Crystal structure of hexa-μ-chlorido-μ4-oxido-tetrakis{[1-(2-hydroxyethyl)-2- methyl-5-nitro-1H-imidazole-κN3]copper(II)} containing short NO2· · ·NO2 contacts

The title tetranuclear copper complex, [Cu4Cl6O(C6H9N3O3)4] or [Cu4Cl6O- (MET)4][MET is 1-(2-hydroxyethyl)-2-methyl-5-nitro-1Η-imidazole or metronidazole], contains a tetrahedral arrangement of copper(II) ions. Each copper atom is also linked to the other three copper atoms in the tetrahedron via bridging chloride ions. A fifth coordination position on each metal atom is occupied by a nitrogen atom of the monodentate MET ligand. The result is a distorted CuCl3NO trigonal–bipyramidal coordination polyhedron with the axial positions occupied by oxygen and nitrogen atoms. The extended structure displays O− H ⋅ ⋅ ⋅O hydrogen bonding, as well as unusual short O⋅ ⋅ ⋅ N interactions [2.775 (4) A ˚ ] between the nitro groups of adjacent clusters that are oriented perpendicular to each other. The scattering contribution of disordered water and methanol solvent molecules was removed using the SQUEEZE procedure [Spek (2015). Acta Cryst. C71, 9–16] in PLATON [Spek (2009). Acta Cryst. D65, 148–155]

Synthesis, Structures, and Reactivity of Zinc, Cadmium, and Magnesium Complexes Supported by Nitrogen Donor and Carboxylate Ligands

The bis(2-pyridylthio)methyl ligand, [Bptm], offers a synthetically convenient alternative to a variety of multidentate ligands, including most notably [Tptm] (tris(2-pyridylthio)methyl) and [BptmSTol] (bis(2-pyridylthio)(p-tolylthio)methyl), and, in contrast with [Tptm], necessarily coordinates to metal centers in a κ³ fashion. As such, numerous [Bptm] complexes of zinc have been synthesized and structurally characterized. In Chapter 1, we describe the reaction of the protonated ligand [Bptm]H with the homoleptic zinc compounds Me₂Zn and Zn[N(SiMe₃)₂]₂ to afford, respectively, [Bptm]ZnMe and [Bptm]ZnN(SiMe₃)₂; the latter has been used as a starting point for a wide range of reactivity.Most notably, the terminal zinc hydride, [Bptm]ZnH, can be accessed via either (i) metathesis of the zinc siloxide, [Bptm]ZnOSiPh₃, with either PhSiH₃ or HBpin, or (ii) direct metathesis of the zinc amide [Bptm]ZnN(SiMe₃)₂ with HBpin; the latter reactivity is not precedented and offers a novel approach for the synthesis of molecular zinc hydrides. Both [Bptm]ZnN(SiMe₃)2 and [Bptm]ZnH provide access to a variety of monomeric derivatives, including the zinc halides [Bptm]ZnX (X = Cl, Br, I) and the zinc isocyanate [Bptm]ZnNCO; the latter can be accessed directly via (i) metathesis of [Bptm]ZnH with Me₃SiNCO or (ii) a multistep reaction of [Bptm]ZnN(SiMe₃)₂ with CO₂. [Bptm]ZnH also undergoes insertion of CO₂ into its Zn—H bond to afford the zinc formate, [Bptm]ZnO₂CH, in which the formate moiety exhibits a monodentate binding mode in the solid state. This reactivity enables it to serve as a catalyst for the hydrofunctionalization of CO₂; specifically, [Bptm]ZnH catalyzes the hydrosilylation of CO₂ by (RO)₃SiH (R = Me, Et) at elevated temperatures to afford the respective silyl formates (RO)3SiO₂CH, as well as the hydroboration of CO₂ by HBpin at room temperature to afford the boryl formate HCO₂Bpin. In the absence of CO₂, [Bptm]ZnH also catalyzes the reduction of HCO₂Bpin to the methanol level, MeOBpin. Similarly, [Bptm]ZnH serves as an effective catalyst for the hydrosilylation and hydroboration of a variety of ketones and aldehydes. In all cases, hydroboration is more facile than the corresponding hydrosilylation. The [Bptm]Zn system has been investigated computationally, and the kinetics of insertion of CO₂ into the Zn—H bond of [Bptm]ZnH as well as the thermodynamics of the catalytic cycle have been examined. Further mechanistic studies examine two noteworthy spectroscopic features of the system, namely rapid exchange (i) between the zinc and boryl formates [Bptm]ZnO₂CH and HCO₂Bpin, as well as (ii) between [Bptm]ZnH and [Bptm]ZnO₂CH. Both of these exchange processes have been investigated with variable-temperature NMR spectroscopy; in particular, the former exchange resolves at low temperatures and can be confirmed by exchange spectroscopy. In addition to the aforementioned monomeric zinc halides [Bptm]ZnX (X = Cl, Br, I), the dimeric bridging zinc fluoride {[Bptm]Zn(μ-F)}₂ has been synthesized via reaction of Me3SnF with either [Bptm]ZnN(SiMe₃)₂ or [Bptm]ZnH, as outlined in Chapter 2. The dimeric nature of the fluoride in contrast with the other monomeric halides can be attributed to the significant polarity of the Zn—F bond. {[Bptm]Zn(μ-F)}2 also reacts with Me₃SiCF₃ to afford an unusual instance of a structurally characterized zinc trifluoromethyl complex, [Bptm]ZnCF₃. Chapter 3 discusses cadmium analogues to the [Bptm]Zn system, which provide a comparison and a contrast both with their zinc counterparts as well as with previously reported [Tptm]Cd complexes. While the cadmium amide [Bptm]CdN(SiMe₃)2 may be synthesized in a manner corresponding to that for its zinc analogue, the siloxides {[Bptm]Zn(μ-OSiR₃)}₂ (R = Me, Ph) form dimers that are distinct from the monomeric [Bptm]ZnOSiPh₃ and [Tptm]CdOSiPh₃, although similar to {[Tptm]Cd(μ-OSiMe₃)}₂. The distinctions between the [Bptm]Zn and [Bptm]Cd siloxides have been investigated computationally, indicating that the cadmium species show a thermodynamic preference for dimer formation, which can be attributed to the larger atomic radius of cadmium relative to zinc. Attempts to synthesize a cadmium hydride are interrupted by a Schlenk-type equilibrium giving way to the bis(ligand) complex [Bptm]2Cd and CdH₂, which in turn decomposes to Cd and H2. However, spectroscopic studies indicate that under CO₂, [Bptm]CdN(SiMe₃)₂ and HBpin react to trap a cadmium hydride species as the bridging formate derivative, [Bptm]Cd(μ-O₂CH)₂Bpin. The interaction of nitrogen-rich ligands with main group metals is further probed in Chapter 4, which describes the investigation of the coordination of 2,2’:6,2”-terpyridine (terpy) to magnesium compounds. Most prominently, unsubsituted terpy forms an adduct, terpyMg[N(SiMe₃)₂]₂, with the monomeric form of the magnesium amide {Mg[N(SiMe₃)₂]₂}₂. The adduct reacts with halide donors to form a series of mixed amide-halide complexes, terpyMg[N(SiMe₃)]X (X = Cl, Br, I), as well as a mixed amide-azide complex, terpyMg[N(SiMe₃)₂]N₃. These complexes represent the first instances of neutral monomeric terpyMg compounds that feature unsubstituted terpyridine. Structural comparisons of these complexes with one another as well as with comparable compounds are undertaken. Complexes of terpy with cadmium and zinc analogues, terpyCd[N(SiMe₃)₂]₂ and terpyZn [N(SiMe₃)₂]₂, are explored further, and DFT calculations are used to explore the strength of the interactions between the ligand and the metals in each case. Finally, in Chapter 5, attention is given to the recently reported zinc bromide complex featuring a zwitterionic carboxylate ligand, (Cbp)2ZnBr₂. The structure reported for this complex features several anomalous features, including abnormally long Zn—Br and Zn—O bonds, unusually small atomic displacement parameters for Zn, and a high R-value. This information led us to synthesize and investigate the cadmium counterpart, (Cbp)₂CdBr₂; we find that the cadmium complex possesses nearly identical structural parameters to the reported zinc complex, and when the cadmium is refined as zinc, the displacement parameter problems are reproduced. Therefore, we conclude that the reported structure is in fact that of (Cbp)₂CdBr₂, and report a revised structure for (Cbp)₂ZnBr₂

Fold Mechanics of Natural and Synthetic Origami Papers

To realize engineered materials and structures via origami methods and other folding construction techniques, fundamental understanding of paper folding mechanics and their dependency on paper micro/nanostructure is needed. Using selected papers commonly used in origami designs, we establish the relationship between the mechanical properties of fibrous paper and their corresponding ability to form and retain simple creases and mountain/valley folds. Using natural fiber paper (abaca), synthetic fiber paper (Tyvek), and a metalfiber laminate paper, we studied how the fold radius depends on the load applied using a controlled rolling apparatus. After folding, we examined the resultant micro- and nanoscale deformation using electron microscopy. In general we found that the fold radius follows a power law, decreasing with the applied rolling force. At a critical strain, each paper exhibits a transition between elastic and plastic behavior, after which the trend asymptotically approaches the minimum fold radius with increased applied force. Finally, we present examples of centimeter-scale two-dimensionally "mountain fold" patterns and relate the folding characteristics observed in these designs to the mechanical properties of the papers in folding. Keywords: Deformation; Fibers; Synthetic fibers; Laminates; Electron microscopy; Metal fibers; Construction; Stress; Mechanical properties; Nanoscale phenomenaNational Science Foundation (U.S.) (Grant EFRI-1240264

Origami Solar-Tracking Concentrator Array for Planar Photovoltaics

Solar-tracking concentrators can potentially lead to low-cost photovoltaic modules that minimize the use of costly semiconductor materials by improving optical collection and coupling. However, solar concentrators and accompanying trackers have proven to be expensive, bulky, and heavy, thereby resulting in increased balance-of-system costs. Here we demonstrate a lightweight and low-profile, and potentially low-cost planar solar-tracking concentrator based on the ancient Japanese art of origami. The tightly packed hexagonal concentrator and tracker arrays are fabricated by cutting and folding thin reflecting sheets that capture and direct concentrated light onto a small, high-efficiency GaAs solar cell. The tracker enables single-axis solar tracking via a simple one-dimensional translational motion of an actuator with minimal energy expense (∼2.9 J/m²/day). Further, we demonstrate stable operation over 10 000 cycles. The solar concentrated cell achieves a 450% increase in diurnal energy output compared with an equivalent, unconcentrated cell. The potentially low cost and low profile of the origami concentrators may lead to their wide deployment on rooftops and other building-integrated applications

Forum : Vol. 33, No. 02 (Summer : 2009)

https://digitalcommons.usf.edu/forum_magazine/1047/thumbnail.jp