Reduction of IU(NHAriPr6)2 (AriPr6 = 2,6-(2,4,6-iPr3C6H2)2C6H3) results in a rare example of a U(II) complex, U(NHAriPr6)2, and the first example that is a neutral species. Here, we show spectroscopic and magnetic studies that suggest a 5f46d0 valence electronic configuration for uranium, along with characterization of related U(III) complexes.
Catalysis by high-valent metals such as titanium(IV) impacts our lives daily through reactions like olefin polymerization. In any catalysis, optimization involves a careful choice of not just the metal but also the ancillary ligands. Because these choices dramatically impact the electronic structure of the system and, in turn, catalyst performance, new tools for catalyst development are needed. Understanding ancillary ligand effects is arguably one of the most critical aspects of catalyst optimization and, while parameters for phosphines have been used for decades with low-valent systems, a comparable system does not exist for high-valent metals. A new electronic parameter for ligand donation, derived from experiments on a high-valent chromium species, is now available. Here, we show that the new parameters enable quantitative determination of ancillary ligand effects on catalysis rate and, in some cases, even provide mechanistic information. Analysing reactions in this way can be used to design better catalyst architectures and paves the way for the use of such parameters in a host of high-valent processes.
A complex with single, double and triple bonds between nitrogen and the same metal center has been synthesized, [NCr(NPh)(NPri2)2]–. The complex shows differential activity, with some electrophiles attacking the imido and others the nitrido.
The first uranium bis(acyl)phosphide (BAP) complexes were synthesized from the reaction between sodium bis(mesitoyl)phosphide (Na( mes BAP)) or sodium bis(2,4,6-triisopropylbenzoyl)phosphide (Na( tripp BAP)) and UI3(1,4-dioxane)1.5. Thermally stable, homoleptic BAP complexes were characterized by single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy, when appropriate, for the elucidation of the electronic structure and bonding of these complexes. EPR spectroscopy revealed that the BAP ligands on the uranium center retain a significant amount of electron density. The EPR spectrum of the trivalent U( tripp BAP) 3 has a rhombic signal near g = 2 (g 1 = 2.03; g 2 = 2.01; and g 3 = 1.98) that is consistent with the EPR-observed unpaired electron being located in a molecular orbital that appears ligand-derived. However, upon warming the complex to room temperature, no resonance was observed, indicating the presence of uranium character.
“Weakly coordinating anions” such as tetraarylborates are ubiquitous in applications of inorganic and organometallic chemistry, with great industrial importance. In this work, we probe the ion-pairing ability of these weakly coordinating anions using the highly sensitive chromium(VI) nitrido bis(diisopropylamido) system NCr(N-i-Pr2)2X, with one variable coordination site (X). This system is being used in the quantification of ligand donor ability to high-valent metal centers and has simply been called the ligand donor parameter (LDP). The donor ability of the variable ligand can be measured by solution-state rotational barrier studies via NMR spectroscopy. If the variable ligand is neutral, the chromium complex is cationic, {NCr(N-i-Pr2)2L}+, with its pendant anion. Despite the weakly coordinating nature of the counteranions employed, a significant amount of ion pairing has been noted in solution, the result of which is substantial enough to perturb the sensitive LDP measurement. These effects have been noted for many commonly used counteranions, including hexafluoroantimonate(V), hexafluorophosphate(V), tetraphenylborate, and tetrakis(bis(3,5-trifluoromethyl)phenyl)borate (BArF24). Using diffusion ordered (DOSY) and rotating-frame Overhauser effect (ROESY) NMR spectroscopy and LDP values, we have shown, predictably, that the extent of ion pairing is solvent dependent and appears to be minimized by increasing the dielectric constant of the NMR solvent utilized. Additionally, we have gained insight into differences in the nature of ion pairing dependent upon the identity of the weakly coordinating anion employed. It was found that the tetraarylborate anions appear to be fully ion paired in CDCl3 but affect amido rotation less in comparison to other anions. We postulate that the smaller effect on the internal rearrangement by these fluorinated tetraarylborate anions is due to a lack of specificity in the interaction with the cation rather than a lack of ion pairing, which may be a general feature of these anions.
A simple one-pot procedure for the production of 2-carboxylpyrroles with 4-alkyl, 5-alkyl, 4aryl, 4-aryl-5-alkyl, or 3,4-diaryl substitution patterns is presented. The procedure involves the titanium-catalyzed multicomponent coupling of alkynes, primary amines and isonitriles to give 1,3-diimines in situ; the multicomponent product is then treated with the ethyl ester of glycine hydrochloride to give the NH-pyrrole. The reaction can be carried out with the neutralized glycine ester or with the hydrochloride salt using DBU as a base. Yields of pyrrole based on starting alkyne varied from 25-65% over the one pot procedure, and in most cases only one regioisomer of the product is observed. Further, it is proposed that the regioselectivities of the reactions are a result of rate-determining ring closure after relatively fast transimination with glycine ethyl ester.
A method to explore head-to-head ϕ back-bonding from uranium f-orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single-crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N-((dimethyl)prop-2-enylsilyl)-2,6-diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI 2 (L1) 2 (1), which was synthesized by using the mono-deprotonated ligand [K][N-((dimethyl)prop-2-enyl)silyl)-2,6-diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2] 2 (2) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligandThis allyl-based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back-bonding. The bonding between U IV and the allyl fragments is best described as ligand-to-metal electron donation from a two carbon fragment-localized electron density into empty forbitals.
A series of rare nitrido cyclopentadienyl complexes have been prepared of chromium(VI). An improved synthesis of NCr(I)(NPr i 2 ) 2 is provided, which reacts with NaCp to give NCr(η 1 -Cp)(NPr i 2 ) 2 (1) in 82% isolated yield. The η 1 -indenyl complex 2 can be prepared using a similar procedure. Replacement of one diisopropylamido ligand was accomplished to prepare NCr(NPr i 2 )Cp(O 2 CPh) (3), NCr-(NPr i 2 )Cp(Cl) (4), and [NCr(NPr i 2 )Cp(NCMe)] + [SbF 6 ] − (5). In the series, the Cr−C interactions increase in bond order, 3 < 4 < 5, as the cyclopentadienyl compensates for the decreasing donor ability of the other ligands on chromium. The Ligand Donor Parameters (LDPs) for the η 1 -cyclopentadienyl and η 1 -indenyl ligands were measured as 13.73 and 13.76 kcal/ mol, respectively. Compounds 1−5 were characterized by single-crystal X-ray diffraction.
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