Novel Acetate Binding Modes in [Na2Cu(CH3COO) 4(H2O)]·H2O

Article abstract of DOI:10.1021/ic034517y

A new copper(II) acetate, [Na₂Cu(CH₃COO) ₄(H₂O)]·H₂O (1), has been crystallized from an aqueous solution containing sodium acetate and copper(II) acetate monohydrate in a 4:1 ratio and the structure determined by X-ray crystallography. 1 crystallizes in the monoclinic space group P2(1)/c, with a = 16.638(3) A, b = 11.781(2) A, c = 15.668(3) A, β = 90.11(3)°, V = 3071.0(11) A³, and Z = 4. In the asymmetric unit, sodium ions bridge two crystallographically unique square planar [Cu(CH₃COO)₄]^2- units to their symmetry-generated neighbors to form corrugated 2D sheets of Na ₂Cu(CH₃COO)₄, which are held together by H-bonding interactions involving the waters of crystallization. In contrast, the structures of known sodium copper acetates are better described as 3D frameworks. The metal centers are bridged by a number of acetates in novel coordination modes. The square planar Cu(II) geometry generated by oxygen atoms from four different acetates is an unexpected feature given the weak ligand field provided by the acetate ligands.

Full text of DOI:10.1021/ic034517y

Inorg. Chem. 2003, 42, 7037−7040
Novel Acetate Binding Modes in [Na2Cu(CH COO)4(H O)]‚H O
3
2
2
,†
Andrew C. Warden,† Milton T. W. Hearn,‡ and Leone Spiccia*
School of Chemistry and Centre for Green Chemistry, Monash UniVersity,
Victoria, Australia, 3800
Received May 14, 2003
A new copper(II) acetate, [Na2Cu(CH COO)4(H O)]‚H O(1), has been crystallized from an aqueous solution containing
3
2
2
sodium acetate and copper(II) acetate monohydrate in a 4:1 ratio and the structure determined by X-ray
crystallography. 1 crystallizes in the monoclinic space group P2(1)/c, with a ) 16.638(3) Å, b ) 11.781(2) Å, c )
3
15.668(3) Å, â ) 90.11(3)°, V ) 3071.0(11) Å , and Z ) 4. In the asymmetric unit, sodium ions bridge two
2-
crystallographically unique square planar [Cu(CH COO)4] units to their symmetry-generated neighbors to form
3
corrugated 2D sheets of Na2Cu(CH COO)4, which are held together by H-bonding interactions involving the waters
3
of crystallization. In contrast, the structures of known sodium copper acetates are better described as 3D frameworks.
The metal centers are bridged by a number of acetates in novel coordination modes. The square planar Cu(II)
geometry generated by oxygen atoms from four different acetates is an unexpected feature given the weak ligand
field provided by the acetate ligands.
Introduction
transition metal complexes. Recent attempts to synthesize
double salts consisting of alkali metal cations and Cu(II)
acetate anions met with limited success in the case of
sodium.10 Although the structures of two compounds of
composition NaH[Cu2(CH3COO)6] have been reported,11,12
both of these structures contain the well-characterized
[Cu2(CH3COO)4] binuclear units,1 linked by acetate-bridged
Na+ ions to generate a 3D network. The Na+ salt of the Cu-
(II) propionate, Na[Cu4(C2H5COO)9(H2O)], also forms a 3D
framework in which the Cu(II) propionate complexes are
linked by Na+ ions.13 Recently, Ng et al.7 reported the
formation of a 1:1 sodium perchorate-[Cu(betaine)4]2+
adduct, [Cu{(CH3)3NCH2CO2}4(NaClO4)](ClO4)2, in which
the Cu(II) center is, perhaps unexpectedly, square planar.
We report the synthesis and X-ray structure of [Na2Cu(CH3-
COO)4(H2O)]‚H2O (1). This complex forms a 2D layered
structure which features a number of novel acetate bind-
ing modes and, to our knowledge, incorporates the first
example of square-planar [Cu(CH3COO)4]2- anions bind-
ing to Na+.
The properties and supramolecular behavior of copper-
(II)/carboxylates have been studied extensively since the
structure of Cu2(CH3COO)4‚2H2O was first reported five
decades ago.1 Although much of the attention has focused
on analogues of this complex, various Cu(II) carboxylate
derivatives that incorporate alkali metal ions, often sodium,
have been reported in the last 20 years for ligands such as
croconate,2 picolinate,3 azamacrocycle acetate derivatives,4,5
amino acids,6,7 crown ethers8 and alkoxo anions.9 These
studies have sparked interest in understanding the influence
of alkali metal ions on the structure and properties of
* To whom correspondence should be addressed. E-mail:
leone.spiccia@sci.monash.edu.au.
† School of Chemistry.
‡ Center for Green Chemistry.
(1) Van Niekerk, J. N.; Schoening, F. K. L. Acta Crystallogr. B 1953,
31, 227.
(2) Wang, C.-C.; Yang, C.-H.; Lee, G.-H. Inorg. Chem. 2001, 41, 1015.
(3) Goher, M. A. S.; Mautner, F. A. Polyhedron 1994, 13, 149.
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C.; Weiss, J. Inorg. Chem. 1982, 21, 1208.
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K.-C.; Guzei, I. A.; Rheingold, A. L. J. Am. Chem. Soc. 2000, 122,
10561.
(6) Antolini, L.; Menabue, L.; Saladini, M.; Sola, M. Inorg. Chim. Acta
1988, 152, 17.
(7) Ng, S. W.; Che, X. M.; Yang, G. Acta Crystallogr., Sect. C 1998, 54,
1389.
(8) Gibney, B. R.; Kessissoglou, D. P.; Kampf, J. W.; Pecoraro, V. L.
Inorg. Chem. 1994, 33, 4840.
(9) Purdy, A. P.; George, C. F. Polyhedron 1995, 14, 761.
(10) Hardt, H. D.; Street, G. Z. Anorg. Allg. Chem. 1967, 350, 84.
(11) Seigrist, T.; Chamberlain, B. L.; Ramirez, A. P.; LoBrutto, R. J. Solid
State Chem. 1996, 121, 61.
(12) Wu, C.; Lu, C.-Z.; Yang, W.-B.; Zhuang, H.-H.; Huang, J. S. Inorg.
Chem. Commun. 2001, 4, 504.
(13) Chung, Y. H.; Wei, H. H.; Liu, Y. H.; Lee, G. H.; Wang, Y.
Polyhedron 1998, 174, 449.
10.1021/ic034517y CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/10/2003
Inorganic Chemistry, Vol. 42, No. 22, 2003 7037

Warden et al.
Table 1. Crystal Structure and Refinement Data for 1
Experimental Section
empirical formula
fw, g mol-1
λ, Å
crystal system
space group
a, Å
C16H32Cu2Na4O20
763.46
Reagents and Physical Techniques. Commercial reagents were
used as supplied without further purification. IR spectra were
recorded on a Perkin-Elmer 1600 series instrument as KBr disks.
Electronic spectra were recorded on a Cary 5G UV-visible-NIR
spectrophotometer. The magnetic moment was measured using a
Faraday balance incorporating a 4 in. Newport electromagnet.
Preparation of [Na2Cu(CH3COO)4(H2O)]‚H2O (1). The fol-
lowing procedure was devised following the discovery of crystals
of 1 in a solution containing 1-benzyl-4-acetato-1,4,7-triazacy-
clononane trihydrochloride, 2 equiv of copper(II) chloride, sodium
hydroxide, and excess sodium acetate. A solution of sodium acetate
(3.0 g, 36 mmol) in distilled water (10 mL) was stirred and heated
to between 80 and 90 °C in a beaker on a hotplate. Copper(II)
acetate monohydrate (1.80 g, 9.0 mmol) was then added in small
portions over a 10 min period with continued stirring and heating,
the solid being allowed to completely dissolve before further
additions, to give a deep blue clear solution from which a light
blue powder started to precipitate. After 30 min the solution was
cooled to room temperature and left open to the atmosphere in an
evaporating dish. The light blue powder continued to precipitate
for several hours. After a few days, small, isolated clusters of deep
blue crystals of 1 formed. The crystals were separated from the
light blue powder by several washings with ethanol (95%) in a
test tube with stirring. This suspended most of the powder in the
liquid, which could be decanted leaving the crystals of 1 at the
bottom of the tube. When exposed to air for several minutes the
crystals appear to lose solvent and develop a fine white film on
the surface. Yield: 1.13 g (32%). Anal. Calcd for 1: C, 25.2; H,
4.2. Found: C, 25.4; H, 4.1. IR (KBr) ν/cm-1: 3474 (s, OH), 3004,
2931 (w, CH), 1617, 1583, 1561, 1406 (s, -COO), 1344 (m), 1050
(w), 1016 (w), 938 (w), 677 (w), 622 (w). UV/visible spectrum
(diffuse reflectance, nm): 280, 350-400 (br), 580-670 (br).
Magnetic moment: µeff ) 1.88 µB at 293 K. Anal. of light blue
powder. Found: Calcd for {[0.85Cu(OH)(CH3COO)‚H2O][0.15Cu-
(OH)2]}: C, 13.7; H, 3.6; C, 13.8; H, 2.6; Na, 0; Cu, 42.5. Na, 0;
Cu, 42.8. IR (KBr) ν/cm-1: 3482, 3199 (s, OH), 3000 (w, CH),
1572, 1524, 1431, (vs, -COO), 1341 (w), 1141 (w), 1028 (w),
980 (w), 909 (m), 680 (m), 650 (w), 619 (w).
X-ray Crystallography. A portion of a crystal of 1 was cut under
a microscope and mounted on a glass fiber. Diffraction data for 1
was collected on an Enraf-Nonius CCD diffractometer with
monochromated Mo KR radiation (λ ) 0.71073 Å) at 123(2) K
using φ and/or ω scans. Data were corrected for Lorentz and
polarization effects, and absorption corrections were applied. All
non-hydrogen atoms were refined anisotropically. Water hydrogen
atoms were located on Fourier difference maps and refined
isotropically without restraint with the exception of one on O(18).
The difficulty in locating the second hydrogen on O(18) may be
due to the presence of only one suitable H-bond acceptor in the
vicinity of this water molecule, in contrast to O(20), which has
both of its hydrogen atoms participating in interactions with an
acetate oxygen (O(11)) and O(18). The structure was solved by
direct methods, and refinement on F2 used the full matrix least-
squares methods of SHELXS-9714 and SHELXL-97,15 respectively.
The program X-Seed16 was used as an interface to the SHELX
programs, and to prepare the figures. Crystal data is given in Table
0.71073
monoclinic
P2(1)/c
16.638(3)
11.781(2)
15.668(3)
90.11(3)
3071.0(11)
4
1.649
1.521
3.00-28.28
28880
b, Å
c, Å
â, deg
V, Å3
Z
F
calcd, g/cm3
µ(Mo KR), mm-1
θ range, deg
no. of measd reflns
no. of obsd reflns [I>2σ(I)]
GOF on F2
7176
1.182
0.0735
0.1385
a
R1
b
wR2
largest diff peak and hole, e Å-3
+0.450, -0.942
2
2
2
a ∑||Fo| - |Fc||/∑|Fo|. b [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
Table 2. Selected Bond Lengths and Intermetallic Distances (Å) and
Angles (deg) for Construct A in 1
Cu(1)-O(19)
Cu(1)-O(9)
Cu(1)-O(6)
1.933(4)
1.962(4)
1.967(4)
1.978(4)
2.342(5)
2.352(4)
2.369(4)
2.375(5)
3.297(4)
3.177(2)
3.531(2)
3.197(3)
3.440(4)
Na(2)-O(6)#5a
Na(2)-O(4)
2.432(4)
2.771(5)
2.323(4)
2.336(4)
2.380(5)
2.397(5)
2.525(5)
2.549(4)
3.289(4)
3.521(2)
3.184(2)
3.193(3)
3.434(4)
Na(4)-O(9)
Cu(1)-O(12)
Na(2)-O(10)#5
Na(2)-O(19)
Na(2)-O(16)#5
Na(2)-O(22)#5
Na(1)‚‚‚Na(3)
Na(1)‚‚‚Cu(2)
Na(1)‚‚‚Cu(2)*
Na(3)‚‚‚Cu(2)
Na(3)‚‚‚Na(3)*
Na(4)-O(4)#6
Na(4)-O(16)#8
Na(4)-O(22)#8
Na(4)-O(4)
Na(4)-O(3)
Na(2)‚‚‚Na(4)
Na(2)‚‚‚Cu(1)
Na(2)‚‚‚Cu(1)*
Na(4)‚‚‚Cu(1)
Na(4)‚‚‚Na(4)*
O(19)-Cu(1)-O(9)
O(19)-Cu(1)-O(6)
O(9)-Cu(1)-O(6)
O(19)-Cu(1)-O(12)
O(9)-Cu(1)-O(12)
O(6)-Cu(1)-O(12)
O(10)#5-Na(2)-O(19)
O(10)#5-Na(2)-O(16)#5 100.3(3) O(4)#6-Na(4)-O(22)#8
O(19)-Na(2)-O(16)#5 154.5(2) O(16)#8-Na(4)-O(22)#8
O(10)#5-Na(2)-O(22)#5 100.3(2) O(9)-Na(4)-O(4)
O(19)-Na(2)-O(22)#5
O(16)#5-Na(2)-O(22)#5
O(10)#5-Na(2)-O(6)#5
O(19)-Na(2)-O(6)#5
O(16)#5-Na(2)-O(6)#5
O(22)#5-Na(2)-O(6)#5
O(10)#5-Na(2)-O(4)
O(19)-Na(2)-O(4)
173.6(2) O(16)#5-Na(2)-O(4)
91.3(2) O(22)#5-Na(2)-O(4)
91.8(2) O(6)#5-Na(2)-O(4)
88.6(2) O(9)-Na(4)-O(4)#6
88.8(2) O(9)-Na(4)-O(16)#8
175.4(2) O(4)#6-Na(4)-O(16)#8
104.6(2) O(9)-Na(4)-O(22)#8
78.1(1)
71.0(2)
108.5(1)
166.9(2)
103.7(2)
87.2(2)
109.8(2)
78.8(2)
80.0(2)
80.6(2)
90.2(2)
167.2(2)
87.2(2)
80.1(1)
89.7(1)
108.4(2)
165.6(2)
84.1(1)
100.0(2) O(4)#6-Na(4)-O(4)
80.7(2) O(16)#8-Na(4)-O(4)
79.3(2) O(22)#8-Na(4)-O(4)
101.8(2) O(9)-Na(4)-O(3)
77.3(2) O(4)#6-Na(4)-O(3)
157.5(2) O(16)#8-Na(4)-O(3)
171.2(2) O(22)#8-Na(4)-O(3)
78.2(1) O(4)-Na(4)-O(3)
a Symmetry operations used to generate equivalent atoms: #5 -x + 1,
1
3
1
1
y + /2, -z + /2, #6 -x + 1, -y + 1, -z + 2, #8 x, -y + /2, z + /2.
1, and selected bond lengths and intermetallic distances and angles
are given in Table 2. Thermal ellipsoids are drawn at 30%
probability.
Results and Discussion
Synthesis. The complex initially formed in an aqueous
solution containing 1-benzyl-4-acetato-1,4,7-triazacyclononane
trihydrochloride, 2 equiv of CuCl2, NaOH, and excess
NaCH3COO. It deposited when the filtrate was allowed to
evaporate, following removal of the desired Cu(II) tacn
derivative. To our surprise, an X-ray structure determination
revealed that a new anionic Cu(II)-acetate complex had been
(14) Sheldrick, G. M. SHELXS-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(15) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(16) Barbour, L. J. X.-SeedsA software tool for supramolecular crystal-
lography. J. Supramol. Chem. 2001, 1, 189.
7038 Inorganic Chemistry, Vol. 42, No. 22, 2003

NoWel Acetate Binding Modes in [Na2Cu(CH3COO)4(H2O)]‚H2O
Table 3. Hydrogen Bond Contacts for 1 (Å and deg)
D-H‚‚‚A
d(D-H)
d(H‚‚‚A) d(D‚‚‚A)
(DHA)
O(21)-H(3)‚‚‚O(2)#2a 0.998(10) 1.869(19) 2.839(5)
163(5)
152(6)
164(7)
154(7)
166(7)
159(8)
143(10)
O(21)-H(4)‚‚‚O(18)
O(20)-H(15)‚‚‚O(11)
O(18)-H(19)‚‚‚O(12)
O(22)-H(1)‚‚‚O(3)#1
O(22)-H(2)‚‚‚O(20)
O(20)-H(20)‚‚‚O(18)
0.998(10) 1.83(3)
0.65(6) 2.19(7)
0.996(10) 1.91(4)
2.748(6)
2.816(8)
2.834(6)
2.836(6)
2.744(7)
2.859(7)
0.66(6)
0.82(9)
0.66(8)
2.19(7)
1.96(9)
2.30(9)
a Symmetry transformations used to generate equivalent atoms: #1 -x
1
3
1
3
+ 1, y - /2, -z + /2, #2 -x, y + /2, -z + /2.
obtained. Following this discovery, a higher yielding syn-
thesis of 1 was designed. The reactions conditions represent
a compromise between competing effects. It was found that
higher proportions of copper(II) acetate returned copper(II)
acetate as the crystalline phase, whereas lower proportions
gave a light blue powdery product but failed to produce the
blue crystalline phase. Heating in excess of 95 °C induced
the formation of the light blue product prior to completing
the addition of copper acetate. More dilute solutions returned
Cu(II) acetate, as did removal of the light blue product prior
to evaporation of the filtrate indicating that the light blue
material is involved in the formation of 1. The light blue
material exhibited strong νasym (1572, 1524 cm-1) and νsym
(1431 cm-1) acetate stretches. The spectrum is different from
that of 1 or copper(II) acetate monohydrate. Elemental
analyses corresponded to {[0.85Cu(OH)(CH3COO)‚H2O]-
[0.15Cu(OH)2]}; this can viewed as mixture of [Cu(OH)-
CH3COO)] and Cu(OH)2. The analytical and spectroscopic
data for 1 supported the proposed composition. In the IR
spectrum, a water OH stretch is evident at 3467 cm-1 and
pairs of νasym and νsym carboxylate stretches are evident at
1617 and 1583 cm-1, and 1405 and 1344 cm-1, respectively.
The solution electronic spectrum of 1 was the same as that
of Cu(II) acetate, both showing a broad band in the 750-
800 nm region. The diffuse reflectance spectrum was
different. It exhibited two of the three transitions expected
for a square planar Cu(II) complex as broad bands in the
600-650 and 350-420 nm regions.
Figure 1. Asymmetric unit of 1 including oxygen atoms in the sodium
coordination sphere. Hydrogen bonds (- - -). C and H atoms not labeled
for clarity. H atoms on methyl and symmetry-related waters not shown.
Figure 2. Square planar [Cu(CH3COO)4]2- anion in construct A.
Structural Description. The molecular structure of [Na2-
Cu(CH3COO)4(H2O)]‚H2O, 1, exhibits two crystallographi-
cally unique but geometrically identical [Na2Cu(CH3COO)4-
(H2O)] constructs (A for Cu(1) and B for Cu(2)) and two
water molecules that participate in H-bonding interactions
only (Figure 1). These two waters of hydration, O(18) and
O(20), are the only components that give rise to the
asymmetry that generates the two separate constructs. The
other two water molecules, O(21) and O(22), are each
coordinated to two sodium atoms and each donate H-bonds
to a water and an acetate. All water hydrogen atoms were
located in Fourier difference maps and refined with the
exception of one on O(18).
Figure 3. Coordination environment of sodium atoms in construct A.
Asterisk (*) indicates symmetry-related atom.
acetates are arranged such that adjacent acetates lie on
opposite sides of the plane defined by the copper coordination
plane, probably to minimize steric interactions (Figure 2).
The SP geometry is unexpected given the weak ligand field
of acetate but has been observed in two [Cu(betaine)4]2+
complexes in which the ligands also adopt an up/down
conformation. This complex is cationic as the betaine ligands
are neutral, combining a negatively charged carboxylate with
a positively charged quaternary ammonium.
In the anionic [Cu(CH3COO)4]2- complexes, four acetate
anions coordinate to a square planar (SP) Cu(II) center and
in turn bridge to one or more sodium ions through either
one or both of their oxygen atoms. Cu-O bond lengths do
vary (1.933(4) and 1.984(4) Å) but are in the expected range
for carboxylates binding equatorially to Cu(II). The four
Since the constructs are essentially identical, the discussion
from hereafter focuses on construct A. The sodium atoms
are in a distorted octahedral geometry defined by five acetate
oxygen atoms and one water ligand (Figure 3) with Na-O
distances in the 2.323(4)-2.771(5) Å range (Table 2). The
acetates act as counterions and connect the symmetry-related
Inorganic Chemistry, Vol. 42, No. 22, 2003 7039

Warden et al.
Figure 4. (a, left) Corrugated 2D sheets bridged by water molecules (viewed along c axis). Acetate groups and H-bonds have been removed for clarity.
(b, right) The H-bonding network unit connecting the 2D sheets (viewed along c axis: bottom sheet, construct A; top sheet, construct B). Aside from the
2D sheets, the bridging waters and acetates from one asymmetric unit are shown. Na ) green, Cu ) blue, O ) red, C ) gray, H ) light blue.
Chart 1. Coordination and Hydrogen-Bonding Modes (I-IV) for
Acetate Anions in 1
Within the 2D sheets, Cu and Na centers assemble via
coordination to acetate and water into nanosized macrocyclic
units consisting of six Na+ and four Cu2+ with dimensions
0.9 × 1.2 nm. Each unit shares an edge with four macro-
cycles packing together into undulating sheets that lie atop
each other 180° “out-of-phase”.
The acetates interact with the sodium, coppers and water
in four different ways (Chart 1). Very recently the structure
of Cu(CH3COO)(CH3O) provided the first example of acetate
acting as a tetradentate ligand in a copper complex.17
A
substructure search of the CCDC revealed only two examples
of acetate anions in a Cu/Na coordination environment in
which the acetate anions in these structures exhibited only
binding mode II.11,12 Inclusion of carboxylates that are part
of a larger ligand assembly revealed one structure, with a
picolinate anion, that exhibited binding mode I.7 No occur-
rences of modes I, III, or IV could be found in structures
incorporating acetate anions.
constructs. For each Na+ center, four Na-O distances are
below the sum of the van der Waals radii while the other
two distances suggest weak Na+‚‚‚O interactions. The longest
contact of 2.78 Å is between Na(2) and an acetate oxygen,
O(4). This oxygen atom is also involved in interactions with
two other sodium atoms, Na(4) and Na(4)* (2.525(5) and
2.336(4) Å, respectively), the former representing the second
weakest Na+‚‚‚O interaction seen in the structure.
Constructs A and B self-assemble into two separate 2D
sheets connected to one another through hydrogen bonds
involving the uncoordinated water molecules (Figure 4a,b).
One acetate and one sodium-bound water molecule per
construct, respectively accept and donate H-bonds to the two
uncoordinated water molecules that separate the sheets. The
bridge can be broken down into three major groups of
interactions: Na-O-H‚‚‚O-H‚‚‚O-Cu (sheet B to sheet
A), Na-O-H‚‚‚(H)O-H‚‚‚O-Cu (sheet A to sheet B), and
the O-H‚‚‚O contact between the uncoordinated waters
(O(20)-H(20)‚‚‚O(18), 2.859(7) Å, 143(10)°) (see Figure
4b).
Acknowledgment. This research was supported by the
Australian Research Council.
Supporting Information Available: X-ray crystallographic file
for 1 in CIF format. This material is available free of charge from
http://pubs.acs.org.
IC034517Y
(17) Koval, I. A.; Gamez, P.; Roubeau, O.; Driessen, W. L.; Lutz, M.;
Spek, A. L.; Reedjik, J. Inorg. Chem. 2003, 42, 868.
7040 Inorganic Chemistry, Vol. 42, No. 22, 2003