Isolation of two different Ni2Zn complexes with an unprecedented cocrystal formed by one of them and a "coordination positional isomer" of the other

Article abstract of DOI:10.1021/ic402415b

A new homometallic trinuclear Ni(II) complex [(NiL)₂Ni(NCS) ₂] (1) and three heterometallic trinuclear Ni(II)-Zn(II)-Ni(II) complexes [(NiL)₂Zn(NCS)₂] (2), [(NiL)₂Zn(NCS) ₂(CH₃OH)₂]·2CH₃OH (3) and {[(NiL)₂Zn(NCS)₂(CH₃OH)₂]} {[(NiL)₂Zn(NCS)₂]} (4) have been synthesized by using the "complex as ligand" approach with the "metalloligand" [NiL] (H₂L = N,N′-bis(salicylidene)-1,3-propanediamine) and thiocyanate in different ratios. All the complexes have been structurally and magnetically characterized. In the isomorphous complexes 1 and 2, the two terminal square planar Ni atoms and the central octahedral nickel atom (in 1) or zinc atom (in 2) are arranged in a bent structure where two cis κN-SCN^- thiocyanate ions are coordinated to the central atom. The chemical composition of 3 is very similar to that of 2 but, in 3, the central Zn atom is tetrahedral and the κN-SCN^- thiocyanate ions occupy an axial position of each terminal nickel atom (which now are octahedral with the sixth position occupied by a methanol molecule). Complex 4 consists of two closely related trinuclear units 4A and 4B. In 4A, the coordination environments of the metals are identical to those of 3 whereas 4B is a "coordination position isomer" of complex 2 with the central square pyramidal Zn and one of the terminal square pyramidal Ni atoms coordinated by two κN-SCN^- thiocyanate ions. Complex 4 is a unique example of a cocrystal formed by two similar trinuclear units (4A and 4B) where 4A is identical to an existing complex (3) and 4B is a "coordination position isomer" of another existing complex (2).

Full text of DOI:10.1021/ic402415b

Article
pubs.acs.org/IC
Isolation of Two Different Ni2Zn Complexes with an Unprecedented
Cocrystal Formed by One of Them and a “Coordination Positional
Isomer” of the Other
Lakshmi Kanta Das,† Apurba Biswas,† Carlos J. Gomez-García,‡ Michael G. B. Drew,§
́
and Ashutosh Ghosh*,†
†Department of Chemistry, University College of Science, University of Calcutta, 92, APC Road, Kolkata-700009, India
‡Instituto de Ciencia Molecular (ICMol), Universidad de Valencia. C/Catedrat
́ ́ ́
ico Jose Beltran, 2, 46980 Paterna, Valencia, Spain
§School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, United Kingdom
S
* Supporting Information
ABSTRACT: A new homometallic trinuclear Ni(II) complex
[(NiL)2Ni(NCS)2] (1) and three heterometallic trinuclear Ni(II)−
Zn(II)−Ni(II) complexes [(NiL)2Zn(NCS)2] (2), [(NiL)2Zn-
( N C S ) 2 ( C H 3 O H ) 2 ] · 2 C H 3 O H ( 3 ) a n d { [ ( N i L ) 2 Z n -
(NCS)2(CH3OH)2]} {[(NiL)2Zn(NCS)2]} (4) have been synthesized
by using the “complex as ligand” approach with the “metalloligand”
[NiL] (H2L = N,N′-bis(salicylidene)-1,3-propanediamine) and thio-
cyanate in different ratios. All the complexes have been structurally and
magnetically characterized. In the isomorphous complexes 1 and 2, the
two terminal square planar Ni atoms and the central octahedral nickel
atom (in 1) or zinc atom (in 2) are arranged in a bent structure where
two cis κN-SCN− thiocyanate ions are coordinated to the central atom.
The chemical composition of 3 is very similar to that of 2 but, in 3, the
central Zn atom is tetrahedral and the κN-SCN− thiocyanate ions occupy an axial position of each terminal nickel atom (which
now are octahedral with the sixth position occupied by a methanol molecule). Complex 4 consists of two closely related
trinuclear units 4A and 4B. In 4A, the coordination environments of the metals are identical to those of 3 whereas 4B is a
“coordination position isomer” of complex 2 with the central square pyramidal Zn and one of the terminal square pyramidal Ni
atoms coordinated by two κN-SCN− thiocyanate ions. Complex 4 is a unique example of a cocrystal formed by two similar
trinuclear units (4A and 4B) where 4A is identical to an existing complex (3) and 4B is a “coordination position isomer” of
another existing complex (2).
the study of organic and of pharmaceutical industries.12−14
They are defined as solids that are crystalline materials
composed of two or more different molecular and/or ionic
INTRODUCTION
■
The isolation, identification and characterization of different
crystal forms represents one of the most active areas of modern
solid state chemistry because these are associated with different
compounds generally in a stoichiometric ratio in the same
crystal lattice.15 If the molecular and/or ionic components
solid state phenomena and properties, which are important in
both academic and industrial purposes.1−6 Isomerism and
within the cocrystal contain similar structures, comparable
potential energies and almost similar crystallization kinetics,
cocrystal formation are fundamental solid state phenomena.
then cocrystallization is easier.16 There are many examples of
Isomerism has been a very important aspect in the develop-
cocrystals of organic molecules,13,14,17−27 and formation of
ment of coordination chemistry.7 It arises when two or more
forms of a complex are found having different structures either
these species can be explained on the basis of noncovalent
interactions. In contrast, cocrystals containing metal complexes
in atom connectivity or in the orientation of atoms in space.
are relatively rare28−33 due to the fact that compounds with
different geometries rarely possess similar lattice packing forces
and exhibit similar crystallization kinetics.
Various type of isomerism, e.g., ionization, hydrate, linkage,8
polymerization, coordination, coordination position, valence,
geometric (cis−trans, fac−mer), optical, etc. have been well
recognized since the time of Werner.9,10 Among these,
“coordination position” isomerism is arguably the least familiar
as examples are found very rarely.11 Such isomerism occurs in
multinuclear complexes with different arrangements of
coordinating groups relative to the metal ions. On the other
hand, cocrystals are a subject of great and growing interest in
Recently, we have focused our attention on the synthesis of
heterometallic polynuclear complexes based on neutral “metal-
loligands” (mononuclear Cu(II) or Ni(II) complexes with salen
Received: September 24, 2013
© XXXX American Chemical Society
A
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Complex 1. Yield: 0.705
g
(83%). Anal. Calcd for
type N2O2 donor tetradentate di-Schiff base ligands, salen =
N,N′-bis(salicylidene)-ethylenediamine) and some pseudoha-
C36H32Ni3N6O4S2: C 50.70, H 3.78, N 9.85%. Found: C 50.59, H
3.87, N 9.88%. UV/vis: λmax (CH3OH) = 592, 407 and 343 nm; λmax
(solid, reflectance) = 630, 510 and 391. IR (KBr): ν (CN) 1610 and
ν (SCN) 2085 cm−1.
lides (N3 ,NCS−,NCO− and N(CN)2 ).34−41 On the course of
our investigations, we succeeded in isolating several trinuclear
molecules with bent and linear shapes and also observed
important solid state phenomena such as “polymorphism”,35,36
“supamolecular isomerism”40 and “linear-bent” isomerism.41 In
most of the trinuclear heterometallic compounds reported so
far, the oxygen atoms of two neutral “metalloligands” chelate
the central heterometal ion (usually divalent) and its cationic
charges are neutralized by either anionic coligands or
counteranions. The molecular geometries of the resulting
trinuclear compounds depend upon the nature of the anionic
coligands. If these anionic coligands bridge two adjacent metal
centers, they will produce linear trinuclear compounds,42−44
whereas, both linear and bent structures may be obtained when
the anions are monodentate45−49 or noncoordinating.50
Here we report the structural and magnetic characterization
of a homometallic trinuclear Ni(II) complex [(NiL)2Ni-
(NCS)2] (1) and three heterometallic trinuclear Ni(II)−
Zn(II)−Ni(II) complexes [(NiL)2Zn(NCS)2] (2), [(NiL)2Zn-
(NCS)2(CH3OH)2]·2CH3OH (3) and {[(NiL)2Zn-
(NCS)2(CH3OH)2]}{[(NiL)2Zn(NCS)2]} (4), which are
synthesized by the “complex as ligand” strategy with the
“metalloligand” [NiL] (where H2L = N,N′-bis(salicylidene)-
1,3-propanediamine) and thiocyanate anion. Complexes 1 and
2 are isostructural bent trinuclear species whereas 3 is a linear
trinuclear complex, and complex 4 is a cocrystal formed by two
linear trinuclear units 4A and 4B. The trinuclear unit 4A is
identical to complex 3 and 4B is a “coordination position
isomer” of complex 2. To the best of our knowledge, complex 4
is the first known example of a dual-component cocrystal
formed by trinuclear heterometallic complexes with any Schiff
base ligand.
−
−
Complex 2. Yield: 0.650
g (76%). Anal. Calcd for
C36H32Ni2N6O4S2Zn: C 50.30, H 3.75, N 9.78%. Found: C 50.41, H
3.55, N 9.71%. Ni:Zn ratio = 66.2:33.8 (determined by electron probe
micro analysis). UV/vis: λmax (CH3OH) = 590, 405 and 337 nm; λmax
(solid, reflectance) = 624, 499 and 390. IR (KBr): ν (CN) 1625 and
ν (SCN) 2088 cm−1. HRMS (m/z, ESI+): found for [(NiL)H]+
=
339.08 (calcd 339.06), [(NiL)Na]+ = 361.10 (calcd 361.05),
[(NiL)2H]+= 677.11 (calcd 677.12), [(NiL)2Na]+ = 699.11 (calcd
699.10).
Synthesis of [(NiL)2Zn(NCS)2(CH3OH)2]·2CH3OH (3). Complex
3 was prepared by mixing the same components as for 2 but with
different stoichiometric ratios. The precursor “metalloligand” [NiL]
(1.284 g, 4 mmol) was dissolved in methanol (20 mL) and then an
aqueous solution (1 mL) of Zn(ClO4)2·6H2O (0.373 g, 1 mmol), and
an aqueous solution (1 mL) of ammonium thiocyanate (0.152 g, 2
mmol) were added to this solution. The solution was stirred for 1 h at
room temperature. In this case, a small amount of light blue product
separated out upon stirring the solution. The blue rhombic shaped X-
ray quality single crystals of 3 were obtained by slow evaporation of
the filtrate. The compound was washed with diethyl ether and dried in
a desiccator containing anhydrous CaCl2.
Complex 3: Yield: 0.618 g, (63%, calculated with respect to
Zn(ClO4)2·6H2O). Anal. Calcd for C40H48Ni2N6O8S2Zn: C 48.64, H
4.90, N 8.51%. Found: C 48.69, H 4.78, N 8.48%. Ni:Zn ratio =
64.2:35.8 (determined by electron probe micro analysis). UV/vis: λmax
(CH3OH) = 588, 405 and 337; λmax (solid, reflectance) = 1023, 576,
410 and 364 nm. IR (KBr): ν(CN) 1626 and ν(SCN) 2093 cm−1.
HRMS (m/z, ESI+): found for [(NiL)H]+ = 339.05 (calcd 339.06),
[(NiL)Na]+ = 361.04 (calcd 361.05), [(NiL)2H]+ = 677.12 (calcd
677.12), [(NiL)2Na]+ = 699.06 (calcd 699.10).
Synthesis of {[(NiL)2Zn(NCS)2(CH3OH)2]}{[(NiL)2Zn(NCS)2]} (4).
Complex 4 was also prepared by mixing the same components as for 2
and 3 but with a smaller proportion of “metalloligand”. The precursor
“metalloligand” [NiL] (0.321 g, 1 mmol) was dissolved in methanol
(20 mL) and then an aqueous solution (1 mL) of Zn(ClO4)2·6H2O
(0.373 g, 1 mmol), and an aqueous solution (1 mL) of ammonium
thiocyanate (0.152 g, 2 mmol) were added to this solution. The
solution was stirred for 1 h at room temperature. Here a red solid
product was separated out. The microcrystalline red product was
separated by filtration. Evaporation at room temperature of the filtrate
yielded red needle shaped X-ray quality single crystals of 4. The
compound was washed with diethyl ether and dried in a desiccator
containing anhydrous CaCl2.
Complex 4. Yield: 0.353 g, (58%, calculated with respect to [NiL]).
Anal. Calcd for C74H72Ni4N12O12S4Zn2: C 48.96, H 4.00, N 9.26%.
Found: C 48.78, H 3.92, N 9.19%. Ni:Zn ratio = 61.1:38.9
(determined by electron probe micro analysis). UV/vis: λmax
(MeOH) = 587, 406 and 338 nm; λmax (solid, reflectance) = 1032,
623, 530 and 371 nm. IR (KBr): ν (CN) 1627 and ν (SCN) 2075
cm−1. HRMS (m/z, ESI+): found for [(NiL)H]+ = 339.09 (calcd
339.06), [(NiL)Na]+ = 361.04 (calcd 361.05), [(NiL)2H]+ = 677.13
(calcd 677.12), [(NiL)2Na]+ = 699.06 (calcd 699.10).
EXPERIMENTAL SECTION
Starting Materials. The salicylaldehyde and 1,3-propanediamine
were purchased from Lancaster and were of reagent grade. They were
used without further purification.
■
Caution! Perchlorate salts of metal complexes with organic ligands
are potentially explosive. Only a small amount of material should be
prepared and it should be handled with care.
Synthesis of the Schiff Base Ligand N,N′-Bis(salicylidene)-
1,3-propanediamine (H2L) and the “Metalloligand” [NiL]. The
Schiff base ligand was synthesized by standard methods: 5 mmol of
1,3-propanediamine (0.42 mL) was mixed with 10 mmol of
salicylaldehyde (1.04 mL) in methanol (20 mL). The resulting
solution was refluxed for ca. 2 h and was allowed to cool. The yellow
methanolic solution was used directly for complex formation. An
aqueous solution (20 mL) of Ni(ClO4)2·6H2O (1.820 g, 5 mmol) and
10 mL of ammonia solution (20%) were added to a methanolic
solution of H2L (10 mL, 5 mmol) to prepare the “metalloligand”
[NiL], as reported earlier.51
Physical Measurements. Elemental analyses (C, H and N) were
performed using a Perkin-Elmer 2400 series II CHN analyzer. IR
spectra in KBr pellets (4000−500 cm−1) were recorded using a Perkin-
Elmer RXI FT-IR spectrophotometer. Electronic spectra in methanol
and in solid state were recorded in a Hitachi U-3501 spectropho-
tometer. Powder X-ray diffraction patterns were recorded on a Bruker
D-8 Advance diffractometer operated at 40 kV voltage and 40 mA
current and calibrated with a standard silicon sample, using Ni-filtered
Cu Kα (λ = 0.154 06 nm) radiation. The electrospray ionization mass
spectrometry (ESI-MS positive) spectra were recorded with a
Micromass Qtof YA 263 mass spectrometer. The results have been
described in the Supporting Information (Figures S1−S3). The Zn
and Ni contents and ratios were measured on a Philips ESEM X230
Synthesis of [(NiL)2Ni(NCS)2] (1) and [(NiL)2Zn(NCS)2] (2). The
previously prepared [NiL] complex (0.642 g, 2 mmol) was dissolved
in methanol (20 mL) and then an aqueous solution (1 mL) of
Ni(ClO4)2·6H2O (0.364 g, 1 mmol), and an aqueous solution (1 mL)
of ammonium thiocyanate (0.152 g, 2 mmol) were added. The mixture
was stirred for 1 h at room temperature when a red solid separated out.
The solid was filtered, and the filtrate was allowed to stand overnight,
resulting in the formation of prismatic red X-ray quality single crystals
of 1. The brown rhombic-shaped single crystals of complex 2 were
obtained in the same manner as 1, except that Zn(ClO4)2·6H2O
(0.372 g, 1 mmol) was used in the synthesis instead of Ni(ClO4)2·
6H2O. Both compounds were washed with diethyl ether and dried in a
desiccator containing anhydrous CaCl2.
B
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Table 1. Crystal Data and Structure Refinement of Complexes 1−4
complexes
1
2
3
4
formula
C36H32Ni3N6O4S2
852.89
C36H32Ni2N6O4S2Zn
859.59
C40H48Ni2N6O8S2Zn
987.75
C74H72Ni4N12O12S4Zn2
1815.26
monoclinic
C2/c
M
crystal system
space group
a, Å
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
12.557(5)
9.989(5)
28.118(5)
100.490(5)
3468(2)
4
12.598(5)
9.960(5)
28.247(5)
101.299(5)
3476(2)
4
12.162(5)
16.153(5)
22.493(5)
94.635(5)
4404(2)
4
27.683(5)
9.304(5)
30.671(5)
95.513(5)
7863(5)
4
b, Å
c, Å
β, deg
V, Å3
Z
Dc, g cm−3
μ, mm−1
F (000)
R(int)
1.633
1.643
1.490
1.533
1.783
1.927
1.538
1.712
1752
1760
2048
3728
0.073
0.075
0.074
0.101
total reflections
unique reflections
I > 2σ(I)
39557
35550
27829
46452
6391
7159
7808
9561
4427
5399
5166
5287
a
b
R1 , wR2 For I > 2σ(I)
0.0405, 0.0856
0.919
0.0670, 0.1767
1.06
0.0717, 0.1980
1.04
0.0596, 0.1832
1.03
c
GOF on F2
a
b
c
2
2
4
2
R1 = ∑||Fo| − |Fc||/∑|Fo| wR2 (Fo ) = ∑[w(Fo − Fc2)2/∑wFo ]1/2 GOF = ∑[w(Fo − Fc2)2/(Nobs − Nparams)]1/2
scanning electron microscope equipped with an EDAX DX-4
microsonde. Variable temperature magnetic susceptibility measure-
ments were carried out in the temperature range 2−300 K with an
applied magnetic field of 0.1 T on polycrystalline samples of
complexes 1, 3 and 4 (with masses of 23.33, 43.61, and 27.89 mg,
respectively) with a Quantum Design MPMS-XL-5 SQUID magneto-
meter. The susceptibility data were corrected for the sample holders
previously measured using the same conditions and for the
diamagnetic contributions of the salt as deduced by using Pascal’s
constant tables (χdia = −427.58 × 10−6, −519.86 × 10−6 and −938.58
× 10−6 emu·mol−1 for 1, 3 and 4, respectively).52
Crystallographic Data Collection and Refinement. Well-
formed single crystals of each complex were mounted on a Bruker-
AXS SMART APEX II diffractometer equipped with a graphite
monochromator and Mo Kα (λ = 0.71073 Å) radiation. The crystals
were positioned 60 mm from the CCD, and frames (360) were
measured with a counting time of 5 s at 293 K. The structures were
solved using the Patterson method through the SHELXS 97 program,
while difference Fourier synthesis and least-squares refinement
confirmed the positions of the non-hydrogen atoms, which were
refined with anisotropic displacement parameters. Hydrogen atoms
were placed in idealized positions and their displacement parameters
were fixed to be 1.2 times larger than those of the atom to which they
were attached. Absorption corrections were carried out using the
SADABS program,53 while all calculations were made via SHELXS
97,54 SHELXL 97,55 PLATON 99,56 ORTEP-3257 and WINGX
system ver-1.64.58 Data collection, structure refinement parameters
and crystallographic data for the four complexes are given in Table 1.
In unit 4B, the two Ni(II) atoms are equivalent due to the presence
of a C2 axis but there is only one κN-SCN− ligand coordinated to the
Ni(II) ions. Accordingly, we found a disorder with an occupancy factor
of 1/2 for this κN-SCN− ligand coordinated to the Ni(II) ions. This
disorder implies that the κN-SCN− ligand is coordinated to one Ni(II)
ion in one-half of the 4B units and to the other Ni(II) ion in the other
half.
Ni(II) complex [(NiL)2Ni(NCS)2] (1). The microcrystalline
red product, which is isolated on stirring, is pure 1, as its
powder XRD pattern is identical to that simulated from the X-
ray structure of 1 (Figure S4, Supporting Information). The
prismatic red single crystals of 1 were obtained on keeping the
filtrate overnight in the open atmosphere. Interestingly, a
reaction of the same “metalloligand” [NiL] and ammonium
thiocyanate with zinc perchlorate, instead of nickel perchlorate,
at room temperature, resulted in three different heterometallic
complexes: [(NiL)2 Zn(NCS)2 ] (2), [(NiL)2 Zn-
(NCS)2(CH3OH)2]·2CH3OH (3) and {[(NiL)2Zn-
(NCS)2(CH3OH)2]} {[(NiL)2Zn(NCS)2]} (4), depending
on the molar ratios of the reactants. When [NiL], Zn(II) and
SCN− were mixed in the same stoichiometric ratio as for 1 i.e.,
in 2:1:2 molar ratio, then a brown microcrystalline product of
complex 2 was obtained. A comparison of the powder XRD
patterns of this brown product with that of the simulated
powder XRD pattern of the single crystal (Figure S4,
Supporting Information) clearly showed that it is pure 2.
Complex 2 crystallizes as brown rhombic-shaped single crystals.
Complex 3 was synthesized by using the same components but
with an increase in the proportion of “metalloligand” [NiL] i.e.,
mixing [NiL], Zn(II) and SCN− in a 4:1:2 molar ratio. The
powder XRD pattern of the isolated light blue microcrystalline
product is identical to that obtained from simulation of the
crystal data of 3, indicating its phase purity (Figure S4,
Supporting Information). Complex 3 crystallizes as blue
rhombic-shaped single crystals. Finally, when [NiL], Zn(II)
and SCN− were mixed in a 1:1:2 molar ratio at room
temperature, i.e., decreasing the proportion of “metalloligand”
[NiL], a red solid was isolated from the reaction mixture. The
isolated red product is pure 4, as is evident from its powder
XRD pattern (Figure S4, Supporting Information). Red needle-
shaped X-ray quality single crystals of 4 were obtained by the
slow evaporation of the filtrate in open atmosphere. It is
important to mention that although we tried to prepare
equivalent Ni(II) complexes, the color and the powder XRD
pattern are unchanged when nickel perchlorate hexahydrate
and ammonium thiocyanate were mixed in either increasing or
RESULTS AND DISCUSSION
■
Syntheses of the Complexes. The Schiff-base ligand
(H2L) and its Ni(II) complex, [NiL], were synthesized using
the reported procedure.51 The Ni(II) complex, on reaction with
nickel perchlorate hexahydrate and ammonium thiocyanate in a
2:1:2 molar ratio at room temperature, resulted in the trinuclear
C
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Scheme 1. Formation of Complexes 1−4 and the Transformation of Complexes 4 and 3 to 2
Figure 1. Electronic spectra of the complexes in MeOH (left) and solid state (right).
decreasing proportions of [NiL], thus 1 is the only complex
that was obtained with Ni(II). In summary, we can easily obtain
complexes 4, 2 and 3 by simply increasing the [NiL] ratio, from
1 to 2 and 4, respectively (fixing the Zn(II):NCS− ratio to 1:2,
Scheme 1).
It should be noted that both complexes 3 and 4 are
transformed to 2 when these are dissolved in methanol
separately and the solutions are kept at room temperature for
slow evaporation. On the other hand, complexes 1 and 2 do not
change to any other form upon recrystallization.
IR and UV−vis Spectra of the Complexes. Besides
elemental analysis, all the complexes were initially characterized
by IR spectra. Like the precursor “metalloligand” [NiL], a
strong and sharp band due to the azomethine υ (CN) group
of the Schiff base appears at 1610, 1625, 1626 and 1627 cm−1
for complexes 1−4, respectively. The precursor “metalloligand”
[NiL] is neutral and obviously is not associated with any
counteranion. Therefore, the characteristics peaks for thio-
cyanate in the region of 2100−2050 cm−1 indicate the
formation of the complexes. These peaks were clearly detected
at 2085, 2088, 2093 and 2075 cm−1 in the IR spectra of 1−4,
respectively (Figures S5−S8, Supporting Information).
The UV−vis spectra of the complexes in methanolic solution
and their solid state diffuse reflectance spectra are shown in
Figure 1 and the spectral parameters are given in Table 2. The
electronic spectra of all the complexes in methanol are almost
identical, but they differ appreciably in the solid state, especially
in the visible region. Thus, they show a sharp single absorption
band near 343, 337, 337 and 338 nm in methanol and 391, 390,
364 and 371 nm in the solid state for 1−4, respectively,
attributed to ligand-to-metal charge transfer transitions. Besides
this band, a broad absorption band (ν1) is observed in the
D
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[(NiL)2Ni(NCS)2] (where H2L = N,N′-bis(salicylidene)-1,3-
propanediamine). The structure contains a nickel(II) ion (Ni2)
in a distorted octahedral environment together with two square
planar nickel(II) ions (Ni1 and Ni3) with similar environments.
The two square planar nickel atoms are bonded to two nitrogen
atoms and two oxygen atoms from the tetradentate Schiff base
ligand with very similar bond lengths (Table 3). The four
donor atoms in the equatorial plane show root-mean-squared
deviations of 0.053 and 0.028 Å for Ni1 and Ni3, respectively,
with the nickel atoms at 0.011(1), 0.021(1) Å from the
appropriate mean plane. The trans angles are all close to 170°
(Table 3), indicating a slight tetrahedral distortion from the
ideal square planar geometry, as confirmed by the low τ4
indexes63 (0.134 for Ni1 and 0.133 for Ni3; τ4 is defined as τ4 =
[360° − (α + β)]/141°, with α and β (in deg) being the two
largest angles around the central metal in the complex with τ4 =
0 for a perfect square planar and τ4 = 1 for a perfect
tetrahedron. The dihedral angle between the two N2−Ni−O2
planes is 22.1(1)°, indicating that the two “metalloligands” are
almost parallel to each other.
The central Ni2 atom has an octahedral environment formed
by four oxygen atoms from the two chelating “metalloligands”
and by two terminal cis κN-SCN− ligands. The geometry is very
distorted, primarily due to the small bite angles of the chelating
[NiL] metalloligands (ca. 67−69°). Interestingly, the two κN-
SCN− ligands present different orientations as reflected by their
Ni−N−C bond angles [Ni2−N1−C1 = 176.9(3)° and Ni2−
N2−C2 = 150.2(4)°]. The Ni1···Ni2, Ni2···Ni3 and Ni3···Ni1
distances are 3.046(1), 3.045(1) and 4.357(2) Å, respectively.
The Ni1−Ni2−Ni3 angle is 91.33(3)°, indicating a nearly
perpendicular arrangement of the metal atoms in the bent Ni3
unit.
Complex 2 is isostructural to 1 but with a central Zn(II) ion,
instead of a Ni(II) one, bridging the external square planar
Ni(II) atoms (Figure 2; right), thus forming a neutral trinuclear
unit of formula [(NiL)2Zn(NCS)2]. As expected, a comparison
of the bond lengths and angles between 1 and 2 shows very
small differences (Table 3). Again, there is a small tetrahedral
distortion in the equatorial planes around nickel with root-
mean-squared deviations of the coordinating atoms of 0.032
and 0.034 Å around Ni1 and Ni3, respectively. The metal
atoms are 0.011(31) and 0.024(3) Å from the planes. The τ4
values (0.130 for Ni1 and 0.136 for Ni3) also confirm a slightly
distorted square planar geometry around the metal centers. The
two “metalloligands” are also nearly parallel to each other, as
indicated by the dihedral angle (20.2(2)°) between the two
N2−Ni−O2 planes.
Table 2. UV−Vis Spectral Parameters of the ‘‘Metalloligand”
and Complexes 1−4
complex λmax (nm) (ε, M−1 cm−1) in CH3OH
λmax (nm) in solid state
[NiL] 592(48), 406(3725) and 343(5227)
387, 508 and 632
1
2
3
4
592 (148), 407(7857) and 343 (11599) 391, 510 and 630
590 (70), 405(2775)and 337 (14724) 390, 499 and 624
588 (108), 405(1988)and 337 (23555) 364, 410, 576 and 1023
587(272), 406(4277) and 338 (25588) 371, 530, 623 and 1032
visible region at 592, 590, 588 and 587 nm along with a less
intense shoulder (ν2) at 407, 405, 405 and 406 nm in methanol
for 1−4, respectively, whereas the “metalloligand” [NiL] shows
band maxima (ν1) at 592 nm along with a less intense shoulder
(ν2) at 406 nm. This band is typical of d−d transitions of Ni(II)
ions with a square planar environment. The electronic
spectrum for a four coordinate nickel(II) compound with a
square planar geometry is expected to exhibit absorption bands
near 610 (ν1) and 500 nm (ν2), corresponding to the spin
l
l
allowed d−d transitions Blg
← ←
lAg and B3g lAg,
respectively.59,60 The observation of the ν1 and ν2 bands
confirms the square planar environment around Ni(II) in
methanol solutions. However, in the solid state, the band
positions of 1 (broad band at 630 nm (ν1) along with a hump
510 nm (ν2)) and 2 (broad band at 624 nm (ν1) along with a
hump 499 nm (ν2)) are almost the same as in the mononuclear
precursor [NiL] (broad band at 623 nm (ν1) along with a
hump 500 nm (ν2)), in agreement with the square planar
geometry around Ni(II). On the other hand, 3 exhibits two
distinct bands at 576 and 1023 nm, which can be assigned to
3
3
3
the spin-allowed d−d transitions T1g(F) ← A2g and T2g
←
3A2g, respectively. These values agree with the literature values
for octahedral Ni(II) compounds.61,62 Moreover, complex 4
shows a weak broad band at 623 nm (ν1) along with a hump at
530 nm (ν2) and another well-separated broad band at 1032
nm. The former two bands (ν1l and ν2) are due to the spin
l
l
l
allowed d−d transitions Blg ← Ag and B3g ← Ag for square
planar Ni(II) and the latter band is assignable to the transition
3T2g ← 3A2g for octahedral Ni(II) geometry, in agreement with
the structural data (see below).
Structures of the Complexes. The structure of 1 is shown
in Figure 2 (left) together with the atomic numbering scheme
in the coordination spheres. Bond lengths and angles in the
metal coordination sphere are given in Table 3. The molecular
structure of 1 consists of a neutral trinuclear entity of formula
The zinc atom, Zn(2) has a similar distorted octahedral
environment to Ni(2) in complex 1. The cis [64.5(2)−
104.7(2)°] and trans [153.9(2)−158.7(2)°] angles also indicate
significant distortions from the ideal octahedral geometry
around the zinc atom. The orientation of the two κN-SCN−
ligands are also different (Zn2−N1−C1 = 177.3(6)° and Zn2−
N2−C2= 150.7(5)°]. The Zn···Ni distances are 3.124(2) and
3.142(2) Å whereas the Ni···Ni distance is 4.424(3) Å. Like in
1, the Ni1−Zn2−Ni3 angle (89.80(3)°) indicates a nearly
perpendicular arrangement of the three metal atoms in the
Ni2Zn unit.
The noncovalent interactions present between the two
trinuclear units in complexes 1 and 2 are also very similar (see
Figures S9 and S10, Supporting Information).
The structure of 3 is shown in Figure 3 together with the
atomic numbering scheme. Bond lengths and angles in the
Figure 2. Structures of complexes 1 (left) and 2 (right) with ellipsoids
at 30% probability. In both structures, the six-membered saturated
chelate ring containing atoms N49 and N53 is disordered and two
positions were refined for the three methylene groups. Only one
possible orientation is shown.
E
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Table 3. Bond Distances (Å) and Angles (deg) for Complexes 1−3
complex 1
complex 2
complex 3
Ni(1)−O(11)
Ni(1)−O(31)
Ni(1)−N(19)
Ni(1)−N(23)
Ni(2)−O(11)
Ni(2)−O(31)
Ni(2)−O(41)
Ni(2)−O(61)
Ni(2)−N(1)
Ni(2)−N(2)
Ni(3)−O(41)
Ni(3)−O(61)
Ni(3)−N(49)
Ni(3)−N(53)
O(11)−Ni(1)−O(31)
O(11)−Ni(1)−N(19)
O(11)−Ni(1)−N(23)
O(31)−Ni(1)−N(19)
O(31)−Ni(1)−N(23)
N(19)−Ni(1)−N(23)
O(11)−Ni(2)−O(31)
O(11)−Ni(2)−O(41)
O(11)−Ni(2)−O(61)
O(11)−Ni(2)−N(1)
O(11)−Ni(2)−N(2)
O(31)−Ni(2)−O(41)
O(31)−Ni(2)−O(61)
O(31)−Ni(2)−N(1)
O(31)−Ni(2)−N(2)
O(41)−Ni(2)−O(61)
O(41)−Ni(2)−N(1)
O(41)−Ni(2)−N(2)
O(61)−Ni(2)−N(1)
O(61)−Ni(2)−N(2)
N(1)−Ni(2)−N(2)
1.873(3)
Ni(1)−O(11)
Ni(1)−O(31)
Ni(1)−N(19)
Ni(1)−N(23)
Zn(2)−O(11)
Zn(2)−O(31)
Zn(2)−O(41)
Zn(2)−O(61)
Zn(2)−N(1)
Zn(2)−N(2)
Ni(3)−O(41)
Ni(3)−O(61)
Ni(3)−N(49)
Ni(3)−N(53)
O(11)−Ni(1)−O(31)
O(11)−Ni(1)−N(19)
O(11)−Ni(1)−N(23)
O(31)−Ni(1)−N(19)
O(31)−Ni(1)−N(23)
N(19)−Ni(1)−N(23)
O(11)−Zn(2)−O(31)
O(11)−Zn(2)−O(41)
O(11)−Zn(2)−O(61)
O(11)−Zn(2)−N(1)
O(11)−Zn(2)−N(2)
O(31)−Zn(2)−O(41)
O(31)−Zn(2)−O(61)
O(31)−Zn(2)−N(1)
O(31)−Zn(2)−N(2)
O(41)−Zn(2)−O(61)
O(41)−Zn(2)−N(1)
O(41)−Zn(2)−N(2)
O(61)−Zn(2)−N(1)
O(61)−Zn(2)−N(2)
N(1)−Zn(2)−N(2)
O(41)−Ni(3)−O(61)
O(41)−Ni(3)−N(49)
O(41)−Ni(3)−N(53)
O(61)−Ni(3)−N(49)
O(61)−Ni(3)−N(53)
N(49)−Ni(3)−N(53)
1.866(4)
1.866(4)
1.902(5)
1.900(5)
2.116(4)
2.189(4)
2.320(4)
2.094(4)
2.004(6)
2.035(6)
1.876(4)
1.866(4)
1.883(5)
1.915(5)
79.0(2)
92.0(2)
171.1(2)
170.8(2)
92.5(2)
96.6(2)
66.9(2)
98.3(2)
158.7(2)
97.5(2)
94.3(2)
76.1(2)
95.5(2)
96.0(2)
153.9(2)
64.5(2)
157.7(2)
89.6(2)
96.2(2)
97.9(2)
104.7(2)
78.3(2)
93.1(2
Ni(1)−O(3)
Ni(1)−O(11)
Ni(1)−O(31)
Ni(1)−N(1)
Ni(1)−N(19)
Ni(1)−N(23)
Zn(2)−O(11)
Zn(2)−O(31)
Zn(2)−O(41)
Zn(2)−O(61)
Ni(3)−O(4)
Ni(3)−O(41)
Ni(3)−O(61)
Ni(3)−N(2)
Ni(3)−N(49)
Ni(3)−N(53)
O(3)−Ni(1)−O(11)
O(3)−Ni(1)−O(31)
O(3)−Ni(1)−N(1)
O(3)−Ni(1)−N(19)
O(3)−Ni(1)−N(23)
O(11)−Ni(1)−O(31)
O(11)−Ni(1)−N(1)
O(11)−Ni(1)−N(19)
O(11)−Ni(1)−N(23)
O(31)−Ni(1)−N(1)
O(31)−Ni(1)−N(19)
O(31)−Ni(1)−N(23)
N(1)−Ni(1)−N(19)
N(1)−Ni(1)−N(23)
N(19)−Ni(1)−N(23)
O(11)−Zn(2)−O(31)
O(11)−Zn(2)−O(41)
O(11)−Zn(2)−O(61)
O(31)−Zn(2)−O(41)
O(31)−Zn(2)−O(61)
O(41)−Zn(2)−O(61)
O(4)−Ni(3)−O(41)
O(4)−Ni(3)−O(61)
O(4)−Ni(3)−N(2)
O(4)−Ni(3)−N(49)
O(4)−Ni(3)−N(53)
O(41)−Ni(3)−O(61)
O(41)−Ni(3)−N(2)
O(41)−Ni(3)−N(49)
O(41)−Ni(3)−N(53)
O(61)−Ni(3)−N(2)
O(61)−Ni(3)−N(49)
O(61)−Ni(3)−N(53)
N(2)−Ni(3)−N(49)
N(2)−Ni(3)−N(53)
N(49)−Ni(3)−N(53)
2.181(5)
1.871(3)
2.044(5)
2.047(5)
2.048(7)
2.022(6)
2.024(6)
1.954(5)
1.947(5)
1.952(5)
1.956(5)
2.204(6)
2.053(5)
2.046(5)
2.043(7)
2.034(7)
2.032(7)
87.3(2)
87.0(2)
179.2(3)
85.6(2)
90.7(2)
79.0(2)
92.3(2)
90.5(2)
168.8(2)
93.6(3)
167.4(2)
89.9(2)
93.7(3)
89.9(3)
100.3(3)
83.7(2)
129.1(2)
119.5(2)
119.8(2)
126.1(2)
84.3(2)
86.4(2)
87.1(2)
178.4(3)
90.7(2)
84.6(2)
79.6(2)
95.2(3)
89.6(2)
166.8(2)
92.8(2)
169.0(2)
90.3(2)
89.8(3)
93.8(3)
100.2(3)
1.901(4)
1.894(4)
2.063(3)
2.125(3)
2.185(3)
2.049(2)
1.994(4)
2.025(4)
1.885(3)
1.866(3)
1.883(3)
1.905(3)
79.16(11)
91.56(13)
170.75(13)
170.29(13)
92.55(13)
96.91(15)
69.41(11)
99.08(10)
162.91(11)
95.42(12)
94.98(12)
79.56(10)
96.68(10)
94.35(14)
159.67(13)
67.73(10)
161.04(12)
90.62(13)
95.49(12)
95.95(12)
100.22(15)
78.06(11)
93.25(14)
170.46(13)
170.79(14)
92.42(13)
96.29(15)
O(41)−Ni(3)−O(61)
O(41)−Ni(3)−N(49)
O(41)−Ni(3)−N(53)
O(61)−Ni(3)−N(49)
O(61)−Ni(3)−N(53)
N(49)−Ni(3)−N(53)
170.3(2)
170.7(2)
92.1(2)
96.6(2)
metal coordination spheres are given in Table 3. The molecular
structure of 3 consists of the neutral trinuclear unit [(NiL)2Zn-
(NCS)2(CH3OH)2] and two noncoordinated methanol mole-
cules. In this trinuclear unit, the three metal atoms (two
terminal Ni atoms and the central Zn atom) are in a nearly
linear disposition. Thus, the geometry is very different from
that observed in 1 and 2. The two terminal Ni(II) ions present
a distorted octahedral coordination sphere. The basal plane of
each Ni(II) ion is formed by the two imine nitrogen atoms and
two phenoxido oxygen atoms from one ‘‘metalloligand”. These
four donors in the equatorial plane show root-mean-squared
deviations from their mean planes of 0.044 and 0.056 Å for Ni1
and Ni3, respectively. The metal atoms are displaced 0.082(3)
and 0.099(3) Å from the mean plane in the direction of axial
F
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Ni1···Zn2 and Ni3···Zn2 distances are 3.032(1) and 3.024(1) Å
and the Ni1···Ni3 distance is 6.056(2) Å. In contrast to 1 and 2,
the three metal atoms in the trinuclear unit are nearly linear as
is evident from the Ni1−Zn2−Ni3 angle (178.93(4)°).
As in 1 and 2, some noncovalent H-bonding and C−H···π
interactions are present in complex 3 (see Figure S11,
Supporting Information).
Complexes 2 and 3 represent rare examples of very closely
related Ni2Zn complexes having different spatial arrangements
(bent, nearly orthogonal in 2 and almost linear in 3, Figure 4).
These different arrangements arise from the different
coordination environments of the Ni(II) and Zn(II) ions: the
terminal Ni(II) ions are square planar in 2 but octahedral in 3.
The central Zn(II) ion also presents different coordination
spheres: octahedral in 2 and tetrahedral in 3 (Figure 4). It
should be noted that the occurrence of linear and bent
molecular geometries in trinuclear compounds is very rare and
only recently has it been found in a couple of isomers with a
similar Schiff base.41 The comparable energy of stabilization in
both geometries of Zn(II) (tetrahedral and octahedral) due to
the lack of crystal field stabilization may explain the presence of
these two closely related Ni2Zn complexes with so different
coordination environments.
A search in the CCDC database (updated May 2013) shows
a total of ca. 30 trinuclear compounds formed by transition
metals and group 12 metals with Schiff bases and κ-N or κ-S
SCN ligands.34−37,47,64−74 Only four present a linear geometry
(as observed in 3): two Cu2Hg trinuclear complexes (with the
SCN ligand, as expected, κ-S coordinated to the central soft Hg
atom),75one Cd3 trinuclear complex72 and one Zn3 trinuclear
complex.76 In the latter two compounds, each of the terminal
metal atoms bears a coordinated SCN− ligand (κ-S for Cd and
κ-N for Zn, in agreement with the softer character of Cd).
Complex 3 is, therefore, the first linear compound of this type
that contains Ni(II) ions.
Figure 3. Structure of complex 3 with ellipsoids at 30% probability.
Two noncoordinated methanol molecules are not shown.
atoms N1 and O4 for Ni1 and Ni3, respectively. The basal Ni−
O and Ni−N bond distances are in similar ranges (Table 3).
The apical positions are occupied by the nitrogen atoms from a
κN-SCN− anion and by an oxygen atom from a coordinated
methanol molecule. The apical Ni−O and Ni−N bond lengths
are also in similar ranges and the axial trans angles are close to
180° (Table 3). In contrast to 1 and 2, the orientation of the
two SCN− ligands is similar with Ni−N−C bond angles of
167.9(87) and 170.1(7)°. Again, in contrast to 1 and 2, the
dihedral angle between the two N2−Ni−O2 planes is 77.7(1)°,
indicating that the two “metalloligands” are almost perpendic-
ular (Figure 3).
Another remarkable difference between in 1 and 2 from 3 is
the tetrahedral environment of the Zn(II) ion, which is bonded
to four bridging phenoxido oxygen atoms from two different
[NiL] units. The four Zn−O bonds lengths are very similar (in
the range 1.947(5)−1.956(5) Å, Table 3) forming a distorted-
tetrahedral with O−Zn−O bond angles in the range 83.7(2)−
129.1(2)° (Table 3). The distorted tetrahedral geometry
around the Zn(II) ion is confirmed by its τ4 index of 0.74
and by the dihedral angle of 82.6(2)° between the two O−Zn−
O planes. The dihedral angle is 0° for a perfectly square planar
arrangement and 90° for a perfect tetrahedral arrangement. The
The structure of the two independent complexes of 4 is
shown in Figure 5 together with the atomic numbering scheme.
Selected bond lengths and angles are summarized in Table 4.
The molecular structure of the cocrystal 4 contains two discrete
neutral trinuclear units [(NiL)2Zn(NCS)2(CH3OH)2] (named
4A) and [(NiL)2Zn(NCS)2] (named 4B) together with two
Figure 4. Coordination environments of complexes of 2 (left) and 3 (right).The bent arrangement of three metal atoms of complex 2 and the
linearity of three metal atoms of complex 3 are indicated by the light blue shadows. Note that 1 is isostructural with 2. Color code: O = red, N =
blue, C = white, S = yellow.
G
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Figure 5. Structure of the two independent complexes (both located on a 2-fold axis) of complex 4 with ellipsoids at 30% probability (symmetry
a
b
transformation = 1 − x, y, 1/2 − z for 4A and = 3/2 − x, 1/2 − y, 2 − z for 4B). Two noncoordinated water molecules are not shown here.
Table 4. Bond Distances (Å) and Angles (deg) for Complex 4
complex 4A
complex 4B
Ni(1)−O(2)
Ni(1)−O(11)
Ni(1)−O(31)
Ni(1)−N(1)
Ni(1)−N(19)
Ni(1)−N(23)
Zn(1)−O(11)
Zn(1)−O(31)
2.159(4)
Ni(2)−N(3)
Ni(2)−O(41)
Ni(2)−O(61)
Ni(2)−N(49)
Ni(2)−N(53)
Zn(2)−N(4)
Zn(2)−O(41)
Zn(2)−O(61)
N(3)−Ni(2)−N(49)
N(3)−Ni(2)−N(53)
N(3)−Ni(2)−O(41)
N(3)−Ni(2)−O(61)
O(41)−Ni(2)−O(61)
O(41)−Ni(2)−N(49)
O(41)−Ni(2)−N(53)
O(61)−Ni(2)−N(53)
O(61)−Ni(2)−N(49)
N(49)−Ni(2)−N(53)
O(41)−Zn(2)−O(61)
2.048(9)
1.928(5)
1.929(5)
1.941(5)
1.933(4)
1.943(9)
2.126(4)
2.140(4)
97.4(3)
2.038(3)
2.044(3)
2.057(5)
2.019(4)
2.022(4)
1.946(3)
1.953(3)
O(2)−Ni(1)−N(19)
O(2)−Ni(1)−O(11)
O(2)−Ni(1)−O(31)
O(2)−Ni(1)−N(1)
O(2)−Ni(1)−N(23)
O(11)−Ni(1)−O(31)
O(11)−Ni(1)−N(1)
O(11)−Ni(1)−N(19)
O(11)−Ni(1)−N(23)
O(31)−Ni(1)−N(1)
O(31)−Ni(1)−N(19)
O(31)−Ni(1)−N(23)
N(1)−Ni(1)−N(19)
N(1)−Ni(1)−N(23)
N(19)−Ni(1)−N(23)
O(11)−Zn(1)−O(31)
90.54(15)
87.74(14)
84.22(13)
177.50(16)
87.58(16)
79.30(12)
92.50(15)
90.80(14)
168.68(14)
93.37(14)
168.94(14)
89.98(15)
91.95(16)
91.74(17)
99.55(16)
83.82(12)
127.58(12)
118.62(12)
130.30(12)
103.6(3)
89.5(3)
90.2(3)
80.74(18)
90.99(18)
164.23(18)
90.39(18)
168.75(19)
95.86(18)
71.69(15)
135.12(19)
90.80(16)
104.5(3)
92.68(16)
136.19(18)
104.3(3)
120.2(3)
119.1(3)
71.59(16)
O(41)−Zn(2)−O(41)b*
O(41)−Zn(2)−O(61)b
O(41)−Zn(2)−N(4)
O(61)−Zn(2)−O(41)b
O(61)−Zn(2)−O(61)b
O(61)−Zn(2)−N(4)
O(41)b−Zn(2)−N(4)
O(61)b−Zn(2)−N(4)
O(61)b−Zn(2)−O(41)b
O(11)−Zn(1)−O(11)a*
O(11)−Zn(1)−O(31)a
O(31)−Zn(1)−O(31)a
*
a
b
Symmetry elements: = 1 − x, y, 1/2 − z for 4A; = 3/2 − x, 1/2 − y, 2 − z for 4B.
disordered water molecules. Both trinuclear units have a
crystallographic 2-fold axis. The structure of the trinuclear unit
4A is identical to that in 3 with all the donor atoms in the
coordination spheres of the terminal Ni and central Zn centers
similarly bonded. All the Ni−O and Ni−N bond lengths are
very similar (Table 4). Again, the root-mean-squared deviation
of these four donor atoms in the basal plane around the Ni
center is 0.010 Å while the metal atom deviates 0.083(1) Å
from this plane in the direction of the N1 atom. The dihedral
angle between the two N2−Ni−O2 planes is 79.53°. The central
Zn atom possesses a distorted-tetrahedral geometry (τ4 = 0.71)
with a dihedral angle between the two O−Zn−O planes of
80.39°, very similar, as expected, to the angle between the N2−
Ni−O2 planes. Like in 3, the Zn−O bond lengths are very close
and the O−Zn−O bond angles are in the range 83.82(12)−
130.30(12)°. The Ni1···Zn1 and Ni1···Ni1a distances are
3.022(2) and 6.044(4) Å, respectively, and the Ni1−Zn1−
Ni1a angle is 178.77(3)°, indicating an almost perfect linear
arrangement of the three metal ions.
The other trinuclear unit (4B), although it has a similar
molecular shape to 4A (almost linear Ni2Zn unit), presents
different coordination environments for the metal atoms
(Figure 5). Interestingly, 4B and complex 2 are “coordination
position isomers” because they have the same formula:
[(NiL)2Zn(SCN)2] but present a different distribution of the
thiocyanato ligands. Thus, in 4B, one of the two terminal Ni
H
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Figure 6. The two Ni2Zn trinuclear units in the cocrystal 4. The linearity of three metal atoms is shown by light blue shadow (left, 4A; right, 4B).
Color code: O = red, N = blue, C = white, S = yellow.
atoms is square planar whereas the other is square pyramidal
although there is a positional disorder with occupancy of 1/2 in
such a way that in half of the complexes the κN-SCN− ligand is
coordinated to one Ni(II) ion and in the other half it is
coordinated to the other (because the two terminal Ni atoms
are equivalent due to the C2 axis). The central atom is a Zn(II)
atom with a square pyramidal geometry. In 2, the two terminal
Ni(II) ions are square planar and the central Zn(II) is
octahedral. The basal plane of the two terminal Ni(II) ions in
4B is formed by the two imine N atoms and the two phenoxido
O atoms of the Schiff base. The Ni−O and Ni−N bond lengths
in the basal plane are slightly shorter than in 3 and 4A (Tables
3 and 4). A κN-SCN− anion completes the square pyramidal
geometry of the terminal Ni(II) atoms occupying the axial
position in both Ni2 atoms with an occupancy factor of 1/2.
The square pyramidal geometry around Ni(2) is slightly
distorted, as indicated by the Addison parameter (τ = 0.075).
The τ is 0 for an ideal square pyramid and 1 for a trigonal
bipyramid.77 The central Zn(II) ions presents a square
pyramidal coordination sphere where the basal plane is
constituted by the four bridging phenoxido oxygen atoms of
the two different [NiL] ‘‘metalloligands”. An axial κN-SCN− ion
completes the square pyramidal geometry. The low Addison
parameter of the Zn(II) atom (τ = 0.018) indicates a very small
distortion of the square pyramidal geometry.
complex 3 is isolated for a [NiL] concentration four times
higher than for complex 4A (whose structure is identical to 3).
Surprisingly, the use of intermediate concentrations does not
yield the linear complexes 3, 4A nor 4B instead, the bent one
(2). Interestingly, if we replace Zn(II) by Ni(II), we always
observe the bent complex 1 (isostructural to 2). This
observation indicates that the central Ni(II) ion prefers the
octahedral geometry (as in 1) in contrast with Zn(II) that may
present different geometries: octahedral (in 2), tetrahedral (in 3
and 4A) and square pyramidal (in 4B), in agreement with the
lack of energy of stabilization of the crystal field in Zn(II)
complexes for any geometry with only σ bonds (d10 ion).
Magnetic Measurements of the Complexes. Because
the X-ray structure determination does not allow an
unambiguous assignment of the Zn(II) and Ni(II) centers (as
they have similar electron densities), we have performed
magnetic measurements in complexes 1−4 to determine the
exact nature of each metal center in each trinuclear unit.
The thermal variation of the χmT product for complex 1 per
Ni3 unit shows at room temperature a value of ca. 1.2 emu·K·
mol−1, which is the expected value for a S = 1 Ni(II) ion with a
g value of ca. 2.2. When the temperature is lowered, χmT
remains constant down to ca. 10 K (Figure 7). Below ca. 10 K,
χmT shows an abrupt decrease to reach a value of ca. 0.4 emu·
K·mol−1 at 2 K. This behavior indicates that complex 1 presents
A surprising fact in the structure of 4 is the presence of two
different (although related) complexes (4A and 4B, Figure 6),
one of them (4A) being identical to complex 3 and the other
one (4B) being a “coordination position isomer” of complex 2.
As far as we know, this is the first example of a cocrystal formed
by a trinuclear unit (4A) that may crystallize as an isolated
trinuclear unit (3) and a second trinuclear unit (4B), which is a
“coordination position isomer” of another trinuclear unit (2)
that can also be isolated.
Another surprising finding is the existence of up to three
different, although closely related, Ni2Zn trinuclear units
formed with the same [NiL] complex, Zn(II) and SCN−.
This fact suggests that the different linear and bent trinuclear
units have very small differences in energy. Although the
driving force to obtain complexes 2, 3, 4A and 4B seems to be
the [NiL]:Zn(II):SCN− ratio (Scheme 1), there must be other
factors determining the crystallized trinuclear unit because
Figure 7. Thermal variation of the χmT product per Ni3 trinuclear unit
for complex 1. Solid line is the best fit to the model (see text).
I
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only one paramagnetic Ni(II) center (the octahedral central
one) and, therefore, the two terminal Ni(II) ions are
diamagnetic, in agreement with their square planar geometry
(d8 configuration). Accordingly, the magnetic properties of 1
have been fitted to a simple isolated S = 1 monomer with a zero
field splitting (ZFS). The fit is very satisfactory in the whole
temperature range (solid line in Figure 7) with g = 2.216 and
|D| = 7.4 cm−1.
Complex 2 is diamagnetic, as expected from its structure that
shows a central octahedral diamagnetic Zn(II) ion and two
terminal square planar diamagnetic Ni(II) ions.
The thermal variation of χmT for complex 3 per Ni2Zn unit
shows a room temperature value of ca. 2.4 emu·K·mol−1, close
to the expected value for two isolated S = 1 Ni(II) ions with a g
value of ca. 2.2. When the temperature is lowered, χmT remains
constant down to ca. 10 K (Figure 8). Below ca. 10 K, χmT
Figure 9. Thermal variation of the χmT product per formula unit (two
Ni2Zn trinuclear units) for complex 4. Solid line is the best fit to the
model (see text).
Note that the fit at low temperatures slightly deviates from the
experimental points, probably due to the assumption that the
three Ni(II) ions present the same ZFS. Because the magnetic
measurements have been performed on powdered samples, we
cannot determine the sign of the ZFS parameter in neither of
the three compounds.
CONCLUSIONS
■
In the present work, we have shown that the complex formed
with Ni(II) and the tetradentate Schiff base ligand N,N′-
bis(salicylidene)-1,3-propanediamine, [NiL], may act as a
“metalloligand” toward Ni(II) and Zn(II) ions to produce
different types of Ni3 (1) and Ni2Zn trinuclear compounds (2,
3 and 4) with SCN− as a coligand. One of the most surprising
results is that by simply changing the [NiL] concentration, up
to three different Ni2Zn compounds with very close
compositions but different geometries can be isolated, namely
the bent species [(NiL)2Zn(NCS)2] (2), the linear ones
[(NiL)2Zn(NCS)2(CH3OH)2] (3 and 4A) and [(NiL)2Zn-
(NCS)2] (4B). Interestingly, the bent complex is the only one
obtained as a Ni3 trinuclear unit (1).
But the most interesting and unexpected result is the
formation of complex 4, which is a cocrystal formed by two
different Ni2Zn trinuclear units (4A and 4B). 4A is identical to
complex 3 whereas 4B is a “coordination position isomer” of
complex 2. This unusual result, a cocrystal formed by an
existing complex and a “coordination position isomer” of
another existing complex is, as far as we know, unprecedented
in coordination chemistry. The “coordination position isomer”
between 2 and 4B is raised due to the migration of one SCN−
ligand from the central octahedral Zn(II) ion to one of the
square planar terminal Ni(II) ions. The flexibility of the
coordination numbers of Ni(II) and Zn(II) ions seems to play
the crucial role in obtaining these isomers. Moreover, the
observation of the “coordination position isomers” with SCN−
anions proves that it can produce isomers other than the well-
known “linkage” isomers.
Figure 8. Thermal variation of the χmT product per Ni2Zn trinuclear
unit for complex 3. Solid line is the best fit to the model (see text).
shows an abrupt decrease and reaches a value of ca. 1.2 emu·K·
mol−1 at 2 K. This behavior indicates that complex 3 presents
two isolated paramagnetic Ni(II) centers (the octahedral
terminal ones) in agreement with its structure. Accordingly,
the magnetic properties of 3 have been fitted to a model for
two isolated S = 1 monomers with a ZFS. This model
reproduces very satisfactorily the magnetic properties of 3 in
the whole temperature range with g = 2.179 and |D| = 6 cm−1
(solid line in Figure 8).
The thermal variation of χmT per formula unit for 4
(containing two trinuclear Ni2Zn units, 4A and 4B) shows a
room temperature χmT value of ca. 3.5 emu·K·mol−1, which is
the expected value for three isolated S = 1 Ni(II) ions with g ≈
2.2. This χmT value remains constant down to ca. 30 K, and
below this temperature, it shows a sharp decrease and reaches a
value of ca. 1.1 emu·K·mol−1 at 2 K (Figure 9). This behavior
indicates that 4 presents a total of 3 paramagnetic Ni(II) ions
per formula unit. Since 4A is identical to complex 3, we can
assume that the contribution of this trinuclear unit 4A must be
ca. 2.4 emu·K·mol−1 (as in 3) and that the remaining ca. 1.1
emu·K·mol−1 must be the contribution of 4B. This value agrees
with the observed structure of 4B that, besides the diamagnetic
central Zn(II) ions, shows a diamagnetic square planar Ni(II)
ion and a paramagnetic square pyramidal Ni(II) ion.
Accordingly, we have fitted the magnetic properties to a
model consisting in three isolated S = 1 monomers with the
same ZFS parameter (to reduce the number of adjustable
parameters). This model reproduces very satisfactorily the
magnetic properties of 4 with g = 2.056 and |D| = 12.2 cm−1.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures of powder XRD pattern, IR spectra and ESI mass
spectra of the complexes (Figures S1−S11), the non-covalent
interactions present in the complexes 1−3, electrospray
ionization mass spectrometry of complexes 2−4 and crystallo-
graphic data in CIF format for all four complexes (1−4). This
J
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Article
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material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*A. Ghosh. E-mail: ghosh_59@yahoo.com.
■
(20) Zhang, X.-L.; Chen, X.-M. Cryst. Growth Des. 2005, 5, 617−622.
(21) Akpinar, H.; Mague, J. T.; Novak, M. A.; Friedmane, J. R.; Lahti,
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Notes
The authors declare no competing financial interest.
(22) Keyes, T. E.; Forster, R. J.; Bond, A. M.; Miao, W. J. Am. Chem.
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ACKNOWLEDGMENTS
■
L. K. Das and A. Biswas are thankful to CSIR, India for
awarding Senior Research Fellowship [Sanction No. 09/028
(0805)/2010-EMR-I for L. K. Das and 09/028 (0717)/2008-
EMR-I for A. Biswas]. We also acknowledge the Spanish
MINECO (project MAT2011-26507) and the Generalitat
Valenciana (Prometeo and ISIC Programs) for financial
support. Thanks are given to the Consejo Superior de
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Investigaciones Cientificas (CSIC) of Spain for the award of
a license for the use of the Cambridge Crystallographic Data
Base (CSD). Crystallography was performed at the DST-FIST,
India funded Single Crystal Diffractometer Facility at the
Department of Chemistry, University of Calcutta. The authors
also thank the Department of Science and Technology (DST),
New Delhi, India, for financial support (SR/S1/IC/0034/
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