K. Travis Holman

Polyoxometal cations within polyoxometalate anions. Seven-coordinate uranium and zirconium heteroatom groups in [(UO2)12(μ3-O)4(μ2-H2O)12(P2W15O56)4]32− and [Zr4(μ3-O)2(μ2-OH)2(H2O)4 (P2W16O59)2]14−

Abstract Two new composite polyoxotungstate anions with unprecedented structural features, [(UO 2 ) 12 (μ 3 -O) 4 (μ 2 -H 2 O) 12 (P 2 W 15 O 56 ) 4 ] 32− ( 1 ) and [Zr 4 (μ 3 -O) 2 (μ 2 -OH) 2 (H 2 O) 4 (P 2 W 16 O 59 ) 2 ] 14− ( 2 ) contain polyoxo-uranium and -zirconium clusters as bridging units. The anions are synthesized by reaction of Na 12 [P 2 W 15 O 56 ] with solutions of UO 2 (NO 3 ) 2 and ZrCl 4 . The structure of 1 in the sodium salt contains four [P 2 W 15 O 56 ] 12− anions assembled into an overall tetrahedral cluster by means of trigonal bridging groups formed by three equatorial-edge-shared UO 7 pentagonal bipyramids. The structure of anion 2 consists of a centrosymmetric assembly of two [P 2 W 16 O 59 ] 12− anions linked by a {Zr 4 O 2 (OH) 2 (H 2 O) 4 } 10+ cluster. Both complexes in solution yield the expected two-line 31 P-NMR spectra with chemical shifts of −2.95, −13.58 and −6.45, −13.69 ppm, respectively.

A Water-Soluble Xe@cryptophane-111 Complex Exhibits Very High Thermodynamic Stability and a Peculiar 129Xe NMR Chemical Shift

The known xenon-binding (±)-cryptophane-111 (1) has been functionalized with six [(η(5)-C(5)Me(5))Ru(II)](+) ([Cp*Ru](+)) moieties to give, in 89% yield, the first water-soluble cryptophane-111 derivative, namely [(Cp*Ru)(6)1]Cl(6) ([2]Cl(6)). [2]Cl(6) exhibits a very high affinity for xenon in water, with a binding constant of 2.9(2) × 10(4) M(-1) as measured by hyperpolarized (129)Xe NMR spectroscopy. The (129)Xe NMR chemical shift of the aqueous Xe@[2](6+) species (308 ppm) resonates over 275 ppm downfield of the parent Xe@1 species in (CDCl(2))(2) and greatly broadens the practical (129)Xe NMR chemical shift range made available by xenon-binding molecular hosts. Single crystal structures of [2][CF(3)SO(3)](6)·xsolvent and 0.75H(2)O@1·2CHCl(3) reveal the ability of the cryptophane-111 core to adapt its conformation to guests.

A Cubic, 12-Connected, Microporous Metal-Organometallic Phosphate Framework Sustained by Truncated Tetrahedral Nodes

A rare example of a microporous metal-organic phosphate, [Co(12)(L)(6)(μ(3)-PO(4))(4)(μ(3)-F)(4)(μ-H(2)O)(6)][NO(3)](2) (1), is synthesized by the reaction of a [(η(5)-C(5)H(5))Fe(II)](+)-functionalized terephthalate ligand with Co(NO(3))(2)·6H(2)O and phosphate and fluoride ions generated from the in situ hydrolysis of hexafluorophosphate. 1 is a cubic, 12-connected, face-centered cubic framework sustained by the linear connection of unprecedented, dodecanuclear truncated tetrahedral coordination clusters.

Reversible phase transitions within self-assembled fibrillar networks of (R)-18-(n-alkylamino)octadecan-7-ols in their carbon tetrachloride gels.

The CCl(4) gel phases of a series of low-molecular-mass organogelators, (R)-18-(n-alkylamino)octadecan-7-ols (HSN-n, where n = 0-5, 18 is the alkyl chain length), appear to be unprecedented in that the fibrillar networks of some of the homologues undergo thermally reversible, gel-to-gel phase transitions, and some of those transitions are evident as opaque-transparent changes in the appearance of the samples. The gels have been examined at different concentrations and temperatures by a wide variety of spectroscopic, diffraction, thermal, and rheological techniques. Analyses of those data and data from the neat gelators have led to an understanding of the source of the gel-to-gel transitions. IR and SANS data implicate the expulsion (on heating the lower-temperature gel) or the inclusion (on cooling the higher-temperature gel) of molecules of CCl(4) that are interspersed between fibers in bundles. However, the root cause of the transitions is a consequence of changes in the molecular packing of the HSN-n within the fibers. This study offers opportunities to design new gelators that are capable of behaving in multiple fashions without entering the sol/solution phase, and it identifies a heretofore unknown transformation of organogels.

Molecule-constructed microporous materials: long under our noses, increasingly on our tongues, and now in our bellies.

Crystalline, microporous materials—that is, exhibiting permanent/accessible pores of molecular (< 2 nm) dimensions— constructed from molecular components have been under our noses now for over four decades. Historical examples arose from the study of crystalline clathrates (e.g., Dianin s compound, Werner clathrates) that were discovered to be formally porous. Increasingly, however, molecule-constructed microporous materials are on our tongues in the context of materials design, as the power of synthetic chemistry is progressively being brought to bear on materials related issues. Indeed, science continues to challenge truths related to our current inability to formally predict the crystalline structure—or, therefore, the properties—of molecular materials from a simple knowledge of their building blocks. While Maddox s famous, two-decade old assertion regarding the problem of crystal structure prediction resonates nearly as true today, most contemporary efforts in the synthesis of molecule-constructed materials circumvent the issue of structure prediction by employing empirical strategies that attempt to exploit known, reproducible, structural building blocks. The empirical approach has been particularly fruitful in the development of new microporous materials. Efforts in crystal engineering have led to molecular crystals that exhibit microposority, “porosity without pores”, or other important properties (e.g. reactivity, polarity). Moreover, in recent years it has become almost routine to connect multi-topic organic ligand struts by inorganic coordination clusters to give crystalline coordination polymers or metal–organic frameworks (MOFs) that, through judicious choice of components, can often be expected to exhibit microporosity. Similarly, crystalline covalent organic frameworks (COFs) are now possible through the reversible condensation of rigid, symmetrical molecular precursors. And if one eschews the issue of crystallinity, polymers of intrinsic microporosity (PIMs) are available by the condensation of rigid molecular units that are unable to close-pack. Many of these approaches are buoyed by coincident advances in the fields of supramolecular chemistry, self-assembly, and coordination chemistry and, while an appreciable level of structural design is possible in certain chemical systems—for example, through the reticular approach—the role of serendipity in materials synthesis remains an important one. Two recent articles by Stoddart, Yaghi and co-workers 7] illustrate the roles of design and serendipity at the crossroads of emerging applications for molecule-constructed microporous materials. At a rudimentary level, crystal microporosity permits an influence on the dynamic behavior of atoms and molecules— for example, rotation/translation, uptake/release, sampling/ recognition, switching, reaction—within ordered, three-dimensional nanoenvironments. It is no wonder then that porosity is coupled to technologically useful properties such as ion exchange, separations, storage, heterogeneous catalysis, etc. In an effort to couple the addressable dynamics of mechanically interlocked molecules with the tunable crystalline arrangements afforded by MOFs, a strategy that has been described as robust dynamics, the Stoddart and Yaghi groups have for the first time successfully anchored a molecular catenane within a three-dimensional (3D) MOF (Figure 1). Possessing unresolved chirality, ethynyl moieties, cationic charge, counteranions, high surface area, and exceptional length (3.3 nm!), the catenated ligand ( )-[H2L][PF6]4 presents a significant challenge to a reticular design strategy.

Laying traps for elusive prey: recent advances in the non-covalent binding of anions

The structure and function of a new class of host molecules for the supramolecular complexation of anionic guest species are analysed within the context of other recent advances in the field. In particular, organometallic hosts basd upon the calixarenes, and the related macrocycle cyclotriveratrylene (CTV), are examined. X-Ray crystallographic results clearly demonstrate the inclusion of anionic guest species such as BF4–, I–, CF3SO3–, ReO4–etc. within the ostensibly electron-rich bowl-shaped cavities of both types of host as a result of cooperative effects arising from the presence of two or more metal centres arranged around a common binding pocket. Solution radiochemical studies show that hosts based upon CTV in particular are selective for large tetrahedral anions such as MO4–(M = Tc, Re). It is anticipated that the ability to discriminate between anions on a size and shape selective basis by means of manipulation of host cavity dimensions will pave the way towards new sensor devices and methods of environmental waste remediation.

An achiral form of the hexameric resorcin[4]arene capsule sustained by hydrogen bonding with alcohols

The well-known hexameric capsules sustained by self-assembly of resorcin[4]arenes 1 with water molecules (1(6).(H2O)8) are shown to assemble similarly with (+/-)-2-ethylhexanol (2EH) as an achiral 1(6).(2EH)6.(H2O)2 species which further encapsulates 2EH.

Enclathration and Confinement of Small Gases by the Intrinsically 0D Porous Molecular Solid, Me,H,SiMe2

The stable, guest-free crystal form of the simple molecular cavitand, Me,H,SiMe2, is shown to be intrinsically porous, possessing discrete, zero-dimensional (0D) pores/microcavities of about 28 Å(3). The incollapsible 0D pores of Me,H,SiMe2 have been exploited for the enclathration and room temperature (and higher) confinement of a wide range of small gases. Over 20 isostructural x(gas/guest)@Me,H,SiMe2 (x ≤ 1) clathrates (guest = H2O, N2, Ar, CH4, Kr, Xe, C2H4, C2H6, CH3F, CO2, H2S, CH3Cl, CH3OCH3, CH3Br, CH3SH, CH3CH2Cl, CH2Cl2, CH3I, CH3OH, BrCH2Cl, CH3CH2OH, CH3CN, CH3NO2, I2), and a propyne clathrate (CH3CCH@Me,H,SiMe2·2CHCl3), have been prepared and characterized, and their single crystal structures determined. Gas enclathration is found to be highly selective for gases that can be accommodated by the predefined, though slightly flexible 0D pore. The structure determinations provide valuable insight, at subangstrom resolution, into the factors that govern inclusion selectivity, gas accommodation, and the kinetic stability of the clathrates, which has been probed by thermal gravimetric analysis. The activation (emptying) of several clathrates (guest = H2O, N2, CO2, Kr, CH3F) is shown to occur in a single-crystal-to-single-crystal (SC → SC) fashion, often requiring elevated temperatures. Akin to open pore materials, water vapor and CO2 gas are shown to be taken up by single crystals of empty Me,H,SiMe2 at room temperature, but sorption rates are slow, occurring over weeks to months. Thus, Me,H,SiMe2 exhibits very low, but measurable, gas permeability, despite there being no obvious dynamic mechanism to facilitate gas uptake. The unusually slow exchange kinetics has allowed the rates of gas (water vapor and CO2) sorption to be quantified by single crystal X-ray diffraction. The data are well fit to a simple three-dimensional diffusion model.

Polymorphism and inclusion properties of three-dimensional metal-organometallic frameworks derived from a terephthalate sandwich compound.

An organometallic sandwich compound of terephthalic acid, namely, [(eta(5)-Cp)Fe(II){eta(6)-(1,4-C(6)H(4)(COOH)(2))}](+) (H(2)1(+)), is reported, along with X-ray single crystal structures of [H1 x H(2)1][PF(6)] and H1. [H(2)1 x H1][PF(6)] was reacted with the nitrate salts of Co(II) and Ni(II) to yield a series of three-dimensional (3D) metal-organometallic framework (MOMF) materials of the composition [M(3)(1)(4)(mu-H(2)O)(2)(H(2)O)(2)][NO(3)](2) x xsolvent (M = Co(II) (2), Ni(II) (3); xsolvent = 4EtOH, or 2DMF x 2 H(2)O). These framework structures were shown by single crystal and powder X-ray diffraction to be polymorphic, possessing identical 3D body-centered tetragonal network topologies, but differing in the manner by which the [CpFe](+) groups are arranged within the two-dimensional, square grid sheets of the 3D networks. alpha-2-EtOH, beta-2-EtOH, alpha-3-EtOH, beta-2-DMF, and beta-3-DMF were thermally desolvated, giving rise to isolable apohosts of composition [M(3)(1)(4)(mu-H(2)O)(2)(H(2)O)(2)][NO(3)](2) (M = Co(II) (2), Ni(II) (3)) that were shown by PXRD to possess different, as yet unknown, crystal structures. The desolvated apohosts were studied with respect to their ability to selectively reabsorb water and/or alcohols. They show a modest preference for the absorption of water and short chain, linear alcohols (

Supramolecular chemistry of [{M(CO)3(µ3-OH)}4](M = Mn or Re): a modular approach to crystal engineering of superdiamondoid networks

The compounds [{M(CO)3(µ3-OH)}4](M = Mn 1 or Re 2), which are cubane-like molecules possessing Td symmetry with four strong hydrogen-bond donor moieties rigidly directed towards the vertices of a tetrahedron, cocrystallized with 2 equivalents of a series of linear difunctional hydrogen-bond acceptor molecules or ‘spacers’ to afford 14 three-dimensional superdiamondoid networks with varying, but predictable, extents of interpenetration according to X-ray crystallography. The extent can be rationalized on the basis of the volume and length of the spacer molecule: benzene, toluene, p-xylene, p-fluorotoluene, naphthalene, 1-methylnaphthalene (all viaπ-hydrogen bonds), N,N,N′,N′-tetramethylethane-1,2-diamine (tmen), 1,2-bis(diphenylphosphoryl)ethane (dppoe) and 1,4-diamino-benzene (dab) afford two-fold networks; 1,2-diaminoethane (en) and 2-chloropyrazine (cpyz) afford three-fold networks; 4,4′-bipyridyl (bipy) and 4,4′-bipiperidine (bipip) afford four-fold networks. The adducts 1·2dppoe, 2·2dab, 1·2bipy, 2·2bipy and 1·2bipip crystallize with enough space in the crystal lattice to enclathrate one, four, two, two and two acetonitrile molecules per molecule of cubane, respectively. All fourteen networks crystallize in space groups that reflect at least some of the symmetry inherently present in 1 and 2.