Synthesis, structure, DFT calculations, and full vibrational analysis of the prototypical thioformaldehyde complex mer-[W(CO)3(Me2PC2H4PMe2)(η2-SCH2)]

Abstract

Reaction of PPN[W(CO)₃(dmpe)(SH)] 2 (PPN = Ph₃PNPPh₃, dmpe = Me₂PC₂H₄PMe₂) with aqueous formaldehyde in the presence of trifluoroacetic acid gives the thioformaldehyde complex mer-[W(CO)₃(Me₂PC₂H₄PMe₂)(η²-SCH₂)] 3 in almost quantitative yield. The isotopomer mer-[W(CO)₃(Me₂PC₂H₄PMe₂)(η²-SCD₂)] 3-D₂2 was obtained analogously. 3 has a slightly distorted pentagonal-bipyramidal structure with one carbonyl group, two P atoms and the C and S atoms of the thioformaldehyde ligand spanning the pentagonal plane. In solution, 3 is in equilibrium with fac-[W(CO)₃(Me₂PC₂H₄PMe₂)(η²-SCH₂)] 4. The structures and energies of 3 and 4 and their rotamers 3′ and 4′ were calculated by DFT methods leading to a very good agreement with experimental data. FT-IR and FT-Raman spectra of 3 and 3-D₂2 were recorded and assigned with the aid of DFT calculations. Calculated vibrational amplitudes emphasize the extensive mixing of modes, particularly in the low-wavenumber region. The HOMO of 3 can approximately be described as a lone-pair at sulfur and is 0.38 eV higher in energy than that of isomer 4. The LUMOs of both isomers are highly delocalized with that of 4 being 0.45 eV lower in energy. On this basis it is expected that electrophiles will add preferentially at the sulfur atom of 3 while nucleophiles if at all will add to the thiocarbonyl carbon of 4.