Experimental Evaluation of (L)Au Electron-Donor Ability in Cationic Gold Carbene Complexes

²⁹ Si NMR spectroscopy was employed to evaluate the electron donor properties of the (L)Au fragments in the cationic gold (β,β-disilyl)vinylidene complexes [(L)Au=C=CSi(Me)₂ CH₂ CH₂ Si(Me)₂ ]⁺ B(C₆ F₅ )₄^- [L=P(tBu)₂ o-biphenyl or NHC] relative to the p-substituted aryl group in the α-aryl-(β,β-disilyl)vinyl cations [(p-C₆ H₄ X)-C= CSi(Me)₂ CH₂ CH₂ Si(Me)₂ ]⁺ B(C₆ F₅ )₄^- . Similarly, ¹⁹ F NMR was employed to evaluate the σ- and π-electron donor properties of the (L)Au fragments in the neutral gold fluorophenyl complexes (L)Au(C₆ H₄ F) and in the cationic (fluorophenyl)methoxycarbene complexes [(L)AuC(OMe)(C₆ H₄ F)]⁺ SbF₆^- [L=P(tBu)₂ o-biphenyl or IPr] relative to the p-substituted aryl group of the protonated monofluorobenzophenones [(p-C₆ H₄ X)(C₆ H₄ F)COH]⁺ OTf^- . The results of these studies indicate that relative to p-substituted aryl groups, the gold (L)Au fragments [L=P(tBu)₂ o-biphenyl or NHC] are significantly more inductively electron donating and are comparable π-donors and for this reason, the extent of (L)Au→C1 electron donation in gold carbene complexes appears to exceed that provided by a p-(dimethyamino)phenyl group. Furthermore, the [L=P(tBu)₂ o-biphenyl]Au fragment is a nominally stronger electron donor than the (IPr)Au fragment, and both are significantly more inductively electron donating than the (PPh₃ )Au and [P(OMe)₃ ]Au fragments.