Structural characterization of ({[tren]Zn(HOMe)} · ClO4 · BPh4) (tren = tris(2-aminoethyl)amine) and CO2 fixation into monomethyl carbonato zinc(II) complex

Article abstract of DOI:10.1016/S1387-7003(03)00175-8

The reaction of both tris(2-aminoethyl)amine and tris(2-dimethylaminoethyl)amine with Zn(ClO⁴)₂ in the presence of equimolar amount of NaBPh₄ in methanol afforded two different zinc(II) complexes, namely ({[tren]Zn(HOMe)}

Full text of DOI:10.1016/S1387-7003(03)00175-8

Inorganic Chemistry Communications 6 (2003) 1030–1034
www.elsevier.com/locate/inoche
Structural characterization of ({[tren]Zn(HOMe)} ꢀ ClO4 ꢀ BPh4)
(tren ¼ tris(2-aminoethyl)amine) and CO2 fixation into
monomethyl carbonato zinc(II) complex
a,
b
c
M.M. Ibrahim *, Kazuhiko Ichikawa , Motoo Shiro
a
Chemistry Department, Faculty of Education, Tanta University, Kafr-Elsheikh Branch, P.O. Box 33516, Kafr-El-Sheikh, Egypt
Division of Material Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
b
c
X-ray Research Laboratory, Rigaku Corporation, Akishima 196-8666, Japan
Received 7 January 2003; accepted 22 April 2003
Published online: 11 June 2003
Abstract
The reaction of both tris(2-aminoethyl)amine and tris(2-dimethylaminoethyl)amine with Zn(ClO4)2 in the presence of equimolar
amount of NaBPh4 in methanol afforded two different zinc(II) complexes, namely ({[tren]Zn(HOMe)} ꢀ ClO4 ꢀ BPh4) 1 and
({[trenMe6]Zn} ꢀ ClO4 ꢀ BPh4) 2. Zinc(II) Complex 1, which has been structurally characterized by X-ray crystallography, takes up
CO2 in methanol in the presence of base to fix it into monomethyl carbonato complex ({[tren]Zn(OCO2Me)} ꢀ BPh4 ꢀ 2MeOH) 3.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Zinc(II) complexes; X-ray crystal structure; CO2 fixation
ROH þ CO2 ! ROCOOH
1. Introduction
ð1Þ
M–OR þ ROCOOH ! M–O2COR þ ROH
The reaction chemistry of CO2 with transition metal
complexes is particularly interesting since CO2 is a
As one of the approaches, various types of successful
combustion product and environmental pollutant, yet it
zinc model complexes have been used to fix CO2 in al-
coholic medium as monoalkyl carbonate [6].
development of an effective chemical method for the
Tris(2-aminoethyl)amine, tren and tris(2-dimethyla-
minoethyl)amine, trenMe6 are well known and widely
various carbonate, formic acid, and other stock chemi-
studied tripodal amines containing four coordination
is also a potentially useful carbon source [1]. Thus, the
fixation and activation of CO2 for its conversion to
cals has been receiving a long-standing interest [2].
sites [7]. They bind Zn(II) ion in aqueous and methan-
Besides direct coordination of CO2 to transition
olic solutions. The species present in solution for both
metals with its intact form [3], many insertion reactions
systems tren/Zn(II) and trenMe6/Zn(II) and the corre-
of CO2 into an M–O bond have been reported for metal
sponding stability constants were previously reported
[8]. In both systems, monohydroxo species are present at
alcohol catalyzed chain mechanism for apparent CO2
alkaline pH.
Recently [9], we have reported the simulation and
Eq. (1). These reactions are in principle simple solvolysis
hydration of CO2 as hydrogen carbonate using zinc
alkoxide complexes [4]. Chisholm et al. [5] proposed an
insertion into a Mo–O bond of Mo2(OR)6 in solution
and hydrolysis processes and involve no redox chemistry
enzyme model complexes. Here we report structural
characterization of ({[tren]Zn(HOMe)} ꢀ ClO4 ꢀ BPh4)
and CO2 fixation into monomethyl carbonato zinc(II)
complex using zinc(II) complexes derived from the tri-
podal ligands tren and trenMe6.
* Corresponding author. Fax: +20-47-223415.
E-mail address: ibrahim652001@yahoo.com (M.M. Ibrahim).
1387-7003/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00175-8

M.M. Ibrahim et al. / Inorganic Chemistry Communications 6 (2003) 1030–1034
1031
2. Experimental
20H, –C6H5), 3.65 (s, 6H, –NH2), 2.84 (t, 6H, –CaH2),
2.76 (t, 6H, –CbH2–), and 2.00 ppm(s, 3H, –CH ). 13C
NMR (DMSO-d6): 164.52 (–OCO2–), 136.02–121.67
3
2.1. Materials
(phenyl C), 52.11–48.93 (–CH2–CH2–), and 37.04 ppm
(–CH3). IR (KBr, cmÀ1): (–OCO2), 1620 and 1307 cmÀ1
.
All reagents and solvent used were of analytical
grade. tren was taken fromTokyo Kasei Organic
Chemical Company. TrenMe6 ꢀ 3HClO4, has been
synthesized by a modification of Ciampolint, synthesis
[7]. 1H and 13C NMR spectra were measured by a
JEOL EX-400 spectrometer. IR measurements were
measured using JASCO FT-IR 300E spectrophotom-
eter.
2.3. X-ray crystallography
The structure of zinc(II) complex 1 was studied by X-
ray crystallography. The diffraction data were collected
using a Rigaku RAXIS-IV Imaging Plate Diffractometer
using graphite monochromated Mo-Ka radiation
ꢀ
(k ¼ 0:7107 A). Crystal data: C31H42N4O5BClZn, Mr ¼
2.2. Syntheses
662.34, monoclinic, space group Pn(#7), a ¼ 10:013ð1Þ
ꢀ
ꢀ
ꢀ
A, b ¼ 10:304ð1Þ A, c ¼ 15:394ð2Þ A, b ¼ 90:36ð1Þ°,
V ¼ 1588:3ð3Þ A , Z ¼ 2, Dcalcd ¼ 1:385 g cmÀ3, F000
¼
3
ꢀ
2.2.1. Synthesis of ({[tren]Zn(HOMe)} ꢀ ClO4 ꢀ BPh4) 1
Zinc(II) complex 1 was synthesized by the reaction of
tren (0.37 ml, 2.48 mmol) with equimolar amounts of
Zn(ClO4)2 ꢀ 6H2O (0.93 gm, 2.5 mmol) and NaBPh4
(0.85 gm, 2.5 mmol) in methanol. The mixed solution
was stirred for 3 h and white precipitate was obtained by
adding diethyl ether. Single crystals suitable for X-ray
crystallography were obtained by the slow evaporation
of diethy lether into acetonitrile solution of the complex.
Yield: 1.36 g (82.9%). Found: C, 55.84%; H, 6.41%; N,
8.51%; Cl, 5.23%. Anal. Calc. for C31H42N4O5BClZn
({[tren]Zn(HOMe)} ꢀ ClO4 ꢀ BPh4): C, 56.21%; H, 6.39%;
696, l ¼ 9:03 cmÀ1. The structure was solved by direct
methods [10] and expanded using Fourier techniques
[11]. The non-hydrogen atoms were refined anisotropi-
cally. Hydrogens were included but not refined. The fi-
nal cycle of full-matrix least-squares refinements [12]
was based on 3146 observed reflections (I > 0:00rðIÞ)
P
and 389 variable parameters gave R ¼ kFoj À jFcjj=
P
2
P
P
2 1=2
jFoj0:043 and Rw ¼½ xðjFojÀ jFojÞ = xðFo2Þ ꢁ
¼
0:059, and GOD on F ¼1:22. the maximum and mini-
mum peaks on the final diffÀerence Fourier map corre-
3
ꢀ
sponded to 0.96 and )0.59 e /A , respectively.
1
N, 8.46%; Cl, 5.35%. H NMR (DMSO-d6): 7.15–6.77
(m, 20H, –C6H5), 3.61 (s, 6H, –NH2), 2.79 (t, 6H, –
CaH2), 2.76 (t, 6H, –CaH2–), and 1.99 ppm(s, 3H, –
2.4. Reactions of both 1 and 2 with CO2
CH3). IR (KBr, cmÀ1): (ClO4À), 1060 and 628 cmÀ1
.
0.166 gmof 1 and 0.178 gmof 2 were placed in two
test tubes under N2 and dissolved in 10 ml of methanol
in the presence of equimolar amount of MeONa (57 ll
(25 mM), 25 wt%). The atmosphere was replaced with
CO2 and the solutions were set at roomtemperature for
4 h. After that the resultant solutions were evaporated to
dryness under vacuum, and the white solids thus ob-
tained were characterized by IR and NMR spectros-
copies. Zinc(II) complex 1 showed almost quantitative
formation of the monomethyl carbonato complex 3
whereas zinc(II) complex 2 showed no change and gave
only the starting material. In view of this facile inter-
conversion, attempts to isolate complex 3 as single
crystals for X-ray measurements were unsuccessful. The
reversibility of this reaction has been confirmed and
removal of CO2 atmosphere by warming the solution to
40 °C results in rapid generation of the methanol zinc(II)
complex 1. In another technique: dissolving both 1 and 2
(50 mM) in DMSO-d6 in the presence of eqimolar
amount of MeONa (114 ll (50 mM), 25 wt%). CO2
was bubbled for 4 h in to these solutions. Reaction of 1
with CO2 was monitored from the product of the
monomethyl carbonato zinc(II) complex using 13C NMR
spectroscopy.
2.2.2. Synthesis of ({[trenMe6]Zn} ꢀ ClO4 ꢀ BPh4) 2
Zinc(II) complex 2 was synthesized in a fashion
similar to complex 1. Yield: 1.49 g (80.3%). Found: C,
60.61%; H, 6.94%; N, 7.95%; Cl, 4.85%. Anal. calcd. For
C36H50N4O4BClZn ({[trenMe6]Zn} ꢀ ClO4 ꢀ BPh4): C,
59.15%; H, 6.89%; N, 7.67%; Cl, 4.85%. 1H NMR
(DMSO-d6): 7.26–6.93 (m, 20H, –C6H5), 2.89 (t, 6H,
CaH2–), 2.73 (t, 6H, CbH2–), and 2.84 ppm(s, 18H, –
CH3). IR (KBr, cmÀ1): (ClO4À), 1100 and 626 cmÀ1
.
2.2.3. Synthesis of ({[tren]Zn(OCO2Me)} ꢀ BPh4 ꢀ
2MeOH) 3
Bubbling of CO2 into a methanolic solution (10 ml) of
zinc(II) complex 1 (0.166 gm, 25 mM) in the presence of
equimolar amount of MeONa (57 ll (25 mM), 25 wt%)
for 4 h at roomtemperature. The resultant solution was
evaporated to dryness under vacuum, and the white solid
thus obtained was characterized as monomethyl carbo-
nato complex. Yield: 0.169 g (94.9%). Found: C, 64.86%;
H, 6.67%; N, 7.54%. Anal. Calc. for C40H49N4O3BZn
({[tren]Zn(OCO2Me)} ꢀ BPh4 ꢀ 2MeOH): C, 64.74%; H,
1
6.65%; N, 7.55%. H NMR (DMSO-d6): 7.19–6.75 (m,

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M.M. Ibrahim et al. / Inorganic Chemistry Communications 6 (2003) 1030–1034
3. Results and discussion
118.5(3)°. The interchelate N–Zn–N(4) bond angles, all
three of which are equal to the value of 83.03(6)°, are
similar to the value of 83.07(2)° [14] and differ fromthe
value of 80.9° [15].
The methanol molecule acts as a monodentate ligand
occupying the fifth site of the coordination sphere
around the zinc atomin trans-position with respect to
the four bridgehead nitrogen atoms. Due to the penta-
coordinate nature of the zinc atom, the Zn(1)–O(1)
3.1. Description of the molecular structure of ({[tren]-
Zn(HOMe)} ꢀ ClO4 ꢀ BPh4) 1
Treatment of tren ligand with equimolar amounts of
Zn(ClO4)2 ꢀ 6H2O and NaBPh4 in methanol followed by
crystallization via diethyl ether into acetonitrile solution
at an ambient temperature yielded ({[tren]Zn(HOMe)} ꢀ
ClO4 ꢀ BPh4) 1 as colorless crystalline blocks [13].
Fig. 1 shows an ORTEP drawing of the molecular
structure of 1. The essential part of 1 consists of zinc(II)
ion, four nitrogen atoms from the coordinated tren, and
one methanol molecule, which is hydrogen-bonded to
the oxygen atoms of the perchlorate anion. A view of the
structure down the Zn(1)–O(1) bond reveals the nearly
threefold symmetry of the coordinated tren molecule
neatly coiled around the zinc atom. The zinc atom
adopts the slightly distorted trigonal-bipyramidal coor-
dination geometry with N(1), N(2), and N(3) on the
basal plane where N(4) and the oxygen atomof coor-
dinated methanol are at the apex.
ꢀ
distance in this complex (2.22(1) A) is longer than that
found in a recently reported S3-ligated tetrahedral zin-
c(II) complex possessing a single methanol ligand
({[TmMeS]Zn(HOMe)} ꢀ ClO4 ꢀ MeOH): 1.993(3) A [16]
ꢀ
and close to that found in the pentacoordinated trigo-
nal-bipyramidal ({[bmapa]Zn(HOMe)} ꢀ (ClO4)2 ꢀ MeO
ꢀ
H):2.077(1) A [17].
Zinc(II) complex 1 exhibits comparable hydrogen-
bonding interactions in the solid state between the hy-
droxyl proton of the coordinated methanol, the primary
amine protons, and the oxygen atoms of the perchlorate
anion: O(1)ꢀ ꢀ ꢀO(2) 3.56(2), O(1)ꢀ ꢀ ꢀO(4) 2.74(2), O(4)ꢀ ꢀ ꢀ
ꢀ
N(2) 3.05(2), and O(5)ꢀ ꢀ ꢀN(3) 3.36(2) A. This short
The axial Zn(1)–N(4) interactions with bond distance
hydrogen-bonding interactions involving the zinc(II)-
bound methanol were also found with other related
zinc(II) complexes [16,17].
ꢀ
of 2.16(1) A is significantly longer than the equatorial
Zn–N distances, which are in the range of 2.05(1)–
ꢀ
2.07(3) A. Similar values have been found in ({[tren]
Zn(NCS)} ꢀ NCS) [14] and ({[tren]Zn(H2O)} ꢀ (ClO4)2)
[15]: 2.058(5) and 2.051(5), respectively, for the equa-
torial Zn–N distances and 2.292(4) and 2.193(4), re-
spectively, for the apical Zn–N distances. The equatorial
N–Zn–N bond angles are all with an average of
3.2. Fixation of CO2 by using zinc(II) complexes 1 and 2
Zinc(II) complex 1 takes up CO2 in methanol at room
temperature to fix it into monomethyl carbonato com-
plex: ({[tren]Zn(OCO2Me)} ꢀ BPh4 ꢀ 2MeOH) 3. Reac-
tion routes are given in Scheme 1. Zinc(II) complex 3 is
obtained by bubbling CO2 through methanolic solution
containing equimolar amounts of Zinc(II) complex 1 in
the presence of an equimolar amount of MeONa as a
base. When CO2 is bubbled in the absence of base, the
reaction gives only the methanol complex 1. The re-
sulting zinc(II) complex 1 also reacts with CO2 in
DMSO-d6 to give the desired zinc(II) complex 3, which
1
has been revealed by NMR spectroscopies. H and 13C
NMR spectra (Fig. 2) indicated the evidence of CO2
fixation. The signal at 164.5 ppmhas been attributed to
the formation of methyl carbonato complex. It also
showed two IR strong bands (mCO) that appeared
around 1620 and 1307 cmÀ1, which are characteristic of
Fig. 1. ORTEP drawing of molecular structure of ({[tren]-
Zn(HOMe)} ꢀ ClO4 ꢀ BPh4) 1. Ellipsoids are depicted at the 50%
ꢀ
probability level. Selected bond lengths (A) and bond angles (°): Zn(1)–
O(1) 2.11(1), Zn(1)–N(1) 2.05(2), Zn(1)–N(2) 2.07(1), Zn(1)–N(3)
2.05(1), Zn(1)–N(4) 2.16(1); O(1)–Zn(1)–N(1) 98.0(7), O(1)–Zn(1)–
N(2) 92.4(6), O(1)–Zn(1)–N(3) 100.5(4), O(1)–Zn(1)–N(4) 175.0(7),
N(1)–Zn(1)–N(2) 122.1(4), N(1)–Zn(1)–N(3) 115.9(7), N(1)–Zn(1)–
N(4) 84.7(6), N(2)–Zn(1)–N(3) 117.6(7), N(2)–Zn(1)–N(4) 82.6(5),
N(3)–Zn(1)–N(4) 81.8(6).
Scheme 1.

M.M. Ibrahim et al. / Inorganic Chemistry Communications 6 (2003) 1030–1034
1033
Fig. 2. 1H and 13C NMR spectra of zinc(II) complex 1 after bubbling CO2 for 4 h. The peak with * was assigned to zinc(II)-bound methyl carbonate
and X points out the solvent peaks.
–OCO2– group, as was found in monomethyl carbonato
complexes of zinc and other metals [4,6].
Supplementary materials
As shown in Scheme 1, the reaction of 1 with CO2 is
reversible. When methanolic solution of the resulting
methyl carbonato complex is warmed to ca. 40 °C, the
starting methanol complex 1 was recovered, which has
been characterized by X-ray crystallography.
Atomic parameters and complete lists of bond lengths
and angles will be deposited at the Cambridge Crystal-
lographic Data Centre, CCDC. Please contact the cor-
responding author for further details.
Efficiency of CO2 uptake depends strongly on the type
of ligand used. When CO2 is bubbled for 3 h into a
similar solution containing zinc(II) complex 2 in the
presence and absence of base, the starting material is
recovered. The inactivity of that complex towards the
fixation of CO2 was in accordance with Canary hy-
pothesis [8b]: zinc ions require the ability to accommo-
date expanded coordination numbers in hydrolytic
catalytic cycle. Since zinc(II) complexes of tren can form
six coordinate complexes but those of trenMe6 cannot.
In five coordinate complexes, both ligands adopt a C3-
symmetric conformation [18] (see Fig. 1). However, in six
coordinate complexes, tren adopts a Cd conformation
[8b] that is sterically inaccessible to trenMe6 due to the
steric hindrance caused by the bulky Me2N substituents.
The formation of zinc(II)-bound methoxy species in
solution seems to play a fundamental role in the process
of fixation of CO2. Most likely, the metal-bound meth-
oxy ion combines with CO2 to produce the carbonate
complex. Previously, Ito et al. [6b,6c] have found that
the hydroxo complex of macrocyclic ligands combines
reversibly with CO2 in methanolic solution giving a
methyl carbonato adduct.
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