Ionic Dissociation of SiCl₄: Formation of [SiL₆]Cl₄ with L=Dimethylphosphinic Acid

Abstract

Reactions of SiCl₄ with R₂PO(OH) (R=Me, Cl) yield compounds with six‐fold coordinated silicon atoms. Whereas R=Me afforded the hexacoordinated tetra‐cationic silicon complex [Si(Me₂PO(OH))₆]⁴⁺ with chloride counter‐ions, R=Cl caused release of HCl with formation of a cyclic dimeric silicon complex [Si(Cl₂PO(OH))(Cl₂PO₂)₃(μ‐Cl₂PO₂)]₂ with bridging bidentate dichlorophosphates.

Ionic dissociation of SiCl₄: The first example of a structurally characterized tetra‐cationic hexacoordinated silicon tetrachloride is reported. It was obtained by reaction of dimethylphosphinic acid with SiCl₄ under ambient conditions. Additionally, the structure of a cyclic dimeric hexacoordinated silicon complex was determined by single‐crystal X‐ray diffraction and verified by solid‐state NMR spectroscopy.

Design of hypercoordinated silicon compounds is of particular interest in organic chemistry, where they may play roles as catalysts and reagents.1, 2 As heavier congener of carbon, silicon and its higher coordination number compounds are of particular academic interest as well. Thus, various neutral, anionic and cationic silicon complexes have been reported.3, 4 A more specific topic are hexacoordinated silicon compounds with [SiO₆] skeletons. In naturally occurring low‐pressure silicates5 and silicophosphate minerals6 the silicon atom is present in fourfold coordination. Only several high‐pressure silicates, for example, stishovite or MgSiO₃ 5 and synthesized (at atmospheric pressure) crystalline silicophosphate compounds, for example, SiP₂O₇ 7 or Si₅O(PO₄)₆ 8 contain hexacoordinated silicon atoms. Molecular silicon compounds with [SiO₆] skeletons include besides anionic species, like the dianions of [Si^VI(PO₄)₆(Si^IV(OEt)₂)₆],9 [Si^VI(S₂O₇)₃]10, 11 or [Si^VI(P₃O₉)₂],12 also neutral complexes13 and cationic complexes. The latter include mainly monocationic species14 while some tetracationic complexes are known in literature only for systems with very soft (“silicophobic”) counter‐ions, for example, hexa(pyridine‐N‐oxide)silicon tetraiodide ([Si(pyO)₆]I₄)15 or hexakis(N,N‐dimethylformamide)silicon tetraiodide ([Si(DMF)₆]I₄).16 As to neutral O‐donor ligands, hypercoordinated adducts of SiF₄ with phosphine oxides, trans‐[SiF₄(OPPh₃)₂],17 and of SiCl₄ with phosphoric amides, for example, [Cl₃Si(OP(NMe₂))₃]‐cation,18 are also known, while OH‐functionalized P=O‐compounds tend to act as anionic ligands.

In this regard, dichlorophosphates ([Cl₂PO₂]⁻) are described as bridging bidentate ligands through the coordination of both oxygens to metal atoms. Additionally, dichlorophosphate can operate as monofunctional and also as trifunctional group.19 In trimethylsilyl dichlorophosphate, Cl₂PO₂SiMe₃, only one oxygen is coordinated to silicon because of the poor hypercoordination tendency of SiMe₃ groups.19, 20 Dimethylphosphinate groups ([Me₂PO₂]⁻) were also handled as bridging bidentate ligands.21, 22, 23 Here, we report the crystal structures of the two different six‐fold coordinated silicon compounds SiPCl and SiPMe (Scheme 1), which enhance the portfolio of coordination motifs. The complexes were prepared by reaction of SiCl₄ with dichlorophosphoric acid in benzene (SiPCl) and dimethylphosphinic acid in chloroform (SiPMe) (see Supporting Information for details).

Scheme 1.

Preparation of the hexacoordinated silicon compounds SiPMe and SiPCl ([SiO₆]‐structural units are bold indicated for clarity).

Figure 1 shows the molecular structure of SiPCl in the crystal (triclinic, space group P‐1). A compound of this formal composition “H[Si(O₂PCl₂)₅]” has been reported in literature (also obtained by the reaction of dichlorophosphoric acid and SiCl₄ in benzene),24 but its structure has not been elucidated so far. Compound SiPCl is a cyclic dimer, which is located about a center of inversion. Two bridging Cl₂PO₂ ligands between two silicon atoms furnish a Si₂O₄P₂ eight‐membered ring. The remaining four coordination sites at each of the octahedrally coordinated Si atoms host monodentate O‐donor ligands, that is, three Cl₂PO₂ ⁻ anions and one (HO)OPCl₂ molecule. Thus, the molecule bears five crystallographically non‐equivalent dichlorophosphate groups. The silicon coordination spheres are slightly distorted octahedra with bond lengths and trans‐O‐Si‐O angle ranges of 1.73–1.79 Å and 175.6–178.4°, respectively. The Si−O bond lengths increase in the order monodentate anionic < bridging anionic < mono‐dentate neutral ligand (O1 and O5 share roles of anionic and neutral ligand as their Cl₂PO₂ moieties are intermolecularly connected by hydrogen bridges (i.e., O10−H10 is directed toward O7* of an adjacent molecule, O7* generated from O7 by symmetry operation x‐1, y, z)). The intermolecular separation O10−O7* (2.48 Å) is in accord with a highly attractive hydrogen bridge, but disorder of the P1O7Cl1Cl2 group hampers any detailed discussion of this hydrogen bridge. The P−O distances range from 1.45 Å (normal double bond lengths) to 1.53 Å and the angles about phosphorus span a range of 105–114° with angles increasing in the order Cl‐P‐Cl<Cl‐P‐O<O‐P‐O (in accordance with VSEPR). P−Cl and P−O distances in SiPCl do not differ significantly from corresponding distances in POCl₃ (P−Cl: 1.98 Å, P=O: 1.45 Å)25 or other related compounds with Cl₂PO₂ ligands, for example, [(Cl₃SnOPCl₃)⁺(PO₂Cl₂)⁻]₂ (P−Cl: 1.98 Å, P−O: 1.50 Å).26

Figure 1.

Molecular structure of SiPCl in the crystal. (ORTEP representation with 50 % probability ellipsoids). The P1O7Cl1Cl2 group was disordered and refined in two positions (site occupancy ratio 0.81(2):0.19(2)). Selected bond lengths and angles are provided in Table 1.

²⁹Si and ³¹P solid‐state NMR spectroscopy points out few impurities from dichlorophosphoric acid and its (silylated) condensation products in the bulk solid material. Nevertheless, we were able to assign the ²⁹Si chemical shift (−214.5 ppm) to the [Si(OP)₆] octahedra and the ³¹P NMR signals to the phosphorous atoms of the crystal structure SiPCl (δ(³¹P)=1.8 ppm (P1), −21.5 ppm (P2), 4.5 ppm (P3), −10.4 ppm (P4), 15.1 ppm (P5)). The combination of ³¹P single pulse (SP), cross polarization (CP) and ³¹P double quantum (DQ)/single quantum (SQ) MAS NMR spectra allows the complete assignment of the ³¹P chemical shifts. The latter spectrum is given in Figure 2. We observe one diagonal DQ peak at −21.5 ppm (for the spatially closest chemically equivalent atoms P2) and furthermore, two DQ peak pairs at 1.8 ppm and −21.5 ppm (P1‐P2) as well as at 4.5 ppm and −21.5 ppm (P3−P2). A detailed explanation of the NMR analysis and the assignment is given in Supporting Information.

Figure 2.

³¹P DQ‐SQ‐MAS‐NMR spectrum of SiPCl.

Following our studies, we changed the phosphorus compound from dichlorophosphoric acid to dimethylphosphinic acid. Dimethylphosphinate groups [(Me₂PO₂]⁻) were also reported as bridging bidentate ligands.21, 22, 23 Known structures with dimethylphosphinate groups, for example, Et₂Sn(O₂PMe₂)₂,21 Me₃SnO₂PMe₂ 22 and Et₂ClSnO₂PMe₂ 23 are polymeric, whereas Ph₂Sn(O₂PMe₂)₂ has a polymeric ring‐chain structure. The Me₂PO₂ ligands functioning as double bridges, like in SiPCl, between tin atoms to give eight‐membered Sn₂O₄P₂ rings.21 Here, we can report a new crystalline product based on dimethylphosphinic acid as a neutral oxygen‐donor ligand, which was obtained by the reaction of tetrachlorosilane with dimethylphosphinic acid in chloroform.

Contrary to our expectation (i.e., substitution with release of HCl), a novel tetra‐cationic silicon complex was created by ionic dissociation of SiCl₄, because the dimethylphosphinic acid (pK=3.1)27 is a weaker acid than for example, methylphosphonic acid (pK=2.3)27 or methylphosphoric acid (pK=1.5).25 In contrast the dichlorophosphoric acid is described as a strong acid, which forces weaker acids out of its salts, for example, HCl from CaCl₂.28 The formula composition in the crystal structure of compound SiPMe (monoclinic, space group C2/c, Figure 3) comprises of a [Si(Me₂PO(OH))₆]⁴⁺ cation, four chloride ions and one chloroform molecule. The Si atom is located on a crystallographic center of inversion, thus the asymmetric unit contains a half cation, two Cl anions and half a molecule CHCl₃. The silicon atom of SiPMe is surrounded by the dimethylphosphinic acid molecules as neutral ligands in a nearly regular octahedral geometry with an average Si−O bond length of 1.76 Å (Table 1). The P−O distances range from 1.52 to 1.55 Å and are in good agreement with normal single bond lengths. One of the Cl anions in the asymmetric unit is hydrogen bonded to the OH groups of two neighboring cations. The other one is hydrogen bonded to only one cation. The H⋅⋅⋅Cl separations of these hydrogen bonds are around 2.1 Å. Additionally, Cl₃CH⋅⋅⋅Cl (2.7 Å) and PCH₃⋅⋅⋅Cl (2.9 Å) contacts were also found. Only a few other tetra‐cationic silicon compounds with neutral ligands were reported in literature, for example, hexa(pyridine‐N‐oxide)silicon tetraiodide, [Si(pyO)₆]I₄,15 hexakis(N,N‐dimethylformamide)silicon tetraiodide, [Si(DMF)₆]I₄,16 hexakis(N,N‐dimethylformamide)‐silicon tris(tribromide) chloride, [Si(DMF)₆][Br₃]₃Cl29 and complexes with six N‐donor atoms.30, 31 The compound SiPMe is insoluble in organic solvents, so the ²⁹Si and ³¹P NMR chemical shifts (δ(²⁹Si)=−204.2 ppm; δ(³¹P)=56.88 ppm) were obtained from solid state NMR experiments.

Figure 3.

Molecular structure of SiPMe. (ORTEP representation with 50 % probability ellipsoids; C‐bonded hydrogen atoms omitted for clarity). The crystal structure is built up from [Si(Me₂PO(OH))₆]⁴⁺‐cations with four chloride ions and one molecule chloroform. Selected bond lengths and angles are summarized in Table 1.

Table 1.

Selected bond lengths [Å] and angles [°] for SiPCl and SiPMe.

Bond lengths [Å]		Angles [°]
SiPCl
Si1−O1	1.770(2)	O3‐Si1‐O5	178.42(10)
Si1−O2	1.763(2)	O4‐Si1‐O2	175.64(10)
Si1−O3	1.730(2)	O2‐Si1‐O5	87.87(9)
Si1−O4	1.736(2)	O6‐P2‐O2	114.06(11)
Si1−O5	1.790(2)	O2‐P2‐Cl4	108.57(8)
Si1−O6	1.773(2)	Cl3‐P2‐Cl4	105.98(4)
SiPMe
Si1−O1	1.7505(11)	O1‐Si1‐O3	89.14(6)
Si1−O3	1.7648(11)	O1‐Si1‐O5	89.77(5)
Si1−O5	1.7710(11)	O3‐Si1‐O5	90.97(5)
P3−O5	1.5230(11)	O5‐P3‐O6	107.99(7)
P3−O6	1.5543(13)	O5‐P3‐C6	113.32(8)
		O6‐P3‐C6	110.10(9)
		O5‐P3‐C5	107.65(8)
		O6‐P3‐C5	108.79(9)
		C6‐P3‐C5	108.88(9)

In order to probe whether the weaker Si−Br bond dissociates in a related manner, SiBr₄ was treated with dimethylphosphinic acid, too. In fact, the corresponding bromide salt SiPMe^Br formed, which shows very similar ²⁹Si and ³¹P NMR signals and crystallizes as an isomorph of SiPMe (with longer unit cell axes because of the larger halide anion). Thus, a comparison of the corresponding data of SiPMe and SiPMe^Br is given in the Supporting Information. The chemistry related to compound SiPMe is subject of ongoing investigations, because SiPMe is the first example of a structurally characterized tetra‐cationic hexacoordinated silicon tetrachloride. Many further structures of such tetra‐cationic silicon complexes may be accessible based on reactions of SiCl₄ and SiBr₄ with other phosphinic acid derivatives.

Furthermore, the activation of SiCl₄ with chiral bisphosphoramides is known in literature32 and leads to cationic silicon species, which are employed as catalysts, for example, for the enantioselective allylation of aldehydes.32, 33, 34, 35 Also, phosphoric amides proved highly active as Lewis base catalysts for the disproportionation of methylchlorodisilanes.36 This activation of the weak Lewis acid SiCl₄ and other chlorosilanes by that special kind of very polar Lewis base (which has an inherent negative connotation because of the toxicity of phosphoric amides) may be transferred to the chlorosilane/phosphinic acid derivatives system, which would represent a non‐toxic alternative to chlorosilane/phosphoramide systems. In addition to the donor qualities, dialkylphosphinic acids offer tools for including chiral information or linkers to solid supports (or both) by their alkyl backbone, rendering OP(OH)(alkyl)₂ a fundamental motif worthwhile being studied in this regard.

CCDC 1967458 (SiPCl), 1967459 (SiPMe) and 1987161 (SiPMe^Br) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

J. Kowalke, J. Wagler, C. Viehweger, E. Brendler, E. Kroke, Chem. Eur. J. 2020, 26, 8003.