Molecular structure of cycloborataphosphonianes. Part II. Crystal structure of 1,1,3,3,5,5-hexaphenylcyclotriborataphosphoniane [(Ph2P)BH2]3

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Abstract
The title compound gives two crystalline modifications: α, m.p. 161 °C, triclinic, a= 13·96, b= 10·27, c= 12·54 Å(all ± 0·04 Å), α= 108·2, β= 96·8, γ= 97·7°(all ± 0·2°), space group P, Z= 2; and β, m.p. 179 °C, monoclinic, a= 20·23 ± 0·06, b= 9·71 ± 0·02, c= 18·65 ± 0·09 Å, β= 113·9°± 0·4°, space group Ic or I2/c, Z= 4. The structure of the α-form was determined by the heavy-atom technique from X-ray diffractometer intensity data and refined by least-squares methods to R 0·075 for 5940 independent reflexions. The six-membered boron-phosphorus ring has the chair conformation with a close approach to symmetry Cs(m), the torsion angles in the ring lying in the range 40–55°. Mean bond lengths are: B–P 1·948, B–H 1·11, P–C 1·823 Å; and bond angles: B–P–B 114·3, P–B–P 112·6, H–B–H 112, and C–P–C 103·1°. The molecular dimensions differ only slightly from those of [(Me2P)BH2]3 and [(Me2N)BH2]3; they provide no evidence for B→P π bonding and are compatible with a simple cyclic structure based on P→B σ bonds.

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