Polyhedron 27 (2008) 3359–3370

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Polyhedron

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An attempt towards coordination supramolecularity from Mn(II), Ni(II)and Cd(II) with a new hexadentate [N4O2] symmetrical Schiff base ligand:Syntheses, crystal structures, electrical conductivity and optical properties

Saikat Sarkar a, Susobhan Biswas b, Meng-Sheng Liao c, Tapas Kar d,*, Yildirim Aydogdu e, Fethi Dagdelen e,Golam Mostafa f, Asoke Prasun Chattopadhyay b, Glenn P.A. Yap g, Rui-Hua Xie h, Abu T. Khan i,Kamalendu Dey b,*

a Department of Chemistry, Santipur College, Santipur 741 404, West Bengal, Indiab Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, Indiac Department of Chemistry, P.O. Box 17910, Jackson State University, Jackson, MS 39217, USAd Department of Chemistry and Biochemistry, Utah State University, Logan, 84322-0300, USAe Department of Physics, Faculty of Arts and Sciences, Firat University, 23169 Elazig, Turkeyf Department of Physics, Jadavpur University, Jadavpur, Kolkata 700 032, Indiag Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USAh Department of Applied Physics, Xi’an Jiaotong University, Xi’an 710049, Chinai Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 May 2008Accepted 20 July 2008Available online 19 September 2008

Keywords:Schiff baseSelf-assemblyCrystal structureActivation energyDFT

0277-5387/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.poly.2008.07.034

* Corresponding authors. Tel.: +91 33 25828750x(K. Dey).

E-mail addresses: saikat_s@rediffmail.com (S.(T. Kar), kdey_chem@rediffmail.com (K. Dey).

To explore the influence of non-covalent weak force interactions, mainly exerted by carboxylic groups, onthe formation of supramolecular architectures of transition metal complexes and their electrical conduc-tion processes, a new symmetrical [N4O2] hexadentate Schiff base ligand, 1,8-N-bis(3-carboxy)disalicy-lidene-3,6-diazaoctane-1,8-diamine, abbreviated to H4fsatrien, and its complexes of Ni(II), Cd(II) andMn(II) have been synthesized using in situ condensation of the ligand components in the presence ofmetal ions. The complexes were structurally characterized by elemental analyses, IR, UV–Vis, NMR,ESR, molar conductivity and magnetic measurements. The crystal structures of all the complexes havebeen determined by a single crystal X-ray diffraction study. The 1-D, 2-D and 3-D networks of the com-plexes are formed by p–p stacking, C–H� � �p interactions and mono or bifurcated H-bonding. The elec-tronic structures of the complexes have been examined using the DFT method. Solid-state properties(e.g. electrical conductivity at different temperatures and optical properties) of the Ni(II) and Mn(II) com-plexes have also been studied and, depending on the temperature, the conductivity of the complexes isfound to be insulating and semiconducting (intrinsic and extrinsic) in nature. The optical band gap (Egd)of complexes (1) and (3) is found to be 2.57 and 2.30 eV, respectively.

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1. Introduction

The chemistry of metallosupramolecular species has been gain-ing considerable interest primarily because of their fascinatingstructural diversities [1] (catenanes, rotaxanes, knots, grids, lad-ders, racks, molecular squares and boxes, cubes, helicates, etc.).Metal ions and organic ligands are the two parent and necessarycomponents for designing discrete or polymeric/supramolecularcoordination architectures. In addition to coordination bonding[2], several types of non-covalent interactions, such as hydrogen

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306; fax: +91 33 25828282

Sarkar), tapas.kar@usu.edu

bonding [3], p–p stacking [4] and CH� � �p interactions [4] areresponsible for the growth of molecular building blocks by linkingthe multicentre discrete monomers. Such self-assembled species,due to their intriguing topologies, have found application in semi-conductivity, superconductivity, charge transfer, magnetism, nano-porous and biomimetic materials [5,6].

Several kinds of organic ligands have been designed quite intel-ligently, which play the key role in creating coordinating supra-molecules [7]. One of these kinds consists of symmetric andasymmetric Schiff bases derived from different amines. Metal com-plexes of such Schiff base ligands are being used as spin cross over(SCO) molecular magnetic materials and bistable molecular mate-rials [8]. Such ‘materials with memory’ [9] have been synthesizedbased on light, pressure and temperature-dependent switchingover phenomenon between high-spin (HS) and low-spin (LS) states

3360 S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370

of the metal ions. Schrauzer and Windgassen [10] found that inor-ganic metal complexes might exhibit conducting properties, whichare nowadays the subject of an active research area both in thefield of solid state physics and inorganic chemistry; especially innano-chemistry [11]. Conducting and semiconducting compoundshave attracted considerable focus because of their useful functionin molecular electronics and vital biological life processes [12].Such semiconducting metal complexes stand on the threshold ofa bright and exciting future. One of the most important aspectsof the studies of electronic characteristics of such compounds isthe relationship to their chemical structures, which is yet to be ex-plored in detail. It is also considered that different applicationsmight be found for supramolecular inorganic semiconductors,which do not necessarily parallel to the present uses for organicand discrete inorganic semiconducting materials. Much attentionhas also been paid to the optical properties of such compounds be-cause of their application in material chemistry, particularly inoptoelectronics. There are very rare reports on Schiff base [13]complexes that have such properties. Their preparation is still ex-tremely challenging.

Keeping in view the aspects stated above, which open up new vis-tas for synthesizing unique metal complexes of both fundamentaland technological importance, our aim is to prepare new Schiff basemetal complexes and to tune their properties. We report here thesynthesis and characterization of a new hexadentate N4O2 donorsymmetrical Schiff base ligand having two carboxylic acid groupsand its metal complexes, paying special attention to their X-ray crys-tal structures, electrical conductivity and optical properties. In allthe three complexes prepared, both carboxylic acid groups remainnon-coordinated to the metal ions, but surely exert their influenceon the formation of mono- and bifurcated hydrogen bondings,which help to build up the supramolecular architecture and its per-suasion in conductivity. This is in contrast to similar disalicylidenetype complexes [14] that have no carboxylic acid moiety.

Density functional theory (DFT) calculations have also been car-ried out to optimize the structures and comparisons have beenmade with the experimental geometries. Moreover, the theoreticaloutcome of the DFT approach gives insight into the electronicstructure and spectral properties of the complexes.

2. Experimental

2.1. Materials and general methods

Triethylenetetramine was purchased from Aldrich and used assuch. 3-Formylsalicylic acid was synthesized according to Duff andBills [15]. Spectrograde solvents were used for physical measure-ments. All the other reagents and solvents were purchased fromcommercial sources, purified and dried by standard procedures [16].

2.2. Physical measurements

The elemental analyses were carried out on Elementar Vario ELIII, Carlo Erba 1108 elemental analyzers at the Sophisticated Ana-lytical Instrument Facility (SAIF), Central Drug Research Institute,Lucknow. The electronic spectra (in DMSO and other media as indi-cated in the text) were recorded on a Shimadzu UV-2401PC spec-trophotometer and infrared spectra (KBr) on a Perkin–ElmerL120-000A spectrophotometer. The ESR spectrum was measuredin an ESR spectrometer at SAIF, IIT Madras. The molar conductance(10�3 M in DMSO) was measured using an Elico conductivitybridge and the magnetic data were measured on a Guoy Balance.

The samples for electrical conductivity measurements wereprepared from the complexes in the form of tablets; their thicknesswas 0.1 cm at a pressure of approximately 1 � 108 Pa. These tablets

were placed between two copper electrodes with silver paste andcontacts were tested to be ohmic. The electrical conductivities ofthe prepared complexes were measured with a Keithley 6514 sys-tem electrometer, by applying a dc voltage using a Keithely 230programmable voltage source. The conductivities were calculatedby using the general equation r = (I/Vc)(d/a), where (I) is the cur-rent in amperes, Vc the potential drop across the sample of cross-sectional area (a) and thickness (d). Optical absorption spectra (inthe solid state) were taken using a Perkin–Elmer Lambda 2S Dou-ble Beam spectrophotometer.

2.3. Computational details

The calculations were carried out using the Amsterdam densityfunctional (ADF) program package [17]. Triple-f STO (Slater-typeorbital) basis sets with one additional polarization function wereused throughout. [Ar3d], [Ne] and [He] core definitions were usedfor Cd, Ni/Mn and C/N/O, respectively. Hence the valence set on themetals includes the (n � 1)s and (n � 1)p shells. The density func-tional chosen for the calculations was BP (Becke’s gradient correc-tion for exchange [18] plus Perdew’s gradient correction forcorrelation [19]), which has been shown to yield satisfactorynumerical results in previous computations. Electron excitationenergies related to the electronic absorption spectra were calcu-lated using the time-dependent density functional response theory(TDDFT) [20] as implemented in the ADF program. The recent imple-mentation of TDDFT in the updated ADF program allows calcula-tions of excitation energies for open-shell systems. Calculationsof open-shell systems were spin-unrestricted. The spin multiplici-ties (2S + 1) for the Ni-, Cd- and Mn-complexes are 3, 1 and 6,respectively.

2.4. Preparation of the ligand. 1,8-N-Bis(3-carboxy)disalicylidene-3,6-diazaoctane1,8-diamine, (H4fsatrien)

3-Formylsalicylic acid (3-fsa) (1.66 g, 10 mmol) was dissolvedin methanol (25 ml), to which triethylenetetramine (trien)(0.73 g, 5 mmol) taken in methanol (20 ml) was added dropwisewith stirring. Immediately, a yellow solution was obtained. Glacialacetic acid (3–4 drops) were then added to it with stirring to facil-itate completion of the condensation reaction; stirring was contin-ued for 6 h at �10 �C until the separation of a yellow powderycompound at room temperature. It was filtered off, washed withmethanol and ether, and dried in vacuo over fused CaCl2. Yield25%. Anal. Calc. for C22H26N4O6 (H4fsatrien): C, 59.72; H, 5.92; N,12.66. Found: C, 59.63; H, 5.89; N, 12.71%. IR (KBr, cm�1): 3412,3290 (mOH, mNH), 1657 (mC@N). UV–Vis (DMSO, kmax, nm (e,L M�1 cm�1)): 265, 360, 495 (1128), 895 (23). 1H NMR (DMSO-d6,300 MHz, ppm, 298 K): d 12.28 (2H, s, COOH), 10.8(2H, s, OH),8.94 (2H, s, CH@N), 7.91 (2H, d, Ar-H), 7.33 (2H, d, Ar-H), 6.67(2H, t, Ar-H), 6.2 (2H, br, NH), 3.00–3.83 (8H, m, CH2), 2.50–2.79(4H, m, CH2). 13C NMR (DMSO-d6, 300 MHz, ppm, 298 K): d 172(COOH), 161.5 (C@N), 167.1, 139.1, 136.6, 130.3, 116.93, 114.09(Ar-C), 51.2, 50.2, 49.15 (CH2).

Due to the poor yield of the ligand, we used the in situ reactionprocedure for the synthesis of the metal complexes.

2.5. Syntheses of the metal complexes. [M(H2fsatrien)] (M = Ni (1), Cd(2) and Mn (3))

A solution of the ligand H4fsatrien was prepared by the methoddescribed above, by reacting 3-formylsalicylic acid (1.66 g,10 mmol) and trien (0.73 g, 5 mmol) in methanol (45 ml). The me-tal(II) acetate (5 mmol), dissolved in minimum volume ofmethanol (15 ml), was then added dropwise to the Schiff base solu-tion with stirring, and the mixture was kept under reflux for 1 h.

S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370 3361

Precipitation occurred for all of the above complexes during therefluxing, and the precipitates were collected by filtration underthe hot conditions. The precipitates were washed thoroughly withmethanol and diethyl ether, and dried in vacuum over fused CaCl2.The concentration, cooling and slow evaporation of the mother li-quor produced crystalline compounds, suitable for single X-raycrystal study, for the nickel(II) and cadmium(II) complexes. TheX-ray quality crystal of the manganese(II) complex was obtainedby slow evaporation of a solution of the complex in methanol–dichloromethane–acetonitrile. Yield 70–85%. Anal. Calc. forC22H24N4O6Ni (1): C, 52.94; H, 4.85; N, 11.22. Found: C, 52.89; H,4.93; N, 11.16%. IR (KBr, cm�1): 3425, 3266 (mOH, mNH), 1644 (mC@N).UV–Vis (DMSO, kmax, nm (e, L M�1 cm�1)): 385 (261), 585 (9.1), 850(20.4), 990 (15.9); (Nujol, kmax, nm): 350, 400 (sh), 540 (sh), 594(sh), 822 (sh), 940, 1025. Molar conductivity (DMSO): KM

= 14.1 X�1 mol�1 cm2. Magnetism (solid state, room temperature):leff = 3.05 BM. Colour: snuff.

Anal. Calc. for C22H24N4O6Cd (2): C, 47.79; H, 4.37; N, 10.13.Found: C, 47.83; H, 4.41; N, 10.19%. IR (KBr, cm�1): 3420, 3275(mOH, mNH), 1637 (mC@N). UV–Vis (DMSO, kmax, nm (e, L M�1 cm�1)):267, 353. Molar conductivity (DMSO): KM = 15.3 X�1 mol�1 cm2.Magnetism (solid state, room temperature): Diamagnetic. 1HNMR (DMSO-d6, 300 MHz, ppm, 298 K): d 8.49 (2H, s, CH@N),7.15 (2H, d, Ar-H), 6.81 (2H, d, Ar-H), 6.62 (2H, t, Ar-H), 6.08 (2H,br, NH), 3.02–3.44 (8H, m, CH2), 2.41–2.70 (4H, m, CH2). 13CNMR (DMSO-d6, 300 MHz, ppm, 298 K): d 176 (COOH), 169.2(C@N), 168.6, 140.1, 133.3, 126.5, 119, 112.2 (Ar-C), 55.3, 49.1, 46(CH2). Colour: off white.

Anal. Calc. for C22H24N4O6Mn (3): C, 53.34; H, 4.88; N, 11.31.Found: C, 53.29; H, 4.92; N, 11.35%. IR (KBr, cm�1): 3451, 3264(mOH, mNH), 1640 (mC@N). UV–Vis (DMSO, kmax, nm (e, L M�1 cm�1)):241 (10889), 262.5 (28752), 348.5 (18894), 867 (87), 874.5 (85);(DMF, kmax, nm (e, L M�1 cm�1)): 400 (21300) (br), 405 (9873)(sh). Molar conductivity (DMSO): KM = 16.2 X�1 mol�1 cm2. Mag-netism (solid state, room temperature): leff = 5.81 BM. ESR (at RTpolycrystalline state): gav = 2.0. Colour: shining yellow.

Table 1Crystal data and structure refinement parameters for 1, 2 and 3

1 2 3

Empirical formula C22H24NiN4O6 C22H24CdN4O6 C22H24MnN4O6

Formula weight 499.16 552.85 495.39Crystal size (mm) 0.28 � 0.17 � 0.06 0.32 � 0.29 � 0.21 0.40 � 0.30 � 0.20Crystal system orthorhombic monoclinic orthorhombicSpace group P2(1)2(1)2(1) P2(1)/c PbcaTemperature (K) 120(2) 120(2) 120(2)a (ÅA

0

) 8.7826(14) 12.686(2) 16.055(2)b (ÅA

0

) 15.209(3) 14.723(3) 15.2764(19)c (ÅA

0

) 15.545(3) 11.620(2) 17.230(2)a (�) 90 90 90b (�) 90 91.279(2) 90c (�) 90 90 90Cell volume (ÅA

03) 2076.4(6) 2169.7(7) 4226.0(9)

F(000) 1040 1120 2056Z 4 4 8Dcalc (Mg m�3) 1.597 1.692 1.557l (mm�1) 0.985 1.055 0.674h Range for data

collection (�)1.87–28.29 1.61–28.18 2.19–28.26

Tmax, Tmin 0.9433, 0.7701 0.8089, 0.7289 0.8770, 0.7742Goodness-of-fit on

F21.038 1.030 1.039

Final R indices[I > 2r(I)]

R1 = 0.0412,wR2 = 0.0977

R1 = 0.0262,wR2 = 0.0685

R1 = 0.0347,wR2 = 0.0963

R indices (all data) R1 = 0.0493,wR2 = 0.1022

R1 = 0.0284,wR2 = 0.0699

R1 = 0.0358,wR2 = 0.0974

Largest differencein peak and hole(e ÅA

0�3)

0.783 and �0.380 1.039 and �0.298 0.419 and �0.251

2.6. Crystallographic data collection and structure determination

The crystal data and refinement details are summarized in Table1. In each case, a single crystal was mounted using Paratone(g) oilonto a glass fiber and cooled to the data collection temperature.Data were collected on a Brüker-AXS APEX CCD diffractometerwith monochromatized Mo Ka radiation (k = 0.71073 Å). Unit cellparameters were obtained from 60 data frames, 0.3� x, from threedifferent sections of the Ewald sphere. The systematic absences inthe diffraction data and the unit cell parameters were uniquelyconsistent with the reported space groups. An inspection of thepacking diagram of complex (1) displays no overlooked symmetryand the cell analysis routines in PLATON suggested no other unit cellor space group [21]. The data-set was treated with SADABS absorp-tion corrections based on redundant multiscan data [22]. Thestructures were refined by full matrix least squares, based on F2.Refinement of the absolute structure parameter in complex (1)yielded the Flack parameter 0.497(16), suggesting a twinned dataset. All non-hydrogen atoms were refined with anisotropic dis-placement parameters. Hydrogen atoms on the hydroxyl groups,H(1) and H(2), were located from the electron density differencemap and refined with a riding model. All other hydrogen atomswere treated as idealized contributions. Structure factors werecontained in the SHELXTL 6.12 program library [23].

3. Results and discussion

The new dibasic hexadentate ligand 1,8-N-bis(3-carboxy)disal-icylalidene-3,6-diazaoctane-1,8-diamine, H4fsatrien, was preparedby the condensation reaction of 3-formylsalicylic acid with trieth-ylenetetramine (2:1 molar ratio) in methanol, as described laterand shown in Scheme 1. The in situ reaction of the ligandcomponents and Ni(CH3COO)2 � 4H2O, Cd(CH3COO)2 � 2H2O andMn(CH3COO)2 � 2H2O in methanol (Scheme 1) yielded the com-plexes (1)–(3) in good yield and purity. The ligand and all the com-plexes gave correct the elemental analyses and were furthercharacterized by IR, UV–Vis, NMR, ESR, molar conductivity, mag-netic measurements and X-ray crystallography.

3.1. Crystal and DFT-optimized structures

All the three complexes (1)–(3) were derived from the samepolydentate ligand utilizing N4O2 donor sites. Complexes (1) and(2) crystallize in different crystallographic systems, orthorhombic(space group P2(1)2(1)2(1)) and monoclinic (space group P2(1)/c), respectively, while complex (3) crystallizes in the same crystalsystem as (1), with the space group Pbca. The crystallographicand refinement data are summarized in Table 1. Selected bondlengths and angles, from both the experiment and the DFT optimi-zation, are given in Table 2. All cartesian coordinates are providedin the Supporting information (S1 and S2).

In general, there is good agreement between the calculated andX-ray crystal structural parameters, especially for the Ni-complex,where the largest deviation is 0.1 Å for the bond lengths and 5� forthe bond angles. Relatively large discrepancies with the X-ray crys-tal measurements are found for the Mn-complex, where the largestdeviation is 0.19 Å for the bond lengths and 8� for the bond angles.While the optimized Mn–N(2) and Mn–N(3) bond lengths aresomewhat larger than the experimental ones, the Mn–N(1) andMn–N(4) bond lengths optimized are notably (up to 0.2 Å) smallerthan those in the crystal structure. Nevertheless, both experimentand theoretical calculations show an octahedral environment forthe central metal ions. In the case of the Cd-complex, the calcu-lated structural parameters compared in most cases favorably withthe experimental ones.

COOH

OH

CHO

2 + H2NNH NH NH2

MeOH

COOH

OH HO

C CN NNH NHH H

H4fsatrien

HOOC

M2+

[M(H2fsatrien)]M = Ni(II) (1)M = Cd(II) (2)M = Mn (II) (3)

Scheme 1.

3362 S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370

The molecular structures of these complexes consist of pseudo-octahedrally coordinated metal ions with a hexadentate ligand. TheORTEP drawings of these molecular structures are depicted in Fig. 1(1), Fig. 2 (2) and Fig. 3 (3). The metal ions are surrounded by twooxygen atoms in cis positions, two imino nitrogen atoms and twoamine nitrogen atoms to complete the distorted octahedron. Thisarrangement is consistent with that of Ni(sal)2trien � 3H2O [24].

Table 2Selected bond lengths (ÅA

0

) and bond angles (�) for 1, 2 and 3

Bonds (in Å)/angles (�) Experimental Theoretical

Ni-complex (1)Ni–N(2) 2.110(3) 2.159Ni–N(3) 2.145(3) 2.249Ni–N(4) 2.041(3) 2.016Ni–N(1) 2.034(3) 2.001Ni–O(4) 2.039(2) 2.049Ni–O(1) 2.053(2) 2.039O(4)� � �O(6)a 2.445(3) (1.663) 2.511 (1.538)O(1)� � �O(2)a 2.456(3) (1.675) 2.513 (1.537)N(4)–Ni–N(1) 172.3 175.0N(3)–Ni–O(4) 167.4 162.3N(2)–Ni–O(1) 165.7 164.5O(6)–H(6)� � �O(4) 153.9 157.7O(2)–H(2)� � �O(1) 153.8 157.5

Cd-complex (2)Cd–N(2) 2.3544(16) 2.500Cd–N(3) 2.3566(16) 2.512Cd–N(4) 2.3041(17) 2.312Cd–N(1) 2.3060(16) 2.311Cd–O(1) 2.3070(14) 2.319Cd–O(4) 2.3024(13) 2.313O(2)� � �O(1)a 2.430(2) (1.477) 2.506 (1.516)O(5)� � �O(4)a 2.455(2) (1.673) 2.513 (1.524)N(4)–Cd–N(1) 155.7 164.3N(2)–Cd–O(1) 143.7 139.1N(3)–Cd–O(4) 144.6 140.1O(2)–H(2)� � �O(1) 159.2 159.6O(5)–H(5A)� � �O(4) 154.0 159.5

Mn-complex (3)Mn–N(2) 2.3116(13) 2.394Mn–N(3) 2.3047(13) 2.378Mn–N(4) 2.2334(13) 2.044Mn–N(1) 2.2387(13) 2.056Mn–O(4) 2.1267(11) 2.006Mn–O(3) 2.1130(11) 2.000O(4)� � �O(5)a 2.462(2) (1.633) 2.555 (1.596)O(3)� � �O(7)a 2.468(2) (1.577) 2.557 (1.601)N(4)–Mn–N(1) 174.7 174.3N(3)–Mn–O(4) 155.4 154.1N(2)–Mn–O(3) 151.7 152.5O(2)–H(1)� � �O(3) 159.0 150.8O(54)–H(2)� � �O(4) 159.1 157.4

a O� � �H bond distance is in parentheses.

Fig. 1. ORTEP view of the Ni-complex with ellipsoids drawn at the 50% probabilitylevel.

Fig. 2. ORTEP view of the Cd-complex with ellipsoids drawn at the 50% probabilitylevel.

The two imino nitrogen atoms [d(Mn–N) = 2.2387(13) and2.2334(13) Å; d(Cd–N) = 2.3041(17) and 2.3060(16) Å; d(Ni–N) = 2.034(3) and 2.041(3) Å] are coordinated to the metal ionsmore strongly than the other two amine nitrogen atoms [d(Mn–N) = 2.3047(13) and 2.3116(13) Å; d(Cd–N) = 2.3544(16) and2.3566(16) Å; d(Ni–N) = 2.110(3) and 2.145(3) Å]. This unequiva-

Fig. 3. ORTEP view of the Mn-complex with ellipsoids drawn at the 50% probabilitylevel.

Table 3Weak force interactions (Å, �) for the Ni-complex

D–H� � �A D–H H� � �A D� � �A \D–H� � �A

Hydrogen bonding interactionsO2–H2� � �O1 0.84 1.67 2.456(3) 154.00O6–H6� � �O4a 0.84 166 2.445(3) 154.00N2–H2B� � �O3 0.93 2.24 3.154(4) 167.00

C–H?p(j) H� � �R (Å) \C–H� � �R (�) C� � �R (Å)

C–H� � �p interactionsC10–H10A?R(1)(ii) 2.60 161 3.548

p(i)?p(j) Dihedral angle (i, j) (�) Distance between the barycenters(Å)

p–p interactions (face-to-face)R(1)?R(2)(i) 11.65 3.968

Symmetry code: (a) 1/2 � x, �y, �1/2 + z; (i) = ½ � x, �y, ½ + z; (ii) �x, ½ + y, ½ � z.R(i)/R(j) denotes the centroids of the ith/jth ring; R(1)?C1–C2–C3–C4–C5–C6.R(2)?C16–C17–C18–C19–C20–C21.

Fig. 4. The 1-D coordination structure of the Ni-complex.

Fig. 5. The 2-D coordination structure of the Ni-complex showing intramolecularC–H� � �p and p–p interactions.

S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370 3363

lency in coordination of the metal-nitrogen bonds can be explainedby the fact that imino nitrogens are better donors than amine nitro-gens; the latter are somewhat influenced by the stereo-chemicalarrangement of the trien and carboxylic acid groups. The axial bondangles of N1–M–N4 vary from 155.7(6)� to 175.0� and agree wellwith distorted octahedral structures. In most cases the metal–oxy-gen bond distances [2.039(2) and 2.053(2) Å for 1; 2.3024(13) and2.3070(14) Å for 2; 2.1130(11) and 2.1267(11) Å for 3] are shownto be shorter than the metal–nitrogen distances as a consequenceof the negative charge borne by the phenolic oxygen atoms. The ste-ric effect of the carboxyl group on the aromatic ring compels the o-phenolato plane to bend away from the central atom, which ishelped by the presence of the highly flexible trien group [24]. Thebinding of the N4O2 hexadentate ligand by the metal ions resultsin the formation of three five-membered and two six-memberedchelate rings, fused together creating large geometrical constraints.These steric strains, caused mainly by the formation of five-mem-bered chelate rings, give rise to larger distortions from the octahe-dral angles, as reflected by numerous bond angles smaller or largerthan 90�.

In the complex [Ni(H2fsatrien)] (1), the adjacent mononuclearunits are locked by face-to-face p–p interactions (distance be-tween the centroids of the interacting rings, 3.968 Å; dihedral an-gle, 11.65�) exerted by terminal phenyl moieties and by hydrogenbonding interactions (Table 3). The molecules are arranged in anoffset manner to maximize such interactions and generate a 1-Dchain along the c-direction (Fig. 4). The adjacent zigzag chainsare interwoven through C–H� � �p interactions with a \C–H� � �p an-gle of 161� and a C� � �p distance of 3.548 Å (Table 3), and areresponsible for the formation of the 2-D supramolecular architec-ture in the bc-plane (Fig. 5).

The solid-state structure of [Mn(H2fsatrien)] (3) is very similarto that of complex (1), but the p–p and C–H� � �p weak interactionsin the former are stronger. Here the adjacent discrete molecularunits are arranged into a 1-D chain along the b-direction (Fig. 6)via strong face-to-face p–p stacking interactions between the phe-nyl groups of the ligand with a centroid–centroid distance of3.626 Å and dihedral angle of 6.50�. Furthermore, the adjacent zig-zag 1-D chains are linked together to generate a supramolecular 2-D sheet in the bc-plane (Fig. 7) through C–H� � �p interactions, hav-ing a \C–H� � �p angle of 162� and a C� � �p distance of 3.428 Å (Table4). The disposition of the adjacent sheets is shown in Fig. 8.

Unlike the complexes of Mn(II) and Ni(II), the supramolecularstructure of the Cd(II) complex exhibits a 3-D architecture. Twoterminal phenyl rings of the two closest mononuclear complexesof (2) are linked through face-to-face p interactions (distance be-tween the barycentres of the interacting rings, 3.912 Å; dihedralangle, 10.05�) and through C–H� � �p interactions (\C– H� � �p angle,158� and C� � �p distance, 3.604 Å), and are responsible for the for-mation of the 1-D chain along the a-direction (Fig. 9). The 1-Dchains pack alongside one another, and the 1-D columns are held

Fig. 7. The 2-D coordination structure of the Mn-complex showing intramolecularC–H� � �p and p–p interactions.

Table 4Weak force interactions (Å, �) for the Mn-complex

D–H� � �A D–H H� � �A D� � �A \D–H� � �A

Hydrogen bonding interactions (Å, �)O2–H1� � �O3 0.93 1.58 2.468(2) 159.00O5–H2� � �O4 0.87 1.63 2.462(2) 159.00N3–H3B� � �O6a 0.93 2.25 3.031(2) 141.00

C–H?p(j) H� � �R (Å) \C–H� � �R (�) C� � �R (Å)

C–H� � �p interactionsC12–H12B?R(2)(ii) 2.47 162 3.428

p(i)?p(j) Dihedral angle (i, j) (�) Distance between the barycenters (Å)

p–p interactions (face-to-face)R(1)?R(2)(i) 6.50 3.626

Symmetry code: (a) �x, �1/2 + y, 1/2 � z; (i) �x, �½ + y, ½ � z; (ii) x, ½ � y, ½ + z.R(i)/R(j) denotes the centroids of the ith/jth ring; R(1)?C2–C3–C4–C5–C6–C7.R(2)?C16–C17–C18–C19–C20–C21.

Fig. 6. The 1-D coordination structure of the Mn-complex.

Fig. 8. Perspective view of the adjacent sheets in the Mn-complex.

Fig. 9. The 1-D coordination structure of the Cd-complex.

Table 5Weak force interactions (Å, �) for the Cd-complex

D–H� � �A D–H H� � �A D� � �A \D–H� � �A

Hydrogen bonding interactions (Å, �)O2–H2� � �O1 0.99 1.48 2.430(2) 159.00N2–H2B� � �O5a 0.93 2.58 3.273(2) 132.00N2–H2B� � �O6a 0.93 2.06 2.938(2) 157.00N3–H3A� � �O2b 0.93 2.39 3.030(2) 126.00N3–H3A� � �O3b 0.93 2.23 3.106(2) 156.00O5–H5A� � �O4 0.84 1.67 2.455(2) 154.00

C–H?p(j) H� � �p(Å) \C–H� � �p (�) C� � �p (Å)

C–H� � �p interactionsC10–H10A?R(1)(i) 2.67 158 3.604C13–H13A?R(2)(ii) 2.72 158 3.656

p(i)?p(j) Dihedral angle (i, j) (�) Distance between the barycenters (Å)

p–p interactions (face-to-face)R(1)?R(2) 10.05 3.912

Symmetry code: (a) x, 3/2 � y, �1/2 + z; (b) x, 5/2 � y, �1/2 + z; (i) 2 � x, 2 � y,1 � z; (ii) 1 � x, 2 � y, 1 � z.R(i)/R(j) denotes the centroids of the ith/jth ring; R(1)?C1–C2–C3–C4–C5–C6;R(2)?C16–C17–C18–C19–C20–C21.

3364 S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370

together through O–H� � �O and N–H� � �O hydrogen bonding interac-tions (Table 5) to form a grid-like structure when viewed along thea-direction (Fig. 10), which also involves bifurcated H-bonding[25]. In this structure the sheets are arranged parallel to the ab-plane to generate a 3-D supramolecular architecture (Fig. 11),where another C–H. . .p interaction (\C–H� � �p angle, 158� andC� � �p distance, 3.656 Å) (Table 5) is mainly responsible for the for-mation of the solid-state structure.

3.2. Electronic structures

The valence molecular orbital (MO) energy level diagrams of theligands and their MII-complexes (M = Ni, Cd, Mn) are illustrated in

Fig. 10. The perspective view of the 2-D structure of the Cd-complex with bifurcated H-bonding.

Fig. 11. A 3-D supramolecular assembly of the Cd-complex.

S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370 3365

Fig. 12, from which we can see the change of the electronic struc-ture on going from the ligand to the MII–ligand complex. Accordingto the calculations, the neutral ligand (H2fsatrien) itself has anopen-shell. This result is expected by examining the orbital energylevel diagram of H2fsatrien, where the HOMO (highest occupiedmolecular orbital) and second HOMO (i.e. HOMO � 1) are close to

each other and therefore two unpaired electrons will enter thetwo orbitals, respectively. To confirm the open-shell ground statefor the ligand, other density functionals (such as PW91, mPW,BLYP, PBE) were also tested and they gave the same results. Wemay note that the orbital energy level diagrams of H2fsatrien areslightly different for different [M(H2fsatrien)]. This is because

Fig. 12. Valence molecular orbital energy level diagrams of the ligands and their MII-complexes (M = Ni, Cd, Mn)).

Table 6UV–Vis data for the complexes

Complex Experimental Calculated

kmaxa Emax

b Ec fd Contributione (%)

Ni-complex 385 3.22 3.35 (3.28) 0.0125 41 (85a?93a);16 (87a?95a)

585 2.12 2.18 (2.15) 0.0015 90 (91a?93a)850 1.46990 1.25

Cd-complex 260 4.77 4.66 (4.71) 0.0075 49 (87a?97a);37 (88a?97a)

352 3.52 3.53 (3.51) 0.0023 50 (94a?97a);34 (94a?98a)

427(sh) 2.9 2.90 (2.92) 0.0574 37 (93a?95a);33 (93a?96a)

Mn-complex 400 3.1 3.12 (3.02) 0.0009 67 (89a?96a);13 (85a?90a)

405 (sh) 3.06 3.03 (3.00) 0.0084 53 (90a?96a);12 (86a?95a)

867 1.43 1.45 (1.07) 0.0039 95 (94a?96a)874 1.42 1.36 (0.79) 0.0274 99 (94a?95a)

a Absorption maxima in nm.b Experimental energy in eV.c TDDFT energy in eV; the values in parentheses are the results calculated for the

optimized structure.

3366 S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370

when we calculated the orbital energies of H2fsatrien, the structureof H2fsatrien in the respective [M(H2fsatrien)] was adopted andthis structure is somewhat different for different M. This also re-flects slight perturbations of the ligand energy states by the differ-ent metals. The Ni- and Mn-complexes are open-shell as well,where some unpaired electrons are on the metal d-orbitals. Uponcomplexation, the singly occupied orbitals of the ligand becomedoubly occupied by obtaining electrons from the metal ns-orbitals.The Cd-complex has a closed-shell ground state because the Cd d-orbitals are fully occupied.

With M = Ni or Mn, the HOMO and LUMO (lowest unoccupiedmolecular orbital) are associated with the metal and ligand compo-nents, respectively. A very large splitting is found between the me-tal d-derived orbitals. The Mn-complex is calculated to be high-spin (2S + 1 = 6); that is, it has five unpaired electrons on the me-tal–d orbitals, in agreement with the magnetic susceptibility mea-surements. The low-spin state (2S + 1 = 2) is about 0.3 eV higher inenergy than the high-spin state according to our calculations. Theenergies of the Mn–d orbitals are higher than those of the Ni–dones so that the energy gap between the HOMO and LUMO is smallin the Mn-complex. In the case of M = Cd, the d-orbitals are espe-cially low in energy, so they do not appear in the valence set ofthe orbitals.

d Oscillator strength.e See Fig. 12 for the orbitals.

3.3. Electronic spectrum and TDDFT calculations

The UV–Vis (in DMSO) spectra were recorded both for the li-gand and its metal complexes. The measured UV–Vis spectral datafor the Ni(II), Cd(II) and Mn(II) complexes are presented in Table 6.To have a better insight into the electronic spectra, TDDFT calcula-

tions were performed for the allowed transitions from the groundstate to the excited states. The theoretical results, obtained for boththe crystal and optimized structures, are provided in the same ta-ble to compare the experimental data. The characters of theabsorption bands are indicated by the contributions listed in the

S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370 3367

last column of the table. More calculated excitation energies (Eexc)for other, various (allowed) transitions are given in the Supportinginformation (S3–S5). Also, the calculated Eexc values for the com-plexes agree quite well with the experimental data.

The present TDDFT method is not able to calculate optical spec-tra of bulk materials. From Figs. 4–11, we see that the molecularunits in all the crystal structures are rather discrete and are assem-bled through different weak force interactions. Therefore, the UV–Vis spectra (in the solution state) of the bulk materials can largelybe represented by the spectra of the discrete molecules; althoughthe optical spectra (in the solid state) show some features of bandstructure (see Section 3.6).

Fig. 13. Temperature dependence of the e

Fig. 14. Temperature dependence of the e

3.4. Magnetic moments, ESR and molar conductance values

The magnetic susceptibilities of the three complexes underinvestigation were measured in the solid state at room tempera-ture. The complexes [Ni(H2fsatrien)] (1) and [Mn(H2fsatrien)] (3)are paramagnetic, while the complex [Cd(H2fsatrien)] (2) is dia-magnetic. Magnetic moments of octahedral nickel(II) complexesare usually found in the range 2.9–3.3 BM [26]. The observedmagnetic moment of 3.10 BM for the nickel(II) complex, [Ni(H2fsa-trien)] (1) falls in the range for octahedral nickel(II) [27]. This elim-inates the possibility of attaining other geometries by nickel(II) inthe complex. The manganese(II) complex exhibits a magnetic mo-

lectrical conductivity of complex (1).

lectrical conductivity of complex (3).

3368 S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370

ment of 5.82 BM at room temperature and is indicative of a high-spin manganese(II) centre. The complex also exhibits a strongbut broad ESR signal centered at g = 2, consistent with manga-nese(II) [28].

The molar conductance values of the complexes (1) to (3) inDMSO solution are found in the range KM = 15.3–16.4X�1 cm2 mol�1, indicating the non-electrolytic nature of the com-plexes in the DMSO solution [29].

3.5. Electrical conductivity

The electrical conductivity variations with temperature for thecomplexes (1) and (3) (shaped as a pellet) were measured andthe lnr � 1000/T graphs are given in Figs. 13 and 14. It can be ob-served that complex (1) shows three distinct regions, depending onthe temperature (Fig. 13). At low temperatures (290–360 K) (large1/T) extrinsic behaviour dominates [22,30]. It can be seen from thefigures that the conductivity is virtually temperature-independentat the beginning of the extrinsic region, where the increase in thenumber of electrons donated from the donor levels in its effect onthe conductivity and is balanced by the decrease in the mean freepath, being fully ionized, i.e., completely emptied. The conductivitystarts dropping as a result of phonon scattering in the temperaturerange 360–410 K, until the onset of intrinsic conduction (at a tem-perature of 410 K), when the conductivity increases again withtemperature [31]. Intrinsic conductivity occurs at high tempera-tures (410–480 K), as shown in the intrinsic region for the complex(1). The extrinsic and intrinsic regions have a positive temperaturecoefficient of electrical conductivity.

In spite of showing insulating properties at low temperatures,complex (3) showed conduction properties at about 390 K(Fig. 14). Complex (3) has a positive temperature coefficient be-tween 390–405 K and 410–480 K. The conductivity is constant inthe region between 405 and 410 K. We can say that the behaviouris similar to the behaviour in the exhausting region.

Fig. 15. The optical absorption

The increasing of conduction with temperature is semiconduc-tor behaviour. The Arrhenius equation for the extrinsic region of n-type semiconductors is

r ¼ r0expð�Ea=kTÞ ð1Þ

where r0 is the pre-exponential factor, Ea is the activation energyand k is the Boltzmann’s constant. By using this equation, the acti-vation energy can be calculated [22,26]. The slope in the extrinsicregion for an n-type semiconductor is given by (Eg � Ed)/k and Eg/2k in the intrinsic region [22]. The calculated activation energiesfor complex (1) were found to be low (0.42 eV) in the lower temper-ature range (290–360 K), but considerably higher (0.81 eV) in thehigh-temperature range (410–480 K). The activation energies ofcomplex (3) in the extrinsic region (390–405 K) and the intrinsic re-gion (410–480 K) are 1.22 eV and 0.72 eV, respectively. Thus theconductivity of the compound shows the typical Arrhenius-typedependence on temperature. Such an n-type electrical semiconduc-tivity mechanism of the present Schiff base complexes may be ex-plained in terms of transfer of electrons in two possible ways.Firstly [32] the conduction process may be assumed to start withthe delocalization of the p-electrons and then the excitation of elec-trons from the HOMO to the LUMO of p-MOs, which actually switchover the metal complexes to the semiconducting state. In a second[33] way, the electrons hop from one localized metal site to thenext, i.e. in this case the holes have little effect on the mobility ofnegative charges. When the electron lands on a new site, it causesthe surroundings to adjust their localization and the electron istrapped temporarily in the potential well and it resides until acti-vated to migrate to another nearby site. Thus the electrons are sup-posed to tunnel from one molecular unit to the comparable unfilledlevel of the adjacent molecular unit. In this case the tunneling ofelectrons is observed through the intermolecular potential barrierto occupy the corresponding orbital in the adjacent unit via p–pstacking, C–H� � �p interactions and extensive H-bonding.

spectra for complex (1).

Fig. 16. The optical absorption spectra for complex (3).

S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370 3369

3.6. Optical properties

To obtain the energy-band gaps and the nature of the opticaltransition involved in the complexes, the optical spectra were re-corded at room temperature and the fundamental absorption edgeis analyzed within the framework of the theory of Mott and Davis[30]. The optical band gaps of the samples were determined byapplying the relation [31,34],

ahm ¼ Aðhm� EgÞn ð2Þ

where a is the linear absorption coefficient, hm is the photon energy,Eg is the energy gap between the top of the valence band and bot-tom of the conduction band at the same value of wave numberand n depends on the kind of optical transitions. Specifically, n is1/2, 3/2, 2 and 3 for transitions which are directly allowed, directlyforbidden, indirectly allowed and indirectly forbidden, respectively.A is a constant involving the properties of the bands.

The energy gap for allowed direct (Egd) and indirect (Egi) transi-tions can be determined by relating

ahm ¼ Aaðhm� EgdÞ1=2 ð3Þ

and

ahm ¼ Aiðhm� EgiÞ2 ð4Þ

where Aa and Ai are the characteristic parameters for the respectivetransitions, independent of m. From Eqs. (3) and (4), Eg can be deter-mined from the extrapolation to zero of the linear parts of the(ahm)2 = f(hm) and (ahm)1/2 = f(hm) curves.

The optical absorption spectra, i.e. the plots of (ahm)2 versus hm,are given in Figs. 15 and 16 for complexes (1) and (3) and analysisof the absorption data was carried out to determine the predomi-nant optical transition. A satisfactory fit was obtained for (ahm)2

as a function of hm, showing the existence of a direct gap. Theextrapolation of the graphs to (ahm)2 = 0 yields the optical bandgap Egd. The values of the optical band gap Egd were determinedfrom Figs. 15 and 16 by least squares fitting of the data. In thisstudy, the optical band gaps, Egd, of complexes (1) and (3) werefound to be 2.57 and 2.30 eV, respectively.

4. Conclusions

The synthesis, characterization and crystal structures of threecomplexes [M(H2fsatrien)] (M = Ni (1), Cd (2) and Mn (3)) with anew symmetrical hexadentate Schiff base (N4O2 donor) as a ligandhave been described. Both the X-ray crystal structure determina-tions and theoretical calculations demonstrate that the complexesare all pseudo-octahedral with some variations in their configura-tions. Some intermolecular weak forces, such as p–p stacking, C–H� � �p interactions, unusual H-bonding interactions also playimportant roles by linking the discrete subunits and low-dimen-sional entities into high-dimensional (1-D, 2-D, 3-D) supramolecu-lar coordination networks. The electrical semiconducting characterof the metal complexes has been nicely correlated with thesestructural patterns. On the other hand, optical studies indicate thatcomplexes (1) and (3) have direct band gaps. DFT/TDDFT calcula-tions agree well with the experimentally observed structures andabsorption spectra of the molecules.

Acknowledgments

One of the authors (K.D.) is thankful to the UGC, New Delhi forawarding Emeritus Fellowship and Financial grants to carry outthis work. We are grateful to the Sophisticated Analytical Instru-ment Facility, Central Drug Research Institute, Lucknow and IITMadras for elemental analyses and some spectroscopic measure-ments and IICB, Kolkata for NMR analysis. We are also grateful tothe reviewers for their valuable comments.

Appendix A. Supplementary data

CCDC 677239, 677240 and 677238 contain the supplementarycrystallographic data for complexes 1, 2 and 3. These data can beobtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Cen-tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-ciated with this article can be found, in the online version, atdoi:10.1016/j.poly.2008.07.034.

3370 S. Sarkar et al. / Polyhedron 27 (2008) 3359–3370

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