Solid-state Circular Dichroism Spectra Research Articles

Noncentrosymmetric Supramolecular Hydrogen-Bonded Assemblies Based on Achiral Pyrazine-Bridged Zinc(II) Coordination Polymers with Pyrazinedione Derivatives

Reaction of M(OAc)2·xH2O (M, x = Zn, 2 and Co, 4), 1,4-dihydro-5,6-dicyano-2,3-pyrazinedione (H2CN2pyzdione), and pyrazine (pyz) affords two compounds of the same molecular formula {[M(H2O)6][M(CN2pyzdione)2(pyz)]·6H2O}n (M = Zn for 1 and Co for 2) in which discrete units of [M(H2O)6]2+ are linked to one-dimensional chains of [M(CN2pyzdione)2(pyz)]2– via multiple O–H···O hydrogen-bonding interactions and M2+-bound H2O molecules in [M(H2O)6]2+ also serve as linkers of hydrogen-bonded interstitial H2O molecules. Remarkably, 1 crystallizes in the monoclinic crystal system, the similar crystal system and unit cell parameters as 2, but with a space group distinct from 1 and 2, i.e., 1 is the noncentrosymmetric space group C2, whereas 2 is the centrosymmetric space group C2/m. This polar structure for 1 is induced by the presence of alternating arrangements of distinguishable two axial Zn–N bonds within [Zn(CN2pyzdione)2(pyz)]2– chains. Indeed, solid-state circular dichroism spectra of 1 exhibit significant Cotton effects, as evidenced by the polar space group C2. Moreover, these Cotton effects show clear temperature-dependence depending on contents of H2O molecules of 1.

Three-dimensional homochiral coordination polymers of Eu(III) and Tb(III): Synthesis, structure determination, and optical properties

Self-assembly of the corresponding europium and terbium salts with designed chiral ligand (S)-5-((2-carboxy-5-oxopyrrolidin-1-yl)methyl)isophthalic acid (H3L) generated two novel 3D coordination polymers {[Eu(L)]·0.5CH3CN}n (1) and {[Tb(L)]·0.5CH3CN}n (2). Single-crystal X-ray diffraction analyses reveal that 1 and 2 are isostructural metal-organic frameworks built from four-stranded helixes. Complexes 1 and 2 are intriguing homochiral porous architectures with 6,6-connected (48∙66∙8)(413∙62) topology, whose homochirality was further confirmed by their solid-state circular dichroism spectra. Thermal analyses disclose that the desolvated frameworks of 1 and 2 can maintain their structural stability below 370 ​°C. Solid-state photoluminescence studies show that 1 and 2 exhibit strong characteristic Eu(III) and Tb(III) emissions with long lifetimes of 1.31 and 1.35 ​ms, respectively. In addition, 1 and 2 also exhibit significant second harmonic generation activities with their intensities of about 0.29 and 0.31 times larger than that of potassium dihydrogenphosphate, respectively. Very importantly, the excellent thermostabilities and strong red and green emissions of 1 and 2 are of great significance for further exploration of their applications in optical devices.

The recent development of multilevel chirality research based on nucleotide coordination complexes

Chirality is a kind of special symmetry that is prevalent in nature. Organisms exhibit the function of multiple-level transformation of chirality. The transfer, induction and amplification of multi-level chirality are fundamental problems and vital to chemistry, biology, physics and materials science. Generally, there are four levels for chirality transformation from molecular chirality to macroscopic chirality, including configuration (molecular) chirality (first level), conformational chirality (second level), phase chirality (third level) and macroscopic chirality (fourth level). Based on the coordination supramolecular assemblies of nucleotides, the combination of X ray single crystal diffraction technique and solid-state circular dichroism spectrum is an effective way to study multiple-level chirality. The comprehensive understanding of the multi-level chirality transfer mechanism can provide guidelines for us not only to cognize the relationship between the structure and function of organisms, but also to design and synthesis of chiral functional materials and artificial molecular machines rationally. In this review, an effective method to study conformational chirality and phase chirality has been described based on the research results of nucleotide chiral coordination supramolecular assemblies, which is a combination of X-ray single crystal diffraction technique and solid-state circular dichroism spectroscopy. It is the first time to develop the concept of “Extending Axial Chirality” creatively based on axial chirality in organic chemistry to 1D coordination polymer. According to the accurate analysis of crystal structures, the roles of metal-ligand coordination bonding, hydrogen bonding, π-π stacking in chirality inducing, chirality assembly and chirality transfer were revealed. Also, the expectant research areas of chiral research in the future were prospected.

A Chiral 3 D Net with 2 D Cairo Pentagonal Tiling Projection in Site-Modified CuCN/CuSCN Networks.

A novel isohedral 3-periodic net (i.e. a net with one kind of tile), showing a Cairo pentagonal tiling projection on the Euclidean plane, has been identified in a series of site-modified CuCN/CuSCN networks, namely, [Cu6 (SCN)2 (CN)6 ⋅Ni(tpy)2 ]n (1), [Cu6 (SCN)(CN)7 ⋅Ni(tpy)2 ]n (2) and [Cu6 (CN)8 ⋅Ni(tpy)2 ]n (3) (tpy=2,2':6'',2''-terpyridine). These infinite inclusion compounds were prepared via a solvothermal sulfur migration reaction with subtle modification to fine-tune the sulfur content. The chiroptical activity of single crystals, which arises from the intrinsically chiral net, was confirmed by X-ray diffraction and solid-state circular dichroism spectra.

An Efficient Strategy for Self-Assembly of DNA-Mimic Homochiral 1D Helical Cu(II) Chain from Achiral Flexible Ligand by Spontaneous Resolution.

Four helical copper complexes Cu[N(CN)2]2(Hhmp) (1), {Cu[N(CN)2]2(Hhmp)}∞ (2), (l-{Cu4[N(CN)2]2(hmp)4(CH3COO)2·CH3CN}∞ (3a), and d-{Cu4[N(CN)2]2(hmp)4(CH3COO)2·CH3CN}∞ (3b) (Hhmp = 2-(hydroxymethyl)pyridine) have been prepared toward a mimic DNA structure. By changing the solvent and supplementary ligand, the structures can be successfully tuned from quasi-double-helical (complex 1) to racemic 1D single helix (complex 2), then the right (3a)-/left (3b)-handed double helices. The topologies of 3a and 3b may be considered as a mimic of DNA, where the Cu-O bonds between the two strands replace the hydrogen-bonding interactions in DNA. Solid-state circular dichroism spectra confirmed that 3a and 3b are optically active, respectively. Magnetic measurements for 1-3 indicated all complexes to be antiferromagnetic interactions. The best fitting results to the magnetic susceptibilities were J = -0.80 cm(-1), g = 2.11 for 1 and J1 = -9.22 cm(-1), J2 = 3.56 cm(-1), J3 = -9.49 cm(-1), g = 2.27 for 3.

L- and D-[LnZn(IN)3(C2H4O2)]n (Ln = Eu, Sm, and Gd): Chiral Enantiomerically 3D 3d-4f Coordination Polymers Constructed by Interesting Butterfly-like Building Units and -[Ln-O-Zn]n- Helices.

A total of six three-dimensional chiral coordination compounds L- and D-[LnZn(IN)3(C2H4O2)]n (Ln = Eu, Sm, and Gd; HIN = isonicotinic acid) have been successfully synthesized under hydrothermal conditions without any chiral auxiliary and characterized by IR, TG, elemental analyses, and solid-state circular dichroism spectra. The structures of 1-6 were determined by single-crystal X-ray structural analysis, which shows that L-[LnZn(IN)3(C2H4O2)]n (Ln = Eu (1), Sm (2), and Gd (3)) crystallize in space group P6522 and are levogyrate. The chiral frameworks of L-[LnZn(IN)3(C2H4O2)]n are constructed from L-helical Ln-O-Zn cluster chains, while adjacent L-type helical -[Ln-O-Zn]n- chains are connected through IN(-) ligands. D-[LnZn(IN)3(C2H4O2)]n (Ln = Eu (4), Sm (5), and Gd (6)) crystallize in space group P6122, and their chiral frameworks consist of D-helical Ln-O-Zn cluster chains. The observed second-harmonic generation efficiencies of [EuZn(IN)3(C2H4O2)]n, [SmZn(IN)3(C2H4O2)]n, and [GdZn(IN)3(C2H4O2)]n are 0.4, 0.3, and 0.3 times that of urea, respectively. We also studied luminescence spectra and luminescence lifetimes of 1 and 2. The luminescence lifetimes of 1 and 2 are 1.18 ms, and 29.6 μs, respectively.

Three complexes with helical frameworks based on l-glutamine and l-asparagine: Crystal structures and circular dichroism properties

Three novel complexes with helical two dimensional (2D) frameworks, [Ni(Asn)2(H2O)] (1) (HAsn=l-asparagine), [Cd(Gln)2]n (2) (HGln=l-glutamine) and [Zn(Gln)2]n (3), have been studied based on X-ray single crystal diffraction analysis. l-asparagine induces two kinds of chirality, the chiral Ni(II) center and the supramolecular M-helixes, into complex 1. l-glutamine induces one kind of chirality, the helixes, into complexes 2 and 3, respectively. Combining the crystal structures of complexes 1–3, the final aggregate chirality induced by coordinated bond interactions can be detected by the liquid state circular dichroism (CD) spectra while the solid state CD spectra of complexes 1–3 additionally reflect the final aggregate chirality induced by weak interactions.

Absolute Asymmetric Photocyclization of Triisopropylbenzophenone Derivatives in Crystals and Their Morphological Changes

Although 4-(2,4,6-triisopropylbenzoyl)benzylbenzamide is an achiral molecule, chiral crystals can form through spontaneous crystallization in a methanol solution. In the M crystal, twofold helical hydrogen-bond chains form in a counterclockwise direction among the molecules along the a axis to generate crystal chirality. The solid-state circular dichroism spectra of the two enantiomorphous crystals as Nujol mulls show a good mirror-image relationship. UV irradiation of the M crystal at >290 nm caused highly enantioselective Norrish type II photocyclization to yield the (R)-cyclobutenol with 94% ee in 100% yield as the sole product, resulting in successful absolute asymmetric synthesis. In contrast, the (S)-cyclobutenol was obtained from the P crystal with 95% ee in 100% yield. The high enantiodifferentiation in the crystalline-state photocyclization is attributed to the shorter distance between the carbonyl oxygen atom and one of the methine γ-hydrogen atoms of the two o-isopropyl groups as well as the smooth transformation with minimum molecular motion because of the similar shapes of the reactant and product molecules. UV irradiation of the platelike crystals resulted in a crack in the direction perpendicular to the long axis (the a axis of the unit cell), likely because the hydrogen-bond chains were broken during the photocyclization.

Strong Intermolecular Exciton Couplings in Solid‐State Circular Dichroism of Aryl Benzyl Sulfoxides

A series of 13 enantiopure aryl benzyl sulfoxides () with different substituents on the two aromatic rings has been previously analyzed by means of electronic circular dichroism (CD) spectroscopy. Most of these compounds are crystalline and their X-ray structure is established. For almost one-half of the series, CD spectra measured in the solid state were quite different from those in acetonitrile solution. We demonstrate that the difference is due to strong exciton couplings between molecules packed closely together in the crystal. The computational approach consists of time-dependent density functional theory (TDDFT) calculations run on "dimers" composed of nearest neighbors found in the lattice. Solid-state CD spectra are well reproduced by the average of all possible pairwise terms. The relation between the crystal space group and conformation, and the appearance of solid-state CD spectra, is also discussed.

Construction of one-dimensional homochiral helical supramolecular stereoismers using chiral macrocyclic nickel(II) complexes as building blocks

One one-dimensional right-handed helical chain of {[Ni(RR-L)][Ni(CN)4]·2H2O}n (Δ-2·(2H2O)n) was successfully constructed by the reaction of the [Ni(α-RR-L)]2+ and [Ni(CN)4]2− in acetonitrile/water (1:1) (L=5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), and another two one-dimensional left-handed helical chains of {[Ni(SS-L)][Ni(CN)4]·2H2O}n (Λ-2·(2H2O)n) and {[Ni(SS-L)][Ni(CN)4]·1.25H2O}n (Λ-3·(1.25H2O)n) were obtained from the reaction of [Ni(α-SS-L)]2+ with [Ni(CN)4]2−. The reaction of [Ni(CN)4]2− building blocks with [Ni(α-SS-L)]2+ in acetonitrile/methanol (1:1) gave a one-dimensional left-handed helical chain of {[Ni(SS-L)][Ni(CN)4]·MeCN}n (Λ-2·(MeCN)n). In all supramolecular isomers, the [Ni(RR-L)]2+/[Ni(SS-L)]2+ cations are alternately bridged by [Ni(CN)4]2− anions through cyano-groups to form one-dimensional helical chains. The homochiral natures of four complexes were confirmed by the results of solid-state circular dichroism spectra measurements.

Spontaneous resolution of novel Zn complexes in the formation of 3D metal–organic frameworks based on 2,2′-biimidazole ligand

The novel three-dimensional chiral framework of the four-coordinate zinc complexes, [Zn4(biim)3(ina)2] (1a and 1b) (H2biim=2,2′-biimidazole, ina=isoniconate), has been prepared via hydrothermal reaction. X-ray single crystal diffraction analysis shows the biim2− in title complexes acts as a tetradentate ligand and links four different Zn2+ ions. Spontaneous chiral resolution occurs in solvo-thermal reaction, which was confirmed by solid-state circular dichroism spectra. Photoluminescent spectra of compound 1 exhibit an intense blue emission at 440nm.

Solid‐state Circular Dichroism and Hydrogen Bonding, Part 2: The Case of Hypothemycin Re‐investigated

The impact of crystalline forces on the solid-state circular dichroism (CD) spectrum of hypothemycin (), a biologically active molecule extracted from natural sources, has been analyzed by means of time-dependent density functional theory CD calculations. Input structures were extracted from the X-ray geometry of and consisted in the isolated molecule, its cluster with five water molecules, and 20 different dimers (plus water molecules) representative of all the closest neighbors found in the crystal. The effects of solid-state intermolecular hydrogen bonds and through-space exciton couplings in determining the solid-state CD spectrum of hypothemycin were evaluated and compared.

Temperature-Controlled Chiral and Achiral Copper Tetrazolate Metal–Organic Frameworks: Syntheses, Structures, and I2 Adsorption

Four tetrazole-based three-dimensional (3D) metal-organic frameworks (MOFs), {[Cu(II)(btz)]·0.5H(2)O}(n) (1), [Cu(II)(btz)](n) (1'), {[Cu(II)(btz)]·0.5I(2)}(n) (1'·0.5I(2)), and [Cu(II)Cu(I)(2)(btz)(2)](n) (2) [H(2)btz = 1,5-bis(5-tetrazolo)-3-oxapentane)], have been successfully obtained and characterized by crystallography. Compound 1 features a chiral porous framework. The bulk crystallization of 1 is composed of enantiomers 1a (P4(1)) and 1b (P4(3)), which has been demonstrated by the crystal structure analyses of nine crystals of 1 randomly selected. The Cotton effect displayed in the solid-state circular dichroism spectrum of 1 is therefore attributed to enantiomeric excess rather than enantiopurity. The completely dehydrated phase of 1, that is, 1', can adsorb 0.5 I(2) molecule per formula unit to yield compound 1'·0.5I(2), which has been supported by single-crystal X-ray diffraction, elemental analysis, and thermogravimetric analysis. The locations of I(2) in the pores were unambiguously determined, and the interactions between I(2) molecules and the pore structures were investigated. Compound 2 crystallizes in an achiral C2/c space group. Interestingly, the formations of chiral 1 and achiral 2 significantly depend on the reaction temperature. Between 80 and 140 °C, we got compound 1 as the only product. At 150 °C, both 1 and 2 were in coexistence in the final product. From 160 to 180 °C, only compound 2 was obtained. More interestingly, treatment of the crystals of 1 or the mixture of 1 and 2 obtained at 150 °C in their mother liquor at 170 °C yielded the crystals of 2 in a single phase.

Dynamic chiral-at-metal stability of tetrakis(d/l-hfc)Ln(iii) complexes capped with an alkali metal cation in solution

Chiral tetrakis(β-diketonate) Ln(III) complexes Δ-[NaLa(d-hfc)(4)(CH(3)CN)] (1) and Λ-[NaLa(l-hfc)(4) (CH(3)CN)] (2) (d/l-hfc(-) = 3-heptafluo-robutylryl-(+)/(-)-camphorate) are a pair of enantiomers and crystallize in the same Sohncke space group (P2(1)2(1)2(1)) with dodecahedral (DD) geometry. Typically positive and negative exciton splitting patterns around 320 nm were observed in the solid-state circular dichroism (CD) spectra of complexes 1 and 2, which indicate that their shell configurational chiralities are Δ and Λ, respectively. The apparent bisignate couplets in the solid-state CD spectra of [CsLn(d-hfc)(4)(H(2)O)] [Ln = La (3), Yb (5)] and [CsLn(l-hfc)(4)(H(2)O)] [Ln = La (4), Yb (6)] show that they are a pair of enantiomers and their absolute configurations are denoted Δ and Λ, respectively. The crystallographic data of 5 reveals that its coordination polyhedron is the square antiprism (SAP) geometry and it undergoes a phase transition from triclinic (α phase, P1) to monoclinic (β phase, C2) upon cooling. The difference between the two phases is brought about by the temperature dependent behaviour of the coordination water molecules, but this did not affect the configurational chirality of the Δ-SAP-[Yb(d-hfc)(4)](-) moiety. Furthermore, time-dependent CD, UV-vis and (19)F NMR were applied to study the solution behavior of these complexes. It was found that the chiral-at-metal stability of the three pairs of complexes is different and affected by both the Ln(3+) and M(+) ion size. The results show that the Cs(+) cation can retain the metal center chirality and stablize the structures of [Ln(d/l-hfc)(4)](-) or the dissociated tris(d/l-hfc)Ln(III) species in solution for a longer time than that of the Na(+) cation, and it is important that the Cs(+) ion successfully lock the configurational chirality around the Yb(3+) center of the complex species in solution. This is reasoned by the short Cs(+)···FC, Cs(+)···O-Yb and Cs(+)···Yb(3+) interactions observed in the crystal structure of α-5 and further confirmed by the chiral self-assembly of 5 or 6 from [Yb(H(2)O)(d/l-hfc)(3)] induced by CsI in a CHCl(3) solution.

Spontaneous resolution of a homochiral helix built from a tetra-nuclear nickel cluster

A chirally helical coordination polymer [Ni4(H2O)7L4·H2O]n (H2222L = 2-(pyridin-2-yl)-1H-imidazole-4,5-dicarboxylic acid) constructed from a Ni4L4 building block exhibits spontaneous chiral resolution and the enantiomers of 1M and 1P have been confirmed by single-crystal analysis and solid-state circular dichroism spectra. The magnetic properties of compound 1 are also discussed.

Structure–chiroptical property relationship of kinetically labile camphor-derivative β-diketone Yb(iii) complexes: do the adducts coexist as diastereomers or not?

The present work examines the relationship between the crystal structures and chiroptical properties of four chiral Yb(III) complexes with camphor-derivative β-diketone ligands by means of solid-state circular dichroism (CD) spectroscopy. For the seven-coordinate complexes, [Yb(H2O)(d-hfc)3] (I) and [Yb(H2O)(l-hfc)3] (II) (d/l-hfc− = 3-heptafluorobutyryl-(+)/(−)-camphorate), the Λ- and Δ-diastereomers coexist in their crystals and no apparent bisignate couplets are observed in their solid-state CD spectra. A theoretical study indicates that the ground-state energy difference between the two diastereomers I and II is only 0.913 kcal mol−1, which explains why they could coexist in a crystal environment with the ratio of 1:1. While, eight-coordinate complexes Δ-[Yb(TPPO)2(d-hfc)3]·CHCl3·3C6H12 (III) and Λ-[Yb(TPPO)2(l-hfc)3]·CHCl3·3C6H12 (IV) (TPPO = triphenylphosphine oxide) are enantiopure in the solid-state, and typically negative and positive exciton splitting patterns around 330 nm are observed in their solid-state CD spectra. The solid-state CD spectra of these four complexes are in accordance with their X-ray single-crystal analyses. Besides, their solution CD spectra show that no particular isomer predominates in solution.

ChemInform Abstract: Absolute Structural Elucidation of Natural Products — A Focus on Quantum‐Mechanical Calculations of Solid‐State CD Spectra

AbstractReview: 184 refs.

Spontaneous resolution of four-coordinate Zn(ii) complexes in the formation of three-dimensional metal–organic frameworks

The three-dimensional chiral helical framework of the four-coordinate zinc complexes, [Zn(bdt)]n (1a and 1b) (H2bdt = 1,4-benzeneditetrazol-5-yl), has been prepared via hydrothermal reaction. At the chiral centers, the tetrahedral zinc centers link with bdt ligands to form a chiral unc-type network, which exhibits an intense blue emission and spontaneous chiral resolution. The enantiomers of 1a and 1b have been confirmed by solid-state circular dichroism spectra.

Zn[Htma][ddm]: An Interesting Three-Dimensional Chiral Nonlinear Optical-Active Zinc-Trimesate Framework

The three-dimensional (3D) chiral helical framework of the new polymeric solid, Zn[Htma][ddm] (1) (tma = trimesate, ddm = p,p′-diamino-diphenyl-methane), has been prepared by the hydrothermal reaction of zinc acetate dihydrate, H3tma, and ddm. X-ray single-crystal structural analysis reveals that 1 (C22H18N2O6Zn) crystallizes in the orthorhombic P212121 space group, Z = 4 (a = 5.4533(3) Å, b = 17.593(1) Å, c = 20.542(1) Å, V = 1970.8(2) Å3). Tetrahedrally coordinated zinc centers in 1 bridged by the rigid trigonal tectonic tma ligands with Δ-configuration plus the flexible ddm molecules give rise to an interesting 3D chiral helical diamondoid network single enantiomer that has been confirmed by solid-state circular dichroism spectra. 1 is stable up to about 297 °C in the ambient environment and exhibits an intense fluorescent emission band at 420 nm upon excitation at 326 nm and the band gap semiconducing character. The theoretical calculations reveal that 1 is a potential nonlinear optical-active material.

Chirality- and Threefold-Symmetry-Directed Assembly of Homochiral Octupolar Metal−Organoboron Frameworks

Homochiral octupolar metal-organoboron frameworks with the general formula [M(2)L(OH)(MeOH)] x 3 H(2)O [M = Co (1), Mn (2), Ni (3), Cu (4), Zn (5), Cd (6)] have been constructed from racemic C(3)-symmetric tris(4-benzoic acid)tridurylborane and the divalent metal ions. Compounds 1-6 are isostructural and crystallize in the chiral cubic space group F432, and they adopt an eightfold-interpenetrating (10,3)-a network formed by linking bimetal building blocks with three bidentate carboxylate groups of bridging Lambda-L ligands. Bulk crystals of each of the six compounds are not a racemic mixture, and their optical activity and enantiomeric nature were demonstrated by solid-state circular dichroism spectra. Consistent with their polar structures, the colorless compounds 5 and 6 exhibit powder second harmonic generation intensities 3-4 times higher than that of potassium dihydrogen phosphate, making them, to the best of our knowledge, the first two examples of NLO-active, homochiral octupolar metal-organic solids.