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Structural and theoretical investigation on two dinuclear Fe(III) complexes of tridentate NNO-donor Schiff base ligands
Polyhedron 73 (2014) 139–145
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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 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 b
a r t i c l e
i n f o
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
a b s t r a c t Two new Fe(III) complexes of formulas [FeL1(N3)(OMe)]2 (1) and [FeL2(N3)(OH)]2H2O (2) have been synthesized by using two tridentate NNO-donor Schiff base ligands HL1{(2-[(3-methylaminoethylimino)-methyl]-phenol)} and HL2 {(2-[1-(2-dimethylaminoethylimino)methyl]-phenol)}, respectively. Substitution 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 environment around each iron center in both complexes. To explain the preferential bridging ability of methoxido 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. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The chemistry of iron(III) complexes is dominated by oxygenbridged species because of the high oxygen affinity of the iron(III) ion [1–8]. This type of dinuclear iron(III) unit with oxido, hydroxido, alkoxido, peroxido, or carboxylato bridge(s) have been extensively 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]. Tridentate NNO donor Schiff base ligands, derived from the mono-condensation of a diamine and salicylaldehyde derivative ⇑ Corresponding author. Tel.: +91 9433344484; fax: +91 33 2351 9755. E-mail address: [email protected] (A. Ghosh). http://dx.doi.org/10.1016/j.poly.2014.01.012 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
have been used extensively along with pseudohalides to synthesize both discrete and infinite polynuclear complexes of CuII and NiII with interesting structures and magnetic properties [17–26]. However, 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 ClO 4, NO 3 , PF6 , etc. [27–29], except in a few cases where di-iron complexes with tridentate NNO donor Schiff base ligands have been reported. Among them, there are three methoxido- [30–32], one ethoxido- [31], one hydroxido- [33], and two oxido-bridged diferric 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. In the present contribution, two tridentate NNO-donor Schiff base ligands, HL1 = (2-[1-(2-dimethylaminoethylimino)methyl] phenol) and HL2 = (2-[(3-methylaminoethylimino)-methyl]-phenol) (Scheme 1), have been used to synthesize two new ferric compounds [FeL1(N3)(OMe)]2 (1) and [[FeL2(N3)(OH)]2H2O (2)
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R H C
N
N
CH3
HL
N = L-
OH
N
O
CH3 N N3
O
Fe N
O
O
O Fe
1. Fe(NO3)3. 4H2O 2. NaN3
1. Fe(NO3)3. 4H2O 2. NaN3
N3 N
HL1: Fe3+: N3- = 1:1:2
HL2: Fe3+: N3- = 1:1:2
N
N N3
H O
CH3 1
O
N
Fe
Fe N
O
O H
N3 N
2 R = H (HL2)
R = CH3 (HL1) 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 l-hydroxido-bridged dimer.
2. Experimental 2.1. Starting materials The diamines and salicyldehyde were purchased from Lancaster Chemical Co. The chemicals were of reagent grade and used without further purification. Caution! Although no incidents were recorded in this study, azido 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.
2.3.2. Synthesis of [FeL2(N3)2(OH)]2H2O (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 solution, red coloured, X-ray quality, square shaped single crystals were obtained after two days. 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, cm1): 2045s, 1627s, 3245mb. 2.4. Physical measurements Elemental analyses (C, H and N) were performed using a Perkin-Elmer 240C elemental analyzer. IR spectra in KBr pellets (4500–500 cm1) were recorded using a Perkin–Elmer RXI FT-IR spectrophotometer. Electronic spectra in methanol (1000200 nm) were recorded in a Hitachi U-3501 spectrophotometer.
2.2. Synthesis of the Schiff base ligands The ligands HL1 and HL2 were prepared by refluxing salicylaldehyde (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 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 solution (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 resulting solution at room temperature, red colored X-ray quality, rectangular 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, cm1): 2048s, 1615s, 3244mb.
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-Calibur 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 structure was solved using direct methods with the SHELXS97 program [39]. A suitable single crystal of the complex 2 was mounted on a Bruker-AXS SMART APEX II diffractometer equipped with a graph0 ite monochromator and Mo Ka (k = 0.71073 Å A) radiation. The crystal 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 difference Fourier synthesis revealed the positions of remaining nonhydrogen 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
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a difference Fourier map and refined with distance constraints. Absorption corrections were carried out for 1 using ABSPACK [40] and for 2 using SADABS programs [41]. Refinements were carried out on F2 with SHELXL97. Other programs 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. 3. Results and discussion 1
2
The two new Fe(III) complexes [FeL (N3)(OMe)]2 (1) and [FeL (N3)2(OH)2]2H2O (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 deprotonated tridentate Schiff base ligand and one terminal azide ion coordinate to the Fe(III) centre but two Fe(III) centres are bridged by two methoxido groups and to hydroxide groups in 1 and 2, respectively (Scheme 1). 3.1. Description of structures of complexes 1 and 2 3.1.1. [FeL1(N3)(OMe)]2 (1) 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
Table 1 Crystal data and details of structure refinement of complexes 1 and 2.
Formula M Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) Rint Total reflections Unique reflections I > 2r(I) R1, wR2 I > 2r(I) T (K) Maximum, minimum electron density (e/Å3)
1 C24H36Fe2N10O4 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 1.948, 0.789
2 C20H28 Fe2N10O4,H2O 602.24 triclinic 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 0.459, 0.383
Table 2 Dimensions in the metal coordination spheres of both the complexes. Compound Bond length (Å) Fe1–O4 Fe1–O11 Fe1–N1 Fe1–N19 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
1
2A
2B
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)
90.87(1) 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)
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) 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.
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 reported previously in di-l-methoxido bridged diferric complexes [30–32]. The l-OMe bridge in this complex leads to a perfectly planar Fe2(l-OMe)2 ring as the dimer sits on a crystallographic inversion 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 Fel-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 H Cg⁄ = 2.91 Å, where symmetry element ⁄ = x, 1/2 y, 1/2 + z) are likely to have significant influence on the 2D packing of the molecules (Fig. 2). 3.1.2. [FeL2(N3)2(OH)2]2H2O (2) Complex 2 consists of l-hydroxido bridged dimeric molecules (Fig. 3). In the structure of 2 there are two centrosymmetric independent 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 meridionally 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
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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. Fig. 2. 2D supramolecular networks of complex 1. Hydrogen bonds are shown as dotted lines.
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)
and 3.084(1) Å for 2A and 2B respectively. The Fe–O–Fe bridge angles 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 hydrogen atoms of the discrete water molecule are involved in two donor
Fig. 3. Structure of complex 2. (a) The centrosymmetric structure with ellipsoids drawn at 50% probability. (b) The edge-sharing dimeric structure of 2.
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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 cm1 for complexes 1 and 2 respectively, confirming the presence of the Schiff base. The peaks at 2048 and 2045 cm1 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 cm1 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). Moreover 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 contributions from less intense hydroxo/alkoxo- to-Fe CT are also expected in this region [47] (Fig. 5).
Fig. 5. UV–Vis spectra of complexes 1 and 2.
O–H O hydrogen bonding interactions with the phenoxo oxygen atom of two neighboring units (2A and 2B), and in addition the oxygen atom of the water molecule forms acceptor O H–N hydrogen bonds with two amine hydrogen atoms of the ligands of neighboring molecules. Further hydrogen bonds are formed by each bridging hydroxo group of dinuclear unit with the nitrogen atom of the terminal azide of neighboring units through O–H N hydrogen bonding interactions. There are also N–H O interactions to the bridging OH group in same molecule. The dimensions of hydrogen bonds are given 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 respectively, 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 108 density-based convergence criterion
Table 3 H-bonding dimensions of complex 2. Donor–H Acceptor
D–H (Å)
H A (Å)
D A (Å)
D–H A (°)
Symmetry
O(1W)–H(1W)O(11B) O(1W)–H(2W)O(11A) O(4A)–H(4A)N(1B) O(4B)–H(4B)N(1A) N(23B)–H(22B)O(1W) N(23A)–H(23A)O(1W)
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, x, 1 y, x, 1 y, x, 1 y, x, 1 + y, z x, 1 y,
Fig. 6. Steric crowding between two dimers in complex 1.
1z 1z 1z 1z 1z
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stable than complex 2 (Ef = 1532.56 kcal/mol). In addition, the theoretical study indicates that the dimerization energies of complex 2 is 10.93 kcal/mol, leads to extra stability through the formation 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–H p), which compensate for the positive dimerization energy. Acknowledgments
Fig. 7. Intra-dimer H-bonding interaction in complex 2.
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 Diffractometer 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. PSW150/11-12 (ERO). S. Biswas is thankful to CSIR, India for research fellowship [Sanction no. 09/028(0732)/2008-EMR-I]. Appendix A. Supplementary data
for all energy calculations. Interaction energies have also been corrected for the Basis Set Superposition Error (BSSE) using the Boys and Bernardi’s method [55,56].
2Fe3þ þ 2HðHL1 Þ þ 2MeOH þ 2NaN3 þ 4H2 O ¼ ½Fe2 ðL1 Þ2 ðN3 Þ2 ðOMeÞ2 þ 2Naþ þ 4H3 Oþ 2Fe3þ þ 2HðHL2 Þ þ 2NaN3 þ 7H2 O ¼ ½Fe2 ðL2 Þ2 ðN3 Þ2 ðOHÞ2:H2 O þ 2Naþ þ 4H3 Oþ 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 relatively more stable than complex 2 (Ef = 1532.56 kcal/mol). From the structure of complex 2, it is clear that hydrogen bonding provides extra stability. To check the additional stability through intermolecular interaction, we have calculated dimerization energies by considering direct 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 complex 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 repulsion 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 crystal structure in 2. Here, the preferential dihydroxy bridges are of prime importance as they facilitate hydrogen bonding with nitrogen 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. 4. Conclusions We have prepared two new Fe(III) complexes with two NNO donor Schiff-base ligands and azido co-ligands. Both complexes 1 and 2 are centrosymmetric dimers where the Fe(III) ions are in distorted octahedral environment, bridged by two l2-methoxide and hydroxide ions, respectively. The theoretical results demonstrate that complex 1 (Ef = 1546.41 kcal/mol) is relatively more
Crystallographic data (excluding structure factors) for compounds, 1 and 2 have been deposited with the Cambridge Crystallographic 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: [email protected] or http://www.ccdc.cam.ac.uk/deposit Telephone: (44) 01223 762910 Facsimile: (44) 01223 336033. References [1] M. Mitra, A.K. Maji, B.K. Ghosh, P. Raghavaiah, J. Ribas, R. Ghosh, Polyhedron 67 (2014) 19. [2] I.M. Wasser, C.F. Martens, C.N. Verani, E. Rentschler, H.-W. Huang, P.M. Moenne-Loccoz, L.N. Zakharov, A.L. Rheingold, K.D. Karlin, Inorg. Chem. 43 (2004) 651. [3] T.C. Berto, A.L. Speelman, S. Zheng, N. Lehnert, Coord. Chem. Rev. 257 (2013) 244. [4] D.M. Kurtz, Chem. Rev. 90 (1990) 585. [5] B.J. Wallar, J.D. Lipscomb, Chem. Rev. 96 (1996) 2625. [6] E.I. Solomon, T.C. Brunold, M.I. Davis, J.N. Kemsley, S.K. Lee, N. Lehnert, F. Neese, A.J. Skulan, Y.-S. Yang, J. Zhou, Chem. Rev. 100 (2000) 235. [7] J. Du Bois, T.J. Mizoguchi, S.J. Lippard, Coord. Chem. Rev. 200–202 (2000) 443. [8] E.Y. Tshuva, S.J. Lippard, Chem. Rev. 104 (2004) 987. [9] M.A. Holmes, R.E. Stenkamp, J. Mol. Biol. 220 (1991) 723. [10] R.E. Stenkamp, L.C. Sieker, L.H. Jensen, J. Am. Chem. Soc. 106 (1984) 618. [11] A.C. Rosenzweig, C.A. Frederick, S.J. Lippard, P. Norlund, Nature 366 (1993) 537. [12] M. Costas, K. Chen, L. Que, Coord. Chem. Rev. 200–202 (2000) 517. [13] Y. Raffard-Pons, N. Moll, F. Banse, K. Miki, M. Nierlich, J.-J. Girerd, Eur. J. Inorg. Chem. (2002) 1941. [14] A.E. True, R.C. Scarrow, C.R. Randall, R.C. Holz, L. Que, J. Am. Chem. Soc. 115 (1993) 4246. [15] P. Nordlund, H. Eklund, J. Mol. Biol. 232 (1993) 123. [16] O.Y. Lyakina, A.A. Shteinman, Kinet. Catal. 53 (2012) 694. [17] C. Adhikary, S. Koner, Coord. Chem. Rev. 254 (2010) 2933. [18] S. Naiya, C. Biswas, M.G.B. Drew, C.J. Gómez-García, J.M. Clemente-Juan, A. Ghosh, Inorg. Chem. 49 (2010) 6616. [19] C. Biswas, M.G.B. Drew, E. Ruiz, M. Estrader, C. Diaz, A. Ghosh, Dalton Trans. 39 (2010) 7474. [20] P. Mukherjee, M.G.B. Drew, M. Estrader, A. Ghosh, Inorg. Chem. 47 (2008) 7784. [21] R. Biswas, P. Kar, Y. Song, A. Ghosh, Dalton Trans. 40 (2011) 5324. [22] P. Manikandan, R. Muthukumaran, K.R. Justin Thomas, B. Varghese, G.V.R. Chandramouli, P.T. Manoharan, Inorg. Chem. 40 (2001) 2378. [23] S.K. Dey, N. Mondal, M.S.E. Fallah, R. Vicente, A. Escuer, X. Solans, M. FontBardía, T. Matsushita, V. Gramlichjj, S. Mitra, Inorg. Chem. 43 (2004) 2427. [24] M.S. Ray, A. Ghosh, R. Bhattacharya, G. Mukhopadhyay, M.G.B. Drew, J. Ribas, Dalton Trans. (2004) 252. [25] S. Naiya, H.-S. Wang, M.G.B. Drew, Y. Song, A. Ghosh, Dalton Trans. 40 (2011) 2744. [26] P. Mukherjee, M.G.B. Drew, C.J. Gómez-García, A. Ghosh, Inorg. Chem. 48 (2009) 4817. [27] M.D. Timken, D.N. Hendrickson, E. Sinn, Inorg. Chem. 24 (1985) 3947.
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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 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 b
a r t i c l e
i n f o
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
a b s t r a c t Two new Fe(III) complexes of formulas [FeL1(N3)(OMe)]2 (1) and [FeL2(N3)(OH)]2H2O (2) have been synthesized by using two tridentate NNO-donor Schiff base ligands HL1{(2-[(3-methylaminoethylimino)-methyl]-phenol)} and HL2 {(2-[1-(2-dimethylaminoethylimino)methyl]-phenol)}, respectively. Substitution 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 environment around each iron center in both complexes. To explain the preferential bridging ability of methoxido 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. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The chemistry of iron(III) complexes is dominated by oxygenbridged species because of the high oxygen affinity of the iron(III) ion [1–8]. This type of dinuclear iron(III) unit with oxido, hydroxido, alkoxido, peroxido, or carboxylato bridge(s) have been extensively 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]. Tridentate NNO donor Schiff base ligands, derived from the mono-condensation of a diamine and salicylaldehyde derivative ⇑ Corresponding author. Tel.: +91 9433344484; fax: +91 33 2351 9755. E-mail address: [email protected] (A. Ghosh). http://dx.doi.org/10.1016/j.poly.2014.01.012 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
have been used extensively along with pseudohalides to synthesize both discrete and infinite polynuclear complexes of CuII and NiII with interesting structures and magnetic properties [17–26]. However, 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 ClO 4, NO 3 , PF6 , etc. [27–29], except in a few cases where di-iron complexes with tridentate NNO donor Schiff base ligands have been reported. Among them, there are three methoxido- [30–32], one ethoxido- [31], one hydroxido- [33], and two oxido-bridged diferric 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. In the present contribution, two tridentate NNO-donor Schiff base ligands, HL1 = (2-[1-(2-dimethylaminoethylimino)methyl] phenol) and HL2 = (2-[(3-methylaminoethylimino)-methyl]-phenol) (Scheme 1), have been used to synthesize two new ferric compounds [FeL1(N3)(OMe)]2 (1) and [[FeL2(N3)(OH)]2H2O (2)
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R H C
N
N
CH3
HL
N = L-
OH
N
O
CH3 N N3
O
Fe N
O
O
O Fe
1. Fe(NO3)3. 4H2O 2. NaN3
1. Fe(NO3)3. 4H2O 2. NaN3
N3 N
HL1: Fe3+: N3- = 1:1:2
HL2: Fe3+: N3- = 1:1:2
N
N N3
H O
CH3 1
O
N
Fe
Fe N
O
O H
N3 N
2 R = H (HL2)
R = CH3 (HL1) 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 l-hydroxido-bridged dimer.
2. Experimental 2.1. Starting materials The diamines and salicyldehyde were purchased from Lancaster Chemical Co. The chemicals were of reagent grade and used without further purification. Caution! Although no incidents were recorded in this study, azido 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.
2.3.2. Synthesis of [FeL2(N3)2(OH)]2H2O (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 solution, red coloured, X-ray quality, square shaped single crystals were obtained after two days. 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, cm1): 2045s, 1627s, 3245mb. 2.4. Physical measurements Elemental analyses (C, H and N) were performed using a Perkin-Elmer 240C elemental analyzer. IR spectra in KBr pellets (4500–500 cm1) were recorded using a Perkin–Elmer RXI FT-IR spectrophotometer. Electronic spectra in methanol (1000200 nm) were recorded in a Hitachi U-3501 spectrophotometer.
2.2. Synthesis of the Schiff base ligands The ligands HL1 and HL2 were prepared by refluxing salicylaldehyde (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 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 solution (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 resulting solution at room temperature, red colored X-ray quality, rectangular 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, cm1): 2048s, 1615s, 3244mb.
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-Calibur 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 structure was solved using direct methods with the SHELXS97 program [39]. A suitable single crystal of the complex 2 was mounted on a Bruker-AXS SMART APEX II diffractometer equipped with a graph0 ite monochromator and Mo Ka (k = 0.71073 Å A) radiation. The crystal 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 difference Fourier synthesis revealed the positions of remaining nonhydrogen 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
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a difference Fourier map and refined with distance constraints. Absorption corrections were carried out for 1 using ABSPACK [40] and for 2 using SADABS programs [41]. Refinements were carried out on F2 with SHELXL97. Other programs 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. 3. Results and discussion 1
2
The two new Fe(III) complexes [FeL (N3)(OMe)]2 (1) and [FeL (N3)2(OH)2]2H2O (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 deprotonated tridentate Schiff base ligand and one terminal azide ion coordinate to the Fe(III) centre but two Fe(III) centres are bridged by two methoxido groups and to hydroxide groups in 1 and 2, respectively (Scheme 1). 3.1. Description of structures of complexes 1 and 2 3.1.1. [FeL1(N3)(OMe)]2 (1) 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
Table 1 Crystal data and details of structure refinement of complexes 1 and 2.
Formula M Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) Rint Total reflections Unique reflections I > 2r(I) R1, wR2 I > 2r(I) T (K) Maximum, minimum electron density (e/Å3)
1 C24H36Fe2N10O4 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 1.948, 0.789
2 C20H28 Fe2N10O4,H2O 602.24 triclinic 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 0.459, 0.383
Table 2 Dimensions in the metal coordination spheres of both the complexes. Compound Bond length (Å) Fe1–O4 Fe1–O11 Fe1–N1 Fe1–N19 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
1
2A
2B
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)
90.87(1) 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)
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) 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.
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 reported previously in di-l-methoxido bridged diferric complexes [30–32]. The l-OMe bridge in this complex leads to a perfectly planar Fe2(l-OMe)2 ring as the dimer sits on a crystallographic inversion 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 Fel-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 H Cg⁄ = 2.91 Å, where symmetry element ⁄ = x, 1/2 y, 1/2 + z) are likely to have significant influence on the 2D packing of the molecules (Fig. 2). 3.1.2. [FeL2(N3)2(OH)2]2H2O (2) Complex 2 consists of l-hydroxido bridged dimeric molecules (Fig. 3). In the structure of 2 there are two centrosymmetric independent 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 meridionally 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
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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. Fig. 2. 2D supramolecular networks of complex 1. Hydrogen bonds are shown as dotted lines.
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)
and 3.084(1) Å for 2A and 2B respectively. The Fe–O–Fe bridge angles 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 hydrogen atoms of the discrete water molecule are involved in two donor
Fig. 3. Structure of complex 2. (a) The centrosymmetric structure with ellipsoids drawn at 50% probability. (b) The edge-sharing dimeric structure of 2.
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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 cm1 for complexes 1 and 2 respectively, confirming the presence of the Schiff base. The peaks at 2048 and 2045 cm1 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 cm1 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). Moreover 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 contributions from less intense hydroxo/alkoxo- to-Fe CT are also expected in this region [47] (Fig. 5).
Fig. 5. UV–Vis spectra of complexes 1 and 2.
O–H O hydrogen bonding interactions with the phenoxo oxygen atom of two neighboring units (2A and 2B), and in addition the oxygen atom of the water molecule forms acceptor O H–N hydrogen bonds with two amine hydrogen atoms of the ligands of neighboring molecules. Further hydrogen bonds are formed by each bridging hydroxo group of dinuclear unit with the nitrogen atom of the terminal azide of neighboring units through O–H N hydrogen bonding interactions. There are also N–H O interactions to the bridging OH group in same molecule. The dimensions of hydrogen bonds are given 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 respectively, 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 108 density-based convergence criterion
Table 3 H-bonding dimensions of complex 2. Donor–H Acceptor
D–H (Å)
H A (Å)
D A (Å)
D–H A (°)
Symmetry
O(1W)–H(1W)O(11B) O(1W)–H(2W)O(11A) O(4A)–H(4A)N(1B) O(4B)–H(4B)N(1A) N(23B)–H(22B)O(1W) N(23A)–H(23A)O(1W)
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, x, 1 y, x, 1 y, x, 1 y, x, 1 + y, z x, 1 y,
Fig. 6. Steric crowding between two dimers in complex 1.
1z 1z 1z 1z 1z
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stable than complex 2 (Ef = 1532.56 kcal/mol). In addition, the theoretical study indicates that the dimerization energies of complex 2 is 10.93 kcal/mol, leads to extra stability through the formation 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–H p), which compensate for the positive dimerization energy. Acknowledgments
Fig. 7. Intra-dimer H-bonding interaction in complex 2.
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 Diffractometer 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. PSW150/11-12 (ERO). S. Biswas is thankful to CSIR, India for research fellowship [Sanction no. 09/028(0732)/2008-EMR-I]. Appendix A. Supplementary data
for all energy calculations. Interaction energies have also been corrected for the Basis Set Superposition Error (BSSE) using the Boys and Bernardi’s method [55,56].
2Fe3þ þ 2HðHL1 Þ þ 2MeOH þ 2NaN3 þ 4H2 O ¼ ½Fe2 ðL1 Þ2 ðN3 Þ2 ðOMeÞ2 þ 2Naþ þ 4H3 Oþ 2Fe3þ þ 2HðHL2 Þ þ 2NaN3 þ 7H2 O ¼ ½Fe2 ðL2 Þ2 ðN3 Þ2 ðOHÞ2:H2 O þ 2Naþ þ 4H3 Oþ 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 relatively more stable than complex 2 (Ef = 1532.56 kcal/mol). From the structure of complex 2, it is clear that hydrogen bonding provides extra stability. To check the additional stability through intermolecular interaction, we have calculated dimerization energies by considering direct 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 complex 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 repulsion 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 crystal structure in 2. Here, the preferential dihydroxy bridges are of prime importance as they facilitate hydrogen bonding with nitrogen 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. 4. Conclusions We have prepared two new Fe(III) complexes with two NNO donor Schiff-base ligands and azido co-ligands. Both complexes 1 and 2 are centrosymmetric dimers where the Fe(III) ions are in distorted octahedral environment, bridged by two l2-methoxide and hydroxide ions, respectively. The theoretical results demonstrate that complex 1 (Ef = 1546.41 kcal/mol) is relatively more
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