A new access route to dimetal sandwich complexes, including a radical anion

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Syntheses and crystal structures of new naphthalene– and anthracene–vanadate salts and an unprecedented dimetallabis(anthracene) sandwich complex: [Na(tetrahydrofuran)3][V2(anthracene)2]

Reduction of bis(naphthalene)vanadium(0) by potassium naphthalene (KNp) in tetrahydrofuran (THF) provides a highly reactive, thermolabile, and so far unisolable brown substance, which affords the first reported derivatives of bis(naphthalene)vanadates. From these solutions, thermally stable (298 K) and structurally characterized compounds have been obtained, including dark-red rods of catena-poly[bis(μ3-η4:η6:η4-naphthalene)tetrakis(tetrahydrofuran)dipotassiumvanadium], [K2V(C4H8O)4(C10H8)2] n or [K(THF)2]2[V(C10H8)2] (3), and red plates of (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane)potassium [1,2-bis(dimethylphosphanyl)ethane]bis(η4-naphthalene)vanadium tetrahydrofuran monosolvate, [K(C18H36N2O6)][V(C10H8)2(C6H16P2)]·C4H8O or [K([2.2.2]cryptand)][V(C10H8)2(dmpe)]·THF [dmpe is 1,2-bis(dimethylphosphanyl)ethane] (4b). Notably, [V(C10H8)2]2− is the only example of a structurally authenticated homoleptic bis(arene)metallate dianion and was obtained by further reduction of the brown material by KNp in THF, in the presence of trimethylphosphane (PMe3). Addition of anthracene (An) to the brown material in THF afforded deep-violet and paramagnetic crystalline (1,4,7,10,13,16-hexaoxacyclooctadecane)bis(tetrahydrofuran)potassium [(η4-anthracene)(tetrahydrofuran)vanadium]-μ-η4:η2-anthracene-[(1,4,7,10,13,16-hexaoxacyclooctadecane)potassium]-μ-η2:η4-anthracene-[(η4-anthracene)(tetrahydrofuran)vanadium] tetrahydrofuran disolvate, [K(C12H24O6)(C4H8O)2][KV2(C12H24O6)(C4H8O)2(C14H10)4]·2C4H8O or [K(18-crown-6)][K(18-crown-6)(THF)2][V(C14H10)2(THF)]2·2(THF) (5), which readily reacted with PMe3 and dmpe to give new vanadate salts. These were structurally characterized as (1,4,7,10,13,16-hexaoxacyclooctadecane)bis(tetrahydrofuran)potassium bis(η4-anthracene)(trimethylphosphane)vanadium tetrahydrofuran monosolvate, [K(C12H24O6)(C4H8O)2][V(C14H10)2(C3H9P)]·C4H8O or [K(18-crown-6)(THF)2][V(C14H10)2(PMe3)]·THF (6), and tetrakis(1,2-dimethoxyethane)potassium bis(η4-anthracene)[1,2-bis(dimethylphosphanyl)ethane]vanadium, [K(C4H10O2)4][V(C14H10)2(C6H16P2)] or [K(DME)4][V(C14H10)2(dmpe)] (DME is 1,2-dimethoxyethane) (7b). The last three structures contain the first known bis(anthracene)vanadates and are thereby derivatives of the unknown bis(anthracene)vanadium(0). Attempts to obtain the sodium salt analog of 5 in THF resulted instead in the formation of a unique substance, (μ3-η6:η6:η6-anthracene)(μ2-η6:η6-anthracene)tris(tetrahydrofuran)sodiumdivanadium, [NaV2(C14H10)2(C4H8O)3] or [Na(THF)3][V2(C14H10)2] (8), containing the first reported dimetallabis(anthracene) sandwich compound.

Actinide arene-metalates: ion pairing effects on the electronic structure of unsupported uranium-arenide sandwich complexes

Addition of [UI₂(THF)₃(μ-OMe)]₂·THF (2·THF) to THF solutions containing 6 equiv. of K[C₁₄H₁₀] generates the heteroleptic dimeric complexes [K(18-crown-6)(THF)₂]₂[U(η⁶-C₁₄H₁₀)(η⁴-C₁₄H₁₀)(μ-OMe)]₂·4THF (118C6·4THF) and {[K(THF)₃][U(η⁶-C₁₄H₁₀)(η⁴-C₁₄H₁₀)(μ-OMe)]}₂ (1THF) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both 118C6·4THF and 1THF are thermally stable in the solid-state at room temperature; however, after crystallization, they become insoluble in THF or DME solutions and instead gradually decompose upon standing. X-ray diffraction analysis reveals 118C6·4THF and 1THF to be structurally similar, possessing uranium centres sandwiched between bent anthracenide ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arenide ion pairing that is seen in 1THF but absent in 118C6·4THF, which is observed to have a significant effect on the electronic characteristics of the two complexes. Structural analysis, SQUID magnetometry data, XANES spectral characterization, and computational analyses are generally consistent with U(iv) formal assignments for the metal centres in both 118C6·4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of 1THF (3.74 μ_B) is significantly lower than that of 118C6·4THF (4.40 μ_B) at 300 K. Furthermore, the XANES data shows the U L_III-edge absorption energy for 1THF to be 0.9 eV higher than that of 118C6·4THF, suggestive of more oxidized metal centres in the former. Of note, CASSCF calculations on the model complex {[U(η⁶-C₁₄H₁₀)(η⁴-C₁₄H₁₀)(μ-OMe)]₂}²⁻ (1*) shows highly polarized uranium–arenide interactions defined by π-type bonds where the metal contributions are primarily comprised by the 6d-orbitals (7.3 ± 0.6%) with minor participation from the 5f-orbitals (1.5 ± 0.5%). These unique complexes provide new insights into actinide–arenide bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.

Use of Chatt metal-arene protocols with uranium leads to the synthesis of the first well-characterized, unsupported actinide–arenide sandwich complexes. The electronic structures of the actinide centres show a key sensitivity to ion pairing effects.

The Chatt reaction: conventional routes to homoleptic arenemetalates of d-block elements

Joseph Chatt was the first to discover in the early 1960s that previously unknown transition metal compounds were accessible and isolable via the reactions of alkali metal arene radical anions with transition metal precursors containing good leaving groups, such as weakly basic neutral or anionic ligands, especially halides. Later Peter Timms confirmed the importance of these early studies with the synthesis of several new bis(arene)metal(0) sandwich compounds by a variant of Chatt's route. Following a brief historical survey of alkali metal arene compounds, first examined in some detail by Wilhelm Schlenk, use of these reagents in the conventional syntheses of anionic homoleptic arene metal complexes of the d-block elements will be described. In several cases these species are quite useful because they function as storable "naked" atomic metal anion reagents, especially in their reactions with carbon monoxide and isocyanides. In view of Chatt's seminal contributions to an often unique route to organometallic and inorganic compounds, it is proposed that this valuable synthetic method be named the "Chatt reaction" in honor of a giant of chemistry.