Synthesis and structure of dimeric anthracene-9-carboxylato bridged dinuclear erbium(III) complex, [Er2(9-AC)6(DMF)2(H2O)2]

Article abstract of DOI:10.1016/j.saa.2008.12.011

We study the influence of the bulky aromatic rings, e.g. anthracence-9-carboxylic acid (9-ACA) with a large conjugated π-system on the structure and spectroscopic properties of [Er₂(9-AC)₆(DMF)₂(H₂O)₂

Full text of DOI:10.1016/j.saa.2008.12.011

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Spectrochimica Acta Part A 72 (2009) 884–889
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and structure of dimeric anthracene-9-carboxylato bridged dinuclear
erbium(III) complex, [Er2(9-AC)6(DMF)2(H2O)2]
Eny Kusrinia,∗, Rohana Adnanb, Muhammad I. Salehb, Lim-Kong Yanb, Hoong-Kun Func
a Faculty of Pharmacy, University of Pancasila, Srengseng Sawah, Jagakarsa, 12640 Jakarta, Indonesia
b School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
c School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
We study the influence of the bulky aromatic rings, e.g. anthracence-9-carboxylic acid (9-ACA) with a
large conjugated ␲-system on the structure and spectroscopic properties of [Er2(9-AC)6(DMF)2(H2O)2]
complex where 9-AC = anthracence-9-carboxylato and DMF = N,Nꢀ-dimethylformamide. The complex has
been prepared from the erbium chloride and 9-ACA in the mixture of H2O:DMF solution (4:1, v/v) fol-
lowed by pH adjustment to 6. The complex is crystallized in a monoclinic system with space group P21/n.
The two Er(III) ions are double bridged by the deprotonated carboxyl groups of two 9-AC anions (O1 and
O1A), forming an eight-coordination number. The chelating bidentate (O,O), chelating–bridging triden-
tate (O,O,Oꢀ) and monodentate of 9-AC anions are observed in the dinuclear [Er2(9-AC)6(DMF)2(H2O)2]
complex. The Er–Er distance is 4.015 Å in the dimeric unit. Intramolecular O–H· · ·O and C–H· · ·O hydrogen
bonds as well as numerous of intermolecular C–H· · ·␲ interactions between the anthracene rings by edge-
to-face interactions linked the dinuclear dimeric units into two-dimensional supramolecular network in
a propeller-arrangement. Electronic absorption spectra of the Er(III) complex and its salt were measured.
The emission spectrum of the complex is composed of a broad band due to the emission of intraligand
␲* → ␲ transition from the 9-AC anions and a shoulder peak originating from the 4f–4f emission transi-
tion of the Er(III) ions. The complex has a high thermal stability which can be attributed to the effectively
increase the rigidity of the 9-AC anions.
Received 2 July 2008
Received in revised form
14 November 2008
Accepted 8 December 2008
Keywords:
Anthracence-9-carboxylic acid
Crystal structure
Dimeric complex
Erbium
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
f–f transitions, has a small molar absorption coefficient in the
UV–vis–near infrared (NIR) spectrum, which makes it unsuitable
Exploration for new materials, an important step in solid-state
chemistry, has led to the discovery of lanthanide complexes with
diverse properties and to better understanding of structure, bond-
ing, and what is possible in the solid state. The lanthanide ions
had a special electronic and spectroscopic properties mainly asso-
ciated with their 4f electronic configuration, represent a unique
series of metals in the periodic table. The paramagnetic and optical
properties of Ln(III) ions, their hard acid character and their high
coordination numbers make Ln(III) complexes attractive for appli-
cations such as in luminescence studies [1,2], magnetic resonance
imaging contrast agents [3], mild reagents and catalysts in organic
synthesis [4], molecular magnetic materials [5].
for pumping efficiency. This is contrast to the organic chromophore
ligand usually has a molar absorption coefficients 3–5 orders of
magnitude larger [9,10]. To achieve an efficient energy transfer, a
matching is required between a triplet state of the organic sensi-
tizer and the erbium levels corresponding to low-energy transitions
of the Er(III) ion, i.e. 4F7/2 (490 nm), 2H11/2 (520 nm), 4S3/2 (545 nm),
4F9/2 (650 nm) or I9/2 (800 nm) [8]. The suitability of the energy
4
gap between the lowest triplet state energy level of the ligand T1
(L) and the lowest excited energy level of Ln(III) is a critical factor for
sensitized luminescence of the central Ln(III) ion [11]. The energy
transfer process can be realized either through direct coordination
of a sensitizer ligand.
The study of possibility the Er(III) emission for planar opti-
cal amplifiers operating at 1500 nm has recently attracted much
attention [6–8]. However, the Er(III) ion has an intrinsically low
miscibility in silica, and due to the forbidden character of its intra-
The Ln(III) ions formed various complexes in which higher coor-
dination numbers, usually greater than 6 and often as high as 9–10,
appear to dominate [12–14]. Coordination of the erbium complexes
have been reported to be between 8 and 9 as shown by [Er(Pic)
2(TTD)(H2O)][Pic]·CH3CN [15], [Er(Pic)2(EO5)][Pic] [16], [Er(NCS)3
{(py)2C(OCH3)(OH)3}] [17], [Er(L1)2(H2O)2]n[ClO4]3n·nH2O [18],
[Er(L2)2(H2O)]n[ClO4]3n·nH2O [18], [Er2(Acc6)4(H2O)8](CIO4)6·
(H2O)11 [19], [(hexacyclen)Er(␮-OH)2Er(hexacyclen)]·(CF3SO3)4
∗
Corresponding author. Tel.: +62 21 864727x562; fax: +62 21 864727.
E-mail address: eny2kus@yahoo.com (E. Kusrini).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.12.011

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[20], [Er2(anth)6(H2O)4]·2H2O [21] and [Er2(NCS)3{(py)2C(OCH3)
O}3(CH3OH)] [19] where TTD = N,N,Nꢀ,Nꢀ-tetraphenyl-3,6,9-trioxa-
undecanediamide, Pic = picrate anion, EO5 = pentaethylene glycol,
(py)2CO = di-2-pyridyl ketone, L1 = N,Nꢀ-dicarboxymethyl-N,N,Nꢀ,N
was filtered using ordinary grade filter paper and allowed to evap-
orate at ambient temperature. The mixture was left to stand for
several days at room temperature and the yellowish orange crys-
tals were obtained after several days with 46.2% yield suitable for
X-ray diffraction determination. Anal. Calc. for C96H72Er2N2O16 : C,
62.40; H, 4.01; N, 1.52. Found: C, 62.45; H, 3.19; N, 1.46%. Significant
peaks in IR (KBr) cm−1: ꢀ(OH) 3418, ꢀ(C O) 1661, ꢀa(COO) 1552,
ꢀs(COO) 1389, ꢀa(C–N) 1114, and ꢀs(C–N) 853.
ꢀ-tetramethyl-l,3-propanediammonium,
L2 = N,Nꢀ-dicarboxy-
Acc6:
methyl-N,N,Nꢀ,Nꢀ-tetramethyl-1,4-butanediammonium,
1-aminocyclohexane-1-carboxylic acid, hexacyclen = 1,4,7,10,13,
16hexaazacyclooctadecane and anth = anthranilate.
Exploratory research in lanthanide–carboxylate complex has
been pursued with the principal objective of expanding the
limitation for the either relatively electron-poor or metal-rich
systems. The monocarboxylic acid ligand with bulky aromatic
skeletons, e.g. anthracene or naphthalene ring to construct
functional the Ln(III)–carboxylate complexes have not yet been
formally described. Some of the transition metal complexes
with anthracene-9-carboxylic acid (9-ACA) have been reported to
construct a series of complexes having mononuclear, dinuclear,
tetranuclear structures [22–25].
The coordination site of the 9-ACA ligand and benzoic acid are
similar because both the ligands have one carboxylic group in the
same position. They are different only due to the bulk of the aro-
matic skeleton of the 9-ACA ligand. To further explore the influence
of the bulky aromatic ring skeleton, e.g. 9-ACA with a large con-
jugated ␲-system on the structure and spectroscopic properties of
its complex in this paper, we prepared the erbium complex with
the 9-ACA ligand in the mixture of H2O:DMF solution. An analogy
of benzoic acid, by taking the advantage of its carboxylate bridging
coordination ability together with the steric bulk of its anthracene
rings from the 9-ACA ligand. The derivatives of anthracene are more
considerable in the preparation of luminescence polymers, pho-
todimerization, and molecular orientation in organic film growth
for in synthesis of non-linear optical materials [26–29].
2.4. Physical measurements
The percentages of carbon, hydrogen, and nitrogen were deter-
mined by using a Perkin-Elmer 2400II elemental analyzer. IR
spectra were recorded on a Perkin-Elmer 2000 FTIR spectropho-
tometer in the region of 4000–400 cm−1 by using the conventional
KBr pellet method for solid samples. For liquid sample, e.g. DMF, a
thin layer of sample was applied to the surface of a KRS-5 (Thallium
bromoiodide). Thermogravimetric analyses (TGA) were performed
on a Perkin-Elmer TGA-7 series thermal analyzer under a nitrogen
atmosphere, with a heating rate of 20 ◦C/min. The UV–vis spectra
in the solid state were determined by using a Perkin-Elmer Lambda
35 UV–vis spectrophotometer.
Photoluminescence (PL) measurements were made at room
temperature by using a Jobin Yvon HR800UV system. The data were
collected and processed with Labspec Version 4 software. An HeCd
laser was used for excitation at 325 nm, and the emission spectra
were scanned from 330 to 1000 nm. An incident laser (20 mW) was
used as the excitation source. A microscope objective lens (UV40×)
was used to focus the laser on the sample surface. The emitted
light was dispersed by a double grating monochromator (0.8 m
focal length) equipped with an 1800 groove/mm holographic plane
grating. Signals were detected with a Peltier-cooled CCD4 array
detector.
2. Experimental
2.5. X-ray crystallographic study
2.1. Materials
X-ray diffraction data was collected from single crystal by
using a Bruker APEX2 area-detector diffractometer with a graphite
monochromated Mo-K␣ radiation source and a detector distance of
5 cm. Data were processed using APEX2 software [30]. The collected
data were reduced by using the SAINT program and the empiri-
cal absorption corrections were applied with the SADABS program
[30]. The structures were solved by direct methods and refined by
least-squares method on F2obs using the SHELXTL program [31]. All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were located from different Fourier maps and were isotropically
refined. The final refinement converged well. Data for publication
were prepared with SHELXTL [31] and PLATON [32].
All chemicals and solvents were of analytical grade and
used without further purification. 9-ACA (C15H10O2, 98% purity)
was purchased from Merck (Germany). Er2O3 (99.9% purity)
was purchased from Sigma–Aldrich (Steinheim, Germany). N,Nꢀ-
Dimethylformamide (DMF) (99% purity) and HCl (37%, v/v) were
purchased from Fisher (USA).
2.2. Preparation of the sodium salt of ACA ligand and ErCl3·6H2O
The sodium salt of 9-ACA ligand was synthesized by adding
equivalent mole in 1:1 ratio of 9-ACA (445 mg, 2 mmol) and NaOH
(80 mg, 2 mmol) in distilled water. The aqueous suspensions of acid
and NaOH were continuously heated at 70 ◦C in water bath and
stirred vigorously for 30 min. The yellow solution was evaporated
until dryness. The product was kept in dessicator filled with blue
indicative silica gel before analysis.
3. Results and discussion
3.1. Preparation and IR analysis
ErCl3·6H2O was prepared by dissolving its oxide (191.3 mg,
0.5 mmol) in the mixture of hydrochloric acid:distilled water (1:5,
v/v), followed by drying.
Analytical data for the newly synthesized complex indicates that
the complex is consistent with a 2:6 (metal-to-ligand) composi-
tion, [Er2(9-AC)6(DMF)2(H2O)2]. The complex is insoluble in water
and other common organic solvents. A small amount of DMF in the
mixture helped to complex form more readily.
2.3. Preparation of the [Er2(9-AC)6(DMF)2(H2O)2] complex
The IR spectrum of the 9-ACA ligand had a strong antisymmet-
ric stretching mode from ꢀ(C O) at 1682 cm−1. The sodium salt of
9-ACA ligand had strong absorption bands from the asymmetric
and symmetric stretching vibration bands of the carboxyl groups at
1538 and 1323 cm−1, respectively. The aromatic ꢀ(C–H) stretching
and the out-of-plane C–H bending vibrations of substituted ben-
zenes near at 3030 and 892 cm−1, respectively. The DMF molecule
A mixture of the sodium salt of 9-ACA (666.7 mg, 3 mmol) and
ErCl3·6H2O (1 mmol) was dissolved in 20 mL deionized water and
added 5 mL of DMF with slowly constant stirring. The pH = 6.0 of
the solution was adjusted by using 5 M NaOH. After a clear solu-
tion was obtained, the mixture solution was heated for 20 min in
water bath at temperature of 70 ◦C. The resulting warm solution

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E. Kusrini et al. / Spectrochimica Acta Part A 72 (2009) 884–889
had a strong absorption band ꢀ(C O) at 1632 cm−1 and the ꢀ(C–H)
Table 1
Crystal data and structure refinement for the Er2(9-AC)6(DMF)2(H2O)2 complex.
for methylene group at 2930 cm−1
.
Upon complexation, the typically absorption stretching bands
were shifted towards the free sodium salt of 9-ACA and DMF lig-
ands. The asymmetric and symmetric stretching vibration band of
COO− group is shifting to the higher frequency by ∼14 and 66 cm−1
after the formation of the complex. The absorption stretching band
ꢀ(C O) is shifted to lower frequency by ∼21 cm−1 in the com-
plex. This shifting is due to the ꢀ(C O) group from the 9-AC ligand
involved in the coordination with the Er(III) ion and hydrogen bond-
ing occur too. The peak positions of antisymmetric and symmetric
stretching modes of COO− group as well as its shifts with respect to
the frequency of the acidic form C O strongly depend on the central
metal ion-carboxylate coordination [8].
The ionic character for chelation between the Er(III) ion and
the carboxyl group in the complex was shown in the difference
in frequency between the asymmetric and symmetric stretching
vibration (ꢁ = ꢀa − ꢀs) of the carboxyl groups is 163 cm−1. The dif-
ference ꢁ also indicates that the carboxyl group is coordinated to
Er(III) as bidentate or bridging modes.
Parameter
Compound
Formula
Molecular weight
Temperature
C96H72Er2N2O16
1844.08
100 K
Crystal system, space group
Monoclinic, P21/n
Unit cell dimensions
a (Å)
b (Å)
13.9387 (4)
13.6552 (4)
20.9439 (6)
90
101.2470(10)
3909.8(2)
2
1.566
2.205
1852
c (Å)
˛ = ꢂ (◦)
ˇ (◦)
Volume (Å3)
Z
Dx (g/cm3)
ꢃ (mm−1
F(0 0 0)
)
Crystal size (mm)
0.36 × 0.24 × 0.23
ꢄ (◦)
2.11–32.50
h, k, l
−18/21, −20/20, −31/31
41,043/13,780 [R(int) = 0.0367]
97.4
Reflections collected/unique
Completeness to theta = 32.50 (%)
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2ꢅ(I)]
R indices (all data)
Full-matrix least-squares on F2
13,780/2/531
3.2. X-ray studies
1.032
R1 = 0.0282, wR2 = 0.0660
R1 = 0.0345, wR2 = 0.0695
1.383, −0.684
The [Er2(9-AC)6(DMF)2(H2O)2] complex is crystallized in mono-
clinic system with space group P21/n (Table 1). In complex studied,
the carboxyl groups of the six 9-ACA ligands are all deprotonated
and adopt three kinds of coordination modes, i.e. monodentate,
chelation bidentate, and chelation–bridging tridentate (Fig. 1). The
structure consists of the centrosymmetric dimeric molecule [Er(9-
AC)3(DMF)(H2O)] moiety, in which two Er(III) ions are bridged via
two carboxylato groups from the 9-AC ligand. Thus, the two metal
ions and the two bridging oxygen atoms of carboxyl groups form
a square plane with four membered-ring, Er1–O1–Er1A–O1A. The
two carboxyl groups of the two 9-AC ligands forms a dimer in the
ꢆ␳max, ꢆ␳min (e Å−3
)
solid state with the O1 and O1A atoms forming bridges between
the two [Er(9-AC)3(DMF)(H2O)] moieties. The chelating bidentate
(O,O), chelating–bridging tridentate (O,O,Oꢀ) and monodentate of
9-AC ligands are observed in dimeric complex. Each of the Er(III) ion
is an eight-coordinate with the two chelating–bridging tridentate,
bidentate and monodentate of 9-AC ligands, one DMF and one
water molecule also participating. The chelating–bridging triden-
Fig. 1. Molecular structure of the [Er2(9-AC)6(DMF)2(H2O)2] complex with 50% probability ellipsoids. The hydrogen atoms are omitted for clarity.

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887
Table 2
Selected bond length (Å) and bond angle (◦) of the Er2(9-AC)6(DMF)2(H2O)2 complex.
Bond
Length (Å)
Bond
Angle (◦)
Er1–O7
Er1–O5
Er1–OW1
Er1–O1
Er1–O4
Er1–O3
Er1–O2
Er1A–O1
O1–C15
O2–C15
O3–C30
O4–C30
C45–O5
C45–O6
C46–O7
2.2479(14)
2.2555(14)
2.2956(14)
2.4949(13)
2.3863(14)
2.3990(15)
2.4087(13)
2.3187(13)
1.281(2)
1.256(2)
1.279(2)
1.255(2)
1.275(2)
O7–Er1–O5
O7–Er1–OW1
O5–Er1–OW1
O7–Er1–O1A
O5–Er1–O1A
OW1–Er1–O1A
O7–Er1–O4
O5–Er1–O4
OW1–Er1–O4
O1A–Er1–O4
O7–Er1–O3
O5–Er1–O3
OW1–Er1–O3
O1A–Er1–O3
O4–Er1–O3
O7–Er1–O2
O5–Er1–O2
OW1–Er1–O2
O1A–Er1–O2
O4–Er1–O2
O3–Er1–O2
O7–Er1–O1
O5–Er1–O1
OW1–Er1–O1
O1A–Er1–O1
O4–Er1–O1
O3–Er1–O1
O2–Er1–O1
84.25(6)
112.29(6)
74.82(5)
80.43(5)
138.25(5)
75.61(5)
78.29(5)
82.23(5)
153.23(5)
131.13(5)
87.82(6)
136.84(5)
145.78(5)
81.22(5)
54.63(5)
156.30(5)
86.65(5)
86.16(5)
119.94(4)
78.82(5)
83.95(5)
144.16(5)
130.19(5)
74.65(5)
67.02(5)
111.85(5)
73.25(5)
52.96(4)
1.249(2)
1.236(3)
Fig. 2. Perspective view coordination polyhedron geometry around the Er1 atom as
a triangular dodecahedron.
tate coordination mode of the 9-AC ligand in our complex studied
is similar compare to the [Ag6(9-AC)6(dmp)2] complex [25] where
dmp = 2,6-dimethylpyridine. The steric bulk of the anthracene
rings skeleton from the 9-AC ligands may play important roles in
the formation of dimeric complex. The Er–Er distance is 4.015 Å
in the dimeric unit. The coordination geometries of the two Er(III)
ions in the [Er2(9-AC)6(DMF)2(H2O)2] complex are similar and can
be described as a triangular dodecahedron with symmetry D2d
(Fig. 2). The DMF molecule has one C O group involved in the
inner coordination sphere of the complex.
The three anthracene cores of the 9-AC ligands in the complex
studied are coplanar and form a 14-membered rings with max-
imum deviation from least-squares plane of −0.128(3), 0.046(3)
and 0.131(3) Å for the C10, C25 and C34 atoms, respectively. This
coplanarity is similar with the free 9-ACA ligand that has been
reported by Fitzgerald and Gerkin [33]. The carboxyl groups, i.e.
(O1–O2–C15–C14), (O3–O4–C30–C29) and (O5–O6–C45–C44) of
the 9-AC ligands are planar. The dihedral angles between the
plane of the carboxyl group from the 9-AC anions and the best-
fit core anthracene planes are 56.10(9), 64.56(9) and 71.79(9)◦ for
the 14-membered rings of (C1–C14), (C16–C29) and (C31–C44),
respectively. These dihedral angle are larger than those found
in the anthracene-9-carboxylic acid of 54.87(6)◦ [33] and the
anthracence-1,8-carboxylic acid of 13.9(2) and 21.6(2)◦ [34]. The
anthracene cores of the 9-AC anions had close conformity to mirror
symmetry with respect to a mirror through the C7, C14; C22, C29;
C37, C44 atoms. The dihedral angle between the plane of the car-
boxyl group from the chelating–bridging tridentate 9-AC anion and
the best-fit core anthracene plane is the smallest. This is due to the
balancing position between two [Er(9-AC)3(DMF)(H2O)] moieties
in a propeller-arrangement.
Note: A = refers to the bridging coordination bond.
9-AC and monodentate 9-AC ligands with bond angles of 138.2(5)
and 130.2(5)◦, respectively (Table 2). The carboxyl group of the
9-AC anion adopting tridentate ꢃ-O,O,Oꢀ mode coordinates to
two distinct Er(III) ions with the Er–O bond lengths of Er1–O1
[2.4949(13) Å], Er1–O2 [2.4087(13) Å] and Er1–O1A [2.3187(13) Å].
In the C15, C30, C45 carboxyl groups, the C–O distances for single
bonds are 1.281(2), 1.278(2) and 1.276(2) Å, while the C–O distances
for double bonds are 1.256(2), 1.255(2) and 1.249(2) Å, respectively.
These distances for single bonds are shorter than those found in
the free 9-ACA molecule of 1.292(2) Å, while for double bonds are
longer than those found in the free 9-ACA molecule of 1.240(2) Å
[33]. The C–O distances for the chelating–bridging tridentate 9-AC
anion is the longest.
The O6 atom of the monodentate 9-AC ligand does not par-
ticipate in the coordination to Er(III) ions, however, this atom
is important in the structure helped to stabilize by a strong
intramolecular hydrogen bonding, OW1–H21W· · ·O6 (Fig. 3). The
anthracene rings and carboxyl group to adjust their steric configu-
rations more freely and allow the 9-AC ligands to display a variety
of coordination modes.
Five intramolecular O–H· · ·O and C–H· · ·O hydrogen bonds
helped to stabilize of the crystal packing of complex, i.e.
OW1–H11W· · ·O3 [1.87(3) Å], OW1–H21W· · ·O6 [1.76(3) Å],
C17–H17A· · ·O4 [2.4483 Å], C27–H27A· · ·O3 [2.4292 Å] and
C48–H48A· · ·O7 [2.3765 Å] (Fig. 3). The structure of [Er2(9-
AC)6(DMF)2(H2O)2] units are like a propeller-arrangement with the
9-AC anions act as a fan (Fig. 4). Besides, the structure also contains
numerous intermolecular C–H· · ·␲ supramolecular interactions
between the anthracene rings with an edge-to-face interac-
tions that further link the dinuclear units into two-dimensional
supramolecular network from the different crystallographic direc-
tions in the ranges of 2.3763–3.3691 Å and angles of 85.95–166.20◦.
The C4–H4A· · ·Cg7 [2.4326 Å; 166.2◦; −1/2 + x,1/2 − y,−1/2 + z] and
C11–H11A· · ·Cg7 [2.5676 Å; 152.56◦; −1 + x,y,z] with Cg7 = aromatic
ring of (C31–C36). Edge-to-face interactions may be regarded as
weak forms hydrogen bonds by an attractive interaction between
The Er–O bond lengths lie between 2.248(1) and 2.495(1) Å,
which are comparable to the erbium complexes [2,18–20,35]
(Table 2). The average Er–O9-AC bond length for the
chelating–bridging tridentate 9-AC ligand is 2.4074(13) Å. The
average Er–O9-AC bond length for the bidentate 9-AC ligand is
2.393(2) Å. The monodentate Er–O bond length is 2.2555(14) Å.
The Er–O bond length for the monodentate DMF is the shortest
[2.2479(14) Å]. The coordinated water molecule was positioned
between O1 and O5 of the carboxyl groups from the tridentate