Structural and theoretical investigation on two dinuclear Fe(III) complexes of tridentate NNO-donor Schiff base ligands

Article abstract of DOI:10.1016/j.poly.2014.01.012

Two new Fe(III) complexes of formulas [FeL¹(N ₃)(OMe)]₂ (1) and [FeL²(N₃)(OH)] ₂·H₂O (2) have been synthesized by using two tridentate NNO-donor Schiff base ligands HL¹

Full text of DOI:10.1016/j.poly.2014.01.012

Polyhedron 73 (2014) 139–145
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structural and theoretical investigation on two dinuclear Fe(III)
complexes of tridentate NNO-donor Schiff base ligands
Subrata Naiya a,b, Sanjib Giri c, Saptarshi Biswas a, Michael G.B. Drew d, Ashutosh Ghosh a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b Sushil Kar College, Ghoshpur, Champahati, Baruipur, 24 Parganas(S), West Bengal 743330, India
c Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
d School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Two new Fe(III) complexes of formulas [FeL1(N3)(OMe)]2 (1) and [FeL2(N3)(OH)]2ꢀH2O (2) have been
synthesized by using two tridentate NNO-donor Schiff base ligands HL1{(2-[(3-methylaminoethylimi-
no)-methyl]-phenol)} and HL2 {(2-[1-(2-dimethylaminoethylimino)methyl]-phenol)}, respectively. Sub-
stitution of the –CH3 group on the secondary amine group of the N,N-dimethyldiamine fragment of
the Schiff base by a H-atom leads to a structural change from a methoxido-bridged dimer (1) to a single
hydroxido-bridged dimer (2). Both complexes have been characterized by single-crystal X-ray analyses,
IR and UV–Vis spectroscopy and DFT studies. Both dimers are centrosymmetric containing two FeIII atoms
which are bridged by two methoxido ions in compound 1 and by two hydroxido ions in compound 2. The
chelating tridentate Schiff base and a terminal azido group complete the distorted octahedral environ-
ment around each iron center in both complexes. To explain the preferential bridging ability of methox-
ido and hydroxido groups in complexes 1 and 2, we have carried out theoretical calculations based on
DFT to compare their formation energies and dimerization energies. In the case of complex 2, it is noticed
that the close proximity of the azido group from the iron centers allow the hydroxido bridged complex to
gain extra stability through H-bonds, whereas due to the steric crowding of the azido group and the
methyl moiety of adjacent molecules, complex 1 disfavors the formation of hydrogen bonds as well as
hydroxido bridging.
Article history:
Received 29 October 2013
Accepted 11 January 2014
Available online 22 January 2014
Keywords:
Schiff base
Fe(III)
Crystal structures
Supramolecular interactions
DFT studies
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
have been used extensively along with pseudohalides to synthesize
both discrete and infinite polynuclear complexes of CuII and NiII
The chemistry of iron(III) complexes is dominated by oxygen-
bridged species because of the high oxygen affinity of the iron(III)
ion [1–8]. This type of dinuclear iron(III) unit with oxido, hydroxi-
do, alkoxido, peroxido, or carboxylato bridge(s) have been exten-
sively studied, mainly because of their performance in a range of
biological activities such as the storage and transport of dioxygen
[9,10], oxidation of alkanes [11–13], phosphate ester hydrolysis
[14] and DNA synthesis [15]. Synthetic efforts to prepare model
complexes of iron proteins have yielded several astonishing and
interesting oxygen-bridged poly-iron compounds. Their structure,
magnetic, spectroscopic, redox properties and chemical reactivities
have been intensely investigated and are summarized in several
reviews [3–8,16].
with interesting structures and magnetic properties [17–26]. How-
ever, such ligands have seldom been used to synthesize complexes
with Fe(III). They usually afforded mononuclear bis(Schiff base)
complexes with non-coordinating counter anions such as ClOꢁ4 ,
NOꢁ3 , PFꢁ6 , etc. [27–29], except in a few cases where di-iron com-
plexes with tridentate NNO donor Schiff base ligands have been re-
ported. Among them, there are three methoxido- [30–32], one
ethoxido- [31], one hydroxido- [33], and two oxido-bridged difer-
ric compounds [33,34]. Although, several iron(III) complexes have
been characterized with salen-type N2O2 donor ligands [35–37], to
our knowledge no double hydroxido bridged iron(III) complexes of
the ligand used (2-[(3-methylaminoethylimino)-methyl]-phenol)
in this study have been reported.
Tridentate NNO donor Schiff base ligands, derived from the
mono-condensation of a diamine and salicylaldehyde derivative
In the present contribution, two tridentate NNO-donor Schiff
base ligands, HL1 = (2-[1-(2-dimethylaminoethylimino)methyl] -
phenol) and HL2 = (2-[(3-methylaminoethylimino)-methyl]-phe-
nol) (Scheme 1), have been used to synthesize two new ferric
compounds [FeL1(N3)(OMe)]2 (1) and [[FeL2(N3)(OH)]2ꢀH2O (2)
⇑
Corresponding author. Tel.: +91 9433344484; fax: +91 33 2351 9755.
E-mail address: ghosh_59@yahoo.com (A. Ghosh).
http://dx.doi.org/10.1016/j.poly.2014.01.012
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

140
S. Naiya et al. / Polyhedron 73 (2014) 139–145
R
CH3
H
C
N
N
HL
N
= L-
O
OH
N
CH3
O
H
O
O
N3
Fe
O
O
N3
Fe
O
N
N
1. Fe(NO3)3. 4H2O
2. NaN3
1. Fe(NO3)3. 4H2O
2. NaN3
N
N
Fe
N3
Fe
N3
N
O
H
2
N
HL1: Fe3+: N3- = 1:1:2
N
O
N
HL2: Fe3+: N3- = 1:1:2
CH3
1
R = CH3 (HL1)
R = H (HL2)
Scheme 1. Synthetic route of the complexes.
respectively. Herein, we present the syntheses, crystal structures,
and DFT calculations of these two compounds where complex 1
is a double methoxido-bridged dimer, and complex 2 is a double
2.3.2. Synthesis of [FeL2(N3)2(OH)]2ꢀH2O (2)
The procedure was the same as that for complex 2, except that
5 mmol of a methanolic solution of the Schiff base ligand HL2
(10 mL) was added instead of HL1. The colour of the solution
turned red immediately. By slow evaporation of the resulting solu-
tion, red coloured, X-ray quality, square shaped single crystals
were obtained after two days.
l-hydroxido-bridged dimer.
2. Experimental
Complex 2: Yield: 1.080 g. (72%). Anal. Calc. for C20H28Fe2N10O5
(600.19): C, 40.02; H, 4.70; N, 23.34. Found: C, 40.11; H, 4.57; N,
23.37%. IR (KBr pellet, cmꢁ1): 2045s, 1627s, 3245mb.
2.1. Starting materials
The diamines and salicyldehyde were purchased from Lancaster
Chemical Co. The chemicals were of reagent grade and used with-
out further purification.
2.4. Physical measurements
Caution! Although no incidents were recorded in this study, azi-
do 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.
Elemental analyses (C, H and N) were performed using a
Perkin-Elmer 240C elemental analyzer. IR spectra in KBr pellets
(4500–500 cmꢁ1) were recorded using a Perkin–Elmer RXI FT-IR
spectrophotometer. Electronic spectra in methanol (1000-
200 nm) were recorded in a Hitachi U-3501 spectrophotometer.
2.2. Synthesis of the Schiff base ligands
2.4.1. Crystallographic studies
4011 independent reflection data of complex 1 were collected
with MoKa radiation at 150 K using the Oxford Diffraction X-Cali-
bur CCD System. The crystal was positioned at 50 mm from the
CCD. 321 frames were measured with a counting time of 10 s. Data
analysis was carried out with the CrysAlis program [38]. The struc-
ture was solved using direct methods with the SHELXS97 program
[39].
The ligands HL1 and HL2 were prepared by refluxing salicylalde-
hyde (0.5 mL, 5 mmol) with N,N-dimethylethane-1,2-diamine
(0.55 mL, 5 mmol) and N-methylethane-1,2-diamine (0.3 mL,
5 mmol) separately in methanol (30 mL) for 30 min. The resulting
dark yellow solutions of the tridentate ligands HL1 and HL2 were
subsequently used for complex formation.
2.3. Synthesis of the complexes
A suitable single crystal of the complex 2 was mounted on a
Bruker-AXS SMART APEX II diffractometer equipped with a graph-
ite monochromator and Mo Ka (k = 0.71073 ÅA) radiation. The crys-
0
2.3.1. Synthesis of [FeL1(N3)(OMe)]2 (1)
At room temperature, a 5 mmol methanolic solution of Schiff
base ligand HL1 (10 mL) was added to a 20 mL methanolic solution
of Fe(NO3)ꢀ4H2O (1.225 g, 5 mmol) followed by an aqueous solu-
tion (5 mL) of NaN3 (0.650 g, 10 mmol). The colour of the solution
turned red and was then filtered. By slow evaporation of the result-
ing solution at room temperature, red colored X-ray quality, rect-
angular shaped single crystals were obtained after few days.
Complex 1: Yield: 1.204 g. (75%). Anal. Calc. for C24H36Fe2N10O4
(640.30): C, 45.02; H, 5.67; N, 21.88. Found: C, 45.21; H, 5.37; N,
21.77%. IR (KBr pellet, cmꢁ1): 2048s, 1615s, 3244mb.
tal was positioned at 60 mm from the CCD. 360 frames were
measured with a counting time of 10s. The structure was solved
using the Patterson method with SHELXS97 [39] and subsequent dif-
ference Fourier synthesis revealed the positions of remaining non-
hydrogen atoms.
For both structures the non-hydrogen atoms were refined with
anisotropic thermal parameters. The hydrogen atoms bonded to
carbon were included in geometric positions and given thermal
parameters equivalent to 1.2 times those of the atom to which they
were attached Hydrogen atoms bonded to oxygen were located in

S. Naiya et al. / Polyhedron 73 (2014) 139–145
141
Table 2
a difference Fourier map and refined with distance constraints.
Dimensions in the metal coordination spheres of both the complexes.
Absorption corrections were carried out for 1 using ABSPACK
[40] and for 2 using SADABS programs [41].
Compound
1
2A
2B
Refinements were carried out on F2 with SHELXL97. Other pro-
grams used were, PLATON 99 [42], ORTEP-32 [43] and WinGX system
Ver-1.64 [44]. Data collection, structure refinement parameters
and crystallographic data for complexes 1 and 2 are given in
Table 1.
Bond length (Å)
Fe1–O4
Fe1–O11
Fe1–N1
Fe1–N19
2.031(2)
1.922(3)
2.032(3)
2.135(3)
2.248(3)
2.007(3)
1.932(3)
1.943(3)
2.047(4)
2.110(3)
2.186(3)
2.032(3)
1.941(3)
1.941(3)
2.072(4)
2.095(4)
2.216(4)
2.012(3)
Fe1–N22
Fe1–O4_a
Bond angle (°)
O4–Fe1–O11
O4–Fe1–N1
O4–Fe1–N19
O4–Fe1–N22
O4–Fe–O4_a
O11–Fe–N1
O11–Fe1–N19
O11–Fe1–N22
O4_a–Fe1–O11
N1–Fe1–N19
N1–Fe1–N22
O4_a–Fe1–N1
N19–Fe1–N22
O4_a–Fe1–N19
O4_a–Fe1–N22
3. Results and discussion
90.87(1)
98.26(13)
95.53(15)
164.81(14)
96.90(13)
75.46(16)
90.89(14)
86.77(13)
164.82(13)
96.53(15)
98.72(14)
88.42(14)
169.04(15)
78.35(13)
89.77(13)
86.52(13)
96.30(13)
94.51(14)
164.93(12)
101.27(11)
91.06(11)
74.23(11)
95.23(12)
85.34(11)
162.68(11)
99.50(11)
92.97(13)
87.21(12)
91.14(12)
77.40(12)
173.34(11)
97.59(11)
The two new Fe(III) complexes [FeL1(N3)(OMe)]2 (1) and [FeL2
(N3)2(OH)2]2ꢀH2O (2) have been prepared with the tridentate Schiff
base ligands HL1 and HL2, respectively, following a similar method:
iron(III) nitrate was allowed to react with the corresponding Schiff
base and sodium azide in 1:1:2 M ratios in methanol at ambient
conditions. The compositions and structures of the compounds
are somewhat different. Although in both of them, one deproto-
nated tridentate Schiff base ligand and one terminal azide ion coor-
dinate to the Fe(III) centre but two Fe(III) centres are bridged by
two methoxido groups and to hydroxide groups in 1 and 2, respec-
tively (Scheme 1).
172.36(14)
98.54(15)
77.45(15)
93.42(15)
86.63(13)
165.03(14)
94.90(13)
92.36(14)
87.33(16)
169.00(15)
78.40(15)
95.30(13)
86.49(15)
Symmetry element a = 1 ꢁ x, ꢁy, 1 ꢁ z for 1, ꢁx, 1 ꢁ y, 1 ꢁ z for 2A, ꢁx, 2 ꢁ y, 1 ꢁ z
for 2B.
3.1. Description of structures of complexes 1 and 2
3.1.1. [FeL1(N3)(OMe)]2 (1)
is 3.220(1) Å and the distance between the two bridging oxygen
atoms is 2.437(4) Å, both of which are comparable with those re-
Both the structures are centrosymmetric dimers in which the
iron atoms have distorted six-coordinate octahedral environments.
Bond lengths and bond angles are given in Table 2. The structure of
1 is shown in Fig. 1 together with the atom numbering scheme in
the metal coordination sphere.
In 1, each iron atom is bonded to three atoms of the meridionally
coordinated tridentate ligand with the following bond lengths: Fe–
O(11) 1.922(3) Å, Fe–N(19) 2.135(3) Å, and Fe–N(22) 2.248(3) Å.
The equatorial plane is completed by a bridging methoxy group
with an Fe–O(4)a a bond length of 2.007(3) Å (symmetry element
a = 1 ꢁ x, ꢁy, 1 ꢁ z). A second methoxy group, O(4) with a bond
length of 2.031(2) Å, and a terminal azido group, N(1) with a bond
length of 2.032(3) Å occupy the axial positions (Fig. 1a). Thus the
two FeIII octahedra are joined together by two bridging methoxy
groups sharing an edge as shown in Fig. 1b. The Fe–Fe separation
ported previously in di-
[30–32]. The -OMe bridge in this complex leads to a perfectly pla-
nar Fe2( -OMe)2 ring as the dimer sits on a crystallographic inver-
l-methoxido bridged diferric complexes
l
l
sion center. In the ring, the Fe–O–Fe angle [105.77(6)°] and O–Fe–O
angle [74.23(6)°] also lie within the ranges of similar compounds
[30–32,45]. The two Fe-l-OMe bond lengths are slightly different
making the Fe2O2 core asymmetric, which is a common feature
for such dimers. The phenyl rings are almost perpendicular to the
Fe2(l-OMe)2 core with a dihedral angle of 86.18°. The longer Fe-
l-OMe [2.032(3) Å] bond is trans to the azido ligand, while the
shorter bond [2.007(3) Å] is trans to the imino nitrogen atom.
In compound 1 there are no significant H-bonds and only the
C–H/
p interactions (between H14 and Cg of phenyl ring of the
neighboring unit with dimension Hꢀ ꢀ ꢀCg⁄ = 2.91 Å, where symme-
try element ⁄ = x, ꢁ1/2 ꢁ y, 1/2 + z) are likely to have significant
influence on the 2D packing of the molecules (Fig. 2).
Table 1
Crystal data and details of structure refinement of complexes 1 and 2.
1
C
2
3.1.2. [FeL2(N3)2(OH)2]2ꢀH2O (2)
Formula
M
Crystal system
Space group
a (Å)
24H36Fe2N10O4
C20H28 Fe2N10O4,H2O
602.24
triclinic
Complex 2 consists of
l-hydroxido bridged dimeric molecules
640.32
monoclinic
P21/c
11.6149(6)
15.9431(6)
7.9325(3)
(90)
101.596(5)
(90)
1438.94(11)
2
1.478
1.057
668
0.054
9648
4154
2853
0.0679, 0.1748
150
(Fig. 3). In the structure of 2 there are two centrosymmetric inde-
pendent dimers, denoted as A and B with similar geometries which
are compared in Table 2. The structure of 2A is shown in Fig. 3.
In complex 2, as in 1, the iron atoms are six-coordinated with
distorted octahedral environments and each metal atom is merid-
ionally coordinated by the three donor atoms of the tridentate
ligand (HL2) with the following bond lengths: Fe–O(11)
1.943(3) Å, Fe–N(19) 2.110(3) Å, and Fe–N(22) 2.186(4) Å (for 2A)
and Fe–O(11) 1.941(3) Å, Fe–N(19) 2.095(4) Å, and Fe–N(22)
2.216(4) Å (for 2B).
The equatorial plane is completed by a bridging hydroxy group
with an Fe–O(4) bond lengths of 1.932(3) Å in 2A and 1.941(4) Å in
2B. Axial positions are occupied by a second hydroxy group, O(4)
(ꢁx, 1 ꢁ y, 1 ꢁ z in A, ꢁx, 2 ꢁ y, 1 ꢁ z in B) with bond lengths of
2.032(3), 2.012(3) Å, and a terminal azido group, N(1) with bond
lengths of 2.047(4),2.072(4) Å in 2A and 2B respectively as usually
found in this type of double hydroxo-bridged Fe(III) dimers. As in 1,
the two FeIII octahedra are joined together by two bridging
ꢀ
P1
9.674(2)
11.715(2)
12.126(3)
80.388(3)
86.153(3)
77.724(3)
1323.3(5)
2
1.512
1.147
620
0.056
10136
5101
2864
0.0523, 0.1
293
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
F(000)
Rint
Total reflections
Unique reflections
I > 2r(I)
R1, wR2 I > 2
r
(I)
T (K)
Maximum, minimum electron
density (e/Å3)
1.948, ꢁ0.789
0.459, ꢁ0.383

142
S. Naiya et al. / Polyhedron 73 (2014) 139–145
Fig. 1. Structure of complex 1. (a) The centrosymmetric structure with ellipsoids drawn at 30% probability. (b) Edge-sharing dimeric structure of 1.
Fig. 4. 1D supra-molecular structure of the complex 2. Only the hydrogen atoms
involved in hydrogen bonds are shown. Hydrogen bonds shown as dotted lines.
and 3.084(1) Å for 2A and 2B respectively. The Fe–O–Fe bridge an-
Fig. 2. 2D supramolecular networks of complex 1. Hydrogen bonds are shown as
dotted lines.
gles are 104.56(16)° for 2A and 102.55(15)° for 2B.
In the crystal lattice of 2, the dinuclear units Fe2L22(OH)2 (both
2A and 2B) are linked through six intermolecular hydrogen-bonding
interactions to form a two-dimensional network (Fig. 4). Two hydro-
gen atoms of the discrete water molecule are involved in two donor
methoxido groups by sharing an edge as shown in Fig. 3b. The two
Fe-l-OH bond lengths are slightly different making the Fe2(OH)2
core asymmetric. The two Fe atoms are separated by 3.135(1)
Fig. 3. Structure of complex 2. (a) The centrosymmetric structure with ellipsoids drawn at 50% probability. (b) The edge-sharing dimeric structure of 2.

S. Naiya et al. / Polyhedron 73 (2014) 139–145
143
3.2. IR and UV–Vis spectra of the complexes
Spectroscopic data and their assignments are given in the
Experimental Section. The IR spectra of the two complexes are very
similar and show a strong sharp peak for corresponding to the
mC
@
N
at 1615 and 1627 cmꢁ1 for complexes 1 and 2 respectively,
confirming the presence of the Schiff base. The peaks at 2048
and 2045 cmꢁ1 in the IR spectra of 1 and 2, respectively, appear
to be due to the stretching of the N-coordinated terminal azide.
The N–H stretching mode is seen at 3244 and 3245 cmꢁ1 as a sharp
band for complexes 1 and 2, respectively.
The electronic spectra of compounds 1 and 2 are recorded in
acetonitrile solution. The spectra of the two complexes are similar.
The spectra show a band in the visible region (415 nm for 1 and
420 nm for 2) assigned to azide-to-FeIII charge transfer (CT). More-
over a moderately strong band (341 nm for 1 and 324 nm for 2) is a
superposition of the amino-to-Fe and oxygen to FeIII CT band
[31,46]. The highest energy bands (225 nm for 1 and 229 nm for
2) are attributed to
p–
p⁄ transitions of the ligand, but contribu-
tions from less intense hydroxo/alkoxo- to-Fe CT are also expected
Fig. 5. UV–Vis spectra of complexes 1 and 2.
in this region [47] (Fig. 5).
O–Hꢀ ꢀ ꢀO hydrogen bonding interactions with the phenoxo oxygen
atom of two neighboring units (2A and 2B), and in addition the oxy-
gen atom of the water molecule forms acceptor Oꢀ ꢀ ꢀH–N hydrogen
bonds with two amine hydrogen atoms of the ligands of neighboring
molecules. Further hydrogen bonds are formed by each bridging hy-
droxo group of dinuclear unit with the nitrogen atom of the terminal
azide of neighboring units through O–Hꢀ ꢀ ꢀN hydrogen bonding inter-
actions. There are also N–Hꢀ ꢀ ꢀO interactions to the bridging OH
group in same molecule. The dimensions of hydrogen bonds are gi-
ven in Table 3.
3.3. Theoretical calculations
To check the reason behind the preferential bridging ability of
methoxido and hydroxido groups in complexes 1 and 2 respec-
tively, we carried out theoretical calculations based on density
functional theory. In each case, we have calculated both formation
energy and dimerization energy using the hybrid B3LYP functional
with the LANL2DZ basis set for ferric atoms and 6-31G(d) basis set
for other atoms as implemented in the GAUSSIAN 03 package [48–54].
We have also considered 10ꢁ8 density-based convergence criterion
Table 3
H-bonding dimensions of complex 2.
Donor–Hꢀ ꢀ ꢀAcceptor
D–H (Å)
Hꢀ ꢀ ꢀA (Å)
Dꢀ ꢀ ꢀA (Å)
D–Hꢀ ꢀ ꢀA (°)
Symmetry
O(1W)–H(1W)ꢀꢀꢀO(11B)
O(1W)–H(2W)ꢀꢀꢀO(11A)
O(4A)–H(4A)ꢀꢀꢀN(1B)
0.87
0.86
0.70(5)
0.76(5)
0.84(4)
0.91
2.01
2.02
2.19(5)
2.11(5)
2.35(4)
2.21
2.845(4)
2.811(4)
2.881(5)
2.862(5)
3.184(6)
3.109(5)
160
152
171(5)
177(8)
171(4)
168
ꢁx, 1 ꢁ y, 1 ꢁ z
ꢁx, 1 ꢁ y, 1 ꢁ z
ꢁx, 1 ꢁ y, 1 ꢁ z
ꢁx, 1 ꢁ y, 1 ꢁ z
x, 1 + y, z
O(4B)–H(4B)ꢀꢀꢀN(1A)
N(23B)–H(22B)ꢀꢀꢀO(1W)
N(23A)–H(23A)ꢀꢀꢀO(1W)
ꢁx, 1 ꢁ y, 1 ꢁ z
Fig. 6. Steric crowding between two dimers in complex 1.

144
S. Naiya et al. / Polyhedron 73 (2014) 139–145
stable than complex 2 (Ef = ꢁ1532.56 kcal/mol). In addition, the
theoretical study indicates that the dimerization energies of com-
plex 2 is ꢁ10.93 kcal/mol, leads to extra stability through the for-
mation of hydrogen bonding network. The complex 1 shows
positive dimerization energy (0.71 kcal/mol) which is energetically
unfavorable due to steric repulsion between the azide group and a
methyl moiety of an adjacent molecule. It is probably stabilized in
the solid state via intermolecular noncovalent interactions
(C–Hꢀ ꢀ ꢀ
p), which compensate for the positive dimerization energy.
Acknowledgments
We thank EPSRC and the University of Reading for funds for the
X-Calibur system used for complex 1. Crystallography for 2 was
performed at the DST-FIST, India-funded Single Crystal Diffractom-
eter Facility at the Department of Chemistry, University of Calcutta.
S. Naiya would like to thank University Grants Commission, India
for funds provided through Minor Research Project No. F. PSW-
150/11-12 (ERO). S. Biswas is thankful to CSIR, India for research
fellowship [Sanction no. 09/028(0732)/2008-EMR-I].
Fig. 7. Intra-dimer H-bonding interaction in complex 2.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors) for com-
pounds, 1 and 2 have been deposited with the Cambridge Crystal-
lographic Data Centre, CCDC No. 965138 (1) and 965139 (2). Copies
of the data can be obtained free of charge on application to The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K, e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/deposit
Telephone: (44) 01223 762910 Facsimile: (44) 01223 336033.
for all energy calculations. Interaction energies have also been cor-
rected for the Basis Set Superposition Error (BSSE) using the Boys
and Bernardi’s method [55,56].
2Fe3þ þ 2HðHL1Þ þ 2MeOH þ 2NaN3 þ 4H2O
¼ ½Fe2ðL1Þ ðN3Þ ðOMeÞ ꢂ þ 2Naþ þ 4H3Oþ2Fe3þ
2
2
2
þ 2HðHL2Þ þ 2NaN3 þ 7H2O
¼ ½Fe2ðL2Þ ðN3Þ ðOHÞ2ꢂ:H2O þ 2Naþ þ 4H3Oþ
References
2
2
To compare the relative stability of those complexes, the above
equations have been used to calculate the formation energies and
the results indicate that complex 1 (Ef = ꢁ1546.41 kcal/mol) is rel-
atively more stable than complex 2 (Ef = ꢁ1532.56 kcal/mol). From
the structure of complex 2, it is clear that hydrogen bonding pro-
vides extra stability.
To check the additional stability through intermolecular inter-
action, we have calculated dimerization energies by considering di-
rect experimentally obtained structures.
In the case of complex 2, the computed energy is ꢁ10.93 kcal/
mol and it represent that the molecules get extra stability through
the formation of hydrogen bonding network. However, the com-
plex 1 shows positive dimerization energy (0.71 kcal/mol) which
may indicate about the distabilization factor due to steric repulsion
between the azide group and the methyl moiety of an adjacent
molecule as shown in Fig. 6. It is worth mentioning that this repul-
sion is not relevant to complex 2 due to the absence of the methyl
group in the ‘‘HL2’’ ligand which leads to a more close packed crys-
tal structure in 2. Here, the preferential dihydroxy bridges are of
prime importance as they facilitate hydrogen bonding with nitro-
gen atom (N1) of neighbouring molecule (Fig. 7). In complex 1,
the possibility of such hydrogen bonding interaction is restricted
due to steric interaction between terminal azide group and methyl
moiety of adjacent molecule as shown in Fig. 6.
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