Cyclometallated Pt(II) Complexes Containing a Functionalized Bis-Urea Alkynyl Ligand: Probing Aggregation Mediated by Hydrogen Bonds versus Pt...Pt and pi-pi Interactions

Neutral cyclometallated Pt(II) complexes [(C<^>Nt-Bu<^>Nt-Bu)Pt-R] (4-5) containing a bis-urea alkynyl (R) ligand have been prepared and fully characterized. The dichotomous nature of 4-5 makes it difficult to anticipate which interactions (hydrogen bonds, pi-pi stacking and Pt...Pt) will dominate their assembly process. The aggregation properties of 4-5 were thus probed by multifarious analytical (UV/Vis, FT-IR, NMR, MS, CD, emission) and computational (DFT, TD-DFT) techniques. CD analyses of sergeants-and-soldiers type mixtures between 4 and an enantiopure Pt-free monomer (S,S)-3 reveal that no amplification of supramolecular helicity occurs in this system. In fact, complex 4 acts as competitor or chain capper, likely through hydrogen bonding, of the homochiral assemblies formed by (S,S)-3. In DMSO, H-1 NMR and DOSY analyses indicate that 4 and 5 form aggregates. Likewise, the aggregation is promoted by the addition of H2O as suggested by red shift of the lowest energy emission band, which might originate from a (MMLCT)-M-3 excited state. TD-DFT calculations confirm that self-aggregation occurs through Pt...Pt and pi-pi interactions yielding head-to-tail aggregates in DMSO and DMSO/H2O mixtures. Our study therefore suggests a (condition dependent) competitive rather than a cooperative mode of action of the different types of interactions present in aggregates of 4-5.