A ruthenium(II) complex having a ligand undergoing partial C = N cleavage and unusual double-bond shift and nonchirality

Article abstract of DOI:10.1016/j.inoche.2013.11.046

A novel six-coordinate ruthenium(II) complex 1, formulated as [Ru(bpy) ₂L_b]PF₆× C₂H₅OH (bpy = 2,2′-bipyridine), has been obtained and characterized. Ligand L_b in 1 comes from a chiral bis-Schiff-base ligand L_a which is condensed between sodium(I) salt of D- or L-phenylanine and terephthaldicarboxaldehyde. It is noted that ligand L_a undergoes partial C = N bond cleavage of Schiff-base unit forming a free aldehyde group at one end of phenyl ring, and an unusual proton migration, a C = N double-bond shift and subsequent nonchirality at the other end simultaneously in the presence of a ruthenium(II) ion.

Full text of DOI:10.1016/j.inoche.2013.11.046

Inorganic Chemistry Communications 40 (2014) 112–115
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Inorganic Chemistry Communications
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A ruthenium(II) complex having a ligand undergoing partial C = N
cleavage and unusual double-bond shift and nonchirality
Jiao Geng a, Kun Zhang a, Yu-Xin Peng a, Li Wang a,b, Wei Huang a,
⁎
a
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel six-coordinate ruthenium(II) complex 1, formulated as [Ru(bpy)2Lb]PF6⋅C2H5OH (bpy = 2,2′-bipyridine),
has been obtained and characterized. Ligand Lb in 1 comes from a chiral bis-Schiff-base ligand La which is
condensed between sodium(I) salt of D- or L-phenylanine and terephthaldicarboxaldehyde. It is noted that
ligand La undergoes partial C = N bond cleavage of Schiff-base unit forming a free aldehyde group at one end
of phenyl ring, and an unusual proton migration, a C = N double-bond shift and subsequent nonchirality at
the other end simultaneously in the presence of a ruthenium(II) ion.
Received 7 November 2013
Accepted 29 November 2013
Available online 7 December 2013
Keywords:
Ruthenium(II) complex
Schiff-base ligand
α-Amino acid
© 2013 Elsevier B.V. All rights reserved.
C = N bond cleavage
Structural elucidation
Investigations on the α-amino acids and their derivatives have at-
tracted much attention because syntheses, structures, reactions, and ap-
plications of these compounds are important in many research fields
such as biochemistry, organic and organometallic chemistry [1–4].
Schiff bases having chiral α-amino acid units and their metal complexes
and metalloproteins can afford useful candidates on the therapeutic
pharmaceuticals of malignant tumor [5,6].
However, the Schiff-base unit is not chemically stable because of the
reversible formation and cleavage of C = N double bond. We have pre-
viously reported a series of enantiomeric and racemic Schiff-base mac-
rocyclic ligands with different cadmium(II), zinc(II), manganese(II),
nickel(II) and copper(II) salts undergoing the cleavage of Schiff-base
imine double bonds and subsequent ring contraction of the macrocyclic
ligands due to the size effects and the spatial restrictions of coordination
geometry of the central metals, the steric hindrance of ligands and the
counterions used [7–11].
In our previous study, an enantiomeric pair of sodium(I) directed
organic/inorganic frameworks formulated [Na4(H2O)12]n⋅[2La]n has
been described, where ligand La is condensed between sodium(I) salt
of D- or L-phenylanine and terephthaldicarboxaldehyde [12]. To further
explore their coordination chemistry, a Ru(II) ion is used to react
with this enantiomeric pair of chiral ligands and herein we report
an unexpected example of novel ruthenium(II) complex [Ru(bpy)2Lb]
PF6⋅C2H5OH (1) where ligand Lb derivates from partial C = N cleavage
of Schiff-base unit and proton migration, double-bond shift and subse-
quent nonchirality at both ends of ligand La at the same time. Actually,
the cleavage of Schiff-base unit in La is easy to be understood as we
mentioned above. Nevertheless, the study is unusual in literature on
the proton migration and subsequent C = N double-bond shift in the
presence of a metal ion, which will make the resultant ligand nonchiral.
Complex 1 can been obtained by refluxing equal molar ratio of Na2La
and [RuCl2(bpy)2] · H2O in methanol and adding an excess saturated
aqueous solution of NH4PF6 [13]. Here a new ligand Lb has been yielded
in Ru(II) complex 1 where partial C = N cleavage of chiral bis-Schiff-
base ligands and subsequent double-bond shift and racemization take
place (Scheme 1). In our experiments, each of sodium(I) salt of
bis-Schiff-base ligand, condensed between D- or L-phenylanine and
terephthaldicarboxaldehyde, produces the same achiral ligand Lb.
Electrospray ionization mass spectrum (ESI-MS) of complex 1 in the
positive ion mode gives a most intense peak centered at m/z = 694.17
(calculated: 694.14), as can be seen in Fig. 1, which is assigned to the
monovalent cationic peak of ([Ru(bpy)2Lb]+) in complex 1. Further-
more, the isotopic distributions of [Ru(bpy)2Lb]+ cation at 100% abun-
dance are calculated by using the numbers and natural isotopic
abundance of atoms and the results are in good agreement with the
experimental data (the Inset of Fig. 1), indicative of the existence of
expected [Ru(bpy)2Lb]+ species.
In addition to the strong C = N Schiff-base absorption at 1638 cm−1
,
Fourier transform infrared (FT-IR) spectrum of complex 1 reveals
the presence of two new peaks at 1686 and 841 cm−1, corresponding
to the characteristic absorptions of free aromatic aldehyde group of
Lb after partial C = N cleavage of bis-Schiff-base ligands and
hexafluorophosphorate anion. Compared with the electronic spectrum
of compound Na2L1 [12], the strong absorption peak at 293 nm in
complex 1, which corresponds to the π–π* transition of the azomethine
chromophore, exhibits a bathochromic shift of 12 nm after the degra-
dation and proton migration of ligand La. Furthermore, a new broad
⁎
Corresponding author. Tel.: +86 25 83686526; fax: +86 25 83314502.
E-mail address: whuang@nju.edu.cn (W. Huang).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.11.046

J. Geng et al. / Inorganic Chemistry Communications 40 (2014) 112–115
113
ONa
R(S)
+
H
O
*
N
N
N
N
Ru
[Ru(bpy)2]Cl
2 /NH PF
6 / reflux
4
O
N
C=N cleavage of the Schiff-base unit
and proton migation and nonchirality
O
O
N
E
N
H
O
H
*
R(S)
ONa
Scheme 1. Schematic illustration of the preparation of Ru(II) complex 1 where the chiral carbon atoms are marked with asterisks.
absorption band centered at 481 nm in complex 1 is found, which can
be assigned to the typical metal-to-ligand charge transfer after the
Ru(II) ion complexation. Moreover, the single-crystal structure of com-
plex 1 has been successfully obtained by using the X-ray diffraction
method, which will be discussed below.
X-ray single-crystal structural analyses of complex 1 [14] reveal that
the coordination geometry for the six-coordinate Ru(II) center is a
distorted octahedron with the Ru–O distance of 2.083(5) Å and Ru–N
distances ranging from 2.024(6) to 2.049(6) Å, as illustrated in Fig. 2. Li-
gand Lb exhibits E configuration in complex 1 and its carboxylic group
adopts a monodentate coordination fashion with the central Ru(II)
ion. Compared with the bis-Schiff-base ligand La, partial cleavage of
C = N double bond occurs and an aldehyde group is uncoordinated
with the O3–C37 double bond length of 1.251(13) Å. The dihedral
angle between the two bpy planes in complex 1 is 87.1(2)°, while that
between the two phenyl rings in ligand Lb is 60.0(2)°. In addition, the
dihedral angle between one of the bpy ligands and the benzaldehyde
plane in ligand Lb is 9.4(2)°, where intramolecular π–π stacking in-
teractions are found between one pyridine ring of this bpy ligand and
the phenyl ring with the centroid-to-centroid separation of 3.650 Å.
Fig. 1. Positive ESI-MS spectrum of Ru(II) complex 1 obtained in the positive mode, together with the calculated IDPs (Inset) corresponding to the peak at 100% abundance for comparison.

114
J. Geng et al. / Inorganic Chemistry Communications 40 (2014) 112–115
bond lengths in complex 1, and subsequent loss of the chirality of Lb.
Compared with the N–C and C = N bond lengths in the starting mate-
rial Na2La, where 1.456(6)–1.467(6) Å for the single bond (phenylanine
moiety) and 1.253(6)–1.269(6) Å for the double bond (Schiff-base
moiety) are observed, the N5–C30 and N5–C22 bond lengths in complex
1 are 1.433(9) and 1.293(10) Å, respectively, indicative of typical single-
bond and double-bond character.
So a unique rearrangement of a Schiff-base ligand in a Ru(II) coordi-
nation sphere takes place involving the proton migration to cause the
shift of a C = N double bond in ligand Lb, which is accompanied by
the disappearance of a chiral center (C22) in this case. Moreover, the
crystallization of complex 1 in a centrosymmetric triclinic space group
P 1 can also be regarded as an evidence for the disappearance of chi-
rality for ligand Lb. In contrast, the two C–O and C = O bond lengths
of carboxylic unit in complex 1 are 1.250(9) Å for the coordinated
one and 1.253(9) Å for the free one, which lie within the range of
1.228(7)–1.264(6) Å in the starting material of Na2La.
Thus far, investigations on the hydrogen migration and subsequent
double-bond shift and racemization of a chiral ligand are very rare in
literature. Nevertheless, there has been considerable interest in the
oxidative dehydrogenation of coordinated amines to the corresponding
imines or nitrile in the presence of certain Ru(II) complexes [15–20]. It is
already known that imines of α-amino ester exist in solution in the
tautomeric equilibrium with corresponding azomethine ylides [21],
which would make possible the proton migration in this case. Further-
more, the electron transfer from the coordination nitrogen atom of
Schiff-base unit to the Ru(II) center after metal–ligand coordination
interactions is suggested to play important roles in the cleavage of
Schiff-base C = N double at one end as well as the proton migration
at the other end of ligand.
In summary, we report herein a novel six-coordinate ruthenium(II)
complex 1, formulated as [Ru(bpy)2Lb]PF6 · C2H5OH, where a unique
nonchiral ligand Lb is formed from a chiral bis-Schiff-base ligand with
two stereogenic centers derived from D- or L-phenylalanine. In the
process of Ru(II) ion complexation, the enantiomeric ligands undergo
half C = N cleavage of Schiff-base unit forming a free aldehyde group
at one end, and at the same time, an unusual proton migration and a
following C = N double-bond shift take place at the other end making
the Lb ligand nonchiral. Moreover, typical intramolecular π–π stacking
interactions are found between adjacent bpy and phenyl rings in com-
plex 1 with the centroid-to-centroid separation of 3.650 Å, and a di-
meric packing structure is formed between the aromatic rings of
neighboring molecules with the centroid-to-centroid separation of
3.897 Å.
Fig. 2. ORTEP diagram (30% thermal probability level ellipsoids) of the cationic structure of
Ru(II) complex 1 with the atom-numbering scheme. Selected bond lengths and bond an-
gles (Å and °): Ru1–O1, 2.083(5), Ru1–N1, 2.046(6), Ru1–N2, 2.034(6), Ru1–N3, 2.026(6),
Ru1–N4, 2.049(6), Ru1–N5, 2.024(6), N5–C30, 1.433(9), N5–C22, 1.293(10), C21–C22,
1.526(11), C22–C23, 1.496(10), O1–C21, 1.250(9), O2–C21, 1.253(9), O3–C37, 1.251(13)
Å; O1–Ru1–N1, 94.7(2), O1–Ru1–N2, 173.4(2), O1–Ru1–N3, 94.0(2), O1,–Ru1–N4,
88.0(2), O1–Ru1–N5, 78.9(2), N1–Ru1–N2, 79.2(2), N1–Ru1–N3, 97.5(2), N1–Ru1–N4,
175.4(2), N1–Ru1–N5, 89.2(2), N2–Ru1–N3, 89.3(2), N2–Ru1–N4, 98.3(2), N2–Ru1–N5,
98.3(2), N3–Ru1–N4, 78.5(2), N3–Ru1–N5, 170.7(2), N4–Ru1–N5, 95.1(2)°.
Furthermore, typical intermolecular π–π stacking interactions are found
between adjacent two molecules forming a dimeric packing structure in
complex 1 with the centroid-to-centroid separation of 3.897 Å (Fig. 3).
It is very interesting to mention that an unusual proton migration is
found to occur from the stereogenic carbon atom of the phenylalanine
component to the carbon atom of the imine moiety (Scheme 1). The
proton migration in the course of the Ru(II) complexation is suggested
to cause the shift of a C = N double bond, as indicated by the related
Fig. 3. Perspective view of the dimeric packing structure in Ru(II) complex 1. Hydrogen atoms, anions and solvent ethanol molecules are omitted for clarity.

J. Geng et al. / Inorganic Chemistry Communications 40 (2014) 112–115
115
obtained by slow evaporation of the filtrate for six days at room temperature in
air with a yield of ~23%. Main FT-IR (KBr pellets, cm−1): 3415 (s), 1686 (m), 1638
(s), 1495 (m), 1463 (m), 1446 (m), 1421 (m), 841 (vs), 765 (m) and 558 (m).
Elemental Analysis: calculated for RuC39H36N5O4PF6 (884.77): C, 52.94; H, 4.10; N,
7.92. Found: C, 53.08; H, 4.31; N, 7.84. Positive ESI-MS: m/z = 694.17 (calculated:
694.14) [Ru(bpy)2Lb]+ (100%). UV–Vis (λmax in methanol): 481 and 293 nm
[14] Single-crystal sample of complex 1 was glue-covered and mounted on the top of a
glass fiber and used for data collection on a Bruker SMART 1 K CCD diffractometer
at 291(2) K using graphite mono-chromated MoKα radiation (λ = 0.71073 Å).
The collected diffraction data were reduced by using the SAINT program and empir-
ical absorption corrections were done by using the SADABS program. Both of the
structures were solved by direct method and refined by least-squares method on
F2obs by using the SHELXTL-PC software package. All non-H atoms were anisotropi-
cally refined and all hydrogen atoms were inserted in the calculated positions
assigned fixed isotropic thermal parameters and allowed to ride on their respective
parent atoms. Crystallographic data for RuC39H36N5O4PF6: M = 884.77 g/mol, crys-
tal size, 0.10 × 0.10 × 0.10 mm, crystal system, triclinic, space group P 1 (No. 2),
a = 9.7749(16), b = 13.770(2), c = 14.138(2) Å, α = 96.192(3), β = 95.220(2),
Acknowledgments
This work was financially supported by the Major State Basic Research
Development Programs (Nos. 2013CB922101 and 2011CB933300), the
National Natural Science Foundation of China (No. 21171088), and the
Natural Science Foundation of Jiangsu Province (Grant BK20130054).
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