An actinide zintl cluster: A Tris(triamidouranium)μ3- η2:η2:η2-heptaphosphanortricyclane and its diverse synthetic utility

Article abstract of DOI:10.1002/anie.201306492

A diuranium(V) arene complex reductively cleaves P₄ to give a triuranium heptaphosphanortricyclane cluster (see picture; Ugreen, Ppurple, Nblue, Siorange). This cluster is the first example of a molecular actinide [P₇] Zintl complex and the first example of uranium-promoted catenation of P₄. This complex undergoes a wide range of reactions under mild conditions to afford liberated P₇R₃ phosphanortricyclanes.

Full text of DOI:10.1002/anie.201306492

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Angewandte
Communications
DOI: 10.1002/anie.201306492
An Actinide Zintl Cluster: A Tris(triamidouranium)m3-h2:h2:h2-
Heptaphosphanortricyclane and Its Diverse Synthetic Utility**
Dipti Patel, Floriana Tuna, Eric J. L. McInnes, William Lewis, Alexander J. Blake, and
Stephen T. Liddle*
Zintl clusters,[1] exemplified by the heptaphosphanortricy-
clane trianion [P7]3ꢀ, are fundamentally interesting and
important structural units in solid-state and molecular
chemistry.[1,2] Their importance derives from the key role
they have played in the development of polyhedral bonding
models and isoelectronic relationships to cycloalkanes as well
as synthetic applications.[3–5]
For 5f metals, reports of P4 activation are exceptionally
rare; there is one report of thorium-mediated activation of P4
at elevated temperature or with co-reagents,[13] and only two
examples of uranium-mediated activation of P4 are known.[14]
However, for both uranium cases it is notable that no
fragmentation or catenation of P4 was observed and instead
only cleavage of two of the P P bonds in P4 to give [P4]2ꢀ rings
ꢀ
There is currently major interest in the activation of
elemental phosphorus.[6] This is because phosphorus-contain-
ing molecules are ubiquitous and form the basis of numerous
industries, yet their synthesis relies on chlorination of P4 to
give PCl3 followed by multistep derivatizations. A highly
attractive concept is to avoid the need for PCl3 and access
organophosphorus compounds directly from elemental phos-
phorus. In principle, [P7]3ꢀ is an attractive precursor to
organophosphorus derivatives; however, although Group 1
derivatives can be prepared straightforwardly in liquid
ammonia, high-temperature melts have a reputation for
detonating in the presence of traces of moisture, and Na/K
reduction of phosphorus in ethers gives non-stoichiometric
mixtures.[7] Unlike main-group and late-transition-metal-
mediated activation of P4,[8,9] examples of early metal-
mediated transformations of P4 are far less common.[10] In
Group 3 and 4f-block chemistry, despite the potentially
strongly reducing nature of these metals, activation of P4 is
surprisingly rare,[11] presumably because of the hard–soft
mismatch between the electropositive metal and soft phos-
phorus.[12]
was observed. Indeed, early metal-mediated conversion of P4
to [P7]3ꢀ is in general a rare occurrence.[6,11c,d] Herein, we
report that a diuranium(V)–arene-tetraanion complex reduc-
tively cleaves P4 to selectively form a triuranium heptaphos-
phanortricyclane cluster under mild conditions. This cluster is
the first example of a molecular actinide [P7] Zintl complex
and the first example of fragmentation and catenation of P4 to
a higher oligomer promoted by uranium. Additionally, it is
notable that no binary uranium phosphides are formed.
Furthermore, we show that this complex is a precursor to
a wide range of facile derivatization reactions in closed
synthetic cycles for the activation and functionalization of P4
under mild conditions.
Treatment of [{U(TsTol)}2(m-h6:h6-C6H5CH3)][15] (1, TsTol
=
HC(SiMe2NAr)3; Ar= 4-MeC6H4) with P4 (1:1.1 of 1:P4)
afforded, after work-up and isolation, brown crystals of the
Zintl complex [{U(TsTol)}3(m3-h2:h2:h2-P7)] (2) in 12% yield of
crystalline product (Scheme 1). This low yield reflects the
[*] Dr. D. Patel, Dr. W. Lewis, Prof. A. J. Blake, Prof. S. T. Liddle
School of Chemistry, University of Nottingham
University Park, Nottingham, NG72RD (United Kingdom)
E-mail: stephen.liddle@nottingham.ac.uk
Dr. F. Tuna, Prof. E. J. L. McInnes
School of Chemistry and Photon Science Institute
University of Manchester
Oxford Road, Manchester, M139PL (United Kingdom)
Scheme 1. Synthesis of 2. Reagents and conditions: i) P4 (1.1 equiv) in
toluene, ꢀtoluene. Ar=p-tolyl.
[**] We thank the Royal Society, the Engineering and Physical Sciences
Research Council, the European Research Council, the University of
Nottingham, the University of Manchester, and the UK National
Nuclear Laboratory for generously supporting this work, and also
the EPSRC UK National EPR Facility.
surprisingly high solubility of 2, but by 1H NMR spectroscopy
we estimate about 65% of the crude reaction mixture is 2,
with some protonated ligand present, presumably from minor
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306492.
ꢀ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
1
decomposition.[16] The H NMR spectrum of crystalline 2 is
broad and the complex is silent in the 31P NMR spectrum in
the range ꢁ 1000 ppm, which is most likely as a result of
a combination of a reduction of the intensities of resonances
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n
owing to extensive JPP couplings (n = 1, 2, 3),[2–5,17] and line-
the presence of the latter supports the uranium(IV) formu-
lation.
broadening that is due to dynamic processes and fast
relaxation from the presence of coordinated uranium centers.
To confirm the structure of 2, the X-ray crystal structure
Variable-temperature H and 31P NMR spectroscopy could
was determined (Figure 1).[21] The salient feature of 2 is the
formation of a [P7]3ꢀ trianion that bridges three [U(TsTol)]+
fragments, where each uranium center coordinates to two
phosphorus centers on the upper rim of the [P7]3ꢀ trianion.
1
not freeze out any dynamic processes or induce coalescence to
one time-averaged species, which is most likely due to the
effects described above. Germane to this point, the [P7]3ꢀ
trianion is well-known to undergo facile and very complicated
Cope-type rearrangements in solution that are similar to
bullvalene,[18] and we suggest this contributes to the origin of
the broad NMR resonances. The magnetic moment of pure 2
in solution was found to be 4.67 mB at 298 K. In reasonable
agreement with this, the magnetic moment of powdered 2 was
found to be 4.20 mB at 298 K; this decreases slowly on cooling
down to ca. 80 K before falling more precipitously, reaching
1.25 mB at 1.8 K and still decreasing. The room-temperature
moment corresponds to 2.42 mB per uranium(IV) ion; this is
ꢀ
The U P bond lengths span the range 2.9486(17)–
3.0308(17) ꢀ, which compares to the sum of the covalent
radii of 2.81 ꢀ for uranium and phosphorus,[22] and most likely
reflects the sterically demanding nature of the {U(TsTol)}+
ꢀ
fragments and the bridging coordination mode. The U N and
ꢀ
P P bond lengths are unexceptional. A common parameter
used to assess the extent of ionic character in [P7]3ꢀ trianions is
the Q value,[1,7e,23] where Q = h/a (h = distance from the apical
P-center to the center-point of the lower rim of three P-
ꢀ
centers; a = average P P distance in the lower rim). For ionic
3
significantly lower than the value calculated for a free H4
systems, the Q value is typically 1.3–1.4 (for example, in
term (3.58 mB), but this is a common observation for
uranium(IV). For example, [U(TsXy)Co(CO)3(PPh3)] (Xy =
xylyl; the Co is diamagnetic) contains a single uranium(IV)
ion with a near identical capping ligand and has a room-
temperature moment of 2.77 mB,[19] close to the value per
uranium ion we observe for 2 and this supports a uranium(IV)
formulation. It is noticeable that the magnetic moment of 2
decreases with decreasing temperature more slowly than is
commonly observed for uranium(IV), but this has been
P7(SiR3)3 derivatives), and for 2 the Q value is 1.39, which is
ꢀ
suggestive of predominantly electrostatic U P bonding. The
computational size of 2 rendered a full DFT analysis of 2
intractable, but a preliminary single-point energy calculation
ꢀ
on 2 revealed the U P interactions to be essentially ionic,
which is in agreement with the structural and NMR spectro-
scopic data.
With 2 in hand, we undertook preliminary experiments to
explore its synthetic utility under ambient conditions,
Scheme 2,[16] because, although the activation of white
observed
before
in
[U(TsXy)Co(CO)3(PPh3)]
and
[UO(N’’)3][CoCp*2] (N’’ = N(SiMe3)2), which both contain
uranium(IV) centers.[19,20] The electronic absorption spectrum
of 2 is dominated by charge transfer bands in the 25000–
12000 cmꢀ1 region, and a number of surprisingly intense
absorptions (e = 120–250 Lmolꢀ1 cmꢀ1) that are as assigned as
f!f transitions are observed in the 12000–5000 cmꢀ1 region;
Scheme 2. Reactions of 2 with electrophiles. Reagents and conditions
(all at RT): i) excess Me3SiCl, C6D6, THF, ꢀ3; ii) 3LiCl, C6D6, THF,
3tmeda, ꢀ3; iii)excess MeI, C6D6, THF, ꢀ3; iv) PhI, C6D6, THF, ꢀ3.
phosphorus is burgeoning, subsequent functionalization and
liberation reactions are not common[6,24] and are often limited
to silyl derivatives.[11c] To benchmark the reactivity of 2, we
treated 2 with three equivalents of Me3SiCl to quantitatively
[25]
afford P7(SiMe3)3
and [(TsTol)U(Cl)(m-Cl)U(THF)2-
(TsTol)][26] (3, after the addition of THF); both of these
compounds are known and were identified by 1H and
31P NMR spectroscopies. Encouraged by the facile reactivity
of 2, we examined more challenging electrophiles. Lithium
chloride reacts quantitatively with 2 to afford 3 (after the
addition of THF and tmeda) and P7[Li(tmeda)]3.[27] This is
notable because alkali-metal derivatives of [P7]3ꢀ can be
difficult to prepare. In the context of organophosphorus
Figure 1. Molecular structure of 2 with ellipsoids set at 40% proba-
bility; hydrogen atoms, minor disorder components, and lattice solvent
are omitted for clarity.
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temperature for 16 h to yield large brown blocks of 1 (Crystalline
yield: 0.10 g, 12%). Inspection of the crude reaction mixture showed
that 2 constitutes about 65% of the reaction mixture. Anal. calcd. (%)
for C84H120N9P7Si9U3·C6H14: C 42.80, H 5.35, N 4.99%; found:
derivative chemistry, we find that excess methyl iodide and
phenyl iodide both react cleanly and quantitatively with 2 to
afford P7(Me)3 and P7(Ph)3,[29] respectively, with concom-
[28]
itant formation of 3 (after the addition of THF), whereas
these phosphanortricyclanes were previously not straightfor-
ward to prepare. This broad palate of reactions establishes
that sp3 and sp2 (aromatic) carbon-based electrophiles can be
1
C 42.63, H 5.34, N 4.82%. H NMR (C6D6): dH = 15–11.50 (27H, vb
s, Ar-CH3), 1.75–0 (36H, vb s, o and m-CH), ꢀ1.75–ꢀ4.50 (54H, v br s,
ꢀ
SiCH3), ꢀ73.31 ppm (3H, s, Si CH). Magnetic moment (Evansꢁ
method, C6D6, 298 K): meff = 4.67 mB. FTIR (Nujol): n˜1604 (w), 1513
(w), 1495 (s), 1403 (w), 1364 (w), 1286 (w), 1251 (m), 1243 (m), 1221
(s), 1171 (w), 1102 (w), 1015 (w), 974 (m), 933 (m), 899 (s), 858 (s), 841
(vs), 810 (s), 765 (w), 708 (m), 697 (w), 543 (w), 503 (m) cmꢀ1. UV/Vis/
NIR (toluene): lmax (e/Lmolꢀ1 cmꢀ1): 1033–1139 (217, 200), 1457
(119), 1622 (130), 1844 (151), 2053–2278 (158, 146).
substituted onto the [P7]3ꢀ framework in P C bond forming
ꢀ
reactions from 2, thus providing extensive opportunities for
subsequent functionalization and derivatization chemistry.
As 3 is the direct precursor to 1, the derivatization
chemistry described herein presents the closure of synthetic
cycles for the activation and functionalization of white
phosphorus (Scheme 3). In practice, two turnovers could be
Received: July 25, 2013
Revised: August 30, 2013
Published online: October 14, 2013
Keywords: actinides · reduction · uranium · white phosphorus ·
.
Zintl phases
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achieved before the mixture of products rendered subsequent
reactions unfeasible. The yields of the polyphosphide deriv-
atives for the first turnover were generally quantitative, but
this dropped to circa 40% in the second turnover, reflecting
the buildup of inorganic salts and the diminishing yields of
1 and 2 in each turnover. However, the diverse and
straightforward nature of these reactions suggests that 2 is
amenable to reactions with a wide range of functional
electrophiles, and reactions to extend the scope and efficacy
of this reactivity are ongoing.
To conclude, the reaction of P4 with [{U(TsTol)}2(m-h6:h6-
C6H5CH3)] affords the first example of an actinide [P7] Zintl
complex and the first example of fragmentation and cate-
nation of P4 promoted by uranium. This Zintl complex is
a precursor to a range of derivatives that represent general
methods for the preparation of alkali-metal-, hydrocarbon-,
ꢀ
ꢀ
aromatic-, and silyl-functionalized P7 derivatives via Li P, P
2
3
ꢀ
C (sp - and sp -hybridized carbon groups), and P Si bond
formation reactions. This offers significant synthetic scope for
the closure of synthetic cycles for the activation and
functionalization of P4 under mild conditions and further
demonstrates the ability of triamido uranium complexes to
activate and liberate functionalized small molecules.[30]
Experimental Section
Synthesis of 2: Toluene (20 mL) was added to a cold (ꢀ788C) stirring
mixture of 1 (0.79 g, 0.50 mmol) and P4 (0.07 g, 0.55 mmol). The
mixture was stirred at ꢀ788C for 5 min, then was allowed to warm to
room temperature, and it was then stirred for a further 16 h. All
volatiles were removed in vacuo. The product was extracted into
hexanes (10 mL), filtered, and the hexanes extract was stored at room
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structure was solved by direct methods and refined by full-matrix
À
Á
least-squares on all F2 values to give wR2 = {S[w Fo2 ꢀ Fc2 2]/
À
Á
2
S[w Fo2 ]}1/2 = 0.1042, conventional R = 0.0439 for F values of
À
Á
17824 with Fo2 > 2s Fo2 , S = 1.030 for 1059 parameters. Residual
electron density extrema were 2.86 and ꢀ1.30 eꢀꢀ3
.
CCDC 952235 (2) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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[21] Crystal data for 2: C84H120N9P7Si9U3·C6H14, Mr = 2525.75, space
group P21/n, a = 17.4611(6), b = 21.0364(8), c = 28.5203(10) ꢀ,
b = 91.2220(10)8, V= 10473.7(7) ꢀ3, Z = 4, 1calcd = 1.602 gcmꢀ3
;
MoKa radiation, l = 0.71073 ꢀ, m = 4.883 mmꢀ1
129658 data (24181 unique, Rint = 0.057, q < 55.28). Data were
,
T= 90 K.
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