Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes

A series of pyridine dipyrrolide actinide(IV) complexes, (MesPDPPh)AnCl2(THF) and An(MesPDPPh)2 (An = U, Th, where (MesPDPPh) is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine), have been prepared. Characterization of all four complexes has been performed through a combination of solid- and solution-state methods, including elemental analysis, single crystal X-ray diffraction, and electronic absorption and nuclear magnetic resonance spectroscopies. Collectively, these data confirm the formation of the mono- and bis-ligated species. Time-dependent density functional theory has been performed on all four An(IV) complexes, providing insight into the nature of electronic transitions that are observed in the electronic absorption spectra of these compounds. Room temperature, solution-state luminescence of the actinide complexes is presented. Both Th(IV) derivatives exhibit strong photoluminescence; in contrast, the U(IV) species are nonemissive.


Synthesis of Th( Ph PDP Ph )2
Caution! 232 Th is a weak α-emitter (4.082 MeV) with a half-life of 1.41 x 10 10 years; manipulations and reactions should be carried out in monitored fume hoods or in an inert atmosphere drybox in a radiation laboratory equipped with α-and β counting equipment.ThCl4(DME)2 was synthesized following a reported procedure. 3 the glovebox, a 20 mL scintillation vial equipped with a magnetic stirrer was loaded with 100 mg (0.195 mmol) of H2 Ph PDP Ph and 3 mL of toluene, affording a yellow solution.In a separate vial, 66 mg (0.395 mmol, 2.03 eq.) of LiHMDS was dissolved in 3 mL of toluene.The LiHMDS solution was added to the suspension of H2 Ph PDP Ph with vigorous stirring, inducing an immediate color change to a luminescent dark orange.The resulting suspension was stirred for 2 hrs.In a separate vial, 54 mg of ThCl4(DME)2 (0.097 mmol, 0.50 eq.) was suspended in 3 mL of toluene and added to the suspension of Li2 Ph PDP Ph , inducing an immediate color change to bright orange.The mixture was stirred for 16 hrs, at which time it was filtered over a 1" pad of celite supported by a glass microfiber plug and collected in a 20 mL scintillation vial.
Removal of volatiles in vacuo, followed by trituration with pentane afforded an orange powder identified as the title compound.

Quantum Yield Determination
The quantum yields of 1b and 2b were determined via comparative method.Rhodamine 6G in ethanol (Φ = 0.95) was used as a standard.The quantum yield was calculated using the following equation: The subscripts X and Y stand for the sample and reference, respectively.Φ is the photoluminescence quantum yield.η is the refractive index of the solvent.

Figure S1. 1 H
Figure S1.1 H NMR spectrum (400 MHz) of the attempt to synthesize U( Mes PDP Ph )2 via a one-pot salt metathesis reaction in benzene-d6.Note, formation of eleven major peaks is inconsistent with the expected number of resonances for the desired product.

Figure S2. 1 HFigure S3. 1 H
Figure S2.1 H NMR spectrum (400 MHz) of Mes PDP Ph UCl2(THF) (1a) in benzene-d6.Identifiable paramagnetic peaks with unique resonances are labeled; those that could not be formally assigned (2H and 4H resonances) were left unlabeled.The unmarked resonances correspond to toluene, pentane, and diethyl ether present in the sample and the starred (*) resonances correspond to a small amount of protonated ligand (H2 Mes PDP Ph ) present in the sample.

*Figure S4 .
Figure S4.Molecular structure of ( Mes PDP Ph )ThCl2(THF) (1b) shown with 30% probability ellipsoids.Hydrogen atoms and solvent molecules have been removed for clarity.Key: dark green, Th; blue, N; gray, C; light green, Cl; red, O.All non-carbon atoms have been labeled in the image.

Figure S11 .
Figure S11.Molecular structure of Th( Mes PDP Ph )2 (2b) shown with 30% probability ellipsoids.Hydrogen atoms and solvent molecules have been removed for clarity.Key: orange, Th; blue, N; gray, C; light green, Cl; red, O.All non-carbon atoms have been labeled in the image.

Figure S13 .
Figure S13.Emission of Rhodamine-6G in anhydrous ethanol plotted with 1b and 2b for determination of quantum yields via the comparative method.

Figure S14 .
Figure S14.Top: Time profile for the phosphorescence intensity (304 µs) of 2b at room temperature in toluene.Inset is time profile plotted on a logarithmic scale.Bottom: Time profile for the phosphorescence intensity (256 µs) of 1b at room temperature in toluene.Inset is time profile plotted on a logarithmic scale.

Table S2 .
Crystallographic parameters for molecular structures of 1a and 1b.

Table S3 .
Crystallographic parameters for molecular structures of 2a and 2b.