Synthesis and structure of a stable 1,3-dihydrotriphosphane and its thermal decomposition leading to the formation of the corresponding phosphine and diphosphene

Article abstract of DOI:10.1021/om0501069

Treatment of dichloroferrocenylphosphine with two molar amounts of a lithium phosphide bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) group afforded the corresponding 1,3-dihydro-2-ferrocenyltriphosphane [1; (TbtHP)₂PFc, Fc = ferrocenyl] as a mixture of three diastereomers in 73% yield. In sharp contrast to the previously reported 1,3-dihydrotriphosphanes [(RHP)₂PR, R - Ph, t-Bu], 1 was quite stable toward air and moisture either in the solid state or in solution at ambient temperature. The structural characterization of 1 was achieved by NMR spectra and X-ray crystallographic analysis. In the ³¹P{¹H} NMR spectrum of the mixture of three diastereomers of 1, the characteristic two A₂B and one ABX system were observed as signals assignable to two meso and one dl isomer, respectively. The X-ray crystallographic analysis for a single crystal obtained from the diastereomer mixture of 1 revealed its molecular structure, having P-P bond lengths of 2.2304(12) and 2.2322(12) A and a P-P-P bond angle of 96.17(5)°, although the configuration could not be determined. Thermolysis of 1 in toluene led to the quantitative formation of TbtPH₂ (2) and (E)-TbtP=PFc (3), as judged by the ¹H and ³¹P NMR spectra. Kinetic studies indicated that the thermolysis of 1 is a first-order reaction including a unimolecular dissociative process, which was reasonablely supported by theoretical calculations.

Full text of DOI:10.1021/om0501069

3074
Organometallics 2005, 24, 3074-3080
Synthesis and Structure of a Stable
1,3-Dihydrotriphosphane and Its Thermal Decomposition
Leading to the Formation of the Corresponding
Phosphine and Diphosphene
Noriyoshi Nagahora, Takahiro Sasamori, Nobuhiro Takeda, and
Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
Received February 16, 2005
Treatment of dichloroferrocenylphosphine with two molar amounts of a lithium phosphide
bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) group afforded the
corresponding 1,3-dihydro-2-ferrocenyltriphosphane [1; (TbtHP)2PFc, Fc ) ferrocenyl] as a
mixture of three diastereomers in 73% yield. In sharp contrast to the previously reported
1,3-dihydrotriphosphanes [(RHP)2PR, R ) Ph, t-Bu], 1 was quite stable toward air and
moisture either in the solid state or in solution at ambient temperature. The structural
characterization of 1 was achieved by NMR spectra and X-ray crystallographic analysis. In
the 31P{1H} NMR spectrum of the mixture of three diastereomers of 1, the characteristic
two A2B and one ABX system were observed as signals assignable to two meso and one dl
isomer, respectively. The X-ray crystallographic analysis for a single crystal obtained from
the diastereomer mixture of 1 revealed its molecular structure, having P-P bond lengths of
2.2304(12) and 2.2322(12) Å and a P-P-P bond angle of 96.17(5)°, although the configuration
could not be determined. Thermolysis of 1 in toluene led to the quantitative formation of
TbtPH2 (2) and (E)-TbtPdPFc (3), as judged by the 1H and 31P NMR spectra. Kinetic studies
indicated that the thermolysis of 1 is a first-order reaction including a unimolecular
dissociative process, which was reasonablely supported by theoretical calculations.
Introduction
Fehlner in 1968 that P3H5 easily underwent thermal
decomposition giving PH3 and P2H2 with a first-order
kinetic constant to the substrate as speculated by a
mass spectrometric study.6 In addition, 1,3-dihydro-
triphosphanes (RHP)2PR (R ) Ph, t-Bu) have been
reported as marginally stable examples of triphosphorus
hydrides (Chart 1).7,8 (PhHP)2PPh (4a) was found to
undergo a disproportionation reaction at room temper-
ature to give homologues of oligophosphorus hydrides
(PhP)nH2 together with cyclic phenylphosphanes (PhP)n
probably via oligomerization of P-H units of 4a.7
Similarly, (t-BuHP)2P(t-Bu) (4b) gave a mixture of the
corresponding phosphine (t-BuPH2) and cyclotetraphos-
phetane (t-BuP)4 via its disproportionation reaction.8
Taking into account that triphosphanes 5a-d (Chart
1), having no phosphorus-hydrogen bond, were isolated
as stable compounds,9 it can be concluded that the
Remarkable progress has been made in the chemistry
of open-chain phosphorus compounds because of their
unique structures and properties.1 According to the
recent reports of triphosphorus compounds, there were
notable candidates for new types of heterocycles as
building blocks containing phosphorus atoms2 and a
new class of triphosphane ligands in the field of homo-
geneous catalysts.3 Since the first indication for the
existence of P3H5,4 which is one of the simple oligophos-
phorus hydrides, there were numerous reports on the
synthesis and characterization of oligo- and polyphos-
phorus hydrides.1 In most cases, open-chain phosphorus
hydrides are thermally unstable and difficult to treat
under ambient temperature due to the weak phospho-
rus-hydrogen bonds.5 For example, it was reported by
* To whom correspondence should be addressed. Phone: +81-774-
38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-
u.ac.jp.
(5) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons: Canada,
1999.
(6) Fehlner, T. P. J. Am. Chem. Soc. 1966, 88, 1819-1821.
(7) (a) Baudler, M.; Koch, D.; Carlsohn, B. Chem. Ber. 1978, 111,
1217-1220. (b) Baudler, M.; Reuschenbach, G. Phosphorus Sulfur
1980, 9, 81-85.
(8) (a) Baudler, M.; Hellmann, J.; Reuschenbach, G. Z. Anorg. Allg.
Chem. 1984, 509, 38-52. (b) Baudler, M.; Tscha¨bunin, H. Z. Anorg.
Allg. Chem. 1992, 617, 31-36.
(9) (a) Bender, H. R. G.; Nieger, M.; Niecke, E. Z. Naturforsch. 1993,
48b, 1742-1752. (b) Kovacs, I.; Krautscheid, H.; Matern, E.; Sattler,
E.; Fritz, G.; Ho¨nle, W.; Borrmann, H.; Schnering, H. G. V. Z. Anorg.
Allg. Chem. 1996, 622, 1564-1572. (c) Urnezius, E.; Lam, K.-C.;
Rheingold, A. L.; Protasiewicz, J. D. J. Organomet. Chem. 2001, 630,
193-197.
(1) For a review on open-chain phosphorus compounds, see: Baudler,
M.; Glinka, K. Chem. Rev. 1994, 94, 1273-1297.
(2) (a) Breen, T. L.; Stephan, D. W. Organometallics 1997, 16, 365-
369. (b) Bashall, A.; Garcia, F.; Lawson, G. T.; McPartlin, M.;
Rothenberger, A.; Woods, A. D.; Wright, D. S. Can. J. Chem. 2002, 80,
1421-1427. (c) Uhl, W.; Benter, M. Dalton. Trans. 2000, 18, 3133-
3135. (d) Bashall, A.; Hopkins, A. D.; Mays, M. J.; McPartlin, M.; Wood,
J. A.; Woods, A. D.; Wright, D. S. Dalton. Trans. 2000, 12, 1825-1826.
(e) Althaus, H.; Breunig, H. J.; Probst, J.; Ro¨sler, R.; Lork, E. J.
Organomet. Chem. 1999, 585, 285-289.
(3) (a) Dou, D.; Duesler, E. N.; Paine, R. T. Inorg. Chem. 1999, 38,
788-793. (b) Caminade, A. M.; Majoral, J. P.; Mathieu, R. Chem. Rev.
1991, 91, 575-612, and references therein.
(4) Royen, P.; Hill, K. Z. Anorg. Allg. Chem. 1936, 229, 97.
10.1021/om0501069 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/14/2005

A Stable 1,3-Dihydrotriphosphane
Organometallics, Vol. 24, No. 13, 2005 3075
Chart 1
Figure 1. 31P{1H} NMR spectrum of 1 in benzene-d6 at
50 °C. One ABX (open circles) and two A2B (open triangles
and squares) systems were observed.
Chart 2
Scheme 1. Synthesis of 1a
a (a) n-BuLi (1.0 equiv), Et2O, -78 °C to rt. (b) FcPCl2 (0.5
equiv), THF, -78 °C to rt.
reactivity of phosphorus-hydrogen bonds inherently
gives 1,3-dihydrotriphosphanes thermal instability.
On the other hand, it has been demonstrated by the
isolation of Mes*PH2 (Mes* ) 2,4,6-tri-tert-butylphenyl)
that an aryl-substituted primary phosphine derivative
can be easily handled as a stable compound in the
open air when it is kinetically well stabilized.10 In the
course of our studies on kinetically stabilized doubly
bonded systems between heavier group 15 elements
using original steric protection groups, Tbt and 2,6-bis-
[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]-
phenyl (Bbt) groups,11 we found that the Tbt group is
applicable to the kinetic stabilization of 1,3-dihydro-
triphosphane (TbtHP)2PFc (1). We report here the
synthesis and structure of kinetically stabilized 1,3-
dihydrotriphosphane 1, which can be isolated as a
crystalline compound stable at room temperature, to-
gether with its unique thermal disproportionation re-
action leading to the formation of TbtPH2 (2) and
(E)-TbtPdPFc (3).
Table 1. 31P{1H} NMR Data of 1 in Benzene-d6 at
50 °C
spin
system
δ (P1)
[ppm]
δ (P2)
[ppm]
δ (P3)
[ppm]
JP1-P2
[Hz]
JP1-P3
[Hz]
JP2-P3
[Hz]
A2B
ABX
A2B
-69.5
-75.6
-62.6
-46.2
-42.2
-52.3
-69.5
-73.0
-62.6
-221
-223
-219
-221
-219
-219
<(5
solution of dichloroferrocenylphosphine12 at -78 °C gave
a diastereomer mixture of 1 as orange crystals in 73%
isolated yield. In sharp contrast to the previously
reported 1,3-dihydrotriphosphanes 4a,b,7,8 1 is stable
in the air and on exposure to daylight in the solid state.
Moreover, 1 was thermally stable without any decom-
position either up to 177 °C in the solid state or at 80
°C in a benzene-d6 solution in a sealed tube, although
1 has two reactive phosphorus-hydrogen bonds. The
remarkable stability of 1 reflects the prominent protect-
ing ability of Tbt groups.
Compound 1 was isolated as a diastereomeric mixture
of 1a (1S*,3R*), 1b (1R*,3R*), and 1c (1R*,3S*) formed
in the ratio of 1:1:1, which showed characteristic signals
of two A2B and one ABX system in the 31P{1H} NMR
spectrum (Chart 2, Figure 1, and Table 1). The two A2B
systems should be assignable to 1a (1S*,3R*) and 1c
(1R*,3S*), and the ABX system should be attributed to
1b (1R*,3R*) in analogy with the case of previously
reported 1,3-dihydrotriphosphane 4a.7,13 It was difficult
to draw a final conclusion for the assignment of the two
A2B signals. In the 1H NMR spectrum for 1, three
singlet signals could be observed separately for the
unsubstituted cyclopentadienyl protons of the three
diastereomers 1a-c in the ratio 1:1:1, although the full
assignment of signals for 1a-c was difficult due to the
complexity of the spectrum. Each diastereoisomer of
1a-c could not be separated by using preparative thin-
layer, gel permeation liquid chromatography, and re-
crystallization.
Results and Discussion
Synthesis and Structural Characterization of
1,3-Dihydrotriphosphane 1. The synthetic route for
1,3-dihydrotriphosphane 1 is shown in Scheme 1. Ad-
dition of an Et2O solution of TbtPHLi11h to a THF
(10) (a) Cowley, A. H.; Kilduff, J. E.; Newman, T. H.; Pakulski, M.
J. Am. Chem. Soc. 1982, 104, 5820-5821. (b) Issleib, K.; Schmidt, H.;
Wirkner, C. Z. Anorg. Allg. Chem. 1982, 488, 75-79. (c) Bertrand, G.;
Couret, C.; Escudie, J.; Majid, S.; Majoral, J. P. Tetrahedron Lett. 1982,
23, 3567-3570. (d) Yoshifuji, M.; Shibayama, K.; Inamoto, N.; Mat-
sushita, T.; Nishimoto, K. J. Am. Chem. Soc. 1983, 105, 2495-2497.
(11) (a) Tokitoh, N.; Arai, Y.; Okazaki, R.; Nagase, S. Science 1997,
277, 78-80. (b) Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase,
S.; Uekusa, H.; Ohashi, Y. J. Am. Chem. Soc. 1998, 120, 433-434. (c)
Sasamori, T.; Takeda, N.; Tokitoh, N. Chem. Commun. 2000, 1353-
1354. (d) Sasamori, T.; Arai, Y.; Takeda, N.; Okazaki, R.; Furukawa,
Y.; Kimura, M.; Nagase, S.; Tokitoh, N. Bull. Chem. Soc. Jpn. 2002,
75, 661-675. (e) Sasamori, T.; Takeda, N.; Fujio, M.; Kimura, M.;
Nagase, S.; Tokitoh, N. Angew. Chem., Int. Ed. 2002, 41, 139-141. (f)
Sasamori, T.; Takeda, N.; Tokitoh, N. J. Phys. Org. Chem. 2003, 16,
450-462. (g) Sasamori, T.; Mieda, E.; Takeda, N.; Tokitoh, N. Chem.
Lett. 2004, 33, 104-105. (h) Nagahora, N.; Sasamori, T.; Takeda, N.;
Tokitoh, N. Chem. Eur. J. 2004, 10, 6146-6451. (i) Sasamori, T.;
Mieda, E.; Nagahora, N.; Takeda, N.; Takagi, N.; Nagase, S.; Tokitoh,
N. Chem. Lett. 2005, 34, 166-167.
(12) Sollott, G. P.; Howard, G. J. Org. Chem. 1964, 29, 2451-2452.
(13) Baudler, M.; Reuschenbach, G.; Koch, D.; Carlsohn, B. Chem.
Ber. 1980, 113, 1264-1271.

3076 Organometallics, Vol. 24, No. 13, 2005
Nagahora et al.
Table 2. Inversion Barriers at a Tricoordinated
Phosphorus Atom
inversion energy
molecule
[kcal/mol]
reference
PH3
35
26.5
22
23.6
27.3
20.3
10-15
14a
14b
14c
14d
14a
14a
13
(H2P)2
(PhHP)2
(PhMeP)2
(H2P)2PH
(H2P)2PH
[Ph(Me3Si)P]2PPha
a In this case, the phosphorus atoms at the terminal positions
may invert due to the low electronegativity of the silicon atom.
Reported inversion barriers of the phosphorus atom
for several types of phosphines shown in Table 2
indicated that the introduction of bulky substituents at
the phosphorus atom decreases the inversion barriers.14
It can be considered that the inversion barrier of 1
should be smaller than those reported for 4a,b due to
the extremely bulky substituent, Tbt, and the inversion
at the phosphorus atom of 1a-c in solution should give
rise to the equilibrium state between 1a, 1b, and 1c
under ambient conditions.15 Therefore, 1 was observed
as a mixture of diastereomers 1a-c in the ratio of
1:1:1 in solution at ambient temperature. This is prob-
ably due to the small difference in thermodynamic
stability between 1a, 1b, and 1c, which was supported
by theoretical calculations for the model molecules
6a-c [(DmpHP)2PFc (Dmp ) 2,6-dimethylphenyl)] (Fig-
ure 2).16
Figure 2. Optimized structures and relative energies of
6a-c calculated at the B3LYP/6-31G(d) level.
The structural characterization of 4a,b has not been
fully accomplished due to their inherent instability,7,8
and the solid state structures have been known for only
four triphosphane derivatives, 5a-d, so far (Chart 1).9
Taking into account these previous reports, compound
1 is the first example of a kinetically stabilized 1,3-
dihydrotriphosphane derivative. Fortunately, single
crystals of 1 suitable for X-ray crystallographic analysis
were obtained by slow recrystallization of an n-hexane
solution of 1. The molecular structure of 1 was deter-
mined by X-ray crystallographic analysis (Table 3), and
the ORTEP drawing and selected structural parameters
are shown in Figure 3 and Table 4, respectively. Since
the geometry of the hydrogen atoms on the phosphorus
atoms of 1 cannot be determined by the X-ray crystal-
lographic analysis in principle, it was not revealed
whether the single crystal might consist of one isomer
or contain three isomers, 1a-c. When single crystals
of 1 were redissolved in benzene-d6 solution, the ratio
Table 3. Crystallographic Data and Experimental
Parameters for the Crystal Structure Analysis of 1
formula
C64H129FeP3Si12
1384.51
0.50 × 0.50 × 0.35
103(2)
fw
cryst dimens/mm
collection temp/K
cryst syst
space group
a/Å
monoclinic
P21/n (#14)
17.626(4)
22.035(5)
22.441(5)
103.146(3)
8487(3)
b/Å
c/Å
â/deg
volume/Å3
Z
4
density/Mg m-3
no. of indep reflns
no. of params
R1 [I > 2σ(I)]
wR2 (all data)
goodness of fit
1.084
14 935
765
0.056
0.113
1.095
tion, probably due to the inversion of the phosphorus
atoms of 1. The phosphorus-phosphorus bond lengths
of 1 are 2.2304(12) and 2.2322(12) Å, which lie in a
range of typical phosphorus-phosphorus single bond
lengths (ca. 2.19-2.24 Å)17 and are comparable to those
of previously reported triphosphanes (2.190-2.268 Å).9
The P-P-P bond angle [96.17(5)°] of 1 is somewhat
smaller than those of the reported triphosphanes 5a-d
(100.0-118.7°).9
Thermolysis of 1,3-Dihydrotriphosphane 1. We
carried out the thermolysis of 1 in toluene-d8 to inves-
tigate the thermal stability of 1. Heating of the solution
of 1 in a sealed tube at 130 °C afforded both TbtPH2
(2)11h and (E)-TbtPdPFc (3)11h in the ratio 1:1 as shown
1
of the diastereomers 1a-c estimated by the H NMR
spectrum was 1:1:1, the same as before the crystalliza-
(14) (a) Glukhovtsev, M. N.; Dransfeld, A.; Schleyer, P. v. R. J. Phys.
Chem. 1996, 100, 13447-13454. (b) Rak, J.; Skurski, P.; Liwo, A.;
Błaz´ejowski, J. J. Am. Chem. Soc. 1995, 117, 2638-2648. (c) Albrand,
J. P.; Gagnaire, D. J. Am. Chem. Soc. 1972, 94, 8630-8632. (d)
Lambert, J. B.; Jackson, G. F.; Mueller, D. C. J. Am. Chem. Soc. 1970,
92, 3093-3097.
(15) Since the transition state for the inversion of a tricoordinated
phosphorus atom should be considered to have a planar geometry with
an sp2-hybridized phosphorus atom, bulky substituents on the phos-
phorus atom are known to stabilize the transition state due to the relief
of steric repulsion; see: Nagase, S. Structures and Reactions of
Compounds Containing Heavier Main Group Elements. In The Transi-
tion State-A Theoretical Approach, Fueno, T., Ed.; Gordon Science:
Amsterdam, 1999; Chapter 8.
(16) (DmpHP)2PFc 6a-c have optimized structures as (1S*,3R*),
(1R*,3R*), and (1R*,3S*) geometries, respectively. In Figure 2 are
shown the relative energies for 6a-c calculated at B3LYP/6-31G(d)
level.
(17) Baudler, M.; Glinka, K. Chem. Rev. 1993, 93, 1623-1667, and
references therein.

A Stable 1,3-Dihydrotriphosphane
Organometallics, Vol. 24, No. 13, 2005 3077
Figure 4. 1H NMR spectral change (3.3-4.9 ppm) for the
thermolysis of 1 in toluene-d8 (a) before heating, (b) after
heating at 130 °C for 1.0 h, (c) for 5.0 h, and (d) for 10 h.
TbtPH2 (2, open squares) and (E)-TbtPdPFc (3, open
triangles) were formed.
Figure 3. ORTEP drawing of 1 with thermal ellipsoid
plots (50% probability). Hydrogen atoms are omitted for
clarity.
Table 4. Selected Bond Lengths (Å), Bond Angles
(deg), and Torsion Angles (deg) of 1
Figure 5. First-order kinetic plots of the thermolysis of 1
in toluene-d8 (9.95 × 10-3 mol/dm3) in a range from 130 to
150 °C. The lines are least-squares fit to the data points
up to three half-lives.
Bond Lengths
P(1)-P(2)
P(1)-P(3)
P(1)-C(1)
P(2)-C(11)
P(3)-C(38)
2.2322(12)
2.2304(12)
1.841(3)
1.841(3)
1.845(3)
Bond Angles
P(2)-P(1)-P(3)
C(1)-P(1)-P(3)
C(1)-P(1)-P(2)
C(11)-P(2)-P(1)
C(38)-P(3)-P(1)
96.17(5)
100.47(11)
95.99(11)
109.61(10)
110.54(10)
Torsion Angles
C(1)-P(1)-P(2)-C(11)
C(1)-P(1)-P(3)-C(38)
P(2)-P(1)-P(3)-C(38)
P(3)-P(1)-P(2)-C(11)
173.37(15)
-70.38(15)
-167.66(11)
-85.38(12)
Figure 6. Eyring plot of the thermolysis of 1 in a range
from 95 to 150 °C.
Table 5. Data for the Thermal Decomposition of 1
Scheme 2. Thermolysis of 1
in Toluene-d8
temperature [°C]
observed kinetic constant [s-1
]
95
105
115
130
140
150
(2.34 ( 0.10) × 10-6
(5.89 ( 0.31) × 10-6
(1.54 ( 0.25) × 10-5
(6.39 ( 0.22) × 10-5
(1.47 ( 0.13) × 10-4
(3.68 ( 0.13) × 10-4
in Scheme 2. The signals for the unsubstituted cyclo-
pentadienyl protons of the three diastereomers 1a-c
equally decreased during the thermolysis, as monitored
by 1H NMR spectroscopy (Figure 4). In addition, we
confirmed that no formation of 1 took place by the
reverse reaction of 2 with 3 under the same conditions.
One can see in Figure 5 that plots of a logarithm of
the concentration of 1 against time show the linear
decay of 1 through >90% consumption in a toluene-d8
solution at 130 °C, indicating that the thermolysis is a
first-order reaction with respect to the concentration of
the substrate with the rate constant k ) (6.39 ( 0.22)
× 10-5 s-1. The rates of the thermolysis of 1 were
monitored at 95, 105, 115, 140, and 150 °C, as sum-
marized in Table 5. The activation parameters were
estimated as ∆Hq ) 28.8 ( 0.86 kcal/mol, ∆Sq ) -6.8
( 1.8 cal/mol‚K, and ∆Gq(298 K) ) 30.8 ( 0.86
kcal/mol by the Eyring plot shown in Figure 6. These
kinetic studies suggested that the thermolysis of 1
should proceed via a unimolecular process with a
relatively tight geometry at the transition state.18
Kinetic studies on the thermolysis of 1 in a toluene
solution were performed using not only NMR but also

3078 Organometallics, Vol. 24, No. 13, 2005
Nagahora et al.
theoretical calculations, the difference of free energies
-
between P3H5 (7a) and the ionic pair of P2H3+ and PH2
was estimated as ∆E ) 132 kcal/mol, which was
overwhelmingly larger than that between 7a and TS7b
(∆E ) 45.2 kcal/mol) (vide infra). We have carried out
theoretical calculations on the favorable path A in detail.
In Figure 8 are shown the optimized structure of 7a,
i.e., the most stable conformation of P3H5,19 together
with its rotational isomer 7b, final products 8 and 9,
and their transition states TS7a and TS7b. The reac-
tion coordinate shown in Figure 9 suggested that the
rotational barrier for the phosphorus-phosphorus bond
of 7a giving 7b was calculated as ∆Gq(298 K) ) 3.4
kcal/mol via the transition state TS7a. In addition, it
was found that 7b could be converted to PH3 (8) and
(E)-HPdPH (9) via the transition state TS7b, having a
four-membered ring geometry with a barrier ∆Gq(298
K) ) 44.9 kcal/mol. A vector of negative frequency of
TS7b corresponds to the P1-H5 stretching mode,
indicating an intramolecular hydrogen-transfer process
giving 8 and 9. These results suggest that the dispro-
portionation reaction of P3H5 giving 8 and 9 is most
likely interpreted in terms of an intramolecular hydrogen-
transfer process via the four-membered ring transition
state TS7b.
Figure 7. UV/vis spectral change during the thermolysis
of 1 in toluene at 130 °C.
Scheme 3. Possible Mechanisms for the
Disproportionation Reaction of P3H5
Reaction Mechanism of 1. Taking into account that
the calculated ∆Sq value (-2.3 cal/mol‚K) in the dispro-
portionation process of P3H5 is quite consistent with
the characteristic negative value of ∆Sq (-6.8 ( 1.8
cal/mol‚K) observed in the thermolysis of 1, the reaction
mechanism of the thermal disproportionation reaction
of 1 might be interpreted in terms of the intramolec-
ular hydrogen-transfer process similar to the case of
P3H5. A possible reaction mechanism is shown in
Scheme 4. The three diastereomers 1a (1S*,3R*), 1b
(1R*,3R*), and 1c (1R*,3S*) should undergo isomeriza-
tion to each other (vide supra) due to the ready inversion
at the phosphorus atoms under the conditions for the
thermolysis of 1. TS1a, TS1b, TS1b′, and TS1c are
conceivable as transition states for the intramolecular
hydrogen-transfer processes in the thermal dispropor-
tionation reaction of 1 based on the theoretical studies
described above. In view of the configurations of the
transition states, TS1b′ and TS1c giving (Z)-TbtPdPFc
should not be favored as compared with TS1a or TS1b
due to the steric repulsion between Tbt and Fc groups.
Although both TS1a and TS1b afford (E)-TbtPdPFc
directly, TS1b should be more favorable than TS1a
because all substituents are placed with an anti-
configuration in TS1a. Consequently, the reaction
mechanism of the thermal disproportionation reaction
of 1 is most likely interpreted in terms of the pathway
from 1b leading to the formation of 2 and 3 via TS1b
with a four-membered ring geometry.
UV/vis spectroscopy. In the UV/vis spectra, the intensi-
ties of the metal-to-ligand charge-transfer (MLCT) band
for 3 (λmax ) 550 nm) were monitored as an index of
the concentration of 3 during the heating of a toluene
solution of 1 in a sealed quartz cell at 130 °C (Figure
7). The rate constant k ) (5.90 ( 0.58) × 10-5 s-1 for
the first-order reaction with respect to the concentration
of 3 obtained by the UV/vis spectroscopic method was
similar to that estimated by the NMR studies.
Theoretical Calculations of P3H5. We have per-
formed theoretical calculations on the reaction mecha-
nism for the disproportionation reaction of parent
triphosphane P3H5 giving PH3 and (E)-HPdPH as a
model system. Thus, two different pathways can be
postulated for the unimolecular process of the dispro-
portionation reaction of P3H5, as shown in Scheme 3.
The first one (path A) is an intramolecular hydrogen-
transfer reaction via the transition state with a four-
membered ring structure. In the second one (path B),
P3H5 undergoes a decomposition giving an ionic pair of
P2H3+ and PH2- followed by a proton-transfer reaction
leading to the formation of PH3 and (E)-HPdPH.
However, a bimolecular route such as path B is incon-
sistent with the observed negative activation entropy
and not suitable as a major mechanistic route. In
Conclusion
In summary, we have succeeded in the synthesis of
(TbtHP)2PFc (1) by taking advantage of kinetic stabi-
lization using extremely bulky substituents, Tbt groups.
(18) (a) Li, Y.; Marks, T. J. Organometallics 1996, 15, 3770-3772.
(b) Dexter, C. S.; Hunter, C.; Jackson, F. W. J. Org. Chem. 2000, 65,
7417-7421. (c) Lin, M.; Spivak, G. J.; Baird, M. C. Organometallics
2002, 21, 2350-2352. (d) Dom´ınguez, R. M.; Herize, A.; Rotinov, A.
Alvarez,-A. A.; Visbal, G. J. Phys. Org. Chem. 2004, 17, 399-408. (e)
Oh, H. K.; Kim, I. K.; Lee, H. W.; Lee, I. J. Org. Chem. 2004, 69, 3806-
3810.
(19) Theoretical investigations for the structures and stabilities of
P3H5 have been reported. The gauche configuration 7a with Cs
symmetry was found to be the most stable geometry for the P3H5
molecule. See: Ding, F. J.; Zhang, L. F. J. Mol. Struct. (THEOCHEM)
1996, 369, 167-172.

A Stable 1,3-Dihydrotriphosphane
Organometallics, Vol. 24, No. 13, 2005 3079
Figure 8. Optimized structures of ground states 7a, 7b, 8, and 9 and transition states TS7a and TS7b at the B3LYP/
6-311+G(2d,p) level. The arrows for TS7a and TS7b indicate the transition vectors.
atoms on the terminal phosphorus atoms of 1 enabled
it to undergo the thermal disproportionation reaction.
Our studies on the thermolysis of 1 gave a concrete
explanation to the previous works on the disproportion-
ation of P3H5 by Fehlner.6 These results may open a door
to a new chemistry of open-chain phosphorus hydrides
and make it possible to synthesize a new type of
diphosphene derivative.
Experimental Section
General Procedures. All reactions were carried out under
an argon atmosphere. All solvents were purified by standard
methods and then dried by using the Ultimate Solvent System
(Glass Contour Company).20 All solvents (benzene-d6, toluene-
d8, and toluene) used in the thermolysis of 1 were dried over
a potassium mirror before use. Preparative thin-layer chro-
matography (PTLC) was performed with Merck Kieselgel 60
PF254. Preparative gel permeation liquid chromatography
(GPLC) was performed on an LC-908 or LC-918 apparatus
equipped with JAI-gel 1H and 2H columns (Japan Analytical
Industry Co., Ltd.) with toluene as an eluent. The 1H NMR
(300 MHz) spectra were measured in C6D6 or C7D8 with a
JEOL AL-300 spectrometer using C6HD5 (δ ) 7.15 ppm) or
C6D5CHD2 (δ ) 2.09 ppm) as an internal standard. 31P{1H}
NMR (120 MHz) spectra were measured using 85% H3PO4 in
water (δ ) 0 ppm) as an external standard. High-resolution
mass spectral data were obtained on a JEOL SX-270 mass
spectrometer. The electronic spectra were recorded on a
JASCO V-570 UV/vis spectrometer. An oil bath used in the
kinetic studies was a LAUDA thermostat K6 KS, the temper-
ature of which was maintained in a range of (0.2 °C. All
melting points were determined on a Yanaco micro melting
point apparatus and were uncorrected. Elemental analyses
were performed by the Microanalytical Laboratory of the
Institute for Chemical Research, Kyoto University. TbtPH2
(2)11h and dichloroferrocenylphosphine12 were prepared ac-
cording to the reported procedures.
Figure 9. Energy diagram for the disproportionation
reaction of P3H5 calculated at the B3LYP/6-311+G(2d,p)
level. Energies are in kcal/mol (relative to 7a).
In contrast to the previously reported 1,3-dihydrotri-
phosphanes [(RHP)2PR, 4a; R ) Ph, 4b; R ) t-Bu], 1
was stable in air without any disproportionation at
ambient temperature. The structural characterization
of 1 was established by the spectroscopic data and X-ray
crystallographic analysis. The 31P{1H} NMR spectrum
for 1 showed two A2B and one ABX system, which
corresponded to two meso (1a and 1c) and one dl (1b)
isomer, respectively. The thermolysis of 1 in solution
above 95 °C resulted in the quantitative formation of
TbtPH2 (2) and (E)-TbtPdPFc (3). The Eyring plot of
the thermolysis of 1 in a range from 95 to 150 °C gave
the activation parameters ∆Hq ) 28.8 ( 0.86 kcal/mol
and ∆Sq ) -6.8 ( 1.8 cal/mol‚K as the first-order kinetic
constants. The thermal disproportionation reaction of
1 was reasonably interpreted in terms of an intramo-
lecular hydrogen-transfer process via a four-membered
ring geometry based on the DFT calculations for the
case of P3H5. It was found that the existence of hydrogen
Synthesis of 1,3-Dihydrotriphosphane 1. To a solution
of TbtPH2 (2; 234.1 mg, 0.400 mmol) in ether (10 mL) was
added n-butyllithium in n-hexane (1.50 M, 267 µL, 0.401
mmol) at -78 °C. The solution of TbtPHLi obtained here was
(20) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.

3080 Organometallics, Vol. 24, No. 13, 2005
Scheme 4. Possible Mechanism for the Thermolysis of 1
Nagahora et al.
warmed to room temperature for 0.5 h and then cooled to -78
°C. A solution of dichloroferrocenylphosphine (57.4 mg, 0.200
mmol) in ether (10 mL) was added to the reaction mixture at
-78 °C. After stirring at the same temperature for 0.5 h, the
solution was allowed to warm to room temperature for 6 h.
After removal of the solvent, n-hexane was added to the
residue and the mixture was filtered through Celite. The
filtrate was purified by GPLC and PTLC (eluting with
n-hexane) to afford (TbtHP)2PFc (1; 200.5 mg, 0.145 mmol,
73% yield) as yellow crystals containing three diastereomers,
1a-c. 31P{1H} NMR (121 MHz, C6D6, 50 °C): δ -75.6, -73.0,
confirmed that the optimized structures have minimum ener-
gies by frequency calculations. In calculations for P3H5, all
geometries were fully optimized at the B3LYP/6-311+G(2d,p)
level, and it was confirmed by frequency calculations that the
optimized structures have minimum energies and the transi-
tion state has one imaginary frequency corresponding to the
expected transition vectors. Energies and thermodynamic
parameters at 298 K were calculated at the same level as
structural optimization. The reaction paths were established
by the connectivity between the reactant and the transition
state by IRC calculations. Computation time was provided by
the Supercomputer Laboratory, Institute for Chemical Re-
search, Kyoto University.
1
-42.2 (ABX system, JPP ) 223, 219 Hz), -69.5, -46.2 (A2B
system, 1JPP ) 221 Hz), -62.6, -52.3 (A2B system, 1JPP ) 219
Hz). HRMS (FAB): found m/z 1383.5906 ([M + H]+), calcd for
C64H130FeP3Si12 1383.5956. Anal. Found: C, 55.54; H, 9.49.
Calcd for C64H129FeP3Si12: C, 55.52; H, 9.39.
Acknowledgment. This work was partially sup-
ported by Grants-in-Aid for Scientific Research (Nos.
14078213 and 16750033) and COE Research on “Ele-
ments Science” (No. 12COE2005), and the 21st Century
COE Program on Kyoto University Alliance for Chem-
istry from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. N.N. thanks Research
Fellowships of the Japan Society for the Promotion of
Science for Young Scientists.
General Procedure for the Kinetic Studies on the
Thermolysis of 1. In a glovebox filled with argon, 1 (55.0 mg,
39.8 µmol) was dissolved in dried toluene-d8 (4.00 mL), and a
600 µL portion of the solution (9.95 × 10-3 mol/dm3) was
transferred to an NMR tube. The tube was degassed and
sealed after three freeze-pump-thaw cycles. The sealed tube
was heated to 130.0 °C in an oil bath controlled in an error
range of (0.2 °C. The thermolysis was interrupted by cooling
to room temperature after regular time intervals to measure
Supporting Information Available: Theoretically opti-
mized coordinates of 6a-c, 7a, 7b, TS7a, TS7b, 8, and 9 in
PDF format. X-ray crystallographic file of 1 in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
the H NMR spectra for over three half-lives. The concentra-
tions of 1a, 1b, 1c, and 3 were determined by the integrals of
the corresponding unsubstituted cyclopentadienyl ring protons
1
in the H NMR spectra.
X-ray Crystallographic Analysis of 1. Single crystals of
1 were obtained by slow crystallization from an n-hexane
solution at room temperature. The intensity data were col-
lected on a Rigaku Mercury CCD diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.71070 Å). The
structure was solved by direct methods (SHELXS-97)21 and
refined by full-matrix least-squares procedures on F2 for all
reflections (SHELXL-97).22 All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms except for those
on phosphorus atoms were placed using AFIX instructions. No
hydrogen atoms on phosphorus atoms were placed. Crystal-
lographic data for 1 are shown in Table 3.
OM0501069
(21) Sheldrick, G. M. Acta Crystallogr. Sect. A 1990, 46, 467-473.
(22) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, 1997.
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, Jr. J.
A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck,
A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.;
Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez,
C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision
A.11; Gaussian, Inc.: Pittsburgh, PA, 2001.
Theoretical Calculations. All theoretical calculations
were carried out using the Gaussian 98 program package23
with density function theory at the B3LYP level. In calcula-
tions for 6a-c, the 6-31G(d) basis sets were used. It was