ANIONIC FIVE-COORDINATED CYCLIC ORGANOFLUOROSILICATES VARYING IN RING SIZE FROM FIVE- TO SEVEN-MEMBERED

Abstract

Synthesis of the potassium crown ether complexes (K, 18-c-6) 3 and 4 of the anionic five-coordinated spirocyclic fluorosilicates from the precursor fluorosilanes has provided additional members of this new class of silicon compounds. Silicates 3 and 4 are monofluoro catecholate derivatives [(CH₂) _n SiF(O₂C₆H₄)]⁻, containing the five-membered tetramethylene (n = 4) and six-membered pentamethylene (n = 5) ring system, respectively. X-ray analyses of these fluorosilicates are reported along with that of the previously synthesized related monocyclic anionic fluorosilicates [BiPhSiF₃][K, 18-c-6] (1) and [BiBzSiF₃][K, 18-c-6] (2). The first structural characterization of a pentacoordinated silicon compound containing a seven-membered ring is provided by 2. The study also yields the first structure of a fluorosilicate containing a six-membered ring (4). The geometries are all trigonal bipyramids. Silicates 2–4 have carbon-containing rings positioned diequatorially, whereas the trifluorosilicate 1, with ligands directly attached to silicon similar to that for 2, has its five-membered biphenyl ring placed in axial-equatorial sites. Ring-strain relief is cited as the principal factor accounting for the structure of 1 compared to location of the more easily accommodated seven-membered ring diequatorially for 2. Ring-strain calculations lead to an estimate of 8 kcal/mol for the stabilization of 1 relative to location of the ring in diequatorial positions. Agreement with an approximate value of the activation energy of 9 kcal/mol from variable-temperature ¹⁹F NMR spectra for ligand exchange in solution is achieved. The biphenyl silicate 1 crystallizes in the monoclinic space group P2₁/n with a = 9.004 (2) Å, b = 24.125 (8) Å, c = 12.286 (2) Å, β = 95.21 (2); and Z = 4. The bibenzyl silicate 2 crystallizes in the monoclinic space group P2₁/c with a = 12.588 (3) Å, b = 16.935 (3) Å, c = 14.374 (4) Å, β = 110.69 (2); and Z = 4. The spirocyclic fluorosilicate 3 crystallizes in the monoclinic space group P2₁/n with a = 9.692 (2) Å, b = 13.899 (3) Å, c = 19.908 (4) Å, β = 102.55 (2); and Z = 4. The spirocyclic fluorosilicate 4 crystallizes in the monoclinic space group P2₁/c with a = 10.082 (2) Å, b = 13.676 (3) Å, c = 19.908 (4) Å, β = 102.55 (2); and Z = 4. The final conventional unweighted residuals are 0.055 (1), 0.042 (2), 0.041 (3), and 0.039 (4).