II) Azido Complexes with Electron-Poor Alkynes: Metal-Dependent Preference for N1 vs N2 Triazolate Coordination and Kinetic Studies with 1H and 19F NMR Spectroscopy

Two square-planar palladium(II) and platinum(II) azido complexes [M(N₃)(L)] with L = N-phenyl-2-[1-(2-pyridinyl)ethylidene]hydrazine carbothioamide reacted with four different electron-poor alkynes R-C≡C-R' with R = R' = COOCH₃, COOEt, COOCH₂CH₂OCH₃ or R = CF₃, R' = COOEt in a [3 + 2] cycloaddition "iClick" reaction. The resulting triazolate complexes [M(triazolate^R,R')(L)] were isolated by simple precipitation and/or washing in high purity and good yield. Six out of the eight new compounds feature the triazolate ligand coordinated to the metal center via the N2 nitrogen atom, but fortuitous solubility properties allowed isolation of the N1 isomer in two cases from acetone. When the solvent was changed to DMSO, the N1 → N2 isomerization could be studied by NMR spectroscopy and took several days to complete. ¹⁹F NMR studies of the iClick reaction with F₃C-C≡C-COOEt led to identification of a putative early linear intermediate in addition to the N1 and N2 isomers, however with the latter as the final product. Rate constants determined by ¹H or ¹⁹F NMR spectroscopy increased in the order Pd > Pt and CF₃/COOEt > COOR/COOR with R = CH₃, Et, CH₂CH₂OCH₃. The second-order rate constant k₂ > 3.7 M^-1 s^-1 determined for the reaction of [Pd(N₃)(L)] with F₃C-C≡C-COOEt is the fastest observed for an iClick reaction so far and compares favorably with that of the most evolved strained alkynes reported for the SPAAC (strain-promoted azide-alkyne cycloaddition) to date. Selected title compounds were evaluated for their anticancer activity on the GaMG human glioblastoma brain cancer cell line and gave EC₅₀ values in the low micromolar range (2-16 μM). The potency of the Pd(II) complexes increased with the chain length of the substituents in the 4- and 5-positions of the triazolate ligand.

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.

Protecting-Group-Free Site-Selective Reactions in a Metal-Organic Framework Reaction Vessel

Site-selective organic transformations are commonly required in the synthesis of complex molecules. By employing a bespoke metal-organic framework (MOF, 1·[Mn(CO)₃N₃]), in which coordinated azide anions are precisely positioned within 1D channels, we present a strategy for the site-selective transformation of dialkynes into alkyne-functionalized triazoles. As an illustration of this approach, 1,7-octadiyne-3,6-dione stoichiometrically furnishes the mono-"click" product N-methyl-4-hex-5'-ynl-1',4'-dione-1,2,3-triazole with only trace bis-triazole side-product. Stepwise insights into conversions of the MOF reaction vessel were obtained by X-ray crystallography, demonstrating that the reactive sites are "isolated" from one another. Single-crystal to single-crystal transformations of the Mn(I)-metalated material 1·[Mn(CO)₃(H₂O)]Br to the corresponding azide species 1·[Mn(CO)₃N₃] with sodium azide, followed by a series of [3+2] azide-alkyne cycloaddition reactions, are reported. The final liberation of the "click" products from the porous material is achieved by N-alkylation with MeBr, which regenerates starting MOF 1·[Mn(CO)₃(H₂O)]Br and releases the organic products, as characterized by NMR spectroscopy and mass spectrometry. Once the dialkyne length exceeds the azide separation, site selectivity is lost, confirming the critical importance of isolated azide moieties for this strategy. We postulate that carefully designed MOFs can act as physical protecting groups to facilitate other site-selective and chemoselective transformations.

Probing the factors that influence the conformation of a guanidinato ligand in [(η5-C5Me5)M(NN)X] (NN = chelating N,N′,N′′-tri(o-substituted aryl)guanidinate(1−); X = chloro, azido and triazolato)

The conformational difference illustrated is ascribed to a subtle repulsive interaction between the o-Cl substituent of two proximal aryl rings in the guanidinate ligand.

Diazoalkane complexes of ruthenium with tris(pyrazolyl)borate and bis(pyrazolyl)acetate ligands

Diazoalkane complexes [Ru(Tp)(N2CAr1Ar2)(PPh3)L]BPh4 ( and ) [Tp = tris(pyrazolyl)borate; L = P(OMe)3, P(OEt)3; Ar1 = Ar2 = Ph; Ar1 = Ph, Ar2 = p-tolyl; Ar1Ar2 = C12H8] were prepared by allowing chloro-compounds RuCl(Tp)(PPh3)L to react with diazoalkane in the presence of NaBPh4. Acrylonitrile CH2[double bond, length as m-dash]C(H)CN reacts with diazoalkane complexes to give 3H-pyrazole derivatives [Ru(Tp){N[double bond, length as m-dash]NC(Ar1Ar2)CH(CN)CH2}(PPh3){P(OMe)3}]BPh4 and [Ru(Tp){N[double bond, length as m-dash]NC(Ar1Ar2)CH2C(H)CN}(PPh3){P(OMe)3}]BPh4 (). Diazoalkane complexes [Ru(bpza)(N2CAr1Ar2)(PPh3)2]BPh4 () [bpza = bis(pyrazolyl)acetate] were also prepared. All complexes were characterised by IR and NMR spectroscopy and X-ray crystal structure determination of [Ru(Tp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}]BPh4 (). The differences exhibited by [Ru(Tp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}](+) and [Ru(Cp){N2C(Ph)(p-tolyl)}(PPh3){P(OMe)3}](+), as regards coordination of the diazoalkane ligand and reactivity towards alkenes, were explained on the basis of a comparative DFT study.

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.

Computational insight into the mechanism of the Pd(0)–Brønsted acid cooperatively catalysed head-to-tail dimerization of terminal alkynes

The Pd(0)–Brønsted acid cooperatively catalysed head-to-tail dimerization of terminal alkynes was computationally studied to gain a mechanistic insight.

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.

Amino acid bioconjugation via iClick reaction of an oxanorbornadiene-masked alkyne with a Mn(I)(bpy)(CO)3-coordinated azide

The catalyst-free room temperature iClick reaction of an unsymmetrically 2,3-disubstituted oxanorbornadiene (OND) as a "masked" alkyne equivalent with [Mn(N3)(bpy(CH3,CH3))(CO)3] leads to isolation of a phenylalanine ester bioconjugate, in which the model amino acid is linked to the metal moiety via a N-2-coordinated triazolate formed in a cycloaddition-retro-Diels-Alder (crDA) reaction sequence, in a novel approach to bioorthogonal coupling reactions based on metal-centered reactivity.

Carboxylate switch between hydro- and carbopalladation pathways in regiodivergent dimerization of alkynes

Experimental and theoretical investigation of the regiodivergent palladium-catalyzed dimerization of terminal alkynes is presented. Employment of N-heterocyclic carbene-based palladium catalyst in the presence of phosphine ligand allows for highly regio- and stereoselective head-to-head dimerization reaction. Alternatively, addition of carboxylate anion to the reaction mixture triggers selective head-to-tail coupling. Computational studies suggest that reaction proceeds via the hydropalladation pathway favoring head-to-head dimerization under neutral reaction conditions. The origin of the regioselectivity switch can be explained by the dual role of carboxylate anion. Thus, the removal of hydrogen atom by the carboxylate directs reaction from the hydropalladation to the carbopalladation pathway. Additionally, in the presence of the carboxylate anion intermediate, palladium complexes involved in the head-to-tail dimerization display higher stability compared to their analogues for the head-to-head reaction.

Microwave-assisted palladium-catalyzed highly regio- and stereoselective head to head dimerization of terminal aryl alkynes in water

A highly regio- and stereoselective oxime palladacycle/imidazolinium-catalyzed head to head dimerization of terminal aryl alkynes in water is presented.

Pyridine-enhanced head-to-tail dimerization of terminal alkynes by a rhodium-N-heterocyclic-carbene catalyst

A general regioselective rhodium-catalyzed head-to-tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh-N-heterocyclic-carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π-alkyne-coordinated Rh(I) species [RhCl(NHC)(η(2)-HC≡CCH2Ph)(py)] (3) and [RhCl(NHC){η(2)-C(tBu)≡C(E)CH=CHtBu}(py)] (4) and the Rh(III)-hydride-alkynyl species [RhClH{-C≡CSi(Me)3}(IPr)(py)2] (5). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C-H oxidative addition, alkyne insertion, and reductive elimination. A 2,1-hydrometalation of the alkyne is the more favorable pathway in accordance with a head-to-tail selectivity.

Strain-promoted azide-alkyne cycloaddition with ruthenium(II)-azido complexes

The reactivity of an exemplary ruthenium(II)-azido complex towards non-activated, electron-deficient, and towards strain-activated alkynes at room temperature and low millimolar azide and alkyne concentrations has been investigated. Non-activated terminal and internal alkynes failed to react under such conditions, even under copper(I) catalysis conditions. In contrast, as expected, rapid cycloaddition was observed with electron-deficient dimethyl acetylenedicarboxylate (DMAD) as the dipolarophile. Since DMAD and related propargylic esters are excellent Michael acceptors and thus unsuitable for biological applications, we investigated the reactivity of the azido complex towards cycloaddition with derivatives of cyclooctyne (OCT), bicyclo[6.1.0]non-4-yne (BCN), and azadibenzocyclooctyne (ADIBO). While no reaction could be observed in the case of the less strained cyclooctyne OCT, the highly strained cyclooctynes BCN and ADIBO readily reacted with the azido complex, providing the corresponding stable triazolato complexes, which were amenable to purification by conventional silica gel column chromatography. An X-ray crystal structure of an ADIBO cycloadduct was obtained and verified that the formed 1,2,3-triazolato ligand coordinates the metal center through the central N2 atom. Importantly, the determined second-order rate constant for the ADIBO cycloaddition with the azido complex (k2=6.9 × 10(-2) M(-1) s(-1)) is comparable to the rate determined for the ADIBO cycloaddition with organic benzyl azide (k2=4.0 × 10(-1) M(-1) s(-1)). Our results demonstrate that it is possible to transfer the concept of strain-promoted azide-alkyne cycloaddition (SPAAC) from purely organic azides to metal-coordinated azido ligands. The favorable reaction kinetics for the ADIBO-azido-ligand cycloaddition and the well-proven bioorthogonality of strain-activated alkynes should pave the way for applications in living biological systems.

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

Metal-azide-metal-acetylide cycloaddition (iClick) reactions to synthesize heterotrimetallics and an unexpected novel tetranuclear gold(I) complex, are described. In addition, a discussion regarding the connection between traditional azide-alkyne cycloaddition reactions and iClick is presented focusing on applications towards linking multiple metal ions.

Probing platinum azido complexes by 14N and 15N NMR spectroscopy

Metal azido complexes are of general interest due to their high energetic properties, and platinum azido complexes in particular because of their potential as photoactivatable anticancer prodrugs. However, azido ligands are difficult to probe by NMR spectroscopy due to the quadrupolar nature of (14)N and the lack of scalar (1)H coupling to enhance the sensitivity of the less abundant (15)N by using polarisation transfer. In this work, we report (14)N and (15)N NMR spectroscopic studies of cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))] (1) and trans,trans,trans-[Pt(N(3))(2)(OH)(2)(X)(Y)], where X=Y=NH(3) (2); X=NH(3), Y=py (3) (py=pyridine); X=Y=py (4); and selected Pt(II) precursors. These studies provide the first (15)N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J((15)N,(195)Pt) couplings for azido and am(m)ine ligands. The (14)N(α) (coordinated azido nitrogen) signal in the Pt(IV) azido complexes is extremely broad (W(1/2)≈2124 Hz for 4) in comparison to other metal azido complexes, attributable to a highly asymmetrical electric field gradient at the (14)N(α) atom. Through the use of anti-ringing pulse sequences, the (14)N NMR spectra, which show resolution of the broad (14)N(α) peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the (14)N(α) signals correlate with the viscosity of the solvent. For (15) N-enriched samples, it is possible to detect azido (15)N resonances directly, which will allow photoreactions to be followed by 1D (15)N NMR spectroscopy. The T(1) relaxation times for 3 and 4 were in the range 5.7-120 s for (15)N, and 0.9-11.3 ms for (14)N. Analysis of the (1)J((15)N,(195)Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral Pt(IV) complexes, within the series 2-pic<py<NH(3)<Cl(-)<N(3)(-)<NO(2)(-)<SCN(-) (2-pic=2-methylpyridine). In addition, an axial Cl(-) appears to weaken an equatorial Pt(IV)-NH(3) bond to a greater extent than an axial OH(-) ligand.