Ludovic Castro

Decamethylscandocinium-hydrido-(perfluorophenyl)borate: fixation and tandem tris(perfluorophenyl)borane catalysed deoxygenative hydrosilation of carbon dioxide

This work was directed at studying the capability of structurally defined, strongly Lewis-acidic metal centers to effect catalytic reductive fixation of the small molecule substrate CO2. Exposing solutions or solid samples of the ion pair [Cp*2Sc][HB(C6F5)3] 1CIP, in which the highly electrophilic decamethyl-scandocene cation and [HB(C6F5)]− as a potentially reactive source of hydride equivalents are associated, to CO2 selectively produces ion pair [Cp*2Sc][HCO2B(C6F5)3] 2CIP. The results of solution and solid state structural analysis of 2CIP imply ionic association of [Cp*2Sc]+ and [HCO2B(C6F5)3]− rather than B(C6F5)3-adduct formation to neutral Cp*2Sc-formate. In the presence of B(C6F5)3 co-catalyst and excess triethylsilane, the formation of 2CIP from 1CIP initiates the catalytic deoxygenative hydrosilation of CO2 to CH4. The roles of ion pairs 1 and 2, borane co-catalyst, and silane in the catalytic reaction were studied mechanistically by NMR spectroscopy. Intermediately formed 3,3,7,7-tetraethyl-3,7-disila-4,6-dioxanonane product was found to exert an accelerating effect on the overall reaction rate by promoting [HCO2B(C6F5)3]− dissociation to give 2SIP through formation of separated ion pairs [Cp*2Sc(κ2-(Et3SiO)2CH2)][HCO2{B(C6F5)3}n], n = 1, 2. DFT calculations show that the formation 2CIP from the reaction of 1CIP with CO2 is exoergic and without significant energy barriers. This work lays the basis for future studies of reactive ion pairs of this kind in the context of small molecule chemistry.

Carbon Monoxide Activation via O-Bound CO Using Decamethylscandocinium–Hydridoborate Ion Pairs

Ion pairs [Cp*(2)Sc](+)[HB(p-C(6)F(4)R)(3)](-) (R = F, 1-F; R = H, 1-H) were prepared and shown to be unreactive toward D(2) and α-olefins, leading to the conclusion that no back-transfer of hydride from boron to scandium occurs. Nevertheless, reaction with CO is observed to yield two products, both ion pairs of the [Cp*(2)Sc](+) cation with formylborate (2-R) and borataepoxide (3-R) counteranions. DFT calculations show that these products arise from the carbonyl adduct of the [Cp*(2)Sc](+) in which the CO is bonded to scandium through the oxygen atom, not the carbon atom. The formylborate 2-R is formed in a two-step process initiated by an abstraction of the hydride by the carbon end of an O-bound CO, which forms an η(2)-formyl intermediate that adds, in a second step, the borane at the carbon. The borataepoxide 3-R is suggested to result from an isomerization of 2-R. This unprecedented reaction represents a new way to activate CO via a reaction channel emanating from the ephemeral isocarbonyl isomer of the CO adduct.

Formation of a Uranium Trithiocarbonate Complex via the Nucleophilic Addition of a Sulfide-Bridged Uranium Complex to CS2

The uranium(IV)/uranium(IV) μ-sulfide complex [{(((Ad)ArO)(3)N)U}(2)(μ-S)] reacts with CS(2) to form the trithiocarbonate-bridged complex [{(((Ad)ArO)(3)N)U}(2)(μ-κ(2):κ(2)-CS(3))]. The trithiocarbonate complex can alternatively be formed in low yields from low-valent [(((Ad)ArO)(3)N)U(DME)] through the reductive cleavage of CS(2).

Controlling selectivity in the reductive activation of CO2 by mixed sandwich uranium(III) complexes

The synthesis and molecular structures of a range of uranium(III) mixed sandwich complexes of the type [U(η8-C8H6(1,4-SiMe3)2)(η5-CpMe4R)] (R = Me, Et, iPr, tBu) and their reactivity towards CO2 are reported. The nature of the R group on the cyclopentadienyl ring in the former has a significant effect on the outcome of CO2 activation: when R = Me, the products are the bridging oxo complex {U[η8-C8H6(1,4-SiMe3)2](η5-CpMe5)}2(μ-O) and the bridging oxalate complex {U[η8-C8H6(1,4-SiMe3)2](η5-CpMe5)}2(μ-η2:η2-C2O4); for R = Et or iPr, bridging carbonate {U[η8-C8H6(1,4-SiMe3)2](η5-CpMe4R)}2(μ-η1:η2-CO3) and bridging oxalate complexes {U[η8-C8H6(1,4-SiMe3)2](η5-CpMe4R)}2(μ-η2:η2-C2O4) are formed in both cases; and when R = tBu the sole product is the bridging carbonate complex {U[η8-C8H6(1,4-SiMe3)2](η5-CpMe4tBu)}2(μ-η1:η2-CO3). Electrochemical studies on both the uranium(III) complexes and the dimeric uranium(IV) CO2 reduction products have been carried out and all exhibit quasi reversible redox processes; in particular, the similarities in the U(III)/U(IV) redox couples suggest that the selectivity in the outcome of CO2 reductive activation by these complexes is steric in origin rather than electronic. The latter conclusion is supported by a detailed computational DFT study on the potential mechanistic pathways for reduction of CO2 by this system.

Insights into the Mechanism of Reaction of [(C5Me5)2SmII(thf)2] with CO2 and COS by DFT Studies

Reaction mechanisms for the oxidative reactions of CO(2) and COS with [(C(5)Me(5))(2)Sm] have been investigated by means of DFT methods. The experimental formation of oxalate and dithiocarbonate complexes is explained. Their formation involve the samarium(III) bimetallic complexes [(C(5)Me(5))(2)Sm-CO(2)-Sm(C(5)Me(5))(2)] and [(C(5)Me(5))(2)Sm-COS-Sm(C(5)Me(5))(2)] as intermediates, respectively, ruling out radical coupling for the formation of the oxalate complex.

Formation of a Bridging Phosphinidene Thorium Complex.

The synthesis and structural determination of the first thorium phosphinidene complex are reported. The reaction of 2 equiv of (C5Me5)2Th(CH3)2 with H2P(2,4,6-(i)Pr3C6H2) at 95 °C produces [(C5Me5)2Th]2(μ2-P[(2,6-CH2CHCH3)2-4-(i)PrC6H2] as well as 4 equiv of methane, 2 equiv from deprotonation of the phosphine and 2 equiv from C-H bond activation of one methyl group of each of the isopropyl groups at the 2- and 6-positions. Transition state calculations indicate that the steps in the mechanism are P-H, C-H, C-H, and then P-H bond activation to form the phosphinidene.

Are 5f Electrons Really Active in Organoactinide Reactivity? Some Insights from DFT Studies

Actinide chemistry is an important field in both academic and industrial research. It is mainly associated with the problem of nuclear waste reprocessing. However, experimental studies mainly focus on thorium (Th) and/or uranium (U) complexes. This is due to the complexity of dealing with transuranium elements, such as neptunium (Np) or plutonium (Pu), and more precisely to the high security requirements to deal with these elements experimentally. In this context, theoretical studies are very timely. Indeed, since the last decade, theoretical studies have shown their ability to describe the structure and the reactivity of actinide-containing molecules. However, numerous studies treat actinyl ions and to a lesser extent organoactinide complexes. The theoretical treatment of such systems has been considered a challenge for a long time. Indeed, pioneer work by Pyykkç et al. has shown that relativistic effects are important to describe the structure of actinidecontaining molecules. Moreover, the presence of 5f unpaired electrons makes the calculations more complicated because the ground state wavefunction is not necessarily represented by a single Slater determinant, so that DFT methods should be used with caution in such calculations. A similar situation was initially found for the lanthanide complexes. However, in the latter case, the 4f orbitals are not active in the bonding so that there is no need to treat them explicitly. The situation is somewhat different for the actinides. Indeed, it was shown that the 5f orbitals play a role in the bonding, explaining the existence of the actinyl ions. Thus, it was recommended to treat them explicitly. Work by Vallet et al. demonstrated that DFT (B3LYP) is not reliable to represent the reduction of uranyl ion by water. On the other hand, it is generally reported that DFT optimized geometries and frequencies compare well with experiment. Recently, some theoretical works have been reported on organoactinide complexes, mainly cyclopentadienyl containing molecules. 37] Barros et al. reported that the Cp2UO and Cp2UNMe complexes could be represented by a single determinant, allowing the safe use of DFT methods. Hence, reaction mechanisms studied have become feasible for organoactinide complexes. In that field, Yang et al. reported a study of the reactivity of 2-picoline with Cp2ThMe2 and Cp2UMe2. Similarly, Yahia and Maron proposed a study of the reactivity of pyridine N-oxide with the same catalysts. Finally, Cantat et al. reported the study of the reactivity of the migratory insertion of diphenyldiazomethane with the same catalysts. All these studies agreed that actinide complexes were using their 5f orbitals to coordinate the substrate, but no active participation of the 5f electrons was reported. A recent work by Moritz et al. proposed the definition of f-in-core effective core potentials to represent actinide centers (5f-incore). In that study on model complexes, it was shown that 5fin-core (implicit treatment) results are in excellent agreement with those obtained with an explicit treatment of the 5f electrons on both geometries, vibrational frequencies and dissociation energies. Based on this work, we report a complete comparative mechanistic study of the reactions of methane and ethylene with Cp2AnMe2 (An = U, Np and Pu). In all cases, explicit and implicit treatment of the 5f electrons results are compared. The computational details can be found in the Supporting Information. In order to clarify the definition of the f-in-core and smallcore ECP, it is necessary to come back to the electronic configurations of the atoms U, Np and Pu. The electronic configurations of these actinides are: U: [Rn] 5f6d7s; Np: [Rn]5f6d7s ; Pu: [Rn]5f6d7s. In the case of an An, it can be written as: U: [Rn]5f6d7s ; Np(IV): [Rn]5f6d7s ; Pu(IV): [Rn]5f6d7s. Thus, 5f-in-core (large-core) effective core potentials are explicitly treating 12 electrons for these complexes: [Core]6s6p6d7s. Small-core ECPs are explicitly treating 32, 33 and 34 electrons for U, Np and Pu respectively : U: [Core]5s5p5d5f6s6p6d7s ; Np: [Core]5s5p5d5f6s6p6d7s ; Pu: [Core]5s5p5d5f6s6p6d7s. For an An(III), the configurations can be written as : U : [Rn]5f6d7s ; Np(III): [Rn]5f6d7s ; Pu(III): [Rn]5f6d7s. Thus, 5f-in-core (large-core) effective core potentials are explicitly treating 11 electrons for these complexes: [Core]6s6p6d7s. Small-core ECPs are explicitly treating 32, 33 and 34 electrons for U, Np and Pu respectively, as for the oxidation state IV. The difference between the use of the two ECPs (large-core and small-core) is not only the lack of explicit treatment of 5f electrons with the largecore ECP, but also the absence of explicit treatment of all the electrons present in the n = 5 shell (which we call the sub-valence shell in the following). The use of the DFT has first been validated by comparing DFT and CASSCF results. Indeed, CASSCF single-point calculations were performed on DFT-optimized geometries for Cp2AnMe2 with An = U, Np and Pu. The chosen active space is the set of 5f orbitals (CASSCF [2,7] for U, [3,7] for Np and [4,7] for Pu). As already found by Barros et al. , these calculations [a] L. Castro, A. Yahia, Prof. Dr. L. Maron Laboratoire de Physique et Chimie des Nano-objets, INSA Universit Paul Sabatier 135 avenue de Rangueil, 31077 Toulouse Cedex (France) Fax: (+ 33) 561 559 697 E-mail : laurent.maron@irsamc.ups-tlse.fr [b] A. Yahia Institut de Chimie S parative de Marcoule, UMR 5257, CEA, CNRS Universit Montpellier II, ENSCM, Centre de Marcoule BP 17171, 30207 Bagnols sur C ze Cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200901035.

Reduction of Carbon Dioxide Promoted by a Dinuclear Tantalum Tetrahydride Complex

The reaction of 1 equiv of carbon dioxide with the dinuclear tetrahydride complex ([NPN]Ta)(2)(μ-H)(4) [where NPN = PhP(CH(2)SiMe(2)NPh)(2)] results in the formation of ([NPN]Ta)(2)(μ-OCH(2)O)(μ-H)(2) via a combination of migratory insertion and reductive elimination. The identity of the ditantalum complex containing a methylene diolate fragment was confirmed by single-crystal X-ray analysis, NMR analysis, and isotopic labeling studies. Density functional theory calculations were performed to provide information on the structure of the initial adduct formed and likely transition states and intermediates for the process.

Insight into the reaction mechanisms of (MeC5H4)3U with isoelectronic heteroallenes CS2, COS, PhN3, and PhNCO by DFT studies: a unique pathway that involves bimetallic complexes.

The mechanisms of the reduction of four isoelectronic heteroallenes (CS(2), COS, PhN(3), and PhNCO) by trivalent uranium complex (MeC(5)H(4))(3)U were determined by using DFT methods. The experimental formation of either the bimetallic CS(2) and the PhNCO uranium(IV) adducts or the bimetallic sulfide complex (COS) and the monometallic uranium(V) phenylimide complex (PhN(3)) were rationalized. The formation of the products was explained by a unique reaction mechanism with a uranium(IV)-bridged heteroallene intermediate.

Concomitant Carboxylate and Oxalate Formation From the Activation of CO2 by a Thorium(III) Complex

Abstract Improving our comprehension of diverse CO2 activation pathways is of vital importance for the widespread future utilization of this abundant greenhouse gas. CO2 activation by uranium(III) complexes is now relatively well understood, with oxo/carbonate formation predominating as CO2 is readily reduced to CO, but isolated thorium(III) CO2 activation is unprecedented. We show that the thorium(III) complex, [Th(Cp′′)3] (1, Cp′′={C5H3(SiMe3)2‐1,3}), reacts with CO2 to give the mixed oxalate‐carboxylate thorium(IV) complex [{Th(Cp′′)2[κ2‐O2C{C5H3‐3,3′‐(SiMe3)2}]}2(μ‐κ2:κ2‐C2O4)] (3). The concomitant formation of oxalate and carboxylate is unique for CO2 activation, as in previous examples either reduction or insertion is favored to yield a single product. Therefore, thorium(III) CO2 activation can differ from better understood uranium(III) chemistry.