Bis-chelates with the Isopropylideneimino Donor

Comparison of 29Si NMR Chemical Shifts (CDC13, 300 K) and Selected Bond Lengths 

The fact that in 64a the monodentate ligands are cis to each other, as has been found in numerous other hexacoordinate bis-acylimidato-7V,(9 chelates 30-38,43-50 suggests that in the absence of special constraints the cis orientation is preferred. It follows that the uncommon trans geometry, found in 65 and 66, is the result of steric repulsion between the bulkier Si-Cl and Si-Br bonds and the isopropylidene methyl group. All three dihalo-complexes (the cis-64 and trans-65 and 66) react readily with 

TMSOTf, exchanging their halogeno ligands by triflate (Eq. 26), to yield the same trans-ditriflate chelates 67a,b (Fig. 33). 

The trans geometry is confirmed in solution for 65b-67b by the appearance of a singlet for the methylene protons in the NMR spectra due to C2h symmetry of the trans chelates. 

The data in Table XX permit an assessment of the donor strength of the new isopropylideneimino ligand.20,52 Comparison of the bond lengths between the two chelate families (Table XX) clearly indicates stronger 

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