James D. Martin

Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis

The thermal-induced and photoinduced valence tautomerism of a series of Co(dioxolene)(2)(4-X-py)(2) complexes (dioxolene = 3,5-di-tert-butylcatecholate or 3,5-di-tert-butylsemiquinonate; 4-X-py = 4-(X)pyridine, X = H (1), OMe (2), Me (3), CN (4), Br (5), NO(2) (6)) is described. The thermal valence tautomerism (ls-Co(III)(SQ)(Cat)(4-X-py)(2) <--> hs-Co(II)(SQ)(SQ)(4-X-py)(2)) is only observed for complexes 4, 5, and 6 where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample of 4-6 is held at 10 K in a SQUID magnetometer and irradiated with white light (lambda = 400-850 nm), the hs-Co(II) tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-Co(II) tautomer persists until ca. 90 K, approximately 40 K higher than the thermal stability of previously reported complexes. Heating and cooling the sample while maintaining irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (DeltaT = ca. 25 K). Below 50 K, the hs-Co(II) tautomer displays temperature-independent relaxation to the ls-Co(III) form, and above 50 K, the relaxation is thermally activated with an activation energy E(a) > ca. 1500 cm(-1). The coordination geometry (trans-pyridines), pyridine substitution, and crystal packing forces conspire to create the comparatively thermally stable photogenerated hs-Co(II) tautomer, thus providing an excellent handle for molecular and crystal engineering studies.

Designing intermediate-range order in amorphous materials

Amorphous materials are commonly understood to consist of random organizations of molecular-type structural units. However, it has long been known that structural organizations intermediate between discrete chemical bonds and periodic crystalline lattices are present even in liquids. Numerous models—including random networks and crystalline-type structures with networks composed of clusters and voids—have been proposed to account for this intermediate-range order. Nevertheless, understanding and controlling structural features that determine intermediate-range order in amorphous materials remain fundamental, yet presently unresolved, issues. The most characteristic signature of such order is the first peak in the total structure factor, referred to as the first sharp diffraction peak or ‘low Q’ structure. These features correspond to large real-space distances in the materials, and understanding their origin is key to unravelling details of intermediate-range order. Here we employ principles of crystal engineering to design specific patterns of intermediate-range order within amorphous zinc-chloride networks. Using crystalline models, we demonstrate the impact of various structural features on diffraction at low values of Q. Such amorphous network engineering is anticipated to provide the structure/property relationships necessary to tailor specific optical, electronic and mechanical properties.

University learning: Improve undergraduate science education

It is time to use evidence-based teaching practices at all levels by providing incentives and effective evaluations, urge Stephen E. Bradforth, Emily R. Miller and colleagues.

The Hydrogen‐Bonded Framework of the First Anti‐Zeotype: [{(H2NEt2)2}2(CuCl4)][AlCl4]

A two-coordinate cationic link between [CuCl4 ]3- tetrahedra by means of two inversion-related dialkylammonium cations yields the hydrogen-bonded A2 X anti-cristobalite framework [{(H2 NEt2 )2 }2 (CuCl4 )]+ . This novel cationic framework is constructed around space-filling and templating [AlCl4 ]- anions resulting in large pores (16.6×6.7 Å).

Magnetic bistability in a cobalt bis(dioxolene) complex: long-lived photoinduced valence tautomerism.

The thermal- and photoinduced valence tautomerism of a cobalt bis(dioxolene) complex is described. The thermal conversion is precipitous, complete within 10 K, and is accompanied by a 5 K hysteresis loop (107 K < T(1/2) < 112 K). Rapid thermal quenching (300 K --> 10 K in ca. 5 s) and photoinduced valence tautomerism result in trapping of the metastable Co(II)-state at low temperatures through intermolecular hydrogen bonding. This lattice stabilization results in unmatched kinetic and thermal stability for a valence tautomer from 10-50 K, with residual hs-Co(II) persisting until about 90 K.

Mixed Anion (Phosphate/Oxalate) Bonding to Iron(III) Materials

A novel phosphate/oxalate inorganic-organic hybrid material has been prepared to elucidate synthesis and bonding characteristics of iron(III) with both phosphate and organic matter (OM). Such mixed anion bonding of inorganic oxyanions and OM to iron(III) and aluminum(III) in environmental systems has been proposed but not proven, mainly because of the complexity of natural geochemical matrices. The compound reported here with the molecular formula of [C(3)H(12)N(2)](2)[Fe(5)(C(2)O(4))(2)(H(x)PO(4))(8)] (I) was hydrothermally synthesized and characterized by single crystal X-ray diffraction and X-ray absorption spectroscopy (XAS). In this new structure, Fe-O octahedra and P-O tetrahedra are connected by corner-sharing to form a 2-D network in the a-b plane. Oxalate anions cross-link these Fe-P layers constructing a 3-D anionic framework. A diprotonated structure-directing template, DAP (1,3-diaminopropane), resides in the oxalate layer of the structure and offsets the negative charge of the anionic framework. Iron K-edge XANES spectra confirmed that the iron in I is Fe(III). The crystal structure of I is used to successfully fit its Fe K-edge EXAFS spectrum, which exhibits spectral signatures that unambiguously identify iron-phosphate and iron-OM bonding. Such molecular spectroscopic features will be invaluable for the evaluation of complex environmental systems. Furthermore, syntheses demonstrated the critical role of the templating amine to mediate whether or not the iron(III) is reduced by the organic acid.

A comparison of heterometallic alkoxide molecules containing copper(I) or copper(II) and zirconium

Abstract The synthesis, characterization and structure and thermal decomposition of ClCuIIZr2(OiPr)9 (1) and Cu2IZr2(OiPr)10 (2) are reported. Compound 1 has a CuZr2(μ3-OR)2(μ2-OR)3 central core, with chloride as a terminal ligand on copper. This paramagnetic CuII species is particularly interesting in that the 1H NMR signals of those alkoxides which are μ2- and μ3-bridged to copper undergo paramagnetic shifts (to 8.66 and 10.41 ppm at 25°C), but those of the remaining alkoxide groups are unperturbed from the region characteristic of the Zr2(OiPr)9− unit. Compound 2 consists of a Zr2(OiPr)9− face-sharing bioctahedron with two μ2-alkoxides bridging to a Cu2IOR+ fragment giving the copper a linear two-coordinate environment. Both 1 and 2 are highly soluble in hydrocarbon solvents. TGA studies of both compounds reveal information on possible mechanisms and products of thermolysis.

The tungsten-tungsten triple bondXVI. Bis(cyclopentadienyl)- and bis(indenyl)tetradimethylamidoditungsten (WW) ☆

Abstract From the reaction between 1,2-W2Cl2(NMe2)4 and NaCp (Cp = C5H5) (two equiv.) in toluene, a yellow-orange, hydrocarbon-soluble, crystalline solid of formula W2Cp2(NMe2)4 has been obtained. NMR data are consistent with a gauche 1,2- W2R2(NMe2)4 molecule having a WW triple bond and restricted rotations about the WN bonds. The NMR data, however, do not uniquely define the nature of the CpW bonding beyond the fact that there are five time-averaged equivalent carbon and hydrogen atoms. No crystals suitable for an X-ray study were obtained. The related reaction involving 1,2-W2Cl2(NMe2)4 and LiC9H7 (C9H7 = indenyl) (two equiv.) gave an orange, hydrocarbon-soluble, crystalline compound. The NMR data are indicative of a gauche 1,2- W2R2(NMe2)4 molecule in solution having a virtual C2 axis of symmetry and restricted rotations about the WN bonds. An X-ray study rev ealed the presence of the gauche rotamer and that the indenyl-metal bonding was η3-C9H7. Pertinent bond distances (A) and angles (°) for 1,2-W2(η3-C9H7)2(NMe2)4 are WW = 2.334(1), WN = 1.96(1) (av.), and W-η3-C3(indenyl) = 2.36(1)–2.54(1), WWN = 102(1) (av.) and WWC(η3-C3(indenyl)) = 91–117°. The reaction between 1,2-W2Cp2(NMe2)4 and EtCOOCOEt (> four equiv.) proceeds in hydrocarbon solvents to give the compound W2Cp2(NMe2)(O2CEt)3 and Me2NCOEt (three equiv.) in contrast to other 1,2-W2R2(NMe2)4 compounds which yield either W2(O2CEt)4 when R contains β-hydrogen atoms or W2R2(O2CEt)4 compounds. The molecular structure of W2Cp2(NMe2)(O2CEt)3 can be viewed as two fused four-legged piano-stools sharing an edge, μ-NMe2 and μ-η1-O2CEt, with a pair of bridging O 2CEt ligands spanning a WW bond of distance 2.78 A. The Cp ligands form W-η5-C5H5 bonds roughly trans to the WW axis.

Crystalline and liquid structure of zinc chloride trihydrate: a unique ionic liquid.

The water/ZnCl(2) phase diagram in the vicinity of the 75 mol % water composition is reported, demonstrating the existence of a congruently melting phase. Single crystals of this 3-equiv hydrate were grown, and the crystal structure of [Zn(OH(2))(6)][ZnCl(4)] was determined. Synchrotron X-ray and neutron diffraction and IR and Raman spectroscopy along with reverse Monte Carlo modeling demonstrate that a CsCl-type packing of the molecular ions persists into the liquid state. Consistent with the crystalline and liquid structural data, IR spectroscopy demonstrates that the O-H bonds of coordinated water do not exhibit strong intermolecular hydrogen ion bonding but are significantly weakened because of the waters coordination to Lewis acidic zinc ions. The O-H bond weakening makes this system a very strong hydrogen-bond donor, whereas the ionic packing along with the nonpolar geometry of the molecular ions makes this system a novel nonpolar, hydrogen-bonding, ionic liquid solvent.

Triple bonds between tungsten atoms with ancillary dimesitylboroalkoxide ligands. Preparations, properties and structures of W2(NMe2)4[OB(Mes)2]2 and W2(OBut)4[OB(Mes)2]2

From the reaction between W 2 (NMe 2 ) 6 and dimesitylborinic acid, (Mes) 2 BOH (2 equiv.), in toluene, the golden- yellow crystalline compound W 2 (NMe 2 ) 4 [OB(Mes) 2 ] 2 ( 1 ) has been isolated and characterized (elemental analysis, 1 H, 13 C{ 1 H}, 11 B NMR spectroscopy, IR spectroscopy and a single crystal X-ray diffraction study). At −160 °C, a =39.379(7), b =13.649(2), c =21.918(4) A, β=123.12(1)°, Z =8 and space group C 2/ c . The compound W 2 (OBu t ) 4 [OB(Mes) 2 ] 2 ( 2 ) has been similarly characterized as a dark red crystalline compound obtained from the reaction between W 2 (OBu t ) 6 and (Mes) 2 BOH (2 equiv.) in toluene. At −159 °C, a =17.164(2), b =19.773(2), c =18.490(2) A, β=102.91(1)°, Z =4 and space group C 2. In both compounds there are unsupported WW bonds of distance 2.3068(13) and 2.3521(15) A for compound 1 and 2 , respectively. In 1 there is a central ethane like W 2 N 4 O 2 core with the gauche conformation, WN=1.94(2) (av.) and WO=1.93(1) A. The coordination geometry at nitrogen is trigonal planar and the NC 2 units are aligned along the WW axis as found for related compounds. In compound 2 the W 2 O 4 O′ 2 skeleton is eclipsed and most surprisingly the WO′ distances to the OB(Mes) 2 ligands are shorter 1.81(1) A than the WO distances 1.94(1) and 1.90(1) A to the alkoxide ligands. In both 1 and 2 the coordination geometry at boron is trigonal planar and the OB distances fall in the range 1.37(3)–1.41(3) A, and are statistically equivalent to that of the free borinic acid. Compound 2 has crystallographically imposed C 2 symmetry and the OB(Mes) 2 groups are syn . These are the first structurally characterized boroalkoxides coordinated to the (WW) 6+ moiety and comparisons of WO π bonding are made with respect to related OR and OSiR 3 compounds.