Increasing the lability of polarised phosphorus-phosphorus bonds

Article abstract of DOI:10.1002/ejic.200700964

Diphosphanes with polarised P-P bonds react readily with Lewis acids like borane, gallium trichloride, or with elemental selenium, to give products arising from electrophilic attack at the more basic phosphorus atom and consecutive bond cleavage. Spectroscopic studies proved that the initial reaction with borane proceeds under preservation of the P-P bond to give a transient phosphane-borane which rearranged below ambient temperature. The results suggest that Lewis acid coordination decisively enhances the weakening of the polarised P-P bond. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200700964

FULL PAPER
DOI: 10.1002/ejic.200700964
Increasing the Lability of Polarised Phosphorus–Phosphorus Bonds
[a]
[b]
ˇ[a]
Sebastian Burck,[a][‡] Dietrich Gudat,* Martin Nieger, and Denis Vindus
Keywords: Phosphorus / Phosphanes / Lewis acids / Bond polarisation / Substituent effects
Diphosphanes with polarised P–P bonds react readily with
Lewis acids like borane, gallium trichloride, or with elemen-
tal selenium, to give products arising from electrophilic at-
tack at the more basic phosphorus atom and consecutive
bond cleavage. Spectroscopic studies proved that the initial
reaction with borane proceeds under preservation of the P–P
bond to give a transient phosphane–borane which re-
arranged below ambient temperature. The results suggest
that Lewis acid coordination decisively enhances the weak-
ening of the polarised P–P bond.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
Unsymmetrical diphosphanes A react easily with electro-
philes by heterolytic cleavage of the phosphorus–phospho-
rus bond.[1] The unusually high degree of reactivity has
been attributed to ionic bond polarisation which was ex-
pressed in terms of bond/no-bond resonance between two
canonical structures A and B (Scheme 1). The bond polari-
sation follows in this picture from a significant contribution
of resonance structure B, which results in the first place
from the high stability of the 1,3,2-diazaphospholenium cat-
ion[2,3] and may be further enhanced by suitable electronic
stabilisation of the anion fragment (e.g. in an aromatic
phospholide[1]). Since attachment of an electrophilic borane
renders a phosphanide anion likewise less nucleophilic,[4]
and thus more stable, it can be assumed that the balance
between resonance structures C and D of a diphosphane–
borane adduct is even further shifted to the side of a ionic
structure D. Similar arguments hold for the resonance
structures E, F of products arising from oxidation of the
exocyclic phosphorus atom by a chalcogen (E = O, S, Se).
In both cases, the increased weight of the ionic resonance
structure implies a further weakening effect on the P–P
bond and suggests that the products should even be more
prone to undergo bond cleavage reactions than the diphos-
phane A. In a formal sense, the bonding situation in a bo-
rane adduct C/D is comparable to that in P–P-bonded
phosphane–phosphenium adducts which are likewise
known to react under P–P bond cleavage and ligand ex-
change.[5]
Scheme 1.
In this contribution we report on studies of the reactions
of diphosphanes A with selenium, borane, and gallium tri-
chloride, and present evidence which proves that the action
of a Lewis acid or the oxidation by a chalcogen renders the
P–P bond indeed more prone for activation reactions.
Results and Discussion
[a] Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70550 Stuttgart, Germany
Fax: +49-711-685-64241
The reaction of the diphosphane 1a[1] with 1 equiv. of
elemental selenium yielded a mixture of the starting mate-
rial and the diselenophosphinate 2 (Scheme 2). Quantitative
formation of 2 was accomplished after addition of a second
equivalent of selenium.
E-mail: gudat@iac.uni-stuttgart.de
[b] Laboratory of Inorganic Chemistry, University of Helsinki,
A. I. Virtasen aukio 1, Helsinki, Finland
[‡] Current address: Section Organic and Inorganic Chemistry,
Vrije Universiteit Amsterdam,
De Boelelaan 1083, Amsterdam, The Netherlands
704
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 704–707

Increasing the Lability of Polarised Phosphorus–Phosphorus Bonds
2.79 Å) in the iminophosphenium diselenophosphinate
[Mes*NP][Se2PPh2] of Niecke et al.[10] In contrast to this
species, the molecular structure of which was deemed to
resemble the “frozen transition state” for a [1,3]-sigmatropic
shift of an iminophosphenium fragment between the two
selenium atoms of a covalent diselenophosphinate,[10] the
P1–Se2 distance in 2 [3.391(1) Å] is clearly nonbonding. Al-
together, the molecular structure of 2 (Figure 1) is best de-
scribed as contact ion pair of a 1,3,2-diazaphospholenium
cation and a η1-bound diselenophosphinate anion. A sim-
ilar view of the bonding situation was recently suggested
for P-chlorodiazaphospholenes.[7]
Scheme 2. Reactions of the diphosphanes 1a/b [1a: R = Mes (2,4,6-
Me3C6H2); 1b: R = 2,6-Me2C6H3; 5, 6: Ar = 2,6-Me2C6H3].
The 31P NMR spectrum of isolated 2 shows signals at δ
= 164.4 and 20.4 ppm attributable to the tri- and the tetra-
coordinate phosphorus atoms, respectively. The upfield sig-
nal displays a single set of 77Se satellites (1JPSe = 617 Hz)
which remains unchanged in the temperature range between
30 and –80 °C and suggests the occurrence of dynamic ex-
change between terminal and bridging selenium atoms [this
view is also in accord with the observation that the magni-
tude of this coupling is intermediate with respect to the two
distinguishable couplings 1JPSe = 368, 762 Hz in tBu2P–Se–
Figure 1. Molecular structure of 2. Thermal ellipsoids are drawn
with 50% probability level, and H atoms have been omitted for
clarity. Selected bond lengths [Å] and angles [°]: P1–N2 1.688(2),
P1–N5 1.689(2), P1–Se1 2.766(1), P1–Se2 3.391(1), P2–Se1
2.193(1), P2–Se2 2.135(1), N2–C3 1.402(3), C3–C4 1.343(3), C4–
N5 1.398(3); N2–P1–N5 88.5(1), N2–P1–Se1 108.3(1), N5–P1–Se1
102.6(1), N2–P1–Se2 104.9(1), N5–P1–Se2 166.6(1), Se1–P1–Se2
72.60(2), P2–Se1–P1 92.76(2), Se2–P2–Se1 116.48(3), P2–Se2–P1
77.99(2).
The reaction leading to 2 is related to known reactions
P(Se)tBu2[6]]. The observation of coupling between the 31P of diphosphanes R4P2 with chalcogens E (E = S, Se) which
nuclei (2JPP = 5.1 Hz) in the spectra of freshly prepared yield, depending on the nature of reactants and reaction
solutions allows to conclude that the exchange is strictly conditions, either diphosphane chalcogenides R2P–P(E)R2,
intramolecular. Ageing of the solutions is usually ac- chalcogenobis(phosphanes) R2P–E–PR2, or phosphan-
companied by some signal broadening which renders the yl dichalcogenophosphinates R2P–E–P(E)R2, respec-
splitting unobservable. This effect can be explained as a tively.[6,11,12] The formation of all products in these reac-
consequence of intermolecular exchange processes that are tions can be rationalised by a common mechanism[10] pro-
presumably catalysed by trace amounts of decomposition ceeding by initial oxidation of one phosphorus atom, subse-
products formed.
quent migratory insertion of the chalcogen atom into the
An X-ray diffraction study of a single-crystal grown from P–P bond, and, finally, oxidation of one phosphorus atom
toluene solution at 4 °C revealed the presence of isolated by a second chalcogen atom. Even if the overall conditions
molecules with a planar diazaphospholene ring (deviation during the formation of 2 are similar to those employed for
of any atom from mean plane Ͻ 0.03 Å). The endocyclic the conversion of sterically congested symmetrical diphos-
bond lengths are similar to those in P-chlorodiazaphos- phanes R4P2 (R = tBu, iPr2N) into phosphanyl diseleno-
pholenes.[7] The P2–Se1/Se2 bond lengths of 2.193(1) and phosphinates R2P–Se–P(Se)R2,[6,12] the present reaction dif-
2.135(1) Å differ perceptibly, but their average comes close fers as no spectroscopically detectable intermediates could
to the mean P–Se distance in ionic diselenophosphinates be observed. This finding implies that the first reaction step
(2.17Ϯ0.03 Å[8]), and the asymmetry in the PSe2 moiety is in this case rate-determining and suggests thus that the
resembles that in the η1-bound ligands in a complex initial selenation of 1 further facilitates the P–P bond-cleav-
[Ga(η2-Se2PiPr2)(η1-Se2PiPr2)2].[9] The P1–Se1 bond age step.
[2.766(1) Å] clearly exceeds a normal single bond in com-
Considering that group-13 element halides form stable
pounds RRЈP–SeRЈЈ (mean distance 2.26Ϯ0.07 Å[8]) but adducts with phosphanes and diphosphanes,[13] we studied
matches the appropriate “inter-ionic” distances (2.64– next the reaction of equimolar amounts of 1a and GaCl3
Eur. J. Inorg. Chem. 2008, 704–707
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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705

S. Burck, D. Gudat, M. Nieger, D. Vindusˇ
FULL PAPER
in the hope to obtain a stable adduct. A 31P NMR spectrum phosphanes which yield normally thermally very stable ad-
of the reaction mixture showed in addition to the signal of ducts with Lewis acids. It appears plausible that all ob-
the 1,3,2-diazaphospholenium cation 3[2] at δ = 205 ppm a served reactions of 1a,b are initiated by coordination of the
further broad signal at δ = –45.5 ppm. The 71Ga NMR Lewis acid to the Ph2P moiety, even if the resulting adduct
spectrum disclosed likewise the formation of two gallium- was detected only in one case by NMR spectroscopy. The
containing species of which one was easily identified as unusually low reaction temperatures for this type of frag-
[GaCl4]– (δ71Ga = 251 ppm).[14] The position of the second mentation support the hypothesis that Lewis acid coordina-
signal (δ71Ga = 266 ppm) is in accord with the reported tion facilitates the bond cleavage step by boosting the pola-
chemical shift of the anion [Ga(PPh2)4]– (4)[15] which ac- risation in the P–P bond. Considering that this conclusion
counts also for the remaining 31P NMR signal. The forma- is likely to hold as well for reactions of diphosphanes like
tion of these species is explained if one assumes that the 1a,b with other types of electrophiles, our new findings per-
reaction proceeds by coordination of the Lewis acid to the mit a conclusive understanding of the previously observed
PPh2 moiety of 1a and concomitant P–P bond cleavage to diphosphanation of electron-deficient alkenes or alkynes[1]
give the cation 3 and an anion [Ph2PGaCl3]–, which then under exceptionally mild conditions.
undergoes redistribution of Ph2P and Cl substituents.
Detection of the postulated Lewis acid adduct of a di-
phosphane was finally feasible in the reaction of 1b with
Experimental Section
1 equiv. of BH3·thf at –10 °C. 31P NMR spectra of reaction
General: All manipulations were carried out under a protective gas
mixtures disclosed that the signals of 1b had been displaced
by a new AX-type spin system with chemical shifts of δ =
(argon) in flame-dried glassware. Solvents were dried by using com-
mon procedures. 1a,b were prepared as described previously.[1]
116.3 and –5 ppm, respectively. The significant increase in
NMR spectra: Bruker Avance 400 (1H: 400.13 MHz; 31P:
161.9 MHz; 13C: 100.4 MHz; 71Ga: 122.0 MHz) at 30 °C; chemical
1
the magnitude of JPP (409 Hz) and the broadening of the
shifts refer to ext. TMS (1H, 13C), 85% H3PO4 (Ξ
=
upfield resonance due to partially resolved spin coupling to
10/11B nuclei prove the coordination of a BH3 unit to the
Ph2P moiety of 1b and lead us to formulate the product as
the diphosphane–borane 5. The product decomposed
within several hours at –10 °C, or more rapidly upon warm-
ing to room temperature, to give a mixture of the cyclic
40.480747 MHz, 31P), Ga(NO3)3 (Ξ = 30.496576 MHz, 71Ga). MS:
Varian MAT 711, EI, 70 eV. Elemental analysis: Perkin–Elmer
2400CHSN/O Analyser. Melting points were determined in sealed
capillaries.
Synthesis of 2: 1a (2 mmol, 1.06 g) and elemental selenium
phosphanylboranes 7 which was identified by the coinci- (4 mmol, 0.32 g) were dissolved in toluene (50 mL), and the mix-
dence of the observed 31P and 11B chemical shifts [δ31P =
ture was heated to 110 °C for 4 h. After cooling to room tempera-
ture, the solvent was reduced to half the volume. Storage at 4 °C
–19.7 (br.) ppm; δ11B = –37.2 (br.) ppm] with literature
data,[16,17] and the P-hydridodiazaphospholene 6. The
afforded yellow needles that were filtered off and dried in vacuo to
1
give 1.28 g (93%) of 2 of m.p. 165 °C. H NMR (C6D6): δ = 8.12–
identity of the latter was derived from the presence of a
7.95 (m, 4 H, Ph), 6.93–6.83 (m, 6 H, Ph), 6.71 (s, 4 H, m-CH),
doublet 31P NMR signal (δ31P = 64.1 ppm) with a charac-
3
5.87 (d, JPH = 0.8 Hz, 2 H, NCH), 2.31 (s, 12 H, o-CH3), 2.10 (s,
teristic splitting of 1JPH = 138 Hz, and was verified by inde-
2
6 H, p-CH3) ppm. 13C{1H} NMR (C6D6): δ = 132.6 (d, JPC
=
pendent synthesis by reduction of the appropriate P-chloro-
1,3,2-diazaphospholene with LiAlH4 as previously re-
ported.[3] A plausible mechanism for the formation of 6, 7
2
2.2 Hz, i-C), 130.6 (d, JPC = 4.3 Hz, o-CH), 130.2 (s, o-C), 127.2
(d, JPC = 11.6 Hz, i-C), 125.3 (d, JPC = 0.9 Hz, m-CH), 125.0 (s,
m-CH), 124.4 (s, p-C), 123.6 (s, p-CH), 117.7 (d, JPC = 9.8 Hz,
1
3
2
6
4
involves cleavage of the P–P bond of 5 with concomitant NCH), 16.0 (d, JPC = 0.8 Hz, p-CH3), 14.5 (d, JPC = 3.2 Hz, o-
CH3) ppm. 31P{1H} NMR (C6D6): δ = 164.4 (d, JPP = 5.1 Hz,
2
transfer of a hydride ion from the BH3 moiety to the diaza-
phospholene phosphorus atom, and trimerisation of the re-
N2P), 20.4 (d, 2JPP = 5.1, JPSe = 617 Hz, PPh2) ppm. Attempts to
1
record a 77Se NMR spectrum were unsuccessful, presumably as
maining Ph2P–BH2 fragment. Such a reaction pattern is
detection of a signal was precluded by extensive exchange-induced
highly unusual, as borane adducts of diphosphanes are nor-
line broadening. (+)-FAB MS (3-nitrobenzyl alcohol): m/z (%) =
mally isolable and thermally highly stable species,[18] and
323.2 (50.8) [M]+, 133.0 (50.0) [M – C11H13NP]+, 119.1 (17.2) [M –
has so far only been reported for the polarised diphosphane
(F3C)2PPH2.[19] The low temperature in the formation of
the oligomers 7 is further remarkable, since generation of
the same species by rhodium catalysed dehydrocoupling of
C11H13N2P]+, 73.0 (100.0) [M – C13H22]+. C32H34N2P2Se2 (666.50):
calcd. C 57.67, H 5.14, N 4.20; found C 57.69, H 5.28, N 4.27.
Reaction of 1a and GaCl3: 1a (0.15 mmol, 76 mg) and GaCl3
(0.15 mmol, 26 mg) were dissolved in CD3CN (1 mL). Quantitative
formation of 3, 4 was confirmed by 1H and 31P NMR spectroscopy.
1H NMR (CD3CN): δ = 8.02 (s, 4 H, m-CH), 7.47 (m, 4 H, o-CH),
a
phosphane–borane requires temperatures around
120 °C.[16]
3
7.30–7.15 (m, 6 H, m/p-CH), 5.41 (d, JPH = 1.0 Hz, 2 H, NCH),
2.36 (s, 6 H, p-CH3), 2.14 (s, 12 H, o-CH3) ppm. 31P{1H} NMR
(CD3CN): δ = 206.0 (N2P), –45.4 [br. s, Ga(PPh2)4] ppm. 71Ga
NMR (CD3CN): δ = 266 [br. s, Ga(PPh2)4], 251 (s, GaCl4) ppm.
Conclusions
It was demonstrated that the diphosphanes 1a,b react
with selenium, gallium trichloride, or borane at or below
ambient temperature under P–P bond cleavage. This behav-
Reaction of 1b and BH3·thf: 1b (0.1 mmol, 48 mg) and BH3·thf
(0.1 mmol, 0.1 mL of 1  solution in thf) were dissolved in [D6]-
benzene (0.6 mL), and the mixture was immediately cooled to
iour contrasts with the known reactivity of symmetrical di- –10 °C. Quantitative formation of 5 was proven by NMR spec-
706 Eur. J. Inorg. Chem. 2008, 704–707
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Increasing the Lability of Polarised Phosphorus–Phosphorus Bonds
1
[2] D. Gudat, A. Haghverdi, H. Hupfer, M. Nieger, Chem. Eur. J.
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troscopy. H NMR (thf/C6D6): δ = 7.47 (m, 4 H, o-Ph), 6.88–6.77
(m, 12 H, m/p-Ph, m/p-C6H3), 5.79 (dd, JPH = 1.1, 1.8 Hz, 2 H,
NCH), 2.45 (s, 6 H, o-CH3), 2.32 (s, 6 H, o-CH3), 1.4 (v. br., 3 H,
BH3) ppm. 31P{1H} NMR (thf/C6D6): δ = 116.2 (d, 1JPP = 409 Hz,
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[8] Results of queries in the CSD data base for compounds of the
type R2P(Se)SeRЈ and R2P–SeRЈ, respectively.
N2P), –5.0 (d, JPP = 409 Hz, Ph2P) ppm. 11B{1H} NMR (thf/
1
C6D6): δ = –35.5 (br.) ppm. Upon warming to ambient tempera-
ture, the signals of 5 were replaced by those of 6, 7. 31P NMR (thf/
C6D6): δ = 64.1 (d, 1JPH = 137 Hz, 6), –20.2 (br. s, 7) ppm. 11B{1H}
NMR (thf/C6D6): δ = –37.2 (br.) ppm. Repetition of the reaction
on a larger scale (1 mmol each of 1b and BH3·thf in 20 mL of
hexane) afforded after warming to ambient temperature a product
mixture which gave a similar 31P NMR spectrum. Attempts to iso-
late the products formed in pure form remained unsuccessful. The
identity of 6 was proven by independent synthesis according to a
literature procedure.[3] Yield 62%, m.p. 38 °C. H NMR (C6D6): δ
= 7.12 (d, JPH = 138 Hz, 1 H, PH), 6.92 (s, 6 H, m/p-CH), 5.74
1
1
3
(d, JPH = 2.0 Hz, 2 H, NCH), 2.29 (br. s, 18 H, o-CH3) ppm.
13C{1H} NMR (C6D6): δ = 140.6 (d, JPC = 13.8 Hz, i-C), 137.3
2
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(br. s, o-C), 128.8 (d, 5JPC = 1.0 Hz, p-CH), 126.3 (d, 4JPC = 1.8 Hz,
2
m-CH), 121.3 (d, JPC = 6.3 Hz, NCH), 19.2 (br. s, o-CH3) ppm.
31P NMR (C6D6): δ = 64.3 (d, JPH = 138 Hz) ppm. C18H21N2P
1
(296.35): calcd. C 72.95, H 7.14, N 9.45; found C 72.86, H 7.31, N
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fractometer at 123(2) K using Mo-Kα radiation (λ = 0.71073 Å).
Direct methods (SHELXS-97[20]) were used for structure solution,
and full-matrix least-squares refinement on F2 (SHELXL-97[21]). H
atoms were localised by difference Fourier synthesis and refined
using a riding model. An empirical absorption correction from
equivalent reflections was applied (min./max. transmission 0.4915/
0.6279). The absolute structure was determined by refinement of
Flack’s parameter, x = 0.079(5).[22] Crystal data: orange crystals,
C32H34N2P2Se2, M = 666.47, crystal size 0.40ϫ0.30ϫ0.20 mm, or-
thorhombic, space group Pna21 (no. 33), a = 22.938(1), b =
11.153(1), c = 11.863(1) Å, V = 3034.9(4) Å3, Z = 4, ρ(calcd.) =
1.459 Mgm–3, F(000) = 1352, µ = 2.566 mm–1, 53036 reflexions
(2θmax = 55.0°), 6883 unique (Rint = 0.029), 349 parameters, 1 re-
straint, R1 [IϾ2σ(I)] = 0.022, wR2 (all data) = 0.057, largest diff.
peak/hole 1.326/–0.296 eÅ–3. CCDC-660525 contains the supple-
mentary 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.
13–16.
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[17] Distinction between trimeric and tetrameric adducts solely on
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Acknowledgments
This work was financially supported by the Deutsche Forschungs-
gemeinschaft.
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Received: September 14, 2007
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Published Online: December 18, 2007
Eur. J. Inorg. Chem. 2008, 704–707
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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