Poster Abstracts - 2014
Synthesis, Characterization and Catalytic Activity of Palladium Indenyl and Cp Complexes, Patrick Melvin, Yale University.
Abstract: Two processes have been shown to impact the efficiency of Pd(II) precatalysts: a) rate of activation from Pd(II) to active Pd(0) species, and b) likelihood of forming a less active Pd(I) dimer. Applying these concepts to precatalyst design, a highly active Pd(II) precatalyst has been synthesized and screened for catalysis. Initial results indicate that Pd(Indenyl)(IPr)Cl outperforms the prevalent Pd(Cinnamyl)(IPr)Cl complex for a simple Suzuki-Miyaura reaction. This increase in efficiency has been credited to the facile activation of Pd(Indenyl)(IPr)Cl to the active monoligated Pd(0) species, despite its high tendency to form Pd(I) dimer. Current efforts are aimed at improving the Pd(Indenyl)(IPr)Cl scaffold by limiting the likelihood of Pd(I) dimer formation, thereby keeping active Pd(0) in solution.
Trapping an Elusive Bügi-Dunitz intermediate: A Crystallographic-Computational Joint Study. Jesus Campos Manzano, Yale University.
Abstract: Hydroxamic acids are important ligands in bioinorganic chemistry and have recently emerged as highly valuable anchors for dye sensitized solar cells. Despite the great attention shown in bioinorganic literature due to their connection with siderophores, there are only few examples of pure coordination chemistry. We have found [Cp*Ir(μ-OH)₃IrCp*]OH reacts with PhCONHOH to give [Cp*Ir(η2-ONCOPh)] in which the doubly deprotonated -NHOH unit binds side-on via N and O, an otherwise unrecorded binding mode. The X-ray structure shows unprecedented pyramidalization at Ir together with secondary bonding between the carbonyl oxygen and Ir (Ir···O = 2.87Å). The related o-hydroxyphenyl hydroxamic acid gives a conventional chelate structure in which both sp3 O atoms are bound in deprotonated form. The distorted conformation in [Cp*Ir(η2-ONCOPh)] is stabilized by weak π-π interactions in the solid state. Computational studies suggest that being a transient conformation in solution that could be classified as a Burgi-Dunitz intermediate. These results demonstrate the importance of weak interactions in the solid state to gain access to otherwise inaccessible structures.
The synthesis and structural characterization of cobalt diazoalkane complexes. Simon Bonyhady, Yale University.
Abstract: The use of bulky β-diketiminate ligands has facilitated the synthesis of compounds featuring cobalt centers in unusual oxidation states and coordination numbers. In particular, it is possible to isolate a cobalt(I) complex that is supported only by a diketiminate, as a "masked two-coordinate" species.1 This species is highly reactive, as demonstrated through the activation of C-F bonds.1,2 This presentation will discuss the reactivity of this cobalt(I) complex toward diazoalkanes. A number of diazoalkane coordination complexes have been synthesized, and the use of Rigaku single crystal X-ray diffractometers has allowed for the unambiguous characterization of these species.
1. Dugan, T. R., Sun, X., Rybak-Akimova, E. V, Olatunji-Ojo, O., Cundari,
T. R., & Holland, P. L., J. Am. Chem. Soc. 2011, 133(32),
12418-12421
2. Dugan, T. R., Goldberg, J. M., Brennessel, W. W., & Holland, P. L.,
Organometallics 2012, 31(4), 1349-1360.
Structural and Electronic Properties of Old and New Late 3d Transition Metal Complexes with Dodecafluoropinacol. Laleh Tahsini, Boston University.
Abstract: There is a wealth of literature describing highly-oxidized and reactive late transition metal complexes with N-donor ligands or mixed N,O-donor ligand systems.[1] These strong-donor ligand environments stabilize such higher oxidation states, and also typically engender low-spin electronic configurations. Far fewer reports exist of high-valent, late transition metal complexes in a weaker-field, all O-donor ligand environment. Homoleptic aryloxides and alkoxides are rare as mononuclear species because the strong basicity of these ligands gives rise to bridging alkoxides in polymeric structures.[2] Decreased bridging can be achieved electronically with fluorinated aryloxide or alkoxide ligands, which maintain open coordination sites and substrate access to the metal center. Highly fluorinated aryloxides and alkoxides further endow transition metal complexes of these ligands with oxidative stability because the oxidative robustness of C-F bonds (117 kcal mol⁻¹) compared to C-H bonds (98 kcal mol⁻¹) diminishes unwanted ligand bond oxidation.
In this report the electron withdrawing power of fluorinated, O-donating groups is combined with the chelate effect in a fully fluorinated bidentate alkoxide ligand, (pinF), to afford electronically and thermodynamically stable complexes. Seven new homoleptic complexes of A₂[M(pinF)₂] have been synthesized, namely (Me₄N)₂[Fe(pinF)₂], 1; (Me₄N)₂[Co(pinF)₂], 2; (Bu₄N)₂[Co(pinF)₂], 3; {K(DME)₂}₂[Ni(pinF)₂], 4; (Me₄N)₂[Ni(pinF)₂], 5; {K(DME)₂}₂[Cu(pinF)₂], 7; and (Me₄N)₂[Cu(pinF)₂], 8 in addition to the previously reported complexes, K₂[Cu(pinF)₂], 6, and K₂[Zn(pinF)₂]·, 9.[3] All compounds have been characterized by UV-vis, cyclic voltammetry, elemental analysis, and, for paramagnetic compounds, Evans method magnetic susceptibility. Single-crystal X-ray crystallographic data were obtained for all complexes except 5. A square-planar geometry about the metal center was confirmed in all Fe (1), Ni (4), and Cu (6, 7, 8) complexes independent of counter cation. The Co species exhibit square-planar (3) or distorted square-planar geometries (2), and the Zn species (9) is tetrahedral. Cyclic voltammetry data reveal reversible redox couples for Ni(III)/Ni(II) in 5 and for Cu(III)/Cu(II) in 8 but quasi-reversible couples for the Fe(III)/Fe(II) in 1 and the Co(III)/Co(II) in 2. Perfluorination of the pinacolate ligand results in an increase in the central C-C bond length due to steric clashes between CF₃ groups, relative to perhydro pinacolate complexes. Both types of pinacolate complexes exhibit O-C-C-O torsion angles around 40°. Together these data demonstrate that perfluoropinacol ligand makes possible highly unusual and coordinatively unsaturated high-spin metal centers with ready thermodynamic access to rare oxidation states such as Ni(III) and Cu(III).
[1] (a) Boisvert, L.; Goldberg, K. I. Acc. Chem. Res. 2012,
45, 899-910. (b) Yin, G. Coord. Chem. Rev. 2010, 254, 1826-1842.
[2] Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. Alkoxo and
Aryloxo Derivatives of Metals, 2001
[3] Tahsini, L.; Specht, S. E.; Lum, J. S.; Nelson, J. J. M.; Long, A. F.;
Golen, J. A.; Rheingold, A. L.; Doerrer, L. H. Inorg. Chem. 2013,
52, 14050-14063.
Enantiselective Synthesis of Atropisomeric Bezamides Through Peptide-Mediated Bromination. Kim Barrett, Yale University.
Abstract: The development of asymmetric methodology utilizing small peptide catalysts remains a pillar of Miller group research. Recently, the Miller group has focused on the generation of enantioenriched axially chiral molecules such as allenes, biaryls, and benzamides. Herein, we report the enantioselective synthesis of various atropisomeric benzamides employing catalytic electrophilic aromatic bromination. The catalyst is a simple tetrapeptide containing a critical tertiary amine thought to function as a Brønsted base. In addition, examination of the initially-formed products at low conversion suggests that the catalyst accesses a unique bromination pathway, with the initial catalytic bromination occuring in both a regioselective and stereochemistry-determining manner.
Synthesis and characterization of iridium hydride clusters containing N-heterocyclic carbene ligands. Liam Sharninghausen, Yale University.
Abstract: N-heterocyclic carbenes (NHCs) are commonly used as spectator ligands in transition metal catalysis, due in part to their low lability and tunable sterics. Our group has recently found that a family of iridium bis-NHC compounds are highly active catalyst precursors for several hydrogen borrowing transformations, including the acceptorless dehydrogenation of glycerol, methanol and sugar alcohols. Analysis of iridium species formed from precursors [Ir(IMe)₂(COD)]BF₄, 1, [Ir(IMe)₂(CO)₂]BF₄, 2, and (Ir(IMe)COD)Cl, 3, under typical reaction conditions showed uncommon NHC exchange processes and formation of rare cluster architectures.
Novel iridium clusters containing NHC ligands were isolated and studied by x-ray crystallography, spectroscopic techniques and DFT calculations. Tetrameric species [Ir₄(IMe)₈H₁₀]²⁺ and [Ir₄(IMe)₇H₁₀]²⁺ are formed when 1 is used as a precursor. The crystal structures of [Ir₄(IMe)₈H₁₀]²⁺ and [Ir₄(IMe)₇H₁₀]²⁺ show tetrahedral and "butterfly" iridium core structures, respectively, with NHC ligands, the first examples of such clusters bearing NHC ligands. A hexameric iridium species, [Ir₆(IMe)₈H₁₀(CO)₂]²⁺, is formed as the major product when bis and mono-NHC iridium precursors, 1 and 3, are combined in the same reaction vessel. The crystal structure shows the six iridium atoms oriented in an unprecedented "bow-tie" structure. NMR and DFT studies are being undertaken to further characterize these unusual clusters.
Toward Scalable Fabrication of Aligned Single-Walled Carbon Nanotube Composites by Magnetic Alignment of Lyotropic Precursors. Marissa Tousley, Yale University.
Abstract: Formation of monodisperse, nanoporous materials for size-based separations using scalable methods remains a challenging problem. In particular, aligned carbon nanotube membranes provide compelling prospects for high selectivity and high permeability separations but scalability and robust control of nanotube size are hurdles to be overcome. Here we extend the concept of templated assembly of single-walled carbon nanotubes (SWNTs) to non-aqueous systems, which afford the creation of nanoporous selective layers for use in aqueous separations. Non-degenerate alignment of a polymerizable, polyoxyethylene alkylphenyl lyotropic template has been achieved through sample rotation in a superconducting magnet. Characterization as a function of temperature using in-situ small angle x-ray scattering (SAXS) has been conducted. By doping the non-polar phase of this template with a non-reactive species, structure and alignment retention has been attained following photopolymerization into a nanoporous polymer. These polymers have been characterized using SAXS and polarized optical microscopy (POM). Stable sequestration of SWNTs in the aqueous channels of this same template has been previously demonstrated, where the presence of individual SWNTs was verified using near-infrared fluorescence and ultraviolet-visible spectroscopies. Our recent work on template alignment and polymerization, in combination with our previous efforts on stable SWNT sequestration, has resulted in the development of a platform suitable for the ultimate fabrication of aligned nanotube membranes using space pervasive magnetic fields and tailored SWNTs of any diameter or purity.
Magnetic Field Aligned Polymerizable Inverted Hexagonal Mesophases: Towards Polymer Membranes with Highly Ordered Nanochannels. Xunda Feng, Yale University.
Abstract: We present the fabrication of larger-area highly ordered polymer nanostructures by magnetic field directed self-assembly and polymerization of inverted hexagonal mesophases. Small angle and wide angle X-ray scattering has been employed to characterize the aligned hexagonally packed nanochannels and the ordering at the molecular lever. Transmission electron microscopy demonstrates the consistent orientation showed by the X-ray scattering. Our studies show that magnetic field alignment offers a facile approach to functional polymer membranes with highly ordered internal nanostructures.
Isolation of All Possible Isomers of Iridium Complexes with Chloride and a Pyridine Alkoxide Ligand: A High Oxidation State-Promoting, Oxidation Resistant Ligand Set. Dimitar Shopov, Yale University.
Abstract: The ligand 2-(2-pyridyl)isopropanol (pyalk) had emerged as a robust, oxidation-resistant ligand in our prior water oxidation studies. Reactions of this ligand with chloride salts of iridium affords varying amounts of mono-, bis- and tris-chelated iridium complexes with chlorides completing the octahedral coordination sphere. The complexes are stable in both the III and IV oxidation states and undergo conversion at remarkably mild potentials. Using chelation, excision, and conversion reactions, all eight possible pyalk-containing species can be prepared in sufficient quantities. Partly owing to the complexes' deep and characteristic colors in the Ir(IV) state, they were fully isolated and crystal structures solved. The importance of specific geometric orientation of ligands in determining the properties of a complex is highlighted by the optical and redox properties of the bis- and tris-pyalk isomer sets. Most notably, the two tris-chelate isomers show dramatically different redox behaviors.
Understanding structure-property relationships in complex oxide heterostructures. Divine Kumah, Yale University.
Abstract: Complex oxide materials possess a wide range of interesting electronic and magnetic properties including ferroelectricity and superconductivity and are of interest for both scientific and technological applications. In these systems, a strong coupling exists between the atomic scale structural properties and their electronic and magnetic properties. Using diffraction techniques, we image the atomic scale structures of thin complex oxide films synthesized using molecular beam epitaxy (MBE), to understand and manipulate their physical properties for device applications.
Stimuli-Responsive Block Copolymer Nanoporous Template by Magnetic-Field Alignment. Youngwoo Choo, Yale University.
Abstract: Block copolymers have attracted intense interest related to their potential application as stimuli-responsive nanoporous membranes based on the selective removal of one component of the system, and stimuli-responsiveness of the remaining material. There are, however, several non-trivial challenges associated with realizing vertically aligned nanopores in stimuli-responsive thin films. Here, we present a facile approach to direct the self-assembled morphology of novel poly(lactide) based brush-block copolymers using a magnetic field. The norbornene backbone of the BCP architecture allows cross-linking of the template while the liquid crystalline majority block provides magnetic anisotropy. In-situ temperature resolved small angle x-ray scattering (SAXS) under a 6 T magnetic field revealed that the system self-assembles into highly ordered hexagonally-packed cylinders of PLA and that this aligned structure is maintained after UV cross-linking of the LC matrix and hydrolytic removal of the PLA material. Furthermore, temperature resolved SAXS showed that the nanopores can be reversibly closed and opened multiple times while retaining their alignment by appropriate heating and cooling in the absence of the field.
New Routes to Reactive Iron-Sulfur Clusters. Richard Lewis, Yale University.
Abstract: The iron molybdenum cofactor of nitrogenase is the site of N₂ reduction, and has a cage structure that is unknown in synthetic compounds. Our group endeavors to gain insight into nitrogenase through the synthesis of new Fe-Sulfur complexes where x-ray crystallography is one of the staple techniques. We have tested new routes for the formation of Fe-Sulfur clusters and our data shows Fe-Sulfur clusters exhibiting various oxidation states. Using information gleaned from x-ray diffraction, we can begin to test the reactivity of these new clusters toward nitrogenase substrates.