Synthesis and some octahedral complexes of a chiral triaza macrocycle

Article abstract of DOI:10.1039/a809125k

The chiral and bulky tacn (1,4,7-triazacyclononane, L¹) analogue chtacn (2,5,8-triazabicyclo[7.4.0^1,9]tridecane, L²), which has a cyclohexane ring fused to the tacn framework, has been synthesized commencing with (±), (+)-

Full text of DOI:10.1039/a809125k

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Synthesis and some octahedral complexes of a chiral triaza
macrocycle
Simon W. Golding,a Trevor W. Hambley,b Geoffrey A. Lawrance,*a Stephan M. Luther,a
Marcel Maedera and Peter Turnerb
a Department of Chemistry, The University of Newcastle, Callaghan 2308, Australia
b School of Chemistry, The University of Sydney, New South Wales 2006, Australia
Received 23rd November 1998, Accepted 7th April 1999
The chiral and bulky tacn (1,4,7-triazacyclononane, L1) analogue chtacn (2,5,8-triazabicyclo[7.4.01,9]tridecane,
L2), which has a cyclohexane ring fused to the tacn framework, has been synthesized commencing with ( ), (ϩ)-
or (Ϫ)-trans-cyclohexane-1,2-diamine. Syntheses and properties of cobalt(), nickel(), chromium() and iron()
complexes are described. In the complex bis(RR-2,5,8-triazabicyclo[7.4.01,9]tridecane)cobalt() chloride hexafluoro-
phosphate the cyclohexane rings and pairs of adjacent secondary amines occupy an approximate plane around the
cobalt ion, with the remaining secondary amines in each tridentate ligand in trans dispositions. A large positive
Cotton effect occurs under the low energy absorption band in the circular dichroism spectrum of this cobalt()
complex of the R,R-(Ϫ)-chtacn ligand. In the dinuclear complex aqua-di-µ-chloro-chlorobis(SS-2,5,8-triaza-
bicyclo[7.4.01,9]tridecane)dinickel() perchlorate each nickel atom is bound to a tridentate macrocyclic ligand
in different dispositions, with the distorted octahedron of each nickel completed by two bridging chloride ions
and either a chloride ion or aqua molecule. For [M(Ln)2]nϩ complexes, electronic maxima are shifted slightly to
lower energy and reduction potentials slightly to more negative potential in the case of L2 compared with L1.
Introduction
CH3
NH
The triazamacrocycle tacn (1,4,7-triazacyclononane, L1) rep-
resents the simplest and most studied example of a cyclic
NH
NH
NH
NH
NH
polyamine capable of co-ordinating facially to an octahedral
complex. First reported by Koyama and Yoshino,1 the co-
ordination chemistry of this molecule has been comprehen-
sively investigated, particularly by Chaudhuri, Weighardt and
co-workers.2 It forms metal complexes which in all cases exceed
in thermodynamic stability those of the closely related acyclic
triamine 3-azapentane-1,5-diamine, an effect associated with
the favourable entropy effects arising from the endodentate
conformation of the tacn ligand where all ligating atoms are
directed towards the macrocycle centre, diminishing rearrange-
ment on co-ordination. Co-ordination of tacn to octahedral
complexes occurs dominantly facially involving all three
secondary amines; with two ligands bound, the metal ion is
effectively ‘sandwiched’ between them. For co-ordination to
square-planar metal ions, chelation by two donors only occurs,
leaving one unbound but available for protonation or other
interactions. The range of complexes prepared involving tacn is
both extensive and diverse.2
A number of other triazamacrocycles have been prepared
and aspects of their co-ordination chemistry examined. These
include examples containing larger rings,1,3 pendants on the
ring,2 or incorporating aromatic units as part of the ring.4
Further, examples with mixed donor sets such as N2O,5,6 N2S7
and NS2,8 as well as O3- and S3-tacn analogues,9,10 have been
reported. We have been examining tetraaza macrocycles incor-
porating cyclohexane rings fused to the macrocycle, which
introduce chirality, bulk and rigidity to the ligand system.11 As
an extension of this study, we are now investigating triaza-
macrocycles with cyclohexane rings fused to the macrocycle
framework. Details of the syntheses of the analogue chtacn
(2,5,8-triazabicyclo[7.4.01,9]tridecane, L2) and aspects of its
co-ordination chemistry to cobalt(), chromium(), iron()
and nickel(), including crystal structure determinations of a
cobalt() and nickel() complex, are reported herein.
NH
NH
NH
L3
L2
L1
Experimental
Syntheses
The polyamine macrocycle (L2) was prepared by a variation of
the well studied Richman–Atkins procedure,3,12–15 as described
for the successive steps below.
N,NЈ-Bis(p-tolylsulfonyl)-trans-cyclohexane-1,2-diamine.
A
combination of 7.58 g (66.4 mmol) of (ϩ)-, (Ϫ)- or ( )-trans-
1,2-diaminocyclohexane and 5.70 g (138.2 mmol) of NaOH
was dissolved in 70 cm3 water and 26.38 g (138.4 mmol) of
purified p-toluenesulfonyl chloride dissolved in 150 cm3 of
diethyl ether were added dropwise over 4 h under vigorous
stirring. After addition was completed, the mixture was stirred
for 45 min before 10 cm3 methanol were added. The mixture
was filtered and the solid washed with water, then methanol,
and recrystallised from hot methanol, giving big colourless
1
crystals (17.7 g, 63%). NMR spectra (CDCl3): H, δ 7.75 (d,
4 H), 7.3 (d, 4 H), 4.97 (br, 2 H), 2.78 (br, 2 H), 2.4 (s, 6 H), 1.81
(br, 2 H), 1.55 (br, 2 H) and 1.11 (br, 4 H); 13C (1H decoupled),
δ 143.5, 137.2, 129.8, 127.3, 56.6, 33.3, 24.2 and 21.6.
N,N-bis[2-(p-tolylsulfonyloxy)ethyl]toluene-p-sulfonamide.14
A mixture of 14 g (132 mmol) of diethanolamine and 130 cm3
of dry pyridine was cooled to <0 ЊC in an ice-salt bath. Next,
76.3 g (400.2 mmol) of purified p-toluenesulfonyl chloride were
added in small portions so that the temperature stayed below
0 ЊC, and the solution was stirred for 2 h at 0 ЊC after completion
J. Chem. Soc., Dalton Trans., 1999, 1975–1980
1975

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of the addition. Then 250 cm3 of 5 mol dmϪ3 HCl were slowly
added so that the temperature always stayed <10 ЊC. A sticky
red oil has formed and the mixture was allowed to stir over-
night. The solid formed was filtered off, washed with ice–water
and recrystallised from methanol (53.8 g, 72%). NMR spectra
crystals suitable for X-ray crystallography, but good quality
crystals were obtained for the RR isomer by rotary evaporation
of the yellow band obtained from column chromatography to
dryness and recrystallisation of this chloride salt from water in
the presence of an excess of sodium hexafluorophosphate,
which yielded crystals of formula [Co(RR-L2)2][PF6]2Clؒ
2.5H2O. The minor product did not crystallise, but was assigned
as trichloro(2,5,8-triazabicyclo[7.4.01,9]tridecane)cobalt() by
spectroscopy. Electronic spectrum (water): λmax 393 and 568
nm. NMR spectra (D2O–water): 13C (1H decoupled), δ 60.1,
45.7, 42.7, 32.1 and 26.5.
1
(CDCl3): H, δ 7.75 (d, 4 H), 7.6 (d, 2 H), 7.33 (d, 4 H), 7.27
(d, 2 H), 4.11 (t, 4 H), 3.37 (t, 4 H), 2.45 (s, 6 H) and 2.41
(s, 3 H); 13C (1H decoupled), δ 145.2, 144.2, 135.4, 132.5, 130.1,
130.0, 128.0, 127.3, 68.3, 48.5, 21.7 and 21.5.
2,5,8-Tris(p-tolylsulfonyl)-2,5,8-triazabicyclo[7.4.01,9]tridec-
ane. N,NЈ-Bis(p-tolylsulfonyl)-trans-cyclohexane-1,2-diamine
(6 g, 14.2 mmol) was dissolved in 190 cm3 of dry DMF, 4.61 g
(33.4 mmol) of dry K2CO3 were added and the mixture was
heated to 50 ЊC for an hour. Then 9.9 g (17.4 mmol) of N,N-
bis[2-(p-tolylsulfonyloxy)ethyl]toluene-p-sulfonamide dissolved
in 100 cm3 of dry DMF were added dropwise within 26 h.
The reaction mixture was kept at 50 ЊC for 7 d. The solution
was evaporated to 50 cm3 and was poured into 2 dm3 of ice–
water acidified with 10 cm3 of 10 mol dmϪ3 HCl. The preci-
pitate was vacuum filtered, washed with ice–water and refluxed
in EtOH for an hour. The solid was filtered off and dried
(2.56 g, 28%).
Aquachloro-di-µ-chloro-bis(2,5,8-triazabicyclo[7.4.01,9]tri-
decane)dinickel(II) perchlorate, [Ni2(L2)2Cl3(OH2)][ClO4]. A
mixture of L2 (486 mg, 2.65 mmol) and NiCl2ؒ6H2O (630 mg,
2.65 mmol) was dissolved in ethanol (50 cm3), 1 mol dmϪ3
NaClO4 (5 cm3) was added and the solution was allowed to
stand for crystallisation. Blue crystals formed and were col-
lected, washed with cold ethanol and recrystallised from
water (0.96 g, 52%) (Found: C, 33.75; H, 6.3; N, 11.6. Calc.
for C20H44Cl4N6Ni2O5: C, 33.9; H, 6.3; N, 11.85%). Elec-
tronic spectrum (water): λmax 352 (18), 433 (sh), 575 (10) and
780 nm (ε 6 dm3 molϪ1 cmϪ1). Crystals employed for X-ray
crystallography contained SS-L2.
Bis(2,5,8-triazabicyclo[7.4.01,9]tridecane)nickel(II) nitrate,
[Ni(L2)2][NO3]2. Compound L2 (425 mg, 2.32 mmol) and
Ni(NO3)2ؒ6H2O (337 mg, 1.16 mmol) were dissolved in water
(75 cm3). The pH was adjusted to 8 with NaOH, and the
solution set aside to crystallise. The pink crystalline product
formed was collected, washed with cold ethanol and recrystal-
lised from water (0.26 g, 45%) (Found: C, 43.65; H, 7.8; N, 20.8.
Calc. for C20H42N8NiO6: C, 43.7; H, 7.7; N, 20.4%). Electronic
spectrum (water): λmax 290 (23), 508 (7) and 812 (ε 9 dm3 molϪ1
cmϪ1) and 880 (sh) nm.
2,5,8-Triazabicyclo[7.4.01,9]tridecane L2. 2,5,8-Tris(p-tolyl-
sulfonyl)-2,5,8-triazabicyclo[7.4.01,9]tridecane (2.32 g, 3.6
mmol) and dry Na2HPO4 (2.58 g, 18.2 mmol) were suspended
in 60 cm3 of dry methanol. After addition of 3% w/w sodium
amalgam (33.54 g, 43.76 mmol) the mixture was refluxed over-
night. The addition of equivalent amounts of Na2HPO4 and
sodium amalgam was repeated two times with further refluxing
overnight after each addition. Water (300 cm3) was added, Hg
decanted, and the solid collected and washed with water. The
solution was evaporated to 100 cm3 and extracted with CH2Cl2
(5 × 100 cm3). The organic phase was dried over Na2SO4,
filtered and evaporated to yield a colourless oil (0.64 g, 97%).
Bis(2,5,8-triazabicyclo[7.4.01,9]tridecane)chromium(III)
chloride perchlorate pentahydrate, [Cr(L2)2]Cl2[ClO4]ؒ5H2O.
The compound CrCl3ؒ6H2O (727 mg, 2.73 mmol) was dissolved
in DMSO (40 cm3) at 190 ЊC and the solution evaporated to 25
cm3. The solution was cooled to 70 ЊC and 1 g (5.45 mmol) of
L2 in 10 cm3 ethanol was added. The temperature was raised to
170 ЊC and the DMSO completely evaporated. The product
was dissolved in water, diluted to 500 cm3 and loaded onto
a Dowex 50W-X2 cation-exchange column (3 × 25 cm). After
washing with 1 mol dmϪ3 HCl to remove unco-ordinated CrIII,
first a red and then a major orange second band were eluted
with 4 mol dmϪ3 HCl. The volume was reduced to 15 cm3, solid
NaClO4 was added and the solution set aside to crystallise (226
mg, 15%; further crops on extended standing) (Found: C, 35.6;
H, 7.8; N, 12.1. Calc. for C20H52Cl3CrN6O9: C, 35.4; H, 7.7; N,
12.35%). Electronic spectrum (water): λmax 345 (55) and 446 nm
(ε 83 dm3 molϪ1 cmϪ1).
1
NMR spectra (CDCl3): H, δ 2.9–2.6 (m, 8 H), 2.35 (br, 3 H),
2.25 (br, 2 H), 1.81 (br, 2 H), 1.68 (br, 2 H) and 1.19 (br, 4 H);
13C (1H decoupled), δ 60.0, 46.9, 44.5, 33.9 and 26.1.
The trihydrochloride salt was prepared by dissolving the
above oily product in 25 cm3 of ethanol and adding 5 cm3 of 10
mol dmϪ3 HCl. The solution was evaporated to dryness. Dry
diethyl ether was added and the product collected by vacuum
filtration (Found: C, 38.5; H, 8.3; N, 13.1. Calc. for C10H24Cl3N3ؒ
H2O: C, 38.6; H, 8.4; N, 13.5%).
Metal complexes. All syntheses below were performed
successfully with either racemic, RR- or SS-L2, with products
differing in chiroptical properties only.
Bis(2,5,8-triazabicyclo[7.4.01,9]tridecane)cobalt(III)
per-
chlorate, [Co(L2)2][ClO4]3. A solution of L2ؒ3HCl (185 mg,
0.63 mmol) and CoCl2ؒ6H2O (75 mg, 0.32 mmol) in water (50
cm3) and 1 mol dmϪ3 HCl (10 cm3) was aerated for 24 h, then
activated charcoal was added and the solution refluxed for 6 h.
The charcoal was filtered off and the solution diluted to
200 cm3 and loaded onto a Dowex 50W-X2 cation-exchange
column (2 × 20 cm). Elution with 3 mol dmϪ3 HCl gave only
two bands, a major orange and a minor red band, which were
collected and evaporated to dryness. The major product was
dissolved in 10 cm3 of water, the pH adjusted to 7, and 4 cm3 of
1 mol dmϪ3 NaClO4 were added. The solution was allowed to
stand for crystallisation. The orange crystals of the major band
were collected, washed with cold ethanol and diethyl ether and
dried (193 mg, 84%) (Found: C, 32.6; H, 5.7; N, 10.9. Calc. for
C20H42Cl3CoN6O12ؒH2O: C, 32.4; H, 6.0; N, 11.3%). Electronic
spectrum (water): λmax 336 (96) and 469 nm (ε 97 dm3 molϪ1
cmϪ1). NMR spectra (D2O): 1H, δ 3.49 (br, 2 H), 3.38–2.76 (br,
18 H), 2.38 (br, 2 H), 2.08 (br, 2 H), 1.79 (br, 8 H) and 1.26 (br,
4 H); 13C (1H decoupled), δ 69.2, 64.1, 56.2, 53.9, 50.9, 44.5,
32.0, 31.5, 26.9 and 26.0. The perchlorate salt did not produce
Bis(2,5,8-triazabicyclo[7.4.01,9]tridecane)iron(III) chloride
hexahydrate, [Fe(L2)2]Cl3ؒ6H2O. The compound FeCl3ؒ6H2O
(740 mg, 2.73 mmol) was dissolved in DMSO (30 cm3) at
140 ЊC, the resultant solution cooled to 25 ЊC, L2 (1 g, 5.45
mmol) in ethanol (10 cm3) added with stirring and the temper-
ature raised to 150 ЊC. The solution was evaporated to 10 cm3
and the yellow-brown precipitate collected. The product was
washed with cold ethanol and recrystallised from water. Orange
crystals were collected, washed with ethanol and diethyl ether
and air dried (413 mg, 29%) (Found: C, 37.2; H, 8.3; N, 12.7.
Calc. for C20H42Cl3FeN6ؒ6H2O: C, 37.7; H, 8.5; N, 13.2%).
Electronic spectrum (water): λmax 347 (281), 435 (ε 73 dm3 molϪ1
cmϪ1) and 512 (sh) nm.
Spectroscopic methods
Absorption spectra were recorded using a Hitachi 150-20
spectrophotometer and circular dichroism spectra using a
JASCO J-710 spectropolarimeter operating with J-700
Windows software, employing aqueous solutions. Infrared
1976
J. Chem. Soc., Dalton Trans., 1999, 1975–1980

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spectra were recorded on a Bio-Rad FTS-7 FT-IR spectrometer
for compounds dispersed in KBr discs, and NMR spectra on a
Bruker Avance DPX300 spectrometer in either CDCl3 solution
(for ligands) or D2O (for complexes). Voltammetry in aqueous
solution (with 0.1 mol dmϪ3 sodium perchlorate as electrolyte)
employed a conventional three-electrode system and nitrogen
purge gas, using a BAS Model CV-27 electrochemical con-
troller. A glassy carbon working electrode polished with
groups replaced by a cyclohexane unit. This is achieved by
commencing with tosylated trans-cyclohexane-1,2-diamine
and reacting with tosylated diethanolamine in a Richman–
Atkins procedure, a process which yields the target small
ring macrocycle with a reasonable yield of approximately 30%.
An alternative procedure of treating dibromocyclohexane
with a tritosylated triamine was not successful. Failure of this
particular route can be interpreted in terms of steric hindrance
of the initial substitution step, similar to arguments proposed
earlier for a bulky nucleophile attacking a sterically hindered
site.15 Steric hindrance is reduced in the successful method
developed here, but the yield remains lower than achieved
for the less bulky tacn. The free amine is obtained from the
tosylated intermediate by employing sodium amalgam as
the detosylating agent, a highly efficient reaction where the
triamine is recovered in essentially stoichiometric yield
provided a large amount of freshly prepared amalgam is
employed. Spectroscopy is consistent with the formulation of
the product as L2; for example, five resonances are detected
in the methylene region of the proton-decoupled 13C NMR
spectrum, as required. Confirmation of the structure comes
in the structural studies reported later. The potentially tri-
dentate ligand L2 possesses two chiral carbon centres, and
resolution of the precursor diamine prior to macrocycle
synthesis yields an optically active macrocycle, since the
chiral centres are not involved in any of the chemistry. In
addition, the cyclohexane ring fused to the ring framework
provides both a bulkier and more rigid macrocycle. As
expected, the co-ordination chemistry (apart from chiroptical
properties) of the racemic, RR- or SS-L2 forms is identical, and
hence in discussion below reference to a particular optical
isomer is made only when discussing circular dichroism
spectroscopy.
Octahedral complexes of cobalt(), chromium(), iron()
and nickel() with two L2 molecules co-ordinated as tridentate
ligands to two opposite octahedral faces have been prepared.
The octahedral cobalt() complex has been characterised by
a crystal structure analysis, and may represent the common
isomer met in the series, although this awaits further structural
studies. No great difficulty in preparing the bis(triamine) com-
plexes was encountered, although it is known that some tri-
azamacrocycles form only 1:1 complexes.2 In addition to the
1:2 nickel() complex, a dinuclear nickel() complex where a
single L2 is co-ordinated to each of two nickel() ions was
isolated and characterised by a crystal structure analysis.
Again, tridentate co-ordination of the macrocycle is observed.
There is no evidence to suggest that the 1:1 complex is pre-
ferred, the dinuclear mono and mononuclear bis complexes
arising from slightly different syntheses. The bis(L2) metal com-
plexes can in principle exist in two geometric isomers (1 and 2
below). We did not detect in the chromatographic isolation of
complexes more than one band assignable to bis(L2) species,
with any minor additional band characteristic of a mono(L2)
complex. This may mean that only one isomer is formed follow-
ing equilibration, or else that separation is not readily achieved.
However, at least for the diamagnetic cobalt() complex, NMR
spectroscopy of the crude evaporated band off the column does
not show multiple sets of resonances, indicative of dominantly
one isomer (1).
alumina before measurement of each voltammogram,
a
Ag–AgCl reference electrode and a platinum wire auxiliary
electrode were used. The potentials reported were converted
into those referred to the normal hydrogen electrode (NHE).
The scan rate for cyclic voltammetry results reported was
usually 100 mV sϪ1. Microanalyses were performed by the
Research School of Chemistry Microanalytical Service at the
Australian National University.
X-Ray crystallography
Cell constants were determined by least-squares fits to the
setting parameters of 25 independent reflections, measured and
refined on a Rigaku AFC7R diffractometer, [Ni2(SS-L2)2-
Cl3(OH2)][ClO4] A, or an Enraf-Nonius CAD4-F diffract-
ometer [Co(RR-L2)2][PF6]2Clؒ2.5H2O B, employing graphite
monochromated Cu-Kα radiation (1.5418 Å) for A and Mo-Kα
(0.71069 Å) for B. Data reduction and application of Lorentz-
polarisation and ψ scan and analytical absorption corrections
(A and B respectively) were carried out using TEXSAN.16 The
structures were solved by direct methods using SHELXS 8617
and refined using full-matrix least-squares methods with
TEXSAN.16 Hydrogen atoms were included at calculated sites
with thermal parameters derived from the parent atoms. The
hydrogen atoms of one of the co-ordinated water molecules of
A were located in the difference map and included at these sites
without further refinement. Non-hydrogen atoms were refined
anisotropically. Scattering factors and anomalous dispersion
terms for Ni and Co (neutral atoms) were taken from ref. 18.
Anomalous dispersion effects were included in Fc;19 the values
for ∆f Ј and ∆f Љ were those of Creagh and McAuley.20 The values
for the mass attenuation coefficients are those of Creagh and
Hubbell.21 All other calculations were performed using
TEXSAN.16 Selected bond lengths and angles are listed in
Tables 2 and 3. The atomic nomenclature is defined in Figs. 2
and 3.22
Crystal data. [Ni2(SS-L2)2Cl3(OH2)][ClO4], C20H44Cl4N6-
Ni2O5, M = 707.80, monoclinic, space group P21, a = 14.406(3),
b = 7.845(1), c = 26.639(41) Å, β = 102.60(1)Њ, U = 2938(1) Å3,
Dc (Z = 4) = 1.600 Mg mϪ3
,
µ(Cu-Kα) = 3.069 mmϪ1
,
F(000) = 1480, T = 294 K. Specimen: purple needles; N = 4953,
No = 3996 [|Fo| > 2.5σ(|Fo|), 38 < θ < 61Њ], hkl 0 to 16, 0 to 8,
Ϫ29 to 29. Final R = 0.0426, RЈ = 0.0475, parameters = 665,
goodness of fit = 2.420. Residual extrema ϩ0.49, Ϫ0.56 e ÅϪ3
.
[Co(RR-L2)2][PF6]2Clؒ2.5H2O, C20H47ClCoF12N6O2.5P2, M =
795.94, monoclinic, space group C2, a = 20.443(2), b = 9.994(2),
c = 8.8967(7) Å, β = 112.78(1)Њ, U = 1682.1(4) Å3, Dc (Z =
2) = 1.577 Mg mϪ3, µ(Mo-Kα) = 0.782 mmϪ1, F(000) = 882,
T = 294 K. Specimen: orange needles; N = 1585, No = 1485
[|Fo| > 2.5σ(|Fo|), 2 < θ < 25Њ], hkl Ϫ24 to 22, 0 to 11, 0 to 10.
Final R = 0.0526, RЈ = 0.0534, parameters = 202, goodness of
fit = 4.442. Residual extrema ϩ0.62, Ϫ0.40 e ÅϪ3
.
CCDC reference number 186/1416.
See http://www.rsc.org/suppdata/dt/1999/1975/ for crystallo-
graphic files in .cif format.
NH
NH
NH
NH
NH
NH
NH
NH
Co
Co
NH
NH
Results and discussion
NH
NH
The molecule L2, as a triazamacrocycle with a nine-membered
saturated ring, is a close analogue of the well known L1,2 but
with one of the CH2CH2 links between two secondary amine
1
2
J. Chem. Soc., Dalton Trans., 1999, 1975–1980
1977

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Fig.
2
A
view of the [Co(RR-L2)2]3ϩ cation, including atom
Fig. 1 Circular dichroism spectrum of [Co(RR-L2)2][ClO4]3.
numbering.
Table 2 Selected bond distances (Å) and angles (Њ) for [Co(RR-L2)2]3ϩ
Table 1 Comparison of the electronic spectra and redox potentials of
[M(L1)2]nϩ and [M(L2)2]nϩ complexes
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
1.976(7)
1.976(8)
1.948(5)
Co(1)–N(1*)
Co(1)–N(2*)
Co(1)–N(3*)
1.977(7)
1.975(8)
1.948(5)
Mnϩ Ligand λmax/nm (εmax/dm3 molϪ1 cmϪ1
)
E1/2/V vs. NHE
Ϫ0.41
Ϫ0.46
Ϫ1.14
Ϫ1.21
ϩ0.13
ϩ0.03
CoIII L1
L2
333 (89), 458 (100)
336 (96), 469 (97)
340 (64), 439 (88)
345 (55), 446 (83)
336 (288), 430 (82), 500 (sh)
347 (281), 435 (73), 512 (sh)
N(1)–Co(1)–N(1*)
N(1)–Co(1)–N(2*)
N(1)–Co(1)–N(3*)
N(1*)–Co(1)–N(2*)
N(1*)–Co(1)–N(3*)
N(2)–Co(1)–N(3)
N(2*)–Co(1)–N(3)
N(3)–Co(1)–N(3*)
92.4(4)
176.0(4)
97.4(3)
84.8(2)
85.3(3)
85.2(4)
92.3(3)
176.1(7)
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(1*)–Co(1)–N(2)
N(1*)–Co(1)–N(3)
N(2)–Co(1)–N(2*)
N(2)–Co(1)–N(3*)
N(2*)–Co(1)–N(3*)
84.8(2)
85.3(3)
176.0(4)
97.4(3)
98.1(4)
92.3(3)
85.1(4)
CrIII L1
L2
FeIII L1
L2
NiII
L1
L2
308 (12), 505 (5), 800 (7), 870 (sh) ϩ0.95
290 (23), 508 (7), 812 (9), 880 (sh) ϩ0.86
Previously, the only substituted chiral analogue of tacn
reported was the R-2-methyl-1,4,7-triazacyclononane (L3).23
Examination of this system arose following the observation
that crystals of the unsubstituted [Co(L1)2]Cl3 spontaneously
resolve on crystallising, the individual chelate rings in a bound
ligand having a preferred common configuration λλλ (or δδδ).
Optical activity cannot arise as a result of disposition of the
ligands in this system, but solely from the dissymmetric chelate
ring conformations, producing a relatively large Cotton effect.
With R-L3, the largest ring-conformation optical activity
reported was observed. Unfortunately, X-ray crystallography
showed orientational disorder of the methyl group,24 and nine
geometrical and configurational isomers are possible, partly
separated by chromatography on SP-Sephadex.25
maxima for the L2 complexes are shifted to slightly lower energy
than those for the L1 complex, indicative of a slightly weaker
ligand field for the new ligand L2 compared with L1. Calcu-
lation of 10Dq for L2 from spectroscopy of the nickel() com-
pounds yields ca. 12 300 cmϪ1 compared with a value of ca.
12 500 cmϪ1 reported for L1.27 The origins of this small differ-
ence are presumably steric, related to the bulk of the cyclo-
hexane ring fused to the macrocycle ring adjacent to two of the
nitrogen donors. Although there is no required relationship
between redox potentials and electronic maxima, it is also
notable that a reasonably consistent shift in redox potentials
from L1 to L2 also occurs (Table 1). All four [M(L2)2]nϩ com-
plexes display reversible or quasireversible couples in the cyclic
voltammetry (50 mV sϪ1 scan rate, glassy carbon working elec-
trode, aqueous 0.1 mol dmϪ3 NaClO4) varying in potential from
ϩ0.86 V (∆E 90 mV) for the NiIII/II couple, ϩ0.03 V (∆E 74 mV)
for FeIII/II, Ϫ0.46 V (∆E 76 mV) for CoIII/II, and to Ϫ1.21 V (∆E
90 mV) for CrIII/II, cited versus the NHE. These range from 50 to
100 mV more negative than the values reported for the corre-
sponding compound in the [M(L1)2]nϩ series.
Orientational disorder is removed and the number of
possible isomers is reduced in the present study with L2. The
CD of the cobalt() complex of the R,R-(Ϫ) isomer of L2 is
dominated by a very strong Cotton effect under the lowest
energy transition at 486 nm of ∆ε ϩ51.4 dm2 molϪ1 (Fig. 1). The
maximum lies to lower energy than the absorption maximum
(469 nm), and is most likely associated with the low-energy
1
1A1 → 1E component under the A1g → 1T1g octahedral
The [Co(RR-L2)2][PF6]2Clؒ2.5H2O complex crystallised in
the C2 space group. The structure (Fig. 2) consists of a complex
cation located on a crystallographic twofold rotation axis, a
chloride anion also on a twofold rotation axis, a hexafluoro-
phosphate anion and two water molecules, one at a general site
and the other on a twofold rotation axis. The complex cation
displays a distorted octahedral geometry and has the cyclo-
hexane groups located trans to one another. There are no obvi-
ous steric reasons for this arrangement being preferred. As with
the nickel complex discussed below, bond lengths and angles
are not abnormal, although the Co–N(3) bond [1.948(5) Å] is
significantly shorter than the two Co–N bond adjacent to the
cyclohexane ring [Co–N(1) 1.976(7), Co–N(2) 1.976(8) Å] and
is at the short end of the range for hexaaminecobalt() systems.
With the two longer bonds associated with the two secondary
nitrogen atoms adjacent to the cyclohexane ring, it is tempting
to assign the elongation to steric effects. However, the longer
and not the shorter distances are very similar to the average
distance reported for [Co(L3)2]3ϩ of 1.974 Å,24 which suggests
that steric influences of the cyclohexane ring are not marked.
transition envelope, similar to the behaviour for [Co(nn)3]3ϩ
complexes (nn = ethane-1,2-diamine, R-propane-1,2-diamine
or R,R-cyclohexane-1,2-diamine).26 For these and related com-
plexes a positive Cotton effect is associated with the  absolute
configuration of the complex. Whereas the distribution of
chelate rings is an important component in these tris(bidentate
ligand) systems, in the L2 complex the large Cotton effect arises
chiefly from the dissymmetric chelate ring conformations. The
δδδ conformations in the L2 complex produce the same sign
of Cotton effect under the low energy d–d transition as the
δδδ conformations in L3, with the size of the transition larger
(by ca. 20%) for the L2 complex. A much less intense set of
bands occurs under the higher energy 1A1g → 1T2g octahedral
transition envelope, with additional strong Cotton effects
between 180 and 300 nm presumably associated with charge
transfer transitions and n → π* transitions of the ligand.
A comparison of the electronic spectra and metal-centred
redox properties of bis(triamine)metal complexes of L1 and L2
appears in Table 1. It is immediately noticeable that, in all cases,
1978
J. Chem. Soc., Dalton Trans., 1999, 1975–1980

View Article Online
The structure (Fig. 3) of the dinickel() complex of SS-L2
consists of two independent dimeric complex cations and two
perchlorate anions. Tridentate co-ordination of the ligand L2 is
confirmed. The dimers consist of two Ni(L2) units bridged by
two µ-chloro ligands. The co-ordination sphere of one of the Ni
atoms is completed by a chloro ligand and the other by an aqua
ligand with an intramolecular hydrogen bond [O(1) ؒ ؒ ؒ Cl(3)
3.316(7), O(2) ؒ ؒ ؒ Cl(6) 3.347 Å] between these two ligands pro-
ducing a bend in the Ni2Cl2 plane. Additional assymetry in
the dimers arises from the orientation of the L2 ligand. In both
of the independent molecules one ligand has the cyclohexane
ring lying in the Ni2Cl2 plane and the other ligand has it lying
perpendicular to this plane. It is not clear whether this is a
consequence of the difference in the nature of the axial ligands
but there are no obvious steric reasons for the difference. It is
possible that a number of isomers form in solution and this is
the one that crystallises preferentially; the solution is very likely
a complex mixture, as the bis(triamine)nickel() ions also
crystallise from a similar reaction solution. The two independ-
ent molecular cations have the same arrangement of the
tridentate ligands but are diastereomers. It is also not clear why
the complex forms with one axial chloro ligand and one axial
aqua ligand. However, if both axial ligands were chlorides the
repulsion between them would lead to a folding in the opposite
direction and a consequent repulsion between the amine
ligands. Bond lengths and angles about the Ni are normal
for amine complexes of this type and do not indicate that
co-ordination of the tridentate ligand is accompanied by the
induction of significant stress.
Fig. 3 A view of the [Ni2(SS-L2)2Cl3(OH2)]ϩ cation, including atom
numbering.
Table 3 Selected bond distances (Å) and angles (Њ) for [Ni2(SS-L2)2-
Cl3(OH2)]ϩ
Ni(1)–Cl(1)
Ni(1)–Cl(3)
Ni(1)–N(2)
Ni(2)–Cl(1)
Ni(2)–O(1)
Ni(2)–N(5)
Ni(3)–Cl(4)
Ni(3)–Cl(6)
Ni(3)–N(8)
Ni(4)–Cl(4)
Ni(4)–O(2)
Ni(4)–N(11)
2.471(3)
2.480(2)
2.060(9)
2.460(2)
2.169(5)
2.056(7)
2.381(2)
2.454(2)
2.081(7)
2.442(3)
2.151(6)
2.087(7)
Ni(1)–Cl(2)
Ni(1)–N(1)
Ni(1)–N(3)
Ni(2)–Cl(2)
Ni(2)–N(4)
Ni(2)–N(6)
Ni(3)–Cl(5)
Ni(3)–N(7)
Ni(3)–N(9)
Ni(4)–Cl(5)
Ni(4)–N(10)
Ni(4)–N(12)
2.404(2)
2.081(7)
2.075(8)
2.421(3)
2.080(7)
2.07(1)
2.518(3)
2.103(9)
2.074(7)
2.444(2)
2.054(7)
2.06(1)
The presence of a cyclohexane ring fused onto the basic tacn
framework clearly influences the co-ordination chemistry
involving this ligand somewhat. Nevertheless, the ability of a
relatively rigid chiral tacn ligand to form 1:2 complexes with
octahedral metal ions has been established, and aspects of the
co-ordination chemistry of these described. We are extending
our studies to examine the effect of increasing the number of
fused rings around the tacn core.
Cl(1)–Ni(1)–Cl(2)
Cl(1)–Ni(1)–N(1)
Cl(1)–Ni(1)–N(3)
Cl(2)–Ni(1)–N(1)
Cl(2)–Ni(1)–N(3)
Cl(3)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
N(2)–Ni(1)–N(3)
Cl(1)–Ni(2)–O(1)
Cl(1)–Ni(2)–N(5)
Cl(2)–Ni(2)–O(1)
Cl(2)–Ni(2)–N(5)
O(1)–Ni(2)–N(4)
O(1)–Ni(2)–N(6)
N(4)–Ni(2)–N(6)
Cl(4)–Ni(3)–Cl(5)
Cl(4)–Ni(3)–N(7)
Cl(4)–Ni(3)–N(9)
Cl(5)–Ni(3)–N(7)
Cl(5)–Ni(3)–N(9)
Cl(6)–Ni(3)–N(8)
N(7)–Ni(3)–N(8)
N(8)–Ni(3)–N(9)
Cl(4)–Ni(4)–O(2)
Cl(4)–Ni(4)–N(11)
Cl(5)–Ni(4)–O(2)
Cl(5)–Ni(4)–N(11)
O(2)–Ni(4)–N(10)
O(2)–Ni(4)–N(12)
N(10)–Ni(4)–N(12)
Ni(1)–Cl(1)–Ni(2)
Ni(3)–Cl(4)–Ni(4)
85.62(9)
98.0(3)
91.8(3)
176.4(3)
97.1(2)
94.6(2)
82.3(3)
83.0(3)
87.3(2)
174.6(3)
88.0(2)
99.4(3)
172.2(3)
96.7(4)
83.6(4)
85.84(9)
96.3(2)
94.4(2)
177.0(2)
95.6(3)
92.0(2)
83.0(3)
81.9(3)
89.1(2)
93.7(3)
88.4(2)
179.2(2)
173.6(3)
90.4(3)
83.9(4)
92.7(1)
95.2(1)
Cl(1)–Ni(1)–Cl(3)
Cl(1)–Ni(1)–N(2)
Cl(2)–Ni(1)–Cl(3)
Cl(2)–Ni(1)–N(2)
Cl(3)–Ni(1)–N(1)
Cl(3)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
Cl(1)–Ni(2)–Cl(2)
Cl(1)–Ni(2)–N(4)
Cl(1)–Ni(2)–N(6)
Cl(2)–Ni(2)–N(4)
Cl(2)–Ni(2)–N(6)
O(1)–Ni(2)–N(5)
N(4)–Ni(2)–N(5)
N(5)–Ni(2)–N(6)
Cl(4)–Ni(3)–Cl(6)
Cl(4)–Ni(3)–N(8)
Cl(5)–Ni(3)–Cl(6)
Cl(5)–Ni(3)–N(8)
Cl(6)–Ni(3)–N(7)
Cl(6)–Ni(3)–N(9)
N(7)–Ni(3)–N(9)
Cl(4)–Ni(4)–Cl(5)
Cl(4)–Ni(4)–N(10)
Cl(4)–Ni(4)–N(12)
Cl(5)–Ni(4)–N(10)
Cl(5)–Ni(4)–N(12)
O(2)–Ni(4)–N(11)
N(10)–Ni(4)–N(11)
N(11)–Ni(4)–N(12)
Ni(1)–Cl(2)–Ni(2)
Ni(3)–Cl(5)–Ni(4)
90.72(9)
174.7(2)
90.03(8)
94.1(2)
90.0(2)
172.6(2)
82.8(3)
85.50(9)
100.4(2)
92.6(3)
92.0(3)
174.8(3)
90.5(2)
81.9(3)
82.7(3)
91.56(9)
176.4(2)
89.1(1)
94.8(2)
93.0(2)
172.6(2)
82.1(3)
86.16(9)
96.4(3)
175.4(3)
95.4(2)
98.4(2)
92.4(2)
83.9(3)
81.8(3)
95.3(1)
91.7(1)
Acknowledgements
Support of the Australian Research Council for this project is
gratefully acknowledged.
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Paper 8/09125K
1980
J. Chem. Soc., Dalton Trans., 1999, 1975–1980