Tris(tert-butyl)­phosphine sulfide, a phosphine sulfide with three bulky substituents

The title compound, C₁₂H₂₇PS, has crystallographic C₃ symmetry. The bond angles at phospho­rus are tetrahedral [C-P-S 109.31 (12)° and C-P-C 109.63 (12)°] and the P-C bond length is 1.899 (4) Å. The shortest intermolecular contacts exist between methyl H atoms and the S atom (3.09, 3.12 and 3.28 Å). A survey of various phosphine sulfides containing three equal ligands (Me₃PS, Et₃PS, Cy₃PS, ^tBu₃PS, etc.) shows the influence of substituents with different steric demand on the geometry at phospho­rus and on the P-C bond length.

S1. Comment

Phosphine sulfides have been well known since the first reports in the early sixties (Zingaro et al., 1961, 1963). Subsequent studies concentrated on complex formation with transition metals and group 13 compounds (i.e. Ga, In: for example GaX₃ * 2 S = PPh₃; X = Cl, Br, I) (Lobana, 1992). Additionally, the formation of charge transfer complexes from phosphine chalcogenides and diiodine in different stoichiometries [Ph₃P = X * I₂; X = O, S, Se (Gofrey et al., 1997) and 2 Ph₃PS * 3 I₂ (Arca et al., 1999)] and their structures have been studied. \sch

Here we report the structure of tris(tert-butyl)phosphine sulfide, (I), which, to the best of our knowledge, is the most sterically strained compound in this class up to now. The X-ray structure is shown in Fig. 1 and the important structural data, together with a series of comparable phosphine sulfides [Me₃PS (X-ray: Eller & Corfield, 1971; electron diffraction: Wilkins et al., 1975], Ph₃PS (monoclinic: Codding & Kerr, 1978; triclinic: Ziemer et al., 2000), Cy₃PS (Kerr et al., 1977), etc.) are summarized in Table 1. In (I), the geometry at the phosphorus atom is tetrahedral within the limits of error [mean C—P—C angle 109.63 (12)°]. The corresponding average S—P—C angle is 109.31 (12)°. As can be seen from Table 1 the C—P—C angle distinctly grows from Me₃PS [105.8 (3)°] to Cy₃PS (108.0°) and (I) [109.63 (12)°] due to the increasing steric demand of the substituents at phosphorus. In spite of the low precision the X-ray structure determination of Et₃PS (van Meerssche & Leonard, 1959) also should be mentioned [P—S 1.865 (40), P—C 1.865 (40) Å; C—P—C angle: 107°, S—P—C angle 112°]. The P—S bond in (I) [1.962 (3) Å], together with 1.966 (2) Å (Cy₃PS), lies at the upper limit for P—S bond distances found in phosphorus sulfides. In Table 1 it is shown that the P—S bond length only differs in a small range [1.9545 (9)/1.950 (3) (Ph₃PS), 1.959 (2) (Me₃PS), 1.962(3 (I), and 1.966 (2) Å (Cy₃PS)].

On the other hand the average S—P—C angle increases with smaller substituents from 109.31 (12) (I) to 110.9° (Cy₃PS) and 113.2 (3)° in Me₃PS. For alkyl and cycloalkyl substituted phosphine sulfides the structural effects are due to the increasing steric demand whereas the influence of electronic effects is small and can be neglected. For Ph₃PS, however, we found that electronic effects from the electron withdrawing phenyl groups gain more importance and influence the molecular structure. There is a correlation between the steric effect, which is lengthening the P—C bond to 1.814 (2) Å [longer than in Me₃PS, 1.798 (2) Å], and an electronic effect shortening the P═S bond to 1.955 (1) Å, which is the shortest one found in the discussed series of phosphine sulfides. Therefore, the C—P—C angle in Ph₃PS [105.6 (8)°], which is equal to 105.8 (3)° in Me₃PS, is decreased by electronic effects too. The increasing electronic effects in triphenylphosphine compounds, in comparison to alkyl phosphine compounds, are visually shown on a steric and electronic map (Tolman, 1977).

In the discussed phosphine sulfides, with increasing steric demand, the P—C bond is dramatically lengthened from 1.798 (2) Å (Me₃PS, with no steric stress) to 1.899 (4) Å in (I). The comparison of the latter value with 1.798 (2) Å (Me₃PS) and 1.844 (2)/1.840 (2) Å in Cy₃PS shows that there is a significant change depending on the steric demand from a primary (methyl) to a secondary (cyclohexyl) and a tertiary alkyl substituent [tert-butyl in (I)]. This affects to lengthening the P—C bond in (I) to 1.899 (4) Å, which, to the best of our knowledge, is the longest P—C bond known in the phosphine sulfide family.

Tert-butyl-2-phenylethenyl-2-phenylethynyl-phosphine sulfide, (II), contains an unstrained tert-butyl group (Mahieu et al., 1997). The two unsaturated substituents are slim ligands and the P—C bond length (P—tert-butyl) is 1.827 (4) Å [mean P—C distance 1.791 (4) Å]. However, longer P—C bond distances, in comparison to (I), are described for tris(tert-butyl)phosphine [mean P—C distance 1.911 (2) Å, mean C—P—C angle 107.5 (3)°; Bruckmann & Krüger, 1995] and for the tetrakis(tert-butyl) phosphonium cation [1.924 (4) Å, mean C—P—C angle 109.47°; Schmidbaur et al., 1980].

The structural data of the complete tris(tert-butyl)phosphine chalcogenides ^tBu₃PX (X = O, S, Se, Te) have been recently summarized (Steinberger et al., 2001). From the comparison of the corresponding P—C distances it can be seen that there is no great difference between the shortest distance in ^tBu₃PO [1.888 (6) Å] and the longest one in ^tBuPSe [1.9079 (14) Å]. The P—C—P angles are also very similar and amount to about 110° (Steinberger et al., 2001). Also for ^tBu₃PNH (X = NH) the corresponding data are in the same order [P—C: 1.913 (6) Å, C—P—C: 109.4 (5)°; Rankin et al., 1985]. This indicates that in the case of the neutral ^tBu₃PX compounds there is no strong steric or electronic influence of the different chalcogenes and the isoelectronic NH group, respectively, to the P—C bond lengths and the C—P—C angles.

Even when ^tBu₃PX is substituted by a proton, i.e. in the case of the tris(tert-butyl)phosphonium ion (^tBu₃PH)⁺, the changes in the corresponding bond data are not very large. Thus, in the tris(tert-butyl)phosphonium salt of the binuclear chloroferrate(III), (^tBu₃PH)₂ [Fe₂(µ-OEt)₂Cl₆], slightly decreasing P—C bond lengths and plausibly increasing C—P—C angles were found in comparison to the neutral ^tBu₃PX compounds [mean P—C 1.87 (1) Å and mean C—P—C 114.3 (5)°; Walker & Poli, 1990]. These results show the dominating steric influence of the bulky tert-butyl groups to the bonding relations in this type of compounds.

The closest intermolecular distances are found between the sulfur atom and methyl H atoms (3.09, 3.12 and 3.28 Å), which belong to three different tert-butyl groups. Because of the threefold symmetry around sulfur, there are nine of such sulfur-hydrogen contacts in the crystal structure. The intermolecular distances correspond about the value of the sum of the van der Waals radii of sulfur and hydrogen (3.00 Å; Bondi, 1964). Hereby, it is evident that only one hydrogen of each methyl group is directed towards the sulfur atom.

S2. Experimental

Tris(tert-butyl)phosphine sulfide is prepared during the reaction of py→ PS₂Cl (640 mg, 3.05 mmol) and tris(tert-butyl)phospine (617 mg, 3.05 mmol) in benzene at room temperature. The reaction mixture is stirred over night and a yellow solid is formed. After filtration the solvent is completely evaporated in vacuum. As product a colorless solid is isolated (700 mg, 98% yield). X-ray suitable crystals are obtained by recrystallization from toluene at -38°C. The compound is completely characterized. ³¹P-NMR(121.472 MHz, CDCl₃, 298 K): δ = 89.67 p.p.m. (³J_P—H = 13.6 Hz); ¹H-NMR(300.130 MHz, CDCl₃, 298 K): δ = 1.31 p.p.m. (d, ³J_H—P = 13.6 Hz, C(CH₃)₃); ¹³C-NMR(75.148 MHz, CDCl₃, 298 K): δ = 41.40 p.p.m. (d, ²J_C—P = 34 Hz, C(CH₃)₃), 30.02 p.p.m. [C(CH₃)₃]; MS(70 eV) [m/e(%)]: 234 (8.3) [M⁺], 178 (18.8) [M⁺ - C₄H₈], 122 (36.2) [M⁺ - 2 C₄H₈], 57 (100) [^tBu⁺], 41 (27.6) [C₃H₅⁺].

S3. Refinement

At the end of the usual refinement all methyl carbons revealed very anisotropic displacement parameters and two sets of split sites for these atoms were introduced. In the proceeding refinement the set occupation ratio converged to about 70/30. Hereby, R₁(gt) and wR₂(all) decreased from 0.095 and 0.196 to 0.063 and 0.136. Additionally, the deviations from the zero level in the corresponding final difmaps dropped from -0.62 and 0.71 to -0.35 and 0.34 e/ų, and the displacement ellipsoids became normal (Fig. 1). On the other side, the P—C1 distance, discussed in this work, only changed insignificantly, from 1.896 (6) to 1.899 (4) Å.

Computing details

Data collection: STADI4-1.06 (Stoe & Cie, 1997); cell refinement: STADI4-1.06; data reduction: X-RED1.07 (Stoe & Cie, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XSTEP2.18 (Stoe & Cie, 1997); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. Molecular structure showing 50% probability displacement ellipsoids of (I). H atoms and the minor disorder component are omitted for clarity.

tris-tert.-butyl-phosphine sulfide

Crystal data

C12H27PS	Dx = 1.080 Mg m^−3
Mr = 234.37	Mo Kα radiation, λ = 0.71073 Å
Cubic, Pa3	Cell parameters from 39 reflections
Hall symbol: -P 2ac 2ab 3	θ = 14.0–15.0°
a = 14.232 (12) Å	µ = 0.30 mm^−1
V = 2883 (4) Å^3	T = 180 K
Z = 8	Cube, colorless
F(000) = 1040	0.68 × 0.60 × 0.57 mm

Data collection

Stoe STADI-4 diffractometer	816 reflections with I > 2σ(I)
Radiation source: fine-focus sealed X-ray tube	Rint = 0.009
Planar graphite monochromator	θmax = 25.5°, θmin = 2.5°
2Θ/ω–scan,ratio=0,width(ω)=3.2–3.2°	h = −12→12
Absorption correction: ψ scan (North et al., 1968)	k = −1→11
Tmin = 0.820, Tmax = 0.846	l = −17→17
1065 measured reflections	3 standard reflections every 120 min
901 independent reflections	intensity decay: 1.7%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.063	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.136	w = 1/[σ^2(Fo^2) + 9.0951P] where P = (Fo^2 + 2Fc^2)/3
S = 1.33	(Δ/σ)max = 0.003
901 reflections	Δρmax = 0.35 e Å^−3
57 parameters	Δρmin = −0.34 e Å^−3
9 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0027 (5)

Special details

Experimental. Recrystallized from toluene. During data collection the crystal was in cold N₂ gas of 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.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.3148 (3)	0.6808 (3)	0.0288 (3)	0.0322 (10)
C2	0.3563 (5)	0.7782 (4)	0.0032 (5)	0.0418 (18)	0.697 (10)
H2A	0.3397	0.8238	0.0520	0.063*	0.697 (10)
H2B	0.3307	0.7989	−0.0573	0.063*	0.697 (10)
H2C	0.4249	0.7735	−0.0013	0.063*	0.697 (10)
C2S	0.3013 (11)	0.7852 (10)	0.0128 (12)	0.0418 (18)	0.303 (10)
H2S1	0.2540	0.8092	0.0567	0.063*	0.303 (10)
H2S2	0.2802	0.7960	−0.0519	0.063*	0.303 (10)
H2S3	0.3610	0.8179	0.0232	0.063*	0.303 (10)
C3	0.3540 (6)	0.6089 (5)	−0.0403 (5)	0.043 (2)	0.697 (10)
H3A	0.4217	0.6017	−0.0299	0.047*	0.697 (10)
H3B	0.3428	0.6305	−0.1047	0.047*	0.697 (10)
H3C	0.3227	0.5483	−0.0306	0.047*	0.697 (10)
C3S	0.3899 (11)	0.6447 (13)	−0.0403 (13)	0.043 (2)	0.303 (10)
H3SA	0.3995	0.5772	−0.0307	0.047*	0.303 (10)
H3SB	0.4491	0.6781	−0.0294	0.047*	0.303 (10)
H3SC	0.3689	0.6559	−0.1050	0.047*	0.303 (10)
C4	0.2092 (5)	0.6917 (6)	0.0152 (5)	0.0440 (19)	0.697 (10)
H4A	0.1851	0.7386	0.0595	0.048*	0.697 (10)
H4B	0.1782	0.6313	0.0266	0.048*	0.697 (10)
H4C	0.1963	0.7123	−0.0493	0.048*	0.697 (10)
C4S	0.2166 (10)	0.6368 (11)	0.0064 (12)	0.0440 (19)	0.303 (10)
H4SA	0.2197	0.5685	0.0145	0.048*	0.303 (10)
H4SB	0.1992	0.6515	−0.0586	0.048*	0.303 (10)
H4SC	0.1695	0.6629	0.0492	0.048*	0.303 (10)
P	0.34295 (6)	0.65705 (6)	0.15705 (6)	0.0210 (4)
S	0.26333 (6)	0.73667 (6)	0.23667 (6)	0.0275 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.038 (2)	0.036 (2)	0.023 (2)	0.0033 (18)	−0.0031 (17)	0.0008 (17)
C2	0.058 (5)	0.041 (3)	0.026 (3)	0.008 (4)	0.007 (4)	0.002 (2)
C2S	0.058 (5)	0.041 (3)	0.026 (3)	0.008 (4)	0.007 (4)	0.002 (2)
C3	0.052 (5)	0.050 (5)	0.026 (3)	0.012 (3)	−0.002 (3)	−0.010 (3)
C3S	0.052 (5)	0.050 (5)	0.026 (3)	0.012 (3)	−0.002 (3)	−0.010 (3)
C4	0.042 (3)	0.055 (5)	0.035 (3)	0.010 (4)	−0.011 (3)	0.001 (4)
C4S	0.042 (3)	0.055 (5)	0.035 (3)	0.010 (4)	−0.011 (3)	0.001 (4)
P	0.0210 (4)	0.0210 (4)	0.0210 (4)	0.0011 (4)	0.0011 (4)	−0.0011 (4)
S	0.0275 (5)	0.0275 (5)	0.0275 (5)	0.0038 (4)	0.0038 (4)	−0.0038 (4)

Geometric parameters (Å, º)

C1—C2S	1.515 (13)	C1—C4S	1.564 (14)
C1—C4	1.524 (7)	C1—P	1.899 (4)
C1—C3	1.524 (7)	P—C1^i	1.899 (4)
C1—C3S	1.540 (14)	P—C1^ii	1.899 (4)
C1—C2	1.551 (7)	P—S	1.963 (3)
C2S—C1—C4	75.9 (7)	C2—C1—C4S	130.6 (7)
C2S—C1—C3	127.4 (8)	C2S—C1—P	110.2 (7)
C4—C1—C3	110.4 (5)	C4—C1—P	110.4 (4)
C2S—C1—C3S	108.6 (9)	C3—C1—P	115.0 (4)
C4—C1—C3S	129.6 (8)	C3S—C1—P	114.1 (8)
C3—C1—C3S	27.3 (5)	C2—C1—P	107.7 (3)
C2S—C1—C2	30.3 (5)	C4S—C1—P	108.2 (7)
C4—C1—C2	104.8 (5)	C1—P—C1^i	109.64 (13)
C3—C1—C2	108.0 (5)	C1—P—C1^ii	109.64 (13)
C3S—C1—C2	83.3 (7)	C1^i—P—C1^ii	109.64 (13)
C2S—C1—C4S	104.4 (8)	C1—P—S	109.30 (13)
C4—C1—C4S	29.9 (5)	C1^i—P—S	109.30 (13)
C3—C1—C4S	85.8 (7)	C1^ii—P—S	109.30 (13)
C3S—C1—C4S	110.9 (9)
C2S—C1—P—C1^i	160.9 (7)	C3S—C1—P—C1^ii	43.8 (7)
C4—C1—P—C1^i	79.1 (5)	C2—C1—P—C1^ii	−46.7 (5)
C3—C1—P—C1^i	−46.6 (4)	C4S—C1—P—C1^ii	167.8 (6)
C3S—C1—P—C1^i	−76.6 (7)	C2S—C1—P—S	41.1 (7)
C2—C1—P—C1^i	−167.1 (3)	C4—C1—P—S	−40.7 (4)
C4S—C1—P—C1^i	47.4 (7)	C3—C1—P—S	−166.4 (4)
C2S—C1—P—C1^ii	−78.6 (7)	C3S—C1—P—S	163.6 (7)
C4—C1—P—C1^ii	−160.5 (4)	C2—C1—P—S	73.1 (4)
C3—C1—P—C1^ii	73.8 (4)	C4S—C1—P—S	−72.4 (7)

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