Dinuclear versus trinuclear complex formation in zinc(II) benzoate/pyridyl oxime chemistry depending on the position of the oxime group

Article abstract of DOI:10.1016/j.poly.2009.05.076

The initial employment of pyridine-3-carbaldehyde oxime, (3-py)C(H)NOH, and pyridine-4-carbaldehyde oxime, (4-py)C(H)NOH, in zinc(II) carboxylate chemistry is reported. The syntheses, crystal structures and IR characterization are described for [Zn_3<

Full text of DOI:10.1016/j.poly.2009.05.076

Polyhedron 28 (2009) 3243–3250
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Polyhedron
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Dinuclear versus trinuclear complex formation in zinc(II) benzoate/pyridyl
oxime chemistry depending on the position of the oxime group
Konstantis F. Konidaris a, Michalis Kaplanis a, Catherine P. Raptopoulou b, Spyros P. Perlepes a,
Evy Manessi-Zoupa a, Eugenia Katsoulakou a,
*
a Department of Chemistry, University of Patras, 265 04 Patras, Greece
b Institute of Materials Science, NCSR ‘‘Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 7 June 2009
The initial employment of pyridine-3-carbaldehyde oxime, (3-py)C(H)NOH, and pyridine-4-carbaldehyde
oxime, (4-py)C(H)NOH, in zinc(II) carboxylate chemistry is reported. The syntheses, crystal structures
and IR characterization are described for [Zn3(O2CPh)6{(3-py)C(H)NOH}2] (1) and [Zn2(O2CPh)4{(4-
py)C(H)NOH}2] (2). The trinuclear molecule of 1 has a linear structure, with one monoatomically bridging
This paper is devoted to Dr. Aris Terzis for
his great contribution to the advancement
of inorganic chemistry in Greece through
single-crystal, X-ray crystallography.
g1 g2 and two syn, syn-g1 g1 benzoate groups spanning each pair of ZnII ions; the terminal metal
: :l : :l
ions are each capped by one (3-py)C(H)NOH ligand coordinating through its pyridyl nitrogen. Complex
2 exhibits a dinuclear paddle–wheel structure with a ZnꢀꢀꢀZn distance of 2.990(2) Å; each ZnII ion has a
square pyramidal geometry with four carboxylate oxygens in the basal plane and the pyridyl nitrogen
of one monodentate (4-py)C(H)NOH ligand at the apex. Both complexes form 1D architectures by virtue
of hydrogen bonding interactions involving the free oxime group as donor and the oxime nitrogen (1) or
carboxylate oxygens (2) as acceptors. IR data are discussed in terms of the known structures and coordi-
nation modes of the ligands.
Keywords:
Crystal structures
IR spectra
Paddle–wheel zinc(II) complexes
Pyridine-3-carbaldehyde oxime complexes
Pyridine-4-carbaldehyde oxime complexes
Zinc(II) benzoate clusters
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
to recreate with synthetic models the structure, properties, reactiv-
ity and/or function of the mononuclear and dinuclear active sites in
Simple carboxylate anions (RCO2ꢁ) are ubiquitous and versatile
ligands in coordination chemistry. Myriads of complexes with a
great variety of metals have been synthesized, many of which have
played key roles in the conceptual development of inorganic chem-
istry [1]. Perhaps most striking in terms of their impact are the so
named dinuclear ‘‘paddle wheel”, the trinuclear ‘‘basic” carboxyl-
ate and the tetranuclear ‘‘butterfly” complexes (Scheme 1). The
ZnII metalloenzymes [2]; (ii) the realization that some ZnII carbox-
ylate complexes based on aromatic ligands are luminescent [3],
and; (iii) the fact that some ZnII carboxylates are active catalysts
for the coupling of CO2 and epoxides [4].
There is currently a renewed interest in the coordination chem-
istry of oximes [5]. The efforts of several research groups are driven
by a number of considerations [6]. Pyridyl oximes (Scheme 2) are
popular ligands [7] whose anions are versatile ligands for a variety
of research objectives. For example, such ligands and especially 2-
pyridyl oximes – have been key ‘‘players” in the areas of single-
molecule [8] and single-chain [9] magnetism. We recently reported
[8] the use of methyl 2-pyridyl ketone oxime, (2-py)C(Me)NOH, to
prepare [MnI3IIO(O2CR)3{(2-py)C(Me)NO}3]+ complexes that were the
initial examples of triangular single-molecule magnets, by switching
the exchange coupling from the usual antiferromagnetic to ferro-
magnetic. Moreover, the activation of 2-pyridyl oximes towards fur-
ther reactivity by 3d-metal centers is also becoming a fruitful area of
research [10].
ꢁ
versatility of the RCO2 ligands is reflected in the variety of metal
binding modes it can adopt; up to four metal ions have been shown
to bind to a single carboxylate ligand. In contrast to the great
number of studies concerning 3dn, 4dn, 5dn (n = 1–9) and 4f-metal
carboxylate chemistry, normally associated with other organic
coligands, relatively little is known about the group 12 metal
carboxylate complexes. One reason for this is the availability of
only a limited number of ZnII, CdII and HgII carboxylate starting
materials in the market.
The current interest in the synthetic, structural and spectro-
scopic studies of new mononuclear, dinuclear and polynuclear ZnII
carboxylate complexes has three main driving forces: (i) the desire
This work describes our initial results in a general programme
that can be considered as an amalgamation of the two, above
mentioned research areas. The project concerns the study of the
reactions of ZnII carboxylate sources and simple pyridyl oximes
* Corresponding author. Tel.: +30 2610 996019; fax: +30 2610 997118.
E-mail address: katsoulakou@upatras.gr (E. Katsoulakou).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.076

3244
K.F. Konidaris et al. / Polyhedron 28 (2009) 3243–3250
Scheme 1. Paddle wheel (left), trinuclear basis carboxylate (middle) and tetranuclear ‘‘butterfly” carboxylate (right) complexes. Curved lines represent bidentate bridging
carboxylate groups. The paddle wheel structural type is associated with a variety of metal–metal interactions ranging from no interaction, weak spin-pairing, various orders
of metal–metal bonding, to the ‘‘super-short” metal–metal bonds. The L groups are often monodentate ligands, but they may represent intercluster association into polymeric
structures or (only in the case of the ‘‘butterfly” complexes) one donor atom of a bidentate chelate.
2.2. Compound preparation
2.2.1. [Zn3(O2CPh)6{(3-py)C(H)NOH}2] (1)
A solution of Zn(O2CPh)2ꢀ2H2O (0.346 g, 1.00 mmol) in MeCN
(10 ml) was added gradually to a stirred solution of (3-py)C(H)-
NOH (0.122 g, 1.00 mmol) in MeCN (10 ml). Slow evaporation of
the resulting solution at room temperature yielded colourless pris-
matic crystals of 1 after three days. The crystals were collected by
filtration under vacuum, washed with MeCN (2 ꢂ 5 ml) and Et2O
(5 ml), and dried in air. Typical yields were in the 50–55% range
(based on the metal). FT-IR (KBr pellet, cmꢁ1): 3354mb, 3064w,
1622sh, 1612vs, 1602m, 1570s, 1540m, 1490m, 1448w, 1400vs,
Scheme 2. General structural formula and abbreviation of the 2-pyridyl oximes
that are used in our laboratories (left); the ligands pyridine-3-carbaldehyde oxime
[3-pyridinealdoxime, (3-py)C(H)NOH] and pyridine-4-carbaldehyde oxime [4-pyr-
1304m, 1192w, 1130w, 1058w, 1026w, 978m, 890w, 874w,
idinealdoxime, (4-py)C(H)NOH] employed in this work are shown in the middle and
right part, respectively, of the scheme.
838w, 814w, 720s, 700m, 670m, 654w, 586w, 528w, 446w. Anal.
Calc. for C54H42N4O14Zn3 (1167.03): C, 55.57; H, 3.63; N, 4.80.
Found: C, 55.29; H, 3.37; N, 4.82%.
(the term ‘‘simple” means here ligands with only one pyridyl and
one oxime group as donors). Our goals are to investigate if clusteꢁrs
2.2.2. [Zn2(O2CPh)4{(4-py)C(H)NOH}2] (2)
and coordination polymers can be prepared from the ZnII/RCO2
/
To a stirred solution of Zn(O2CPh)2ꢀ2H2O (0.138 g, 0.40 mmol)
(py)C(R0)NOH reaction systems (R0=H, Me, Ph) and to isolate, if pos-
sible, photoluminescent complexes. Reactions of Zn(O2CR)2ꢀ2H2O
(R = Me, Ph) with the more complex ligand di-2-pyridyl ketone
oxime, (2-py)C(2-py)NOH, have led to compounds [Zn4(O-
H)2(O2CR)2{(2-py)C(2-py)NO}4] [11] with the extremely rare in-
verse 12-metallacrown-4 structural motif. We report here the
preparation and characterization of two complexes derived from
the reactions of Zn(O2CPh)2ꢀ2H2O with (3-py)C(H)NOH and (4-
py)C(H)NOH (see Scheme 2). Efforts to date with the isomeric pyr-
idine-x-carbaldehyde oximes [x = 2, (2-py)C(H)NOH, R0 = H in
Scheme 2; x = 3, (3-py)C(H)NOH; x = 4, (4-py)C(H)NOH] have al-
most completely neglected Zn chemistry; no ZnII complexes of
(3-py)C(H)NOH and (4-py)C(H)NOH have been reported, while
the only structurally characterized ZnII/(2-py)C(H)NOH or (2-
py)C(H)NOꢁ compound is [Zn4(OH)2Cl2{(2-py)C(H)NO}4] [12].
in MeCN (10 ml) was added
a solution of (4-py)C(H)NOH
(0.049 g, 0.40 mmol) in the same solvent (10 ml). The reaction
solution was stirred for 15 min during which time a white solid
precipitated. The precipitate was collected by filtration and the fil-
trate was allowed to slowly evaporate at room temperature. After
Table 1
Crystallographic data for complexes 1 and 2.
Parameter
1
2
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C54H42N4O14Zn3
1167.09
C40H32N4O10Zn2
859.48
monoclinic
P21/n
16.485(8)
13.008(6)
19.982(9)
90
113.31(2)
90
triclinic
ꢀ
P1
10.414(4)
10.830(4)
12.690(5)
81.66(1)
79.73(1)
66.40(1)
1286.2(9)
1
b (Å)
c (Å)
a
(°)
2. Experimental
b (°)
c
(°)
V (Å3)
3935(3)
4
2.1. General and spectroscopic measurements
Z
T (K)
298
0.71073
1.507
1.459
4800/4529 (0.014)
3719
298
0.71073
1.451
1.281
6082/5893 (0.031)
3735
All manipulations were performed under aerobic conditions
using materials (reagent grade) and solvents as received.
Zn(O2CPh)2ꢀ2H2O was prepared by the reaction of Zn(O2C-
Me)2ꢀ2H2O with and excess of PhCO2H in CHCl3 under reflux.
Microanalyses (C, H, N) were performed by the University of Ioann-
ina (Greece) Microanalytical Laboratory using an EA 1108 Carlo
Erba analyzer.
Radiation, wavelength Mo K
a
qcalc (g cmꢁ3
)
l
(mmꢁ1
Data collected/unique (Rint
Data with I > 2 (I)
)
)
r
a
R1 [I > 2
r
> (I)]
0.0361
0.1159
0.0516
0.1203
b
wR2 [unique data]
P
P
IR spectra (4000–450 cmꢁ1) were recorded on a Perkin–Elmer
16 PC FT-spectrometer with samples prepared as KBr pellets.
a
R1
=
(|Fo| ꢁ |Fc|)/ (|Fo|).
P
P
wR2 = { [w(F2o ꢁ F2c )2]/ [w(F2o)2]}1/2
.
b

K.F. Konidaris et al. / Polyhedron 28 (2009) 3243–3250
3245
three days, X-ray quality colourless crystals of 2 were collected by
mains invariant during 360° rotation around the
U
axis, thus
filtration under vacuum, washed with MeCN (2 ꢂ 5 ml) and Et2O
(5 ml), and dried in air. Typical yields (including the white solid)
were in the 50–55% range. FT-IR (KBr pellet, cmꢁ1): 3221mb,
3066w, 1634vs, 1573s, 1409vs, 1301m, 1225w, 1175vw, 1067vw,
1030w, 990m, 932w, 832m, 714s, 673m, 560w, 536m, 472w,
460w. The IR spectra of the white powder and the colourless crys-
tals (in the form of a powder) are identical. Anal. Calc. for
C40H32N4O10Zn2 (859.44): C, 55.90; H, 3.76; N, 6.52. Found: C,
55.62; H, 3.78; N, 6.55%.
absorption correction is not required.
The structures were solved by direct methods using SHELXS-86
[13] and refined by full-matrix least-squares techniques on F2 with
SHELXL-97 [14]. Further experimental crystallographic details for 1:
2hmax = 50°; 424 parameters refined; (
D
/r
)
max = 0.000; (
D
q)
max
/
(
D
q
)
min = 0.557/ꢁ0.712 e/Å3; R1 (for all data), 0.0581. All hydrogen
atoms were located by difference maps and refined isotropically.
All non-hydrogen atoms were refined anisotropically. Further
experimental crystallographic details for 2: 2hmax = 47.2°; 633
parameters refined;
(D/r)max = 0.002; (Dq)max/(Dq)min = 0.292/
2.3. Single-crystal X-ray crystallography
ꢁ0.354 e/Å3; R1 (for all data), 0.1075. All hydrogen atoms were
located by difference maps and were refined isotropically. All
non-hydrogen atoms were refined anisotropically.
A colourless crystal of 1 (0.10 ꢂ 0.20 ꢂ 0.60 mm) and a colour-
less crystal of 2 (0.20 ꢂ 0.30 ꢂ 0.50 mm) were mounted in air
and covered with epoxy glue. Diffraction measurements were
made on a Crystal Logic Dual Goniometer diffractometer using
3. Results and discussion
graphite monochromated Mo Ka radiation. Details for data collec-
3.1. Brief synthetic comments and IR spectra
tion and processing are listed in Table 1. Unit cell dimensions were
determined and refined by using the angular settings of 25 auto-
matically centered reflections in the range 11 < 2h < 23°. Three
standard reflections monitored every 97 reflections showed less
than 3% variation and no decay. Lorentz, polarization corrections
The chemical and structural identities of the products from the
Zn(O2CPh)2ꢀ2H2O/LH reaction systems in MeCN, where LH is (3-
py)C(H)NOH or (4-py)C(H)NOH, depends on the pyridyl oxime li-
gand. Use of (3-py)C(H)NOH gives the trinuclear cluster 1, whereas
employment of (4-py)C(H)NOH led to the dinuclear compound 2.
were applied using Crystal Logic software.
W-scan data were col-
lected for both structures. The intensity of a certain reflection re-
Fig. 1. Partially labeled plot of the molecular structure of 1. H atoms have been omitted for clarity. Colour code: Zn, orange; O, red; N, blue; C, gray. Primes are used for
symmetry related (ꢁx, 1 ꢁ y, ꢁz) atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. A part of the 1D structure of 1 due to H-bonding interactions (green dashed lines); colour code as in Fig. 1. H atoms and the phenyl rings of the benzoates have been
omitted for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3246
K.F. Konidaris et al. / Polyhedron 28 (2009) 3243–3250
2ZnðO2CPhÞ2 ꢀ 2H2O þ 2ð4-pyÞCðHÞNOH
MeCN
! ½Zn2ðO2CPhÞ4fð4-pyÞCðHÞNOHg2ꢃ þ 4H2O
ð2Þ
Two features of the reactions represented by the chemical (1)
and (2) deserve comments. First, the ‘‘wrong” reaction ratio (1:1)
employed for the preparation of 1, compared to the stoichiometric
ratio (1.5:1) required by Eq. (1), obviously did not prove detrimen-
tal to the formation of the complex. With the identity of 1 crystal-
lographically established (vide infra), the ‘‘correct” stoichiometric
ratio, i.e. ZnII:(3-py)C(H)NOH = 1.5:1, was employed and led to
the pure compound in very good yield (60–65%). Second, com-
plexes 1 and 2 seem to be the only isolable products from the
Zn(O2CPh)2ꢀ2H2O/LH reaction systems. The ZnII to ligand reaction
ratio (we used 1:2 and 1:3 ratios to ‘‘force” the formation of mono-
nuclear complexes, e.g. [Zn(O2CPh)2(LH)x] with x = 2 or 3), the
ꢁ
ꢁ
presence of counteranions (we added ClO4 or PF6 in some reac-
tion mixtures to ‘‘force” the isolation of cationic complexes), the
nature of the solvent (use of alcohols gives the same products)
and the precipitation/crystallization method have no influence on
the identity of the products.
The IR spectra of 1 and 2 exhibit medium intensity bands at
ꢄ3350 and 3320 cmꢁ1, respectively, assignable to the
m(OH) vibra-
tion of the coordinated pyridyl oxime ligands [6a,15]. The broad-
ness and low frequency of these bands are both indicative of
hydrogen bonding. The medium intensity bands at 1636 and
1124 cmꢁ1 in the spectrum of the free ligand (3-py)C(H)NOH are
assigned to
m(C@N)oxime and m(N–O)oxime, respectively [6a,16].
The corresponding vibrations in the spectrum of free (4-py)C(H)-
NOH appear at 1566 and 1096 cmꢁ1. In the spectra of the com-
plexes, these typical bands of the neutral oxime group are
situated at approximately the same wavenumbers to those in the
spectra of the corresponding free ligands, confirming the crystallo-
graphically established (vide infra) non-participation of the oxime
Fig. 3. Crystal packing diagram of complex 1. The metal coordination spheres are
represented as polyhedra [Zn(1), green; Zn(2), purple]. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
group in coordination. For example, the
m(C@N)oxime and m(N–
O)oxime vibrations appear at 1622 and 1130 cmꢁ1, respectively, in
the spectrum of 1. The in-plane deformation band of the pyridyl
ring of the free ligands [638 cmꢁ1 in (3-py)C(H)NOH, 646 cmꢁ1 in
(4-py)C(H)NOH] shifts upwards in the complexes (654 cmꢁ1 in 1,
673 cmꢁ1 in 2), confirming the involvement of the ring-N atom
in coordination [17]. The strong bands at 1573 and 1409 cmꢁ1 in
The preparation of the two complexes is summarized in the bal-
anced Eqs. (1) and (2):
3ZnðO2CPhÞ2 ꢀ 2H2O þ 2ð3-pyÞCðHÞNOH
MeCN
! ½Zn3ðO2CPhÞ6fð3-pyÞCðHÞNOHg2ꢃ þ 6H2O
ð1Þ
Fig. 4. Partially labeled plot of the molecular structure of 2. Colour code as in Fig. 1. H atoms have been omitted for clarity. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

K.F. Konidaris et al. / Polyhedron 28 (2009) 3243–3250
3247
Fig. 5. A small portion of the ladder-like 1D architectures of 2 due to H-bonding interactions (green dashed lines). Colour code as in Fig. 1. The H atoms and the phenyl rings of
the benzoates have been omitted for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Crystal packing diagram of complex 2. The metal coordination spheres are represented as polyhedra (purple colour). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
signed to
acter. The
m
as(CO2); both may also involve a pyridyl stretching char-
s(CO2) modes appear at 1400 and 1448 cmꢁ1. The
Table 2
m
Selected interatomic distances (Å) and angles (°) for complex 1.
appearance of two distinct bands for each mode reflects the pres-
ence of two different types of PhCOOꢁ groups in the complex (vide
Distances
infra) [19]. The 1612, 1400 cmꢁ1 pair (
D
= 212 cmꢁ1) are assigned
Zn(1)–O(11)
Zn(1)–O(21)
Zn(1)–O(31)
Zn(1)–O(110)
Zn(1)–O(210)
Zn(1)–O(310)
2.084(3)
2.069(3)
2.144(3)
2.084(3)
2.069(3)
2.144(3)
Zn(2)–O(12)
Zn(2)–O(22)
Zn(2)–O(31)
Zn(2)–O(32)
Zn(2)–N(1)
Zn(1)ꢀꢀꢀZn(2)
1.955(3)
1.951(3)
2.147(3)
2.202(3)
2.073(4)
3.392(5)
[18–20] to the g1
:
g2
:l
2 benzoate, while the 1570, 1448 cmꢁ1 pair
2) benzoates.
(D
= 122 cmꢁ1) to the bidentate bridging (g1 g1
: :l
3.2. Description of structures
Angles
O(11)–Zn(1)–O(21)
O(11)–Zn(1)–O(31)
O(11)–Zn(1)–O(110)
O(11)–Zn(1)–O(210)
O(11)–Zn(1)–O(310)
O(21)–Zn(1)–O(31)
O(110)–Zn(1)–O(21)
O(21)–Zn(1)–O(210)
O(21)–Zn(1)–O(310)
O(110)–Zn(1)–O(31)
O(210)–Zn(1)–O(31)
O(31)–Zn(1)–O(310)
O(110)–Zn(1)–O(210)
93.6(1)
90.2(1)
180.0
86.4(1)
89.8(1)
90.3 (1)
86.4(1)
180.0
89.7(1)
89.8(1)
89.7(1)
180.0
O(110)–Zn(1)–O(310)
O(210)–Zn(1)–O(310)
O(12)–Zn(2)–O(22)
O(12)–Zn(2)–O(31)
O(12)–Zn(2)–O(32)
O(12)–Zn(2)–N(1)
O(22)–Zn(2)–O(31)
O(22)–Zn(2)–N(1)
O(31)–Zn(2)–N(1)
O(22)–Zn(2)–O(32)
O(31)–Zn(2)–O(32)
O(32)–Zn(2)–N(1)
90.2(1)
90.3(1)
113.9(1)
107.9(1)
105.7(1)
99.5(1)
94.4(1)
93.2(2)
145.5(1)
138.1(1)
59.9(1)
93.1(1)
Aspects of the molecular and crystal structures of complexes 1
and 2 are shown in Figs. 1–6. Selected interatomic distances and
angles are listed in Tables 2 and 4, while important hydrogen
bonding interactions are presented in Table 5.
ꢀ
Complex 1 crystallizes in the triclinic space group P1. Its crystal
structure consists of trinuclear [Zn3(O2CPh)6{(3-py)C(H)NOH}2]
molecules. The central metal center [Zn(1)] is located on a crystal-
lographic inversion center and thus the trinuclear molecule adopts
a strictly linear structure. Zn(1) is coordinated by six oxygen atoms
ꢁ
belonging to six PhCO2 groups. One g1
:
g2
:l
and two syn, syn-
93.6(1)
g1 g1 carboxylate groups span each pair of neighbouring ZnII
: :l
Symmetry operations used to generate equivalent atoms: (0) ꢁx, 1 ꢁ y, ꢁz.
ions. The terminal metal ions [Zn(2), Zn(20)] are each capped by
one monodentate (3-py)C(H)NOH ligand; the donor atom of (3-
py)C(H)NOH is the pyridyl nitrogen. The ZnO6 octahedron is almost
perfect with the Zn(1)–O bond lengths and the cis O–Zn(1)–O bond
angles being in the ranges 2.069(3)–2.144(3) Å and 86.4(1)–
93.6(1)°, respectively. The coordination geometry about the termi-
nal ZnII ions is described as distorted square pyramidal with the
carboxylate oxygen atoms O(12) and O(120) occupying the apical
positions for Zn(2) and Zn(20), respectively. Analysis of the shape-
the spectrum of 2 are assigned to the
of the benzoato ligands, respectively [18]; the former may also in-
volve a pyridyl stretching character. The difference , where
mas(CO2) and ms(CO2) modes
D
D
=
m
as(CO2) ꢁ
m
s(CO2), is 164 cmꢁ1, less than that for NaO2CPh
(184 cmꢁ1), as expected for bidentate bridging ligation [18]. In
the IR spectrum of 1, the bands at 1570 and 1612 cmꢁ1 are as-

3248
K.F. Konidaris et al. / Polyhedron 28 (2009) 3243–3250
Table 3
Pentinent structural propertiesa for trinuclear ZnII complexes containing the [ZnI3I(g1 g1 -J2CPh)4(g1 g2
: :l : :l-J2CPh)2] unit.
Complex
B (Å)
D (Å)
A (Å)
ZnꢀꢀꢀZn (Å)
C–Ob (Å)
C–Od (Å)
h (°)
b (°)
a (°)
[Zn3(O2CPh)6(py)2]b,c
[Zn3(O2CPh)6 (2,3-Me2pyz)2]d
Complex 1e
2.328
2.103
2.147
2.121
2.264
2.202
2.141
2.207
2.144
3.444
3.370
3.392
1.248
1.278
1.271
1.241
1.242
1.247
100.73
102.85
101.46
86.43
94.03
91.38
139.45
139.05
145.52
Abbreviations: 2,3-Me2pyz = 2,3-dimethylpyrazine; py = pyridine.
a
See Scheme 3 for the definition of the various structural parameters given in this table.
Ref. [22b].
This discrete cluster cocrystallizes with [Zn2(O2CPh)4(py)2] in the complex.
Ref. [22b].
This work.
b
c
d
e
to trinuclear ZnII benzoate complexes is demonstrated in Scheme 3
and Table 3. The monoatomic bridging mode is postulated to be an
important intermediate between the other, more common carboxyl-
ate binding modes, based on observed variation of the geometry for
monoatomic bridges in structurally characterized metal complexes.
The ‘‘movement” from monoatomic bridging to the other binding
modes has been termed ‘‘carboxylate shift” [23]; such a shift is con-
sidered to be kinetically important in carboxylate-containing metal-
loproteins. Complex 1 is characterized by a moderate D value, D > B,
A, a rather long ZnꢀꢀꢀZn distance, a difference between C–Od and C–Ob
Table 4
Selected interatomic distances (Å) and angles (°) for complex 2.
Distances
Zn(1)–O(31)
Zn(1)–O(21)
Zn(1)–N(1)
Zn(1)–O(41)
Zn(1)–O(51)
Zn(1)ꢀꢀꢀZn(2)
2.007(5)
2.014(5)
2.028(4)
2.061(4)
2.132(5)
2.990(2)
Zn(2)–O(32)
Zn(2)–N(11)
Zn(2)–O(52)
Zn(2)–O(42)
Zn(2)–O(22)
2.019(5)
2.028(4)
2.070(4)
2.105(5)
2.017(5)
Angles
O(31)–Zn(1)–O(21)
O(21)–Zn(1)–N(1)
O(21)–Zn(1)–O(41)
O(31)–Zn(1)–O(51)
N(1)–Zn(1)–O(51)
O(31)Zn(1)N(1)
O(31)Zn(1)O(41)
N(1)–Zn(1)–O(41)
O(21)–Zn(1)–O(51)
O(41)–Zn(1)–O(51)
157.2(2)
100.5(2)
89.4(2)
89.6(2)
93.4(2)
102.2(2)
86.5(2)
107.2(2)
86.5(2)
159.5(2)
O(22)–Zn(2)–O(32)
O(32)–Zn(2)–N(11)
O(32)–Zn(2)–O(52)
O(22)–Zn(2)–O(42)
N(11)–Zn(2)–O(42)
O(22)–Zn(2)–N(11)
O(22)–Zn(2)–O(52)
N(11)–Zn(2)–O(52)
O(32)–Zn(2)–O(42)
O(52)–Zn(2)–O(42)
158.8(2)
98.8(2)
89.0(2)
88.1(2)
94.7(2)
102.0(2)
88.6(2)
107.6(2)
86.2(2)
157.7(2)
and a distinct tilt (a : :l carboxylate group towards
–b) of the g1 g2
Zn(2); thus, this compound can be placed in class II in Lippard’s ‘‘car-
boxylate shift” classification scheme [23].
Compound 1 joins only a handful of structurally characterized
metal complexes of (3-py)C(H)NOH [24]; in all the previously re-
ported complexes the 3-pyridinealdoxime ligand is attached to
the metal ions through the pyridine nitrogen atom (like in 1) with
determining angles using the approach of Reedijk and co-workers
[21] yields a trigonality index, , value of 0.12 ( = 0 and 1 for per-
s
s
fect sp and tbp geometries, respectively). The coordination of the
bridging benzoate oxygen [O(31), O(310)] is slightly pyramidal
P
[
J(31) = 341.4(1)°]. The Zn(1)–O(31)–Zn(2) angle is 104.5(1)°.
In the crystal lattice of 1, the molecules interact through hydro-
gen bonds building 1D chains parallel to the [2 0 ꢁ1] vector.
Complex 1 is the third structurally characterized ZnI3I complex
containing the [ZnI3I(g1 g1 2-O2CPh)4(g1 g2 2-O2CPh)2]0 unit.
: :l : :l
The other two complexes are [Zn3(O2CPh)6(2,3-Me2pyz)2] [22a]
and the trinuclear part of complex [Zn3(O2CPh)6(py)2]ꢀ[Zn2
(O2CPh)4(py)2] [22b], where 2,3-Me2pyz and py are 2,3-dimethyl-
pyrazine and pyridine, respectively. Such complexes have been
important in the establishment of the ‘‘carboxylate shift” concept
[23]. Of the various types of metal-carboxylate binding modes,
monoatomic bridging between metal ions offers great flexibility.
Analysis [23] of the structural characteristics of dinuclear and trinu-
clear complexes containing monoatomically bridging carboxylate
groups reveals that they can be grouped into three classes, depend-
ing upon the strength of the interaction of the non-bridging, or dan-
gling (Od), oxygen atom [O(32) in compound 1] with one of the
bridged metal centers [Zn(2) in 1]. The application of this analysis
Scheme 3. Schematic structure of complex
1 showing the dangling (Od) and
bridging (Ob) oxygen atoms of the g1
:
g2
:
l2 carboxylate groups, and defining the
geometry of monoatomic carboxylate-bridged polymetallic centers in relation to
the ‘‘carboxylate shift” mechanism proposed by Lippard and co-workers [23]; the
values of the relevant structural parameters for 1 and the other two, structurally
characterized complexes containing the [Zn3II
unit are listed in Table 3.
( : :l : :l
g1 g1 -O2CPh)4(g1 g2 -O2CPh)2]0
Table 5
Hydrogen bonding interactions in 1 and 2.
Interaction D–HꢀꢀꢀA
DꢀꢀꢀA (Å)
HꢀꢀꢀA (Å)
D–HꢀꢀꢀA (°)
Symmetry operation of A
1
O1–HO1ꢀꢀꢀN2
2.894(6)
1.94(8)
159(7)
2 ꢁ x, 1 ꢁ y, ꢁ1 ꢁ z
2
O1–HO1ꢀꢀꢀO52
2.727(7)
2.781(7)
1.92(8)
2.05(11)
169(8)
157(11)
x, ꢁ1 + y, z
x, 1 + y, z
O11–HO11ꢀꢀꢀO41

K.F. Konidaris et al. / Polyhedron 28 (2009) 3243–3250
3249
the free oxime group participating in hydrogen bonding interac-
tions. Complex 1 is the first ZnII complex of (3-py)C(H)NOH.
Complex 2 crystallizes in thꢁe monoclinic space group P21/n. The
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
four syn, syn-g1 g1
: :l PhCO2 ligands display a paddle wheel
arrangement about the ZnꢀꢀꢀZn axis. Each ZnII center has a square
pyramidal coordination geometry [s = 0.04 for Zn(1) and 0.02 for
Acknowledgement
Zn(2)], with the apex provided by coordination of the pyridyl nitro-
gen atom of a monodentate (4-py)C(H)NOH ligand. The metal to
apical atom distances are the same, i.e., 2.028(4) Å. The mean
Zn–O(carboxylate) bond lengths are 2.054(5) Å for Zn(1) and
2.053(5) Å for Zn(2). These distances are typical and unremarkable
[25]. Zn(1) lies 0.386 Å and Zn(2) lies 0.387 Å out of their respec-
tive least-squares basal planes towards N(1) and N(11), respec-
tively. The Zn(1)ꢀꢀꢀZn(2)–N(11) and Zn(2)ꢀꢀꢀZn(1)–N(1) angles are
the same, i.e. 160.8°. Dinuclear [Zn2(O2CR)4L2] complexes
(L = monodentate ligand) have previously been observed to be
either collinear [26] or similarly offset [25]. The tetracarboxylate
bridging framework can accommodate metal–metal separations
of up to ꢄ3.5 Å [25]. The ZnꢀꢀꢀZn separation in 1 [2.990(2) Å] is
shorter than this maximum, but longer than the CuꢀꢀꢀCu distances
in structurally similar CuI2I complexes [27].
E.M.-Z and K.F.K thank the Research Committee of the Univer-
sity of Patras for funding this work (C. Karatheodory Grant 2008,
No. C.584).
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lographic data for complexes 1 and 2. These data can be obtained

3250
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