X/Y platinum(II) complexes: some features of supramolecular assembly via halogen bonding

Four series of new luminescent cyclometalated complexes [Pt(C^N)(IPy)Y] (HC^N = 2-phenylpyridine (Hppy), 2-(1-benzofuran-3-yl)pyridine (Hbfpy), methyl-2-phenylquinoline-4-carboxylate (Hmpqc), 2-(1-benzothiophen-3-yl)pyridine (Hbtpy), IPy = 4-iodopyridine, and Y = Cl, Br, I) have been investigated as X/Y 'building blocks' for the construction of a supramolecular network utilizing the I atom in IPy as a halogen bond (XB) donor (the X atom). The σ-hole of the X atom was found to provide non-covalent X⋯Y, X⋯Pt and X⋯π (π system of the metalated chelate ring) interactions for the complexes in the crystal state. NBO analysis confirms donation of the platinum electron density to iodine upon the X⋯Pt interaction. The nature of the X counterpart in XB depends on the nature of the Y atom and the cyclometalating ligand of the Pt(II) complex. DFT calculations show that the HOMO of [Pt(C^N)(IPy)Y] in the S₀ state is delocalized over Pt, Y and a C-coordinating fragment of C^N, while the LUMO in most complexes is formed by the Py orbitals of IPy. However, the α-HOMO in the lowest triplet state of [Pt(C^N)(IPy)Y] contains no contribution of the IPy wavefunctions. All Pt(II) complexes exhibited triplet luminescence in solution and in the solid state (Φ up to 0.129), which is determined by the nature of the C^N ligand. The emission profile is independent of the nature of the ligand Y, while the quantum yield decreases from Cl to I. Accordingly, on the basis of DFT calculation, this emission is interpreted as a C^N intraligand charge transfer predominantly. The XB formation did not show an effect on the luminescence of the complexes in the solid phase, however grinding of crystals results in an increase of brightness.