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

Abstract

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[Ni^IIL₂]·H₂O (1), [Et₄N][Ni^IIIL₂]·H₂O (2), and [Ni^IVL₂]·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, α = 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-N_amide 2.131(8) Å and Ni-N_py 1.994(7) Å for 1 and Ni-N_amide 1.946(8) Å and Ni-N_py 1.846(8) Å for 3. The observation of short axial M-N_py and long equatorial M-N_amide 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 =½, 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 (i_p,a/i_p,c ≈ 1) and electrochemically quasireversible (ΔE_p = 100 mV) oxidative responses: a Ni^III-Ni^II couple (E_½ = 0.05 V vs SCE) and a Ni^IV-Ni^III couple (E_½ = 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 Ni^III-Ni^II reduction potentials and the reciprocal of solvent dielectric constants is obtained.