The Synthesis and Structure of a Scandium Nitrate Hydroxy-Bridged Dimeric Complex Supported by Bipyridyl Ligands.

The current discussion on whether scandium, yttrium and lanthanum should represent Group 3 in the Periodic Table or whether lutetium should replace lanthanum in the group has prompted us to further explore the structural chemistry of the Group 3 elements and compare the coordination numbers and coordination geometries adopted. The steric and electronic properties of the coordinated ligands have a major influence on the structures adopted. We report the synthesis and crystal structure determination of an unusual dinuclear scandium complex [(bipy)(NO3)2Sc(µ-OH)2Sc(NO3)2(bipy)] obtained by the reaction of hydrated scandium nitrate with 2,2'-bipyridyl (bipy) in either ethanol or nitromethane. The crystal structure of the complex shows that the scandium centers are eight coordinate, and the structure obtained contrasts with related complexes found in the lanthanide series [Ln(bipy)2(NO3)3] and [Ln(phen)2(NO3)3] (phen = phenanthroline) and in [M(terpy)(NO3)3] (M = Sc, Er-Lu), where these complexes are all mononuclear.

Sponge-Like Behaviour in Isoreticular Cu(Gly-His-X) Peptide-Based Porous Materials.

We report two isoreticular 3D peptide-based porous frameworks formed by coordination of the tripeptides Gly-L-His-Gly and Gly-L-His-L-Lys to Cu(II) which display sponge-like behaviour. These porous materials undergo structural collapse upon evacuation that can be reversed by exposure to water vapour, which permits recovery of the original open channel structure. This is further confirmed by sorption studies that reveal that both solids exhibit selective sorption of H2 O while CO2 adsorption does not result in recovery of the original structures. We also show how the pendant aliphatic amine chains, present in the framework from the introduction of the lysine amino acid in the peptidic backbone, can be post-synthetically modified to produce urea-functionalised networks by following methodologies typically used for metal-organic frameworks built from more rigid "classical" linkers.

Coordination polymer flexibility leads to polymorphism and enables a crystalline solid-vapour reaction: a multi-technique mechanistic study.

Despite an absence of conventional porosity, the 1D coordination polymer [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)3 ] (1; TMP=tetramethylpyrazine) can absorb small alcohols from the vapour phase, which insert into AgO bonds to yield coordination polymers [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)3 (ROH)2 ] (1-ROH; R=Me, Et, iPr). The reactions are reversible single-crystal-to-single-crystal transformations. Vapour-solid equilibria have been examined by gas-phase IR spectroscopy (K=5.68(9)×10(-5) (MeOH), 9.5(3)×10(-6) (EtOH), 6.14(5)×10(-5) (iPrOH) at 295 K, 1 bar). Thermal analyses (TGA, DSC) have enabled quantitative comparison of two-step reactions 1-ROH→1→2, in which 2 is the 2D coordination polymer [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)2 ] formed by loss of TMP ligands exclusively from singly-bridging sites. Four polymorphic forms of 1 (1-A(LT) , 1-A(HT) , 1-B(LT) and 1-B(HT) ; HT=high temperature, LT=low temperature) have been identified crystallographically. In situ powder X-ray diffraction (PXRD) studies of the 1-ROH→1→2 transformations indicate the role of the HT polymorphs in these reactions. The structural relationship between polymorphs, involving changes in conformation of perfluoroalkyl chains and a change in orientation of entire polymers (A versus B forms), suggests a mechanism for the observed reactions and a pathway for guest transport within the fluorous layers. Consistent with this pathway, optical microscopy and AFM studies on single crystals of 1-MeOH/1-A(HT) show that cracks parallel to the layers of interdigitated perfluoroalkyl chains develop during the MeOH release/uptake process.

Chemical and structural stability of zirconium-based metal-organic frameworks with large three-dimensional pores by linker engineering.

The synthesis of metal-organic frameworks with large three-dimensional channels that are permanently porous and chemically stable offers new opportunities in areas such as catalysis and separation. Two linkers (L1=4,4',4'',4'''-([1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoic acid, L2=4,4',4'',4'''-(pyrene-1,3,6,8-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid) were used that have equivalent connectivity and dimensions but quite distinct torsional flexibility. With these, a solid solution material, [Zr6 O4 (OH)4 (L1)2.6 (L2)0.4 ]⋅(solvent)x , was formed that has three-dimensional crystalline permanent porosity with a surface area of over 4000 m(2) g(-1) that persists after immersion in water. These properties are not accessible for the isostructural phases made from the separate single linkers.

Solid-state interconversions: unique 100 % reversible transformations between the ground and metastable states in single-crystals of a series of nickel(II) nitro complexes.

The solid-state, low-temperature linkage isomerism in a series of five square planar group 10 phosphino nitro complexes have been investigated by a combination of photocrystallographic experiments, Raman spectroscopy and computer modelling. The factors influencing the reversible solid-state interconversion between the nitro and nitrito structural isomers have also been investigated, providing insight into the dynamics of this process. The cis-[Ni(dcpe)(NO2)2] (1) and cis-[Ni(dppe)(NO2)2] (2) complexes show reversible 100 % interconversion between the η(1)-NO2 nitro isomer and the η(1)-ONO nitrito form when single-crystals are irradiated with 400 nm light at 100 K. Variable temperature photocrystallographic studies for these complexes established that the metastable nitrito isomer reverted to the ground-state nitro isomer at temperatures above 180 K. By comparison, the related trans complex [Ni(PCy3)2(NO2)2] (3) showed 82 % conversion under the same experimental conditions at 100 K. The level of conversion to the metastable nitrito isomers is further reduced when the nickel centre is replaced by palladium or platinum. Prolonged irradiation of the trans-[Pd(PCy3)2(NO2)2] (4) and trans-[Pt(PCy3)2(NO2)2] (5) with 400 nm light gives reversible conversions of 44 and 27 %, respectively, consistent with the slower kinetics associated with the heavier members of group 10. The mechanism of the interconversion has been investigated by theoretical calculations based on the model complex [Ni(dmpe)Cl(NO2)].

Guest-adaptable and water-stable peptide-based porous materials by imidazolate side chain control.

The peptide-based porous 3D framework, ZnCar, has been synthesized from Zn(2+) and the natural dipeptide carnosine (ß-alanyl-L-histidine). Unlike previous extended peptide networks, the imidazole side chain of the histidine residue is deprotonated to afford Zn-imidazolate chains, with bonding similar to the zeolitic imidazolate framework (ZIF) family of porous materials. ZnCar exhibits permanent microporosity with a surface area of 448 m(2) g(-1) , and its pores are 1D channels with 5 Šopenings and a characteristic chiral shape. This compound is chemically stable in organic solvents and water. Single-crystal X-ray diffraction (XRD) showed that the ZnCar framework adapts to MeOH and H2 O guests because of the torsional flexibility of the main His-ß-Ala chain, while retaining the rigidity conferred by the Zn-imidazolate chains. The conformation adopted by carnosine is driven by the H bonds formed both to other dipeptides and to the guests, permitting the observed structural transformations.

A polar corundum oxide displaying weak ferromagnetism at room temperature.

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO(3) (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO(3) has a weak ferromagnetic ground state below 356 K-this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO(3).

Enhanced stability in rigid peptide-based porous materials.

Pepped up: Notwithstanding the intrinsic conformational flexibility of peptides, [Zn(Gly-Thr)(2)] behaves as a robust porous metal-organic framework thanks to the rigidity introduced by the use of Gly-Thr (see scheme). This rigidity arises from the sequence of amino acids in the dipeptide that locks its conformational flexibility in the framework.

Bis(tert-butyl isocyanide-κC)[4-fluoro-N-({2-[N-(4-fluorophenyl)carboximidoyl]cyclopenta-2,4-dien-1-ylidene}methyl)anilinido-κ2N,N']copper(I).

The solid-state structure of the title compound, [Cu(C(19)H(13)F(2)N(2))(C(5)H(9)N)(2)], shows that the Cu(I) centre adopts a distorted tetrahedral coordination geometry, being coordinated by two N atoms of the 6-aminofulvene-2-aldimine (AFA) chelating ligand and by the bridgehead C atoms of the two isocyanide ligands. The cyclopentadienyl and imine components of the AFA ligand are approximately coplanar, with an angle between the planes of 5.00 (3)°. The Cu atom lies 0.6460 (3) Å above the imine plane defined by the N and C atoms of the seven-membered metallocycle. There is also an uncommon C-H···Cu anagostic interaction, with an intramolecular Cu···H distance of 2.67 Å, which is less than the sum of the van der Waals radii.

Spin-spin interactions in porphyrin-based monoverdazyl radical hybrid spin systems.

The spin-spin interactions in a complex consisting of a metalloporphyrin with a verdazyl radical attached at one of the beta positions of the porphyrin ring are investigated. The X-ray crystal structure of the copper porphyrin complex shows that the plane of the verdazyl moiety is oriented such that it is nearly perpendicular to the plane of the porphyrin ring so that weak magnetic interactions between the metal and radical are expected. Consistent with this expectation, magnetic susceptibility and continuous-wave electron paramagnetic resonance (EPR) measurements of the copper (d(9)) and vanadyl (d(1)) versions of the porphyrin show that the metal and radical are weakly antiferromagnetically coupled. Thus, the ground state is a singlet, but the triplet state is thermally accessible above approximately 5 K. Spin-polarized transient EPR measurements of the free-base analogue show that its lowest excited state is a quartet, indicating that the verdazyl radical couples ferromagnetically to the triplet excited state of the porphyrin. Low-temperature transient EPR measurements on the vanadyl porphyrin reveal that the lowest excited quintet state is populated. This implies that the antiferromagnetic coupling between the metal and radical observed in the ground state is switched to a ferromagnetic arrangement in the excited state by the presence of the unpaired electrons in the pi and pi* orbitals of the porphyrin.

Probing flexibility in porphyrin-based molecular wires using double electron electron resonance.

A series of butadiyne-linked zinc porphyrin oligomers, with one, two, three, and four porphyrin units and lengths of up to 75 A, have been spin-labeled at both ends with stable nitroxide TEMPO radicals. The pulsed EPR technique of double electron electron resonance (DEER) was used to probe the distribution of intramolecular end-to-end distances, under a range of conditions. DEER measurements were carried out at 50 K in two types of dilute solution glasses: deutero-toluene (with 10% deutero-pyridine) and deutero-o-terphenyl (with 5% 4-benzyl pyridine). The complexes of the porphyrin oligomers with monodentate ligands (pyridine or 4-benzyl pyridine) principally adopt linear conformations. Nonlinear conformations are less populated in the lower glass-transition temperature solvent. When the oligomers bind star-shaped multidentate ligands, they are forced to bend into nonlinear geometries, and the experimental end-to-end distances for these complexes match those from molecular mechanics calculations. Our results show that porphyrin-based molecular wires are shape-persistent, and yet that their shapes can deformed by binding to multivalent ligands. Self-assembled ladder-shaped 2:2 complexes were also investigated to illustrate the scope of DEER measurements for providing structural information on synthetic noncovalent nanostructures.

Interlocked host anion recognition by an indolocarbazole-containing [2]rotaxane.

The design, synthesis, structure, and anion-binding properties of the first indolocarbazole-containing interlocked structure are described. The novel [2]rotaxane molecular structure incorporates a neutral indolocarbazole-containing axle component which is encircled by a tetracationic macrocycle functionalized with an isophthalamide anion recognition motif. (1)H NMR and UV-visible spectroscopies and X-ray crystallography demonstrated the importance of pi-donor-acceptor, CH...pi, and electrostatic interactions in the assembly of pseudorotaxanes between the electron-deficient tetracationic macrocycle and a series of pi-electron-rich indolocarbazole derivatives. Subsequent urethane stoppering of one of these complexes afforded a [2]rotaxane, which was shown by (1)H NMR spectroscopic titration experiments to exhibit enhanced chloride and bromide anion recognition compared to its non-interlocked components. Computational molecular dynamics simulations provide further insight into the mechanism and structural nature of the anion recognition process, confirming it to involve cooperative hydrogen-bond donation from both macrocycle and indolocarbazole components of the rotaxane. The observed selectivity of the [2]rotaxane for chloride is interpreted in terms of its unique interlocked binding cavity, defined by the macrocycle isophthalamide and indolocarbazole N-H protons, which is complementary in size and shape to this halide guest.

Enhanced control over metal composition in mixed Ga/Zn and Ga/Cu coordinated pyrogallol[4]arene nanocapsules.

The stepwise synthesis of mixed metal-organic C-alkylpyrogallol[4]arene nanocapsules allows control over the metal ratios in the resulting assemblies via sequential incorporation and ejection of different metals.

Building multistate redox-active architectures using metal-complex functionalized perylene bis-imides.

A series of multistate redox-active architectures has been synthesized, structurally characterized, and their optical and redox properties investigated. Specifically, two redox-active ferrocene or cobalt-dithiolene moieties have been introduced to the "bay" region of perylene-bisimides. Three of these disubstituted perylene-bisimide species have been structurally characterized by single crystal X-ray diffraction, confirming the twisted nature of the central perylene core. The first isomeric pair of disubstituted perylene-bisimide isomers, N,N'-di-(n-butyl)-1,7-diferrocenyl-perylene-3,4:9,10-tetracarboxylic acid bisimide (2) and N,N'-di-(n-butyl)-1,6-diferrocenyl-perylene-3,4:9,10-tetracarboxylic acid bisimide (3), structurally characterized by single crystal X-ray diffraction are reported and compared. Structural characterization of the cobalt-dithiolene substituted perylene-bisimide, N,N'-di-(n-butyl)-1,7-dicyclopentadienyl-cobalt(II)-dithiolenyl-perylene-3,4:9,10-tetracarboxylic acid bisimide (4), reveals the expected twisting of the perylene core and confirms the ene-dithiolate geometry of the cobalt dithiolene moiety. Cyclic voltammetry measurements, coupled with spectroelectrochemcial and electron paramagnetic resonance studies, of 1-4, where 1 is N,N'-di-(n-butyl)-1,7-diethynylferrocenyl-perylene-3,4:9,10-tetracarboxylic acid bisimide, reveal the two anticipated perylene-bisimide based reductions. In addition, for the ferrocene substituted compounds, 1-3, a single reversible two-electron oxidation is seen with only a small degree of communication between the ferrocene groups observed in the 1,6-isomer where the two ferrocene groups are attached to the same naphthyl moiety. In the case of 4, two reversible reductions associated with the cobalt-dithiolene moieties are observed, confirming communication across the reduced perylene core.

Synthesis, characterization, and electrochemistry of a series of iron(II) complexes containing self-assembled 1,5-diaza-3,7-diphosphabicyclo[3.3.1]nonane ligands.

The reaction between PPh(CH(2)OH)(2), iron(II) sulfate, ammonium sulfate, and formaldehyde in aqueous solution gives the iron(II) complex [Fe(kappa(2)-O(2)SO(2))L(2)] (1), where L is the bidentate phosphine ligand 3,7-diphenyl-1,5-diaza-3,7-diphosphabicyclo[3.3.1]nonane. During the course of the reaction, the ligand L self-assembles on the metal center. The reaction between PPh(CH(2)OH)(2), iron(II) chloride, ammonium chloride, and formaldehyde under similar conditions gives cis-[FeCl(2)L(2)] (cis-2). The complex cis-2 is converted into trans-2 in Et(2)O, whereas in water it is converted into cis-[Fe(OH(2))(2)L(2)](2+), though both of these interconversions are reversible. The chloro ligands in cis-2 are readily displaced by reaction with thiocyanate, azide, and carbonate to give cis- and trans-[Fe(NCS)(2)L(2)] (cis- and trans-3), cis- and trans-[Fe(N(3))(2)L(2)] (cis- and trans-4), and [Fe(kappa(2)-O(2)CO)L(2)] (5), respectively. The complex cis-2 reacts with CO in water to give trans-[FeCl(CO)L(2)]Cl (trans-6), whereas trans-2 reacts with CO in diethyl ether to give cis-[FeCl(CO)L(2)]Cl (cis-6), though cis-6 isomerizes in water to form trans-6. The reaction of cis-2 with sodium borohydride gives the hydride chloride complex trans-[FeCl(H)L(2)] (7). Electrochemical studies have been undertaken on complexes 1, cis-2, and 7. These reveal reversible oxidations for cis-2 and 7, with the latter giving rise to an unusual 17-electron iron(III) hydride chloride complex. Crystal structures have been obtained for 1, trans-2, trans-3, 5, and 7.

A time-resolved infrared vibrational spectroscopic study of the photo-dynamics of crystalline materials.

Time-resolved infrared vibrational spectroscopy is a structurally sensitive probe of the excited-state properties of matter. The technique has found many applications in the study of molecules in dilute solution phase but has rarely been applied to crystalline samples. We report on the use of a sensitive pump-probe time-resolved infrared spectrometer and sample handling techniques for studies of the ultrafast excited-state dynamics of crystalline materials. The charge transfer excited states of crystalline metal carbonyls and the proton transfer of dihydroxyquinones are presented and compared with the solution phase.

Comparison of the effects of pressure on three layered hydrates: a partially successful attempt to predict a high-pressure phase transition.

We report the effect of pressure on the crystal structures of betaine monohydrate (BTM), L-cysteic acid monohydrate (CAM) and S-4-sulfo-L-phenylalanine monohydrate (SPM). All three structures are composed of layers of zwitterionic molecules separated by layers of water molecules. In BTM the water molecules make donor interactions with the same layer of betaine molecules, and the structure remains in a compressed form of its ambient-pressure phase up to 7.8 GPa. CAM contains bi-layers of L-cysteic acid molecules separated by water molecules which form donor interactions to the bi-layers above and below. This phase is stable up to 6.8 GPa. SPM also contains layers of zwitterionic molecules with the waters acting as hydrogen-bond donors to the layers above and below. SPM undergoes a single-crystal to single-crystal phase transition above 1 GPa in which half the water molecules reorient so as to form one donor interaction with another water molecule within the same layer. In addition, half of the S-4-sulfo-L-phenylalanine molecules change their conformation. The high-pressure phase is stable up to 6.9 GPa, although modest rearrangements in hydrogen bonding and molecular conformation occur at 6.4 GPa. The three hydrates had been selected on the basis of their topological similarity (CAM and SPM) or dissimilarity (BTM) with serine hydrate, which undergoes a phase transition at 5 GPa in which the water molecules change orientation. The phase transition in SPM shows some common features with that in serine hydrate. The principal directions of compression in all three structures were found to correlate with directions of hydrogen bonds and distributions of interstitial voids.

X-ray crystal structures of diacetates of 6-s-cis and 6-s-trans astaxanthin and of 7,8-didehydroastaxanthin and 7,8,7',8'-tetradehydroastaxanthin: comparison with free and protein-bound astaxanthins.

The crystal structures of the 6-s-cis [s-cis-(1)] and 6-s-trans [s-trans-(1)] conformers of the diacetates of astaxanthin (AXT) and those of (3S,3'S)-7,8-didehydroastaxanthin [(3S,3'S)-3,3'-dihydroxy-7,8-didehydro-beta,beta-carotene-4,4'-dione (2)] and (3S,3'S)-7,8,7',8'-tetradehydroastaxanthin [(3S,3'S)-3,3'-dihydroxy-7,8,7',8'-tetradehydro-beta,beta-carotene-4, 4'-dione (3)] are reported. The conformations of these four molecules vary in particular with the angle of twist of the end rings out of the plane of the polyene chain; for s-cis-(1), the end rings are twisted out of the plane of the polyene chain by an angle of -49.0 (5)degrees , and the conformation is therefore similar to that found for unesterified AXT as well as for the carotenoids, canthaxanthin and beta,beta-carotene. For s-trans-(1), the end rings are coplanar with the polyene chain and its conformation is much more similar to that of the protein-bound AXT in the blue protein, crustacyanin, which is found in the shell of lobsters, although s-trans-(1) shows much less bowing of the polyene chain. In (2) and (3) the end rings are also almost coplanar with the polyene chain with the end rings in (2) in the s-cis conformation, and in (3) in the s-trans conformation. Thus, an extensive ensemble of the possible beta end-ring conformations has been determined. These structures are compared with one another as well as unbound, unesterified AXT and protein-bound AXT. Also, the effect of the end-ring conformations on the colour and UV-vis spectra of the crystals was established.

Nitrogen and hydrogen adsorption by an organic microporous crystal.

Quick on the uptake: Following its identification during a targeted search, the intriguing crystal structure of 3,3',4,4'-tetra(trimethylsilylethynyl)biphenyl was investigated. Simple removal of the included solvent provides an organic crystal with an open microporous structure that has a striking similarity to that of zeolite A (see picture). Reversible adsorption of nitrogen and hydrogen gases at 77 K confirms that the microporosity is permanent.

Noncovalent interactions under extreme conditions: high-pressure and low-temperature diffraction studies of the isostructural metal-organic networks (4-chloropyridinium)2[CoX4] (X = Cl, Br).

The crystal structures of the two isostructural metal-organic salts, (4-chloropyridinium) 2[CoX 4], X = Cl ( 1), Br ( 2), have each been determined at nine temperatures from 30 to 300 K and at nine pressures from atmospheric pressure to 4.2 GPa. A 5% reduction in unit cell volume is observed upon temperature reduction, whereas an 18-19% reduction is observed upon increasing the pressure over the ranges studied. The structures adopt a tape arrangement propagated by bifurcated N-H...X 2Co hydrogen bonds and by Co-X...Cl-C halogen bonds. Intertape interactions include type I Co-X...Cl-C and C-Cl...Cl-C halogen-halogen interactions as well as offset pi-stacking between the aromatic rings of the cations. Although little anisotropy in compression is seen upon temperature reduction, marked anisotropy in compression is observed upon pressure increase. Compression between tapes far exceeds compression along the tape, consistent with the strong attractive nature of the intratape N-H...X 2Co and Co-X...Cl-C interactions and the weaker dispersion dominated Co-X...Cl-C and C-Cl...Cl-C halogen-halogen interactions and pi-stacking interactions. Increased distortion of the [CoX 4] (2-) anions from idealized tetrahedral geometry arises upon pressure increase, consistent with local changes in the electric field that result from compression of the pairs of pi-stacked cations. The study of the isostructural pair of compounds permits a rare opportunity for quantitative evaluation of the "internal" or "chemical" pressure exerted by changing the [CoBr 4] (2-) anion for the smaller [CoCl 4] (2-) anion. Thus, crystal structures of 1 and 2 with equivalent unit cell volumes require an additional pressure of ca. 1 GPa exerted upon the structure containing the larger [CoBr 4] (2-) anion ( 2). This internal pressure increases to ca. 1.9 GPa at the highest pressures used in this study. Most significant is that examination of the isovolumetric pairs of structures shows that the structures containing the [CoCl 4] (2-) anion are contracted along the <101> vector, the direction of tape propagation, by ca. 1.2% and correspondingly expanded in other directions, relative to those containing the [CoBr 4] (2-) anion. Since the effect of difference in anion size has been removed by application of pressure, this anisotropy in dimensions clearly indicates that the N-H...X 2Co hydrogen bonds and Co-X...Cl-C halogen bonds are more strongly attractive for X = Cl rather than X = Br. Use of internal pressure thereby provides unique insight into the relative strength of intermolecular interactions.