Reversible, photoinduced activation of P4 by low-coordinate main group compounds

Article abstract of DOI:10.1002/chem.201402031

Two unique systems based on low-coordinate main group elements that activate P₄ are shown to quantitatively release the phosphorus cage upon short exposure to UV light. This reactivity marks the first reversible reactivity of P₄, and the germanium system can be cycled 5 times without appreciable loss in activity. Theoretical calculations reveal that the LUMO is antibonding with respect to the main group element-phosphorus bonds and bonding with respect to reforming the P₄ tetrahedron, providing a rationale for this unprecedented activity, and suggesting that the process is tunable based on the substituents.

Full text of DOI:10.1002/chem.201402031

DOI: 10.1002/chem.201402031
Full Paper
&
Bond Activation
Reversible, Photoinduced Activation of P4 by Low-Coordinate
Main Group Compounds
Jonathan W. Dube,[a] Cameron M. E. Graham,[a] Charles L. B. Macdonald,[b]
Zachary D. Brown,[c] Philip P. Power,[c] and Paul J. Ragogna*[a]
Abstract: Two unique systems based on low-coordinate
main group elements that activate P4 are shown to quantita-
tively release the phosphorus cage upon short exposure to
UV light. This reactivity marks the first reversible reactivity of
P4, and the germanium system can be cycled 5 times with-
out appreciable loss in activity. Theoretical calculations
reveal that the LUMO is antibonding with respect to the
main group element–phosphorus bonds and bonding with
respect to reforming the P4 tetrahedron, providing a rationale
for this unprecedented activity, and suggesting that the pro-
cess is tunable based on the substituents.
The investigation of p-block elements in unusual bonding en-
vironments has been a prominent area of research over the
past two decades. Justification for the continued study of
these compounds is born from their potential for unprecedent-
ed reactivity, which previously was thought to be reserved
only for transition metals.[1] Dominating this area of main
group chemistry are the “frustrated Lewis pairs” (FLPs),[2] stable
singlet carbenes,[3] and low-coordinate or multiply bonded
main group compounds,[4] all of which have been shown to ac-
tivate a variety of small molecule substrates, most notably H2
and NH3. Furthermore, reversible activation of small molecule
substrates by p-block elements is extremely rare, limited to H2,
CO2, and ethylene by special FLP systems or uniquely bonded
heavy main group elements.[5] Furthering the discovery of
main group systems capable of activating and releasing sub-
strates is paramount for the continued development of chemi-
cal modification of organic molecules without the aid of transi-
tion metals.
pounds. The controlled activation of this reagent provides
access to atomically precise phosphorus fragments for the
building of more complex molecules or for use as a single-
source precursor for subsequent chemistry. Early and late tran-
sition metals have a long history of reactivity with white phos-
phorus; an array of products have been isolated, and transfer
of phosphorus fragments to other substrates has also been
achieved.[6] Two exceptional examples of this are by anionic
niobium compounds reported by Cummins (A, B), which can
transfer phosphorus to main group and transition metal elec-
trophiles, giving unique structures.[7] Metal-free activation of P4
has only recently been accomplished, most commonly with
low-coordinate, amphiphilic, main group centers.[8] Insertion
into one, or multiple, P–P bonds of the P4 tetrahedron has
been observed for dicoordinate mimics of singlet carbenes.
The neutral group 13 or 14 compounds controllably break one
or two PÀP bonds depending on the main group element (C,
D in Figure 1),[9] while cationic phosphenium ions can activate
one, two, or three PÀP bonds with the product ratio depend-
ing on the reaction stoichiometry (E, F).[10] N-heterocyclic car-
benes (NHCs) have shown diverse reactivity involving the deg-
radation or aggregation of P4-producing base-stabilized P1,
P2,[11a] P4,[11b] P8,[11c,d] or P12 fragments,[11e] with the product de-
pending on the electronic nature of the carbene (G, H, I). The
utility of these compounds has been limited to very special
cases where the phosphorus atoms can be functionalized by
conjugated dienes in situ,[11 ] transition metal Lewis acids,[9f] or
selenation with elemental selenium.[12] A crucial development
in this field is the controlled release or transfer of the activated
phosphorus fragment to a new substrate, while pushing for-
ward to the catalytic functionalization of P4 to a single product
is considered the ultimate goal. While Weigand et al. have re-
cently demonstrated that a compound similar to E can be stoi-
chiometrically fragmented into two phosphorus containing
products by the addition of an NHC, the phosphenium ion re-
quired for the initial activation remains in one of the prod-
White phosphorus, P4, is a commodity reagent used for the
production of the majority of phosphorus-containing com-
[a] J. W. Dube, C. M. E. Graham, Prof. P. J. Ragogna
Department of Chemistry and Center for Advanced Materials
and Biomaterials Research (CAMBR)
The University of Western Ontario
1151 Richmond St., N6A 5B7 London, Ontario (Canada)
Fax: (+1)519-661-3022
E-mail: pragogna@uwo.ca
[b] Prof. C. L. B. Macdonald
Department of Chemistry and Biochemistry
The University of Windsor
601 Sunset Ave., N9B 3P4 Windsor, Ontario (Canada)
[c] Dr. Z. D. Brown, Prof. P. P. Power
Department of Chemistry
The University of California
1 Shields Ave., 95616 Davis, California (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402031.
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reaction can be accelerated by
heating a 1:1 stoichiometric mix-
ture of GeAr2 and P4 at 608C for
24 h, however the isolated yield
is comparable to the room tem-
perature reaction (cf. 65%) as
heating the germanium starting
material for prolonged periods
of time results in decomposition.
The reaction rate also increases
when four stoichiometric equiva-
lents of P4 are used, with a 75%
isolated yield obtained after
4 days at room temperature. It
should be noted that 1 is the
only
phosphorus-containing
observed in the
product
31P{1H} NMR spectra of the reac-
tion mixture over the range of
conditions employed. Com-
pound 1 is very insoluble in ali-
phatic hydrocarbon solvents,
Figure 1. Two examples of transition metal compounds resulting from P4 activation that can also transfer the
phosphorus fragment to a new substrate (A, B; Ar=3,5-dimethylphenyl), and selected examples of the products
from reaction of low coordinate main group compounds and NHCs with P4 (C–I). Dipp=2,6-diisopropylphenyl.
ucts.[13] To date there are no examples of products from metal-
free P4 activation releasing the phosphorus fragment from the
p-block element containing starting material. This considerable
problem could persist due to the strength of the newly
formed main group element–phosphorus bonds, resulting in
a reluctance to release the phosphorus component after acti-
vation. In this context, we report the controlled activation of
a single PÀP bond in the white phosphorus tetrahedron by
a diarylgermylene,[14] producing 1 (Scheme 1), which was isolat-
ed in good yields and comprehensively characterized. This
novel product, in addition to a known phosphorus-based
system, quantitatively release P4 upon short exposure to UV
light, representing an unprecedented mode of reactivity for
metal-free white phosphorus activation.
The 1:1 stoichiometric addition of the diarylgermylene,
GeAr2 (Ar=2,6-Mes2C6H3, Mes=2,4,6-(CH3)3C6H2) to P4 at room
Scheme 1. Top: The synthesis of 1 from P4 and GeAr2 in toluene as well as
the reverse reaction upon exposure to UV light. Bottom: The solid-state
structure of 1 with a side view on the left and the end on view on the right.
Ellipsoids are drawn to 50% probability and hydrogen atoms have been re-
moved for clarity. Selected bond lengths [ꢁ] and angles [8]: Ge(1)–P(1)
2.3433(7), Ge(1)–P(2) 2.3509(9), Ge(1)–C(1) 1.9975(12), Ge(1)–C(25) 1.9932(12),
P(1)–P(3) 2.2091(7), P(1)–P(4) 2.2432(8), P(2)–P(3) 2.2470(7), P(2)–P(4)
2.2146(7), P(3)–P(4) 2.1746(9), C(1)–Ge(1)–C(25) 111.39(5), P(1)–Ge(1)–P(2)
82.29(2), P(3)–P(1)–P(4) 58.47(3), P(1)–P(3)–P(4) 61.55(2), P(1)–P(4)–P(2)
87.71(2).
temperature results in the gradual appearance of two doublet
of doublets, consistent with an A2X2 spin system (dP =À275,
135 ppm; 1JP-P =155 Hz in toluene), in the 31P{1H} NMR spec-
trum (Scheme 1).[15] The reaction does not go to completion in
one week as evidenced by the characteristic purple color of
GeAr2 in the reaction mixture and the distinctive signal of P4 in
the 31P{1H} NMR spectrum (dP =À521 ppm). After removing the
reaction solvent the remaining starting materials are isolated
by extraction into pentane leaving a white powder. The
1H NMR spectrum of the colorless solids show two sets of sig-
nals for the terphenyl substituents, differing from that of the
free GeAr2. An X-ray diffraction study on a single crystal grown
from a saturated benzene solution at room temperature con-
firms the identity of the product to be 1, isolated consistently
between 60–65% as a purified powder. The combined pentane
fractions give back the appropriate mass balance of the reac-
tants, which can be reused for the repeated synthesis of 1. The
however is reasonably soluble in aromatic solvents. This com-
pound is also extremely stable in the solid state, melting at
2408C, and shows no signs of decomposition when stored at
room temperature under a nitrogen atmosphere.
Compound 1 represents the first activation of white phos-
phorus by a germanium-containing compound, which has
been recognized as an obvious omission during the diverse re-
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activity studies of N-heterocyclic carbenes and N-heterocyclic
silylenes with P4.[8a] A germanium analogue of these systems
has been noted to not react with P4 in refluxing toluene.[9b] The
rationale for the difference in reactivity is the lower reduction
potential of germanium compared to silicon due to the inert
pair effect. This behavior clearly differs greatly from that of the
diarylgermylene, 1, which mildly activates white phosphorus at
room temperature, albeit over several days. It is likely that the
lack of electronically stabilizing nitrogen atoms allows the ger-
manium atom to be electrophilic enough to initiate the activa-
tion process. There is only one example of a low-coordinate tin
center controllably reacting with white phosphorus,[16] however
the reaction of SnAr2 and P4 results in decomposition of the
stannylene and no phosphorus containing products are ob-
served in the 31P{1H} NMR spectrum.
is slightly shorter at 2.1746(9) ꢁ. These are all consistent with
PÀP single-bond lengths in the literature, which typically range
from 2.15 to 2.30 ꢁ. The CÀGeÀC bond angle has decreased
slightly from the free GeAr2 at 111.39(5)8 (cf. 114.4(2)8) while
the PÀGeÀP bond angle is much smaller at 82.29(2)8.[14] The
steric bulk of the terphenyl substituents likely prevents the ac-
tivation of a second PÀP bond as the flanking aryl rings extend
around the P4 fragment. This considerable steric pocket could
also partially explain the slow reaction times.
The potential for this reaction to be reversible was first ob-
served visually with an NMR sample that had been left under
natural light for several days. The solution was originally color-
less and gradually turned the characteristic purple/blue color
of GeAr2, while the presence of P4 was also confirmed by
31P{1H} NMR spectroscopy. Irradiating 1 with UV light,[18] instead
of ambient light, accelerates the reverse reaction considerably,
while thermal conditions have no effect on the reaction. Irradi-
ation of a saturated solution of 1 (30 mgmLÀ1 in toluene) for
two hours results in a color change to an intense purple color.
The rate is clearly dependent on concentration with normal
The solid-state structure of 1 is also shown in Scheme 1 and
confirms the single insertion of GeAr2 into one PÀP bond.[17]
The P4 unit exists in a butterfly confirmation while the germa-
nium atom is in a distorted tetrahedral geometry. This struc-
ture resembles that of other heavy p-block element carbenoids
(i.e., D, E) in which a single PÀP bond is broken and two main
group element–phosphorus bonds are formed. The GeÀP
bond lengths are nearly identical at 2.3433(7) and 2.3509(9) ꢁ.
The phosphorus atoms adjacent to the germanium atom pos-
sess PÀP bond lengths of 2.2091(7), 2.2432(8), 2.2146(7), and
2.2470(7) ꢁ. The PÀP bond on the exterior of the tetrahedron
1
samples suitable for H NMR spectroscopy (10 mgmLÀ1) requir-
ing less than 30 min of irradiation (23.2 mW). The quantitative
production of P4 is easily identified by monitoring the reaction
by 31P{1H} NMR spectroscopy (Figure 2A) while the clean refor-
1
mation of GeAr2 was confirmed by H NMR spectroscopy (Fig-
ure S7 in the Supporting Information). The reaction reverts
Figure 2. A) 31P{1H} NMR spectra of: purified 1 in toluene, 1 after 30 min radiation with UV light, 1 h UV light, 1.5 h UV light, 2 h UV light, and left standing for
5 days in the dark, from top to bottom, respectively. B) 31P{1H} NMR spectra of: purified 1 in toluene, 1 after 2 h radiation with UV light, left standing for
5 days in the dark, repeated 5 times, from top to bottom, respectively. C) UV/Vis spectra of a dilute solution of purified 1, and 1 after exposure to UV light for
30 min from left to right, respectively. Insets show concentrated NMR samples. D) A plot of the percentage of 1 observed in the 31P{1H} NMR spectra for each
cycle shown in (B). Note: UV light is generated from a low-pressure, high-intensity single-arc mercury lamp, 254 nm.
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back to an appreciable amount of product (79% by integration
of the 31P{1H} NMR spectrum) after 5 days at room temperature
in the dark. The yield obtained from the integrations in NMR
spectrum is comparable to what is observed in the reaction of
GeAr2 and P4 in a 1:1 stoichiometry for the same time period.
The starting materials are regenerated with nearly no sign of
decomposition upon further radiation with UV light for 45 min.
Cycling the reaction again it is observed that compound 1 is
produced in comparable amounts, 77% by integration, after
5 days in the dark. This process can be cycled 5 times (25 days
in solution) without an appreciable reduction in the produc-
tion of 1 (14% loss over the 5 cycles; Figure 2B, D). The per-
centage decrease can likely be attributed to the small decom-
position of GeAr2, which is inherently very sensitive in solution,
over the 25 days of the experiment.
The UV/Vis spectrum of 1 shows an absorbance with a lmax
of 294 nm (e=7933 LmolÀ1 cmÀ1; Figure 2C left). After irradiat-
ing a sample of 1 with UV light for 20 min, a broad absorption
consistent with the presence of GeAr2 is observed with a lmax
of 579 nm (Figure 2C right) comparing well to the literature
value (cf. 578 nm).[14] To understand the origin of the absorb-
ance, time-dependent DFT calculations were performed on
1 and on a series of less-substituted model clusters derived
from diarylgermylenes (Ph2GeP4 and (2,6-Ph2C6H3)2GeP4). For all
of the model complexes, the calculations reveal that the ab-
sorptions are primarily attributable to excitations from the
highest-occupied MO’s, which are bonding with respect to the
PÀGeÀP fragment in the molecule, to the lowest-unoccupied
MO’s, all of which are antibonding with respect to the GeÀP
bonds in the PÀGeÀP fragment and bonding in regard to for-
mation of a PÀP bond (orbitals and interactions are illustrated
for Ph2GeP4 in Figure 3). The lmax values for the series become
increasingly redshifted as the substitution on the aryl group is
increased, but all remain in the range of 300–350 nm. For com-
parative purposes, TD-DFT calculations were also performed on
the diarylgermylene precursors and reproduce the experimen-
tal observations well: for example, the maximum absorbance
for (2,6-Mes2C6H3)2Ge is calculated to be at 567 nm even in the
absence of a solvent correction.
Figure 3. Top: Illustration of the important frontier orbitals for the model
compound Ph2GeP4. The arrows indicate the important orbital interactions
that give rise to the transitions calculated using TD-DFT; the calculated spec-
trum is illustrated in the inset and the transition with the largest oscillator
strength is highlighted. Bottom: Reaction coordinate diagram of mechanism
for the insertion of the model germylene Ph2Ge into P4 to produce Ph2GeP4.
The stationary point listed D–A is the donor–acceptor intermediate and the
transition state for the insertion process (TS) is boxed.
In addition to the subtle substituent-dependent changes
predicted for the UV/Vis spectra, thermochemical data ob-
tained from models of all of the clusters, the precursor germy-
lenes, and P4 reveal that the insertion reaction becomes less fa-
vorable as the substituent on Ge is changed from Ph to a ter-
phenyl group. For example, the insertion reaction of Ph2Ge
into P4 to yield Ph2GeP4 has a DG of À109 kJmolÀ1, whereas
the analogous insertion reaction involving (2,6-Ph2C6H3)2GeP4
has a DG of À88 kJmolÀ1. Further DFT calculations provide in-
sight into mechanistic details of the insertion process, which is
summarized graphically in Figure 3. The process begins with
the formation of a donor–acceptor complex between P4 and
the germylene involving a single very long PÀGe bond of
2.614 ꢁ (no transition state was identified for that process).
This is followed by the asymmetrical insertion of germanium
atom into the most proximate PÀP bond; examination of
imaginary vibrational mode for this transition state confirms
that the motion involves the breaking of the PÀP bond and
the formation of two GeÀP bonds (2.507 and 2.615 ꢁ). The
transition state leads directly to the final inserted compound,
which has a nearly C2 symmetric structure and much shorter
GeÀP bonds (both are 2.334 ꢁ). It should be noted that we
also looked at the triplet surface and found it to be much
higher in energy.
After observing and rationalizing the unique behavior of the
diarylgermylene we were very curious to see if this reversible
behavior persists in other systems known to activate P4. A di-
coordinate, zwitterionic, phosphenium ion was the logical
choice as it is isolobal to the germylene, while also being net
charge neutral. Compound 2 is easily prepared and cleanly in-
serts into one PÀP bond of the P4 tetrahedron, resulting in
compound 3 which is structurally similar to 1 (Scheme 2).[10b]
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Experimental Section
All experimental details, including general methods, methods for
X-ray crystallography and the theoretical work, synthesis of 1, and
complete NMR spectra for characterization and reversibility studies
are included in the Supporting Information.
Scheme 2. The reaction of the zwitterionic phosphenium ion (2) inserting
into P4 (3), and the previously unknown reverse reaction using UV light for
only 15 min.
Acknowledgements
The authors are grateful toNatural Science and Engineering Re-
search Council (NSERC), the Ontario Government, the Canadian
Foundation of Innovation (CFI), and the U.S. Department of
Energy (FG02-07ER46475) for their generous funding. In addi-
tion, the University of Western Ontario, the University of Wind-
sor, and the University of California at Davis are thanked for
their funding contributions. Prof. C. D. Martin (Baylor Universi-
ty) is thanked for useful discussions.
Much to our amazement, isolated 3 reverts back to the starting
materials, as observed by 31P{1H} NMR spectroscopy, after irra-
diation with UV light for only 15 min (Figure S25, and S26 in
the Supporting Information). This finding provides the very
real possibility that many other main group systems that acti-
vate white phosphorus could do so reversibly. While this par-
ticular zwitterionic phosphenium ion system is not nearly as ef-
ficient as GeAr2 at inserting into white phosphorus, and doing
so repeatedly, calculations on model phosphenium systems in-
volving Ph2P+ and Cl2Al(NSiMe3)2P and P4 (see the Supporting
Information) reveal that it is possible to tune the energetics of
activation and release by altering the main group element car-
benoid or the substituents on the ligand framework. Overall,
the controllable insertion and release reactivity described
herein is a completely unprecedented mode of chemistry for
white phosphorus and it is ripe for further development and
optimization.
Keywords: bond activation
· germanium · main group
chemistry · reversible reactivity · white phosphorus
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Received: March 4, 2014
Published online on && &&, 0000
&
&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Rooooxanne, put on the UV light: A
low-coordinate germanium compound
and a phosphenium ion are shown to
quantitatively release a P4 fragment
upon exposure to UV light (see figure,
Mes=2,4,6-trimethylphenyl). Theoretical
work reveals the LUMO to be antibond-
ing with respect to the main group ele-
ment–phosphorus bonds and bonding
for reforming the P4 tetrahedron, ration-
alizing this unique reactivity.
&
Bond Activation
J. W. Dube, C. M. E. Graham,
C. L. B. Macdonald, Z. D. Brown,
P. P. Power, P. J. Ragogna*
&& – &&
Reversible, Photoinduced Activation
of P4 by Low-Coordinate Main Group
Compounds
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ