Fine-Tuning Electronic Properties of Luminescent Pt(II) Complexes via Vertex-Differentiated Coordination of Sterically Invariant Carborane-Based Ligands

We report the synthesis of two isomeric Pt(II) complexes ligated by doubly deprotonated 1,1′-bis(o-carborane) (bc). The κ²-C,C-bound isomer (3a) is generated through salt metathesis of K₂[bc] and Pt(dtb-bpy)Cl₂ (dtb-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine), whereas the κ²-B,C-bound isomer (3b) forms after the addition of K[H-bc] to a reaction mixture containing the Pt(II) starting material and an alkyl lithium reagent. The structures of both isomers were confirmed with ¹H NMR spectroscopy and single crystal X-ray crystallography. A cathodic shift is observed for both oxidation (>260 mV) and reduction (∼130 mV) potentials of 3b versus those of 3a, which is corroborated by DFT calculations of both complexes. While photophysical studies reveal similar quantum yields and excited state lifetimes for both isomers, emission spectra for 3b in a PMMA film, frozen solution at 77 K, and as a neat solid are slightly blue-shifted relative to the corresponding emission spectra for 3a. Ultimately, this work provides a potential route to fine-tune the electronic properties of luminescent metal complexes by virtue of vertex-differentiated coordination chemistry of carborane-based ligands.