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Cyclic nanometre-scale sandwich complexes assembled from individual building blocks were synthesized. Sandwich complexes, in which a metal ion is π-coordinated by two planar aromatic organic rings belong to the foundations of organometallic chemistry. They have been successfully used in a wide variety of applications ranging from catalysis, synthesis and electrochemistry to nanotechnology, materials science and medicine1,2. Extending the sandwich structural motif leads to linear multidecker compounds, in which aromatic organic rings and metal atoms are arranged in an alternating fashion. However, the extension to a cyclic multidecker scaffold is unprecedented. Here we show the design, synthesis and characterization of an isomorphous series of circular sandwich compounds, for which the term 'cyclocenes' is suggested. These cyclocenes consist of 18 repeating units, forming almost ideally circular, closed rings in the solid state, that can be described by the general formula [cyclo-MII(µ-η8:η8-CotTIPS)]18 (M = Sr, Sm, Eu; CotTIPS = 1,4-(iPr3Si)2C8H62-). Quantum chemical calculations lead to the conclusion that a unique interplay between the ionic metal-to-ligand bonds, the bulkiness of the ligand system and the energy gain on ring closure, which is crucially influenced by dispersion interactions, facilitate the formation of these cyclic systems. Up to now, only linear one-dimensional multidecker sandwich compounds have been investigated for possible applications such as nanowires3-10. This textbook example of cyclic sandwich compounds is expected to open the door for further innovations towards new functional organometallic materials.
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Lanthanide metal clusters excel in combining molecular and material chemistry properties. Here, we report an efficient cooperative sensitization UC phenomenon of a Eu3+/Yb3+ nonanuclear lanthanide cluster in CD3OD. The synthesis and characterization of the heteronuclear cluster in the solid state and solution are described together with the UC phenomenon showing Eu3+ luminescence in the visible region upon 980 nm NIR excitation of Yb3+ at concentrations as low as 100 nM. Alongside being the Eu/Yb cluster to display UC (with a quantum yield value of 4.88 × 10-8 upon 1.13 W cm-2 excitation at 980 nm), the cluster exhibits downshifted light emission of Yb3+ in the NIR region upon 578 nm visible excitation of Eu3+, which is ascribed to sensitization pathways for Yb through the 5D0 energy levels of Eu3+. Additionally, a faint emission is also observed at ca. 500 nm upon 980 nm excitation, originating from the cooperative luminescence of Yb3+. The [Eu8Yb(BA)16(OH)10]Cl cluster (BA = benzoylacetonate) is also a field-induced single-molecular magnet (SMM) under 4K with a modest Ueff/kB of 8.48 K, thereby joining the coveted list of Yb-SMMs and emerging as a prototype system for next-generation devices, combining luminescence with single-molecular magnetism in a molecular cluster.
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Cucurbit[7]uril (CB7), a supramolecular host, is employed to control the pathway of photolysis of an aryl azide in an aqueous medium. Normally, photolysis of aryl azides in bulk water culminates predominantly in the formation of azepine derivatives via intramolecular rearrangement. Remarkably, however, when this process unfolds within the protective confinement of the CB7 cavity, it results in a carboline derivative, as a consequence of a CâH amination reaction. The resulting carboline caged by CB7 reveals long-lived room temperature phosphorescence (RTP) in the solid state, with lifetimes extending up to 2.1 s. These findings underscore the potential of supramolecular hosts to modulate the photolysis of aryl azides and to facilitate novel phosphorescent materials.
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The platinum hydride cluster Pt12H24- is studied in gas phase by a combination of trapped ion electron diffraction and density functional theory computations. We find a cuboctahedral platinum cage with bridge bound hydrogen atoms. This unusual structure is stabilized by Pt-H-Pt multicenter bonds and shows characteristics of spherical aromaticity.
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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(CîCR)18]2-, M = Ni, Pd, or Pt, and their associated fragment ions. The three different alkynyl ligand-stabilized (CîCR, 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(CîCR)18]2- species fragment by sequential diyne loss to form [MAu24(CîCR)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(CîCR)18]2- also fragments to generate alkyne-oligomers, an inference supported by the parallel observation of precursor dianion isomerization as collision energy is increased.
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Some of the authors of the present research group have previously reported mass spectrometric detection of [PdAu9(PPh3)8(CN)]2+ (PdAu9CN) by atmospheric pressure plasma (APP) irradiation of [MAu8(PPh3)8]2+ (PdAu8) in methanol and proposed based on density functional theory (DFT) calculations that PdAu9CN is constructed by inserting a CNAu or NCAu unit into the Au-PPh3 bond of PdAu8 [Emori et al., J. Chem. Phys. 155, 124312 (2021)]. In this follow-up study, we revisited the structure of PdAu9CN by high-resolution ion mobility spectrometry on an isolated sample of PdAu9CN with the help of dispersion-corrected DFT calculation. In contradiction to the previous proposal, we conclude that isomers in which an AuCN unit is directly bonded to the central Pd atom of PdAu8 are better candidates. This assignment was supported by Fourier transform infrared and ultraviolet-visible spectroscopies of isolated PdAu9CN. The simultaneous formation of [Au(PPh3)2]+ and PdAu9CN suggests that the AuCN species are formed by APP irradiation at the expense of a portion of PdAu8. These results indicate that APP may offer a unique method for transforming metal clusters into novel ones by generating in situ active species that were not originally added to the solution.
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A coumarin functionalized aminodiphosphine has been introduced as a bidentate ligand in coinage metal chemistry. Mono-, di-, and trimetallic copper and silver complexes were synthesized with this ligand. The hybrid character of the ligand led to compounds with rich luminescence properties. These include coumarin-based blue fluorescence, observed as a sole emission in solution at room temperature, and green phosphorescence, which is efficient at low temperatures and dominates the spectra of the metal complexes. In the rigid environment of frozen solutions, the green phosphorescence shows an unusually long (for metal complexes) decay on the seconds timescale in high quantum yield. In addition, a red phosphorescence, which may be assigned to the triplet state localized in the phosphine-M3 Cl2 (M=Cu, Ag), is observed for the trinuclear complexes at low temperature. Neither the second-long phosphorescence nor the red emission is observed for the coumarin ligand, thus they must be a result of the coordination to coinage metal clusters. The excited states in these compounds were also investigated by femtosecond transient absorption spectroscopy and quantum chemical calculations.
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The synthesis of Câ§Câ§P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of PtII and PdII via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of PtII as well as PdII emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S0 â T1) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S0 â T1 excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S0 â Sn â T1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.
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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.
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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.
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Hidrocarbonetos Aromáticos com Pontes , Imidazóis , Compostos Heterocíclicos com 2 Anéis , Humanos , Imidazolidinas , Compostos MacrocíclicosRESUMO
We report the first vibrationally well-resolved absorption and laser-induced fluorescence spectra of the radical cations of free-base phthalocyanine (H2Pc+) and zinc phthalocyanine (ZnPc+) isolated in 5 K neon matrices and compare them to the spectral properties of the corresponding neutrals. The samples were generated by low-energy deposition of the mass-selected ions. The spectra are also discussed in terms of time-dependent density functional theory calculations and compared with recently reported scanning tunneling microscopy-induced single-molecule luminescence of the same species adsorbed on NaCl-covered Au(111) or Ag(111) single crystal supports.
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We compare the intersystem crossing rate, kISC, of Rose Bengal (RB) in an aqueous pH 12 solution with the corresponding relaxation rates of four different RB-derived anion and dianion species isolated in the gas phase: the doubly deprotonated dianion ([RB-2H]2-), the singly deprotonated monoanion ([RB-H]-), and the corresponding singly negatively charged sodium and cesium adducts ([RB-2H + Na]- and [RB-2H + Cs]-, respectively). Each of them was probed following photoexcitation of their first singlet excited states (S1) at or near room temperature. The solution was studied by transient absorption spectroscopy, whereas the mass-selected anions were characterized by time-resolved photoelectron spectroscopyâall with ca. 50 femtosecond temporal resolution. [RB-H]- shows an S1 lifetime of ca. 80 ps; the solution ensemble, thought to consist primarily of solvated dianion chromophores, shows a similar lifetime of ca. 70 ps. By contrast, the isolated dianion, [RB-2H]2-, has a much longer lifetime. Superimposed on S1 decay attributable mainly to intersystem crossing, all four isolated anions also show some rapid oscillatory features of the transient photoelectron signal on a 4-5 ps timescale after excitation. Interestingly, an analogous phenomenon is also seen in the transient absorption measurements. We attribute it to a librational oscillation as the S1 state, initially populated in the S0 geometry, relaxes into its excited state equilibrium structure. Some implications of these observations for RB photophysics and interpretation of solution measurements are discussedâalso in terms of density functional theory and time-dependent density functional theory calculations of ground and excited states.
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Rosa Bengala , Teoria da Densidade FuncionalRESUMO
The structures of platinum cluster anions Pt6--Pt13- have been investigated by trapped ion electron diffraction. Structures were assigned by comparing experimental and simulated scattering functions using candidate structures obtained by density functional theory computations, including spin-orbit coupling. We find a structural evolution from planar structures (Pt6-, Pt7-) and amorphous-like structures (Pt7--Pt9-) to structures based on distorted tetrahedra (Pt9--Pt11-). Finally, Pt12- and Pt13- are based on hcp fragments. While the structural parameters are well described by density functional theory computations for all clusters studied, the predicted lowest energy structure is found in the experiment only for Pt6-. For larger clusters, higher energy isomers are necessary to obtain a fit to the scattering data.
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Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state-dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser-induced fluorescence (LIF) spectra of samples comprising both the mass-selected neutral molecule (VOPcâ , a stable radical) and its cation produced upon electron ionization (EI) isolated in 5â K neon matrices. Ionization of the essentially non-emissive VOPcâ forms a high-spin diradical cation (VOPc+.. ) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR-emitting electro-optic devices.
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We investigated the structural and spectroscopic properties of singly deprotonated biliverdin anions in vacuo, using a combination of cryogenic ion spectroscopy, ion mobility spectrometry, and density functional theory. The ion mobility results show that at least two conformers are populated, with the dominant conformer at 75-90% relative abundance. The vibrational NH stretching signatures are sensitive to the tetrapyrrole structure, and they indicate that the tetrapyrrole system is in a helical conformation, consistent with simulated ion mobility collision cross sections. The vibrational spectrum in the fingerprint region of this singly deprotonated species shows that the two propionate groups share the remaining acidic proton. The S1 band of the electronic spectrum in vacuo is broad, despite ion trap temperatures of 20 K during ion preparation, with a congested Franck-Condon envelope showing partially resolved vibrational features. The vertical transition exhibits a small solvatochromic red shift (-320 cm-1) in aqueous solution.
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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.
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A ligand exchange strategy has been employed to understand the role of ligands on the structural and optical properties of atomically precise 29 atom silver nanoclusters (NCs). By ligand optimization, â¼44-fold quantum yield (QY) enhancement of Ag29(BDT)12-x(DHLA)x NCs (x = 1-6) was achieved, where BDT and DHLA refer to 1,3-benzene-dithiol and dihydrolipoic acid, respectively. High-resolution mass spectrometry was used to monitor ligand exchange, and structures of the different NCs were obtained through density functional theory (DFT). The DFT results from Ag29(BDT)11(DHLA) NCs were further experimentally verified through collisional cross-section (CCS) analysis using ion mobility mass spectrometry (IM MS). An excellent match in predicted CCS values and optical properties with the respective experimental data led to a likely structure of Ag29(DHLA)12 NCs consisting of an icosahedral core with an Ag16S24 shell. Combining the experimental observation with DFT structural analysis of a series of atomically precise NCs, Ag29-yAuy(BDT)12-x(DHLA)x (where y, x = 0,0; 0,1; 0,12 and 1,12; respectively), it was found that while the metal core is responsible for the origin of photoluminescence (PL), ligands play vital roles in determining their resultant PLQY.
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A unique feature of the work carried out in the Collaborative Research Center 3MET continues to be its emphasis on innovative, advanced experimental methods which hyphenate mass-selection with further analytical tools such as laser spectroscopy for the study of isolated molecular ions. This allows to probe the intrinsic properties of the species of interest free of perturbing solvent or matrix effects. This review explains these methods and uses examples from past and ongoing 3MET studies of specific classes of multicenter metal complexes to illustrate how coordination chemistry can be advanced by applying them. As a corollary, we will show how the challenges involved in providing well-defined, for example monoisomeric, samples of the molecular ions have helped to further improve the methods themselves thus also making them applicable to many other areas of chemistry.
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Complexos de Coordenação , Análise EspectralRESUMO
We present a straightforward and generally applicable synthesis route for cofacially linked homo- and heterobimetallic porphyrin complexes. The protocol allows the synthesis of unsymmetrical aryl-based meso-meso as well as ß-meso-linked porphyrins. Our method significantly increases the overall yield for the published compound known as o-phenylene-bisporphyrin (OBBP) by a factor of 6.8. Besides the synthesis of 16 novel homobimetallic complexes containing MnIII , FeIII , NiII , CuII , ZnII , and PdII , we achieved the first single-crystal X-ray structure of an unsymmetrical cofacial benzene-linked porphyrin dimer containing both planar-chiral enantiomers of a NiII 2 complex. Additionally, this new methodology allows access to heterobimetallic complexes such as the FeIII -NiII containing carbon monoxide dehydrogenase active site analogue. The isolated species were investigated by various techniques, including ion mobility spectrometry, DFT calculations, and UV/Vis spectroscopy. This allowed us to probe the influence of interplane distance on Soret band splitting.
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A novel bis(diphenylphosphino)methane (DPPM) functionalized amidine ligand (DPPM-C(N-Dipp)2 H) (Dipp=2,6-diisopropylphenyl) was synthesized. Subsequent deprotonation with suitable alkali metal bases resulted in the corresponding complexes [M{DPPM-C(N-Dipp)2 }(Ln )] (M=Li, Na, K, Rb, Cs; L=thf, Et2 O). The alkali metal complexes form monomeric species in the solid state, exhibiting intramolecular metal-π-interactions. In addition, a caesium derivative [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 was obtained by cleavage of a diphenylphosphino moiety, forming an unusual six-membered ring structure in the solid state. All complexes were fully characterized by single crystal X-ray diffraction, NMR spectroscopy, IR spectroscopy as well as elemental analysis. Furthermore, the photoluminescent properties of the complexes were thoroughly investigated, revealing differences in emission with regards to the respective alkali metal. Interestingly, the hexanuclear [Cs{PPh2 CH2 -C(N-Dipp)2 }]6 metallocycle exhibits a blue emission in the solid state, which is significantly red-shifted at low temperatures. The bifunctional design of the ligand, featuring orthogonal donor atoms (N vs. P) and a high steric demand, is highly promising for the construction of advanced metal and main group complexes.