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.

Di-tert-butyldiphosphatetrahedrane as a Source of 1,2-Diphosphacyclobutadiene Ligands

Reactions of di‐tert‐butyldiphosphatetrahedrane (1) with cycloocta‐1,5‐diene‐ or anthracene‐stabilised metalate anions of iron and cobalt consistently afford complexes of the rarely encountered 1,2‐diphosphacyclobutadiene ligand, which have previously been very challenging synthetic targets. The subsequent reactivity of 1,2‐diphosphacyclobutadiene cobaltates toward various electrophiles has also been investigated and is compared to reactions of related 1,3‐diphosphacyclobutadiene complexes. The results highlight the distinct reactivity of such isomeric species, showing that the 1,2‐isomers can act as precursors for previously unknown triphospholium ligands. The electronic structures of the new complexes were investigated by several methods, including NMR, EPR and Mößbauer spectroscopies as well as quantum chemical calculations.

Low-oxidation state cobalt-magnesium complexes: ion-pairing and reactivity

Magnesium cobaltates (Arnacnac)MgCo(COD)2 (1-3) were synthesised by reacting (Arnacnac)MgI(OEt2) with K[Co(η4-COD)2] (COD = 1,5-cyclooctadiene) [Arnacnac = CH(ArNCMe)2; Ar = 2,4,6-Me3-C6H2 (Mes), 2,6-Et2-C6H3 (Dep), 2,6-iPr2-C6H3Mes (Dipp)]. Compounds 1-3 form contact ion-pairs in toluene, while solvent separated ion-pairs are formed in THF. The effect of ion-pairing on the reactivity is illustrated by reaction of 2 with tert-butylphosphaalkyne, which affords distinct 1,3-diphosphacyclobutadiene complexes. The heteroleptic sandwich complex [(Depnacnac)MgCo(P2C2tBu2)]2 (4) is selectively formed in toluene, while the homoleptic bis(1,3-diphosphacyclobutadiene) complex [(Depnacnac)Mg(THF)3][Co(P2C2tBu2)2] (5) is obtained in THF. Complex 4 is a precursor to further unusual phosphaorganometallic compounds. Substitution of the labile COD ligand in 4 by white phosphorus (P4) enabled the synthesis of the phosphorus-rich sandwich compound [(Depnacnac)MgCoP4(P2C2tBu2)]2 (6). The heterobimetallic complex (Cp*NiP2C2tBu2)Co(COD) (7) was isolated after treatment of 4 with Cp*Ni(acac) (Cp* = C5Me5, acac = acetylacetonate).

1,3-Diphosphacyclobutene Cobalt Complexes

The synthesis and characterization of rare 1,3-diphosphacyclobutene transition-metal complexes is described. Reactions of the cobalt-hydride complex [Co(P₂ C₂ tBu₂ )₂ H] (G) with nBuLi, tBuLi, or PhLi afforded [Li(solv)_x {Co(η³ -P₂ C₂ tBu₂ HR)(η⁴ -P₂ C₂ tBu₂ )}] (1: R=nBu, (solv)_x =(Et₂ O)₂ ; 2: R=tBu, (solv)_x =(thf)₂ ; 3: R=Ph, (solv)_x =(Et₂ O)(thf)₂ ), with an η³ -coordinated 1,3-diphosphacyclobutene ligand as a result of organyl-anion attack at one of the phosphorus atoms of the bis(1,3-diphosphacyclobutadiene) backbone. In contrast to the reactions with PhLi, the aryl-magnesium compounds p-tolyl magnesium chloride and p-fluorophenyl magnesium bromide deprotonate [Co(P₂ C₂ tBu₂ )₂ H] to give the magnesium salt [Mg(MeCN)₆ ][Co(η⁴ -P₂ C₂ tBu₂ )₂ ]₂ (4), which contains a bis(1,3-diphosphacyclobutadiene)-cobaltate anion. The [Co(η⁴ -P₂ C₂ tBu₂ )₂ ]^- anions are well separated from the octahedral [Mg(MeCN)₆ ]²⁺ cation in the molecular structure of 4. Compound 1 reacts with Me₃ SiCl to give neutral [Co(η³ -P₂ C₂ tBu₂ HnBu)(η⁴ -P₂ C₂ tBu₂ SiMe₃ )] (5, 52 % yield) with an SiMe₃ group attached to one of the P atoms of the previously unfunctionalized backbone.

Functionalization of 1,3-diphosphacyclobutadiene cobalt complexes via Si–P bond insertion

We report the synthesis of functionalized 1,3-bis(diphosphacyclobutadiene) complexes via the insertion of carbon-oxygen bonds of ethers, esters, aldehydes and amides into the P–Si bond of silylated complexes. Reactions of [K(tol)2][Co(η 4-P2C2R2)2] [[K(tol)2][1a]: R=tBu, [K(tol)2][1b]: R=tPent (=tert-pentyl)] with Me3SiCl afford the trimethylsilyl-substituted derivatives [Co(η 4-P2C2R2SiMe3)(η 4-P2C2R2)] (2a, b, R=tBu, tPent). The Me3Si group is connected to a phosphorus atom of one of the 1,3-diphosphacyclobutadiene ligands. 2a, b readily react with organic substrates containing C–O single and C=O double bonds at ambient temperature. [Co(η 4-P2C2R2(CH2)4OSiMe3)(η 4-P2C2R2)] (3a, b) are formed by reaction of 2a, b with traces of THF. They can also be isolated by reacting the THF solvates [K(thf)2{Co(P2C2 tBu2)2}] ([K(thf)2][1a]) and [K(thf)3{Co(P2C2 tPent2)2}] ([K(thf)3][1b]) with Me3SiCl in toluene or THF. The adamantyl-substituted complex [Co(η 4-P2C2Ad2(CH2)4OSiMe3)(η 4-P2C2Ad2)] (3c) was prepared analogously from [K(thf)4{Co(P2C2Ad2)2}] and Me3SiCl. [K(thf)2][1a] reacts cleanly with Ph3SnCl affording [Co(η 4-P2C2 tBu2SnPh3)(η 4-P2C2 tBu2)] (4) in high yield. Reaction of 2a with styrene oxide affords [Co(η 4-P2C2 tBu2PhC2H3OSiMe3)(η 4-P2C2 tBu2)] (5) as a single regioisomer. By contrast, multinuclear NMR spectroscopic studies indicate mixtures of two isomeric insertion products 6/6′ and 7/7′, respectively, which result from the insertion of 1,2-epoxy-2-methylpropane and 1,2-epoxyoctane. Moreover, these monitoring studies show that reactions of 2a with acyclic ethers afford alkyl substituted complexes such as [Co(η 4-P2C2 tBu2Et)(η 4-P2C2 tBu2)] (8) and alkylsilyl ethers. Reaction of 2a with γ-butyrolactone gives [Co(η 4-P2C2 tBu2(CH2)3C(O)OSiMe3)(η 4-P2C2 tBu2)] (9) via cleavage of the endocyclic C–O single bond of the lactone. Benzaldehyde and acetone cleanly react with 2a to [Co(η 4-P2C2 tBu2CH(Ph)OSiMe3)(η 4-P2C2 tBu2)] (10) and [Co(η 4-P2C2 tBu2CMe2OSiMe3)(η 4-P2C2 tBu2)] (11), while the sterically more demanding ketones 3-pentanone and acetophenone selectively yield the known hydride complex [Co(η 4-P2C2 tBu2)2H] (A). Phenyl isocyanate reacts with 2a at elevated temperature to form [Co(η 3-P2C2 tBu2CON(Ph)SiMe3)(η 4-P2C2 tBu2)] (12) with a functionalized η 3-coordinated ligand. [K(tol)2][1a], [K(tol)2][1b], 2a, 2b, 3a–c, 4, 5, and 9–12 were isolated and characterized by multinuclear NMR spectroscopy, UV/Vis spectroscopy and elemental analysis. [K(tol)2][1b], 2a, 2b, 3c, 4, 5, and 9–12 were additionally characterized by X-ray crystallography.

Recent NMR developments applied to organic-inorganic materials

In this contribution, the latest developments in solid state NMR are presented in the field of organic-inorganic (O/I) materials (or hybrid materials). Such materials involve mineral and organic (including polymeric and biological) components, and can exhibit complex O/I interfaces. Hybrids are currently a major topic of research in nanoscience, and solid state NMR is obviously a pertinent spectroscopic tool of investigation. Its versatility allows the detailed description of the structure and texture of such complex materials. The article is divided in two main parts: in the first one, recent NMR methodological/instrumental developments are presented in connection with hybrid materials. In the second part, an exhaustive overview of the major classes of O/I materials and their NMR characterization is presented.

Diverse silver(i) sulfobenzoate coordination polymers and their recycling property as homogeneous catalyst in oxygenation of sulfide

Four silver sulfobenzoate coordination polymers with bpmb ligands were prepared. The variable positions of sulfonate groups on benzene rings gave rise to different coordination polymers and variable chemical properties.

Solid state NMR studies and chemical shift calculations of a gold(I) complex with a diphosphacyclobutadiene cobaltate sandwich anion

The (31)P and (13)C solid state nuclear magnetic resonance spectra of a neutral gold(I) complex with bis(diphosphacyclobutadiene) cobaltate anions, [Au{Co(P2C2tPent2)2}2(PMe3)2], are reported. Complete (31)P resonance assignments have been derived from saturation transfer, radio-frequency driven recoupling (RFDR) and RTOBSY experiments and confirmed further by ab-initio calculations of magnetic shielding tensors by density functional theory, with consideration of relativistic effects. Coordination of the diphosphacyclobutadiene ring with gold(I) results in a high-frequency shift of the (31)P signal of the directly coordinated P atom, whereas a low-frequency shift is observed for the P atom at the opposite end of that ring. Based on these results, a previous assignment made for the complex salts [Au(PMe3)4][Au{Co(P2C2Ad2)2}2] and [K(18-crown-6)(thf)2][Au{Co(P2C2Ad2)2}2] (Ad=adamantyl) must be corrected.