Formation of a diphosphine: Synthesis and molecular structure of bis(tetraphenylbutadienyl)diphosphine, (Ph4C4)P-P(C4Ph4)

Article abstract of DOI:10.1039/b001257m

Reaction of 1,4-dilithiotetraphenylbutadiene with phosphorus(III) bromide affords bis(tetraphenylbutadienyl)diphosphine, (Ph₄C₄)P-P(C₄Ph₄), with a phosphorus-phosphorus bond length of 2.2051(11) A.

Full text of DOI:10.1039/b001257m

Formation of a diphosphine: synthesis and molecular structure of
bis(tetraphenylbutadienyl)diphosphine, (Ph4C4)P–P(C4Ph4)
Jason K. Vohs, Pingrong Wei, Jianrui Su, Brent C. Beck, Sonya D. Goodwin and Gregory H. Robinson*
Department of Chemistry, The University of Georgia, Athens, GA 30602-2556, USA.
E-mail: robinson@sunchem.chem.uga.edu
Received (in Columbia, MO, USA) 14th February 2000, Accepted 5th April 2000
Published on the Web 25th May 2000
Reaction of 1,4-dilithiotetraphenylbutadiene with phos-
phorus(III) bromide affords bis(tetraphenylbutadienyl)di-
phosphine, (Ph4C4)P–P(C4Ph4), with a phosphorus–phos-
phorus bond length of 2.2051(11) Å.
spectroscopy elemental analyses and single-crystal X-ray
diffraction (Fig. 1), is significant as it represents an interestingly
facile, if unexpected, one-pot route to a diphosphine.
The past two decades have witnessed a number of studies
concerning compounds containing phosphorus–phosphorus
bonds.9 Indeed, this laboratory has had an interest in the
preparation of diphosphines having previously reported the
synthesis and molecular structure of [(Me3Si)2P{Me2-
Ga}2PMe]2,10 isolated from reaction of Me3P–GaMe3 with
P(SiMe3)3, having a P–P single bond length of 2.25(3) Å.
Relative to diphosphenes it is important to note the previously
reported s-bonded pentamethylcyclopentadienyl-based diph-
osphene, bis(pentamethylcyclopentadienyl)diphosphene, (s-
C5Me5)PNP(s-C5Me5).11 The phosphorus atoms in (s-
C5Me5)PNP(s-C5Me5) are two-coordinate (s-bonding to one of
the carbon atoms of the ring and to the other phosphorus atom)
with a P–P double bond length of 2.031(3) Å.
A number of points regarding the structure and bonding in
(Ph4C4)P–P(C4Ph4) are relevant. As expected, the five-mem-
bered butadienyl rings are planar about the phosphorus atoms
with the phenyl rings arranged in a propeller-like fashion.
Indeed, the molecule resides in a trans configuration. The
environment about the phosphorus atoms must be considered
distorted pyramidal thereby precluding significant interaction
of the phosphorus lone electron pairs with the conjugated
system of the tetraphenylbutadienyl moieties. The P–C bond
lengths are generally unremarkable. Clearly. the phosphorus–
phosphorus bond of 2.2051(11) Å in (Ph4C4)P–P(C4Ph4) must
be considered a single bond. Moreover, the 31P NMR resonance
of d 215.98 in (Ph4C4)P–P(C4Ph4) is also consistent with a P–P
single bond. For comparison, the 31P resonance of (s-
C5Me5)PNP(s-C5Me5) was reported at d 504 (121 MHz, C6D6)
Bis(2,4,6-tri-tert-butylphenyl)diphosphene, (But3C6H2)PNP-
(C6H2But2),1 the first compound shown to contain a formal
phosphorus–phosphorus double bond [P–P 2.034(2) Å], was
prepared by the magnesium metal reduction of (But3C6H2)PCl2.
Indeed, alkali (or alkaline earth) metal reduction is one of three
general routes to diphosphines (the other two being heating a
phosphinous halide, R2PCl, with a secondary phosphine or with
an alkali metal phosphine),2 a significant development in main
group chemistry. Furthermore, as evidenced by subsequent
studies concerning (But3C6H2)PNP(C6H2But3),3 the chemistry
of diphosphenes and diphosphines continues to be of interest to
both synthetic and computational chemists. Crucial in the
stabilization of (But3C6H2)PNP(C6H2But3) was the utilization of
the sterically demanding 2,4,6-tri-tert-butylphenyl ligand. In a
related vein, this laboratory has recently examined the organo-
group 13 (III) chemistry of 1,4-dilithiotetraphenylbutadiene
resulting in two interesting complexes: (a) a spirogallane
anion,4 [(Ph4C4)Ga(C4Ph4)]2 I; and (b) an In2C8 ten-membered
ring, (Et2O)2Li(Br)2In{(C4Ph4)}2In(Br)2Li(OEt2)2 II.5
Herein, we report the synthesis6 and molecular structure7 of
bis(tetraphenylbutadienyl)diphosphine,
(Ph4C4)P–P(C4Ph4)
III, readily isolated from reaction of 1,4-dilithiotetraphenylbu-
tadiene,8 prepared by the action of an excess of metallic lithium
on diphenylacetylene, with phosphorus(III) bromide (Scheme
1
1). This compound, characterized by H, 13C and 31P NMR
Fig. 1 Molecular structure of (Ph4C4)P–P(C4Ph4). Bond distances (Å) and
angles (°): P(1)–C(22) 1.797(3), P(1)–C(1) 1.799(3), P(1)–P(2) 2.2051(11),
P(2)–C(29) 1.796(3), P(2)–C(50) 1.799(3); C(22)–P(1)–C(1) 91.16(12),
C(22)–P(1)–P(2) 108.13(9), C(1)–P(1)–P(2) 100.16(9), C(29)–P(2)–C(50)
91.26(14), C(29)–P(2)–P(1) 106.14(10), C(50)–P(2)–P(1) 99.92(10).
Scheme 1
DOI: 10.1039/b001257m
Chem. Commun., 2000, 1037–1038
This journal is © The Royal Society of Chemistry 2000
1037

dissolved in hexane (40 mL). The resulting solution was filtered
immediately and dried in vacuo. The solid was recrystallized in diethyl
ether and resulted in yellow–orange crystals (0.56 g, 34% yield): mp
245–246 °C (decomp.). Anal. (E+R Microanalytical Labroatories,
Parsipanny, NJ). Calc. (found) for C59H47P2: C, 86.60 (86.51); H, 5.80
(5.76)%. dH(300 MHz, 298 K, CDCl3): 0.46 [q, 3H, CH3 (hexane)], 0.84
[m, 4H, CH2 (hexane)], 7.26–7.56 [m, 40H, CH (aromatic)] dC(300
MHz, 298 K, CDCl3): 14.01, 22.53, 31.47, 126.32, 127.09, 127.82,
129.60, 130.26, 136.52, 137.33, 143.85, 147.82, 151.75. dP(300 MHz,
298 K, CDCl3): 215.98.
7 X-Ray intensity data were measured at room temperature on a Bruker
SMART™ CCD-based X-ray diffractometer system with graphite-
monochromated Mo-Ka radiation (l = 0.71073 Å). The structure was
solved by direct methods using the SHELXTL 5.1 system of computer
programs. The non-hydrogen atoms were refined anisotropically while
hydrogen atoms were placed in idealized positions with their coor-
dinates and thermal parameters riding on the attached carbon atoms.
Crystallographic data: monoclinic, space group C2/c (no. 15), a =
28.092(4), b = 13.6821(16), c = 24.311(2) Å, b = 98.078(5)°, V =
9251.4(19) Å3 and Z = 8. The asymmetric unit contains one (Ph4C4)P–
P(C4Ph4) molecule and one-half hexane unit (situated about an
inversion center). Refinement converged at R1 = 0.056 and wR2 =
0.132. CCDC 182/1625. See http://www.rsc.org/suppdata/cc/b0/
b001257m/ for crystallographic files in .cif format.
8 A. Orechoff, Chem. Ber., 1914, 47, 89; W. Schlenk and E. Bergmann,
Annales, 1928, 463, 71; E. Bergmann and O. Zwecker, Annales, 1931,
487, 155; E. Bergmann and W. Schreiber, Annales, 1933, 500, 118; L.
Smith and H. H. Hoehn, J. Am. Chem. Soc., 1941, 63, 1184.
9 (a) J. E. Richman, R. O. Day and R. R. Holmes, J. Am. Chem. Soc.,
1980, 102, 3955; (b) K. M. Flynn, M. M. Olmstead and P. P. Power,
J. Am. Chem. Soc., 1983, 105, 2085: (c) D. C. Pestana and P. P. Power,
J. Am. Chem. Soc., 1989, 111, 6887; (d) M. Drieß, P. Frankhauser, H.
Pritzkow and W. Siebert, Chem. Ber., 1991, 124, 1497.
10 J. A. Burns, W. T. Pennington and G. H. Robinson, Organometallics,
1995, 14, 1533.
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while that for the [trans-{[Fe(CO4)]2[PCH(SiMe3)2]2}] diph-
osphene was reported at d 384.55 (external 85% H3PO4) [P–P
2.039(1) Å].9b
It is significant that the tetraphenylbutadienyl moiety con-
tinues to find utility in main group chemistry. It is noteworthy
that the title compound bears some resemblance to the
previously reported bis[1-(trimethylsilyl)-2,3,4,5-tetraphenyl-
1-silacyclopentadienyl] compound isolated in 54% yield from
the sonication of 1,1-dichloro-2,3,4,5-tetraphenyl-1-silacyclo-
pentadiene in the presence of three equivalents of metallic
lithium.12
We are grateful to the National Science Foundation (G. H. R.:
CHE-95-9520162) and to the donors of the Petroleum Research
Fund, administered by the American Chemical Society, for
support of this work.
Notes and references
1 M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and T. Higuchi, J. Am.
Chem. Soc., 1981, 103, 4578.
2 D. E. C. Corbridge, Phosphorus: An Outline of its Chemistry,
Biochemistry and Technology, Elsevier, New York, 1985, p. 435.
3 A. H. Cowley, A. Decken, N. C. Norman, C. Kru¨ger, F. Lutz, H.
Jacobson and T. Ziegler, J. Am. Chem. Soc., 1997, 119, 3389.
4 J. Su, S. D. Goodwin, X.-W. Li and G. H. Robinson, J. Am. Chem. Soc.,
1998, 120, 12 994.
5 B. C. Beck, J. Su, P. Wei, X.-W. Li and G. H. Robinson,
Organometallics, 2000, 19, 1214.
6 Inside a drybox (M Braun Labmaster 130) a reaction vessel was charged
with phosphorus(III) bromide (1.08 g, 4 mmol), and diethyl ether (30
mL). Upon returning to the benchtop, 1,4-dilithiotetraphenylbutadiene,
prepared by reaction of lithium (0.09 g, 12.96 mmol) and diph-
enylacetylene (1.74 g, 9.78 mmol) in diethyl ether, was slowly added to
the reaction vessel at 278 °C over a period of 30 min via an addition
funnel. The system was allowed to warm to room temperature and stir
overnight. The solvent was removed in vacuo, and the solid was re-
12 J.-H. Hong, P. Boudjouk and S. Castellino, Organometallics, 1994, 13,
3387.
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Chem. Commun., 2000, 1037–1038