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 (L1, (N,N′)-1,1′-(1,4-phenylene)­bis­(N-(pyridin-2-ylmethylene)­methanamine)) or Me substituents
(L2, (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 L1 with 2 equiv of Ni­(COD)2 forms dinuclear Ni2(L1)­(COD)2, whereas the reaction of L2 with 2 equiv of Ni­(COD)2 leads to Ni2(L2)2, along
with 1 equiv of Ni­(COD)2. The compounds were characterized
by 1H and 13C NMR spectroscopy, mass spectrometry,
and elemental analysis; the structure of Ni2(L2)2 was determined by XRD. Ni2(L2)2 exists as syn and anti stereoisomers in the solid state
and in solution. DFT calculations suggest Ni­(I) for both Ni2(L1)­(COD)2 and Ni2(L2)2, with the radical anion localized on one iminopyridine
fragment in Ni2(L1)­(COD)2 and delocalized
over two iminopyridine fragments in Ni2(L2)2. Both Ni2(L1)­(COD)2 and
Ni2(L2)2 undergo a reaction with
excess diphenylacetylene, forming diphenylacetylene complexes. However,
whereas Ni2(L1)­(diphenylacetylene)2 decomposes upon removal of the excess diphenylacetylene, Ni2(L2)2 demonstrates a reversible disassembly/reassembly
sequence upon the addition/removal of diphenylacetylene.