First-row transition metal complexes of 3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate)

Article abstract of DOI:10.1016/S0020-1693(00)00350-9

Reaction of 3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate) with M(NO₃)₂·xH₂O (M = Mn, Co, Ni, Zn) in acetonitrile solutions produced a linear polymer [Mn(μ-C₆H₅C₄O₃)(C₆ H₅C₄O₃) (H₂O)₃]_n (1) and a series of isomorphous monomeric complexes [M(C₆H₅C₄O₃)₂ (H₂O)₄] [M = Co (2), Ni (3), Zn (4)]. The complex 1 contains octahedral manganese with each metal centre being bridged μ-1,3 by a phenylsquarate ligand to two neighbouring metal atoms and further coordinated to a pendant phenylsquarate ligand group and three aqua ligands. The isomorphous monomeric complexes 2-4 all contain two phenylsquarate ligand groups oriented trans to each other, but with the substituent phenyl group on the ligand oriented cis to the ligating oxygen atom. The coordination polyhedron in each of these monomers is completed by four aqua ligands. The polymeric manganese complex shows no significant antiferromagnetic coupling and no semiconductivity.

Full text of DOI:10.1016/S0020-1693(00)00350-9

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Inorganica Chimica Acta 313 (2001) 56–64
First-row transition metal complexes of
3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate)
Avril Williams a, Bert D. Alleyne a, Hazel-Ann Hosein a, Hema Jaggernauth a,
Lincoln A. Hall a,*, Bruce M. Foxman b, Laurence K. Thompson c, Charles Agosta d
a Department of Chemistry, The Uni6ersity of the West Indies, St. Augustine, Trinidad and Tobago, West Indies
b Department of Chemistry, Brandeis Uni6ersity, Waltham, MA 02454-9110, USA
c Department of Chemistry, Memorial Uni6ersity, St. John’s, Newfoundland A1B 3X7, Canada
d Department of Physics, Clark Uni6ersity, Worcester, MA 01610, USA
Received 2 October 2000; accepted 10 October 2000
Abstract
Reaction of 3-phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenylsquarate) with M(NO3)2·xH2O (M=Mn, Co, Ni, Zn) in
acetonitrile solutions produced a linear polymer [Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1) and a series of isomorphous
monomeric complexes [M(C6H5C4O3)2(H2O)4] [M=Co (2), Ni (3), Zn (4)]. The complex 1 contains octahedral manganese with
each metal centre being bridged m-1,3 by a phenylsquarate ligand to two neighbouring metal atoms and further coordinated to
a pendant phenylsquarate ligand group and three aqua ligands. The isomorphous monomeric complexes 2–4 all contain two
phenylsquarate ligand groups oriented trans to each other, but with the substituent phenyl group on the ligand oriented cis to the
ligating oxygen atom. The coordination polyhedron in each of these monomers is completed by four aqua ligands. The polymeric
manganese complex shows no significant antiferromagnetic coupling and no semiconductivity. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Phenylsquarate; Crystal structures; Transition metal complexes; Polymeric complex
1. Introduction
and the complexes prepared from them, we decided to
synthesise a series of complexes with 3-phenyl-4-hy-
droxycyclobut-3-ene-1,2-dione (phenylsquarate) in
which the substituent was changed from the smaller
aliphatic methyl group to the aromatic and more steri-
cally demanding phenyl moiety. Here, we report on the
structure and bonding in a series of first-row transition
metal complexes of the phenylsquarate ligand and the
results of the magnetochemical and semiconducting
studies of the manganese phenylsquarate polymer 1.
Owing to our continued interest in investigating the
complexing properties of monosubstituted squarate lig-
ands and the magnetochemical and semiconducting
properties of their metal complexes, we recently synthe-
sised and reported on several series of first-row transi-
tion metal and lanthanide complexes with a variety of
these ligands, including the ligand 3-methyl-4-hydroxy-
cyclobut-3-ene-1,2-dione (methylsquarate) [1,2]. In or-
der to expand our knowledge of the effects of different
types of substituent on the properties of these ligands
2. Experimental
2.1. Preparation of the ligand
3-Phenyl-4-hydroxycyclobut-3-ene-1,2-dione (phenyl-
squarate) was prepared according to the method of
Liebeskind et al. [3].
* Corresponding author. Tel.: +1-868-662 2002; fax: +1-868-645
3771.
E-mail address: lhall@chem.uwi.tt (L.A. Hall).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(00)00350-9

A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
57
2.2. Preparation of the complexes [Mn(v-C6H5C4O3)-
(C6H5C4O3)(H2O)3]n (1) and [M(C6H5C4O3)2(H2O)4]
[M=Co (2), Ni (3) and Zn (4)]
analyses were done by flame atomic absorption spec-
trophotometry using a Unicam 929 atomic absorption
spectrometer.
40 ml of a solution of phenylsquarate (0.10 g, 5.80×
10−4 mol) in acetonitrile was mixed with an equal volume
of an equimolar solution of M(NO3)2·6H2O for M=Mn,
Co and Zn and for M=Ni a 1.15×10−3 molar solution
of Ni(NO3)2·6H2O in the same solvent. (Note: the 2:1
metal:ligand ratio in the preparation of the Ni complex
afforded superior quality crystals). The resulting mixture
in each case was filtered and left to evaporate slowly at
about 15°C until crystallisation was complete.
[Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1). Yield 15%.
Anal. Found: C, 51.48; H, 3.61; Mn, 12.10. Calc. for
C20H16MnO9: C, 52.76; H, 3.54; Mn,12.10%.
[Co(C6H5C4O3)2(H2O)4] (2). Yield 15%. Anal. Found:
C, 50.26; H, 3.68; Co, 12.24. Calc. for C20H18CoO10: C,
50.32; H, 3.77; Co, 12.36%.
[Ni(C6H5C4O3)2(H2O)4] (3). Yield 12%. Anal. Found:
C, 50.40; H, 3.79; Ni, 12.50. Calc. for C20 H18NiO10: C,
50.40; H, 3.80; Ni, 12.31%.
2.4. Crystallographic analyses
Crystallographic data for compounds 1–4 are sum-
marised in Table 1. Data for 1 and 2 were collected on
a Nonius CAD4-Turbo diffractometer; the structure was
solved by direct methods (SIR92) [4]. Data for structure
3 were collected on a Nonius Kappa CCD diffractometer
in the Department of Chemistry, University of Toronto,
and data for structure 4 were collected on a Siemens P4
diffractometer in the Department of Chemistry, Imperial
College, London. The latter two structures were solved
by direct methods (SHELXS-86) [5]. For all four structures,
full-matrix least-squares refinement was carried out using
the Oxford University CRYSTALS-PC system [6]. All
non-hydrogen atoms were refined using anisotropic dis-
placement parameters; hydrogen atoms were refined
using isotropic displacement parameters. Drawings were
produced using the Oxford University program
CAMERON [7]. Hydrogen bonding was analysed using
PLATON [8]. A full report on structures 1–4 is available
from the authors as a CIF file.
[Zn(C6H5C4O3)2(H2O)4] (4). Yield 12%. Anal. Found:
C, 46.25; H, 4.25; Zn, 12.63. Calc. for C20H18O10Zn·
2H2O: C, 46.20; H, 4.24; Zn, 12.59%. (This complex
apparently absorbs water on standing similar to other
phenylsquarate complexes.)
2.5. Variable-temperature magnetic measurements
The low yields are due to repeated filtration of the
reaction mixtures prior to crystal formation in order to
remove fine particulate material that is formed initially.
Variable-temperature magnetic data for 1 (2–300 K)
were obtained using a Quantum Design MPMS5S
SQUID magnetometer operating at 0.1 T. Calibrations
were carried out with a palladium standard cylinder,
and [H2TMEN][CuCl4] (H2TMEN=(CH3)2HNCH2-
CH2NH(CH3)22+) [9]. The sample was placed in a gelatin
capsule held within a drinking straw attached to
2.3. Elemental analyses
C and H analyses were done by MEDAC Limited,
Brunel Science Centre, Egham, Surrey, UK. The metal
Table 1
Crystallographic data for [Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1) and [M(C6H5C4O3)2(H2O)4] [M=Co(2), Ni (3), Zn (4)]
Chemical formula
Formula weight
1, C20H16MnO9
455.27
2, C20H18CoO10
477.29
3, C20H18NiO10
477.06
4, C20H18O10Zn
483.73
,
a (A)
7.5795(4)
8.2222(5)
15.5452(9)
84.847(5)
88.050(4)
81.516(4)
954.1
10.0773(20)
13.2445(24)
7.4491(11)
10.0832(4)
13.0546(7)
7.3658(3)
10.1156(8)
13.2142(14)
7.4519(7)
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
98.461(14)
97.266(3)
98.149(7)
3
,
V (A )
983.5
2
961.8
2
986.0
2
Z
2
Space group
T (K)
P1 [Ci, no. 2]
P21/c [C25h; no. 14]
294(1)
P21/c [C25h; no. 14]
150(2)
P21/c [C25h; no. 14]
293(2)
(
294(1)
0.710 73
1.585
,
u (A)
0.710 73
1.612
0.924
0.710 73
1.612
1.07
0.710 73
1.629
1.39
zcalc (g cm−3
)
v (mm−1
)
0.714
Transmission factors
0.82–0.91
0.0381
0.0405
0.63–0.74
0.0478
0.0501
0.78-0.93
0.0347
0.0351
0.61-0.90
0.0417
0.0445
Ra (I/|(I)]1.96)
b
Rw (I/|(I)]1.96)
a R=ꢀꢁꢁFoꢁ−ꢁFcꢁꢁ/ꢀꢁFoꢁ.
2
b Rw={ꢀw[ꢁFoꢁ−ꢁFcꢁ]2/ꢀwꢁFoꢁ }1/2
.

58
A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
Fig. 1. Environment of the Mn atoms in the structure of [Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1).
the sample-transport rod. Diamagnetic corrections for
the sample holder assembly and compound were ap-
plied in the usual way.
(Figs. 1 and 2). The MnꢀO bond lengths range from
,
2.153(2) to 2.220(3) A, and the cis and trans OꢀMnꢀO
angles range from 80.8(1) to 104.8(1)° and 165.8(1) to
170.8(1)° respectively. The phenylsquarate ligand in this
complex adopts two different coordination roles. Each
Mn atom in the polymer chain is linked to its neigh-
bour by a m-1,3 bridging phenylsquarate ligand [ligating
atoms O(11) and O(13)] while being simultaneously
coordinated to a monodentate pendant phenylsquarate
group [ligating atom O(1)]. The two phenylsquarate
moieties are nearly coplanar, with a dihedral angle of
21.9°; dihedral angles between the phenyl substituent
and the squarate ring are 2.3° and 11.1° for the bridg-
ing and pendant ligands respectively. The phenyl sub-
stituent on the pendant ligand is oriented cis to the
ligating oxygen atom. The coordination sphere about
the Mn is completed by three aqua ligands. As shown
in Fig. 2 and Table 2, short OꢀH···O hydrogen bonds
2.6. Semiconducti6ity measurements
Resistivity measurements were made using a stan-
dard four lead configuration to test the semiconducting
properties of the sample. Silver paste was used to fix
four 15 mm diameter gold leads to the corners of a
sample. In some of our experiments we used a direct
current of 1 mA and a voltmeter to make the resistance
measurements, and in others we used an alternating
current of 0.1 mA and a lock-in amplifier operating at
17.7 Hz. The samples were placed in an insulated probe
with helium exchange gas while the end of the probe
was lowered into liquid nitrogen or helium, depending
on the temperature range we intended to cover. The
temperature was measured with a calibrated cernox
thermometer from Lake Shore Cryotronics. All mea-
surements were made with the sample at a constant
temperature by using a standard PID temperature
controller.
,
involving X···Y pairs O(4)···O(12) (2.74 A), O(5)···O(12)
,
,
(2.71 A), and O(5)···O(2) (2.77 A) link the coordination
polymer along the crystallographic a direction.
The crystal structure of 1, shown in Fig. 3, exhibits a
bilayer motif, with alternating strongly hydrogen-
bonded layers and hydrophobic layers containing the
organic ligand tails. This arrangement is rather similar
to that observed for triaquabis(trans-2-pentenoato)-
calcium [10]. Short hydrogen bonds involving X···Y
3. Results and discussion
The
manganese
complex
[Mn(m-C6H5C4O3)-
,
,
atoms O(6)···O(3) (2.84 A) and O(4)···O(3) (2.69 A)
connect polymer chains along the crystallographic b
and c axes respectively. We can expect a bilayer pack-
ing motif to promote (i) parallel or nearly parallel
(C6H5C4O3)(H2O)3]n (1), exists as a one-dimensional
neutral coordination polymer in which chain propaga-
tion occurs along the crystallographic a direction, with
the Mn atom in a distorted octahedral environment

A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
59
Fig. 2. Part of the polymer chain of [Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1) showing the hydrogen bonding interactions.
alignment of organic side chains and (ii) relatively short
contacts between atoms in the organic region of the
bilayer [11]. As shown in Fig. 3, there is an alternating
pattern between the C4-rings and/or phenyl moieties.
There are sets of ligands in a nearly parallel arrange-
ment; left to right, the interplanar angles between pairs
of phenyl groups are 12.2°, 0.0°, and 12.2°, related to
the first pair by a centre of symmetry. The shortest
contacts between phenyl moieties are in the vicinity of
spectively. For the pendant ligand there is little, if any,
further extension of this delocalisation into the adjacent
,
MnꢀO bond [Mn(1)ꢀO(1)=2.190(2) A]. However, for
the bridging ligands the MnꢀO bond lengths indicate
that the delocalisation continues into one of the two
adjoining MnꢀO bonds and extends into the trans
,
MnꢀOaqua bond as well [Mn(1)ꢀO(11)=2.153(2) A,
,
,
Mn(1)ꢀO(4)=2.157(2) A; cf. Mn(1)ꢀO(1)=2.190(2) A].
A similar migration of electron density into the trans
MꢀO bond was assumed responsible for the cis-direct-
ing effect of the methoxysquarate ligand in a number of
,
,
3.6 A, whereas there is a contact of 3.40 A between
neighbouring C4-rings.
Although the phenylsquarate ligand in 1 adopts two
different coordination modes there are no significant
differences in the corresponding CꢀC bond lengths on
the respective C4-rings. The shorter C4-ring bond dis-
tances in both the pendant and bridging ligands are
Table 2
,
Selected bond lengths (A) and bond angles (°) for [Mn(m-
C6H5C4O3)(C6H5C4O3)(H2O)3]n (1)
Mn(1)ꢀO(1)
Mn(1)ꢀO(4)
Mn(1)ꢀO(5)
Mn(1)ꢀO(6)
Mn(1)ꢀO(11)
Mn(1)ꢀO(13)
O(1)ꢀC(1)
O(2)ꢀC(2)
O(3)ꢀC(3)
O(11)ꢀC(11)
2.190(2)
2.157(2)
2.193(2)
2.220(3)
2.153(2)
2.187(2)
1.246(3)
1.213(3)
1.244(3)
1.241(3)
O(12)ꢀC(12)
O(13)ꢀC(13)
C(1)ꢀC(2)
C(1)ꢀC(4)
C(2)ꢀC(3)
1.215(3)
1.241(3)
1.519(4)
1.426(3)
1.505(3)
1.423(3)
1.506(3)
1.434(3)
1.514(3)
1.433(3)
,
approximately 1.43 A, whereas the longer distances are
,
approximately 1.51 A. It should be noted that these
CꢀC bond lengths are within experimental error of the
corresponding bond lengths in the coordinated methyl-
squarate ligands in either the polymeric or monomeric
complexes of the first-row transition metals reported
recently [2b].
C(3)ꢀC(4)
C(11)ꢀC(12)
C(11)ꢀC(14)
C(12)ꢀC(13)
C(13)ꢀC(14)
In the Mn complex 1 there is a pattern of delocalisa-
tion that extends over the assembly of atoms consisting
of the pair of shorter ring CꢀC bonds and their adja-
O(1)ꢀMn(1)ꢀO(4)
O(1)ꢀMn(1)ꢀO(5)
O(4)ꢀMn(1)ꢀO(5)
O(1)ꢀMn(1)ꢀO(6) 169.63(9)
O(4)ꢀMn(1)ꢀO(6)
O(5)ꢀMn(1)ꢀO(6)
88.85(8)
89.64(8)
82.97(8)
O(5)ꢀMn(1)ꢀO(11)
O(6)ꢀMn(1)ꢀO(11)
O(1)ꢀMn(1)ꢀO(13)
O(4)ꢀMn(1)ꢀO(13)
O(5)ꢀMn(1)ꢀO(13) 170.83(7)
O(6)ꢀMn(1)ꢀO(13) 96.6(1)
O(11)ꢀMn(1)ꢀO(13) 94.00(6)
93.08(7)
85.57(9)
82.91(7)
91.51(7)
,
cent CꢀO bonds (approximately 1.24 A) on both the
bridging and pendant phenylsquarate ligands. This is
amply demonstrated by comparing the CꢀC and CꢀO
bond lengths in this assembly of atoms with the longer
80.8(1)
89.7(1)
O(1)ꢀMn(1)ꢀO(11) 104.81(7)
O(4)ꢀMn(1)ꢀO(11) 165.80(7)
,
C4-ring CꢀC bonds (approximately 1.51 A) and the
,
uncoordinated CꢀO bonds (approximately 1.21 A) re-

60
A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
(Fig. 4). The following description of structural features
is taken from data for the Zn complex (4), for which
the data are of the highest precision; differences be-
tween complexes 4 and 2/3 are not chemically signifi-
cant (Table 3). There is a short intramolecular OꢀH···O
,
hydrogen bond (O···O, 2.82 A); this feature has also
been observed in tetraaquabis(carboxylato)metal com-
plexes [12,13]. The phenyl substituent, which is oriented
cis with respect to the ligating oxygen atom, is nearly
coplanar (6.4°) with the C4-ring. The tendency for
tetraaquabis(carboxylates) to pack in a bilayer structure
[11] is once again observed here (Fig. 5). The squarate
ligands pack in an infinite stack, with pairs of ligands
related by the operation [x, −3/2−y, −1/2−z]. The
shortest contact between adjacent C4-rings (dihedral
,
angle 13.0°) is C(1)ꢀC(2), 3.28 A, whereas the shortest
contact between the phenyl substituents (dihedral angle
,
2.0°) is C(6)ꢀC(9), 3.44 A. Short hydrogen bonds be-
,
tween X···Y pairs O(4)···O(3) (2.81 A), O(5)···O(2)
,
,
(2.69 A) and O(5)···O(3) (2.87 A) link complexes in bc
planes, as shown in Fig. 5.
There are no statistically significant differences in
bond lengths between corresponding CꢀC bonds in the
C4-rings of complexes 2–4. The values for the zinc
complex are illustrative. Again, in each ring there are
,
two short CꢀC bonds of about 1.43–1.44 A and two
,
longer ones of about 1.50–1.51 A as observed in the
polymeric manganese complex 1. Analysis of the CꢀC,
CꢀO and metalꢀO bond lengths in complexes 2–4
reveals a pattern of delocalisation extending from one
of the ligands to the other through the trans metalꢀO
bonds by which they are attached. The zinc phenyl-
squarate will be used to illustrate this phenomenon.
Additionally, useful comparisons can be made with the
analogous zinc methylsquarate, in which a different
pattern of delocalisation is observed. Each of the two
shorter CꢀC bonds on the ligand C4-ring in zinc phenyl-
Fig.
3.
The
crystal
structure
of
[Mn(m-C6H5C4O3)-
(C6H5C4O3)(H2O)3]n (1) showing the bilayer motif and hydrogen-
bonded layers.
first-row transition metal complexes reported before
[2c]. There is no evidence for a similar extended delocal-
isation involving the other adjoining MnꢀO bond
,
squarate
4
[C(1)ꢀC(4)=1.432(3) A; C(3)ꢀC(4)=
1.437(3) A], which have significant double bond
character, is attached to long CꢀO bond
,
,
[Mn(1)ꢀO(13A)=2.187(2) A,Mn(1)ꢀO(5)=2.193(2) A].
The marked disparity in the lengths of the MnꢀO bonds
to the two ligating oxygen atoms of the bridging
phenylsquarate arises due to the unsymmetrical distri-
bution of the differently coordinated ligand groups
along the polymer chain. This disparity is not observed
in the cobalt (II) and nickel (II) methylsquarate poly-
mers in which chain propagation occurs via a combina-
tion of the crystallographic 21 screw axis and the mirror
plane passing through the methyl and diametrically
disposed carbonyl group on the methylsquarate ligand
[2b].
,
a
,
,
[C(1)ꢀO(1)=1.240(3) A, C(3)ꢀO(3)=1.235(3) A; cf.
,
C(2)ꢀO(2)=1.216(3) A]. The ZnꢀO bonds by which the
ligands are coordinated to the metal are short at
,
2.094(3) A [ZnꢀO(1) and ZnꢀO(1%)] when compared
,
with the corresponding ZnꢀO bonds [2.178(2) A] in zinc
methylsquarate [2b]. It is clear that the cis orientation
of the phenyl substituent to the ligating oxygen in 4
facilitates this extended delocalisation, whereas the
trans orientation of the methyl substituent in zinc
methylsquarate restricts the delocalisation to the two
short ring CꢀC bonds and the uncoordinated CꢀO
bonds.
3.1. Structures of [M(C6H5C4O3)2(H2O)4] [M=Co (2),
Ni (3), Zn (4)]
Although in complexes 2–4 and the transition metal
methoxysquarates the respective C4-ring substituents
are oriented cis to the ligating oxygen atom, the ligands
are, however, oriented trans to each other in 2–4 but
cis to each other in the methoxysquarates. Thus, the
Complexes 2–4 are isomorphous, monomeric, octa-
hedral species each containing two phenylsquarate lig-
ands oriented trans to each other; the octahedron about
each metal centre is completed by four aqua ligands

A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
61
Fig. 4. Environment of the Zn atoms in the structure of [Zn(C6H5C4O3)2(H2O)4] (4)].
row transition metals. They assumed that the enhanced
overlap was responsible for the antiferromagnetic cou-
pling observed in Cu2(salNEt2)2(H2O)(C4O4)·H2O. We
had also speculated that monosubstitution would have
a similar effect, since a monosubstituted ligand auto-
matically has symmetry lower than D4h. Now, we have
evidence to suggest that the energies of the orbitals on
the phenylsquarate ligand, which has C26 symmetry, are
different than those of the unsubstituted squarate ion.
cis-directing effect of the methoxysquarate ligand is not
observed for the phenylsquarate ligand. We speculate
that this may be due to the instability that would result
from the enhanced steric hindrance caused by the two
juxtaposed phenyl substituents in a cis configuration.
The complexes described in this paper, and the first-
row transition metal methylsquarate complexes re-
ported previously, all exhibit similar C4-ring geometries
,
and multiple bond localisation with a D(CꢀC)=0.06 A
Table 3
similar to that observed in Cu2(salNEt2)2(H2O)-
(C4O4)·H2O [13] but greater than that observed in
NH4[V2O2(OH)(C4O4)2·(H2O)3]H2O [15] and the vana-
dium complexes synthesised by Hilbers et al. [16], all of
which are m-1,2-squarato bridged. Thus, the ligand ring
geometry is apparently unaffected when the aliphatic
methyl substituent is replaced by the aromatic phenyl
substituent. This we assumed is due to the fact that,
although the obvious conjugation between the phenyl
,
Selected bond lengths (A) and bond angles (°) for
[M(C6H5C4O3)2(H2O)4] (2–4)
2, M=Co
3, M=Ni
4, M=Zn
M(1)ꢀO(1)
M(1)ꢀO(4)
M(1)ꢀO(5)
O(1)ꢀC(1)
O(2)ꢀC(2)
O(3)ꢀC(3)
C(1)ꢀC(2)
C(1)ꢀC(4)
C(2)ꢀC(3)
C(3)ꢀC(4)
2.095(3)
2.069(4)
2.084(4)
1.246(6)
1.232(5)
1.242(6)
1.476(6)
1.428(6)
1.499(6)
1.445(6)
2.062(2)
2.058(2)
2.060(2)
1.253(3)
1.232(3)
1.244(3)
1.501(4)
1.436(4)
1.516(4)
1.446(4)
2.094(2)
2.089(2)
2.086(2)
1.240(3)
1.216(3)
1.235(3)
1.501(3)
1.432(3)
1.513(3)
1.437(3)
,
substituent and the C4-cycle (CphenylꢀC4=1.45 A) sug-
gests that there is interaction of the two p-systems,
there is no available mechanism for the migration of
electron density into the C4-cycle as in the
methoxysquarate ligand.
O(1)ꢀM(1)ꢀO(1)
O(1)ꢀM(1)ꢀO(4)
O(4)ꢀM(1)ꢀO(4)
O(1)ꢀM(1)ꢀO(5)
O(4)ꢀM(1)ꢀO(5)
O(5)ꢀM(1)ꢀO(5)
180
85.30(17)
180
86.72(16)
92.79(18)
180
180
86.04(8)
180
85.61(9)
93.31(9)
180
180
85.62(8)
180
86.32(8)
92.79(9)
180
Xanthopoulos et al. [14] suggested that when the
squarate ligand changed its bridging mode from m-1,3
to m-1,2 the resultant increase in energy of its orbitals
due to the lowering of its symmetry from D4h to C26
enhanced the overlap with the metal orbitals of first-

62
A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
Fig. 5. The structure of [M(C6H5C4O3)2(H2O)4] [M=Mn (1), Co (2), Ni (3), Zn (4)] showing the bilayer motif and hydrogen-bonded layers.
and a monomer obtained at 15°C. However, it is possi-
ble that the d orbital electron distribution in the metal
might be a factor in determining whether monomeric or
polymeric species are formed. We are also unable to
explain why we were unsuccessful in preparing a cop-
per(II) complex with the phenylsquarate ligand.
This evidence is provided by luminescence spec-
troscopy of europium(III) phenylsquarate, which shows
that the phenyl substituent sensitises Eu3+(5D0“7Fj)
emissions; this phenomenon is typical of ligands con-
taining low-energy orbitals [17]. Similar emissions are
absent in the spectrum of europium(III) squarate.
These low-energy orbitals are attributable to the inter-
action between the p-orbitals on the phenyl ring and
those on the C4-cycle in the phenylsquarate ligand, as
indicated by the short CphenylꢀC4 bond length. Since
short CphenylꢀC4 bonds are also present in the first-row
transition metal phenylsquarates described herein, we
assume that the orbital energies of the phenylsquarate
ligands in these complexes are different from and lower
than those in the unsubstituted squarate ligand.
3.2. Magnetic and semiconducti6ity studies on
[Mn(v-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1)
The magnetic and conducting properties of 1 were
investigated, since one-dimensional polymers contain-
ing paramagnetic centres have potential for demon-
strating such properties.
It is not clear why a polymeric species was obtained
only in the case of manganese whilst monomeric species
were obtained for Co, Ni and Zn. It might be that the
steric demands of the substituent phenyl rings hinder
chain propagation in the case of the smaller Co, Ni and
Zn ions or alternatively different physical conditions
may be necessary for polymer formation. An example
of the latter is provided by the methylsquarate com-
plexes of cobalt, where a polymer was obtained at 28°C
3.2.1. Variable-temperature magnetic determinations
Variable-temperature (2–300 K) magnetic data for
[Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1) were ob-
tained in a 0.1 T field. The magnetic moment varied
from 5.8 mB at 300 K to 5.6 mB at 2 K, indicating that
the manganese(II) centres in the one-dimensional chains
are uncoupled. The small drop in moment at very low
temperature cannot be considered significant in terms
of any intra-chain exchange coupling. The absence of

A. Williams et al. / Inorganica Chimica Acta 313 (2001) 56–64
63
coupling interactions suggests that the bridging phenyl-
squarate ligands in this complex are not functioning as
efficient mediators for such interactions. This is explica-
ble in terms of the fact that, based on the X-ray data,
the delocalisation that begins within the bridging
phenylsquarate ligands only extends into the orbitals of
one of the two Mn atoms to which these ligands are
attached and not into the other [Mn(1)ꢀO(13A)=
monosubstituted squarate ligands that coordinate with
the substituent cis to the ligating oxygen. Increased
electron density on the C4-ring in polymeric complexes
containing m-1,3 bridging monosubstituted squarate lig-
ands is apparently required for the mediation of cou-
pling interactions between metal centres in order to
counteract the adverse effect of the long six-bond con-
nections. We intend to continue our investigation into
the properties of monosubstituted squarate ligands in
order to develop a capability for synthesising (i) poly-
meric species with a greater degree of certainty and (ii)
complexes that exhibit molecular magnetism and
semiconductivity.
,
2.187(2), Mn(1)ꢀO(11)=2.153(2) A]. Thus, the ex-
tended delocalisation along the entire polymer chain,
which is necessary for the mediation of coupling inter-
actions, is absent. This is exacerbated by the long
six-bond connections between the metal centres. In
addition, although the conjugation between the p-sys-
tems on the phenyl substituent and C4-cycle has been
shown to produce ligand orbitals available for interac-
tion with the metal orbitals (vide supra), there is appar-
ently insufficient electron density available for
facilitating communication between the metal centres.
This is demonstrated by the fact that the polymeric
manganese(II) diphenylaminosquarate (which contains
symmetrically oriented m-1,3 bridging ligands and addi-
tional electron density on the ligand of the C4-cycle due
to the migration of the nitrogen lone pair, unlike
[Mn(m-C6H5C4O3)(C6H5C4O3)(H2O)3]n (1)) does show
antiferromagnetic coupling, although it is very weak
[J= −0.27(5) cm−1, J%= −0.40(5) cm−1; manuscript
in preparation] [18].
5. Supplementary material
A CIF file containing data for the structures of 1–4
and a table of hydrogen bonding information for com-
plexes 1 and 4 (two pages) are available from the
authors on request.
Acknowledgements
L.A.H. wishes to thank the St. Augustine Campus
Committee on Graduate Studies, The University of the
West Indies, for financial support; B.M.F. wishes to
thank the National Science Foundation (grants DMR-
9629994 and CHE-9305789), and Polaroid Corporation
for the partial support of this research. L.K.T. wishes
to thank the Natural Sciences and Engineering Re-
search Council of Canada (NSERC) for financial
support.
3.2.2. Semiconducti6ity studies
The
manganese
complex
[Mn(m-C6H5C4O3)-
(C6H5C4O3)(H2O)3]n (1) shows no conductivity (resistiv-
ity \5×10−3 V m). This, we assume, is due to the
absence of the extended delocalisation along the poly-
mer chain which would be required to produce the
band gaps necessary for semiconduction.
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