Solvent Effect on the Photoinduced Intramolecular Charge-Transfer and Redox Potentials of Three Difuranones.

We report solvent-dependent excited state properties of three difuranone derivatives with a quinoidal backbone by steady-state and lifetime fluorescence measurements and theoretical calculations. Remarkable bathochromic shifts in fluorescence with diminished intensity indicate the occurrence of strong intramolecular charge-transfer transitions in high polar solvents. Cyclic voltammetric redox potentials reveal an interesting variation of biradical characters of the compounds with increasing solvent polarity. Solvent polarity also significantly modulates the energy levels of the charge-transfer (CT) states, as observed from the combined analyses of redox potentials and photophysical data via the Rehm-Weller equation. When high polar solvents favor forward CT by a more exoergic driving force and stabilize the charge-separated states, the reverse CT process diminishes. Estimated free energies of activation for CT suggest that high polar solvents lessen the activation barrier. Calculated excited state energies of the compounds at the CAM-B3LYP/6-31+G* level fulfill the primary conditions required for singlet fission, a process that can substantially increase the efficiency of solar cells, and the crystal packing for compound 1 also reveals a favorable geometry for singlet fission.

A [4+2] Condensation Strategy to Imine-Linked Single-Crystalline Zeolite-Like Zinc Phosphate Frameworks.

Double-4-ring zinc phosphate (D4R), [Zn(dipp)(4-Py-CHO)]4 (2) (dipp=diiminopyridine), bearing four formyl groups, has been utilized as a building block (SBU) for the synthesis of a new class of imine-linked [4+2] COF-like polycrystalline zinc phosphate frameworks. Reactions of 2 with a series of linear aromatic diamines results in the formation of polycrystalline frameworks [Zn4 (dipp)4 (L)2 ]n (3-6) (L=L1 to L4 , diimines formed by condensation of 4-pyridine carboxaldehyde with diamines). Employing an alternative synthetic strategy, through a diffusion-controlled slow reaction of 2 with the pre-synthesized 4,4'-bispyridyl bisimine (L3 ), [Zn4 (dipp)4 (L3 )2 ]n (5') has been obtained as single crystals. Complex 5' is a 3D-framework, exhibiting a rare eightfold interpenetrated diamondoid network. The long spacer length (19.6 Å) results in extensive entanglement in 5'. Powder diffraction data suggest that these compounds are isoreticular 3D-frameworks. To study the effect of the relative position of pyridyl donors with respect to the central benzidine moiety, 3,3'-bispyridyl bisimine (L5 ) was investigated as the spacer. A slow reaction of 1 b with L5 leads to the isolation of a 2D-boxed-sheet coordination polymer [Zn4 (dipp)4 (L5 )2 ]n (7). Selective formation of 3D-framework 5' from L3 and the 2D-framework 7 from L5 is due to the angles created by the coordination of para- and meta-pyridyl nitrogen centers at the zinc centers of the D4R cubane. Compound 5' has been utilized as a catalyst for Knoevenagel condensation.

Elusive Double-Eight-Ring Zeolitic Secondary Building Unit.

The double-eight-ring (D8R), an elusive secondary building unit of zeolites, has been stabilized for the first time, both in solution and solid-state. The present study further establishes that any of the three double-ring building blocks of zeolites, viz. D4R, D6R and D8R ([ArPO3Zn(L)]n (n = 4, 6 or 8)), can be preferentially isolated (over the other two) through a careful choice of metal source, aryl phosphate and ancillary ligand, apart from maintaining a meticulous control on the reaction conditions.

Complex Structural Landscape of Titanium Organophosphonates: Isolation of Structurally Related Ti4, Ti5, and Ti6 Species and Mechanistic Insights.

[Ti(acac)2(OiPr)2] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti4(acac)4(µ-O)2(µ-tBuPO3)2(µ-tBuPO3H)4]·2CH3CN (1), pentanuclear [Ti5(acac)5(µ-O)2(OiPr)(µ-tBuPO3)4(µ-tBuPO3H)2] (2), hexanuclear [Ti6(acac)6(µ-O)2(OiPr)2(µ-tBuPO3)6] (3), or [Ti6(acac)6(µ-O)3(OiPr)(µ-tBuPO3)5(µ-tBuPO3H)]·2CH3CN (4). The isolation of each of these products in pure form depends on the molar ratio of the reactants or the solvent medium. Among these, 3 is obtained as the only product when the reaction is conducted in CH2Cl2. The structural analysis reveals that a simple cluster growth route relates the clusters 1-4 to each other and that a reactive cyclic single-4-ring titanophosphonate [Ti(acac)(OiPr)2(tBuPO3H)]2 is the fundamental building block. While the tetranuclear 1 has structural resemblance to the D4R building block of zeolites, the hexanuclear clusters 3 and 4 have the shape of zeolitic D6R building blocks. The presence of adventitious water in the phosphonic acid (arising from small quantities of hydrogen-bonded water) results in the formation of µ-O2- bridges across an adjacent pair of titanium centers in clusters 1-4. To further verify the stability of the hexanuclear cluster over other structural forms, the reaction of tBuPO3H2 was performed with [Ti(acac)2(O)], instead of Ti(acac)2(OiPr)2, in CH3CN to yield [Ti6(acac)6(µ-O)4(µ-tBuPO3)4(µ-tBuPO3H)2]·2CH3CN (5). Compound 5 exhibits a core structure similar to those of 3 and 4 with small variations in the intracluster Ti-O-Ti linkage. Compound 3 is an efficient and selective catalyst for olefin epoxidation under both homogeneous and heterogeneous conditions.

A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions.

Reaction of Zn(OAc)2 ⋅2 H2 O with 2,6-diisopropylphenyl phosphate (dippH2 ) in the presence of pyridine-4-carboxaldehyde (Py-4-CHO) in methanol resulted in the isolation of a tetrameric zinc phosphate cluster [Zn(dipp)(Py-4-CH(OH)(OMe))]4 ⋅4 MeOH (1) with four hemiacetal moieties stabilized on the double-4-ring inorganic cubane cluster. The change of solvent from methanol to acetonitrile leads to the formation of [Zn(dipp)(Py-4-CHO)]4 (2), in which the coordinated Py-4-CHO retains its aldehydic form. Dissolution of 1 in CD3 CN readily converts it to the aldehydic form and yields 2. Similarly 2, which exists in the aldehyde form in CD3 CN, readily converts to the hemiacetal form in CD3 OD/CH3 OH. Compound 1 is an unprecedented example in which four hemiacetals have been stabilized on a single molecule in the solid state retaining its stability in solution as revealed by its (1) H NMR spectrum in CD3 OD. The solution stability of 1 and 2 has further been confirmed by ESI-MS studies. To generalize the stabilization of multiple hemiacetals on a single double-four-ring platform, pyridine-2-carboxaldehyde (Py-2-CHO) was used as the auxiliary ligand in the reaction between zinc acetate and dippH2 , leading to isolation of [Zn(dipp)(Py-2-CH(OH)(OMe))]4 (3). Understandably, recrystallization of 3 from acetonitrile yields the parent aldehydic form, [Zn(dipp)(Py-2-CHO)]4 (4). Single-crystal X-ray diffraction studies reveal that supramolecular bonding, aided by hydrogen-bonding interactions involving the hemiacetal functionalities (C-OH, C-OMe, and C-H), are responsible for the observed stabilization. The hemiacetal/aldehyde groups in 1 and 2 readily react with p-toluidine, 2,6-dimethylaniline, and 4-bromoaniline to yield the corresponding tetra-Schiff base ligands, [Zn(dipp)(L)]4 (L=4-methyl-N-(pyridin-4-ylmethylidene)aniline (5), 2,6-dimethyl-N-(pyridin-4-ylmethylene)-aniline (6), and 4-bromo-N-(pyridin-4-ylmethylene)aniline (7)). Isolation of 5-7 opens up further possibilities of using 1 and 2 as new supramolecular synthons and ligands.

Accessing a Biologically Relevant Benzofuran Skeleton by a One-Pot Tandem Heck Alkynylation/Cyclization Reaction Using Well-Defined Palladium N-Heterocyclic Carbene Complexes.

Well-defined palladium N-heterocyclic carbene (NHC) complexes were employed in the one-pot tandem Heck alkynylation/cyclization sequence for preparing biologically relevant benzofuran compounds under copper-free conditions in a time-efficient step-reduced fashion. In particular, a series of binuclear palladium complexes, 1b-1e and 2b-2e, of the alkyl-bridged NHC ligands, namely, {1,1'-di-R1-4,4'-R2-di-1,2,4-triazoline-5,5'-diylid-2-ene] (R1 = i-Pr; R2 = -(CH2)2-, -(CH2)3-), and their mononuclear analogues, trans-(NHC)PdBr2(pyridine) (3b) and cis-(NHC)PdBr2(PPh3) (3c), successfully catalyzed the one-pot tandem Heck alkynylation/cyclization reaction of 2-iodophenol with a variety of terminal alkyne substrates, yielding 2-substituted benzofuran derivatives. The mononuclear complexes 3b and 3c were nearly half as active as the representative dinuclear analogue 1c under analogous reaction conditions, thereby implying that, at the same mole percent of the palladium loading, the monometallic 3b and 3c and the bimetallic 1c complexes were equally effective as catalysts. The two sites of the bimetallic complex 1c performed as two separate independent catalytic sites, displaying no cooperativity effect in the catalysis. Finally, the practical utility of the aforementioned catalysts was demonstrated for a representative catalyst 1c through the convenient synthesis of a key intermediate, 3-[2-(benzo[d][1,3]dioxol-5-yl)-7-methoxybenzofuran-5-yl]propan-1-ol, in a total-synthesis protocol of the natural product Egonol.

Fluoride ion sensing and caging by a preformed molecular D4R zinc phosphate heterocubane.

Double-4-ring (D4R) zinc phosphate [Zn(dipp)(DMSO)]4 (1, dipp = 2,6-di-iso-propylphenylphosphate, DMSO = dimethyl sulfoxide), on treatment with a free fluoride ion source, exhibited ability to sense and capture fluoride ions from a variety of sources, as evidenced by extensive solution (31)P and (19)F NMR spectral titration studies. The fluoride ion-encapsulated cage [(n)Bu4N][F@{Zn(dipp)(DMSO)}4] (2) was isolated in good yield from an equimolar reaction between 1 and (n)Bu4NF in methanol and characterized by analytical and spectroscopic methods. When 1-methyl-4,4'-bipyridin-1-ium fluoride (MeQ-F) was used as the fluoride ion source a zwitterionic cage [F@{Zn4(dipp)4(MeQ)(DMSO)3}] (3) was isolated. Crystal structure determination for 3 confirmed not only fluoride incorporation inside the D4R cage but also a weak interaction of the central fluoride ion with all four zinc centers of the cubane, resulting in a trigonal bipyramidal geometry around the zinc centers. To establish the selectivity, cubane 1 was treated with 2 equiv of MeQ-X (X = various anions) under similar conditions to isolate [F@{Zn4(dipp)4(MeQ)2(MeOH)2}][X] (X = I 4; BF4 5; PF6 6) in good yields. The crystal structure determination of 4 and 5 showed that the iodide and tetrafluoroborate anions are found outside the cage while fluoride ion has entered the cavity. The final fluoride encapsulated D4R cage is anionic in 2, neutral in 3, and cationic in 4-6, showing the versatility of the cubane framework to stabilize fluoride ions in all three forms. NMR titrations showed that 1 can sense even 1 ppm level of fluoride ions and sequester them from fluoridated water and toothpaste extract.

A Schiff base and its copper(II) complex as a highly selective chemodosimeter for mercury(II) involving preferential hydrolysis of aldimine over an ester group.

The syntheses of a new Schiff base, diethyl-5-(2-hydroxybenzylidene)aminoisophthalate (HL), and a copper complex, [Cu(L2)] (1), imparting L(-), have been described. Both the ligand HL and complex 1 have been thoroughly characterized by elemental analyses, electrospray ionization mass spectrometry, FT-IR, NMR ((1)H and (13)C), electronic absorption, and emission spectral studies and their structures determined by X-ray single-crystal analyses. Distinctive chemodosimetric behavior of HL and 1 toward Hg(2+) has been established by UV/vis, emission, and mass spectral studies. Comparative studies further revealed that the chemodosimetric response solely originates from selective hydrolysis of the aldimine moiety over the ester group and 1 exhibited greater selectivity toward Hg(2+) relative to HL while the sensitivity order is reversed. Further, these followed different hydrolytic pathways but ended up with the same product analyzed for diethyl-5-aminoisophthalate (DEA). Hg(2+)-induced displacement of Cu(2+) and subsequent hydrolysis of the -HC═N- moiety in 1 affirmed the identity of the actual species undergoing hydrolysis as HL. The occurrence of Cu(2+) displacement and Hg(2+) detection via hydrolytic transformation has been supported by various physicochemical studies.

Ab initio chemical synthesis of designer metal phosphate frameworks at ambient conditions.

Stepwise hierarchical and rational synthesis of porous zinc phosphate frameworks by predictable and directed assembly of easily isolable tetrameric zinc phosphate [Zn(dipp)(solv)]4 (dippH2 = diisopropylphenyldihydrogen phosphate; solv = CH3OH or dimethyl sulfoxide) with D4R (double-4-ring) topology has been achieved. The preformed and highly robust D4R secondary building unit can be coordinatively interconnected through a varied choice of bipyridine-based ditopic spacers L1-L7 to isolate eight functional zinc phosphate frameworks, [Zn4(dipp)4(L1)1.5(DMSO)]·4H2O (2), [Zn4(dipp)4(L2)1.5(CH3OH)] (3), [Zn4(dipp)4(L1)2] (4), [Zn4(dipp)4(L3)2] (5), [Zn4(dipp)4(L4)2] (6), [Zn4(dipp)4(L5)2] (7), [Zn4(dipp)4(L6)2] (8), and [Zn4(dipp)4(L7)2] (9), in good yield. The preparative procedures are simple and do not require high pressure or temperature. Surface area measurements of these framework solids show that the guest accessibility of the frameworks can be tuned by suitable modification of bipyridine spacers.

Rhenium(I)-based bridgeless double metallocalix[4]arenes.

Bridgeless double metallocalix[4]arenes possessing two orthogonally arranged dinuclear cavitands were obtained from a Re2(CO)10, rigid bis-chelating OO donor (H2-L), and a flexible bis-ditopic NN donor (L') by a one-pot approach.

Fluoride-free Hiyama coupling by palladium abnormal N-heterocyclic carbene complexes.

A series of palladium complexes of the abnormal N-heterocyclic carbene ligands of the type (a-NHC)PdI2(L) [L = NC5H5(1-3)b and PPh3(1-3)c] effectively catalyzed the Hiyama coupling of aryl bromides and iodides with PhSi(OMe)3 under the highly desired fluoride-free conditions. Interestingly enough, the pyridine based trans-(1-3)b complexes and a PPh3 derived cis-3c complex exhibited higher yields than the related PPh3 derived trans-(1-2)c complexes. The superior performances of the pyridine based trans-(1-3)b complexes and the PPh3 derived cis-3c complex have been correlated to a tighter binding of the a-NHC ligand to the palladium center in these complexes, leading to a greater (a-NHC) ligand influence on the metal center partaking in the catalysis.

Cationic D4R zinc phosphate-anionic polyoxometalate hybrids: synthesis, spectra, structure and catalytic studies.

A facile synthetic strategy for the generation of a new class of inorganic-organic-inorganic hybrids is reported. Replacement of labile DMSO ligands from the preformed Double-Four-Ring (D4R) zinc phosphate cluster [Zn(dipp){(CH3)2SO}]4 by the addition of 4-aminopyridine (4-ampyr) results in the isolation of another discrete D4R cluster [Zn(dipp)(4-ampyr)]4 (1), which forms a hydrogen-bonded framework in solid-state. If 1-methyl-4,4'-bipyridinium salts are employed instead, tetra-cationic D4R cluster [Zn(dipp)(L)]4[PF6]4 (2) and [Zn(dipp)(L)]4[ClO4]4 (3) (L = 1-methyl-4,4'-bipyridinium, dipp = 2,6-di-iso-propylphenylphosphate) are isolated. Compound 2 was reacted with three different polyoxometalates, [TBA]4[Mo8O26] (POM-1), [TBA]4[PMo11VO40] (POM-2), and [TBA]4[SiMo12O40] (POM-3) to obtain amorphous hybrids, [Zn(dipp)(L)]4[Mo8O26] (4), [Zn(dipp)(L)]4[PMo11VO40] (5), and [Zn(dipp)(L)]4[SiMo12O40] (6), respectively. All the hybrid materials have been characterized by analytical and spectroscopic studies. The molecular structure of 1 has also been determined by single-crystal XRD measurements. N2 gas sorption analyses show moderate BET surface area and also establish the mesoporous nature of the hybrids 4-6. The catalytic potential of hybrids 4, 5 and 6 has been explored towards epoxidation of cyclohexene.