Vinyltrifluoroborate Complexes of Silver Supported by N‐Heterocyclic Carbenes

Abstract

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 AgI 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 AgI→[C2H3BF3]− π‐backbonding is relatively weak but not insignificant.