Structure of Cr II Sites

At this point, we can schematically represent the structure of Cr(II) sites as (SiO)2Cr L , where L represents a weak ligand (oxygen of a SiOSi bridge) and n is a not fully known figure which increases upon activation at high temperature. The adsorption of CO at room temperature on grafted Cr(II) sites... 

The examination of the vco bands in the 2200-2179 cm region at room temperature reveals that Cr(II) sites are distributed in two basic structural configurations, namely CrA and Cre. These results confirm fhe view illus-frafed before concerning the structural complexity of the Cr(ll) system. CrA sites seem to correspond to the first family of chromates represented in Scheme 4, while Cre sites correspond to a family characterized by a larger aocro bond angle. It is important to underline here that, when we speak about CrA and Cre sites, we are referring to two families of structures instead of simply to two different well-defined sites. 

Espelid and B0rve [100] have recently explored the structure, stabihty, and vibrational properties of carbonyls formed at low-valent chromium boimd to sibca by means of simple cluster models and density fimctional theory (DFT) [101]. These models, although reasonable, do not take into consideration the structural situations discussed before but they are a useful basis for discussion. They foimd that the pseudo-tetrahedral mononuclear Cr(II) site is characterized by the highest coordination energy toward CO. 

A T structure with the strongest ct-donor D trans to the empty site (I in Scheme 1) is preferred in the case of three pure cr-donor ligands. The presence of a ir-acceptor ligand also makes the T structure more stable. When one of the ligands is a tt-donor, X, a Y structure of type II (Scheme 1) is observed. This structure permits the formation of a w bond between the empty metal d orbital and the lone pair of X. No such tt bond is present in the T structure since all symmetry adapted d orbitals are filled. This partial M—X multiple bond stabilizes Y over T. 

The aim of this Section is to discuss the experimental methods, problems, and recent improvements in the determination of the exact nature of the precursor species Yi, Y2, Y3, etc. (i.e., the determination of the initiation mechanism for the ethene polymerization). Facing this topic, we must be aware that, besides the problems related to the determination of the Cr(II) structure (vide supra Section VI.A.2), the identification of the species formed during the initial stages of the reaction has been prevented so far for two other reasons (a) only a fraction of the Cr(II) sites are active in the polymerization under the usually adopted conditions (225), so that almost all the characterization techniques give information about the inactive majority Cr sites and (b) the active sites are characterized by a very high polymerization rate (high turnover frequency, TOF). It is thus clear that any experimental efforts devoted to the detection of the precursor and/or intermediate species must solve these two problems (vide infra Section VI.C). 

Rhus vernicifera stellacyanin is a blue copper glycoprotein that reacts rapidly with many inorganic redox agents (37, 117, 169). In the chromous reduction of the protein (37), ET is believed to involve binding of Cr(II) to y-COi of Asp-49 and e-NH3 of Lys-50 the Cr(II) to Cu(II) ET would then take place over a Cr-Cu distance of —11 A. Since a model of the structure of stellacyanin indicates that there are no other negative residues near Asp-49, it can be concluded that the labeling is primarily due to favorable ET reactivity at the 49-50 site (190). 

The speculated presence of dicarbonyl and monocarbonyl species on the silica surface was further confirmed by ONIOM calculations. The model cut from the (100) face of p-cristobalite was applied to mimic the local structures of the silica surface. Two different molecular models with replaceable and irreplaceable silox-ane ligand were built for the dicarbonyl and monocarbonyl species, respectively, as shown in Fig. 18. The calculated relative shifting for the symmetric and asymmetric CO stretching was 11 cm, very close to the experimental value of 12 cm, which revealed information on the local coordination environment of the Cr(II) site (see structures 5e and fie in Scheme 11). 

Zhong L, Lee MY, Liu Z, et al Spectroscopic and structural characterization of Cr(II)/Si02 active site precursors in model Phillips polymerization catalysts, J Catal 293 1—12, 2012a. 

We mentioned earlier (see Section V,C,1) that for the two-dimensional compounds of formula (catliMnCrCoxlg], with cat+ standing for a monovalent cation, each metal site of a given chirality (A or A) is surrounded by three metal sites of the opposite chirality (A or A), so that within a layer all the Mn(II) sites have the same chirality and all the Cr(III) sites have the other chirality. If the Mn(II) and Cr(III) sites had the same chirality, the hexagons of the honeycomb structure could no longer be closed, and the structure would be three- instead of two-dimensional. This structure as a whole would obviously be chiral 86). 

This increased efficiency probably derives from two causes. First, as noted above, Cr(II)/silica itself is considerably more efficient than Cr(VI)/silica. (One might say that the copolymerization, which always favors ethylene, is less selective for ethylene.) Second, the sites producing ot-olefin are in close proximity to the sites consuming 1-hexene within the catalyst/polymer pore structure. Therefore, the consuming sites probably experience a higher local concentration of comonomer than exists in the reactor as a whole, and consequently more monomer is incorporated. 

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