Direct synthesis of an anionic 13-vertex closo-cobaltacarborane cluster

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.

Rich redox-activity and solvatochromism in a family of heteroleptic cobalt complexes

The combination of redox-active metals with redox-active ligands can lead to interesting charge transfer behaviours, including valence tautomerism and solvatochromism. With the aim of investigating a relatively underexplored redox-active metal/redox-active ligand combination, complexes [Co^II(acac)₂(X-BIAN)] (acac^- = acetylacetonate; X-BIAN = bis(4-X-phenyl)iminoacenaphthene; 1: X = -CF₃, 2: X = -Cl, 3: X = -H, 4: X = -Me) and [Co^III(acac)₂(Me-BIAN)]⁺ (5+) have been synthesised and characterised. At all temperatures investigated, and in both the solid and solution state, complexes 1-4 exist in a Co^II-BIAN⁰ charge distribution, while 5+ adopts a Co^III-BIAN⁰ charge distribution. In the case of 1-4, the potential Co^III-BIAN˙^- valence tautomer is inaccesible; the energy ordering between the ground Co^II-BIAN⁰ state and the excited Co^III-BIAN˙^- state must be reversed in order for an entropically driven interconversion to be possible. The energy gap between the states can be monitored via metal-to-ligand charge transfer bands in the visible region. We demonstrate tuning of this energy gap by varying the electronic properties of the BIAN ligand, as well as by controlling the molecular environment through solvent choice. Solvatochromic analysis, in combination with crystallographic evidence, allows elucidation of the specific solvent-solute interactions that govern the molecular behaviour of 1-4, affording insights that can inform potential future applications in sensing and switching.

Low-Valence Anionic α-Diimine Iron Complexes: Synthesis, Characterization, and Catalytic Hydroboration Studies

The synthesis of rare anionic heteroleptic and homoleptic α-diimine iron complexes is described. Heteroleptic BIAN (bis(aryl)iminoacenaphthene) complexes 1-[K([18]c-6)(thf)_0.5] and 2-[K([18]c-6)(thf)₂] were synthesized by reduction of the [(BIAN)FeBr₂] precursor complex using stoichiometric amounts of potassium graphite in the presence of the corresponding olefin. The electronic structure of these paramagnetic species was investigated by numerous spectroscopic analyses (NMR, EPR, ⁵⁷Fe Mössbauer, UV-vis), magnetic measurements (Evans NMR method, SQUID), and theoretical techniques (DFT, CASSCF). Whereas anion 1 is a low-spin complex, anion 2 consists of an intermediate-spin Fe(III) center. Both complexes are efficient precatalysts for the hydroboration of carbonyl compounds under mild reaction conditions. The reaction of bis(anthracene) ferrate(1-) gave the homoleptic BIAN complex 3-[K([18]c-6)(thf)], which is less catalytically active. The electronic structure was elucidated with the same techniques as described for complexes 1-[K([18]c-6)(thf)_0.5] and 2-[K([18]c-6)(thf)₂] and revealed an Fe(II) species in a quartet ground state.