Self-assembled metallasupramolecular cages towards light harvesting systems for oxidative cyclization

Designing artificial light harvesting systems with the ability to utilize the output energy for fruitful application in aqueous medium is an intriguing topic for the development of clean and sustainable energy. We report here facile synthesis of three prismatic molecular cages as imminent supramolecular optoelectronic materials via two-component coordination-driven self-assembly of a new tetra-imidazole donor (L) in combination with 180°/120° di-platinum(ii) acceptors. Self-assembly of 180° trans-Pt(ii) acceptors A1 and A2 with L leads to the formation of cages Pt₄L₂(1a) and Pt₈L₂(2a) respectively, while 120°-Pt(ii) acceptor A3 with L gives the Pt₈L₂(3a) metallacage. PF₆⁻ analogues (1b, 2b and 3b) of the metallacages possess a high molar extinction coefficient and large Stokes shift. 1b–3b are weakly emissive in dilute solution but showed aggregation induced emission (AIE) in a water/MeCN mixture as well as in the solid state. AIE active 2b and 3b in aqueous (90% water/MeCN mixture) medium act as donors for fabricating artificial light harvesting systems via Förster resonance energy transfer (FRET) with organic dye rhodamine-B (RhB) with high energy efficiency and good antenna effect. The metallacages 2b and 3b represent an interesting platform to fabricate new generation supramolecular aqueous light harvesting systems with high antenna effect. Finally, the harvested energy of the LHSs (2b + RhB) and (3b + RhB) was utilized successfully for efficient visible light induced photo-oxidative cross coupling cyclization of N,N-dimethylaniline (4) with a series of N-alkyl/aryl maleimides (5) in aqueous acetonitrile with dramatic enhancement in yields compared to the reactions with RhB or cages alone.

Synthesis of Pt(ii) based metallacages as aggregation induced emissive supramolecular architectures for fabricating artificial light harvesting systems for cross coupling cyclization under visible light is achieved.     

Topological prediction of palladium coordination cages

The preparation of functionalized, heteroleptic Pd_xL_2x coordination cages is desirable for catalytic and optoelectronic applications. Current rational design of these cages uses the angle between metal-binding (∠B) sites of the di(pyridyl)arene linker to predict the topology of homoleptic cages obtained via non-covalent chemistry. However, this model neglects the contributions of steric bulk between the pyridyl residues—a prerequisite for endohedrally functionalized cages, and fails to rationalize heteroleptic cages. We describe a classical mechanics (CM) approach to predict the topological outcomes of Pd_xL_2x coordination cage formation with arbitrary linker combinations, accounting for the electronic effects of coordination and steric effects of linker structure. Initial validation of our CM method with reported homoleptic Pd₁₂LFu₂₄ (LFu = 2,5-bis(pyridyl)furan) assembly suggested the formation of a minor topology Pd₁₅LFu₃₀, identified experimentally by mass spectrometry. Application to heteroleptic cage systems employing mixtures of LFu (∠B = 127°) and its thiophene congener LTh (∠B = 149° ∠B_exp = 152.4°) enabled prediction of Pd₁₂L₂₄ and Pd₂₄L₄₈ coordination cages formation, reliably emulating experimental data. Finally, the topological outcome for exohedrally (LEx) and endohedrally (LEn) functionalized heteroleptic Pd_xL_2x coordination cages were predicted to assess the effect of steric bulk on both topological outcomes and coordination cage yields, with comparisons drawn to experimental data.

A molecular mechanics approach enables the accurate prediction of polyhedral topology for homoleptic and heteroleptic palladium M_xL_2x coordination cages, allowing for new insight and design when considering endo- and exo-hedral functionalization.     

NMR (195Pt and 13C) contribution to the study of some Pt(II), Pt(IV) and mixed-valence thioamido complexes

The ¹⁹⁵Pt and ¹³C chemical shifts (δ_Pt and δ_c) are reported for platinum(II), platinum(IV) and class II mixed-valence complexes, with general formula [PtL₄]X₂, cis- and trans-PtL₂X₂, PtL₂X₄ and Pt₂L₄X₆ (where L may be thiourea, 2-imidazolidine-thione, tetrahydro 2-pyrimidinethione, thiocaprolactam, pyridine-2-thione and tetramethylthiourea, and X may be Cl or Br). The ¹⁹⁵Pt chemical shifts can be understood in view of ¹³C data in terms of variations of electronegativities and σ-donor abilities of ligands attached to platinum.     

Stereochemical non-rigidity of a metal polyhedron; carbon-13 and platinum-195 fourier transform nuclear magnetic resonance spectra of [Ptn(CO)2n]2- (n = 3, 6, 9, 12 or 15)

Platinum-195 spectra are reported for [Pt_n(CO)_2n]^2- (n = 3, 6, 9, 12 and 15) and carbon-13 spectra are reported for n = 6, 9 and 12 over a range of temperatures. The spectra provide evidence for (a) intramolecular rotation of the Pt₃-triangles about the principal three-fold axis, (b) inter-exchange of Pt₃-triangles, (c) lack of terminal/edge carbonyl exchange within the Pt₃(CO)₃(μ-CO)₃ group. Evidence is also presented for the formation of [Ni₃Pt₃(CO)₁₂]^2- on mixing [Pt₆(CO)₁₂]^2- [Ni₆(CO)₁₂]^2-.     

氧-硼共修饰多壁碳纳米管载铂催化剂的制备及氧还原活性

采用一种简便的方法,合成了氧-硼共修饰的多壁碳纳米管材料,以此为载体制备的铂基催化剂具有更小的铂粒径、更高的电化学表面积(40 m²·g_Pt^-1)和更高的氧还原活性(0.3 A·mg_Pt^-1)。氧、硼在提高碳纳米管的载体分散性、控制铂颗粒的均匀性和粒径、促进氧还原反应的氧吸附/解离方面发挥着重要的作用。     

Electrochemistry of platinum(II) coordination compounds I. Electroreduction of dihalogenobis(isocyanides)-platinum(II) complexes

Electroreduction of cis and trans dihalogeobis(isocyanide)platinum(II) complexes PtL₂X₂ (X = Cl, Br, I; L = cyclohexyl isocyanide or tert-butyl isocyanide) on gold and platinum electrodes has been studied in propylene carbonate and acetonitrile containing tetraethylammonium perchlorate as supporting electrolyte. The first, monoelectronic, reduction step is irreversible, the half-wave potential being critically dependent on the chemical nature of X in PtL₂X₂. Potentiostatic reductive coulometry of PtX2L2 in the cavity of an ESR spectrometer produces a signal attributed to a monomeric paramagnetic Pt^I complex. It is also shown that one halide anion is liberated per PtX₂L₂ in this first reduction step. A second monoelectronic reduction step is observed, which results in the decomposition of the complex to form a solid deposit containing platinum on the electrode. A reduction scheme is proposed for PtX₂L₂, on the basis of these results.     

氧-硼共修饰多壁碳纳米管载铂催化剂的制备及氧还原活性

采用一种简便的方法,合成了氧-硼共修饰的多壁碳纳米管材料,以此为载体制备的铂基催化剂具有更小的铂粒径、更高的电化学表面积(40 m²·g_Pt^-1)和更高的氧还原活性(0.3 A·mg_Pt^-1)。氧、硼在提高碳纳米管的载体分散性、控制铂颗粒的均匀性和粒径、促进氧还原反应的氧吸附/解离方面发挥着重要的作用。     

A density functional study of bare and hydrogenated platinum clusters

We perform density functional theory calculations using Gaussian atomic-orbital methods within the generalized gradient approximation for the exchange and correlation to study the interactions in the bare and hydrogenated platinum clusters. The minimum-energy structures, binding energies, relative stabilities, vibrational frequencies and the highest occupied and lowest unoccupied molecular-orbital gaps of Pt_nH_m (n = 1–5, m = 0–2) clusters are calculated and compared with previously studied pure platinum and hydrogenated platinum clusters. We investigate any magic behavior in hydrogenated platinum clusters and find that Pt₄H₂ is more stable than its neighboring sizes. The lowest energy structure of Pt₄ is found to be a distorted tetrahedron and that of Pt₅ is found to be a bridge site capped tetrahedron which is a new global minimum for Pt₅ cluster. The successive addition of H atoms to Pt_n clusters leads to an oscillatory change in the magnetic moment of Pt₃–Pt₅ clusters.     

Complex three-dimensional platinum-indium networks in the ternary indides Dy2Pt7In16 and Tb6Pt12In23

Well crystallized samples of Dy₂Pt₇In₁₆ and Tb₆Pt₁₂In₂₃ were synthesized by an indium flux technique. Arc-melted precursor alloys with the starting compositions ∼DyPt₃In₆ and ∼TbPtIn₄ were annealed with a slight excess of indium at 1200 K followed by slow cooling (5 K/h) to 870 K. Both indides were investigated by X-ray diffraction on powders and single crystals: Cmmm, a=1211.1(2), b=1997.8(3), c=439.50(6) pm, wR2=0.0518, 1138 F² values, 45 variable parameters for Dy₂Pt₇In₁₆ and C2/ma=2834.6(4), b=440.05(7), c=1477.1(3) pm, β=112.37(1)°, wR2=0.0753, 2543 F² values, 126 variable parameters for Tb₆Pt₁₂In₂₃. The platinum atoms in the terbium compound have a distorted trigonal prismatic coordination. In Dy₂Pt₇In₁₆, trigonal and square prismatic coordination occur. The shortest interatomic distances are observed for Pt-In followed by In-In contacts. Considering these strong interactions, both structures can be described by complex three-dimensional [Pt₇In₁₆] and [Pt₁₂In₂₃] networks. The networks leave distorted pentagonal channels in Dy₂Pt₇In₁₆, while pentagonal and hexagonal channels occur in Tb₆Pt₁₂In₂₃. The crystal chemistry and chemical bonding of the two indides are briefly discussed.     

Kinetics of nucleophilic attack upon coordinated organic moieties.: XXIII. Attack by cyclohexylamine upon the diene ligands in [MX2(1,5-cyclooctadiene)] (M = Pd,Pt; X = Cl,Br) complexes

Kinetic and spectroscopic studies of the reactions of cyclohexylamine with the complexes [MX₂(1,5-cyclooctadiene)] (I) (M = Pd; X = Cl, Br; M = Pt, X = Br) in acetone reveal the rate law, k_obs = K₁k₂[amine]², for the rapid sequence

For X = Br, the palladium(II) complex is ca. 70 times more reactive than its platinum(II) analogue. This is the first quantitative comparison reported to date for nucleophilic attack upon olefins coordinated to Pd^II and Pt^II centres. The reactivity order Pd^II ⪢ Pt^II may arise from the higher ionization potential of Pd²⁺ compared to Pt²⁺, which makes Pd^II a less effective back-π-bonder. Replacing the bromo ligands in [PdBr₂(1,5-COD)] by chloro ligands lowers the rate of formation of III by a factor of 8.     

Interaction of platinum and molybdophosphoric heteropoly acid under conditions of catalyst preparation for benzene oxidation to phenol with an O2-H2 gas mixture

The transformations of platinum and a heteropoly acid (HPA) in binary systems prepared from H₂PtCl₆ or H₂PtCl₄ and H₃PMo₁₂O₄₀ were studied using IR and UV-VIS spectroscopy, elemental analysis, XPS, EXAFS, TPR, and HREM. The calcination of platinum chloride with the HPA to 450°C resulted in the formation of a platinum salt of the HPA along with decomposition products (mixture I). The reduction of calcined samples containing Pt: HPA = 1: 1 with hydrogen at 300°C (mixture II) followed by exposure to air resulted in the regeneration of the HPA structure. The resulting solid samples of Pt _1?n ⁰ Pt _n ^II Cl_mO_xH_y) (H_3+p PMo _12?p ^VI Mo _p ^V O₄₀) (III) contained platinum and molybdenum in both oxidized and reduced states. The following association species were isolated from mixtures I and II by dissolving in water: [Pt _n ^II PMo₁₂O₄₀] (I _s) (n = 0.3?0.8) and [Pt _n ⁰ PMo ₁₂ ^red O₄₀] (II _s) (n ≈ 1). Under exposure to air, the solutions of I _s were stable (pH ～2), whereas Pt^met was released from II _s. After the drying of I _s, the solid association species (Pt _n ^II Cl_mO_xH_y). (H₃PMo₁₂O₄₀), where n = 0.3?0.8, m = 0.2?1, and x = 3?0, (I _solid) were obtained. The I _solid/SiO₂ supported samples were prepared by impregnating SiO₂ with a solution of I _s and drying at 100°C. Platinum metal particles of size ～20 Å and a mixed-valence association species of platinum with the HPA were observed after the reduction of I _solid/SiO₂ with hydrogen at 100–250°C. These samples were active in the gas-phase oxidation of benzene to phenol at 180°C with the use of an O₂-H₂-N₂ mixture.     

Synthesis and crystal structures of Nd6Pt13In22, Sm6Pt12.30In22.70, and Gd6Pt12.48In22.52

The rare earth-platinum-indides Nd₆Pt₁₃In₂₂, Sm₆Pt_12.30In_22.70, and Gd₆Pt_12.48In_22.52 were synthesized from the elements by arc-melting of the components. Single crystals were grown using special annealing sequences. The three indides were investigated by X-ray powder and single crystal diffraction: Tb₆Pt₁₂In₂₃ type, C2/m, Z=2, a=2811.9(6), b=441.60(9), , β=112.10(3)°, wR₂=0.0629, 3645 F² values, 126 variables for Nd₆Pt₁₃In₂₂, a=2821.9(6), b=443.06(9), , β=112.39(3)°, wR₂=0.0543, 3679 F² values, 127 variables for Sm₆Pt_12.30In_22.70, and a=2818.5(6), b=439.90(9), , β=112.29(3)°, wR₂=0.0778, 3938 F² values, 127 variables for Gd₆Pt_12.48In_22.52. Most platinum atoms in these structures have a distorted trigonal prismatic coordination by rare earth metal and indium atoms. Together, the platinum and indium atoms build up a complex three-dimensional [Pt_12+xIn_23−x] polyanionic network in which the rare earth metal atoms fill distorted pentagonal and hexagonal channels. The 2c Wyckoff site in these structures plays a peculiar role. This site is occupied by indium in the prototype Tb₆Pt₁₂In₂₃, while platinum atoms fill the 2c site in Nd₆Pt₁₃In₂₂, leading to a linear Pt₃ chain with Pt-Pt distances of 275 pm. The crystals with samarium and gadolinium as rare earth metal component show mixed Pt/In occupancies.     

Thermal behavior of platinum fluorides: II. Higher fluorides

Proceeding from analysis of literature calorimetric data and the investigation of equilibria of higher platinum florides, the following values of the enthalpies of formation at 298 K (kJ/mol) were recommended: for PtF_{4 cr}, ?679.5 ± 6.0; for PtF_{4 gas}, ?518.2 ± 6.2; Pt₂F_{10 gas}, ?1323.1 ± 6.2; for Pt₃F_{15 gas}, ?2055.9 ± 13.2; and for PtF₆ = ?672.0 ± 6.3. The partial pressures and activities of the disproportionation and degradation products of platinum pentafluoride and platinum hexafluoride have been calculated; their correlation with the experimental data has been demonstrated.     

Structural and electrocatalytic properties of Pt/C and Pt-Ni/C catalysts prepared by electrochemical dispersion

The structural and electrocatalytic properties of Pt/C and Pt-Ni/C catalysts prepared by the electrochemical dispersion of metals under the action of pulse alternating current in a solution of NaOH were studied. Using X-ray diffraction analysis and scanning and transmission electron microscopy, it was found that the synthesized Pt/C catalysts contained active constituent particles with the average size D ₁₁₁ = 10.6 nm with a predominantly cubic shape. Upon the dispersion of a Pt₃Ni alloy, the Pt-Ni/C catalyst containing the particles of a stoichiometric metal phase of Pt₃Ni (D ₁₁₁ = 9.6 nm) and also Pt _x Ni particles (x > 3) enriched in platinum (D ₁₁₁ = 8.1 nm). The synthesized catalysts possessed high electrocatalytic activity and stability in the reaction of methanol oxidation. The characteristics of these catalysts as anodes in the membrane-electronic unit of a hydrogen-air solid-polymer fuel cell were studied.     

Surface site density and utilization of platinum group metal (PGM)-free Fe–NC and FeNi–NC electrocatalysts for the oxygen reduction reaction

Pyrolyzed iron-based platinum group metal (PGM)-free nitrogen-doped single site carbon catalysts (Fe–NC) are possible alternatives to platinum-based carbon catalysts for the oxygen reduction reaction (ORR). Bimetallic PGM-free M₁M₂–NC catalysts and their active sites, however, have been poorly studied to date. The present study explores the active accessible sites of mono- and bimetallic Fe–NC and FeNi–NC catalysts. Combining CO cryo chemisorption, X-ray absorption and ⁵⁷Fe Mössbauer spectroscopy, we evaluate the number and chemical state of metal sites at the surface of the catalysts along with an estimate of their dispersion and utilization. Fe L_3,2-edge X-ray adsorption spectra, Mössbauer spectra and CO desorption all suggested an essentially identical nature of Fe sites in both monometallic Fe–NC and bimetallic FeNi–NC; however, Ni blocks the formation of active sites during the pyrolysis and thus causes a sharp reduction in the accessible metal site density, while with only a minor direct participation as a catalytic site in the final catalyst. We also use the site density utilization factor, ϕ_{SDsurface/bulk}, as a measure of the metal site dispersion in PGM-free ORR catalysts. ϕ_{SDsurface/bulk} enables a quantitative evaluation and comparison of distinct catalyst synthesis routes in terms of their ratio of accessible metal sites. It gives guidance for further optimization of the accessible site density of M–NC catalysts.

The gravimetric surface density and ORR catalytic turnover frequency of Fe–NC and Fe/Ni–NC catalysts were investigated. Both catalysts feature chemically identical Fe sites, but the presence of Ni lowered the gravimetric surface density of Fe sites.     

Orientational self-sorting in cuboctahedral Pd cages

Cuboctahedral coordination cages of the general formula [Pd₁₂L₂₄]²⁴⁺ (L = low-symmetry ligand) were analyzed theoretically and experimentally. With 350 696 potential isomers, the structural space of these assemblies is vast. Orientational self-sorting refers to the preferential formation of particular isomers within the pool of potential structures. Geometric and computational analyses predict the preferred formation of cages with a cis arrangement at the metal centers. This prediction was corroborated experimentally by synthesizing a [Pd₁₂L₂₄]²⁴⁺ cage with a bridging 3-(4-(pyridin-4-yl)phenyl)pyridine ligand. A crystallographic analysis of this assembly showed exclusive cis coordination of the 3- and the 4-pyridyl donor groups at the Pd²⁺ ions.

Cuboctahedral [Pd₁₂L₂₄]²⁴⁺ cages based on low-symmetry ligands can potentially form a vast number of isomers, but the self-assembly process was found to be highly selective.     

Activation energy of hydrogen migration by relay in the O2/Pt19/SnO2/H2 + n H2O system

In the framework of investigation of active and stable electrocatalysts for fuel cells, the hydrogen migration by relay with the consecutive formation of H₂O molecules in the O₂/Pt₁₉/SnO₂/H₂·nH₂O → O/Pt₁₉/SnO₂·nH₂O + H₂O system was simulated. The simulations were performed by the density functional theory (DFT) method with the generalized gradient adjustment (GGA=PBE) under periodic boundary conditions in the projector augmented plane wave (PAW) basis set with a pseudo-potential using the VASP program package. At the cathode on the platinum cluster surface, the oxygen molecules without a barrier form peroxide complexes that dissociate with an energy decrease. The protons transferred via the proton-conducting channels from the anode to cathode form first OH groups bound to the platinum cluster and then H₂O molecules that are easily separated from the cluster (～0.2 eV). The proton transfer process proceeds by relay and involves several water molecules.     

Pt-Se纳米空球修饰玻碳电极上甲酸的电催化氧化

以无定形硒溶胶为模板制备了不同硒覆盖度(θSe)(θSe=0.49,0.39,0.06,0)的Pt-Se和Pt纳米空球(分别记为(Pt-Se)HN和PtHN),发展了利用亚硫酸盐彻底除去核壳纳米粒子上Se的方法.对获得的纳米空球进行了形貌和结构的表征,结果表明所制备的(Pt-Se)HN粒径均匀,分散性好,球壳呈多孔结构.以其作为电催化剂制备了(Pt-Se)HN修饰的玻碳(GC)电极((Pt-Se)HN/GC),利用常规电化学方法比较该电极与PtHN/GC和商用碳载铂(Pt/C)修饰GC(Pt/C/GC)电极对甲酸的催化氧化作用,发现对甲酸氧化的活性顺序为(Pt-Se)HN/GCPtHN/GCPt/C/GC.三种电极催化甲酸氧化的机理有所不同:前者更倾向于通过弱吸附中间体直接氧化成CO2的单途径机理进行,后两者则通过强吸附和弱吸附中间体的双途径机理进行.在一定Se覆盖度条件下,(Pt-Se)HN/GC对甲酸的氧化有助催化作用.     

Conformational control of Pd2L4 assemblies with unsymmetrical ligands

With increasing interest in the potential utility of metallo-supramolecular architectures for applications as diverse as catalysis and drug delivery, the ability to develop more complex assemblies is keenly sought after. Despite this, symmetrical ligands have been utilised almost exclusively to simplify the self-assembly process as without a significant driving foa mixture of isomeric products will be obtained. Although a small number of unsymmetrical ligands have been shown to serendipitously form well-defined metallo-supramolecular assemblies, a more systematic study could provide generally applicable information to assist in the design of lower symmetry architectures. Pd₂L₄ cages are a popular class of metallo-supramolecular assembly; research seeking to introduce added complexity into their structure to further their functionality has resulted in a handful of examples of heteroleptic structures, whilst the use of unsymmetrical ligands remains underexplored. Herein we show that it is possible to design unsymmetrical ligands in which either steric or geometric constraints, or both, can be incorporated into ligand frameworks to ensure exclusive formation of single isomers of three-dimensional Pd₂L₄ metallo-supramolecular assemblies with high fidelity. In this manner it is possible to access Pd₂L₄ cage architectures of reduced symmetry, a concept that could allow for the controlled spatial segregation of different functionalities within these systems. The introduction of steric directing groups was also seen to have a profound effect on the cage structures, suggesting that simple ligand modifications could be used to engineer structural properties.

Steric and geometric constraints were used to design unsymmetrical ditopic ligands that form single Pd₂L₄ cage isomers with high fidelity.