What a Difference a 5f Element Makes: Trivalent and Tetravalent Uranium Halide Complexes Supported by One and Two Bis[2-(diisopropylphosphino)-4-methylphenyl]amido (PNP) Ligands

The coordination behavior of the bis[2-(diisopropylphosphino)-4-methylphenyljamido ligand (PNP) toward Ul(3)(THF)(4) and UCl4 has been investigated to access new uranium(III) and uranium(IV) halide complexes supported by one and two PNP ligands. The reaction between (PNP)K (6) and 1 equiv of Ul(3)(THF)(4) afforded the trivalent halide complex (PNP)Ul(2)(4-Bu-t-pyhdine)(2) (7) in the presence of 4-tert-butylpyridine. The same reaction carded out with UCl4 and no donor ligand gave [(PNP)[(PNP)UCl3](2) (8), in which the uranium coordination sphere in the (PNP)UCl3 unit is completed by a bridging chloride ligand. When UCl4 is reacted with 1 equiv (PNP)K (6) in the presence of THF, trimethylphosphine oxide (TMPO), or triphenylphosphineoxide (TPPO), the tetravalent halide complexes (PNP)UCl3(THF) (9), (PNP)UCl3(TMPO)(2) (10), and (PNP)UCl3(TPPO) (11), respectively, are formed in excellent yields. The bis(PNP) complexes of uranium(III), (PNP)(2)Ul (12), and uranium(IV), (PNP)(2)UCl2 (13), were easily isolated from the analogous reactions between 2 equiv of 6 and Ul(3)(THF)(4) or UCl4, respectively. Complexes 12 and 13 represent the first examples of complexes featuring two PNP ligands coordinated to a single metal center. Complexes 7-13 have been characterized by single-crystal X-ray diffraction and H-1 and P-31 NMR spectroscopy. The X-ray structures demonstrate the ability of the PNP ligand to adopt new coordination modes upon coordination to uranium. The PNP ligand can adopt both pseudo-meridional and pseudo-facial geometries when it is kappa(3)-(P,N,P) coordinated, depending on the steric demand at the uranium metal center. Additionally, its hemilabile character was demonstrated with an unusual K-2-(P,N) coordination mode that is maintained in both the solid-state and in solution. Comparison of the structures of the mono(PNP) and bis(PNP) complexes 7, 9, 11-13 with their respective C3Me5 analogues 1-4 undoubtedly show that a more sterically congested environment is provided by the PNP ligand. The electronic influence of replacing the C5Me5 ligands with PNP was investigated using electronic absorption spectroscopy and electrochemistry. For 12 and 13, a chemically reversible wave corresponding to the U-IV/U-III redox transformation comparable to that for 3 and 4 was observed. However, a similar to 350 mV shift of this couple to more negative potentials was observed on substitution of the bis(C5Me5) by the bis(PNP) framework, therefore pointing to a greater electronic density at the metal center in the PNP complexes. The UV-visible region of the electronic spectra for the mono(PNP) and bis(PNP) complexes appear to be dominated by PNP ligand-based transitions that are shifted to higher energy in the uranium complexes than in the simple ligand anion (6) spectrum, for both the U-VI and U-III oxidation states. The near IR region in complexes 1-4 and 7, 9, 11-13 is dominated by f-f transitions derived from the 5f(3) and 5f(2) valence electronic configuration of the metal center. Though complexes of both ligand sets exhibit similar intensities in their f-f bands, a somewhat larger ligand-field splitting was observed for the PNP system, consistent with its higher electon donating ability.