Katherine L. Hull

Homo-and Heterodimetallic Geminal Dianions Derived from the Bis(phosphinimine) (Ph2P(NSiMe3)}2CH2 and the Alkali Metals Li, Na, and K

The geminal organodimetallic complexes [({Ph2P(NSiMe3)}2C)2M4], where M4=Na4, 3; Li2Na2, 4; LiNa3, 5; Li2K2, 6; Na2K2, 7, and Na3K, 8, have been prepared through a variety of methods including direct or sequential deprotonation of the neutral ligand with strong bases (tBuLi, nBuNa, (Me3Si)2NNa, PhCH2K or (Me3Si)2NK), transmetalation of the homometallic derivatives (M4=Li4, 2 or Na4, 3) with tBuONa or tBuOK, and by cation exchange upon mixing the homometallic complexes in an arene solution. Complexes 3-8 have been characterized by single-crystal X-ray diffraction and are found to form a homologous series of dimeric structures in the solid-state, in accord with the previously reported structure of 2. Each complex is composed of a plane of four metals, M4, in which the ligands adopt capping positions to form distorted M4C2 octahedral cores. The metals in homometallic complexes 2 and 3 define an approximate square, whereas the heterometallic derivatives 4-8 have distinctly rhombic arrangements. The lighter metals in 4-8 interact strongly with the carbanions and the heavier metals are pushed towards the periphery of the structures. 1H, 13C, 7Li, 31P, and 29Si multinuclear NMR spectroscopic studies, cryoscopic measurements, and electrospray ionization-mass spectroscopic studies are consistent with the dimers being retained in solution. Dynamic solution behavior was discovered for Li2Na2 complex 4, in which all five possible tetrametallic derivatives Li4, Li3Na, Li2Na2, LiNa3 and Na4 coexist. Density functional theory (DFT) and natural bond order (NBO) calculations in association with natural population analyses (NPA) reveal significant differences in the electronic structures of the variously metalated dianions. The smaller cations are more effective in localizing the double negative charge on the carbanion (in the form of two lone pairs), leading to differences in the distribution of the electron density within the ligand backbones. In turn, a complex interplay of hyperconjugation, electrostatics and metal-ligand interactions is found to control the resulting electronic structures of the geminal organodimetallic complexes.

Seeded solution synthesis of straight and branched CdSe nanowires

The solution phase synthesis of narrow diameter (< 10 nm) CdSe NWs is described. Crystalline NWs with lengths between 1-10 mm are obtained using a seeded solution approach, whereby NW growth is catalyzed by Au/Bi core/shell NPs. A gold biphasic reduction step results in 1.5 (3) nm diameter Au NPs and is followed by the thermolysis of trialkylbismuthines to yield low melting, bimetallic particles with diameters less than 3 nm. These Au/Bi NPs are catalytically active towards the growth of similar diameter CdSe NWs (~7 nm) that exhibit unique quantum confinement effects since the bulk exciton Bohr radius of CdSe is 5.6 nm. Manipulating the Cd:Se ratio results in a transition from straight to branched NWs, yielding v-shapes, tripods, y-shapes, as well as higher order structures. Structural characterization shows NW growth along either the [111] or [0001] directions of zinc blende (ZB) and wurtzite (W) phases respectively for both straight and branched NWs. High resolution TEM imaging reveals that the NWs alternate between ZB and W along their length. A similar reaction scheme can be used to produce PbSe NWs with diameters less than 5 nm, demonstrating the generality of the technique.

Solution Phase Synthesis of Semiconductor Nanowires

The solution phase synthesis of narrow diameter CdSe and PbSe nanowires (NWs) is described. Crystalline NWs with lengths between 1–10 μm are obtained using a seeded solution approach, whereby NW growth is catalyzed by Au/Bi core/shell nanoparticles (NPs). A gold biphasic reduction step results in 1.5 (3) nm diameter Au NPs and is followed by the thermolysis of trialkylbismuthines to yield low melting, bimetallic particles with diameters less than 3 nm. These Au/Bi NPs are catalytically active towards the growth of similar diameter CdSe NWs (∼7 nm) that exhibit quantum confinement effects. By varying the ratio of Cd (or Pb) to Se, both straight and branched NWs can be obtained, with branched structures including v-shapes, tripods, and y-shapes in the case of CdSe and t-shapes in the case of PbSe. Structural characterization shows that both straight and branched CdSe NWs grow along either the zinc blende (ZB) or wurtzite (W) directions. Conversely, PbSe has a rocksalt crystal structure, and both straight and branched NWs grow along directions.