Bivalent, Trivalent, and Tetravalent Nickel Complexes with a Common Tridentate Deprotonated Pyridine Bis-Amide Ligand. Molecular Structures of Nickel(II) and Nickel(IV) and Redox Activity

Using a tridentate bis-amide ligand 2,6-bis[N-(phenyl)carbamoyl]pyridine (H2L), in its deprotonated form, nickel
complexes in three consecutive oxidation states [Et4N]2[NiIIL2]·H2O (1), [Et4N][NiIIIL2]·H2O (2), and [NiIVL2]·0.75H2O (3) have been prepared, and 1 and 3 structurally characterized. These X-ray structures represent first
crystallographically characterized NiN6 coordination sphere, with a common pyridine bis-amide ligand. Complex
1 crystallizes in the orthorhombic space group Pccn, a = 10.175(2) Å, b = 20.834(3) Å, c = 23.765(4) Å, Z =
4, and 3 crystallizes in the monoclinic space group P21/a, a = 14.874(7) Å, b = 13.300(4) Å, c = 16.604(5) Å,
β = 99.678(3)°, Z = 4. Considerable distortion is observed with the average distances being Ni−Namide 2.131(8)
Å and Ni−Npy 1.994(7) Å for 1 and Ni−Namide 1.946(8) Å and Ni−Npy 1.846(8) Å for 3. The observation of
short axial M−Npy and long equatorial M−Namide bonds (tetragonally compressed octahedral geometry) is caused
by the steric requirement of the ligand. Magnetic susceptibility measurements (63−300 K) reveal that the spin
states of nickel centers in 1 and 2 are S = 1 and S = 1/2, respectively. Complex 3 is diamagnetic. In their absorption
spectra (MeCN), 1 exhibits a d−d transition at 854 nm; 2 and 3 display LMCT transitions at 449 nm with a
shoulder at 636 nm and at 480 nm with a shoulder at 730 nm, respectively. The nickel(III) complex 2 exhibits
a rhombic EPR signal (g values:  2.149, 2.115, and 2.034), showing that the metal center is the primary residence
site of the unpaired electron. Cyclic voltammetric measurements of 1 in MeCN solution at a glassy carbon electrode
exhibit two chemically reversible (ip,a/ip,c ≈ 1) and electrochemically quasireversible (ΔEp = 100 mV) oxidative
responses:  a NiIII−NiII couple (E1/2 = 0.05 V vs SCE) and a NiIV−NiIII couple (E1/2 = 0.51 V vs SCE). A
one-electron chemical oxidation of yellowish brown 1 was achieved in a two-phase solvent mixture H2O−CH2Cl2 with [Fe(η5-C5H5)2][PF6], which led to the isolation of reddish brown 2. A two-electron chemical oxidation
of 1 was readily achieved in MeCN with ceric ammonium nitrate to afford dark violet crystals of 3. For 1 a linear
correlation between the NiIII−NiII reduction potentials and the reciprocal of solvent dielectric constants is obtained.