Claude Villiers

Lanthanide(III) and Actinide(III) Complexes [M(BH4)2(THF)5][BPh4] and [M(BH4)2(18-crown-6)][BPh4] (M = Nd, Ce, U): Synthesis, Crystal Structure, and Density Functional Theory Investigation of the Covalent Contribution to Metal-Borohydride Bonding

Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3.C4H8S, 4.THF, 5, and 6.THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7.C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U...B with respect to the Ce...B distance, which is accompanied by a lengthening of the U-Hb bonds and an opening of the Hb-B-Hb angle (Hb = bridging hydrogen atom of the eta3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 -->M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M-Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the pi bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.

Reactions of Aliphatic Ketones R2CO (R=Me, Et,iPr, andtBu) with the MCl4/Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions

Analysis of the products of the reactions of ketones R2CO (R = Me, Et, iPr, tBu) with the MCl4/Li(Hg) system (M = U, Ti) at 20 degrees C revealed significant differences. For R = Me, the reaction proceeded exclusively (M = U) or preferentially (M = Ti) via a metallopinacol intermediate resulting from dimerization of ketyl radicals. Pinacol was liberated by hydrolysis, and tetramethylethylene was obtained after further reduction at 65 degrees C. For R=iPr, formation of iPr2C=CiPr2 as the only coupling product, the nonproduction of this alkene by reduction of the uranium pinacolate [U]-OCR2CR2O-[U] (R= iPr) at 20 degrees C, and the instability of the corresponding titanium pinacolate towards rupture of the pinacolic C-C bond indicated that reductive coupling of iPr2CO did not proceed by dimerization of ketyl radicals. Formation of 2,4-dimethyl-2-pentene was in favor of a carbenoid intermediate resulting from deoxygenative reduction of the ketyl. These results revealed that for sterically hindered ketones, McMurry reactions can be viewed as Wittig-like olefination reactions. For R=tBu, no coupling product was obtained and the alkane tBu2CH2 was the major product. The involvement of the carbenoid species [M]=CtBu2 was confirmed by its trapping with H2O, leading to tBu2CH2, and with the aldehydes RCHO, giving the cross-coupling products tBu2C=C(R)H (R = Me, tBu). Therefore, in the case of severely congested ketones, McMurry reactions present strong similarities to the Clemmensen reduction of ketones, owing to the involvement in both reactions of carbenoid species which exhibit similar reactivity.

Anionic triscyclopentadienyluranium(III) hydrides

Abstract Addition of H − to [(C 5 H 4 R) 3 U] (R  H, Me, SiMe 3 , or t Bu) or sodium amalgam reduction of the U IV hydrides [(C 5 H 4 R) 3 UH] (R  SiMe 3 , or t Bu) afforded the hydrido-bridged anions [(C 5 H 4 R) 3 UHU(C 5 H 4 R) 3 ] − (R  H or Me) or the monomeric anions [(C 5 H 4 R) 3 UH] − (R  SiMe 3 or t Bu). Crystals of [Na(18-crown-6)(THF) 2 ][(C 5 H 4 SiMe 3 ) 3 UHU(C 5 H 4 SiMe 3 ) 3 ] were obtained from an equimolar mixture of [Na(18-crown-6)][(C 5 H 4 SiMe 3 ) 3 UH] and [(C 5 H 4 SiMe 3 ) 3 U] and their structure determined.

Synthesis, structure and oxidative addition reactions of triscyclopentadienyluranium(III) nitrile complexes

Abstract The U III nitrile complexes [U(cp) 3 (NCR)1 (cp =η-C 5 H 5 ; R = Me, n Pr, i Pr or t Bu) have been prepared by treatment of [U(cp) 3 (THF)] (THF, tetrahydrofuran) with the corresponding nitrile; the crystal structures of [U(cp) 3 (NC n Pr)] and [U(cp) 3 (NC i Pr)] have been determined. Reaction of [U(cp) 3 (THF)] with benzonitrile at room temperature or thermolysis of the adducts [U(cp) 3 (NCR)] (R = Me or n Pr) afforded an equimolar mixture of the U IV compounds [U(cp) 3 (CN)] and [U(cp) 3 (R)] (R = Me, n Pr or Ph).

The First cis-Dioxido Uranyl Compound under Scrutiny

The discovery of a cis-dioxido uranyl compound, an exciting event and a milestone in the history of actinide chemistry, was recently reported in this journal by P. B. Duval et al. However, several peculiar points in the synthesis and structure of this compound, [UO2(fcdc)(thf)·fc]n (fcdc = 1,2-ferrocenedicarboxylate, fc = ferrocene), appeared questionable to us. The particularly easy and reversible formation of this polymeric product from uranyl acetate dihydrate and ferrocenecarboxylic acid (fccH), occurring at room temperature and involving for each uranyl unit the activation, breaking, and building of two C H and two C C bonds, seemed to us extraordinary and, it must be said, quite difficult to believe. The H NMR spectra in CD3OD, and their interpretation concerning the fluxional behavior of the compound in solution, were not understandable to us. The crystal structure was refined in the Ama2 space group to a R1 factor of 4.99%. However, examination of the Crystallographic Information File deposited at the Cambridge Crystallographic Data Centre revealed several anomalies. The THF moiety is affected by very large disorder effects and some displacement ellipsoids are clearly anomalous. This last point is particularly apparent in the coordinated fcdc molecule, in which some atoms of the aromatic ring bearing the acid groups are affected with extremely anisotropic displacement parameters, whereas the acid groups themselves have large, but quite isotropic parameters. The C4 C7 bond, which connects these two parts, is out of the usual range, as indicated by Duval et al. It is also surprising for the O-U-O angle of 708, which is considerably smaller than the O-M-O angles in cis-dioxido transitionmetal complexes, to be associated with U O distances equal to those in trans uranyl complexes. All these observations indicated that a closer look at the structure, which is an essential part of the paper, was warranted, considering the importance of the claim at stake. To settle these doubts, we decided to reproduce the synthesis of this compound. By following the same procedure as described by Duval et al, starting from a 1:2 mixture of [UO2(OAc)2(H2O)2], where OAc is acetate, and fccH in CH2Cl2/THF, we did not obtain the 80–83% yield of red crystals as reported in ref. [1]. We repeatedly observed instead the formation of a few small red microcrystals, not suitable for X-ray diffraction, along with a reddishorange powder and other dark material. Dissolution of this mixture in pyridine or methanol gave red solutions which, upon addition of pentane, deposited red crystals of [UO2(fcc)2(py)2]·py (1) and [UO2(fcc)2(MeOH)]2·2MeOH (2), respectively, in approximately 80% yield. The crystal structures of 1 and 2 are shown in Figure 1 and Figure 2, respectively, together with selected bond lengths and angles; the dimeric structure of 2 is quite similar to those of [{UO2(OAc)2(L)}2] (L = Ph3AsO, [2a] Ph3PO, [2b] and AcOH), the trans uranyl units being bound to one terminal (chelating) and two bridging carboxylate ligands. It can be noted that compound 2 was obtained in the very solvent used for the NMR spectroscopic study by Duval et al. A large quantity of red crystals, also of sufficient quality for an accurate and unambiguous crystal-structure determination, was obtained when an excess of fccH (3 or 4 molar equivalents) was used in its reaction with [UO2(OAc)2(H2O)2] in THF; indeed, this excess should shift the equilibrium postulated by Duval et al towards the formation of the cis-dioxido uranyl compound. The one-dimensional polymeric crystal structure of the compound obtained from this reaction, [UO2(fcc)2(H2O)·thf]n (3 ; Figure 3), is quite similar

Anionic tricyclopentadienyluranium(III) complexes. Crystal structure of [Cp3UClUCp3][Na(18-crown-6)(THF)2] (Cp=η-C5H5, THF=tetrahydrofuran)

Abstract Sodium amalgam reduction of the complexes Cp 3 UX(Cp = η-C 5 H 5 , X = Cl, BH 4 , Me, n-Bu) led to the formation of the corresponding Cp 3 UX - anions which were alternatively prepared by addition of X - to Cp 3 U(THF)(I). Electron transfer or ligand exchange reactions were found to occur between Cp 3 UX and Cp 3 UX - , Cp 3 UX or Cp 3 UX - and I. The crystal structure of [Cp 3 UClUCp 3 ][Na(18-crown-6)(THF) 2 ](XI) revealed the bent and symmetrical UClU bridge.

Oxidative addition of organic halides to (η-C5H5)3U(THF) (THF = tetrahydrofuran). A convenient new synthesis of triscyclopentadienyl uranium(IV) hydrocarbyl complexes

Abstract Cp 3 U(THF) (Cp = η-C 5 H 5 ; THF = tetrahydrofuran) reacts with organic halides RX to give the equimolecular mixture of the Cp 3 UX and Cp 3 UR compounds. The features of the reaction are characteristic of an atom abstraction oxidative addition mechanism. Treatment of Cp 3 UCl with RX in the presence of sodium amalgam leads to quantitative formation of the Cp 3 UR complexes.

The first urea azine molecule and its coordination to uranium in the first actinide guanidinate complexes

The urea azine molecule (CyNH)(2)C=N-N=C(HNCy)(2) (1) was easily prepared by reaction of the carbodiimide CyN=C=NCy and H(2)NNH(2) and this novel type of bis-guanidine proved useful in affording chelating and bridging ligands for the building of polynuclear compounds, as illustrated by the synthesis of the first uranium guanidinate complexes [(THF)(2)Li(mu-Cl)(2)UCl(mu-L)](2) (2) and [UCl(mu-L)(2)UCl(2)(micro-Cl)(2)UCl(mu-L)](2) (3) (L = - 2H); the X-ray crystal structures of compounds - were determined.

Formation of carbenoid intermediates in the reaction of ditertiobutyl ketone with low-valent titanium reagents

Abstract Treatment of t Bu 2 CO with TiCl 4 and Li(Hg) in THF gave the hydrocarbon t Bu 2 CH 2 as the major product (40% yield); 60% of the total quantity of t Bu 2 CH 2 was liberated after hydrolysis of the reaction mixture. Similar experiments with THF- d 8 and D 2 O afforded t Bu 2 CHD and t Bu 2 CD 2 , indicating that carbenoid species t Bu 2 C[Ti] are likely intermediates in the reaction of t Bu 2 CO with low-valent titanium reagents.

Pinacol versus para coupling of aromatic ketones

Abstract Aromatic ketones PhCOR were coupled with uranium complexes to give, after deuterolysis, the pinacol 3 and the keto-alcohol 4 resulting respectively from the pinacol and para coupling of the carbonyl substrate. The organometallic precursors of 3 and 4 were in equilibrium. Pinacols were obtained in higher yields by using the less sterically hindered complexes.