Crystal structure of 1-(2,6-diiso-propyl-phen-yl)-1H-imidazole.

The crystal structure of the title compound, C15H20N2 or DippIm, is reported. At 106 (2) K, the mol-ecule has monoclinic P21/c symmetry with four mol-ecules in the unit cell. The imidazole ring is rotated 80.7 (1)° relative to the phenyl ring. Inter-molecular stabilization primarily results from close contacts between the N atom at the 3-position on the imidazole ring and the C-H bond at the 4-position on the neighboring DippIm, with ar-yl-aryl distances outside of the accepted distance of 5 Šfor π-stacking.

Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis.

The application of bimolecular reductive elimination to the activation of iron catalysts for alkene-diene cycloaddition is described. Key to this approach was the synthesis, characterization, electronic structure determination, and ultimately solution stability of a family of pyridine(diimine) iron methyl complexes with diverse steric properties and electronic ground states. Both the aryl-substituted, (MePDI)FeCH3 and (EtPDI)FeCH3 (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N), and the alkyl-substituted examples, (CyAPDI)FeCH3 (CyAPDI = 2,6-(C6H11N═CMe)2C5H3N), have molecular structures significantly distorted from planarity and S = 3/2 ground states. The related N-arylated derivative bearing 2,6-di-isopropyl aryl substituents, (iPrPDI)FeCH3, has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N2 atmosphere, both (MePDI)FeCH3 and (EtPDI)FeCH3 underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [(MePDI)Fe(N2)]2(µ-N2) and [(EtPDI)Fe(N2)]2(µ-N2), respectively, while (iPrPDI)FeCH3 proved inert to C-C bond formation. By contrast, addition of butadiene to all three iron methyl complexes induced ethane formation and generated the corresponding iron butadiene complexes, (RPDI)Fe(η4-C4H6) (R = Me, Et, iPr), known precatalysts for the [2+2] cycloaddition of olefins and dienes. Kinetic, crossover experiments, and structural studies were combined with magnetic measurements and Mössbauer spectroscopy to elucidate the electronic and steric features of the iron complexes that enable this unusual reductive elimination and precatalyst activation pathway. Transmetalation of methyl groups between iron centers was fast at ambient temperature and independent of steric environment or spin state, while the intermediate dimer underwent the sterically controlled rate-determining reaction with either N2 or butadiene to access a catalytically active iron compound.

Effect of methyl-ene versus ethyl-ene linkers on structural properties of tert-butyl and mesityl bis-(imidazolium) bromide salts.

The crystal structures of ligand precursor bis-(imidazolium) salts 1,1'-methyl-enebis(3-tert-butyl-imidazolium) dibromide monohydrate, C15H26N4 +·2Br-·H2O or [ tBuNHC2Me][Br]2·H2O, 1,1'-(ethane-1,2-di-yl)bis-(3-tert-butyl-imidazolium) dibromide dihydrate, C16H28N4 +·2Br-·2H2O or [ tBuNHC2Et][Br]2·2H2O, 1,1'-methyl-enebis[3-(2,4,6-tri-methyl-phen-yl)imidazolium] dibromide dihydrate, C25H30N4 2+·2Br-·2H2O or [MesNHC2Me][Br]2·2H2O, and 1,1'-(ethane-1,2-di-yl)bis-[3-(2,4,6-tri-methyl-phen-yl)imidazolium] dibromide tetra-hydrate, C26H32N4 2+·2Br-·4H2O or [MesNHC2Et][Br]2·4H2O, are reported. At 293 K, [ tBuNHC2Me][Br]2·H2O crystallizes in the P21/c space group, while [ tBuNHC2Et][Br]2·2H2O crystallizes in the P21/n space group at 100 K. At 112 K, [MesNHC2Me][Br]2·2H2O crystallizes in the ortho-rhom-bic space group Pccn while [MesNHC2Et][Br]2·4H2O crystallizes in the P21/c space group at 100 K. Bond distances and angles within the imidazolium rings are generally comparable among the four structures. All four bis-(imidazolium) salts co-crystallize with one to four mol-ecules of water.

Crystal structures of two dioxomolybdenum complexes stabilized by salan ligands featuring phenyl and cyclo-hexyl backbones.

Two cis-dioxomolybdenum complexes based on salan ligands with different backbones are reported. The first complex, dioxido{2,2'-[l,2-phenyl-enebis(imino-methyl-ene)]bis-(phenolato)}molybdenum(VI) di-methyl-formamide disolvate, [Mo(C20H18N2O2)O2]·2C3H7NO (PhLMoO2, 1b), features a phenyl backbone, while the second complex, (6,6'-{[(cyclo-hexane-1,2-di-yl)bis(aza-nedi-yl)]bis-(methyl-ene)}bis-(2,4-di-tert-butyl-phenolato))dioxidomolybdenum(VI) methanol disolvate, [Mo(C36H56N2O2)O2]·2CH3OH (CyLMoO2, 2b), is based on a cyclo-hexyl backbone. These complexes crystallized as solvated species, 1b·2DMF and 2b·2MeOH. The salan ligands PhLH2 (1a) and CyLH2 (2a) coordinate to the molybdenum center in these complexes 1b and 2b in a κ2 N,κ2 O fashion, forming a distorted octa-hedral geometry. The Mo-N and Mo-O distances are 2.3475 (16) and 1.9567 (16) Å, respectively, in 1b while the corresponding measurements are Mo-N = 2.3412 (12) Å, and Mo-O = 1.9428 (10) Šfor 2b. A key geometrical feature is that the N-Mo-N angle of 72.40 (4)° in CyLMoO2 is slightly less than that of the PhLMoO2 angle of 75.18 (6)°, which is attributed to the flexibility of the cyclo-hexane ring between the nitro-gen as compared to the rigid phenyl ring in the PhLMoO2.

Synthesis, structural analysis, and docking studies with SARS-CoV-2 of a trinuclear zinc complex with N-phenylanthranilic acid ligands.

The structure of a trinuclear zinc complex, hexakis(µ2-2-anilinobenzoato)diaquatrizinc(II), [Zn2(C13H10NO2)6(H2O)2] or (NPA)6Zn3(H2O)2 (NPA is 2-anilinobenzoate or N-phenylanthranilate), is reported. The complex crystallizes in the triclinic space group P-1 and the central ZnII atom is located on an inversion center. The NPA ligand is found to coordinate via the carboxylate O atoms with unique C-O bond lengths that support an unequal distribution of resonance over the carboxylate fragment. The axial H2O ligands form hydrogen bonds with neighboring molecules that stabilize the supramolecular system in rigid straight chains, with an angle of 180° along the c axis. π stacking is the primary stabilization along the a and b axes, resulting in a highly ordered supramolecular structure. Docking studies show that this unique supramolecular structure of a trinuclear zinc complex has potential for binding to the main protease (Mpro) in SARS-CoV-2 in a different location from Remdesivir, but with a similar binding strength.

Quantification of Ni-N-O Bond Angles and NO Activation by X-ray Emission Spectroscopy.

A series of ß-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

Direct Observation of Transmetalation from a Neutral Boronate Ester to a Pyridine(diimine) Iron Alkoxide.

Transmetallation of the neutral boronate esters, (2-benzofuranyl)BPin and (2-benzofuranyl)BNeo (Pin = pinacolato, Neo = neopentylglycolato) to a representative pyridine(diimine) iron alkoxide complex, (iPrPDI)FeOEt (iPrPDI = 2,6-(2,6-iPr2-C6H3N=CMe)2C5H3N; R = Me, Et, SiMe3), to yield the corresponding iron benzofuranyl derivative was studied. Synthesis of the requisite iron alkoxide complexes was accomplished either by salt metathesis between (iPrPDI)FeCl and NaOR (R = Me, Et, SiMe3) or by protonation of the iron alkyl, (iPrPDI)FeCH2SiMe3, by the free alcohol R'OH (R' = Me, Et). A combination of magnetic measurements, X-ray diffraction, NMR and Mössbauer spectroscopies and DFT calculations identified each (iPrPDI)FeOR compound as an essentially planar, high-spin, S = 3/2 compound engaged in antiferromagnetic coupling with a radical anion on the chelate (S Total = 3/2; S Fe = 2, S PDI = 1/2). The resulting iron benzofuranyl product, (iPrPDI)Fe(2-benzofuranyl) was characterized by X-ray diffraction and in combination with magnetic measurements, spectroscopic and computational data, was identified as an overall S = 1/2 compound, demonstrating that a net spin-state change accompanies transmetallation (S Fe = 1, S PDI = 1/2). These findings may be relevant to further development of iron-catalyzed Suzuki-Miyaura cross-coupling with neutral boronate esters and alkoxide bases.

A five-coordinate cobalt bis-(di-thiol-ene)-phosphine complex [Co(pdt)2(PTA)] (pdt = phenyl-dithiol-ene; PTA = 1,3,5-tri-aza-7-phosphaadamantane).

The title compound, bis-(1,2-diphenyl-2-sulfanyl-idene-ethane-thiol-ato-κ2 S,S')(1,3,5-tri-aza-7-phosphaadamantane-κP)cobalt(II) dichloromethane hemisolvate, [Co(pdt)2(PTA)]·0.5C2H4Cl2 or [Co(C14H10S2)2(C6H12N3P)]·0.5C2H4Cl2, contains two phenyl-dithiol-ene (pdt) ligands and a 1,3,5-tri-aza-7-phosphaadamantane (PTA) ligand bound to cobalt with the solvent 1,2-di-chloro-ethane mol-ecule located on an inversion center. The cobalt core exhibits an approximately square-pyramidal geometry with partially reduced thienyl radical monoanionic ligands. The supra-molecular network is consolidated by hydrogen-bonding inter-actions primarily with nitro-gen, sulfur and chlorine atoms, as well as parallel displaced π-stacking of the aryl rings. The UV-vis, IR, and CV data are also consistent with monoanionic di-thiol-ene ligands and an overall CoII oxidation state.

C(sp3)-H Fluorination with a Copper(II)/(III) Redox Couple.

Despite the growing interest in the synthesis of fluorinated organic compounds, few reactions are able to incorporate fluoride ions directly into alkyl C-H bonds. Here, we report the C(sp3)-H fluorination reactivity of a formally copper(III) fluoride complex. The C-H fluorination intermediate, LCuF, along with its chloride and bromide analogues, LCuCl and LCuBr, were prepared directly from halide sources with a chemical oxidant and fully characterized with single-crystal X-ray diffraction, X-ray absorption spectroscopy, UV-vis spectroscopy, and 1H nuclear magnetic resonance spectroscopy. Quantum chemical calculations reveal significant halide radical character for all complexes, suggesting their ability to initiate and terminate a C(sp3)-H halogenation sequence by sequential hydrogen atom abstraction (HAA) and radical capture. The capability of HAA by the formally copper(III) halide complexes was explored with 9,10-dihydroanthracene, revealing that LCuF exhibits rates 2 orders of magnitude higher than LCuCl and LCuBr. In contrast, all three complexes efficiently capture carbon radicals to afford C(sp3)-halogen bonds. Mechanistic investigation of radical capture with a triphenylmethyl radical revealed that LCuF proceeds through a concerted mechanism, while LCuCl and LCuBr follow a stepwise electron transfer-halide transfer pathway. The capability of LCuF to perform both hydrogen atom abstraction and radical capture was leveraged to enable fluorination of allylic and benzylic C-H bonds and α-C-H bonds of ethers at room temperature.

The duality of electron localization and covalency in lanthanide and actinide metallocenes.

Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C8H8)2, have provided evidence for non-negligible 4f-electron density on Ce and implied that charge transfer from the ligands occurs as a result of covalent bonding. Strong correlations of the localized 4f-electrons to the delocalized ligand π-system result in emergence of Kondo-like behavior and other quantum chemical phenomena that are rarely observed in molecular systems. In this study, Ce(C8H8)2 is analyzed experimentally using carbon K-edge and cerium M5,4-edge X-ray absorption spectroscopies (XAS), and computationally using configuration interaction (CI) calculations and density functional theory (DFT) as well as time-dependent DFT (TDDFT). Both spectroscopic approaches provide strong evidence for ligand → metal electron transfer as a result of Ce 4f and 5d mixing with the occupied C 2p orbitals of the C8H8 2- ligands. Specifically, the Ce M5,4-edge XAS and CI calculations show that the contribution of the 4f1, or Ce3+, configuration to the ground state of Ce(C8H8)2 is similar to strongly correlated materials such as CeRh3 and significantly larger than observed for other formally Ce4+ compounds including CeO2 and CeCl6 2-. Pre-edge features in the experimental and TDDFT-simulated C K-edge XAS provide unequivocal evidence for C 2p and Ce 4f covalent orbital mixing in the δ-antibonding orbitals of e2u symmetry, which are the unoccupied counterparts to the occupied, ligand-based δ-bonding e2u orbitals. The C K-edge peak intensities, which can be compared directly to the C 2p and Ce 4f orbital mixing coefficients determined by DFT, show that covalency in Ce(C8H8)2 is comparable in magnitude to values reported previously for U(C8H8)2. An intuitive model is presented to show how similar covalent contributions to the ground state can have different impacts on the overall stability of f-element metallocenes.

Nitrosyl Linkage Isomers: NO Coupling to N2O at a Mononuclear Site.

Linkage isomers of reduced metal-nitrosyl complexes serve as key species in nitric oxide (NO) reduction at monometallic sites to produce nitrous oxide (N2O), a potent greenhouse gas. While factors leading to extremely rare side-on nitrosyls are unclear, we describe a pair of nickel-nitrosyl linkage isomers through controlled tuning of noncovalent interactions between the nitrosyl ligands and differently encapsulated potassium cations. Furthermore, these reduced metal-nitrosyl species with N-centered spin density undergo radical coupling with free NO and provide a N-N coupled cis-hyponitrite intermediate whose protonation triggers the release of N2O. This report outlines a stepwise molecular mechanism of NO reduction to form N2O at a mononuclear metal site that provides insight into the related biological reduction of NO to N2O.

Quantitative Evidence for Lanthanide-Oxygen Orbital Mixing in CeO2, PrO2, and TbO2.

Understanding the nature of covalent (band-like) vs ionic (atomic-like) electrons in metal oxides continues to be at the forefront of research in the physical sciences. In particular, the development of a coherent and quantitative model of bonding and electronic structure for the lanthanide dioxides, LnO2 (Ln = Ce, Pr, and Tb), has remained a considerable challenge for both experiment and theory. Herein, relative changes in mixing between the O 2p orbitals and the Ln 4f and 5d orbitals in LnO2 are evaluated quantitatively using O K-edge X-ray absorption spectroscopy (XAS) obtained with a scanning transmission X-ray microscope and density functional theory (DFT) calculations. For each LnO2, the results reveal significant amounts of Ln 5d and O 2p mixing in the orbitals of t2g (σ-bonding) and eg (π-bonding) symmetry. The remarkable agreement between experiment and theory also shows that significant mixing with the O 2p orbitals occurs in a band derived from the 4f orbitals of a2u symmetry (σ-bonding) for each compound. However, a large increase in orbital mixing is observed for PrO2 that is ascribed to a unique interaction derived from the 4f orbitals of t1u symmetry (σ- and π-bonding). O K-edge XAS and DFT results are compared with complementary L3-edge and M5,4-edge XAS measurements and configuration interaction calculations, which shows that each spectroscopic approach provides evidence for ground state O 2p and Ln 4f orbital mixing despite inducing very different core-hole potentials in the final state.

Comparing the 2,2'-Biphenylenedithiophosphinate Binding of Americium with Neodymium and Europium.

Advancing our understanding of the minor actinides (Am, Cm) versus lanthanides is key for developing advanced nuclear-fuel cycles. Herein, we describe the preparation of (NBu4 )Am[S2 P((t) Bu2 C12 H6 )]4 and two isomorphous lanthanide complexes, namely one with a similar ionic radius (i.e., Nd(III) ) and an isoelectronic one (Eu(III) ). The results include the first measurement of an Am-S bond length, with a mean value of 2.921(9) Å, by single-crystal X-ray diffraction. Comparison with the Eu(III) and Nd(III) complexes revealed subtle electronic differences between the complexes of Am(III) and the lanthanides.

Spectroscopic and computational investigation of actinium coordination chemistry.

Actinium-225 is a promising isotope for targeted-α therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, Ac(III) reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac-Cl and Ac-OH2O distances are measured to be 2.95(3) and 2.59(3) Å, respectively. The X-ray absorption spectroscopy comparisons between Ac(III) and Am(III) in HCl solutions indicate Ac(III) coordinates more inner-sphere Cl(1-) ligands (3.2±1.1) than Am(III) (0.8±0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique Ac(III) chemical behaviour.

Redox Non-Innocence of Nitrosobenzene at Nickel.

Nitrosobenzene (PhNO) serves as a stable analogue of nitroxyl (HNO), a biologically relevant, redox-active nitric oxide derivative. Capture of nitrosobenzene at the electron-deficient ß-diketiminato nickel(I) complex [(i) Pr2 NNF6 ]Ni results in reduction of the PhNO ligand to a (PhNO)(./-) species coordinated to a square planar Ni(II) center in [(i) Pr2 NNF6 ]Ni(η(2) -ONPh). Ligand centered reduction leads to the (PhNO)(2-) moiety bound to Ni(II) supported by XAS studies. Systematic investigation of structure-reactivity patterns of (PhNO)(./-) and (PhNO)(2-) ligands reveals parallels with superoxo (O2 )(./-) and peroxo (O2 )(2-) ligands, respectively, and forecasts reactivity patterns of the more transient HNO ligand.

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029.

A tetravalent cerium macrocyclic complex (CeLK4) was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce4+, with log ß110 = 61(2) and ΔG = -348 kJ/mol, compared with log ß110 = 32.02(2) for the analogous Pr3+ complex. In addition, L exhibits an unusual preference for forming CeL4- relative to formation of the analogous actinide complex, ThL4-, which has ß110 = 53.7(5). The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce3+/Ce4+ couple of the complex (measured at -0.454 V vs SHE). The unprecedented behavior prompted an electronic structure analysis using L3- and M5,4-edge X-ray absorption near-edge structure (XANES) spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK4 has partial covalent character due to ligand-to-metal charge transfer (LMCT) in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK4 is similar to that observed for [Et4N]2[CeCl6] and CeO2 and significantly less than that for the organometallic sandwich compound cerocene, (C8H8)2Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.

Acrylate formation from CO2 and ethylene: catalysis with palladium and mechanistic insight.

We report the first catalyst based on palladium for the reaction of CO2, alkene and a base to form sodium acrylate and derivatives. A mechanism similar to a previously reported Ni(0)-catalyst is proposed based on stoichiometric in situ NMR experiments, isolated intermediates and a parent palladalactone. Our palladium catalyst was applied to the coupling of CO2 with conjugated alkenes.

Covalency in lanthanides. An X-ray absorption spectroscopy and density functional theory study of LnCl6(x-) (x = 3, 2).

Covalency in Ln-Cl bonds of Oh-LnCl6(x-) (x = 3 for Ln = Ce(III), Nd(III), Sm(III), Eu(III), Gd(III); x = 2 for Ln = Ce(IV)) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6(x-) (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce(III) and Ce(IV)). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce(III) to Ce(IV) had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl6(3-) (formally Ln(III)), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce(IV) 4f-orbital mixing (t1u* + t2u*) in CeCl6(2-). This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl6(2-) (M = Ti, Zr, Hf, U).

Electronic Structure Determination of Pyridine N-Heterocyclic Carbene Iron Dinitrogen Complexes and Neutral Ligand Derivatives.

The electronic structures of pyridine N-heterocyclic dicarbene (iPrCNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example (iPrCNC)Fe(N2)2 and the related pyridine derivative (iPrCNC)Fe(DMAP)(N2) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the iPrCNC chelate functioning as a classical π acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of N,N-diallyl-tert-butylamine to (iPrCNC)Fe(N2)2 yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a π acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis(imino)pyridine iron complexes and may account for the lack of catalytic [2π + 2π] cycloaddition reactivity.