Reactions of Low-Valent Titanocene(II) Fragments with trans-4,4′-Azobispyridine (RN═NR, R = C5H4N): Formation of Tetranuclear Molecular Squares by trans−cis Isomerization

Titanium(II) as a Fuel Atom in Energetic Materials

The energetic content of the compounds MgTp₂, FeTp₂, MnTp₂, and TiTp₂ is measured by bomb calorimetry and compared to theoretical calculations (Tp = trispyrazoylborate). TiTp₂ had the largest heat of combustion of the four compounds. Comparison of the heat of combustion of the Ti complex to those of Mg and Mn complexes suggests an effective combustion energy of Ti^II of between 1400 and 3000 kJ/mol, affirming the role of Ti^II as a strong fuel atom.

Iron complexes of bridging azo ligands in aqueous solution: changes in the thermal switching mechanism on coordination and oxidation state of metal centres

Three azobenzenes CN(C₆H₄)-NN-(C₅H₄N) (py-iso), CN(C₆H₄)-NN-(C₆H₄)CN (cyano-iso) and CN(C₆H₄)-NN-(C₆H₄)NC (iso-iso) with good coordinating groups (pyridine, phenylcyano or phenylisocyano) at the ends of the diazenyl unit have been synthesized and fully characterised. These compounds have been used as ligands in the synthesis of water-soluble metallic species by coordination to {Fe^II(CN)₅^3-} units, either in one or two of the anchoring groups of the derivatives. Both the azo derivatives and their complexes are photochemically active with respect to their trans-to-cis isomerisation process. Their cis-to-trans reverse thermal reaction has been thoroughly studied as a function of the donor groups, solvent, temperature and pressure, in order to gain insight into the rotation or inversion mechanisms involved in the process. A comparison of the isomerisation mechanism between the iron complexes and the corresponding free ligands revealed an interesting fine tuning of the process on coordination of the {Fe^II(CN)₅^3-} moieties, which may even produce, in some cases, non-photoswitchable species containing typically photoactive units.

Crystal structure of 4,4′-(diazenediyl)dipyridinium nitrate perchlorate

The title compound was obtained unexpectedly by the reaction of Co(ClO₄)₂·6H₂O and cytidine-5′-monophosphate with 4,4′-azo­pyridine in an aqueous solution of nitric acid. In the crystal, N—H⋯O hydrogen bonds between dications and anions lead to the formation of chains.

The title compound, C₁₀H₁₀N₄²⁺·NO₃⁻·ClO₄⁻, was obtained unexpectedly by the reaction of Co(ClO₄)₂·6H₂O and cytidine-5′-monophosphate with 4,4′-azo­pyridine in an aqueous solution of nitric acid. The mol­ecular structure comprises two planar 4,4′-diazenediyldipyridinium dications lying on inversion centres and perchlorate and nitrate anions in general positions. In the crystal, N—H⋯O hydrogen bonds between dications and anions lead to the formation of [232] chains.

Latest News: Reactions of Group 4 Bis(trimethylsilyl)acetylene Metallocene Complexes and Applications of the Obtained Products

Recently published reactions of group 4 metallocene bis(trimethylsilyl)acetylene (btmsa) complexes from the last two years are reviewed. Complexes like Cp'2 Ti(η2 -Me3 SiC2 SiMe3 ) and Cp2 Zr(py)(η2 -Me3 SiC2 SiMe3 ) with Cp' as Cp (cyclopentadienyl) and Cp* (pentamethylcyclopentadienyl) have been considered (py=pyridine). These complexes can liberate a reactive low-valent titanium or zirconium center by dissociation of the ligands and act as ''masked'' MII complexes (M=Ti, Zr). They represent excellent sources for the clean generation of the reactive coordinatively and electronically unsaturated complex fragments [Cp'2 M]. This is the reason why they were used for many synthetic and catalytic reactions during the last years. As an update to several review articles on this topic, this contribution provides an update with recent examples of preparative organometallic and organic chemistry of these complexes, acting as reagents for a wide range of coordinating and coupling reactions. In addition, applications and investigations concerning reaction products derived from this chemistry are mentioned, too.

Photo-/Electroinduced Irreversible Isomerization of 2,2'-Azobispyridine Ligands in Arene Ruthenium(II) Complexes

Novel arene Ru^II complexes containing 2,2'-azobispyridine ligands were synthesized and characterized by using ¹ H and ¹³ C NMR spectroscopy, UV/vis spectroscopy, electrochemistry, DFT calculations and single-crystal X-ray diffraction. Z-configured complexes featuring unprecedented seven-membered chelate rings involving the nitrogen atom of both pyridines were isolated and were shown to undergo irreversible isomerization to the corresponding E-configured five-membered chelate complexes in response to light or electrochemical stimulus.

Advantages of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes as Metallocene Sources Towards Other Synthetically used Systems

Active species for synthetic and catalytic applications are formed from well defined complexes or mixtures of compounds. For group 4 metallocenes, three pathways for the formation of the reactive complex fragment [Cp'2M] are known: (i) reductive mixtures and well defined complexes which are able to form the metallocene fragments either by (ii) addition or (iii) substitution reactions. In this account for each of theses systems (i)-(iii) a prominent example will be discussed in detail, (i) the Negishi reagent Cp2ZrCl2/n-BuLi, (ii) bis(η5 : η1-pentafulvene) complexes and (iii) metallocene bis(trimethylsilyl)acetylene complexes, to show the advantages and the disadvantages for each of these methods for synthetic applications. This account summarizes some main advantages of group 4 metallocene bis(trimethylsilyl)acetylene complexes as metallocene generating agents over other synthetically used systems. For each of the special purposes, all described systems have advantages as well as disadvantages. The aim of this overview is to help synthetic chemists in selecting the most effective system on the basis of [Cp'2M] (M=Ti, Zr) for synthetic or catalytic puposes.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

Coordination driven self-assembly of [2 + 2 + 2] molecular squares: synthesis, crystal structures, catalytic and luminescence properties

Ten molecular squares were prepared through coordination-driven self-assembly and their catalytic activity and luminescence properties are reported.

Set of Multifunctional Azo Functionalized Semiconducting Cd(II)-MOFs Showing Photoswitching Property and Selective CO2 Adsorption

Syntheses, structural characterizations, photoluminescence, and adsorption properties of three new azo-functionalized Cd(II)-MOFs, namely, {[Cd(azbpy)(msuc)]·2.5(H₂O)}_n (2), {[Cd(azbpy)(mglu)]·5(H₂O)}_n (3), and {[Cd_1.5(azbpy)₂(glu)]·(NO₃)·MeOH}_n (4) [where msuc^2- = methylsuccinate; mglut^2- = methylglutarate; glut^2- = glutarate; azbpy = 4,4'-azobispyridine] have been reported. The compounds show different structures only with the variation of aliphatic dicarboxylates. The photoswitching behavior for the above-mentioned newly synthesized Cd(II)-MOFs along with one of our previously reported other azo-functionalized Cd(II)-MOF, namely, {[Cd(azbpy)(suc)]·2(H₂O)}_n (1), has been studied extensively. At photoilluminated condition, the conductivity values can draw a clear structure-property relationship among the structures of compounds 1-4. Single crystal structural analysis reveals that all the compounds exhibit a three-dimensional (3D) framework connected by azbpy linker and respective aliphatic dicarboxylate through their bis-chelating mono/bis oxo-bridging fashion. Compounds 1-3 exhibit an iso-structural honeycomb like 3D framework showing the same coordination environments, where the metal-carboxylate 2D sheets of compounds 1-3 are pillared by N,N'-donor azbpy linkers. On the other hand, compound 4 exhibits a 2-fold interpenetrated 3D framework with a little difference in its coordination environment and the pillaring of 1D metal-carboxylate ladder by azbpy linkers. All the compounds significantly demonstrate their enhanced sensitivity under light rather than the dark condition. The gas and solvent vapor sorption studies have been performed for the synthesized compounds 2-4. Moreover, compound 2 exhibits an enhanced type IV selective CO₂ adsorption isotherm over N₂ along with the appearance of gate opening phenomena in that.

Self-assembly of a fac-Mn(CO)3-core containing dinuclear metallacycles using flexible ditopic linkers

Flexible dimanganese metallacycles have been achieved using Mn(CO)₅Br and adaptable ditopic pyridyl linkers. The host–guest chemistry of Mn(i)-dinuclear metallacycles has been explored.

From chromium-chromium quintuple bonds to molecular squares and porous coordination polymers

Reaction of the quintuply bonded chromium(I) dimer [ApCrCrAp] (Ap = sterically demanding 2-aminopyridinate) with pyrazine yields a chromium(II) complex with a η(4):η(4) face-on coordinated pyrazine dianion. Reaction with 4,4'-bipyridine, on the other hand, completely cleaves the metal-metal bond, leading to a chromium(II)-based molecular square. XRD and magnetic measurements show ligand radical anions and a ferrimagnetic alignment of alternating metal and ligand magnetic moments. Controlled polymerization of the molecular square with pyrazine yields a porous coordination polymer featuring both reduced and nonreduced linkers.

Self-assembled metalla-rectangles bearing azodipyridyl ligands: synthesis, characterization and antitumor activity

Sixteen arene-Ru based molecular-rectangles were self-assembled in high yields by the equimolar mixing of arene-Ru acceptors (Aa-Ad) with various azopyridyl ligands (1,2-di(pyridyl-4yl)diazene (L1), 1,2-bis(pyridin-4-ylmethylene)hydrazine (L2), 1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine (L3), 1,2-bis(pyridin-4-ylmethylene)hydrazine (L4)) in nitromethane-methanol solutions. These new molecular-rectangles were fully characterized by a host of analytical techniques including elemental analysis, (1)H and (13)C NMR and HR-ESI-MS. The solid-state structures of two molecular-rectangles (1b and 4d) were determined by single crystal X-ray diffraction data. UV-visible and fluorescence studies were also carried out for the entire suite of rectangles. As with recent studies of similar arene-Ru complexes, the anti-proliferative activities of these complexes were evaluated against SK-hep-1 (liver cancer) and A-549 (lung cancer) human cancer lines. Additionally, the cellular pharmacology and intracellular localizations in AGS (gastric cancer) human cancer cells were determined for selected complexes (1c, 1d and 4c) by apoptosis and fluorescence microscopy studies. These studies confirm that arene-Ru molecular-rectangles inhibit cell cycle progression to the G0 phase, in contrast to that of cisplatin which arrests cell growth in the G2 phase.

Construction of coordination-driven self-assembled [5 + 5] pentagons using metal-carbonyl dipyridine ligands

The coordination-driven self-assembly of two metal-carbonyl-cluster-coordinated dipyridyl donors, (4-C(5)H(4)N)(2)C[triple bond]CCo(2)(CO)(6) (1) and (4-C(5)H(4)N)(2)C[triple bond]CMo(2)Cp(2)(CO)(4) (2), with a linear diplatinum(II) acceptor ligand was investigated. The structures of the resulting self-assembled polygons were found to be controlled by the steric bulk of the metal-carbonyl cluster adduct. The use of a sterically less imposing ligand 1 resulted in a pentagon-hexagon mixture, which was characterized by electrospray ionization time-of-flight mass spectroscopy. The exclusive formation of a [5 + 5] pentagon was achieved by the self-assembly of the bulkier molybdenum donor ligand 2 with a linear organoplatinum(II) acceptor ligand. Molecular force field modeling was used to study the structural details of the pentagonal and hexagonal architectures. The first Fe(3)-Co(6)-Pt(6) trimetal [3 + 3] hexagon was also synthesized via the combination of 1 with a 120 degrees ferrocenyldiplatinum(II) acceptor.