Synthesis and characterization of heterobimetallic Ni(II)-Zn(II) complexes from bis(2-hydroxy-1 -naphthaldehyde)succinoyldihydrazone

Article abstract of DOI:10.1080/00958971003759077

Monometallic zinc(II) and nickel(II) complexes, [Zn(H₂nsh)(H₂O)] (1) and [Ni(H₂nsh)(H₂O)₂] (2), have been synthesized in methanol by template method from bis(2-hydroxy-1-naphthaldehyde)succinoyldihydrazone (H4nsh). Reaction of monometallic complexes with alternate metal(II) acetates as a transmetallator in 1: 3 molar ratio resulted in the formation of heterobimetallic complexes [NiZn(nsh)(A)₃] and [ZnNi(nsh)(A′)₂] (A′ = H₂O (3), py (4), 2-pic (5), 3-pic (6), 4-pic (7)), (A' = H₂O (8), py (9), 2-pic (10), 3-pic (11), and 4-pic (12)). The complexes have been characterized by elemental analyzes, mass spectra, molar conductance, magnetic moments, electronic, EPR, and IR spectroscopies. All of the complexes are non-electrolytes. Monometallic zinc(II) is diamagnetic while monometallic nickel(II) complex and all heterobimetallic complexes are paramagnetic. The metal centers in heterobimetallic complexes are tethered by dihydrazone and naphthoxo bridging. Zinc(II) is square pyramidal; nickel(II) is six-coordinate distorted octahedral except [ZnNi(nsh)(A)₂], in which nickel(II) has square-pyramidal geometry. The displacement of metal center in monometallic complexes by metal ion has been observed in the resulting heterobimetallic complexes.

Full text of DOI:10.1080/00958971003759077

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Synthesis and characterization
of heterobimetallic Ni(II)–Zn(II)
complexes from bis(2-hydroxy-1 -
naphthaldehyde)succinoyldihydrazone
a
a
a
a
R.A. Lal , M. Chakraborty , O.B. Chanu , S. Choudhury , R.
a
a
b c
Borthakur , S. Copperfield & A. Kumar
a
Department of Chemistry , North-Eastern Hill University ,
Shillong–793022, Meghalaya, India
b
Institute of Chemistry , Academia Sinica 128, Academia Road,
Sec. 2, Taipei 115, Taiwan, ROC
c
Department of Chemistry, Faculty of Science and Agriculture ,
The University of West Indies , St. Augustine, Trinidad and
Tobago, West Indies
Published online: 07 Apr 2010.
To cite this article: R.A. Lal , M. Chakraborty , O.B. Chanu , S. Choudhury , R. Borthakur , S.
Copperfield & A. Kumar (2010) Synthesis and characterization of heterobimetallic Ni(II)–Zn(II)
complexes from bis(2-hydroxy-1 -naphthaldehyde)succinoyldihydrazone, Journal of Coordination
Chemistry, 63:7, 1239-1251, DOI: 10.1080/00958971003759077
To link to this article: http://dx.doi.org/10.1080/00958971003759077
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Journal of Coordination Chemistry
Vol. 63, No. 7, 10 April 2010, 1239–1251
Synthesis and characterization of heterobimetallic
Ni(II)–Zn(II) complexes from bis(2-hydroxy-1
-naphthaldehyde)succinoyldihydrazone
R.A. LAL*y, M. CHAKRABORTYy, O.B. CHANUy,
S. CHOUDHURYy, R. BORTHAKURy,
S. COPPERFIELDyand A. KUMARzx
yDepartment of Chemistry, North-Eastern Hill University,
Shillong – 793022, Meghalaya, India
zInstitute of Chemistry, Academia Sinica 128, Academia Road, Sec. 2,
Taipei 115, Taiwan, ROC
xDepartment of Chemistry, Faculty of Science and Agriculture, The University
of West Indies, St. Augustine, Trinidad and Tobago, West Indies
(
Received 31 August 2009; in final form 16 October 2009)
Monometallic zinc(II) and nickel(II) complexes, [Zn(H
2), have been synthesized in methanol by template method from bis(2-hydroxy-
-naphthaldehyde)succinoyldihydrazone (H nsh). Reaction of monometallic complexes with
2 2 2 2 2
nsh)(H O)] (1) and [Ni(H nsh)(H O) ]
(
1
4
alternate metal(II) acetates as a transmetallator in 1 : 3 molar ratio resulted in the formation of
0
heterobimetallic complexes [NiZn(nsh)(A)
0
3
] and [ZnNi(nsh)(A )
2
O (8), py (9), 2-pic (10), 3-pic (11), and 4-pic (12)).
2
] (A ¼H
2
O (3), py (4), 2-pic
(
5), 3-pic (6), 4-pic (7)), (A ¼H
The complexes have been characterized by elemental analyzes, mass spectra, molar conduc-
tance, magnetic moments, electronic, EPR, and IR spectroscopies. All of the complexes are
non-electrolytes. Monometallic zinc(II) is diamagnetic while monometallic nickel(II) complex
and all heterobimetallic complexes are paramagnetic. The metal centers in heterobimetallic
complexes are tethered by dihydrazone and naphthoxo bridging. Zinc(II) is square pyramidal;
2
nickel(II) is six-coordinate distorted octahedral except [ZnNi(nsh)(A) ], in which nickel(II) has
square-pyramidal geometry. The displacement of metal center in monometallic complexes by
metal ion has been observed in the resulting heterobimetallic complexes.
Keywords: Nickel(II);
Zinc(II);
Heterobimetallic;
succinoyldihydrazone; Magnetic moment; Spectroscopic studies
Bis(2-hydroxy-1-naphthaldehyde)-
1. Introduction
Homo and heteropolymetallic complexes containing two or more metal centers in close
proximity are involved in a variety of important biochemical processes [1, 2] and are
important in materials of technological and industrial significance [3, 4]. Metal ions in
enzymes give rise to monometallic, homometallic, and heterometallic enzymes. For
example, Mo, Zn, and Ni are present in xanthine oxidase [5], carbonic anhydrase [6],
*
Corresponding author. Email: ralal@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958971003759077

1
240
R.A. Lal et al.
and bacterial hydrogenase [7], respectively, in monometallic enzymes. Copper and
manganese are found in haemocyanin and tyrosinase [8] and oxygen evolving complex
(
OEC) [9] in photosystem II, as multimetallic enzymes. Further, Mo is found in
nitrogenase [10] in combination with iron, and Zn and Cu are found together in
superoxide dismutase [11] and Ni occurs in combination with iron in jack bean urease
[
12] and Ni–Fe hydrogenase [13]. Zn occurs in kidney bean phosphatase in combination
with iron [14] and in combination with copper in proteins obtained from rat liver [15].
The efficacy of heterobimetallic complexes in asymmetric activation of carbon dioxide
and related molecules has been demonstrated [16]. Further, heterobimetallic complexes
have potential to mediate chemical reactions of industrial relevance either more
efficiently than or, in a different manner to, isolated metal centers [17]. Heterobimetallic
complexes offer an opportunity to study cooperative interaction between metal ions.
Dihydrazones derived from condensation of o-hydroxyaromatic aldehydes and
ketones with acyldihydrazones, aroyldihydrazines, and pyridoyl dihydrazines are
potential polyfunctional ligands which can bind with metal ions in various ways giving
rise to monometallic, homopolymetallic, and heteropolymetallic complexes depending
on the preferred stereochemical disposition of metal and the bonds formed in
coordination. A survey of literature reveals that although metal complexes of monoacyl
hydrazones, aroyl hydrazones, and pyridoyl hydrazones have been studied in
detail [18–21], those of acyl-, aroyl-, and pyridoyl-dihydrazones have received
attention in recent years only [22–25]. Heterobimetallic complexes derived from
dihydrazones are quite meager [23, 26] with not a single report of heterobimetallic
complexes of dihydrazones containing succinoyl and bulky naphthyl fragments in their
molecular skeleton. This article describes the synthesis and characterization of
heterobimetallic nickel–zinc complexes derived from bis(2-hydroxy-1-naphthaldehyde)-
succinoyldihydrazone (H nsh) (figure 1). Monometallic zinc(II) and nickel(II)
4
complexes have also been synthesized and characterized as precursor in the synthesis
of heterobimetallic complexes.
2
2
. Experimental
.1. Materials
Zinc acetate (Zn(OAc) ꢀ2H O), nickel acetate (Ni(OAc) ꢀ4H O), diethyl succinate
2
2
2
2
(
(
(CH ) (CO Et) ), hydrazine hydrate (N H ꢀH O), 2-hydroxy-1-naphthaldehyde
2
2
2
2
2
4
2
C H (OH)(CHO)), pyridine, 2-picoline, 3-picoline, 4-picoline, and methanol were
1
0
6
procured commercially and used without purification.
H
N
H
N
H
C
H
C
(
CH )
2 2
N
H
C
O
C
O
N
H
O
O
4
Figure 1. Bis(2-hydroxy-1-naphthaldehyde)succinoyldihydrazone (H nsh).

Succinoyldihydrazone
1241
2.2. Physical measurements
All conductance measurements were made at 1 kHz using a Wayne Kerr B905
Automatic Precision Bridge. This LCR meter has 0.01 ns resolution and measures
conductance with an accuracy of 0.05%. A dip-type conductivity cell having platinized
platinum electrode was used. The cell constant was determined using a standard KCl
ꢁ1
solution. Infrared spectra from 4000 to 400/500 cm were recorded on a BX-III/FT-IR
1
Perkin Elmer spectrophotometer. H-NMR spectra were recorded on a Bruker AC-F
3
00 MHz and AMX 400 High Resolution Multinuclear FT-NMR spectrometer in
DMSO-d with tetramethylsilane (TMS) as internal standard. Room temperature (RT)
6
magnetic susceptibility measurements were made on Sherwood Magnetic Susceptibility
Balance MSB-Auto. EPR spectra of the complexes in powdered form were recorded
at X-band frequency on a Varian E-112 E-Line Century Series EPR spectrometer using
TCNQ (g ¼2.0027) as an internal field maker. Atmospheric pressure chemical
ionization (APCI) mass spectra of the complexes were recorded on a Waters ZQ4000
Micromass spectrometer.
2.3. Analysis of the complexes
Elemental analyzes (C, H, and N) were performed on a Perkin Elmer 2400 CHNS/O
Analyzer 11. Zinc and nickel contents were determined by literature method [27].
Water, ethanol, pyridine, 3-picoline, and 4-picoline were determined by heating the
ꢂ
ꢂ
ꢂ
samples in an electronic oven for 4 h at 110 C or 180 C or 220 C each, respectively, and
determining the weight loss. Water was identified by passing the vapors through a test
tube containing anhydrous copper sulfate which turned blue, while pyridine, 3-picoline,
and 4-picoline molecules were determined by passing the vapors through a test tube
containing (1) a solution of sodium hydroxide and iodine; (2) a solution of CHCl3
containing a drop of 5 M NaOH solution and (3) a test tube containing cyanogen
bromide solution followed by treatment with phloroglucinol solution. The solutions
turned red, green, violet, and blue [28], respectively.
2.4. Preparation of the ligand
Bis(2-hydroxy-1-naphthaldehyde)succinoyldihydrazone was prepared in two steps. In
the first step, succinoylhydrazine was prepared by reacting diethyl succinate with
hydrazine hydrate in 1 : 2.5 molar ratio. In the second step, bis(2-hydroxy-1-
naphthaldehyde)succinoyldihydrazone (H nsh) was prepared by reacting succinoyldi-
4
hydrazine (1.0 g, 6.84 mmol) in dilute ethanol with 2-hydroxy-1-naphthaldehyde (2.59 g,
1
was purified by washing with hot ethanol and dried over anhydrous CaCl . m.p. 254 C
5.04 mmol) in ethanol under reflux for 30 min. The yellow precipitate, thus, obtained
ꢂ
2
(Found (%): C, 68.81; H, 4.80; N, 12.65). Required for C H O N (%): C,
26 22 4 4
6
8.72;H, 4.84; N, 12.33.
2
.5. Preparation of [Zn(H nsh)(H O)] (1) and [Ni(H nsh)(H O) ] (2)
2 2 2 2 2
Succinoyldihydrazine (0.50 g, 3.42 mmol) was dissolved in hot methanol–water mixture
90 : 10) and a solution of Zn(OAc) ꢀ2H O (0.75 g, 3.45 mmol) in methanol (50 mL) was
(
2
2

1
242
R.A. Lal et al.
added accompanied by gentle stirring for 15 min. This solution was added to a solution
of 2-hydroxy-1-naphthaldehyde (1.47 g, 8.54 mmol) in methanol (100 mL) drop-by-drop
ꢂ
at 70 C over a period of 10 min accompanied by gentle stirring. The reaction mixture
was refluxed for 1 h precipitating a bright yellow compound, which was filtered while
hot, washed repeatedly with hot methanol, finally with ether, and dried over anhydrous
CaCl2.
Complex 2 was prepared by the above method using Ni(OAc) ꢀ4H O instead of
2
2
Zn(OAc) ꢀ2H O.
2
2
2
.6. Preparation of [NiZn(nsh)(A) ] (where A ^ H O (3); pyridine (py, 4);
3 2
2
-picoline (2-pic, 5); 3-picoline (3-pic, 6); 4-picoline (4-pic, 7))
[
panied by gentle stirring for 10 min at 60 C. This was added to a solution of
Zn(H nsh)(H O)] (1.00 g, 1.81 mmol) was suspended in methanol (100 mL) accom-
ꢂ
2
2
Ni(OAc) ꢀ4H O (1.35 g, 5.43 mmol) in methanol (50 mL) drop-by-drop over a period
2
2
of 10–15 min accompanied by gentle stirring. The reaction mixture thus obtained was
refluxed for 5 h, precipitating a light brown compound, which was isolated in the usual
way.
In order to prepare 4, compound 3 (1.00 g, 1.55 mmol) was suspended in methanol
ꢂ
(
1
100 mL) accompanied by gentle stirring for 10 min at 60 C and pyridine (1.30 mL,
5.52 mmol) was added maintaining [NiZn(nsh)(H O) ] and pyridine molar ratio
2 3
at 1 : 10. The resulting reaction mixture was refluxed for 3 h, precipitating orange
compound which was collected in the usual way.
Complexes 5–7 were prepared in a similar way using 2-picoline, 3-picoline, and
4
-picoline instead of pyridine.
0 0
.7. Preparation of [ZnNi(nsh)(A ) ] (where A ^ H O (8); pyridine (py, 9);
2 2
2
2
-picoline (2-pic, 10); 3-picoline (3-pic, 11); 4-picoline (4-pic, 12))
[
[
ZnNi(nsh)(H O) ] was prepared by following essentially the same method as for
2
2
NiZn(nsh)(H O) ] by reaction between [Ni(H nsh)(H O) ] and Zn(OAc) ꢀ2H O
2
3
2
2
2
2
2
instead of [Zn(H nsh)(H O)] and Ni(OAc) ꢀ4H O.
2
2
2
2
The remaining complexes were prepared by following essentially the same method
as used for the complexes [NiZn(nsh)(A) ] using [ZnNi(nsh)(H O) ] instead of
3
2
2
[
NiZn(nsh)(H O) ].
2 3
3
. Results and discussion
Synthesis of pure heterobimetallic complexes is difficult to avoid considerable quantities
of undesired binuclear products. Heterobimetallic products are accessible by strategies
developed by Lintvedt et al. [29] from monometallic complex using polyfunctional
ligands and Davies et al. [30] via transmetallation beginning with a complex of discrete
molecularity as target and another metal complex as transmetallator. Monometallic
complexes [Zn(H nsh)(H O)] (1) and [Ni(H nsh)(H O) ] (2) were synthesized by
2
2
2
2
2
reacting metal acetate with succinoyldihydrazine in 1 : 1 molar ratio and reacting the

Succinoyldihydrazone
1243
resultant solution with excess 2-hydroxy-1-naphthaldehyde. When 1 and 2 reacted with
Ni(OAc) ꢀ4H O and Zn(OAc) ꢀ2H O, respectively, in 1 : 3 molar ratio in methanol,
2
2
2
2
the resulting precipitates were heterobimetallic complexes [NiZn(nsh)(H O) ] (3) and
2
3
[
ZnNi(nsh)(H O) ] (8), respectively. Reaction of these heterobimetallic complexes with
2 2
pyridine, 2-picoline, 3-picoline, and 4-picoline in 1 : 10 molar ratio led to substitution of
water by pyridine and substituted pyridines.
The complexes formula, color, decomposition point, percent yield, analytical data,
molar conductance, magnetic moment, and electronic spectral data are given in table 1.
All of the complexes are insoluble in water, and common organic solvents, such as
ethanol, methanol, acetone, chloroform, carbon tetrachloride, benzene, and ether but
are soluble in DMSO and DMF. All of the complexes are air stable solid powders,
ꢂ
decomposing above 300 C.
Since the complexes are soluble in DMSO and DMF efforts were undertaken to
crystallize the complexes in both saturated and dilute solutions in CH CN, DMSO,
3
DMF, DMSO–CH CN, DMSO–CH Cl , DMF–CH CN, and DMF–CH Cl
2.
3
2
2
3
2
ꢂ
The solutions were gently evaporated either at RT or at 40, 50, and 60 C in hot air
electric oven to promote crystal growth. The solutions were also kept in a freezer to
grow crystals. In all our efforts, only amorphous compounds precipitated which
prevented analysis of the complexes by X-ray crystallography.
3
.1. Mass spectra
Mass spectra of 1 and 2 show m/z values of 518 and 1059.41, respectively. The m/z value
of 518 for 1 is close to the value 517.38 obtained by loss of coordinated water from the
þ
complex. Hence, this signal is assigned to [Zn(H nsh)] species, suggesting that 1 is
2
monomeric. On the other hand, the m/z value 1059.4 for 2 is close to the value 1057.38
corresponding to dimer from loss of water from nickel(II). Accordingly, this signal is
þ
assigned to ½NiðH nshÞðH OÞꢃ species.
2
2
2
3
.2. Thermal studies
ꢂ
None of the complexes showed weight loss below 160 C ruling out the possibility
of water in the lattice. Complexes 2 and 8 showed weight loss corresponding to two
ꢂ
waters at 180 C, while 1 and 3 showed weight loss corresponding to one and three
waters, respectively. This suggests water in the first coordination sphere around the
metal in these complexes. Complexes 4–7 showed weight loss corresponding to three
pyridine/2-picoline/3-picoline/4-picoline molecules while 9–12 showed weight loss
ꢂ
corresponding to two pyridine/2-picoline/3-picoline/4-picoline molecules at 220 C.
Expulsion of these neutral electron donors at high temperature indicates that they are
coordinated to metal [28].
3
.3. Molar conductance
The molar conductance values (in DMSO) for 1–12 lie in the range
2
ꢁ ꢁ1
1
2
.81–3.21 ohm cm mol (table 1) suggesting all complexes are non-electrolytes [31].

1
244
R.A. Lal et al.

Succinoyldihydrazone
1245
3.4. Magnetic moment
Complex 1 is diamagnetic as expected, while the remaining complexes are
paramagnetic. The ꢀeff values for 2–7 lie in the range 2.87–3.15 B.M. while ꢀeff
for 8–12 fall in the range 3.35–3.42 B.M. The ꢀeff values for nickel(II) complexes fall in
the regions 3.00–3.30, 3.00–3.45, and 3.45–4.00 B.M., respectively, for octahedral,
square-pyramidal, and tetrahedral geometries [32], while square-planar nickel(II)
complexes are diamagnetic. The magnetic moments rule out tetrahedral or square-
planar structures. Values for 3–7 lie in the range 2.95–3.15 B.M. indicating distorted
octahedral stereochemistry [32, 33]. Ni(II) complexes having square-pyramidal
stereochemistry have magnetic moment values lying in the region 3.25–3.45 [34]; 8–12
at 3.35–3.43 B.M. are in the range for high-spin, square-pyramidal complexes.
Hence, 8–12 are suggested to have square-pyramidal geometry.
3.5. Electronic spectra
The important electronic spectral bands along with their molar extinction coefficient
are summarized in table 1. The electronic spectra of 3 and 8 are shown in
‘
‘Supplementary material’’. The ligand exhibits bands at 311, 322, 355, and 371 nm
assigned to ꢂ !ꢂ* and n !ꢂ* transitions. In 1 and 2, the ligand bands are red-shifted
by 8–77 nm indicating bonding of dihydrazone to the metal. In addition to the ligand
bands, complex 1 shows an additional band at 479 nm with molar extinction coefficient
3
ꢁ1
ꢁ1
of 2272 dm mol cm ; this band is assigned to ligand-to-metal charge transfer
LMCT), probably from naphtholate oxygen to zinc(II). [Ni(H nsh)(H O) ] (2)
has absorption bands at 845 and 564 nm in addition to the ligand and charge-transfer
(
2
2
2
3
3
3
3
bands, assigned to A !T (F) (ꢃ ) and A !T (F) (ꢃ ) transitions, respectively.
2
3
g
2g
3
1
2g
1g
2
The band due to A !T (P) (ꢃ ) appears to be masked by strong intra-
2
g
1g
3
ligand/charge-transfer transition at 410 nm. The bands observed at 845 and 564 nm
are characteristic of nickel(II) in distorted octahedral environment, consistent with the
presence of nickel(II) with NNOO coordination [35].
+In heterobimetallic complexes, the ligand bands are shifted to longer wavelength by
–60 nm indicating strong bonding between dihydrazone and metal. In addition, the
8
heterobimetallic complexes show a new band in the region 472–481 nm, similar to the
charge-transfer band at 479 nm observed in 1. Accordingly, this band is assigned to
LMCT originating from naphtholate oxygen. Complexes 3–7 exhibit new bands
at 931–940 nm (ꢃ ) and 615–628 nm (ꢃ ), respectively. The position of these bands is
1
2
typical of octahedral Ni(II). A comparison of the position of the bands in the regions
31–940 and 615–628 nm with the corresponding bands in the spectra [30] of
9
[
2
þ 2þ
Ni(H O) ] at 1175 and 740 nm and of [Ni(NH3)6] at 935 and 570 nm suggests
2 6
that ꢃ bears similarity with that of NH while the second band, ꢃ , is intermediate
1
3
2
between those observed for H O and NH . Nickel thus may occupy NNOO
2
3
coordination of the ligand, suggesting that Zn(II) is displaced by Ni(II).
Hence, Ni(II) occupies NNOO coordination while zinc(II) has OOOO coordination
chamber. Complexes 8–12 derived from [Ni(H nsh)(H O) ] exhibit only one band
2
2
2
at 965–967 nm, similar to high-spin square-pyramidal Ni(II) [36, 37] and the band is
3
3
assigned to B !E(F). A comparison of the position of this band in the region
1
2
þ
65–987 nm with the corresponding band in spectra of [Ni(H O) ] at 1175 (ꢃ ) and
2 6 1
9

1
246
R.A. Lal et al.
2
Ni(NH ) ] at 935 (ꢃ ) and those in 3–7 suggests that the ꢃ band in these complexes is
þ
[
shifted away from position of ꢃ in complexes with nitrogen donors and toward the
3
6
1
1
1
position of ꢃ with oxygen donors. Thus, nickel in 8–12 is probably coordinated to
1
carbonyl oxygen in enol form, suggesting that Ni(II), which originally occupies NNOO
coordination, is displaced by Zn(II) in the heterobimetallic complexes. Thus, it is
suggested that in these complexes the nickel(II) occupies OOOO coordination chamber,
while zinc(II) occupies NNOO coordination chamber. Various ligand field parameters,
namely Racah inter-electronic repulsion parameter (B), ligand field splitting energy
(10 Dq), covalence factor (ꢄ) and ligand field stabilization energy for 3–7 are provided
in ‘‘Supplementary material’’. Calculated values of ꢃ /ꢃ (1.49–1.51) lie in the range
reported for an octahedral environment around Ni(II) [38].
2
1
3
.6. EPR spectra
EPR spectra of 3 and 8 have been recorded as representative examples of the geometry
of the nickel(II) in heterobimetallic complexes. EPR spectra of 3 at 9.1 GHz show a
single signal at g ¼2.004 in polycrystalline form both at RT and liquid nitrogen
temperature (LNT), while those of 8 are featureless. The signal at g ¼2.004 is attributed
to arise from the double quantum transition [39]. In the Ni(II) complex, the zero-field
splitting (ZFS) ranges from a few [40] to some tens of wave numbers [41]. Consequently,
the conventional X-band and Q-band EPR spectroscopy using electromagnetic quanta
ꢁ1
around 0.3 and 1.5 cm , respectively, find it difficult to excite transitions between the
states of the spin-triplet manifold [42]. Four-coordinate square-planar/five-coordinate
nickel(II) centers are present in hydrogenase enzyme [43, 44] and the reduced states of
C-cluster in the bacterial carbon monoxide dehydrogenase [45]. The nickel(II) centers
are EPR silent in these biological systems, while four-coordinate tetrahedral and
six-coordinate octahedral complexes are EPR active. The fact that 3 shows an intense
signal with g ¼2.004 suggests that nickel(II) has octahedral geometry, while the
featureless spectrum of
8
suggests that the nickel(II) is five-coordinate
square-pyramidal. Further, the g-value for nickel complexes depends on the donor
set as well as on the coordination geometry of the complex. Complexes with oxygen
donors (donor set NiO ) have g-values greater than 2.2, whereas those with the nitrogen
6
donors (donor set NiN ) generally have g value either equal to 2.2 or less than this.
6
The g value in 3 is less than 2.2, as is expected for NiN O donor set [46, 47].
4
2
The intensity of the signal at g ¼2.004 in 3 remains unchanged at RT as compared to
LNT, ruling out the possibility of impurity [48].
1
.7. H-NMR spectra
3
The ligand shows two proton doublets at ꢅ 11.61 and 12.73 ppm assigned to ꢅ(OH)
protons (Supplementary material). These doublets appear as singlets at 12.21 and
1
2.80 ppm in the H-NMR spectrum of 1 with downfield shift by 0.60 and 0.07 ppm,
1
respectively, showing coordination of naphtholic –OH group to zinc. ꢅ(NH) signal,
which appears at ꢅ 9.99 and 11.14 ppm in the free dihydrazone disappears in 1
indicating the collapse of amide structure of the dihydrazone and coordination to zinc
in the enol form. This is also supported by IR spectroscopy which does not show any

Succinoyldihydrazone
1247
band characteristic of 4C ¼O. The azomethine proton signals at ꢅ 8.60 and 9.05 ppm
as doublets in the free dihydrazone appear at ꢅ 8.99 and 9.13 ppm as doublets, showing
coordination of azomethine nitrogen to zinc. Azomethine proton signals in the
uncoordinated dihydrazone and 1 as two resonances only suggests that the dihydrazone
is coordinated to zinc in the anti-cis configuration [49].
3
.8. Infrared spectra
Some of the structurally significant IR spectral bands for the free dihydrazone, the
precursor Zn(II) and Ni(II) complexes and the heterobimetallic Ni–Zn and Zn–Ni
complexes are listed in ‘‘Supplementary material’’. Comparison of IR spectra of the
monometallic, heterobimetallic, and uncoordinated dihydrazone suggests that the
dihydrazone is coordinated to the metal center in the enol form in all of the complexes.
The uncoordinated dihydrazone shows medium intensity bands at 3423 and
ꢁ
1
3244 cm
dihydrazone and ꢃ(NH) of secondary –NH, respectively. IR spectra of 1 and 2 show
assigned to ꢃ(OH) vibration of 2-hydroxy-1-naphthaldehyde of the
ꢁ
1
a medium intensity band in the region 3000–3500 cm attributed to water absorbed by
KBr during pellet preparation and contribution from coordinated water in the
complexes. The band at 3000–3500 cm in 1 and 2 also appears to have contribution
from the stretching vibration of naphtholic –OH. Neither complex shows ꢃ(NH)
ꢁ
1
ꢁ
1
ꢁ1
at 3244 cm or ꢃ(C ¼O) vibration at 1672 cm in the uncoordinated dihydrazone,
suggesting enolization of the ligand in the complexes.
The ꢃ(C ¼N) is a couple of bands at 1622–1602 cm
ꢁ
1
similar to that in the
ꢁ
1
uncoordinated dihydrazone in which these bands appear at 1633 and 1593 cm
.
ꢁ
1
The average negative shift of 2-3 cm indicates coordination of dihydrazone through
azomethine nitrogen to metal [50]. The existence of two ꢃ(C ¼N) bands shows that the
two azomethine nitrogens are inequivalent, suggesting that the two M N bonds are
not the same; the differences between ꢃ(C ¼N) stretching frequencies of the order of
ꢁ
1
1
3–18 cm fall in the range for anti-cis configuration [49, 51]. Thus, dihydrazone exists
1
in the anti-cis configuration in the complexes as also deduced from H-NMR
spectroscopy of 1.
ꢁ1
A band of medium intensity at 1540 cm in the infrared spectrum of the free
dihydrazone is attributed to joint contribution of amide II þꢃ(C–O) (naphtholic).
This band splits into two bands in IR spectra of the complexes and at 1540 and
ꢁ1
ꢁ1
1
508 cm , respectively. The position of the band at 1540 cm
remains almost
ꢁ1
unchanged in 2–7 and shifts to higher frequency by 19-40 cm in the remaining
complexes. The intensity of this band is considerably increased in 8–12. A new intense
band at 1508 cm in IR spectra of the complexes is attributed to stretching vibration of
newly created NCO produced as a result of deprotonation of the enolic form of the
ligand [52]. The complexes show new weak bands at 528 and 558 cm [53] due to
coordinated naphtholic –OH. The IR spectra of the heterobimetallic complexes 3–12
are almost the same as those of 1 and 2 suggesting the mode of coordination of
dihydrazone remains essentially the same as in 1 and 2. Complexes 4–7 and 9–12 show a
ꢁ
1
ꢁ1
ꢁ
1
new but very weak band at 1012–1014 and 1051–1072 cm , respectively, assigned to
ring breathing mode of pyridine and substituted pyridine in the complexes [54]. A new
1
band at 773–780 cm in spectra of all heterobimetallic complexes, which is observed
ꢁ
neither in the IR spectrum of the uncoordinated dihydrazone nor in 1 and 2, is assigned

1
248
R.A. Lal et al.
(
a)
(b)
HC
N
H
HC
N
C
C
N
N
OH
O
C
C
N
N
OH
OH
Zn
O
Ni
N
OH
O
N
O
HC
O
HC
Figure 2. Tentative structure of (a) [Zn(H
2
nsh)(H
2
O)] (1) and (b) [Ni(H
2
nsh)(H
2
O)
2
] (2).
O
to the stretching vibration of the tetraatomic species [M
M] resulting from bridging
O
naphtholate oxygen [55]. A new non-ligand band at 500–525 cm
ꢁ
1
is assigned to
is assigned to
ꢁ
1
ꢃ(M–O)(naphtholate) [56] and the band at 420–482 cm
ꢃ(M–O)(carbonyl) [57], indicating coordination of naphtholate oxygen and enolized
carbonyl oxygen to the metal. On the basis of results obtained from various
physico-chemical and spectral studies, structures for the complexes have been suggested
as shown in figures 2–4.
4. Conclusion
This article describes two monometallic zinc(II) and nickel(II) complexes and 10
heterobimetallic nickel(II)–zinc(II) complexes derived from bis(2-hydroxy-1-
naphthaldehyde)succinoyldihydrazone. The dihydrazone is coordinated in enol form
in all of the complexes in anti-cis configuration. In monometallic complexes 1 and 2, the
dihydrazone is a dibasic tetradentate ligand coordinating through two azomethine
nitrogens and two protonated naphtholic –OH groups. One and two waters are
coordinated to zinc(II) and nickel(II), respectively. The zinc(II) and nickel(II) in 1 and 2
are proposed to have square-pyramidal and distorted-octahedral stereochemistry,
respectively. In all heterobimetallic complexes, the dihydrazone is a tetrabasic
hexadentate ligand coordinating to the metals in enol form in anti-cis configuration.
On reaction of monometallic complexes with the alternate metal salts, the first metal
center is displaced by the transmetallating metal. In [NiZn(nsh)(A) ], nickel(II) is
3

Succinoyldihydrazone
1249
A
C
C
N
N
O
O
Ni
A
N
O
O
N
O
A
Zn
Zn
O
A
O
A
N
N
C
Ni
O
N
N
C
A
Figure 3. Tentative structure of [NiZn(nsh)(A)
3-picoline (6), and 4-picoline (7).
3
] where A ¼H
2
O (3), pyridine (4), 2-picoline (5),
A
C
C
N
N
O
O
O
Zn
N
N
O
A
Ni
A
Ni
O
O
N
N
C
C
Zn
A
O
O
N
N
2
Figure 4. Tentative structure of [ZnNi(nsh)(A) ].
six-coordinate, while zinc(II) is five-coordinate. In [ZnNi(nsh)(A) ], both metal centers
2
have square-pyramidal geometry.
Acknowledgments
We are thankful to the Head, NMR Research Center, Indian Institute of Science,
1
Bangalore, India, for providing H-NMR spectral data and Head, SAIF, North-Eastern

1
250
R.A. Lal et al.
Hill University, Shillong, India, for elemental analyzes and mass spectral data. We would
like to thank Head, SAIF, IIT, Mumbai, India, for providing us the EPR spectral data.
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