Selective preparation of the [3,5-tBU2-1,2,4-C2P3]- ion and synthesis and structure of the cationic species nido-[3,5-tBU2-1,2,4-C2P3]+, isoelectronic with [C5R5]+

Article abstract of DOI:10.1002/anie.200351096

Building pyramids: The first P-C cluster, namely, the novel nido species 2, is readily prepared from 1 (? = CtBu), which is also an excellent precursor for the selective synthesis of the anion 3. DFT calculations show that there is a switch in the global minimum, on the introduction of three P atoms (CH) into the [C₅H₅]⁺ ion, from cyclic-unsaturated to saturated cage species, such as 2.

Full text of DOI:10.1002/anie.200351096

Communications
Phosphorus–Carbon Compounds
Selective Preparation of the [3,5-tBu2-1,2,4-C2P3]ꢀ
Ion and Synthesis and Structure of the Cationic
Species nido-[3,5-tBu2-1,2,4-C2P3]+, Isoelectronic
with [C5R5]+**
Jason M. Lynam, May C. Copsey, Michael Green,*
John C. Jeffery, John E. McGrady,
Christopher A. Russell,* John M. Slattery, and
Anthony C. Swain
Dedicated to Professor Manfred Regitz
There has been significant progress in the chemistry of
transition-metal complexes carrying the h5-3,5-tBu2-1,2,4-
C2P3 ligand.[1] Although the isolobal relationship PMCH
and diagonal relationship P/C suggest that complexes carrying
this ligand should behave like the corresponding h5-cyclo-
pentadienyl-substituted analogues, recent work has shown
that the phosphorus-containing systems exhibit subtle differ-
ences in both bonding and reactivity.[2–4] This situation
suggests that further investigations could be rewarding.In a
new approach to this chemistry we focused on the reactivity of
the interesting tricyclic C2P3 compound 1 (Scheme 1), first
reported by Regitz et al.[5] First, we used this species as a
convenient starting point for a new selective synthetic
approach to the aromatic ion [3,5-tBu2-1,2,4-C2P3]ꢀ (3).
Second, and again reflecting on the consequences of replacing
CH groups by isolobal P atoms, we explored the possibility of
utilizing 1 as a precursor to cations [C2tBu2P3]+, isoelectronic
with [C5R5]+.
The published synthetic pathways to the anion 3,[6–8] which
derive from earlier work by Becker et al.,[9,10] involve reaction
ꢁ
of P CtBu with alkali metals (M).However, this procedure is
*
Scheme 1. a) PCl3; b) M, THF, ꢀMCl; c) Ph3SnCl, ꢀMCl ( =CtBu;
unselective, and to obtain pure 3 it is necessary to repeatedly
recrystallize the moisture-sensitive alkali-metal salts to
remove the second product of these reactions, the [2,4,5-
tBu3-1,3-C3P2]ꢀ ion.Therefore, we recognized that if the full
potential of this area was to be realized, it was important to
.. =lone pair; M=Li, Na.
devise a more efficient and selective pathway to 3.We
reasoned that if the tricyclic compound 1, which is readily
formed in good to excellent yield by treating the readily
accessible zirconium complex 2 with PCl3,[5] was treated with
an alkali metal in THF, the anionic species A should be
generated (Scheme 1).We then hypothesized that such a
species should undergo consecutive ring-opening reactions
(A!B!3) and thus selectively provide the desired anion.
Reaction (RT, 2 h, THF) of 1 with Li or Na metal leads to
quantitative formation of the lithium or sodium salt of 3,
which confirms our ring-opening hypotheses.Both Li· 3 and
Na·3 react quantitatively with Ph3SnCl to give the known
compound 3,5-tBu2-1-SnPh3-1,2,4-C2P3 (4), which has been
shown to be a valuable reagent for the syntheses of complexes
of the h5-C2P3 ligand.[7]
[*] Prof. M. Green, Dr. C. A. Russell, Dr. J. M. Lynam, M. C. Copsey,
Dr. J. C. Jeffery, J. M. Slattery, Dr. A. C. Swain
School ofChemistry
University ofBristol
Cantock's Close, Bristol BS81TS (UK)
Fax: (+44)117-929-0509
E-mail: michael.green@bristol.ac.uk
chris.russell@bristol.ac.uk
Dr. J. E. McGrady
Department ofChemistry
University ofYork
Heslington, York, YO105DD (UK)
[**] We thank the Leverhulme Trust (Emeritus Research Fellowship for
M.G.), the Royal Society (University Research Fellowship for C.A.R.),
the Ramsay Memorial Fellowship Trust (Research Fellowship for
J.M.L.), the University ofBristol (M.G., C.A.R., J.M.L.), and the
EPSRC (A.C.S., M.C.C., J.M.S.) for financial support. We would also
like to thank Dr. Simon Duckett (York) for helpful discussions
concerning the interpretation ofNMR spectra.
We believed that 1 might react with Lewis acids to provide
an entry point to the cationic [C2tBu2P3]+ manifold, related to
the isoelectronic pentamethylcyclopentadienyl cation, a spe-
cies that has recently attracted considerable attention.[11–15] In
particular, in view of the isolobal relationship PMCH, and
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Stohrer and Hoffmann's conclusion[16] that a dynamic Jahn–
Teller system of C2v [CH]5+ ion geometries can be accessed by
interaction of an empty p orbital centered on C(3) with the s*
levels of a tricyclo[2.1.0.02,5]pentane unit, abstraction of a
ꢀ
chloride anion from the P Cl bond of 1 might lead via a
phosphenium cation to the formation of the unknown
[C2tBu2P3]+ ions and potentially interesting structures.
Our attempt to achieve this by treating 1 with SbCl5 was
ꢀ
unsuccessful in that chlorination of the P P bond occurred to
give SbCl3 and the known compound 5 (Scheme 2).[5] How-
ever, reaction (208C, CH2Cl2, 10 min) of 2 with a stoichio-
metric amount of AlCl3 produced a colorless solution from
which a small amount of white precipitate separated.This
solid redissolved on gentle warming, and storage of the
resulting solution at 208C for 24 h furnished colorless
crystalline blocks.A single-crystal X-ray diffraction study of
this compound showed it to have the Wadeian structure nido-
[3,5-tBu2-1,2,4-C2P3]+[AlCl4]ꢀ (6) (Figure 1).The molecule is
essentially ion-separated, with [AlCl4]ꢀ ions loosely linked by
long-range Cl···P interactions (ranging from 3.37 to 3.58 Š) to
a cluster cation that is isoelectronic with the hypothetical
[C5Me5]+ cation.
The cation consists of three phosphorus and two tBuC
units arranged in a pseudo square-based-pyramidal geometry,
consisting of a diamond-shaped P2C2 base (with alternating P
and C units) capped by an apical four-coordinate P atom.This
is a 14-electron nido cluster, based on an octahedron with one
missing vertex, in which the carbon atoms occupy the sites of
Figure 1. Crystal structure of 6. H atoms have been omitted for clarity.
Only one ofthe two independent molecules in the unit cell is shown.
ꢀ
ꢀ
Selected bond lengths [Š] and angles [8]: P(1) C(1) 1.8009(15), P(1)
ꢀ
ꢀ
ꢀ
C(6) 1.8037(15), P(1) P(3) 2.2367(7), P(2) C(1) 1.8011(15), P(2)
ꢀ
ꢀ
ꢀ
C(6) 1.8020(16), P(2) P(3) 2.2517(8), P(3) C(1) 1.9955(16), P(3)
C(6) 2.0022(15); C(1)-P(1)-C(6) 79.30(7), C(1)-P(1)-P(3) 58.04(5),
C(6)-P(1)-P(3) 58.24(5), C(1)-P(2)-C(6) 79.34(7), C(1)-P(2)-P(3)
57.72(5), C(6)-P(2)-P(3) 57.93(5), C(1)-P(3)-C(6) 70.25(6), C(1)-P(3)-
P(1) 49.97(4), C(6)-P(3)-P(1) 49.99(5), C(1)-P(3)-P(2) 49.73(5), C(6)-
P(3)-P(2) 49.70(5), P(1)-P(3)-P(2) 75.93(2), P(1)-C(1)-P(2) 100.10(7),
P(1)-C(1)-P(3) 71.99(6), P(2)-C(1)-P(3) 72.55(6). Selected bond
lengths [Š] and angles [8] for the second independent molecule in the
ꢀ
lowest connectivity.The basal P C bonds are all approx-
ꢀ
imately equal (ca.18.0 Š), whereas the P
C bonds to the
apical P atom are significantly longer (2.00 Š). The P-C-P
angles in the basal unit are approximately 1008, whereas the
C-P-C angles are considerably more compressed (798).All the
bond angles subtended at the apical phosphorus atom by
adjacent basal atoms are extremely acute (ca.50 8).
ꢀ
unit cell (P(6) is the apical phosphorus atom): P(4) C(16) 1.7981(16),
ꢀ
ꢀ
ꢀ
P(4) C(11) 1.8013(16), P(4) P(6) 2.2540(8), P(5) C(16) 1.8018(16),
ꢀ
ꢀ
ꢀ
P(5) C(11) 1.8083(15), P(5) P(6) 2.2287(7), P(6) C(16) 1.9934(16),
ꢀ
P(6) C(11) 1.9955(16); C(16)-P(4)-C(11) 79.29(7), C(16)-P(4)-P(6)
The structure of 6 is an analogue of that predicted by
Hoffmann[16] for the [C5H5]+ ion and also to the species nido-
57.63(5), C(11)-P(4)-P(6) 57.67(5), C(16)-P(5)-C(11) 79.01(7), C(16)-
P(5)-P(6) 58.14(5), C(11)-P(5)-P(6) 58.13(5), C(16)-P(6)-C(11)
70.30(6), C(16)-P(6)-P(5) 50.14(5), C(11)-P(6)-P(5) 50.32(5), C(16)-
P(6)-P(4) 49.63(5), C(11)-P(6)-P(4) 49.70(5), P(5)-P(6)-P(4) 76.19(2),
P(4)-C(11)-P(5) 100.03(7), P(4)-C(11)-P(6) 72.63(6), P(5)-C(11)-P(6)
71.54(6).
ꢁ
2-tBu-1,2-PCB3H5 prepared by the reaction of P CtBu with
Scheme 2. a) SbCl5, CH2Cl2, ꢀSbCl3; b) AlCl3, CH2Cl2 ( =CtBu, .. =lone pair).
*
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Figure 2. Optimized structural parameters and relative energies E of[C 2H2P3]+ isomers.
B4H10.[17] Recent calculations have shown, however, that the
vinylcyclopropenyl and cyclopentadienyl cations are more
stable isomers of the [C5H5]+ ion than a square-based-
pyramidal structure.[18,19] These observations encouraged us
to survey systematically the potential-energy surface (PES) of
the model cation [C2H2P3]+ at the B3LYP/6-311G** level.
Optimized structural parameters and relative energies of the
most stable isomers are summarized in Figure 2.In contrast to
the isomers of the [C5H5]+ ion the most stable structures on
the PES for [P3C2H2]+ ions are square-based pyramids, the
lowest lying of which, I, has two basal phosphorus atoms in a
2,3-arrangement.Structure II, which has the same connectiv-
ity as 6, is 24.4 kJmolꢀ1 higher in energy, and its optimized
structural parameters are remarkably similar to those in the
crystal structure of 6.The third possible square-based
pyramid, structure III, with a CH group in the apical position,
is 99.8 kJmolꢀ1 higher in energy than I.A survey of species
located on the PES of [C5H5]+ led us to investigate additional
isomers based on the cyclopentadienyl (IV and V) methyl-
enecyclobutenyl (VI) and vinylcyclopropenyl (VII, VIII, and
IX) structures.[*] However, even the most stable of these
alternative structures (VII) lies 83.5 kJmolꢀ1 above the global
minimum I.The calculations therefore imply that 6 is not the
most stable isomer of the [C2tBu2P3]+ ion, despite the fact that
it is formed in the experiment.We cannot rule out the
possibility that 6 crystallized preferentially, and that ana-
logues of I and II, undergoing rapid exchange, are present in
our experiments.However, if one considers that 6 is formed
by abstraction of a chloride anion from 1, then it would be the
kinetic product of the reaction.It might be possible to access
structure I from II if III is invoked as an intermediate.
However, the barrier for the conversion of II into III is
calculated to be rather high (115 kJmolꢀ1), and this implies
that 6 is the only species of the [P3C2R2]+ manifold that is
present.
Phosphorus compounds have a striking structural resem-
blance to carbon compounds.However, as is shown here and
in valence isomers of tetraphosphabenzene and phosphorus
analogues of amidinium salts, any isomers that are high in
energy as a result of the constraint of bond angles (i.e., strain
energy) will be much more stable on the hypersurface for the
phosphorus analogues.[20] Most significantly, the stability of
the planar cyclopentadienyl, methylenecyclobutenyl, and
vinylcyclopropenyl analogues is much reduced in the phos-
phorus-containing systems because of the inefficient p–p
overlap in double bonds involving atoms of the second period.
As a result, there is a distinct switch in global minimum from
cyclic unsaturated species to saturated cages with high
connectivity.
The 1H, 13C, and 27Al NMR spectra of 6 in CD2Cl2 solution
are consistent with the solid-state structure.Curiously, how-
ever, no signals attributable to 6 could be observed in the
31P NMR spectrum of 6 over the temperature range ꢀ100 to
+ 308C.Further calculations shed some light on this rather
[*] We considered many alternative linkage isomers ofthe methylene-
cyclobutenyl and vinylcyclopropenyl structures; only the most stable
are shown in Figure 2.
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puzzling observation.The NMR spectroscopic chemical-shift
values of the 31P nuclei in structure II were calculated to be
d = ꢀ532 (apical) and d = 246 (basal) ppm.The value for the
apical phosphorus atom is in good agreement with that found
for nido-2-tBu-1,2-PCB3H5.[17] The lack of signals at these
chemical shifts in our spectra led us to conclude that a
fluxional process was occurring, the result of which was to
exchange the apical and basal phosphorus atoms.A similar
process was predicted by Stohrer and Hoffmann[16] in their
analysis of bond-stretch isomerism and polytopal rearrange-
ments within the [C5H5]+ manifold.Here, exchange of basal
and apical carbon atoms is predicted to occur via a C2v
transition state.In the case of the [P 3C2H2]+ manifold, a
similar C2v-symmetric transition state was located (one
imaginary frequency), only 37.0 kJmolꢀ1 higher in energy
than II and thus allowing facile exchange of apical and basal
phosphorus atoms (Figure 3).
cation of 6 obeys Wade's rules and is in fact the first example
of a cationic phosphorus–carbon cluster.
In conclusion we have shown that the tricyclic C2P3
compound 1 can selectively ring open to give in high yield
the anion 3 and the novel nido cation 6.
Experimental Section
3·M: In a typical experiment, compound 1 (172 mg, 0.65 mmol) was
dissolved in THF (5 mL) and added to a suspension of Na pieces
(100 mg, 4.34 mmol) in THF (5 mL). The resulting mixture was
stirred for 2 h, after which time it had turned deep red, which
indicated formation of 2: an NMR spectrum recorded at this time
indicated that conversion into 2 was essentially quantitative.The
solution was then filtered, cooled to ꢀ308C, and a solution of Ph3SnCl
(272 mg, 0.71 mmol) in THF (10 mL) added in portions, which caused
an instant color change to yellow.Conversion into compound 4 was
again quantitative, as shown by 31P NMR spectroscopy.
The reaction with Li could be accomplished in the same manner
by using an excess of Li metal that had been activated by sonication.
6: AlCl3 (87 g, 0.65 mmol) was added to a stirred solution of 1
(172 mg, 0.65 mmol) in CH2Cl2 (5 mL).After stirring the mixture for
10 min, a white solid was observed to form in the colorless solution.
The solid redissolved on gentle heating.Storage for 16 h at 20 8C
yielded a good crop of colorless blocks of 6. 1H NMR (258C,
299.9 MHz, CD2Cl2): d = 1.01 ppm (s, C(CH3)3); 27Al NMR (258C,
78.2 MHz, CD2Cl2): d = 102.2 ppm (s, AlCl4ꢀ); 13C NMR (258C,
75.4 MHz, CD2Cl2): d = 29.5 (apparent q, 3.0 Hz, C(CH3)3), 31.2 (br,
C(CH3)3); 32.8 ppm (br, P3C2tBu).
All density functional calculations were performed with the
Gaussian98 program.[23] The natures of the minima and transition
states were confirmed by vibrational analysis (zero or one imaginary
frequency, respectively).Shielding parameters s were calculated by
using the GIAO formalism,[24] and converted into chemical shifts d by
using d = 328.5ꢀs ppm, where 328.5 ppm is the absolute shielding
parameter for the 31P reference standard, 85% H3PO4.[25]
Crystal structure for 6: C20H36Al2Cl8P6, Mr = 799.87, T= 173(2) K,
monoclinic, space group P2(1)/n, a = 10.529(2), b = 21.520(4), c =
16.354(3) Š, b = 95.95(3)8, V= 3685.6(13) Š3, Z = 4, 1calcd
=
Figure 3. Facile exchange ofapical and basal phosphorus atoms.
1.442 Mgmꢀ3, m = 0.933 mmꢀ1, 3.14 ꢂ 2q ꢂ 54.968.X-ray measure-
ments were made using a Bruker APEX area-detector diffractometer
with MoKa radiation (l = 0.71073 Š)[26] on a single crystal of 6 of size
0.2 ” 0.2 ” 0.1 mm that was coated in perfluoropolyether oil and
This result allowed us to calculate the rate constant for
this exchange process for a range of temperatures, and
31P NMR spectra were simulated using these data.At 298 K
the simulated spectrum exhibited a single resonance at d =
ꢀ12 ppm with a line width at half height (w1/2) of about
3000 Hz, and at 183 K two peaks at d = ꢀ532 and 246 ppm
were predicted, with a line width of about 100 Hz.The poor
solubility of 6 and the calculated broadening of the signals
provides an explanation for our inability to observe, even at
low temperature, any signal in the 31P NMR spectrum of this
species.Consistent with this rapid exchange of apical and
basal phosphorus atoms, signals arising from 6 in the
13C NMR spectrum all show significant broadening.
mounted on a glass fiber.Of the 25669 reflections collected, 8453
[27]
were independent.Intensities were integrated
from several series
of exposures, each exposure covering 0.38 in w, and the total data set
was a hemisphere.Absorption corrections were applied, based on
multiple and symmetry-equivalent measurements.[28] The structure
was solved by direct methods[29] and refined by least-squares methods
on F2 values for all reflections[30] with weighting wꢀ1 = s2(F2o) +
(0.0267P)2, where P = [max(Fo2, 0) + 2F2c]/3, to final values of R1 =
0.0256 [I > 2s(I)] and wR2 = 0.0575 (all data). Largest peak and hole
in the final difference map were 0.462 and ꢀ0.179 eŠꢀ3.All non-
hydrogen atoms were assigned anisotropic displacement parameters
and refined without positional constraints.All hydrogen atoms were
constrained to ideal geometries and refined with fixed isotropic
displacement parameters.CCDC-202457 contains the supplementary
crystallographic data for this paper.These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@ccdc.cam.
ac.uk).
From a synthetic standpoint, the synthesis of 6 by a Lewis
acid promoted ring-opening reaction of 1 represents a new
approach to phosphorus–carbon clusters.These compounds
ꢁ
were previously prepared from P CtBu, either by thermolysis
or by reaction with various transition-metal species or Lewis
acids,[21,22] synthetic approaches which have to date not
provided access to molecules such as 6.Interestingly, the
Received: February 4, 2003 [Z51096]
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