A novel samarium(ii) complex bearing a dianionic bis(phenolate) cyclam ligand: synthesis, structure and electron-transfer reactions

Synthesis and Characterization of Solvated Lanthanide(II) Bis(triisopropylsilyl)phosphide Complexes

Lanthanide (Ln) silylamide chemistry is well-developed, but the corresponding silylphosphide chemistry is immature; there are only ten structurally characterized examples of Ln(II) bis(trimethylsilyl)phosphide complexes to date and no reported derivatives with bulkier R-groups. Here, we report the synthesis of the first f-block bis(triisopropylsilyl)phosphide complexes, [Ln{P(SiiPr3)2}2(THF)x] (1-Ln; Ln = Sm, Eu, x = 3; Ln = Yb, x = 2), by the respective salt metathesis reactions of parent [LnI2(THF)2] with 2 equiv of [Na{P(SiiPr3)2}]n in toluene. Complexes 1-Ln were characterized by a combination of NMR, EPR, ATR-IR, electronic absorption and emission spectroscopies, elemental analysis, SQUID magnetometry, and single crystal X-ray diffraction. These data contrast with those obtained for related Ln(II) bis(trimethylsilyl)phosphide complexes due to the bulkier ligands in 1-Ln and also with Ln(II) bis(triisopropylsilyl)amide complexes due to a combination of longer Ln-P vs. Ln-N bonds and the softer nature of P- vs. N-donor ligands.

Isolation and Electronic Structures of Lanthanide(II) Bis(trimethylsilyl)phosphide Complexes

While lanthanide (Ln) silylamide chemistry is mature, the corresponding silylphosphide chemistry is underdeveloped, with [Sm{P(SiMe3)2}{μ-P(SiMe3)2}3Sm(THF)3] being the sole example of a structurally authenticated Ln(II) silylphosphide complex. Here, we expand the Ln(II) {P(SiMe3)2} chemistry through the synthesis and characterization of nine complexes. The dinuclear "ate" salt-occluded complexes [{Ln[P(SiMe3)2]3(THF)}2(μ-I)K3(THF)] (1-Ln; Ln = Sm, Eu) and polymeric "ate" complex [KYb{P(SiMe3)2}3{μ-K[P(SiMe3)2]}2]∞ (2-Yb) were prepared by the respective salt metathesis reactions of parent [LnI2(THF)2] (Ln = Sm, Eu, Yb) with 2 or 3 equiv of K{P(SiMe3)2} in diethyl ether. The separate treatment of these complexes with either pyridine or 18-crown-6 led to the formation of the mononuclear solvated adducts trans-[Ln{P(SiMe3)2}2(py)4] (3-Ln; Ln = Sm, Eu, Yb) and [Ln{P(SiMe3)2}2(18-crown-6)] (4-Ln; Ln = Sm, Eu, Yb), with concomitant loss of K{P(SiMe3)2}. The complexes were characterized by a combination of NMR, electron paramagnetic resonance (EPR), attenuated total reflectance infrared (ATR-IR), electronic absorption and emission spectroscopies, elemental analysis, SQUID magnetometry, and single crystal X-ray diffraction. We find that these complexes contrast with those of related Ln(II) bis(silyl)amide complexes due to differences in ligand donor atom hardness and ligand steric requirements from Ln-P bonds being longer than Ln-N bonds. This leads to higher coordination numbers, shorter luminescence lifetimes, and smaller easy-axis magnetic anisotropy parameters.

Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential and pKa

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N₂ reduction reaction (N₂RR) in particular, parameters that dictate selectivity for either the 6H⁺/6e^- product ammonia (NH₃) or the 4H⁺/4e^- product hydrazine (N₂H₄) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (Fe), with which NH₃ is typically the major product of N₂R, to instead favor N₂H₄ as the sole observed fixed-N product (>99:1). This dramatic shift is achieved by replacing moderate reductants and strong acids with a very strongly reducing but weakly acidic Sm^II-(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (Sm^II-PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N₂H₄ per Fe and 67% fixed-N yield per H⁺). However, by generating N₂H₄ as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH₃-selective conditions with Fe. Mechanistic data support assignment of iron hydrazido(2-) species FeNNH₂ as selectivity-determining: we infer that protonation of FeNNH₂ at N_β, favored by strong acids, releases NH₃, whereas one-electron reduction to FeNNH₂^-, favored by strong reductants such as Sm^II-PH, produces N₂H₄ via reactivity initiated at N_α. Spectroscopic data also implicate a role for Sm^III-binding to anionic FeN₂^- (via an Fe-N₂- -Sm^III species) with respect to catalytic efficacy.

Sm(II)-Mediated Proton-Coupled Electron Transfer: Quantifying Very Weak N-H and O-H Homolytic Bond Strengths and Factors Controlling Them

Coordination of alcohols to the single-electron reductant samarium diiodide (SmI2) results in substantial O-H bond weakening, affording potent proton-coupled electron transfer (PCET) reagents. However, poorly defined speciation of SmI2 in tetrahydrofuran (THF)/alcohol mixtures limits reliable thermodynamic analyses of such systems. Rigorous determination of bond dissociation free energy (BDFE) values in such Sm systems, important to evaluating their reactivity profiles, motivates studies of model Sm systems where contributing factors can be teased apart. Here, a bulky and strongly chelating macrocyclic ligand ((tBu2ArOH)2Me2cyclam) maintains solubility, eliminates dimerization pathways, and facilitates clean electrochemical behavior in a well-defined functional model for the PCET reactivity of SmII with coordinating proton sources. Direct measurement of thermodynamic parameters enables reliable experimental estimation of the BDFEs in 2-pyrrolidone and MeOH complexes of ((tBu2ArO)2Me2cyclam)SmII, thereby revealing exceptionally weak N-H and O-H BDFEs of 27.2 and <24.1 kcal mol-1, respectively. Expanded thermochemical cycles reveal that this bond weakening stems from the very strongly reducing SmII center and the formation of strong SmIII-alkoxide (and -pyrrolidonate) interactions in the PCET products. We provide a detailed analysis comparing these BDFE values with those that have been put forward for SmI2 in THF in the presence of related proton donors. We suggest that BDFE values for the latter systems may in fact be appreciably higher than the system described herein. Finally, protonation and electrochemical reduction steps necessary for the regeneration of the PCET donors from SmIII-alkoxides are demonstrated, pointing to future strategies aimed at achieving (electro)catalytic turnover using SmII-based PCET reagents.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Delivery of a Masked Uranium(II) by an Oxide-Bridged Diuranium(III) Complex

Oxide is an attractive linker for building polymetallic complexes that provide molecular models for metal oxide activity, but studies of these systems are limited to metals in high oxidation states. Herein, we synthesized and characterized the molecular and electronic structure of diuranium bridged U^III /U^IV and U^III /U^III complexes. Reactivity studies of these complexes revealed that the U-O bond is easily broken upon addition of N-heterocycles resulting in the delivery of a formal equivalent of U^III and U^II , respectively, along with the uranium(IV) terminal-oxo coproduct. In particular, the U^III /U^III oxide complex effects the reductive coupling of pyridine and two-electron reduction of 4,4'-bipyridine affording unique examples of diuranium(III) complexes bridged by N-heterocyclic redox-active ligands. These results provide insight into the chemistry of low oxidation state metal oxides and demonstrate the use of oxo-bridged U^III /U^III complexes as a strategy to explore U^II reactivity.

Heteroleptic Samarium(III) Chalcogenide Complexes: Opportunities for Giant Exchange Coupling in Bridging σ- and π-Radical Lanthanide Dichalcogenides

The introduction of (N2)3-• radicals into multinuclear lanthanide molecular magnets raised hysteresis temperatures by stimulating strong exchange coupling between spin centers. Radical ligands with larger donor atoms could promote more efficient magnetic coupling between lanthanides to provide superior magnetic properties. Here, we show that heavy chalcogens (S, Se, Te) are primed to fulfill these criteria. The moderately reducing Sm(II) complex, [Sm(N††)2], where N†† is the bulky bis(triisopropylsilyl)amide ligand, can be oxidized (i) by diphenyldichalcogenides E2Ph2 (E = S, Se, Te) to form the mononuclear series [Sm(N††)2(EPh)] (E = S, 1-S; Se, 1-Se, Te, 1-Te); (ii) S8 or Se8 to give dinuclear [{Sm(N††)2}2(μ-η2:η2-E2)] (E = S, 2-S2; Se, 2-Se2); or (iii) with Te═PEt3 to yield [{Sm(N††)2}(μ-Te)] (3). These complexes have been characterized by single crystal X-ray diffraction, multinuclear NMR, FTIR, and electronic spectroscopy; the steric bulk of N†† dictates the formation of mononuclear complexes with chalcogenate ligands and dinuclear species with the chalcogenides. The Lα1 fluorescence-detected X-ray absorption spectra at the Sm L3-edge yielded resolved pre-edge and white-line peaks for 1-S and 2-E2, which served to calibrate our computational protocol in the successful reproduction of the spectral features. This method was employed to elucidate the ground state electronic structures for proposed oxidized and reduced variants of 2-E2. Reactivity is ligand-based, forming species with bridging superchalcogenide (E2)-• and subchalcogenide (E2)3-• radical ligands. The extraordinarily large exchange couplings provided by these dichalcogenide radicals reveal their suitability as potential successors to the benchmark (N2)3-• complexes in molecular magnets.

Synthesis of a dinuclear europium(iii) complex through deprotonation and oxygen-atom transfer of trimethylamine N-oxide

A dinuclear europium complex was synthesized via unprecedented deprotonation and oxygen-atom transfer of Me₃NO.

Synthesis of dinuclear aluminum complexes bearing bis-phenolate ligand and application in ring-opening polymerization of ε-caprolactone

Dinuclear zwitterionic aluminum complexes were prepared and applied as efficient catalysts toward ROP of ε-CL in a controlled manner.