Synthesis and properties of cobalt(III) complexes of tripodal ligands containing amide functional groups

Article abstract of DOI:10.1039/a809316d

Two new, trigonally symmetrical, triamide tripodal ligands and their complexes with the cobalt(III) ion, [1,1,1-tris-(4-amino-1-oxo-2-azabutyl)ethanato]cobalt(III)trihydrate 1 and [1,1,1-tris(4-amino-4-methyl-3-oxo-2-azapentyl)-ethanato]cobalt(III)-water- ethanol (1/8/0.5) 2 have been prepared. The hexadentate structures of the neutral, trigonal, triamido complexes have been established by an X-ray study of 1 and ¹³C NMR spectrometry of both molecules. The chemical, spectroscopic and electrochemical properties of the complexes have been investigated as well as those of the monoamido tripodal complex [1,9-diamino-5-(4-amino-2-azabutyl)-5-ethoxycarbonyl-4-oxo-3,7-diazanonanato] cobalt(III) chloride.

Full text of DOI:10.1039/a809316d

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Synthesis and properties of cobalt(III) complexes of tripodal
ligands containing amide functional groups
Patricia M. Angus,*ab Anthony J. Elliott,*a Alan M. Sargeson*ab and Anthony C. Willis*a
a Research School of Chemistry, Australian National University, Canberra ACT 0200,
Australia
b Department of Chemistry, Faculty of Science, Australian National University,
Canberra ACT 0200, Australia
Received 30th November 1998, Accepted 27th January 1999
Two new, trigonally symmetrical, triamide tripodal ligands and their complexes with the cobalt() ion, [1,1,1-tris-
(4-amino-1-oxo-2-azabutyl)ethanato]cobalt()trihydrate 1 and [1,1,1-tris(4-amino-4-methyl-3-oxo-2-azapentyl)-
ethanato]cobalt()–water–ethanol (1/8/0.5) 2 have been prepared. The hexadentate structures of the neutral,
trigonal, triamido complexes have been established by an X-ray study of 1 and 13C NMR spectrometry of both
molecules. The chemical, spectroscopic and electrochemical properties of the complexes have been investigated as
well as those of the monoamido tripodal complex [1,9-diamino-5-(4-amino-2-azabutyl)-5-ethoxycarbonyl-4-oxo-3,7-
diazanonanato]cobalt() chloride.
monoamide tripodal complex, [1,9-diamino-5-(4-amino-2-aza-
Introduction
butyl)-5-ethoxycarbonyl-4-oxo-3,7-diazanonanato]cobalt()
Varieties of tripodal ligands exist and most form stable com-
chloride, [Co(CO2Et-2-oxosen–H)]Cl2,24 were examined and
plexes with various transition metal ions.1 In fact, these com-
compared with those of its polyamide analogues. These tripods
plexes are capable of displaying high thermodynamic stability,
all have the potential to be capped at the open end to form a
kinetic inertness and unusual co-ordination modes. A typical
metal ion cage and this aspect was also investigated.
example of a tripodal ligand is sen [1,1,1-tris(4-amino-
2-azabutyl)ethane].2–4 The cobalt() complex of this ligand has
been studied3–5 and it is a precursor to some interesting macro-
bicyclic cage complexes.6,7 Also, a range of different metal com-
plexes with this ligand has been reported and the structure of
the nickel() ion, [NiII(sen)]2ϩ, has been described.8 Lions and
co-workers1,2 pointed the way to the rich chemistry and stability
of such molecules in the 1950s and 1960s. Since then numerous
hexaamine tripodal ligands and their complexes have been
reported,9 including tripods with either 2-pyridylmethyl10 or
3-aminopropyl11 arms. Furthermore, the quaternary cap car-
bon can be replaced by a tertiary amine atom.12–14 Also other
analogues exist where donor atoms such as S15–18 and O14 have
been substituted for some or all of the amines within the ligand.
Varying the nature of the donor atoms produces ligands whose
complexes have different cavity sizes and physical and chemical
properties. Despite the wide variation among tripodal ligands
there seem to be few examples containing amide groups.
Results
Syntheses
The synthesis of the neutral complex [CoIII(2,2Ј,2Љ-trioxosen Ϫ
3H)]ؒ3H2O 1 (Scheme 1) was achieved through a six step pro-
O
O
O
O
O
O
NH
O
NH
O
NH
O
H5C2
C2H5
C2H5
NH2
NH2
NH2
+
3
H2N
NH2
Co(II), air
charcoal
Amide groups usually bind through nitrogen as anions19 and
their complexes have somewhat smaller metal–nitrogen bond
lengths than their amine analogues. Such ligands are therefore
useful in stabilising transition metal ions in higher oxidation
states,20,21 and in polyamide-bearing ligands this stabilisation,
especially of NiIII and CuIII, is ascribed mainly to the strong
σ-donor properties of the deprotonated amide group.21,22 Also,
incorporation of amido groups into macrocyclic ligands leads
to chemical properties different to those observed for the
analogous hexaamines. For instance, the ligands are more
selective in their interactions with different metal ions and dif-
ferent electronic properties are observed for the resultant metal
complexes.23 In this paper the syntheses of two different tri-
amide tripodal ligands are described along with those of their
cobalt() complexes. The chemical, redox and spectral charac-
teristics of these two complexes were also examined and are
compared to those of [CoIII(sen)]3ϩ. In addition, the spectro-
scopic and electrochemical properties of a previously reported
O
O
O
N
N
N
Co
N
N
N
H2
H2
H2
1
Scheme 1
cedure. Diethyl malonate was treated with ethyl chloroformate
to replace one of the methylene hydrogen atoms with a third
ester residue to produce a tripod with an apical hydrogen.25 This
was subsequently converted into the sodium salt by treating the
tripod with sodium metal to eliminate dihydrogen. Methylation
at the central carbon atom was ultimately achieved by treating
the sodium salt with methyl iodide.26 The methylated triester
J. Chem. Soc., Dalton Trans., 1999, 1131–1136
1131

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was then treated with an excess of ethanediamine to produce the
triamide ligand (Scheme 1); using an excess of amine reagent
helped to limit the reaction of two ester groups with one
diamine. Purification was achieved through complexation with
CoIII as shown in Scheme 1. The yield of the triamide was not
high among other by-products, and converting the ligand into
the cobalt() triamido complex using Co2ϩ and O2 in the pres-
ence of charcoal was the easiest method of purification.
Removal of the other products was achieved using an ion
exchange resin and the desired product was crystallised from
the effluent. Microanalysis indicated that the product was a
non-electrolyte and this was consistent with its elution charac-
teristics from the resin, i.e. the hexadentate binds as a trianion.
Attempts to produce a cage complex from the triamido
species [CoIII(2,2Ј,2Љ-trioxosen Ϫ3H)] with methanal and nitro-
methane under basic conditions were unsuccessful. Variations
in solvents and reaction conditions produced none of the
desired product. In all instances, either starting material or
decomposition products or a combination of both arose from
the respective reactions. Although the complex possessed three
primary amines on one octahedral face which should, in prin-
ciple, be susceptible to the capping process, no evidence for
reaction of methanal with the tripodal amines was obtained
using methods which have been productive in capping such
facial amine groups in other instances.6,15 Even the use of
methanal alone failed to generate the facial triimine.
Fig. 1 Thermal ellipsoid diagram of [CoIII(2,2Ј,2Љ-trioxosen Ϫ 3H)]ؒ
3H2O showing atomic labelling. Ellipsoids show 50% probability levels,
except for the hydrogen atoms which are drawn as circles of arbitrary
radius.
The synthesis of [CoIII{5,5Ј,5Љ-tri(Me2)-4,4Ј,4Љ-trioxosen Ϫ
3H}] followed the procedures summarised in Scheme 2 and
microanalytical data indicated that the amide ligand was bind-
ing as a trianion.
X-Ray crystallography
H2N
NH
O
NH2
O
NH
Br
O
NH
Br
NH2
+
The structure of [CoIII(2,2Ј,2Љ-trioxosen-3H)]ؒ3H2O is shown
in Fig. 1 and the bond distances and angles are given in Table
1. The crystallography confirms that the ligand binds as a
hexadentate. Three facial co-ordination sites are occupied by
deprotonated amide nitrogen atoms and the other three by the
primary amine groups. Each of the five membered chelate rings
has adopted an ob conformation about the C3 axis through the
metal.28 The triamide cap conformation also fixes the orien-
tation of the trigonal primary amine face giving the complex an
overall conformation of C3ob3. This structure is different to that
observed for the tripodal hexadentate complex [CoIII(sen)]3ϩ
which has a lel3 conformation.5
base
O
Br
3
Br
Br
NaN3
Pd–C
O
NH
O
NH
O
O
NH
O
NH
NH
O
NH
The three Co–N(amine) distances [1.989(4), 2.003(4) and
1.995(4) Å] are significantly longer than the average Co–N(amine)
bond length observed for [CoIII(sen)]3ϩ, 1.971(3) Å,5 and of ob3
sar type cages such as [CoIII{(NMe3)2-sar}]5ϩ which has an
average Co–N(amine) distance of 1.961(6) Å.29 This was not
unexpected, however, as bonding the amido-N group to the
cobalt() ion leads to a lengthening of the bond trans to it,
since the anionic amido-N group is a stronger σ donor than an
amine. The Co–N(amido) bonds [1.895(4), 1.902(4) and 1.895(4)
Å] are all significantly shorter than the three Co–N(amine) bonds
and comparable to those observed in cobalt() amido cage
complexes.24 The Co–N(amido) bond lengths of the triamide
compound are shorter than the comparable distance [1.911(8)
Å]30 observed for the [CoIII(CH3CONH)(NH3)5]2ϩ cation, but
longer than that quoted for the bis(glycylglycinato)cobalt()
ion (1.87 Å).31 In addition the N–C(amido) and the C–O(amido)
bond lengths (Table 1) are similar (within 3σ) to the expected
co-ordinated N–C(peptide) (1.30 Å)19 and C-O(peptide) (1.27 Å)19
bond distances (where all of the amide groups are deproton-
ated).
NH2
N3
H2N
N3
NH2
N3
Co(II), air
charcoal
O
N
O
N
N
O
Co
N
N
N
H2
H2
H2
2
Scheme 2
was adapted from a published polyamide macrocycle syn-
thesis.27 Initially, 1,1,1-tris(aminoethyl)ethane was treated with
2-bromo-2-methylpropionyl bromide and the next step involved
substituting azide for bromide groups. Reduction of the triazide
in ethanolic Pd/C/H2 then afforded the triamidetriamine tripod
(Scheme 2) in high yield. Production of the cobalt() complex
2 (Scheme 2) followed using Co2ϩ, the ligand, oxygen as the
oxidant and charcoal as the catalyst. Isolation of this complex
from the ion exchange column as a non-electrolyte and the
The N–Co–N bond angles (Table 1) of [CoIII(2,2Ј,2Љ-
trioxosen Ϫ 3H)]ؒ3H2O deviate slightly from the angles
required for octahedral symmetry of the CoN6 core (≈2–7Њ). In
addition, the average twist angle between the two sets of N3
planes, perpendicular to the C3 axis, is 51Њ; also the two N3
1132
J. Chem. Soc., Dalton Trans., 1999, 1131–1136

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Table
1
Bond distances (Å) and angles (Њ) for [CoIII(2,2Ј,2Љ-
trioxosen Ϫ 3H)]ؒ3H2O
Table 2 Electrochemical data for CoIII–CoII couples of triamido
monoamido and saturated tripodal complexes (vs. SCE). HMDE elec-
trode; 1 mM solutions of complex in 0.05 M KH3(C2O4)2ؒ2H2O, 0.1 M
NaClO4 and 0.05 M Na3PO4 at 20 ЊC with a scan rate of 20 mV sϪ1
Co–N(1)
Co–N(2)
Co–N(5)
1.989(4)
Co–N(2)
Co–N(4)
Co–N(6)
1.895(4)
1.902(4)
1.895(4)
1.267(6)
1.491(7)
1.312(6)
1.469(6)
1.466(7)
1.296(6)
1.561(7)
1.575(7)
1.564(7)
2.003(4)
1.995(4)
1.260(6)
1.254(6)
1.480(7)
1.457(7)
1.291(6)
1.480(6)
1.522(8)
1.487(8)
1.514(9)
1.505(7)
Complex
pH
E1/2/V
Epc/mV
O(1)–C(3)
O(3)–C(9)
N(2)–C(2)
N(3)–C(4)
N(4)–C(6)
N(6)–C(8)
C(1)–C(2)
C(4)–C(5)
C(7)–C(8)
C(10)–C(11)
O(2)–C(6)
N(1)–C(1)
N(2)–C(3)
N(4)–C(5)
N(5)–C(7)
N(6)–C(9)
C(3)–C(10)
C(6)–C(10)
C(9)–C(10)
[CoIII(2,2Ј,2Љ-trioxosen Ϫ 3H)]
1.68a
—
—
—
—
Ϫ565
Ϫ954
Ϫ1100
Ϫ782
≈7b
12.04c
1.68a
[CoIII{5,5Ј,5Љ-tri(Me2)-
4,4Ј4Љ-trioxosen Ϫ 3H}]
≈7b
—
—
—
—
Ϫ0.76
Ϫ0.5
Ϫ1330
—
Ϫ462
Ϫ500
Ϫ795e
—
12.04c
1.68a
≈7b
d
[CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ
[CoIII(sen)]3ϩ
12.04c
7.0
N(1)–Co–N(2)
N(1)–Co–N(4)
N(1)–Co–N(6)
N(2)–Co–N(4)
N(2)–Co–N(6)
N(3)–Co–N(5)
N(4)–Co–N(5)
N(5)–Co–N(6)
Co–N(2)–C(2)
C(2)–N(2)–C(3)
Co–N(4)–C(5)
C(5)–N(4)–C(6)
Co–N(6)–C(8)
C(8)–N(6)–C(9)
N(2)–C(2)–C(1)
O(1)–C(3)–C(10)
N(3)–C(4)–C(5)
O(2)–C(6)–N(4)
N(4)–C(6)–C(10)
N(6)–C(8)–C(7)
O(3)–C(9)–C(10)
C(3)–C(10)–C(6)
C(3)–C(10)–C(11)
C(6)–C(10)–C(11)
86.0(2)
171.5(2)
96.6(2)
89.8(2)
88.6(2)
93.9(2)
93.5(2)
83.3(2)
112.7(3)
118.9(4)
113.0(3)
119.6(4)
112.5(3)
120.5(4)
106.3(4)
118.2(5)
108.8(4)
127.0(5)
114.8(4)
105.2(4)
118.4(4)
109.4(4)
111.6(4)
111.0(4)
N(1)–Co–N(3)
N(1)–Co–N(5)
N(2)–Co–N(3)
N(2)–Co–N(5)
N(3)–Co–N(4)
N(3)–Co–N(6)
N(4)–Co–N(6)
Co–N(1)–C(1)
Co–N(2)–C(3)
Co–N(3)–C(4)
Co–N(4)–C(6)
Co–N(5)–C(7)
Co–N(6)–C(9)
N(1)–C(1)–C(2)
O(1)–C(3)–N(2)
N(2)–C(3)–C(10)
N(4)–C(5)–C(4)
O(2)–C(6)–C(10)
N(5)–C(7)–C(8)
O(3)–C(9)–N(6)
N(6)–C(9)–C(10)
C(3)–C(10)–C(9)
C(6)–C(10)–C(9)
C(9)–C(10)–C(11)
89.4(2)
91.7(2)
94.5(2)
171.2(2)
83.6(2)
173.2(2)
90.6(2)
108.4(3)
119.7(3)
110.0(3)
121.6(3)
111.9(3)
122.2(3)
108.1(4)
126.1(5)
115.6(4)
106.2(4)
118.2(4)
109.4(4)
127.9(5)
113.7(4)
105.6(4)
107.5(4)
111.6(4)
a 0.05 M KH3(C2O4)2ؒ2H2O, b 0.1 M NaClO4. c 0.05 M Na3PO4. d Not
observable by HMDE. e Epa Ϫ720 mV, ipc/ipa 0.98, ∆Ep 75 mV.
methyl groups of each arm of the tripod are chemically differ-
ent. One signal occurs for each type of methyl group and AB
coupling might be expected. However, the coupling between the
two methyl groups is small so the peak intensities are distorted
and deviate from the classical AB pattern. The two expected AB
doublet pairs merge and appear as a broad multiplet at δ 1.40.
The two methylene protons within the cap of this complex are
also inequivalent and coupled to each other, giving rise to two
AB doublets centred on δ 2.47 and 3.13. The NMR spectra
of [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ were reported previously24
and because that complex has only one amide group in the
ligand it lacks the threefold symmetry of the other tripods.
Infrared spectroscopy
A single, strong absorption band attributed to the C᎐O stretch-
᎐
ing vibration of the deprotonated co-ordinated amide group is
displayed at 1609 cmϪ1 for [CoIII(2,2Ј,2Љ-trioxosen Ϫ 3H)] and
at 1568 cmϪ1 for [CoIII{5,5Ј,5Љ-tri(Me2)-4,4Ј,4Љ-trioxosen
Ϫ3H}]. This is consistent with the C3 symmetry of both
molecules. The complex [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ has an
planes are not exactly parallel (tipped ≈3Њ), which indicates the
metal possesses a pseudo-octahedral character resulting largely
from the rotation of the two N3 planes. Lastly, the torsion
angles of the three five-membered rings of the triamide are
Ϫ46.4(5)Њ for N(1)–C(1)–C(2)–N(2), Ϫ45.9(6)Њ for N(3)–C(4)–
C(5)–N(4) and Ϫ41.2(6)Њ for N(5)–C(7)–C(8)–N(6) respectively.
The small CoN6 angle deformations, the slightly non-parallel
N3 planes and the variations in the torsion angles are probably
due to crystallographic influences rather than electronic effects
within the molecule. The low-spin d6 cobalt() electronic con-
figuration and the tripodal ligand would be expected to confer a
C3 symmetric configuration on the complex overall. Suitable
crystals of the other complex were not obtained.
amide C᎐O stretching vibration at 1600 cmϪ1 24
.
᎐
Electronic absorption spectroscopy
The d–d absorption bands occur at 358 and 490 nm for [CoIII-
(2,2Ј,2Љ-trioxosen Ϫ 3H)] and 350 and 482 nm for [CoIII{5,5Ј,5Љ-
tri(Me2)-4,4Ј,4Љ-trioxosen Ϫ 3H}]. The spectra do not differ
greatly from that of the [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ ion
whose d–d bands occur at 342 and 492 nm (ε = 163 and 159 MϪ1
cmϪ1 respectively). Comparison between the two systems is
valid because the apical substituent has little influence on the
position of the d–d bands for molecules of this kind.32 In each
instance, characteristic absorption bands arise from the
NMR Spectroscopy
1
Laporte forbidden transitions of origin T1g ← 1A1g and
The fully N-deuteriated, proton-decoupled 13C NMR spectra
of both triamide complexes show that each possesses average
C3 symmetry at least for that NMR timescale. The carbon
atoms within each arm of the tripods are inequivalent to each
other but equivalent to the corresponding carbon atoms on the
other arms. Consequently, five signals appear in the spectrum
of [CoIII(2,2Ј,2Љ-trioxosen-3H)] while seven signals occur for the
1T2g ← 1A1g (Oh parentage) for low spin CoIII and a red shift
occurs in the lower energy d–d transition of both complexes
compared to that of [CoIII(sen)]3ϩ. In addition the band inten-
sities of the triamides are significantly greater than those of
[CoIII(sen)]3ϩ, characteristic of some charge transfer compon-
ent. The spectra were unchanged when recorded in acid
solution.
[CoIII{5,5Ј,5Љ-tri(Me2)-4,4Ј,4Љ-trioxosen Ϫ 3H}]. The H NMR
1
spectra are not so simple: for [CoIII(2,2Ј,2Љ-trioxosen Ϫ 3H)] a
singlet arises from the methyl group in the apical position of the
complex (δ 1.17) while a complex ABCD spin system arises
from the ethylene link protons over δ 2.20–3.60. For [CoIII-
{5,5Ј,5Љ-tri(Me2)-4,4Ј,4Љ-trioxosen Ϫ 3H}] a singlet occurs at
δ 1.02 for the apical methyl protons while a distorted AB doub-
let pair arises for the remaining gem-methylene hydrogens. The
protons within each methyl group located on the five membered
chelate rings appear equivalent to each other, but the gem
Electrochemistry
The cyclic voltammetry reduction potentials, measured in
aqueous solution with a hanging mercury drop electrode, of the
triamides and [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ and [CoIII(sen)]2ϩ
appear in Table 2. For [CoIII(2,2Ј,2Љ-trioxosen Ϫ 3H)] the reduc-
tion wave occurs at Epc = Ϫ0.95 V (vs. SCE) at pH ≈7 while for
[CoIII{5,5Ј,5Љ-tri(Me2)-4,4Ј,4Љ-trioxosen Ϫ 3H}] the potential
appears at Epc = Ϫ1.33 V under the same conditions. Both
J. Chem. Soc., Dalton Trans., 1999, 1131–1136
1133

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complexes displayed irreversible behaviour in all media at scan
rates of 20 mV sϪ1 and higher. These results indicate however
that the electrode potentials of both triamido complexes are
much more negative and much less reversible than that of [CoIII-
(sen)]3ϩ (E1/2 = Ϫ0.5 V vs. SCE)33 The potential of the CoIII–CoII
reduction wave varied with the pH of the medium employed for
each complex. In more basic solution the redox couples shifted
to more negative values. The largely irreversible behaviour in
acidic, neutral and basic solutions indicates that protons are
involved. Partial ligand dissociation is likely in the cobalt()
state of the triamides which would account for the irreversi-
bility. Given the substantial irreversibility involved, it is diffi-
cult to relate the reduction wave potential to the chemical
changes. From the oxidation scan it also appeared that CoIV
was inaccessible in water with these triamide complexes. No
oxidation waves were observed for either complex when
scanned to 1.4 V with a carbon electrode. This was unexpected
since higher oxidation states of nickel,21 copper22 and possibly
cobalt20 appear to have been obtained with macrocyclic poly-
amido-N complexes. The [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ
molecule has a reduction potential 0.2–0.3 V (according to
pH) more negative than that for [CoIII(sen)]3ϩ. The reduction
potential for a triamido complex therefore could be 0.6–0.9 V
more negative, and the results in this study sustain this
estimate.
the difference between the eg* and t2g energy levels. In relevant
hexaamine cage systems the absorption bands for the ob3 con-
formation are 30 nm lower in wavelength and the intensities
are less than those in the corresponding lel3 conformation.36
This wavelength shift however is not observed in this case
because the opposing effects of the amido-N donors dominate
the issue.
The CoIII–CoII reduction waves of the triamido complexes in
water appear to be the most negative of the reported cobalt()
amido complexes doubtless because the three amido groups
within the tripodal ligand reduce the formal charge on the
complex to zero. This lack of charge leads to stabilisation of the
higher oxidation state and the charge at the metal centre is also
reduced by charge donation from the anionic ligand. Both
effects influence the redox potential, making it easier to stabilise
the metal ion in a higher oxidation state. These effects taken
threefold were anticipated to make the cobalt() state access-
ible, however this oxidation state was not detected in any of
these studies. This was surprising since CuIII, NiIII and NiIV 37
are readily accesssible in water in complexes in which the ligand
is an open-chain triamido species.
The tripodal triamido complexes differ from those of open-
chain and macrocyclic amido systems in two ways. First, the
latter type ligands tend to favour square-planar co-ordination
of metal ions. This is mostly due to the planarity of the amido
groups and in those cases the atoms of the amido groups
are commonly coplanar with the metal ion of the complex.
Secondly in square-planar systems the d–d absorption bands
are usually blue-shifted when compared to those of equivalent
polyamine complexes. In contrast, the two tripodal triamido
ligands bind the cobalt() ion in a near-octahedral manner that
results in the planar amido groups being not co-planar with the
central cobalt() ion. Also, with the octahedrally co-ordinated
triamido complexes the electronic transitions are red-shifted
when compared to those of [Co(sen)]3ϩ. Hence the observed
differences in co-ordination between the two forms of amide
ligand may be a factor in the differing electronic properties of
the two systems.
Discussion
The synthesis of two new tripodal triamide ligands and their
cobalt() complexes has permitted the study of the chemical
and physical properties of octahedral cobalt() complexes
containing multiple amido-N groups and the structure of one
of these complexes has been confirmed by X-ray analysis. The
amide groups in these tripods produce a smaller cavity for the
metal ion than do hexaamine tripodal ligands and deproton-
ation of the three amides on co-ordination to cobalt() leads to
the formation of neutral complexes. The complexes [CoIII-
(2,2Ј,2Љ-trioxosen Ϫ 3H)], [CoIII{5,5Ј,5Љ-tri(Me2)-4,4Ј,4Љ-trioxo-
sen Ϫ 3H}] and [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ were resistant
to nucleophilic attack by OHϪ at the amide group, as is
common in amido-N-bonded complexes.19 In addition proton-
ation at the amide oxygen or nitrogen was not observed in these
complexes, a property also shown by amide-functionalised
cobalt() cage complexes.24
In conclusion, two new tripodal triamide ligands have been
synthesized, and they formed octahedral cobalt() complexes.
We were unable to prepare triamido cage complexes from them
and clearly the presence of the three amido groups within the
ligands affects their ability to react with methanal, presumably
the amine groups are less acidic in the neutral complexes. Previ-
ous work has shown that a complex of a tripod with a single
amide group could be capped.24 However, the triamido com-
plexes themselves display some interesting properties that will
merit further investigation, especially in relation to promoting
higher oxidation states and non-square-planar configurations
in complexes of Fe, Cu and Ni.
The lack of reactivity of the amide functional groups may be
attributed to the changes in their electronic structure brought
about by co-ordination to cobalt(). The nature of these
changes has been probed by spectroscopy and electrochemistry
of
[CoIII(2,2Ј,2Љ-trioxosen Ϫ 3H)],
[CoIII{5,5Ј,5Љ-tri(Me2)-
4,4Ј,4Љ-trioxosen Ϫ 3H}] and [CoIII(CO2Et-2-oxosen Ϫ H)]2ϩ
.
In the infrared spectra the C᎐O stretching vibrations fall well
᎐
below those expected for unbound secondary amides (1680–
1630 cmϪ1) and this is the predicted consequence of co-
ordination and deprotonation.34 In the electronic spectra the
absorption bands have greater intensities than those found in
the [CoIII(sen)]3ϩ spectrum and also more metal-ligand charge
transfer properties; in addition the bands occur at lower
frequencies.
The triamide complexes have three amine and three amido
functional groups, making the environment about the metal ion
distorted along the C3 axis. Consequently, the Oh symmetry
restrictions are somewhat diminished and the intensity of the
observed transitions has increased. There may also be a charge
transfer component arising from the trianionic ligand. The red
shift arises from the π interaction of the lone pair of electrons
on the amido nitrogen with the metal-based t2g non-bonded
electrons; this causes an increase in the energy of the t2g orbitals
and they are consequently closer to the eg* set.35 The deproton-
ated ligand can be expected to be a better ligand in a σ sense
than an amine but this is not detected in the value of ∆ which is
Experimental
The 1H and 13C NMR spectra were acquired on a Varian
300 MHz spectrometer and referenced externally against 1,4-
dioxane (δ 3.70 {1H} and 67.4 {13C} vs. TMS), infra-red spectra
with a Perkin-Elmer 1800 FTIR spectrometer as KBr disks.
Standard electrochemical measurements were performed on a
BAS 100 Electrochemical Analyser using 1 × 10Ϫ3 M solutions
under a nitrogen atmosphere. In all cases, a three electrode
system was used with full iR compensation. The auxiliary elec-
trode consisted of a platinum wire and the reference electrode
was Hg–Hg2Cl2–KCl (saturated) (SCE), 241 mV vs. SHE. The
working electrode consisted of either an edge plane pyrolytic
graphite (EPG) carbon or mercury electrode. The mercury elec-
trode (Metrohm 663 VA stand, interfaced with an RSC Model-
411 interface unit) was generally used as a hanging mercury
drop electrode (HMDE). Electronic absorption spectra were
obtained with a Hewlett-Packard 8450A UV/Visible spectro-
photometer in 1 cm quartz cells.
1134
J. Chem. Soc., Dalton Trans., 1999, 1131–1136

View Article Online
Syntheses
tion funnel to avoid boiling the solvent since the reaction was
VERY exothermic. The solution became cloudy upon produc-
tion of [Et3NH]Br. After stirring the solution for 1 h, the reac-
tion mixture was washed twice with dilute aqueous HCl and
twice with dilute aqueous Na2CO3. The resultant CH2Cl2 solu-
tion was dried over Na2SO4 to give a clear, pale yellow solution.
The CH2Cl2 was removed under reduced pressure to leave a
yellow oil (13.6 g, 97%). δH(CDCl3) 0.90 (s, 3 H, CqCH3), 1.95 [s,
18 H, C(CH3)2], 2.99 (s, 3 H, CH2 cap), 3.02 (s, 3 H, CH2 cap)
and 7.72 (s, 3 H amide NH); δC(CDCl3) 19.2 (CH3Cq), 31.9
[C(CH3)2], 41.8 (CH3Cq), 42.0 (CH3CqCH2), 60.4 [C(CH3)2] and
CAUTION: Reactions involving methanal in basic solution were
carried out in a well ventilated fumehood and quenched with
acetic acid to avoid formation of carcinogenic halogenoethers.
Whilst no problems were encountered in the course of this
work, azide compounds are potentially explosive and should be
handled with appropriate care and on a small scale.
The complex [CoIII(CO2Et-2-oxosen Ϫ H)]Cl2ؒ3H2O was
prepared according to a published method:24 λmax/nm (ε/MϪ1
cmϪ1) (H2O) 342 (163) and 492 (159).
172.8 (C᎐O).
᎐
Methanetricarboxylic acid triethyl ester. This compound was
prepared by reaction of diethyl malonate and ethyl chloro-
formate.25 δH(CDCl3) 1.30 (t, 9 H, CH3 ester), 4.27 (q, 6 H, CH2
ester) and 4.41 (s, 1 H, CH); δC(CDCl3) 13.4 (CH3CH2), 58.5
1,1,1-Tris(4-amino-4-methyl-3-oxo-2-azapentyl)ethane. 1,1,1-
Tris(4-bromo-4-methyl-3-oxo-2-azapentyl)ethane (13.6 g, 0.025
mol) was dissolved in ethanol (250 cm3), an aqueous solution of
NaN3 (5.85 g, 0.09 mol) (fumehood, CAUTION) was added
and the mixture was heated under reflux (12 h). Solvent was
removed under reduced pressure until precipitation was initi-
ated. The aqueous solution was extracted twice with CH2Cl2
and the extract dried over anhydrous Na2SO4. It is important to
execute this procedure to remove the NaN3, since the product is
exposed to Pd/C in the next step and care should be taken to
avoid the formation of a heavy metal azide. The CH2Cl2 was
again removed under reduced pressure until precipitation
was initiated. Ethanol (≈500 cm3) was added and the solution
was again concentrated under reduced pressure. CAUTION: a
small sample (≈5 mg) of 1,1,1-tris(4-azido-4-methyl-3-oxo-2-
azapentyl)ethane was isolated as a solid for spectroscopic
analysis. δH(CDCl3) 0.74 (s, 3 H, CH3Cq), 1.49 [s, 18 H, CH3
C(CH3)2], 2.86 (s, 3 H, CH2 cap), 2.88 (s, 3 H, CH2 cap) and 7.67
(s, 3 H amide NH); δC(CDCl3) 18.6 (CH3Cq), 24.5 [C(CH3)2],
41.3 (CH3Cq), 42.3 (CH3CqCH2), 64.2 [C(CH3)2] and 173.8
(C C᎐O), 61.8 (CH CH ) and 163.5 (C᎐O).
᎐
᎐
q
3
2
Sodium methanetricarboxylic acid triethyl ester. This com-
pound was prepared by reaction of methanetricarboxylic acid
triethyl ester with sodium.26 δH(D2O) 1.19 (t, 9 H, CH3 ester)
and 4.04 (q, 6 H, CH2 ester); δC(D2O) 14.7 (CH3CH2), 58.3
(C C᎐O), 60.7 (CH CH ) and 172.7 (C᎐O).
᎐
᎐
q
3
2
1,1,1-Ethanetricarboxylic acid triethyl ester. This compound
was prepared by reaction of sodium methanetricarboxylic acid
triethyl ester with methyl iodide.26 δH(CDCl3) 1.29 (t, 9 H, CH3
ester), 1.71 (s, 3 H, CqCH3) and 4.26 (q, 6 H, CH2 ester);
δC(CDCl3) 13.3 (CH3CH2), 18.2 (CH3Cq), 61.5 (CH3Cq), 61.6
(CH CH ) and 167.2 (C᎐O).
᎐
3
2
1,1,1-Tris(4-amino-1-oxo-2-azabutyl)ethane. Ethanetricarb-
oxylic acid triethyl ester (60 g; 0.24 mol) in ethanol (250 cm3)
was added dropwise to a refluxing solution of ethane-1,2-
diamine (600 cm3; 9.0 mol) under nitrogen. The solution was
then refluxed for 72 h and cooled; excess of ethane-1,2-diamine
was removed by rotary evaporation yielding an impure dark
brown oil (74.4 g). The isolation of pure material (1 g) involved
absorbing crude product (3 g) on silica gel and elution with
CHCl3–CH3OH–28% aqueous ammonia (8:3:1). δH(D2O)
1.27 (s, 3 H, CqCH3), 2.69 (t, 6 H, CH2 en) and 3.21 (t, 6 H, CH2
en); δC(D2O) 14.7 (CH3Cq), 40.5 (H2NCH2), 42.1 (CH2-
(C᎐O). The bulk triazide product was reduced in portions with
᎐
H2 in an autoclave (50 psi) with 10% Pd/C. Since the reaction
evolves one N2 molecule for every H2 molecule absorbed,
frequent evacuation of the bomb and refilling with H2 is
advantageous (i.e. H2 from the high pressure reservoir is not
efficiently consumed). The mixture was filtered to remove
the catalyst and the filtrate evaporated to dryness yielding
a grey-white solid consistent with the desired product (7.4 g,
79%). δH(D2O) 0.72 (s, 3 H, CH3Cq), 1.36 [s, 18 H, C(CH3)2] and
3.01 (s, 6 H, CH2 cap); δC(D2O) 19.8 (CH3Cq), 28.2 [C(CH3)2],
41.9 (CH3Cq), 42.9 (CH3CqCH2), 55.8 [C(CH3)2] and 181.7
NHC᎐O) 48.6 (CH C ) and 174.3 (C᎐O).
᎐
᎐
3
q
[1,1,1-Tris(4-amino-1-oxo-2-azabutyl)ethanato]cobalt(III)-
trihydrate, [CoIII(2,2Ј,2Љ-trioxosen ؊ 3H)]ؒ3H2O 1. Crude 1,1,1-
Tris(4-amino-1-oxo-2-azabutyl)ethane (6 g) was dissolved in
water (100 cm3) and CoIICl2ؒ6 H2O (6 g) dissolved in water (100
cm3) and charcoal (10 g) were added; the solution was then
aerated for 12 h. After filtration, the orange solution was
diluted to 4 L and sorbed onto a column (20 × 5 cm) of SP
Sephadex C-25 cation exchange resin. An orange band not
retained by the resin was reduced to 20 cm3 by rotary evapor-
ation and then allowed to evaporate in air until dark red-orange
crystals (1 g) were formed (Found: C, 32.7; H, 6.8; Co, 14.4; N,
20.7. C11H21CoN6O3ؒ3H2O requires C, 33.2; H, 6.8; Co, 14.8; N,
21.1%). δH(D2O) 1.17 (s, 3 H, Cq-CH3), 2.20, 2.60, 2.71 and 3.60
(ABCD spin system, 12 H, CH2 en); δC(D2O) 16.2 (CH3Cq),
(C᎐O).
᎐
[1,1,1-Tris(4-amino-4-methyl-3-oxo-2-azapentyl)ethanato]-
cobalt(III)–water–ethanol (1/8/0.5),
[CoIII{5,5Ј,5Љ-tri(Me2)-
4,4Ј,4Љ-trioxosen ؊ 3H}]ؒ8H2O.0.5C2H5OH 2. 1,1,1-Tris(4-
amino-4-methyl-3-oxo-2-azapentyl)ethane (1 g, 2.68 × 10Ϫ3
mol) was dissolved in water (200 cm3), a solution of CoII-
Cl2ؒ6H2O (0.65 g, 2.68 × 10Ϫ3 mol) in water (200 cm3) and char-
coal (10 g) were added and the mixture aerated for 12 h. The
cobalt() complex was isolated in the same way as that for
[Co(2,2Ј,2Љ-trioxosen Ϫ 3H)]ؒ3H2O except that the eluate was
evaporated to dryness and the product recrystallised from
aqueous ethanol to yield a dark red solid (1.1 g, 68%). (Found:
C, 35.7; H, 8.1; N, 13.9. C17H33CoN6O3ؒ8H2O.0.5C2H5OH
requires C, 35.8; H, 8.8; N, 13.9%). δH(D2O) 1.02 (s, 3 H,
CqCH3), 1.40 [m, 18 H, CH3 H2NC(CH3)2], 2.47 and 3.13 (AB
doublet of doublets, 6 H, CH2 cap); δC(D2O) 21.7 (CH3Cq), 27.1
[H2NC(CH3)2], 30.2 [H2NC(CH3)2], 44.6 (CH3Cq), 51.9 (CH3Cq-
CH ), 61.5 [H NC(CH ) ] and 184.0 (C᎐O); νmax/cmϪ1 (C᎐O
44.8 (H NCH ), 47.8 (CH NC᎐O), 67.1 (CH C ) and 178.5
᎐
2
2
2
3
q
(C᎐O); νmax/cmϪ1 (C᎐O amide stretch) 1609; λmax/nm (ε/MϪ1
᎐
᎐
cmϪ1) (H2O) 358 (323) and 490 (398).
1,1,1-Tris(4-bromo-4-methyl-3-oxo-2-azapentyl)ethane. 1,1,1-
tris(aminomethyl)ethane (tame) (3 g; 0.026 mol) and Et3N (12.5
cm3; 0.09 mol) were added to CH2Cl2 (200 cm3). The mixture
was dried over Na2SO4 (1 h) and then filtered into a three-neck
round-bottomed flask (1 dm3) equipped with a reflux condenser
and a pressure-equalising addition funnel. Under a nitrogen
atmosphere, 2-bromo-2-methylpropionyl bromide (9.5 cm3;
0.077 mol) was carefully added to the solution from the addi-
᎐
᎐
2
2
3 2
amide stretch) 1568; λmax/nm (ε/MϪ1 cmϪ1) (H2O) 350 (214) and
482 (292).
Attempted cage syntheses using [CoIII(2,2Ј,2Љ-trioxo-
sen ؊ 3H)]ؒ3H2O. The complex [CoIII(2,2Ј,2Љ-trioxosen Ϫ
3H)]ؒ3H2O (a) (0.5 g, 1.45 × 10Ϫ3 mol), (b) (1.0 g, 2.9 × 10Ϫ3
mol) or (c) (0.5 g, 1.45 × 10Ϫ3 mol), was dissolved in (a) aceto-
J. Chem. Soc., Dalton Trans., 1999, 1131–1136
1135

View Article Online
nitrile (30 cm3)–water (25 cm3), (b) methanol (50 cm3) or (c)
water (100 cm3) and to each of these solutions was added (a)
36% aqueous methanal (5 cm3; 0.06 mol), (b) paraformaldehyde
(0.8 g, 0.027 mol) or (c) 36% aqueous methanal (2.35 g, 0.08
mol), nitromethane (a) (5 cm3, 0.09 mol), (b) (0.2 g, 3.3 × 10Ϫ3
mol) or (c) (1.25 g, 0.02 mol) and triethylamine {(a) (3 cm3,
0.022 mol) or (b) (0.2 cm3, 1.45 × 10Ϫ3 mol} or NaOH {(c), 1 M
added until reaction solution pH ≈12}. The mixtures were (a)
stirred for 4 d during which time methanal (0.5 cm3) and
nitromethane (0.5 cm3) were added at regular intervals, (b)
refluxed for 4 d or (c) stirred for 5 d, then treated with acetic
acid, diluted to 1 dm3 with water and chromatographed on a
50W × 2 Dowex column. The single red-orange band that eluted
straight from the column with water in each instance was
collected and taken to dryness. Investigation of the residues by
13C NMR spectroscopy indicated the bands were starting
material.
7 A. M. Sargeson, Chem. Br., 1979, 15, 23 and refs. therein.
8 A. McAuley, S. Subramanian and T. W. Whitcombe, J. Chem. Soc.,
Dalton Trans., 1993, 2209.
9 G. A. Lawrance and P. G. Lye, Comments Inorg. Chem., 1994, 15,
339 and refs. therein.
10 A. H. R. Al-Obaidi, K. B. Jensen, J. J. McGarvey, H. Toftlund,
B. Jensen, S. E. J. Bell and J. G. Carroll, Inorg. Chem., 1996, 35, 5055.
11 A. M. Sargeson, Pure Appl. Chem., 1986, 58, 1511.
12 I. J. Clark, R. J. Geue, L. M. Engelhardt, J. MacB. Harrowfield,
A. M. Sargeson and A. H. White, Aust. J. Chem., 1993, 46, 1485.
13 M. P. Suh and D. Kim, Inorg. Chem., 1985, 24, 3712.
14 T. Sato, H. Takeda, K. Sakai and T. Tsubomura, Inorg. Chim. Acta,
1996, 246, 413.
15 L. R. Gahan, T. W. Hambley, A. M. Sargeson and M. R. Snow,
Inorg. Chem., 1982, 21, 2699.
16 J. I. Bruce, L. R. Gahan, T. W. Hambley and R. Stranger, Inorg.
Chem., 1993, 32, 5997.
17 K. G. Ragunathan and P. K. Bharadwaj, J. Chem. Soc., Dalton
Trans., 1992, 2417.
18 C. M. Thorne, S. C. Rawle, G. A. Admans and S. R. Cooper,
J. Chem. Soc., Chem. Commun., 1987, 306.
19 H. Sigel and R. B. Martin, Chem. Rev., 1982, 82, 385.
20 T. J. Collins, R. D. Powell, C. Slebodnick and E. S. Uffelman, J. Am.
Chem. Soc., 1991, 113, 8419.
Crystal structure analysis of [CoIII(2,2Ј,2Љ-trioxosen ؊ 3H)]ؒ
3H2O
Crystal data. C11H27CoN6O6, M = 398.30, hexagonal, space
group P65 (no. 170), a = 8.040(3), c = 43.730(4) Å, U = 2447.8(5)
Å3, T = 23 1 ЊC, Z = 6, µ(Cu-Kα) = 86.50 cmϪ1, 3956 reflec-
tions measured (Rigaku AFC6R diffractometer), 1259 unique
(Rint = 0.050) and 1090 > 3σ which were used in all calculations.
The final R was 0.027, RЈ = 0.029. Refinement in space group
P61 with inverted co-ordinates gave a much higher R factor,
indicating that the absolute configuration of the molecule is as
shown in the Figure.
21 D. W. Margerum, Pure Appl. Chem., 1983, 55, 23.
22 D. W. Margerum and G. D. Owens, Metal Ions in Biological
Systems, 1981, 12, 75.
23 E. Kimura, J. Coord. Chem., 1986, 15, 1.
24 R. J. Geue, W. R. Petri, A. M. Sargeson and M. R. Snow, Aust. J.
Chem., 1992, 45, 1681.
25 H. Lund and A. Voigt, Org. Synth., 1948, Coll. Vol. II, 594.
26 H. Böhme and L. Häfer, Chem. Ber., 1966, 99, 281; Chem. Ber.,
1966, 99, 879.
27 T. J. Collins, K. L. Kostka, E. S. Uffelman and T. L. Weinberger,
Inorg. Chem., 1991, 30, 4204.
28 For an explanation of the lel and ob terminology, see Inorg. Chem.,
1970, 9, 1.
29 P. V. Bernhardt, A. M. T. Bygott, R. J. Geue, A. J. Hendry, B. R.
Korybut-Daszkiewicz, A. M. Sargeson and A. C. Willis, Inorg.
Chem., 1994, 33, 4553.
30 M. L. Schneider, G. Ferguson and R. J. Balahura, Can. J. Chem.,
1973, 51, 2180.
31 M. T. Barnet, D. A. Buckingham, H. C. Freeman, I.-N. Hsu and
D. van der Helm, Chem. Commun., 1970, 367.
32 G. A. Lawrance, P. A. Lay and A. M. Sargeson, Inorg. Chem., 1990,
29, 4808.
CCDC reference number 186/1337.
See http://www.rsc.org/suppdata/dt/1999/1131/ for crystallo-
graphic files in .cif format.
Acknowledgements
This work was partially supported by the Australian Research
Council. A. J. E. gratefully acknowledges the award of a
PhD scholarship from the Australian National University
(ANU). We also thank the Microanalytical Services Unit,
ANU, Dr K. Brown for assistance with the electrochemistry
and Dr P. V. Bernhardt for helpful advice.
33 A. M. Bond, G. A. Lawrance, P. A. Lay and A. M. Sargeson, Inorg.
Chem., 1983, 22, 2010.
34 M. K. Kim and A. E. Martell, J. Am. Chem. Soc., 1966, 88, 914;
Biochemistry, 1964, 3, 1169.
35 D. J. Bull, I. I. Creaser, A. M. Sargeson, B. W. Skelton and A. H.
White, Inorg. Chem., 1987, 26, 3040 and refs. therein.
36 R. J. Geue, A. J. Hendry and A. M. Sargeson, J. Chem. Soc., Chem.
Commun., 1989, 1646.
37 D. W. Margerum, K. L. Chellappa, F. P. Bossu and G. L. Burce,
J. Am. Chem. Soc., 1975, 97, 6894; E. J. Subak, Jr., V. M. Loyola and
D. W. Margerum, Inorg. Chem., 1985, 24, 4350; T. L. Pappenhagen,
W. R. Kennedy, C. P. Bowers and D. W. Margerum, Inorg. Chem.,
1985, 24, 4356.
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Paper 8/09316D
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J. Chem. Soc., Dalton Trans., 1999, 1131–1136