Preparation and photochemistry of cobalt(III) amino and amino acidato complexes containing tripodal polypyridine ligands

Article abstract of DOI:10.1039/b311809f

The photochemical reactivities of cobalt(III)-diamine and cobalt(III)-amino acid compounds have been compared using complexes that also contain polypyridyl ligands. Metallacyclic complexes result from UV-induced photodecarboxylation reactions of the amino acid complexes. UV-irradiation of closely related complexes with amine donors replacing the carboxylate donors does not lead to the production of the same metallacyclic products. The reported UV-induced fragmentation of amine donors and subsequent metallacycle formation appears not to be a general reaction. Nine cobalt(III) complexes of polypyridyl ligands have been structurally characterised, including four that also contain amino acid ligands and one that contains a three-membered metallacyclic ring.

Full text of DOI:10.1039/b311809f

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Preparation and photochemistry of cobalt(III) amino and amino
acidato complexes containing tripodal polypyridine ligands†
Carl A. Otter and Richard M. Hartshorn*
Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch,
New Zealand. E-mail: r.hartshorn@chem.canterbury.ac.nz
Received 24th September 2003, Accepted 5th November 2003
First published as an Advance Article on the web 25th November 2003
The photochemical reactivities of cobalt()–diamine and cobalt()–amino acid compounds have been
compared using complexes that also contain polypyridyl ligands. Metallacyclic complexes result from UV-induced
photodecarboxylation reactions of the amino acid complexes. UV-irradiation of closely related complexes with
amine donors replacing the carboxylate donors does not lead to the production of the same metallacyclic products.
The reported UV-induced fragmentation of amine donors and subsequent metallacycle formation appears not to be
a general reaction. Nine cobalt() complexes of polypyridyl ligands have been structurally characterised, including
four that also contain amino acid ligands and one that contains a three-membered metallacyclic ring.
reported (based substantially on spectroscopic studies) from the
Introduction
photolysis of the following pairs of complexes: [Co(en)3]3ϩ and
Photodecarboxylation of amino acids coordinated to cobalt()
[Co(en)2(gly)]2ϩ, [Co(bpy)2(en)]3ϩ and [Co(bpy)2(gly)]2ϩ, and
gives rise to metallacyclic species containing cobalt–carbon
[Co(trien)(phen)]3ϩ and [Co(dtma)(phen)]2ϩ (dtma = 8-amino-
bonds.1 The stepwise mechanistic pathway for this process has
3,6-diazaoctanoate).1 Tris(diamine)cobalt() systems have
been proposed to involve cobalt–ligand bond homolysis, de-
carboxylation, and then radical trapping by cobalt() to form
the cobalt–carbon bond. Several such metallacycles have been
reported, although those that have been structurally character-
been reported to produce organic material on UV photolysis
that is consistent with ligand-based carbon–carbon bond
homolysis and decomposition of the complexes.4,5
We are interested in establishing whether cobalt() com-
plexes of coordinated amines do indeed exhibit photochemistry
that gives the same complexes as those produced from co-
ised are either part of a polydentate framework or contain good
π-acceptor ligands in the coordination sphere.2
Similar chemistry has been postulated for the photolysis of
ordinated amino acids. The photochemistry of [Co(bpg)-
coordinated polyamines (Scheme 1),1 and we have recently
(phen)]2ϩ (bpg = N,N-bis(2-pyridylmethyl)glycine) has been
explored the chemistry of some oxime based systems as part of
studied previously in this laboratory, and produces a metalla-
a study of this possibility.3 Identical products have been
cyclic product when irradiated with UV light.2j The product is
stable, has been structurally characterised, and it is possible to
follow the reaction using NMR spectroscopy. In this paper we
describe the preparation and photochemistry of the related
cobalt() complex (Scheme 2) containing the ligand N,N-bis(2-
pyridylmethyl)-1,2-ethanediamine) (bpen), in which a primary
amine donor replaces the carboxylate donor of bpg. We also
report the results from studies of a new series of complexes
containing the tripodal ligand tris(2-pyridylmethyl)amine (tpa).
Experimental
Materials
Reagent grade materials were used without further purification
for all syntheses. -Alanine, 2-aminoisobutyric acid, sarcosine,
1,2-ethanediamine and D2O were obtained from Aldrich Chem.
Co. Glycine, -phenyl glycine and 1,10-phenanthroline were
obtained from BDH and ion exchange resins from Sigma.
The
ligands
N,N-bis(2-pyridylmethyl)-1,2-ethanediamine
(bpen),6 N,N-bis(2-pyridylmethyl)-1,3-propanediamine (bptn),7
and tris(2-pyridylmethyl)amine perchlorate8 were prepared as
previously described. [Co(tpa)(NO2)2](ClO4) was prepared
according to a literature procedure.8
Measurements
1
The H spectra were taken at 300 MHz and 13C spectra at 75
MHz on a Varian Unity-300 spectrometer at 23 ЊC. D2O was
used as a solvent, unless stated otherwise, and sodium 3-tri-
methylsilylpropanesulfonate (TMPS, δ 0, singlet) was used as
an internal reference. Microanalyses were performed at the
University of Otago.
Scheme 1
† Electronic supplementary information (ESI) available: Synthetic,
spectroscopic and crystallographic details. See http://www.rsc.org/
suppdata/dt/b3/b311809f/
D a l t o n T r a n s . , 2 0 0 4 , 1 5 0 – 1 5 6
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
150

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Scheme 2
CAUTION: perchlorate salts of metal complexes containing
organic ligands are potentially explosive and should be handled
with care and in small quantities.
The syntheses and the characterization data for the following
compounds are deposited in the ESI:† [Co(bpen)(NO2)2]NO3ؒ
H2O, [Co(bpen)Cl2](ClO4), [Co(bpen)(phen)](ClO4)3ؒ2H2O,
5.74 (half of AB system, 2H, CH2N), 4.91 (half of AB system,
2H, CH2N), 4.64 (s, H, CH2N), 4.24 (s, CH2Co). The following
signals are assigned to the major component: δ 9.98 (d, H, pyr-
idyl), 6.04 (half of AB system, 2H, CH2N), 5.61 (half of AB
system, 2H, CH2N), 5.26 (s, 2H, CH2N), 4.77 (s, 2H, CH2Co).
13C NMR (D2O): The following signals were assigned on peak
height. Major component: δ 165.60, 165.56, 156.75. 154.92,
142.47, 141.32, 128.58, 128.47, 125.47, 123.99, 71.57, 68.84,
37.56. Minor component: δ 163.75, 163.36, 154.56, 153.22,
142.19, 141.70, 128.94, 128.11, 125.16, 123.92, 73.00, 70.72,
37.73. Anal. Required for C19H22Cl2CoN5O8: C, 39.46; H, 3.83;
N, 12.11%. Found: C, 39.22; H, 3.89; N, 12.00%.
[Co(bpen)(gly)](ClO4)2ؒ2H2O,
[Co(bpen)(gly)][ZnCl4]ؒ3H2O,
[Co(bptn)(NO2)2]NO3ؒ0.5H2O, [Co(bptn)Cl2](ClO4), [Co(tpa)-
Cl2](ClO4), [Co(tpa)(en)](ClO4)3ؒ0.5H2O, [Co(tpa)(gly)](ClO4)2ؒ
H2O, [Co(tpa)(gly)][ZnCl4]ؒ1.5H2O, [Co(tpa)(-ala)](ClO4)2ؒ
H2O, [Co(tpa)(aib)](ClO4)2ؒH2O, [Co(tpa)(β-ala)](ClO4)2ؒH2O,
[Co(tpa)(pgly)](ClO4)2ؒ1.5H2O, [Co(tpa)(sar)](ClO4)2ؒH2O.
1H NMR photolysis experiments
[Co(tpa)(CH2NH2)](BPh4)2ؒ(CH3)2CO
Samples of millimolar concentration in D2O were irradiated in
5 mm NMR tubes using a 500 W high pressure mercury vapour
lamp. Solutions of [Co(tpa)(-ala)](ClO4)2ؒH2O, [Co(tpa)-
(aib)](ClO4)2ؒH2O and [Co(tpa)(pgly)](ClO4)2ؒ2H2O were irradi-
To a solution of [Co(tpa)(CH2NH2)](ClO4)2 (0.04 g, 7 × 10Ϫ6
mol) in water (1.5 mL) and methanol (10 mL), a solution of
NaBPh4 (0.5 g, 15 mmol) in methanol (10 mL) was added. The
solution was refrigerated and after 4 h a crude tetraphenyl-
borate salt of the metallacycle precipitated, was collected by
filtration and air dried. Yield 0.03 g. X-Ray quality crystals
were obtained from the vapour diffusion of diethyl ether into a
solution of the crude salt in acetone at 4 ЊC. Microanalytical
data was not obtained as the crystalline material loses solvent in
air.
1
ated for 75 min. H NMR spectra were recorded at 10, 20, 30,
50 and 75 min. After irradiation, the solutions were acidified
with DCl and the spectra rerecorded. The acidified, photolysed
solution of [Co(tpa)(pgly)](ClO4)2ؒ2H2O was extracted with
1
CDCl3, and the H NMR spectrum of the organic phase was
recorded. Solutions of [Co(tpa)(gly)](ClO4)2ؒH2O, [Co(tpa)-
(gly)][ZnCl4]ؒ2H2O, were irradiated over 17 h and the 1H NMR
spectra were recorded after 6, 10 and 24 h. [Co(bpen)(gly)]-
(ClO4)2ؒH2O and [Co(bpen)(gly)][ZnCl4]ؒ2H2O were irradiated
Preparation of [Co(tpa)(CH2NHMe)](ClO4)2ؒH2O
A solution of [Co(tpa)(sar)](ClO4)2ؒH2O (0.2 g, 3 mmol) in
water (35 mL) was irradiated over 13 h using an immersible
mercury vapour lamp in a jacketed cell. Water was passed
through the jacket, cooling the solution to around 10 ЊC.
The crude photosylate was loaded onto a column of CM-25
Sephadex (14 cm × 3.5 cm, Naϩ form). Elution with water
(250 mL) and aqueous NaClO4 (250 mL, 0.05 M) developed a
red band which was collected using aqueous NaClO4 (0.15 M).
The band was concentrated on a rotary evaporator at 35 ЊC to
approximately 10 mL and placed in a fridge overnight. Red
microcrystalline material formed over this time that was
collected by filtration and washed with ethanol and diethyl
ether. Yield 0.02 g. 1H NMR (D2O): δ 9.95 (d, H, pyridyl), 8.13
(d, H, pyridyl), 8.03 (d, H, pyridyl), 7.80–7.69 (m, 3H, pyridyl),
7.55–7.49 (m, 3H, pyridyl), 7.25–7.18 (m, 3H, pyridyl), 6.10–
6.00 (m, 2H, CH2N), 5.68–5.61 (m, 2H, CH2N), 5.26 (s, 2H,
CH2N), 4.92 (s, H, CoCH), 4.59 (s, H, CoCH). 13C NMR
(D2O): 165.56, 165.33, 165.26, 156.58, 155.39, 154.63, 142.77,
142.64, 141.41, 128.74, 128.72, 128.69, 126.10, 125.67, 123.99,
71.77, 71.73, 69.27, 45.96, 38.04. ESMS: m/z 394 (Mϩ ϩ H), 349
(Mϩ Ϫ CH2NHCH3). Anal. Required for C20H24Cl2CoN5O8: C,
40.56; H, 4.08; N, 11.82%. Found: C, 40.31; H, 3.99; N, 11.73%.
1
over 48 h and the H NMR spectra were recorded after 6, 24
and 48 h. Solutions of [Co(bpen)(phen)](ClO4)3ؒ3H2O and
[Co(tpa)(en)](ClO4)3ؒH2O were irradiated over 4 days, and the
NMR spectra were recorded every 24 h.
Preparation of [Co(tpa)(CH2NH2)](ClO4)2
A solution of [Co(tpa)(gly)](ClO4)2ؒH2O (0.2 g, 3 mmol) in
water (35 mL) was irradiated over 24 h using an immersible
mercury vapour lamp in a jacketed cell. Water was passed
through the jacket, cooling the solution to around 10 ЊC. The
crude photosylate was loaded onto a column of CM-25 Sepha-
dex (14 cm × 3.5 cm, Naϩ form). Elution with water (250 mL)
and aqueous NaClO4 (250 mL, 0.05 M) developed a red band
which was collected using aqueous NaClO4 (0.15 M). The band
was concentrated on a rotary evaporator at 35 ЊC until an
intensely coloured orange solid precipitated. The precipitate
was collected by filtration and washed with ethanol and diethyl
ether. The crude metallacycle was recrystallised from water.
1
Yield 0.05 g. H NMR (D2O): Two isomers are present in the
crystalline material in the ratio of 1 : 1.12. The following signals
are assigned to the minor component: δ 10.13 (d, 1H, pyridyl),
D a l t o n T r a n s . , 2 0 0 4 , 1 5 0 – 1 5 6
151

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Crystal structure determinations
and subsequent ion-exchange chromatography, we were able
to isolate the metallacyclic species [Co(tpa)(CH2NH2)](ClO4)2.
The metallacycle co-elutes with unreacted starting material, but
when the eluted band is concentrated on a rotary evaporator,
the perchlorate salt of the metallacyclic complex precipitates
from the solution before that of the unreacted starting material.
Both isomers are present in this isolated material. A reaction
time of 17 h was found to provide the optimum yield from
the bulk photolyses. Using a similar procedure, we were
able to isolate a metallacyclic species from the photolysis of
Experimental details are provided in the ESI.† Crystallographic
data are shown in Table 1.
CCDC reference numbers 220354–220362.
See http://www.rsc.org/suppdata/dt/b3/b311809f/ for crystal-
lographic data in CIF or other electronic format.
Results
[Co(tpa)(sar)]2ϩ
.
Complex syntheses
Complexes containing amino acids substituted at the
α-carbon were more reactive. After 75 min of irradiation, only a
small amount of starting material could be detected by NMR
spectroscopy in the photolysates of [Co(tpa)(-ala)]2ϩ and
[Co(tpa)(pgly)]2ϩ and none was present in the photolysate of
[Co(tpa)(aib)]2ϩ. In general, the loss in intensity of resonances
assigned to starting material was accompanied by the growth of
new resonances assigned to the metallacyclic product.
The reaction of Co(NO3)2ؒ6H2O with bpen or bptn using
standard conditions9 afforded good yields of the [Co(bpen)-
(NO2)2]NO3 and [Co(bptn)(NO2)2]NO3 complexes. The dinitro
complex of the bpen system is the more symmetrical trans iso-
mer. In the case of bptn, however, the reaction produces major
and minor products that both contain the less symmetrical
cis-coordinated bptn. We suspect that the material is a mixture
of linkage isomers in which both the nitro and nitrito co-
ordination modes are present. An incomplete X-ray structure
analysis of poor quality crystals revealed that the nitro group
trans to the pyridine donor was disordered, and the residual
electron density in the area was consistent with the presence of
O-bound nitrito ligands in some molecules.
Decomposition of the metallacycles resulted, ultimately, in
formation of the carbonyl compounds derived from the amino
acid, presumably via the related imines. In some cases, the
decomposition was sufficiently rapid that metallacyclic complex
resonances had only rather low intensity. The carbonyl com-
pounds could usually be detected in the photolysate, but in the
case of the phenylglycine complex, the benzaldehyde was
detected following extraction into CDCl3. The complexity of
the aromatic region and the low stability of the metallacyclic
complexes limited the degree to which resonances could be
assigned to the metallacyclic complexes, but some could be
assigned by analogy to the stable complex derived from the
glycinato complex (Table 2). Resonances due to free tpa and
free amino acid could also be identified, along with another set
of resonances for an unidentified compound that was formed,
in small amounts, from all of these substrates.
The substitution reaction of [Co(bpen)Cl2]ClO4 with 1,10-
phenanthrolene gave [Co(bpen)(phen)]3ϩ. Only a moderate
yield was obtained since [Co(phen)3]3ϩ is also produced under
the reaction conditions. [Co(bpen)(gly)]2ϩ was prepared from
[Co(bpen)Cl2]ClO4, and a similar route was used to prepare
tpa-amino acid complexes, [Co(tpa)(aa)]2ϩ
.
Following the reactions of [Co(tpa)Cl2]ClO4 and [Co(bpen)-
Cl2](ClO4) with amino acids, ion exchange chromatography of
the reaction mixtures affords one major product band. In each
case, 1H NMR spectra of the dried eluates were consistent with
the formation of only one diastereoisomer. Several of these
amino acid complexes were structurally characterised, either as
their perchlorate or tetrachlorozincate salts, while studies of a
number of others did not produce structural solutions of pub-
lishable standard. In the [Co(tpa)(aa)]2ϩ complexes prepared
under these conditions, the amino acid is coordinated with the
carboxylate group trans to the tertiary amine of tpa. Similarly,
in [Co(bpen)(gly)]2ϩ, the glycine ligand is coordinated with the
carboxylate group trans to the tertiary amine of bpen.
[Co(bpen)(phen)]3ϩ is reasonably stable to UV irradiation,
and a significant amount of starting material is present even
after 4 days. New peaks do appear in the spectrum; the most
abundant species in the solution after this time is [Co(phen)3]2ϩ
.
A new AB system appears at 5.11 and 5.43 ppm, and several
singlets appear at 4.49, 4.14 and 3.97 ppm. The singlet at 3.97
ppm is much larger than the other two after four days of photo-
lysis. A large singlet is also present at 0.16 ppm. We are unable
to identify the compounds giving rise to these peaks, but it is
clear that they are not the metallacyclic complex that is
Photolysis experiments
obtained on photolysis of [Co(bpg)(phen)]2ϩ
.
The effect of UV irradiation on samples of [Co(bpen)(gly)]2ϩ
,
The complex [Co(tpa)(en)](ClO4)3 is comparatively stable
toward UV radiation. After 4 days of photolysis there are few
[Co(bpen)(phen)]3ϩ, [Co(tpa)(en)]2ϩ and [Co(tpa)(aa)]2ϩ dissol-
ved in D2O was followed by NMR spectroscopy. Generally, the
amino acid containing complexes required less irradiation than
their amine analogues before a change in the NMR spectrum
was noticeable.
1
changes in the H NMR spectrum. The only significant new
peak is a singlet appearing at 3.40 ppm. Free en was identified
in the photolysate, following spiking experiments. A dark
material begins to precipitate from the solution after 24 h. We
note also that, when the solution has stood for 7 days following
1H NMR spectra recorded during the photolysis of [Co-
(tpa)(gly)]2ϩ show the gradual appearance of pairs of new
peaks. The most obvious new peaks in the downfield region are
a pair of doublets that appear at 10.12 and 9.98 ppm, repro-
ducibly integrating with a ratio of 1 : 1.1. The aromatic region
between 7.50 and 8.50 ppm rapidly becomes complicated, and
three new AB systems appear between 5.58 and 6.07 ppm. A
fourth AB system is mostly obscured by a broad HOD peak in
the crude photolysate. Two new singlets appear at 4.32 and 4.72
ppm and these also integrate at 1 : 1.1. After 24 h of irradiation,
starting material is still present in the photolysate (approx. 60%
conversion).
1
the photolysis, the H and 13C NMR spectra show that the
picolyl protons on the starting material have exchanged. Similar
observations have proven significant in a recent study of ligand
substitution mechanism.10 There is no indication of any peaks
that can be assigned to the metallacyclic complex, [Co(tpa)-
(CH2NH2)]2ϩ
.
Photolyses on bulk samples of the amine complexes were
performed in a similar manner to those of the related amino
acid complexes. No metallacyclic complexes could be detected
following use of the chromatographic procedures that were
developed for their isolation from photolysates of the amino
acid based systems.
Two isomers of the metallacyclic complex are possible (with
the amine bound cis or trans to the tertiary amine of the tpa
ligand) and we believe, based on these NMR results, that both
are present in the isolated material. The relative yield of these
two isomers appears not to depend on temperature.
Description of structures
The complex cations [Co(bpen)(NO2)2]ϩ, [Co(bpen)(phen)]3ϩ
,
From a larger scale photolysis of a sample of [Co(tpa)(gly)]2ϩ
[Co(bpen)(gly)]2ϩ and [Co(bptn)Cl2]ϩ all have near octahedral
D a l t o n T r a n s . , 2 0 0 4 , 1 5 0 – 1 5 6
152

Table 1 Crystallographic data
Compound
Formula
Mr
[Co(tpa)Cl2](ClO4)ؒH2O
C18H18Cl3CoN4O4
519.64
[Co(tpa)(gly)][ZnCl4]ؒ2H2O
C20H26Cl4CoN5O4Zn
666.56
[Co(tpa)(-ala)](ClO4)2ؒH2O
C21H26Cl2CoN5O11
654.30
[Co(tpa)(aib)](ClO4)2ؒH2O
C22H28Cl2CoN5O11
668.32
[Co(bptn)Cl2](ClO4)
C15H20Cl3CoN4O4
485.63
Crystal system
Space group
Orthorhombic
P212121
Orthorhombic
P212121
Orthorhombic
P212121
Orthorhombic
P212121
Monoclinic
P21/n
a/Å
b/Å
c/Å
8.597(10)
15.162(13)
15.214(17)
8.380(2)
12.014(3)
25.633(6)
11.351(4)
13.701(5)
16.721(6)
11.517(5)
13.490(6)
17.594(7)
12.493(4)
12.111(4)
13.068(4)
107.804(4)
1882.7(11)
4
β/Њ
V/Å3
1983(4)
4
2580.8(12)
4
2600.5(17)
4
2733(2)
4
Z
T/K
168(2)
1.305
24201
4039
2240
271
168(2)
2.024
15486
5019
4813
316
168(2)
0.935
33790
5301
4942
398
168(2)
0.891
17782
5541
4858
416
168(2)
1.368
23580
3851
3397
244
0.034
0.091
µ/mmϪ1
Reflections collected
Independent reflections
Observed
Parameters refined
R [I > 2σ(I )]
Rw [I > 2σ(I )]
0.043
0.060
0.029
0.067
0.023
0.055
0.030
0.070
Compound
Formula
Mr
[Co(bpen)(NO2)2](NO3)ؒH2O
C14H20CoN7O8
473.30
[Co(bpen)(phen)](ClO4)3ؒ3H2O
C26H32Cl3CoN6O15
833.86
[Co(bpen)(gly)][ZnCl4]ؒ3H2O
C16H28Cl4CoN5O5Zn C12H23CoN4O7
636.53
[Co(tpa)(CH2NH2)](BPh4)2ؒ(CH3)2CO
C70H68B2CoN5O
1075.84
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Monoclinic
P21/n
Triclinic
P1
¯
a/Å
b/Å
c/Å
α/Њ
15.77(3)
8.229(13)
15.77(3)
12.010(9)
15.139(11)
18.554(12)
9.5884(19)
14.168(3)
18.429(5)
11.338(3)
13.516(4)
18.982(6)
83.967(4)
84.605(5)
88.122(4)
2879.1(15)
2
β/Њ
106.93(4)
98.70(2)
102.541(4)
γ/Њ
V/Å3
1959(6)
4
3335(4)
4
2443.9(10)
4
r
Z
T/K
168(2)
0.935
8674
3694
2797
271
168(2)
0.836
23349
6689
3112
532
168(2)
2.135
19602
4732
3876
309
168(2)
0.347
36499
11492
7084
726
0.057
0.100
µ/mmϪ1
Reflections collected
Independent reflections
Observed
Parameters refined
R [I > 2σ(I )]
Rw [I > 2σ(I )]
0.026
0.059
0.047
0.083
0.030
0.070

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Table 2 Results from photolyses of [Co(tpa)(aa)]2ϩ complexes
Aminoisobutyric
acid
Coordinated amino acid
-Alanine
-Phenylglycine
Sarcosine
1H NMR resonances assigned to
metallacyclic species
C᎐O Compound detected in
᎐
10.15 (d), 5.09–5.80 (2 × AB),
4.20 (m), 0.83 (d)
Acetaldehyde
10.19 (d), 5.6 (AB),
0.72 (s)
Acetone
10.01, 5.2–5.8
(AB systems)
Benzaldehydea
10.19 (d), 6.3 (m),
5.9 (m), 4.8 (s), 1.8 (s)
photolysate
a Following extraction into CDCl3.
Fig. 1 Structures of some complex cations containing the bpen or bptn ligands: [Co(bpen)(NO2)2]ϩ, top left; [Co(bpen)(phen)]3ϩ, top right;
[Co(bpen)(gly)]2ϩ, bottom left; and [Co(bptn)Cl2]ϩ, bottom right.
geometry (Fig. 1), as do the tpa complexes, [Co(tpa)Cl2]ϩ,
[Co(tpa)(gly)]2ϩ, [Co(tpa)(-ala)]2ϩ and [Co(tpa)(aib)]2ϩ that are
shown in Fig. 2. The largest distortion from a perfect octa-
hedron arises from accommodation of the fused five-membered
rings containing the trans pyridine donors.
present in the crystal. We do not believe that the data and
refinement are sufficiently good to make meaningful
determination of C/N partial occupancy at each site.
a
Discussion
A satisfactory structure of the metallacyclic complex, [Co-
(tpa)(CH2NH2)]2ϩ, was finally obtained for the tetraphenyl-
borate salt (Fig. 3). This complex cation also adopts a distorted
octahedral geometry. The distortions can again be attributed to
the constraints put on the structure by the chelate rings. The
three five-membered chelate rings subtend angles at cobalt of
83–87Њ, and the angle in the three-membered metallacyclic ring,
43.5Њ, is much smaller than the 90Њ angle of the perfect octa-
hedron. The carbon–nitrogen bond of the metallacycle is of
similar length (1.432(4) Å) to those observed in other similar
structures (1.35–1.45 Å), and is indicative of a bond order
somewhat greater than one, as has been noted previously.2j The
refinement was marginally better for the isomer shown than for
the structure where N5 and C19 are exchanged but, given that
both isomers were observed by NMR techniques, we consider it
very likely that there is some disorder and both isomers are
The intention of this work was to compare the photochemistry
of coordinated primary amines with the photochemistry of co-
ordinated amino acids. First, study of the [Co(bpen)(phen)]3ϩ
cation would allow a direct comparison with photochemistry of
the [Co(bpg)(phen)]2ϩ that we have studied previously. Second,
we sought to compare the photochemistry of the new com-
plexes [Co(tpa)(en)]3ϩ and [Co(tpa)(gly)]2ϩ. We have also exam-
ined a variety of [Co(tpa)(aa)]2ϩ complexes to establish that
their photochemistry is the same decarboxylation chemistry as
seen for other amino acid systems.
Structural studies
In the complexes [Co(tpa)(gly)]3ϩ, [Co(tpa)(-ala)]3ϩ, [Co(tpa)-
(aib)]3ϩ and [Co(bpen)(gly)]3ϩ the observed coordination mode
D a l t o n T r a n s . , 2 0 0 4 , 1 5 0 – 1 5 6
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Fig. 2 Structures of some complex cations containing the tpa ligand: [Co(tpa)Cl2]ϩ, top left; [Co(tpa)(gly)]2ϩ, top right; [Co(tpa)(-ala)]2ϩ, bottom
left; and [Co(tpa)(aib)]2ϩ, bottom right.
nitrogen atom and carbon backbone. In the less stable
isomer, the amine hydrogen atoms are predicted to lie only
2.13 Å from the adjacent primary amine hydrogen atoms of the
tren ligand, based on the results of a molecular mechanics
study.12
A similar argument may be considered for the [Co(tpa)(aa)]2ϩ
complexes, but with a change in isomer preference due to the
different structures of the tetradentate ligands. The closest
hydrogen atoms to the hydrogen atoms on N(5) of the amino
acid lie 2.206 Å away, on the picolyl arms of the pyridine rings
that are also cis to the amino acid nitrogen atom (and trans to
each other). However, if the glycinato amine occupied the pos-
ition of the carboxylate oxygen (i.e. trans to the tertiary nitro-
gen atom, as is seen in the tren systems), its hydrogen atoms
would lie close to the hydrogen atoms in the 6-position of those
pyridine rings that are trans to each other. The torsion angles
between the vertical Co–N(1) bond and the pyridine rings are
approximately 16Њ for the trans pyridine groups. These pyridine
hydrogen atoms are forced to occupy that region of space
because of geometrical restrictions imposed by coordination
of the tripodal ligand (and formation of three fused five-
membered rings).
Fig. 3 The structure of the [Co(tpa)(CH2NH2)]2ϩ complex cation.
Photochemical studies
of the amino acids, with the carboxylate donor trans to the
tertiary amine, is different to that observed in the [Co(tren)-
(aa)]2ϩ systems (tren = tris(2-aminoethyl)amine), where the
amine donor is coordinated trans to the tertiary amine.
In the tren systems, the relative stabilities of the two
isomers have been discussed previously in relation to possible
non-bonding interactions between hydrogen atoms on the
glycine nitrogen atom and nearby hydrogen atoms on the tren
From the photolysis experiments with the complex pairs
[Co(bpen)(phen)]3ϩ, [Co(bpg)(phen)]2ϩ and [Co(tpa)(gly)]2ϩ
,
[Co(tpa)(en)]3ϩ it is clear that the amino acid based systems
and the analogous amine systems give different products. Stable
metallacycles are produced from photolyses of [Co(bpg)-
(phen)]2ϩ and [Co(tpa)(gly)]2ϩ, and these have been isolated and
fully characterised.
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The presence of metallacyclic products is easily recognised in
1H NMR spectra as a downfield chemical shift is observed for
the resonance assigned to the 6-proton on the pyridyl ring trans
to the metallacycle. This type of downfield shift has been
observed on photolysis of [Co(bpg)(phen)]2ϩ. Spectra obtained
following photolysis of [Co(tpa)(gly)]2ϩ contain two such peaks,
which is indicative of the presence of two such compounds,
while [Co(tpa)(sar)]2ϩ and [Co(tpa)(-ala)]2ϩ produce only one
metallacyclic complex.
The two isomeric metallacyclic products that are produced
from the [Co(tpa)(gly)]2ϩ system presumably arise because the
amine end of the metallacyclic ligand can be coordinated either
cis or trans to the tertiary amine donor of the tpa ligand. There
is little preference for either isomer on steric (or any other)
grounds until a methyl group is introduced, as in the sarcosine
and alanine systems; in which case only one isomer is detected.
One might predict, on steric grounds, that the opposite isomer
preference might be observed in the sarcosine and alanine sys-
tems, and that the methyl-substituted end of the ligand would
be bound at the less hindered site, which is cis to the tertiary
amine and trans to a pyridine donor (see Fig. 3).
The downfield resonances assigned to the 6-proton on the
pyridyl ring trans to the metallacycle occur at 10.13 and 9.98
ppm in the two isomers of the glycine derived system. The dif-
ference in chemical shift will result from this proton being close
to either the coordinated methylene group or the coordinated
amine group of the metallacyclic complex, depending on the
isomer. If the methyl substituent is introduced cis to the tertiary
amine, the pyridine 6-proton resonance should not be greatly
affected, and the chemical shift should depend principally upon
whether the pyridine proton is close to an amine group (as it
would be in the alanine derived metallacycle) or a methylene
group (as it would be in the sarcosine derived metallacycle)
of a metallacycle. The observation of resonances at 10.15 and
9.95 ppm for the alanine and sarcosine derived metallacycles,
respectively, is consistent with this proposal.
Variable-temperature NMR experiments provided no evi-
dence of fluxionality at temperatures up to 80 ЊC, and the initial
isomeric ratio was retained on cooling the sample to 23 ЊC. The
results do, however, demonstrate the stability of the metalla-
cyclic isomers that were present, as there was no evidence of
any decomposition of the sample following this treatment. In
the absence of any fluxionality in the system, the two isomers
must be formed during the photochemical reaction. The lability
of the cobalt() intermediates that have been proposed for
such systems1 would presumably allow sufficient configur-
ational flexibility in the formation of the cobalt–carbon bond
to account for the production of these isomers.
(phen)]3ϩ. It is possible that the amine systems are simply very
much less reactive, but no metallacycle was detected (by NMR)
in the photolysates, even over long periods of time. Bulk photo-
lyses using an immersion lamp gave similarly negative results.
Given the observed stability of the metallacycles, it seems
unlikely that the amine systems are producing the same product
as the glycine derived systems.
Conclusion
We have established that if amine ligands are photochemically
cleaved, as is reported in the literature, the formation of
metallacyclic species is not a general reaction. Indeed, our
attempts to characterise metallacycles following photolyses of
amine complexes, as described in the literature, have proven
unsuccessful as well. We have prepared the metallacyclic com-
plexes via the photodecarboxylation of chelated amino acids,
shown they are relatively stable under the conditions of
the experiments, and established that the products from the
irradiation of the amine complexes are different. We have also
isolated and structurally characterised nine cobalt() com-
plexes of polypyridyl ligands, including four that also contain
amino acid ligands and one containing a three-membered
metallacyclic ring.
Acknowledgements
The authors would like to thank Professor W. T. Robinson and
Dr J. Wikaira for their help with X-ray structural investigations
associated with this project. We are particularly indebted to
them for the time they invested in ultimately unsuccessful
attempts to obtain structural solutions from poor data sets.
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The photochemistry of [Co(tpa)(-ala)]2ϩ, [Co(tpa)(aib)]2ϩ
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Only small amounts of metallacyclic complexes could be
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be formed in all these cases when the photolysed solutions are
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[Co(bpen)(phen)]3ϩ and [Co(tpa)(en)]3ϩ require longer
periods of irradiation before significant changes are present in
their NMR spectra. New species are produced which do not
have NMR spectra that correspond to the metallacyclic product
that would be predicted based on the literature (which we have
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