Continuous flow synthesis of atom-precise platinum clusters.

Subnanometer clusters with precise atom numbers hold immense potential for applications in catalysis, as single atoms can significantly impact catalytic properties. Typically, inorganic clusters are produced using batch processes with high dilutions, making the scale-up of these processes time-consuming and its reproducibility challenging. While continuous-flow systems have been employed for organic synthesis and, more recently, nanoparticle preparation, these approaches have only rarely been applied to cluster synthesis. In a flexible, continuous flow synthesis platform, we integrate multiple continuous stirred tank reactors (CSTR) into a cascade to synthesize clusters with a precise number of atoms, demonstrating the potential of this approach for atom precise cluster synthesis and expanding the application of continuous-flow systems beyond organic synthesis.

Cyclic ion mobility of doped [MAu24L18]2- superatoms and their fragments (M = Ni, Pd and Pt; L = alkynyl).

Collision-induced dissociation and high-resolution cyclic ion mobility mass spectrometry, along with quantum chemical calculations and trajectory simulations, were used to compare the structures of isolated [MAu24(CCR)18]2-, M = Ni, Pd, or Pt, and their associated fragment ions. The three different alkynyl ligand-stabilized (CCR, R = 3,5-(CF3)2C6H3), transition metal-doped, gold cluster dianions showed mutually resolvable collision cross sections (CCS), which were ordered consistently with their molecular structures from X-ray crystallography. All three [MAu24(CCR)18]2- species fragment by sequential diyne loss to form [MAu24(CCR)18-n]2-, with n up to 12. The resultant fragment isomer distributions are significantly n- and M-dependent, and hint at a process involving concerted elimination of adjacent ligands. In particular [NiAu24(CCR)18]2- also fragments to generate alkyne-oligomers, an inference supported by the parallel observation of precursor dianion isomerization as collision energy is increased.

Tailoring Vacancy Defects in Isolated Atomically Precise Silver Clusters through Mercury-Doped Intermediates.

Vacancy defects are known to have significant effects on the physical and chemical properties of nanomaterials. However, the formation and structural dynamics of vacancy defects in atomically precise coinage metal clusters have hardly been explored due to the challenges associated with isolation of such defected clusters. Herein, we isolate [Ag28(BDT)12]2- (BDT is 1,3-benzenedithiol), a cluster with a "missing atom" site compared to [Ag29(BDT)12]3-, whose precise structure is known from X-ray diffraction. [Ag28(BDT)12]2- was formed in the gas-phase by collisional heating of [Ag28Hg(BDT)12]2-, a Hg-doped analogue of the parent cluster. The structural changes resulting from the loss of the Hg heteroatom were investigated by trapped ion mobility mass spectrometry. Density functional theory calculations were performed to provide further insights into the defect structures, and molecular dynamics simulations revealed defect site-dependent structural relaxation processes.

Elucidating the Structures of Intermediate Fragments during Stepwise Dissociation of Monolayer-Protected Silver Clusters.

Fragmentation dynamics of ligated coinage metal clusters reflects their structural and bonding properties. So far methodological challenges limited probing structures of the fragments. Herein, we resolve the geometric structures of the primary fragments of [Ag29 L12 ]3- , i.e. [Ag24 L9 ]2- , [Ag19 L6 ]- and [Ag5 L3 ]- (L is 1,3-benzene dithiolate). For this, we used trapped ion mobility mass spectrometry to determine collision cross sections of the fragments and compared them to structures calculated by density functional theory. We also report that following two sequential [Ag5 L3 ]- elimination steps, further dissociation of [Ag19 L6 ]- also involves a new channel of Ag2 loss and Ag-S and C-S bond cleavages. This reflects a competition between retaining the electronic stability of 8 e- superatom cluster cores and increasing steric strain of ligands and staples. These results are also of potential interest for future soft-landing deposition studies aimed at probing catalytic behavior of Ag clusters on supports.

Exploring Isomerism in Isolated Cyclodextrin Oligomers through Trapped Ion Mobility Mass Spectrometry.

Cyclodextrin (CD) macrocycles are used to create a wide range of supramolecular architectures which are also of interest in applications such as selective gas adsorption, drug delivery, and catalysis. However, predicting their assemblies and identifying the possible isomers in CD oligomers have always remained challenging due to their dynamic nature. Herein, we interacted CDs (α, ß, and γ) with a divalent metal ion, Cu2+, to create a series of Cu2+-linked CD oligomers, from dimers to pentamers. We characterized these oligomers using electrospray ionization mass spectrometry and probed isomerism in each of these isolated oligomers using high resolution trapped ion mobility spectrometry. Using this technique, we separated multiple isomers for each of the Cu2+-interlinked CD oligomers and estimated their relative population, which was not accessible previously using other characterization techniques. We further carried out structural analysis of the observed isomers by comparing the experimental collision cross sections (CCSs) to that of modeled structures. We infer that the isomeric heterogeneity reflects size-specific packing patterns of individual CDs (e.g., close-packed/linear). In some cases, we also reveal the existence of kinetically trapped structures in the gas phase and study their transformation to thermodynamically controlled forms by examining the influence of activation of the ions on isomer interconversion.

Further Dimensions for Sensing in Biofluids: Distinguishing Bioorganic Analytes by the Salt-Induced Adaptation of a Cucurbit[7]uril-Based Chemosensor.

Insufficient binding selectivity of chemosensors often renders biorelevant metabolites indistinguishable by the widely used indicator displacement assay. Array-based chemosensing methods are a common workaround but require additional effort for synthesizing a chemosensor library and setting up a sensing array. Moreover, it can be very challenging to tune the inherent binding preference of macrocyclic systems such as cucurbit[n]urils (CBn) by synthetic means. Using a novel cucurbit[7]uril-dye conjugate that undergoes salt-induced adaptation, we now succeeded in distinguishing 14 bioorganic analytes from each other through the facile stepwise addition of salts. The salt-specific concentration-resolved emission provides additional information about the system at a low synthetic effort. We present a data-driven approach to translate the human-visible curve differences into intuitive pairwise difference measures. Ion mobility experiments combined with density functional theory calculations gave further insights into the binding mechanism and uncovered an unprecedented ternary complex geometry for CB7. TThis work introduces the non-selectively binding, salt-adaptive cucurbit[n]uril system for sensing applications in biofluids such as urine, saliva, and blood serum.

Kinetics of Intercluster Reactions between Atomically Precise Noble Metal Clusters [Ag25(DMBT)18]- and [Au25(PET)18]- in Room Temperature Solutions.

The kinetics of intercluster metal atom exchange reactions between solvated [Ag25(DMBT)18]- and [Au25(PET)18]- (DMBT and PET are 2,4-dimethylbenzenethiol and 2-phenylethanethiol, respectively, both C8H10S) were probed by electrospray ionization mass spectrometry and computer-based modeling. Anion mass spectra and collision induced dissociation (CID) measurements show that both cluster monomers and dimers are involved in the reactions. We have modeled the corresponding kinetics assuming a reaction mechanism in which metal atom exchange occurs through transient dimers. Our kinetic model contains three types of generic reactions: dimerization of monomers, metal atom exchange in the transient dimers, and dissociation of the dimers to monomers. There are correspondingly 377 discrete species connected by in total 1302 reactions (i.e., dimerization, dissociation and atom exchange reactions) leading to the entire series of monomeric and dimeric products [AgmAu25-m]- (m = 1-24) and [AgmAu50-m]2- (m = 0-50), respectively. The rate constants of the corresponding reactions were fitted to the experimental data, and good agreement was obtained with exchange rate constants which scale with the probability of finding a silver or gold atom in the respective monomeric subunit of the dimer, i.e., reflecting an entropic driving force for alloying. Allowing the dimerization rate constant to scale with increasing gold composition of the respective reactants improves the agreement further. The rate constants obtained are physically plausible, thus strongly supporting dimer-mediated metal atom exchange in this intercluster reaction system.

Probing the structure of giant fullerenes by high resolution trapped ion mobility spectrometry.

We present high-resolution trapped ion mobility spectrometry (TIMS) measurements for fullerene ions in molecular nitrogen. Three different charge states were studied (monocations, monoanions and dianions) with fullerenes ranging in size from C60 to C150. Ions were prepared by either electrospray ionization (ESI, for mono- and dianions) or by atmospheric pressure chemical ionization (APCI, for monocations) of a preformed fullerene soot extract solution. We demonstrate that TIMS allows to identify (and separate) constituent isomers in favorable cases. Using DFT calculations based on known condensed phase structures and trajectory method (TM) calculations we can reproduce the experimental TIMSCCSN2 for fullerenes up to C108 to within 0.5%. Using candidate structures based on quantum chemical predictions, we have also obtained structural information for fullerenes C110-C150- a size range not previously accessed in condensed phase studies. We find that soluble fullerenes in this size have near-spherical rather than tubular structures. While the TM programs presently available for CCS modelling do a remarkably good job at describing the ion mobility of high (and even giant) fullerenes we observe a slight but systematic size-dependent deviation between TIMSCCSN2 values and our best computational fits which may reflect systematic bonding changes as the cage size increases.

Strong impact of wildfires on the abundance and aging of black carbon in the lowermost stratosphere.

Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (∼140 ng·m-3, standard temperature and pressure) were over 20 times higher than the background concentration (∼6 ng·m-3) with more than 100-fold enhanced peak values (up to ∼720 ng·m-3). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was ∼120 nm with a mean coating thickness of ∼150 nm in the BB plume and ∼90 nm with a coating of ∼125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of ∼1 nm·h-1 due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate.

The reaction rates of O2 with closed-shell and open-shell Al(x)⁻ and Ga(x)⁻ clusters under single-collision conditions: experimental and theoretical investigations toward a generally valid model for the hindered reactions of O2 with metal atom clusters.

In order to characterize the oxidation of metallic surfaces, the reactions of O2 with a number of Al(x)(-) and, for the first time, Ga(x)(-) clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O2 pressures of ~10(-8) mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O2 could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [M(x)O2](-), proceeds via the less stable peroxo species, [M(x)(+)···O2(2-)](-), which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O2. This could be one of the main reasons why O2 reactions with metal surfaces proceed more slowly than Cl2 reactions with such surfaces, even though O2 reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl2 with them. Furthermore, our results indicate that the spin-forbidden reactions of (3)O2 with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M(x)(+)···O2(2-)](-) are the root cause for the observed even/odd differences in reactivity.

Luminescent cell-penetrating pentadecanuclear lanthanide clusters.

A novel pentadecanuclear lanthanide hydroxy cluster [{Ln15(µ3-OH)20(PepCO2)10(DBM)10Cl}Cl4] (Ln = Eu (1), Tb (2)) featuring the first example with peptoids as supporting ligands was prepared and fully characterized. The solid-state structures of 1 and 2 were established via single-crystal X-ray crystallography. ESI-MS experiments revealed the retention of the cluster core in solution. Although OH groups are present, 1 showed intense red fluorescence with 11(1)% absolute quantum yield, whereas the emission intensity and the quantum yield of 2 were significantly weaker. In vitro investigations on 1 and 2 with HeLa tumor cells revealed an accumulation of the clusters in the endosomal-lyosomal system, as confirmed by confocal microscopy in the TRLLM mode. The cytotoxicity of 1 and 2 toward the HeLa cells is moderate.

{Sn9[Si(SiMe3)3]3}− and {Sn8Si[Si(SiMe3)3]3}−: variations of the E9 cage of metalloid group 14 clusters.

The disproportionation reaction of the subvalent metastable halide SnBr proved to be a powerful synthetic method for the synthesis of metalloid cluster compounds of tin. Hence, the neutral metalloid cluster compound Sn(10)[Si(SiMe(3))(3)](6) (3) was synthesized from the reaction of SnBr with LiSi(SiMe(3))(3). In the course of the reaction anionic clusters might also be present, and we now present the first anionic cluster compound {Sn(8)E[Si(SiMe(3))(3)](3)}(-) (E = Si, Sn), where one position in the cluster core is occupied by a silicon or a tin atom, giving further insight into structural variations of E(9) cages in metalloid group 14 cluster compounds.

The formal combination of three singlet biradicaloid entities to a singlet hexaradicaloid metalloid Ge14[Si(SiMe3)3]5[Li(THF)2]3 cluster.

The reaction of GeBr with LiSi(SiMe(3))(3) leads to the metalloid cluster compound [(THF)(2)Li](3)Ge(14)[Si(SiMe(3))(3)](5) (1). After the introduction of a first cluster of this type, in which 14 germanium atoms form an empty polyhedron, [(THF)(2)Li](3)Ge(14)[Ge(SiMe(3))(3)](5) (2), we present here further investigations on 1 to obtain preliminary insight into its chemical and bonding properties. The molecular structure of 1 is determined via X-ray crystal structure solution using synchrotron radiation. The electronic structure of the Ge(14) polyhedron is further examined by quantum chemical calculations, which indicate that three singlet biradicaloid entities formally combine to yield the singlet hexaradicaloid character of 1. Moreover, the initial reactions of 1 after elimination of the [Li(THF)(2)](+) groups by chelating ligands (e.g., TMEDA or 12-crown-4) are presented. Collision induced dissociation experiments in the gas phase, employing FT-ICR mass spectrometry, lead to the elimination of the singlet biradicaloid Ge(5)H(2)[Si(SiMe(3))(3)](2) cluster. The unique multiradicaloid bonding character of the metalloid cluster 1 might be used as a model for reactions and properties in the field of surface science and nanotechnology.

Sandwich-type tetranuclear lanthanide complexes with cucurbit[6]uril: from molecular compounds to coordination polymers.

Sandwich-type lanthanide complexes with macrocyclic ligand cucurbit[6]uril (C 36H 36N 24O 12, CB[6]) were synthesized under hydrothermal conditions from aqueous solutions of lanthanide(III) bromides, CB[6], and 4-cyanopyridine. According to X-ray analysis (Ln = La, Pr, Dy, Ho, Er, and Yb), the compounds with different structural types of lanthanide cores have a common fragment where the tetranuclear hydroxo complex is sandwiched between two macrocycles {(IN@CB[6])Ln 4(mu 3-OH) 4(IN@CB[6])} (6+) (IN = isonicotinate). The photoluminescence (for Ln = Eu) and Fourier transform ion cyclotron resonance mass spectra (for Ln = Pr, Dy, and Er) were studied. The compounds are used for the first time as precursors for the synthesis of lanthanide-silver heterometallic coordination polymers. The chainlike crystal structure of polymers (Ln = La, Pr, and Dy) is constituted by the sandwich complexes linked via the coordination of IN nitrogen atoms to the silver atoms.

Binding energy and preferred adsorption sites of CO on gold and silver-gold cluster cations: adsorption kinetics and quantum chemical calculations.

We revisit the reactivity of trapped pure gold (Au(n)+, n < 26) and silver gold alloy cluster cations (Ag(m)Au(n)+, m + n < 7) with carbon monoxide as studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The experimental results are discussed in terms of ab initio computations which provide a comprehensive picture of the chemical binding behaviour (like binding energy, adsorption sites, associated vibrational frequencies) of CO to the noble metal as a function of cluster size and composition. Starting from results for pure gold cluster cations for which an overall decrease of CO binding energy with increasing cluster size was experimentally observed--from about 1.09 +/- 0.1 eV (for n = 6) to below 0.65 +/- 0.1 eV (for n > 26) we demonstrate that metal--CO bond energies correlate with the total electron density and with the energy of the lowest unoccupied molecular orbital (LUMO) on the bare metal cluster cation as obtained by density functional theory (DFT) computations. This is a consequence of the predominantly sigma-donating character of the CO-M bond. Further support for this concept is found by contrasting the predictions of binding energies to the experimental results for small alloy cluster cations (Ag(m)Au(n)+, 4 < m + n < 7) as a function of composition. Here, binding energy drops with increasing silver content, while CO still binds always in a head-on fashion to a gold atom. Finally we show how the CO stretch frequency of Ag(m)Au(n)CO+ may be used to identify possible adsorption sites and pre-screen favorable isomers.

Electron emission from laser-heated fullerene dianions: probing the repulsive Coulomb barrier.

Electron detachment from fullerene dianions Cm2- (m=76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm(2-) surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm(-) generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.

Reactions of mixed silver-gold cluster cations AgmAun + (m+n=4,5,6) with CO: radiative association kinetics and density functional theory computations.

Near thermal energy reactive collisions of small mixed metal cluster cations Ag(m)Au(n) (+) (m+n=4, 5, and 6) with carbon monoxide have been studied in the room temperature Penning trap of a Fourier transform ion-cyclotron-resonance mass spectrometer as a function of cluster size and composition. The tetrameric species AgAu(3) (+) and Ag(2)Au(2) (+) are found to react dissociatively by way of Au or Ag atom loss, respectively, to form the cluster carbonyl AgAu(2)CO(+). In contrast, measurements on a selection of pentamers and hexamers show that CO is added with absolute rate constants that decrease with increasing silver content. Experimentally determined absolute rate constants for CO adsorption were analyzed using the radiative association kinetics model to obtain cluster cation-CO binding energies ranging from 0.77 to 1.09 eV. High-level ab initio density functional theory (DFT) computations identifying the lowest-energy cluster isomers and the respective CO adsorption energies are in good agreement with the experimental findings clearly showing that CO binds in a "head-on" fashion to a gold atom in the mixed clusters. DFT exploration of reaction pathways in the case of Ag(2)Au(2) (+) suggests that exoergicities are high enough to access the minimum energy products for all reactive clusters probed.

Electron transfer collisions between isolated fullerene dianions and SF6.

Electron transfer collisions of trapped doubly charged fullerene anions C76(2-), C(78)2-, and C84(2-) with SF6 are studied in a Fourier transform ion cyclotron resonance mass spectrometer at center-of-mass collisional energies ranging from thermal energy to 77 eV. Collision energy dependencies manifest threshold energies for (nominally exoergic) single electron transfer onto SF6 of 1.46+/-0.3 eV, 1.56+/-0.3 eV, and 1.63+/-0.3 eV for C(76)2-, C78(2-), and C(84)2-, respectively. Kinetics studies reveal charge-transfer cross sections of up to 430+/-200 A2 for C84(2-) at a collision energy of 77 eV. The mechanism and the energetics are discussed in terms of classical electrostatic model calculations. Additionally, we rationalize the collision energy dependencies of the charge-transfer cross sections using the two-state Landau-Zener formalism to describe the associated resonant electron tunneling probability.

Binding energies of CO on gold cluster cations Au n + (n=1-65): a radiative association kinetics study.

Room temperature CO adsorption on isolated gold cluster cations is studied over a wide size range (Au(n) (+),126), with notable exceptions at n=30, 31 and 48, 49 which manifest local binding energy maxima. For the smallest sizes (3