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

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.