Palladium structure models

Metal-n-allyl complexes are important in a number of stereoselective catalytic reactions and are therefore attractive for computational chemists (see also Chapter 13, Section 13.2). An empirical force field study based on the MM2 parameterization scheme aimed at predicting stereoselective nickel(0)-catalyzed cycloadditions was recently conducted 56. As in a similar study 57, where a force field for the structure optimization of palladium-allyl systems was developed, dummy atoms were needed to define the structural model. Based on the assumptions required to model a catalytic process, the results obtained have to be interpreted with caution. 

While the formation of the palladium species was shown to be fast (and in better than 90% yield), the kinetically less labile platinum(II) ion initially gave a mixture of oligomers however, these kinetic products slowly rearranged (over a month) to the required thermodynamic product 9. Models predict that free rotation of the 4,4 -bipyridine units is restricted such that they will prefer to be orientated perpendicular to the plane of the square such an arrangement has been confirmed for the above tetra-palladium structure. ... 

G.D. Waddill and L.L. Kesmodel. Benzene Chemisorption on Palladium Surfaces. I. High Resolution Electron Energy Loss Vibrational Spectra and Structural Models. Phys. Rev. 5 31 4940 (1985). 

The screened proton model of nickel or palladium hydrides and Switendick s concept of the electronic structure do not constitute a single approach sufficient to explain the observed facts. In this review, however, such a model will be used as the basis for further discussions. It allows for the explanation and general interpretation of the observed change of catalytic activity of the metals, when transformed into their respective hydrides. 

A key feature of this study was the structural information available on the model palladium nanoparticle catalyst. The mean particle size is 5.5 nm, containing on average 3000 atoms the majority of the particles are well formed with a (111) orientation and terminated by (111) facets with only a small fraction of (100) facets exposed. 

A few additional points have also been raised by specific surface-science work concerning the catalytic reduction of NO. For instance, it has been widely recognized that the reaction is sensitive to the structure of the catalytic surface. It was determined that rough surfaces such as (110), or even (100), planes enhance NO dissociation over flatter (111) surfaces, and also favor N2 desorption instead of N20 production. On the other hand, NO dissociation leads to poisoning by the resulting atomic species, hence the faster reaction rates seen with medium-size vs. larger particles on model rhodium supported catalyst (the opposite appears to be true on palladium). Also, at least in the case of palladium, the formation of an isocyanate (-NCO) intermediate was identified... 

The cyclooctapyrroles shown in Figure 55 appear predestined to form binuclear metal complexes since the loop-shaped conformation of these macrocycles exhibits two structurally identical, helical N4 cavities. Enantiomers of such complexes, which are presumably generally very stable towards racemization owing to the rigidity of the molecule imposed by the incorporation of the metal, are of interest as possible models for binuclear metalloenzymes and as potential catalysts in asymmetric synthesis. The first two ligands as well as their recently obtained palladium complexes601 were... 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

Additionally to nickel ICC, chelates of the types 467, 468, 472, and 473, containing either cobalt, iron, copper, palladium, zinc, cadmium, or mercury, have also been obtained [269,270,804]. Special interest amongst them is provoked by copper ICC containing an azomethinic N,S-ligand environment. These compounds are one of the several biomimetic models of nonporphyrine metal proteins [448,804,834]. The majority of these ICC have trimetallocyclic structures of the types shown by struc-... 

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