Iron(II) Complexes Supported by Sulfonamido Tripodal Ligands: Endogenous versus Exogenous Substrate Oxidation

High-valent iron species are known to act as powerful oxidants in both natural and synthetic systems. While biological enzymes have evolved to prevent self-oxidation by these highly reactive species, development of organic ligand frameworks that are capable of supporting a high-valent iron center remains a challenge in synthetic chemistry. We describe here the reactivity of an Fe­(II) complex that is supported by a tripodal sulfonamide ligand with both dioxygen and an oxygen-atom transfer reagent, 4-methylmorpholine-<i>N</i>-oxide (NMO). An Fe­(III)–hydroxide complex is obtained from reaction with dioxygen, while NMO gives an Fe­(III)–alkoxide product resulting from activation of a C–H bond of the ligand. Inclusion of Ca²⁺ ions in the reaction with NMO prevented this ligand activation and resulted in isolation of an Fe­(III)–hydroxide complex in which the Ca²⁺ ion is coordinated to the tripodal sulfonamide ligand and the hydroxo ligand. Modification of the ligand allowed the Fe­(III)–hydroxide complex to be isolated from NMO in the absence of Ca²⁺ ions, and a C–H bond of an external substrate could be activated during the reaction. This study highlights the importance of robust ligand design in the development of synthetic catalysts that utilize a high-valent iron center

Solvent-Free Organometallic Reactivity: Synthesis of Hydride and Carboxylate Complexes of Uranium and Yttrium from Gas/Solid Reactions

Gas/solid reactions involving H₂ and CO₂ with the metallocenes (C₅Me₅)₂UMe₂ and (C₅Me₅)₂U­(allyl)₂ as solids in the absence of solvent provide an improved method to make organouranium hydride and carboxylate products. Decomposition products that can form in solution from the reactive hydrides can be avoided by this method, and this approach can also provide intermediates too reactive to isolate in some solution reactions. In contrast to the variable nature of the hydrogenolysis reaction of (C₅Me₅)₂UMe₂ in toluene that forms byproducts along with the mixture of [(C₅Me₅)₂UH₂]₂ and [(C₅Me₅)₂UH]₂, a byproduct-free hydrogenolysis occurs when (C₅Me₅)₂UMe₂ in the solid state is treated with H₂ gas to form predominantly [(C₅Me₅)₂UH₂]₂. H₂ reacts with solid (C₅Me₅)₂U­(C₃H₅)₂ and (C₅Me₅)₂U­(C₃H₅) similarly. The reaction of CO₂ (80 psi) with solid (C₅Me₅)₂UMe₂ forms the monocarboxylate (C₅Me₅)₂U­(O₂CCH₃-κ²<i>O</i>,<i>O</i>′)­Me, in contrast to the solution reaction that forms the diacetate (C₅Me₅)₂U­(O₂CCH₃-κ²<i>O</i>,<i>O</i>′)₂ in minutes. The reaction of H₂ with solid (C₅Me₅)₂Y­(C₃H₅) provided (C₅Me₅)₂Y­(μ-H)­YH­(C₅Me₅)₂ without the decomposition products that it forms in solution such that single crystals suitable for X-ray diffraction could be isolated for the first time

Solvent-Free Organometallic Reactivity: Synthesis of Hydride and Carboxylate Complexes of Uranium and Yttrium from Gas/Solid Reactions

Gas/solid reactions involving H₂ and CO₂ with the metallocenes (C₅Me₅)₂UMe₂ and (C₅Me₅)₂U­(allyl)₂ as solids in the absence of solvent provide an improved method to make organouranium hydride and carboxylate products. Decomposition products that can form in solution from the reactive hydrides can be avoided by this method, and this approach can also provide intermediates too reactive to isolate in some solution reactions. In contrast to the variable nature of the hydrogenolysis reaction of (C₅Me₅)₂UMe₂ in toluene that forms byproducts along with the mixture of [(C₅Me₅)₂UH₂]₂ and [(C₅Me₅)₂UH]₂, a byproduct-free hydrogenolysis occurs when (C₅Me₅)₂UMe₂ in the solid state is treated with H₂ gas to form predominantly [(C₅Me₅)₂UH₂]₂. H₂ reacts with solid (C₅Me₅)₂U­(C₃H₅)₂ and (C₅Me₅)₂U­(C₃H₅) similarly. The reaction of CO₂ (80 psi) with solid (C₅Me₅)₂UMe₂ forms the monocarboxylate (C₅Me₅)₂U­(O₂CCH₃-κ²<i>O</i>,<i>O</i>′)­Me, in contrast to the solution reaction that forms the diacetate (C₅Me₅)₂U­(O₂CCH₃-κ²<i>O</i>,<i>O</i>′)₂ in minutes. The reaction of H₂ with solid (C₅Me₅)₂Y­(C₃H₅) provided (C₅Me₅)₂Y­(μ-H)­YH­(C₅Me₅)₂ without the decomposition products that it forms in solution such that single crystals suitable for X-ray diffraction could be isolated for the first time

Synthesis and CO₂ Insertion Reactivity of Allyluranium Metallocene Complexes

The U⁴⁺ metallocene allyl chloride complexes (C₅Me₅)₂U­[η³-CH₂C­(R)­CH₂]Cl (R = H, Me) can be synthesized by reaction of (C₅Me₅)₂UCl₂ with 1 equiv of the corresponding allyl Grignard reagents, [CH₂C­(R)­CH₂]­MgCl, in hydrocarbon solvents. Bis­(allyl)­uranium complexes can also be obtained in this manner using 2 equiv of the corresponding allyl Grignard, and X-ray crystallographic studies reveal the presence of both η³- and η¹-allyl ligands: (C₅Me₅)₂U­[η³-CH₂C­(R)­CH₂]­[η¹-CH₂C­(R)CH₂]. Sodium amalgam reduction of (C₅Me₅)₂U­[η³-CH₂C­(R)­CH₂]Cl generates the U³⁺ metallocene allyl complexes (C₅Me₅)₂U­[η³-CH₂C­(R)­CH₂]. Carbon dioxide reacts with the U⁴⁺ allyl complexes to form the U–C insertion products (C₅Me₅)₂U­[κ²<i>O</i>,<i>O</i>′-O₂CCH₂CHCH₂]_2–<i>x</i>Cl_x (<i>x</i> = 0, 1). The dicarboxylate (C₅Me₅)₂U­[κ²<i>O</i>,<i>O</i>′-O₂CCH₂CHCH₂]₂, which has a 171.98(5)° O–U–O angle, reacts with Me₃SiCl to regenerate (C₅Me₅)₂UCl₂ and liberate Me₃SiOC­(O)­CH₂CHCH₂

Reactivity of U³⁺ Metallocene Allyl Complexes Leads to a Nanometer-Sized Uranium Carbonate, [(C₅Me₅)₂U]₆(μ‑κ¹:κ²‑CO₃)₆

The U³⁺ allyl complexes (C₅Me₅)₂U­[CH₂C­(R)­CH₂] (R = H, Me) display three different types of reactivity, as exemplified by reactions with PhNNPh, cyclooctatetraene, and CO₂. Two equivalents of (C₅Me₅)₂U­[CH₂C­(R)­CH₂] effect a four-electron reduction of PhNNPh to form the bis­(imido) complex (C₅Me₅)₂U­(NPh)₂ and the bis­(allyl) species (C₅Me₅)₂U­[CH₂C­(R)­CH₂]₂. Two-electron reduction of C₈H₈ occurs to form (C₅Me₅)­(C₈H₈)­U­[CH₂C­(R)­CH₂] products that contain only one cyclopentadienyl ring per metal. With CO₂ at 80 psi, both reduction and insertion occur. A hexametallic uranium carbonate, [(C₅Me₅)₂U]₆(μ-κ¹:κ²-CO₃)₆, is isolated as well as the bis­(carboxylate) complexes (C₅Me₅)₂U­[κ²-<i>O</i>,<i>O</i>′-O₂CCH₂C­(R)CH₂]₂. The polymetallic carbonate complex can also be synthesized from [(C₅Me₅)₂U]₂(μ-η⁶:η⁶-C₆H₆) and [(C₅Me₅)₂U]­[(μ-Ph)₂BPh₂] and CO₂

Reactivity of U³⁺ Metallocene Allyl Complexes Leads to a Nanometer-Sized Uranium Carbonate, [(C₅Me₅)₂U]₆(μ‑κ¹:κ²‑CO₃)₆

The U³⁺ allyl complexes (C₅Me₅)₂U­[CH₂C­(R)­CH₂] (R = H, Me) display three different types of reactivity, as exemplified by reactions with PhNNPh, cyclooctatetraene, and CO₂. Two equivalents of (C₅Me₅)₂U­[CH₂C­(R)­CH₂] effect a four-electron reduction of PhNNPh to form the bis­(imido) complex (C₅Me₅)₂U­(NPh)₂ and the bis­(allyl) species (C₅Me₅)₂U­[CH₂C­(R)­CH₂]₂. Two-electron reduction of C₈H₈ occurs to form (C₅Me₅)­(C₈H₈)­U­[CH₂C­(R)­CH₂] products that contain only one cyclopentadienyl ring per metal. With CO₂ at 80 psi, both reduction and insertion occur. A hexametallic uranium carbonate, [(C₅Me₅)₂U]₆(μ-κ¹:κ²-CO₃)₆, is isolated as well as the bis­(carboxylate) complexes (C₅Me₅)₂U­[κ²-<i>O</i>,<i>O</i>′-O₂CCH₂C­(R)CH₂]₂. The polymetallic carbonate complex can also be synthesized from [(C₅Me₅)₂U]₂(μ-η⁶:η⁶-C₆H₆) and [(C₅Me₅)₂U]­[(μ-Ph)₂BPh₂] and CO₂

Modulating the Primary and Secondary Coordination Spheres within a Series of Co^IIOH Complexes

The interplay between the primary and secondary coordination spheres is crucial to determining the properties of transition metal complexes. To examine these effects, a series of trigonal bipyramidal Co–OH complexes have been prepared with tripodal ligands that control both coordination spheres. The ligands contain a combination of either urea or sulfonamide groups that control the primary coordination sphere through anionic donors in the trigonal plane and the secondary coordination sphere through intramolecular hydrogen bonds. Variations in the anion donor strengths were evaluated using electronic absorbance spectroscopy and a qualitative ligand field analysis to find that deprotonated urea donors are stronger field ligands than deprotonated sulfonamides. Structural variations were found in the Co^II–O bond lengths that range from 1.953(4) to 2.051(3) Å; this range in bond lengths were attributed to the differences in the intramolecular hydrogen bonds that surround the hydroxido ligand. A similar trend was observed between the hydrogen bonding networks and the vibrations of the O–H bonds. Attempts to isolate the corresponding Co^III–OH complexes were hampered by their instability at room temperature

Synthesis and Characterization of a Redox-Active Bis(thiophenolato)amide Ligand, [SNS]^3–, and the Homoleptic Tungsten Complexes, W[SNS]₂ and W[ONO]₂

A new tridentate redox-active ligand platform, derived from bis­(2-mercapto-<i>p</i>-tolyl)­amine, [SNS^cat]­H₃, has been prepared in high yields by a four-step procedure starting from commericially available bis­(<i>p</i>-tolyl)­amine. The redox-active pincer-type ligand has been coordinated to tungsten to afford the six-coordinate, homoleptic complex W­[SNS]₂. To benchmark the redox behavior of the [SNS] ligand, the analogous tungsten complex of the well-known redox-active bis­(3,5-di-<i>tert</i>-butylphenolato)­amide ligand, W­[ONO]₂, also has been prepared. Both complexes show two reversible reductions and two partially reversible oxidations. Structural, spectroscopic, and electrochemical data all indicate that W­[ONO]₂ is best described as a tungsten­(VI) metal center coordinated to two [ONO^cat]^3– ligands. In contrast, experimental data suggests a higher degree of S→W π donation, giving the W­[SNS]₂ complex non-innocent electronic character that can be described as a tungsten­(IV) metal center coordinated to two [SNS^sq]^2– ligands

Expanding Yttrium Bis(trimethylsilylamide) Chemistry Through the Reaction Chemistry of (N₂)^2–, (N₂)^3–, and (NO)^2– Complexes

The reaction chemistry of the side-on bound (N₂)^2–, (N₂)^3–, and (NO)^2– complexes of the [(R₂N)₂Y]⁺ cation (R = SiMe₃), namely, [(R₂N)₂(THF)­Y]₂(<i>μ</i>-η²:η²-N₂), <b>1</b>, [(R₂N)₂(THF)­Y]₂(<i>μ</i>-η²:η²-N₂)­K, <b>2</b>, and [(R₂N)₂(THF)­Y]₂(<i>μ</i>-η²:η²-NO), <b>3</b>, with oxidizing agents has been explored to search for other (E₂)^<i>n</i>−, (E = N, O), species that can be stabilized by this cation. This has led to the first examples for the [(R₂N)₂Y]⁺ cation of two fundamental classes of [(monoanion)₂Ln]⁺ rare earth systems (Ln = Sc, Y, lanthanides), namely, oxide complexes and the tetraphenylborate salt. In addition, an unusually high yield reaction with dioxygen was found to give a peroxide complex that completes the (N₂)^2–, (NO)^2–, (O₂)^2– series with <b>1</b> and <b>3</b>. Specifically, the (<i><i>μ</i>-</i>O)^2– oxide-bridged bimetallic complex, [(R₂N)₂(THF)­Y}₂(<i><i>μ</i>-</i>O), <b>4</b>, is obtained as a byproduct from reactions of either the (N₂)^2– complex, <b>1</b>, or the (N₂)^3– complex, <b>2</b>, with NO, while the oxide formed from <b>2</b> with N₂O is a polymeric species incorporating potassium, {[(R₂N)₂Y]₂(<i><i>μ</i>-</i>O)₂K₂(<i><i>μ</i>-</i>C₇H₈)}_<i>n</i>, <b>5</b>. Reaction of <b>1</b> with 1 atm of O₂ generates the (O₂)^2– bridging side-on peroxide [(R₂N)₂(THF)­Y]₂(<i>μ</i>-η²:η²-O₂), <b>6</b>. The O–O bond in <b>6</b> is cleaved by KC₈ to provide an alternative synthetic route to <b>5</b>. Attempts to oxidize the (NO)^2– complex, <b>3</b>, with AgBPh₄ led to the isolation of the tetraphenylborate complex, [(R₂N)₂Y­(THF)₃]­[BPh₄], <b>7</b>, that was also synthesized from <b>1</b> and AgBPh₄. Oxidation of the (N₂)^2– complex, <b>1</b>, with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl, TEMPO, generates the (TEMPO)⁻ anion complex, (R₂N)₂(THF)­Y­(η²-ONC₅H₆Me₄), <b>8</b>

Cocrystallization of (μ‑S₂)^2– and (μ-S)^2– and Formation of an [η²‑S₃N(SiMe₃)₂] Ligand from Chalcogen Reduction by (N₂)^2– in a Bimetallic Yttrium Amide Complex

The reactivity of the (N₂)^2– complex {[(Me₃Si)₂N]₂Y­(THF)}₂(μ-η²:η²-N₂) (<b>1</b>) with sulfur and selenium has been studied to explore the special reductive chemistry of this complex and to expand the variety of bimetallic rare-earth amide complexes. Complex <b>1</b> reacts with elemental sulfur to form a mixture of compounds, <b>2</b>, that is the first example of cocrystallized complexes of (S₂)^2– and S^2– ligands. The crystals of <b>2</b> contain both the (μ-S₂)^2– complex {[(Me₃Si)₂N]₂Y­(THF)}₂(μ-η²:η²-S₂) (<b>3</b>) and the (μ-S)^2– complex {[(Me₃Si)₂N]₂Y­(THF)}₂(μ-S) (<b>4</b>), respectively. Modeling of the crystal data of <b>2</b> shows a 9:1 ratio of <b>3</b>:<b>4</b> in the crystals of <b>2</b> obtained from solutions that have 1:1 to 4:1 ratios of <b>3</b>/<b>4</b> by ¹H NMR spectroscopy. The addition of KC₈ to samples of <b>2</b> allows for the isolation of single crystals of <b>4</b>. The [S₃N­(SiMe₃)₂]⁻ ligand was isolated for the first time in crystals of [(Me₃Si)₂N]₂Y­[η²-S₃N­(SiMe₃)₂]­(THF) (<b>5</b>), obtained from the mother liquor of <b>2</b>. In contrast to the sulfur chemistry, the (μ-Se₂)^2– analogue of <b>3</b>, namely, {[(Me₃Si)₂N]₂Y­(THF)}₂(μ-η²:η²-Se₂) (<b>6</b>), can be cleanly synthesized in good yield by reacting <b>1</b>, with elemental selenium. The (μ-Se)^2– analogue of <b>4</b>, namely, {[(Me₃Si)₂N]₂Y­(THF)}₂(μ-Se) (<b>7</b>), was synthesized from Ph₃PSe