Polyhedron 59 (2013) 69–75

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Polyhedron

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Palladium(II) complexes with symmetrical dihydroxy-2,20-bipyridine ligands:Exploring their inter- and intramolecular interactions in solid-state

Eder Tomás-Mendivil, Josefina Díez, Victorio Cadierno ⇑Laboratorio de Compuestos Organometálicos y Catálisis (Unidad Asociada al CSIC), Departamento de Química Orgánica e Inorgánica,Instituto Universitario de Química Organometálica ‘‘Enrique Moles’’, Universidad de Oviedo, Julián Clavería 8, 33006 Oviedo, Spain

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

Article history:Received 8 January 2013Accepted 24 April 2013Available online 3 May 2013

Keywords:Palladium complexesBipy ligandsDihydroxy-2,20-bipyridinesCrystal structuresHydrogen bonds

0277-5387/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.poly.2013.04.043

⇑ Corresponding author.E-mail address: vcm@uniovi.es (V. Cadierno).

The structures of complexes [PdCl2{3,30-(OH)2-2,20-bipy}] (1), [PdCl2{4,40-(OH)2-2,20-bipy}] (2),[PdCl2{5,50-(OH)2-2,20-bipy}] (3) and [PdCl2{6,60-(OH)2-2,20-bipy}] (4) were determined by means ofX-ray diffraction studies, employing in all cases crystals grown from DMF solutions. We have found that,depending on the exact location of the OH groups on the 2,20-bipyridine skeleton, the chloride ligands areforced to interact with different C(sp2)-hydrogens of the bipy ligands in neighboring molecules, and theseintermolecular interactions seem to govern the formation of the corresponding networks. In general, theOH groups interact by H-bonding with DMF molecules of crystallization or other OH groups, thus com-pleting the crystals.

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1. Introduction

2,20-Bipyridine (bipy) is one of the most popular chelating li-gands in coordination and organometallic chemistry, complexeswith virtually all transition metals being presently known. Reflect-ing the popularity of this ligand design, many substituted variantsof bipy have been described [1]. Among them, dihydroxy-2,20-bipyridines have emerged in recent years as promising auxiliary li-gands for homogeneous catalysis given the solubility in water thatimpart to their complexes, their enhanced donor properties, andtheir ability to generate hydrogen bonds with the substrates nearthe active metal center (metal–ligand bifunctional catalysis) [2].In this context, we have recently described the preparation andspectroscopic characterization of a series of palladium(II) com-plexes 1–4 containing symmetrical dihydroxy-2,20-bipyridine li-gands (Fig. 1), which behaved as efficient catalysts in theconjugate addition of arylboronic acids to a,b-unsaturated car-bonyl compounds in water [3].

On the other hand, the harnessing of intermolecular forces forthe rational assembly of molecular building blocks, with the ulti-mate goal of designing extended supramolecular structures, is afundamental challenge in chemistry [4]. In addition to the coordi-native-bond approach, widely used in the design of supramolecu-lar coordination compounds and polymers [5], intermolecularhydrogen-bonding represents an essential force for the self-organi-zation of metal complexes into extended 3D networks [6]. This fact

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prompted us to undertake crystallographic studies on complexes1–4 since the presence in these compounds of both hydrogen-bonddonor (OH units) and acceptor (chloride ligands) [7] groups withinthe same molecule, along with its rigid square-planar nature, couldlead to the formation of extended structures in the solid-state. Thedifferent location of the hydroxyl groups in the bipy ligand skele-ton should also result in different hydrogen-bonded networks.The crystallographic results obtained are presented herein. Wemust stress at this point that, to the best of our knowledge, thisis the first systematic structural study performed on a family ofisomeric metal-complexes with symmetrically disubstituted 2,20-bipyridine ligands.

2. Experimental

2.1. General information

Complexes [PdCl2{3,30-(OH)2-2,20-bipy}] (1), [PdCl2{4,40-(OH)2-2,20-bipy}] (2), [PdCl2{5,50-(OH)2-2,20-bipy}] (3) and [PdCl2{6,60-(OH)2-2,20-bipy}] (4) were prepared by following the methodsrecently described by us [3].

2.2. X-ray crystallography

Crystals of complexes 1–4 suitable for X-ray diffraction analysiswere grown in dimethylformamide (DMF) by slow cooling of a hotsolution to r.t. (4), or by slow diffusion of toluene (1–2) or diethylether (3) into a saturated solution of the corresponding compound.The most relevant crystal and refinement data are collected in

Fig. 1. Structure of the Pd(II) complexes 1–4.

70 E. Tomás-Mendivil et al. / Polyhedron 59 (2013) 69–75

Table 1. In all the cases, data collection was performed on a OxfordDiffraction Xcalibur Nova single crystal diffractometer, using Cu Karadiation (k = 1.5418 Å). Images were collected at a 65 (1) or63 mm (2–4) fixed crystal-detector distance, using the oscillationmethod, with 1� oscillation and variable exposure time per image(15–50 s for 1, 1.5–5 s for 2 and 1.5 s for 3–4). Data collection strat-egy was calculated with the program CRYSALIS PRO CCD [8]. Data reduc-tion and cell refinement were performed with the program CRYSALIS

PRO RED [8]. An empirical absorption correction was applied usingthe SCALE3 ABSPACK algorithm as implemented in the program CRYSALIS

PRO RED [8].

Table 1Crystal data and structure refinement for compounds 1–4.

1 2

Empirical formula 2(C10H8Cl2N2O2Pd)�DMF C1

Formula weight 804.06 51T (K) 100(2) 12k (Å) 1.5418 1.5Crystal system monoclinic triSpace group P21/c P�1Unit cell dimensionsa (Å) 7.6294(2) 9.3b (Å) 30.0905(7) 14c (Å) 11.4805(3) 17a (�) 90 68b (�) 90.944(2) 76c (�) 90 75V (Å3) 2635.25(12) 20Z 4 4Dcalc (g cm�3) 2.027 1.6Absorption coefficient (mm�1) 15.153 10F(000) 1584 10Crystal size (mm) 0.108 � 0.034 � 0.021 0.1h (�) 2.94–74.29 3.3Index ranges �9 6 h 6 9,

�37 6 k 6 35,�11 6 l 6 13

�1�1�2

Reflections collected 9794 14Independent reflections (Rint) 4762 (0.0249) 78Completeness to theta max. 88.5% 95Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 4762/11/347 78Goodness-of-fit (GOF) on F2 1.041 1.0Weight function (a, b) 0.0960, 18.6751 0.1R1

a [I > 2r(I)] 0.0535 0.0wR2

a [I > 2r(I)] 0.1496 0.1R1 (all data) 0.0592 0.0wR2 (all data) 0.1553 0.1Largest difference in peak and hole (e �3) 1.682 and �5.184 4.3

a R1 = R(|Fo| � |Fc|)/R|Fo|; wR2 = {R[w(Fo2 � Fc

2)2]/R[w(Fo2)2]}½.

The software package WINGX [9] was used in all the cases forspace group determination, structure solution and refinement.For 1 and 4, the structures were solved by Patterson interpretationand phase expansion using DIRDIF [10]. For 2 and 3, the structureswere solved by direct methods using SIR92 [11] or SIR2004 [12],respectively. Isotropic least-squares refinement on F2 usingSHELXL97 [13] was performed. During the final stages of the refine-ments, all the positional parameters and the anisotropic tempera-ture factors of all the non-H atoms were refined. The H atoms weregeometrically located and their coordinates were refined riding ontheir parent atoms. The H1O, H2O, H3O and H4O atoms of 1, andthe H2 atom of 3, were found from different Fourier maps andincluded in a refinement with isotropic parameters. In the crystals1 and 2 two independent molecules of the complex were found inthe asymmetric unit. In the crystal of 1, a highly disordered DMFmolecule of solvation per two molecules of the complex wasfound. In the crystals of 2 and 3, two DMF molecules of solvationper formula unit of the complex were found. On the other hand,in the crystal of 2 the Pd and Cl atoms were disordered, and theywere located in two positions with occupancy of 86% and 14%. Inall cases, the maximum residual electron density is located nearto heavy atoms (i.e. palladium and chlorine atoms). The functionminimized was [RwFo

2 � Fc2)/Rw(Fo

2)]1/2 where w = 1/[r2(Fo2) +

(aP)2 + bP] (a and b values are collected in Table 1) with r2(Fo2)

from counting statistics and P = (Max(Fo2 + 2Fc

2)/3. Atomic scatter-ing factors were taken from reference [14]. Geometrical calcula-tions were made with PARST [15]. The crystallographic plots weremade with ORTEP-3 [16].

3 4

0H8Cl2N2O2Pd�2DMF C10H8Cl2N2O2Pd�2DMF C10H8Cl2N2O2Pd1.68 511.68 365.483(1) 123(1) 297(7)418 1.5418 1.5418

clinic orthorhombic monoclinicPbcn P21/n

322(4) 7.6129(2) 10.2443(2).2849(7) 18.0718(4) 6.8522(1).0598(7) 14.5222(4) 16.6101(2).071(4) 90 90.565(4) 90 96.473(2).789(4) 90 9019.95(16) 1997.95(9) 1158.53(3)

4 483 1.701 2.095.107 10.219 17.10432 1032 71218 � 0.09 � 0.043 0.423 � 0.153 � 0.043 0.186 � 0.125 � 0.0319–74.29 5.77–74.24 4.84–74.441 6 h 6 11,7 6 k 6 17,1 6 l 6 18

�9 6 h 6 9,�22 6 k 6 21,�17 6 l 6 16

�12 6 h 6 12,�7 6 k 6 8,�20 6 l 6 19

876 4385 662647 (0.0275) 1984 (0.0546) 2283 (0.0365).1% 96.6% 96.3%

47/0/540 1984/0/127 2283/0/15438 1.044 1.117078, 3.1562 0.1524, 0.6205 0.0762, 0.0000572 0.0672 0.0477603 0.1961 0.1112673 0.0744 0.0514709 0.2095 0.117692 and �1.146 1.708 and �1.455 0.784 and �1.672

Fig. 2. ORTEP-type views of the molecular structures of 1–4 with the crystallographic labelling schemes. Thermal ellipsoids are drawn at the 30% probability level.

Table 2Comparative bond lengths (Å), angles (�) and deviations from the mean PdCl2N2 plane (Å) for complexes 1–4 and [PdCl2(bipy)].

1 2 3 4 [PdCl2(bipy)]a

Bond lengthsPd(1)–Cl(1) 2.304(3) 2.303(2) 2.2914(13) 2.3062(14) 2.317(3)Pd(1)–Cl(2) 2.304(3) 2.298(2) 2.2914(13)b 2.3082(14) 2.277(3)Pd(1)–N(1) 2.007(11) 2.051(5) 2.026(4) 2.058(4) 2.03(1)Pd(1)–N(2) 2.015(12) 1.995(5) 2.026(4)c 2.054(4) 2.03(1)C–O(1) 1.334(17) 1.327(7) 1.348(8) 1.317(6)C–O(2) 1.344(17) 1.318(7) 1.348(8)d 1.306(7)

Bond anglesCl(1)–Pd(1)–Cl(2) 88.15(12) 90.51(9) 89.19(7)e 82.60(7) 89.9(1)Cl(1)–Pd(1)–N(1) 95.7(3) 94.43(14) 95.33(13) 98.23(11) 95.3(3)Cl(1)–Pd(1)–N(2) 176.1(3) 174.51(15) 175.40(13)f 178.67(11) 175.0(3)Cl(2)–Pd(1)–N(1) 176.1(3) 173.90(15) 175.40(13)g 177.08(11) 174.5(3)Cl(2)–Pd(1)–N(2) 95.7(3) 94.72(15) 95.33(13)h 98.49(12) 94.4(4)N(1)–Pd(1)–N(2) 80.4(4) 80.46(19) 80.2(2)i 80.65(15) 80.5(4)

DeviationsPd(1) �0.0009(1) 0.0140(1) 0.0000(1) 0.0014(3) 0.0034(1)Cl(1) �0.0057(1) 0.0415(1) �0.0130(1) �0.0035(14) �0.0390(1)Cl(2) 0.0061(1) �0.0491(1) 0.0130(1)j �0.0122(13) 0.0380(1)N(1) 0.0076(1) �0.0599(1) 0.0160(1) �0.0869(33) �0.0488(1)N(2) 0.0061(1) 0.0534(1) �0.0160(1)k �0.0211(33) �0.0464(1)

a Data taken from Ref. [10].b Pd(1)–Cl(10).c Pd(1)–N(10).d C(20)–O(10).e Cl(1)–Pd(1)–Cl(10).f Cl(1)–Pd(1)–N(10).g Cl(10)–Pd(1)–N(1).h Cl(10)–Pd(1)–N(10).i N(1)–Pd(1)–N(10).j Deviation of Cl(10).k Deviation of N(10).

E. Tomás-Mendivil et al. / Polyhedron 59 (2013) 69–75 71

Fig. 3. View of the intermolecular interactions present in the structure of complex [PdCl2{3,30-(OH)2-2,20-bipy}] (1) (Pd, light brown; Cl, green; C, grey; N, dark blue; O, red; H,light blue). (Colour online).

Fig. 4. Interactions within the repetitive units present in the structure of[PdCl2{3,30-(OH)2-2,20-bipy}] (1).

72 E. Tomás-Mendivil et al. / Polyhedron 59 (2013) 69–75

3. Results and discussion

Crystals of complexes 1–4 suitable for X-ray diffraction analysiswere grown in dimethylformamide (DMF) by slow cooling of a hotsolution to r.t. (4), or by slow diffusion of toluene (1–2) or diethylether (3) into a saturated solution of the corresponding compound.DMF was used as solvent since it was the only one that provided agood solubility and stability of all the complexes studied. Exceptfor [PdCl2{6,60-(OH)2-2,20-bipy}] (4), crystals containing DMF mol-ecules of solvation were in all the cases obtained. Thus, for[PdCl2{3,30-(OH)2-2,20-bipy}] (1), two crystallographically indepen-dent molecules of the complex and one DMF molecule were foundin the asymmetric unit. For its side, in the asymmetric unit of[PdCl2{4,40-(OH)2-2,20-bipy}] (2), two independent molecules ofthe complex solvated with four molecules of DMF were found. Inthe case of [PdCl2{5,50-(OH)2-2,20-bipy}] (3), two DMF moleculesper molecular unit of the complex were also present in the crystal.

ORTEP plots of the molecular geometries of complexes 1–4 areshown in Fig. 2 and selected bonding parameters collected inTable 2.1 For all of them, the geometry around the Pd atom is almostideal square planar, with a maximum deviation from the meanPdCl2N2 plane of 0.0869(33) Å for the N(1) atom of 4. The Pd-coordi-nation is characterized by metal-centered angles between 80.2(2)�and 98.49(12)�, with the two chloride ligands mutually cis disposed.These values, along with the Pd–Cl and Pd–N bond distances ob-served, fit well with those previously reported for [PdCl2(bipy)][17]. As observed for this model compound, the 2,20-bipyridine skel-etons in complexes 1–4 are almost planar, with a maximum twistangle of 6.52� for the pyridine rings of complex 4 (0.57–4.02� forcomplexes 1–3 and 2.98� for [PdCl2(bipy)]). It is worthy of note that,in the structure of [PdCl2{3,30-(OH)2-2,20-bipy}] (1), the close prox-imity of the two hydroxyl substituents enables the formation of anintramolecular hydrogen bond between both groups (see Fig. 2)[18,19].2,3 According to the classification of Jeffrey [20], the distancesand angle of the O(2)–H� � �O(1) contact (O(2)–H = 0.899 Å, H–

1 For [PdCl2{3,30-(OH)2-2,20-bipy}] (1) and [PdCl2{4,40-(OH)2-2,20-bipy}] (2) we onlyshow and give the data of one of the two independent molecules present in theasymmetric unit.

2 Such an intramolecular interaction has been observed in the structures of[Ru(bipy)2{3,30-(OH)2–2,20-bipy}][BPh4] and [RuCl(g6-tha){3,30-(OH)2–2,20-bipy}](tha = tetrahydroanthracene), which represent the only examples of metal complexeswith a 3,30-dihydroxy-2,20-bipyridine ligand characterized to date by single-crystal X-ray diffraction:

3 We must note that, in complexes containing the 3,30-dihydroxy-2,20-bipyridineligand, the steric strain associated to the close proximity of the two hydroxyl groupsis usually released by the spontaneous deprotonation of one the OH groups. This leadsto the formation of a stable seven-membered ring through a strong O� � �H� � �Ointeraction. See ref. [18] and [19].

O(1) = 1.571 Å, O(2)–O(1) = 2.417 Å and O(2)–H–O(1) = 155.17�) al-low it to be classified as ‘‘moderate’’ among the H-bonds consideredmost common in chemical systems. Intramolecular H-bonds of mod-erate intensity are also established between the hydroxyl groups ofthe bipy unit and the chloride ligands of complex [PdCl2{6,60-(OH)2-2,20-bipy}] (4) (O(1)–H = 0.820 Å, H–Cl(1) = 2.064 Å, O(1)–Cl(1) = 2.856 Å and O(1)–H–Cl(1) = 162.11�; O(2)–H = 0.820 Å,H–Cl(2) = 2.064 Å, O(2)–Cl(2) = 2.861 Å and O(2)–H–Cl(2) = 164.27�)(Fig. 2). It is important to emphasize at this point that, to our knowl-edge, [PdCl2{5,50-(OH)2-2,20-bipy}] (3) is the first example of a metalcomplex containing the 5,50-dihydroxy-2,20-bipyridine ligand struc-turally characterized by X-ray diffraction methods [21].4

With regard to the intermolecular interactions present in thecrystals, in contrast to our expectations, H-bonds between thechloride ligands acceptors and the bipy–OH donors of adjacentmolecules were in no case observed. Thus, in the case of complex[PdCl2{3,30-(OH)2-2,20-bipy}] (1), it forms bimolecular aggregatesin which the two molecules of 1 interact through the OH groupsby H-bonding (see Figs. 3 and 4). The strength of this intermolecu-lar O(1)–H� � �O(2)0 H–bond is comparable with that of theintramolecular one found in the structure of this derivative(O(1)–H = 0.906 Å, H–O(2)0 = 1.532 Å, O(1)–O(2)0 = 2.405 Å andO(1)–H–O(2)0 = 156.15�). An additional H-bond is also establishedbetween one of the [PdCl2{3,30-(OH)2-2,20-bipy}] molecules ofthese aggregates and the DMF molecule of solvation. In the crystal,

4 A search in the Cambridge Structural Database (CSD) revealed only one previousexample containing the 6,60-dihydroxy-2,20-bipyridine ligand (see Ref. [2e]), andthree others with 4,40-dihydroxy-2,20-bipyridine: See Refs. [2b] and [21]

Fig. 5. View of the intermolecular interactions present in the structure of complex [PdCl2{4,40-(OH)2-2,20-bipy}] (2) (Pd, light brown; Cl, green; C, grey; N, dark blue; O, red; H,light blue). (Colour online).

Fig. 6. Interactions within the repetitive units present in the structure of[PdCl2{4,40-(OH)2-2,20-bipy}] (2).

Fig. 7. View of the intermolecular interactions present in the structure of complex[PdCl2{5,50-(OH)2-2,20-bipy}] (3) (Pd, light brown; Cl, green; C, grey; N, dark blue; O,red; H, light blue). DMF molecules of solvation have been omitted for clarity.(Colour online).

Fig. 8. The angle between the layers formed by complex [PdCl2{5,50-(OH)2-2,20-bipy}] (3).

E. Tomás-Mendivil et al. / Polyhedron 59 (2013) 69–75 73

these bimolecular units are linked together through weak intermo-lecular interactions, involving the chloride ligands and the aro-matic hydrogen atoms located at the 5,50-positions of the bipy

ligands in immediately neighboring molecules (Pd–Cl� � �H–C(sp2)distances within the range 2.776–2.828 Å), thus forming aextended 2D network (see Fig. 3) [22].

Bimolecular units can also be identified in the structure of[PdCl2{4,40-(OH)2-2,20-bipy}] (2) (see Figs. 5 and 6). In them, thetwo molecules of the complex are now connected through weakinteractions between the chloride ligands and the aromatic hydro-gen atoms at the 5,50-positions of the bipy skeletons (Pd–Cl� � �H–C(sp2) distances within the range 2.804–3.216 Å), closely relatedto those found in the crystal lattice of [PdCl2{3,30-(OH)2-2,20-bipy}](1). In addition, both molecules of [PdCl2{4,40-(OH)2-2,20-bipy}] areH-bonded to DMF molecules, one of them through the two chlorideligands (with one molecule of DMF), and the other one through theOH groups (with two molecules of DMF) and one chloride ligand(with one molecule of DMF). One of the DMF molecules of solva-tion acts as bridge between these bimolecular units, through aweak H-bond interaction between its C(@O)–H hydrogen and oneof the hydroxyl groups of an adjacent bipy (C–H = 0.930 Å, H–O(1) = 2.612 Å, C–O(1) = 3.525 Å and C–H–O(1) = 167.20�), leadingto polymeric 1D chains (see Fig. 5).

As for complexes 1 and 2, the molecules of [PdCl2{5,50-(OH)2-2,20-bipy}] (3) also interact in the solid state through weakPd–Cl� � �H–C(sp2) bonds (Figs. 7 and 8). However, since the 5,50-positions of the bipy ligand are in this case occupied by the hydro-xyl groups, the chloride ligands are now forced to interact withalternative aromatic hydrogens, in particular with those located

Fig. 9. View of the intermolecular interactions present in the structure of complex [PdCl2{6,60-(OH)2-2,20-bipy}] (4) (Pd, light brown; Cl, green; C, grey; N, dark blue; O, red; H,light blue) (Colour online).

Fig. 10. The Pd� � �Pd interactions within the layers of complex [PdCl2{6,60-(OH)2-2,20-bipy}] (4).

74 E. Tomás-Mendivil et al. / Polyhedron 59 (2013) 69–75

in the 3,30- and 4,40-positions of neighbouring molecules (distanceswithin the range 2.999–3.030 Å) (see Fig. 7). The molecules of 3 arearranged in the crystal lattice in layers, interconnected throughthis type of Pd–Cl� � �H–C(sp2) contacts, that form between theman angle of 52.44� (Fig. 8). Similarly to [PdCl2{4,40-(OH)2-2,20-bipy}](2), the hydroxyl groups of [PdCl2{5,50-(OH)2-2,20-bipy}] (3) onlyinteract with the DMF molecules of solvation through H-bonds(for clarity the DMF molecules are not shown in Figs. 7 and 8; acomplete figure including these DMF molecules can be found inthe Supplementary material file).

Finally, with regard to complex [PdCl2{6,60-(OH)2-2,20-bipy}](4), in which DMF molecules are not present in the crystals,5 it gen-erates a 2D network in which the molecules of the complex interactagain through weak Pd–Cl� � �H–C(sp2) contacts, now involving thehydrogen atoms located on the carbons at the 3,30-positions of thebipy ligand (distances in the range 2.802–2.892 Å) (Fig. 9). The linearchains thus formed are interconnected by H-bonds between the OHgroups and the aromatic hydrogens at the 4,40-positions of the bipyligands in neighbouring chains (O� � �H–C(sp2) distances in the range2.583–2713 Å).

5 This compound shows a much lower solubity in DMF in comparison with itsisomers 1–3. This fact, along with the presence of intramolecular O(1)–H� � �Cl(1) andO(2)–H� � �Cl(2) H-bonds (see Fig. 2), could explain its void affinity by DMF.

It is also worthy of note that the molecules of [PdCl2{6,60-(OH)2-2,20-bipy}] (4) stack in the crystal in an alternating A� � �B� � �A� � �Bfashion along the crystallographic a axis (see Fig. 10). A shortPd� � �Pd separation of 3.45 Å was found within these stacks (theassociated Pd� � �Pd� � �Pd angle is 166.80�), suggesting the possibilityof Pd–Pd bonding [23].6 This Pd� � �Pd interaction could explain themost intense yellow colour of the crystals of 4 in comparison withthose of 1–3.

4. Conclusion

In summary, the solid-state structures of four Pd(II) complexescontaining 2,20-bipyridine ligands symmetrically disubstitutedwith hydrogen-bond donor OH units have been determined bymeans of single-crystal X-ray diffraction techniques. As expected,depending on the exact location of the OH groups on the 2,20-bipyridine skeleton, the arrangement of the molecules in the crys-tal is different. In all the cases, the intermolecular interactions thatthe chloride ligands establish with the aromatic C(sp2)–H hydro-gens of neighbouring molecules govern the formation of thecorresponding networks. In general, the OH groups interact byH-bonding only with DMF molecules of solvation or other OHgroups, but not with the chloride ligands as initially anticipated.To the best of our knowledge, this is the first systematic structuralstudy of a family of isomeric metal-complexes with 2,20-bipyridineligands symmetrically disubstituted in the four positions of thepyridyl skeletons.

Acknowledgements

This work was supported by the Spanish MINECO (projectsCTQ2010-14796/BQU and CSD2007-00006). E.T.-M. thanks MECDof Spain and the European Social Fund for the award of a Ph.D.grant (FPU program).

Appendix A. Supplementary data

CCDC 914127–914130 contain the supplementary crystallo-graphic data for compounds 1–4. These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: depos-

6 Related arrangements in stacks with metal–metal interactions have beenpreviously described in the solid-state crystal structures of compounds [MCl2(bipy)](M = Pd, Pt), the distance of 3.45 Å in 4 being identical to that found in [PtCl2(bipy)]and remarkably shorter than that of [PdCl2(bipy)] (4.58 Å). See Refs. [17] and [23].

E. Tomás-Mendivil et al. / Polyhedron 59 (2013) 69–75 75

it@ccdc.cam.ac.uk. Supplementary data associated with this articlecan be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2013.04.043.

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[22] These interactions are commonly observed in palladium–chloride complexeswith bipy-type ligands. For a very recent example, see: N. Lu, Y.-M. Ou, T.-Y.Feng, W.-J. Cheng, W.-H. Tu, H.-C. Su, X. Wang, L. Liu, M.D. Hennek, T.S. Sayler,J.S. Thrasher, J. Fluorine Chem. 137 (2012) 54.

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