Synthesis and Solution Phase Characterization of Strongly Photooxidizing Heteroleptic Cr(III) Tris-Dipyridyl Complexes

We report the preparation and characterization of Cr(III) coordination complexes featuring the dimethyl 2,2′-bipyridine-4,4′-dicarboxylate (4-dmcbpy) ligand: [(phen)2Cr(4-dmcbpy)](OTf)3 (1), [(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4), [(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7), and [Cr(4-dmcbpy)3](BF4)3 (8), where phen is 1,10-phenanthroline, Ph2phen is 4,7-diphenyl-1,10-phenanthroline, and Me2bpy is 4,4′-dimethyl-2,2′-bipyridine. Static and nanosecond time-resolved absorption and emission properties of these complexes dissolved in acidic aqueous (1 M HCl) solutions are reported. Emission spectra collected at 297 K show a narrow spectrum with an emission maximum ranging from 732 nm (1) to 742 nm (4). The emissive state is thermally activated and decays via first order kinetics at all temperatures explored (283 to 353 K). At 297 K the observed lifetime ranges from 7.7 μs (8) to 108 μs (4). The photophysical data suggest that in these acidic aqueous environments these complexes store ∼1.7 eV for multiple microseconds at room temperature. Of the heteroleptic species, complex 4 shows the greatest absorption of visible wavelengths (ε = 1270 M−1 cm−1 at 491 nm), and homoleptic complex 8 has improved absorption at visible wavelengths over [Cr(bpy)3]3+. The electrochemical properties of 1, 4, 7, and 8 were investigated by cyclic voltammetry. It is found that inclusion of 4-dmcbpy shifts the “CrIII/II” E1/2 by +0.22 V compared to those of homoleptic parent complexes, with the first reduction event occurring at −0.26 V versus Fc+/Fc for 8. The electrochemical and photophysical data allow for excited state potentials to be determined: for 8, CrIII*/II lies at +1.44 V versus ferrocenium/ferrocene (∼+2 V vs NHE), placing it among the most powerful photooxidants reported.