Drawing Mononuclear Octahedral Coordination

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Drawing Mononuclear Octahedral Coordination Compounds Containing Tridentate Chelating Ligands Aminou Mohamadou, Karen Ple, and Arnaud Haudrechy* Institut de Chimie Mol eculaire de Reims (ICMR), Universit e de Reims Champagne-Ardenne, Facult e des Sciences, Moulin de la Housse, BP 1039-51687 Reims Cedex 2, France *[email protected]

In our previous article (1), we proposed an easy way to draw all possible stereoisomers of octahedral complexes with monodentate chelating ligands, yielding a total of 75 stereoisomers, which could be classified into five families. To further this didactical study, an interesting case appears with tris-heteroleptic complexes of the type [M(A)2(B)2(C)2] (Figure 1, 1a), where hydrocarbon chains connect the three donor atoms giving two tridentate chelating ligands with an imposed bonding sequence A-B-C (Figure 1, 1b). These compounds give octahedral complexes with two tridentate chelating ligands of the type [M(A-B-C)2]. In principle, these bis(tridentate) complexes can form six geometric isomers: five of them are facial (fac) and one is meridional (mer). It is to be noted that when the central donor atom B of the tridentate A-B-C ligand is a secondary or tertiary amine, it becomes a stereogenic center upon complexation, thus, giving seven pairs of enantiomers and one achiral diastereomer. We describe a simple method of drawing all possible stereoisomers of these bis(tridentate) complexes. Several examples of these types of complexes exist in the literature, Mohamadou et al. (2) have reported the solid-state characterization of the enantiomeric pair of fac-ΔSR all cis and fac-ΛRS all cis [Co(pynso)2]þ [pynso = 3-(2-pyridylmethylamino)ethanesulfinate, pyCH2NHCH2CH2SO2-], crystallizing in the same crystal lattice (Figure 2). Raymond et al. (3) have detected all isomers of [Co(PMG)2]3complex [PMG = N-(phosphonomethyl)glycine, (O2CCH2NHCH2PO3)3-] in solution, but only one achiral diastereoisomer fac-RS all trans cobalt(III) compound has been isolated in the solid state (Figure 3). The introduction of stereogenic centers on the lateral arms is obviously one way to generate chirality, but this subject will not be treated in the present article. We were more interested that the achiral pynso and PMG ligands could generate indirect chirality on the metal atom. This special case of octahedral geometry introduces interesting stereochemical variations, with only one constraint: the ligating atoms A, B, and C must obey a defined sequential arrangement. Although detailed discussions have been described by von Zelewsky in an excellent reference book (4), these different approaches are not easy for practical teaching. In this article, we simplify the presentation of all possible stereoisomers of these bis(tridentate) complexes as well as detail the resulting chiral subtleties. In all of our drawings, only the Newman-like projections are used, omitting the central metal for clarity. The covalent bonds between the A, B, and C donor atoms are emphasized as bold or 302

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Figure 1. Tridentate chelating ligands with an imposed sequence A-B-C.

Figure 2. Pynso chelating ligand and isolated structures.

Figure 3. N-(Phosphonomethyl)glycine chelating ligand and isolated structure.

dashed lines, to give the orientations above or below, in front or in back of the omitted central atom (Figure 4). Binding the two tridentate chelating ligands around the central metal can provide several possible stereoisomers. The stereoisomers can be viewed along the axis placing a given atom, here chosen as atom A, at the summit, and having either A, B, or C in the lower axial position. We will consider the five diastereomeric forms (4 achiral, one chiral) of a general [M(A)2(B)2(C)2] complex(1, 2) ,then consider the possible stereochemical arrangements of the A-B-C bonds and eliminate any duplicates.

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Vol. 88 No. 3 March 2011 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100159a Published on Web 12/16/2010

In the Classroom

Figure 4. Newman-like projection.

Figure 7. Through an A-A axis (B and C in trans positions).

Figure 5. Through an A-A axis (B or C in cis or trans positions).

Figure 8. Through an A-B axis (the other A and B in cis positions).

Figure 6. Through an A-A axis (B and C in cis positions): mirror images.

Trans-AA Stereoisomers

Figure 9. Through an A-B axis (the other A and B in trans positions).

In this example, the two A atoms are place at the summit and the lower axial position. Two possibilities then appear in the equatorial plane: in the first one (Figure 5), the matching donor atoms (BB and CC) are cis (I) or trans (II). These two reference diastereoisomers of the general octahedral [M(A)2(B)2(C)2] complex allow us to derive several possible stereoisomers by considering the A-B-C linkages. In case I, it is easy to see that two symmetry planes exist and that in II, an inversion center is present. That is the reason why these parent structures with no special binding between the chelating ligands are achiral. In case I, we can immediately deduce that a symmetrybreaking operation (such as binding A to B for example) would lead to the creation of an enantiomeric pair. Considering I in the scope of the tridentate chelating ligand study, the upper axial donor atom A (Figure 6) can be bound to the atom B on the left side (case I-1) or the right side (case I-2) of the metal atom. Consequently, because the ligating atoms are disposed in the defined sequence A-B-C, these structures are enantiomers. In cases derived from II, all possible bonding patterns preserve the inversion center, which shows that they are achiral and, in fact, identical (Figure 7). To demonstrate this point, starting from the upper axial atom A, we can bind the B atom on the top and then either C atom, giving the daughter sets II-1 and II-2 or bind the B atom on the bottom, and then either C atom, giving the daughter sets II-3 and II-4. A simple 180° rotation shows that II-1 is identical to II-4 and II-2 is identical to II-3, and flipping II-1 shows that it is identical to its mirror image II-2!

Trans-AB Stereoisomers

r 2010 American Chemical Society and Division of Chemical Education, Inc.

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In this case, atom A is once again at the summit and atom B in the lower axial position. Figure 8 shows that the remaining A atom can be positioned in an equatorial cis position to the second B atom (structure III). Case III lacks symmetry elements and can therefore exist in two enantiomeric forms. Because there is only one possible way to bind atom A to atom B, only the chiral complex III-1 and its mirror image III-2 exist. For structure IV, atom A is at the summit and atom B in the lower axial position, and the remaining A atom is trans to the B atom. Looking through the A-B axis, the parent “unconnected” structure possesses a symmetry plane, but the operation of binding A, B, and C leads to the creation of an enantiomeric pair. Starting from the summit A atom, the bond with atom B gives the two cases IV-1 and IV-2. We can easily draw their mirror images (respectively IV-3 and IV-4); however, a 90° rotation shows that IV-2 is identical to IV-3, restricting this example to the pair of enantiomers IV-1 and IV-3 (Figure 9). Trans-AC Stereoisomers In this case, Figure 10 shows one possibility where the remaining A atom can be positioned in an equatorial cis position to the second C atom (structure V). However, it is not necessary to discuss this case, because another Newman-like projection clearly shows that V is identical to the case III, which we have detailed before.

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In the Classroom Table 1. Seven Pairs of Enantiomers and One Achiral Diastereoisomer

Figure 10. Through an A-C axis (the other A and C in cis positions).

Figure 11. Through an A-C axis (the other A and C in trans).

a subtlety, because they become stereogenic upon complexation, and the absolute configuration of the amine can be either R or S. Four possible configurations (VI-1-1 to VI-1-4 and their mirror images VI-3-1 to VI-3-4) need to be carefully detailed to identify the different symmetries (Figure 12). The case VI-1-2, seen through another A-C axis, reveals that it is identical to VI-1-3 (Figure 13). This means that only three pairs of enantiomers are possible when atom B is an amine nitrogen, which becomes a chiral center upon complexation.

Figure 12. A secondary amine generating new stereogenic centers.

Figure 13. Equivalence of the two isomers VI-1-2 and VI-1-3.

Through an A-C axis, the remaining A atom can also be positioned in trans to the second C atom (structure VI). Starting with the summit A atom, bonding with atom B underneath gives the two cases VI-1 and VI-2. We can easily draw their mirror images (VI-3 and VI-4). Moreover, starting from the same framework, if atom A is bonded to atom B on the top, it is possible to draw two structures VI-5 and VI-6, with their mirror images VI-7 and VI-8. However, by a simple 90° rotation, we can see that VI-5 is identical to VI-3, and similarly, VI-6 is identical to VI-4, thus, simplifying all possibilities to the enantiomer couples VI-1-VI-3 and VI-2-VI-4 (Figure 11). Trans-AC Stereoisomers: The Special Case of a Stereogenic Amine Positioned on B In this last example, when the central donor atom B is a secondary (or tertiary) amine nitrogen, cases VI-1 and VI-3 hide 304


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Conclusion We have presented a didactical approach for presenting mononuclear octahedral coordination complexes containing tridentate chelating ligands. Even if it is possible to show three-dimensional figures in the classroom, in our experience, this method is easier for students to understand. It should to be noted that with these kinds of complexes, one can distinguish basically two cases: • If the chelating ligand coordinated to the metal is achiral, five pairs of enantiomers and one achiral diastereoisomer are obtained. When the chelating ligand atom can become a chiral center upon complexation (for example B = NH or NR0 ), two additional pairs of enantiomers derived from the new stereogenic centers can be obtained. Thus, a total of seven pairs of enantiomers and one achiral diastereoisomer are possible. • If the chelating ligand coordinated to the metal is chiral entity and it is used in enantiomerically pure form [for example,


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In the Classroom

N-(phosphonometyl)-(L)-alanine], all pairs of enantiomers given above become diastereoisomers. In this case, the total of 15 diastereoismers can be obtained.

The fac and mer diastereomers, in general, have different chemical potentials in the thermodynamic sense. They represent local minima on the energy hypersurface. Even though predicting which configuration corresponds to the global minimum is not trivial, it can be seen that, in the chelate formed by each tridentate ligand in the mer configuration, the three donor atoms have a mixed position to each other (two cis and one trans position). Thus, the mer configuration has a higher statistical weight as compared with the fac configuration (in which all the donor atoms of the same chelating ligand are cis) and is therefore entropically preferred. In summary, the mononuclear metal complexes of two tridentate chelating ligands having A, B, and C donor atoms [ML2], with B atom becoming a chiral center upon complexation, give seven pairs of enantiomers and one achiral diastereoisomer. All of these isomers have been detailed in the text and are summarized again in Table 1.


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In any kind of highly organized molecular assemby, stereochemistry must be well defined. Nature demonstrates this very clearly, for example, by the microbial transport process that generally involves geometry recognition and chirality at the metal center of the siderophore complex. By defining the nature of the donor atoms of the achiral chelating ligands, one can built up chiral complexes with the aim of generating catalytically active metal compounds for enantioselective reactions. It is very important to recognize this complexity to fully understand catalytic phenomena. Literature Cited 1. Mohamadou, A.; Haudrechy, A. J. Chem. Educ. 2008, 85, 436–440. 2. Mohamadou, A.; Jubert, C.; Barbier, J.-P. Inorg. Chim. Acta 2006, 359, 273–282. 3. Heineke, D.; Franklin, S. J.; Raymond, N. K. Inorg. Chem. 1994, 33, 2413–2421. 4. von Zelewsky, A. Stereochemistry of Coordination Compounds; John Wiley and Sons: Chichester, U.K., 1996; pp 116-176.


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Drawing Mononuclear Octahedral Coordination

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