Mark J. Hampden-Smith

The synthesis and characterization of Pb(O2CCH3)2(18-crown-6) · 3H2O, a monomeric lead(II) carboxylate compound

Abstract The compound Pb(O2CCH3)2(18-crown-6) · 3H2O was synthesized via the reaction of either an aqueous solution of lead(II) acetate with 18-crown-6 or by reaction of lead(II) carbonate with acetic acid and 18-crown-6 in water. The product was shown to be monomeric in the solid state, as determined by single-crystal X-ray diffraction. The molecule is markedly asymmetric with both acetate ligands chelating to the same side of the 10-coordinate lead(II) centre. Solid-state FTIR of a crystalline sample revealed a difference, Δ, in νasym(CO) and νsym(CO) of 612 cm−1. Solution 207Pb NMR spectroscopy was consistent with coordination of the 18-crown-6 ligand to the lead centre in aqueous solution. Thermogravimetric analysis studies showed that Pb(O2CCH3)2(18-crown-6)·3H2O thermally decomposed in air to form crystalline PbO at 380°C.

Ester elimination versus ligand exchange: the role of the solvent in tin–oxo cluster-building reactions

Ester elimination between Sn(OBUt)4 and Sn(OAc)4 in refluxing toluene yields Sn6(O)6(OCMe3)6(O2CMe)6 which is characterized in solution and in the solid state by single-crystal X-ray diffraction; reaction of the same species in pyridine results in the formation of the ligand exchange products, Sn(OBut)X(OAc)4–x where x= 1–3.

Copper Etching: New Chemical Approaches

There is a tremendous demand for improved performance and speed in consumer electronics that is likely to continue as new applications and developments occur. This demand necessitates a reduction in the critical dimensions and an increase in the density of devices in microelectronic circuits. As a result, new materials must be considered for integration into microelectronics technology. In particular, the metal wiring or interconnects that connect different components in silicon-based semiconductor devices is a subject of great interest. As the dimensions of transistors shrink below the 0.5 μm level, their speed will become limited by the delays in the existing interconnect material, Al-Si-Cu alloy ( p ~ 3 μΩ cm). Therefore, to avoid problems associated with RC (“resistance/capacitance”) time delays and voltage drops, it will be necessary to construct interconnections of materials that possess lower resistivities, resistance to electromigration and hillock formation, and resistance to diffusion into other materials (see Table I). A number of materials are possible candidates to replace the Al-Si-Cu alloy, including W, Ag, Au, and Cu. Tungsten has excellent resistance to electromigration and hillock formation, but has higher resistivity compared with the Al-Si-Cu alloy. Thus, applications of W are likely to be found where short interconnection distances are necessary. Silver has the lowest resistivity of all metals, but is easily corroded and diffuses rapidly into many materials used in semiconductor devices. However, some specific applications for silver are viable, such as the formation of contacts on ceramic superconductors. Gold has a lower resistivity than the Al-Si-Cu alloy and is inert to chemical corrosion. As a result Au is used where device reliability is the primary concern-for example, for wiring in GaAs-based semiconductors and electrical contacts in packaging.

The tungsten-tungsten triple bondXVI. Bis(cyclopentadienyl)- and bis(indenyl)tetradimethylamidoditungsten (WW) ☆

Abstract From the reaction between 1,2-W2Cl2(NMe2)4 and NaCp (Cp = C5H5) (two equiv.) in toluene, a yellow-orange, hydrocarbon-soluble, crystalline solid of formula W2Cp2(NMe2)4 has been obtained. NMR data are consistent with a gauche 1,2- W2R2(NMe2)4 molecule having a WW triple bond and restricted rotations about the WN bonds. The NMR data, however, do not uniquely define the nature of the CpW bonding beyond the fact that there are five time-averaged equivalent carbon and hydrogen atoms. No crystals suitable for an X-ray study were obtained. The related reaction involving 1,2-W2Cl2(NMe2)4 and LiC9H7 (C9H7 = indenyl) (two equiv.) gave an orange, hydrocarbon-soluble, crystalline compound. The NMR data are indicative of a gauche 1,2- W2R2(NMe2)4 molecule in solution having a virtual C2 axis of symmetry and restricted rotations about the WN bonds. An X-ray study rev ealed the presence of the gauche rotamer and that the indenyl-metal bonding was η3-C9H7. Pertinent bond distances (A) and angles (°) for 1,2-W2(η3-C9H7)2(NMe2)4 are WW = 2.334(1), WN = 1.96(1) (av.), and W-η3-C3(indenyl) = 2.36(1)–2.54(1), WWN = 102(1) (av.) and WWC(η3-C3(indenyl)) = 91–117°. The reaction between 1,2-W2Cp2(NMe2)4 and EtCOOCOEt (> four equiv.) proceeds in hydrocarbon solvents to give the compound W2Cp2(NMe2)(O2CEt)3 and Me2NCOEt (three equiv.) in contrast to other 1,2-W2R2(NMe2)4 compounds which yield either W2(O2CEt)4 when R contains β-hydrogen atoms or W2R2(O2CEt)4 compounds. The molecular structure of W2Cp2(NMe2)(O2CEt)3 can be viewed as two fused four-legged piano-stools sharing an edge, μ-NMe2 and μ-η1-O2CEt, with a pair of bridging O 2CEt ligands spanning a WW bond of distance 2.78 A. The Cp ligands form W-η5-C5H5 bonds roughly trans to the WW axis.

Preparation and characterization of hexacyclohexoxido- and trispinacolato-ditungsten (MM). A comparison of staggered and eclipsed ethane-like O3WWO3 units

Abstract The reactions between W2(OBut)6 and each of pinacol (3 equiv) and cyclohexanol (> 6 equiv) yield the title compounds W2(OCMe2CMe2O)3, I, and W2(OCy)6, II (Cy = cyclohexyl) as yellow needles, which are sparingly soluble in hydrocarbon solvents. In the solid-state both compounds form infinite chains. Compound I has three pinacolate ligands which span the WW bond: WW = 2.2738(8) A, WO = 1.90 A (av) and WWO = 99° (av). The three bridging ligands lead to a near eclipsed W2O6 skeleton having OWWO torsion angles = 10° (av). The infinite chain is caused by weak intermolecular oxygen-to-tungsten bonding, W … O = 2.9 A. The W2(OCy)6 molecule has crystallographically imposed C2h symmetry and the central W2O6 moiety is staggered (virtual D3d) with WW = 2.340(1) A, WO = 1.87 A (av) and WWO = 107° (av). The infinite chain in the solid-state results from the stacking of the cyclohexyl ligands. The intermolecular contacts are essentially identical to the intramolecular ones. In solution W2(OCMe2CMe2O)3 undergoes rapid enantiomerization such that a time-averaged eclipsed W2O6 moiety is seen on the NMR time-scale. The cyclohexyl rings in W2(OCy)6 are not inverting (chair ⇌ boat ⇌ chair) on the NMR time-scale.

The tungsten-tungsten triple bond18. Bridging and terminal allyl ligands in complexes of the formula W2(R)2(NMe2)4, where R C3H5 and C4H7

Abstract The reaction between RMgCl (two equivalents) and 1,2-W2Cl2(NMe2)4 in hydrocarbon solvents affords the compounds W2R2(NMe2)4, where R = allyl and 1− and 2-methyl-allyl. In the solid state the molecular structure of W2(C3H5)2(NMe2)4 has C2 symmetry with bridging allyl ligands and terminal WNMe2 ligands. The WW distance 2.480(1) A and the CC distances, 1.47(1) A, imply an extensive mixing of the allyl π-MOs with the WW π-MOs, and this is supported by an MO calculation on the molecule W2(C3H5)2(NH2)4 employing the method of Fenske and Hall. The most notable interaction is the ability of the (WW)6+ centre to donate to the allyl π*-MO (π3). This interaction is largely responsible for the long WW distance, as well as the long CC distances, in the allyl ligand. The structure of the 2-methyl-allyl derivative W2(C4H7)2(NMe2)4 in the solid state reveals a gauche-W2C2N4 core with WW = 2.286(1) A and WC = 2.18(1) A, typical of WW and WC triple and single bonds, respectively. In solution (toluene-d8) 1H and 13C NMR spectra over a temperature range −80°C to +60°C indicate that both anti- and gauche- W2C2N4 rotamers are present for the 2-methyl-allyl derivative. In addition, there is a facile fluxional process that equilibrates both ends of the 2-methyl-allyl ligand on the NMR time-scale. This process leads to a coalescence at 100°C and is believed to take place via an η3-bound intermediate. The 1-methyl-allyl derivative also binds in an η1 fashion in solution and temperature-dependent rotations about the WN, WC and CC bonds are frozen out at low temperatures. The spectra of the allyl compound W2(C3H5)2(NMe2)4 revealed the presence of two isomers in solution—one of which can be readily reconciled with the presence of the bridging isomer found in the solid state while the other is proposed to be W2(η3-C3H5)2(NMe2)4. The compound W2R2(NMe2)4 where R = 2,4-dimethyl- pentadiene was similarly prepared and displayed dynamic NMR behaviour explainable in terms of facile η1 = η3 interconversions.

Alcohol–alkoxide exchange between Sn(OBut)4 and HOBut in co-ordinating and non-co-ordinating solvents

Proton NMR magnetization-transfer experiments have been utilized to measure the kinetic parameters of alcohol interchange between the homoleptic tin(IV) alkoxide Sn(OBut)4 and ButOH in various solvents. The reaction was studied in pyridine with rate constants measured over the temperature range 24–112 °C (k1= 0.22 s–1 at 24 °C to 12 s–1 at 112 °C) from which activation parameters were derived (ΔG‡298= 18.8 kcal mol–1, ΔH‡= 9.5 kcal mol–1 and ΔS‡=–30 cal K–1 mol–1). These data along with variable-temperature 119Sn-{1H} NMR data are consistent with a five-co-ordinate intermediate such as [Sn(OBut)4·HOBut] and suggest that the metal, even in sterically encumbered metal alkoxide compounds such as Sn(OBut)4, is sufficiently co-ordinatively and electronically unsaturated to react with bulky alcohols. In non-co-ordinating solvents such as benzene the exchange rate is faster (k= 1.93 s–1 at 24 °C). Room-temperature solution 119Sn-{1H} NMR spectroscopy of Sn(OBut)4 dissolved in pyridine (py) shows evidence for formation of Sn(OBut)4·py, consistent with an exchange mechanism in which py competes with ButOH for co-ordination sites at tin(IV). Unambiguous evidence for the co-ordination of donor molecules to tin(IV) in homoleptic tin(IV) alkoxide compounds was obtained from the isolation and structural characterization of Sn(OSiPh3)4(NC5H5)2·0.5NC5H5, the first example of a donor adduct of a homoleptic tin(IV) alkoxide. Single-crystal X-ray diffraction showed that this compound is monomeric and approximately octahedral with trans pyridine groups.

Solvent dependent ester elimination reactions in the preparation of mixed-metal oxo clusters: the synthesis of PbSn2(µ3-O)(OBut)4(OAc)4

The reaction between Sn(OBut)4 and Pb(OAc)4 has been studied as a function of the nature of the reaction solvent: in refluxing toluene, ester elimination results in the quantitative formation of PbSn2(µ3-O)(OBut)4(OAc)4 which has been characterized in solution by 1H, 119Sn and 207Pb NMR and in the solid state by single-crystal X-ray diffraction, whereas in pyridine, reaction of the same two species results in ligand exchange forming Sn(OBut)3(OAc)·pyridine and Pb(OAc)3(OBut).

Synthesis and characterization of perovskite-phase mixed-metal oxides: lead titanate

The reaction of Pb(O2CCMe2OH)2 and (acac)2Ti(OPri)2, where acac = CH3COCHCOCH3, in a 1 : 1 stoichiometry results in elimination of two equivalents of HOPri and formation of a species with empirical formula [Pb(O2CCMe2O)2Ti(acac)2·xC5H5N]. This species can act as a single-source precursor to PbTiO3. 1H NMR studies of the formation of this species in pyridine reveal that the alkoxide ligands are completely eliminated and that the reaction appears to occur in two steps. The species [Pb(O2CCMe2O)2Ti(acac)2] does not react with HOPri, indicating that this reaction is irreversible. Thermolysis of [Pb(O2CCMe2O)2Ti(acac)2] results in loss of the organic ligands and the initial formation of amorphous material. Part of this amorphous material crystallizes at 310 °C with 2 nm sized crystallites which can be attributed to the presence of either pyrochlore-phase PbTiO3 or PbO, based on powder X-ray diffraction data. When the sample is heated further to 330 °C, the remainder of the amorphous material starts to crystallize as perovskite-phase PbTiO3 with a much larger crystallite size of 30 nm. X-Ray powder diffraction and transmission electron microscopy data are consistent with the presence of a small amount of 2 nm sized crystallites in perovskitephase PbTiO3 at 410 °C. Analytical data (atomic absorption spectroscopy) were consistent with a 1 : 1 Pb : Ti atomic ratio, which we interpret as being more consistent with the presence of pyrochlore-phase PbTiO3 than PbO. We believe that there is little or no phase transformation under these conditions. As a result we conclude that single-source precursors to PbTiO3 result in lower crystallization temperatures than multiple sources of Pb and Ti and that pyrochlorephase PbTiO3 does not transform into crystalline perovskite-phase PbTiO3 under these conditions.

Generation of metal titanate powders by spray pyrolysis using single-source precursors

Crystalline, phase-pure, submicrometre sized PbTiO3 and BaTiO3 particles have been produced via aerosol decomposition of single-source mixed metal–organic precursors A(O2CCMe2O)2Ti(OPri)2(A = Pb or B) at 600–900 °C. The particles were hollow, ranged in size from 0.1 to 1 µm and consisted of 30–50 nm crystallites.