Steric parameters ligands

Fio. 9. Detenninatiun of two steric parameters (Ai and Ah) associated with carbene ligands in Cp Ru(L)Cl complexes. 

The nucleophilic carbcnes are phosphine-mimics but they are much more. They reside at the upper end of the Tolman electronic and steric parameter scales. Much remains to be explored with these ligands. With a rudimentary understanding of ligand stereoelectronic properties, we feel confident much exciting chemistry remains to be explored. 

Many attempts have been undertaken to define a reliable steric parameter complementary to the electronic parameter. Most often Tolman s parameter 0 (theta) is used. Tolman proposed to measure the steric bulk of a phosphine ligand from CPK models in the following way. From the metal centre, located at a distance of 2.28 A from the phosphorus atom in the appropriate direction, a cone is constructed which embraces all the atoms of the substituents on the phosphorus atom (see Figure 1.6). 

The formation of inclusion complexes is influenced mainly by steric parameters of ligands (geometry and size) and by the possibility of creating hydrophobic interactions and hydrogen bonds (physicochemical properties). Hydrophobic interactions predominate in the cavity, and they can act in concert with polar interactions that occur with hydroxyl groups located at the outer lip of the CDs. There are no covalent bonds (14). 

Scheme 3.5-2. Ligand property-specific control as a function of the electronic (xj) and steric (0j) ligand parameters for the system COD2Ni/P-ligands/butadiene (1.0 1.0 170),...

The known crystal structures of these complexes show no undue strain upon the geometry inflicted by the tether length. Apparently, an ethyl bridge between the indenyl (flu-orenyl) part of the ligand and the NHC unit is sufficiently long. In a comparative study between a tethered [135] and an untethered [194] nickel(II) complex (see Figure 4.64), no significant differences in the steric parameters were reported. However, a downfield shift of A8 = 4.1 ppm (from 166.8 to 170.9 ppm) in the C-NMR spectrum for the tethered complex was considered by the authors to be due to a chelate effect [135]. 

Figure 11 Sphere dimensions for steric parameter determination (%Vbut) of NHC ligands...

Table 14 Steric parameters associated with selected NHC and sterically demanding PR3 ligands...

The quantitation of steric effects is complex at best and challenging in all other situations, particularly at the molecular level. An added level of confusion comes into play when attempts are made to delineate size and shape. Nevertheless, sterics are of overwhelming importance in ligand-receptor interactions as well as in transport phenomena in cellular systems. The first steric parameter to be quantified and used in QSAR studies was Taft s constant (157). is defined as... 

For this reason, a huge number of steric parameters were defined and used from the beginning to represent the steric properties of a molecule. Steric properties influence molecule energy, reaction and conformational paths, reaction rates and equilibria, binding affinity between ligand and receptor and other thermodynamic properties. 

Basic trialkyl phosphines are best suited as ligands for the palladium-catalyzed cyclotrimerization of 1 and CO2, and a strong influence of the ligand structure on the performance of the catalyst is observed. As noted earlier [4 c], the Tolman concept [10 a] of electronic (T ) and steric (0) parameters is obviously not sufficient to explain the observed ligand effects. The steric parameter Er, recently developed for phosphine ligands on the basis of molecular mechanics [11], also failed to show any correlation with the experimental results. The understanding of these... 

To successfully correlate thermodynamic or kinetic parameters with stereo-electronic effects, there must be a clean separation of the electronic from the steric effects (Eq. 1). Since there is no necessary reason for an experimentally based steric parameter to be free of electronic effects, workers have turned to applications of computational chemistry to achieve a quantitative measure of pure steric influence of a ligand. 

Figure 3 Percent buried volume (%VgJ, steric parameter of some common NHC ligands computed from the DFT-optimized geometries of [(NHC)lr(CO)fl] complexes in which d=2. iO A and R = 3.5 For comparison purposes, the values of two...

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