Unusual coordination mode of 3-methoxysalicylaldehyde in mononuclear zinc(II) complexes with nitrogenous bases: Synthesis, structural characterization and theoretical studies

Polyhedron 87 (2015) 275–285

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Unusual coordination mode of 3-methoxysalicylaldehyde in mononuclear zinc(II) complexes with nitrogenous bases: Synthesis, structural characterization and theoretical studies Despoina Anastasiadou a, Ariadni Zianna a, Maria Gdaniec b, Michael P. Sigalas c, Evdoxia Coutouli-Argyropoulou d, Agnieszka Czapik b, Maria Lalia-Kantouri a,⇑ a

Aristotle University of Thessaloniki, Department of Chemistry, Laboratory of Inorganic Chemistry, Thessaloniki 54 124, Greece Faculty of Chemistry, Adam Mickiewicz University, 61-614 Poznan, Poland c Aristotle University of Thessaloniki, Department of Chemistry, Laboratory of Applied Quantum Chemistry, Thessaloniki 54 124, Greece d Department of Organic Chemistry and Biochemistry, Faculty of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece b

a r t i c l e

i n f o

Article history: Received 17 October 2014 Accepted 28 November 2014 Available online 8 December 2014 Keywords: Zinc 3-Methoxysalicylaldehyde Crystal structure Thermal study DFT calculations

a b s t r a c t The reaction of the new precursor compound [Zn(3-OCH3-salo)2(H2O)2] (1) with the nitrogenous bases enR afforded the Zn(II) compounds [Zn(3-OCH3-salo)2(enR)] (2–5), where 3-OCH3-salo is the anion of 3-methoxysalicylaldehyde (o-vanillin) and enR the neutral bipy, phen, neoc and dpamH ligands. The new compounds were characterized by physicochemical and spectral (FT-IR, UV–Vis, 1H NMR) data. The X-ray diffraction study of [Zn(3-OCH3-salo)2(bipy)]CH3OH (2) and [Zn(3-OCH3-salo)2(dpamH)] (5) confirmed the coordination mode of the 3-OCH3-salo ligand to the zinc cation through the phenolate and methoxy oxygen atoms, as predicted from the spectroscopic data. The thermal stability of the compounds [Zn(3-OCH3-salo)2(enR)] was investigated by the simultaneous TG/DTG-DTA technique and compared with the precursor (1). The residue at 1000 °C was estimated from TG curves as a carbonaceous mixture of ZnO. The molecular structure, the alternate coordination mode of 3-OCH3-salo ligand, the isomerism and the energetics of the metal–ligand interactions for compounds 2 and 5 have been studied by means of density functional theory (DFT) calculations. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Zinc is an essential trace element for growth and development in all forms of life and the second most abundant trace metal in the human body. It tends to be tightly bound within over 300 metalloenzymes [1]. Beneficial, therapeutic and preventive effects of zinc in infectious diseases have been reported [2,3]. Zinc complexes with diverse ligands used as drugs have been structurally characterized and have been shown to possess noteworthy biological activity [4,5]. Strong coordinating properties of 2-hydroxybenzaldehydes with transition metals have stimulated research on these compounds showing their importance for both the pure [6,7] and applied chemistry fields, especially in extractive metallurgy as analytical reagents [8,9]. More specifically, 3-methoxysalicylaldehyde ⇑ Corresponding author. Tel./fax: +30 2310 997844. E-mail addresses: [email protected] (M. Gdaniec), [email protected] (M.P. Sigalas), [email protected] (E. Coutouli-Argyropoulou), [email protected] (M. Lalia-Kantouri). http://dx.doi.org/10.1016/j.poly.2014.11.030 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

(3-OCH3-saloH), commonly known also as o-vanillin (o-van) can be used as a multidentate ligand for the construction of polymers with defined geometry and special properties [10,11]. This compound has also been used in coordination chemistry of transition metals [12–14]. For example, in Ni(II) [12] and VO(IV) [13] complexes 3-OCH3-saloH behaves as a uninegative ligand coordinating in a bidentate chelating manner, through the phenolate and aldehyde oxygen atoms and this is the normal coordination mode of this ligand in mononuclear complexes. In polynuclear complexes 3-OCH3-salo ligand can act as tri- or tetradentate ligand. For example, in the dimeric mixed-ligand copper(II) complex [Cu(3OCH3-salo)(bipy)(OClO3)]2 [14] it chelates the metal center through the phenolate and aldehyde oxygen atoms and simultaneously the phenolate O atoms serves as a bridge between two copper ions. In the heptanuclear complex [Fe2(3-OCH3-salo)8Na5]3OH8H2O [15] all 3-OCH3-salo ligands act in a tetradentate bis-chelating mode with the phenolate O atoms used as bridges between the metal centers. Recently, we have initiated a research project focused on the complexes of 3d transition metals and salicylaldehyde ligands

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(X-salo, where X = Cl, Br, NO2, CH3, OCH3) formed in the absence or presence of bidentate nitrogenous bases (enR). So far neutral compounds of the general formula [M(X-salo)2(enR)] [16–19] and cationic complexes of the type [Co(3-OCH3-salo)(dpamH)2]+ X, where X = Cl or NO3 [20,21] have been obtained. As a continuation of this research, we present here the complexes formed in the reaction of the precursor compound [Zn(3-OCH3-salo)2(H2O)2] (1) with the nitrogenous bases enR, where 3-OCH3-salo is the anion of 3methoxysalicylaldehyde and enR stands for the neutral 2,20 -bipyridine (bipy), 2,20 -dipirydylamine (dpamH), phenanthroline (phen) and neocuproine (neoc). This reaction resulted in the compounds with formula [Zn(3-OCH3-salo)2(enR)] (2–5), which were characterized by physicochemical and spectral (FT-IR, UV–Vis, 1H NMR) data. The X-ray diffraction analysis of 2 and 5 has been used to determine stereochemistry of the complex molecules and coordination geometry of the zinc(II) ion. The thermal stability and decomposition mode for three of the new compounds have been studied in nitrogen atmosphere by using the simultaneous technique (TG/DTG-DTA). Results of density functional theory (DFT) calculations for compounds 2 and 5 have been used to explore the coordination modes of the o-vanillin ligand, the energetics of metal–ligand interactions and stereoisomerism of [Zn(3-OCH3salo)2(enR)] complexes. The ligands used in this work are shown in Schemes 1 and 2. 2. Experimental 2.1. Materials The ligands, 3-OCH3-saloH, the nitrogenous bases enR (2,20 bipyridine, 1,100 phenanthroline, 2,9-dimethyl-1,10-phenanthroline and the 2,20 -dipyridylamine) and the salts CH3ONa and ZnCl2 were obtained as reagent grade from Aldrich and used as received. Solvents for preparation and physical measurements of ‘‘extra pure’’ grade were obtained from Fluka and used without further purification. 2.2. Instrumentation/analytical procedures Infrared (IR) spectra (400–4000 cm1) were recorded on a Nicolet FT-IR 6700 spectrometer with samples prepared as KBr pellets. UV–Vis spectra were recorded as Nujol mulls and in MeOH solutions at concentrations in the range 105–103 M on a Hitachi U-2001 dual beam spectrophotometer. 1H NMR spectra were recorded at 300 MHz on a Bruker AVANCEIII 300 spectrometer using CDCl3 or DMSO-d6 as solvent. C, H and N elemental analyses were performed on a Perkin–Elmer 240B elemental microanalyzer. Molecular conductivity measurements were carried out in DMF solutions with a Crison Basic 30 conductometer. Melting point measurements were performed at Buchi apparatus in silicon oil or at heating plate of a Reichert apparatus. The thermal behavior of the title compounds was studied in nitrogen using the simultaneous TG/DTG-DTA technique, over the temperature range

ambient-to-1000 °C, using a Setaram Model Setsys-1200 thermogravimetric analyzer. The samples, with a mass of about 10 mg were heated in platinum crucibles, at a heating rate 10 °C min1. 2.2.1. Computational details The electronic structure and geometries of the complexes studied were computed within the density functional theory (DFT) using gradient corrected functional, at the Becke3LYP computational level [22,23]. The effective core potential (ECP) approximation of Hay and Wadt was used [24–26]. For Zn atom, the electrons described by the ECP were those of 1s, 2s and 2p shells. The basis set used was of valence double-f quality [27]. Full geometry optimizations were carried out without symmetry constraints. A frequency calculation after each geometry optimization ensured that the calculated structures are real minima or transition states in the potential energy surface of the molecules and allowed the calculation of the zero point energy included in all reported relative energies. In calculating the interaction energies between molecular fragments in the complexes, we accounted for the basis set superposition error (BSSE) by recalculating the monomer energies using the full dimer basis at the optimized geometry of the dimer, using the counterpoise method [28,29]. Time-dependent density functional theory (TD-DFT) calculations were performed employing the same functional and basis set for the analysis of the excitation energies for both compounds. Solvent (methanol) effects were computed by using the integral equation formulation-polarized continuum model IEF-PCM [30,31]. All the calculations were performed using the GAUSSIAN-03W ver. 6.0 package [32]. 2.3. X-ray crystal structure determination Slow evaporation of the reaction mixtures yielded yellow single crystals of compounds [Zn(O-CH3-salo)2(bipy)].CH3OH (2) and [Zn(O-CH3-salo)2(dpamH)] (5) suitable for X-ray structural analysis. The diffraction data were collected at 130 K with an Oxford Diffraction Xcalibur E diffractometer using Mo Ka radiation for compound 2, and at 293 K with an Oxford Diffraction SuperNova diffractometer using Cu Ka radiation for compound 5. The intensity data were collected and processed using CrysAlisPro software [33]. The crystals of 5 were twinned with the twin law corresponding to 180° rotation about the [1 0 0] direction of the direct lattice. The volume ratio of the twin domains was 13/87. The structures were solved by direct methods with the program SHELXS-97 [34] and refined by full-matrix least-squares method on F2 with SHELXL-97 [34]. The carbon-bound hydrogen atoms were refined as riding on their carriers and their displacement parameters were set equal to 1.5Ueq(C) for the methyl groups and 1.2Ueq(C) for the remaining H atoms. The O–H and N–H hydrogen atoms were located in electron density difference maps and freely refined. A summary of the crystallographic data is given in Table 1. Molecular graphics were generated with ORTEP-3 for Windows [35] and Mercury 3.3 software [36]. 2.4. Synthesis of the zinc(II) Compounds 2.4.1. Synthesis of [Zn(3-OCH3-salo)2(H2O)2] (1) Complex 1 was prepared according to previous procedure [19], expressed by the Eq. (1).

ZnCl2 þ 2 3-OCH3  saloH þ 2CH3 ONa ! ½Znð3-OCH3 -saloÞ2 ðH2 OÞ2  þ 2NaCl þ 2CH3 OH

Scheme 1. Structural formula of the o-vanillin ligand (3-OCH3-saloH).

ð1Þ

A solution of 3-OCH3-saloH (152 mg, 1 mmol) deprotonated with CH3ONa (54 mg, 1 mmol) in MeOH (10 mL), was added drop-wise to a 20 mL solution (MeOH/H2O) of ZnCl2 (68 mg, 0.5 mmol) at room temperature. The reaction mixture was stirred

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Scheme 2. (A–D) The nitrogenous bases enR: (A) bipy, (B) dpamH, (C) phen and (D) neoc and H atoms labeling.

Table 1 Crystallographic data, data collection and refinement details for compounds [Zn(3-OCH3-salo)2(bipy)]CH3OH (2) and [Zn(3-OCH3-salo)2(dpamH)] (5).

Empirical formula CCDC No. Formula weight Crystal system Temperature (K) Radiation Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z, Z0 Absorption coefficient (mm1) Crystal density (g cm3) Crystal size (mm3) h range for data collection (°)/completeness (%) Measured reflections/Independent reflections/Rint Parameters/restraints Goodness-of-fit (GOF) Final R indices R1, wR2 [I > 2r(I)] R1, wR2 (all data) Largest difference peak/hole (e Å3)

2

5

[Zn(3-OCH3-salo)2(bipy)]CH3OH 1027404 555.87 monoclinic 130 Mo Ka P21/c

[Zn(3-OCH3-salo)2 (dpamH)] 1027405 538.84 triclinic 293 Cu Ka  P1

12.8465(10) 14.6347(11) 13.7262(10) 90 108.612(8) 90 2445.6(3) 4, 1 1.056 1.510 0.5  0.35  0.25 4.13–26.37/99.1 34 156/4950/0.0504 342/0 1.034

11.1166(7) 14.0023(11) 16.7176(13) 112.412(7) 99.103(6) 90.265(6) 2369.4(3) 4, 2 1.855 1.511 0.15  0.12  0.04 2.90–66.59/99.1 18 016/8294/0.0504 662/0 1.031

0.0384/0.0856 0.0543/0.0938 0.44/0.45

0.0589, 0.1554 0.0852, 0.1767 0.66, 0.62

for 2 h and then turned yellowish and a yellow solid was formed, collected by filtration, washed with cold water and air-dried (250 mg, yield 62%). M.p. 128–131 °C, conductivity in DMF solution 10.5 lS/cm. The microcrystalline compound is soluble in MeOH and Me2CO and analyzed as [Zn(3-OCH3-salo)2(H2O)2], (C16H18 O8Zn1) (MW 403.4): C, 47.59; H, 4.46. Found: C, 47.72; H, 4.52%. IR spectrum (KBr): selected peaks in cm1: 3430 m v(O–H) of coordinated water, 2828 m v(C–H) of OCH3, 1635 vs v(C@O), 1345 s v(C–O-Zn), 522 m v(Zn–O); UV–Vis: k/nm (e/M1 cm1) in MeOH: 227(14 270), 270(9370), 340(2500), 390(2780). 1H NMR spectrum in DMSO-d6 (d/ppm): 9.88 (2H, s, H7-3-OCH3-salo), 6.97 (2H, br, H6-3-OCH3-salo), 6.75 (2H, br, H4-3-OCH3-salo), 6.37 (2H, br, H5-3-OCH3-salo), 3.66 s (6H, OCH3). 2.4.2. Synthesis of [Zn(3-OCH3-salo)2(enR)] (2–5) Compounds 2–5 were prepared according to previous procedure, [18], expressed by the Eq. (2).

½Znð3-OCH3 -saloÞ2 ðH2 OÞ 2 þ enR ! ½Znð3-OCH3 -saloÞ2 enR þ 2H2 O ð2Þ

A methanolic solution (10 mL) of the enR base was added slowly to a methanolic solution (10 mL) of [Zn(3-OCH3-salo)2 (H2O)2] (1) (1 mmol, 403.4 mg) under stirring at room temperature. The reaction mixture was stirred for 2 h, reduced in volume and left for slow evaporation. A pale yellow microcrystalline product was formed, filtered off and air-dried in every case. 2.4.3. Synthesis of [Zn(3-OCH3-salo)2(bipy)]CH3OH (2) 1 mmol of bipy (156 mg) was reacted with 1 mmol of compound 1. Pale yellow crystals of 2 suitable for X-ray structure determination were deposited over 10 days. The pale yellow microcrystalline product (344 mg, yield 62%) analyzed as [Zn(3OCH3-salo)2(bipy)]CH3OH, (C27H26Zn1N2O7) (MW = 555.4): C, 58.34; H, 4.68; N 5.04; Found: C, 57.88; H, 4.63; N, 4.84%. M.p. 197–200 °C, conductivity in DMF solution 14.5 lS/cm. IR spectrum (KBr): selected peaks in cm1: 3554(m) v(O–H) of crystallized MeOH, 2833(m) v(C–H) of OCH3, 1634(s) v(C@O), 1603(m) v(C@N), 1341(m) v(C–O-Zn), 549(m) v(Zn–O), 416(weak, (w) v(Zn–N), 855(m) and 767(m), d(C–H)pyridyl; UV–Vis: k/nm (e/M1 cm1) in MeOH: 228(15 898), 276(13 580), 293(sh), 306(9454),

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340(4000), 395(4780). 1H NMR spectrum in CDCl3/DMSO-d6 (d/ppm), 50 °C: 10.02 (2H, s, H7-3-OCH3-salo), 6.99 (2H, dd, J = 7.7, 1.5 Hz, H6-3-OCH3-salo), 6.83 (2H, d, J = 7.7, 1.5 Hz, H4-3-OCH3salo), 6.32 (2H, t, J = 7.7 Hz, H3-3-OCH3-salo), 3.61 (6H, s, OCH3) 0 and 8.70 (2H, d, J = 4.4 Hz, H3- and H3 -bipy), 8.52 (2H, d, 6 60 J = 7.9 Hz, H - and H -bipy), 8.09 (2H, dd as t, J = 7.2 Hz, H5- and 0 0 H5 -bipy), 7.59 (2H, dd as t, J = 6.0 Hz, H4- and H4 -bipy). 2.4.4. Synthesis of [Zn(3-OCH3-salo)2(phen)] (3) 1 mmol of phen (180 mg) was reacted with 1 mmol of compound 1. The pale yellow product (328 mg, yield 60%) analyzed as [Zn(3-OCH3-salo)2(phen)], (C28H22Zn1N2O6) (MW = 547.4): C, 61.38; H, 4.02; N 5.12; Found: C, 60.58; H, 3.97; N, 5.09. M.p.250–252 °C, conductivity in DMF solution 36.7 lS/cm. IR spectrum (KBr): selected peaks in cm1: 2836(m) v(C-H) of OCH3, 1641(s) v(C@O), 1605(m) v(C@N), 1390(m) v(C–O-Zn), 595(m) v(Zn–O), 442(weak, (w)) v(Zn-N), 849(m) and 727(m) d(C-H)pyridyl; UV–Vis: k/nm (e/M1cm1) in MeOH: 228(15 116), 269(11 480), 290(sh), 344(4260), 393(3260). 1H NMR spectrum in CDCl3 (d/ ppm): 9.78 s (2H, s, H7-3-OCH3-salo), 7.01 (2H, d, J = 7.5 Hz, H63-OCH3-salo), 6.79 (2H, d, J = 7.5 Hz, H4-3-OCH3-salo), 6.39 (2H, t, J = 7.5 Hz, H5-3-OCH3-salo), 3.70 (6H, s, OCH3) and 9.15 (2H, d, J = 4.6 Hz, H2- and H9-phen), 8.42 (2H, d, J = 8.0 Hz, H4- and H7phen), 7.88 (2H, s, H5- and H6-phen), 7.79 (2H, dd, J = 8.0, 4.6 H3and H8-phen). 2.4.5. Synthesis of [Zn(3-OCH3-salo)2(neoc)] (4) 1 mmol of neoc (208 mg) was reacted with 1 mmol of compound 1. The pale yellow product (334 mg, yield 58%) analyzed as [Zn(3-OCH3-salo)2(neoc)], (C30H26Zn1N2O6) (MW = 575.4): C, 62.57; H, 4.52; N 4.87; Found: C, 61.44; H, 4.54; N, 4.88. M.p. 220–221 °C, conductivity in DMF solution 33.5 lS/cm. IR spectrum (KBr): selected peaks in cm1: 2836(m) v(C-H) of OCH3, 1645(s) v(C@O), 1601(m) v(C@N), 1389(m) v(C–O-Zn), 505(m) v(Zn–O), 436(weak, (w)) v(Zn-N), 858(m) 770(m), 744(m), 731(s) d(C-H)pyridyl; UV–Vis: k/nm (e/M1cm1) in MeOH: 231(18 316), 270(16 200), 290(sh), 343(4640), 394(4230). 1H NMR spectrum in DMSO-d6

Scheme 3. Structural isomers of [Zn(O-CH3-salo)2(enR)].

(d/ppm) in 40 °C: 1H NMR spectrum in CDCl3 (d/ppm): 10.05 s (2H, s, H7-3-OCH3-salo), 7.19 (2H, br, H6-3-OCH3-salo), 6.78 (2H, br, H4-3-OCH3-salo), 6.52 (2H, br, H5-3-OCH3-salo), 3.47 (2H, s, OCH3) and 8.36 (2H, br, H4 and H7-neoc), 7.86 (2H, s, H5 and H6-neoc), 7.69 (2H, br, H3 and H8-neoc), 3.08 (s, 2H, CH3). 2.4.6. Synthesis of [Zn(3-OCH3-salo)2(dpamH)] (5) 1 mmol dpamH (171 mg) was reacted with 1 mmol of compound 1. Pale yellow crystals of 5 suitable for X-ray structure determination were deposited over 2 weeks. The pale yellow crystalline product (355 mg, yield 67.4%) analyzed as [Zn(3OCH3-salo)2(dpamH)], (C26H23 Zn1N3O6) (MW = 538.4): C, 57.95; H, 4.27; N 7.80; Found: C, 56.58; H, 4.31; N, 7.40. M.p. 230– 232 °C, conductivity in DMF solution 35.0 lS/cm. IR spectrum (KBr): selected peaks in cm1: 3326(m), 3205(m) and 3084(m) v(N–H)dpamH, 1634(s) d(N–H)dpamH, 1640(s) v(C@O), 1602(s) v(C@N), 1383(m) v(C–O-Zn), 775(m), 744(m) and 723(s) d(C-H)pyridyl, 549(m) v(Zn–O), 420(w) v(Zn-N); UV–Vis: k/nm (e/M1cm1) in MeOH: 217(15 898), 260(13 320), 317(7848), 400(4250). 1H NMR spectrum in CDCl3/DMSO-d6 (d/ppm): 10.04 s (2H, s, H7-3-

Fig. 1. 1H NMR spectrum of 5, [Zn(3-OCH3-salo)2(dpamH)] in DMSO-d6.

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279

Fig. 2. The molecular structures of [Zn(O-CH3-salo)2(bipy)].CH3OH (left) and [Zn(O-CH3-salo)2(dpamH) (right). Displacement ellipsoids are shown at the 30% probability level.

Table 2 Selected bond lengths [Å] and angles [°] for 2 and 5. 2

Bond distance Zn(1)–N(1) Zn(1)–N(8,9)* Zn(1)–O(2A) Zn(1)–O(3A) Zn(1)–O(2B) Zn(1)–O(3B) Bond angle N(1)–Zn(1)–N(8,9)* N(1)–Zn(1)–O(2A) N(1)–Zn(1)–O(3A) N(1)–Zn(1)–O(2B) N(1)–Zn(1)–O(3B) N(8,9)*–Zn(1)–O(2A) N(8,9)*–Zn(1)–O(3A) N(8,9)*–Zn(1)–O(2B) N(8,9)*–Zn(1)–O(3B) O(2A)–Zn(1)–O(3A) O(2A)–Zn(1)–O(2B) O(2A)–Zn(1)–O(3B) O(3A)–Zn(1)–O(2B) O(3A)–Zn(1)–O(3B) O(2B)–Zn(1)–O(3B) *

5 Molecule 1

Molecule 2

2.110(2) 2.110(2) 1.970(2) 2.274(2) 2.003(2) 2.255(2)

2.095(4) 2.087(4) 1.984(3) 2.341(3) 2.002(3) 2.387(3)

2.108(4) 2.081(4) 1.984(3) 2.348(3) 2.005(3) 2.372(4)

78.28(8) 98.66(7) 92.66(7) 101.77(7) 171.04(7) 104.64(7) 170.88(7) 95.05(7) 93.44(7) 75.45(7) 154.08(7) 86.65(6) 87.76(7) 95.67(6) 75.33(6)

90.10(14) 105.37(14) 175.53(13) 96.63(14) 95.96(14) 97.59(14) 94.36(13) 104.86(13) 173.75(13) 74.30(13) 148.41(14) 82.31(13) 82.01(13) 79.58(13) 72.97(12)

89.94(15) 104.25(13) 174.53(14) 97.22(13) 95.23(14) 99.21(15) 95.43(14) 102.68(14) 173.71(13) 73.90(12) 149.20(14) 83.01(14) 82.64(12) 79.48(13) 73.16(12)

N(8) in 2 corresponds to N(9) in 5.

OCH3-salo), 6.99 (2H, d, J = 7.8 Hz, H6-3-OCH3-salo), 6.77 (2H, d, J = 7.8 Hz, H4-3-OCH3-salo), 6.33 (2H, t, J = 7.8 Hz, H5-3-OCH3-salo), 3.63 (6H, s, OCH3).and 9.57 (1H, br s, H7-dpamH), 8.20 (2H, d, 0 J = 4.7 Hz, H3- and H3 -dpamH), 7.72–7.47 (4H, m, J = 7.1 Hz, H5, 0 50 6 60 H , H and H - dpamH), 6.75 (2H, br d, H4- and H4 - dpamH). 3. Results and discussion 3.1. Synthesis and characterization The reaction of the salt ZnCl2 with deprotonated 3-methoxysalicylaldehyde in methanol afforded in good yield the precursor compound [Zn(3-OCH3-salo)2(H2O)2] (1). Reactions of compound 1 with the nitrogenous bases enR (1:1 stoichiometric ratio) led to the formation of yellow microcrystalline compounds. The

elemental analyses and the absence of electrical conductivity in DMF solutions indicated neutral compounds, with the general formulae [Zn(3-OCH3-salo)2(enR)] (2–5). The coordination mode of the ligands in the new zinc compounds was examined using spectroscopic methods (IR, electronic excitation (UV–Vis) spectra and 1 H NMR). 3.2. Spectroscopy 3.2.1. Infrared (IR), electronic excitation (UV–Vis) and 1H NMR spectra IR spectroscopy has been used in order to confirm the deprotonation and binding mode of the 3-OCH3-saloH and the enR bases. In the IR spectrum of the free 3-OCH3-saloH the bands stemming from the stretching and bending vibrational modes of the phenolic OH around 3200 cm1 and 1410 cm1, respectively, disappear from the spectra of all complexes indicating the ligand deprotonation [37]. Moreover, the band at 1245 cm1 originating from the m(C– O(H)) stretching vibration, in the studied complexes exhibits positive shift (100 cml) and its intensity is enhanced appreciably, denoting coordination through the phenolate oxygen (C–O(Zn)). In turn, the band at 1640 cm1, attributable to the v(HC@O) stretching vibration of the aldehyde carbonyl group is not shifted for the complexes towards lower frequencies, as it was expected [38,39], but remains at the unchanged position, showing that the aldehyde oxygen is not involved in the metal coordination. In the IR of the title complexes, the band originating from the m(C–H) stretching vibration of the OCH3 group shows small negative shift (6 cml) with respect to the corresponding band at 2839 cml of the free ligand, pointing out to a possible metal coordination through the oxygen atom of the methoxy group (Zn OCH3). The intense bands at 1602 cm1 attributed to the v(C@N)aromatic stretching vibrations are present in the compounds (2–5), denoting the coordination through the nitrogen atoms of the enR bases. In the case of [Zn(3-OCH3-salo)2(dpamH)] (5), the observed bands at 3326, 3205 and 3084 cm1 are attributed to the m(N–H)dpamH stretching vibration, while the band at 1634 cm1 to the bending mode d(N–H)dpamH of the same bond. These bands are found at the same positions as in the free dpamH, denoting the neutral character of the dpamH in the complexes. The band at 720 cm1, ascribed to the rocking vibrations of the pyridyl C-H bonds and the band at  820 cm1, attributable to the deformation vibrations of the pyridyl C-H bonds, disclosed the occurrence of the

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Fig. 3. The centrosymmetric dimer in 5 formed via N–H  O hydrogen bonds and p–p stacking interactions.

nitrogenous bases in the studied compounds. The medium-to-low intensity bands at 570 cm1 and  420 cm1 are attributed to the coordination bonds Zn–O and Zn–N, respectively, according to the literature [40]. In the visible region of the electronic spectra of zinc(II) complexes d?d transitions are not expected. The experimental absorption spectra (UV–Vis) of the title compounds in methanol solution show five dominant bands at (227, 270 and 340 nm) and (293(sh) and 310 nm) arising from intra-ligand pp⁄ transitions within the 3-OCH3-salo and enR ligands, respectively. One more excitation at  395 nm could be assigned to a charge transfer from the 3-OCH3-salo to the zinc ion. 1 H NMR spectroscopy has also been used in order to confirm the deprotonation of the salicylaldehyde in the title compounds, its coordination mode and the stability in solution. The deprotonation of the phenol group can be easily seen from the absence of the –OH signal, which is obvious to the 1H NMR spectrum of the free ligand, appearing as broad peak at d = 11.08 ppm [41–43]. The 1H NMR spectra of the title compounds (Fig. 1 is given representatively for compound 5) are consistent with the obtained structures of 1–5. All sets of signals related to the presence of the ligands in the corresponding compounds are present (five for the 3-OCH3salo and four for the enR ligands). In particular, the signals of protons in the complexes, attributable to the aldehyde group at d  9.88 ± 0.1 ppm, are practically at the same position as in the free ligand (d = 9.90 ppm), denoting the existence of non-coordinated aldehyde group. The signals of protons from the benzene ring are observed at 6.32–7.17 ppm, while those of the pyridine ring at somewhat higher d values (6.75–8.70 ppm). The peak attributable to the protons of the methoxy group is observed at d = 3.66 ppm in the complexes, showing a small downfield shift with respect to the free ligand (d = 3.92 ppm). Finally, in the compound (5) the peak attributable to the NH proton of the dpamH is observed as a single broad peak at d = 9.57 ppm.

3.3. Crystal structures For the complexes with the formula [Zn(O-CH3-salo)2(enR)] numerous structural isomers and stereoisomers can exist. Under assumption of octahedral coordination of Zn(II) and bidentate chelating mode of binding of the O-CH3-salo and enR ligands to the metal center, the following three structural isomers (I, II, III) can exist (Scheme 3): (I) the isomer having both O-CH3-salo ligands chelating the metal center via phenolate and aldehyde oxygen atoms, (II) the isomer having both O-CH3-salo ligands chelating

via phenolate and methoxy oxygen atoms and (III) the isomer having one ligand chelating via phenolate and methoxy oxygen atoms and the other one chelating via phenolate and aldehyde oxygen atoms. Moreover, for the structural isomers (I) and (II) eight chiral stereoisomers can be derived. These are two enantiomeric pairs with the C2 molecular symmetry (either two metoxy or two aldehyde O atoms situated trans to the enR N atoms) and two pairs of asymmetric stereoisomers, with phenolate and methoxy or phenolate and aldehyde O atoms located trans to the enR N atoms. For isomer (III) sixteen stereoisomers (eight enantiomeric pairs) exist. Taking into account rich structural diversity of the studied compounds with the formula [Zn(O-CH3-salo)2(enR)], X-ray crystallography had to be applied to determine the structural isomer and stereoisomer for the compounds 2 and 5. The molecular structures of [Zn(O-CH3-salo)2(bipy)].CH3OH (2) and [Zn(O-CH3-salo)2(dpamH)] (5) with the atom numbering scheme are shown in Fig. 2 and selected bond distances and angles are given in Table 2. Both compounds crystallize in centrosymmetric space groups (P21/c and P-1) and therefore are racemic. The asymmetric unit cell of 2 consists of one complex molecule and one methanol sovent molecule that is linked to the phenolate oxygen atom O2B via O–H  O hydrogen bond. The crystal of 5 contains two symmetry independent molecules. The Zn-O bond lengths to the phenolate O atoms [1.970(2)-2.005(3) Å] are significantly shorter than the corresponding bonds to the methoxy oxygen atom [2.255(2)-2.387(3) Å]. According to our expectations, the enR and O-CH3-salo ligands coordinate the zinc(II) cation in a bidentate chelating mode and the metal center is in a distorted octahedral environment formed by two nitrogen atoms from the enR ligand and four oxygen atoms from two chelating O-CH3-salo ligands. The unusual feature of the zinc(II) complexes 2 and 5 is that both consist of structural isomers of type II, i.e. the methoxy groups and not the aldehyde groups are involved in the formation of the chelate rings. In the known mononuclear metal complexes of 2-hydroxy-3-methoxybenzaldehyde (3OCH3-salo, o-vanillin) ligand with nickel(II), cobalt(II), copper(II) and vanadium(IV), aldehyde and phenolate groups were always responsible for chelation [12,13,16–17,20–21,44–48]. However, in the known mononuclear cobalt(II) and manganese(II) complexes of 4-hydroxy-3-methoxybenzaldehyde (vanillin), the vanillinate anion chelates to the divalent metal cation through the methoxy and phenolate groups, the M-Omethoxy bond being longer than the M-phenolate bond by a difference of 0.26 (Å) [49,50]. These findings are in agreement with our results, where an average difference has been found equal to 0.32 (Å).

D. Anastasiadou et al. / Polyhedron 87 (2015) 275–285

Moreover, both complexes consist of the same stereoisomer with the methoxy O atoms and the N atoms of the enR ligand defining equatorial plane and the phenolate O atoms occupying the axial positions. In 2 the complex molecule exhibits non-crystallographic C2 symmetry, however in 5 this symmetry is observed only for the ZnN2O4 coordination unit, because the symmetry of the complex molecule is broken due to different orientation of the aldehyde groups in the O-CH3-salo ligands. This change in orientation of the aldehyde groups results from the three-center hydrogen bonding interactions between the amino N–H group and phenolate and aldehyde oxygen atoms. These interactions together with p–p stacking interactions of dpamH pyridine rings lead to the formation of centrosymmetric dimers in 5 (Fig. 3).

Optimized geometry for 2 (structural isomer II)

Optimized geometry for 2a (structural isomer I)

281

3.4. Computational Studies Using DFT The molecular structure of complexes [Zn(3-OCH3-salo)2(bipy)] (2) and [Zn(3-OCH3-salo)2(dpamH)] (5) in their singlet ground state has been optimized under no symmetry constrains at the density functional level of theory using the B3LYP. Fig. 4 shows the optimized structures, whereas selected calculated bond lengths and angles concerning the coordination sphere are given in Table 3. Mean experimental bond lengths determined by X-ray structure analysis are also given for comparison. In both cases the final optimized structure has essentially C2 symmetry with the twofold rotational axis passing through the Zn atom and bisecting the NN line of the bipy and dpamH ligands. An overall agreement has been

Optimized geometry for 5 (structural isomer II)

Optimized geometry for 5a (structural isomer I)

Fig. 4. Optimized geometries of complexes [Zn(3-OCH3-salo)2(bipy)] (2) and [Zn(3-OCH3-salo)2(dpamH)] (5) and their structural isomers 2a and 5a. Only aldehyde H atoms are shown for clarity.

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Table 3 Selected bond distances (Å) and bond angles (°) calculated for complexes [Zn(3-OCH3salo)2(bipy)] (2) and [Zn(3-OCH3-salo)2(dpamH)] (5) and their structural isomers 2a and 5a.a

Zn–N(1) Zn–N(2) Zn–O(1) Zn–O(2) Zn–O(3) Zn–O(4) Zn–O(5) Zn–O(6) N(1)–Zn–N(2) N(1)–Zn–O(1) N(1)–Zn–O(2) N(1)–Zn–O(3) N(1)–Zn–O(4) N(1)–Zn–O(5) N(1)–Zn–O(6) N(2)–Zn–O(1) N(2)–Zn–O(2) N(2)–Zn–O(3) N(2)–Zn–O(4) N(2)–Zn–O(5) N(2)–Zn–O(6) O(1)–Zn–O(2) O(1)–Zn–O(3) O(1)–Zn–O(4) O(1)–Zn–O(5) O(1)–Zn–O(6) O(2)–Zn–O(3) O(2)–Zn–O(4) O(2)–Zn–O(5) O(2)–Zn–O(6) O(3)–Zn–O(4) O(3)–Zn–O(5) O(3)–Zn–O(6) O(4)–Zn–O(5) O(4)–Zn–O(6) O(5)–Zn–O(6) a b

2

5

2a

5a

2.195(2.110)b 2.195(2.111) 2.011(2.003) 2.281(2.255)

2.179(2.085) 2.173(2.091) 1.998(1.985) 2.330(2.337)

2.201 2.201 2.061

2.192 2.193 2.057

2.011(1.970) 2.280(2.274)

1.993(2.001) 2.416(2.386)

2.138 2.061

2.170 2.058

75.6(78.3) 98.7(95.0) 92.5(93.4)

87.7(90.0) 101.5(97.6) 93.7(94.5)

2.139 74.8 100.1

2.169 86.1 101.3

99.3(104.6) 166.5(170.9)

99.2(104.9) 176.8(173.7)

93.5 85.2

93.3 84.6

99.2(101.8) 166.5(171.0)

99.8(105.4) 171.8(175.5)

162.1 85.2

166.7 84.6

98.8(98.6) 92.4(92.7)

100.2(96.6) 92.6(96.1)

162.0 99.9

166.7 101.6

93.6

93.6

76.0(75.4)

75.1(74.2)

157.1(154.1) 89.3(87.8)

154.4(148.3) 87.6(82.3)

83.4 173.4

82.5 171.7

92.3

91.8

92.6

91.5

100.7

90.3

83.4

83.5

89.3(86.6) 100.0(95.7)

76.0(75.5)

87.7(82.0) 77.9(79.4)

72.7(72.9)

Numbering scheme as in Fig. 4. Mean experimental values for 2 and 5 in parentheses.

(Structural isomer 2b)

found between the calculated and experimental structure in each case with the largest deviation of bond distances being about 0.08 Å, while this of bond angles being about 6°. In both complexes the 3-OCH3-salo ligand coordinates to the zinc cation though the phenolate and methoxy oxygen atoms, whereas in a series of related complexes the ligand coordinates though the phenolate and aldehydo oxygen atoms like a normal salicylaldehydo ligand [12,13,20,21]. A full optimization of the studied Zn complexes where the 3-OCH3-salo ligands adopt the later coordination mode gave the structures 2a and 5a shown in Fig. 4, which are 1.6 and 6.7 kcal/mol higher in energy than structures 2 and 5. These energy differences, although small, are in agreement with what is experimentally found. The stability of the coordination mode adopted in complex 2 is also reflected in the adiabatic interaction energies calculated including the BSSE error. Thus, the interaction energy of the two 3-OCH3-salo ligands with the metal fragment [Zn(bipy)]2+ in the experimental structure 2 has been calculated as 391.2 kcal/mol, which is slightly higher than this calculated for structure 2a (389.5 kcal/mol). In both complexes 2 and 5 the two 3-OCH3-salo ligands adopt a mutual orientation where the two methoxy oxygen atoms adopt a cis orientation and are almost coplanar with the two nitrogen atoms of the a-diimine chelate ligand, consisting thus the [NNOO] basal plane of the octahedron, while the two phenolate oxygen atoms occupy the axial trans positions. To explore any other possible mutual orientation of the two salicylaldehydo ligands a series of calculations have been carried out for alternate configurations of complex 2. Starting from appropriate initial structures full optimizations at the same level of theory gave two stereoisomers shown in Fig. 5, being minima in the potential energy hypersurface of the molecule according to the hessian calculations. In the stereoisomer 2b, being only 3.5 kcal/mol higher in energy than stereoisomer 2, the two phenolate oxygen atoms adopt a cis orientation consisting along with the diimine nitrogen atoms the [NNOO] basal plane of the octahedron, while the two methoxy oxygen atoms occupy the axial trans positions. In the stereoisomer 2c, being 5.6 kcal/mol higher in energy than stereoisomer 2, the [NNOO]

(Structural isomer 2c)

Fig. 5. Optimized geometries of the two stereoisomers, 2b and 2c of the complex [Zn(3-OCH3-salo)2(bipy)] (2). (Hydrogens other than aldehydo not shown for clarity).

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basal plane of the octahedron includes a methoxy and a phenolate oxygen atom of each ligand, while the axial positions are occupied also by a methoxy and a phenolate oxygen atom. 3.5. Thermal studies The thermal behavior for the title compounds (1, 2, 3 and 5) was studied in nitrogen atmosphere by using the simultaneous TG/ DTG-DTA technique. The studied compounds are unstable upon heating, decomposing in many successive stages. The temperature ranges, the determined percentage mass losses, and the thermal effects accompanying the decomposition are given in Table 4. Representative thermal curves are depicted for the precursor

compound [Zn(3-OCH3-salo)2(H2O)2] (1) in Fig. 6 and for [Zn(3OCH3-salo)2(bipy)]CH3OH (2) in Fig. 7. For compound 1, in the first decomposition stage (DTGmax = 120 °C) the mass loss coincides with the release of two coordinated water molecules, while the DTA curve shows one sharp endothermic peak at 125 °C, which is attributed to melting followed by decomposition. The melting points of the studied compounds were also determined by automated melting point capillary tube system in static air, confirming the melting points found on the DTA curves. Upon further heating, the elimination of the 3-methoxy-salicyladehyde ligand takes place in fragments, mainly at DTGmax 250 °C (Table 4, Fig. 6), leading to a carbonaceous ZnO at 1000 °C. The complex nature of thermal decomposition for

Table 4 Thermoanalytical results (TG/DTG-DTF) for the zinc complexes [Zn(3-OCH3-salo)2(H2O)2] (1) and [Zn(3-OCH3-salo)2(enR)] (2, 3, 5) in nitrogen atmosphere. a/a

Stage

Trange (°C)

Evolved moiety formula

DTGmax (°C)

DTA() (°C)

Mass loss exp /% (Mass loss calc/%)

1

I II III IV Residue

30–180 200–320 320–490 490–1000 >1000

2H2O PhO {CO + OCH3} {L-O} Solid (ZnO + C)

120 250 385 520

125 250 390 520(+)

9.0 (8.9) 23.0 (23.0) 13.0 (14.6) 22.0 (33.5) 33.0 (20.2 + x)

2

I IIa IIb III IV Residue

30–120 160–220 220–350 350–700 700–1000 >1000

1 MeOH H2CO L {CH3OC6H3) bipy Solid (ZnO + C)

95 195(sh) 280 380, 510 820, 950

95 195 280 400, 480 950

5.0 (5.7) 5.0 (5.4) 27.0 (27.1) 17.5 (19.0) 20.0 (28.3) 30.0 (14.6 + x)

3

Plateau I(a + b) II Residue

30–210 210–500 500–800 >800

HL Unknown Solid (ZnO + C)

270, 280, 290

250, 270 620

27.0 (27.7) 16.0 (–) 57.0 (14.8 + x)

Plateau I(a + b) II III Residue

30–200 200–280 280–400 400–800 >800

HL {H2CO + OCH3} Unknown Solid(ZnO + C)

247, 266 322 576

231, 266 322 610

28.5 11.5 15.0 45.0

5

(28.2) (11.1) (–) (15.1 + x)

x = Non-combusted carbon, (–) endothermic thermal effect, a and b = decomposition steps. PhOH = C6H5OH, L = 3-methoxy-salicylaldehyde, L-O = Ligand-oxygen.

Fig. 6. Thermoanalytical curves (TG/DTG-DTA) for the compound [Zn(3-OCH3-salo)2(H2O)2] (1), with heating rate 10 °C min1 in N2 atmosphere.

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Fig. 7. Thermoanalytical curves (TG/DTG-DTA) for the compound [Zn(3-OCH3-salo)2(bipy)]CH3OH (2), with heating rate 10 °C min1 in N2 atmosphere.

analogous transition metal complexes has also recently been referred [19,51–53], while the kind of carbonaceous residue (ZnO and un-pyrolized compounds as organic part) has been observed in analogous zinc complexes with the Schiff base 5-bromosalicylaldehyde isonicotinoylhydrazone [53]. The thermal decomposition of compounds 2, 3 and 5 is even more complicated. For compound 2 (Fig. 7), in the first decomposition stage (DTGmax = 95 °C) the mass loss coincides with the release of one crystallized molecule of methanol, evidence arisen also from the endothermic peak on the DTA curve at 95 °C. Upon further heating, the decomposition takes place suddenly (along with the melting of [Zn(3-OCH3-salo)2(bipy)], sharp endothermic peak on DTA curve at 195 °C), with the successive elimination of one formaldehyde molecule (H2CO) and one 3-methoxy-salicylaldehyde molecule (L) at DTGmax 195 and 280 °C, respectively (Table 4, Fig. 7). The decomposition continues further with gradually mass loss of the salicylaldehyde fragments and bipy in the temperature range 400–1000 °C, leading to the mixture of carbonaceous metal oxide (ZnO + C). The compounds [Zn(3-OCH3-salo)2(phen)] (3) and [Zn(3-OCH3-salo)2(dpamH)] (5) are stable until 200 °C and suddenly decompose after melting (250 for comp. 3 and 231 °C for comp. 5) with the elimination in fragments of one molecule of the o-vanillin ligand (L) at DTGmax 270,280,290 and 247, 266 °C, respectively. Upon further heating up to 800 °C, the decomposition proceeds with gradually mass losses, which cannot be attributed with certainty to any specific moiety of the ligands in compounds 3 and 5. 4. Conclusions One precursor compound [Zn(3-OCH3-salo)2(H2O)2] (1) and four octahedral compounds [Zn(3-OCH3-salo)2(enR)] (2–5) were synthesized and characterized. It has been shown that in these complexes the 3-OCH3-salo ligands chelate the zinc(II) atom via phenolate and methoxy O atoms. This coordination mode has been predicted by 1H NMR spectroscopy and confirmed by X-ray crystallography for 2 and 5. The DFT calculations with GAUSSIAN-03 also confirm that for zinc(II) cation, the isomer with the above chelating mode of the ligand is slightly more stable than the isomers usually observed for other metal cations, with the 3-OCH3-salo ligand coordinating through the aldehyde and phenolate oxygen atoms.

The compounds are stable at ambient temperature, but unstable upon heating in nitrogen (TG/DTG-DTA technique) decomposing in several stages, giving at 1000 °C a carbonaceous mixture of ZnO. Appendix Supplementary. data CCDC 844426, 870053, 870054, 844425 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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