Mono- and Dinuclear Heteroleptic Cobalt Complexes with α-Diimine and Polyarene Ligands

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.

α-Diimines as Versatile, Derivatizable Ligands in Ruthenium(II) p-Cymene Anticancer Complexes

α-Diimines are among the most robust and versatile ligands available in synthetic coordination chemistry, possessing finely tunable steric and electronic properties. A series of novel cationic ruthenium(II) p-cymene complexes bearing simple α-diimine ligands, [(η⁶- p-cymene)RuCl{κ² N-(HCNR)₂}]NO₃ (R = Cy, [1]NO₃; R = 4-C₆H₁₀OH, [2]NO₃; R = 4-C₆H₄OH, [3]NO₃), were prepared in near-quantitative yields as their nitrate salts. [2]NO₃ displays high water solubility. The potential of the α-diimine ligand in [3]NO₃ as a carrier of bioactive molecules was investigated via esterification reactions with the hydroxyl groups. Thus, the double-functionalized derivatives [(η⁶- p-cymene)RuCl{κ² N-(HCN(4-C₆H₄OCO-R))₂}]NO₃ (R = aspirinate, [5]NO₃; valproate, [6]NO₃) and also [4]Cl (R = Me) were obtained in good-to-high yields. UV-vis and multinuclear NMR spectroscopy and cyclic voltammetric studies in aqueous solution revealed only minor ruthenium chloride hydrolytic cleavage, biologically accessible reduction potentials, and pH-dependent behavior of [3]NO₃. Density functional theory analysis was performed in order to compare the Ru-Cl bond strength in [1]⁺ with the analogous ethylenediamine complex, showing that the higher stability observed in the former is related to the electron-withdrawing properties of the α-diimine ligand. In vitro cytotoxicity studies were performed against tumorigenic (A2780 and A2780cisR) and nontumorigenic (HEK-293) cell lines, with the complexes bearing simple α-diimine ligands ranging from inactive to IC₅₀ values in the low micromolar range. The complexes functionalized with bioactive components, i.e., [5]NO₃ and [6]NO₃, exhibited a marked increase in the cytotoxicity with respect to the precursor [3]NO₃.

Activation of CN bonds by high-valent group 6 metal chlorides, including the conversion of an α-diimine into a functionalized imidazolium

The direct interactions of WCl₆and MoCl₅with α-diimines (and also a carbodiimide in the case of MoCl₅) have been explored, leading to unusual reaction pathways.

Allowing the direct interaction of N-aryl α-diimines with a high valent metal chloride: one-pot WCl6-promoted formation of quinoxalinium salts

The full potential of a high valent metal chloride as both a chlorinating and an oxidative agent was explored by allowing WCl₆ to react with N-(2,6-diisopropylphenyl) α-diimines, in CH₂Cl₂ at room temperature. These α-diimines underwent unprecedented conversion to quinoxalinium cations via intramolecular C-N coupling.

Noninnocent ligands: heteroleptic nickel complexes with α-diimine and 1,2-diketone derivatives

Reaction of the Ni-Ni-bonded compound [(Ni^IL˙^-)₂] (1, L = [(2,6-iPr₂C₆H₃)NC(Me)]₂) with various 1,2-diketones afforded a series of heteroleptic complexes: [LNi(PhC(O)-C(O)Ph)] (2), [LNi(PhC(O)-C(O)Me)] (3), [LNi(3,5-tBu₂C₆H₂O₂)] (4), and [(LNi){μ-η²,η²-(MeC(O)-C(O)Me)}(NiL)] (5). Furthermore, the complex [Na(Et₂O)][LNi{PhC(O)-C(O)Ph}] (6) was obtained by the reduction of 2 with 1.0 equiv. of Na metal. These complexes, which contain three potential redox-active centers, nickel and both α-diimine and 1,2-diketone ligands, were characterized by X-ray crystallography, NMR, EPR, and UV-vis-NIR spectra, magnetic susceptibility measurements, and DFT computations to elucidate their electronic structures.