Computational and experimental approaches to the molecular structure of the HCl adduct of Me3PO.

The reaction of anhydrous HCl(g) with trimethyl phosphane oxide yields trimethylhydroxy phosphonium chloride. A crystal structure analysis showed that the prevalent mesomeric structure in the solid state is the phosphonium chloride ion pair. Ab initio calculations in the gas phase cannot reproduce these findings, whereas higher correlated methods (CISD) and solvation models predict the experimental structure correctly.

Introduction

The bonding situation in different phosphanes, phosphane oxides, and ylides has been investigated over the last decades [1-3]. Earlier investigations of differently substituted formal “hydroxy phosphonium halides” showed the influence of the counter ion with respect to the contributions of the mesomeric structures 1,2 and their isomer 3 (Scheme 1). The corresponding triphenyl substituted phosphonium cation was studied crystallographically with different formal counter ions, like for e.g. F−, [4], Cl− [5], Br− [6], H2O*Br− [7]. For X = F the best description is formula 1, which can be attributed to the strong H–F bond. For the softer atoms (X = Cl and Br), which are also weaker bases, the crystal structures revealed the corresponding hydroxytriphenyl phosphonium halides (2) as the prevalent tautomeric structure. For X = Br also, the formation of a water adduct (type 4) was observed. Whereas the phenyl substituted phosphonium compounds have been studied extensively, only little is known about their aliphatic congeners. Some earlier spectroscopic results from NMR and IR investigations suggested, however, a similar trend for the aliphatic compounds [8]. Nevertheless, only a single crystal structure with trialkyl substitution was reported in the literature, so far (type 2: R = Pr, X = I) [9], and a further compound having an indenyl and two methyl substituents in the form of its water adduct (type 4: X = Cl) [10]. By contrast, the soft–soft interactions of various iodo-phosphonium salts with iodine (I−, I3−) have been studied extensively [9,11-13]. In our work, we wanted to investigate the bonding situation of adducts of hydrogen halides with the smallest trialkyloxophosphorane, i.e. trimethyl phosphane oxide.

Scheme 1

Isomeric structures of hydrogen halide adducts of oxophosphoranes 1–3, including a water containing variety (4).

Results and discussion

In order to get insight into the bonding situation of the alkyl substituted compounds R3POHX, we performed an exploratory ab initio study on the simplest representatives (R=CH3, X = F, Cl, Br). Our initial attempts to analyze these molecules by means of DFT calculations (B3LYP) with a triple ζ basis set (6–311G**) revealed that only the bromine derivative exists as a type 2 structure. We attributed these findings to the incompleteness of the basis set not able to describe diffuse electron distribution. Addition of diffuse functions (B3LYP/6-311++G**) did not alter the findings. The fluorine and the bromine derivatives show the phosphane oxide adduct (2) and the phosphonium ion (1), respectively. A summary of selected geometric parameters is given in Table 1. Nevertheless, the PO bond lengths for the fluorine derivative (1.513 Å) is significantly longer than for trimethylphosphane oxide (1.489(6) Å [14] and 1.500 Å) suggesting some interaction between the P=O and the HF moiety.1 Interestingly, for the chlorine derivative, the calculations also predict the HCl adduct rather than the phosphonium ion as the minimum energy structure. Interaction of the HCl molecule with trimethylphosphane oxide results in a stabilization by ca. 3.2 kcal/mol (ΔGinteraction). Only the bromine derivative shows the expected phosphonium ion structure having a shortened O–H (1.069 Å) and elongated H–Br (1.935 Å) distance.

Table 1

Structural changes on adduct formation. Comparison of selected geometric parameters of (CH3)3POHX (calculated) with (CH3)3PO (calculated and experimental). Distances in [Å] and angles in [°].

(CH3)3POHX				(CH3)3PO
X =	F	Cl	Br	–	–
	B3LYP/6-311++G(d,p)			B3LYP/6-311++G(d,p)	X-ray [14]
P–O	1.513	1.512	1.568	1.500	1.489(6)
P–Ctrans	1.821	1.822	1.807	1.830	1.772(6)
P–Ccis	1.823	1.823	1.808	1.830	1.772(6)
O–H	1.581	1.619	1.069	–	–
O…X	2.534	2.960	2.973	–	–
H–X	0.959	1.344	1.935	–	–
C–P–OH	10.0	34.1	180.0	–	–
POX	121.7	127.5	99.4	–	–

This is, in fact, the opposite of the experimental findings with R = phenyl. It seems, however, very unlikely that changing phenyl to alkyl substituents would switch the preferred structure from type 2 to 1. Therefore, we synthesized this compound to check the bonding situation experimentally. General access to phosphonium halides is given by reaction of the phosphane oxide 6 with the corresponding anhydrous HX. We were also able to obtain 5 after gentle hydrolysis of 7 [15] (Scheme 2). In the 31P NMR spectra, a signal at +66.9 ppm is observed. This is in good agreement with similar compounds [16]. The proton spectra show one doublet at 1.93 ppm (3JPH = 18 Hz) and a broad singlet at 8.74 ppm. In the 13C NMR spectra, a doublet at 15.53 ppm (d, 1JPC = 91.6 Hz) is observed. Based on the spectroscopic data, the presence of the pentavalent isomer 3 seems unlikely; however, no unambiguous assignment between the remaining possible isomeric structures 1 and 2 can be made.

Scheme 2

Synthesis of 5 starting from trimethyl phosphane oxide (6) or the chlorotrimethyl phosphonium chloride (7).

We were also able to grow single crystals suitable for X-ray diffraction from a saturated chloroform solution. The crystal structure analysis clearly shows the formation of a hydroxytrimethyl phosphonium chloride. All atoms are on general positions. All non-hydrogen atoms are refined with anisotropic displacement parameters, while the hydrogen atoms were refined isotropically without any geometric constraints. The hydrogen bond [O1–H1⋯Cl1 179.5(14)°, O1⋯Cl1 2.8763(4) Å] connects the ion pair which shows almost Cs symmetry. The chloride anion is surrounded by two methyl groups and the next oxygen atom is as near as 3.7421(5) Å having no significant contributions to the stabilization of the phosphonium ion pairs. Interestingly, the P–O bond is very short (P1–O1 1.5600(4) Å), which is only 4.7% longer than in Me3P=O (Fig. 1).

Fig. 1

ORTEP plot of the unit cell content of 5 showing the four nearest molecules. Probability level of the thermal ellipsoids is 50%, hydrogen atoms are drawn at arbitrary radii. The hydrogen bond of the ion pairs is displayed with dotted lines. Selected distances [Å] and angles [°]: P1–O1 1.5600(4), P1–C1 1.7740(5), P1–C2 1.7725(5), P1–C3 1.7798(6), O1–H1 0.90(1), H1–Cl1 1.98(1), O1–H1···Cl1 179.5(14).

In order to account for the observed discrepancy of the calculated and the X-ray structure, we tried to involve the surroundings of the molecule in a very simple way. The very common IEF-PCM formalism for modeling a solvent by means of a polarizable continuum was used to mimic the crystallographic surroundings. For these calculations, one needs to specify a “solvent” by its dipole moment (μ) and the mean polarizability (). Those parameters were obtained from preliminary calculations of the phosphonium ion in a highly polar medium modeled by the IEF-PCM2 using the Onsager equation (Eq. (4)) for polar liquids. Accounting for the different orientations of the dipole moment in the solid state gives the Kirkwood correlation factor. By using this approach, we were able model the observed structure. Interestingly also a higher correlated method – CISD – showed good agreement with the X-ray structure, but not 2nd and 3rd order Møller-Plesset calculations (MP2, and MP3). Table 2 gives relevant structural parameters of the calculated and the X-ray structures.

Table 2

Comparison of calculated and measured structural parameters of Me3POHCl.

	X-ray	B3LYP					CISD	MP2	MP3a
		6–311G**	6–311++G**	6–311++G**			6–311++G**	6–311++G**	6–311++G**
g.pb./PCMc		g.p.	g.p.	PCM: 0.5	PCM: 80	PCM: 98	g.p.	g.p.	g.p.
P–O	1.560(1)	1.512	1.515	1.507	1.585	1.590	1.547	1.515	1.497
P–Ctrans	1.780(1)	1.822	1.823	1.825	1.796	1.795	1.791	1.821	1.812
P–Ccis	1.773(1)	1.823	1.821	1.826	1.803	1.802	1.791	1.823	1.811
O⋯H	0.897(14)	1.619	1.615	1.709	1.024	1.020	1.041	1.615	1.686
O⋯Cl	2.876(1)	2.960	2.580	3.020	2.916	2.931	2.787	2.958	2.989
H–Cl	1.979(14)	1.344	1.346	1.331	1.892	1.912	1.779	1.346	1.309
C–P–O–H	−176.7(9)	−145.9	−172.0	178.3	−179.7	179.9	180.0	171.9	−0.1
P–O–Cl	113.6(1)	127.5	125.9	111.5	114.4	115.3	98.2	123.4	128.5
rmsd (dist)d	–	0.40	0.41	0.43	0.07	0.06	0.11	0.39	0.43
rmsd (ang)e	–	23.9	9.3	1.9	2.2	2.6	11.2	7.7	125.3

Calculations resulted in a staggered orientation of the C–P–O–H moiety.

Gas phase calculations.

Solvation model IEF-PCM with respective permittivity and rsolv. = 5.32 Å.

Root mean square deviation (rmsd) of selected distances [Å].

rmsd of selected angles [°].

A detailed analysis of the molecular orbitals of trimethyl phosphine oxide (6) and 5 at the B3LYP/6-311++G** level of theory with IEF-PCM (ɛR = 98 and rsolv. = 5.32 Å) was carried out in order to understand the rather short P–O single bond. In 6 the s-type orbitals at the oxygen and phosphorus atoms form a highly polarized σ-bond (Fig. 2 a) and two p-type lone pairs of O show significant negative hyperconjugation with the P–C antibonding orbitals (Fig. 2 b + c) resulting in a short P–O bond. In compound 5, the two σ-bonds (P–O and O–H) form two linear combinations (σO−H + σP−O (d) and σO−H – σP−O), with less polarization towards the oxygen atom compared to 6. Furthermore, the p-type orbitals of the oxygen atom show slightly diminished, but still significant negative hyperconjugation (Fig. 2 e + f). Interestingly, the interaction energy in 5 gives similar results for the solvation models (ΔGinteraction 3.9 kcal/mol) and the gas phase (ΔGinteraction 3.2 kcal/mol) despite their significantly different structures.

Fig. 2

Graphical representation of selected molecular orbitals for 6 (top a–c) and 5 (bottom (d–f) at a contour level of 0.1 a.u. a: HOMO-15. b: HOMO-1. c: HOMO. d: HOMO-19. e: HOMO-4. f: HOMO-3.

Conclusion

We were able to synthesize and structurally characterize the hydroxymethyl phosphonium chloride, which proves the presence of distinct ion pairs in the solid state. Different calculations referring to gas phase, solution, and “solid” state reveal a large influence of the molecular surroundings on the bonding situation besides the counter-ion which plays a crucial role for the prevailing structural isomer of these compounds. Widely implemented continuum models like the PCM were used to model the environment in the solid state for an easy prediction and confirmation of the solid state structure.

Crystal data
Empirical formula	C3H10OP+Cl−
Formula weight	128.53
Crystal description	Block, colorless
Crystal size	0.28 × 0.28 × 0.18 mm
Crystal system, space group	Monoclinic, P 21/n
Unit cell dimensions
a	6.3264(2) Å
b	9.1397(3) Å
c	11.3681(4) Å
β	105.8420(10) °
Volume	632.35(4) Å3
Z	4
Calculated density	1.350 Mg/m3
F(000)	272
Linear absorption coefficient μ	0.735 mm−1
Absorption correction	Semi-empirical from equivalents
Max. and min. transmission	0.748 and 0.643
Unit cell determination	2.90 < Θ < 40.67°
	6625 reflections used at 100K
Data collection
Radiation source	Fine-focus sealed tube
Radiation and wavelength	MoKα, 0.71073 Å
Scan type	ϕ and ω scans
Θ range for data collection	3.36 to 40.00°
Index ranges	−11 ≤ h ≤ 11, −15 ≤ k ≤ 16, −15 ≤ l ≤ 20
Reflections collected/unique	10607/3906
Significant unique reflections	3507 with I > 2σ(I)
R(int), R(sigma)	0.0177, 0.0194
Completeness to Θ = 40.0°	99.4%
Refinement
Refinement method	Full-matrix least-squares on F2
Data/parameters/restraints	3906/95/0
Goodness-of-fit on F2	1.052
Final R indices [I > 2σ(I)]	R1 = 0.0211, wR2 = 0.0568
R indices (all data)	R1 = 0.0245, wR2 = 0.0585
Extinction expression	None
Weighting scheme	w = 1/[σ2(Fo2) + (aP)2 + bP] where P = (Fo2 + 2Fc2)/3
Weighting scheme parameters a, b	0.0315, 0.0495
Largest Δ/σ in last cycle	0.001
Largest difference peak and hole	0.437 and −0.356e/Å3