Intramolecular Formation of a Cr^I(bis-arene) Species via TEA Activation of [Cr(CO)₄(Ph₂P(C₃H₆)PPh₂)]⁺: An EPR and DFT Investigation

Abstract

Activation of the catalytically relevant complex [Cr(CO)4(1)]+ (1 = Ph2P(C3H6)PPh2) by Et3Al (TEA) leads to formation of the Cr(I) bis-arene complex [Cr(1-bis-η6-arene)]+, as revealed by EPR and DFT calculations. This bis-arene complex is formed by intramolecular rearrangement and coordination of Cr(I) to the ligand phenyl groups in aliphatic solvents following loss of CO, preventing release of Cr(I) into solution. By comparison in aromatic solvents (toluene), the [Cr(bis-tolyl)]+ complex is preferentially formed.

Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study

“…With the exception of one proton, it should be noted that the Cp 1 H hyperfine couplings in all three complexes are quite large, and this is largely due to the combination of the relatively short Ni-1 H(Cp) distances (these vary from 2.70 to 3.07 Å giving dipolar values from A|| = 8.0, A  = -4.0 MHz to A|| = 5.5, A  = -2.7 MHz in a classic point dipole approximation;Table S4) along with the appreciable Fermi contact contribution. Indeed, the 1 H hyperfine couplings for  5 coordinated Cp rings or  6 coordinated arene rings in paramagnetic first row transition metal complexes are generally large, so the current findings are consistent with previous reports [119][120][121]. The contribution of the dominant anisotropic hyperfine terms coupled with the appreciable aiso terms for the Cp 1 H's confirm the small amount of unpaired spin associated with the Cp ring, which is consistent with the calculated SOMOs of the complexes (Figure S17).…”

The Role of Catalyst Support, Diluent and Co-Catalyst in Chromium-Mediated Heterogeneous Ethylene Trimerisation

“…Entries 3, 4) leads to a considerable drop in activity and an associated switch in product selectivity from oligomerisation towards PE formation. It has been reported previously that treatment of Cr III complexes with alkyl aluminium reagents in aromatic solvents, for example Cr(acac) 3 /AlMe 3 in toluene [ 38 ], leads to the formation of reduced chromium(I) sandwich complexes of the type [Cr(η 6 -arene) 2 ] + [ 22 ], something that has been ascribed to account for the deactivation of homogeneous chromium oligomerisation systems in such solvents [ 29 , 37 ]. Hence, it is likely that analogous Cr I arene species are also formed during the activation of the oxide-bound chromium amide species, something that is consistent with the observed drop in activity.…”

“…However, one barrier to the development of such heterogeneous systems is their complexity. Even for established homogeneous chromium-based selective olefin oligomerisation systems, where aspects of the general catalytic mechanism have been elucidated [ 1 , 20 ], most notably the role of a metallacyclic reaction pathway (Scheme 1 ) [ 1 , 21 ], there remains considerable debate about their precise mode of operation, including the formal oxidation states of the catalytically-active chromium species and specific aspects of ligand control [ 22 – 25 ]. Indeed, it is well-documented that catalytic performance relies on a complex interplay of factors including not only the structure of the molecular precursor and its supporting ligands, but also the nature of the aluminium activator, reaction solvent, and process conditions [ 26 ].…”

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

“…The same group also studied the activation when higher levels of TEA were employed. 118 It was clear that the TEA was responsible for the complete removal of all CO groups from the Cr 11 complex and this reaction occurs via a dominant pathway involving a series of Cr 11 intermediates, including a cis-[Cr(CO) 3 (Ph 2 PN(i-Pr)PPh 2 )] 1 complex and a 'piano-stool' type complex [Cr(CO) 2 (Ph 2 PN(i-Pr)PPh 2 )] 1 . Each of these paramagnetic complexes produced a distinctive set of spin Hamiltonian parameters as characterised by CW EPR, and verified by DFT.…”