Syntheses and Structures of Cationic Bis(gold) Cyclopentadienyl Complexes

The reaction of (P)AuOTf [P = P(t-Bu)2o-biphenyl] with the cyclopentadienyl lithium reagents C5H5Li, C5H4MeLi, and C5HMe4Li forms the corresponding gold η1-cyclopentadienyl complexes (P)Au(η1-C5H5) (3a), (P)Au(η1-C5H4Me) (3b), and (P)Au(η1-C5HMe4) (3c), respectively, in >60% isolated yield. Treatment of complexes 3a or 3b with (P)AuNTf2 forms the corresponding cationic bis(gold) cyclopentadienyl complexes trans-{[(P)Au]2(η1,η1-cyclopentadien-1,3-yl)}+ NTf2– (4a) and trans-{[(P)Au]2(η1,η1-methylcyclopentadien-1,3-yl)}+ NTf2– (4b), respectively, in near-quantitative yield, which were characterized in solution and by X-ray crystallography. Both 4a and 4b undergo migration of the gold atoms about the cyclopentadienyl ring, which is fast on the NMR time scale at −80 °C. In the solid state, the gold atoms of 4a and 4b are positioned on the opposite faces of the cyclopentadienyl ring above and below the C1 and C3 carbon atoms. The reaction of 3c with (P)AuNTf2 similarly forms the cationic bis(gold) tetramethylcyclopentadienyl complex {[(P)Au]2(C5HMe4)}+ NTf2– (4c) although the structure of this complex remains obscure. The binding affinity of mono(gold) complexes 3a–3c toward exogenous (P)Au+ exceeds that of the corresponding unmetalated cyclopentadienes by more than 3 orders of magnitude. Protodeauration of bis(gold) complexes 4 with acetic acid at −80 °C is >1500 times slower than the protodeauration of the corresponding monogold complexes 3 under identical conditions.