Use of Acidic Solvents to Minimize Catalyst Inhibition by Ground State Destabilization

2 Use of Acidic Solvents to Minimize Catalyst Inhibition by Ground State Destabilization 

One approach that could be considered is to carry out the reaction in Uquid as solvent. Under these conditions essentially aU the methanol formed should complex to the Lewis acid and free methanol would be unavailable for coordination to the metal, minimizing ground state stabihzation and catalyst inhibition. The key issue with this strategy is that B(C6Fs)3 is expensive and unstable and most likely it would not be cost effective to attempt to separate the MeOH B(C6F5)3 complex and recycle the B(C6p5)3. However, if the Lewis acid utilized is inexpensive, thermally robust and the methanol Lewis acid adduct formation can be made inexpensively reversible this could be potentially useful strategy. 

Weak coordinating 1 igands tbai can he displaced by msfiane 

Calculations show [10] that replacement of the HSO4 Ugand by methane in the Pt(bpym)Cl2/H2S04 system is uphill by 24 kcal/mol with a barrier of -33 kcal/mol (Fig. 7.22). This is comparable to the -28-30 kcal/mol barrier obtained experimentally for the CH activation step by carrying out the reaction in D2SO4. Theoretical and experimental studies show that the overall activation energy for CH activation with the Pt(bpym)Cl2/H2S04 system results from the two key 

The basis for this large difference in rate can be explained by calculations [10] (Fig. 7.22), that show that the water complex, [(Hbpym)PtCl (H20)] is -9 kcal/ mol more stable than the [(Hbpym)PtCl(HS04)] complex. Consistent with the expected dependence on solvent acidity, as can be seen from Fig. 7.25, the drop off in rate below 85% sulfuric acid solvent correlates well with the solvent acidity. As shown schematically in Fig. 7.26, this decrease in rate is consistent with a kinetic model that show that the added ground state stabilization in the presence of water (-9 kcal/mol) would be expected to decrease the TOF by a preequilibrium factor of -10 .  

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