PENTACOORDINATED MOLECULES .32. STRUCTURAL CHARACTERIZATION OF SPIROPHOSPHORANES EXISTING AS NEARLY PERFECT TRIGONAL BIPYRAMIDS

The single-crystal x-ray characterization of 2,3:7,8-dibenzo-l,4,6-trioxa-9-aza-5λ5-phosphaspiro[4.4]nona-2,7-diene, IV, and 2,3:7,8-dibenzo-9-methyl-l,4,6-trioxa-9-aza-5λ5-phosphaspiro[4.4]nona-2,7-diene, V, revealed relatively undistorted trigonal bipyramids with the unique proton ligand occupying an equatorial site. The structures lie along the coordinate connecting idealized trigonal-bipyramidal and rectangular-pyramidal geometries. They are displaced from the trigonal bipyramid by 5.5 (IV) and 11.2% (V) as measured by dihedral-angle differences from polytopal faces. Data for both compounds were collected by using Mo K radiation on an Enraf-Nonius CAD4 automated diffractometer. IV crystallizes in the orthorhombic space group P212121, with a = 5.729 (2) Å, b = 11.218 (5) Å, c = 17.234 (7) Å, and Z = 4. Full-matrix least-squares refinement gave R = 0.032 and Rw = 0.042 for the 1393 reflections having I ≥ 3σ1, and 2° ≤ 2θMoK± 59°. V crystallizes in the monoclinic space group P21/c, with a = 9.142 (2) Å, b = 16.491 (6) Å, c = 8.133 (2) Å, β = 95.90 (2)°, and Z = 4. Full-matrix least-squares refinement gave R = 0.038 and Rw = 0.048 for the 2062 reflections having I ≥ 3σI and 2° ≤ 2θMoK± ≤ 55°. Examination of the structural data on these and related spirophosphoranes suggests that the small size of the phosphorus-bound proton acts as a principal factor in producing conformations of IV and V little distorted from a trigonal bipyramid. The presence of only one equatorial ring nitrogen atom in IV and V also appears to allow the ideal trigonal bipyramid to be closely approached. © 1979, American Chemical Society. All rights reserved.