Nature Of Metal Center Research Articles

Dinuclear Group 4 Metal Complexes Bearing Anthracene-Bridged Bifunctional Amido-Ether Ligands: Remarkable Metal Effect and Cooperativity toward Ethylene/1-Octene Copolymerization.

Two types of bifunctional amido-ether ligands (syn-L and anti-L) with the rigid anthracene skeleton were designed to support dinuclear group 4 metal complexes. All organic ligands and organometallic complexes (syn-M2 and anti-M2; M = Hf, Zr, and Ti) were fully characterized by 1H and 13C NMR spectroscopies and elemental analyses. The anti-Hf2 complex showed two confirmations at room temperature with C2-symmetry or S2-symmetry that can inter-exchange, as indicated by VT NMR, while only a C2-symmetric isomer was observed for syn-Hf2 complex at room temperature. However, for Zr and Ti analogues, both syn and anti complexes exhibited only one conformation at room temperature. The molecular structures of complexes syn-Hf2, anti-Hf2, and syn-Ti2 in the solid state were further determined by single-crystal X-ray diffraction, revealing the distances between two metal centers in syn-M2 from 7.138 Å (syn-Ti2) to 7.321 Å (syn-Hf2) but a much farther separation in anti-M2 (8.807 Å in C2-symmetric anti-Hf2). The mononuclear complex (2-CH3O-C6H4-N-C14H9)Zr(NMe2)3 (mono-Zr1) was also prepared for control experiments. In the presence of alkyl aluminum (AlEt3) as the alkylating agent and trityl borate ([Ph3C][B(C6F5)4]) as the co-catalyst, all metal complexes were tested for copolymerization of ethylene with 1-octene at high temperature (130 °C). The preliminary polymerization results revealed that the activity was highly dependent upon the nature of metal centers, and syn-Zr2 showed the highest activity of 9600 kg(PE)·mol-1 (Zr)·h-1, which was about 17- and 2.2-fold higher than those of syn-Hf2 and syn-Ti2, respectively. Benefitting from both steric proximity and electronical interaction of two metal centers, syn-Zr2 exhibited significant cooperativity in comparison to anti-Zr2 and mono-Zr1, with regard to activity and molecular weight and 1-octene incorporation of resultant copolymers.

Supramolecular Coordination Cages for Artificial Photosynthesis and Synthetic Photocatalysis.

Because sunlight is the most abundant energy source on earth, it has huge potential for practical applications ranging from sustainable energy supply to light driven chemistry. From a chemical perspective, excited states generated by light make thermodynamically uphill reactions possible, which forms the basis for energy storage into fuels. In addition, with light, open-shell species can be generated which open up new reaction pathways in organic synthesis. Crucial are photosensitizers, which absorb light and transfer energy to substrates by various mechanisms, processes that highly depend on the distance between the molecules involved. Supramolecular coordination cages are well studied and synthetically accessible reaction vessels with single cavities for guest binding, ensuring close proximity of different components. Due to high modularity of their size, shape, and the nature of metal centers and ligands, cages are ideal platforms to exploit preorganization in photocatalysis. Herein we focus on the application of supramolecular cages for photocatalysis in artificial photosynthesis and in organic photo(redox) catalysis. Finally, a brief overview of immobilization strategies for supramolecular cages provides tools for implementing cages into devices. This review provides inspiration for future design of photocatalytic supramolecular host-guest systems and their application in producing solar fuels and complex organic molecules.

Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes

A series of new charge transfer (CT) chromophores of "α-diimine-MII-catecholate" type (where M is 3d-row transition metals-Cu, Ni, Co) were derived from 4,4'-di-tert-butyl-2,2'-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) in accordance with three modified synthetic approaches, which provide high yields of products. A square-planar molecular structure is inherent for monomeric [CuII(3,6-Cat)(bipytBu)]∙THF (1) and NiII(3,6-Cat)(bipytBu) (2) chromophores, while dimeric complex [CoII(3,6-Cat)(bipytBu)]2∙toluene (3) units two substantially distorted heteroleptic D-MII-A (where D, M, A are donor, metal and acceptor, respectively) parts through a donation of oxygen atoms from catecholate dianions. Chromophores 1-3 undergo an effective photoinduced intramolecular charge transfer (λ = 500-715 nm, extinction coefficient up to 104 M-1·cm-1) with a concomitant generation of a less polar excited species, the energy of which is a finely sensitive towards solvent polarity, ensuring a pronounced negative solvatochromic effect. Special attention was paid to energetic characteristics for CT and interacting HOMO/LUMO orbitals that were explored by a synergy of UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT study. The current work sheds light on the dependence of CT peculiarities on the nature of metal centers from various groups of the periodic law. Moreover, the "α-diimine-MII-catecholate" CT chromophores on the base of "late" transition elements with differences in d-level's electronic structure were compared for the first time.

Modulation of coordinative unsaturation degree and valence state for cerium-based adsorbent to boost phosphate adsorption

The electronic structure and associated chemical characteristics of metal-based adsorbent are directly relevant to the selectivity and efficiency of phosphate uptake. However, few studies focus on the nature of metal centers’ electronic orbit (i.e., the coordination number and valence state). Herein, we report a coordinatively unsaturated Ce(III)-based materials, which exhibits excellent potential in effective phosphate removal. Via controlled partial thermolysis and the following reduction process, the valence state and coordination number of original Ce(III)-MOF (denoted as CM) can be tuned and optimized. The manufacture of more coordination vacancy was fulfilled through total release of solvent molecules under annealing at 300 °C in air. Meanwhile, the reduction procedure could precisely tuned the ratio of Ce(III)/Ce(IV). The result shows that samples only annealed induce a sharp decrease of the phosphate capacity due to the high amount of Ce(IV) state. After the reduction process, the XPS spectra reveal the growth of oxygen vacancy content calculated as 11.6% and the increase of Ce(III)/Ce(IV) values from 0.79:1 to 1.36:1. Based on those great improvement of the unsaturated coordination numbers and the recovery of Ce(III) content for metal centers, the maximum capacity of CM-300(R) to adsorb phosphate is up to 273 mg/g, 2.6 times larger than that of pristine CM. Those new insights provide a novel strategy for synthesizing a highly active adsorbent by controlling the electronic structure of metal centers for efficient phosphate removal.

Interdependence between structure of nitro-substituted palladium and zinc porphyrinates and its spectral, coordination and acid-base properties

Self-porphyrins and corresponding transition metal complexes are used as components of sensory, optically-useful and catalytically active materials. Current work provides data on synthesis and spectrophotometric experiment using meso-dinitrosubstituted derivatives of 5,15-diphenyl-2,3,7,8,12,13,17,18-octaalkylporphyrin being compared with 2,3,7,8,12,13,17,18-octaethylporphyrin and 2,3,7,8,12,13,17,18-octaethyl-5,15-dinitroporphyrin. Influence of macrocycle deformation, nature of metal center, electronic effects of substituents and acidity of medium on coordination and acid-base properties of the porphyrins is analyzed. As result is possible to conclude that the formation of metal porphyrins proceed rather faster in the case of anionic porphyrins than in the case of molecular porphyrins, which is accompanied with the decrease of energy parameters. The process is accompanied with decreasing energy parameters. Significantly increasing acid properties of porphyrin macrocycles are observed upon introducing nitro-groups into meso-positions in contrary to introducing it into para-positions of the phenyl rings. Such transformation results in negligible fall of base properties compared to meso-modification. Reaction of zinc and porphyrins proceeds faster compared to palladium. Moreover, last one is unable to interact with dianionic porphyrin form due to formation of highly stable metal-DBU coordination complex.

A series of transition metal coordination polymers with mixed ligands: Specific sensing and removal of metal ions

Starting from the transition metal salts and mixed organic ligands of 4,4′-{[1,2-phenylenebis-(methylene)]bis(oxy)}dibenzoic acid (H2L) and different N-containing linkers, six new coordination polymers, [Cd(L)(2,2′-bipy)]n (1); [Cu(L)(2,2′-bipy)]n (2); {[Zn(L)(4,4′-bipy)][Zn(L)(H2O)]}n (3); [Cu(L)(4,4′-bipy)0.5 (H2O)]n (4), [Zn2(L)2(bpp)]n (5) and [Cd3(L)2(OAc)2(Him)2]n (6) (2,2′-bipy=2,2′-bipyridine, 4,4′-bpy=4,4′-bipyridine and bpp=1,3-bis(4-pyridyl)propane), were synthesized and characterized. The structural differences of 1–6 indicate the effect of the auxiliary N-donor ligands and nature of metal centers could modulate the resulting architectures. The photoluminescent sensing behaviours of 1 and 5 toward Cu2+ cation and nitro explosives have been investigated in detail. Interestingly, the immobilized Cu2+ cation can be easily separated by ethylenediaminetetraacetic acid (EDTA).

Structural and magnetic properties of 1D coordination polymers constructed by isophthalate and 2,4′-bipyridine

Three new complexes {[Cu(ip)(2,4′-bpy) 2] · H 2O} n ( 1), {[Co(ip)(2,4′-bpy) 2] · H 2O} n ( 2) and [Ni(ip)(2,4′-bpy) 2(H 2O)] n ( 3) were prepared in the reaction of isophthalic acid (H 2ip) with different metal ions such as Cu II, Co II, and Ni II under hydrothermal conditions in the presence of 2,4′-bipyridine (2,4′-bpy) ligand. The metal ions are bridged by ip 2− anions to form 1D ribbon with 8-membered and 16-membered ring in complexes 1 and 2, while nickel ions are bridged by ip 2− anions and water molecules forming 1D ribbon with 8-membered and 20-membered ring in complex 3. It was found that antiferromagnetic interactions between metal ions can be tuned by the nature of metal center and/or the coordinated water molecule.