Complexes of L1 and L2:
Both the complexes of L1 crystallise in the same space group: monoclinic, C2/c; and the complexes of L2 also crystallise in the equivalent space group: monoclinic, P21/n. The only short contacts seen for PtL1, PdL1 and PtL2 are those to the PF6
- counter ions which pack in-between the cations as seen in Figures 3.30 and 3.31. The F···H intermolecular contactsobserved here allow for the formation of relatively stable crystal lattices. The strange behaviour of fluorine in non-bonded interactions has been recognised in the literature.326 Mostly, fluorine will rarely form hydrogen bonds, which is explained by many electrostatic and steric factors (e.g. its low polarisability and tightly contracted lone pairs).327 Despite these interactions involving fluorine being much
weaker than conventional hydrogen bonds the role which they play in molecular packing cannot be ignored.327 A search of the CSD reveals the common occurrence of C–H···F interactions in organic crystals. Althoff et al. have shown that even a single C–H···F–C contact may be strong enough to pair two molecules.328
Figure 3.30: Crystal packing viewed down the b-axis for PtL1 (Z = 4) and PdL1 (Z = 4).
Figure 3.31: Crystal packing viewed down the b-axis for PtL2 (Z = 4) and PdL2 (Z = 4).
The metal cations and hexafluorophosphate ions of PtL1, PdL1 and PtL2 are connected via C–H···F hydrogen bonding with H···F distances ranging from 2.200 to 2.628 Å. These short C–H···F contacts are presented in Table 3.5 and are consistent with relatively strong hydrogen bonds between the hydrogen atoms of the cationic metal chelates and
the fluorine atoms of the anions. A three-dimensional hydrogen-bonded network is created between the interacting cations and anions. Crystal structures were not obtained for most of these metal complexes with smaller or different anions (Cl- or SbF6
-). This fact alone clearly suggests the importance of these particular counter ions and their intermolecular interactions that evidently allow the formation of these crystals.
Table 3.5: Selected C–H···F interaction distances (Å) observed for PtL1, PdL1, PtL2 and PdL2.
Hydrogen atom involved
in the interaction
PtL1 PdL1 PtL2 PdL2
H
a 2.594 2.650(40) – –H
c2.421 2.488
2.559(40) 2.536(40)
2.435 2.504
2.459 2.516
H
d – – 2.619 2.603H
e 2.4372.318
2.379(40) 2.195(40)
2.373 2.399
2.436 2.417
H
f2.649
2.521 – 2.618 2.600
2.658
H
g – – 2.6422.469
2.517 2.631
For PdL2 there are complementary short contacts of 2.848 Å between Cf-Hf···Cc. Once more, as seen for the respective ligands, there is no experimentally significant π–π stacking involving the pyridine rings of adjacent molecules. One important reason for this, despite the aromatic nature of the pyridyl groups and the fact that they are well known to π-stack,303 is that the cationic chelates in the present salts are divalent. Hunter and Sanders have shown that π-π interactions occur when attractive contacts that take place between π-electrons and the σ-framework prevail over unfavourable contributions, e.g. π-electron repulsion.329 Thus the complexes in this work possess significant Coulombic repulsion which is likely between cations in the lattice320 and these forces of repulsion presumably outweigh any attractive London/dispersion forces that contribute to π-π stacking.
The observed short contacts may be explained for the L1 complexes by the size of the bridging group. Due to the bridging group being rather small the cation is mostly flat and hence there is barely any out-of-plane tilting of the pyridyl groups. Thus there is not much contact between the cations themselves, but somewhat more between the cations and their PF6
- counter ions. These anions are positioned between the metal complexes in the horisontal plane as well as in the vertical plane between the cations. The conformation of the propyl bridge linking the two pyridyl groups is not the regular staggered arrangement for an aliphatic chain. It is clear that the bridging group adopts a more curved conformation, which allows the two imine nitrogen atoms to point towards the metal centre. The structure of PtL2 is nonetheless predominantly flat with only minor distortions from the N4 plane.
For PdL2 the distortion out of the N4 plane is slightly greater (visible in the packing in Figure 3.31). This is due to the difference in size between the metal centres as the ligand has to slightly ruffle in order to fit around the smaller palladium atom. This therefore allows the complimentary short contact observed between two cations for Cf-Hf···Cc. Due to the similarities between platinum(II) and palladium(II) it is obvious that these two metals are likely to have similar preferences with respect to their ligand selectivity and the symmetry of their coordination compounds.330 Each divalent metal centre is encircled by a distorted square planar array of four nitrogen atoms with the metal ion located in the mean square plane formed by these four ligating atoms (PtL1 and PdL1) or only slightly distorted from it (PtL2 and PdL2). The two complexes of each ligand are therefore isomorphous; they have the same space group, and remarkably similar bond distances and angles as well as similar unit cell parameters. This can be easily observed from their packing arrangements which are almost identical for each group of complexes.The only difference between them is the position of the cations of the Pd(II) chelates. For the Pd(II) chelates, the alkyl bridges of the cations point along a negative b-axis direction, while those of the Pt(II) chelates point along the positive b-axis direction. In effect, the cation orientations differ about an axis that lies in the ac plane and bisects the a-O-c angle (where O is the cell origin) for the Pt(II) and Pd(II) salts, by a 180º rotation.
Complexes of L4, L4m and L4b:
PtL4 and PdL4 each have two molecules with different conformations, one saddled and the other „twisted‟. This out-of-plane tilt of the pyridine rings (relative to the N4 plane) may
be accredited to alleviation of unfavourable steric interactions between the adjacent pyridine rings. These molecules crystallise in alternating layers of the two types of cations (saddled and twisted) with the anions forming a barrier between them. Both PtL4 and PdL4 crystallise in the monoclinic space group P21/c. Their short contacts (not including those to the counter ions, PF6
-) are shown in Figure 3.32. For PtL4 there is again no π–π stacking; however, the structure of PdL4 is loosely stabilised by π–π interactions between pyridine rings of adjacent molecules along the stack. Since the complexes PtL4 and PdL4 do not have isomorphous structures, the packing is distinct in the two salts. This leads to different cation-cation interactions for each complex.
Figure 3.32: Illustration of the short intermolecular contacts for PtL4 (top left) and PdL4 (right).
The π–π stacking observed for PdL4 will be rather weak due to the lack of significant ring-ring overlap. This is, however, consistent with the 2+ charge on each cation and the significant Coulombic repulsion between these cations. An offset overlap of two pyridine rings (Figure 3.33) is more common than complete overlap; complete overlap is indeed considered to be a rather rare phenomenon.303 The distances for the Cp-Cc short contacts are 3.384(7) and 3.392(7) Å due to the tilted orientation of the two PdL4 cations towards each other. For the complexes with the 2,-2-dimethylpropane bridge the greater steric bulk causes a much greater distortion from a planar chelate geometry. This allows the molecules to be tilted enough for the short C-C contacts observed. Finally, due to this more distorted conformation and less-ordered packing some π–π interactions are possible. This has also been identified for PdL4m.
3.202(10) Å
3.392(7) Å 3.384(7) Å
Figure 3.33: The overlap of the observed π–π stack in PdL4 as viewed down the c-axis.
PdL4m only has a saddled structure (unlike PtL4 and PdL4) which crystallises in the monoclinic, P21/n space group. The cations form an interesting alternating conformation as shown in Figure 3.34, which demonstrates an inversion pair. The centre of inversion exists halfway between the two Pd(II) ions in the dimer. The complementary π–π stacking of the pyridine rings of neighbouring cations is also slightly offset, as observed for PdL4. Short contacts are present between the pyridine carbon atoms of neighbouring molecules with a Cc···Cc distance of 3.353(8) Å. The perpendicular distance between the mean planes calculated for the four nitrogen atoms of adjacent molecules is measured as 3.938(10) Å.
Figure 3.34: Illustration of the alternating packing for PdL4m (left) with a view of the π-π stacking and the overall interplanar distance (right).
The larger, more bulky structure of PdL4b shown in Figure 3.35 exhibits no π–π stacking. The alkyl C-H groups are not good H-bond donors so C-H···π interactions between cations, which are weakly stabilising, are unlikely. Here, the only short contacts observed between the metal cations are: C3···Hh-Ch (2.826 Å) and C4···Hh-Ch (2.714 Å).
These short, nonbonded contacts between cations form a crystallographcally-generated chain of cations along a 2-fold screw axis in the unit cell. These chains are in turn held in position by short contacts to the PF6
- anions present in the crystal. This is a very different packing to either that of PdL4 or PdL4m, which have the same bridging groups. It therefore seems that the change in bulk of the group on the imine carbon and the subsequent distortion from planarity cause great changes in the way these cations stack in the crystal lattice.
3.938(10) Å
Figure 3.35: Illustration of the one-dimensional chains formed for PdL4b, as viewed down the b-axis (left) and the short contacts between cations within a chain (right).
2.826 Å
2.714 Å
2.826 Å
2.714 Å C3
C4
Hh
Hh C3
C4