Enantiomeric programming in tripodal transition metal scaffolds

Article abstract of DOI:10.1039/b703761a

A new route to the isolation of the enantiopure tris-chelate complex (Δ/Λ)-fac-[Ru(L¹)₃]²⁺ (where L¹ is 2,2′-bipyridine-5-carboxylic acid) is demonstrated, where the transition metal centre retains the memory of

Full text of DOI:10.1039/b703761a

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Enantiomeric programming in tripodal transition metal scaffolds
´
Nicholas C. Fletcher,* Ciaran Martin and Heather J. Abraham
Received (in Durham, UK) 13th March 2007, Accepted 8th May 2007
First published as an Advance Article on the web 25th May 2007
DOI: 10.1039/b703761a
A new route to the isolation of the enantiopure tris-chelate
complex (D/K)-fac-[Ru(L1)3]21 (where L1 is 2,20-bipyridine-5-
carboxylic acid) is demonstrated, where the transition metal
centre retains the memory of the chirality present in a simple
tripodal tether used to control the metal centred geometry.
natural products to direct the metal-centred chirality in caged
2,20-bipyridine complexes has given rise to excellent control
over the metal centred stereochemistry, using either
terpenoids26–28 or simple amino acids.29 However in all of
the previously reported examples, the final product contains
the organic fragment used to govern the metal centred chir-
ality, giving considerable steric bulk to the material. In this
paper we explore the possible development of a new chiral
tether, and its disconnection, leaving an enantiopure complex
bearing only the memory of the tether.
The isolation of enantiomerically pure coordination com-
plexes has become increasingly important in recent years
arising from their continued use in material science,1–3 asym-
metric catalysis4 and medicinal chemistry.5–7 In particular,
tris-chelate diimine complexes of Ru(II), Os(II), Ir(III) and
Rh(III) have proved to be extremely useful in this respect due
to their stability to racemization and optical properties, pre-
senting as two enantiomers D and L.8 The isolation of metal
complexes as a single asymmetric form has not received the
same level of attention as that paid to the tetrahedral carbon
atom.9 Overwhelmingly, the majority of examples have relied
upon diastereotopic crystallization with a chiral counter-ion, a
method exploited by Werner in 1911.10 The parent complex
[Ru(bipy)3]21 (where bipy is 2,20-bipyridine) was enantiomeri-
cally resolved by diastereomeric crystallization with antimonyl
tartrate by Dwyer in 1949,11 and subsequently a number of
other related species have been isolated by similar meth-
ods.12–15 Chromatographic techniques, either using a chiral
stationary phase,16 or a chiral anion in the eluent (cation-
exchange chromatography),17,18 have allowed enantiomeric
separation to be achieved in a number of cases on a moderate
scale and a number of review articles have explored this
rapidly expanding topic over recent years.19–22
In the preceding communication24 we reported the prepara-
tion of fac-[Ru(L1)3]21 (where L1 is 2,20-bipyridine-5-
carboxylic acid) by tethering the three ligands together with
the triethanolamine. To direct the metal centred stereochem-
istry, the tether itself must contain the chirality, which can
then be removed subsequent to the complexation step. Fol-
lowing a literature procedure, two enantiopure trialkanol-
amines N(CH2–(S/R)–CHROH)3 (where R ¼ Me or Ph) were
prepared by reaction of either S- or R-propylene oxide or
R-styrene oxide with ammonia.30 From these, the tripodal
ligands S- or R-LMe and R-LPh were isolated in reasonable
yield (72 and 38%, respectively) from L1 via the acyl chloride
(Scheme 1). Both ligands were unstable with respect to sapo-
nification and rapidly decomposed on silica, presumably due
In addition to the metal centred stereochemistry, if the
bidentate ligand has Cs-symmetry, arising from a different
substitution pattern of the two halves of the chelate, meridio-
nal (mer) and facial (fac) isomerism will be present. In the
majority of the diimine ligands investigated, this diastereo-
meric difference will favour the less sterically hindered mer
form and subsequent separation can be problematic.23 To
overcome the difficulty of isolating solely the fac isomer, we
have reported a tripodal cage-like system to orientate the three
functional groups along the C3-axis present in the fac-iso-
mer.24 By using mild base hydrolysis, the ester linkages con-
necting the tris-chelates can then be removed to give the
desired geometric isomer. A similar procedure has been em-
ployed by Weizman et al. with the additional benefit that with
the inclusion of three L-alanine groups in the structure, the
chirality at the metal centre can also be directed.25 The use of
School of Chemistry and Chemical Engineering, Queen’s University
Belfast, Stranmillis Road, Belfast, Northern Ireland, UK BT9 5AG.
E-mail: n.fletcher@qub.ac.uk
Scheme
1
(ia) SOCl2 reflux, (ib) N(CH2-(R)-CH(Ph)OH)3,
NEt3–THF reflux, (ii) [Ru(DMSO)4Cl2], AgNO3–ethanol in high
dilution, reflux, (iii) KOH–water.
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to the considerable steric strain put on the system. Conse-
quently, purification was achieved by the use of a large excess
of the bipyridine precursor, and recrystallization from acetone
(the inorganic salts and starting acid/acyl chloride being
sparingly soluble).
The complexation of the ligand to Ru(II) was achieved by
the slow addition of the precursors to a large volume of
refluxing ethanol, containing an excess of silver nitrate giving
an almost immediate colour change to the characteristic red of
the complex. The product was purified on a short cation-
exchange column to give the mononuclear product, and the
two complexes [Ru(R-LPh)]21 and [Ru(S-LMe)]21 isolated as
Scheme 2 Conversion of fac-[Ru(L1)3](PF6)2 to fac-[Ru(L2)3](PF6)2
to (ia) SOCl2–CH3CN reflux, (ib) 3,4-dimethoxyanaline, NEt3–THF
reflux, (ic) KPF6(aq).
1
the hexafluorophosphate salts. The H NMR spectra of both
complexes (Fig. 1) indicated the C3-symmetry of the fac
isomer. Only one set of signals was observed for the bipyridine
peaks. For both of the complexes, the CH protons adjacent to
the carboxylate group presented as a multiplet since the CH2 is
inequivalent due to the restricted configuration. However,
these CH2 signals indicate the presence of one dominant
diastereomer (de in excess of 90%), in both complexes pre-
senting as two doublets of doublets at 2.80 and 2.26 ppm for
[Ru(R-LPh)]21 and at 2.78 and 2.33 ppm for [Ru(R-LMe)]21
(12.0 Hz J1 coupling). Compared to the 85% yield of the
racemic mixture obtained using the achiral triethanolamine
tether,24 the unoptimized yield of up to 22% was disappoint-
ing although anticipated given the ligand’s stability. Signifi-
cantly, a large quantity of red material could not be
precipitated or extracted following the purification, which is
typical of the presence of free carboxylate groups as a result of
deesterification.
retaining an excess of ammonium salts), indicating the absence
of the characteristic aliphatic signals. Due to the problems in
isolating (D or L)-[Ru(L1)3](PF6)2, and its propensity to ester
formation, it was reacted without isolation to form the tri-amide
complex (Dor L)-[Ru(L2)3](PF6)2 (Scheme 2). The formation of
the tri-acyl chloride proved problematic, and the direct use of
thionyl chloride gave only intractable oils. However, the use of a
10% mixture of thionyl chloride in acetonitrile proved success-
ful, followed by the addition of the aromatic amine 3,
4-dimethoxyaniline (4-aminoveratrole), and the product could
be achieved in a reasonable yield of 67% from [Ru-
(R-LMe)](PF6)2 as the fac isomer, as demonstrated by 1H
NMR spectroscopy and mass spectrometry.
The UV/Vis spectra of all of the complexes show the
characteristic p–p* transitions at approximately 290 nm, and
the strong metal to ligand charge transfer (MLCT) absorption
at 484 nm for [Ru(R-LMe)](PF6)2 and 486 nm for [Ru-
(R-LPh)](PF6)2. The MLCT absorption is red shifted for both
complexes relative to [Ru(bipy)3)](PF6)2 (445 nm) presumably
due to the electron withdrawing nature of the ligand, and a
steric strain causing deviation from the ideal coordination
geometry. On conversion to fac-[Ru(L2)3](PF6)2 the MLCT
moves back to 460 nm, in keeping with the removal of the
constraints. A contribution from aromatic transitions, attrib-
uted to the 3,4-dimethoxyaniline group, is apparent at 460 nm.
The fluorescence observed for complex [Ru(R-LMe)](PF6)2 and
fac-[Ru(L2)3](PF6)2 were similarly red shifted when compared
to [Ru(bipy)3)](PF6)2, with significantly lower quantum yields,
as would be expected from the electron withdrawing carbonyl
group (Table 1).
Both complexes [Ru(R-LPh)](PF6)2 and [Ru(R-LMe)](PF6)2
were dissolved in acetonitrile, and stirred with an excess of an
aqueous solution of KOH. The resulting solution was ob-
served to darken significantly in colour, as the complexes
hydrolysed. Following acidification, and the addition of
NH4PF6, the complex (D or L)-[Ru(L1)3](PF6)2 could be
isolated with difficulty. The extraction procedure proved
problematic, given the hydrophilicity of the carboxylic acid
groups. Drying the extraction with anhydrous magnesium
sulfate and recrystallization from dichloromethane and hexane
1
gave sufficient product for H NMR characterization (despite
An investigation of the circular dichroism (CD) spectra of
all of the complexes indicates that a strong Cotton effect is
observed, typical of a dominant single enantiomer. In keeping
with the NMR data, the size of the Cotton effect would
indicate that a single diastereomer is present for both of the
tripodal ligand systems with [Ru(S-LMe)](PF6)2 and [Ru-
(R-LPh)](PF6)2 adopting
a
D-configuration, and [Ru-
(R-LMe)](PF6)2 the opposite L form (by comparison of the
sign of the Cotton effect in the p–p* ligand transitions).32 The
addition of sodium hydroxide to the CD sample (in an
aqueous–acetonitrile mixture) resulted in conversion to fac-
[Ru(L1)3](PF6)2 with complete retention of the metal centred
stereochemistry (Fig. 2). The disconnection of the tether is
accompanied by a blue shift in the MLCT Cotton effect,
consistent with the visible absorption spectrum. Disappoint-
ingly, the conversion of complex [Ru(R-LMe)](PF6)2 to
Fig. 1 1H NMR spectra of (a) R-LMe (300 MHz, CDCl3, 25 1C), (b)
[Ru(R-LMe)](PF6)2 (500 MHz, acetone-d6, 25 1C).
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Fig. 2 Circular dichroism spectrum of (a) L-[Ru(R-LMe)](PF6)2
(black) and L-[Ru(L1)](PF6)2 (grey), (b) D-[Ru(R-LPh)](PF6)2 (black)
and D-[Ru(L1)](PF6)2 (grey); (5 ꢂ 10ꢁ5 mol dmꢁ3, 50% aqueous
CH3CN at 25 1C).
fac-[Ru(L2)3](PF6)2 resulted in a significant drop in the Cotton
effect, despite retaining the fac geometry, indicating that in the
process of the amidification a degree of racemization can
occur. It is assumed that this occurs via a ligand dissociation
of the complex following amidification, due to the increased
steric bulk of the ligand in keeping with observations made
with similar complexes.33
In summary we have managed to demonstrate for the first
time, an elegant synthetic route to the isolation of a single
enantiomeric form of fac-[Ru(L1)3]21, with the complex re-
taining the memory of a chiral auxiliary used to control the
geometry. While the yields are currently a little disappointing,
we are in the process of optimising the synthetic procedures
and extending the methodology to other metal centres. This
may permit the isolation of enantiopure materials that are not
readily accessible by traditional techniques.
Experimental
(þ)-(2S,20S,200S) and (ꢁ)-(2R,20R,200R)-triisopropanolamine
and (þ)-(2R,20R,200R)-triphenylethan-2-olamine were pre-
pared according to a literature procedure from the appropriate
epoxide.30 2,20-Bipyridine-5-carboxylic acid was isolated by
oxidation of 5-methyl-2,20-bipyridine.23
Ligand R-LMe (S-LMe prepared similarly)
2,20-Bipyridine-5-carboxylic acid (1.003 g, 5.0 mmol) was
refluxed in thionyl chloride (40 ml) for 3 h. The thionyl
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chloride was removed by distillation and the acyl chloride
dried in vacuo. The residue was dissolved in dry THF (60 ml)
and brought to reflux under nitrogen. To this a mixture of
(2R,20R,200R)-triisopropanolamine (0.275 g, 1.44 mmol), THF
(10 ml) and triethylamine (2 ml) were added dropwise over
1 h and then refluxed for a further 16 h. The solvent was
removed and the solid dissolved in DCM (100 ml). The
solution was washed with water (5 ꢂ 50 ml) and the organic
layer collected and dried over magnesium sulfate, filtered and
evaporated to dryness. The solid was dissolved in acetone, and
an insoluble white material was removed by filtration. The
solvent was removed giving the product as a syrupy brown oil.
[Ru(R-LPh)](PF6)2
The complex was prepared following a similar procedure to
[Ru(R-LMe)](PF6)2 and purified using SP Sephadexs C-25
cation exchange column with the divalent product obtained
when eluted with aqueous toluenesulfonic acid sodium salt
(0.20 M, 10% acetone) solution. The product was isolated by
the addition of ammonium hexafluorophosphate (0.30 g) to
the major fraction and recrystallized from acetone water.
Yield 10%. Analysis Found: C 47.32; H 4.35; N 5.88%,
C57H45N7O6RuP2F12 ꢄ 7H2O requires
C 47.51; H 4.13;
1
N 6.80%, H NMR (500 MHZ, d6 acetone) dH 9.11 (1H, s,
bipyH6), 8.92 (1H, d, J ¼ 8.6 Hz, BpyH3), 8.88 (1H, d, J ¼ 8.2
Yield: 0.764 g (72%), 1H NMR (500 Hz, CDCl3) dH 9.28 (1H,
30
Hz, BpyH ), 8.70 (1H, d, J ¼ 8.6 Hz, BpyH4), 8.21 (1H, dd, J
0
s, BpyH6), 8.71 (1H, d, J ¼ 4.7 Hz, BpyH6 ) 8.51 (1H, d, J ¼
0
¼ 8.2, 7.7 Hz, BpyH4 ), 7.87 (1H, d, J ¼ 5.7 Hz, BpyH6), 7.65
0
8.2 Hz, BpyH3), 8.48 (1H, d, J ¼ 8.2 Hz, BpyH3 ), 8.42 (1H, d,
0
(1H, m, BpyH5 ), 7.42–7.20 (5H, m, Ph), 6.22 (1H, m, OCH),
0
J ¼ 8.2 Hz, BpyH4), 7.85 (1H, dd, J ¼ 8.2, 7.6 Hz, BpyH4 ),
0
2.80 (1H, m, J ¼ 12.0, 12.0, NCHa), 2.26 (1H, m, J ¼ 3.3, 12.0,
7.36 (1H, dd, J ¼ 4.6, 7.6 Hz, BpyH5 ), 5.46 (1H, m, OCH,
2.56–2.39 (2H, m, NCH2), 1.37 (3H, d, J ¼ 6.4 Hz, CH3);
ESMS: [M]1 738.4.
NCHb), ESMS. m/z 1170.3 [M ꢁ PF6]1, 512.7 [M ꢁ 2PF6]21
.
24
K-[Ru(L1)3](PF6)2
[Ru(R-LMe)](PF6)2 (103 mg, 91.2 mmol) was dissolved in
acetonitrile (30 ml) and mixed with an aqueous solution
(30 ml) containing potassium hydroxide (0.20 g, 3.6 mmol)
for 72 h. The volume of solvent was reduced to 25 ml, acidified
with 2 M aqueous HCl to pH 3 and 0.2 g of ammonium
hexafluorophosphate added. The product was extracted with
DCM (3 ꢂ 30 ml), dried over anhydrous sodium sulfate, and
dried in vacuo. The product retained a high degree of water
and salt preventing detailed analysis. The product was
used without further characterisation (impure: 1H NMR
Ligand R-LPh
The ligand was prepared following a similar procedure to that
used for R-LMe using (2R,20R,200R)-triphenylethan-2-olamine
and purified by column chromatography eluted with DCM
containing 2% methanol, collecting the second major fraction
1
(unoptimized yield 38%). H NMR (500 Hz, CDCl3) dH 9.20
0
(1H, s, BpyH6), 8.68 (1H, d, J ¼ 5.0 Hz, BpyH6 ) 8.43 (1H, d, J
¼ 8.2 Hz, BpyH3), 8.34 (1H, d, J ¼ 8.2 Hz, BpyH4), 8.28 (1H,
0
0
d, J ¼ 8.2 Hz, BpyH3 ), 7.78 (1H, dd, J ¼ 8.2,7.6 Hz, BpyH4 ),
0
(500 MHZ, d6 acetone) dH 8.98 (1H, d, J ¼ 8.4 Hz, BpyH3),
7.42–7.20 (6H, m, Ph þ BpyH5 ), 6.28 (1H, d, J ¼ 9.1 Hz,
OCH), 3.40 (1H, m, NCH2), 1.93 (1H, m, NCH2); ESMS:
[M]1 924.5.
0
8.93 (1H, d, J ¼ 8.2 Hz, BpyH3 ), 8.66 (1H, d, J ¼ 8.4 Hz,
BpyH4), 8.49 (1H, s, bipyH6), 8.27 (1H, dd, 0J ¼ 8.2, 7.7 Hz,
0
BpyH4 ), 8.13 (1H, d, J ¼ 5.7 Hz, BpyH6 ), 7.66 (1H, m,
0
BpyH5 ).
[Ru(R-LMe)](PF6)2
24
K-[Ru(L2)3](PF6)2
Silver nitrate (1.01 g, 60 mmol) was dissolved in refluxing
ethanol (500 ml) under nitrogen. To this, a mixture of R-LMe
(0.570 g, 0.773 mmol) and [Ru(DMSO)4Cl2] (0.36 g, 0.744
mmol) dissolved in ethanol (30 ml) and DMSO (20 ml) was
slowly added by mechanical pumping over 4 h and refluxed for
an additional 2 h. The mixture was cooled and sodium
chloride (B1 g) was added. The brown solution was filtered
under gravity and the ethanol removed at reduced pressure.
The residues were suspended in water (150 ml), filtered onto a
SP Sephadexs C-25 cation exchange column, and the divalent
product eluted with aqueous toluenesulfonic acid sodium salt
(0.15 M, 10% acetone) solution. The product was isolated by
the addition of ammonium hexafluorophosphate (0.30 g) to
the major fraction and recrystallized from acetone–water.
Yield: 0.190 g (22%). Analysis Found: C 45.31; H 4.11;
N 7.56%, C42H39N7O6RuP2F12 ꢄ 2(CH3)2CO ꢄ 2H2O requires
The crude product [Ru(L1)3](PF6)2 was dissolved in dry
acetonitrile (20 ml) and thionyl chloride (5 ml) was added
over 5 minutes. The mixture was refluxed for 6 h, and the
solvent removed in vacuo. The residues were dissolved in
CH3CN (30 ml) and added over 1 h to a mixture of 3,4-
dimethoxyaniline (4-aminoveratrole) (0.15 g 9.8 mmol) and
triethylamine (1 ml) in acetonitrile (30 ml). The resulting red
solution was heated at reflux for 2 h and stirred for 14 h at
room temperature. The solvent was removed and the crude
product dissolved in water (50 ml), neutralised with a satu-
rated aqueous Na2CO3 solution, and purified using a SP
Sephadexs C-25 cation exchange column. The divalent product
was collected having been eluted with aqueous toluenesulfonic
acid sodium salt (0.25 M, 20% acetone) solution. The product
was isolated by the addition of NH4PF6 (0.30 g) and recrystal-
lized from acetone–water and further purified by passage down
Sephadex LH20 eluted with methanol–acetonitrile. (Yield from
103 mg of [Ru(S-LMe)](PF6)2 85 mg, 67%). Analysis Found: C
47.58; H 4.17; N 7.19%, C57H51N9O9RuP2F12 ꢄ 3H2O requires
1
C 45.01; H 4.33; N 7.65%, H NMR (500 MHZ, d6 acetone)
0
d
H 8.95 (3H, m, bipyH3,3 ,6), 8.70 (1H, d, J ¼ 8.6 Hz, BpyH4),
0
8.30 (1H, dd, J ¼ 8.2, 7.7 Hz, BpyH4 ), 7.86 (1H, d, J ¼ 5.7 Hz,
0
BpyH6), 7.73 (1H, dd, J ¼ 5.7, 7.7 Hz, BpyH5 ), 5.39 (1H, m,
OCH), 2.78 (1H, dd, J ¼ 12.0, 12.0, NCHa), 2.33 (1H, dd, J ¼
3.3, 12.0, NCHb), 1.24 (3H, d, J ¼ 6.0 Hz, CH3), ESMS. m/z
1
C 47.18; H 3.96; N 8.69%, H NMR (500 MHZ, d3 acetoni-
trile) dH 11.09 (1H, s, NH), 8.72 (1H, s, bipyH6), 8.53 (1H, d,
0
984.2 [M ꢁ PF6]1, 838.2 [MH ꢁ 2PF6]1, 419.5 [M ꢁ 2PF6]21
.
J ¼ 8.6 Hz, BpyH3), 8.52 (1H, d, J ¼ 8.0 Hz, BpyH3 ), 8.35
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(1H, d, J ¼ 8.6 Hz, BpyH4), 8.06 (2H, m, BpyH40and BpyH6),
7.40 (1H, m, BpyH50), 7.65 (1H, d, J ¼ 8.6 Hz, ver), 7.53 (1H,
s, ver), 6.80 (1H, d, J ¼ 8.6 Hz, ver), 3.73 (3H, s, OMe), 3.50
(3H, s, OMe); ESMS. m/z 1252.3 [M ꢁ PF6]1, 553.6 [M ꢁ
15 R. Caspar, H. Amouri, M. Gruselle, C. Cordier, B. Malezieux, R.
Duval and H. Leveque, Eur. J. Inorg. Chem., 2003, 499–505.
16 W. R. Browne, D. Hesek, J. F. Gallagher, C. M. O’Connor, J. S.
Killeen, F. Aoki, H. Ishida, Y. Inoue, C. Villani and J. G. Vos,
Dalton Trans., 2003, 2597–2602.
2PF6]21
.
17 N. C. Fletcher, P. C. Junk, D. A. Reitsma and F. R. Keene, J.
Chem. Soc., Dalton Trans., 1998, 133–138.
18 E. Holder, G. Trapp, J. C. Grimm, V. Schurig and E. Lindner,
Tetrahedron: Asymmetry, 2002, 13, 2673–2678.
Acknowledgements
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This research was supported by the EPSRC, and we thank the
EPSRC Mass Spectrometry Service, Swansea for the MS.
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