Inorganica Chimica Acta 383 (2012) 204–212

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Inorganica Chimica Acta

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Ortho-, meta-, and para-pyridyl oximinoacetoacetates andsilver(I)-oximinoacetoacetate networks: Synthesis and X-ray crystallographicanalyses

Muhammad Altaf ⇑, Yi Wang, Ingrid M. Socorro, Helen Stoeckli-Evans ⇑Institute of Physics, University of Neuchâtel, Rue Emile-Argand 11, CH-2000 Neuchâtel, Switzerland

a r t i c l e i n f o

Article history:Received 2 October 2010Received in revised form 2 November 2011Accepted 3 November 2011Available online 12 November 2011

Keywords:Silver(I) coordination compoundsSupramolecular networksOximinoacetoacetatesPyridyl-oximeHydrogen bonding

0020-1693/$ - see front matter � 2011 Published bydoi:10.1016/j.ica.2011.11.010

⇑ Corresponding authors. Tel.: +41 32 718 24 26; faE-mail addresses: altaf_dr@hotmail.com (M. A

unine.ch (H. Stoeckli-Evans).

a b s t r a c t

A series of novel oximinoacetoacetate ligands, ortho-, meta-, and para-pyridyloximinoacetoacetates, HL1,HL2 and HL3, respectively, have been synthesized and structurally characterized. Ligands HL2 and HL3 werefurther used for the synthesis of three new silver(I) complexes: [Ag(HL2)2]�NO3 (1) a pincer-type mononu-clear complex, [Ag2(L2)2(HL2)2] (2) a centrosymmetric dimer complex, and [Ag(L3)(HL3)]n (3) a two-dimen-sional coordination polymer. The ligands, with multiple hydrogen donor and acceptor atoms for theformation of non-covalent hydrogen bonding interactions in the solid state, resulted in the formation ofone-dimensional hydrogen bonded chains and helices. The structural studies of the AgI compounds revealthat these ligands can be efficiently used to construct hydrogen bonded two-dimensional networks. Incomplex 1 an extended two-dimensional network was formed through hydrogen bonds involving the OHgroup of the oximinoacetoacetate ligand and an O-atom of the NO3

� counter anion. Complex 2 assembled intoa two-dimensional multi-layered supramolecular network via O–H� � �O hydrogen bonds involving two ligandmolecules. In the case of complex 3 a two-dimensional coordination polymer was formed. Unlike incomplexes 1 and 2, here no classical hydrogen bonding interactions were observed in the crystal. Thepresence of non-classical C–H� � �O and C–H� � �N hydrogen bonds leads to the formation of three-dimensionalnetworks in the crystal structures of all three complexes.

� 2011 Published by Elsevier B.V.

1. Introduction cations with organic ligands, has given rise to a wide variety of

The synthesis of the solid state structures of pyridine–oxime li-gands and their silver(I) complexes relies on an understanding oftheir potential O–H� � �O and O–H� � �N intra- and intermolecularinteractions [1–6]. These non-covalent interactions compete andeventually guide the assembly of the building blocks to form ex-tended solid state low-dimensional architectures [7–9]. The chemis-try of coordination polymers has attracted intense attention as theyhave great potential as functional materials in the fields of physi-and chemical-sorption, catalysis and optics, for example [10–12].Elemental silver and silver salts have been used for decades as anti-microbial agents in curative and preventive health care [13–20].Silver(I) complexes and coordination polymers having silver(I)–Nand silver(I)–O bonds show potential antimicrobial activitiesagainst many microorganisms [21–26]. Their effective antimicrobialactivity is thought to be due to the weak silver(I)–N/O bonds, whichare easily replaced by biomolecules especially those having thiolgroups. The rapidly growing area of coordination polymers andpolymeric materials based on non-covalent interactions of metal

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fascinating one-, two-, and three-dimensional polymeric structures.The high degree of design arises from the coupling of the well under-stood coordination properties of the individual metal ions andhighly developed ligand syntheses within the newer areas of supra-molecular chemistry and crystal engineering [27–32]. In the synthe-sis of crystalline materials by design, the assembly of molecularunits in predefined arrangements is a key goal. Directional intra-and inter-molecular interactions are primary tools in achieving thisgoal, with hydrogen bonding and p–p interactions currently beingthe best amongst them [33,34]. Normally rigid components (ligandand metal) are most commonly used for the rational construction ofvarious molecular architectures [35–40]. This is a prolific area ofcurrent crystal engineering, since such materials have potentialapplications as versatile host–guest materials for catalysis or sepa-ration [41–43]. These classical intermolecular interactions havebeen employed previously as a part of an effective assembly strategyof extended supramolecular networks containing coordinationcomplexes [44,45].

A search of the Cambridge Structural Database [46], indicatedthe presence of numerous pyridine–oxime ligands, but only threecrystal structure analyses of oximinoacetoacetate type ligands[47–49]. One in particular is similar to the title compounds,2-(hydroxyimino)-3-oxo-3-phenylpropionic acid ethyl ester [47],

M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212 205

but none have been used in coordination chemistry. Here, we pre-pared three new pyridine substituted oximinoacetoacetate type li-gands and have found that two of them are particularly usefulcomponents for the construction of some interesting coordinationpolymers.

2. Experimental

2.1. Material and physical measurements

All the reactants and solvents were commercial available(Aldrich and ACROS) and were used without further purification.In all cases, the crystalline products were obtained by the slow evap-oration method. Microanalyses were carried out by Mr. D. Mooser(Ecole d’ingénieurs de Fribourg, Filière de chimie). The IR spectrawere recorded as KBr plates on a Perkin–Elmer Spectrum One FT-IR instrument. 1H and 13C NMR spectra were recorded on a BrukerAMX 200 MHz.

Warning: Perchlorate salts are dangerous and should be handledwith care in very small quantities.

2.2. Synthesis of 2-hydroxyimino-3-oxo-3-(2-pyridyl)-propanoic acidethyl ester (HL1)

To a mechanically stirred solution of 3-oxo-3-(2-pyridyl)-prop-anoic acid ethyl ester (6.4 g, 0.033 mol) and glacial acetic acid(4.7 g) was added 6.5 ml aqueous solution of sodium nitrite(2.5 g) during a period of 1 h, maintaining the temperature of thesolution at 5 �C. The solution was stirred for an additional 2 h untila large quantity of a white solid had been formed. The temperatureof the solution was raised to room temperature on leaving thesolution to stand in ambient temperature. The mixture was thenfiltered, well washed with water, and dried. Recrystallization fromwater/ethanol affords HL1 (7.1 g, 96%) as colourless crystals; mp138–139 �C; Anal. Calc. for C10H10N2O4: C, 54.05; H, 4.54; N,12.61. Found: C, 53.90; H, 4.58; N, 12.18%. mmax(KBr)/cm�1 3123,2998, 2754, 1729, 1691, 1631, 1589, 1471, 1442, 1328, 1272,1140, 1024, 941, 839, 789; 1H NMR (200 MHz; CD3OD; Me4Si):dH 8.69–8.65 (m, 1H, PyH), 8.10–7.98 (m, 2H, PyH), 7.68–7.61 (m,1H, PyH), 4.27 (q, J = 7.1 Hz, 2H, CH2), 1.23 (t, J = 7.1 Hz, CH3) ppm.

2.3. Synthesis of 2-hydroxyimino-3-oxo-3-(3-pyridyl)-propanoic acidmethyl ester (HL2)

To a mechanically stirred solution of methyl nicotinoyl acetate(1.35 g, 7.5 mmol) and glacial acetic acid (1.06 g) was added a1.5 ml aqueous solution of sodium nitrite (0.58 g, 8.4 mmol) duringa period of 1 h, maintaining the temperature of the solution at 5 �C.The solution was stirred for an additional 1 h until a large quantityof a white solid had been formed. The temperature of the solutionwas raised to room temperature on the leaving the solution to standin ambient temperature. The mixture was filtered, well-washed withwater, and dried to give crude product. Recrystallization from water/ethanol affords HL2 (1.3 g, 83%) as colourless crystals; mp 167–168 �C; Anal. Calc. for C9H8N2O4: C, 51.93; H, 3.87; N, 13.46. Found:C, 51.66; H, 3.84; N, 13.43%. mmax(KBr)/cm�1: 3110, 3005, 2497,1916, 1727, 1687, 1597, 1494, 1474, 1438, 1334, 1260, 1139, 1030,964, 833, 796, 756; 1H NMR (400 MHz; CDCL3; Me4Si): dH 15.94 (brs, 1H, NOH), 9.63–9.62 (m, 1H, PyH), 8.71–8.70 (m, 1H, PyH), 8.42–8.40 (m, 1H, PyH), 7.65–7.58 (m, 1H, PyH), 3.99 (s, 3H, CH3) ppm.

2.4. Synthesis of 2-hydroxyimino-3-oxo-3-(4-pyridyl)-propanoic acidethyl ester (HL3)

To a mechanically stirred solution of ethyl nicotinoyl acetate(19.3 g, 0.1 mol) and glacial acetic acid (14.1 g) was added 20 ml

aqueous solution of sodium nitrite (7.7 g, 0.11 mol) during a periodof 1 h, maintaining the temperature of the solution at 5 �C. The solu-tion was stirred for an additional 2 h until large quantity of thewhite solid had been formed. The temperature of the solutionwas raised to room temperature on leaving the solution to standin ambient temperature. The mixture was filtered, well-washedwith water and dried to give crude product of HL3. Recrystallizationfrom water/ethanol yielded colourless crystals (17.6 g, 80%); mp171–172 �C; Anal. Calc. for C10H10N2O4: C, 54.05; H, 4.54; N,12.61. Found: C, 53.88; H, 4.52; N, 12.32%. mmax(KBr)/cm�1: 3079,2990, 1732, 1651, 1625, 1597, 1555, 1501, 1475, 1445, 1382,1339, 1203, 1028, 995, 827, 746; 1H NMR (200 MHz; CD3OD;Me4Si): dH 84–8.81 (m, 2H, PyH), 7.79–7.76 (m, 2H, PyH), 4.30 (q,J = 7.2 Hz, 2H, CH2), 1.27 (t, J = 7.2 Hz, CH3) ppm.

2.5. Synthesis of complex [Ag(HL2)2] NO3 (1)

The solution of AgNO3 (0.34 g, 0.2 mmol) in water (10 ml) wasadded to a solution of HL2 (0.833 g, 0.4 mmol) in ethanol (20 ml)with continued stirring for 30 min. The colourless solution ob-tained was filtered to avoid any impurity and kept undisturbedfor crystallization by slow evaporation. After 3 days colourless wellshaped rod-like crystals of complex (1) were obtained. The crystalswere isolated by filtration, washed with water and air dried (0.91 g,78%); Anal. Calc. for C18H16AgN5O11: C, 36.83; H, 2.72; N, 11.94.Found: C, 36.88; H, 2.69; N, 11.93%. mmax(KBr)/cm�1: 3073, 2984,1722, 1631, 1605, 1581, 1485, 1455, 1372, 1319, 1223, 1009,971, 823, 742.

2.6. Synthesis of complex [Ag2(L2)2(HL2)2] (2)

The reaction of HL2 (0.833 g, 0.4 mmol) in ethanol (20 ml) withAgX (0.2 mmol) in water (10 ml) (X = ClO4

�, BF4�, SbF6

�, PF6�,

CH3COO�) with continued stirring at room temperature for20 min resulted in the formation of a colourless solution. The solu-tion was filtered and kept at room temperature for crystallization.After 5 days pale yellow plate-like crystals of complex (2) were ob-tained. The extra solvent was decanted off and the yellow crystal-line product was air dried and weighed (0.71 g, 68% for the reactionof HL2 with silver perchlorate); Anal. Calc. for C36H30Ag2N8O16: C,41.28; H, 2.86; N, 10.70. Found: C, 41.17; H, 2.76; N, 10.75%.mmax(KBr)/cm�1: 3071, 2986, 1723, 1635, 1606, 1583, 1489, 1445,1378, 1325, 1228, 1017, 989, 835, 735.

2.7. Synthesis of complex [Ag(L3)(HL3)]n (3)

Following the same procedure as described above for complex 2a crystalline product was obtained by the reaction of HL3 (0.889 g,0.4 mmol) in ethanol (20 ml) with AgNO3 (0.34 g, 0.2 mmol) inwater (10 ml). The light yellow crystalline product of complex (3)was obtained (0.88 g, 80%); Anal. Calc. for C20H19AgN4O8: C, 43.53;H, 3.45; N, 10.15. Found: C, 43.66; H, 3.41; N, 10.22%. mmax(KBr)/cm�1: 3068, 2987, 1721, 1645, 1608, 1598, 1543, 1512, 1477,1453, 1386, 1349, 1217, 1017, 982, 835, 743. The same productwas obtained by the reaction of HL3 with other silver(I) saltscontaining non-coordinating anions, under the same condition asdescribed for complex 2.

2.8. X-ray crystallography

The intensity data were collected at 173 K on either, a one circle(/ scans) [50], or a two circle (/ and x scans) [51] Stoe Image PlateDiffraction System, using MoKa graphite monochromated radia-tion. The structures were solved by Direct methods using the pro-gram SHELXS-97 [52]. The refinement and all further calculationswere carried out using SHELXL-97 [52]. The H-atoms were either

Table 2Selected classical hydrogen bond distances and angles (Å, �) for HL1, HL2 and HL3 andcomplexes 1, 2 and 3.

Donor–H� � �acceptor D–H D� � �A H� � �A D–H� � �A (�)

L1O2–H2O� � �O3i 0.921(19) 1.787(19) 2.7024(11) 172(2)L2O2–H2O� � �N1ii 0.90(4) 1.79(4) 2.691(2) 172(3)L3O2–H2O� � �N1iii 0.96(2) 1.70(2) 2.6505(13) 177(2)1O2–H2O� � �O5iv 0.89 1.8 2.6797(19) 170O2–H2O� � �O6v 0.89 2.45 3.112(2) 1312O2–H2O� � �O6vi 0.91(2) 1.59(2) 2.4968(17) 179(3)O2–H2O� � �N4vi 0.91(2) 2.37(2) 3.1721(17) 148(2)3O2–H2O� � �O2vii 0.84 1.62 2.443(3) 164O2–H2O� � �N2vii 0.84 2.43 3.067(3) 134

Symmetry operations: (i) �x + 1/2, y � 1/2, �z + 1/2; (ii) �x + 2, y � 1/2, �z + 1/2;(iii) x, �y � 1/2, z � 1/2; (iv) �x, �y + 2, �z; (v) x, �y + 2, z�1/2; (vi) x + 1/2, �y + 1/2, �z; (vii) �x, �y + 1, z.

206 M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212

located from Fourier difference maps and freely refined or includedin calculated positions and treated as riding atoms using SHELXL

default parameters. The non-H atoms were refined anisotropically,using weighted full-matrix least-squares on F2. Semi-empiricalabsorption corrections were applied using the MULscanABS routinein PLATON [53]. A summary of the crystal data and refinement detailsfor ligands HL1, HL2, and HL3 and compounds 1, 2 and 3 are givenin Table 1. Details of the classical hydrogen bonding are given inTable 2. Further geometric parameters for compounds 1, 2 and 3are given in Supplementary material. The figures were drawn usingthe programs PLATON [53], ORTEP3 [54] and Mercury [55].

3. Results and discussion

3.1. Structural descriptions of the ligands

The new organic ligand molecules ortho-, meta-, and para-pyridyloximinoacetoacetates (HL1, HL2 and HL3), were preparedare shown in Scheme 1. They were originally synthesized as pre-cursors for the synthesis of new substituted pyrazine ligands, forexample, pyrazine 2,5-(20-pyridyl)-3,6-dicarboxylic acid.

Suitable single crystals of ligands HL1, HL2 and HL3 for X-raydiffraction analysis were prepared by recrystallization from solu-tions in ethanol and a small amount of water. The X-ray crystallo-graphic results are consistent with the elemental analysis resultsand spectroscopic data. Their potential hydrogen bonding patternsare shown in Scheme 2.

3.1.1. Structure of 2-hydroxyimino-3-oxo-3-(2-pyridyl)-propanoicacid ethyl ester (HL1)

The molecular structure of HL1 is illustrated in Fig. 1a. The mol-ecule is T-shaped with the pyridine ring inclined to the mean planethrough the hydroxyimino propanoic acid ethyl ester moiety[atoms O2/O4/N2/C7/C8/C9, planar to within 0.101(1) Å] by79.26(9)�. The 3-oxo and the acetate C@O groups are trans to oneanother.

Table 1Crystal data and structure refinement details for ligands HL1, HL2 and HL3 and complexe

HL1 HL2a HL3

Formula C10H10N2O4 C9H8N2O4 C10H1

M 222.2 208.17 222.2Wavelength (Å) 0.71073 0.71073 0.710Temperature (K) 173 173 173Crystal system monoclinic orthorhombic monoSpace group C2/c (no. 15) P212121 (no. 19) P21/ca (Å) 20.7192(19) 5.7247(9) 13.03b (Å) 8.1493(4) 9.5387(12) 5.664c (Å) 14.3365(13) 17.044(3) 14.41a (�) 90 90 90b (�) 118.732(6) 90 99.17c (�) 90 90 90V (Å3) 2122.6(3) 930.7(2) 1050.Z 8 4 4Dcalc (Mg m�3) 1.391 1.486 1.405l(MoKa) (mm�1) 0.109 0.119 0.111F(000) 928 432 464Crystal size (mm) 0.50 � 0.26 � 0.14 0.40 � 0.30 � 0.08 0.45 �h Limits (�) 1.62–29.59 1.39–26.14 1.80–Measured reflections 14073 5930 1447Unique reflections (Rint) 2859, 0.041 1029, 0.077 2837Observed reflections [I > 2r(I)] 2364 1029 2237Goodness of fit (GOF) on F2 1.052 1.050 1.046R1(F)b [I > 2r(I)] 0.0370 0.0369 0.038wR2(F2)c [I > 2r(I)] 0.0930 0.0895 0.095Flack X parameter

a Friedel pairs were merged.b R1 =

P||Fo| � |Fc||/

P|Fo|.

c wR2 = [P

w(Fo2 � Fc

2)2/P

wFo4]1/2.

In the crystal structure of HL1 intermolecular O–H� � �O hydro-gen bonds, involving the oxime OH group and the carbonyl O-atomof the acetate group, results in the formation of a ribbon-like poly-mer, extending in the b direction, as shown in Fig. 1b.

3.1.2. Structure of 2-hydroxyimino-3-oxo-3-(3-pyridyl)-propanoicacid methyl ester (HL2)

Ligand HL2 crystallized in the non-centrosymmetric ortho-rhombic space group P21 21 21 and is illustrated in Fig. 2a. The mol-ecule is T-shaped with the with the pyridine ring inclined to themean plane through the hydroxyimino propanoic acid methyl estermoiety [atoms O2/O4/N2/C7/C8/C9, planar to within 0.016(2) Å] by74.42(17)�. As in HL1 the 3-oxo and the acetate C@O groups aretrans to one another.

s 1, 2 and 3.

1 2 3

0N2O4 C18H16AgN5O11 C36H30Ag2N8O16 C20H19AgN4O8

586.23 1046.4 551.2673 0.71073 0.71073 0.71073

173 173 173clinic monoclinic orthorhombic orthorhombic(no. 14) C2/c (no. 15) Pbca (no. 61) Fdd2 (no. 43)34(14) 17.8112(10) 8.5697(4) 34.376(2)6(3) 8.4799(6) 21.4134(7) 16.8249(9)11(14) 14.9098(8) 21.3474(8) 7.7317(5)

90 90 905(8) 100.616(4) 90 90

90 90 9034(16) 2213.4(2) 3917.4(3) 4471.8(5)

4 4 81.759 1.774 1.6380.982 1.085 0.9551176 2096 2224

0.35 � 0.14 0.40 � 0.10 � 0.10 0.30 � 0.30 � 0.09 0.27 � 0.18 � 0.1429.58 2.33–29.51 1.91–29.64 2.37–29.571 14819 46616 14969, 0.061 2989, 0.037 5293, 0.0403 2986, 0.0338

2595 4718 28370.678 1.043 1.075

7 0.0267 0.0225 0.02971 0.0787 0.0528 0.0776

– �0.03(3)

Scheme 1. Synthesis of ortho-, meta-, and para-pyridyloximinoacetoacetates.

(a) (b)Scheme 2. (a) The O–H� � �O oxime� � �acetoacetate hydrogen bonding interaction, as observed in HL1. (b) The O–H� � �N oxime� � �pyridine hydrogen bonding interaction, asobserved in HL2 and HL3.

Fig. 1. (a) The molecular structure of HL1, with displacement ellipsoids drawn at the 50% probability level. (b) A view along the a-axis of the O–H� � �O hydrogen bondinginteractions in HL1, leading to a helical chain propagating in [010].

M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212 207

In the crystal structure intermolecular O–H� � �N hydrogen bond-ing interactions, involving the oxime O–H moiety and the pyridinenitrogen atom, results in the formation of a helical chain propagat-ing in [010] (Fig. 2b).

The most notable difference between the molecular structuresof HL2 and HL1 is the presence of O–H� � �N hydrogen bondinginteractions instead of an O–H� � �O as found in HL1 (Fig. 3a). TheO–H� � �N hydrogen bonding in HL2 is comparable to that found in

Fig. 2. (a) The molecular structure of HL2, with displacement ellipsoids drawn at the 50% probability level. (b) A partial view, along the a-axis, of the formation of the O–H� � �Nhydrogen bonded helical chain arrangement propagating in [010].

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previously reported oxime structures involving pyridine deriva-tives [1–9].

3.1.3. Structure of 2-hydroxyimino-3-oxo-3-(4-pyridyl)-propanoicacid ethyl ester (HL3)

In the molecular structure of HL3 (Fig. 3a) the ester moiety isnow rotated by almost 180� from the position observed in HL1and HL2, and the 3-oxo and the acetate C@O groups are now cisto one another. Here the pyridine ring is inclined to the mean planethrough the hydroxyimino propanoic acid ethyl ester moiety[atoms O2/O4/N2/C7/C8/C9, planar to within 0.022(1) Å] by72.88(7)�.

Fig. 3. (a) The molecular structure of HL3, with displacement ellipsoids drawn at the 50%lines) helical chains propagating in [001].

In the crystal structure of HL3 (Fig. 3b) an O–H� � �N hydrogenbonding interaction is present and leads to the formation of a heli-cal chain propagating in [001]. This type of hydrogen bondinginteraction is similar to that observed for HL2 (Fig. 2b) and in otherpyridine–oxime derivatives [1–9].

3.2. Structural description of the complexes

Reactions of HL2 and HL3 with silver(I) salts resulted in the for-mation of complexes 1–3. It was not possible to identify any com-plexes formed from the reaction of HL1 with the silver(I) salts. This

probability level. (b) A view along the a-axis of O–H� � �N hydrogen bonded (dotted

Fig. 4. View of the molecular structure of complex 1, with displacement ellipsoids drawn at the 50% probability level [atoms Ag1, O5 and N3 lie on a 2-fold rotation axis;symmetry code: (i) = �x, y, 0.5 � z].

M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212 209

may be due to steric reasons owing to the proximity of the pyridineN-atom with respect to the oximinoaceto moiety.

3.2.1. Structure of silver(I) complex [Ag(HL2)2] NO3 (1)Reaction of meta-pyridyl oximinoacetoacetate (HL2) with

AgNO3 in ethanol/water gave complex [Ag(HL2)2]�NO3 (1). LigandHL2 acts as a monodentate ligand coordinating via the pyridineN-atom. The molecular structure of this pincer-like complex (1)is shown in Fig. 4.

The geometrical environment around the silver(I) atom is linearand the central metal atom lies on a 2-fold rotation axis. The Ag1–N1 bond distance is 2.2104(16) Å and the N1–Ag1–N1i bond angle,which shows considerable deviation from the ideal linear angle of180�, is 162.07(6)�. The nitrate counter ion is also located on a 2-fold rotation axis, seemingly trapped between the methyl groupsof the acetate moieties, but it is not coordinated to the metal cen-tre. In the crystal structure the nitrate anion bridges symmetry re-lated molecules via O–H� � �O hydrogen bonds involving the oximeOH H-atom. This leads to the formation of a double-stranded helixpropagating along the c axis direction (Fig. 5).

3.2.2. Structure of silver(I) complex [Ag2(L2)2(HL2)2] (2)The reaction of HL2 in ethanol with aqueous solutions of AgX

(X = ClO4�, BF4

�, SbF6�, PF6

�, CH3COO�) salts gave pale-yellowplate-like crystals of the dimeric complex [Ag2(L2)2(HL2)2] (2)

Fig. 5. A partial view along c-axis of the crystal packing in complex 1, showing O–H� � �Odouble-stranded helix propagating in [001]. C-bound H-atoms have been omitted for clareferred to the web version of this paper.)

(Fig. 6). For the synthesis of compound 2 systematic variations,including the ratio of the reactants and solvents, did not affectthe end product of the reaction. In each attempt deprotonation ofone ligand molecule occurs with this anion, L2�, coordinating withthe metal centre. All the reactions we have tried, with different sil-ver(I) salts containing non-coordinating anions (except AgNO3), re-sulted in the formation of one product only, that is, compound 2.The ligand HL2 did not show any reaction with silver(I) salts con-taining coordinating anions like AgI or AgBr, as shown in our recentstudies, using the same experimental conditions.

In complex 2 the Ag–N4oxime bond distance is 2.3882(13) Å andthe Ag1–N1pyridene and Ag1–N3pyridene bond distances are2.3240(13) and 2.2928(13) Å, respectively, while the Ag� � �O7 bonddistance is 2.6794(13) Å. In this centrosymmetric dimeric complexthe geometrical environment around the silver(I) metal centre ispseudo trigonal pyramidal. The N–Ag–N bond angles vary from119.59(4)� to 119.73(4)� and the O–Ag–N bond angles range from65.97()� to 109.37()�. It can be seen that the molecular structure ofcomplex 2 is considerably different to that of complex 1. In this di-mer complex the Ag� � �Ag distance is ca. 6.845 Å. The maximumand minimum distances between different parts of the caging li-gand molecules coordinating with the two metal centres of the di-mer are ca. 4.1 and 10.3 Å. These distances between the metalcentres and the ligand molecules are considerable for a vacantguest cavity in this complex molecule.

hydrogen bonding interactions (dashed blue lines) leading to the formation of therity. (For interpretation of the references to color in this figure legend, the reader is

Fig. 6. View of the molecular structure of the centrosymmetric dimeric complex 2, with displacement ellipsoids drawn at the 50% probability level [the molecule possesses acrystallographic inversion centre; symmetry code: (i) = �x, �y, �z].

210 M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212

In the crystal O–H� � �O hydrogen bonds, involving the oxime OHH-atom and the deprotonated NO� moiety, result in the formationof an infinite two-dimensional structure, as shown in Fig. 7. Theseclassical O–H���O hydrogen bonding interactions are the same ashave been found previously in metal–oxime complexes [7–9].

3.2.3. Structure of silver(I) complex [Ag(L3)(HL3)]n (3)The synthesis of silver(I) complex 3 was carried out following

the same procedure as described above for complex 2, but usingthis time AgNO3, and other silver(I) salts containing non-coordinat-ing anions. The X-ray single crystal analysis revealed that complex3, which crystallizes in the orthorhombic space group Fdd2, is atwo-dimensional coordination polymer. The silver(I) atom lies on

Fig. 7. A view along c-axis of the crystal packing in complex 2, showing O–H� � �O hyddimensional network. C-bound H-atoms have been omitted for clarity. (For interpretativersion of this paper.)

a 2-fold rotation axis and the geometrical environment aroundthe silver(I) atoms is distorted tetrahedral, as shown in Fig. 8.

The Ag–Npyridine bond distance is 2.212(2) Å, while theAg–Noxime bond distance is 2.581(2) Å. The N–Ag–N bond anglesvary from 76.99(7)� to 159.05(9)�. The most notable feature in thispolymeric chain structure is that each ligand molecule coordinateswith two metal centres. This bis-monodentate coordination behav-iour of the pyridine–oxime ligand HL3 is entirely different to thanobserved for ligand HL2 and other pyridine–oxime ligands re-ported previously [7–9]. Each HL3 molecule is 50% deprotonated,and the hydrogen atom is shared by the two oxygen atoms of theoxime groups of two ligand HL3 molecules related by the 2-foldrotation axis. This behaviour of the ligand molecule towards the

rogen bonding interactions (dashed blue lines) leading to the formation of a two-on of the references to color in this figure legend, the reader is referred to the web

Fig. 8. A partial view of the molecular structure of complex 3, with displacement ellipsoids drawn at the 50% probability level [the silver atom, Ag1, is situated on a 2-foldrotation axis; symmetry codes: (i) �x, �y, z; (ii) x, �0.5 + y, �0.5 + z; (iii) �x, 0.5 � y, �0.5 + z].

Fig. 9. A view along a-axis of the crystal packing of compound 3, showing the two-dimensional polymer sheets running parallel to the bc-plane. H-atoms have been omittedfor clarity.

M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212 211

AgI atom results in the formation of a novel two-dimensionalsupramolecular polymeric complex.

For complex 3, unlike in complexes 1 and 2, there are no inter-molecular O–H� � �O or N–H� � �O hydrogen bonds involved in theconstruction of the two-dimensional polymeric network (Fig. 9).The novelty here in this two-dimensional architecture is the pres-ence of the symmetric intramolecular hydrogen bonding interac-tion involving the oxime OH moieties of the ligand moleculescoordinated to the AgI atoms.

3.3. IR and NMR spectroscopic studies

The IR spectra of ligands HL1, HL2 and HL3 and the silver(I)complexes 1–3 were recorded in the range of 4000–450 cm�1

and are characterized by a number of absorption bands of theuncoordinated and coordinated pyridine–oxime ligands, apartfrom the characteristic bands for m(C@N, C@O, aromatic and non-aromatic hydrogen 1500–1600, 1700–1750, 2900–2990 and3000–3100 cm�1, respectively). Compared to those in the ligandsHL2 and HL3 a low frequency shift in the m(C@N and C@O) bandsof the complexes 1–3 was observed, which is characteristic of com-plex formation. A sharp absorption band around 1300 cm�1 forNO3

� bending was observed for complex 1, indicating the presenceof a non-coordinated NO3

� counter ion [56–59]. The 1H and 13CNMR spectra of ligands HL1, HL2 and HL3 did not show any unu-sual peaks, and were consistent with the single crystal X-ray struc-tures. For the silver(I) complexes, due to their very low solubility in

common organic solvents, it was not possible to carry out NMRspectroscopic analysis.

4. Conclusions

The three new pyridine substituted oximinoacetoacetates HL1,HL2 and HL3 have been synthesized and found to form helicalchains in the solid state, via O–H� � �O or O–H� � �N hydrogen bonds.Simple and systematic reactions of silver(I) salts with HL2 gave amononuclear complex (1) and a dimeric complex (2), while atwo-dimensional coordination polymer (3) was obtained using li-gand HL3. The coordination spheres of the silver(I) atoms werefound to be linear in 1, pseudo trigonal pyramidal in 2, and distortedtetrahedral in 3. As a result of the hydrogen bonding capabilities ofthese ligands in complexes 1 and 2 two-dimensional supramolecu-lar networks were obtained. Complex 3 was shown to be a two-dimensional coordination polymer, and in the crystal structure noclassical hydrogen bonds were present. In the crystal structures ofall three complexes non-classical C–H� � �O and C–H� � �N hydrogenbonds are present (see Supporting information Tables S4–S6). Theygive rise to the formation of three-dimensional networks for 1, 2and 3 (see Supporting information, Figs. S1–S3). It is interestingto note that both the organic ligands and especially the silver(I)coordination compounds are light stable. However, the low solubil-ity of the silver(I) complexes did not allow us to carry out othertests to explore their physical, biochemical or chemical properties.

212 M. Altaf et al. / Inorganica Chimica Acta 383 (2012) 204–212

Acknowledgement

This work was supported by the Swiss National Science founda-tion (Grants FN 20.111738 and 20-119924).

Appendix A. Supplementary material

CCDC 726516, 726517, 726518, 726519, 726520 and 726521contain the supplementary crystallographic data for HL1, HL2,HL3, 1, 2 and 3. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2011.11.010.

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