Z‐DNA Recognition in B‐Z‐B Sequences by a Cationic Zinc Porphyrin

For the first time it has been shown by spectroscopic studies such as circular dichroism and UV/Vis that cationic zinc porphyrin serves as a selective spectroscopic sensor that is able to recognize short left‐handed Z‐DNA tracts embedded in the B‐Z‐B sequences.     

Novel Properties of Supramolecular Complexes Formed by Pairing Cationic Porphyrin and Anionic Metal-Oxo Cluster

There are numpous examples of supramolecular assemblies bound by ion pair attTngtion insolution l~8. In most cases, the properties of such complexes differ notably from those ofthe parent individual compounds. It is well known that charged porphyrins bindoppositely charged molecules with the aid of electfostatic interactions 8~ic. We nowreport some novel properties of supramolecular complexes formed byspontaneous association of cationic porphyrins (MTBPyP'+, M=H,, Zn) and anionicmetal-oxo …     

Formation of Discrete Ladders and a Macroporous Xerogel Film by the Zipperlike Dimerization of Meso–Meso‐Linked Zinc(II) Porphyrin Arrays with Di(pyrid‐3‐yl)acetylene

Metal porphyrins assemble to form a supramolecular architecture with a characteristic structure and characteristic properties and functions upon complexation with appropriate ligands. However, there are few applications of these assembly processes to the construction of polymeric porphyrin arrays with useful functionalities. In this study, we found that meso–meso‐linked Zn^II porphyrin arrays underwent zipperlike dimerization upon complexation with di(pyrid‐3‐yl)acetylene (DPA) in chloroform to form discrete double‐stranded porphyrin ladders. Similarly, the assembly of poly(zinc(II) porphyrinylene) with DPA gave a thermoresponsive gel, whose three‐dimensional network structure was so strong that a macroporous xerogel film was obtained.     

DNA‐Accelerated Catalysis of Carbene‐Transfer Reactions by a DNA/Cationic Iron Porphyrin Hybrid

A novel DNA‐based hybrid catalyst comprised of salmon testes DNA and an iron(III) complex of a cationic meso‐tetrakis(N‐alkylpyridyl)porphyrin was developed. When the N‐methyl substituents were placed at the ortho position with respect to the porphyrin ring, high reactivity in catalytic carbene‐transfer reactions was observed under mild conditions, as demonstrated in the catalytic enantioselective cyclopropanation of styrene derivatives with ethyl diazoacetate (EDA) as the carbene precursor. A remarkable feature of this catalytic system is the large DNA‐induced rate acceleration observed in this reaction and the related dimerization of EDA. It is proposed that high effective molarity of all components of the reaction in or near the DNA is one of the key contributors to this unique reactivity. This study demonstrates that the concept of DNA‐based asymmetric catalysis can be expanded into the realm of organometallic chemistry.     

Metal‐Free Activation in the Anionic Ring‐Opening Polymerization of Cyclopropane Derivatives

The successful activation observed when using Bu^tP₄ phosphazene base and thiophenol or bisthiols for the anionic ring opening polymerization (ROP) of di‐n‐propyl cyclopropane‐1,1‐dicarboxylate is described. Well‐defined monofunctional or difunctional polymers with a very narrow molecular weight distribution were obtained through a living process. Quantitative end‐capping of the propagating malonate carbanion was accessible by using either an electrophilic reagent such as allyl bromide or a strong acid such as HCl. Kinetics studies demonstrated a much higher reactivity compared to the conventional route using alkali metal thiophenolates.

     

On the Reactivity of Tetrakis(trifluoromethyl)cyclopentadienone towards Carbon‐Based Lewis Bases

The reactivitiy of tetrakis(trifluoromethyl)cyclopentadienone towards different C‐based Lewis bases, such as N‐heterocyclic carbenes (NHC), ylides and isonitriles, are reported. While sterically not hindered carbenes were found to yield kinetic adducts by regiospecific nucleophilic attack at the position adjacent to the carbonyl group of the ketone, bulkier nucleophiles afforded the thermodynamically more stable O‐bridged zwitterions. Interestingly, isonitriles were found to dimerize and trimerize under the same reaction conditions, forming bicyclic products that evolve differently depending on the nature of the substituents.     

Aza‐nido‐dodecaboranes and ‐borates from Aza‐closo‐dodecaboranes by the Addition of Neutral or Anionic Bases: Mechanism

The addition of neutral (L = py, NEt₃, NHEt₂, NH₂tBu) and anionic Lewis bases (X^‐ = OH^‐, Br^‐, N₃^‐, Me^‐, NHBu ^‐, NHtBu^‐, [FeCp(CO)₂]^‐) to aza‐closo‐dodecaboranes RNB₁₁H₁₁ ( 1 ) or to derivatives thereof with boron bound non‐hydrogen ligands yields nido‐clusters RNB₁₁H₁₁L or [RNB₁₁H₁₁X]^‐ or derivatives thereof, respectively, the non‐planar pentagonal aperture N—B4—B9—B8—B5 of which hosts a B8—B9 hydrogen bridge. The base is either bound to B8 ( 3 )or B4 ( 5 )or B2( 7 ). The structures of these adducts are concluded from ¹H and ¹¹B NMR including 2D‐NMR spectra, and in the case of MeNB₁₁H₁₁(NHEt₂) (type 3 ) also by a crystal structure analysis. With two of the adducts MeNB₁₁H₁₁L (L = py, NHEt₂), isomers of the type 3 , 5 , and 7 , and with two of the adducts, MeNB₁₁H₁₁(NH₂tBu) and {MeNB₁₁H₁₁[FeCp(CO)₂]}^‐, isomers of the type 3 and 7 could be identified. The position of boron‐bound ligands during the addition of bases to 1 shows, that only vertices of the ortho‐belt of 1 are involved in the opening process. A mechanism is made plausible that starts by the attack of the base at B2 of 1 and opening of the N‐B2 bond, denoted as a [3c, 1c]‐collocation, to give 2 with an endo‐H atom, whose migration into an adjacent bridge position and opening of a second B—N bond, denoted as a [3c, 2c]‐translocation, gives 3 ; this isomer can be transformed into 7 by a sequence of [3c, 2c]‐translocations via the isomers 4 , 5 , and 6 . The chiral type 3 species MeNB₁₁H₁₁L with L = NHEt₂, NH₂tBu undergo a rapid enantiomerization, for whose mechanism the exchange of the bridging and the amine‐H atom has been made plausible.     

The Structures of Uncommon Cationic N‐alkenyl Purine and Pyrimidine Bases

The structures of 1,3 bis‐(allyl)cytosinium bromide ( 1 ), 7,9 bis‐(allyl)adeninium bromide hydrate ( 2 ) and 3‐(butenyl)adeninium bromide ( 3 ) have been determined. 1 is dimeric via N―H…Br hydrogen bonds, which further associate into ribbons via weaker C―H…Br interactions. 2 is also dimeric, now from N―H…N hydrogen bonds, which build up further through a complex array of hydrogen bonds involving both the anion and water of crystallization (O―H…N, O―H…Br, C―H…Br). 3 does not dimerise but forms polymeric sheets via a series of N―H…N, C―H…N and C―H…Br hydrogen bonds.     

Water‐Soluble Electrostatic Complexes of C60‐Anchored Anionic Polymers and Cationic Porphyrins

The preparation of novel highly water‐soluble electrostatic complexes from C₆₀‐anchored multi‐armed poly(acrylic acid)s and cationic porphyrin derivatives was demonstrated. The UV‐vis absorption and photoluminescence (PL) of these complexes show that the absorption of soret band exhibits a remarkable red shift and the emission of the Q band indicates an apparent quenching effect in comparison with that of the parent cationic porphyrin. These results imply a remarkable ground state and excited state interaction between the porphyrin ring and the C₆₀ moiety.

     

One‐Dimensional,Cofacial Porphyrin Polymers Formed by Self‐Assembly of meso‐Tetrakis(ERE Donor) Zinc(II) Porphyrins

A series of meso‐tetrakis‐(ERE donor) zinc(II) porphyrins n Zn (ERE donor=4‐R‐3,5‐bis[(E)‐methyl]phenyl; 1 Zn: E=NMe₂, R=Br; 2 Zn: E=NMe₂, R=H; 3 Zn: E=OMe, R=Br; 4 Zn: E=OMe, R=H) have been synthesized in excellent yields. As a result of the combination of a Lewis acidic site and eight Lewis basic sites within one molecule, monomeric molecules of n Zn self‐assemble to form one‐dimensional porphyrin polymers [ n Zn]_∞ in the solid state, as confirmed for 1 Zn and 3 Zn by X‐ray crystallography. The coordination environment around the zinc(II) ions in these polymers is octahedral. They are ligated by four equatorial nitrogen atoms of the porphyrin and two apical E atoms (E=N, O) provided by the EBrE donor groups of adjacent n Zn molecules. Complexes 2 Zn and 4 Zn did not form single crystals, but solid‐state UV/Vis analysis points to the formation of similar structures. Solution UV/Vis and ¹H NMR spectroscopy indicated that interactions between 1 Zn and 2 Zn monomers in the polymers are stronger than between 3 Zn and 4 Zn monomers. Interestingly, they also revealed that the presence of a neighboring bromine atom in the EBrE donor groups has a considerable influence on the coordination properties of the benzylic N or O atoms. The zinc(II) ions of the porphyrins most likely adopt only hexacoordination in the solid state, owing to the unique predisposition of Lewis acidic and basic sites in the n Zn molecules. Several parameters of the aggregates, for example, the interplanar separation between porphyrins and the zinc–zinc distances, change as a function of the coordinating E groups. The high degree of modularity in their synthesis makes these zinc(II) porphyrins an interesting new entry in noncovalent multiporphyrin assemblies.     

Synthesis of a Phlorin from a Meso‐Fused Anthriporphyrin by a Diels–Alder Strategy

An anthracene‐containing meso‐fused carbaporphyrin, which has extended π‐conjugation pathways as compared to the corresponding naphthalene‐containing carbaporphyrin, has been synthesized. The weak global aromaticity of the anthriporphyrin also allowed its use as the diene for a Diels–Alder reaction with dimethyl acetylenedicarboxylate (DMAD). The resulting phlorin contains an interesting bicyclic structure. Moreover, to the best of our knowledge, this phlorin is the first Diels–Alder adduct of a diene forming part of the global π‐conjugation pathway of an aromatic porphyrinoid.     

Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C‐Metalated N‐Methylquinoxalinium or N‐Methylpyrazinium Ligands

The cationic cluster complexes [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L¹Me)]⁺ ( 3 ⁺; HL¹=quinoxaline) and [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L²Me)]⁺ ( 5 ⁺; HL²=pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐Lⁿ)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N‐heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris‐sec‐butylborohydride (K‐selectride) to give neutral products that contain novel nonaromatic N‐heterocyclic ligands. The following are the products that have been isolated: [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹Me₂)] ( 6 ; from 3 ⁺ and methyl lithium), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹HMe)] ( 7 ; from 3 ⁺ and K‐selectride), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²Me₂)] ( 8 ; from 5 ⁺ and methyl lithium), and [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²HMe)] ( 11 ; from 5 ⁺ and K‐selectride). Whereas the reactions of 3 ⁺ lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N‐methyl group, the reactions of 5 ⁺ give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N‐methyl group. The LUMOs and the atomic charges of 3 ⁺ and 5 ⁺ confirm that the reactions of these clusters with anionic nucleophiles are orbital‐controlled rather than charge‐controlled processes. The N‐heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face‐capping modes. Those of compounds 6 – 8 have the atoms of a ligand C?N fragment σ‐bonded to two Ru atoms and π‐bonded to the other Ru atom, whereas the ligand of compound 11 has a C? N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable‐temperature ¹H NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above ?93 °C, the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through a conformational change of the ligand CH₂ group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180° rotation of the entire organic ligand over a face of the metal triangle.