Preparation and Reactivity of the Versatile Uranium(IV) Imido Complexes U(NAr)Cl₂(R₂bpy)₂ (R = Me, ^<i>t</i>Bu) and U(NAr)Cl₂(tppo)₃

Uranium tetrachloride undergoes facile reactions with 4,4′-dialkyl-2,2′-bipyridine, resulting in the generation of UCl₄(R₂bpy)₂, R = Me, ^<i>t</i>Bu. These precursors, as well as the known UCl₄(tppo)₂ (tppo = triphenylphosphine oxide), react with 2 equiv of lithium 2,6-di-isopropylphenylamide to provide the versatile uranium­(IV) imido complexes, U­(NDipp)­Cl₂(L)_<i>n</i> (L = R₂bpy, <i>n</i> = 2; L = tppo, <i>n</i> = 3). Interestingly, U­(NDipp)­Cl₂(R₂bpy)₂ can be used to generate the uranium­(V) and uranium­(VI) bisimido compounds, U­(NDipp)₂­X­(R₂bpy)₂, X = Cl, Br, I, and U­(NDipp)_2­I₂(^<i>t</i>Bu₂bpy), which establishes these uranium­(IV) precursors as potential intermediates in the syntheses of high-valent bis­(imido) complexes from UCl₄. The monoimido species also react with 4-methylmorpholine-N-oxide to yield uranium­(VI) oxo-imido products, U­(NDipp)­(O)­Cl₂(L)_<i>n</i> (L = ^<i>t</i>Bu₂bpy, <i>n</i> = 1; L = tppo, <i>n</i> = 2). The aforementioned molecules have been characterized by a combination of NMR spectroscopy, X-ray crystallography, and elemental analysis. The chemical reactivity studies presented herein demonstrate that Lewis base adducts of uranium tetrachloride function as excellent sources of U­(IV), U­(V), and U­(VI) imido species

Preparation and Reactivity of the Versatile Uranium(IV) Imido Complexes U(NAr)Cl₂(R₂bpy)₂ (R = Me, ^<i>t</i>Bu) and U(NAr)Cl₂(tppo)₃

Uranium tetrachloride undergoes facile reactions with 4,4′-dialkyl-2,2′-bipyridine, resulting in the generation of UCl₄(R₂bpy)₂, R = Me, ^<i>t</i>Bu. These precursors, as well as the known UCl₄(tppo)₂ (tppo = triphenylphosphine oxide), react with 2 equiv of lithium 2,6-di-isopropylphenylamide to provide the versatile uranium­(IV) imido complexes, U­(NDipp)­Cl₂(L)_<i>n</i> (L = R₂bpy, <i>n</i> = 2; L = tppo, <i>n</i> = 3). Interestingly, U­(NDipp)­Cl₂(R₂bpy)₂ can be used to generate the uranium­(V) and uranium­(VI) bisimido compounds, U­(NDipp)₂­X­(R₂bpy)₂, X = Cl, Br, I, and U­(NDipp)_2­I₂(^<i>t</i>Bu₂bpy), which establishes these uranium­(IV) precursors as potential intermediates in the syntheses of high-valent bis­(imido) complexes from UCl₄. The monoimido species also react with 4-methylmorpholine-N-oxide to yield uranium­(VI) oxo-imido products, U­(NDipp)­(O)­Cl₂(L)_<i>n</i> (L = ^<i>t</i>Bu₂bpy, <i>n</i> = 1; L = tppo, <i>n</i> = 2). The aforementioned molecules have been characterized by a combination of NMR spectroscopy, X-ray crystallography, and elemental analysis. The chemical reactivity studies presented herein demonstrate that Lewis base adducts of uranium tetrachloride function as excellent sources of U­(IV), U­(V), and U­(VI) imido species

Positional Selectivity in C–H Functionalizations of 2‑Benzylfurans with Bimetallic Catalysts

Metal-catalyzed carbon–carbon bond-forming reactions are a mainstay in the synthesis of pharmaceutical agents. A long-standing problem plaguing the field of transition metal catalyzed C–H functionalization chemistry is control of selectivity among inequivalent C–H bonds in organic reactants. Herein we advance an approach to direct site selectivity in the arylation of 2-benzylfurans founded on the idea that modulation of cooperativity in bimetallic catalysts can enable navigation of selectivity. The bimetallic catalysts introduced herein exert a high degree of control, leading to divergent site-selective arylation reactions of both sp² and sp³ C–H bonds of 2-benzylfurans. It is proposed that the selectivity is governed by cation−π interactions, which can be modulated by choice of base and accompanying additives [MN­(SiMe₃)₂, M = K or Li·12-crown-4]

A Linear <i>trans</i>-Bis(imido) Neptunium(V) Actinyl Analog: Np^V(NDipp)₂(<i>^<i>t</i></i>Bu₂bipy)₂Cl (Dipp = 2,6‑^<i>i</i>Pr₂C₆H₃)

The discovery that imido analogs of actinyl dioxo cations can be extended beyond uranium into the transuranic elements is presented. Synthesis of the Np­(V) complex, Np­(NDipp)₂(^<i>t</i>Bu₂bipy)₂Cl (<b>1</b>), is achieved through treatment of a Np­(IV) precursor with a bipyridine coligand and lithium-amide reagent. Complex <b>1</b> has been structurally characterized, analyzed by ¹H NMR and UV–vis−NIR spectroscopies, and the electronic structure evaluated by DFT calculations

A Step beyond the Feltham–Enemark Notation: Spectroscopic and Correlated <i>ab Initio</i> Computational Support for an Antiferromagnetically Coupled M(II)–(NO)⁻ Description of Tp*M(NO) (M = Co, Ni)

Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}⁹ species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me₂-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co–NO bonding interaction as a Co(II) (<i>S</i>_Co = ³/₂) metal center, antiferromagnetically coupled to a triplet NO^– anion (<i>S</i>_NO = 1), an interpretation of the electronic structure that was validated by <i>ab initio</i> multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant <i>g</i>-anisotropy in the <i>S</i> = ¹/₂ ground state, but the linear-response DFT performed poorly at calculating the <i>g</i>-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin–orbit coupling were required for obtaining accurate modeling of the molecular <i>g</i>-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (<i>S</i>_Ni = 1) metal center antiferromagnetically coupled to an <i>S</i>_NO = 1 NO^–. The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO⁰ formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and <i>ab initio</i> computational methods for their ability to predict various spectroscopic and molecular features

A Step beyond the Feltham–Enemark Notation: Spectroscopic and Correlated <i>ab Initio</i> Computational Support for an Antiferromagnetically Coupled M(II)–(NO)⁻ Description of Tp*M(NO) (M = Co, Ni)

Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}⁹ species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me₂-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co–NO bonding interaction as a Co(II) (<i>S</i>_Co = ³/₂) metal center, antiferromagnetically coupled to a triplet NO^– anion (<i>S</i>_NO = 1), an interpretation of the electronic structure that was validated by <i>ab initio</i> multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant <i>g</i>-anisotropy in the <i>S</i> = ¹/₂ ground state, but the linear-response DFT performed poorly at calculating the <i>g</i>-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin–orbit coupling were required for obtaining accurate modeling of the molecular <i>g</i>-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (<i>S</i>_Ni = 1) metal center antiferromagnetically coupled to an <i>S</i>_NO = 1 NO^–. The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO⁰ formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and <i>ab initio</i> computational methods for their ability to predict various spectroscopic and molecular features

Diniobium Inverted Sandwich Complexes with μ‑η⁶:η⁶‑Arene Ligands: Synthesis, Kinetics of Formation, and Electronic Structure

Monometallic niobium arene complexes [Nb­(BDI)­(N^<i>t</i>Bu)­(R-C₆H₅)] (<b>2a</b>: R = H and <b>2b</b>: R = Me, BDI = <i>N</i>,<i>N</i>′-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)] (<b>7a</b>: R = H and <b>7b</b>: R = Me) in solution. The kinetics of this reaction were followed by ¹H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds <b>7a</b>-<b>b</b> showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H­(OEt₂)]­[B­(C₆F₅)₄], resulting in the monoprotonated cationic inverted sandwich complex <b>8</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]­[(BDI)­Nb­(N^<i>t</i>Bu)]­(μ-C₆H₅)]­[B­(C₆F₅)₄] and the dicationic complex <b>9</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)]­[B­(C₆F₅)₄]₂ (BDI^# = (ArNC­(Me))₂CH₂). NMR, UV–vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb–arene interactions. The X-ray crystal structures, UV–vis spectra, arene ¹H NMR chemical shifts, and large <i>J</i>_CH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L_3,2-edge XANES spectra and the lack of sp³ hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d² electronic configurations are appropriate to describe the Nb atoms in all four complexes

Diniobium Inverted Sandwich Complexes with μ‑η⁶:η⁶‑Arene Ligands: Synthesis, Kinetics of Formation, and Electronic Structure

Monometallic niobium arene complexes [Nb­(BDI)­(N^<i>t</i>Bu)­(R-C₆H₅)] (<b>2a</b>: R = H and <b>2b</b>: R = Me, BDI = <i>N</i>,<i>N</i>′-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)] (<b>7a</b>: R = H and <b>7b</b>: R = Me) in solution. The kinetics of this reaction were followed by ¹H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds <b>7a</b>-<b>b</b> showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H­(OEt₂)]­[B­(C₆F₅)₄], resulting in the monoprotonated cationic inverted sandwich complex <b>8</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]­[(BDI)­Nb­(N^<i>t</i>Bu)]­(μ-C₆H₅)]­[B­(C₆F₅)₄] and the dicationic complex <b>9</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)]­[B­(C₆F₅)₄]₂ (BDI^# = (ArNC­(Me))₂CH₂). NMR, UV–vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb–arene interactions. The X-ray crystal structures, UV–vis spectra, arene ¹H NMR chemical shifts, and large <i>J</i>_CH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L_3,2-edge XANES spectra and the lack of sp³ hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d² electronic configurations are appropriate to describe the Nb atoms in all four complexes

Ligand-Directed Reactivity in Dioxygen and Water Binding to <i>cis</i>-[Pd(NHC)₂(η²‑O₂)]

Reaction of [Pd­(IPr)₂] (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) and O₂ leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex <i>cis</i>-[Pd­(IPr)₂(η²-O₂)]. At temperatures ≳ −40 °C, <i>cis</i>-[Pd­(IPr)₂(η²-O₂)] adds a second O₂ to form <i>trans</i>-[Pd­(IPr)₂(η¹-O₂)₂]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex. In addition to reaction with O₂, <i>cis</i>-[Pd­(IPr)₂(η²-O₂)] reacts at low temperature with H₂O in methanol/ether solution to form <i>trans</i>-[Pd­(IPr)₂(OH)­(OOH)]. The crystal structure of <i>trans</i>-[Pd­(IPr)₂(OOH)­(OH)] is reported. Neither reaction with O₂ nor reaction with H₂O occurs under comparable conditions for <i>cis</i>-[Pd­(IMes)₂(η²-O₂)] (IMes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene). The increased reactivity of <i>cis</i>-[Pd­(IPr)₂(η²-O₂)] is attributed to the enthalpy of binding of O₂ to [Pd­(IPr)₂] (−14.5 ± 1.0 kcal/mol) that is approximately one-half that of [Pd­(IMes)₂] (−27.9 ± 1.5 kcal/mol). Computational studies identify the cause as interligand repulsion forcing a wider C–Pd–C angle and tilting of the NHC plane in <i>cis</i>-[Pd­(IPr)₂(η²-O₂)]. Arene–arene interactions are more favorable and serve to further stabilize <i>cis</i>-[Pd­(IMes)₂(η²-O₂)]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O₂ binding. A complete reaction diagram is constructed for formation of <i>trans</i>-[Pd­(IPr)₂(η¹-O₂)₂] and leads to the conclusion that kinetic factors inhibit formation of <i>trans</i>-[Pd­(IMes)₂(η¹-O₂)₂] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate <i>trans</i>-[Pd­(NHC)₂(η¹-O₂)] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O₂ is constructed. A range of low-energy pathways at different angles of approach are present and blur the distinction between pure “side-on” or “end-on” trajectories for oxygen binding