Difference in the Reactivities of H- and Me-Substituted Dinucleating Bis(iminopyridine) Ligands with Nickel(0)

The reactivity of dinucleating bis(iminopyridine) ligands bearing H (L-1, (N,N')-1,1'-(1,4-phenylene)bis(N-(pyridin-2-ylmethylene)methanamine)) or Me substituents (L-2, (N,N')-1,1'-(1,4-phenylene)bis(N-(1-(pyridin-2-yl)ethylidene)methanamine)) on the imine carbon atom with Ni(COD)(2) (COD = 1,5-cyclooctadiene) has been investigated. Treatment of L-1 with 2 equiv of Ni(COD)2 forms dinuclear Ni-2(L-1)(COD)(2), whereas the reaction of L-2 with 2 equiv of Ni(COD)(2) leads to Ni-2(L-2)(2), along with 1 equiv of Ni(COD)(2). The compounds were characterized by H-1 and C-13 NMR spectroscopy, mass spectrometry, and elemental analysis; the structure of Ni-2(L-2)(2) was determined by XRD. Ni-2(L-2)(2) exists as syn and anti stereoisomers in the solid state and in solution. DFT calculations suggest Ni(I) for both Ni-2(L-1)(COD)(2) and Ni-2(L-2)(2), with the radical anion localized on one iminopyridine fragment in Ni-2(L-1)(COD)(2) and delocalized over two iminopyridine fragments in Ni-2(L-2)(2). Both Ni-2(L-1)(COD)(2) and Ni-2(L-2)(2) undergo a reaction with excess diphenylacetylene, forming diphenylacetylene complexes. However, whereas Ni-2(L-1)(diphenylacetylene)(2) decomposes upon removal of the excess diphenylacetylene, Ni-2(L-2)(2) demonstrates a reversible disassembly/reassembly sequence upon the addition/removal of diphenylacetylene.