Electronic: Versus steric effects of pyridinophane ligands on Pd(III) complexes

Article abstract of DOI:10.1039/c7dt04366j

Several new Pd^II and Pd^III complexes supported by electronically and sterically tuned tetradentate pyridinophane ligands ^MeN4^OMe, ^MeN4, and ^tBuN4 were isolated and fully characterized (^MeN4^OMe: N,N′-dimethyl-2,11-diaza[3,3](2,6)-para-methoxypyridinophane; ^MeN4: N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane; ^tBuN4: N,N′-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane). Cyclic voltammetry studies, UV-vis and EPR spectroscopy, and X-ray crystallography were employed to reveal that the steric properties of the N-substituents of the ^RN4 ligands have a pronounced effect on the electronic properties of the corresponding Pd^III complexes, while the electronic tuning of the ligand pyridyl groups has a surprisingly minimal effect. An explanation for these observations was provided by DFT and TD-DFT calculations which suggest that the electronic properties of the Pd^III complexes are mainly dictated by their frontier molecular orbitals that have major atomic contributions from the Pd center (mainly the Pd dz² atomic orbital) and the axial N atom donors.

Full text of DOI:10.1039/c7dt04366j

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PAPER
Electronic versus steric effects of pyridinophane
ligands on Pd(III) complexes†
Cite this: DOI: 10.1039/c7dt04366j
a
a
b
a
Fengzhi Tang,‡ Sungho V. Park,‡ Nigam P. Rath and Liviu M. Mirica
*
II
III
Several new Pd and Pd complexes supported by electronically and sterically tuned tetradentate pyri-
Me
OMe Me
tBu
Me
OMe
dinophane ligands
N4
,
N4, and
N4 were isolated and fully characterized ( N4
Me
: N,N’-
dimethyl-2,11-diaza[3,3](2,6)-para-methoxypyridinophane;
tBu
N4: N,N’-dimethyl-2,11-diaza[3,3](2,6)pyri-
dinophane;
N4: N,N’-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane). Cyclic voltammetry studies, UV-
vis and EPR spectroscopy, and X-ray crystallography were employed to reveal that the steric properties of
R
the N-substituents of the N4 ligands have a pronounced effect on the electronic properties of the
III
corresponding Pd complexes, while the electronic tuning of the ligand pyridyl groups has a surprisingly
Received 20th November 2017,
Accepted 11th December 2017
minimal effect. An explanation for these observations was provided by DFT and TD-DFT calculations
III
which suggest that the electronic properties of the Pd complexes are mainly dictated by their frontier
DOI: 10.1039/c7dt04366j
molecular orbitals that have major atomic contributions from the Pd center (mainly the Pd d
orbital) and the axial N atom donors.
z
2 atomic
rsc.li/dalton
cyclic voltammetry (CV), UV-Vis and EPR spectroscopy, and
Introduction
1
9
X-ray crystallography. By decreasing the steric bulk of the
Me
iPr
Ligand modification via electronic and steric tuning has been N-substituents, the ligands N4 and N4 were found to dra-
III
broadly employed to understand the fundamentals of ligand matically influence the properties of the corresponding Pd
1
–4
design in organometallic chemistry and catalysis.
The ana- complexes such as their redox potentials, UV-vis absorptions,
lysis of such steric and electronic effects can provide important and EPR parameters. The X-ray structures of these isolated
III
insights into the development of novel catalytic systems with Pd complexes also reveal shorter Pd–Naxial bond distances
5
–10
tBu
iPr
Me
high reactivity and regio- and enantioselectivities.
In the
(
N4 >
N4 >
N4) due to reduced steric interactions
1
8,26
context of Pd catalysis, it has been shown recently that high- between the equatorial ligands and the N-substituents.
valent Pd and Pd species are active intermediates in several Continuing the study of such uncommon mononuclear Pd
stoichiometric and catalytic C–C and C–heteroatom bond complexes,
III
IV
III
1
8,25
we report herein the synthesis and characteriz-
1
1–18
III
19–27
II
III
Me
OMe
transformations.
Given our interest in Pd chemistry,
ation of Pd and Pd complexes supported by the N4
probing the ligand effects onto the properties of the corres- ligand (N,N′-dimethyl-2,11-diaza[3,3](2,6)-p-methoxypyridino-
III
ponding Pd complexes should provide insights toward the phane) and the systematical comparison of their spectroscopic
III
understanding of Pd reactivity and the development of cata- and structural properties with those of the analogous com-
III
Me
tBu
lysts involving Pd species.
plexes supported by N4 and
N4 ligands, along with sup-
In 2010, we have reported that a tetradentate ligand N,N′-di- porting DFT calculations.
tBu
tert-butyl-2,11-diaza[3,3](2,6)pyridinophane ( N4) can stabil-
III
ize the paramagnetic Pd complexes that show unique pro-
7
perties due to the d electronic structure of the Pd center, as
shown through various characterization methods such as
Results and discussion
Me
OMe
Synthesis of the N4
ligand and the Pd complexes
Our previous studies showed that the N-substituents of the
R
a
N4 ligand have a dramatic effect on the stability and reactivity
Department of Chemistry, Washington University, One Brookings Drive, St Louis,
III
IV
18
Missouri 63130-4899, USA. E-mail: mirica@wustl.edu
of the corresponding Pd and Pd complexes. In addition
to this strong steric effect, the manipulation of the electronic
properties of N4 ligands was also considered to further tune
the properties of high-valent Pd complexes. Thus, we sought to
b
Department of Chemistry and Biochemistry, University of Missouri-St Louis,
One University Boulevard, St Louis, Missouri 63121-4400, USA
R
†
Electronic supplementary information (ESI) available. CCDC 1583174–1583176.
For ESI and crystallographic data in CIF or other electronic format see DOI:
Me
OMe
employ an electron-rich
N4
ligand containing an elec-
1
0.1039/c7dt04366j
‡
These authors contributed equally to this work.
tron-donating methoxy group at the para position of the
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Me
OMe
II
II=III
that the ( N4
)Pd Cl
2
complex 1 exhibits an Epa oxi-
II=III
dation potential that is only 20 mV lower than the Epa for the
Me
II
tBu
II
(
N4)Pd Cl
2
complex 2, while the ( N4)Pd Cl
2
complex 3
II=III
has an Epa oxidation potential that is ∼180 mV higher than
III/IV
that for 2. In addition, the E1/2Pd
within 20 mV of each other, while the E1/2Pd
300 mV higher than those for 1 and 2. A similar trend is
values for 1 and 2 are
III/IV
value for 3 is
∼
II
II=III
observed for the Pd MeCl complexes 4–6, where the Epa oxi-
tBu
dation potential for the
than those for complexes 4/5, while the E1/2Pd
N4 complex 6 is ∼170 mV higher
III/IV
value for
tBu
the
N4 complex 6 is ∼380 mV higher than those for com-
plexes 4/5 (Fig. 1 and Table 1). Overall, these results suggest
that increasing the electron-donating ability of the pyridyl
groups has a minimal effect on the electrochemical properties
of the corresponding Pd complexes, while the nature of the
N-substituent has a more pronounced effect on these redox
properties of Pd complexes, most likely by directly modulating
the energy and atomic orbital (AO) contributions of the fron-
tier molecular orbitals (FMOs) of these Pd complexes (see
below).
Me
OMe
Scheme 1 Synthesis of the N4
ligand.
Me
OMe
pyridyl groups. The synthesis of N4
acid as the starting material and followed a slightly modified
employed chelidamic
synthetic
route
to
afford
the
target
ligand
Me
OMe
28,29
N4
(Scheme 1).
R
II
The dichloride and methyl chloride ( N4)Pd complexes
–6 were prepared by mixing the corresponding N4 ligand
with the Pd precursors (COD)PdCl2 or (COD)PdMeCl in
III
UV-Vis absorption spectroscopy of the Pd complexes. The
R
1
+
+
UV-Vis spectra of complexes 1 –6 exhibit two strong absorp-
tion bands at 578–723 nm and 303–395 nm that are assigned
II
tBu
II
CH
and 6
reported previously, the ( N4
2
Cl
2
1
or Et
2
O, respectively. While the ( N4)Pd complexes 3
19
as LMCT bands (Table 1), as reported previously. The steric
9
Me
II
26
and ( N4)Pd complexes 2 and 5
have been
manipulation of the N-substituents greatly influences the
Me
OMe
II
)Pd complexes 1 and 4 are
III
tBu
absorption properties of the Pd complexes, as the ( N4)
Pd Cl
reported herein for the first time. Given the accessible oxi-
dation potentials for all of these complexes (see below), they
can be oxidized easily by one electron using either controlled
potential electrolysis (CPE) or chemical oxidation with 1 equiv.
of ferrocenium hexafluorophosphate (Fc PF
corresponding Pd complexes (Scheme 2). These Pd com-
plexes are stable at room temperature and X-ray quality crystals
III
III
+
+
2
and Pd MeCl complexes 3 and 6 show transitions
that are red shifted by 100–150 nm vs. those for the analogous
+
+
Me
complexes 2 and 5 supported by the less bulky
N4
26
+
+
ligand. By comparison, the Pd complexes 1 and 4 of the
+
−
6
) to yield the
more electron-rich
similar to those of analogous ( N4)Pd complexes 2 and 5
Fig. 2 and Table 1), suggesting that the MOs involved in the
transitions of these Pd complexes have negligible contri-
butions from the pyridine-based AOs.
EPR spectroscopy of the Pd complexes. The EPR spectra
collected at 77 K for the Pd complexes reveal axial signals for
the Pd Cl complexes 1 –3 and rhombic patterns for the
2
Pd MeCl complexes 4 –6 , yet all suggesting a Pd center
Me
N4
OMe
ligand exhibit UV-Vis spectra
III
III
Me
+
+
(
can be obtained by Et
2
O diffusion into the solutions of the
complexes in DCM or MeCN at −20 °C.
III
3
0
III
II
III
III
Comparison of the electronic properties of the Pd and Pd
III
+
+
complexes
III
+
+
III
II
Electrochemical studies of the Pd complexes. The CV
characterization of the dichloride Pd complexes 1–3 reveals
II
with a d
z
ground state (Fig. 3). The axial superhyperfine coup-
2
III
Me
OMe
ling constants of the Pd complexes supported by N4
Me
+
+
and N4 ligands were identical (25.5 G for 1 and 2 , 23.0 G
+
+
for 4 and 5 , Table 1), implying a similar extent of interaction
Fig. 1 Overlay of CV traces (0.1 M Bu
4
NClO
4
III
/DCM, 100 mV s− scan
1
III
19,25,26
III
+
+
+
+
Scheme 2 Synthesis of the Pd complexes.
rate) for (a) Pd Cl
2
complexes 1 –3 and (b) Pd MeCl complexes 4 –6 .
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Table 1 Spectroscopic and electrochemical properties of the Pd complexes
Paper
III
III=II
II=III
III=IV
=2
a
UV-vis (MeCN) λ, nm (ε, M−1 cm−1
b
c
Epc , Epa , E1
(ΔE
p
) , mV
)
x y z
EPR g , g , g , (A2N, G)
+
+
+
+
+
+
1
2
3
4
5
6
−195, −40, 595 (110)
583 (960), 365 (sh, 2260), 325 (sh, 3060)
578 (1420), 362 (2440)
716 (1820), 395 (2780)
615 (560), 495 (380), 303(4700)
606 (615), 475 (480), 316 (3550)
723 (1100), 545 (490), 368 (3300)
2.129, 2.129, 2.004 (25.5)
2.126, 2.126, 2.004 (25.5)
2.152, 2.142, 2.000 (23.0)
2.219, 2.108, 2.016 (23.0)
2.212, 2.101, 2.014 (23.0)
2.239, 2.134, 2.005 (19.5)
−175, −45, 610 (115)
d
e
−18, 117/626 , 918 (76)
e
−650/−185 , −50, 195 (100)
d
d
e
e
−620/−135 , −480/−20 , 205 (130)
−464, 134, 587 (63)
a
+
/MeCN or DCM, scan rate 100 mV s−1; ΔE
Redox potentials are reported vs. Fc/Fc in 0.1 M Bu
4
NClO
4
p
is the peak potential separation for the
III
IV
b
c
d
+
Pd /Pd couple. In 3 : 1 PrCN : MeCN glass, 77 K. Superhyperfine coupling to the two axial N donors (I = 1). The CV and UV-vis data for 3 ,
5
+
+
e
II=III
II=III
, and 6 are from ref. 25, 26, and 19, respectively. The duplicate Epa and Epc values are likely due to two conformations present in solution
(ref. 25).
III
+
2
Fig. 2 Overlay of UV-Vis spectra in MeCN for (a) Pd Cl complexes 1 –
+
III
+
+
3
and (b) Pd MeCl complexes 4 –6 .
+
Fig. 4 ORTEP representation (50% probability ellipsoids) of 1 . Selected
+
bond distances (Å) and angles (°): 1 : Pd1–N1 2.0009(19), Pd1–N1_i
2.0009(18), Pd1–N2 2.300(3), Pd1–N3 2.308(3), Pd1–Cl1 2.3018(6), Pd1–
Cl1_i 2.3018(6), N1–Pd1–N1_i 82.67(11), N2–Pd1–N3 152.89(10).
Fig. 3 Overlay of EPR spectra (frozen glasses in 1 : 3 MeCN : PrCN, 77 K)
III
+
+
III
+
+
for (a) Pd Cl
2
complexes 1 –3 and (b) Pd MeCl complexes 4 –6 .
III
between the Pd center and the axial N donors, despite the
different electronic properties of the pyridyl groups for the two
+
Fig. 5 ORTEP representation (60% probability ellipsoids) of 2 . Selected
tBu
III
+
ligands. In contrast, the
N4-supported Pd complexes 3
+
bond distances (Å) and angles (°): 2 : Pd1–N1 2.029(8), Pd1–N2 2.002(8),
+
and 6 exhibit superhyperfine coupling constants that are ∼3 Pd1–N3 2.310(9), Pd1–N4 2.311(9), Pd1–Cl1 2.303(3), Pd1–Cl2 2.322(3),
G smaller than those of the other complexes, suggesting a N1–Pd1–N2 82.3(3), N3–Pd1–N4 153.5(3).
III
Me
OMe
weaker Pd –Naxial interaction. In addition, the N4
and
N4 Pd complexes showed smaller differences between g /g
N4 supported ana- almost identical to those of the
Me
III
x
y
tBu
Me
III
values and g values than those of the
N4 Pd
complexes,
z
logues, which could be attributed to a more symmetric geome- suggesting
a
minimal effect of the electron-donating
III
III
try around the Pd center (see below).
Structural comparison of the Pd complexes. All the single bond distances in the N4-supported Pd complexes 3 and
p-methoxy group on the Pd center. However, the Pd–Nax
III
tBu
III
+
+
+
+
Me
crystal X-ray characterizations of complexes [1 ]ClO
4
, [2 ]ClO
4
,
6 are ∼0.1 Å longer than those in N4 supported analogs –
+
III
and [4 ]ClO
4
revealed Pd centers in a distorted octahedral likely due to the increased steric clash between the
III
26
geometry, similar to the other reported analogous Pd com- N-substituents and equatorial CH or Cl ligands. In addition,
3
+
+
+
plexes [3 ]ClO
4
, [5 ]ClO
4 4
, and [6 ]ClO (Fig. 4–6 and Fig. S25, this steric hindrance also leads to an overall distortion of com-
1
9,26
III
+
+
S26†).
centers in the
Surprisingly, all the metrical parameters of the Pd
plexes 3 and 6 such that the pyridyl is tilted in the opposite
-supported complexes 1 and 4 are direction relative to the equatorial plane, in order to accommo-
Me
OMe
+
+
N4
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+
Fig. 6 ORTEP representation (50% probability ellipsoids) of 4 . Selected
+
bond distances (Å) and angles (°): 4 : Pd1–N1 2.055(3), Pd1–N2 2.130(3),
Pd1–N3 2.297(3), Pd1–N4 2.340(3), Pd1–C1 2.040(4), Pd1–Cl1
2.3298 (11), N1–Pd1–N2 81.09(12), N3–Pd1–N4 149.19(11).
date the bulkier t-butyl N-substituents (see the space-filling
30
models in Fig. S25 and S26†). Overall, these structural charac-
III
teristics of the Pd complexes further suggest that the steric
modulation of the N-substituent has a more pronounced effect
than the electronic donation ability of the pyridyl groups.
Computational studies. It is known that the electronic and
chemical properties of a metal complex are closely related to
3
1
its frontier molecular orbitals (FMOs).
Therefore, DFT
studies were employed in order to investigate the ground state
III
electronic structures of our family of Pd complexes, with
special emphasis on the FMO energies and the atomic contri-
bution to the FMOs α-HOMOs and β-LUMOs. In addition,
TD-DFT calculations were used to calculate the absorption
III
spectra assigned to the observed transitions for these Pd
complexes.
III
+
+
For all Pd complexes 1 –6 , the electron densities for both Fig. 7 Comparison of the DFT-calculated α-HOMOs, β-LUMOs, and
the α-HOMOs and β-LUMOs are localized mainly along the
axial direction, with ∼30% contribution from the Pd center
III
+
+
their atomic contributions for Pd Cl
and 3 (bottom).
2
complexes 1 (top), 2 (middle),
+
(
almost exclusively the d
z
2
atomic orbital) and ∼40% contri-
3
0
bution from the Naxial atoms (Fig. 7 and Tables S1–S8†). In
tBu
III
contrast, the contributions from the equatorial atom donors Naxial bond distances for the
N4Pd complexes are ∼0.1 Å
Me
Me
OMe
are minimal (<6%). This further supports the experimental longer than those of the corresponding
N4 and
N4
observation that altering the N-substituents of the axial N complexes (Table 2). The less covalent Pd–Naxial interactions in
tBu
III
donor atoms has a more prominent effect than modifying the the N4Pd complexes can also explain their smaller super-
equatorial pyridyl groups. Interestingly, the atomic contri- hyperfine coupling constants observed in the EPR spectra
+
+
butions to the α-HOMOs and β-LUMOs for all complexes 1 –6
(Table 1), as well as the red shift observed in the UV-vis spectra
Me
III
Me
OMe
III
+
+
are almost identical for the
N4Pd
and
N4
Pd
of complexes 3 and 6 (Fig. 2). The TD-DFT calculated UV-Vis
+
+
complexes, revealing that the electronic tuning of the pyridyl spectra and electronic transitions for complexes 1 –6 match
groups has a negligible effect. This also provides an expla- well with the experimental spectra (Fig. S27 and S38†), and
nation why the electronic properties of the two series of Pd
complexes are similar. In contrast, the N4Pd complexes 3
3
0
III
the absorptions at 578–723 nm are assigned to β-HOMO to
β-LUMO LMCT transitions. The DFT calculations indeed show
tBu
III
+
+
Me
Me
OMe
III
and 6 exhibit less covalent metal–axial ligand interactions, as that the HOMO–LUMO gaps for the N4 and N4
Pd
suggested by the slightly smaller contributions from the complexes are similar, while the HOMO–LUMO gaps for the
tBu
III
Pd centers and Naxial ligands to the FMOs (Fig. 7 and Tables
N4Pd analogs are ∼0.7 eV smaller (Fig. 8), which leads to
3
0
S1–S8†), which is likely due to the steric clash between the the 100–150 nm red shift for the corresponding absorption
bulky Bu N-substituents and the equatorial ligands that leads bands. Finally, the HOMOs of complexes 3 and 6 are slightly
to a distortion of the Pd complex geometry (Fig. S25†). This higher in energy than those of the other complexes, which sup-
also correlates with the metrical parameters obtained from the ports their observed higher Pd
t
+
+
III
30
III/IV
II/III
(and Pd
) redox poten-
X-ray characterization of these complexes showing that Pd– tials (Fig. 1 and Table 1).
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Table 2 Selected bond distances and angles for the Pd complexes 1 –6
Paper
III
+
+
Pd–Nax
Pd–Neq
Pd–Cl
Pd–C
Nax–Pd–Nax
Neq–Pd–Neq
+
+
+
+
+
+
a
1
2
3
4
5
6
2.304(3)
2.310(9)
2.417(2)
2.318(3)
2.320(3)
2.4268(16)
2.0009(19)
2.016(8)
2.003(2)
2.092(3)
2.085(2)
2.0585(16)
2.3018(6)
2.312(3)
2.3160(7)
2.3298(11)
2.344(3)
NA
NA
NA
152.89(10)°
153.5(3)°
145.07(7)°
149.19(11)°
149.12(11)°
142.37(5)°
82.67(11)°
82.3(3)°
88.06(9)°
81.09(12)°
81.44(12)°
86.61(6)°
a
b
a
2.040(4)
2.021(11)
2.0921(18)
b
b
2.3403(5)
a
b
+
+
+
Not applicable. Bond distances and angles for 3 , 5 , and 6 are from ref. 25, 26, and 19, respectively.
Experimental section
General experimental details
All operations were performed under a nitrogen atmosphere
using standard Schlenk and glove box techniques if not indi-
cated otherwise. All reagents for which the syntheses are not
given were purchased from Fisher Scientific, Sigma-Aldrich,
Acros, STREM, or Pressure Chemical and were used as received
without further purification. Solvents were purified prior to
use by passing them through a column of activated alumina
using an MBRAUN SPS. The synthesis and characterization of
Me
II
Me
II
+
Me
complexes
(
2
N4)Pd Cl , [( N4)Pd MeCl] , and [( N4)
III
+
27
Pd MeCl] have been reported previously. NMR spectra were
obtained on a Varian Mercury-300 spectrometer (300.121 MHz)
or a Varian Unity Inova-600 spectrometer (599.746 MHz).
Chemical shifts are reported in parts per million (ppm) with
Fig. 8 Pictorial representation of the FMO energy levels (β AOs) for
+
+
complexes 1 –6 .
4
residual solvent resonance peaks as internal references.
Abbreviations for the multiplicity of NMR signals are singlet
(
s), doublet (d), triplet (t), quartet (q), septet (sep), multiplet
Conclusion
(m), and broad resonance (br). UV-vis spectra were recorded on
a Varian Cary 50 Bio spectrophotometer and are reported as
In summary, we report herein the synthesis of the electroni-
−1
−1
λmax, nm (ε, M cm ). EPR spectra were recorded on a JEOL
JES-FA X-band (9.2 GHz) EPR spectrometer in 3 : 1
PrCN : MeCN at 77 K or 293 K. Solution magnetic susceptibility
Me
OMe
cally tuned pyridinophane ligand
N4
and the corres-
II
III
ponding Pd and Pd complexes, as well as a systematic com-
III
III
parison of the properties of the Pd Cl
plexes supported by the
Remarkably, the
almost identical spectroscopic and structural properties, as
observed by cyclic voltammetry studies, UV-vis and EPR spec-
troscopy, and X-ray crystallography. In contrast, the steric pro-
perties of the N-substituents of the N4 ligands have a pro-
nounced effect on the electronic and structural properties of
2
and Pd MeCl com-
III
measurements for the Pd complexes were obtained at 293 K
Me
OMe Me
tBu
N4
and
,
N4, and
N4 ligands.
32
with the Evans method, including the corresponding dia-
Me
OMe
Me
III
N4
N4 Pd complexes have
33
magnetic corrections. ESI-MS experiments were performed
using a linear quadrupole ion trap Fourier transform ion cyclo-
tron resonance mass spectrometer (LTQ-FTMS, Thermo, San
Jose, CA) or a Bruker Maxis Q-TOF mass spectrometer with an
electrospray ionization source. ESI mass-spectrometry was pro-
vided by the Washington University Mass Spectrometry
Resource. Elemental analyses were carried out by the
Columbia Analytical Services Tucson Laboratory and Intertek
Pharmaceutical Services.
R
tBu
the corresponding Pd complexes, as observed for the
N4-
III
supported Pd complexes. To support these experimental
observations, DFT and TD-DFT calculations were employed to
III
reveal that the electronic properties of the Pd complexes are
mainly dictated by their frontier molecular orbitals that have
major atomic contributions from the Pd center (mainly the
Electrochemical studies
Pd dz
2
atomic orbital) and the Naxial atom donors. Overall, these Cyclic voltammetry (CV) studies were performed with a BASi
2
3,24
results suggest that the properties as well as the nature
of EC
Epsilon
electrochemical
workstation
or
CHI
the axial atom donors have a pronounced effect on the stability Electrochemical Analyzer 660D. Electrochemical grade
III
and ultimately reactivity of the corresponding Pd centers, and Bu NClO from Fluka was used as the supporting electrolyte.
4
4
d
7
ions in general, and further expansion of the ligand systems The electrochemical measurements were performed under a
employed should be mainly aimed at modifying the coordi- blanket of nitrogen, and the analyzed solutions were deaerated
nation properties and identity of the axial atom donors.
by purging with nitrogen. A glassy carbon disk electrode (GCE)
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1
was used as the working electrode, and a Pt wire as the auxili- (85%). H NMR (300 MHz, CDCl
3
), δ: 6.89 (s, 2H, arom-H),
ary electrode. The non-aqueous Ag-wire reference electrode 4.48 (s, 4H, –CH –), 3.88 (s, 3H, p-OCH ).
2
3
assembly was filled with 0.01 M AgNO
solution. The reference electrodes were calibrated against 4-Methoxy-2,6-bis(bromomethyl)pyridine (4.41 g) in 40 mL
ferrocene for each CV measurement. anhydrous DMF was added dropwise to 2.884 g TsNHNa in
3
/0.1 M Bu
4
NClO
4
/MeCN
Methoxy N,N′-ditosyl-2,11-diaza[3.3](2,6)pyridinophane.
Bulk electrolysis oxidations were performed in a two-com- 300 mL anhydrous DMF at 80 °C. After the addition was com-
partment bulk electrolysis cell (BASi) separated by a fine-frit plete, stirring was continued at 80 °C for another hour. Another
glass junction at room temperature. A reticulated vitreous 2.884 g of TsNHNa was added all at once and heated at 80 °C
carbon working electrode was used in the anodic compartment overnight. After 24 h, the reaction mixture was cooled down to
equipped with a magnetic stirring bar. A Pt gauze (25 mm × room temperature and the solvent was removed under reduced
1
0 mm) was used as the auxiliary electrode in the cathodic pressure. The brown residue was stirred with 150 mL MeOH for
compartment. A non-aqueous Ag/0.01 M AgNO electrode was 10 minutes. The resulting suspension was filtered, then washed
used as the reference electrode. Prior to electrolysis, a CV of with water and a small amount of methanol. The light yellow
3
II
1
the Pd precursor was performed in the same setup. The solid was dried in vacuo. Yield: 0.75 g (33%). H NMR
potential of electrolysis was set to be 100 mV–200 mV, more (300 MHz, CDCl ), δ: 7.68 (d, J = 8 Hz, 4H, tosyl-arom-H), 7.35
positive than the corresponding oxidation peak potential (d, J = 8 Hz, 4H, tosyl-arom-H), 6.94 (s, 4H, pyr-arom-H), 4.92
3
observed in the CV scan. Electrochemical grade Bu NClO
from Fluka was used as the supporting electrolyte.
(s, 8H, –CH –), 4.00 (s, 6H, p-OCH ), 2.42 (s, 6H, tosyl-CH ).
2 3 3
4
4
H
OMe
Methoxy 2,11-diaza[3.3](2,6)pyridinophane ( N4
.75 g of p-methoxy N,N′-ditosyl-2,11-diaza[3.3](2,6)pyridino-
phane, 13.7 mL of 90% H SO was added and the solution was
). To
1
2
4
Synthetic procedures
stirred at 110 °C for 3 hours. After cooling down to room temp-
erature, the solution was diluted with 65 mL water and treated
2
8
4
-Methoxypyridine-2,6-dicarboxylic acid dimethyl ester.
Chelidamic acid monohydrate (1.31 g) was refluxed in with 3 M NaOH to adjust the pH to 12–13. The resulting solu-
300 mL dry methanol in the presence of 6 mL concentrated tion was extracted with DCM 3 times, dried over Na SO , and
∼
2
4
1
sulfuric acid for 4 days. After cooling down to RT, sodium car- dried under vacuum. Yield: 0.77 g (92%). H NMR (300 MHz,
bonate (1 equiv. relative to the amount of sulfuric acid) was CDCl ), δ: 6.06 (s, 4H, arom-H), 3.86 (s, 8H, –CH –), 3.66 (s,
3
2
3
added and the solvent was removed. The residue was dissolved 6H, p-OCH ).
in saturated aqueous sodium bicarbonate solution and then
extracted with dichloromethane (DCM). The organic layer was
dried over sodium sulfate and filtered, the solvent was by dissolving 0.77 g of N4
Methoxy
N,N′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane
Me
OMe
(
N4
). An Eschweiler–Clarke methylation was conducted
H
OMe
in 114 mL HCOOH and 25 mL
removed and the white solid was dried in vacuo. Yield: 0.81 g 37% CH O. The mixture was refluxed under nitrogen for 1 day.
2
1
(
55%). H NMR (300 MHz, CDCl ), δ: 7.82 (s, 2H, arom-H), After cooling, the reaction mixture was treated with 13.6 mL
3
4
.01 (s, 6H, –COOCH
3
), 3.98 (s, 3H, p-OCH
3
).
37% HCl and the solvent was removed under reduced
of pressure. The residue was diluted with 10 mL H O and basi-
-methoxypyridine-2,6-dicarboxylic acid dimethyl ester in fied with 1 M NaOH to pH 13. The mixture was extracted with
0 mL methanol, sodium borohydride (5 equiv., 1.86 g) was DCM 3 times, and the organic layer was dried over Na SO , fil-
4
-Methoxy-2,6-bis(hydroxymethyl)pyridine. To 2.22
g
2
4
5
2
4
slowly added. As the mixture refluxed, the color changed from tered, and dried under vacuum. The white product was further
colorless to yellow, and then to white. The resulting solution purified by extraction into hexane and removal of the solvent.
1
was stirred at room temperature for 2 days. The solvent was Yield: 580 mg (58%). H NMR (300 MHz, CD
3
CN), δ: 6.35 (s,
–), 3.63 (s, 6H, p-OCH ), 2.63 (s,
solved in 30 mL of saturated NaHCO solution. The solution 6H, N–CH3). C NMR (300 MHz, CDCl ), δ: 165.39 (py-Cortho),
then removed under reduced pressure and the residue was dis- 4H, arom-H), 3.64 (s, 8H, –CH
2
3
1
3
3
3
was extracted with DCM, and the organic and aqueous layers 159.04 (py-Cmeta), 109.16 (py-Cpara), 65.94 (–CH
2
–), 54.94
were dried separately. The aqueous layer residue was extracted (p-OCH ), 49.06 (N-methyl-C).
3
Me
OMe
II
Me
OMe
with 100 mL chloroform using a Soxhlet extractor for 2 days at
(
N4
)Pd Cl , 1.
N4
(41.0 mg, 0.144 mmol) were dissolved in 3 mL of
DCM, separately. The ligand solution was added dropwise to
-Methoxy-2,6-bis(bromomethyl)pyridine. 4-Methoxy-2,6-bis the precursor solution under stirring. After the addition was
hydroxymethyl)pyridine (2.97 g) was suspended in 125 mL dry complete, the reaction mixture was stirred for an additional
(47.2 mg, 0.144 mmol) and
2
1
II
8
0 °C. Yield: 0.90 g (54%). H NMR (300 MHz, CDCl
3
), δ: 6.71 (COD)Pd Cl
).
2
(
s, 2H, arom-H), 4.71 (s, 4H, –CH –), 3.86 (s, 3H, p-OCH
2
3
4
(
DCM. A mixture of thionyl bromide (5 mL, 3.5 equiv.) in 24 hours in the dark. The clear, dark yellow solution eventually
5 mL DCM was added dropwise at 0 °C. The mixture was turned cloudy. The solvent was removed and the residue was dis-
stirred for 4 days in the dark, and then it was poured into solved in a minimum (2 mL) of DCM. This yellow mixture was
00 mL of ice-cold water. The aqueous layer was neutralized to filtered, washed with ether and pentane, and dried in vacuo.
3
1
1
pH 8 with 3 M NaOH on an ice bath. The organic layer was Yield: yellow powder, 62.9 mg, 86%. H NMR (300 MHz, CDCl ):
3
separated, washed with 100 mL 1 M NaOH and then with 6.65 (d, J = 15 Hz, 4H, –CH
2
–), 6.61 (s, 4H, arom-H), 4.25 (d, J =
), 2.33 (s, 6H, N–CH ).
1
00 mL water. The DCM layer was then dried over sodium 15 Hz, 4H, –CH –), 3.81 (s, 6H, p-OCH
2
3
3
Me
OMe
II
sulfate, filtered and concentrated to dryness. Yield: 4.41 g ESI-MS of ( N4
)Pd Cl in MeCN, m/z 469.0643; calculated
2
Dalton Trans.
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Paper
+
Me
III
+
for [M − Cl] , C18
H
24ClN
4
O
2
Pd, 469.0643. Anal. found: C, 41.26; 444.0107; calculated for [( N4)Pd Cl
2
] ,
16 2 4
C H20Cl N Pd,
Me OMe
II
1
H, 4.46; N, 10.07; calcd for
(
N4
)Pd Cl · CH Cl
2 2 2
2
444.0100. Anal. found: C, 36.87; H, 4.14; N, 11.54; calcd for
1
2
Me
III
(
C
18
Me
H
24Cl
2
N
4
O
2
Pd)· CH
2
Cl
2
: C, 40.53; H, 4.60; N, 10.22.
[( N4)Pd Cl
2
](ClO
4
)·MeCN (C18
CN) = 1.91μ .
)Pd MeCl]ClO , [4 ]ClO . Ferrocenium hexa-
4 4
3 4 4
H23Cl N O Pd): C, 36.88; H,
OMe
II
(
N4
)Pd MeCl, 4. Under a nitrogen atmosphere, the 3.95; N, 11.95. μeff (CD
ligand (44.3 mg, 0.135 mmol) and (COD)Pd MeCl
3
B
Me
OMe
II
Me
OMe
III
+
N4
[( N4
precursor (35.5 mg, 0.135 mmol) were loaded in a round fluorophosphate (18.1 mg, 0.055 mmol) and 4 (26.0 mg,
bottom flask. Anhydrous ethyl ether (13.7 mL) was added, and 0.054 mmol) were combined in 5 mL MeCN and stirred for
a light yellow suspension immediately formed. This suspen- 30 minutes. The resulting solution was washed with pentane
sion was stirred for 48 hours in the dark. The pale yellow pre- and concentrated with a rotary evaporator. The resulting dark
cipitate was filtered, washed with ether and pentane and dried purple residue was dissolved in a minimum amount of MeCN,
1
in vacuo. Yield: 45.5 mg, 70%. H NMR (300 MHz, CDCl ): 6.75 and a concentrated solution of LiClO4 was added and ether
3
(
s, 2H, arom-H), 6.75 (s, 2H, arom-H), 6.07 (d, J = 12 Hz, 2H, was then diffused at −20 °C. Yield: 6.3 mg, 60%. UV-vis
−
1
−1
–
–
2
2 2
CH –), 5.89 (d, J = 12 Hz, 2H, –CH –), 4.21 (d, J = 12 Hz, 2H, (MeCN), λ, nm (ε, M cm ): 615 (560), 495 (380), 303 (4700).
Me
OMe
III
+
CH –), 4.08 (d, J = 12 Hz, 2H, –CH –), 3.77 (s, 6H, p-OCH ), ESI-MS [( N4
)Pd MeCl] in MeCN, m/z 504.0293; calcd
2
2
3
Me
OMe
Me
OMe
III
+
.31 (s, 6H, N–CH
3
), 0.41 (s, 3H, Pd–CH
3
). ESI-MS of ( N4
)
for [( N4
)Pd MeCl] , 504.0311. Anal. found: C, 40.71; H,
Pd·CH CN]: C, 40.30;
II
+
Pd MeCl in MeCN, m/z 449.1105; calculated for [M − Cl] , 4.82; N, 10.84; calcd for [C19
H
27Cl
2
N
4
O
6
3
C H N O Pd, 449.1169. Anal. found: C, 47.52; H, 5.66; N, H, 4.83; N, 11.19. μeff (CD CN) = 1.88μ .
1
9
27
4
2
3
B
Me
II
1
4
1.29; calcd for ( N4OMe)Pd MeCl (C19
7.02; H, 5.61; N, 11.54.
4 2
H27ClN O Pd): C,
X-ray crystallography
Me
OMe
III
+
+
+
[
(
N4
electrolysis of (
formed in 12 mL of 0.03 M Bu
working compartment of the electrolysis cell, while 12 mL 0.03 mounted on MiTeGen loops in random orientations.
M Bu NClO /MeCN solution was inside the auxiliary compart- Preliminary examination and data collection were performed
)Pd Cl ]ClO , [1 ]ClO . The controlled potential X-ray quality crystals of [1 ]ClO and [2 ]ClO were obtained by
2
4
4
4
4
Me OMe
II
N
4
)Pd Cl
2
(80 mg, 0.166 mmol) was per- slow ether vapor diffusion into the corresponding acetonitrile
NClO /MeCN inside the solutions. Suitable crystals of appropriate dimensions were
4
4
4
4
ment. The electrolysis potential was set as 260 mV vs. Ag/0.01 using a Bruker Kappa Apex-II Charge Coupled Device (CCD)
M AgNO based on the CV measurement in the same electroly- detector system single crystal X-Ray diffractometer equipped
3
sis cell. The color of the solution turned dark purple quickly with an Oxford Cryostream LT device. The data were collected
during the electrochemical oxidation. The electrolysis was using graphite monochromated Mo Kα radiation (λ
=
stopped after the charge corresponding to a one-electron oxi- 0.71073 Å) from a fine focus sealed tube X-ray source.
dation has passed through the cell. The remaining dark purple Preliminary unit cell constants were determined with a set of
solution was filtered and then diethyl ether was diffused at 36 narrow frame scans. Typical data sets consist of a combi-
−20 °C for a few days to afford dark blue crystals suitable for nation of ϖ and ϕ scan frames with a typical scan width of
X-ray crystallography. The crystals were filtered, washed with 0.5° and a counting time of 15–30 seconds per frame at a
ether and pentane and dried in vacuo. Yield: 40 mg, 40%. UV- crystal to detector distance of ∼4.0 cm. The collected frames
−
1
−1
vis (MeCN), λ, nm (ε, M cm ): 583 (960), 365 (2260), 325 were integrated using an orientation matrix determined from
Me
OMe
III
III
+
(
3060). ESI-MS [( N4
)Pd Cl
2
]
in MeCN, m/z 484.0851; the narrow frame scans. Apex II and SAINT software
Me
OMe
+
34
calcd for [( N4
)Pd Cl
2
] , 484.0857. Anal. found: C, 37.46; packages were used for data collection and data integration.
H, 4.43; N, 10.51; calcd for [C H Cl N O Pd·CH CN]: C, The analysis of the integrated data did not show any decay.
1
8
24
3
4
6
3
3
7.17; H, 4.21; N, 10.84. μeff (CD
3
CN) = 1.86μ
B
.
Final cell constants were determined by the global refinement
Me
III
+
[(
N4)Pd Cl
2
Me
](ClO
4
), [2 ]ClO . The controlled potential of reflections from the complete data set. The data were cor-
4
II
34
electrolysis of ( N )Pd Cl (74.1 mg, 0.166 mmol) was per- rected for systematic errors using SADABS based on the Laue
4
2
formed in 12 mL 0.03 M Bu
4 4
NClO /MeCN inside the working symmetry using equivalent reflections. Structure solutions and
compartment of the electrolysis cell, while 12 mL 0.03 M refinements were carried out using the SHELXTL-PLUS soft-
3
5
Bu NClO /MeCN solution was inside the auxiliary compart- ware package. The structures were refined with full matrix
4
4
2
2 2
ment. The electrolysis potential was set as 470 mV vs. Ag/0.01 least-squares refinement by minimizing ∑w(F
o
− F
c
) . All
M AgNO based on the CV measurement in the same electroly- non-hydrogen atoms were refined anisotropically to conver-
3
sis cell. The color of the solution turned dark purple quickly gence. Typically, H atoms are added at the calculated positions
during the electrochemical oxidation process. The electrolysis in the final refinement cycles.
was stopped when the electrolysis current decreased to <5 μA.
DFT and TD-DFT calculations
The dark purple solution in the working compartment was fil-
tered and the filtrate was set for ether diffusion at −20 °C, to The density functional theory (DFT) calculations were per-
afford dark purple needle crystals suitable for X-ray crystallo- formed with the program package Gaussian 09. Single point
graphy after a few days. The crystals were filtered, washed with and geometry optimization calculations were performed using
3
6,37
ether and pentane and dried in vacuo. Yield: 41.4 mg, 45%. the B3LYP functional
along with the Stevens CEP-31G(d)
−
1
−1
III
UV-vis (CH
3
CN), λ, nm (ε, M cm ): 578 (1420), 362 (2440). valence basis sets and effective core potentials were
Me
38,39
ESI-MS of solution of [( N4)Pd Cl ](ClO ) in MeCN, m/z employed.
The CEP-31G(d) valence basis set is valence
2
4
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Paper
Dalton Transactions
triple-ζ for palladium and double-ζ for main group elements, 15 T. W. Lyons and M. S. Sanford, Chem. Rev., 2010, 110, 1147.
with an additional d polarization function for Cl. This func- 16 P. Sehnal, R. J. K. Taylor and I. J. S. Fairlamb, Chem. Rev.,
tional/basis set combination has been shown previously to
2010, 110, 824.
4
0,41
reproduce good experimental parameters of Pd complexes.
Single point calculations were performed using the crystallo-
17 D. C. Powers and T. Ritter, Top. Organomet. Chem., 2011,
35, 129.
+
+
graphic coordinates for 1 –6 , with solvent and counteranion 18 L. M. Mirica and J. R. Khusnutdinova, Coord. Chem. Rev.,
molecules being excluded. The calculated ground state wave- 2013, 257, 299.
functions were investigated by analyzing the frontier molecular 19 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, J. Am.
orbitals and the atomic contributions to the spin density. The Chem. Soc., 2010, 132, 7303.
atomic contributions to frontier molecular orbitals were calcu- 20 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, Angew.
4
2
lated using the program Chemissian. TD-DFT calculations
were employed to obtain the predicted absorption bands and 21 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, J. Am.
their major contributions transitions. The calculated UV-vis Chem. Soc., 2012, 134, 2414.
Chem., Int. Ed., 2011, 50, 5532.
4
3
spectra were generated using the program GaussSum, with a 22 J. R. Khusnutdinova and L. M. Mirica, in C-H and C-X Bond
full width at half maximum (FWHM) value of 3750 cm−
1
.
Functionalization: Transition Metal Mediation, ed. X. Ribas,
Royal Society of Chemistry, 2013, p. 122.
2
2
2
2
2
2
2
3 J. Luo, J. R. Khusnutdinova, N. P. Rath and L. M. Mirica,
Chem. Commun., 2012, 48, 1532.
4 J. Luo, N. P. Rath and L. M. Mirica, Organometallics, 2013,
Conflicts of interest
The authors declare no competing financial interest.
31, 3343.
5 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, Inorg.
Chem., 2014, 53, 13112.
6 F. Tang, F. Qu, J. R. Khusnutdinova, N. P. Rath and
L. M. Mirica, Dalton Trans., 2012, 41, 14046.
7 F. Tang, Y. Zhang, N. P. Rath and L. M. Mirica,
Organometallics, 2012, 31, 6690.
8 A.-L. Gassner, C. l. Duhot, J.-C. G. Bünzli and
A.-S. Chauvin, Inorg. Chem., 2008, 47, 7802.
Acknowledgements
We thank the DOE Catalysis Science Program (DE-FG02-
1ER16254) for support. S. V. P. thanks the Office of
1
Undergraduate Research at Washington University for support.
9 F. Bottino, M. Di Grazia, P. Finocchiaro, F. R. Fronczek,
A. Mamo and S. Pappalardo, J. Org. Chem., 1988, 53, 3521.
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Dalton Trans.
This journal is © The Royal Society of Chemistry 2017