Difference in the Reactivities
of H- and Me-Substituted
Dinucleating Bis(iminopyridine) Ligands with Nickel(0)
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Abstract
The reactivity of dinucleating bis(iminopyridine) ligands
bearing
H (L<sup>1</sup>, (<i>N</i>,<i>N</i>′)-1,1′-(1,4-phenylene)bis(<i>N</i>-(pyridin-2-ylmethylene)methanamine)) or Me substituents
(L<sup>2</sup>, (<i>N</i>,<i>N</i>′)-1,1′-(1,4-phenylene)bis(<i>N</i>-(1-(pyridin-2-yl)ethylidene)methanamine)) on the imine
carbon atom with Ni(COD)<sub>2</sub> (COD = 1,5-cyclooctadiene) has
been investigated. Treatment of L<sup>1</sup> with 2 equiv of Ni(COD)<sub>2</sub> forms dinuclear Ni<sub>2</sub>(L<sup>1</sup>)(COD)<sub>2</sub>, whereas the reaction of L<sup>2</sup> with 2 equiv of Ni(COD)<sub>2</sub> leads to Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub>, along
with 1 equiv of Ni(COD)<sub>2</sub>. The compounds were characterized
by <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy, mass spectrometry,
and elemental analysis; the structure of Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub> was determined by XRD. Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub> exists as syn and anti stereoisomers in the solid state
and in solution. DFT calculations suggest Ni(I) for both Ni<sub>2</sub>(L<sup>1</sup>)(COD)<sub>2</sub> and Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub>, with the radical anion localized on one iminopyridine
fragment in Ni<sub>2</sub>(L<sup>1</sup>)(COD)<sub>2</sub> and delocalized
over two iminopyridine fragments in Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub>. Both Ni<sub>2</sub>(L<sup>1</sup>)(COD)<sub>2</sub> and
Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub> undergo a reaction with
excess diphenylacetylene, forming diphenylacetylene complexes. However,
whereas Ni<sub>2</sub>(L<sup>1</sup>)(diphenylacetylene)<sub>2</sub> decomposes upon removal of the excess diphenylacetylene, Ni<sub>2</sub>(L<sup>2</sup>)<sub>2</sub> demonstrates a reversible disassembly/reassembly
sequence upon the addition/removal of diphenylacetylene
