Predictive Gold Nanocluster Formation Controlled by Metal‐Ligand Complexes

Open AccessCCS ChemistryMINI REVIEW1 Mar 2022Multidimensional Mass Spectrometry Assisted Metallo-Supramolecular Chemistry Heng Wang, Chenxing Guo and Xiaopeng Li Heng Wang College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 Shenzhen University General Hospital, Shenzhen University Clinical Medical Academy, Shenzhen 518055 , Chenxing Guo College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 and Xiaopeng Li *Corresponding author: E-mail Address: [email protected] College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 Shenzhen University General Hospital, Shenzhen University Clinical Medical Academy, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202101408 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metallo-supramolecular chemistry based on the reversible, directional, and predictable noncovalent interaction of metal–ligand coordination has been widely applied in constructing a myriad of two- and three-dimensional supramolecules with sophisticated structures. The development of metallo-supramolecular chemistry highly relies on facile characterization methods. For advancement of structural complexity with desired functions, however, conventional crystallographic and spectroscopic characterizations are limited by crystal growth, sample purity, and severe signal overlaps. With high sensitivity and low requirement for purity, multidimensional mass spectrometry (MS), including electrospray ionization-MS (ESI-MS), tandem MS (MS2), and ion mobility-MS (IM-MS), is emerging as an essential analytical tool for characterization of the chemical compositions, connectivity, stability, shape, and size of metallo-supramolecules, as well as kinetic and thermodynamic features of self-assembly processes. Thus, this review mainly focuses on the recent development of multidimensional MS in metallo-supramolecular characterization and discusses how ESI-MS, MS2, and IM-MS assist the development of metallo-supramolecular chemistry. Download figure Download PowerPoint Introduction Supramolecular self-assembly, inspired by the precise organization of bio-macromolecules in living systems, allows for the spontaneous formation of ordered complexes from a chaos state.1 Among various noncovalent driving forces, metal–ligand coordination plays a vital role in the self-assembly of structurally and functionally sophisticated metallo-supramolecules, benefitting from the high directionality and predictable feature of coordination bonds.2–6 As a result, metallo-supramolecular chemistry has witnessed an explosion in constructing a myriad of artificial supramolecular architectures, including polygons,7–11 knots,12–15 links,16–19 fractals,20–22 as well as Platonic,23–27 Archimedean,28–34 Goldenberg polyhedrons,35,36 and so on. And these complexes show great potentials among different fields, including catalysis,37–46 nano-electronics,47,48 stabilization of air- or water-sensitive molecules,49–51 disease diagnosis and therapy,52–57 and so on. Furthermore, various delicate assembling approaches, inter alia, directional bonding,2 symmetry interactions,58 molecular paneling,59 and sub-components,60 have been developed to assist the exploration of metallo-supramolecular chemistry. Compared to the flourish of self-assembly strategies, however, the lack of efficient characterization methods always limits the investigation of assembled complexes at equilibrium and the kinetic details of self-assembly processes in solution because of the weak interaction and dynamic nature of metallo-supramolecules. Among the conventional characterizations of metallo-supramolecules, single-crystal X-ray diffraction (SC-XRD) is a powerful and direct method that allows for detailed structural information at the atomic level.61 However, it remains a formidable challenge to obtain high-quality single crystals, particularly for metallo-supramolecules with large voids filled by disordered solvent molecules or counterions. In addition, the refinement of the crystal structures for large metallo-supramolecules is difficult because the current crystal refinement approaches were mainly developed for small molecules. Apart from those issues the structural information obtained through SC-XRD in solid phase may not correlate with the study of metallo-supramolecular systems in solution or gas phase. Another well-established characterization method is NMR spectroscopy, whereby various nuclei in metallo-supramolecular systems, such as 1H, 13C, 19F, 31P, and so on can be detected, allowing the chemical environment of these nuclei in metallo-supramolecules to be elucidated.62,63 Through two-dimensional (2D) NMR spectroscopic techniques, for example, 1H–1H correlation spectroscopy (COSY), nuclear Overhauser effect spectroscopy (NOESY), and diffusion ordered spectroscopy (DOSY), diverse structural information about the metallo-supramolecules can be further obtained, including metal–ligand coordinations, geometric distributions of nuclei, and details of the size and shape of assembled architectures. However, the application of NMR is also limited in analyzing samples with low purity, severe overlapping signals, or those containing paramagnetic metal ions. As complementary characterization techniques, various microscopic methods, including atomic force microscopy (AFM), scanning tunneling microscopy (STM), and transmission electron microscopy (TEM), directly image the analytes. AFM and STM, in which tips probe the surface of the target molecules on substrates, reveal the size and shape of an individual molecule64 and distinguish the metal ions by using scanning tunneling spectroscopy (STS).65 However, high image resolution relies on sharp tips, as well as high-quality samples without impurities, severe aggregations, or dissociations of the target supramolecules. TEM uses electrons as the "probe" and can offer atomic resolution of the entire molecular structure assisted by computer-assisted three-dimensional (3D) reconstruction, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) techniques.66 Unfortunately, in addition to the challenges of preparing a high-quality sample, TEM also suffers from the intrinsic instability of metallo-supramolecules under electron beam or during sample preparation. The mass-to-charge ratio (m/z) information of the intact metallo-supramolecular ions obtained by mass spectrometry (MS) measurements in the gas phase enables the determination of the chemical composition of analytes. Furthermore, MS characterization provides higher sensitivity but a lower sample purity requirement than crystallographic analysis and NMR. Such features along with high analytical speed make it possible to monitor the dynamic process of self-assembly before the system in question reaches equilibrium. Thus, MS is emerging as an essential analytical tool for determining structures and monitoring reaction processes. However, traditional MS approaches also suffer from the fragmentation of metallo-supramolecules bound by weak coordination interaction even when a soft ionization method, that is, electrospray ionization (ESI), is applied. Cold-spray ionization (CSI),67 equipped with an ESI source cooled by liquid nitrogen, was used to overcome the problem by lowering the internal energy gained during the ionization. Due to the labile connectivity, matrix-assisted laser desorption/ionization (MALDI) could cause very severe fragmentation and thus is not preferred for the characterization of metallo-supramolecules. In addition to the m/z information obtained from MS, tandem MS (MS2) and ion mobility-MS (IM-MS) can elucidate more detailed information of the analytes. First, as a controllable fragmentation technique, MS2 offers deeper insight into the analytes, such as the information about the stability and connectivity of the structures. Furthermore, as an orthogonal in situ separation method, IM-MS can identify different species with the same m/z; more importantly, it provides structural information in terms of the shape and size of selected ions. During the past two decades, multidimensional MS with the combination of MS, IM-MS, and MS2 has played an important role in the development of metallo-supramolecular chemistry. Here, we discuss how ESI-MS, MS2, and IM-MS can be applied in (1) determining chemical compositions, connectivity, shape, and size of metallo-supramolecules, (2) distinguishing the isomers and by-products present in a dynamic combinatorial library, and (3) investigating kinetic and thermodynamic features of self-assembly processes. More importantly, we anticipate that multidimensional MS would not only act as an analytical method, but also serve as a powerful tool to assist the design of next-generation metallo-supramolecules, and even advance the development of new synthetic strategies. ESI-MS Studies Charged coordination complexes are ideal analytes for ESI-MS studies because of the easy-to-ionize feature of these supramolecules in which the counterions are readily stripped off, especially in polar and volatile solvents. Another advantage of ESI-MS is its intrinsic capability of reducing the signals corresponding to the multiply charged analytes with high molar mass (<104 Da) to the mass range of the conventional mass analyzers, since it only measures the m/z values of these ensembles. Consequently, a series of peaks is usually observed, representing different charge states of the same intact analyte through successive loss of counterions. To minimize the fragmentation during ionization, it is crucial to ionize the fragile analytes under mild conditions, including careful selection of the focusing and ionization voltages, low operation temperature, and fast ion extraction process through increasing source pressure (or a low vacuum).68 Additionally, highly concentrated metallo-supramolecular samples are often required to inhibit the potential entropy-driven dissociation in solution, although the ESI source needs more extensive cleaning after each measurement. Without annoying fragmentation, ESI-MS is suitable for detecting the chemical compositions of the ionic species in the metallo-supramolecular analytes, as well as providing information regarding the following aspects of the self-assembly: (1) fast screening the assembled structures; (2) monitoring the dynamic processes during the self-assembly; and (3) detecting the minor products to guide further design. Acting as a fast-screening characterization approach for the self-assembly Fast screening of the analytes' structural information by ESI-MS does not require extensive purification processes that are otherwise inevitable for NMR measurement and single-crystal growth. In most cases, after simply removing the inorganic salts to reduce ion suppression, the resulting sample solution is ready for direct measurement. With careful control of stoichiometry, the solution containing the self-assembled species can be directly subjected to ESI-MS without any further purification. Additionally, its capability of detecting m/z values enables the characterization of multiple assemblies with different numbers of building blocks, paramagnetic metallo-supramolecular analytes, and host–guest adducts. Therefore, ESI-MS could serve as a complementary or even superior method to perform structural characterization compared to NMR. The first example showcasing the fast screening by ESI-MS is the characterization of different generations of metallo-dendrimers S1/ S2 (Figure 1a) comprised of polyazaaromatic Ru(II) complexes reported by Kirsch-De Mesmaeker et al.69 ESI-MS directly generated clear and simple patterns that allowed the unambiguous determination of the chemical formula, whereas the other spectroscopic characterization just provided nonconclusive data (Figure 1b). Second, in the characterization of metallo-macrocycles reported by Newkome and co-workers,701H NMR gave very similar chemical shifts for the hexagon, octagon, and decagon ( S3- S5) assembled from the Ru(II)-connected bitopic 2,2′:6′,2″-terpyridyl (TPY) subunit L3 (Figure 1c) because of the similar symmetry and chemical environments of the detected protons. In contrast, these analogs with distinct molar mass were unambiguously identified by ESI-MS, where three distinct sets of peaks were detected (Figures 1d and 1e).70 Figure 1 | ESI-MS acting as a fast-screening approach for the preparation of (a and b) metallo-dendrimers comprised of polyazaaromatic Ru(II) complexes S2 and its corresponding ESI-MS spectrum; (c–e) TPY-Ru(II)/Fe(II)-based metallo-hexagon S3, -octagon S4, and -decagon S5 with corresponding 1H NMR spectra and ESI-MS spectra; (f–h) self-assembly of supramolecular fractals S6 based on the metal complexation between a TPY-based ligand and paramagnetic metal ions with corresponding 1H NMR and ESI-MS spectrum of the Co(II) complex (S6-Co). (a and b) Adapted with permission from ref 69. Copyright 1996 American Chemical Society. (c–e) Adapted with permission from ref 70. Copyright 2011 Wiley-VCH. (f–h) Adapted with permission from ref 71. Copyright 2020 American Chemical Society. Download figure Download PowerPoint Metallo-supramolecules containing paramagnetic transition metal ions with unpaired electrons remain a formidable challenge for NMR characterization. In contrast, ESI-MS has no problem characterizing these samples. For example, Shionoya et al.25 constructed a series of metallo-supramolecular octahedra by mixing a tritopic pyridinyl building block with different transition metal ions. ESI-MS spectra of the paramagnetic analytes containing Cu(II), Ni(II), Co(II), and Mn(II) ions clearly showed a dominant set of peaks ascribed to the designed structures; whereas, NMR spectra only displayed broad peaks with limited information.25 Similarly, with the help of ESI-MS characterization, coordination-driven self-assembly based on a TPY-paramagnetic metal ion complex was applied to construct a series of supramolecular fractals reported by Li et al.,71 which not only expanded the library of metal ions for assembling TPY-based architectures but also set up an ideal platform for studying the kinetic and thermodynamic features of these metallo-supramolecules (Figures 1f–1h). When characterizing the paramagnetic Co(II) and Ni(II) cages assembled using the sub-component self-assembly strategy, the reliable ESI-MS method also played a vital role in identifying the formation of target structures.72,73 Investigation of host–guest complexation is an important subfield of metallo-supramolecular chemistry, which is related to the fundamental study of molecular recognition and the potential applications of these metal–organic containers. NMR spectroscopy is extensively applied to provide evidence for the interaction with diagnostic chemical shifts of the protons on guest molecules. However, it cannot always accurately address how many guest molecules are encapsulated in the containers. ESI-MS can properly examine the exact change in mass upon the encapsulation of the guest molecules, thus, providing an accurate m/z value for deducing the number of guest molecules. In a self-assembled M8L6 (M = the metal ion, L = the ligand) cube with porphyrin faces, Nitschke et al.74 described that large guests such as coronene and fullerene can be selectively encapsulated (Figure 2a). Compared with the complicated NMR spectra that only provided a 2:1 ratio of guests in two different environments, ESI-MS spectrum of the coronene-encapsulated cube directly provided the number of the guest molecules via deconvolution of the dominant set of peaks (Figures 2b and 2c). The ESI-MS spectrum also clearly confirmed that the cube allowed for selective encapsulation of C70, C76, C82, and C84 over C60 from the fullerene soot, as evidenced by the absence of any discernible peaks assignable to the C60-encapsulated or guest-free cube. Figure 2 | ESI-MS-based fast screening for studying the guest encapsulation of metallo-supramolecular cages. (a–c) Self-assembly of porphyrin-faced cube S8 for encapsulation of three coronene with corresponding characterization by 1H NMR and ESI-MS; (d and e) a PBI-edged tetrahedron capable of encapsulating two molecules of C60S9 and the corresponding ESI-MS spectrum; (f–h) self-assembly of a metallo-supramolecular peanut S10 for simultaneous encapsulation of one diamantane and two phenanthrenes (S11) with corresponding characterization by 1H NMR and ESI-MS. (a–c) Adapted with permission from ref 74. Copyright 2011 Wiley-VCH. (d and e) Adapted with permission from ref 76. Copyright 2013 American Chemical Society. (f–h) Adapted with permission from ref 83. Copyright 2017 Springer Nature. Download figure Download PowerPoint Thus, ESI-MS is also a feasible and rapid method for qualitatively studying the encapsulation selectivity of a certain metallo-supramolecule with different guests. In another example of selective encapsulation, when mixing a metallo-supramolecular tetragonal prism with a mixture of C60 and its derivatives, the selectivity could be determined qualitatively by comparing the relative abundance of the peaks corresponding to the host–guest complexes containing different guests. Among them, C60 and PCBM-C60 have higher affinity with the host than N-methylpyrrolidine-C60.75 However, it is still challenging to quantitatively determine the selectivity by ESI-MS because of the different ionization efficiency of those complexes, although the host molecules are the same. Ideally, quantitative data could be obtained if the ionization efficiencies of different complexes are calibrated. In addition to encapsulating one fullerene molecule, Würthner et al.76 applied ESI-MS to unequivocally prove the encapsulation of one or two molecules of C60 inside the cavity of perylene-bisimide (PBI) edged tetrahedron S9 (Figures 2d and 2e). Note that the C60 guest does not bear any protons so that it can only be identified by 13C NMR. Although the 13C NMR could confirm the encapsulation of the C60, it was unable to determine the number of guest molecules because conventional 13C NMR does not allow for quantitative analysis as opposed to 1H NMR. Metallo-supramolecular trigonal prisms, wherein C3-symmetrical tritopic pyridyl ligands, linear ditopic pyridyl ligands, and an end-capped Pd(II) moiety served as the base faces, pillars, and vertices, respectively, were found to be an excellent molecular container capable of encapsulating a variety of guest molecules, including, coronene,77 triphenylene,78 pyrene-4,5-dione,79,80 and metal–organic complexes.81,82 More intriguingly, the number of the encapsulated guest molecules can be precisely manipulated via tuning the length of the organic pillars. The accurate number of encapsulated guests could be unambiguously determined by ESI-MS. A more complicated situation was reported by Yoshizawa et al.83 The self-assembled metallo-supramolecular peanut S10 heteroleptically encapsulated two different types of guests, that is, one diamantane and two phenanthrenes, inside its two cavities (Figure 2f). Without an SC-XRD result or clear NMR spectrum (Figure 2g), such an unusual encapsulation was identified by ESI-MS (Figure 2h), where peaks with m/z values of the expected host–guest complex were found. Monitoring the dynamic processes for unveiling self-assembly details Metallo-supramolecules bonded via labile coordination interactions exhibit dynamic features in solution due to the dissociation and reassociation of the weak bonds. Thus, it is hard to investigate the kinetic processes by traditional characterization techniques in solution. Nevertheless, in an ESI-MS measurement, each complex ion is a separated species in gas phase. Therefore, the dynamic process is "frozen" to display the kinetic instead of thermodynamic feature of the system. As one type of dynamic process, ligand exchange involves the disassociation of a ligand from the assembled supramolecule and subsequent replacement by another ligand. Such a process is related to the coordination bond strength and the activation energies during the exchange processes. If the time scales of such dynamic exchanges are comparable for MS measurement, one should be able to study the exchange process of two kinds of pre-assembled supramolecules with distinct molar masses of ligands by MS. Once the exchange occurs, the m/z values corresponding to each possible re-assembled species as well as the initial ensembles would be different, all of which can be specifically recorded by ESI-MS measurement. For example, Fujita et al.84 reported the studies of ligand exchange between two M12L24 spheres via mixing two topologically identical spheres bearing different length of alkyl chains (–OC3H7 vs –OC6H13) and then monitoring under time-dependent ESI-MS (Figures 3a and 3b). Due to the difference in mass, all the peaks corresponding to the initial two spheres and the exchanged ones were easily identified in the spectra. Time-dependent measurements revealed a quite remarkable kinetic stability of the spheres regarding ligand exchange. The half-time for the first ligand exchange was measured to be approximately 20 days. After evaluating the ionization efficiency of two spherical supramolecules, the estimated kinetic ligand exchange rate was found to be 2 × 10−2 M−1·s−1, which is far less than the exchange rate between a free ligand with a sphere (half-time of 23 min). Therefore, the ESI-MS studies semi-quantitatively revealed that the fully closed framework slowed down the dynamic exchange processes through a cooperative effect.84 A similar method was also utilized to carefully study the dynamic nature of metallo-polygons assembled from organoplatinum motifs and pyridyl ligands with 1H/2D isotope labeling.85 In addition to the quantitative determination of the ligand exchange rates, further understanding about the kinetic processes in coordination-driven self-assembly was provided by time-dependent ESI-MS, which showed the ligand exchange rate highly depended on the experimental conditions such as temperature, solvent, and counterions. Figure 3 | ESI-MS of ligand exchange. (a) Ligand exchange between two structurally identical spheres (Pd12L724 and Pd12L824) with different alkyl chain lengths (-n-C3H7 vs -n-C6H13), and (b) the time-dependent ESI-MS spectra; (c) self-assembly of four types of supramolecular snowflakes (S12–S15) with different lengths of the ligands, and time-dependent ESI-MS spectra of the mixture of and (a and b) Adapted with permission from ref Copyright American Chemical Society. and e) with permission from ref Copyright Wiley-VCH. with permission from ref Copyright 2017 American Chemical Society. Download figure Download PowerPoint In addition to the pyridyl ESI-MS also provided insight into the ligand exchange processes of metallo-supramolecular systems based on analyzing the peaks to the complexes in ESI-MS it was revealed that the exchange processes a certain of to the size and shape of the ligand via the of the structure when the Through a time-dependent ESI-MS measurement, such a was estimated in the metallo-supramolecular snowflakes with different length of ligand The exchange rate was found to directly correlate with that is, the the the ligand exchange. When = the exchange equilibrium = required for whereas = in no exchange (Figures Furthermore, the ligand exchange rate was also found to correlate with the coordination metal ion in TPY-based To quantitatively these exchange rates, different metal ions (M = were applied. of these metal ions was used to two structures with the same molecular size and shape but different molar After mixing the of pre-assembled supramolecules, time-dependent ESI-MS was utilized to the relative abundance of each After the ionization efficiency of these supramolecules, ESI-MS data provided exchange of these metallo-supramolecules at with a of Mn(II) Co(II), whereas the and fractals were found to be to the ligand exchange. The exchange rate of fractals was determined to be × M−1·s−1, which is higher than that of fractals × Monitoring the in the self-assembly processes is also a kinetic investigation that can be carefully by ESI-MS. When using NMR to monitor the complexation of ligands with or Ni(II) shifts of the peaks were observed, the of an before the formation of However, the limited information provided by NMR spectra not elucidate the ESI-MS was then used to the and the that is, linear by the m/z value of the intact Thus, ESI-MS can reveal the reaction via monitoring the Such capability was also used in monitoring the formation of a supramolecular wherein ligand and self-assembly were in a The time-dependent the of the coordination and processes (Figures and Figure | (a) The formation of supramolecular via a ligand and self-assembly (b) time-dependent ESI-MS monitoring the (b) Adapted with permission from ref Copyright American Chemical Society. Download figure Download PowerPoint With m/z values of the more details about the self-assembly processes could be allowing for deeper understanding of the assembling For in the of ligands with ions to and an with metal ion in the ligand with instead of was through ESI-MS by increasing ligand or metal Such an that the complexation to the A similar of complexation was also in other systems by ESI-MS, its ESI-MS of the species during the formation of prism with a ligand revealed the of the The ESI spectra the of self-assembly and showed the formation of the base with a Such a the design of a building block with two types of two distinct geometric that is, as base and respectively, allowing for the self-assembly of a structures to further design With the reliable m/z ESI-MS can identify the desired before other and characterization. For based on the directional an tritopic ligand was designed to metallo-supramolecules by coordination with or to the assembled could be or Unfortunately, NMR characterization because of the Nevertheless, ESI-MS provided evidence for the formation of a which the of the ligand to the of the complex for structural on

Colloidal CdSe quantum dots (QDs) are suitable as electron acceptors in polymer/nanoparticle bulk heterojunction hybrid solar cells. For this application, a thick organic ligand shell which is typically surrounding the QDs after synthesis needs to be removed. Ligand exchange with pyridine is the most widely used method for this purpose. Although this approach is already 15 years old, detailed studies on the effectiveness of ligand exchange with pyridine for solar cell applications are still missing. In the present work hybrid solar cells were prepared from CdSe QDs initially capped with oleic acid (OA), and the impact of single and multiple pyridine treatment was thoroughly investigated. NMR was applied to determine the composition of the ligand shell as well as to distinguish the bound and free ligands before and after ligand exchange. It is shown that after a single pyridine treatment some amount of OA is still present in the samples. By using thermal gravimetric analysis (TGA) we could obtain also quantitative information about the effectiveness of subsequent pyridine treatments. In a series of one-, two-, and threefold ligand exchange, the estimated surface coverage by OA decreased from 26% to 12%, whereas that of pyridine increased from 54% to 80%. Laboratory solar cells with pyridine-capped CdSe QDs and poly(3-hexylthiophene) (P3HT) were characterized by current−voltage (I−V) measurements, and in order to get deeper insight into charge carrier generation and recombination processes, CdSe/P3HT blends were studied by light-induced electron spin resonance (l-ESR). Although repeated pyridine treatment was found to have a beneficial effect in the sense that more complete ligand exchange was achieved, which in turn enabled more efficient charge transfer, the performance of the solar cells was found to be reduced. This fact correlates with increased aggregation tendency of repeatedly pyridine-treated particles, negatively influencing the morphology of the blends, as well as with a larger amount of surface defects in particles stabilized by the weak pyridine ligand shell.

Colloidal quantum dots (CQDs), which benefit from a size-tuned bandgap, are a solution-processed material for infrared energy harvesting. This characteristic enables the fabrication of solar cells that form tandem devices with silicon. Unfortunately, in the case of CQDs having diameters sufficiently large (>4 nm) so that the nanoparticles absorb light well beyond silicon’s bandgap, conventional ligand exchanges fail. Here we report a strategy wherein short-chain carboxylates, used as a steric hindrance controller, facilitate the ligand exchange process on small-bandgap CQDs. We demonstrate that the net energy barrier to replace original capping ligands with lead halide anions is decreased when short carboxylates are involved. The approach produces more complete ligand exchange that enables improved packing density and monodispersity. This contributes to a 2-fold reduction in the trap state density compared to the best previously reported exchange. We demonstrate solar cells that achieve a record infrared photon-to-electron conversion efficiency at the excitonic peak.

Formation of gold nanoclusters in MgO by ion implantation at elevated temperatures

This study reports the gas phase complexation behavior of Pu(IV) with N,N-dialkylamide ligands and tributylphosphate by electrospray ionization mass spectrometry. In-source collision induced dissociation (CID) and ion trap collision induced dissociation were used to investigate the kinetic stability of Pu species in the gas phase. With both class of ligands, N,N-dialkylamide and trialkylphosphate, oxidation of Pu from the tetravalent state to the hexavalent state was observed at both high skimmer voltages and during ion trap CID. The results suggest that oxidation is due to the presence of a nitrate group in the coordination sphere of the metal and is independent of the ligand.

As a versatile post-synthesis modification method, ligand exchange reaction exhibits great potential to extend the space of accessible nanoclusters. In this review, we summarized this process for thiolate-protected gold nanoclusters. In order to better understand this reaction we will first provide the necessary background on the synthesis and structure of various gold clusters, such as Au25(SR)18, Au38(SR)24, and Au102(SR)44. The previous investigations illustrated that ligand exchange is enabled by the chemical properties and flexible gold–sulfur interface of nanoclusters. It is generally believed that ligand exchange follows a SN2-like mechanism, which is supported both by experiments and calculations. More interesting, several studies show that ligand exchange takes place at preferred sites, i.e. thiolate groups –SR, on the ligand shell of nanoclusters. With the help of ligand exchange reactions many functionalities could be imparted to gold nanoclusters including the introduced of chirality to achiral nanoclusters, size transformation and phase transfer of nanoclusters, and the addition of fluorescence or biological labels. Ligand exchange was also used to amplify the enantiomeric excess of an intrinsically chiral cluster. Ligand exchange reaction accelerates the prosperity of the nanocluster field, and also extends the diversity of precise nanoclusters.

A simple competition assay to probe pentacopper(I)-thiolato cluster ligand exchange

Biological molecular imaging and sensing requires fluorophores that are not only stable and bright, but also small enough to allow the unencumbered observation of the movement of proteins. Toward this goal, we have studied the formation of fluorescent metallic gold nanoclusters stabilized by small molecule ligands. The morpholine and piperazine backbones of Good’s buffers were used to template fluorescent clusters, through a process of etching of nanoparticles first formed in the reaction. The clusters are found to be subnanometer sized, with nanosecond fluorescence lifetimes and as bright, or brighter, than the commercial dye norharmane.

New insights into the constitution of solutions containing labile, polynuclear compounds: an electrospray mass spectrometric study of mercury- and cadmium-rich dithiocarbamato cations

Organoimido functionalized trinuclear gold(I) clusters with fluorescent chromophore

Multinuclear ( 31P, 77Se, 113Cd, 199Hg) magnetic resonance, electrospray mass spectrometric and electrochemical studies on the reactions of M(S 2P{OEt} 2) 2 (M=Cd, Hg) with potentially bidentate mixed Group 15/Group 16 and Group 16 donor ligands

Complex speciation and exchange kinetics of labile ligands are critical parameters for understanding the reactivity of metal complexes in solution. We present a novel approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). The introduction of isotopically labelled ligands to a solution of metal host and unlabelled ligands allows the quantitative investigation of the solution-phase equilibria. Furthermore, ion mobility separation can target individual isomers, such as ligands bound at specific sites. As a proof of concept, we investigate the solution equilibria of labile pyridine ligands coordinated in the cavity of macrocyclic porphyrin cage complexes bearing diamagnetic or paramagnetic metal centres. The effects of solvent, porphyrin coordination sphere, transition metal, and counterion on ligand dissociation are discussed. Rate constants and activation parameters for ligand dissociation in the solution can be derived from our ESI-MS approach, thereby providing mechanistic insights that are not easily obtained from traditional solution-phase techniques.

A series of novel (C∧N∧N) cyclometalated AuIII complexes of general formula [Au(bipydmb-H)X][PF6] (bipydmb-H = C∧N∧N cyclometalated 6-(1,1-dimethylbenzyl)-2,2'-bipyridine) were prepared with a range of anionic ligands X in the fourth coordination position, featuring C (alkynyl)-, N-, O-, or S-donor atoms. The X ligands are varied in nature and include three coumarins, 4-ethynylaniline, saccharine, and thio-β-d-glucose tetraacetate, the tripeptide glutathione (GSH), and a coumarin-substituted amide derived from 4-ethynylaniline. The gold(I) complex [Au(C2ArNHCOQ)(PPh3)] (HC2ArNHCOQ = N-(4-ethynylphenyl)-2-oxo-2 H-chromene-3-carboxamide) was also prepared for comparison. The new compounds were fully characterized by means of analytical techniques, including NMR, absorption, and emission spectroscopy. The crystal structures of three cyclometalated AuIII complexes and of the AuI derivative were solved by single-crystal X-ray diffraction. The antiproliferative activity of the new AuIII cyclometalated derivatives was evaluated against cancer cells in vitro. According to the obtained results, only complexes 3-PF6 and 5-PF6, featuring coumarins as ancillary ligands and endowed with high redox stability in solution, display antiproliferative effects, with 5-PF6 being the most potent, while all of the others are scarcely active to nonactive in the selected cell lines. In order to study the reactivity of the compounds with biomolecules, the interaction of complexes 3-PF6 and 5-PF6 with the protein cytochrome c and the amino acids cysteine and histidine was analyzed by electrospray ionization mass spectrometry (ESI MS), showing adduct formation only with Cys after at least 1 h incubation. Furthermore, the parent hydroxo complex [Au(bipydmb-H)(OH)][PF6] (1OH-PF6) was investigated in a competitive assay to determine the protein vs oligonucleotide binding preferences by capillary zone electrophoresis (CZE) coupled to ESI-MS. Of note, the compound was found to selectively form adducts with the oligonucleotide over the protein upon ligand exchange with the hydroxido ligand. Adduct formation occurred within the first 10 min of incubation, demonstrating the preference of 1OH-PF6 for nucleotides in this setup. Overall, the obtained results point toward the possibility to selectively target DNA with gold(III) organometallics.

We present an example of an interparticle reaction between atomically precise nanoclusters (NCs) of the same metal, resulting in entirely different clusters. In detail, the clusters [Ag12(TBT)8(TFA)5(CH3CN)]+ (TBT = tert-butylthiolate, TFA = trifluoroacetate, CH3CN = acetonitrile) and [Ag18(TPP)10H16]2+ (TPP = triphenylphosphine) abbreviated as Ag12 and Ag18, respectively, react leading to [Ag16(TBT)8(TFA)7(CH3CN)3Cl]+ and [Ag17(TBT)8(TFA)7(CH3CN)3Cl]+, abbreviated as Ag16 and Ag17, respectively. The two product NCs crystallize together as both possess the same metal chalcogenolate shell, composed of Ag16S8, making them indistinguishable. The occupancies of Ag16 and Ag17 are 66.66 and 33.33%, respectively, in a single crystal. Electrospray ionization mass spectrometry (ESI MS) of the reaction product and a dissolved crystal show the population of Ag16 and Ag17 NCs to be in a 1:1 and 2:1 ratio, respectively. This suggests selective crystallization in the cocrystal. Time-dependent ESI MS was employed to understand the formation of product clusters by monitoring the reaction intermediates formed in the course of the reaction. We present an unprecedented growth mechanism for the formation of silver NCs mediated by silver thiolate intermediates.

Charge reduction reactions are important for native mass spectrometry (nMS) because lower charge states help retain native-like conformations and preserve noncovalent interactions of protein complexes. While mechanisms of charge reduction reactions are not well understood, they are generally achieved through the addition of small molecules, such as polyamines, to traditional nMS buffers. Here, we present new evidence that surface-active, charge reducing reagents carry away excess charge from the droplet after being emitted due to Coulombic repulsion, thereby reducing the overall charge of the droplet. Furthermore, these processes are directly linked to two mechanisms for electrospray ionization, specifically the charge residue and ion evaporation models (CRM and IEM). Selected protein complexes were analyzed in solutions containing ammonium acetate and selected trialkylamines or diaminoalkanes of increasing alkyl chain lengths. Results show that amines with higher surface activity have increased propensities for promoting charge reduction of the protein ions. The electrospray ionization (ESI) emitter potential was also found to be a major contributing parameter to the prevalence of charge reduction; higher emitter potentials consistently coincided with lower average charge states among all protein complexes analyzed. These results offer experimental evidence for the mechanism of charge reduction in ESI and also provide insight into the final stages of the ESI and their impact on biological ions.