Actinide–Silicon Multiradical Bonding: Infrared Spectra and Electronic Structures of the Si(μ-X)AnF₃ (An = Th, U; X = H, F) Molecules

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 C_3v 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.