Dimer 1a Research Articles

Chiral Cu(ii)-amino alcohol based complexes for asymmetric aza-Henry reaction of N-Ts imines

A series of chiral dimeric ligands 1A–C, 2A–B, 3A–B and 4A derived from (S)/(R) 1,1′-bi(2-naphthol)-bis-aldehyde/piperazine-bis-aldehyde and various aminoalcohols viz., (1R,2S)-(−)-2-aminodiphenylethanol, (1S,2R)-(−)-2-aminodiphenylethanol, (1R,2S)-1-amino-2,3-dihydro-1H-inden-2-ol and (R)-valinol were synthesized. In situ generated complexes 1A–C–, 2A–B–, 3A–B–, 4A–Cu(II)/Cu(I) of dimeric chiral ligands with different copper salts were used as catalysts for the asymmetric aza-Henry reaction of a variety of N-tosylimines as substrates with different nitroalkanes at RT to afford good yields of aza-Henry products (80% with respect to the imines) with excellent enantioselectivity (ee > 99%) in 24 h with nitromethane and high syn selective products with excellent enantioselectivity with nitroethane. The dimeric chiral Cu(II) complex 1A–Cu(II) retained its performance at the gram level and was expediently recycled for a number of times. The enantio-pure aza-Henry product was further used for the synthesis of (S)-levamisole (an anthelminthic agent) in good yield and ee in three steps.

Formation of 1,3‐Diphosphacyclobutadiene Complexes from Phosphaalkynes and Their Coordination Behavior

AbstractBy using [(Cp″′Co)2tol] (Cp″′ = η5‐C5H2tBu3, tol = toluene), a new synthesis of the 1,3‐diphosphete complexes [Cp″′Co(η4‐P2C2tBu2)] (1a) and [Cp″′Co(η4‐P2C2iPr2)] (1b) was found, and their reactions with the Lewis acids W(CO)4 and CuI halides were investigated. The reaction of 1b with W(CO)4(nbd) (nbd = norbornadiene) leads to the complex [(Cp″′Co)(μ,η4:1‐{PCiPr}2)]2W(CO)4 (2) with the two diphosphete complexes 1b coordinated trans to a central W(CO)4 moiety. The reactions of 1a with CuI halides result in different products depending on the stoichiometry. In a 1:1 ratio, the dimeric compounds 3a–c [(Cp″′Co)(μ,η4:1‐{PCtBu}2)Cu(μ‐X)]2 (3a: X = Cl; 3b: X = Br, 3c: X = I) were isolated, whereas an excess of CuX gave the two‐dimensional polymers [({Cp″′Co}{μ3,η4:1:1‐(PCtBu)2})2(Cu{μ3‐X})4]n (6a: X = Cl, 6b: X = Br, 6c: X = I). From the reaction of 1a and 1b with CuBr and CuI in a 1:2 or 2:3 ratio, the one‐dimensional polymers [({Cp″′Co}{μ3,η4:1:1‐(PCR)2})({Cp″′Co}{μ,η4:1‐(PCR)2})(Cu{μ3‐X})4]n (4a: X = Br, R = iPr, 4b: X = I, R = iPr, 4c: X = I, R = tBu) are formed. From the reaction of 1a with CuI, a second product, [({Cp″′Co}{μ3,η4:1:1‐(PCtBu)2})(Cu{μ3‐I})4(MeCN)2]n (5), was isolated by using CH3CN for crystallization. The products were comprehensively characterized by spectroscopic methods, X‐ray crystallography, and 31P magic angle spinning (MAS) NMR spectroscopy.

Structure, Luminescence, and Vapochromism of Bridged Cationic Copper(I) Dimers and Polymers

The dimeric complex of Cu(I) [Cu2(PPh3)4(MeCN)2(Bpy)](BF4)2 (1a, Bpy = 4,4′-dipyridyl) self-assembles in CH2Cl2 or acetone and shows intense photoluminescence (excitation λmax = 356 nm, emission λmax = 486 nm, ϕ = 0.47). The MeCN ligands are readily removed from 1a, producing [Cu2(PPh3)4(Bpy)](BF4)2 (1b) and altering the photophysical behavior (excitation λmax = 336 nm, emission λmax = 568 nm, ϕ = 0.07). The desolvated compound 1b reversibly absorbs many vapor phase nucleophiles, as revealed by thermogravimetry. Intense luminescence emission is restored for 1b/Nu, Nu = MeCN (1a), acetone, tetrahydrothiophene (THT), and Et2S. Films of 1b are produced when CH2Cl2 solutions of 1a are cast onto glass. The films also react with Nu vapor, again producing intense emission. The pyrazine-bridged dimer [Cu2(PPh3)4(MeCN)2(Pyz)](BF4)2 (2a) is produced in CH2Cl2, while [Cu2(PPh3)4(MeCN)(acetone)(Pyz)](BF4)2∙½[Cu2(PPh3)4(MeCN)2(Pyz)](BF4)2 (2b) is formed in acetone. The crystal structure of the polymer {[Cu(PPh3)2(Bpy)](BF4)∙acetone} n (3) is reported, as is the mixed polymer/dimer structure {[Cu(PPh3)2(Pyz)](BF4)} n ∙½n[Cu2(PPh3)4(BF4)2(Pyz)] (4b) in which the dimer units show Cu–FBF3 coordination. The Pyz dimer and the Pyz and Bpy polymers show much weaker luminescence behavior than that of Bpy dimers 1a and the related 1b/Nu adducts. Nucleophile adduct structures [Cu2(PPh3)4(THT)2(Bpy)](BF4)2∙½ethyl ether (5) and [Cu2(PPh3)4(Py)2(Bpy)](BF4)2∙CH2Cl2 (6) confirmed the coordination of one Nu per Cu to 1b.

Highly Efficient Liquid Membrane Transport of Dyes Using Calix[4]arene-Linked Triphenylene Dimers as Carriers

The extraction of dyes from aqueous phase to organic phase using calix[4]arene-linked triphenylene dimers (1a and 1b) with large π-cavity as extractants were examined. 1a and 1b showed outstanding extraction percentage for four common dyes [Orange G sodium salt (OG), Brilliant ponceau (BP), Methyl Green (MG), and Xylenol Orange disodium salt (XO)]. The highest extraction percentage of 1a was 95% for MG. Moreover, the abilities of liquid membrane transport of 1a and 1b for dyes were studied. The influences of pH value, concentration, solvent, stirring speed, and the temperature on liquid membrane transport was investigated to optimize the transport condition. The results of liquid membrane transport suggested 1a and 1b were excellent carriers for tested dyes with high fluxes. The high concentration in the source phase or membrane phase, CH2Cl2, high stirring speed, and low temperature were favorable for the efficiency of liquid membrane transport. The high extraction percentages and outstanding liquid membrane transport abilities indicated that 1a and 1b possessed excellent complexation for dyes based on the π-π stacking action between dyes and triphenylene units.

A Series of Asymmetrical Phthalocyanines: Synthesis and Near Infrared Properties

We report here the preparation of asymmetrical phthalocyanine dimers 1a–3a, which are endowed with novel charge transfer bands at 1,151–1,154 nm and strong NIR luminescences at 840–860 nm and 1,600–1,650 nm. Through H-bonding interaction, 1a–3a are inclined to self-assemble into hexrod nanotubes at the interface of CHCl3 and CH3OH. Our results provide further insights into the interaction in molecular dimers, and suggest that 1a–3a have potential application in magnets and supramolecular architectures.

Symmetric mesogenic twins derived from salicylaldimines

The synthesis, characterization, and mesomorphic properties of a series of symmetric dimeric twins derived from mesogenic salicylaldimines 1a–e are reported. In order to understand the structure–property relations, the bridged spacer (m=5, 6, 7, 8, 9), and alkoxy chains (n=6, 8, 10, 12, 14) were systematically varied. Except for 1a, all derivatives 1b–e exhibited mesophases whose mesomorphic properties strongly depended on the spacer parity and the alkoxy chains. Dimers 1a exhibited crystal or SmX phases. Other dimers 1b–e were truly mesogenic, giving enantiotropic SmC, SmA or/and N phases. Odd–even effect was observed on phase temperatures, and nematic phases were observed for derivatives with odd spacer, and a value of d/l=0.92 (1a; m=5, n=8)–0.84 (1c; m=7, n=14) was obtained by powder X-ray diffraction data, therefore, a monolayer structure in mesophase was proposed.

Undecenyl resorc[4]arene in the chair conformation as preorganized synthon for olefin metathesis

Tetramerization of (E)-2,4-dimethoxycinnamic acid ω-undecenyl ester with ethereal BF3 gave three stereoisomers 1a, 1b, and 1c, which were assigned as the chair, cone, and 1,2-alternate conformations, respectively. The chair conformation of 1a was confirmed by X-ray diffraction analysis, which also showed a peculiar self-assembly behavior in the crystal lattice, forming intercalated hydrophilic and hydrophobic layers (6–7 A thickness) as a consequence of strong CH–π interactions. Undecenyl resorc[4]arene 1a, which featured the simplest pattern of substituents, was submitted to olefin metathesis using the second-generation Grubbs complex as the catalyst. Depending on the reaction conditions, different products were isolated: a bicyclic alkene 2a (46%), a linear dimer 3a (5%), and a cross-linked homopolymer P1a (44%).

Carbohydrate-conjugated porphyrin dimers: Synthesis and photobiological evaluation for a potential application in one-photon and two-photon photodynamic therapy

We report the synthesis of bioconjugated zinc porphyrin dimers 1a–e designed as photosensitizers for one-photon and two-photon excited photodynamic therapy. These macrocycles are substituted with carbohydrate units (glucose, mannose, lactose) in order to target tumor cells over-expressing lectin membrane receptors. Polarity, singlet oxygen production and in vitro photocytotoxicity are studied to determine their photodynamic therapy potentiality.

Synthesis and Characterization of Triphenylphosphine Adducts of Ferrocene-Based Palladacycles and Their Performance in the Suzuki and Sonogashira Reactions with Bromo- and Chloroarenes

A new triphenylphosphine adduct of cyclopalladated ferrocenylpyridazine containing a chloride anion, 2a, has been synthesized from the reaction of the chloride-bridged palladacyclic dimer 1a with triphenylphosphine. The corresponding adducts 3a,b containing iodide anion have been readily prepared through anion exchange reactions of 2a,b with NaI in acetone. These complexes were characterized by elemental analysis, IR and 1H-NMR. Additionally, their crystal structures have been determined by X-ray diffraction and intermolecular C–H···X (Cl, Br, I) bonds were found in the crystals. The use of these palladacycles as catalysts for the Suzuki and Sonogashira reactions was examined. The complexes 2a,b exhibited higher catalytic activity than the corresponding 3a,b in the Suzuki reaction. However, the order of activity of adducts with varying halogen anions is 3a~3b > 2a~2b in the Sonogashira reaction.

Resorcinol Based Acyclic Dimeric and Cyclic Di- and Tetrameric Cyclodiphosphazanes: Synthesis, Structural Studies, and Transition Metal Complexes

The condensation reaction of resorcinol with cis-[ClP(μ-N(t)Bu)(2)PN(H)(t)Bu] produced a difunctional derivative 1,3-C(6)H(4)[OP(μ-N(t)Bu)(2)PN(H)(t)Bu](2) (1), whereas the similar reaction with [ClP(μ-N(t)Bu)](2) resulted in the formation of a 1:1 mixture of dimeric and tetrameric species, [{P(μ-N(t)Bu)}(2){1,3-(O)(2)-C(6)H(4)}](2) (2a) and [{P(μ-N(t)Bu)}(2){1,3-(O)(2)-C(6)H(4)}](4) (2b), which were separated by repeated fractional crystallization and column chromatography. The reaction of dimer 2a with H(2)O(2) and selenium produces tetrachalcogenides [{(O)P(μ-N(t)Bu)}(2){1,3-(O)(2)-C(6)H(4)}](2) (3) and [{(Se)P(μ-N(t)Bu)}(2){1,3-(O)(2)-C(6)H(4)}](2) (4), respectively. The reaction between the dimer (2a) and [Pd(μ-Cl)(η(3)-C(3)H(5))](2) or AuCl(SMe(2)) yielded the corresponding tetranuclear complexes, [{((Cl)(η(3)-C(3)H(5))Pd)P(μ-N(t)Bu)}(2){1,3-(O)(2)-C(6)H(4)}](2) (5) and [{(ClAu)P(μ-N(t)Bu)}(2){1,3-(O)(2)-C(6)H(4)}](2) (6) in good yield. The complexes 5 and 6 are the rare examples of phosphorus macrocycles containing two or more exocyclic transition metal fragments. Treatment of 1 with copper halides in 1:1 molar ratio resulted in the formation of one-dimensional (1D) coordination polymers, [(CuX){1,3-C(6)H(4){OP(μ-N(t)Bu)(2)PN(H)(t)Bu}}(2)](n) (7, X = Cl; 8, X = Br; 9, X = I), which showed the helical structure in solid state because of intramolecular hydrogen bonding, whereas similar reactions of 1 with 4 equiv of copper halides also produced 1D-coordination polymers, [(Cu(2)X(2))(2){1,3-C(6)H(4){OP(μ-N(t)Bu)(2)PN(H)(t)Bu}(2)}](n) (10, X = Cl; 11, X = Br; 12, X = I), but containing Cu(2)X(2) rhomboids instead of CuX linkers. The crystal structures of 1, 2a, 2b, 4, 7-9, and 12 were established by X-ray diffraction studies.

Antiferromagnetic coupling across a tetrametallic unit through noncovalent interactions

Three paramagnetic heterobimetallic lantern complexes of the form [PtM(tba)4(OH2)] (M = Fe, 1; Co, 2; Ni, 3; tba = thiobenzoate) have been prepared in a single-step, bench-top procedure. In all three cases, a lantern structure with Pt–M bonding is observed in solution and in the solid state. Compound 1 is a monomer whereas 3 exists as a dimer in the solid state via a Pt⋯Pt metallophilic interaction. Compound 2 has been characterized in forms with (2a, purple) and without (2b, yellow) Pt⋯Pt metallophilic interactions. The dimers 2a (J = −10 cm−1, based on the spin Hamiltonian Ĥ = −2J(SA·SB)) and 3 (J = −60 cm−1) exhibit antiferromagnetic coupling between the two first-row metal ions in the solid state via a Pt⋯Pt non-covalent metallophilic interaction. The electronic structure of C4v [PtM(tba)4], C2 [PtM(tba)4(OH2)], (M = Fe, Co, Ni) and D2 symmetry [PtM(tba)4(OH2)]2 M = Co, Ni, units have been studied with DFT calculations, confirming the relative spin-state energies observed and the antiferromagnetic exchange pathway through four dz2 orbitals. The compounds 2a and 3 are the first examples of antiferromagnetic coupling through an unbridged M⋯M contact.

Study of the different behaviour of thiazolin and thiazin indazole derivatives with palladium(II) acetate

The study of the reactivity of the ligands 2-(indazol-1-yl)-2-thiazoline ( 1) and 2-(indazol-1-yl)-1,3-thiazine ( 2) with palladium(II) acetate has allowed the isolation of different compounds. For the thiazolin indazole derivative ( 1) a cyclopalladated compound: (μ-OAc) 2[Pd( k 2- C, N-TnInA)] 2 ( 1a) with a bidentate [C(sp 2,phenyl),N] − ligand and a central “Pd(μ-OAc) 2Pd” unit was obtained. Treatment of the dimeric compound 1a with sodium acetylacetonate produced the mononuclear compound [Pd( k 2- O, O′-acac)( k 2- C, N-TnInA)] ( 1c). The reaction of 1a with LiCl followed by the addition of PPh 3 gave the compound [PdCl(PPh 3)( k 2- C, N-TnInA)] ( 1e) where the acetate groups were replaced by Cl − and PPh 3 ligands, and with the PPh 3 in a cis-arrangement to the metallated carbon atom. The solid-state structure of 1e was determined by single-crystal X-ray diffraction methods. The reaction of 2 with palladium(II) acetate produced, in contrast to ligand 1, the complex cis-[Pd(OAc) 2( k 2- N, N′-TzIn)] ( 2a) with a bidentate [ N, N′] ligand. Compound 2a reacted with LiCl giving cis-[PdCl 2( k 2- N, N′-TzIn)] ( 2b), where the acetate groups were exchanged by chloride ligands. DFT calculations suggest that the most favoured compound obtained from the reaction between 1 and palladium(II) acetate should be the cyclopalladated one, while for 2 the formation of the [ N, N′] coordinated complex is less unfavoured.

N‐Acetyl‐5‐arylalkoxytryptamine Analogs: Probing the Melatonin Receptors for MT1‐Selectivity

A series of melatonin analogs obtained by the replacement of the ether methyl group with larger arylalkyl and aryloxyalkyl substituents was prepared in order to probe the melatonin receptors for MT(1) -selectivity. The most MT(1) -selective agents 11 and 15 were substituted with a Ph(CH(2) )(3) or a PhO(CH(2) )(3) group. Compounds 11 and 15 displayed 11.5-fold and 11-fold higher affinity for the MT(1) receptors than for the MT(2) subtype. Interestingly, in our binding assay 11 and 15 have shown considerably higher MT(1) -affinity and selectivity than the reference ligand, the dimeric agomelatine 1a.

Regiospecific [2 + 2] photocyclodimerization of trans-4-styrylpyridines templated by cucurbit[8]uril

Addition of HCl accelerated the photocyclodimerization of trans-4-styrylpyridine 1a in methanol and increased the yield of syn-head-to-tail (syn-HT) dimer 2a through the effect of cation-π interactions between the pyridinium ion of one molecule and the phenyl group of the other. We examined the photoirradiation products of derivatives of 1a having alkyl substituents on the phenyl group (1b-1f). The effect of the alkyl substituent on product distribution was rather limited for the photoreaction in MeOH solutions. However, the substituents had a distinct effect on the product distribution for the photoreaction of the inclusion complexes of hydrochloride salts of trans-4-styrylpyridines with cucurbit[8]uril in aqueous solutions. Introducing an alkyl group at the 2- or 3-position of the phenyl group completely shifted the major product from the syn-HT dimer to the syn-head-to-head (syn-HH) dimer. By adjusting the balance of host-guest interactions and cation-π interactions between guest molecules through systematic changes in the substituents on the phenyl ring of trans-4-styrylpyridine, we could change the orientation of the reactant molecules in the host cavity, resulting in a change of the major regioisomer of the photocyclodimerization products.

Tri-p-tolylphosphine-cyclopalladated ferrocenylpyrimidine complexes: synthesis, crystal structures, catalytic activity, and a palladacycle cocrystallization adduct

Two new tri-p-tolylphosphine-cyclopalladated ferrocenylpyrimidine complexes 1 and 2 have been prepared in good yields by reaction of the corresponding chloride-bridged palladacyclic dimers 1a and 2a with tri-p-tolylphosphine. Complexes 1 and 2 were characterized by elemental analysis, IR, 1H NMR, and ESI–MS. Additionally, their crystal structures have been determined by X-ray diffraction, and intermolecular C–H···Cl hydrogen bonds were found in their crystals. Complex 1 was found to be a palladacycle cocrystallization adduct with {PdCl2[P(C6H4-p-Me)3]2}. These complexes were found to be efficient catalysts for the Suzuki reaction.

Dimeric and Trimeric Tetrathiafulvalenes with Strong Intramolecular Interactions in the Oxidized States

New dimeric and trimeric TTF derivatives with methylenedithio spacers (1a,b, 2a, and 2b) have been synthesized. X-ray structure analysis revealed that TTF units of the dimer 1b adopted distorted face-to-face overlapping arrangement both in intra- and intermolecular stacking. Cyclic voltammetric study indicated that trimeric 2a was in favor of taking di- and tetracationic states, while the dimeric 1a was in favor of taking a monocation. The absorption spectroscopic study suggested an existence of the strong face-to-face interaction particularly in di-, tri-, and tetracationic state of the trimeric TTF derivatives.

Ligand Self-Assembling through Complementary Hydrogen-Bonding in the Coordination Sphere of a Transition Metal Center: The 6-Diphenylphosphanylpyridin-2(1H)-one System

Motivated by previous findings which had shown that transition metal catalysts based on the 6-diphenylphosphanylpyridone ligand (6-DPPon, 2) display properties as a self-assembling bidentate ligand-metal complex, we have performed a thorough study on the bonding situation of this ligand, alone and in the coordination sphere of a late transition metal. Thus, combining a number of spectroscopic methods (UV-vis, IR, NMR, X-ray), we gained insights into the unique structural characteristics of 2. These experimental studies were corroborated by DFT calculations, which were in all cases in good agreement with the experimental results. The free ligand 2 prefers to exist as the pyridone tautomer 2A and dimerizes to the pyridone-pyridone dimer 4A in solution as well as in the crystal state. The corresponding hydroxypyridine tautomer 2B is energetically slightly disfavored (ca. 0.9 kcal/mol within the up-conformer relevant for metal coordination); hence, hydrogen bond formation within the complex may easily compensate this small energy penalty. Coordination properties of 2 were studied in the coordination sphere of a platinum(II) center. As a model complex, [Cl(2)Pt(6-DPPon)(2)] (11) was prepared and investigated. All experimental and theoretical methods used prove the existence of a hydrogen-bonding interligand network in solution as well as in the crystal state of 11 between one 6-DPPon ligand existing as the pyridone tautomer 2A and the other ligand occupying the complementary hydroxypyridine form 2B. Dynamic proton NMR allowed to determine the barrier for interligand hydrogen bond breaking and, in combination with theory, enabled us to determine the enthalpic stabilization through hydrogen-bonding to contribute 14-15 kcal/mol.

Synthesis of Polyhalogenated 4,4′-Bipyridines via a Simple Dimerization Procedure

Polyhalogenated 4,4'-bipyridines were conveniently synthesized in a single step starting from dihalopyridines. A mechanism was proposed on the basis of experiments performed with 2-chloro-5-bromopyridine 1a. 2-Chloro-4-lithio-5-bromopyridine A1 was produced via ortholithiation of 1a by using either LDA or t-BuLi bases. When LDA was used, dimer 3a containing two chlorines and two bromine atoms was formed predominantly accompanied by several byproducts whose structure and mechanism of formation are discussed. In the case of t-BuLi, although the major product was 2-chloropyridine 7, a new pyridone product 8 was formed that is probably the result of the dihydropyridine intermediate hydrolysis. The dimerization procedure involving LDA was employed to prepare a large number of halogenated 4,4'-bipyridines in moderate to good yields. In some specific cases, halogenated 3,4' and 2,4'-bipyridines were obtained in lower yields and their structures were unambiguously assigned by X-ray diffraction analysis.

Homogeneous Catalytic Dehydrocoupling/Dehydrogenation of Amine−Borane Adducts by Early Transition Metal, Group 4 Metallocene Complexes

The efficient catalytic dehydrocoupling of a range of amine-borane adducts, R'RNH x BH(3) (R' = R = Me 1a; R' = R = (i)Pr 1b; R' = Me, R = CH(2)Ph 1c) by a series of group 4 metallocene type precatalysts has been demonstrated. A reduction in catalytic activity was detected upon descending the group and also on substitution of the cyclopentadienyl (Cp) ligands with sterically bulky or electron-donating substituents. Precatalysts Cp(2)TiCl(2)/2(n)BuLi and Cp(2)Ti(PMe(3))(2), which are believed to act as precursors to [Cp(2)Ti], were found to promote the transformation of 1a to [Me(2)N-BH(2)](2) (3a) in a homogeneous catalytic process. Mechanistic studies identified the linear dimer Me(2)NH-BH(2)-NMe(2)-BH(3) (2a) as a reaction intermediate, which subsequently undergoes further catalytic dehydrogenation to form cyclic dimer 3a. Synthesis of the (2)H-isotopologues of 1a allowed the extraction of phenomenological kinetic isotope effects for 1a --> 2a and 2a --> 3a from initial rate data, which permitted the proposal of a catalytic cycle with plausible intermediates. Support for the presence of an active Ti(II) catalyst was provided by the lack of reactivity of Ti(III) complexes TiCl(3) and Cp(2)TiCl or Ti(0) in the form of THF soluble colloids or bulk Ti powder toward 1a or 1b. Modeling of the rates of consumption of 1a and formation of 3a during catalysis by Cp(2)Ti(PMe(3))(2) supported this conclusion and allowed the proposal of a two cycle, four step reaction mechanism. The proposed first cycle generates 2a in a two step process. In the second cycle, interaction of 2a with the same catalyst then results in a catalytic dehydrogenative ring closing reaction to form 3a, also in a two step process.

How light affects 5,10,15-tris(pentafluorophenyl)corrole

5,10,15-Tris(pentafluorophenyl)corrole slowly undergoes oligomerization when left in solution at room temperature, under air and ambient light. This transformation was studied in different solvents (chloroform, dichloromethane, and toluene) and the main resulting products were identified as the 3,3′-corrole dimer 2a and the 3,3′,17′,3′′-corrole trimer 3a.