Rh(III) porphyrins as building blocks for porphyrin coordination arrays: from dimers to heterometallic undecamers

The coordination chemistry of a Rh(III) porphyrin building block was investigated with a view to the construction of heterometallic arrays of porphyrins. The Rh(III) porphyrin was found to coordinate methanol in the solid state and weakly in CDCl(3) solution. Crystallization afforded five coordinate pi stacked Rh(III) porphyrins. The distribution of products from reaction of Rh(III) porphyrin with DABCO, 4,4'-bipyridine, and 4,4'-bipyrimidine could be displaced toward dimeric species by silica gel column chromatography or recrystallization which served to remove excess ligand. Weak coordination to nitriles was observed, although it was sufficiently strong to organize a dimeric complex of 5,5'-dicyano-2,2'-bipyridine in the solid state. Complexes with 4,4'-bipyrimidine and 5,5'-dicyano-2,2'-bipyridine possess uncoordinated chelating nitrogen atoms. Larger heterometallic porphyrin arrays were assembled using a combination of Sn(IV) and Rh(III) porphyrin coordination chemistry. A Sn(IV) porphyrin acted as a core around which were coordinated two isonicotinate groups, carboxylic acid functionalized porphyrins, or porphyrin trimer dendrons. Rh(III) porphyrins were coordinated to pyridyl groups at the periphery of these entities. In this way an eleven porphyrin array, with four different porphyrin metalation states, was assembled. The diamagnetic nature of both the Rh(III) and Sn(IV) porphyrins, the slow ligand exchange kinetics on the NMR time scale, and tight ligand binding permitted the porphyrin arrays to be analyzed by two-dimensional (1)H NMR techniques.     

Supramolecular tetrads containing Sn(IV) porphyrin, Ru(II) porphyrin, and expanded porphyrins assembled using complementary metal-ligand interactions

Two examples of supramolecular tetrads containing Sn(IV) porphyrin, expanded thiaporphyrins such as sapphyrin and rubyrin, and Ru(II) porphyrin assembled using non-interfering cooperative tin(IV)-oxygen and ruthenium(II)-nitrogen coordination properties are described. These are the first examples in which the expanded porphyrins are used as axial ligands. The tetrads were prepared by adopting one step as well as stepwise approaches. In a one pot approach, the mono meso-pyridyl dihydroxy Sn(IV) porphyrin, meso-hydroxyphenyl expanded thiaporphyrin, and Ru(II) porphyrin were reacted in benzene under refluxing conditions followed by column chromatographic purification on alumina to afford tetrads. In a stepwise approach, the axial bonding type of triads containing Sn(IV)porphyrin as central unit and expanded thiaporphyrins as axial ligands were synthesized first by reacting meso-pyridyl dihydroxy Sn(IV) porphyrin with meso-hydroxyphenyl expanded thiaporphyrin in benzene at refluxing temperature. In the next step, the triads were reacted with Ru(II) porphyrin under mild reaction conditions to afford tetrads in decent yields. Both methods worked efficiently and produced stable, soluble tetrads in decent yields. One-dimensional (1D) and two-dimensional (2D) NMR techniques were used to confirm the identity of these novel tetrads. Absorption and electrochemical studies indicated that the components in tetrads interact weakly and retain their individual characteristic features. The steady state photophysical studies revealed that the quantum yield of Sn(IV) porphyrin in tetrads was reduced significantly because of non-radiative decay pathways operating in these systems.     

Structural chemistry of organotin(IV) complexes

The amazing structural diversity in organotin compounds is discussed in the systems containing -O and -S donor ligands. It is demonstrated that there exist a fascinating range of structural diversity for organotin(IV) complexes, including differences in coordination number and molecular geometry. The difference in structure is correlated with the nature of tin and ligand bonded R groups. Despite the large number of different structures found in organotin(IV) carboxylates, there is limited range of coordination geometries about the Sn atom. The four coordinated Sn atom in triorganotin(IV) complexes is invariably distorted tetrahedral and five coordinated Sn is distorted trigonal bipyramidal. A large range has been observed for diorganotin carboxylate structures, where five, six and seven coordinate geometries have been reported. The Sn atom in mono-organotin has only been demonstrated to exist in distorted octahedral geometries (the single exception being a pentagonal bipyramidal geometry). In the case of organotin(IV) complexes of S donor ligands, it has been shown that there exists a rich diversity in Sn atom geometries and coordination modes of the sulfur donor ligands themselves. As in related carboxylate systems, the assignment of coordination numbers to the Sn centers in some compounds is controversial. As a general trend, it has been shown that, the overall coordination number at the Sn atom decreases with the increasing number of organic substituents at the Sn atom. This phenomenon is usually achieved by increased asymmetry in the mode of coordination of the sulfur donor ligands.     

Exploring Supramolecular Self‐Assembly of Tetraarylporphyrins by Halogen Bonding: Crystal Engineering with Diversely Functionalized Six‐Coordinate Tin(L)2–Porphyrin Tectons

This study targets the construction of porphyrin assemblies directed by halogen bonds, by utilizing a series of purposely synthesized Sn(axial ligand)₂–(5,10,15,20‐tetraarylporphyrin) [Sn(L)₂‐TArP] complexes as building units. The porphyrin moiety and the axial ligands in these compounds contain different combinations of complimentary molecular recognition functions. The former bears p‐iodophenyl, p‐bromophenyl, 4′‐pyridyl, or 3′‐pyridyl substituents at the meso positions of the porphyrin ring. The latter comprises either a carboxylate or hydroxy anchor for attachment to the porphyrin‐inserted tin ion and a pyridyl‐, benzotriazole‐, or halophenyl‐type aromatic residue as the potential binding site. The various complexes were structurally analyzed by single‐crystal X‐ray diffraction, accompanied by computational modeling evaluations. Halogen‐bonding interactions between the lateral aryl substituents of one unit of the porphyrin complex and the axial ligands of neighboring moieties was successfully expressed in several of the resulting samples. Their occurrence is affected by structural (for example, specific geometry of the six‐coordinate complexes) and electronic effects (for example, charge densities and electrostatic potentials). The shortest intermolecular I???N halogen‐bonding distance of 2.991 Å was observed between iodophenyl (porphyrin) and benzotriazole (axial ligand) moieties. Manifestation of halogen bonds in these relatively bulky compounds without further activation of the halophenyl donor groups by electron‐withdrawing substituents is particularly remarkable.     

Regioselective reactivity of an asymmetric tetravalent di[dihydroxotin(IV)] bis-porphyrin host driven by hydrogen-bond templation

Local molecular environment effects on the rates of ligand exchange at an asymmetric di[dihydroxotin(IV)] bis-porphyrin 5 are examined. The host 5 possesses four non-equivalent tin(IV)-ligand binding sites that are distinguished by their position relative to a shallow cavity, by the steric environment at each binding site and by electronic-structure differences between the constituent porphyrin and quinoxalinoporphyrin macrocycles. These design features of the asymmetric host are confirmed by X-ray crystal structure analysis. Binding experiments with monodentate carboxylic acids and bidentate dicarboxylic acids show significant differences in the rate of ligand exchange at each of the four tin(IV) binding sites. For monodentate carboxylic acids, binding preferentially occurs at the exterior porphyrin site. Further addition of carboxylic acid results in sequential binding at the quinoxalinoporphyrin sites and lastly at the interior site on the porphyrin, with high regioselectivity. These selective binding outcomes are immediately apparent by NMR spectroscopy. A series of 2D NMR spectroscopy experiments allowed identification of the preferred binding sites at the host. This positively identifies that steric hindrance and electron-withdrawing functionality on the porphyrin macrocycle impede ligand exchange. However, these effects are overcome by dicarboxylic acid guests, which form ditopic hydrogen-bond interactions between the intracavity hydroxo ligands in the initial stage of ligand exchange, leading to regioselective binding between the tin(IV) sites within the cavity. It is envisaged that the factors identified herein that define regioselective ligand exchange at host 5 will find wider application in supramolecular systems incorporating tin(IV) porphyrins.     

Synthesis,X-ray crystal structure,and electrochemistry of trans-bis(ferrocenecarboxylato)(tetraphenylporphyrinato)tin(IV)

trans-Bis(ferrocenecarboxylato)(5,10,15,20-tetraphenylporphyrinato)tin(IV) complex Sn(TPP)(FcCOO)₂ has been synthesized and fully characterized. The X-ray structural analysis of Sn(TPP)(FcCOO)₂ reveals that the tin(IV) center is octahedrally coordinated by the porphyrin occupying the square base and axial coordination of two ferrocenecarboxylato ligands in an anti orientation with respect to each other. The Fe(II) center of the ferrocenecarboxylato ligand lies 5.7 Å from the tin(IV) center of the porphyrin ring. The cyclic voltammogram of Sn(TPP)(FcCOO)₂ exhibits three distinctive redox couples consisting of one oxidative wave and two reductive waves due to the ferrocenecarboxylato ligands and the porphyrin ring, respectively.     

Quantum Chemical Study on the Structure and Energetics of Binary Ionic Porphyrin Complexes

We study the structural and energetic properties of binary ionic porphyrin molecular complexes [H₄TPPS₄]²⁻∙∙∙SnTP using quantum chemical techniques. As the axial ligands and the protonation of pyridine sites highly influence the structure and coordination of metal‐containing porphyrin, various structures of SnTP in the presence and absence of axial ligands and pyridine protons were considered. The constructed porphyrins were then made to interact face to face, and the formed complexes were optimized at the HF/STO‐3G level of theory. The stability and stack‐like interaction of the complexes were analyzed through interplanar spacing, planar angle, and edge‐to‐edge distance. The structural parameters emphasize the importance of axial ligands for the formation of stack‐like structures. The complex that contains axial ligands with pyridine protons, namely [H₄TPPS₄]²⁻∙∙∙[X'SnXTPH]⁴⁺, shows a perfectly stacked layer with a reasonable interplanar distance, which is confirmed from the calculated counterpoise interaction and deformation energies. The energetic parameters were found to correlate well with the obtained geometries. The molecular electrostatic potential (MEP) maps were obtained to infer the presence of nonbonded interaction between the binary ionic porphyrins.     

Structure elucidation of dicarboxylate complex of Sn porphyrin with a ring current effect model

A new simple model of porphyrin ring current effect was proposed based on a line current approximation. It can reproduce the porphyrin-induced shifts for several Sn(IV)(tpp) and Sn(IV)(oep) dicarboxylate complexes quite satisfactorily. Perpendicular arrangement of the aromatic rings in the diaromatic-carboxylate complexes of Sn(IV)(tpp) and Sn(IV)(oep) was clarified with this porphyrin ring current effect model. There are two structures, exo and endo, in solution in dinaphthalene-1- and 2- carboxylate complexes of Sn(IV)(tpp) and Sn(IV)(oep). The exo conformer is in dynamic equilibrium with the endo form in solution. Thermodynamic data of these conformational equilibria are given.     

Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.     

Noncovalent binding between fullerenes and protonated porphyrins in the gas phase

Noncovalent interactions between protonated porphyrin and fullerenes (C?? and C??) were studied with five different meso-substituted porphyrins in the gas phase. The protonated porphyrin-fullerene complexes were generated by electrospray ionization of the porphyrin-fullerene mixture in 3:1 dichloromethane/methanol containing formic acid. All singly protonated porphyrins formed the 1:1 complexes, whereas porphyrins doubly protonated on the porphine center yielded no complexes. The complex ion was mass-selected and then characterized by collision-induced dissociation with Xe. Collisional activation exclusively led to a loss of neutral fullerene, indicating noncovalent binding of fullerene to protonated porphyrin. In addition, the dissociation yield was measured as a function of collision energy, and the energy inducing 50% dissociation was determined as a measure of binding energy. Experimental results show that C?? binds to the protonated porphyrins more strongly than C??, and electron-donating substituents at the meso positions increase the fullerene binding energy, whereas electron-withdrawing substituents decrease it. To gain insight into π-π interactions between protonated porphyrin and fullerene, we calculated the proton affinity and HOMO and LUMO energies of porphyrin using Hartree-Fock and configuration interaction singles theory and obtained the binding energy of the protonated porphyrin-fullerene complex using density functional theory. Theory suggests that the protonated porphyrin-fullerene complex is stabilized by π-π interactions where the protonated porphyrin accepts π-electrons from fullerene, and porphyrins carrying bulky substituents prefer the end-on binding of C?? due to the steric hindrance, whereas those carrying less-bulky substituents favor the side-on binding of C??.     

Facile formation of a meso-meso linked porphyrin dimer catalyzed by a manganese(IV)-oxo porphyrin

A manganese(IV)-oxo porphyrin catalyzes C-C bond formation between zinc porphyrins at the meso-position with a two-electron oxidant to afford the meso-meso linked porphyrin dimer efficiently. The meso-meso linked dimer is formed via formation of the porphyrin radical cation, and the rate-determining step in the catalytic cycle is the formation of a manganese(IV)-oxo porphyrin with a two-electron oxidant.     

Preparation and structural characterization of [Ph3Sn(IV)] complexes with pyridine-carboxylic acids or hydroxypyridine, -pyrimidine and -quinoline

A number of [Ph₃Sn(IV)]⁺ complexes formed with ligands containing -OH (-CO), or -COOH group(s) and aromatic {N} donor atom have been prepared. The binding sites of the ligands were identified by FT-IR spectroscopic measurements. In the complexes containing hydroxy and carboxylate functions, the carboxylato group is coordinated to the organotin(IV) centres in monodentate or bridging bidentate manner. It was also found that in the hydroxypyridine and -pyrimidine complexes the [Ph₃Sn(IV)]⁺ moiety in most cases reacts with the phenolic form of the ligands. The rationalisation of the experimental ¹¹⁹Sn Mössbauer nuclear quadrupole splittings, |Δ_exp| - according to the point charge model formalism - together with the FT-IR data support the formation of trigonal bipyramidal (Tbp) or octahedral (O_h) molecular structures. Furthermore, X-ray diffraction analysis has been performed on the triphenyltin(IV)-3-phenolato-2(1H)-pyridinone-O,O′ single crystals. The penta-coordinated tin center exhibits a Tbp geometry. In case of 2-picolinic acid, a trans-phenylation was observed during the complexation, resulting [Ph₂Sn(IV)]²⁺ complex and Ph₄Sn(IV).     

钌卟啉轴向配合物的合成与结构表征

采用一种简单的合成方法, 以钌羰基卟啉(1)和二氧钌卟啉(2)为原料, 与不同的氨基酸酯作用, 得到系列单氨基酸酯羰基和双氨基酸酯为轴向配体的钌卟啉配合物(4a~4c和5a~5d). 所有化合物均经过红外光谱、 核磁共振谱和质谱等表征手段确证为目标化合物, 并用X射线单晶衍射测定了化合物4a的分子结构, 这对研究钌卟啉与生物大分子的相互作用具有重要的意义.     

Weak-field anions displace the histidine ligand in a synthetic heme peptide but not in N-acetylmicroperoxidase-8: possible role of heme geometry differences

We have recently reported that aquo and thioether complexes of the ferric cytochrome c heme peptide N-acetylmicroperoxidase-8 (FeIII-1) exhibit greater low-spin character than do the corresponding complexes of a synthetic, water-soluble, monohistidine-ligated heme peptide (FeIII-2; Cowley, A. B.; Lukat-Rodgers, G. S.; Rodgers, K. R.; Benson, D. R. Biochemistry 2004, 43, 1656-1666). Herein we report results of studies showing that weak-field ligands bearing a full (fluoride, chloride, hydroxide) or partial (phenoxide, thiocyanate) negative charge on the coordinating atom trigger dissociation of the axial His ligand in FeIII-2 but not in FeIII-1. We attribute the greater sensitivity of His ligation in FeIII-1 to weak-field anionic ligands than to weak-field neutral ligands to the following phenomena: (1) anionic ligands pull FeIII further from the mean plane of a porphyrin than do neutral ligands, which will have the effect of straining the His-Fe bond in FeIII-2, and (2) heme in FeIII-2 is likely to undergo a modest doming distortion following anion binding that will render the His-ligated side of the porphyrin concave, thereby increasing porphyrin/ligand steric interactions. We propose that ruffling of the heme in FeIII-1 is an important factor contributing to its ability to resist His dissociation by weak-field anions. First, ruffling should allow His to more closely approach the porphyrin than is possible in FeIII-2, thereby reducing bond strain following anion binding. Second, the ruffling deformation in FeIII-1, which is enforced by the double covalent heme-peptide linkage, will almost certainly prevent significant porphyrin doming.     

Properties of polyethylene glycol supported tetraarylporphyrin in aqueous solution and its interaction with liposomal membranes

5,10,15,20-Tetrakis(4-hydroxyphenyl)porphyrin was functionalized by covalent attachment of poly(ethylene glycol) (PEG) chains of various molecular weights, 350, 2000, and 5000 Da. The properties of PEG-functionalized tetraarylporphyrins in aqueous solution and their interactions with liposomes have been studied. Electronic absorption spectroscopy, dynamic light scattering, atomic force microscopy, and fluorescence quenching were used to monitor aggregation of porphyrin chromophores and behavior of the attached PEG chains in the aqueous solution. The tendency for aggregation of porphyrin chromophores in aqueous solution and the efficiency of fluorescence quenching by KI decrease with increasing length of PEG chain linked to the porphyrin ring. The experimental results indicate that polymer clusters are present in aqueous solution of all pegylated porphyrins. The interactions between the pegylated porphyrins and phosphatidylcholine liposomes in the aqueous solution were studied using the fluorescence methods. The apparent binding constants of porphyrin chromophores to liposomes were determined. The degree of binding was found to be dependent on the molecular weight of the attached polymer.     

Investigations of rotamers in diaxial Sn(IV)porphyrin phenolates—towards a molecular timepiece

An approach to the formation of molecular timepieces is outlined based on differentiating between rotamers in diaxial Sn(IV) porphyrin phenolates. Two models are explored in detail. The first explores how the rates of rotation of the diaxial ligands is discriminated based on steric hindrance of the two porphyrin macrocycle faces at low temperature. The second model explores a ‘stopwatch’ function based on the ligation of Ag(I) ions to a 5,15-dipyridylporphyrinato tin(IV) complex bearing 3-hydroxypyridine ligands. The complexation inhibits rotation of the axial ligand, a result, which can be reversed by precipitation of Ag(I) using tetraethylammonium bromide. X-ray crystallography has also been used to characterize two Ag(I) 5,15-dipyridylporphyrinato tin(IV)complexes. The two isoforms differ in their supramolecular organization. One structure is formed through a cofacial stack linking each porphyrin by Ag(I) coordination. The other displays a sheet-like coordination polymer structure.     

Reactions of collman complex with tin(IV) and germanium(IV) porphyris,formation of [tin(II) and germanium(II) porphyrins] tetracarbonyliron and crystal structure of [2,3,7,8,12,13,17,18-octaethylporphinato tin(II)] tetracarbonyliron

The action of Na₂Fe(CO)₄ with tin(IV) and germanium(IV) porphyrins affords metal(II) porphyrin complexes [(por)M(II)Fe(CO)₄] (por = porphyrinate, M - Sn(II) or Ge(II)). The molecular structure of [(oep)Sn(II)Fe(CO)₄] was solved by X-ray diffraction techniques. The molecular structure of [(oep)Sn(II)Fe(CO)₄] was solved by X-ray diffraction techniques : the Sn coordination is square pyramidal with the iron in axial position (Sn-Fe = 2.492(1)Å) whereas the Fe coordination is trigonal bipyramidal. Mössbauer parameters provide convincing evidence for the formal zero oxidation state of the iron atom.     

Fluorescence and absorption spectroscopic studies on the interaction of porphyrins with snake gourd (Trichosanthes anguina) seed lectin

The interaction of several free-base porphyrins and their corresponding copper(II) and zinc(II) derivatives with the galactose-specific lectin from snake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two apparently equivalent binding sites for the porphyrins. Association constants obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 x 10(5) M(-1), with the metalloporphyrins being seen to have higher affinity for the lectin compared with the free-base analogues. Both positively charged and negatively charged porphyrins bind to snake gourd seed lectin (SGSL) with comparable affinities, suggesting that binding occurs primarily via hydrophobic interactions. Further, binding of porphyrins is found to be largely unaffected by the presence of the sugar ligand, lactose, indicating that the binding sites for the carbohydrate and porphyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in vivo, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic therapy, which involve the use of porphyrins.     

Interfacial Molecular Assemblies of Metalloporphyrins with Two Trans or One Axial Ligands

Molecular assemblies of metalloporphyrins trans‐dichloro(5,10,15,20‐tetra‐p‐tolylporphyrinato)tin(IV) (SnCl2TPPMe) and trans‐dihydroxo(5,10,15,20‐tetra‐p‐tolylporphyrinato)tin(IV) (Sn(OH)2TPPMe), which have two trans axial ligands, as well 5,10,15,20‐tetrakis(4‐methoxyphenyl)‐21H,23H‐porphine iron(III) chloride (FeClTPPOMe) and 5,10,15,20‐tetraphenyl‐21H,23H‐porphine manganese(III) chloride (MnClTPP), which have one axial ligand, are interfacially organized by Langmuir and Langmuir–Blodgett (LB) techniques. SnCl2TPPMe and Sn(OH)2TPPMe form nanofibrous structures which can display distinct supramolecular chirality, although the molecular units themselves are achiral, while FeClTPPOMe and MnClTPP form irregular nanoparticles that display negligible supramolecular chirality. An interpretation in terms of the effects of the axial ligands is proposed for this interesting phenomenon. Moreover, compared with assemblies of the diprotonated free‐base porphyrins, which are fabricated by interfacial (air/2.4 M HCl) organization of free‐base porphyrin, those of SnCl2TPPMe and Sn(OH)2TPPMe display higher stability in terms of supramolecular chirality. These results indicate that the assembly properties of metalloporphyrins can essentially be affected by the axial ligands that are attached to their chromophores, and that stable chiral porphyrin supramolecular associations can be easily produced by using achiral metalloporphyrins bearing two trans axial ligands.     

Synthesis of urea picket porphyrins and their use in the elucidation of the role buried solvent plays in the selectivity and stoichiometry of anion binding receptors

The synthesis of alpha,alpha-5,10-diurea and alpha,alpha,alpha-5,10,15-triurea picket porphyrins are detailed in this report. In previous reports, these porphyrins, along with alpha,alpha,alpha,alpha-5,10,15,20-tetraurea picket porphyrin, were used to demonstrate the important role one buried solvent molecule plays in the selectivity and stoichiometry of binding inorganic anions. Building on prior work, this report discusses the results of acetate anion binding studies between tetra- and diurea picket porphyrins (the latter does not contain a buried solvent molecule in the anion-receptor complex), compares differences in thermodynamic data obtained from van't Hoff plots of a porphyrin anion receptor able to utilize buried solvent in its binding motif with one that does not, and compares the crystal structure of a tetraurea porphyrin 1-chloride anion complex that contains buried solvent with new X-ray crystal structures of tetraurea porphyrin 1-dichloride or bisdihydrogenphosphate anion complexes that contain no buried solvent. Data from our previous work, and the work described herein, demonstrates that one buried solvent molecule provides stability to the receptor-anion complex that is similar in energy to a moderately strong hydrogen bond.