Pyridylalkylamine ligands and their palladium complexes: Structure and reactivity revisited by NMR

Article abstract of DOI:10.1002/mrc.4058

Pyridylmethylamines or pma are versatile platforms for different catalytic transformations. Five pma-ligands and their respective Pd complexes have been studied by liquid state NMR. By comparing ¹H, ¹³C and ¹⁵N chemical shifts for each pma/pma-Pd couple, a general trend for the metallacycle atoms concerns variations of the electronic distribution at the pendant arm, especially at the nitrogen atom of the ligand. Moreover, the increase of the chemical shift of the pendant arm nitrogen atom from primary to tertiary amine is also related to the increase of crowding within the complex. This statement is in good agreement with X-ray data collected for several complexes. Catalytic results for the Suzuki-Miyaura reaction involving the pma-Pd complexes showed within this series that a sterically crowded and electron-rich ligand in the metallacycle was essential to reach the coupling product with a good selectivity. In this context, NMR study of chemical shifts of all active nuclei especially in the metallacycle could give a trend of reactivity in the studied family of pma-Pd complexes. Copyright

Full text of DOI:10.1002/mrc.4058

Research article
Received: 4 December 2013
Revised: 28 January 2014
Accepted: 17 February 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.4058
Pyridylalkylamine ligands and their palladium
complexes: structure and reactivity revisited
by NMR
Alexandre Requet,a Olivier Colin,a Flavien Bourdreux,a Salim M. Salim,a
Sylvain Marque,a Christine Thomassigny,a Christine Greck,a
Jonathan Farjonb** and Damien Prima*
Pyridylmethylamines or pma are versatile platforms for different catalytic transformations. Five pma-ligands and their
respective Pd complexes have been studied by liquid state NMR. By comparing 1H, 13C and 15N chemical shifts for each
pma/pma–Pd couple, a general trend for the metallacycle atoms concerns variations of the electronic distribution at the
pendant arm, especially at the nitrogen atom of the ligand. Moreover, the increase of the chemical shift of the pendant
arm nitrogen atom from primary to tertiary amine is also related to the increase of crowding within the complex. This
statement is in good agreement with X-ray data collected for several complexes. Catalytic results for the Suzuki–Miyaura
reaction involving the pma–Pd complexes showed within this series that a sterically crowded and electron-rich ligand in
the metallacycle was essential to reach the coupling product with a good selectivity. In this context, NMR study of chemical
shifts of all active nuclei especially in the metallacycle could give a trend of reactivity in the studied family of pma–Pd
complexes. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: NMR; 15N; 1H; 13C; pyridylmethylamine; N-heterocyclic benzhydrylamines; N,N-bidentate ligands; palladium
took place. Results in solution have been compared with solid
state data, evidencing a clear impact of the substitution pattern
Introduction
The installation of a pendant methylamine arm at pyridine
rings generates an ideal 1,2-N,N-bidentate ligand and confers to
the so-called pyridylmethylamine (pma) scaffold interesting
complexation properties. Such motif can be encountered in
numerous bioinspired Cu-, Fe-, Re-, or Zn-based coordination
compounds.[1] The combination of pma and transition metals
revealed especially valuable as catalytic systems in synthetic
transformations such as various oxidative processes,[2] cycloaddi-
tion of dienes,[3] Henry reactions,[4] and Friedel–Crafts
alkylations[5–7] driving the interest of the scientific community
for new and robust pma-based catalytic systems. We recently
contributed to this area and reported on the synthesis,
complexation, and catalysis using pma–Pd[8–11] and pma–Cu
combinations.[8] In the context of Suzuki–Miyaura C–C bond
formation using pma–Pd catalytic systems, it has been shown
that the substitution at the benzylic position of the pendant
arm was of major importance to obtain high conversions (Fig. 1). These
first sets of results were completed by solid state X-ray data arising from
several pma–Pd complexes, which evidenced a close overall geometry
and a similar spatial arrangement of substituents in all complexes.
Our objectives are to gain further information and
complementary structure–reactivity data in the context of
catalytic activity of the pma family. With these aims, liquid state
NMR techniques were implemented to fully probe various
pyridylalkylamine ligands and their respective Pd complexes. In
this context, heteronuclear 1H–X (X: 15N, 13C) NMR correlations
through well-known HMBC[12] experiments revealed a useful
and crucial tool to estimate the contribution of surrounding
substituents of the nitrogen atom in solution where the reaction
around the pendant arm nitrogen atom including the class of
the amine on characteristic metal-N bonds in Pd complexes.
Within the pma family, attempts to determine whether a
tertiary-based or a secondary amine-based catalyst and the
nature of substituents would be beneficial to catalytic properties
were realized in biphenyl and binaphthyl model series. In the
following, the synthesis of different ligands will be described.
Results and Discussion
Synthesis of ligands and complexes
We first focused on the preparation of five pma ligands and their
corresponding complexes (Fig. 2). Ligands 1–3 were obtained in high
yields from pyridine-2-carbonitrile and phenylmagnesium chloride,
pyridine-2-carboxaldehyde and benzylamine or methylbenzyl-
amine, respectively (Scheme 1).
*
Correspondence to: Damien Prim, Université de Versailles Saint-
Quentin-en-Yvelines, Institut Lavoisier de Versailles UMR CNRS 8180, 45,
avenue des Etats-Unis, 78035 Versailles, France. E-mail: damien.prim@uvsq.fr
** Correspondence to: Jonathan Farjon, Equipe RMN en milieu Orienté, Institut
de Chimie Moléculaire et des Matériaux d’Orsay (ICMMO), UMR CNRS 8182,
bât. 410, Université Paris-Sud, 91405 Orsay, France. E-mail: jonathan.
farjon@u-psud.fr
a Université de Versailles Saint-Quentin-en-Yvelines, Institut Lavoisier de Versailles,
UMR CNRS 8180, 45, avenue des Etats-Unis, 78035 Versailles, France
b Equipe RMN en milieu Orienté, Institut de Chimie Moléculaire et des Matériaux
d’Orsay (ICMMO), UMR CNRS 8182, bât. 410, Université Paris-Sud, 91405 Orsay, France
Magn. Reson. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

A. Requet et al.
Figure 1. First sets of characteristic data in the pma family.
Figure 2. Series of pma ligands 1–5 and complexes 6–10.
Ligands 4 and 5 were prepared through methylation starting
of already reported pma derivatives.[7] The pendant arm within
this series consists of one primary, two secondary, and two
tertiary amines. The primary amine 1 displays a phenyl group at
the benzylic position. Secondary amines in one part and tertiary
amines in the other part differ each other by the increasing steric
hindrance of the additional substituent (2 vs 3 and 4 vs 5). At
least, we focused on 5/10 as a new ligand-complex couple that
combines both a phenyl group at the benzylic position and two
additional methyl groups. Ligands 1–5 were then reacted with
Na2PdCl4 in freshly distilled MeOH to afford the complexes 6–10.
As already observed for ligands bearing nonracemic groups, the
presence of stereodefined carbon centers in ligand 3[8] controls
the central chirality on the nitrogen atom and induces the
formation of the two diastereomers 8a and 8b during the com-
plexation process. Although they could not be separated, 1H
NMR spectra display two characteristic sets of signals (Table 1 SI).
It is worth noting that the increase of steric crowding at the pen-
dant arm nitrogen atom allowed complexation without noticeable
impacts on yield, complex 10 being isolated in a fair 88% yield.
NMR is a well-known technique that will help to rationalize
structure, electronic distribution, and steric effects with reactivity
of the complexes under study.
(complex)-δi(ligand).[12] Tables 1–4 SI gather chemical shifts but
also Δi for characteristic protons and carbons. Further data could
arise from analysis of 15N chemical shifts. However, classical 1D
15N NMR analysis appears time consuming because of low
isotopic natural abundance (0.36%) and long T1 relaxation times.
Thus, evaluation of coordination and analysis of chemical shift
variation between complex and ligand have been obtained
through more sensitive 1H-detected heteronuclear 1H–15N
HMBC[13] correlations (Fig. 1 SI and Table 1).
1H, 13C and 15N NMR studies for a full characterization of
ligands 1–5 and complexes 6–10
Numbering system for all ligands and complexes is shown in
Fig. 3.
By the way of chemical shifts, the effect of the metal chelation
has been investigated by comparing the chemical shifts of the
ligand and the complex for each active atom i through Δi = δi
Scheme 1. Preparation of ligands 1–5.
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Magn. Reson. Chem. (2014)

Pyridylalkylamine ligands and their palladium complexes: structure and reactivity revisited by NMR
major impact of the presence of the Pd atom depending on
substituents of the amine atom. Δδ for C7 that is embedded into
the metallacycle decreases from the primary to the tertiary amine
from 5.4 to 3.7 ppm from ligand/complex couples 1/6 to 4/9. In
addition, the largest Δδ of 14.5 and 17.3 ppm is observed
between structures 5 and 10 characterized by the presence of a
tertiary amine (Table 3 SI).
We next turned our attention to the impact of the complexa-
tion on both nitrogen atoms being the most affected nuclei
due to their close contact to the metal. Table 1 gathers 15N chem-
Figure 3. Numbering system adopted in the pma family.
1
ical shifts and Fig. 1 SI shows examples of heteronuclear H-15N
Careful examination of NMR spectra and calculated Δδ war-
rants some additional comments. In a general manner whatever
the nuclei, Δi is not zero, meaning that the Pd is chelated to the
correlations on primary, secondary and tertiary ligand-complex
couples. Overlapping of both ligand and complex heteronuclear
1H-15N correlations clearly evidenced influence of complexation
over 15N chemical shifts. For ligands 1 to 5, δN1 changes in a
range of 1 to 4 ppm showing that this site is only poorly affected
by substituents at the pendant arm. In contrast, δN8 exhibit a
large shielding up to +13 ppm (between ligands 1 and 3) most
plausibly arising from the presence of increasing donating
groups from a primary to a tertiary amine. It is worth noting that
the comparison of chemical shifts of the pyridinic nitrogen atom
N1 in the ligand-complex couples shows a large deshielding of
restraint range from À88 to À93 ppm. In deep contrast, the pen-
dant arm nitrogen atom N8 appears less affected by the presence
of the palladium atom. Indeed, Δδ for N8: ΔN8 ranged from -20 to
+8 ppm. This is due to the fact that only N1 is involved in the
pyridine ring and π-backbonding allows stabilizing the Pd-N1
bond. Moreover, Δδ for N1 are almost insensitive to the pendant
arm substitution, it changes up to 5 ppm from 1/6 to 5/10 couples.
This trend results from contributions of two main factors. First,
the presence of additional electron donating groups that shield
N8. In the following, we will detail the contribution of each
ornamental moiety of the pending arm of the ligand. ΔN8
increase up to 8 ppm, meaning that all added groups enrich the
N8 and therefore directly impacted the Pd–N8 bond strength.
Some additional trends and effects of each substituents can be
given within these series of couples: (i) from 1 to 8 ppm shielding
effect of benzylic methyl groups on chemical shifts (Table 1,
compare couples 2/7 vs 3/8a/8b and 4/9 vs 5/10), (ii) combined
effect of benzyl and N-methyl groups leads to large shielding up
to 20.6 ppm (Table 1, compare couples 1/6 vs 4/9), (iii) combined
effect of phenyl and N-methyl groups also shows positive
shielding of 14.1 ppm (Table 1, compare 2/7 vs 4/9), and (iv)
the presence of N-methyl group results in shielding of 6 ppm
(Table 1, compare 4/9 vs 2/7 and vs 1/6). Consequently, the Ph
leads to a shielding of 8 ppm and the benzyl group to a shielding
of 14 ppm. The second effect explaining the increase of the ΔN8
is the strong enhancement of the steric crowding from 1/6 to 5/10
at the pendant arm nitrogen atom that plausibly concomitantly
increases the Pd–N8 strength.
1
ligand and that all nuclei (i.e. H, 13C, and 15N) are spies feeling
the complexation. As expected, almost all protons undergo the
influence of the presence of the metal in the complexes 6–10.
Pyridinic protons (H2–5) show Δδ averaged ranging from 0.03
to 0.60 ppm. Probe protons, namely H7 and H9 exhibit character-
istic-enhanced Δδ values from 1/6 to 5/10 couples. First, H7 are
privileged spectator vis-à-vis the complexation to the palladium
atom being localized into the metallacycle. Second, the increase
of the substitution at the pendant arm nitrogen atom impacts
Δδ. A clear shielding from a ligand to another one is observed
and induces an increasing of the Δδ from primary (Δδ = 0.35) for
couple 1/6, to tertiary amine (Δδ = 0.81) for the couple 4/9. A similar
trend can be observed for H9, albeit in a lesser extent (secondary for
3/8a, 8b vs tertiary for 4/9: Δδ ≈0.5 vs Δδ > 0.6) (Table 1 SI). More-
over, the chelation of the Pd affects aromatic nuclei. Deshieldings
1
for benzylic H are in the same range for complexes 7 and 8a/8b.
The more impacted protons in these two complexes are H7 and
H9, being localized in α position to N8. However, for the complexes
9 and 10, benzylic protons H17, H18, and H19 have a highest
deshielding than the complexes 7, 8a, and 8b. This is plausibly
due to the fact that electrons are more dispersed in the complexes
9 and 10 due to a supplementary Ph group. In the case of the
phenyl moiety, H14 and H15 have almost the same deshielding in
the complexes 6 and 9 (Table 2 SI).
Effect of complexation can also be observed at carbon atoms.
Indeed, if C3–C6 are characterized by a Δaveraged ranging from
1.2 to 4.2 ppm, C2 is poorly affected by the presence of the metal
(Δaveraged = 0.2–0.5 ppm). In contrast, C7, C9, and C10 undergo
Table 1. 15N chemical shifts for free ligands and complexes, Δ = δN
(complex)-δN(ligand) is given between brackets
Ligand/complex
N1[a]
N8[a]
1
À67
À342
6
À155 (À88)
À65
À362 (À20)
À342
Solid state study confirmed the liquid state NMR analytical
picture.
2
7
À154 (À89)
À63
À356 (À14)
À332
Within this series, five crystal structures of pma–Pd com-
plexes,[14] have already been reported by us, evidencing that all
complexes adopt similar 3D geometry whatever the nature of
substituents at the pendant arm and of amine class (Table 2).
Although secondary and tertiary amine-based complexes present
similar molecular arrangements, noticeable differences exist
between both types of complexes. As evidenced in Table 3, length
of Pd–N1 bonds are close (2.020–2.025 Angströms) regardless of
the amine class within this series. In deep contrast, Pd–N8
bonds are much longer for tertiary amines by comparison with
4
9
À156 (À93)
À66
À332 (0)
À329
3
8a/8b
5
À155/À153 (À89/À88) À342/À342 (À13/À13)
À63
À330
10
À156 (À93)
À322 (+8)
aNegative 15N chemical shifts are referred to external pure CH3NO2 (its
δ
15N = 0 ppm). The 1H–15N HMBC has been used for detecting the 15
chemical shift of NO2 due to the evolution of 2JH–N coupling.
N
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A. Requet et al.
Table 2. X-ray details and selected crystallographic data
Spatial arrangement for pma-based complex
Complex
CCDC (Ref.)
677 262 (6)
Pd–N1(Angströms)
Pd–N8(Angströms)
2.021
2.031
812 300 (8)
812 301 (8)
812 302 (8)
677 264 (6)
2.020
2.025
2.020
2.020
2.043
2.038
2.043
2.083
secondary amines (2.083 Angströms vs 2.031–2.043Angströms) in
full agreement with an increase of steric crowding at the tertiary
amine detected by NMR through an increasing of the ΔN8 value.
(3.77 ppm) could be clearly identified in 1H NMR spectra of
crude material (Scheme 2). The presence of the side product
14 plausibly arises from debromination of the starting mate-
rial during catalytic process after the oxidative addition step.
The latter process plausibly evolves concomitantly to the
further transmetallation step, which is mandatory to the
formation of the expected product 12. Both competitive pro-
cesses are worth to evaluate. Within this context, 11/12 and
11/14 ratios allow an easy and direct interpretation of both
catalytic efficiency and selectivity respectively. Complex 5
could not be compared with all other complexes due to low
solubility in the toluene/EtOH/H2O solvent system used.
Reactions were realized using 5% of catalytic system loading,
2 eq. of the boronic acid 13 and 4 eq of Cs2CO3 as the base
in toluene/EtOH/H2O: 1/1/0.5 as the solvent.
Complex 7 was first evaluated as the catalytic system for the
obtention of binaphthyl derivative 11. Disappointingly, 1/8 and
1/2 ratios of 11/12 and 11/14, respectively, were observed after
3 h reaction course (Table 3, entry 1). If the 11/12 ratio remained
unchanged after 13 h, a large increase of the amount of
debromination the side product 14 was observed (entry 2).
Moving from 7 to complex 8 bearing an additional methyl group
revealed beneficial to the formation of 11 by comparison with 7.
Pma–Pd complexes as catalytic systems for Suzuki–Miyaura
reaction
Both data in solution and in the solid state indicate that the Pd–N
bond length is deeply depending on the steric crowding at the
pendant arm nitrogen atom. Chemical shifts observed and Δδ
for protons, carbons, and nitrogen atoms calculated in the
presence of additional electron donating groups are also
consistent with an electronic enrichment of the N8 atom. These
features are expected to impact catalytic activities of pma–Pd
complexes within the Suzuki–Miyaura coupling context. We
recently evidenced that pma-based catalysts could be used for
the formation of carbon–carbon bonds through Suzuki–Miyaura
reaction.[6,8] The evaluation of the catalytic efficiency and
the role of substitution pattern at the pma core have been
realized using 2-methoxy-1,1′-binaphthyl 11 as model com-
pound. As shown below, the latter represents a convenient
target as methoxy groups of the reactant 12 (4.05 ppm), the
side product 14 (3.94 ppm), and the binaphthyl target 11
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Pyridylalkylamine ligands and their palladium complexes: structure and reactivity revisited by NMR
Table 3. Comparison of catalytic activity
]
Entry
1
Time (h)
3
Catalytic system
Ratio 11/12[a]
1/8
Ratio 11/14[b
1/2
7
8
2
13
1/8
1/4.5
3
4
3
1/5
1/5
1/1.8
1/1.8
13
]
5[b
24
1/0
1/0.3
6
7
8
9
3
13
20
30
1/2
1/0.3
1/0.3
1/0.2
1/0.2
1/1
9
1/0.7
1/0.6
10
11
12
13
3
13
20
24
1/1.8
1/0.35
1/0.1
1/0.1
1/0.3
1/0.15
1/0.15
1/0.15
10
1
aDetermined by H NMR on the crude material.
bSee the work by Grach et al.[8]
Scheme 2. Evaluation of the catalytic activity of pma-based Pd complexes.
However, initial 11/12 and 11/14 ratios did not evolve over a 13-h
period (entries 3–4).
In addition, the use of 9 also allowed limiting the formation of
the side product 14 to a 1/0.2 ratio. A further increase of substi-
tution as displayed in the complex 10 was finally examined.
The use of 10 led to a clear improvement of catalytic efficiency
(entries 10–13) showing a decrease of the starting bromide 12
until residual ratio of 1/0.1. Furthermore, the formation of 14
could also be limited to a 1/0.15 ratio, again largely favoring
Interestingly, the joint presence of phenyl and methyl groups
at both benzylic position and pendant arm nitrogen atom in
the complex 9 allowed reaching higher efficiency and selectivity.
Indeed, the formation of the target 11 gradually increased until it
was the major product formed (entries 6–9).
Magn. Reson. Chem. (2014)
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A. Requet et al.
the formation of the expected coupling product. The presence of
a tertiary amine has thus a clear beneficial impact on the catalytic
efficiency of pma-based series of palladium complexes shown in
this study.
Experimental Section
Experimental details for the preparation of the ligand 5 and the
complex 10, 1H NMR, 13C NMR spectra, and detailed NMR
Tables 1, 2, 3, and 4 are given in the Supporting Information.
These results have however to be compared with previously
reported data (entry 5) in which the NH analogue of 10 furnished
a complete conversion and an 11/14 ratio of 1/0.3 within 24h
reaction course. If a complete conversion using complex 10 could
not be obtained in 24 h, our results suggest that 10 would
generate lesser amount of the side product 14 being thus compet-
itive as catalytic system. Our results allow confirming preliminary
results and raising a general trend in the context of pma-based
Pd complexes and Suzuki–Miyaura reaction. At first, a large
substituent at the benzylic position was required for the
obtention of fair to complete conversions. Second, the
presence of a tertiary amine with all electron donor groups
(Ph and Bn) induced a clear electronic enrichment of the
pendant arm nitrogen atom in full agreement with liquid
state NMR together with an increase of the steric crowding
in accordance with solid state data and liquid NMR. Third,
installation of additional substituents at pma-ligands
revealed competitive in the context of catalytic efficiency
and beneficial to selectivity by decreasing the formation of
side products.
Acknowledgements
The authors are grateful to the CNRS, the Universities of Paris Sud and
Versailles-St Quentin-en-Yvelines, ANR (ANR-11-BS07-030-01). This work
is supported by a public grant overseen by the French National Research
Agency (ANR) as part of the «Investissements d’Avenir » program
n° ANR -11- IDEX-0003-02 and CHARMMMAT ANR - 11-LABEX-0039.
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Conclusions
A complete analysis of a set of five pma ligands and their
respective Pd complexes have been investigated. A full chemical
1
shift study for H, 13C, and 15N has been envisioned for ligands
and complexes. 15N chemical shifts coming from sensitive
15N–1H HMBC have been the most interesting data for feeling
the design of the ligand on the strength of Pd–N bonds. NMR
showed its potential to probe Pd surrounding at atomic
resolution. This technique allows a clear identification of
electronic distribution by comparing ligands to complexes
chemical shifts at atomic resolution. The electronic contribution
of each group on the pendant arm can be quantified by NMR.
The most sensitive nuclei to detect the effect of the ligand design
changes are constitutive of the metallacycle. The general trend is
an increase of the Δδ for spectators 1H, 13C, and 15N of the
metallacycle because of the increase of the ligand electron density
and especially on the pendant arm nitrogen atom. Moreover,
the effect of the increasing steric hindrance by adding Ph,
Me, and benzyl moieties tends on one hand to decrease the
Pd–N bonds strength and on another hand to increase Δδ.
These results are in complete agreement with the X-ray data.
Pma–Pd complexes have been successfully used as catalysts
within the binaphthyl series. The comparison of five pma–Pd
complexes showed that conversion and selectivity are
strongly related to the substitution pattern at the pendant
arm fitting with X-ray data. The presence of sterically
hindered moieties (Ph, Me, and benzyl) has been showed
beneficial to catalytic activity. Such substituents also account
for an overall increase of electronic contribution of the
ligand, which is in complete agreement with measured chem-
ical shifts observed. Our study evidenced NMR as a pertinent
and insightful tool to design ligands at atomic resolution in
order to probe atoms of the ligand being the most involved
in the reactivity of the Pd complexes.
[14] CCDC 677262 see ref 6/ CCDC 677264 see ref 6/ CCDC 812300 see
ref 8/ CCDC 812301 see ref 8/ CCDC 812302 see ref 8.
Supporting Information
Additional supporting information may be found in the online
version of this article at the publisher’s website.
wileyonlinelibrary.com/journal/mrc
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2014)