Ming-Hua Zeng

Tracking the Formation of a Polynuclear Co16 Complex and Its Elimination and Substitution Reactions by Mass Spectroscopy and Crystallography

We present the syntheses and structures of the biggest chiral cobalt coordination cluster, [Co16(L)4(H3L)8(N3)6](NO3)2·16H2O·2CH3OH (1, where H4L = S,S-1,2-bis(1H-benzimidazol-2-yl)-1,2-ethanediol). 1 consists of two Co4O4 cubes (Co4(L)2(H3L)2) alternating with Co2(EO-N3)2Co2 (Co4(L)2(H3L)2(N3)2), bridged by the benzimidazole and azide nitrogen atoms to form a twisted ring. The ligand adopts both cis and trans forms, and all the rings have the same chiralilty. ESI-MS of 1 from a methanol solution of crystals reveals the fragment [Co16(L)4(H3L)8(N3)6+2H](4+), suggesting the polynuclear core is stable in solution. ESI-MS measurements from the reaction solution found smaller fragments, [Co4(H3L)4-H](3+), [Co4(H3L)4-2H](2+), [Co4(H3L)4(N3)2](2+), and [Co2(H3L)2](2+), and ESI-MS from a methanol solution of the solid deposit found in addition the Co16 core. These results and the dependence on the synthesis time allow us to propose the process for the formation of 1, which opens up a new way for the direct observation of the ligand-controlled assembly of clusters. In addition, the isolation of [Co4(H3L)4](NO3)4·4H2O (2) consisting of separate Co4O4 cubes with the ligands being only cis in crystalline form supports the proposal. Interestingly, N3(-) is replaced by either CH3O(-) or OH(-), and this is the first time that high-resolution ESI-MS is successfully utilized to examine both the step-by-step elimination and substitution of inner bridging ligands in such a high nuclear complex. Increasing the voltage results in stepwise elimination of azide from the parent cluster. The preliminary magnetic susceptibility of 1 indicates ferromagnetic cubes antiferromagnetically coupled to the squares within the cluster, though in a field of 2.5 kOe, weak and slow relaxation is observed below 4 K.

Tandem Postsynthetic Modification of a Metal–Organic Framework by Thermal Elimination and Subsequent Bromination: Effects on Absorption Properties and Photoluminescence

Inside and outside: Two consecutive postsynthetic modifications, first an elimination reaction in the channels and then bromination at the surface, were realized in a new hybrid metal-organic framework. The dramatic effects of the different groups in the channels and at the surface were studied using gas sorption and the loading/release of solvent and iodine.

A Porous Coordination Polymer Assembled from 8-Connected {CoII3(OH)} Clusters and Isonicotinate: Multiple Active Metal Sites, Apical Ligand Substitution, H2 Adsorption, and Magnetism

A microporous coordination polymer, namely, [Co(3)(ina)(4)(OH)(C(2)H(5)OH)(3)](NO(3))·C(2)H(5)OH·(H(2)O)(3) (1, or MCF-38, ina = isonicotinate), with 8-connected {Co(3)(OH)} clusters as the structural secondary building units, has been solvothermally synthesized. The hydroxo-centered Co(II) cluster involves multiple active metal sites. The interesting apical ligand substitutions have been directly observed, and the corresponding products of [Co(3)(ina)(4)(OH)(G)(x)(H(2)O)(n)](NO(3))·G·(H(2)O)(m) (1 ⊃ PrOH, G = PrOH, x = 2, n = 1, m = 3; 1 ⊃ BuOH, G = BuOH, x = 2, n = 1, m = 1, and 1 ⊃ MeOH, G = MeOH, x = 3, n = 0, m = 7) have also been obtained by solvothermal syntheses or crystal-to-crystal transformations. High-pressure H(2) adsorption measurement at 77 K reveals that activated 1 can absorb 2.2 wt % H(2) at 5 bar. The relative H(2) absorption at low pressure (86% of the storage capacity at 1 bar) is higher than the corresponding values reported for some typical porous coordination polymers. The magnetic studies of 1 show a dominant antiferromagnetic coupling between Co(II) ions of intra- and inter-cluster.

A Porous 4-Fold-Interpenetrated Chiral Framework Exhibiting Vapochromism, Single-Crystal-to-Single-Crystal Solvent Exchange, Gas Sorption, and a Poisoning Effect

The synthesis and characterization of a 4-fold-interpenetrated pseudodiamond metal-organic framework (MOF), Co(II)(pybz)2·2DMF [pybz = 4-(4-pyridyl)benzoate], are reported. N,N-Dimethylformamide (DMF) of the channels can be removed to give the porous framework, and it can also be exchanged for methanol, ethanol, benzene, and cyclohexane. It is a rare example of a stable MOF based on a single octahedral building unit. The single-crystal structures of Co(II)(pybz)2·2DMF, Co(II)(pybz)2, Co(II)(pybz)2·4MeOH, and Co(II)(pybz)2·2.5EtOH have been successfully determined. In all of them, the framework is marginally modified and contains a highly distorted and strained octahedral node of cobalt with two pyridine nitrogen atoms and two chelate carboxylate groups. In air, the crystals of Co(II)(pybz)2·2DMF readily change color from claret red to light pink. Thermogravimetric analysis and Raman spectroscopy indicate a change in coordination, where the carboxylate becomes monodentate and an additional two water molecules are coordinated to each cobalt atom. In a dry solvent, this transformation does not take place. Tests show that Co(II)(pybz)2 may be a more efficient drying agent than silica gel and anhydrous CuSO4. The desolvated Co(II)(pybz)2 can absorb several gases such as CO2, N2, H2, and CH4 and also vapors of methanol, ethanol, benzene, and cyclohexane. If Co(II)(pybz)2 is exposed to air and followed by reactivation, its sorption capacity is considerably reduced, which we associate with a poisoning effect. Because of the long distance between the cobalt atoms in the structure, the magnetic properties are those of a paramagnet.

Unprecedented Marriage of a Cationic Pentanuclear Cluster and a 2D Polymeric Anionic Layer Based on a Flexible Tripodal Ligand and a CuII Ion

The anionic CdI(2)-type topological net, [Cu(2)(tci)(2)](2-), and the pentanuclear copper cluster cation [Cu(5)(tci)(2)(OH)(2)(H(2)O)(8)](2+) [tci = tris(2-carboxyethyl)isocyanurate] form a complementary 3D supramolecular framework. Interestingly, there exist centrosymmetric cyclic (H(2)O)(18) clusters in the cavities.

Microwave-assisted synthesis, crystal structure and properties of a disc-like heptanuclear Co(II) cluster and a heterometallic cubanic Co(II) cluster

The reaction of Co(ClO4)2·6H2O with a disodium 2-[(2-hydroxy-3-methoxy-enzylidene)-amino]-ethanesulfonic acid (Na2L) in a mixture solution (methanol : acetonitrile = 1 : 1 (v/v)) or an acetonitrile solution under microwave and controlled pressure/temperature, or solvothermal conditions leads to the formation of two completely different CoII complexes: heptanuclear cluster [Co7(immp)6(CH3O)6]·2ClO4 (1) (immp is 2-iminomethyl-6-methoxy-phenolic anion) and tetra-heteronuclear cubane cluster [NaCo3(L)3(CH3CN)3(OH)]2·CH3CN (2). The microwave-assisted reactions can improve the reaction rate and yield compared to simple solvothermal synthesis. The magnetic investigation shows that complexes 1 and 2 exhibit a ferromagnetic coupling between the CoII ions. AC susceptibilities of the two complexes reveal no frequency-dependent out-of-phase signals and the corresponding magnetic properties were discussed.

A unique 2D framework containing linear trimeric cobalt(II) of mixed Td-Oh-Td geometries linked by two different single-carboxylate-aromatic amine ligands: structure and magnetic properties.

A new 2D coordination polymer Co3(OH)2(pa)2(ina)2 (1, pa = 3-(1H-benzimidazol-2-yl) propanoic carboxylate, ina = isonicotinate) contained uncommon, linear Co(ii) trimers of mixed Td-Oh-Td geometries, exhibits spin canting below 20 K. Such magnetic behavior mainly arises from the Dzyaloshinski-Moriya interaction from the anisotropic, mixed geometries trimeric Co(II) ions to the crimpled 2D network based on the nature of the binding modes of Co(II)-carboxylate trimer and the effect of the intertrimers arrangement of 1. The mixed single-carboxylate-aromatic amine ligands bridged metal systems display a new structurally authenticated example of a thick 2D layer, and also indicate homometallic Co(II) clusters with Td-Oh-Td mixed-geometries can result in relatively obvious noncompensation moments, according to different efficient spins of Co(II) at very low temperature, in spite of antiferromagnetic intracluster interactions.

Mn(II)-Based Porous Metal–Organic Framework Showing Metamagnetic Properties and High Hydrogen Adsorption at Low Pressure

A Mn(II)-based homometallic porous metal-organic framework, Mn(5)(btac)(4)(μ(3)-OH)(2)(EtOH)(2)·DMF·3EtOH·3H(2)O (1, btac = benzotriazole-5-carboxylate), has been solvothermally synthesized and structurally characterized by elemental analysis, thermogravimetric analysis, and X-ray crystallographic study. 1 is a 3D neutral framework featuring 1D porous channels constructed by {Mn-OH-Mn}(n) chains and btac linkers. Magnetic studies show that 1 is a 3D metamagnet containing 1D {Mn-OH-Mn}(n) ferrimagnetic chains. High-pressure H(2) adsorption measurement at 77 K reveals that activated 1 can absorb 0.99 wt % H(2) at 0.5 atm and reaches a maximum of 1.03 wt % at 5.5 atm. The steep H(2) absorption at lower pressure (98.2% of the storage capacity at 0.5 atm) is higher than the corresponding values of some MOFs (MIL-100 (16.1%), MOF-177 (57.1%), and MOF-5 (22.2%)). Furthermore, activated 1 can adsorb CO(2) at room temperature and 275 K. The adsorption enthalpy is 22.0 kJ mol(-1), which reveals the high binding ability for CO(2). Detailed gas sorption implies that the exposed Mn(II) coordination sites in the activated 1 play an important role to improve its adsorption capacities.

Magneto−Structural Correlation in a Metamagnetic Cobalt(II)-Based Pillared Trilayer Motif Constructed by Mixed Pyridyl-Type Carboxylate Ligands

The first structurally authenticated example of coordinated polymer featuring homometallic pillared-trilayer structure, [Co(3)(ina)(2)(pico)(2)(H(2)O)(2)](n) (1), was built from mixed pyridyl-type monocarboxylates, isonicotinate (ina) and 3-hydroxypicolinate (pico), containing Kagome-type [Co(3)(pico)(2)(H(2)O)(2)](n)(2n+) trilayers. The magnetic phase diagram of 1 shows a metamagnetic transition below 3.2 K, arising from the competing interactions between the antiferromagnetic intralayer couplings in different amplitudes with obviously noncompensated moments versus weak AF interlayer coupling.

Synthesis, crystal structure and magnetic properties of two 3-D gadolinium complexes

Two new gadolinium coordination polymers, Gd(tci)(H2O)]·DMF·2H2O (1) and [Gd(bci)(H2O)]·2H2O(2) (H3tci = tri(2-carboxyethyl)isocyanurate, H3bci = bis(2-carboxyethyl)isocyanurate), have been synthesized under solvothermal conditions by the reaction of gadolinium nitrate and H3tci or H3bci with presence of DMF and H2O. 1 and 2 feature an interesting 3D metal–organic framework which contains infinite linear Gd-(µ-O)2-Gd chains in the structures. The carboxylate groups bridge two Gd(III) centers in µ2-carboxylato-κ1O: κ1O′ and µ2O;κ2O,O′ patterns for 1 and µ2O;κ2O,O′ fashion for 2. The magnetic studies indicate that there exist antiferromagnetic interactions in 1 and ferromagnetic interactions in 2. The best fittings to the experimental magnetic susceptibilities gave J = −0.015 cm−1, g = 1.98 for 1 and J = 0.055 cm−1, g = 1.98 for 2.