Carbonate binding to copper(II) in solution: Mixed-ligand complex formation and its application to the isolation and separation of the three isomers of [Cu(bpp)(H2O)][ClO4]2 [bpp = 2,6-bis(pyrrolidin-2-yl)pyridine]

Article abstract of DOI:10.1039/b111501b

The binding of the carbonate anion to [Cu(meso-bpp)(H₂O)]²⁺ and rac-[Cu(bpp)(H₂O)]²⁺ [bpp = 2,6-bis(pyrrolidin-2-yl)pyridine] in aqueous solution has been investigated. Formation constants of the carbonato compl

Full text of DOI:10.1039/b111501b

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Carbonate binding to copper(II) in solution: mixed-ligand complex
formation and its application to the isolation and separation
of the three isomers of [Cu(bpp)(H2O)][ClO4]2
[bpp ؍ 2,6-bis(pyrrolidin-2-yl)pyridine]
Klaus Bernauer,* Amel Cabort, Nathalie Guicher, Helen Stoeckli-Evans and Georg Süss-Fink
Institut de Chimie, Université de Neuchâtel, Case postale 2, CH-2007 Neuchâtel, Switzerland
Received 17th December 2001, Accepted 21st February 2002
First published as an Advance Article on the web 4th April 2002
The binding of the carbonate anion to [Cu(meso-bpp)(H2O)]2ϩ and rac-[Cu(bpp)(H2O)]2ϩ [bpp = 2,6-bis(pyrrolidin-
2-yl)pyridine] in aqueous solution has been investigated. Formation constants of the carbonato complexes
[Cu(meso-bpp)(CO3)] and rac-[Cu(bpp)(CO3)] (1.02 × 103 MϪ1 and 1.77 × 103 MϪ1, respectively, µ = 0.70 M)
have been calculated from spectrophotometric measurements. The formation of these Cu2ϩ complexes can also
be used for an improved synthesis and an easy isolation of the three diastereoisomers of bpp. The mixture of
[Cu(meso-bpp)(H2O)]2ϩ and rac-[Cu(bpp)(H2O)]2ϩ is separated by elution from SP Sephadex C-25, either as
hydroxo or carbonato derivatives. rac-[Cu(bpp)(H2O)]2ϩ is then resolved into the enantiomers [Cu(S,S-bpp)(H2O)]2ϩ
and [Cu(R,R-bpp)(H2O)]2ϩ, again on SP Sephadex C-25, by means of -(ϩ)-tartrate as chiral eluent. The three
stereoisomers, meso-bpp, (S,S)-bpp and (R,R)-bpp are liberated from the corresponding copper() complexes by
ligand displacement using trans-1,2-diaminocyclohexane-N,N,NЈ,NЈ-tetraacetic acid (H4cdta). The structure of the
meso isomer was solved by a single crystal X-ray analysis using the perchlorate salt [meso-bppH2][ClO4]2ؒ2H2O.
high stereoselectivity in the formation of homoleptic 1 : 2
Introduction
complexes.10 Nevertheless, the method used for the synthesis
Binding of HCO3Ϫ/CO32Ϫ to metal centres has been a subject of
showed several disadvantages. The separation of the two dia-
intense research, given the interest in the enzymatic hydration/
stereoisomers has been achieved by chromatography of the
dehydration reaction of carbon dioxide catalysed by carbonic
p-nitrobenzoyl derivatives, but deprotection of the ligand
causes significant loss of the product. Finally, the racemate was
separated into the two enantiomers by crystallisation as the
O,O-dibenzoyltartate.
anhydrase.1 In order to get a better understanding of the
enzymatic activity, Cu2ϩ complexes have been used as model
compounds.2 It has been demonstrated that the ability of these
complexes to react with HCO3Ϫ depends on the position of the
In the present paper we report on the formation constants of
coordinated water molecule in the coordination sphere of the
the binding equilibria of the carbonate anion to [Cu(meso-
Cu2ϩ centre. Bi-, tri- and tetra-dentate ligands have been used
bpp)(H2O)]2ϩ and rac-[Cu(bpp)(H2O)]2ϩ and the use of these
for these model compounds, and various binding modes in
equilibria for the chromatographic separation of the three
mono- or oligo-meric compounds have been identified.2a Linear
complexes [Cu(meso-bpp)(H2O)]2ϩ, [Cu(S,S-bpp)(H2O)]2ϩ and
triamines and their derivatives have also found a steadily
[Cu(R,R-bpp)(H2O)]2ϩ which, after decomplexation, allows the
increasing interest as auxiliary ligands in asymmetric synthesis,3
preparation of the three stereoisomers meso-bpp [i.e. (R,S)-
bpp], (S,S)-bpp and (R,R)-bpp.
as ligands for complexes with predetermined chirality,4,5 or as
building blocks in the formation of heteronuclear,6 helical7 and
dendrimeric complexes.8 Often these ligands contain a central
pyridine unit to ensure meridional coordination and to reduce
the number of possible structural isomers. Furthermore this
aromatic unit allows the formation of manifold derivatives by
substitution of the aromatic moiety.
Several years ago, we reported the synthesis and the separ-
ation of the three stereoisomers of the triamine 2,6-bis-
(pyrrolidine-2-yl)pyridine (bpp) (Fig. 1).9 The measurements of
complex formation equilibria with a chiral isomer of this ligand
and different metal ions such as Ni(), Co(), Zn() revealed
Experimental
Spectroscopic measurements
UV/Vis absorption spectra were recorded on a Varian Cary-1E.
Circular dichroism (CD) spectra were recorded on spectro-
polarimeters JASCO J-700 and J-500-C. 1H and 13C NMR
measurements were performed on a Varian Gemini spectro-
meter (200 MHz).
Materials
SP and QAE Sephadex were obtained from Amersham-
Pharmacia-Biotech. The precursor of bpp, 2,6-bis(3,4-dihydro-
2H-pyrrol-5-yl)pyridine (bdpp), was prepared as previously
described.9
Syntheses
2,6-Bis(pyrrolidin-2-yl)pyridine (bpp). The mixture of rac-
and meso-bpp was obtained as reported earlier.9 However,
Fig. 1 Steroisomers of 2,6-bis(pyrrolidin-2-yl)pyridine (bpp).
DOI: 10.1039/b111501b
J. Chem. Soc., Dalton Trans., 2002, 2069–2073
This journal is © The Royal Society of Chemistry 2002
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instead of a 1 : 2 rac : meso mixture, a 1 : 1 mixture of the two
diastereoisomers can be obtained by the following modification
of the reduction step of the precursor 2,6-bis(3,4-dihydro-2H-
pyrrol-5-yl)pyridine (bdpp): 2.54 g (1.19 × 10Ϫ2 mol) of bdpp
were dissolved in 500 ml dried ethanol, and NaCNBH3 (2.21 g;
3.58 × 10Ϫ2 mol) was added under N2. The pH of the solution
was kept at 4 by addition of 2 M HCl. After 4 hours the pH was
raised to 12 by the addition of aqueous 2 M NaOH, and the
organic fraction was extracted with CH2Cl2, washed with a
saturated solution of NaCl and dried over MgSO4. The solvent
was removed by evaporation, and the residue was dried in
vacuo. The resulting mixture of the bpp diastereoisomers (1.3 g,
53%) was directly used for the synthesis of the copper
complexes.
ee < 90% were submitted to repeated separation. [Cu(R,R-
bpp)(H2O)][ClO4]2: [α]589 = ϩ689Њ (c = 0.015 M, H2O); ∆ε650
=
Ϫ1.00, ∆ε547 = 0; ∆ε510 = ϩ0.14, ∆ε650 = 0.
Isolation of the three bpp isomers
The three isomers of bpp are liberated from the corresponding
copper complexes by a ligand displacement reaction. To the
aqueous solution of [Cu(bpp)(H2O)]2ϩ, 1.5 equivalents of
trans-1,2-diaminocyclohexane-N,N,NЈ,NЈ-tetraacetic acid were
added, and the pH of the solution adjusted to pH = 7.8 with
NaOH. The exchange reaction was monitored by UV/Vis for
the meso complex and by CD for the optically active complexes.
After completion of the reaction, the diluted solution con-
taining not more than 1 g dmϪ3 of the initial compound
[Cu(bpp)(H2O)][ClO4]2, was introduced into a QAE Sephadex
anion-exchange column loaded with ClO4Ϫ. The column was
then eluted with water. The effluent solution was monitored by
UV absorption (λ = 262 nm) or by polarimetric measurements.
Upon concentration of the solutions using a rotatory evapor-
ator, the perchlorate salts of the protonated bpp isomer was
obtained as a crystalline solid. The solid was filtered and dried
under vacuum. The purity of the products was checked by
comparing the specific rotation of the enantiomers of bpp with
data given in the literature.9 The structure of the isomer meso-
bpp was solved by X-ray crystal structure analysis.
Warning: Perchlorates can be dangerous and should be
handled in small amounts with screen protection!
[Cu(bpp)(H2O)][ClO4]2. The crude mixture of bpp (1.3 g,
6.3 × 10Ϫ3 mol) was dissolved in water (100 ml), and an excess
of aqueous copper sulfate was added. The solution was stirred
for 1 h at pH = 8.0 in order to precipitate unreacted copper as
Cu(OH)2. The mixture was filtered over Celite, and the pH of
the solution fixed at a value between 5.0 and 6.0 by addition of
diluted HClO4 (ca. 10%). The deep blue solution of [Cu-
(bpp)(H2O)][ClO4]2 was further diluted with water to five to ten
times its volume in order to enhance the retention of the
bivalent copper complexes. The solution was introduced
into a SP Sephadex C-25 column which was then extensively
washed with water. After complete elution of the copper com-
plexes with a 0.2 M buffer (NaHCO3/Na2CO3, pH = 10), 3 g
of the pure diastereoisomeric mixture were obtained almost
quantitatively as solid perchlorate salts which crystallised after
acidification to pH = 5.0 with HClO4 (10%).
1H-NMR (200 MHz, CDCl3): δ 1.74 (m, 2H), 1.85 (q, 4H),
2.20 (m, 2H), 2.60 (s, 2H), 3.00 (m, 2H), 3.22 (m, 2H), 4.22 (t,
2H), 7.17 (d, 2H), 7.58 (t, 1H). Specific rotation of (ϩ)bpp
[(Ϫ)bpp]: [α]365 (c = 0.10 M, H2O) ϩ378Њ [Ϫ340Њ], [α]589 (c =
0.10 M, H2O) ϩ126Њ [Ϫ113Њ].
Equilibrium data measurements
For the determination of the equilibrium constants by UV/Vis,
a series of eight samples (at constant pH) of [Cu(bpp)-
(H2O)]2ϩ (2.5 × 10Ϫ3 M) containing a carbonate buffer
([HCO3Ϫ] = [CO32Ϫ]) were prepared; the total concentration
varying between 5 × 10Ϫ3 and 0.1 M. The ionic strength was
kept at 0.70 M by addition of the calculated amount of KNO3.
The pH of the solution was adjusted, if necessary, to 9.74 by
NaOH. The samples were kept at 25 ЊC for 24 h, and the pH
was controlled prior to measuring the UV/Vis spectra. The
measurements were repeated after 24 h in order to ensure the
stability of the solutions.
Separation of [Cu(meso-bpp)(H2O)]2؉ and rac-[Cu(bpp)-
(H2O)]2؉
First method. A solution of the perchlorate salts of the
mixture of diastereoisomers (2 g) in water (2000 ml) was intro-
duced into a SP Sephadex C-25 column (l = 120 cm, Ø = 8 cm).
Elution with a buffer (H3BO3 0.1 M, Na2SO4 0.5% at pH = 8.35)
allowed quantitative separation; the first fraction was identified
as meso, and the second as rac. The fractions were separately
acidified by diluted HClO4 and crystallised from water.
For the measurements at variable pH, the solutions were
prepared in a similar way with constant total carbonate buffer
Second method. A solution of the perchlorate salts of the
mixture of diastereoisomers (2 g) in water (2000 ml) was intro-
duced into a SP Sephadex C-25 column (l = 120 cm, Ø = 8 cm).
Elution with a buffer (NaHCO3/Na2CO3 0.02 M, pH = 10)
allowed quantitative separation; the first fraction was identified
as rac, and the second as meso. The fractions were separately
acidified with diluted HClO4 and treated as indicated below.
2Ϫ
concentration (0.05 M) the HCO3Ϫ/CO3 ratio being varied
from 40 to 0.02 corresponding to a pH range from 8.0 to 11.2
and a pKa value of 9.74 (µ = 0.70 M and T = 25 ЊC).
X-Ray crystallography
Suitable crystals of [meso-bppH2][ClO4], colourless, rod-like
single crystals, were grown from water. Intensity data were col-
lected at 233 K on a Stoe AED2 4-circle diffractometer using
Mo-Kα graphite monochromated radiation (λ = 0.71073 Å)
with ω/2Θ scans in the 2Θ range 5–51Њ (see Table 1). The struc-
ture was solved by direct methods using the program SHELXS-
97.11 The refinement and all further calculations were carried
out using SHELXL-97.11 H atoms were included in calculated
positions and treated as riding atoms using SHELXL-97
default parameters, except the water H atoms which were
located from difference Fourier maps and refined isotropically
with a fixed O–H bond length of 0.90(1) Å. The non-H atoms
were refined anisotropically, using weighted full-matrix least-
squares on F 2.
Chiral resolution of rac-[Cu(bpp)(H2O)]2؉
A solution of 1.5 g of rac-[Cu(bpp)(H2O)][ClO4]2 in 2000 ml
of water were introduced into a SP Sephadex C-25 column
(l = 35 cm, Ø = 3 cm) which was then washed with water. The
complexes were eluted with a solution of sodium -(ϩ)-tartrate
(0.05 M), the pH of which was fixed to 5.0 with NaOH. The S,S
isomer moves faster than the R,R isomer, the two bands were
collected separately. The solutions of the two enantiomers were
again introduced into a small SP Sephadex C-25 column, and
the tartrate was washed out with water. The compounds were
then eluted with a 0.2 M carbonate buffer and the perchlorate
salts isolated as described before. The optical purity of the
effluent was checked by measuring the ratio of visible absorp-
tion to optical rotation and/or CD intensity. The enantiomeric
excess (ee) of the samples (90 > ee > 100%) increased up to
constant values by recrystallisation from water. Samples of
CCDC reference number 175425.
See http://www.rsc.org/suppdata/dt/b1/b111501b/ for crystal-
lographic data in CIF or other electronic format.
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plex, ZnII-aminopropyl-[12]aneN3.12 The spectral differences
Table 1 Summary of crystal structure data for [meso-bppH2][ClO4]2ؒ
2H2O
between the three species of bpp complexes involved in the two
equilibra (1) and (2) allow the determination of the correspond-
ing formation constants using the pKa of the meso- and rac-
complexes at µ = 0.70 M (8.61 for [Cu(meso-bpp)(H2O)]2ϩ and
8.85 for rac-[Cu(bpp)(H2O)]2ϩ).
Empirical formula
Formula weight
C13H25Cl2N3O10
454.26
Crystal system
Space group
Monoclinic
C2/m
a/Å
b/Å
c/Å
α/Њ
14.313(6)
11.416(4)
12.481(5)
90.00(3)
91.15(3)
90.00(3)
2038.9(14)
4
(1)
β/Њ
γ/Њ
V/Å3
(2)
Z
T /K
233(2)
0.71073
0.0374
Wavelength, λ(Mo-Kα)/Å
µ/cmϪ1
Two types of measurements have been performed, one at
constant pH with variable concentrations of an equimolar
Reflections collected
Unique reflections
Rint
4231
2001
0.0289
0.0575, 0.1545
2Ϫ
HCO3Ϫ/CO3 buffer, and one with a constant total carbonate
R1, wR2 [I > 2.0σ(I )]a
concentration at variable pH between 8.0 to 11.2. The second
type of measurements allow the exclusion of significant binding
of HCO3Ϫ. The formation constants reported in Table 2 are
calculated from the visible absorption at λ = 639 nm. The acid
dissociation constants of the two diasteroisomeric copper com-
plexes at µ = 0.70 M [pKa(meso) = 8.61; pKa(rac) = 8.85] are
slightly different compared to the values at µ = 0.10 M, given in
the literature.9
a R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|, wR2 = [Σ(w(Fo2 Ϫ Fc2)2)/Σw(Fo )]1/2
.
4
Results and discussion
Carbonate binding by [Cu(bpp)(H2O)]2؉ in aqueous solution
The isomer mixture of [Cu(bpp)(H2O)]2ϩ is only slowly eluted
from a SP Sephadex cation-exchange column by a NaClO4
solution even under basic conditions. However, it can be dis-
placed easily by a moderately concentrated (0.2 M) carbonate
buffer at pH = 10, suggesting the formation of carbonato com-
plexes. This hypothesis is supported by the visible absorption
spectra shown in Fig. 2.
The values given in Table 2 show that the formation of
the hydroxo complex is more favourable with [Cu(meso-bpp)-
(H2O)]2ϩ than with the racemic mixture of [Cu(S,S-bpp)-
(H2O)]2ϩ and [Cu(R,R-bpp)(H2O)]2ϩ, whereas the opposite is
true for the carbonato complexes. If the coordinated water
molecule is more strongly bound in [Cu(meso-bpp)(H2O)]2ϩ
,
one of the protons is abstracted more easily than in the optic-
ally active complexes of the racemic mixture. On the other
hand, the coordination of the carbonate anion requires the
elimination of the whole H2O ligand, which is easier in
rac-[Cu(bpp)(H2O)]2ϩ
.
Another possible explanation of the higher stability of
rac-[Cu(bpp)(CO3)] is based on structural considerations: In
the meso complex both pyrrolidine rings are located on the
same side of the CuN3O plane. This complex therefore offers
one favourable and one unfavourable apical position for the
coordination of the carbonate anion. In the complex with rac-
bpp, on the other hand, the pyrrolidine rings are located on
opposite sides with respect to the CuN3O plane. The crystallo-
graphic analysis of the coordination polymer [Cu(bpp)(SO4)]ؒ
2/3H2Oؒ1/3L [L = di-O,OЈ-(4-toluyl)--tartaric acid]10 shows
that both sides are easily accessible in the racemic (or optically
active) complex for the coordination of the sulfate oxygen atom.
Given the carbonate being a chelating ligand in a pentacoord-
inate square pyramidal coordination geometry, the carbonate
binding in the racemic compound is favoured for statistical
reasons. This pentacoordinate pyramidal structure has been
found in the above-mentioned coordination polymer. For this
reason, we tentatively suggest the structures represented in
Fig. 3 for the carbonato-bpp copper() complexes in solution.
Fig. 2 Visible absorption spectra of [Cu(meso-bpp)(H2O)]2ϩ (a) and
rac-[Cu(bpp)(H2O)]2ϩ (b) in HCO3Ϫ/CO3 0.2 M, pH = 10.3 (1); H2O
2Ϫ
(2) and NaOH 0.01 M (3).
Fig. 3 Suggested structure for [Cu(R,R-bpp)(CO3)] and [Cu(meso-
Even though numerous carbonato complexes are known with
their molecular structures,2 only a few quantitative equilibrium
data are known for CO3 binding to metal complexes in
solution. Thus, Kimura et al. have reported a value of log
K = 2.8 for the formation constant of the carbonato zinc com-
bpp)(CO3)].
2Ϫ
The formation of the carbonato complex is in competition
with the formation of the hydroxo complex and depends on the
pH and on the carbonate buffer concentration. Fig. 4 shows the
J. Chem. Soc., Dalton Trans., 2002, 2069–2073
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Table 2 Molar extinction coefficients and formation constants of the Cu(bpp) complexes
ε636/cmϪ1 molϪ1 l
log K (µ = 0.1 M)
log K (µ = 0.7 M)
Complex
rac
meso
rac
meso
rac
meso
[Cu(bpp)(H2O)]2ϩ
[Cu(bpp)(OH)]ϩ
[Cu(bpp)(CO3)]
129.1
113.2
147.2
128.2
109.1
145.6
5.51
5.74
5.15
5.39
3.24a
3.01a
a Maximum error 0.2 log units.
Fig. 4 Calculated distribution of the species for racemic (—) and meso
2Ϫ
(---) complexes; [HCO3Ϫ/CO3
10Ϫ3 M.
]
= 0.02 M and [Cu(bpp)]total =
total
Fig. 5 Relative concentration difference ∆ = ([rac] Ϫ [meso])/c (a) and
the chromatographic factor f (b) of meso and rac [Cu(bpp)(CO3)];
pH = 10.0, [Cu(bpp)]total = 10Ϫ3 M.
distribution of the different species as a function of pH for a
constant total carbonate concentration (0.02 M), suggesting the
maximum carbonate binding to occur approximately at pH =
9.0 for both, the meso isomer and the racemic mixture of the
two enantiomers.
defined by f are as seen from Fig. 5, realised by using a carbon-
ate concentration which is roughly twice as high as required to
obtain the maximum difference between the concentrations of
the two diastereoisomers, e.g. c = 0.02–0.04 M for pH = 10.0.
Contrary to the elution as hydroxo complexes with the borate
buffer at pH = 8.37 (H3BO3 0.1 M, Na2SO4 0.5%), the elution as
carbonato complexes HCO3Ϫ/CO32Ϫ yields the racemic mixture
first, due to the higher affinity of the chiral complexes to the
carbonate anion, with respect to the meso complex.
Separation of the diasteroisomers
The separation of [Cu(meso-bpp)(H2O)]2ϩ and rac-[Cu(bpp)-
(H2O)]2ϩ can be achieved by two methods. The first method is
based on the difference between the pKa of the two species:
(8.25 for [Cu(meso-bpp)(H2O)]2ϩ and 8.49 for rac-[Cu(bpp)-
(H2O)]2ϩ, µ = 0.10 M). According to the ratio of the acidity
constants Ka(meso)/Ka(rac) = 1.70, the equilibrium (3) is shifted to
the right hand side. At pH = 8.37, corresponding to 1/2[pKa(meso)
ϩ pKa(rac)], the relative amount of the hydroxo species is 42.2%
for the racemic and 55.1% for the meso-compound. The latter is
therefore eluted first by 0.5% Na2SO4 at pH = 8.35, due to the
lower affinity of the monocationic hydroxo complex.
Chromatographic separation of rac-[Cu(bpp)(H2O)]2؉ into the
enantiomers
The racemic mixture of [Cu(bpp)(H2O)]2ϩ, fixed on a SP
Sephadex cation exchanger, can be eluted with a -(ϩ)-tartrate
buffer solution at pH = 5, giving two well separated bands, the
ratio of the Rf values being about 1.5 in favour of the S,S
isomer which is eluted first. The quality of the separation
depends on both the rate of elution and the concentration of
the eluent. Unfortunately, to some extent ligand exchange
seems to occur on the column. The separation of the enanti-
omers might therefore be due to the formation of mixed bpp–
tartrato complexes, rather than due to ion pair formation. Best
separation is therefore achieved by rapid elution with buffers of
low concentration. The optical purity of the separated (S,S)-
and (R,R)-[Cu(bpp)(H2O)]2ϩ enantiomers can be checked by
comparison of the visible and the CD spectra (Fig. 6). The
(3)
The second method takes advantage of the different stabil-
ities of the carbonato complexes. The conditions for the separ-
ation are given by two factors: Firstly, the relative difference
between the concentrations of the two diastereoisomers, which
determines the separation into two bands. Secondly, the sum of
the uncharged carbonato species and the charged aqua and
hydroxo complexes determine the displacement rate of the two
bands in the column. The influence of these two factors are
combined to a chromatographic factor f which is given by eqn.
(4), in which c represents the identical total concentration of the
two diastereoisomers.
f = ([rac] Ϫ [meso]) × ([rac] ϩ [meso])/c2
(4)
The calculated value of the chromatographic factor f, which
theoretically lies between 0 and 1, is represented in Fig. 5,
together with the relative difference of the two diastereo-
isomeric carbonato complexes for variable carbonate concen-
trations at pH = 10. The optimum conditions for the separation,
Fig. 6 CD spectra of [Cu(S,S-bpp)(H2O)][ClO4]2 and [Cu(R,R-bpp)-
(H2O)][ClO4]2; pH = 6.27.
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Table
3 Selected bond lengths (Å) and angles (Њ) for [meso-
cationic complexes are finally transformed into the perchlorate
salts; recrystallisation from water gives the enantiomerically
pure perchlorate salts.
bppH2][ClO4]2
C(4)–C(3)
C(3)–C(2)
C(2)–N(1)
C(2)–C(5)
C(5)–C(6)
1.371(4)
1.384(4)
1.337(4)
1.496(5)
1.537(5)
C(6)–C(7)
C(7)–C(8)
C(8)–N(2)
C(5)–N(2)
1.482(7)
1.473(7)
1.499(5)
1.496(4)
Isolation of the free ligands
The three isomers meso-bpp, (R,R)-bpp and (S,S)-bpp are
liberated from the corresponding complexes by ligand displace-
ment with trans-1,2-diaminocyclohexane-N,N,NЈ,NЈ-tetra-
acetic acid (H4cdta), the solution being previously adjusted to
pH = 7.8 by addition of NaOH [eqn. (5)] followed by anion
exchange on a QAE Sephadex C-25 anion exchange column
[eqn. (6)].
N(1)–C(2)–C(3)
N(1)–C(2)–C(5)
C(2)–C(5)–N(2)
C(2)–C(5)–C(6)
C(6)–C(5)–N(2)
122.4(3)
117.5(3)
111.9(3)
114.4(3)
102.9(3)
C(5)–N(2)–C(8)
N(2)–C(8)–C(7)
C(8)–C(7)–C(6)
C(7)–C(6)–C(5)
109.4(3)
103.7(4)
105.2(4)
103.8(3)
C(8) 1.473(7), C(8)–N(2) 1.499(5) Å] (see Table 3). The carbon–
carbon bond connecting the pyridine and the pyrrolidine cycles
[C(2)–C(5)] measures 1.496(5) Å. The pyrrolidine rings are
not planar, the ring atoms being formally sp3 hybridised. The
pyramidalisation of C(5) and C(5Ј) [C(2)–C(5)–C(6) 114.4(3),
C(2)–C(5)–N(2) 111.9(3), C(6)–C(5)–N(2) 102.9(3)Њ] however,
deviates considerably from tetrahedral geometry due to ring
restrictions. The torsion angle between the ring systems N(1)–
C(2)–C(5)–N(2) measures 15.84(4)Њ.
(5)
(6)
Concentration of the effluent solution yields the correspond-
ing isomers of bpp in the protonated form as the perchlorate
salt. The isolated compounds [meso-bppH2][ClO4]2, [(S,S)-
bppH2][ClO4]2 and [(R,R)-bppH2][ClO4]2 are white, air-stable
and water-soluble crystalline materials. The structure of the
isolated meso isomer was solved by X-ray crystal structure
analysis using a single crystal of [meso-bppH2][ClO4]ؒ2H2O
recrystallised from water.
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Fig.
7
Molecular structure (ORTEP plot)13 of [meso-bppH2]2ϩ
.
Displacement ellipsoids are drawn at the 50% probability level.
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