Evaluating binuclear copper(II) complexes for glycoside hydrolysis

Article abstract of DOI:10.1021/ic9014064

Three binuclear copper(II) complexes were characterized as solids by X-ray diffraction and in solution by UV/vis spectrophotometric titration, and subsequently evaluated for their glycosidase-like activity. The structure analysis revealed comparable intermetallic Cu ? ? ? Cu distances (~3.5 A) for the complexes 2 and 3. Despite this similarity, the composition of the complexes differs significantly in aqueous solution as revealed by spectrophotometric titrations. The hydrolysis of selected nitrophenylglycopyranosides is up to 11,000-fold accelerated over background in the presence of the copper(II) complexes in 3-(cyclohexylamino)-1- propanesulfonic acid (CAPS) buffer at pH 10.5 and 30 °C.

Full text of DOI:10.1021/ic9014064

Inorg. Chem. 2010, 49, 2639–2648 2639
DOI: 10.1021/ic9014064
Evaluating Binuclear Copper(II) Complexes for Glycoside Hydrolysis
Susanne Striegler,* Natasha A. Dunaway, Moses G. Gichinga, James D. Barnett, and Anna-Gay D. Nelson
Department of Chemistry and Biochemistry, 179 Chemistry Building, Auburn University, Auburn,
Alabama 36849
Received July 17, 2009
Three binuclear copper(II) complexes were characterized as solids by X-ray diffraction and in solution by UV/vis
spectrophotometric titration, and subsequently evaluated for their glycosidase-like activity. The structure analysis
˚
revealed comparable intermetallic Cu Cu distances (∼3.5 A) for the complexes 2 and 3. Despite this similarity, the
3 3 3
composition of the complexes differs significantly in aqueous solution as revealed by spectrophotometric titrations. The
hydrolysis of selected nitrophenylglycopyranosides is up to 11,000-fold accelerated over background in the presence
of the copper(II) complexes in 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) buffer at pH 10.5 and 30 °C.
Introduction
during the reaction, and/or lack catalytic activity when
altering the model substrate from a 4-nitrophenyl-glycoside
to a 2-nitrophenyl analogue.3,6 Thus, other catalytic systems
with broader applicability are still needed.
In recent years, several artificial enzyme mimics for glyco-
side hydrolysis have been studied to model the glycosyl
transfer reaction observed in Nature.1-6 The importance of
this reaction is evidenced by the fact that around two-thirds
of the carbon in the biosphere exists in the form of carbo-
hydrates. Application of glycosyl transfer reactions is parti-
cularly interesting for the synthesis of non-natural carbo-
hydrate entities which are sorely needed as antiviral agents,
food fillers, or new antibiotics circumventing the observed
increased bacterial resistance to currently available drugs.
Although a glycosyl transfer is conveniently achieved with
glycosyl transferases, the use of these enzymes is mostly
restricted by their limited availability.7 Glycosidases used as
substitutes often lack selectivity and/or provide low reaction
yields.
Metal ions including Cu(II), Ni(II), Co(II), Al(III), and
La(III) have been shown to accelerate the cleavage of
disaccharides and other glycosides efficiently.8,9 So far,
studies on the hydrolysis of glycosides using metal ions have
largely focused on substrates that provide a binding site for
the metal ion close to the glycosidic bond to allow a
nucleophilic attack of water on the anomeric carbon. This
strategy, however, narrows the pool of substrates to glyco-
sides with strong metal ligating properties and excludes many
naturally occurring glycosides, such as disaccharides and
oligosaccharides. As a result, a selective cleavage of a specific
glycosidic bond in substrates that do not possess such strong
metal ion ligating properties is out of reach with the currently
available methods.
We therefore investigated the use of binuclear transition
metal complexes with defined geometry as new tools for the
cleavage of glycosidic bonds to open a new direction toward
the development of selective artificial glycosidase mimics.
For the initial catalyst design, we focused on the evaluation of
the structure-activity relationship for binuclear copper(II)
complexes with similar pentadentate backbone ligands. The
study summarized herein discloses preliminary results on
their structures in the solid state, their composition in aque-
ous solution, and the evaluation of the catalytic hydrolysis
of model aryl glycosides in water. A water-soluble pyri-
dine carboxaldehyde-based complex N,N0-{1,3-bis[(pyridin-
2-ylmethyl)amino]propan-2-ol}ato dicopper(II) (μ-acetato)
Extensive studies aimed at overcoming these shortcomings
with artificial enzyme mimics have employed cyclodextrins
derivatized with dicyanohydrins,2,5 diacids,6 or trifluoro-
methyl groups.1 Although these catalysts show many desir-
able properties as functional enzyme mimics, they also require
elevated temperatures for catalysis,4 tend to decompose
*To whom correspondence should be addressed. E-mail: susanne.striegler@
auburn.edu.
(1) Bjerre, J.; Fenger, T. H.; Marinescu, L. G.; Bols, M. Eur. J. Org.
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2010 American Chemical Society
Published on Web 02/18/2010
pubs.acs.org/IC

2640 Inorganic Chemistry, Vol. 49, No. 6, 2010
Striegler et al.
Figure 1. Structures of binuclear copper(II) complexes 1, 2, and 3.
Scheme 1. Synthesis of Asymmetric Complex 2
15-19
diperchlorate, Cu2bpdpo (1),10-12 its asymmetric analogue
N,N0-{bis(2-pyridylmethyl)-1,4-diaminobutan-2-ol}ato dicop-
per(II) (μ-acetato) diperchlorate, Cu2bpdbo (2), and a
symmetric salicylaldehyde-based analogue N,N0-{1,3-bis[{2-
hydroxy-4-[2-(2-methoxyethoxy)-ethoxy]-ethoxy}benzylidene-
amino]-propan-2-ol}ato dicopper (μ-acetate), Cu2TEGbsdpo
(3),12,13 were employed toward this end (Figure 1).
intramolecular Cu Cu distances of 3.418-3.541 A.
˚
3 3 3
Thus, the distance between the Cu(II) ions in 1a (3.499(1)
14
A) is within this typical range for acetato-bridged bi-
˚
nuclear complexes, but is considerably longer than in other
phosphato-bridged complexes, for example, [Cu2(L1)-
(O2P(OAr))](ClO4)2, HL1 = 2,6-bis[bis(2-pyridylethyl)-
aminomethyl]phenol; Ar = p-nitrophenol, Cu Cu =
3 3 3
3.001 A;20,21 We consequently assume that 1 behaves like
˚
other acetato-bridged complexes which renders the geo-
metry of the Cu(II) ions and the intramolecular
Cu Cu distances in the acetato-bridged complex 1
Results and Discussion
Complex Structures in the Solid State. To elucidate the
connection between intermetallic distance and catalytic
performance during glycoside hydrolysis, the binuclear
copper(II) complexes 2 and 3 were studied in the solid
state via X-ray diffraction of single crystals. The inter-
metallic distances and the geometry of the backbone
ligands around the metal cores were determined and
compared to the structural data of 1a,14 which is a known
phosphate analogue of 1; our efforts to crystallize 1 itself
were futile, its synthesis and the sugar binding ability were
described previously.10-12,22,23
3 3 3
(discussed herein) and the known diphenylphosphato-
bridged complex 1a14 comparable.
Complex 2 was prepared by condensing commercially
available 2-pyridine carboxaldehyde (4) with 1,4-diami-
nobutan-2-ol at ambient temperature following a proto-
col for the preparation of 1 that we developed previously
(Scheme 1).10-12,22,23 The Schiff-base ligand obtained
was reduced in situ with sodium borohydride supplying
the pentadentate ligand N,N0-bis(2-pyridylmethyl)-1,4-
diaminobutan-2-ol, bpdbo, (5). The coordination of
copper(II) acetate monohydrate to 5 in the presence of
sodium perchlorate in an aqueous ethanol-methanol
solution yielded blue crystals of 2 after slow evaporation
of the solvent.
Elemental analysis suggests a composition of C18H26-
Cl2Cu2N4O12 for 2, while the complex formation between
5 and copper(II) acetate monohydrate is evident from
infrared spectra (see Supporting Information). The C-H
deformation vibrations of the pyridyl groups shift from
1475 and 1433 cm-1 in 5 to 1480 and 1451 cm-1 in 2,
Both complexes, 1 and 1a,14 possess the same backbone
ligand and differ only in the bridging counteranion
(1: acetate; 1a:14 diphenylphosphate). The structure
analyses of binuclear Cu(II) complexes with related back-
bone ligands and monobridging acetato groups show
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J. Am. Chem. Soc. 1984, 106(17), 4765–72.
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(23) Striegler, S. Anal. Chim. Acta 2005, 539(1-2), 91–95.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2641
respectively, while the N-H deformation vibration of
the secondary amino groups changes from 1592 to 1612
cm-1. The aliphatic C-O stretching vibrations found at
1048 cm-1 in 5 cannot be unambiguously assigned to a
vibration in 2 and overlap badly with very strong per-
chlorate stretching vibrations at 1098 cm-1. Nevertheless,
comparable assignments of the observed infrared bands
were made previously for related complexes.24
The second metal ion, Cu(2), is bound in a square
pyramidal environment with a N2O3 donor set consisting
of ligand 5, the exogenous acetate group, and the oxygen
atom O(12) of a strongly coordinated water molecule
in axial position. By contrast, the basal plane N(4)-
N(3)-O(11)-O(9) is nearly perfect, and Cu(2) lies 0.129
˚
A above this plane; again, this displacement is similar to
that in 1a.14 The angle Cu(1)-O(11)-Cu(2) is 123.5(2)° in
2, and is considerably smaller than in 1a (132.9(2)°).14
This observation is ascribed to the distortion of 2 because
of the asymmetry of ligand 5. The structure of 2 in the
solid state is accordingly determined as [Cu2(5-H)(OAc)-
(H2O)(ClO4)]ClO4. The distortion of 2 decreases the
The structural study of 2 reveals an intermetallic
˚
Cu Cu distance of 3.39 A and a tridentate N2O donor
3 3 3
set of 5 to each metal ion.24 Both Cu(II) ions, Cu(1) and
Cu(2), are bridged by the endogenous μ-alkoxo oxygen
atom O(11) of 5 and the oxygen atoms O(9) and O(10) of
an exogenous acetate group (Figure 2).
˚
˚
intermetallic distance (Cu Cu: 3.39 A) by about 0.1 A
3 3 3
in comparison to 1a (Cu Cu: 3.499(1) A),14 but remains
˚
The equatorial donor atoms N(2)-N(1)-O(11)-
O(10) around Cu(1) strongly deviate from their best plane
leading to significant torsion of the N2O2 donor set by
19.2°. The coordination sphere of Cu(1) is completed by
the oxygen atom O(3) of a perchlorate counteranion that
is weakly bound in an axial position. The average Cu-N
and Cu-O distances in the equatorial plane of Cu(1) are
3 3 3
in the typical distance observed for other monobridged
binuclear complexes.15-19 While this small difference in
the distance between the two Cu(II) ions alone might only
have a minor effect on the catalytic turnover, the overall
distorted geometry of 2 in comparison to 1a might have a
profound effect on a putative use of the complexes as
transition metal catalysts. Selected bond distances, angles,
and dihedral angles of 2 are listed in Table 1.
˚
1.998 and 1.922 A, respectively. These distances are
similar to the corresponding averages in the symmetric
analogue 1a.14 The distance of Cu(1) to the axial per-
Prior to investigating the relevance of the intermetallic
distance and the overall complex geometry for catalytic
hydrolysis reactions, we prepared a symmetric Schiff-
base complex 3 (Figure 1) derived from salicylaldehyde
derivatives to alter the hydrogen donor and acceptor
properties of the ligand backbone while preserving the
geometry of the coordination sites of the Cu(II) ions in
comparison to 1. Diffraction-quality single crystals of 3
were not obtained despite all efforts; the flexibility of its
water solubility-enhancing triethylene glycol side chains
most likely prevents crystallization. Instead, complex 3a
with a methoxyethylene glycol side chain was prepared
and examined. This minor alteration at the periphery
of the ligand backbone is very unlikely to affect the geo-
metry of the binuclear metal core in 3a versus 3. A
comparison of the structure-activity relationship for
complexes 1 and 3, however, will provide valuable in-
sights for the catalyst design and the choice of electron
donating or withdrawing backbone ligands (see below) at
a given intermetallic distance.
˚
chlorate oxygen atom O(3) is 2.542 A in 2, and compar-
able to the average distance observed for weakly
coordinating perchlorate oxygen atoms in 1a.14
Figure 2. ORTEP325-POV Ray representation for the structure of 2,
Cu2bpdbo, depicted in ellipsoids with 60% probability; carbon atoms in
gray, copperatoms inyellow, oxygenatoms inred, nitrogenatoms inblue,
and chlorine atoms in green; hydrogen atoms and a second perchlorate
anion are omitted for clarity.
The synthesis of 3a followed a protocol for the regio-
selective alkylation of hydroxysalicylaldehydes that we
˚
Table 1. Selected Bond Lengths l [A], Angles Φ [deg] and Torsion Angles τ [deg] for 2
˚
[A]
distance l
angle Φ
[deg]
dihedral angle τ
[deg]
Cu(1)-O(11)
Cu(1)-O(10)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(2)-O(9)
Cu(2)-O(11)
Cu(2)-N(3)
Cu(2)-N(4)
Cu(2)-O(12)
1.908(3)
1.935(3)
1.987(4)
2.008(4)
1.929(3)
1.941(3)
1.985(4)
2.002(4)
2.303(4)
2.542
O(11)-Cu(1)-O(10)
O(11)-Cu(1)-N(1)
O(10)-Cu(1)-N(1)
O(11)-Cu(1)-N(2)
O(10)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
O(9)-Cu(2)-O(11)
O(9)-Cu(2)-N(4)
O(9)-Cu(2)-N(3)
O(11)-Cu(2)-N(4)
O(11)-Cu(2)-N(3)
N(3)-Cu(2)-N(4)
O(9)-Cu(2)-O(12)
O(11)-Cu(2)-O(12)
N(3)-Cu(2)-O(12)
N(4)-Cu(2)-O(12)
Cu(1)-O(11)-Cu(2)
94.7(1)
97.1(2)
162.2(2)
170.5(2)
88.5(1)
81.8(2)
98.7(1)
91.4(1)
167.2(2)
165.3(1)
86.3(2)
81.8(2)
92.7(1)
98.2(2)
98.3(2)
91.8(1)
123.5(2)
N(3)-Cu(2)-O(11)-Cu(1)
N(4)-Cu(2)-O(11)-Cu(1)
N(1)-Cu(1)-O(11)-Cu(2)
N(2)-Cu(1)-O(11)-Cu(2)
159.4
162.7
164.2
80.8
Cu(1) O(3)
3 3 3
Cu(1) Cu(2)
3 3 3
3.390

2642 Inorganic Chemistry, Vol. 49, No. 6, 2010
Scheme 2. Synthesis of Schiff Base Complex 3a
Striegler et al.
developed earlier;22 in short, the hydroxyl group of 2-
methoxyethanol (6) was activated by tosylation yielding
7, which was subsequently reacted with 2,4-dihydroxy-
benzaldehyde (8) in a Williamson ether synthesis provid-
ing a regioselectively alkylated aldehyde 9. Condensation
of 9 with 1,3-diaminopropanol supplied pentadentate
ligand N,N0-1,3-bis[{2-hydroxy-4-[2-(methoxyethoxy)]}-
benzylideneamino]propan-2-ol, EGbsdpo, (10a) (Scheme 2).
The reaction of 10a with copper(II) acetate monohydrate in
the presence of triethylamine yielded a green solid that was
crystallized from ethanol to obtain 3a after slow evaporation
of the solvent.
Elemental analysis revealed the composition of com-
plex 3a as C25H31Cu2N2O9. Infrared spectra showed a
shift of the azomethine band in 10a upon coordination of
copper(II) acetate monohydrate from 1634 cm-1 to 1630
cm-1 in 3a. Similarly, the phenoxy C-O stretch vibration
band is shifted from 1116 cm -1 in 10a to 1123 cm-1 in 3a
(see Supporting Information). Comparable shifts have
been observed for the formation of other Schiff-base
complexes previously.26
The structure analysis of 3a by X-ray diffraction in-
dicates square planar coordination environments for
both Cu(II) ions, which are bridged by an endogenous
μ-alkoxo oxygen atom of 10a and an exogenous μ-1,3-
coordinating acetate group, providing N1O3 donor sets
(Figure 3). Very similar bond lengths and bond angles are
reported for related binuclear Cu(II) complexes derived
from other salicylaldehyde (derivatives) and 1,3-diami-
nopropanol.27 A discussion of the structural features of
3a is consequently shortened herein. Notably, the inter-
bond angles, distances, and torsion angles are listed in
Table 2. The structure of Cu2(EGbsdpo) (3a) in the solid
state is confirmed as Cu2(10a-3H)(OAc).28
Complex Structures in Aqueous Solution. The composi-
tion of complexes 2 and 3 was determined in aqueous
solution prior to investigating a potential glycosidase-like
activity of all dinuclear copper(II) complexes. Toward
this end, spectrophotometric titrations using the method
€
from Zuberbuhler et al. were employed using the global
analysis model.29 The compositions of 1 in water and of 3
in aqueous methanol were established previously.11,12,14
For complex 3, a solution of the water-soluble ligand 10
and copper(II) acetate monohydrate (molar ratio 1:2) was
titrated with a sodium hydroxide solution from pH 4 to
13. The water-soluble ligand 10 (L = TEGbsdpo) is an
analogue of 10a (L=EGbsdpo) and different only in the
length of its ethylene glycol side chains at the periphery of
the ligand. The pentadentate ligands 10 and 10a will
therefore provide identical metal ion coordination sites
in the complexes 3 and 3a. From the alteration of the UV/
vis spectra upon base addition, the distribution of species
was computed (Figure 4).29
The titration experiment revealed the formation of a
mononuclear copper(II) complex 3b [CuL]2þ between
pH 5 and 7, whereas several binuclear complexes are
observed at alkaline pH. A binuclear complex 3c,
(28) X-ray structural analyses: Cu2(EGbsdpo)(OAc) (3a):C25H31Cu2-
N2O9, green rectangular, crystal dimensions 0.128 ꢀ 0.076 ꢀ 0.024 mm,
˚
triclinic, P1, Z = 2, a = 6.381(2), b = 10.723(4), c = 19.732(7) A,
3
˚
R =94.606(7)°, β = 91.898(7)°, γ =107.159(7)°, V = 1283.5(8) A (T =
193 K), μ = 17.14 cm-1, R1 = 0.0523, ωR2 = 0.1151; Cu2(bpdbo)(OAc)-
(ClO4)2 (2): C18H26Cl2Cu2N4O12, blue platelet, crystal dimensions 0.304 ꢀ
˚
metallic Cu Cu bond distance in 3a is 3.506 A and thus
3 3 3
comparable to 1a14 and 1 (see above) despite the different
electronic nature of the backbone ligand 10a in compar-
ison to 11, N,N0-1,3-bis[(pyridin-2-ylmethyl)amino]pro-
pan-2-ol (bpdpo, backbone ligand of 1a14 and 1). Selected
0.250 ꢀ 0.168 mm, monoclinic, P21/n, Z = 4, a = 15.0763(17), b =
3
˚
˚
12.2502(14), c = 15.3221(18) A, β = 116.020(2)°, V = 2543.0(5) A (T =
153 K), μ = 19.51 cm-1, R1 = 0.0584, ωR2 = 0.1830; Bruker APEX CCD
˚
diffractometer: θmax = 56.62° (3a) and 56.68° (2), MoKR, λ = 0.71073 A,
0.3° ω scans, 13126 (3a) and 24008 (2) reflections measured, 6310 (3a) and
6303 (2) independent reflections all of which were included in the refinement.
The data was corrected for Lorentz-polarization effects and for absorption
(SHELXPREP), solutions were solved by direct methods, anisotropic
refinement of F2 by full-matrix least-squares, 343 parameters. Further details
of the crystal structure investigations may be obtained from the CIF files.
(24) Striegler, S.; Dittel, M. Inorg. Chem. 2005, 44(8), 2728–2733.
(25) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30(5, Pt. 1), 565.
(26) Jana, S.; Dutta, B.; Bera, R.; Koner, S. Langmuir 2007, No. 23, 2492–
2496.
(27) Butcher, R. J.; Diven, G.; Erickson, G.; Jasinski, J.; Mockler, G. M.;
Pozdniakov, R. Y.; Sinn, E. Inorg. Chim. Acta 1995, 239(1-2), 107–16.
€
(29) Binstead, R. A.; Jung, B.; Zuberbuhler, A. D. SPECFIT/32 Global
Analysis System, 3.0; Spectrum Software Associates: Marlborough, MA, 2000.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2643
Figure 3. ORTEP25-POV ray representation for the structure of 3a, Cu2(EGbsdpo), depicted in ellipsoids with 60% probability; carbon atoms in gray,
copper atoms in yellow, oxygen atoms in red, nitrogen atoms in blue, and chlorine atoms in green; all hydrogen atoms are omitted for clarity.
˚
Table 2. Selected Bond Lengths [A] and Bond Angles [deg] for Complex 3a, Cu2EGbsdpo
˚
[A]
distance l
angle Φ
[deg]
dihedral angle τ
[deg]
Cu(1)-O(5)
Cu(1)-O(6)
Cu(1)-N(1)
Cu(1)-O(2)
Cu(2)-O(7)
Cu(2)-O(6)
Cu(2)-N(2)
Cu(2)-O(1)
1.897(3)
1.910(3)
1.921(3)
1.931(3)
1.893(2)
1.921(2)
1.925(3)
1.942(3)
3.506
O(5)-Cu(1)-O(6)
O(5)-Cu(1)-N(1)
O(6)-Cu(1)-N(1)
O(5)-Cu(1)-O(2)
O(6)-Cu(1)-O(2)
N(1)-Cu(1)-O(2)
O(7)-Cu(2)-O(6)
O(7)-Cu(2)-N(2)
O(6)-Cu(2)-N(2)
O(7)-Cu(2)-O(1)
O(6)-Cu(2)-O(1)
N(2)-Cu(2)-O(1)
Cu(1)-O(6)-Cu(2)
173.7(1)
93.6(1)
84.8(1)
87.8(1)
94.5(1)
173.6(2)
175.2(1)
93.3(1)
83.9(1)
87.6(1)
94.6(1)
171.3(1)
132.5(1)
N(2)-Cu(2)-O(6)-Cu(1)
N(1)-Cu(1)-O(6)-Cu(2)
179.81
166.48
Cu(1) Cu(2)
3 3 3
[Cu2L-2H]
2þ, dominates in solution above pH 6.6 and
is further deprotonated above pH 8.6 forming 3d,
[Cu2L-3H]þ; the species 3e, [Cu2L-3H(OH)], dominates
above pH 11.2. Plausible structures for the complexes
3b-3e are suggested in Scheme 3; binding constants and
species considered during computation are given in the
Supporting Information. The pKa for the coordination of
water is calculated from these values as 11.66.
To characterize the composition of the asymmetric
complex 2 in aqueous solution, a solution of ligand 5
(L = bpdbo) and copper(II) acetate monohydrate was
titrated with base as described for 3 (vide infra). The
species distribution data reveal the presence of free Cu(II)
ions and 5, and the formation of a mononuclear species
Figure 4. Distribution of species for binuclear copper(II) complex 3
derived from 10 and Cu(OAc)2 H2O (molar ratio = 1:2) in aqueous
solution between pH 5 and 13 at 30 °C.
[CuL]2þ (2a) between pH 5 and 8 (Figure 5).
3
3þ
The formation of a binuclear species [Cu2L-H
]
(2b)
competing with 2a is noted above pH 6. Further depro-
tonation of 2b in its backbone ligand 5 in alkaline pH
is unlikely due to the high pKa values typically observed
for secondary amines,30 although equilibrium struc-
tures may still exist. Any subsequent loss of protons
in alkaline solution is thus ascribed to the deproto-
nation of water molecules coordinating to the metal
center of 2.
binding constants of 2b and 2c. Further deprotonation of
[Cu2L-H (OH)]2þ above pH 13 leads to the formation of
[Cu2L-H (OH)2]þ (2d). The large intermetallic Cu Cu
3 3 3
˚
distance (3.39 A) in 2 suggests terminal coordination of
the resulting hydroxide ions to one of the Cu(II) ions
rather than μ-coordination to both.14,31-33
In view of the subsequent evaluation of the binuclear
complexes as catalysts for the hydrolysis of glycosides,
the speciation data also reveal the composition of the
complexes and a possible catalytic form under the cho-
sen conditions (CAPS buffer, pH 10.5). Accordingly,
The loss of a proton of a water molecule coordinat-
ing to [Cu2L-H]
3þ above pH 9.5 results in the formation
of a [Cu2L-H (OH)]2þ species (2c), which is the predomi-
nant form of 2 above pH 10.7 (Scheme 4). The pKa for
the water coordination is calculated as 10.66 from the
(31) Frey, S. T.; Murthy, N. N.; Weintraub, S. T.; Thompson, L. K.;
Karlin, K. D. Inorg. Chem. 1997, 36(6), 956–957.
(30) Martell, A. E.; Smith, R. M. NIST - Critically selected stability
constants of metal complexes database, Version 5.0; U. S. Department of
Commerce, National Institute of Standards and Technology: Gaithersburg, MD,
1998.
(32) Gajda, T.; Kramer, R.; Jancso, A. Eur. J. Inorg. Chem. 2000, No. 7,
1635–1644.
(33) Mazurek, W.; Berry, K. J.; Murray, K. S.; O’Connor, M. J.; Snow,
M. R.; Wedd, A. G. Inorg. Chem. 1982, 21(8), 3071–80.

2644 Inorganic Chemistry, Vol. 49, No. 6, 2010
Striegler et al.
Scheme 3. Structures for the Copper(II) Species 3b-3e Derived from 10 and Cu(OAc)2 H2O between pH 4 and 13
3
disclose the influence of an overall planar (1) and dis-
torted (2) complex geometry on the Lewis acidity of the
metal centers.
In a model reaction, the catalytic hydrolysis of 12 was
monitored by the formation of the yellow p-nitrophen-
oxide anion using UV/vis spectroscopy at 410 nm
(Scheme 5). The reaction depends linearly on the concen-
tration of the catalyst under the provided conditions (see
Supporting Information).
Attempts to increase the substrate concentration to
reach saturation of the catalytic sites were limited by the
solubility of 12 in aqueous solution. Nevertheless, pseudo-
first order conditions were provided with excess substrate
in concentrations between 5 and 50 mM and a catalyst
concentration of 0.1 mM. The hydrolysis of 12 was
followed under these conditions for 26 h without any
evidence for catalyst deactivation by product inhibition
(see Supporting Information). The related substrates
p-nitrophenyl-β-D-galactopyranoside (13), p-nitrophenyl-
R-D-glucopyranoside (14), and p-nitrophenyl-β-D-gluco-
pyranoside (15) were found to have an even lower solubi-
lity in aqueous solution than 12, which limited their
examination to concentrations between 5 and 35 mM.
When rate constants (kcat) and Michaelis-Menten con-
stants (KM) for the catalytic hydrolysis of 12-16 could not
be obtained by a non-linear regression fit of the p-nitro-
phenolate formation (Table 3), the catalytic efficiency
(kcat/KM) was estimated from a linear fit of the data.
All binuclear complexes show significant accelerations
of the hydrolysis of 12 (by at least a factor of 103). Of the
three complexes, the catalytic efficiency (kcat/Km) of 1 is
about twice that of 2 and about 2.5 times that of 3. The
catalytic efficiency of 1 to catalyze the hydrolysis of 12
is about 2-fold higher than the cleavage of the glycosi-
dic bonds in 13, 14, or 15; a similar trend is observed
when using 2 as catalyst under otherwise identical condi-
tions. The catalytic efficiency of 2 during the catalytic
hydrolysis of o-nitrophenyl-R-D-galactopyranoside (16)
is about 6-fold lower than for 12 because of increased
autocatalytic hydrolysis of this substrate. These results
overall suggest a slight preference for a pyridyl ring (1 and
2) over a phenoxy group (3) in the ligand framework of
the catalyst, and for a symmetrical (1) over an unsym-
metrical (2) backbone ligand for the future design of
Figure 5. Distribution of species derived from 5 and Cu(OAc)2 H2O
3
(molar ratio 1: 2) in aqueous solution between pH 5 and 13 at 30 °C.
complex 1 exists primarily as [Cu2(11-H)(OH)]2þ species
(88%), whereas the remaining ligand is forming a Cu2-
[(11-H)(OH)2]þ species, with 11 (bpdpo) as backbone
ligand of 1.11,14 Both forms may be catalytically active.
By contrast, the asymmetric analogue 2 exists predomi-
nantly as [Cu2(5-H)]3þ species (56%) and to about 44%
as [Cu2(5-H)(OH)]2þ species, with 5 (bpdbo) as backbone
ligand of 2. Complex 3 forms mainly a [Cu2(10-3H)]þ
species (93%) at the given pH, while the [Cu2(10-3H)-
(OH)] species exists in less than 7%, with 10 (TEGbsdpo)
as backbone ligand of 3. Hypothesizing that the hydro-
lysis of glycosides requires the presence of a hydroxyl
group-containing species, the apparent catalytic activity
of 1 should be higher than that of 2 or 3 in 3-(cyclo-
hexylamino)-1-propanesulfonic acid (CAPS) buffer at
pH 10.5.
Hydrolysis of a Model Glycoside. To probe the struc-
tural and electronic effects of the ligand backbone on the
catalytic performance of the binuclear Cu(II) complexes,
the ability of 1, 2, and 3 to hydrolyze p-nitrophenyl-R-D-
galactopyranoside (12) in CAPS buffer at pH 10.5 was
studied as a model reaction. The intermetallic distances in
˚
the binuclear complexes are comparable (∼3.5 A) as
revealed by the discussion on their structure analyses
(vide infra). Any difference in the catalytic performance
of the complexes is therefore related to the electronic
features of the ligand backbones and, consequently, to the
species derived therefrom. A comparison of the catalytic
performance of complexes 1 and 2 will furthermore

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2645
Scheme 4. Structures for Copper(II) Species 2a-2d Derived from 5 and Cu(OAc)2 H2O between pH 5 and 13
3
Scheme 5. Catalytic Hydrolysis of p-Nitrophenyl-R-D-galactopyra-
noside (12)
mononuclear complex 2a derived from 5 and Cu(II)
chloride behaves alike and promotes the hydrolysis of
12 about 1 order of magnitude slower than 2. In related
studies, binuclear transition metal complexes were shown
to hydrolyze phosphodiesters and carboxylic acid esters
very efficiently and superior to their mononuclear ana-
logues.14,34-42 Efforts to investigate the hydrolysis of
12 with free Cu(II) ions were futile as a precipitate
(presumably Cu(II) hydroxide) is formed immediately
upon dissolving Cu(II) chloride or Cu(II) acetate as
catalyst precursor compounds in CAPS buffer at pH 10.5.
The presented experimental data allow some prelimi-
nary conclusions on the mechanism of the glycoside
hydrolysis promoted by 1, 2, and 3. The hydrolysis of
the glycosides is linearly dependent on the catalyst con-
centration for all complexes under the conditions studied;
however, an increase of the rate for the product formation
is noted during prolonged reaction time and will be
investigated in future studies (see Supporting Informa-
tion). Saturation conditions allow treatment of the data
as pseudo-first order kinetics and reveal the formation of
a substrate-catalyst complex. The p-nitrophenolate pro-
duct does not deactivate the catalyst during a 26 h time
period. The hydrolysis proceeds at pH 10.5, but not at pH
9, indicating a contribution of the metal-bound hydroxide
ion in base catalysis (Scheme 6). The catalyst may reform
upon substitution of the coordinated sugar with water
and subsequent proton loss in alkaline solution.
binuclear complexes for the hydrolysis of glycosides. The
electronic features of the ligand backbone control the
Lewis acidity of the metal core in 1, 2, and 3 and determine
the amount of the resulting catalytically active species
containing deprotonated water.
In combination with the speciation data, we suggest
[Cu2(11-H)(OH)]2þ, [Cu2(5-H)(OH)]2þ, and [Cu2(10-3H)-
(OH)] as the catalytic active forms of 1, 2, and 3 at pH 10.5,
respectively. However, the speciation data of 2 and 3
indicate the presence of [Cu2(5-H)]3þ and [Cu2(10-3H)]þ
as major species under these conditions (vide infra). To
clarify the role of the major species versus a putative
catalytically active minor species, the catalytic hydrolysis
of 12 by 2 and 3 was additionally studied in borate and
TAPS buffer at pH 9. Under these conditions, complex 2
exists as [Cu2(5-H)]3þ (97.5%) and as [Cu2(5-H)(OH)]2þ
(2.5%) species (Figure 5). The catalytic hydrolysis of 12 is
slowed down by about 1.5 orders of magnitude (Table 3,
entry 11) indicating a decrease in the amount of the
catalytically active species. Similarly, complex 3 exists
as [Cu2(10-3H)]þ species (73.1%), [Cu2(10-2H)]þ species
(26.6%) and as hydroxyl group containing species
[Cu2(10-3H)(OH)] (0.3%) (Figure 4). At pH 9, the cata-
lytic hydrolysis of 12 proceeds in the same order of
magnitude as the background reaction (without catalyst)
even when the concentration of 3 is increased by 10-fold.
Thus, the species [Cu2(5-H)]3þ and [Cu2(10-3H)]þ are not
catalytically active despite their predominant presence
at pH 10.5, and the catalytic hydrolysis of 12 is solely
promoted by [Cu2(5-H)(OH)]2þ and [Cu2(10-3H)(OH)] as
proposed above.
The hydrolysis of 12 catalyzed by 1 is overall about
11,000-fold accelerated over the background reaction
(kcat/knon) at 30 °C in CAPS buffer. A comparable
performance of artificial glycosidase mimics based on
cyclodextrins has been reported; however, they require
elevated temperatures (∼60 °C).4 As complex 1 has been
(34) Gajda, T.; Duepre, Y.; Toeroek, I.; Harmer, J.; Schweiger, A.;
Sander, J.; Kuppert, D.; Hegetschweiler, K. Inorg. Chem. 2001, 40(19),
4918–4927.
(35) Koevari, E.; Kraemer, R. J. Am. Chem. Soc. 1996, 118(50), 12704–
12709.
The catalytic hydrolysis of 12 by complex N,N0-1,3-
bis[(pyridin-2-ylmethyl)amino]propan-2-ol copper(II)
diperchlorate, Cubpdpo, 1b,14 was then studied to corre-
late the performance of the binuclear complex 1 to its
mononuclear analogue. The hydrolysis of 12 is more than
1 order of magnitude slower with 1b14 than with 1
(Table 3) despite an increased catalyst concentration from
0.1 to 1 mM. This observation suggests a cooperative
effect of both Cu(II) ions in the binuclear complexes
during the transformation of 12. An in situ prepared
(36) Yamada, K.; Takahashi, Y.-i.; Yamamura, H.; Araki, S.; Kawai, M.;
Saito, K. Chem. Commun. 2000, No. 14, 1315–1316.
(37) Feng, G.; Natale, D.; Prabaharan, R.; Mareque-Rivas, J. C.;
Williams, N. H. Angew. Chem., Int. Ed. 2006, 45(42), 7056–7059.
(38) Jancso, A.; Mikkola, S.; Lonnberg, H.; Hegetschweiler, K.; Gajda, T.
Chem.;Eur. J. 2003, 9(21), 5404–5415.
(39) Jurek, P. E.; Martell, A. E. Chem. Commun. 1999, No. 16, 1609–1610.
(40) Selmeczi, K.; Giorgi, M.; Speier, G.; Farkas, E.; Reglier, M. Eur. J.
Inorg. Chem. 2006, No. 5, 1022–1031.
(41) Zhou, Y.-H.; Zhao, M.; Li, J.-H.; Mao, Z.-W.; Ji, L.-N. J. Mol.
Catal. A 2008, 293(1-2), 59–64.
(42) Liu, C. T.; Neverov, A. A.; Brown, R. S. J. Am. Chem. Soc. 2008,
130(42), 13870–13872.

2646 Inorganic Chemistry, Vol. 49, No. 6, 2010
Striegler et al.
Table 3. Summary of Kinetic Parameters for the Catalytic Hydrolysis of the Glycopyranosides 12-16;
k
cat (ꢀ10-4
)
k
non (ꢀ10-7
)
]
k
cat/Km (ꢀ10-3
)
entry
pH
cat
S
[min-1
]
Km [mM]
[M-1 min-1
[M-1 min-1
]
kcat/knon [M]
kcat/(Km ꢀ knon
)
1
2
3
4
5
6
7
8
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
10.5
9.0
1
1
1
1
1b
2
2
2
2
2
2
3
12
13
14
15
12
12
13
14
15
16
12
12
43.4 ( 9.6
13.2 ( 2
10.5 ( 4.5
a
3 ( 0.8
21.4 ( 3.3
20.0 ( 2.7
2.4 ( 1
5.0 ( 0.4
2.5 ( 0.3
a
213 ( 57
80 ( 15
138 ( 69
a
220 ( 70
185 ( 35
256 ( 38
77 ( 12
211 ( 20
132 ( 20
a
3.8
7.5
2.8
2.2
3.8
3.8
7.5
2.8
2.2
32
20.4
16.5
6.7
7.0
1.4
11.6
7.8
3.1
2.4
1.9
0.7
8.4
11,000
1,800
3,700
a
54,000
22,000
24,000
32,000
3,700
30,000
10,000
11,000
11,000
600
800
5,600
2,700
860
2,300
80
9
10
11
12
0.6
3.8
a
3,300
12,000
22,000
10.5
12.7 ( 2.9
152 ( 43
a Not determined.
Scheme 6. Putative Mechanism for the Hydrolysis of 12 by 1 (n = 1) or 2 (n = 2)a
a -OR = p-nitrophenolate.
previously shown to discriminate among carbohy-
drates,11 a study to evaluate the selective and catalytic
transformation of glycosides by binuclear copper(II)
complexes will be topic of future investigations.
Conclusions. Three binuclear copper(II) complexes 1, 2,
and 3 were synthesized and characterized in solid state by
X-ray diffraction and by spectrophotometric titration in
aqueous solution. The complexes show comparable inter-
of magnitude slower when catalyzed by a mononuclear
complex. Overall, the study has given very valuable in-
sights and direction for the future design of transition
metal complexes for the non-natural transformation of
glycosides.
Experimental Section
Instrumentation. 1H and 13C NMR spectra were recorded on a
Bruker AV 400 spectrometer (400.2 MHz for 1H and 100.6 MHz
for 13C). Chemical shifts (δ) are expressed in parts per million
and coupling constants (J) in hertz. Signal multiplicities are
denoted as s (singlet), d (doublet), t (triplet), and m (multiplet).
Deuterated chloroform was used as a solvent, and chemical shift
values are reported relative to the residual signals of this solvent
(δ = 7.24 for 1H and δ = 77.0 for 13C NMR). UV-vis spectra
were recorded at 30.0 ( 0.1 °C over a range of 200-800 nm on a
metallic Cu Cu distances despite electronically differ-
3 3 3
ent backbone ligands. This observation is important for
the comparison of distorted complex 2 to symmetric 1
during sugar binding and transformation. All complexes
were found to moderately catalyze the hydrolysis of
model glycosides in aqueous alkaline solution with up
to 11,000-fold acceleration of the reaction over back-
ground. The glycoside hydrolysis is more than 1 order

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 2647
Varian Cary 50 with WinUV Analysis Suite software, version
3.0, using disposable Brandtech macro cells (220-900 nm) of
1 cm thickness and 4.5 mL volume for the determination of the
distribution of species. Disposable 1.5 mL semimicro Brandtech
UV cuvettes (220-900 nm) of 10 mm light path with caps were
used for the hydrolysis studies. The pH was measured using a
Beckman Φ 250 pH meter equipped with refillable long Futura
pH electrode of 0.7 mm thickness. The pH meter was calibrated
before each set of readings (3-point calibration). The IR spectra
were obtained on a Shimadzu IR Prestige-21 FT-IR spectro-
photometer with IR solution software version 1.10 as thin films
or KBr pellets (ν in cm-1). X-ray diffraction data were collected
on a Bruker SMART APEX CCD X-ray diffractometer. Sam-
ples for elemental analysis were sent to Atlantic Microlab Inc.,
Atlanta, GA. ESI-MS data were obtained on a VG Trio-2000
Fisons Instruments mass spectrometer with VG MassLynx
software, version 2.00, or sent to the Laboratory for Biological
Mass Spectrometry, Texas A&M University, College Station,
TX, for analysis. Thin layer chromatography (TLC) was pre-
formed on silica gel TLC plates from SORBENT Technologies,
200 μm, 4 ꢀ 8 cm, aluminum backed, with fluorescence indicator
F254 with detection by charring with anthrone sulfate, and by
UV light when applicable. Column chromatography was carried
out using silica gel 60 as stationary phase from Silicycle (40-
63 μm, 230-240 mesh). All melting points were recorded on a Mel-
Temp melting point apparatus, and the values are uncorrected.
Chemicals. The ligand TEGbsdpo (10),43 2-hydroxy-4-(2-
methoxyethoxy)benzaldehyde (9),22 N,N0-{1,3-bis[(pyridin-
2-ylmethyl)amino]propan-2-ol}ato dicopper(II) (μ-acetato)
diperchlorate 1,44 N,N0-1,3-bis[(pyridin-2-ylmethyl)amino]pro-
pan-2-ol (bpdpo, 11),11,14 and 1,4-diaminobutan-2-ol dihydro-
chloride45 were synthesized as described; 2-pyridine carboxalde-
hyde was obtained from Aldrich, distilled under reduced pressure
and stored in the dark at 10 °C prior to use; 4-nitrophenol was
obtained from Fluka. All other reagents were obtained from
Fisher and Sigma-Aldrich and were used without further puri-
fication.
Synthesis of N,N0-1,3-bis{[2-hydroxy-4-(2-methoxyethoxy)]-
benzylideneamino}propan-2-ol, EGbsdpo (10a). A solution of
1,3-diaminopropanol (115 mg, 1.24 mmol) in 10 mL of ethanol
was added to a solution of 2-hydroxy-4-(2-methoxyethoxy)-
benzaldehyde (9)22 in 150 mL of ethanol/methanol (1/1, v/v)
at ambient temperature and allowed to stir for 48 h. The
resulting solution was evaporated to give a crude yellow product
that was recrystallized from MeOH/EtOH (1/1 v/v) to give a
yellow powder. Yield 23% (230 mg, 0.52 mmol); mp 120-
121 °C; δH (CDCl3) 8.16 (s, 2H, -CH=N-), 7.06 (d, 8.3, 2H,
ArH), 6.37 (m, 4H, ArH), 4.15 (m, 1H, -CHOH-), 4.06 (t, 4.7,
4H, ArOCH2-), 3.70 (m, 4H, -CH2-), 3.59 (m, 2H, -CH2-);
δC (CDCl3, 100.6 MHz) 166.1, 165.1, 163.2, 132.9, 112.2, 107.0,
101.8, 70.7, 70.2, 67.2, 61.3, 59.2; νmax (KBr)/cm-1 3356 (br,
OH), 3075w (=CH arom.) 2880s (C-H), 1634v (CdN), 1115s
(C-O). Positive ion ESI MS, calcd for (C23H30N2O7 þH)þ
446.21, found 446.16.
ambient temperature. A green powder was isolated by filtration
after 7 days and recrystallized from ethanol to obtain dark green
crystals. Yield 44% (70 mg, 0.11 mmol). Found: C, 47.63; H,
4.80; N, 4.40%. C25H31Cu2N2O9 requires: C, 47.62, H, 4.95, N,
4.44%; νmax (KBr)/cm-1 3455br (OH), 2905s (C-H), 1630v
(CdN), 1128s (C-O).
N,N0-bis(2-pyridylmethyl)-1,4-diaminobutan-2-ol, bpdbo (5).
To a solution of 1,4-diaminobutan-2-ol dihydrochloride45 (0.5 g,
2.82 mmol) in 20 mL of methanol was added a solution of
sodium hydroxide (226 mg, 5.64 mmol) in 10 mL of methanol.
The resulting solution was stirred at ambient temperature for
30 min and then filtered to remove precipitated NaCl. A solution
of distilled 2-pyridine carboxaldehyde (0.6 g, 5.64 mmol) in
10 mL of methanol was added dropwise to the filtrate, and the
resulting yellow solution stirred overnight at ambient tempera-
ture. Sodium borohydride (158 mg, 5.64 mmol) was then added
in small portions after which the solution was refluxed for 1 h. It
was then cooled to ambient temperature and evaporated to
dryness by rotary evaporation. The crude product was dissolved
in 20 mL of nanopure water and extracted using dichloro-
methane (2 ꢀ 50 mL). The separated organic layer was dried
using anhydrous sodium sulfate and evaporated to obtain a
yellow oil that was purified by chromatography on Sephadex
LH-20-100 using methanol as eluent. Yield: 40% (320 mg,
1.1 mmol), yellow oil; Rf (SiO2/MeOH): 0.3; δH (400.2 MHz,
CDCl3) 8.54 (m, 2H, -CH = N-), 7.75-7.55 (m, 2H, ArH),
7.35-7.25 (m, 2H, ArH), 7.22-7.06 (m, 2H, ArH), 4.04-3.83
(m, 5H, ArOCH2, -CHOH), 3.40-3.05 (m, 2H, NH),
3.04-2.76 (m, 2H, CH2CH2NH), 2.75-2.58 (m, 2H, CH2NH),
1.77-1.52 (m, 2H, CH2CH2NH); δC (100.6 MHz, CDCl3)
160.1, 159.3, 149.4, 149.3, 136.7, 136.6, 122.5, 122.3, 122.2,
122.0; IR (thin film on KBr disk) νmax/cm-1 3303br (OH),
3063w (N-H), 2927s, 2830s, 729s; HRMS calcd for (C16H22-
N4O þ H)þ 287.1872; found 287.1874.
N, N0-{bis(2-pyridylmethyl)-1,4-diaminobutan-2-ol}ato dicop-
per(II) (μ-acetato) Diperchlorate, Cu2bpdbo, (2). Copper(II)
acetate monohydrate (1.4 g, 7.0 mmol) was dissolved in 10 mL
of nanopure water and mixed with a solution of sodium
perchlorate (2.8 g, 22.8 mmol) in 5 mL of water. The resulting
solution was diluted with 140 mL of methanol and added to 5
(716 mg, 2.5 mmol) dissolved in 20 mL of ethanol at ambient
temperature. The resulting solution immediately turned deep
blue, and was concentrated to 30 mL under vacuum at 40 °C.
The remaining solution was allowed to stand at ambient tem-
perature for 4 days. A blue crystalline solid formed that was
isolated by filtration, air-dried and not further purified. Yield
28% (480 mg, 0.7 mmol): Caution! perchlorate salts are poten-
tially explosive. However, we did not experience any difficulty in
handling or drying the compound below 40 °C; found: C, 31.52; H,
3.70; N, 8.09%. C18H26Cl2Cu2N4O12 requires C, 31.36; H, 3.95;
N, 8.13%; νmax (KBr)/cm-1 3443br (OH) 3247w (N-H), 2930w
(C-H), 1564s (N-H def), 1095s (C-O)
Species Distribution Studies. In a typical experiment, a 2 mL
aqueous solution containing 1 mM copper(II) acetate monohy-
drate and 0.5 mM pentadentate ligand 5 or 10 were titrated with
10 μL of freshly prepared sodium hydroxide solution at 30.0 (
0.1 °C. The ionic strength of the aqueous solution was main-
tained constant with 0.1 M NaClO4. The pH was recorded after
mixing the solutions thoroughly. UV-vis spectra were then
taken as a function of the pH and the resulting data were
computed by the global fitting model provided by the program
Specfit.29,46 The concentration of the sodium hydroxide solu-
tion titrated was increased gradually from 0.005 to 5 M.
Hydrolysis Studies. In a typical experiment, the concentration
range of the substrate was 4-50 mM with a catalyst concentra-
tion of 0.1 mM in a solution with a total volume of 1 mL.
N,N0-{1,3-bis{[2-hydroxy-4-(2-methoxyethoxy)]benzylidene-
amino}-propan-2-ol}ato dicopper (μ-acetate), Cu2EGbsdpo, (3a).
A solution of copper(II) acetate monohydrate (100 mg, 0.5
mmol) in 10 mL of N,N-dimethylformamide (DMF) was added
dropwise to a solution of 8 (110 mg, 0.25 mmol) in 30 mL of
DMF. Triethyl amine (76 mg, 0.75 mmol) was then added, and
the mixture allowed to stir at 60 °C for 24 h. The solution was
then concentrated to half of its volume under vacuum at 60-
70 °C, and the remaining solution kept in an open flask at
(43) Gichinga, M. G.; Striegler, S. J. Am. Chem. Soc. 2008, 130(15), 5150–
5156.
(44) Mazurek, W.; Kennedy, B. J.; Murray, K. S.; O’Connor, M. J.;
Rodgers, J. R.; Snow, M. R.; Wedd, A. G.; Zwack, P. Inorg. Chem. 1985, 24
(20), 3258–64.
(46) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbuehler, A. D. Talanta
1985, 32(2), 95–101.
(45) Murase, I.; Ueno, S.; Kida, S. Inorg. Chim. Acta 1984, 87(2), 155–7.

2648 Inorganic Chemistry, Vol. 49, No. 6, 2010
Striegler et al.
Experiments were conducted in 50 mM CAPS buffer at pH 10.5
or 50 mM borate buffer at pH 9. The reaction was monitored
over time by formation of 4-nitrophenolate using UV-vis
spectroscopy at 410 nm (or 450 nm for formation of 2-nitro-
phenolate).47-49 The background reaction was performed in a
similar manner but without addition of the catalyst. Stock
solutions of the mononuclear complexes 1b were prepared in
situ from equimolar amounts of copper(II) chloride and the
ligands bpdpo (11)11,14 and bpdbo (5), respectively; stock solu-
tions of the binuclear complexes 1, 2, and 3 were prepared by
dissolving appropriate amounts of the isolated complexes. The
extinction coefficient for the product formation was determined
by a calibration curve and used to convert the absorbance of the
reaction product into molar amounts (4-nitrophenolate in bo-
rate buffer: ε = 16160 M-1 cm-1; CAPS buffer: ε = 16190 M-1
cm-1; 2-nitrophenolate in CAPS buffer: ε = 2310 M-1 cm-1).
Acknowledgment. A CAREER award from the National
Science Foundation to S.S. (CHE-0746635) to support this
work is gratefully acknowledged. The authors thank T. E.
Albrecht-Schmitt for help with the structural analysis of
complexes 2 and 3a, and T. Webb for helpful discussion during
the preparation of the manuscript.
Supporting Information Available: Kinetic profiles for the
hydrolysis of 12 and 16 by 1-3; 1H and 13C NMR spectra, IR
spectra and MS chromatograms of 5 and 10a. This material is
available free of charge via the Internet at http://pubs.acs.org.
CCDC 699736 (2), and 699735 (3a) contain supplementary
crystallographic data and can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
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