Evaluating f-element bonding from structure and thermodynamics.

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.

Covalent lanthanide chemistry near the limit of weak bonding: observation of (CpSiMe3)3Ce-ECp* and a comprehensive density functional theory analysis of Cp3Ln-ECp (E = Al, Ga).

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.

Theoretical investigation of the rubicordifolin cascade.

The results of a theoretical investigation on the complex cascade reaction leading to the natural product rubicordifolin are reported. These computations analyze the discrete transformations that are required during the conversion of the vinyl naphthoquinone starting material into the natural product, including two pseudopericyclic cyclizations as well as a diastereoselective, hetero-Diels-Alder reaction.

A comparison of 4f vs 5f metal-metal bonds in (CpSiMe3)3M-ECp* (M = Nd, U; E = Al, Ga; Cp* = C5Me5): synthesis, thermodynamics, magnetism, and electronic structure.

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.

Mechanistic studies on Au(I)-catalyzed [3,3]-sigmatropic rearrangements using cyclopropane probes.

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.

A heterobimetallic complex with an unsupported uranium(III)-aluminum(I) bond: (CpSiMe3)3U-AlCp* (Cp* = C5Me5).

A heterobimetallic complex with the first unsupported bond between an actinide and a group 13 element, (CpSiMe3)3U-AlCp* (Cp* = C5Me5) (1), was synthesized by reaction of (CpSiMe3)3U and 1/4(Cp*Al)4 in toluene. Density functional theory calculations indicate that the U-Al bond exhibits some covalent character resulting from a Cp*Al-->U charge-transfer.

Scope and mechanism of formal S(N)2' substitution reactions of a monomeric imidozirconium complex with allylic electrophiles.

The zirconium imido complex Cp2(THF)Zr=NSi(t-Bu)Me2 (1) reacts with allylic ethers, chlorides, and bromides to give exclusively the products of the SN2' reaction; i.e., attack at the allylic position remote from the leaving group with migration of the double bond. The primary amine products can be isolated in excellent yields, after in situ Cbz protection, in the presence of variety of functional groups. Good diastereoselectivity and complete stereoselectivity allowed the formation of enantioenriched allylic amines from enantioenriched allylic ethers. Regiospecific substitution with 1 has also been achieved with allylic fluorides, which are notoriously poor substrates in other substitution reactions. On the basis of rate and kinetic isotope effect studies, we propose a general mechanism for the allylic substitution reactions with 1 which involves dissociation of THF and binding of the substrate, followed by the substitution step. In a DFT study of the substitution reaction, we identified a six-membered closed transition state for the substitution step and other relevant stationary points along the reaction coordinate. This study shows that the substitution reaction can be described as a concerted asynchronous [3,3]-sigmatropic rearrangement. This detailed knowledge of the reaction mechanism provides a rationale for the origins of the observed regio-, diastereo-, and stereoselectivity and of the unusual reactivity profile observed in the reaction.

Oxygen-centered hexatantalum tetradecaimido cluster complexes.

The syntheses and characterization of several octahedral hexatantalum cluster compounds of formula (ArN)14Ta6O are described (Ar=Ph, p-MeC6H4, p-MeOC6H4, p-t-BuC6H4, p-BrC6H4, m-ClC6H4). Treatment of Bn3Ta=N-t-Bu (Bn=CH2C6H5) or pentakis(dimethylamido)tantalum with an excess of the appropriate aniline and stoichiometric water or tantalum oxide afforded varying yields of arylimido clusters. The structures of two species were confirmed by X-ray diffraction (XRD), while the identity of the central oxygen atom was elucidated by electrospray mass spectrometry (MS) using 17O/18O-enriched material. The title species are very air- and moisture-sensitive but quite thermally stable in solution. Experimentally determined optical properties and oxidation/reduction potentials, as well as some computational results, indicate that they possess an electronic structure wherein the highest occupied molecular orbitals are ligand-centered, while the lowest unoccupied orbitals are metal-centered and delocalized throughout the tantalum cage. Whereas chemical oxidation resulted in cluster decomposition, reduction with decamethylcobaltocene yielded stable salts of formula [Cp*2Co][(ArN)14Ta6O] (Ar=Ph, Ar=p-MeC6H4). Small-molecule reactivity studies on one of these clusters showed that its imido functionalities are moderately reactive toward oxide donors but inert with respect to metallaheterocycle-forming processes. Clean imido/oxo exchange was observed with aldehydes and ketones, leading cleanly to organic imines with no soluble byproducts being observed. This exchange was also observed with a rhenium oxo compound (generating an imidorhenium complex as the only soluble species). All 14 imido groups were transferred in these reactions, and no mixed-ligand cluster intermediates were ever observed.