1,1′-(Diselanediylbis{[P,P-diphenyl-N-(trimethylsilyl)phosphorimidoyl]methanylylidene})bis[1,1-diphenyl-N-(trimethylsilyl)-λ5-phosphanamine] pentane disolvate

The title compound, C62H78N4P4Se2Si4·2C5H12, is made up of two [SeC(PPh2NSiMe3)(PPh2NHSiMe3)] units related by an inversion center situated at the mid-point of the diselenide bond. It crystallized with two disordered molecules of pentane used as solvent of crystallization. It is a rare example of an antiperiplanar diselenide and exhibits a long Se—Se bond of 2.4717 (8) Å. The Se—C bond length of 1.876 (5) Å is short in comparison with the range of values found for other diselenides (1.91–1.97 Å). The molecule exhibits two intramolecular N—H⋯N hydrogen bonds. In the crystal, there are no significant intermolecular interactions present. One of the Me3Si– groups is disordered over two positions with a refined occupancy ratio of 0.708 (8):0.292 (8). The contribution of the disordered solvent to the scattering was removed with the SQUEEZE option of PLATON [Spek (2009 ▶). Acta Cryst. D65, 148–155]. The solvent contribution has been included in the reported molecular weight and density.

The title compound, C 62 H 78 N 4 P 4 Se 2 Si 4 Á2C 5 H 12 , is made up of two [SeC(PPh 2 NSiMe 3 )(PPh 2 NHSiMe 3 )] units related by an inversion center situated at the mid-point of the diselenide bond. It crystallized with two disordered molecules of pentane used as solvent of crystallization. It is a rare example of an antiperiplanar diselenide and exhibits a long Se-Se bond of 2.4717 (8) Å . The Se-C bond length of 1.876 (5) Å is short in comparison with the range of values found for other diselenides (1.91-1.97 Å ). The molecule exhibits two intramolecular N-HÁ Á ÁN hydrogen bonds. In the crystal, there are no significant intermolecular interactions present. One of the Me 3 Si-groups is disordered over two positions with a refined occupancy ratio of 0.708 (8):0.292 (8). The contribution of the disordered solvent to the scattering was removed with the SQUEEZE option of PLATON [Spek (2009). Acta Cryst. D65, 148-155]. The solvent contribution has been included in the reported molecular weight and density.

Experimental
In this work we unexpectedly isolated a small amount of the title diselenide as red crystals from the reaction of {Li 2 [C(PPh 2 NSiMe 3 ) 2 ]} with SeCl 4 (1:1 molar ratio) in pentane.
The crystal structure analysis of the title compound revealed a centrosymmetric dimer in which a diselenido (-Se-Se-) unit bridges two monoprotonated units (Fig. 1). The torsion angle C25-Se-Se i -C25 i is 180 °, as a result of the inversion centre, and the two anionic ligands are in a trans orientation with respect to -Se-Se-bridge. The Se-Se bond length of 2.4717 (8) Å is substantially longer than those in typical diaryl diselenides (range 2.29-2.35 Å) [Dickson et al., 1999]. At the same time the Se-C bond of 1.876 (5) Å is short in comparison with the range of values found for other diselenides (1.91-1.97 Å) (Back & Codding, 1983) and the calculated value of 1.91 Å (Pyykkö & Atsumi, 2009). This bonding arrangement is reminiscent of the alternation of the S-S bond lengths in cycloheptasulfur, which has been rationalized in terms of p lone pair repulsions of the neighbouring sulfur atoms due to the torsion angle of 0° and hyperconjugation (Steudel et al., 1980). We also note that the Se-Se bond lengths of 2.492 (2) Å found for the dication [(Ph 3 P) 2 CSe-SeC(PPh 3 ) 2 ] 2+ (Schmidbaur et al., 1983) and 2.508 (1) Å exhibited by the dilithium complex {[Li(TMEDA)] 2 [(SPh 2 P) 2 CSe-SeC(PPh 2 S) 2 ]} (Konu et al., 2010) are comparable to that of the title compound. In the latter case, the Se-Se bonding interaction is due solely to the poor overlap of the SOMOs of the anion radicals.
Consequently the calculated binding energy is small (90 kJ mol -1 ) compared to typical values for organic diselenides, e.g.  (Konu et al., 2010) is also comparable with that in the title compound.
The protonation of one of the nitrogen atoms, N1, is evident from the disparity of ca 0.07 Å in the P-N bond lengths, and smaller differences in the Si-N and P-C(Se) distances of 0.038 Å and 0.036 Å, respectively. Intramolecular hydrogen bonding between the N-H functionality and two-coordinate nitrogen atom (1.981 (15) Å) is observed ( Table   1). The geometry at the three-coordinate carbon atom is almost planar (Σ<C(25) = 356.1°).
In the crystal, there are no significant intermolecular interactions present.
at 195 K. The solution was then allowed to reach room temperature and stirred for a further 8 h after which it was filtered, to remove LiCl, and gave an orange solution. The 31 P NMR spectrum of the filtrate (400 MHz, pentane, 298 K) revealed a complex mixture of products with resonances at δ 4.99 (s), 1.14(s), 3.87(s), 5.40 (d, J = 5.0 Hz), 7.83(d, J = 5.0 Hz), 10.23(s), and 18.51(s). The filtrate was allowed to evaporate slowly at room temperature under an argon atmosphere to produce a few red crystals of the title compound after 45 days. Further details of the synthetic procedure are available in the archived CIF.

Refinement
The hydrogen atom bound to atom N2 was located in the difference Fourier map and refined with U iso (H) = 0.05 Å 2 . All C-bound H atoms were placed in idealized positions and treated as riding atoms: C-H = 0.95 and 0.98 Å for CH and CH 3 H atoms, respectively, with U iso (H) = 1.5U eq (C-methyl) and = 1.2U eq (C) for other H atoms. The Me 3 Si group involving Si2 is disordered with two alternative orientations, atoms C30/C31:C32/C33 with a refined occupany ratio of 0.708 (8)  A view of the title molecule, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular N-H···N hydrogen bonds are shown as dashed lines (see Table 1 for details; only the major components of the disordered methyl groups on Si2 are shown; the C-bound H atoms have been omitted for clarity).

Special details
Experimental. Some details of the procedure for the synthesis of the title compound: Solvents were dried over and distilled from Na/benzophenone (toluene). t-BuLi (1.7 M in pentane), CH 2 (PPh 2 ) 2 , and azidotrimethylsilane were purchased from Aldrich Chemical Co. and were used without further purification. 31 P NMR spectra were recorded on Bruker 400 NMR spectrometers. 31 P NMR chemical shifts were referenced externally to 85% H 3 PO 4 (0 p.p.m.). Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.