A cw EPR and ENDOR investigation on a series of Cr(i) carbonyl complexes with relevance to alkene oligomerization catalysis: [Cr(CO)4L]+ (L = Ph2PN(R)PPh2, Ph2P(R)PPh2)

Abstract

The preparation and characterisation of the Cr(I) complexes [Cr(CO)₄L]⁺ (L = Ph₂PN(R)PPh₂, Ph₂P(R)PPh₂), which are used as pre-catalysts for the selective oligomerization of ethylene, are reported. The electronic properties and structural features of these complexes in frozen solution have been established via continuous wave X-band Electron Paramagnetic Resonance (cw-EPR) and continuous wave ¹H, ¹⁴N and ³¹P Electron Nuclear Double Resonance (cw-ENDOR) spectroscopy. The EPR spectra are dominated by the g anisotropy, with notably large ^PA couplings from the two equivalent ³¹P nuclei. The spin Hamiltonian parameters (g_⊥ (g_xx = g_yy) > g_e > g_∥ (g_zz)) are consistent with a low-spin d⁵ system possessing C_2v symmetry, with a SOMO where the metal contribution is primarily d_xy for all complexes. The isotropic Fermi contact term (^Pa_iso, determined by EPR and ENDOR) was found to be largest for complexes containing ligands e, d, f and g, indicating that the ³¹P 3 s character in the SOMO is higher for the PNP type ligands than the PCP type. Subtle structural differences in the complexes were also identified through variations in the Δg shifts (identified by EPR), and through differences in the phenyl ring conformations (identified by ¹H ENDOR). Attempts to correlate trends in EPR-derived parameters with data measured for catalysis using these pre-catalysts are also made, but no clear connections were found.