Reduced and excited states of the intermediates (α-diimine)(C5R5)Rh in hydride transfer catalysis schemes: EPR and resonance Raman spectroscopy, and comparative DFT calculations of Co, Rh and Ir analogues

Abstract

The electronic structures of the highly air-sensitive intermediates (N⁁N)(C₅Me₅)Rh, (N⁁N = 2,2′-bipyridine (bpy), 2,2′-bipyrimidine (bpym), 2,2′-bipyrazine (bpz) and 3,3′-bipyridazine (bpdz)) of hydride transfer catalysis schemes were studied through resonance Raman (rR) spectroscopy and through EPR of the reduced forms [(N⁁N)(C₅Me₅)Rh]˙⁻. The rR results are compatible with a predominant MLCT character of the lowest excited states [(N⁁N)(C₅Me₅)Rh]*, and the EPR spectra of the reduced states reveal the presence of anion radical ligands, (N⁁N)˙⁻, coordinated by unusually electron rich rhodium(I) centres. The experimental results, including the assignments of electronic transitions, are supported by DFT calculations for the model compounds [(N⁁N)(C₅H₅)Rh]^o/˙⁻, N⁁N = bpy or bpym. The calculations confirm a significant but not complete mixing of metal and ligand orbitals in the lowest unoccupied MO which still retains about ¾ π*(N⁁N) character. DFT calculations on (bpy)(C₅H₅)M and [(bpy)(C₅H₅)ClM]⁺, M = Co, Rh, Ir, agree with the experimental results such as the differences between the homologues, especially the different LUMO characters of the precursor cations in the case of Co (→d_M) and Rh or Ir (→π*(bpy)).