Luminescent Rhenium/Palladium Square Complex Exhibiting Excited State Intramolecular Electron Transfer Reactivity and Molecular Anion Sensing Characteristics

TLDR: In this article, a light-emitting square complex, 1; was prepared by combining the chromophore fac-Re(CO)F1(4,4'-bpy)2 (2)6 (105 mg, 0.170 m o l ) (dppp = 1,3-(diphenylphosphino)propane) in 50 mL of dry CH2C12.

Abstract: Rigid macrocycles based on cis bridging ligation of transition metals represent an unusual,'-4 and largely new,5 class of compounds having tremendous promise in host-guest, inclusion, and molecular recognition chemistry. Most of the available systems are (1) derived from pt(I1) or Pd(I1) triflate species, (2) tetranuclear with respect to metal content, and therefore, (3) arranged in square or box-like geometries, with metal atoms defining the comers. We report a simple, but potentially very useful, variant and extension of this theme: incorporation of visible-light-addressable, luminescent metal-ligand components within a square assembly. Induction of photoluminescent characteristics is particularly attractive in the context of eventual molecular sensing applications because it suggests an altemative to 'H NMR spectroscopy for detection of guest inclusion. Induction of luminescence also opens up the possibility of electronic excited state reactivity and possible manipulation of reactivity by encapsulated guests. The light-emitting square complex, 1; was prepared by combining the chromophore fac-Re(CO)F1(4,4'-bpy)2 (2)6 (105 mg, 0.170 "01) (4,4'-bpy = 4,4'-bipyridine) with Pd(dppp)(triflate)zId (139 mg, 0.170 m o l ) (dppp = 1,3-(diphenylphosphino)propane) in 50 mL of dry CH2C12. After 12 h of stirring, the pale yellow product was collected as a powder on a glass frit, washed with methylene chloride, and dried under vacuum. The yield was 83%. Elemental analysis data are consistent with the formation of either 1 or a larger assembly with 1:l Re:Pd stoichiometry.' FAB+ mass spectral measurements (m-NBA) yielded intense peaks at m/z = 2871.7 (2871.2 expected for l*H+), 2721.0 (1 triflate) and 2564.9 (1 triflate 4,4'bpy), thereby establishing the tetranuclear metal content of the compound. No peaks for compounds of higher mass (e.g., hexanuclear or octanuclear complexes) were found. FT-IR and 'H NMR measurements (supporting information) yielded additional data supportive of molecular square formation.* Figure 1 shows that the lowest energy transition in the electronic absorption spectrum of 2 is shifted substantially red by formation of the square. The shift is similar to that observed upon ligand protonation in 26 and is consistent with assignment