Endocyclic P-P bond cleavage in carbaborane-substituted 1,2-diphosphetane: A new route to secondary phosphinocarbaboranes

Article abstract of DOI:10.1039/c2cc34860h

Carbaborane-substituted 1,2-diphosphetane reacts with elemental lithium and hydrogen chloride to give exclusively secondary mono- and bis(phosphino) carbaboranes. The latter reacts with two equivalents of formaldehyde and one equivalent of aniline to give a carbaborane-substituted 1-aza-3,6-diphosphepane.

Full text of DOI:10.1039/c2cc34860h

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Endocyclic P–P bond cleavage in carbaborane-substituted
1,2-diphosphetane: a new route to secondary phosphinocarbaboraneswz
¨
Anika Kreienbrink, Peter Lonnecke, Matthias Findeisen and Evamarie Hey-Hawkins*
a
a
b
a
Received 27th April 2012, Accepted 31st July 2012
DOI: 10.1039/c2cc34860h
Carbaborane-substituted 1,2-diphosphetane reacts with elemental
lithium and hydrogen chloride to give exclusively secondary
mono- and bis(phosphino)carbaboranes. The latter reacts with
two equivalents of formaldehyde and one equivalent of aniline to
give a carbaborane-substituted 1-aza-3,6-diphosphepane.
While the chemistry of 1,2-diphosphetes is well developed, little is
known about their saturated relatives, the 1,2-diphosphetanes.1
The facile high-yield synthesis of carbaborane-substituted
1,2-diphosphetanes by reduction of bis(halophosphino)-dicarba-
closo-dodecaborane(12)s now allows their synthetic potential
to be studied. Thus, elemental iodine reacts in an oxidative ring-
opening reaction to give the first chiral 1,2-bis(iodophosphino)-
1,2-dicarba-closo-dodecaborane(12)s.2 In this preliminary
account, we present a new synthetic route to secondary mono- (1)
and bis(phosphino)carbaboranes (2) by reductive P–P bond
cleavage of carbaborane-substituted 1,2-diphosphetane with
elemental lithium and subsequent reaction with hydrogen chloride.
Secondary phosphines are very useful starting materials for
numerous phosphorus-based compounds.3 However, secondary
mono- and bis(phosphino)carbaboranes are rare, as problems
such as cleavage of the cluster carbon–phosphorus bond are
usually encountered in the reaction of the corresponding
mono- and bis(halophosphino)-dicarba-closo-dodecaborane(12)s
with LiAlH4.4 Thus, this reaction seemed to be limited to
phenylphosphino5 and ferrocenylphosphino6 derivatives, but
using 1,2-diphosphetanes as starting material offers access to
secondary mono- and bis(phosphino)carbaboranes that were
inaccessible before.
Scheme 1 Synthesis of tert-butyl-substituted secondary phosphino-
carbaboranes 1 and 2.
tert-butyl-substituted 1,2-diphosphetane is cleaved by elemental
lithium to give a deep red solution of the lithium diphosphane-
diide species in THF. Addition of hydrogen chloride in diethyl
ether gave equal amounts of rac- and meso-1,2-bis(tert-butyl-
phosphino)-1,2-dicarba-closo-dodecaborane(12) (2) in moderate
yield (56%). Furthermore, cleavage of one P–C bond is observed
with formation of the corresponding 1-tert-butylphosphino-
1,2-dicarba-closo-dodecaborane (1) (10% yield) and primary
phosphine (Scheme 1). A 31P NMR spectrum of the reaction
mixture already shows the presence of both species, but
the products 1 and 2 can be separated by fractional sublima-
tion and are obtained as air- and water-sensitive colourless
solids.y
The 31P NMR spectrum of 1 in C6D6 exhibits a doublet at
26.2 ppm with a P–H coupling constant (1JPH) of 220 Hz. In
the 13C{1H} NMR spectrum, two doublets are observed for
the carbon cluster atoms of the mono-substituted carbaborane
at 65.8 and 67.6 ppm with 2JCP and 1JCP of 22.2 and 66.9 Hz,
respectively. The 31P NMR spectrum of 2 is more complex and
exhibits two multiplets at 9.5 and 10.3 ppm (ratio ca. 1 : 0.8)
for the HPC–CPH fragments of the two diastereomers
(AA‘XX’ spin system). The coupling constants were obtained
from a simulated 1H NMR spectrum of one species (1JPH = 222,
3JPP = 112 Hz).8 The 13C{1H} NMR spectrum shows a complex
coupling pattern for the ABX spin system of the PCCP moiety
at 78.5 ppm, as reported earlier.9 P–H stretching modes for
both compounds were observed in the IR spectra (2316 cmꢀ1
for 1 and 2311 cmꢀ1 for 2).
Alkali metals, lithium alkyls and platinum(0) complexes
were reported to cleave the P–P bond of phosphorus hetero-
cycles, e.g., 1,2-dihydro-1,2-diphosphetes.7 With lithium, the
resulting dianionic lithium salts offered access to new, open-
chain diphosphaethene derivatives. Similarly, the P–P bond of
a Institut fur Anorganische Chemie, Universitat Leipzig,
Johannisallee 29, 04103 Leipzig, Germany.
¨
¨
E-mail: hey@rz.uni-leipzig.de; Fax: +49 341-9739319
¨
b Institut fur Analytische Chemie, Universitat Leipzig, Linne´straße 3,
04103 Leipzig, Germany
¨
An X-ray structure analysis was carried out for 1 (Fig. 1).10
The P1–C2 bond length (1.864(2) A) is similar to those in the
tert-butyl-substituted 1,2-diphosphetane (1.883(6) A), the
C1carb–C2carb bond (1.696(2) A) is slightly longer (cf. 1.645(4) A).2
The P1–H1p bond length (1.31(2) A) compares well with
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available. CCDC
877502 and 891099. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc34860h
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9385–9387 9385

0
0
Fig. 2 (a) Molecular structure of (RP RP/SP SP)-4,5-(ortho-dicarba-
closo-dodecaboranyl)-3,6-di-tert-butyl-1-phenyl-1-aza-3,6-diphosphepane;
(b) central 7-membered heterocycle with selected bond lengths (A)
and angles (1): C1–C2 1.750(7), C1–P1 1.915(6), C3–P1 1.860(6),
C3–N1 1.472(7), N1–C4 1.447(7), C4–P2 1.869(6), P2–C2 1.895(6),
C2–C1–P1 121.1(4), C2–P1–C4 101.1(3), P2–C4–N1 113.1(4),
C4–N1–C3 111.3(5) (ellipsoids are shown at 50% probability).
Fig. 1 Molecular structure of 1-tert-butylphosphino-1,2-dicarba-closo-
dodecaborane (1) with selected bond distances (A) and angles (1):
C1–C2 1.696(2), C1–H1 1.03(2), C2–P1 1.864(2), P1–H1p 1.31(2),
P1–C3 1.863(2), B–B 1.767–1.789, B–C 1.715–1.739, C1–C2–P1
112.6(1), C2–P1–C3 109.90(7), C2–P1–H1p 106(1) (ellipsoids are
shown at 50% probability).
similar bonds found in secondary phosphinocarbaboranes4 or
phosphonium salts.11 As expected, the CCarb–CCarb–P bond angle
(112.6(1)1) in 1 is much larger than in the 1,2-diphosphetane
(97.9(7)1).2
Mannich-type reaction of 2 with formaldehyde and a primary
amine. The secondary bis(phosphino)carbaborane 2 and the
P,N-heterocycle 3 will now be employed as a chelating ligand
in transition metal complexes.
Support from the Konrad-Adenauer Stiftung (doctoral grant
for A.K.), the Graduate School Leipzig School of Natural
Sciences – Building with Molecules and Nano-objects (BuildMoNa)
and the COST Action CM0802 PhoSciNet is gratefully
acknowledged. We thank Prof. Dr. S. Berger (Universitat
Leipzig) for valuable advice on the NMR spectroscopic studies,
and Chemetall GmbH and BASF SE for generous donations
of chemicals.
Aminomethylation of secondary phosphines is well known
and allows easy access to tertiary phosphino amino derivatives.12
Formaldehyde and secondary amines give linear condensation
products,13 whereas the reaction with primary amines results in
P,N-heterocyclic compounds.14 Accordingly, 2 reacts with two
equivalents of formaldehyde and one equivalent of aniline in
DMF at 60 1C (3 h) to give the seven-membered air- and water-
stable 4,5-(dicarba-closo-dodecaboranyl)-3,6-di-tert-butyl-1-
phenyl-1-aza-3,6-diphosphepane (3) in 67% yield (Scheme 2).y
The 31P{1H} NMR spectrum of 3 shows two singlets at 30.2
and 36.7 ppm (ratio 30 : 1 for rac : meso) for the two expected
Notes and references
y NMR spectroscopic data of 1–3 (in C6D6 for 1 and 2): 1: 1H NMR:
1
diastereomers, while the H, 13C and 11B NMR spectra show
3
d = 0.87 (d, JPH = 14.2 Hz, 9H, C(CH3)3), 2.80 (s, 1H, CCarb–H),
3.76 (d, JPH = 220 Hz, 1H, P–H), 1.20–3.60 ppm (m, 10H, B10H10);
1
identical signals for both diastereomers.
11B{1H} NMR: d = ꢀ0.7 (br s, 1B), ꢀ1.4 (br s, 1B), ꢀ7.2 (br s, 2B),
The P–CH2–N protons appear as two multiplets (2JHH = 15 Hz,
2JPH = 7–11 Hz). The 13C{1H} NMR spectrum shows a
complex coupling pattern for the ABX spin system of the
PCCP moiety at 84.6 ppm and a multiplet at 52.6 ppm for the
CH2–P moiety (1JCP = 16 Hz). Compound rac-3 crystallizes
from n-hexane and was structurally characterized (Fig. 2).10
In summary, a potentially versatile route to secondary tert-
butyl-substituted mono- and bis(phosphino)carbaboranes was
developed by reductive P–P bond cleavage of 1,2-diphosphetanes
with lithium and subsequent reaction with hydrogen chloride.
The phosphorus-containing heterocycle 3 was synthesized by a
ꢀ8.9 (br s, 1B), ꢀ10.3 (br s, 1B), ꢀ11.6 (br s, 2B), ꢀ12.8 ppm (br s,
2
2B); 13C{1H} NMR: d = 28.5 (d, JCP = 14.9 Hz, C(CH3)3), 31.9
=
(m, C(CH3)3), 65.8 (d, 2JCP = 22.2 Hz, CCarb–H), 67.6 ppm (d, 1JCP
1
66.9 Hz, CCarb–P); 31P NMR: d = 26.2 ppm (d, JPH = 220 Hz). 2:
1H NMR: d = 1.00 (m, 3JPH = 8 Hz, 18H, C(CH3)3), 3.93 (m, 1JPH
222 Hz, JPP = 112 Hz, 2H, P–H), 1.50–3.50 ppm (m, 10H, B10H10);
11B{1H} NMR: d = 0.3 (br s, 2B), ꢀ6.3 (br s, 5B), ꢀ9.0 ppm (br s, 3B);
13C{1H} NMR: d = 29.2 (m, C(CH3)3), 32.6 (m, C(CH3)3), 78.5 ppm
=
3
1
3
(m, C2B10H10); 31P NMR: d = 9.51 (m, JPH = 222 Hz, JPP
=
112 Hz), 10.27 ppm (m, 1JPH = 222 Hz, 3JPP = 112 Hz). 3: 1H NMR
3
(CDCl3): d = 1.16 (d, JPH = 13.4 Hz, 18H, C(CH3)3), 3.86 (m,
2JHH = 15 Hz, JPH = 7 Hz, 2H, CH2P), 3.91 (m, JHH = 15 Hz,
2JPH = 11 Hz, 2H, CH2P), 6.82–7.24 (m, 5H, C6H5), 1.80–3.20 ppm
(m, 10H, B10H10); 11B{1H} NMR (CDCl3): d = 0.2 (br s, 2B), ꢀ5.1
2
2
(br s, 2B), ꢀ9.2 (br s, 6B); 13C{1H} NMR (C6D6): d = 26.9 (d, 2JCP
=
1
14.8 Hz, C(CH3)3), 33.5 (d, JCP
=
(m, JCP = 16.0 Hz, CH2P), 84.6 (m, JCP
27.6 Hz, C(CH3)3), 52.8
1
+
2JCP = 105 Hz,
1
C2B10H10), 118.0, 120.7, 129.2, 149.9 ppm (s, C6H5); 31P NMR
(CDCl3): d = 30.2 (s), 36.7 (s) ppm.
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Scheme 2 Condensation reaction of 2 with formaldehyde and aniline.
9386 Chem. Commun., 2012, 48, 9385–9387
c
This journal is The Royal Society of Chemistry 2012

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b = 112.994(5)1, V = 1379.22(9) A3, Z = 4, rcalc = 1.119 Mg mꢀ3
,
m = 0.162 mmꢀ1, 2ymax = 30.511, R = 0.0541, Rw = 0.1091. T =
130(2) K, 4206 independent reflections, 243 parameters, 1 restraint,
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M = 437.53, orthorhombic, space group Pbca, a = 11.8524(8) A,
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,
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9385–9387 9387