Tethered complexes

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

Fig. 16.4 Some examples of tethered complexes on supported metals (as described by Angelici).

Figure 2.12 MS data of tethered complexes, (a) Tethering experiment between TS and 10 disulfides equilibrated for 1 hour, (b) tethering with varying concentrations of 2-mercaptoethanol, and (c) tethering with varying pool size of disulfides. Reproduced from Reference 31 with permission of the National Academy of Sciences, USA. Copyright (2000) National Academy of Sciences, U.S.A.

Tethered arene complexes of ruthenium(II) are also formed readily when the conformational freedom of the tether is restricted, for which purpose the biphenyl unit has proved ideal. Thus, (2-dicyclohexylphosphino)biphenyl and its derivatives react with [RuCl2(r 6-C6H6)]2 in DMF at 100 °C to give directly the tethered complexes 67-69 in yields of 31-96% without detection or isolation of the presumed P-coordinated intermediates.65... 

The complexed arene rings in tethered complexes of ruthenium(II) are close to planar, though the ipso-carbon atom is often pulled slightly towards the metal atom. In the phosphine complexes, the Ru-C(arene) distances trans to the P-donor (2.22-2.29 A) are significantly greater than those trans to the Ru-Cl bonds (2.15-2.25 A). This feature is also evident in non-tethered complexes of the type [RuCl2(r 6-arene)(PR3)] and can be attributed to the higher trans-influence of PR3 relative to that of Cl-.88... 

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]. 

---