Time-resolved analysis of photoluminescence at a single wavelength for ratiometric and multiplex biosensing and bioimaging

Simultaneous analysis of luminescence signals of multiple probes can improve the accuracy and efficiency of biosensing and bioimaging. Analysis of multiple signals at different wavelengths usually suffers from spectral overlap, possible energy transfer, and difference in detection efficiency. Herein, we reported a polymeric luminescent probe, which was composed of a phenothiazine-based fluorescent compound and a phosphorescent iridium(iii) complex. Both luminophores emitted at around 600 nm but their luminescence lifetimes are 160 times different, allowing time-resolved independent analysis. As the fluorescence was enhanced in response to oxidation by hypochlorite and the phosphorescence was sensitive toward oxygen quenching, a four-dimensional relationship between luminescence intensity, fluorescence/phosphorescence ratio, hypochlorite concentration, and oxygen content was established. In cellular imaging, time-resolved photoluminescence imaging microscopy clearly showed the independent fluorescence response toward hypochlorite and phosphorescence response toward oxygen in separated time intervals. This work opens up a new idea for the development of multiplex biosensing and bioimaging.

A single-wavelength dual-emissive polymeric probe shows fluorescence enhancement toward ClO⁻ and phosphorescence quenching toward O₂, allowing simultaneously imaging cellular ClO⁻ and O₂via time-resolved photoluminescence imaging microscopy.     

Complexation and redox chemistry of neptunium,plutonium and americium with a hydroxylaminato ligand

There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-(^tBuNO)py]⁻(interchangeable hereafter with [(^tBuNO)py]⁻), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[(^tBuNO)py]₄ (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, ¹H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [(^tBuNO)py]⁻ ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E_1/2 = −2.26 V vs. Fc^+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of U^IV[(^tBuNO)py]₄ that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [(^tBuNO)py]⁻ ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [Am^III(μ₂-(^tBuNO)py)((^tBuNO)py)₂]₂ (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-(^tBuNO)py]⁻, in good agreement with experimental observations.

The coordination modes and electronic properties of a strongly coordinating hydroxylaminato ligand with Np, Pu and Am were investigated.Complexes were characterized by a range of experimental and computational techniques.     

Accessing lanthanide-based,in situ illuminated optical turn-on probes by modulation of the antenna triplet state energy

Luminescent lanthanides possess ideal properties for biological imaging, including long luminescent lifetimes and emission within the optical window. Here, we report a novel approach to responsive luminescent Tb(iii) probes that involves direct modulation of the antenna excited triplet state energy. If the triplet energy lies too close to the ⁵D₄ Tb(iii) excited state (20 500 cm⁻¹), energy transfer to ⁵D₄ competes with back energy transfer processes and limits lanthanide-based emission. To validate this approach, a series of pyridyl-functionalized, macrocyclic lanthanide complexes were designed, and the corresponding lowest energy triplet states were calculated using density functional theory (DFT). Subsequently, three novel constructs L3 (nitro-pyridyl), L4 (amino-pyridyl) and L5 (fluoro-pyridyl) were synthesized. Photophysical characterization of the corresponding Gd(iii) complexes revealed antenna triplet energies between 25 800 and 30 400 cm⁻¹ and a 500-fold increase in quantum yield upon conversion of Tb(L3) to Tb(L4) using the biologically relevant analyte H₂S. The corresponding turn-on reaction can be monitored using conventional, small-animal optical imaging equipment in presence of a Cherenkov radiation emitting isotope as an in situ excitation source, demonstrating that antenna triplet state energy modulation represents a viable approach to biocompatible, Tb-based optical turn-on probes.

The rational, analyte-mediated modulation of the relative energy of the lanthanide-sensitizing triplet state produces Tb-based luminescence, observable by a conventional optical imager in presence of the Cherenkov radiation emitting radioisotope ¹⁸F.     

Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer

The current investigation demonstrates highly efficient photochemical upconversion (UC) where a long-lived Zr(iv) ligand-to-metal charge transfer (LMCT) complex serves as a triplet photosensitizer in concert with well-established 9,10-diphenylanthracene (DPA) along with newly conceived DPA–carbazole based acceptors/annihilators in THF solutions. The initial dynamic triplet–triplet energy transfer (TTET) processes (ΔG ∼ −0.19 eV) featured very large Stern–Volmer quenching constants (K_SV) approaching or achieving 10⁵ M⁻¹ with bimolecular rate constants between 2 and 3 × 10⁸ M⁻¹ s⁻¹ as ascertained using static and transient spectroscopic techniques. Both the TTET and subsequent triplet–triplet annihilation (TTA) processes were verified and throughly investigated using transient absorption spectroscopy. The Stern–Volmer metrics support 95% quenching of the Zr(iv) photosensitizer using modest concentrations (0.25 mM) of the various acceptor/annihilators, where no aggregation took place between any of the chromophores in THF. Each of the upconverting formulations operated with continuous-wave linear incident power dependence (λ_ex = 514.5 nm) down to ultralow excitation power densities under optimized experimental conditions. Impressive record-setting η_UC values ranging from 31.7% to 42.7% were achieved under excitation conditions (13 mW cm⁻²) below that of solar flux integrated across the Zr(iv) photosensitizer''s absorption band (26.7 mW cm⁻²). This study illustrates the importance of supporting the continued development and discovery of molecular-based triplet photosensitizers based on earth-abundant metals.

The LMCT photosensitizer Zr(^MesPDP^Ph)₂ paired with DPA-based acceptors enabled low power threshold photochemical upconversion with record-setting quantum efficiencies.     

Core–shell structured phosphorescent nanoparticles for detection of exogenous and endogenous hypochlorite in live cells via ratiometric imaging and photoluminescence lifetime imaging microscopy

We report a ratiometric phosphorescence sensory system for hypochlorite (ClO^–) based on core–shell structured silica nanoparticles. Two phosphorescent iridium(iii) complexes were immobilised in the inner solid core and outer mesoporous layer of the nanoparticles, respectively. The former is insensitive to ClO^– and thus serves as an internal standard to increase the accuracy and precision, while the latter exhibits a specific and significant luminogenic response to ClO^–, providing high selectivity and sensitivity. Upon exposure to ClO^–, the nanoparticles display a sharp luminescence colour change from blue to red. Additionally, intracellular detection of exogenous and endogenous ClO^– has been demonstrated via ratiometric imaging and photoluminescence lifetime imaging microscopy. Compared to intensity-based sensing, ratiometric and lifetime-based measurements are independent of the probe concentration and are thus less affected by external influences, especially in intracellular applications.     

A redox-mediator pathway for enhanced multi-colour electrochemiluminescence in aqueous solution

The classic and most widely used co-reactant electrochemiluminescence (ECL) reaction of tris(2,2′-bipyridine)ruthenium(ii) ([Ru(bpy)₃]²⁺) and tri-n-propylamine is enhanced by an order of magnitude by fac-[Ir(sppy)₃]³⁻ (where sppy = 5′-sulfo-2-phenylpyridinato-C²,N), through a novel ‘redox mediator’ pathway. Moreover, the concomitant green emission of [Ir(sppy)₃]³⁻* enables internal standardisation of the co-reactant ECL of [Ru(bpy)₃]²⁺. This can be applied using a digital camera as the photodetector by exploiting the ratio of R and B values of the RGB colour data, providing superior sensitivity and precision for the development of low-cost, portable ECL-based analytical devices.

A water-soluble Ir(iii) complex is shown to enhance the ‘remote’ mechanism of the most widely used co-reactant ECL reaction of tris(2,2′-bipyridine)ruthenium(ii) with tripropylamine.     

Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number,charge, oxidation states and stability of manganese complexes throughout redox cycling

Reactivity assays previously suggested that two quinol-containing MRI contrast agent sensors for H₂O₂, [Mn(H2qp1)(MeCN)]²⁺ and [Mn(H4qp2)Br₂], could also catalytically degrade superoxide. Subsequently, [Zn(H2qp1)(OTf)]⁺ was found to use the redox activity of the H2qp1 ligand to catalyze the conversion of O₂˙⁻ to O₂ and H₂O₂, raising the possibility that the organic ligand, rather than the metal, could serve as the redox partner for O₂˙⁻ in the manganese chemistry. Here, we use stopped-flow kinetics and cryospray-ionization mass spectrometry (CSI-MS) analysis of the direct reactions between the manganese-containing contrast agents and O₂˙⁻ to confirm the activity and elucidate the catalytic mechanism. The obtained data are consistent with the operation of multiple parallel catalytic cycles, with both the quinol groups and manganese cycling through different oxidation states during the reactions with superoxide. The choice of ligand impacts the overall charges of the intermediates and allows us to visualize complementary sets of intermediates within the catalytic cycles using CSI-MS. With the diquinolic H4qp2, we detect Mn(iii)-superoxo intermediates with both reduced and oxidized forms of the ligand, a Mn(iii)-hydroperoxo compound, and what is formally a Mn(iv)-oxo species with the monoquinolate/mono-para-quinone form of H4qp2. With the monoquinolic H2qp1, we observe a Mn(ii)-superoxo ↔ Mn(iii)-peroxo intermediate with the oxidized para-quinone form of the ligand. The observation of these species suggests inner-sphere mechanisms for O₂˙⁻ oxidation and reduction that include both the ligand and manganese as redox partners. The higher positive charges of the complexes with the reduced and oxidized forms of H2qp1 compared to those with related forms of H4qp2 result in higher catalytic activity (k_cat ∼ 10⁸ M⁻¹ s⁻¹ at pH 7.4) that rivals those of the most active superoxide dismutase (SOD) mimics. The manganese complex with H2qp1 is markedly more stable in water than other highly active non-porphyrin-based and even some Mn(ii) porphyrin-based SOD mimics.

Manganese complexes with polydentate quinol-containing ligands are found to catalyze the degradation of superoxide through inner-sphere mechanisms. The redox activity of the ligand stabilizes higher-valent manganese species.     

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices

A new class of C^C^N ligand-containing carbazolylgold(iii) dendrimers has been designed and synthesized. High photoluminescence quantum yields of up to 82% in solid-state thin films and large radiative decay rate constants in the order of 10⁵ s⁻¹ are observed. These gold(iii) dendrimers are found to exhibit thermally activated delayed fluorescence (TADF), as supported by variable-temperature emission spectroscopy, time-resolved photoluminescence decay and computational studies. Solution-processed organic light-emitting diodes (OLEDs) based on these gold(iii) dendrimers have been fabricated, which exhibit a maximum current efficiency of 52.6 cd A⁻¹, maximum external quantum efficiency of 15.8% and high power efficiency of 41.3 lm W⁻¹. The operational stability of these OLEDs has also been recorded, with the devices based on zero- and second-generation dendrimers showing maximum half-lifetimes of 1305 and 322 h at 100 cd m⁻², respectively, representing the first demonstration of operationally stable solution-processed OLEDs based on gold(iii) dendrimers.

A new class of carbazolylgold(iii) C^C^N dendrimers with thermally activated delayed fluorescence properties has been designed and synthesized for the realzaqtion of operationally stable solution-processed organic light-emitting devices.     

Mechanistic study of photocatalytic CO2 reduction using a Ru(ii)–Re(i) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)₃]²⁺ photosensitiser and fac-[Re(diimine)(CO)₃{OC(O)OC₂H₄NR₂}] catalyst units can be used to reduce CO₂ to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO₂ to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO₂, e.g., 10% CO₂, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO₂ was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)−, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)− is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)− produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)− proceeds with an observed first-order rate constant of approximately 1.8 s⁻¹ at 298 K and is a unimolecular reaction, independent of the concentrations of both CO₂ and RuC2(Re)−.

Formation processes and reactivity of an important intermediate of photocatalytic CO₂ reduction, one-electron reduced species of a Ru(ii)–Re(i) supramolecular photocatalyst with a carbonate ester ligand, were investigated in detail.     

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)₃(THF)₃] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P2₁/c space group with a slight compression of the unit cell from 3396.4(2) ų to 3373.2(4) ų along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT²⁻ ligands with an end-bound {Ca²⁺(THF)₃} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT²⁻ ligands act in a η⁸-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)₃(THF)₃] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)₃(THF)₃] in contrast to [ErKCa(COT)₃(THF)₃], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations.

A new class of heterotrimetallic COT-based self-assemblies accommodates metals from groups I–III in three different oxidation states and enables tuning of electronic and magnetic properties.     

Molecular design of efficient yellow- to red-emissive alkynylgold(iii) complexes for the realization of thermally activated delayed fluorescence (TADF) and their applications in solution-processed organic light-emitting devices

Here, we report the design and synthesis of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which show tunable emission colors spanning from the yellow to red region in the solid state and exhibit thermally activated delayed fluorescence (TADF) properties. These complexes display high photoluminescence quantum yields of up to 0.87 and short excited-state lifetimes in sub-microsecond timescales, yielding high radiative decay rate constants on the order of up to 10⁶ s⁻¹. The observation of the drastic enhancement in the emission intensity of the complexes with insignificant change in the excited-state lifetime upon increasing the temperature from 200 to 360 K indicates an increasing radiative decay rate. The experimentally estimated energy splitting between the lowest-lying singlet excited state (S₁) and the lowest-lying triplet excited state (T₁), ΔE_S1–T1, is found to be as small as ∼0.03 eV (250 cm⁻¹), comparable to the value of ∼0.05 eV (435 cm⁻¹) obtained from computational studies. The delicate choice of the cyclometalating ligand and the fused heterocyclic ligand is deemed the key to induce TADF through the control of the energy levels of the intraligand and the ligand-to-ligand charge transfer excited states. This work represents the realization of highly emissive yellow- to red-emitting gold(iii) TADF complexes incorporated with fused heterocyclic alkynyl ligands and their applications in organic light-emitting devices.

We report the design of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which shows tunable emission colors spanning yellow to red region and exhibits thermally activated delayed fluorescence (TADF) properties.     

Chelation-assisted C–C bond activation of biphenylene by gold(i) halides

A chelation-assisted oxidative addition of gold(i) into the C–C bond of biphenylene is reported here. The presence of a coordinating group (pyridine, phosphine) in the biphenylene unit enabled the use of readily available gold(i) halide precursors providing a new, straightforward entry towards cyclometalated (N^C^C)- and (P^C)-gold(iii) complexes. Our study, combining spectroscopic and crystallographic data with DFT calculations, showcases the importance of neighboring, weakly coordinating groups towards the successful activation of strained C–C bonds by gold.

Pyridine and phosphine directing groups promote the C–C activation of biphenylene by readily available gold(i) halides rendering a new entry to (N^C^C)- and (P^C)-gold(iii) species.

Activation of C–C bonds by transition metals is challenging given their inertness and ubiquitous presence alongside competing C–H bonds.¹ Both the intrinsic steric hindrance as well as the highly directional character of the p orbitals involved in the σ_C–C bond impose a high kinetic barrier for this type of processes.^2,3 Biphenylene, a stable antiaromatic system featuring two benzene rings connected via a four-membered cycle, has found widespread application in the study of C–C bond activation. Since the seminal report from Eisch et al. on the oxidative addition of a nickel(0) complex into the C–C bond of biphenylene,⁴ several other late transition metals have been successfully applied in this context.⁵ Interestingly, despite the general reluctance of gold(i) to undergo oxidative addition,⁶ its oxidative insertion into the C–C bond of biphenylene was demonstrated in two consecutive reports by the groups of Toste^7a and Bourissou,^7b respectively. The high energy barrier associated with the oxidation of gold could be overcome by the utilization of gold(i) precursors bearing ligands that exhibit either a strongly electron-donating character (e.g. IPr = [1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene])^7a or small bite angles (e.g. DPCb = diphosphino-carborane).^7b,8 In line with these two approaches, more sophisticated bidentate (N^C)- and (P^N)-ligated gold(i) complexes have also been shown to aid the activation of biphenylene at ambient temperature (Scheme 1a).^7c,dOpen in a separate windowScheme 1(a) Previous reports on oxidative addition of ligated gold(i) precursors onto biphenylene. (b) This work: pyridine- and phosphine-directed C–C bond activation of biphenylene by commercially available gold(i) halides.In this context, we hypothesized that the oxidative insertion of gold(i) into the C–C bond of biphenylene could be facilitated by the presence of a neighboring chelating group.⁹ This approach would not only circumvent the need for gold(i) precursors featuring strong σ-donor or highly tailored bidentate ligands but also offer a de novo entry towards interesting, less explored ligand templates. However, recent work by Breher and co-workers showcased the difficulty of achieving such a transformation.¹⁰Herein, we report the oxidative insertion of readily available gold(i) halide precursors into the C–C bond of biphenylene. The appendage of both pyridine and phosphine donors in close proximity to the σ_C–C bond bridging the two aromatic rings provides additional stabilization to the metal center and results in a de novo entry to cyclometalated (N^C^C)- and (P^C)gold(iii) complexes (Scheme 1b).Our study commenced with the preparation of 5-chloro-1-pyridino-biphenylene system 2via Pd-catalyzed Suzuki cross coupling reaction between 2-bromo-3-methylpyridine and 2-(5-chlorobiphenylen-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 1 (Scheme 2).¹¹ To our delight, the reaction of 2 with gold(i) iodide in toluene at 130 °C furnished complex κ³-(N^C^C)Au(iii)–I 3 in 60% yield.^12,13 Complex 3 was isolated as yellow plate-type crystals from the reaction mixture and its molecular structure was unambiguously assigned by NMR spectroscopy, high-resolution mass spectrometry (HR-MS) and crystallographic analysis. Complex 3 exhibits the expected square-planar geometry around the metal center, with a Au–I bond length of 2.6558(3) Å.¹⁴ The choice of a neutral weakly bound gold(i)-iodide precursor is key for a successful reaction outcome: similar reactions in the presence of [(NHC)AuCl + AgSbF₆] failed to deliver the desired biscyclometalation adducts, as reported by Breher et al. in ref. 10. The oxidative insertion of gold(i) iodide into the four-membered ring of pyridino-substituted biphenylene provides a novel and synthetically efficient entry to κ³-(N^C^C)gold(iii) halides. These species have recently found widespread application as precursors for the characterization of highly labile, catalytically relevant gold(iii) intermediates,^15a–d as well as for the preparation of highly efficient emitters in OLEDs.^15e–g Previous synthetic routes towards these attractive biscyclometalated gold(iii) systems involved microwave-assisted double C–H functionalization reactions that typically proceed with low to moderate yields.^15aOpen in a separate windowScheme 2Synthesis of complex 3via oxidative addition of Au(i) into the C–C bond of pyridine-substituted biphenylene. X-ray structures of complex 3 with atoms drawn using 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Additional selected bond distances [Å]: N–Au = 2.126(2), C1–Au = 1.973(2), C2–Au = 2.025(2), Au–I = 2.6558(3) and bond angles [deg]: N–Au–I = 99.25(6), N–Au–C1 = 79.82(9), C1–Au–C2 = 81.2(1), C2–Au–I = 99.73(8). For experimental details, see ESI.^†Encouraged by the successful results obtained with the pyridine-substituted biphenylene and considering the prominent use of phosphines in gold chemistry,^6,16 we wondered whether the same reactivity would be observed for a P-containing system. To this end, both adamantyl- and tert-butyl-substituted phosphines were appended in C1 position of the biphenylene motif. Starting from 5-chlorobiphenylene-1-carbaldehyde 4, phosphine-substituted biphenylenes 5a and 5b could be accessed in 3 steps (aldehyde reduction to the corresponding alcohol, Appel reaction and nucleophilic displacement of the corresponding benzylic halide) in 64 and 57% overall yields, respectively.¹³ The reactions of 5a and 5b with commercially available gold(i) halides (Me₂SAuCl and AuI) furnished the corresponding mononuclear complexes 7a–b and 8a–b, respectively (Scheme 3).¹³ All these complexes were fully characterized and the structures of 7a, 7b and 8a were unambiguously characterized by X-ray diffraction analysis.¹³ Interestingly, the nature of the halide has a clear effect on the chemical shift of the phosphine ligand so that a Δδ of ca. 5 ppm can be observed in the ³¹P NMR spectra of 7a–b (Au–Cl) compared to 8a–b (Au–I), the latter being the more deshielded. The Au–X bond length is also impacted, with a longer Au–I distance (2.5608(1) Å for 8a) compared to that measured in the Au–Cl analogue (2.2941(7) Å for 7a) (Δd = 0.27 Å).¹³Open in a separate windowScheme 3Synthesis and reactivity of complexes 7a–b, 8a–b, 9 and 10. X-ray structure of complexes 11b, 12 and 14 with atoms drawn using 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. For experimental details and X-ray structures see ESI.^†Despite numerous attempts to promote the C–C activation in these complexes,^10,13 all reactions resulted in the formation of highly stable cationic species 11a–b and 12, which could be easily isolated from the reaction media. In the case of cationic mononuclear-gold(i) complexes 11, a ligand scrambling reaction in which the chloride ligand is replaced by a phosphine in the absence of a scavenger, a process previously described for gold(i) species, can be used to justify the reaction outcome.¹⁷ The formation of dinuclear gold complex 12 can be ascribed to the combination of a strong aurophilic interaction between the two gold centers (Au–Au = 2.8874(4) Å) and the stabilizing η²-coordination of the metal center to the aromatic ring of biphenylene. Similar η²-coordinated gold(i) complexes have been reported but, to the best of our knowledge, only as mononuclear species.¹⁸Taking into consideration the observed geometry of complexes 7a–b in the solid state,¹³ the facile formation of stable cationic species 11 and 12 and the lack of reactivity of the gold(i) iodides 8a–b, we hypothesized that the free rotation around the C–P bond was probably restricted, placing the gold(i) center away from the biphenylene system and thus preventing the desired oxidative insertion reaction. To overcome this problem, we set out to elongate the arm bearing the phosphine unit with an additional methylene group, introduced via a Wittig reaction from compound 4 to yield ligand 6, prepared in 4 steps in 27% overall yield. Coordination with Me₂SAuCl and AuI resulted in gold(i) complexes 9 and 10, respectively (Scheme 3). The structure of 9 was unambiguously assigned by X-ray diffraction analysis and a similar environment around the metal center to that determined for complex 7a was observed for this complex.¹³With complexes 9 and 10 in hand, we explored their reactivity towards C–C activation of the four-membered ring of biphenylene.¹⁹ After chloride abstraction and upon heating at 100 °C for 5 hours, ring opening of the biphenylene system was observed for complex 9. Interestingly, formation of mono-cyclometalated adduct 13 was exclusively observed (the structure of 13 was confirmed by ¹H, ¹³C, ³¹P, ¹⁹F, ¹¹B and 2D NMR spectroscopy and HR-MS).¹³ The solvent appears to play a major role in this process, as performing the reaction in non-chlorinated solvents resulted in stable cationic complexes similar to 11.^13,20,21 The presence of adventitious water is likely responsible for the formation of the monocyclometalated (P^C)gold(iii) complex 13 as when the reaction was carried out in C₂H₄Cl₂ previously treated with D₂O, the corresponding deuterated adduct 13-d could be detected in the reaction media. These results showcase the difficulties associated with the biscyclometalation for P-based complexes as well as the labile nature of the expected biscyclometalated adducts. Interestingly though, these processes can be seen as a de novo entry towards relatively underexplored (P^C)gold(iii) species.²²The C–C activation was further confirmed by X-ray diffraction analysis of the phosphonium salt 14, which arise from the reductive elimination at the gold(iii) center in 13 upon exchange of the BF₄⁻ counter-anion with the weakly coordinating sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBAr^F).^13,23 The phosphorus atom is four-coordinate, with weak bonding observed to the distant counter-anion and a distorted tetrahedral geometry (C1–P–C2 = 95.05(17), C2–P–C3 = 112.1(1), C3–P–C4 = 116.6(1), C4–P–C1 = 107.4(2) deg). These results represent the third example in which the C(sp²)–P bond reductive elimination at gold(iii) has been reported.²⁴Further, it is important to note that, in contrast to the reactivity observed for the pyridine-substituted biphenylene, neither P-coordinated gold(i) iodo complexes 8a, 8b nor 10 reacted to give cyclometalated products despite prolonged heating, which highlights the need for highly reactive cationized gold(i) species to undergo oxidative addition when phosphine ligands are flanking the C–C bond.¹³To get a deeper understanding on the observed differences in reactivity for the N- vs. P-based directing groups, ground- and transition-state structures for the oxidative insertion of gold(i) halides in C1-substituted biphenylenes were computed by DFT calculations. The reactions of Py-substituted 2 with AuI to give 3 (I) and those of P-substituted 7a (II) and 9 (III) featuring the cationization of the gold(i) species were chosen as models for comparative purposes with the experimental conditions (Fig. 1 and S1–S10 in the ESI^†).^25–27 The computed activation energies for the three processes are in good agreement with the experimental data. The pyridine-substituted biphenylene I exhibits the lowest activation barrier for the oxidative insertion process (ΔG^‡ = 34.4 kcal mol⁻¹). The reaction on the phosphine-substituted derivatives II and III proved to be, after cationization of the corresponding gold(i) halide complexes (II-BF₄, III-BF₄) higher in energy (ΔG^‡ = 39.6 and 46.3 kcal mol⁻¹ respectively), although the obtained values do not rule out the feasibility of the C–C activation process. The transition state between I and I′ exhibits several interesting geometrical features: (a) the biphenylene is significantly bent, (b) the cleavage of the C–C bond is well advanced (d_C–C = 1.898 Å in TSIvs. d_C–C = 1.504 Å in I), and (c) the two C and the I atoms form a Y-shape around gold with minimal coordination from the pyridine (d_N–Au = 2.742 Å in TSIvs. d_N–Au = 2.093 Å in I and 2.157 Å in I′, respectively). The transition-state structures found for the P-based ligands (TSII and TSIII) also show an elongation of the C–C bond and display a bent biphenylene. However, much shorter P–Au distances (d_P–Au = 2.330 Å for TSII and 2.314 Å for TSIII) can be observed compared to the pyridine-based system, as expected due to the steric and electronic differences between these two coordinating groups. Analogously, longer C–Au distances were also found for the P-based systems (d_C1–Au = 2.152 Å for TSIvs. 2.235 Å and 2.204 Å for TSII and TSIII; d_C2–Au = 2.143 Å for TSIvs. 2.219 Å and 2.162 Å for TSII and TSIII), with a larger deviation of square planarity for Au in TSIII compared to TSII.^28,29 These results suggest that, provided the appropriate distance to the C–C bond is in place, the strong coordination of phosphorous to the gold(i) center does not prevent the C–C activation of biphenylene but other reactions (i.e. formation of diphosphine gold(i) cationic species, protodemetalation) can outcompete the expected biscyclometalation process. In contrast, a weaker donor such as pyridine offers a suitable balance bringing the gold in close proximity to the C–C bond and enables both the oxidative cleavage as well as the formation of the double metalation product.Open in a separate windowFig. 1Energy profile (ΔG and ΔG^‡ in kcal mol⁻¹), optimized structures, transition states computed at the IEFPCM (toluene/1,2-dichloroethane)-B3PW91/DEF2QZVPP(Au,I)/6-31++G(d,p)(other atoms) level of theory for the C–C activation of biphenylene with gold(i) iodide from I and gold(i) cationic from II and III. Computed structures of the transition states (TSI, TSII and TSIII) and table summarizing relevant distances.     

C(sp3)–H oxygenation via alkoxypalladium(ii) species: an update for the mechanism

Pd-catalyzed C(sp³)–H oxygenation has emerged as an attractive strategy for organic synthesis. The most commonly proposed mechanism involves C(sp³)–H activation followed by oxidative addition of an oxygen electrophile to give an alkylpalladium(iv) species and further C(sp³)–O reductive elimination. In the present study of γ-C(sp³)–H acyloxylation of amine derivatives, we show a different mechanism when tert-butyl hydroperoxide (TBHP) is used as an oxidant—namely, a bimetallic oxidative addition-oxo-insertion process. This catalytic model results in an alkoxypalladium(ii) intermediate from which acyloxylation and alkoxylation products are formed. Experimental and computational studies, including isolation of the putative post-oxo-insertion alkoxypalladium(ii) intermediates, support this mechanistic model. Density functional theory reveals that the classical alkylpalladium(iv) oxidative addition pathway is higher in energy than the bimetallic oxo-insertion pathway. Further kinetic studies revealed second-order dependence on [Pd] and first-order on [TBHP], which is consistent with DFT analysis. This procedure is compatible with a wide range of acids and alcohols for γ-C(sp³)–H oxygenation. Preliminary functional group transformations of the products underscore the great potential of this protocol for structural manipulation.

Alkoxypalladium(ii) species lead to γ-C(sp³)–H acyloxylation and alkoxylation products using tert-butyl hydroperoxide as the oxidant.     

The switchable phosphorescence and delayed fluorescence of a new rhodamine-like dye through allenylidene formation in a cyclometallated platinum(ii) system

A new rhodamine-like alkyne-substituted ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. The chemo-induced “ON–OFF” switching capabilities on the spirolactone ring of the Rhodyne ligand with an end-capping platinum(ii) metal centre can modulate the interesting acetylide–allenylidene resonance. The long-lived ³IL excited state of Rhodyne in its ON state as an optically active opened form was revealed via steady-state and time-resolved spectroscopy studies. Exceptional near-infrared (NIR) phosphorescence and delayed fluorescence based on a rhodamine-like structure were observed at room temperature for the first time. The position of the alkyne communication bridge attached to the platinum(ii) unit was found to vary the lead(ii)-ion binding mode and also the possible resonance structure for metal-mediated allenylidene formation. The formation of a proposed allenylidene resonance structure was suggested to rationalize these phenomena.

A new rhodamine-like ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. Allenylidene formation could trigger NIR phosphorescence at 740 nm originating from Rhodyne ³IL, as well as delayed fluorescence at 620 nm.     

Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N₂ splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium(iii) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH₂)₂Cl]⁺, or results in dehydrohalogenation to the rhenium(iii) amido complex, (PNP)Re(NH₂)Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol⁻¹, while DFT computations indicate a substantially weaker N–H bond of the putative rhenium(iv)-imide intermediate (BDE = 38 kcal mol⁻¹). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH₃ generation.

Rhenium–PNP complexes split N₂ to nitrides, but the nitrides do not give ammonia. Here, the thermodynamics of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, showing that the first H addition is the bottleneck.     

Influence of the primary and secondary coordination spheres on nitric oxide adsorption and reactivity in cobalt(ii)–triazolate frameworks

Nitric oxide (NO) is an important signaling molecule in biological systems, and as such, the ability of porous materials to reversibly adsorb NO is of interest for potential medical applications. Although certain metal–organic frameworks are known to bind NO reversibly at coordinatively unsaturated metal sites, the influence of the metal coordination environment on NO adsorption has not been studied in detail. Here, we examine NO adsorption in the frameworks Co₂Cl₂(bbta) (H₂bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) and Co₂(OH)₂(bbta) using gas adsorption, infrared spectroscopy, powder X-ray diffraction, and magnetometry. At room temperature, NO adsorbs reversibly in Co₂Cl₂(bbta) without electron transfer, with low temperature data supporting spin-crossover of the NO-bound cobalt(ii) centers of the material. In contrast, adsorption of low pressures of NO in Co₂(OH)₂(bbta) is accompanied by charge transfer from the cobalt(ii) centers to form a cobalt(iii)–NO⁻ adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data indicate additional uptake of the gas and disproportionation of the bound NO to form a cobalt(iii)–nitro (NO₂⁻) species and N₂O gas, a transformation that appears to be facilitated by secondary sphere hydrogen bonding interactions between the bound NO₂⁻ and framework hydroxo groups. These results provide a rare example of reductive NO binding in a cobalt-based metal–organic framework, and they demonstrate that NO uptake can be tuned by changing the primary and secondary coordination environment of the framework metal centers.

Nitric oxide (NO) shows differences in adsorption and reactivity in two related cobalt(ii)–triazolate frameworks, demonstrating how the primary and secondary coordination sphere of metal centers in adsorbents can be designed for targeted delivery.     

DFT insight into asymmetric alkyl–alkyl bond formation via nickel-catalysed enantioconvergent reductive coupling of racemic electrophiles with olefins

A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K₃PO₄. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K₃PO₄ greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.

The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K₃PO₄ plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species.     

Kinetic trapping of a cobalt(ii) metallocage using a carbazole-containing expanded carbaporphyrinoid ligand

The meso-unsubstituted expanded porphyrinoid 3, incorporating two carbazole moieties, acts as an effective ligand for Co(ii) and permits the isolation and X-ray diffraction-based characterization of a 6 : 3 metal-to-ligand metallocage complex that converts spontaneously to the constituent 2 : 1 metal-to-ligand metalloring species in chloroform solution. The discrete metalloring is formed directly when the Co(ii) complex is crystallized from supersaturated solutions, whereas crystallization from more dilute solutions favors the metallocage. Studies with two other test cations, Pd(ii) and Zn(ii), revealed exclusive formation of the monomeric metalloring complexes with no evidence of higher order species being formed. Structural, electrochemical and UV-vis-NIR absorption spectral studies provide support for the conclusion that the Pd(ii) complex is less distorted and more effectively conjugated than its Co(ii) and Zn(ii) congeners, an inference further supported by TD-DFT calculations. The findings reported here underscore how expanded porphyrins can support coordination modes, including bimetallic complexes and self-assembled cage structures, that are not necessarily easy to access using more traditional ligand systems.

Carbazole containing expanded carbaporphyrinoid ligand supports the formation of 2 : 1 metal-to-ligand complexes with Pd, Co, and Zn. Solid-state studies also revealed formation of a 6 : 3 metal-to-ligand metallocage in the case of Co complexation.     

Phosphorescent [3 + 2 + 1] coordinated Ir(iii) cyano complexes for achieving efficient phosphors and their application in OLED devices

A series of neutral [3 + 2 + 1] coordinated iridium complexes bearing tridentate bis-NHC carbene chelates (2,6-bisimidazolylidene benzene), bidentate chelates (C^N ligands, e.g. derivatives of 2-phenylpridine), and monodentate ions (halides and pseudo-halides, such as Br, I, OCN and CN ions) have been systematically designed and synthesized. X-ray single crystal structure characterization revealed that the nitrogen atom in C^N ligands is located trans to the carbon atom in the benzene ring in tridentate chelates, while the coordinating carbon atom in C^N ligands is located trans to the monodentate ligands. Photophysical studies reveal that the C^N ligands play a vital role in tuning the UV absorption and emission properties, while the tridentate bis-NHC carbene chelates influence the lowest absorption band and emission energy when compared to heteroleptic Ir(ppy)2(acac) [i.e. molar absorptivities at ∼450 nm for ppy-OCN and Ir(ppy)2(acac) are 350 M⁻¹ cm⁻¹ and 1520 M⁻¹ cm⁻¹ and emission maximum peaks are at 465 nm and 515 nm respectively]. Among monodentate ligands that the complexes bear, the group containing the cyanide ligand displays higher emission energy, higher photophysical quantum yields, longer triplet lifetimes and better electrochemical and thermal stabilities than those of cyanate and bromide. Particularly, a blue organic light-emitting diode (OLED) based on dfppy-CN exhibited a maximum external quantum efficiency of 22.94% with CIE coordinates of (0.14, 0.24). Furthermore, a small efficiency roll-off of 5.7% was observed for this device at 1000 cd m⁻².

Construction of [3 + 2 + 1] coordinated iridium(iii) cyano complexes for achieving high-efficiency phosphors and their application in blue OLEDs with low efficiency roll-off.     

Genomics-driven discovery of chiral triscatechol siderophores with enantiomeric Fe(iii) coordination

Ferric complexes of triscatechol siderophores may assume one of two enantiomeric configurations at the iron site. Chirality is known to be important in the iron uptake process, however an understanding of the molecular features directing stereospecific coordination remains ambiguous. Synthesis of the full suite of (DHB^L/DLys^L/DSer)₃ macrolactone diastereomers, which includes the siderophore cyclic trichrysobactin (CTC), enables the effects that the chirality of Lys and Ser residues exert on the configuration of the Fe(iii) complex to be defined. Computationally optimized geometries indicate that the Λ/Δ configurational preferences are set by steric interactions between the Lys sidechains and the peptide backbone. The ability of each (DHB^L/DLys^L/DSer)₃ diastereomer to form a stable Fe(iii) complex prompted a genomic search for biosynthetic gene clusters (BGCs) encoding the synthesis of these diastereomers in microbes. The genome of the plant pathogen Dickeya chrysanthemi EC16 was sequenced and the genes responsible for the biosynthesis of CTC were identified. A related but distinct BGC was identified in the genome of the opportunistic pathogen Yersinia frederiksenii ATCC 33641; isolation of the siderophore from Y. frederiksenii ATCC 33641, named frederiksenibactin (FSB), revealed the triscatechol oligoester, linear-(DHB^LLys^LSer)₃. Circular dichroism (CD) spectroscopy establishes that Fe(iii)–CTC and Fe(iii)–FSB are formed in opposite enantiomeric configuration, consistent with the results of the ferric complexes of the cyclic (DHB^L/DLys^L/DSer)₃ diastereomers.

Synthesis and genome mining leverage access to diastereomers of the siderophore cyclic trichrysobactin. Computational modeling and CD spectroscopy address effects that ligand-based stereochemistry exerts on the configuration at Fe(iii).