Structure of the telluroketone TeC(PPh2NSiMe3) 2 coordinated to an Li8O6 cluster

Article abstract of DOI:10.1016/j.poly.2013.02.005

The reaction of Li₂[C(PPh₂NSiMe₃) ₂] with elemental tellurium in THF was carried out in an attempt to generate the tellurocarbonyl dianion [TeC(PPh₂NSiMe₃) ₂]^2- as the dilithium derivative. This thermally unstable tridentate ligand was characterized by ³¹P and ⁷Li NMR spectroscopy. Attempted recrystallization gave yellow crystals, which were shown by X-ray crystallography to be a neutral, centrosymmetric complex comprised of two molecules of the novel telluroketone [TeC(PPh₂NSiMe ₃)₂] stabilized by N,Te,N coordination to LiOLi units of the rhombic dodecahedral cluster [Li₈(μ₅-O) ₂(μ₅-OH)₄(C₄H₈O) ₄].

Full text of DOI:10.1016/j.poly.2013.02.005

Polyhedron 53 (2013) 230–234
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structure of the telluroketone TeC(PPh2NSiMe3)2 coordinated to an Li8O6 cluster
⇑
Ramalingam Thirumoorthi, Tristram Chivers
Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of Li2[C(PPh2NSiMe3)2] with elemental tellurium in THF was carried out in an attempt to
generate the tellurocarbonyl dianion [TeC(PPh2NSiMe3)2]2À as the dilithium derivative. This thermally
unstable tridentate ligand was characterized by 31P and 7Li NMR spectroscopy. Attempted recrystalliza-
tion gave yellow crystals, which were shown by X-ray crystallography to be a neutral, centrosymmetric
complex comprised of two molecules of the novel telluroketone [TeC(PPh2NSiMe3)2] stabilized by N,Te,N
Received 18 October 2012
Accepted 4 February 2013
Available online 10 February 2013
Keywords:
coordination to LiOLi units of the rhombic dodecahedral cluster [Li8(
l5-O)2(l5-OH)4(C4H8O)4].
Methanediides
Lithium–oxygen cluster
Telluroketone
Ó 2013 Elsevier Ltd. All rights reserved.
X-ray structure
1. Introduction
Ph2
P
Ph2
P
S
PPh2
C
I
C
Recently Davies et al. reported the synthesis of the bis(dite-
llurophosphinate) [PhP(Te)2CH2CH2P(Te)2Ph]2À as its dilithium
derivative [1]. Attempted recrystallization of this extremely air-
and moisture-sensitive product produced a small amount of yellow
crystals that were identified by X-ray crystallography. The compo-
sition was established to consist of the dianion [PhP(Te)2CH2CH2-
P(Te)2Ph]2À and the dication [Li8(OH)6(THF)8]2+ (1) (Fig. 1), which
exhibits a rhombic dodecahedral cluster arrangement of Li and O
atoms [1]. However, discussion of the structural parameters of this
interesting complex were prevented by the inferior quality of the
X-ray data (R1 = 12.55%).
S
S
Li
Se
(TMEDA)
Se
Li
Li
PPh2
(3)
S
TMEDA
TMEDA
(2)
In contrast to the reactions with sulfur or selenium, the treat-
ment of Li2[C(PPh2S)2] with tellurium does not proceed cleanly
to give
a single product [3]. Consequently, we turned our
attention to the reaction of the well-known methanediide
Li2[C(PPh2NSiMe3)2] [4–6] with tellurium to determine whether
the presence of bulky SiMe3 substituents on the ligating nitrogen
atoms might have a stabilizing influence on the C–Te functional-
ity. In this contribution we describe the formation and X-ray
structure of a dimeric complex that incorporates two molecules
of the neutral ligand TeC(PPh2NSiMe3)2 (4), a rare example of a
telluroketone (tellone) [7], which is stabilized by N,Te,N-coordi-
nation to LiOLi units of the neutral rhombic dodecahedral cluster
In recent work we have synthesized dilithium salts of the chac-
ogenocarbonyl dianions [EC(PPh2S)2]2À (E = S, Se) by the reactions
of Li2[C(PPh2S)2] with elemental chalcogens in the presence of tet-
ramethylethylenediamine (TMEDA) [2]. The solid-state structure of
the selenium derivative 2 was shown to be bicyclic with the Li+ cat-
ions bis-S,Se-chelated by the dianionic ligand. Two-electron oxida-
tion of this dianion with I2 yielded the neutral selone stabilized as a
complex with lithium iodide {[Li(TMEDA)][I(Se)C(PPh2S)2]} (3); the
attempted removal of LiI from 3 resulted in decomposition of the
selone [2].
Li8(l5-O)2(l5-OH)4(C4H8O)4 (vide infra). The latter is related to
the dicationic Li8O6 cluster illustrated in Fig. 1 [1] by the re-
moval of two protons. Previously, the structural characterization
of telluroketones had been limited to 2-telluroimidazoline (5) [8]
and the tungsten pentacarbonyl complex of 1,1,3,3-indantellone
(6) [9].
⇑
Corresponding author. Tel.: +1 403 220 5741; fax: +1 403 289 9488.
E-mail address: chivers@ucalgary.ca (T. Chivers).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.02.005

R. Thirumoorthi, T. Chivers / Polyhedron 53 (2013) 230–234
231
(THF)8]2+ are now taken up by nitrogen donor atoms of the two
telluroketone ligands 4. As was the case with the polyhedral dica-
tion [Li8(OH)6(THF)8]2+ [1], the neutral cluster Li8(
l5-O)2(l5-OH)4-
(C4H8O)4 is likely formed by the interaction of lithium cations with
small amounts of moisture that were absorbed by the glass vessel
during the prolonged recrystallization of 7. The dication [Li8(OH)6-
(THF)8]2+ is viewed to be comprised of a hexameric (LiOH)6
aggregate which is bi-capped by two Li+ cations [1] based on the
well-known propensity of lithium alkoxides to form hexameric
(LiOR)6 clusters [11]. The formation of Li2O from the hydrolytic
degradation of highly moisture-sensitive lithium reagents by
adventitious moisture is a well-established phenomenon. Some
recent examples include the sandwich complexes {Li{nBuC(Nt-
Fig. 1. The Li8O6 rhombic dodecahedron in [Li8(OH)6(THF)8]2+(1) [1].
Te
W(CO)5
Te
Ph2
P
Ph2
P
iPr
SiMe3
iPr
Me3Si
C
N
N
N
N
Te
4
( )
(5)
(6)
2. Results and discussion
Bu)2}}4ÁLi2O [12] and [3-furyl-C(NTMS)2Li]4ÁLi2O [13], [2-FC6H4-
C(NTMS)2Li]4ÁLi2O [14]. Earlier representatives can be found in
Refs. [15–19]. While hydrolysis is the likely source of LiOH in 8,
the formation of Li2O could also result from reduction of adventi-
tious O2 by the highly reducing dianion [(Te)C(PPh2NSiMe3)2]2À
in 7, which would also explain the formation of the telluroketone 4.
The quality of the X-ray structure of 8 (R1 = 8.54%) is sufficient
to allow a detailed discussion of the structural parameters, which
are summarized in Table 1. The central carbon in the telluroketone
The reaction ofLi2[C(PPh2NSiMe3)2]with telluriumpowderinTHF
initially produced a green solution which became dark purple after
several hours at 23 °C. The 31P NMR spectrum of the reaction mixture
showed two major resonances at d 17.1 and 34.2 , both of which were
singlets, together with a minor resonance at d À4.9, which is attribut-
able to the neutral precursor H2C[PPh2(NSiMe3)]2, cf. lit value:
À3.0 ppm in CDCl3 [10]. When the reaction was monitored by 31P
NMR spectroscopy the major peak observed was the singlet at d
17.1, but a substantial amount of unreacted Li2[C(PPh2NSiMe3)2]
remained after 1 h, in addition to the resonance at d À4.9 for H2-
C[PPh2(NSiMe3)]2 and some minor resonances in the d 32–34 region.
Removal of the solvent from a reaction mixture that had been stirred
for ca. 24 h gave an extremely air- and moisture-sensitive sticky
black residue, which was extracted with heptane to give a yellow
solution. This solution exhibited only the resonances at d 17.1 and
À4.9 in the 31P NMR spectrum; the 7Li NMR spectrum showed a sin-
glet atd 1.06, but attemptstorecordthe 125TeNMR spectrumresulted
in decomposition as manifested by the formation of elemental tellu-
rium on the sides of the NMR tube. These observations suggest that
the 31P NMR resonance at d 17.1 can be attributed to the expected
product [Li(THF)]2[TeC(PPh2NSiMe3)2] (7) (Scheme 1); the resonance
observed at d 34.2 after longer reactions times is likely due to a
decomposition product resulting from the thermal lability of the C–
Te bond.
TeC(PPh2NSiMe3)2 is essentially planar (
R
\C13 ꢀ 357.8°), consis-
tent with a three-coordinate carbon centre. The P–C (1.729(8) Å)
and P–N (1.583(7) Å) bond distances are shorter and longer,
respectively, than those in [CH2(PPh2NSiMe3)2] (P–C 1.825(1) and
P–N 1.536(2) Å) [20] suggesting electron delocalization within
the NPCPN backbone of the tellone 4. The C–Te bond length of
2.065(7) Å in 8 is comparable with reported values of 2.087(4) Å
for the 2-telluroimidazolines 5 [8] and 2.04–2.05 Å in telluroa-
mides [21], but significantly longer than the distance 1.987(5) Å
found for the metal complex 6 in which the telluroketone ligand
is weakly coordinated to tungsten [9]. The calculated carbon–
tellurium double bond distance in Me2C@Te is 1.968 Å [22], while
C-single bond values is 2.158 Å [23].
The Te–O distance of 1.9456(5) Å falls within single and double
bond values. A Te@O distance of 1.829(1) Å has been reported re-
cently for a monomeric telluroxane [24], cf. a calculated value of
1.814 Å for Ph2Te@O [25], while the mean Te–O single-bond value
in the polymer [(4-MeOC6H4)2TeO]n is 2.063 Å [26]. Thus the tellu-
rium center is bonded quite strongly to the Li8O6 cluster in 8. The
O–Te–C bond angle is 97.1(3)°.
Attempts to grow crystals of [Li(THF)]2[TeC(PPh2NSiMe3)2] (7)
were unsuccessful, presumably owing to thermal instability. How-
ever, storage of the yellow heptane solution in an argon-filled
glove box for 1 week yielded a few yellow crystals that were iden-
The Li–O distances within the Li8O6 cluster fall within the range
1.894(15)–2.049(14) Å. All Li atoms in this cage structure are four-
coordinate, but there are two different lithium environments. Each
Li atom in the central lithium core (Li1, Li2, Li1⁄, Li2⁄) is connected
to an oxide, two hydroxides and a THF molecule. By contrast, the
THF molecule is replaced by an imino nitrogen donor from the
(Te)C(PPh2NSiMe3)2 ligand for each of the other four lithium atoms
(Li3, Li4, Li3⁄, Li4⁄). The Li–O distances for the latter quartet of lith-
ium atoms (1.937(16)–2.049 (14) Å are significantly longer than
those exhibited by Li1, Li2, Li1⁄ and Li2⁄ (1.894(15)–1.959
(15) Å). The Li–N distances of 2.049(14) and 2.057(14) Å are
tified by X-ray crystallography as {[TeC(PPh2NSiMe3)2] [Li4(l5
O)( [8(C7H16)2] (Scheme 1 and Fig. 2a).
5-OH)2(C4H8O)2]ÁC7H16
Complex 8 is a centrosymmetric dimer comprised of two
molecules of the telluroketone TeC(PPh2NSiMe3)2 (4) N,Te,N-
coordinated to two LiOLi units of the dimeric Li8O6 cluster
-
l
}
2
Li8(l5-O)2(l5-OH)4(C4H8O)4 (Fig. 2). As can be seen from a compar-
ison of Figs. 1 and 2b, this neutral cluster is derived from the dica-
tion [Li8(OH)6(THF)8]2+ [1] through the replacement of two
hydroxyl (OHÀ) groups by two oxide (O2À) ligands; additionally,
four of the coordination sites occupied by THF ligands in [Li8(OH)6

232
R. Thirumoorthi, T. Chivers / Polyhedron 53 (2013) 230–234
Scheme 1. Synthesis of Li2[TeC(PPh2NSiMe3)2] (7) and subsequent formation of 8.
slightly shorter than the values of 2.108(9) and 2.128(9) Å reported
for Li2[C(PPh2NSiMe3)2], which also adopts a dimeric structure
[27].
Bruker 400 spectrometers. The 1H, 31P and 7Li chemical shifts are
given relative to TMS, 85% H3PO4 and aqueous LiCl, respectively.
H2C(PPh2NSiMe3)2 was obtained in 87% yield by the reaction of
Ph2PCH2PPh2 with an excess of neat trimethylsilyl azide at 130 °C
according to the literature procedure [10]. The reagent Li2[C(PPh2-
NSiMe3)2] was prepared by the treatment of CH2[PPh2(NSiMe3)]2
with two equivalents of tert-butyllithium in benzene [28]. Yield:
60%. 31P NMR (162 MHz, C6D6, 25 °C): d 14.6 (s), cf. lit value:
14.4 ppm [4]; 7Li NMR (155.5 MHz, C6D6, 25 °C): d 2.98 (s).
3. Experimental
3.1. General procedures
All reactions and manipulations of reagents and products were
carried out under an argon atmosphere by using standard Schlenk
or glove-box techniques. Solvents were dried over and then dis-
3.1.1. Reaction of Li2[C{PPh2(NSiMe3)}2] with tellurium
t
tilled from Na/benzophenone (THF, benzene and heptane). BuLi
A mixture of Li2[C(PPh2NSiMe3)2] (0.33 g, 0.58 mmol) and tellu-
rium powder (0.074 g, 0.58 mmol) in a 100 mL flask was cooled to
À78 °C. Tetrahydrofuran (15 mL) was slowly added to the flask and
the reaction mixture was stirred for 15 min to give a green solution
(1.7 M in pentane), tellurium powder, azidotrimethylsilane and
Ph2PCH2PPh2 were purchased from Aldrich Chemical Co. and used
without further purification. 31P NMR spectra were recorded on
Fig. 2. (a) Molecular structure of 8(C7H16)2. Solvent heptane molecule is not shown. Hydrogen atoms of phenyl, methyl and hydroxyl groups have been omitted for clarity (b)
the Li8O6 cluster in 8.

R. Thirumoorthi, T. Chivers / Polyhedron 53 (2013) 230–234
233
Table 1
Selected bond lengths (Å) and bond angles (°) for 8.
8
Te1–O1
Te1–C13
Li1–O1
Li1–O3
Li1–O2
Li1–O5
Li2–O1
Li2–O4
1.946(5)
2.065(7)
Li2–O3
Li2–O2
Li3–O1
Li3–O2
Li3–O3
Li3–N1
Li4–O2
Li4–O1
1.937(16)
1.951(15)
1.937(16)
2.009(15)
2.018(14)
2.057(14)
1.997(15)
2.043(16)
Li4–O3
Li4–N2
P1–N1
P1–C13
P1–C7
P1–C1
P2–N2
P2–C13
2.049(14)
2.142(14)
1.589(7)
1.730(8)
1.813(8)
1.816(8)
1.577(7)
1.728(8)
P2–C20
P2–C14
Si1–N1
Si2–N2
N2–Li4
O1–Li4
O2–Li3
O3–Li2
1.816(8)
1.838(8)
1.694(7)
1.700(7)
2.142(14)
2.043(16)
2.009(15)
1.937(16)
1.919(13)
1.925(15)
1.959(15)
1.961(14)
1.894(15)
1.923(17)
O1–Te1–C13
N1–P1–C13
N1–P1–C7
C13–P1–C7
N1–P1–C1
C13–P1–C1
C7–P1–C1
N2–P2–C13
N2–P2–C20
C13–P2–C20
N2–P2–C14
C13–P2–C14
C20–P2–C14
N1–Si1–C26
97.1(3)
117.7(4)
108.1(4)
109.5(4)
110.4(4)
103.8(4)
106.9(4)
116.1(4)
111.8(4)
104.8(4)
112.5(4)
109.7(4)
100.6(4)
111.0(4)
N1–Si1–C27
C26–Si1–C27
N1–Si1–C28
C26–Si1–C28
C27–Si1–C28
N2–Si2–C30
N2–Si2–C31
C30–Si2–C31
N2–Si2–C29
C30–Si2–C29
C31–Si2–C29
P2–C13–P1
P2–C13–Te1
P1–C13–Te1
110.1(4)
106.0(5)
116.3(4)
106.7(4)
106.2(4)
116.5(4)
111.1(4)
105.2(4)
108.6(4)
109.3(4)
105.5(5)
133.6(5)
114.1(4)
110.1(4)
Table 2
4. X-ray crystallography
Crystallographic data for 8(C7H16)2.
8
Crystallographic data for 8(C7H16 2
were collected with a Nonius Kappa CCD diffractometer with use
of monochromated Mo K radiation (k = 0.71073 Å) at 120 K. The
structure was solved by direct methods using the program SHELXS
97 and refined with SHELXL-97 [29] and by full-matrix least-squares
with anisotropic thermal parameters for the non-hydrogen atoms.
Hydrogen atoms of hydroxy groups (O2 and O3) were not located
in Fourier maps for 8. Hydrogen atoms were included in calculated
positions and were refined by riding mode.
) are given in Table 2. Data
Empirical formula
Formula weight
T (K)
C92H144P4N4Si4Li8O10Te2
2013.07
120(2)
0.71073
triclinic
P1
12.233(2)
14.883(3)
15.788(3)
74.70(3)
a
-
k (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
ꢀ
a
(°)
b (°)
68.57(3)
c
(°)
87.34(3)
2576.8(9)
1
1.297
0.728
5. Conclusions
V (Å3)
Z
In addition to providing a rare example of a structurally charac-
terized telluroketone, the title compound embodies a neutral
Li8O2(OH)4 cluster that is related by the removal of two protons
to the dicationic cluster [Li8(OH)6]2+ reported recently by Davies
et al. [1]. In contrast to that precedent, the structural determina-
tion in the present instance is sufficiently precise to allow a mean-
ingful discussion of geometrical parameters.
Dcalc (mg mÀ3
)
Absorption coefficient (mmÀ1
F(000)
)
1048
Crystal size (mm3)
h (°)
0.19 Â 0.13 Â 0.09
2.27–25.05
À14 6 h 6 14,
À17 6 k 6 17,
À18 6 l 6 18
Limiting indices
Reflections collected/unique (Rint
Completeness to theta
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices
)
16871/9040 (0.0910)
98.8%
Acknowledgment
0.9373 and 0.8740
Full-matrix least-squares on F2
9040/0/567
The authors are grateful to NSERC (Canada) for continuing
financial support.
1.138
R1, wR2 [I > 2
R1, wR2 (all data)
Largest difference in peak and hole (e ÅÀ3
r
(I)]
0.0869, 0.1608
0.1234, 0.1809
0.882 and À0.689
Appendix Supplementary. material
)
CCDC 906326 contains the supplementary crystallographic data
for complex 8. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
and then it was allowed to warm to room temperature and stirred
for a further 24 h to produce a dark purple solution. NMR data for
the THF solution: 31P NMR (162 MHz, 25 °C: d À4.9 (s), 17.1 (s),
34.2 (s).
Removal of solvent from the reaction mixture under vacuum
gave a dark (black) sticky solid, which was extracted with heptane
to give a yellow solution: 31P NMR (162 MHz, 25 °C) À4.9 (s), 17.1
(s); 7Li NMR (155.5 MHz, 25 °C): d 1.06 (s). After ca. 1 week in an
argon-filled glove box at room temperature, a few pale yellow crys-
tals were formed and identified as 8 by X-ray crystallography.
References
[1] R.P. Davies, M.G. Martinelli, L. Patel, A.J.P. White, Inorg. Chem. 49 (2010) 4626.
[2] J. Konu, T. Chivers, H.M. Tuononen, Chem. Eur. J. 16 (2010) 12977.
[3] J. Konu, R. Thirumoorthi, T. Chivers, unpublished observations.
[4] C.M. Ong, D.W. Stephan, J. Am. Chem. Soc. 121 (1999) 2939.
[5] A. Kasani, R.P.K. Babu, R. McDonald, R.G. Cavell, Angew. Chem., Int. Ed. 38
(1999) 1483.

234
R. Thirumoorthi, T. Chivers / Polyhedron 53 (2013) 230–234
[6] For a recent review of main-group and transition-metal complexes, see T.K.
Panada, P.W. Roesky, Chem. Soc. Rev. 38 (2009) 2782.
[17] M. Driess, H. Pritzkow, S. Martin, S. Rell, D. Fenske, G. Baum, Angew. Chem., Int.
Ed. 35 (1996) 986.
[7] For a brief review of telluroketones (tellones), see N. Kuhn, G. Verani, in: F.A.
Devillanova (Ed.), Handbook of Chalcogen Chemistry: New Perspectives in
Sulfur, Selenium and Tellurium, RSC Publishing, Cambridge, UK, 2007, p. 131
(Chapter 2.3).
[18] W. Clegg, L. Horsburgh, P.R. Dennison, F.M. Mackenzie, R.E. Mulvey, Chem.
Commun. (1996) 1065.
[19] F.M. Mackenzie, R.E. Mulvey, W. Clegg, L. Horsburgh, Polyhedron 17 (1998)
993.
[8] N. Kuhn, G. Henkel, T. Kratz, Chem. Ber. 126 (1993) 2047.
[9] (a) M. Minoura, T. Kawashima, R. Okazaki, J. Am. Chem. Soc. 115 (1993) 7019;
(b) M. Minoura, T. Kawashima, N. Tokitoh, R. Okazaki, Chem. Commun. (1996)
123.
[20] A. Müller, M. Möhlen, B. Neumüller, N. Faza, W. Massa, K. Dehnicke, Z. Anorg.
Allg. Chem. 625 (1999) 1748.
[21] G.M. Li, R.A. Zingaro, M. Segi, J.H. Reibenspies, T. Nakajima, Organometallics 16
(1997) 756.
[10] A. Hagenbach, S. Athenstädt, H.E. Daróczi, U. Abram, R. Alberto, Z. Anorg. Allg.
Chem. 630 (2004) 2709.
[22] M. Minoura, T. Kawashima, R. Okazaki, N. Shimizu, S. Kumakara, unpublished
work cited in Ref. [9].
[11] K.M. Fromm, E.D. Gueneau, Polyhedron 23 (2004) 1479. and other references
to [LiOR]6 structures cited in Ref. [1].
[23] F.H. Allen, O. Kennard, D.G. Watson, L.B. Brammer, A.G. Orpen, R. Taylor, J.
Chem. Soc., Perkin Trans. 2 (1987) S1.
[12] (a) T. Chivers, A. Downard, G.P.A. Yap, J. Chem. Soc., Dalton Trans. (1998)
2603;
[24] T.M. Klapötke, B. Krumm, M. Scherr, Phosphorus, Sulfur, Silicon Relat. Elem.
184 (2009) 1347.
(b) A. Downard, T. Chivers, Eur. J. Inorg. Chem. (2001) 2193.
[13] S. Aharonovich, M. Botoshanski, Z. Rabinovich, R.M. Waymouth, M.S. Eisen,
Inorg. Chem. 49 (2010) 1220.
[25] N.W. Alcock, W.D. Harrison, J. Chem. Soc., Dalton Trans. (1982) 709.
[26] J. Beckmann, D. Dakternieks, A. Dutihie, F. Ribot, M. Schürmann, N.A.
Lewcenko, Organometallics 22 (2003) 3257.
[14] C. Knapp, E. Lork, R. Mews, Z. Anorg. Allg. Chem. 629 (2003) 1511.
[15] H.-J. Gais, J. Vollhardt, J. Am. Chem. Soc. 110 (1988) 978.
[16] S.C. Ball, I. Cragg-Hine, M.G. Davidson, R.P. Davies, M.I. Lopez-Solera, P.R.
Raithby, D. Reed, R. Snaith, E.M. Vogl, J. Chem. Soc., Chem. Commun. (1995)
2147.
[27] A. Kasani, R.P.K. Babu, R. McDonald, R.G. Cavell, Angew. Chem., Int. Ed. 38
(1999) 1483.
[28] K.L. Hull, I. Carmichael, B.C. Noll, K.W. Henderson, Chem. Eur. J. 14 (2008)
3939.
[29] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.