Pt(II) uptake by dendrimer outer pockets: 1. Solventless ligand exchange reaction

Density functional theory is used to elucidate molecular-level details of the complexation of Pt(II) metal compounds with PAMAM dendrimers. Particular attention is given to the ligand exchange reaction (LER). Binding of Pt(II) complexes to one dendrimer atom site (monodentate binding) is found to be thermodynamically feasible. Tertiary amine nitrogen (N3) is found to be the most favorable binding site in agreement with previous experimental work. Comparing the binding of Pt(II) species to atom sites in simple molecules with those to similar sites in dendrimer outer pockets allowed us to assess the impact of dendrimer branches on the binding. The impact of branches is manifested in more complex reaction profiles for complexation of Pt(II) species, because of the numerous ways in which a single molecule could be hosted by an outer dendrimer pocket. It is found that branches slightly improve the binding strength to all sites, particularly to N3. However, they could also be responsible for the increase of the activation energy for direct LER of PtCl₄ ^2- and PtCl₃(H₂O)^- at the N3 site. Considering the thermodynamics of both complexation steps, namely noncovalent binding (NCB) and LER, it is found that to have a PtCl₃^- moiety bound to N3, as a result of NCB + LER operating on PtCl₄ ^2-, is more likely than to have any other ion hosted in the outer pockets. However, the activation energy for direct LER of PtCl₄ ^2- at the N3 site is found to be the largest among all Pt(II) metal complexes and even larger than the barrier to its own aquation yielding PtCl₃(H₂O)^-.