Synthesis and structure of the first stable phosphabismuthene

Article abstract of DOI:10.1002/1521-3773(20020104)41:1<139::AID-ANIE139>3.0.CO;2-3

The exceedingly different size of p orbitals is overcome in the novel doubly bonded system between phosphorus and bismuth atoms that occurs in the first stable phosphabismuthene 1. Compound 1 was obtained from the condesation reaction of Mes*PH₂ with an overcrowed dibromobismuthane by using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base (see scheme).

Full text of DOI:10.1002/1521-3773(20020104)41:1<139::AID-ANIE139>3.0.CO;2-3

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heavier main group elements. While some examples of
phosphaarsenes and phosphastibenes are reported,[8, 9] no
heteronuclear double bonds between bismuth and the other
Group 15 elements have been described probably because of
the weakness of p bond between Bi and another atom due to
the difference of the size of the p orbitals. Very recently,
however, we succeeded in the synthesis of the first stable
Synthesis and Structure of the First Stable
Phosphabismuthene**
Takahiro Sasamori, Nobuhiro Takeda, Mizue Fujio,
Masahiro Kimura, Shigeru Nagase, and
Norihiro Tokitoh*
There has been much interest in doubly bonded compounds
between Group 15 elements, that is, heavier congeners of azo
compounds, since the first isolation of a stable diphosphene,
stibabismuthene (BbtSb BiBbt),[10] which is the first example
ˆ
of a heteronuclear doubly bonded system containing a
bismuth atom, by taking advantage of a new type of steric
protection group, 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(tri-
methylsilyl)methyl]phenyl (denoted as Bbt).[11] Herein, we
report the first synthesis of a stable phosphabismuthene, that
is, a novel doubly bonded system between phosphorus and
bismuth atoms, in which the exceedingly different size of p
orbitals is overcome.
ˆ
Mes*P PMes* (Mes* ˆ 2,4,6-tri-tert-butylphenyl), by Yoshi-
fuji et al. in 1981.[1] In 1980s, several examples of kinetically
[2]
ˆ
stabilized diphosphenes
(RP PR)
and diarsenes
(RAs AsR)[3] have been synthesized and fully characterized.
In contrast, much heavier analogues, that is, distibene
(RSb SbR) and dibismuthene (RBi BiR),[5] have only been
reported very recently by us, where the first isolation of stable
distibene and dibismuthene was achieved by taking advantage
of an efficient steric protection group, 2,4,6-tris[bis(trimethyl-
silyl)methyl]phenyl (denoted as Tbt) (Scheme 1).[6] Later on,
ˆ
[4]
ˆ
ˆ
The condensation reaction of BbtBiBr2 with Mes*PH2 in
the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in
hexane at room temperature afforded a deep orange powder
(Scheme 2), the 31P NMR spectrum of which showed a broad
R
R
Mes*
DBU
Mes* : R = R' = tBu
Tbt : R = R' = CH(SiMe3)2
Bbt : R = CH(SiMe3)2, R' = C(SiMe3)3
Mes*PH2
P
Bi
Bbt
BbtBiBr2
+
hexane
RT
R'
1
Scheme 1. Some steric protection groups.
Scheme 2. Synthesis of phosphabismuthene 1.
Power and co-workers also described the syntheses of another
series of distibenes and dibismuthene containing bulky 2,6-
Ar2C6H3 groups (Arˆ mesityl or 2,4,6-triisopropylphenyl).[7]
Thus, homonuclear doubly bonded systems between heavier
Group 15 elements are no longer imaginary species even in
the case of bismuth, and the next target molecules are
heteronuclear doubly bonded systems, which may be key
compounds of great importance in the systematic elucidation
of the intrinsic nature of low-coordinate compounds of
singlet at d ˆ 612along with a small peak for Mes*PH (d ˆ
2
À130). Since the low-field chemical shift of d ˆ 612is
characteristic of a low-coordinate phosphorus atom, this
result strongly suggests the formation of phosphabismuthene
1 in this reaction. In addition, the broadening of the signal,
which is considered to be due to the 9/2nuclear spin of the
ˆ
adjacent Bi atom, is also regarded as an evidence for the P Bi
double bond of 1. The yield of 1 was estimated to be 91% on
1
the basis of a comparison of the integrals of the H NMR
signals compared with those of TbtH added as an internal
standard. Although phosphabismuthene 1 was very difficult to
purify by chromatographic separation owing to its extreme
high reactivity toward moisture and air, recrystallization of
the orange powder from hexane at À408C afforded 1 ¥
0.5 hexane as green single crystals. It is noteworthy that
compound 1 is not only a new member of a novel class of
heteronuclear doubly bonded systems between heavier
Group 15 elements but also the first example of a stable
species with a double bond between the third and sixth row
main group elements.
[*] Prof. N. Tokitoh, Dr. N. Takeda
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
Fax : (‡81)774-38-3209
E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
T. Sasamori
Graduate School of Science, Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581 (Japan)
Dr. M. Fujio
Institute for Fundamental Research of Organic Chemistry,
Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581 (Japan)
M. Kimura
The molecular structure of 1 was determined by X-ray
crystallographic analysis (Figure 1).[12] This molecule has a
Graduate School of Science, Tokyo Metropolitan University
Hachioji, Tokyo 192-0397 (Japan)
trans configuration with
179.79(9)8. The length of the P Bi bond of 1 is 2.4541(6) ä,
which is the midpoint between the E E bond lengths of the
a
C-Bi-P-C torsion angle of
Prof. S. Nagase
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
À
À
[**] This work was partially supported by a Grant-in-Aid for COE
Research on Elements Science (No. 12CE2005) and a Grant-in-Aid
for Scientific Research on Priority Areas (A) (No. 11166250) from the
Ministry of Education, Culture, Sports, Science and Technology of
Japan. T.S. thanks Research Fellowships of the Japan Society for the
Promotion of Science for Young Scientists.
ˆ
corresponding
diphosphene
(Mes*P PMes*
(2),
2.034(2) ä)[1] and dibismuthenes (TbtBi BiTbt (3),
ˆ
2.8206(8) ä[5] and BbtBi BiBbt (4), 2.8699(6) ä[13]). This
ˆ
À
P Bi bond length is about 8% shorter than the sum of the
covalent radii of the phosphorus and bismuth atoms (2.66 ä)
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ˆ
(94.18) as that of PhP BiPh (5). In addition, the P-Bi-C angle
ˆ
of HP BiBbt (7) system was also found to be widened to
103.18, which is almost the same value as that of 1. These
results indicate that the wider angle at the Bi atom deter-
mined from the X-ray crystallographic analysis of 1 is
probably due to the steric repulsion not between the
substituents (Mes* and Bbt group) but between the bulky
ˆ
Bbt group and the central P Bi moiety.
The UV/Vis spectrum of 1 in hexane shows characteristi-
cally red-shifted absorption maxima at 455 (e 10000), 540 (sh,
1000), and 670 (300) nm, while the previously known heavier
congeners of azo compounds show only two absorption
maxima corresponding to the symmetry-allowed p !p* and
symmetry-forbidden n !p* electron transitions at wave-
lengths longer than 300 nm in all cases. The lmax value of
455 nm, which is attributable to the p !p* transition of the
Figure 1. Left: Molecular structure of
1 (ORTEP drawing; thermal
ellipsoid plot (50% probability)). The fragment of solvated hexane was
omitted for clarity. Right: Schematic drawing of 1 with some selected bond
lengths and angles.
ˆ
P Bi chromophore, lies in the middle between the p !p*
transition of Mes*P PMes* (2: lmax 340 nm (e 7690))[1] and
ˆ
that of BbtBi BiBbt (4: lmax 537 nm (e 6000)).[13] Although
ˆ
ˆ
as well as the E E bond lengths of the doubly bonded system
the latter two lmax values observed for 1 (540 and 670 nm) are
most likely due to the n !p* transitions and one of them at
540 nm is reasonably found in the middle between the lmax
value of 2 (460 nm (e 1360)) and that of 4 (670 nm (sh, e 20)),
another one at 670 nm lies at a strangely longer wavelength.
Although we have no clear explanation for the additional
n !p* transition observed in longer wavelength region for 1,
it could be consistent with the assumption that heteronuclear
doubly bonded systems between heavier Group 15 elements
should have two weak n !p* transitions due to two chemi-
cally distinct lone pairs on the phosphorus and the bismuth
atoms in addition to one strong p !p* transition. These
outcomes for phosphabismuthene 1 discussed above are also
of great importance for understanding the spectroscopic
between heavier Group 15 elements, which are known to be
À
about 8% shorter than the corresponding E E single
bonds.[14] On the other hand, the Bi-P-C and P-Bi-C angles
of 1 were found to be 96.40(7)8 and 102.16(5)8 respectively.
The considerably wider angle at the Bi atom than that at the P
atom may be surprising, because the angles at pnictogens
generally become smaller with increasing atomic number
(from N to Bi). It is well known that the heavier pnictogens
have a stronger tendency to maintain the (ns)2(np)3 valence
electron configuration because of the larger size difference
and energy gap between the valence s and p orbitals.[15] The
[16]
ˆ
results of theoretical calculations for RP BiR' systems,
which are summarized in Table 1, lead us to a reasonable
understanding. All the optimized structures for RP BiR'
systems show very similar bond lengths for their P Bi bond
ˆ
[10]
ˆ
À
properties of BbtSb BiBbt, which we reported previously.
Thus, 1 features a double bond between the phosphorus
and the bismuth atom in the solid state and even in solution, as
suggested by the NMR and UV/Vis spectra and X-ray
crystallographic analysis. Further investigation of the physical
and chemical properties of 1 is currently in progress. Now, as
for heavier Group 15 elements, not only the homonuclear
doubly bonded compounds but also the heteronuclear systems
have been obtained as stable compounds, even in the case of
elements from rows that differ by more than one (i.e. P and
Bi), when they are kinetically well stabilized. The concept of
kinetic stabilization should be very helpful for the synthesis of
Table 1. Calculated structures of phosphabismuthenes.[16]
ˆ
RP BiR'
Angle [8]
Length [ä]
À À
À À
R P Bi
P Bi R'
[a]
À ˆ
À
Ph P Bi Ph
5
1
6
7
97.0
95.3
104.8
90.0
96.6
104.4
94.1
2.427
2.440
2.442
2.428
À ˆ
À
Mes* P Bi Bbt
À ˆ
À
Bbt P Bi Mes*
À ˆ
À
H P Bi Bbt
103.0
[a] The phenyl groups are fixed perpendicularly to the C-P-Bi plane.
ˆ
ˆ
unprecedented doubly bonded systems such as As Sb, As Bi,
(ca. 2.43 ä), which are almost consistent with the experimen-
ˆ
ˆ
À
Sb Si, and Bi Si.
tally obtained P Bi bond lengths of 1. By contrast, the bond
angles at the phosphorus and bismuth atoms vary in these
ˆ
systems. The optimized geometry of PhP BiPh (5) showed no
Experimental Section
distinct difference between angles at the phosphorus atom
(97.08) and the bismuth atom (96.68) in contrast to the
experimentally obtained values of 1. On the other hand, a
theoretically optimized structure of the more sterically
1: DBU (15.0 mL, 0.10 mmol) was added dropwise by means of a syringe to
a solution of Mes*PH2 (14.7 mg, 0.053 mmol) and BbtBiBr2 (51.0 mg,
0.051 mmol) in hexane (10 mL) at room temperature (ca. 258C). The
resulting orange solution was stirred at the same temperature for 14 h.
After filtration through Celite, the filtrate was evaporated. The residue
(orange powder, 57.0 mg) contained 1 as a main product (0.046 mmol, 91%,
estimated by 1H NMR spectroscopy by using TbtH as an internal standard),
along with a small amount of Mes*PH2 and DBU. 1: m.p. 120.0 121.28C
(decomp); 1H NMR (400 MHz, C6D6): d ˆ 0.25 (s, 36H), 0.40 (s, 27H), 1.41
(s, 9H), 1.68 (s, 2H), 1.70 (s, 18H), 7.58 (s, 2H), 7.84 (s, 2H); 13C NMR
(100 MHz, C6D6): d ˆ 2.29 (q), 6.11 (q), 22.80 (s), 32.96 (q), 33.98 (s), 37.02
ˆ
congested system, Mes*P BiBbt (1), was found to have
structural parameters (95.38 for P and 104.48 for Bi) almost
the same as the experimentally obtained values. The opti-
mized structural parameters for the reversibly substituted
model system BbtP BiMes* (6) showed a widened angle at
the P atom (104.88) and almost the same angle at the Bi atom
ˆ
140
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(q), 38.46 (s), 40.61 (d), 119.79(d), 128.52 (s), 131.05 (d), 144.59 (s), 150.66
(s), 152.30 (s), 159.59 (s), a signal assignable to ipso-13C of Bbt group was
not observed.; 31P NMR (121 MHz, C6D6): d ˆ 612.0; UV/vis (n-hexane):
lmax (e) ˆ 455 (10000), 540 (sh, 1000), and 670 nm (300); high-resolution
Erodible Conducting Polymers for Potential
Biomedical Applications**
Alexander N. Zelikin, David M. Lynn, Jian Farhadi,
Ivan Martin, Venkatram Shastri,* and Robert Langer*
‡
MS (FAB): m/z: 1109.5511([M‡H] ), calcd for C48H97Si7PBi 1109.5516.
Slow evaporation of a hexane solution of the residue in a refrigerator at
À408C fixed in a glovebox filled with argon gave green crystals of 1 ¥
0.5 hexane.
Electrically conducting polymers have been investigated
for numerous applications, including organic substitutes for
metals in electronic circuits,[1] coatings for electromagnetic
Received: October 1, 2001 [Z18006]
5]
shielding,[2] analytical and biological sensing devices,[3 and
[1] M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, T. Higuchi, J. Am.
Chem. Soc. 1981, 103, 4587.
as substrates for the manipulation of mammalian cell growth
and function.[6] From a separate standpoint, the development
and application of biodegradable polymers has had a pro-
found impact in numerous medical and surgical applications.[7]
It occurred to us that the creation of biocompatible, degrad-
able conducting polymers could open the door to a number of
new biomedical applications.[8] Several groups have reported
the synthesis and characterization of conducting poly(thio-
phene) derivatives containing hydrolyzable ester groups in
the polymer backbone.[9] However, to our knowledge, the
degradability and biocompatibility of these polymers has not
been established, and the reduced environmental stability of
oxidized polythiophenes could limit their application under
physiological conditions.
Of the various electrically conducting polymers, polypyr-
role (Ppy) has been the most widely studied material for
potential biomedical applications because it is relatively
stable to air and water and can be readily synthesized through
chemical and electrochemical routes.[10] Additionally, Ppy is a
suitable substrate for cell attachment and proliferation[6] and
possesses excellent biocompatibility in vivo.[8, 11] Our attempts
to synthesize biodegradable Ppy derivatives through the
incorporation of backbone ester moieties (in analogy to the
polythiophenes discussed above) have thus far been unsuc-
cessful because of extensive side reactions accompanying
pyrrole coupling chemistry and the generally poor oxidative
stability of the requisite oligopyrrole intermediates. Herein,
we report an alternative strategy for the design of erodible
Ppy materials based on the chemical and electrochemical
polymerization of b-substituted pyrrole monomers containing
ionizable and/or hydrolyzable side groups [Eq. 1]. These
polymers can be fabricated into conductive materials that
erode slowly under physiological conditions and support the
growth, proliferation, and differentiation of primary human
cells in in vitro cell culture assays.
[2] M. Yoshifuji in Multiple Bonds and Low Coordination in Phosphorus
Chemistry (Eds.: M. Regitz, O. J. Scherer), Thieme, Stuttgart, 1990,
p. 321; L. Weber, Chem. Rev. 1992, 92, 1839; A. H. Cowley, J. E.
Kilduff, J. G. Lasch, S. K. Mehrotra, N. C. Norman, M. Pakulski, B. R.
Whittlesey, J. L. Atwood, W. E. Hunter, Inorg. Chem. 1984, 23, 2582;
P. P. Power, Chem. Rev. 1999, 99, 3463; N. Tokitoh, J. Organomet.
Chem. 2000, 611, 217.
¬
[3] C. Couret, J. Escudie, Y. Madaule, H. Ranaivonjatovo, J.-G. Wolf,
Tetrahedron Lett. 1983, 24, 2769; A. H. Cowley, J. G. Lasch, N. C.
Norman, M. Pakulski, J. Am. Chem. Soc. 1983, 105, 5506; A. H.
Cowley, N. C. Norman, M. Paulski, J. Chem. Soc. Dalton Trans. 1985,
383.
[4] N. Tokitoh, Y. Arai, T. Sasamori, R. Okazaki, S. Nagase, H. Uekusa, Y.
Ohashi, J. Am. Chem. Soc. 1998, 120, 433.
[5] N. Tokitoh, Y. Arai, R. Okazaki, S. Nagase, Science 1997, 277, 78.
[6] R. Okazaki, M. Unno, N. Inamoto, Chem. Lett. 1987, 2293; R.
Okazaki, N. Tokitoh, T. Matsumoto in Synthetic Methods of Organo-
metallic and Inorganic Chemistry, Vol. 2 (Ed.: W. A. Herrmann, N.
Auner, U. Klingebiel), Thieme, New York, 1996, p. 260.
[7] B. Twamley, C. D. Sofield, M. M. Olmstead, P. P. Power, J. Am. Chem.
Soc. 1999, 121, 3357.
[8] A. H. Cowley, J. G. Lasch, N. C. Norman, M. Pakulski, B. R. Whitt-
lesey, J. Chem. Soc. Chem. Commun. 1983, 881.
[9] B. Twamley, P. P. Power, Chem. Commun. 1998, 1979.
[10] T. Sasamori, N. Takeda, N. Tokitoh, Chem. Commun. 2000, 1353.
[11] N. Kano, N. Tokitoh, R. Okazaki, Organometallics 1998, 17, 1241.
[12] Crystal data for 1 ¥ 0.5 hexane (C51H103BiPSi7): Mr ˆ 1152.91, Tˆ
≈
93(2) K, triclinic, P1 (no. 2), a ˆ 12.7372(2), b ˆ 14.6943(5), c ˆ
18.5933(5) ä, a ˆ 107.5367(14), b ˆ 95.2385(15), g ˆ 107.9837(5)8,
Vˆ 3090.27(14) ä3, Z ˆ 2, 1calcd ˆ 1.239 gcmÀ3, m ˆ 3.044 mmÀ1, l ˆ
0.71070 ä, 2qmax ˆ 50.0, 35164 measured reflections, 10813 indepen-
dent reflections, 572refined parameters, GOF ˆ 1.034, R1 ˆ 0.0201
and wR2 ˆ 0.0483 (I > 2s(I)), R1 ˆ 0.0225 and wR2 ˆ 0.0489 (for all
data), largest difference peak and hole 1.106 and À0.476 eäÀ3
.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-168594. Copies of the data can be obtained free of charge on
application to CCDC, 12Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk). The intensity
data were collected on a Rigaku/MSC Mercury CCD diffractometer.
The structure was solved by direct methods (SHELXS-97) and refined
by full-matrix least-squares procedures on F 2 for all reflections
(SHELXL-97). All hydrogens were placed using AFIX instructions.
[13] T. Sasamori, N. Takeda, N. Tokitoh, unpublished results.
[14] S. Nagase, S. Suzuki, T. Kurakake, J. Chem. Soc. Chem. Commun.
1990, 1724.
[*] V. Shastri, R. Langer, A. N. Zelikin, D. M. Lynn
Department of Chemical Engineering, Room E25-342
Massachusetts Institute of Technology
[15] S. Nagase in The Chemistry of Organic Arsenic, Antimony and
Bismuth Compounds (Ed.: S. Patai), Wiley, New York, 1994, chap. 1,
p. 1.
Cambridge, Massachusetts 02139 (USA)
Fax : (‡1) 617-258-8827
E-mail: shastriv@seas.upenn.edu, rlanger@mit.edu
[16] Geometry optimization was carried out using the Gaussian 98
program with density functional theory at the B3LYP level. The
triple zeta basis set ([3s3p]) augmented by two sets of d polarization
functions for Bi (d exponents 0.229 and 0.069) and P (d exponents
0.537 and 0.153) and double zeta basis ([2s2p])set for Si were used
with effective core potential; the 3-21G basis set was used for C and H.
J. Farhadi, I. Martin
Department of Surgery, Research Division
Kantonsspital Basel, Basel, (Switzerland)
[**] Financial support was provided by the Department of the Army
(Cooperative Agreement No. DAMD17-99-2-9001 to the Center for
Innovative Minimally Invasive Therapy) and the NIH (GM26698).
DML wishes to thank the NIH for a Postdoctoral Fellowship (NRSA
Fellowship No. 1F32GM20227-01).
Angew. Chem. Int. Ed. 2002, 41, No. 1
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1433-7851/02/4101-0141 $ 17.50+.50/0
141