Kinetico-Mechanistic Insights on the Assembling Dynamics of Allyl-Cornered Metallacycles: The Pt-Npy Bond is the Keystone

The square-like homo- and heterometallamacrocycles [{Pd((3)-2-Me-C3H4)(L-n)(2)}(2){M(dppp)}(2)](CF3SO3)(6) (dppp=1,3-bis(diphenylphosphino)propane) and [{Pd((3)-2-Me-C3H4)(L-1)(2)}(2){M(PPh3)(2)}(2)](CF3SO3)(6) [py=pyridine, M=Pd, Pt, L(n=)4-PPh(2)py (L-1), 4-C(6)F(4)PPh(2)py (L-2)] containing allyl corners were synthesised by antisymbiotic self-assembly of the different palladium and platinum metallic corners and the ambidentate N,P ligands. All the synthesised assemblies displayed a complex dynamic behaviour in solution, the rate of which is found to be dependent on the electronic and/or steric nature of the different building blocks. A kinetico-mechanistic study by NMR line shape analysis of the dynamics of some of these assemblies was undertaken in order to determine the corresponding thermal activation parameters. Both an enhanced thermodynamic stability and slower dynamics were observed for platinum-pyridine-containing species when compared with their palladium analogues. Time-dependent NMR spectroscopy in combination with ESI mass spectrometry was used to study the exchange between the assemblies and their building blocks, as well as that occurring between different metallamacrocycles. Preliminary studies were carried out on the activity of some of the metallamacrocyclic compounds as catalytic precursors in the allylic substitution reaction, and the results compared with that of the monometallic allylic corner [Pd(eta(3)-2-Me-C3H4)(L-1)(2)](+).