Dirhodium(II,II) Paddlewheel Complexes

Abstract

This minireview summarizes synthetic approaches towards homoleptic dirhodium(II,II) paddlewheel complexes with the general formula Rh2A4. These complexes have found numerous applications in a wide range of chemical research and industry as catalysts, detectors, enzymatic inhibitors or building blocks for molecular scaffolds. In organic synthesis they are commonly used to transfer electron‐deficient species, they act as Lewis acids to activate unsaturated bonds, serve as hydrogenation catalysts and participate in oxidation/reduction processes. Dirhodium paddlewheel complexes are composed of the Rh‐Rh backbone and four bridging anions, which surround the core. According to the application, the electrochemical potential of the Rh atom can be modulated, as can the geometry and physical and chemical properties of the metal complex. Dirhodium complexes can be prepared in one step from basic inorganic precursors or by post‐functionalization of the paddlewheel structures.

Adducts of Rhodium(II) Acetate and Rhodium(II) Pivalate with 1,8-Diazabicyclo[5.4.0]undec-7-ene

“…In general, Rh(II) complexes consist of four equatorial ligands that bridge the dirhodium core and two axial ligands that occupy the remaining coordination site on each rhodium atom (Figure 1). The equatorial units can be exchanged for a variety of other ligands to either influence the compound's steric environment or to adjust the electron density within the metal core [17,18]. Ligand substitution at the equatorial sites often requires high boiling temperatures and long reaction times, while axial ligand exchange is more facile and occurs quickly at room temperature.…”

Melding of Experiment and Theory Illuminates Mechanisms of Metal-Catalyzed Rearrangements: Computational Approaches and Caveats

“…In general, chiral ligands in this field are quite large, so ligand conformations can become important. [119][120][121] For instance, ligand blocking groups around the metal dimer catalyst core can adopt either up () or down () configurations leading to multiple arrangements (Figure 2) that should be considered. 120 And the ligands in each of these arrangements may be able to adopt several different conformations.…”

Effects of Axial Solvent Coordination to Dirhodium Complexes on Reactivity and Selectivity in C–H Insertion Reactions: A Computational Study

“…These bimetallic complexes have a "paddle wheel" (sometimes called "lantern") structure, containing a Rh-Rh single bond, the details of which has been subject to experimental and theoretical interrogations for decades. [1][2][3][4][5][6][7][8] These complexes-which have applications spanning from catalysis 9,10 and biology [11][12][13] to supramolecular chemistry [14][15][16][17] -are potent catalysts in organic chemistry because of their ability to promote nitrogen extrusion from diazo compounds to generate transient rhodium carbene intermediates (Scheme 1, center, L = CR2). These intermediates are capable of engaging in a wide range of chemical reactions including (2 + 1) cycloadditions (e.g., cyclopropanation, cyclopropenation, insertion into X-H bonds), various (n + 1) cycloaddition reactions where n > 2, and a diverse array of ylide reactions.…”

“…However, this direction is still in its infancy compared to that of modifying electronic and steric properties of the bridging ligands. 7 Some have explored N-heterocyclic carbenes (NHCs) as external axial coordinating ligands. In one example, Snyder et al isolated a dirhodium carboxylate complex with tetramethylimidazolidene coordinated to the axial position in order to obtain a dirhodium carbene structure suitable for X-ray structure characterization.…”

“…The reaction starts with nucleophilic attack of the diazo compound (1) on dirhodium tetraformate (3) to generate an ylide (6). Dirhodium carbene (7) is formed by N2 extrusion from 6. A weakly bound complex (8) is then formed, from which the carbene carbon inserts into a C-H bond of propane (2).…”