Bivalent transition metal complexes of 4,5-diazafluorene-9-one benzoylhydrazone (HL) and the characterization of weak interaction in CoL2(H2O)2

Article abstract of DOI:10.1016/S0022-2860(00)00568-8

A new ligand 4,5-diazafluorene-9-one benzoylhydrazone (HL) and six of its transition metal complexes ML₂·nH₂O [M(II) = Mn, n = 2, 1; Co, 2, 2; Ni, 2, 3; Cu, 3, 4; Zn, 1.5, 5; Pb, 4, 6] have been prepared and characterized. Elemental analyses, IR, UV-Vis, ¹H NMR, molar conductivity and thermal analyses measurements showed that the ligand coordinates to the metal ion with the enolic O and azomethine N atoms of the acylhydrazone group while the 4,5-diazafluorene moiety is free. Single crystal X-ray diffraction studies showed that CoL₂(H₂O)₂, 2, has a centrosymmetric octahedral geometry of N₂O₄ with the two bidentate ligands in the basal plane and the two water molecules in the axial sites. The molecules of 2 are linked together by intermolecular hydrogen bonds N(diazafluorene)···HOH···N'(diazafluorene) and π-π stacking interactions between the diazafluorene rings to form 2D sheets. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0022-2860(00)00568-8

Journal of Molecular Structure 553 (2000) 91–99
www.elsevier.nl/locate/molstruc
Bivalent transition metal complexes of 4,5-diazafluorene-9-one
benzoylhydrazone (HL) and the characterization of weak
interaction in CoL2(H2O)2
Wen Xiaoa,b, Zhong-Lin Lua,b, Cheng-Yong Sua, Kai-Bei Yuc, Liang-Rong Denga,
Han-Qin Liua, Bei-Sheng Kanga,
*
aSchool of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, People’s Republic of China
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
cChengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China
Received 14 February 2000; accepted 24 March 2000
Abstract
A new ligand 4,5-diazafluorene-9-one benzoylhydrazone (HL) and six of its transition metal complexes ML2·nH2O ‰MꢀII† ˆ
Mn; n ˆ 2; 1; Co, 2, 2; Ni, 2, 3; Cu, 3, 4; Zn, 1.5, 5; Pb, 4, 6] have been prepared and characterized. Elemental analyses, IR,
UV–Vis, 1H NMR, molar conductivity and thermal analyses measurements showed that the ligand coordinates to the metal ion
with the enolic O and azomethine N atoms of the acylhydrazone group while the 4,5-diazafluorene moiety is free. Single crystal
X-ray diffraction studies showed that CoL2(H2O)2, 2, has a centrosymmetric octahedral geometry of N2O4 with the two
bidentate ligands in the basal plane and the two water molecules in the axial sites. The molecules of 2 are linked together
0
…
N
…
by intermolecular hydrogen bonds Ndiazafluorene HOH
and p–p stacking interactions between the diazafluorene
diazafluorene
rings to form 2D sheets. ᭧ 2000 Elsevier Science B.V. All rights reserved.
Keywords: 4,5-Diazafluorene-9-one benzoylhydrazone; Transition metal complexes; Intermolecular hydrogen bonds; p–p Stacking inter-
actions; Enol form
1. Introduction
metals [5], lanthanide metals [6], main group metals
[7] and even non-metals [8] with versatile coordina-
tion modes also prompted the interests of inorganic
chemists. These ligands can coordinate either in the
keto form or the enol form depending on the acidity,
the solvent of the reaction and the nature of the metal
salts. In general, aroylhydrazones chelate in the keto-
form in strongly acidic solutions and in the enol-form
in strongly basic solutions [9]. Without adjusting the
pH value of the reaction, they coordinate in the keto
form on reaction with metal sulphates, nitrates, or
perchloates [10], in the enol-form with metal acetates,
and in either keto- or enol-form with metal chlorides
Though aroylhydrazones have been investigated
extensively, they continue to attract considerable
attention due to their wide biological and pharmaco-
logical activities, such as antitumor [1], antidiabetic
[2], antitubercular [3] activities. However, the corre-
lations between structures and bioactivities are still
not very clear and much work needs to be done [4].
Secondly, their easy coordination with transition
* Corresponding author. Fax: ϩ86-20-8403-4056.
E-mail address: ceslh9@zsu.edu.cn (B.-S. Kang).
0022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00568-8

92
W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
Table 1
Some physical properties and elemental analysis data of HL·H2O and 1–6
Formula
Mr
Color
Lm (s cm2 molϪ1
)
Yield (%)
Found (Calcd.) %
C
H
N
HL·H2O
C18H14N4O2
318.33
689.59
693.58
693.34
716.21
691.03
877.88
Pale
Yellow
1.64
25.65
9.45
70
82
86
84
78
76
78
67.55
(67.91)
4.49
(4.43)
17.48
(17.60)
1
2
3
4
5
6
MnL2·2H2O
(C36H26N8O4Mn)
Yellowish
Red
62.52
(62.70)
3.75
(3.80)
16.22
(16.25)
CoL2·2H2O
(C36H26N8O4Co)
Yellow
Yellow
Black
63.27
(62.33)
3.61
(3.78)
15.90
(16.16)
NiL2·2H2O
(C36H26N8O4Ni)
4.80
62.56
(62.36)
3.50
(3.78)
16.09
(16.17)
CuL2·3H2O
(C36H28N8O5Cu)
8.00
60.70
(60.36)
4.07
(3.94)
15.44
(15.64)
ZnL2·1.5H2O
(C36H25N8O3.5Zn)
Yellow
Yellow
5.08
62.53
(62.56)
3.19
(3.65)
16.34
(16.21)
PbL2·4H2O
15.30
49.00
3.40
12.38
(C36H30N8O6Pb)
(49.25)
(3.44)
(12.77)
[11–13]. Acylhydrazones formed by condensation of
acid hydrazides with aromatic aldehydes or ketones
containing coordination sites in ortho positions should
possess multiple coordination modes. For example,
salicylaldehyde benzoylhydrazone can form four
types of coordination [14] and 2,6-diformyl-4-methyl-
phenol di(benzoylhydrazone) has three tautomeric
forms: keto (mononegative charge), mixed keto–
enol (dinegative charges), and enol forms (trinegative
charges) [15]. Thirdly, rich nonlinear optical proper-
ties [16] and other applications such as analytical and
metal extracting reagents [17] also caused intensive
studies of the aroylhydrazones.
Especially, Ru complexes with 4,5-diazafluorene-9-
one or its derivatives as bidentate [21,22] or bridging
tetradentate ligands [23,24] have been investigated
extensively due to their rich electrochemical, photo-
physical and photochemical properties.
In order to compare the coordination abilities of the
4,5-diazafluorene moiety and the acylhydrazone
group and to further study the coordination modes
of aroylhydrazones, the condensed products 4,5-
diazafluorene-9-one aroylhydrazones have been
designed and synthesized and their transition metal
complexes prepared. Here we report a new ligand
4,5-diazafluorene-9-one benzoylhydrazone (HL) and
six of its transition metal complexes with the crystal
structure of CoL2(H2O)2 examined to depict the coor-
dination mode of the ligand.
In the meantime, 4,5-diazafluorene-9-one and its
derivatives are also strong chelating agents to
transition metals [18,19] and lanthanide metals [20].

W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
93
Table 2
Crystallographic data and data collection parameters for 2
Formula
Color
C36H26N8O4Co
Red
0.56 × 0.50 × 0.30
Monoclinic
C2/c
17.892(5)
7.3310(10)
24.166(5)
90
98.77(2)
90
Mr
Z
693.58
4
1.471
0.604
1428
3.42–54.00
3430
2485
Size (mm)
Crystal system
Space group
r (g cmϪ3
)
)
m (mmϪ1
F (000)
˚
a (A)
2u range (Њ)
˚
b (A)
Reflections collected
Observed reflections ꢀI Ͼ 2sꢀI††
No. of variables
˚
c (A)
a (Њ)
b (Њ)
g (Њ)
232
Ra, wRb
Goodness of fit
0.0338, 0.0794
0.938
0.268,Ϫ0.234
3
3
˚
˚
V (A )
3132.7(12)
D(r)max., min. (eA )
a
R ˆ S͉͉Fo͉ Ϫ ͉Fc͉͉=S͉Fo͉:
2
2
b
2
wR ˆ ‰Swꢀ͉Fo͉ Ϫ ͉Fc͉ =S͉wꢀFo2† ͉Š1=2; w ˆ ‰s2ꢀFo2† ϩ ꢀ0:0467P†2 ϩ 0:0000PŠϪ1; P ˆ ꢀFo2 ϩ 2 Fc2†=3:
2. Experimental
refluxed for 4 h to ensure the completion of reaction.
The precipitate was filtered while hot, washed several
times with ethanol and dried in vacuo. The formula,
formula weight, color, yield, molar conductance and
elemental analyses data are also listed in Table 1.
2.1. Reagent
Transition metal salts and other reagents were
commercially available and of analytical grade.
Benzoylhydrazine was prepared by condensation of
methyl benzoate with hydrazine hydrate in methanol.
4,5-Diazafluorene-9-one was prepared by the reported
method [25].
2.4. Physical measurements
Elemental analyses were performed on a Perkin–
Elmer 240C analytical instrument. IR spectra were
recorded on a Nicolet FT-IR-170SX spectrophot-
ometer in KBr pellets in the 4000–400 cmϪ1 region.
Conductance measurements were made with a DDS-
12A digital conductometer in DMSO solution
2.2. Synthesis of 4,5-diazafluorene-9-one
benzoylhydrazone HL
1
(ϳ10Ϫ4 M) at room temperature. H NMR spectra
Benzoylhydrazine (15 mmol, 2.04 g) dissolved in
15 cm3 of EtOH was added to 4,5-diazafluorene-9-
one (15 mmol, 2.73 g) in 80 cm3 of hot ethanol with
stirring in the presence of several drops of acetic acid.
Yellow precipitate began to appear within several
minutes. The reaction mixture was continued to reflux
for 4 h. Upon cooling, the precipitate was collected,
washed with cold EtOH and dried in vacuo, m.p.
166ЊC. Analytical data and yield are listed in Table 1.
were recorded on a Bruker Aspect AM300 spectro-
meter in (CD3)2CO for HL and a Varian Unity INOVA
500 spectrometer in DMSO-d6 for 5 and 6 at room
temperature using TMS as the internal reference.
Electronic spectra were recorded on a Shimadzu 240
spectrophotometer in DMF solution (ϳ10Ϫ5 M).
Thermal analyses were carried out on a Perkin–
Elmer TGS-2 thermogravimetric analyzer with a
heating rate of 10ЊC minϪ1 between room temperature
and 650ЊC in air.
2.3. Syntheses of complexes ML2·nH2O 1–6
All the complexes were prepared by the following
general procedure: the EtOH solution of
M(OAc)2·xH2O (0.25 mmol) (M ˆ Mn, Co, Ni, Cu,
Zn, Pb; x ˆ 4, 4, 4, 1, 2, 3, respectively) was added
to HL (0.50 mmol, 0.16 g) in 20 cm3 of EtOH and
precipitates appeared immediately. The solution was
2.5. Crystal structure determination
Dark red crystals of 2 suitable for X-ray measure-
ment were obtained by slow evaporation of a solution
of 2 in DMF in air. A single crystal with suitable
dimensions was mounted on a glass fiber and the

94
W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
intensity data were collected on a Siemens P4 diffract-
ometer by v –2u scan technique with graphite-mono-
ꢀ
chromated Mo Ka radiation ꢀl ˆ 0:71073 A† at
ꢀ2†
296 K. The data collection and the refinement para-
meters are summarized in Table 2. The reflection data
were corrected for Lorentz and polarization effects
during data reduction using xscans [26] and an
empirical absorption correction based on a series of
c-scans was applied to the data. The structure was
solved by direct methods and refined on F2 by full-
matrix least-squares methods. All non-hydrogen
atoms were refined anisotropically. All the hydrogen
atoms were generated geometrically and refined using
the riding model with fixed isotropic thermal para-
meters. All calculations were carried out on IBM PC
with Siemens shelxtl program package [27].
.
3.2. Molar conductivity
The molar conductance values of the ligand and the
complexes at 10Ϫ4 mol dmϪ3 in DMSO solution are
listed in Table 1. The molar conductivity of a
compound in dilute solution is mainly dependent on
its stoichiometry. Theoretically, the number of anions
and cations in a compound can be predicted from the
measured molar conductivity values by comparison
with the known data [28]. A review on the use of
conductivity data in various organic solvents for the
characterization of coordination compounds by Geary
in 1971 [29] has been cited extensively as a criteria for
non-electrolytic and electrolytic nature. Thus,
compared to those reported [29], the low molar
conductivity of the ligand and the complexes in the
range 1–26 s cm2 molϪ1 indicated that they are non-
electrolytes. The neutrality of the complexes is a
result of the coordination of the monoanionic form
of the ligand to the metal(II) ions. The slightly higher
values for 1 and 6 might be an indication of certain
degree of dissociation of the complexes in strongly
polar solvents.
3. Results and discussion
3.1. Syntheses
The ligand HL was prepared by the condensation of
4,5-diazafluorene-9-one with benzoylhydrazine in
molar ratio 1:1 (Eq. (1)). Elemental analyses and the
following spectral analyses indicate that on reaction
with transition metal acetates (Eq. (2)) the acylhydra-
zone moiety of the ligand HL coordinates to the metal
ion in the enolic LϪ form to give complexes with
the formula ML2·nH2O, while the diazafluorene part
of the molecule does not coordinate. The ligand is
insoluble in non-polar or weakly polar solvents
such as benzene, alkanes and ether, soluble in
moderately polar solvents such as methanol, ethanol
and acetone, and easily soluble in strongly polar
DMF and DMSO, while complexes 1–6 are only
soluble in DMF and DMSO. The synthetic routes
of the ligand and the complexes are shown as
follows:
3.3. IR spectra
Important IR frequencies of the ligand and the
complexes along with respective assignments are
listed in Table 3. Characteristic band due to n ꢀCyO†
of the free ligand appearing at 1693 cmϪ1 is an indi-
cation that the ligand exists in the keto-form in the
solid state. In all the complexes, the bands n ꢀCyO†;

W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
95
Table 3
more flexible to form stable five-membered chelate
ring, while the two pyridyl N atoms are structurally
rigid and cannot rotate freely.
Some important IR absorption (cmϪ1) of HL·H2O and 1–6
n(OH)
nꢀCyO† nꢀCyN† nꢀCyN–NyC† n(NCO) n(C–O)
The appearance of n(H2O) in range 3300–
3430 cmϪ1 in the compounds HL·H2O and 1–6 as
compared to “free” hydroxyl groups in range 3580–
3650 cmϪ1 is an indication of the presence of
hydrogen bonded water molecules. Hydrogen bonds
between the Ndiazafluorene atom and H2O have been
reported [32], as also confirmed by the crystallo-
graphic studies on HL·H2O [33] and 2.
HL·H2O 3353br 1693vs 1532s
–
1497vs
1482vs
1500vs
1500s
–
–
1
2
3
4
5
6
3300br
3409br
3357br
3405br
3357br
3428br
–
–
–
–
–
–
–
–
–
–
–
–
1484vs 1296s
1293s
1483vs 1297s
1483vs 1297s
1484vs 1297s
1477vs 1292s
1497s
1493s
n ꢀCyN† and n (C–N) (1250 cmϪ1) of the ligand have
disappeared, which indicates that the ligand under-
goes tautomerization from the keto-form to the
enol-form. As a result, three new strong bands are
observed around 1500, 1480, and 1296 cmϪ1 due to
n ꢀCyN–NyC†; n ꢀNyC–O† [5], and n (C–O) [30],
respectively. The medium bands at 1588, 1567, 814,
and 751 cmϪ1 assigned to the absorptions based on the
pyridyl and phenyl rings [31] in the free ligand have
not changed in either intensities or frequencies in all
the complexes, indicating the non-coordination of the
diazafluorene moiety.
All of these facts indicate that the ligand coordi-
nates to the metal center through the deprotonated
enolic aroylhydrazone group while the diazafluorene
moiety is free. The reason for this can be attributed to
two aspects. Firstly, the bidentate N:OϪ chelation is
more electronegative than the N:N chelation.
Secondly, the acylhydrazone group yN–NyCꢀO†– is
3.4. Electronic spectra
The ligand exhibits four bands at 276, 309, 345 and
402 nm, which are generally assigned to the p–pء
transitions [11], which are slightly red shifted in the
complexes. The band at 345 nm can be assigned to
transition based on the acylhydrazone group [11]
which undergoes a bathochromic shift of 3–13 nm.
The reason is that the d-orbital of the metal ion over-
laps with the p-orbital of the ligand to form a large
conjugate system after enolization and complexation,
which makes the energy level difference between the
p and pء orbitals, i.e. p–p ء transition energy
required, decreases. These factors led to a batho-
chromic shift of these bands in the complexes [34].
Unfortunately, attempts to obtain information on the
stereochemical structures of the complexes were
unsuccessful because MLCT bands were not observed
Table 4
1H NMR spectral data (d/ppm, J/Hz) (the proton numberings are shown in the structural scheme in Section 1) for HL in acetone and 5 and 6 in
DMSO-d6 at room temperature. (d: doublet; dd: doublet of doublets; m:multiplet)
a 0
a
b 0
b
g 0
g
a
b
c
a
HL
d
8.776
(dd,1H)
8.759
(dd,1H)
5.1,0.9
7.526
(dd,1H) (d,1H)
5.1, 7.2
8.566
8.248
(d,1H)
7.2
8.048
(d,2H)
7.2
7.705–7.586
(m,4H)
Jb 5.1,0.9
7.8
5
6
d
8.771
(m,1H)
8.728
(m,1H)
7.638
(m,1H)
7.556
(m,1H)
9.398
(d,1H)
8.900
(d,1H)
8.125
(d,2H)
7.496
(m,3H)
c
d
8.640
8.521
7.448
9.029
8.274
8.137
7.212
(d,1H)
(d,1H)
(m,4H)
(d,1H)
(d,1H)
(d,2H)
(m,1H)
a
Merged in the signals of protons b and c.
J values represent the coupling constants of ortho-protons except the value 0.9 Hz represents J value for meta-protons in the three aromatic
b
rings.
Merged in the signals of protons b and b 0.
c

96
W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
cates unsymmetric electron density distribution [21]
due to the different electron-attracting effect of the
two pyridyl N atoms and the sp2 hybridization of the
CyN bond in the hydrazone moiety [24,32,35], which
is consistent with the unsymmetric chelations of 4,5-
diazafluorene-9-one [19] and 4,5-diazafuorene-9-
hydrazine [36]. It is noted that the peaks of protons
g and g 0 have shifted downfield while the changes of
the other proton signals are small because the former
are closer to the CyNazomethine and therefore being
affected more on the coordination of the Nazomethine
atom to the metal ion.
3.6. Thermal analysis
Fig. 1. An ORTEP drawing of 2 at the 30% probability level.
In order to examine the character of the water mole-
cules and the thermal properties of the complexes, the
weight loss and decomposition temperature were
measured for 1 and 5 by thermogravimetric method.
Complex 1 loses 2.89% of its weight at 158–175ЊC,
corresponding to one water molecule (calcd. 2.61%),
then it loses weight of 3.04% at 185–210ЊC by the
loss of another water molecule, implying that both
water molecules might be coordinated to the Mn
ion. From 300ЊC up, it decomposes gradually and
loses weight of 23.56% before 400ЊC due to the loss
of one diazafluorene group (calcd. 24.10%). The
complex decomposes completely at 570ЊC, with the
residual weight of 10.4% (calcd. 10.3% for MnO).
Complex 5 loses about 1.22% of its weight before
owing to the spin-forbidden nature of the d–d trans-
itions and to the low solubility of the complexes.
1
3.5. H NMR spectra
1
The H NMR spectral data of HL, 5 and 6 and their
assignments are listed in Table 4 together with the
coupling constants of HL. The proton signal of HN
at 12.178 ppm in the free ligand disappeared in 5
and 6, indicating obviously that the ligand has
enolized and deprotonated in the latter. The non-
equivalency of the chemical shifts of the protons on
the two pyridyl rings of the diazafluorene moiety indi-
Table 5
˚
Selected atomic lengths (A) and bond angles (Њ) of 2
Co–O(1)
Co–N(2)
N(1)–C(7)
N(2)–C(8)
N(3)–C(9)
N(4)–C(14)
1.9945(13)
2.3023(16)
1.326(2)
1.311(2)
1.329(3)
1.334(2)
Co–O(2)
2.0970(15)
1.268(2)
1.382(2)
1.327(2)
1.331(3)
1.491(3)
O(1)–C(7)
N(1)–N(2)
N(3)–C(13)
N(4)–C(18)
C(6)–C(7)
O(1)–Co–O(2)
O(1)–Co–N(2)
O(2)a–Co–N(2)
C(7)–O(1)–Co
C(8)–N(2)–N(1)
N(1)–N(2)–Co
O(1)–C(7)–C(6)
N(2)–C(8)–C(15)
90.83(6)
74.83(5)
85.37(6)
113.46(11)
112.10(16)
107.18(10)
118.00(15)
124.70(17)
O(1)a–Co–O(2)
O(1)a–Co–N(2)
O(2)–Co–N(2)
C(7)–N(1)–N(2)
C(8)–N(2)–Co
O(1)–C(7)–N(1)
N(1)–C(7)–C(6)
N(2)–C(8)–C(12)
89.17(6)
105.17(5)
94.63(6)
111.85(15)
140.72(14)
127.15(17)
114.83(16)
128.49(18)
a
Ϫx ϩ 1=2; Ϫy ϩ 1=2; Ϫz:

W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
97
3.7. Crystal structure of CoL2(H2O)2 2
The molecular structure of 2 is shown in Fig. 1. The
selected atomic distances and bond angles are listed in
Table 5. The molecule of 2 is centrosymmetric with
the cobalt ion in a distorted octahedral environment
with two azomethine N and two enolic O atoms from
two ligands in the equational plane and two water O
donors in the opposite axial sites. The five-membered
chelate ring CoO(1)C(7)N(1)N(2) is nearly planar with
˚
a mean deviation of 0.1145 A from the least square
plane and a bite angle O(1)CoN(2) of 74.83(5)Њ. The
planar phenyl ring forms a dihedral angle of 29.2Њ with
the chelate ring. The diazafluorene moiety also shows
˚
good planarity with the mean deviation of 0.0358 A
and is inclined by 13.6 and 38.3Њ from the chelate
ring and the phenyl ring, respectively.
It is noted that the Camide–N bond distance
˚
(1.326(2) A) in 2 is shorter than the C···N distance
˚
of the diazafluorene moiety (1.327(2)–1.334(2) A),
indicating its double bond nature. Compared to the
relevant structural parameters of the ligand HL·H2O
˚
[33], the Cdiazafluorene–N (1.311(2) A) and Camide–O
˚
(1.268(2) A) bonds are elongated and Camide–N bond
˚
is shortened (1.290(3), 1.216(3), 1.378(3) A in HL–
H2O, respectively). Besides, the bond Camide–O is
longer while Camide–N is shorter than the corresponding
bond lengths in a Co2ϩ complex of benzoylhydrazone
Fig. 2. (a) Crystal packing of 2 showing intermolcular hydrogen
bonds and p–p stacking interactions. (b) Diagram of the diaza-
fluorene units in 2 showing the intermolecular p–p stacking.
˚
(1.211, 1.365 A, respectively) [39] in which the aroyl-
hydrazone group was reported to coordinate in the keto
form. These facts confirm the phenomenon that the
ligand is enolized in 2, which is also in agreement
with the justification in the literature [15] about the
coordination modes of acylhydrazones: C–O distance
53ЊC corresponding to half a water molecule (calcd.
1.30%), indicating that the water molecule is solvated.
After keeping its weight constant to 400ЊC, the
complex decomposes quickly and exothermically
and loses about 38.5% of its weight till 450ЊC, caused
by the loss of the other water molecule, one diaza-
fluorene and one phenyl ring (calcd. 37.8%). The
decomposition is complete at 560ЊC with the residual
weight of 12.1% (calcd. 11.7% for ZnO). It can be
concluded that the two water molecules in 1 should be
coordinated, and hence the coordination geometry is
octahedral around the Mn ion. For 5, the half a water
molecule should be of absorption, and the other one
coordinated, suggesting that the Zn ion is five-coordi-
nated, which is often observed for Zn complexes
[37,38].
˚
of 1.23–1.25 A for keto-form coordination and of
˚
1.29–1.31 A for enol-form coordination.
The crystal packing showing intermolecular
hydrogen bonds and p–p stacking interactions is
shown in Fig. 2(a). The different distances of
hydrogen bonds are indications of the different charge
density of the two nitrogen atoms in the diazafluorene
moiety. The water molecule H2O(2a) coordinated to
Co(a) uses its two hydrogens to form two hydrogen
bonds with the diazafluorene N(4ag) and N(3d) atoms
…
…
with distances Oꢀ2a† Nꢀ4ag† and Oꢀ2a† Nꢀ3d† of
˚
2.835 and 2.854 A, respectively. Fig. 2(b) shows
three neighboring diazafluorene moieties depicting
the p–p stackings. The three diazafluorene moieties

98
W. Xiao et al. / Journal of Molecular Structure 553 (2000) 91–99
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of neighboring molecules are parallel to each other and
there exist two types of stacking. For the diazafluorene
moietyincludingN(3a), threeandtworingsareinvolved
in stacking with the diazafluorene moieties containing
N(3af) and N(4ag), respectively, and the shortest center-
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˚
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intermolecular H bonds and four intermolecular p–p
stacking interactions forming a 2D layered sheet.
4. Conclusions
The keto-form ligand HL in the solid state enolizes in
solution when reacting with transition metal acetates to
produce the LϪ coordinated complexes with the release
of acetic acid. A large electronically delocalized system
throughout the whole molecule is formed after enoliza-
tion and complexation. The non-coordinationof the 4,5-
diazafluorene moiety shows that its chelating ability is
weaker than the acylhydrazone group. If proper reaction
conditions were chosen, the mononuclear complexes
could be used as building blocks to further construct
polynuclear complexes, particularly heteropolynuclear
ones, which would possess rich and interesting physical
and chemical properties to be applied in material
research. The centrosymmetric octahedral complex
CoL2 (H2O)2 is stabilized by weak intermolecular inter-
actions of both hydrogen bondings and p–p stackings
between the diazafluorene moieties.
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Doctoral Program of Higher Education for financial
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