Electronic Effect Promoted Visible-Light-Driven CO2-to-CO Conversion in a Water-Containing System.

The design of unsaturated nonprecious metal complexes with high catalytic performance for photochemical CO2 reduction is still an important challenge. In this paper, four coordinatively unsaturated Co-salen complexes 1-4 were explored in situ using o-phenylenediamine derivatives and 5-methylsalicylaldehyde as precursors of the ligands in 1-4. It was found that complex 4, bearing a nitro substituent (-NO2) on the aromatic ring of the salen ligand, exhibits the highest photochemical performance for visible-light-driven CO2-to-CO conversion in a water-containing system, with TONCO and CO selectivity values of 5300 and 96%, respectively. DFT calculations and experimental results revealed that the promoted photocatalytic activity of 4 is ascribed to the electron-withdrawing effect of the nitro group in 4 compared to 1-3 (with -CH3, -F, and -H groups, respectively), resulting in a lower reduction potential of active metal centers CoII and lower barriers for CO2 coordination and C-O cleavage steps for 4 than those for catalysts 1-3.

Russian Doll-like 3d-4f Cluster Wheels with Slow Relaxation of Magnetization.

The solvothermal reactions of LnCl3·6H2O and MCl2·6H2O (M = Co, Ni) with 2,2'-diphenol (H2L1) and 5,7-dichloro-8-hydroxyquinoline (HL2) gave three 3d-4f heterometallic wheel-like nano-clusters [Ln7M6(L1)6(L2)6(µ3-OH)6(OCH3)6Cl(CH3CN)6]Cl2·xH2O (Ln = Dy, M = Co, x = 3 for 1; Ln = Dy, M = Ni, x = 0 for 2; Ln = Tb, M = Ni, x = 0 for 3) with similar cluster structure. The innermost Ln(III) ion is encapsulated in a planar Ln6 ring which is further embedded in a chair-conformation M6 ring, constructing a Russian doll-like 3d-4f cluster wheel Ln(III)⸦Ln6⸦M6. 2 and 3 show obvious slow magnetic relaxation behavior with negligible opening of the magnetic hysteresis loop. Such a Russian doll-like 3d-4f cluster wheel with the lanthanide disc isolated by transition metallo-ring is rarely reported.

Electronic effects promoted the catalytic activities of binuclear Co(II) complexes for visible-light-driven CO2 reduction in a water-containing system.

Under the action of a catalyst, the photoinduced reduction of CO2 to chemicals and fuels is one of the greenest and environment-friendly approaches for decreasing atmospheric CO2 emissions. Since the environment was affected by the greenhouse effect, scientists have never stopped exploring efficient photoinduced CO2 reduction systems, particularly the highly desired non-noble metal complexes. Most of the currently reported complexes based on non-noble metals exhibit low catalytic activity, selectivity, and stability in aqueous systems under the irradiation of visible light. Herein, we report a new binuclear cobalt complex [Co2(L1)(OAc)2](OAc) (Co2L1, HL1 = 2,6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-methoxyphenol), which accelerates the visible-light-driven conversion of CO2 to CO in acetonitrile/water (4/1, v/v) nearly 40% more than that for the previously reported [Co2(L2)(OAc)2](OAc) (Co2L2, HL2 = 2, 6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-(tert-butyl)phenol) by our research group. It has an excellent CO selectivity of 98%, and the TONCO is as high as 5920. Experimental results and DFT calculations showed that the enhanced catalytic performance of Co2L1 is due to the electron-donating effect of a methoxy group (-OCH3) in Co2L1 compared to a tertiary butyl group (-C(CH3)3) in Co2L2, which reduces the energy barrier of the rate-limiting CO2 coordination step in the visible-light-driven CO2 reduction process.

Highly Efficient Visible-Light-Driven CO2-to-CO Conversion by Coordinatively Unsaturated Co-Salen Complexes in a Water-Containing System.

The development of cost-effective catalysts for CO2 reduction is highly desired but remains a significant challenge. The unsaturated coordination metal center in a catalyst is favorable for the process of catalytic CO2 reduction. In this paper, two asymmetric salen ligands were used to synthesize two coordinatively unsaturated Co-salen complexes. The two Co-salen complexes exhibit an unsaturated coordination pattern and display high activity and CO selectivity for visible-light-driven CO2 reduction in a water-containing system. The photocatalytic performance of 2 is higher than that of 1 because the reduction potential of the catalytic CoII center and the energy barrier of the catalytic transition states of 2 are lower than those of 1, with turnover numbers (TONCO), turnover frequencies (TOF), and CO selectivity values of 8640, 0.24 s-1, and 97% for 2, respectively. The photocatalytic reduction of CO2 to CO for 2 is well supported by control experiments and density functional theory (DFT) calculations.

Synthesis and antitumor activities of five Cu(II) complexes of bis(5-halosalicylidene)-1,3-propanediamine derivatives.

The development of metal complexes of Schiff base has attracted much attention due to their DNA binding properties and extensive biological activities. We reported here five copper(II) complexes [Cu(L1)] (1), [Cu(L2)] (2), [Cu(L3)] (3), [Cu2(L4)(OAc)] (4), and [Cu2(L5)(HCOO)] (5) bearing the bis-Schiff base ligands of bis(5-chlorosalicylidene)-1,3-propanediamine (H2L1), bis(5-chlorosalicylidene)-2-methyl-1,3-propanediamine (H2L2), bis(5-bromosalicylidene)-2-methyl-1,3-propanediamine (H2L3), bis(5-chlorosalicylidene)-2-hydroxyl-1,3-propanediamine (H3L4), and bis(5-bromosalicylidene)-2-hydroxyl-1,3-propanediamine (H3L5), respectively. The single crystal X-ray diffraction analysis results revealed that complexes 1-3 present mononuclear structures and complexes 4 and 5 show dinuclear structures. It was also shown that all of these complexes are stable under physiological conditions. The in vitro antitumor activities of the five complexes were evaluated. Anticancer selectivity was also found for complex 2 on different cell lines with the lowest IC50 value on Hela cells. Further mechanistic studies showed that the three mononuclear Cu(II) complexes can induce apoptosis through the mitochondrial pathway by decreasing mitochondrial membrane potential and increasing the reactive oxygen species (ROS) and Ca2+ levels. They can activate caspase-3 and caspase-9, and can also regulate the expression of pro-apoptotic protein and anti-apoptotic protein in cells. All of these results showed that complex 2 is a potential anticancer drug.

Heterometallic Metal-Organic Framework Based on [Cu4I4] and [Hf6O8] Clusters for Adsorption of Iodine.

Heterometallic metal-organic framework (MOF) as a kind of porous material is very important because of its excellent properties in catalysis, magnetic, sensor, and adsorption fields, but the reasonable design and syntheses of these are still challenging. Herein, we prepared one heterometallic MOF with the formula [Hf6(µ 3-OH)8(OH)8][(Cu4I4) (ina)4]2·22DMF (NS-1, ina = isonicotinate). Single-crystal X-ray diffraction analysis revealed that NS-1 is a three-dimensional network with flu topology, constructed from 8-connected [Hf6(µ 3-OH)8(OH)8]8+ and 4-connected [Cu4I4] clusters as second building units (SBUs). To our best knowledge, NS-1 is a rare example with two different metal clusters as SBUs in heterometallic Hf-based MOFs. Interestingly, NS-1 exhibits a reversible adsorption performance for iodine in the cyclohexane solution, the adsorption kinetics fits well with the pseudo-second-order equation, and the Freundlich model relating to multilayer adsorption better describes the process of iodine absorption.

Anticancer activity of four trinuclear cobalt complexes bearing bis(salicylidene)-1,3-propanediamine derivatives.

Schiff base and its complexes are being paid more and more interests for their great prospects in biological applications. We reported here four cobalt(II) complexes [Co3(L1)2(HCOO)2] (1), [Co3(L2)2(HCOO)2(CH3OH)2]·2CH3OH (2), [Co3(HL3)2(OAc)2(DMF)2] (3) and [Co3(HL4)2(HCOO)2(DMF)2]·2H2O (4) bearing the bis-Schiff base ligand of bis(5-bromosalicylidene)-1,3-propanediamine (H2L1), bis(5-bromosalicylidene)-2-methyl-1,3-propanediamine (H2L2), bis(5-chlorosalicylidene)-2-hydroxyl-1,3-propanediamine (H3L3) and bis(5-bromosalicylidene)-2-hydroxyl-1,3-propanediamine (H3L4), respectively. The anti-tumor activities of the four titled complexes were screened on a series of tumor cell lines. After an overall consideration of their cytotoxicity on cancer cells and normal cells in comparison to those for cisplatin, complex 3 shows the best anticancer effect among the four titled complexes. It revealed the influence of anti-cancer effects of the substitution groups of H, Me and OH, as well as Cl and Br. Anticancer selectivity was also found for complex 3 on different cancer cell lines with the lowest IC50 value on T-24 cells. Complex 3 induces cell apoptosis through mitochondrial pathway as demonstrated by increasing the level of reactive oxygen species, decreasing mitochondrial membrane potential, activating caspase 3/9 and arresting cell cycle in G1 phase.

Two Heterometallic Nanoclusters [DyIII4NiII8] and [DyIII10MnIII4MnII2]: Structure, Assembly Mechanism, and Magnetic Properties.

A full understanding of the assembly mechanisms of coordination complexes is of great importance for a directional synthesis under control. We thus explored here the formation mechanisms of the two new heterometallic nanoclusters [DyIII4NiII8(µ3-OH)8(L)8(OAc)4(H2O)4]·3.25EtOH·4CH3CN (1) and [DyIII10MnIII4MnII2O4(OH)12(OAc)16(L)4(HL)2(EtOH)2]·2EtOH·2CH3CN·2H2O (2) with different cubane-based squarelike ring structures, which were obtained from the reactions of 4-bromo-2-[(2-hydroxypropylimino)methyl]phenol (H2L) with Dy(NO)3·6H2O and the transition metal salt Ni(OAc)2·4H2O or Mn(OAc)2·4H2O. The high-resolution electrospray ionization mass spectrometry (HRESI-MS) tests showed that the skeletons of clusters 1 and 2 have a high stability under the measurement conditions for HRESI-MS. The intermediates formed in the reaction courses of clusters 1 and 2 were tracked using time-dependent HRESI-MS, which helped to determine the proposed hierarchical assembly mechanisms for 1 (H2L → NiL → Ni2L2 → Ni3L4 → Ni4L4 → DyNi4L5 → Dy2Ni6L6 → Dy3Ni6L6 → Dy3Ni7L7 → Dy4Ni8L8) and 2 (H2L → MnL → DyMnL → DyMn2L → Dy2Mn2Lx → Dy8Mn2L2 → Dy10Mn2L2 → Dy10Mn6Lx and H2L → DyL → Dy4L2 → Dy6L2 → Dy8Mn2L2 → Dy10Mn2L2 → Dy10Mn6Lx). This is one of the rare examples of investigating the assembly mechanisms of 3d-4f heterometallic clusters. Magnetic studies indicated that the title complexes both show slow magnetic relaxation behaviors and cluster 1 is a field-induced single-molecule magnet.

Superb Alkali-Resistant DyIII2NiII4 Single-Molecule Magnet.

A superb alkali-resistant single-molecule-magnet (SMM) material with the molecular formula [Dy2Ni4(L)8(CH3COO)4(NO3)2] (1) (HL = 8-hydroxyquinoline) has been structurally and magnetically characterized. Single-crystal X-ray diffraction revealed that 1 possesses a hexanuclear [DyIII2NiII4] cluster, which is built by two triangular [DyIIINiII2] cores double-bridged through two CH3COO- ions. Interestingly, 1 can keep its original structure in dilute acid and common basic solutions (e.g., triethylamine and NaOH). More importantly, 1 is still stable after treatment with a 20 M NaOH aqueous solution for 1 month at room temperature. Magnetic measurements uncovered that 1 is an SMM under zero applied field with Ueff = 7.43 K. To the best of our knowledge, 1 is the first example of a 3d-4f SMM with such extreme alkali resistance. This work will broaden the vision of preparing SMM materials with excellent chemical stability.

Synthesis and anticancer activity of mixed ligand 3d metal complexes.

Our previously reported copper-based complexes of tropolone show nice antitumor effects, but with high cytotoxicity to normal cells, which is presumably caused by copper ions. Here, we managed to achieve this challenge by using other 3D metals to replace copper ions. We thus prepared four mononuclear 3D metal complexes [M(phen)L2] (M = Mn, Co, Ni, and Zn for 1-4, respectively). Complexes 1 and 4 show selectivity on different cancer cell lines with much lower cytotoxicity to normal cells than cisplatin. The anticancer effects for complexes 2 and 3 on the tested cancer cell lines are very poor. It revealed a tuning effect of different metal ions on the anticancer activities with those for Mn(II) and Zn(II) being much higher than those for Co(II) and Ni(II) in this system. Among them, complex 1 presents a best anticancer effect on HeLa cells comparable to cisplatin. It overcame the afore-mentioned shortage of high cytotoxicity to normal cells for the reported Cu(II) complexes. It revealed from the mechanistic studies that complex 1 mainly induces apoptosis through the mitochondrial pathway by increasing intracellular reactive oxygen species, releasing Ca2+, and activating Caspase 9 and proapoptotic gene Bax.

Two Decanuclear DyIIIxCoII10-x (x = 2, 4) Nanoclusters: Structure, Assembly Mechanism, and Magnetic Properties.

The aggregation and formation of heterometallic nanoclusters usually involves a variety of complex self-assembly processes; thus, the exploration of their assembly mechanisms through process tracking is more challenging than that for homometallic nanoclusters. We explored here the effect of solvent on the formation of heterometallic clusters, which gave two heterometallic nanoclusters, [Dy2Co8(µ3-OCH3)2(L)4(HL)2(OAc)2(NO3)2(CH3CN)2]·CH3CN·H2O (1) and [Dy4Co6(L)4(HL)2(OAc)6(OCH2CH2OH)2(HOCH2CH2OH)(H2O)]·9CH3CN (2), with the H3L ligand formed from the in situ condensation reaction of 3-amino-1,2-propanediol with 2-hydroxy-1-naphthaldehyde in the presence of Co(OAc)2·4H2O and Dy(NO)3·6H2O. It is worth noting that the skeleton of cluster 1 has a high stability under high-resolution electrospray ionization mass spectrometry (HRESI-MS) conditions with a gradually increasing energy of the ion source. Cluster 2 underwent a multistep fragmentation even under a zero ion-source voltage for the measurement of HRESI-MS. Further analysis showed that cluster 2 underwent a possible fragmentation mechanism of Dy4Co6L6 → Dy2Co6L5/DyL → DyCo2L3/DyCo2L → DyL/Co2L2. Most notably, the species emerging in the formation process of cluster 1 were tracked using time-dependent HRESI-MS, from which we proposed its possible formation mechanism of H2L → Co2L2 → Co2DyL2/Co3L2 → Co3DyL2 → Co4DyL2 → Co5Dy2L4 → Co8Dy2L6. As far as we know, it is the first time to track the formation process of Dy-Co heterometallic clusters through HRESI-MS with the proposed assembly mechanism. The magnetic properties of the two titled DyIIIxCoII10-x (x = 2, 4) clusters were studied. Both of them exhibit slow magnetic relaxation, and 1 is a single-molecule magnet at zero direct-current field.

Guest-Induced Switching of a Molecule-Based Magnet in a 3d-4f Heterometallic Cluster-Based Chain Structure.

With the motivation of controlling a magnetic switch by external stimuli, we report here an infinite chain structure formed from the secondary building units of Cu3Tb2 clusters through the linkage of nitrate ions. It behaves as a molecule-based magnet with the highest energy barrier among isolated Tb/Cu-based single-molecule magnets and single-chain magnets, which is close to a dimer of a Cu3Tb2 cluster unit from a magnetic point as revealed by its correlation length of 2.23 Cu3Tb2 units. This kind of molecule-based magnet in a chain structure is rare. The removal of its guest ethanol molecules leads to the complete disappearance of slow magnetic relaxation behavior. Interestingly, the capture and removal of guest ethanol molecules are reversible, mediating a rare ON/OFF switching of a 3d-4f heterometallic molecule-based magnet, which was interpreted by the theoretical calculations based on the structural difference upon desolvation.

Synthesis and antitumor activities of transition metal complexes of a bis-Schiff base of 2-hydroxy-1-naphthalenecarboxaldehyde.

The complexes of Schiff base have attracted much attention for their potential biological activities. In this research, five transition metal complexes TM3L2(OAc)2 (TM = Cu, 1; Ni, 2; Co, 3; Mn, 4; Fe, 5) were prepared using a bis-Schiff base of N,N'-bis[(2-hydroxy-1-naphthalenyl)methylene]-propane-1,3-diamine (H2L), which present similar linear trinuclear structures with their three metal ions consolidated by two bis-Schiff base ligands and two acetate ligands. Their antitumor activities in vitro were screened through seven human cancer cell lines by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. It revealed that complexes 1, 2 and 5 show much higher antitumor activities than the bis-Schiff base ligand and complexes 3 and 4, and even than cisplatin. Among them, complex 1 has the highest inhibitory effects on tumor cells with its IC50 value (half-inhibitory concentration) being less than 0.5 µM for human bladder cancer cell line T-24, at which concentration complex 1 shows nearly no toxicity to the normal cell HL-7702 as revealed by flow cytometry. All of these demonstrate a potential anti-cancer candidate for complex 1, which induces tumor cell apoptosis by blocking T-24 tumor cells at the G2/M phase of the cell cycle, reducing mitochondrial membrane potential, increasing the concentration of reactive oxygen species and Ca2+ in the cell, and changing the expression of proteins.

Tuning slow magnetic relaxation behaviour in a {Dy2}-based one-dimensional chain via crystal field perturbation.

Two novel {Dy2}-based one dimensional chain compounds {[Dy2(H3L)4(OAc)6]·2MeOH} n (1) and {[Dy2(H3L)4(OAc)4(NCS)2]·2MeOH} n (2) (H3L = 1,3-bis(2-hydroxynaphthalenemethyleneamino)-propan-2-ol) have been prepared under solvothermal conditions. Crystal structure analyses indicate that 1 and 2 feature similar 1D chain structures bearing dinuclear secondary building units. The difference between these two structures is that one chelated acetate ligand of Dy(iii) ion in 1 is replaced by one monodentate coordinated NCS- ion in 2, leading to their different coordination numbers and geometry configurations to Dy(iii) ion. Magnetic properties indicate that 1 and 2 display slow magnetic relaxation behavior with an effective energy barrier of 16.44(2) K in 1 and 8.02(2) K in 2, respectively, which is maybe attributed to the subtle crystal field perturbation of Dy(iii) ions.

Two mononuclear dysprosium(iii) complexes with their slow magnetic relaxation behaviors tuned by coordination geometry.

Two mononuclear dysprosium(iii) complexes [Dy(H3NAP)2Cl2]Cl·EtOH (1) and [Dy(H3NAP)2(H2O)Cl2]Cl·-2CH3CN·MeOH·0.5H2O (2) were obtained by coordinating an in situ formed Schiff base ligand of 1,3-bis(2-hydroxynaphthalenemethyleneamino)-propan-2-ol (H3NAP) to the dysprosium(iii) ion. Their Dy(iii) centers are six and seven-coordinated in octahedral and pentagonal-bipyramidal coordination geometries, respectively. Their structural difference is caused by the additional coordinated water molecule in the equatorial positions of complex 2 in comparison with that of complex 1. The well designed semi-rigid ligand contributes significantly to the low coordination numbers of Dy(iii) ions in the two title complexes, as well as to their structural difference. Magnetic investigations revealed that complexes 1 and 2 are both field-induced single-ion magnets (SIMs) with their effective energy barriers being 22.9(6) and 153.9(5) K, respectively. They are typical SIM examples with their performances tuned by the coordination geometries of metal ions.

Metal-Organic Frameworks Based on Multicenter-Bonded [MI ]8 (M=Mn, Zn) Clusters with Cubic Aromaticity.

A new concept for constructing metal-organic frameworks (MOFs) based on multicenter-bonded [MI ]8 (M=Mn, Zn) clusters with cubic aromaticity is discussed. In principle, intermolecular/intramolecular hydrogen-bonding, π-π stacking, coordinated covalent bonding and ionic bonding usually account for the structures of MOFs, and multicentered bonds generally exist in cation, anion, neutral and zwitterionic radicals, while rarely appear in MOFs. Meanwhile, aromaticity became one of the most vexing yet fascinating key concepts in chemistry since the proposition of the structure of benzene molecule in 1865. In this context, this concept article exhibits the preparation, structures, characterization methods, theoretical analysis of current research for MOFs which feature with multicentered bonding [MI ]8 (M=Mn, Zn) cluster with cubic aromaticity, followed by their fluorescence probe and magnetic properties are also summarized. Finally, prospective outlook in this field is given.

Stable ZnI -Containing MOFs with Large [Zn70 ] Nanocages from Assembly of ZnII Ions and Aromatic [ZnI8 ] Clusters.

Two unique ZnI -containing MOFs {[ZnI8 ZnII3 (H2 O)x (HL)12 ](OH)2 ⋅13 H2 O}n (x=6, 1; x=2, 2) (HL=tetrazole monoanion) with high-nuclearity Zn-cages were prepared successfully. These Zn-cages are constructed from [ZnI8 ] clusters with multi-centered ZnI -ZnI bonds and ZnII ions. [ZnI8 ] clusters in 1 and 2 display Oh and D4h symmetry, respectively. Importantly, eight [ZnI8 ] clusters and six ZnII ions form a large [Zn70 ] nanocage in 1. To our knowledge, this is the first MOF based on polynuclear Zn-cages consisting of ZnI and ZnII ions. Compared with reported ZnI -species, 1 and 2 display high thermal and solvent stabilities. Theoretical investigations based on DFT calculations uncover that effective 4s-4s orbital overlap and electron delocalization through these a1g +t1u molecular orbitals on [ZnI8 ] clusters lead to considerable aromatic stabilization, which can explain well the intrinsic stability of 1 and 2. Interestingly, 1 can behave as a luminescent probe to detect the toxic CrVI ions in aqueous.

Metal-Organic Frameworks (MOFs) of a Cubic Metal Cluster with Multicentered Mn(I)-Mn(I) Bonds.

MOFs with both multicentered metal-metal bonds and low-oxidation-state (LOS) metal ions have been underexplored hitherto. Here we report the first cubic [Mn(I) 8 ] cluster-based MOF (1) with multicentered Mn(I)-Mn(I) bonds and +1 oxidation state of manganese (Mn(I) or Mn(I)), as is supported by single-crystal structure determination, XPS analyses, and quantum chemical studies. Compound 1 possesses the shortest Mn(I)-Mn(I) bond of 2.372 Å. Theoretical studies with density functional theory (DFT) reveal extensive electron delocalization over the [Mn(I) 8 ] cube. The 48 electrons in the [Mn(I) 8 ] cube fully occupy half of the 3d-based and the lowest 4s-based bonding orbitals, with six electrons lying at the nonbonding 3d-orbitals. This bonding feature renders so-called cubic aromaticity. Magnetic properties measurements show that 1 is an antiferromagnet. This work is expected to inspire further investigation of cubic metal-metal bonding, MOF materials with LOS metals, and metalloaromatic theory.

First tetrazole-bridged d-f heterometallic MOFs with a large magnetic entropy change.

A novel 3D tetrazole-bridged 3d-4f heterometallic MOF {(H3O)3[Gd3Mn2(Trz)4]·12H2O}n (1) with a hexanuclear [Gd6] cluster was obtained via in situ [2+3] cycloaddition reaction and structurally characterized, possessing good solvent and thermal stabilities, as well as a large magnetic entropy change -ΔS(m) = 40.3 J kg(-1) K(-1) for ΔH = 7 T at 2.0 K. To our knowledge, it is the first example of tetrazole-bridged 3d-4f heterometallic MOFs.

Lanthanide organic framework as a regenerable luminescent probe for Fe(3+).

A unique three-dimensional Tb-BTB framework (1) with two types of one-dimensional channels was obtained and structurally characterized, exhibiting high thermal stability. Luminescent investigations reveal that 1 can detect Fe(3+) with relatively high sensitivity and selectivity. Importantly, 1 as the luminescent probe of Fe(3+) can be simply and quickly regenerated, which represents a rare example in reported luminescent sensors of Fe(3+).