Cis- , coordination

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other paHadium-cataly2ed oxidations of olefins ia various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

C) Reactions involving ligands in two adjacent (i.e., cis) coordination positions. They include certain insertion reactions (e.g., the insertion of cyanide into the Co—C bond) and the reverse elimination reactions. 

All ligands and metal complexes were characterized spectroscopically. The 31P-NMR spectra of ligands 1-4 appear in a region of 5 111.6-140.6. Upon metal complexation an upfield shift is observed. Table 1 summarizes the selected spectroscopic properties of the new phosphinite ligands (1-4) and corresponding metal complexes (5-12). With exception of 8 and 12 all complexes have a cis coordination geometry as judged from JP.Pt and the IR spectra (341 cm"1 and 355 cm"1 for 8 and 12, respectively). Due to the steric pressure imposed on the... 

A related N-methylated zinc methoxide (323) also polymerizes cis- and trans- BO with inversion of stereochemistry. In this case, coordination of the monomer trans to the axial alkoxide is hindered by the N-Me group, so a modified mechanism has been proposed based on cis-coordination of the monomer, but which still involves the simultaneous participation of two metal centers. 

Several aluminum biphenolate complexes have been investigated as initiators for the ROP of PO.810,935 Unlike the TPP and salen-based systems, a cis coordination site is realistically accessible and in theory an alternative cis-migratory mechanism to the backside attack pathway might operate. However, NMR analyses on the resultant PPO show that stereochemical inversion still occurs when the biphenolate initiators are used (Scheme 22). It has also been confirmed that the same process occurs with the Union Carbide calcium alkoxide-amide initiator for both PO and CHO.810... 

In the field of porous supramolecular metal complexes, both molecular and extended-solid materials have been extensively studied in recent years. A particularly well-studied class of compounds is the metal-containing molecular squares, that is, square-shaped porous tetrameric structures (30,108). These have been prepared by several approaches, the most common being the reaction of an organic bridging ligand with a metal complex that has available cis-coordination sites (109-113) (Fig. 13). However, the resulting metal centers are usually coordinatively saturated, which makes it difficult for guest molecules to interact directly with the metal atoms. 

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

After formation of an O-coordinated ketyl radical anion and a cis coordinated tyrosin via hydrogen abstraction, a rapid intramolecular one-electron redox reaction occurs with release of the product aldehyde and formation of the fully reduced active site containing a Cu(I) ion, which then reacts with 02 to give H202 and the active enzyme. The above sequence represents Nature s mechanistic blueprint for coordination chemists. 

The general rule earlier proposed by Drent et al., that Pd(II)-diphoshine complexes, better with a bite angle near 90°, capable of maintaining cis-coordination throughout the copolymerisation process, give high molecular... 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

Figure 15.8 a simple example is presented of a subsequent insertion of CO and methanolysis of the palladium acyl intermediate [14], This is not a very common reaction, because both the ligand requirements and the redox conditions for Wacker and carbonylation chemistry are not compatible. For insertion reactions one would use cis coordinating diphosphines or diimines, which makes the palladium centre more electron-rich and thus the nucleophilic attack in the Wacker part of the scheme will be slowed down. In addition, the oxidants present may lead to catalytic oxidation of carbon monoxide. 

On mixed ligated complexes of palladium(II) and platinum(II), the cis coordination of the NHC and a phosphine ligand is thermodynamically favored. i27,261,262 Thermal isomerization was reported for tra i -[(NHC)2Cr(CO)4] (M = Cr, Mo) to the ds-complex. For [(NHC)2Mo(CO)4] this proceeds even in the solid state. A barrier of rotation due to a double-bond character of the metal-NHC bond could not be determined so far. This is in agreement with the single-bond character of the metal-carbon bond. Barriers of rotation determined so far are due to steric hindrance. 

A cis-coordinating ligand is apparently required to bind and activate MeOH so that a methoxy group is transferred to the polyketone chain and a hydride remains on palladium. Two mechanisms are possible for this reaction (i) nucleophilic attack by the oxygen at the acyl carbonyl with concerted formation of Pd-H (ii) formation of a Pd(acyl) (methoxy) complex and H, followed by reductive elimination and subsequent proton attack on a Pd center. No experimental evidence favoring either mechanism in ethene/CO copolymerisation has been provided so far. 

Of importance, these Schiff base metal complexes that contain cis coordination sites allow binding of a substrate (monomer) and the growing polymer chain or can capture a bidentate substrate (growing carbonate polymer chain). Indeed, the use of... 

Fig. 20 Bidentate binding of a carbonate chain end that requires two cis coordination sites...

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