Reaction of (CpSiMe(3))(3)U or (CpSiMe(3))(3)Nd with (Cp*Al)(4) or Cp*Ga (Cp* = C(5)Me(5)) afforded the isostructural complexes (CpSiMe(3))(3)M-ECp* (M = U, E = Al (1); M = U, E = Ga (2); M = Nd, E = Al (3); M = Nd, E = Ga (4)). In the case of 1 and 2 the complexes were isolated in 39 and 90% yields, respectively, as crystalline solids and were characterized by single-crystal X-ray diffraction, variable-temperature (1)H NMR spectroscopy, elemental analysis, variable-temperature magnetic susceptibility, and UV-visible-NIR spectroscopy. In the case of 3 and 4, the complexes were observed by variable-temperature (1)H NMR spectroscopy but were not isolated as pure materials. Comparison of the equilibrium constants and thermodynamic parameters DeltaH and DeltaS obtained by (1)H NMR titration methods revealed a much stronger U-Ga interaction in 2 than the Nd-Ga interaction in 4. Competition reactions between (CpSiMe(3))(3)U and (CpSiMe(3))(3)Nd indicate that Cp*Ga selectively binds U over Nd in a 93:7 ratio at 19 degrees C and 96:4 at -33 degrees C. For 1 and 3, comparison of (1)H NMR peak intensities suggests that Cp*Al also achieves excellent U(III)/Nd(III) selectivity at 21 degrees C. The solution electronic spectra and solid-state temperature-dependent magnetic susceptibilities of 1 and 2, in addition to X-ray absorption near-edge structure (XANES) measurements from scanning transmission X-ray microscopy (STXM) of 1, are consistent with those observed for other U(III) coordination complexes. DFT calculations using five different functionals were performed on the model complexes Cp(3)M-ECp (M = Nd, U; E = Al, Ga), and empirical fitting of the values for Cp(3)M-ECp allowed the prediction of binding energy estimates for Cp*Al compounds 1 and 3. NBO/NLMO bonding analyses on Cp(3)U-ECp indicate that the bonding consists predominantly of a E-->U sigma-interaction arising from favorable overlap between the diffuse ligand lone pair and the primarily 7s/6d acceptor orbitals on U(III), with negligible U-->E pi-donation. The overall experimental and computational bonding analysis suggests that Cp*Al and Cp*Ga behave as good sigma-donors in these systems.

A comparative study of the Au(I)-catalyzed [3,3]-sigmatropic rearrangement of propargylic esters and propargyl vinyl ethers is described. Stereochemically defined cyclopropanes are employed as mechanistic probes to provide new synthetic and theoretical data concerning the reversibility of this type of rearrangement. Factors controlling the structure-reactivity relationship of Au(I)-coordinated allenes have been examined, thereby allowing for controlled access to orthogonal reactivity.

The discovery of molecular metal-metal bonds has been of fundamental importance to the understanding of chemical bonding.1 For the actinides, examples of unsupported metalmetal bonds are relatively uncommon, consisting of Cp 3 USnPh 3 , and several actinide-transition metal complexes. Traditionally, bonding in the f-elements has been described as electrostatic; however, elucidating the degree of covalency is a subject of recent research. 3 In carbon monoxide complexes of the trivalent uranium metallocenes, decreased ν CO values relative to free CO suggest that the U(III) atom acts as a π-donor. 4 Ephritikhine and coworkers have demonstrated that π-accepting ligands can differentiate trivalent lanthanide and actinide ions, an effect that renders this chemistry of interest in the context of nuclear waste separation technology.

The practical goal to measure and understand the thermodynamic properties of molecules and materials containing f-elements is often achieved through indirect methods. Of the characterization tools available to inorganic chemists, few are more powerful than X-ray crystallography. Yet for lanthanides and actinides, interpretation of a bond length is a challenging undertaking that involves a complex interplay of steric and electronic forces. In this Concept article, we perform an analysis of selected examples in which structural criteria alone have been used to draw qualitative conclusions about chemical bonding. In other instances for which such an analysis is not valid, thermodynamic information is evaluated side by side with structural data to provide reasonable interpretations of a covalent/ionic mode of bonding. A geometric variation larger than 3σ is not necessarily correlated to a change in bonding, nor is an increase in bond energy related to a bond with more covalent character. However, careful consideration of thermodynamic information can lead to reasonable interpretations of electronic structure, and may provide a more reliable benchmark for the theoretical methods which can describe f-elements.

Experimental evidence for the existence of two new lanthanide-metalloligand adducts (CpSiMe(3))(3)Ce-ECp* (E = Al, Ga) is presented. Paramagnetic (1)H NMR titration experiments were employed to derive thermodynamic parameters for Ce-Ga dative bond formation, and competition experiments with the U analogue were performed. Density functional theory calculations were undertaken using model complexes Cp(3)Ln-ECp where Ln = La-Lu and E = Al, Ga. The Ln-E bond distances were predicted to decrease more sharply across the Ln series than those involving hard Lewis bases; however, local increases were observed at Eu and Yb. Electronic analyses were performed in the natural bond orbital-natural localized molecular orbital (NBO/NLMO) formalism, indicating that the E→Ln acceptor orbital is primarily of d character in all cases. The Cp(-) ligands donate significant electron density to the Ln d manifold and thus in its bonding interactions with a dative ligand the Ln center may be considered to be Ln(2+) in the f((n-3))d(1) electronic configuration (n = 3 for La, etc.). Molecular dipole moments, NLMO and natural population analyses, bond order indices, measures of E→Ln charge transfer, and calculated Ln-E heterolytic bond disruption enthalpies were found to follow saw-tooth trends, which correlate to varying degrees with the ionization potentials of the Ln(+) ions (corrected for their ground state-to-f((n-3))d(2) excitations). It is proposed that a steric-strain component which increases with the lanthanide contraction in this case balances the Ln-E bond stabilizing effect of core-orbital contraction. All data indicate that the Ln-E bonding interactions are predominantly of covalent or nonpolar donor-acceptor character. However, the formation of a strong covalent bond is not observed because of resistance to reduction of an effectively divalent Ln center.

A generalized approach to the optimization and implementation of Buchwald-Hartwig reactions in flow is reported, through the combination of three key factors: a highly active palladium catalyst; a universal approach for continuous work-up and purification, and a methodology for catalyst recycling and reuse. The palladium N-heterocyclic carbene (NHC) pre-catalyst [Pd(IPr*)(cin)Cl] 4 (IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene; cin = η 3 -cinnamyl) is an excellent choice for continuous Buchwald-Hartwig reactions, due to its inherent high activity and stability. In preparation for running this reaction in flow (published concurrently), a detailed study has been carried out into its water stability, ultimately allowing the recycling of the catalyst in the organic phase up to 3 times in batch mode. A "right-firsttime" work-up methodology has also been developed, resulting in a universal protocol that allows the selective extraction of the Buchwald-Hartwig product into the aqueous stream as a salt, while retaining the aryl bromide starting material in the organic stream with the catalyst, thus negating the requirement for further purification. It is therefore envisaged that this approach will particularly amenable to exploitation in the Pharmaceutical industry. An optimized, scalable synthesis of [Pd(IPr*)(cin)Cl] is also reported on multi-hundred gram scale.

Immobilised Pd–NHC catalysts were successfully applied in Suzuki–Miyaura reaction under batch and flow conditions.

The coordination chemistry of the potentially semilabile tridentate ligand 2-pyridylbis(diphenylphosphino)methane (NPP) has been investigated. Bidentate (N, P) coordination occurs in CoCl(2)(NPP) (1) and [CdX(mu-X)(NPP)](2) (X = Cl (2); OAc (3)), prepared from the corresponding metal salts, in fac-Re(CO)(3)Br(NPP) (4) and in Fe(CO)(2)(MA)(NPP) (6). The last is one of three products from the reaction of Fe(CO)(4)(MA) (MA = maleic anhydride) with NPP, the other two being Fe(CO)(3)(NPP) (7; P, P coordinated) and the unusual cyclic ylid Ph2PC(2-C5H4N)PPh2C(CH2CO2H)C(=O)(5). The ligand shows tridentate coordination in Cr(CO)(3)(NPP) (9), RuCl(2)(PPh(3))(NPP) (10), and possibly in PtCl(2)(NPP) (8). Carbon monoxide displaces one phosphorus arm of the ligand in 10. Anhydrous NiCl(2) and NPP react in the presence of methanol to give NiCl(2)(P(OMe)Ph(2))(Ph(2)PCH(2)py) (12) in which the NPP ligand has been cleaved. This in turn reacts with O(2) to form trans-NiCl(2)(Ph(2)P(O)CH(2)py)(2) (13). The methine proton of NPP is transferred to the metal on reaction with Pt(C(2)H(4))(PPh(3))(2) and [Ir(COD)(NPP)]BF(4) to form the hydride complexes Pt(H)(PPh(3))(NPP-H) (14) and [Ir(H)(NPP)(NPP-H)]BF(4) (15). In 15 the intact NPP ligand is tridentate. The structures of 1 - 7 and 12 - 15 have been determined.