Tolman cone angle ligands

Unfortunately, for all these reasons the conclusions cannot be applied quantitatively for description of the pH effects in the RCH-RP process. There are gross differences between the parameters of the measurements in [97] and those of the industrial process (temperature, partial pressure of H2, absence or presence of CO), furthermore the industrial catalyst is preformed from rhodium acetate rather than chloride. Although there is no big difference in the steric bulk of TPPTS and TPPMS [98], at least not on the basis of their respective Tolman cone angles, noticable differences in the thermodynamic stability of their complexes may still arise from the slight alterations in steric and electronic parameters of these two ligands being unequally sulfonated. Nevertheless, the laws of thermodynamics should be obeyed and equilibria like (4.2) should contribute to the pH-effects in the industrial process, too. 

It is worth mentioning that Muller and Vogt have recently reintroduced the A, A concept for phosphinine ligands that have similar steric characteristics to NHC [119]. They call the two different angles the occupancy angles a and p, but the definitions are almost identical and they point out that the arithmetic average of a and p is very close to the Tolman cone angle 0 for tertiary phosphanes. 

Figure 15 Schematic depiction of ligand cone angle for a generic phosphine ligand and representative Tolman cone angle values...

A series of monocarbonyl complexes were tested with phenyl groups on the diamine and various substituents on the phosphorus atoms (complexes 64-72). The electronic effects of the ligand can be quantified by use of the carbonyl stretching wavenumber of the complex while the steric effects can be expressed by the Tolman cone angle for groups around the phosphorus atoms. Figure 8 shows a plot of these two parameters versus the TOF for the ATH of acetophenone in isopropanol at 30 °C catalyzed by complexes 64-72 (0.02 mol%) which have been activated by KOtBu (0.16 mol%). 

Figure 1 Illustration of the definition of the Tolman cone angle for a typical aryl phosphine ligand...

Table 23.2 Tolman cone angles for selected phosphine and phosphite ligands.

TABLE 18.4 Tolman cone angles 0) for selected phosphine ligands. 

A recurrent problem in this area and for other substitution reactions is the separation of the electronic and steric effects of the nonreacting ligands. In organometallic systems, the steric effect is usually measured by the Tolman cone angle, 0, but the electronic effect is a mixture of the a-donor and n-acceptor abilities of the nonreacting ligands. These features have been discussed previously in Section 3.5.6. 

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