Synthesis, structure and palladium coordination of ambiphilic, pyridine- and phosphine-tethered N -boryl imine ligands

Article abstract of DOI:10.1039/c8dt05100c

We describe here the synthesis and structural characterization of two new classes of ambiphilic, N-boryl imine ligands, wherein boron is associated with a Lewis basic imine nitrogen. These ligands can be easily generated in two steps from the corresponding pyridinyl- and phosphinyl-tethered aldehydes. ¹¹B NMR analysis suggests the association of the Lewis acidic boron to either the pyridine unit or via intermolecular acid/base interactions with the imine. Both of these ligands can coordinate to palladium, and their structures were confirmed by X-ray crystallography..

Full text of DOI:10.1039/c8dt05100c

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DOI: 10.1039/C8DT05100C
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ARTICLE
Synthesis, Structure and Palladium Coordination of Ambiphilic,
Pyridine- and Phosphine-Tethered N-Boryl Imine Ligands
Received 00th January 20xx,
Accepted 00th January 20xx
Fabio Lorenzini,a Pierre-Louis Lagueux-Tremblay,a Laure Kayser,a Ethan Anderson,a and Bruce A. Arndtsena,*
DOI: 10.1039/x0xx00000x
We describe here the synthesis and structural characterization of two new classes of ambiphilic, N-boryl imine ligands,
wherein boron is associated to a Lewis basic imine nitrogen. These ligands can be easily generated in two steps from the
corresponding pyridinyl- and phosphinyl-tethered aldehydes. 11B NMR analysis suggests the association of the Lewis acidic
boron to either the pyridine unit or via intermolecular acid/base interactions with the imine. Both of these ligands can
coordinate to palladium, and their structures were confirmed by X-ray crystallography.
www.rsc.org/
metal-bonding atom (Figure 1b), although in these cases the
stability and coordination mode can be less straightforward to
control. For example, Bourissou has reported to coordination
chemistry of boryl-phosphines.7 This ligand displays significant
B-P -bonding due to interaction of the boron empty -orbital
with the phosphorus lone pair, thus limiting access to the
Lewis acid site of other bases. Alternatively, Nakamura has
suggested that in situ metallated alkoxide ligands can facilitate
nickel-catalyzed cross coupling reactions, although the precise
metal-ligand-Lewis acid structure in these catalysts is only
postulated.8,9
Introduction
The design of ligands that can directly participate in metal-
based reactions has become a rapidly growing field of research
in organometallic chemistry.1 One promising area involves the
incorporation of Lewis acidic sites onto metal-coordinated
ligands (i.e. ambiphilic ligands).2 Recent studies has
demonstrated that Lewis acid containing ligands can influence
metal-based transformations in a number of directions, such
as forming metal-Lewis acid interactions, association to other
metal-bound ligands, and stabilizing reaction products (Figure
1a). An application where Lewis acid tethered ligands may
prove particularly powerful in orienting potentially reactive
Lewis bases near metal centers. Such systems offer the
potential to exploit the well-established ability of Lewis acids
to activate substrates towards metal-based transformations,
and have been the subject of several recent reports.3
a) Potential coordination modes in tethered lewis acid ligands
L
MLn
B
A
L
MLn
L
MLn
X
L MLn
LA
LA
LA
S
LA
Metal/LA
interaction
Ligand/LA
interaction
Product
stabilization
(e.g. A-B = H2)
Substrate
activation
b) Lewis acids on metal-bonding atoms
A challenge in exploiting Lewis acid containing ligands in
metal-based reactions is their breadth of reactivity. For
example, while Lewis acids can potentially associate to
substrates for use in catalysis, their ability to bind to the metal
center itself or another metal-bound ligand would not only
inhibit this affect, but may even serve to deactivate the metal
toward reaction.4 One solution is to design Lewis acid-
tethered ligands where metal-acid interactions are inhibited by
their structure. Several variants of these systems have been
successfully employed in catalysis, including the incorporation
of Lewis acids into ring systems,5 or the use of a single atom
R2
P
PPh3
R2P
Pt
NiLn
Ar2B
PPh3
O
MgCl
Nakamura 2009
Bourissou 2012
c) This work: synthesis of N-boryl imine ligand
BPh2
BPh2
R2P
N
N
N
R
linker between the ligand and Lewis acid.6 An alternative Figure 1. Ambiphilic ligands and potential role of Lewis acid
would involve placing the Lewis acid directly on a potentially
association.
In light of these features, the design of new classes of
a. Department of Chemistry, McGill University, 801 Sherbrooke St. W.; Montreal, QC
H3A 0B8 Canada.
Electronic Supplementary Information (ESI) available: [Experimental procedures,
compound characterization and crystallographic data]. CCDC: 1886655, 1886652,
1886650, 1886651, 1886653, 1886654. See DOI: 10.1039/x0xx00000x
ambiphilic ligands that place Lewis acids into structurally
distinct environments can prove useful. One such system that
we considered is to use N-boryl imines (Figure 1c). The latter
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structures can provide a method to access boron-tethered ( -0.02 (5H), and  0.06 (4H)). The latter behavioVriepwreArstiuclme Oanblinlye
DOI: 10.1039/C8DT05100C
ligands that are: a) well-defined, b) incorporate a Lewis acidic arises from the reversible association of LiOTMS ( 0.02 (s,
boron adjacent to a Lewis basic atom, c) do not show 9H)) to 2a in solution. The picoline imine 2b can be generated
diminished Lewis acidic character, yet d) are easily generated and isolated via a similar procedure. In this case, the imine
and tuned. Boryl-imines were described by Wade, Lappert, product is more soluble in hexanes, and collected in 25% yield,
Schmid and others in the 1960-1970s,10 where it was shown and its spectroscopic data is consistent with previous
they can be easily generated via N-TMS imines and even reports.12c
associate to palladium salts,10f but have not been significantly
.
BPh2
TMSOLi
N
TMS
R
N
O
exploited thereafter. A useful feature of these imines is the
position of the boron on a sp2-hybridized nitrogen, which has
been postulated to limit -bonding between these units.10
Nevertheless, it is unclear if this would be sufficient to allow
the boron to interact with Lewis bases. In addition, since these
ligands incorporate a boryl unit within close proximity to the
metal binding motif, they open the potential for
intramolecular cyclization of the Lewis acid onto this position,
and it remains uncertain what would be the correct structure
to allow for metal coordination. To address these questions,
we report here our efforts towards the generation and
structural characterization of this class of ligand. These
demonstrate that pyridine- or phosphine-derived ligands
incorporating N-boryl imines can be generated in two steps
from the appropriate aldehyde. In addition, the structure of
these ambiphilic systems, and in particular inter- and
intramolecular bonding to the boron Lewis acid, can be
modulated by the ligand framework.
1M LiNTMS2 in
hexanes
0 oC, 0.5 h
N
BPh2Cl
N
N
R
R
CH2Cl2
4-15h
R = Ph, 1a (80%)
R = H, 1b (86%)
R = Ph, 2a (82%)
R = H, 2b (25%)
Scheme 1. Synthesis of pyridine-tethered boryl imines 1.
In analogy to related N-boryl imine synthesis,10 the
reaction of the N-TMS imine 2a with BPh2Cl at ambient
temperature leads to the formation of N-(diphenylboryl)-2-
benzoylpyridinketimine (1a) over the course of 15 h (Scheme
1). A white precipitate is generated during the formation of 1a,
which is presumably LiCl arising from the reaction of LiOTMS
with the TMS-Cl by-product of the reaction. Removal of this
solid by filtration and solvent evaporation lead to 1a in 80%
yield together with trace TMS impurities.14 1a is an air- and
moisture-sensitive pink solid, and stable when stored under
N2. The 1H and 13C NMR spectra display the expected
resonances in the aromatic region, together with an upfield
shift of the imine C(Ph)=N carbon ( 160.3) arising from the
association of the electron releasing boron atom to nitrogen.
The N-boryl aldimine 1b can be generated by an analogous
reaction of 2b with BPh2Cl. This less sterically encumbered
imine is more labile, and begins to decompose with hours at
ambient temperature. It was therefore collected as a brown
solid upon filtration and solvent evaporation with TMS
impurities. In light of their lability, the structures of these
ligands were fully determined by the preparation of
protonated and metal-coordinated adducts (vide infra).
B. Structure of Pyridyl Ligands 1. 11B NMR spectra of 1a in
CDCl3 reveals an upfield signal at  10.46 (br s). The latter is in
the typical range for a quaternary boron.15 Mass spectral data
suggests that the complex is monomeric, while IR analysis
reveals a C=N band for 1a at 1593 cm-1, which is shifted from
acyclic imines (ca. 1650 cm-1), and instead in a similar region to
that noted for heterocyclic imine structures.16 Thus, we
postulate that the quaternized boron arises from the
intramolecular Lewis acid/base interaction between boron and
the basic pyridine nitrogen to generate a 5-membered boron
heterocycle.17 The generation of a cyclized structure with N-
diphenylboryl ketimine 1a was confirmed by X-ray structural
analysis. 1a can be crystallized from chloroform/CH3CN as its
protonated 1a.HCl salt (Figure 2).18 Anion exchange with
AgOTf affords the analogous 1a.HOTf. In both the structures,
the pyridyl N-diphenylboryl ketimine is in its cyclized form
resulting from the intramolecular interaction between the
boron atom and the Lewis-base pyridine nitrogen. Of note, the
B-N(1) bond length (e.g. 1a.HCl: 1.5987(19) Å) is typical of a
Results and discussion
A. Synthesis of Pyridyl Ligands 1. Our initial studies involved
the preparation of pyridyl-based ligands 1. BF2 and BEt2
substituted variants of 1a have recently been described by
Rawson, where they were isolated as cyclized protonated or
BF3 adducts.11 2-Formyl- and 2-keto-pyridines are
commercially available building blocks, and well-established as
substrates for the generation of bidentate pyridyl/imine
ligands. To adapt this towards incorporating a Lewis acid, we
examined the generation of the N-TMS substituted imine 2a
(Scheme 1). N-TMS picolinaldimine, and the related 2-furyl
imine, have been previously synthesized by Georg following
the chemistry developed by Hart and others,12 but used in situ
for further reaction.13 These procedures were therefore
modified as outlined below.
The reaction of 2-benzoyl pyridine with LiNTMS2 in hexane
leads to the in situ generation of the N-TMS imine 2a, which
undergoes spontaneous precipitation. Filtration under an inert
atmosphere affords 2a as a white solid as what has been
assigned as a LiOTMS adduct in good yield (82%). Imine 2a
decomposes over the course of several hours at ambient
temperature in CDCl3 even under an argon atmosphere. We
postulate that the ability to isolate 2a as a solid precipitate in
the procedure above is due to its stability as a LiOTMS
complex, and it is indefinitely stable when immediately
1
precipitated from hexane and stored at -35 C. The H and 13C
NMR data of 2a are consistent with the structure shown. Of
note, the imine C(Ph)=N carbon is observed at  173.8 (CDCl3),
1
while the N-TMS H NMR signal appears at two broad singlets
2 | J. Name., 2012, 00, 1-3
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borane/pyridine adduct, while the imine bond length (1a.HCl: once again in its cyclized form derived from the intramolecular
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1.2902(17) Å) is also typical of a C=N double bond.19 All other interaction between the Lewis-acidic boron and the pyridine
DOI: 10.1039/C8DT05100C
bond lengths and angles within these salts are within the nitrogen (N(2)-B(2): 1.609(4) Å). Thus, unliked related P-B
expected region, and there are no close contacts between the ligands,21 association to palladium does not disrupt N-B
counterion and the pyridine salt.
association in these five-membered ring adducts. The
palladium coordinates via the imine nitrogen (Pd(1)-N(1):
2.110(3) Å), and the allyl ligand is in the N-Pd-Cl plane. The N-
Pd-Cl bond angle (97.53(7)o) is consistent with a pseudo-
square planar structure for the Pd(II) complex.
Ph
Ph
BPh2
R
N
N
rt, 1h
B
N

[PdCl(C H )]
5 2
1/2
3
N
CHCl3
Pd
R
Cl
1a,b
R = Ph, 3a (80%)
R = H, 3b (81%)
1aHCl
Scheme 2. Synthesis of palladium complexes 3a and 3b.
The palladium complex 3b derived from the ligand 1b can
be generated by a similar procedure to that noted above
(Scheme 2). The crystal structure of this complex (Figure 3)
once again shows the cyclized ligand coordinated in a
monodentate fashion to palladium via the imine nitrogen
(Pd(1)-N(1) 2.124(3)Å). As with 3a, there is a strong boron-
pyridine interaction in the complex (N(1)-B(1) 1.597(4), N(2)-
B(1) 1.648(4)Å), and the N-Pd-Cl bond angle (94.51(8)o) is
consistent with a pseudo-square planar geometry. Unlike in
3a, the chloride ligand is directed away from the imine carbon
in 3b, presumably to minimize interactions with the ketone
aryl unit.
1aHOTf
Figure 2. X-Ray crystal structures of protonated 1a with
chloride and triflate counterions. 50% probability ellipsoids, H
atoms and solvent omitted for clarity. Selected distances (Å)
and angles (deg) for 1aHCl: B(1)-N(1): 1.5987(19); B(1)-N(2):
1.5594 (19); N(2)-C(17): 1.2902(17); C(17)-N(2)-B(1):
115.49(12); N(1)-B(1)-C(7), 110.93(11). 1aHOTf: N(2)-B(1):
1.594(3); N(1)-B(1): 1.580(3); N(1)-C(1): 1,289(2); C(1)-N(1)-
B(1): 115.23(16); N(2)-B(1)-C(13): 111.33(16).
C. Palladium Complexes of 1 We have also probed the
behavior of 1 upon metal coordination. The addition of N-
diphenylboryl ketimine 1a to [Pd(allyl)Cl]2 results in the
immediate formation of [Pd{1-N-[N-(diphenylboryl)-2-
benzoylpyridinketimine]}Cl(C3H5)] (3a) (Scheme 2). 3a can be
isolated from TMS containing impurities upon precipitation
1
with diethyl ether as a yellow powder in 80% yield. The H
3a
3b
NMR spectrum of 3a shows the expected resonances for the
allyl and boryl-imine ligands,20 and the 11B NMR displays a
single peak at  10.25. The latter is similar in chemical shift to
the cyclized form of the ligand in protonated 1a ( 10.46),
suggesting again the presence of a boron-pyridine interaction.
The latter is consistent with IR analysis of 3a, which shows a
band at 1611 cm-1 that correlates with a cyclized imine.16
In order to confirm the structure of 3a, crystals were
prepared by the slow diffusion of diethyl ether into a
chloroform solution of the in situ formed complex, which
resulted in the precipitation of 3a in orange needle-shaped
crystals. X-ray crystallography confirms the structure as shown
(Figure 3). Of note, the N-diphenylboryl ketamine ligand is
Figure 3. X-Ray crystal structures of 3a and 3b, with 30% and
50% probability ellipsoids, H atoms omitted for clarity. Only
the major isomer of the disordered allyl ligand shown.
Selected distances (Å) and angles (deg) for 3a: Pd(1)-N(1):
2.110(3); N(1)-B(2): 1.567(4); N(1)-C(1): 1.275(4); N(2)-B(2):
1.609(4). N(1)-Pd(1)-Cl(1): 97.53(7); C(1)-N(1)-Pd(1): 122.8(2);
C(1)-N(1)-B(2): 111.4(3). 3b: Pd(1)-N(1): 2.124(3); N(1)-B(1):
1.597(4); N(1)-C(4): 1.297(4); N(2)-B(1): 1.648(4). N(1)-Pd(1)-
Cl(1): 94.51(8); C(4)-N(1)-Pd(1): 120.6(2); C(4)-N(1)-B(1):
110.4(3).
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D. Synthesis of Phosphine Ligands 4. In order to address the diphenylboryl imine 4b in 74% yield (Scheme 3). As with the
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DOI: 10.1039/C8DT05100C
challenge of intramolecular acid/base interactions between other ligands above, 4b is collected with trace TMS containing
boron and pyridine, we next examined the incorporation of a impurities, and its structure was fully assigned instead upon
boryl-imine unit into a phosphine structure (4). Relative to metal coordination. 11B NMR analysis once shows evidence for
ligands 1, the phosphine-containing ligands 4 are expected to Lewis acid/base adduct formation with this ligand ( 7.2 ppm).
display a diminished propensity to cyclize due to the tertiary The 31P NMR signal for 4b is in a similar region to that noted
steric bulk of the phosphine and cyclization would lead to a for dialkylaryl phosphines ( 12.1 ppm).22
highly substituted, partially saturated 6-membered ring. As E. Palladium Coordination of 4 In order to determine if the
such, these may better favour metal coordination through the larger phosphine in these ligands can inhibit intramolecular
soft phosphine ligand. Ligand 4 can be generated in a similar cyclization, the structure of these ligands were again examined
fashion to that described above for 1. The reaction of 2- via their coordination to palladium. The reaction of 4a with
(diphenylphosphanyl)-benzaldehyde16 with LiNTMS2 at 0 C [Pd(allyl)Cl]2 results in the immediate formation of 6a (Scheme
leads to the formation of N-TMS imine 5a in 81% yield 4). 11B NMR data shows the presence of a single resonance at
(Scheme 3). As with the pyridine-based 2, 5a is stable only 6.64 ppm, which is suggestive of a tetrahedral boron arising
when kept under nitrogen and away from moisture, which from acid/base interactions. Interestingly, in situ NMR analysis
precludes purification via chromatography, and it is necessary shows the presence of two isomeric version of 6a in solution in
to use an exact stoichiometric amount of LiNTMS2 in its an approximate 1 : 1 ratio. This includes 31P NMR signals of
formation. The 1H, 13C, and 31P NMR spectra display equal intensity ( 18.2 and 18.3 ppm), as well as signals for two
resonances confirming the isolation of 5a as a LiOTMS adduct, allyl and imine ligands (1H NMR: C(H)=N:  9.74 and 9.75
including those due to the imine (C(H)=N: 1H NMR:  9.40 (s, ppm). In addition, mass spectral data show that 6a dimeric.
1H); 13C NMR:  167.3) and triarylphosphine (31P NMR:  10.9 Together, these data are consistent with the formation of 6a
ppm) units.22
either an asymmetric dimeric complex or two similar energy
rotomers. The tBu2P-substituted ligand 4b can be coordinated
to palladium to generate the analogous complex 6b via
reaction with [Pd(allyl)Cl]2. As with 6a, we observe two signals
in the 31P NMR spectrum of equal intensity ( 55.0 and 55.7),
.
TMS
BPh2
TMSOLi
O
1M LiNTMS2
in hexanes
N
N
BPh2Cl
0 oC, 0.5 h
CH2Cl2
rt, 2 h
1
along with two separate imine resonances in the H NMR
PR2
PR2
PR2
spectrum ( 10.12 and 10.46), suggestive of two isomeric
complexes. Complex 6b is not stable in solution and
undergoes decomposition over the course of 1 h.
R = Ph, 4a (82%)
R = tBu, 4b (73%)
R = Ph, 5a (81%)
R = tBu, 5b (89%)
Scheme 3. Synthesis of phosphine-tethered boryl imines 4
Cl
BPh2
Ph2
B
N
Pd
R2P
N
5a undergoes a rapid reaction with BPh2Cl, leading to the
consumption of the N-TMS imine and generation of the
analogous boryl-imine 4a, which can be collected along upon
filtration to remove LiCl, and solvent evaporation. As with the
previously boryl-imine complexes, this ligand is stable only
when stored under nitrogen.14 11B NMR analysis of 4a shows
once again a signal corresponding to a tetra-substituted boron
( 7.9 ppm). Interestingly, however, 31P NMR analysis shows a
single peak for 4a in the region expected for a simple
triarylphosphine ( 17.1 ppm),23 suggesting the boron is not
associated to phosphorus. IR analysis shows a C=N = 1632 cm-
1. This, together with the unperturbed 31P NMR and signal for a
tetrahedral boron in the 11B NMR, suggests the formation of a
dimeric ligand with boron association to a second imine (vide
infra).10
N
B
rt, 30 min
toluene
Pd
Cl
PR2
Ph2
PR2
+
R = Ph, 6a (86%)
R = tBu, 6b (58%)
1/2 [Pd(allyl)Cl]2
Scheme 4. Synthesis of palladium complexes 6a and 6b.
Crystals of both 6a and 6b were prepared by diffusion of
diethyl ether into chloroform solutions of the complexes. The
X-ray crystal structure of 6a is shown in Figure 4. This clearly
demonstrates that, unlike the pyridyl-based 1, the boron Lewis
acid does not cyclized to interact with the phosphine. Instead,
phosphorus coordination to palladium creates a square planar
complex. The palladium in the dimeric structure are in two
different rotational environments, wherein the chloride ligand
is oriented either towards or away from the bridging arene
(Cl1-Pd1-P1-C6 dihedral angle: -33.7(1); Cl2-Pd2-P2-C6’
dihedral angle: 172.0(1)). This is presumably reflected as well
in the observation of two different palladium complexes in a
1:1 ratio by solution NMR. The Pd-P bond lengths (2.2970(8) to
2.3109(8) Å) and P-Pd-Cl angles (94.80(3) and 96.45(3)) are
consistent with a normal phosphine coordinated square planar
complex.
t
The more sterically encumbered Bu2P version of this
ligand, 4b, was also generated. The starting aldehyde for this
reaction, 2-(di-o-tert-butylphosphino)benzaldehyde has been
previously reported by Uozumi,24 but was not isolated in that
report, and instead used in situ for reaction. In order to collect
6, it was purified by column chromatography under a nitrogen
atmosphere (see supplementary information for details). The
aldehyde was subsequently converted to the N-TMS imine (5b,
90%), followed by reaction with Ph2BCl to form the N-
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DOI: 10.1039/C8DT05100C
Figure 4. X-Ray crystal structure of 6a with 50% probability
ellipsoids, H atoms have been omitted for clarity. Only the
major isomer of disordered allyl is shown. Selected distances
(Å) and angles (deg): Pd(1)-P(1): 2.3109(8); Pd(2)-P(2):
2.2970(8); N(2)-C(4’): 1.274(3); N(2)-B(2): 1.589(4); N(2)-B(1):
1.613(4); N(1)-C(4): 1.279(3); N(1)-B(2): 1.586(4); N(1)-B(1):
1.619(4); P(1)-Pd(1)-Cl(1): 94.80(3); P(2)-Pd(2)-Cl(2): 96.45(3);
B(2)-N(1)-B(1): 92.85(19); B(1)-N(2)-B(2): 93.00(19); N(1)-B(1)-
N(2): 86.08(19); N(1)-B(2)-N(2): 88.00(19).
Figure 5. X-Ray crystal structure of 6b with 50% probability
ellipsoids, H atoms and chloroform have been omitted for
clarity. Only the major isomer of disordered allyl is shown.
Selected distances (Å) and angles (deg): Pd(1)-P(1), 2.3517(14);
Pd(2)-P(2): 2.3369(14); C(2)-N(2); 1.279(6); N(2)-B(2): 1.500(7);
N(2)-B(1): 1.613(7); N(1)-C(1): 1.278(6); N(1)-B(2): 1.589(6);
N(1)-B(1): 1.618(7); P(1)-Pd(1)-Cl(1): 96.60(5); P(2)-Pd(2)-Cl(2):
92.66(5); B(2)-N(1)-B(1): 92.7(3); B(2)-N(2)-B(1): 92.4(3); N(1)-
B(2)-N(2): 88.1(3); N(1)-B(1)-N(2): 86.7(3).
Interestingly, the boron in 6a is stabilized through an
intermolecular interaction with an imine nitrogen atom of
another ligand, leading to the formation of the dimeric
structure. These bonds are asymmetric, with a shorter B-N
bond (ca. 1.58 Å) that is similar to those noted in the pyridyl
systems (Figure 3), while the second B-N bond (ca. 1.62 Å) is
elongated, and presumably reflects a strained acid/base
interaction. This strain is seen as well in the bond angles at
nitrogen and boron, which are significantly distorted from the
idealized sp3 hybridization (between 86o and 93o).25 It is
notable that these complexes show no evidence for intra- or
intermolecular interactions with the palladium-bound
chloride.26
Conclusions
In summary, two new classes of N-boryl imine ligands can be
easily generated in two steps from the appropriate aldehyde.
The pyridinyl-based ligands 1 were found to undergo
intramolecular
acid/base
association
between
the
diphenylboron unit and pyridine nitrogen, and remains in this
form upon protonation or coordination to palladium.
However, the phosphine-variants of these ligands (4) do not
cyclize, and instead the boron forms a dimeric complex via
intermolecular association with an imine nitrogen, leaving the
PR2 available for association to palladium. The lack of any
intramolecular acid/base interactions in these latter suggests
these ligands may prove useful in catalysis or in association to
metals where chelation is possible. Studies directed toward
the latter are in progress in our laboratory.
An analogous behaviour is observed in complex 6b, where
t
the Bu2P unit coordinates to palladium to form a dimeric
complex (Figure 5).
Thus, even with the more basic
phosphorus does not undergo intramolecular cyclization, and
instead boron associates to a second imine nitrogen to form a
4-membered ring. The bond lengths (B-N 1.58-1.62 Å) and
angles (between 86 and 92o) are similar to those in complex
6a. Despite the strain in this interaction, 11B NMR of 6a shows
this dimer is present in solution, suggesting that even with its
distortion, imine dimerization is more stabilizing than any
potential B-N -bonding within the ligand itself. The latter
presumably arises in part from the sp2 hybridization of the
imine nitrogen, which leaves the nitrogen and boron atom
capable of bonding to other substrates.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank NSERC and the FQRNT supported Centre for Green
Chemistry and Catalysis for funding this research. We also
thank Prof. Scott Bohle (McGill Univ.) for assistance in crystal
structure refinements.
Notes and references
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DOI: 10.1039/C8DT05100C
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ARTICLE
TOC graphic:
Lewis
Acid
Metal
binding
Ph
Ph
N
B
BPh2
N
2
N
B
N
R2P
Cl
Ph2
L =
C-PR2
L
L =
N
Pd
Pd
R
R
Cl
This journal is © The Royal Society of Chemistry 20xx
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