The power of electron paramagnetic resonance to study asymmetric homogeneous catalysts based on transition-metal complexes

Asymmetric synthesis involves the preparation of chiral compounds with well-defined three-dimensional molecular structure or stereochemistry. Many of these enantioselective transformations involve chiral ligands incorporating transition-metal ions. Over the years numerous synthetic, spectroscopic and theoretical approaches have been applied to understand and probe the inner workings and mode of chiral transfer in these homogeneous based catalysts. For the paramagnetic based asymmetric complexes, electron paramagnetic resonance (EPR) can provide a wealth of detail and information on the structure and mechanism of the catalysts. Despite this potential, the application and uptake of advanced EPR methodologies to this field of endeavor has been limited. The purpose of this review is to explain and illustrate, through representative examples, the enormous amount of information that can be obtained from an EPR study of the chiral metal-based complexes, ranging from electronic structure and symmetry to spatial arrangement of interacting ligands and substrates

Multidentate Lewis acids: synthesis, structure and mode of action of a redox-based fluoride ion sensor

The mode of action of the bidentate bis( boronate) Lewis acid 2 as a fluoride ion sensor is shown to involve selective anion binding together with an electrochemical response

Structural origin of the gradual spin transition in a mononuclear iron(II) complex

Variable temperature single crystal X-ray diffraction and SQUID magnetometry experiments have revealed a gradual spin transition in [Fe II(L)](ClO4)2 (where L=1,4,7-tris(2- aminophenyl)-1,4,7-triazacyclononane), centred around room temperature. The gradual nature of the spin transition has been attributed to the lack of significant intermolecular interactions between iron centres and the propensity of the counter ions to accommodate the internal strain in the crystal caused by spin crossover. © 2011 Elsevier Ltd. All rights reserved

Sterically Encumbered Iridium Bis(N-heterocyclic carbene) Complexes: Air-Stable 14-Electron Cations and Facile Degenerate C–H Activation

Cationic Ir(III) systems supported by a bis(expanded NHC) framework and featuring both agostic C–H and cis alkyl/hydride ligand sets have been targeted by protonation of the corresponding bis(alkyl) hydride complexes. Remarkably, the steric shielding afforded by the NHC substituents is such that these and related putative 14-electron cations are air and moisture stable. In solution, degenerate fluxional exchange is brought about by reversible σ-bond activation within the agostic alkyl C(sp3)–H bond; a non-dissociative mechanism is implied by the activation parameters ΔH⧧ = 8.8(0.4) kcal mol–1 and ΔS⧧ = −12.2(1.7) eu

Structure and pulsed EPR characterization of N,N\u2032-bis(5-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide and its adducts with propylene oxide

The role of steric hindrance in controlling the binding mode of propylene oxide to a novel vanadyl salen-type complex N,N′-bis(5-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide, [VO(3)], has been investigated using CW/pulse EPR, ENDOR and HYSCORE spectroscopy and compared to that of the parent complex N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide, [VO(1)]. The single-crystal X-ray structure of [VO(3)]·HCCl3 has been determined by X-ray analysis and is complemented by DFT calculations and circular dichroism measurements. The structure of the complex in frozen solution, as revealed by the EPR methods, is in good agreement with the X-ray and DFT analyses. Removal of the ‘inner’ tert-butyl groups from the salicylidene rings reduces the steric hindrance between the ligand and epoxide substrate. As a result the selectivity for binding single enantiomers of propylene oxide in these complexes is reversed in [VO(3)] relative to [VO(1)]