Gauging the donor strength of iron(0) complexes via their N-heterocyclic carbene gold(I) adducts.

We isolate and characterize the gold(I)-iron(0) adducts [( i Pr 2 -bimy)Au-Fe(CO) 3 (PMe 3 ) 2 ][BAr F 4 ] and [Au-{Fe(CO) 3 (PMe 3 ) 2 } 2 ][BAr F 4 ] ( i Pr 2 -bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, BAr F 4 = tetrakis(pentafluorophenyl)borate). DFT analysis reveals that the gold-iron interaction in [( i Pr 2 -bimy)Au-Fe(CO) 3 (PMe 3 ) 2 ][BAr F 4 ] is predominantly a σ-donation from iron to gold. We further extend this class of compounds to include [( i Pr 2 -bimy)Au-Fe(CO) 3 (PR 3 ) 2 ][BAr F 4 ] (PR 3 = PPh 3 , PCy 3 , PCyPh 2 , PMePh 2 , PMe 2 Ph, P(4-C 6 H 4 F) 3 ) and [( i Pr 2 -bimy)Au-Fe(CO) 4 (PPh 3 )][BAr F 4 ] and correlate the i Pr 2 -bimy carbenic 13 C NMR signal with the relative donor strength of the iron(0) ligand. This approach allows for a fast and simple approach to gauge relative donor strength of Fe(0) donors.