Steric and Electronic Effects in the Formation and Carbon Disulfide Reactivity of Dinuclear Nickel Complexes Supported by Bis(iminopyridine) Ligands

Abstract

We are developing bimetallic platforms for the cooperative activation of heteroallenes. Toward this goal, we designed a new family of bis(iminopyridine) ((N,N′-1,1′-(1,4-phenylene)bis(N-(pyridin-2-ylmethylene)methanamine) and N,N′-1,1′-(1,4-phenylene)bis(N-(1-(pyridin-2-yl)ethylidene)methanamine)) dinickel complexes, synthesized their CS2 compounds, and studied their reactivity. Bis(iminopyridine) ligands L react with Ni(COD)2 to form Ni2(L)2 complexes or Ni2(L)(COD)2 complexes as a function of the steric and electronic properties of the ligand precursor. Product structures disclosed an anti geometry in the Ni2(L)(COD)2 species and helical (anti) structures for Ni2(L)2 complexes. Carbon disulfide adducts Ni2(L)(CS2)2 were obtained in good yields upon addition of CS2 to Ni2(L)(COD)2 or in a one-pot reaction of L with 2 equiv of both Ni(COD)2 and CS2. Ni2(L)(CS2)2 complexes are highly flexible, displaying both syn and anti conformations (shortest S- - -S separations of 5.0 and 9.5 Å, respectively) in the solid state. DFT calculations demonstrate virtually no energy difference between the two conformations. Electrochemical studies of the Ni2(L)(CS2)2 complexes displayed two ligand-based reductions and a broad CS2-based oxidation. Chemical oxidation with [FeCp2]+ liberated free CS2. The addition of NHC (NHC = 1,3-di-tert-butylimidazolin-2-ylidene) to Ni2(L)(CS2)2 yielded Ni2(NHC)2(CS2)2, in which both carbon disulfide ligands are bridging two Ni centers.