Geometric selectivity, hydrogen-bonding interaction, and solvatochromism of Bis{N-(aminoalkyl)salicylamidato(2-)}cobaltate(III)

Article abstract of DOI:10.1246/cl.2011.696

Cobalt(III) complexes with N-(aminoalkyl)salicylamide dianions, Ln ^2- (n = 14), have been prepared and their molecular and crystal structures have been determined. The geometric (mer- or fac-) selectivity of [Co(Ln)₂]^- com

Full text of DOI:10.1246/cl.2011.696

doi:10.1246/cl.2011.696
6
96
Published on the web June 4, 2011
Geometric Selectivity, Hydrogen-Bonding Interaction, and Solvatochromism
of Bis{N-(aminoalkyl)salicylamidato(2¹)}cobaltate(III)
Ryoji Mitsuhashi, Takayoshi Suzuki,* Yukinari Sunatsuki, and Masaaki Kojima
Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530
(
Received March 31, 2011; CL-110272; E-mail: suzuki@cc.okayama-u.ac.jp)
3
c
Cobalt(III) complexes with N-(aminoalkyl)salicylamide amines were used (for H L1 and H L2). In the preparation of
2
2
2¹
dianions, Ln
(n = 1­4), have been prepared and their
H2L4, if a mixture of methyl salicylate and 1,3-diaminopropane
was refluxed without any solvent, then the intramolecular
condensation of H L4 took place to give a 2-hydroxyphenyl-
substituted N,N¤-propanediyl-bridged amidine, whose crystal
structure and coordination behaviors will be reported elsewhere.
The cobalt(III) complexes bearing Ln (n = 1­4) were
prepared by addition of a stoichiometric amount of t-BuOK to a
methanolic mixture of H2Ln and Co(OAc)2¢4H2O (or CoCl2¢
6H2O) in the molar ratio of 2:1, followed by stirring the mixture
molecular and crystal structures have been determined. The
¹
geometric (mer- or fac-) selectivity of [Co(Ln)2] complexes
2
8
was dependent on the number of the amine­amidato chelate ring
member. Intermolecular homochiral hydrogen-bonds and sol-
vatochromic behavior of the complexes were also observed.
2¹
¹
Nitrogen-ligating amidate anions, [RNC(O)R¤] -¬N, are
classified as strong ·- and ³-donors. In addition, the capability
1
6
in air, affording a gradual color change of the reaction solution.
of the uncoordinated amidate-O atom as a hydrogen-bonding
When the reaction was performed under a nitrogen atmosphere,
no color change of the reaction solution was observed,
indicating that air oxidation was necessary to form stable
2
acceptor is a key factor in many enzymatic systems, and would
3
be fascinating for construction of new functional materials.
2¹
Hence, in combination with an aminoalkyl group that can act as
a hydrogen-bonding donor upon coordination, transition-metal
complexes with amine­amidato type chelate ligands which
result from deprotonation of N-(aminoalkyl)amides would be
good candidates for specific molecular recognition using multi-
cobalt(III) complexes. The resulting complexes with L1 and
2¹
L2 could be isolated as dark purple crystals of K[Co(L1)2]¢
CH3OH (1) and K[Co(L2)2]¢5H2O¢CH3OH (2), respectively, by
concentration of the reaction mixture and diffusion of diethyl
2
¹
2¹
ether vapor. In the cases of L1 and L3 complexes, cation
exchange using PPh4Br and recrystallization of the crude
products from acetonitrile/diethyl ether deposited dark brown
platelet crystals of PPh4[Co(L1)2]¢1.5CH3CN¢2H2O (1P) and
purple block crystals of PPh4[Co(L3)2]¢0.5CH3CN (3P). It
should be noted that, while both crystals of 1 and 1P were
deeply colored, their powdered samples showed different colors,
as seen in their diffuse-reflectance spectra (Figure 1a).
4
ple hydrogen-bonds. We are interested in such amine­amidato
type ligands, particularly unsymmetrical tridentate ligands
having an additional phenoxo donor group. Despite of their
simple molecular structures, transition-metal complexes with N-
(
aminoalkyl)salicylamidates(2¹) have not been widely inves-
4
,5
tigated, although they have often been used as precursors for
3
c
unsymmetrical tetradentate amidate­Schiff base complexes. In
this study, we deal with such tridentate ligands, Ln2 (n = 1­4)
shown in Scheme 1. In order to reveal their fundamental
coordination and hydrogen-bonding behaviors, we have pre-
¹
1
The H NMR spectrum of the crude reaction product of
K[Co(L1)2] indicated the isolation of the sole product. The
spectrum showed four kinds of resonances due to phenyl protons
and three sets of two signals for methyl, methylene, and amine
¹
pared and characterized their cobalt(III) complexes, [Co(Ln)2] .
6
The ligands, H Ln (n = 1­4), were prepared by a reaction of
protons, respectively (Figure S1 ), with a geminal coupling for
2
phenyl salicylate and the appropriate diamines (1,2-diamino-2-
methylpropane, 1,2-diaminopropane, trans-1,2-diaminocyclo-
the latter two sets. These spectral features suggest the chelate
ring formation of amine­amidato moiety and the molecular
3
c,6
¹
hexane, and 1,3-diaminopropane, respectively) in 2-propanol.
For H2L3 and H2L4, other preparative procedures had been
symmetry of C or C for the resulting [Co(L1) ] complex. In
2
i
2
1
addition, a remarkable solvent-dependency of the H NMR and
UV­vis spectra of this complex salt 1 or 1P was observed. As
seen from Figure 1b, two absorption bands appearing in the
2
a,7
reported, but the present method was much simpler and gave
higher yields. In this method, the desired N-(aminoalkyl)salicyl-
amides were precipitated from the reaction mixture due to their
low solubility in 2-propanol, which prevents further reaction to
give symmetric tetradentate diamides. The products collected by
filtration were reasonably pure, even the unsymmetrical di-
Figure 1. (a) Diffuse-reflectance spectra of 1 (blue) and 1P (red).
(b) Absorption spectra of 1P in CH3OH (blue), CHCl3 (green), and
CH3CN (orange).
Scheme 1. The neutral (doubly protonated) forms of the ligands used
in this study.
Chem. Lett. 2011, 40, 696­698
© 2011 The Chemical Society of Japan
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Figure 2. (a) ORTEP (50% probability level, H-atoms omitted) of
the anionic part in PPh4[Co(L1)2]¢1.5CH3CN¢2H2O (1P). (b) A
schematic view of the heterochiral dimer connected by double
hydrogen-bonds in 1P.
Figure 3.
A perspective view illustrating the interaction of
+
¹
[
Co(L1)2] , K , and CH3OH in 1.
region of 500­650 nm became closer in energy and the band
with longer wavelength became more intense in the order of
CH CN < CHCl < CH OH. Also, the difference in chemical
3
3
3
shifts for two methylene resonances became larger in the same
6
order (Figure S1 ).
In order to interpret the color difference between crystals
1
and 1P and the solvatochromic behavior observed in the
complex salts, crystal structures of 1 and 1P have been
Figure 4. (a) ORTEP (50% probability level, H-atoms omitted) of the
anionic part in PPh [Co(L3) ]¢0.5CH CN (3P). (b) A schematic view
of the heterochiral dimer connected by double hydrogen-bonds in 3P.
9
determined by X-ray analysis. It was revealed that both crystals
4
2
3
¹
consisted of the mer-isomer of [Co(L1)2] complex anion. The
molecular structure of the anion in 1P is shown in Figure 2a.
2
¹
¹
6
Owing to the planarity of salicylamidato moiety in L1 , the
tridentate ligand forming an aminoethylamidato five-membered
chelate ring is sterically restricted to the mer-coordination mode.
The mer-isomer is chiral, and a pair of clockwise (C-) and
anticlockwise (A-) enantiomers is possible. The space group of
The molecular structure of [Co(L2)2] in 2 (Figure S2 ) is
¹
similar to that of [Co(L1)2] in 1. Because we have used a
racemic mixture of 1,2-diaminopropane (=pn) for the prepara-
tion of H2L2, an equimolar mixture of R- and S-L2 would be
incorporated into the complex. Thus, C-[Co(R-L2) ] , C-[Co(S-
L2)2] , and C-[Co(R-L2)(S-L2)] diastereomers are possible (in
addition to their A-enantiomers). The crystal structure analysis of
2 indicated a positional disorder for the methyl groups attached
to the C atom next to the amine group over two possible
positions. The H NMR spectrum of 2 in CD3OD was also
indicative of a mixture of the above diastereomers. We also
prepared a similar complex using enantiomerically pure R-pn. In
this case it is expected to form only two isomers, C-[Co(R-
L2)2] and A-[Co(R-L2)2] , and the H NMR spectrum of the
products exhibited resonances due to the two complexes with
almost equal intensities (Figure S3 ). These facts suggest that
there is no selectivity in the formation of bis(L2)-type
complexes, as far as their methyl substituents are concerned.
2¹
¹
2
ꢀ
¹
¹
the crystal 1P is P1, indicating that the crystal is racemic. In this
crystal structure, there are intermolecular hydrogen bonds
between the coordinated amino (NH ) groups and the amidato-
2
O atoms, which form a hetero-chiral (C- and A-) dimer as
illustrated in Figure 2b. In contrast, the crystal structure of
K[Co(L1) ]¢CH OH (1) exhibited an explicit interaction be-
6
1
2
3
+
tween one of the amidate-O atoms and a K cation, which was
also surrounded by two coordinated phenoxo-O atoms and an O
atom of the methanol of crystallization (Figure 3). The other
amidato-O atom was hydrogen-bonded to an amino group of the
neighboring complex anion. We expected that such an inter-
¹
¹
1
6
+
action with K would make the amidato C=O bond longer, and
hence, the ligand-field strength of the amidato-N donor would be
weakened. However, the C=O and the coordination (Co­N)
¹
For the preparation of H2L3 and its complex, [Co(L3)2] , we
9
bond lengths in 1 did not differ much from those in 1P. This
used a racemic mixture (RR- and SS-) of trans-1,2-diamino-
cyclohexane. Because of the stable diequatorial-chair conforma-
tion of the diaminocyclohexane ring, in addition to the planarity
of the salicylamidato fragment, the tridentate amine­amidato­
would be restricted to the mer-
configuration in the [Co(L3)2] complex. Similar to the L2
complex mentioned above, three diastereomers of mer-C-[Co-
is consistent with the almost unchanged transition energies
observed in the range of 500 to 650 nm in the diffuse-reflectance
spectra of 1 and 1P (Figure 1a). Thus, the apparent color change
of the crystals would originate not from the transition-energy
shift, but from the intensity change of the absorption bands
presumably due to the lowering of the molecular symmetry.
Similarly, the solvent-dependency of the absorption spectrum
of (1 or) 1P (Figure 1b) seems to be related to the acceptor
numbers of the solvents,10 because the solvent molecules would
interact with the amidato-O atom of the complex anion, likewise
2¹
phenoxo ligand of L3
¹
¹
¹
(RR-L3)2] , mer-C-[Co(SS-L3)2] , and mer-C-[Co(RR-L3)(SS-
¹
1
L3)] (and their A-enantiomers) are possible. The H NMR
spectrum of the crude products of K[Co(L3)2] (3) was indicative
of the formation of three products in nearly equal ratios.
+
K . At this moment, we cannot fully interpret the solvatochro-
However, the X-ray crystallographic analysis of its PPh salt,
4
1
mic behavior of UV­vis and H NMR spectra, but this result is
PPh4[Co(L3)2]¢0.5CH3CN (3P), revealed the specific crystalli-
zation of one of the products, mer-PPh4[Co(RR-L3)(SS-L3)]
(Figure 4a). In this crystal heterochiral dimerization (Figure 4b)
took place by the amine£amidato double hydrogen-bonds, as
an example of the importance of the uncoordinated amidate-O
moiety in the chemical and physical properties of the amidato-N
complexes.
Chem. Lett. 2011, 40, 696­698
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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similarly found in the crystal of 1P. Interestingly, the hydrogen
2
¹
bonds are formed between the homochiral L3 ligands (i.e., an
2¹
amine group of RR-L3 with an amidato-O of the neighboring
RR-L3 and the analogs of SS-L32¹ ligands). In other words, the
double hydrogen bonds between amine and amidato groups can
recognize the chirality of the ligand itself and the chirality of the
metal center, forming a selective homochiral/heterochiral ag-
gregation. We have attempted to compare the formation energies
of the three diastereomers (monomeric anions) by the DFT
2¹
6
method. The results indicated that the X-ray-derived structure,
Figure 5. ORTEPs (50% probability level, H-atoms omitted) of two
¹
crystallographically independent anionic complexes in K[Co(L4) ]¢
2
mer-C-[Co(RR-L3)(SS-L3)] , is slightly more stable by 2.25
¹
1
¹
¹
5H2O (4).
kcal mol than mer-C-[Co(RR-L3)2] , but mer-C-[Co(SS-L3)2]
¹
1
is more stable by 3.31 kcal mol
than the X-ray-derived
structure. This fact also suggests the importance of the
intermolecular hydrogen bonds.
Differing from the above-mentioned ligands, H2L4 has a
trimethylene group connecting the amine and amidate donor
groups, and this would give a different tendency to stabilize a
specific geometric isomer. Aerobic reaction of Co(OAc)2¢4H2O,
mer-isomer exclusively. The X-ray analyses revealed that
the coordinated amino (­NH2) group and the uncoordinated
amidato-O atom were acting as a hydrogen-bonding donor and
an acceptor, respectively. With these hydrogen-bonds, homo-
chiral aggregation between the tridentate ligands was achieved
in the L3 complex 3P, forming a heterochiral dimer consisting
of a pair of C- and A-enantiomers. This kind of chiral recog-
nition using hydrogen-bonds of amine­amidato moieties may be
applicable for the selective molecular recognition of nucleobases
or nucleosides. Such studies using complex fragments with a
N-(aminoalkyl)salicylamide dianion are now in progress.
2¹
H L4, and t-BuOK (molar ratio of 1:2:4) in methanol at room
2
1
temperature for 2 h gave a green solution, but H NMR spec-
troscopy confirmed that this mixture contained at least two
products. On stirring the mixture at room temperature for two
weeks, or refluxing the mixture for 7 h, the reaction solution
turned brown, and brown columnar crystals of K[Co(L4)2]¢5H2O
(
4) were deposited by concentration of the reaction solution,
This work was partly supported by a Grant-in-Aid for
Scientific Research No. 20550064 from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan.
followed by diffusion of diethyl ether vapor. The crystal structure
of 4 was determined by X-ray analysis. In the asymmetric unit of
space group P1 with Z = 2 there are two halves of complex
ꢀ
anions, which are similar in their molecular structures (Figure 5).
References and Notes
Both complex anions have an inversion center at the Co atom
with fac-configuration of L42¹ ligands (fac-Ci-isomer). This is an
1
For examples: a) R. Carrasco, J. Cano, X. Ottenwaelder, A.
Aukauloo, Y. Journaux, R. Ruiz-García, Dalton Trans. 2005, 2527.
b) D.-L. Popescu, A. Chanda, M. Stadler, F. T. de Oliveira, A. D.
Ryabov, E. Münck, E. L. Bominaar, T. J. Collins, Coord. Chem. Rev.
2008, 252, 2050.
2¹
explicit difference from the corresponding complexes with Ln
n = 1­3) in the above, and presumably due to the six-membered
(
amine­amidato chelate ring. The C8 (and C18) atom has a
conformational restriction from the planar salicylamidato moiety,
but the six-membered ring is flexible enough to form a stable
chair-like conformation with the amine-N atom in the cis-
position to the phenoxo-O atom.
2
3
For example: a) K. Cheng, Q.-Z. Zheng, Y. Qian, L. Shi, J. Zhao,
H.-L. Zhu, Bioorg. Med. Chem. 2009, 17, 7861. b) V.
Mathrubootham, J. Thomas, R. Staples, J. McCraken, J. Shearer,
E. L. Hegg, Inorg. Chem. 2010, 49, 5393.
For example: a) O. Rotthaus, O. Jarjayes, F. Thomas, C. Philouze,
C. P. D. Valle, E. Saint-Aman, J.-L. Pierre, Chem.®Eur. J. 2006, 12,
2293. b) T. Kido, Y. Ikuta, Y. Sunatsuki, Y. Ogawa, N. Matsumoto,
N. Re, Inorg. Chem. 2003, 42, 398. c) J.-P. Costes, F. Dahan, B.
Donnadieu, M.-J. R. Douton, M.-I. F. Garcia, A. Bousseksou, J.-P.
Tuchagues, Inorg. Chem. 2004, 43, 2736.
In order to compare the stability of the possible geometric
¹
isomers for [Co(L4)2] , we have calculated the formation
energies of a mer- and five fac-isomers by the DFT method.
The results indicated two stable isomers; the most favorable one
was the X-ray-derived fac-Ci-isomer, and the secondary stable
4
5
J. McGinley, V. McKee, H. Toftlund, J. M. D. Walsh, Dalton Trans.
2009, 8406.
¹
1
6
one (4.29 kcal mol above) was the fac-C2-trans(NH2)-isomer.
Other isomers were higher in energy more than 10 kcal mol 1.
¹
For the metal complexes containing an amine­amidato chelate with
an additional donor group other than phenoxo, see: a) M. Nonoyama,
K. Yamasaki, Inorg. Chim. Acta 1973, 7, 373. b) G. A. van Albada,
A. Mohamadou, A. M. M. Lanfredi, I. Mutikainen, U. Turpeinen, F.
Ugozzoli, J. Reedijk, Z. Anorg. Allg. Chem. 2004, 630, 1594.
The experimental details, see: Supporting Information (SI), which
is available electronically on the CSJ-Journal Web site, http://
www.csj.jp/journals/chem-lett/index.html.
Thus, it was suggested that the isomers with trans(NH2)-
configuration would be energetically favorable, presumably due
to the distribution of the ligand negative charges over the
equatorial plane. From these calculations, we presumed that in the
6
¹
preparation of [Co(L4)2] the green intermediate formed kineti-
cally would be the fac-C2-trans(NH2)-isomer, which converted
7
8
9
J. Y. Yang, D. G. Nocera, Tetrahedron Lett. 2008, 49, 4796.
P. Stoss, Chem. Ber. 1978, 111, 314.
gradually to the thermodynamically more stable fac-C -isomer.
i
In summary, the amine­amidato­phenoxo tridentate ligands
exhibited geometric selectivity on coordination, owing to the
Crystallographic data reported in this manuscript have been
deposited with Cambridge Crystallographic Data Centre as supple-
mentary publication Nos. CCDC-822937­822941. Copies of the data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; fax: +44
1223 336033; or deposit@ccdc.cam.ac.uk).
planarity of salicylamidate moiety and the number of chelate
ring members of aminoalkylamidate. The ligand, L42¹, with
a 3-aminopropyl substituent afforded the fac-Ci-isomer of
¹
2¹
2¹
[
Co(L4) ] , while the other ligands (L1 ­L3 ) forming a five-
2
membered amine­amidato chelate ring gave the corresponding
10 U. Mayer, Pure Appl. Chem. 1979, 51, 1697.
Chem. Lett. 2011, 40, 696­698
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/