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]+.
Highlights
Cr-based complexes are important catalysts for the selective trimerization/tetramerization of ethylene to yield highly desirable linear α-olefins.[1−7] These catalysts typically employ ligands based on bis(phosphino)amines and bis(sulfanyl)amines, whereby the activity and selectivity of the catalytic reactions can in principle be tuned by varying the nature of the ancillary ligands.[5]
While electron paramagnetic resonance (EPR) studies have been used to study the oxidation states of chromium following the activation step,[7,8] previous work from our group revealed the important role that the ligand itself plays in adopting various conformations of the partially or fully decarbonylated complexes.[9−11] For example, addition of triethylaluminum (Et3Al) in nonaromatic solvents to [Cr(CO)4(Ph2P(C2H4)PPh2)]+ (1,2-bis(diphenylphosphino)ethane, Ph2P(C2H4)PPh2 = dppe) or [Cr(CO)4(Ph2PN(iPr)PPh2)]+ (1,2-bis(diphenylphosphino)-isopropylamine, Ph2PN(iPr)PPh2) resulted in the formation of the Cr(I)-bis-η6-arene complex, formed via intramolecular rearrangement and coordination of
Following UV irradiation of the [Cr(CO)4(dppp)]+ complex at 298 K, the intense blue coloration of the solution changed to pale yellow and this was accompanied by a significant change to the anisotropic EPR spectrum (Figure 1b)
Summary
Cr-based complexes are important catalysts for the selective trimerization/tetramerization of ethylene to yield highly desirable linear α-olefins.[1−7] These catalysts typically employ ligands based on bis(phosphino)amines and bis(sulfanyl)amines, whereby the activity and selectivity of the catalytic reactions can in principle be tuned by varying the nature of the ancillary ligands.[5] Since the first reports of ethylene tetramerization toward 1-octene using chromium-bis(diphenylphosphino)amine (Cr-PNP) catalysts, many studies have focused on the mechanism, with particular reference to the oxidation states of the active chromium centers In most cases, these Cr-based complexes are generally activated prior to catalysis by addition of an alkylaluminum cocatalyst, such as triethylaluminum (Et3Al) or methylaluminoxane (MAO).[5].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
