Structural features of mono- and tri-nuclear Zn(ii) complexes with a non-steroidal anti-inflammatory drug as ligand

Article abstract of DOI:10.1039/c2dt30547j

The interaction of Zn(ii) with the non-steroidal anti-inflammatory drug tolfenamic acid leads to the formation of the structurally characterized trinuclear [Zn₃(tolfenamato)₆(CH₃OH) ₂] complex. In the presence of the N,N′-donor heterocyclic ligands 1,10-phenanthroline and 2,2′-bipyridine at a range of ratios, the mononuclear Zn complexes of the general formulae [Zn(tolfenamato)(N,N′- donor)Cl] and [Zn(tolfenamato)₂(N,N′-donor)] have been isolated and structurally characterized by X-ray crystallography. The deprotonated tolfenamato ligands are coordinated to the Zn(ii) ion through carboxylato oxygen atoms. Tolfenamic acid and its complexes exhibit good binding propensity to human or bovine serum albumin protein having relatively high binding constant values.

Full text of DOI:10.1039/c2dt30547j

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PAPER
Structural features of mono- and tri-nuclear Zn(II) complexes with a
non-steroidal anti-inflammatory drug as ligand†
Alketa Tarushi,a Xanthippi Totta,a Catherine P. Raptopoulou,b Vassilis Psycharis,b George Psomasa and
Dimitris P. Kessissoglou*a
Received 7th March 2012, Accepted 4th April 2012
DOI: 10.1039/c2dt30547j
The interaction of Zn(II) with the non-steroidal anti-inflammatory drug tolfenamic acid leads to the
formation of the structurally characterized trinuclear [Zn3(tolfenamato)6(CH3OH)2] complex. In the
presence of the N,N′-donor heterocyclic ligands 1,10-phenanthroline and 2,2′-bipyridine at a range of
ratios, the mononuclear Zn complexes of the general formulae [Zn(tolfenamato)(N,N′-donor)Cl] and
[Zn(tolfenamato)2(N,N′-donor)] have been isolated and structurally characterized by X-ray
crystallography. The deprotonated tolfenamato ligands are coordinated to the Zn(II) ion through
carboxylato oxygen atoms. Tolfenamic acid and its complexes exhibit good binding propensity to human
or bovine serum albumin protein having relatively high binding constant values.
NSAIDs and their metal complexes with DNA is of interest
Introduction
since their potential anticancer as well as anti-inflammatory
activities may be explained.14 The chemical classes of NSAIDs
Zinc is an element of great biological interest being the second
most abundant trace element in the human body.1 Its role is
comprise salicylate derivatives, phenylalkanoic acids, oxicams,
anthranilic acids, sulfonamides and furanones.15 The NSAID tol-
important in various biological systems since it is critical for
numerous cell processes and is a major regulatory ion in the
metabolism of cells.2 The administration of zinc in children suf-
fering from deadly diarrhoea in the form of the preparation
“Baby Zinc” resulted in significant reduction of child mortality
in underdeveloped countries.3 In the literature, zinc complexes
with drugs used for the treatment of Alzheimer’s disease4 and
others showing antibacterial/antimicrobial,5 anticonvulsant,6
antidiabetic,7 anti-inflammatory8 or antiproliferative/antitumor5,8
activity are structurally characterized. In addition, a number of
workers have reported the structures of binuclear, trinuclear and
polynuclear Zn(II) complexes containing carboxylato bridging
ligands.9–11
Non-steroidal anti-inflammatory drugs (NSAIDs) are the most
frequently used analgesic, anti-inflammatory and antipyretic
agents.12 NSAIDs inhibit the cyclo-oxygenase (COX)-mediated
production of prostaglandins, have exhibited anti-tumorigenic
activity by reducing the number and size of carcinogen-induced
colon tumors or through inducing the apoptosis of several cancer
cell lines as well as a synergistic role on the activity of certain
antitumor drugs.13 In this context, the direct interaction of
fenamic acid (= Htolf, Scheme 1) belongs to the derivatives of
N-phenylanthranilic acid and chemically resembles mefenamic
and niflumic acid and other fenamates in clinical use.15 Htolf is
found in analgesic, anti-inflammatory, antipyretic and antirheu-
matoid drugs and is also used for veterinary purposes.16 To the
best of our knowledge, the crystal structures of two tin(IV),17 two
copper(II)18 and two cobalt(II)19 complexes have been reported in
the literature.
Our recent studies have been focused on the coordination
chemistry of carboxylate-containing anti-inflammatory19–22 or
antimicrobial23,24 drugs with metal ions (Co2+, Ni2+, Cu2+ and
Zn2+) in an attempt to examine their mode of binding and poss-
ible biological relevance. In addition, we have reported studies
on the interaction of these metal complexes with biomolecules
(nucleic acids and serum albumin proteins) as well as their tenta-
tive biological (antimicrobial, anticancer, antioxidant) activity.
Taking into consideration the biological role and activity of zinc
and its complexes as well as the significance of NSAIDs in
aDepartment of General and Inorganic Chemistry, Faculty of Chemistry,
Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece.
E-mail: kessisog@chem.auth.gr; Fax: +30+2310997738;
Tel: +30+2310997723
bInstitute of Materials Science, NCSR “Demokritos”, GR-15310 Aghia
Paraskevi Attikis, Greece
†Electronic supplementary information (ESI) available. CCDC
851684–851687. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30547j
Scheme 1 Tolfenamic acid.
7082 | Dalton Trans., 2012, 41, 7082–7091
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medicine, we present the synthesis and the structural characteriz-
ation of neutral zinc(II) complexes with the NSAID tolfenamic
acid in the presence of: methanol (complex 1, [Zn3(tolf)6-
(CH3OH)2]); a nitrogen-donor heterocyclic ligand such as 1,10-
phenanthroline (= phen) (complexes 2, [Zn(tolf)(phen)Cl] and
4·Et2O, [Zn(tolf)2(phen)]·Et2O); and 2,2′-bipyridine (= bipy)
(complexes 3, [Zn(tolf)(bipy)Cl] and 5, [Zn(tolf)2(bipy)]). The
crystal structures of [Zn3(tolf)6(CH3OH)2] (1), [Zn(tolf)(phen)-
Cl] (2), [Zn(tolf)2(phen)]·Et2O (4·Et2O) and [Zn(tolf)2(bipy)]
(5) complexes are also reported. Due to the biological impact of
these compounds we plan to report later upon completion antiox-
idant and cytotoxicity studies along with the binding properties
with calf-thymus DNA and DNA-competitive binding studies
with ethidium bromide. In this report the affinity for bovine
(BSA) and human serum albumin (HSA) proteins involved in
the transport of metal ions and metal complexes with drugs
through the blood stream, performed with fluorescence spec-
troscopy is also reported.
(–COOH) respectively, have shifted in the IR spectra of com-
plexes 1–5, in the range 1578–1583 cm−1 and 1375–1397 cm−1
assigned to antisymmetric, νasym(CvO), and symmetric,
νsym(CvO), stretching vibrations of the carboxylato group,
respectively. The difference Δ [= νasym(CvO) − νsym(CvO)], a
useful characteristic tool for determining the coordination mode
of the carboxylato ligands, gives a value falling in the range
185–203 cm−1 indicative for an asymmetrical binding mode of
the tolfenamato ligand.19,25
The UV-vis spectra of the complexes have been recorded as
nujol mull and in DMSO solution and are similar, suggesting
that the complexes retain their structure in solution. In addition,
the fact that complexes 1–5 have the same UV-Vis spectral
pattern in nujol and in DMSO solution as well as in the presence
of the buffer solution (150 mM NaCl and 15 mM trisodium
citrate at pH 7.0) at ratios used in the biological experiments in
combination with the fact that they are non-electrolytes in
DMSO solution suggests that the compounds keep their integrity
in solution. A thorough study of similar Cu(II) and Co(II) com-
plexes with NSAIDs as ligands supporting the stability of those
complexes in solution has been recently reported by our
lab.20a,21b
Results and discussion
Synthesis and spectroscopic study of the complexes
Description of the crystal structure of [Zn3(tolf)6(CH3OH)2]
The synthesis of the complexes in high yield was achieved via
the aerobic reaction of tolfenamic acid (C6H3(Cl)(CH3)-
NH-C6H4(COOH)) and KOH with ZnCl2 or Zn(NO3)2·6H2O in
the absence, (eqn (1)) for 1, or presence of the corresponding N,
N′-donor heterocyclic ligand (L = C12H8N2 or C10H8N2), (eqn
(2) and (3)) for complexes 2, 3 and 4, 5, respectively, according
to the equations:
The crystal structure of [Zn3(tolf)6(CH3OH)2] is shown in
Fig. 1, and selected bond distances and angles are listed in
Table 1. Complex 1 is a centrosymmetric trinuclear double-
wheel Zn complex with Zn(1) lying on the inversion center. The
six tolfenamato ligands behave as deprotonated ligands in the
bidentate mode and are coordinated via two carboxylato oxygen
atoms to two zinc atoms, the central Zn, Zn(1), and a terminal
one, Zn(2), forming six bidentate carboxylate bridges.
3ZnCl2 þ 6C6H3ðClÞðCH3Þ ꢀ NH ꢀ C6H4ðCOOHÞ
þ 6KOH þ 2CH3OH
The central Zn atom, Zn(1), is six-coordinate with a distorted
octahedral geometry. The basal plane of the octahedron is
formed by four coordinated carboxylato oxygen atoms Ο(1),
Ο(22), Ο(1)′ and Ο(22)′ of four different tolfenamato bridging
ligands while at the axial positions the carboxylato oxygen
atoms Ο(42) and Ο(42)′ of the remaining two tolfenamato brid-
ging ligands are lying at further distances (Zn(1)–O(42) = 2.207
(3) Å) than O(1) (Zn(1)–O(1) = 2.110(3) Å) and O(22) (Zn–
O(22) = 2.044(2) Å). The six coordinated tolfenamato ligands
form syn–syn bidentate carboxylato bridges between the central
zinc atom, Zn(1), and the two terminal zinc atoms, Zn(2) and Zn
(2)′; three bridges between Zn(1) and each terminal zinc atom.
Each of the terminal zinc atoms, Zn(2) or Zn(2)′, is four-co-
ordinate. It is coordinated to three oxygen atoms O(2), O(21) and
O(41) from the three tolfenamato bridging ligands [Zn(2)–Ο(2)
= 1.940(3) Å, Zn–Ο(21) = 1.901(3) Å and Zn(2)–O(41) = 1.913
(3) Å] and an oxygen atom from a terminal coordinated metha-
nol [Zn(2)–Ο(1M) = 2.030(3) Å]. The tetrahedrality calculated
for Zn(2) atoms gives a dihedral angle of 81.90° (the tetrahedral-
ity for a four-coordinate metal complex can be determined from
the angle subtended by two planes, each encompassing the metal
and two adjacent atoms;26a for strictly square planar complexes
with D4h symmetry, the tetrahedrality is 0°; for tetrahedral com-
! ½Zn3fC6H3ðClÞðCH3Þ ꢀ NH
ꢀ C6H4ðCOOÞg6ðCH3OHÞ2ꢁ þ 6KCl þ 6H2O ð1Þ
ZnCl2 þ C6H3ðClÞðCH3Þ ꢀ NH ꢀ C6H4ðCOOHÞ
þ KOH þ L
ð2Þ
! ½ZnfC6H3ðClÞðCH3Þ ꢀ NH ꢀ C6H4ðCOOÞgðLÞClꢁ
þ KCl þ H2O
ZnðXÞ2 þ 2C6H3ðClÞðCH3Þ ꢀ NH ꢀ C6H4ðCOOHÞ
þ 2KOH þ L
! ½ZnfC6H3ðClÞðCH3Þ ꢀ NH ꢀ C6H4ðCOOÞg2ðLÞꢁ ð3Þ
þ 2KX þ 2H2O
ꢀ
ðX ¼ NO3 or Clꢀ; L ¼ C12H8N2 or C10H8N2Þ
All complexes are soluble in DMSO, DMF and non-electro-
lytes (for 1 mM solutions, ΛM ≤ 1–8 mho cm2 mol−1).
IR spectroscopy has been used in order to confirm the depro-
tonation and binding mode of tolfenamic acid. In the IR spec-
trum of Htolf, the observed absorption band at 3355(br,m) cm−1
,
attributed to the ν(H–O) stretching vibration has disappeared
upon binding to the metal ion. The bands at 1661(s) cm−1
and 1265(s) cm−1 attributed to the stretching vibrations
ν(CvO)carboxylic and ν(C–O)carboxylic of the carboxylic moiety
plexes with D2d symmetry, the tetrahedrality equals 90°) support-
26b
ing a tetrahedral geometry for ZnO4
with the three
tolfenamato oxygen atoms and the methanol oxygen lying at the
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Fig. 1 (A) The molecular structure and partial labeling of [Zn3(tolf)6(CH3OH)2]. (B) Intra- and inter-molecular H-bonding network in 1.
Table 1 Selected bond distances and angles for complex 1
Table 2 Coordination modes and Zn(II)⋯Zn(II) separation (Å) for zinc
carboxylato-bridged complexes
Bond distance
(Å)
Bond distance
(Å)
Coord.
mode
Zn(II)⋯Zn(II)
Zn(1)–O(22)
Zn(1)–O(1)
Zn(1)–O(42)
Zn(1)⋯Zn(2)
2.044(2)
2.110(3)
2.207(3)
3.515
Zn(2)–O(1M)
Zn(2)–O(41)
Zn(2)–O(2)
2.030(3)
1.913(3)
1.940(3)
1.901(3)
Compound
separation (Å) Ref.
1
2
3
4
[Zn(PTCH)(phen)(H2O)]2
3.519
9a
9b
9c
Zn(2)–O(21)
[Zn2(1,4-BDC)2(IP)], 3D polymer 4.39
[Zn(pht)A]n
3.467(2),
3.470(2)
3.31
Bond angle
(°)
Bond angle
(°)
[Zn2(oNBz)4(AQ)2(H2O)2]
[Zn2L1(μ1,3-OAc)2](ClO4)
[Zn7(μ4-O)2(OAc)10(3pdb)]n
[Zn7(μ4-O)2(OAc)10(4pdb)]n
[Zn2L2(μ1,3-OAc)2](ClO4)
[Zn3(benz)6(nia)2],
3.281
9d
10a
O(1)–Zn(1)–O(22)′
Ο(1)–Zn(1)–O(22)
O(1)–Zn(1)–O(42)′
O(1)–Zn(1)–O(42)
Ο(21)–Zn(2)–Ο(41)
86.02(10)
93.98(10)
83.74(10)
96.26(10)
O(22)–Zn(1)–O(42)
O(22)–Zn(1)–O(42)′ 81.89(10)
Ο(2)–Zn(2)–O(41)
Ο(2)–Zn(2)–O(21)
98.11(10)
3.0016(8)
2.9727(8)
3.281
3.1845(2)
3.347(1)
117.06(12)
123.01(13)
90.95(12)
5
6
9d
10d
10d
11b
11a
This
work
10c
8
113.85(13) Ο(2)–Zn(2)–O(1M)
[Zn3(OAc)6(bppz)2](H2O),
Ο(21)–Zn(2)–O(1M) 95.78(13)
Ο(41)–Zn(2)–O(1M) 108.71(13)
[Zn3(MeCHvCHCO2)6(C9H7N)2] Not available
a Symmetry operation: (′) 2 − x, −y, 1 − z.
[Zn3(O2CCH3)6(py)2]
[Zn3(tolf)6(CH3OH)2]
Not available
3.515
7
8
[Zn(L3)2(MeOH)2]3
[Zn2(Indo)4(py)2],
[Zn2(μ-oNBz)4(AMP)2]
[Zn2(O2CCH3)4(py)2]
4.188
2.969(1)
3.00
vertices of the tetrahedron. The bond angles around Zn(2) lie in
the range 90.95(12)°–123.01(13)°.
9b
11a
11b
2.8921(10)
A thorough review of the literature has revealed the coordi-
nation number and modes of the carboxylato bridges in Zn com-
plexes as given in Table 2. The number of the carboxylato
bridges between Zn atoms varies from 1 to 4 while they are
found as monodentate monoatomic coordinated groups or biden-
tate triatomic bridges as well as in diverse combinations of these
two modes. In the case of one or two carboxylato groups, all the
possible combinations have been reported, while four carboxy-
lato bridges are arranged around Zn atoms according to the
paddle-wheel pattern.
For three carboxylato groups, as found in 1, two possible
arrangements have been observed; one monodentate monoatomic
coordinated carboxylato group and two bidentate triatomic car-
boxylato bridges (mode 6), or three bidentate triatomic carboxy-
lato bridges (coordination mode 7) (Fig. 2). For coordination
mode 6, a series of trinuclear Zn complexes of the general
formula [Zn3(RCOO)6(L)2] (where RCOO and L = benzoate and
nicotinamide;11d MeCHvCHCO2 and C9H7N;21 O2CCH3 and
2,5-bis(2-pyridyl)pyrazine10d or pyridine,11a respectively) have
been reported but 1 is a rare example of the general formula
[Zn2(MeCHvCHCO2)4(C9H7N)2] Not available
a 1,4-H2BDC = 1,4-benzendicarboxylic acid; 3pdb = 1,4-bis(3-pyridyl)-
2,3-diaza-1,3-butadiene; 4pdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene;
A = 4-methylpyridine; AQ = 8-aminoquinoline; AMP = 2-aminopyrimi-
dine; benz = benzoate anion; bppz = 2,5-bis(2-pyridyl)pyrazine; HL3 =
9-acridinecarboxylic acid; IndoH = 1-(4-chlorobenzoyl)-5-methoxy-2-
methyl-1H-indole-3-acetic acid); IP = 1H-imidazo[4,5-f][1,10]-phenan-
throline; L1 = 2,6-bis(N-2-(2′-pyridylethyl)-formimidoyl)4-methylphenol;
L2 = 2,6-bis(N-2-(2′-pyridylethyl)-formimidoyl)4-methylphenol; nia =
nicotinamide; OAc = acetate; oNBz = 2-nitrobenzoato anion; phen = 1,10-
phenanthroline; pht2− = dianion of o-phthalic acid; PTCH3 = 3-propane-
tricarboxylic acid.
[Zn3(RCOO)6(L)2] that contains three bidentate triatomic car-
boxylato bridges per pair of Zn atoms. The distance Zn(1)⋯
Zn(2) (3.515 Å) is the longest Zn⋯Zn distance found in the
trinuclear complexes of this formula (Table 2). Three bidentate
triatomic carboxylato bridge modes have been also reported for
the compound [Zn3(9-acridinecarboxylato)6(MeOH)6] with a
Zn⋯Zn separation of 4.188 Å.10c Although the binding mode of
7084 | Dalton Trans., 2012, 41, 7082–7091
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Fig. 2 Coordination modes for zinc carboxylato-bridged complexes.
Table 3 Selected bond distances and angles for 2
the carboxylato bridges is similar to 1, the geometrical features
around the Zn atoms are significantly different. In [Zn3(9-acridi-
necarboxylato)6(MeOH)6], the central Zn atom exhibits almost
perfect octahedral geometry and the terminal Zn atoms are six-
coordinate, presenting a distorted octahedral geometry;10c in 1
the central Zn presents a distorted octahedral geometry with non-
equal Zn–O distances, while the terminal Zn atoms are four-
coordinate with a distorted tetrahedral geometry. The compound
shows an extended intra- and inter-molecular H-bonding
network creating an infinite chain consisting of trinuclear repeat
units along the crystallographic b-axis (Fig. 1(B), Table S1†).
2A
2B
Bond distance
(Å)
Bond distance
(Å)
Zn(1)–O(21)
Zn(1)–O(22)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–Cl(1)
2.343(2)
2.003(2)
2.095(2)
2.056(2)
2.2292(8)
Zn(2)–O(62)
Zn(2)–O(61)
Zn(2)–N(41)
Zn(2)–N(42)
Zn(2)–Cl(3)
2.184(2)
2.095(2)
2.084(2)
2.082(2)
2.2395(8)
Bond angle
(°)
Bond angle
(°)
O(21)–Zn(1)–O(22)
O(21)–Zn(1)–N(1)
O(21)–Zn(1)–N(2)
O(21)–Zn(1)–Cl(1)
O(22)–Zn(1)–N(1)
Ο(22)–Zn(1)–N(2)
O(22)–Zn(1)–Cl(1)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(1)
59.65(8)
139.97(9)
93.33(8)
105.15(6)
98.74(9)
136.95(9)
110.45(7)
80.74(9)
114.30(7)
108.67(7)
O(61)–Zn(2)–O(62)
O(61)–Zn(2)–N(41)
O(61)–Zn(2)–N(42)
O(61)–Zn(2)–Cl(3)
O(62)–Zn(2)–N(41)
O(62)–Zn(2)–N(42)
O(62)–Zn(2)–Cl(3)
N(41)–Zn(2)–N(42)
N(42)–Zn(2)–Cl(3)
N(41)–Zn(2)–Cl(3)
61.28(8)
148.51(9)
92.79(9)
102.73(7)
102.92(8)
135.28(9)
105.61(6)
80.16(9)
115.88(7)
107.97(7)
Description of the crystal structure of [Zn(tolf)(phen)Cl]
The complex [Zn(tolf)(phen)Cl] 2, is a mononuclear Zn(II)
complex and the tolfenamato ligand behaves as a deprotonated
ligand in the bidentate mode and is coordinated to zinc ions via
two carboxylato oxygen atoms.
There are two crystallographically independent molecules in
the asymmetric unit of 2, denoted as 2A and 2B respectively,
which may be considered as related through a 180° pseudo-
rotation (Fig. S1†). A careful inspection of bond distances and
angles (Table 3) reveals severe differences between 2A and 2B
which along with an apparent rotation of the tolfenamato ligand
around the Ccarboxylato–Cphenyl bond justify their simultaneous
presence in the asymmetric unit of 2. For clarity reasons, only
the molecular structure of 2A is shown in Fig. 3.
(φ1–φ2)/60°, where φ1 and φ2 are the largest angles in the coordi-
nation sphere; τ = 0 for a perfect square pyramid, and τ = 1 for a
perfect trigonal bipyramid] τ = (139.97–136.95)/60 = 0.05 (for
2A) and τ = (148.51–135.28)/60 = 0.22 (for 2B), show slight
distortion from the regular square-based pyramidal geometry for
2A27c and more severe distortion for 2B, justifying the presence
of the two crystallographically independent molecules.
The two carboxylate oxygen atoms Ο(21)/O(22) for 2A and
O(61)/O(62) for 2B of the tolfenamato ligand and the nitrogen
atoms N(1)/N(2) for 2A and N(41)/N(42) for 2B of the phen
The zinc atom is five-coordinate and could be described
as having a distorted square pyramidal geometry. The tetragona-
lity27a T5 = 0.953 (for 2A) and 0.943 (for 2B), based on the
changes in bond lengths, along with the trigonality index27b [τ =
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ligand occupy the four positions in the basal plane around Zn(1)
and Zn(2), respectively, while the Cl atom occupies the apical
position. An arrangement similar to that of the N,N′-donor
ligand has also been observed in [Zn(oxo)(bipy)Cl]·MeOH23a
and in a series of copper compounds, [Cu(L)2(bipy)(H2O)] and
[Cu(L)2(phen)(H2O)] (L = a monodentate phenoxyalkanoato
ligand),28 [Cu(oxolinato)(phen)Cl]·MeOH,29a [Cu(enrofloxaci-
nato)(bipy)(H2O)]Cl,29b [Cu(propyl-norfloxacinato)(bipy)Cl]29c
and [Cu(ciprofloxacinato)(phen)Cl]29d with the Owater or Cl
atoms occupying the apical position, where oxolinic acid,
enrofloxacin, propyl-norfloxacin and ciprofloxacin are bidentate
quinolone antimicrobial drugs. The Zn–N distances, [Zn(1)–
N(1) = 2.095(2) Å and Zn(1)–N(2) = 2.056(2) Å for 2A and
Zn(2)–N(41) = 2.084(2) Å and Zn(2)–N(42) = 2.082(2) Å for 2B]
are almost equal with those found in the five-coordinate com-
plexes [Zn(S2CNMe2)2(py)] (2.079(6) Å),30a [Zn(bipy)2(pybet)]
(ClO4) (2.081(5)–2.140(6) Å, pybet− = pyridinioacetate)30b and
[Zn(bipy)(CC13CO2)2(H2O)] (2.086(2) and 2.151(2) Å).30c
In both 2A and 2B, the ligand atoms forming the basal plane
are almost coplanar (for 2A, N(2) and O(22) lie 0.018 Å and
0.024 Å above the plane towards the apex and O(21) and N(1)
are at 0.023 Å and 0.024 Å below the plane; for 2B, N(41) and
O(61) lay above the plane towards the apex by 0.097 Å and
0.104 Å, respectively, and O(62) and N(42) are at 0.093 Å and
0.108 Å below the plane). The zinc atom is displaced towards
the chlorine atom by 0.72 Å in 2A and 0.66 Å in 2B. The trans
angles in the basal plane of 2B [O(61)–Zn(2)–N(41) = 148.51
(9)° and O(62)–Zn(2)–N(42) = 135.28(9)°] show significantly
different values compared to those in 2A [Ο(21)–Zn(1)–N(1) =
139.97(9)° and Ο(22)–Zn(1)–N(2) = 136.95(9)°], justifying the
more distorted square pyramidal geometry around Zn(2). The N–
Zn–N angle observed is 80.74(9)° and 80.16(9)° for 2A and 2B,
respectively, and is similar to reported values of other chelating
polycyclic diimines.20 The phen ligands are planar with the zinc
atoms almost lying in this plane (largest deviation 0.16 Å for Zn
(1) and 0.09 Å for Zn(2)).
Crystal structure of [Zn(tolf)2(phen)]·Et2O, 4·Et2O and
[Zn(tolf)2(bipy)], 5
Diagrams of 4 and 5 are shown in Fig. 4(A) and 4(B), and
selected bond distances and angles are listed in Table 4. Due to
the similarities of the two structures we describe the structural
features of compound 5 that also reflect analogous discussion of
compound 4. Complex 5 is mononuclear and the tolfenamato
ligands behave as deprotonated ligands coordinated to the zinc
atom via the carboxylato groups which adopt the asymmetric
chelating mode (C(1)–O(1) = 1.254(4) Å and C(1)–O(2) =
1.269(3) Å, C(21)–O(21) = 1.271(3) Å and C(21)–O(22) =
1.262(3) Å).
In 5, the zinc atom is six-coordinate and is surrounded by two
tolfenamato ligands and a bidentate 2,2′-bipyridine ligand
showing a distorted trigonal prismatic geometry, with the six ver-
tices occupied by two nitrogen and four oxygen atoms, giving a
ZnN2O4 chromophore. The plane of each trigonal base of the tri-
gonal prism is formed by two oxygen atoms from the two tolfe-
namato ligands (O(1) and O(21) form participate in base 1 and
O(2) and O(22) in base 2) and a nitrogen atom of bipy (N(41)
and N(42), respectively) (Fig. S2†). The two almost parallel tri-
gonal basal planes form an angle of 5.87° and lie at a distance of
2.197 Å with the zinc atom being closer to base 1 (1.095 Å)
than to base 2 (1.136 Å).
The bond distances around the Zn atom are not equal, with
the carboxylate oxygen atoms O(2) and O(21) being closer to Zn
(Zn–O(2) = 1.971(2) Å and Zn–O(21) = 2.0620(18) Å) than
the bipy nitrogens (Zn–N(41) = 2.088(2) Å and Zn–N(42) =
Fig. 3 The molecular structure and partially labeling of 2A.
Fig. 4 The molecular structures and partial labeling of (A) 4 and (B) 5.
7086 | Dalton Trans., 2012, 41, 7082–7091 This journal is © The Royal Society of Chemistry 2012

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2.099(2) Å). These Zn–O and Zn–N bond distances are relatively
shorter than those found in analogous six-coordinate complexes
[Zn(bipy)2(ONO)](NO2) (2.122(9) Å),31a [Zn(phen)2(MeCO2)]-
(ClO4) (2.100(5)–2.160(5) Å)30b and [Zn(pipdtc)2(bipy)]
(2.196(3) Å, pipdtc− = piperidine–carbodithioato anion).31b The
two longer Zn–Ocarboxylate distances (Zn–O(22) = 2.269(2) Å
and Zn–O(1) = 2.575(2) Å), Zn–O(1) are at the borderline of
bond distances for Zn(II).
diimines.23,24 The bipy ligand is almost planar with the zinc
atom lying ∼0.25 Å out of this plane.
Interaction of the compounds with serum albumins
Serum albumin (SA) is the most abundant protein in blood
plasma. Therefore, it is important to consider the interactions of
drugs with plasma proteins particularly with SA. Binding to
these proteins may lead to loss or enhancement of the biological
properties of the original drug, or provide paths for drug trans-
portation. Bovine serum albumin (BSA) is the most extensively
studied serum albumin, due to its structural homology with
human serum albumin (HSA).19–21 BSA and HSA solutions
exhibit an intense fluorescence emission with a peak at 343 nm
and 351 nm, respectively, due to the tryptophan residues (Trp-
134 and Trp-212 in BSA and Trp-212 in HSA) when excited at
295 nm.19,20 The changes and the quenching occurring in the
fluorescence emission spectra of tryptophan in BSA or HSA
upon addition of complexes 1–5 are primarily due to a change in
protein conformation, subunit association, substrate binding or
denaturation.32a Htolf and complexes 1–5 in buffer solutions
exhibit a maximum emission at 330 nm under the same exper-
imental conditions and the SA fluorescence spectra have been
corrected before the experimental data processing.
The N(41)–Zn–N(42) angle observed is 78.19(9)° and is
similar to reported values of other chelating polycyclic
Table 4 Selected bond distances and angles for 4 and 5
Bond distance
(Å)
Zn–O(1)
Zn–O(2)
{1.966(2)}4 or {1.970(2)}5
{2.555(3)}4 or {2.575(2)}5
{2.083(3)}4 or {2.088(2)}5
{1.287(4)}4 or {1.269(3)}5
{1.249(4)}4 or {1.254(4)}5
{2.478(2)}4 or {2.062(2)}5
{1.973(2)}4 or {2.269(2)}5
{2.088(3)}4 or {2.099(2)}5
{1.261(4)}4 or {1.262(3)}5
{1.273(4)}4 or {1.271(3)}5
Zn–N(41)
C(1)–O(1)
C(1)–O(2)
Zn–O(21)
Zn–O(22)
Zn–N(42)
C(21)–O(21)
C(21)–O(22)
Bond angle
(°)
The quenching provoked by Htolf and complexes 1–5 to the
BSA fluorescence signal at 343 nm is significant (up to 12% of
the initial fluorescence intensity for Htolf, 2.5% for 1, 4% for 2,
25% for 3, 1% for 4 and 10% for 5, Fig. 5(A)) because of poss-
ible changes in protein secondary structure leading to changes in
tryptophan environment of BSA, and thus indicating the binding
of each complex to BSA.32b
The Stern–Volmer and Scatchard graphs may be used in order
to study the interaction of a quencher with serum albumins.
According to the Stern–Volmer quenching equation:33a
Ο(1)–Zn–O(22)
O(1)–Zn–N(41)
O(1)–Zn–N(42)
O(1)–Zn–O(21)
O(1)–Zn–O(2)
O(21)–Zn–O(22)
O(22)–Zn–N(41)
N(41)–Zn–N(42)
O(2)–Zn–N(42)
O(2)–Zn–O(21)
O(2)–Zn–O(22)
O(2)–Zn–N(41)
O(21)–Zn–N(41)
O(21)–Zn–N(42)
O(22)–Zn–N(42)
{113.14(9)}4 or {120.33(7)}5
{123.62(10)}4 or {83.95(8)}5
{107.65(10)}4 or {140.49(8)}5
{92.37(9)}4 or {96.71(7)}5
{56.81(9)}4 or {55.98(7)}5
{57.96(9)}4 or {60.42(7)}5
{104.89(10)}4 or {152.86(9)}5
{79.95(10)}4 or {78.19(9)}5
{145.00(9)}4 or {102.23(9)}5
{123.16(8)}4 or {121.68(8)}5
{89.30(9)}4 or {88.51(8)}5
{84.71(9)}4 or {116.97(9)}5
{143.78(9)}4 or {108.09(9)}5
{85.67(9)}4 or {122.23(8)}5
{124.96(10)}4 or {87.80(8)}5
I0
¼ 1 þ kqτ0½Qꢁ ¼ 1 þ KSV½Qꢁ
ð4Þ
I
a { }4: for compound 4; { }5: for compound 5.
where I0 = the initial tryptophan fluorescence intensity of SA,
I = the tryptophan fluorescence intensity of SA after the addition
Fig. 5 (A) Plot of % relative fluorescence intensity at λem = 343 nm (I/I0%) vs. r (r = [compound]/[BSA]) for Htolf and complexes 1–5 in buffer sol-
ution (150 mM NaCl and 15 mM trisodium citrate at pH 7.0). (B) Plot of % relative fluorescence intensity at λem = 351 nm (I/I0%) vs. r (r = [com-
pound]/[HSA]) for Htolf and complexes 1–5 in buffer solution (150 mM NaCl and 15 mM trisodium citrate at pH 7.0).
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7082–7091 | 7087

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Table 5 The BSA and HSA binding constants and parameters (Ksv, kq, K, n) derived for Htolf and complexes 1–5
Compound
Ksv (M−1
)
kq (M−1 s−1
)
K (M−1
)
n
BSA
HSA
Htolf19
2.18( 0.12) × 105
1.87( 0.06) × 106
9.99( 0.60) × 105
1.15( 0.04) × 105
1.00( 0.01) × 106
3.67( 0.15) × 105
0.61( 0.04) × 105
1.39( 0.05) × 105
1.41( 0.07) × 105
0.86( 0.03) × 105
4.27( 0.28) × 105
2.84( 0.12) × 105
2.18( 0.12) × 1013
1.87( 0.06) × 1014
9.99( 0.60) × 1013
1.15( 0.04) × 1013
1.00( 0.01) × 1014
3.67( 0.15) × 1013
0.61( 0.04) × 1013
1.39( 0.05) × 1013
1.41( 0.07) × 1013
0.86( 0.03) × 1013
4.27( 0.28) × 1013
2.84( 0.12) × 1013
1.60 × 105
2.07 × 105
5.70 × 104
1.81 × 105
1.88 × 105
6.20 × 104
3.12 × 105
4.12 × 105
4.37 × 105
1.43 × 105
1.36 × 105
1.49 × 105
1.11
1.44
2.09
0.84
1.31
1.70
0.63
0.79
0.65
0.86
1.20
1.16
[Zn3(tolf)6(CH3OH)2]
[Zn(tolf)(phen)Cl]
[Zn(tolf)(bipy)Cl]
[Zn(tolf)2(phen)]
[Zn(tolf)2(bipy)]
Htolf19
[Zn3(tolf)6(CH3OH)2]
[Zn(tolf)(phen)Cl]
[Zn(tolf)(bipy)Cl]
[Zn(tolf)2(phen)]
[Zn(tolf)2(bipy)]
of the quencher, kq = the quenching rate constants of SA, KSV
=
s−1) suggesting a static quenching mechanism. From the Scatch-
ard graph (Fig. S7†) and eqn (6), the association binding con-
stant of each compound has been calculated (Table 5) with
complexes 2 and 1 exhibiting the highest K values. A compari-
son of the n values shows that the n value of Htolf increases
when it is coordinated to Zn(II) in complexes 1–5.
the dynamic quenching constant, τo = the average lifetime of
SA without the quencher, [Q] = the concentration of the
quencher respectively,
KSV ¼ kqτo
ð5Þ
Comparing the affinity of the compounds for BSA and HSA
(K values), it can be concluded (Table 5) that complexes 3 and 4
show higher affinity for BSA than HSA, while Htolf and com-
plexes 1, 2 and 5 exhibit higher binding constants for HSA than
for BSA. Considering that the binding constant (K) between a
compound and BSA/HSA should be at an optimum and
less than one of the highest protein–ligand binding affinity
(Kavidin–ligands) ≈ 1015 M−1 observed, it is obvious that K values,
between 5.70 × 104 and 4.37 × 105 (Table 5) are quite below this
value suggesting a possible release from the serum albumin to
the target cells. If K is too high, the compound won’t get
released from the SA to the target cells.34 Therefore, the inter-
action of tolfenamic acid and its complexes with SA provides
useful information concerning their potential application.
and, taking as fluorescence lifetime (τo) of tryptophan in SA at
around 10−8 s,33b the dynamic quenching constant (KSV, M−1
can be obtained by the slope of the diagram I0/I vs. [Q], and sub-
sequently the approximate quenching constant (kq, M−1 s−1
)
)
may be calculated. The values of Ksv and kq for the interaction of
Htolf and complexes 1–5 with BSA have been calculated from
(Fig. S4†) according to eqn (5). They are given in Table 5 and
indicate good BSA binding propensity of the complexes with 1
exhibiting the highest quenching ability (kq = 1.87( 0.06) × 1014
M−1 s−1). The kq values (>1013 M−1 s−1) are higher than diverse
kinds of quenchers for biopolymer fluorescence (2.0 × 1010 M−1
s−1) indicating the existence of static quenching mechanism.32
Using the Scatchard equation:33
In relation to previously reported Cu(II)20a and Co(II) mefena-
mato20b as well as the Co(II)-tolfenamato19 complexes, the
Zn(II)-tolfenamato complexes 1–5 present similar or lower
K values for BSA and similar or higher K values for HSA.
ΔI=I0
½Qꢁ
ΔI
¼ nK ꢀ K
ð6Þ
I0
where n is the number of binding sites per albumin and K is the
association binding constant, K (M−1), may be calculated from
the slope in plots (ΔI/I0)/[Q] versus ΔI/I0 and n is given by the
ratio of the y intercept to the slope.33a K and values for Htolf and
complexes 1–5 have been obtained from the corresponding
Scatchard plots (Fig. S5†) and are given in Table 5. It is obvious
that Htolf exhibits a lower K value than its complexes,
suggesting that its coordination to Zn(II) results in an increased
affinity for BSA. Additionally, the n value of Htolf increases
when it is coordinated to Zn(II).
Addition of Htolf or complexes 1–5 to HSA leads to a moder-
ate decrease of the fluorescence signal at 351 nm (Fig. 5(B)) (up
to 44% of the initial fluorescence intensity for Htolf, 23% for
complexes 1 and 2, 33% for 3, 10% for 4 and 14% for 5) which
indicates that their binding to HSA quenches the intrinsic fluor-
escence of the single tryptophan in HSA.32
Conclusions
The synthesis and characterization of the neutral zinc(II) com-
plexes with the non-steroidal anti-inflammatory drug tolfenamic
in the absence or presence of the N,N′-donor heterocyclic
ligands 2,2′-bipyridine or 1,10-phenanthroline has been
achieved. The interaction of Zn(II) with the non-steroidal anti-
inflammatory drug tolfenamic acid in the absence of the N,N′-
donor ligands leads to the formation of the trinuclear Zn
complex [Zn3(tolfenamato)6(CH3OH)2] where all the tolfena-
mato ligands act as bidentate ligands forming three asymmetric
triatomic carboxylato bridges between the central octahedral Zn
and each terminal tetrahedral Zn atom. This arrangement of the
carboxylato bridging ligands around the central Zn atom is
unique for complexes bearing the formula [Zn3(carboxylato)6-
(ligand)2]. Of course it must be noted that the existence of three
symmetric triatomic carboxylato bridges between Zn atoms has
been reported once in a complex of the type [Zn3(carboxylato)6-
(CH3OH)6] where all the Zn atoms are octahedral.
The calculated values of KSV and kq as obtained by the slope
of the Stern–Volmer diagram (Fig. S6†), eqn (4) and (5), Htolf
and 1–5 are given in Table 5 and indicate their good HSA
binding propensity and complex 4 exhibits the highest quenching
ability. The kq values (>1012 M−1 s−1) are higher than diverse
kinds of quenchers for biopolymer fluorescence (2.0 × 1010 M−1
7088 | Dalton Trans., 2012, 41, 7082–7091
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[Zn(tolf)(phen)Cl], 2 and [Zn(tolf)(bipy)Cl], 3. Tolfenamic
acid (0.4 mmol, 105 mg) was dissolved in methanol (15 mL) fol-
lowed by the addition of KOH (0.4 mmol, 22 mg). After 1 h stir-
ring, the solution was added slowly and simultaneously with a
methanolic solution of phen (0.4 mmol, 72 mg) or bipy
(0.4 mmol, 62 mg) to a methanolic solution (10 mL) of ZnCl2
(0.4 mmol, 54 mg) and stirred for 30 min. The solutions were
left for slow evaporation. Pale yellow crystals of [Zn(tolf)-
(phen)Cl], 2 (120 mg, 57%) suitable for X-ray structure deter-
The interaction of equimolar quantities of ZnCl2, tolfenamato
and N,N′-donor ligand results in the formation of complexes of
the formula [Zn(tolfenamato)(N,N′-donor)Cl], while when
double molar quantity of the tolfenamato ligand is used, com-
plexes of the formula [Zn(tolfenamato)2(N,N′-donor)] have been
isolated. The crystal structures of the complexes [Zn3(tolf)6-
(CH3OH)2] 1, [Zn(tolf)(phen)Cl] 2, [Zn(tolf)2-(phen)]·Et2O,
4·Et2O and [Zn(tolf)2(bipy)], 5 have been determined by X-ray
crystallography. Comparing the affinity of the compounds for
BSA and HSA (K values), it can be concluded that complexes 3
and 4 show a higher affinity for BSA than HSA, while Htolf and
complexes 1, 2 and 5 exhibit higher binding constants for HSA
than for BSA. Considering that the binding constant (K)
between a compound and BSA/HSA should be at an optimum
and less than one of the highest protein–ligand binding affinities
(Kavidin–ligands) ≈ 1015 M−1 observed, it is obvious that the K
values observed are quite below suggesting a possible release
from the serum albumin to the target cells. If K is too high, the
compound won’t get released from the SA to the target cells. There-
fore, the interaction of tolfenamic acid and its complexes with SA
provide useful information concerning their potential application.
mination or microcrystalline product [Zn(tolf)(bipy)Cl],
(140 mg, 70%) were deposited after a few days.
3
[Zn(tolf)(phen)Cl], 2 (Found C, 57.54; H, 3.80; N, 7.47;
C26H19Cl2N3O2Zn (MW = 541.71) requires C, 57.65; H, 3.54;
N, 7.76%). IR: νmax/cm−1; νasym(CO2): 1578 (vs); νsym(CO2):
1375 (vs); Δ = 203 cm−1 (KBr disk); UV-vis: λ/nm (ε/M−1
cm−1) as nujol mull: 340, 307(sh); in DMSO: 339(sh) (2150),
309 (13 400). Soluble in DMSO (ΛM = 1 mho cm2 mol−1, 1 mM
in DMSO), DMF, CH3CN, ethanol and CHCl3.
[Zn(tolf)(bipy)Cl], 3 (Found C, 55.40; H, 3.83; N, 7.95;
C24H19Cl2N3O2Zn (MW = 517.72) requires C, 55.08; H, 3.70;
N 8.12%). IR: νmax/cm−1; νasym(CO2): 1582 (vs); νsym(CO2):
1397 (vs); Δ = 185 cm−1 (KBr disk); UV-vis: λ/nm (ε/M−1
cm−1) as nujol mull: 336(sh), 306; in DMSO: 337(sh) (4900),
Experimental
304(sh) (15 400). Soluble in DMSO (ΛM = 7 mho cm2 mol−1
,
1 mM in DMSO), DMF and ethanol.
Materials: instrumentation and physical measurements
Tolfenamic acid, ZnCl2, Zn(NO3)2·6H2O, bipy, phen, KOH, tri-
sodium citrate, NaCl, BSA and HSA were purchased from
Sigma-Aldrich Co. and all solvents were purchased from Merck.
All the chemicals and solvents were reagent grade and were used
as purchased.
[Zn(tolf)2(phen)]·Et2O, 4·Et2O. The complex was prepared
by the addition of a methanolic solution (20 mL) of Htolf
(0.4 mmol, 105 mg) and KOH (0.4 mmol, 22 mg), after 30 min
of stirring, to a methanolic solution (10 mL) of Zn(NO3)2·6H2O
(0.2 mmol, 59 mg) followed by the addition of 20 mL of diethyl-
ether and the solution was stirred for 30 min. The resultant pre-
cipitate of KNO3 was removed by filtration. In the solution, a
methanolic solution (5 mL) of phen (0.2 mmol, 36 mg) was
added and the mixture was left for slow evaporation. Colorless
crystals of [Zn(tolf)2(phen)]·Et2O, 4·Et2O (97 mg, 65%) suit-
able for X-ray structure determination, were deposited after a
week. (Found C, 62.53; H, 4.88; N, 7.05; C44H40Cl2N4O5Zn
Infrared (IR) spectra (400–4000 cm−1) were recorded on a
Nicolet FT-IR 6700 spectrometer with samples prepared as KBr
pellets. UV-visible (UV-vis) spectra were recorded as nujol mulls
and in solution at concentrations in the range 10−5–10−3 M on a
Hitachi U-2001 dual beam spectrophotometer. C, H and N
elemental analysis were performed on a Perkin-Elmer 240B
elemental analyzer. Molar conductivity measurements were carried
out in 1 mM DMSO solution of the complexes with a Crison
Basic 30 conductometer. Fluorescence spectra were recorded in
solution on a Hitachi F-7000 fluorescence spectrophotometer.
(MW = 841.07) requires C, 62.83; H, 4.79; N, 6.66%). IR: νmax
/
cm−1; νasym(CO2): 1583 (vs); νsym(CO2): 1387 (vs); Δ =
196 cm−1 (KBr disk); UV-vis: λ/nm (ε/M−1 cm−1) as nujol
mull: 337, 303(sh); in DMSO: 335(sh) (3100), 305 (18 500).
Soluble in DMSO (ΛM = 8 mho cm2 mol−1, 1 mM in DMSO),
DMF, CH3CN, ethanol and CHCl3.
Synthesis of the complexes
[Zn3(tolf)6(CH3OH)2], 1. A methanolic solution (20 mL) con-
taining tolfenamic acid (1 mmol, 260 mg) and KOH (1 mmol,
56 mg) was stirred for 1 h and then was added dropwise to a
methanolic solution (10 mL) of ZnCl2 (0.5 mmol, 68 mg). The
reaction mixture was stirred for 1 h, filtered and left for slow
evaporation. Colorless crystals of [Zn3(tolf)6(CH3OH)2], 1
(215 mg, 65%) suitable for X-ray structure determination were
deposited after a few days. (Found C, 56.28; H, 4.28; N, 4.38;
C86H74Cl6N6O14Zn3 (MW = 1824.32) requires C, 56.62; H,
4.09; N, 4.61%). IR: νmax/cm−1 νasym(CO2), 1582 (vs);
νsym(CO2), 1397 (vs); Δ = νasym(CO2)–νsym(CO2): 185 cm−1
(KBr disk); UV-vis: λ/nm (ε/M−1 cm−1) as nujol mull: 339, 305
(sh(shoulder)); in DMSO: 337(sh) (3400), 303 (15 700). Soluble
in DMSO (ΛM = 4 mho cm2 mol−1, 1 mM in DMSO), DMF,
CH3CN and ethanol.
[Zn(tolf)2(bipy)], 5. A methanolic solution (10 mL) of tolfe-
namic acid (0.4 mmol, 105 mg) and KOH (0.4 mmol, 22 mg)
after 1 h stirring was added dropwise slowly and simultaneously
with a methanolic solution (10 mL) of bipy (0.2 mmol, 31 mg)
to a methanolic solution (10 mL) of ZnCl2 (0.2 mmol, 27 mg).
The resultant solution was left for slow evaporation. Colorless
crystals of [Zn(tolf)2(bipy)], 5 (90 mg, 60%) suitable for X-ray
structure determination, were collected after a few days. (Found
C, 61.05; H, 4.25; N, 7.42; C38H30Cl2N4O4Zn (MW = 742.93)
requires C, 61.43; H, 4.07; N, 7.54%). IR: νmax/cm−1
;
νasym(CO2): 1583 (vs); νsym(CO2): 1394 (vs); Δ = 189 cm−1
(KBr disk); UV-vis: λ/nm (ε/M−1 cm−1) as nujol mull: 333, 304
(sh); in DMSO: 329(sh) (5100), 307(sh) (6400). Soluble in
DMSO (ΛM = 3 mho cm2 mol−1, 1 mM in DMSO) and DMF.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7082–7091 | 7089

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Table 6 Crystallographic data for complexes 1, 2, 4·Et2O and 5
1
2
4·Et2O
5
Formula
Fw
T (Κ)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
86H74Cl6N6O14Zn3
C26H19Cl2N3O2Zn
541.71
160(2) K
Monoclinic
P21/n
9.2336(1)
16.8821(3)
30.6278(5)
90.00
95.596(1)
90.00
C44H40Cl2N4O5Zn
841.07
160(2) K
Monoclinic
P21/c
12.9035(2)
22.1385(4)
14.6228(2)
90.00
108.377(1)
90.00
C38H30Cl2N4O4Zn
742.93
180(2) K
1824.32
160(2) K
Monoclinic
P21/n
14.3955(2)
11.1192(2)
24.9784(4)
90.00
97.750(1)
90.00
3961.7(1)
2
Triclinic
ˉ
P1
10.4238(2)
10.7481(2)
15.3980(3)
102.937(1)
99.004(1)
92.532(1)
1654.99(5)
2
γ (°)
Volume (Å3)
Z
4751.6(1)
8
1.514
3964.2(1)
4
1.409
D(calc), Mg m−3
Abs. coef., μ, mm−1
GOF on F2
R1 =
1.529
3.503
1.491
2.914
3.746
2.519
1.054
1.034
1.134
1.166
0.0609a
0.1335a
0.0429b
0.1083b
0.0531c
0.1429c
0.0420d
0.1124d
wR2 =
a 4420 reflections with I > 2σ(I); b 7291 reflections with I > 2σ(I); c 5119 reflections with I > 2σ(I); d 4260 reflections with I > 2σ(I).
e Å−3; R1/wR2 (for all data), 0.0463/0.1109. Further experimen-
tal crystallographic details for 4·Et2O: 2θmax = 130°; reflections
collected/unique/used, 27 185/6546 [Rint = 0.0493]/6546; 613
parameters refined; (Δ/σ)max = 0.003; (Δρ)max/(Δρ)min = 1.670/
−0.599 e Å−3; R1/wR2 (for all data), 0.0653/0.1562. Further
experimental crystallographic details for 5: 2θmax = 130°; reflec-
tions collected/unique/used, 19 749/5367 [Rint = 0.0272]/5367;
Albumin binding experiments
The protein binding study was performed by tryptophan fluor-
escence quenching experiments using bovine (BSA, 3 μM) or
human serum albumin (HSA, 3 μM) in buffer (containing
15 mM trisodium citrate and 150 mM NaCl at pH 7.0). The
quenching of the emission intensity of tryptophan residues of
BSA at 343 nm or HSA at 351 nm was monitored using Htolf or
complexes 1–5 as quenchers with increasing concentration.19–21
Fluorescence spectra were recorded from 300 to 500 nm at an
excitation wavelength of 295 nm. The fluorescence spectra of
Htolf and complexes 1–5 in buffer solutions were recorded
537 parameters refined; (Δ/σ)max = 0.002; (Δρ)max/(Δρ)min
=
0.525/−0.451 e Å−3; R1/wR2 (for all data), 0.0521/0.1213. Hydro-
gen atoms were located by difference maps and were refined isotro-
pically or were introduced at calculated positions as riding on
bonded atoms. All non-hydrogen atoms were refined anisotropically.
under the same experimental conditions and exhibited
a
maximum emission at 330 nm. Therefore, the quantitative studies
of the serum albumin fluorescence spectra were performed after
their correction by subtracting the spectra of the compounds.
Abbreviations:
bipy
2,2′-bipyridine
BSA
COX
DMF
bovine serum albumin
cyclo-oxygenase
N,N-dimethylformamide
X-Ray crystal structure determination
A crystal of 1 (0.05 × 0.14 × 0.18 mm), 2 (0.12 × 0.44 ×
0.49 mm), 4 (0.09 × 0.11 × 0.58 mm) and 5 (0.07 × 0.25 ×
0.40 mm) was taken from the mother liquor and immediately
cooled to −113 °C (for 1, 2 and 4) and to −93 °C (for 5). Dif-
fraction measurements were made on a Rigaku R-AXIS SPIDER
Image Plate diffractometer using graphite monochromated Cu
Kα radiation (Table 6). Data collection (ω-scans) and processing
(cell refinement, data reduction and empirical absorption correc-
tion) were performed using the CrystalClear program package.35
The structures were solved by direct methods using36
SHELXS-97 and refined by full-matrix least-squares techniques
on F2 with SHELXL-97.37 Further experimental crystallographic
details for 1: 2θmax = 130°; reflections collected/unique/used,
26 010/6453 [Rint = 0.0896]/6453; 623 parameters refined;
DMSO dimethylsulfoxide
Et2O
HSA
Htolf
diethylether
human serum albumin
Tolfenamic acid
= 2-[(3-Chloro-2-methylphenyl)-
amino]benzoic acid
NSAID non-steroidal anti-inflammatory drug
phen
SA
sh
1,10-phenanthroline
serum albumin
shoulder
vs
very strong
Acknowledgements
This research has been co-financed by the European Union
(European Social Fund-ESF) and Greek national funds through
the Operational Program “Education and Lifelong Learning” of
the National Strategic Reference Framework (NSRF) – Research
Funding Program: Heracleitus II. Investing in knowledge society
through the European Social Fund.
(Δ/σ)max = 0.001; (Δρ)max/(Δρ)min = 0.755/−1.273 e Å−3
;
R1/wR2 (for all data), 0.0863/0.1512. Further experimental crys-
tallographic details for 2: 2θmax = 130°; reflections collected/
unique/used, 33 486/8014 [Rint = 0.0439]/8014; 765 parameters
refined; (Δ/σ)max = 0.001; (Δρ)max/(Δρ)min = 1.232/−1.319
7090 | Dalton Trans., 2012, 41, 7082–7091
This journal is © The Royal Society of Chemistry 2012

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22 (a) S. Fountoulaki, F. Perdih, I. Turel, D. P. Kessissoglou and G. Psomas,
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A. Papakyriakou, Y. Sanakis, N. Katsaros and G. Psomas, J. Inorg.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7082–7091 | 7091