Unusual coordination mode of tetradentate Schiff base cobalt(iii) complexes

Article abstract of DOI:10.1039/c2dt11871h

Contrary to the stereotype, Jacobsen's catalyst, chiral (salcy)Co(iii)OAc adopts an unusual binding mode. The tetradentate {ONNO} ligand does not form a square plane but wraps cobalt in a cis-β fashion while acetate is chelating.

Full text of DOI:10.1039/c2dt11871h

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COMMUNICATION
Unusual coordination mode of tetradentate Schiff base cobalt(III) complexes†
Anish Cyriac,a Jong Yeob Jeon,a Jobi Kodiyan Varghese,a Ji Hae Park,a Soo Young Choi,b Young Keun Chungb
and Bun Yeoul Lee*a
Received 4th October 2011, Accepted 23rd November 2011
DOI: 10.1039/c2dt11871h
Contrary to the stereotype, Jacobsen’s catalyst, chiral
(salcy)Co(III)OAc adopts an unusual binding mode. The
tetradentate {ONNO} ligand does not form a square plane
but wraps cobalt in a cis-b fashion while acetate is chelating.
Metal complexes constructed using tetradentate Schiff bases are
very potent catalysts in various reactions.1–5 Jacobsen discovered
the hydrolytic kinetic resolution (HKR) of epoxides using chi-
ral (tBu-salcy)Co(III)OAc (1) (tBu-salcy = N,N¢-bis(3,5-di-tert-
butylsalicylidene)-1,2-trans-diaminocyclohexane) as a catalyst.6,7
A highly efficient catalyst for CO2/epoxide copolymerizations was
developed, which is also a Co(III) complex of the salcy-type ligand
tethered by four quaternary ammonium salts (2).8,9
In most metal complexes of tetradentate Schiff bases, the
Fig. 1 Catalysts for HKR and CO2/epoxide copolymerization and their
{ONNO} ligand forms a square plane with the metal, creating
1H NMR spectra.
axial sites to which substrate or the fifth ligand binds.10 However,
a question arises about the structure of 1 or 2 when their 1H NMR
spectra are investigated (Fig. 1). In the spectrum of 1 in CD2Cl2,
each symmetric half unit in the salcy ligand gives rise to different
signals.11 In the spectrum of 2 in dmso-d6, two sets of signals are
observed in a ratio of 1 : 0.18. A feature of the major set (marked
with “*”) is the same as that observed for 1 in CD2Cl2; the two
symmetric parts in the ligand are diastereotopic. The minor set
(marked with “#”) is assignable to the species in which the two
parts are equivalent.
Fig. 2(a) shows the X-ray structure. The binding mode is the
conventional one; salcy-ligand forms a square plane with cobalt
while an axial site is occupied by acetate anion, which is stabilized
˚
by a hydrogen bond with acetic acid (d[O–O], 2.597(5) A). The
other axial site is occupied by another acetic acid of which a
carbonyl oxygen coordinates to cobalt, while a hydroxyl group
on acetic acid forms a hydrogen bond with phenoxy oxygen.
Hydrogen position determined directly on diffraction indicates
a fairly strong hydrogen bond (O–H–O angle, 158(5)◦; d[O–O],
Complex 1 is highly soluble in most common organic sol-
vents, hampering the growth of single crystals. An analogue
complex bearing methyl instead of tert-butyl substituent, (Me-
salcy)Co(III)OAc (Me-salcy = N,N¢-bis(3,5-dimethylsalicylidene)-
1,2-trans-diaminocyclohexane) is prepared simply by reacting
the corresponding ligand with Co(OAc)2 in CH2Cl2 under O2
atmosphere in the presence of 5 equivalents of acetic acid. Single
crystals‡ are obtained by diffusing pentane onto CH2Cl2 solution.
2.517(5) A).12 In the 1H NMR spectrum in CD2Cl2, each symmetric
˚
half unit in the ligand is equivalent, with three signals at 7–8 ppm
and a signal at 4.01 ppm for –CH–N proton. The methyl signal
of acetate and acetic acid is observed to be collapsed at 1.77 ppm.
Both the coordinated acetic acid and the hydrogen-bonded acetic
acid cannot be removed by evacuation even at 60 ◦C for 12 h. The
([AcO + AcOH]/[Me-salcy] ratio calculated from the integration
values persistently remains as 3 by the treatment. However, both
acetic acids can be removed by triturating in diethyl ether.
aDepartment of Molecular Science and Technology, Ajou University, Suwon,
443-749, Korea. E-mail: bunyeoul@ajou.ac.kr; Fax: 82 31 219 2394; Tel: 82
31 219 1844
When the crystals of (Me-salcy)Co(OAc)·2(HOAc) are dis-
solved in CH2Cl2 and recrystallization is conducted once more,
bDepartment of Chemistry, Seoul National University, Silimdong Gwanak-
gu, Seoul, 151-747, Korea
2
the two acetic acids are shed off yielding (Me-salcy)Co(k -OAc).
The X-ray structure shows an unexpected binding mode (Fig.
2(b)). The salcy-ligand does not form a square plane, but wraps
cobalt in a cis-b fashion (Fig. 3), while acetate acts as a chelating
ligand. A phenoxyimine situated in mer-arrangement forms a
strict plane with cobalt, while the other phenoxyimine situated
in a fac-arrangement deviates from the stable coplanar structure.
† Electronic supplementary information (ESI) available: Experimen-
tal procedures, compound characterization data, and X-ray crys-
tallography data. CCDC 840673, 840674, 840675, 840676, and
2
840677 for (Me-salcy)Co(OAc)·2(HOAc), (Me-salcy)Co(k -OAc), (Me-
2
salen)Co(OAc)(H2O), (Me-salcy)Co(k -NO3), and (Me-sal-cis-cy)Co(II),
respectively. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt11871h
1444 | Dalton Trans., 2012, 41, 1444–1447
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2
Fig. 2 X-ray structures of (Me-salcy)Co(OAc)·2(HOAc) (a), (Me-salcy)Co(k -OAc) (b), (Me-salcy)Co(k -NO3) (c), (Me-salen)Co(OAc)(H2O) (d), and
(Me-sal-cis-cy)Co(II) (e). Thermal ellipsoids are shown at 30 % probability level. Hydrogen atoms and solvent molecules are omitted for clarity.
notion that Jacobsen’s catalyst 1 adopts a square pyramidal or
an octahedral structure, especially in the presence of an additional
donor, where the salcy-ligand forms a square plane with cobalt.
The X-ray structure of (tBu-salcy)Co(III)Cl was reported to be
a distorted square pyramidal.13 Fundamentally, the tetradentate
Schiff base ligand can adopt three configurations in the formation
of the octahedral complexes: trans, cis-b, and cis-a.14 Most of the
Fig. 3 Three possible configurations of the octahedral complexes formed
complexes adopt a trans configuration; the other two configura-
by the tetradentate Schiff base ligand.
tions have been seldom reported. The (salen)(acac)Co(III) complex
(salen = N,N¢-bis(salicylidene)-1,2-diaminoethane) adopts a cis-
b configuration due to the strong chelating action of acety-
lacetonate (acac) ligand.15 The cis-b configuration was found in
cobalt(III) acetate complex of flexible salpn ligand (salpn = N,N¢-
bis(salicylidene)-1,3-diaminopropane).16 The titanium complexes
adopting the cis-b configuration were reported.14,17–20 Other metal
complexes of the cis-b configuration were also found.21–23
The strain associated with the desire of the imines and aromatic
rings to be coplanar is compensated with adoption of the stable
chair conformation on cyclohexane ring. In this binding mode,
trans-diaminocyclohexane unit is situated in an unstrained chair
conformation. The NCCN dihedral angle is 47.5(3)◦ and the
CCCC dihedral angles on the cyclohexane ring are 54–60◦.
On the contrary, the trans-diaminocyclohexane unit is under
some torsional strain in the conventional binding mode of (Me-
salcy)Co(OAc)·2(HOAc). The NCCN dihedral angle is 33.2(9)◦
and the CCCC dihedral angles are 40–44◦, which significantly
deviate from the unstrained angle of 60◦.
2
In the 1H NMR spectrum of (Me-salcy)Co(k -OAc) in dmso-d6,
two sets of signals are observed in the ratio of 1 : 0.09, as is observed
for 2 (Fig. 1). The spectrum can be assigned to an equilibrium
between two species. The minor set is an acetate-decoordinated one
[(Me-salcy)Co(dmso)2]+AcO-, in which the two symmetric parts in
the ligand are equivalent. Because of the observation of different
signals for each symmetric half of the ligand, the major species
is assignable to an acetate-coordinated complex; either to the
2
The pattern of the 1H NMR spectrum of (Me-salcy)Co(k -OAc)
in CD2Cl2 is identical to that of Jacobsen’s catalyst 1 (Fig. 1 and
ESI†), indicating that the binding mode of 1 is identical to that
2
2
aforementioned (Me-salcy)Co(k -OAc) of cis-b configuration or
of (Me-salcy)Co(k -OAc) shown in Fig. 2(b). It is a common
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1
to (Me-salcy)Co(k -OAc)(dmso) of the conventional trans config-
unit is readily reduced and the majority of crystals are the
reduced complex (Me-sal-cis-cy)Co(II) when recrystallization
is carried out slowly for several days in the CH2Cl2/hexane
phase. In the (Me-sal-cis-cy)Co(II) structure (Fig. 2(e)) where
the {ONNO} ligand forms a square plane with cobalt, the
cis-diaminocyclohexane unit is situated in a fairly unstrained
chair conformation. The NCCN dihedral angle is 43.6(2)◦ and
the average CCCC dihedral angle in the cyclohexane ring is 54.2◦.
Adopting the cis-b configuration seems to relate with a high
activity in CO2/epoxide copolymerization, where a key interme-
diate is the complex coordinated by the carbonate ligand.25,26
Carbonate anion is able to act as a chelate ligand as acetate anion.
It was reported that (salcy)Co(III)Br is active for CO2/propylene
oxide copolymerization, while (salen)Co(III)Br is not active at
all.27 In the binary system composed of the cobalt(III) complex
of salen-type ligand and [PPN]Cl, the turnover frequency (TOF)
is reduced to almost half from 570 to 330 h-1 when the trans-1,2-
diaminocyclohexane unit is replaced with 1,2-diaminopropane.28
A similar trend is also observed in the catalytic system of 2.
Complex 2 shows an extremely high TOF (16 000 h-1) enabling
construction of a pilot plant for a continuous process. When
the trans-1,2-diaminocyclohexane unit in 2 is replaced with
1,2-diaminoethane or cis-1,2-diaminocyclohexane, the TOF is
reduced to 6000 h-1 or 3000 h-1, respectively, and the selectivity
also deteriorated from >99% to 94% and 87%, respectively.
Better performance was also achieved by employing chromium(III)
complexes constructed with salan- or salalen-ligand (the reduced
version of salen ligand). The (salan)Cr(III) complex can adopt
a cis-a configuration while (salen)CrCl adopts the conventional
trans configuration.29,30
uration. It was proposed that the paramagnetic square-pyramidal
(tBu-salcy)Co(III)Cl complex is in equilibrium in THF at low
temperature with diamagnetic (tBu-salcy)Co(III)Cl(THF) of the
conventional trans configuration, for which the two symmetric
parts in the ligand are also diastereotopic.24 However, in this
structure of the conventional trans configuration, most of the
diastereotopic protons are observed as collapsed and, if observed
separately, the separation is not as severe as in the cis-b structure.
Because the separation of the signals is not so severe in dmso-d6 as
in CD2Cl2, the major species can preferably be assignable to (Me-
salcy)Co(k -OAc)(dmso) of the conventional trans configuration.
In THF-d8 at room temperature, the H NMR signal pattern is
almost identical to that in CD2Cl2 indicating preservation of the
cis-b configuration of (Me-salcy)Co(k -OAc) in THF.
1
1
2
Formation of the cis-b configuration is quite favourable; even a
weakly coordinating nitrate ligand also triggers formation of the
cis-b configuration as an acetate ligand. The X-ray structure of
2
(Me-salcy)Co(NO3) is almost identical to that of (Me-salcy)Co(k -
OAc) (Fig. 2(c)). The NCCN dihedral angle is 47.5(3)◦ and the
average CCCC dihedral angle on the cyclohexane ring is 56.0◦,
indicating unstrained chair conformation. In dmso-d6, a set of
signals is observed, which is identical to the minor set observed
for (Me-salcy)Co(k -OAc) in dmso-d6. In the 1H NMR spectrum
in CD2Cl2, it displays paramagnetic signals observed as broad
signals above 10 ppm and below 0 ppm.
The trans-diaminocyclohexane unit plays a critical role in the
formation of the unusual cis-b configuration. The analogue com-
plex is prepared using Me-salen ligand (Me-salen = N,N¢-bis(2,4-
dimethylsalicylidene)-1,2-diaminoethane) by the same procedure
applied for construction of (Me-salcy)Co(III)OAc; reacting (Me-
salen)H2 and Co(OAc)2 in CH2Cl2 under an O2 atmosphere in the
2
In conclusion, the chiral (salcy)Co(III) complex can adopt
an unusual cis-b configuration, especially in the presence of a
chelating additional donor. The trans-diaminocyclohexane unit
plays a critical role in the formation of such an unusual cis-b
configuration. The possibility to form such an unusual cis-b con-
figuration should be judiciously considered in developing catalytic
reactions using the pervasive chiral (salcy)metal complexes.
1
presence of 5 equivalents of acetic acid. The H NMR spectrum
of the crystals isolated by diffusing pentane onto CH2Cl2 solution
indicates the structure of (Me-salen)Co(III)OAc·2(HOAc). The
[AcO + AcOH]/[Me-salen] ratio calculated from the integration
values is ~3. The integration ratio is not reduced to 1 but stays
at ~2 by several successive recrystallizations, indicating that acetic
acid coordinates persistently to cobalt resisting the formation of
Acknowledgements
2
the unusual k -acetate cis-b complex.
The structure revealed from a single crystal obtained after five
successive crystallizations is (Me-salen)Co(III)(OAc)(H2O) of the
conventional trans configuration (Fig. 2(d)). Water is incorporated
from air during the crystallizations. The NCCN dihedral angle
is 42◦, which is closer to the unstrained angle of 60◦ than that
observed for (Me-salcy)Co(III)(OAc)·2(HOAc) of the same bind-
ing mode (33.2(9)◦). By adoption of the trans configuration, the
tetradentate Schiff base ligand constructed using diaminoethane
does not feel such a severe torsional strain as the complex con-
structed with trans-diaminocyclohexane, not favouring formation
of the unusual cis-b configuration. In the 1H NMR spectrum
in dmso-d6, two sets of signals are also observed (1:0.39 ratio).
The major set is assignable to (Me-salen)Co(III)(OAc)(dmso), for
which the two aromatic rings are equivalent but the two protons
attached on a -CH2- are diasterotopic to each other to give a
characteristic AA¢B¢B¢ signal pattern for the -CH2CH2- unit. The
minor set is assignable to [(Me-salcy)Co(III)(dmso)2]+(AcO-).
This work was supported by Construction Technology Innovation
Program (CTIP) funded by the MLTM of Korean government and
by Priority Research Centres Program (2010-0028294) through the
NRF grant funded by the MEST of Korean government.
Notes and references
‡ Crystal data for (Me-salcy)Co(OAc)·2(HOAc): C30H39CoN2O8, M =
˚
614.56, monoclinic, a = 11.6690(6), b = 17.5399(13), c = 16.0611(11) A,
◦
3
˚
b = 110.763(4) , V = 3073.8(3) A , T = 293(2) K, space group P 21/c, Z =
4, 9388 reflections measured, 5539 unique (Rint = 0.0335) which were used
2
in all calculations. The final wR2 was 0.1623 (I > 2s(I)). (Me-salcy)Co(k -
OAc): C26H31CoN2O4·CH2Cl2, M = 579.38, tric◦linic, a = 11.0257(4), b
◦
˚
= 12.2862(4), c = 12.3223(4) A, a = 82.506(2) , b = 65.358(2) , g =
◦
3
˚
66.712(2) , V = 1392.42(8) A , T = 293(2), space group P -1, Z = 2, 9154
reflections measured, 6350 unique (Rint = 0.0242) which were used in all
2
calculations. The final wR2 was 0.1499 (I > 2s(I)). (Me-salcy)Co(k -NO3):
C24H28CoN3O5, M = 497.42, monoclinic, a = 12.1174(9), b = 10.9880(8),
◦
3
˚
˚
c = 17.4499(11) A, b = 95.855(4) , V = 2311.3(3) A , T = 293(2), space
group P 21/c, Z = 4, 9108 reflections measured, 5274 unique (Rint = 0.0762)
which were used in all calculations. The final wR2 was 0.0977 (I > 2s(I)).
(Me-salen)Co(OAc)(H2O): C45H59Cl2Co2N4O11.50, M = 1028.72, triclinic,
Complex (Me-sal-cis-cy)Co(III)(OAc) bearing
a
cis-
diaminocyclohexane instead of trans-diaminocyclohexane
a
1446 | Dalton Trans., 2012, 41, 1444–1447
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◦
˚
a = 11.9561(9), b = 15.1436(11), c = 15.2435(8) A, a = 92.675(4) , b =
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