Transition metal-catalyzed formation of phosphorus-boron bonds a new route to phosphinoborane rings, chains, and macromolecules

Article abstract of DOI:10.1021/ja000732r

A novel catalytic dehydrocoupling route for the synthesis of linear, cyclic, and polymeric phosphinoboranes has been developed. The dehydrocoupling of neat Ph₂PH·BH₃, which is otherwise very slow below 170 °C, is catalyzed by [{Rh(μ-

Full text of DOI:10.1021/ja000732r

J. Am. Chem. Soc. 2000, 122, 6669-6678
6669
Transition Metal-Catalyzed Formation of Phosphorus-Boron Bonds:
A New Route to Phosphinoborane Rings, Chains, and
Macromolecules
Hendrik Dorn, Ryan A. Singh, Jason A. Massey, James M. Nelson,† Cory A. Jaska,
Alan J. Lough, and Ian Manners*
Contribution from the Department of Chemistry, UniVersity of Toronto, 80 St. George Street,
Toronto, Ontario M5S 3H6, Canada
ReceiVed February 29, 2000
Abstract: A novel catalytic dehydrocoupling route for the synthesis of linear, cyclic, and polymeric
phosphinoboranes has been developed. The dehydrocoupling of neat Ph2PH‚BH3, which is otherwise very
slow below 170 °C, is catalyzed by [{Rh(µ-Cl)(1,5-cod)}2] or [Rh(1,5-cod)2][OTf] (0.5-1 mol % Rh) to give
the linear compound Ph2PH-BH2-PPh2-BH3 (1) at 90 °C, and a mixture of the cyclic trimer [Ph2P-BH2]3
(2a) and tetramer [Ph2P-BH2]4 (2b) at 120 °C. In addition, the catalytic potential of other (e.g., Ti, Ru, Rh,
Ir, Pd, Pt) complexes toward the dehydrocoupling of Ph2PH‚BH3 was investigated and was in many cases
demonstrated. The molecular structures of 1 and 2b, and of the primary phosphine-borane adduct PhPH2‚
BH3, were determined by single-crystal X-ray analysis. The dehydrogenative coupling of PhPH2‚BH3 gave
low-molecular-weight poly(phenylphosphinoborane) [PhPH-BH2]n (3) when performed in toluene (110 °C)
with ca. 0.5 mol % [Rh(1,5-cod)2][OTf] as catalyst. The absolute weight-average molecular weight was
determined by static light scattering (SLS) in THF which showed that Mw ) 5600. Samples of high-molecular-
weight polymer 3 (Mw ) 31 000 or 33 300 by SLS) were synthesized using neat conditions at 90-130 °C in
the presence of [{Rh(µ-Cl)(1,5-cod)}2], anhydrous RhCl3, or RhCl3 hydrate (ca. 1 mol % Rh). Poly-
(phosphinoborane) 3 was thereby obtained in ca. 75% yield as an air-stable, off-white solid and was structurally
1
characterized by H, 11B, 13C, and 31P NMR and IR spectroscopy and elemental analysis. The hydrodynamic
diameters for polymers 3 in THF were also determined by dynamic light scattering (DLS). Catalytic
dehydrocoupling of the alkyl-substituted phosphine-borane adduct iBuPH2‚BH3 was also investigated and
was found to be much slower than that of PhPH2‚BH3. This produced poly(isobutylphosphinoborane) [iBuPH-
BH2]n (4) under neat conditions at 120 °C in the presence of [{Rh(µ-Cl)(1,5-cod)}2] in 80% yield. Multinuclear
NMR spectroscopy and DLS were also used to characterize polymer 4, and the latter indicated that Mw ) ca.
10 000-20 000. Prolonged heating of polymers 3 and 4 at elevated temperatures in the presence of catalyst
led to insoluble but solvent-swellable gels possibly due to light interchain cross-linking through P-B bonds.
In the absence of rhodium catalyst thermally induced dehydrocoupling of PhPH2‚BH3 and iBuPH2‚BH3 proceeds
very slowly and forms only low-molecular-weight materials with complex NMR spectra and which probably
possess a branched structure.
Introduction
or sulfur-based9 polymers, illustrate the potential for accessing
materials with unusual properties as well as significant applica-
tions. The reasons for the interest in such polymers is apparent
from simple chemical considerations. Compared to C-C bonds,
bonds between main group elements can be stronger and more
Macromolecules based on main group elements are of
considerable interest,1 and the relatively few polymer systems
currently known, such as poly(siloxanes) [R2Si-O]n,2 poly-
phosphazenes [R2PdN]n,3 polysilanes [R2Si]n,1,4 polysilynes
[RSi]n,5 and more recent examples which include polystannanes
[R2Sn]n,6 polyborazylenes [B3N3R4]n,7 and other boron-based8
(3) (a) Allcock, H. R.; Kugel, R. L. J. Am. Chem. Soc. 1965, 87, 4216.
(b) Allcock, H. R. Chem. Mater. 1994, 6, 1476. (c) Neilson, R. H.; Wisian-
Neilson, P. Chem. ReV. 1988, 88, 541. (d) Montague, R. A.; Matyjaszewski,
K. J. Am. Chem. Soc. 1990, 112, 6721. (e) Honeyman, C. H.; Manners, I.;
Morrissey, C. T.; Allcock, H. R. J. Am. Chem. Soc. 1995, 117, 7035. (f)
De Jaeger, R.; Gleria, M. Prog. Polym. Sci. 1998, 23, 179.
(4) (a) Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1986,
108, 4059. (b) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22.
(5) Bianconi, P. A.; Weidman, T. W. J. Am. Chem. Soc. 1988, 110, 2342.
(6) (a) Imori, T.; Tilley, T. D. J. Chem. Soc., Chem. Commun. 1993,
1607. (b) Imori, T.; Lu, V.; Cai, H.; Tilley, T. D. J. Am. Chem. Soc. 1995,
117, 9931. (c) Babcock, J. R.; Sita, L. R. J. Am. Chem. Soc. 1996, 118,
12481.
* To whom correspondence should be addressed. E-mail: imanners@
alchemy.chem.utoronto.ca.
† Present address: 3M Corporate Process Technology Center, St. Paul,
MN 55114.
(1) For general reviews of this area, see, for example: (a) Mark, J. E.;
Allcock, H. R.; West, R. Inorganic Polymers; Prentice Hall: Englewood
Cliffs, NJ, 1992. (b) Inorganic and Organometallic Polymers IIsAdVanced
Materials and Intermediates; Wisian-Neilson, P., Allcock, H. R., Wynne,
K. J., Eds.; ACS Symposium Series 572; American Chemical Society:
Washington, DC, 1994. (c) Manners, I. Angew. Chem., Int. Ed. Engl. 1996,
35, 1602.
(7) Fazen, P. J.; Beck, J. S.; Lynch, A. T.; Remsen, E. E.; Sneddon, L.
G. Chem. Mater. 1990, 2, 96.
(2) Clarson, S. J.; Semlyen, J. A. Siloxane Polymers; Prentice Hall:
Englewood Cliffs, NJ, 1993.
(8) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
5112.
10.1021/ja000732r CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/27/2000

6670 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Dorn et al.
oxidatively stable or provide additional conformational flex-
ibility (e.g., Si-O bonds in [R2Si-O]n) or can possess interest-
ing electron delocalization effects (e.g., σ-conjugation in [R2Si]n
and heavier group 14 analogues). In addition to novel properties,
versatile reactivity can be accessed (e.g., the substitution of
chlorine in [Cl2PdN]n).
day standards and, where reported, the yields and molecular
weights were generally very low. Since that time the area of
phosphorus-boron polymer chemistry has been neglected.
We were initially encouraged to reinvestigate this area by
the earlier reports (which in some cases we have subsequently
verified) that cyclic structures such as the six-membered rings
[R2P-BH2]3 show high thermal stability and resistance to
oxidation and hydrolysis. Such species were first synthesized
by Burg and Wagner in 1953 via dehydrogenative thermolysis
of Me2PH‚BH3 (150 °C, 40 h) to give a mixture of 90% [Me2P-
BH2]3 and 9% [Me2P-BH2]4.19 The trimer is reported to be
stable up to 300 °C, even in the presence of hydrochloric acid.
This suggested that analogous high polymers might, indeed, have
useful properties in addition to their intrinsic scientific interest
if efficient synthetic routes to these materials could be devel-
oped.
We have previously briefly described our preliminary work
on the rhodium-catalyzed dehydrocoupling of Ph2PH‚BH3 and
PhPH2‚BH3, as well as the synthesis of the first, well-
characterized, high-molecular-weight poly(phosphinoborane),
[PhPH-BH2]n.20 In this paper we report full details of this
chemistry and the extension of our studies to other catalysts
for dehydrogenative coupling and to the alkylphosphine-borane
adduct iBuPH2‚BH3. We describe efficient transition metal-
catalyzed synthetic routes to a variety of linear, cyclic, and
polymeric phosphinoboranes.
The development of new synthetic methods for the formation
of homonuclear or heteronuclear bonds between main group
elements is of critical importance for the construction of
inorganic polymer chains. Transition metal-catalyzed routes,
which are of widespread significance in organic chemistry,
represent an attractive approach to this problem. Previously,
well-characterized transition metal-catalyzed dehydrocoupling
reactions to form Si-Si,4 Ge-Ge,10 and Sn-Sn6 as well as
P-P,11 B-Si,12 and P-Si13 bonds have been reported.14
As part of our continuing program to develop novel extended
structures based on main group elements,15 we are exploring
compounds with four-coordinate phosphorus and boron atoms.16
The attempted synthesis of polymeric materials based on
skeletons of alternating phosphorus and boron atoms, poly-
(phosphinoboranes), received significant attention in the 1950s
and 1960s as such materials seemed likely candidates for
accessing valuable, high-performance properties such as high-
temperature stability and flame retardancy.17 However, the open
literature on phosphinoborane polymer chemistry is very limited,
and most of this work is documented in patents and technical
reports. The main synthetic methodology studied involved
thermally induced dehydrocoupling of phosphine-borane ad-
ducts R2PH‚BH3 at ca. 200 °C and above, which was often
performed in the presence of additives such as amines which
were claimed to prevent cyclization.18 Although in a few cases
low yields of “polymeric” materials were claimed, none of the
products were convincingly structurally characterized by present
Results and Discussion
I. Catalytic Dehydrocoupling of Ph2PH-BH3. Synthesis
and Characterization of Ph2PH-BH2-PPh2-BH3 (1), [Ph2P-
BH2]3 (2a), and [Ph2P-BH2]4 (2b). For our investigations, we
chose Ph2PH‚BH3 as starting material. It is cheaper than Me2-
PH‚BH3, readily prepared from commercially available diphe-
nylphosphine and borane-dimethyl sulfide adduct, and stable
toward air and hydrolysis.
Ph2PH-BH2-PPh2-BH3 (1). Previous work has shown that
neat Ph2PH‚BH3 undergoes thermally induced dehydrocoupling
at 190-200 °C to yield the cyclic trimer [Ph2P-BH2]3
(hereinafter referred to as 2a).21 However, on addition of [{Rh-
(µ-Cl)(1,5-cod)}2] or [Rh(1,5-cod)2][OTf] as catalyst (ca. 0.3
mol %) and lowering the temperature to 90 °C, cyclization is
prevented and the novel, linear compound Ph2PH-BH2-PPh2-
BH3, 1, is formed (eq 1). Under these conditions, formation of
(9) (a) Dodge, J. A.; Manners, I.; Allcock, H. R.; Renner, G.; Nuyken,
O. J. Am. Chem. Soc. 1990, 112, 1268. (b) Liang, M.; Manners, I. J. Am.
Chem. Soc. 1991, 113, 4044. (c) Chunechom, V.; Vidal, T. E.; Adams, H.;
Turner, M. L. Angew. Chem., Int. Ed. 1998, 37, 1928. (d) Roy, A. K. J.
Am. Chem. Soc. 1992, 114, 1530.
(10) For catalytic demethanative coupling, see: Katz, S. M.; Reichl, J.
A.; Berry, D. H. J. Am. Chem. Soc. 1998, 120, 9844.
(11) Etkin, N.; Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1997,
119, 2954.
(12) Jiang, Q.; Carroll, P. J.; Berry, D. H. Organometallics 1993, 12,
177.
(13) (a) Xin, S.; Woo, H. G., Harrod, J. F.; Samuel, E.; Lebuis, A. J.
Am Chem. Soc. 1997, 119, 5307. (b) Shu, R.; Hao, L.; Harrod, J. F.; Woo,
H.-G.; Samuel, E. J. Am. Chem. Soc. 1998, 120, 12988.
(14) For a recent review on catalytic dehydrocoupling, see: Gauvin, F.;
Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem. 1998, 42, 363.
(15) Gates, D. P.; Manners, I. J. Chem. Soc., Dalton Trans. 1997, 2525.
(16) For recent work on novel molecular phosphorus-boron compounds,
see, for example: (a) Power, P. P. Angew. Chem., Int. Ed. Engl. 1990, 29,
449. (b) Paine, R. T.; No¨th, H. Chem. ReV. 1995, 95, 343.
(17) (a) Parshall, G. W. In The Chemistry of Boron and its Compounds;
Muetterties, E. L., Ed.; Wiley: New York, 1967; p 617. (b) Haiduc, I. The
Chemistry of Inorganic Ring Systems; Wiley: New York, 1970; p 349.
(18) During this early pioneering stage of research on phosphorus-boron
compounds the highest molecular weight polymers were formed by
dehydrogenative thermolysis when, e.g., secondary phosphine-borane
adducts R2PH‚BH3 were heated in the presence of a base, usually a tertiary
amine. It was postulated that cyclization is prevented by coordination of
the amine to the terminal BH2 group. For example, thermolysis of Me2P-
PMe2‚BH3 or RMePH‚BH3 (R ) Me or Et) at 175-200 °C in the presence
of amines was reported to give polymers [RMeP-BH2]n with molecular
weights of 1800-6000 (where determined): (a) Wagner, R. I.; Caserio, F.
F. J. Inorg. Nucl. Chem. 1959, 11, 259. (b) Burg, A. B. J. Inorg. Nucl.
Chem. 1959, 11, 258. A patent claims a maximum molecular weight of
13 632 when the thermolysis of Me2PH‚BH3 is carried out at 200 °C for
17 h with a difunctional base, but no further product characterization was
reported in this particular case: Burg, A. B.; Wagner, R. I. U.S. Patent
3,071,553, 1963. In addition, partially characterized, insoluble materials
with three-coordinate phosphorus and boron atoms have been described:
Coates, G. E.; Livingstone, J. G. J. Chem. Soc. 1961, 5053.
1 is quantitative and subsequent recrystallization from diethyl
ether gives colorless, air-stable crystals in 85% yield.It should
be noted that in the absence of the rhodium catalyst no
conversion of Ph2PH‚BH3 was observed at this temperature (see
also Table 1, entry 1).
Two phosphorus environments are detected in the 31P{1H}
NMR spectrum (CDCl3) of 1, centered at δ -3.3 ppm (Ph2PH
group) and δ -17.7 ppm (Ph2P group). The 11B{1H} NMR
spectrum also shows two broad lines centered at δ -33.2 ppm
1
(BH2 group) and δ -37.3 ppm (BH3 group). In the H NMR
(19) Burg, A. B.; Wagner, R. I. J. Am. Chem. Soc. 1953, 75, 3872.
(20) Dorn, H.; Singh, R. A.; Massey, J. A.; Lough, A. J.; Manners, I.
Angew. Chem., Int. Ed. 1999, 38, 3321.
(21) Gee, W.; Holden, J. B.; Shaw, R. A.; Smith, B. C. J. Chem. Soc.
1965, 3171.

Transition Metal-Catalyzed Formation of P-B Bonds
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6671
a
Table 1. Dehydrocoupling of Ph2PH‚BH3
temp conversion
(°C)
entry
catalyst
(%)b
productc
1
2
3
none
none
none
90
120
170
0
<5
95
-
1
2a/2b
(ca. 8:1 ratio)d
1
4
5
6
[{Rh(µ-Cl)(1,5-cod)}2]
[{Rh(µ-Cl)(1,5-cod)}2]
[{Rh(µ-Cl)(1,5-cod)}2]
60
90 100
120 100
10
1
2a/2b
(ca. 2:1 ratio)d
1/2a/2be
7
8
9
[{Rh(µ-Cl)(1,5-cod)}2]
[{Rh(µ-Cl)(1,5-cod)}2]
[Rh(1,5-cod)2][OTf]
120 100
(4h)
170 100
Figure 1. Molecular structure of Ph2PH-BH2-PPh2-BH3 (1).
Selected bond lengths (Å) and angles (deg): P1-B2 1.932(2), P1-B1
1.944(2), P2-H1P 1.349(19), P2-B1 1.923(2); B2-P1-B1 113.01-
(11), H1P-P2-B1 112.6(8), P2-B1-P1 109.23(12). Hydrogen atoms
attached to phenyl rings are omitted.
2a/2b
(ca. 2:1 ratio)d
1
60
70
10 [Rh(1,5-cod)2][OTf]
11 [Rh(1,5-cod)2][OTf]
90 100
120 100
1
2a/2b
(ca. 2:1 ratio)d
1/2a/2be
12 [Rh(1,5-cod)2][OTf]
120 100
(4h)
Compound 1 is the first well-characterized example of a linear
phosphinoborane with four-coordinate boron and phosphorus
atoms. In 1989, Imamoto and Oshiki described the preparation
of linear phosphinoboranes via nucleophilic substitution reac-
tions at the boron atom. The reaction of Me3P‚BH2(OSO2Me)
with Ph2PH‚BH3/NaH was reported to give the four-membered
compound Me3P-BH2-PPh2-BH3 as well as six- and eight-
membered chains.25 However, characterization of these species
by 31P or 11B NMR spectroscopy or by single-crystal X-ray
analysis was not reported.
[Ph2P-BH2]3 (2a) and [Ph2P-BH2]4 (2b). When neat Ph2-
PH‚BH3 and a catalytic amount of [{Rh(µ-Cl)(1,5-cod)}2] or
[Rh(1,5-cod)2][OTf] were heated to 120 °C overnight, the linear
compound 1 was completely consumed and a mixture of the
cyclic trimer [Ph2P-BH2]3 (2a) and tetramer [Ph2P-BH2]4 (2b)
was formed (eq 2).
13 [Rh(PPh3)3Cl]
14 [Rh(1,5-cod)(dmpe)][PF6]
15 [Rh(CO)(PPh3)3H]
16 anhydrous RhCl3
17 RhCl3 hydrate
18 [{Cp*Rh(µ-Cl)Cl}2]
19 [{Ir(µ-Cl)(coe)2}2]
20 [Ir(1,5-cod)2][BF4]
21 Cp2TiMe2
22 Ru3(CO)12
23 Ru3(CO)12
24 [Pd(PPh3)4]
25 [Pt(1,5-cod)2]
26 PdCl2
90
90
90
60
60
20
1
1
1
1
1
1
1
1
1
-
1
-
-
1
90
90
90
90
120
90
90
90-100
90-100
90-100
25
25
15
0
120
90
90
15
0
0
90
90
60
40
27 PtCl2
1
a All reactions were performed without solvent, using ca. 250-350
mg of Ph2PH‚BH3 and a catalyst concentration of ca. 0.5-1.5 mol %
transition metal, reaction time 14-15 h (unless otherwise noted).
b Estimated by 31P NMR spectroscopy. c Traces of Ph2PH and Ph2P(O)H
were sometimes detected by 31P NMR spectroscopy. d Based on
integration of 1H NMR spectra. e In the 31P NMR spectra, observation
of additional signals of low intensity around δ -5.5 and -7.5 ppm
may indicate formation of intermediate species; relative product ratios
1
could not be extracted from the H NMR spectra.
spectrum of 1, particularly noteworthy is the doublet of
multiplets at δ 6.71 ppm (PH proton, JHP ) 397 Hz) and two
sets of broad resonances assigned to the two and three BH
protons; the latter appears approximately as a 1:1:1:1 quartet.
In the EI mass spectrum of 1, the most intense peak comes
from the loss of BH3 (m/z ) 384, [M - BH3]+).
A single-crystal X-ray analysis of compound 1 confirms the
chainlike structure of the molecule, and a SHELXTL drawing
is shown in Figure 1. The geometry around B1, P1, and P2 is
approximately tetrahedral, with bond angles varying from 101.6-
(12) to 112.7(12)°, 105.18(9) to 113.01(1)°, and 99.0(8) to
117.73(10)°, respectively. All three P-B distances are in the
single-bond range (1.90-2.00 Å). The bond distances P1-B2
(1.932(2) Å) and P2-B1 (1.923(2) Å) are comparable to P-B
distances found in other simple phosphine-borane adducts such
as Ph3P‚BH3 (av 1.917 Å),22 (C6H11)2PH‚BH3 (1.919(3) Å),23
or PhPH2‚BH3 (1.924(4) Å, vide infra). The central P1-B1 bond
distance (1.944(2) Å) is typical of a phosphorus-boron bond
length found in cyclic phosphinoboranes, e.g., [Ph2P-BH2]3,
2a, (av 1.948 Å)24 or [Ph2P-BH2]4, 2b, (av 1.939(5) Å, vide
infra).
Both the 31P and 11B NMR spectra revealed only single broad
resonances centered around δ -18.2 ppm and δ -33.8 ppm,
respectively, which are indicative for cyclic phosphinoborane
compounds. The 1H NMR spectrum in CDCl3 showed two sets
of aromatic protons which were assigned to 2a and 2b,
respectively (ca. 2:1 ratio). The signal for the BH2 protons is
1
very broad and is found in the δ 3-1 region of the H NMR
spectrum.
The mixture of 2a/2b can partially be separated upon addition
of diethyl ether. It is thus possible to isolate small amounts of
pure tetramer 2b, which is less soluble in diethyl ether than the
trimer 2a. Compound 2b is a colorless, air-stable solid which
is very soluble in CHCl3, THF, and toluene, less soluble in
diethyl ether, and almost insoluble in hexanes. Again, virtually
no conversion of Ph2PH‚BH3 was observed over 14 h at 120
°C in a blank experiment (Table 1, entry 2).
As mentioned above, the synthesis of 2a was described earlier,
from the uncatalyzed pyrolysis of Ph2PH‚BH3, and the com-
pound was characterized by IR, elemental, and molecular weight
analysis,21 and later by single-crystal X-ray analysis.24 To our
(22) Huffman, J. C.; Skupinski, W. A.; Caulton, K. G. Cryst. Struct.
Commun. 1982, 11, 1435.
(24) Bullen, G. J.; Mallinson, P. R. J. Chem. Soc., Dalton Trans. 1973,
1295.
(23) Day, M. W.; Mohr, B.; Grubbs, R. H. Acta Crystallogr. 1996, C52,
3106.
(25) (a) Imamoto, T.; Oshiki, T. Tetrahedron Lett. 1989, 30, 383. (b)
Oshiki, T.; Imamoto, T. Bull. Chem. Soc. Jpn. 1990, 63, 2846.

6672 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Dorn et al.
Catalyst Activities for the Dehydrocoupling of Ph2PH‚
BH3. To our knowledge, the reactions described above represent
the first examples of transition metal-catalyzed formation of
phosphorus-boron bonds. We have therefore surveyed and
compared a variety of transition metal complexes as catalysts
for the dehydrocoupling of Ph2PH‚BH3, and the results are
recorded in Table 1. The reactions were performed over 14-
15 h without solvent, and 0.5-1.5 mol % of the transition metal
complex was used.
We found that neat Ph2PH‚BH3 is stable at 120 °C (entries 1
and 2) and slowly forms a mixture of 2a/2b (ca. 8:1 ratio) when
heated at 170 °C for 15 h (entry 3) in the absence of catalysts.
The most efficient dehydrocoupling catalysts we found were
[{Rh(µ-Cl)(1,5-cod)}2] and [Rh(1,5-cod)2][OTf], for which full
conversion of Ph2PH‚BH3 to 1 was observed at 90 °C (entries
5 and 10) and to 2a/2b (ca. 2:1 ratio) at 120 °C (entries 6 and
11). At 60 °C, only 10% of Ph2PH‚BH3 was converted to 1 in
the presence of [{Rh(µ-Cl)(1,5-cod)}2] (entry 4), but ap-
proximately 70% conversion was found for [Rh(1,5-cod)2][OTf]
(entry 9). If the dehydrocoupling at 120 °C is performed for 4
h instead of 15 h, Ph2PH‚BH3 is still completely consumed,
but mixtures of 1 and 2a/2b result in both cases (entries 7 and
12). Phosphine ligands at rhodium appear to reduce the catalytic
activity, as was found for Wilkinson’s catalyst, [Rh(1,5-cod)-
(dmpe)][PF6] and [Rh(CO)(PPh3)3H], presumably due to the
lower lability of these complexes (entries 13-15). Efficient
catalytic activity was also observed with Rh(III) complexes.
Thus, 90-100% conversion was found when anhydrous RhCl3,
RhCl3 hydrate, or [{Cp*Rh(µ-Cl)Cl}2] was tested at 90 °C
(entries 16-18). It is possible, however, that Rh(III) is initially
reduced to a low-valent form. Interestingly, the iridium com-
plexes [{Ir(µ-Cl)(coe)2}2] and [Ir(1,5-cod)2][BF4] are less ef-
ficient dehydrocoupling catalysts in comparison to the rhodium
analogues (entries 19 and 20).
Cp2TiMe2, which catalyzes the heterodehydrocoupling of
phosphines with silanes,13b shows only moderate activity, as
does Ru3(CO)12 (entries 21-23). Among the palladium and
platinum compounds tested, no catalysis was observed for the
d10 complexes [Pd(PPh3)4] and [Pt(1,5-cod)2] (entries 24 and
25), but dehydrocoupling occurs with PdCl2 or PtCl2 as catalysts
(entries 26 and 27).
In summary, P-B bond formation via the dehydrocoupling
of Ph2PH‚BH3 is clearly promoted by certain transition metal
complexes, especially [{Rh(µ-Cl)(1,5-cod)}2] or [Rh(1,5-cod)2]-
[OTf]. Another interesting result is that no evidence for
homodehydrocoupling (i.e., P-P or B-B bond formation) was
detected for any of the reactions. The mechanism of these
reactions is unclear at present. The most likely possibility,
however, appears to be a sequence of oxidative addition/
reductive elimination and/or σ-bond metathesis reactions involv-
ing P-H, B-H, M-P, M-B, and M-H bonds.28,29
Figure 2. Molecular structure of [Ph2P-BH2]4 (2b). Of the two
independent molecules in the asymmetric unit, only one is shown.
Selected bond lengths (Å) and angles (deg): P1A-B1A 1.951(5),
P2A-B1A 1.942(5), P2A-B2A 1.937(5), P4A-B2A 1.932(5), P4A-
B3A 1.940(5), P3A-B3A 1.931(5), P3A-B4A 1.939(5), P1A-B4A
1.945(5); B4A-P1A-B1A 120.6(2), B2A-P2A-B1A 121.6(2), B3A-
P3A-B4A 120.2(2), B2A-P4A-B3A 119.6(2), P2A-B1A-P1A
113.3(3), P4A-B2A-P2A 115.8(3), P3A-B3A-P4A 113.7(2), P3A-
B4A-P1A 113.8(3). Hydrogen atoms are omitted.
knowledge, NMR data for 2a have not so far been reported in
the literature. To compare spectroscopic data of the new tetramer
2b with the trimer 2a, a sample of compound 2a was prepared
from neat Ph2PH‚BH3 at 170 °C (15 h, uncatalyzed, see also
Table 1, entry 3).
Multinuclear NMR studies of 2a and 2b revealed that there
are virtually no differences in the 31P and 11B spectra and only
subtle but distinctive differences in the 1H and 13C spectra. Mass
spectroscopic investigations of 2a and 2b were carried out in
order to obtain information about the molecular mass (and thus
the ring size) of the compounds. In the EI mass spectrum of
trimeric 2a the peak with the highest molecular mass is observed
at m/z ) 592 and corresponds to the [M - 2H]+ ion. The EI
mass spectrum for 2b clearly indicates formation of a tetramer,
with the base peak corresponding to the [M - 4H]+ ion (m/z )
788).
The eight-membered cyclic structure of 2b was conclusively
proved by an X-ray diffraction study on a single crystal grown
from a toluene solution at room temperature (Figure 2). The
ring adopts the boat-boat conformation. The P-B single-bond
distances range from 1.922(5) to 1.962(5) Å, with an average
distance of 1.939(5) Å. The B-P-B and P-B-P bond angles
fall in the range 119.5(2)-121.6(2)° (av 120.4(2)°) and 113.3-
(3)-115.8(3)° (av 114.2(3)°), respectively, and differ slightly
from the tetrahedral values.
Only one example of the eight-membered ring family has
been previously crystallographically characterized. In 1962, the
preliminary low-accuracy X-ray structure of [Me2P-BH2]4 was
reported and revealed a puckered ring with very long P-B
distances (2.08(5) Å) and average B-P-B and P-B-P bond
angles of 125(1)° and 104(2)°, respectively.26 The six-membered
ring [Ph2P-BH2]3, 2a, has a chair conformation in the solid
state, a mean P-B bond length of 1.948 Å, and mean bond
angles of 114.3° (B-P-B) and 112.6° (P-B-P).24 For further
comparison, the isoelectronic octaphenylcyclotetraphosphazene
[Ph2PN]4 consists of an eight-membered ring in the twist-boat
conformation with ring angles of 119.8° at phosphorus and
127.8° at nitrogen.27
II. Synthesis and Characterization of Poly(phenylphos-
phinoborane), [PhPH-BH2]n (3), and Poly(isobutylphosphi-
noborane), [iBuPH-BH2]n (4). An important goal on under-
taking this work was the synthesis of high-molecular-weight
(28) For recent work on transition metal-boron compounds, see: (a)
Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786. (b) Irvine, G. J.;
Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E.
G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV. 1998, 98, 2685.
(c) Smith, M. R., III. Prog. Inorg. Chem. 1999, 48, 505. (d) Muhoro, C.
N.; He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 5033. (e) Kawano,
Y.; Yasue, T.; Shimoi, M. J. Am. Chem. Soc. 1999, 121, 11744.
(29) For recent work on P-H bond activation by transition metals, see,
for example: (a) Baker, R. T.; Calabrese, J. C.; Harlow, R. L.; Williams,
I. D. Organometallics 1993, 12, 830. (b) Han, L.-B.; Choi, N.; Tanaka, M.
Organometallics 1996, 15, 3259. (c) Kourkine, I. V.; Sargent, M. D.; Glueck,
D. S. Organometallics 1998, 17, 125.
(26) Goldstein, P.; Jacobsen, R. A. J. Am. Chem. Soc. 1962, 84, 2457.
(27) Begley, M. J.; Sowerby, D. B.; Tillott, R. J. J. Chem. Soc., Dalton
Trans. 1974, 2527.

Transition Metal-Catalyzed Formation of P-B Bonds
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6673
Figure 3. Molecular structure of PhPH2‚BH3 and its symmetry-related
molecule (symmetry code #1: 1 - x, 1 - y, 1 - z), showing weak
intermolecular H‚‚‚H interactions. Selected bond lengths (Å) and angles
(deg): P1-B1 1.924(4), P1-H1P 1.33(4), P1-H2P 1.33(4), B1-H1B
1.12(4), B1-H2B 1.10(4), B1-H3B 1.12(4); H1P-P1-B1 112.9(18),
H2P-P1-B1 114.0(16), H1B-B1-P1 106(2), H2B-B1-P1 105(2),
H3B-B1-P1 103(2).
Figure 4. 31P NMR spectrum of [PhPH-BH2]n (3) in CDCl3 (121
1
1
MHz): (a) H-decoupled; (b) H-coupled, JPH ) 360 Hz.
Synthesis and Characterization of Poly(phenylphosphi-
noborane), [PhPH-BH2]n (3). Early work on the pyrolysis of
PhPH2‚BH3 by Korshak et al. in 1963 suggested that low-
molecular-weight polymers of possible formula [PhPH-BH2]n
could exist. For instance, heating of PhPH2‚BH3 at temperatures
between 100 and 150 °C for a period of 13 h was reported to
give a benzene-soluble polymer with an average composition
unit PhPH-BH2, as determined by elemental analysis, and a
molecular weight (Mn) of 2150, which was determined ebul-
lioscopically. Longer heating and elevated temperatures (up to
250 °C) led to the formation of insoluble material, without
significant molecular weight increase of the benzene-soluble
fraction (maximum Mn ) 2630).32 We have reinvestigated the
dehydrocoupling chemistry of PhPH2‚BH3. Strong motivation
came from the expectation that the rhodium catalysts would
promote P-B coupling in much the same way as for Ph2PH‚
BH3 but beyond dimers, trimers, or tetramers.
phosphinoborane polymers. Encouraged by the rhodium-
catalyzed dehydrocoupling of Ph2PH‚BH3, and based on our
view that the prospective polymer [Ph2P-BH2]n might be
insoluble and that the anticipated steric congestion at phosphorus
could disfavor polymer formation, we moved on to explore the
dehydrocoupling of the analogous primary phosphine-borane
adduct PhPH2‚BH3, as well as the previously unknown alkyl-
substituted adduct iBuPH2‚BH3.
Single-Crystal X-ray Structure of PhPH2‚BH3. Primary
phosphine-borane adducts are an interesting but poorly studied
class of organophosphorus compounds. These species tend to
be unstable and decompose over time or upon heating. To further
investigate the nature of the bonding in this class of compounds,
we decided to examine the molecular structure of PhPH2‚BH3
in detail and undertook a single-crystal X-ray study on a crystal
obtained by sublimation at room temperature. To our knowledge
this represents the first X-ray structure of a primary phosphine-
borane adduct. A SHELXTL plot is depicted in Figure 3.
The PhPH2‚BH3 molecule has a staggered geometry with a
P-B bond length of 1.924(4) Å. The hydrogen atoms at
phosphorus and boron were located and refined (P1-H1P, 1.33-
(4) Å; P1-H2P, 1.33(4) Å; B1-H1B, 1.12(4) Å; B1-H2B,
1.10(4) Å; B1-H3B, 1.12(4) Å). The closest intermolecular
contacts were found between two molecules which create a
centrosymmetric dimer and were determined to be 2.47(6) Å
for H1P‚‚‚H1B (1 - x, 1- y, 1- z) and H1B‚‚‚H1P (1 - x, 1
- y, 1 - z) (Figure 3). However, these distances are underes-
timated due to the systematic error in determining hydrogen
positions in X-ray structures. When a value of 1.21 Å is used
for B-H bonds (the bond length established by neutron
diffraction)30 and 1.40 Å for P-H bonds (determined by
microwave spectroscopy),31 the H‚‚‚H distances are found to
be 2.375 Å, slightly shorter than the sum of the van der Waals
radii for two hydrogen atoms (2.4 Å). This may suggest a weak
attractive PsHδ+‚‚‚δ-HsB interaction as a result of the
oppositely charged hydrogen atoms. Furthermore, the PsH‚‚‚
H angle (148(3)°) is more linear and the BsH‚‚‚H angle (111-
(3)°) is more bent which is also observed for most compounds
comprising an NsHδ+‚‚‚δ-HsB arrangement.30
When PhPH2‚BH3 is refluxed in toluene overnight in the
presence of approximately 0.5 mol % of [Rh(1,5-cod)2][OTf],
dehydrogenative coupling was indeed observed (eq 3). After
precipitation into hexanes an off-white product was isolated and
subsequently identified as poly(phenylphosphinoborane) (3).
The 31P NMR spectrum of 3 in CDCl3 shows a broad signal
at δ -48.9 ppm (ν1/2 ca. 135 Hz) which is in close proximity
to the PhPH2‚BH3 signal (δ -47 ppm). The singlet splits into
1
a doublet with JPH ≈ 360 Hz in the H-coupled spectrum and
is thus characteristic of a single hydrogen substituent at
1
phosphorus (Figure 4). The H NMR spectrum of 3 is also
consistent with the assigned structure (Figure 5): it contains
broad peaks for the phenyl group (δ 7.90-6.65 ppm) and the
BH2 protons (δ 2.20-0.65 ppm) as well as a broad doublet
centered around δ 4.25 ppm for the PH group (JPH ≈ 360 Hz),
somewhat upfield of that in PhPH2‚BH3 (δ 5.51 ppm). The 11
B
NMR spectrum of 3 shows a single broad resonance at δ -34.7
ppm, which is characteristic for a four-coordinate boron center
attached to two phosphorus atoms.33 The 13C NMR resonances
for 3 appear in the expected region; however, the ipso-carbon
of the phenyl ring could not be detected. In the IR spectrum of
3, strong absorptions were observed around 2421 and 2381
Due to the packing of the molecules, all other H‚‚‚H distances
are considerably longer than 2.4 Å. Nevertheless, these H‚‚‚H
interactions should be borne in mind when some of the
properties of [PhPH-BH2]n are discussed (vide infra).
(32) Korshak, V. V.; Zamyatina, V. A.; Solomatina, A. I. IzV. Akad. Nauk
SSSR, Ser. Khim. 1964, 8, 1541.
(33) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectros-
copy of Boron Compounds; Springer: Berlin, 1978; p 357.
(30) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T.
B.; Crabtree, R. H. J. Am. Chem. Soc. 1999, 121, 6337.
(31) Bryan, P. S.; Kuczkowski, R. L. Inorg. Chem. 1972, 11, 553.

6674 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Dorn et al.
by SLS. The material thus obtained was found to have a
molecular weight of Mw ) 31 000 (DPw ) 254), which is a
comparable value.
The SLS studies on solutions of polymers 3 in THF also
revealed that the second virial coefficient A2, which is obtained
from the slope of the line in Figure 6, equals zero. This indicates
that THF is a poor solvent for [PhPH-BH2]n at 22 °C and that
the polymer is on the verge of precipitation (θ conditions).
Indeed, our attempts to analyze the molecular weight distribution
of samples of 3 by gel permeation chromatography (GPC) using
styragel columns and THF as eluent were unsuccessful and
reproducible values for Mw and Mn could not be obtained. An
explanation for this might be the formation of aggregates and/
or adsorption of the polymer chains to the GPC columns which
is likely as THF is a θ solvent at 34 °C.
Figure 5. 1H NMR spectrum of [PhPH-BH2]n (3) in CDCl3 (300
MHz).
Prior to the SLS studies we also carried out dynamic light
scattering (DLS) experiments on THF solutions of 3. DLS is
capable of obtaining the distribution of sizes in a polymer
solution and provides a measurement of the hydrodynamic
diameter (Dh) of the polymer. A freshly prepared sample of
high-molecular-weight 3 (with Mw ) 33 300) in THF showed
a bimodal size distribution with approximate peak values of Dh
≈ 10 nm and Dh ≈ 30 nm. After 24 h, however, the size
distribution obtained by DLS revealed only a single broad peak
with a hydrodynamic diameter Dh ≈ 10 nm. The same size
distribution was obtained when the polymer solution was
remeasured after 3 days. We believe that the two peaks observed
in the freshly prepared solutions arise from the presence of single
polymer chains (Dh ≈ 10 nm) and larger aggregates (Dh ≈ 30
nm). The aggregates may be partially held together by interac-
tions between the polymer chains, possibly of the type
PsHδ+‚‚‚δ-HsB, as was observed to some extent in the X-ray
structure of PhPH2‚BH3 (vide supra). However, these aggregates
break up over a period of 24 h and produce mainly single
polymer chains. At that point the SLS measurement was
performed on the polymer solution, and this afforded high-
quality data at five different concentrations (Figure 6).
Figure 6. Low-angle laser light scattering plot for high-molecular-
weight [PhPH-BH2]n (3) in THF at 22 °C (K ) optical constant, c )
concentration, Rθ ) Rayleigh ratio). The absolute molecular weight
Mw is determined as the inverse of the intercept on the y axis (Mw
)
33 300).
cm-1, which is the region typical for both the P-H and B-H
stretching vibrations; exact assignments could not be made with
certainty. An elemental analysis for C and H afforded data
consistent with the formula [PhPH-BH2]n.
Similar behavior as a function of time was noted by DLS for
other samples of 3. Furthermore, a small Dh value (ca. 1 nm)
was measured for the sample of polymer 3 prepared by catalytic
dehydrocoupling in solution which is consistent with the
relatively low-molecular-weight subsequently determined by
SLS (Mw ) 5600).
The molecular weight of 3 was determined by static light
scattering (SLS) in THF. However, the absolute weight-average
molecular weight (Mw) of 3 prepared by this solution method
was found to be relatively low (Mw ) 5600), corresponding to
a weight-average degree of polymerization (DPw) of 46.
To obtain higher molecular weights by increasing the extent
of reaction, PhPH2‚BH3 and a rhodium catalyst were heated in
the absence of solvent at slightly more elevated temperatures.
Neat PhPH2‚BH3 and a catalytic amount of [{Rh(µ-Cl)(1,5-
cod)}2] (ca. 0.6 mol % rhodium) were heated at 90 °C for 3 h
and then at 130 °C for 3 h. The reaction mixture gradually
became viscous at 90 °C and was completely solid after 3 h at
130 °C. Vigorous gas evolution (H2) was observed when the
temperature was raised to 130 °C. Dissolution of the product
in THF and subsequent precipitation into hexanes gave polymer
3 (ca. 75% yield) which was spectroscopically identical to that
prepared in toluene. However, in this case the high-molecular-
weight nature of 3 was confirmed by SLS, which afforded an
absolute value of Mw ) 33 300, corresponding to DPw ) 273
(Figure 6).
Poly(phenylphosphinoborane) (3) is air- and moisture-stable
in the solid state. It is soluble in THF and chlorinated
hydrocarbons, moderately soluble in toluene and benzene, and
insoluble in aliphatic hydrocarbons such as hexanes or in
hydrophilic solvents such as methanol or water. Prolonged
storage of polymer 3 in THF solution in air results in slow
decomposition, which is indicated by a number of additional
1
resonances in the 31P and H NMR spectra. This process may
involve peroxide-mediated oxidation of the P-H or B-H
groups.
When the reaction mixture for the preparation of high-
molecular-weight 3 was heated for 5 h rather than 3 h at 130
°C, the final product was insoluble in CH2Cl2 or THF, but it
swelled significantly in these solvents to form a colorless gel.
However, the 31P and 1H NMR spectra of this material
(immersed and swollen in CDCl3) were similar to those obtained
for CDCl3-soluble polymer 3. As branching positions could not
be detected in the 31P and 11B NMR spectra, we assume that
polymer 3 either becomes weakly cross-linked (through ad-
ditional interchain P-B coupling) or increases in molecular
weight above the solubility limit when heated for prolonged
To ensure reproducibility, another polymerization of PhPH2‚
BH3 was performed using the same conditions as described
above and the poly(phosphinoborane) 3 subsequently analyzed

Transition Metal-Catalyzed Formation of P-B Bonds
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6675
Figure 7. 31P{1H} NMR spectrum (in CDCl3) of the material obtained
from the uncatalyzed thermolysis of PhPH2‚BH3 (121 MHz).
periods above 130 °C in the presence of catalyst. If the material
is indeed cross-linked, it is not surprising that branching points
could not be detected; even in the case of the low-molecular-
weight sample of 3 (Mw ) 5600) we were unable to observe
Ph2PH- or BH3- end groups in the NMR spectra.
Figure 8. 1H NMR spectrum of [iBuPH-BH2]n (4) in CDCl3 (300
A final result of interest regarding the dehydrogenative
coupling of PhPH2‚BH3 concerns the catalytic performances of
the different rhodium complexes. We found that catalytic
amounts of [{Rh(µ-Cl)(1,5-cod)}2], anhydrous RhCl3, or even
RhCl3 hydrate (ca. 1 mol % rhodium, 3 h at 90 °C and 3 h at
130 °C) gave high polymeric 3 of comparable molecular weight,
which was confirmed by DLS experiments in THF (Dh ≈ 10
nm, after 2 days). Under the same polymerization conditions,
the Rh(I) salt [Rh(1,5-cod)2][OTf] gave a polymer 3 for which
a significantly smaller hydrodynamic diameter (Dh ≈ 5 nm, after
1 day) was found. This suggests that the molecular weight is
significantly lower in this case and, although an absolute value
for Mw was not determined, a value of Mw between 10000 and
20 000 is suggested by the DLS data.
Investigation of the Uncatalyzed Dehydrocoupling of
PhPH2‚BH3. To confirm the catalytic effect of the added
rhodium complexes, neat PhPH2‚BH3 was reacted under the
conditions for the synthesis of high-molecular-weight 3 (3 h at
90 °C and 3 h at 130 °C) but without added catalyst. A colorless
semisolid was thus obtained which was then dissolved in THF
and precipitated into hexanes. Finally, a colorless solid was
isolated in 27% yield. The 31P NMR spectrum of the product
from the uncatalyzed thermolysis of PhPH2‚BH3 is shown in
Figure 7. A comparison of this spectrum with that recorded for
3 (Figure 4) indicated that the two materials possessed different
structures.
The 31P{1H} NMR spectrum in Figure 7 shows the signal
of polymer 3 (ca. δ -49 ppm), but another main peak around
δ -52 ppm is also present. In the 1H NMR spectrum a number
of poorly resolved signals are found in the PH region, in contrast
to the distinct, well-defined doublet at δ 4.25 ppm for 3 (Figure
5). This suggests the presence of low-molecular-weight oligo-
mers (and perhaps cyclics or branched species) in the un-
catalyzed thermolysis of PhPH2‚BH3. Also, hydrodynamic
diameters obtained by DLS measurements in THF solution
clearly revealed the low-molecular-weight nature of the material
(Dh ≈ 1 nm), consistent with the previous work by Korshak
and co-workers, where molecular weights Mn of ca. 2000 were
reported.32
Synthesis and Characterization of Poly(isobutylphosphi-
noborane), [iBuPH-BH2]n (4). To study the reactivity and the
polymerization behavior of primary phosphine-borane adducts
containing an alkyl group at phosphorus, we investigated the
dehydrocoupling of iBuPH2‚BH3, which we prepared in quan-
titative yield from the free phosphine and Me2S‚BH3 in THF
solution. The resulting colorless liquid was fully characterized
by multinuclear NMR and mass spectrometry.
MHz).
4). However, the reaction time was much longer (13 h at 120
°C) compared to the case for the aryl-substituted polymer 3.
Polymer 4 was isolated as a tacky solid after precipitation into
a mixture of 2-propanol/water (1:1) from THF.
The 31P{1H} NMR spectrum of 4 showed a very broad signal
around δ -69 ppm (ν1/2 ca. 300 Hz), in contrast to the relatively
sharp δ -48.9 ppm resonance (ν1/2 ca. 135 Hz) obtained for
[PhPH-BH2]n (3). It is shifted upfield with respect to the
monomer (δ -59.6 ppm). Due to the broadness of the 31P NMR
signal, no clear splitting was observed when the proton-coupled
1
spectrum was recorded. The H NMR spectrum of 4 (Figure
8), on the other hand, was very well resolved and showed a
broad doublet at δ 3.86 ppm (JHP ) 335 Hz) for the PH proton,
which is also shifted upfield compared to the singlet at δ 4.46
ppm for the initial adduct. The BH2 signal appears just above
the baseline and overlaps with the range of the CH, CH2, and
CH3 resonances of the isobutyl group. The 11B NMR signal of
4 is a single broad peak around δ -36 ppm. The signals in the
13C NMR spectrum appear at δ 30.2 (CH, JCP ) 29 Hz), 25.6
(CH3), and 23.7 (CH2) ppm.
In the IR spectrum of 4, a broad, unresolved absorption
around 2421 cm-1 presumably arises from overlapping ν(P-
H) and ν(B-H) bands. Poly(isobutylphosphinoborane) (4) is
stable to both oxygen and moisture and is soluble in THF,
chlorinated solvents, toluene, and even hexanes but is insoluble
in water and methanol.
Conclusive evidence for the polymeric character of 4 was
obtained from DLS experiments in THF solution. As in the case
of 3, a bimodal size distribution was observed for a freshly
prepared sample of 4 in THF (Dh values ca. 7 and 25 nm). After
2 days the distribution became monomodal (peak value of Dh
≈ 7 nm). This behavior is qualitatively similar to that for poly-
(phenylphosphinoborane) 3 and suggests the presence of larger
aggregates in freshly prepared solutions of 4 which break up
over time. However, the hydrodynamic diameters of polymer
4 are slightly smaller than those measured for high polymeric
3. Making the approximation that polymers 3 and 4 exhibit
similar hydrodynamic behavior in THF, the DLS data for 4
suggest a molecular weight of 10 000-20 000.
Dehydropolymerization of neat iBuPH2‚BH3 using [{Rh(µ-
Cl)(1,5-cod)}2] as the catalyst produced [iBuPH-BH2]n (4) (eq
As was found for polymer 3, prolonged heating of iBuPH2‚
BH3 (72 h at 90 °C and 6 h at 130 °C) also afforded an insoluble

6676 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Dorn et al.
[Rh(1,5-cod)2][OTf] (1,5-cod ) 1,5-cyclooctadiene), [Rh(1,5-cod)-
(dmpe)][PF6] (dmpe ) 1,2-bis(dimethylphosphino)ethane), [Rh(CO)-
(PPh3)3H], [Ir(1,5-cod)2][BF4], Ru3(CO)12, [Pd(PPh3)4], PdCl2, PtCl2
(Strem Chemicals), RhCl3, RhCl3 hydrate (Pressure Chemical Co.), and
[Rh(PPh3)3Cl], Me2S‚BH3 (Aldrich) were purchased and used as
received. iBuPH2 was obtained from Cytec Canada Inc. PhPH2‚BH3,36
[{Rh(µ-Cl)(1,5-cod)}2],37 [{Cp*Rh(µ-Cl)Cl}2],38 [{Ir(µ-Cl)(coe)2}2] (coe
) cyclooctene),39 Cp2TiMe2,40 and [Pt(1,5-cod)2]41 were prepared
following literature procedures. Ph2PH‚BH3 was prepared using a
procedure analogous to that used for PhPH2‚BH3.36
polymer 4 which was found to swell significantly upon addition
of THF or CH2Cl2. NMR spectra of a CDCl3-swollen polymer
sample were similar to those of soluble polymer 4. Light P-B
cross-linking or a higher molecular weight above the solubility
limit are possible explanations for the swelling behavior of these
materials.
Investigation of the Uncatalyzed Dehydrocoupling of
iBuPH2‚BH3. A control experiment without catalyst was run
using neat iBuPH2‚BH3 adduct and the same time-temperature
profile as for the synthesis of polymer 4. Numerous signals in
the 31P NMR spectrum of the colorless viscous product were
present in the range of δ -60 to -96 ppm which indicated
formation of different phosphinoborane species. Accordingly,
the 1H NMR spectrum displays several peaks in the PH region
as well as in the isobutyl region. The 11B NMR spectrum
contains two broad, overlapping resonances as the main signals.
However, DLS studies in THF showed that the products possess
a very low molecular weight (Dh < 1 nm). It appears likely
that the product has a branched, oligomeric structure similar to
that formed in the case of PhPH2‚BH3.
Equipment. NMR spectra were recorded on a Varian Gemini 300
MHz or Unity 400/500 MHz spectrometer. Chemical shifts are
referenced to solvent peaks or internal TMS (1H, 13C), or external BF3‚
Et2O (11B) or H3PO4 (31P). Mass spectra were obtained with a VG 70-
250S mass spectrometer operating in electron impact (EI) mode.
Infrared spectra were taken on a Nicolet Magna 550 FT-IR instrument
as Nujol mulls between KBr plates. Elemental analyses were performed
by Quantitative Technologies, Inc., Whitehouse, NJ. Absolute molecular
weights of polymer 3 in THF were determined by static light scattering
(SLS) performed on a Chromatix KMX-6 instrument at a wavelength
of 632.8 nm and a scattering angle of 6-7°. Measurements were carried
out at 22 °C using a metal cell 4.93 mm in length. Each solution was
filtered three times through a MILEX filter with an average pore size
of 0.2 µm before injection into the cell. The refractive index increment,
dn/dc, of the solutions of polymer 3 in THF was obtained by using a
Chromatix KMX-16 differential refractometer operating at a wavelength
of 632.8 nm and determined to be 0.24 mL/g. The instrument was
calibrated with NaCl solutions. Dynamic light scattering (DLS)
experiments were carried out on a wide-angle laser light scattering
photometer from Brookhaven Instruments Corp. A Lexel Excel 3000
argon ion laser with a wavelength of 514.5 nm was used as the light
source. The concentration of the solutions was ca. 7 mg/mL, using
commercially available THF (ACS grade) as solvent. The solutions
were filtered through disposable 0.2 µm filters from Millipore into glass
scattering cells with a diameter of 12.3 mm. The cells were placed
into the BI-200SM goniometer and set in a vat of thermostated toluene
which matched the index of refraction of the glass cells. All samples
were measured at a scattering angle of 90°. The scattered light was
detected by a photomultiplier interfaced to the BI-2030AT digital
correlator with 136 channels and the correlation function measured in
real time. The instrument was controlled by a 486AT computer, and
the data were analyzed by using software supplied by Brookhaven.
Static Light Scattering. Static light scattering (SLS) experiments
in the low-angle regime were used to determine the weight-average
molecular weight, Mw. The Mw was obtained from the Rayleigh-Debye
relationship in the limit of low scattering angle,
In comparison to the dehydrocoupling of PhPH2‚BH3, more
drastic reaction conditions are necessary to polymerize iBuPH2‚
BH3. For example, if iBuPH2‚BH3 is reacted for 3 h at 90 °C/3
h at 130 °C in the presence of [{Rh(µ-Cl)(1,5-cod)}2], the
conditions used to generate high-molecular-weight 3, unreacted
monomer is still present. The dehydrocoupling of iBuPH2‚BH3
is even more sluggish in the absence of rhodium catalyst.
Summary
Novel and facile transition metal-catalyzed P-B bond forma-
tion reactions have been discovered and the first well-character-
ized, high-molecular-weight poly(phosphinoboranes) 3 and 4
have been isolated as air- and moisture-stable solids. Future
work will focus on detailed studies of the physical properties
of poly(phosphinoboranes), which can be regarded as analogues
of poly(R-olefins) with a phosphorus-boron backbone. This
should be very revealing from a fundamental perspective, and
useful properties such as flame retardancy and oxidative stability
may emerge. Substantial differences in physical properties would
be anticipated as the skeletal P-B bonds (typically > 1.92 Å)
are much longer than C-C single bonds (1.54 Å). Opportunities
also exist for the facile structural modification of poly-
(phosphinoboranes) such as 3 and 4 and analogues via exploita-
tion of the P-H functionality using hydrophosphination chem-
istry or deprotonation/electrophilic addition reaction sequences.34
The dehydrocoupling mechanism is also of considerable interest;
the catalytic chemistry may involve M-P and/or M-B bonds,
and studies in this area are in progress.28,29,35a The extension of
the synthetic methodology to other skeletons based on alternat-
ing atomic sequences of Group 13 and Group 15 elements may
also be possible by analogous chemistry, and such materials
would be of potential interest as precursors to III/V semiconduc-
tor materials.35b
Kc/Rθ ) 1/Mw + 2A2c
where c is the concentration of the polymer, Rθ is the measured Rayleigh
ratio, A2 is the second virial coefficient, and K is an optical constant
defined as
4
K ) [4π2n2/N0λ0 ](dn/dc)2
where n is the refractive index of the solvent, λ0 is the wavelength of
the laser light in a vacuum, N0 is Avogadro’s number, and dn/dc is the
refractive index increment of the polymer in THF. Refractive index
increment measurements were carried out at five different concentrations
in THF at 22 °C.
Experimental Section
Dynamic Light Scattering. In dynamic light scattering (DLS) the
experimentally determined intensity autocorrelation function G(2)(τ) is
related to the autocorrelation function representative of the motions of
General Procedures and Materials. All reactions were performed
under an atmosphere of dinitrogen using dry solvents. Workup of all
phosphinoborane compounds was carried out in air. Ph2PH, PhPH2,
(34) See, for example: (a) Han, L.-B.; Tanaka, M. J. Am. Chem. Soc.
1996, 118, 1571. (b) Wicht, D. K.; Kourkine, I. V.; Lew, B. M.; Nthenge,
J. M.; Glueck, D. S. J. Am. Chem. Soc. 1997, 119, 5039. (c) Costa, E.;
Pringle, P. G.; Worboys, K. Chem. Commun. 1998, 49. (d) Wolfe, B.;
Livinghouse, T. J. Am. Chem. Soc. 1998, 120, 5116. (e) Gaumont, A.-C.;
Hursthouse, M. B.; Coles, S. J.; Brown, J. M. Chem. Commun. 1999, 63.
(35) (a) For some recent work see: Dorn, H.; Jaska, C. A.; Singh, R.
A.; Lough, A. J.; Manners, I. Chem. Commun. 2000, 1041. (b) See, for
example: Stone, F. G. A.; Burg, A. B. J. Am. Chem. Soc. 1954, 76, 386.
(36) Bourumeau, K.; Gaumont, A.-C.; Denis, J.-M. J. Organomet. Chem.
1997, 529, 205.
(37) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218.
(38) White, C.; Yates, A.; Maitlis, P. M. Inorg. Synth. 1992, 29, 228.
(39) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18.
(40) Petasis, N. A. In Encyclopedia of Reagents for Organic Synthesis;
Paquette, L. A., Ed.; Wiley: Chichester, 1995; p 470.
(41) Spencer, J. L. Inorg. Synth. 1979, 19, 213.

Transition Metal-Catalyzed Formation of P-B Bonds
Table 2. Crystallographic Data and Structure Refinement
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6677
at a 30% probability level. In all structures hydrogens bonded to carbon
atoms were included in calculated positions and treated as riding atoms.
Hydrogens attached to B1 and P2 in compound 1 as well as B1 and P1
in PhPH2‚BH3 were refined with isotropic thermal parameters, while
in 2b the positions of the hydrogens bonded to boron atoms were refined
with thermal parameters tied to the parent boron atoms. Crystallographic
data were deposited in the Cambridge Crystallographic Data Centre
with codes CCDC-114009 (1), CCDC-140990 (2b), and CCDC-140991
(PhPH2‚BH3).
1
2b
PhPH2‚BH3
formula
C24H26B2P2
398.01
C48H48B4P4
791.98
150.0(1)
triclinic
P1h
12.4731(8)
17.7114(11)
21.70110(10)
87.409(3)
73.293(3)
69.386(2)
4289.4(4)
4
C6H10BP
123.92
100.0(1)
monoclinic
P2(1)/c
5.7037(2)
7.4676(4)
16.9791(6)
90
97.455(3)
90
717.08(5)
4
formula weight
temperature, K
crystal system
space group
a, Å
b, Å
c, Å
150.0(1)
orthorhombic
P2(1)2(1)2(1)
10.247(2)
13.616(3)
15.684(3)
90
90
90
2188.3(7)
4
1.208
Ph2PH-BH2-PPh2-BH3 (1). Neat Ph2PH‚BH3 (0.63 g, 3.12 mmol)
and [Rh(1,5-cod)2][OTf] (ca. 4 mg, 0.3 mol %) were stirred at 90 °C
for 15 h. The reaction mixture became liquid upon heating and solidified
when cooled to room temperature. Recrystallization from diethyl ether
(10 mL) gave colorless crystals of 1 which were also suitable for single-
crystal X-ray analysis. Yield: 0.53 g (85%). 1H NMR (300 MHz,
CDCl3): δ ) 7.68-7.17 (m, aromatic), 6.71 (dm, JHP ) 397 Hz, PH),
3.1-1.5 (br, BH2), 1.6-0.4 (br q, BH3). 11B{1H} NMR (160 MHz,
CDCl3): δ ) -33.2 (br, BH2), -37.3 (br, BH3). 31P{1H} NMR (121
MHz, CDCl3): δ ) -3.3 (br, Ph2PH), -17.7 (br, Ph2P). 31P NMR
(121 MHz, CDCl3): δ ) -3.3 (br d, JPH ) 397 Hz, Ph2PH), -17.7
R, deg
â, deg
γ, deg
volume, Å3
Z
density, g cm-3
absorp coeff,
mm-1
1.226
0.210
1.148
0.274
0.206
F(000)
840
1664
264
crystal size, mm 0.28 × 0.17 × 0.17 × 0.13 × 0.30 × 0.30 ×
0.12
4.18-26.36
0.08
4.09-25.03
0.30
4.54-26.33
0 e h e 7,
(br, Ph2P). MS (EI, 70 eV): m/z (relative intensity) 395 (16) [M+
3H], 384 (100) [M+ - BH3].
-
θ range, deg
index ranges
-12 e h e 12, 0 e h e 14,
Dehydrocoupling of Ph2PH‚BH3. Neat Ph2PH‚BH3 (0.27 g, 1.35
mmol) and [{Rh(µ-Cl)(1,5-cod)}2] (4 mg, ca. 1 mol % rhodium) were
heated at 120 °C for 15 h, and the resulting solid material was analyzed
-17 e k e 16, -19 e k e 21, 0 e k e 9,
-19 e l e 19
16 513
4455
-24 e l e 25
33 054
14 871
-21 e l e 20
6287
1440
reflns collected
independ reflns
1
without further purification or removal of the catalyst. H NMR (300
(Rint ) 0.061)
(Rint ) 0.121)
(Rint ) 0.059)
MHz, CDCl3): δ ) 7.51 (m, aromatic), 7.34 (m, aromatic), 7.23 (m,
aromatic), 7.14 (m, aromatic), 7.03 (m, aromatic), 6.90 (m, aromatic),
3.4-1.2 (br, BH2). 11B{1H} NMR (160 MHz, CDCl3): δ ) -33.8
(br). 13C{1H} NMR (75 MHz, CDCl3): δ ) 132.7, 132.6, 129.3, 128.5,
127.9, 127.4. 31P{1H} NMR (121 MHz, CDCl3): δ ) -18.2 (br). MS
GoF on F2
1.059
0.926
1.211
R1a (I > 2σ(I))
wR2b (all data)
0.0342
0.0820
0.0604
0.1556
0.293/-0.325
0.0564
0.1447
0.486/-0.298
peak/hole, e Å-3 0.283/-0.200
+
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.
the particles G(1)(τ),
(EI, 70 eV): m/z (relative intensity) 788 (30) [M4 - 4H], 592 (16)
+
+
[M3 - 2H], 409 (100) [M3 - PPh2].
Characterization of [Ph2P-BH2]3 (2a). Compound 2a was prepared
by a procedure similar to the reported procedure: neat Ph2PH‚BH3 (0.48
g, 2.40 mmol) was heated at 170 °C overnight (15 h) to give a colorless
solid. Diethyl ether (10 mL) was then added and the suspension filtered.
Crystals of pure 2a were grown from the filtrate at room temperature.
Yield: 0.38 g (80%). 1H NMR (300 MHz, CDCl3): δ ) 7.51 (m,
aromatic), 7.23 (m, aromatic), 7.14 (m, aromatic), 2.8-1.2 (br, BH2).
11B{1H} NMR (160 MHz, CDCl3): δ ) -32.9 (br, ν1/2 ca. 230 Hz).
13C{1H} NMR (75 MHz, CDCl3): δ ) 132.7 (Cortho), 129.3 (Cpara),
127.9 (Cmeta), Cipso not observed. 31P{1H} NMR (121 MHz, CDCl3): δ
) -17.9 (br, ν1/2 ca. 220 Hz). MS (EI, 70 eV): m/z (relative intensity)
409 (100) [M+ - PPh2], 592 (15) [M+ - 2H].
G(1)(τ)
G(1)(0)
2
2
2
2
G(2)(τ) ) Iθ 1 +
) Iθ [1 + e-2Dq τ
]
|
|
[
]
where the normalized intensity autocorrelation function which is
determined experimentally can be expressed as an exponential with a
decay of Γ,
g(1)(τ) ) G(1)(τ)/G(1)(0) ) A e-Γτ
Real systems can rarely be described by a single decay, and therefore
CONTIN, a program supplied by Brookhaven, was used deconvolute
the autocorrelation function and determine the decay from which the
effective diffusion coefficient, Dz, can be calculated:
[Ph2P-BH2]4 (2b). To a mixture of 2a/2b was added ca. 10 mL of
diethyl ether, and the mixture was stirred for 1 h until all of the material
was dissolved or finely suspended in the solution. The suspension was
filtered and colorless compound 2b collected and dried under vacuum.
Yield: ca. 20%. Single crystals for X-ray analysis were obtained from
a toluene solution at room temperature over a period of several days.
1H NMR (300 MHz, CDCl3): δ ) 7.34 (m, aromatic), 7.03 (m,
aromatic), 6.90 (m, aromatic), 3.4-1.3 (br, BH2). 11B{1H} NMR (160
MHz, CDCl3): δ ) -31.2 (br, ν1/2 ca. 290 Hz). 13C{1H} NMR (75
MHz, CDCl3): δ ) 132.6 (Cortho), 128.5 (Cpara), 127.4 (Cmeta), Cipso not
observed. 31P{1H} NMR (121 MHz, CDCl3): δ ) -19.1 (br, ν1/2 ca.
Γ1/q2 ) Dz
where q is the magnitude of the scattering vector, q ) (4πn/λ0) sin(Θ/
2).
From the diffusion coefficient the effective hydrodynamic diameter,
DH, can be determined from the Stokes-Einstein relation,
170 Hz). MS (EI, 70 eV): m/z (relative intensity) 788 (100) [M+
-
DH ) kT/3πηDz
4H].
where η is the solvent viscosity.
Synthesis of Low-Molecular-Weight [PhPH-BH2]n (3). PhPH2‚
BH3 (1.03 g, 8.31 mmol) and [Rh(1,5-cod)2][OTf] (ca. 10 mg, 0.3 mol
%) were dissolved in toluene (15 mL), and the resulting solution was
refluxed for 15 h. The reaction mixture was then concentrated under
vacuum to ca. 5 mL, filtered, and precipitated into 120 mL of hexanes.
The off-white polymeric product 3 was washed with hexanes, decanted,
and dried in vacuo. Yield: 0.68 g (67%). 1H NMR (300 MHz,
CDCl3): δ ) 7.90-6.65 (br, aromatic), 4.25 (br d, JHP ) 360 Hz,
PH), 2.20-0.65 (br, BH2). 11B{1H} NMR (160 MHz, CDCl3): δ )
-34.7 (br, ν1/2 ca. 800 Hz). 13C{1H} NMR (75 MHz, CDCl3): δ )
132.5 (Cortho), 129.3 (Cpara), 128.2 (Cmeta), Cipso not observed. 31P{1H}
NMR (121 MHz, CDCl3): δ ) -48.9 (br, ν1/2 ca. 135 Hz). 31P NMR
(121 MHz, CDCl3): δ ) -48.9 (br d, JPH ) 360 Hz). IR (Nujol):
X-ray Structural Characterization. Crystal data and details of the
data collection are provided in Table 2. Diffraction data were collected
on a Nonius Kappa-CCD using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å). A combination of 1° φ and ω (with κ offsets)
scans were used collect sufficient data. The data frames were integrated
and scaled using the Denzo-SMN package.42 The structures were solved
and refined with the SHELXTL-PC V5.1 software package.43 Refine-
ment was by full-matrix least squares on F2 using all data (negative
intensities included). Molecular structures are presented with ellipsoids
(42) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(43) Sheldrick, G. M. SHELXTL-PC V5.1; Bruker Analytical X-ray
Systems Inc.: Madison, WI, 1997.

6678 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Dorn et al.
ν(B-H) and ν(P-H) ) 2421 (vs) and 2381 (vs) cm-1. Anal. Calcd
for C6H8BP: C, 59.1; H, 6.6. Found: C, 58.9; H, 6.5. Static light
scattering (THF): Mw ) 5600, DPw ) 46, A2 ) 0.
sticky product was dissolved in THF (3 mL) and precipitated into
2-propanol/water (40 mL/40 mL). The solution was decanted and the
product redissolved in CH2Cl2 (3 mL). Removal of the volatiles under
vacuum (50 °C/24 h) left a light yellow sticky solid. Yield: 0.35 g
Synthesis of High-Molecular-Weight [PhPH-BH2]n (3). Neat
PhPH2‚BH3 (4.10 g, 33.1 mmol) and [{Rh(µ-Cl)(1,5-cod)}2] (ca. 50
mg, 0.6 mol % rhodium) were stirred for 3 h at 90 °C and then for 3
h at 130 °C. When the temperature reached 130 °C, vigorous gas
elimination was observed, and after 3 h the contents of the flask were
completely solid. After cooling to room temperature, the dark yellow
material was dissolved in THF (40 mL), filtered, and precipitated into
hexanes (700 mL). The off-white polymeric product was washed with
hexanes, decanted, and dried under vacuum at 50 °C for 48 h. Yield:
3.03 g (75%).
1
(80%). H NMR (300 MHz, CDCl3): δ ) 3.86 (br d, JHP ) 335 Hz,
PH), 1.98 (m, CH), 1.51 (m, CH2), 0.95 (d, JHH ) 5.5 Hz, CH3), 2.0-
0.8 (br, BH2). 11B{1H} NMR (160 MHz, CDCl3): δ ) -36.3 (br, ν1/2
ca. 630 Hz).l 13C{1H} NMR (75 MHz, CDCl3): δ ) 30.2 (d, JCP ) 29
Hz, CH), 25.6 (CH3), 23.7 (CH2). 31P{1H} NMR (121 MHz, CDCl3):
δ ) -69.2 (br, ν1/2 ca. 300 Hz). IR (Nujol): ν(B-H) and ν(P-H) )
2421 (vs) cm-1. Anal. Calcd for C4H12BP: C, 47.1; H, 11.9. Found:
C, 46.9; H, 11.3.
Synthesis of Cross-Linked [iBuPH-BH2]n (4). Neat iBuPH2‚BH3
(0.44 g, 4.23 mmol) and [{Rh(µ-Cl)(1,5-cod)}2] (ca. 10 mg, 1.0 mol
% rhodium) were stirred for 72 h at 90 °C and then for 6 h at 130 °C.
The product was found to be insoluble in common organic solvents.
Subsequent 1H, 11B, and 31P NMR spectroscopic analyses of the product
as a CDCl3-swollen gel afforded data similar to those for CDCl3-soluble
polymer 4.
Uncatalyzed Dehydrocoupling of iBuPH2‚BH3. Heating of neat
iBuPH2‚BH3 (0.41 g, 3.94 mmol) for 13 h at 120 °C resulted in a
colorless, viscous oil which was analyzed without further purification.
1H NMR (300 MHz, CDCl3): δ ) 5.7-3.3 (many signals, PH and
PH2), 2.0-1.7 (br, CH), 1.5-1.2 (br, CH2), 1.1-0.9 (br, CH3), 2.0-
0.1 (br, BH2 and BH3). 11B{1H} NMR (160 MHz, CDCl3): δ ) -31.6
(br), -34.5 (br), -42.3 (m, iBuPH2‚BH3). 31P{1H} NMR (121 MHz,
CDCl3): δ ) -59.6 (m, iBuPH2‚BH3), -61.2 (br), -62.7 (br), -65.2
(br), -69.5 (br), -87.2 (br), -95.7 (br).
The 1H, 11B, 13C, and 31P NMR and IR spectra are as described above.
Static light scattering (THF): Mw ) 33 300, DPw ) 273, A2 ) 0.
A second polymerizaton was conducted following the same proce-
dure: 1.22 g of PhPH2‚BH3. (9.84 mmol) and ca. 15 mg of [{Rh(µ-
Cl)(1,5-cod)}2] (0.6 mol % rhodium) gave polymer 3 in 75% yield
(0.90 g). Static light scattering (THF): Mw ) 31 000, DPw ) 254, A2
) 0.
Synthesis of Cross-Linked [PhPH-BH2]n (3). Heating of neat
PhPH2‚BH3 (0.53 g, 4.28 mmol) and [{Rh(µ-Cl)(1,5-cod)}2] (ca. 11
mg, 1.0 mol % rhodium) for 3 h at 90 °C and then for 5 h at 130 °C
resulted in an insoluble material. However, the solid was found to swell
in THF and chlorinated solvents. 1H, 11B, and 31P NMR spectra of the
CDCl3-swollen gel were identical with those of the CDCl3-soluble
polymer 3.
Uncatalyzed Dehydrocoupling of PhPH2‚BH3. Neat PhPH2‚BH3
(0.49 g, 3.95 mmol) was stirred for 3 h at 90 °C and then for 3 h at
130 °C. The resulting semisolid was dissolved in THF (3 mL),
precipitated into hexanes (100 mL), filtered, and dried under vacuum
overnight at 50 °C to give a colorless powder. Yield: 0.13 g (27%).
1H NMR (300 MHz, CDCl3): δ ) 7.90-6.40 (br, Ph), 6.20-3.30
(many broad signals, PH), 2.30-0.65 (br, BH2). 11B{1H} NMR (160
MHz, CDCl3): δ ) -34.0 (br). 31P{1H} NMR (121 MHz, CDCl3) δ
) -48.3 (br), -50.0 (br), -51.8 (br).
Acknowledgment. This research was supported by the
Natural Science and Engineering Research Council of Canada
(NSERC) and the Petroleum Research Fund administered by
the American Chemical Society. H.D. thanks the Deutsche
Forschungsgemeinschaft (DFG) for a Postdoctoral Fellowship.
I.M. thanks NSERC for an E.W.R. Steacie Fellowship (1997-
1999), the University of Toronto for a McLean Fellowship
(1997-2003), and the Ontario Government for a PREA Award
(1999-2003). The authors also thank Prof. Robert Morris for
useful discussions and Ms. Raluca Barjovanu for help with the
DLS measurements. Cytec Canada Inc. is acknowledged for a
donation of iBuPH2.
Synthesis of iBuPH2‚BH3. To a solution of iBuPH2 (4.59 g, 50.9
mmol) in THF (40 mL) was added dropwise Me2S‚BH3 (5.1 mL, 1.05
equiv) at -25 °C. After stirring of the solution at room temperature
for 15 h, all volatiles were removed under vacuum, and spectroscopi-
cally pure iBuPH2‚BH3 was obtained in quantitative yield as a colorless
oil. 1H NMR (300 MHz, CDCl3): δ ) 4.46 (dm, JHP ) 362 Hz), 1.93
(m, CH), 1.71 (m, CH2), 0.99 (d, JHH ) 6.7 Hz, CH3), 1.1 to -0.1 (br
Supporting Information Available: 1H NMR spectrum
(300 MHz, CDCl3) of the phenyl region of 2a and 2b, a low-
angle laser light scattering plot for low-molecular-weight
[PhPH-BH2]n (3) in THF (Mw ) 5600), and a 1H NMR
spectrum (300 MHz, CDCl3) of the material obtained from the
uncatalyzed thermolysis of PhPH2‚BH3 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
q, BH3). 11B{1H} NMR (160 MHz, CDCl3): δ ) -42.3 (br d, JBP
)
37 Hz). 13C{1H} NMR (75 MHz, CDCl3): δ ) 26.1 (d, JCP ) 5.4 Hz,
CH), 25.5 (d, JCP ) 34.3 Hz, CH2), 23.0 (d, JCP ) 8.0 Hz, CH3). 31P-
{1H} NMR (121 MHz, CDCl3): δ ) -59.6 (m, JPB ) 37 Hz). MS
(EI, 70 eV): m/z (relative intensity) 90 (46) [M+ - BH3], 41 (100)
[C3H5].
Synthesis of [iBuPH-BH2]n (4). Neat iBuPH2‚BH3 (0.45 g, 4.33
mmol) and [{Rh(µ-Cl)(1,5-cod)}2] (ca. 11 mg, 1.0 mol % rhodium)
were stirred for 13 h at 120 °C. After cooling to room temperature, the
JA000732R