Hypervalent neutral O-donor ligand complexes of silicon tetrafluoride, comparisons with other group 14 tetrafluorides and a search for soft donor ligand complexes

Article abstract of DOI:10.1039/c0dt01115k

The hypervalent adducts of SiF₄, trans-[SiF₄(R ₃PO)₂] (R = Me, Et or Ph), cis-[SiF₄{R ₂P(O)CH₂P(O)R₂}] (R = Me or Ph), cis-[SiF ₄(pyNO)₂] and trans-[SiF₄(DMSO)₂] have been prepared from SiF₄ and the ligands in anhydrous CH ₂Cl₂, and characterised by microanalysis, IR and VT multinuclear (¹H, ¹⁹F, ³¹P) NMR spectroscopy. The NMR studies show extensive dissociation at ambient temperatures in non-coordinating solvents, but mixtures of cis and trans isomers of the monodentate ligand complexes were identified at low temperatures. Crystal structures are reported for trans-[SiF₄(R₃PO)₂] (R = Me or Ph), and cis-[SiF₄(pyNO)₂]. The GeF ₄ analogues cis-[GeF₄{R₂P(O)(CH ₂)_nP(O)R₂}] (R = Me or Ph, n = 1; R = Ph, n = 2) were similarly characterised and the structures of cis-[GeF ₄{R₂P(O)CH₂P(O)R₂}] (R = Me or Ph) determined. The reaction of R₃AsO (R = Me or Ph) with SiF₄ does not give simple adducts, but forms [R₃AsOH]⁺ cations as fluorosilicate salts. SiF₄ adducts of some ether ligands (including THF, 12-crown-4) were also characterised by ¹⁹F NMR spectroscopy in solution at low temperatures (～190 K), but are fully dissociated at room temperature. Attempts to isolate, or even to identify, SiF₄ adducts with phosphine or thioether ligands in solution at 190 K were unsuccessful, contrasting with the recent isolation and detailed characterisation of GeF₄ analogues. The chemistry of SiF₄ with these oxygen donor ligands, and with soft donors (P, As, S or Se), is compared and contrasted with those of GeF₄, SnF₄ and SiCl₄. The key energy factors determining stability of these complexes are discussed.

Full text of DOI:10.1039/c0dt01115k

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PAPER
Hypervalent neutral O-donor ligand complexes of silicon tetrafluoride,
comparisons with other group 14 tetrafluorides and a search for soft donor
ligand complexes†
Kathryn George, Andrew L. Hector, William Levason,* Gillian Reid, George Sanderson, Michael Webster and
Wenjian Zhang
Received 26th August 2010, Accepted 23rd November 2010
DOI: 10.1039/c0dt01115k
The hypervalent adducts of SiF4, trans-[SiF4(R3PO)2] (R = Me, Et or Ph),
cis-[SiF4{R2P(O)CH2P(O)R2}] (R = Me or Ph), cis-[SiF4(pyNO)2] and trans-[SiF4(DMSO)2] have been
prepared from SiF4 and the ligands in anhydrous CH2Cl2, and characterised by microanalysis, IR and
VT multinuclear (1H, 19F, 31P) NMR spectroscopy. The NMR studies show extensive dissociation at
ambient temperatures in non-coordinating solvents, but mixtures of cis and trans isomers of the
monodentate ligand complexes were identified at low temperatures. Crystal structures are reported for
trans-[SiF4(R3PO)2] (R = Me or Ph), and cis-[SiF4(pyNO)2]. The GeF4 analogues cis-[GeF4{R2P(O)-
(CH2)nP(O)R2}] (R = Me or Ph, n = 1; R = Ph, n = 2) were similarly characterised and the structures of
cis-[GeF4{R2P(O)CH2P(O)R2}] (R = Me or Ph) determined. The reaction of R3AsO (R = Me or Ph)
with SiF4 does not give simple adducts, but forms [R3AsOH]+ cations as fluorosilicate salts. SiF4
adducts of some ether ligands (including THF, 12-crown-4) were also characterised by 19F NMR
spectroscopy in solution at low temperatures (~190 K), but are fully dissociated at room temperature.
Attempts to isolate, or even to identify, SiF4 adducts with phosphine or thioether ligands in solution at
190 K were unsuccessful, contrasting with the recent isolation and detailed characterisation of GeF4
analogues. The chemistry of SiF4 with these oxygen donor ligands, and with soft donors (P, As, S or Se),
is compared and contrasted with those of GeF4, SnF4 and SiCl4. The key energy factors determining
stability of these complexes are discussed.
octahedral polymer.5 These three fluorides readily behave as Lewis
acids towards F-, forming [MF6]2- (M = Si, Ge, Sn) and [MF5]-
Introduction
anions.8,9 We have recently reported the synthesis, spectroscopic
and structural characterisation of a range of neutral ligand
adducts of SnF4 and GeF4 with hard N- or O-donor ligands,10–13
and the first examples with soft P- or S-donor ligands.11,14,15
Thoroughly characterised complexes of SiF4 with neutral ligands
are relatively few and the majority are with N-donor ligands,
including [SiF4(2,2¢-bipy)], [SiF4(1,10-phen)], trans-[SiF4(py)2],
[SiF4(Me2NCH2SiF3)2], [SiF4{Me2N(CH2)2NMe2}],16,17 and we
have recently reported the first example of displacement of a
fluoride ion in [SiF3(Me3-tacn)]+ (Me3-tacn = 1,4,7-trimethyl-
1,4,7-triazacyclononane).17 Five- and six-coordinate adducts of
SiF4 with N-heterocyclic carbenes have been described very
recently and calculations suggest these should be more stable
than amine or phosphine adducts,18 but relatively little is known
about O-donor ligand complexes, which appear to be of limited
stability.19,20 There are also examples of secondary coordination
of crown ethers (i.e. the crown is hydrogen bonded to the silicon-
coordinated water molecules) in [SiF4(H2O)2]·crown (crown = 15-
crown-5, 18-crown-6).21 The only report of soft donor ligand
complexes with SiF4 is SiF4·nPMe3 (n = 1 or 2) formed at very low
Hypervalent silicon compounds remain an area of great research
activity. The majority of examples are organosilicon derivatives
or contain anionic bi- or poly-dentate nitrogen or oxygen donor
ligands.1–3 Silicon tetrahalides are strong Lewis acids and their
complexes with neutral donor ligands constitute a further group of
hypervalent compounds,4 although their study, particularly those
with SiF4, has been neglected in recent years. The tetrafluorides
of the Group 14 elements exhibit the extremes of chemical
behaviour.4 The lightest member, CF4, is an extremely inert gas,
whilst the heaviest, PbF4, is a polymeric solid containing six-
coordinate lead, only obtained pure by high pressure fluorination.5
PbF4 is a potent fluorinating agent and is incompatible with neu-
tral ligands, and [PbF6]2- is the only derivative known. The three
intermediate members are SiF4 and GeF4, which are molecular
monomers in gas, liquid and solid phases,6,7 and SnF4 which is an
School of Chemistry, University of Southampton, Southampton, UK SO17
1BJ. E-mail: wxl@soton.ac.uk
† CCDC reference numbers 791398–791404. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt01115k
1584 | Dalton Trans., 2011, 40, 1584–1593
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Scheme 1 Selected reactions of SiF4.
o
˚
Table 2 Selected bond lengths (A) and angles ( ) for [SiF4(OPPh3)2]
temperatures in the absence of solvent and identified by tensimetric
measurements and Raman spectroscopy.22 The aim of the present
study was to investigate the formation of the SiF4 adducts with O-
donor ligands, to establish if soft donor ligands (P, As or S) form
adducts, and to draw comparisons with results from our recent
work on SnF4 and GeF4 adducts; there are notable differences
compared with SiCl4 adducts which will also be discussed.
Si1–F1
Si1–F2
Si1–F3
Si1–F4
Si2–F5
Si2–F6
1.667(2)
1.654(2)
1.660(3)
1.659(2)
1.657(2)
1.654(2)
Si1–O1
Si1–O2
P1–O1
P2–O2
Si2–O3
P3–O3
1.832(3)
1.842(3)
1.519(3)
1.519(3)
1.848(3)
1.518(3)
F1–Si1–F2 89.71(13)
F2–Si1–F3 89.87(13)
F3–Si1–F4 90.47(13)
F4–Si1–F1 89.94(13)
O1–Si1–F
89.09(13)–91.10(13)
O2–Si1–F
88.96(13)–90.85(13)
Results and discussion
P1–O1–Si1 137.86(18)
F5–Si2–F6 89.76(12)
F6–Si2–O3 90.16(12)
P2–O2–Si1 138.66(18)
F5–Si2–O3 89.02(12)
P3–O3–Si2 142.47(19)
In our previous studies of GeF4 complexes,13–15 we found that
the complex [GeF4(MeCN)2], made by bubbling GeF4 through
anhydrous MeCN, was a convenient solid molecular synthon
for other germanium fluoride complexes. However, a suitable
analogue does not exist for SiF4—no solid complex forms between
SiF4 and MeCN at ambient temperatures, and as noted below,
ether adducts are only formed at low temperatures. Hence
synthesis of SiF4 adducts was normally achieved by bubbling SiF4
into a solution of the appropriate ligand in anhydrous CH2Cl2 or
toluene (see Scheme 1).
O-Donor ligands—phosphine oxides. Passing SiF4 into solu-
tions of OPMe3 or OPEt3 in anhydrous CH2Cl2 or OPPh3 in
toluene precipitated [SiF4(OPR3)2] (R = Me, Et, Ph) in good yield
as white solids. Crystals of the same complexes were obtained
by refrigerating the filtrates from the preparations. The crystal
structures of two examples [SiF4(OPMe3)2] and [SiF4(OPPh3)2]
(Tables 1 and 2, Fig. 1 and 2) showed them to contain six-
coordinate silicon with very close to octahedral geometry and to
Fig. 1 Structure of the centrosymmetric trans-[SiF4(OPMe3)2] showing
the atom numbering scheme. Displacement ellipsoids are shown at the 50%
probability level and H atoms are omitted for clarity. Symmetry operation:
a = 1 - x, 2 - y, 2 - z.
be trans isomers. The former is isomorphous with the germanium
analogue,12 but the [SiF4(OPPh3)2] crystallised in space group
o
˚
Table 1 Selected bond lengths (A) and angles ( ) for [SiF4(OPMe3)2]
¯
P1 with, unusually, Z = 3, whereas [GeF4(OPPh3)2]·2CH2Cl2
crystallised in the space group P21/n.12 The Si–F distances
Si1–F1
Si1–F2
P1–C
F1–Si1–F2
F1–Si1–O1
1.6719(9)
1.6660(9)
1.780(2)–1.790(2)
90.56(5)
88.92(5)
Si1–O1
P1–O1
—
P1–O1–Si1
F2–Si1–O1
1.8218(11)
1.5267(11)
—
131.44(7)
90.66(5)
˚
fall within a narrow range, 1.654(2)–1.6719(9) A, with Si–O at
˚
˚
1.8218(11) A in trans-[SiF4(OPMe3)2] and 1.832(3)–1.848(3) A in
trans-[SiF4(OPPh3)2]. The P–O distances are essentially identical to
those in the germanium analogues12 and markedly longer than in
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Table 3 Selected bond lengths (A) and angles (o) for
[GeF4{Me2P(O)CH2P(O)Me2}]a
˚
Ge1–F1
Ge1–F2
Ge1–F3
Ge1–O1
F1–Ge1–F2
F1–Ge1–F3
F1–Ge1–F1a
O1–Ge1–O1a
P1–C3–P1a
1.787(4)
1.772(5)
1.780(5)
1.930(4)
91.71(18)
91.04(17)
92.1(2)
P1–O1
P1–C1
P1–C2
P1–C3
F1–Ge1–O1
F2–Ge1–O1
F3–Ge1–O1
P1–O1–Ge1
—
1.532(4)
1.774(7)
1.774(7)
1.792(5)
87.98(17)
88.64(18)
88.60(17)
132.6(3)
—
91.9(2)
114.3(4)
a Symmetry operation: a = -x, y, z.
Table 4 Selected bond lengths (A) and angles (o) for
[GeF4{Ph2P(O)CH2P(O)Ph2}]·CH2Cl2
˚
Fig. 2 Structure of trans-[SiF4(OPPh3)2]. There are two molecules in the
asymmetric unit. The Si1 centred molecule shown here has no symmetry,
and the Si2 centred molecule has a crystallographic centre. Displacement
ellipsoids are shown at the 50% probability level and H atoms are omitted
for clarity. Only the ipso C atoms are labelled with the numbering going
cyclically round the ring.
Ge1–F1
Ge1–F2
Ge1–F3
Ge1–F4
F1–Ge1–F2
F1–Ge1–F3
F1–Ge1–F4
F1–Ge1–O1
F2–Ge1–O1
F3–Ge1–O1
O1–Ge1–O2
1.770(4)
1.772(4)
1.757(4)
1.771(5)
91.1(2)
92.1(2)
94.2(2)
88.9(2)
88.8(2)
86.4(2)
87.0(2)
Ge1–O1
Ge1–O2
P1–O1
P2–O2
F2–Ge1–F4
F3–Ge1–F4
F2–Ge1–O2
F3–Ge1–O2
F4–Ge1–O2
P1–O1–Ge1
P2–O2–Ge1
1.928(5)
1.926(5)
1.525(5)
1.519(5)
92.0(2)
92.6(2)
87.9(2)
88.5(2)
89.9(2)
131.3(3)
129.8(3)
the ligands (OPMe3 1.489(6), OPPh3 1.483(2) A).23 The IR spectra
˚
show strong P–O stretches, significantly lowered from the values
in the ligands,12 and intense broad n(SiF) ~800 cm-1, consistent
with expectation for trans isomers (theory, Eu). Solution NMR
measurements show the complexes are extensively dissociated
in solution at 295 K, and we note that evaporation of CH2Cl2
solutions of [SiF4(OPPh3)2] by gentle heating results in a residue
identified as OPPh3, suggesting this complex is largely dissociated
into its constituents in solution at room temperature. trans-
to PR3—oxidochlorosilanes are the other products24—and similar
behaviour for SiCl4 is not unexpected, but contrasts starkly with
that of SiF4.
Two diphosphine dioxide complexes [SiF4{R2P(O)CH2-
P(O)R2}] (R = Ph or Me) were isolated by reaction of the ligands
with SiF4 in CH2Cl2 solution. The [SiF4{Ph2P(O)CH2P(O)Ph2}]
showed several overlapping n(SiF) and two n(PO) vibrations in the
IR spectrum, consistent with the expected cis isomer. In solution
at ambient temperature, the complex is extensively dissociated,
1
[SiF4(OPPh3)2] fails to exhibit a 19F{ H} spectrum at 295 K, but
on cooling to 243 K, a singlet appears at d = -139 (s) and at 190 K
silicon satellites on the resonance are resolved (1JSiF = 145 Hz). The
1
31P{ H} NMR spectrum shows only some free OPPh3 at 295 K,
1
but at 190 K, a mixture of free and coordinated (d = 36) phosphine
oxide are present. There is no evidence for the presence of a cis
isomer, possibly disfavoured on steric grounds on the small silicon
showing no 19F{ H} NMR resonance, but on cooling the solution
to 273 K, a broad line appears at d = -125.0 and by 180 K two
resonances of equal intensity are seen at d = -112.2, -133.6, but
centre. The 19F{ H} and 31P{ H} NMR spectra of [SiF4(OPR3)2]
(R = Me or Et) in CH2Cl2 solution show broad singlets consistent
with exchanging systems at 295 K, but on cooling the lines split
and sharpen. For [SiF4(OPMe3)2] at 200 K, we observed in the
even at this temperature no JFF couplings were resolved. The
1
1
2
1
31P{ H} NMR spectrum at 295 K showed only free diphosphine
dioxide (d = 25), but at 180 K the resonance had shifted to d = 39,
consistent with coordination. The [SiF4{Me2P(O)CH2P(O)Me2}]
complex was very poorly soluble in chlorocarbons, acetone or
MeCN, and decomposed by DMSO or DMF with displacement
of the diphosphine dioxide. Hence no useful solution data
were obtained. The reaction of SiCl4 with Me2P(O)CH2P(O)Me2
produced mixtures containing chlorophosphonium(V) species and
was not pursued.
1
1
19F{ H} spectrum a singlet at d = -116.6 (s, JSiF = 130 Hz), and
two triplets at -115.2 (t, 2JFF ~7 Hz, 1JSiF = 140 Hz), -128.5 (br,t,
2JFF = 7 Hz, 1JSiF = 130 Hz), consistent with trans and cis isomers in
the ratio ~5 : 1, and two corresponding 31P{ H} resonances were
1
observed at 60.2 and 61.4 ppm. The spectra of [SiF4(OPEt3)2]
(experimental section) were similar except that the trans : cis isomer
2
ratio was ~10 : 1 and the small JFF couplings in the cis isomer
In view of the limited data obtainable on the silicon complexes,
we also prepared three GeF4 adducts with diphosphine dioxides,
[GeF4{Me2P(O)CH2P(O)Me2}], [GeF4{Ph2P(O)CH2P(O)Ph2}]
and [GeF4{Ph2P(O)(CH2)2P(O)Ph2}], which formed in high
yields from reaction of [GeF4(MeCN)2] in CH2Cl2 solution with
the appropriate ligand. Crystal structures were obtained for the
first two examples and show (Tables 3 and 4, Fig. 3 and 4) the
expected cis-chelate geometries. The bond lengths and angles
are unexceptional, showing a near to octahedral arrangement
of the donor atoms. The structure of Me2P(O)CH2P(O)Me2
was also determined (Fig. 5); it proves to be a strongly
coordinating O-donor ligand, although it has been very little
were not resolved. Overall the behaviour resembles that observed
for [GeF4(OPR3)2],12 but the silicon systems are markedly more
dissociated in solution. Some R3PO adducts of SiCl4 with 2 : 1
and 4 : 1 R3PO : SiCl4 have been reported to be formed from
benzene solutions, and identified by microanalysis.19c,20 In CH2Cl2
solution under similar conditions to those used above with SiF4,
we find that SiCl4 reacts with R3PO to form R3PCl2, PR3 and
1
[SiCl4(PR3)2] identified by in situ 31P{ H} NMR studies, with no
resonances attributable to phosphine oxide adducts evident (see
experimental section); the systems are also extremely sensitive to
moisture. SiHCl3 and Si2Cl6 are commonly used to convert R3PO
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Fig. 5 Structure of [Me2P(O)CH2P(O)Me2] showing the atom numbering
scheme. Displacement ellipsoids are shown at the 50% probability level and
◦
˚
H atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
P1–O1 = 1.489(3), P2–O2 = 1.492(3), P1–C3 = 1.819(4), P2–C3 = 1.805(4),
P–C(Me) = 1.781(4)–1.805(4), P1–C3–P2 = 116.0(2).
Fig. 3 Structure of [GeF4{Me2P(O)CH2P(O)Me2}] showing the atom
numbering scheme. Displacement ellipsoids are shown at the 50% proba-
bility level and H atoms are omitted for clarity. The molecule has mirror
symmetry. Symmetry operation: a = -x, y, z.
involvement of the solvent in the speciation is ruled out by
the observation of similar spectra in DMF solution with the
chemical shifts differing only slightly. The probable explanation
is that the first set of resonances belong to the octahedral
[GeF4{Me2P(O)CH2P(O)Me2}] monomer, and the second to
the dimer [GeF4{m-Me2P(O)CH2P(O)Me2}2GeF4]. Even near
ambient temperatures the 1H NMR spectrum in CD3CN
shows two CH3 and two CH2 resonances, consistent with this
explanation.
Other O-donor ligands. The reaction of OAsPh3 with SiF4 in
either toluene or CH2Cl2 produced a white precipitate identified
as [Ph3AsOH][SiF5] by microanalysis, IR spectroscopy (n = 743
1
(vs,br) (AsOH), 894, 800 ([SiF5]-) cm-1 8b,26,27 and 19F{ H} NMR
spectroscopy (CD2Cl2, 295 K) d = -138.1 [SiF5]- 8b). Some samples
also contained varying amounts of [SiF6]2- (d = -126). Colourless
crystals obtained separately from the filtrate were identified by
1
their unit cell28 and 19F{ H} NMR spectrum which is a singlet at
d = -91, as Ph3AsF2.29 The corresponding reaction using OAsMe3
gave a mixture, the major components of which were similarly
identified as [SiF6]2-, [SiF5]- and [Me3AsOH]+ and there is an
unidentified species with d(19F) = -79.9. In this case, Me3AsF2
(d(19F) = -53.7)30 was not detected among the products. The
structure of colourless crystals grown from CH2Cl2 solution
showed them to be [Me3AsOH]2[SiF6] (Fig. 6). The cation has
been structurally characterised once before as an iodide salt.31
The reactions were repeated several times and produced the
same products, although the relative amounts of the fluorosilicate
anions varied from preparation to preparation. The same products
formed when the reaction was conducted in plastic equipment,
eliminating the involvement of the glass vessels in the reaction.
The reaction of SiCl4 with Me3AsO gives Me3AsCl2 cleanly. Whilst
the reaction of metal and non-metal halides with phosphine
oxides usually results in simple adducts, the products with arsine
oxides are more variable. Thus, SnX4 (X = F, Cl, Br, I),10 GeF4,12
Fig. 4 Structure of [GeF4{Ph2P(O)CH2P(O)Ph2}]·CH2Cl2 showing the
atom numbering scheme. Displacement ellipsoids are shown at the 50%
probability level and H atoms and the solvate molecule are omitted for
clarity.
used since it was first reported.25 The P–O distances show
the expected increase on coordination. The multinuclear
NMR spectra obtained for [GeF4{Ph2P(O)CH2P(O)Ph2}]
and [GeF4{Ph2P(O)(CH2)2P(O)Ph2}] (experimental section)
show reversible dissociation/exchange in solution at ambient
temperatures, which slows on cooling and, by 273 K, the
expected features of cis isomers are resolved. The solution
behaviour of [GeF4{Me2P(O)CH2P(O)Me2}] was unexpectedly
more complicated. At 295 K in CD3CN solution (the complex is
1
32
33
insoluble in chlorocarbons) the 19F{ H} NMR spectrum shows
TiF4 and VOF3 give simple adducts, but GeX4 (X = Cl or
Br) give R3AsX2,12 and Ph3AsF2 was a decomposition product of
[VOF3(Ph3AsO)] in solution.33 Aqueous HF also converts Ph3AsO
to Ph3AsF2.34
The complexes [SiF4(pyNO)2] and [SiF4(dmso)2] have been
described several times,19b,19c,35 but with limited data. Both are
readily isolated as hygroscopic white crystalline solids by reaction
two broad resonances of equal intensity and the corresponding
1
31P{ H} NMR spectrum is a singlet. On cooling the solution to
~260 K the fluorine resonances are resolved into the expected
triplets, but on further cooling, two new resonances, also
triplets of 1 : 1 intensity, appear, as does a second phosphorus
resonance; the behaviour is reversible with temperature. The
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Fig. 6 Structure of one of the cations in [Me3AsOH]2[SiF6] showing the
atom numbering scheme, and the H-bond to F5 of the [SiF6]2- anion.
There are two cations in the asymmetric unit and the figure shows the As1
centred cation. The other cation is very similar and forms an H-bond to
F3. Displacement ellipsoids are shown at the 50% probability level and H
bonds to C atoms are omitted for clarity. S◦ymmetry operation: a = x, y +
Fig. 7 Structure of cis-[SiF4(PyNO)2] showing the atom numbering
scheme. Displacement ellipsoids are shown at the 50% probability level
and H atoms are omitted for clarity.
various ethers in CD2Cl2 solution over the temperature range 295–
180 K. No adduct formation was evident above ~200 K, but from
the SiF4/THF mixture a singlet at d = -128.0 appeared at 200 K
and at 185 K two additional broader resonances (1 : 1 intensity)
appeared at d = -130.2, -139.2. The behaviour is reversible with
temperature, leading to an assignment of these resonances to trans-
and cis-[SiF4(THF)2]. Similarly, from mixtures of SiF4 and 12-
crown-4 in CD2Cl2 solution, new resonances appeared on cooling
to 180 K at d = -125.0, -127.6 and -135.7, the last two with some
evidence of poorly resolved couplings. However, even at 180 K in
CD2Cl2 there was no evidence for the formation of adducts with
MeO(CH2)2OMe or 15-crown-5. We note that GeF4-ether adducts
are more stable but still dissociate at ambient temperatures.12
˚
1, z. Selected bond lengths (A) and angles ( ): As1–C = 1.890(6)–1.910(6),
As1–O1 = 1.731(5), O1–H1 = 0.84(2), O1 ◊ ◊ ◊ F5a = 2.616(6), Si1–F5 =
1.701(5), Si1–F3 = 1.720(5), Si1–F(rest) = 1.666(4)–1.694(5), C–As1–C =
111.3(3)–116.9(3), O1–As1–C = 102.2(3)–107.9(3).
of SiF4 with the ligands in CH2Cl2 or toluene solution. In
solution in CD2Cl2 at 295 K neither complex shows a 19F
NMR resonance, but at 180 K resonances of both cis and trans
isomers are present, with the cis the more abundant form in
each (cis : trans ~10 : 1). The crystal structure of [SiF4(pyNO)2]
has been determined (Table 5 and Fig. 7) and this shows it to be
the cis isomer with approximate two-fold symmetry (O1–Si1–O2
87.9(1)◦ and cis F–Si–F greater than 90◦). The isolation of the
cis isomer in the solid state was unexpected compared with the
structures of the phosphine oxide examples above. The reaction of
the stable radical TEMPO, (2,2,6,6-tetramethylpiperidine-N-oxyl)
with SiCl4 affords [Si(TEMPO)Cl3], in which the Si–O bond is
Soft donor ligand complexes. Based upon previous success-
ful characterisations of GeF4 complexes,14,15 the reaction of
SiF4 with alkylphosphines, including Et2P(CH2)2PEt2, PMe3, o-
C6H4(PMe2)2, and alkyldithioethers, RS(CH2)2SR (R = Me, Et),
were explored. The reactions were carried out in anhydrous
CH2Cl2 or toluene solutions with rigorous exclusion of oxygen
and moisture. The work-up involved removing volatile materials
in vacuo, by slow evaporation of the solvent at room temperature in
a glove box, or refrigeration at -18 ◦C. No phosphine or thioether
adducts were isolated from any of these systems; occasionally
very small amounts of white solids formed, which on examination
proved to be phosphine oxides or phosphonium fluorosilicates.‡
The possible formation of adducts was also explored by VT
NMR spectroscopy over the range 295–190 K. Mixtures of SiF4
˚
much shorter (1.619(1) A) and is viewed as a hydroxylammonium
derivative.36 In contrast the white [SiF4(DMSO)2] has single
strong and broad features at n = 1018 and 801 cm-1, which we
assign to the SO and SiF stretches of a trans isomer (theory
D4h = Eu(SiF4) and A2u (SO), respectively). Attempts to obtain
a crystal structure of the complex were unsuccessful, all crystals
examined appeared to be twinned. The reaction of SiCl4 with
DMSO results in deoxygenation, with formation of Me2S and
MeSCH2Cl.37 In contrast, the reaction with GeCl4 gives the
structurally authenticated trans-[GeCl4(DMSO)2].38
Ether complexes of SiF4 are unstable. Tensimetric studies of
SiF4/R2O (R2 = Me2, (CH2)4, (CH2)5 etc.) showed the formation
of adducts at low temperatures (~200 K),19a which dissociated on
warming, and in some cases polymerisation of the ether occurred.
1
1
and the phosphines showed 31P{ H} and H NMR resonances
of ‘free’ ligands (with small temperature drifts), whilst in the
1
19F{ H} spectra the only new resonances seen were attributed
1
to trace amounts of [SiF5]-. The 31P{ H} NMR results are
1
We recorded 19F{ H} NMR spectra on mixtures of SiF4 and
potentially slightly ambiguous since in other systems, including
the tetrafluoride complexes of Ge, Sn or Ti, the coordination
shifts were small and erratic,11,14,15,32 but the absence of new
o
˚
Table 5 Selected bond lengths (A) and angles ( ) for [SiF4(pyNO)2]
Si1–F1
Si1–F2
Si1–F3
Si1–F4
F1–Si1–F2
F1–Si1–F3
F1–Si1–F4
F1–Si1–O1
F1–Si1–O2
O1–Si1–O2
1.658(2)
1.660(2)
1.658(2)
1.651(2)
91.87(11)
93.75(11)
171.33(11)
90.90(10)
83.34(10)
87.91(11)
Si1–O1
Si1–O2
O1–N1
O2–N2
F2–Si1–F3
F2–Si1–F4
F3–Si1–F4
Si1–O1–N1
Si1–O2–N2
—
1.863(2)
1.878(2)
1.354(3)
1.359(3)
93.14(11)
93.89(11)
92.39(11)
120.57(19)
116.46(18)
—
1
resonances attributable to the target complexes in the 19F{ H}
NMR spectra suggest any interaction is very weak even at the
lowest temperatures.
We also attempted to extend the range of ligand types co-
ordinated to GeF4, but were unable to isolate any adducts
‡ Alkylphosphines slowly react with chlorocarbons by quaternisation at P,
although this is very slow for CH2Cl2 at ambient temperatures or below.
1588 | Dalton Trans., 2011, 40, 1584–1593
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with MeSe(CH2)2SeMe, phosphine sulfides or phosphine se-
not form, at least under the conditions used in this study. As
reorganisation energies fall, complex formation becomes more
favourable and this is confirmed experimentally.14,15 Finally, we
note that our inability to isolate phosphine complexes of SiF4, or to
detect such species at low temperature in solution by multinuclear
NMR spectroscopy, does not necessarily conflict with the report
of SiF4/PMe3 adducts22 formed at very low temperature in the
absence of a solvent. Matrix-isolation IR/Raman studies of the
PMe3/SiF4 system would seem worthwhile.
1
lenides. Low temperature 19F{ H} NMR studies of the system
GeF4/MeSe(CH2)2SeMe in anhydrous CD2Cl2, showed no new
resonances above ~220 K, but on further cooling two singlets
appeared, which at 185 K had resolved into two 1 : 2 : 1 triplets with
1 : 1 intensities, the effects being reversible with temperature. Com-
parison of the chemical shifts d = -72.5 (t), -115.7 (t) J = ~82 Hz,
with those in [GeF4{MeS(CH2)2SMe}] (d = -87.0 (t), -123.0 (t)
2JFF = 77 Hz)11, strongly suggest some [GeF4{MeSe(CH2)2SeMe}]
forms at very low temperatures, although uncomplexed GeF4 is
1
still the major feature in the 19F{ H} spectrum. We were unable to
Conclusions
observe a 77Se NMR spectrum from the solution at 185 K, which
could be due either to the presence of some dynamic process or to
the rather low sensitivity of the isotope.
The first detailed characterisation of neutral O-donor ligand
adducts of SiF4 has been achieved with phosphine oxides, pyNO
and DMSO ligands and evidence for complex formation in
solution at very low temperatures for some ethers presented.
Arsine oxides do not form isolable complexes, but react to produce
mixtures of R3AsF2, [R3AsOH]+ and fluorosilicate anions. This
work completes the series of Group 14 tetrafluorides with neutral
amine, N-heterocycles, phosphines, arsines, thioethers and their
oxides. The combination of these experimental data and the
theoretical work available from other research groups has provided
a basis for understanding this chemistry in some detail.
Comparisons of the acceptor properties within the Group 14
tetrafluorides. Comparison of the abilities of SnF4, GeF4 and
SiF4 to form complexes with neutral donor ligands shows the
ability to decrease SnF4 > GeF4 ꢀ SiF4.10–19 Coordination ability
falls very rapidly down both Groups 15 and 16. Hard N- and O-
donor ligands form complexes readily with all three fluorides, with
N-donors giving the more robust examples, while O-donor adducts
are often hygroscopic and are usually partially dissociated in
solution at ambient temperature. Softer phosphines and thioethers
give isolable complexes only with SnF4 and GeF4, whereas with
alkylarsines no pure complexes have been isolated, although some
reaction was evident for SnF4 and GeF4.10,11 A major factor behind
these experimental observations is the increasing mismatch of
orbital size and energy between the hard contracted Lewis acid
acceptor orbitals and the more diffuse ligand donor orbitals as
Groups 15 and 16 are descended. Theoretical investigations have
shown that the reorganisation energy to convert a tetrahedral
MF4 unit to the four-coordinate fragment of the octahedron is
substantial, particularly for SiF4 where the reorganisation severely
disrupts the very strong Si–F bonding.39–41 Fleischer39 has shown
by DFT calculations that the generally stronger Lewis acidity of
GeF4 vs. SiF4 is not due to inherently stronger donor–acceptor
bonds (which actually fall in bond energy from Si to Ge), but is due
to the lower energy required to rearrange/deform the tetrahedral
GeF4. These calculations are for the gas-phase molecules and so
do not take into account solvent or lattice energy factors. Both of
these are important in the real system since the calculations suggest
the rearrangement energies and donor–acceptor bond energies
almost cancel out in some cases.
The very strong Si–F bond (~582 kJmol-1), which is the strongest
single bond in Group 14 compounds, plays a key role and is
retained in the complexes formed with R3PO or DMSO, whereas
with the weaker Si–Cl bond in SiCl4, Cl/O exchange occurs with
the same ligands. This chemistry remains unpredictable in detail,
since both GeF4 and GeCl4 give simple adducts with R3PO or
DMSO, whereas arsine oxides form adducts with all four SnX4’s
and with GeF4, but undergo Cl/O exchange to form R3AsCl2
with SiCl4 and GeCl4. The SiF4/R3AsO systems differ again in
that protonation of R3AsO occurs to form [R3AsOH]+.
The mismatch in orbital size and energy between SiF4 and
soft phosphines or thioethers mean that donor–acceptor bond
energies will be smaller (although still likely to be inherently
favourable). Presumably these are not sufficient to pay back
the high reorganisation energy of SiF4, hence the complexes do
Experimental
All reactions were conducted using Schlenk, vacuum line and
glove-box techniques and under a dry dinitrogen atmosphere.
Solvents were dried by distillation from sodium benzophenone-
ketyl (toluene, THF) or CaH2 (DMSO, CH2Cl2). Electronic
grade SiF4 and GeF4 were obtained from Aldrich and used
as received. OPMe3 and OPEt3 (Strem) were freshly sublimed
in vacuo. OPPh3, pyNO and OAsPh3 (Aldrich) were heated
in vacuo. OAsMe3 was made as described42 and freshly sub-
limed in vacuo. Me2P(O)CH2P(O)Me2 was made by H2O2 ox-
idation of Me2PCH2PMe2.25 o-C6H4(PMe2)2, o-C6H4(AsMe2)2,
EtS(CH2)2SEt and MeS(CH2)2SMe were made by literature
methods,43–45 and R2P(CH2)2PR2 (R = Me or Et) and PMe3
were obtained from Strem and used as received. IR spectra were
recorded as Nujol mulls on Perkin Elmer PE 983G or Spectrum
100 spectrometers, 1H NMR spectra in CDCl3, CD2Cl2 or CD3CN
1
1
solutions on a Bruker AV300, 31P{ H} and 19F{ H} NMR spectra
on a Bruker DPX400 and referenced to 85% H3PO4 and CFCl3,
respectively. Microanalytical measurements on new complexes
were performed by Medac Ltd.
[SiF4(OPMe3)2]. OPMe3 (0.046 g, 0.5 mmol) was dissolved in
anhydrous CH2Cl2 (10 mL) under dinitrogen, and then the solution
frozen in liquid N2. The apparatus was evacuated, then allowed to
warm to room temperature and a slow stream of SiF4 passed into
the solution for 20 min. The mixture was stirred overnight, and the
white solid collected by filtration and dried in vacuo. Yield: 0.073 g,
51%. Required for C6H18F4O2P2Si·CH2Cl2: C, 22.5; H, 5.6. Found:
C, 22.1; H, 5.9%. 1H NMR (400 MHz, CD2Cl2, 293 K): d = 1.60
2
2
(d, JHP = 13.5 Hz); (200 K): d = 1.69 (d, JHP = 13 Hz), 1.71 (d,
2JHP = 13 Hz), ratio ~5 : 1. 31P{ H} NMR (CD2Cl2/CH2Cl2, 293
1
K): d = 51.2 (br, s); (200 K): 60.2 (s), 61.4 (s) ratio ~5 : 1. 19F{ H}
1
NMR (CD2Cl2/CH2Cl2, 293 K): d = -116.5 (s); (243 K): -115.0,
-116.6, -128.8; (200 K): -116.6 (s, 1JSiF = 130 Hz), -115.2 (t, 2JFF
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~7 Hz, 1JSiF = 140 Hz), -128.5 (br,t 2JFF = 7 Hz, 1JSiF = 130 Hz). IR
(Nujol): u = 1097 (vs, br), u(P O), 800 (br, s) u(SiF) cm-1.
30 min. The solution was concentrated to ~5 mL, and the white
solid formed was filtered off and dried in vacuo. Yield: 0.12 g,
83%. Required for C26H24F4GeO2P2·1/3CH2Cl2: C, 52.1; H, 4.1.
[SiF4(OPEt3)2]. [SiF4(OPEt3)2] was prepared as described for
1
Found: C, 52.1; H, 4.2%. H NMR (400 MHz, CDCl3, 293 K):
the OPMe3 analogue. Yield: 82%. Required for C12H30F4O2P2Si:
1
d = 2.93 (m, [4H], CH2), 7.61–7.91 (m, [20H], Ph). 31P{ H} NMR
1
C, 38.7; H, 8.1. Found: C, 38.3; H, 8.2%. H NMR (400 MHz,
(CD2Cl2/CH2Cl2, 293 K): no resonance; (253 K): d = 46.4 (s).
CD2Cl2, 293 K): d = 1.16 (t, [3H], Me), 1.94 (br, [2H], CH2);
1
19F{ H} NMR (CD2Cl2, 293 K): d = -106.4 (br), -129.5 (br); (273
1
no change at 190 K. 31P{ H} NMR (CD2Cl2/CH2Cl2, 293 K):
K): -101.3 (t, 2JFF = 58 Hz), -129.7 (t). IR (Nujol): u = 1150, 1096
(s, br), u(P O), 648, 634, 624(s) u(GeF) cm-1.
1
d = 66.3 (br s); (200 K): 72.3 (s), 71.8 (s), ratio ~10 : 1. 19F{ H}
NMR (CD2Cl2/CH2Cl2, 293 K): d = -137 (vbr, s); (223 K): -114.5,
1
-116.3, -128.0; (200 K): -116.6 (s, JSiF = 128 Hz), -114.5 (br s,
[GeF4{Me2P(O)CH2P(O)Me2}].
[GeF4{Me2P(O)CH2P(O)-
1JSiF = 136 Hz), -127.8 (br s, 1JSiF = 128 Hz). IR (Nujol): u = 1114
(vs, br), u(P O), 798, u(SiF) cm-1.
Me2}] was made similarly to the above. Yield: 90%. Required for
1
C5H14F4GeO2P2: C, 19.0; H, 4.4. Found: C, 18.9; H, 3.6%. H
NMR (CD3CN, 295 K): 1.80 (br), 2.11 (br), 2.96 (br), 3.05 (m);
[SiF4(OPPh3)2]. Into a solution of OPPh3 (0.13 g, 0.48 mmol)
in toluene (10 mL) SiF4 was bubbled for 20 min. The white
precipitate was filtered off and dried in vacuo. Yield: 0.09 g, 57%.
Crystals were deposited on refrigerating the filtrate. Required for
2
(245 K): 1.89 (m), 2.11 (m, both CH3), 3.00, 3.02 (2 ¥ t, JPH
=
1
13 Hz, CH2). 31P{ H} NMR (CD3CN, 295 K): d = 62.8(s); (227
1
K): 64.5 (s), 64.6 (s) ~2 : 1 ratio. 19F{ H} NMR (CD3CN, 295
K): d = -102.6 (br), -129.1 (s); (253 K): -103.8 (t, 2JFF = 55 Hz),
-128.6 (t 2JFF = 55 Hz), -117.4 (br), -134.6 (br); (225 K): -103.1
(t, 2JFF = 55 Hz), -127.8 (t, 2JFF = 55 Hz), -117.3 (t, 2JFF = 58 Hz),
1
C36H30F4O2P2Si: C, 65.5; H, 4.6. Found: C, 65.2; H, 4.7%. H
1
NMR (400 MHz, CD2Cl2, 293 K): d = 7.2–7.7 (m). 31P{ H} NMR
(CD2Cl2/CH2Cl2, 293 K): d = 29.4 (br s); (180 K): 29.6 (s), 36.0
2
-133.2 (t, JFF = 58 Hz), ratio of species ~2 : 1. IR (Nujol): u =
1
(s), ratio ~3 : 1.19F{ H} NMR (CD2Cl2/CH2Cl2, 293 K): d = not
1126 (vbr) u(P O), 644, 610, 579(s) u(GeF) cm-1.
observed; (243 K): -139 (s); (190 K): -139.0 (s, 1JSiF = 145 Hz). IR
(Nujol): u = 1089 (vs, br), u(P O), 801, u(SiF) cm-1.
In situ NMR experiments. Reactions were conducted in 5 mm
OD NMR tubes loaded in a glove box. To dry CD2Cl2 (2 mL)
was added R3PO and SiCl4 in 1 : 1 or 2 : 1 molar ratios, the tubes
[SiF4{Ph2P(O)CH2P(O)Ph2}]. SiF4 gas was bubbled slowly
through a solution of Ph2P(O)CH2P(O)Ph2 (0.21 g, 0.5 mmol)
in 15 mL of anhydrous CH2Cl2 for 1 h. The mixture was left to
stir for 1 h which resulted in a white precipitate. The solid was
isolated through filtration and dried in vacuo. Yield: 0.19 g, 75%.
Required for C25H22F4O2P2Si: C, 57.7; H, 4.3. Found: C, 58.7; H,
1
were sealed and the 31P{ H} and 31P-1H NMR spectra recorded
immediately, and again after 3 h. The relative amounts of the
products varied with the ratio of reactants and with time, but
no other products were identified in significant amounts. From
Me3PO the products were Me3PCl2 dP = 96.0, [SiCl4(Me3P)2] dP =
-6.0, Me3P dP = -62.0. Ph3PO gave Ph3PCl2 dP = 66.5, Ph3P dP =
-6.0, [SiCl4(Ph3P)2] dP = 5.5. Traces of water lead to the generation
of [R3PH]+, easily identified by the chemical shifts and large 1JPH
couplings. The chemical shift of [SiCl4(Me3P)2] dP = -6.0 was
established from an in situ mixture of Me3P and SiCl4; the complex
has been authenticated by a crystal structure.46
1
4.0%. H NMR (CD2Cl2, 295 K): d = 3.6 (t, [2H], CH2), 7.97–
1
7.27 (m, [20H], Ph); (185 K): 3.88 (br), 7.93–7.21 (m). 19F{ H}
NMR (CD2Cl2 293 K): no resonance; (273 K): -125.0 (vbr); (180
1
K): -112.2 (br s), -133.6 (br s) (couplings unclear). 31P{ H} NMR
(CD2Cl2/CH2Cl2, 293 K): d = 25.6 (s); (180 K): 39.6 (s). IR (Nujol):
u = 1154 (s), 1096(m) u(P O), 810(sh), 802(s) 790(sh) u(SiF) cm-1.
[SiF4{Me2P(O)CH2P(O)Me2}]. SiF4 gas was bubbled slowly
through a solution of Me2P(O)CH2P(O)Me2 (0.085 g, 0.5 mmol)
in anhydrous CH2Cl2 (15 mL) for 1 h. The mixture was left to stir
for a further 2 h, and then the solvent was removed in vacuo to yield
a white solid. Yield: 0.125 g, 92%. Required for C5H14F4O2P2Si: C,
22.1; H, 5.2. Found: C, 22.2; H, 4.7%. IR (Nujol): u = 1124, 1098
(s, br), u(P O), 820(m), 792(s) u(SiF) cm-1.
[Ph3AsOH][SiF5]. Ph3AsO (0.16 g, 0.5 mmol) was dissolved
in anhydrous toluene (15 mL) and SiF4 gas was slowly bubbled
through the solution for 1 h. After a further 30 min a large
amount of white precipitate had formed, which was isolated by
filtration and dried in vacuo. Yield: 0.07 g, 31%. Required for
1
C18H16AsF5OSi: C, 48.4; H, 3.6. Found: C, 47.9; H, 3.5%. 19F{ H}
NMR (CD2Cl2, 295 K): d = -138.1 (s). IR (Nujol): n = 743 (vs,br)
(u As-OH), 894, 800 u(SiF) cm-1.
[GeF4{Ph2P(O)CH2P(O)Ph2}]. [GeF4(MeCN)2]11 (0.15 g, 0.5
mmol) and Ph2P(O)CH2P(O)Ph2 (0.21 g, 0.5 mmol) were dissolved
in anhydrous CH2Cl2 (15 mL), and the mixture was stirred for
30 min. The solution was concentrated to ~5 mL and the white
precipitate isolated by filtration and dried in vacuo. Yield: 0.22 g,
78%. Required for C25H22F4GeO2P2: C, 53.2; H, 3.9. Found: C,
Reaction of OAsMe3 with SiF4. The ligand OAsMe3 (0.068 g,
0.50 mmol) was dissolved in anhydrous toluene (15 mL) and SiF4
gas was bubbled through the solution for 1 h. A further 30 min
of stirring yielded a large amount of white precipitate. The white
1
1
solid was isolated through filtration. H NMR (CD2Cl2, 223 K):
52.7; H, 4.0%. H NMR (400 MHz, CDCl3, 293 K): d = 3.81 (t,
1
d = 2.12 (s). 19F{ H} NMR (CD2Cl2, 295 K): d = -79.7 (s), -128.1
2
1
[2H], CH2, JHP = 13.5 Hz), 7.45–7.76 (m, [20H], Ph). 31P{ H}
(s), -137 (w). IR (Nujol): n = 747 (u As–OH), 879, 795 (SiF) cm-1.
NMR (CD2Cl2/CH2Cl2, 293 K): no resonance; (253 K): d = 41.0
(s). 19F{ H} NMR (CD2Cl2, 293 K): d = -102.7 (br), -132.4 (br);
1
Crystals were grown from CH2Cl2.
(273 K): -102.6 (t, 2JFF = 61 Hz), -131.9 (t). IR (Nujol): u = 1157,
1074 (s, br), u(P O), 648, 634, 625(s) u(GeF) cm-1.
[SiF4(pyNO)2]. [SiF4(pyNO)2] was made similarly to the phos-
phine oxide examples by bubbling SiF4 though a solution of pyNO
in CH2Cl2. Yield 80%. Colourless small crystals were formed by
slow evaporation of a CH2Cl2 solution of the complex. Required
for C10H10F4N2O2Si: C, 40.8; H, 3.4; N, 9.5. Found: C, 40.6; H,
[GeF4{Ph2P(O)(CH2)2P(O)Ph2}]. [GeF4(MeCN)2] (0.075 g,
0.25 mmol) and Ph2P(O)(CH2)2P(O)Ph2 (0.14 g, 0.25 mmol) was
dissolved in anhydrous CH2Cl2 (15 mL) and the mixture stirred for
1590 | Dalton Trans., 2011, 40, 1584–1593
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Dalton Trans., 2011, 40, 1584–1593 | 1591
©

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3.3; N, 9.3%. 1H NMR (CD2Cl2, 295 K): d = 7.53 (s, [2H]), 7.64 (s,
(c) I. Warf and M. Onyszchuk, Can. J. Chem., 1970, 48, 2250; (d) B.
Neumueller and K. Dehnicke, Z. Anorg. Allg. Chem., 2008, 634, 2567.
9 D. Babel and A. Tressaud in Inorganic Solid Fluorides, P. Hagenmuller
(Ed) Academic Press NY, 1985, Chapter 3.
1
[H]), 8.36 (s, [2H]); (180 K): 7.34 (br, [3H]), 8.17 (br, [2H]). 19F{ H}
NMR (CD2Cl2, 295 K): no resonance; (180 K): d = -133.6 (s),
-136.7 (t,2JFF = 13 Hz), -140.9 (t, 2JFF = 13 Hz). IR (Nujol): n =
862 (sh), 840 (s), 814 (s) (SiF) cm-1.
10 M. F. Davis, W. Levason, G. Reid and M. Webster, Polyhedron, 2006,
25, 930.
11 M. F. Davis, M. Clarke, W. Levason, G. Reid and M. Webster,
Eur. J. Inorg. Chem., 2006, 2773.
[SiF4(DMSO)2]. [SiF4(DMSO)2] was prepared similarly to the
phosphine oxide adducts. White powder. Yield 93%. Required for
C4H12F4O2S2Si·CH2Cl2: C, 17.4; H, 4.1. Found: C, 17.4; H, 4.5%.
12 F. Cheng, M. F. Davis, A. L. Hector, W. Levason, G. Reid, M. Webster
and W. Zhang, Eur. J. Inorg. Chem., 2007, 2488.
13 F. Cheng, M. F. Davis, A. L. Hector, W. Levason, G. Reid, M. Webster
and W. Zhang, Eur. J. Inorg. Chem., 2007, 4897.
14 M. F. Davis, W. Levason, G. Reid, M. Webster and W. Zhang, Dalton
Trans., 2008, 533.
15 M. F. Davis, W. Levason, G. Reid and M. Webster, Dalton Trans., 2008,
2261.
1
1H NMR (CD2Cl2, 295 K): d = 2.66 (s); (180 K): 2.66 (s). 19F{ H}
NMR (CD2Cl2, 295 K): no resonance; (200 K): d = -123.0 (s);
(180 K): -122.8 (s), -123.0 (s), -135.9 (s). IR (Nujol): n = 1018 (s,
br), (SO), 801 (vs) (SiF) cm-1.
16 (a) A. D. Adley, P. H. Bird, A. R. Frazer and M. Onyszchuk, Inorg.
Chem., 1972, 11, 1402; (b) T. Q. Nguyen, F. Qu, X. Huang and A. F.
Janzen, Can. J. Chem., 1992, 70, 2089; (c) V. A. Bain, R. C. G. Killean
and M. Webster, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst.
Chem., 1969, 25, 156; (d) A. N. Cheklov, V. V. Tkachev and S. A.
Lermontov, Zh. Strukt. Khim., 2003, 44, 1165; (e) V. O. Gel’mbol’dt,
V. Koroeva and A. A. Ennan, Russ. J. Coord. Chem., 2007, 33, 160;
(f) N. W. Mitzel, K. Vojinovic, T. Foerster, H. E. Robertson, K. B.
Borisenko and D. W. H. Rankin, Chem.–Eur. J., 2005, 11, 5114.
17 F. Cheng, A. L. Hector, W. Levason, G. Reid, M. Webster and W.
Zhang, Chem. Commun., 2009, 1334.
X-Ray crystallography. Details of the crystallographic data
collection and refinement parameters are given in Table 6. Crystals
suitable for single crystal X-ray analysis were obtained as described
above. Data collections used a Bruker-Nonius Kappa CCD
˚
diffractometer fitted with Mo Ka radiation (l = 0.71073 A)
and either a graphite monochromator or confocal mirrors, with
the crystals held at 120 K in a nitrogen gas stream. Structure
solution and refinement were straightforward,47–49 with a few
points of interest described below; H atoms bonded to C were
introduced into the models in idealised positions using the default
C–H distance. The Z = 3 arose for [SiF4(OPPh3)2] from one
centrosymmetric molecule and one general molecule in space
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¯
group P1. The two distinct molecules had very similar bond
lengths. The H atoms bonded to O in [Me3AsOH]2[SiF6] were
clearly identified in a late difference electron-density map as the
two highest peaks. They showed sensible O–H and As–O–H
parameters, together with plausible O–H ◊ ◊ ◊ F H-bonds and were
introduced into the model with refined coordinates and a DFIX
restraint on the O–H distance. The structure of [SiF4(pyNO)2]
proved curiously intractable. The systematic absences pointed to
the space group P21/c, but attempts with several packages to find
a solution failed. However, a solution in Pc readily emerged (with
two molecules in the asymmetric unit) with R1 = 0.06, suggesting
possible pseudo-symmetry. However, the vector set of the four Si
atoms in the Pc solution was consistent with the vectors of P21/c,
and introducing the calculated Si atom position into a structure-
factor/electron-density calculation readily yielded the solution in
this space group.
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Acknowledgements
We thank EPSRC for support (EP/H007369/1, EP/C006763/1).
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