Reactivity of [M(η4-P2C2tBu 2)] (M= Ge, Sn) with tert-butylphosphaethyne, P≡CtBu: Synthesis, structural characterisation and computational studies of the novel zwitterionic organophosphorus cage compounds [MP4C 4tBu4] (M = Ge, Sn)

Article abstract of DOI:10.1002/ejic.200700915

Treatment of [M(η⁴-P₂C₂tBu ₂)] with the phosphaalkyne P≡CtBu leads to the formation of the unusual zwitterionic cage compounds [M(P₄C₄tBu ₄)] (M = Ge, Sn) which have been fully characterised in solution by multinuclear NMR spectroscopy and the solid-state structure of [GeP ₄C₄tBu₄] has been elucidated by a single-crystal X-ray diffraction study. DFT calculations support the zwitterionic formulation and suggest the likely reaction pathway. Whilst [GeP₄C₄tBu₄] exhibits considerable stability, [SnP₄C₄tBu₄] shows gradual decomposition in solution within a matter of hours. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200700915

FULL PAPER
DOI: 10.1002/ejic.200700915
Reactivity of [M(η4-P2C2tBu2)] (M = Ge, Sn) with tert-Butylphosphaethyne,
PϵCtBu: Synthesis, Structural Characterisation and Computational Studies of
the Novel Zwitterionic Organophosphorus Cage Compounds [MP4C4tBu4]
(M = Ge, Sn)
Matthew D. Francis,*[a] Peter B. Hitchcock,[a] John F. Nixon,[a] and László Nyulászi[b]
Keywords: Cage compounds / Phosphorus / Tin / Germanium / NMR spectroscopy / DFT calculations / Zwitterion
Treatment of [M(η4-P2C2tBu2)] with the phosphaalkyne
PϵCtBu leads to the formation of the unusual zwitterionic
cage compounds [M(P4C4tBu4)] (M = Ge, Sn) which have
been fully characterised in solution by multinuclear NMR
spectroscopy and the solid-state structure of [GeP4C4tBu4]
has been elucidated by a single-crystal X-ray diffraction
study. DFT calculations support the zwitterionic formulation
and suggest the likely reaction pathway. Whilst [GeP4C4tBu4]
exhibits considerable stability, [SnP4C4tBu4] shows gradual
decomposition in solution within a matter of hours.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
ing the reaction of the phosphaalkyne PϵCtBu with the
group 14 1,3-diphosphacyclobutadienyl complexes [M(η4-
P2C2tBu2)] (M = Ge, 9;[5] Sn, 10[6]).
Introduction
Organo-phosphorus cage compounds have been the fo-
cus of considerable attention in recent years,[1] the two
major synthetic routes generally involving oxidative coup-
ling of polyphospholyl anions CnRnP5–n (n = 0–4) or the
thermal or metal-mediated oligomerization of phosphaal-
kynes PϵCR.[2] Our interest in this area has been on the
development of organo-phosphorus cages containing one
or more additional heteroatoms and a number of examples
of such cages have been described containing antimony 1,[3]
silicon 2 or germanium 3[4] and selenium 4 or tellurium 5[2n]
as shown in Figure 1. Cage 1 was derived from the diphos-
phastibolyl anion [P2SbC2tBu2]– and 2 and 3 from the tri-
phospholyl anion [P3C2tBu2]– (Scheme 1). The chalcogen-
substituted organo-phosphorus systems 4 and 5 were pre-
pared by an unusual facile insertion of the chalcogen into
a specific P–P bond of the hexaphospha-pentaprismane 6,
the latter being itself derived from the oxidative coupling of
two [P3C2tBu2]– anions.[2l] In this paper we wish to describe
the synthesis and characterisation of two new organophos-
phorus cage compounds [MP4C4tBu4] (M = Ge, 7; Sn, 8)
containing germanium and tin, by an unusual route involv-
Scheme 1.
Results and Discussion
Treatment of a pale yellow ether solution of [Ge(η4-
P2C2tBu2)] (9) with a 2.5 to threefold excess of PϵCtBu at
–70 °C followed by warming to ambient temperature led to
a deep red solution from which the unusual cage compound
[GeP4C4tBu4] (7), which could be isolated in 43% yield, af-
ter crystallisation from toluene, as a very dark red crystal-
line solid which melts without decomposition between 108
and 110 °C (Scheme 2). Compound 7 has been fully charac-
terised by multinuclear NMR spectroscopy and in the solid
state by a single-crystal X-ray diffraction study (see Fig-
ure 1, Table 1), which leads us to view it as a zwitterionic
structure (vide infra). The analogous tin compound
[a] Department of Chemistry, School of Life Sciences, University
of Sussex,
Brighton, BN1 9QJ, Great Britain
Fax: +44-1273677196
E-mail: matthew@electron-spin.com
J.Nixon@sussex.ac.uk
[b] Department of Inorganic and Analytical Chemistry, Budapest
University of Technology and Economics,
1521 Budapest, Szt. Gellért tér 4, Hungary
Fax: +36-14633642
E-mail: nyulaszi@mail.bme.hu
Eur. J. Inorg. Chem. 2008, 1761–1766
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1761

M. D. Francis, P. B. Hitchcock, J. F. Nixon, L. Nyulászi
FULL PAPER
[SnP4C4tBu4] 8 can be synthesised in the same way but,
Despite this, we feel confident that 7 and 8 are iso-
unlike 7 its comparative instability in solution has frus- structural even in the absence of a crystal structure of the
trated all attempts to isolate it in pure form and we have latter. Geometry optimization of at the B3LYP/
8
been unable to obtain structural characterisation although LANL2DZ(p) level resulted in a structure which is similar
a full NMR spectroscopic study has been carried out. to that of 7, however, the computed SnC16 bond is consider-
Moreover, attempts to form a stable derivative 8 by coordi- ably longer (2.327 Å) than the usual SnC single bond. The
nation of one or more P-lone pairs or the P=C double weakness of this bond might be related to the instability of
bonds to a metal fragment were unsuccessful.
the compound. The multinuclear NMR spectra of the two
compounds exhibit very similar features. The 31P{1H}
NMR spectrum of 7 shows four multiplet resonances con-
sistent with four distinct phosphorus environments. The
two low field resonances at δ = 450.1 and 271.6 ppm are
consistent with the phosphaalkenyl resonances P(3) and
P(4) whilst the higher field resonances at 76 and 49.6 ppm
are in the expected region for the saturated phosphorus
centres P(1) and P(2). At this point there exists possible
ambiguity in assigning these resonances. To facilitate the
assignment, the chemical shifts were calculated at the
B3LYP/cc-PVTZ//B3LYP/6-31+G* level for 7. Further cal-
culations have been carried out also at the B3LYP/6-
311+G**//B3LYP/6-31+G*, B3LYP/6-31+G*//B3LYP/6-
31+G* and B3PW91/6-31+G*//B3LYP/6-31+G* levels, the
ordering and the chemical shift difference of the two signals
remained unchanged for 7. The data in Table 2 show that
the P(3) signal should appear at δ = 200 ppm lower field
than the P(4) signal, suggesting that P(3) should be assigned
to the 450.1 ppm signal and the 271.6 ppm resonance
should be attributed to P(4). Although the small difference
between the P(1) and P(2) shifts does not allow decisive
predictions solely on the basis of the computations of the
chemical shifts, assignment of the phosphorus signals for
P(1) and P(2) can be logically deduced from their mutual P-
P coupling constants. Thus the resonance at δ = 450.1 ppm
[attributed to P(3) on the basis of the computations] shows
Scheme 2.
1
a large JP,P coupling (174.0 Hz) which is also exhibited by
the resonance at δ = 49.6 ppm, suggesting that the latter
signal belongs to P(1) which is directly bonded to P(3). The
remaining resonance at δ = 76.0 ppm should be attributed
to P(2), which is the neighbour of the unsaturated phospho-
rus P(4) (assigned to the 271.6 ppm signal) as evidenced by
their common JP,P coupling (379.3 Hz). Further confirm-
ation of this assignment can be obtained from the computa-
tion of the spin–spin coupling.
Figure 1. X-ray molecular structure of 7. Hydrogen atoms are omit-
ted for clarity.
Table 1. Selected bond lengths[a] [Å] and angles [°] in 7.
Distances
1
P3–C11
P3–P1
C11–P2
C1–Ge
C2–Ge
C2–P2
1.6914(12)
2.3196(16)
1.8470(10)
2.0633(5)
2.133(4)
P(2)–P(4)
P(4)–C(16)
C(16)–Ge
C(2)–P(1)
P(1)–C(1)
C(1)–P(2)
2.2018(16)
1.685(5)
2.103(5)
1.895(5)
1.8959(12)
1.8081(10)
Table 2. NMR spectroscopic data for 7 and 8. (The numbering of
the P atoms is given in Scheme 2.)
1.804(4)
Angles
δ [ppm]
1JP–P [Hz]
7
JP–Sn [Hz]
8
7[a]
8
8
P1–P3–C11
P3–C11–P2
P2–P4–C16
P4–C16–Ge
C1–P1–C2
P3–P1–C2
C11–P2–C2
C2–P2–P4
94.59(6)
107.61(5)
87.30(16)
121.1(2)
75.07(14)
98.29(14)
109.59(14)
112.91(15)
C1–Ge–C2
C1–Ge–C16
C2–Ge–C16
P3–P1–C1
C11–P2–P4
C1–P2–P4
C1–P2–C2
C1–P2–C11
66.76(12)
93.16(3)
P1
P2
P3
P4
49.6 (73.1)
76.0 (77.2)
450.1 (507.6) 445.0
271.6 (309.5) 284.8
55.5
84.6
174.0
379.2
174.1
379.3
169.8
394.5
169.8
394.4
297
12
85
–
94.35(17)
100.05(6)
123.97(6)
111.26(6)
79.50(14)
110.91(6)
[a] B3LYP/cc-PVTZ//B3LYP/6-31+G*-computed chemical shifts
for 7 are given in parenthesis.
[a] B3LYP/6-31+G*-computed bond lengths of 7 for comparison:
P(3)–C(11) 1.690, P(3)–P(1) 2.363, C(11)–P(2) 1.879, C(1)–Ge
2.135, C(2)–Ge 2.135, C(2)–P(2) 1.823, P(2)–P(4) 2.244, P(4)–C(16)
1.700, C(16)–Ge 2.114, C(2)–P(1) 1.909, P(1)–C(1) 1.909, C(1)–P(2)
1.823.
At the B3LYP/6-31+G* level values of 128 Hz and
338 Hz have been obtained for JP(1)–P(3) and JP(2)–P(4)
respectively, in reasonable agreement with the observed
1
1
1762
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1761–1766

Zwitterionic Organophosphorus Cage Compounds
data. It is expected that further information can be ob- mula [MP5C5tBu5], are almost certainly the result of a facile
tained from the 31P{1H} NMR spectrum of the tin ana- capture of a phosphaalkyne unit by M+ in the gas phase.
logue 8, the chemical shifts and P–P couplings of which are This observation was consistently reproducible and also oc-
almost identical to those in 7 (see Table 2). The similarity curred on analytically pure samples of 7. The molecular
of the chemical shifts and JP–P coupling constants is also structure of 7 is shown in Figure 1 with important bond
predicted computationally at the B3LYP/3-21G(*)//B3LYP/ lengths and distances collected in Table 1. The distances
LANL2DZ(p) level. A further informative feature is the ap- P(3)–C(11) and P(4)–C(16) of 1.6914(12) and 1.685(5) Å
pearance of 119Sn satellites on some of the 31P resonances. respectively, are consistent with localised P=C double
Of the two low-field resonances in the 31P{1H} NMR spec- bonds which typically lie between 1.61 and 1.71 Å.[8]
trum of 8, only that at δ = 445 ppm [assigned to P(3) on
The other P–C bond lengths in the structure which vary
the basis of the shifts computed for 7] shows 119Sn satellites between 1.804(4) and 1.8959(12) Å are in the range for nor-
(JP–Sn of 85 Hz). This observation is completely unexpected, mal P–C single bonds which are expected to be in the region
since P(3) is more distant from the Sn atom than P(4). of around 1.85 Å.[2f] Examination of the structure reveals a
Computation of the coupling constants of 8 at B3LYP/3- trigonal pyramidal germanium centre and a distorted tetra-
21G(*), however, results in values of 48 Hz for JP(3)–Sn and hedral phosphorus P(2). In view of this situation, and in
6 Hz for JP(4)–Sn in agreement with the assignment proposed order to assign meaningful formal oxidation states to the
above (Table 2). A possible explanation might be that the P(2) and Ge centres we are inclined to describe the molecule
zwitterionic electronic structure is responsible for the un- as being zwitterionic in the sense of P(2)+, Ge–. Thus the
usual coupling. It is also noteworthy that the coupling con- molecule can be thought of as a phosphonium salt with a
stants of the two tricoordinate phosphorus atoms [P(1) and germyl anion as the counter anion.
P(2), each being separated from the tin atom by two bonds],
The Ge–C bond lengths range between 2.0633(5) and
are also markedly different. JP(1)–Sn (297 Hz) is much 2.133(4) Å which are only marginally longer than those seen
larger than JP(2)–Sn (12 Hz). The B3LYP/3-21G(*)//B3LYP/ in other structurally characterised germyl anions e.g. 2.042
LANL2DZ(p) coupling constants also follow a similar to 2.054 Å in [Li(thf)2][Ge(C6H4-o–NMe2)3][9] and 2.000 to
trend [JP(2)–Sn129 Hz and JP(1)–Sn 5 Hz], indicating that even 2.024 Å in [Li(Et2O)3][Ge(C6H5)3].[9] Two of the C–Ge–C
with the small basis set the NMR properties of 8 are quali- bond angles in 7 are 94.35(17), 93.16(3)°. These angles are
tatively properly predicted.
also typical of those seen in pyramidal germyl anions e.g.
The 119Sn NMR spectrum of 8 shows a doublet of dou- 96.847–99.241° in [Li(thf)2][Ge(C6H4-o–NMe2)3][9] and
blets of doublets at δ = –206.8 ppm (rel. to SnMe4). The 97.284–98.985° in [Li(Et2O)3][Ge(C6H5)3].[10] The angle
two large couplings of 297 Hz and 85 Hz measured from C(1)–Ge–C(2) of 66.76(12)° in [GeP4C4tBu4] is significantly
the satellites in the 31P{1H} NMR spectrum are mirrored smaller than the other two and is likely to be the result of
in the 119Sn spectrum. In addition a smaller coupling of the geometrical constraints imposed by the strained cage
ca. 12 Hz is also visible, but the satellites representing this structure.
coupling are not visible in the 31P{1H} NMR spectrum,
perhaps being obscured by the main signals. The chemical
shift of δ = –206.8 ppm gives some precedent for the zwit-
Mechanistic and Computational Aspects
terionic interpretation of 7 and 8. This shift is very similar
To understand the formation and the structure of 7 den-
to that of δ = –221 ppm seen in the stannyl anion sity functional calculations were carried out. It is reason-
[Sn(CH2tBu)3]– which has also been structurally character- able to consider that [Ge(η4-P2C2tBu2)] reacts first with one
1
ised as its potassium salt.[7] The 13C and H NMR spectra molecule of PϵCtBu, and the intermediate so formed reacts
of 7 are similar to those of 8 and show resonances consis- with the second molecule of PϵCtBu. Since during the
1
tent with their proposed structures. The H NMR spectra NMR monitoring of the reaction no signal was attributable
of both 7 and 8 show three resonances in the ratio of 1:1:2 to the intermediate, the second reaction step should be fac-
thereby indicating that two of the tBu groups have coinci- ile. It seemed reasonable that either the PϵC triple bond or
dental chemical shifts. This is reflected in the 13C NMR the phosphorus lone pair electrons can interact with the Ge
spectra which show three resonances for the tBu carbon atom, which is unshielded, and has electron deficiency.
atoms instead of four. Likewise, the GIAO B3LYP/cc- Thus, we have considered several ways to bind a PϵCtBu
PVTZ//B3LYP/6-31+G* shieldings show that the two tBu unit to the Ge atom of [Ge(η4-P2C2tBu2)], but upon op-
groups attached at the GeCP1C ring have equivalent chemi- timisation the PϵCtBu was repelled by the rest of the sys-
cal shifts, in agreement with the (pseudo) Cs arrangement tem. The only stable structure, resulting from a 1:1 ratio of
of the molecule. In the case of 7 all carbon signals could be the reactants was 9 (Figure 2), in which a C=P bond bridges
seen whereas in the case of 8 only one of the cage quater- the original four-membered ring. Furthermore, the energy
nary carbon atoms could be detected. The EI mass spectra of this structure was higher by 3.5 kcal/mol (B3LYP/6-
of both compounds exhibit peaks for their molecular ions 31+G*) than that of the starting materials. We were also
with the expected isotopic distribution patterns. Curiously, able to locate the transition structure corresponding to the
and in both cases, a peak was also seen at exactly 100 mass loss of the PϵCtBu unit from 9 resulting in [Ge(η4-
units above M+ which corresponds to an extra PϵCtBu P2C2tBu2)] and PϵCtBu, and furthermore this transition
unit. These extra peaks, which correspond to units of for- structure lies only 11.0 kcal/mol above the energy of 9.
Eur. J. Inorg. Chem. 2008, 1761–1766
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1763

M. D. Francis, P. B. Hitchcock, J. F. Nixon, L. Nyulászi
FULL PAPER
tation energies at 612 nm (f = 0.0002), 520 nm (f = 0.0009)
and 476 nm (f = 0.0034), in agreement with the observed
red colour of 7.
Figure 2. B3LYP/6-31+G* transition states and intermediate on the
reaction path leading to the formation of 7.
Figure 3. Electrostatic potential map of 7 computed at the B3LYP/
6-31+G* level. Red colour denotes positive, blue negative charge.
Addition of a second PϵCtBu molecule to 9 furnishes 7
directly, and we were able to locate the corresponding tran-
sition structure TS97, which is only 18.8 kcal/mol less stable
than the product (7), and is only slightly less stable than the
energy of [Ge(η4-P2C2tBu2)] and two PϵCtBu units. Be-
cause this second transition structure on the reaction path
has a low energy, it is understandable that no intermediate
has been observed during the experiments. We have also
investigated the addition of the second PϵCtBu molecule
in a different regio-chemistry (with the P being attached to
Ge). The product of this reaction is less stable than 7 (by
17.3 kcal/mol) and also the corresponding transition struc-
ture is less stable than TS97 (by 8.9 kcal/mol).[11] Thus the
computational studies of the entire reaction path are in ac-
cordance with the observed product, furthermore the low
energy transition structures in both reaction steps are in
agreement with the rather facile reaction observed at room
temperature. Although the first reaction step is slightly en-
dothermic (interaction with the solvent might influence the
energetics somewhat), the stabilisation in the second step
provides the necessary driving force for the reaction.
Conclusions
We have found that the group-14 1,3-diphosphacyclo-
butadienyl complexes [M(η4-P2C2tBu2)] (M = Ge, Sn) react
smoothly with the phosphaakyne PϵCtBu to afford the
group 14 organophosphorus cage compounds [MP4C4tBu4]
which have been characterised by multinuclear NMR spec-
troscopy, mass spectrometry and a single-crystal X-ray dif-
fraction study in the case of 7. Consideration of these data
lead us to view the compounds as zwitterionic cages, with
an inverted charge distribution at the group 14 element.
These conclusions are fully supported by detailed density
functional computational studies. Finally, there is a marked
difference in the stability of 7 and 8, with the latter showing
considerable degradation in solution over a number of
hours (in accordance with the increased charge inversion)
whereas 7 appears to be stable in solution for at least several
days with no apparent change in its NMR spectra.
It is worthy to note that zwitterionic structures are well
known in solutions, (e.g. amino acids in water), however,
these are stabilized by hydrogen bonds and rearrange in the
gas phase by proton shift to their non ionic form. In the
case of 7 and 8 – likewise in case of ylides (as R3P = CRЈ2)
no such rearrangement is possible, however, in the present
case the negative and positive charges are not stabilized as
being localised in neighbouring atoms.
We have also investigated the computed electronic prop-
erties of the unusual zwitterionic product 7. The B3LYP/6-
31+G*-optimized structural parameters match favourably
with the X-ray structural data (see footnote in Table 1). The
electrostatic potential map of 7 is shown in Figure 3. and
clearly supports the zwitterionic structure formulation. It is
clearly evident that the phosphorus atoms (printed in red)
carry a positive charge, while the germanium atom (shown
in blue) is rather negative. This negative charge is under-
standable, since the HOMO[12] is located nearly entirely on
the germanium atom. It is worthy noting that germanium
is more electropositive than phosphorus (and carbon), thus
7 has an inverted charge distribution. Since tin is even more
electropositive than germanium, the “charge inversion” is
even larger in 8 than in 7 explaining the destabilization (see
above) in case of 8. The high energy HOMO[12] also makes
the colour of 7 understandable, furthermore TD DFT com-
Experimental Section
General Remarks: All procedures were carried out using conven-
tional Schlenk techniques under high purity argon in flame-dried
glassware or in a Miller–Howe glovebox. Toluene and diethyl ether
were dried by heating to reflux over NaK alloy for at least 10 h
before being distilled under nitrogen. C6D6 was dried by over
molten potassium before being vacuum transferred into a storage
ampoule fitted with a greaseless tap. 31P{1H}, 1H and 13C NMR
spectra were recorded with a Bruker DPX-300 spectrometer and
were referenced to external 85% H3PO4, the residual 1H resonances
putations at the B3LYP/6-31+G* level predict vertical exci- of the deuterated solvent and the 13C resonances of the solvent
1764 Eur. J. Inorg. Chem. 2008, 1761–1766
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Zwitterionic Organophosphorus Cage Compounds
1
respectively. 119Sn NMR spectra were recorded with a Bruker
AMX-500 spectrometer and were referenced to external SnMe4. EI
mass spectra were recorded with a VG Autospec instrument at
70 eV. Microanalyses were performed by Medac Ltd, Surrey, Eng-
land. [Ge(η4-P2C2tBu2)][5] and [Sn(η4-P2C2tBu2)][6] were prepared
according to the literature. PϵCtBu was prepared by a modifica-
tion[13] of the original literature procedure.[14]
= 25.7 Hz, 4JP(3)–P(4) = 6.3 Hz], 445.0 [ddd, P(4), JP(4)–P(2) = 169.8,
3JP(4)–P(1) = 14.2 Hz, JP(4)–P(3) = 6.6 Hz]. 119Sn{1H} (186.36 MHz,
4
298 K, C6D6): δ = –206.8 (ddd, JP–Sn = 297, 85 and 12 Hz) ppm.
EI mass spectrum (main peaks) (70 eV): m/z (%) = 620 (7) [M +
PCtBu]+, 520 (9) [M]+, 400 (89) [M – Sn]+, 169 (100) [PC2tBu2]+,
57 (70) [tBu]+, 41 (83) [CH3–C=CH2]+.
X-ray Structure Determination of 7: X-ray quality crystals were ob-
tained from a toluene solution at –85 °C. Intensity data were col-
lected with a KappaCCD diffractometer and the structure was
solved by direct methods and refined on F2 using full-matrix least-
squares with SHELX-97.[15] An empirical absorption correction
was applied. The structure contains one toluene solvent molecule
disordered across an inversion centre. Formula C20H36GeP4·
[GeP4C4tBu4] (7): PϵCtBu (0.22 g, 2.2 mmol, 355 µL) was added
dropwise to a solution of [Ge(η4-P2C2tBu2)] (0.2 g, 0.73 mmol) in
diethyl ether (10 mL) at –70 °C with stirring. The resultant solution
was warmed to room temperature and was stirred for 18 h during
which time it became deep red in colour. Volatiles were removed in
vacuo and the residue recrystallised from toluene at –85 °C to af-
ford [GeP4C4tBu4] as a dark red solid m.p. 108–110 °C (0.15 g,
43%). The crystals so formed were suitable for an X-ray diffraction
¯
0.5(C7H8), M = 519.03, T = 173(2) K, triclinic space group P1, a
= 9.5148(2) Å, b = 10.1061(2) Å, c = 16.3853(4) Å, α = 96.046(1)°,
1
β = 95.661(1)°, γ = 117.485(1)°, V = 1370.46(6) Å3, λ = 0.71073 Ä,
study. H NMR (C6D6, 298 K, 300 MHz): δ = 1.03 (s, 18 H, tBu),
1.39 (s, 9 H, tBu), 1.48 (s, 9 H, tBu) ppm. 13C NMR (C6D6,
75.43 MHz, 298 K): δ = tBu methyl groups: 30.97 (dd, JP–C = 5.07
and 9.41 Hz), 31.66 [dd, JP–C = 2.63 and 13.4 Hz, -C(CH3)3], 33.69
[dd, JP–C = 4.34 and 7.25 Hz, -C(CH3)3] ppm. tBu quaternaries by
refocused INEPT: 36.45 (dd, JP–C = 1.57 and 4.77 Hz), 41.27 [dd,
Z
= 2, dcalcd. = µ = =
1.26 Mgm–3, 1.36 mm–1, size
0.3ϫ0.3ϫ0.3 mm, θ range 3.81 to 25.05°, reflections collected
10215, independent reflections: 4507 (Rint = 0.037), reflections with
IϾ2σ(I) 4092, completeness to θ = 25.05° 93.0%, Tmax = 0.691,
Tmin = 0.615, GooF on F2 = 1.070, R [IϾ2σ(I)]: R1 = 0.052, wR2
= 0.131, R (all data): R1 = 0.058, wR2 = 0.136 largest diff.peak and
hole 1.38 and –0.98 eÅ3 (near disordered solvate).
-C(CH3)3, JP–C = 6.33 and 14.85 Hz], 49.92 [dd, -C(CH3)3, JP–C
=
14.53 and 15.78 Hz]. Cage quaternaries by refocused INEPT:
114.39 (dd, JP–C = 4.6 Hz and 40 Hz), 227.14 (ddd, JP–C = 6.4, 58
and 70.4 Hz), 332.6 (ddd, JP–C = 4.5, 13.4 and 31.3 Hz), 333.3 (ddd,
JP–C = 4.3, 13.3, 31.2 Hz). 31P{1H} NMR (C6D6, 121.68 MHz,
CCDC-207900 (for 7) contains supplementary crystallographic
data. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2
3
298 K): δ = 49.6 [ddd, 1JP(2)–P(4) = 174.0, JP(2)–P(1) = 4.9, JP(2)–P(3)
1
2
= 26.6 Hz, P(2)], 76.0 [ddd, JP(1)–P(3) = 379.2, JP(1)–P(2) = 4.9,
3JP(1)–P(4) = 12.4 Hz, P(1)], 271.6 [ddd, JP(3)–P(1) = 379.2, JP(3)–P(2)
1
3
Computations: Density functional calculations were carried out by
using the Gaussian 03 suite of programs.[16] Geometries were fully
optimized at the B3LYP/3-21G(*) level of the density functional
theory[17] followed by calculation of the second derivatives to char-
acterise the stationary points obtained. For minima all eigenvectors
of the second derivative matrix were positive, while for the transi-
tion structures a single negative eigenvector was obtained. In case
of the transition structures subsequent IRC calculations located the
minima corresponding to the transition state. Further optimiza-
tions were carried out at the B3LYP/6-31+G* level of the theory
making use the B3LYP/3-21G(*) force constants. No further sec-
ond derivative calculations were carried out at the B3LYP/6-31+G*
optimized structures. NMR chemical shifts for 7 were computed at
the B3LYP/cc-PVTZ//B3LYP/6-31+G* level.
4
1
= 26.6, JP(3)–P(4) = 6.5 Hz, P(3)], 450.1 [ddd, JP(4)–P(2) = 174.0,
3JP(4)–P(1) = 12.4, JP(4)–P(3) = 6.5 Hz, P(4)] ppm. EI mass spectrum
4
(main peaks) (70 eV): m/z (%) = 574 (26) [M + PCtBu]+, 474 (16)
[M]+, 417 (7) [M – tBu]+, 343 (55) [M – P2CtBu]+, 305 (46) [M –
PC2tBu2]+, 169 (45) [PC2tBu2]+, 57 (48) [tBu]+, 41 (100) [CH3
–
C=CH2]+. Microanalysis: C20H36GeP4: calcd. C 50.79, H 7.67;
found C 50.64, H 7.68.
[SnP4C4tBu4] (8): [Sn(η4-P2C2tBu2)] (0.050 g, 0.16 mmol) was dis-
solved in C6D6 (ca. 0.6 mL) in an NMR tube and PϵCtBu
(0.040 g, 63 µL, 0.39 mmol) was added slowly with a micro-syringe
and the solution was shaken periodically. Upon complete addition
the solution became a deep red colour. 31P{1H} and 1H NMR spec-
troscopic monitoring at this point showed only resonances corre-
sponding to the product [SnP4C4tBu4] as well as excess PϵCtBu
and so it is reasonable to assume that initially the product is formed
quantitatively. Over a period of hours, monitoring by 31P{1H}
NMR spectroscopy shows considerable decomposition to a number
of unidentified phosphorus containing products as well as starting
material. Due to this facile decomposition, preparative scale
attempts to isolate [SnP4C4tBu4] have been unsuccessful. A mass
spectrum was obtained by examining a sample of the residue after
removal of volatiles from a freshly prepared solution. 1H NMR
(C6D6, 298 K, 300 MHz): δ = 1.03 (s, 18 H, tBu), 1.39 (s, 9 H, tBu),
1.47 (s, 9 H, tBu) ppm. 13C NMR (75.43 MHz, C6D6, 298 K): δ =
tBu methyl groups: 31.4 (dd, JP–C = 6.77 and 9.8 Hz), 32.08 [dd,
JP–C = 2.67 and 14.08 Hz, -C(CH3)3], 35.06 [dd, JP–C = 4.39 and
7.24 Hz, -C(CH3)3] ppm. tBu quaternaries: 36.3 (dd, JP–C = 2.53
and 6.34 Hz), 41.6 [dd, -C(CH3)3, JP–C = 6.13 and 16.95 Hz], 49.90
[dd, -C(CH3)3, JP–C = 14.6 and 18.18 Hz]. Only one cage quater-
nary signal could be found: 117.0 (dd, JP–C = 5.42 Hz and
44.13 Hz). 31P{1H} NMR (C6D6, 298 K, 121.68 MHz): δ = 55.5
The structures and the electrostatic potential surface were visual-
ized by the MOLDEN program.[18]
Acknowledgments
The Leverhulme Trust is gratefully acknowledged for financial
support through a Special Research Fellowship (to M. D. F.). The
authors are thankful for the assistance with NMR spectroscopy by
the late Dr Tony Avent. J. F. N. and L. N. thank the Royal Society
for their support for collaborative projects between Sussex and
Budapest chemistry departments. Financial support from the
Hungarian Scientific Research Fund (OTKA T049258) is gratefully
acknowledged.
[1] a) K. B. Dillon, F. Mathey, J. F. Nixon, Phosphorus: The Car-
bon Copy: From Organophosphorus to Phospha-organic Chemis-
try, Wiley, 1998; b) Phosphorous-Carbon Heterocyclic Chemis-
try: The Rise of a New Domain (Ed.: F. Mathey), Pergamon,
Oxford, 2001; c) F. Mathey, Angew. Chem. Int. Ed. 2003, 42,
1578.
1
2
3
[ddd, P(2), JP(2)–P(4) = 169.9 Hz, JP(2)–P(1) = 2.86 Hz, JP(2)–P(3)
=
25.7 Hz], 84.6 [ddd, P(1), 1JP(1)–P(3) = 394.5 Hz, 2JP(1)–P(2) = 3.0 Hz,
3JP(1)–P(4) = 14.0 Hz], 284.8 [ddd, P(3), JP(3)–P(1) = 394.4, JP(3)–P(2)
1
3
Eur. J. Inorg. Chem. 2008, 1761–1766
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1765

M. D. Francis, P. B. Hitchcock, J. F. Nixon, L. Nyulászi
FULL PAPER
[2]
[9] A. Kawachi, Y. Tanaka, K. Tamao, Eur. J. Inorg. Chem. 1999,
461.
a) R. Streubel, Angew. Chem. Int. Ed. Engl. 1995, 34, 436 and
references cited therein; b) A. Mack, M. Regitz, in Carbocyclic
and Heterocyclic Cage Compounds and Their Building Blocks
(Ed.: K. K. Laali), J. A. I. Press, Stamford, CT, USA, 1999, pp.
199; c) J. F. Nixon, Carbocyclic and Heterocyclic Cage Com-
pounds and Their Building Blocks, pp. 257; d) L. Weber, Adv.
Organomet. Chem. 1997, 41, 1; e) M. Regitz, A. Hoffmann, U.
Bergsträsser, in Modern Acetylene Chemistry (Eds.:P. J. Stang,
F. Diederich), VCH, Weinheim, 1995; f) R. Appel, in Multiple
Bonds and Low Coordination in Phosphorus Chemistry (Eds.:
M. Regitz, O. Scherer) Georg Thieme Verlag, New York, 1990
pp. 153; g) R. Bartsch, P. B. Hitchcock, J. F. Nixon, J. Or-
ganomet. Chem. 1989, 375, C31; h) V. Caliman, P. B. Hitch-
cock, J. F. Nixon, M. Hofmann, P. von R. Schleyer, Angew.
Chem. Int. Ed. Engl. 1994, 33, 2202; i) B. Geissler, T. Wettling,
S. Barth, P. Binger, M. Regitz, Synthesis 1994, 1337; j) F. Tabel-
lion, A. Nachbauer, S. Leininger, C. Peters, F. Preuss, M. Re-
gitz, Angew. Chem. Int. Ed. 1998, 37, 1233; k) T. Wettling, J.
Schneider, O. Wagner, C. G. Kreiter, M. Regitz, Angew. Chem.
Int. Ed. Engl. 1989, 28, 1013; l) B. Geissler, S. Barth, U.
Bergsträsser, M. Slany, J. Durkin, P. B. Hitchcock, M. Hof-
mann, P. Binger, J. F. Nixon, P. von R. Schleyer, M. Regitz, An-
gew. Chem. Int. Ed. Engl. 1995, 34, 484; m) M. M. Al-Ktaifani,
W. Bauer, U. Bergsträsser, B. Breit, M. D. Francis, F. W. Heine-
mann, P. B. Hitchcock, A. Mack, J. F. Nixon, H. Pritzkow, M.
Regitz, M. Zeller, U. Zenneck, Chem. Eur. J. 2002, 8, 2622; n)
A. G. Avent, F. G. N. Cloke, M. D. Francis, P. B. Hitchcock,
J. F. Nixon, Chem. Commun. 2000, 879; o) M. M. Al-Ktaifani,
D. P. Chapman, M. D. Francis, P. B. Hitchcock, J. F. Nixon, L.
Nyulászi, Angew. Chem. Int. Ed. 2001, 40, 3474; p) M. M. Al-
Ktaifani, P. B. Hitchcock, J. F. Nixon, Inorg. Chim. Acta 2003,
356, 103.
[10] T. Habereder, H. Nöth, Z. Anorg. Allg. Chem. 2001, 627, 1003.
[11] Computing the two isomeric structures with H instead of the
tBu groups at the carbon atoms results in a much smaller
(5.6 kcal/mol) energy difference, indicating that the repulsion
of the tBu groups destabilizes the unobserved isomer.
[12] The KS HOMO – as usual – is similar to the corresponding
canonical MO. The Koopmans ionisation energy at the HF/6-
31+G*//B3LYP/6-31+G* level is 7.4 eV.
[13] M. D. Francis, Ph.D Thesis, University of Wales, Swansea,
1998.
[14] G. Becker, G. Gresser, W. Uhl, Z. Naturforsch., Teil B 1981, 36,
16.
[15] G. M. Sheldrick, SHELX-97, University of Göttingen, 1997.
[16] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr, T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tom-
asi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratch-
ian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dap-
prich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.
Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komáromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayak-
kara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Revision
C.02, Gaussian Inc., Wallingford CT, 2004.
[3]
[4]
S. J. Black, M. D. Francis, C. Jones, Chem. Commun. 1997, 305.
A. G. Avent, F. G. N. Cloke, M. D. Francis, P. B. Hitchcock,
J. F. Nixon, Chem. Commun. 2000, 879.
[17] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; b) C. Lee, W.
Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.
[18] A pre- and post-processing program for molecular and elec-
tronic structures, Molden. G. Schaftenaar, J. H. Nordik, J.
Comput. Aided Mol. Des. 2000, 14, 123.
[5] M. D. Francis, P. B. Hitchcock, Organometallics 2003, 21, 2891.
[6] M. D. Francis, P. B. Hitchcock, Chem. Commun. 2002, 86.
[7] P. B. Hitchcock, M. F. Lappert, G. A. Lawless, B. Royo, J.
Chem. Soc. Chem. Commun. 1993, 554.
Received: September 4, 2007
Published Online: January 23, 2008
[8] See ref.[2f], p. 96.
1766
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1761–1766