Host-guest chemistry of 1,2-bis(chloromercurio)tetrafluorobenzene: Chelation of the carbonyl oxygen atom of acetone by a bidentate Lewis acid

Incremental addition of DMF or DMSO to a solution of 1,2-bis(chloromercurio)tetrafluorobenzene (3) in acetone results in a change of the Hg-199 NMR chemical shift of 3. The titration curves thus obtained can be fitted and indicate the formation of the 1:1 host-guest complexes 3 .mu(2)-DMF (4) and 3 .mu(2)-DMSO (5). The stability constants of 4 and 5 have been determined and are respectively equal to 1.8 (+/-0.2) M-1 and 8.0 (+/-0.8) M-1. The X-ray crystal structures of 4 and 3 .mu(2)-acetone (6) have been determined. In the structure of 4, the carbonyl oxygen atom is coordinated to three mercury atoms and has a distorted-tetrahedral geometry. In 6 however, the acetone oxygen is coordinated to only two mercury atoms and exhibits a trigonal-planar geometry. While it is difficult to detect any metrical changes undergone by the organic substrates upon chelation, the IR carbonyl stretching frequency of 4 (v(CO) 1635 cm(-1)) and 6 (v(CO) 1639 cm(-1)) indicates that the carbon oxygen bond is weakened. Accordingly, the C-13 CP/MAS NMR carbonyl carbon chemical shifts of 4 (167 ppm) and 6 (215 ppm) provide evidence for a substantial polarization of the carbonyl. By comparison of the C-13 CP/MAS NMR carbonyl carbon chemical shift, the Lewis acidity of 3 toward acetone can be evaluated to be just inferior to that of MgCl2. Finally, both 4 and 6 have complex and highly networked supramolecular structures which result from intermolecular Hg ... Cl and Hg ... F or Hg ... O contacts.