Influence of bridging groups on the reactivity of dinuclear platinum (II) complexes with bis(2-pyridylmethyl)amine chelate headgroups.
The influence on the reactivity of both the length as well as the structural nature of diamine bridges linking dinuclear Pt(II) complexes with homotopic bis(2-pyridylmethyl)amine headgroups has been investigated. For this purpose, three sets of square-planar Pt(II) complexes sharing a common non-labile bis(2-pyridylmethyl)amine chelate were synthesized and characterized by various spectroscopic methods. The substitution of the coordinated aqua ligands by three thiourea nucleophiles of different steric demands was studied in acidic aqueous medium under pseudo first-order conditions. The reactions were studied as a function of concentration, temperature and in some cases under an applied pressure using the standard stopped-flow technique and UV-visible spectrophotometry. Their thermodynamic properties were investigated by studying the acid-base equilibria of the coordinated aqua ligands using a spectrophotometric titration method. DFT Quantum mechanical calculations were also performed to determine their geometry-optimized structures and energies of the frontier molecular orbitals. The first set of Pt(II) complexes comprise dinuclears, all bridged by a flexible α,ω-alkyldiamines. The second set of complexes is Pt(II) amphiphilic mononuclear analogues of the former set, formed intuitively by excising off one of the Pt(II) chelate headgroups. The last set of complexes comprises Pt(II dinuclear complexes which are structurally related to the first set, but are linked by relatively rigid linkers, which are made up of either phenyldiamine or diaminocyclohexane fragments. In two of the complexes, a single methylene spacer (CH2) group is incorporated between the rigid moieties of the diamine bridge so as to elongate the average distances separating their Pt(II) atoms as well as to modulate the rigidity of the complexes. For comparison purposes, two monomeric analogues bearing the phenyl and cyclohexyl appended groups, respectively, were studied and reported together with these complexes. In general, the substitution reactions of the coordinated aqua ligands of all the Pt(II) complexes by the three sulfur donor nucleophiles (Nu) proceed via a two-step reaction pathway. The first step, whose rate constant is denoted in subsequent text as k2(1st), involves the substitution of the aqua ligands. The second step, induced by the coordination of the strong labilizing thiourea nucleophiles and whose rate constant is denoted in the text as k2(2nd), is ascribed to the dechelation of the one of the cis-coordinated pyridyl units. Thus, the substitution of the aqua ligands and the subsequent dechelation of the pyridyl units, can be expressed as kobs(1st/2nd) = k2(1st/2nd)[Nu] for all the reactions. Negative entropy of activation, negative volume of activation (in cases where measurements were carried out) and second-order kinetics for the substitution reactions all support an associative mode of activation. The substitution reactivity of all the dinuclear complexes is influenced to a greater extent by the steric influences conferred by the bridge as well as a weak electronic effect. The steric influences are mutual, axially exerted and seemingly unique to the square-planar terdentate chelate headgroups. The steric influences depend strictly on length of the diamine (i.e., the average distances separating the Pt centres of the dinuclears) as well as molecular symmetries and shapes of the complexes. The molecular symmetries and hence the shapes of the complexes depend on the parity of the connecting bonds in the diamine (whether even or not). If the connecting bonds of the bridges are even, C2h structures and hence slip-up molecular geometry are preferred. Their overlap geometries cause mutual and axial steric influences on the Pt(II) square-planar chelates which retard substitutional reactivity when the bridge is short. When odd, bowl-shaped complexes of the C2v point group symmetry are preferred in which the axial steric influences are absent at their Pt(II) chelates. In addition their bowl geometry causes an entrapment of the incoming nucleophiles, causing unusually high reactivity when compared to their even-bridged counterpart. For both molecular symmetries (C2h or C2v), the reactivity of the dinuclear complex depends on the average distances separating the Pt(II) centres of the dinuclears. In the former type of complexes, when the average distances separating their Pt(II) centres are long, the axial steric influences at each Pt(II) chelate due to their C2h overlap geometry is weakened, leading to enhanced reactivity as the chain length is increased. In the latter type of complexes, this weakens the ‘entrapment’ effect of their bowl-shaped geometry, resulting in a steady decrease in reactivity when the chain length of the linker is increased. In addition rigidity and planarity within the backbone of the diamine bridge has been found to distort the bowl cavity causing weakening of the ‘entrapment effect’ resulting in the lower rates than expected. The chain length as well as the structural make-up of the linker also determines the amount of electron density donated inductively from the linker to the Pt ions as well as the effective nuclear charge at each Pt(II) centre due to charge addition. These are two opposing factors which also influence the rate of substitution in these complexes to some extent. The inductive effect as well as the presence of a domineering steric influence in the C2h overlap geometry was verified by studying the reactivity of the analogous amphiphilic Pt(II) complexes.