Linear Hybrid Aminoborane/Phosphinoborane Chains: Synthesis, Proton-Hydride Interactions, and Thermolysis Behavior

Article abstract of DOI:10.1021/ic034938f

The reaction of the lithiated phosphine-borane adducts Li[PPhR· BH₃] or Li[CH₂-PR₂·BH₃] with Me₂NH·BH₂Cl afforded the hybrid linear species Me₂NH-BH₂-PPhR-BH₃ (1, R = Ph; 2, R = H) or Me₂NH-BH₂-CH₂-PR₂-BH₃ (3, R = Ph; 4, R = Me). Single-crystal X-ray diffraction studies on 1 and 3, the first for linear hybrid aminoborane/phosphinoborane adducts, confirmed the expected four-coordinate N-B-P-B and N-B-C-P-B frameworks. In addition, interactions between the protic N-H and hydridic B-H hydrogen atoms resulted in short intermolecular H?H contacts for 1, whereas 3 was found to possess an exceptionally short intramolecular H?H distance of 1.95 A?. Solution and solid state infrared studies on 3 and 4 also suggest that these dihydrogen interactions were maintained even in dilute solution. Hydrogen bond strengths in the range of 7.9 to 10.9 kJ mol^-1 indicate the presence of a relatively weak interaction. The thermal and catalytic dehydrocoupling reactivities of 1-4 were also investigated. Chain cleavage reactions were observed for 1 and 2 upon thermolysis at 130 °C to afford species such as Me₂NH·BH₃, [Me₂N-BH₂] ₂, PhPRH·BH₃ (R = Ph, H), PhPRH (R = Ph, H), Ph₂PH-BH₂-PPh₂-BH₃, and also the low molecular weight polyphosphinoborane [PhPH-BH₂]_n (M_w ～ 5000). Similar products were observed for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 °C). Thermolysis of 3 at 130 °C yielded the six-membered ring [BH ₂-CH₂-PPh₂]₂ (5), which presumably results from the dissociation of Me₂NH·BH₃ from 3. Thermolysis of 4 at 90 °C afforded Me₂NH·BH₃ and Me₃P·BH₃, in addition to a product tentatively assigned as [BH₂-CH₂-PMe₂]₂ (6).

Full text of DOI:10.1021/ic034938f

Inorg. Chem. 2004, 43, 1090−1099
Linear Hybrid Aminoborane/Phosphinoborane Chains: Synthesis,
Proton-Hydride Interactions, and Thermolysis Behavior
Cory A. Jaska, Alan J. Lough, and Ian Manners*
Department of Chemistry, UniVersity of Toronto, 80 St. George Street,
Toronto, Ontario, Canada M5S 3H6
Received August 6, 2003
The reaction of the lithiated phosphine−borane adducts Li[PPhR‚BH ] or Li[CH−PR ‚BH ] withMe NH‚BH Cl afforded
3
2
2
3
2
2
the hybrid linear species Me2NH−BH −PPhR−BH (1, R ) Ph; 2, R ) H) or Me2NH−BH −CH −PR −BH (3, R
2
3
2
2
2
3
) Ph; 4, R ) Me). Single-crystal X-ray diffraction studies on 1 and 3, the first for linear hybrid aminoborane/
phosphinoborane adducts, confirmed the expected four-coordinate N−B−P−B and N−B−C−P−B frameworks. In
addition, interactions between the protic N−H and hydridic B−H hydrogen atoms resulted in short intermolecular
H‚‚‚H contacts for 1, whereas 3 was found to possess an exceptionally short intramolecular H‚‚‚H distance of
1.95 Å. Solution and solid state infrared studies on 3 and 4 also suggest that these dihydrogen interactions were
maintained even in dilute solution. Hydrogen bond strengths in the range of 7.9 to 10.9 kJ mol-1 indicate the
presence of a relatively weak interaction. The thermal and catalytic dehydrocoupling reactivities of 1−4 were also
investigated. Chain cleavage reactions were observed for 1 and 2 upon thermolysis at 130 °C to afford species
such as Me2NH‚BH , [Me2N−BH ] , PhPRH‚BH (R ) Ph, H), PhPRH (R ) Ph, H), Ph2PH−BH −PPh2−BH , and
3
2 2
3
2
3
also the low molecular weight polyphosphinoborane [PhPH−BH ] (M ∼ 5000). Similar products were observed
2 n
w
for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 °C). Thermolysis of
3 at 130 °C yielded the six-membered ring [BH −CH −PPh2]2 (5), which presumably results from the dissociation
2
2
of Me2NH‚BH from 3. Thermolysis of 4 at 90 °C afforded Me2NH‚BH and Me3P‚BH , in addition to a product
3
3
3
tentatively assigned as [BH −CH −PMe2]2 (6).
2
2
Introduction
been prepared from the Zr-catalyzed homodehydrocoupling
of bis(phosphine) precursors.3 In addition, tetraphenyldiphos-
phine (Ph2P-PPh2) has been found to result from the
dehydrocoupling of Ph2PH in the presence of a Rh catalyst,4
Thermal and transition-metal-catalyzed dehydrocoupling
of main-group hydrides has recently emerged as a promising
synthetic method for the synthesis of new cyclic and linear
species involving group 13 and group 15 elements.1-11 For
example, phosphorus macrocycles such as (C6H4P2)8 have
(5) Vogel, U.; Timoshkin, A. Y.; Scheer, M. Angew. Chem., Int. Ed. 2001,
40, 4409.
(6) (a) Fazen, P. J.; Remsen, E. E.; Beck, J. S.; Carroll, P. J.; McGhie, A.
R.; Sneddon, L. G. Chem. Mater. 1995, 7, 1942. (b) Fazen, P. J.; Beck,
J. S.; Lynch, A. T.; Remsen, E. E.; Sneddon, L. G. Chem. Mater.
1990, 2, 96.
* Author to whom correspondence should be addressed. Fax: (+1) 416-
978-2450. E-mail: imanners@chem.utoronto.ca.
(1) For a general review of catalytic dehydrocoupling routes to main group
species, see: Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet.
Chem. 1998, 42, 363.
(7) (a) Dorn, H.; Singh, R. A.; Massey, J. A.; Lough, A. J.; Manners, I.
Angew. Chem., Int. Ed. 1999, 38, 3321. (b) Dorn, H.; Singh, R. A.;
Massey, J. A.; Nelson, J. M.; Jaska, C. A.; Lough, A. J.; Manners, I.
J. Am. Chem. Soc. 2000, 122, 6669. (c) Dorn, H.; Vejzovic, E.; Lough,
A. J.; Manners, I. Inorg. Chem. 2001, 40, 4327. (d) Dorn, H.; Rodezno,
J. M.; Brunnho¨ffer, B.; Rivard, E.; Massey, J. A.; Manners, I.
Macromolecules 2003, 36, 291. (e) Jaska, C. A.; Dorn, H.; Lough, A.
J.; Manners, I. Chem. Eur. J. 2003, 9, 271.
(8) For a recent report of B(C6F5)3 catalyzed formation of P-B bonds
via dehydrocoupling, see: Denis, J.-M.; Forintos, H.; Szelke, H.;
Toupet, L.; Pham, T.-N.; Madec, P.-J.; Gaumont, A.-C. Chem.
Commun. 2003, 54.
(2) For metal-catalyzed dehydrocoupling routes involving group 13 or
group 15 elements, see the following. B-Si: (a) Jiang, Q.; Carroll,
P. J.; Berry, D. H. Organometallics 1993, 12, 177. Si-P: (b) Shu,
R.; Hao, L.; Harrod, J. F.; Woo, H. G.; Samuel, E. J. Am. Chem. Soc.
1998, 120, 12988. Si-N: (c) He, J.; Lui, H. Q.; Harrod, J. F.; Hynes,
R. Organometallics 1994, 13, 336. (d) Lui, H. Q.; Harrod, J. F.
Organometallics 1992, 11, 822. B-C: (e) Coapes, R. B.; Souza, F.
E. S.; Thomas, R. L.; Hall, J. J.; Marder, T. B. Chem. Commun. 2003,
614.
(3) (a) Hoskin, A. J.; Stephan, D. W. Angew. Chem., Int. Ed. 2001, 40,
1865. (b) Etkin, N.; Fermin, M. C.; Stephan, D. W. J. Am. Chem.
Soc. 1997, 119, 2954.
(4) Bo¨hm, V. P. W.; Brookhart, M. Angew. Chem., Int. Ed. 2001, 40,
4694.
(9) Jaska, C. A.; Bartole-Scott, A.; Manners, I. Dalton Trans. 2003, 4015.
(10) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun.
2001, 962.
(11) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2003, 125, 9424.
1090 Inorganic Chemistry, Vol. 43, No. 3, 2004
10.1021/ic034938f CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/08/2004

Linear Hybrid Aminoborane/Phosphinoborane Chains
and the thermal dehydrocoupling of P-H and Al/Ga-H
bonds has been employed to yield donor stabilized phos-
phanylalane and -gallane species.5 Thermal heterodehydro-
coupling of borazine has also been shown to afford poly-
borazylene, which is a useful polymeric precursor for the
synthesis of boron nitride ceramics.6 Our group has recently
reported that the catalytic dehydrocoupling of secondary
phosphine-borane adducts yields either linear or cyclic
oligophosphinoboranes, while the primary adducts give high
molecular weight polyphosphinoboranes of the type [RPH-
and would be expected to influence the thermal or catalytic
dehydrocoupling reactivity. Correlation between proton-
hydride bonding and the observed dehydrocoupling chemistry
is of clear interest and will also be discussed.
Results and Discussion
Synthesis and Characterization of Me2NH-BH2-
PPhR-BH3 (1, R ) Ph; 2, R ) H). Recently, we have
found that reaction of R2NH‚BH2Cl with Li[NR2‚BH3] results
in the linear adducts R2NH-BH2-NR2-BH3 (R ) Me; R
) 1,4-C4H8) in moderate yields.11 This appears to be a
general method for the synthesis of linear adducts with
various N-substituents, particularly on the internal nitrogen
atom. When the same method was applied but a lithiated
phosphine-borane adduct was used, the linear hybrid adducts
Me2NH-BH2-RPPh-BH3 (1, R ) Ph; 2, R ) H) were
synthesized (eq 1). For example, the treatment of Ph2PH‚
BH3 with nBuLi gave Li[PPh2‚BH3], which was then reacted
with Me2NH‚BH2Cl to give 1 in 48% yield. Compound 1
was fully characterized by 1H, 11B, 13C, and 31P NMR, mass
spectrometry, infrared spectroscopy, elemental analysis, and
also single-crystal X-ray diffraction. The 11B NMR spectrum
of 1 shows two resonances at δ -11.3 and -39.9 ppm, which
were assigned to the internal (N-BH2-P) and terminal (P-
BH3) boron atoms, respectively. These chemical shifts agree
with those found for R3N-BH2-PMe2-BH3, which vary
between δ -6.2 and -11.2 ppm for the BH2 group, and δ
-35.2 and -37.6 ppm for the BH3 group.12 The 31P NMR
spectrum displays one resonance at δ -19.9 ppm, which is
indicative of a PPh2 unit between two boron atoms.7a-c
i
BH2]n (R ) Ph, Bu, etc.).7-9 We have also shown that
secondary amine-borane adducts undergo dehydrocoupling
at 25 °C in the presence of Rh precatalysts to yield the cyclic
dimers [R2N-BH2]2, while reactions with primary adducts
or NH3‚BH3 yield borazine [RN-BH]3 derivatives.9-11
During the course of our work on amine-boranes, we
discovered that the catalytic dehydrocoupling of the linear
adducts R2NH-BH2-NR2-BH3 yields the cyclic [R2N-
BH2]2.11 This suggested that if the dehydrocoupling of
analogous linear species could be induced by either thermal
or catalytic methods, the formation of novel heterocyclic or
polymeric products would result. In the early 1970s, Keller
and co-workers reported the synthesis of the hybrid adducts
R3N-BH2-PMe2-BH3 (R ) H, Me) from the reaction of
the appropriate amine R3N with the salt Li[BH3-PMe2-
BH3].12 High temperature (150-240 °C) thermolysis of these
compounds resulted in the formation of [Me2P-BH2]3 in all
cases, and the appropriate amine-borane adduct R3N‚BH3
or their thermal decomposition products (e.g. [Me2N-BH2]2,
[MeN-BH]3, [HN-BH]3). However, the compounds were
not structurally characterized, and lower temperature (e.g.
<150 °C) thermal and catalytic dehydrocoupling reactions
were not investigated. In this paper, we present the synthesis
and characterization of a series of new linear hybrid adducts
with N-B-P-B (1 and 2) and N-B-C-P-B (3 and 4)
frameworks, as well as an exploration of their thermal and
catalytic dehydrocoupling chemistry.
Further characterization of 1 was performed using X-ray
crystallography, and this species represents the first example
of a structurally characterized, linear hybrid adduct with a
N-B-P-B backbone. The molecular structure of 1 is shown
in Figure 1a, while a list of the selected bond lengths and
angles can be found in Table 1. Bond lengths of 1.598(2),
1.964(2), and 1.935(2) Å were found for the terminal N-B,
internal B-P, and terminal P-B bonds, respectively. A
similar arrangement of longer internal/shorter terminal B-P
bonds was found in the crystal structures of other linear
phosphine-borane adducts.7a-c,19 The angles around the four-
coordinate nitrogen, boron, and phosphorus atoms were all
found to be approximately tetrahedral. An interesting feature
is the association of the chains in the solid state. Proton-
hydride bonds (Hδ+‚‚‚δ-H) between N-H and B-H hydro-
As a further consideration, the presence of unconventional
proton-hydride13 bonds (Hδ+‚‚‚δ-H) have been detected in
the solid state structures of the linear amine-borane adducts
R2NH-BH2-NR2-BH3.11,14 This unique type of hydrogen
bonding between protic and hydridic hydrogen atoms has
been found to be highly directional in nature and moderately
strong (13-29 kJ mol-1) and can exert an influence on solid
state physical and chemical properties.15-18 Such proton-
hydride interactions would be anticipated in species 1-4,
(12) Schwartz, L. D.; Keller, P. C. Inorg. Chem. 1972, 11, 1931.
(13) Hδ+‚‚‚δ-H interactions have also been described as dihydrogen bonds.
(14) No¨th, H.; Thomas, S. Eur. J. Inorg. Chem. 1999, 1373.
(15) For some recent reviews, see: (a) Desiraju, G. R. Acc. Chem. Res.
2002, 35, 565. (b) Custelcean, R.; Jackson, J. E. Chem. ReV. 2001,
101, 1963.
(16) (a) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson,
T. B.; Crabtree, R. H. J. Am. Chem. Soc. 1999, 121, 6337. (b) Crabtree,
R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle,
T. F. Acc. Chem. Res. 1996, 29, 348.
(17) For NMR studies on H‚‚‚H interactions, see: (a) Gu¨izado-Rodr´ıguez,
M.; Ariza-Castolo, A.; Merino, G.; Vela, A.; No¨th, H.; Bakhmutov,
V. I.; Contreras, R. J. Am. Chem. Soc. 2001, 123, 9144. (b) Gu¨izado-
Rodr´ıguez, M.; Flores-Parra, A.; Sa´nchez-Ruiz, S. A.; Tapia-Bena-
vides, R.; Contreras, R.; Bakhmutov, V. I. Inorg. Chem. 2001, 40,
3243.
(18) For example, cyclotrigallazane [H2Ga-NH2]3 possesses N-Hδ+‚‚‚
δ-H-Ga interactions in the solid state, which leads to the formation
of cubic GaN upon pyrolysis: (a) Campbell, J. P.; Hwang, J. W.;
Young, V. G., Jr.; Von Dreele, R. B.; Cramer, C. J.; Gladfelter, W.
L. J. Am. Chem. Soc. 1998, 120, 521. (b) Jegier, J. A.; McKernan, S.;
Purdy, A. P.; Gladfelter, W. L. Chem. Mater. 2000, 12, 1003.
(19) Greenwood, N. N.; Kennedy, J. D.; McDonald, W. S. J. Chem. Soc.,
Dalton Trans. 1978, 40.
Inorganic Chemistry, Vol. 43, No. 3, 2004 1091

Jaska et al.
14
observed in the structures of Me2NH-BH2-NMe2-BH3
and (1,4-C4H8)NH-BH2-N(1,4-C4H8)-BH3.11
For compound 2, which was synthesized in 83% yield,
similar chemical shifts were observed in the 11B (δ -12.8
and -41.5 ppm) and 31P (δ -54.8 ppm) NMR spectra. In
addition, further evidence for a P-H bond was found in the
1H-coupled 31P NMR spectrum, in which the resonance at δ
-54.8 ppm was found to split into a doublet (JPH ) 344
1
Hz). In the H NMR spectrum, the P-H resonance occurs
as a doublet of sextets resulting from coupling to one
phosphorus nucleus and five adjacent protons on boron (JHH
) 6 Hz). The crystal structure of 2 was also of interest as
N-H‚‚‚H-B and P-H‚‚‚H-B contacts may be present
simultaneously in the solid state, leading to a highly
aggregated, proton-hydride bonded network. A low resolu-
tion X-ray structure was obtained which showed that the
packing of the molecules did not allow for a favorable
alignment for any P-H‚‚‚H-B or N-H‚‚‚H-B proton-
hydride interactions. However, numerous attempts to obtain
a more accurate X-ray analysis were hampered by the
formation of low quality crystals.
Attempted Thermal and Catalytic Dehydrocoupling of
1 and 2. Previous work has shown that thermolysis of the
linear hybrid adducts R3N-BH2-PMe2-BH3 (R ) H or Me)
at temperatures of 150-240 °C results in skeletal bond
cleavage (vide supra).12 We found that thermolysis of 1 at
130 °C for 20 h also resulted in backbone cleavage, with
the formation of a complex mixture of products (eq 2a).
Ph2PH (trace), Ph2PH-BH2-PPh2-BH3 (ca. 15%), Me2NH‚
BH3 (ca. 10%), [Me2N-BH2]2 (ca. 40%), Ph2PH‚BH3 (ca.
35%), and unreacted 1 were all identified by their respective
11B and 31P NMR shifts. In contrast, we found that the
thermolysis of 2 was complete in only 2 h at 130 °C, resulting
solely in the formation of [Me2N-BH2]2 and the polymer
[PhPH-BH2]n (eq 2c), which was isolated in quantitative
yield after precipitation into pentane. The polymer [PhPH-
BH2]n was characterized by 1H, 11B, and 31P NMR spectros-
copy, which gave spectra identical with those of samples
previously synthesized in our group by the Rh-catalyzed
dehydrocoupling of PhPH2‚BH3 at 90-130 °C.7a,b,d Dynamic
light scattering (DLS) on a solution of [PhPH-BH2]n in THF
resulted in a bimodal size distribution with peak values of
Dh ≈ 1.8 and 163 nm. As previously observed, the two peaks
may arise from the presence of single polymer chains (Dh
≈ 1.8 nm) and large polymer aggregates (Dh ≈ 163 nm).7b
Indeed, after 4 days only a single size distribution was
observed with Dh ≈ 1.4 nm, indicating that the larger
aggregates had dissociated in a manner similar to that noted
previously.7b The small Dh values of 1-2 nm suggest that
low molecular weight polymer is formed. Previous work on
[PhPH-BH2]n has shown that samples with a Dh of 1 nm
correspond to an absolute weight-average molecular weight
(Mw) value of ca. 5000 by static light scattering (SLS).7b We
hoped that by utilizing a transition-metal catalyst, dehydro-
coupling of 1 might proceed at lower temperatures to yield
cyclic or polymeric [Me2N-BH2-RPPh-BH2]n species.
However, when 1 was treated with 1.5 mol % of [Rh(1,5-
cod)(µ-Cl)]2 (1,5-cod ) 1,5-cyclooctadiene), chain cleavage
Figure 1. Molecular structure of 1. All hydrogen atoms bonded to carbon
are omitted for clarity. Selected bond lengths and angles are presented in
Table 1.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1 and [1]2
N(1)-B(1)
B(1)-P(1)
P(1)-B(2)
N(1)-H(1N)
N(1)-C(1)
N(1)-C(2)
P(1)-C(3)
P(1)-C(9)
1.598(2)
1.964(2)
1.935(2)
0.81(2)
1.493(2)
1.471(2)
1.828(1)
1.821(1)
N(1)-B(1)-P(1)
B(1)-P(1)-B(2)
C(1)-N(1)-C(2)
C(1)-N(1)-B(1)
C(2)-N(1)-B(1)
B(1)-P(1)-C(9)
B(1)-P(1)-C(3)
B(2)-P(1)-C(9)
B(2)-P(1)-C(3)
110.25(9)
115.19(7)
110.6(1)
109.8(1)
114.3(1)
107.48(6)
109.49(7)
110.16(7)
111.15(7)
H(1N)-H(3*)
H(1N)-H(4*)
H(1N)-H(5*)
2.24a
2.06a
2.99a
N(1*)-H(1N)-H(3)
N(1*)-H(1N)-H(4)
B(2)-H(4)-H(1N*)
B(2)-H(3)-H(1N*)
131.3a
159.2a
85.4a
94.0a
a Using the normalized bond distances of d(N-H) ) 1.03 Å and d(B-
H) ) 1.21 Å.
gen atoms link two molecules together to form a dimeric
structure as shown in Figure 1b. Normalizing the N-H and
B-H bond distances to 1.03 and 1.21 Å,16a respectively, two
short interaction distances of 2.24 and 2.06 Å were found.
A third distance of 2.99 Å was found but is well beyond the
sum of the van der Waals radii for two hydrogen atoms (2.40
Å).20 Linear N-H‚‚‚H angles of 131.3° and 159.2° and bent
B-H‚‚‚H angles of 85.4° and 94.0° were found and are
consistent with other N-H‚‚‚H-B proton-hydride bonded
structures.11,14,16a Proton-hydride bonding interactions to
yield dimeric species in the solid state have also been
(20) Bondi, A. J. Phys. Chem. 1964, 68, 441.
1092 Inorganic Chemistry, Vol. 43, No. 3, 2004

Linear Hybrid Aminoborane/Phosphinoborane Chains
products were again observed (eq 2b). Thus, after 24 h at
50 °C, signals associated with Me2NH‚BH3 (ca. 5%),
[Me2N-BH2]2 (ca. 45%), and Ph2PH‚BH3 (ca. 50%) as well
as unreacted 1 were observed in the 11B and 31P NMR
spectra. After 48 h, an additional resonance due to free Ph2PH
(trace) was observed in the 31P NMR. Compound 1 was
found to be completely consumed after 10 days. In contrast,
under the same conditions in the absence of catalyst, only
35% conversion of 1 was detected. Similarly, when 2 was
treated with 1.3 mol % of [Rh(1,5-cod)(µ-Cl)]2 at 50 °C,
complete conversion to Me2NH‚BH3 (ca. 10%), PhPH2
(trace), [Me2N-BH2]2 (ca. 40%), and PhPH2‚BH3 (ca. 50%)
was detected after 10 days (eq 2d). In contrast, under the
same conditions in the absence of catalyst, only 10%
conversion of 2 was detected.
Figure 2. Molecular structure of 3. All hydrogen atoms bonded to carbon
in the phenyl rings are omitted for clarity. Selected bond lengths and angles
are presented in Table 2.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 3
N(1)-B(1)
B(1)-C(1)
C(1)-P(1)
P(1)-B(2)
N(1)-H(1N)
B(2)-H(5)
H(1N)-H(5)
H(1N)-H(5)
1.619(2)
1.634(2)
1.801(2)
1.928(2)
0.85(2)
1.12(2)
2.13(2)
1.95a
N(1)-B(1)-C(1)
B(1)-C(1)-P(1)
C(1)-P(1)-B(2)
N(1)-H(1N)-H(5)
N(1)-H(1N)-H(5)
B(2)-H(5)-H(1N)
B(2)-H(5)-H(1N)
112.4(1)
114.6(1)
113.40(8)
151.8(2)
149.5a
105.0(1)
102.9a
a Using the normalized bond distances of d(N-H) ) 1.03 Å and d(B-H)
) 1.21 Å.
spectra of 3 and 4 also show the presence of a broad
resonance due to the internal CH2 group at δ 1.31 and 0.54
ppm, respectively. Further characterization of 3 was per-
Synthesis and Characterization of Me2NH-BH2-CH2-
PR2-BH3 (3, R ) Ph; 4, R ) Me). The incorporation of a
spacer unit into the skeleton of compounds 1 and 2 may
prevent chain cleavage and might potentially allow the
formation of larger cyclic species or polymeric products via
dehydrocoupling. A methylene group can be easily incor-
porated through the use of a tertiary phosphine-borane
adduct containing a methyl group MePR2‚BH3 (R ) Ph, Me).
Treatment with nBuLi affords Li[CH2-PR2‚BH3], which was
then reacted with Me2NH‚BH2Cl to yield Me2NH-BH2-
CH2-PR2-BH3 (3, R ) Ph; 4, R ) Me) in yields of 20%
and 32%, respectively (eq 3). The low yields result from a
side reaction between the N-H proton of Me2NH‚BH2Cl
and Li[CH2-PR2‚BH3] regenerating MePR2‚BH3 and form-
ing Li[NMe2‚BH2Cl]. The latter product then eliminates LiCl
to form [Me2N-BH2]2, which was frequently detected in the
reaction mixture. The 31P NMR spectra of 3 and 4 showed
broad quartets and at δ 16.6 (JPB ) 79 Hz) and 3.1 ppm
(JPB ) 70 Hz), respectively. The 11B NMR spectrum of 3
showed two resonances for the BH2 and BH3 groups at δ
-11.7 and -40.2 ppm, respectively. Similar 11B shifts were
observed for 4, but with resolved B-H couplings; a triplet
at δ -9.9 (JBH ) 98 Hz) and a quartet of doublets at δ -40.2
ppm (JBP ) 70 Hz, JBH ) 94 Hz). The most interesting
feature of the 13C NMR spectra arises from the resonance
associated with the internal CH2 group, which gave broad
signals at δ 10.9 and 14.2 ppm for 3 and 4, respectively,
due to the nearby quadrupolar boron nucleus. The 1H NMR
formed using X-ray crystallography, the molecular structure
of which is shown in Figure 2. A list of the selected bond
lengths and angles for 3 can be found in Table 2. The
terminal N-B and P-B adduct bonds were determined to
be 1.619(2) and 1.928(2) Å, respectively, and are typical of
N-B and P-B adduct bond distances. The internal B-C
and C-P distances were found to be 1.634(2) and 1.801(2)
Å, respectively, which are also normal distances for these
types of bonds. One noteworthy feature of interest was the
presence of an intramolecular proton-hydride interaction
between the terminal N-H and B-H hydrogen atoms,
effectively giving a seven-membered heterocycle. Using
normalized B-H and N-H bond distances (vide supra), a
H‚‚‚H contact distance of 1.95Å was found. This is much
shorter than the H‚‚‚H distances found in 1 (2.06 and 2.24
Å), in which the bonding interaction is intermolecular in
nature. This distance is also shorter than the shortest H‚‚‚H
distance (2.02(3) Å) found in the neutron diffraction study
of NH3‚BH3.16a
Attempted Thermal and Catalytic Dehydrocoupling of
3 and 4. Thermolysis of 3 at 130 °C for 6 h was found to
result in solidification of the initially liquid reactant. The
NMR spectra of the reaction mixture indicated partial
Inorganic Chemistry, Vol. 43, No. 3, 2004 1093

Jaska et al.
conversion to a new product 5, with the presence of new
resonances at δ 7.7 (31P) and -33.6 ppm (11B). The 11B NMR
spectrum also showed resonances at δ 3.5 and -13.4 ppm
which were assigned to [Me2N-BH2]2 and Me2NH‚BH3,
respectively. Further heating of the solid for 24 h was found
to result in full conversion to the six-membered cyclic dimer
[BH2-CH2-PPh2]2 (5) in 98% yield (eq 4a). The 13C NMR
of 5 showed a broad resonance at δ 6.3 ppm attributed to
the internal CH2 group. The mass spectrum of 5 was also
indicative of a dimeric species, with the parent ion for both
the dimer and monomer occurring at m/z ) 424 (10%) and
212 (6%), respectively. Heating of 4 at 90 °C in the absence
Figure 3. Molecular structure of 5. All hydrogen atoms bonded to carbon
in the phenyl rings are omitted for clarity. Selected bond lengths and angles
are presented in Table 3.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 5
P(1)-B(1)
P(1)-C(1)
P(1)-C(2)
P(1)-C(8)
B(1)-C(1)*
1.937(2)
1.800(2)
1.816(2)
1.815(2)
1.645(3)
C(1)-P(1)-B(1)
C(1)-P(1)-C(2)
C(1)-P(1)-C(8)
B(1)-P(1)-C(2)
B(1)-P(1)-C(8)
P(1)-B(1)-C(1)*
P(1)-C(1)-B(1)*
110.4(1)
108.28(9)
108.3(1)
111.5(1)
113.2(1)
109.4(2)
112.1(1)
of solvent for 3 days was found to result in mainly
sublimation of the volatile starting material. However, H,
1
11B, and 31P NMR spectra of the reaction mixture indicated
partial conversion (ca. 50%) of 4 to Me2NH‚BH3, Me3P‚
BH3, and a product that may be the cyclic dimer [BH2-
CH2-PMe2]2 (6). Evidence for the formation of 6 was
boron. There are three previous examples describing the
synthesis of six-membered BCP rings,21 with [EtBR-CH2-
RPPh]2 (R ) 1,3-EtCdCMe-SiMe2) being the lone example
which has been characterized by X-ray crystallography to
date.21a Interestingly, the synthesis of [BH2-CHPh-PHMes*]2
(Mes* ) 2,4,6-tri-tert-butylphenyl) is facilitated via dis-
sociation of BH3 and THF from the linear adduct BH3-
PHMes*-CHPh-BH2-THF.21c
Evaluation of Solid State Intra- or Intermolecular
Proton-Hydride Interactions in 1-4 by Infrared Spec-
troscopy. To confirm the presence of proton-hydride
bonding interactions in 1-4, solid state and solution IR
spectroscopy was used. It has been acknowledged that the
presence of hydrogen bonding in molecules can be inferred
from the appearance of broadened, lower frequency stretch-
ing absorptions when compared with non-H-bonded systems.
In particular, amine-borane adducts have been shown to
participate in proton-hydride bonding through either self-
association11,14,16a or the presence of protic H atom sources
(e.g. O-H or N-H).22-25 In these cases, the hydrogen bond
acceptor becomes the hydridic H atoms attached to boron
(or the B-H bond itself),26 despite the lack of a lone pair of
1
obtained from the H (δ 1.52 (d), Me), 11B (δ -30.5 (dt)),
13C (δ 10.6 (d), Me), and 31P NMR (δ 6.0 (q)) spectra. This
assignment is tentative, as the anticipated broad 1H and 13
C
resonances for the CH2 group were not observed due to
overlapping signals. Attempts to isolate 6 from the reaction
mixture by fractional sublimation were unsuccessful. Heating
of 4 at 130 °C in solution was found to result in complete
conversion to [Me2N-BH2]2 and Me3P‚BH3. The presence
of Me3P‚BH3 in the thermolysis reaction of 4 suggests that
a different chain cleavage pathway is operating compared
to that of 3, as MePPh2‚BH3 was not observed in the latter
case.
Catalytic dehydrocoupling trials performed on both 3 and
4 with [Rh(1,5-cod)(µ-Cl)]2 (1-5 mol %) at either 25 or 50
°C were found to result in no observable reaction. Upon
heating to 90 °C, 3 and 4 were completely consumed after
20 h. The only products that could be identified by NMR
were MePPh2‚BH3 (for 3) and Me3P‚BH3 (for 4). Blank
reactions for both 3 and 4 at 90 °C in the absence of catalyst
were found to result in no significant conversion after 20 h.
The cyclic 5 was further characterized by X-ray crystal-
lography, the molecular structure of which is shown in Figure
3. A list of the selected bond lengths and angles for 5 can
be found in Table 3. The six-membered ring adopts a chair
conformation, with typical B-P, P-C, and B-C bond
lengths of 1.937(2), 1.800(2), and 1.645(3)Å, respectively.
The angles around the four-coordinate P and B atoms are
all approximately tetrahedral, ranging from 104.89(9)° to
113.2(1)° for phosphorus and 102.8(1)° to 113.5(1)° for
(21) (a) Ko¨ster, R.; Seidel, G.; Mu¨ller, G.; Boese, R.; Wrackmeyer, B.
Chem. Ber. 1988, 121, 1381. (b) Ionkin, A. S.; Ignate´va, S. N.;
Nekhoroshkov, V. M.; Efremov, J. U. J. A.; Arbuzov, B. A.
Phosphorus, Sulfur Silicon Relat. Elem. 1990, 53, 1. (c) Yoshifuji,
M.; Takahashi, H.; Toyota, K. Heteroatom Chem. 1999, 10, 187.
(22) Brown, M. P.; Heseltine, R. W. J. Chem. Soc., Chem. Commun. 1968,
1551.
(23) Brown, M. P.; Heseltine, R. W.; Smith, P. A.; Walker, P. J. J. Chem.
Soc. A 1970, 410.
(24) Burg, A. B. Inorg. Chem. 1964, 3, 1325.
(25) Patwari, G. N.; Ebata, T.; Mikami, N. J. Phys. Chem. A 2001, 105,
8642.
1094 Inorganic Chemistry, Vol. 43, No. 3, 2004

Linear Hybrid Aminoborane/Phosphinoborane Chains
Table 4. Selected IR Stretching Frequencies for Compounds 1-4 a
These ∆ν values are reasonable as the solid state structure
of NH3‚BH3 has been shown to be highly associated, with
the presence of many long and short-range H‚‚‚H inter-
actions.16a,30
In the case of 1, the solid state IR spectrum shows
absorptions at 3178 cm-1 due to νNH, and at 2337 and 2260
cm-1 due to νBH (asym and sym) vibrations, respectively.
Compared with unassociated Me2NH‚BH3 in CCl4, there is
a large frequency shift of ∆ν ) 122 cm-1 in the value of
compound
ν(NH)
ν(BH)asym
ν(BH)sym
Me3N‚BH3
2382
2372
2366
2263
2260
2265
Me2NH‚BH3
3204
3180
3300
3317
3386
Me2NH‚BH3 (pure liquid)b
Me2NH‚BH3 (0.0021 M in CCl4)c
d
NH3‚BH3
NH3‚BH3
2329
2427
2281
2340
e
1
3178
3224
2337
2376
2260
2279
1 (0.005 M)f
ν
NH. When compared with Me3N‚BH3, ∆ν values of 45 and
2
3178
3232
2342
2391
2267
2271
3 cm-1 are obtained for νBH (asym and sym, respectively).
It has been noted that the asymmetric B-H stretch is affected
by H-bonding to a greater extent than the symmetric stretch,
resulting in a larger ∆ν value.23 When the solution IR
spectrum of 1 was obtained in CH2Cl2 (0.005 M), there was
an observed increase in the frequencies of the stretching
absorptions to 3224 cm-1 for νNH, and to 2376 and 2279
cm-1 for νBH (asym and sym), respectively. This results in
∆ν values between the solid and solutions samples of 46,
39, and 19 cm-1, respectively. Thus, in dilute solution the
intermolecular proton-hydride interactions in 1 are disrupted,
as the stretching frequencies approach values for that of the
unassociated molecules. For 2, similar ∆ν values of 54 cm-1
for νNH, and 49 and 4 cm-1 for νBH (asym and sym),
respectively, between the solid state and solution IR spectra
were obtained, indicating the presence of proton-hydride
interactions only in the solid state.
2 (0.005 M)f
3
3215
3209
2370
2380
2282
2284
3 (0.005 M)f
4
3200
3204
2345
2363
2286
2285
4 (0.005 M)f
a Spectra were obtained as Nujol mulls unless otherwise specified. ν
reported in cm-1 b Reference 24. c Reference 22. d Spectra obtained as a
.
pressed KBr pellet. e Spectra obtained as isolated NH3‚BH3 in an argon
matrix on a CsI window at liquid hydrogen temperature. Reference 28.
f Solution spectra were obtained in dry CH2Cl2.
electrons or a π system. However, to establish these proton-
hydride interactions, values for the non-H-bonding N-H and
B-H stretching absorption were required. This was ac-
complished by obtaining the νNH and νBH stretching absorp-
tions for the amine-borane adducts Me3N‚BH3, Me2NH‚
BH3, and NH3‚BH3, and also 1-4 (Table 4). For Me3N‚BH3,
which lacks νNH, non-H-bonded values of νBH were observed
at 2382 and 2263 cm-1, for the asymmetric (asym) and
symmetric (sym) modes, respectively. For Me2NH‚BH3, in
which intermolecular N-H‚‚‚H-B interactions are present
in the solid state, values of 3204 cm-1 and of 2372 and 2260
cm-1 were found for νNH and νBH (asym and sym),
respectively. Brown has reported a value of 3300 cm-1 for
the non-H-bonded νNH for Me2NH‚BH3 in dilute CCl4
(0.0021 M).22 Thus, there is a frequency shift of ∆ν ) 96
cm-1 for νNH in H-bonded and non-H-bonded samples of
Me2NH‚BH3.27 There appears to be little difference in the
values of νBH between Me3N‚BH3 and Me2NH‚BH3, with
∆ν values of 10 (asym) and 3 cm-1 (sym). The proton-
hydride interactions in Me2NH‚BH3 are also preserved in
the liquid state, with values of 3180 cm-1 for νNH, and 2366
and 2265 cm-1 for νBH (asym and sym), respectively.24 To
confirm that νBH does shift to lower frequency upon
association via dihydrogen bonding, the spectrum of NH3‚
BH3 was obtained as a KBr disk yielding values of 2329
and 2281 cm-1.28 This was compared with that of an
unassociated sample obtained in a rare gas matrix, which
yielded values of 2427 and 2340 cm-1.29 Thus, there is a
large frequency shift of ∆ν ) 98 and 59 cm-1 for νBH on
going from unassociated to associated samples of NH3‚BH3.
In the case of 3, the solid state IR spectrum shows
absorptions at 3215 cm-1 for νNH, and at 2370 and 2282
cm-1 for νBH (asym and sym), respectively. Again, there
is a large ∆ν value of 85 cm-1 for νNH when compared to
the value of the free N-H stretch in Me2NH‚BH3. However,
a small ∆ν value of 12 cm-1 results for the asymmetric
νBH, while the symmetric νBH shows an increase in frequency
(∆ν ) -19 cm-1) when compared to the values obtained
for Me3N‚BH3. When the solution IR spectrum was obtained
in CH2Cl2 (0.005 M), only minor frequency shifts are
observed. In fact, the frequency of νNH decreases to 3209
cm-1, resulting in a ∆ν value of -6 cm-1, while the
frequency of νBH increases to 2380 and 2284 cm-1, resulting
in ∆ν values of 10 and 2 cm-1 for the asymmetric and
symmetric modes, respectively. For 4, similar minor shifts
in the stretching frequencies (∆ν ) 4 cm-1 for νNH, 18 and
-1 cm-1 for νBH (asym and sym), respectively) were also
observed between the solid state and solution samples. One
explanation for the small changes in ν from solid state to
solution may be due to the type of proton-hydride interaction
present in 3 and 4 compared to 1 and 2. In 1 and 2,
intermolecular proton-hydride bonding is present between
molecules, which can easily be disrupted by high dilution.
However, in 3 and 4, intramolecular proton-hydride bonding
is present within the same molecule. High dilution would
have little effect as the N-H and B-H bonds are still in
close proximity to one another and the proton-hydride
bonding would be unaffected.
In addition, a large shift of ∆ν ) 69 cm-1 results for νNH
.
(26) Li, J.; Zhao, F.; Jing, F. J. Chem. Phys. 2002, 116, 25.
(27) The frequency shift (∆ν) can be defined as ∆ν ) ν(XH)free
-
ν(XH)H-bonded where X ) N or B.
(28) A reviewer suggested the possible formation of K[BH3Br] from KBr
and NH3‚BH3. However, evidence for a N-B stretch was observed
at ca. 950 cm-1 in the IR spectrum. In addition, solid state CP-MAS
11B NMR on samples of NH3‚BH3 and KBr/NH3‚BH3 (from the KBr
pellet) showed identical resonances at δ ) -35 ppm.
(30) Additional evidence for Hδ+‚‚‚δ-H interactions in NH3‚BH3 has been
provided by Raman spectroscopy: Trudel, S.; Gilson, D. F. R. Inorg.
Chem. 2003, 42, 2814.
(29) Smith, J.; Seshadri, K. S.; White, D. J. Mol. Spectrosc. 1973, 45, 327.
Inorganic Chemistry, Vol. 43, No. 3, 2004 1095

Jaska et al.
The frequency shifts (∆ν) can be used to estimate the
hydrogen bond strengths of 1-4 in the solid state by using
an empirical equation proposed by Iogansen (eq 5),31 which
correlates ∆H° with the change in the stretching absorption
upon hydrogen-bond formation. For 1 and 2, values of ∆ν
) 122 cm-1 result in proton-hydride bond strengths of ∆H°
) -2.61 kcal mol-1 (-10.9 kJ mol-1). Compounds 3 and 4
(∆ν ) 85 and 100 cm-1, respectively) yield values of -1.9
and -2.20 kcal mol-1 (-7.9 and -9.20 kJ mol-1) for ∆H°,
respectively. These hydrogen bond strengths are on the lower
end of the range of values (3-7 kcal mol-1; 12.6-29.3 kJ
mol-1) determined for unconventional Hδ+‚‚‚δ-H hydrogen
bonds, and are thus considered to be weak interactions.
ends may effectively prevent dissociation at lower temper-
atures, and also help to sterically shield the active sites from
the catalyst, thereby preventing chain cleavage. At higher
temperatures (>90°C), the proton-hydride interactions may
be disrupted, enabling dissociation and/or catalytic chain
cleavage to occur.
Summary
Novel examples of linear hybrid aminoborane/phosphino-
borane species have been synthesized and structurally
characterized by X-ray crystallography. The compounds 1
and 2 possess intermolecular proton-hydride interactions in
the solid state, as indicated by both single-crystal X-ray
diffraction and IR studies, which are disrupted in solution.
Compounds 3 and 4 possess intramolecular proton-hydride
interactions in the solid state, which are preserved even in
dilute solution as indicated by negligible IR frequency shift
(∆ν) values for the N-H and B-H stretching absorptions
between solid state and solution samples. The hydrogen bond
strengths of 1-4 have been estimated to be in the range of
7.9 to 10.9 kJ mol-1, which is indicative of weak interactions.
The thermolysis of 2 and 3 yields, for example, low
molecular weight poly(phenylphosphinoborane) [PPhH-
BH2]n (Mw ) ca. 5000) and the six-membered ring 5 as chain
cleavage products, respectively. Rather than catalytic dehy-
drocoupling, catalytic chain cleavage reactions were observed
for 1-4 in the presence of [Rh(1,5-cod)(µ-Cl)]2, yielding
mixtures of amine-boranes, aminoboranes, and phosphine-
boranes. Interestingly, these results suggest that transition
metals may be capable of inducing the cleaVage of B-N
and B-P bonds, and this possibility will be the subject of
more detailed studies.
-18∆ν
∆ν + 720
∆H° )
(5)
Mechanism for the Skeletal Cleavage Reactions for
1-4. Although the thermolysis chemistry of 1-4 led to a
complex range of products, many of these can be explained
by simple mechanistic considerations. The mechanism for
the chain cleavage reactions of 1-4 in the presence or
absence of catalyst may involve the initial dissociation of
Me2NH and BH3 from the termini. The dissociation may
occur simultaneously, or as a two-step process, to yield the
monomers PhPR-BH2 (R ) Ph, H; from 1 and 2) or BH2-
CH2-PR′2 (R′ ) Ph, Me; from 3 and 4) as transient species
(eq 6). Oligomerization of these species would then give,
for example, low molecular weight [PhPH-BH2]n (from 2)
or the cyclic [BH2-CH2-PPh2]2 (from 3). The free Me2NH
and BH3 would be expected to combine to form Me2NH‚
BH3, which has been previously shown to undergo dehy-
drocoupling at 130 °C32 or in the presence of Rh precata-
lysts10,11 to yield [Me2N-BH2]2. In the uncatalyzed chain
cleavage reactions, products such as PhPH2‚BH3 and Ph2PH‚
BH3 cannot be explained by the dissociative mechanism
postulated. These species may form as a result of complex
thermally induced hydrogen transfer reactions. In the case
of the catalytic chain cleavage reactions, the mechanistic
scenario is even more complex as oxidative addition/
reductive elimination reactions at the transition-metal center
may also lead to the formation of the observed products.
Experimental Section
General Procedures and Materials. All reactions and product
manipulations were performed under an atmosphere of dry nitrogen
using standard Schlenk techniques or in a glovebox. All solvents
were dried over the appropriate drying agents such as Na/
benzophenone (toluene, hexanes, THF, Et2O) or CaH2 (CH2Cl2)
and distilled prior to use. BH3‚SMe2, BH3‚THF (1.0 M in THF),
nBuLi (1.6 M in hexane), HCl (1.0 M in Et2O) (Aldrich), Me2NH‚
BH3, Me3P, MePPh2, Ph2PH, and PhPH2 (Strem Chemicals) were
purchased and used as received. Ph2PH‚BH3,7b PhPH2‚BH3,7b Me3P‚
34
BH3,33 and [Rh(1,5-cod)(µ-Cl)]2 were synthesized via literature
procedures.
Equipment. NMR spectra were recorded on either a Varian
Gemini 300 MHz or a Unity 400 MHz (13C) spectrometer. Chemical
shifts are reported relative to residual protonated solvent peaks (1H,
13C) or external BF3‚Et2O (11B) and H3PO4 (31P) standards. Spectra
were obtained at 300 MHz (1H), 96 MHz (11B), 75 or 100 MHz
(13C), and 121 MHz (31P). Infrared spectra were obtained on a
Perkin-Elmer Spectrum One FT-IR spectrometer using KBr win-
dows. Mass spectra were obtained with a VG 70-250S mass
spectrometer operating in electron impact (EI) mode. Melting point
determinations were performed in sealed capillaries and are
uncorrected. Elemental analyses were performed by Quantitative
The lack of catalytic chain cleavage at 50 °C for
compounds 3 and 4, in contrast to the cases of 1 and 2, may
be due to the intramolecular proton-hydride bonding present
in solution. The N-H‚‚‚H-B interactions between the chain
(31) Iogansen, A. V. Hydrogen Bond; Nauka: Moscow, 1981, as cited in:
Beringhelli, T.; D’Alfonso, G.; Panigati, M.; Mercandelli, P.; Sironi,
(33) Schmidbaur, H.; Weiss, E.; Mu¨ller, G. Synth. React. Inorg. Met.-Org.
Chem. 1985, 15, 401.
A. Chem. Eur. J. 2002, 8, 5340. Equation has units of kcal mol-1
.
(32) Burg, A. B.; Randolph, C. L., Jr. J. Am. Chem. Soc. 1949, 71, 3451.
(34) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218.
1096 Inorganic Chemistry, Vol. 43, No. 3, 2004

Linear Hybrid Aminoborane/Phosphinoborane Chains
Table 5. Crystallographic Data and Summary of Data Collection and
Refinement for Structures 1, 3, and 5
overnight. The volatiles were removed in vacuo to give MePPh2‚
BH3 as a colorless oil, which occasionally crystallized upon standing
at 25 °C. Yield: 13.146 g, 90%. Mp: 48-50 °C. 1H NMR
1
3
5
(CDCl3): δ 7.71-7.64 (m, Ph), 7.50-7.45 (m, Ph), 1.88 (d, JHP
)
emp formula
fw
temp (K)
λ (Å)
cryst syst
space group
cryst size (mm)
C14H22B2NP
256.92
150(1)
0.71073
monoclinic
P21/c
C15H24B2NP
270.94
150(1)
0.71073
orthorhombic
Pbca
C13H14BP
424.04
150(1)
0.71073
monoclinic
P21/n
10 Hz, Me), 1.02 (q, br, JBH ) 93 Hz, BH3). 11B{1H} NMR
(CDCl3): δ -38.2 (d, JBP ) 56 Hz). 13C{1H} NMR (75 MHz,
CDCl3): δ 131.9 (d, JCP ) 10 Hz, Ar), 131.4 (d, JCP ) 2 Hz, Ar),
130.7 (d, JCP ) 56 Hz, ipso-C), 129.0 (d, JCP ) 10 Hz, Ar), 12.1
(d, JCP ) 40 Hz, Me). 31P{1H} NMR (CDCl3): δ 11.2 (q, JPB
)
0.30 × 0.30 ×
0.40 × 0.35 ×
0.22 × 0.10 ×
56 Hz). EI-MS (70 eV): 211 (M+ - 3H, 17%), 200 (MePPh2,
100%), 185 (PPh2, 30%).
0.20
0.30
0.08
a (Å)
b (Å)
c (Å)
11.2250(3)
9.3830(3)
15.3070(5)
90
106.4140(13)
90
1546.49(8)
4
1.103
11.9203(2)
9.6239(3)
28.7897(7)
90
90
90
3302.75(14)
8
1.090
0.153
1168
2.81-27.48
0 e h e 15
0 e k e 12
-37 e l e 0
20296
3776
0.042
1.020
0.0405
6.7219(3)
21.27300(10)
8.4366(5)
90
102.033(2)
90
1179.88(9)
2
1.194
Synthesis of Me2NH‚BH2Cl. To a solution of Me2NH‚BH3
(6.570 g, 111.5 mmol) in Et2O (125 mL) cooled to 0 °C was added
a solution of HCl in Et2O (113 mL, 113 mmol) dropwise over 30
min. The mixture was stirred for 1 h at 0 °C, then warmed to 25
°C, and stirred for a further 1 h. The solvent was removed in vacuo
to give a pale yellow oil, which was transferred to the bottom of a
sublimation apparatus. The oil was distilled under dynamic vacuum
(ca. 10-3 mmHg) and condensed onto a -15 °C coldfinger, which
solidified to give Me2NH‚BH2Cl as a white solid. The purified
product was stored at -30 °C in the glovebox to prevent melting.
Yield: 7.663 g, 74%. 1H NMR (CDCl3): δ 4.52 (br, NH), 2.61 (d,
JHH ) 5.8 Hz, Me), 2.56 (q, br, JBH ) 120 Hz, BH2). 11B NMR
(CDCl3): δ -3.4 (t, JBH ) 120 Hz). 13C{1H} NMR (75 MHz,
CDCl3) δ 40.7 (Me).
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
C (g/cm3)
µ (mm-1
F(000)
)
0.160
552
0.195
448
θ range (deg)
index ranges
3.61-27.49
-14 e h e 14
-10 e k e 12
-19 e l e 19
11694
3523
0.0404
1.034
0.0373
2.65-25.09
0 e h e 8
0 e k e 25
-10 e l e 9
8458
2083
0.067
1.056
0.0422
reflns collected
indep reflns
Rint
GOF on F2
Synthesis of Me2NH-BH2-PPh2-BH3 (1). A solution of nBuLi
in hexanes (3.0 mL, 4.8 mmol) was added dropwise to a solution
of Ph2PH‚BH3 (0.967 g, 4.83 mmol) in THF (15 mL) cooled to 0
°C. The reaction mixture was stirred at 0 °C for 30 min, followed
by warming to 25 °C. After 2 h, the reaction mixture showed
complete conversion to Li[PPh2‚BH3]. 11B{1H} NMR (THF): δ
R1a (I > 2σ(I))
wR2b (all data)
peak/hole (e Å-3
0.0982
0.244/-0.319
0.1038
0.252/-0.266
0.1082
0.268/-0.312
)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc ) ]/∑[w(Fo )2]}1/2
.
2 2
2
Technologies Inc., Whitehouse, NJ, or were obtained on a Perkin-
Elmer Series 2400 CHNS analyzer maintained by the Analest
facility at the University of Toronto. Dynamic light scattering was
performed using a wide-angle laser light scattering photometer from
Brookhaven Instruments Corp. See ref 7b for details on sample
preparation and data analysis.
-31.6 (d, JBP ) 45 Hz). 31P{1H} NMR (THF): δ -32.4 (q, JPB
)
45 Hz). The mixture was recooled to 0 °C, and a solution of
Me2NH‚BH2Cl (0.464 g, 4.97 mmol) in THF (10 mL) was added
dropwise over 5 min. The mixture was allowed to warm to 25 °C
and stirred overnight. The solvent was removed in vacuo, and the
resulting solid was dissolved in toluene (50 mL) and filtered to
remove LiCl. The solvent was removed to give 1 as a white powder.
X-ray quality crystals of 1 were obtained from a toluene/hexanes
(10:1) solution cooled to -30 °C. Yield: 0.591 g, 48%. Mp: 121-
X-ray Structural Characterization. Crystallographic data and
the summary of data collection and refinement for structures 1, 3,
and 5 are presented in Table 5. Diffraction data were collected on
a Nonius Kappa-CCD using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å). The data were integrated and scaled
using the Denzo-SMN package.35 The structures were solved and
refined with the SHELXTL-PC V5.1 software package.36 Refine-
ment was by full-matrix least squares on F2 using all data (negative
intensities included). Molecular structures are presented with
thermal ellipsoids at a 30% probability level. In all structures,
hydrogen atoms bonded to carbon were included in calculated
positions and treated as riding atoms, while those attached to
nitrogen or boron were located and refined with isotropic thermal
parameters. CCDC-211991 (1), CCDC-211992 (3), and CCDC-
211993 (5) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.;
fax, (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
1
123 °C. H NMR (CDCl3): δ 7.76-7.68 (m, Ph), 7.36-7.30 (m,
Ph), 4.76 (br, NH), 2.53 (d, JHH ) 5.1 Hz, Me), 2.3 (br, BH2), 0.93
(q, br, JBH ) 106 Hz, BH3). 11B{1H} NMR (CDCl3): δ -11.3 (d,
JBP ) 76 Hz, BH2), -39.9 (d, JBP ) 59 Hz, BH3). 13C{1H} NMR
(75 MHz, CDCl3): δ 133.6 (d, JCP ) 48 Hz, ipso-C), 133.0 (d, JCP
) 8 Hz, Ar), 129.5 (d, JCP ) 2 Hz, Ar), 128.5 (d, JCP ) 9 Hz, Ar),
45.0 (d, JCP ) 7 Hz, Me). 31P{1H} NMR (CDCl3): δ -19.9 (br,
no resolved JPB coupling). EI-MS (70 eV): m/z 243 (M+ - BH3,
99%), 186 (Ph2PH, 100%). Elemental anal. Calcd (%) for C14H22B2-
NP (256.92): C 65.45, H 8.63, N 5.45. Found: C 65.81, H 8.86,
N 5.33.
Synthesis of Me2NH-BH2-PPhH-BH3 (2). 2 was synthesized
by a procedure analogous to that used for 1. After filtration and
solvent removal, the residue was washed with hexanes (15 mL)
and extracted with Et2O (20 mL). Recrystallization from toluene/
hexanes (10:1) at -30 °C afforded 2 as a white solid.
Synthesis of MePPh2‚BH3. To a solution of MePPh2 (13.616
g, 68.0 mmol) in THF (200 mL) cooled to -30 °C was added BH3‚
SMe2 (6.7 mL, 5.367 g, 70.6 mmol) dropwise via syringe over 5
min. The mixture was slowly warmed to 25 °C and stirred
For Li[PPhH‚BH3]. 11B{1H} NMR (THF): δ -34.6 (d, JBP
)
32 Hz). 31P NMR (THF): δ -93.8 (dq, JPB ) 34 Hz, JPH ) 202
Hz).
For 2. Yield: 0.922 g, 83%. Mp: 70-71 °C. 1H NMR (CDCl3):
δ 7.74-7.64 (m, Ph), 7.40-7.34 (m, Ph), 4.67 (br, NH), 4.61 (d
(sextet), JHP ) 344 Hz, JHH ) 6 Hz, PH), 2.61 (dd, JHP ) 35 Hz,
JHH ) 5.8 Hz, Me), 2.0 (br, BH2), 0.74 (q, br, JBH ) 98 Hz, BH3).
(35) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(36) Sheldrick, G. M. SHELXTL-PC V5.1; Bruker Analytical X-Ray
Systems Inc.: Madison, WI, 1997.
Inorganic Chemistry, Vol. 43, No. 3, 2004 1097

Jaska et al.
11B{1H} NMR (CDCl3): δ -12.8 (d, JBP ) 70 Hz, BH2), -41.5
(d, JBP ) 50 Hz, BH3). 13C{1H} NMR (75 MHz, CDCl3): δ 133.4
(d, JCP ) 7 Hz, Ar), 129.7 (d, JCP ) 2 Hz, Ar), 128.9 (d, JCP ) 50
Hz, ipso-C), 128.6 (d, JCP ) 9 Hz, Ar), 44.7 (d, JCP ) 8 Hz, Me).
31P NMR (CDCl3): δ -54.8 (d, br, JPH ) 344 Hz, no resolved
JPB coupling). EI-MS (70 eV): m/z 180 (M+ - H, 9%), 167
(M+ - BH3, 58%), 123 (PhPH-BH3, 4%), 110 (PhPH2, 51%), 58
(Me2NH-BH2, 100%). Elemental anal. Calcd (%) for C8H18B2NP
(180.83): C 53.14, H 10.03, N 7.75. Found: C 52.40, H 10.10, N
8.00.
dropwise via syringe. The mixture was stirred for 30 min at 0 °C,
then warmed to 25 °C, and stirred for a further 90 min. 11B and 31
P
NMR of the reaction mixture indicated complete conversion to
Li[CH2-PPh2‚BH3]: 11B{1H} NMR (THF), δ -36.1 (d, JBP ) 76
Hz); 31P{1H} NMR (THF), δ 19.2 (q, JPB ) 76 Hz). The mixture
was recooled to -30 °C, and a solution of Me2NH‚BH2Cl (0.526
g, 5.63 mmol) in THF (5 mL) was added. The mixture was allowed
to warm slowly to 25 °C and stirred overnight. The volatiles were
removed in vacuo, and the resulting oil was redissolved in toluene
(15 mL) and filtered to remove LiCl. The solution was concentrated
to ca. 5 mL, added to hexanes (20 mL), and filtered. Upon standing
at 25 °C, colorless crystals of X-ray quality were formed over 3-4
Thermolysis of 1. A sample of 1 (0.151 g, 0.588 mmol) was
heated at 130 °C under nitrogen for 20 h in the absence of solvent.
The reaction mixture showed the presence of the following products
by NMR (CDCl3): Ph2PH (31P{1H} NMR δ -40.3; trace), Ph2PH‚
BH3 (11B{1H} NMR δ -40.0; 31P{1H} NMR δ 1.5; ca. 35%),
Ph2PH-BH2-PPh2-BH3 (11B{1H} NMR δ -33.3 (BH2), -37.4
(BH3); 31P{1H} NMR δ -3.5 (Ph2PH), -18.6 (PPh2); ca. 15%),
[Me2N-BH2]2 (11B NMR δ 4.9 (t); ca. 40%), Me2NH‚BH3 (11B
NMR δ -14.1 (q); ca. 10%), unreacted 1 and an unidentified
product (11B{1H} NMR δ -35.6; 31P{1H} NMR δ -26.8; ca. 5%).
Thermolysis of 2. A sample of 2 (2.569 g, 14.21 mmol) was
heated at 130 °C under nitrogen for 2 h in the absence of solvent.
The 11B and 31P NMR spectra of the reaction mixture showed the
presence of only [Me2N-BH2]2 and [PhPH-BH2]n. The semisolid
was evacuated at 25 °C for 24 h to remove [Me2N-BH2]2. The
residue was then dissolved in CHCl3 and precipitated into hexanes,
1
days. Yield: 0.294 g, 20%. Mp: 106-108 °C. H NMR (CDCl3):
δ 7.74-7.66 (m, Ph), 7.44-7.36 (m, Ph), 5.12 (br, NH), 2.53 (d,
JHH ) 5.7 Hz, Me), 1.31 (m, br, CH2), 0.97 (m, br, BH2 and BH3).
11B{1H} NMR (CDCl3): δ -11.7 (BH2), -40.2 (BH3). 13C{1H}
NMR (100 MHz, CDCl3): δ 132.9 (d, JCP ) 55 Hz, ipso-C), 132.1
(d, JCP ) 8 Hz, Ar), 130.5 (d, JCP ) 2 Hz, Ar), 128.6 (d, JCP ) 10
Hz, Ar), 42.4 (s, Me), 10.9 (s, br, CH2). 31P{1H} NMR (CDCl3):
δ 16.6 (q, JPB ) 79 Hz). EI-MS (70 eV): m/z 257 (M+ - BH3,
75%). Elemental anal. Calcd (%) for C15H24B2NP (270.94): C
66.49, H 8.93, N 5.17. Found: C 66.44, H 8.95, N 5.03.
Synthesis of Me2NH-BH2-CH2-PMe2-BH3 (4). A procedure
analogous to that of 3 was used. After filtration to remove LiCl,
the oily residue was recrystallized (Et2O/hexanes (5:1), -30 °C)
to afford a mixture of 4 and Me3P‚BH3. The solid mixture was
fractionally sublimed at 25 °C: Me3P‚BH3 was first removed under
static vacuum, followed by sublimation of the residue under
dynamic vacuum, which afforded 4 as a white solid.
For Li[CH2-PMe2‚BH3]. 11B{1H} NMR (THF): δ -34.1 (d,
JBP ) 85 Hz). 31P{1H} NMR (THF): δ 1.7 (q, JPB ) 85 Hz).
For 4. Yield: 0.506 g, 32%. Mp: 71-73 °C. 1H NMR (C6D6):
δ 4.89 (br, NH), 2.16 (q, br, BH3), 1.88 (d, JHH ) 5.7 Hz, NMe),
1.14 (q, br, BH2), 1.00 (d, JHP ) 10.6 Hz, PMe), 0.54 (m, br, CH2).
11B NMR (C6D6): δ -9.9 (t, JBH ) 98 Hz, BH2), -40.2 (dq, JBP
) 70 Hz, JBH ) 94 Hz, BH3). 13C{1H} NMR (100 MHz, C6D6):
δ 41.8 (s, NMe), 14.2 (m, br, CH2), 13.4 (d, JCP ) 38 Hz, PMe).
31P{1H} NMR (C6D6): δ 3.1 (q, JPB ) 70 Hz). EI-MS (70 eV):
m/z 133 (M+ - BH3, 100%), 88 (CH2-PMe2-BH2, 24%).
Elemental anal. Calcd (%) for C5H20B2NP (146.82): C 40.90, H
13.73, N 9.54. Found: C 41.14, H 13.49, N 9.68.
1
giving [PhPH-BH2]n as a white solid. Yield: 1.724 g, 99%. H
NMR (CDCl3): δ 7.5-6.9 (br, Ph), 4.4 (br d, JHP ) 309 Hz, PH),
2.1-1.1 (br, BH2). 11B{1H} NMR (CDCl3): δ -33.6 (br). 31P NMR
(CDCl3): δ -49.5 (br d, JPH ) 309 Hz). Dynamic light scattering
(THF): after 3 h, Dh ) 1.8 nm, 163 nm; after 4 days: 1.43 nm.
The peak at Dh ) 1.4-1.8 nm corresponds to an absolute value of
Mw of ca. 5000.7b
Attempted Catalytic Dehydrocoupling of 1. (a) To a solution
of 1 (0.319 g, 1.24 mmol) in toluene (2 mL) was added [Rh(1,5-
cod)(µ-Cl)]2 (0.009 g, 0.02 mmol, 1.5 mol %). The mixture was
heated to 50 °C and monitored periodically by 11B and 31P NMR
spectroscopy. Complete conversion of 1 to form [Me2N-BH2]2 (ca.
45%), Me2NH‚BH3 (ca. 5%), Ph2PH‚BH3 (ca. 50%), and Ph2PH
(trace) was observed after 10 days. (b) Blank reaction: A sample
of 1 heated at 50 °C in toluene in the absence of catalyst indicated
the presence of chain cleavage products (35%) after 10 days. The
chain cleavage products were found to be the same as those
identified above, with the exception of [Me2N-BH2]2, which was
not formed from Me2NH‚BH3 in the absence of a transition-metal
catalyst.11
Thermolysis of 3; Synthesis of [BH2-CH2-PPh2]2 (5). A
sample of 3 (0.166 g, 0.613 mmol) was heated at 130 °C for 24 h
in the absence of solvent. The mixture was cooled to 25 °C, giving
5 as a white solid. X-ray quality crystals were grown from an ether
1
solution at -20 °C. Yield: 0.127 g, 98%. Mp: 253 °C. H NMR
Attempted Catalytic Dehydrocoupling of 2. (a) To a solution
of 2 (0.086 g, 0.48 mmol) in toluene (2 mL) was added [Rh(1,5-
cod)(µ-Cl)]2 (0.003 g, 0.006 mmol, 1.3 mol %). The mixture was
heated to 50 °C and monitored periodically by 11B and 31P NMR
spectroscopy. Complete conversion of 2 to form [Me2N-BH2]2 (ca.
40%), Me2NH‚BH3 (ca. 10%), PhPH2‚BH3 (11B{1H} NMR δ -40.8
(br); 31P{1H} NMR δ -49 (v br); ca. 50%), and PhPH2 (31P{1H}
NMR δ -123.8, trace) was observed after 10 days. (b) Blank
reaction: A sample of 2 heated at 50 °C in toluene in the absence
of catalyst indicated the presence of chain cleavage products (ca.
10%) after 10 days. The chain cleavage products were found to be
the same as those identified above, with the exception of [Me2N-
BH2]2, which was not formed from Me2NH‚BH3 in the absence of
a transition-metal catalyst.11
(CDCl3): δ 7.71 (m, Ph), 7.38 (m, Ph), 1.62 (br, CH2), 2.3-0.8
(br, BH2). 11B{1H} NMR (CDCl3): δ -33.6 (br, BH2). 13C{1H}
NMR (75 MHz, CDCl3): δ 132.0 (d, JCP ) 9 Hz, Ar), 131.4 (d,
JCP ) 54 Hz, ipso-C), 130.6 (s, Ar), 128.8 (d, JCP ) 9 Hz, Ar), 6.3
(br, CH2). 31P{1H} NMR (CDCl3): δ 7.7 (br, PPh2). EI-MS (70
eV): m/z 424 (M+, 10%), 346 (M+ - C6H6, 33%), 313 (M+
-
PPhH - H2, 36%), 237 (M+ - PPh2 - H2, 100%), 212 (M+/2,
6%). Elemental anal. Calcd (%) for C13H14BP (212.02): C 73.64,
H 6.65. Found: C 73.23, H 6.57.
Attempted Catalytic Dehydrocoupling of 3. (a) A sample of
3 (0.035 g, 0.13 mmol) and [Rh(1,5-cod)(µ-Cl)]2 (0.003 g, 0.006
mmol, 5 mol %) were dissolved in toluene in a 5 mm NMR tube
with a D2O insert. No reaction was observed by 11B or 31P NMR
after 24 h at 25 °C, or after 24 h at 50 °C. Heating at 90 °C for 20
h resulted in chain cleavage, with the only identified product being
MePPh2‚BH3. (b) Blank reaction: Heating of 3 in toluene at 90 °C
in the absence of catalyst resulted in no reaction after 20 h.
Synthesis of Me2NH-BH2-CH2-PPh2-BH3 (3). To a solution
of MePPh2‚BH3 (1.168 g, 5.46 mmol) in THF (10 mL) cooled to
0 °C was added a solution of nBuLi in hexanes (3.6 mL, 5.8 mmol)
1098 Inorganic Chemistry, Vol. 43, No. 3, 2004

Linear Hybrid Aminoborane/Phosphinoborane Chains
Thermolysis of 4. (a) A sample of 4 (0.143 g, 0.974 mmol)
was heated at 90 °C for 3 days in the absence of solvent. Colorless
crystals of 4 were observed to sublime to the top of the reaction
vessel. Partial decomposition (ca. 50%) to give Me2NH‚BH3 (ca.
20%), Me3P‚BH3 (11B NMR δ -37.4 (qd); 31P{1H} NMR δ -1.89
(q); ca. 20%) and a product tentatively assigned as [BH2-CH2-
PMe2]2 (6) was observed. Selected NMR characterization for 6:
Blank reaction: Heating of 4 in toluene at 90 °C in the absence of
catalyst resulted in no reaction after 20 h.
Acknowledgment. This work was funded by the Natural
Sciences and Engineering Research Council of Canada
(NSERC) Discovery Grants program. C.A.J. thanks the
Ontario and Canadian Governments for an Ontario Graduate
Scholarship in Science and Technology (OGS-ST) (2001-
2002) and an NSERC Scholarship (2002-2004), and I.M.
thanks the Ontario Government for a PREA award (1999-
2004), the University of Toronto for a McLean Fellowship
(1997-2003), and the Canadian Government for a Canada
Research Chair.
1
yield (11% by NMR); H NMR (C6D6) δ 1.52 (d, JHP ) 13 Hz,
Me); 11B NMR (C6D6) δ -30.5 (dt, JBP ) 61 Hz, JBH ) ca. 91
Hz); 13C{1H} NMR (C6D6) δ 10.6 (d, JCP ) 37 Hz, Me); 31P{1H}
NMR (C6D6) δ 6.0 (q, br). (b) A sample of 4 (0.048 g, 0.33 mmol)
was heated at 130 °C in DMF-d7. After 5 h, the mixture contained
only [Me2N-BH2]2 and Me3P‚BH3.
Attempted Catalytic Dehydrocoupling of 4. (a) A sample of
4 (0.061 g, 0.42 mmol) and [Rh(1,5-cod)(µ-Cl)]2 (0.003 g, 0.006
mmol, 1 mol %) were dissolved in THF and stirred at 25 °C for 2
days. No reaction was observed according to the 11B and 31P spectra
obtained. Heating at 90 °C in toluene for 20 h resulted in chain
cleavage, with the only identified product being Me3P‚BH3. (b)
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC034938F
Inorganic Chemistry, Vol. 43, No. 3, 2004 1099