Fac-Specific syntheses of homochiral [Fe(NN′)3] 2+ complexes (NN′ = pyridine keto-hydrazone); Origins of the stereoselectivity

Article abstract of DOI:10.1039/c000815j

The 2-pyridinehydrazones (from condensation of pyridine-2-carbaldehyde and hydrazines) have previously been noted to have poor ligating ability as a result of a sterically demanding planar conformation. Destabilisation of this conformation is achieved through simple use of the ketohydrazones, and as a result the diamagnetic chiral tris-bidentate diimine complexes fac-[FeL ₃]²⁺ are readily isolated. In the solid state, inter-ligand π-π stacking and complex/counter-ion H-bonding are apparent, and these features persist in solution according to dynamic NMR spectra, which also indicate extremely high stereoselectivity for the fac isomers (>200:1). The compounds crystallise as conglomerates, and time-resolved CD spectra of non-racemic samples indicate a high degree of persistence of chirality (racemisation t_1/2ca 77 min). Variations of solvent and counter-ion indicate that H-bonding is unimportant in determining the structure of the cation. The fac-selectivity arises in the induction of a chiral conformation in the coordinated ligand, and the fact that such non-planar ligands can only be accommodated about the Fe(ii) centre if they all have the same absolute configuration. Adding a hydrazine N-methyl group increases the steric demand further, while retaining the novel non-planar conformation, and as a result paramagnetic chiral bis-bidentate complexes such as [FeL⁷ ₂(CH₃CN)₂]²⁺ are readily available.

Full text of DOI:10.1039/c000815j

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PAPER
www.rsc.org/dalton | Dalton Transactions
fac-Specific syntheses of homochiral [Fe(NN¢)3]2+ complexes (NN¢ = pyridine
keto-hydrazone); origins of the stereoselectivity†
Cynthia Paul Sebli, Suzanne E. Howson, Guy J. Clarkson and Peter Scott*
Received 13th January 2010, Accepted 17th March 2010
First published as an Advance Article on the web 1st April 2010
DOI: 10.1039/c000815j
The 2-pyridinehydrazones (from condensation of pyridine-2-carbaldehyde and hydrazines) have
previously been noted to have poor ligating ability as a result of a sterically demanding planar
conformation. Destabilisation of this conformation is achieved through simple use of the
ketohydrazones, and as a result the diamagnetic chiral tris-bidentate diimine complexes fac-[FeL3]2+ are
readily isolated. In the solid state, inter-ligand p–p stacking and complex/counter-ion H-bonding are
apparent, and these features persist in solution according to dynamic NMR spectra, which also indicate
extremely high stereoselectivity for the fac isomers (>200 : 1). The compounds crystallise as
conglomerates, and time-resolved CD spectra of non-racemic samples indicate a high degree of
persistence of chirality (racemisation t1/2 ca 77 min). Variations of solvent and counter-ion indicate that
H-bonding is unimportant in determining the structure of the cation. The fac-selectivity arises in the
induction of a chiral conformation in the coordinated ligand, and the fact that such non-planar ligands
can only be accommodated about the Fe(II) centre if they all have the same absolute configuration.
Adding a hydrazine N-methyl group increases the steric demand further, while retaining the novel
non-planar conformation, and as a result paramagnetic chiral bis-bidentate complexes such as
[FeL7 (CH3CN)2]2+ are readily available.
2
Introduction
Stereochemically pure tris-chelate compounds are of great cur-
rent interest, not least because of the potential for applica-
tions in supramolecular chemistry,1-4 molecular sensors,5,6 anion
recognition,7,8 DNA targeting9-12 and protein probes.13 Synthesis of
such compounds is however very challenging principally because
of the number of possible isomers present.
In a standard synthesis of a tris-chelate M(a-b)3 complex, the
number of possible orientations leads to an expected mer : fac ratio
of 3 : 1. In addition, the mer and fac isomers are chiral with respect
to the configuration at the metal (D/K). Separation of the four pos-
sible isomers has been achieved for inert systems such as Ru(II) by
crystallisation, chromatography and other techniques,14-17 but this
is laborious, inherently low yielding, subject to racemisation and
leads to uncertainty in optical purity.18 For labile metal systems
there exists the possibility of thermodynamic diastereoselection,
and we recently showed that simple diimine ligands I derived
from 2-iminopyridine and e.g. phenylglycinol gave optically and
stereochemically pure fac tris complexes [Fe(I)3]2+ (X = H, OH,
19
≡
OMe, OCH2Ph, OCH2Py, OCH2C CH) with dr >200 : 1. The
origins of this unprecedented stereoselectivity are evident from
(i) molecular models of the unobserved isomers which show
Numerous coordination compounds of pyridinylhydrazones II
unfavourable steric interactions and (ii) the molecular structure
determinations which show favourable arene p–p stacking.
have been reported.20-27 We asked the question: might phenylhydra-
zones IIa—notionally derived from substituting the stereogenic
CH in I with “N:”—give exclusively the racemic fac series
[M(IIa)3]2+ for the same reasons? This would be particularly
interesting because of the possibility that such complexes could
be formed enantioselectively in the presence of chiral non-
Department of Chemistry, University of Warwick, Gibbet Hill Road,
Coventry, UK CV4 7AL. E-mail: peter.scott@warwick.ac.uk; Fax: +44
(0)24 7657 2710; Tel: +44 (0)24 7652 3238
racemic anions, and thereby act as chiral sensors reporting
in the visible (charge transfer) region. In an early insightful
† CCDC reference numbers 761423–761425. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c000815j
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study of the coordination chemistry of hydrazones II28 Busch
prepared a range of complexes, and while no structural data or
NMR studies were possible, it was convincingly concluded from
synthetic observations, IR and UV data that the anomalously poor
coordinating ability of IIb was due to there being a substantial
contribution from the polar resonance form shown in Fig. 1(a).
This would be prevalent on coordination to a positively charged
metal. The resulting planarisation of the alkylated N atom would
project a methyl group towards the metal centre causing untenable
steric compression in the target complexes. Busch also asserted
that the inability of phenylhydrazone IIa to form tris complexes
resulted from similar steric effects. We fully agree (vide infra)
but nevertheless report here that as a result of some readily
implemented ligand design, tris complexes of phenylhydrazones
with Fe(II) are accessible. These complexes show extraordinary
stereoselectivity for the fac structures, are isolated in non-racemic
form, and are unusually inert to inversion of stereochemistry at
Fe(II). The origins of the stereoselectivity are investigated using
X-ray crystallography, NMR, circular dichroism and synthetic
studies.
L1 and L2 with Fe(II) perchlorate in acetonitrile also gave brown
solutions and solids. Attempted crystallisation of products via
e.g. addition of anti-solvents (diethyl ether, ethyl acetate) to
the solution gave no precipitation but led instead to formation
of paler yellow solutions. An examination of simple molecular
models confirms Busch’s assertion that there is some difficulty
in arranging three such ligands around a first row metal. We
proposed however that for the ketohydrazone series L3–7 since
there would be substantial intra-ligand steric compression in
the planar conformation [see Fig. 1(b)], a non-planar structure
would be favoured. We were thus pleased to find that the reaction
of Fe(ClO4)2·6H2O with three molar equivalents each of 2-
acetylpyridine and phenylhydrazine in acetonitrile gave an intense
purple solution, and the purple diamagnetic solid [FeL3 ][ClO4]2
3
was isolated in good yield on addition of ethyl acetate. Large
single crystals were obtained in another experiment by adding a
small amount of ethyl acetate to the reaction mixture and leaving
to stand overnight. The tetrafluoroborate salt [FeL3 ][BF4]2 was
3
prepared and crystallised in a similar manner. We were not able to
isolate the analogous Zn(II) complexes.
The crystal of [FeL3 ][ClO4]2 selected for X-ray diffraction had
3
the chiral space group P63 and contained the fac-K isomer only,
i.e. the compound crystallises as a conglomerate. The molecular
structure is shown in Fig. 2 along with key bond lengths and angles.
The asymmetric unit contains one pyridyl hydrazone ligand, one
third of an Fe atom, one third of a ClO4 unit, and a further ClO4
refined at a one third occupancy to balance the charge on Fe and
to give it meaningful thermal parameters. The Fe sits on a three-
fold axis as do Cl(1) and O(2) of the perchlorate. In the unit cell
there are two complexes, two perchlorates on the three-fold axis,
Fig. 1 Resonance and conformational issues in pyridinylhydrazone
proligands.
Results and discussion
Synthesis and characterisation of ligands
The pyridinylarylhydrazone ligands used in this study L1–7 were
synthesised using slight modifications of literature procedures.27-36
NMR spectra of samples in d3-acetonitrile and other data are
nevertheless reported herein for comparison with the subsequent
complexes.
Fig. 2 Molecular structure of fac-K-[FeL3 ][ClO4]2 viewed approximately
3
down the C3 axis, showing tripodal arrangement of the hydrazone
NH units, attendant bifurcated hydrogen bonds with ClO4 ion, and inter-
ligand p-stacking. Hydrogen atoms not involved in H-bonding have been
◦
˚
removed for clarity. Selected bond length (A) and angles ( ): Fe(1)-N(2)
1.946(3), Fe(1)-N(1) 1.978(3), N(2)-N(3) 1.409(4), N(2)-Fe(1)-N(2B)
95.38(12), N(2)-Fe(1)-N(1) 80.36(11), N(2B)-Fe(1)-N(1) 89.06(11),
N(2A)-Fe(1)-N(1) 174.14(10), N(1)-Fe(1)-N(1A) 95.47(12). Hydrogen
◦
˚
bond lengths (A) (DH ◊ ◊ ◊ A) and angles ( ) (<DHA): N(3)-H(3) ◊ ◊ ◊ O(1)
2.19, N(3)-H(3) ◊ ◊ ◊ (O1A) 2.41, N(3)-H(3)-O(1) 143.3, N(3)-H(3)-O(1A)
139.5.
Robinson and Busch28 mentioned an impure brown paramag-
netic bis complex [FeL1 ]I2. In our hands treatment of ligands
2
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and six perchlorates at a third occupancy, hence two complexes
and four perchlorates in total.
The molecular structure for this pyridine hydrazone system is
analogous to that observed for the optically and diastereomerically
pure a-methylbenzylimine series e.g. [Fe(I)3]2+ with fac coordina-
tion of the bidentate ligand,19 but is nevertheless unusual since
reported structures of M(a-b)3 complexes are dominated by the
presence of mer isomers.
Three inter-related features are apparent from examination of
these structures: (i) as for [Fe(I)3]2+, p–p interactions are present
between each of the three phenyl groups and the pyridine ring
of an adjacent ligand.19 The closest interatomic contact C(5)-
˚
C(8) is ca 3.25 A and the angle between mean pyridine and
phenyl planes involved in the p-stacking is ca 19◦. The centroid -
˚
centroid distance is ca 3.83 A; (ii) the mean plane of this
p-stacked hydrazone phenyl ring is also nearly orthogonal to
that of the pyridine ring in the same ligand [inter-plane angle ca
79◦]. This minimises the steric interactions with acetylpyridine
hydrazone methyl group C(7) as shown in Fig. 1(b); (iii) the
three fold arrangement of hydrazone NH units at N(3) present a
complimentary H-bonding arrangement to the three oxygen atoms
on one face of the perchlorate tetrahedron. Each of these oxygen
atoms is held in a bifurcated arrangement with two H atoms of the
NH triad, consisting of one close contact and one slightly longer
interaction with a neighbouring NH. In addition, the oxygen O(2)
of the perchlorate ion has a three point interaction with the a
pyridyl CH of the chelated pyridines from a neighbouring complex
Fig. 4 Molecular structure of fac-K-[FeL3 ][BF4]2 viewed approxi-
3
mately down the C3 axis. See also Fig. 2 and 3 o◦f isomorphic
3
˚
fac-K-[FeL ][ClO4]2. Selected bond length (A) and angles ( ): Fe(1)-N(2)
3
1.957(3), Fe(1)-N(1) 1.967(3), N(2)-N(3) 1.409(4), N(2)-Fe(1)-N(2B)
94.13(14), N(2)-Fe(1)-N(1) 80.97(13), N(2B)-Fe(1)-N(1) 89.77(13),
N(2A)-Fe(1)-N(1) 173.96(13), N(1)-Fe(1◦)-N(1A) 95.40(14). Hydrogen
˚
bond lengths (A) (DH ◊ ◊ ◊ A) and angles ( ) (<DHA): N(3)-H(3) ◊ ◊ ◊ F(1)
2.14, N(3)-H(3) ◊ ◊ ◊ F(1A) 2.38, N(3)-H(3)-F(1) 145.9, N(3)-H(3)-F(1A)
139.4.
˚
in the crystal [C–H ◊ ◊ ◊ O(2) 2.513 A]. An infinite chain is thus
formed of alternating perchlorate and complex ions along the
three fold axis (Fig. 3).
Fig.
5
CD spectra of fac-DFe-SC-[Fe(I)3][ClO4]2 (X
=
H) and
fac-DFe-[FeL3 ][BF4]2.
3
in all regions, consistent with the solutions structures being, as
expected, very similar. The decay in the CD signal of fac-DFe-
[FeL3 ][ClO4]2 was monitored for the full spectrum as shown in
3
Fig. 6, and also by acquiring spectra over 5 h in the 500 nm region.
The data were analysed following Brunner.37 The log plot shown
in Fig. 6 is consistent with the expected unimolecular kinetics
for isomerisation of the complex, with an observed rate constant
kobs = 2.82 ( 0.07)¥10-4 s-1. This corresponds to t1/2 for the decay
of 77 ( 2) min.
Fig. 3 Catenation of ions in the crystal structure of fac-K-[FeL3 ][ClO4]2.
Bifurcated N(3)-H ◊ ◊ ◊ O(1) and C(1)-H ◊ ◊ ◊ O(2) hydrogen bonding motifs
3
indicated.
The crystal structure of [FeL3 ][BF4]2 (Fig. 4) is isomorphic with
The 1H and 13C NMR spectra of [FeL3 ][ClO4]2 and
3
3
that of [FeL3 ][ClO4]2, and the crystal selected also contained the
[FeL3 ][BF4]2 in dry acetonitrile were essentially superimposable,
3
3
complex in K configuration. The hydrogen bonding motifs were
also analogous.
with the exception of the hydrazinimino NH signals which were
-
-
observed at 7.89 (ClO4 salt) and 7.77 ppm (BF4 salt) compared
to the free ligand at 8.26 ppm. Most importantly, only one set
of coordinated ligand peaks were observed. We noticed however
that in the spectra at ca 298 K there was significant broadening
of resonances. VT NMR studies were carried out in the accessible
range 233-343 K (Fig. 7). At 343 K all signals were sharp, and on
We were readily able to isolate optically pure crystals of fac-
[FeL3 ][BF4]2 from the racemic mixture. The circular dichroism
3
(CD) spectra of one such sample fac-DFe-[FeL3 ][BF4]2 in acetoni-
3
trile are shown in Fig. 5 along with that of authentic fac-DFe-SC-
[Fe(I)3][ClO4]2 (X = H).19 The spectra have very similar features
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H-bonding. The fac structure observed in the solid state also
thus persists in solution, and indeed there is an unusually high
thermodynamic preference for this structure, with a fac : mer ratio
>200 : 1.
Probing the importance of H-Bonding. In order to probe
the importance of H-bonding on the observed stereoselectivity
we have further varied the counter-ion, examined the effects
of solvents and attempted some electronic variation of the
hydrazone unit.
On mixing three equivalents of L3 in d3-acetonitrile with
[Fe(CH3CN)6][BPh4]2,‡ containing the more weakly coordinat-
ing tetraphenylborate anion, the complex fac-[FeL3 ][BPh4]2 was
3
1
formed exclusively. The H-NMR spectra obtained were almost
Fig. 6 Racemisation of fac-KFe-[FeL3 ][ClO4]2 in acetonitrile at 298 K.
3
identical to those above. The NH resonance was observed at
7.75 ppm. Unfortunately, we were unable to crystallise this
complex.
NMR spectra of these compounds obtained in a range of
solvents also showed similar signals pattern, although in the case
of protic systems there was the expected competitive solvolysis.
For example, a sample of pure [FeL3 ][ClO4]2 was dissolved in dry
lowering the temperature some broadening occurred throughout
but much more so for those resonances assigned to the phenyl
meta and ortho substituents Ha/a¢ and Hb/b¢. The signal for Hc was
relatively unaffected. At 233 K, the lower limit imposed by the
melting point of the solvent, the signals for Ha and Hb broadened
into the baseline and very broad new peaks are detectable for
Ha/Ha¢ (ca 7.2 and 6.55 ppm) and Hb/Hb¢ (ca 7.2 & 5.2 ppm).
The CD studies above show that coordination isomerisation (e.g.
fac/mer-D/K exchange) processes are occurring several orders
of magnitude slower than the NMR chemical shift timescale,
and are thus excluded as a mechanism here. Restricted NH–
CPh bond rotation is thus responsible for averaging the magnetic
environments of Ha/Ha¢ and Hb/Hb¢. The origins of this behaviour
are apparent from the structural studies above; p–p coordination,
the steric effect of the acetyl methyl group and the inter-ion
3
1
d4-methanol to give a purple solution. The H NMR spectrum
acquired after a few minutes showed a mixture of the complex
and free ligand (as compared with an authentic sample). After a
few hours the complex had fully decomposed. Nevertheless, the
NMR spectra of the complex did not show the presence of mer
isomers.
‡ Similar compounds have previously been mentioned.41-43 [Fe(MeCN)6]-
[BPh4]2 was synthesised analogously to [Fe(MeCN)6][B{C6H3(CF3)2}4]2
(see experimental section).41
Fig. 7 NH/aromatic region of VT 1H NMR spectra of [FeL3 ][ClO4]2, in d3-acetonitrile (inset—structure of [FeL3 ]2+).
3
3
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The p-nitro compound L4 was synthesised successfully, but on
treatment with Fe(II) salts deposited an insoluble yellow solid.
The p-methoxy compound L5 has previously been described,38
but in our hands (and using the published method) the orange
salt [HL5]Cl·H2O was obtained, according to microanalysis. The
1H-NMR spectrum in d6-DMSO was the same as that previously
assigned to L5.38 Attempts to synthesise Fe(II) complexes of this
ligand under suitably basic conditions were unsuccessful.
Conclusions
The poor ligating ability of hydrazones L1 and L2, previously
proposed to result from the high steric demand of their planar
conformation [Fig. 1(a)], is confirmed here. Destabilisation of
this conformation as shown in Fig. 1(b) is achieved through
use of ketohydrazones such as L3-7 and this in turn allows
the ready isolation of chiral tris complexes fac-[FeL3 ]2+. The
3
solid state structures from X-ray crystallography indicate three
related features; inter-ligand p–p stacking, the steric effect
of acetyl hydrazone methyl group and complex/counter-ion
H-bonding. These features persist in solution, and there is very
high stereoselectivity for the fac isomers (as indicated by dynamic
NMR behaviour). The degree of persistence of chirality for these
compounds (as demonstrated by a measured racemisation t1/2 of
ca. 77 min) is also striking for a labile Fe(II) centre. Given that the
Further ligand modifications. In confirmation of a previous
report,39 treatment of Fe(II) with L6 led to diamagnetic red bis
tridentate complexes [FeL6 ][ClO4]2 and [FeL6 ][BF4]2 regardless
2
2
of the stoichiometry. The yellow Zn(II) compounds [ZnL6 ][ClO4]2
2
and [ZnL6 ][BF4]2 were also isolated here.
2
In contrast with the unpromising behaviour of L2, treatment
of Fe(ClO4)2 in acetonitrile with ketohydrazone L7 forms a
red/purple solution from which racemic crystals of the octahedral
-
stereoselectivity persists in methanol and is the same for the BPh4
C2-symmetric bis complex [FeL7 (NCMe)2][ClO4]2[MeCN]2 are
salt, it would seem that H-bonding is unimportant in determining
the structure. The origin of the fac-selectivity is thus primarily
steric; the presence of the hydrazone C-methyl group induces an
element of chirality in the coordinated ligand (the conformation
is chiral) and the only sterically tenable combination of three such
ligands around the metal is homochiral. In other words, L3 behaves
like a prochiral version of optically pure I.19
The stability of the octahedral tris complexes is nevertheless
finely balanced: Zn(II) complexes, for which there would be no
OSPE, are not isolated, despite the existence of related [Zn(I)3]2+,19
and electron withdrawing (NO2) substituents in the hydrazone are
not tolerated. The presence of ligating atoms in the hydrazone unit
of L6 leads in contrast to stable, diamagnetic octahedral bis
complexes of both Fe(II)39 and Zn(II).
2
readily isolated. The molecular structure of this compound is
shown in Fig. 8. The Fe atom lies on a two-fold axis. The cis
angles for acetonitrile [N(4)-Fe–N(4A)] and hydrazone [N(2)-Fe–
N(2A)] ligands are 89.66(12) and 95.29(12)◦ respectively, with
the trans pyridine angle [N(1)-Fe–N(1A)] of 177.11(13)◦. In a
similar manner to the structures of L3 complexes above, the
hydrazone unit is non-planar with torsion angle C(6)-N(2)-N(3)-
C(8) of ca 82.4◦. As a result, the non-bonded p-orbital of N(3)
cannot be considered to be in conjugation with the imine and
this atom is correspondingly pyramidalised (sum of angles 349◦).
Again, as for the tris structures, there is reciprocal inter-ligand
p-stacking between phenyl groups and adjacent pyridines. The
˚
closest atomic contact N(1)-C(9A) is ca 3.05 A.
Finally, the more sterically-demanding N-methyl sy-
stem L7 gives
a fascinating paramagnetic chiral complex
[FeL7 (CH3CN)2]2+. There is also a fine balance here in terms
2
of the magnetic properties of the bis(diimine) systems, with
[FeL7 (CH3CN)2]2+ and the previously reported [Fe(I)2]2+ (X =
2
OH) being paramagnetic while [FeL6 ]2+ is diamagnetic. It may
2
be possible to exploit this in the synthesis of chiral spin-crossover
materials.
Experimental section
Starting materials for ligand syntheses were purchased from
Sigma Aldrich and Acros Organics and used without further
purification. Schlenck techniques were applied in handling the
air sensitive compounds. 2-Acetylpyridine phenylhydrazone (L3)
was synthesised as previously reported.23,27,30
NMR spectra were recorded at 298 K on a Bruker DPX300 and
Bruker DPX400 spectrometers. VT NMR spectra were obtained
by Dr Adam J. Clarke on Bruker DRX500. Proton and carbon
NMR assignments were routinely confirmed by 1H-13C (HMQC)
and COSY experiments. Elemental analyses were performed by
Warwick Analytical Services, Coventry, UK and Medac Limited,
Surrey, UK. Mass Spectra were recorded on Bruker Esquire2000
and micrOTOF by the Department of Chemistry Mass Spectrom-
etry Service, University of Warwick. IR data were recorded on
Perkin Elmer Spectrum 100. CD spectra were recorded at 298 K on
Jasco J-815 circular dichroism spectropolarimeter and UV spectra
were obtained on Jacso V-550 UV-visible absorption spectrometer.
Fig.
8
Structure of the cationic unit of [FeL7 (NCMe)2][ClO4]2.
2
2(CH3CN). Hydrogen atoms have been removed for clarity. Selected
bond length (A) and angles (◦):N(4)1-Fe(1)-N(4A) 89.66(12),
˚
N(4A)1-Fe(1)-N(1A) 87.35(8), N(4)-Fe(1)-N(1A) 94.70(8), N(4A)-Fe(1)-
N(1)
177.11(13), N(4A)-Fe(1)-N(2) 175.28(8), N(4)-Fe(1)-N(2) 87.65(8),
N(1A)-Fe(1)-N(2) 96.73(8), N(1)-Fe(1)-N(2) 81.30(8), N(4A)-
Fe(1)-N(2A) 87.65(8), N(4)-Fe(1)-N(2A) 175.28(8), N(1A)-Fe(1)-N(2A)
81.30(8), N(1)-Fe(1)-N(2A) 96.73(8), N(2)-Fe(1)-N(2A) 95.29(12).
94.70(8),
N(4)-Fe(1)-N(1)
87.35(8),
N(1A)-Fe(1)-N(1)
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Crystallography
120.7 (p-Ph), 123.2 (Py), 130.1 (Ph), 136.9 (CH), 138.2 (Py), 146.1
(Cq), 150.1 (Py), 156.5 (Cq). IR (neat) n cm-1 3181 (NH), 1597
[FeL3 ][ClO4]2. C39H39N9Cl2O8Fe, M = 888.54, purple block
+
3
=
(C N), 1565 (pyridine ring), 996 (N–N). MS (ESI ): m/z 198.00
0.32 ¥ 0.10 ¥ 0.10 mm, hexagonal, space group P63, a = b =
[C12H11N3+H]+.
3
˚
˚
˚
15.8761(4) A, c = 10.5809(3) A, V = 2309.63(10) A , Z = 2,
Dc = 1.278 g cm-3, T = 100(2) K, with MoKa (0.71073), 11159
reflections collected, 2842 unique (Rint = 0.0568), R1 [I > 2s(I)] =
0.0778, wR2 (F, all data) = 0.1520, GoF = 0.947, Flack 0.03(3).
CCDC 761423.
2-Pyridinecarboxaldehyde-N-methyl-N-phenylhydrazone29 (L2).
The N-methyl-N-phenylhydrazine (1.20 g, 10.0 mmol) was dis-
solved in absolute ethanol (5 mL), giving a pale yellow solu-
tion. The addition of the pale yellow solution of 2-pyridine-
carboxaldehyde (1.00 g, 10.0 mmol) in absolute ethanol (5 mL)
to the N-methyl-N-phenylhydrazine solution caused the colour
to change to bright yellow. The solution was stirred overnight
at reflux. All the solvent was removed completely under reduced
pressure and a yellow solid was obtained, L2. The yellow solid
was then collected and dried overnight in vacuo at 30 ◦C (1.83 g,
88%). M.p. 76–78 ◦C. Anal. Found (Calcd. for C13H13N3)% C
73.92 (73.91), H 6.18 (6.20), N 19.89 (19.89). 1H NMR (300 MHz,
298 K, Acetone-d6) d 3.55 (s, 3H, CH3), 6.95 (td, J = 7.5, 2 Hz,
1H, p-Ph), 7.19 (ddd, J = 1, 5, 7.5 Hz,1H, Py), 7.34 (m, 2H,
[FeL3 ][BF4]2. C39H39N9B2F8Fe, M = 863.26, purple block
3
0.20 ¥ 0.08 ¥ 0.05 mm, hexagonal, space group P63, a = b =
3
˚
˚
˚
15.8761(4) A, c = 10.5809(3) A, V = 2309.63(10) A , Z = 2,
Dc = 1.241 g cm-3, T = 100(2) K, with MoKa (0.71073), 8792
reflections collected, 2578 unique (Rint = 0.0599), R1 [I > 2s(I)] =
0.0812, wR2 (F, all data) = 0.1535, GoF = 0.960, Flack 0.03(3).
CCDC 761424.
[FeL7 (NCMe)2][ClO4]2.2(CH3CN). C36H42N10O8Cl2Fe, M =
2
869.55, orange plate 0.08 ¥ 0.05 ¥ 0.01 mm, monoclinic, space
=
m-Ph), 7.49 (d, J = 8 Hz, 2H, o-Ph), 7.64 (s, 1H, N CH), 7.75
˚
˚
group C2/c, a = 12.0681(6) A, b = 16.8580(12) A, c = 19.7972(9)
(td, J = 8, 2.5 Hz, 1H, Py), 8.02 (d, J = 8 Hz, 1H, Py), 8.53 (d,
◦
3
-3
˚
˚
A, b = 104.057(5) , V = 3907.0(4) A , Z = 2, Dc = 1.478 g cm ,
T = 100(2) K, with MoKa (0.71073), 23664 reflections collected,
2763 unique (Rint = 0.0400), R1 [I > 2s(I)] = 0.0564, wR2 (F, all
data) = 0.0966, GoF = 0.901. CCDC 761425.
1
J = 5 Hz, 1H, Py). 13C{ H} NMR (75 MHz, 298 K, Acetone-d6)
d 33.3 (CH3), 116.2 (o-Ph), 119.4 (Py), 121.8 (p-Ph), 122.8 (Py),
129.8 (m-Ph), 133.7 (Py), 136.9 (Py), 150.0 (Py). IR (neat) n cm-1
+
=
1599 (C N), 1560 (pyridine ring), 994 (N–N). MS (ESI ): m/z
The crystals were mounted on a glass fibre using an inert oil
prior to transfer to a cold nitrogen gas stream from an Oxford
Cryosystems Cobra on a Oxford Diffraction Gemini four-circle
diffractometer system with a Ruby CCD area detector using Mo-
212.00 [C13H13N3+H]+.
2-Acetylpyridine-4-nitrophenylhydrazone (L4). A brown sus-
pension of 4-nitrophenylhydrazine (1.86 g, 12.16 mmol) in ab-
solute ethanol (25 mL) was added to 2-acetylpyridine (1.47 g,
12.16 mmol) at ambient temperature. Two drops of acetic acid
was added. The solution was stirred at reflux overnight and
then filtered. Distilled water was added to the yellow filtrate.
The crystallised yellow solid, L4, was collected and dried in
vacuo (0.90 g, 29%). M.p. 262-263 ◦C. Anal. Found (Calcd. for
C13H12N4O2)% C 60.74 (60.93), H 4.58 (4.72), N 21.13 (21.86). 1H
NMR (400 MHz, 298 K, Acetone-d6) d 2.47 (s, 3H, CH3), 7.34
(dd, J = 1 Hz, 1H, Py), 7.48 (d, J = 9 Hz, 2H, Ph), 7.82 (t, J =
7.5 Hz, 1H, Py), 8.18 (d, J = 9 Hz, 2H, Ph), 8.27 (d, J = 8 Hz, 1H,
Py), 8.59 (d, J = 4.5 Hz, 1H, Py), 9.70 (br s, 1H, NH). 13C NMR
(400 MHz, 298 K, Acetone-d6) d 10.9 (CH3), 113.3 (Ph), 120.8
(Py), 123.9 (Py), 126.6 (Ph), 137.0 (Py), 147.7 (Cq), 149.5 (Py),
˚
Ka radiation (= 0.71073 A). Diffractometer control and data
collection were with CrysAlis CCD (Oxford Diffraction 2008).
Data were collected with 1◦ frame exposures using phi and omega
scans with data reduction by CrysAlis RED (Oxford Diffraction,
2008). Intensities were corrected semi-empirically for absorption,
based on symmetry-equivalent and repeated reflections with AB-
SPACK (CrysAlis, Oxford Diffraction, 2008). The structures were
solved by direct methods (SHELXS) with additional light atoms
found by Fourier methods. All non-hydrogen atoms were refined
anisotropically. H atoms were placed at calculated positions and
constrained with a riding model. U(H) was set at 1.2 (1.5 for
methyl hydrogen atoms as applicable) times Ueq for parent atom.
SHELXTL was used for structure solution, refinement, and
molecular graphics.
151.8 (Cq), 156.7 (Cq). IR (neat) n cm-1 3323 (NH), 1601 (C N),
=
1575 (pyridine ring). MS (ESI+): m/z 257.0 [C13H12N4O2+H]+.
Syntheses
2-Acetylpyridine-4-methoxyphenylhydrazone
hydrochloride
2-Pyridinecarboxaldehyde phenylhydrazone28 (L1). A pale yel-
low solution of phenylhydrazine (2.10 g, 19.0 mmol) in absolute
ethanol (5 mL) was added to a pale yellow solution of 2-pyridine
carboxaldehyde (2.00 g, 19.0 mmol) in absolute ethanol (5 mL)
at ambient temperature, resulting in a bright yellow solution and
yellow precipitate. Stirring was continued for 24 h at reflux. After
cooling, the yellow solid, L1, was obtained by vacuum filtration
and dried in vacuo at 30 ◦C (2.91 g, 74%). M.p. 189–190 ◦C. Anal.
Found (Calcd. for C12H11N3)% C 72.88 (73.07), H 5.60 (5.62), N
mono hydrate, [HL5]Cl·H2O.
A purple suspension of 4-
methoxyphenylhydrazine mono hydrochloride (0.5 g, 2.86 mmol)
in absolute ethanol (25 mL) was added to 2-acetylpyridine (0.35 g,
2.86 mmol) in absolute ethanol (10 mL) ambient temperature,
followed by glacial acetic acid (2 drops). The resultant orange
solution was stirred at reflux for 3 h before cooling to ambient
temperature. The solvent was reduced to one third of the original
volume. The crystallised orange solid was collected and dried
in vacuo. The orange solid, L5, was recrystallised from hot
ethanol (0.45 g, 65%). M.p. 218-220 ◦C. Anal. Found (Calcd. for
C14H18N3ClO2)% C 56.47 (56.85), H 6.09 (6.13), N 14.08 (14.21).
1H NMR (400 MHz, 298 K, DMSO-d6) d 2.38 (s, 3H, CH3),
3.73 (s, 3H, CH3), 6.90 (d, J = 9 Hz, 2H, Ph), 7.55 (d, J = 9 Hz,
2H, Ph), 7.71 (br t, J = 6.5 Hz, 1H, Py), 8.22 (d, J = 8.5 Hz,
1
21.30 (21.30). H NMR (300 MHz, 298 K, Acetone-d6) d 6.84
(tt, J = 7, 1.5 Hz, 1H, Py), 7.24 (m, 5H, Ph), 7.76 (td, J = 6,
1.5 Hz, 1H, Py), 7.91 (s, 1H, CH), 8.03 (d, J = 8 Hz, 1H, Py),
8.52 (dt, J = 1.5, 4.5 Hz, 1H, Py), 9.85 (br s, 1H, NH). 13C{ H}
1
NMR (75 MHz, 298 K, Acetone-d6) d 113.4 (Ph), 119.6 (Py),
4452 | Dalton Trans., 2010, 39, 4447–4454
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1H, Py), 8.39 (br t, J = 7.5 Hz, 1H, Py), 8.67 (d, J = 4.5 Hz,
cannulae and dried in vacuo (0.15 g, 14%). Anal. Found (Calcd.
for C60H58N6B2Fe)% C 75.58 (76.61), H 6.88 (6.21), N 9.22 (8.93).
1H NMR (400 MHz, 298 K, Acetonitrile-d3) d 6.69 (t, J = 6 Hz,
1H, p-Ph), 6.83 (t, J = 7 Hz, 2H, m-Ph), 7.12 (br d, 2H, o-Ph).
1
1H, Py), 10.34 (s, 1H, NH). 13C{ H} NMR (100 MHz, 298 K,
DMSO-d6) d 11.6 (CH3), 55.2 (CH3), 114.3 (Ph), 115.5 (Ph), 122.5
(Py), 123.2 (Py), 142.3 (Py), 143.9 (br Py). IR (neat) n cm-1 3383
+
1
(NH), 1606 (C N), 1552 (pyridine ring). MS (ESI ): m/z 242.0
13C{ H} NMR (100 MHz, 298 K, Acetonitrile-d3) d 120.9 (p-Ph),
=
[C14H15N3O+H]+.
124.7 (m-Ph), 134.8 (o-Ph), 162.6 (Cq).
2-Acetylpyridine-2-pyridylhydrazone (L6). The pale yellow so-
lution of hydrazinopyridine (1.00 g, 9.17 mmol) in ethanol (15 mL)
was added to the colourless solution of 2-acetylpyridine (1.11 g,
9.17 mmol) and stirred at reflux for 24 h. The solvent was
reduced to ~ 1/3 before adding distilled water dropwise to an
ice cold solution. A yellow solid precipitated and was collected by
filtration. The yellow crystals, L6, were then collected and dried
in vacuo (1.39 g, 71%). M.p.: ca 58 ◦C. Anal. found (Calcd. for
fac-[FeL3 ][ClO4]2. Under an atmosphere of dinitrogen, a
3
colourless solution of 2-acetylpyridine (0.20 g, 1.80 mmol) and
phenylhydrazine (0.18 g, 1.80 mmol) in MeCN (10 mL) was treated
with a solution of Fe(ClO4)2·6H2O (0.20 g, 0.60 mmol) in MeCN
(5 mL) at ambient temperature with stirring. The resultant purple
solution was stirred overnight and the solvent was evaporated
to approximately 1/3 volume. The dropwise addition of ethyl
acetate caused a purple solid to precipitate which was filtered,
collected and dried in vacuo (0.39 g, 79%). Suitable crystals
for X-ray crystallography were obtained by slow evaporation
of the supernatant. M.p. 186–188 ◦C. Anal. Found (Calcd. for
C39H39N9FeCl2O8)% C 51.87 (52.72), H 4.17 (4.42), N 13.82
(14.19). 1H NMR (300 MHz, 298 K, Acetonitrile-d3) d 2.09, (9H, s,
CH3), 6.23 (br, 6H, o-Ph), 6.85 (t, J = 7 Hz, 3H, p-Ph), 6.91 (br t,
6H, m-Ph), 7.05 (d, J = 5 Hz, 3H, Py), 7.53 (t, J = 6 Hz, 3H, Py),
7.74 (d, J = 8 Hz, 3H, Py), 7.89 (s, 3H, NH), 8.03 (t, J = 7.5 Hz, 3H,
1
C12H12N4)% C 66.94 (67.90), H 5.74 (5.70), N 25.69 (26.40). H
NMR (400 MHz, 298 K, Acetonitrile-d3) d 2.37 (s, 3H, CH3), 6.83
(dd, J = 7, 5 Hz, 1H, Py), 7.25 (dd, J = 3.5, 1 Hz, 1H, Py), 7.40
(d, J = 8.5 Hz, 1H, Py), 7.66 (t, J = 8.5 Hz, 1H, Py), 7.73 (t, J =
5 Hz, 1H, Py), 8.17 (dd, J = 11, 4.5 Hz, 2H, Py), 8.53 (d, J =
1
4.5 Hz, 1H, Py), 8.55 (br s, 1H, NH). 13C{ H} NMR (100 MHz,
298 K, Acetonitrile-d3) d 9.9 (CH3), 107.2 (Py), 116.1 (Py), 119.6
(Py), 122.7 (Py), 136.1 (Py), 138.1 (Py), 147.7 (Py), 148.4 (Py). IR
(neat) n cm-1 3234 (NH), 1602 (C N), 1572, 1514 (pyridine ring).
Py). 13C{ H} NMR(75 MHz, 298 K, Acetonitrile-d3) d 15.4 (CH3),
1
=
MS (ESI+): m/z 213.00 [C12H12N4+H]+. lmax nm (emax M-1cm-1):
114.4 (o-Ph), 122.4 (p-Ph), 127.8 (Py), 128.3 (Py), 129.1 (m-Ph),
322 (22105), 469 (988).
138.3 (Py), 143.2 (Cq), 154.1 (Py), 158.3 (Cq), 178.8 (Cq). IR (neat)
n cm-1 3319 (NH), 1600 (C N), 1563 (pyridine ring), 1081 (N–
=
40
2-Acetylpyridine-N-methyl-N-phenylhydrazone
(L7). The
N). MS (ESI+): m/z 688.26 [FeL3 -H]+, 577.00 [(FeL3 )(ClO4)]+,
3
2
solution of N-methyl-N-phenylhydrazine (1.11 g, 9.09 mmol) was
treated with HCl (37%, 2 drops) in EtOH (10 mL), stirred at reflux
for 20 mins. A solution of 2-acetylpyridine (0.86 g, 7.10 mmol) in
ethanol (10 mL) was added dropwise. The colour changed from
pale yellow to bright red-brown and the solution was refluxed
for a further 3 h. The solvent was removed completely and left
a brown oil. The brown oil was extracted into chloroform and
washed with distilled water (5 ¥ 50 mL). The organic portion was
dried over sodium sulfate overnight. The solution was filtered and
the solvent was removed in vacuo. The resulting product, L7, was
purified via column chromatography with eluent petroleum ether:
diethyl ether (7 : 3 v/v) and was obtained as an orange oil (1.00 g,
63%). Anal. Found (Calcd. for C14H15N3)% C 74.40 (74.64), H 6.77
(6.71), N 18.26 (18.65). 1H NMR (400 MHz, 298 K, Acetone-d6)
d 2.44 (s, 3H, CH3), 3.23 (s, 3H, CH3), 6.89 (t, J = 7.5 Hz, 1H,
p-Ph), 7.07 (d, J = 8 Hz, 2H, o-Ph), 7.27 (t, J = 7 Hz, 2H, m-Ph),
7.36 (td, J = 5, 1 Hz, 1H, Py), 7.79 (td, J = 7.5, 2 Hz, 1H, Py),
477.15 [FeL3 -H]+. UV/vis lmax nm (emax M-1cm-1): 335 (9113), 524
2
(5503).
fac-[FeL3 ][BF4]2. A colourless solution of the Fe(BF4)2·6H2O
3
(0.53 g, 1.57 mmol) in acetonitrile (5 mL) was added to solution
of 2-acetylpyridine phenylhydrazone (L3) (1.00 g, 4.73 mmol) in
acetonitrile (60 mL) at ambient temperature. The resulting purple
solution was stirred for 24 h at ambient temperature. The solvent
was concentrated under reduced pressure to around 15 ml and
diethyl ether was added. The precipitated solid was collected and
dried in vacuo (1.18 g, 87%) at 30 ◦C. M.p. 190–193 ◦C. Anal. found
(Calcd. for C39,H39N9FeB2F8)% C 52.91 (54.26), H 4.38 (4.55), N
14.17 (14.60). 1H NMR (300 MHz, 298 K, Acetonitrile-d3) d 2.10,
(9H, s, CH3), 6.21 (br s, 6H, Ph), 6.81 (t, J = 7 Hz, 3H, Ph) 6.88 (m,
6H, Ph), 7.03 (d, J = 5 Hz, 3H, Py), 7.52 (t, J = 6.5 Hz, 3H, Py),
7.74 (d, J = 8 Hz, 3H, Py), 7.91 (s, 3H, NH), 8.03 (t, J = 7.5 Hz,
1
3H, Py). 13C{ H} NMR (75 MHz, 298 K, Acetonitrile-d3) d 15.4
8.24 (d, J = 8 Hz, 1H, Py), 8.61 (d, J = 4 Hz, 1H, Py). 13C{ H}
(CH3), 114.4 (o-Ph), 122.3 (p-Ph), 127.8 (Py), 128.3 (Py), 129.1 (m-
1
NMR (100 MHz, 298 K, Acetone-d6) d 16.0 (CH3), 43.3 (CH3),
116.6 (o-Ph), 121.0 (Py), 121.5 (p-Ph), 124.9 (Py), 129.6 (m-Ph),
Ph), 138.3 (Py), 143.2 (Cq), 154.1 (Py), 158.3 (Cq), 178.8 (Cq). IR
(neat) n cm-1 3339 (NH), 1601 (C N), 1564 (pyridine ring), 1058
=
136.9 (Py), 149.6 (Py), 152.4 (Cq), 157.6 (Cq), 163.9 (Cq). IR (neat)
(N–N). MS (ESI+): m/z 477.10 [FeL3 -H]+, 343.60 [FeL3 -H]2+.
2
3
n cm-1 1598 (C N), 1573 (pyridine ring), 987 (N–N). MS (ESI ):
lmax nm (emax M-1cm-1): 335 (6664), 270 (3247).
+
=
m/z 226.00 [C14H15N3+H]+.
[ZnL6 ][ClO4]2. A yellow solution of L6 (0.10 g, 0.48 mmol)
2
[Fe(CH3CN)6][BPh4]. In the dark, a mixture of AgBPh4
(1.00 g, 2.35 mmol) and FeBr2 (0.25 g, 1.17 mmol) in dry
acetonitrile (15 mL) was stirred overnight at ambient temperature.
The excess AgBPh4 and AgBr were filtered off and the solution
left to settle before re-filtering. The solvent was reduced slowly
to ~5 mL. A beige crystalline solid began to precipitate and
the solution was cooled to -40 ◦C (cooling bath of acetonitrile
and dry ice) for 4 h. The solid was collected by filtration via
in acetonitrile (10 mL) was added to a colourless solution of
Zn(ClO4)2·6H2O (0.009 g, 0.24 mmol) in acetonitrile (15 mL) at
ambient temperature. The resulting bright yellow solution was
stirred at reflux for 24 h. The solvent was reduced to a third of
the original volume and the addition of diethyl ether resulted
in a yellow precipitate, which was collected and dried in vacuo
(0.11 g, 67%). Anal. Found (Calcd. for C24H24Cl2ZnN8O8)% C
41.35 (41.85), H 3.42 (3.51), N 15.98 (16.27). 1H NMR (400 MHz,
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Dalton Trans., 2010, 39, 4447–4454 | 4453
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298 K, Acetonitrile-d3) d 2.69 (s, 6H, CH3), 6.87 (t, J = 6.5 Hz, 2H,
Py), 7.23 (d, J = 8.5 Hz, 2H, Py), 7.39 (t, J = 6.5 Hz, 2H, Py), 7.67
(d, J = 5 Hz, 2H, Py), 7.82 (t, J = 8.5 Hz, 2H, Py), 8.16 (m, 6H, Py),
13 K. K. W. Lo, K. H. K. Tsang, K. S. Sze, C. K. Chung, T. K. M. Lee,
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1
10.34 (s, 2H, NH). 13C{ H} NMR (100 MHz, 298 K, Acetonitrile-
d3) d 12.2 (CH3), 110.6 (Py), 118.1 (Py), 122.9 (Py), 126.1 (Py),
140.8 (Py), 141.6 (Py), 144.7 (Cq), 147.9 (Py), 151.4 (Cq). IR (neat)
n cm-1 3300 (NH), 1618 (C N), 1599 (pyridine ring), 1086 (N–N).
=
MS (ESI+): m/z 244.0 [ZnL6 ]2+. lmax nm (emax M-1cm-1): 336
2
(43779), 475 (26266).
[FeL7 (MeCN)2][ClO4]2. The solution of L7 (0.20 g,
2
0.89 mmol) was added with the solution of Fe(ClO4)2·6H2O
(0.17 g, 0.47 mmol) in acetonitrile (10 ml) at ambient temperature
to give a red/purple solution. The mixture was stirred overnight
and solvent was reduced by half under reduced pressure. Addition
of diethyl ether g◦ave purple solid which was isolated and dried
over vacuo at 30 C, leading to loss of solvent of crystallisation
(0.34 g, 84%). Anal. Found (Calcd. for C32H36Cl2FeN8O8)% C
48.32 (48.81), H 4.52 (4.61), N 15.06 (14.23). IR (neat) n cm-1
+
7
2+
=
1596 (C N), 1084 (N–N). MS (ESI ): m/z 253.1 [FeL ] .
2
Acknowledgements
We thank EPSRC for support, the Tunku Abdul Rahman Sarawak
Scholarship Scheme (CPS) and study leave granted by Universiti
Putra Malaysia (CPS).
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4454 | Dalton Trans., 2010, 39, 4447–4454
This journal is
The Royal Society of Chemistry 2010
©