Investigation of the zinc chloride/methyl(2-pyridyl)ketone oxime reaction system: A mononuclear complex and an inverse 12-metallacrown-4 cluster

Article abstract of DOI:10.1515/znb-2006-0108

The reactions of methyl(2-pyridyl)ketone oxime, (py)C(Me)NOH, with zinc chloride have been investigated. An excess of the ligand in EtOH/MeCN affords the mononuclear complex [ZnCl₂{(py)C(Me)NOH}₂] (1) in moderate yield. The metal ion is coordinated by two chloro ligands and two N,N'-chelating (py)C(Me)NOH molecules. The six-coordinate molecule is the cis-cis-trans isomer considering the positions of the coordinated chlorine, pyridyl and oxime nitrogen atoms, respectively. The reaction between equimolar quantities of ZnCl₂, (py)C(Me)NOH and LiOH · H₂O in EtOH/MeCN leads to the tetranuclear cluster [Zn₄(OH) ₂Cl₂{(py)C(Me)NO}₄] (2) in high yield. The molecule lies on a crystallographic inversion center and has an inverse 12-metallacrown-4 topology. Two triply bridging hydroxides are accommodated within the metallacrown ring. Each (py)C(Me)NO^- ligand adopts the 2.111 coordination mode (Harris notation), chelating one Zn^II ion and bridging a Zn^II₂ pair. Two metal ions are in a distorted O₂N₄ octahedral environment, whereas the other two are in a severely distorted tetrahedral O₃Cl environment. Complex 2 joins a small family of structurally characterized zinc(II) metallacrown complexes. The IR data are discussed in terms of the nature of bonding and the structures of the two complexes.

Full text of DOI:10.1515/znb-2006-0108

Investigation of the Zinc Chloride / Methyl(2-pyridyl)ketone
Oxime Reaction System: A Mononuclear Complex and an
Inverse 12-Metallacrown-4 Cluster
a
b
b
Constantina Papatriantafyllopoulou , Catherine P. Raptopoulou , Aris Terzis ,
a
a
Evy Manessi-Zoupa , and Spyros P. Perlepes
a
Department of Chemistry, University of Patras, 265 04 Patras, Greece
Institute of Materials Science, NCSR “Demokritos”, 153 10 Aghia Paraskevi Attikis, Greece
b
Reprint requests to Prof. S. P. Perlepes or to Assoc. Prof. E. Manessi-Zoupa.
E-mail: perlepes@patreas.upatras.gr or emane@upatras.gr
Z. Naturforsch. 61b, 37 – 46 (2006); received September 28, 2005
This paper is dedicated to the memory of Dimitris Katakis, Professor of Inorganic Chemistry in the
Chemistry Department of the University of Athens
The reactions of methyl(2-pyridyl)ketone oxime, (py)C(Me)NOH, with zinc chloride have
been investigated. An excess of the ligand in EtOH/MeCN affords the mononuclear complex
[
ZnCl {(py)C(Me)NOH} ] (1) in moderate yield. The metal ion is coordinated by two chloro lig-
2
2
ands and two N,N’- chelating (py)C(Me)NOH molecules. The six-coordinate molecule is the cis-
cis-trans isomer considering the positions of the coordinated chlorine, pyridyl and oxime nitro-
gen atoms, respectively. The reaction between equimolar quantities of ZnCl , (py)C(Me)NOH and
2
LiOH·H O in EtOH/MeCN leads to the tetranuclear cluster [Zn (OH) Cl {(py)C(Me)NO} ] (2) in
2
4
2
2
4
high yield. The molecule lies on a crystallographic inversion center and has an inverse 12-metalla-
crown-4 topology. Two triply bridging hydroxides are accommodated within the metallacrown ring.
−
Each (py)C(Me)NO ligand adopts the 2.111 coordination mode (Harris notation), chelating one
II
II
Zn ion and bridging a Zn 2 pair. Two metal ions are in a distorted O N octahedral environment,
2
4
whereas the other two are in a severely distorted tetrahedral O Cl environment. Complex 2 joins a
3
small family of structurally characterized zinc(II) metallacrown complexes. The IR data are discussed
in terms of the nature of bonding and the structures of the two complexes.
Key words: Crystal Structures, Infrared Spectra, Inverse 12-Metallacrown Complexes,
Methyl(2-pyridyl)ketone Oxime Complexes, Zinc(II) Chloro Complexes
Introduction
ylene carbon atoms. The original MC structures de-
scribed contain a -[M-N-O]n- repeat unit, where n = 3,
The study of metallamacrocycles has been intensi-
fied over the last 15 years due to the fact that this
class of compounds is relevant to a variety of mod-
ern chemical interests [1], including catalysis [2], sen-
sors [3], molecular recognition [4] and chiral build-
ing blocks for 1D, 2D and 3D solids [5]. Metalla-
macrocycles include complexes such as metallacrowns
4
and 5 [6]. The nomenclature for metallacrowns is
derived from that of crown ethers and has been dis-
cussed in detail [6]. It should be mentioned at this
point that some researchers prefer to consider MCs
simply as one sub-area of metal wheel (ring) chem-
istry, avoiding the use of their specialized nomen-
clature. These metallamacrocycles are now quite di-
verse and include expanded metallacrowns (carbon
atoms are included in the repeat unit) [14], metalla-
coronates (both of the heteroatoms are oxygen atoms
[
[
6], metallahelicates [7], molecular squares and boxes
8, 9], metallacalixarenes [10], and metallacryptates
and metallacryptands [1, 11].
Metallacrowns (MCs) [6] were first described by with carbon atom extenders) [15], azametallacrowns
Pecoraro’s group [12] sixteen years ago. MCs, the in- (the oxygen atoms replaced by nitrogen atoms to
organic structural and functional analogues of crown give a -[M-N-N]- repeat unit) [16] and expanded aza-
ethers [13], are usually formed with a transition metal metallacrowns (carbon atoms are included in the re-
ion and a nitrogen donor ligand replacing the meth- peat unit) [17]. Now diverse MCs with varying ring
0
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C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
ands and to examine the possibility of creating novel
cluster topologies within this chemistry, we report here
studies of the reactions of methyl 2-pyridyl ketone
oxime [(py)C(Me)NOH, Fig. 1] with ZnCl , which
2
have demonstrated that the anionic form of the ligand
can give MCs. One synthetically relevant, mononu-
Fig. 1. General structure and abbreviations of simple 2- clear zinc(II) complex is reported.
pyridyl oximes, including methyl 2-pyridyl ketone oxime
[(py)C(Me)NOH].
Results and Discussion
sizes are known [1, 6], including 9-MC-3, 12-MC-3, Brief synthetic comments
1
1
3
2-MC-4, 12-MC-6, 15-MC-5, 15-MC-6, 16-MC-4,
6-MC-8, 18-MC-6, 22-MC-8, 24-MC-6, 24-MC-8,
0-MC-10 and 36-MC-12. Metallacrowns (in their
The chemical and structural identities of the prod-
ucts from the ZnCl /(py)C(Me)NOH reaction sys-
2
−
tem depend on the OH concentration. The mononu-
broad sense) have been studied for their selective
cation and anion binding [6, 18], ability to stabilize
active molecules [19], as sensors [3], as antibacterial
compounds [20], as building blocks for extended solids
clear complex [ZnCl {(py)C(Me)NOH} ] (1) was
2
2
isolated from the direct reaction between ZnCl
2
and (py)C(Me)NOH, whereas employment of hy-
droxides lead to the tetranuclear cluster [Zn (OH) -
4
2
[5], and as single-molecule magnets [1].
{
(py)C(Me)NO} ] (2). The preparation of the two
4
Restricting further discussion to classical MCs, i. e.
complexes is summarized in the balanced eqs (1)
and (2):
those containing the -[M-N-O]n- repeat unit, it is
known that their synthesis requires the employment of
tri- and tetradentate ligands containing hydroxamate or
oximate functionalities to provide a scaffolding within
which the desired metal-containing core can be real-
ized. Very popular ligands for the construction of MCs
are 2-pyridyl ketone oximes [(py)C(R)NOH, Fig. 1] in
their anionic form [6, 21 – 29].
EtOH/MeCN
ZnCl2 +2 (py)C(Me)NOH −− −−−−−−− −→
(
1)
2)
[
ZnCl2{(py)C(Me)NOH}2]
1
EtOH/MeCN
4
ZnCl2 +4 (py)C(Me)NOH+4 LiOH·H2O −− −−−−−−− −→
[
Zn4(OH)2Cl2{(py)C(Me)NOH}4]+4 LiCl+2 HCl+6 H2O
(
We have been exploring the chemistry of MCs
2
[28, 29] and the coordination chemistry of 2-pyridyl
oximes [(py)C(R)NOH, Fig. 1] [28 – 34], the latter not
necessarily in relation to the former. There is cur-
rently a renewed interest in the coordination chem-
istry of 2-pyridyl oximes [35, 36]. The anionic forms
of these molecules are versatile ligands for a variety
Some features of the reactions represented by eq. (1)
and eq. (2) deserve comments. First, the “wrong”
ZnCl2 to (py)C(Me)NOH reaction ratio (1:1) em-
ployed for the preparation of 1 (see Experimental Sec-
tion), compared to that required by eq. (1) [1:2], did
not prove detrimental to the formation of the complex.
With the identity of 1 established by single-crystal
X-ray crystallography, the “correct” stoichiometry was
employed and led to the pure compound in high yield.
Second, the addition of a base is absolutely necessary
for the preparation of 2, which contains the deproto-
nated ligand, due to the high pKa value (10.87) of the
of objectives, including µ and µ behaviour, forma-
2
3
tion of polynuclear metal complexes (clusters), isola-
tion of coordination polymers, mixed-metal chemistry
and significant magnetic properties; the activation of
2
-pyridyl oximes by 3d-metal centers towards further
reactions is also becoming a fruitful area of research.
The majority of the metal complexes of these ligands
have been prepared in the last 15 years and much of
their chemistry remains to be explored in more detail
oxime group of (py)C(Me)NOH [36]. Note that Et N
3
can also act as a base to facilitate formation of the de-
protonated ligand (the relevant procedue is not men-
[35, 36].
Zinc(II) metallacrown complexes based on 2-pyr- tioned in the Experimental Section). Third, the reac-
idyloximate ligands have attracted some prior inter- tion mixture which leads to 2 should be filtered be-
est [21, 25, 28, 37]; details will be given in “Results fore crystallization to remove LiCl which is insolu-
and Discussion”. Seeking to extend the family of ble in EtOH/MeCN; filtration is not necessary if Et N
3
Zn(II) MCs incorporating new 2-pyridyloximate lig- is employed as a base. The product also needs to be
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C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
39
˚
◦
˚
◦
Table 1. Selected bond lengths (A) and angles ( ) relevant to Table 2. Selected bond lengths (A) and angles ( ) relevant to
a
the zinc coordination sphere for complex 1.
the zinc coordination spheres in complex 2·2 Et O .
2
Zn(1)...Zn(1’)
Zn(1)...Zn(2)
Zn(1)...Zn(2’)
Zn(2)...Zn(2’)
Zn(1)-Cl(1)
5.782(4) Zn(1)-O(21)
3.307(4) Zn(2)-O(21)
3.327(4) Zn(2)-O(21’)
3.252(4) Zn(2)-N(1)
2.208(2) Zn(2)-N(2)
1.946(5) Zn(2)-N(11)
1.960(4) Zn(2)-N(12)
1.982(4)
2.105(4)
2.134(4)
2.193(5)
2.113(5)
2.214(6)
2.109(5)
Zn(1)-Cl(1)
Zn(1)-Cl(2)
Zn(1)-N(1)
2.448(1)
2.369(1)
2.276(2)
Zn(1)-N(2)
Zn(1)-N(11)
Zn(1)-N(12)
2.131(2)
2.213(2)
2.154(2)
Cl(1)-Zn(1)-Cl(2)
Cl(1)-Zn(1)-N(1)
Cl(1)-Zn(1)-N(2)
Cl(1)-Zn(1)-N(11)
Cl(1)-Zn(1)-N(12)
Cl(2)-Zn(1)-N(1)
Cl(2)-Zn(1)-N(2)
Cl(2)-Zn(1)-N(11)
97.4(1)
158.3(1)
87.6(1)
87.4(1)
108.1(1)
94.7(1)
105.8(1)
161.7(1)
Cl(2)-Zn(1)-N(12)
N(1)-Zn(1)-N(2)
N(1)-Zn(1)-N(11)
N(1)-Zn(1)-N(12)
N(2)-Zn(1)-N(11)
N(2)-Zn(1)-N(12)
N(11)-Zn(1)-N(12)
88.7(1)
71.8(1)
86.8(1)
90.1(1)
92.0(1)
157.3(1)
73.0(1)
Zn(1)-O(1)
Zn(1)-O(11)
Zn(1)-O(21)-Zn(2)
Zn(1)-O(21)-Zn(2’)
Zn(2)-O(21)-Zn(2’)
Cl(1)-Zn(1)-O(1)
Cl(1)-Zn(1)-O(11)
Cl(1)-Zn(1)-O(21)
O(1)-Zn(1)-O(11)
O(1)-Zn(1)-O(21)
O(11)-Zn(1)-O(21)
108.0(2) O(21)-Zn(2)-N(11)
107.8(2) O(21)-Zn(2)-N(12)
100.2(2) O(21’)-Zn(2)-N(1)
114.5(2) O(21’)-Zn(2)-N(2)
106.4(2) O(21’)-Zn(2)-N(11) 162.3(2)
122.6(1) O(21’)-Zn(2)-N(12) 91.0(2)
106.2(2) N(1)-Zn(2)-N(2)
102.5(2) N(1)-Zn(2)-N(11)
103.1(2) N(1)-Zn(2)-N(12)
N(2)-Zn(2)-N(11)
159.3(2) N(2)-Zn(2)-N(12)
90.3(2) N(11)-Zn(2)-N(12)
95.0(2)
107.3(2)
90.7(2)
106.9(2)
74.8(2)
99.2(2)
91.1(2)
89.9(2)
157.0(2)
74.3(2)
O(21)-Zn(2)-O(21’) 79.8(2)
O(21)-Zn(2)-N(1)
O(21)-Zn(2)-N(2)
a
Primed atoms are related to the unprimed ones by the symmetry
transformation −x, 1−y, 1−z.
Fig. 2. Partially labelled plot of the molecular structure of
complex 1.
washed with a small amount of H O to ensure com-
2
plete removal of LiCl. Fourth, the simultaneous pres-
ence of a hydroxo cluster and HCl in the products of
the reaction represented by eq. (2) could suggest that
the latter can decompose the former. The high yield
(60– 70%) of the preparation of 2 is against such a hy-
pothesis. The yield of 2 is not increased employing a
ZnCl : (py)C(Me)NOH : LiOH·H O ratio of 4:4:6, see
2
2
eq. (3):
Fig. 3. Partially labelled ball-and-stick presentation of the
tetranuclear molecule of 2 · 2 Et O. Primed and unprimed
EtOH/MeCN
2
4
ZnCl2 +4 (py)C(Me)NOH+6 LiOH·H2O −− −−−−−−− −→
atoms are related by the crystallographic inversion center.
O(21) and O(21’) are the hydroxo oxygen atoms. The inverse
metallacrown ring is highlighted.
[
Zn4(OH)2Cl2{(py)C(Me)NO}4]+6 LiCl+10 H2O
(3)
2
And fifth, complexes 1 and 2 seem to be the only tonation state of the ligand), led us to suspect that 1
products from the ZnCl /(py)C(Me)NOH reaction sys- could be transformed into 2; this has, indeed, turned
2
tem. The ZnCl : (py)C(Me)NOH ratio, the nature of out to be the case. Treatment of 1 with [NMe ]OH in
2
4
the solvent (the solvents MeOH, and MeOH/MeCN MeOH/MeCN leads to the high-yield (> 70%) isola-
were also used), the addition of counter anions such as tion of pure 2. The identity of the product from this
−
−
ClO4 and PF6 (for the isolation of potentially exist- transformation was confirmed by microanalysis and IR
ing cationic complexes) and the crystallization method spectroscopy comparing the results with those for the
have no influence on the identity of the products.
authentic sample prepared by the method reported in
The fact that complexes 1 and 2 contain basically the the Experimental Section and represented by eq. (2).
same ligand set (the major difference being the pro- The Br o¨ nsted acidity of 1 is summarized in eq. (4):
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C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
There are two intramolecular hydrogen bonds with
the uncoordinated oxime oxygen atoms as donors and
MeOH/MeCN
4
ZnCl2{(py)C(Me)NOH}+6 [NMe4]OH −− −−−−−−−− −→
1
(
4) the chloro ligands as acceptors. Their dimensions are
[Zn4(OH)2Cl2{(py)C(Me)NO}4] (2)
O(1)...Cl(1) 3.009 A˚ , H(O1)-Cl(1) 2.246 A˚ , O(1)-
◦
˚
+
6 [NMe4]Cl +4 (py)C(Me)NOH+ 4 H2O
H(O1)...Cl(1) 155.5 and O(11)...Cl(2) 3.114 A,
˚
H(O11)...Cl(2) 2.391 A, O(11)-H(O11)...Cl(2)
◦
1
48.6 .
Compound 1 is the second structurally character-
ized zinc(II) complex of (py)C(Me)NOH, see Table 3.
Description of structures
Selected interatomic distances and angles for com-
The first one was [Zn(NO ) {(py)C(Me)NOH} ] [41].
3
2
2
plexes 1 and 2 · 2 Et O are listed in Tables 1 and 2,
2
The molecular structures of the two complexes are very
similar, the only essential difference being the replace-
ment of the terminal chlorides of the former by mon-
respectively. The molecular structures of the two com-
plexes are shown in Figs 2 and 3.
Complex 1 crystallizes in the monoclinic space odentate nitrato ligands in the latter; an analogous cis-
group P2 /n. Its structure consists of well-separated cis-trans arrangement has also been observed in the
1
II
[
ZnCl {(py)C(Me)NOH} ] molecules. The Zn ion structure of the nitrato complex.
2
2
is coordinated by two chloro ligands and two N,N’-
Somewhat to our surprise, complex 1 is only just
chelating (py)C(Me)NOH molecules. The ligating the third member of a small family that comprises
atoms of (py)C(Me)NOH are the nitrogen atoms of the mononuclear, six-coordinate [ZnCl (L-L) ] complexes
2
2
neutral oxime and the 2-pyridyl groups. Thus, adopt- with a cis-cis-trans arrangement of donor atoms,
ing the Harris notation [38], (py)C(Me)NOH behaves where L-L is an asymmetric N,N’-chelating ligand
as an 1.011 ligand, see Fig. 4. The Harris notation de- [40, 42].
scribes the binding mode as X.Y Y Y ...Yn, where X
1
2
3
The tetranuclear molecule of 2 · 2 Et O lies on
2
is the overall number of metals bound by the whole lig-
and, and each value of Y refers to the number of metal
atoms (ions) attached to the different donor atoms. The
ordering of Y is listed by Cahn-Ingold-Prelog priority
rules, hence here O before N.
a crystallographic inversion center and has a planar,
nearly rhombic arrangement of the metal centers. The
two diagonals of the rhombus correspond to the in-
teratomic distances Zn(1)...Zn(1’) and Zn(2)...Zn(2’)
˚
which are equal to 5.782(4) and 3.252(4) A, respec-
The six-coordinate molecule is the cis-cis-trans iso- tively, the short diagonal being similar to the sides
˚
mer considering the positions of the coordinated chlo- of the rhombus [3.307(4), 3.327(4) A]. Due to the
rine, pyridyl nitrogen and oxime nitrogen atoms, re- presence of the crystallographic inversion center, the
2
+
spectively. The cis arrangement of the oxime groups {Zn(2)O(21)Zn(2’)O(21’)} sub-core is strictly pla-
seems unfavourable, most probably due to the steric nar. Using metallacrown nomenclature [6], the formula
hindrance arising from the methyl group upon oxime of the cluster is {(OH)[inv12-MC
-
ZnN{(py)C(Me)NO}
coordination. The molecule has no imposed symme- 4]Cl } ·2 Et O. The metallacrown is characterized as
2
2
try, although it approximates closely C2 symmetry, inverse because the Zn atoms, rather than the oximato
with the C axis bisecting the Cl(1)-Zn(1)-Cl(2) angle. oxygen atoms, are oriented towards the center of the
2
˚
The two Zn-Cl bond lengths [2.448(1), 2.369(1) A] cavity. For 12-MC-4 complexes, two structural motifs
are rather similar, while the Zn-N bonds [Zn- have been reported: regular [6, 23] and inverse [6, 21,
N = 2.131(2), 2.154(2) A] are slightly stronger than 25, 28, 29, 37], with most 12-MC-4 compounds be-
oxime
˚
˚
the Zn-Npyridyl bonds [Zn-N = 2.276(2), 2.213(2) A]. longing to the former type. In the regular motif, there
The Zn-Cl, Zn-Noxime and Zn-Npyridyl bond lengths in is an -N-O-M-N-O-M- linkage, i. e., an -[M-N-O]n- re-
1
compare well with the corresponding distances in peat unit, with the oxygen atoms oriented towards the
other structurally characterized six-coordinate zinc(II) center of the cavity and capable of binding cations [6].
complexes containing terminal chloro ligands [39, 40], In the inverse motif, which has been realized only for
pyridyl nitrogen [28, 39– 41] or oxime nitrogen atoms Zn [21, 25, 28, 37] and Co [29], the ring metal cations,
[28, 41]. The distortion from perfect octahedral geom- rather than the anionic oxygen atoms, are oriented to-
etry is a consequence of the relatively small bite an- wards the center of the cavity which is now capable of
gle of the chelating ligand [N(1)-Zn(1)-N(2) = 71.8(1), encapsulating anions, with the connectivity transposed
◦
N(11)-Zn(1)-N(12) = 73.0 ].
to -N-O-M-O-N-M-.
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C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
41
Table 3. Selected structural parameters for the inverse 12-
a
MC-4 complexes [Zn (OH) Cl {(py) CNO} ] and 2.
Parameter [A] or [ ]
4
2
2
2
4
˚
◦
[Zn4(OH)2Cl2{(py)2CNO}4] 2·2 Et2O
Zntetr...Zntetr
5.826
3.231
3.326
2.207
1.963
1.960
2.118
2.178
2.144
109.4
99.4
5.782
3.252
3.317
2.208
1.982
1.953
2.120
2.204
2.111
107.9
100.2
316.0
Znoct...Znoct
Zntetr...Znoct
b
Zntetr...Cl
Zntetr...Ohydroxo
Zntetr...Ooximate
Znoct...Ohydroxo
Znoct...Npyridyl
Znoct...Noximate
Zntetr-Ohydroxo-Znoct
Znoct-Ohydroxo-Znoct
Σ(Zn-Ohydroxo-Zn)
b
b
b
b
b
318.1
a
−
The anion (py)2CNO behaves as a 2.1110 ligand in this complex,
b
i. e., one of the 2-pyridyl rings remains uncoordinated;
ues; oct = octahedral Zn ion; tetr = tetrahedral Zn ion.
mean val-
II
II
Fig. 4. The crystallographically established coordination the diethylether oxygen atom [O(31) and its symmetry-
−
modes of (py)C(Me)NOH and (py)C(Me)NO in the hith- related partner O(31’)] as acceptor. Its dimensions are
erto structurally characterized metal complexes.
˚
˚
O(21)...O(31) 2.903 A, H(O21)...O(31) 2.020 A and
◦
O(21)-H(O21)...O(31) 172.6 .
The Zn centers in 2 · 2 Et O are bridged along
2
The molecular structure of 2 · 2 Et O is very sim-
2
each side of the rhombus by one µ -hydroxide
3
ilar to the structures of [Zn (OH) Cl {(py) CNO} ]
4
2
2
2
4
[
O(21), O(21’)] and one oximato (-1) group. The
[
25] and [Zn (OH) Cl {(py)CHNO} ] [37]. Rather
4
2
2
4
−
(py)C(Me)NO ligands adopt the 2.111 [38] coordina-
than discuss similarities, we list in Table 3 typical
structural parameters for the representative complex
tion mode. The coordination about the hydroxo oxygen
atom, O(21), is markedly pyramidal [sum of Zn-O(21)-
[
Zn (OH) Cl {(py) CNO} ] [25] and for 2 · 2 Et O.
4
2
2
2
4
2
◦
Zn angles = 316.0(2) ].
The remarkable similarity of the molecular struc-
II
The Zn ions exhibit two coordination geome- tures is clearly evident. Obviously, the replacement of
−
−
−
tries. Zn(2) and Zn(2’) are in a distorted cis-cis- (py)C(Me)NO by (py)CHNO and (py) CNO has
2
trans (O
) (N
hydroxo 2
) (N
pyridyl 2
) octahedral envi- little structural effect.
oximate 2
ronment, whereas Zn(1) and Zn(1’) are in a severely
distorted (Ohydroxo)(Ooximate)2Cl tetrahedral environ- structurally characterized zinc(II) metallacrown com-
Complex 2 · 2 Et O extends to ten the number of
2
ment. Each hydroxo oxygen atom bridges two octa- plexes that are based on 2-pyridyloximate ligands.
II
II
hedral Zn ions and one tetrahedral Zn ion. The oc- These complexes are summarized in Table 4. The com-
tahedrally coordinated metal ions are bound to two plexes belong to two families. The first family con-
−
(
py)C(Me)NO moieties forming two five-membered sists of complexes [Zn (OH) X {(py)C(R)NOH} ],
4
2
2
−
4
−
−
3
chelate rings. The two µ -hydroxide ions are accom- where X is a monodentate (Cl [25, 37], N [28],
modated in the center of the MC ring. The ligands NCO [28]), asymmetric bidentate (MeCO
form a propeller configuration that imposes absolute PhCO2 [28]) or a bidentate (acac [28]) terminal
3
−
−
[21],
2
−
−
stereoisomerism, with Λ chirality at Zn(2) and ∆ chi- ligand. These complexes have similar structures with
rality at Zn(2’). The longer Zn(2)-O(21, 21’) bonds an inverse 12-MC-4 topology. The second family con-
˚
[
2.105(4), 2.134(4) A] compared to the Zn(1)-O(21) sists of two pentanuclear cationic complexes [28] in
˚
II
−
one [1.982(4) A] are a consequence of the higher co- which the five Zn are held together by six (py) CNO
2
ordination number of Zn(2). The Zn(1)-Cl(1) bond ligands that adopt three different coordination modes;
˚
length [2.208 A] in 2·2 Et O is typical for such a ter- the chloro and isothiocyanato ligands are terminal.
2
II
minal bond in tetrahedral zinc(II) complexes [25, 37].
The five Zn ions define two nearly equilateral tri-
The crystal structure of 2 · 2 Et O is stabilized by angles sharing a common apex, and the novel Zn5
2
one crystallographically independent, intermolecular topology can be described as two “collapsed” 9-MC-
hydrogen bond with the hydroxo oxygen atom [O(21) 3 motifs sharing a common metal apex. Such struc-
−
and its symmetry-related partner O(21’)] as donor and tures can not be adopted by using (py)CHNO or
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2
C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
Complexa
Table 4. Structurally characterized zinc(II)
metallacrown complexes based on 2-pyrid-
yloximate ligands.
Topology
Ref.
[37]
this work
[25]
[21]
[28]
[28]
[28]
[28]
[
[
[
[
[
[
[
[
[
[
Zn4(OH)2Cl2{(py)CHNO}4]
Zn4(OH)2Cl2{(py)C(Me)NO}4]
Zn4(OH)2Cl2{(py)2CNO}4]
inv 12-MC-4
inv 12-MC-4
inv 12-MC-4
inv 12-MC-4
inv 12-MC-4
inv 12-MC-4
inv 12-MC-4
inv 12-MC-4
b
a
b
Zn4(OH)2(O2CMe)2{(py)2CNO}4]
Solvate molecules have been omitted; There
b
II
Zn4(OH)2(O2CPh)2{(py)2CNO}4]
Zn4(OH)2(N3)2{(py)2CNO}4]
Zn4(OH)2(NCO)2{(py)2CNO}4]
are two 5-coordinate Zn ions instead of two
tetrahedrally coordinated Zn ions; c The two
“collapsed” 9-MC-3 structures share a common
II
b
−
Zn4(OH)2(acac)2{(py)2CNO}4]
Zn apex; acac = acetylacetonato ligand; inv 12-
Zn5Cl2{(py)2CNO}6][ZnCl(NCS)3]
two “collapsed” 9-MC-3c [28]
MC-4 = inverse 12-metallacrown-4.
Zn5(NCS)2{(py)2CNO}6(MeOH)][Zn(NCS)4] two “collapsed” 9-MC-3c [28]
−
Table 5. Structurally characterized metal complexes containing (py)C(Me)NOH and/or (py)C(Me)NO as ligands.
Complexa
Coordination Coordination spheres; coordination geometries
Ref.
mode(s)
II
[
[
[
[
[
[
[
[
[
[
[
[
[
a
NiBr2{(py)C(Me)NOH}2]
Zn(NO3)2{(py)C(Me)NOH}2]
ZnCl2{(py)C(Me)NOH}2]
1.011
1.011
1.011
cis, cis, trans-Ni Br2(Npy)2(Nox)2; oct
[43]
[41]
this work
[43]
[44]
[45]
[31]
[46]
[47]
II
cis, cis, trans-Zn (Onitr)2(Npy)2(Nox)2; oct
II
cis, cis, trans-Zn Cl2(Npy)2(Nox)2; oct
II
Ni{(py)C(Me)NO}{(py)C(Me)NOH}(H2 O)2](NO3) 1.011; 1.011 trans, cis, cis-Ni O2(Npy)2(Nox)2; oct
II
Ni{(py)C(Me)NO}{(py)C(Me)NOH}(H2 O)2](ClO4) 1.011; 1.011 trans, cis, cis-Ni O2(Npy)2(Nox)2; oct
III
RhCl2{(py)C(Me)NO}{(py)C(Me)NOH}]
Co{(py)C(Me)NO}3]
1.011; 1.011 trans, cis, cis-Rh Cl2(Npy)2(Nox)2; oct
III
1.011
1.011
1.100
fac-Co N6; oct
III
Pt{(py)C(Me)NO}2]
trans-Pt N4; sp
cis, trans-Sb C3(Oox)2; tbp
V
Ph3Sb{(py)C(Me)NO}2]
n
1.100, 2.101 Sn C2(µ3-O)(Oox)(Nox), SnIVC2(µ3-O)2(Oox); spy, tbp [48]
IV
Bu 4Sn4O2{(py)C(Me)NO}4]
IV
IV
Et8Sn4O2{(py)C(Me)NO}4]
Me8Sn4O2{(py)C(Me)NO}4]
Zn4(OH)2Cl2{(py)C(Me)NO}4]
1.100, 2.101 Sn C2(µ3-O)(Oox)2, Sn C2(µ3-O)2(Nox); tbp, spy
[48]
IV
II
IV
II
1.100, 2.111 Sn C2(µ3-O)(Oox)2, Sn C2(µ3-O)2(Npy)(Nox); tbp, oct [49]
2.111
Zn (µ3-OH)(Oox)2Cl, Zn (µ3-OH)2N4; tetr, oct
this work
Solvate and other lattice molecules have been omitted; Nox = oxime or oximato nitrogen; Npy = 2-pyridyl nitrogen; Onitr = nitrate
oxygen; Oox = oximato oxygen; sp = square planar; spy = square pyramidal; tbp = trigonal bipyramidal; tetr = tetrahedral.
−
(
py)C(Me)NO , because these ligands lack the sec- (Fig. 4) is the exclusive one for the metal complexes
ond 2-pyridyl ring. It is worth mentioning that an- containing the neutral ligands. Of particular note are
other zinc(II) metallacrown complex containing a 2- the simultaneous presence of 1.011 (py)C(Me)NOH
−
pyridyloximate ligand has been structurally character- and (py)C(Me)NO ligands in nickel(II) [43, 44] and
ized: this is [Zn (shi) {(py) CNO} (MeOH) ], where rhodium(III) complexes [45]. The nature of R affects
8
4
2
4
2
3−
shi is the trianion of salicylhydroxamic acid [25]. the structural motif of the organometallic complexes
The molecule contains a 12-MC-4 core constructed by [R Sn O {(py)C(Me)NO} ] [48, 49]. It should also
8
4
2
4
3
−
four metal ions and four shi ligands; the MC core be mentioned that the coordination modes 1.100 and
accommodates a dinuclear Zn {(py) CNO} compo- 2.101, observed in organometallic compounds of anti-
2
2
4
nent, while two of the ring metals create dinuclear mony(V) [47] and tin (IV) [48, 49], are unique for the
II
−
units with the remaining two Zn ions through oxygen (py)C(R)NO ligands (R = H, Me) [36].
bridges. Since the MC ring is not constructed by the
(
−
py) CNO ligands, this octanuclear cluster has not
2
Infrared characterization
been included in Table 4.
A characteristic feature in the IR spectrum of
Complexes 1 and 2 join a small family of struc- 1 is the appearance of a strong broad band at
−
1
turally characterized metal complexes containing the 3200 cm assignable [33] to ν(OH) of the neutral
neutral or the anionic forms of methyl 2-pyridyl ke- (py)C(Me)NOH ligand. The presence of hydroxo lig-
tone oxime as ligands. Since many of these complexes ands in 2 is manifested by a medium broad band at
−
1
have been reported recently, we felt it timely to col- 3414 cm [32]. The broadness and relatively low fre-
lect all metal complexes of (py)C(Me)NOH and/or quency of these two bands are both indicative of strong
−
(
py)C(Me)NO in Table 5, together with the ligands’ hydrogen bonding.
−
1
coordination modes for convenient comparison. In-
Several bands appear in the 1620– 1350 cm
spection of Table 5 shows that the 1.011 ligation mode range in the spectra 1 and 2. Contributions from the
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C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
43
ν(C=N) and δ(CH ) modes would be ex- two new zinc (II) chloro complexes, one mononuclear
oxime/oximate
3
pected in this region, but overlap with the stretching vi- (1), and the other tetranuclear (2). In the former case,
brations of the aromatic ring renders assignments and the ligand is neutral and chelates through its nitrogen
the discussion of the coordination shifts difficult. The atoms; in the latter case, the ligand is anionic, chelat-
−
1
II
medium bands at 1566 and 1116 cm
trum of the free ligand (py)C(Me)NOH are assigned to a valuable addition to the small, but growing, fam-
ν(C=N) and ν(N-O)oxime, respectively [50]. The ily of inverse metallacrown complexes. The isolation
in the spec- ing and bridging a Zn pair (Fig. 4). Complex 2 is
2
oxime
−
1
1
116 cm band is shifted to a lower wavenumber and characterization of 2 also extends the body of
1102 cm ) in 1 which contains the neutral form of results that emphasize the ability of the anionic lig-
the ligand. This shift is attributed to the coordination and (py)C(Me)NO (and, in general, of the anionic 2-
of the oxime nitrogen atom, in accord with the slightly pyridyl oximes) to form interesting structural types in
longer N-O bonds in 1 (average length 1.390 A) com- coordination chemistry.
pared to this bond (1.406 A) in the free ligand [48].
Somewhat to our surprise the 1566 cm
shifted to a higher wavenumber (1596 cm , overlap- atives of 2 have an inverse 12-MC-4 structure. The
ping with an aromatic stretch) in 1. This shift may be terminal chloro ligands of 2 could have future utility
indicative of the oxime nitrogen coordination [51]. Ex- as sites for facile incorporation of bis(monodentate)
tensive studies on Schiff-base complexes (which also bridging organic ligands as a means to get access
−
1
(
−
˚
˚
Work is in progress to clarify if the isothiocyanate,
band is selenocyanate, cyanate, azide and dicyanamide deriv-
−
1
−
1
II
contain a C=N bond) have shown [52] that a change in to higher-nuclearity Zn clusters; experiments us-
the s character of the nitrogen lone pair occurs upon co- ing dicarboxylate ligands are planned. Analogues
ordination such that the s character of nitrogen orbital of compound 2, [Zn (OH) Cl {(py)CHNO} ] and
4
2
2
4
involved in the C=N bond increases; this change in [Zn (OH) Cl {(py) CNO} ] (see Table 4) with the re-
4
2
2
2
4
hybridization produces a greater C=N stretching force maining – commercially available – simple 2-pyridyl-
constant relative to the free neutral ligand. In accord aldoxime (Fig. 1), i. e. phenyl 2-pyridyl ketone oxime,
with this concept the C=N bond lengths in 1 (average are not known to date, and it is currently not evident
˚
1
.277 A) are not larger relative to the length of this whether the stability of these species is dependent on
˚
bond in free [48] (py)C(Me)NOH (1.278 A). We assign the particular nature of the oxime ligand. We are cur-
−
1
the 1544 and 1148 cm bands of 2 to ν(C=N)oximate rently studying this matter.
and ν(N-O)oximate, respectively [50]. The frequency
of the C=N stretching vibration has decreased from
Experimental Section
− −1
1
1
596 cm in 1 to 1544 cm in 2, whereas the NO
stretching vibration has increased from 1102 cm in
to 1148 cm in 2. These inverse shifts are in ac-
cord with the concept [50] that upon deprotonation of
the oxime group there is a higher contribution of N=O
and a lower contribution of C=N to the electronic struc-
ture; consequently the ν(CN) vibration shifts to a lower
frequency and ν(NO) to a higher frequency in 2, both
−
1
All manipulations were performed under aerobic condi-
tions using materials and solvents (Merck, Aldrich) as re-
ceived. The free ligand (py)C(Me)NOH was synthesized
by the reaction of equimolar quantities of 1-pyridin-2-
−
1
1
yl-ethanone (2-acetylpyridine), (py)C(Me)O, H NOH·HCl
2
and NaOEt in EtOH, as described by Saarinen and co-
workers [54]. The crude material was recrystallized twice
◦
◦
from benzene, m. p. 119 C (literature [54] 120 C). Ele-
shifts relative to 1. Indeed, the N-O bonds in 2 (aver- mental analyses (C, H, N) were conducted at the Univer-
age length 1.353 A) are shorter than those in 1 (average sity of Ioannina, Greece, Microanalytical Service using an
length 1.390 A).
˚
˚
−1
)
EA 1108 Carlo Erba analyser. IR spectra (4000 – 500 cm
were recorded on a Perkin-Elmer 16PC FT spectrometer with
samples prepared as KBr pellets.
The in-plane deformation band of the 2-pyridyl ring
−
1
of free (py)C(Me)NOH at 632 cm shifts upwards in
− −1
1
1
(638 cm ) and 2 (640 cm ), confirming the in-
[
ZnCl {(py)C(Me)NOH} ] (1)
2
2
volvement of the ring N-atom in coordination [53].
A colourless solution of ZnCl2 (0.14 g, 1.0 mmol) in
EtOH (10 ml) was added to a slurry of (py)C(Me)NOH
0.14 g, 1.0 mmol) in MeCN (10 ml). The resulting colour-
Concluding Comments and Perspectives
(
The second use (see Table 4) of methyl 2-pyridyl less solution was stirred for about 4 h and was then allowed
ketone oxime in Zn chemistry has provided access to to slowly concentrate by solvent evaporation at room tem-
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4
C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
Table 6. Summary of crystal data, data collection and struc- [Zn Cl (OH) {(py)C(Me)NO} ]·2 Et O(2·2 Et O)
4
2
2
4
2
2
ture refinement for the X-ray diffraction study of complexes
A colourless solution of ZnCl2 (0.14 g, 1.0 mmol) in
EtOH (10 ml) was added to a slurry of (py)C(Me)NOH
1
and 2·2 Et O.
2
Complex
1
2·2 Et2O
(
0.14 g, 1.0 mmol) and LiOH·H O (0.04 g, 1.0 mmol) in
Chemical formula
Formula weight
Colour, habit
Crystal system
Space group
C14 H16 Cl2N4O2Zn C36H50Cl2N8O8Zn4
2
408.58
1055.22
colourless prisms
monoclinic
P21/n
12.230(10)
14.640(10)
12.730(10)
90.96(3)
2279(3)
4
MeCN (10 ml). The reaction mixture was stirred for about
colourless prisms
monoclinic
P21/n
8.775(4)
10.771(5)
18.304(9)
92.44(2)
1728.4(14)
4
1.570
1.743
832
Mo-Kα
3 h, and then filtered and layered with Et O. Slow mixing
2
gave well-formed, X-ray quality colourless prismatic crys-
tals, which were collected by filtration, washed with cold
H O (2 × 1 ml), EtOH (2 ml) and Et O (3 ml) and dried
˚
a [A]
2
2
˚
b [A]
in vacuo over silica gel. Typical yields were in 60 – 70%
range. IR (KBr pellet): ν = 3414, 3070, 3025, 1598, 1544,
1476, 1440, 1376, 1256, 1148, 1072, 978, 784, 702, 640, 558,
˚
c [A]
β [deg]
˚
3
V [A ]
−
506 cm . The dried solid analysed satisfactorily as Et O-
2
1
Z
Dcalc [g cm 3]
−
1.538
2.252
1080
Mo-Kα
free. C28H30N O Cl Zn (906.96): calcd. C 37.08, H 3.34,
N 12.36; found C 36.90, H 3.40, N 12.20.
8
6
2
4
−
1
µ [mm
F (000)
Radiation [A]
]
˚
Crystal structure determinations
(
λ = 0.71073)
(λ = 0.71073)
Temperature [K]
Scan mode/speed
293(2)
θ-2θ/5
293(2)
Colourless prismatic crystals of 1 (0.25×0.25×0.70 mm)
θ-2θ/5
and 2·2 Et O (0.20×0.30×0.35 mm) were mounted in air;
2
−
1
[deg min
]
the crystals of 2 · 2 Et O had been previously covered with
2
θ Range [deg]
2.19 – 25.00
−10 → 0
2.17 – 24.94
−13 → 14
0 → 16
epoxy glue. Diffraction measurements were made on a Crys-
tal Logic Dual Goniometer diffractometer using graphite
monochromated Mo radiation. Crystal data and full details of
the data collection and data processing are listed in Table 6.
Unit cell dimensions were determined and refined by using
the angular settings of 25 automatically centered reflections
Ranges h
k
l
−12 → 0
−21 → 21
0 → 13
Measured reflections 3260
3246
Unique reflections
3049
Rint = 0.0181)
3099
(Rint = 0.0503)
(
Reflections used
◦
in the range 11 < 2θ < 23 . Three standard reflections moni-
[
I > 2σ(I)]
2727
272
a = 0.0146;
b = 0.5923
1.055
0.0256
0.0665
0.477/−0.327
2608
280
a = 0.1085;
b = 4.2129
1.038
0.0559
0.1537
0.981/−0.695
tored every 97 reflections showed less than 3% variation and
no decay. Lorentz, polarization corrections were applied us-
ing Crystal Logic software.
Parameters refined
w
a
2
GoF (on F )
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
∆
[
a
b
The structures were solved by direct methods using
SHELXS-86 [55] and refined by full-matrix least-squares
techniques on F with SHELXL-97 [56]. All H atoms for
structure 1 and some of the H atoms for structure 2 were lo-
cated by difference maps and refined isotropically. For both
structures all non-H atoms were refined using anisotropic
displacement parameters. Two X-ray crystallographic files
b
c
2
ρ)max/(∆ρ)min
˚
−3
]
e A
2
2
2
2
2
w = 1/[σ (Fo ) + (aP) + bP] and P = (max(Fo , 0) + 2Fc )/3;
c
2
2 2
R1 = Σ(|Fo| − |Fc|)/Σ(|Fo|);
wR2 = {Σ[w(Fo − Fc ) ]/
2
2
1/2
Σ[w(Fo ) ]}
.
for 1 and 2 · 2 Et O in CIF format have been deposited
2
with the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK, under the numbers 283299
perature for a period of 1 – 2 d. Well-formed X-ray qual-
ity colourless prismatic crystals appeared which were col-
(
1) and 283300 (2 · 2 Et O). Copies may be requested free
2
lected by filtration, washed with Et O (3 ml) and dried in
2
of charge from the Director of CCDC (E-mail: deposit@
ccdc.cam.ac.uk).
air. Typical yields were in the 50 – 60% range. IR (KBr pel-
let): ν = 3200, 3120, 3068, 1596, 1480, 1448, 1380, 1324,
Acknowledgements
1
4
258, 1164, 1140, 1102, 1042, 964, 786, 748, 682, 638, 552,
−1
51 cm – C H N O Cl Zn (408.58): calcd. C 41.15,
S.P.P. and C.P. thank the European Social Fund (ESF),
the Operational Program for Educational and Vocational
Training II (EPEAEK II) and particularly the Program
PYTHAGORAS (Grant b.365.037), for funding this work.
1
4
16
4
2
2
H 3.96, N 13.72; found C 41.33. H 3.88, N 14.60.
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C. Papatriantafyllopoulou et al. · A Mononuclear Complex and an Inverse 12-Metallacrown-4 Cluster
45
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