Syntheses and crystal structures of mono-, di- and trinuclear cobalt complexes of a salen type ligand

Article abstract of DOI:10.1016/j.ica.2008.06.023

Three cobalt complexes containing the salen type ligand, bis(salicylidene)-meso-1,2-diphenylethylenediaminato (mdpSal^2-), are reported. The complexes differ in nuclearity and include the mononuclear, Co(mdpSal) (1), which contains a Co(II) meta

Full text of DOI:10.1016/j.ica.2008.06.023

Inorganica Chimica Acta 362 (2009) 1405–1411
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Syntheses and crystal structures of mono-, di- and trinuclear cobalt complexes of a
salen type ligand
Jenna Welby a, Linah N. Rusere a, Joseph M. Tanski b, Laurie A. Tyler a,
*
a Department of Chemistry and Biochemistry, Union College, Schenectady, NY 12308, United States
b Department of Chemistry, Vassar College, Poughkeepsie, NY 12604, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 April 2008
Received in revised form 17 June 2008
Accepted 18 June 2008
Available online 25 June 2008
Three cobalt complexes containing the salen type ligand, bis(salicylidene)-meso-1,2-diphenylethylenedi-
aminato (mdpSal2ꢀ), are reported. The complexes differ in nuclearity and include the mononuclear,
Co(mdpSal) (1), which contains a Co(II) metal center bound to one mdpSalꢀ2 ligand frame in a square pla-
nar geometry. The second complex is the dinuclear [Co(mdpSal)Cl]2 (2) in which both cobalt ions have
been oxidized to the +3 oxidation state. The overall geometry of complex 2 is an edge-sharing bioctahe-
dron with the coordination sphere around each cobalt metal center consisting of one mdpSalꢀ2 ligand
and one Clꢀ ion. The shared edge between the Co(III) ions contains two bridging phenolate groups, one
Keywords:
Cobalt
Salen
from each ligand frame. Complex 3 is a linear, mixed valence, trinuclear species, [Co(mdpSal)(OAc)(l-
OAc)]2Co, with the oxidation states of the metal centers assigned as Co(III)–Co(II)–Co(III). The terminal
Co(III) centers are equivalent with the central Co(II) lying on the inversion center of the molecule. Each
cobalt ion in 3 adopts an octahedral geometry with the terminal Co(III) ions being bound to one mdpSal2ꢀ
ligand each. All phenolate groups bridge to the central Co(II). The coordination sphere about each metal
Imine
Phenolate
Dinuclear
Dimer
Trinuclear
center in the trinuclear complex is completed by four acetate groups, two of which bind in a l-fashion
bridging from the terminal Co(III) metal centers to the central Co(II). The complexes have been charac-
terized by X-ray crystallography as well as UV–Vis and IR spectroscopy.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
The success of Salen type ligands in coordination compounds is
due in part to the ease of which a variety of substituents can be ap-
Organic molecules containing imine groups have been of inter-
est to inorganic chemists for some time. These types of molecules
have been shown to be good ligands for transition metals and have
found utility in a broad range of applications. Undoubtedly, the
Schiff base ligands receiving the most attention have been those
classified as Salen type ligands. Ligands categorized in
this class are tetradentate, consisting of two imine nitrogen and
two phenolic oxygen donors that coordinate in the basal plane of
the metal ion, and are readily prepared from the condensation of
a salicylaldehyde and a diamine. Factors that have contributed to
the widespread and continued interest in Salen type ligands are
(a) the initial recognition that metal complexes containing these
types of ligands can reversibly bind O2 [1–4], (b) the similarity be-
tween Salen type ligands and heme and hence their use in model
complexes [5,6] and finally, (c) the more recent discovery of Salen
complexes as efficient chiral catalyst, a field which continues to be
at the forefront of research efforts [7–11].
pended to the ligand periphery and the affinity of this donor set for
many different metal ions [12–16]. These inherent properties have
made this ligand class a unique platform to perform systematic
studies on in order to achieve a desired outcome. Indeed, changing
one of these two factors has great influence on the characteristics
of the coordination complex. For example Jacobsen employed the
same Salen type ligand system with both Cr and Mn and found
the Cr analogue to be an effective catalyst for the enantioselective
ring opening of epoxides while the Mn derivative catalyzes the
enantioselective epoxidation of olefins (Fig. 1) [17–19].
Another characteristic of the Salen type ligands, albeit less stud-
ied, is their potential to form multinuclear complexes. This feature
arises due to the susceptibility of deprotonated phenolic oxygen
donors to bind more than one metal ion [20–22]. Interest in
homo-dinuclear complexes continues to develop owing to their
discovery in such biological systems as the di-nickel containing
ureases [23] and the bifunctional enzyme ACS/CODH [24], the di-
iron core of the hydroxylase unit of methane monooxygenases
[25] and the di-copper hemocyanins [26] to name a few. In addi-
tion, homo- and hetero-multinuclear complexes display novel
magnetic, structural and redox properties which continue to make
them of interest [27–30].
* Corresponding author.
E-mail address: tylerl@union.edu (L.A. Tyler).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.06.023

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J. Welby et al. / Inorganica Chimica Acta 362 (2009) 1405–1411
(s, 2H, OH). Selected IR bands: (cmꢀ1) 1620, 1576 (m,
mNC), 1493
(m), 1278 (m), 1052 (m), 758 (s), 701 (s).
2.3.1. Synthesis of Co(mdpSal) (1)
N
O
N
O
A batch of 0.194 g (0.499 mmol) of mdSalH2 was suspended in
20 mL of MeOH. After 20 min, 0.124 g (0.449 mmol) of
Co(OAc)2 ꢁ 4H2O dissolved in 10 mL of MeOH were added to it.
Upon addition of the metal, the solution immediately turned red-
brown and a bright orange precipitate began to form. The mixture
was stirred for 3 h and the solid collected by gravity filtration and
dried under high vacuum. Yield: 0.15 g (63%). Selected IR bands:
M
t-Bu
t-Bu
Cl
t-Bu
t-Bu
(cmꢀ1) 1599, 1578 (m,
(s), 701 (s). Electronic absorption spectrum in CH2Cl2: kmax (nm)
Mꢀ1 cmꢀ1
415 (10248), 350 (9798), 265 (30580), 240
mNC), 1524 (m), 1428 (m), 1146 (m), 757
Fig. 1. Salen type ligand employed by Jacobsen et al. M = Cr or Mn.
(e
,
)
(40071). Anal. Calc. for C28H22CoN2O2: C, 70.44; H, 4.64; N, 5.87.
Found: C, 70.30; H, 4.62; N, 5.87%.
Ph
N
Ph
N
H
H
2.3.2. Synthesis of [Co(mdpSal)Cl]2 (2)
Complex 1 (0.125 g, 0.262 mmol) was dissolved in CH2Cl2 and
refluxed. After 30 min, 1 drop of concentrated HCl was added to
it. The solution immediately changed from orange to red-orange
in color. Slow diffusion of diethyl ether (Et2O) into the solution re-
sulted in the isolation of 2 as red blocks. Yield: 67 mg (50%). Se-
OH
HO
lected IR bands: (cmꢀ1) 1603, 1597 (m,
m
NC), 1443 (m), 1323 (m),
1238 (w), 908 (w), 700 (s). Electronic absorption spectrum in
CH2Cl2: kmax (nm) (
, Mꢀ1 cmꢀ1) 340 (11193), 250 (43005). Anal.
Fig. 2. N,N0-Bis(salicylidene)-meso-1,2-diphenylethylenediamine (mdpSalH2).
e
Calc. for C28H22Co2N2O2Cl2: C, 65.71; H, 4.64; N, 5.38. Found: C,
65.28; H, 4.46; N, 5.30%.
We have isolated three new cobalt containing complexes with
the Salen type ligand N,N0-bis(salicylidene)-meso-1,2-diphenyle-
thylenediamine (mdpSalH2) (Fig. 2) which differ in nuclearity,
namely the mononuclear Co(mdpSal) (1), the dinuclear [Co(mdp-
Sal)Cl]2 (2) and the linear, mixed valence, trinuclear species
2.3.3. Synthesis of [Co(mdpSal)(OAc)(l-OAc)]2Co (3)
Complex 3 was isolated as a byproduct from the reaction which
resulted in the formation of complex 1. Whilst complex 1 was fil-
tered off from the reaction mixture, slow evaporation of the meth-
anolic mother liquor afforded isolation of complex 3 as orange
microcrystalline solid. The solid was collected and dried under
high vacuum for 12 h. Yield: 0.11 g (36%, based on Co). Selected
[Co(mdpSal)(OAc)(l-OAc)]2Co (3). This work describes the synthe-
sis, characterization and crystal structure of each complex, 1–3.
IR bands: (cmꢀ1) 1622 (m), 1601, (s,
1293 (s), 1206 (w), 907 (m), 739 (s), 701 (s). Electronic absorption
spectrum in CH2Cl2: kmax (nm) (
, Mꢀ1 cmꢀ1) 400(sh) (10991), 375
mNC), 1548 (m), 1447 (m),
2. Experimental
e
2.1. Materials
(18412), 374 (16402), 300(sh) (24965), 275 (sh) (50805), 240
(124661). Anal. Calc. for C64H56Co3N4O12: C, 61.50; H, 4.52; N,
4.48. Found: C, 61.10; H, 4.65; N, 4.47%.
Salicylaldehyde (98%), meso-1,2-diphenylethylenediamine (98%),
Co(OAc)2 ꢁ 4H2O and anhydrous solvents were purchased
from Sigma–Aldrich Chemical Co. and used without further
purification.
2.4. X-ray data collection and structure solution and refinement
Crystals suitable for X-ray analysis were obtained using the fol-
lowing procedures at room temperature: Red blocks of Co(mdp-
Sal) (1) were obtained within 24 h by diffusion of Et2O into a
solution of 1 in CH2Cl2. Red blocks of the dietherate of [Co(mdp-
Sal)Cl]2 (2 ꢁ 2Et2O) were obtained by slow diffusion Et2O into a
solution of 2 in CH2Cl2 after 48 h. Slow diffusion of Et2O into a
2.2. Physical measurements
Infrared spectra were obtained with a Thermoelectron, Avatar
330 FT-IR spectrophotometer equipped with a Smart Orbit reflec-
tance insert, diamond window. Absorption spectra were measured
on a Hewlett-Packard 8453 diode array spectrophotometer. 1H
spectra were recorded on a Varian 200 MHz spectrometer.
methanolic solution of [Co(mdpSal)(OAc)(l-OAc)]2Co yielded or-
ange needles of 3 as a solvate (3 ꢁ 8CH3OH) within 4 h. X-ray dif-
fraction data were collected on a Bruker APEX 2 CCD platform
2.3. Preparation of compounds
diffractometer (Mo Ka (k = 0.71073 Å)) equipped with and Oxford
liquid nitrogen cold stream. Suitable crystals were mounted in a
nylon loop with Paratone-N cryoprotectant oil. The structures
were solved using direct methods and standard difference map
techniques, and were refined by full-matrix least-squares proce-
dures on F2 with SHELXTL (Version 6.14) [32]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms on carbon
were included in calculated positions and were refined using a
riding model. Hydrogen atoms on oxygen were refined semi-
freely using a distance restraint. Crystal data and refinement de-
tails are presented in Table 1 for complexes 1–3 while selected
The ligand, mdpSalH2, was prepared using a method similar to
those used to synthesize other Salen type ligands [31]. The starting
materials (salicylaldehyde and meso-1,2-diphenylethylenedi-
amine) were dissolved in a minimum amount of EtOH and added
together in a 1:1 ratio. Upon mixing, an immediate bright yellow
precipitate formed. The reaction was stirred for 2 h and the solid
collected and dried under high vacuum. Yield: 85%. 1H NMR (CDCl3,
200 MHz, 25 °C, d from TMS): 4.75 (s, 2H, ethylene), 6.81 (t, 2H),
6.92 (d, 2H), 7.07 (d, 2H), 7.15 (m, 12H), 8.09 (s, 2H, imine), 13.1

J. Welby et al. / Inorganica Chimica Acta 362 (2009) 1405–1411
1407
bond distances for the complexes are listed in Table 2 and bond
angles are given in Table 3.
The structure of 2 contains disordered diethyl ether solvate. The
atoms of the disordered ether molecules were included in the
refinement as a diffuse contribution to the scattering using the
program SQUEEZE in the PLATON suite of programs [33].
this reaction mixture as well. After removal of the solid Co(mdpSal)
via filtration, slow evaporation of the homogeneous mother liquor
resulted in the formation of orange microcrystalline solid within
24 h. These crystals had a tinge of green color to the eye as well
as a different morphology from that of crystalline 1 (needle versus
block) which led us to believe that it was a different product. This
resulted in the subsequent isolation and full characterization of the
In the case of 3, several crystals of [Co(mdpSal)(OAc)(l-OAc)]2-
Co were screened and found to be multiply twinned. X-ray diffrac-
tion data were collected at 125 K on a crystal that was determined
with CELL_NOW to contain two twin domains, the second rotated
by 180° around the 0 0 1 reciprocal axis of the first. Both domains
were integrated with SAINT using the two-component orientation
matrix produced by CELL_NOW. The data were absorption cor-
rected and scaled with TWINABS (version 2008/1). Initial solutions
were found and refined with merged and roughly detwinned HKLF
4 format data before final refinement against HKLF 5 format twin
data. The twin ratio (BASF) refined to 0.331(2). The structure con-
tains four molecules of methanol solvate per asymmetric unit, one
of which is disordered over two sites. The methanol solvate was re-
fined with the help of similarity restraints on displacement param-
eters (SIMU) and rigid bond restrains on 1–2 and 1–3 distances and
anisotropic displacement parameters (DELU).
linear, trinuclear complex, [Co(mdpSal)(OAc)(l-OAc)]2Co (3). Com-
bining the yield of complex 1 with that of 3 from the reaction ac-
counts for 99% of the Co starting material.
Only two classes of linear, trinuclear cobalt complexes have
been reported; those that have oxidation states assigned as
Co(II)–Co(II)–Co(II) and those with a Co(III)–Co(II)–Co(III) configu-
ration. The first class of linear, trinuclear Co(II) complexes has re-
ceived considerably more attention due to the discovery that
Co3(dppa)4Cl2 exists in two different forms [35,36]. In addition,
the Co(II) centers in these types of complexes can exist in different
spin states, which have made for interesting temperature depen-
dent magnetic studies [37,38]. There are notably fewer examples
of the second class available in the literature [39–41]. Complex 3
is a neutral species that contains four, mono-anionic acetate
groups and two di-negative mdpSalꢀ2 ligands. Summing the anio-
nic groups gives rise to a total ꢀ8 charge. The +8 charge must
therefore be assigned among the three Co ions. Because the termi-
nal Co centers are equivalent and contain Co–N bond lengths less
than 1.89 Å, they are assigned as low spin, Co(III) centers. The bond
lengths to the cobalt(III) centers in complex 3 are similar to other
Co(III)–Nimine and Co(III)–Ophen bond lengths previously reported
(see Table 2 for a list of bond lengths). The central Co–Ophen bond
distances in complex 3 are longer than those observed for the ter-
minal Co(III) centers (each longer than 2.09 Å) and by comparison
of the bonding parameters of the central Co ion to similar com-
plexes previously reported, it is assigned a +2 oxidation state. We
therefore assign complex 3 to the latter class of linear, trinuclear
complexes, Co(III)–Co(II)–Co(III), based on (1) charge consider-
ations and (2) bond length analysis and comparison to other
examples.
3. Results and discussion
3.1. Synthesis
The synthesis of complex 1 followed closely to other Co(II)–
Salen syntheses previously reported starting with Co(OAc)2 ꢁ 4H2O
as the metal salt in MeOH [13,16]. Complex 1 is slightly soluble in
MeOH with 63% of the product that immediately precipitates out of
the reaction mixture upon the addition of the Co(OAc)2 ꢁ 4H2O to
the dissolved ligand. This complex was first reported by Costa et
al. who investigated how the sterics of tetradentate ligand frames
affect the mechanism and rate of coordination and redox reactions
of Co(II) complexes [34]. These studies did not include X-ray anal-
ysis and the structure is reported here for the first time. Interest-
ingly, the slight solubility of mononuclear 1 in the methanolic
solution led us to the isolation of the trinuclear complex 3 from
The isolation of dimeric 2 from a solution of 1 was obtained by
chance. Upon setting up crystallization of complex 1 in CH2Cl2, one
Table 1
Summary of crystal data and intensity collection and structure refinement parameters for complexes 1–3
[Co(mdpSal)] (1)
[Co(mdpSal)Cl]2 (2 ꢁ 2Et2O)
[Co(mdpSal)Co(OAc)4Co(mdpSal)] (3 ꢁ 8MeOH)
Empirical formula
Molecular weight
Crystal color, habit
Crystal size (mm)
Temperature (K)
Crystal system
Space group
C28H22CoN2O2
477.41
red, block
0.23 ꢂ 0.17 ꢂ 0.14
125(2)
C56H44Cl2Co2N4O4 ꢁ 2(C4 H10O)
C64H56Co3N4O12 ꢁ 8(CH3OH)
1173.95
1506.25
orange, plate
0.25 ꢂ 0.08 ꢂ 0.02
125(2)
orange, needle
0.15 ꢂ 0.10 ꢂ 0.08
125 (2)
monoclinic
C2/c
monoclinic
P21/n
triclinic
ꢀ
P1
Unit cell dimensions
a (Å)
b (Å)
12.4870(7)
11.3889(7)
16.263(1)
90
106.252(1)
90
2220.4(2), 4
1.428
0.802
8.5153(7)
11.9213(9)
12.791(1)
95.201(1)
91.576(1)
94.986(1)
1287.4(2), 1
1.514
28.247(6)
12.039(3)
24.538(5)
90
110.770(3)
90
7802(3), 4
1.282
0.699
c (Å)
a
(°)
b (°)
c
(°)
V (Å3), Z
Dcalc (mg mꢀ3
)
Absorption coefficient (
l
, mmꢀ1
)
0.810
U
Range collected (°)
1.83–26.37
99.9
1.60–26.73
99.5
1.54–24.81
93.9
Completeness to
U max (%)
Reflections collected/unique (Rint
Data/restraints/parameters
R1, wR2 (I > 2rI)
)
23764/4532 (0.0365)
4532/0/298
0.0306, 0.0743
0.0377, 0.0790
1.029
14684/5445 (0.0390)
5445/0/307
0.0358, 0.0763
0.0536, 0.0801
1.016
13460/6321 (0.0714)
6321/40/468
0.0789, 0.2192
0.1072, 0.2357
1.058
R1, wR2 (all data)
Goodness of fit on F2
Largest diff peak/hole (e/Å3)
0.379/ꢀ0.318
0.351/ꢀ0.328
1.675/ꢀ0.648

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J. Welby et al. / Inorganica Chimica Acta 362 (2009) 1405–1411
batch resulted in the formation of orange plates, not red blocks as
expected. Crystallographic analysis of this material confirmed it to
be that of dimeric 2. These findings prompted us to investigate the
reaction conditions which resulted in the formation of [Co(mdp-
Sal)Cl]2. It has been shown that ClÅ radical exists in chlorinated sol-
vents, especially in the presence UV light1 [42] and that certain
Co(II) salen complexes can react with organic halides [43]. This led
us to consider the likelihood of complex 1 undergoing an oxidative
addition reaction with CH2Cl2 and concomitant phenolate bridge for-
mation. To investigate this possibility we tried several different reac-
tions employing both UV irradiation [44] as well as radical initiators
(AIBN and benzoyl peroxide) however we were unable to isolate 2
from any of these attempts. We also carried out several reactions
that added a slurry of NEt4Cl in CH2Cl2 to a solution of 1 in CH2Cl2
as well as a similar reaction in MeOH. These attempts were also
unsuccessful, possibly due to the low solubility of either the chloride
salt in CH2Cl2 or that of complex 1 in MeOH. We were able to find
two similar Co(II)(salen) complexes in the literature that, when oxi-
dized, formed dimeric species. The first report found oxidation oc-
curred after heating a solution of the monomeric Co(II) complex in
ethanol/water to ꢃ40° C for 48 h [44]. We carried out similar studies
in CH2Cl2 and CHCl3 (due to very low solubility in H2O) with no
apparent oxidation occurring (monitored by UV–Vis spectroscopy).
The second study reports conversion of a cobalt salen monomer to
a dicobalt(III) dichloride complex after a chloroform solution of it
was left standing in air for several weeks [45]. Our solutions standing
in CH2Cl2 and CHCl3 resulted in re-crystallization of monomeric 1 in
less than 48 h. We were however able to isolate dimeric 2 by adding
a drop of HCl to a boiling solution of 1 in CH2Cl2. The crystal struc-
ture of the product of this synthesis was analogous to the first report
and confirmed that complex 2 is reproducibly isolated from these
syntheses. We therefore conclude that our initial isolation of com-
plex 2 was due most probably to contamination from cleaning glass-
ware with HCl.
3.3. Structure of complexes
3.3.1. Structure of Co(mdpSal) (1)
The structure of 1 is shown in Fig. 4. The neutral species con-
tains one Co(II) ion ligated to one mdpSal2ꢀ ligand. The complex
Table 2
Selected bond distances (Å) of complexes 1–3
[Co(mdpSal)] (1)
Co(1)–N(1)
Co(1)–O(1)
O(1)–C(1)
1.858(2)
1.846(1)
1.314(2)
Co(1)–N(2)
Co(1)–O(2)
N(1)–C(7)
1.867(1)
1.841(1)
1.303(2)
[Co(mdpSal)Cl]2 ꢁ 2Et2O (2 ꢁ 2C4H10O)
Co(1)–N(1)
Co(1)–O(1)
O(1)–C(1)
1.892(2)
1.931(1)
1.361(2)
2.2382(6)
Co(1)–N(2)
Co(1)–O(2)
N(1)–C(7)
1.885(2)
1.875(1)
1.280(2)
2.9842(6)
Co(1)–Cl(1)
Co(1)–Co(2)
[(Co(mdpSal)(OAc)(
Co(1)–N(1)
Co(1)–O(1)
O(1)–C(1)
l-OAc))2Co] ꢁ 8MeOH (3 ꢁ 8CH3OH)
1.881(7)
1.903(6)
1.30(1)
Co(1)–N(2)
Co(1)–O(2)
N(1)–C(7)
Co(1)–O(5)
Co(2)–O(2)
Co(1)–Co(2)
1.892(7)
1.912(6)
1.28(1)
1.879(6)
2.096(6)
3.090(1)
Co(1)–O(3)
Co(2)–O(1)
Co(2)–O(4)
1.926(6)
2.141(6)
2.052(6)
Table 3
Selected bond angles (°) of complexes 1–3
[Co(mdpSal)] (1)
O(2)–Co–O(1)
O(2)–Co–N(1)
N(1)–C(7)–C(6)
85.91(5)
173.36(6)
125.0(2)
N(1)–Co—-N(2)
O(1)–Co–N(2)
C(1)–O(1)–Co
86.39(6)
169.32(6)
128.24(1)
[Co(mdpSal)Cl]2 ꢁ 2Et2O (2 ꢁ 2C4H10O)
O(2)–Co–O(1)
O(2)–Co–N(1)
N(1)–C(7)–C(6)
Co–O(1)–Co
87.08(6)
178.46(7)
123.8(2)
98.80(6)
N(1)–Co–N(2)
O(1)–Co–N(2)
C(1)–O(1)–Co
O(1)–Co–O(1A)
85.14(7)
171.75(7)
118.7(1)
81.20(6)
3.2. Spectral characterization
[(Co(mdpSal)(OAc)(
O(2)–Co–O(1)
O(2)–Co–N(1)
N(1)–C(7)–C(6)
Co(1)–O(1)–Co(2)
O(3)–Co(1)–O(5)
l
-OAc))2Co] ꢁ 8MeOH (3 ꢁ 8CH3OH)
82.1(2)
177.5(3)
125.7(9)
99.5(2)
N(1)–Co–N(2)
84.9(3)
178.6(3)
123.6(6)
82.1(2)
O(1)–Co–N(2)
The IR spectrum of the ligand shows stretching frequencies at
C(1)–O(1)–Co(1)
O(1)–Co(1)–O(2)
O(1)–Co(2)–O(2)A
1620 cmꢀ1 and 1278 cmꢀ1 which are assigned as the mN@C and
mC–O
m
,
respectively. Once reacted with Co(OAc)2 ꢁ 4 H2O, the
177.7(3)
108.7(2)
N@Cband shifts by ꢃ20 cmꢀ1 to lower energy in complexes 1–3
to 1600, 1597 and 1601 cmꢀ1, respectively [46,47]. Conversely,
the mC–O is seen at ꢃ20 cmꢀ1 higher energy when compared to
the ligand which indicates the phenolic oxygen groups are bonded
to the metal center [48]. The overlap of the stretching frequencies
of the acetate groups (bridging versus non-bridging) in complex 3
gives rise to broad bands in the 1622–1580 cmꢀ1 range. However,
1
0.8
0.6
0.4
0.2
the frequencies at ca. 1620 and 1445 cmꢀ1 are assigned masm
coo-
and msym coo- and are characteristic of bridging acetate groups [49].
The electronic spectra of the ligand and complexes 1–3 were re-
corded in CH2Cl2 and can be seen in Fig. 3. The spectrum of the li-
gand exhibits intense absorption bands in the high energy range
(265 and 320 nm) which are identified as
p ?
p* transitions. The
higher energy band arises from the phenyl rings while the lower
energy transition is due to the azomethine chromophore [46,47].
Complexation of the ligand results in a slight blue shift of the
p
?
p* transition of the phenyl rings by 15–25 nm and a red shift
of the
p
?
p
* azomethine chromophore by ꢃ25 nm, which is most
notable in complexes 1 and 3. In addition, complex 1 exhibits a
band at 415 nm which supports the square planar geometry in
solution [48].
0
225
275
325
375
425
475
525
Wavelength (nm)
1
The experimental apparatus used was an Atlas Suntest XLS+ Solar Simulator with
Fig. 3. Electronic absorption spectra of complexes 1–3 in MeCl2. mdpSalH2 (—-—-)
Co(mdpSal) (—), [Co(mdpSal)Cl]2 (–-–-) and [Co(mdpSal)(OAc)( -OAc)]2Co (----).
750 W intensity light.
l

J. Welby et al. / Inorganica Chimica Acta 362 (2009) 1405–1411
1409
adopts a slightly distorted square planar geometry with identical
N(1)–Co–O(1) and N(2)–Co–O(2) bond angles (94.47(6)°) and
nearly 10° larger than the corresponding N(1)–Co–N(2) and
O(1)–Co–O(2) bond angles (86.39(6)° and 85.91(5)°, respectively).
The average Co–N bond distance is 1.862(6) Å while the Co–O bond
distances are slightly shorter with an average of 1.843(4) Å. Both
distances are similar to other Co(II)–salen bond lengths previously
reported [50].
3.3.2. Structure of [Co(mdpSal)Cl]2 (2)
The structure of the neutral, dimeric 2 is shown in Fig. 5. Each
coordination sphere for the metal ions is comprised of one
mdpSal2ꢀ (N2O2) ligand frame, one Clꢀ, and one phenolate oxygen
bridge, giving rise to an overall octahedral geometry around each
Co(III) center. The distance between the two equivalent Co(III) me-
tal centers is 2.9842(6) Å. The average Co(III)–N bond distance is
1.888(5) Å, which is slightly longer (0.026 Å) than the average
Co(II)–N bond distance in complex 1. Likewise, the non-bridging
Co–O(2) bond length in 2 is 1.8754(1) Å, a distance that is
0.0320 Å longer than in complex 1. The Co–O(1) and Co–O(1 A)
bridging bond distances in complex
2
are both longer
Fig. 4. Thermal ellipsoid plot (30% probability level) of complex 1 showing the
numbering scheme. H atoms have been omitted for clarity.
(1.9312(1) Å and 1.9989(1) Å, respectively) than the non-bridging
Co–O(2) bond length. The difference of 0.0677 Å between the
two lengths is due to one ligand bound to one Co (shorter) bridging
to the other (longer) Co center. All Co–O bonds in 2 are similar in
length to previously reported complexes [44,45]. The Co–Cl bond
length is 2.2382(6) Å and is within the range of other Co(III)–Cl
bond lengths [46]. The Co–ligand bond angles for complex 2 are
slightly smaller than the analogous angles found in complex 1 with
the exception of the N(2)–Co–O(2) bond angle which is slightly
larger.
3.3.3. Structure of [Co(mdpSal)(OAc)(
(3 ꢁ 8CH3OH)
l-OAc)]2Co ꢁ 3CH3OH
The complete structure of 3 is shown in Fig. 6 while the asym-
metric unit of the complex is given in Fig. 7. The neutral, trinuclear
species consists of three cobalt metal centers, two mdpSal2ꢀ ligand
frames and four acetate ions, giving rise to an octahedral environ-
ment around each metal ion. The two terminal Co(III) ions are
equivalent giving the molecule C2 symmetry. Each outer Co(III) is
bound to one mdpSal2ꢀ ligand in the equatorial plane with two
acetate groups occupying the axial sites. The middle Co(II) is lo-
cated at the crystallographic inversion center and one acetate from
each of the outer Co(III) ions coordinates in a
-acetate ligands occupy two sites that are located cis to each other
on the Co(II). The coordination sphere around the central cobalt is
l-fashion to it. These
l
Fig. 5. Thermal ellipsoid plot (30% probability level) of complex 2 showing the
numbering scheme. H atoms have been omitted for clarity.
Fig. 6. Thermal ellipsoid plot (30% probability level) of complex 3 showing the numbering scheme. H atoms have been omitted for clarity.

1410
J. Welby et al. / Inorganica Chimica Acta 362 (2009) 1405–1411
lengths. As expected, the non-bridging Co–Ophenbond distance in
complex 3 is 0.0558(14) Å shorter than the bridging Co–Ophen
length [41]. Interestingly, the bridging Co(III)–Ophenbond distances
in complex 2 are longer than the those found in complex 3
(1.931(1) Å versus an average 1.908(6) Å) which is likely due to
the fact that in the dimer (2), the phenolate bridges to another
Co(III) while in the trinuclear species (3), the bridge is to a Co(II)
ion [42].
The other three types of Co–O bonds present in 3 are formed
between the metal and the acetate groups. We found the bond
distances to lengthen as follows: Co(II)–O -acetate < Co(III)–
l
O
l
-acetate < Co(III)–Oacetate,
which, unlike the mdpSal2ꢀ ligand
frame, exhibit shorter Co(III)–ligand bond lengths.
Acknowledgements
The Jerome A. Schiff Charitable Trust and Union College are
thanked for their generous financial support (LAT). We also thank
the National Science Foundation (NSF 0521237) for supporting
the X-ray diffraction facility at Vassar College (JMT).
Fig. 7. Thermal ellipsoid plot (30% probability level) of the asymmetric unit of
complex 3 showing the numbering scheme. H atoms have been omitted for clarity.
Appendix A. Supplementary material
CCDC 683123, 683125, and 683124 contains the supplementary
crystallographic data for Co(mdpSal) (1), [Co(mdpSal)Cl]2
completed by four bridging phenolate oxygens, two from each of
the mdpSal2ꢀ ligand frames. The greatest deviations from octahe-
(2 ꢁ 2Et2O), and [Co(mdpSal)(OAc)( -OAc)]2Co (3 ꢁ 8CH3OH). These
l
dral geometry is in the center O(4) -acetate-Co(II)–O(2A)phen bond
l
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2008.06.023.
angle which measures 162.6(2)°. The average Co(III)–N bond
length is 1.887(7) Å, a value that is nearly identical to the analo-
gous bond lengths in complex 2. The average Co(III)–Ophen bond
length is 1.908(6) Å, which is shorter than the bridging Co–O
bond lengths in 2. Both the Co(III)–Oacetate and the Co(III)–O
l-acetate
References
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