Synthesis of electrochemically responsive TTF-based molecular tweezers: evidence of tight intramolecular TTF pairing in solution

We report the synthesis and conformational studies of TTF-containing molecular tweezers based on a 1,2,4,5-tetramethylbenzene scaffold. In the neutral form the tweezers are expected to adopt the closed conformation, while, upon oxidation, the open conformation should be preferred due to electrostatic repulsion between the oxidized TTF moieties. Cyclic voltammetry studies demonstrate electronic pairing with formation of mixed-valence [TTF]₂⁺ species and opening of the tweezers upon the full oxidation of the TTF groups. Variable-temperature (VT) NMR studies evidence tight intramolecular TTF pairing at low temperature. Molecular modeling studies showed clear preference for an open conformation of tweezers in a fully oxidized state.     

Donor-acceptor-donor triads incorporating tetrathiafulvalene and perylene diimide units: synthesis, electrochemical and spectroscopic studies

Three donor-acceptor-donor triads 1-3 consisting of tetrathiafulvalene units attached to perylene diimides by flexible and rigid spacers were synthesized and characterized. UV/vis spectroscopic and cyclic voltammetric results indicate that they all show negligible intramolecular charge transfer interaction in their ground states. As compared to the reference compound 21, triads 1-3 display reduced fluorescence and their fluorescence lifetimes are shortened, which is probably owing to the photoinduced electron transfer interactions between the PI units and TTF units. The different photophysical behaviors between 1 and 2 (and 3) might be due to their difference in the spatial separation of TTF and PI units. It is preliminarily found that the steric hindrance of the groups attached to TTF units can affect their photostability.     

Synthesis and characterization of half-sandwich iridium complexes containing 2,6(7)-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene and ancillary ortho-carborane-1,2-dichalcogenolato ligands

The binuclear half-sandwich iridium complexes {Cp^∗IrCl₂}₂(μ-2,6(7)-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene) (3) and {Cp^∗Ir[E₂C₂(B₁₀H₁₀)]}₂(μ-2,6(7)-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene) (E = S(5a), Se(5b)) were prepared from the reaction of [Cp^∗IrCl(μ-Cl)]₂ or the “pseudo-aromatic” half-sandwich iridium complex Cp^∗Ir[E₂C₂(B₁₀H₁₀)] (E = S(4a), Se(4b)) with a tetrathiafulvalene (TTF) derivative 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (2) at room temperature. The complexes (3, 5a and 5b) have been fully characterized by IR and NMR spectroscopy, as well as elemental analysis. And the molecular structures of 2 and 5a were established through X-ray crystallography. It is interesting that infinite tunnels are created by repeating ‘buckled bowl’ molecules of 5a.     

Binding studies of tetrathiafulvalene-calix[4]pyrroles with electron-deficient guests

The neutral meso-octamethylporphyrinogen derivative, tetraTTF-calix[4]pyrrole 1 (TTF=tetrathiafulvalene), acts as a multi-faceted receptor in that it interacts with an assortment of different guests in different ways. The conformation of receptor 1 can be reversibly switched between the 1,3-alternate conformation (i.e., 1, Fig. 1) and the cone conformation (i.e., 1·Cl⁻, Fig. 2) by the repetitive addition of chloride and sodium ions. In this paper, the results of detailed and systematic complexation studies involving both 1 and its chloride-bound complex, 1·Cl⁻, with a variety of guests are described. Receptor 1 binds quasi-planar nitroaromatic guests in its 1,3-alternate conformation, while release of these guests takes place upon addition of chloride anions. On the other hand, spherical fullerene guests are strongly bound by 1·Cl⁻. Finally, it was found that a bidentate guest, consisting of a quasi-planar 2,5,7-trinitro-9-dicyanomethylenefluorene moiety tethered to a spherical C₆₀ fullerene, could be recognized by receptor 1 in either its 1,3-alternate or its chloride-bound cone conformation, albeit through very different binding modes.     

The first methyl antimony linked dimeric tetrathiafulvalene and tetraselenafulvalenes

In quest of novel organic donors, dimeric tetrathiafulvalene (TTF) and tetraselenafulvalenes (TSFs) linked by a single or double methyl antimony bridge, MeSb(TTF)₂ (1), (MeSb)₂(TSF)₂ (cis-2 and trans-2), and MeSb(TSF)₂ (3), have been synthesized. Singly bridged 1 and 3 show three pairs of redox waves, whereas doubly bridged cis-2 and trans-2 show two pairs of redox waves similarly to TSF. The X-ray structural analyses of neutral crystals, 1, cis-2 and trans-2, have succeeded. In their structures, antimony and chalcogen atoms form close intermolecular contacts useful in constructing supramolecular networks.     

Synthesis and properties of new π-conjugated pyridine ligands with tetrathiafulvalene derivatives and the rhenium(I) tricarbonyl complexes

Three new π-conjugated pyridine ligands with redox-active tetrathiafulvalene (TTF) derivatives, L₁-L₃, have been synthesized and characterized. The Diels-Alder reaction is the key step for this multistep synthetic strategy. The crystal structures of L₁ and L₃ have been studied. The electrochemical and spectroscopic properties of these new ligands, as well as the corresponding tricarbonyl rhenium(I) complexes (ReL₁(CO)₃X, X = Cl, 5; X = Br, 6) have also been investigated.     

Synthesis and characterization of half-sandwich Rh, Ir complexes containing mono-thiolate-ortho-carborane ligand

Two binuclear complexes [Cp^∗M(Cl)Carb^S]₂ (Cp^∗ = η⁵-C₅Me₅, M = Rh (1a), Carb^S = SC₂(H)B₁₀H₁₀, Ir (1b)) were synthesized by the reaction of LiCarb^S with the dimeric metal complexes [Cp^∗MCl(μ-Cl)]₂ (M = Rh, Ir). Four mononuclear complexes Cp^∗M(Cl)(L)Carb^S (L = BuⁿPPh₂, M = Rh (2a), Ir (2b); L = PPh₃, M = Rh (4a), Ir (4b)) were synthesized by reactions of 1a or 1b with L (L = BuⁿPPh₂ (2); PPh₃ (4)) in moderate yields, respectively. Complexes 3a, 3b, 5a, 5b were obtained by treatment of 2a, 2b, 4a, 4b with AgPF₆ in high yields, respectively. All of these compounds were fully characterized by IR, NMR, and elemental analysis, and the crystal structures of 1a, 1b, 2a, 2b, 4a, 4b were also confirmed by X-ray crystallography. Their structures showed 3a, 3b and 5a, 5b could be expected as good candidates for heterolytic dihydrogen activation. Preliminary experiments on the dihydrogen activation driven by these half-sandwich Rh, Ir complexes were done under mild conditions.     

An investigation of the role of the disparate redox states of the tetrathiafulvalene unit in modulating hydrogen bonding interactions in solution

We have investigated the electrochemically controlled hydrogen bonding interactions between tetrathiafulvalene host 3 and guests 4 or 5. Stabilisation of the 3⁺ state is dependent upon the nature of the guest species, whereas both guests prevent precipitation of the electrochemically generated 3²⁺ species at the working electrode via hydrogen bonded molecular recognition processes.     

New synthetic route to polyphosphine ligands bearing 1,2,4,5-tetrakis(phosphino)benzene framework: structural characterizations of 1,4-(PPh2)2-2,5-(PR2)2-C6F2 (R = Ph, Pr, Et)

Reactions of 1,4-dibromo-2,5-difluorobenzene with two equivalents of lithium diisopropylamide at low temperature (T < −90 °C) followed by a quench with a slight excess of ClPPh₂ afford 1,4-dibromo-2,5-bis(diphenylphosphino)-3,6-difluorobenzene (1) in good yields. Reacting 1 with two equivalents of BuLi followed by a quench with a slight excess of ClPR₂ yield novel 1,2,4,5-tetrakis(phosphino)-3,6-difluorobenzenes 1,4-(PPh₂)₂-2,5-(PR₂)₂-C₆F₂ (R = Ph (2a); R = ⁱPr (2b); R = Et (2c)) in moderate yields. Compounds 1 and 2a-c were characterized by multinuclear NMR spectroscopy and elemental analyses. In addition, molecular structures of 2a-c have been determined by single crystal X-ray crystallography. Phosphorus atoms of PPh₂/PR₂ substituents in 2a-c are displaced from the plane of the central phenyl ring due to steric interactions with neighboring groups.     

Chiral molecular polygons based on the Pt-alkynyl linkage: Self-assembly, characterization, and functionalization

Treatment of 2,2′-diacetoxy-1,1′-binaphthyl-6,6′-bis(ethyne), L-H₂, with one equiv of trans-Pt(PEt₃)₂Cl₂ led to a mixture of different sizes of chiral metallocycles [trans-(PEt₃)₂Pt(L)]_n (n = 3-8, 1-6). Each of the chiral molecular polygons 1-6 was purified by silica-gel column chromatography and characterized by ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy, MS, IR, UV-Vis, and circular dichroism (CD) spectroscopies, size exclusion chromatography, and microanalysis. Chiral molecular square 2 was also characterized by single-crystal X-ray diffraction. The acetyl groups of 2 were readily deprotected under mild conditions to generate 2a which possesses exposed chiral dihydroxy functional groups. The dihydroxy groups were functionalized with n-octadecyl chains or Fréchet-type dendrons to generate dendritic molecules built on a chiral molecular square core. This work shows the potential of generating interesting functional supramolecular systems based on Pt-alkynyl chiral molecular polygons.     

A molecular-clip-based approach to cofacial zinc-porphyrin complexes

We have synthesized molecular-clip-based cofacial zinc-porphyrin complexes. The complexes are shown to be new efficient receptors for the complexation of 1,4-diazabicyclo[2,2,2]octane, acridinium ions, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 1,2,4,5-benzenetetracarboxylic dianhydride (BTDA). Large binding constants (K_asso) in the range of 2.0 × 10⁴-1.1 × 10⁸ M⁻¹ were obtained for the 1:1 complexes of molecular tweezers (3) and rectangle (8) by inserting the hosts between the two porphyrin rings. The values of K and the spectroscopic results suggest that 3 and 8 serve as effective building blocks to capture the hosts. Semi-empirical calculations show that the guests position themselves within the cleft of the bis-porphyrins.     

Synthesis, characterization, and ethylene polymerizations of various N-O chelated mono Cp titanium complexes

Various mono Cp^∗ type titanium mononuclear complexes (1-5) and dinuclear complexes (6 and 7) containing non-Cp type chelate ligand, picolinate group, which has multi-binding sites of N and O atoms were synthesized and fully characterized by ¹H and ¹³C NMR spectroscopy, mass spectroscopy, elemental analysis, and X-ray diffraction study. The 1 and 2/MMAO catalytic systems for ethylene polymerization exhibited a moderate activity and gave the polyethylenes with broad molecular weight distributions.     

CpCo(dithiolene) complexes of highly flexible oxddt ligand with two different Z-shaped and U-shaped structures

[CpCo(oxddt)] complex (2, oxddt = o-xylenediyldithioethylene-1,2-dithiolate, Cp = η⁵-cyclopentadienyl) was obtained from o-xylenediyldithioethylene-1,3-dithiol-2-one (OC(oxddt)) (1). 2 further reacted with diazoalkanes (N₂CHR) to form some alkylidene-bridged adducts [CpCo(CHR)(oxddt)] (R = H (3a), SiMe₃ (3b)). Adduct 3a further reacted with protic acids (HX) to give some S-methylated adducts [CpCo(X)(oxddt)(S-Me)] (X = Cl (4a), OCOCF₃ (4c)), followed by the Co-C bond cleavage in the three-membered cobaltathiirane ring. Two different Z-shaped and U-shaped molecular structures were observed by X-ray diffraction studies. In the former structure (Z), the dithiolene and o-xylylene planes are located at almost parallel position each other, and in the latter structure (U), both planes are not parallel but the o-xylylene moiety is located closer to the dithiolene plane than the Z-shaped one. The Z-shaped structure involves 1 and 2. The U-shaped structure involves 3a, 3b, 4a and 4c. Complex 1 showed a one-dimensional chain through intermolecular π-π interaction in the crystal. Complex 2 had a dimeric interaction between dithioethylenedithiolate moieties (S₂C₂S₂) in the oxddt. The SiMe₃ group in 3b was placed at an exo-position with respect to the cobaltadithiolene ring due to a steric hindrance from the U-shaped oxddt ligand. In 4a, the X and Me groups are located at the opposite side of the dithiolene plane (anti-form) but in 4c, both groups are presented at the same side of the dithiolene plane (syn-form). The NMR analysis of 4a in solution indicated existence of both anti- and syn-isomers (7:1).     

Regioselectivity in the 1,3-dipolar cycloaddition of adamantylidenefulvene and its modification by inclusion in cyclodextrins' solutions

The 1,3-dipolar cycloaddition of adamantylidenefulvene (1) with 2 equiv of nitrile oxides 2a-d gave 1/1 cycloadducts, 3a-d and 4a-d, as the major products, and four other 1/2 minor cycloadducts 5-8a,b. The ratios of 1/1 cycloadducts 3a-d to 4a-d in THF solution were about 1/1 in the four different nitrile oxides 2a-d studied and microwave was found to accelerate the reactions and enhance their yields. It is noteworthy that the regioselectivity of 3a/4a was enhanced to 71/29 in β-cyclodextrin (β-CD) aqueous solution compared to that of 40/60 in the absence of β-CD. The regioselectivity of 3b/4b was further enhanced to 99/1 when 4-tert-butylphenyl hydroximinoyl chloride (9b) was complexed with β-CD and then proceeded to react with 1; this is in sharp contrast with that of 33/67 in the absence of β-CD. The binding constant of 1·β-CD in acetone-d₆/D₂O (1/1) was determined to be 188±9 M⁻¹ by ¹H NMR titration experiments. The binding mode of 1·β-CD was further determined by ROESY experiment. Furthermore, molecular dynamic simulations were carried out to provide information of the complexation modes of 1·β-CD, 3a·β-CD, 4a·β-CD, 9a·β-CD, and 9b·β-CD. It was found that both steric and electrostatic effects play important roles in determining the regio- and stereochemistry of 1,3-dipolar cycloaddition of 1. Finally, β-CD is shown to serve as a chiral shift reagent to differentiate the enantiomers of 4a in ¹H NMR.     

Synthesis and characterization of heterometallic Rh/Ru complexes supported by 1,2-dichalcogenolato-o-carborane ligands

The 16-electron half-sandwich complexes Cp^∗Rh[E₂C₂(B₁₀H₁₀)] (E = S, 1a; Se, 1b) react with [Ru(COD)Cl₂]_x under different conditions to give different types of heterometallic complexes. When the reactions were carried out in THF for 24 h, the binuclear Rh/Ru complexes [Cp^∗Rh(μ-Cl)₂(COD)Ru][E₂C₂(B₁₀H₁₀)] (E = S, 2a; Se, 2b) bridged by two Cl atoms and the binuclear Rh/Rh complexes (Cp^∗Rh)₂[E₂C₂(B₁₀H₁₀)] (E = S, 3a; Se, 3b) with direct Rh-Rh bond can be isolated in moderate yields. [Ru(COD)Cl₂] fragments in 2a and 2b have inserted into the Rh-E bond. If the [Ru(COD)Cl₂]_x was reacted with 1a in the presence of K₂CO₃ in methanol solution, the product [Cp^∗Rh(COD)]Ru[S₂C₂(B₁₀H₁₀]] (4a), K[(μ-Cl)(μ-OCH₃)Ru(COD)]₄ (5a) and 3a were obtained. The B(3)-H activation in complex 4a was found. However, when the reaction between 1b and [Ru(COD)Cl₂]_x was carried out in excessive NaHCO₃, the carborane cage opened products {Cp^∗Rh[S₂C₂(B₉H₁₀)]}Ru(COD) (6b), {Cp^∗Rh[S₂C₂(B₉H₉)]}Ru(COD)(OCH₃) (7b) and 3b were obtained. All complexes were fully characterized by their IR, ¹H NMR and elemental analyses. The molecular structures of 2a, 2b, 3b, 4a, 5a, and 7b have been determined by X-ray crystallography.     

Zur Elektronenstruktur metallorganischer Komplexe der f-Elemente LIX. Spektrochemische und nephelauxetische Effekte sowie experimentorientierte MO-Schemata (im f-Bereich) pseudo-trigonal-planarer (substituierter) Tris(η-cyclopentadienyl)neodym(III)-Komplexe und eines THF-Monoaddukts

The absorption spectra of pseudo (ψ) trigonal planar Nd( η⁵-C₅H₄^tBu)₃ (1) and Nd(η⁵-C₅H₄SiMe₃)₃ (2) as well as ψ trigonal pyramidal [Nd(η⁵-C₅H₄^tBu)₃(THF)] (3) have been measured at room and low temperatures.From the spectra obtained, truncated crystal field (CF) splitting patterns of these compounds could be derived, and simulated by fitting the parameters of a phenomenological Hamiltonian.For 60, 57 and 74 assignments, reduced r.m.s.deviations of 24.7, 23.1 and 29.0 cm⁻¹ were achieved for complexes 1-3, respectively.On the basis of the CF parameters used, the global CF strengths experienced by the Nd³⁺ central ions of complexes 1-3, as well as the individual CF strengths associated with one [C₅H₄^tBu]⁻ or [C₅H₄SiMe₃]⁻ ligand, respectively, of homoleptic compounds 1 and 2 are estimated.The obtained Slater parameters F² and the spin-orbit coupling parameters ζ_4f allow the insertion of compounds 1-3 into truncated nephelauxetic and relativistic nephelauxetic series.Whereas adduct 3 exhibits a pronouncedly lower global ligand field strength as well as increased F² and ζ_4f values, the compounds 1 und 2 show (within experimental error) nearly identical values.Besides, the experimentally-based non-relativistic and relativistic molecular orbital schemes (in the f range) of complexes 1 and 3 are set up and compared with the as yet available results of quantum chemical model calculations.     

A close inspection of Ag(I) coordination to molecular baskets. A study of solvation and guest encapsulation in solution and the solid state

Four molecular baskets 1-4, each comprising a modular bowl-shaped platform and a set of distinctive aromatic gates, were synthesized. The gates were made to incorporate a nitrogen heteroatom at different positions, as in 2-4, or without a coordinating nitrogen, as in 1 (Fig. 1). For polydentate 3 and 4, the location of the nitrogen was revealed to have an effect on directing the basket's coordination to Ag(I) cation, and subsequent folding to enclose space. The folded geometries were shown to encompass a helical, Ag(I):3, or C_3v/C_2v symmetrical, Ag(I):4, dynamic arrangement of the gates (from density functional theory). For 2 and 1, however, the presence of Ag(I) caused the sole formation of oligomers and the absence of coordinating interactions, respectively. Variable temperature (VT) ¹H and ¹⁹F NMR measurements of Ag(I):3 did not provide direct evidence for the solvation of its inner space and the encapsulation of the BF_4⁻ counterion. Moreover, CH₃CN or CH₃NC substrates were not found inside of Ag(I):3. The finding is in contrast with the behavior of Cu(I):3, which is known to encapsulate these guests. The intriguing guest selectivity was accounted for by small structural and electronic differences of Ag(I)/Cu(I) folded baskets. The X-ray solid-state structural studies of 2, 3, and 4 revealed the basket's capacity to fill its inner space with small compounds. Thus, 3 was found with an ordered molecule of chloroform, while 4 contained molecules of CH₃OH and H₂O. The basket selectivity for enclosing and positioning guests, in the solid state, was deduced to be guided by their size and weak host-guest and guest-guest noncovalent interactions.     

Extended TTF-type donors fused with pyrazine units; synthesis and characterization

Pyrazinedicyanoethylthiotetrathiafulvalene, (pzdc-TTF) (1a), an extended TTF fused with a pyrazine moiety and also a dithiolene ligand precursor, was synthesized through a cross-coupling with triethyl phosphite between pyrazine-1,3-dithiole-2-thione (I) and 4,5-bis(2-cyanoethylthio)-1,3-dithiole-2-one (III). This reaction also yields to dipyrazine TTF derivative (1b) and 2,3,6,7-Tetrakis(2-cyanoethylthio)TTF (1c), resulting from the self-coupling reactions of the thione (I) and ketone (III). The crystal structure of 1a is composed by pairs of head to head donor stacks of pzdc-TTF molecules along b in opposite orientations. Single crystals of 1b revealed a new polymorph with a face-to-face π-stacking motif. The electrochemical properties of 1a studied by cyclic voltammetry (CV) in DMF, shows two single electron oxidation processes typical of TTF-based donors; one pair of asymmetric redox waves centered at 627 mV versus Ag/Ag⁺ is ascribed to the couple [pzdc-TTF]⁺/[pzdc-TTF]²⁺, and one pair of quasi-reversible redox waves centered at 430 mV is ascribed to the couple [pzdc-TTF]⁰/[pzdc-TTF]⁺.     

Thieno[2,3-a]carbazole-based donor–π–acceptor organic dyes for efficient dye-sensitized solar cells

New donor–π–acceptor organic dyes K-1 and K-2 containing thieno[2,3-a]carbazole as an electron donor were designed and synthesized for dye-sensitized solar cells (DSCs). Photophysical and electrochemical properties of K-dyes were investigated. DSCs based on K-dyes showed a high conversion efficiency of 6.6–6.7% with a J_sc of 12.40–12.49 mA cm⁻² and a V_oc of 0.70–0.71 V. The molecular geometry calculation indicated that the existence of thienocarbazole donor in K-dyes enhanced the molecular planarity compared to the carbazole analogue dye MK-3. As a result, DSCs based on K-dyes showed high IPCEs, perhaps due to efficient intramolecular charge transfer and electron injection from excited dye to TiO₂ conduction band.