Stereoselectivity in the formation of tris-diimine complexes of Fe(ii), Ru(ii), and Os(ii) with a C2-symmetric chiral derivative of 2,2′-bipyridine

Article abstract of DOI:10.1039/b512116g

A C₂-symmetric enantiopure 4,5-bis(pinene)-2, 2′-bipyridine ligand (-)-L was used to investigate the diastereoselectivity in the formation of [ML₃]²⁺ coordination species (M = Fe(ii), Ru(ii), Os(ii), Zn(ii), Cd(ii), Cu(ii), Ni(ii)), and [ML₂Cl₂] (M = Ru(ii), Os(ii)). The X-ray structures of the [ML₃]²⁺ complexes were determined for Δ-[FeL₃](PF₆)₂, Δ-[RuL ₃](PF₆)₂, Λ-[RuL₃](PF ₆)₂, Δ-[OsL₃](PF₆) ₂, and Λ-[OsL₃](TfO)₂. All of these compounds were also characterized by NMR, CD and UV/VIS absorption spectroscopy. The [FeL₃]²⁺ diastereoisomers were studied in equilibrated solutions at various temperatures and in several solvents. The [RuL₃]²⁺ complexes, which are thermally stable up to 200 °C, were photochemically equilibrated. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512116g

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Stereoselectivity in the formation of tris-diimine complexes of Fe(II), Ru(II),
and Os(II) with a C2-symmetric chiral derivative of 2,2ꢀ-bipyridine†
Dusˇan Drahonˇovsky´,a Ulrich Knof,a Laurence Jungo,a Thomas Belser,a Antonia Neels,b Gae¨l Charles Labat,b
Helen Stoeckli-Evansb and Alex von Zelewsky*a
Received 25th August 2005, Accepted 28th October 2005
First published as an Advance Article on the web 29th November 2005
DOI: 10.1039/b512116g
A C2-symmetric enantiopure 4,5-bis(pinene)-2,2ꢀ-bipyridine ligand (−)-L was used to investigate the
diastereoselectivity in the formation of [ML3]2+ coordination species (M = Fe(II), Ru(II), Os(II), Zn(II),
Cd(II), Cu(II), Ni(II)), and [ML2Cl2] (M = Ru(II), Os(II)). The X-ray structures of the [ML3]2+ complexes
were determined for D-[FeL3](PF6)2, D-[RuL3](PF6)2, K-[RuL3](PF6)2, D-[OsL3](PF6)2, and
K-[OsL3](TfO)2. All of these compounds were also characterized by NMR, CD and UV/VIS
absorption spectroscopy. The [FeL3]2+ diastereoisomers were studied in equilibrated solutions at var◦ious
temperatures and in several solvents. The [RuL3]2+ complexes, which are thermally stable up to 200 C,
were photochemically equilibrated.
Introduction
In the third edition of his book from 1913,1 Alfred Werner
mentions a series of substitution reactions at metal centres
containing the CoIII(en)2 moiety. Werner assumes that in all these
transformations, e.g. in eqn (1), the absolute configuration is
retained.
(−)-[CoCl2(en)2]+ + CO3 → (+)-[Co(CO3)(en)2]+
(1)
2−
Scheme 1 Bailar inversion at an octahedral centre.
At that time, a definitive proof of the stereochemical course
of this type of reaction could not be easily given. It seems
that Werner has assumed that chiral octahedral coordination
centres undergo substitution reactions in which the configuration
is always retained (except for cases when racemization occurs). An
analogous statement for tetrahedral reaction centres had already
been dispelled by Walden in 1896 and 1897.2 For octahedral
centres the first observation of an inversion in a substitution
reaction was reported by John C. Bailar Jr in 19343 for the
reaction of chiral CoIII complexes (Scheme 1). This reaction has
been studied by W. G. Jackson4 in detail and it was shown that two
Cl− ligands are simultaneously substituted. But even 70 years after
Bailar’s report, a general mechanistic explanation of an inversion
process at an octahedral centre seems to be missing,4 whereas
inversion at tetrahedral centres is a well understood process.
The observation of opposite configurations at the metal centre
in the diastereoselective formation of tris-complexes with an
enantiopure derivative of bipyridine (−)-L (Scheme 2) prompted
us to investigate the stereochemical course of the reactions leading
to ML3 species. Special emphasis was put on the problem of
whether inversion could occur upon substitution at octahedral
centres under certain conditions with this type of ligand.
Scheme 2
Recently, interesting investigations on the diastereoselectivity of
the formation of tris-bpy type complexes have been published.25
These cases, however, are not directly comparable to our system,
since the selective interactions are mainly determined by polar
substituents of the bpy core.
Results and discussion
aDepartment of Chemistry, University of Fribourg, Chemin du Muse´e 9,
CH 1700, Fribourg, Switzerland. E-mail: Alexander.vonZelewsky@unifr.ch;
Fax: +41 26 300 9738; Tel: +41 26 300 8732
Ligand synthesis
bInstitute of Chemistry, University of Neuchaˆtel, Avenue de Bellevaux 51,
CH 2000, Neuchaˆtel, Switzerland
The ligand (−)-L 4,5-bis(pinene)-2,2ꢀ-bipyridine was prepared ac-
cording to a previously described strategy5 using double Kro¨hnke
cyclization6 of pyridinium salts (Scheme 3).
† Electronic supplementary information (ESI) available: Experimental
details for the synthesis of the (−)-L ligand. See DOI: 10.1039/b512116g
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Scheme 3 Synthesis of the ligand (−)-L. (i) Br2, 0 ◦C; (ii) 50% aq. NH2OH, 0 ◦C; (iii) pyridine, Et2O, rt; (iv) (−)-myrtenal, NH4OAc, formamide, rt, 5
d; (v) aq. HCl, reflux; (vi) I2, pyridine; (vii) (−)-myrtenal, NH4OAc, formamide, rt, 5 d.
The synthesis consists of the monobromination of butane-2,3-
dione (I),24 formation of the monooxime (III), and alkylation of
pyridine giving the first Kro¨hnke salt (IV), which undergoes a con-
densation reaction with (−)-myrtenal in presence of ammonium
acetate to yield the oxime intermediate (V). The latter is hydrolyzed
to ketone (VI), the second Kro¨hnke salt (VII) is formed, and
final cyclization with (−)-myrtenal gives the desired ligand (−)-L.
The protocol presented here (see ESI†) brings some improvement
to our formerly published approach,7 and it also represents an
alternative to a synthesis reported by Kocˇovsky´.8
three metals, the [M(bpy)3]2+ complexes formed as racemates, D-
[M(bpy)3]2+, and K-[M(bpy)3]2+ which can be resolved by chiral
auxiliaries. However, [Ru(bpy)3]2+ and [Os(bpy)3]2+ are highly
stable in their optically active forms, while D-[Fe(bpy)3]2+ and K-
[Fe(bpy)3]2+ racemize within minutes in aqueous solutions at room
temperature.9
Ligand (−)-L forms the complex [FeL3]2+ as a mixture of two
diastereoisomers, D-[FeL3]2+ (1a) and K-[FeL3]2+ (1b). The D-(1a)
diastereoisomer is the more abundant species in the reaction
product, and can be obtained in crystalline form, and its structure
was determined by X-ray diffraction. The iron centre is relatively
labile, and the isomeric ratio changed quite drastically upon
variation of the temperature and solvent. The isomerization of
D-[FeL3]2+ (1a) was observed by CD and NMR spectroscopy
after the pure crystalline diastereoisomer had been dissolved
Formation of the complexes and analysis of their configurations
A distinct difference in the behaviour of tris-diimine complexes
of the group 8 metals Fe(II), Ru(II) and Os(II) is the lability
with respect to ligand exchange reactions. For example, for all
1
at room temperature in various solvents. Fig. 1 shows the H
Fig. 1 1H NMR spectra of D/K-[FeL3]2+ in acetone-d6: (a) D-[FeL3]2+, 5 min after dissolving; (b) D-[FeL3]2+, equilibrated solution after ca. 1 h.
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NMR spectrum of a solution obtained by dissolving deep red
crystals of D-[FeL3](PF6)2 in acetone-d6 after ca. 5 min, and
after equilibration (ca. 1 h). The NMR pattern is relatively
simple due to the high symmetry (D3) of the complexes. The two
diastereoisomers are clearly discernible, and their assignment to
the two species D-[FeL3]2+ (1a), and K-[FeL3]2+ is straightforward.
The following diastereomeric ratios were observed in equilibrated
solutions (eqn (2)), after ca. 3 d at room temperature; D/K = 1.7
(acetone), 2.8 (DMSO), 6.3 (acetonitrile), and 25 (ethylene glycol).
surprisingly, shows a preference for either the D-, or the K-
configured species, depending on reaction conditions (Scheme 5).
Scheme 5
If [RuL3]2+ is prepared in a one step reaction starting from
Ru(DMSO)4Cl2 with a small excess of (−)-L (Ru : L = 1 : 3.3) in
ethylene glycol, the same ratio of K/D = 1.7 is obtained, as shown
in Fig. 3, which is the 1H NMR spectrum of the raw product after
K-[FeL3]2+ ꢀ D-[FeL3]2+
(2)
Fig. 2 shows the CD spectrum of an equilibrated solution of
D/K-[FeL3]2+ in acetonitrile at room temperature. The sign of the
exciton couplet in the p–p* region (275–320 nm) corresponds to
the contribution of the preponderant D-configured complex.10 The
CD activity in the MLCT region (430–580 nm) is much smaller,
but has the same sign.
−
precipitation of its PF6 salt.
Thus, under these conditions (reaction medium: ethylene gly-
col), the opposite configuration is preferred, as compared to the
[FeL3]2+ complex, whereas the same configuration is preferred
when the reaction is carried out starting from the predominantly
D-configured intermediate cis-[RuL2Cl2] in methanol.
Are these differences due to thermodynamic or kinetic control?
Could it be possible that in one of the substitution steps an inversion
occurs? In order to answer these two questions, several other
experiments were carried out.
Separation of the two diastereoisomers D-[RuL3]2+ (2a), and K-
[RuL3]2+ (2b) by crystallization led to the pure PF6 salts, which
−
could be completely characterised by X-ray structure analysis and
NMR, CD, and UV/VIS spectroscopy.
Fig. 4 shows the CD spectra of the two pseudo-enantiomers
D-[RuL3]2+ and K-[RuL3]2+, respectively.
Fig. 2 CD spectra of an equilibrated acetonitrile solution of [FeL3]2+ in
favour of D-diastereoisomer (D/K = 6.3).
A comparative discussion of the UV/VIS spectra of the FeII,
RuII, and OsII complexes will be given later in this publication.
Both D-, and K-[RuL3]2+ were dissolved in ethylene glycol and
heated in a pressure tube to 200 ◦C for one week. After that
time no change in the NMR spectra could be observed. Thus,
thermal equilibrium could not be established between the two
species. However, under the influence of relatively weak irradiation
in the visible region◦by a 50 W tungsten lamp, equilibrium has been
established at 135 C in ethylene glycol after ca. 7 d. A ratio of
K/D = 1.5 was reached from either side under these conditions
(Scheme 6).
Thus for FeII, D is the more stable diastereoisomer in all
solvents examined. The free energy differences vary between
1.32 kJ mol−1 for acetone, and 7.98 kJ mol−1 for ethylene glycol.
The temperature dependence of equilibrium (2) was determined by
NMR spectroscopy in acetonitrile-d3 over the range 0–40 ◦C. The
data fit a van’t Hoff plot with the values of DH◦ = − 8.9 kJ mol−1
and DS◦ = − 15 J mol−1 K−1. Although we were not able to
crystallize the thermodynamically less stable K-[FeL3]2+ (1b), we
can unambiguously assign the D-[FeL3]2+ (1a) complex as the
thermodynamically preferred product.
While the [FeL3]2+ complexes form at room temperature, and
equilibrium between diastereoisomers is reached within hours, as
reported above, RuII, and, as will be shown later, OsII complexes
can only be synthesized at higher temperatures. Convenient
starting materials for the RuII complexes are Ru(DMSO)4Cl2 and
Ru(MeCN)4Cl2, respectively. After reaction of these compounds
with 2 (−)-L in methanol, an intermediate compound cis-
[RuL2Cl2] can be isolated (Scheme 4).
Scheme 6
This photodiastereoisomerization is analogous to the well
known photoracemization of [Ru(bpy)3]2+.11 The activation of this
process is believed to proceed through a thermal population of a
3
3MC (metal-centred) state from the lowest excited MLCT state
of the complexes.11 The photon in Scheme 6 can be considered to
catalyse the reaction, leading to thermodynamic equilibrium.
Accordingly for [RuL3]2+, the K-configured species seems to
Scheme 4
Its CD spectrum indicates clearly a predominance of the
D-configured diastereoisomer. Further substitution of the two
remaining chloride ligands by (−)-L leads to [RuL3]2+, which,
be 1.38 kJ mol−1 more stable, which corresponds to a Kequilib.
=
1.5. Photocatalytic diastereoisomerization was repeated for D,K-
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−
Fig. 3 1H-NMR spectra of [RuL3]2+ in acetone-d6 (raw product after precipitation of its PF6 salt, K/D = 1.7).
Fig. 4 CD spectra of the two diastereomeric complexes D-[RuL3]2+ (black
line) and K-[RuL3]2+ (grey line).
[Ru(bpy)2(L-trp)]+ (where L-trp is the L-tryptophane anion) by
Williams et al.12 The process was called “photochemical inver-
sion”. In our opinion this rearrangement does notrepresent a
true inversion, as e.g. that observed by Bailar,3 but rather an
equilibration between two diastereoisomers.
Scheme 7
occurs, although, starting from D-[RuL2Cl2], an excess of K-
[RuL3]2+ is formed. This, however, is due to thermodynamic equi-
libration between diastereoisomeric species and not to a genuine
inversion process. The same can be stated for the photocatalysis
(Scheme 6), mentioned above, where the pure D form transforms
into an equilibrium mixture, which contains an excess of the K-
isomer. The difference in reactivity in the two solvents (methanol
and ethylene glycol) is due to the substitution of a chloride ligand
by MeOH, clearly indicated by a colour change from violet to red
when [RuL2Cl2] is dissolved in methanol. This can be also shown by
MS measurements (see Experimental). Thus, methanol facilitates
the leaving of the chloride ligand. Recently, the influence of the
solvent in isomerization reactions of RuII complexes has been
studied in several cases.26 A detailed non-speculative mechanistic
explanation in our system is not possible, however.
The occurrence of the intermediate complex cis-[RuL2Cl2]
mentioned above offers the possibility to probe deeper into the
stereoselectivity of the substitution reaction. The complex cis-
[RuL2Cl2] can be separated into its two diastereoisomers D-
, and K-[RuL2Cl2], respectively, by chromatographic methods
(see Experimental). Their substitution reactions under various
conditions are given in Scheme 7. All reactions proceed with
the indicated stereoselectivities of at least 75%, approaching
100% in several cases. It clearly emerges that the substitution of
two chloride ligands by (−)-L in methanol at 60 ◦C proceeds
in a kinetically controlled reaction with complete retention of
configuration. In ethylene glycol no reactivity is observed below
100 ◦C. Above this temperature both diastereoisomers, K- and D-
[RuL2Cl2], yield preferentially the K-configured [RuL3]2+ complex.
The most probable explanation of this outcome is the assumption
of a thermodynamically controlled process. Thus, no real inversion
The corresponding OsII complexes with (−)-L were prepared
according to Scheme 8. As expected, the reactions require
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[M(bpy)3]2+ (see also the ESI†). Table 2 (see Experimental) gives
the general results for the structure measurement, whereas Table 1
compares same specific measures within the coordination species.
As an example, an ORTEP28 presentation of D-[OsL3]2+ is given in
Fig. 6.
Scheme 8
a higher temperature than in the case of RuII. Reactions in
solvents with lower boiling points than ethylene glycol such as
methanol or ethanol did not yield the corresponding products. The
stereochemical outcome is very similar for OsII and RuII. Again,
both diastereoisomers give similar CD spectra (Fig. 5) and the
complexes appear to be thermally stable at 200 ◦C. In contrast◦to
the RuII complexes, the OsII compounds are even stable at 200 C
and under irradiation from a 1000 W lamp (the RuII complexes
photoisomerize under irradiation from a 50 W lamp).
Fig. 6 ORTEP presentation of D-[OsL3]2+
.
The values given in Table 1 indicate slight distortions in the
[ML3]2+ complexes, as compared to their [M(bpy)3]2+ analogues.
For all of the four parameters given (trans, chelate, prismatic and
antiprismatic), the distortions in the cases of D/Kdiastereoisomers
of [RuL3]2+ and [OsL3]2+ are in opposite directions from the
respective values for the [M(bpy)3]2+ complexes. Fo◦r example, the
trans angles are D-[RuL3]2+ 174.9◦; K-[RuL3]2+ 171.7 ; [Ru(bpy)3]2+
172.6◦; etc. Also, the distortions in the D-[FeL3]2+ have the same
signs as in D-[RuL3]2+ and D-[OsL3]2+. Thus, the “distortions” due
to the chiral pinene groups, albeit small, are highly systematic.
Fig. 5 CD spectra of the two diastereomeric complexes D-[OsL3]2+ (black
line) and K-[OsL3]2+ (grey line).
UV/VIS absorption spectra
These spectra are surprisingly variable, especially in the visible
region. The p–p* absorptions in the UV around 300 nm are given
in Fig. 7. They are almost identical for the D and Kpairs of [OsL3]2+
and [RuL3]2+(data are given in the Experimental). Fig. 8 gives the
MLCT bands, including the relatively strong spin forbidden triplet
absorption at long wavelength (OsII). While the characteristic
1MLCT transitions for D-[FeL3]2+, and D-[RuL3]2+ show the typical
shoulder, which is also observed in [Fe(bpy)3]2+ and [Ru(bpy)3]2+,
D-[OsL3]2+ exhibits a prominent splitting of this band, which is
We concluded that the OsII and RuII complexes behave very
similarly, albeit the activation energies are significantly higher for
OsII.
X-Ray structures
The data for the five complexes examined, D-[FeL3]2+, D-, and K-
[RuL3]2+, D-, and K-[OsL3]2+ are given in Tables 1 and 2 together
with data from literature for the corresponding tris-bpy species
Table 1 Average structural data (3 × trans, 3 × chelate, 3 × prismatic, 6 × antiprismatic angles; 6 × M–N bond lengths) indicating the distortion of
diastereomeric species with respect to the parent [M(bpy)3]2+ complexes
b
c
d
Anglea
D-[FeL3]2+
[Fe(bpy)3]2+
K-[RuL3]2+
D-[RuL3]2+
[Ru(bpy)3]2+
K-[OsL3]2+
D-[OsL3]2+
[Os(bpy)3]2+
trans/◦
173.4(10)
81.8(9)
93.5(7)
91.5(9)
1.966(2)
175.2(2)
81.6(1)
95.0(1)
88.7(1)
1.956(2)
174.9(1)
78.6(1)
97.8(1)
86.0(1)
2.055(2)
171.7(2)
79.0(1)
95.1(1)
91.3(1)
2.051(2)
172.6(2)
78.6(2)
95.7(12)
89.4(2)
175.0(3)
77.8(3)
98.6(2)
85.2(3)
2.057(5)
170.7(3)
78.2(3)
95.3(2)
91.9(3)
2.064(5)
172.4
chelate/◦
77.8(4)
96.3(3)
90.1(4)
2.056(8)
prismatic/◦
antiprismatic/◦
˚
M–N/A
2.053(2)
a Defined as in ref. 21. b Ref. 22. c Ref. 21. d Ref. 23.
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Fig. 10 VIS absorption spectra of K-[OsL3]2+ in acetonitrile and chloro-
form, respectively.
Fig. 7 UV absorption spectra of D-[FeL3]2+, D-[RuL3]2+, and D-[OsL3]2+
in acetonitrile.
Fig. 8 VIS absorption spectra of D-[FeL3]2+, D-[RuL3]2+, and D-[OsL3]2+
in acetonitrile.
Fig. 11 VIS absorption spectra of K-[RuL3]2+ in acetonitrile and chloro-
form, respectively.
also present in [Os(bpy)3]2+, but in a much less pronounced manner
(Fig. 9).
do not show this solvatochromic effect. Most of these phenomena
can be observed by the eye. While a D-[OsL3]2+ solution is brown,
K-[OsL3]2+ in acetonitrile and other polar solvents, is deep green.
In order to extend these investigations to other coordination
centres, preliminary measurements (CD and 1H NMR) were
carried out with the series of labile metal ions Zn(II), Cd(II), Cu(II),
and Ni(II). For all of these metals, CD spectra (see Experimental)
clearly indicate a diastereoselectivity in favour of the D isomer.
At room temperature, the diamagnetic ions Zn(II) and Cd(II)
yield observable 1H NMR spectra of the complexes [ML3]2+.
In the case of Zn(II), the two diastereoisomers show resolved
splittings indicating a value D/K = 1.5, whereas in the case of
Cd(II), ligand exchange obviously averages the signals of the two
diastereoisomers.
Conclusions
Fig. 9 VIS absorption spectra of (a) [Os(bpy)3]2+, (b) D-[OsL3]2+, and (c)
K-[OsL3]2+ in acetonitrile.
The C2-symmetric ligand (−)-L used in the present investigation
induces stereoselectivity in the formation of the chiral metal
complexes [ML3]2+ (M = Fe(II), Ru(II), Os(II), Zn(II), Cd(II),
Cu(II), Ni(II)). The diastereoselectivity is, however, quite weak,
representing energy differences between the D and K isomers
of few kJ mol−1. In all cases except Ru(II) and Os(II), the D
isomer is the thermodynamically preferred configuration, using
the (−)-L ligand. The detailed investigations of the reaction course,
Apparently, this must be due to the distortions observed
in the structures, since the diastereomeric K-[OsL3]2+ does not
show this splitting to the same extent. For the latter a distinct
solvatochromism is observed (Fig. 10), which is also present in
the K-[RuL3]2+ case (Fig. 11). On the contrary, the D complexes
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especially in the case of Ru(II) and Os(II) gave no evidence for true
inversion processes, although coordination species with opposite
configurations have been observed for intermediate species.
148.84 (C-3, aromatic CH), 147.49, 149.24, 159.32 (C-5, C-4, C-
2); K diastereoisomer: d 20.93 (C-12, endo CH3), 25.71 (C-13, exo
CH3), 31.44 (C-9, CH2), 33.48 (C-7, CH2), 39.39 (C-11), 40.40 (C-8,
CH), 45.26 (C-10, CH), 123.37 (C-6, aromatic CH), 150.02 (C-3,
aromatic CH), 147.28, 148.72, 158.87 (C-5, C-4, C-2). UV-VIS
(c = 2 × 10−5 mol dm−3, acetonitrile equlibrated solution): 267
(39 100), 277 (37 830), 309 (80 600), 366 (8960), 533 nm (12 420);
CD (c = 2 × 10−5 M, ethylene glycol): 319 (−296), 299 nm (150).
MS (ESI): m/z 1233.61 (62%, M+–PF6−), 544.32 (100%, M+–2 ×
PF6−). Anal.: Calcd for C72H84N6FeP2F12: C, 62.70; H, 6.14; N,
6.09; Found: C, 62.51; H, 5.83; N, 6.33.
Experimental
General
Unless otherwise specified, materials were purchased from com-
mercial suppliers and used without further purification. Ruthe-
nium trichloride and ammonium hexachloroosmate were pur-
chased from Johnson Matthey and Branderberger AG. NMR spec-
tra were measured on a Varian Gemini 300, Bruker DRX 500 or
Bruker Avance 400 in acetonitrile-d3 (d, 99.8%), dichloromethane-
d2 (d, 99.9%), chloroform-d (d, 99.8%), acetone-d6 (d, 99.9%),
ethylene glycol-d6 (d, 99%), or dimethylsulfoxide-d6 (d, 99.8%)
(deuterated solvents were purchased from Cambridge Isotope
Laboratories and Armar Chemicals (glycol)). Varian Gemini 300
(1H: 300.075 MHz), Bruker Avance 400 (1H: 400.13 MHz, 13C:
100.62 MHz), Bruker DRX 500 (1H: 500.13 MHz, 13C: 125.76
MHz). Chemical shifts (d scale, ppm) are given relative to the
internal Me4Si (1H, 13C) standard or the solvent itself was used
as internal standard. Assignment of the NMR signals is based
cis-[RuL2Cl2]. The freshly prepared [Ru(MeCN)4Cl2]
(18.4 mg, 0.055 mmol), ligand (−)-L (37.8 mg, 0.11 mmol),
and anhydrous lithium chloride were dissolved in dry methanol
(80 mL), and the mixture was refluxed for 40 h under argon. The
volume of the violet solution was reduced to 40 mL, a portion
of water was added (20 mL), and the polar phase was extracted
with toluene until the latter was nearly colourless. The organic
phase was dried over sodium sulfate, the solvent was evaporated,
and the remaining black powder was washed with hexane to
remove traces of non-coordinated ligand (−)-L giving the product
[RuL2Cl2] predominantly with D configuration at the ruthenium
centre (yield: 30 mg, 0.0035 mmol). This labile species has been
1
1
on H, 13C{ H}, 13C APT, COSY, 13C HMQC. UV/VIS spectra
were recorded on a Perkin–Elmer Lambda 25 spectrometer. CD
spectra were recorded on a Jasco J-715 spectropolarimeter. Mass
spectra were recorded on a Bruker Bio APEX II (ESI) and on
a VG-Instruments 7070E (FAB). A diffractometer STOE IPDS-
2 was used to record X-ray diffractions. The Ecole d’inge´nieurs
et d’architectes de Fribourg performed the elemental analysis.
[Ru(DMSO)4Cl2],13 [Ru(MeCN)4Cl2],14 and rac-[Os(bpy)3](PF6)2
1
characterized unambiguously by its H NMR, and MS spectra.
Elemental analysis yielded in most cases unsatisfactory results. 1H
NMR (300.075 MHz, CDCl3): d 0.18, 0.58, 0.71, 0.75, 0.90, 1.12,
1.20, 1.23, 1.30, 1.38, 1.40, 1.42, 2.30, 2.35, 2.50, 2.60, 2.78, 2.90,
2.93, 3.08, 3.20, 3.78, 6.80, 6.83, 7.69, 7.70, 7.85, 7.90, 9.03, 9.08.
MS (FAB): m/z 861.29 (100%, M+ + H). MS (ESI) (in MeOH):
m/z 825.35 (100%, [RuL2Cl]+), 857.38 (90%, [RuL2Cl(MeOH)]+),
860.38 (80%, M+); (in THF): m/z 860.38 (100%, M+).
15
were synthesized using literature procedures.
Separation of the diastereoisomers by column chromatography
(neutral aluminium oxide, isopropyl alcohol : hexane mixture
of 1 : 20) gave highly enriched complexes D-[RuL2Cl2] and K-
[RuL2Cl2] in a poor preparative yield of 18% ([RuL2Cl2] partially
reacted with Al2O3). UV-VIS: D diastereoisomer (c = 1.4 ×
10−6 mol dm−3, dichloromethane): 268 (25 800), 279 (23 600), 311
(48 000), 382 (8700), 550 nm (8000); K diastereoisomer (c = 1.9 ×
10−6 mol dm−3, dichloromethane): 267 (22 900), 278 (21 800), 312
(45 800), 380 (7700), 548 nm (6700). CD: D diastereoisomer (c =
1.4 × 10−6 mol dm−3, dichloromethane): 485 (9), 405 (−9.8), 319
(−105), 301 nm (49); K diastereoisomer (c = 1.9 × 10−6 mol dm−3,
dichloromethane): 480 (−6.5), 408 (6.8), 318 (59), 299 nm (−28).
Preparations
4,5-Bis(pinene)-2,2ꢀ-bipyridine-ligand (−)-L. was synthesized
according to improved procedure (see ESI†), and analytical data
agree with those previously described in literature.7,8
[FeL3](PF6)2. A mixture of the ligand (−)-L (50 mg, 0.15 mmol)
and (NH4)2Fe(SO4)2·6H2O (20 mg, 0.051 mmol) was heated
in 5 mL of a 9 : 1 mixture of ethylene glycol : water containing
one drop of 1 M HCl at 150 ◦C for 4 h. The resulting mixture was
cooled to 100 ◦C, and NH4PF6 (250 mg, 1.5 mmol) in 3 mL of water
was added. The solution was kept at 100 ◦C for 1 h, and allowed
to cool to room temperature. Deep red crystals of D-[FeL3](PF6)2
[RuL3](PF6)2. A mixture of the ligand (−)-L (50 mg, 0.15 mmol)
and [Ru(DMSO)4Cl2] (24 mg, 0.050 mmol) was heated in 5 mL
of a 9 : 1 mixture of ethylene glycol : water containing one
drop of 1 M HCl at 135 ◦C for 5 h. After cooling the solution
to 100 ◦C, a solution of NH4PF6 (250 mg, 1.5 mmol) in water
(3 mL) was added. Precipitated orange red complex [RuL3](PF6)2
was collected on a filter (yield: 65 mg, 91%). The raw product was
recrystallized from chloroform. The Kdiastereoisomer crystallized
first. After repeated crystallization D-[RuL3](PF6)2 was enriched
1
were filtered off in quantitative yield. H NMR (500.13 MHz,
acetone-d6); D diastereoisomer: d 0.76 (s, 3H-12, endo CH3), 1.08
(d, 1H-9, endo CH2, J = 10.0 Hz), 1.37 (s, 3H-13, exo CH3), 2.39
(m, 1H-8, CH), 2.48 (dd, 1H-10, CH, J = 5.5 Hz, J = 5.5 Hz), 2.66
(m, 1H-9, exo CH2), 3.27 (d, 2H-7, CH2, J = 2.4 Hz), 6.85 (s, 3H-3,
aromatic CH), 8.53 (s, 3H-6, aromatic CH); K diastereoisomer: d
0.28 (s, 3H-12, endo CH3), 1.23 (d, 1H-9, endo CH2, J = 9.9 Hz),
1.26 (s, 3H-13, exo CH3), 2.30 (m, 1H-8, CH), 2.56 (dd, 1H-10, CH,
J = 5.4 Hz, J = 5.4 Hz), 2.66 (m, 1H-9, exo CH2), 3.17 (ddd, 2H-7,
CH2, J = 66.9 Hz, J = 18.8 Hz, J = 2.6 Hz), 7.24 (s, 3H-3, aromatic
1
in the mother liquor. H NMR (500.13 MHz, acetone-d6); D-
diastereoisomer: d 0.76 (s, 3H-12, endo CH3), 1.08 (d, 1H-9, endo
CH2, J = 10.0 Hz), 1.38 (s, 3H-13, exo CH3), 2.38 (m, 1H-8, CH),
2.52 (dd, 1H-10, CH, J = 5.5 Hz, J = 5.5 Hz), 2.67 (m, 1H-9,
exo CH2), 3.25 (br s, 2H-7, CH2), 7.24 (s, 3H-3, aromatic CH),
8.52 (s, 3H-6, aromatic CH); K diastereoisomer: d 0.29 (s, 3H-12,
CH), 8.49 (s, 3H-6, aromatic CH). 13C{ H}NMR (125.76 MHz,
1
acetone-d6); D diastereoisomer: d 21.96 (C-12, endo CH3), 25.86
(C-13, exo CH3), 31.49 (C-9, CH2), 33.55 (C-7, CH2), 39.65 (C-11),
40.36 (C-8, CH), 45.90 (C-10, CH), 124.28 (C-6, aromatic CH),
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endo CH3), 1.24 (d, 1H-9, endo CH2, J = 9.9 Hz), 1.26 (s, 3H-13,
exo CH3), 2.31 (m, 1H-8, CH), 2.57 (dd, 1H-10, CH, J = 5.5 Hz,
J = 5.5 Hz), 2.66 (m, 1H-9, exo CH2), 3.16 (ddd, 2H-7, CH2, J =
59.2 Hz, J = 18.8 Hz, J = 2.9 Hz), 7.54 (s, 3H-3, aromatic CH),
(s, 3H-6, aromatic CH); K diastereoisomer: d 0.29 (s, 3H-12, endo
CH3), 1.24 (d, 1H-9, endo CH2, J = 10.0 Hz), 1.25 (s, 3H-13, exo
CH3), 2.29 (m, 1H-8, CH), 2.54 (dd, 1H-10, CH, J = 5.4 Hz, J =
5.4 Hz), 2.66 (m, 1H-9, exo CH2), 3.26 (ddd, 2H-7, CH2, J =
64.1 Hz, J = 18.7 Hz, J = 2.9 Hz), 7.42 (s, 3H-3, aromatic CH),
1
8.47 (s, 3H-6, aromatic CH). 13C{ H}NMR(125.76MHz, acetone-
1
d6); D diastereoisomer: d 21.82 (C-12, endo CH3), 25.87 (C-13, exo
CH3), 31.56 (C-9, CH2), 33.60 (C-7, CH2), 39.68 (C-11), 40.38
(C-8, CH), 45.65 (C-10, CH), 124.52 (C-6, aromatic CH), 146.27
(C-3, aromatic CH), 147.63, 148.10, 155.39 (C-5, C-4, C-2); K
diastereoisomer: d 20.97 (C-12, endo CH3), 25.72 (C-13, exo CH3),
31.46 (C-9, CH2), 33.51 (C-7, CH2), 39.67 (C-11), 40.41 (C-8,
CH), 45.04 (C-10, CH), 123.65 (C-6, aromatic CH), 147.59 (C-3,
aromatic CH), 147.30, 147.65, 156.50 (C-5, C-4, C-2). UV-VIS:
D diastereoisomer (c = 8.4 × 10−6 mol dm−3, acetonitrile): 265
(37 500), 273 (38 400), 297 (90 200), 465 nm (14300); K diaster-
eoisomer (c = 1.2 × 10−5 mol dm−3, acetonitrile): 265 (38 400), 275
(40 800), 301 (100 000), 452 nm (17 100); (c = 1.2 × 10−5 mol dm−3,
chloroform): 443 nm (15 000). CD: D diastereoisomer (c = 8.4 ×
10−6 mol dm−3, acetonitrile): 482 (−24), 429 (26), 310 (−190), 293
(162), 271 nm (−32); K diastereoisomer (c = 1.2 × 10−5 mol dm−3,
acetonitrile): 479 (10), 422 (−17), 309 (304), 288 nm (−202).
MS (ESI): m/z 1279.56 (88%, M+–PF6−), 567.31 (100%, M+–2 ×
PF6−). Anal.: Calcd for C72H84N6RuP2F12·2H2O: C, 59.21; H, 6.07;
N, 5.75; Found: C, 58.86; H, 6.15; N, 5.74.
8.46 (s, 3H-6, aromatic CH). 13C{ H}NMR(125.76 MHz, acetone-
d6); D diastereoisomer: d 21.76 (C-12, endo CH3), 25.88 (C-13, exo
CH3), 31.64 (C-9, CH2), 33.40 (C-7, CH2), 39.80 (C-11), 40.40
(C-8, CH), 45.58 (C-10, CH), 124.69 (C-6, aromatic CH), 145.08
(C-3, aromatic CH), 147.17, 148.11, 158.98 (C-5, C-4, C-2); K
diastereoisomer: d 20.97 (C-12, endo CH3), 25.73 (C-13, exo CH3),
31.55 (C-9, CH2), 33.35 (C-7, CH2), 39.50 (C-11), 40.41 (C-8,
CH), 44.96 (C-10, CH), 123.81 (C-6, aromatic CH), 146.72 (C-3,
aromatic CH), 146.89, 147.79, 158.41 (C-5, C-4, C-2). UV-VIS:
D diastereoisomer (c = 8.1× 10−6 mol dm−3, acetonitrile): 265
(41 100), 273 (39 800), 300 (99 000), 399 (11 400), 411 (11 300),
442 (14 300), 502 (14 600), 588 (4400), 657 (3800), 683 nm (4000);
K diastereoisomer (c = 5.3 × 10−6 mol dm−3, acetonitrile): 265
(51 000), 275 (49 200), 303 (136 200), 378 (15 000), 397 (15 600), 467
(20 000), 596 nm (5300); (c = 5.9 × 10−6 mol dm−3, chloroform):
378 (13 900), 397 (14 100), 452 (19 000), 587 (4500), 622 nm (4400).
CD: D diastereoisomer (c = 8.1 × 10−6 mol dm−3, acetonitrile):
508 (−34), 440 (42), 313 (−245), 296 (187), 263 nm (−24); K
diastereoisomer (c = 5.3 × 10−6 mol dm−3, acetonitrile): 489 (44),
435 (−41), 311 (299), 297 (−213), 263 nm (34). MS (ESI): m/z
1370.74 (50%, M+–PF6−), 612.36 (100%, M+− 2 × PF6−). Anal.:
Calcd for C72H84N6OsP2F12: C, 56.47; H, 5.59; N, 5.55; Found: C,
56.09; H, 5.67; N, 5.43.
cis-[OsL2Cl2]. A mixture of the ligand (−)-L (157 mg,
0.576 mmol) and (NH4)2OsCl6 (100 mg, 0.228 mmol) was heated
in 10 mL of a 9 : 1 mixture of ethylene glycol : water containing
one drop of 1 M HCl at 180 ◦C for 3 h. After cooling the
solution to 100 ◦C, a saturated water solution of sodium dithionite
(10 mL) was added. The purple-black precipitate that had formed
was isolated by filtration, washed with water to remove [OsL3]2+
and other ionic byproducts, and washed with large volumes of
hexane to give the product in 88% yield in favour of the D-
configured diastereoisomer. This species has been characterized
unambiguously by its 1H NMR, and MS spectra. Elemental
[ZnL3](PF6)2. A mixture of the ligand (−)-L (31 mg, 0.09 mmol)
and ZnCl2 (4 mg, 0.03 mmol) in ethanol (7 mL) was heated
under reflux for 2 h. A solution of NH4PF6 (250 mg) in water
(3 mL) was added, and the precipitate was collected on a filter
1
to yield the product (41 mg) in 98% preparative yield. H NMR
(400.13 MHz, acetone-d6); D diastereoisomer: d 0.71 (s, 3H-12,
endo CH3), 1.09 (d, 1H-9, endo CH2, J = 12.0 Hz), 1.40 (s, 3H-
13, exo CH3), 2.38 (m, 1H-8, CH), 2.53 (dd, 1H-10, CH, J =
5.5 Hz, J = 5.5 Hz), 2.68 (m, 1H-9, exo CH2), 3.20 (br s, 2H-7,
CH2), 7.09 (s, 3H-3, aromatic CH), 8.24 (s, 3H-6, aromatic CH); K
diastereoisomer: d 0.29 (s, 3H-12, endo CH3), 1.25 (d, 1H-9, endo
CH2, J = 12.0 Hz), 1.33 (s, 3H-13, exo CH3), 2.34 (m, 1H-8, CH),
2.65 (dd, 1H-10, CH, J = 5.5 Hz, J = 5.5 Hz), 2.68 (m, 1H-9, exo
CH2), 3.14 (m, 2H-7, CH2), 7.41 (s, 3H-3, aromatic CH), 8.12 (s,
3H-6, aromatic CH). UV-VIS: predominantly D diastereoisomer
(c = 5.4 × 10−5 mol dm−3, acetonitrile): 215 (72 800), 270 (35 300),
311 (53 900), 324 nm (46 500). CD: predominantly D diastereoiso-
mer (c = 5.4 × 10−5 mol dm−3, acetonitrile): 325 (−148), 301 nm
(41). MS (ESI): m/z 1241.57 (90%, M+–PF6−), 548.30 (100%, M+−
2 × PF6−). Anal.: Calcd for C72H84N6ZnP2F12: C, 61.50; H, 6.09;
N, 6.05; Found: C, 61.09; H, 6.20; N, 5.93.
1
analysis yielded in most cases unsatisfactory results. H NMR
(300.075 MHz, CDCl3): d 0.20, 0.55, 0.75, 0.78, 0.88, 1.11, 1.22,
1.24, 1.30, 1.37, 1.40, 1.42, 2.32, 2.37, 2.51, 2.62, 2.79, 2.92, 2.90,
3.10, 3.24, 3.77, 6.81, 6.83, 7.70, 7.72, 7.85, 7.90, 9.04, 9.09. UV-
VIS (c = 2.5 × 10−5 moldm−3, dichloromethane): 275 (32 800), 312
(29 700), 455 (8100), 577 nm (10 000). CD (c = 2.5 × 10−5 mol dm−3,
dichloromethane): 363 (−15), 320 nm (17), MS (ESI): m/z 950.33
(100%, M+).
[OsL3](PF6)2.
A
mixture of the ligand (−)-L (36 mg,
0.105 mmol) and cis-[OsL2Cl2] (100 mg, 0.105 mmol) was heated
in 5 mL of a 9 : 1 mixture of ethylene glycol : water containing
one drop of 1 M HCl at 180 ◦C for 5 h under argon. After cooling
◦
the solution to 100 C, a solution of NH4PF6 (250 mg) in water
(3 mL) was added. The precipitate was collected on a filter (yield:
142 mg, 90%). The diastereoisomers were separated by column
chromatography (neutral aluminium oxide, acetonitrile : toluene,
1 : 3) to yield pure K-[OsL3](PF6)2 and D-[OsL3](PF6)2 as deep
[CdL3](PF6)2. The same procedure used to prepare
[ZnL3](PF6)2 complex was applied, and CdCl2·2H2O (6 mg,
0.03 mmol) was used for this complex. The product was filtered
off (yield: 38 mg, 88%). 1H NMR (400.13 MHz, acetone-d6): d 0.59
(br s, 3H-12, endo CH3), 1.14 (d, 1H-9, endo CH2, J = 12.0 Hz),
1.39 (br s, 3H-13, exo CH3), 1.58 (m, 1H-8, CH), 2.37 (m, 1H-10,
CH), 2.69 (m, 1H-9, exo CH2), 3.16 (br s, 2H-7, CH2), 7.56
(s, 3H-3, aromatic CH), 8.14 (s, 3H-6, aromatic CH). UV-VIS:
predominantly D diastereoisomer (c = 3.5 × 10−5 mol dm−3,
1
green, and dark brown crystalline solids, respectively. H NMR
(500.13 MHz, acetone-d6); D diastereoisomer: d 0.77 (s, 3H-12,
endo CH3), 1.07 (d, 1H-9, endo CH2, J = 9.9 Hz), 1.37 (s, 3H-
13, exo CH3), 2.37 (m, 1H-8, CH), 2.49 (dd, 1H-10, CH, J =
5.4 Hz, J = 5.4 Hz), 2.66 (m, 1H-9, exo CH2), 3.25 (dd, 2H-7,
CH2, J = 5.0 Hz, J = 2.7 Hz), 7.20 (s, 3H-3, aromatic CH), 8.48
1452 | Dalton Trans., 2006, 1444–1454
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acetonitrile): 213 (87 900), 264 (32 600), 308 (53 900), 322 nm
(42 500). CD: predominantly D diastereoisomer (c = 3.5 ×
10−5 mol dm−3, acetonitrile): 324 (−71), 298 nm (18). MS (ESI):
m/z 1289.52 (33%, M+–PF6−), 573.29 (100%, M+− 2 × PF6−).
Anal.: Calcd for C72H84N6CdP2F12: C, 60.23; H, 5.90; N, 5.85;
Found: C, 59.98; H, 5.95; N, 5.47.
D-[RuL3](PF6)2 (2a). The compound crystallized with two
PF6− anions, one being strongly disordered with occupancies of 0.5
for all fluorine atomic positions (F7 to F12, and F7A to F12A),
and five chloroform molecules. Two of these solvent molecules
are disordered, having occupancies of 0.5 for Cl1, Cl1A, Cl2,
Cl2A, Cl4, Cl4A, Cl5, Cl5A, Cl6, Cl6A, Cl7, Cl7A, Cl8, Cl8A,
−
C56 and C56A. P–F distances for the disordered PF6 anion
[NiL3](PF6)2. The same procedure used to prepare
[ZnL3](PF6)2 complex was applied, and NiCl2·6H2O (7 mg,
0.03 mmol) was used for this preparation. The light pink product
was filtered off (yield: 35 mg, 84%). UV-VIS: predominantly D
diastereoisomer (c = 3.4 × 10−5 mol dm−3, acetonitrile): 220
(80 200), 265 (46 900), 311 (52 200), 324 nm (45 200). CD:
predominantly D diastereoisomer (c = 4.4 × 10−5 mol dm−3,
acetonitrile): 326 (−165), 298 nm (34). MS (ESI): m/z 1235.57
(41%, M+–PF6−), 545.31 (100%, M+− 2 × PF6−). Anal.: Calcd for
C72H84N6NiP2F12: C, 61.85; H, 6.13; N, 6.08; Found: C, 61.19; H,
6.18; N, 5.99.
and C–Cl distances for the disordered chloroform molecules were
constrained to their theoretical values20 with estimated standard
deviations of 0.05. No absorption correction was applied.
K-[RuL3](PF6)2 (2b). This compound crystallized with two
−
PF6 anions, one being strongly disordered with occupancies of
0.5 for all of the fluorine atomic positions (F1 to F6, and F1A to
F6A), and six chloroform molecules, one of them being disordered
(occupancy 0.75 for C56, Cl8, Cl9 and 0.25 for C56A, Cl8A
and Cl9A). All atoms having partial occupancies were refined
isotropically. P–F distances in the disordered PF6− anion and C–Cl
distances in the disordered chloroform molecule are constrained
to their theoretical values20 with estimated standard deviations of
0.05. No absorption correction was applied.
[CuL3](PF6)2. The same procedure used to prepare
[ZnL3](PF6)2 complex was applied, and CuCl2·2H2O (5 mg,
0.03 mmol) was used for this preparation. The green product
was filtered off (yield: 37 mg, 90%). UV-VIS: predominantly D
diastereoisomer (c = 4.4 × 10−5 mol dm−3, acetonitrile): 215
(76 700), 264 (46 900), 312 (40 400), 324 nm (44 700). CD:
predominantly D diastereoisomer (c = 4.4 × 10−5 mol dm−3,
acetonitrile): 327 (−37), 305 nm (12). MS (ESI): m/z 1240.61
(70%, M+–PF6−), 547.80 (100%, M+–2xPF6−). Anal.: Calcd for
C72H84N6CuP2F12: C, 61.67; H, 6.10; N, 6.06; Found: C, 61.24; H,
6.22; N, 5.90.
D-[OsL3](PF6)2 (3a). For this compound the atoms of the
−
counterions PF6 were refined isotropically. A semiempirical ab-
sorption correction was applied using MULABS (PLATON99,18
Tmin = 0.632, Tmax = 0.830).
K-[OsL3](TfO)2 (3b). For X-ray structural analysis K-
[OsL3](PF6)2 was converted to the triflate K-[OsL3](TfO)2 via water
soluble chloride salt (via Dowex 1 × 2–100 ion exchange resin),
and precipitation with lithium triflate. The molecular formula of
this compound is [Os(C24H28N2)3](CF3SO3)2 (C3H6O)4 . As a result
of the high disorder found in the anions and the solvent molecules,
the SQUEEZE instruction in PLATON033 was used to calculate
Crystal structure determinations
Intensity data were colleted using a Stoe Imaging Plate Diffrac-
tometer System (Stoe & Cie, 1995) equipped with a one-circle u
goniometer and a graphite monochromator. Data collection was
the potential accessible area for anions and solvent molecules
3
˚
in the unit cell; 2095.0 A were calculated containing ca. 560
electrons. Therefore, two molecules of trifluoromethanesulfonate
(2 × 73 electrons), and four acetone molecules (4 × 32 electrons)
per asymmetric unit were included in all further calculations. The
highest residual peak of 2.02 e A−3 was located near to the osmium
˚
atom. The thermal parameters of the non-hydrogen atoms in the
ligands were constrained to be equal. A semiempirical absorption
◦
◦
performed at −50 C (−100 C for 3b), using Mo Ka radiation
˚
(k = 0.71073 A). 190 or 200 exposures (3 min per exposure) were
obtained at an image plate distance of 70 mm with 0 < u <
190 or 200◦ and with the crystal oscillating through 1◦ in u. The
˚
resolution was Dmin–Dmax 12.45–0.81 A. The structures were solved
by direct methods using the program SHELXS-9716 and refined
by full matrix least squares on F2 with SHELXL-97.17 The non-
hydrogen atoms were refined anisotropically, unless stated below.
The hydrogen atoms were included in calculated positions and
treated as riding atoms using SHELXL-97 default parameters.
The atomic coordinates of the various structures correspond to
the absolute structures of the molecules in the crystals. This is
indicated by the refined Flack27 x parameters and also corresponds
to the absolute structure of the ligand used. Structures 1a and
2a have the same space group and similar cell parameters and
appear to be isostructural. More crystallographic details are given
in Table 2 and significant bond lengths and angles are listed in
Table 1. The figures were drawn with the program PLATON.18
correction was applied using MULABS (PLATON03,19 Tmin
0.576, Tmax = 0.781).
=
CCDC reference numbers: 281496 (D-[FeL3](PF6)2), 281497 (D-
[RuL3](PF6)2), 281498 (K-[RuL3](PF6)2), 281499 (D-[OsL3](PF6)2),
and 281500 (K-[OsL3](TfO)2).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b512116g
Acknowledgements
This work was supported by the Swiss National Science Founda-
tion.
D-[FeL3](PF6)2 (1a). This compound crystallized with one
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This journal is
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Dalton Trans., 2006, 1444–1454 | 1453
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View Article Online
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1454 | Dalton Trans., 2006, 1444–1454
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