Synthesis of the {Co[(6-amino-6-(4-aminobenzyl)-1,4,8,11-tetra-azacyclotetradecane)]-[PO 4]}, a potential phosphate-binding site for molecular recognition

Article abstract of DOI:10.1016/S0020-1693(99)00261-3

The preparation of Co(NH₃)₅(PO₄) and Co(L⁰)(PO₄) [L⁰: 6-amino-6-(4-aminobenzyl)-1,4,8,11-tetra-azacyclotetradecane] is reported. ³¹P NMR resonances for the two complexes are pH dependent, and are shifted downfield compared to that of unbound phosphate at the same pH. Co(NH₃)₅(PO₄): pH 12.0, δ_P 8.17 ppm; pH 7.2, δ_P 7.50 ppm; Co(L⁰)(PO₄): pH 7.2, δ_P 7.20 ppm. Co(L⁰)(PO₄) is a remarkable robust complex. Over a period of five weeks at 25°C, pH 7.2, less than 1% hydrolysis (aquation) occurred, so that k₁<2E-15 s^-1.

Full text of DOI:10.1016/S0020-1693(99)00261-3

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Inorganica Chimica Acta 293 (1999) 245–247
Note
Synthesis of the
{Co[(6-amino-6-(4-aminobenzyl)-1,4,8,11-tetra-azacyclotetradecane)]-
[PO4]}, a potential phosphate-binding site for molecular recognition
Giulio A. Amadei, Joseph E. Earley *
Department of Chemistry, Georgetown Uni6ersity, Washington, DC 20057-1227, USA
Received 26 March 1999; accepted 25 June 1999
Abstract
The preparation of Co(NH3)5(PO4) and Co(L0)(PO4) [L0: 6-amino-6-(4-aminobenzyl)-1,4,8,11-tetra-azacyclotetradecane] is
reported. 31P NMR resonances for the two complexes are pH dependent, and are shifted downfield compared to that of unbound
phosphate at the same pH. Co(NH3)5(PO4): pH 12.0, lP 8.17 ppm; pH 7.2, lP 7.50 ppm; Co(L0)(PO4): pH 7.2, lP 7.20 ppm.
Co(L0)(PO4) is a remarkable robust complex. Over a period of five weeks at 25°C, pH 7.2, less than 1% hydrolysis (aquation)
occurred, so that k1B2E−15 s−1. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Cobalt complexes; Azamacrocycle complexes; Phosphate complexes
1. Introduction
We have recently reported the synthesis and charac-
terization of a new pentadentate amino-tetraazamacro-
cyclic ligand (L0, 6-amino-6-(4-aminobenzyl)-1,4,8,11-
tetra-azacyclotetradecane)] (Fig. 1), and its Co(III)
complex, having the metal ion center pentacoordinate
to the five nitrogen donors, and the sixth coordinative
Fig. 1. Fully saturated tetraazamacrocyclic ligand L0.
position filled by a water molecule [1]. We pointed out
poses, such as screening synthetic phosphate-ion recep-
the potentiality of our new ligand to be able to be
tors [2].
immobilized to a solid support via the free amino group
on the aromatic pendant attached to the tetraazamacro-
cyclic core. We now report the preparation of a Co(III)
complex of that ligand in which the sixth coordination
2. Experimental
position of the metal center ion is filled with a phos-
phato group. Co(L0)(PO4) is a remarkable robust com-
plex. This new system could be used as an immobilized
phosphato binding site for molecular recognition pur-
2.1. Methods and materials
Electronic spectra were recorded using a Hewlett-
Packard 8451A diode array spectrophotometer. Infra-
red spectra were recorded using a Mattson Instruments
2020 Galaxy Series FT-IR. 31P NMR spectra were
recorded using
a Varian FT-NMR Mercury-300
* Corresponding author. Tel.: +1-202-687 6073; fax: +1-202-687
6209.
NMR spectrometer working at 121.473 MHz,
0020-1693/99/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00261-3

246
G.A. Amadei, J.E. Earley / Inorganica Chimica Acta 293 (1998) 245–247
chemical shifts cited are given versus 85% H3PO4 (exter-
nal standard). Trifluoromethylsulfonic acid (99%) was
purchased from Aldrich and vacuum distilled prior to
use (85–86°C, 5 mmHg). The cation resin exchanger
Dowex 50X8-100 was purchased from Across. Phos-
phoric acid (85%) and phosphorus pentoxide (99%)
were purchased from Fisher Chemical. Cobalt chloride
hexahydrate (99%) from J.T. Baker Chemical. All sol-
vents (GR) and sodium hydroxide (99%) were pur-
chased from EM Science. The precursor complexes
[Co(NH3)5(Cl)]Cl2 [3], [Co(NH3)5(OSO2CF3)](CF3SO3)2
[4] and [Co(L0)(Cl)]Cl2·HCl [1] were synthesized by
reported methods. Satisfactory microanalyses and spec-
troscopic data consistent with the literature were ob-
tained in each case.
added dropwise under vigorous stirring. The pellets
thus formed were filtered under vacuum through a
sintered glass funnel, and repeatedly washed with Et2O.
The purple solid (1.3 g, 86.4%) was dried in vacuo over
P2O5. Electronic spectrum. Solvent: CF3SO3H. umax 524
nm (m 45.8 M−1 cm−1) and 345 nm (m 40.6 M−1
cm−1). IR spectrum (KBr disk): 1350 cm−1 (coordi-
nated CF3SO3−) and 1270 cm−1 (ionic CF3SO3−).
2.4. Synthesis of phosphato(L0)cobalt(III). Co(L0)(PO4)
A
500 mg (0.62 mmol) sample of [Co(L0)-
(OSO2CF3)](CF3SO3)2·CF3SO3H was stirred with 25 ml
of polyphosphoric acid (PPA) at room temperature
(23°C) until all the salt was dissolved (12 h). The
solution was then diluted to 200 ml with deionized,
distilled water and sorbed onto a column of Dowex
50×8-100 (H+ form, ca. 100 ml) cation exchange
resin. Washing with deionized, distilled water removed
unbound phosphate ions (ca. 350 ml, phosphomolyb-
date test, final eluate pH 5.0). Elution with 0.02 mol
l−1 NaOH yielded the phosphato complex solution.
The eluate was then treated with concentrated NH3 (20
ml) and C2H5OH (75 ml). A fine powder was obtained
after 2 days in the freezing compartment of a refrigera-
tor. The powder was filtered, washed with cold water,
cold C2H5OH and Et2O (58 mg, 20%). Electronic spec-
trum. Solvent: HCl 1.0E-03 M. umax 488 nm (m 64.0
M−1 cm−1) and 356 nm (m 48.0 M−1 cm−1). Micro-
analysis. Found: Co, 12.38; P, 6.85; N, 17.92. Calc.: Co,
12.43; P, 6.54; N, 17.73%. 31P NMR spectrum (D2O,
H3PO4 external reference): pH 7.2, lP 7.20 ppm. 1H
NMR spectrum (D2O): 1.61 ppm (q, 2H, CH2), 2.58
ppm (t, 4H, CH2ꢀNH), 2.20–2.45 ppm (unresolved
multiplet 12H). 13C NMR off-resonance decoupled
spectrum (D2O): cyclam portion: 29.0 ppm (t, J=122
Hz), 51.9 ppm (t, J=135 Hz), 52.0 ppm (t, J=135
Hz), 52.6 ppm (t, J=135 Hz), 58.5 ppm (s), 68.2 ppm
(t, J=130 Hz). Benzylic CH2: 42.0 ppm (t, J=122 Hz).
Aromatic carbons: 129 ppm (ipso, s), 122 ppm (o, d,
J=155 Hz), 135 ppm (m, d, J=155 Hz), 137 ppm (p,
CꢀNH2, s).
2.2. Synthesis of phosphatopentamminocobalt(III).
Co(NH3)5(PO4)
A total of 79.5 mg (0.152 mmol) of [Co(NH3)5-
(OSO2CF3)](CF3SO3)2 was stirred with 0.5 ml of
polyphosphoric acid (PPA)1 at room temperature
(23°C) until all the salt was dissolved (95 h). The
solution was then diluted to 50 ml with deionized,
distilled water and sorbed onto a column of Dowex
50X8-100 (H+ form, ca. 20 ml) cation exchange resin.
Washing with deionized, distilled water removed un-
bound phosphate ions (ca. 150 ml, phosphomolybdate
test, final eluate pH 5.0). Elution with 0.02 mol l−1
NaOH yielded the phosphato complex solution. The
eluate was then treated with concentrated NH3 (20 ml)
and C2H5OH (75 ml). A fine purple powder was ob-
tained after 3 h in the freezing compartment of a
refrigerator. The powder was filtered, washed with cold
water, cold C2H5OH and Et2O (7.5 mg, 21%). Elec-
tronic spectrum. Solvent: HCl 1.0E-03 M. umax 518 nm
(m 64 M−1 cm−1) and 356 nm (m 48 M−1 cm−1) [5].
Microanalysis. Found: Co, 21.00; P, 10.85; N, 25.01.
Calc.: Co, 21.42; P, 11.26; N, 25.46%. 31P NMR spec-
trum (D2O, 85% H3PO4 external reference): pH 12, lP
8.17 ppm; pH 7.2, lP 7.5 ppm.
2.3. Synthesis of [Co(trifluoromethanesulfonato)-
(6-amino-6-(4-aminobenzyl)-1,4,8,11-tetra-
azacyclotetradecane)]trifluoromethanesulfonate.
[Co(L0)(OSO2CF3)](CF3SO3)2·CF3SO3H
3. Results and discussion
We chose to synthesize Co(III) phosphato complexes
by treating corresponding labile trifluoromethanesul-
fonato complexes with a suitable source of phosphate
ions. Labile trifluoromethanesulfonato complexes were
synthesized according to a method developed by Dixon
et al. [4]. Consistent with the literature [4], the
[Co(L0)(OSO2CF3)](CF3SO3)2]·CF3SO3H complex show-
The method described by Dixon et al. [4] was fol-
lowed. To [Co(L0)(Cl)]Cl2·HCl (1.0 g, 1.85 mmol)
freshly distilled trifluoromethanesulfonic acid (25.0 ml)
was added. The solution was stirred and warmed at
90°C until evolution of HCl ceased (1.5 h). Then, the
solution was cooled to 0°C, and Et2O (100 ml) was
ed absorption maxima at 520 nm (42.0 M−1 cm−1
)
and 340 nm (38.5 M−1 cm−1) recorded in an-
hydrous CF3SO3H at room temperature (23°C), and
1 PPA was prepared by mixing equal weight amount of P2O5 and
85% solution of H3PO4.

G.A. Amadei, J.E. Earley / Inorganica Chimica Acta 293 (1998) 245–247
247
bound phosphate ions at the same pH (HPO4 2−, lP 3.5
ppm [7], lP 3.24 ppm2). The 31P NMR resonance (Table
1) for the coordinated phosphato group in the
Co(NH3)5(PO4) complex at pH 12.0, showed a singlet
at 8.17 ppm, downfield shifted compared to the reso-
nance of unbound phosphate ions at the same pH
(PO4 3−, lP 6.0 ppm [7], lP 5.45 ppm2). The signal due
to the coordinated phosphato group remained un-
changed over a period of five weeks at 25°C and at pH
7.2, for both the Co(III)-phosphato complexes, without
any detectable amount of unbound phosphate (HPO42−
, pH 7.2), which strongly suggests negligible aquation
(B1%), with an upper limit of the aquation rate con-
Table 1
Assignment of the 31P NMR spectra of different Co(III)-phosphato
complexes
Complex
Concentration
(ppm)
31P NMR
(ppm)
pH
Co(NH3)5(PO4)
Co(NH3)5(PO4)
Co(L0) (PO4)
1000
1000
1000
8.17
7.50
7.20
12
7.2
7.2
infrared spectrum (KBr disk) exhibiting bands at 1350
and 1270 cm−1 due to coordinated and ionic tri-
fluoromethanesulfonate moiety, respectively [4]. Two
different sources of phosphate were employed. Treat-
ment of the two triflato complexes with 85% H3PO4
yielded mainly the aquo complex, whereas treatment of
the triflato complex with polyphosphoric acid yielded
21% Co(NH3)5(PO4) and 18% Co(L0)(PO4), respec-
tively. Separation of the phosphato complex from the
aquo was achieved by following the method developed
by Schmidt and Taube [6]. The mixture of phosphato
and aquo complexes was placed onto a column of
Dowex 50X8-100 (H+ form) cation exchange resin, and
eluted with dilute alkali (0.02 M NaOH). The base
converted the aquo complexes to dipositive cation spe-
cies that were held on the resin, whereas the phosphato
complexes were converted to neutral species that were
eluted with alkali.
stant estimated to be k1B2E−15 s−1
.
Our
Co(L0)(PO4) complex could be immobilized to a solid
support via the free amino group on the aromatic
pendant attached to the tetraazamacrocyclic core [8],
leading to an immobilized phosphato binding site for
molecular recognition purposes, such as screening syn-
thetic phosphate ion receptors [2].
2 This experimental value has been determined by neutralizing 85%
H3PO4 solution with a calculated amount of NaOH.
References
[1] G.A. Amadei, M.H. Dickman, R.A. Wazzeh, P. Dimmock, J.E.
Earley, Inorg. Chim. Acta 288 (1999) 40.
[2] W.S. Yeo, J.I. Hong, Tetrahedron Lett. 39 (1998) 8137.
[3] C.R. Schlessinger, Inorg. Synth. 9 (1967) 160.
[4] N.E. Dixon, G.W. Jackson, M.J. Lancaster, G.A Lawrence, A.M.
Sargeson, Inorg. Chem. 20 (1981) 470.
[5] W.G. Jackson, C.N. Hookey, M.L. Randall, P. Comba, A.M.
Sargeson, Inorg. Chem. 23 (1984) 2473.
[6] W. Schmidt, H. Taube, Inorg. Chem. 2 (1963) 698.
[7] E. Fluck, Die Kernmagnetische Resonanz und ihre Anwendung in
der anorganischen Chemie, Springer, Berlin, 1963, p. 255.
[8] A. Bayada, G.A. Lawrance, M. Meader, M.A. O’Leary, J. Chem.
Soc., Dalton Trans. 21 (1994) 3107.
31P NMR spectroscopy was employed to gain addi-
tional experimental evidence for the coordination of the
phosphato moiety to the metal ion center in both
Co(NH3)5(PO4) and Co(L0)(PO4) complexes, and to
estimate the stability of these species to hydrolysis. The
31P NMR resonance (Table 1) for the coordinated
phosphato group in the Co(NH3)5(PO4) and
Co(L0)(PO4) complexes at pH 7.2, showed a singlet at
7.50 and 7.20 ppm, respectively. This resonance is
shifted downfield compared to the resonance of un-
.