Equilibrium Studies in Solution Involving Nickel(II) Complexes of Flexidentate Schiff Base Ligands: Isolation and Structural Characterization of the Planar Red and Octahedral Green Species Involved in the Equilibrium

Article abstract of DOI:10.1021/ic0346174

Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L¹-OH-L³-OH) based on 1-(2-aminoethyl)-piperazine (X = H, L¹-OH; X = NO₂, L²-OH; and X = Br, L ³-OH) and their nickel(II) complexes (1a, 1b, 2, and 3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the complexes have profound influences on the coordination geometry of the isolated products. With the parent ligand L¹-OH, the product obtained is either a planar red compound [Ni(L¹-O)] ⁺, isolated as tetraphenylborate salt (1a), or an octahedral green compound [Ni(L¹-NH)(H₂O)₃]²⁺, isolated with sulfate anion (1b); both have been crystallographically characterized. In aqueous solution, both these planar (S = 0) and octahedral (S = 1) forms are in equilibrium that has been followed in the temperature range 298-338 K by ¹H NMR technique using the protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L¹-O)] ⁺ + 3H₂O + H⁺ ? [Ni(L ¹-NH)(H₂O)₃]²⁺ (ΔH° = -46 ± 0.2 kJ mol^-1 and ΔS° = -133 ± 5 J K ^-1 mol^-1) reflects formation of three new Ni-OH ₂ bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L²-OH, the sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L³-OH, the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been structurally characterized by X-ray diffraction analysis.

Full text of DOI:10.1021/ic0346174

Inorg. Chem. 2003, 42, 8439−8445
Equilibrium Studies in Solution Involving Nickel(II) Complexes of
Flexidentate Schiff Base Ligands: Isolation and Structural
Characterization of the Planar Red and Octahedral Green Species
Involved in the Equilibrium
Suman Mukhopadhyay,† Debdas Mandal,† Dipesh Ghosh,†,§ Israel Goldberg,‡ and
,†
Muktimoy Chaudhury*
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
Kolkata 700 032, India, and School of Chemistry, Tel AViV UniVersity, Tel AViV 69978, Israel
Received June 4, 2003
Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L1-OH−L3-OH) based on 1-(2-aminoethyl)-
piperazine (X ) H, L1-OH; X ) NO , L2-OH; and X ) Br, L3-OH) and their nickel(II) complexes (1a, 1b, 2, and
2
3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations
that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral
complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the
complexes have profound influences on the coordination geometry of the isolated products. With the parent
ligand L1-OH, the product obtained is either a planar red compound [Ni(L1-O)]+, isolated as tetraphenylborate
salt (1a), or an octahedral green compound [Ni(L1-NH)(H O)3]2+, isolated with sulfate anion (1b); both have
2
been crystallographically characterized. In aqueous solution, both these planar (S ) 0) and octahedral (S ) 1)
forms are in equilibrium that has been followed in the temperature range 298−338 K by 1H NMR technique using
+
the protocol of Evans’s method. The large exothermicity of the equilibrium process [Ni(L1-O)]+ + 3H O + H h
2
[Ni(L1-NH)(H O)3]2+ (∆H° ) −46 ± 0.2 kJ mol-1 and ∆S° ) −133 ± 5 J K-1 mol-1) reflects formation of three
2
new Ni−OH bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, the
2
sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L3-OH,
the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been
structurally characterized by X-ray diffraction analysis.
Introduction
analyses.3,4 Over the years, several other such equilibria have
been reported using macrocyclic5-12 and open chain
The possibility of the existence of an equilibrium in
solution between the octahedral and planar forms of nickel-
(II) complexes was reported for the first time with some
sterically constrained ligands, viz. stilbenediamine (stien) by
Lifschitz et al.1,2 The proposed equilibrium [Ni(stein)X2] h
[Ni(stien)2+] + 2X- (X- ) various anions) was latter
established by magnetic, spectroscopic, and X-ray structure
(3) Higginson, W. C. E.; Nyburg, S. C.; Wood, J. S. Inorg. Chem. 1964,
3, 463.
(4) Nyburg, S. C.; Wood, J. S. Inorg. Chem. 1964, 3, 468.
(5) Fabbrizzi, L.; Paoletti, P.; Clay, R. M. Inorg. Chem. 1978, 17, 1042.
(6) Sabatini, L.; Fabbrizzi, L. Inorg. Chem. 1979, 18, 438.
(7) Swisher, R. G.; Dayhuff, J. P.; Stuehr, D. J.; Blinn, E. L. Inorg. Chem.
1980, 19, 1336.
(8) Hay, R. W.; Bembi, R.; Sommerville, W. Inorg. Chim. Acta 1982,
59, 147.
* Author to whom correspondence should be addressed. E-mail: icmc@
mahendra.iacs.res.in.
(9) Steenland, M. W. A.; Dierck, I.; Herman, G. G.; Devreese, B.; Lippens,
W.; Van Beeumen, J.; Goeminne, A. M. J. Chem. Soc., Dalton Trans.
1997, 3637.
(10) Sakata, K.; Wada, S.; Sato, N.; Kurisu, M.; Hashimoto, M.; Kato, Y.
Inorg. Chim. Acta 1986, 119, 111.
(11) Bembi, R.; Bhardwarj, V. K.; Singh, R.; Singh, R.; Taneja, K.; Aftab,
S. Inorg. Chem. 1984, 23, 4153.
(12) Hay, R. W.; Jeragh, B.; Ferguson, G.; Kaitner, B.; Ruhl, B. L. J. Chem.
Soc., Dalton Trans. 1982, 1531.
† Indian Association for the Cultivation of Science.
‡ Tel Aviv University.
§ Present address: Department of Chemistry, University of Missouris
Kansas City, Kansas City, MO 64110.
(1) Lifschitz, I.; Bos, J. G.; Dijkema, K. M. Z. Anorg. Allg. Chem. 1939,
24, 97.
(2) Lifschitz, I.; Bos, J. G. Recl. TraV. Chim. 1940, 5, 407.
10.1021/ic0346174 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/11/2003
Inorganic Chemistry, Vol. 42, No. 25, 2003 8439

Mukhopadhyay et al.
Scheme 1
Chart 1
square-planar-octahedral equilibrium in solution is followed
1
in detail by H NMR and electronic spectroscopies.
polyamines and Schiff base ligands.13-16 These reported
equilibria are generally of two types (Scheme 1). The most
common one involves axial attachment of ligand X (X )
anions or solvent molecules, predominantly water) to square-
planar species NiL, largely of macrocyclic ligands L (Scheme
1A) to generate the octahedral counterpart NiLX2.5-14 The
second type of equilibrium (Scheme 1B), on the other hand,
involves flexidentate ligands (L′) which form bis complexes
of composition Ni(L′)2. At lower temperatures, these ligands
bind nickel(II) in tridentate fashion to render octahedral
geometry which reduces to a planar one at elevated temper-
atures due to steric constraints of the associated ligands.15,16
Most of these equilibrium studies were followed by spec-
troscopic techniques, and only in a few cases were the species
involved characterized in the solid state by X-ray diffraction
analysis.4,12
Experimental Section
Materials. 1-(2-Aminoethyl)piperazine, 5-bromosalicylaldehyde,
and 5-nitrosalicylaldehyde were obtained from Aldrich. Salicyl-
aldehyde (Aldrich) was distilled before use. Solvents were reagent
grade, dried from appropriate reagents,21 and distilled under nitrogen
prior to their use. Spectroscopic grade D2O was also obtained from
Aldrich. All other chemicals were commercially available and used
as received.
Syntheses. Ligands. The Schiff base ligands (L1-OH-L3-OH)
were prepared following a general procedure. Synthesis of 1-(2-
salicylaldiminoethyl)piperazine (L1-OH) is described below as a
prototype.
To a stirred solution of salicylaldehyde (2.44 g, 20 mmol) in
absolute ethanol (20 mL) was added dropwise an equimolar amount
(2.58 g) of 1-(2-aminoethyl)piperazine, and the resulting mixture
was refluxed for 1 h to obtain an orange solution. The solvent was
then removed by rotary evaporation to obtain a red oil which on
standing for days in a vacuum produced a yellow hygroscopic solid.
Several attempts to recrystallize the compound were unsuccessful.
Both ligands L2-OH and L3-OH are also extremely hygroscopic
yellow solids, which eluded our repeated attempts at recrystalli-
zation. The ligands were then used for metalation without further
purification.
Recently, reports have been made on pH-dependent
reversible translocation of Ni(II) ion from octahedral to the
square-planar site in ditopic ligand systems.17 Such a
translocation phenomenon can be used effectively as a light
emission switch by tagging the ditopic ligand systems with
fluorescent probes.18,19
Complexes. [Ni(L1-O)](BPh4) (1a). To a stirred solution of L1-
OH (0.23 g, 1 mmol) in methanol (20 mL) was added a solution
of Ni(ClO4)2‚6H2O (0.36 g, 1 mmol) taken also in methanol. The
resultant orange solution was refluxed for 2 h. Solid NaBPh4 (0.50
g) was then added to the solution while it was hot and filtered
immediately. The filtrate was kept in the air for ca. 12 h to obtain
an orange microcrystalline solid. It was collected by filtration,
washed with Et2O (5 × 2 mL), and dried in a vacuum. The
compound was recrystallized from CH2Cl2/n-hexane mixture.
Diffraction quality crystals were obtained by diffusing n-hexane
into a dichloromethane solution of the compound. Yield: 0.34 g
(55%). Anal. Calcd for C37H38BNiN3O: C, 72.76; H, 6.23; N, 6.88.
Found: C, 73.03; H, 6.29; N, 6.70. IR (KBr disk, cm-1): ν(N-H),
3263s; ν(CdN), 1618s; ν(C-O/phenolate), 1530s; ν(phenyl ring),
738s, 706s.
Herein, we report the nickel(II) complexes of three new
flexidentate Schiff base ligands (L1-OH-L3-OH; Chart 1).20
The piperazinyl arm of these ligands can, in principle, have
both boat and chair conformations that force these molecules
to display ambidentate ligation behavior, leading to both
octahedral and square-planar geometry for the Ni(II) com-
plexes. The structures of these complexes in the solid state
have been examined by X-ray diffraction analysis. The
(13) Anichini, A.; Fabbrizzi, L.; Paoletti, P. Inorg. Chim. Acta 1977, 24,
L21.
(14) Evans, D. F.; Missen, P. H. J. Chem. Soc., Dalton Trans. 1985, 1451.
(15) Sacconi, L.; Nannelli, P.; Nardi, N.; Campigli, U. Inorg. Chem. 1965,
4, 943.
(16) Sacconi, L.; Nardi, N.; Zanobini, F. Inorg. Chem. 1966, 5, 1872.
(17) Amendola, V.; Fabbrizzi, L.; Mangano, C.; Pallavicini, P. Acc. Chem.
Res. 2001, 34, 488.
(18) Fabbrizzi, L.; Foti, F.; Licchelli, M.; Poggi, A. Inorg. Chem. 2002,
41, 4612.
[Ni(L1-NH)(H2O)3](SO4)‚6H2O (1b). An aqueous solution (25
mL) of NiSO4‚6H2O (0.26 g, 1 mmol) was added dropwise to a
solution of the ligand L1-OH (0.23 g, 1 mmol) taken in methanol
(10 mL). The resultant green solution was stirred for 30 min and
(19) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P.; Parodi, L. Angew. Chem.,
Int. Ed. 1998, 37, 800.
(20) In the ligand L-OH, the dissociable proton is attached to the phenolic
oxygen atom. L-NH, on the other hand, represents the N (piperazine)-
protonated, O (phenol)-deprotonated form of the ligand.
(21) Perin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: Oxford, England, 1980.
8440 Inorganic Chemistry, Vol. 42, No. 25, 2003

Ni(II)-Flexidentate Schiff Base Ligand Complexes
Table 1. Relevant Crystal Data for [Ni(L1-O)](BPh4) (1a), [Ni(L1-NH)(H2O)3](SO4)‚6H2O (1b), [Ni(L2-NH)(H2O)3](SO4)‚2H2O (2), and
[Ni(L3-O)](BPh4) (3)
1a
1b
2
3
empirical formula
formula weight
cryst size (mm3)
cryst system
space group
T (K)
C37H38BNiN3O
610.22
C13H37NiN3O14S
550.23
C13H28NiN4O12S
523.16
C37H37BBrNiN3O
689.13
0.10 × 0.10 × 0.20
hexagonal
P65
0.30 × 0.30 × 0.45
0.10 × 0.10 × 0.30
0.10 × 0.15 × 0.25
triclinic
triclinic
triclinic
P1h
293
P1h
110
P1h
110
110
a (Å)
b (Å)
c (Å)
9.6549(5)
9.7345(5)
18.0363(9)
98.6550(10)
99.3440(10)
110.5200(10)
1526.57(13)
2
8.8690(1)
11.9180(2)
12.1220(2)
100.3600(6)
109.7881(7)
93.5550(10)
1175.49(3)
2
9.0570(1)
9.5280(1)
14.2900(2)
77.6760(9)
72.6110(10)
64.2060(2)
1054.55(2)
2
9.7290(1)
9.7290(1)
56.9660(13)
90.00
90.00
120.00
4669.63(13)
6
1.470
R (deg)
â (deg)
γ (deg)
V (Å)3
Z
F
calcd (g cm-3
)
1.328
644
1.554
584
1.648
548
F(000)
2136
radiation used
Mo KR
Mo KR
Mo KR
Mo KR
R1, wR2 (all data)
R1, wR2 (data with I > 2σI)
GOF
0.061, 0.102
0.042, 0.095
1.010
0.036, 0.086
0.033, 0.083
0.975
0.027, 0.064
0.025, 0.062
1.024
0.093, 0.138
0.057, 0.118
0.942
filtered. The filtrate was concentrated under reduced pressure to
drive off methanol and filtered again. The filtrate was layered with
acetone and kept at 4 °C for several days to obtain a moss-green
crystalline compound along with some X-ray diffraction quality
crystals. The product was collected by filtration and dried over fused
CaCl2. Yield: 0.14 g (25%). Anal. Calcd for C13H37NiN3O14S: C,
28.35; H, 6.72; N, 7.63. Found: C, 29.16; H, 6.71; N, 7.85. IR
(KBr disk, cm-1): ν(O-H), 3406b; ν(CdN), 1648s; ν(C-O)/
phenolate, 1548s; ν(SO4), 1111s and 620s. µeff ) 3.32 µB at 25 °C.
[Ni(L2-NH)(H2O)3](SO4)‚2H2O (2). An aqueous solution (25
mL) of NiSO4‚6H2O (0.26 g, 1 mmol) was added dropwise to a
stirred solution of the ligand L2-OH (0.28 g, 1 mmol), dissolved in
methanol (10 mL). A yellow compound was precipitated almost
immediately, was collected by filtration, and washed with ethanol
(5 × 2 mL). For recrystallization, the compound was dissolved in
hot water (ca. 40 °C) and kept at 4 °C for several days to get a
green crystalline compound along with some X-ray diffraction
quality crystals. The product was collected by filtration, washed
with ethanol (4 × 10 mL), and dried over fused CaCl2. Yield: 0.24
g (45%). Anal. Calcd. for C13H28NiN4O12S: C, 29.84; H, 5.36; N,
10.71. Found: C, 29.41; H, 5.07; N, 10.87. IR (KBr disk, cm-1):
ν(O-H), 3575s; ν(N-H), 3250b; ν(CdN) 1656s; ν(C-O/
phenolate), 1548s; ν(SO4), 1107s, and 620s; ν(C-NO2), 844m. µeff
) 3.33 µB at 25 °C.
[Ni(L3-O)](BPh4) (3). A solution of Ni(ClO4)2‚6H2O (0.36 g, 1
mmol) in methanol (20 mL) was added dropwise to a stirred solution
of the ligand L3-OH (0.31 g, 1 mmol), also taken in methanol (10
mL). The resultant red solution was refluxed for 1 h. Sodium
tetraphenylborate (0.5 g) in solid was then added to this hot solution
and filtered. The filtrate was kept at room temperature for several
days to obtain an orange microcrystalline compound. It was
collected by filtration, washed with Et2O (2 × 5 mL), and finally
dried in a vacuum. The compound was recrystallized from an
acetone/petroleum ether (60-80 °C) mixture. X-ray diffraction
grade crystals were obtained by diffusing n-hexane into an acetone
solution of the compound. Yield: 0.38 g (55%). Anal. Calcd for
C37H37BrNiN3O: C, 64.50; H, 5.37; N, 6.10. Found: C, 64.10; H,
5.41; N, 6.10. IR (KBr disk, cm-1): ν(N-H), 3281s; ν(CdN),
1623s; ν(C-O/phenolate), 1527s; ν(BPh4), 735s, and 707s.
with a Bruker Model Avance DPX 300 apparatus. Elemental
analyses (for C, H, and N) were performed in this laboratory (at
IACS) using a Perkin-Elmer 2400 analyzer.
X-ray Crystallography. Selected crystallographic data are given
in Table 1, and complete data are given in the supplementary CIF
file (see Supporting Information). Intensity data for 1a were
collected at room temperature on a Bruker 1K SMART CCD
diffractometer using θ-2θ technique, while for 1b, 2, and 3 data
were collected on a Nonius Kappa CCD diffractometer, with both
diffractometers using Mo KR X-radiation (λ ) 0.710 73 Å). No
crystal decay was observed during the data collections. The analyzed
crystals of 1b, 2, and 3 were covered with a thin layer of light oil
and cooled at 110 K in order to minimize structural disorder and
thermal motion effect and to increase the precision of the results.
Intensity data for all the complexes were measured within the θ
range of 2.42-27.90°. The structures were solved by direct and
Patterson methods and refined by a full matrix least-squares
procedure based on F2 using the SHELXS-97 (1a), SIR-92 (1b and
2), DIRDIF-96 (3), and SHELXL-97 software.24-26
Variable Temperature Magnetic Susceptibility Measure-
ments. The octahedral-square-planar equilibrium of 1a-1b in
solution was monitored by susceptibility (øg) measurement at
variable temperatures following Evans’s method.27 The working
solution in D2O containing 2% tert-butyl alcohol as a reference
compound was prepared by dissolving 0.0427 g of 1b per milliliter
of the solvent. The solution was taken in a NMR tube. The same
combination of solvent (2% tert-butyl alcohol in D2O) was also
used as external standard and taken in a capillary tube of 2 mm
outer diameter. The latter was placed inside the NMR tube, and
the combination was used for ∆f measurements in the temperature
(22) Dutta, S. K.; Kumar, S. B.; Bhattacharyya, S.; Tiekink, E. R. T.;
Chaudhury, M. Inorg. Chem. 1997, 36, 4954.
(23) Dutta, S. K.; McConville, D. B.; Youngs, W. J.; Chaudhury, M. Inorg.
Chem. 1997, 36, 2517.
(24) Sheldrick, G. M. SHELX-97: Programs for Crystal Structure Analysis,
Release 97-2; University of Go¨ttingen: Go¨ttingen, Germany, 1998
(includes SHELXS-97 and SHELXL-97).
(25) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. SIR-92; J. Appl. Crystallogr. 1994,
27, 435.
(26) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia-
Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF-
96 Program System; Technical Report of the Crystallography Labo-
ratory; University of Nijmegen: Nijmegen, The Netherlands, 1996.
(27) Evans, D. F. J. Chem. Soc. 1959, 2003.
Physical Measurements. Room-temperature magnetic moments,
UV-vis spectra, and IR spectra (as KBr disk) were obtained as
described elsewhere.22,23 1H NMR measurements were performed
Inorganic Chemistry, Vol. 42, No. 25, 2003 8441

Mukhopadhyay et al.
range 298-338 K, using a Bruker DPX 300 NMR spectrometer,
operated at 300 MHz frequency. The spectral width was 7.5 ppm
(2269 Hz), and for each spectrum, 300 scans were recorded.
1
The H NMR spectra of the reference compound in the two
coaxial tubes, due to the difference in their volume susceptibilities,
exhibit chemical-shift differences (∆f), which are measured (in
hertz) and used to calculate the mass susceptibility (øg) of the
dissolved paramagnetic molecule from
øo(do - ds)
3∆f
2πfm
øg )
+ øo +
(1)
m
where f is the operating radio frequency (in hertz) of the
spectrometer, m is the concentration of the paramagnetic substance
in grams per milliliter, and øo is the mass susceptibility of the pure
solvent; do and ds are the densities of the pure solvent and the
solution, respectively. The last term in eq 1 can be neglected in all
calculations (as it would introduce an error much less than 1%),
the details of which are described elsewhere.28
Figure 1. Molecular structure of complex 1a showing the atom numbering
scheme.
ligand is tridentate with piperazine moiety in chair conforma-
tion, making only a single ring nitrogen atom available for
coordination to the nickel center, while the other in the
protonated form stays away from coordination. The remain-
ing coordination sites in 1b are taken up by three water
molecules. The overall composition of 1b is [Ni(L1-NH)-
(H2O)3](SO4)‚6H2O, in which L1-NH stands for the N-
protonated form of the ligand L1-OH.20 Compounds 2 and 3
have geometry and coordination environments closely similar
to those of 1b and 1a, respectively.
IR spectra of the complexes have a couple of prominent
bands appearing at ca. 1640 and 1535 cm-1 regions,
assignable to ν(CdN) and ν(C-O/phenolate) stretching
modes, respectively. In the high-frequency region, a broad
medium intensity band in the 3500-3400 cm-1 range due
to ν(O-H) vibration indicates the presence of coordinated
water molecules in 1b and 2. Compounds 1a and 3, on the
other hand, show a strong sharp band due to ν(N-H)
stretching at ca. 3270 cm-1. In addition, the octahedral
complexes 1b and 2 display two strong bands at 1100 and
620 cm-1, confirming the presence of ionic sulfate.29
Likewise, 1a and 3 show two strong bands at ca. 710 and
610 cm-1 due to phenyl ring vibrations, associated with
tetraphenylborate anion.
Description of Crystal Structures. Figures 1 and 2 show
the perspective views of the square-planar complexes of 1a
and 3, respectively. Their selected bond distances and angles
are given in Table 2. Complex 1a crystallizes in the triclinic
space group P1h with two molecules per unit cell, while 3
has the hexagonal space group P65 with six molecules in
the unit cell. The observed crystal structures confirm the boat
conformation of the associated piperazine ring in the ligands.
The latter binds to Ni(II) in tetradentate fashion, providing
an N3O donor set. In both molecules, the Ni-N bond
distances are slightly different, with the piperazine associated
Ni-N(3) bond being the longest: 1.931(2) Å in 1a and
1.915(6) Å in 3. The trans angles O(1)-Ni-N(2) and N(1)-
Ni-N(3) in 1a are 172.88(7)° and 163.79(7)°, respectively.
Corresponding angles in 3 are almost identical (Table 2).
The two halves of the molecule (Ni, O(1), and N(1)) and
Results and Discussion
Synthesis. Three new ligands (L1-OH-L3-OH) have been
prepared by Schiff base condensation between 1-(2-amino-
ethyl)piperazine and salicylaldehyde or its 5-substituted nitro
and bromo derivatives. The conformational flexibility of the
piperazine ring (from boat to chair form and vice versa)
makes one of these ligands (L1-OH) capable of coordinating
in ambidentate fashion, generating interesting chemistry of
Ni(II) ion in solution as well as in the solid state. Thus with
the ligand L1-OH the complex formation process is very
much dependent on the counteranion. The solution appears
to be green at room temperature in the presence of Cl- and
2-
-
-
SO4 ions, while with ClO4 , Br-, and BPh4 anions the
solutions are yellow to orange in color. We have been
successful in isolating the red compound 1a in the solid state
as a tetraphenylborate salt from a boiling methanolic solution.
The corresponding green compound 1b could be obtained
in the solid state only in the presence of sulfate anion by
cooling at subambient temperature. With the nitro derivative
ligand L2-OH, however, the only product obtained is the
green crystalline compound 2, which in solution can with-
stand temperatures up to 100 °C without showing any change
in color. The latter compound, like 1b, has been isolated in
the solid state only in the presence of sulfate anion. With
the bromo derivative ligand L3-OH, the exclusive product
is a red crystalline compound 3, isolated only as BPh4 salt
as in the case of 1a, which shows only a marginal color
change on cooling at ca. 77 K. The presence of other anions
has no effect on the compositions of 2 and 3, nor do the
anions have any impact on the isolation of these compounds.
The sterically constrained ligand L1-OH plays an interest-
ing role in the formation of 1a and 1b, with the former being
a diamagnetic square-planar compound while the latter is a
paramagnetic (µeff ) 3.32 µB) octahedral one. As revealed
from X-ray crystallographic analysis (see below), the ligand
L1-OH is tetradentate in 1a with two of its four donor atoms
being contributed by the nitrogen atoms of the piperazine
moiety in boat conformation. In 1b, on the other hand, the
(29) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compunds, 3rd ed.; Wiley-Interscience: New York, 1978.
(28) Crawford, T. H.; Swanson, J. J. Chem. Educ. 1971, 48, 382.
8442 Inorganic Chemistry, Vol. 42, No. 25, 2003

Ni(II)-Flexidentate Schiff Base Ligand Complexes
Figure 2. Molecular structure of complex 3 showing the atom numbering
scheme. The atomic displacement parameters of the non-hydrogen atoms
are represented by 50% probability thermal ellipsoids (darkened for non-C
atoms).
Figure 3. ORTEP drawing and crystallographic numbering scheme for
complex 1b. The atomic displacement parameters of the non-hydrogen atoms
are represented by 50% probability thermal ellipsoids (darkened for non-C
atoms).
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes
1a and 3
1a
3
(a) Bond Lengths
1.8148(14)
Ni-O(1)
Ni-N(1)
Ni-N(2)
Ni-N(3)
1.798(5)
1.836(6)
1.888(6)
1.915(6)
1.8232(16)
1.8738(16)
1.9312(17)
(b) Bond Angles
97.21(7)
O(1)-Ni-N(1)
O(1)-Ni-N(2)
O(1)-Ni-N(3)
N(1)-Ni-N(2)
N(1)-Ni-N(3)
N(2)-Ni-N(3)
96.8(2)
173.8(3)
98.7(2)
87.8(3)
163.9(3)
77.0(3)
172.88((7)
98.00(7)
88.62(7)
163.79(7)
76.65(8)
Figure 4. ORTEP drawing and crystallographic numbering scheme for
complex 2. The atomic displacement parameters of the non-hydrogen atoms
are represented by 50% probability thermal ellipsoids (darkened for non-C
atoms).
(Ni, N(2), and N(3)) are twisted by 7.9° in 1a. The
corresponding angle for 3 is 6.8°.
The molecular structures and atom numbering schemes
for the cations in the complexes 1b and 2 are shown in
Figures 3 and 4, respectively, and the selected geometric
parameters are listed in Table 3. Both complexes crystallize
in triclinic space group P1h and have two molecular units
accommodated in the unit cell. Unlike in 1a and 3, the
ligands here behave in a tridentate fashion due to a
conformational change (from boat to chair form) of the
associated piperazine ring, generating distorted octahedral
structures for 1b and 2 (Figures 3 and 4). Three atoms O(1),
N(1), and N(2) from the tridentate ligand, along with an
oxygen atom O(3) from a coordinated water molecule,
occupy the meridian plane, while the apical positions are
taken up by two other oxygen atoms O(2) and O(4) also
coming from the coordinated water molecules. The trans
angles O(1)-Ni-N(2), 173.36(6)° (170.77(5)° for 2); N(1)-
Ni-O(3), 175.45(6)° (176.85(5)°); and O(2)-Ni-O(4),
176.34(6)° (172.94(4)°), are close to linearity. The Ni atom
lies essentially in the meridian plane, being formally
displaced by 0.01 Å (0.02 Å) from the least-squares plane
of O(1), N(1), N(2), and O(3), toward the apical oxygen atom
O(2).
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complexes
1b and 2
1b
2
(a) Bond Lengths
2.0232(15)
1.9997(16)
2.2321(16)
2.0844(13)
2.0534(14)
2.0915(14)
Ni-O(1)
Ni-N(1)
Ni-N(2)
Ni-O(2)
Ni-O(3)
Ni-O(4)
2.0295(13)
2.0080(13)
2.2153(15)
2.0863(12)
2.0912(12)
2.0566(11)
(b) Bond Angles
91.57(6)
173.36(6)
82.72(6)
89.99(6)
90.02(6)
93.56(6)
91.01(6)
175.45(6)
89.78(6)
86.75(6)
95.93(6)
89.81(6)
93.25(5)
176.34(6)
85.36(6)
O(1)-Ni-N(1)
O(1)-Ni-N(2)
N(1)-Ni-N(2)
O(1)-Ni-O(2)
O(1)-Ni-O(3)
O(1)-Ni-O(4)
N(1)-Ni-O(2)
N(1)-Ni-O(3)
N(1)-Ni-O(4)
N(2)-Ni-O(2)
N(2)-Ni-O(3)
N(2)-Ni-O(4)
O(2)-Ni-O(3)
O(2)-Ni-O(4)
O(3)-Ni-O(4)
90.08(5)
170.77(5)
82.49(6)
89.17(5)
88.49(5)
90.60(5)
93.55(5)
176.85(5)
93.11(5)
96.71(5)
97.12(5)
86.47(5)
83.39(5)
172.94(4)
89.98(5)
Of particular interest in the octahedral complexes is the
zwitterionic nature of the tridentate ligands arising from the
protonation of the N(3) piperazine ring atom, which stays
away from coordination due to the chair conformation of
the ring. The overall electroneutrality of the coordinated
tridentate ligands generates cationic nickel(II) complexes that
require sulfate anion for their crystallization. There are six
and two water molecules present in 1b and 2, respectively,
as crystallization solvent. Extensive hydrogen bonding among
these and coordinated water molecules, the sulfate ion, and
the N-H protons of the piperazinium ring play a significant
role in the stabilization of the octahedral products leading
Inorganic Chemistry, Vol. 42, No. 25, 2003 8443

Mukhopadhyay et al.
Table 4. Hydrogen Bonding Parameters for Compounds 1b and 2
A
H
B
A‚‚‚B
A-H
H‚‚‚B
A-H‚‚‚B
1ba
O10
O11
O13
N3
O14
N3
O4
O4
O9
H2#
H3#
H8#
H83A
H14B
H83B
H94A
H94B
H99A
O6
O6
O5
O5
O8
O7
O8
O5
O7
2.873(2)
2.759(2)
2.810(2)
2.816(2)
2.895(2)
2.788(2)
2.717(2)
0.8547
0.8081
0.7960
0.9000
0.8979
0.8868
0.8289
0.8653
0.9245
2.0413
1.9744
2.0615
1.9637
2.0009
1.9030
1.9366
1.9193
1.8410
164.08
163.63
154.54
157.48
173.59
175.41
156.53
178.83
164.51
2.784((2)
2.743(2)
2b
N3
N3
N3
O10
O2
O3
O4
O9
H3A
H3B
H3B
H90A
H92B
H93B
H94A
H99A
O7
O7
O8
O6
O7
O6
O8
O5
2.698(2)
0.9112
0.9385
0.9385
0.8478
0.8618
0.9064
0.8328
0.8628
1.7990
2.5846
1.8259
1.9578
1.8508
1.8053
1.8599
1.8786
168.47
123.79
172.23
178.47
166.06
176.68
172.22
176.48
3.2030(18)
2.7588(17)
2.805(2)
2.6952(17)
2.7107(17)
2.6874(19)
2.7402(19)
a O2-O4 represent water molecules coordinated to the Ni cation, O5-
O8 are part of the SO4 anion, and O9-O14 represent other free water
molecules incorporated into the crystal lattice. b O9 and O10 represent free
water molecules incorporated into the crystal lattice.
Figure 5. Electronic absorption spectra of 1b (concentration 4.7 × 10-3
M) in aqueous solution as a function of temperature: (i) 298, (ii) 308, (iii)
318, (iv) 328, and (v) 338 K.
Table 5. Summary of Electronic Spectral Data in Solution
complex solvent
λ
max/nm (ꢀmax/mol-1 cm2)
square-planar d8 species.33 The second ligand-field band, 1A1g
1a
1b
CH3CN 286 (38 900), 318 (12 100), 384 (4450), 451 (sh)
1
f B1g, expected to appear at higher energy is probably
H2O
H2O
233 (19 100), 264 (7850), 354 (5950), 617 (7),
746 (5), 944 (15)
230 (17 550), 247 (12 200), 384 (14 600), 610 (10),
746 (8), 938 (18)
obscured by the tail of a near-UV absorption present in 1a
and 3.
2
3
The green octahedral complexes 1b and 2, isolated as their
sulfate salts, are only soluble in water. In the visible region,
aqueous solutions of these complexes display three weak
band maxima (ꢀ, 5-18 mol-1 cm2) at 944 (938 for 2), 746
(746), and 617 nm (610 nm) (Figure 5i) corresponding to
CH3CN 239 (25 900), 313 (4450), 396 (3250), 466 (sh)
to their crystallization. Details of the hydrogen bonding
parameters are listed in Table 4, while Figure S1 (in the
Supporting Information) provides a complete view of the
atoms participating in hydrogen bonding in 1b. In the
octahedral complexes, the Ni-O and Ni-N distances are
in the ranges 2.0232(15)-2.0915(14) and 1.9997(16)-
2.2321(16) Å, respectively, which are as expected for high-
spin Ni(II) complexes reported so far.30,31 When these
distances are compared with the corresponding bond lengths
of the square-planar complexes, an interesting correlation
emerges between the structural features of 1a and 1b. The
bond lengths in 1b are systematically larger compared to
the corresponding distances in 1a. This is, however, not
unexpected32 considering in the high-spin complex 1b the
3
3
3
3
the spin-allowed transitions A2g f T2g, A2g f T1g, and
3A2g f T1g(P), respectively, as expected for Ni(II) in a
distorted octahedral environment.33
3
When the temperature of an aqueous solution of 1b is
slowly increased, its color gradually changes from green to
light orange with a concomitant change in spectral intensity.
In the visible region, all the d-d bands show a decrease in
spectral intensities with the rise in temperature as shown in
Figure 5. The appearance of isobestic points at 568 nm is a
clear indication of the existence of an equilibrium in solution
between the octahedral species 1b and its square-planar
counterpart 1a formed at higher temperatures (vide supra)
and is represented by eq 2.
2
2
presence of a singly populated dx -y orbital, which is
antibonding in nature. This, in consequence, results in an
expansion of the basal core, leading to longer bond distances
in 1b compared to the square-planar analogue 1a.
Electronic Spectra. Electronic spectral data for the
complexes are summarized in Table 5. In acetonitrile, 1a
displays a medium intensity band at 451 nm (λmax, for the
corresponding band in 3 is 466 nm) in the form of a shoulder
1
1
due to the spin-allowed A1g f A2g transition, typical of
(30) Jeffery, J. C.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1992, 2119.
(31) Psomas, G.; Stemmler, A. J.; Dendrinou-Samara, C.; Bodwin, J. J.;
Schneider, M.; Alexiou, M.; Kampf, J. W.; Kessissoglou, D. P.;
Pecoraro, V. L. Inorg. Chem. 2001, 40, 1562.
Equilibrium Studies. Since one component of this equi-
librium is a paramagnetic species while the other is a
(32) (a) Duval, H.; Bulach, V.; Fischer, J.; Weiss, R. Inorg. Chem. 1999,
38, 5495. (b) Fabbrizzi, L.; Micheloni, M.; Paoletti, P. J. Chem. Soc.,
Dalton Trans. 1980, 134.
(33) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier
Science: Amsterdam, 1984.
8444 Inorganic Chemistry, Vol. 42, No. 25, 2003

Ni(II)-Flexidentate Schiff Base Ligand Complexes
Table 6. Magnetic and Equilibrium Data for the Planar h Octahedral
Equilibrium in Aqueous Solution Involving 1a and 1b
temp (K)
µ
% planar
% octahedral
Keq
298
308
318
328
338
3.18
3.09
2.93
2.72
2.51
8.3
13.4
22.1
32.9
42.9
91.7
86.6
77.9
67.1
57.1
11.04
6.46
3.52
2.04
1.33
diamagnetic one, this offers an opportunity14,28 to follow the
above equilibrium by magnetic measurements in solution
1
using H NMR technique as proposed by Evans.27 With an
aqueous solution of 1b, the magnetic moments have been
measured in the temperature range 298-338 K following
this methodology as summarized in the Experimental Section.
A gradual decrease in the magnetic moment value, as
displayed in Table 6, with the rise in temperature is a clear
indication of the transformation of 1b into the diamagnetic
species 1a. The percentage of diamagnetic species present
in solution has been calculated using15,28
Figure 6. Plot of log Keq versus 1/T for the octahedron-square-planar
equilibrium in aqueous solution of Ni(II)-L1-OH complex.
associated anions have profound influences on the coordina-
tion geometry of the complexes. With the ligand L1-OH, both
square-planar (1a) and the octahedral (1b) complexes have
been isolated and crystallographically characterized. In
solution, these compounds are in equilibrium (eq 2), which
is unique in the sense that both solvation and the change of
ligand denticity are simultaneously in operation here. All the
other equilibria involving square-planar and octahedral
nickel(II) complexes reported so far5-16 are either due to
solvation5-14 or due to change in ligand denticity.15,16 Also,
unlike the previous examples,15,16 the ligand denticity here
increases as we go up the temperature scale when the
octahedral species (1b) is gradually converted to the square-
planar one (1a). Also, this is one of the rare examples4,12
where both forms involved in equilibrium have been isolated
in the solid state and characterized crystallographically.
% diamagnetic species ) 100[1 - µ2/(3.32)2]
(3)
where µ is the magnetic moment of the solution at any
particular temperature and 3.32 µB is the magnetic moment
of pure 1b in the solid state.
The equilibrium constant Keq for eq 2 is defined as Keq
)
[octahedral]/[planar]. Since eq 2 is pH dependent, Keq here
is a composite parameter that includes the contribution due
to temperature dependence of pH. The percentage of the
planar species increases from 8.3 at 298 K to 42.9 at 338 K
with Keq ) 11.04 and 1.33 at the above two temperatures,
respectively. A plot of log Keq versus 1/T is linear (Figure
6), giving ∆H° ) -46 ( 0.2 kJ mol-1 from the least-
squares slope (correlation coefficient 0.999 58). The values
of ∆S° obtained at each temperature from the equation
∆G° ) ∆H° - T∆S° give an average value of -133 ( 5
J K-1 mol-1. However, the sizes of these thermodynamic
parameters appear questionable considering the composite
nature of the equilibrium constants.
Acknowledgment. Financial support received from the
Council of Scientific and Industrial Research (CSIR), New
Delhi, is gratefully acknowledged. Two of us (D.M. and
D.G.) thank CSIR for the award of research fellowships. We
also thank Professor R. J. Butcher and Dr. Gautom K. Patra
for their help in the crystal structure determination of 1a
and 1b, respectively.
Concluding Remarks
Structural and equilibrium studies of nickel(II) complexes
of flexidentate salicylaldimino Schiff base ligands (L1-OH-
L3-OH) have been reported here. The piperazinyl arm of
these ligands can in principle have either a boat or a chair
conformation, leading to square-planar or octahedral geom-
etry, respectively, of the reported complexes. Both remote
substitution in the ligand aromatic ring and the nature of the
Supporting Information Available: Figure S1; X-ray crystal-
lographic files in CIF format for compounds 1a, 1b, 2, and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC0346174
Inorganic Chemistry, Vol. 42, No. 25, 2003 8445