Lanthanide(III) and Actinide(III) Complexes [M(BH4)2(THF)5][BPh4] and [M(BH4)2(18-crown-6)][BPh4] (M = Nd, Ce, U): Synthesis, Crystal Structure, and Density Functional Theory Investigation of the Covalent Contribution to Metal-Borohydride Bonding

Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3·C4H8S, 4·THF, 5, and 6·THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7·C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U···B with respect to the Ce···B distance, which is accompanied by a lengthening of the U−Hb bonds and an opening of the Hb−B−Hb angle (Hb = bridging hydrogen atom of the η3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 → M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M−Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B−H−U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the π bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.