Synthesis, Properties, and Reactivity of Palladium and Nickel NHC Complexes Supported by Combinations of Allyl, Cyclopentadienyl, and Indenyl Ligands

The synthesis of a series of Pd and Ni complexes containing combinations of 2-methylallyl (C4H7), cyclopentadienyl (C5H5, Cp), and indenyl (C7H9, Ind) ligands is reported. In all cases these complexes are supported by the electron-donating N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr). The mixed Cp/2-methylallyl complexes (eta(1)-Cp)(eta(3)-2-methylallyl)Pd(IPr) (Pd-CpAlI) and (eta(5)-Cp)(eta(1)-2-methylallyl)Ni(IPr) (Ni-CpAlI) were synthesized through the reaction of IPr with (Cp)(2-methylallyl)M (M = Ni, Pd). The binding mode of the ligands is different in the two complexes, and as a result the total valence electron count around the metal is 18 for the Ni complex and only 16 for the Pd species. In the case of Pd, an analogue of Pd-CpAlI containing an indenyl ligand, (eta(1)-Ind)(eta(3)-2-methylallyl)Pd(IPr) (Pd-IndAlI), was synthesized through the reaction of (eta(3)-Ind)Pd(IPr)Cl (Pd-IndCl) with (2-methylallyl)magnesium chloride. The corresponding Ni complex (eta(5)-Ind)(eta(1)-2-methylallyl)Ni(IPr) (Ni-IndAlI) could not be isolated. The binding modes of the ligands in the mixed indenyl/Cp complexes (eta(1)-Ind)(eta(5)-Cp)M(IPr) (M = Ni (Ni-IndCp), Pd (Pd-IndCp)) were the same for both Ni and Pd. Pd-IndCp was prepared through the reaction of Pd-IndCl with NaCp, while Ni-IndCp was synthesized through the reaction of (eta(5)-Cp)Ni(IPr)Cl (Ni-CpCl) with lithium indenyl. Similarly, the structures of the bis(Cp) complexes (eta(5)-Cp)(eta(1)-Cp)Ni(IPr) (Ni-CpCp) and (eta(5)-Cp)(eta(1)-Cp)Pd(IPr) (Pd-CpCp) were identical for the two different metals. In contrast to Pd-CpCp, which is an 18-electron complex, the related bis(indenyl) Pd complex (eta(3)-Ind)(eta(1)-Ind)Pd(IPr) (Pd-IndInd) is a 16-electron species, while no Ni analogue of Pd-IndInd was characterized. Preliminary reactivity studies with electrophiles indicate that, in all systems with mixed ligands, the eta(1)-ligand is nucleophilic and reacts selectively. The complexes Pd-CpAlI, Ni-CpAlI, Pd-CpCp, Ni-CpCp, Pd-IndInd, Pd-IndAlI, and Pd-IndInd were characterized by X-ray crystallography.