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A comparative structural and property investigation of four new bivalent transition metal complexes based on 4-nitrophenylacetic acid with rigid 4-nitrobenzoate analogues
Accepted Manuscript Research paper A comparative structural and property investigation of four new bivalent transition metal complexes based on 4-Nitrophenylacetic acid with rigid 4-Nitrobenzoate analogues Kiran T Dhavskar, Raymond J. Butcher PII: DOI: Reference:
S0020-1693(16)30824-6 http://dx.doi.org/10.1016/j.ica.2017.05.027 ICA 17596
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
7 November 2016 10 April 2017 11 May 2017
Please cite this article as: K.T. Dhavskar, R.J. Butcher, A comparative structural and property investigation of four new bivalent transition metal complexes based on 4-Nitrophenylacetic acid with rigid 4-Nitrobenzoate analogues, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.05.027
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A comparative structural and property investigation of four new bivalent transition metal complexes based on 4-Nitrophenylacetic acid with rigid 4-Nitrobenzoate analogues a*
b
Kiran T Dhavskara* and Raymond J. Butcherb
Department of Chemistry, Goa University, Goa 403206, India E-mail: [email protected] , Telephone: +91-8390910141 Department of Chemistry, Howard University Washington DC 20059 E-mail: [email protected] , Telephone: Office: (202) 806-9892 Fax: (202) 806-5442
Abstract The synthesis, spectral, thermal, magnetic properties, and structural features of four new onedimensional (1-D) polymers having formula [M(4-npa)2(µ2-H2O)(H2O)2]·2H2O where (M=Mn 1; M=Co 2; M=Zn 3; M=Ni 4; 4-npa=4-nitrophenylacetate) are investigated and compared with corresponding 4-nitrobenzoate (4-nba) analogues. The single crystal studies reveal that compounds 1-3 are isostructural and crystallize in the centrosymmetric monoclinic space group P21/c. Their structure consists of distorted octahedra of an unique central metal ion, two crystallographically unique monodentate 4-npa ligands, two terminal water molecules, a µ2-bridging bidentate water and two lattice water molecules. The central metal exhibits a distorted {MO6} octahedron with two O atoms from terminal water molecules disposed cis to each other, two O atoms from the monodentate carboxylates also disposed in a cis geometry and the remaining two O atom from the µ2-bridging bidentate water disposed trans. The µ2-brindging mode of water leads to the formation of an extended 1-D chain structure. The lattice water links parrallel 1-D chains with the aid of H-bonding interactions. The IR, XRD powder pattern, and thermal characterization reveal that the structure of polycrystalline compound 4 is nearly identical with that of other 4-npa analogues.
Keywords: Extended structure, 4-nitrophenylacetic acid, Zinc, Manganese, isostructural, bridging water
1. Introduction The study of metal carboxylates is a topical area of research due to their biological significance [1, 2] and usefulness in material science [3-5]. As part of our metal-carboxylate research program, we are investigating the structural chemistry of substituted aromatic carboxylates of the s- and d-block metals. Based on our research studies, we have observed so far that the flexible nature of the oxophilic s-block cations to adopt to higher coordination assists in the formation of compounds with an extended structure even when monotopic aromatic carboxylic acids are used as main linkers [6-8]. Although, first row transition metal centers are preferred due to their well-known coordination behaviors, the extended structures are often built using multitopic primary ligands instead of monotopic aromatic carboxylate linkers as these normally lead to the formation of monomeric structures [9-15]. Hence it was a challenging task to synthesize extended structures using d-block metal cations in the media of such O-donor ligands. The construction of intriguing structural topologies depends upon how the primary ligand of given chemical nature and stereochemistry arranges itself around the central metal besides the coordination geometries of central metal. Since flexible coordination behavior of s-block metal cation with rigid ligand could yield extended structures, it was of interest to investigate the structural outcome when flexible ligand would combine with metals preferring fixed geometry. Hence replacement of 4-nba with 4-nitrophenylacetic acid (Fig. 1) where the –COOH group is more flexible and is attached to a –CH2 unit was a rational approach to serve this purpose as it did lead to a successful synthesis of compounds having extended structure. The synthesis, properties and structural investigation of four new compounds of relatively less explored 4-npa system are described in this paper.
Fig. 1
2. Experimental 2.1 Materials and Methods All the chemicals used in this study were of reagent grade and were used as received without any further purification. Infrared (IR) spectra of the solid samples diluted with KBr were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer from 4000-400 cm-1 at a resolution of 4 cm-1 . UV-Vis diffuse-reflectance spectra were recorded on a Shimadzu UV-2450 double beam spectrophotometer. BaSO4 powder was used as reference (100 % reflectance). Absorption data were calculated from the reflectance data using the Kubelka-Munk function (a/S=(1-R2/2R where a is the absorption coefficient, R the reflectance and S the scattering coefficient). Isothermal weight loss studies were performed in a temperature controlled electric furnace. TG-DTA study was performed in flowing oxygen in Al2O3 crucibles at heating rate of 10 K min-1 using a STA-409 PC simultaneous thermal analyser from Netzsch. Magnetization measurements were carried out on a Quantum design Versa Lab vibrating sample magnetometer in an external field up to 3 Tesla and from 50 K to 300 K. ESR study was carried out on a Bruker EMx/micro-x instrument at room temperature. The morphology of the zinc oxides was investigated by scanning electron microscope (Carl Zeiss Model No EVO18). N2 adsorption-desorption studies were performed on a Quantachrome NOVA 1000 surface and porosity instrument using N2 as a probe molecule at -196 o C. Prior to the measurements, the sample were degassed at 300 o C for 12 hours to remove any moisture and volatiles. The Brunauer-Emmett-Teller (BET) model was used to determine the specific surface area of the metal oxide samples. The pore size and pore volume of the samples were measured using the Barrett-Joyner-Halenda (BJH) method. .
2.2 Preparation of the complexes 2.2.1 Synthesis of [Mn(4-npa)2(µ2-H2O)(H2 O)2]·2H2O (1) A solution of sodium bicarbonate (0.840 g, 10 mmol) in water (30 mL) was added into 4-npaH (1.812 g, 10 mmol) to obtain the sodium salt of carboxylic acid. The solution was warmed on waterbath in order to dissolve the acid faster. To this solution, [Mn(H2O)4]Cl2 (0.990 g, 5 mmol) in water (20 mL) was slowly added. The resultant solution mixture was filtered and left aside for crystallization.
Transparent, pale brownish crystals appeared within 3-4 days which were collected by filtration in ~ 83% yield.
2.2.2 Synthesis of [Co(4-npa) 2(µ2-H2O)(H2O)2]·2H2O (2) The use of [Co(H2O)6]Cl2 instead of the Manganese salt gives compound 2 in 78 % yield. However during synthesis of compound 2 excess water (>200mL) should be used to avoid immediate precipitation of product. Alternatively compound 2 can be obtained by heating CoCO3 with 4-npaH in excess water. The reaction mixture is then filtered and left for crystallisation. Within a week, red coloured crystals suitable for single crystal analyses appear in the mother liquor which is then isolated by filtration.
2.2.3 Synthesis of [Zn(4-npa)2(µ2-H2O)(H2O)2]·2H2 O (3) A mixture of 4-npaH (1.812g, 10 mmol) and zinc oxide (0.407 g, 5 mmol) was taken in excess distilled water (~250 mL) and the reaction mixture was heated on a water bath. Brisk effervescence was observed and the heating was continued till most of the oxide reacted with acid. During the reaction, slight excess of ZnO was added to ensure that no 4-npaH remains unreacted. The hot reaction mixture was then filtered and heated to concentrate the volume to 100 mL and left aside for crystallization. White transparent crystals which separated in a few days were collected by filtration in ~ 90 % yield.
2.2.4 Synthesis of [Ni(4-npa)2(µ2-H2O)(H2O)2]·2H2O (4) The analogous nickel compound (compound 4) was also obtained in polycrystalline form in a similar manner like compound 3 by heating nickel carbonate with 4-npaH in excess water or by reacting NiCl2·6H2 O with sodium salt of 4-npaH in water.
2.3 X-ray crystallography Single crystal X-ray analysis of compounds 1-3 was done at the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology (IIT) Madras. X-ray intensity data were collected
using Bruker AXS Kappa Apex II CCD Diffractometer equipped with graphite monochromated Mo (Kα) (λ = 0.7107 Å) radiation, through an optimized strategy which gave an average 4-fold redundancy. The program APEX2-SAINT (Bruker, 2004) was used for integrating the frames. Fourfold redundancy per reflection was utilized for achieving good multi-scan absorption correction using SADABS (Bruker, 2004). Besides absorption, Lorentz, polarization and decay corrections were applied to intensity during data reduction. The structures were solved by direct methods using SIR92 and refined by full-matrix least squares techniques using SHELXL-97 [16, 17]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms attached to the aromatic ring were introduced in calculated positions and included in the refinement by riding on their respective parent C atoms. Crystallographic data and selected refinement results for complexes 1–3 are summarized in Table 1-3. Table 1- Crystal data and structure refinement for 1- 3 Empirical formula Formula weight (g mol-1) Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) β (°) Volume (Å3) Z Dcalc (mg/m3) Absorption coefficient (mm-1) F(000) Crystal size (mm3) θ range for data collection (°) Index ranges Reflections collected / unique Completeness to θ Absorption correction
Max. and min. Transmission Refinement method Data / restraints /
C16H22N2MnO13 (1) 505.30 296(2) 0.71073 Monoclinic P21/c
C16H22N2CoO13 (2) 509.28 293(2) 0.71073 Monoclinic P21 /c
C16H22N2ZnO13 (3) 515.73 293(2) 0.71073 Monoclinic P21/c
10.7951(3) 8.1873(2) 24.9212(6) 98.924(2) 2175.94(10) 4 1.542 0.676
10.7802(15) 8.0299(9) 24.629(3) 98.877(6) 2106.5(5) 4 1.606 0.887
10.8262(3) 8.0624(2) 24.6227(6) 98.7820(10) 2124.00(9) 4 1.613 10228
1044 0.35 x 0.35 x 0.32 1.91 to 26.00
1052 0.30 x 0.20 x 0.20 2.339 to 25.434
1064 0.35 x 0.30 x 0.25 1.90 to 26.00
-13 ≤ h ≤ 13 -10 ≤ k ≤ 10 -30 ≤ l ≤ 30 25996 / 4274 (R(int) = 0.0257) 100% Semi-empirical from equivalents 0.8193 and 0.7917
-12 ≤ h ≤ 12 -9 ≤ k ≤ 9 -0 ≤ l ≤ 29 3663 / 3663 (R(int) = 0.000) 94.1% Semi-empirical from equivalents 0.822 and 0.642
-13 ≤ h ≤ 13 -9 ≤ k ≤ 9 -30 ≤ l ≤ 30 34348 / 4168 (R(int) = 0.0652) 100% Semi-empirical from equivalents 0.7497 and 0.6710
Full-matrix leastsquares on F2 2538 / 12 / 174
Full-matrix leastsquares on F2 3663 / 63 / 353
Full-matrix leastsquares on F2 4168 / 15 / 328
parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole (e Å -3)
1.145 R1 = 0.0393, wR2 = 0.0910 R1 = 0.0499, wR2 = 0.1010 0.0070(5) 0.394 and -0.337
1.134 R1 = 0.0629, wR2 = 0.1888 R1 = 0.1181, wR2 = 0.1961 n/a 0.516 and -0.675
1.082 R1 = 0.0315, wR2 = 0.0800 R1 = 0.0488, wR2 = 0.0937 0.0054(5) 0.569 and -0.567
Table 2- Selected bond lengths [Å] and bond angles [o] for 1-3 Bond lengths [Mn(4-npa)2(H2 O)3]·2H2O (1) Mn1-O11 2.1372(18) Mn1-O5 2.1446(17) Mn1-O1 2.1496(17) Mn1-O10 2.1504(18) Mn1-O9 i 2.2116(16) Mn1-O9 2.3447(16)
[Co(4-npa)2(H2O)3]·2H2O (2) Co1-O3W 2.045(4) Co1-O1 2.046(4) Co1-O2W 2.049(4) Co1-O5 2.051(5) Co1-O1W 2.232(4) Co1-O1Wii 2.232(4)
[Zn(4-npa)2(H2O)3 ]·2H2 O (3) Zn1-O6 2.0575(16) Zn1-O2 2.0576(17) Zn1-O3 2.0577(18) Zn1-O4 2.0613(17) Zn1-O1 2.1252(15) Zn1-O1iii 2.2962(16)
Bond angles O11-Mn1-O9 O11-Mn1-O10 O1-Mn1-O9 O10-Mn1-O9 O5-Mn1-O9 i O11-Mn1-O5 O5-Mn1-O9 O5-Mn1-O1
85.52(6) 87.62(8) 87.97(6) 88.85(7) 89.25(6) 89.36(8) 89.79(6) 91.25(7)
O1-Mn1-O9 i O1-Mn1-O10 O10-Mn1-O9i O11-Mn1-O9i O11-Mn1-O1 O5-Mn1-O10 O9-Mn1-O9 i
91.43(6) 91.62(8) 92.14(7) 95.08(7) 173.47(7) 176.78(8) 178.86(4)
O3W-Co1-O1 O3W-Co1-O2W O1-Co1-O2W O3W-Co1-O5 O1-Co1-O5 O2W-Co1-O5 O3W-Co1-O1W O1-Co1-O1W
176.12(18) 86.57(18) 90.17(18) 91.99(18) 91.32(17) 177.86(15) 91.56(15) 86.54(15)
O2W-Co1-O1W O5-Co1-O1W O3W-Co1-O1Wii O1-Co1-O1Wii O2W-Co1-O1Wii O5-Co1-O1Wii O1W-Co1-O1Wii
93.60(16) 88.02(15) 88.74(15) 93.14(15) 86.08(15) 92.31(15) 179.54(4)
O2-Zn1-O1 iii O2-Zn1-O3 O6-Zn1-O1 O3-Zn1-O1 iii O4-Zn1-O1 O6-Zn1-O2 O4-Zn1-O1 iii O6-Zn1-O4
85.34(7) 86.20(8) 87.74(6) 88.06(7) 89.84(7) 90.25(8) 90.29(6) 91.14(7)
O6-Zn1-O1iii O3-Zn1-O4 O3-Zn1-O1 O2-Zn1-O1 O2-Zn1-O4 O6-Zn1-O3 O1-Zn1-O1iii
91.57(6) 92.40(8) 92.63(7) 94.55(7) 175.45(7) 176.45(8) 179.30(2)
Symmetry transformations used to generate equivalent atoms: i) -x+2, y+1/2, -z+1/2; ii) -x+2, y-1/2, -z+1/2; iii) -x+1, y+1/2, -z+1/2
Table 3- Hydrogen bonding geometry [Å and °] for 1-3 D-H···A
d(D-H)
d(H···A)
d(D···A)
Symmetry code
2.56 1.803(18) 1.775(18) 2.63(3) 1.764(17) 1.843(19) 1.822(17) 1.788(17) 2.11(2) 1.94(2) 1.97(3) 2.06(2)
3.431(4) 2.680(2) 2.647(3) 3.124(2) 2.630(2) 2.697(2) 2.703(3) 2.676(3) 2.934(4) 2.752(3) 2.733(3) 2.873(4)
155.6 175(3) 171(3) 117(2) 166(3) 161(3) 179(3) 174(3) 155(4) 154(4) 145(4) 156(4)
-x+2, -y, -z+1 -x+2, y-1/2, -z+1/2 -x+2, y+1/2, -z+1/2 x, y, z x, y, z x, y, z x, y, z -x+2, y-1/2, -z+1/2 -x+1, -y, -z -x+2, y+1/2, -z+1/2 x, y, z x-1, -y-1/2, z-1/2
1.98(4) 2.54(5) 1.806(19) 1.888(18) 1.94(2) 1.88(2) 1.84(4) 1.954(19) 1.89(2) 2.53 2.38 2.62 2.12(13) 2.35(6) 1.94(3) 1.94(3) 2.20(5) 2.24(6) 2.33(9)
2.678(6) 3.091(5) 2.621(6) 2.661(5) 2.669(16) 2.673(13) 2.58(4) 2.729(9) 2.662(6) 3.387(10) 3.18(2) 3.308 2.58(4) 2.85(3) 2.725(9) 2.728(15) 2.89(2) 2.729(13) 2.88(2)
143(5) 125(4) 168(4) 164(5) 151(4) 171(3) 154(4) 165(3) 159(5) 153.7 139.4 131.3 115(11) 120(6) 157(7) 160(8) 141(7) 119(6) 125(8)
-x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2, y-1/2, -z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2, y-1/2, -z+1/2 -x+2, -y, -z -x+3, y+1/2, -z+1/2 -x+3,-y,-z+1 -x+2, y-1/2, -z+1/2 x-1,-y-1/2,z-1/2 x,y,z x,y,z x+1,-y-1/2,z+1/2 x,y,z x,y,z
2.62 2.55 1.91(2) 2.03(2) 2.11(2) 1.826(18) 1.90(2) 1.774(17) 1.816(17) 1.799(17) 1.821(18) 2.59(3) 1.744(18)
3.359(5) 3.406(4) 2.724(3) 2.853(4) 2.918(4) 2.700(3) 2.726(3) 2.657(3) 2.686(3) 2.675(3) 2.671(2) 3.093(2) 2.617(2)
133.1 153.8 154(4) 158(4) 154(4) 173(3) 154(4) 178(3) 177(3) 177(3) 161(3) 117(3) 168(3)
-x, y-1/2, -z+1/2 -x+1, -y+1, -z+1 -x+1, y-1/2, -z+1/2 -x+2, -y+1, -z -x, -y+1, -z x, y, z -x+1, y-1/2, -z+1/2 x, y, z -x+1, y+1/2, -z+1/2 -x+1, y+1/2, -z+1/2 -x+1, y-1/2, -z+1/2 -x+1, y-1/2, -z+1/2 -x+1, y-1/2, -z+1/2
[Mn(4-npa)2(H2 O)3]·2H2O (1) C13-H13···O6 O11-H11A···O1 O11-H11B···O13 O9-H9A···O1 O9-H9A···O2 O9-H9B···O6 O10-H10C...O12 O10-H10D···O5 O12-H12A···O3 O12-H12B···O6 O13-H13A···O2 O13-H13B···O7
0.93 0.878(18) 0.878(18) 0.885(17) 0.885(17) 0.888(17) 0.882(17) 0.891(17) 0.876(18) 0.879(18) 0.880(18) 0.863(19)
[Co(4-npa)2(H2O)3]·2H2O (2) O1W-H1W1···O2 O1W-H1W2···O5 O1W-H1W2···O6 O2W-H2W1···O5 O2W-H2W2···O5WA O2W-H2W2···O5WB O3W-H3W1···O4WA O3W-H3W1···O4WB O3W-H3W2···O1 C6-H6···O2 C7-H7B···O7A C13-H13···O8 O4WA-H4W2···O3W O4WB-H4W3···O8A O4WB-H4W4···O2 O5WA-H5W1···O6 O5WA-H5W2···O3 O5WB-H5W3···O6 O5WB-H5W4···O3A
0.815(18) 0.828(18) 0.828(18) 0.794(17) 0.803(18) 0.803(18) 0.794(18) 0.794(18) 0.811(17) 0.93(2) 0.97(2) 0.93 0.82(2) 0.826(19) 0.832(19) 0.819(19) 0.82(2) 0.82(2) 0.82(2)
[Zn(4-npa)2(H2O)3 ]·2H2 O (3)
C2-H2A···O10 C5-H5···O7 O13-H13A···O5 O13-H13B···O8 O12-H12A···O11 O3-H3A···O12 O12-H12B···O7 O2-H2D···O13 O2-H2C···O4 O3-H3B···O6 O1-H1A···O7 O1-H1B···O4 O1-H1B···O5
0.97 0.93 0.879(19) 0.863(19) 0.868(18) 0.878(16) 0.881(18) 0.883(17) 0.871(17) 0.877(17) 0.884(17) 0.887(17) 0.887(17)
D= Donor and A= Acceptor
3. Results and Discussion 3.1 Synthetic aspects, spectral and thermal studies Compounds 1, 2, and 3 are prepared by reacting the corresponding metal chlorides with sodium salt of 4-npaH in water. Alternatively, compound 2 can also be prepared by direct reaction of metal carbonate with free acid, where the reaction mixture was heated in water to obtain clear solution. Compound 3 is obtained by heating ZnO with 4-npaH in water. The Infrared and Raman spectra (Fig. S1& S2) of 1-3 are identical; therefore is a first indication of possible structural similarity in all four compounds. A comparison of IR spectra of complexes with that of the free acid reveals that the band due –COOH group of the free acid occurring at 1711 cm-1 is shifted to lower energies due to the formation of metal carboxylate . The presence of water in 1-3 can be evidenced by the characteristic profile of the spectra in the 3500-3000 cm-1 region. The inverted overlap of infrared and Raman spectra (Fig. 2) shows the presence of strong bands in both the spectra’s and is assigned to symmetric stretch of -NO2 vibrations occurring at 1339 cm-1. Comparatively less intense band of asymmetric NO2 stretch in IR appears as second intense band in Raman at 1597cm-1. The third intense band seen in Raman spectra represents the C-N stretching vibrations occurring at 1103 cm-1. All these bands are present in the free acid as well at 1601, 1343, and 1103 cm-1 and not major changes in these values are seen when acid is complexed with metal.
Fig. 2 The inverted overlap of IR and Raman spectra showing strong νs NO2
The TG-DTA curves (Fig. S3-S5) of 1, 2 and 3, exhibit three thermal events for all compounds. The first endothermic event (96 oC for 1; 116 oC for 2; 95 oC for 3) can be assigned to the dehydration process. Upon heating at 120 oC in an oven, these compounds show weight loss, between 16-17%, that corresponds to loss of five water molecules. The formation of an anhydrous compound is confirmed by devoid of the strong O-H signal at 3600 cm-1 in their respective IR spectra (Fig. S6-S8) which reappear when compounds are equilibrated over water for 4-5 days showing successful rehydration. The last event is exothermic and represents the decomposition of organic ligand. Due to the absence of associated mass spectral data of the emitted fragments, the exact nature of ligand decomposition process cannot be further commented. The high decomposition temperature (366 oC for 1; 367 oC for 2; 465 oC for 3) of Zn complex could be due to more stability of these metal in tetrahedral coordination compared to Mn and Co. The Diffused reflectance spectra (DRS) of free acid show two charge transfer bands at 257 and 325 nm, and these bands also appear in all three complexes of acid (Fig. S9). At lower intensities (Fig. S10), d-d absorption become visible showing very weak absorption for compound 1 due to doubly forbidden (Laporte and Spin) nature of transition bands associate with d5 system. For compound 2, which is a d7 system, bands are comparatively intense, and three spin allowed d-d transitions namely 4 T1g (F) → 4 T2g (F), 4 T1g (F) → 4 T1g (P), and 4 T1g (F) → 4 A2g(F) occur in visible region at 530, 484, and 450 nm respectively. No d-d transitions are associated with compound 3 which is a d10 system. 3.2 X-ray crystal structure determination Compounds 1-3 crystallize in the centrosymmetric monoclinic space group P21/c and are isostructural. In view of the isostructural nature, the structure is described for the Mn(II) analogue. The crystal structure (Fig. 3) of 1 consists of a central divalent Mn(II), two crystollographically unique monodentate 4-npa ligands, two terminal water molecules, a bridging water molecule and two lattice water molecules (O12 and O13) with all atoms located on general positions. The central metal is bonded to six O atoms resulting in a {MO6} (M=Mn, Co or Zn) coordination sphere. Of these two O atoms (O1 and O5) come from the unique 4-npa ligands which are disposed cis to each other. The oxygens of the two terminal water molecules (O10 and O11) function as monodentate lignads and
adopt a cis orientation. The bridging water (O11) functions as a µ 2-bidentate bridge and is linked to two symmetry related Mn(II) ions. Thus each Mn(II) is bonded to O9 and O9i which are disposed trans at Mn-O distances of 2.3447(16) and 2.2116(16) Å respectively. It is this binding mode which is responsible for the 1-D structure of this compound (and the analogues 2 and 3) as can be evidenced by the formation of a water linked chain connecting {Mn(4-npa)2(H2 O)2} units along b axis (Fig. 4). The Mn-O bond distances vary between 2.1372(18) and 2.3447(16) Å. The cis O-Mn-O angles range from 85.52(6) to 91.62(8) while the trans O-Mn-O angles scatter in a range from 173.47(7) to 178.86(4) indicating the distortion of the {MnO6} octahedron. The M-O bond distances and O-M-O bond angles (Table 2) in 2 and 3 can be explained similarly. An analysis of the crystal structure reveals that the H atoms attached to the coordinated waters O9-O11 and the lattice water O12 and O13 and the O atoms of the -COO- and -NO2 groups of the 4-npa anions function as H-acceptors (Table 3). The most interesting aspect of the H-bonding interaction is the role of O123 and O13 which link adjacent 1-D chains with the aid of interchain O-H···O interactions (Fig. 4)
Fig. 3 The crystal structure of [Mn(H2O)2(4-npa)2(µ-H2O)]·2H2O (1) showing the atom labelling scheme and the six coordination around Mn(II). Displacement ellipsoids are drawn at the 50% probability level excepting for the H atoms, which are shown as circles of arbitrary radius. Symmetry code: i) -x+2, y+1/2, -z+1/2
Fig. 4 A view of an infinite chain along b-axis due to the µ2-bridging bidentate coordination modes of water (top); Linking of two infinite chains along b-axis by means of lattice (O12, O13) water (bottom).
3.3 Characterization and structural investigations of Ni-4npa complex (4) The IR spectra (Fig. S11) of compound is identical with rest of the compounds and TG-DTA curves (Fig. S12) show the similar profile with three DTA events where the first event can be assigned to dehydration process which can be evidenced from weight loss of 16.78% in TG curve. The diffused reflectance spectra (Fig. S13) shows intra ligand charge transfer bands at 261 and 308 nm which is typical of free acid chromophore. Spin allowed d-d transitions are seen at lower intensities as merged broad band between 600-800nm. The Ni (II) compound has composition and formula same as other analogues as indicated by elemental analysis. The X-ray powder pattern of Ni-4npa complex (Fig. 5) was compared with theoretically generated powder pattern from cif file of compound 2. A comparison shows high similarity between two patterns. Therefore base on elemental composition, IR, thermal and XRD powder pattern Ni (II) compound is expected to have structure very similar with the remaining isostructural analogues. However the Ni compound is obtained in polycrystalline form and is expected to have deformaties in its ordered arrangement which are absent in the remaining crystalline analogues having same composition
Fig. 5 A comparison of X-ray powder pattern of Ni-4npa complex (top) with that of theoretically generated powder pattern from cif file of compound 2 (bottom)
3.5 Similarity and differences between 1-4 and corresponding 4-nba compounds A comparative study of the structural chemistry of corresponding complexes of rigid 4-nba (Table 4) with Mn, Co, Ni, Zn is presented. The coordination numbers of the bivalent 3d metals excepting Zn is six when complexed with both rigid 4-nba as well as flexible 4-npa ligand which is a typical of dblock metal. It is interesting to note that Zn(II) which is well known for tetra coordination exhibits six coordination with flexible ligand forming water bridged 1-D extended chain, isostructural with remaining 4-npa analogues of Mn, Co and Ni. In case of 4-nba, Mn, Co, and Ni form monomeric and isostructural compounds with formula [M(4-nba)2(H2O)2]·2H2O in which 4-nba ligand is monodentate while a recently redetermined structure by Necefoglu et al [15] of [Zn(4-nba)2(H2O)2] reveals that in addition to monodentate coordination, 4-nba coordinates to Zn(II) cation with considerably weaker secondary bond thereby giving zinc(II) intermediate coordination geometry between octahedral and tetrahedral. The flexible ligands are disposed cis while rigid ligands lie trans around hexacoordinated metal in the their corresponding complexes. In the case of 4-npa compounds only bridging waters are disposed trans to one another in the coordination sphere of central metal Table 4 Structural details of selected transition metal 4-nba complexes Compound S. G. C.N Binding modes of Structure ligand type
Reference
[Mn(4-nba)2(H2O)4]·2H2O [Co(4-nba)2(H2O)4]·2H2O [Ni(4-nba)2(H2O)4]·2H2O [Ni (H2O)6] (4-nba)2·2H2O [Zn(4-nba)2(H2O)2]
Pī Pī Pī C2/c C2/c
6 6 6 6 4-6
Monomer Monomer Monomer Monomer Monomer
11 12 11 13 15
[Mn(4-npa)2(H2O)3]·2H2O [Co(4-npa)2(H2O)3]·2H2O [Zn(4-npa)2(H2O)3]·2H2O
P21/c P21/c P21/c
6 6 6
1-D Polymer 1-D Polymer 1-D Polymer
This work This work This work
Mondentate Mondentate Mondentate Ionic Mondentate with weak secondary interactions Monodentate Monodentate Monodentate
S. G. = Space group; C.N. = Coordination number
The ESR spectrum of 1 and reported 4-nba analogue [Mn(4-nba)2(H2O)2]·2H2O (Mn-4nba) exhibits a isotropic signal centered at g = 2.00 (Fig. 6). This indicates that metal centered are far apart for any interactions in solid state. However the broadness of the signal in case of Mn-4nba is due to spin exchange and dipolar interactions.
Fig. 6 ESR spectra of 1 (For ESR spectra reported [Mn(4-nba)2 (H2O)4]·2H2O see Fig. S14)
The magnetic studies of 1 (Fig. 7, 8) and corresponding Mn-4nba complex (Fig. S15, S16) reveal paramagnetic behavior for at and below room temperature as shown in magnetization verses temperature and field i.e. M-T and M-H curves of both the compounds. The same behavior was also observed in case of complex 2 and corresponding Co-4nba complex in their respective magnetization curves (Fig. S17-S20). A magnetic moment of ~5.83BM and 4.90 BM is found for 1 and 2 respectively indicating a high spin nature of Mn(II) and Co(II). It is found that magnetization values are slightly higher for corresponding 4-nba complexes at lower temperatures than that of 4-npa complexes. The magnetization curves of Ni-4nba complexes also have the similar trend (Fig. S21, S22) showing paramagnetic behavior at and below room temperature. The magnetic moment of 2.03 BM was observed at and below room temperature for Ni-4nba complex
Fig. 7 Magnetization vs Temperature (M-T) curve of compound 1
Fig. 8 Magnetization vs Field (M-H) curve of compound 1
However, in case of complex 4 (Fig. 9) the splitting of FC and ZFC curve at low temperature clearly indicate some magnetic interaction in the complex. However these interactions are visible only at low magnetic field of 0.5 T and disappear when M-T measurement is carried out at higher field of 1 T. These could be due to presence or clustering of magnetic domains in certain regions of solids. Weak hysteresis loop is observed for this sample when M-H measurement is carried out at 50 K indicating weak ferromagnetic interactions at low temperature (Fig. 9). The values of magnetic moment obtained for these complexes are very low than expected i.e 1.13 BM at 300K, 1.37BM at 150K and 1.58BM at 50K. The magnetic moment value is found to increase as the temperature is lowered.
Fig. 9 The M vs T plot (top) and M vs H plot (bottom) of compound 4
The surface morphologies of zinc oxides obtained from the decomposition of complex 3 and [Zn(4-nba)2(H2O)2] are studied using SEM. These images are also compared with the SEM image of standard ZnO used for the synthesis of these complexes (Fig. 10). SEM images reveal that the standard Zinc oxide (ZnOS) material which is used as a starting material is non-porous with firmly held or consolidated particles. The SEM image of Zinc oxide (ZnOA) obtained from decomposition of [Zn(4-nba)2(H2O)2] show a porous morphology.
Fig. 10 The SEM images of standard ZnO (top), ZnO from [Zn(4-nba)2(H2O)2] (middle), and ZnO from compound 3 (bottom)
Similarly zinc oxide (ZnOB) obtained from decomposition of 3 also shows porous nature. In addition to this the SEM image reveals that particles have definite geometric shape. The morphology of oxide was dependent on ligand size which was further supported by BET and pore data (Table 4). During calcinations the escaping ligand created hollow voids in the ZnO structure. A comparative surface analysis of ZnOA and ZnOB with that of standard ZnOS was done and interestingly the size of ligand and pore diameter, pore volume were in good agreement. The N2 adsorption-desorption isotherms revealed the mesoporous nature of the prepared Zinc oxides (ZnOA and ZnOB). Specifically isotherms were identified as type IV isotherms with a H2 hysteresis loop for ZnOA and H3 hysteresis loop for ZnOB which is characteristic of ink-bottle and slit-shaped pores respectively[18]. Table 5. BET surface area and pore data Zinc Oxides BET Surface area (m2/g)
Pore diameter (nm)
Pore volume (mL/g)
ZnOS ZnOA ZnOB
1.736 2.815 3.115
0.005 0.014 0.032
4 14 28
Conclusion All three complexes of 4-npa with Mn, Co, and Zn are isotructural and based on powder pattern, IR, and thermal data corresponding polycrystalline Ni analogue is also expected to have structure very similar to the remaining analogues. Use of flexible 4-npa ligand in place of rigid 4-nba has led to successful synthesis of 1-D polymeric structure as evidenced from comparative table which shows monomeric nature for reported 4-nba compounds of same bivalent d-block cation. The ZnO obtained from decomposition of corresponding 4-nba and 4-npa complexes shows porous nature.
Supplementary Information CCDC 1437821–1437823 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via the Cambridge Crystallographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:
Acknowledgments The author like to thank the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology (IIT) Madras for single crystal X-ray analysis of 1, 2 and 3 reported in this paper .The author also acknowledges helpful discussions with research supervisor Prof. B. R. Srinivasan and research colleagues Kedar Narvekar, Daniel Cutinho and Celia Bragranza from Goa University. Financial support under the Special Assistance Programme (DSA-1) of the University Grants Commission, New Delhi is gratefully acknowledged.
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The investigation of the structural outcome upon reaction of flexible ligand with metals preferring fixed geometry
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The replacement of rigid 4-nitrobenzoic acid by flexible 4-nitrophenyl acetic acid lead to successful synthesis of 1-D polymeric structure
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All four compounds of 4-nitrophenyl acetic with corresponding Mn, Co, Ni, Zn have similar structure. Comparative study of structure and properties with corresponding 4-nitrobenzoate analogues is reported.
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S0020-1693(16)30824-6 http://dx.doi.org/10.1016/j.ica.2017.05.027 ICA 17596
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
7 November 2016 10 April 2017 11 May 2017
Please cite this article as: K.T. Dhavskar, R.J. Butcher, A comparative structural and property investigation of four new bivalent transition metal complexes based on 4-Nitrophenylacetic acid with rigid 4-Nitrobenzoate analogues, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.05.027
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A comparative structural and property investigation of four new bivalent transition metal complexes based on 4-Nitrophenylacetic acid with rigid 4-Nitrobenzoate analogues a*
b
Kiran T Dhavskara* and Raymond J. Butcherb
Department of Chemistry, Goa University, Goa 403206, India E-mail: [email protected] , Telephone: +91-8390910141 Department of Chemistry, Howard University Washington DC 20059 E-mail: [email protected] , Telephone: Office: (202) 806-9892 Fax: (202) 806-5442
Abstract The synthesis, spectral, thermal, magnetic properties, and structural features of four new onedimensional (1-D) polymers having formula [M(4-npa)2(µ2-H2O)(H2O)2]·2H2O where (M=Mn 1; M=Co 2; M=Zn 3; M=Ni 4; 4-npa=4-nitrophenylacetate) are investigated and compared with corresponding 4-nitrobenzoate (4-nba) analogues. The single crystal studies reveal that compounds 1-3 are isostructural and crystallize in the centrosymmetric monoclinic space group P21/c. Their structure consists of distorted octahedra of an unique central metal ion, two crystallographically unique monodentate 4-npa ligands, two terminal water molecules, a µ2-bridging bidentate water and two lattice water molecules. The central metal exhibits a distorted {MO6} octahedron with two O atoms from terminal water molecules disposed cis to each other, two O atoms from the monodentate carboxylates also disposed in a cis geometry and the remaining two O atom from the µ2-bridging bidentate water disposed trans. The µ2-brindging mode of water leads to the formation of an extended 1-D chain structure. The lattice water links parrallel 1-D chains with the aid of H-bonding interactions. The IR, XRD powder pattern, and thermal characterization reveal that the structure of polycrystalline compound 4 is nearly identical with that of other 4-npa analogues.
Keywords: Extended structure, 4-nitrophenylacetic acid, Zinc, Manganese, isostructural, bridging water
1. Introduction The study of metal carboxylates is a topical area of research due to their biological significance [1, 2] and usefulness in material science [3-5]. As part of our metal-carboxylate research program, we are investigating the structural chemistry of substituted aromatic carboxylates of the s- and d-block metals. Based on our research studies, we have observed so far that the flexible nature of the oxophilic s-block cations to adopt to higher coordination assists in the formation of compounds with an extended structure even when monotopic aromatic carboxylic acids are used as main linkers [6-8]. Although, first row transition metal centers are preferred due to their well-known coordination behaviors, the extended structures are often built using multitopic primary ligands instead of monotopic aromatic carboxylate linkers as these normally lead to the formation of monomeric structures [9-15]. Hence it was a challenging task to synthesize extended structures using d-block metal cations in the media of such O-donor ligands. The construction of intriguing structural topologies depends upon how the primary ligand of given chemical nature and stereochemistry arranges itself around the central metal besides the coordination geometries of central metal. Since flexible coordination behavior of s-block metal cation with rigid ligand could yield extended structures, it was of interest to investigate the structural outcome when flexible ligand would combine with metals preferring fixed geometry. Hence replacement of 4-nba with 4-nitrophenylacetic acid (Fig. 1) where the –COOH group is more flexible and is attached to a –CH2 unit was a rational approach to serve this purpose as it did lead to a successful synthesis of compounds having extended structure. The synthesis, properties and structural investigation of four new compounds of relatively less explored 4-npa system are described in this paper.
Fig. 1
2. Experimental 2.1 Materials and Methods All the chemicals used in this study were of reagent grade and were used as received without any further purification. Infrared (IR) spectra of the solid samples diluted with KBr were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer from 4000-400 cm-1 at a resolution of 4 cm-1 . UV-Vis diffuse-reflectance spectra were recorded on a Shimadzu UV-2450 double beam spectrophotometer. BaSO4 powder was used as reference (100 % reflectance). Absorption data were calculated from the reflectance data using the Kubelka-Munk function (a/S=(1-R2/2R where a is the absorption coefficient, R the reflectance and S the scattering coefficient). Isothermal weight loss studies were performed in a temperature controlled electric furnace. TG-DTA study was performed in flowing oxygen in Al2O3 crucibles at heating rate of 10 K min-1 using a STA-409 PC simultaneous thermal analyser from Netzsch. Magnetization measurements were carried out on a Quantum design Versa Lab vibrating sample magnetometer in an external field up to 3 Tesla and from 50 K to 300 K. ESR study was carried out on a Bruker EMx/micro-x instrument at room temperature. The morphology of the zinc oxides was investigated by scanning electron microscope (Carl Zeiss Model No EVO18). N2 adsorption-desorption studies were performed on a Quantachrome NOVA 1000 surface and porosity instrument using N2 as a probe molecule at -196 o C. Prior to the measurements, the sample were degassed at 300 o C for 12 hours to remove any moisture and volatiles. The Brunauer-Emmett-Teller (BET) model was used to determine the specific surface area of the metal oxide samples. The pore size and pore volume of the samples were measured using the Barrett-Joyner-Halenda (BJH) method. .
2.2 Preparation of the complexes 2.2.1 Synthesis of [Mn(4-npa)2(µ2-H2O)(H2 O)2]·2H2O (1) A solution of sodium bicarbonate (0.840 g, 10 mmol) in water (30 mL) was added into 4-npaH (1.812 g, 10 mmol) to obtain the sodium salt of carboxylic acid. The solution was warmed on waterbath in order to dissolve the acid faster. To this solution, [Mn(H2O)4]Cl2 (0.990 g, 5 mmol) in water (20 mL) was slowly added. The resultant solution mixture was filtered and left aside for crystallization.
Transparent, pale brownish crystals appeared within 3-4 days which were collected by filtration in ~ 83% yield.
2.2.2 Synthesis of [Co(4-npa) 2(µ2-H2O)(H2O)2]·2H2O (2) The use of [Co(H2O)6]Cl2 instead of the Manganese salt gives compound 2 in 78 % yield. However during synthesis of compound 2 excess water (>200mL) should be used to avoid immediate precipitation of product. Alternatively compound 2 can be obtained by heating CoCO3 with 4-npaH in excess water. The reaction mixture is then filtered and left for crystallisation. Within a week, red coloured crystals suitable for single crystal analyses appear in the mother liquor which is then isolated by filtration.
2.2.3 Synthesis of [Zn(4-npa)2(µ2-H2O)(H2O)2]·2H2 O (3) A mixture of 4-npaH (1.812g, 10 mmol) and zinc oxide (0.407 g, 5 mmol) was taken in excess distilled water (~250 mL) and the reaction mixture was heated on a water bath. Brisk effervescence was observed and the heating was continued till most of the oxide reacted with acid. During the reaction, slight excess of ZnO was added to ensure that no 4-npaH remains unreacted. The hot reaction mixture was then filtered and heated to concentrate the volume to 100 mL and left aside for crystallization. White transparent crystals which separated in a few days were collected by filtration in ~ 90 % yield.
2.2.4 Synthesis of [Ni(4-npa)2(µ2-H2O)(H2O)2]·2H2O (4) The analogous nickel compound (compound 4) was also obtained in polycrystalline form in a similar manner like compound 3 by heating nickel carbonate with 4-npaH in excess water or by reacting NiCl2·6H2 O with sodium salt of 4-npaH in water.
2.3 X-ray crystallography Single crystal X-ray analysis of compounds 1-3 was done at the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology (IIT) Madras. X-ray intensity data were collected
using Bruker AXS Kappa Apex II CCD Diffractometer equipped with graphite monochromated Mo (Kα) (λ = 0.7107 Å) radiation, through an optimized strategy which gave an average 4-fold redundancy. The program APEX2-SAINT (Bruker, 2004) was used for integrating the frames. Fourfold redundancy per reflection was utilized for achieving good multi-scan absorption correction using SADABS (Bruker, 2004). Besides absorption, Lorentz, polarization and decay corrections were applied to intensity during data reduction. The structures were solved by direct methods using SIR92 and refined by full-matrix least squares techniques using SHELXL-97 [16, 17]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms attached to the aromatic ring were introduced in calculated positions and included in the refinement by riding on their respective parent C atoms. Crystallographic data and selected refinement results for complexes 1–3 are summarized in Table 1-3. Table 1- Crystal data and structure refinement for 1- 3 Empirical formula Formula weight (g mol-1) Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) β (°) Volume (Å3) Z Dcalc (mg/m3) Absorption coefficient (mm-1) F(000) Crystal size (mm3) θ range for data collection (°) Index ranges Reflections collected / unique Completeness to θ Absorption correction
Max. and min. Transmission Refinement method Data / restraints /
C16H22N2MnO13 (1) 505.30 296(2) 0.71073 Monoclinic P21/c
C16H22N2CoO13 (2) 509.28 293(2) 0.71073 Monoclinic P21 /c
C16H22N2ZnO13 (3) 515.73 293(2) 0.71073 Monoclinic P21/c
10.7951(3) 8.1873(2) 24.9212(6) 98.924(2) 2175.94(10) 4 1.542 0.676
10.7802(15) 8.0299(9) 24.629(3) 98.877(6) 2106.5(5) 4 1.606 0.887
10.8262(3) 8.0624(2) 24.6227(6) 98.7820(10) 2124.00(9) 4 1.613 10228
1044 0.35 x 0.35 x 0.32 1.91 to 26.00
1052 0.30 x 0.20 x 0.20 2.339 to 25.434
1064 0.35 x 0.30 x 0.25 1.90 to 26.00
-13 ≤ h ≤ 13 -10 ≤ k ≤ 10 -30 ≤ l ≤ 30 25996 / 4274 (R(int) = 0.0257) 100% Semi-empirical from equivalents 0.8193 and 0.7917
-12 ≤ h ≤ 12 -9 ≤ k ≤ 9 -0 ≤ l ≤ 29 3663 / 3663 (R(int) = 0.000) 94.1% Semi-empirical from equivalents 0.822 and 0.642
-13 ≤ h ≤ 13 -9 ≤ k ≤ 9 -30 ≤ l ≤ 30 34348 / 4168 (R(int) = 0.0652) 100% Semi-empirical from equivalents 0.7497 and 0.6710
Full-matrix leastsquares on F2 2538 / 12 / 174
Full-matrix leastsquares on F2 3663 / 63 / 353
Full-matrix leastsquares on F2 4168 / 15 / 328
parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole (e Å -3)
1.145 R1 = 0.0393, wR2 = 0.0910 R1 = 0.0499, wR2 = 0.1010 0.0070(5) 0.394 and -0.337
1.134 R1 = 0.0629, wR2 = 0.1888 R1 = 0.1181, wR2 = 0.1961 n/a 0.516 and -0.675
1.082 R1 = 0.0315, wR2 = 0.0800 R1 = 0.0488, wR2 = 0.0937 0.0054(5) 0.569 and -0.567
Table 2- Selected bond lengths [Å] and bond angles [o] for 1-3 Bond lengths [Mn(4-npa)2(H2 O)3]·2H2O (1) Mn1-O11 2.1372(18) Mn1-O5 2.1446(17) Mn1-O1 2.1496(17) Mn1-O10 2.1504(18) Mn1-O9 i 2.2116(16) Mn1-O9 2.3447(16)
[Co(4-npa)2(H2O)3]·2H2O (2) Co1-O3W 2.045(4) Co1-O1 2.046(4) Co1-O2W 2.049(4) Co1-O5 2.051(5) Co1-O1W 2.232(4) Co1-O1Wii 2.232(4)
[Zn(4-npa)2(H2O)3 ]·2H2 O (3) Zn1-O6 2.0575(16) Zn1-O2 2.0576(17) Zn1-O3 2.0577(18) Zn1-O4 2.0613(17) Zn1-O1 2.1252(15) Zn1-O1iii 2.2962(16)
Bond angles O11-Mn1-O9 O11-Mn1-O10 O1-Mn1-O9 O10-Mn1-O9 O5-Mn1-O9 i O11-Mn1-O5 O5-Mn1-O9 O5-Mn1-O1
85.52(6) 87.62(8) 87.97(6) 88.85(7) 89.25(6) 89.36(8) 89.79(6) 91.25(7)
O1-Mn1-O9 i O1-Mn1-O10 O10-Mn1-O9i O11-Mn1-O9i O11-Mn1-O1 O5-Mn1-O10 O9-Mn1-O9 i
91.43(6) 91.62(8) 92.14(7) 95.08(7) 173.47(7) 176.78(8) 178.86(4)
O3W-Co1-O1 O3W-Co1-O2W O1-Co1-O2W O3W-Co1-O5 O1-Co1-O5 O2W-Co1-O5 O3W-Co1-O1W O1-Co1-O1W
176.12(18) 86.57(18) 90.17(18) 91.99(18) 91.32(17) 177.86(15) 91.56(15) 86.54(15)
O2W-Co1-O1W O5-Co1-O1W O3W-Co1-O1Wii O1-Co1-O1Wii O2W-Co1-O1Wii O5-Co1-O1Wii O1W-Co1-O1Wii
93.60(16) 88.02(15) 88.74(15) 93.14(15) 86.08(15) 92.31(15) 179.54(4)
O2-Zn1-O1 iii O2-Zn1-O3 O6-Zn1-O1 O3-Zn1-O1 iii O4-Zn1-O1 O6-Zn1-O2 O4-Zn1-O1 iii O6-Zn1-O4
85.34(7) 86.20(8) 87.74(6) 88.06(7) 89.84(7) 90.25(8) 90.29(6) 91.14(7)
O6-Zn1-O1iii O3-Zn1-O4 O3-Zn1-O1 O2-Zn1-O1 O2-Zn1-O4 O6-Zn1-O3 O1-Zn1-O1iii
91.57(6) 92.40(8) 92.63(7) 94.55(7) 175.45(7) 176.45(8) 179.30(2)
Symmetry transformations used to generate equivalent atoms: i) -x+2, y+1/2, -z+1/2; ii) -x+2, y-1/2, -z+1/2; iii) -x+1, y+1/2, -z+1/2
Table 3- Hydrogen bonding geometry [Å and °] for 1-3 D-H···A
d(D-H)
d(H···A)
d(D···A)
Symmetry code
2.56 1.803(18) 1.775(18) 2.63(3) 1.764(17) 1.843(19) 1.822(17) 1.788(17) 2.11(2) 1.94(2) 1.97(3) 2.06(2)
3.431(4) 2.680(2) 2.647(3) 3.124(2) 2.630(2) 2.697(2) 2.703(3) 2.676(3) 2.934(4) 2.752(3) 2.733(3) 2.873(4)
155.6 175(3) 171(3) 117(2) 166(3) 161(3) 179(3) 174(3) 155(4) 154(4) 145(4) 156(4)
-x+2, -y, -z+1 -x+2, y-1/2, -z+1/2 -x+2, y+1/2, -z+1/2 x, y, z x, y, z x, y, z x, y, z -x+2, y-1/2, -z+1/2 -x+1, -y, -z -x+2, y+1/2, -z+1/2 x, y, z x-1, -y-1/2, z-1/2
1.98(4) 2.54(5) 1.806(19) 1.888(18) 1.94(2) 1.88(2) 1.84(4) 1.954(19) 1.89(2) 2.53 2.38 2.62 2.12(13) 2.35(6) 1.94(3) 1.94(3) 2.20(5) 2.24(6) 2.33(9)
2.678(6) 3.091(5) 2.621(6) 2.661(5) 2.669(16) 2.673(13) 2.58(4) 2.729(9) 2.662(6) 3.387(10) 3.18(2) 3.308 2.58(4) 2.85(3) 2.725(9) 2.728(15) 2.89(2) 2.729(13) 2.88(2)
143(5) 125(4) 168(4) 164(5) 151(4) 171(3) 154(4) 165(3) 159(5) 153.7 139.4 131.3 115(11) 120(6) 157(7) 160(8) 141(7) 119(6) 125(8)
-x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2, y-1/2, -z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2,y+1/2,-z+1/2 -x+2, y-1/2, -z+1/2 -x+2, -y, -z -x+3, y+1/2, -z+1/2 -x+3,-y,-z+1 -x+2, y-1/2, -z+1/2 x-1,-y-1/2,z-1/2 x,y,z x,y,z x+1,-y-1/2,z+1/2 x,y,z x,y,z
2.62 2.55 1.91(2) 2.03(2) 2.11(2) 1.826(18) 1.90(2) 1.774(17) 1.816(17) 1.799(17) 1.821(18) 2.59(3) 1.744(18)
3.359(5) 3.406(4) 2.724(3) 2.853(4) 2.918(4) 2.700(3) 2.726(3) 2.657(3) 2.686(3) 2.675(3) 2.671(2) 3.093(2) 2.617(2)
133.1 153.8 154(4) 158(4) 154(4) 173(3) 154(4) 178(3) 177(3) 177(3) 161(3) 117(3) 168(3)
-x, y-1/2, -z+1/2 -x+1, -y+1, -z+1 -x+1, y-1/2, -z+1/2 -x+2, -y+1, -z -x, -y+1, -z x, y, z -x+1, y-1/2, -z+1/2 x, y, z -x+1, y+1/2, -z+1/2 -x+1, y+1/2, -z+1/2 -x+1, y-1/2, -z+1/2 -x+1, y-1/2, -z+1/2 -x+1, y-1/2, -z+1/2
[Mn(4-npa)2(H2 O)3]·2H2O (1) C13-H13···O6 O11-H11A···O1 O11-H11B···O13 O9-H9A···O1 O9-H9A···O2 O9-H9B···O6 O10-H10C...O12 O10-H10D···O5 O12-H12A···O3 O12-H12B···O6 O13-H13A···O2 O13-H13B···O7
0.93 0.878(18) 0.878(18) 0.885(17) 0.885(17) 0.888(17) 0.882(17) 0.891(17) 0.876(18) 0.879(18) 0.880(18) 0.863(19)
[Co(4-npa)2(H2O)3]·2H2O (2) O1W-H1W1···O2 O1W-H1W2···O5 O1W-H1W2···O6 O2W-H2W1···O5 O2W-H2W2···O5WA O2W-H2W2···O5WB O3W-H3W1···O4WA O3W-H3W1···O4WB O3W-H3W2···O1 C6-H6···O2 C7-H7B···O7A C13-H13···O8 O4WA-H4W2···O3W O4WB-H4W3···O8A O4WB-H4W4···O2 O5WA-H5W1···O6 O5WA-H5W2···O3 O5WB-H5W3···O6 O5WB-H5W4···O3A
0.815(18) 0.828(18) 0.828(18) 0.794(17) 0.803(18) 0.803(18) 0.794(18) 0.794(18) 0.811(17) 0.93(2) 0.97(2) 0.93 0.82(2) 0.826(19) 0.832(19) 0.819(19) 0.82(2) 0.82(2) 0.82(2)
[Zn(4-npa)2(H2O)3 ]·2H2 O (3)
C2-H2A···O10 C5-H5···O7 O13-H13A···O5 O13-H13B···O8 O12-H12A···O11 O3-H3A···O12 O12-H12B···O7 O2-H2D···O13 O2-H2C···O4 O3-H3B···O6 O1-H1A···O7 O1-H1B···O4 O1-H1B···O5
0.97 0.93 0.879(19) 0.863(19) 0.868(18) 0.878(16) 0.881(18) 0.883(17) 0.871(17) 0.877(17) 0.884(17) 0.887(17) 0.887(17)
D= Donor and A= Acceptor
3. Results and Discussion 3.1 Synthetic aspects, spectral and thermal studies Compounds 1, 2, and 3 are prepared by reacting the corresponding metal chlorides with sodium salt of 4-npaH in water. Alternatively, compound 2 can also be prepared by direct reaction of metal carbonate with free acid, where the reaction mixture was heated in water to obtain clear solution. Compound 3 is obtained by heating ZnO with 4-npaH in water. The Infrared and Raman spectra (Fig. S1& S2) of 1-3 are identical; therefore is a first indication of possible structural similarity in all four compounds. A comparison of IR spectra of complexes with that of the free acid reveals that the band due –COOH group of the free acid occurring at 1711 cm-1 is shifted to lower energies due to the formation of metal carboxylate . The presence of water in 1-3 can be evidenced by the characteristic profile of the spectra in the 3500-3000 cm-1 region. The inverted overlap of infrared and Raman spectra (Fig. 2) shows the presence of strong bands in both the spectra’s and is assigned to symmetric stretch of -NO2 vibrations occurring at 1339 cm-1. Comparatively less intense band of asymmetric NO2 stretch in IR appears as second intense band in Raman at 1597cm-1. The third intense band seen in Raman spectra represents the C-N stretching vibrations occurring at 1103 cm-1. All these bands are present in the free acid as well at 1601, 1343, and 1103 cm-1 and not major changes in these values are seen when acid is complexed with metal.
Fig. 2 The inverted overlap of IR and Raman spectra showing strong νs NO2
The TG-DTA curves (Fig. S3-S5) of 1, 2 and 3, exhibit three thermal events for all compounds. The first endothermic event (96 oC for 1; 116 oC for 2; 95 oC for 3) can be assigned to the dehydration process. Upon heating at 120 oC in an oven, these compounds show weight loss, between 16-17%, that corresponds to loss of five water molecules. The formation of an anhydrous compound is confirmed by devoid of the strong O-H signal at 3600 cm-1 in their respective IR spectra (Fig. S6-S8) which reappear when compounds are equilibrated over water for 4-5 days showing successful rehydration. The last event is exothermic and represents the decomposition of organic ligand. Due to the absence of associated mass spectral data of the emitted fragments, the exact nature of ligand decomposition process cannot be further commented. The high decomposition temperature (366 oC for 1; 367 oC for 2; 465 oC for 3) of Zn complex could be due to more stability of these metal in tetrahedral coordination compared to Mn and Co. The Diffused reflectance spectra (DRS) of free acid show two charge transfer bands at 257 and 325 nm, and these bands also appear in all three complexes of acid (Fig. S9). At lower intensities (Fig. S10), d-d absorption become visible showing very weak absorption for compound 1 due to doubly forbidden (Laporte and Spin) nature of transition bands associate with d5 system. For compound 2, which is a d7 system, bands are comparatively intense, and three spin allowed d-d transitions namely 4 T1g (F) → 4 T2g (F), 4 T1g (F) → 4 T1g (P), and 4 T1g (F) → 4 A2g(F) occur in visible region at 530, 484, and 450 nm respectively. No d-d transitions are associated with compound 3 which is a d10 system. 3.2 X-ray crystal structure determination Compounds 1-3 crystallize in the centrosymmetric monoclinic space group P21/c and are isostructural. In view of the isostructural nature, the structure is described for the Mn(II) analogue. The crystal structure (Fig. 3) of 1 consists of a central divalent Mn(II), two crystollographically unique monodentate 4-npa ligands, two terminal water molecules, a bridging water molecule and two lattice water molecules (O12 and O13) with all atoms located on general positions. The central metal is bonded to six O atoms resulting in a {MO6} (M=Mn, Co or Zn) coordination sphere. Of these two O atoms (O1 and O5) come from the unique 4-npa ligands which are disposed cis to each other. The oxygens of the two terminal water molecules (O10 and O11) function as monodentate lignads and
adopt a cis orientation. The bridging water (O11) functions as a µ 2-bidentate bridge and is linked to two symmetry related Mn(II) ions. Thus each Mn(II) is bonded to O9 and O9i which are disposed trans at Mn-O distances of 2.3447(16) and 2.2116(16) Å respectively. It is this binding mode which is responsible for the 1-D structure of this compound (and the analogues 2 and 3) as can be evidenced by the formation of a water linked chain connecting {Mn(4-npa)2(H2 O)2} units along b axis (Fig. 4). The Mn-O bond distances vary between 2.1372(18) and 2.3447(16) Å. The cis O-Mn-O angles range from 85.52(6) to 91.62(8) while the trans O-Mn-O angles scatter in a range from 173.47(7) to 178.86(4) indicating the distortion of the {MnO6} octahedron. The M-O bond distances and O-M-O bond angles (Table 2) in 2 and 3 can be explained similarly. An analysis of the crystal structure reveals that the H atoms attached to the coordinated waters O9-O11 and the lattice water O12 and O13 and the O atoms of the -COO- and -NO2 groups of the 4-npa anions function as H-acceptors (Table 3). The most interesting aspect of the H-bonding interaction is the role of O123 and O13 which link adjacent 1-D chains with the aid of interchain O-H···O interactions (Fig. 4)
Fig. 3 The crystal structure of [Mn(H2O)2(4-npa)2(µ-H2O)]·2H2O (1) showing the atom labelling scheme and the six coordination around Mn(II). Displacement ellipsoids are drawn at the 50% probability level excepting for the H atoms, which are shown as circles of arbitrary radius. Symmetry code: i) -x+2, y+1/2, -z+1/2
Fig. 4 A view of an infinite chain along b-axis due to the µ2-bridging bidentate coordination modes of water (top); Linking of two infinite chains along b-axis by means of lattice (O12, O13) water (bottom).
3.3 Characterization and structural investigations of Ni-4npa complex (4) The IR spectra (Fig. S11) of compound is identical with rest of the compounds and TG-DTA curves (Fig. S12) show the similar profile with three DTA events where the first event can be assigned to dehydration process which can be evidenced from weight loss of 16.78% in TG curve. The diffused reflectance spectra (Fig. S13) shows intra ligand charge transfer bands at 261 and 308 nm which is typical of free acid chromophore. Spin allowed d-d transitions are seen at lower intensities as merged broad band between 600-800nm. The Ni (II) compound has composition and formula same as other analogues as indicated by elemental analysis. The X-ray powder pattern of Ni-4npa complex (Fig. 5) was compared with theoretically generated powder pattern from cif file of compound 2. A comparison shows high similarity between two patterns. Therefore base on elemental composition, IR, thermal and XRD powder pattern Ni (II) compound is expected to have structure very similar with the remaining isostructural analogues. However the Ni compound is obtained in polycrystalline form and is expected to have deformaties in its ordered arrangement which are absent in the remaining crystalline analogues having same composition
Fig. 5 A comparison of X-ray powder pattern of Ni-4npa complex (top) with that of theoretically generated powder pattern from cif file of compound 2 (bottom)
3.5 Similarity and differences between 1-4 and corresponding 4-nba compounds A comparative study of the structural chemistry of corresponding complexes of rigid 4-nba (Table 4) with Mn, Co, Ni, Zn is presented. The coordination numbers of the bivalent 3d metals excepting Zn is six when complexed with both rigid 4-nba as well as flexible 4-npa ligand which is a typical of dblock metal. It is interesting to note that Zn(II) which is well known for tetra coordination exhibits six coordination with flexible ligand forming water bridged 1-D extended chain, isostructural with remaining 4-npa analogues of Mn, Co and Ni. In case of 4-nba, Mn, Co, and Ni form monomeric and isostructural compounds with formula [M(4-nba)2(H2O)2]·2H2O in which 4-nba ligand is monodentate while a recently redetermined structure by Necefoglu et al [15] of [Zn(4-nba)2(H2O)2] reveals that in addition to monodentate coordination, 4-nba coordinates to Zn(II) cation with considerably weaker secondary bond thereby giving zinc(II) intermediate coordination geometry between octahedral and tetrahedral. The flexible ligands are disposed cis while rigid ligands lie trans around hexacoordinated metal in the their corresponding complexes. In the case of 4-npa compounds only bridging waters are disposed trans to one another in the coordination sphere of central metal Table 4 Structural details of selected transition metal 4-nba complexes Compound S. G. C.N Binding modes of Structure ligand type
Reference
[Mn(4-nba)2(H2O)4]·2H2O [Co(4-nba)2(H2O)4]·2H2O [Ni(4-nba)2(H2O)4]·2H2O [Ni (H2O)6] (4-nba)2·2H2O [Zn(4-nba)2(H2O)2]
Pī Pī Pī C2/c C2/c
6 6 6 6 4-6
Monomer Monomer Monomer Monomer Monomer
11 12 11 13 15
[Mn(4-npa)2(H2O)3]·2H2O [Co(4-npa)2(H2O)3]·2H2O [Zn(4-npa)2(H2O)3]·2H2O
P21/c P21/c P21/c
6 6 6
1-D Polymer 1-D Polymer 1-D Polymer
This work This work This work
Mondentate Mondentate Mondentate Ionic Mondentate with weak secondary interactions Monodentate Monodentate Monodentate
S. G. = Space group; C.N. = Coordination number
The ESR spectrum of 1 and reported 4-nba analogue [Mn(4-nba)2(H2O)2]·2H2O (Mn-4nba) exhibits a isotropic signal centered at g = 2.00 (Fig. 6). This indicates that metal centered are far apart for any interactions in solid state. However the broadness of the signal in case of Mn-4nba is due to spin exchange and dipolar interactions.
Fig. 6 ESR spectra of 1 (For ESR spectra reported [Mn(4-nba)2 (H2O)4]·2H2O see Fig. S14)
The magnetic studies of 1 (Fig. 7, 8) and corresponding Mn-4nba complex (Fig. S15, S16) reveal paramagnetic behavior for at and below room temperature as shown in magnetization verses temperature and field i.e. M-T and M-H curves of both the compounds. The same behavior was also observed in case of complex 2 and corresponding Co-4nba complex in their respective magnetization curves (Fig. S17-S20). A magnetic moment of ~5.83BM and 4.90 BM is found for 1 and 2 respectively indicating a high spin nature of Mn(II) and Co(II). It is found that magnetization values are slightly higher for corresponding 4-nba complexes at lower temperatures than that of 4-npa complexes. The magnetization curves of Ni-4nba complexes also have the similar trend (Fig. S21, S22) showing paramagnetic behavior at and below room temperature. The magnetic moment of 2.03 BM was observed at and below room temperature for Ni-4nba complex
Fig. 7 Magnetization vs Temperature (M-T) curve of compound 1
Fig. 8 Magnetization vs Field (M-H) curve of compound 1
However, in case of complex 4 (Fig. 9) the splitting of FC and ZFC curve at low temperature clearly indicate some magnetic interaction in the complex. However these interactions are visible only at low magnetic field of 0.5 T and disappear when M-T measurement is carried out at higher field of 1 T. These could be due to presence or clustering of magnetic domains in certain regions of solids. Weak hysteresis loop is observed for this sample when M-H measurement is carried out at 50 K indicating weak ferromagnetic interactions at low temperature (Fig. 9). The values of magnetic moment obtained for these complexes are very low than expected i.e 1.13 BM at 300K, 1.37BM at 150K and 1.58BM at 50K. The magnetic moment value is found to increase as the temperature is lowered.
Fig. 9 The M vs T plot (top) and M vs H plot (bottom) of compound 4
The surface morphologies of zinc oxides obtained from the decomposition of complex 3 and [Zn(4-nba)2(H2O)2] are studied using SEM. These images are also compared with the SEM image of standard ZnO used for the synthesis of these complexes (Fig. 10). SEM images reveal that the standard Zinc oxide (ZnOS) material which is used as a starting material is non-porous with firmly held or consolidated particles. The SEM image of Zinc oxide (ZnOA) obtained from decomposition of [Zn(4-nba)2(H2O)2] show a porous morphology.
Fig. 10 The SEM images of standard ZnO (top), ZnO from [Zn(4-nba)2(H2O)2] (middle), and ZnO from compound 3 (bottom)
Similarly zinc oxide (ZnOB) obtained from decomposition of 3 also shows porous nature. In addition to this the SEM image reveals that particles have definite geometric shape. The morphology of oxide was dependent on ligand size which was further supported by BET and pore data (Table 4). During calcinations the escaping ligand created hollow voids in the ZnO structure. A comparative surface analysis of ZnOA and ZnOB with that of standard ZnOS was done and interestingly the size of ligand and pore diameter, pore volume were in good agreement. The N2 adsorption-desorption isotherms revealed the mesoporous nature of the prepared Zinc oxides (ZnOA and ZnOB). Specifically isotherms were identified as type IV isotherms with a H2 hysteresis loop for ZnOA and H3 hysteresis loop for ZnOB which is characteristic of ink-bottle and slit-shaped pores respectively[18]. Table 5. BET surface area and pore data Zinc Oxides BET Surface area (m2/g)
Pore diameter (nm)
Pore volume (mL/g)
ZnOS ZnOA ZnOB
1.736 2.815 3.115
0.005 0.014 0.032
4 14 28
Conclusion All three complexes of 4-npa with Mn, Co, and Zn are isotructural and based on powder pattern, IR, and thermal data corresponding polycrystalline Ni analogue is also expected to have structure very similar to the remaining analogues. Use of flexible 4-npa ligand in place of rigid 4-nba has led to successful synthesis of 1-D polymeric structure as evidenced from comparative table which shows monomeric nature for reported 4-nba compounds of same bivalent d-block cation. The ZnO obtained from decomposition of corresponding 4-nba and 4-npa complexes shows porous nature.
Supplementary Information CCDC 1437821–1437823 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via the Cambridge Crystallographic Data Centre (CCDC) via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:
Acknowledgments The author like to thank the Sophisticated Analytical Instrument Facility (SAIF), Indian Institute of Technology (IIT) Madras for single crystal X-ray analysis of 1, 2 and 3 reported in this paper .The author also acknowledges helpful discussions with research supervisor Prof. B. R. Srinivasan and research colleagues Kedar Narvekar, Daniel Cutinho and Celia Bragranza from Goa University. Financial support under the Special Assistance Programme (DSA-1) of the University Grants Commission, New Delhi is gratefully acknowledged.
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The investigation of the structural outcome upon reaction of flexible ligand with metals preferring fixed geometry
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The replacement of rigid 4-nitrobenzoic acid by flexible 4-nitrophenyl acetic acid lead to successful synthesis of 1-D polymeric structure
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All four compounds of 4-nitrophenyl acetic with corresponding Mn, Co, Ni, Zn have similar structure. Comparative study of structure and properties with corresponding 4-nitrobenzoate analogues is reported.
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