Evidence of Chelate−Chelate Stacking Interactions in Crystal

DOI: 10.1021/cg100312r

Evidence of Chelate-Chelate Stacking Interactions in Crystal Structures of Transition-Metal Complexes

2010, Vol. 10 3901–3908

Dusan N. Sredojevic,† Zoran D. Tomic,‡ and Snezana D. Zaric*,† †

Department of Chemistry, University of Belgrade, Studentski trg 16, 11000 Belgrade, ca Institute of Nuclear Sciences, Laboratory of Theoretical Physics and Condensed Serbia, and ‡Vin Matter Physics, P.O. Box 522, 11001 Belgrade, Serbia Received March 10, 2010; Revised Manuscript Received June 30, 2010

ABSTRACT: Evidence of chelate-chelate stacking interactions was obtained by analyzing crystal structures of square-planar transition-metal complexes from the Cambridge Structural Database. The analysis showed that chelate-chelate stacking interactions occur in a large number of the crystal structures of neutral square-planar complexes. We found 955 structures with 1866 chelate-chelate stacking interactions. In most of the structures, chelate rings are fused with other chelate rings or with organic aromatic rings; however, chelate-chelate stacking interactions of isolated rings were also found. In order to describe the geometry of the chelate-chelate stacking interactions, several geometric parameters were analyzed. In most of the interactions, two chelate rings are in parallel or antiparallel orientation. The normal distances are similar to the distances of stacking interactions observed in other systems, while the offset of two interacting chelate rings can be different from that observed in other systems.

Introduction Noncovalent interactions of aromatic and other π-systems, including stacking interactions, are very important in various molecular systems, from biomolecules to crystal packing.1 However, the characterization of stacking interactions is a challenge for both experimental and theoretical work. Recently, several new methods have been developed and used intensively to study stacking interactions.2-4 Most of these studies consider organic aromatic molecules; however, other planar molecules and fragments can also be involved in stacking interactions.5-10 We showed that a water molecule can form a parallel alignment interaction with C6 aromatic rings.5 Analyses of crystal structures from the Cambridge Structural Database (CSD) showed that a water molecule or one of its O-H bonds can be found parallel to the aromatic ring plane at distances typical for stacking interactions. The interaction energies obtained by ab initio calculations performed on model systems are as large as 1.60 kcal mol-1. The calculations and the data observed in the crystal structures suggest that the water molecule prefers a position above the C-H region over one above the ring.5 In transition-metal complexes, planar chelate rings with delocalized π-bonds can form stacking6-9 interactions similar to those of aromatic organic molecules.4 These observations could be connected with speculations of aromatic character in planar chelate rings with delocalized π-bonds.11 Our previous results show that there are stacking interactions between chelate and C6-aromatic rings in crystal structures of squareplanar transition-metal complexes.6-8 The crystal structures show mutual slipped-parallel (offset face-to-face) orientation between chelate and C6-aryl rings similar to the slipped-parallel orientation of two benzene rings.4 However, C6-aromatic rings can also form CH-π interactions with chelate rings, where hydrogen atoms of C6-aryl rings are perpendicular to the π-system of the chelate ring.12 The analyses of the tendency

for stacking vs CH-π interactions in crystal structures of square-planar transition-metal complexes showed that the number of stacking interactions is a few times larger than the number of CH-π interactions. We also found out that most of the chelate rings in the crystal structures are fused with aromatic or other π-delocalized chelate rings. This observation is in agreement with the fact that condensed aromatic systems with extended π-delocalization tend to orient their planes parallel.3 Based on the fact that chelate rings with delocalized π-bonds can form a noncovalent interaction similar to those of aromatic organic molecules, one can anticipate that two chelate rings could form a mutual stacking interaction. Here we present evidence of chelate-chelate stacking interactions by analyzing crystal structures from the Cambridge Structural Database. To the best of our knowledge, this is the first study of the stacking interactions between two chelate rings in crystal structures from the Cambridge Structural Database (CSD). Methodology

*Corresponding author. Telephone: 381 11 3336 605. Fax: 381-11-638785. E-mail: [email protected].

It was shown that the analysis of the data from the Cambridge Structural Database (CSD)13 can be used to obtain important conclusion.5-8,14,15 A CSD search was performed using the Quest 3D16 program, to extract all structures of square-planar transitionmetal complexes with chelate rings. This program was used to retrieve structures satisfying the following criteria: (a) the crystallographic R factor e10%; (b) error-free coordinates according to the criteria used in the CSD system; (c) no crystallographic disorder; (d) no polymeric structures; (e) the metal atom is coordinated by exactly four atoms according to the criteria used in the CSD. The search was based on the fragments defined in Figure 1. To ensure square-planar coordination, trans angles in the ML4 fragment were restricted to lie in the range 150-180°. In order to ensure planarity in the chelate rings, the torsion angles in the ring were restricted to a maximum value of 5°. To avoid the possible influence of charges on the interactions of two complexes with chelate rings, we examined only neutral complexes. In the study of isolated chelate rings, the structures with aromatic organic fragments were excluded to avoid the possible influence of these delocalized systems on chelate-chelate interactions.

r 2010 American Chemical Society

Published on Web 08/06/2010

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The data related to the intermolecular chelate-chelate stacking interaction were analyzed by using the geometrical parameters in Figure 1. It was considered that an interaction existed if the dihedral angle between the mean planes of two chelate rings (P1 and P2), j, was less than 10°, the distance between the centers of two chelate rings, Ct, was less than 4.6 A˚, and the angle between the normal to the chelate ring and the line that connects the centers of two chelate rings, β, was less than 35°. To characterize the stacking interactions, the normal distances (R) from the centroid of one chelate ring to the mean plane of another and the torsion angles τ (angle M-Ω-Ω0 -M0 , Figure 1) were also analyzed.

Results and Discussion Our analysis of the data in the CSD showed that chelatechelate stacking interactions occur in a large number of the crystal structures of neutral square-planar complexes. We analyzed only square-planar complexes without charge in order to avoid the influence of the charges on the interactions of two complexes. The number of the structures with charged

Figure 1. Geometrical parameters describing the interactions between two chelate rings of square-planar complexes. The chelate ring can be five- or six-membered. (a) Ct is the distance between the centers of two chelate rings, P1 and P2 are the mean planes of the chelate rings, and β is the angle between the normal to the chelate ring and the line that connects the centers of the two chelate rings. (b) D is the distance between the two metals, and τ is the torsion angle metal-centroid-centroid0 -metal0 (M-Ω-Ω0 -M0 ).

and neutral square-planar complexes with different types of chelate rings found in the CSD is shown in Table 1, while the number of the chelate-chelate interactions is shown in Table 2. In the CSD we found 717 structures with charged complexes and 2473 structures with neutral complexes. In 955 structures (38.6%) with neutral complexes, we found chelate-chelate stacking interactions. In a large number of these structures, there are two or more interactions; hence, we found in these structures 1866 chelate-chelate stacking interactions (Table 2). These interactions exist in the structures of complexes with various metals and ligands, and they are ubiquitous in structures of square-planar complexes. Many of the complexes analyzed in this work have more than one chelate ring in the coordination sphere; these chelate rings are in most cases fused. Chelate rings can also be fused with aromatic organic rings. Since it was shown that the size of the planar system has an influence on the stacking interactions,3 crystal structures of complexes with isolated and fused chelate rings were analyzed separately. The observation of the stacking interactions of isolated chelate rings can demonstrate that the stacking interactions are not just the consequence of the overlap of the organic fragments and large delocalized systems. Stacking Interactions of Isolated Chelate Rings. We found 151 crystal structures of neutral square-planar complexes with isolated chelate rings where aromatic organic fragments were excluded to avoid their possible influence on the chelatechelate interactions. The complexes had one isolated chelate ring or two isolated chelate rings in trans positions. In 31 of the 88 structures with five-membered chelate rings and in 32 of the 63 structures with six-membered rings, we observed stacking interactions between chelate rings (Table 1). The data show that in a large number of structures stacking interactions between chelate rings do not occur. We screened all structures in order to find the differences in the constitution

Table 1. Number of Structures with Charged and Neutral Square-Planar Complexes Containing Different Types of Chelate Rings

structures with isolated chelates

structures with fused chelates

227

2953

charged complexes

positively charged complexes

negatively charged complexes

five-membered

64

29

35

88

31

six-membered five- and sixmembered overall number of structures five-membered

12 0

12 0

0 0

63 1

32 1

76

41

35

151

63

502

399

103

1704

676

139

127

12

655

250 80

641

526

115

2322

892

717

567

150

2473

955

six-membered five and sixmembered overall number of structures overall

neutral complexes with chelate-chelate interactions

3180

neutral complexes

Table 2. Number of Interactions in Structures of Neutral Square-Planar Complexes Containing Different Types of Chelate Rings interactions in neutral complexes interactions of isolated chelates

interactions of fused chelates

89

1777

five-membered six-membered five and six-membered overall five-membered six-membered five- and six-membered overall

46 42 1 89 1229 410 138 1777

parallel

cross

antiparallel

15 4 1 20 139 118 21 278

3 0 0 3 201 53 78 332

23 38 0 61 655 176 2 833

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of complexes with and without these interactions. Analysis of the constitution of five-membered chelate rings shows that these rings can be planar and, at the same time, have the Table 3. Number of Chelate-Chelate Stacking Interactions for Certain Types of Five- and Six-Membered Isolated Chelate Rings five-membered chelate rings Au(SCCS) Ni(NCCN) Ni(ONCN) Ni(SCCS) Ni(SSCS) Ni(NCNN) Ni(SNSN) Ni(SCNO) Pt(OCCO) Pt(NCCN) Pt(SCCS) Pt(SCNN) Pd(NOCN)

2 13 1 3 1 1 4 2 2 6 1 1 1

Cu(ONCS) Pd(NCCN) Pd(CNNC)

six-membered chelate rings 2 5 1

Au(OCCCO) Ni(SCCCS) Ni(SCCCO) Ni(OCCCO) Ni(NCCCO) Pd(OCCCO) Pd(SCCCO) Pd(NCCCO) Pt(OCCCO) Pt(OCCCN) Cu(OCCCO) Cu(NCCCO) Cu(NCCCN)

1 1 4 4 2 4 1 1 1 1 13 7 2

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sp3-atoms in the ring. These chelate rings do not form stacking interactions. Bulky groups as substituents on the chelate ring can make the stacking interactions of two chelates impossible. Even the remote voluminous ligands, in trans positions with respect to ligator chelate atoms, could also prevent stacking interactions of chelate rings. However, some complexes with bulky groups form stacking interactions if the groups can adopt conformations that enable stacking interactions. All the structures with six-membered chelate rings possess R,β-unsaturated ligands as analogs or derivatives of acetylacetonato complexes. In some structures, ligator atoms are nitrogen or sulfur instead of oxygen atoms as in acac. We inspected structures with six-membered rings and recognized that the chelate-chelate interactions do not occur if unsaturated groups such as CN, OCCH3 , NCCH3, and OCOEt are subsituents on the R- or β-carbon atom of these chelates. It seems that these groups disturb the delocalization in

Figure 2. Histograms showing the distribution of torsion angle τ values in crystal structures with isolated (a) five-membered and (b) sixmembered chelate rings.

Figure 3. Three most frequent conformations in crystal structures with chelate-chelate stacking interactions.

Figure 4. Histograms showing the distribution of normal distances (R) for (a) five-membered and (b) six-membered isolated chelate rings.

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Figure 5. Histograms showing the distribution of angle β for (a) five-membered and (b) six-membered isolated chelate rings.

Figure 6. Two views of the six-membered chelate-chelate stacking interaction in the antiparallel conformation (τ ∼ 180°) in the structure bis(2,4-pentanedionato)palladium(II).18 Arrangements of two molecules are drawn using the program ORTEP.19.

Figure 7. Plots of the distances between the two metals D versus the torsion angle τ for structures with five- (a) and six- (b) membered isolated chelate rings.

the chelate rings and make them unable to form stacking interactions. According to the criteria defined in the Methodology section, in a number of structures, we observed more than one interaction. Hence, the 63 crystal structures with isolated chelate rings revealed 46 interactions between five-membered, 42 interactions between six-membered, and one interaction between five- and six-membered isolated chelate rings (Table 2) . There are 5 types of metal atoms and 29 types of chelate rings involved in chelate-chelate stacking interactions (Table 3). In order to show that close contacts of two molecules are the consequence of attractive chelate-chelate interactions and not a consequence of the other attractive interactions between two molecules, all interactions of isolated chelate

rings were visually inspected. We could not find any hydrogen bond between two interacting molecules. In four structures, short metal-metal distances were observed; in three, CH/O interactions; and in one structure, a CH/π interaction. Because of the small number of the structures with additional interactions as well as the fact that the found additional interactions are not considered to be strong, one can consider that attractive chelate-chelate interactions are responsible for the close contacts of two molecules. To describe the geometry of the chelate-chelate stacking interactions, the geometric parameters depicted in Figure 1 were analyzed. The distribution of the torsion angles (τ), that describe the mutual orientation of two chelate rings (Figure 1), is shown in Figure 2. The histograms show that in most of the interactions the angle τ is less than 10° or near

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180°; that is, parallel and antiparallel conformations (Figure 3) dominate. For interactions of five-membered chelate rings, the number of parallel and antiparallel orientations is similar; however, in the case of six-membered chelate rings, there are only a few examples of parallel orientation, with most of them being antiparallel (Figure 2). Data for five-membered rings (Figure 2a) show that there are a few cases of structures with the cross conformation (τ ∼ 90°) (Figure 3). Histograms showing the distribution of the normal distances for the stacking interaction of five- and six-membered chelate rings are shown in Figure 4. Although the peak of the distribution is at somewhat smaller distances for five-membered rings, substantial numbers of the interactions occur with the normal distances over 3.4 A˚. The differences in normal distances for five- and six-membered chelate rings are a consequence of differences in typical stacking conformations for five- and six-membered chelate rings discussed later. The normal distances for two types of chelate rings are in the range typical for stacking interactions.15 The distributions of angles β (Figure 1) are very different for the stacking interactions of five- and six-membered rings (Figure 5). For five-membered rings, values of angle β below 10° are not observed; most of the interactions have values between 18 and 35°, with no clear preference for any value. The larger values of angle β for five-membered chelates show that these complexes have a large tendency for slippedparallel orientations, which is in accordance with the most stable conformation of two benzene rings4 and as we

Figure 8. Two views of the stacking interactions in the structure (N-(methoxymethyl)disulfur-dinitrogen)-(N-hydrogen-disulfurdinitrogen)nickel(II).20 In this structure there are three chelatechelate stacking interactions; two of them are in parallel conformation (showed with dashed lines), and one is in antiparallel (showed with solid line). Arrangements of two molecules are drawn using the program ORTEP.19

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observed for stacking interactions of chelate rings with organic aromatic rings.6-8 On the other hand, for six-membered rings, values of the angle β are quite low; most of them are below 12°, with the peak in the range 4-6°. The small values of angle β indicate that the two rings are in face-to-face orientation with a very small offset. In order to understand the small values of angle β, we visually inspected the structures. Most of the complexes have two six-membered chelate rings in trans positions with substituents on the rings. In order to enable stacking in spite of the substituents on the chelate rings, the interactions have an antiparallel conformation and small values of angle β to avoid substituents clashing (Figure 6). The plots of metal-metal distance (D) for different torsion angles (τ) are presented in Figure 7. Although a small number of interactions have parallel conformations (τ ∼ 0°), it is evident that metal-metal distances are short (3.5-4.75 A˚) for both five- and six-membered chelates. For interactions with the cross conformations of five-membered rings, the distances between two metals are short, with the exception of one interaction (Figure 7a). The distances for antiparallel conformations are in a wide range for both fiveand six-membered chelates. For five-membered chelates the distances can be quite short, from 3.5 A˚, while for sixmembred chelates the distances are not below 4.2 A˚. Longer distance for six-membered rings can be anticipated based on the typical conformations discussed above and shown in Figure 6. On the other hand, five-membered rings tend to form antiparallel conformations with very short metal-metal distances. Short distances appear in structures with interacting

Figure 10. Torsion angles for cross conformations: 100 and 90° are a consequence of the geometry of five- and six-membered chelate rings, respectively. In these systems, there are two chelate-chelate stacking interactions: first with a torsion angle of 180° and second with a torsion angle near 100 (a) or 90° (b).

Figure 9. Histograms showing the distribution of torsion angle τ, in crystal structures with (a) five-membered and (b) six-membered fused chelate rings.

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Figure 11. Histograms showing the distribution of normal distances (R) for three conformations in crystal structures with (a) five-membered and (b) six-membered fused chelate rings.

conformations shown in Figure 8. Complexes with two trans five-membered chelate rings such as these form simultaneously two parallel interactions and one antiparallel interaction. This type of conformation is prevented in complexes with sixmembered chelates by their substituents (Figure 6). The shorter normal distances for interactions of five-membered chelate rings can be understood by the conformation in Figure 8. Stacking Interactions of Fused Chelate Rings. By searching the CSD, 2322 structures were found of square-planar complexes in which chelate rings are fused with other chelate or/ and organic aromatic rings and, in that way, part of an extended π-system. In 892 of these structures, we observed stacking interactions between two chelate rings (Table 1). Hence, in a large number of structures, chelate rings do not form mutual stacking interactions. The main reason is the large size of the planar ligand. In complexes with large planar ligands (for example, bipy, phen, terpy), it is probable that aromatic organic fragments overlap mutually or with chelate rings, while two chelate rings do not overlap mutually. This is shown in the analysis of the stacking interactions of heteroaromatic ligands15 and of terpy complexes.17 In the 892 structures, we observed 1777 stacking interactions between two chelate rings: 1229 interactions between five-membered chelate rings, 410 interactions between sixmembered rings, and 138 interactions between five- and sixmembered chelate rings (Table 2). The numbers indicate that in most of the structures there is more than one interaction, which could be anticipated since in most of the complexes there are two or three fused chelate rings. The data in Figure 9 show the distribution of the torsion angle (τ) values. As in isolated chelate rings the torsion angle has preferred values, in a substantial number of interactions, the torsion angle has values close to 180°. However, there are a number of interactions where this angle is less than 10°, and there are fewer interactions where these values range from 90 to 110° for five-membered and from 80 to 100° for sixmembered chelates. These results indicate that in crystal

structures with fused chelate rings there are also three main conformations of interacting chelate rings; parallel, antiparallel, and crossed (Figure 3). It is interesting to note that in the cross conformation the values of the torsion angle τ are different for five- and sixmembered chelates, around 100° and 90°, respectively. We visually inspected these structures and found that in all complexes there are two fused chelate rings. When chelate rings are fused, there are two simultaneous chelate-chelate stacking interactions: one with a torsion angle of 0 or 180° and another with a torsion angle near 100 or 90° (Figure 10). Thus, the value of angle τ for the cross interaction is a consequence of parallel or antiparallel interaction with another ring. Figure 10 shows the simultaneous antiparallel and cross conformations and why angles τ for cross conformations are near 100 and 90° for five- and six-membered chelates, respectively. The distributions of normal distances of two interacting chelate rings for the three conformations for five- and sixmembered fused chelate rings are shown in Figure 11. The normal distances in all histograms are in the range typical for stacking interactions.15 The histograms for both five- and sixchelate rings do not show important differences for different conformations. However, the data indicate shorter normal distances for all conformations of six-membered rings. The peaks of the distributions for the interactions of six-membered rings are in the range 3.3-3.4 A˚ for cross and antiparallel conformations and in the range 3.4-3.5 A˚ for parallel conformations. For the interactions of five-membered rings, the peaks are in the range 3.4-3.5 A˚ for all conformations, with large numbers of interactions with R above 3.5 A˚. The histograms showing the distribution of the values of angle β are presented in Figure 12. While fused six-membered rings show a tendency for somewhat smaller values of angle β, the difference between five- and six-memebered rings is much less pronounced than in the case of isolated five- and six-membered chelate rings (Figure 5). The values of angle β for fused chelate rings are in a large range, indicating

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Figure 12. Histograms showing the distribution of angle β for (a) five-membered and (b) six-membered fused chelate rings.

Figure 13. Plots of the distances between the two metals D versus the torsion angle τ for structures with (a) five-membered and (b) sixmembered fused chelate rings.

slipped-parallel orientations (values of angle β above 15) but also almost face-to-face orientations (small values of angle β), that are not typical for stacking interactions between benzene molecules and between chelate rings with C6 aromatic rings.4,6-8 The large range of values of angle β for fused chelate rings is a consequence of the stacking of a whole fused, large planar system. The plots of metal-metal distance (D) for different torsion angles (τ) are presented in Figure 13. In parallel conformations (τ ∼ 0°), metal-metal distances are short, in the range from 3.0 to 4.7 A˚. In structures where the torsion angle is about 100°, distances between two metals can be both short and longer; they are in the range from 3.0 to 6.0 A˚. In antiparallel conformations (τ ∼ 180°), metal-metal distances are never below 3.5 A˚; they have values from 3.5 to 7.0 A˚. While for isolated chelate five- and six-memebered rings there are pronounced differences in histograms and plots (Figures 5 and 7), the histograms and plots are quite similar for five- and six-membered fused rings (Figures 12 and 13). These similarities for fused five- and six-membered chelate rings are a consequence of interactions of whole large delocalized planar systems. Conclusions The analysis of the data in the Cambridge Structural Database (CSD) showed evidence of chelate-chelate stacking interactions in a large number of the crystal structures of neutral square-planar complexes. We found 955 structures with chelate-chelate stacking interactions. In a large number

of these structures, there are two or more interactions; hence, we found in these structures 1866 chelate-chelate stacking interactions. The data from crystal structures of complexes with isolated and fused chelate rings were analyzed separately. The observation of the isolated chelate rings stacking interactions demonstrated that the chelate-chelate stacking interactions are not just the consequence of the overlap of the organic fragments and large delocalized systems. The analysis of the geometries of the interactions shows that in most of the interactions two chelate rings are in parallel or antiparallel orientation. The normal distances are similar to the distances of stacking interactions observed in other systems.4,6-8,15 However, the offset of two interacting chelate rings, defined by angle β, is different than that observed in other systems. Namely, in other systems, two interacting rings are in mutual slipped-parallel (offset face-to-face) orientation, while two interacting chelate rings can be found also in faceto-face (or very close to it) orientation. The large range of angle β values for chelate-chelate interactions is a consequence of the stacking of a whole large planar system. Acknowledgment. This work was supported under Project No. 142037 by the Ministry of Science of the Republic of Serbia. S.D.Z. acknowledges the support of the Humboldt Foundation. Supporting Information Available: Tables of geometrical parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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