Ligand Arm Length Governs Geometry, Spin State, and Reactivity in Unsymmetrical β-Diketiminato Nickel(II) and Nickel(I) Complexes

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Unsymmetrical N-aryl-N′-alkylpyridyl β-diketiminate ligands (HL1 and HL2) enable fine control over the coordination geometry, spin-state, and reactivity of nickel(II) and nickel(I) complexes through variation in pyridyl chelating arm length. Treatment of NiCl2(2,4-lutidine)2 with deprotonated ligands HL1 and HL2 in THF afforded the corresponding complexes L1NiIICl and L2NiIICl, respectively. X-ray crystallography reveals that L1NiIICl adopts a distorted square-planar geometry and is diamagnetic, whereas L2NiIICl exhibits the rare paramagnetic seesaw geometry due to the extended six-membered chelate ring. Reduction with KC8 yields the three-coordinate nickel(I) complexes L1NiI (T-shaped) and L2NiI (Y-shaped), with rhombic EPR signals (g1 = 2.288–2.349) reflecting ligand-modulated spin density. Most notably, L2NiIICl undergoes unprecedented slow activation of the β-diketiminate backbone in acetonitrile, producing the high-spin five-coordinate adduct [L2MeCNHNiCl][Cl], providing evidence for nitrile-induced ligand modification, a reactivity absent in the shorter-arm analogue L1NiIICl. In contrast, hydroxide coordination affords the mononuclear square-planar L1NiIIOH with a terminal OH group, whereas L2NiIICl forms the diamagnetic dimer L2Ni(μ–OH)2NiL2 with bridging hydroxides, demonstrating ligand-dependent nuclearity control. These results highlight how chelate ring size and ligand flexibility serve as powerful levers for tuning electronic structure, coordination geometry, spin multiplicity, and ligand-centered reactivity.