Tris(tert-butyl)phosphine selenide, the missing link in tris(tert-butyl)­phosphine chalcogenide structures ^tBu₃P=X (X = O, S, Se, Te)

In tris(tert-butyl)­phosphine selenide, C₁₂H₂₇PSe, all the methyl ligands are disordered over two sites in the ratio 70/30. The mol­ecule displays crystallographic C₃ symmetry. The bond angles at the P atom are distorted tetrahedral [C-P-C 110.02 (5)° and Se=P-C 108.91 (5)°]. The P-C and P=Se bond lengths are 1.908 (1) and 2.1326 (6) Å, respectively. A comparison of the structural data of the complete series of tris(tert-butyl)­phosphine chalcogenides (^tBu₃PO, ^tBu₃PS, ^tBu₃PSe and ^tBu₃PTe) with the corresponding data of other phosphine chalcogenides substituted by smaller organic groups shows the great influence of the bulky tert-butyl ligands.

S1. Comment

Phosphine chalcogenides are well known compounds and have been fully studied (Patai & Hartley, 1992). Here we report the structure of tris(tert-butyl)phosphine selenide, (I). Together with the recently published molecular structure of ^tBu₃PS (Steinberger et al., 2001), the series of X-ray structures of tris(tert-butyl)phosphine chalcogenides has now been completed. The previously reported data are summarized in a review by Gilheany (1992). \sch

Compound (I) was first prepared from tris(tert-butyl)phosphine and Se in the late seventies (DuMont et al., 1976; Philip & Polenski, 1980), but the molecular structure was not determined. In order to compare the structural differences, the characteristic data for (I), together with the corresponding values for ^tBu₃PO (Rankin et al., 1985), ^tBu₃PS (Steinberger et al., 2001), ^tBu₃PTe (Kuhn et al., 1987) and the starting material, ^tBu₃P (Krüger & Bruckmann, 1995), are listed in Table 1.

The crystal structure of (I) is disordered. However, this disorder only involves the methyl groups, and in the structure refinement a 'split model' for these atoms appeared to handle the problem adequately. Similar disorder was observed for the isomorphous sulfur derivative. We assume that this disorder has not significantly affected the geometry determined for the inner part of the molecule.

In (I), the geometry at the P atom is distorted tetrahedral [C—P—C 110.02 (5) and Se═P—C 108.91 (5)°] and the P—C and P═Se bond lengths are 1.9079 (14) and 2.1326 (6) Å, respectively. The P═Se bond in (I), together with the value in tris(2,4-dimethoxyphenyl)phosphine selenide [2.135 (3) Å; Allen et al., 1990] represents an upper limit for phosphorus selenides.

From the data in Table 1 it can be seen that there is a marked steric influence of the tert-butyl group. In comparison with phosphine chalcogenides substituted by less bulky groups at the phosphorus, such as Me₃PO [1.772 (6) and 1.770 (10) Å; Engelhardt et al., 1986], Me₃PS [1.798 (2) Å; Eller & Corfield, 1971] or Me₃PSe [1.786 (14) Å; Cogne et al., 1980], a distinct lengthening of the P—C bonds [1.888 (6)–1.9079 (14) Å] is observed. Furthermore, Gilheany (1992) concluded that bulky substituents lengthen the P═X bond distance (X is O, S, Se or Te) by ca 0.002–0.004 Å. In (I), the increase of the P═Se distance is about 0.002 Å, compared with Me₃PSe [2.111 (3) Å (X-ray data; Cogne et al., 1980) and 2.091 (3) Å (electron diffraction; Jacob & Samdal, 1977)] and Ph₃PSe [2.106 (2) Å (Codding & Kerr, 1979)]. However, in the case of the phosphine sulfides, such as ^tBu₃PS [1.962 (3) Å; Steinberger et al., 2001] and Me₃PS [1.959 (2) Å; Eller & Corfield, 1971], the difference in the P—S bond length is only small.

The P—X distances in phosphine chalcogenides obtained by X-ray structure determination (XR) are about 0.0015–0.002 Å longer than those obtained by electron diffraction in the gas phase (ED), as can be seen from the examples of Me₃PO [1.489 (6) Å (XR; Engelhardt et al., 1986) and 1.476 (2) Å (ED; Wilkins et al., 1975)], Me₃PS [1.959 (2) Å (XR; Eller & Corfield, 1971) and 1.940 (2) Å (ED; Wilkins et al., 1975)] and Me₃PSe [2.111 (3) Å (XR; Cogne et al., 1980) and 2.091 (3) (ED; Jacob & Samdal, 1977)]. To the best of our knowledge, no corresponding data for phosphine tellurides are available.

Related literature

For related literature, see: Allen et al. (1990); Codding & Kerr (1979); Cogne et al. (1980); DuMont, Kroth & Schumann (1976); Eller & Corfield (1971); Engelhardt et al. (1986); Gilheany (1992); Jacob & Samdal (1977); Krüger & Bruckmann (1995); Kuhn et al. (1987); Patai & Hartley (1992); Rankin et al. (1985); Sheldrick (1997); Steinberger et al. (2001); Wilkins et al. (1975).

S2. Experimental

To a solution of tris(tert-butyl)phosphine (0.97 g, 4.82 mmol) in toluene (10 ml), grey selenium granules (0.39 g, 4.92 mmol; 2% excess) were added and the mixture heated for 16 h at 338 K. After removal of excess Se by filtration, the solvent was completely evaporated in high vacuum, yielding ^tBu₃PSe, (I) (yield 1.31 g, 97%). The title compound was characterized by ³¹P NMR [121.472 MHz, toluene, 298 K: d = 92.92 p.p.m. (³J_P—H = 13.6 Hz; ¹J_P—Se = 691 Hz)] and mass spectroscopy. Recrystallization from toluene at 238 K afforded single crystals of (I).

S3. Refinement

At the end of the conventional refinement all methyl C atoms revealed very anisotropic displacement parameters. Split sites for these atoms and their associated H atoms were introduced. The major methyl component was refined as a rigid group and allowed to rotate but not tip; the minor component was refined with a riding model assuming ideally staggered torsion angles. Moreover, nine restraints were used [DFIX = 1.54 for the six C1—C_methyl distances and SADI for the three C_methyl—C_methyl,split disorder pairs; SHELXL97 (Sheldrick, 1997)?]. The site occupancy converged to about 70/30. Hereby, R₁ and wR₂ decreased from 0.040 and 0.100 to 0.019 and 0.047, respectively. Additionally, the deviations from the zero level in the corresponding final difference maps changed from -0.53 and 0.62 to -0.19 and 0.62 e/ų, respectively, and the displacement ellipsoids became normal. The P—C1 distance discussed in this work changed insignificantly, from 1.907 (3) to 1.908 (2) Å.

Computing details

Data collection: IPDS (Stoe & Cie, 1997); cell refinement: IPDS; data reduction: IPDS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XSTEP32 (Stoe & Cie, 1997); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. The molecular structure of (I) showing 50% probability displacement ellipsoids. Only the main disorder component is shown. H atoms have been omitted for clarity.

Tris(tert-butyl)phosphine selenide

Crystal data

C12H27PSe	Dx = 1.231 Mg m^−3
Mr = 281.27	Mo Kα radiation, λ = 0.71073 Å
Cubic, Pa3	Cell parameters from 5000 reflections
Hall symbol: -P 2ac 2ab 3	θ = 5–20°
a = 14.4773 (18) Å	µ = 2.55 mm^−1
V = 3034.3 (7) Å^3	T = 180 K
Z = 8	Cube, colourless
F(000) = 1184	0.60 × 0.56 × 0.52 mm

Data collection

Stoe IPDS diffractometer	993 independent reflections
Radiation source: fine-focus sealed X-ray tube	867 reflections with I > 2σ(I)
Planar graphite monochromator	Rint = 0.049
Detector resolution: 6.667 pixels mm^-1	θmax = 26.0°, θmin = 3.5°
ϕ–rotation, steps of 1.2°, 177 exposures scans	h = −17→17
Absorption correction: numerical (X-RED; Stoe & Cie, 1997)	k = −17→17
Tmin = 0.310, Tmax = 0.351	l = −17→17
20131 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.018	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.046	H-atom parameters not refined
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0301P)^2 + 0.3113P] where P = (Fo^2 + 2Fc^2)/3
993 reflections	(Δ/σ)max = 0.011
74 parameters	Δρmax = 0.62 e Å^−3
9 restraints	Δρmin = −0.19 e Å^−3

Special details

Experimental. Recrystallized from toluene. During data collection the crystal was in cold N₂ gas from a cryostream cooler (Oxford Cryosystems, 1992).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

All crystal faces were indexed by microscope on the diffractometer and thereafter a Gaussian integration for absorption correction was carried out, as implemented in X-RED (Stoe & Cie, 1997).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.31865 (11)	0.68057 (10)	0.02827 (10)	0.0321 (3)
C2	0.3603 (3)	0.7760 (2)	0.0040 (2)	0.0448 (9)	0.698 (9)
H2A	0.3397	0.8219	0.0494	0.054*	0.698 (9)
H2B	0.3398	0.7946	−0.0578	0.054*	0.698 (9)
H2C	0.4279	0.7721	0.0051	0.054*	0.698 (9)
C2S	0.3102 (9)	0.7853 (5)	0.0135 (5)	0.046 (2)	0.302 (9)
H2S1	0.2640	0.8105	0.0561	0.055*	0.302 (9)
H2S2	0.2912	0.7977	−0.0503	0.055*	0.302 (9)
H2S3	0.3701	0.8147	0.0252	0.055*	0.302 (9)
C3	0.3561 (3)	0.6083 (3)	−0.0398 (2)	0.0422 (8)	0.698 (9)
H3A	0.4226	0.6003	−0.0300	0.051*	0.698 (9)
H3B	0.3450	0.6292	−0.1033	0.051*	0.698 (9)
H3C	0.3245	0.5493	−0.0297	0.051*	0.698 (9)
C3S	0.3922 (7)	0.6412 (8)	−0.0403 (5)	0.046 (2)	0.302 (9)
H3SA	0.4002	0.5749	−0.0292	0.055*	0.302 (9)
H3SB	0.4513	0.6729	−0.0307	0.055*	0.302 (9)
H3SC	0.3714	0.6511	−0.1039	0.055*	0.302 (9)
C4	0.2124 (2)	0.6899 (3)	0.0141 (2)	0.0423 (9)	0.698 (9)
H4A	0.1879	0.7363	0.0568	0.051*	0.698 (9)
H4B	0.1827	0.6303	0.0260	0.051*	0.698 (9)
H4C	0.1998	0.7091	−0.0496	0.051*	0.698 (9)
C4S	0.2246 (5)	0.6397 (8)	0.0025 (4)	0.044 (2)	0.302 (9)
H4SA	0.2263	0.5725	0.0101	0.053*	0.302 (9)
H4SB	0.2104	0.6547	−0.0620	0.053*	0.302 (9)
H4SC	0.1768	0.6658	0.0428	0.053*	0.302 (9)
P	0.34528 (2)	0.65472 (2)	0.15472 (2)	0.02013 (14)
Se	0.260232 (9)	0.739768 (9)	0.239768 (9)	0.02804 (10)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0382 (8)	0.0351 (8)	0.0230 (7)	0.0052 (6)	−0.0013 (6)	−0.0008 (6)
C2	0.063 (3)	0.0385 (16)	0.0333 (14)	0.0033 (15)	0.0060 (15)	0.0119 (11)
C2S	0.074 (7)	0.038 (3)	0.025 (3)	0.012 (4)	0.002 (3)	0.007 (2)
C3	0.050 (2)	0.053 (2)	0.0242 (12)	0.0093 (14)	−0.0022 (14)	−0.0099 (14)
C3S	0.058 (6)	0.055 (6)	0.025 (3)	0.016 (4)	0.003 (3)	−0.005 (3)
C4	0.0406 (15)	0.051 (2)	0.0348 (14)	0.0116 (14)	−0.0121 (10)	−0.0015 (13)
C4S	0.045 (4)	0.057 (5)	0.029 (3)	0.007 (3)	−0.004 (2)	−0.004 (3)
P	0.02013 (14)	0.02013 (14)	0.02013 (14)	0.00120 (12)	0.00120 (12)	−0.00120 (12)
Se	0.02804 (10)	0.02804 (10)	0.02804 (10)	0.00518 (5)	0.00518 (5)	−0.00518 (5)

Geometric parameters (Å, º)

C1—C4S	1.531 (7)	C1—C3S	1.564 (8)
C1—C3	1.537 (4)	C1—P	1.9079 (14)
C1—C2S	1.537 (7)	P—C1^i	1.9079 (14)
C1—C2	1.548 (3)	P—C1^ii	1.9079 (14)
C1—C4	1.558 (3)	P—Se	2.1326 (6)
C4S—C1—C3	83.9 (3)	C4—C1—C3S	128.4 (4)
C4S—C1—C2S	106.1 (4)	C4S—C1—P	109.8 (3)
C3—C1—C2S	127.7 (3)	C3—C1—P	114.26 (16)
C4S—C1—C2	129.5 (3)	C2S—C1—P	110.1 (3)
C3—C1—C2	109.0 (2)	C2—C1—P	108.29 (14)
C2S—C1—C2	28.2 (4)	C4—C1—P	110.08 (13)
C4S—C1—C4	28.7 (3)	C3S—C1—P	113.6 (3)
C3—C1—C4	108.8 (2)	C1^i—P—C1	110.02 (5)
C2S—C1—C4	79.5 (4)	C1^i—P—C1^ii	110.02 (5)
C2—C1—C4	106.1 (2)	C1—P—C1^ii	110.02 (5)
C4S—C1—C3S	108.1 (4)	C1^i—P—Se	108.91 (5)
C3—C1—C3S	26.4 (3)	C1—P—Se	108.91 (5)
C2S—C1—C3S	109.0 (4)	C1^ii—P—Se	108.91 (5)
C2—C1—C3S	85.2 (4)
C4S—C1—P—C1^i	167.6 (5)	C2—C1—P—C1^ii	−167.6 (2)
C3—C1—P—C1^i	75.4 (2)	C4—C1—P—C1^ii	76.8 (2)
C2S—C1—P—C1^i	−76.0 (5)	C3S—C1—P—C1^ii	−74.9 (5)
C2—C1—P—C1^i	−46.2 (2)	C4S—C1—P—Se	−73.1 (5)
C4—C1—P—C1^i	−161.8 (2)	C3—C1—P—Se	−165.3 (2)
C3S—C1—P—C1^i	46.5 (5)	C2S—C1—P—Se	43.3 (5)
C4S—C1—P—C1^ii	46.3 (5)	C2—C1—P—Se	73.1 (2)
C3—C1—P—C1^ii	−46.0 (2)	C4—C1—P—Se	−42.5 (2)
C2S—C1—P—C1^ii	162.6 (5)	C3S—C1—P—Se	165.8 (5)

Symmetry codes: (i) −z+1/2, −x+1, y−1/2; (ii) −y+1, z+1/2, −x+1/2.