Recommended reading: 16.7-16.10 (3rd/4th edition) Ch 102 Problem Set 2 Due: Thursday, April 19, 2018 Before Class Problem 1. (1 point) Metal-containing polymers have been studied for a wide variety of applications. For all of the complexes below, determine the 1-D symmetry class. Indicate the unit cell and asymmetric unit for each, and label the symmetry elements contained in the unit cell. Metal-containing polymers often have rigid bidentate ligands bridging metals. Gold and platinum are commonly used in the synthesis of such materials, as shown below (Puddephatt, Chem. Comm. 1998. 1055). Metal-organic polymers containing π-delocalization along their entire length have been investigated for their conductive properties, as well as for nonlinear optics and crystal fluids. For example, Frazier et al. reported a large nonlinear refractive index for a palladium poly-yne (Frazier et al. Polymer. 1987. 28, 553). Metal-containing polymers have also been studied for the controlled modification of surfaces. The polymer below was deposited on graphite (Surin et al. Angewandte Chemie. 2006. 46(1-2), 245). Problem 2 (3 points) Ladder polymers have unusual structures in that their backbones consist of fused rings. As a result, their conformational freedom is greatly limited. Rigid ladder polymers have been studied for potential applications, including in the fields of membrane gas separation and organic photonics/electronics. For all examples shown below, determine the monomers point groups and the polymers 1-D symmetry classes. Indicate the unit cell and asymmetric unit for each polymer. Label the symmetry elements contained in each unit cell and compare them with the symmetry elements of the corresponding monomer. 1
1) There are two general routes for synthesizing ladder polymers: a) direct ladder polymerization, (Cyclic units are joined during polymerization) b) zipping of a linear preformed polymer (functional side groups of normal linear polymers are connected to form the ladder structure). Same color and shape represent same chemical structure. For your symmetry assignments, only consider the shape and color of the boxes as shown. Do not assign point groups to the monomers here. 2) Polyacene-type polymers can be prepared in multifunctional polycondensation reactions by several methods. For example, (triethylsilyl)-butadiyne can be polymerized using a Ziegler-Natta catalyst, and the resulting polymer desilylated with Bu 4 NF to obtain a conjugated polymer with pendant acetylene groups. This can be thermally converted to a highly conductive ladder polymer. Methyl groups should be considered spherical. 3) Cyanoacetylene can be polymerized into linear polymers at low temperature by anionic, Ziegler-Natta, or metathesis polymerization catalysts. Polymers obtained by anionic polymerization are rich in the trans-transoid motif (top) and can be cyclized thermally. However, polymers featuring cis-transoid motifs (bottom) are hard to cyclize due to the relative position of the nitrile groups. 4) A graphene ribbon-like polymer was prepared by the condensation of 1,4,5,8-tetraaminoanthraquinone and 1,3,6,8-tetraketo-1,2,3,6,7,8-hexahydro-pyrene. 2
Problem 3 (2 points) Covalent Organic Frameworks (COFs) are two or three dimensional polymers that have attracted attention as a result of their porosity, leading to applications in gas storage, photoelectricity, and catalysis. These are frequently synthesized by imine condensation, boronic acids dehydration, and triazine cyclotrimerization. For the 2-D COFs shown below, assign the 2-D symmetry group, label the symmetry elements, and mark the smallest possible unit cell and corresponding asymmetric unit. A) Shown below is COF-366-Co, which was investigated as use as catalysts for CO 2 reduction in water. 3
B) Recently Jiang and coworkers have investigated a series of COFs which feature multiple linker types, an example of which is shown below. 4
C) Two component COFs with similar structures can also be synthesized that feature both pyrene and thienothiophene motifs. Complete the following structural diagrams using a 1:2 ratio of pyrene to thienothiophene, considering only the pm isomer then indicate the symmetry operations, unit cell, and asymmetric unit. 5
D) Another class of COFs features multiple pore sizes and one example is shown below. This particular example has been reported Zhao and coworkers to uptake volatile iodine. 6
Problem 4 (2 points) Self-assembled monolayers (SAM) of molecules on surfaces are of great interest due to their potential for tailored modification or enhancement of material properties. In one study, tetra- or di-substituted pyrene molecules were deposited onto a gold surface and visualized by scanning tunneling microscopy (STM). This technique revealed that between the two different molecules, there were three different phases of molecular arrangement. For monolayers shown below, assign the 2-D symmetry group, label the symmetry elements, and mark the smallest possible unit cell and corresponding asymmetric unit. A) 7
B) 8
C) 9
D) Consider a hypothetical SAM of a 1:1 mixture of 2,7-diazapyrene and 4,9-dibromopyrene. Use the two templates below, which already have the 2,7-diazapyrene molecules drawn, to complete SAMs of space groups pmm and pmg. Draw the two bromine substituents onto the unsubstituted pyrenes to indicate the orientation(s) of the 4,9-dibromopyrene molecules. Label the symmetry elements, and mark the smallest possible unit cell and corresponding asymmetric unit. Space group pmm: 10
Space group pmg: Problem 5. (2 points) Pick a topic of interest from the recommended reading in bold. Prepare two power point slides including relevant descriptive chemistry (background on synthesis, applications, reactivity, properties, trend, etc, as applicable), some concepts presented in class (point group assignment, symmetry elements, etc) and some application of the provided software (for example, highlight symmetry elements / operations). Turn in a printout of the slides with your problem set, and email the TAs the slides in pdf format by 12:00 noon on the due date. Please format file names as FirstName_LastName_PSET# and include your name on the first slide. 11