Dynamic Rotational Motions of Vaulted Chiral trans-Bis(salicylaldiminato)palladium(II) Complexes Bearing Rigid or Flexible Carbon Chain Linkers

Article abstract of DOI:10.1002/ejic.201800666

Planar chiral, trans-bis(salicylaldiminato)palladium(II) complexes having a macrocyclic vaulting structure consisting of either a heptamethylene-butadiynylene-heptamethylene (1) or polymethylene (2) bridge were synthesized and subsequently characterized by NMR, IR, high-resolution mass spectrometry, and single crystal XRD. The dynamic rotational behaviors of these vaulted complexes resulting from a skipping-rope-like molecular motion were examined on the basis of kinetic studies of the racemization of the optically pure (100 % ee) complexes. The chiral inversion rate was found to be significantly affected by the conformational flexibility of the complexes, which in turn could be controlled by adjusting the linker length, the linker rigidity, and the ligation properties of the solvent.

Full text of DOI:10.1002/ejic.201800666

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A Journal of
Accepted Article
Title: Dynamic Rotational Motions of Vaulted Chiral trans-
Bis(salicylaldiminato)palladium(II) Complexes Bearing Rigid or
Flexible Carbon Chain Linkers
Authors: Masahiro Ikeshita and Takeshi Naota
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800666
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Dynamic Rotational Motions of Vaulted Chiral trans-
Bis(salicylaldiminato)palladium(II) Complexes Bearing Rigid or Flexible
Carbon Chain Linkers
Masahiro Ikeshita[a] and Takeshi Naota*[a]
Abstract:
Planar
chiral,
trans-bis(salicylaldiminato)palladium(II)
The rotational behaviors of these complexes were compared based on
the results of kinetic studies of the chirality inversions of the optically
pure forms in the solution state. Such studies of the molecular mobility
based on the integration of multipoint axis rotations remains to be
ascertained, although molecular shaft rotation with single pivot point
has been extensively studied using various motifs, such as the water-
mill,[28] propeller,[29] gyroscope,[30] and double-decker structures.[31]
This paper describes the synthesis, structure and chirality inversion of
complexes 1 and 2, bearing rigid and flexible vaulting linkers,
respectively. In particular, we focus on the effects of the linker and
solvent (as internal and external factors) on the inversion rate.
complexes having a macrocyclic vaulting structure consisting of either a
heptamethylene-butadiynylene-heptamethylene (1) or polymethylene
bridge (2) were synthesized and subsequently characterized by NMR, IR,
high-resolution mass spectrometry and single crystal XRD. The dynamic
rotational behaviors of these vaulted complexes resulting from a skipping
rope-like molecular motion were examined on the basis of kinetic studies of
the racemization of the optically pure (100% ee) complexes. The chiral
inversion rate was found to be significantly affected by the conformational
flexibility of the complexes, which in turn could be controlled by adjusting
the linker length, the linker rigidity, and the ligation properties of the
solvent.
Introduction
Transition metal complexes vaulted with carbon chains have been
extensively studied as functional materials due to their specific three-
dimensional structures based on bridged ligands and coordination
planes.[1–4] A number of such vaulted complexes have been prepared by
the reaction of transition metals with tether-linked ligands,[5–27] and the
correlations between various structures and functions have been
studied. This prior research has focused on the association[8,10,12,13] and
aggregation[14,15,16,20] properties of these compounds, as well as their
[9,15,17,18,21]
optical characteristics
transformation.[5–7,25,27]
and catalytic activity during organic
As part of our program aimed at developing flexible trans-ligating
complexes with new functionalities, we have examined the molecular
mobility of vaulted trans-bis(salicylaldiminato) d8 transition metal
complexes, in which the d-π conjugation of the flexible coordination
platforms can be tuned.[12–21] Our group is also investigating the effects
of the rigidity[13,19,20] or flexibility[12,14,15,16,17,18,21] of the linker on the
overall molecular mobility and related characteristics. In the present
study, we focused on the dynamic rotational behaviors of vaulted
complexes resulting from a skipping rope-like molecular motion
(Scheme 1). For this purpose, novel mononuclear chiral trans-
bis(salicylaldiminato)palladium(II) complexes incorporating either a
rigid diyne-linked spacer (1) or flexible polymethylene spacers (2; with
linkers shorter, equal to or longer than that in 1) were synthesized.
Scheme 1. Chirality inversion of planar, chiral, vaulted complexes 1 and 2
based on a skipping rope-like rotational motion.
Results and Discussion
Synthesis and Structure of Pd Complexes with Rigid or Flexible
Carbon Chain Bridges
Complexes 1, 2c (n = 18) and 2d (n = 20) were newly synthesized
by the reaction of Pd(OAc)2 with an equimolar amount of the
corresponding diimine (7, 11c, and 11d, respectively) in the presence
of Et3N (4 equiv) in boiling toluene. The deca- and dodecamethylene-
bridged analogues 2a and 2b were prepared based on a similar
method using commercially available diamines.[32] The synthetic
routes are shown in Scheme 2. The diyne-linked diimine 7 was
obtained from the sequence in Scheme 2a, which included
isomerization of 3-nonyn-1-ol (3) with NaH,[33] a palladium-catalyzed
M. Ikeshita,a Prof. Dr. T. Naotaa
[a]
Department of Chemistry, Graduate School of Engineering Science
Osaka University
Machikaneyama, Toyonaka, Osaka 560-8531, Japan
E-mail: naota@chem.es.osaka-u.ac.jp
http://www.soc.chem.es.osaka-u.ac.jp/index.html
Supporting information and ORCID from the author for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201800666
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homocoupling reaction of the resulting terminal alkyne 4,[34] the
Mitsunobu reaction of alcohol 5 with phthalimide,[35] hydrolysis of the
diimide intermidiate with aqueous hydrazine[35] and subsequent
treatment of diamine 6 with salicylaldehyde. The polymethylene-linked
diamines 11c and 11d were prepared as depicted in Scheme 2b. These
sequences included the amidation of dicarboxylic acid 8, the reduction
of 9 with LiAlH4, and similar treatment of the resulting diamine 10
with salicyaldehyde. Complexes 1, 2c, and 2d were characterized by
1H and 13C nuclear magnetic resonance (NMR) spectroscopy, infrared
(IR) spectroscopy, high-resolution mass spectroscopy (HRMS), H-H
correlation spectroscopy (COSY) and Nuclear Overhauser effect
spectroscopy (NOESY). Optically pure (100% ee) complexes of 1, 2a,
2b, and 2c were obtained from the racemic mixtures using a
preparative high performance liquid chromatography (HPLC) system
incorporating a chiral column. Specific rotations []D in CHCl3 were
determined to be +287 ± 2 (c 0.130, 25°C) for (+)-1, +561±2 (c 0.100,
22°C) for (+)-2a, +510±1 (c 0.102, 21°C) for (+)-2b, and +463±1 (c
0.141, 18°C) for (+)-2c. It was not possible to perform optical
resolution of the long-linked complex 2d by the HPLC method because
the complex underwent rapid racemization even at room temperature.
Circular dichroism (CD) spectra of these complexes in CHCl3 are
shown in Figure 1.
(a)
(b)
1) CDI
The trans-coordination and vaulted three-dimensional overall
structure of the vailted complexes were unequivocally established by
single-crystal X-ray diffraction (XRD) analysis of the racemic 1, 2c
and optically pure (+)-1 crystals obtained by recrystallization from
ethyl acetate and CHCl3/hexane (1:1), respectively. The
crystallographic data for (±)-1, (+)-1, and (±)-2c are summarized in
Table 1. The molecular structure of (+)-1 indicates that the absolute
configuration of the (+)-form can be assigned as R. ORTEP drawings
of vaulted complexes (±)-1, (+)-1, and (±)-2c are presented in Figure 2.
Here, C(5)–Pd(1)–C(11) angles and C(13)–N(1)–Pd(1)–O(1)/C(14)–
CH3CN, r.t.
HO2C(CH2)n-2CO2H
H2NOC(CH2)n-2CONH2
2) NH3 aq., 0 °C
8
9
CHO
OH
LiAlH4
H2N(CH2)nNH2
10
THF, reflux
EtOH, reflux
Pd(OAc)2
2
N(2)–Pd(1)–O(2) dihedral angles are provided to express
a
Et3N
N (CH )
2 n N
c: n = 18
toluene, reflux
OH
HO
n = 20
d:
macroscopic view of the coordination planarity of each trans-
bis(salicylaldiminato)palladium moieties. In the case of diyne-linked
complex 1, the planarity of the coordination blade is much lower than
that in the octadecamethylene-linked complexes 2c. The C(5)–Pd(1)–
C(11) angles in (±)-1 (139°) and (+)-1 (162°) are significantly smaller
than that in (±)-2c (unit 1: 175°; unit 2: 178°), and the average C(13)–
N(1)–Pt(1)–O(1)/C(14)–N(2)–Pt(1)–O(2) dihedral angles in (±)-1 (25°)
and (+)-1 (7.9°) are much larger than those in (±)-2c (unit 1: 4.1°; unit
11
Scheme 2. The synthetic routes to the [Pd(L)] complexes(a) 1 and (b) 2.
(a)
(b)
(c)
(d)
Figure 1. CD spectra of 2.00×10–4 M solutions of optically pure (a) 1, (b) 2a and (c) 2b, and (d) 2c in CHCl3.
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(a)
Table 1. Crystal Data and Structural Refinement Details for Compounds (±)-1, (+)-1 and
(±)-2c.
(±)-1
C32H38N2O2Pd
589.06
(+)-1
C32H38N2O2Pd
589.06
(±)-2c
C32H46N2O2Pd
597.13
Formula
MF
T [K]
113
113
113
Crystal color, habit
Crystal size [mm]
Crystal system
Space group
a [Å]
yellow, chip
0.20×0.15×0.10
orthorhombic
Pbcn (#60)
15.881(2)
10.4357(12)
34.010(4)
90
yellow, platelet
0.30×0.20×0.10
monoclinic
P21 (#4)
10.02240
16.39600
11.02040
90
yellow, chip
0.09×0.07×0.05
monoclinic
P21/c (#14)
10.59850
22.91880
25.40500
90
(b)
b [Å]
c [Å]
α [°]
β [°]
90
112.35000
90
100.54200
90
γ [°]
90
V [Å3]
5636.4(17)
8
1674.91027
2
6064.98607
8
(c)
Z
Dcalcd [gcm-3]
μ(MoKα) [cm-1]
F(000)
1.388
1.168
1.308
6.897
5.802
6.416
2488.00
55.1
612.00
2512.00
55.2
2θmax [°]
55.1
No. of reflections measured
No. observed reflections
No. variables
R1 (I > 2σ(I))[a]
22898
16015
115119
6162
7023
13990
667
334
334
Figure 2. ORTEP representations of (a,b) (R)-1 and (c) (R)-2c in the
form of racemic (a,c) and optically pure (b, (+)-form) crystals. Side
(left) and overhead (right) views are provided. Thermal ellipsoids are
shown at the 50% probability level. The C(5)–Pd(1)–C(11) angles and
C(13)–N(1)–Pd(1)–O(1)/C(14)–N(2)–Pd(1)–O(2) dihedral angles (in
parentheses) are included under each structure.
0.0799
0.0729
0.0496
0.0945
1.081
wR2 (all reflections)[b]
Goodness of fit
0.1170
1.005
0.1903
1.036
0.05(6)
2
Flack parameter
[a] R1 = (|Fo|–|Fc|)/ (|Fo|). [b] wR2 = [[w(Fo –Fc2)2]/ w(Fo )2]1/2
.
2
was found to undergo a slow chiral inversion at 351 K in toluene,
with the enantiomer excess of 2c decreasing to 98.7, 98.2 and
97.9%ee, after stirring for 10, 20, and 60 min, respectively
(Supporting Information). This time-dependent change in enantiomer
excess can be described using an equation for a reversible first-order
relationship [ln(1/ee)=2kt][36] (Figure 3a), and the reversible rate
constant k is estimated to be 1.68(6)×10–6 s–1. From this k value, the
half-life (t1/2) and Gibbs free energy (G‡351K) were estimated to be
57.2 h and 124.9±0.4 kJ·mol–1 based on the equations t1/2 = ln2/2k
and G‡ = RTln(kbT/kh), respectively. In contrast, diyne-linked
(+)-1 (Figure 3a) and short polymethylene-linked (+)-2a (n = 10) and
(+)-2b (n = 12) did not undergo racemization in toluene under the
same reaction conditions. Almost same values of kinetic parameters;
2: 4.3°). The diyne moieties of (±)-1 and (+)-1 are located just above
the palladium atoms, thus maintaining a highly symmetric linker
conformation. These observations indicate that the linearity of the
four sp-hybridized carbon atoms in the diyne linker of 1 decreases the
planarity of the coordination blade and increase the regularity of the
linker conformation in the crystalline state. This, in turn, strongly
suggests differences in the molecular mobility of 1 and 2c in the
solution state, as described further on.
Kinetic Studies of the Chirality Inversion of Optically Active
Complexes Based on a Skipping Rope-Like Movement
k = 1.82(1)×10–6 s–1, t1/2 = h, and G‡
= 124.7±0.2 kJ·mol–1
351K
The chirality inversion of these chiral vaulted complexes in the
solution state was examined by means of HPLC analysis using an
optically active column. Optically pure (100% ee) (+)-2c (n = 18)
were obtained in the rotation of (+)-2c in 1,2-dichloroethane at 351 K
(Figure 3b). Also, complexes (+)-1 (Figure 3b), (+)-2a, and (+)-2b
did not rotate in 1,2-dichloroethane under the same measurement
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