Axial Site Occupancy by the Least Electronegative Ligands in Trigonal Bipyramidal Tetraoxyphosphoranes1

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Abstract
Analogous to the formation of CH2[(t-Bu)2C6H2O]2P(Ph)(O2C6Cl4) (1), the new bicyclic tetraoxyphosphoranes CH2[(t-Bu)2C6H2O]2P(Et)(O2C6Cl4) (3) and CH2[ClC6H3O]2P(Ph)(O2C6Cl4) (4) were synthesized by the oxidative addition of the appropriate cyclic phosphines with o-tetrachlorobenzoquinone. For the formation of CH2[(t-Bu)2C6H2O]2P(Ph)(O2C2Ph2) (2), a similar reaction was followed with the use of benzil (PhCOCOPh) in place of o-tetrachlorobenzoquinone. X-ray analysis of 1 − 3 revealed trigonal bipyramidal geometries and provided evidence for the first series of complexes in the absence of ring strain in which the least electronegative group, ethyl or phenyl, is located in an axial position, in violation of the electronegativity rule. Thus, the two oxygen-containing ring systems occupied two different sets of positions in the trigonal bipyramid (TBP) with the eight-membered rings at diequatorial sites. X-ray analysis of 4 revealed a trigonal bipyramidal geometry with electron-withdrawing chlorine substituents on each ring assumed the more conventional geometry with the rings occupying axial−equatorial positions and the phenyl group located in the remaining equatorial site. The fact that molecular mechanics calculations favorably reproduced the observed geometries suggests that a steric contribution associated with the ring tert-butyl groups for 1 − 3 is partly responsible in favoring diequatorial ring occupancy for the eight-membered ring. NMR data supported rigid pentacoordinated structures in solution at 23 °C. Phosphorane 1 crystallizes in the orthorhombic space group Fdd2 with a = 44.787(5) Å, b = 34.648(8) Å, c = 10.3709(9) Å, and Z = 16. Phosphorane 2 crystallizes in the orthorhombic space group Pna21 with a = 20.658(8) Å, b = 10.342(2) Å, c = 19.879(6) Å, and Z = 4. Phosphorane 3 crystallizes in the orthorhombic space group Pcmn with a = 9.807(2) Å, b = 16.632(4) Å, c = 23.355(3) Å, and Z = 4. Phosphorane 4 crystallizes in the monoclinic space group C2/c with a = 35.699(5) Å, b = 12.187(2) Å, c = 14.284(3) Å, β = 107.08(1)°, and Z = 8. The final conventional unweighted residuals are 0.0395 (1), 0.0518 (2), 0.0540 (3), and 0.0868 (4).