Reactions of boron amidinates with CO2 and CO and other small molecules

Article abstract of DOI:10.1021/ja1064153

Reaction of Piers' borane, HB(C₆F₅)₂, with either tert-butyl or isopropyl carbodiimide cleanly affords the boron amidinates HC(RN)₂B(C₆F₅)₂ [R = iPr (1), tBu (2)]. These species undergo a variety of insertion reactions. For example, treatment of 1 with CO₂ or excess carbodiimide gives HC(iPrN)₂(CO₂)B(C₆F₅)₂ (3) or HC(iPrN)₂C(iPrN)₂B(C₆F₅) ₂ (4), respectively. Similarly, exposure of 1 or 2 to 1 atm CO gives HC(RN)₂(CO)B(C₆F₅)₂ [R = iPr (5), tBu (6)], while reaction of 1 with CNtBu gives HC(RN)₂(CNtBu)B(C ₆F₅)₂ [R = iPr (7), tBu (8)]. Compounds 1 and 2 also react with benzaldehyde, resulting in the formation of HC(RN) ₂(PhHCO)B(C₆F₅)₂ [R = iPr (9), tBu (10)]. Compound 1 also reacts with MeCN to give HC(iPrN)₂(MeCN) B(C₆F₅)₂ (11) and effects heterolytic C-H cleavage to afford HC(iPrN)(iPrNH)(PhCC)B(C₆F₅) ₂ (12). In contrast, the species PhC(iPrN)₂B(C ₆F₅)₂ (14) derived from PhC(iPrN) ₂BCl₂ (13) failed to react with any of the above substrates. These data, in addition to the isolation of HC(iPrN) ₂(C₆F₄)BF(C₆F₅) (15), the product of thermolysis of 1, provide further support for the notion that the transient open-chain form of these amidinates is present in solution.

Full text of DOI:10.1021/ja1064153

Published on Web 09/01/2010
Reactions of Boron Amidinates with CO2 and CO and Other
Small Molecules
Meghan A. Dureen and Douglas W. Stephan*
Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario,
Canada M5S 3H6
Received July 20, 2010; E-mail: dstephan@chem.utoronto.ca
Abstract: Reaction of Piers’ borane, HB(C6F5)2, with either tert-butyl or isopropyl carbodiimide cleanly affords
the boron amidinates HC(RN)2B(C6F5)2 [R ) iPr (1), tBu (2)]. These species undergo a variety of insertion
reactions. For example, treatment of 1 with CO2 or excess carbodiimide gives HC(iPrN)2(CO2)B(C6F5)2 (3)
or HC(iPrN)2C(iPrN)2B(C6F5)2 (4), respectively. Similarly, exposure of 1 or 2 to 1 atm CO gives
HC(RN)2(CO)B(C6F5)2 [R ) iPr (5), tBu (6)], while reaction of 1 with CNtBu gives HC(RN)2(CNtBu)B(C6F5)2
[R ) iPr (7), tBu (8)]. Compounds 1 and 2 also react with benzaldehyde, resulting in the formation of
HC(RN)2(PhHCO)B(C6F5)2 [R ) iPr (9), tBu (10)]. Compound 1 also reacts with MeCN to give HC(iPrN)2-
(MeCN)B(C6F5)2 (11) and effects heterolytic C-H cleavage to afford HC(iPrN)(iPrNH)(PhCC)B(C6F5)2 (12).
In contrast, the species PhC(iPrN)2B(C6F5)2 (14) derived from PhC(iPrN)2BCl2 (13) failed to react with any
of the above substrates. These data, in addition to the isolation of HC(iPrN)2(C6F4)BF(C6F5) (15), the product
of thermolysis of 1, provide further support for the notion that the transient “open-chain” form of these
amidinates is present in solution.
acceptor adducts of CO such as R3B·CO [R ) H,17 F, CF3,
C6F5, BX2 (X ) Cl, F)]18 are also known. The most well
Introduction
The oxides of carbon are rather notorious molecules. Carbon
monoxide, which is widely known for its toxicity, constitutes a
major atmospheric pollutant derived from the exhaust of internal
combustion engines. At the same time, the increasing levels of
carbon dioxide in the atmosphere arising from the industrial
revolution have contributed to global warming and climate
change. Efforts to reduce the emissions of these gases have
involved the development of more efficient combustion pro-
cesses and the development of protocols for the sequestration
of CO2.1 However, from a fundamental chemical perspective,
the problem can be distilled to the limited reactivity known for
these oxides.
While many studies have probed the reactivity of transition-
metal species with CO and (to a lesser extent) CO2, studies of the
corresponding reactivity of main-group species have been rather
limited. The combination of organolithium and Grignard reagents
with CO to produce metal acyls has been well-documented.2-5
Donor-acceptor adducts for the Al and Ga Lewis acids have
been observed in matrix-isolation studies, while examples of
CO insertion into B-B6 and Al-C,7-9 Ga-C,10 and Ge-C11
bonds have been reported. CO has also been shown to react
with carbenes12,13 and silylenes14,15 to afford ketenes and silyl
ketenes, respectively, although the analogous reactions with
N-heterocyclic carbenes have not been observed.16 Donor-
developed main-group chemistry involving CO involves reac-
tions with organoboranes, which proceed via a transient adduct
of CO and subsequent alkyl transfer to give aldehydes, ketones,
and alcohols.19,20 In a related sense, boron-bound CO has been
shown to undergo nucleophilic attack by amines21 or phos-
phines,22 affording zwitterionic species of the form R3B(CO)-
ER′3 (E ) N, P; R ) CF3; R′ ) Me).22
(6) Teichmann, J.; Stock, H.; Pritzkow, H.; Siebert, W. Eur. J. Inorg.
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A R T I C L E S
Dureen and Stephan
Scheme 1. Reactivity of the Four-Membered FLP with CO2, Olefin,
and H2
Scheme 2. Syntheses of 1-4
we focused on the potential reactivity of boron amidinates.
While sterically bulky amidinate ligands have been exploited
for polymerization catalysts37-42 and the stabilization of reactive
main-group species,43-46 the reactivity of boron amidinates38,47,48
has drawn little attention.49 In this manuscript, we describe the
synthesis and reactivity of new boron amidinates and demon-
strate that although these species are isolated as four-membered
chelate compounds, they are reactive. This affords new and rare
examples of main-group species that react with a variety of small
molecules, including CO2 and CO.
On the other hand, CO2 is known to react with amines to
afford carbamic acids23,24 and to insert into Al-C bonds25 as
well as N-E bonds of the amido complexes of P,26 As, Si,27
and B.28 Phosphines and N-heterocyclic carbenes, both of which
are strong main-group nucleophiles, can be used as catalysts in
the fixation of CO2 with propargyl alcohols to afford cyclic
carbamates.29 Recently, we developed phosphine-borane frus-
trated Lewis pair (FLP) systems30-32 and showed that such inter-
or intramolecular systems are capable of reversible CO2 fixa-
tion.33 In related work, O’Hare and co-workers34 described the
use of the FLP based on tetramethylpiperidine and B(C6F5)3 to
effect the stoichiometric reduction of CO2 in the presence of
H2. In subsequent work, we described the formation of the CO2-
bound species (C6H2Me3)3P(CO2)(AlCl3)2 and its stoichiometric
reduction to methanol by treatment with NH3BH3 and water.35
In seeking new FLP systems that might also capture CO2
and perhaps even CO, we took particular note of systems that
react as FLPs and for which the Lewis acid-base adducts are
accessible. One example is the FLP system pioneered by Erker
and co-workers,36 which is based on the phosphinoborane
(C6H2Me3)2PC2H4B(C6F5)2 (Scheme 1). This species exists in
solution as an equilibrium mixture of the open-chain FLP form
and the four-membered-ring P-B adduct. With this in mind,
Results and Discussion
Conventional syntheses of boron amidinates involve the
metathetical treatment of BCl3 with the Li salt of the ligand or
the elimination of ClSiMe3 from the silyl amidinate.50 Alter-
natively, insertion of a carbodiimide into the B-N bond of
R2NBCl251,49 or the B-C bond of PhBCl247 has been employed.
Herein we employed an approach based on hydroboration, using
Piers’ borane, HB(C6F5)2.52 This species reacts with isopropyl
carbodiimide and with tert-butyl carbodiimide in CH2Cl2,
affording the new products 1 and 2, respectively, as colorless
crystals (Scheme 2). These products were isolated in 80 and
84% yield, respectively. The 11B NMR resonances of 1 and 2
were observed at 2.2 and 1.1 ppm, respectively, while the
corresponding 19F NMR spectra showed similar sets of signals
at -133.8, -157.9, and -164.3 ppm and -131.9, -158.0, and
-164.4 ppm. In both cases, these data reflect a four-coordinate
B center. Both products exhibit an 1H NMR singlet at 7.8 ppm
consistent with the formation of an amidinate fragment, prompt-
ing the formulation of the products as HC(RN)2B(C6F5)2 [R )
iPr (1), tBu (2)]. These formulations were subsequently con-
firmed via X-ray crystallographic studies (Figures 1 and 2). The
(23) Tolman, W. B. ActiVation of Small Molecules; Wiley-VCH: Weinheim,
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13560 J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010

Reactions of Boron Amidinates with Small Molecules
A R T I C L E S
Figure 1. POV-ray depiction of 1. Color code: C, black; F, pink; N, blue-
green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N(1)-C(1), 1.310(3);
N(1)-B, 1.590(3); N(2)-C(1), 1.318(3); N(2)-B, 1.583(3); C(8)-B,
1.623(3); C(14)-B, 1.613(3); C(1)-N(1)-B, 87.50(17); N(1)-C(1)-N(2),
103.7(2); C(1)-N(2)-B, 87.52(17); N(2)-B-N(1), 81.28(15).
Figure 3. POV-ray depiction of 3. Color code: C, black; F, pink; O, red;
N, blue-green; B, yellow-green. Hydrogen atoms have been omitted for
clarity. Selected bond distances (Å) and angles (deg): O(1)-B, 1.493(5);
O(1)-C(8), 1.292(5); O(2)-C(8), 1.208(5); N(1)-C(1), 1.293(5); N(1)-C(2),
1.509(6); N(1)-B, 1.544(6); N(2)-C(1), 1.327(6); C(8)-O(1)-B, 129.2(3);
C(1)-N(1)-B, 119.6(3); C(1)-N(2)-C(8), 121.4(4); N(1)-C(1)-N(2),
125.9(4); O(2)-C(8)-O(1), 123.4(4); O(2)-C(8)-N(2), 121.0(4); O(1)-
C(8)-N(2), 115.6(4); O(1)-B-N(1), 107.2(3).
efforts to observe these species by variable-temperature NMR
spectroscopy were undertaken. At 80 °C in C6D5Br, no evidence
of an “open-chain” form was observed. Despite the lack of
evidence of access to an FLP-like form, the reactivity of 1 with
CO2 was probed. Placing a solution of 1 under an atmosphere
of CO2 for 16 h ultimately resulted in the formation of needlelike
crystals of 3 in 96% yield (Scheme 2). This product exhibited
a 11B signal at 0.3 ppm and 19F resonances at -134.9, -156.4,
and -163.6 ppm. This species also exhibited a 13C signal at
146.1 ppm attributable to the incorporation of CO2. The
corresponding CO IR stretching frequency was observed at 1742
cm-1, giving rise to the formulation of 3 as HC(iPrN)2-
(CO2)B(C6F5)2.
An X-ray structural study of 3 confirmed the insertion of the
CO2 fragment into the B-N bond, affording a six-membered
heterocycle (Figure 3). The resulting B-N bond length is
1.544(6) Å, with a O-B-N angle of 107.2(3)°. The resulting
B-O bond distance was determined to be 1.493(5) Å. This B-O
bond distance in 3 is slightly shorter than those found in the
P/B systems tBu3P(CO2)B(C6F5)3 and (C6H2Me3)2PC2H4B-
(C6F5)2(CO2) [1.5474(15) and 1.550(4) Å, respectively33],
whereas the CdO and C-O bond lengths of 1.208(5) and
1.292(5) Å in 3 are similar to those seen in P/B-CO2
complexes. Although the B center in 3 is expected to be less
Lewis acidic those than in B(C6F5)3 and (C6H2Me3)2-
PC2H4B(C6F5)2, the stronger B-O bond implies greater donation
from N to the CO2 fragment.
Figure 2. POV-ray depiction of 2. Color code: C, black; F, pink; N, blue-
green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N(1)-C(1), 1.321(2);
N(1)-C(2), 1.4718(19); N(1)-B, 1.586(2); N(2)-C(1), 1.314(2); N(2)-B,
1.606(2); B-C(16), 1.626(2); B-C(10), 1.628(2); C(1)-N(1)-B, 87.97(11);
C(1)-N(2)-B, 87.37(11); N(1)-C(1)-N(2), 103.70(13); N(1)-B-N(2),
80.95(10).
four-coordinate geometry about B gives rise to B-C and B-N
distances in the ranges 1.613(3)-1.628(3) and 1.583(3)-1.606(2)
Å, respectively, while the four-membered BNCN rings contain
acute N-B-N angles of 81.28(15) and 80.95(10)° for 1 and 2,
respectively. These geometries are similar to those reported for
PhC(NSiMe3)2BX2 (X ) Cl, Br),50 C6H4(C(NCy)2BCl2)2,53 and
BuC(NtBu)2BPh(Cl).54 However, the B-N bond lengths in 1
and 2 are significantly longer than the ones found in those boron
halide species [1.559(4) and 1.580(5) Å, respectively, for
PhC(NSiMe3)2BX2 (X ) Cl, Br)50 and 1.571(5) Å for PhB(Cl)-
(NtBu)2CBu54]. This is perhaps unexpected, as the B centers in
1 and 2 are expected to be more Lewis acidic, but it appears
that steric interactions between the N substituents and the C6F5
rings play a role. Further evidence of the steric congestion is
provided by the short transannular B-C distances in 1 and 2
[2.015(3) and 2.028(3) Å, respectively].
The treatment of 1 (generated in situ) with excess carbodi-
imide was undertaken, as the NdCdN moiety in the latter is
isoelectronic with CO2. The formation of a new product, 4, was
evidenced by the shift in the 11B resonance to -3.7 ppm and
shifts in the 19F signals to -132.4, -158.9, and -165.1 ppm.
1
The H NMR spectrum showed the amidinate-type CH proton
at 7.5 ppm as well as isopropyl methine signals at 4.2, 3.7, 3.6,
As we had speculated that ring congestion and strain would
provide access to the η1 “open-chain” form of these amidinates,
(52) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17,
5492–5503.
(50) Hill, N. J.; Findlater, M.; Cowley, A. H. Dalton Trans. 2005, 3229–
(53) Lu, Z.; Hill, N. J.; Findlater, M.; Cowley, A. H. Inorg. Chim. Acta
2007, 360, 1316–1322.
3234.
(51) Findlater, M.; Hill, N. J.; Cowley, A. H. Polyhedron 2006, 25, 983–
(54) Blais, P.; Chivers, T.; Downard, A.; Parvez, M. Can. J. Chem. 2000,
78, 10–15.
988.
9
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A R T I C L E S
Dureen and Stephan
Scheme 3. Syntheses of 5-12
Figure 4. POV-ray depiction of 4. Color code: C, black; F, pink; N, blue-
green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): B-N(1), 1.535(2); B-N(4),
1.573(2); N(1)-C(1), 1.3648(19); N(2)-C(1), 1.2674(19); N(3)-C(2),
1.326(2); N(4)-C(2), 1.2942(19); N(1)-B-N(4), 107.63(12); C(1)-N(1)-B,
125.02(12); C(2)-N(3)-C(1), 120.59(13); C(2)-N(4)-B, 120.84(13);
N(1)-C(1)-N(3), 113.62(13); N(4)-C(2)-N(3), 125.49(14).
resulting B-C and B-N bond distances in the newly formed
ring were 1.6365(14) and 1.5794(13) Å, respectively. The
corresponding C-B-N angle was 97.57(7)°, while the CdO
bond length was typical at 1.2169(12) Å. Although insertion of
CO into B-C bonds of boranes is well-documented, the
formation of 5 and 6 presents the first examples of insertion of
CO into B-N bonds, to the best of our knowledge. It is
noteworthy that no reports have described CO sequestration by
FLPs.
This reactivity prompted an examination of the reactions of
1 and 2 with tert-butylisonitrile, a small molecule containing a
CtN moiety that is isoelectronic with CO. These reactions
afforded the products 7 and 8 in 85 and 90% yield, respectively
(Scheme 3), and the NMR spectral properties of these com-
pounds were similar to those described for 5, prompting their
formulation as HC(RN)2(CNtBu)B(C6F5)2 [R ) iPr (7), tBu (8)].
X-ray-quality crystals of 7 afforded structural confirmation of
this formulation (Figure 6). The C-B-N angle in 7 was
determined to be 97.26(13)°, while the exocyclic CdN bond
length was 1.255(2) Å. Once again, this observation stands in
contrast to the corresponding reaction of (C6H2Me3)2PC2-
H4B(C6F5)2, where a classical Lewis acid adduct is formed.55
and 3.2 ppm. These data together with the remaining 1H NMR
resonances were consistent with the formulation of 4 as
HC(iPrN)2C(iPrN)2B(C6F5)2 (Scheme 2). A crystallographic
study (Figure 4) revealed the nature of 4 as a six-membered
ring formed by insertion of a carbodiimide fragment into one
of the B-N bonds of 1. The resulting B-N bond distances in
4 were found to be 1.535(2) and 1.573(2) Å, while the N-B-N
angle was broadened to 107.63(12)°. The length of the exocyclic
CdN bond was determined to be 1.2674(19) Å. This compound
was often formed as a byproduct in the synthesis of 1, especially
in syntheses on a larger scale, reflective of the rapidity of this
insertion.
It is noteworthy that the formation of 4 stands in marked
contrast to the reported reaction of (C6H2Me3)2PC2H4B(C6F5)2
with a carbodiimide, where a classical Lewis acid-base adduct
between one of the imido N atoms and the B center is formed.55
If it is assumed that a related adduct is formed as a transient
species en route to 4, it is reasonable to suggest that the greater
basicity and steric accessibility result in nucleophilic attack of
the central C by the pendant imido N atom, prompting ring
closure.
In view of the ability of these boron amidinates to react with
substrates that the P/B systems do not activate, the species 1
and 2 were exposed to ∼1 atm carbon monoxide, and spectro-
scopic monitoring revealed the slow formation of the products
5 and 6, respectively (Scheme 3). In the case of 6, the reaction
was exceptionally slow, affording a yield of 97% after 3 weeks,
whereas the reaction of 1 afforded 5 in 88% yield in 36 h. The
11B NMR spectrum of 5 showed a signal at -11.0 ppm, and
19F resonances appeared at -133.2, -158.0, and -164.2 ppm.
Compound 6 exhibited similar shifts, and both compounds
exhibited 1H resonances consistent with inequivalent N substit-
uents. Thus, the data are consistent with the formulations
HC(RN)2(CO)B(C6F5)2 [R ) iPr (5), tBu (6)]. IR spectra showed
absorptions at 1714 and 1713 cm-1, respectively, attributable
to the expected CdO band. X-ray-quality crystals of 5 confirmed
the formulation of the five-membered ring (Figure 5). The
Figure 5. POV-ray depiction of 5. Color code: C, black; F, pink; O, red;
N, blue-green; B, yellow-green. Hydrogen atoms have been omitted for
clarity. Selected bond distances (Å) and angles (deg): O-C(1), 1.2169(12);
N(1)-C(2), 1.3560(12); N(1)-C(1), 1.4229(12); N(2)-C(2), 1.2981(13);
N(2)-B, 1.5794(13); C(1)-B, 1.6365(14); C(2)-N(1)-C(1), 109.99(8);
C(2)-N(2)-B, 109.90(8); O-C(1)-N(1), 121.13(9); O(1)-C(1)-B,
132.82(9); N(1)-C(1)-B, 105.91(7); N(2)-C(2)-N(1), 116.02(8); N(2)-
B-C(1), 97.57(7).
(55) Mo¨mming, C. M.; Fro¨mel, S.; Kehr, G.; Fro¨hlich, R.; Grimme, S.;
Erker, G. J. Am. Chem. Soc. 2009, 131, 12280–12289.
9
13562 J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010

Reactions of Boron Amidinates with Small Molecules
A R T I C L E S
Figure 6. POV-ray depiction of 7. Color code: C, black; F, pink; N, blue-
green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N(1)-C(1), 1.293(2);
N(1)-B, 1.603(2); N(3)-C(2), 1.255(2); N(2)-C(1), 1.325(2); N(2)-C(2),
1.455(2); N(3)-C(2), 1.255(2); B-C(2), 1.667(3); C(1)-N(1)-B, 109.91(14);
C(1)-N(2)-C(2), 111.56(15); N(1)-B-C(2), 97.26(13); N(3)-C(2)-N(2),
113.12(16); N(3)-C(2)-B, 142.74(17); N(2)-C(2)-B, 103.97(14);
N(1)-C(1)-N(2), 117.00(17).
Figure 8. POV-ray depiction of 11. Color code: C, black; F, pink; N, blue-
green; B, yellow-green. Hydrogen atoms except for the NH proton have
been omitted for clarity. Selected bond distances (Å) and angles (deg):
N(1)-C(1), 1.2962(10); N(1)-B, 1.5831(12); N(2)-C(1), 1.3399(10);
N(2)-C(8), 1.4464(11); N(3)-C(8), 1.2551(11); N(3)-B, 1.5147(11);
C(8)-C(9), 1.5013(11); C(1)-N(1)-B, 120.19(7); C(1)-N(2)-C(8),
118.27(7); C(8)-N(3)-B, 124.57(7); N(3)-C(8)-N(2), 122.15(7).
1.2551(11) Å, respectively. It is also the case that this reactivity
is not observed in P/B FLP systems, where simple adduct
formation with the borane is observed.
To probe other FLP-type reactions of the boron amidinates,
compounds 1 and 2 were exposed to 4 atm H2. No reaction
was observed, even upon heating to 60 °C. This observation is
consistent with the results of previous efforts to activate H2 using
sterically demanding phosphines and aminoboranes of the form
R2NB(C6F5)2, where no reaction occurred. This observation is
consistent with the notion that a combined total Lewis acidity
and basicity of the components of the FLP must reach a
relatively high threshold to effect the heterolytic cleavage of
H2.
In contrast, heterolytic cleavage of the CH bond of a terminal
alkyne was readily achieved. Combination of 1 with phenyl-
acetylene resulted in the formation of the product 12 in 75%
yield. This species exhibits a 11B chemical shift of -12.9 ppm
and 19F resonances consistent with the formation of a four-
Figure 7. POV-ray depiction of 10. Color code: C, black; F, pink; O, red;
N, blue-green; B, yellow-green. Hydrogen atoms except for the NH proton
have been omitted for clarity. Selected bond distances (Å) and angles (deg):
O(1)-B, 1.4671(11); O(1)-C(1), 1.3955(10); N(1)-C(8), 1.3120(11);
N(1)-B, 1.5962(11); N(2)-C(8), 1.3307(11); N(2)-C(1), 1.4779(11);
C(1)-O(1)-B, 119.04(6); N(1)-C(8)-N(2), 126.29(8).
1
coordinate boron center. The H NMR spectrum shows in-
equivalent isopropyl fragments as well as single resonances
corresponding to C-H and N-H fragments. These data,
together with the 13C resonances as well as crystallographic data,
confirmed the formulation of 12 as HC(iPrN)(iPrNH)-
(PhCC)B(C6F5)2 (Scheme 3 and Figure 9). The B-C bond
length to the acetylide fragment is 1.581(3) Å, while the B-N
bond distance is 1.589(3) Å. The B-N separation for the
pendant N is 2.928 Å. Similar alkyne deprotonation reactions
have been reported previously for phosphine/borane FLPs.56
Mechanistic Considerations. The above reactions demonstrate
the remarkable reactivity of boron amidinates 1 and 2. Moreover,
these reactions suggest that the boron amidinates behave as
FLPs, implying that the strained four-membered ring is in
equilibrium with an “open-chain” form to some degree. In this
latter form, where one of the N atoms dissociates from B, the
unquenched Lewis acidity of the B center and the nucleophilicity
of the pendant N atom allow for reactions with substrates. It is
Compounds 1 and 2 also reacted with benzaldehyde, yielding
the insertion products 9 and 10 in 82 and 70% yield, respec-
tively. The products can be formulated as HC(RN)2(PhHCO)-
1
B(C6F5)2 [R ) iPr (9), tBu (10)] (Scheme 3). The H, 19F, and
11B NMR data were similar to those for the insertion products
described above and are consistent with these formulations. In
particular, the 19F data show that the C6F5 rings are inequivalent
(i.e., diastereotopic) as a result of the presence of the chiral
(racemic) C atom upon incorporation of the carbonyl fragment.
The formulation of 9 was unambiguously confirmed crystallo-
graphically (Figure 7).
Compound 1 also reacted with acetonitrile, affording the
insertion product 11 in 70% yield. The product was formulated
1
as HC(iPrN)2(MeCN)B(C6F5)2 (Scheme 3) on the basis of H,
19F and 11B NMR data. The crystal structure of 11 (Figure 8)
reveals B-N(3) and N(3)-C(8) distances of 1.5147(11) and
(56) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396–
8397.
9
J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010 13563

A R T I C L E S
Dureen and Stephan
Figure 9. POV-ray depiction of 12. Color code: C, black; F, pink; N, blue-
green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N(1)-C(9), 1.298(3);
N(2)-C(9), 1.309(3); N(1)-B, 1.589(3); C(1)-B, 1.581(3); C(1)-C(2),
1.199(3); C(9)-N(1)-B, 121.57(17); C(2)-C(1)-B, 172.4(2); N(1)-
C(9)-N(2), 126.7(2).
Figure 11. POV-ray depiction of 15 Color code: C, black; F, pink; N,
blue-green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): F(1)-B, 1.414(2); N(1)-B,
1.567(3); N(1)-C(1), 1.299(3); N(2)-C(1), 1.338(3); N(2)-C(8), 1.421(2);
C(8)-C(9), 1.405(3); C(9)-B, 1.608(3); C(1)-N(1)-B, 123.41(17);
C(1)-N(2)-C(8), 120.49(16); N(1)-C(1)-N(2), 125.76(18); C(8)-C(9)-B,
121.91(17); F(1)-B-N(1), 106.53(16); F(1)-B-C(9), 109.45(16); N(1)-
B-C(9), 107.02(16).
Scheme 4. Synthesis of 15
Figure 10. POV-ray depiction of 14. Color code: C, black; F, pink; N,
blue-green; B, yellow-green. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): N(1)-C(1), 1.3276(18);
N(1)-B, 1.596(2); N(2)-C(1), 1.3319(19); N(2)-B, 1.597(2); C(1)-C(2),
1.476(2); C(14)-B, 1.633(2); C(20)-B, 1.624(2); C(1)-N(1)-B, 88.42(11);
C(1)-N(2)-B, 88.20(11); N(1)-C(1)-N(2), 102.41(12); N(1)-B-N(2),
80.96(10).
We also examined the thermolysis of 1 above 80 °C, which
produced a new species, 15, that did not revert to 1 upon cooling
to room temperature. Spectroscopic studies of 15 revealed a 11B
NMR resonance at -0.6 ppm and 19F NMR data that were
consistent with the formation of BF and C6F4 fragments. These
reasonable to suggest that the activation of CO2 or CO and their
isoelectronic analogues may proceed via mechanisms involving
an initial interaction with either the Lewis acidic B center or
the pendant nucleophilic N or perhaps involving the concerted
action of the acid and base, as with other FLPs.33 Nonetheless,
the precise details of these processes are not known. The
postulate of FLP behavior is consistent with the observation
that 2 undergoes insertion reactions much more slowly than 1.
Presumably, the enhanced basicity of the N atoms slows ring
opening, and the additional steric crowding slows the reactions
with substrates.
To evaluate the impact of steric demands on the reactivity,
the species PhC(iPrN)2BCl2 (13) was prepared in a conventional
fashion and converted to the boron amidinate PhC(iPrN)2-
B(C6F5)2 (14). This species was isolated and characterized (Figure
10). In contrast to 1 and 2, no reaction of 14 was observed in any
of the efforts to effect insertion reactions with the variety of
substrates described above. These observations further support the
notion that steric crowding inhibits access to the “open-chain” form
of the boron amidinate, thus precluding reactivity.
1
data, together with H and 13C NMR data, were consistent with
the formulation of 15 as HC(iPrN)2(C6F4)BF(C6F5). X-ray analysis
confirmed the formation of a new species containing a six-
membered ring in which one of the amidinate N atoms effects
aromatic substitution at the ortho position of one of the B-bound
C6F5 rings, with concurrent transfer of the F atom to B (Figure
11). The formation of 15 further supports the notion that a transient
“open-chain” form of the amidinate is accessible, as this results in
nucleophilic attack of an aromatic o-CF bond and subsequent F
transfer to B to give 15 (Scheme 4). In a related sense, we recently
reported an analogous o-CF bond attack by a pendant phosphine
that led to the formation of C6F4B(C6F5)2CH2PtBu2, which contains
a five-membered BCPC2 ring.57
Conclusions
Herein we have described a new synthetic pathway to boron
amidinates. These species react with CO2, CO, carbodiimide,
isocyanide, acetonitrile, and benzaldehyde to effect insertion
9
13564 J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010

Reactions of Boron Amidinates with Small Molecules
A R T I C L E S
Data for 2: Yield: 116 mg, 80%. 1H NMR: δ 7.8 (s, 1H, HCdN),
of these substrates into a B-N bond, affording new boron
heterocycles. One boron amidinate also reacts with phenylacety-
lene to effect C-H activation, affording a boron acetylide
species, while thermolysis of this boron amidinate results in
ortho substitution of N on one of the B-bound fluoroarene rings.
All of this reactivity points to reactions of the boron amidinates
as masked FLPs. The general utility of boron amidinates as
synthons for novel heterocycles continues to be of interest in
our laboratories, with particular interest in the reactivity of the
lighter dihalo analogues of these compounds.
t
1.1 (s, 18H, Bu). 11B NMR: δ 1.1 (s). 13C{1H} NMR, partial: δ
158.7 (s, NdC(H)-N), 148.8 (dm, 1JC-F ) 250 Hz, o-C6F5), 140.6
(dm, 1JC-F ) 253 Hz, p-C6F5), 138.6 (dm, 1JC-F ) 257 Hz, m-C6F5),
53.4 (s, C(CH3)3), 29.7 (s, C(CH3)3). 19F NMR: δ -131.9 (dd, 4F,
3JF-F ) 24 Hz, 4JF-F ) 9 Hz, o-C6F5), -158.0 (tm, 2F, 3JF-F ) 20
Hz, p-C6F5), -164.4 (m, 4F, 3JF-F ) 19 Hz, m-C6F5). Anal. Calcd
for C21H19BN2F10 (500.192): C, 50.43; H, 3.83; N, 5.60. Found: C,
50.21; H, 4.11; N, 5.80. X-ray-quality crystals were grown from
slow cooling of a solution in pentane.
Synthesis of HC(iPrN)2(CO2)B(C6F5)2 (3). A solution of 1 (20
mg, 0.04 mmol) in CD2Cl2 (∼0.8 mL) in a Teflon-sealed NMR
tube was degassed via the freeze-pump-thaw technique and
pressurized with ∼1 atm CO2. The solution was left for 16 h, at
Experimental Section
1
which point H, 11B, and 19F spectra revealed the reaction to be
General Remarks. All manipulations were carried out under
an atmosphere of dry, O2-free N2 employing an Innovative
Technology glovebox and a Schlenk vacuum line. Solvents were
purified with a Grubbs-type column system manufactured by
Innovative Technology and dispensed into thick-walled Schlenk
glass flasks equipped with Teflon-valve stopcocks (pentane, toluene,
CH2Cl2) or dried over the appropriate agents and distilled into the
same kind of storage flasks (C6H5Br). All of the solvents were
thoroughly degassed after purification (repeated freeze-pump-thaw
cycles). Deuterated solvents were dried over the appropriate agents,
vacuum-transferred into storage flasks with Teflon stopcocks, and
degassed accordingly (C6D5Br, CD2Cl2). Toluene and pentane were
stored over potassium mirrors, while bromobenzene and dichlo-
complete and quantitative. The reaction mixture was transferred to
a tared vial, and pentane (3 mL) was added and the reaction mixture
cooled to -35 °C. After the formation of fine needlelike crystals,
the supernatant was decanted, and the crystals were dried in vacuo
1
(21 mg, 96%). H NMR: δ 7.8 (s, 1H, HCdN), 4.7 (septet, 1H,
3
3JH-H ) 6 Hz, CH(CH3)2), 3.8 (septet, 1H, JH-H ) 6 Hz,
CH(CH3)2), 1.4 (d, 6H, 3JH-H ) 6 Hz, CH(CH3)2), 1.2 (d, 6H, 3JH-H
) 6 Hz, CH(CH3)2). 11B NMR: δ 0.3 (s). 3C{1H} NMR, partial: δ
152.1 (s, NdC(H)-N), 148.2 (dm, 1JC-F ) 245 Hz, o-C6F5), 146.1
(s, CO2), 140.6 (dm, 1JC-F ) 255 Hz, p-C6F5), 137.2 (dm, 1JC-F
)
250 Hz, m-C6F5), 52.5 (s, CH(CH3)2), 50.5 (s, CH(CH3)2), 22.9 (s,
CH(CH3)2), 21.5 CH(CH3)2). 19F NMR: δ -134.9 (dd, 4F, 3JF-F
)
1
romethane were stored over 4 Å molecular sieves. H, 11B, 13C,
4
3
24 Hz, JF-F ) 9 Hz, o-C6F5), -156.4 (t, 2F, JF-F ) 21 Hz,
p-C6F5), -163.6 (m, 4F, m-C6F5). Anal. Calcd for C20H15BN2O2F10
(516.147): C, 46.54; H, 2.92; N, 5.42. Found: C, 46.49; H, 3.04;
N, 5.55. X-ray-quality crystals were grown from slow cooling of a
solution in 3:1 pentane/dichloromethane.
19F, and 31P NMR spectra were recorded at 25 °C on Varian 300
and 400 MHz and Bruker 400 MHz spectrometers. Chemical shifts
are given relative to SiMe4 and referenced to the residual solvent
signal (1H, 13C) or relative to an external standard (11B, (Et2O)BF3;
19F, CFCl3; 31P, 85% H3PO4). Spectra were recorded in CD2Cl2
unless otherwise noted. In some instances, signal and/or coupling
assignments were derived from two-dimensional NMR experiments.
Chemical shifts are reported in parts per million and coupling
constants as scalar values in hertz. Combustion analyses were
performed in-house employing a PerkinElmer CHN analyzer.
Synthesis of HC(iPrN)2C(iPrN)2B(C6F5)2 (4). HB(C6F5)2 (69
mg, 0.2 mmol) was dissolved in dichloromethane (8 mL), and
isopropyl carbodiimide (51 mg, 0.4 mmol) in dichloromethane (1
mL) was added. The reaction mixture was stirred for 48 min, after
which the solvent was concentrated to ∼1 mL under reduced
pressure. Pentane (2 mL) was added, and subsequent cooling to
-35 °C afforded colorless crystals (92 mg, 79%). 1H NMR: δ 7.5
(s, 1H, HCdN), 4.2 (m, br, 1H, CH(CH3)2), 3.7 (septet, 1H, 3JH-H
59
HB(C6F5)2,52 Ph(iPrN)2Li,58 and PhMe ·Zn(C6F5)2 were synthe-
sized via literature procedures. All of the other reagents were
purchased from Aldrich; liquids were stored over 4 Å molecular
sieves, and gases and solutions were used as received.
Synthesis of HC(RN)2B(C6F5)2 [R ) iPr (1), tBu (2)]. These
compounds were prepared in a similar fashion, and thus, only the
preparation of 2 is detailed. HB(C6F5)2 (100 mg, 0.3 mmol) was
dissolved in dichloromethane (8 mL), and tert-butyl carbodiimide
(44.5 mg, 0.3 mmol) in dichloromethane (1 mL) was added
dropwise. The reaction mixture was stirred for 30 min, after which
the solvent was removed under reduced pressure. The resultant
white solid was recrystallized from pentane to afford colorless
crystals of 2.
3
) 6 Hz, CH(CH3)2), 3.6 (septet, 1H, JH-H ) 6 Hz, CH(CH3)2),
3
3
3.2 (m, 1H, JH-H ) 7 Hz, CH(CH3)2), 1.4 (d, 6H, JH-H ) 7 Hz,
3
CH(CH3)2), 1.1 (d, 6H, JH-H ) 7 Hz, CH(CH3)2), 1.05 (d, 6H,
3JH-H ) 7 Hz, CH(CH3)2), 1.0 (d, 6H, CH(CH3)2). 11B NMR: δ
-3.7 (s). 13C{1H} NMR, partial: δ 150.8 (s, NdC(H)-N), 149.6
(dm, 1JC-F ) 240 Hz, o-C6F5), 140.3 (dm, 1JC-F ) 250 Hz, p-C6F5),
138.8 (s, N-C(N)dN), 137.7 (dm, 1JC-F ) 245 Hz, m-C6F5), 54.1
(s, CH(CH3)2), 50.2 (s, CH(CH3)2), 49.3 (s, CH(CH3)2), 49.2 (s,
CH(CH3)2), 25.6 (s, CH(CH3)2), 23.5 (s, CH(CH3)2), 22.9 (s,
CH(CH3)2), 20.2 (s, CH(CH3)2). 19F NMR: δ -132.4 (s, br, 4F,
o-C6F5), -158.9 (tm, 2F, 3JF-F ) 20 Hz, p-C6F5), -165.1 (dd, 4F,
Data for 1: Colorless crystals, 281 mg, 84%. 1H NMR: δ 7.8 (s,
3JF-F ) 23 Hz, JF-F ) 4 Hz, m-C6F5). Anal. Calcd for
4
3
1H, HCdN), 3.8 (septet, 2H, JH-H ) 7 Hz, CH(CH3)2), 1.2 (d,
C26H29BN4F10 (598.340): C, 52.19; H, 4.89; N, 9.36. Found: C,
51.88; H, 5.11; N, 9.60. The crystalline product was suitable for
X-ray analysis.
12H, 3JH-H ) 7 Hz, CH(CH3)2). 11B NMR: δ 2.2 (s). 13C{1H} NMR,
1
partial: δ 159.4 (s, NdC(H)-N), 148.1 (dm, JC-F ) 240 Hz,
o-C6F5), 140.5 (dm, 1JC-F ) 260 Hz, p-C6F5), 137.8 (dm, 1JC-F
)
Synthesis of HC(RN)2(CO)B(C6F5)2 [R ) iPr (5), tBu (6)].
These compounds were prepared in a similar fashion, and thus,
only the preparation of 5 is detailed. A solution of 1 (22 mg, 0.04
mmol) in CD2Cl2 (∼ 0.8 mL) in a Teflon-sealed NMR tube was
degassed via the freeze-pump-thaw technique and pressurized
with ∼1 atm CO. The solution was left for 36 h, at which point
1H, 11B, and 19F spectra revealed the reaction to be complete and
quantitative. The reaction mixture was transferred to a tared vial,
and pentane (3 mL) was added. After the formation of fine
needlelike crystals, the supernatant was decanted, and the crystals
were dried in vacuo (20 mg, 88%).
260 Hz, m-C6F5), 47.2 (s, CH(CH3)2), 23.4 (s, CH(CH3)2). 19F NMR:
δ -133.8 (d, 4F, 3JF-F ) 23 Hz, o-C6F5), -157.9 (tm, 2F, 3JF-F
)
3
20 Hz, p-C6F5), -164.3 (m, 4F, JF-F ) 19 Hz, m-C6F5). Anal.
Calcd for C19H15BN2F10 (472.138): C, 48.34; H, 3.20; N, 5.93.
Found: C, 48.15; H, 3.23; N, 6.01. X-ray-quality crystals were
grown from slow cooling of a solution in pentane.
(57) Zhao, X.; Gilbert, T. M.; Stephan, D. W. Chem.sEur. J. [Online early
access]. DOI: 10.1002/chem.201001575. Published Online: July 21,
2010.
(58) Hayashida, T.; Kondo, H.; Terasawa, J.-i.; Kirchner, K.; Sunada, Y.;
Nagashima, H. J. Organomet. Chem. 2007, 692, 382–394.
(59) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M.
Organometallics 2001, 20, 3772–3776.
1
Data for 5: Yield: 20 mg, 88%. H NMR (CD2Cl2): δ 8.3 (s,
1H, HCdN), 4.4 (septet, 1H, 3JH-H ) 6 Hz, CH(CH3)2), 3.8 (septet,
3
3
1H, JH-H ) 6 Hz, CH(CH3)2), 1.3 (d, 6H, JH-H ) 6 Hz,
9
J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010 13565

A R T I C L E S
Dureen and Stephan
3
CH(CH3)2), 1.2 (d, 6H, JH-H ) 6 Hz, CH(CH3)2). 11B NMR
(CD2Cl2): δ -11.0 (s). 13C{1H} NMR (CD2Cl2), partial: δ 157.7
(s, NdC(H)-N), 148.6 (dm, 1JC-F ) 248 Hz, o-C6F5), 140.6 (dm,
(s, NdC(H)-N), 148.2 (dm, 1JC-F ) 249 Hz, o-C6F5), 138.6 (dm,
1
1JC-F ) 256 Hz, p-C6F5), 138.1 (s), 137.4 (dm, JC-F ) 252 Hz,
m-C6F5), 129.3 (s), 128.5 (s), 127.6 (s), 84.6 (s, CH(Ph)), 50.5 (s,
CH(CH3)2), 50.3 (s, CH(CH3)2), 23.7 (s, CH(CH3)2), 22.7 (s,
CH(CH3)2), 21.9 (s, CH(CH3)2), 20.9 (s, CH(CH3)2). 19F NMR: δ
-132.4 (m, 2F, o-C6F5), -135.3 (s, br, 2F, o-C6F5), -158.9 (t, 1F,
1JC-F ) 266 Hz, p-C6F5), 137.8 (dm, JC-F ) 255 Hz, m-C6F5),
1
51.6 (s, CH(CH3)2), 43.6 (s, CH(CH3)2), 23.3 (s, CH(CH3)2), 22.2
(s, CH(CH3)2). 19F NMR (CD2Cl2): δ -133.2 (m, 4F, o-C6F5),
3
3
3JF-F ) 18 Hz, p-C6F5), -159.1 (t, 1F, JF-F ) 19 Hz, p-C6F5),
-158.0 (t, 2F, JF-F ) 20 Hz, p-C6F5), -164.2 (m, 4F, m-C6F5).
Anal. Calcd for C20H15BN2OF10 (500.148): C, 48.03; H, 3.02; N,
5.60. Found: C, 47.85; H, 2.63; N, 5.89. X-ray-quality crystals were
grown from slow diffusion of pentane into a solution in
dichloromethane.
-164.8 (m, 2F, m-C6F5), -165.1 (m, 2F, m-C6F5). Anal. Calcd for
C26H21BN2OF10 (578.262): C, 54.00; H, 3.66; N, 4.84. Found: C,
53.89; H, 3.77; N, 4.59.
1
Data for 10: Colorless microcrystalline solid, 17 mg, 70%. H
Data for 6: The reaction was complete after 3 weeks under ∼1
NMR: δ 8.3 (s, 1H, HCdN), 7.1-7.0 (m, 5H, Ph), 6.2 (s, 1H,
CH(Ph)), 1.5 (s, 9H, C(CH3)3), 1.3 (s, 9H, C(CH3)3). 11B NMR: δ
-3.1 (s). 13C{1H} NMR, partial: δ 153.5 (s, NdC(H)-N), 138.6
(s), 1838.5 (s), 127.9 (s, br), 81.3 (s, CH(Ph)), 59.95 (s, C(CH3)3),
59.4 (s, C(CH3)3), 30.5 (s, C(CH3)3), 29.3 (s, C(CH3)3). 19F NMR:
1
atm CO. Microcrystalline solid, 5 mg, 97%. H NMR: δ 8.4 (s,
1H, HCdN), 1.5 (s, 9H, C(CH3)3), 1.3 (s, 9H, C(CH3)3). 11B NMR:
δ -11.5 (s). 13C{1H} NMR, partial: δ 159.5 (s, NdC(H)-N), 148.8
(dm, 1JC-F ) 239 Hz, o-C6F5), 140.5 (dm, 1JC-F ) 261 Hz, p-C6F5),
1
3
4
138.0 (dm, JC-F ) 257 Hz, m-C6F5), 59.2 (s, C(CH3)3), 57.5 (s,
δ -132.3 (dd, 1F, JF-F ) 26 Hz, JF-F ) 8 Hz, o-C6F5), -133.3
C(CH3)3), 30.1 (s, C(CH3)3), 28.7 (s, C(CH3)3). 19F NMR: δ -132.1
3
3
(d, 1F, JF-F ) 24 Hz, o-C6F5), -133.6 (d, 1F, JF-F ) 22 Hz,
o-C6F5), -132.4 (dd, 1F, JF-F ) 24 Hz, JF-F ) 8 Hz, o-C6F5),
(s, br, 4F, o-C6F5), -157.9 (t, 2F, 3JF-F ) 20 Hz, p-C6F5), -164.1
3
4
3
4
3
3
(td, 4F, JF-F ) 20 Hz, JF-F ) 6 Hz, m-C6F5). Anal. Calcd for
C22H19BN2OF10 (528.202): C, 50.03; H, 3.63; N, 5.30. Found: C,
50.15; H, 3.44; N, 5.01.
-159.7 (t, 1F, JF-F ) 19 Hz, p-C6F5), -162.0 (t, 1F, JF-F ) 24
Hz, p-C6F5), -165.6 (m, 2F, m-C6F5), -167.5 (m, 1F, m-C6F5),
-167.7 (m, 1F, m-C6F5). Anal. Calcd for C28H25BN2OF10 (606.316):
C, 55.47; H, 4.16; N, 4.62. Found: C, 55.52; H, 4.33; N, 4.73. X-ray-
quality crystals were grown from slow diffusion of pentane into a
solution in dichloromethane.
Synthesis of HC(RN)2(CNtBu)B(C6F5)2 [R ) iPr (7), tBu
(8)]. These compounds were prepared in a similar fashion, and thus,
only the preparation of 7 is detailed. A solution of tert-butylisonitrile
(8.8 mg, 0.11 mmol) in pentane (1 mL) was added in one portion
to a solution of 1 (50 mg, 0.11 mmol) in bromobenzene (2 mL).
The solvent was removed under reduced pressure and the residue
recrystallized from pentane to afford clear, colorless crystals (50
mg, 85%).
Synthesis of HC(iPrN)2(MeCN)B(C6F5)2 (11). Acetonitrile (20
mg, 0.05 mmol) in pentane (1 mL) was added in one portion to a
solution of 1 (25 mg, 0.05 mmol) in dichloromethane (2 mL). The
solvent was removed under reduced pressure and the residue washed
with cold pentane (0.5 mL) to afford a white powder (25 mg, 87%).
1
3
1H NMR: δ 7.6 (s, 1H, HCdN), 4.2 (septet, 1H, JH-H ) 7 Hz,
Data for 7: H NMR: δ 7.9 (s, 1H, HCdN), 4.8 (septet, 1H,
3
3JH-H ) 7 Hz, CH(CH3)2), 3.8 (septet, 1H, JH-H ) 6 Hz,
CH(CH3)2), 3.6 (septet, 1H, 3JH-H ) 7 Hz, CH(CH3)2), 2.1 (s, 3H,
CH(CH3)2), 1.3 (d, 6H, 3JH-H ) 7 Hz, CH(CH3)2), 1.1 (d, 6H, 3JH-H
) 6 Hz, CH(CH3)2), 0.9 (s, 9H, C(CH3)3). 11B NMR: δ -10.0 (s).
13C{1H} NMR, partial: δ 153.8 (s, NdC(H)-N), 149.2 (dm, 1JC-F
3
3
Me), 1.4 (d, 6H, JH-H ) 7 Hz, CH(CH3)2), 1.1 (d, 6H, JF-F ) 7
Hz, CH(CH3)2). 11B NMR: δ -4.4 (s). 13C{1H} NMR, partial: δ
1
150.2 (s, NdC(H)-N), 149.6 (dm, JC-F ) 240 Hz, o-C6F5), 141
1
0.4 (s, MeC) 140.2 (dm, 1JC-F ) 245 Hz, p-C6F5), 137.6 (dm, 1JC-F
) 260 Hz, m-C6F5), 52.1 (s, CH(CH3)2), 51.0 (s, CH(CH3)2), 23.5
(s, CH(CH3)2), 23.3 (s, CH(CH3)2), 23.0 (s, CH3). 19F NMR: δ
-135.3 (dd, 4F, 3JF-F ) 24 Hz, 4JF-F ) 8 Hz, o-C6F5), -159.8 (t,
2F, 3JF-F ) 21 Hz, p-C6F5), -165.1 (m, 4F, m-C6F5). Anal. Calcd
for C21H18BN3F10 (519.19): C, 49.15; H, 3.54; N, 8.19. Found: C,
48.81; H, 3.96; N, 7.87. X-ray-quality crystals were grown from
diffusion of pentane into a solution in dichloromethane.
) 232 Hz, o-C6F5), 140.2 (dm, JC-F ) 260 Hz, p-C6F5), 138.0
1
(dm, JC-F ) 273 Hz, m-C6F5), 54.8 (s, C(CH3)3), 47.9 (s,
CH(CH3)2), 43.2 (s, CH(CH3)2), 30.7 (s, C(CH3)3), 23.8 (s,
CH(CH3)2), 22.4 (s, CH(CH3)2). 19F NMR: δ -129.9 (s, br, 4F,
o-C6F5), -158.5 (tm, 2F, 3JF-F ) 18 Hz, p-C6F5), -164.6 (td, 4F,
4
3JF-F ) 20 Hz, JF-F ) 6 Hz, m-C6F5). Anal. Calcd for
C24H24BN3F10 (555.272): C, 51.92; H, 4.36; N, 7.57. Found: C,
52.05; H, 4.62; N, 7.52. X-ray-quality crystals were grown from
slow cooling of a solution in pentane.
Synthesis of HC(iPrN)(iPrNH)B(C6F5)2(CCPh) (12). Phenyl-
acetylene (0.1 mL, 0.9 mmol) was added in one portion to a solution
of 1 (11 mg, 0.02 mmol) in CD2Cl2 (0.8 mL). The reaction was
monitored by 11B NMR spectroscopy and appeared to reach ∼80%
completion after 1 week, at which point pentane (1 mL) was added
and the reaction mixture cooled to -35 °C overnight to afford
1
Data for 8: Colorless microcrystalline solid, 21 mg, 90%. H
NMR: δ 8.1 (s, 1H, HCdN), 1.6 (s, 9H, C(CH3)3), 1.1 (s, 9H,
C(CH3)3), 0.9 (s, 9H, C(CH3)3). 11B NMR: δ -10.07 (s). 13C{1H}
1
NMR, partial: δ 156.2 (s, NdC(H)-N), 149.0 (dm, JC-F ) 241
Hz, o-C6F5), 140.6 (dm, 1JC-F ) 245 Hz, p-C6F5), 137.9 (dm, 1JC-F
) 260 Hz, m-C6F5), 58.2 (s, C(CH3)3), 58.0 (s, C(CH3)3), 55.2 (s,
C(CH3)3), 30.8 (s, C(CH3)3), 30.7 (s, C(CH3)3), 29.3 (s, C(CH3)3).
19F NMR: δ -128.3 (s, br, 4F, o-C6F5), -158.8 (t, 2F, 3JF-F ) 21
Hz, p-C6F5), -164.9 (m, 4F, m-C6F5). Anal. Calcd for C26H28BN3F10
(583.326): C, 53.54; H, 4.84; N, 7.20. Found: C, 53.84; H, 4.92;
N, 7.10.
1
colorless crystals (10 mg, 75%). H NMR: δ 7.4 (d, 1H, HC )
NH), 7.3 (m, 2H, Ph), 7.3-7.2 (m, 3H, Ph), 7.0 (m, br, dNH),
3.93 (septet, 1H, 3JH-H ) 6 Hz, CH(CH3)2), 3.6 (m, 1H, CH(CH3)2),
3
3
1.3 (d, 6H, JH-H ) 7 Hz, CH(CH3)2), 1.2 (d, 6H, JH-H ) 7 Hz,
CH(CH3)2). 11B NMR: δ -12.9 (s). 13C{1H} NMR, partial: δ 154.2
(s, NdC(H)-N), 148.3 (dm, 1JC-F ) 240 Hz, o-C6F5), 140.0 (dm,
1JC-F ) 248 Hz, p-C6F5), 137.9 (dm, JC-F ) 250 Hz, m-C6F5),
1
Synthesis of HC(RN)2(PhHCO)B(C6F5)2 [R ) iPr (9), tBu
(10)]. These compounds were prepared in a similar fashion, and
thus, only the preparation of 9 is detailed. A solution of benzal-
dehyde (6.5 mg, 0.05 mmol) in pentane (1 mL) was added in one
portion to a solution of 1 (29 mg, 0.05 mmol) in toluene (2 mL).
The solvent was removed under reduced pressure, and the residue
was washed with pentane (2 mL) and dried in vacuo to afford a
white powder (29 mg, 82%).
131.7 (s), 128.8 (s), 128.0 (s), 125.5(s), 50.9 (s, CH(CH3)2), 23.9
(s, CH(CH3)2), 23.8 (s, CH(CH3)2). 19F NMR: δ -133.4 (dd, 4F,
3JF-F ) 23 Hz, JF-F ) 7 Hz, o-C6F5), -159.4 (t, 2F, JF-F ) 21
Hz, p-C6F5), -165.0 (m, 4F, m-C6F5). Anal. Calcd for C27H21BN2F10
(574.274): C, 56.47; H, 3.69; N, 4.88. Found: C, 56.35; H, 4.05;
N, 5.01. X-ray-quality crystals were grown from slow diffusion of
pentane into a solution in dichloromethane.
4
3
1
Synthesis of PhC(iPrN)2BCl2 (13). A solution of PhC(iPrN)2Li
(432 mg, 2.2 mmol) in toluene (8 mL) was cooled to -35 °C, at
which point boron trichloride (2.2 mL, 1.0 M in heptane, 2.2 mmol)
was added in one portion. The reaction mixture was stirred for 5
min, after which the solvent was removed under reduced pressure.
The residue was extracted into pentane (15 mL) and filtered through
Data for 9: H NMR: δ 7.8 (s, 1H, HCdN), 7.4-7.3 (m, 5H,
Ph), 5.5 (s, 1H, CH(Ph)) 3.5 (septet, 1H, 3JH-H ) 7 Hz, CH(CH3)2),
3
3
3.3 (septet, 1H, JH-H ) 7 Hz, CH(CH3)2), 1.3 (d, 3H, JH-H ) 7
3
Hz, CH(CH3)2), 1.2 (d, 3H, JH-H ) 7 Hz, CH(CH3)2), 1.15 (d,
3H, JH-H ) 7 Hz, CH(CH3)2), 1.05 (d, 3H, JH-H ) 7 Hz,
CH(CH3)2). 11B NMR: δ -1.5 (s). 13C{1H} NMR, partial: δ 150.4
3
3
9
13566 J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010

Reactions of Boron Amidinates with Small Molecules
A R T I C L E S
Table 1. Crystallographic Data
1
2
3·1/2CH2Cl2
4
5
7
formula
formula weight
crystal system
space group
a (Å)
C19H15BN2F10
472.14
monoclinic
P21/n
13.2107(10)
11.2408(9)
13.749(1)
90
91.133(2)
90
2041.3(3)
4
C21H19BN2F10
500.19
monoclinic
P21/c
11.4966(6)
11.8061(6)
16.2558(9)
90
101.241(3)
90
2164.1(2)
4
C20.5H16BN2O2F10Cl
558.11
monoclinic
C2/c
34.510(3)
11.4156(10)
12.7933(11)
90
109.377(8)
90
4754.5(7)
8
C26H29BN4F10
598.34
monoclinic
P21/n
11.2343(5)
16.3811(7)
15.2027(7)
90
97.190(2)
90
2775.8(2)
4
C20H15BN2OF10
500.15
monoclinic
P21/n
16.1775(8)
9.8189(5)
12.9563(6)
90
95.769(1)
90
2047.62(17)
4
C24H24BN3F10
555.27
monoclinic
P21/n
10.3891(4)
11.6890(4)
21.6116(7)
90
96.453(1)
90
2607.85(16)
4
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
T (K)
150(2)
1.536
0.155
16705
0.0447
4105
150(2)
1.535
0.150
19726
0.0365
4988
150(2)
1.559
0.261
48727
0.1114
5561
331
0.0686
0.2653
0.968
170(2)
1.432
0.132
44638
0.0587
6373
150(2)
1.622
0.163
50738
0.0369
6751
150(2)
1.414
0.134
74894
0.0381
5995
dcalc (g/cm3)
abs. coeff, Γ (mm-1
data collected
Rint
)
data used
variables
R (>2σ)
wR2
297
307
370
311
0.134
0.0425
0.1135
0.997
0.0388
0.0944
1.016
0.0396
0.0989
1.011
0.0358
0.1005
1.026
0.0453
0.1359
1.029
GOF
10
11
12
14
15
formula
formula weight
crystal system
space group
a (Å)
C28H25BN2OF10
606.31
monoclinic
P21/n
9.7338(3)
14.8731(4)
18.2826(5)
90
93.477(2)
90
2641.93(13)
4
C21H18BN3F10
519.19
C27H21BF10N2
574.27
C25H19BN2F10
C19H15BF10N2
472.14
548.23
orthorhombic
Pcba
14.6255(12)
16.4744(16)
20.2803(18)
90
90
90
4886.5(8)
8
triclinic
triclinic
triclinic
j
j
j
P1
P1
P1
8.2916(12)
10.6546(15)
12.5718(18)
100.759(7)
98.769(7)
101.432(7)
1048.6(3)
2
8.4288(2)
9.3068(4)
17.6394(8)
76.475(2)
85.958(2)
89.111(2)
1342.00(10)
2
14.3554(2)
15.9382(2)
20.1115(4)
89.5980(10)
74.1920(10)
64.3460(10)
3958.26(14)
8
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
T (K)
150(2)
1.524
0.141
68526
0.0424
12607
150(2)
1.625
0.159
10708
0.0306
7464
321
0.0429
150(2)
1.421
0.132
25676
0.0286
6154
369
0.0531
0.1387
1.014
150(2)
1.490
0.141
82565
0.0584
5598
150(2)
1.585
0.159
103071
0.0517
21261
1169
0.0517
0.0947
dcalc (g/cm3)
abs. coeff, Γ (mm-1
data collected
Rint
data used
variables
R (>2σ)
wR2 (>2σ)
GOF
)
385
348
0.0405
0.1176
1.026
0.0374
0.0875
0.966
0.1282
1.027
1.025
1
Celite. The reaction mixture was concentrated to ∼1 mL in vacuo
and cooled to -35 °C to afford colorless crystals (351 mg, 56%).
1H NMR: δ 7.7-7.6 (m, 3H, Ph), 7.5 (m, 2H, Ph) 3.9 (m, 2H,
3JH-H ) 7 Hz, CH(CH3)2), 1.3 (d, 12H, 3JH-H ) 7 Hz, CH(CH3)2).
11B NMR: δ 6.0 (s). 13C{1H} NMR, partial: δ 174.0 (s, br,
NdC(Ph)-N), 132.8(s), 129.9 (s), 128.3(s), 125.9 (s, br), 46.9 (s,
CH(CH3)2), 22.9 (s, CH(CH3)2). Anal. Calcd for C13H19BN2Cl2
(285.025): C, 54.78; H, 6.72; N, 9.83. Found: C, 54.53; H, 6.64;
N, 9.81.
1JC-F ) 251 Hz, p-C6F5), 137.3 (dm, JC-F ) 251 Hz, m-C6F5),
131.4 (s), 129.1 (s), 127.7 (s), 127.5 (s), 46.2 (s, CH(CH3)2), 22.2
(s, CH(CH3)2). 19F NMR: δ -132.5 (dd, 4F, 3JF-F ) 24 Hz, 4JF-F
3
) 9 Hz, o-C6F5), -158.2 (t, 2F, JF-F ) 21 Hz, p-C6F5), -164.4
(m, 4F, JF-F ) 19 Hz, m-C6F5). Anal. Calcd for C25H19BN2F10
(548.256): C, 54.77; H, 3.49; N, 5.11. Found: C, 54.69; H, 3.53;
N, 5.29. X-ray-quality crystals were grown from slow cooling of a
solution in pentane.
3
Synthesis of HC(iPrN)2(C6F4)BF(C6F5) (15). A solution of 1
(15 mg, 0.026 mmol) in C6D5Br (0.8 mL) was heated to 120 °C
for 5 min and then cooled to room temperature. The reaction did
Synthesis of PhC(iPrN)2B(C6F5)2 (14). A solution of 13 (351
mg, 1.2 mmol) was dissolved in dichloromethane (5 mL) and cooled
to -35 °C, after which a solution of PhMe ·Zn(C6F5)2 (605 mg,
1.2 mmol) in dichloromethane (3 mL) was added, resulting in the
immediate formation of a white precipitate. After the reaction
mixture was warmed to ambient temperature, the solvent was
removed under reduced pressure, and the residue was extracted into
pentane (10 mL) and filtered through Celite. The solution was
concentrated to ∼2 mL and cooled to -35 °C to afford colorless
1
not proceed below 80 °C according to H NMR analysis. Pentane
(2 mL) was added and the reaction mixture cooled to -35 °C
overnight to afford colorless crystals that were then dried under
1
4
reduced pressure (10 mg, 67%). H NMR: δ 7.8 (d, 1H, JH-H
)
)
2 Hz, HCdN), 4.9 (septet of doublets, 1H, 3JH-H ) 7 Hz, 4JH-H
3
2 Hz, CH(CH3)2), 4.0 (septet, 1H, JH-H ) 7 Hz, CH(CH3)2), 1.6
3
3
(d, 3H, JH-H ) 7 Hz, CH(CH3)2), 1.5 (d, 3H, JH-H ) 7 Hz,
CH(CH3)2), 1.4 (d, 3H, 3JH-H ) 7 Hz, CH(CH3)2), 1.2 (d, 6H, 3JH-H
) 7 Hz, CH(CH3)2). 11B NMR: δ -0.6 (d, 1JB-F ) 59 Hz). 13C{1H}
1
crystals (635 mg, 94%). H NMR: δ 7.7-7.6 (m, 5H, Ph), 3.8
(septet, 2H, 3JH-H ) 7 Hz, CH(CH3)2), 0.9 (d, 12H, 3JH-H ) 7 Hz,
CH(CH3)2). 11B NMR: δ 1.1 (s). 13C{1H} NMR, partial: δ 171.6
(s, NdC(Ph)-N), 148.1 (dm, 1JC-F ) 245 Hz, o-C6F5), 139.9 (dm,
1
NMR, partial: δ 148.4 (dm, JC-F ) 241 Hz, o-C6F5), 148.3 (s,
NdC(H)-N), 141.4 (dm, JC-F ) 249 Hz), 140.6 (dm, JC-F
1
1
)
9
J. AM. CHEM. SOC. VOL. 132, NO. 38, 2010 13567

A R T I C L E S
Dureen and Stephan
1
1
249 Hz), 138.3 (dm, JC-F ) 251 Hz), 137.7 (dm, JC-F ) 248
Fourier map calculations. The refinements were carried out using
2
full-matrix least-squares techniques on F, minimizing the function
Hz), 122.3 (d, JC-F ) 12 Hz, N-CAr), 119.3 (s, br, B-C), 55.4
w(Fo - Fc)2, where the weight w is defined as 4Fo /2σ(Fo ) and Fo
and Fc are the observed and calculated structure factor amplitudes,
respectively. In the final cycles of each refinement, all of the non-
hydrogen atoms were assigned anisotropic temperature factors in
the absence of disorder or insufficient data. In the latter cases, atoms
were treated isotropically. C-H atom positions were calculated and
allowed to ride on the carbon to which they were bonded, assuming
a C-H bond length of 0.95 Å. H-atom temperature factors were
fixed at 1.10 times the isotropic temperature factor of the C atom
to which they were bonded. The H-atom contributions were
calculated but not refined. The locations of the largest peaks in the
final difference Fourier map calculation as well as the magnitude
of the residual electron densities in each case were of no chemical
significance. The results of the crystallographic analyses are shown
in Table 1.
2
2
(s, CH(CH3)2), 55.2 (s, CH(CH3)2), 51.4 (s, CH(CH3)2), 25.1 (s,
CH(CH3)2), 24.1 (s, CH(CH3)2), 23.0 (s, CH(CH3)2), 22.96 (s,
CH(CH3)2). 19F NMR: δ -134.4 (m, 1F), -135.3 (td, 2F, 3JF-F
)
4
23 Hz, JF-F ) 9 Hz, o-C6F5), 148.3 (t, 1F, JF-F ) 14 Hz), 157.5
3
(t, JF-F ) 18 Hz), -158.7 (t, 1F, JF-F ) 21 Hz, p-C6F5), -159.7
(dd, 1F, JF-F ) 23 Hz, JF-F ) 21 Hz), -165.0 (m, 4F, m-C6F5),
169.3 (m, br, B-F). Anal. Calcd for C19H15BN2F10 (472.138): C,
48.33; H, 3.20; N, 5.93. Found: C, 47.83; H, 3.42; N, 5.82. X-ray-
quality crystals were grown from slow diffusion of pentane into a
solution in dichloromethane.
X-ray Data Collection and Reduction. Crystals were coated
in Paratone-N oil in the glovebox, mounted on a MiTegen
Micromount, and placed under a N2 stream, thus maintaining a dry,
O2-free environment for each crystal. The data were collected on a
Bruker Apex II diffractometer employing Mo KR radiation (λ )
0.71073 Å). Data collection strategies were determined using Bruker
Apex software and optimized to provide >99.5% complete data to
a 2θ value of at least 55°. The data were collected at 150 ( 2 K
for all of the crystals except 4, for which the data were collected
at 170 K. The frames were integrated with the Bruker SAINT
software package using a narrow-frame algorithm. Data were
corrected for absorption effects using the empirical multiscan
method (SADABS).
Acknowledgment. D.W.S. gratefully acknowledges financial
support from NSERC of Canada. D.W.S. is grateful for the support
of a Canada Research Chair and a Killam Research Fellowship
from the Killam Foundation.
Supporting Information Available: Crystallographic data
(CIF) for compounds 1-5, 7, 10-12, 14, and 15. This material
is available free of charge via the Internet at http://pubs.acs.org.
X-ray Data Solution and Refinement. Non-hydrogen atomic
scattering factors were taken from literature tabulations.60 The
heavy-atom positions were determined using direct methods
employing the SHELXTL direct-methods routine. The remaining
non-hydrogen atoms were located from successive difference
JA1064153
(60) Cromer, D. T.; Waber, J. T. Int. Tables X-Ray Crystallogr. 1974, 4,
71–147.
9
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