Preparation and structure of the unique silicon(iv) cation [SiF 3(Me3tacn)]+

Article abstract of DOI:10.1039/b822236c

The trifluorosilicon(iv) cation [SiF₃(Me₃tacn)] ⁺, isolated as salts with [SiF₅]^-, [B{3,5-(CF₃)₂C₆H₃}₄] ^- or Cl^-, is obtain

Full text of DOI:10.1039/b822236c

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Preparation and structure of the unique silicon(IV) cation
[SiF3(Me3tacn)]+w
Fei Cheng, Andrew L. Hector, William Levason,* Gillian Reid, Michael Webster
and Wenjian Zhang
Received (in Cambridge, UK) 10th December 2008, Accepted 26th January 2009
First published as an Advance Article on the web 10th February 2009
DOI: 10.1039/b822236c
The trifluorosilicon(IV) cation [SiF3(Me3tacn)]+, isolated as
salts with [SiF5]ꢀ, [B{3,5-(CF3)2C6H3}4]ꢀ or Clꢀ, is obtained
by reaction of SiF4 with Me3tacn under anhydrous conditions,
and compared with the tetrafluoro complexes [SiF4(tmeda)]
and [SiF4{j2-Me2N(CH2)2NMe(CH2)2NMe2(CH2Cl)}][SiF5]
obtained with the acyclic di- and tri-amines.
with Na[B{3,5-(CF3)2C6H3}4] in CH2Cl2 solution affords
[SiF3(Me3tacn)][B{3,5-(CF3)2C6H3}4] cleanly.
Crystals of [SiF3(Me3tacn)]Cl, obtained from a sample
made in CH2Cl2, show (Fig. 1)y a distorted octahedral cation,
based upon facially coordinated Me3tacn and three fluoride
ligands, the cation having three-fold crystallographic symmetry.
The Clꢀ anion results from attack on the solvent by the
liberated ‘‘naked’’ fluoride—we have observed similar genera-
tion of chloride anion in some GeF4 chemistry.2 The Si–F
bond length in the cation (1.629(3) A) is slightly shorter than
in [SiF4(tmeda)] (below), 1.6432(10)–1.6541(12) A, or in the
anion in [Ph4P]2[SiF6] (1.689(1)–1.703(1) A).8 The displace-
ment of the fluoride ion from SiF4 by Me3tacn appears to be
unprecedented, and to explore this reaction further, we reacted
SiF4 with Me2N(CH2)2N(Me)(CH2)2NMe2 (pmdta), which is
the nearest acyclic analogue of the Me3tacn macrocycle,
Recently there has been much renewed interest in the coordination
chemistry of the p-block metals, in part resulting from the
importance of these elements in electronics (e.g. III–V
semi-conductors), organic synthesis and medicine.1 Our own
recent work has involved studies of Ge(IV) and Ge(II) complexes
with a variety of N-, O-, P- and S-donor ligands.2 Similar studies
of the non-metallic elements are much rarer, and although
silicon(IV) halides are well known as Lewis acids and hypervalent
silicon compounds continue to attract considerable interest,3
isolated complexes with neutral ligands are very limited. Most
are of type [SiX4L] or [SiX4L2], for example the [SiF4(py)2],
[SiF4(N–N)] (N–N = 2,20-bipyridyl or 1,10-phenanthroline).4
We now report that reaction of SiF4 with the triaza macrocycle
Me3tacn (N,N0,N00-trimethyl-1,4,7-triazacyclononane) under
anhydrous conditions in CH2Cl2 or toluene results in the forma-
tion of the stable cation [SiF3(Me3tacn)]+, isolated as the [SiF5]ꢀ
and also prepared [SiF4(tmeda)] as
spectroscopic model.
a structural and
The reaction of SiF4 with the linear triamine, pmdta, was
extremely sensitive to trace moisture, leading to the isolation
of [SiF5]ꢀ or [SiF6]2ꢀ salts with protonated triamine cations
(identified by 19F NMR spectroscopy and sometimes crystallo-
graphically). However, in rigorously anhydrous CH2Cl2
solution the major product was [SiF4{k2-Me2N(CH2)2N-
(Me)(CH2)2NMe2(CH2Cl)}][SiF5], and the crystal structure
salt.z The average silicon–fluorine bond energy is 582 kJ molꢀ1 5
,
which constitutes the strongest single bond involving a group
14 element. Therefore, displacement of Fꢀ by neutral ligands
under such mild conditions was very unexpected. Although
[SiF3(Me3tacn)]+ must be made under anhydrous conditions to
avoid protonation of the amine, once formed, the cation is
surprisingly robust—the complex is air stable as a solid, and is
not decomposed by common donor solvents (MeCN, thf). The
positive and negative ion electrospray mass spectra (MeCN) show
cation and anion respectively as the only significant features. The
IR spectrum shows four Si–F stretching vibrations at 882(s), 798(s)
(assigned to [SiF5]ꢀ)6 and 770(sh), 751(m) (assigned as the a1 and e
modes of the cation). The 19F{1H} NMR spectrum shows a
sharp singlet at d = ꢀ147.6 with 29Si satellites (1JSiF = 208 Hz)
due to the cation, and a broad feature at d = ꢀ137, assigned
to the fluxional [SiF5]ꢀ anion.7 The spectra from some prepara-
tions also show small amounts of [SiF6]2ꢀ (d = ꢀ128) and Fꢀ
(for which d varies considerably with solvent).7 Metathesis
Fig. 1 View of the cation in [SiF3(Me3tacn)]Cl with atom numbering
scheme. Ellipsoids are drawn at the 50% probability level and H
atoms are omitted for clarity. Symmetry operations: a = 1 ꢀ y,
x ꢀ y, z; b = 1 ꢀ x + y, 1 ꢀ x, z. Selected bond lengths (A) and
angles (1): Si1–F1 = 1.629(3), Si1–N1 = 2.013(3); F1–Si1–F1a =
School of Chemistry, University of Southampton, Southampton,
UK SO17 1BJ. E-mail: wxl@soton.ac.uk
w CCDC 713184–713186. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b822236c
95.24(15), N1–Si1–N1a
F1b–Si1–N1 = 88.14(12).
= 85.32(14), F1a–Si1–N1 = 90.90(12),
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Notes and references
z [SiF3(Me3tacn)][SiF5]: SiF4 gas was slowly bubbled into a solution of
Me3tacn (173 mg, 1.0 mmol) in 15 mL toluene with stirring for 2 h,
resulting in slow formation of a white precipitate. The volatiles were
removed under vacuum to leave a white solid, which was then washed
with hexane and dried. Yield: 4 90%. The NMR spectra show small
amounts of [SiF6]2ꢀ and Fꢀ (a common problem in fluorosilicate
systems);7 hence, satisfactory microanalytical data were not obtained.
ES+ mass spectrum (MeCN): m/z = 256 [SiF3(Me3tacn)]+; ESꢀ mass
spectrum (MeCN): m/z = 123 [SiF5]ꢀ. 1H NMR (300 MHz, 295 K,
CD2Cl2): 3.37 (m) [6H], 3.09 (m) [6H], 2.80 (s) [9H]. 19F{1H} NMR
(376.5 MHz, 295 K, CD2Cl2): ꢀ137.0 (br) [SiF5]ꢀ, ꢀ147.6 (s) 1JSiF
=
208 Hz. IR (Nujol/cmꢀ1): 882(s), 798(s), 770(sh), 751(m).
The preparation was also carried out in CH2Cl2, producing a
spectroscopically identical product. Mixing equivalent amounts of
[SiF3(Me3tacn)][SiF5] and Na[B{3,5-(CF3)2C6H3}4] in CH2Cl2,
followed by removal of NaF by filtration, gave [SiF3(Me3tacn)]-
[B{3,5-(CF3)2C6H3}4]. 19F{1H} NMR (376.5 MHz, 295 K,
Fig. 2 View of the structure of the cation in [SiF4{Me2N(CH2)2N-
(Me)(CH2)2NMe2(CH2Cl)}][SiF5] with atom numbering scheme.
Ellipsoids are drawn at the 50% probability level and H atoms are
omitted for clarity. Selected bond lengths (A) and angles (1): Si1–F1 =
1.634(3), Si1–F2 = 1.656(3), Si1–F3 = 1.645(3), Si1–F4 = 1.655(3),
Si1–N1 = 2.026(4), Si1–N2 = 2.069(4); N1–Si1–N2 = 86.02(17),
F–Si1–F (cis) = 91.05(15)–95.87(16).
1
CD2Cl2): ꢀ63.2 (s), ꢀ147.0 (s) JSiF = 205 Hz. ES+ mass spectrum
(MeCN): m/z = 256 [SiF3(Me3tacn)]+, ESꢀ mass spectrum (MeCN):
m/z = 863 [B{3,5-(CF3)2C6H3}4]ꢀ.
[SiF4{j2-Me2N(CH2)2N(Me)(CH2)2NMe2(CH2Cl)}][SiF5]: SiF4 was
bubbled slowly into a solution of pmdta (173 mg, 1.0 mmol) in 15 mL
CH2Cl2 with stirring for 1 h. The solvent was removed in vacuo, to
leave a white solid, which was washed with hexane and dried in vacuo.
1H NMR (295 K, CD2Cl2): 2.70 (br,s) [2H] CH2Cl, 2.43 (m) [4H] CH2,
2.34 (m) [4H] CH2, 2.21 (s) [3H], MeN, 2.18 (br,s) [12H] Me2N.
19F{1H} NMR (CD2Cl2): ꢀ125.4 (t), ꢀ126.2 (t), ꢀ137.0 (s), ꢀ153.7
(Fig. 2)z shows an octahedral silicon centre coordinated to
four fluorines with similar d(Si–F) to those in [SiF4(tmeda)]
(below), and with the triamine bound in a k2-mode. The ‘‘free’’
amine function is quaternised by a CH2Cl group resulting
from reaction with the solvent. An [SiF5]ꢀ anion pro-
vides electroneutrality. The spectroscopic data are also con-
sistent with this formulation, and there was no evidence
for formation of an SiF3N3 cation analogous to the Me3tacn
complex.
2
(t) JFF = 18 Hz. Colourless crystals of [SiF4{k2-Me2N(CH2)2-
N(Me)(CH2)2NMe2(CH2Cl)}][SiF5] were obtained by cooling
dichloromethane solution of the complex in a freezer.
a
[SiF4(tmeda)]: SiF4 gas was slowly bubbled into a solution of tmeda
(116 mg, 1.0 mmol) in 15 mL of CH2Cl2 for 30 min with stirring. The
solvent was removed under vacuum to leave a white solid, which was
recrystallised from CH2Cl2 solution. Yield: 495%. Anal. calc. for
C6H16F4N2Siꢂ1/4CH2Cl2: C, 31.1; H, 6.7; N, 11.6. Found: C, 31.4; H,
7.5; N, 11.9%. 1H NMR (300 MHz, CD2Cl2): 2.72 (s) [4H], 2.50 (s)
[SiF4(tmeda)]8 contains a distorted six-coordinate silicon centre
(Fig. 3) with chelating tmeda, and the 19F{1H} NMR spectrum
(CD2Cl2) shows two triplets, d = ꢀ132.6 (2JFF = 20 Hz,
1
1JSiF = 188 Hz), ꢀ150.9 (2JFF = 20 Hz, JSiF = 150 Hz), the
2
[12H]. 19F{1H} NMR (295 K, CD2Cl2): ꢀ132.6 (t) JFF = 20 Hz,
2
1
1JSiF = 188 Hz), ꢀ150.9 (t) JFF = 20 Hz, JSiF = 150 Hz). IR
(Nujol/cmꢀ1): 843(s), 801(s), 725(m). Colourless crystals of [SiF4(tmeda)]
were obtained from CH2Cl2 solution by cooling in a freezer for
several days.
higher frequency resonance being due to Si–FtransN
.
y Crystal data for [SiF3(Me3tacn)]Cl: a few air-stable crystals of
[SiF3(Me3tacn)]Cl were obtained by slow evaporation of the solvent
from an MeCN solution of the product obtained from a reaction in
CH2Cl2. C9H21ClF3N3Si, M = 291.83, trigonal, space group R3c
(no. 161), a = 8.779(2), c = 29.255(2) A, U = 1952.7(6) A3, Z = 6,
m(Mo-Ka) = 0.405 mmꢀ1, T = 120 K, 2633 reflections measured,
911 unique reflections, Rint = 0.053, 53 parameters refined, R1
(all data) = 0.065, R1 [I 4 2s(I)] = 0.059, wR2 (all data) = 0.156,
wR2 [I 4 2s(I)] = 0.151. Structure solution and refinement were
routine.9,10
z Crystal
NMe2(CH2Cl)}][SiF5]: C10H25ClF9N3Si2, M = 449.96, triclinic, space
data
for
[SiF4{j2-Me2N(CH2)2N(Me)(CH2)2-
ꢀ
group P1 (no. 2), a = 6.9707(15), b = 11.533(2), c = 12.264(2) A, a =
107.672(10), b = 95.625(10), g = 98.336(10)1, U = 918.9(3) A3, Z = 2,
m(Mo-Ka) = 0.425 mmꢀ1, T = 120 K, 17 682 reflections measured,
4177 unique reflections, Rint = 0.081, 231 parameters refined, R1
(all data) = 0.125, R1 [I 4 2s(I)] = 0.082, wR2 (all data) = 0.237,
wR2 [I 4 2s(I)] = 0.201. Structure solution and refinement were
routine.9,10
Fig. 3 View of the structure of [SiF4(tmeda)] with atom numbering
scheme. Ellipsoids are drawn at the 40% probability level and H atoms
are omitted for clarity. Selected bond lengths (A) and angles (1):
Si1–F1 = 1.6541(12), Si1–F2 = 1.6477(11), Si1–F3 = 1.6432(10),
Si1–F4 = 1.6448(12), Si1–N1 = 2.0409(17), Si1–N2 = 2.0463(14);
N1–Si1–N2 = 85.10(7), F–Si1–F (cis) = 92.68(5)–96.42(6).
8 Crystal data for [SiF4(tmeda)]: C6H16F4N2Si, M = 220.30, monoclinic,
space group P21 (no. 4), a = 6.3146(15), b = 10.648(2), c = 7.6840(15) A,
b = 112.180(10)1, U = 478.40(17) A3, Z = 2, m(Mo-Ka) = 0.265 mmꢀ1
T = 120 K, 5735 reflections measured, 2049 unique reflections, Rint =
,
Thus, the formation of a [SiF3(ligand)]+ cation appears to
be unique to Me3tacn, reflecting the very strong tendency of
this macrocycle to coordinate as a tridentate ligand, and
leading to a remarkably stable complex. In contrast to the
results2 with GeF4, attempts to obtain thioether complexes of
Si(IV), including with [9]aneS3, did not lead to complexation.
We thank RCUK for support (EP/C006763/1).
0.027, 122 parameters refined, R1 (all data) = 0.030, R1 [I 4 2s(I)] =
0.028, wR2 (all data) = 0.071, wR2 [I 4 2s(I)] = 0.069. Structure solution
and refinement were routine.9,10
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