Bimetallic complexes of d- and f-block metals with pentalene ligands

The focus of this thesis is the synthesis and characterisation of organometallic complexes incorporating the silylated pentalene ligand, [C8H4{SiiPr3-1,4}2]2- (= Pn†), bound to more than one metal centre. In general, metals in low oxidation states from the d- and f-block of the periodic table have been selected for these bimetallic complexes, as they are potentially reactive with small molecule substrates. Chapter One introduces the pentalene molecule and its derivatives, and discusses their use as ligands in organometallic chemistry. Particular emphasis is given to the application of organometallic pentalene complexes, ranging from conducting polymers in materials chemistry to small molecule activation and catalysis. In Chapter Two the silylated pentalene ligand Pn† is used to bridge two lanthanide(II) centres in anti-bimetallic sandwich complexes of the type [Cp*Ln]2(μ-Pn†) (Ln = Yb, Eu and Sm). Magnetic measurements and electrochemical methods are used to investigate the extent of intermetallic communication in some of these systems, which show potential for the design of organometallic 'molecular-wires'. Chemical oxidation of [Cp*Yb]2(μ-Pn†) leads to dissociation into mononuclear fragments (η8-Pn†)YbCp* and [Cp*Yb]+, and reaction of [Cp*Sm]2(μ-Pn†) with CO yields (η8-Pn†)SmCp*. Rational synthetic routes to mononuclear mixed-sandwich Pn†/Cp* compounds with trivalent f-block ions (Dy, Tb and U) are also developed, and their magnetic properties are studied by SQUID magnetometry including variable-frequency ac susceptibility measurements. These studies identified (η8-Pn†)DyCp* as the first known example of a pentalene based single molecule magnet, with a closed-waist hysteresis loop observed up to 2 K. Chapter Three describes the synthesis of iron(II) complexes with silylated pentalene ligands, and efforts towards incorporating them into extended organometallic arrays and heteronuclear anti-bimetallic complexes. Six complexes have been structurally characterised including the triple-decker homobimetallic [Cp*Fe]2(μ-Pn†), and the potassium salt [Cp*Fe(η5-Pn†)][K] which is an organometallic polymer in the solid state. Chapter Four documents efforts towards the synthesis of syn-bimetallic pentalene complexes, including the first row d-block metals V, Ti and Sc. A novel synthetic route to the di-titanium bis(pentalene) 'double-sandwich' complex (Pn†)2Ti2 is developed, via chloride-bridged dimers [(η8-Pn†)Ti]2(μ-Cl)x (x = 2 and 3). The electronic and magnetic properties of the latter are investigated using EPR spectroscopy and SQUID magnetometry, and the structure and bonding in (Pn†)2Ti2 is examined using spectroscopic, crystallographic, electrochemical and computational techniques. Preliminary studies toward the synthesis of an analogous di-scandium complex were unsuccessful, however three novel complexes have been synthesised including (η8- Pn†)ScCp* which is first example of a Sc complex bearing a Pn† ligand to be characterised by X-ray diffraction. Chapter Five explores the reactivity of the double-sandwich compound (Pn†)2Ti2 prepared in Chapter Four, with small molecules which are of industrial and environmental importance. The relatively open structure of (Pn†)2Ti2 allows the formation of adducts with unsaturated small molecules CO, MeNC and CO2. In the latter case the adduct formed is unstable at room temperature and the coordinated CO2 molecule is reduced to give a bis(oxo) bridged dimer and a di-carbonyl complex. This provides the first example of small molecule activation by a di-metal bis(pentalene) double-sandwich complex. The reactivity survey of (Pn†)2Ti2 is extended in Chapter Six to other substrates; including unsaturated heteroallenes as model molecules for CO2. In the case of nonpolar heteroallenes CS2 and carbodiimide, thermally stable adducts are isolated and have been structurally characterised. Polar heteroallenes COS and organic isocyanates undergo reductive transformations to give sulfide- and carbonimidate-bridged complexes respectively. The reactivity of (Pn†)2Ti2 with organic molecules containing heteroatom-heteroatom bonds is also described; the reactions with diphenyldichalcogenides and azobenzene show the ability of the double-sandwich complex to act as a 2e- and 4e- reducing agent respectively. The rich and varied chemistry shown by (Pn†)2Ti2 is evaluated and future work is suggested

Carbon dioxide activation by a uranium(III) complex derived from a chelating bis(aryloxide) ligand

The new dianionic ligand, C6H4{p-C(CH3)2C6H2Me2O−}2 (= p-Me2bp), featuring two aryloxide donors and a central arene ring, has been synthesized, and used to prepare the mixed-ligand U(III) compound, [U(Cp*)(p-Me2bp)] which exhibits an η6-interaction with the uranium center. Reductive activation of CO2 was investigated using [U(Cp*)(p-Me2bp)] in supercritical CO2, which gave a dinuclear uranium carbonate complex,{U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO2 showed the formation of a rare U(IV) oxalate complex, {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2), alongside {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3). The relative ratio of the latter two products is temperature dependent: at low temperatures (-78 ˚C) oxalate formation is favored, whilst at room temperature the carbonate is the dominant product. The U(IV) iodide, [U(Cp*)(p-Me2bp)I], was also synthesized and used as part of an electrochemical study, the results of which showed that [U(Cp*)(p-Me2bp)] has a UIV/UIII redox couple of −2.18 V vs FeCp2+/0 as well as an possible electrochemically accessible UIII/UII reduction process at −2.56 V vs FeCp2+/0

Reactivity of a dititanium bis(pentalene) complex toward heteroallenes and main-group element–element bonds

The reactivity of the Ti═Ti double bond in (μ,η5:η5-Pn†)2Ti2 (1; Pn† = 1,4-{SiiPr3}2C8H4) toward isocyanide and heteroallene substrates, and molecules featuring homonuclear bonds between main-group elements (E–E) has been explored. Reaction of 1 with methyl isocyanide or 1,3-N,N′-di-p-tolylcarbodiimide resulted in the formation of the 1:1 adducts (μ,η5:η5-Pn†)2Ti2(μ,η2-CNMe) (2) and (μ,η5:η5-Pn†)2Ti2(μ-C{N(4-C6H4CH3)}2) (3), respectively, which are thermally stable up to 100 °C in contrast to the analogous adducts formed with CO and CO2. Reaction of 1 with phenyl isocyanate afforded a paramagnetic complex, [(η8-Pn†)Ti]2(μ,κ2:κ2-O2CNPh) (4), in which the “double-sandwich” architecture of 1 has been broken and an unusual phenyl-carbonimidate ligand bridges two formally Ti(III) centers. Reaction of 1 with diphenyl dichalcogenides, Ph2E2 (E = S, Se, Te), led to the series of Ti–Ti single-bonded complexes (μ,η5:η5-Pn†)2[Ti(EPh)]2 (E = S (5), Se (6), Te (7)), which can be considered the result of a 2e– redox reaction or a 1,2-addition across the Ti═Ti bond. Treatment of 1 with azobenzene or phenyl azide afforded [(η8-Pn†)Ti]2(μ-NPh)2 (8), a bridging imido complex in which the pentalene ligands bind in an η8 fashion to each formally Ti(IV) center, as the result of a 4e– redox reaction driven by the oxidative cleavage of the Ti═Ti double bond. The new complexes 2–8 were extensively characterized by various techniques including multinuclear NMR spectroscopy and single-crystal X-ray diffraction, and the experimental work was complemented by density functional theory (DFT) studies

Ethene activation and catalytic hydrogenation by a low-valent uranium pentalene complex

The reaction of the uranium(III) complex [U(η8-Pn††)(η5-Cp*)] (1) (Pn†† = C8H4(1,4-SiiPr3)2, Cp* = C5Me5) with ethene at atmospheric pressure produces the ethene-bridged diuranium complex [{(η8-Pn††)(η5-Cp*)U}2(μ-η2:η2-C2H4)] (2). A computational analysis of 2 revealed that coordination of ethene to uranium reduces the carbon–carbon bond order from 2 to a value consistent with a single bond, with a concomitant change in the formal uranium oxidation state from +3 in 1 to +4 in 2. Furthermore, the uranium–ethene bonding in 2 is of the δ type, with the dominant uranium contribution being from f–d hybrid orbitals. Complex 2 reacts with hydrogen to produce ethane and reform 1, leading to the discovery that complex 1 also catalyzes the hydrogenation of ethene under ambient conditions

The reductive activation of CO2 across a Ti═Ti double bond: synthetic, structural, and mechanistic studies

[Image: see text] The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti(2)Pn(†)(2) (1) (Pn(†) = 1,4-{Si(i)Pr(3)}(2)C(8)H(4)) with CO(2) is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO(2) reaction is performed at −78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO(2) molecule bound symmetrically to the two Ti centers in a μ:η(2),η(2) binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO(2) is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized. 5 is thermally unstable with respect to a μ-O dimer in which the Ti–Ti bond has been cleaved and one pentalene ligand binds in an η(8) fashion to each of the formally Ti(III) centers. The molecular structure of 7 shows a “side-on” bound carbonyl ligand. Bonding of the double-sandwich species Ti(2)Pn(2) (Pn = C(8)H(6)) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO(2) reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-Ti(III) species to yield di-Ti(II) and di-Ti(IV) products. 1 forms a structurally characterized, thermally stable CS(2) adduct 8 that shows symmetrical binding to the Ti(2) unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(μ-S) mono(CO) species 10. Ph(3)PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(μ-S) complex 11 with a double-sandwich structure and bis(μ-S) dimer 12 in which the Ti–Ti bond has been cleaved

Bonding in complexes of bis(pentalene)di-titanium, Ti2(C8H6)2

Bonding in the bis(pentalene)di-titanium ‘double-sandwich’ species Ti2Pn2 (Pn = C8H6) and its interaction with other fragments have been investigated by xdensity functional calculations and fragment analysis. Ti2Pn2 with C2v symmetry has two metal-metal bonds and a low-lying metal based empty orbital, all three frontier orbitals having a1 symmetry. The latter may be regarded as being derived by symmetric combinations of the classic three frontier orbitals of two bent bis(cyclopentadienyl) metal fragments. Electrochemical studies on Ti2Pn†2 (Pn† = C8H4{SiiPr3-1,4}2) reveal a one-electron oxidation, and the formally mixed-valence Ti(II)-Ti(III) cationic complex [Ti2Pn†2][B(C6F5)4] has been structurally characterised. Theory indicates an S = ½ ground state electronic configuration for the latter, confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti2Pn2 preserving C2v symmetry, as does carbon disulfide. The dominant interaction in Ti2Pn2CO2 is σ donation into the LUMO of bent CO2 and donation from the O atoms to Ti2Pn2 is minimal, whereas in Ti2Pn2CS2 there is significant interaction with the S atoms. The bridging O atom in the mono(oxo) species Ti2Pn2O, however, employs all three O 2p orbitals in binding and competes strongly with Pn, leading to weaker binding of the carbocyclic ligand, and the sulfur analog Ti2Pn2S behaves similarly. Ti2Pn2 is also capable of binding one, two and three molecules of carbon monoxide. The bonding demands of a single CO molecule are incompatible with symmetric binding and an asymmetric structure is found. The dicarbonyl adduct Ti2Pn2(CO)2 has Cs symmetry with the Ti2Pn2 unit acting as two MCp2 fragments. Synthetic studies show, that in the presence of excess CO a tricarbonyl complex Ti2Pn†2(CO)3 is formed, which optimises to an asymmetric structure with two terminal CO ligands and one semi-bridging. Low temperature 13C NMR spectroscopy reveals a rapid dynamic exchange between the two bound CO sites and free CO

Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum