Synthesis and structural chemistry of non-cyclopentadienyl organolanthanide complexes

Rare earth stibolyl and bismolyl sandwich complexes

The design of molecular rare earth complexes to achieve unique magnetic and bonding properties is a growing area of research with possible applications in advanced materials and molecular magnetics. Recent efforts focus on developing ligand frameworks that can enhance magnetic characteristics. Here we show the synthesis and characterization of a class of rare earth complexes, [(η5-C4R4Sb)Ln(η8-C8H8)] and [(η5-C4R4Bi)Ln(η8-C8H8)], featuring η5-coordinated stibolyl and bismolyl ligands. The ligand aromaticity and bonding situation within these complexes are investigated by quantum chemical calculations. Magnetic studies of the ErIII analogues reveal large barriers and intriguing properties, including waist-restricted hysteresis and slow relaxation of the magnetization, making them single-molecule magnets. Comparison between the experimental barrier and CASSCF-SO calculations indicates that relaxation in all systems occurs through high-energy excited states. These findings suggest that stibolyl and bismolyl ligands can be promising candidates for achieving high-energy barriers in Er-based SMMs, offering a pathway to molecular designs with enhanced magnetic properties.

Synthesis and characterization of borohydride rare-earth complexes supported by 2-pyridinemethanamido ligands and their application in the ring-opening polymerization of cyclic esters

The synthesis of 2-pyridinemethanamido borohydride complexes of yttrium and neodymium was achieved through the in situ deprotonation of the protio-ligand 2-pyridinemethanamine C5H3R1N-C(CH3)R2-NH(2,6-iPr2C6H3), denoted as PyAH (with PyAH1: R1 = R2 = H; PyAH2: R1 = CH3, R2 = H; PyAH3: R1 = C(CH3)N-(2,6-iPr2C6H3), R2 = CH3), in the presence of trisborohydride RE(BH4)3(THF)3 (RE = Y and Nd) as a precursor and a base. The isolation of various molecular structures, nine of which were structurally characterized by X-ray diffraction analysis, was achieved and revealed to depend not only on (i) the nature of the 2-pyridinemethanamido ligand and (ii) the rare-earth element but also on (iii) the reaction conditions, notably the type of base used. These include seven mono-substituted species, eventually also comprising the cation derived from the base reagent, such as [(PyA1)Y(BH4)3]2[Mg(THF)6] (1Y), [(PyA1)Nd(BH4)3Mg(PyA1)](THF)4 (1Nd), (PyA1)Nd(BH4)2(THF)2 (1'Nd), [(PyA1)Nd(THF)(BH4)(μ-BH4)]2 (1''Nd), [(PyA2)Nd(BH4)3]2[Mg(THF)6] (3Nd), (PyA2)Nd(BH4)2(THF)2 (3'Nd) and (PyA3)Nd(BH4)2 (4Nd), as well as two bis-substituted complexes (PyA1)2Y(BH4) (2Y) and (PyA1)2Nd(BH4) (2Nd). On the other hand, the unexpected amido/ene-amido derivative [(PyA(EA))Y(BH4)2][Li(THF)4] (5Y) (PyA(EA): R1 = CCH2-N(2,6-iPr2C6H3), R2 = CH3), where the PyAH3 protio-ligand underwent double deprotonation, was recovered from the reaction carried out with nBuLi in the yttrium series. In some cases, the synthesis led to the isolation of borohydride 2-pyridinemethanamido-supported magnesium complexes (PyA2)Mg(BH4)(THF) and (PyA3)Mg(BH4)(THF). In parallel, the PyAH2 pro-ligand could be structurally analyzed, and an unprecedented adduct of the type [KN(SiMe3)2·PyAH1]2 was isolated and characterized by X-ray diffraction analysis. Preliminary investigations of the ring-opening polymerization of L-lactide and ε-caprolactone with some of the complexes synthesized are finally presented, demonstrating moderate to high catalytic activities.

Tris-Silanide f-Block Complexes: Insights into Paramagnetic Influence on NMR Chemical Shifts

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

Rare-Earth Metal Complexes Supported by A Tridentate Amidinate Ligand: Synthesis, Characterization, and Catalytic Comparison in Isoprene Polymerization

To systematically investigate the dependence of the initiating group and metal size on polymerization performance, a family of rare-earth metal bis(alkyl)/bis(benzyl)/bis(amide) complexes supported by a monoanionic tridentate amidinate ligand [(2,6-iPr2C6H3)NC(Ph)N(C6H4-2-OMe]- (HL) were synthesized and well-characterized. Treatment of rare-earth metal tris(alkyl)/tris(benzyl)/tris(amide) complexes Y(CH2C6H4NMe2-o)3 or Y(CH2SiMe3)3(THF)2 or Ln[N(SiHMe2)2]3(THF)x (Ln = Sc, x = 1; Ln = Y, La, Sm, Lu, x = 2) with 1 equiv of HL gave the corresponding mono(amidinate) rare-earth metal bis(alkyl)/bis(benzyl)/bis(amide) complexes [(2,6-iPr2C6H4)NC(Ph)N(C6H4-2-OMe)]Y(CH2C6H4NMe2-o)2 (1), [(2,6-iPr2C6H4)NC(Ph)N(C6H4-2-OMe)]Y(CH2SiMe3)2(THF) (2), and [(2,6-iPr2C6H4)NC(Ph)N(C6H4-2-OMe)]Ln[N(SiHMe2)2]2(THF)n (Ln = Y, n = 1 (3); Ln = La, n = 1 (4); Ln = Sc, n = 0 (5); Ln = Lu, n = 0 (6); Ln = Sm, n = 0 (7)) in good isolated yields. These complexes were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction. In the presence of excess AlMe3 and on treatment with 1 equiv of [Ph3C][B(C6F5)4], these complexes could serve as precatalysts for cationic polymerization of isoprene, in which the dependence of the polymerization activity and regioselectivity on the initiating group and metal size was observed.

Role of Bis(phosphinimino)methanides as Universal Ligands in the Coordination Sphere of Metals across the Periodic Table

The coordination chemistry of bis(phosphinimino)methanide ligands is widespread and accompanies a large number of metal ions in the periodic table ranging from lithium to neptunium. This unique class of ligand systems show copious coordination chemistry with the main-group, transition, rare-earth, and actinide metals and are considered to be among the most attractive ligand systems to researchers. The bis(phosphinimino)methanide metal complexes offer an extensive range of applications in various fields and have been demonstrated as one of the universal ligand systems to stabilize the metal ions in not only their usual but also their unusual oxidation states. The main-group and transition metal chemistry using bis(phosphinimino)methanides as ligands was last updated almost a decade ago. In this review, we provide a comprehensive overview of various state-of-the-art bis(phosphinimino)methanide-supported metal complexes by dealing with their synthesis, characterization, reactivity, and catalytic studies which were not included in the last critical reviews.

Gas-phase formation of Grignard-type organolanthanide (III) ions RLnCl3 - : The influences of lanthanide center and hydrocarbyl group

RATIONALE

Compared with organomagnesium compounds (Grignard reagents), the Grignard-type organolanthanides (III) exhibit several utilizable differences in reactivity. However, the fundamental understanding of Grignard-type organolanthanides (III) is still in its infancy. Decarboxylation of metal carboxylate ions is an effective method to obtain organometallic ions that are well suited for gas-phase investigation using electrospray ionization (ESI) mass spectrometry in combination with density functional theory (DFT) calculations.

METHODS

The (RCO₂ )LnCl₃ ^- (R = CH₃ , Ln = La-Lu except Pm; Ln = La, R = CH₃ CH₂ , CH₂ CH, HCC, C₆ H₅ , and C₆ H₁₁ ) precursor ions were produced in the gas phase via ESI of LnCl₃ and RCO₂ H or RCO₂ Na mixtures in methanol. Collision-induced dissociation (CID) was employed to examine whether the Grignard-type organolanthanide (III) ions RLnCl₃ ^- can be obtained via decarboxylation of lanthanide chloride carboxylate ions (RCO₂ )LnCl₃ ^- . DFT calculations can be used to determine the influences of lanthanide center and hydrocarbyl group on the formation of RLnCl₃ ^- .

RESULTS

When R = CH₃ , CID of (CH₃ CO₂ )LnCl₃ ^- (Ln = La-Lu except Pm) yielded decarboxylation products (CH₃ )LnCl₃ ^- and reduction products LnCl₃ ·^- with a variation in the relative intensity ratio of (CH₃ )LnCl₃ ^- /LnCl₃ ·^- . The trend is as follows: (CH₃ )EuCl₃ ^- /EuCl₃ ·^-  < (CH₃ )YbCl₃ ^- /YbCl₃ ·^-  ≈ (CH₃ )SmCl₃ ^- /SmCl₃ ·^-  < other (CH₃ )LnCl₃ ^- /LnCl₃ ·^- , which complies with the trend of Ln (III)/Ln (II) reduction potentials in general. When Ln = La and hydrocarbyl groups were varied as CH₃ CH₂ , CH₂ CH, HCC, C₆ H₅ , and C₆ H₁₁ , the fragmentation behaviors of these (RCO₂ )LaCl₃ ^- precursor ions were diverse. Except for (C₆ H₁₁ CO₂ )LaCl₃ ^- , the four remaining (RCO₂ )LaCl₃ ^- (R = CH₃ CH₂ , CH₂ CH, HCC, and C₆ H₅ ) ions all underwent decarboxylation to yield RLaCl₃ ^- . (CH₂ CH)LaCl₃ ^- and especially (CH₃ CH₂ )LaCl₃ ^- are prone to undergo β-hydride transfer to form LaHCl₃ ^- , whereas (HCC)LaCl₃ ^- and (C₆ H₅ )LaCl₃ ^- are not. A minor reduction product, LaCl₃ ·^- , was formed via C₆ H₅ radical loss of (C₆ H₅ )LaCl₃ ^- . The relative intensities of RLaCl₃ ^- compared to (RCO₂ )LaCl₃ ^- decrease as follows: HCC > CH₂ CH > C₆ H₅  > CH₃  > CH₃ CH₂  >> C₆ H₁₁ (not visible).

CONCLUSION

A series of Grignard-type organolanthanide (III) ions RLnCl₃ ^- (R = CH₃ , Ln = La-Lu except Pm; Ln = La, R = CH₃ CH₂ , CH₂ CH, HCC, and C₆ H₅ ) were produced from (RCO₂ )LnCl₃ ^- via CO₂ loss, whereas (C₆ H₁₁ )LaCl₃ ^- did not. The experimental and theoretical results suggest that the reduction potentials of Ln (III)/Ln (II) couples as well as the bulkiness and hybridization of hydrocarbyl groups play important roles in promoting or limiting the formation of RLnCl₃ ^- via decarboxylation of (RCO₂ )LnCl₃ ^- .

Trimethyltriazacyclohexane coordination chemistry of simple rare-earth metal salts

Reactions of 1,3,5-trimethyl-triazacyclohexane (Me₃tach) with common rare-earth metal iodide, chloride, and triflate salts were examined to determine the capacity of this inexpensive chelate to provide alternative precursors for THF-free reactions. The reaction of LaI₃(THF)₄ and CeI₃(THF)₄ with 1,3,5-trimethyl-triazacyclohexane in THF generated toluene soluble (Me₃tach)₂LnI₃, 1-Ln, in which the Ln center has a tri-capped trigonal prismatic geometry with two eclipsed Me₃tach rings. Reaction with NdI₃(THF)_3.5 forms the analogous 1-Nd, but a different structure with one outer sphere iodide, [(Me₃tach)₂NdI₂][I], 2-Nd, is also accessible and has a structure reminiscent of bent metallocenes. The reaction of LaCl₃ and Me₃tach forms the less soluble (Me₃tach)₂LaCl₃, which has a structure analogous to 1-Ln with eclipsed Me₃tach rings. The mono-ring yttrium complex, (Me₃tach)YCl₃(THF)₂, could be isolated from the reaction of YCl₃ with Me₃tach. Reactions of La(OTf)₃ with Me₃tach were sensitive to the presence of residual proton sources as exemplified by the isolation of {[(Me₃tach)La(μ-OH)(μ-OTf)]₂(μ-OTf)₂}₂, 5-La, and [HMe₃tach][(Me₃tach)₂La-(OTf)₄], 6-La. SmI₂ reacts with Me₃tach to produce the Sm(II) complex, (Me₃tach)₂SmI₂(THF), 7-Sm, but 2-Sm can also form in this reaction. Complexes of the larger 1,4,7-trimethyltriazacyclononane (Me₃tacn) ligand, namely (Me₃tacn)LaI₃(THF), (Me₃tacn)YCl₃, and (Me₃tacn)SmI₂(THF) were synthesized for comparison. Several examples of the protonated ligands with simple counteranions, [HMe₃tach][X] (X = Cl, Br, I) and [HMe₃tacn][OTf], were identified in the course of these studies.

Isolation and Reactivity of Stannylenoids Stabilized by Amido/Imino Ligands

The reaction of the lithium aryl(silyl)amide Dipp(ⁱ Pr₃ Si)NLi (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ) with one equivalent of SnCl₂ in THF gave a novel stannylenoid Dipp(ⁱ Pr₃ Si)NSnCl⋅LiCl(THF)₂ . Heating the solution of amidostannylenoid in toluene to 80 °C resulted in dimeric amido(chloro)stannylene [Dipp(ⁱ Pr₃ Si)NSnCl]₂ , which can be converted to bis(amido)stannylene Sn[N(Dipp)(ⁱ Pr₃ Si)]₂ and amido(imino)stannylene Sn[N(Dipp)(ⁱ Pr₃ Si)][IPrN] (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-imino). Treatment of bis(imino)stannylenoid [IPrN]₂ Sn(Cl)Li with N₂ O resulted in the dimeric complex [IPrNSn(Cl)OLi]₂ . All compounds were characterized by NMR, elementary analysis, and X-ray structural determination.

π-Carbazolyl supported bis(alkyl) complexes of Sc, Y and La for α-olefin polymerization and hydrogenation

A series of new half-sandwich bis(alkyl) rare-earth metal complexes coordinated by a sterically demanding 1,3,6,8-tetra-tert-butyl-carbazol-9-yl ligand [tBu₄Carb]La(CH₂C₆H₅)₂(THF) (1-La), [tBu₄Carb]Ln(o-NMe₂C₆H₄CH₂)₂ (Ln = Sc (2-Sc), Y (2-Y), La (2-La), [tBu₄Carb]Ln(CH₂SiMe₃)₂(THF) (Ln = Sc (3-Sc), Y (3-Y)), were synthesized. 1-La, 2-La, and 2-Y were prepared by an alkane elimination protocol, while 2-Sc, 3-Sc, and 3-Y became accessible only when salt metathesis reactions of tBu₄CarbK with R₂Ln(THF)_n⁺[BPh₄]^- were employed. X-ray analysis revealed that in all complexes the carbazolyl ligand exhibits π-coordination with metal ions. 2-Sc and 3-Sc when activated with [Ph₃C][B(C₆F₅)₄] demonstrate excellent activity in α-olefin (octene-1, nonene-1, decene-1 and 1,1-diphenyl-but-1-ene) polymerization. When H₂ was used as a chain transfer agent (1 bar, rt) in the presence of 3-Sc/[Ph₃C][B(C₆F₅)₄] or 2-Y, 2-La olefin hydrogenation occurred with quantitative conversion.

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

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 UI2 (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 ligand [K]2 [N-((dimethyl)propenidesilyl)-2,6-diisopropylanilide] (L2). This 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 UIV and the allyl fragments is best described as ligand-to-metal electron donation from a two carbon fragment-localized electron density into empty f-orbitals.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Late Lanthanide Macrocyclic Tetra-NHC Complexes

An isostructural set of macrocyclic tetra-N-heterocyclic carbene (NHC) complexes were synthesized on late lanthanides including Lu, Yb, Ho, Dy, and Gd. They were characterized by single-crystal X-ray diffraction, multinuclear NMR, electrochemistry, and SQUID magnetometry. Solid-state structures show that all complexes are in a highly distorted square-pyramidal geometry with an axial HMDS ligand. ¹H NMR for Lu, Yb, and Dy demonstrates that these geometries are maintained in solution. Electrochemical measurements on the Yb complex show that the NHCs are very strong σ-donors compared to other organometallic Yb complexes. Magnetic measurements of the Yb and Dy complexes reveal slow relaxation of the magnetization in both complexes. The highly anisotropic Dy complex possesses an energy barrier to spin reversal of 52.42 K/36.43 cm^-1 and waist-restricted hysteresis up to 2.8 K. Finally, an 18-atom macrocycle variant of the Lu complex was synthesized for comparison in reactivity and stability. These complexes are the first lanthanides prepared with macrocyclic NHCs and suggest that NHCs may be a promising ligand for developing single-molecule magnets.

Synthesis and photophysical properties of rare earth complexes bearing silanediamido ligands Me2Si(NAryl)22− (Aryl = Dipp, Mes)

Luminescence of the newly obtained silanediamides of rare earths was investigated. The triplet states of the ligands {Me2Si(NAryl)2}2− (Aryl = Dpp, Mes) were determined by the emission of Gd complexes, and the bright emission of Tb3+ was observed.

A simple one-pot route to stable formamidinatoiodidolanthanoid(III) complexes from lanthanoid metals

A number of formamidinatoiodidolanthanoid(III) complexes, [Ln(DFForm)₂I(thf)₂] and [Ln(DFForm)I₂(thf)₃] (DFFormH = N,N'-bis(2,6-difluorophenyl)formamidine) and [Ln(DippForm)I₂(thf)₃] (DippFormH = N,N'-bis(2,6-diisopropylphenyl)formamidine) have been synthesized in good yields by one-pot direct reactions of the corresponding free metals with iodine and DFFormH or DippFormH in suitable ratios and are stable to rearrangement.

Bis(4-benzhydryl-benzoxazol-2-yl)methane - from a Bulky NacNac Alternative to a Trianion in Alkali Metal Complexes

A novel sterically demanding bis(4-benzhydryl-benzoxazol-2-yl)methane ligand 6 (4-BzhH2 BoxCH2 ) was gained in a straightforward six-step synthesis. Starting from this ligand monomeric [M(4-BzhH2 BoxCH)] (M=Na (7), K (81 )) and dimeric [{M(4-BzhH2 BoxCH)}2 ] (M=K (82 ), Rb (9), Cs (10)) alkali metal complexes were synthesised by deprotonation. Abstraction of the potassium ion of 8 by reaction with 18-crown-6 resulted in the solvent separated ion pair [{(THF)2 K@(18-crown-6)}{bis(4-benzhydryl-benzoxazol-2-yl)methanide}] (11), including the energetically favoured monoanionic (E,E)-(4-BzhH2 BoxCH) ligand. Further reaction of 4-BzhH2 BoxCH2 with three equivalents KH and two equivalents 18-crown-6 yielded polymeric [{(THF)2 K@(18-crown-6)}{K@(18-crown-6)K(4-Bzh BoxCH)}]n (n→∞) (12) containing a trianionic ligand. The neutral ligand and herein reported alkali complexes were characterised by single X-ray analyses identifying the latter as a promising precursor for low-valent main group complexes.

Renaissance of Homogeneous Cerium Catalysts with Unique Ce(IV/III) Couple: Redox-Mediated Organic Transformations Involving Homolysis of Ce(IV)-Ligand Covalent Bonds

Recent advances in the catalytic application of cerium complexes were achieved through controlling the Ce(IV/III) redox couple. Although Ce(IV) complexes have been extensively investigated as stoichiometric oxidants in organic synthesis on the basis of their highly positive redox potentials, these complexes can be used as catalysts, not only by introducing supporting ligands around the coordination sphere of cerium, but also by taking advantage of the photoresponsive properties of Ce(IV) and Ce(III) species. Cerium is highly abundant, comparable to that of some first-row transition metals such as copper, nickel, and zinc. Cerium complexes are new and promising homogeneous catalyst candidates for a variety of organic transformations under mild reaction conditions. They are typically used to activate dioxygen to oxidize organic compounds and applied for organic radical generation using the photoresponsive character of Ce(IV) carboxylates and alkoxides as well as electronic transition of Ce(III), in which homolysis of Ce(IV)-ligand covalent bonds is an important step for the overall catalytic cycle. In this Perspective, we first review the early discovery of Ce(OAc)₄-mediated oxidative transformations to emphasize the importance of Ce(IV)-OAc bond homolysis in various C-C bond-forming reactions and its relation to recent developments. We then focus on the fundamental importance of Ce(IV) reactivity involving thermal and photoassisted homolysis of the Ce(IV)-ligand covalent bond and the developments regarding Ce(IV/III) redox changes in catalytic reactions together with our recent findings on cerium-based catalysis.

Yttrium germole dianion complexes with Y-Ge bonds

The reactions of dipotassium 3,4-dimethyl-2,5-bis(trimethylsilyl)-germole dianion K2[1] with YCl3 and Cp*YCl2 (Cp* = cyclopentadienyl) in THF at room temperature afforded the dianion salt [(K-cryptand-222)2][1-YCl3] (K2[2]) and the dimeric complex [1-Y-Cp*]2 (3), respectively. While the polymeric complex {[(1)2-Y-K(toluene)]2}n (4) was obtained from the reaction of K2[1] and half molar equivalent of YCl3(THF)3.5 in toluene at 80 °C. The germole dianions in complexes 3 and 4 feature η5/η1 coordination interactions with the yttrium atoms. They represent the first examples of rare earth (RE) complexes containing RE-Ge bonds other than the RE-GeR3 structural type.

Recent Advances in Radiometals for Combined Imaging and Therapy in Cancer

Nuclear medicine is defined as the use of radionuclides for diagnostic and therapeutic applications. The imaging modalities positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are based on γ-emissions of specific energies. The therapeutic technologies are based on β^- -particle-, α-particle-, and Auger electron emitters. In oncology, PET and SPECT are used to detect cancer lesions, to determine dosimetry, and to monitor therapy effectiveness. In contrast, radiotherapy is designed to irreparably damage tumor cells in order to eradicate or control the disease's progression. Radiometals are being explored for the development of diagnostic and therapeutic radiopharmaceuticals. Strategies that combine both modalities (diagnostic and therapeutic), referred to as theranostics, are promising candidates for clinical applications. This review provides an overview of the basic concepts behind therapeutic and diagnostic radiopharmaceuticals and their significance in contemporary oncology. Select radiometals that significantly impact current and upcoming cancer treatment strategies are grouped as clinically suitable theranostics pairs. The most important physical and chemical properties are discussed. Standard production methods and current radionuclide availability are provided to indicate whether a cost-efficient use in a clinical routine is feasible. Recent preclinical and clinical developments and outline perspectives for the radiometals are highlighted in each section.

An Unsymmetric Imino-Phosphanamidinate Ligand and its Y(III) Complex: Synthesis, Characterization, and Catalytic Hydroboration of Carbonyl Compounds

An imino-phosphanamide ligand, [NHI^iPr2Me2P(Ph)NH-2,6-ⁱPr₂C₆H₃] (LH), containing two different N-substituents was prepared by the direct reaction of the lithium salt of N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenyl phosphanamine, PhP(Cl)NH-2,6-ⁱPr₂-C₆H₃. Reaction of LH with Y(N(SiMe₃)₂)₃ afforded the heteroleptic complex, [{L}Y(N(SiMe₃)₂)₂] (1), by elimination of HN(SiMe₃)₂. Compound 1 was characterized by multinuclear NMR and X-ray crystallography. In the complex, the Y(III) center was found to be tetracoordinate in a distorted tetrahedral geometry. The ligand, imino-phosphanamidinate, [L]^-, functions in a chelating manner, and its coordination to Y(III) results in a distorted 4-membered YPN₂ ring. As a proof of principle of its activity, 1 was used as a precatalyst for the hydroboration of various aldehydes and ketones using HBpin as the hydrogen source. The hydroboration reaction was rapid and clean even with low catalyst loadings (0.01-0.1 mol %). In addition, a very good functional group tolerance was observed in these reactions.

Ln(ii) alkyl complexes: from elusive exotics to catalytic applications

The synthesis, structures and reactivity of isolable Ln^II (Ln = Sm, Eu, Yb) alkyl complexes are discussed. The application of Ln^II alkyl derivatives in a variety of catalytic reactions is considered as well.

Rare-Earth Metal Complexes Supported by Polydentate Phenoxy-Type Ligand Platforms: C-H Activation Reactivity and CO2/Epoxide Copolymerization Catalysis

Mono- and dinuclear group 3 metal complexes incorporating polydentate bis(imino)phenoxy {N²O}^- and bis(amido)phenoxy {N²O}^3- ligands were synthesized by alkane elimination reactions from the tris(alkyl) M(CH₂SiMe₃)₃(THF)₂ and M(CH₂C₆H₄-o-NMe₂)₃ (M = Sc, Y) precursors. Complex 1a-Y was used for the selective C-H activation of 2-phenylpyridine at the 2'-phenyl position affording the corresponding bis(aryl) product 3a-Y, which was found to be reacted reluctantly with weak electrophiles (styrene, imines, hydrosilanes). The mechanism of formation of 3a-Y was established by DFT calculations, which also corroborated high stability of the complex toward insertion of styrene, apparently stemming from the inability to form the corresponding adduct. Copolymerization of cyclohexene oxide and CO₂ promoted by 1a-Y (0.1-0.5 mol %) was demonstrated to proceed under mild conditions (toluene, 70 °C, P_CO2 = 12 bar) giving polycarbonates with high efficiency (maximal TON of 460) and selectivity (97-99% of carbonate units).

f-Element silicon and heavy tetrel chemistry

The last three decades have seen a significant increase in the number of reports of f-element carbon chemistry, whilst the f-element chemistry of silicon, germanium, tin, and lead remain underdeveloped in comparison. Here, in this perspective we review complexes that contain chemical bonds between f-elements and silicon or the heavier tetrels since the birth of this field in 1985 to present day, with the intention of inspiring researchers to contribute to its development and explore the opportunities that it presents. For the purposes of this perspective, f-elements include lanthanides, actinides and group 3 metals. We focus on complexes that have been structurally authenticated by single-crystal X-ray diffraction, and horizon-scan for future opportunities and targets in the area.

Electronegativity and location of anionic ligands drive yttrium NMR for molecular, surface and solid-state structures

Yttrium is present in various forms in molecular compounds and solid-state structures; it typically provides specific mechanical and optical properties. Hence, yttrium containing compounds are used in a broad range of applications such as catalysis, lasers and optical devices. Obtaining descriptors that can provide access to a detailed structure-property relationship would therefore be a strong base for the rational design of such applications. Towards this goal, ⁸⁹Y (100% abundant spin ½ nucleus), is associated with a broad range of NMR chemical shifts that greatly depend on the coordination environment of Y, rendering ⁸⁹Y NMR an attractive method for the characterization of yttrium containing compounds. However, to date, it has been difficult to obtain a direct relationship between ⁸⁹Y chemical shifts and its coordination environment. Here, we use computational chemistry to model the chemical shift of a broad range of Y(iii) molecular compounds with the goal to reveal the underlying factors that determine the ⁸⁹Y chemical shift. We show through natural chemical shift (NCS)-analysis that isotropic chemical shifts can easily help to distinguish between different types of ligands solely based on the electronegativity of the central atom of the anionic ligands directly bound to Y(iii). NCS-analysis further demonstrates that the second most important parameter is the degree of pyramidalization of the three anionic ligands imposed by additional neutral ligands. While isotropic chemical shifts can be similar due to compensating effects, investigation of the chemical shift anisotropy (CSA) enables discriminating between the coordination environment of Y.

2-Butene Tetraanion Bridged Dinuclear Samarium(III) Complexes via Sm(II)-Mediated Reduction of Electron-Rich Olefins

While reduction reactions are ubiquitous in chemistry, it is very challenging to further reduce electron-rich compounds, especially the anionic ones. In this work, the reduction of 1,3-butadienyl dianion, the anionic conjugated olefin, has been realized by divalent rare-earth metal compounds (SmI₂), resulting in the formation of novel 2-butene tetraanion bridged disamarium(III) complexes. Density functional theory (DFT) analyses reveal two features: (i) the single electron transfer (SET) from 4f atomic orbitals (AOs) of each Sm center to the antibonding π*-orbitals of 1,3-butadienyl dianion is feasible and the new HOMO formed by the bonding interaction between Sm 5d orbitals (AOs) and the π*-orbitals of 1,3-butadienyl dianion can accept favorably 2e^- from 4f AOs of Sm(II); (ii) the 2-butene tetraanionic ligand serves as a unique 10e^- donating system, in which 4e^- act as two σ-donation bonding interactions while the rest 6e^- as three π-donation bonding interactions. The disamarium(III) complexes represent a unique class of the bridged bis-alkylidene rare-earth organometallic complexes. The ligand-based reductive reactivity of 2-butene tetraanion bridged disamarium(III) complexes demonstrates that 2-butene tetraanionic ligand serves as a 3e^- reductant toward cyclooctatetraene (COT) to provide doubly COT-supported disamarabutadiene complexes. The reaction of the disamarium(III) complexes with Cp*Li produces the doubly Cp*-coordinated Sm(III) complexes via salt metathesis. In addition, the reaction with Mo(CO)₆ affords the oxycyclopentadienyl dinuclear complex via CO insertion.

Half-sandwich rare-earth metal complexes bearing a C5Me4-C6H4-o-CH2NMe2 ligand: synthesis, characterization and catalytic properties for isoprene, 1-hexene and MMA polymerization

A new ortho-dimethylaminomethylphenyl-tetramethylcyclopentadienyl ligand C₅Me₄H-C₆H₄-o-CH₂NMe₂ (HL) and a series of rare-earth metal complexes bearing this ligand were synthesized. Of these complexes, two binuclear alkyl complexes [(C₅Me₄-C₆H₄-o-CH₂N(Me)CH₂-μ)Ln(CH₂SiMe₃)]₂ (Ln = Sc (1a) and Y (1b)) were obtained from the alkane elimination reaction of the free ligand with Ln(CH₂SiMe₃)₃(THF)₂, followed by an intramolecular C-H activation process of a NMe group in the ligand with a CH₂SiMe₃ group, two binuclear dichloro complexes (C₅Me₄-C₆H₄-o-CH₂NMe₂)₂Y₂Cl₄[LiCl(THF)₂] (2a) and [(C₅Me₄-C₆H₄-o-CH₂NMe₂)LuCl(μ-Cl)]₂ (2b) were synthesized by the reaction of anhydrous yttrium or lutetium trichloride with the lithium salt of the ligand LiL, and the binuclear bis(borohydrido) complexes [(C₅Me₄-C₆H₄-o-CH₂NMe₂)Ln(μ-BH₄)BH₄]₂ (Ln = Sm (3a) and Nd (3b)) were synthesized by the reaction of Ln(BH₄)₃(THF)₃ (Ln = Sm and Nd) with the lithium salt of the ligand. The molecular structures of all complexes 1a, 1b, 2a, 2b, 3a and 3b were determined by single-crystal X-ray crystallography. Upon activation with AlR₃/Ph₃CB(C₆F₅)₄, MAO or MMAO, the binuclear alkyl complexes 1a and 1b show good catalytic activity for isoprene cis-1,4 enriched regioselective polymerization and moderate catalytic activity for 1-hexene polymerization. Complexes 3a and 3b were studied as catalysts for methyl methacrylate polymerization reaction under different conditions and were found to show moderate to high catalytic activity.

An imido ligand significantly enhances the effective energy barrier of dysprosium(iii) single-molecule magnets

We report herein an imido ligand 1,3-bis(2,6-diisopropylphenyl) imidazolin-2-imine (Im^DippNH) that can form a very short Dy-N bond (2.12 Å) with the dysprosium(iii) ion, which leads to a much larger effective energy barrier for magnetisation reversal (803 K) compared to the analogous alkoxide ligand (53 K). Moreover, we predict that a linear two-coordinate [Dy(Im^DippN)₂]⁺ complex may have an effective energy barrier larger than 4000 K.

Alternative (κ1-N:η6-arene vs.κ2-N,N) coordination of a sterically demanding amidinate ligand: are size and electronic structure of the Ln ion decisive factors?

The amine elimination reaction of equimolar amounts of ansa-bis(amidine) C₆H₄-1,2-{NC(tBu)NH(2,6-iPr₂C₆H₃)}₂ (L¹H) and [(Me₃Si)₂N]₂Yb(THF)₂ affords a bis(amidinate) Yb^II complex [C₆H₄-1,2-{NC(tBu)N(2,6-iPr₂C₆H₃)}₂]Yb(THF) (1) in 68% yield. Complex 1 features a rather rare η¹-amido:η⁶-arene coordination of both amidinate fragments to the Yb^II ion, resulting in the formation of a bent bis(arene) structure. Oxidation of 1 by I₂ regardless of the molar ratio of reagents (2 : 1 or 1 : 1) leads to the formation of the Yb^III species [{(2,6-iPr₂C₆H₃)[double bond, length as m-dash]NC(tBu)NH}-C₆H₄-1,2-{NC(tBu)N(2,6-iPr₂C₆H₃)}]YbI₂(THF)₂ (2) in which only one amidinate fragment is coordinated to the ytterbium ion in κ²-N,N'-chelating coordination mode, while the second NCN fragment is protonated in the course of the reaction and is not bound to the metal ion. The outcome of the salt metathesis reaction of LaCl₃ with lithium amidinates [C₆H₄-1,2-{NC(tBu)N(2,6-R₂C₆H₃)}₂Li₂] (R = Me, iPr) is proven to be strongly affected by the substituent 2,6-R₂C₆H₃ on the amidinate nitrogens. When R = iPr, the salt metathesis reaction occurs smoothly and results in the formation of an ate-chloro-amidinate complex [C₆H₄-1,2-{NC(tBu)N(2,6-iPr₂C₆H₃)}₂]La(μ²-Cl)Li(THF)(μ²-Cl)₂Li(THF)₂ (3) in which the La^III ion is coordinated by both amidinate fragments in a "classic"κ²-N,N'-chelating fashion. In the case of R = Me, the reaction requires prolonged heating for completion. Moreover, the salt metathesis reaction is accompanied by the fragmentation of the ligand and affords a trinuclear chloro-amidinate complex [C₆H₄-1,2-{NC(tBu)N(2,6-Me₂C₆H₃)}₂]La{[(tBu)C(N-2,6-Me₂C₆H₃)₂]La(THF)}₂(μ²-Cl)₄(μ³-Cl)₂ (4) containing one dianionic ansa-bis(amidinate) and two monoanionic [(tBu)C(N-2,6-Me₂C₆H₃)₂] amidinate fragments. DFT calculations are conducted to determine the factor that governs this change in coordination mode and, in particular, the effect of the metal oxidation state.

Synthesis and molecular structure of enantiomerically pure pentadienyl complexes of the rare-earth metals

The enantiomerically pure pentadienyl (Pdl*) ligand derived from the natural product (1R)-(-)-myrtenal forms with MCl₃ (M = La, Ce, Pr, and Nd) the corresponding homoleptic [(η⁵-U-Pdl*)₃M compounds (1-M). These complexes were fully characterised by ¹H NMR spectroscopy, elemental analyses and X-ray diffraction. They exhibit in solution and solid state idealized C₃ symmetry, and their molecular structures also reveal that the Pdl* ligand adopts a U-conformation and coordinates exclusively with its less sterically encumbered face to the rare-earth metal atom. For the paramagnetic representatives an assignment of the ¹H NMR resonances based on a simple dipolar model gave satisfactory results.

Structural diversity in a homologous series of donor free alkali metal complexes bearing a sterically demanding triazenide

The isolation and structural characterization of donor free alkali metal complexes of a bis(terphenyl) substituted triazenide are presented.