An unusually flexible expanded hexaamine cage and its CuII complexes: Variable coordination modes and incomplete encapsulation

Article abstract of DOI:10.1021/ic201326d

The bicyclic hexaamine "cage" ligand Me ₈tricosaneN₆ (1,5,5,9,13,13,20,20-octamethyl-3,7,11,15,18, 22-hexaazabicyclo[7.7.7]tricosane) is capable of encapsulating octahedral metal ions, yet its expanded cavity allows the complexed met

Full text of DOI:10.1021/ic201326d

ARTICLE
pubs.acs.org/IC
An Unusually Flexible Expanded Hexaamine Cage and Its CuII Complexes:
Variable Coordination Modes and Incomplete Encapsulation
Chang-Jin Qin,† Lloyd James,‡ Jy D. Chartres,§ Leighton J. Alcock,‡ Kimberley J. Davis,‡ Anthony C. Willis,†
||
Alan M. Sargeson,†, Paul V. Bernhardt,*,§ and Stephen F. Ralph*,‡
†Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia
‡School of Chemistry, University of Wollongong, New South Wales 2522, Australia
§School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, Queensland 4072, Australia
S Supporting Information
b
ABSTRACT: The bicyclic hexaamine “cage” ligand Me8tricosaneN6 (1,5,5,9,13,13,20,20-
octamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane) is capable of encapsulating
octahedral metal ions, yet its expanded cavity allows the complexed metal to adopt a variety
of geometries comprising either hexadentate or pentadentate coordination of the ligand.
When complexed to CuII the lability of the metal results in a dynamic equilibrium in
solution between hexadentate- and pentadentate-coordinated complexes of Me8tricosa-
neN6. Both [Cu(Me8tricosaneN6)](ClO4)2 (6-coordinate) and [Cu(Me8tricosane-
N6)](S2O6) (5-coordinate) have been characterized structurally. In weak acid (pH 1) a
singly protonated complex [Cu(HMe8tricosaneN6)]3+ has been isolated that finds the
ligand binding as a pentadentate with the uncoordinated amine being protonated.
visÀNIR and electron paramagnetic resonance (EPR) spectroscopy show that the
predominant solution structure of [Cu(Me8tricosaneN6)]2+ at neutral pH comprises a
five-coordinate, square pyramidal complex. Cyclic voltammetry of the square pyramidal
[Cu(Me8tricosaneN6)]2+ complex reveals a reversible CuII/I couple. All of these structural, spectroscopic, and electrochemical
features contrast with the smaller cavity and well studied “sarcophagine” (sar, 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane) CuII
complexes which are invariably hexadentate coordinated in neutral solution and cannot stabilize a CuI form.
’ INTRODUCTION
expanded macrobicyclic hexaamine Me5tricosaneN6 (1,5,9,13,20-
pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane)
whose complexes indeed exhibited remarkable structural, electronic,
and spectroscopic properties16À20 compared with the smaller cavity
sar (and sep) cages. The CoIII complex [Co(Me5tricosaneN6)]3+
exhibits unusually long CoIIIÀNbondlengths (2.010(4)À2.032(4)
Å)16 and a weak ligand field in comparison with [Co(sep)]3+
(CoÀN 1.96 Å).1 In addition, the [Co(Me5tricosaneN6)]3+/2+
redox couple16 is found at a significantly more positive potential
than [Co(sep)]3+/2+.4 All of these features highlight the influence of
the larger cavity of the Me5tricosaneN6 ligand on the properties of
the metal ion it encapsulates.
The octa-methylated analogue Me8tricosaneN6 (1,5,5,9,13,13,-
20,20-octamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane,
Chart 1), bears exactly the same ligand framework as Me5tricosa-
neN6 yet the subtle addition of methyl groups on each strap alters its
coordination chemistry markedly. Unusually high CoIII/II redox
potentials and optical properties have been reported for the Co
complexes of Me8tricosaneN6 and its triimine.21 Oxidation of pink
[Co(Me8tricosaneN6)]2+ yields two different CoIII complexes. One
of these is an orange complex obtained from aqueous solution and
The macrocyclic cage ligands sepulchrate (sep or 1,3,6,8,10,-
13,16,19-octaazabicyclo[6.6.6]icosane)1 and sarcophagine (sar
or 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane)2 and substi-
tuted analogues (Chart1) occupy a pivotal place in coordination
chemistry. Through systematic investigations of the transition
metal coordination chemistry of sep and sar, unique physical
properties of their complexes have emerged such as extreme
resistance to dissociation3 and racemization.4 These properties
have led to some important applications particularly in medicinal
chemistry through the development of a functionalized ligandbased
on (NH2)2sar (Chart1) through complexation of 64CuII for diag-
nostic positron emission tomography (PET) imaging.5,6
It has been well established that truly novel properties can
arise through tuning the macrocyclic ring size, and these have led
to many applications of macrocycles such as the expanded
porphyrin family,7À9 crown ethers,10 and cryptands11,12 whose
ring size has been varied to suit the size of the guest ion or
molecule. There are many other well studied families of organic
hosts whose selectivity has been fine-tuned for guest binding by
adjustment of the macrocyclic size and shape.13À15
A natural evolution of our research was to expand the
macrocyclic ring size of sar by lengthening the three “straps”
connecting each triamine “cap” of the ligand. This led to the
Received: June 21, 2011
Published: August 01, 2011
r
2011 American Chemical Society
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|

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ARTICLE
Chart 1
has been characterized by X-ray crystallography, with an average
CoIIIÀN distance of 1.99(1) Å;22 intermediate between [Co-
(Me5tricosaneN6)]3+16 and [Co((NH3)2sar)]5+.23 The second
CoIII complex of Me8tricosaneN6 was blue but gave identical
NMR spectra to that of the orange compound in water. The
structural basis of these unusual properties is still poorly understood
and to date no systematic study of Me8tricosaneN6 has revealed why
its complexes exhibit such unusual properties compared with
analogues from the sep and sar family.
1H NMR spectrum (D2O): δ 1.27 (s, 3H, CH3); δ 3.22 (s, 6H, CH2CH3);
δ 4.28 (s, 6H, CH2Ph); δ 7.4À7.6 (overlapping multiplet, 15H, ArH).
1,1,1-Tris(aminomethyl)ethane Trihydrochloride (tame 3HCl).
1,1,1-Tris((benzylamino)methyl)ethane 3HCl (2.50 g, 5.0 mmol)
3
3
was dissolved in methanol (150 mL) in a round bottomed flask and
purged with nitrogen for 3 min. Palladium on carbon (10% (w/w); 0.53
g) was added followed by ammonium formate (4.73 g, 75 mmol), and
then a reflux condensor with rubber septum was attached. The apparatus
was evacuated and then filled with nitrogen. After this process was
repeated a further two times, the reaction mixture was brought to reflux
for 4 h. The reaction mixture was cooled and then filtered through a pad
of Celite, which was washed with methanol. The combined filtrates were
evaporated to give the product as a colorless solid (1.0 g, 88%). 1H NMR
spectrum (D2O): δ 1.25 (s, 3H, CH3); δ 3.17 (s, 6H, CH2).
Herein we report the synthesis and isolation of Me8tricosa-
neN6 as its free base in addition to its copper(II) coordination
chemistry. This study, in comparison with our previous investi-
19
gations of the CuII coordination chemistry of Me5tricosaneN6
and (NH2)2sar,24 highlights the fact that Me8tricosaneN6 is an
atypical cage that, unlike sar and sep which demand a rigid and
well-defined coordination environment, is extremely flexible and
can accommodate a variety of coordination geometries and
coordination numbers. This flexibility of coordination opens
up a new area of cage ligand coordination chemistry whereby
incompletely encapsulated complexes are the norm.
[Co(Me8tricosaneN6)](NO3)2. [Co(tame)2]Cl3 (10.2 g; 0.0255 mol)
and NaClO4 (46 g; 0.38 mol) were stirred with 250 mL of acetonitrile for
10 min. Paraformaldehyde (3.0 g; 0.10 mol) and isobutyraldehyde (71.9
g; 1.00 mol) were added to the solution, followed by 20 mL of
triethylamine. The reaction mixture was stirred for 2 h at room
temperature, during which time it changed color from orange to brown,
and finally dark purple. Dilute HCl (1 M, 300 mL) was added to quench
the reaction, and the resulting solution diluted to ∼4 L with water and
loaded onto a Dowex 50W X-2 cation exchange column (15 Â 6 cm
resin bed). The dark red band was first washed with 700 mL of water,
then 2 L of 1 M HCl. After these washings only the mixture of CoIII
complexes formed in the reaction remained bound to the column. These
were then eluted with 5 M HCl, and the eluate was evaporated to dryness
using a rotary evaporator.
The resulting solid residue containing [Co(Me8tricosanetriimine-
N6)]Cl3 was dissolved in 400 mL of water and adsorbed onto a SP
Sephadex C-25 cation exchange column (57 Â 9 cm resin bed) and
eluted using 0.2 M K2SO4. The last pink band to elute was collected and
adsorbed onto a Dowex 50W X-2 cation exchange column (7 Â 4.5 cm
resin bed). The column was washed with 600 mL of water, followed by 2
L of 1 M HCl. Finally the column was eluted using 2 L of 5 M HCl to give
a pink solution containing [CoMe8tricosanetriimineN6)]3+, which was
evaporated to dryness.
’ EXPERIMENTAL SECTION
Reagents. 1,1,1-tris(p-toluenesulfonyloxy)methyl)ethane was pre-
pared according to a literature procedure.25 The complex [Co(tame)2]
Cl3 (tame = 1,1,1-tris(aminomethyl)ethane) was prepared as des-
cribed.26 An improved synthesis of the free ligand tame is described below.
1,1,1-Tris((benzylamino)methyl)ethane. 1,1,1-Tris(p-toluenesulfo-
nyloxy)methyl)ethane (11.7 g, 20.0 mmol) was suspended in benzyl-
amine (26 mL, 240 mmol) and heated to 180 °C for 2 h. The reaction
mixture was then allowed to cool, resulting in precipitation of benzy-
lammonium tosylate. Excess benzylamine was removed by vacuum
distillation. After cooling, hexane (200 mL) was added to the remaining
solid mass, and the mixture brought to reflux for 15 min. The reaction
mixture was cooled to room temperature and then refrigerated over-
night. The resulting suspension was filtered, and the solid washed with
hexane. The combined filtrates were washed with water, then dried over
magnesium sulfate and evaporated to give 1,1,1-tris((benzylamino)
methyl)ethane as a colorless oil. (6.6 g, 85%). 1H NMR spectrum
(CDCl3): δ 0.88 (s, 3H, CH3); δ 2.56 (s, 6H, CH2CH3); δ 3.75 (s, 6H,
CH2Ph); δ 7.1À7.4 (overlapping multiplet, 15H, ArH).
The resulting pink solid was dissolved in 30 mL of water, and the pH
of the solution adjusted to ∼10 with sat. Na2CO3. A solution containing
NaBH4 (0.38 g; 9.9 mmol) dissolved in 40 mL of water (pH previously
adjusted to ∼10 with Na2CO3) was then added with stirring. After 20
min the reaction was quenched by addition of 120 mL of sat. NaHCO3.
After a further 30 min stirring the solution was diluted to ∼1.5 L with
water and then adsorbed onto a SP Sephadex C-25 cation exchange
column (55 Â 6 cm resin bed) and eluted using 0.2 M NaNO3. A single
light purple band was collected and evaporated on a hot plate at
40À50 °C to ∼30 mL, resulting in a pink precipitate of [Co(Me8-
1,1,1-Tris((benzylamino)methyl)ethane Trihydrochloride. The tria-
mine free base (6.6 g) was converted to the corresponding hydrochlor-
ide salt by dissolving the compound in methanol (50 mL) then adding
conc. HCl (10 mL). The solvent was evaporated, and residual water
removed as an azeotrope with ethanol on a rotary evaporator. The
resulting solid was recrystallized from ethanol to give 1,1,1-tris
tricosaneN6)](NO3)2 H2O, which was filtered offand dried in air. Yield 450
3
mg (3%). Anal. Calc. for C25H56CoN8O7: ([Co(Me8tricosaneN6)](NO3)2
((benzylamino)methyl)ethane 3HCl as a colorless solid. (8.5 g, 100%).
3
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Table 1. Crystal Data
[Cu(Me8tricosaneN6)]
[Cu(Me8tricosane)]
(S2O6)
[Cu(HMe8tricosaneN6)]
Me8tricosaneN6
(NO3)2 2H2O
(NO3)3 H2O
3
3
formula
Fw
C25H54N6
438.74
C25H58CuN8O8
662.33
C25H54CuN6O6S2
666.40
C25H57CuN9O10
707.34
cryst. syst.
space group
a /Å
triclinic
tetragonal
monoclinic
P21/c (No. 14)
10.527(1)
monoclinic
P1 (No. 2)
10.2473(1)
11.4242(2)
12.9246(2)
96.516(1)
112.770(1)
100.383(1)
1343.98(4)
2
P42/n (No. 86)
10.8480(8)
P21/n (No. 14, variant)
18.996(1)
b /Å
15.585(2)
10.7350(6)
c /Å
28.720(3)
19.257(3)
19.297(1)
R /deg
β /deg
γ /deg
V /Å3
90.68(1)
119.174(9)
3379.8(5)
4
3159.1(7)
4
3435.8(4)
4
Z
T /K
200(2)
293(2)
0.71073
0.701
200(2)
0.71073
0.86
293(2)
0.71073
0.699
λ/Å
μ/cmÀ1
0.71073
0.065
Fcalc
1.084
1.302
1.393
0.038
0.035
1.367
R(obs data)a
wR2(all data)b
0.0402
0.0689
0.0412
0.0902
0.1138
2
0.1867
2 1/2
a R(Fo) = ∑||Fo| À |Fc||/∑|Fo|. b Rw(Fo ) = [∑w(Fo À Fc2)/∑wFo ]
.
2
H2O): C 46.9; H 8.8; N 17.5; Co 9.2. Found C 46.7; H 9.2; N 17.4;
([Cu(Me8tricosaneN6)](ClO4)2 3H2O): C 39.8; H 8.0; N 11.1; Cl
3
3
À
Co 9.3.
9.4. Found C 39.6; H 7.5; N 10.5; Cl 9.3. The CF3SO3À, PF6 and
Anion exchange chromatography (Dowex 1-X8, ClÀ form) of
S2O62À salts were obtained similarly by addition of the sodium salt of the
respective anion. The dithionate salt was obtained as X-ray quality
crystals from dilute ammonia solution.
[Co(Me8tricosaneN6)](NO3)2 H2O yielded the corresponding chlor-
3
ide salt, which was isolated after evaporation of the eluate. Anal. Calc. for
C25H60CoN6Cl2O3: ([Co(Me8tricosaneN6)]Cl2 3H2O): C 48.2; H
9.7; N 13.5; Cl 11.4; Co 9.5. Found C 49.0; H 9.6; N 13.3; Cl 11.9;
Co 9.5.
[Cu(HMe8tricosaneN6)](NO3)3.
[Cu(Me8tricosaneN6)](NO3)2
3
(5 mg) was dissolved in DMF/H2O (1:2) (1 mL). Dilute nitric acid
was added dropwise to pH 1 which led to a color change from blue to
purple. Slow evaporation of the solution at room temperature afforded
purple crystals of the complex suitable for X-ray work.
Me8tricosaneN6. [Co(Me8tricosaneN6)](NO3)2 (1.14 g, 1.83
mmol) was dissolved in 500 mL of water. The solution was then
adsorbed onto a column of Dowex 50W X-2 cation exchange resin
(7.5 Â 4.5 cm resin bed, H+ form), and the column washed with 0.5 M
HCl (500 mL) and then 1 M HCl (1 L) which resulted in complex
dissociation and elution of pink Coaq2+. A faint white band of the free
ligand was apparent on the top of the light brown column, and this eluted
with 5 M HCl (1.5 L). The eluate was evaporated to dryness, and the
resulting colorless powder redissolved in approximately 20 mL of water.
Sufficient saturated NaOH solution was then added to adjust the pH to
∼12, resulting in a brown precipitate which was isolated by filtration and
dried in air. The precipitate was recrystallized using a minimum amount
(250 mL) of hot MeCN to yield pure Me8tricosaneN6. Yield 640 mg
(80%).
Physical Methods. Positive ion electrospray ionization (ESI) mass
spectra were obtained using a Waters VG Quattro mass spectrometer
and aqueous solutions containing either Me8tricosaneN6 or its copper-
(II) complex. The absorption spectrum of [Cu(Me8tricosaneN6)]2+ was
recorded using a Varian Cary 500 UVÀvisÀNIR spectrophotometer
and quartz cuvettes. Cyclic voltammetry was performed on a BAS100B/
W potentiostat employing a glassy carbon working electrode, Pt auxiliary
electrode, and Ag/AgCl reference electrode. The supporting electrolyte
was 0.1 NaNO3, and all solutions were purged with Ar before measure-
ment. Elemental analysis was performed by the Microanalytical Unit at
the Australian National University. 1H and 13C NMR spectra were
recorded on a Varian Unity-300 (300 MHz 1H, 75 MHz 13C) spectro-
meter in either D2O or CDCl3. Electron paramagnetic resonance (EPR)
spectra were measured with a Bruker ER200 instrument at X-band
frequency (∼9.3 GHz) as 2 mM DMF/H2O 1:2 frozen solutions at 140
K. Spin Hamiltonian parameters were determined by spectral simulation
with the program EPR50F.27
Crystallography. For Me8tricosaneN6 the X-ray data were col-
lected with a Nonius KappaCCD diffractometer at 200 K using the
COLLECT software (Nonius B.V.) while both data reduction and cell
refinement were accomplished using DENZO/SCALEPACK.28 For
[Cu(Me8tricosaneN6)](S2O6) data were collected on a Rigaku AFC6S
while data reduction was carried out with the teXsan package (Rigaku/
MSC). The structures were solved with SIR9229 and refined by full-matrix
least-squares analysis using the program CRYSTALS30 in the case of
[Cu(Me8tricosaneN6)](S2O6) and teXsan in the case of Me8tricosaneN6.
Anal. Calc. for C25H56N6O (Me8tricosaneN6 H2O): C 65.7; H 12.4;
3
1
N 18.4. Found C 64.9; H 12.6, N 18.0. H NMR spectrum (CDCl3):
δ 0.70 (s, 6H, cap CH3); δ 0.89 (s, 18H, strap CH3); δ 2.35 (s, 12H, strap
CH2); δ 2.52 (s, 12H, cap CH2). 13C NMR spectrum (CDCl3): δ 25.31
(cap CH3); δ 25.62 (strap CH3); δ 35.18 (strap quaternary C); δ 38.66
(cap quaternary C); δ 61.73 (cap CH2); δ 62.66 (strap CH2).
[Cu(Me8tricosaneN6)](NO3)2. Me8tricosaneN6 (100 mg, 0.23 mmol)
was dissolved in 10 mL of ethanol with gentle heating. To this was added
a solution of 56 mg of Cu(NO3)2 2.5H2O (0.24 mmol) in 6 mL of
3
ethanol resulting in an immediate color change to dark blue. The
solution was heated at ∼50 °C for 15 min and then allowed to evaporate
to near dryness, resulting in dark blue crystals of the desired compound
which were suitable for X-ray work. The perchlorate salt was obtained
by addition of sat. NaClO4 to a concentrated solution of [Cu
(Me8tricosaneN6)](ClO4)2. Anal. Calc. for C25H60CuN6Cl2O11
:
For [Cu(Me8tricosaneN6)](NO3)2 2H2O and [Cu(HMe8tricosaneN6)]
3
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(NO3)3 H2O crystallographic data were acquired at 293 K on an Oxford
disorder in the sites of the amine protons attached to atoms N15, N21,
N24, and N30.
3
Diffraction Gemini CCD diffractometer and operating within the range 2 <
2θ < 50 Å. Data reduction and empirical absorption corrections (multiscan)
were performed with Oxford Diffraction CrysAlisPro software (Oxford
Diffraction, vers. 171.33.42). Structures were solved by direct methods with
SHELXS and refined by full-matrix least-squares analysis with SHELXL-9731
(Table 1). All non-H atoms were refined with anisotropic thermal para-
meters. In all structures graphite-monochromated MoÀKR radiation
(0.71073 Å) was used.
’ RESULTS AND DISCUSSION
Improved Synthesis of 1,1,1-Tris(aminomethyl)ethane
(tame). The synthesis of Me8tricosaneN6 (Scheme 1) proceeds
via a metal directed template reaction using [Co(tame)2]3+
(tame = 1,1,1-tris-(aminomethyl)ethane) as the precursor. The
triamine tame is well-known25,34À36 and also is the precursor for
the related cage ligand Me5tricosaneN6.16 Most synthetic routes
toward tame utilize 1,1,1-tris(hydroxymethyl)ethane as its tris-
(benzenesulfonate) or tris(tosylate) ester. In one of the earliest
reported procedures, the tris(benzenesulfonate) ester was re-
acted with potassium phthalimide in xylene to obtain the
corresponding tris(phthalamido) derivative, which was then
subjected to hydrolysis with aqueous KOH for 2À3 days.25 After
hydrolysis, distillation at high temperatures (>200 °C) was
required to obtain impure tame ligand. Geue and Searle later
reported an improved version of this procedure, in which
potassium phthalimide was reacted with the tris(benzene-
sulfonate) ester of 1,1,1-tris(hydroxymethyl)ethane in dimethyl-
formamide (DMF), instead of xylene, and the subsequent alka-
line hydrolysis was performed in a high pressure autoclave.35
Subsequent separation of the protonated tame ligand from
various other polyamine byproducts requires cation exchange
column chromatography. Although this procedure is able to be
performed on a relatively large scale, the chromatographic
purification procedure is laborious and the need to perform the
hydrolysis under conditions of high temperature and pressure is
an added drawback. A commonly used alternative method for
obtaining tame relies on the preparation, and subsequent reduction
with LiAlH4, of 1,1,1-tris(azidomethyl)ethane.34,36 However, owing
to the potential explosive hazards of organic azides, this method is
not recommended for large scale preparations.
Molecular structure diagrams in Figures 2À4 were produced with
ORTEP3,32 while space filling and packing diagrams were produced
with Mercury (vers 2.4) and disorder in the structure of [Cu(Me8-
tricosaneN6)](NO3)2 2H2O (Supporting Information Figure S2) was
3
shown with PLUTON.33 This comprised complete rotational disorder
of the complex cation about a crystallographic 2-fold axis with only the
metal atom occupying this axis. Because of the proximity of the two
disordered components, the ligand C- and N-atoms were refined
isotropically (each at half occupancy). Restraints on the CÀN and
CÀC bonds were applied early in refinement but were lifted for the final
stages until convergence. The disordered nitrate anions were restrained
to be trigonal planar. In the structure of Me8triconsaneN6 there was
The new method we report herein avoids both hazardous
triazide intermediates and cation exchange column chromatog-
raphy, affording tame as its hydrochloride salt in good yield. A
further advantage is that the method is readily able to be scaled up.
When large-scale reactions are being performed the ammonium
formate is best added gradually as a methanolic solution rather than
as a single quantity of solid reagent.
Figure 1. ORTEP view of the free ligand Me8tricosaneN6. Only one of
the two contributors to NH disorder is shown (30% probability
ellipsoids).
Figure 2. ORTEP views (from the same aspect) of (A) six-coordinate [Cu(Me8tricosaneN6)](NO3)2 2H2O and (B) five-coordinate
3
[Cu(Me8tricosaneN6)](S2O6). 30% probability ellipsoids are shown, and anions have been omitted for clarity. Note the same configuration of the
coordinated N-donors is present in both complexes while a H-bond to N36 in B replaces the coordinate bond in A.
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Figure 4. visÀNIR spectra of [Cu(Me8tricosaneN6)]2+ (pH 1.2 and
7.0) and [Cu(sar)]2+ (pH 7.0).
Scheme 1
Figure 3. ORTEP view of five-coordinate [Cu(HMe8tricosaneN6)]
(NO3)3 H2O. 30% probability ellipsoids are shown, and anions and
3
water molecules have been omitted for clarity.
Synthesis and Characterization of Me8tricosaneN6. The
synthesis of Me8tricosaneN6 (Scheme 1) proceeds via a metal
template reaction of [Co(tame)2]3+ with formaldehyde and
isobutyraldehyde (2-methylpropanal), yielding the structurally
characterized CoIII tri-imine complex [Co(Me8tricosanetriim-
ineN6)]Cl3.21 This is the lowest yielding step (ca. 10%) of the
reaction sequence as the mixed aldehyde reaction has many other
possible outcomes including methylene-linked amines (aminals)
and other polyimines. Separation and characterization of all CoIII
complexes formed in the formaldehyde/isobutyraldehyde con-
densation reaction has not been achieved, but 13C NMR
indicates that there are many species and they are typically
asymmetric. Unreacted [Co(tame)2]3+ is typically recovered as
about 10% of all Co complexes after column chromatography.
The inherently low yield of this reaction is in contrast to the
metal directed Mannich reactions leading to the sar and sep CoIII
cages1,2 which proceed in quantitative yields. Reduction of
[Co(Me8tricosanetriimineN6)]3+ affords two N-based diastere-
omeric CoII complexes of Me8tricosaneN6;21 one pink and one
violet which are both remarkably air stable unlike all other CoII
hexaamines.
The cobalt(II) complexes of smaller cavity ligands (e.g.,
[Co(sep)]2+ and [Co(sar)]2+) are exceptionally resistant to dis-
sociation in acidic solution. For example, divalent [Co((NH3)2-
sar))]4+ shows no detectable loss of the metal ion in strongly
acidic solution at 20 °C after several days.2 Procedures for pre-
paring the metal-free hexaamine cage ligands sar and (NH2)2sar
require forcing conditions, such as high temperatures in con-
centrated HBr (in a sealed tube) or reaction with hot aque-
ous cyanide to remove the metal from its cage.3 In contrast,
[Co(Me8tricosaneN6)]2+ dissociates rapidly in dilute acid (1 M
with the ligand exhibiting effective D3h symmetry in solution.
Complete assignment of all NMR signals was achieved from a
gHMBC spectrum of the compound (Supporting Information,
Figure S1), which enabled the proton and carbon resonances arising
from the two different types of methylene groups in the caps and
straps to be identified.
Single crystals of Me8tricosaneN6 suitable for X-ray crystal-
lography were obtained from evaporation of an acetonitrile
solution of the free ligand. Figure 1 shows the asymmetric
conformation of the ligand. A feature is an array of four
2+
HCl), and the liberated Coaq ions separate readily from the
protonated free ligand on a cation exchange column. This
resembles the behavior of [Co(Me5tricosaneN6)]2+, which un-
dergoes rapid and essentially quantitative demetalation after
being treated with 5 M HCl for 1 h at 70 °C. 16
The positive ion ESI mass spectrum of Me8tricosaneN6 in water
only contained signals (m/z 439), assigned to (Me8tricosaneN6 +
H)+. Only four resonances were evident in the 1H NMR spectrum
of Me8tricosaneN6 in CDCl3, and six 13C NMR peaks, consistent
intramolecular H-bonds (NÀH N) that link two of the three
3 3 3
straps and orient the four N-atoms in an approximately planar
array ready for metal coordination. The third strap is in a
completely different conformation, and in this state neither of
its N-donors are organized for metal binding. The conformation
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Table 2. Selected Bond Lengths (Å) and Angles (deg)
[Cu(Me8tricosaneN6)]2+
(6-coord., NO3Àsalt)
[Cu(Me8tricosaneN6)]2+
(5-coord., S2O62À salt)
[Cu(HMe8tricosaneN6)]3+
(5-coord., NO3À salt)
CuÀN12
2.06(1)
2.43(1)
2.00(1)
2.27(1)
2.037(9)
2.54(1)
80.6(5)
94.2(5)
79.7(5)
152.2(5)
108.1(3)
111.3(4)
2.037(3)
2.038(3)
2.338(3)
2.034(3)
2.030(3)
2.051(2)
2.037(2)
2.406(2)
2.039(2)
2.093(2)
CuÀN22
CuÀN32
CuÀN16
CuÀN26
CuÀN36
N12ÀCuÀN22
N12ÀCuÀN32
N12ÀCuÀN16
N12ÀCuÀN26
N12ÀCuÀN36
N22ÀCuÀN16
τa
88.5(1)
80.0(1)
90.6(1)
174.9(1)
88.34(9)
90.71(8)
96.00(8)
159.10(9)
147.2(1)
0.46
164.77(9)
0.09
a τ = RÀβ/60 where R and β are the two largest coordinate angles.
of Me8tricosaneN6 is quite similar to that of diprotonated
(H2Me5tricosaneN6)(CF3SO3)2.20
octahedral D3 SSSSSS isomers (NiII, CoIII, CuII, ZnII,
PtIV)16,18,19,39 and C3h SSSRRR trigonal prismatic forms (CdII,
HgII).20
Structural Characterization of CuII Complexes of Me8tri-
cosaneN6. Dark blue solutions of [Cu(Me8tricosaneN6)]2+
formed rapidly by mixing free ligand and a CuII salt in methanol
or ethanol. The complex could then be isolated as a variety of
salts (e.g., nitrate, perchlorate, triflate, dithionate, or hexafluoro-
phosphate) by addition of an excess of the sodium salt of the
appropriate anion. Two subtly different complexes were crystal-
lized from aqueous ethanol (nitrate) and dilute aqueous ammo-
nia (dithionate). Both were characterized crystallographically.
The structure of the nitrate salt (Figure 2A) finds the cage
coordinated as a hexadentate. One pair of trans CuÀN bonds (to
N22 and N36) are longer than the remaining four coordinate
bonds as expected for axially elongated six-coordinate CuII
complexes because of the JahnÀTeller effect.37,38 The structure
is in fact disordered with the Cu ion occupying a 2-fold axis that
passes through no other atoms (Supporting Information, Figure
S2). These two orientations of the cation evidently may pack
equally well as each other. This is reminiscent of the CuII,19 NiII,
and ZnII structures18 of Me5tricosaneN6 reported to date where
disorder of the cation has been a feature.
Crystallization of [Cu(Me8tricosaneN6)]2+ as its S2O62À salt
from weakly basic solution (dilute ammonia) afforded crystals
suitable for X-ray studies, and the structure of the complex cation
is shown in Figure 2B. Yet another mode of coordination is seen
where the ligand is bound as a pentadentate, and the noncoordi-
nated secondary amine (N36) accepts an intramolecular H-bond
(N16ÀH N36 2.34 Å). The five-coordinate geometry of the
3 3 3
Cu ion falls between that of square pyramidal and trigonal
bipyramidal. This degree of distortion may be quantified by the
parameter τ (see Table 2 for definition).40 For an ideal square
pyramid τ = 0 while for an ideal trigonal bipyramid τ = 1. In the
case of [Cu(Me8tricosaneN6)](S2O6) we obtain a value τ = 0.46.
There is extensive H-bonding between the complex cation and
the dithionate anion (Supporting Information, Figure S3).
In solution there should be little impediment to conversion
between the six-coordinate and five-coordinate forms of the
cation [Cu(Me8tricosaneN6)]2+ defined in Figure 2 as they share
the same configuration of their coordinated N-donors. Hexadentate
coordinated [Cu(Me8tricosaneN6)](NO3)2 2H2O exhibits an
3
The mode of coordination in [Cu(Me8tricosaneN6)](NO3)2
is asymmetric. This is best described by the absolute configura-
tion of the six chirotopic N-donors; in this case SSSRSS (for N12,
N22, N32, N16, N26, and N36, respectively) as drawn in
Figure 2. (The structure is of a racemate so this relative
configuration naturally includes the enantiomeric RRRSRR con-
figuration) The SSSSSS diastereomer possesses D3 symmetry,
and this isomer was identified in [Co(Me8tricosaneN6)]
(NO3)2.21 In the same paper a different N-based isomeric form
(SRSRSS) was found for [Co(Me8tricosaneN6)][ZnCl4]. The
observation of three different N-based isomers from the few
extant structures of crystallographically characterized hexaden-
tate complexes of Me8tricosaneN6 is testimony to the cavity of
the ligand being exceptionally flexible, which is in stark contrast
to the sep and sar family where a single N-based isomeric form
(SSSSSS/RRRRRR) has been identified in each of the 136 crystal
structures of N6 coordinated complexes currently in the Cam-
bridge Structural Database. Complexes from the [M(Me5trico-
saneN6)]n+ family presently fall into two categories: pseudo-
SSSRSS (RRRSRR) configuration while [Cu(Me8tricosaneN6)]
(S2O6) is SSSRS (RRRSR), that is, they share the same N-based
configuration of their five common coordinated amines. Moreover,
if a vector were drawn between Cu and the sixth uncoordinated
amine (N36) in Figure 2B then this amine would retain the same
absolute (S) configuration as the hexadentate form in Figure 2A. On
this basis we assume that the five- and six-coordinate cations in
Figure 2 are interconvertible in solution, and H-bonding with the
anions and packing forces dictate which solid state structure is
observed. In the structure of [Cu(Me8tricosaneN6)](S2O6) the
dithionate anion forms a number of H-bonds with the cation
(Supporting Information, Figure S3), and these are sufficient to
favor the 5-coordinate geometry.
A solution of [Cu(Me8tricosaneN6)]2+ was acidified to pH 1
with dilute nitric acid which afforded a color change from blue to
purple indicative of a change in coordination mode. Slow
evaporation of this solution led to isolation of X-ray quality
crystals of the monoprotonated complex [Cu(HMe8tricosane-
N6)](NO3)3 H2O, and the structure of the complex cation is
3
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Figure 6. X-band EPR spectra of (A) [Cu(Me8tricosane)]2+ (pH 7)
and (B) [Cu(HMe8tricosane)]3+ (pH 1). Both solutions contained
2 mM complex in DMF/water (1:2), and spectra were acquired at 140 K.
The spectral frequency was 9.337 GHz.
Figure 5. Spectrophotometric (visÀNIR) pH titration of [Cu(Me8-
tricosaneN6)]2+ in water (0.1 M KNO3). The pH values are shown in
the legend.
A solution of [Cu(Me8tricosaneN6)]2+ acidified to pH 1.2
with nitric acid became immediately purple (λmax 570 nm,
Figure 4). These are the same conditions used to prepare the
structurally characterized protonated complex shown in Figure 3.
Given that the structure of the compound crystallized at pH 1 has
been established as five-coordinate [Cu(HMe8tricosaneN6)]3+
(Figure 3) the visible spectrum measured at pH 1 may be
assigned to this same five-coordinate structure.
shown in Figure 3. The ligand is coordinated in a pentadentate
mode with one amine protonated. There is a change in the
N-based isomeric form relative to 5-coordinate [Cu(Me8-
tricosaneN6)](S2O6) upon protonation (SSSRS to SSRRR for
N12, N22, N32, N16, and N26, respectively). As a result of the
two N-inversions (at N32 and N26), the coordinate angles and
geometry both change. The five coordinate geometry of [Cu-
(HMe8tricosaneN6)](NO3)3 H2O is close to (axially elongated)
A spectrophotometric pH titration on [Cu(Me8tricosane-
N6)]2+/[Cu(HMe8tricosaneN6)]3+ was carried out comprising
spectra acquired at 19 different pH values. A selection of
these data are presented in Figure 5, which shows the gradual
spectral changes between pH 6 and 4. Global analysis of the
entire set of 19 spectra with SPECFIT43 gave a pKa for the
protonation reaction of 5.2(1). This value is typical of a nonco-
ordinated amine in proximity to a dipositively charged metal.41,44
The spectral changes (both in wavelength and peak intensity) are
consistent with the solid state structures shown in Figure 2B for
[Cu(Me8tricosaneN6)]2+ (distorted square pyramidal) and
[Cu(HMe8tricosaneN6)]3+ in Figure 3 (square pyramidal).
Electron Paramagnetic Spectroscopy. EPR spectroscopy
was also undertaken to further characterize the solution struc-
tures of [Cu(Me8tricosaneN6)]2+ and [Cu(HMe8tricosane-
N6)]3+ (Figure 6) as the spin Hamiltonian (g and A) values
of CuII complexes are very sensitive to both coordination
number and geometry.45 Clearly both spectra are very similar,
and each is consistent with an axially symmetric CuII ion (gz > gy
= gx; Az > Ay = Ax). The spin Hamiltonian parameters were ob-
tained by spectral simulation27 (Supporting Information, Figure
S4). The g and A values are actually very similar to those
reported19 for [Cu(Me5tricosaneN6)]2+ and distinct from the
trigonally twisted (and tetragonally elongated) smaller N6 cage
complex [Cu((NH2)2sar)]2+ (see Table 3) where gz > gy > gx and
Az > Ay > Ax.24 The EPR spectrum of the protonated complex
[Cu(HMe8tricosaneN6)]3+ (Figure 6B) was only slightly differ-
ent to that of [Cu(Me8tricosaneN6)]2+ (Figure 6A); the main
change being a decrease in gz (from 2.240 to 2.205) and increase
in Az (Table 3).
3
square pyramidal (τ = 0.09).40 The axial elongation is again typical
of JahnÀTeller distorted 5-coordinate CuII complexes where the
axial bond (CuÀN32) is at least 0.3 Å longer than the four
remaining CuÀN bonds in the equatorial plane.37,41
Electrospray Ionization Mass Spectrometry. The positive
ion ESI mass spectra of various salts of [Cu(Me8tricosaneN6)]2+
resembled strongly those obtained previously for metal ion
complexes of other cage ligands.42 In each case the mass
spectrum showed ions of high abundance at m/z 251.1, which
are attributable to the divalent cation. The only other ions
present were generally of much lower abundance, and attribu-
table to ion pairs formed either in the gas phase or solution. For
example, the mass spectrum of the triflate salt showed ions at m/z
650.6 from ([Cu(Me8tricosaneN6)]2+ + CF3SO3À)+, while for
the hexafluorophosphate salt the corresponding ions arising from
([Cu(Me8tricosaneN6)]2+ + PF6À)+ were present at m/z 646.6.
Electronic Spectroscopy. The electronic absorption spectra
of [Cu(Me8tricosaneN6)]2+ and [Cu(sar)]2+ in water (pH 7) are
compared in Figure 4. Although both complexes are blue, the
spectrum of the larger cavity [Cu(Me8tricosaneN6)]2+ complex
showed a broad d-d band in the visible region at 604 nm, a
shoulder at about 900 nm and a more intense charge transfer
band at 279 nm. By comparison, the electronic spectrum of
[Cu(sar)]2+ exhibits a pronounced NIR absorption band at
1189 nm which is characteristic of a CuIIN6 complex in a
tetragonally elongated ligand field (dx2Ày2 ground state).19,24,37,38
The absence of a prominent NIR absorption band for
[Cu(Me8tricosaneN6)]2+ suggests that this complex may not
be six-coordinate in solution, or at least that any 6-coordinate
form is a minor component of an equilibrium mixture. When the
spectrum of [Cu(Me8tricosaneN6)]2+ was measured in acetoni-
trile the visible maximum shifted to 641 nm. This pronounced
shift suggests inclusion of solvent molecules in the coordination
sphere (H2O or MeCN) and supports the view that the cage is
only partially coordinated to the metal.
The spin Hamiltonian parameters for [Cu(Me8tricosaneN6)]2+,
[Cu(HMe8tricosaneN6)]3+, and [Cu(Me5tricosaneN6)]2+ (pub-
lished previously, Table 3)19 are quite similar and suggest a common
solution structure. Given that [Cu(HMe8tricosaneN6)]3+ is un-
doubtedly five-coordinate and its square pyramidal structure is
shown in Figure 3, by implication [Cu(Me8tricosaneN6)]2+ and
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[Cu(Me5tricosaneN6)]2+ are also dominantly five-coordinate
in solution. The somewhat larger Az and smaller gz values
found for [Cu(HMe8tricosaneN6)]3+ are consistent with a
structure closer to ideal square pyramidal (essentially coplanar
equatorial N-donors) relative to five-coordinate [Cu(Me8tri-
cosaneN6)]2+ where a solid state structure intermediate
of square pyramidal and trigonal bipyramidal was found
(Figure 2B).
The voltammetry of [Cu(Me8tricosaneN6)]2+ is strongly pH
dependent. Bearing in mind the pKa of 5.2(1) determined spectro-
photometrically for [Cu(Me8tricosaneN6)]2+ the results reflect the
electrochemistry of [Cu(Me8tricosaneN6)]2+ and its conjugate acid
[Cu(HMe8tricosaneN6)]3+. As the pH is lowered (Figure 8), an
irreversible cathodic peak emerges at À420 mV which is due to
reduction of [Cu(HMe8tricosaneN6)]3+ followed by rapid acid
catalyzed dissociation liberating Cuaq+ from the cage. Below ∼pH 4
a cathodic H+ reduction wave (ca. À650 mV vs Ag/AgCl) becomes
dominant and obscures all other signals to lower potential. At pH
values between 7 and 10 the voltammetry is unchanged.
Recently Àit has been shown that [Cu(Me5tricosaneN6)]2+ (as
its CF3SO3 salt) is also capable of adopting a pentadentate
coordinated structure, even without ligand protonation, although
in that case the CF3SO3À anion was found to be coordinated in
the solid state.46 Previously [Cu(Me5tricosaneN6)]2+ was crys-
tallized and structurally characterized in a six-coordinate form,19
so it also appears that five and six-coordinate [Cu(Me5trico-
saneN6)]2+ complexes are also in facile equilibrium in solution,
which parallels the behavior of the new [Cu(Me8tricosaneN6)]2+
complexes reported herein.
Structural Aspects and Crystal Packing. Apart from the
greater degree of conformational flexibility seen in com-
plexes from the [M(Me5tricosaneN6)]n+ and [M(Me8trico-
sanceN6)]n+ family, the disorder also seen commonly in the
limited set of crystal structures from these expanded cages
appears to be a consequence of their more “spherical” shape
relative to the [M(sar)]n+ and [M(sep)]n+ analogues. This is
illustrated in Figure 9 where three “representative” crystal
structures of complexes (two CoIII and one CoII) from each
family are shown in space filling representation. The
[M((NH3)2sar)]n+ complex cation (Figure 9A) has an elongated,
ellipsoidal shape with the three ethylenediamine chelates resulting in
a narrow equatorial “belt” encircling the metal. In contrast the
[M(Me5tricosaneN6)]n+ (Figure 9B) and [M(Me8tricosanceN6)]n+
(Figure 9C) cations, with six-membered chelate rings and methyl
groups attached to the central C-atom define a more oblate shape.
To aid comparisons, all three complexes bear an isosteric substituent
Electrochemistry. Previous cyclic voltammetry studies of the
small cavity cage complex [Cu(sar)]2+ demonstrated that it
undergoes an irreversible reduction in aqueous solution at all
scan rates.47 This may be attributed to the CuI ion being
incompatible with the preferred hexadentate binding mode of
the cage. No structurally characterized six-coordinate CuI com-
plexes are known, and CuI complexes with coordination numbers
greater than 4 are very rare. The cyclic voltammogram of a
neutral, aqueous solution of [Cu(Me8tricosaneN6)]2+ shows a
facile and quasi-reversible CuII/I couple at À690 mV vs Ag/AgCl
with the anodic/cathodic peak current ratio (ipa/ipc) being
+
(ÀNH3 or ÀCH3) on the apical cap C-atoms, so are essentially the
essentially unity at all sweep rates from 20 to 1000 mV sÀ1
.
same vertical length as drawn. Apart from their more isotropic
shape, the NH groups in [M(Me5tricosaneN6)]n+ and [M(Me8-
tricosanceN6)]n+ aremoreobscured(blueNatoms) bytheadjacent
methyl groups rendering them less accessible to H-bond acceptors
in the crystal lattice. We believe that these features are responsible
for the disorder we have seen so far in many complexes from the
Me5tricosaneN6 and Me8tricosaneN6 family.
Representative voltammograms are shown in Figure 7 recorded
at pH 7. The peak separation (ΔE) increased only slightly from
65 mV (at a 20 mV À1 sweep rate) to 98 mV (1000 mV sÀ1 sweep
rate) indicative of fast heterogeneous electron transfer. For compar-
ison the electrochemical data for [Cu(Me5tricosaneN6)]2+19 and
the smaller cavity cage [Cu(NH2)2sar)]2+ are included in Table 3.
Figure 7. Cyclic voltammetry of [Cu(Me8tricosaneN6)]2+ (0.1 M
aqueous NaNO3, pH 7) at different sweep rates (20 to 500 mV sÀ1).
Figure 8. Cyclic voltammetry of 1 mM [Cu(Me8tricosaneN6)]2+ (0.1
M aqueous NaNO3, 100 mV sÀ1 sweep rate) at different pH values.
Table 3. EPR Spin Hamiltonian Parameters, Optical Spectroscopy, and Electrochemical Data for CuII Complexes in This Work
gz (Az, G)
gy (Ay, G)
gx (Ax)
λmax (nm)
ECu(II/I) (mV vs Ag/AgCl)
[Cu(Me8tricosaneN6)]2+
[Cu(HMe8tricosaneN6)]3+
[Cu(Me5tricosaneN6)]2+
[Cu(NH2)2sar)]2+
2.240 (159)
2.205 (177)
2.250 (160)
2.220 (130)
2.052 (20)
2.050 (24)
2.060 (À)
2.12 (55)
2.052 (20)
2.050 (24)
2.060 (À)
2.07 (15)
604, 900(sh)
570
À690
À420 (cathodic peak, irrev.)
À760
616
658, 1177
À750 (cathodic peak, irrev.)
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ARTICLE
Figure 9. Space filling representations of the complex cations (A) [Co((NH3)2sar)]5+ (bis(antimonyl tartrate) chloride salt),23 (B) [Co(Me5-
tricosaneN6)]3+ (PF6À salt),16 and (C) [Co(Me8tricosaneN6)]2+ (nitrate salt).21 Coordinates taken from Cambridge Structural Database and displayed
in Mercury (vers 2.4).
’ CONCLUSIONS
Universities of Wollongong and Queensland. We also thank Dr.
Chris Noble (University of Queensland) for assistance with the
EPR measurements.
We have shown that the ligand Me8tricosaneN6 is unique
among the hexaamine cage family in being able to accommodate
a number of different coordination modes in this case to the same
metal CuII. The larger and flexible cavity offered by Me8tricosa-
neN6 is in contrast to ligands from the sar family which bind all
six-coordinate transition metals tightly and in most cases irre-
versibly. Fortunately, we have been able to isolate and structurally
characterize two distinct forms of [Cu(Me8tricosane)]2+; one
six-coordinate and the other five-coordinate. In solution all
evidence (EPR, electrochemistry, and optical spectroscopy)
leads to the conclusion that [Cu(Me8tricosaneN6)]2+ (at neutral
or basic pH) is predominantly in a five-coordinate form resem-
bling that shown in Figure 2B, that is, only five of the six
N-donors are bound to the metal at any given time. Given the
remarkably similar spectroscopy of [Cu(Me8tricosaneN6)]2+
and [Cu(Me5tricosaneN6)]2+ (Table 3), we believe it is likely
that [Cu(Me5tricosaneN6)]2+ too is five-coordinate in solution
and can also equilibrate between 6- and 5-coordinate forms
rapidly. Further studies of this remarkable new addition to the
hexaamine cage family are underway.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystal structure data in CIF
b
format, 2D NMR spectra of Me8tricosaneN6, views of the
disordered [Cu(Me8tricosaneN6)]2+ cation, packing diagram
for [Cu(Me8tricosaneN6)](S2O6) H2O, and simulated and
experimental EPR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
3
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: p.bernhardt@uq.edu.au (P.V.B.), sralph@uow.edu.au
(S.F.R.).
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J.-E.; Lee, S. S. Coord. Chem. Rev. 2010, 254, 1713–1725.
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Notes
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’ ACKNOWLEDGMENT
We gratefully acknowledge financial support from the Aus-
tralian Research Council (DP1096029 to P.V.B.) as well as the
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ARTICLE
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