CRYSTAL GROWTH & DESIGN
Stacking Interactions between Chelate and Phenyl Rings in Square-Planar Transition Metal Complexes
2006 VOL. 6, NO. 1 29-31
Zoran D. Tomic´,† Dusˇan Sredojevic´,‡ and Snezˇana D. Zaric´*,‡ ‘Vincˇ a’ Institute of Nuclear Sciences, Laboratory of Theoretical Physics and Condensed Matter Physics, 11001, Belgrade, P.O. Box 522, Serbia and Montenegro, and Department of Chemistry, UniVersity of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia and Montenegro ReceiVed August 8, 2005
ABSTRACT: Analysis of geometrical parameters in the crystal structure of square-planar complexes, with and without chelate rings, of all transition metals from Cambridge Structural Database shows that there are stacking interactions between the phenyl ring and the chelate ring with delocalized π-bonds. Short distances between the metal and carbon atom of the phenyl ring and parallel orientation of phenyl ring with respect to the coordination plane were found in the crystal strucutres of complexes with chelate rings. There is a correlation of the distances between the centers of chelate and phenyl rings and the metal-carbon distances showing that the chelate and phenyl rings tend to overlap. In the crystal structures, there are mutual slipped-parallel orientations of the phenyl and chelate rings. The data show that the geometry of the stacking interaction between phenyl and chelate rings is similar to the geometry of the stacking interaction of two benzene rings, indicating that chelate rings can behave similarly to organic aromatic rings. The importance of noncovalent interactions of π-systems has been shown in recent years.1 In transition metal complexes, there are specific types of noncovalent interactions. Metal complexes, actually ligands coordinated to a metal, can interact with the π-system. These interactions, named metal ligand aromatic cation-π interactions (MLACπ),2 also can be considered as a type of XH/π hydrogen bond.3 Recently, a few studies of noncovalent interactions with a chelate ring as a π-system were published.4-6 The delocalized π-system of chelate rings can be considered as a soft base, similar to double, triple bonds or aromatic rings and could be connected with the assumption that planar chelate rings with delocalized π-bonds can have aromatic character.7 Chelate rings can be involved in CH/π interactions with organic moieties and in stacking interactions with phenyl rings.4-6 The previous results5,6 show that there are stacking interactions between chelate and phenyl rings in square-planar Cu(II) complexes. By searching the crystal structures of square-planar complexes of Cu(II) in the Cambridge Structural Database (CSD), a number of structures were found with a distance between copper(II) and carbon atom of the phenyl ring below the sum of van der Waals radii.5 Analysis of geometrical parameters shows that there is correlation between the presence of a chelate ring and short copper(II)-carbon contacts and that there is a correlation between the copper(II)carbon distances and the distances between the centers of the chelate and phenyl rings. The two rings are in mutual slipped-parallel orientation. Hence, chelate and phenyl rings in square-planar Cu(II) complexes interact in a similar way as aromatic organic molecules.5 It is interesting to investigate whether these interactions are specific for Cu(II), or they can exist in complexes with other transition metals. Here we present results of analyzing interactions of chelate and phenyl rings by searching crystal structures of squareplanar complexes of all transition metals from the CSD.8 To the best of our knowledge, this is the first result that shows that phenylchelate stacking interactions are ubiquitous in metal complexes. To find out whether interactions between chelate and phenyl rings exist in square-planar complexes of different transition metals, geometrical parameters were compared for complexes with and without chelate rings. The CSD search program Quest 3D8 was used to retrieve structures satisfying the following criteria: (a) the * To whom correspondence should be addressed. Tel.: +381-11-3282111. Fax: +381-11-638-785. E-mail:
[email protected]. † ‘Vinc ˇ a’ Institute of Nuclear Sciences. ‡ University of Belgrade.
Table 1. Number of Chelate Rings with Certain Metal Atom Types and with Certain Ligand Types ligand atoms in chelate ring
metal Au Co Cr Cu Fe Ir Ni Pd Pt Rh
8 13 1 52 1 1 82 77 48 2
CCCC CCCO CNCN NCCC OCNO ONNO SCCN SCCS SCSS NCCN NCCO
ligand atoms in chelate ring 1 3 2 13 4 1 17 16 2 105 27
NNCC NNCN NNCO NNCS NNNO NCCCN NCCCO NNCCN NNCCO OCCCO
6 13 4 17 10 5 35 1 1 2
crystallographic R factor e 10%, (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 system, (f) a phenyl ring is present, (g) only one metal atom is present in asymmetric unit. It was frequently observed that complex molecules crystallize by incorporating the solvent or other chemical species in the crystal lattice. To minimize the possible influence of other structural fragments on the interaction between the complex molecules, we use the additional criterion that only one chemical species (i.e., only complex molecule) is present in the asymmetric unit. To ensure relative planarity of the complex molecule, the trans angles formed by the metal and coordinated atoms have been restricted to lie between 150 and 180°. To ensure planarity of the chelate ring, we measured the torsion angles in the chelate ring and used only those with a maximum value of torsion angle less than 5°. Geometrical parameters of the searched contact was extracted by using the instructions implemented in program Quest and additionally analyzed using locally written programs. In the cases in which more than one chelate ring is present in the molecule, we assumed that the one whose centroid is closer to the centroid of the phenyl ring interacts with the phenyl ring. We found 604 crystal structures of square-planar complexes and phenyl rings satisfying the above conditions. A total of 285 of these structures contained a chelate ring, and 319 structures did not have a chelate ring. There were 16 types of metal atoms in extracted data set, 10 types of metal atoms, and 21 types of ligands in chelate rings (Table 1). All ligands that form chelate rings have π-bonds prior to coordination to the metal atom and could be involved in stacking interactions with phenyl rings. Metal atoms can make π-bonds with ligands; hence, delocalization could be improved by coordination.
10.1021/cg050392r CCC: $33.50 © 2006 American Chemical Society Published on Web 12/09/2005
30 Crystal Growth & Design, Vol. 6, No. 1, 2006
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Figure 1. Geometrical parameters describing the interaction of squareplanar complexes with the phenyl ring. D is distance between the metal and the closest phenyl carbon, Ct is the distance between the centers of the chelate and phenyl rings, P1 is mean plane of metal and coordinated atoms, P2 is mean plane of the phenyl ring, and β is the angle between the normal to the phenyl ring and the line that connect the centers of the chelate and phenyl rings.
Figure 3. Histograms showing the distribution of the dihedral angle θ (°) between the mean plane of the phenyl ring (P2) and the mean plane of the metal with coordinated atoms (P1) with (a) and without (b) chelate rings.
Figure 2. Histograms showing the distribution of parameter ∆ (Å) in crystal structures with (a) and without (b) chelate rings.
Crystal structures of complexes with and without chelate rings were analyzed, to investigate whether the presence of a chelate ring has an influence on the metal-phenyl carbon distance. Histograms that show the distributions of the number of structures versus the metal-carbon distance, actually parameter ∆, are presented in Figure 2, in the structures with (panel a) and without chelate rings (panel b). The parameter ∆ is difference of the sum of the van der Waals radii and the distance between the metal and the closest phenyl carbon (D) (Figure 1) (∆ ) ∑vdW - D). The parameter ∆ enables us to use the criteria common for all metal atom types. There is a different distribution of the number of structures with and without chelate rings. In the complexes with chelate rings, there is a larger percentage of structures with a metal-carbon distance below the sum of the van der Waals radii (∆ > 0) than in the complexes without chelate rings. These data are similar to data previously found for Cu(II) complexes5 and show that the presence of a chelate ring with delocalized π-bonds has an influence on the close contact between the metal atom and the carbon atom of the phenyl ring. The other important geometrical parameter is the dihedral angle θ, the angle between the mean plane of the phenyl ring (P2) and the mean coordination plane consisting of metal and coordinated atoms (P1). The planes are shown in Figure 1. The distribution of this angle in structures with and without a chelate ring is shown in Figure 3. In the structures with a chelate ring, the phenyl ring has a large tendency to be oriented approximately parallel to the mean coordination plane (Figure 3a). In contrast, in the structures without a chelate ring the phenyl ring has a tendency to be oriented orthogonal to the coordination plane (Figure 3b). These data, short metal-carbon distances and approximately mutual parallel orientation of the phenyl and chelate rings, indicate phenyl-chelate stacking interactions.
Figure 4. Plot of the distances between the centers of the chelate and phenyl rings Ct (Å) versus the metal-carbon distance D (Å).
One of the consequences of the overlapping of two rings is correlation of the distances between the centers of the chelate and phenyl rings (Ct) and metal-carbon distances (D) (Figure 1). This correlation can be observed as shown in Figure 4. The data show that when the distance between centers of these two rings is short then the metal-carbon distance is also short. This correlation can only exist if the chelate and phenyl rings tend to overlap. Positions of phenyl rings with respect to chelate rings show that in all cases with small dihedral angles φ (the angle between the mean planes of chelate and phenyl rings) the two rings are in mutual slipped-parallel orientations (offset face to face). The values of the dihedral angle φ plotted versus angle β [the angle between normal on phenyl ring and line connecting the centers of two rings (Figure 1)] are shown in Figure 5. The plot shows that when the two rings are parallel (small dihedral angle φ), angle β has, in most cases, values between 15 and 35°. This indicates a slipped-parallel orientation of the two rings. All data in this work, position of the peak of the distribution of the metal-phenyl carbon distances in the region below the sum of van der Waals radii, positive correlation of metal-carbon distances with the distances between centers of chelate and phenyl rings, as well as mutual slipped-parallel orientation of two rings, show that stacking interactions between phenyl and chelate rings can exist in all transition metal square-planar complexes, similar to squareplanar Cu(II) complexes.5 The data also show that the geometry of the stacking interaction between phenyl and chelate rings is similar
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Crystal Growth & Design, Vol. 6, No. 1, 2006 31
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(3) Figure 5. Plot of the dihedral angle φ (angle between the mean planes of chelate and phenyl rings), versus angle β [angle between the normal to the phenyl ring and the line connecting the centers of the two rings (Figure 1)].
to the geometry of the stacking interaction of two benzene rings,9 indicating that chelate rings can behave similarly to organic aromatic rings. Acknowledgment. This work was supported by Ministry of Science of the Republic of Serbia (Grant IO1795). The authors would like to thank Prof. E. W. Knapp for support.
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CG050392R
