Methylene spacer-regulated structural variation in cobalt(II/III) complexes with bridging acetate and salen- or salpn-type Schiff-base ligands

Article abstract of DOI:10.1002/ejic.200701025

Two linear, trinuclear mixed-valence complexes, [Co^II{(μ- L¹)-(μ-OAc)Co^III(OAc)}₂] (1) and [Co ^II{(μ-L²)(μ-OAc)Co^III(OAc)}₂] (2) and two mononuclear Co^III complexes [Co^III{L ³}(OAc)] (3), and [Co^III{L⁴}(OAc)] (4) were prepared and the molecular structures of 1, 2 and 4 elucidated on the basis of X-ray crystallography [OAc = Acetate ion, H₂L¹ = H ₂Salen = 1,6-bis(2-hydroxyphenyl)-2,5-diazahexa-1,5-diene, H ₂L² = H₂Me₂-Salen = 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene, H₂L³ = H₂Salpn = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene, H ₂L⁴ = H₂Me₂Salpn = 2,8-bis(2-hydroxyphenyl)-3,7-diazanona-2,7-diene]. In complexes 1 and 2, the acetate groups show both monodentate and bridging bidentate coordination modes, whereas chelating bidentate acetate is present in 4. The terminal Co ^IIIN₂O₄ centres in 1 and 2 exhibit uniform facial arrangements of both non-bridged N₂O and bridging O ₃ donor sets and the Co^II centre is coordinated to six (four phenoxo and two acetato) oxygen atoms of the bridging ligands. The effective magnetic moment at room temperature corresponds to the presence of high-spin Co^II in both 1 and 2. The complexes 1 and 2 are thus Co^III(S = 0)-Co^II(S = 3/2)-Co^III(S = 0) trimers. Complexes 3 and 4 are monomeric and diamagnetic containing low-spin Co^III(S = 0) with chelating tetradentate Schiff base and bidentate acetate. Calculations based on DFT rationalise the formation of trinuclear or mononuclear complexes. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701025

FULL PAPER
DOI: 10.1002/ejic.200701025
Methylene Spacer-Regulated Structural Variation in Cobalt(II/III) Complexes
with Bridging Acetate and Salen- or Salpn-Type Schiff-Base Ligands
Shouvik Chattopadhyay,[a,b] Michael G. B. Drew,[c] and Ashutosh Ghosh*[a]
Keywords: Cobalt / Mixed valence / Schiff base / N2O2 ligands / Crystal structures / DFT calculations
Two linear, trinuclear mixed-valence complexes, [CoII{(µ-L1)-
(µ-OAc)CoIII(OAc)}2] (1) and [CoII{(µ-L2)(µ-OAc)CoIII(OAc)}2]
(2) and two mononuclear CoIII complexes [CoIII{L3}(OAc)] (3),
and [CoIII{L4}(OAc)] (4) were prepared and the molecular
structures of 1, 2 and 4 elucidated on the basis of X-ray crys-
tallography [OAc = Acetate ion, H2L1 = H2Salen = 1,6-bis(2-
uniform facial arrangements of both non-bridged N2O and
bridging O3 donor sets and the CoII centre is coordinated to
six (four phenoxo and two acetato) oxygen atoms of the
bridging ligands. The effective magnetic moment at room
temperature corresponds to the presence of high-spin CoII in
both 1 and 2. The complexes 1 and 2 are thus CoIII(S = 0)–
CoII(S = 3/2)–CoIII(S = 0) trimers. Complexes 3 and 4 are mo-
nomeric and diamagnetic containing low-spin CoIII(S = 0)
with chelating tetradentate Schiff base and bidentate ace-
tate. Calculations based on DFT rationalise the formation of
trinuclear or mononuclear complexes.
hydroxyphenyl)-2,5-diazahexa-1,5-diene, H2L2
Salen 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene,
= H2Me2-
=
H2L3 = H2Salpn = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-
1,6-diene, H2L4 = H2Me2Salpn = 2,8-bis(2-hydroxyphenyl)-
3,7-diazanona-2,7-diene]. In complexes 1 and 2, the acetate
groups show both monodentate and bridging bidentate coor-
dination modes, whereas chelating bidentate acetate is pres-
ent in 4. The terminal CoIIIN2O4 centres in 1 and 2 exhibit
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
auto-oxidation of hydrocarbons.[13] Linear trinuclear CoIII
-
Introduction
(S = 0)–CoII(S = 3/2)–CoIII(S = 0) complexes containing
deprotonated Schiff base are rare and to the best of our
knowledge only three such structures have been reported so
far.[14] One is [CoIICoIII2(SO3)2(Me2Salpn)2(nPrOH)2]·
nPrOH[14a] which was obtained by the reduction of corre-
sponding CoIII complexes by SO2. The other two complexes
are [CoII{(µ-Salen)(µ-OAc)CoIII(NCS)}2][14b] and [CoII(µ-
Salpn)2(µ-OAc)2Co2III(NCS)2],[14c] both of which have been
prepared by oxidation of the CoII salt [H2Salen = 1,6-bis-
(2-hydroxyphenyl)-2,5-diazahexa-1,5-diene, H2Salpn = 1,7-
Cobalt-Salen related systems have a wide range of appli-
cations: they are used as catalysts for the oxygenation of
organic molecules, they can act as antiviral agents, due to
their ability to interact with proteins and nucleic acids, and
they have been used to mimic biological co-factors, such as
cobalamin.[1–7] Salen-type ligands are also significant be-
cause of the fascinating ability of the phenoxo oxygen
atoms to form µ2-bridges thus affording high-nuclearity
complexes which could act as promising candidates to offer
valuable insights into various natural electron-transfer
events.[8] High-nuclearity coordination complexes achieved
by these types of ligand along with other appropriate co-
ligands provide tractable examples to rationalise magnetic
exchange phenomenon[9,10] emerging from temperature-de-
pendent interaction between the spin states of individual
magnetic centres. Amongst multinuclear complexes of tran-
sition metals, cyclic trinuclear cobalt complexes form an im-
portant class[11] as they have been found to show some cata-
lytic activity in the epoxidation of olefins[12] and in the
bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene,
H2Me2-
Salpn = 2,8-bis(2-hydroxyphenyl)-3,7-diazanona-2,7-diene].
Our attempts with Salen (H2L1) and H2Me2Salen, i.e. 7-
bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene (H2L2), as
representatives of the quadridentate bis(salicylidene)-type
Schiff-base ligands family, furnished novel CoIII–CoII–CoIII
mixed-valence linear trinuclear species, whereas the ligands,
Salpn (H2L3) and Me2Salpn (H2L4) afforded mononuclear
CoIII complexes (Scheme 1). X-ray crystal structure deter-
minations of three of these complexes have established their
structures. Thus, the use of bis(salicylidene) ligands with
[a] Department of Chemistry, University College of Science, Uni- polymethylene spacers of varying flexibility allowed us to
versity of Calcutta,
regulate their electronic and steric demands which could ef-
fectively modulate structural versatility enlightening spin
and valence change phenomena.
Herein, we report the synthesis, spectroscopic characteri-
sations and room-temperature magnetic moments of four
92 A.P.C. Road, Kolkata 700009, India
E-mail: ghosh_59@yahoo.com
[b] Department of Chemistry, Jhargram Raj College,
Jhargram, West Midnapore 721507, West Bengal, India
[c] School of Chemistry, The University of Reading,
P. O. Box 224, Whiteknights, Reading RG6 6AD, UK
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S. Chattopadhyay, M. G. B. Drew, A. Ghosh
FULL PAPER
gands H2L3 and H2L4 we increased the metal/ligand ratio
(up to 2:1) but we always obtained as product the mononu-
clear complexes 3 and 4. On the other hand, for the ligands
H2L1 and H2L2 even if we decreased the metal/ligand ratio
to 1:1, we did not obtain any mononuclear CoIII complexes
analogous to 3 and 4, instead trinuclear complexes (1 and
2) resulted invariably. Thus, irrespective of the molar ratio
of metal and ligand, H2Salen and H2Me2Salen formed tri-
nuclear complexes whereas H2Salpn and H2Me2Salpn
yielded mononuclear complexes.
To investigate the importance of air oxidation, the reac-
tions were performed under N2 atmosphere leading to a
red compound being obtained as the product, which was
identified as a mononuclear CoII complex ([CoL], L = L1,
L2, L3, L4) by IR spectra, elemental analysis and mass spec-
tra. The formation of no complex with CoIII logically
tempted us to believe that atmospheric oxygen here acted
as an external oxidant according to the Equations (1) and
(2).
Scheme 1.
complexes. Three of them have been characterised by X-ray
crystallography. Density Functional Theory with the
B3LYP methodology is used to rationalise the formation of
trimeric or monomeric complexes with the incorporation of
a CH2 spacer in the diamine chain.
(1)
Results and Discussion
(2)
Syntheses
Again, 1, 2, 3 and 4 could be reduced by N2H4 to a
reddish brown gelatinous precipitate. These were mono-
meric cobalt(II) complexes. Amongst these, diffraction
quality dark red-brown single crystals of a cobalt(II)-Salen
complex were collected by maintaining the reaction flask at
about 40 °C for two days under N2 atmosphere. The di-
meric structure (Scheme 2) of this compound has been re-
ported elsewhere by another group.[15]
Facile condensation of 1,2-diaminoethane or 1,3-diami-
nopropane in 1:2 molar ratio with salicylaldehyde and 2-
hydroxyacetophenone separately, afforded four neutral con-
densates, H2Salen, H2Salpn, H2Me2Salen and H2Me2Salpn
(general abbreviations H2L1, H2L2, H2L3 and H2L4, respec-
tively, where H atoms are dissociable phenolic hydrogens),
which have been employed as the tetradentate chelating li-
gands in the present work (Scheme 1).
Methanol solution of Co(OAc)2·4H2O was added sepa-
rately in a methanol solution of H2L1 and H2L2 in 3:2 mo-
lar ratio with constant stirring at ambient temperature. It
was kept for 2 days to separate brown compounds. Subse-
quent crystallisation of the product from dichloromethane
solution gave brown crystals of the trinuclear mixed-valence
cobalt complexes (1 and 2) as authenticated by X-ray struc-
ture determinations (Scheme 1).
However, when we added methanol solution of Co-
(OAc)2·4H2O to the ligands H2L3 and H2L4 in the same
molar ratio (3:2), monomeric compounds 3 and 4 were
formed (instead of the formation of trimeric analogues of 1
and 2), both of which contained CoIII with chelating acetate
(Scheme 1) as was evident from the X-ray structure deter-
mination of 4. We failed to grow the single crystal of com-
plex 3 and it was characterised by IR and UV/Vis spectra,
elemental analysis, mass spectra and magnetic moment
Scheme 2. A sketch of the dimeric Co(Salen) compounds.
Structure Description: 1 and 2
The trinuclear structures of 1 and 2 as shown in Fig-
measurement. To obtain the trinuclear compounds with li- ures 1 and 2, each contain three cobalt atoms in an approxi-
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Spacer-Regulated Structural Variation in Co(II/III) Complexes
mately linear arrangement. The data for 2 are poor because O(30) from another. These oxygen atoms are also bridging
the crystal gave a very weak diffraction pattern but it is to one of the two outer cobalt atoms Co(2) and Co(3). Mo-
clear that the dimensions are very similar to those found in lecular dimensions are given in Tables 1 and 2 and confirm
1. Each cobalt atom has a six-coordinate octahedral envi- that Co(1) has oxidation state (II) and Co(2) and Co(3)
ronment. In both structures the central cobalt atom Co(1) oxidation state (III). This is indicated by the fact that these
is bonded to four oxygen atoms forming an equatorial oxygen atoms are bound more strongly to the Co(2) and
plane, O(31) and O(50) from one ligand and O(11) and Co(3) with distances ranging from 1.840(5)–1.924(4) Å in
Figure 1. The structure of 1 with ellipsoids at 25% probability.
Figure 2. The structure of 2 with ellipsoids at 25% probability.
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S. Chattopadhyay, M. G. B. Drew, A. Ghosh
FULL PAPER
the two structures compared to Co(1)–O distances of Table 2. Selected bond angles for compounds 1 and 2.
2.070(6)–2.173(5) Å. To complete the metal coordination
1
2
sphere of Co(1), there are two bridging acetates with oxygen
atoms O(61) and O(81) in mutually cis positions at dis-
tances ranging from 2.023(6)–2.082(6) Å. These acetates
O(61)–Co(1)–O(81)
O(61)–Co(1)–O(11)
O(81)–Co(1)–O(11)
88.46(13)
87.84(14)
165.52(15)
164.56(14)
87.29(12)
99.74(13)
92.63(15)
86.64(13)
107.51(15)
72.31(12)
84.22(15)
92.93(14)
72.77(15)
110.81(13)
176.83(14)
89.0(2)
84.8(2)
163.2(2)
162.1(2)
83.2(2)
106.9(2)
90.8(2)
87.9(2)
107.8(2)
72.9(2)
88.9(2)
92.6(2)
71.8(2)
107.5(2)
179.5(2)
form bridges to the adjacent cobalt atoms such that Co(3) O(61)–Co(1)–O(31)
O(81)–Co(1)–O(31)
is bonded to O(63) at 1.924(4), 1.923(6) Å in the two struc-
O(11)–Co(1)–O(31)
tures and Co(2) is bonded to O(83) at 1.908(3), 1.900(6) Å
O(61)–Co(1)–O(50)
in the two structures. The coordination spheres of Co(2)
O(81)–Co(1)–O(50)
and Co(3) are completed with two nitrogen atoms of the
ligand [1.810(9)–1.902(8) Å] and an oxygen atom from a
monodentate acetate with distances Co(2)–O(91) 1.901(4),
1.884(6) Å and Co(3)–O(71) 1.898(4), 1.846(7) Å in 1 and
2, respectively.
O(11)–Co(1)–O(50)
O(31)–Co(1)–O(50)
O(61)–Co(1)–O(30)
O(81)–Co(1)–O(30)
O(11)–Co(1)–O(30)
O(31)–Co(1)–O(30)
O(50)–Co(1)–O(30)
Table 1. Selected bond lengths for compounds 1 and 2.
N(42)–Co(2)–O(50)
N(42)–Co(2)–N(39)
O(50)–Co(2)–N (39)
N(42)–Co(2)–O(91)
O(50)–Co(2)–O(91)
N(39)–Co(2)–O(91)
N(42)–Co(2)–O(83)
O(50)–Co(2)–O(83)
N(39)–Co(2)–O(83)
O(91)–Co(2)–O(83)
N(42)–Co(2)–O(31)
O(50)–Co(2)–O(31)
N(39)–Co(2)–O(31)
O(91)–Co(2)–O(31)
O(83)–Co(2)–O(31)
95.4(2)
85.2(3)
176.85(15)
90.9(2)
87.42(15)
95.66(17)
84.8(2)
92.60(15)
84.37(16)
175.74(18)
177.0(2)
82.83(13)
96.42(19)
91.34(15)
92.90(15)
94.6(13)
87.2(3)
178.2(3)
93.8(3)
86.3(2)
93.3(3)
85.7(3)
93.9(2)
86.5(3)
179.4(3)
176.4(3)
84.3(2)
93.9(3)
89.6(3)
91.0(3)
1
2
Co(1)–O(61)
Co(1)–O(81)
Co(1)–O(11)
Co(1)–O(31)
Co(1)–O(50)
Co(1)–O(30)
2.037(3)
2.038(3)
2.112(3)
2.123(3)
2.137(3)
2.143(3)
2.082(6)
2.023(6)
2.151(7)
2.173(5)
2.086(6)
2.070(6)
Co(2)–N (42)
Co(2)–O(50)
Co(2)–N (39)
Co(2)–O(91)
Co(2)–O(83)
Co(2)–O(31)
1.858(5)
1.888(4)
1.888(5)
1.901(4)
1.908(3)
1.911(3)
1.825(8)
1.887(5)
1.871(7)
1.884(6)
1.900(6)
1.882(6)
N(22)–Co(3)–N(19)
N(22)–Co(3)–O(30)
N(19)–Co(3)–O(30)
N(22)–Co(3)–O(71)
N(19)–Co(3)–O(71)
O(30)–Co(3)–O(71)
N(22)–Co(3)–O(11)
N(19)–Co(3)–O(11)
O(30)–Co(3)–O(11)
O(71)–Co(3)–O(11)
N(22)–Co(3)–O(63)
N(19)–Co(3)–O(63)
O(30)–Co(3)–O(63)
O(71)–Co(3)–O(63)
O(11)–Co(3)–O(63)
82.8(4)
97.8(3)
178.9(2)
97.4(3)
93.6(2)
87.20(16)
174.9(3)
96.3(3)
83.04(16)
87.66(17)
83.3(3)
88.5(4)
95.1(3)
176.1(3)
92.6(3)
93.3(3)
88.1(3)
176.9(3)
93.6(3)
82.8(3)
89.7(3)
86.1(3)
84.7(3)
94.0(3)
177.6(3)
91.7(2)
Co(3)–N (22)
Co(3)–N (19)
Co(3)–O(30)
Co(3)–O(71)
Co(3)–O(11)
1.810(9)
1.865(6)
1.895(4)
1.898(4)
1.914(4)
1.902(8)
1.840(8)
1.840(6)
1.846(7)
1.904(5)
The intramolecular separations between Co(2)···Co(1)
are 3.074 and 3.059 Å in 1 and 2 respectively, whereas Co(1)···
Co(3) separations are 3.075 in 1 and 3.051 Å in 2. None of
these distances are sufficiently short to imply any metal–
metal bonding or allow intra-metal spin exchange through
mutual interaction. The bridging angles, Co(1)–O(50)–
Co(2), Co(1)–O(31)–Co(2), Co(1)–O(30)–Co(3) and Co(1)–
O(11)–Co(3), 99.5(2), 99.1(1), 99.0(2) and 99.5(2) in com-
plex 1 and 100.6(3), 97.7(3), 102.4(3) and 97.4(2) in complex
2, indicate non-collinear CoIII–O–CoII fragments, not suit-
able for significant electronic exchange.
85.6(2)
93.54(17)
178.9(2)
91.60(18)
Table 3. Hydrogen bond lengths [Å] and angles [°] for complex 1.
D–H···A
D···A
H···A
ЄD–H···A
O100–H101···O73
O100–H102···O93
2.759(8)
2.818(8)
1.87(6)
1.99(7)
167(8)
157(7)
In complex 1, O(73) of one of the terminal acetate moie-
ties is involved in hydrogen bonding with H(101) of a water
molecule present in the crystal (Table 3). The other hydro-
gen of the same water molecule, H(102), forms an H-bond
with a symmetry related (1 –y, x, 2 –z) oxygen atom, O(93Ј)
of a terminal acetate ligand. Similarly, H(102Ј), H(102ЈЈ)
Description of the Structure of 4
The structure of 4 is shown in Figure 4. The structure
and H(102ЈЈЈ) are hydrogen bonded with O(93ЈЈ), O(93ЈЈЈ) shows the cobalt atom in a distorted octahedral environ-
and O(93) (ЈЈ = y, 1 –x, 2 –z, ЈЈЈ = 1 –x, 1 –y, z) respectively ment bonded to two oxygen atoms of an acetate group and
to form a cyclic dodecanuclear tetramer of complex 1 (Fig- four donor atoms of ligand L4. The distortion to the octa-
ure 3). There is no such H-bond present in complex 2.
hedron is primarily caused by the small bite angle to the
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Spacer-Regulated Structural Variation in Co(II/III) Complexes
Figure 3. The tetrameric H-bonded structure in 1.
acetate of 65.9(1)°. The bond lengths to the acetate are Table 4. Selected bond lengths [Å] and bond angles [°] for com-
pound 4.
1.996(1), 1.982(1) Å and for the ligand to oxygen 1.860(1),
1.872(1) Å and to nitrogen 1.935(1), 1.917(1) Å (Table 4).
Bond lengths
Co(1)–O(11)
Co(1)–O(31)
Co(1)–O(41)
Co(1)–O(43)
Co(1)–N(19)
Co(1)–N(23)
1.860(1)
1.872(1)
1.996(1)
1.982(1)
1.935(1)
1.917(1)
Bond angles
O(11)–Co(1)–O(31)
O(11)–Co(1)–N(23)
O(31)–Co(1)–N(23)
O(11)–Co(1)–N(19)
O(31)–Co(1)–N(19)
N(23)–Co(1)–N(19)
O(11)–Co(1)–O(43)
O(31)–Co(1)–O(43)
N(23)–Co(1)–O(43)
N(19)–Co(1)–O(43)
O(11)–Co(1)–O(41)
O(31)–Co(1)–O(41)
N(23)–Co(1)–O(41)
N(19)–Co(1)–O(41)
O(43)–Co(1)–O(41)
90.07(5)
90.60(5)
90.37(5)
90.24(5)
176.83(5)
92.79(6)
99.48(5)
87.67(5)
169.72(5)
89.17(5)
165.04(5)
86.47(5)
103.96(5)
92.43(5)
65.86(5)
Figure 4. The structure of 4 with ellipsoids at 35% probability.
The conformation of the saturated six-membered ring
[Co(1)–N(19)–C(20)–C(21)–C(22)–N(23)–Co(1)] is interme-
diate between boat and twist-boat with puckering param-
eters[16,17] q3 = –0.049(2), q2 = Q = 0.871(1), θ = 93.2(1), Φ donors occupy the four equatorial positions, the other in
= 196.22(11). The N(19)–Co(1)–N(23) angle is 92.8(1)° and which one oxygen is displaced from the equatorial plane,
is typical of a six-membered chelate ring.[18] In the octahe- occupying an axial position of the coordination polyhe-
dral complex of a Salen-type ligand, when it acts as a tetra- dron.[19] In the present case, because of the presence of the
dentate ligand, two different arrangements of four donor bidentate acetate, the ligand necessarily assumes the second
atoms around the metal atom is found; one in which the alternative.
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Room-Temperature Magnetic Moment
inert atmosphere and CoII is stabilised by the weaker field
strength of the phenoxo-bridged ligand. Besides the ligand
field strength, other factors, e.g. flexibility of the Schiff base
and the role of co-ligands should also be taken into con-
sideration for the formation of such complexes with dif-
ferent nuclearities. The Salpn ligand stabilises both the
square-planar coordination geometry and the twisted ge-
ometry like complex 4 owing to its flexibility. Although the
less-flexible Salen ligand also stabilises the meridional coor-
dination geometry, it does not usually stabilise the twisted
geometry owing to its structural restriction. Generally, CuII
and NiII complexes with these kinds of tetradentate Schiff-
The effective magnetic moment at room temperature of
the two trinuclear complexes (1 and 2) was 5.36 and
5.10 BM, which corresponds to the presence of a high-spin
CoII in the molecules. The complexes thus contain low spin
CoIII and high spin CoII, i.e. a CoIII(S = 0)–CoII(S = 3/2)–
CoIII(S = 0) trimer. Complexes 3 and 4 are monomers with
only one low-spin CoIII and thus are diamagnetic.
Discussion
There are two types of trinuclear linear cobalt complexes base ligands are able to coordinate to other metal ions in a
with these types of ligands reported in the literature.[14,20] di-µ-phenoxo bridging manner as the complex ligands. But
The first type includes CoII–CoII–CoII complexes[20] CoIII, MnIII or CrIII complexes with the same ligands are
whereas the second variety contains mixed-valence CoIII
–
hard to coordinate to other metal ions in the same manner.
CoII–CoIII complexes.[14] The structural data reveal that However, a second bridging group (e.g. an acetate ion) may
CoIII–N and CoIII–O distances in Co-Salen-type complexes reinforce the bridging mode of the phenoxo group of such
vary from 1.82–1.88 Å and 1.88–1.91 Å, respectively, metallo-ligands to result in the formation of polynuclear
whereas those distances in Co-Salpn-type complexes range complexes such as 1 and 2. Moreover, the coordination
from 1.92–1.94 Å and 1.86–1.90 Å.[14]
mode of the co-ligands is also important in controlling the
These variations in bond length have been investigated nuclearity of the product, especially in the case of flexible
using Quantum Mechanics methods. The two structures ligands L3 and L4. It seems that a monodentate co-ligand
Co(Me2Salpn)(OAc) (4) and Co(Me2Salen)(OAc) were in- (e.g. SCN– or Py) favours a trinuclear product [R] while a
vestigated with the Gaussian03 program.[21] The crystal chelating bidentate co-ligand (e.g. CH3COO–) gives a mo-
structure of 4 was used as a starting model and that of nonuclear one.
Co(Me2Salen)(OAc) was built from that by removing a CH2
We investigated these reactions further using Quantum
moiety and optimisation with molecular mechanics. The Mechanics methods with the DFT methodology described
two structures were then optimised using Density Func- above. Thus, the enthalpy of the reaction Co(OAc)2+ + L2–
tional Theory with the B3LYP methodology in which the = Co(OAc)L was calculated for both L = Salen and L =
Co atom was given the LANL2DZ basis set and the re- Salpn by geometry optimisation of all the structures in the
maining atoms the 6-31+G* basis set. After geometry op- reactions. The starting model for Co(OAc)2+ was obtained
timisation, the two structures had equivalent Co–O dis- by removing L from the Co(OAc)L complex and for the
tances but the Co–N distances were 1.92, 1.92 Å in ligands was built using the all-trans conformations. Values
Co(Me2Salen)(OAc) and 1.95, 1.96 Å in Co(Me2Salpn)- for ∆H of –68.18 kcalmol–1 for Salen and –61.50 kcalmol–1
(OAc).
for Salpn were obtained. The significant difference of
Both the X-ray structural data and theoretical calcula- 6.68 kcalmol–1 suggests that the Salen mononuclear com-
tion indicate that in Salpn-type ligands the CoIII–N bond plex is easier to form than the mononuclear Salpn complex.
lengths are longer than Salen-type ligands presumably due However, as demonstrated in this work, in practice the mo-
to the presence of a bulkier trimethylene group between the nonuclear Salpn complex is formed but the mononuclear
N atoms causing a weaker ligand field for the Schiff bases Salen complex is not as instead it forms the trinuclear com-
derived from 1,3-propanediamine and its derivative. There- plex. This calculation therefore strongly suggests that for
fore, it is reasonable to expect that strong field Salen-type Salen, it is the preference for the trimer that is the driving
ligands can stabilise CoIII and thereby form mixed-valence force for its formation rather than any instability in the
CoIII–CoII–CoIII complexes easily but with weaker field monomer. However, calculations on the trimers were incon-
Salpn-type ligands, the formation of such a mixed-valence clusive due to convergence problems.
complex is unfavourable until another strong field mono-
dentate ligand such as SCN– or Py is coordinated to the
terminal cobalt atoms.[14b,14c] On the other hand, the Salpn
IR and Electronic Spectra
ligands also form either the mononuclear CoIII complexes
or trinuclear CoII–CoII–CoII complexes depending on the
The most important aspects concerning the infrared
method of preparation and the nature of other co-ligands. spectra of the four compounds 1, 2, 3 and 4 deal with the
The CoIII complexes are formed in the aerobic condition possibility of characterising the different coordination
and become stable as in the mononuclear complex the che- modes for the acetate ligand to a transition metal, which is
lated Salpn ligand has a shorter Co–O bond length than its usually detected by the differences (∆ν) in the antisymmetric
phenoxo bridging counterpart and thus can exert stronger νasym (COO) and symmetric stretching frequencies νsym
-
crystal field stabilisation energy. The CoII–CoII–CoII com- (COO).[22] The infrared spectrum of complex 1 displays sev-
plexes are formed when the reaction is carried out under an eral intense bands at 1650, 1597, 1447 and 1425 cm–1 and
1698
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Spacer-Regulated Structural Variation in Co(II/III) Complexes
they are assigned to ν(COO–) frequencies. The appearance Schiff base can regulate the nuclearity and oxidation state
of four bands corroborates the presence of two types of of the exhaustively studied cobalt–Schiff base complexes.
acetate groups. Separations of 150 cm–1 (between the bands
at 1597 and 1447 cm–1) and 225 cm–1 (between 1650 and
1425 cm–1) suggest their assignment to bridging and uni-
Experimental Section
dentate carboxylate coordinations, respectively (163 cm–1
Materials and Methods: All chemicals were of reagent grade and
used without further purification. Elemental analysis (C, H, N) was
for the free acetate ion). A similar structure is indicated for
complex 2 because of the similarity of the pattern of the IR
performed with a Perkin–Elmer 240C elemental analyser. IR spec-
spectrum.
tra (4500–500 cm–1) were recorded as KBr pellets with a Perkin–
Elmer RXI FT-IR spectrophotometer. Electronic spectra in aceto-
nitrile (1200–350 nm) were recorded with a Hitachi U-3501 spectro-
photometer. The magnetic susceptibility measurements were per-
formed with an EG & PAR vibrating sample magnetometer, model
155 at room temperature and diamagnetic corrections were made
using Pascal’s constants.
Two intense bands at 1586 and 1474 cm–1 in the IR spec-
trum of complex 3 are assigned to ν(COO–) frequencies.
Separations between frequencies of 112 cm–1 indicate the
chelating bidentate coordination of the acetate moiety.
In the IR spectra of all four complexes a distinct band
due to the azomethine (C=N) group at 1649–1573 cm–1 is
routinely seen and the red shift of this C=N absorption of
about 15 cm–1 on going from the free ligands to the com-
plexes suggests the weak π-accepting ability of the coordi-
nated ligands. A broad band around 3400 cm–1 indicates
the presence of an H2O molecule in the crystals of 1 and 2.
The electronic spectra of 1 and 2 display one low energy
absorption band at 703 and 547 nm, respectively, attribut-
able to a transition in the visible region of a low-spin co-
balt(III) in octahedral geometry, obscuring the transitions
of the divalent metal ions. Bands from CoII are Laporte-
forbidden transitions and are assumed to be too weak to be
visible. The electronic spectra of 3 and 4 also contain low-
energy bands at 658 and 569 nm.
Synthesis of the Ligands: The ligands H2L1 and H2L2 were synthe-
sised by condensation of 1,2-diaminoethane with salicylaldehyde or
2-hydroxyacetophenone, respectively, in dry methanol following the
literature method.[24] The ligands H2L3 and H2L4 were prepared in
the same way using 1,3-diaminopropane instead of 1,2-diamino-
ethane.
Synthesis of Complexes 1 and 2: To prepare complexes 1 and 2, a
methanol solution (20 mL) of cobalt(II) acetate tetrahydrate
(3 mmol, 747 mg) was added to the ligand solution of H2L1
(2 mmol, 536 mg) and H2L2 (2 mmol, 624 mg), respectively, and
the mixture was stirred vigorously for 3 h. Deep brown precipitates
of complexes 1 and 2 were separated out after 2 days and were
collected by filtration. Diffraction quality single crystals were ob-
tained after a few days by slow evaporation of the dark brown
dichloromethane solution of the compounds in an open atmo-
sphere.
Luminescence Studies
Compound 1: Yield 0.812 g (82%). C40H45Co3N4O14.5 (990.59):
calcd. C 50.81, H 4.26, N 5.93; found C 51.23, H 4.93, N 6.01.
UV/Vis (acetonitrile): λmax (εmax, dm3 mol–1 cm–1) = 703 (1879),
364(8058) nm. Magnetic moment µ = 5.36 BM.
The spectroscopic data of the four complexes and that
for the free ligands in acetonitrile solution are listed in
1
Table 5. These are assigned to intraligand (π-π*) fluores-
cence.[23]
Compound 2: Yield 0.877 g (86%). C44H50Co3N4O13 (1019.67):
calcd. C 52.76, H 4.83, N 5.59; found C 52.90, H 5.74, N 6.31.
UV/Vis (acetonitrile): λmax (εmax, dm3 mol–1 cm–1) = 547 (1159), 355
(5185) nm. Magnetic moment µ = 5.10 BM.
Table 5. Photophysical data for complexes 1–4 and also for the free
ligands.
Sample
Absorption
Emission
Synthesis of Complexes 3–4: Complexes 3 and 4 were prepared by
adding a methanol solution (20 mL) of cobalt(II) acetate hexahy-
drate (2 mmol, 498 mg) into the methanol solution of H2L3
(2 mmol, 564 mg) and H2L4 (2 mmol, 620 mg), respectively, and
stirring vigorously. Brown precipitates separated out immediately
and were collected by filtration. Diffraction quality single crystals
of 4 were obtained after a few days by slow evaporation of a brown
dichloromethane solution in a refrigerator, but it was not possible
to isolate suitable single crystals of 3 for X-ray diffraction.
1
364
355
371
359
406
398
410
414
517
502
523
506
472
454
473
480
2
3
4
H2Salen
H2Salpn
H2Me2Salen
H2Me2Salpn
Compound 3: Yield 0.34 g (85%). C19H19CoN2O4 (398.07): calcd.
C 57.29, H 4.81, N 7.03; found C 57.18, H 4.56, N 7.32. UV/Vis
(acetonitrile): λmax (εmax
, = 658 (1055), 371
dm3 mol–1 cm–1)
Conclusions
(8045) nm. MS: m/z (%) = 400.08 [M+], 401.09 [M + 1]+, 402.09
[M + 2]+. Magnetic moment, diamagnetic.
In this study, it has been shown that the mixed oxidation
state tricobalt(III/II/III) complex is produced when a Salen-
type Schiff base is allowed to react with CoII acetate in the
presence of air. On the other hand, Salpn-type Schiff bases
yield mononuclear CoIII complexes under the same condi-
tions. These examples thus demonstrate for the first time
Compound 4: Yield 0.374 g (88%). C21H23CoN2O4 (426.34): calcd.
C 59.16, H 5.44, N 6.57; found C 59.24, H 5.56, N 6.52. UV/Vis
(acetonitrile): λmax (εmax
(9876) nm. Magnetic moment, diamagnetic.
, = 569 (972), 359
dm3 mol–1 cm–1)
X-ray Crystallography: Crystal data for all the three crystals are
that the methylene spacer in the diamine fragment of the given in Table 6. Crystal data for the three crystals were measured
Eur. J. Inorg. Chem. 2008, 1693–1701
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1699

S. Chattopadhyay, M. G. B. Drew, A. Ghosh
FULL PAPER
Table 6. Crystal data for compounds 1, 2 and 4.
1
2
4
Formula
Formula weight
Temperature [K]
C40H45Co3N4O14.5
990.59
150
C44H50Co3N4O13
1019.67
150
C21H23CoN2O4
426.34
150
Crystal system
Space group
tetragonal
I4
monoclinic
P21/a
triclinic
P1
¯
a [Å]
b [Å]
c [Å]
α [°]
23.426(1)
23.426(1)
16.328(1)
(90)
(90)
(90)
8960.0(8)
4
1.469
17.989(3)
11.664(2)
21.578(4)
(90)
97.59(1)
(90)
4487.8(1)
4
1.509
8.604(1)
10.071(1)
11.311(1)
99.64(1)
94.80(1)
106.33(1)
918.4(2)
2
β [°]
γ [°]
V [Å3]
Z
Dcalcd. [mgm–3]
1.542
0.97
µ [mm–1]
1.17
1.17
F(000)
4080
0.05ϫ0.05ϫ0.30
30785
13073
0.060
2108
0.03ϫ0.03ϫ0.22
30745
13070
0.205
444
Crystal size [mm]
Total reflections
Independent reflections
Rint
0.05ϫ0.05ϫ0.30
6514
5141
0.014
Collected reflections [I Ͼ 2σ(I)]
R1, wR2 (all data)
R1, wR2 [I Ͼ 2σ(I)]
5144
0.1543, 0.1302
0.0544, 0.1142
1975
0.3319, 0.2175
0.0793, 0.1901
4304
0.0391, 0.0877
0.0314, 0.0861
[6]
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with Mo-Kα radiation at 150 K on an Oxford Diffraction X-Cali-
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of 10 s, 50 s and 2 s, respectively, for 1, 2 and 4. Data analysis was
carried out with the CRYSALIS program[25] to provide 13073,
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Non-hydrogen atoms were refined with anisotropic thermal param-
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tively.
[7]
[8]
[9]
[10]
CCDC-656434 (for 1), -656435 (for 2) and -656436 (for 4) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11]
Acknowledgments
We like to thank the Engineering and Physical Sciences Research
Council (EPSRC) and the University of Reading for funds for the
Marresearch Image Plate and Oxford Diffraction CCD systems.
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[25]
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[27]
Received: September 21, 2007
Published Online: February 13, 2008
Eur. J. Inorg. Chem. 2008, 1693–1701
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1701