Tetrazine Ligands Research Articles

Triazine Chalcogenones from Thiocyanate or Selenocyanate Addition to Tetrazine Ligands in Ruthenium Arene Complexes.

The chemistry of 1,2,4,5-tetrazines has attracted considerable interest both from a synthetic and applicative standpoint. Recently, regioselective reactions with alkynes and alkenes have been reported to be favored once the tetrazine ring is coordinated to Re(I), Ru(II), and Ir(III) centers. Aiming to further explore the effects of metal coordination, herein, we unveil the unexplored reactivity of tetrazines with chalcogenocyanate anions. Thus, ruthenium(II) tetrazine complexes, [RuCl{κ2N-3-(2-pyridyl)-6-R-1,2,4,5-tetrazine}(η6-arene)]+ (arene = p-cymene, R = H, [1a]+, R = Me, [1b]+, R = 2-pyridyl, [1c]+; arene = C6Me6, R = H, [1d]+, R = Me, [1e]+; PF6- salts), reacted quantitatively and in mild conditions with M(ECN) salts (M = Na, K, Bu4N; E = O, S, Se). The addition of thiocyanate or selenocyanate to the tetrazine ligand is regioselective and afforded, via N2 release, 1,2,4-triazine-5-chalcogenone heterocycles, the one with selenium being unprecedented. The novel ruthenium complexes [RuCl{κ2N-(2-pyridyl)}{triazine chalcogenone}(η6-arene)] 2a-e (sulfur), 3b, 3d, and 3e (selenium) were characterized by analytical (CHNS analyses, conductivity), spectroscopic (IR, multinuclear and two-dimensional (2D) NMR), and spectrometric (electrospray ionization mass spectrometry (ESI-MS)) techniques. According to density functional theory (DFT) calculations, the nucleophilic attack of SCN- on the tetrazine ring is kinetically driven. Compound 2b is selectively and reversibly mono-protonated on the triazine ring by HCl or other strong acids, affording a single tautomer. When reactions of chalcogenocyanates were performed on the 2,2'-bipyridine (bpy) complex [RuCl(bpy)(η6-p-cymene)]+, the chloride substitution products [Ru(ECN)(bpy)(η6-p-cymene)]+ (E = O, [4]+; E = S, [5]+; E = Se, [6]+) were obtained in 82-90% yields (PF6- salts). Combined spectroscopic data (IR, 1H/13C/77Se NMR) was revealed to be a useful tool to study the linkage isomerism of the chalcogenocyanate ligand in [4-6]+.

Computational screening of chemically active metal center in coordinated dipyridyl tetrazine network

Creation, stabilization, characterization, and control of single transition metal (TM) atoms may lead to significant advancement of the next-generation catalyst. Metal organic network (MON) in which single TM atoms are coordinated and separated by organic ligands is a promising class of material that may serve as a single atom catalyst. Our density functional theory-based calculations of MONs in which dipyridyl tetrazine (DPTZ) ligands coordinate with a TM atom to form linear chains leads to two types of geometries of the chains. Those with V, Cr, Mo, Fe, Co, Pt, or Pd atoms at the coordination center are planar while those with Au, Ag, Cu, or Ni are non-planar. The formation energies of the chains are high (∼2.0–7.9 eV), suggesting that these MON can be stabilized. Moreover, the calculated adsorption energies of CO and O2 on the metal atom at center of the chains with the planar configuration lie in the range 1.0–3.0 eV for V, Cr, Mo, Fe, and Co at the coordination center, paving the way for future studies of CO oxidation on TM-DPTZ chains with the above five atoms at the coordination center.

Tough tetrazine-functionalized styrene–butadiene rubber with self-adhesion through zinc–nitrogen coordination

This paper reports a feasible system for the direct adhesion of cross-linked rubbers, by incorporating tetrazine ligands into a styrene–butadiene rubber followed by the addition of zinc dimethacrylate with the aim of forming reversible cross-links.

A Dicobalt(II) Single-Molecule Magnet via a Well-Designed Dual-Capping Tetrazine Radical Ligand

The recent years have witnessed the glory development for the construction of high-performance mononuclear single molecule magnets (SMMs) within a specific coordination geometry, which, however, is not well applied in cluster-based SMMs due to the synthetic challenges. Given that the monocobalt(II) complexes within a trigonal-prismatic (TPR) coordination geometry have been classified as excellent SMMs with huge axial anisotropy (D ≈ -100 cm-1), here we designed and synthesized a new dual-capping tetrazine ligand, 3,6-bis(6-(di(1H-pyrazol-1-yl)methyl)pyridin-2-yl)-1,2,4,5-tetrazine (bpptz), and prepared a novel dicobalt(II) complex, [Cp2CoIII][{(hfac)CoII}2(bpptz•-)][hfac]2·2Et2O (1, hfac = hexafluoroacetylacetonate). In the structure of 1, the bpptz•- radical ligand enwraps two Co(II) centers within quasi-TPR geometries, which are further bridged by the tetrazine radical in the trans mode. The magnetic study revealed that the interaction between the Co centers and the tetrazine radical is strongly antiferromagnetic with a coupling constant (J) of -65.8 cm-1 (in the -2J formalism). Remarkably, 1 exhibited the typical SMM behavior with an effective energy barrier of 69 cm-1 under a 1.5 kOe dc field, among the largest for polynuclear transition metal SMMs. In addition, DFT and ab initio calculations suggested that the presence of a strong Co(II)-radical magnetic interaction effectively quenches the QTM effect and enhances the barrier height for the magnetization reversal.

Formation of 1D coordination polymers by reaction of a tetrazine ligand and PbX2 (X: Br, I) salts: Spectral, structural and theoretical studies

Two lead(II) 1D-coordination polymers, [Pb3(μ-BPTZ)2(μ4-Br)(μ3-Br)(μ-Br)3Br]n (1) (BPTZ = 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine) and [Pb(HPMPHz)(μ-I)2]n (2) (HPMPHz = zwitterionic form of N-(pyridin-2-ylmethylene)picolinohydrazonic acid, HPMPH) were prepared and identified by elemental analysis, FT-IR spectroscopy and single-crystal X-ray diffraction. The crystal structure of 1, is the first example for coordination of a tetrazine ligand to the lead(II) ion. The 1D chain in 1 is expanding by BPTZ and bromide bridges. Three different coordination environments were found for lead atoms: PbN4(μ4-Br)(μ3-Br)(μ-Br)2 with cube geometry; PbN2(μ4-Br)(μ3-Br)(μ2-Br)Br and PbN2(μ4-Br)(μ3-Br)2(μ2-Br)2 with capped octahedral geometries. Ligands around these lead(II) ions are arranged in hemidirected mode. X-ray analysis of 2 revealed a ring opening hydrolysis reaction that converts BPTZ ligand to the HPMPHz. This structure is the first example for a PbN2OI4 coordination environment which is extended by iodide bridges and contains hemidirected pentagonal bipyramidal coordinated lead(II) ions. Also 2 is the first example containing zwitterionic form of HPMPH. In the crystal network of the complexes, in addition to hydrogen bonds there are π–π stacking interactions between pyridine rings. Optical properties of the compound 1 were investigated by UV–Vis diffuse reflectance and emission spectroscopy methods in solid state. According to the DFT calculated electronic band structures, the compounds are indirect band gap semiconductors. Linear optical properties of the compounds (including the dielectric function, absorption coefficient, reflectivity, refractive index, optical conductivity and energy loss function) are also investigated by DFT method.

Unprecedented intramolecular pancake bonding in a {Dy2} single-molecule magnet

The first example of unique coordination induced intramolecular pancake bonding was achieved through the reduction of two bis(pyrazolyl)-tetrazine ligands.

Structural, spectral and theoretical study of the coordination of 3,6-bis(2-pyridyl)tetrazine ligand with zinc(II) and mercury(II)

We report the synthesis and single crystal X-ray structures of three zinc(II) and mercury(II) coordination polymers (1D-zigzag-{[Zn(μ-BPTZ)Cl2]·DMSO}n (1), 1D-zigzag-{[Zn(μ-BPTZ)Br2]·CH3CN}n (2) and 2D-herringbone [Hg2(μ-BPTZ)(μ-CN)2(CN)2]n (3)) and a binuclear 3D-supramolecular coordination compound of mercury(II), [Hg2(μ-BPTZ)(SCN)4] (4), with 3,6-di(pyridin-2-yl)-1,2,4,5-tetrazine (BPTZ) ligand. The compounds are characterized by elemental analysis and by FT-IR, 1H NMR spectroscopies. Stability of syn and anti conformers of BPTZ and the energy barrier for converting the isomers was calculated by DFT method. 1 and 2 are isostructural coordination polymers in which the zinc(II) ion has a distorted octahedral coordination environment N4X2 (X: Cl (1), Br (2)). The coordination environment of mercury(II) ion in 3 (with N3C2 donor set) and 4 (with N2S2 donor set) is close to square-pyramidal and seesaw geometries, respectively. The binuclear [Hg2(μ-BPTZ)(SCN)4] units in 4 are linked via Hg……Nthiocyanate interactions to form a 3D-supramolecular coordination network. In addition, all coordination modes of BPTZ derivatives, reported in crystal structure database (CSD), were extracted and discussed.

Laser Initiation of Fe(II) Complexes of 4-Nitro-pyrazolyl Substituted Tetrazine Ligands.

The synthesis and characterization of new 1,2,4-triazolyl and 4-nitro-pyrazolyl substituted tetrazine ligands has been achieved. The strongly electron deficient 1,2,4-triazolyl substituted ligands did not coordinate Fe(II) metal centers, while the mildly electron deficient 4-nitro-pyrazolyl substituted ligands did coordinate Fe(II) metal centers in a 2:1 ratio of ligand to metal. The thermal stability and mechanical sensitivity characteristics of the complexes are similar to the conventional explosive pentaerythritol tetranitrate. The complexes had strong absorption in the visible region of the spectrum that extended into the near-infrared. In spite of having improved oxygen balances, increased mechanical sensitivity, and similar absorption of NIR light to recently reported Fe(II) tetrazine complexes, these newly synthesized explosives were more difficult to initiate with Nd:YAG pulsed laser light. Specifically, the complexes required lower densities (0.9 g/cm3) to initiate at the same threshold utilized to initiate previous materials at higher densities (1.05 g/cm3).

Iodide and triiodide anion complexes involving anion-π interactions with a tetrazine-based receptor.

Protonated forms of the tetrazine ligand L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) interact with iodide in aqueous solution forming relatively stable complexes (ΔG° = -11.6(4) kJ mol-1 for HL2+ + I- = (HL2)I and ΔG° = -13.4(2) kJ mol-1 for H2L22+ + I- = [(H2L2)I]+). When solutions of [(H2L2)I]+ are left in contact with air, crystals of the oxidation product (H2L2)2(I3)3I·4H2O are formed. Unfortunately, the low solubility of I3- complexes prevents the determination of their stability constants. The crystal structures of H2L2I2·H2O (1), H2L2(I3)2·2H2O (2) and (H2L2)2(I3)3I·4H2O (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I- and I3- with H2L22+ is dominated by anion interactions with the π electron density of the receptor. Only in the case of 1, the iodide anions involved in close anion-π interactions with the ligand tetrazine ring form an additional H-bond with the protonated morpholine nitrogen of an adjacent ligand molecule. Conversely, in crystals of 2 and 3 there are alternate segregated planes which contain only protonated ligands hydrogen-bonded to cocrystallized water molecules or I3- and I- forming infinite two-dimensional networks established through short interhalogen contacts, making these crystalline products good candidates to behave as solid conductors. In the solid complexes, the triiodide anion displays both end-on and side-on interaction modes with the tetrazine ring, in agreement with density functional theory calculations indicating a preference for the alignment of the I3- molecular axis with the molecular axis of the ligand. Further information about geometries and structures of triiodide anions in 2 and 3 was acquired by the analysis of their Raman spectra.

An air stable radical-bridged dysprosium single molecule magnet and its neutral counterpart: redox switching of magnetic relaxation dynamics.

A pair of related dinuclear lanthanide compounds, viz., [Dy(tmhd)3]2(bptz) (1) and {Cp2Co}{[Dy(tmhd)3]2(bptz)} (2) (tmhd = 2,2,6,6-tetramethyl-3,5-heptane dionate; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) are reported. These molecules represent the first study of rare earth ions in which an organic ligand bridged species is isolated in both the neutral and radical anion forms. Both compounds exhibit out-of-phase signals below 4 K. The radical compound is highly air stable, with minimal changes in magnetic behavior after exposure to the atmosphere for two weeks.

Photoactive Excited States in Explosive Fe(II) Tetrazine Complexes: A Time-Dependent Density Functional Theory Study

Time-dependent density functional theory was used to investigate optical absorption of novel Fe(II) coordination complexes with tetrazine ligands. These octahedral compounds absorb near-infrared (NIR) light and can be applied as secondary explosives with low laser-initiation thresholds compared to pentaerythritol tetranitrate. Herein, numerous ligand architectures are studied to determine relationships between molecular structure and optical absorption in order to tune the low-energy charge transfer (CT) band. Geometrical structures and vertical excitation energies calculated with the TPSSh density functional and 6-311G basis set are in excellent agreement with experiment, with a maximum deviation from UV–vis spectra of 0.10 eV. By altering molecular substituents of the ligand scaffold, the CT band can be tuned between 500 and 1100 nm. Additional conjugation in the ligand scaffold pushes the CT band into the NIR region of the spectrum. Triazolo-tetrazine ligands shift the CT band by approximately 0.70 eV ...

Correlating the Structural, Electronic, and Explosive Sensitivity Properties of CuII Tetrazine Complexes

The synthesis and characterization of explosive coordination complexes of CuII with nitrogen‐rich tetrazine ligands and nitrate counter‐anions have been achieved. The complexes adopt a variety of coordination geometries including monomeric and dimeric architectures with five and six coordinate CuII centers. The thermal stabilities of the complexes correlate to the strength of the Cu–nitrate interaction such that more strongly bound nitrates exhibit higher thermal stability. Exchanging amine groups on the coordinating ligands with 3,3′‐dinitroazetidine groups led to changes in the solid‐state structures and increases to the impact sensitivity of the resulting complexes relative to the corresponding amine derivatives.

Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes: Cooperativity and Mechanism of Motion.

The existence of two rings in [3]pseudorotaxanes presents opportunities for those rings to undergo double switching and cooperative mechanical coupling. To investigate this capability, we identified a new strategy for bringing two rings into contact with each other and conducted mechanistic studies to reveal their kinetic cooperativity. A redox-active tetrazine ligand bearing two binding sites was selected to allow for two mobile copper(I) macrocycle ring moieties to come together. To realize this switching modality, ligands were screened against their ability to serve as stations on which the rings are initially parked, ultimately identifying 5,5'-dimethyl-2,2'-bipyridine. The kinetics of switching a macrocycle in a single-site [2]pseudorotaxane between bipyridine and single-site tetrazine stations were examined using electrochemistry. The forward movement was rate-limited by the bimolecular reaction between reduced tetrazine and bipyridine [2]pseudorotaxane. Two bipyridines were then used with a double-site tetrazine to verify double switching of two rings. Our results indicated stepwise movements, with the first ring moving 4 times more frequently (faster) than the second. While this behavior is indicative of anticooperative kinetics, positive thermodynamic cooperativity sets the two rings in motion even though just one tetrazine is reduced with one electron. Double switching in this [3]pseudorotaxane uniquely demonstrates how a series of independent thermodynamic states and kinetic paths govern an apparently simple mechanical motion.

A dinuclear [{(p-cym)Ru(II)Cl}2(μ-bpytz˙(-))](+) complex bridged by a radical anion: synthesis, spectroelectrochemical, EPR and theoretical investigation (bpytz = 3,6-bis(3,5-dimethylpyrazolyl)1,2,4,5-tetrazine; p-cym = p-cymene).

The reaction of the chloro-bridged dimeric precursor [{(p-cym)Ru(II)Cl}(μ-Cl)]2 (p-cym = p-cymene) with the bridging ligand 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine (bpytz) in ethanol results in the formation of the dinuclear complex [{(p-cym)Ru(II)Cl}2(μ-bpytz˙(-))](+), [1](+). The bridging tetrazine ligand is reduced to the anion radical (bpytz˙(-)) which connects the two Ru(II) centres. Compound [1](PF6) has been characterised by an array of spectroscopic and electrochemical techniques. The radical anion character has been confirmed by magnetic moment (corresponding to one electron paramagnetism) measurement, EPR spectroscopic investigation (tetrazine radical anion based EPR spectrum) as well as density functional theory based calculations. Complex [1](+) displays two successive one electron oxidation processes at 0.66 and 1.56 V versus Ag/AgCl which can be attributed to [{(p-cym)Ru(II)C}2(μ-bpytz˙(-))](+)/[{(p-cym)Ru(II)Cl}2(μ-bpytz)](2+) and [{(p-cym)Ru(II)Cl}2(μ-bpytz)](+)/[{(p-cym)Ru(III)Cl}2(μ-bpytz)](2+) processes (couples I and II), respectively. The reduction processes (couple III-couple V), which are irreversible, likely involve the successive reduction of the bridging ligand and the metal centres together with loss of the coordinated chloride ligands. UV-Vis-NIR spectroelectrochemical investigation reveals typical tetrazine radical anion containing bands for [1](+) and a strong absorption in the visible region for the oxidized form [1](2+), which can be assigned to a Ru(II) → π* (tetrazine) MLCT transition. The assignment of spectroscopic bands was confirmed by theoretical calculations.

Two- and Three-Electron Oxidation of Single-Site Vanadium Centers at Surfaces by Ligand Design.

Rational, systematic tuning of single-site metal centers on surfaces offers a new approach to increase selectivity in heterogeneous catalysis reactions. Although such metal centers of uniform oxidation states have been achieved, the ability to control their oxidation states through the use of carefully designed ligands had not been shown. To this end, tetrazine ligands functionalized by two pyridinyl or pyrimidinyl substituents were deposited, along with vanadium metal, on the Au(100) surface. The greater oxidizing power of the bis-pyrimidinyltetrazine facilitates the on-surface redox formation of V(3+), compared to V(2+) when paired with the bis-pyridinyltetrazine, as determined by X-ray photoelectron spectroscopy. This demonstrates the ability to control metal oxidation states in surface coordination architectures by altering the redox properties of organic ligands. The metal-ligand complexes take the form of one-dimensional polymeric chains, resolved by scanning tunneling microscopy. The chain structures in the first layer are very uniform and are based on the same quasi-square-planar coordination geometry around single-site V with either ligand. Formation of a different, dimer structure is observed in the early stages of the second layer formation. These systems offer new opportunities in controlling the oxidation state of single-site transition metal atoms at a surface for new advances in heterogeneous catalysts.

Redox-active on-surface assembly of metal-organic chains with single-site Pt(II).

The formation and stabilization of well-defined transition-metal single sites at surfaces may open new routes to achieve higher selectivity in heterogeneous catalysts. Organic ligand coordination to produce a well-defined oxidation state in weakly reducing metal sites at surfaces, desirable for selective catalysis, has not been achieved. Here, we address this using metallic platinum interacting with a dipyridyl tetrazine ligand on a single crystal gold surface. X-ray photoelectron spectroscopy measurements demonstrate the metal-ligand redox activity and are paired with molecular-resolution scanning probe microscopy to elucidate the structure of the metal-organic network. Comparison to the redox-inactive diphenyl tetrazine ligand as a control experiment illustrates that the redox activity and molecular-level ordering at the surface rely on two key elements of the metal complexes: (i) bidentate binding sites providing a suitable square-planar coordination geometry when paired around each Pt, and (ii) redox-active functional groups to enable charge transfer to a well-defined Pt(II) oxidation state. Ligand-mediated control over the oxidation state and structure of single-site metal centers that are in contact with a metal surface may enable advances in higher selectivity for next generation heterogeneous catalysts.

A Pretargeted PET Imaging Strategy Based on Bioorthogonal Diels–Alder Click Chemistry

The specificity of antibodies have made immunoconjugates promising vectors for the delivery of radioisotopes to cancer cells; however, their long pharmacologic half-lives necessitate the use of radioisotopes with long physical half-lives, a combination that leads to high radiation doses to patients. Therefore, the development of targeting modalities that harness the advantages of antibodies without their pharmacokinetic limitations is desirable. To this end, we report the development of a methodology for pretargeted PET imaging based on the bioorthogonal Diels-Alder click reaction between tetrazine and transcyclooctene. A proof-of-concept system based on the A33 antibody, SW1222 colorectal cancer cells, and (64)Cu was used. The huA33 antibody was covalently modified with transcyclooctene, and a NOTA-modified tetrazine was synthesized and radiolabeled with (64)Cu. Pretargeted in vivo biodistribution and PET imaging experiments were performed with athymic nude mice bearing A33 antigen-expressing, SW1222 colorectal cancer xenografts. The huA33 antibody was modified with transcyclooctene to produce a conjugate with high immunoreactivity, and the (64)Cu-NOTA-labeled tetrazine ligand was synthesized with greater than 99% purity and a specific activity of 9-10 MBq/μg. For in vivo experiments, mice bearing SW1222 xenografts were injected with transcyclooctene-modified A33; after allowing 24 h for accumulation of the antibody in the tumor, the mice were injected with (64)Cu-NOTA-labeled tetrazine for PET imaging and biodistribution experiments. At 12 h after injection, the retention of uptake in the tumor (4.1 ± 0.3 percent injected dose per gram), coupled with the fecal excretion of excess radioligand, produced images with high tumor-to-background ratios. PET imaging and biodistribution experiments performed using A33 directly labeled with either (64)Cu or (89)Zr revealed that although absolute tumor uptake was higher with the directly radiolabeled antibodies, the pretargeted system yielded comparable images and tumor-to-muscle ratios at 12 and 24 h after injection. Further, dosimetry calculations revealed that the (64)Cu pretargeting system resulted in only a fraction of the absorbed background dose of A33 directly labeled with (89)Zr (0.0124 mSv/MBq vs. 0.4162 mSv/MBq, respectively). The high quality of the images produced by this pretargeting approach, combined with the ability of the methodology to dramatically reduce nontarget radiation doses to patients, marks this system as a strong candidate for clinical translation.

Bond elongation in the anion radical of coordinated tetrazine ligands: A crystallographic, spectroscopic and computational study of a reduced {Re(CO)3Cl} complex

The binuclear complex [(μ-Me2BPTZ)(Re(CO)3Cl)2] (1), where Me2BPTZ=3,6-(5-methyl-pyridyl)-1,2,4,5-tetrazine, can be reduced by addition of bis(η5-pentamethylcyclopentadienyl) iron(II) (decamethylferrocene, Fc∗), to obtain a stable radical anion form 1−. A single-crystal X-ray diffraction study of the radical anion (1−)(Fc∗+) was conducted and compared with a computational model of the same compound in the neutral and reduced states. As such, this work presents the first structural analysis of a reduced diimine ligand that is coordinated to {Re(CO)3Cl} moieties. Bond-length changes within the tetrazine ring system were consistent with previously reported examples of tetrazine radicals and with calculated structures that show clear elongation of the azo-type NN bond. Consistently atomic charge calculations indicate that the extra electron in the radical anion resides largely at the tetrazine core. A negligible change in the Re–Cl bond length is observed and computed.

On the laser ignition and initiation of explosives

Energetic materials are conventionally ignited by the application of heat to some part of an explosive target. This is most often provided by a flame or by electrical heating using a resistive wire. The material responds by thermally heating and starting a burning zone, which spreads out from the ignition point generating gas. In some cases this zone can accelerate (due to the effect of this gas) into a reactive shock wave, termed detonation. Lasers are used in a variety of applications to thermal heat a range of materials, and thus seem an obvious candidate to trigger chemical reaction in energetic ones. A light pulse offers several advantages over an electrical one, since it may be delivered down a path both immune to electrical effects and chemically stable. Thus, the triggering of safety apparatus (such as the firing of bolts on aircraft exits) represents a major thrust in the development of laser–triggered explosive devices. The energy of the pulse may be used in one of two ways to achieve these effects. In the first, the laser is shone directly upon the chemical medium, which absorbs the light at discrete wavelengths. In the second, it is used to vaporize a metallic flyer that is launched to impact on a target creating a high–pressure zone. Either mechanism starts the required reaction. However, when the pulse is delivered directly to the material, detonation is found to proceed immediately with no transition via burning. This makes the process inexplicable using present concepts. This review will address a range of the experiments conducted and theories developed for this novel field. However, it should be emphasized that the fundamental mechanisms remain to be fully explained making the application both academically stimulating as well as industrially important.

Metallomacrocycle complex bridged polymers: electronic structures of –[MacM(L)

The electronic structures of one-dimensional metallomacrocycles with bidentate bridging ligands, such as –[MacM(L)]n (Mac=phthalocyaninato dianion, tetrabenzoporphyrinato dianion; M=Fe(II), Ru(II), Os(II); L=dabco, pyrazine, triazine, tetrazine (tz), 4,4′-bipyridine, bipyridyacetylene, bis(4-pyridyl)ethylene, bis(4-pyridyl)bencene) have been studied using the tight-binding solid state extended Hückel. The results of this study have shown that the intrinsic semiconductivity properties depend on several factors: (i) the valence band is composed largely of the transition metal dxy orbital; and (ii) the conduction band is composed of a mixture between the macrocycle and bridged orbitals for systems formed by pyrazine, triazine, bipyridine, bipyridyacetylene, etc. However, this composition is different when the ligand tetrazine is used. Such a band is formed by the π system of tetrazine ligand.