Structural characterization of potassium salts of the decamethylmanganocene anion Cp*2Mn. Dalton Trans 2019;48:17078-17082. [PMID: 31701989 DOI: 10.1039/c9dt03551f]

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.

Metastable Charged Dimers in Organometallic Species: A Look into Hydrogen Bonding between Metallocene Derivatives

Multiply charged complexes bound by noncovalent interactions have been previously described in the literature, although they were mostly focused on organic and main group inorganic systems. In this work, we show that similar complexes can also be found for organometallic systems containing transition metals and deepen in the reasons behind the existence of these species. We have studied the structures, binding energies, and dissociation profiles in the gas phase of a series of charged hydrogen-bonded dimers of metallocene (Ru, Co, Rh, and Mn) derivatives isoelectronic with the ferrocene dimer. Our results indicate that the carboxylic acid-containing dimers are more strongly bonded and present larger barriers to dissociation than the amide ones and that the cationic complexes tend to be more stable than the anionic ones. Additionally, we describe for the first time the symmetric proton transfer that can occur while in the metastable phase. Finally, we use a density-based energy decomposition analysis to shine light on the nature of the interaction between the dimers.

The Redox Journey of Iconic Ferrocene: Ferrocenium Dications and Ferrocenate Anions

The recent discoveries of both dicationic and monoanionic ferrocene derivatives throw light on the effect of the substituents on the C₅ ring as well as the choice of redox agents and solvent system in the preparation of previously believed to be difficult synthetic targets. These oxidized and reduced forms of ferrocene are of interest to spectroscopists, magnetochemists, and theoreticians.

Low-spin 1,1'-diphosphametallocenates of chromium and iron

We report two anionic diphosphametallocenates, [K(2.2.2-crypt)][M(PC4Me4)2] (M = Cr, 2-Cr; Fe, 2-Fe). Both are low-spin (S = ½) by EPR spectroscopy and SQUID magnetometry. This contrasts the high-spin (S = 3/2) ferrocenate, [K(2.2.2-crypt)][Fe(C5H2-1,2,4-tBu)2] (4-Fe). Quantum chemical calculations suggest this is due to significant differences in ligand field splitting of the d-orbitals which also explain structural features in the 2-M complexes.

Isolation and electronic structures of derivatized manganocene, ferrocene and cobaltocene anions

The discovery of ferrocene nearly 70 years ago marked the genesis of metallocene chemistry. Although the ferrocenium cation was discovered soon afterwards, a derivatized ferrocenium dication was only isolated in 2016 and the monoanion of ferrocene has only been observed in low-temperature electrochemical studies. Here we report the isolation of a derivatized ferrocene anion in the solid state as part of an isostructural family of 3d metallocenates, which consist of anionic complexes of a metal centre (manganese, iron or cobalt) sandwiched between two bulky Cp^ttt ligands (where Cp^ttt is {1,2,4-C₅H₂ ^tBu₃}). These thermally and air-sensitive complexes decompose rapidly above -30 °C; however, we were able to characterize all metallocenates by a wide range of physical techniques and ab initio calculations. These data have allowed us to map the electronic structures of this metallocenate family, including an unexpected high-spin S = 3/2 ground state for the 19e^- derivatized ferrocene anion.