Vinyltrifluoroborate Complexes of Silver Supported by N-Heterocyclic Carbenes

Silver complexes involving vinyltrifluoroborate, (IPr)Ag(CH₂=CHBF₃) (4), [(IPr)₂Ag][CH₂=CHBF₃] (5), (SIPr)Ag(CH₂=CHBF₃) (6) and (IPr*)Ag(CH₂=CHBF₃) (7) have been synthesized by using (NHC)AgNO₃ [NHC = IPr (1), SIPr (2) and IPr* (3)] and K(CH₂=CHBF₃) in a metathesis process. NMR spectroscopic details of 1–7 and X-ray crystal structures of compounds 5–7 as well as that of a precursor (IPr)AgNO₃ are reported. Compounds 6 and 7 are more stable in solution compared to 4 (which forms 5) indicating a supporting ligand effect in the stability of monomeric (NHC)Ag(CH₂=CHBF₃). Calculated metal–ligand binding energies show that [CH₂=CHBF₃]⁻ is a significantly better ligand for Ag^I than CH₂=CH₂ or isoelectronic CH₂=CHCF₃. IPr, SIPr and IPr* carbenes coordinate to Ag(CH₂=CHBF₃) moiety forming silver adducts 4, 6 and 7, with binding energies of about –72.1, –72.1 and –83.8 kcal/mol, respectively. The C=C bond length of [CH₂=CHBF₃]⁻ (calculated value of 1.341 Å) increases upon coordination to the silver ion. Experimental data show a similar trend for structurally characterized 6 and 7. Electrostatic attraction dominates the bonding between the alkene ligand [C₂H₃BF₃]⁻ and the metal fragments [Ag(IPr)]⁺ and [Ag(SIPr)]⁺ in 4 and 6. The strongest orbital interaction involves σ-donation from the occupied π-orbital of the [C₂H₃BF₃]⁻ ligand to the vacant orbital of the metal fragment, while Ag^I→[C₂H₃BF₃]⁻ π-backbonding is relatively weak but not insignificant.