N-substituted aminobiphenyl palladacycles stabilized by dialkylterphenyl phosphanes: Preparation and applications in C[sbnd]N cross-coupling reactions

Article abstract of DOI:10.1016/j.ica.2020.120214

Neutral and cationic N-methyl- and N-phenyl-2-aminobiphenyl methanesulfonate palladacycles stabilized with dialkylterphenyl phosphanes have been prepared and characterized. Neutral structures are favored with the less bulky phosphane PMe₂Ar^Xyl2, L1, while more sterically demanding ligands PiPr₂Ar^Xyl2, L3, and PCyp₂Ar^Xyl2 (Cyp = cyclopentyl), L4, lead to cationic complexes in which the phosphane exhibits a bidentate κ¹-P, η¹-C_arene coordination mode involving one of the ipso carbon atoms of a flanking terphenyl aryl ring. The complexes were evaluated for activity in C[sbnd]N cross-coupling reactions and [Pd(N-methyl-2-aminobiphenyl)L4](OMs) (OMs = mesylate) was identified as the most efficient precatalyst, facilitating the coupling of aryl chlorides with secondary and primary amines and indoles.

Full text of DOI:10.1016/j.ica.2020.120214

Inorganica Chimica Acta 518 (2021) 120214
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
N-substituted aminobiphenyl palladacycles stabilized by dialkylterphenyl
phosphanes: Preparation and applications in C N cross-coupling reactions
–
a
,
1
a
,
1
a
a,
b
b
´
Andrea Monti , Raquel J. Rama , Beatriz G o´ mez , Celia Maya , Eleuterio Alvarez ,
b
a,*
Ernesto Carmona , M. Carmen Nicasio
a
Departamento de Química Inorg a´ nica, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain
Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorg a´ nica and Centro de Innovaci o´ n en Química Avanzada (ORFEO-CINQA), Consejo Superior
b
de Investigaciones Científicas (CSIC) and Universidad de Sevilla, Avenida Am ´e rico Vespucio 49, 41092 Sevilla, Spain
A R T I C L E I N F O
A B S T R A C T
Neutral and cationic N-methyl- and N-phenyl-2-aminobiphenyl methanesulfonate palladacycles stabilized with
Dedicated to Prof. Maurizio Peruzzini on the
occasion of his 65th birthday.
dialkylterphenyl phosphanes have been prepared and characterized. Neutral structures are favored with the less
Xyl2
Xyl2
Xyl2
bulky phosphane PMe
2
Ar
, L1, while more sterically demanding ligands PiPr
2
Ar
, L3, and PCyp
2
Ar
(Cyp
1
1-Carene co-
=
cyclopentyl), L4, lead to cationic complexes in which the phosphane exhibits a bidentate κ -P,
ordination mode involving one of the ipso carbon atoms of a flanking terphenyl aryl ring. The complexes were
η
Keywords:
Cross-coupling
Aryl amination
Palladium catalysis
Phosphane ligands
Palladacycles
evaluated for activity in C
–
N cross-coupling reactions and [Pd(N-methyl-2-aminobiphenyl)L4](OMs) (OMs =
mesylate) was identified as the most efficient precatalyst, facilitating the coupling of aryl chlorides with sec-
ondary and primary amines and indoles.
1
. Introduction
In modern synthetic chemistry, Pd-catalyzed cross-coupling meth-
efficient Pd(II) precatalysts have been developed (Fig. 1).[7b,7c]
Of these, stands out the family of 2-aminobiphenyl N,C-palladacycles
introduced by Buchwald,[8] which has been applied to a wide variety of
C C and C-heteroatom cross-coupling reactions.[8d,9] The remarkable
–
odologies are indispensable tools for building molecular complexity.[1]
A range of specialized tertiary phosphanes [2] and, to a lesser degree, N-
heterocyclic carbenes [3] have emerged as privileged ligand platforms
to generate highly active palladium catalysts. Although structurally
different, these ligands share the ability of stabilizing monoligated
palladium(0) species LPd(0), which is proposed to be the true catalyti-
cally active species in most cross-coupling reactions.[4] Inefficient
production of such LPd(0) intermediates under catalytic conditions
result of a decrease in the activity and the selectivity of the catalyst
system.[5] Despite the operational simplicity of in situ catalyst gener-
ation, i.e. by mixing the ligand of choice in excess with the appropriate
palladium precursor, problems associated with formation of Pd(0) spe-
cies with different composition and reactivity[6] make this procedure
less appealing. Accordingly, the use of well-defined Pd precatalysts with
the optimal palladium to ligand ratio (1:1) has proved to be a more
convenient and cost-effective approach for the generation of the desired
active monoligated LPd(0).[7] In recent years, several types of highly
catalytic activity of these palladacycle precatalysts rely on facile acti-
vation, in the presence of base, to yield the desired monoligated palla-
dium(0) species.[10] However, the activation reaction also produced
catalytic amount of carbazole as by-product (Scheme 1). This NH-
heterocycle is also susceptible of being arylated under reaction condi-
tions, complicating product purification and diminishing the yield.[11]
Moreover, it has been observed that carbazole can hinder catalyst per-
formance due to coordination, as carbazolyl ligand, to in-cycle Pd(II)
intermediates.[11] To address these shortcomings, palladacycle pre-
catalysts with N-substituted-2-aminobiphenyl ligands have been
described.[12] Their activation render the formation of N-substituted
carbazoles by-products that are unreactive towards further functionali-
zation by cross-coupling. In addition, the absence of the NH-
functionality in these molecules limits their coordination capabilities.
Very recently, we have reported on the preparation of 2-aminobi-
phenyl palladacycles supported by
a variety of dialkylterphenyl
*
Corresponding author.
E-mail address: mnicasio@us.es (M.C. Nicasio).
These authors contributed equally to this work.
1
https://doi.org/10.1016/j.ica.2020.120214
Received 27 October 2020; Received in revised form 14 December 2020; Accepted 17 December 2020
Available online 25 December 2020
0
020-1693/© 2020 Elsevier B.V. All rights reserved.

A. Monti et al.
Inorganica Chimica Acta 518 (2021) 120214
P). 13C{ H} NMR (75 MHz, CDCl
1
◦
(
d, 3H, JHP = 10.0 Hz, CH
47.3 (d, JCP = 10 Hz, C ), 144.3 (d, JCP = 3 Hz, C
Hz, C ), 140.6 (C ), 138.4 (C
), 141.0 (d, JCP = 4 Hz, C
38.2 (CHar), 136.9 (C ), 136.4 (C ), 131.3 (CHar), 131.2 (CHar), 128.6
CHar), 128.5 (CHar), 128.3 (CHar), 128.1 (CHar), 128.0 (CHar), 126.6
CHar), 125.2 (CHar), 122.2 (CHar), 40.3 (CH N), 39.7 (OMs), 22.0
P), 15.1 (d, JCP = 29 Hz,
3
3
, 25 C): δ
1
q
q
), 142.2 (d, JCP = 3
q
q
q
q
), 138.2 (CHar),
1
q
q
(
(
(
3
CH
3
), 22.0 (CH
3
), 16.4 (d, JCP = 33 Hz, CH
3
CH
3
P). 31P{ H} NMR (121 MHz, CDCl
1
3
, 25 C): δ 1.9. Anal. Calc. for
◦
Fig. 1. Examples of outstanding Pd(II) coupling precatalysts.
C
H
42NO Cl
PPdS⋅½ CH
2
: C, 59.85; H, 5.61; N, 1.81; S, 4.15. Found: C,
38
3
2
5
9.90; H, 5.89; N, 1.68; S, 4.20. ESI-MS (Orbitrap): m/z calculated for
+
+
C
37
H39NPPd [M – (CH
3 3
O S)] 634.1849. Found 634.1854.
2
.2.2. Pd(N-phenyl-2-aminobiphenyl)(PMe
CH Cl (5 mL) was added to a mixture of [Pd(N-phenyl-2-amino-
biphenyl)(µ-OMs)]
.20 mmol). The reaction mixture was stirred 2 h at room temperature.
2
ArXyl2)(OMs), 2⋅L1
2
2
Xyl2
2
(89.2 mg, 0.10 mmol) and PMe
2
Ar
(69.1 mg,
0
After filtration through a Celite plug, the solvent was evaporated under
reduced pressure and the crude reaction product was purified by
Scheme 1. Pathway for the activation of 2-aminobiphenyl palladacycles.
◦
recrystallization from petroleum ether: CH
2
Cl
2
(3:1) mixtures at 5 C.
1
◦
Yield: 118.5 mg (76%). H NMR (300 MHz, CDCl
3
, 25 C): δ 9.90 (d, 1H,
phosphane ligands developed in our research group,[13] and demon-
–
strated their competence as precatalysts in a variety of C N cross-
J
HP = 6.0 Hz, NH), 7.57 (td, 1H, JHH = 7.6 Hz, JHP = 2 Hz, CHar), 7.50
(
dd, 1H, JHH = 7.4 Hz, JHP = 1.9 Hz, CHar), 7.37–7.27 (m, 2H, CHar),
coupling reactions.[14] This work extends these studies to N-
substituted 2-aminobiphenyl palladacycle analogues. Herein, we
described the synthesis and the structural characterization of a series of
N-methyl- and N-phenyl-substituted-2-aminobiphenyl palladacycle
bearing terphenyl phosphanes. The catalytic activity of these pre-
catalysts in the arylation of a range of N-nucleophiles including primary
and secondary amines and indoles is also presented.
7
7
.24–7.17 (m, 4H, CHar), 7.11–7.00 (m, 8H, CHar), 6.95 (t, 2H, JHH
=
.3 Hz, CHar), 6.77 (d, 2H, JHH = 7.9 Hz, CHar), 6.55 (td, 1H, JHH = 7.5
Hz, JHP = 1.6 Hz, CHar), 5.95 (t, 1H, JHH = 6.9 Hz, CHar), 2.64 (s, 3H,
OMs), 2.09 (s, 12H, CH
3
), 1.29 (d, 3H, JHP = 10.8 Hz, CH
3
P), 0.61 (d,
13
1
◦
3
H, JHP = 10.2 Hz, CH
47.5 (d, JCP = 10 Hz, C
), 143.0 (C
), 141.0 (d, JCP = 4 Hz, C
Hz, C
3
P). C{ H} NMR (75 MHz, CDCl
), 145.5 (d, JCP = 2 Hz, C
), 138.7 (C
), 136.5 (C
3
, 25 C): δ
1
q
q
), 143.6 (d, JCP = 3
Hz, C
q
q
q
q
), 138.4 (d, JCP
=
3
q
), 137.9 (d, JCP = 13 Hz, CHar), 136.8 (C
q
q
), 131.4
2
. Experimental section
(
(
(
CHar), 131.3 (CHar), 131.2 (d, JCP = 2 Hz, CHar), 128.7 (CHar), 128.6
CHar), 128.4 (CHar), 128.3 (CHar), 128.2 (CHar), 127.9 (CHar), 127.5
CHar), 126.9 (d, JCP = 5 Hz, CHar), 125.2 (CHar), 124.6 (d, JCP = 2 Hz,
2
.1. General considerations
CHar), 124.3 (CHar), 120.5 (CHar), 40.0 (OMs), 22.1 (CH
3
), 21.9 (CH
3
),
P). P{ H} NMR
44NO PPdS: C,
All preparations and manipulations were carried out under oxygen-
31
1
1
7.2 (d, JCP = 34 Hz, CH
3
P), 15.6 (d, JCP = 29 Hz, CH
3
free nitrogen, using conventional Schlenk techniques. Solvents were
◦
(
121 MHz, CDCl
3
, 25 C): δ 2.0. Anal. Calc. for C43
H
3
rigorously dried and degassed before use. Terphenyl phosphanes L1-L4,
6
3
5.19; H, 5.60; N, 1.77; S, 4.05. Found: C, 65.23; H, 5.90; N, 1.53; S,
[
13a,13b] and [Pd(N-R-2-aminobiphenyl)(µ-OMs)]
2
(R = Me, Ph)[12]
+
.78. ESI-MS (Orbitrap): m/z calculated for C42
H
41NPPd [M
–
were synthesized by following previously reported procedures. Reagents
were purchased from commercial suppliers and used without further
purification. Solvents were dried and degassed before use. Solution NMR
spectra were recorded on a Bruker Avance 300 MHz spectrometer. The
+
(
CH
3
O
3
S)] 696.2006. Found 696.2015.
2
.3. General catalytic procedure for aryl amination reactions
1
13
H and C resonances of the solvent were used as the internal standard
The base was placed into a vial equipped with a J Young tap con-
taining a magnetic bar. The aryl halide (0.5 mmol), the N-nucleophile
3
1
and the chemical shifts are reported relative to TMS while P was
referenced to external H PO . Elemental analyses were performed by the
3
4
(
(
0.6 mmol), the GC internal standard (dodecane, 50 µL) and the solvent
Servicio de Microan a´ lisis of Instituto de Investigaciones Químicas (IIQ).
High resolution mass spectra were registered on Orbitrap Elite Mass
Spectrometer at the Centro de Investigaci o´ n Tecnología e Innovaci o´ n,
CITIUS (Universidad de Sevilla). X-ray diffraction studies were accom-
plished at CITIUS and Instituto de Investigaciones Químicas (IIQ).
0.5 mL) were added in turn, under a nitrogen atmosphere. The reaction
◦
mixture was placed in an oil bath at 110 C and the precatalyst was
added (0.5 mL, 0.01 M in dioxane) under N
2
flow. The reaction mixture
◦
was stirred for 18 h in an oil bath at 110 C. The reaction was allowed to
cool to room temperature, diluted with ethyl acetate and filtered. The
conversion was determined by GC analysis. Purification of products
were attained by flash chromatography.
2
2
.2. Synthesis of representative palladacycles
.2.1. Pd(N-methyl-2-aminobiphenyl)(PMe
CH Cl (5 mL) was added to a mixture of [Pd(N-methyl-2-amino-
biphenyl)(µ-OMs)]
.20 mmol). The reaction mixture was stirred 2 h at room temperature.
2
ArXyl2)(OMs), 1⋅L1
2.4. X-ray crystallographic determinations
2
2
Xyl2
2
(77.1 mg, 0.10 mmol) and PMe
2
Ar
(69.1 mg,
Suitable crystals for X-ray diffraction were coated with per-
fluoropolyether and mounted on a glass fibber and fixed in cold nitrogen
stream to the goniometer head. Data collection have been performed on
two difractometers: a Bruker-Nonius X8 Apex-II CCD diffractometer,
using a graphite monochromator Mo radiation (λ = 0.71073 Å) and fine-
0
After filtration through a Celite plug, the solution was taken to dryness.
The crude reaction product was purified by recrystallization from pe-
◦
troleum ether: CH
2
Cl
2
(8:1) mixtures at 5 C, rendering the title com-
1
◦
◦
pound as yellow crystals. Yield: 98.7 mg (67%). H NMR (300 MHz,
sliced
ω and φ scans (scan widths 0.30 to 0.50 ) [15a] under a flow of
◦
CDCl
3
, 25 C): δ 7.59 (t, 1H, JHH = 7.1 Hz, CHar), 7.39–7.37 (m, 1H,
cold nitrogen (used with 1⋅L1, 1⋅L2, 1⋅L3, 1⋅L4 and 3) and a Bruker-
AXSX8Kappa diffractometer equipped with an Apex-II CCD area detec-
CHar), 7.32–7.09 (m, 13H, CHar and NH), 7.01 (t, 1H, JHH = 7.3 Hz,
CHar), 6.73 (t, 1H, JHH = 7.3 Hz, CHar), 6.14 (t, 1H, JHH = 6.7 Hz, CHar),
tor, using a graphite monochromator Ag K
α1 (λ = 0.56086 Å) and with a
2
.58 (s, 3H, OMs), 2.40 (dd, 3H, JHH = 5.1 Hz, JHP = 2.7 Hz, CH
3
N), 2.23
P), 0.73
Bruker Cryo-Flex low-temperature device (used with 2⋅L1). Data ob-
tained were reduced (SAINT) and corrected for absorption effects by the
(
3
s, 6H, CH ), 2.17 (s, 6H, CH
3
), 1.15 (d, 3H, JHP = 10.3 Hz, CH
3
2

A. Monti et al.
Inorganica Chimica Acta 518 (2021) 120214
Scheme 2. General synthesis of palladacycles.
◦
multiscan method (SADABS).[15b] The structures were solved by direct
process is taking place in solution. Cooling the samples from 25 C to
2
◦
methods (SIR2002,[16] SHELXS) and refined against all F data by full-
ꢀ 30 C leads to the observation of two resonances at ca. 47 and 27 ppm
2
matrix least squares techniques (SHELXL-2018/3) minimizing w[F
o
-
(see Fig. S1 in the Supplementary Data), approximately at the same
2
2
31
F
c
] .[17] All non-hydrogen atoms were refined with anisotropic
ratio. The P chemical shift of the lower frequency resonance is indic-
1
displacement parameters. Hydrogen atoms were included in calculated
positions and allowed to ride on their carrier atoms with the isotropic
temperature factors Uiso fixed at 1.2 times (1.5 times for methyl groups)
of the Ueq values of the respective carrier atoms. A summary of cell
parameters, data collection, structures solution, and the refinement of
crystal structures are provided in the Supporting Information (Tables S1
and S2). Atomic coordinates, anisotropic displacement parameters and
bond lengths and angles can be found in the cif files which have been
deposited in the Cambridge Crystallographic Data Centre with no.
ative of a classical κ -P coordination of the phosphane ligand, whereas
that of higher frequency signal (Δδ ca. 55 ppm with respect to free
ligand) suggests a bidentate coordination mode of the phosphane,
involving additional Pd⋅⋅⋅Carene interactions with one of the flanking ring
of the terphenyl fragment.[13c,14] Presumably, they correspond to
neutral and cationic palladacycles that interconvert each other by co-
ordination/decoordination of the mesylate ligand. Accordingly, upon
switching the original solvent (CDCl
3
) to a less polar solvent such as
6 6
C D
, only the species with the lower frequency resonance (δ 26.8 and
3
1
1
2
,039,701 (1⋅L1), 2,039,702 (1⋅L2), 2,039,703 (1⋅L3), 2,039,704
1⋅L4), 2,039,457 (2⋅L1) and 2,039,705 (3). The data can be obtained
free of charge via: https://www.ccdc.cam.ac.uk/structures/
26.4 for 1⋅L2 and 2⋅L2, respectively) was observed in P{ H}NMR
spectra. Furthermore, both H and C{ H} NMR spectra of these species
recorded in C at room temperature resemble those corresponding to
1 13 1
(
6 6
D
1
⋅L1 and 2⋅L1 described above, indicating that neutral palladacycles are
3
. Results and discussion
6 6 3
favored in C D . Conversely, the more polar solvent CD OD shifts the
equilibrium mixture towards the cationic palladacycles [Pd(N-R-2-
Xyl2
+
3
.1. Synthesis and NMR characterization of palladacycles
aminobiphenyl)(PEt
2
Ar
)] (see Supplementary Data). Unfortu-
OD were unstable preventing
nately, solutions of these complexes in CD
3
The synthesis of complexes 1 and 2 was accomplished by the equi-
their full characterization. However, the most prominent NMR features
of such cationic species with terphenyl phosphanes other than L2 will be
discussed below.
molar reaction of N-substituted-2-aminobiphenyl-Pd methanesulfonate
dimers with dialkylterphenyl phosphanes L1-L4, in dichloromethane at
room temperature (Scheme 2).[12] For the less bulky phosphanes, i.e.
those having methyl (L1) or ethyl substituents (L2) on the phosphorous,
reactions proceeded smoothly furnishing neutral N-methyl- and N-
phenyl-2-aminobiphenyl palladacycles in high yields. Palladacycles
When reactions shown in Scheme 2 were undertaken using sterically
Xyl2
Xyl2
crowded phosphanes PiPr
2
Ar
(L3) and PCyp
2
Ar
(Cyp = cyclo-
pentyl) (L4), only N-methyl-2-aminobiphenyl palladacycles 1⋅L3 and
1⋅L4 could be isolated, as air-stable, analytically pure solids in good
1
13
1
1
⋅L1, 2⋅L1, 1⋅L2 and 2⋅L2 were isolated as air-stable, analytically pure
yields. Both, in the H and C{ H} NMR spectra of palladacycles 1⋅L3
and 1⋅L4 the non–equivalence of the two flanking rings of the terphenyl
moieties is clearly observable. Thus, for complex 1⋅L3, differentiated
signals appear for the four methyl substituents of the xylyl rings in the
pale-brown solids.
3
1
1
P{ H} NMR spectra of palladacycles 1⋅L1 and 2⋅L1 in CDCl
3
comprise a sharp singlet (δ 1.9 for 1⋅L1 and 2.0 for 2⋅L1), which is
shifted ca. 40 ppm to higher frequency relative to the free phosphane.
Comparison with NMR data found for other transition metals terphenyl
phosphanes complexes, including palladium,[13,14,18] suggests mon-
odentate coordination of phosphane in 1⋅L1 and 2⋅L1. Hence, it is safe to
assume that the mesylate anion is coordinated to the metal center in
these two palladacycles. Room-temperature 1H NMR spectra of these
species are consistent with fast rotation of the phosphane around the P-
aliphatic region of its 1H NMR spectrum recorded in CDCl
at room-
3
temperature. In addition, distinctive pattern of signals for each of the
isopropyl groups bonded to phosphorus are found in the expected range.
Moreover, 31P{ H} NMR spectra of 1⋅L3 and 1⋅L4 show single reso-
nances around 62.3 and 54.4 ppm, respectively, shifted by ca. 49 ppm to
higher frequency values relative to uncoordinated phosphanes. Such a
shift seems to be consistent with a bidentate binding mode of the
phosphane in these complexes. Taken together, these observations
support a cationic formulation for palladacycles 1⋅L3 and 1⋅L4.
1
C
aryl bond, since only two resonances are observed for the four methyl
substituents on the terphenyl moiety. In addition, the two P-Me groups
2
give rise to differentiated doublets at 1.15 and 0.73 ppm ( JHP = 10.2
Reactions conducted to obtain N-phenyl-2-aminobiphenyl pallada-
cycles with bulky ligands L3 and L4 led to the formation, in both in-
stances, of a mixture of products consisting of two main components
along with minor species. According to 31P{ H} NMR spectroscopy
2
Hz) for 1⋅L1 and 1.29 and 0.61 ppm ( JHP = 10.5 Hz) for 2⋅L1. More-
over, the methyl group bound to the N atom in palladacycle 1⋅L1 leads to
a doublet of doublets centered at 2.40 ppm due to additional coupling
with the 31P nucleus ( JHP = 2.7 Hz), in agreement with a mutually trans
disposition of the phosphane and the amino group in this complex.
Likewise, the resonance due to the NH group in complex 2⋅L1 is strongly
1
4
3
recorded in CDCl , one of the major species seems to be the expected
3
1
1
palladacycle, since its P{ H} resonance appears at similar chemical
shifts (ca. 65 and 55 ppm) to those found for the N-methyl analogues.
The second main component of the mixture shows a single resonance at
higher frequency, close to 80 ppm, which was tentatively attributed to a
Pd(II)-phosphane complex bearing the N-phenyl-2-aminobiphenyl
ligand solely bonded through the carbon atom. Unfortunately, the rapid
3
deshielded and appears at 9.90 ppm as a doublet ( JHP = 6.0 Hz).
Unlike the above, 31P{ H} NMR spectra in CDCl
1
of palladacycles
3
Xyl2
1
⋅L2 and 2⋅L2, bearing ethyl-substituted ligand PEt
very broad resonances at room temperature, indicating that an exchange
2
Ar
(L2), exhibit
3

A. Monti et al.
Inorganica Chimica Acta 518 (2021) 120214
Fig. 2. Molecular structures of 1⋅L1 (left) and 2⋅L1
(
right). Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at 50% probability.
◦
Selected bond lengths (Å) and angles ( ) for 1⋅L1: Pd-P
2
.2623(4), Pd-N 2.1156(12), Pd-C1 1.9898(15), Pd-
O1 2.1558(12), N-Pd-P 172.59(4), C1-Pd-O1 176.00
6), C1-Pd-P, 92.11(4), P-Pd-O1 90.49(4), C1-Pd-N
(
8
6.82(6), N-Pd-O1 90.21(5); for 2⋅L1: Pd-P 2.2545
(
(
6), Pd-N 2.1403(19), Pd-C1 1.990(2), Pd-O1 2.1670
16), N-Pd-P 167.79(6), C1-Pd-O1 175.43(7), C1-Pd-
P, 91.93(6), P-Pd-O1 92.36(5), C1-Pd-N 87.00(8), N-
Pd-O1 89.15(6).
Fig. 3. Molecular structures of 1⋅L2. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at 50% probability. Selected bond lengths (Å) and
◦
angles ( ) for 1⋅L2: Pd-P 2.2787(8), Pd-N 2.1200(14), Pd-C1 1.9807(15), Pd-O1
2
.2019(13), N-Pd-P 167.02(4), C1-Pd-O1 177.49(6), C1-Pd-P, 94.04(5), P-Pd-
O1 87.87(4), C1-Pd-N 85.72(6), N-Pd-O1 92.06(5).
degradation of the mixture in chlorinated solvents precluded further
characterization. However, red crystals of a degradation product resul-
ted from the reaction with phosphane L4 could be isolated by layering a
dichloromethane solution with petroleum ether. The X-ray diffraction
Fig. 4. Molecular structures of 1⋅L4. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at 50% probability. Selected bond lengths (Å) and
◦
angles ( ) for 1⋅L4: Pd-P 2.2767(5), Pd-N 2.1587(16), Pd-C1 1.9982(19), Pd-
C20 2.411(2), N-Pd-P 161.22(5), C1-Pd-C20 171.27(8), C1-Pd-P, 90.82(5), P-
Pd-C20 83.97(5), C1-Pd-N 82.86(7), N-Pd-C20 104.22(7).
analysis reveals this is a neutral Pd(II) complex of composition Pd(Cl)
Xyl2
(
OMs)(PCyp
2
Ar
), 3 (see Fig. S2, Supplementary Data), in which the
loss of the N-phenyl-2-aminobiphenyl ligand is compensated by the
◦
◦
1
1
actually of 8 for 1⋅L1 and 12 for 2⋅L1. These notable distortions can be
reasonably ascribed to the crowded coordination environment around
the metal center. The phosphane lies opposite to the amino group of the
chelating organic ligand, as usually encountered in analogous
phosphane-stabilized N,C-palladacycles.[12,19] The mesylate ligand in
these complexes is involved in intermolecular hydrogen bonding in-
teractions with the NH function (NH⋅⋅⋅O 2.98 Å for 1⋅L1 and 2.82 Å for
adoption of a κ -P,
η
-Carene coordination mode of the terphenyl phos-
phane ligand. This finding appears to support that N-phenyl-2-amino-
biphenyl platform cannot accommodate very large ligands such as
Xyl2
Xyl2
PiPr
2
Ar
, L3 and PCyp
2
Ar
, L4. A similar remark has been reported
for reactions of N-phenyl-2-aminobiphenyl-palladacycles methanesul-
fonate dimers with sterically encumbered dialkylbiaryl phosphanes.[12]
2
⋅L1). Moreover, the terphenyl phosphane ligand adopts, in both com-
plexes, a conformation [13b] in which one of the P-Me bonds is nearly
coplanar with the central ring of the terphenyl moiety, resulting in two
3
.2. Molecular structures of palladacycles
Neutral structures postulated for palladacycles 1⋅L1 and 2⋅L1 on the
◦
unequal P-Cipso-Cortho bond angles (average value: ca. 118 and 124 ).
basis of NMR spectroscopy were confirmed by X-ray diffraction studies.
The two complexes share akin structural features (Fig. 2). Thus, in both
species, the Pd center resides in a slightly distorted square-planar co-
ordination geometry formed by the chelating N-substituted-2-amino-
biphenyl ligand, the phosphane and the mesylate anion. The angle
between the two planes defined by atoms N, Pd, OMs and P, Pd, C1, is
Orange crystals of palladacyle 1⋅L2 suitable for single-crystal X-ray
diffraction were obtained by layering a dichloromethane solution of
◦
1
⋅L2 with petroleum ether at 5 C. As shown in Fig. 3, in the solid-state
the palladacycle adopts a four-coordinate distorted square-planar ge-
ometry in which the phosphane ligand is coordinated in a classical
4

A. Monti et al.
Inorganica Chimica Acta 518 (2021) 120214
Fig. 5. Catalytic performance of precatalysts 1 and 2 in the N-arylation of morpholine with 4-chlorotoluene. Reaction conditions: 4-chlorotoluene (0.5 mmol),
◦
morpholine (0.6 mmol), NaOtBu (0.6 mmol), dioxane (1 mL), T = 110 C, reaction time 18 h. Conversions were determined by GC analysis of the reaction mixtures
a
using dodecane as internal standard. Isolated yield of product, see ref. 14.
1
manner (κ -P) and the mesylate anion is also bonded to the metal center.
Table 1
The structural properties of this complex are similar to those already
a.
Substrate scope for precatalyst 1⋅L4 in C
–
N cross-coupling reactions.
described for palladacycles 1⋅L1 and 2⋅L1 and will not be further
discussed.
For complexes 1⋅L3 and 1⋅L4 bearing larger phosphanes PiPr
2
ArXyl2
, respectively, X-ray diffraction analyses reveal they are
Xyl2
2
and PCyp Ar
Xyl2
+
composed of a [Pd(N-methyl-2-aminobiphenyl)(PR
2
Ar
)] cation
with a mesylate counterion (Fig. 4 and Fig. S3). In the cationic com-
plexes, the Pd(II) center is formally four-coordinate with the N,C-
1
1
chelating ligand and the phosphane (in a bidentate κ -P,
η
-Carene
bonding mode) forming the coordination sphere. The Pd⋅⋅⋅Cipso separa-
tion of ca. 2.40 Å for 1⋅L3 and 2.41 Å for 1⋅L4 suggests the existence of a
weak bonding interaction between the two atoms involved,[20] and it is
comparable to those of other known 2-aminobiphenyl palladacycles
stabilized by biaryl phosphane[8c] or terphenyl phosphane ligands.[14]
To facilitate this interaction, the central aryl ring of the terphenyl
fragment bends towards the metal, resulting in a narrowing of the cor-
◦
◦
responding P-Cipso-Cortho angle (114.4 and 115.8 for 1⋅L3 and 1⋅L4,
◦
◦
respectively) at the expense of the other (127.8 and 125.7 for 1⋅L3 and
1
⋅L4, respectively). Moreover, the two P-R groups are located at oppo-
site sides with respect to the plane defined by the central aryl ring of the
terphenyl moiety. The Pd-P and Pd-N bond lengths are within the range
found for analogous cationic palladacycles.[8c,14]
3
.3. Catalytic applications in C
–
N cross-coupling reactions
With a set of N-substituted-2-aminobiphenyl palladacycles in hand,
a
Reaction conditions: aryl chloride (0.5 mmol), N-nucleophile (0.6 mmol),
their catalytic activity in the Buchwald-Hartwig reaction[21] was then
assessed. The reaction between 4-chlorotoluene and morpholine was
chosen as the model system to test precatalysts performance. We began
applying the reaction conditions found for parent 2-aminobiphenyl
◦
NaOtBu (0.6 mmol), dioxane (1 mL), T = 110 C, reaction time 18 h. Isolated
b
yield of products. Reaction performed in toluene.
5

A. Monti et al.
Inorganica Chimica Acta 518 (2021) 120214
palladacycle precatalysts, namely, NaOtBu as the base, THF as the sol-
Acknowledgements
◦
vent, 0.5 mol% Pd loading and 80 C as the reaction temperature.[14]
The formation of the aryl amine was measured by GC after 18 h. For
We thank MICINN (CTQ2017-82893-C2-2-R) and FEDER-
Universidad de Sevilla (US-1262266) for financial support. AM and
RJR thank MICINN and the Universidad de Sevilla (V Plan Propio de
Investigaci o´ n), respectively, for research fellowships. Thanks are also
due to Centro de Investigaci o´ n, Tecnología e Innovaci o´ n de la Uni-
versidad de Sevilla (CITIUS).
comparative purposes, the catalytic activity of [Pd(2-aminobiphenyl)
Xyl2
(
PCyp
2
Ar
)](OMs), the best performing precatalyst of the unsub-
stituted 2-aminobiphenyl-derived series, was also included in this study.
None of the N-substituted palladacycles promoted the C N cross-
–
coupling under these conditions. When rising the temperature up to
◦
1
10 C the two precatalysts 1⋅L1 and 2⋅L1 bearing the less sterically
Xyl2
encumbered phosphane PMe
2
Ar
displayed very low activity (Fig. 5).
Appendix A. Supplementary data
As expected, the catalytic performance improved significantly with
increasing the ligand bulkiness and precatalyst 1⋅L4 demonstrated to be
the most effective (Fig. 5), but in no case reaching or exceeding the yield
found for the unsubstituted 2-aminobiphenyl palladacycle. Full con-
versions were attained with precatalyst 1⋅L4 when using 1 mol% cata-
lyst loading. Interestingly, it was noted that precatalyst 2⋅L2 was more
effective than precatalysts 1⋅L2 arising from the N-methyl-2-amino-
biphenyl scaffold. Presumably, the larger size of the phenyl substituent
as compare to the methyl could facilitate the precatalyst activation.
Under the optimized reaction conditions, the competence of pre-
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2020.120214.
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7