The synthesis of 4′-aryl substituted terpyridines by Suzuki cross-coupling reactions: substituent effects on ligand fluorescence

Abstract

Several 4′-aryl-substituted 2,2′:6′,2″-terpyridines (tpy-C₆H₄R) have been prepared by palladium-catalysed cross-coupling of 4′-bromoterpyridine or 4′-triflate-terpyridine (triflate = trifluoromethylsulfonyloxy) with aryl boronic acids or esters, RC₆H₄B(OR′)₂ (R = H, m-NH₂, p-CHO, -NO₂, -CN, -NMe₂, -NPh₂). The new ligand 4′-mesitylterpyridine (mesityl = 2,4,6-trimethylphenyl) was prepared in the same way. Similarly, 4′-bromophenylterpyridine (tpy-ϕ-Br) has been cross-coupled with aryl halides to generate several new biaryl-substituted terpyridines (tpy-ϕ-C₆H₄R where R = H, p-CN, NMe₂, NPh₂), together with two related compounds with pendent 3- or 4-pyridyl groups (tpy-ϕ-C₆H₄-py). For selected compounds, the alternative coupling strategy of reaction of a terpyridine-4-boronate or terpyridine-4-phenylboronate with the appropriate aryl halide has also been investigated (e.g. to prepare tpy-ϕ-C₆H₄NO₂), but was generally found to be less satisfactory. All of the compounds are fluorescent in the UV region of the spectrum, the biaryl-substituted compounds being only slightly red-shifted compared to the monoaryl systems, but with the further red-shift that accompanies protonation being more significant for the former. Fluorescence lifetimes in solution are in the range 1–5 ns. The emission spectra of the aminobiphenyl-substituted compounds (tpy-ϕ-C₆H₄NR″₂, where R″ = Me or Ph) display a large red-shift with increasing solvent polarity, suggesting the involvement of an intramolecular charge transfer state, as found previously for the two analogues omitting the phenyl ring (tpy-C₆H₄NR″₂). In contrast to the latter, however, protonation or binding of a Lewis acidic metal ion to the aminobiphenyl compounds serves to quench almost completely their emission.