Para-Derivatized Pybox Ligands As Sensitizers in Highly Luminescent Ln(III) Complexes

New complexes of pyridine-bis(oxazoline) derivatized with -H, -OMe, and -Br at the para position of the pyridine ring with Eu(III) and Tb(III) have been isolated. These are highly luminescent in the solid state, regardless of the ligand-to-metal ratio. Several of the metal complexes were isolated and characterized by single crystal X-ray diffraction, showing the rich diversity of structures that can be obtained with this family of ligands. [Eu(PyboxOMe)3](NO3)3·3CH2Cl2, 1, crystallizes in the monoclinic space group P21/n and has the cell parameters a = 14.3699(10) Å, b = 13.4059(9) Å, c = 25.8766(18) Å, β = 95.367(1)°, and V = 4963.1(6) Å3. The isostructural [Tb(PyboxOMe)3](NO3)3·3CH2Cl2, 2, crystallizes with the parameters a = 14.4845(16) Å, b = 13.2998(15) Å, c = 25.890(3) Å, β = 94.918(2)°, and V = 4969.1(10) Å3. 3, a 1:1 complex with the formula [Eu(PyboxBr)(NO3)3(H2O)], crystallizes in the monoclinic P21/c space group with a = 11.649(2) Å, b = 8.3914(17) Å, c = 20.320(4) Å, β = 100.25(3)°, and V = 1954.5(7) Å3. 4, a product of the reaction of PyboxBr with Tb(NO3)3, is [Tb(PyboxBr)2(η2-NO3)(η1-NO3]2[Tb(NO3)5]·5H2O. It crystallizes in the monoclinic space group P21 with a = 15.612(3) Å, b = 14.330(3) Å, c = 16.271(3) Å, β = 92.58(3)°, and V = 3636.5(13) Å3. [Tb(Pybox)3](CF3SO3)3·3CH2CN, 5, crystallizes in the triclinic space group P1̅ with a = 12.3478(2) Å, b = 15.0017(2) Å, c = 16.1476(4) Å, α = 100.252(1)°, β = 100.943(1)°, γ = 113.049(1)°, and V = 2594.80(8) Å3. Finally, compound 6, [Tb(Pybox)2(NO3)(H2O)](NO3)2·CH3OH, crystallizes in the triclinic P1̅ space group with a = 9.7791(2) Å, b = 10.1722(2) Å, c = 15.3368(3) Å, α = 83.753(1)°, β = 78.307(1)°, γ = 85.630(1)°, and V = 1482.33(5) Å3. In solution, the existence of 3:1, 2:1, and 1:1 species can be observed through absorption and luminescence speciation measurements as well as NMR spectroscopy. The stability constants in acetonitrile, as an average obtained from absorption and emission titrations, are log β11 = 5.4, log β12 = 8.8, and log β13 = 12.8 with Eu(III) and log β11 = 4.5, log β12 = 8.4, and log β13 = 11.7 for the Tb(III) species with PyboxOMe. Pybox displayed stability constants log β11 = 3.6, log β12 = 9.1, and log β13 = 12.0 with Eu(III) and log β11 = 3.7, log β12 = 9.3, and log β13 = 12.2 for the Tb(III) species. Finally, PyboxBr yielded log β11 = 7.1, log β12 = 12.2, and log β13 = 15.5 for the Eu(III) species and log β11 = 6.2, log β12 = 11.0, and log β13 = 15.4 with Tb(III). Photophysical characterization was performed in all cases on solutions with 3:1 ligand-to-metal ion stoichiometry and allowed determination of quantum yields and lifetimes of emission for PyboxOMe of 23.5 ± 1.6% and 1.54 ± 0.04 ms for Eu(III) and 21.4 ± 3.6% and 1.88 ± 0.04 ms for Tb(III). For Pybox these values were 25.6 ± 1.1% and 1.49 ± 0.04 ms for Eu(III) and 23.2 ± 2.1% and 0.44 ± 0.01 ms for Tb(III) and for PyboxBr they were 35.8 ± 1.6% and 1.46 ± 0.03 ms for Eu(III) and 23.3 ± 1.3% and a double lifetime of 0.79 ± 0.05/0.07 ± 0.01 ms for Tb(III). A linear relationship between the triplet level energies and the Hammett σ constants was found. Lifetime measurements in methanol as well as the NMR data in both methanol and acetonitrile indicate that all complexes are stable in the 3:1 stoichiometry in solution and that there is no solvent coordination to the metal ion.