Synthesis, crystal structures and DNA-binding properties of dinuclear complexes with macrocyclic ligands

Two symmetrical macrocyclic dinuclear complexes, [Cu₂L¹(ClO₄)₂(H₂O)₂][Cu₂L¹(H₂O)₂] (ClO₄)₂ (1) and [Cu₂L²(ClO₄)₂] (2), (where H₂L¹ and H₂L² are the [2?+?2] condensation products of 1,3-diaminopropane with 2,6-diformyl-4-methylphenol and 2,6-diformyl-4-flurophenol, respectively), have been synthesized and characterized. The electronic and magnetic properties of the complexes were studied by cyclic voltammetry and magnetic susceptibility. There are strong antiferromagnetic couplings between the two copper(II) centers in both complexes. The strongly electron-withdrawing fluorine groups in H₂L² weaken the antiferromagnetic exchange, but make the metal centers more easily reduced than its analog H₂L¹. The interactions of the complexes with calf thymus DNA were studied by UV?CVis and CD spectroscopic techniques.     

Strong convergence of the empirical distribution of eigenvalues of sample covariance matrices with a perturbation matrix

Let , where is a random symmetric matrix, a random symmetric matrix, and with being independent real random variables. Suppose that , and are independent. It is proved that the empirical spectral distribution of the eigenvalues of random symmetric matrices converges almost surely to a non-random distribution.     

Bare and ligand protected planar hexacoordinate silicon in SiSb3M3+ (M = Ca,Sr, Ba) clusters

The occurrence of planar hexacoordination is very rare in main group elements. We report here a class of clusters containing a planar hexacoordinate silicon (phSi) atom with the formula SiSb₃M₃⁺ (M = Ca, Sr, Ba), which have D_3h (¹A₁′) symmetry in their global minimum structure. The unique ability of heavier alkaline-earth atoms to use their vacant d atomic orbitals in bonding effectively stabilizes the peripheral ring and is responsible for covalent interaction with the Si center. Although the interaction between Si and Sb is significantly stronger than the Si–M one, sizable stabilization energies (−27.4 to −35.4 kcal mol⁻¹) also originated from the combined electrostatic and covalent attraction between Si and M centers. The lighter homologues, SiE₃M₃⁺ (E = N, P, As; M = Ca, Sr, Ba) clusters, also possess similar D_3h symmetric structures as the global minima. However, the repulsive electrostatic interaction between Si and M dominates over covalent attraction making the Si–M contacts repulsive in nature. Most interestingly, the planarity of the phSi core and the attractive nature of all the six contacts of phSi are maintained in N-heterocyclic carbene (NHC) and benzene (Bz) bound SiSb₃M₃(NHC)₆⁺ and SiSb₃M₃(Bz)₆⁺ (M = Ca, Sr, Ba) complexes. Therefore, bare and ligand-protected SiSb₃M₃⁺ clusters are suitable candidates for gas-phase detection and large-scale synthesis, respectively.

The global minimum of SiSb₃M₃⁺ (M = Ca, Sr, Ba) is a D_3h symmetric structure containing an elusive planar hexacoordinate silicon (phSi) atom. Most importantly, the phSi core remains intact in ligand protected environment as well.

Exploring the bonding capacity of main-group elements (such as carbon or silicon) beyond the traditional tetrahedral concept has been a fascinating subject in chemistry for five decades. The 1970 pioneering work of Hoffmann and coworkers¹ initiated the field of planar tetracoordinate carbons (ptCs), or more generally, planar hypercoordinate carbons. The past 50 years have witnessed the design and characterization of an array of ptC and planar pentacoordinate carbon (ppC) species.^2–14 However, it turned out to be rather challenging to go beyond ptC and ppC systems. The celebrated CB₆²⁻ cluster and relevant species^15,16 were merely model systems because C avoids planar hypercoordination in such systems.^17,18 In 2012, the first genuine global minimum D_3h CO₃Li₃⁺ cluster was reported to have six interactions with carbon in planar form, although electrostatic repulsion between positively charged phC and Li centers and the absence of any significant orbital interaction between them make this hexacoordinate assignment questionable.¹⁹ It was only very recently that a series of planar hexacoordinate carbon (phC) species, CE₃M₃⁺ (E = S–Te; M = Li–Cs), were designed computationally by the groups of Tiznado and Merino (Fig. 1; left panel),²⁰ in which there exist pure electrostatic interactions between the negative C^δ− center and positive M^δ+ ligands. These phC clusters were achieved following the so-called “proper polarization of ligand” strategy.Open in a separate windowFig. 1The pictorial depiction of previously reported phC CE₃M₃⁺ (E = S–Te; M = Li–Cs) clusters and the present SiE₃M₃⁺ (E = S–Te and N–Sb; M = Li–Cs and Ca–Ba) clusters. Herein the solid and dashed lines represent covalent and ionic bonding, respectively. The opposite double arrows illustrate electrostatic repulsion.The concept of planar hypercoordinate carbons has been naturally extended to their next heavier congener, silicon-based systems. Although the steric repulsion between ligands decreases due to the larger size, the strength of π- and σ-bonding between the central atom and peripheral ligands dramatically decreases, which is crucial for stability. Planar tetracoordinate silicon (ptSi) was first experimentally observed in a pentaatomic C_2v SiAl₄⁻ cluster by Wang and coworkers in 2000.²¹ Very recently, this topic got a huge boost by the room-temperature, large-scale syntheses of complexes containing a ptSi unit.²² A recent computational study also predicted the global minimum of SiMg₄Y⁻ (Y = In, Tl) and SiMg₃In₂ to have unprecendented planar pentacoordinate Si (ppSi) units.²³ Planar hexacoordinate Si (phSi) systems seem to be even more difficult to stabilize. Previously, a C_2v symmetric Cu₆H₆Si cluster was predicted as the true minimum,²⁴ albeit its potential energy surface was not fully explored. A kinetically viable phSi SiAl₃Mg₃H₂⁺ cluster cation was also predicted.²⁵ However, these phSi systems^24,25 are only local minima and not likely to be observed experimentally. In 2018, the group of Chen identified the Ca₄Si₂²⁻ building block containing a ppSi center and constructed an infinite CaSi monolayer, which is essentially a two-dimensional lattice of the Ca₄Si₂ motif.²⁶ Thus, it is still an open question to achieve a phSi atom to date.Herein we have tried to find the correct combination towards a phSi system as the most stable isomer. Gratifyingly, we found a series of clusters, SiE₃M₃⁺ (E = N, P, As, Sb; M = Ca, Sr, Ba), having planar D_3h symmetry with Si at the center of the six membered ring, as true global minimum forms. Si–E bonds are very strong in all the clusters, and alkaline-earth metals interact with the Si center by employing their d orbitals. However, electrostatic repulsion originated from the positively charged Si and M centers for E = N, P, and As dominates over attractive covalent interaction, making individual Si–M contacts repulsive in nature. This makes the assignment of SiE₃M₃⁺ (E = N, P, As; M = Ca, Sr, Ba) as genuine phSi somewhat skeptical. SiSb₃M₃⁺ (M = Ca, Sr, Ba) clusters are the sole candidates which possess genuine phSi centers as both electrostatic and covalent interactions in Si–M bonds are attractive. The d orbitals of M ligands play a crucial role in stabilizing the ligand framework and forming covalent bonds with phSi. Such planar hypercoordinate atoms are, in general, susceptible to external perturbations. However, the present title clusters maintain the planarity and the attractive nature of the bonds even after multiple ligand binding at M centers in SiSb₃M₃(NHC)₆⁺ and SiSb₃M₃(Bz)₆⁺. This would open the door for large-scale synthesis of phSi as well.Two major computational efforts were made before reaching our title phSi clusters. The first one is to examine SiE₃M₃⁺ (E = S–Po; M = Li–Cs) clusters, which adopt D_3h or C_3v structures as true minima (see Table S1 in ESI^†), being isoelectronic to the previous phC CE₃M₃⁺ (E = S–Po; M = Li–Cs) clusters. In the SiE₃M₃⁺ (E = S–Po; M = Li–Cs) clusters, the Si center always carries a positive charge ranging from 0.01 to +1.03|e|, in contrast to the corresponding phC species (see Fig. 1). Thus, electrostatic interactions between the Si^δ+ and M^δ+ centers would be repulsive (Fig. 1). Given that the possibility of covalent interaction with an alkali metal is minimal, it would be a matter of debate whether they could be called true coordination. A second effort is to tune the electronegativity difference between Si and M centers so that the covalent contribution in Si–M bonding becomes substantial. Along this line, we consider the combinations of SiE₃M₃⁺ (E = N, P, As, Sb; M = Be, Mg, Ca, Sr, Ba). The results in Fig. S1^† show that for E = Be and Mg, the phSi geometry has a large out-of-plane imaginary frequency mode, which indicates a size mismatch between the Si center and peripheral E₃M₃ (E = N–Bi; M = Be, Mg) ring. On the other hand, the use of larger M = Ca, Sr, Ba atoms effectively expands the size of the cavity and eventually leads to perfect planar geometry with Si atoms at the center as minima. In the case of SiBi₃M₃⁺, the planar isomer possesses a small imaginary frequency for M = Ca. Although planar SiBi₃Sr₃⁺ and SiBi₃Ba₃⁺ are true minima, they are 2.2 and 2.5 kcal mol⁻¹ higher in energy than the lowest energy isomer, respectively (Fig. S2^†). Fig. 2 displays some selected low-lying isomers of SiE₃M₃⁺ (E = N, P, As, Sb; M = Ca, Sr, Ba) clusters (see Fig. S3–S6^† for additional isomers). The global minimum structure is a D_3h symmetric phSi with an ¹A₁′ electronic state for all the twelve cases. The second lowest energy isomer, a ppSi, is located more than 49 kcal mol⁻¹ above phSi for E = N. This relative energy between the most stable and nearest energy isomer gradually decreases upon moving from N to Sb. In the case of SiSb₃M₃⁺ clusters, the second-lowest energy isomer is 4.6–6.1 kcal mol⁻¹ higher in energy than phSi. The nearest triplet state isomer is very high in energy (by 36–53 kcal mol⁻¹, Fig. S3–S6^†) with respect to the global minimum.Open in a separate windowFig. 2The structures of low-lying isomers of SiE₃M₃⁺ (E = N, P, As, Sb; M = Ca, Sr, Ba) clusters. Relative energies (in kcal mol⁻¹) are shown at the single-point CCSD(T)/def2-TZVP//PBE0/def2-TZVP level, followed by a zero-energy correction at PBE0. The values from left to right refer to Ca, Sr, and Ba in sequence. The group symmetries and electronic states are also given.Born–Oppenheimer molecular dynamics (BOMD) simulations at room temperature (298 K), taking SiE₃Ca₃⁺ clusters as case studies, were also performed. The results are displayed in Fig. S7.^† All trajectories show no isomerization or other structural alterations during the simulation time, as indicated by the small root mean square deviation (RMSD) values. The BOMD data suggest that the global minimum also has reasonable kinetic stability against isomerization and decomposition.The bond distances, natural atomic charges, and bond indices for SiE₃Ca₃⁺ clusters are given in † for M = Sr, Ba). The Si–E bond distances are shorter than the typical Si–E single bond distance computed using the self-consistent covalent radii proposed by Pyykkö.²⁷ In contrast, the Si–M bond distance is almost equal to the single bond distance. This gives the first hint of the presence of covalent bonding therein. However, the Wiberg bond indices (WBIs) for the Si–M links are surprisingly low (0.02–0.04). We then checked the Mayer bond order (MBO), which can be seen as a generalization of WBIs and is more acceptable since the approach of WBI calculations assumes orthonormal conditions of basis functions while the MBO considers an overlap matrix. The MBO values for the Si–M links are now sizable (0.13–0.18). These values are reasonable considering the large difference in electronegativity between Si and M, and, therefore, only a very polar bond is expected between them. In fact, the calculations of WBIs after orthogonalization of basis functions by the Löwdin method gives significantly large bond orders (0.48–0.55), which is known to overestimate the bond orders somewhat. The above results indicate that the presence of covalent bonding cannot be ruled out only by looking at WBI values.Bond distances (r, in Å), different bond orders (WBIs) {MBOs} [WBI in orthogonalized basis], and natural atomic charges (q, in |e|) of SiE₃Ca₃⁺ (E = N, P, As, Sb) clusters at the PBE0/def2-TZVP level

Fluorinated nanoporous SiO2 films with ultra-low dielectric constant

Fluorinated nanoporous silica (denoted as SiO₂:F) thin films with low dielectric constant were prepared by a sol-gel method and spin coating technique. The leakage current densities of the SiO₂:F thin films were 10⁻⁸ and 3 × 10⁻⁶ A/cm² respectively for the as-deposited films and for those subjected to annealing at a temperature of 450 °C. These currents are more than one order of magnitude lower than those of the common SiO₂ films. Photoluminescent results showed strong blue-light emission and a small blue shift in the SiO₂:F films that were related to the increment of the porosity. The dielectric properties were also characterized and the k value of the annealed SiO₂:F film was found to be about 1.67. The hole size in the films is small and the size distribution is uniform for the annealed SiO₂:F samples due to the effects of fluorination. The underlying mechanism for fluorination is discussed in this paper.     

具源项的奇异扩散方程解的存在性及渐近性

讨论了一类具源项的奇异扩散方程的第二初边值问题,证明了经典解的存在性以及时间充分大时解的渐近性质.     

First total synthesis of yanuthones: novel farnesylated epoxycyclohexenoid marine natural products

The total synthesis of the recently isolated marine natural products of mixed biosynthetic origin, yanuthones A, B, C and 22-deacylyanuthone A, has been accomplished following a short regio- and stereocontrolled approach involving the key intermediacy of 2-farnesyl-p-benzoquinone.     

Enhancement of SrTiO<Subscript>3</Subscript>/BiPO<Subscript>4</Subscript> heterostructure for simulated organic wastewater degradation under UV light irradiation

A novel SrTiO₃/BiPO₄ heterostructure with different amounts of SrTiO₃ have been successfully prepared through the hydrothermal process. The photocatalysts were characterized by X-ray powder diffraction, UV–Vis diffuse reflectance spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic performance was evaluated by degrading the methylene blue dye solution under UV light. Results showed that the samples displayed excellent photocatalytic degradation efficiency due to the highly efficient suppression of the recombination of electron–hole pairs. A possible mechanism of SrTiO₃/BiPO₄ heterojunctions was discussed. The research indicated that the as-prepared SrTiO₃/BiPO₄ heterogeneous photocatalyst can be used as an effective material for degrading industrial organic wastewater.     

梯形结构二(对甲基苄基)锡氧(氯)簇合物的合成、结构及量子化学研究

二(对甲基苄基)二氯化锡和氢氧化钠溶液反应,合成了梯形结构二(对甲基苄基)锡氧(氯)簇合物,经X-射线衍射测定了其晶体结构。属三斜晶系,空间群为P1,晶体学参数a=0.979 52(16) nm,b=1.313 8(2) nm,c=1.419 5(2) nm,α=62.965(10)°,β=88.551(12)°,γ=73.709(11)°,V=1.550 5(5) nm³,Z=1,D_c=1.563 g·cm^-3,μ(Mo Kα)=17.38 cm^-1,F(000)=727,R=0.024 2,wR=0.057 9。簇合物为由Sn₂O₂构成的平面四元环形成的1个中心内环和2个由Sn₂OCl(O)构成的平面四元外环组成的梯形结构。锡原子均为五配位三角双锥构型。对其结构进行量子化学从头计算,探讨了配合物的稳定性、分子轨道能量以及一些前沿分子轨道的组成特征。     

用稳定自由基聚合及光引发聚合制备嵌段共聚物

介绍了稳定自由基聚合的反应原理、引发剂设计，以及用稳定自由基聚合制备嵌段共聚物的几种方法：连续加料法、双官能团引发剂法和一步法。对于光引发聚合的原理及硫自由基的稳定性对聚合反应的影响也进行了讨论。