Nickel(I) monomers and dimers with cyclopentadienyl and indenyl ligands

The reaction of (μ-Cl)2Ni2(NHC)2 (NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr) or 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported Ni(I) dimers of the form (μ-Cp)(μ-Cl)Ni2(NHC)2 (NHC = IPr (1 a) or SIPr (1 b); Cp = C5H5) or (μ-Ind)(μ-Cl)Ni2(NHC)2 (NHC = IPr (2 a) or SIPr (2 b); Ind = C7H9), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ-Cl)2Ni2(NHC)2 (NHC = IPr or SIPr) generates unusual 17 valence electron Ni(I) monomers of the form (η(5)-Cp)Ni(NHC) (NHC = IPr (3 a) or SIPr (3 b)) or (η(5)-Ind)Ni(NHC) (NHC = IPr (4 a) or SIPr (4 b)), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the Ni(I) monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired-single-electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ-Cp)(μ-Cl)Ni2(NHC)2 undergoes homolytic cleavage of the Ni-Ni bond and is in equilibrium with (η(5)-Cp)Ni(NHC) and (μ-Cl)2Ni2(NHC)2. There is no evidence that this equilibrium occurs for (μ-Ind)(μ-Cl)Ni2(NHC)2. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the Ni(I) monomers (η(5)-Cp)Ni(NHC) or (η(5)-Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active Ni(I) precatalysts for the Suzuki-Miyaura reaction. Complexes 1 a, 2 b, 3 a, 4 a and 4 b have been characterized by X-ray crystallography.