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
(η1-Cp)­(η3-2-methylallyl)­Pd­(IPr)
(CpAllPd) and (η5-Cp)­(η1-2-methylallyl)­Ni­(IPr) (CpAllNi) 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 CpAllPd containing an indenyl ligand, (η1-Ind)­(η3-2-methylallyl)­Pd­(IPr) (IndAllPd), was synthesized through the reaction of (η3-Ind)­Pd­(IPr)Cl (IndClPd) with (2-methylallyl)magnesium chloride. The corresponding Ni complex
(η5-Ind)­(η1-2-methylallyl)­Ni­(IPr)
(IndAllNi) could not be isolated.
The binding modes of the ligands in the mixed indenyl/Cp complexes
(η1-Ind)­(η5-Cp)­M­(IPr) (M = Ni (IndCpNi), Pd (IndCpPd)) were the same for both Ni and Pd. IndCpPd was prepared through the reaction
of IndClPd with NaCp, while IndCpNi was synthesized through the
reaction of (η5-Cp)­Ni­(IPr)Cl (CpClNi) with lithium indenyl. Similarly, the structures
of the bis­(Cp) complexes (η5-Cp)­(η1-Cp)­Ni­(IPr) (CpCpNi) and (η5-Cp)­(η1-Cp)­Pd­(IPr) (CpCpPd) were identical for the two different metals.
In contrast to CpCpPd, which
is an 18-electron complex, the related bis­(indenyl) Pd complex (η3-Ind)­(η1-Ind)­Pd­(IPr) (IndIndPd) is a 16-electron species, while no Ni analogue
of IndIndPd was characterized.
Preliminary reactivity studies with electrophiles indicate that, in
all systems with mixed ligands, the η1-ligand is
nucleophilic and reacts selectively. The complexes CpAllPd, CpAllNi, CpCpPd, CpCpNi, IndClPd, IndAllPd, and IndIndPd were characterized by X-ray crystallography.