In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

A Continuous-Wave EPR Investigation into the Photochemical Transformations of the Chromium(I) Carbonyl Complex [Cr(CO)4bis(diphenylphosphino)]+ and Reactivity with 1-hexene

Chromium complexes containing a bis(diphenylphosphino) ligand have attracted significant interest over many years due to their potential as active catalysts for ethylene oligomerisation when combined with suitable co-catalysts such as triethylaluminium (TEA) or methylaluminoxane (MAO). While there has been considerable attention devoted to the possible reaction intermediates and the nature of the Cr oxidation states involved, the potential UV photoactivity of the Cr(I) complexes has so far been overlooked. Therefore, to explore the photoinduced transformations of bis(diphenylphosphino) stabilized Cr(I) complexes, we used continuous-wave (CW) EPR to study the effects of UV radiation on a cationic [Cr(CO)4(dppp)]+[Al(OC(CF3)3)4]- complex (1), where dppp represents the 1,3 bis-(diphenylphosphino)propane ligand, Ph2P(C3H6)PPh2. Our preliminary investigations into the photochemistry of this complex revealed that [Cr(CO)4(dppp)]+ (1) can be readily photo-converted into an intermediate mer-[Cr(CO)3(κ1-dppp)(κ2-dppp)]+ complex (2) and eventually into a trans-[Cr(CO)2(dppp)2]+ complex (3) in solution at room temperature under UV-A light. Here, we show that the intermediate species (2) involved in this transformation can be identified by EPR at much lower temperature (140 K) and at a specific wavelength (highlighting the wavelength dependency of the reaction). In addition, small amounts of a 'piano-stool'-type complex, namely [Cr(CO)2(dppp-η6-arene)]+ (4), can also be formed during the photoconversion of [Cr(CO)4(dppp)]+ using UV-A light. There was no evidence for the formation of the [Cr(L-bis-η6-arene)]+ complex (5) in these UV irradiation experiments. For the first time, we also evidence the formation of a 1-hexene coordinated [Cr(CO)3(dppp)(1-hexene)]+ complex (6) following UV irradiation of [Cr(CO)4(dppp)]+ in the presence of 1-hexene; this result demonstrates the unprecedented opportunity for exploiting light activation during Cr-driven olefin oligomerisation catalysis, instead of expensive, difficult-to-handle, and hazardous chemical activators.

Active Sites in Cr(III)-Based Ethylene Polymerization Catalysts from Machine-Learning-Supported XAS and EPR Spectroscopy

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry, EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls and identified the presence of several surface species in high and low-spin states, associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Highly Efficient Ethylene Tetramerization Using Cr Catalysts Constructed with Trifluoromethyl-Substituted N-Aryl PNP Ligands

Tetramerization of ethylene by chromium catalysts stabilized with functionalized N-aryl phosphineamine ligands C₆H₄(m-CF₃)N(PPh₂)₂ (1), C₆H₄(p-CF₃)N(PPh₂)₂ (2), C₆H₄(o-CF₃)N=PPh₂-PPh₂ (3), and C₆H₃(3,5-bis(CF₃))N(PPh₂)₂ (4) was evaluated. The parameter optimization includes temperature, co-catalyst, and solvent. Upon activation with MMAO-3A, the new catalyst system especially with m-functional PNP ligand (1) exhibited high 1-octene selectivity and productivity while giving minimum undesirable polyethylene and C₁₀ ⁺ olefin by-products. Using PhCl as a solvent at 75 °C led to a remarkable α-olefin (1-C₆ + 1-C₈) selectivity (>90 wt %) at a reaction rate of 2000 kg·g_Cr ^-1·h^-1. Under identical conditions, analogous PNP ligands bearing -CH₃, -Et, and -Cl functional moieties at the meta position of the N-phenyl ring displayed significantly lower reactivity. The catalyst with p-functional ligand (2) exhibited lower activity and comparable selectivities, while the Cr/PPN (with ligand 3) system gave no noticeable reactivity. The molecular structure of the precatalyst (1-Cr), exhibiting a monomeric structural feature, was elucidated with the aid of single-crystal X-ray diffraction study.

Trifluoromethoxy-substituted nickel catalysts for producing highly branched polyethylenes: impact of solvent, activator and N,N′-ligand on polymer properties

Highly branched and narrowly dispersed polyethylenes with high or ultra-high molecular weights are accessible using the depicted nickel precatalyst/activator combinations.

Chromium complexes supported by bidentate thioether-imine [N,S] ligands: synthesis and ethylene oligomerization studies

Chromium complexes bearing thioether-imine ligands were synthesized and their catalytic behavior in ethylene oligomerization has been investigated, evaluating the effect of the ligand and the experimental parameters on the activity, selectivity, and product distribution.

Role of the ligand and activator in selective Cr–PNP ethene tri- and tetramerization catalysts – a spectroscopic study

The reaction of the ethene tetramerization catalyst ((C₆H₅)₂P)₂N^iPrCrCl₃(THF), and trimerization catalyst ((o-C₆H₄OMe)₂P)₂N^MeCrCl₃, with AlMe₃ and MMAO was investigated using spectroscopy (Cr K-edge XAS, X-band EPR and UV-vis) and catalytic studies.