1,1,1-Tris(dimethylamino)-2-[tris(dimethylamino)phosphoranylidene]diphosphinium tetraphenylborate tetrahydrofuran monosolvate

In the tetrahydrofuran solvate of the title salt, C12H36N6P3 +·C24H20B−·C4H8O, the cation features short P—P bond lengths [2.1111 (11) and 2.1364 (10) Å] and a distinctly bent P—P—P angle [104.67 (4)°] that confirm that the molecule is not allene-like. In the crystal, the solvent molecule is linked to the cation via a weak C—H⋯O hydrogen bond.

In the tetrahydrofuran solvate of the title salt, C 12 H 36 N 6 P 3 + ÁC 24 H 20 B À ÁC 4 H 8 O, the cation features short P-P bond lengths [2.1111 (11) and 2.1364 (10) Å ] and a distinctly bent P-P-P angle [104.67 (4) ] that confirm that the molecule is not allene-like. In the crystal, the solvent molecule is linked to the cation via a weak C-HÁ Á ÁO hydrogen bond.

Comment
The triphosphenium salt [P(P[NMe 3 ]) 2 ][BPh 4 ], which was first reported by Schmidpeter & Lochschmidt (1986), has been used as a reagent and a source of "P + " for decades but no structure of any salt containing the cation has ever been reported. As part of our ongoing investigations of low valent group 15 compounds, we were able to obtain crystals of the tetrahydrofuran solvate of the salt (1) from the slow evaporation of a THF solution of the salt.
The molecular structure of (1) is shown in Fig. 1. The P-P distances in the cation in (1) of 2.1111 (11) Å and 2.1364 (10) Å are significantly shorter than typical P-P single bonds ca 2.24 (2) Å that have been reported in the Cambridge Structural Database (as determined from the 14 examples of diorganodiphosphines that are found in CSD Version 5.35) (Allen, 2002) but are consistent with those reported for salts of other triphosphenium cations (Ellis & Macdonald, 2007). More importantly, the P-P-P angle of 104.67 (4)° clearly indicates that the geometry about the dicoordinate phosphorus atom in (1) is best-described as being bent and thus resembles that of the only other structurally characterized triphosphenium (Ellis & Macdonald, 2006). The bent geometry in (1) is in stark contrast to the perfectly linear allene-like P-C-P arangement in the analogous carbodiphosphorane C(P[NMe 3 ]) 2 (Appel et al., 1983).
The metrical parameters of the tetraphenylborate anion and the THF solvent of crystallization are unexceptional and there are no unusually short cation-anion contacts. Details of a weak hydrogen bond between the cation and the THF oxygen atom (1) is listed in Table 1.

Experimental
The salt (1) was synthesized using the method described by Schmidpeter and Lochschmidt (1986). Suitable crystals were obtained by the slow evaporation of a tetrahydrofuran solution of the salt in a nitrogen-filled glove box. The crystal used for data collection was coated in mineral oil, mounted and placed in the cold stream on the diffractometer.

Refinement
All non-H atoms were refined anisotropically and H atoms were initially located in the difference Fourier maps. The H atoms were subsequently modeled as riding atoms with a C-H distance of 0.98 Å and U(H) of 1.5 times that of the carbon atom to which they are attached for each methyl hydrogen atom; each rigid methyl group was allowed to rotate in order to maximize the sum of electron density at the calculated H atom positions. The H atoms on the phenyl groups were modeled with a C-H distance of 0.95 Å and U(H) of 1.2 times that of the carbon atom to which they are attached and each methylene hydrogen atom was modeled with a C-H distance of 0.99 Å and U(H) of 1.2 times that of the carbon atom to which they are attached.