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.