title: Actinide-silicon multiradical bonding: infrared spectra and electronic structures of the Si(μ-X)AnF3 (An = Th, U; X = H, F) molecules.
abstract: We report a series of Si(μ-X)AnF3 (An = Th, U; X = H, F) complexes with silicon-actinide(IV) single bonds and unexpected multiradical features that form rare triplet silylenes. These bridged molecules have been prepared in microscopic scale through reactions of laser-ablated uranium and thorium atoms with silicon fluorides and identified from infrared spectra in argon and neon matrixes and relativistic quantum chemical calculations. Similar neon matrix experiments for the reactions of uranium with CF4 and CHF3 were carried out for comparison. Our density functional theory calculations show that the Si-U single-bonded species Si(μ-X)UF3 (X = H, F) with U(IV) oxidation state and the quasi-agostic bridge ligand of H or F are most stable among all the isomers, whereas the naively anticipated triple-bonded species XSi≡UF3 with U(VI) oxidation state and the double-bonded species XSi(•)═(•)UF3 with U(V) oxidation state lie markedly higher in energy. Similar thorium products from reactions with XSiF3 are also found to prefer the Si(μ-X)ThF3 structures with Si-Th single bonds and bridged H or F ligands. High level ab initio wave function theory calculations with the CCSD(T) and CASPT2 methods confirm that the ground states are quintet for Si(μ-X)UF3 and triplet for Si(μ-X)ThF3 with two unpaired electrons on the silylene group. These silicon-bearing molecules as the lowest-energy isomer of XSiAnF3 represent the first silicon-actinide systems with unusual "triplet" silylenes and Si-An single bonds with multiradical character. They are in dramatic contrast to the uranium-carbon analogs, XC≡UF3, which form triple-bonded singlet ground states with C3v symmetry. The calculated vibrational frequencies of the Si(μ-X)AnF3 complexes agree well with experimental observations. These results accentuate the critical difference of chemical bonding of 3p- and 2p-row main-group elements with actinides. The Lewis electron-pair model and the octet rule break down for these silicon compounds.
authors.affiliation: Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University , Beijing 100084, China.
authors.forename: Han-Shi
authors.initials: HS
authors.lastname: Hu
authors.forename: Fan
authors.initials: F
authors.lastname: Wei
authors.forename: Xuefeng
authors.initials: X
authors.lastname: Wang
authors.forename: Lester
authors.initials: L
authors.lastname: Andrews
authors.forename: Jun
authors.initials: J
authors.lastname: Li