7.3 1,1,3-Triphenyl Allyl Palladium Complexes
8 Introduction to Chirality in Coordination Compounds
8.2 Introduction to Chirality of Transition Metal Complexes
The history of chiral organometallic and coordination complexes goes back as far as 1893, when Alfred Werner adapted the principle of handedness to octahedral complexes, which was till then only established for tetrahedral carbon atoms.[210] He further succeeded in separating the two enantiomers for complexes of the type [Co(III)(en)2(NH3)X]X2 (X = Cl, Br) (Figure 8.4),[211] which all in all, won him the nobel prize for chemistry in 1913.
Figure 8.4: Chiral Werner complexes; Λ- and Δ-enantiomers of XLII = [Co(III)(en)2(NH3)X]X2 (X = Cl, Br).
Since then various principal elements of chirality have been identified in organometallic and coordination chemistry. First of all, metals coordinated by four different types of ligands, such as for organometallic cyclopentadienyl complexes, resemble the classical chiral centers as found at sp3-hydrized carbon atoms (see Figure 8.5; example a)).[212] Another example for central chirality are tetrahedral clusters with four different vertices, for instance example b) in Figure 8.5.[213]
Figure 8.5: Examples for different types of chirality possible for coordination or organometallic compounds; a) and b) represent central chirality; c) example for planar chirality; d) example for axial chirality e) helical chirality, the Δ isomer is depicted.
Another type of chirality, namely planar chirality, can be found in hetero-disubstituted metallocene or other organometallic arene complexes (see Figure 8.5; example c)).[214] For a metal ion that is coordinated by four monodentate or two bidentate ligands, a chiral axis can be created, as previously discussed for the square planar complex 67d (example d). Here, steric effects can prevent free rotation along one axis, resulting in a stable chiral conformation (atropisomerism). In some special cases, even planar chirality can arise for such square planar complexes.[215] The last principal element of chirality, which is a type of axial chirality, are helical geometries, that are often found in octahedral complexes with at least two bidentate ligands occupying cis-positions (or three bidentate ligands, see Figure 8.5, e)).[216] These species are generally described with a prefix Λ or Δ, depending on whether a left-handed (Λ) or a right-handed (Δ) propeller twist is present.
An important field, in which those chiral elements are frequently found, is supramolecular chemistry. For his work in this field, Jean-Marie Lehn was awarded the nobel prize in 1987, together with Donald J. Cram and Charles J. Pedersen.[217] Lehn investigated the self-assembly of various metal-ions and bridging ligands, leading inter alia to a variety of chiral supramolecular structures.
One prominent, visually appealing class of these supramolecular compounds are helicates (greek: helix; winding, convolution, spiral), a term introduced by Lehn and coworkers.[218] It describes polymetallic architectures, self-assembled by organic ligands wrapped around various metal ions along a helical axis.[219] These compounds have received great attention, not only due to their striking resemblance to naturally occurring supramolecular helical structures such as the double-stranded, right-handed DNA,[220] but also due to a variety of interesting applications, which will be outlined later. Further, significant efforts have been made to elucidate the driving forces behind their formation.[221]
A helical structure is characterized by three parameters (see Figure 8.6). First to mention is its chirality, which describes the sense of screw along the helical axis. A right-handed helix is described with (P) (for plus), while the left-handed enantiomer represents a (M)-configured (for minus) helix. Next, the radius gives information about the extent of the helix in x and y direction, while the pitch quantifies the distances between one turn of the helix and the next along the z-axis.
Figure 8.6: Helical screw about a defined axis along the x-axis, radius and pitch represent relevant parameter for characterization of a given helix.
One prominent example for helicates, are Lehn’s double-stranded helicates formed by self-assembly of polypyridine-ligands and copper(I) salts, with constant pitch lengths of about 12 Å and radii of around 6 Å (one example shown in Scheme 8.1).[222] By linking from two up to five 2,2’-bipyridine units together, various helicates could be obtained. As in most reported cases, two homochiral helices (for instance, XLIV) are formed (self-recognition). As result of the achiral nature of the ligand (for XLIII), racemic mixtures were obtained.
Scheme 8.1: One example for self-recognition in the self-assembly of racemic homochiral double-stranded helicates XLIV from oligobipyridine strands XLIII.[222]
This self-organization of homochiral helices, often described as positive cooperativity, was believed to be dictated by one copper(I) center and then transferred to the others metal centers. One major cause of this effect is generally believed to be the conformational rigidity of the ligand backbone. However, Albrecht and co-workers made an interesting discovery, as for their ligands (differing in the number of methylene groups for the spacer group) either
the homochiral or heterochiral arrangements were preferred. The latter resulted in the formation of meso-helicates (or better mesocates).[223] Thus, it is often not easily predictable whether a homochiral or heterochiral combination is formed.
If, however, a chiral ligand is used, not only the chirality at the metal center, but also the self-organization might be influenced. This “enantioselective approach” towards asymmetric coordination compounds was pioneered by Smirnoff in 1920, who succeed in isolating chiral hexacoordinated platinum(IV) complexes, using enantiopure 1,2-diaminopropanes as ligands.[224] In these experiments different ligand to metal ratios were applied and their influence on the optical rotation was evaluated. Interestingly, for the [PtL3]4+ stoichiometry, the optical rotation exceeded the expected values for a linear relationship, thus, indicating another source of chirality. In comparison to the transfer from a chiral ligand to an organic substrate, here, due to the low activation barriers, the formation processes are most frequently under thermodynamic control, thus often leading to great stereoselectivity.
Various synthetic strategies for the stereoselective synthesis of coordination compounds have subsequently been developed in general[225] and for helicates in particular.[226] The first example was reported by Carrano and Raymond in 1978,[227] who isolated a dinuclear iron(III) triple stranded helicate using the dihydroxamate siderophore rhodotorulic acid XLV.
In the following two more “helical ferric ions binders” were reported.[228] In 1991 Lehn and co-workers introduced a slightly modified C2-symmetric version XLVI of their previously established tris(bipyridine) ligands (see Scheme 8.1; XLIII), which allowed the highly stereoselective formation of trinuclear copper(I) and silver(I) double helicates.[229]
Figure 8.7: Selected pioneering ligands XLV and XLVI in the field of stereoselective helicate formation.
It took, however, until the mid-to-late 1990s for the attention to focus on their diastereoselective synthesis. Since then various attempts employing chiral ligands in order to isolate primarily double and triple stranded enantiopure helicates have been reported, following two major approaches. By introducing a chiral group either in proximity to the
coordinating donor atoms or in a peripheral spacer unit, predetermination of the chirality at the metal center has been observed.
Figure 8.8: Selected examples of ligand classes used for diastereoselective formation of enantiopure helicates.
Prominent examples of the first approach are the terpene-derived CHIRAGEN (from CHIRAlity GENerator) ligands,[230] such as ter-[231] and quaterpyridines[232] (see Figure 8.8) as well as related ligand scaffolds.[233] By combination of a 2,2’-bipyridine unit with a chiral backbone, such as atropisomeric biaryls,[234] enantiopure 1,2-diamines[235] or other chiral building blocks,[236] thediastereoselective formation of helicates could be accomplished.
While early studies were dominated by copper(I) and silver(I) ions, later investigations included zinc(II),[232c,233a,234c,f-i,235a]
Apart from these polypyridines, dicatecholligands[237,238] were able to provide enantiopure gallium(III) and titanium(IV) helicates. Further, bis(β-diketonates),[239] di-Schiff-base[240] and P-donor-ligands[241] have been successfully applied in this field.
Although a plethora of enantiopure helicates have been reported,[231b,232a,b,d,e,234c,h,235b, 236a,c,240b-d] after best knowledge, no example exists where the spacer unit acts as a direct bridge between two metal centers within these multimetallic arrangements. Consequently, it might be interesting to investigate the effect of relatively rigid ligand scaffolds, such as the PyrBOX ligand scaffold, for those supramolecular compounds.
One interesting field of application in which the chirality of helicates is of crucial importance is medicinal chemistry.[242] Due to the principle of chiral recognition, these supramolecular structures are potential protein α-helix mimics, making them interesting for diagnostic or therapeutic applications. As opposed to the normally applied rigid ligand backbones, Scott
and co-workers recently introduced a more flexible type of helicates that was coined
“flexicates”.[243] They further outlined four criterias helicates have to fulfill in order to be practically relevant for medicinal purposes:
Non-racemizing material with high optical purities
Solubility and stability in aqueous media for biological studies
Availability on a practical scale
Synthetic flexibility for further structural optimization
Their flexicates approach can be described as follows. Instead of using a rigid backbone in which the coordination chemistry for the metal centers are mechanically coupled and therefore can reduce diastereoselectivity of the helix formation,[244] they used a combination of a flexible spacer with a variety of coordinating end-groups which, in presence of zinc(II) and iron(II) salts, gave essentially optically pure, non-racemizing triple-stranded helicates.
For these helicates the chirality at both metal centers is predetermined independently.
Figure 8.9: a) Conventional helicate synthesis starting from three rigid chiral ligands (BA-AB, different color for each strand) bridging to metal centers resulting in a triple-stranded helix;
b) ‘Flexicates’ approach, starting from two monometallic complexes, with a ligand that contains end-groups (green), and spacer (blue) with connecting functionality (red), which predetermines independently the chirality at both metal center.[242]
These compounds were further shown to interact with DNA and having antibiotic, as well as anti-cancer activity.[245] Due to their capability of enantioselective inhibition of Aβ-aggregation and passing the blood-brain-barrier, the authors suggested their approach as new therapeutic strategy against Alzheimer’s disease.[246]
Apart from these currently developing fascinating medicinal applications, helicates have found various other applications and their self-organization and self-recognition processes have been intensively investigated.[219,247]
Next, in order to evaluate the potential of PyrBOX ligands to form chiral supramolecular coordination compounds, the chiral recognition phenomena (Figure 8.10) found for their bidentate BOX and tridentate PyBOX and TRISOX (tris(oxazoline) analogs will be briefly discussed.
Figure 8.10: a) General structure of PyBOX, BOX and TRISOX ligands b) heterochiral and c) homochiral combination of two chiral ligands; grey balls represent R-substituents at the chiral centers, olive balls represent coordinated metal center.
Previously reported single stranded helical structures for BOX ligands with silver(I) and copper(I) in solid state would indicate a preference for chiral helicates over mesocates, however in both cases fluxional exchange processes were presumed in solution.[248]
Furthermore, for PyBOX ligands homochiral recognition in solution was established for a variety of silver(I) and copper(I) helicates,[249] as well as for a supramolecular capsule formed by self-assembly of two TRISOX ligands and three silver(I) ions.[250]
On the other hand, studies performed in the context of non-linear effects with racemic mixtures of oxazoline ligands do not allow a general assessment, whether homo- or heterochiral combinations are preferred for a particular ligand.[251]
It can be concluded that by implementation of chiral ligands, one can frequently exclude the formation of the heterochiral combinations, potentially leading to mesocates. Nonetheless, the substituent specific interactions in homochiral pairings may destabilize the overall structure.
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