Molecular-Level Insight into Cr/Silica Phillips-Type Catalysts: Polymerization-Active Dinuclear Chromium Sites

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates

“…This cluster model was designed to account for the heterogeneity of the amorphous silica surface and the presence of mainly large (Si-O) n rings in the SiO network on silica (n > 6) (36, 37): it presents a Cr(III) bonded to three siloxide oxygen atoms of a siloxane network making a cage which exposes two types of faces containing the Cr atom, one face with six-membered ring (Cr-O)(Si-O) 2 and two with eight-membered ring (Cr-O)(Si-O) 3 cycles. The silicon atoms of the clusters were saturated with fluorine atoms, which are good models for silica surfaces (14,34,38,39). The calculated Cr-O bond distances in 3a M are ∼1.80 Å, similar to the Cr-O distances in 1 and somewhat shorter than EXAFS distances in 3.…”

An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium‐Based Catalysts: Binuclear Metallacycles as a Reason for 1‐Octene Selectivity?

“…[35] This postulated mechanism is supported by kinetic investigations [36] as well as density functional theory (DFT) studies (Scheme 4). [29] The conceivability of the concept of a binuclear center for selective ethylene oligomerization was supported by a synthesized complex consisting of two chromium centers bridged by a 1,4-butandiyl unit by Theopold et al [37] Although the proposed catalytic cycle for selective oligomerization (Scheme 5) has not been realized so far, this example shows that two linked chromium atoms are able to build up a metallacycle.…”

“…[21][22][23] If tetramerization is not explainable by the monometallic mechanism, alternatively a bimetallic mechanism based on two chromium centers, as discussed in the following section, appears conceivable. [28,29] Already Kçhn stated in his highlight: "Although a monometallic mechanism is generally assumed, analogous bimetallic mechanisms based on two chromium(II) sites can also be formulated." [28] Scheme 2.…”

Reactivity of Chromium Complexes under Spin Control

“…[8] Although a monometallic mechanism is generally assumed, analogous bimetallic mechanisms based on two chromium(II) sites can also be formulated. [9] Attempts have been made to determine the oxidation states during catalysis by experimental and computational studies. Magnetic-moment measurements using the Evans method for different catalysts were consistent with the oxidation states II, III, or IV in the resting state, although large errors associated with this method warrant some caution.…”