Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

Abstract

The chiral-at-metal diastereomers (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal] (P−P′ = (R)-Prophos and (R,R)-Norphos, Hal = Cl, Br, and I) were synthesized, separated, and characterized by X-ray crystallography. Halide exchange and epimerization reactions were studied in 9:1 and 1:1 chloroform/methanol mixtures proceeding at room temperature or slightly above according to first-order. The rate-determining step in the Hal exchange reactions was the dissociation of the Ru−Hal bond, forming the pyramidal 16-electron intermediates (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+, which maintain the metal configuration. These intermediates can invert their metal configuration or react with nucleophiles with retention of the metal configuration. The measured competition ratios showed that the inversion of the intermediates was slow compared to quenching with nucleophiles, indicating a high pyramidal stability of the 16-electron fragments (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+ toward inversion in agreement with a basilica-type energy profile. Stereochemically this implies that substitution reactions in (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal] occur with predominant retention of configuration, however, accompanied by a well-defined share of inversion, a point overlooked when (R Ru,R C)- and (S Ru,R C)-[CpRu(Prophos)Cl] were extensively used as starting materials in the synthesis of a variety of organometallic derivatives. The rates of the approach to the epimerization equilibrium were much smaller than those of the Hal exchange reactions, because in the basilica-type energy profile the intermediates (R Ru,R C)-/(S Ru,R C)-[CpRu(P−P′)]+, formed in the cleavage of the Ru−Cl bond, have to cross another barrier of appreciable height for inversion. Increasing the methanol content of the solvent increased the rates of the Hal exchange and epimerization reactions. Obviously, the pyramidality of the fragments [CpRu(P−P′)]+ is enforced by the small P−Ru−P angles (83° in the Prophos derivatives and 86° in the Norphos derivatives of (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal]). Due to these small angles, intermediates (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+ resist planarization and thus inversion of the metal configuration.

The Stereospecific Ligand Exchange at a Pseudo‐Benzylic T‐4 Iridium Centre in Planar‐Chiral Cycloiridium (η6‐Arene)tricarbonylchromium Complexes

“…Keywords: chirality · metallacycles · organometallic compounds · stereoisomerization · stereoselectivity strained system. [19,20] This principle has been evaluated in many ways in attempts to constrain the configuration of the metal centre at pseudo-tetrahedral transition-metal centres of half-sandwich metallocenes within a limited number of configurations. [21][22][23][24] Complementary effects can be introduced to increase the stereoselectivity such as an overall molecular electrostatic unbalance around the metal centre subjected to ligand exchange.…”

Co‐Crystallization of Half‐Sandwich (RM,RC)/(SM,RC) Diastereomers in Single Crystals

“…( S Ru , R C )‐[CpRu(Prophos)Cl], ( S Ru , R C )‐[CpRu(Prophos)Br], and ( R Ru , R C )‐[CpRu(Prophos)I], the parent compounds to the methyl‐substituted analogues presented here, do not show diastereomer co‐crystallization.…”

Change of the Fe Configuration in Chiral Half‐Sandwich Complexes Within the Solvent Cage

“…coordination position. [10][11][12] In the cross-over experiment with (S Fe ,R C )-1 and (S Fe ,R C )-2, exchange of ligands NCPh and NCMe competes with pyramidal inversion. Ligand exchange must proceed via individual species in the bulk solution, whereas pyramidal inversion may occur within the contact pair.…”