Christine Stern

Supramolecular Chemistry of Metalloporphyrins

Supramolecular chemistry, defined as “chemistry outside a molecule”, is at the heart of the development of chemistry of complex systems, molecular devices, ensembles, and nanochemistry.1 This is the chemistry where molecules are able to self-organize, self-assemble, and self-control into systems and the components are often analogues to biological molecules. Metalloporphyrins and metallophthalocyanines are remarkable precursors in supramolecular chemistry, and the rapid development of this chemistry led to assemblies possessing various architectures and properties (photo-, electro-, and catalytic properties and others). Metalloporphyrins are one of the cornerstones on which the existence of life is based, and major biochemical, enzymatic, and photochemical functions depend on the special properties of a tetrapyrrolic macrocycle. However, metalloporphyrins are the only molecules as key elements that require assembly with other elements to form the supramolecular structure, that is, the working device. In natural systems, polypeptides define a given structural organization and hold all the moieties together. Such complex natural devices are not accessible by direct chemical synthesis so far, but their modeling, using simplified designs, has been actively exploited during the last decades. The rapid development of this new area of chemistry has promoted the understanding of the concepts of design and strategies of self-assembly of structures based on intermolecular interactions to result in natural and synthetic supramolecular complexes of metalloporphyrins. Synthetic metalloporphyrin complexes are often used as analogues of natural systems found in photosynthesis, oxygen carriers, and catalysts.2,3 Such research also led to the discovery of new applications of these systems, for example in photodynamic therapy, information storage devices or photoelectrical devices that transform energy in both directions (photocells and lightemitting diodes).4-6 An application of increasing importance is the use of metalloporphyrins as receptors, exploiting their ability to selectively form complexes which can sharply change the spectral properties.7,8 Using molecules that combine different receptor units such as porphyrins and † A.N. Frumkin Institute of Physical Chemistry and Electrochemistry. ‡ Université de Bourgogne. Irina Beletskaya was born in 1933 in Leningrad (USSR). She received her Diploma in 1955, her Ph.D. in 1958, and her Doctor of Chemistry degree in 1963, all at M.V. Lomonosov Moscow State University. The subject for the latter was “Electrophilic Substitution at Saturated Carbon”. She became a Full Professor in 1970. She is currently a head of the Laboratory of Organoelement Compounds at M.V. Lomonosov Moscow State University and a full member (Academician) of Russian Academy of Sciences. She was a recipient of the Lomonosov Prize (1979), the Mendeleev Prize (1982), and the Nesmeyanov Prize (1991). Irina Beletskaya is a chief editor of the Russian Journal of Organic Chemistry. She is the author of more than 1000 articles and a number of monographs. Her current scientific interests are focused on organoelement compounds, transition metal catalysis, and organocatalysis. Chem. Rev. 2009, 109, 1659–1713 1659

Bimetallic silicon chemistry: New opportunities in coordination and organometallic chemistry

Abstract Bimetallic complexes containing silyl or siloxy ligands may display unique structures and reactivity patterns that are directly related to a subtle interplay between the metals and the ligands. Access to this class of compounds will be discussed. The recently discovered hemilabile behaviour of the bridging –Si(OR)3 ligand in heterobimetallic complexes has led to a number of developments in our group and others that are reviewed here. This will include the chemistry of Ph2PCH2PPh2 (dppm)-stabilized Fe–Pd and Fe–Pt alkyl complexes which allow, under mild conditions, controlled insertion reactions of isonitriles, alkynes or CO/olefins into the metal–carbon bond. In some cases, silicon migration reactions leading to new μ-siloxycarbene complexes have been observed. Other reactions that will be presented are the alkyne insertion into metal–hydride bonds, fluorination of the Si(OR)3 ligand and the catalytic dehydrogenative coupling of stannanes HSnR3. By altering the nature of the assembling ligand (μ-PR2 vs. μ-dppm) but keeping the metals and the silyl ligand unchanged, the first examples of intramolecular silyl migration from one metal to another were discovered. Finally, the use of aminosilyl in place of alkoxysilyl ligands led to the formation of new silylene complexes, to unprecedented examples of metal-mediated substituent exchange reactions between phosphorus and silicon, and to the characterization of the first complexes containing a bridging aminosilyl ligand. Many of these reactions involve steps that are directly relevant to the mechanisms of currently investigated catalytic systems.

Photophysical Properties of a Rhodium Tetraphenylporphyrin-tin Corrole Dyad. The First Example of a Through Metal–Metal Bond Energy Transfer†

Abstract The luminescence spectroscopy study and the determination of the photophysical parameters for the M-M′-bonded rhodium meso-tetraphenylporphyrin-tin(2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) complex, (TPP)Rh-Sn(Me6Et2Cor) 1, was investigated. The emission bands as well as the lifetimes (τe) and the quantum yields (Φe; at 77 K using 2MeTHF as solvent) were compared with those of (TPP)RhI 2 (TPP = tetraphenylporphyrin) and (Me6Et2Cor)SnCl 3 (Me6Et2Cor = 2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) which are the two chemical precursors of 1. The energy diagram has been established from the absorption, fluorescence and phosphorescence spectra. The Rh(TPP) and Sn(Me6Et2Cor) chromophores are the energy donor (D) and acceptor (A), respectively. The total absence of fluorescence in 1 (while fluorescence is observed in the tin derivative 3) indicates efficient excited state deactivation, presumably due to heavy atom effect and intramolecular energy transfer (ET). The large decreases in τP and ΦP of the Rh(TPP) chromophore going from 2 to 1 indicate a significant intramolecular ET in the triplet states of 1 with an estimated rate ranging between 106 and 108 s−1. Based on the comparison of transfer rates with other related dyads that exhibit similar D-A separations and no M-M′ bond, and for which slower through space ET processes (102–103 s−1) operate, a through M-M′ bond ET has been unambiguously assigned to 1.

Unusual Formation of a Stable 2D Copper Porphyrin Network

Copper(II) 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrin was obtained and characterized by means of cyclic voltammetry, electron paramagnetic resonance, Fourier transform infrared, and UV-visible spectroscopy. Three crystalline forms were grown and studied by means of X-ray diffraction methods (single crystal and powder). The highly electron-withdrawing effect of phosphoryl groups attached directly to the porphyrin macrocycle results in a self-assembling process, with formation of a stable 2D coordination network, which is unusual for copper(II) porphyrins. The resulting 2D structure is a rare example of an assembly based on copper(II) porphyrins where the copper(II) central metal ion is six-coordinated because of a weak interaction with two phosphoryl groups of adjacent porphyrins. The other polymorph of copper(II) 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrin contains individual (isolated) porphyrin molecules with four-coordinated copper(II) in a distorted porphyrin core. This polymorph can be obtained only by slow diffusion of a copper acetate/methanol solution into solutions of free base 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrin in chloroform. It converts to the 2D structure after dissolution in chloroform followed by consecutive crystallizations, using slow diffusion of hexane. A six-coordinated copper(II) porphyrin containing two axially coordinated dioxane molecules was also obtained and characterized by X-ray diffraction crystallography. The association of copper(II) 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrin in solution was also studied.

Synthesis of meso-polyphosphorylporphyrins and example of self-assembling

Pd-catalyzed coupling reactions have been used to prepare meso-phosphorylporphyrins. A 2D metal-organic network formed via P horizontal lineO...Zn axial supramolecular coordination of 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrin is the first example of a 2D framework based on phosphorylporphyrin derivatives.

The skeleton of the staghorn coral Acropora millepora: molecular and structural characterization

The scleractinian coral Acropora millepora is one of the most studied species from the Great Barrier Reef. This species has been used to understand evolutionary, immune and developmental processes in cnidarians. It has also been subject of several ecological studies in order to elucidate reef responses to environmental changes such as temperature rise and ocean acidification (OA). In these contexts, several nucleic acid resources were made available. When combined to a recent proteomic analysis of the coral skeletal organic matrix (SOM), they enabled the identification of several skeletal matrix proteins, making A. millepora into an emerging model for biomineralization studies. Here we describe the skeletal microstructure of A. millepora skeleton, together with a functional and biochemical characterization of its occluded SOM that focuses on the protein and saccharidic moieties. The skeletal matrix proteins show a large range of isoelectric points, compositional patterns and signatures. Besides secreted proteins, there are a significant number of proteins with membrane attachment sites such as transmembrane domains and GPI anchors as well as proteins with integrin binding sites. These features show that the skeletal proteins must have strong adhesion properties in order to function in the calcifying space. Moreover this data suggest a molecular connection between the calcifying epithelium and the skeletal tissue during biocalcification. In terms of sugar moieties, the enrichment of the SOM in arabinose is striking, and the monosaccharide composition exhibits the same signature as that of mucus of acroporid corals. Finally, we observe that the interaction of the acetic acid soluble SOM on the morphology of in vitro grown CaCO3 crystals is very pronounced when compared with the calcifying matrices of some mollusks. In light of these results, we wish to commend Acropora millepora as a model for biocalcification studies in scleractinians, from molecular and structural viewpoints.

Acceleration of the through space S1 energy transfer rates in cofacial bisporphyrin bio-inspired models by virtue of substituents effect on the Förster J integral and its implication in the antenna effect in the photosystems

The singlet k(ET) for cofacial β-octaalkylporphyrin/bis(meso-aryl)porphyrin dyads increases linearly with the gap between the donor-acceptor 0-0 fluorescence peaks at 77 K.

Acetylenes Rearranging on Ruthenium–Porphyrinogen and Leading to Vinylidene and Carbene Functionalities

Through a proton-transfer reaction a porphyrinogen assists the transformation of terminal acetylenes into Ru-vinylidenes, which are the entry point to a variety of Ru-carbenes and Ru-cumulenes. The scheme (in which the porphyrinogen is stylized) shows the reversible interconversion of an acetylide into a divinylidene unit.

Electrochemical and Spectroelectrochemical Studies of Diphosphorylated Metalloporphyrins. Generation of a Phlorin Anion Product

Two series of diphosphoryl-substituted porphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The investigated compounds are 5,15-bis(diethoxyphosphoryl)-10,20-diphenylporphyrins (Ph)2(P(O)(OEt)2)2PorM and 5,15-bis(diethoxyphosphoryl)-10,20-di(para-carbomethoxyphenyl)porphyrins (PhCOOMe)2(P(O)(OEt)2)2PorM where M = 2H, Co(II), Ni(II), Cu(II), Zn(II), Cd(II), or Pd(II). The free-base and five metalated porphyrins with nonredox active centers undergo two ring-centered oxidations and two ring-centered reductions, the latter of which is followed by a chemical reaction of the porphyrin dianion to give an anionic phlorin product. The phlorin anion is electroactive and can be reoxidized by two electrons to give back the starting porphyrin, or it can be reversibly reduced by one electron at more negative potentials to give a phlorin dianion. The chemical conversion of the porphyrin dianion to a phlorin anion proceeds at a rate that varies with the nature of the central metal ion and the solvent. This rate is slowest in the basic solvent pyridine as compared to CH2Cl2 and PhCN, giving further evidence for the involvement of protons in the chemical reaction leading to phlorin formation. Calculations of the electronic structure were performed on the Ni(II) porphyrin dianion, and the most favorable atoms for electrophilic attack were determined to be the two phosphorylated carbon atoms. Phlorin formation was not observed after the two-electron reduction of the cobalt porphyrins due to the different oxidation state assignment of the doubly reduced species, a Co(I) π anion radical in one case and an M(II) dianion for all of the other derivatives. Each redox reaction was monitored by thin-layer UV-visible spectroelectrochemistry, and an overall mechanism for each electron transfer is proposed on the basis of these data.

Synthesis and Self-Organization of Zinc β-(Dialkoxyphosphoryl)porphyrins in the Solid State and in Solution

The first synthesis and self-organization of zinc β-phosphorylporphyrins in the solid state and in solution are reported. β-Dialkoxyphosphoryl-5,10,15,20-tetraphenylporphyrins and their Zn(II) complexes have been synthesized in good yields by using Pd- and Cu-mediated carbon-phosphorous bond-forming reactions. The Cu-mediated reaction allowed to prepare the mono-β-(dialkoxyphosphoryl)porphyrins 1 Zn-3 Zn starting from the β-bromo-substituted zinc porphyrinate ZnTPPBr (TPP = tetraphenylporphyrin) and dialkyl phosphites HP(O)(OR)(2) (R = Et, iPr, nBu). The derivatives 1 Zn-3 Zn were obtained in good yields by using one to three equivalents of CuI. When the reaction was carried out in the presence of catalytic amounts of palladium complexes in toluene, the desired zinc derivative 1 Zn was obtained in up to 72% yield. The use of a Pd-catalyzed C-P bond-forming reaction was further extended to the synthesis of β-poly(dialkoxyphosphoryl)porphyrins. An unprecedented one-pot sequence involving consecutive reduction and phosphorylation of H(2)TPPBr(4) led to the formation of a mixture of the 2,12- and 2,13-bis(dialkoxy)phosphorylporphyrins 5 H(2) and 6 H(2) in 81% total yield. According to the X-ray diffraction studies, 1 Zn and 3 Zn are partially overlapped cofacial dimers formed through the coordination of two Zn centers by two phosphoryl groups belonging to the adjacent molecules. The equilibrium between the monomeric and the dimeric species exists in solutions of 1 Zn and 3 Zn in weakly polar solvents according to spectroscopic data (UV/Vis absorption and NMR spectroscopy). The ratio of each form is dependent on the concentration, temperature, and traces of water or methanol. These features demonstrated that zinc β-phosphorylporphyrins can be regarded as new model compounds for the weakly coupled chlorophyll pair in the photosynthesis process.