Synthesis and charge-transfer dynamics in a ferrocene-containing organoboryl aza-BODIPY donor-acceptor triad with boron as the hub.

A N,N'-bis(ferroceneacetylene)boryl complex of 3,3'-diphenylazadiisoindolylmethene was synthesized by the reaction of an N,N'-difluoroboryl complex of 3,3'-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex was characterized by a variety of spectroscopic techniques, electrochemistry, and ultrafast time-resolved methods. Spectroscopy and redox behavior was correlated with the density functional theory (DFT) and time-dependent DFT calculations. An unexpected degree of coupling between the two Fc ligands was observed. Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7 ± 0.1 ps) photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a lifetime of 4.8 ± 0.1 µs.

Redox and photoinduced electron-transfer properties in short distance organoboryl ferrocene-subphthalocyanine dyads.

Reaction between ferrocene lithium or ethynylferrocene magnesium bromide and (chloro)boronsubphthalocyanine leads to formation of ferrocene- (2) and ethynylferrocene- (3) containing subphthalocyanine dyads with a direct organometallic B-C bond. New donor-acceptor dyads were characterized using UV-vis and magnetic circular dichroism (MCD) spectroscopies, NMR method, and X-ray crystallography. Redox potentials of the rigid donor-acceptor dyads 2 and 3 were studied using the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches and compared to the parent subphthalocyanine 1 and conformationally flexible subphthalocyanine ferrocenenylmethoxide (4) and ferrocenyl carboxylate (5) dyads reported earlier. It was found that the first oxidation process in dyads 2 and 3 is ferrocene-centered, while the first reduction as well as the second oxidation are centered at the subphthalocyanine ligand. Density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to probe the electronic structures and explain the UV-vis and MCD spectra of complexes 1-5. DFT-PCM calculations suggest that the LUMO, LUMO+1, and HOMO-3 in new dyads 2 and 3 are centered at the subphthalocyanine ligand, while the HOMO to HOMO-2 in both dyads are predominantly ferrocene-centered. TDDFT-PCM calculations on compounds 1-5 are indicative of the π → π* transitions dominance in their UV-vis spectra, which is consistent with the experimental data. The excited state dynamics of the parent subphthalocyanine 1 and dyads 2-5 were investigated using time-resolved transient spectroscopy. In the dyads 2-5, the initially excited state is rapidly (<2 ps) quenched by electron transfer from the ferrocene ligand. The lifetime of the charge transfer state demonstrates a systematic dependence on the structure of the bridge between the subphthalocyanine and ferrocene.

Electrochemistry and catalytic properties for dioxygen reduction using ferrocene-substituted cobalt porphyrins.

Cobalt porphyrins having 0-4 meso-substituted ferrocenyl groups were synthesized and examined as to their electrochemical properties in N,N'-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte. The examined compounds are represented as (Fc)n(CH3Ph)(4-n)PorCo, where Por is a dianion of the substituted porphyrin, Fc and CH3Ph represent ferrocenyl and/or p-CH3C6H4 groups linked at the four meso-positions of the macrocycle, and n varies from 0 to 4. Each porphyrin undergoes two reversible one-electron reductions and two to six one-electron oxidations in DMF, with the exact number depending upon the number of Fc groups on the compound. The first electron addition is metal-centered to generate a Co(I) porphyrin. The second is porphyrin ring-centered and leads to formation of a Co(I) π-anion radical. The first oxidation of each Co(II) porphyrin is metal-centered to generate a Co(III) derivative under the given solution conditions. Each ferrocenyl substituent can also be oxidized by one electron, and this occurs at more positive potentials. Each compound was investigated as a catalyst for the electoreduction of dioxygen when adsorbed on a graphite electrode in 1.0 M HClO4. The number of electrons transferred (n) during the catalytic reduction was 2.0 for the three ferrocenyl substituted compounds, consistent with only H2O2 being produced as a product of the reaction. Most monomeric cobalt porphyrins exhibit n values between 2.6 and 3.1 under the same solution conditions, giving a mixture of H2O and H2O2 as a reduction product, although some monomeric porphyrins can give an n value of 4.0. Our results in the current study indicate that appending ferrocene groups directly to the meso positions of a porphyrin macrocycle will increase the selectivity of the oxygen reduction, resulting in formation of only H2O2 as a reaction product. This selectivity of the electrocatalytic oxygen reduction reaction is explained on the basis of steric hindrance by the ferrocene substituents which prevent dimerization.

Hydroxo-bridged dicopper(II,III) and -(III,III) complexes: models for putative intermediates in oxidation catalysis.

A macrocyclic ligand (L(4-)) comprising two pyridine(dicarboxamide) donors was used to target reactive copper species relevant to proposed intermediates in catalytic hydrocarbon oxidations by particulate methane monooxygenase and heterogeneous zeolite systems. Treatment of LH4 with base and Cu(OAc)2·H2O yielded (Me4N)2[L2Cu4(µ4-O)] (1) or (Me4N)[LCu2(µ-OH)] (2), depending on conditions. Complex 2 was found to undergo two reversible 1-electron oxidations via cyclic voltammetry and low-temperature chemical reactions. On the basis of spectroscopy and theory, the oxidation products were identified as novel hydroxo-bridged mixed-valent Cu(II)Cu(III) and symmetric Cu(III)2 species, respectively, that provide the first precedence for such moieties as oxidation catalysis intermediates.

Synthesis, characterization, and electron-transfer processes in indium ferrocenyl-containing porphyrins and their fullerene adducts.

Three new indium(III) tetra- and penta(ferrocenyl)-substituted porphyrins of the general formula XInTFcP [X = Cl(-), OH(-), or Fc(-); TFcP = 5,10,15,20-tetraferrocenylporphyrin(2-); Fc = ferrocene] have been prepared and characterized by UV-vis, magnetic circular dichroism (MCD), (1)H, (13)C, 2D, and variable-temperature NMR spectroscopy, as well as elemental analysis. Molecular structures of the ClInTFcP, FcInTFcP, and FcInTFcP@4C60 complexes were determined by X-ray crystallography with the last compound being not only the first example of a C60 adduct to the organometallic porphyrins but also the first structure in which organometallic porphyrin antennas intercalated into four electron-transfer channels. The electronic structures and relative energies of individual atropisomers, as well as prospective electron-transfer properties of fullerene adducts of XInTFcP complexes, were investigated by the Density Functional Theory (DFT) approach. Redox properties of XInTFcP complexes were investigated using electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation approaches. Electrochemical experiments conducted in low-polarity solvent using noncoordinating electrolyte were crucial for the sequential oxidation of ferrocene substituents in XInTFcP compounds. In agreement with DFT calculations, the axial ferrocene ligand in FcInTFcP, with direct In-C σ-bond has a 240 mV lower oxidation potential compared to the first oxidation potential for equatorial ferrocene substituents connected to the porphyrin core. The first equatorial ferrocene oxidation process in all XInTFcP complexes is separated by at least 150 mV from the next three ferrocene based oxidations. The second, third, and fourth redox processes in the ferrocene region are more closely spaced. The addition of the bulky axial ferrocene ligand results in significantly larger rotational barriers for equatorial ferrocene substituents in FcInTFcP compared to the other complexes and leads to better defined redox waves in cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments. Mixed-valence compounds of the general formula [XInTFcP](n+) (n = 1, 2) were observed and characterized by spectroelectrochemical and chemical oxidation approaches. In all cases, the presence of the intense intervalence charge transfer (IVCT) bands associated with the oxidation of a single equatorial ferrocene substituent were detected in the NIR region confirming the presence of the iron-based mixed-valence species and suggesting long-range metal-metal coupling in the target systems. The resulting data from the mixed-valence [XInTFcP](n+) (n = 1, 2) complexes matched very closely to the previously reported MTFcP and metal-free poly(ferrocenyl)porphyrins and were assigned as Robin and Day Class II mixed-valence compounds.

Dithiolopyranthione Synthesis, Spectroscopy and an Unusual Reactivity with DDQ.

The bicyclic pyran thiolone tetrahydro-3αH-[1,3]dithiolo[4,5-ß]pyran-2-thione (3a) engages in a highly unusual fragmentation in the presence of DDQ. The pyran thiolone, 3a, was synthesized by chlorination of 3,4-dihydro-2H-pyran (1), followed by condensing with CS2 and NaSH. Reaction of 3a with DDQ generates the isomerized pyran thiolone tetrahydro-3αH-[1,3]dithiolo[4,5-ß]pyran-2-thione (3b) and 4-benzyl-5-(3-hydroxypropyl)-1,3-dithiole-2-thione (4) via a deep-seated rearrangement. The identity of 3b was confirmed by single crystal X-ray analysis: P21/c, a=5.807(9) Å, b = 12.99(2) Å, c = 11.445(15), ß=113.23(6)°. Mechanistic experiments and computational insight is used to explain the likely sequence of events in the highly unusual formation of 4. Collectively, these results establish fundamental reactivity patterns for further research in this area.

Cadmium(II) chloride, bromide and iodide complexes with 4,4'-bipyridazine: when are diazine and halide bridges (in)compatible?

In poly[di-µ-chlorido-µ-(4,4'-bipyridazine)-κ(2)N(1):N(1')-cadmium(II)], [CdCl(2)(C(8)H(6)N(4))](n), (I), and its isomorphous bromide analogue, [CdBr(2)(C(8)H(6)N(4))](n), (II), the halide atom lies on a mirror plane and the Cd(II) ion resides at the intersection of two perpendicular mirror planes with m2m site symmetry. The pyridazine rings of the ligand lie in a mirror plane and are related to each other by a second mirror plane perpendicular to the first. The compounds adopt the characteristic structure of the [M(II)X(2)(bipy)] type (bipy is bipyridine) based on crosslinking of [Cd(µ-X)(2)](n) chains [Cd-Cl = 2.5955 (9) and 2.6688 (9) Å; Cd-Br = 2.7089 (4) and 2.8041 (3) Å] by bitopic rod-like organic ligands [Cd-N = 2.368 (3)-2.380 (3) Å]. This feature is discussed in terms of supramolecular stabilization, implying that the periodicity of the inorganic chain [Cd···Cd = 3.7802 (4) Å in (I) and 3.9432 (3) Å in (II)] is favourable for extensive parallel π-π stacking of monodentate pyridazine rings, with centroid-centroid distances of 3.7751 (4) Å in (I) and 3.9359 (4) Å in (II). This is not the case for the longer iodide bridges, which cannot stabilize such a pattern. In poly[tetra-µ-iodido-µ(4)-(4,4'-bipyridazine)-κ(4)N(1):N(2):N(1'):N(2')-dicadmium(II)], [Cd(2)I(4)(C(8)H(6)N(4))](n), (III), the ligands are situated across a centre of inversion; they are tetradentate [Cd-N = 2.488 (2) and 2.516 (2) Å] and link successive [Cd(µ-I)(2)](n) chains [Cd-I = 2.8816 (3)-3.0069 (4) Å] into corrugated layers.

Comparative electronic structures and UV-vis spectra of tribenzosubporphyrin, tribenzomonoazasubporphyrin, tribenzodiazasubporphyrin, and subphthalocyanine: insight from DFT and TDDFT calculations.

Electronic structures, geometries, and vertical excitation energies of chloroboron subphthalocyanine, tribenzodiazasubporphyrin, tribenzomonoazasubporphyrin, and tribenzosubporphyrin were calculated using density functional theory (DFT) and time-dependent (TD) DFT coupled with polarized continuum model (PCM) approach. Molecular geometries calculated at the BP86/6-311G(d) level reveal bowl-shape, trigonal prismatic conformations for all compounds with a variable bowl-depth that depends on the number of meso-nitrogen atoms in corresponding molecule. TDDFT-PCM calculations predict that the Q-band should undergo gradual high-energy shift, while the B-band should undergo low-energy shift upon stepwise substitution of the meso-nitrogen atoms in subphthalocyanine toward tribenzosubporphyrin. The TDDFT-PCM predicted trend was rationalized on the basis of electronic structures of target macrocycles. When comparison between theory and experiment is available, TDDFT-PCM calculations are in qualitative and quantitative agreement with experimental data.

Photoinduced charge transfer in short-distance ferrocenylsubphthalocyanine dyads.

Two new ferrocenylsubphthalocyanine dyads with ferrocenylmethoxide (2) and ferrocenecarboxylate (3) substituents directly attached to the subphthalocyanine ligand via the axial position have been prepared and characterized using NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopies as well as X-ray crystallography. The redox properties of the ferrocenyl-containing dyads 2 and 3 were investigated using the cyclic voltammetry (CV) approach and compared to those of the parent subphthalocyanine 1. CV data reveal that the first reversible oxidation is ferrocene-centered, while the second oxidation and the first reduction are localized on the subphthalocyanine ligand. The electronic structures and nature of the optical bands observed in the UV-vis and MCD spectra of all target compounds were investigated by a density functional theory polarized continuum model (DFT-PCM) and time-dependent (TD)DFT-PCM approaches. It has been found that in both dyads the highest occupied molecular orbital (HOMO) to HOMO-2 are ferrocene-centered molecular orbitals, while HOMO-3 as well as lowest unoccupied molecular orbital (LUMO) and LUMO+1 are localized on the subphthalocyanine ligand. TDDFT-PCM data on complexes 1-3 are consistent with the experimental observations, which indicate the dominance of π-π* transitions in the UV-vis spectra of 1-3. The excited-state dynamics of the dyads 2 and 3 were investigated using time-correlated single photon counting, which indicates that fluorescence quenching is more efficient in dyad 3 compared to dyad 2. These fluorescence lifetime measurements were interpreted on the basis of DFT-PCM calculations.

Facile access to a series of large polycondensed pyridazines and their utility for the supramolecular synthesis of coordination polymers.

Domino cyclization of ketoenols and hydrazine leads to a series of polycondensed pyridazines, which reveal potential as rigid N-donor multidentate ligands for supramolecular synthesis of open coordination polymers.

Unexpected fluorescence properties in an axially sigma-bonded ferrocenyl-containing porphyrin.

Molecular structure, redox, and unexpected fluorescence properties of a tetraphenylporphyrin tin(iv) complex axially sigma-bonded with two ferrocene substituents were investigated using UV-vis, MCD, electro- and spectroelectrochemical methods as well as DFT calculations and X-ray crystallography.

1,2,4,5-tetrazine: an unprecedented micro4-coordination that enhances ability for anion pi interactions.

A series of framework coordination polymers reveals the use of 1,2,4,5-tetrazines as efficient bridging ligands towards silver(i) and copper(i) ions. All four nitrogen atoms were functional as lone pair donors leading to an unprecedented mu(4)-coordination of the ligands (1,2,4,5-tetrazine, ttz; 3,6-dimethyl-1,2,4,5-tetrazine, Me(2)ttz) in [Ag(ttz)(X)] (X = NO(3), ; ClO(4), ), [Ag(2)(Me(2)ttz)(NO(3))(2)] (), [Ag(2)(Me(2)ttz)(H(2)O)(2)(ClO(4))(2)] (), [Ag(3)(Me(2)ttz)(H(2)O)(2)(CF(3)SO(3))(3)] () and [Cu(4)Cl(4)(Me(2)ttz)] (). In and , micro(4)-tetrazines and silver ions (AgN(4), Ag-N 2.42-2.53 A) compose a 3D framework of {4(2);8(4)} topology. Structures and were based on disilver-tetrazine ribbons, while in mu(4)-tetrazines interconnect silver-triflate chains. In , micro(4)-ligands connect inorganic layers sustained by copper-chloride squares, hexa- and octagons (1.974(2) and 1.981(2) A). Multiple N-coordination to metal ions enhances the ability of the electron deficient tetrazine system for anionpi binding. Compounds and exhibit very short interactions of this type with corresponding OC(N) separations down to a record value of 2.78 A and Opi 2.61 A. For , pi-acidity of tetrazine was reflected by contacts Clpi of 3.30 A. Results of high level ab initio calculations (RI-MP2/aug-cc-pVTZ) were in good agreement with experimental results, and were suggestive of the progressive enhancement of the pi-acidity by increasing the number of Ag(i) ions N-coordinated to tetrazine.

Copper(II) bromide and copper(II) acetate complexes of 4,4'-(p-phenylene)bipyridazine.

4,4'-(p-Phenylene)bipyridazine, C(14)H(10)N(4), (I), and the coordination compounds catena-poly[[dibromidocopper(II)]-mu-4,4'-(p-phenylene)bipyridazine-kappa(2)N(2):N(2')], [CuBr(2)(C(14)H(10)N(4))](n), (II), and catena-poly[[[tetrakis(mu-acetato-kappa(2)O:O')dicopper(II)]-mu-4,4'-(p-phenylene)bipyridazine-kappa(2)N(1):N(1')] chloroform disolvate], {[Cu(2)(C(2)H(3)O(2))(4)(C(14)H(10)N(4))].2CHCl(3)}(n), (III), contain a new extended bitopic ligand. The combination of the p-phenylene spacer and the electron-deficient pyridazine rings precludes C-H...pi interactions between the lengthy aromatic molecules, which could be suited for the synthesis of open-framework coordination polymers. In (I), the molecules are situated across a center of inversion and display a set of very weak intermolecular C-H...N hydrogen bonds [3.399 (3) and 3.608 (2) A]. In (II) and (III), the ligand molecules are situated across a center of inversion and act as N(2),N(2')-bidentate [in (II)] and N(1),N(1')-bidentate [in (III)] long-distance bridges between the metal ions, leading to the formation of coordination chains [Cu-N = 2.005 (3) A in (II) and 2.199 (2) A in (III)]. In (II), the copper ion lies on a center of inversion and adopts CuN(2)Br(4) (4+2)-coordination involving two long axial Cu-Br bonds [3.2421 (4) A]. In (III), the copper ion has a tetragonal pyramidal CuO(4)N environment. The uncoordinated pyridazine N atom and two acetate O atoms provide a multiple acceptor site for accommodation of a chloroform solvent molecule by trifurcated hydrogen bonding [C-H...O(N) = 3.298 (5)-3.541 (4) A].

Hydrate frameworks involving the pyridazino[4,5-d]pyridazine unit as a multiple hydrogen-bond acceptor.

1,4,5,8-Tetramethylpyridazino[4,5-d]pyridazine trihydrate, C(10)H(12)N(4) x 3 H(2)O, (I), and 1,2,3,6,7,8-hexahydrocinnolino[5,4,3-cde]cinnoline tetrahydrate, C(12)H(12)N(4) x 4 H(2)O, (II), exhibit exceptional functionality of the condensed N(4)-heteroaromatic frame as a symmetric acceptor of four hydrogen bonds [N...O = 2.843 (2)-2.8716 (10) A]. Thus, all the N atoms of the electron-deficient and highly pi-acidic polynitrogen heterocycles function as lone-pair donors. In (I), all the molecular components lie on or across special positions; the site symmetry is 2/m for the organic and m2m and m for the two water molecules. In (II), the organic polycycle lies across a crystallographic inversion center. Both structures involve a hydrogen-bonded centrosymmetric water-pyridazine dimer as the basic supramolecular unit, which is integrated into two-dimensional [in (I)] and three-dimensional [in (II)] hydrate frameworks by hydrogen bonding with the additional water molecules [O...O = 2.744 (2)-2.8827 (19) A]. The hydrate connectivity exists in the form of an (H(2)O)(3) trimer in (I) and as a one-dimensional zigzag (H(2)O)(n) chain in (II).

Silver(I) ions bridged by pyridazine: doubling the ligand functionality for the design of unusual 3D coordination frameworks.

Nitrogen donor tetradentate ligands 4,4'-bipyridazine (bpdz) and pyridazino[4,5-d]pyridazine (pp) were prepared by inverse electron demand Diels-Alder cycloaddition reactions of 1,2,4,5-tetrazine. Examination of their behaviour towards silver(I) ions revealed a special potential of the ligands for the design of 3D coordination frameworks involving characteristic polynuclear and polymeric silver(I)-pyridazine motifs and multiple coordination of the ligands. Ag4(pp)5(ClO4)4 and Ag4(pp)5(SiF6)(BF4)(2).4H2O adopt a unique 3D trinodal 4,4,5-connected topology based upon five-fold coordination of the metal ions and tetradentate bridging function of the organic modules. Complexes Ag3(L)3(SO3CF3)3.nH2O and Ag4(L)3(X)4.nH2O (L = bpdz, pp; X = BF4-, 0.5SiF(6)(2-)) illustrate formation of highly-connected frameworks incorporating trinuclear clusters as an origin of the net connectivity. In the carboxylate complexes Ag2(L)(R(F)COO)2 (R(F) = CF3, C2F5, C3F7) the pyridazine and acido ligands act as complementary linkers for generation of 3D frameworks involving helicate motifs. Fused bicyclic pyridazine pp is a unique system combining very efficient sigmaN-donor ability and pronounced pi-acidity. The coordination frameworks commonly exhibit strong anion-pi interactions, including unprecedented examples of double anion-pi,pi binding that occur between pyridazino[4,5-d]pyridazine as a double pi,pi-receptor for geometry complementary SiF(6)(2-) anions.

4,4'-Bipyridazine: a new twist for the synthesis of coordination polymers.

A new polydentate ligand 4,4'-bipyridazine (4,4'-bpdz) was prepared by employing inverse electron demand cycloaddition of 1,2,4,5-tetrazine. A unique combination of structural simplicity, ampolydentate character and efficient donor properties towards Cu(I), Cu(II) and Zn(II) provide wide new possibilities for the synthesis of coordination polymers incorporating the 4,4'-bpdz module either as a bi-, tri- or tetradentate connector between the metal ions. 1D coordination polymers Cu(2)(4,4'-bpdz)(CH(3)CO(2))(4) x 4H(2)O and Zn(4,4'-bpdz)(NO(3))(2), and interpenetrated (4,4)-nets in [Cu(4,4'-bpdz)(2)(H(2)O)(2)]S(2)O(6) were closely related to 4,4'-bipyridine compounds. 1D "ladder-like" polymer Cu(2)(4,4'-bpdz)(3)(CF(3)CO(2))(4) and the unprecedented 3D binodal net ({8(6)}{6(3);8(3)}) in [Cu(3)(4,4'-bpdz)(6)(H(2)O)(4)](BF(4))(6) x 6H(2)O were based upon a combination of linear and angular organic bridges. Complex [Cu(3)(OH)(2)(4,4'-bpdz)(3)(H(2)O)(2){CF(3)CO(2)}(2)](CF(3)CO(2))(2) x 2H(2)O has a "NbO-like" 3D topology incorporating discrete dihydroxotricopper(II) clusters linked by tri- and tetradentate ligands. The tetradentate function of the 4,4'-bpdz ligand was especially relevant for copper(I) complexes, which adopt layered Cu(2)X(2)(4,4'-bpdz) (X = Cl, Br) or 3D chiral framework (X = I) structures based upon infinite (CuX)(n) chains. The electron deficient character of the ligand was manifested by short anion-pi interactions (O-pi 3.02-3.20; Cl-pi 3.35 A), which may be involved as a factor for controlling the supramolecular structure.

Silver(I) sulfate coordination polymers with 4,4'-bipyridazine and pyridazino[4,5-d]pyridazine.

Poly[[micro(4)-4,4'-bipyridazine-micro(5)-sulfato-disilver(I)] monohydrate], [[Ag(2)(SO(4))(C(8)H(6)N(4))].H(2)O](n), (I), and poly[[aqua-micro(4)-pyridazino[4,5-d]pyridazine-micro(3)-sulfato-disilver(I)] monohydrate], [[Ag(2)(SO(4))(C(6)H(4)N(4))(H(2)O)].H(2)O](n), (II), possess three- and two-dimensional polymeric structures, respectively, supported by N-tetradentate coordination of the organic ligands [Ag-N = 2.208 (3)-2.384 (3) A] and O-pentadentate coordination of the sulfate anions [Ag-O = 2.284 (3)-2.700 (2) A]. Compound (I) is the first structurally examined complex of the new ligand 4,4'-bipyridazine; it is based upon unprecedented centrosymmetric silver-pyridazine tetramers with tetrahedral AgN(2)O(2) and trigonal-bipyramidal AgN(2)O(3) coordination of two independent Ag(I) ions. Compound (II) adopts a typical dimeric silver-pyridazine motif incorporating two kinds of square-pyramidal AgN(2)O(3) Ag(I) ions. The structure exhibits short anion-pi interactions involving noncoordinated sulfate O atoms [O...pi = 3.041 (3) A].

Cadmium nitrate coordination polymers with substituted pyridazino[4,5-d]pyridazines.

catena-Poly[[aquabis(nitrato-kappa2O,O')cadmium(II)]-mu-1,2,3,6,7,8-hexahydrocinnolino[5,4,3-cde]cinnoline-kappaN1:kappaN6], [Cd(NO3)2(C12H12N4)(H2O)]n, (I), and catena-poly[[[bis(nitrato-kappa2O,O')cadmium(II)]-mu-2,2,7,7-tetramethyl-1,2,3,6,7,8-hexahydrocinnolino[5,4,3-cde]cinnoline-kappaN1:kappaN6] chloroform solvate], {[Cd(NO2)2(C12H12N4)].CHCl3}n, (II), are the first structurally examined cadmium-pyridazine coordination compounds. They possess one-dimensional polymeric structures supported by the bidentate bridging function of the cinnolino[5,4,3-cde]cinnoline ligands, which lie about inversion centres. The Cd atoms are seven-coordinated in (I) and six-coordinated in (II), involving two bidentate nitrate groups [Cd-O = 2.229 (2)-2.657 (2) A], two N atoms of the cinnoline ligands [Cd-N = 2.252 (2)-2.425 (2) A], and, additionally, a water O atom in (I) [Cd-O = 2.284 (2) A]. In (I), the coordinated organic and aqua ligands form an intramolecular O-H...N hydrogen bond [O...N = 2.730 (3) A].

Metal-organic frameworks exhibiting strong anion-pi interactions.

Coordination polymers offer a significant potential for applications in adsorption, guest and anion recognition and sensing. Their structure commonly provides binding sites for such specific interactions as pi-pi stacking and XH...pi hydrogen bonding. The latter reflects the ability of the pi-cloud to interact with positively polarized atoms. An electrostatic interaction between anionic species and electron deficient heterocycles, which parallels the above binding scheme, is also possible and very recently the existence of anion-pi interactions was proved in the solid state and in solution. This effect may be significant also for biomolecule/solution interfaces, as it occurs in protein structures. In fact, such interactions could be especially relevant for host-guest chemistry of coordination polymers, particularly for functionalization of hydrophobic crystal cavities and for the design of geometrically rigid anion receptors. However, typical electron deficient heterocycles such as 1,3,5-triazines and 1,2,4,5-tetrazines are very weak donors and they are hardly suitable for bridging metal ions and the generation of coordination frameworks. As a system that combines efficient donor properties towards transition metal ions and a pronounced ability for anion-pi interactions we have developed unsubstituted pyridazino[4,5-d]pyridazine, which was readily accessible by a novel one-pot synthesis involving inverse electron demand Diels-Alder cycloaddition (Scheme 1). Unusual anion binding properties of the ligand may be clearly related to its electron-deficiency (LUMO energy -1.591 vs. -0.288 eV for the parent pyridazine), influenced also by N-coordination to such Lewis acids as metal ions.

Self-assembly sodalite-like framework.

The dense packing of shape complementary N-donor molecules dominates the self-assembly of coordination polymers possessing sodalite-like topology and supports nanosize molecular cages.