Solid-state CD Spectra Research Articles

Metal-directed and ligand-distorted assembly of chiral/achiral one-dimensional coordination polymers: Syntheses, structures and physical properties

The hydrothermal reactions of 5-bromoisophthalic acid (H 2BIPA) and metal salts in the presence of pyridine (py) result in three new coordination polymers, namely, M(BIPA)(py) 3 [M = Co(II) ( 1), Ni(II) ( 2)] and Zn(BIPA)(py) 2·H 2O ( 3), all of which have been characterized by single-crystal X-ray, elemental analysis, infrared spectroscopy, and thermogravimetric analysis. Compounds 1 and 2 are isostructural and contain 2 1 helical chains made up of metal ions and BIPA 2− ligand, the chains are further formed to the supramolecular structures by the π⋯π interactions between the adjacent parallel py rings. Compound 3 has a 1D chiral left-handed helical chain structure. It is notable that 3 is obtained by second order spontaneous resolution using achiral starting materials. Moreover, the solid-state CD spectrum of single crystals and powder samples of 3 is investigated. The nonlinear optical measurement shows that 3 displayed a second-harmonic-generation (SHG) response of 0.7 times that for urea.

Observation of Long-Range Vicinal Effect in Chiral Cu(II)-Cr(VI) or Cu(II)-W(VI) Bimetallic Coordination Polymers

We have prepared some diastereomers of [CuL2][M2O7] (L is 1,2-diaminocyclohexane and its derivatives; M = Cr and W) bimetallic coordination polymers and confirmed their structural similarity and inner electronic states by means of XRD and XAS, respectively. For the first time, we have successfully observed distant vicinal effect of which the chiral source is only chiral organic ligands of [CuL2]2+ moieties (acting as ligand complex), while probe bands for solid state CD spectra are charge transfer (CT) bands of [M2O7]2− moieties (achiral complex) with the d0 electronic configuration. The new concept (interpretation) of this observation will be important for supramolecular chirality of coordination polymers built by ligand complexes.

Synthesis, Magnetic Properties, and Structural Investigation of Mixed-Ligand Cu(II) Helical Coordination Polymers with an Amino Acid Backbone and N-Donor Propping: 1-D Helical, 2-D Hexagonal Net (hcb), and 3-D ins Topologies

Three Cu(II) coordination polymers, {[Cu(l-cysteate)(1,4-Bix)(H2O)]·H2O}n (1), {[Cu(l-aspartate)(1,4-Bix)0.5]·H2O}n (2), and {[Cu(l-aspartate)(4,4′-Bpy)0.5]H2O}n (3), have been synthesized solvothermally, employing enantiopure l-amino acids and bidentate N-donor ligands, 1,4-bis(imidazol-1-ylmethyl)benzene (1,4-Bix) and 4,4′-bipyridyl(4,4′-Bpy), to yield 1-D, 2-D, and 3-D helical coordination polymers, respectively. All the three neutral frameworks are well-characterized by various physicochemical methods (C,H,N analysis, IR, TGA) including the single crystal X-ray diffraction technique. The chiral nature of complexes 2 and 3 was established using solid state CD spectra. Structural investigation of complex 1 revealed 1-D helical chains, whereas that of complex 2 confirms a chiral and helical two-dimensional 63-hcb net, with helicity propagating in both the dimensions. Cu(II) nodes with the mixed ligands in complex 3 generate a three-dimensional framework by the pillaring of Cu(l-aspartate) 2-D helical lay...

A novel helical chainlike cobalt(II) coordination polymer containing 1D trigonal channels: Synthesis, crystal structure and properties

A novel helical-chain coordination polymer [Co(tau)2(4,4′-bipy)]n·nH2O 1 (tau = taurine, 4,4′-bipy = 4,4′-bipyridine) has been hydrothermally synthesized and characterized by infrared spectroscopy, and thermogravimetric analysis, magnetic measurements, solid-state CD spectra and single crystal X-ray diffraction analysis. The intertwist trigonal-fold helical chains are further connected through hydrogen bonds to give a three-dimensional supramolecular network, in which trigonal microporous channels filled with water guest molecules exist within the polymer coils. Compound 1 shows weak antiferromagnetic interactions among metal ions.

Concentration effects in solid-state CD spectra of chiral atropisomeric compounds

Atropisomerism is one of the basic concepts in stereochemistry. Chiral crystals of stereochemically labile atropisomers that originated from Mirror Symmetry Breaking (MSB) can only be characterized by solid-state chiroptical techniques. Herein, solid-state circular dichroism and UV-Vis spectra of six atropisomeric compounds (most of them were obtained from MSB) have been studied. A concentration effect including a wavelength shift and inverse concentration-dependence has been found and preliminarily explained by the absorption flattening effect, scattering effect and the torsion in the molecular structures.

Structural diversity in two dimensional chiral coordination polymers involving 4,4′-bipyridine and l-cysteate as bridging ligands with Zn and Cd metal centres: Synthesis, characterization and X-ray crystallographic studies

Two chiral coordination polymers involving amino acid backbone l-cysteic acid (H 2 l-cys) and N-donor ligand 4,4′-bipyridine (4,4′-bpy) [{Cd( l-cys)(4,4′-bpy)(H 2O)}3.5H 2O] n 1, [{Zn 2( l-cys) 2(4,4′-bpy) 2(H 2O) 4}·(H 2O)·(4,4′-bpy)] n 2 with rectangular grids have been synthesized. Both compounds are insoluble in common polar/non-polar solvents and well characterized by various physico-chemical techniques such as CHN, IR, TGA, NMR, solid state CD and single crystal X-ray diffraction methods. Complexes 1 and 2 comprise l-cysteate dianions self assembled into chiral coordination polymers by bridging the adjacent metal centres through the carboxylate oxygen generating one-dimensional helical chains. The helical chains are pillared by 4,4′-bpy generating two dimensional network. Complex 1 generates two dimensional (4,4) rectangular grid network with dimension 4.77 Å × 11.74 Å (based on d Cd···Cd) involving with the edge sharing l-cys and 4,4′-bpy ligands, respectively. Complex 2 possesses a brick-wall type (6,3) network topology with edge lengths of the grids 11.42 Å × 10.11 Å based on d Zn···Zn. Lattice water molecules are encapsulated between the adjacent rectangular grids via hydrogen bonding interactions. One 4,4′-bpy moiety is stacked between the adjacent layers of brick-wall network via weak π···π interaction with the edge sharing N-donor ligand. Even though both the complexes are rigid and stable, N 2 adsorption studies by these complexes revealed not much promising results. The terminal protruding sulphonate groups, angular orientation of the grids within the two-dimensional network and interpenetration of the adjacent offset sheets concomitantly prevent the formation of through tubular channels. Flexible coordination geometry around the metal centre and the versatile coordination mode of the amino carboxylate group from the l-cys moiety are accountable for the variation of the appealing network topology in these complexes. Chiral nature is established by solid state CD spectrum which shows a positive cotton effect for both complexes.

Homochiral Metal−Organic Frameworks for Heterogeneous Asymmetric Catalysis

Homochiral crystallizations of two enantiomeric metal-organic frameworks (MOFs) Ce-MDIP1 and Ce-MDIP2 were achieved by using L- or D-BCIP as chiral inductions, respectively, where the chiralities were characterized by solid state CD spectra. Ce-MDIPs exhibit excellent catalytic activity and high enantioselectivity for the asymmetric cyanosilylation of aromatic aldehydes; the homochiral Cd-TBT MOF having L-PYI as a chiral adduct exhibits stereochemical catalysis toward the Aldol reactions.

Metabolites from the Fungus Phoma sp. 7210, Associated with Aizoon Canariense

A new metabolite, 3,16-diketoaphidicolan (1), was isolated together with four known compounds: aphidicolin (2), 17-acetyl-aphidicolin (3), (+)-eupenoxide (4), and phomoxanthone A (5) from the endophytic fungus Phoma sp. The structure of the new compound 1 was determined by spectroscopic methods (mainly extensive 1D and 2D NMR experiments and by mass spectral measurements) and confirmed by X-ray crystallography. Its absolute configuration was assigned by means of the solid-state CD/TDDFT approach comparing the solid-state CD spectrum with the TDDFT-calculated one on the X-ray geometry.

Systematic Investigation of CD Spectra of Aryl Benzyl Sulfoxides Interpreted by Means of TDDFT Calculations

The CD spectra of 13 crystalline aryl benzyl sulfoxides 1a-m with various substituents on the two aromatic rings were recorded in solution and in the solid state. Solution CD spectra were very homogeneous along the series, consisting in most cases of a couplet-like feature in the 200-300 nm region. The red-shifted component of the couplet, corresponding to the sulfoxide-centered n-pi* transition, is always positive for (R) absolute configuration in accordance with Mislow's rule. The presence of a strong electron-withdrawing substituent on the phenyl ring (nitro or ester group) alters the shape of the CD spectrum. CD calculations using the TDDFT method were run for eight representative compounds using DFT-optimized geometries. In all cases, calculated spectra were in very good agreement with experimental ones and allowed for rationalization of the diverse spectral behaviors. It is demonstrated that TDDFT//DFT calculations represent a reliable option for assigning the absolute configuration of this class of compounds whenever crystals suitable for X-ray are not available. Solid-state CD spectra recorded with the KCl pellet technique were in some cases in agreement with those in solution. However, in other cases new and strong CD signals appeared which were interpreted as being due to intermolecular couplings in the crystals.

Generation of chirality in 4,4′-azopyridine by co-crystallization with optically active dicarboxylic acids

The co-crystals of anti-4,4′-azopyridine apy with optically active 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid 1 and trans-1,2-cyclohexanedicarboxylic acid 2 were prepared and their solid state CD spectra were measured. The positive Cotton effect sign, corresponding to the lowest energy n–π ∗ transition, was correlated with the M helicity of the twisted Ar–N N chromophore. The absolute sense of the twist of the guest apy molecule was deduced from the X-ray structures of the ( S,S)- 1· apy and ( S,S)- 2· apy complexes.

Stereochemistry of chiral pentacoordinate spirophosphoranes correlated with solid-state circular dichroism and 1H NMR spectroscopy

Two sets of diastereomers of pentacoordinate spirophosphoranes separately derived from l-valine (or d-valine) and l-leucine (or d-leucine) were synthesized, isolated, and structurally characterized both in the solid state (X-ray crystallography and solid-state CD spectroscopy) and in the solution ( 1H NMR). The Λ and Δ absolute configurations of a pair of enantiomers 3a and 4a with distorted trigonal bipyramids (TBPs) geometry are directly determined by X-ray diffraction analysis, respectively. The chiral-at-phosphorus features of the related diastereomers were correlated with their solid-state CD and 1H NMR spectra.

Enantiopure and Racemic Sandwich-like Networks with Dehydration, Readsorption, and Selective Guest-Exchange Phase Transformations

The room-temperature solution reactions of R- or S-H2opa (H2opa = 2-hydroxy-2-phenylacetic acid) with freshly prepared [Ag(NH3)2](OH) in the presence of 1,3-di(4-pyridyl)propane (bpp) yielded two enantiomeric compounds, [Ag2(R-Hopa)(bpp)2](R-Hopa)·3H2O (1·3H2O) and [Ag2(S-Hopa)(bpp)2](S-Hopa)·3H2O (2·3H2O). While a similar solution reaction was carried out with racemic H2opa instead of R- or S-H2opa, a racemic compound [Ag2(R-Hopa)(S-Hopa)(bpp)2]·4H2O (3·4H2O) was formed. When bulk crystals of 3·4H2O were exposed to the air at ambient temperature, a partially dehydrated compound [Ag2(R-Hopa)(S-Hopa)(bpp)2]·H2O (4·H2O) was found, which would transform to [Ag2(R-Hopa)(S-Hopa)(bpp)2]·2MeOH (5·2MeOH) by exposing the crystals of 4·H2O in mixed MeOH−EtOH−DMF solvent or in methanol. Compounds 1−5 have characteristic hydrogen-bonding-directed sandwich-like networks consisting of Ag−bpp layers and Hopa−−ROH chains (R = H for 1−4 and Me for 5). The different structural features come from different hydrogen bonding interactions between Hopa− and the lattice water or MeOH molecules. In 1·3H2O and 2·3H2O, R-Hopa− or S-Hopa− form left-handed or right-handed helices with water molecules running in parallel to the Ag-bpp layers while in 3·4H2O, the centrosymmetric hydrogen bonding chains are composed of two enantiomers of Hopa−. Upon dehydration, the Ag···Ag dimeric units in [Ag2(bpp)2]n2n+ in 3·4H2O were broken and the positions of the Ag-bpp chains were adjusted to form new Hopa−−H2O chains between the silver layers. After absorbing methanol, the Ag-bpp chains restored to their original layer structures and 5·2MeOH is pseudopolymorphic to 3·4H2O. The reversible dehydration, readsorption, and guest-exchange among 3·4H2O, 4·H2O, and 5·2MeOH is an interesting example of phase transformation for racemic coordination polymers. The chirality of 1·3H2O and 2·3H2O was further conformed by solid-state CD spectra.

Polymorphism and Pseudopolymorphism of an Aromatic Amide: Spontaneous Resolution and Crystal-to-Crystal phase Transition

1,2-Bis(N-p-fluorobenzoyl-N-methylamino)benzene (1) afforded 13 types of crystals upon crystallization from various solvents. Eleven of them are pseudopolymorphs that have crystallizing solvents in the crystalline lattice, whereas the others are polymorphs. Both the polymorphs and pseudopolymorphs included conformational polymorphs based on cis/trans isomerization of the amide bonds. Three of the pseudopolymorphs showed spontaneous resolution (total asymmetric transformation), belonging to the chiral space group P31 or P32. The absolute configuration of each chiral crystal was determined by single crystal X-ray analysis. Solid-state CD spectrum was also measured in the KBr matrix for the same crystal used for X-ray analysis. Phase transition between the polymorphs and pseudopolymorphs was observed by simultaneous measurement of DSC and X-ray powder diffraction.

Self‐Assembled Amphotericin B Is Probably Surrounded by Ergosterol: Bimolecular Interactions as Evidenced by Solid‐State NMR and CD Spectra

Amphotericin B (AmB) is thought to exert its pharmacological effects by forming a barrel-stave assembly with ergosterol in fungal membranes. To examine the interaction between AmB and ergosterol (Erg) or cholesterol (Cho), (13)C- and (19)F-labelled covalent conjugates were prepared as reported previously (N. Matsumori et al. Chem. Biol. 2004, 11, 673-679). The CD spectra of the conjugates in a membrane-bound form suggested that the distance between the heptaene moieties of the ergosterol conjugates AmB-C(2)-(6-F)Erg 2 and AmB-C(2)-Erg 3 is similar to that of AmB in ergosterol-containing membranes, but significantly larger than that of AmB in nonsterol or cholesterol-containing membranes. These observations suggest that, as is the case with ergosterol-containing membranes, the conjugated sterol moiety prevents the close contact between the heptaene moieties within the membrane that would reduce channel conductivity of the AmB assemblies. To further investigate this bimolecular interaction, we recorded the solid-state NMR spectra of conjugates 2 and AmB-C(2)-(6-F)Cho 4, which are composed of uniformly (13)C-labelled AmB and 6-fluorinated ergosterol or cholesterol; the conjugates were expected to facilitate the estimation of distances between the fluorine and carbon atoms. By using rotor-synchronous double resonance (rotational echo double resonance of X cluster; RDX) experiments, we deduced the distance between the fluorine atom and its nearest carbon atom in the heptaene moiety of 2 to be less than 8.6 A. This indicates that the B ring of ergosterol comes close to the AmB polyene moiety. A conformational search of the AmB-ergosterol conjugate by using distance constraints derived from the RDX results suggested that ergosterol molecules possibly surround the AmB assembly, which is in contrast with the conventional image in which ergosterol is inserted into AmB molecules.

Helicates, Boxes, and Polymers from Simple Pyridine-Alcohol Ligands: the Impact of the Identity of the Transition Metal Ion

The coordination chemistry of 6-methylpyridine-2-methanol (1) and enantiopure (R)-1-(6-methylpyridin-2-yl)ethanol (2) with a range of divalent first-row transition metal salts has been investigated in an effort to determine whether hydrogen-bonded helicates will form, as observed for cobalt(II) salts. Hydrogen-bonded helicates, [Cu2(1)2(1-H)2X2] (X = Cl, Br), were only observed upon combining 1 with CuCl2 and CuBr2 in MeOH solution. Other metal salts led to alternative products, viz. Cu(ClO4)2 in the presence of base gives [Cu2(1)2(1-H)2](ClO4)2, ZnCl2 and ZnBr2 give the 1-D helical coordination polymers [Zn(1-H)Cl]infinity and [Zn(1-H)Br]infinity, a mixture of NiCl2 and Ni(OAc)2 produces the [Ni4(1-H)4Cl2(OAc)2(MeOH)2] cubane, NiCl2 leads to the [Ni4(1-H)4Cl4(MeOH)4] cubane, while MnCl2 gives the known cubane [Mn4(1-H)6Cl4]. The reaction of 2 with CuCl2 produces the mononuclear complex Lambda-[Cu(2)2Cl]Cl, while reaction with CuBr2 leads to a dimer, Lambda,Lambda-[Cu2(2)3(2-H)Br2]Br, which is held together by a single hydrogen bond between the monomeric subunits. The solid-state CD spectra of these latter complexes were recorded and found to be very similar. The temperature-dependent magnetic behavior of [Cu2(1)2(1-H)2X2] (X = Cl, Br), [Cu2(1)2(1-H)2](ClO4)2, [Cu2(2)3(2-H)Br2]Br, and [Ni4(1-H)4Cl2(OAc)2(MeOH)2] was investigated. Weak antiferromagnetic coupling between the copper(II) centers is mediated by the hydrogen bonds in the [Cu2(1)2(1-H)2X2] (X = Cl, Br) complexes.

Absolute configuration of 1β,10β-epoxydesacetoxymatricarin isolated from Carthamus oxycantha by means of TDDFT CD calculations

The absolute configuration of 1β,10β-epoxydesacetoxymatricarin 1, a sesquiterpenoid isolated from Carthamus oxycantha, was established by TDDFT CD calculations in combination with single crystal X-ray analysis. Since the solid-state CD spectrum shows contributions from intermolecular interactions in the crystal, 1 represents a test case for our solid-state CD/TDDFT approach.

Circular dichroism of diglycosyl dichalcogenides in solution and solid state

Solution and solid-state CD spectra of nine peracetylated and deacetalyted diglycosyl disulfides were measured to study the relationship between the low-energy CD transitions (n1-->sigma*(S--S) and n2-->sigma*(S--S)) and helicity of the inherently chiral disulfide chromophore as perturbed by chiral carbohydrate moieties. The solid-state CD spectra were directly correlated with the reported X-ray structures of Ac4GlcSSGlcAc4 and Ac4GlcSSGalAc4, and the CD data revealed that all the disulfides have M helicity with C1--S--S--C1' dihedral angles -90 degrees < phi < 0 degrees both in solution and in the solid state. A TDDFT CD study was carried out on dimethyl diselenide which confirmed that the same quadrant rule is relevant between the signs of the low-energy CD transitions and the diselenide torsional angle as formulated previously for the disulfide chromophore. The CD spectra of Ac4GlcSeSeGlcAc4 measured in solution and in the solid state were correlated with its X-ray structure and reproduced well by TDDFT CD calculations performed on its tetra-O-acetyl derivative.

Molecular self-assembly and optical activity of chiral thionooxalamic acid esters

Three chiral bisthionooxalamides were synthesized by acylation with ethyl or (1 R)-menthyl chloroxoacetate of the corresponding diamines and subsequent thionation with Lawesson’s reagent. Single crystal X-ray diffraction analysis revealed that products 4b– 7b self-assemble in the solid state by the ring [N–H⋯O C, R 2 2 ( 10 ) ] or chain [N–H⋯S C, C(4)] hydrogen-bond motifs. Only in the case of 4b was a helical superstructure formed. In racemic compound 6b, the molecules are connected via N–H⋯S C hydrogen bonds into homochiral chains, similar to those formed in 7b. The solid state CD spectra of chiral bisthionooxalamides are characterized by strong Cotton effects in the region of the thioamide n–π ∗ transition. Their sign is determined by the helicity of the S C–C O unit.

Absolute configuration of hypothemycin and 5′- O-methylhypothemycin from Phoma sp.—a test case for solid state CD/TDDFT approach

Two metabolites, the known antitumor macrolide hypothemycin 1 and its new 5′- O-methyl ether, 5′- O-methylhypothemycin 2, and the known steroid ergosterol 3, were isolated from a Phoma sp. The structures were elucidated by means of spectroscopic data analysis, and the absolute configuration of hypothemycin 1 was confirmed by single crystal X-ray analysis in combination with the new solid-state CD/TDDFT methodology. Since the solid-state CD spectrum shows contributions from intermolecular interactions in the crystal, 1 represents a critical test case for our solid-state CD/TDDFT approach.

Spontaneous Resolution of Aromatic Sulfonamides: Effective Screening Method and Discrimination of Absolute Structure

An effective screening method combining parallel synthesis and solid-state CD measurements was established to identify achiral aromatic sulfonamides that show spontaneous resolution with rapidity. We found that 4 of the 12 achiral sulfonamides crystallized as chiral crystals through this method. The chirality of each sulfonamide was discriminated by solid-state CD spectra and Flack parameter in an X-ray analysis. Correspondence between the observed Cotton effect and the absolute configuration could be confirmed by time-dependent DFT calculations. [structure: see text]