Inorganic Chemistry, Vol.57, No.7, 3699-3712, 2018
Pseudohalide Tectons within the Coordination Sphere of the Uranyl Ion: Experimental and Theoretical Study of C-H center dot center dot center dot O, C-H center dot center dot center dot S, and Chalcogenide Noncovalent Interactions
A series of uranyl thiocyanate and selenocyanate of the type [R4N](3)[UO2(NCS)(5)] (R-4 = Bu-n(4), Me(3)Bz, Et(3)Bz), [Ph4P][UO2(NCS)(3)(NO3)] and [R4N](3)[UO2(NCSe)(5)] (R-4 = Me-4, Pr-n(4), Et(3)Bz) have been prepared and structurally characterized. The resulting noncovalent interactions have been examined and compared to other examples in the literature. The nature of these interactions is determined by the cation so that when the alkyl groups are small, chalcogenide center dot center dot center dot chalcogenide interactions are present, but this "switches off" when R = Pr-n and charge assisted U=O center dot center dot center dot H-C and S(e)center dot center dot center dot H-C hydrogen bonding remain the dominant interaction. Increasing the size of the chain to Bu-n results in only S center dot center dot center dot H-C interactions. The spectroscopic implications of these chalcogenide interactions have been explored in the vibrational and photophysical properties of the series [R4N](3)[UO2(NCS)(5)] (R-4 = Me-4, Et-4, Pr-n(4), Bu-n(4), Me(3)Bz, Et(3)Bz), [R4N](3)[UO2(NCSe)(5)] (R-4 = Me-4, Pr-n(4), Et(3)Bz) and [Et4N](4)[UO2(NCSe)(5)]-[NCSe]. The data suggest that U=O center dot center dot center dot H-C interactions are weak and do not perturb the uranyl moiety. While the chalcogenide interactions do not influence the photophysical properties, a coupling of the U=O and delta(NCS) or delta(NCSe) vibrational modes is observed in the 77 K solid state emission spectra. A theoretical examination of representative examples of Se center dot center dot center dot Se, C-H center dot center dot center dot Se, and C-H center dot center dot center dot O=U by molecular electrostatic potentials and NBO and AIM methodologies gives a deeper understanding of these weak interactions. C-H center dot center dot center dot Se are individually weak but C-H center dot center dot center dot O=U interactions are even weaker, supporting the idea that the -yl oxo's are weak Lewis bases. An Atoms in Molecules study suggests that the chalcogenide interaction is similar to lone pair center dot center dot center dot pi or fluorine center dot center dot center dot fluorine interactions. An oxidation of the NCS ligands to form [(UO2)(SO4)(2)(H2O)(4)]center dot 3H(2)O was also noted.