Synthesis and CO₂ Insertion Chemistry of Uranium(IV) Mixed-Sandwich Alkyl and Hydride Complexes

A series of U­(IV) mixed-sandwich alkyls of the form [U­(COT^TIPS2)­Cp*R] (R = Me, CH₂Ph, CH₂TMS, CH­(TMS)₂; COT^TIPS2 = C₈H₆(Si^<i>i</i>Pr₃-1,4)₂; Cp* = C₅Me₅; TMS = SiMe₃) have been synthesized and structurally characterized, and their reactivity toward H₂ and CO₂ has been investigated. The alkyls R = Me, CH₂Ph, CH₂TMS react at room temperature with a stoichiometric amount of CO₂ to form κ²-carboxylate complexes. Reaction of all four alkyls with H₂ yields a monomeric, terminal hydride complex, [U­(COT^TIPS2)­Cp*H], which is unstable with respect to hydrogen loss and reacts with CO₂ to give the κ²-formate complex [U­(COT^TIPS2)­Cp*­(κ²-O₂CH)]. Additionally, a common decomposition product of the alkyls and hydride complexactivation of a Cp* methyl group to give a “tucked-in” alkylhas been isolated and structurally characterized and its insertion chemistry toward CO₂ has been examined

The Reductive Activation of CO₂ Across a TiTi Double Bond: Synthetic, Structural, and Mechanistic Studies

The reactivity of the bis­(pentalene)­dititanium double-sandwich compound Ti₂Pn^†₂ (<b>1</b>) (Pn^† = 1,4-{SiⁱPr₃}₂C₈H₄) with CO₂ is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO₂ reaction is performed at −78 °C, the 1:1 adduct <b>4</b> is formed, and low-temperature spectroscopic measurements are consistent with a CO₂ molecule bound symmetrically to the two Ti centers in a μ:η²,η² binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO₂ is quantitatively reduced over a period of minutes to give the bis­(oxo)-bridged dimer <b>2</b> and the dicarbonyl complex <b>3</b>. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (<b>5</b>) and monocarbonyl (<b>7</b>) double-sandwich complexes, which have been independently synthesized and structurally characterized. <b>5</b> 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 η⁸ fashion to each of the formally Ti^III centers. The molecular structure of <b>7</b> shows a “side-on” bound carbonyl ligand. Bonding of the double-sandwich species Ti₂Pn₂ (Pn = C₈H₆) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO₂ 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. <b>1</b> forms a structurally characterized, thermally stable CS₂ adduct <b>8</b> that shows symmetrical binding to the Ti₂ unit and supports the formulation of <b>4</b>. The reaction of <b>1</b> with COS forms a thermally unstable complex <b>9</b> that undergoes scission to give mono­(μ-S) mono­(CO) species <b>10</b>. Ph₃PS is an effective sulfur transfer agent for <b>1</b>, enabling the synthesis of mono­(μ-S) complex <b>11</b> with a double-sandwich structure and bis­(μ-S) dimer <b>12</b> in which the Ti–Ti bond has been cleaved

Synthesis and CO₂ Insertion Chemistry of Uranium(IV) Mixed-Sandwich Alkyl and Hydride Complexes

A series of U­(IV) mixed-sandwich alkyls of the form [U­(COT^TIPS2)­Cp*R] (R = Me, CH₂Ph, CH₂TMS, CH­(TMS)₂; COT^TIPS2 = C₈H₆(Si^<i>i</i>Pr₃-1,4)₂; Cp* = C₅Me₅; TMS = SiMe₃) have been synthesized and structurally characterized, and their reactivity toward H₂ and CO₂ has been investigated. The alkyls R = Me, CH₂Ph, CH₂TMS react at room temperature with a stoichiometric amount of CO₂ to form κ²-carboxylate complexes. Reaction of all four alkyls with H₂ yields a monomeric, terminal hydride complex, [U­(COT^TIPS2)­Cp*H], which is unstable with respect to hydrogen loss and reacts with CO₂ to give the κ²-formate complex [U­(COT^TIPS2)­Cp*­(κ²-O₂CH)]. Additionally, a common decomposition product of the alkyls and hydride complexactivation of a Cp* methyl group to give a “tucked-in” alkylhas been isolated and structurally characterized and its insertion chemistry toward CO₂ has been examined

Bonding in Complexes of Bis(pentalene)dititanium, Ti₂(C₈H₆)₂

Bonding in the bis­(pentalene)­dititanium “double-sandwich” species Ti₂Pn₂ (Pn = C₈H₆) and its interaction with other fragments have been investigated by density functional calculations and fragment analysis. Ti₂Pn₂ with <i>C</i>_2<i>v</i> 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^† = 1,4-{SiⁱPr₃}₂C₈H₄) revealed a one-electron oxidation, and the formally mixed-valence Ti­(II)–Ti­(III) cationic complex [Ti₂Pn^†₂]­[B­(C₆F₅)₄] has been structurally characterized. Theory indicates an <i>S</i> = ¹/₂ ground-state electronic configuration for the latter, which was confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti₂Pn₂, preserving the <i>C</i>_2<i>v</i> 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 analogue Ti₂Pn₂S behaves similarly. Ti₂Pn₂ is also capable of binding one, two, or 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 <i>C_s</i> symmetry with the Ti₂Pn₂ unit acting as two MCp₂ fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti₂Pn^†₂(CO)₃ is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature ¹³C NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO

Steric Effects in the Reductive Coupling of CO by Mixed-Sandwich Uranium(III) Complexes

The selectivity of the mixed-sandwich U­(III) complexes of the type [U­(η-C₈H₆{Si^<i>i</i>R₃-1,4}₂)­(η-Cp^R′)] (R = Me, ⁱPr; R′ = Me₄H, Me₅, Me₄ⁱPr, Me₄SiMe₃, Me₄Et) toward the reductive coupling of CO to form uranium-bound oxocarbons has been explored. In this context, the new U­(III) mixed-sandwich complexes [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4TMS)], [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4iPr)], [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4Et)], [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp*)], and [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp^Me4TMS)] have been prepared and structurally characterized. The reactivity toward CO is dominated by the “global” sterics around the uranium center, while selectivity for oxocarbon formation is largely regulated by the steric bulk of the Cp^R′ ligand. Accordingly, with excess CO [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4TMS)] and [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4iPr)] show no reactivity, [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp^Me4TMS)] is completely selective for the formation of the ynediolate complex [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp^Me4TMS)]₂(μ-η¹:η¹-¹³C₂O₂), [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp*)] affords only the deltate complex [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp*)]₂(μ-η²:η²-C₃O₃), and [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4H)] gives solely the squarate complex [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4H)]₂(μ-η²:η²-C₄O₄). Additionally, the squarate moiety has been removed from the uranium center in the last complex by reaction with Me₃SiCl to afford the TMS ester of squaric acid, C₄O₂(OTMS)₂

Steric Effects in the Reductive Coupling of CO by Mixed-Sandwich Uranium(III) Complexes

The selectivity of the mixed-sandwich U­(III) complexes of the type [U­(η-C₈H₆{Si^<i>i</i>R₃-1,4}₂)­(η-Cp^R′)] (R = Me, ⁱPr; R′ = Me₄H, Me₅, Me₄ⁱPr, Me₄SiMe₃, Me₄Et) toward the reductive coupling of CO to form uranium-bound oxocarbons has been explored. In this context, the new U­(III) mixed-sandwich complexes [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4TMS)], [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4iPr)], [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4Et)], [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp*)], and [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp^Me4TMS)] have been prepared and structurally characterized. The reactivity toward CO is dominated by the “global” sterics around the uranium center, while selectivity for oxocarbon formation is largely regulated by the steric bulk of the Cp^R′ ligand. Accordingly, with excess CO [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4TMS)] and [U­(η-C₈H₆{Si^<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4iPr)] show no reactivity, [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp^Me4TMS)] is completely selective for the formation of the ynediolate complex [U­(η-C₈H₆{SiMe₃-1,4}₂)­(η-Cp^Me4TMS)]₂(μ-η¹:η¹-¹³C₂O₂), [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp*)] affords only the deltate complex [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp*)]₂(μ-η²:η²-C₃O₃), and [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4H)] gives solely the squarate complex [U­(η-C₈H₆{Si<i>i</i>Pr₃-1,4}₂)­(η-Cp^Me4H)]₂(μ-η²:η²-C₄O₄). Additionally, the squarate moiety has been removed from the uranium center in the last complex by reaction with Me₃SiCl to afford the TMS ester of squaric acid, C₄O₂(OTMS)₂

Comparison of the Reactivity of the Low Buried-Volume Carbene Complexes (ITMe)₂Pd(PhCCPh) and (ITMe)₂Pd(PhNNPh)

The novel Pd(0)-azobenzene complex (ITMe)₂Pd­(PhNNPh) (<b>5</b>) (ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) has been isolated and characterized in the solid state and by cyclic voltammetry. Its reactivity toward E–E′ bonds (E, E′ = Si, B, Ge) has been compared with that of the known carbene complex (ITMe)₂Pd­(PhCCPh) (<b>2</b>). Whereas <b>2</b> reacts with all E–E′ bonds studied, <b>5</b> only reacted with B–B and B–Si moieties, echoing our previous catalytic studies on azobenzenes

Double-Sandwich Pentalene Complexes M₂(pent^†)₂ (M = Rh, Pd; pent^† = 1,4-Bis(triisopropylsilyl)pentalene): Synthesis, Structure, and Bonding

The bis­(pentalene) complexes M₂(pent^†)₂ (M = Rh (<b>1</b>), Pd (<b>2</b>); pent^† = 1,4-bis­(triisopropylsilyl)­pentalene) have been synthesized and structurally characterized. In both <b>1</b> and <b>2</b> the metals have a formal electron count in excess of 18 per metal center, and DFT calculations indicate antibonding metal–metal interactions are present in <b>1</b>, whereas <b>2</b> involves antibonding metal–ligand interactions