Unravelling the Photochemical Transformations of Chromium(I) 1,3 Bis(diphenylphosphino), [Cr(CO)₄(dppp)]⁺, by EPR Spectroscopy

Abstract

UV-induced photochemical transformations of the paramagnetic [Cr­(CO)4(Ph2PCH2CH2CH2PPh2)]+ complex (abbreviated [Cr­(CO)4(dppp)]+) in dichloromethane was investigated by CW EPR spectroscopy. Room-temperature UV irradiation results in the rapid transformation of [Cr­(CO)4(dppp)]+ into trans-[Cr­(CO)2(dppp)2]+. However, low-temperature (77–120 K) UV irradiation reveals the presence of an intermediate mer-[Cr­(CO)3(κ1-dppp)­(κ2-dppp)]+ complex which photochemically transforms into trans-[Cr­(CO)2(dppp)2]+. The derived spin Hamiltonian parameters for these complexes were confirmed by DFT calculations. The photoinduced reaction is shown to be concentration-dependent, leading to a distribution of the three complexes ([Cr­(CO)4(dppp)]+, mer-[Cr­(CO)3(κ1-dppp)­(κ 2-dppp)]+, and trans-[Cr­(CO)2(dppp)2]+). A bimolecular photoinduced mechanism is proposed to account for the formation of mer-[Cr­(CO)3(κ1-dppp)­(κ2-dppp)]+ and trans-[Cr­(CO)2(dppp)2]+.

Spectroscopic Interrogation of the Reduction of Model Chromium Precatalysts for Olefin Oligomerization

“…Together these results imply that the major reduction products obtained by using Cp* 2 Co and Al-based reductants are integer-spin, non-Kramers Cr II (d 4 , S = 2) species that are not directly observable in our cases by X-band EPR. We conclude that the small signal observed near g = 2 upon chemical reduction, however, is a minor Cr I product that arises via a minor reaction pathway about which little evidence is available. , …”

“…The collected EPR data for the in situ chemical reductions were analyzed by using the validated QEPR assay to determine the concentration of the Cr I species , formed upon reduction ( vide supra ). Our findings show that reduction results in <1% conversion to the species assigned as Cr I with respect to the concentration of the Cr III starting material used in each case.…”

“…These narrower signals can be assigned to d 5 Cr I species with S = 1/2 in all three cases on the basis of spectral profiles for Cr I systems in the literature. 46,47 Notably, the d 4 Cr II species expected as the major reduction products would be difficult to observe with the EPR instrumentation used in these studies in perpendicular mode; this is due to the high likelihood of these Cr II species having non-Kramers S = 2 ground states. 48 In the detectable Cr I species, no significant hyperfine coupling to ligands in the secondary shell (e.g., phosphorus) was detected.…”

“…We conclude that the small signal observed near g = 2 upon chemical reduction, however, is a minor Cr I product that arises via a minor reaction pathway about which little evidence is available. 54,55 Electronic Absorption Spectroscopy. UV−vis electronic absorption (EA) spectra were collected for the isolated Cr III materials, yielding similar absorption spectra for 1, 2, and 3 that feature two d−d absorption bands at ca.…”

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

“…A CW EPR study was therefore recently reported on the UV activation of a [Cr(CO) 4 (Ph 2 P(CH 2 ) 3 PPh 2 )] 1 complex. 120 In general there is very little literature on the photochemistry of Cr 11 systems, 120 despite the copious amount of literature available on the analogous Cr 0 complexes; this is likely due to the air sensitive nature of the Cr 11 systems. UV photolysis of the [Cr(CO) 4 (Ph 2 P(CH 2 ) 3 PPh 2 )] 1 complex breaks both the Cr-CO and Cr-P bonds, ultimately leading to the formation of a dicarbonyl [Cr(CO) 2 (Ph 2 P(CH 2 ) 3 PPh 2 ) 2 ] 1 complex and a mer-[Cr(CO) 3 -(Ph 2 P(CH 2 ) 3 PPh 2 ) 2 ] 1 complex which is an intermediate observed only at low temperature.…”