Enantiomeric Separations of Ruthenium (II) Polypyridyl Complexes Using HPLC With Cyclofructan Chiral Stationary Phases

Article abstract of DOI:10.1002/chir.22389

The enantiomeric separation of 21 ruthenium (II) polypyridyl complexes was achieved with a novel class of cyclofructan-based chiral stationary phases (CSPs) in the polar organic mode. Aromatic derivatives on the chiral selectors proved to be essential for enantioselectivity. The R-napthylethyl carbamate functionalized cyclofructan 6 (LARIHC CF6-RN) column proved to be the most effective overall, while the dimethylphenyl carbamate cyclofructan 7 (LARIHC CF7-DMP) showed complementary selectivity. A combination of acid and base additives was necessary for optimal separations. The retention factor vs. acetonitrile/methanol ratio plot showed a U-shaped retention curve, indicating that different interactions take place at different polar organic solvent compositions. The separation results indicated that π-π interactions, steric effects, and hydrogen bonding contribute to the enantiomeric separation of ruthenium (II) polypyridyl complexes with cyclofructan chiral stationary phases in the polar organic mode.

Full text of DOI:10.1002/chir.22389

CHIRALITY 27:64–70 (2015)
Enantiomeric Separations of Ruthenium (II) Polypyridyl Complexes
Using HPLC With Cyclofructan Chiral Stationary Phases
YANG SHU,1,2 ZACHARY S. BREITBACH,2 MILAN K. DISSANAYAKE,2 SIRANTHA PERERA,2 JOSEPH M. ASLAN,2 NAGHAM ALATRASH,2
FREDERICK M. MACDONNELL,2 AND DANIEL W. ARMSTRONG2,3
*
1College of Life and Health Sciences, Northeastern University, Shenyang, China
2Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX, USA
3AZYP LLC, Arlington, TX, USA
ABSTRACT
The enantiomeric separation of 21 ruthenium (II) polypyridyl complexes was
achieved with a novel class of cyclofructan-based chiral stationary phases (CSPs) in the polar
organic mode. Aromatic derivatives on the chiral selectors proved to be essential for
enantioselectivity. The R-napthylethyl carbamate functionalized cyclofructan 6 (LARIHC CF6-RN)
column proved to be the most effective overall, while the dimethylphenyl carbamate cyclofructan
7 (LARIHC CF7-DMP) showed complementary selectivity. A combination of acid and base addi-
tives was necessary for optimal separations. The retention factor vs. acetonitrile/methanol ratio plot
showed a U-shaped retention curve, indicating that different interactions take place at different
polar organic solvent compositions. The separation results indicated that π–π interactions, steric
effects, and hydrogen bonding contribute to the enantiomeric separation of ruthenium (II)
polypyridyl complexes with cyclofructan chiral stationary phases in the polar organic mode.
Chirality 27:64–70, 2015. © 2014 Wiley Periodicals, Inc.
KEY WORDS: cyclofructan; ruthenium (II) polypyridyl complexes; enantiomeric separation;
chiral stationary phase; LARIHC
INTRODUCTION
Cyclofructans (CFs) are structural isomers of CDs. They
are naturally occurring chiral crown ethers which consist of
β-(2-1) linked D-fructofuranose units. Recently, isopropyl
carbamate CF6 (LARIHC CF6-P), R-naphthylethyl-carbamate
CF6 (LARIHC CF6-RN), and dimethylphenyl carbamate CF7
(LARIHC CF7-DMP) have been developed as bonded chiral
stationary phases for HPLC. These CF CSPs provide excel-
lent selectivities for many racemic compounds (spiroindoline
phytoalexins, binaphthyl catalysts, tetrahydrobenzimidazoles,
chiral acids, amines, amino compounds, metal complexes,
neutral compounds, etc).23–29 The LARIHC CF6-P shows
unique selectivity and broad applicability for amine-containing
racemates. LARIHC CF6-RN, and LARIHC CF7-DMP exhibit
complementary selectivities. Previous studies showed that
CFs behave very differently compared to CDs when used
as chiral selectors. Sulfated cyclofructan 6 was superior to
sulfated cyclodextrins in the enantiomeric separation of
four basic pharmaceuticals by capillary electrophoresis.30
The R-naphthylethyl functionalized CF6 CSP proved more
suitable for the enantiomeric separation of binaphthyl cata-
lysts when compared to the R-naphthylethyl functionalized
CD CSP.25 It is necessary to develop new CSPs and apply
them comprehensively to strive for better or complemen-
tary enantiomeric separations.
A plethora of ruthenium(II) polypyridyl complexes have
been synthesized due to their robust nature and distinctive
electrochemical and photophysical characteristics. Recently,
these complexes have been widely used for cellular imaging
and therapeutics, as they are known to have specific interac-
tions with DNA.1–4 In addition, ruthenium(II) complexes are
effective catalysts for organic synthesis and dye sensitizers
for solar cells.5–8 They are also used to construct various su-
pramolecular assemblies.9–11 Ruthenium(II) polypyridyl com-
plexes exhibit axial chirality and the right- and left-handed
configurations of the octahedral complexes are referred to as
Δ- and Λ-enantiomers. Enantiomers of ruthenium(II) complexes
exhibit very different biological activities when used as DNA
intercalating agents, stabilizers of G-Quadruplex DNA, and
inhibitors of enzyme activity.12–14 As a catalyst, enantiomers of
ruthenium(II) complexes dramatically influence the stereochem-
istry of chiral products.15 Therefore, there is a great need for
analytical methods by which ruthenium(II) polypyridyl complex
enantiomers can be separated and evaluated.
The separation of geometric isomers, diastereomers, and
enantiomers of ruthenium complexes have been achieved
by chromatographic methods and capillary electrophoresis
using chiral selectors.16–22 Capillary electrophoresis is not
suitable for preparative-scale separations and, as such, high-
performance liquid chromatography (HPLC) with chiral
stationary phases (CSPs) has proven to be the best way to
separate enantiomers of organometallic compounds due to
the technique’s broad selectivity, high efficiency, and ability
to transition to preparative-scales. For example, the enantio-
meric separation of ruthenium polypyridyl complexes has
been obtained with macrocyclic glycopeptide CSPs and cyclo-
dextrin (CD) CSPs.20–22
*Correspondence to: Daniel W. Armstrong, Department of Chemistry and Bio-
chemistry, University of Texas at Arlington, 700 Planetarium Place, Arlington,
TX 76019. E-mail: sec4dwa@uta.edu
Received for publication 30 April 2014; Accepted 13 August 2014
DOI: 10.1002/chir.22389
Published online 7 October 2014 in Wiley Online Library
(wileyonlinelibrary.com).
© 2014 Wiley Periodicals, Inc.

ENANTIOMERIC SEPARATIONS OF RUTHENIUM (II) POLYPYRIDYL COMPLEXES
65
unless stated otherwise. For all HPLC experiments, the injection volume
was 5 μL and the flow rate was 1.0 mL/min in isocratic mode. The UV
wavelength of 254 nm was employed for detection. The chloride (Cl-) salts
and hexafluorophosphate (PF-6) salts of ruthenium(II) polypyridyl com-
plexes were dissolved in methanol and acetonitrile, respectively. Samples
were dissolved at 1.0 mg/mL concentration and subsequently diluted
with methanol or acetonitrile for LC injection. Each sample was analyzed
in duplicate. The “dead time” t0 was determined by the peak of the refrac-
tive index change due to the unretained sample solvent.
In the present work, LARIHC CF6-P, LARIHC CF6-RN, and
LARIHC CF7-DMP columns were utilized for the enantiomeric
separation of 21 ruthenium(II) polypyridyl complexes, 19 of
which have not been separated previously by any means. The
dependence of separation performance on the chiral selector
structure, mobile phase composition, type, and concentration
of additives and the structure of analytes were investigated to
explore the chiral separation mechanism.
EXPERIMENTAL
Materials and Chemicals
RESULTS AND DISCUSSION
Enantiomeric Separations of Ruthenium(II) Polypyridyl
Complexes With LARIHC CSPs
Triethylamine (TEA), trimethylamine, ethanolamine, butylamine,
acetic acid (AA), trifluoroacetic acid, formic acid, ammonium nitrate,
tetramethylammonium nitrate, ammonium trifluroacetate, and
tetramethylammonium acetate were purchased from Aldrich Chemical
(St. Louis, MO) and used without further purification. Water was ob-
tained from Millipore (Billerica, MA). Acetonitrile (ACN) and methanol
(MeOH) of HPLC grade were purchased from VWR (Boston, MA).
The 21 racemic ruthenium (II) polypyridyl complexes used in this
study were produced according to the literature.31–37 The structures of
the ligands and ruthenium(II) polypyridyl complexes are depicted in
Figure 1A,B, respectively. Figure 1C gives the structures and the names
of each individual ruthenium(II) polypyridyl complex.
The enantiomeric separation of 21 ruthenium(II) polypyridyl
complexes was evaluated in the polar organic mode using
LARIHC CF6-P, LARIHC CF6-RN, and LARIHC CF7-DMP
CSPs. The results showed that the racemates of ruthenium
(II) polypyridyl complexes are only separated by aromatic
derivatized cyclofructans (i.e., LARIHC CF6-RN and LARIHC
CF7-DMP) but not by nonaromatic derivatized cyclofrutans
(i.e., LARIHC CF6-P). This indicates that π–π interaction be-
tween the aromatic groups of the CSPs and polypyridyl groups
of the analytes is one major factor for chiral recognition. This is
consistent with previous studies on the enantiomeric separa-
tion of ruthenium(II) polypyridyl complexes with three native
and six derivatized β-cyclodextrin CSPs, where only the three
aromatic derivatized CSPs showed enantiomeric selectivity.22
In the case of the R- and S-naphthylethyl derivatized CD CSPs,
it has been shown that the stereogenic configuration of the de-
rivative group on the oligosaccharide is more important than
the cyclodextrin molecule, even though the attached chiral
Isopropyl carbamate CF6 (LARIHC CF6-P), R-naphthylethyl-carbamate
CF6 (LARIHC CF6-RN), and dimethylphenyl carbamate CF7 (LARIHC
CF7-DMP) columns, 25 × 0.46 cm (i.d.), were obtained from AZYP
(Arlington, TX).
Chromatographic Conditions
An Agilent 1200 HPLC (Agilent Technologies, Palo Alto, CA) was used
in this study. It consisted of a 1200 diode array detector, autosampler, and
quaternary pump. All separations were carried out at room temperature
Fig. 1. Structures of (A) stereochemistry of [Ru(phen)3]2+, (B) the polypyridyl ligands, and (C) the cation of ruthenium (II) polypyridyl complexes.
Chirality DOI 10.1002/chir

66
SHU ET AL.
Fig. 1. (Continued)
Chirality DOI 10.1002/chir

ENANTIOMERIC SEPARATIONS OF RUTHENIUM (II) POLYPYRIDYL COMPLEXES
67
moiety and cyclodextrin molecule contribute to the chiral rec-
ognition in a synergistic or antagonistic fashion.38 Similarly,
the need for aromatic derivatized cyclofructan (CF) chiral
selectors indicated that the external interaction (i.e., outer
portion of the macrocycle) between the analytes and derivative
group play a significant role in chiral recognition as well.
Table 1 shows the optimized enantiomeric separation con-
ditions for the 21 ruthenium(II) polypyridyl complexes on
the LARIHC CF6-RN and CF7-DMP columns. The aromatic
derivatized columns provided excellent enantiomeric separa-
tions. Both LARIHC columns yielded a 100% success rate in
separating these analytes, meaning every compound was at
least partially separated on each column. Further, RS values
as high as 6.2 (LARIHC CF6-RN, compound 9) were
obtained. All but one of these racemates (compound 17)
was greater than baseline separated on one or both CSPs.
The LARIHC CF6-RN generally gave higher selectivity
values (1.08–2.28) than the CF7-DMP CSP (1.04–1.26). How-
ever, enantiomers of compound 2 [Ru(bpy)2(phendione)]
(Cl2) were baseline separated on the CF7-DMP CSP with
good selectivity (α = 1.30), while only partially separated by
the CF6-RN CSP (α = 1.08). This indicates that LARIHC CF6-
RN and CF7-DMP show complementary enantioselectivities
in the chiral separation of ruthenium(II) polypyridyl com-
plexes. This complementary nature has been noted before,
as the CF6-RN CSP provided a higher enantioselectivity for
chiral amines and the CF7-DMP CSP had greater success
separating chiral acids.24 The complementary characteristic
between the CF6-RN and CF7-DMP columns facilitates sepa-
rating a large number of different compounds.
Fig. 2. The separation of [Ru(bpy)2(o-OCH3-dppz](PF6)2 enantiomers on
LARIHC CF6-RN at various AA/TEA (v/v) ratio. Molar excess of acid or base
listed in parenthesis. Flow rate: 1 mL/min, UV detection: 254 nm.
solubility of the analytes in the mobile phase. The mobile
phase for the polar organic mode usually is composed of ace-
tonitrile, methanol, and small amounts of triethylamine (TEA)
and acetic acid (AA). In order to evaluate the effects of addi-
tives, the separation of compound 14 [Ru(bpy)2(o-OCH3-
dppz](PF6)2 was investigated using both the LARIHC
CF6-RN and LARIHC CF7-DMP columns and a mobile phase
of ACN/MeOH = 30/70, in which baseline separation can be
achieved. Figure 2 shows the chromatograms of compound
14 on the LARIHC CF6-RN column with different amounts
of TEA and AA in the mobile phase. When no additive was
used (Fig. 2G), the analyte did not elute within 1 h. The same
was true when just an acid or a base was used (Fig. 2E,F).
Therefore, it was found to be necessary to use a combination
of acid (AA) and base (TEA) in the mobile phase. As shown in
Figure 2A–C, having a molar excess of base (Fig. 2A) or acid
(Fig. 2B,C) did not eliminate the peak tailing and selectivity
did not change. However, when equimolar amounts of acid
and base were used (Fig. 2D), efficiency increased greatly,
resulting in the great resolution. Next, keeping the acid/base
Effect of Additives on the Enantiomeric Separation
Enantiomeric separations of ruthenium(II) polypyridyl
complexes were observed mainly in polar organic mode and
were not successful in the normal phase due to the lack of
TABLE 1. Summary of the optimized enantiomeric separations of 21 ruthenium (II) polypyridyl complexes on LARIHC CF6-RN and
LARIHC CF7-DMP
LARIHC CF6-RN
LARIHC CF7-DMP
Code
Name
Mobile phase
k1
α
RS
k1
α
RS
1
2
3
4
5
6
7
8
[Ru(phen)2(phendione)](Cl2)
[Ru(bpy)2(phendione)](Cl2)
[Ru(di-phenylphen)2(phendione)](Cl2)
[Ru(phen)2(pbtp β)](PF6)2
[Ru(phen)2(pbtp α)](PF6)2
[Ru(phen)2(dppz)](PF6)2
[Ru(phen)2(p-CN-dppz)](PF6)2
[Ru(phen)2(o-CN-dppz)](PF6)2
[Ru(phen)2(p-Br-dppz)](PF6)2
[Ru(phen)2(o-Br-dppz)](PF6)2
[Ru(phen)2(o-F-dppz)](PF6)2
[Ru(phen)2(o-Cl-dppz)](PF6)2
[Ru(Phen)2(o-OCH3-dppz)](PF6)2
[Ru(bpy)2(o-OCH3-dppz)](PF6)2
[Ru(bpy)2(o-F-dppz)](PF6)2
[Ru(bpy)2(o-Cl-dppz)](PF6)2
[Ru(bpy)3](PF6)2
100MeOH/1.6AA/2.4TEA
100MeOH/1.6AA/2.4TEA
100MeOH/1.6AA/2.4TEA
2.50
1.31
1.00
1.27
1.31
4.52
4.28
5.38
5.17
5.55
4.24
4.38
4.59
4.58
3.97
4.10
6.79
7.86
8.44
0.86
1.24
1.30
1.08
1.21
1.54
1.50
1.53
1.87
1.44
2.00
1.59
1.53
1.54
1.51
1.23
1.23
1.24
1.06
1.12
1.22
1.56
2.28
2.3
0.6
2.5
2.2
3.0
3.7
5.5
3.1
6.2
4.0
3.3
3.9
3.7
1.8
1.5
1.7
0.6
1.0
1.9
1.9
4.3
1.44
0.82
0.51
3.35
3.10
2.37
3.38
3.08
4.23
3.60
2.42
3.43
2.30
1.59
2.11
3.72
1.44
2.52
2.88
1.57
2.27
1.14
1.30
1.24
1.25
1.26
1.25
1.19
1.21
1.24
1.27
1.25
1.26
1.20
1.11
1.17
1.18
1.04
1.07
1.13
1.06
1.20
1.6
2.2
3.0
3.9
4.2
4.4
3.0
3.3
3.5
3.6
3.3
4.0
3.2
1.9
2.5
3.1
0.5
0.8
1.5
1.0
0.9
95MeOH / 5ACN/0.05 M N(CH3)4NO3
95MeOH / 5ACN/0.05 M N(CH3)4NO3
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
30ACN / 70MeOH/ 1.6AA / 4TEA
100MeOH/1.6AA/4TEA
9
10
11
12
13
14
15
16
17
18
19
20
21
[Ru(phen)(bpy)2](PF6)2
[Ru(phen)2(bpy)](PF6)2
[Ru(di-phenylphen)3](PF6)2
[Ru(tetra-methlyphen)3](PF6)2
100MeOH/1.6AA/4TEA
100MeOH/1.6AA/4TEA
100MeOH/1.6AA/4TEA
100MeOH/1.6AA/4TEA
Chirality DOI 10.1002/chir

68
SHU ET AL.
TABLE 2. Effect of types of bases and acids in the mobile
TABLE 3. Effect of salt type in the mobile phase on enantio-
meric separation of [Ru(bpy)2(o-OCH3-dppz](PF6)2 with
LARIHC CF6-RN and LARIHC CF7-DMP chiral stationary
phases
phase (ACN/MeOH/acid/base) on enantiomeric separation of
[Ru(bpy)2(o-OCH3-dppz](PF6)2 with LARIHC CF6-RN and
LARIHC CF7-DMP chiral stationary phases*
LARIHC CF6-RN
LARIHC CF7-DMP
LARIHC CF6-RN
LARIHC CF7-DMP
k1
α
RS
k1
α
RS
Salts
k1
α
RS
k1
α
RS
BASE ADDITIVES
Trimethylamine
Ethanolamine
Butylamine
Triethylamine
ACID ADDITIVES
Acetic acid
NH4NO3
1.17
0.52
0.62
7.52
1.40
1.35
1.22
1.15
1.5
1.3
0.8
0.9
3.34
3.09
2.75
6.50
1.11
1.12
1.10
1.14
1.1
1.4
1.0
1.6
1.59
1.44
1.76
1.31
1.33
1.30
1.29
1.33
1.6
1.5
1.5
1.6
3.45
4.96
4.48
2.34
1.13
1.15
1.14
1.12
1.8
1.9
1.7
1.9
N(CH3)4NO3
NH4COOCF3
N(CH3)4COOCH3
MeOH/ACN: 70/30, concentration of salt: 0.025 mol/L, flow rate: 1 mL/min,
UV detection: 254 nm.
1.31
1.63
4.56
1.33
1.26
1.24
1.6
1.5
1.8
2.34
1.31
2.20
1.12
1.11
1.13
1.9
1.6
2.2
Trifluoroacetic acid
Formic acid
complexes on cyclodextrin and macrocyclic glycopeptides
CSPs.21,22 Ammonium nitrate, tetramethylammonium nitrate,
ammonium trifluroacetate, and tetramethylammonium ace-
tate were tested as additives in a 70/30 methanol/acetonitrile
mobile phase. Compound 14 was enantiomerically separated
with all the above-mentioned additives and the separation
results are listed in Table 3. Tetramethylammonium nitrate
resulted in the best compromise of short analysis time and
good resolution and was determined to be the most useful
salt. This is in agreement with past reports using macrocyclic
glycopeptide CSPs and cyclodextrin CSPs.21,22 However, for
the CF-based CSPs used here, AA/TEA additives are most
useful, so the tetramethylammonium nitrate was only used
as a secondary option.
*MeOH/ACN: 70/30, concentration of acid: 1.0%, molar ratio of acid/base: 1,
acetic acid used when testing bases and TEA used when testing acids, flow
rate: 1 mL/min, UV detection: 254 nm.
molar ratio at one, an evaluation of several different bases and
acids (Table 2) was performed to understand the influence on
the separation of compound 14. Overall, comparing the data
in Table 2, it was determined that a mix of acetic acid and
triethylamine resulted in the greatest selectivities and efficien-
cies in the shortest amount of time. Also, it appears that the
acidic additives used in the polar organic mode have a greater
influence on the selectivity, efficiency, and retention of the
analytes than basic additives.
To further improve peak efficiency, mobile phases with in-
creasing ionic strength (equal molar concentrations of AA
and TEA) were prepared. Figure 3A–D shows that an
increase in ionic strength decreased the retention time and
led to improved peak efficiencies. The combination of
280 mM AA and 280 mM TEA was found to be the optimized
additive ratio for 80% of the analytes in this study.
Considering that equimolar amount of acids and bases gave
the optimum separation, the effect of simple ammonium salts
added to the mobile phase on the enantiomeric separation
was also investigated on both the LARIHC CF6-RN and
LARIHC CF7-DMP columns. Note that these ammonium
and tetraalkylammonium additives can only be tested in their
equimolar salt form due to the nonaqueous mobile phases be-
ing used. Further, such ammonium salts have proven to be
the best additives for separating similar chiral ruthenium(II)
Effects of Mobile Phase Composition on the Enantiomeric
Separation
It has been reported that the use of acetonitrile as a modi-
fier produced the best resolution in the polar organic mode.23
Hence, the effect of the percentage of acetonitrile in the
mobile phase containing AA/TEA additives was examined.
Figure 4A shows that the retention factor of compound 14
slightly decreased with increased the concentrations of
ACN until it reached 50/50 ACN/MeOH. Then retention
gradually increased in the range of 50–80% ACN. Subse-
quently, the retention time steeply increased when the con-
centration of ACN exceeded 80%. The increase of the
retention factor with the concentration of ACN is probably at-
tributed to the competition of the methanol with the analyte
for hydrogen bonding sites on the stationary phase. Similar
competition between methanol and analytes occurred on
cyclodextrin CSPs in the polar organic mode.39
The dependence of retention factor on the concentration of
acetonitrile was also carried out by using another additive,
ammonium nitrate (see Fig. 4B). Here, a U shape retention
curve was observed and the retention factor decreased with
increasing acetonitrile concentration from 0–40%. It reached
a minimum in the range of 40–60% and then increased again
with acetonitrile concentration increasing from 60% to 85%
(ammonium nitrate does not dissolve at higher acetonitrile
concentrations). The retention factor increased with acetoni-
trile percentage due to the competition of the methanol with
the analyte for hydrogen bonding sites at high concentrations
of acetonitrile. However, the opposite trend is observed when
the ACN is below 40%. This suggests that other interactions
dominated in the chromatographic separation. The reason
for increased retention in high methanol concentrations when
using an ammonium salt is unclear, but this trend was
Fig. 3. The separation of [Ru(bpy)2(o-OCH3-dppz](PF6)2 enantiomers on
LARIHC CF6-RN column at various AA:TEA (v:v) amount. Molar ratio of
AA/TEA: 1, flow rate: 1 mL/min, UV detection: 254 nm.
Chirality DOI 10.1002/chir

ENANTIOMERIC SEPARATIONS OF RUTHENIUM (II) POLYPYRIDYL COMPLEXES
69
Fig. 4. The retention factor of compound [Ru(bpy)2(o-OCH3-dppz](PF6)2 vs. acetonitrile concentration on LARIHC CF6-RN. The mobile phase composition: (A)
ACN/MeOH/AA/TEA (v/v), (B) ACN/MeOH/NH4NO3 (w/w).
observed on CD CSPs as well.22 It should be noted that this
“dual” retention behavior provides a second means of method
development, separation, and optimization.
were separated on both columns. The retention factors and
selectivities of these analytes increased in the order of
17 < 18 < 19. Thus, as the size of the conjugated system
increases, so does enantioselectivity.
Influence of the Enantiomer Structure on the Enantiomeric
Separation
CONCLUSION
The dependence of retention and separation on the struc-
ture of the racemic ruthenium(II) polypyridyl complexes
was investigated to explore the enantiomeric separation
We found greater than baseline separations for 20 of 21 chi-
ral ruthenium (II) polypyridyl complexes (of which 19 have
never been separated) using cyclofructan-based CSPs. Addi-
tives played an essential role in the enantioselectivity and
retention. It appears that π–π interactions are of great impor-
tance for these separations, as aliphatic derivatives of
cyclofructan were unable to discriminate between the enan-
tiomers. It is anticipated that these separations will be very
important with the growing interest in chiral metallo-organic
complexes and future studies will focus on determining elu-
tion patterns of Δ- and Λ-enantiomers which will be helpful
in preparative separations.
-
mechanism. The PF6 anion of the ruthenium(II) polypyridyl
complexes was exchanged to Cl- and the analysis was carried
out on an LARIHC CF6-RN column under the same mobile
phase conditions. When the separation of [Ru(phen)2(o-Cl-
dppz)](PF6)2 is compared with that of ([Ru(phen)2(o-Cl-dppz)]
(Cl)2, the chromatographic data, such as retention time, selec-
tivity, and efficiency did not change. This is likely attributed
to the exchange of anions from the mobile phase additives with
initial counter anions of the complex.
Table 1 summarizes the chromatographic data for all ruthe-
nium (II) polypyridyl complexes. Compounds 1 and 3 are
structurally related compounds, which contain one phendione
ligand but different aromatic moieties for the remaining
ligands. Compound 1 was retained much longer than com-
pound 3, while compound 3 had much greater resolution
values. Apparently, the steric bulk of the “diphenylphen”
ligands on compound 3 decreased nonenantioselective
retention interactions, while enhancing enantioselectivity.
The effect of the positions of substituent groups (ortho and
para) on a given ligand (dppz) was also investigated. Analytes
7 and 8 have cyano (CN) substituents at the ortho (o) and
para (p) positions in the dppz ligand, respectively. Analytes
9 and 10 have bromo (Br) substituents in those positions.
Interestingly, higher resolutions were obtained for the para
substituted analytes on the LARIHC CF6-RN CSP, whereas
the analytes with ortho substitutions were better separated
on the LARIHC CF7-DMP CSP. This highlights the comple-
mentary nature of these two CSPs.
Analytes which have ortho substituted electron-withdrawing
groups (compounds 10, 11, 12, 15, 16) generally showed
higher selectivity and resolution than the analytes which have
electron-donating groups (compounds 13, 14) on the
CF7-DMP CSP. It appears that the electron-withdrawing
groups (-CN, -Br, -Cl, -F) on the dppz ligands makes these
analytes more π-acidic and the π–π interaction between π-acidic
analytes and the π-basic dimethylphenyl groups is enhanced.
This leads to improved separation of those analytes.
ACKNOWLEDGMENTS
DWA acknowledges that research in this publication was
supported by the National Institute of General Medical Sci-
ence of the National Institute of Health under Award Number
R44GM103359. The content is solely the responsibility of the
authors and does not necessarily represent the official views
of the National Institutes of Health. DWA also thanks the
Robert A. Welch Foundation (Y-0026) for support. Further,
the authors appreciate financial support from the China
Scholarship Council, the Natural Science Foundation of China
(No. 21105008) and the Chinese Fundamental Research
Funds for the Central Universities (N110805001). FMM
acknowledges support from the Robert A. Welch Foundation
(Y-1301) and NIH 2R15CA113747-02.
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