Pt(II)-Coordinated Tricomponent Supramolecular Assemblies of Tetrapyridyl Porphyrin and Dicarboxylate Ligands: Are They 2D Bow Ties or 3D Prisms?

Abstract

Thermodynamically favored heteroleptic coordination of one aza- and another oxo-coordinating ligand with Pt(II) ions yield tricomponent supramolecular coordination complexes (SCCs) that have much greater structural complexity and functional diversity than the traditional bicomponent SCCs containing only one of the ligands. Herein, we demonstrate that heteroleptic coordination of tetrapyridyl porphyrins (M¢TPP, M¢ = Zn or H2) and various dicarboxylate ligands (XDC) having different lengths and rigidity with cis- (Et3P)2PtII corners actually yields bow tie (⋈)-shaped tricomponent [{cis-(Et3P)2Pt}4(M¢TPP)(XDC)2] 4+ complexes featuring a M¢TPP core and two parallel XDC linkers held together by four heteroligated PtII(N,O) corners. Although previous reports have claimed that the self-assembly of these three components produced tetragonal prisms having two cofacial M¢TPP planes connected by four XDC linkers via eight PtII(N,O) corners, our extensive 1 H, 31P, and 2D NMR, ESI-MS, X-ray crystallographic, and computational studies unequivocally demonstrated that in reality, no such prism was formed because instead of connecting two cofacial M¢TPP ligands, the XDC linkers actually bridged two adjacent pyridyl termini of an M¢TPP ligand via shared PtII(N,O) corners, forming bow tie complexes. In addition to direct crystallographic evidence, the NMR spectra of these complexes revealed that the M¢TPP ligands contained two distinct pyrrole protons (4 each)—those located inside the triangles were shielded by and coupled to adjacent XDC linkers, whereas the exposed ones were not—an unmistakable sign of their bow tie structures. Thus, this work not only unveiled novel bow tie-shaped coordination complexes, but also accurately defined the actual structures and compositions of M¢TPP-based tricomponent SCCs.