Structural Comparison

Although the four anthracene derivatives, as well as their gold complexes, are structurally closely related, only 23 and 27 are isostructural. In none of the structures lattice solvent molecules are co-crystallized, which ensures good comparability. For all compounds the asymmetric unit contains one molecule, independent of the space group. In consideration of the packing behaviour of substituted anthracenes which have been described in previous publications, a packing motif with a large π-π overlap and bulky phosphoryl substituents facing in opposite directions appears to be most favourable.^{[54, 59]} For explanations on quantification of structural properties such as folding angle, twist deformation or C-H^…π bonds, please see 3.2.1.

MeAnPS(NMe2)2 (21) crystallizes in the monoclinic space group P21/n. The anthracene moiety is not perfectly planar, because it is slightly folded along the

9,10-vector with the dimethylamino and the methyl group both located at the wider side of the anthracene plane. The folding angle adds up to 9.9°, while the twist deformation is weak at 2.0°.

Figure 3-35: Crystal structures of MeAnPS(NMe2)2 (21, left) and MeAnPS(NEt2)2 (23, right), hydrogen atoms are omitted.

The phosphorus atom in 9-position and the methyl group in 10-position are located almost exactly in the C9-C10-axis. The torsion angle of the C8-C9-P˗S-bond to the anthracene plane 70.9°, hence the molecule is not symmetrical, which would require a 90° angle. Packing plots reveal the intermolecular interactions of 21 in the solid state.

Every two molecules show nearly exact parallel orientation of the anthracene planes with the convex sides facing each other and the phosphoryl substituents pointing to the outside. The observed offset in the face-to-face π-stacking results in a π-π overlap of approximately 35% with a π-π distance of 3.51 Å (Figure 3-36). Additionally, a methyl C-H^…π interaction between an aminomethyl group and an outer ring of the anthracene π system can be found.

Figure 3-36: π-π overlap in the structure of MeAnPS(NMe2)2 (21): top view (left) and side view (right).

In the solid state structure of 21, sp³ C-H^…π distances of 2.946 Å and 2.993 Å, following from two methyl C-H-bonds, are observed. The angles to the anthracene ring-plane measure 48.8° and 25.7°, respectively, which is considerably more acute than the optimum T-shaped arrangement. Therefore these interactions can be considered comparatively weak.

Figure 3-37: Low angled C-H^…π interaction in the structure of 23 without π-π overlap.

Compared to MeAnPS(NMe2)2 (21), the steric bulk of the phosphoryl substituent is increased in MeAnPS(NEt2)2 (23) by replacing the dimethylamino groups with diethylamino groups. This is reflected by a larger folding angle of the anthracene moiety of 13.8°. As in 21, both amino-substituents are located on the same side of the

anthacene plane, leading to a stronger folding and also a slight twist angle of 8.8° of the ring system. Also, the phosphorus atom is forced out of the C9^…C10-axis.

Furthermore, the intermolecular interactions in the solid state structure of 23, which crystallizes in the monoclinic space group P21/c, differ significantly from those found in 21. In 23, no noteworthy π-π interactions are found: the shortest distance between two π-systems measures nearly 7 Å, which is negligible. Though a “head-to-tail”

arrangement is also found in 23, the molecules are shifted so far that the π systems no longer overlap and the only significant interaction found is an sp³ C-H^…π interaction of the methyl group in 10-position with the central ring of the adjacent anthracene moiety. One C-H-bond is directed towards the anthracene π system showing a distance/angle of 2.899 Å/31.2°, which again can be considered fairly weak (Figure 3-37).

Figure 3-38: Crystal structures of MeAnPSe(NMe2)2 (25, left) and MeAnPSe(NEt2)2 (27, right), hydrogen atoms are omitted.

Although MeAnPS(NMe2)2 (21) and MeAnPSe(NMe2)2 (25) are identical molecules apart from the P-bound chalcogen, their solid state structures differ significantly. As in 21 and 23, both bulky amino-groups are located on the same side of the anthracene plane, leading to a folding of the anthracene moiety of 16.7°, which is nearly twice as large as the folding angle in 21. Moreover a distinctly stronger twist deformation of 8.0° is found. Also, the phosphorus atom is – as observed in MeAnPS(NEt2)2 (23) – forced out of the C9^…C10-axis. The intermolecular interactions found for 25 combine the packing phenomena already discussed for 21 and 23. Two different “head-to-tail”

type arrangements occur, one of them shows a π-π overlap of approximately 35% and

a distance of 3.60 Å with aromatic hydrogen atoms located above the ring centres of the opposite π system. The second interaction found is between a C-H bond of the methyl group in 10-position and an outer anthracene ring. The C-H^…π distance here is 2.917 Å, the angle to the ring plane measures 41.2°. Additionally, an aromatic sp² C-H^…π bond in 2-position of the anthracene moiety to an adjacent π system is present.

It is also fairly accute (40.8°) and emulates a distance of 3.096 Å.

As mentioned before, MeAnPS(NEt2)2 (23) and MeAnPSe(NEt2)2 (27) are isostructural. While the same exchange of the chalcogen in 21 and 25 completely changed the packing motif, the exchange of the sulfur atom to selenium does not influence the packing arrangement of 27 compared to 23. This shows that the steric demand of the diethylamino groups outnumbers the effect of the heavier chalcogen in 27. Hence, the deviations of cell parameters and intermolecular interactions are marginal. The folding- and twist angle of the anthracene moiety is identical to the angle found in 23. The methyl C-H^…π interaction exhibits minimal deviations from the one found in 23, measuring 2.978 Å (29.5°).

Figure 3-39: Crystal structures of [MeAnP(NMe₂)₂(S)AuCl] (22, left) and [MeAnP(NEt₂)₂(S)AuCl] (24, right); hydrogen atoms are omitted.

All gold complexes show a linear E-Au-Cl coordination geometry, which is characteristic of gold(I) compounds.^[69] However, the orientation of the linear E-Au-Cl fragment differs among the four compounds. Surprisingly no gold-gold interactions were found in any of the complexes.

Due to the coordination of gold(I) the structure of [MeAnP(NMe2)2(S)AuCl] (22) clearly is different from the structure of MeAnPS(NMe2)2 (21). In 22, the typical “head-to-tail” orientation is also observed, with the anthracene moieties shifted in a similar way as described for MeAnPS(NEt2)2 (23), leading to virtually no π-π overlap.

Nevertheless aromatic C-H bonds are located over the ring centres of the adjacent anthracene moiety in parallel orientation. As in 21, both amino groups are located at the same side of the anthracene plane. The linear S-Au-Cl fragment is directed away from the anthacene moiety. Although the phosphorus atom is located distinctly further outside of the C9^…C10-axis, the folding angle of the anthracene ring system is smaller than in 21, enclosing only 6.0°. The twist deformation of the fluorophore is however slightly stronger (4.4°). The “head-to-tail” arrangement produces a C-H^…π bond from the 10-methyl C-H bond to a peripheral ring of the anthracene moiety. The distance of 2.776 Å and an angle of 48.2° to the ring plane suggest that this C-H^…π interaction can be considered stronger than the ones described before. A second C-H^…π bond resulting from the interaction of an aromatic C-H bond in 2-position with a neighbouring anthracene ring is also found, measuring 2.980 Å and 53.7°.

Figure 3-40: Left: sp³ C-H^…π bonding in the structure of [MeAnP(NMe2)2(S)AuCl] (22), S-Au-Cl fragments are omitted for clarity; right: sp² C-H^…π bonding in the structure of 22, substituents are omitted.

[MeAnP(NEt2)2(S)AuCl] (24) again shows a different arrangement. Though, like [MeAnP(NMe2)2(S)AuCl] (22), 24 also crystallizes in P21/c, the orientation of the linear S-Au-Cl fragment is antipodal to the one observed in 22.While in 22 this fragment points away from the anthracene moiety, here it is located behind the anthracene ring system on the reverse side of the bulky diethylamino groups. Driven by the steric demand of these groups, this arrangement leads to a notable distortion of the anthracene moiety. A folding angle of 20.7° is reached, accompanied by a twist angle of 4.4°, and the phosphorus atom is clearly displaced from the C9^…C10-axis, to which

the P-C9-bond encloses an angle of 13.5°. The “head-to-tail” arrangement of every two molecules is also found in 24. They face each other with their concave side, while the π-π distance measures only 3.21 Å, achieving an overlap of approximately 25%.

Additionally an aromatic sp² C-H^…π interaction very similar to the one found in 22 is observed. Again originating from a C-H bond in 2-position, it measures 3.055 Å and 42.6° to the adjacent π system.

Figure 3-41: differences in fluorophore deformation between [MeAnP(NMe₂)₂(S)AuCl] (22) (red) and [MeAnP(NEt₂)₂(S)AuCl] (24) (blue).

The structure of [MeAnP(NMe2)2(Se)AuCl] (26) is the first structure among the compounds in this chapter that shows a rotated phosphoryl substituent with the amino groups located on opposite sides of the anthracene plane. However the arrangement is not symmetric which would require the P-Se-bond to be located in the anthracene plane. In fact, one N-P-bond has a torsion angle of nearly 90° with respect to the anthracene plane. The supposedly more relaxed arrangement with both bulky amino substituents located on different sides surprisingly does not lead to less deformation of the ring system. The folding angle adds up to 12.7°, but the deviation of the phosphorus atom from the C9^…C10-axis is significantly smaller than the one found in 24. The twist angle of the fluorophore is in the same range as observed for 24, measuring 5.4°. As in 22, the linear Se-Au-Cl fragment is directed away from the anthracene moiety. While there is virtually no π-π overlap, a C-H^…π interaction of the methyl group in 10-position is present. The short distance of only 2.617 Å and the fairly acute angle of 65.5° to the π-system makes this C-H^…π bond the supposed strongest found among 21-28.

Figure 3-42: sp³ C-H^…π bonding in the structure of [MeAnP(NMe2)2(Se)AuCl] (26): top view (left) and side view (right).

Like in [MeAnP(NMe2)2(Se)AuCl] (26), the bulky amino groups are also located on opposite sides of the anthracene plane in [MeAnP(NEt2)2(Se)AuCl] (28). Though in the structure of [MeAnP(NEt2)2(S)AuCl] (24), the sulfur oxidized equivalent of 28, this was not the case, the linear Se-Au-Cl fragment is directed away from the anthracene moiety. This results in less deformation of the ring system, reflected by a folding angle of 11.1° (20.7° in 24) and a twist angle of 5.1°.

Figure 3-43: Left: Superposition of 24 and 28 (transparent), ethyl groups are omitted for clarity; right:

superposition of 26 and 28 (transparent).

The π-π distance in the typical “head-to-tail” arrangement measures 3.53 Å, though the overlap of approximately 15% is not very strong.

Moreover a C-H^…π interaction (2.772 Å/63.1°) between an aminoethyl group and an outer ring of the anthracene moiety is observed, as well as a similar interaction of the 10-methyl group with the central anthracene ring (2.935 Å/64.8°). One further fairly long C-H^…π interaction of 3.074 Å is also found, but the combination of this long distance and the acute angle of 35.7° to the π system renders it less important.

As anticipated, the bulky phosphoryl substituents of every two molecules are oriented in opposite directions in all eight structures, leading to a “head-to-tail”

packing motif in all cases. The resulting π-π overlap, however, varies considerably, as well as the distortion of the fluorophores and the number and strength of C-H^…π type interactions. Although MeAnPS(NEt2)2 (23) and MeAnPSe(NEt2)2 (27) are isostructural, their corresponding gold complexes [MeAnP(NEt2)2(S)AuCl] (24) and [MeAnP(NEt2)2(Se)AuCl] (28) are not. All gold complexes feature a linear E-Au-Cl fragment directed away from the anthracene moiety except for 24, where the fragment is located behind the anthracene plane leading to a significantly stronger distortion than found in the other structures. Despite the close relation among the eight compounds and the similarity of the substituents, a prediction of the resulting packing motif and the corresponding intermolecular interactions is not possible.

Nevertheless the information acquired from the detailed evaluation of the solid state structures can be consulted to understand and explain solid state fluorescence properties of the compounds at hand.