Alexander S. Filatov

Understanding and Curing Structural Defects in Colloidal GaAs Nanocrystals

GaAs is one of the most important semiconductors. However, colloidal GaAs nanocrystals remain largely unexplored because of the difficulties with their synthesis. Traditional synthetic routes either fail to produce pure GaAs phase or result in materials whose optical properties are very different from the behavior expected for quantum dots of direct-gap semiconductors. In this work, we demonstrate a variety of synthetic routes toward crystalline GaAs NCs. By using a combination of Raman, EXAFS, transient absorption, and EPR spectroscopies, we conclude that unusual optical properties of colloidal GaAs NCs can be related to the presence of Ga vacancies and lattice disorder. These defects do not manifest themselves in TEM images and powder X-ray diffraction patterns but are responsible for the lack of absorption features even in apparently crystalline GaAs nanoparticles. We introduce a novel molten salt based annealing approach to alleviate these structural defects and show the emergence of size-dependent excitonic transitions in colloidal GaAs quantum dots.

Synthesis of Alternating Donor–Acceptor Ladder-Type Molecules and Investigation of Their Multiple Charge-Transfer Pathways

We describe the synthesis as well as the optical and charge-transport properties of a series of donor-acceptor (D-A) ladder-type heteroacenes. These molecules are stable, soluble, and contain up to 24 fused rings. Structural analyses indicated that the backbones of Su200910r and Seu200910r are bent in single crystals. The three 10-ring heteroacenes were functionalized with thiol anchoring groups and used for single-molecular conductance measurements. The highest conductance was observed for molecular wires containing a benzoselenadiazole (BSD) moiety, which exhibits the narrowest band gap. Multiple charge-transport pathways were observed in molecular wires containing either benzothiadiazole (BTD) or BSD. The conductance is a complex function of both energy gap and orbital alignment.

Expanding the Structural Motif Landscape of Heterometallic β-Diketonates: Congruently Melting Ionic Solids

The first example of ionic β-diketonates in which both the cation and anion are octahedral coordinatively saturated metal diketonate moieties are reported. Heterometallic tin-transition-metal heteroleptic diketonates were obtained through solid-state redox reactions and are formulated as {[SnIV(thd)3]+[MII(hfac)3]-} (MII = Mn (1), Fe (2), Co (3); thd = 2,2,6,6-tetramethyl-3,5-heptanedionate, hfac = hexafluoroacetylacetonate). X-ray single-crystal structural investigations along with DART mass spectrometry, multinuclear NMR, and magnetic susceptibility measurements have been used to confirm an assignment of metal oxidation states in compounds 1-3. Ionic compounds were found to melt congruently at temperatures below the decomposition point. As such, they represent prospective materials that can be utilized as ionic liquids as well as reagents for the soft transfer of diketonate ligands. An unexpected volatility of ionic compounds 1-3 was proposed to occur through a transport reaction, in which the transport agent is one of the products of their partial decomposition in the gas or condensed phase.

Binary Transition Metal Oxide Hollow Nanoparticles for Oxygen Evolution Reaction

Low-cost transition metal oxides are actively explored as alternative materials to precious metal-based electrocatalysts for the challenging multistep oxygen evolution reaction (OER). We utilized the Kirkendall effect allowing the formation of hollow polycrystalline, highly disordered nanoparticles (NPs) to synthesize highly active binary metal oxide OER electrocatalysts in alkali media. Two synthetic strategies were applied to achieve compositional control in binary transition metal oxide hollow NPs. The first strategy is capitalized on the oxidation of transition-metal NP seeds in the presence of other transition-metal cations. Oxidation of Fe NPs treated with Ni (+2) cations allowed the synthesis of hollow oxide NPs with a 1-4.7 Ni-to-Fe ratio via an oxidation-induced doping mechanism. Hollow Fe-Ni oxide NPs also reached a current density of 10 mA/cm2 at 0.30 V overpotential. The second strategy is based on the direct oxidation of iron-cobalt alloy NPs which allows the synthesis of hollow Fe xCo100- x-oxide NPs where x can be tuned in the range between 36 and 100. Hollow Fe36Co64-oxide NPs also revealed the current density of 10 mA/cm2 at 0.30 V overpotential in 0.1 M KOH.

Crystal structure of the inverse crown ether tetra­kis­[μ2-bis­(tri­methyl­sil­yl)amido]-μ4-oxido-dicobalt(II)disodium, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O)

The first cobalt-containing inverse crown ether, [Co2Na2{μ2-N(SiMe3)2}4](μ4-O), features a central μ4-oxido ligand. Weak intermolecular Na⋯H3C—Si interactions form an infinite chain extending along [010] in the crystal.

CCDC 1848263: Experimental Crystal Structure Determination

Related Article: McKenna K. Goetz, Ethan A. Hill, Alexander S. Filatov, John S. Anderson|2018|J.Am.Chem.Soc.|140||doi:10.1021/jacs.8b07399

Isolation of a Terminal Co(III)-Oxo Complex

Late transition metal oxo complexes with high d-electron counts have been implicated as intermediates in a wide variety of important catalytic reactions; however, their reactive nature has often significantly limited their study. While some examples of these species have been isolated and characterized, complexes with d-electron counts >4 are exceedingly rare. Here we report that use of a strongly donating tris(imidazol-2-ylidene)borate scaffold enables the isolation of two highly unusual CoIII-oxo complexes which have been thoroughly characterized by a suite of physical techniques including single crystal X-ray diffraction. These complexes display O atom and H atom transfer reactivity and demonstrate that terminal metal oxo complexes with six d-electrons can display strong metal-oxygen bonding and sufficient stability to enable their characterization. The unambiguous assignment of these complexes supports the viability of related species that are frequently invoked, but rarely observed, in the types of catalytic reactions mentioned above. The studies described here change our understanding of the reactivity and bonding in late transition metal oxo complexes and open the door to further study of the properties of this class of elusive and important intermediates.

Redox Activity, Ligand Protonation, and Variable Coordination Modes of Diimino-Pyrrole Complexes of Palladium

Ligand-based functionality is a prominent method of increasing the reactivity or stability of metal centers in coordination chemistry. Some of the most successful catalysts use ligand-based redox activity, pendant protons, or hemilability in order to specifically accelerate catalysis. Here we report the diimino-pyrrole ligand Tol,CyDIPyH (Tol,CyDIPy = 2,5-bis( N-cyclohexyl-1-( p-tolyl)methanimine)pyrrolide), which exhibits all three of these ligand properties. Metalation of Tol,CyDIPy to Pd gives the pseudo-square planar complex (Tol,CyDIPy)PdCl, which upon reduction forms a mixture of products, including a Pd(I)-Pd(I) dimer wherein Tol,CyDIPy bridges the dimeric unit. Upon addition of PMe3, the imine arms of (Tol,CyDIPy)PdCl are displaced to yield (Tol,CyDIPy)Pd(PMe3)2Cl, where the Tol,CyDIPy ligand binds in a monodentate fashion. This complex can be reduced to generate a ligand-based radical, as shown by EPR spectroscopy. Finally, (Tol,CyDIPy)PdCl also can be protonated at the imine arm, exhibiting a total of three different coordination modes across this series of complexes. Taken together, these studies show that Tol,CyDIPy exhibits notable flexibility in its coordination and redox chemistry.

Isolable iodosylarene and iodoxyarene adducts of Co and their O-atom transfer and C–H activation reactivity

We report an unusual series of discrete iodosyl- and iodoxyarene adducts of Co(ii) including detailed studies of their O-transfer reactivity and mechanism.

Crystal structure of 3-[(2-acetamido­phen­yl)imino]­butan-2-one

In the title compound, the imine C=N bond is essentially coplanar with the ketone C=O bond in an s-trans conformation. In the crystal, molecules are connected into chains along the c axis through C—H⋯O hydrogen bonds, with two adjacent chains hinged by C—H⋯O hydrogen bonds.