2,4,6-Tri(hydroxy)-1,3,5-triphosphinine, P3C3(OH)3: The Phosphorus Analogue of Cyanuric Acid

Article abstract of DOI:10.1002/anie.201610156

Cyanuric acid (C₃H₃N₃O₃) is widely used as cross-linker in basic polymers (often in combination with other crosslinking agents like melamine) but also finds application in more sophisticated materials such as in supramolecular assemblies and molecular sheets. The unknown phosphorus analogue of cyanuric acid, P₃C₃(OH)₃, may become an equally useful building block for phosphorus-based polymers or materials which have unique properties. Herein we describe a straightforward synthesis of 2,4,6-tri(hydroxy)-1,3,5-triphosphinine and its derivatives P₃C₃(OR)₃which have been applied as strong π-acceptor η⁶-ligands in piano stool Mo(CO)₃complexes.

Full text of DOI:10.1002/anie.201610156

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International Edition: DOI: 10.1002/anie.201610156
German Edition: DOI: 10.1002/ange.201610156
Phosphorus Heterocycles
Hot Paper
2,4,6-Tri(hydroxy)-1,3,5-triphosphinine, P3C3(OH)3: The Phosphorus
Analogue of Cyanuric Acid
˝
Riccardo Suter,* Yanbo Mei, Matthew Baker, Zoltan Benko,* Zhongshu Li, and
Hansjçrg Grꢀtzmacher*
Abstract: Cyanuric acid (C3H3N3O3) is widely used as cross-
linker in basic polymers (often in combination with other
crosslinking agents like melamine) but also finds application in
more sophisticated materials such as in supramolecular
assemblies and molecular sheets. The unknown phosphorus
analogue of cyanuric acid, P3C3(OH)3, may become an equally
useful building block for phosphorus-based polymers or
materials which have unique properties.[1] Herein we describe
a straightforward synthesis of 2,4,6-tri(hydroxy)-1,3,5-triphos-
phinine and its derivatives P3C3(OR)3 which have been applied
as strong p-acceptor h6-ligands in piano stool Mo(CO)3
Scheme 1. Classic synthesis of cyanuric acid I and relevant reactions of
complexes.
Na(OCP).
C
yanuric acid I was discovered by Wçhler[2] nearly two
centuries ago and became a useful basic chemical. Its
industrial production on a million kilogram scale is still
based on the pyrolysis of urea (Scheme 1). In this reaction
ammonia is released and isocyanic acid (HNCO) is formed
which has been identified as the key intermediate.[3] Deriv-
atives of the cyanuric acid[4] are employed in the synthesis of
herbicides,[5] dyes,[6] flame retardants[7] and are employed as
components in supramolecular assemblies.[8] The keto form Ia
is thermodynamically preferred over the tautomeric form Ib.
To the best of our knowledge, the phosphorus analogues
of cyanuric and isocyanuric acid have neither been synthe-
sized nor observed by spectroscopic methods. The six-
membered P3C3 ring is known from 1,3,5-triphosphinines
which are isolated with sterically demanding alkyl substitu-
ents to achieve kinetical stability (e.g. R = tert-butyl, 1,1-
dimethylpropyl, 1-methylcyclohexyl, 1-methylcyclopentyl,
adamantyl).[9] Although the aromatic stabilization in the
parent 1,3,5-triphosphinine is similar to that in benzene, even
the heavily substituted analogues undergo easily chemical
transformations in which the low-coordinate phosphorus
centers change to ones with a higher coordination number.
For example, P3C3 Bu3 reacts reversibly with dihydrogen or
t
forms inorganic cage compounds when reacted with pnicto-
gen trihalides.[10] 1,3,5-Triphosphinines are excellent ligands
to bind metals in both h1 and h6 coordination modes.[11] Very
recently the first aromatic pnictogen–silicon congeners of
benzene E3Si3R3 [E = P, As ; R = (PhC(NtBu)2)3] have been
isolated.[12]
A straightforward approach to P3C3(OH)3 would be the
À ꢀ
trimerization of HO C P. But attempts to produce any of the
HPCO isomers in solution by protonation of phosphaethy-
nolate salts, M+(OCP)À (M = Na, K) failed so far.[13] The
phosphaketene HP C O is by 20.3 kcalmolÀ1 [14] more stable
= =
À ꢀ
than the tautomer HO C P and was only detected by IR and
microwave spectroscopy in the gas phase as product of the
photolysis of PH3 and CO.[15] An alternative synthesis of
P3C3(OH)3 could consist in the thermolysis of the phosphorus
analogue of urea, namely H2P(CO)NH2 II (Scheme 1), which
however, decomposes to PH3 and isocyanic acid upon
heating.[10a] It is known that phosphaketenes, R P C O,
À = =
[16]
dimerize to cyclic 1,3-diketones (RP)2(CO)2
which are
reluctant to undergo a ring expansion to the desired trimers.
We therefore reasoned that a suitable oxyphosphaalkyne
[*] R. Suter, Y. Mei, M. Baker, Prof. Dr. H. Grꢀtzmacher
Department of Chemistry and Applied Biosciences, ETH Zurich
8093 Zurich (Switzerland)
À ꢀ
(RO C P) is needed which in the first step may dimerize to
E-mail: suter@inorg.chem.ethz.ch
a 1,3-diphosphabutadiene. Because this species is still a reac-
tive diene component, it may further react with a third
hgruetzmacher@ethz.ch
˝
Dr. Z. Benko
À ꢀ
equivalent of RO C P to give P3C3(OR)3 in analogy to the
Budapest University of Technology and Economics
Szent Gellꢁrt tꢁr 4, 1111 Budapest (Hungary)
E-mail: zbenko@mail.bme.hu
few known examples of transition metal promoted trimeriza-
tion of alkyl-substituted phosphaalkynes to triphosphi-
nines.[17] The stability of R O C P versus R P C O is
À À ꢀ
À = =
Dr. Z. Li, Prof. Dr. H. Grꢀtzmacher
Lehn Institute of Functional materials (LIFM), Sun Yat-Sen University
510275 Guangzhou (China)
À
À
ꢀ
=
mainly dictated by the R O versus R P and C P versus C O
bond dissociation energies.[18] To date only lanthanide com-
plexes were found in which the OCP unit binds via the oxygen
center[19] while all tetrel-substituted oxyphosphaalkynes R3E-
OCP with E = C, Si, Ge, Sn, Pb are thermodynamically less
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610156.
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stable than the corresponding phosphaketene
R3E-PCO isomers. Only the siloxy phospha-
alkyne III has been observed as transient
product in a kinetically controlled reaction
before it isomerized to the thermodynamically
preferred isomer IV. Boron is more oxophilic
À
than silicon (bond dissociation energies: B O:
809 kJmolÀ1, Si O: 799.6 Æ 13.4 kJmolÀ1)
À
À
while the B P dissociation energy is smaller
À
À
in comparison with that of the Si P bond (B
À1
À1 [20]
À
P: 347 Æ 16.7 kJmol , Si P 363.6 kJmol ).
Composite calculations at the CBS-QB3 level
À À ꢀ
show that Me2B O C P is only slightly more
stable (by 1.4 kcalmolÀ1) than its isomer
À = =
Me2B P C O (for details and substituent
effects see Table S1 in the Supporting Infor-
mation) which means that the formation of
either constitutional isomer can be expected.
An equimolar solution of the readily available
(À)-B-chlorodiisopinocampheylborane
Scheme 3. Deprotection of 4 to give 5. Protection with tBuPh2SiCl formed the silyl ether
functionalized compound 6. Syntheses of complexes 7, 8, and 9.
(ipc)2B-Cl
2
[(À)-DIP-Chloride] and Na-
(OCP)(dioxane)2.5 1 has been analyzed by
31P-NMR spectroscopy at À408C and shows, beside the
signal of unreacted Na(OCP) (d = À391.2 ppm), three new
resonances at d = À298.0 ppm, d = À321.2 ppm, and d =
À317.4 ppm. We tentatively assign the resonance at d =
227 ppm) using a triple resonance broad-band (TBI) probe
with 1H/2D/31P/BB channels. The three O-BR2 functionalities
in 4 can be easily cleaved when an excess of alcohol like
methanol or tert-butanol is added to a solution of 4 in
[D8]THF (Scheme 3). The quantitative formation of P3C3-
(OH)3 (5) is indicated by a new singlet in the 31P-NMR
spectrum at d = 133.7 ppm (without observable JP-H coupling).
The presence of a higher order multiplet in the 13C-NMR
spectrum at d = 231.3 ppm verified the existence of a P3C3
core with magnetically inequivalent carbon atoms. The
resonance for the OH groups in the 1H-NMR spectrum
could not be unambiguously identified but after reaction of 4
with [D1]methanol one singlet resonance in the 2H-NMR
spectrum at d = 10.0 ppm indicates the presence of three
deuteroxy (O-D) groups of the triol [D3]5. We have no
spectroscopic evidence for the presence of other tautomers of
P3C3(OH)3 although CBS-QB3 calculations in the gas phase
indicate that the 1,3,5H-2,4,6-trione, (PH)3(CO)3 lies practi-
cally at the same energy as the triol tautomer (energy
difference: 0.6 kcalmolÀ1, which can be considered in the
range of the computational error). This is in marked contrast
to the mono-hydroxy derivative, phosphinin-2-ol, which is
significantly more stable by 18.1 kcalmolÀ1 than the keto
form.[22] These calculations refer to isolated molecules. In
order to explain why only the triol tautomer is observed in
solution, we assume that this isomer may be stabilized in THF
solution via O···H···O hydrogen bonds (note that the PH
group is a poor hydrogen donor). This assumption is
supported by the fact that 1,3,5-triphosphinine-2,4,6-triol 5
is stable in THF solution for several days but decomposes in
benzene over hours to form PH3 and further unidentified
products. Also upon removal of the solvent compound 5
decomposes. The aromaticity was evaluated by NICS calcu-
lations at the B3LYP/6-311+G** level of theory which show
that P3C3(OH)3 [NICS(1): À8.1 ppm] is best described as an
aromatic compound comparable to benzene [NICS(1):
À À ꢀ
À298.0 ppm to (ipc)2B O C P [3a] and the other two to
[18,21]
À = =
isomers of (ipc)2B P C O [3b] (Scheme 2).
When the
solution was warmed to room temperature, a new singlet in
the 31P-NMR spectrum at d = 182.7 ppm appeared, which is in
the range of triphosphinines such as P3C3(tBu)3 [d(31P) =
211.8 ppm], along with several persistent resonances indicat-
ing side products.
Scheme 2. The formation of [3a], [3b] and 4 from the reaction of
Na(OCP) 1 with (ipc)2B-Cl 2.
When
a solution of one equivalent of Na(OCP)-
(dioxane)2.5 in THF was added at room temperature to
a solution of 2, a very clean reaction occurs and only the signal
at d = 182.7 ppm is observed in the 31P-NMR spectrum of the
reaction mixture. Note that inverting the sense of addition—
that is, adding 2 to 1—leads to a mixture of several products
which have not been identified. Characterization by single-
crystal X-ray crystallography shows that the product with
d(31P) = 182.7 ppm is the desired P3C3[OB(ipc)2]3 (4). We
assume that 4 is formed by trimerization of the borinic ester
À À ꢀ
functionalized phosphaalkyne (ipc)2B O C P [3b] although
we could not detect any intermediates. The 13C-NMR
resonance for the aromatic carbon atoms could be observed
at d = 226.6 ppm which is similar to the one of P3C3(tBu)3 (d =
2
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À10.2 ppm] or 1,3,5-triphosphinine P3C3H3 [NICS(1):
À8.6 ppm].
The reaction between 5 and tBuPh2SiCl in presence of 1,8-
(nCO [cmÀ1] = 2022.4, 1971.6), 8 (nCO [cmÀ1] = 1992.8 (s),
1941.4), 9 (nCO [cmÀ1] = 1988.4 (s), 1924.7), [Mo(Mes)(CO)3]
(nCO [cmÀ1] = 1941.1 (s), 1869.9). The data are not available
t
t
diazabicycloundec-7-ene (DBU) cleanly leads to the silylated
compound P3C3(OSitBuPh2)3 6 as indicated by one singlet in
the 31P spectrum (d = 177.5 ppm). Similarly to the triol 5,
compound 6 could not be isolated due to its sensitivity and
was only observed in solution.
for [Mo(P3C3 Bu3)(CO)3] but for [Cr(P3C3 Bu3)(CO)3] the
stretching absorptions in a cyclohexane solution are observed
at nCO [cmÀ1] = 1979 (s), 1926 (s).[25] These data indicate the
strong electron-accepting properties of the phosphacyanuric
acid derivatives, especially for the borylated complex 7, which
diminishes the donation of electron density from Mo to CO.
Because of their increased p-accepting properties, p-
conjugated heterocycles with sp2-hybridized phosphorus cen-
ters are remarkable ligands often leading to very stable
complexes.[23] Therefore the reactions between the borylated
and silylated tri(oxy)triphosphinines 4 or 6 and [Mo(Mes)-
(CO)3] were investigated. In both cases, a clean displacement
of the mesitylene (Mes) group occurs, leading to products 7 or
8 which show singlet resonances at d = 8.1 ppm or d =
16.1 ppm in the 31P-NMR spectrum, respectively. The chem-
ical shift of the related complex [Mo{P3C3(tBu)3}(CO)3] is
observed at d = 56.7 ppm.[11d] Reaction of the borylated
complex 7 in THF with an excess of water leads to 9 which
was characterized in solution (31P NMR: d = À4.9 ppm).
ATR-IR spectra were obtained from the crystalline products
of 7 and 8 and the one of [Mo(Mes)(CO)3] was measured
under the same conditions for comparison.[24] The IR
spectrum of 9 was measured in solution. The two stretching
frequencies of the carbonyl groups decrease in the order 7
À
This effect leads to a low Mo C bond order close to one and
ꢀ
consequently a high C O bond order causing elevated nCO
stretching frequencies. In line with this assumption are the
resonances in the 95Mo-NMR spectrum which are signifi-
cantly deshielded for
7
and
8
(d = À1567 ppm; d =
À1578 ppm) in comparison to [Mo(Mes)(CO)3] (d =
À1903 ppm).[26]
Single crystals of the uncoordinated borylated trioxy-
phosphinine P3C3[OB(ipc)2]3 4 (light yellow) and the dark
yellow complexes [Mo(P3C3(OX)3)(CO)3] 7 [X = B(ipc)2] and
8 (X = SitBuPh2) were subjected to X-ray diffraction analyses
in order to determine the structures which are displayed in
À
Figure 1a–c. The P3C3 ring in 4 is planar with an average P C
bond length of 1.736 ꢀ. The averaged bond angles C-P-C
104.08 and P-C-P 136.08 are very similar to the ones in
P3C3(tBu)3.
Figure 1. a) Structure of 4. Thermal ellipsoids are shown at a 50% probability level. For clarity, hydrogen atoms and solvent molecules are omitted
and the camphor groups in 4 are only shown as ball-and-stick models. Ct=centroid of the P3C3 ring. Interatomic distances [ꢂ] and angles [8]:
P–Caverage 1.736; P-C-Paverage 136.0, C-P-Caverage 104.0. b) Structure of 7. P–Caverage 1.765, Mo–Cring
2.342, Mo–Pring
2.602, Mo–Ct 1.733; P-C-
average
average
Paverage 138.7, C-P-Caverage 101.18. c) Structure of 8. P–Caverage 1.768, Mo–Cring
2.364, Mo–Pringaverage 2.623, Mo–Ct 1.753; P-C-Paverage 137.86, C-P-
average
Caverage 101.92.
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À
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In summary a remarkably simple synthesis of 2,4,6-oxy
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Acknowledgements
The work was funded by the ETH Zꢁrich and the Swiss
Science Foundation (SNF). Z.B. is grateful a NKFIH grant
(No. 116329) and a Jꢂnos Bolyai Research Fellowship.
Conflict of interest
The authors declare no conflict of interest.
˝
Tate, P. B. Hitchcock, G. A. Lawless, Z. Benko, L. Nyulaszi, J. F.
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Keywords: coordination compounds · cyanuric acid ·
heterocycles · phosphinines · phosphorus
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[21] A similar phenomenon was observed for the reaction of
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i
Na(OCP) with Pr3Si(OTf) which leads to three resonances in
31
À = =
the P NMR spectrum which were assigned to R3Si P C O
À À ꢀ
(d = À371.3 and À368.4 ppm) and R3Si O C P (d =
À307.3 ppm).
Manuscript received: October 17, 2016
Revised: December 5, 2016
[22] X. Chen, S. Alidori, F. F. Puschmann, G. Santiso-Quinones, Z.
˝
Benko, Z. Li, G. Becker, H.-F. Grꢁtzmacher, H. Grꢁtzmacher,
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Phosphorus Heterocycles
˝
R. Suter,* Y. Mei, M. Baker, Z. Benko,*
Z. Li, H. Grꢁtzmacher*
&&&& — &&&&
2,4,6-Tri(hydroxy)-1,3,5-triphosphinine,
P3C3(OH)3: The Phosphorus Analogue of
Cyanuric Acid
Done in three ticks: The trimerization of
boryl-substituted phosphaalkynes allows
the straightforward synthesis of triphos-
phabenzenes on a multi-gram scale.
These compounds give stable metal
complexes and can be further function-
alized at the oxygen centers.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!