Structural investigation of anionic cyclometalated Pt(II)-tetrabromocatecholate complexes: quasi-halogen bonding and elusive polymorphism at play

Four unusual anionic heteroleptic cyclometalated monomeric Pt(II) complexes bearing the aromatic tetrabromocatecholate dianion (Br4cat) and either benzo[h]quinoline H(bzq) (1), (2,4-difluorophenyl)-pyridine H(F2ppy) (2), 2-phenylpyridine H(ppy) (3) or 2,2’-thienylpyridine H(thpy) (4) as the cyclometalating ligand were synthesized, precipitated as NBu4+ salts and recrystallized from acetone solutions. Single-crystal XRD investigation indicated that in the crystal structures of 1–4 the monoanionic [(C∧N)Pt(Br4cat)]− [(C∧N) = bzq (1), F2ppy (2), ppy) (3), thpy (4α, 4β)] moieties engage in various intermolecular interactions, including π-π stacking and Br···Br contacts (mostly of quasi-type II), which ultimately determine their aggregation into 2D (1) or 3D (2–4) supramolecular networks, albeit without short Pt···Pt contacts. A combined PXRD and DSC analysis of both microcrystalline powders and single-crystals of 1–4 unequivocally revealed the occurrence of solid-state polymorphism for 1, 3 and 4. Efforts to grow single-crystals of more than just one form of 1 and 3 were unsuccessful. Conversely, two polymorphs of 4 (4α and 4β) could be isolated and studied. The deep structural investigation of 1–4 put forward herein contributes validating the hypothesized reasons behind the significant enhancement of luminescence in the solid-state previously reported for this series of heteroleptic cyclometalated Pt(II) monomers.