Results and Discussion

CHAPTER 3 X-ray Crystallography

3.4 Results and Discussion

3.4.1 General

R C

_i

N

_p

N

_i

R

⁼

or where

or

H

e

H_e H_e H_e

C

_e

M

M = Pt or Pd

C C C

C C

e

C

e₂ e₃ e₄

e₅ e₆

1

C

_h

C

_g

C

_f

C

_h

C

_g

C

_f

L1-L6

L1m-L6m

L1b-L6b

bridging groups =

C

h

C

h

C

_f

C

_g

C

_f

C

_i

R

N C

_a

C

_p

C

_b

C

_d

C

_c

N

Figure 3.13: The notation used for labelling the chemically unique atoms in the basic framework of the Schiff base ligands and complexes.

Figure 3.13 shows the notation that has been used to label the atom framework of the structures throughout this work. Cf, Cg, Ch etc. are used to denote the bridging group carbon atoms, while Ca–d denotes the carbon atoms in the pyridine ring. Ci and Cp refer to the carbon atoms involved in the imine bond and the quaternary carbon in the pyridine ring, respectively. The hydrogen atoms will be named according to the atom to which

they are bonded, except for the hydrogen on the imine group which is labelled He. When there is a methyl group on Ci the atoms are labelled Ce and He and when it is a phenyl ring the carbons are labelled Ce1, Ce2 etc. (and consequently, He1, He2 etc.). The nitrogen atoms involved in the imine bond are labelled Ni and those in the pyridine ring, Np.

Mononuclear and dinuclear complexes have been reported for many metal complexes of these types of ligands (as discussed earlier). The complexes in this work are expected to be mononuclear by design. Platinum(II) and palladium(II) are considered isoelectronic and isosymmetric²⁹⁶ and we therefore expect to see similarities in the conformation of the ligands around these metal centres, as well as for the bond lengths and angles. The main difference in comparison to the literature structures presented here will be that platinum and palladium form four-coordinate square planar complexes rather than six- coordinate octahedral complexes.

The preferred geometry is correlated to the electronic configuration. A square planar configuration will be imposed not only by the ligand, but as a result of additional splitting of the orbital levels due to lack of z-axis ligands. This additional splitting is most advantageous as it allows for the pairing of all electrons (the value of Δo is sufficiently high).²⁹⁷ Four ligands that each donate a pair of electrons to the central metal will fill the sigma bonding MOs (molecular orbitals) and subsequently the d electrons must be divided between the eg, b1g and b1g*

MOs. This reduced symmetry results in no unique energy splitting analogous to Δo or Δt. The four approximately nonbonding MOs lie much lower in energy than b1g*

and therefore this d⁸ system will be more stable as four- coordinate square planar than in an octahedral complex where two electrons will be in the eg*

MOs.²⁹⁸

The crystal field stabilisation energy (in terms of Δo) is therefore much greater for square planar coordination than for octahedral coordination. The d-orbital splitting generally increases per period down a group with around 30–50% going from one transition series to the next.²⁹⁹ This subsequently promotes the square planar geometry for complexes of the 5^th and 6^th period (e.g., Pt and Pd, respectively) transition metals. The loss in bond energy caused by an octahedral complex being transformed to a square planar complex may be made up by this greater energy difference for platinum and palladium; however, it is not usually enough to ensure square planar complexes for nickel. Overall, square

configuration in this work is likely to be square planar (similar to the configuration of the ligand seen for the trans isomers of the tetragonally distorted octahedral structures reviewed earlier) with the use of platinum(II) and palladium(II) as the coordinating metals.

An important objective of this work is therefore to describe the structures and properties of square planar d⁸ PGM complexes of this group of ligands.

Despite the predominantly planar tendency of the tetradentate chelating ligands in this work some distortions from the N4 plane (formed by the two imine (Ni) and two pyridine (Np) nitrogen atoms) are to be expected. The bonded central metal ion will resist undue radial expansion or contraction but it can readily distort from a planar geometry; it has been known to be quite flexible toward out-of-plane deformation.³⁰⁰ Consequently, the central metal ion may be easily displaced in a direction perpendicular to the mean plane;

however, this is not necessarily the case. The metal may lie in the N4 plane causing the distortion elsewhere in the structure. This has been observed for the trans isomer copper complexes presented from the literature.193,177,190

Distortions may also be due to packing effects in the crystal lattice or an attempt to reduce the strain in the bonding network around the metal. Electrostatic repulsion of the inner hydrogen atoms on the pyridine ring, which applies to solution as well as solid state species, may cause the pyridine rings to tilt out of the N4 plane created by the ligand and metal centre.²⁴⁹ The pyridine ring subunits are themselves always planar but can be tilted in order to reduce steric strain. Structures with larger substituents on the imine carbon are likely to have a greater tendency towards this type of distortion. This reduction of steric crowding may cause substantial nonplanarity for the structures. Thus it is possible for either or both of these factors to play a role in the out-of-plane deviations for the structures presented in this work.

Lastly, altering the length of the carbon bridge between the imine groups in these complexes not only allows change in the molecular conformation, but also facilitates different crystal packing architectures for each derivative. Thus the distortion from the plane and the nonplanarity of the bridging group are complimentary to each other. The chelating ligand makes an arc around the metal centre in order to offer the four nitrogen atoms for bonding, resulting in limited space available for the other atoms linking these nitrogen atoms. The more atoms in this space, i.e. the larger the bridging group, the greater the extent of distortion of the chelate. This will result in the bridging group being

pulled out of the N4 plane. The extent to which it is removed from the plane will depend on the nonplanarity seen around the metal and the size and bulk of the bridging group.

3.4.2 X-ray data for the free ligands