Designing Molecular Catalysts for Selective CH Functionalization

Abstract

The synthesis, structural characterization, photoluminescence, and excited state absorption properties of a series of platinum(II) terpyridyl complexes bearing a bipyridyl acetylide subunit are presented. The [tBu3tpyPtCtCbpy]þ (1) complex displays a broad and structureless emission profile at room temperature (RT), a lifetime of 5.8 μs, and transient absorption (TA) difference spectra characteristic of a charge transfer (CT) excited state. Upon coordination of Fe2þ to 1, producing tetranuclear 2, the CT emission was quantitatively quenched presumably through the low-lying iron-based ligand field states present. Surprisingly, the addition of Zn2þ to solutions of 1 produces a higher energy emissive state with a substantially longer excited state lifetime of 16.1 μs. The combined spectroscopic data measured for the zinc titration product (3) suggests that the overall excited state is dominated by a CT manifold, albeit at higher energy relative to 1. The photophysics of a bis-phosphine complex bearing two trans-disposed bpy-acetylide subunits (4) produced a model chromophore possessing an intraligand triplet excited state with a lifetime of 26 μs at RT. The bipyridyl analogue of 1, tBu2bpyPt(CtCbpy)2 (5), was also prepared and its photophysics are consistent with a lowest CT parentage at RT. The 77 K emission spectra measured for complexes 1, 3, 4, and 5 are all consistent with a triplet bpy-acetylide localized excited state; the E00 energies vary over a modest 344 cm-1 across the series. However, the shorter 77 K excited state lifetimes observed for 1, 3, and 5 in comparison to 4 suggests that the energetically proximate CT state in the former compounds significantly influences excited state decay at low temperature.